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Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

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Review of Hybrid Ion Capacitors: From Aqueous to Lithium to Sodium Jia Ding,*,† Wenbin Hu,‡ Eunsu Paek,§ and David Mitlin*,§ †

Chemistry and Materials, State University of New York, Binghamton, New York 13902, United States Key Laboratory of Advanced Ceramics and Machining Technology (Ministry of Education), School of Material Science and Engineering, Tianjin University, Tianjin 300072, China § Chemical & Biomolecular Engineering and Mechanical Engineering, Clarkson University, Potsdam, New York 13699, United States Downloaded via KAOHSIUNG MEDICAL UNIV on June 29, 2018 at 11:23:58 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: In this critical Review we focus on the evolution of the hybrid ion capacitor (HIC) from its early embodiments to its modern form, focusing on the key outstanding scientific and technological questions that necessitate further in-depth study. It may be argued that HICs began as aqueous systems, based on a Faradaic oxide positive electrode (e.g., Co3O4, RuOx) and an activated carbon ion-adsorption negative electrode. In these early embodiments HICs were meant to compete directly with electrical double layer capacitors (EDLCs), rather than with the much higher energy secondary batteries. The HIC design then evolved to be based on a wide voltage (∼4.2 V) carbonate-based battery electrolyte, using an insertion titanium oxide compound anode (Li4Ti5O12, LixTi5O12) versus a Li ion adsorption porous carbon cathode. The modern Na and Li architectures contain a diverse range of nanostructured materials in both electrodes, including TiO2, Li7Ti5O12, Li4Ti5O12, Na6LiTi5O12, Na2Ti3O7, graphene, hard carbon, soft carbon, graphite, carbon nanosheets, pseudocapacitor T-Nb2O5, V2O5, MXene, conversion compounds MoS2, VN, MnO, and Fe2O3/Fe3O4, cathodes based on Na3V2(PO4)3, NaTi2(PO4)3, sodium super ionic conductor (NASICON), etc. The Ragone chart characteristics of HIC devices critically depend on their anode−cathode architectures. Combining electrodes with the flattest capacity versus voltage characteristics, and the largest total voltage window, yields superior energy. Unfortunately “flat voltage” materials undergo significant volume expansion/contraction during cycling and are frequently lifetime limited. Overall more research on HIC cathodes is needed; apart from high surface area carbon, very few positive electrodes demonstrate the necessary 10 000 or 100 000 plus cycle life. It remains to be determined whether its lithium ion capacitors (LICs) or sodium ion capacitors (NICs) are superior in terms of energy−power and cyclability. We discuss unresolved issues, including poorly understood fast-charge storage mechanisms, prelithiation and presodiation, solid electrolyte interface (SEI) formation, and high-rate metal plating.

CONTENTS 1. Introduction 2. Hybrid Ion Capacitors: Motivation 3. History: HICs As an Outgrowth of Asymmetric Aqueous Ultracapacitors 4. HICs As 4.2 V Carbonate Electrolyte-Based Devices 5. Next-Generation HICs: “Parallel” Electrodes through Nanostructuring 6. Unresolved Issues: SEI and CEI Formation and Metal Plating 7. Motivation for Sodium Ion Capacitors 8. Prelithiation and Presodiation 9. LIC and NIC Electrode Materials: Architectures and Performance 9.1. Titanium Oxide Compound-Based Li Anodes 9.2. Titanium Oxide Compound-Based Na Anodes 9.3. Architectures Employing Titanium Compounds 9.4. Carbons for Li Electrodes © XXXX American Chemical Society

9.5. Carbons for Na Electrodes 9.6. All Carbon Architectures 9.7. Emerging Architectures: Pseudocapacitive Oxides 9.8. Emerging Architectures: MXenes 9.9. Emerging Architectures: Conversion Compounds 9.10. Emerging Architectures: Battery-Related Intercalation Ceramics 10. Ragone Chart Comparisons and Concluding Thoughts Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments References

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Received: February 22, 2018

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DOI: 10.1021/acs.chemrev.8b00116 Chem. Rev. XXXX, XXX, XXX−XXX

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1. INTRODUCTION The Review will discuss both lithium ion capacitors (LICs) and sodium ion capacitors (NICs), tracing the scientific evolution from the former to the latter. We assume that the readers are versed in ultracapacitor and secondary ion battery (LIB, NIB, and SIB) literature, which in our opinion is a prerequisite to being productive in the HIC field. The concept of a hybridized device is rooted in an aqueous asymmetrical ultracapacitor, where the negative electrode is an inert activated carbon while the positive electrode is a surface redox-active oxide. Such systems were designed to compete with electrical double layer (EDLC) ultracapacitors and were not envisioned to be a substitute for the much higher energy but lower power secondary batteries. We strongly argue that this historical view is indeed correct: Given the performance advantages and limitations of HICs, their technological niche lies in ultracapacitor-like high power−high cyclability applications, but where 3−4 times higher energy versus an ultracap warrants the additional device complexity and reduced cycling life. In section 2, we detail this point, providing numerical performance comparisons between conventional LIBs, ultracapacitors, and emerging HIC devices. We also identify applications where HICs may excel over either ultracaps or LIBs and discuss why we believe that HICs are a competitor only to the former. In section 3, we go through some of the overlooked history, tracing the origin of HICs to the classic Conway and coworkers concepts of asymmetric aqueous ultracapacitor devices equipped with a redox oxide positive electrode. In section 4, we discuss the first modern embodiment of a HIC, which was a lithium ion capacitor (LIC) based on a Li4Ti5O12 intercalation material anode and an activated carbon cathode. This concepta high-voltage battery carbonate electrolyte, an ion insertion anode, and an ion adsorption cathodehas largely remained since then, with subsequent rate improvements being achieved for the anode though nanostructuring. Section 5 addresses these performance improvements explicitly. We argue that one may view the transition to “intrinsically parallel”, i.e., high-energy and high-power dual-material electrodes, as a natural evolution of the ongoing drive to overcome the solid-state diffusional limitation intrinsic of bulk ion storage. Section 6 concerns a topic that in our opinion has not received sufficient attention. HIC devices are supposed to survive 100 000 cycles or even more, so the Coulombic efficiency (CE) is of the essence. Even a 0.01% loss per cycle is unacceptable. Moreover, HICs typically possess more limited supplies of Li or Na ions than conventional LIBs, making the solid electrolyte interface (SEI)-induced CE loss very problematic. Likewise it is absolutely critical that Li or Na metal plating does not occur, because the danger of a catastrophic dendrite event may be much higher than with LIBs, due to the order(s) of magnitude higher cycle number and charge rates. Section 7 provides the rationale for the new and intense focus on Na-based technology. The rationale may be viewed as 2-fold: First, the long-term economics and geographic availability of Na are more favorable than with Li. Second, there is growing evidence that Na has lower propensity than Li to metallically plate into nanopores at low voltages. This has the potential to make the NIC system much safer than the LIC system in terms of avoiding cycle-induced dendrite formation. Section 8 covers an important albeit overlooked issue in HIC

literature. Unlike a standard lithium or sodium ion battery, where the charge carrier ions are stored in the oxide-based cathode, Li or Na ions are not intrinsic to the cathode (or the anode). Hence, either the device relies exclusively on the ions within the electrolyte (like a conventional ultracapacitor) or there is a secondary process to introduce the ions into the cathode and/or anode. Broadly speaking, the two key approaches are based on either introducing a separate lithiation/sodiation step into the electrode fabrication process or adding a secondary phase that is the source of ions into one or both electrodes. Section 9 discusses the wide range of electrode materials and architectures that are rapidly appearing in the scientific literature. Here we provide some critical perspectives regarding their overall prospects as viewed from a voltage, cyclability, and rate-capability perspective. We provide extensive comparisons and an associated discussion of the rate capability of electrode materials, as well as of energy−power characteristics of devices. Section 9.1 is titanium oxide compound based Li anodes; section 9.2 is titanium oxide compound based Na anodes; section 9.3 is architectures employing titanium compounds; section 9.4 is hard carbons for Li anodes; section 9.5 is hard carbons for Na anodes; section 9.6 is all carbon architectures; section 9.7 is pseudocapacitive oxides; section 9.8 is MXenes; section 9.9 is conversion compounds; and section 9.10 is intercalation compounds. Section 10 places these comparisons in graphical format, providing a series of quantitative Ragone charts from the compiled literature data. We also provide a concluding outlook, pointing out the important yet insufficiently explored topics in the fundamental science of HIC materials. We hope that this Review may be useful to the energy-storage community as a roadmap regarding the past, the present, and the future of highly scientifically exciting and potentially technologically game-changing hybridized ion capacitors. Concerning the electrode-labeling convention employed in this Review: In a symmetric carbon−carbon device there are no anode and cathode per se because the thermodynamic state of the two electrodes is identical. This is distinct from a ceramic cathode versus lithium metal half-cell, where the ceramic possesses an intrinsically higher redox stability and is at a positive voltage relative to Li/Li+ (ΔG = −nFE). When an ultracapacitor is charged, one electrode is negatively polarized whereas the opposite one is positively polarized, with the two voltage excursions being equal and opposite for the case of a symmetric device. In this Review we will employ the terms “anode−cathode” and “positive electrode−negative electrode” interchangeably, agreeing with what is done in the ultracap and HIC communities. However, we recognize that, for symmetric cells, the terms positive electrode−negative electrode are more accurate, because the polarity of the test cell may be reversed while obtaining in principle the same electrochemical response.

2. HYBRID ION CAPACITORS: MOTIVATION Electrical energy-storage systems play a crucial role in consumer electronics, automotive, aerospace, and stationary markets. There are primarily two types of devices for reversible electrochemical energy storage, secondary batteries, and electrochemical capacitors (ultracapacitors and supercapacitors). The former offers a high energy density, while the latter offers high power and high cyclability. The dominant current technology is the lithium ion battery (LIB), which is based on a Li-containing ceramic oxide cathode and a graphite anode. B


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Figure 1. Schematic illustration of wayside regenerative braking energy storage for subway trains.8 Regen braking may significantly benefit from hybrid ion capacitor (HIC) devices due their up to 4 times higher energy density as compared to conventional ultracapacitors. Reprinted with permission from ref 8. Copyright 2013 Elsevier Ltd.

Figure 2. (A) Performance of a Li−based hybrid ion capacitor (HIC), as illustrated by a Ragone chart. The HIC is based on MnO on carbon nanosheet anode and a nanosheet cathode and is labeled “3D-MnO/CNS||3D-CNS”.11 (B) Representative charge−discharge curves for a HIC device, with differing anode-to-cathode mass ratios. (A, B) Reproduced with permission from ref 11. Copyright 2014 American Chemical Society.

cost, i.e., kWh/$. Conversely, LIBs do not possess sufficient power to harvest even a notable fraction of the stopping energy, which requires a charging rate as high as 200C (1C is 1 h charge, 200C is 18 s charge). Hybrid ion capacitors (HICs) represent an emerging class of devices that may bridge the performance of commercial EDLC ultracapacitors and conventional ion batteries.12−15 In our opinion, a HIC will never compete with lithium or sodium ion batteries (LIBs or NIBs) in terms of energy per weight or per volume. Hence, it will not displace a LIB or a NIB when energy density or specific energy is key. As indicated, sodium- or lithium-based ion capacitors (HICs and LICs) are capable of yielding energy values 4 or 5 times higher than EDLC ultracapacitors. This superior energy is achieved by relying on a carbonate-based battery electrolyte that gives a higher device voltage than standard ultracap acetonitrile (∼4.2 vs ∼2.7 V) and having at least one of the electrodes store charge by bulk mechanisms. Hence, HICs appear suited to compete head on with electrochemical ultracapacitor technology. The performance of HIC is illustrated by a Ragone chart shown in Figure 2A, which shows a typical energy−power performance of a carbon nanosheet (CNS)-based hybrid ion capacitor (HIC), labeled “3D-MnO/CNS||3D-CNS”.11 As may be observed, such a device operates in the 25−50 Wh/kg specific energy range (roughly 1/10th to 1/5th of the state-of-the-art LIB), while delivering specific powers in the 1 000−10 000 W/kg range (5× to 50× that of a LIB). Although researchers have viewed HICs to represent the extreme end of high-power secondary ion batteries, their voltage profiles are not analogous to conventional batteries even when batteries are operated at maximum rate. As Figure 2B illustrates, the voltage versus

For instance, commercial Panasonic lithium ion batteries (LIBs) deliver a specific energy upward of 200 Wh kg−1, but with a maximum specific power being below 350 W kg−1. By contrast, most commercial electrochemical capacitors possess specific power values as high as 10 kW kg−1, but with specific energies in the 5 Wh kg−1 range. An emerging target for an advanced electrical energy-storage devices is to deliver both high energy and high power in a single system.1−7 A hybrid ion capacitor (HIC) is a relatively new device that is intermediate in energy between batteries and supercapacitors, while in principle offering supercapacitor-like power and cyclability values. One important potential end-use of HIC devices is in regenerative braking. Regenerative braking energy harvesting from trains, heavy automotive, and ultimately light vehicles represents a huge potential market that remains not fully exploited due to the limitations of existing secondary battery and supercapacitor (electrochemical capacitor and ultracapacitor) technologies. For instance, The New York City (NYC) Metropolitan Transportation Authority (MTA) uses ∼2 150 GWh of energy per year for train locomotion, with a total cost of more than $250 000 000 per year.9 On-board regenerative braking, as illustrated in Figure 1, is a potential path for recovering a major fraction of that energy.8 A recent highly publicized study by Dayton T. Brown concluded that neither battery nor supercapacitor solutions are able to meet the requirements, due to energy density limitations, heat dissipation problems, and overall implementation cost.10 Supercapacitors do not possess the intrinsic energy-storage density required for economical and space-efficient deployment. In effect they are too bulky by weight, by volume, or by C


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Figure 3. (A) Scanning elecron microscopy (SEM), conventional transmission electron microscopy (TEM), and high-resolution TEM micrographs of graphene−nickel cobaltite (NiCo2O4) positive electrodes labeled GNCC-250 and GNCC-350.16 (B) Galvanostatic charging−discharging profiles of the asymmetric capacitor with different negative electrode (activated carbon (AC)) to positive electrode mass ratios, tested in 6 M KOH at a high current of 1 A g−1.16 (C) Ragone plots related to energy density and power density of GNCC//AC asymmetric capacitor with various mass ratios, GNCC//GNCC and AC//AC symmetric capacitors.16 (A−C) Reproduced with permission from ref 16. Copyright 2012 Springer Nature.

process could vary too and may include reversible ion intercalation, adsorption at bulk defects including heteroatoms, bulk conversion reactions, bulk alloying reactions, bulk formation of intermediate compounds, etc. The core premise, however, is to combine a bulk electrode and a surface electrode. Yet at high rates and with modern highly nanostructured materials, surface and bulk contributions begin to blur, especially in the anode. Figure 3 shows the microstructure, charge−discharge curves, and Ragone chart characteristics of an asymmetric ultracapacitor based on an activated carbon negative electrode and a graphene−nickel cobaltite (NiCo2O4) positive electrode.16 It may be observed that, apart from the lower voltage window required by an aqueous (6 M KOH) electrolyte, the galvanostatic performance of such asymmetric devices is much closer to that of HICs than it is to conventional EDLC capacitors. For a pure EDLC system, by definition there is a linear voltage versus capacity behavior on both charging and discharging. The charge−discharge curve must then be triangular, with the ΔQ/ΔV being constant at all voltages. In an ideal case, the Coulombic efficiency (CE) of an EDLC device is 100%, and the charge−discharge curves are fully symmetric both in shape and in time to charge versus time to discharge. Except in rare cases such as for MnOx (to be discussed shortly), faradic systems will have notable deviation from linearity and an asymmetry between charge and discharge. In addition to the intrinsic nonlinearity of the redox processes, asymmetric reduction−oxidation overpotentials and parasitic side reactions will contribute to the nonEDLC like behavior.

capacity profile of HICs is closer to that of a classical ultracapacitor, i.e., nearly triangular without obvious plateaus. The performance matrices and design principles for commercial ultracapacitors are well-established. Conversely, scientific literature-based HIC performance standards are often a combination of seemingly arbitrary battery and ultracapacitor metrics, e.g., 5 000−10 000 cycles at full charge−discharge. This is not optimum, as compromising certain ultracapacitor criteria may render hybrid devices uncompetitive for most applications. For example, a nonoptimum compromise is choosing a target cycling lifetime value intermediate between long-cycle lithium ion batteries (∼2 000) and ultracaps (>100 000). While 5 000 cycles is a very impressive lifetime for a LIB, it is far below any targeted application of an ultracapacitor that may continuously cycle at tens, hundreds, or even thousands of charges per day. Hence, the first goal of this Review is to provide an argument that hybrid ion devices, be it Na- or Li-based, should be judged according to ultracapacitor criteria.

3. HISTORY: HICS AS AN OUTGROWTH OF ASYMMETRIC AQUEOUS ULTRACAPACITORS The core of a classic hybrid ion capacitor (HIC) is the concept that an electrode that stores charge based on reversible surface adsorption of ions may be combined in a series with an electrode that stores charge by a bulk process. The nature of the surface charge storage process may vary, being one or a combination of pure capacitive EDLCs, reversible ion adsorption at defects/heteroatoms, reversible surface redox reactions, or even reversible surface plating of metallic nanoparticles. Likewise, the nature of the bulk ion storage D


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strictly speaking pseudocapacitive in their galvanostatic profiles or their CVs.

Early versions of a hybridized bulk−surface device was put forth by Conway and co-workers,17,18 using an acidic aqueous electrolyte and a ruthenium oxide-based positive electrode. Although modern HIC devices exclusively employ organic electrolytes with up to 3.5 V windows, the early sulfuric acid (etc.) based conceptions captured the core of the process. Unlike many other redox-active oxides that work primarily through changing valence states at or near their surface, ruthenium actually reversibly intercalates H+ ions into its bulk. This results in a much higher capacitance/capacity for a given particle size but high rate limitations associated with solid-state diffusion. Other redox-active systems such as the cobaltites have also been shown to be bulk redox-active as well.19 Through neutron reflectometry it has been confirmed that in an aqueous electrolyte cobaltite undergoes reversible bulk redox reactions in addition to surface reactions.19 This may be also confirmed by considering the capacity and capacitance of unsupported cobaltite. For example, spherical nanoparticles of Co3O4 with an average diameter of 8 nm were reported to have a specific capacitance of 1100 F/g in a 0.8 V window. While 8 nm appears small, the resultant surface area is actually much lower than for an activated carbon: A 8 nm particle will have a volume of 2.68 × 10−25 m3, a surface area of 2 × 10−16 m2, and a weight of 1.64 × 10−18 g (density of Cobalt(II, III) oxide is 6.11 × 106 g/m3). This results in a specific surface area of 122 m2/g, which is far too low to have an appreciable EDLC contribution to charge storage. The theoretical specific capacitance of Co3O4 is 3 × 96 500 Coul mol−1/240.8 g mol−1/0.8 V = 1 503 F/g, which assumes every cobalt ion in the lattice undergoes a single electron redox reaction. A measured specific capacitance of 1100 F/g in 0.8 V swing, with a surface area of 122 m2/g, clearly indicates that much of the bulk Co3+/Co2+ undergoes redox transitions too. Therefore, one may strongly argue that many aqueous asymmetric supercapacitors, starting with systems based on RuOx and continuing to the modern alloy cobaltites,20−23 are in effect HICs as well because one electrode stores charge by bulk processes.17,18,24−26 A 2012 review by Cericola and Kötz provided an in-depth discussion of aqueous asymmetrical systems, and readers are encouraged to review that work for more detail.27 Other interesting redox-active cathode aqueous capacitors include activated carbon−phosphotungstate and polyoxometalate−carbon architectures (2012).28−30 These offer promising energy−power characteristics and should be explored further. It should also be emphasized that this surface versus bulk distinction is different from the distinction between true pseudocapacitive oxides versus oxides that generally store charge by reversible faradic surface reactions. There has been an excellent publication recently dealing with that specific issue.31 Oxides like MnO2 are true pseudocapacitors in a sense that their galvanostatic charge−discharge profiles are in fact nearly perfect triangles (ΔV/ΔQ = constant), while their CV curves are boxlike without distinct redox peaks. This actually does not say anything about the charge−discharge mechanism. For instance, for MnO2 in an aqueous acidic electrolyte the boxlike shape is due to the large number of oxidation states and intermediate MnOx structures. Conversely for a MnO2 with templated nanoporosity in a carbonate electrolyte with a Li salt, the boxlike shape is derived from Li reversible intercalation into the ordered nanopores.2 In aqueous electrolytes materials such as Ni(OH)2 and Co3O4 are not

4. HICS AS 4.2 V CARBONATE ELECTROLYTE-BASED DEVICES An embodiment of a HIC based on a wide voltage carbonatebased electrolyte (up to 4.2 V) is roughly 15 years old. An early commercial embodiment of an ultracapacitor−battery hybrid was published by Telcordia Technologies in 2001.32,33 A related work also employed pseudocapacitive conductive polymer cathode to boost the capacity of the electrode and the overall device.34 The key innovation was replacing an activated carbon anode, normally employed in a symmetrical EDLC ultracapacitor, with Li4Ti5O12 intercalation material, while switching to a higher-voltage (∼3.5 V) battery electrolyte. There are several advantages of Li4Ti5O12 over other materials employed as LIB anodes. The theoretical capacity of Li4Ti5O12 is 175 mAh/g. This is low for a LIB anode but satisfactory for a HIC device with much lower total energy. Very importantly the oxide possesses nearly zero strain upon Li insertion−extraction and has terminal lithiation voltage of 1.55 V vs Li/Li+, which is above that for solid electrolyte interface (SEI) formation in a carbonate electrolyte.35−39 That group of researchers went on to design what is effectively a high-power battery, employing a Li4Ti5O12 anode and a composite activated carbon−micron LiCoO2 cathode.40 In our opinion, while such a system was highly scientifically interesting, it does not qualify as a true hybrid device because in effect both electrodes stored ions primarily in the bulk. As would be expected, the device energy was improved to 40 Wh/ kg, while its cyclability was degraded to the level of a highperformance LIB, i.e., 20% capacity loss after 9 000 cycles. We would attribute the degraded cycling ability to the volume expansion at the cathode. Because of its high power (4 000 W/ kg), such a system would be useful for battery-type fast charge−discharge applications but would not be useful for replacing commercial ultracapacitors that require hundreds of thousands of full charge−discharge cycles. As will be demonstrated in the next section, in some cases, by truly nanostructuring an insertion cathode it is possible to achieve extended cyclability. Whether the resultant system remains a HIC or becomes a high power−high cyclability battery becomes difficult to discern, although the calculation performed in the previous section does indicate that even for truly nanostructured electrodes the surface capacitive contribution to charge storage is not large. The material Li4Ti5O12 is such a zero-expansion electrode, making it well-suited for hybrid devices. At high rates, Li4Ti5O12 stores roughly 100 mAh/g of charge, which is more than what is expected for a typical 100 F/g carbon, which would be equivalent to 28 mAh/g in a 1 V window. The material is cycled between Li4Ti5O12 and Li7Ti5O12, employing either a secondary source of Li or the Li present in the electrolyte only. The sloping voltage plateau for Li4Ti5O12 is between 2.75 and 1.5 V vs Li/Li+, preventing SEI formation. This further promotes cycling integrity of the electrode in a hybrid device. It should be pointed out that this consideration would not be relevant where a traditional 2.7 AC ultracap electrolyte was employed because in principle SEI does not form on either electrode. However, acetonitrile and other solvents employed for traditional supercapacitors are liable to catalytically decompose on noncarbon surfaces at high E


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Figure 4. (A) Discharge voltage profile versus energy and differential capacity versus voltage (insert) for a 500 F hybrid device discharged at currents ranging from 4 to 40 A.33 (B) Ragone plots of hybrid cells repeatedly held for 4 h at various voltages.33 (C) Cycling of hybrid device at 3.0 V maximum voltage and various minimum cutoff voltages.33 (A−C) Reproduced with permission from ref 33. Copyright 2003 Elsevier Science B.V.

higher cutoff voltages (i.e., narrower total voltage windows) lead to a drastically improved cycling lifetime. While this observation was not discussed in detail in the original manuscript, we equate a wider total voltage window with greater level of SEI formation and more extensive Li intercalation into the anode. This leads to Li loss and an increase in impedance due to the SEI, as well as structural damage due to insertion−extraction volume changes. Early modeling work on the energy of HIC devices was performed by Zheng in 2003.47 The author derived a formula describing the energy density of hybrid cells, specifically with an intercalation-type anode and an adsorption-type cathode. It was observed that, with an organic battery electrolyte containing a dissolved LiPF6 salt, it was the ion concentration that wound up limiting the energy density. This was the case with either Li-free WO2 anodes or with LixTi5O12. An energy density of 32 Wh/kg (55 Wh/L) was obtained at the maximum 1 mol ion concentration, which is the typical solubility limit of LiPF6 in various combination of carbonates. Energy of 32 Wh/kg (55 Wh/L) by active materials (not including pore volume) agrees well with experimental studies around the same time period. However, it is generally a factor of 2−4 lower than the experimental values reported for systems over the last several years. We argue that much of the improvement may be attributed to having a supplementary source of charge carriers (Li or Na) in the system, which helps to overcome ion concentration limitations.

voltages, which would lead to similar accelerated parasitic product and pore plugging.41−44 In refs 33 and 45, the authors tested a 500 F 1.9 mm thick prismatic cell and 700 F pouch cells, respectively, making their hybrid device data much more directly comparable to commercial ultracaps versus the usual button cells tested in the scientific literature. Figure 4A shows the discharge profiles of the device at currents ranging from 4 to 40 A. It should be noted that, at lower currents, the cell behaves battery-like, with discernible albeit sloping voltage plateaus. This is due to the Li4Ti5O12 anode, because the voltage versus capacity profile of the ion adsorption cathode is expected to be triangular during charge−discharge. However, at higher currents, the device displays nearly a 45° discharge profile. It should be pointed out that, even at the higher rates, the capacitance is not constant with voltage, making the standard EDLC energy approximation E = 1/2CVave2 less useful. These devices are not discharged to nearly 0 V because, analogously to commercial EDLC capacitors, little useful energy is achieved below 1 V. Figure 4B shows the Ragone plots based on device weight (not active material weight), at different maximum voltages.33 At power values representative of the maximum power for a modern conventional LIB such as a Panasonic 18650 (e.g., 300 W/kg), the hybrid cells are about an order of magnitude less energetic. This shortcoming is the reason why we argue that HICs are really suited to compete with ultracapacitors, not with LIBs. However, at power values representative of modern ultracapacitors (e.g., >1000 W/kg), these 15-year-old “generation one” hybrid devices are more than twice as energetic as a state-of-the-art modern commercial ultracap, e.g., Maxwell K2.46 Figure 4C shows the cycling life of the hybrid devices as a function of the minimum cutoff voltage. As may be observed, F


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Figure 5. Role of increasing the device voltage window on propylene carbonate (PC) electrolyte solvent decomposition on both the negative and positive carbon electrodes.51 Reproduced with permission from ref 51. Copyright 2010 Wiley-VCH Verlag GmbH & Co.

5. NEXT-GENERATION HICS: “PARALLEL” ELECTRODES THROUGH NANOSTRUCTURING During the past decade the HIC field has really blossomed. According to the Web of Science, in 2008 there were 15 publications on the topic “hybrid ion capacitor”, in 2009 there were 20, by 2013 there were 104, while in 2016 there were 223. In 2011 Kötz and co-workers explicitly described a parallel versus serial approach for the design of HIC devices.48 Conceptually it is based on having bimaterial electrode, where one material stores charge by EDLC while the second material in the same electrode stores charge by a Faradaic process. In effect the electrode would consist of a physical mixture of a battery and an ultracapacitor. The authors in ref 48 demonstrated significantly improved power and energy with internally parallel electrodes versus with internally serial hybrids. The internally parallel electrodes displayed batterylike flat current versus voltage profile. However, at intermediate and low powers, all hybrid devices displayed up to an order of magnitude lower energy density at as compared to a standard Li4Ti5O12 anode versus LiMn2O4 cathode cells fabricated using the same techniques. Therefore, unless truly high power (e.g., >1 000 W/kg) is required, fast-charge batteries are preferable. No cycling data was performed in that study, so one cannot specify whether the internally parallel electrodes possessed improved longevity. Perhaps an obvious shortcoming of bimaterial electrodes is that, at very high rates,

only the ion adsorption capacitor materials are actually active, while the battery materials dissipate energy primarily by heat. Another early bimaterial device architecture is (LiMn2O4 + AC) − Li4Ti5O12 (2009),49 which gave a 5 000-cycle lifetime. Devices based on a Li4Ti5O12 anode and a mixture of LiFePO4 and activated carbon cathode were an early (2007) version of the intrinsically parallel approach.50 It was demonstrated in both studies that indeed the electrode acts like a parallel device: The fast rate pulse current is drawn from the activated carbon while the much higher capacity oxide actually recharges the carbon when the pulse is off. In 2010 Naoi published an early combined review−new data manuscript on “next-generation” hybrids.51 A follow-up publication by Naoi et al. expanded on a similar concept of a nanostructured carbon-supported Li4Ti5O12 anode.52 Later, Naoi et al. summarized the concept of “nanohybrid”, providing insightful perspective on the topic.53 A recent review also emphasized nanostructuring by comparing the performance between bulk and nano electrode materials.54 It was argued that a lithium titanate (Li4Ti5O12 in this case) anode was essential to achieving supercapacitor-like power densities. The authors stated that a lithium titanate anode-based “nanohybrid capacitor” is distinct51 and more novel as compared to a lithium ion capacitor that used prelithiated graphite, published in 2008.55 However, we feel that this distinction may not be very strong, as workers in 2003 (discussed in previous section) published a titanate electrode while calling the device a G


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pore plugging and a loss of active surface area in an activated carbon. A high-end (i.e., pure, hence minimal, parasitic reactions) activated carbon in a 2.7 V acetonitrile supercapacitor will keep its surface area during cycling or at fully charged holds. However, this is not expected to be the case with a carbonate electrolyte HIC, at a typical charged cell voltage of 4.2 V. As indicated, in our opinion, even on the positive HIC electrode it is not correct to attribute all the charge storage to pure EDLC. While EDLC certainly has some contribution even after the carbon is overgrown with CEI, we actually believe other surface charge-storage mechanisms gain higher importance after cycle 1. Possible charge-storage mechanisms on carbons at high voltages will be discussed later in this text. It is expected that the SEI products due to carbonate electrolyte reduction on supercapacitor carbons in a hybrid device should be akin to what is observed with other types of carbons employed in LIBs, such as graphite or hard carbon.68,69 Both the solvent and the salt of the electrolyte solution are thermodynamically unstable and undergo reduction on the anode, which may operate at low potentials close to metallic lithium.70,71 A material with a large surface-tovolume ratio will therefore cause higher irreversible capacity losses.72,73 These reduction films passivate the anode surface and prevent further decomposition of the electrolyte solution. However, any volume changes experienced during electrochemical cycling can continuously weaken and fracture the SEI layer, exposing fresh material to the electrolyte with each cycle.74−77 This will form a new SEI layer. The SEI is mostly composed of electrolyte-reduction products such as Li2CO3. The instability of the SEI can eventually lead to overall capacity loss and failure of the battery or the HIC device.75,78−80 Besides solvent-reduction products such as Li2CO3 and alkyl carbonates, the anode SEI also partially consists of LiF, which is a (electroless) decomposition product of the LiPF6 salt but can also be formed through reaction with trace amounts of water to HF and eventually LiF.81,82 Figure 6A shows a classic depiction of SEI in graphite or hard carbon, which is perceived as a composite of various redox and chemical-decomposition product phases.83,84 More recent work (e.g., Figure 6B) on the subject points to compositional and structural radial gradients within the SEI layer, as well as a dynamic growing−shrinking character with charge−discharge.83,85−88 Although the SEI structure/chemistry has been less explored for the case of Na ions, qualitatively it appears similar to Li. Irreversible reduction of solvent molecules leads to reduction of the solvent molecules to Na2CO3, Na alkyl carbonates, Na alkoxides, etc.89−91 One additional point regarding SEI formation in hybrid ion capacitor anode materials is related to the cycling-induced capacity gain often reported in the literature for a range of carbon-supported nanomaterials, such as oxides or sulfides. A cycling-induced capacity increase is not unusual for Li-based anodes, especially for oxides.11,93−98 However, it is not observed for similar anode microstructures when tested against Na, at least to an extent where it would be a significant contributor to the total reversible capacity.93,98,99 Researchers have attributed this phenomenon to a charge-storage contribution through surface adsorption as a result of the “extra” surface area created through cycling of conversion compounds.97,100 We disagree with this interpretation based on the observation that the surface area of both electrodes (especially anode) markedly decreases rather than increases

battery−supercapacitor hybrid. We do fully agree with Naoi that a nanostructured lithium titanate anode is advantageous. The nanometer scale of the particles greatly helps to overcome the kinetic limitation of the slow solid-state diffusivity of Li in the oxide.56 Employing a carbon fiber support allowed Naoi to partially compensate for the known poor electrical conductivity (hence, poor power) of Li4Ti5O12, which is 1.5 V. This Faradaic process may be mechanistically akin to the reversible and high Coulombic efficiency redox reactions in polymer-based electrochemical capacitors.101−103 A parasitic oxidation product, i.e., the cathode electrolyte interlayer (CEI), is very likely formed on positive electrodes in HIC devices. The most intuitive evidence for this oxidation layer is the commonly observed first-cycle capacity loss (i.e., first CE < 100%) for positive carbon electrodes in carbonate electrolytes, both with lithium cells60,104−106 and with sodium cells.107−111 Once formed, a stable passivation layer could in principle eliminate any further electrolyte oxidation on the electrode surface.112−115 It has been demonstrated that the chemical nature of the passivation layer (e.g., thickness, organic/inorganic species) on the ceramic cathodes signifi-

Figure 7. Formation of Li metal (circled) on hard carbon particles in a LIC device that have undergone 10 000 charge−discharge cycles. The cycling voltage was fairly shallow (1.9−3.8 V), but the low cycling temperature (−10 °C) favored Li metal formation on the surface instead of Li ion insertion into the bulk.52 Reproduced with permission from ref 52. Copyright 2012 Royal Society of Chemistry.

We fully agree with the authors in ref 52 that a nanostructured lithium titanate should significantly reduce the extent of the Li metal plating concern. However, Li4Ti5O12 by itself is not sufficiently electrically conductive to achieve the high rates needed for HICs. A supporting carbon phase is therefore almost mandatory, which then creates a similar risk of Li plating on its surface if a wide device voltage window is employed. The only way to really avoid metal plating is to be sure that the anode stays sufficiently far from 0 V vs Li/Li+, especially at high charge rates. This in turn requires a three- or four-electrode cell (one or two reference electrodes) for quantification, something which is rarely done. It should also be pointed out that, in a true two-electrode cell (rather than a half-cell with one electrode being Li metal), the voltage swing of the individual electrodes may only be estimated from their mass−capacity ratio. This creates further difficulty in accurately controlling both metal and SEI formation. I


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Figure 8. (A) Diagram showing an electrochemical method for prelithiation of graphite or other carbons.146 (B) Electrode products in a pouch cell after a chemical prelithiation procedure.146 (A, B) Reproduced with permission from ref 146. Copyright 2015 Elsevier B.V. (C) Three-electrode setup of a LIC device using stabilized lithium powder as lithium source.119 Reproduced with permission from ref 119. Copyright 2012 Elsevier B.V. (D) Schematic illustration of the charge−discharge profiles of type 1 and type 2 ion-donating additives.147 Reproduced with permission from ref 147. Copyright 2017 Elsevier B.V.

7. MOTIVATION FOR SODIUM ION CAPACITORS The first argument for NICs research is a geo-economic one concerning the uneven global distribution of lithium reserves, as well as the potential for unsustainable price spikes due to even short-term production scarcities. Similar to extraction of known oil reserves, increasing the scale of lithium carbonate production is a timely, long-term process that is likely not flexible enough to meet spikes in demand due to, for example, multiple new Gigafactories. This creates difficulties in predicting Li pricing and hence the dollars per kWh of LIB and LIC technology.128−131 Conversely, the precursors for Nabased electrodes are universally abundant, including on land (salt, sodium carbonate, and sodium hydroxide) and in salt water.132 In the United States salt is produced through mining (44%) and from brine (38%),133 which may then be separated to form Na metal through electrolysis or ion-exchanged into various oxides. As an immediate cost-saving feature, both sodium ion batteries and NICs may employ inexpensive Al current collectors for both electrodes because Al is Na-inert. Conversely, lithium ion batteries and LICs must employ a costlier copper anode current collector because Li alloys with Al at anode voltages. As seen by the sloping shape of the Al−Li charge−discharge curve,134,135 a limited but potentially catastrophic reaction between Al and Li may occur even

when high insertion potential titania anodes are employed. In terms of a price comparison, the difference between Al and Cu is not trivial and can tangibly impact the cost of cell production.136 A full techno-economic analysis of NICs versus LICs is not currently available. However, qualitatively one could translate the findings for LIBs versus NIBs to visualize the differences: Authors have recently examined a broad range of techno-economics of NIB production and concluded that indeed there is a cost saving to be had even with the current relatively low Li precursor costs and without having to extrapolate to adjust for new Gigafactories, etc.137 A key safety motivation for NICs is that, when nongraphitic carbons are employed as anodes, Na atom underpotential deposition into the carbons’ pores (a.k.a. metal nanopore filling, nanoplating) appears to be minimally active as compared to Li plating in the same materials.127 This point will be elaborated further in the subsequent sections of this Review. However, for now it should be indicated that lack of metal plating at low potential may in principle lead to much safer electrodes during cycling because of the suppression of metal dendrite growth that occurs as Li accumulates and strips over thousands of cycles.138−140 Because metallic particles are likely to be catalytic toward SEI formation, the cycling CE of NICs may ultimately be superior as well.107 J


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the downside is that lithiation (sodiation) is in effect “blind”, with it not being possible to control or even monitor the state of charge, i.e., the lithium (sodium) content in the electrode. Of course, such analysis may be done after the fact; however, that again adds a level of complexity to the fabrication process, which may not be economically scalable. To both avoid the complex cell-reassembling process and be able to monitor the electric signals during the predoping reaction, some researchers built hybrid devices with multielectrode configurations, with one component being the source of the ions.144,152,159,160 Figure 8C shows a three-electrode hybrid setup, which utilized the stabilized lithium powers (SLPs) as the lithium source.119 On the basis of this principle, other lithium sources, e.g., lithium silicide, lithium-rich transition metal oxides, etc., have been employed for various anodes (e.g., carbon, silicon, etc.).143,154,161,162 Historically, prelithiation has been performed on the anode, which is in fact contrary to how LIBs are configured. Recently prelithiation (presodiation) has been achieved on the cathode.147 This may be intrinsically safer, if the device then starts out thermodynamically stable as fabricated (which would not be the case for a symmetric cell, e.g., carbon−carbon). Such an approach for HICs could leverage a number of recent studies on ion-donating materials for conventional ceramic LIB cathodes, designed to compensate the first cycle loss.163−170 The ion-donating lithium-rich (sodium-rich) materials display a large hysteresis between charge and discharge. This in effect creates an irreversible delithation (desodiation) process at the first charge of the cell. The ions are injected into the electrolyte and shuttle back and forth between the primary storage phases during subsequent cycling. Materials such as Li 3 N, 14 8, 16 5 Li6CoO4,146,164 Na2NiO2,145,163 etc. have been employed as cathode additives to reduce cycle 1 CE loss in this manner. The concept should be directly transferable to LICs and NICs. As shown in Figure 8D, materials may have more negative lithiation or sodiation voltage than the minimum potential of the active carbon (type 1).154 Otherwise, they may act as a mini primary cell, possessing minimal capability that is reversible (type 2).144,147,171 Both types are suitable for the cathode additive in hybrid devices. Brousse and co-workers recently reported an extremely powerful example of a chemical additive ion source concept, one that may be transformative in advancing the HIC technology due to its effectiveness and scalability.172 A sacrificial organic lithium salt (3,4-dihydroxybenzonitrile dilithium) was incorporated into the HIC activated carbon cathode. During the first charge, the Li2DHBN compound irreversibly decomposed with the remnants dissolving into the electrolyte. It served as a reservoir of lithium ions into the graphite anode, without negatively affecting other aspects of electrochemical performance. In our opinion, prelithiation and presodiation via cathode sacrificial chemical additives required further exploration because hybrid devices really cannot achieve optimum performance based on electrolyte ions alone. Of all the precharging methods, the sacrificial additives approach has the most promise for technological applications due to the straightforwardness of translating this concept to large cells and continuous manufacturing. Clearly more research activity on this topic would be beneficial.

8. PRELITHIATION AND PRESODIATION The prelithiation or presodiation of one or both electrodes is an indispensable step for building high-performance hybrid LIC and NIC devices. A key purpose of the process is to compensate for the ion loss during early SEI/CEI layer formation. Because of the limited solubility of salts such as LiPF6 or NaClO4 in carbonate solvents (∼1 mol in 1:1 volume ratio, at room temperature), irreversible loss of ions results in a degradation of ionic conductivity. It has been demonstrated that systems based on nonlithiated (nonsodiated) electrodes will diminish the lithium (sodium) ion content in the electrolyte during extended cycling and, thus, greatly deteriorate the overall capacity.52,141 In addition to SEI, both Li and Na may be trapped within the bulk of the electrode materials such as hard carbons, especially during the first several cycles.127 Precharging at least one electrode is a crucial factor in determining the performance of a hybrid device in terms of initial energy−power and cycling retention. A survey of both LIC and HIC Ragone characteristics generally points to devices at the top of the energy−power spectrum being those that employ a secondary source of ions in addition to the electrolyte.107,141−143 The high-performance architectures that do not explicitly prelithiate or presodiate the electrodes to a notable capacity “condition” the electrodes in half cells.144 What conditioning amounts to is cycling the electrodes versus Li or Na metal prior to assembly into a full device but leaving them at near-zero capacity. Such conditioning makes up for the Li/Na ion loss due to initial SEI formation and irreversible bulk trapping. As long as the SEI growth and bulk trapping are not significant during cycling, the electrolyte concentration then remains fairly stable. Prelithiation (presodiation) has also been successfully employed to widen the voltage window and extend cyclability of devices by controlling the voltage swing of the anode, thus reducing the depth of charge and the associated SEI growth, volumetric expansion, and ion trapping.52,141,145 The most common technique for prelithitation (presodiation) is electrochemical. As shown by the diagram in Figure 8A, a closed electrical circuit forms connecting the electrode (usually anode) and the lithium (sodium) metal.146 An electrically insulating−ionically conducting separator is employed, analogously to what is used in battery cells. The external circuit can include an electronic charger or a resistor, or even just a short circuit, depending on the controllability requirement of the predoping process.145,146,148−151 The charging process can be controlled by applying a programmable procedure to the terminals.152,153 The major downside to the method is necessary extra assembly/disassembly cell procedures, where in effect the device has to be fabricated twice. While scientifically useful and to some extent convenient, it is not obvious how such an approach may be done economically for a large number of commercial cells. A potentially more scalable approach may be based on chemical (rather than electrochemical) charging through the direct reaction between the electrode material and lithium (sodium) metal while in the presence of an electrolyte.154−156 Such reactions may be accelerated by milling the regents to achieve improved contact.157 Figure 8B shows the prelithiated graphite electrode film obtained by this direct contact method in a pouch cell.146,158 The major advantage of this approach is its elegant simplicity and the potential for scale-up. However, K


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Figure 9. Plot of the average working potential versus practical capacity of promising electrode materials for lithium ion capacitors (LICs).

Figure 10. Plot of the average working potential versus practical capacity of promising electrode materials for sodium ion capacitors (NICs).

9. LIC AND NIC ELECTRODE MATERIALS: ARCHITECTURES AND PERFORMANCE Figures 9 and 10 highlight various materials that are electrode candidates for LIC and NIC applications, respectively. It may be observed that asymmetric insertion carbon anode− adsorption carbon cathode architectures may in principle give the widest operating voltage window. It is doubtful that 10 000 or 100 000 full charge−discharge cycles are possible with classic LIB or NIB ceramic cathodes. Sodium ion ceramic cathodes are known to give worse cycling lifetime that Li ceramic cathodes,173−176 neither of which typically survives more that several thousand cycles, even when fully optimized. However, in the high-voltage region (e.g., 3−4.5 V vs Li/Li+ or 2.7−4.2 V vs Na/Na+), the capacity of an adsorption carbon is 5 000 W kg−1 with a charge/discharge time span of tens of seconds or even several seconds.59,195−199 Moreover, due to the very small volume change of the intercalation anodes and the absence of the SEI layer formation, the energy of the TiO2and Li4Ti5O12-based LIC can be well-maintained upon extremely long cycling from 5 000 to 20 000 cycles.59,159,197,200 As indicated earlier, we know of no cathode besides highsurface-area carbons that could match such cyclability. 9.2. Titanium Oxide Compound-Based Na Anodes

Titanium-based materials are a major class of the sodium anodes. After carbons, the most studied systems for Na capacitors are based on TiO2 (anatase, rutile, brookite, amorphous, etc.). Researchers are discovering that TiO2 exhibits different redox chemistry and charge-storage mechanisms with Na as compared to its well-studied Li counterparts. With anatase TiO 2 , charge storage may involve a combination of Na+ intercalation into the lattice201,203,204 and a conversion reaction to reversibly form Ti and Na2O in its terminally sodiated state.202 As shown in Figure 12A, Mitlin and co-workers were the first to propose that the stacked edgesharing TiO6 octahedral framework is expected to form possible interstitial sites for Na accommodation.201 This proposed reversible intercalation is different from the twophase reaction that is characteristic of the lithiation reaction TiO2 + 0.5Li+ + e− = Li0.5TiO2.191,205,206 In our opinion the existing experimental evidence for Na intercalation reaction requires more confirmation. For instance, the weak paramagnetic interaction between Na+ and Ti3+ ions observed in 23 Na NMR spectra may point against a true intercalation reaction.207 At low charge rates the conversion reaction is better supported. This is based on the detection of reversible switching between trivalent and tetravalent Ti during charge/ discharge in XPS204 and X-ray absorption spectroscopy (XAS)202 spectra (Figure 12C). Researchers have also published extensive TEM results to support the conversion reaction.202,204,208 Importantly, TiO2 of various structures is another material where the low-rate sodiation sequence likely does not apply to charging rates representative for HICs. It is not obvious how at 5C, 10C, 20C, etc. various TiO2 electrodes would have the diffusion time necessary to undergo reversible

Figure 11. Improvement in the rate capability of TiO2 and Li4Ti5O12 Li anodes. The C rates plotted are the maximum values at which >100 mAhg−1 of reversible capacity is achieved.

listed delivered specific capacity of >100 mA hg−1 under the corresponding C rate in the vertical axis. Starting from the charge/discharge in hours (below 1C)180,181 for the early proof-of-concept work, the time span decreased to several minutes (10−60C) 182−186 and even seconds (100− 300C)187,188 for the state-of-the-art studies. The breakthrough performance improvements are based on progressively finer nanoscopic mixtures of the oxides and high electrical conductivity carbons. It should be pointed out that nanostructuring of each phase should not change the bulk ionic conductivity of the materials but will give a greater overall surface and interface contribution. Room-temperature Li ion conduction on TiO2 and Li4Ti5O12 surfaces and interfaces should be orders of magnitude higher than in the bulk.189−194 With increasing surface-to-volume ratio, this contribution to the total Li flux will become progressively more important. The vast majority of carbons employed in such hybrid electrodes are Li- (and Na)-active at the relevant voltages. Hence, it is expected that they will also significantly contribute to the Li flux, in effect acting as “ion highways” while supporting the oxides and imbuing electrical conductivity.

Figure 12. (A) Possible sodium sites in the anatase TiO2 lattice. Reproduced with permission from ref 201. Copyright 2013 Royal Society of Chemistry. (B) First charge and discharge curves and the corresponding lattice parameters. (C) XANES Ti k-edge of an anatase TiO2 electrode at the charge/discharge data points marked in (B). Reproduced with permission from ref 202. Copyright 2014 American Chemical Society. M


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Figure 13. Examples of nanostructured TiO2 for high-rate anode applications. (A−C) Graphene-wrapped anatase TiO2 nanofibers as Na anodes.218 Reproduced from an open access journal, ref 218. (D−F) Anatase TiO2 nanocubes as Na anodes.229 Reproduced with permission from ref 229. Copyright 2015 Royal Society of Chemistry. (G−I) Carbon dots supported upon N-doped TiO2 nanorods, as Na and Li anodes.223 Reproduced with permission from ref 223. Copyright 2015 Royal Society of Chemistry.

challenging in the sodium system. Specifically Na ion diffusion appears to be more sluggish in the titania systems.210−213 The early proof-of concept studies on TiO2 can acquire ∼150 mAhg−1 capacity at the low rate (e.g., 50 mAg−1); however, negligible capacity can maintain as the current density is increased (e.g., 2 Ag−1).201,214 The following studies developed strategies in optimizing the performance, especially the rate capability. While nanostructuring of the active material is an established way to improve the kinetics, with TiO2 a carbon support phase is also necessary. Past studies include extremely small TiO2 particles/nanodots (0D),215−217 nanorods/nanowires/nanotubes (1D),202,218,219 and thin TiO2 sheets/sphere shells (2D).203,220 The downsizing of the TiO2 can greatly shorten the diffusion distance of sodium ion within the material bulk. Introducing electronic conductive second phases such as graphene199,215,221 or graphitic carbon217,222 is also effective. The intrinsic electronic conductivity of TiO2 could also be modified by aliovalent doping, for instance, nitrogen,223 tin,224 boron,225 and niobium.226 On the basis of these strategies, the state-of-the-art TiO2 material could deliver >300 mAhg−1 at low rate, and >50 mAhg−1 capacity was maintained at current density as high as 30 Ag−1. The substantial improvement in rate capability enables TiO2 to be a promising anode for NICs.227,228

conversions over many cycles. With sodium, such reactions are known to be sluggish even at charging rates of several hours and with highly nanostructured particle sizes.99 Researchers have argued for fast-rate Na+ intercalation pseudocapacitance, which may be the case.209 It is straightforward to distinguish between diffusional control (maximum rate is proportional to square root of time) versus reaction control (maximum rate is linear with time). However, establishing reaction control is literally analogous to measuring activation polarization in a galvanostatic experiment; it tells very little regarding the actual limiting process or the dominant charge storage mechanism. The process may be Na surface adsorption or it may be a reaction in the bulk, yielding the exact same linear time dependence. For the case of fast-rate Na with TiO2, more experimental and modeling work is needed to understand which fast-rate mechanisms would actually account for the observed voltage profiles and the measured reversible capacities. One advantage with NICs over LICs is that the working voltage of TiO2 against Na/Na+ is lower than that against Li/ Li+ (ca. 0.75 V vs ca. 1.5 V).202,204 This gives devices higher energy but creates issues related to SEI formation, which occurs below ∼1 V vs Na/Na+. However, the factors limiting the rate capability of TiO2 still exist and become even more N


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Figure 14. High-resolution annular bright field micrograph and linear intensity profiles of a sodiated Li4Ti5O12 nanoparticle, with the phase boundaries being highlighted. Reproduced with permission from ref 210. Copyright 2013 Springer Nature.

Figure 15. Schematic of the change in the Na2Ti3O7 structure upon sodiation. Reproduced with permission from ref 238. Copyright 2014 Royal Society of Chemistry.

capacity. However, it would give the same issues with SEI as would be preset in a bare carbon, because in either case there would be significant low-voltage capacity. Lithium titanium oxide Li4Ti5O12 is also an anode material employed with NICs. In the sodium system the potential is near 0.9 V, which is lower than the 1.55 V working potential in the lithium system.230,231 Different from the biphasic mechanism for lithium storage, with Na the material undergoes a three-phase separation mechanism among Li4Ti5O12, Li7Ti5O12, and Na6LiTi5O12, giving a theoretical capacity of 175 mAhg−1.210,232 The three-phase mechanism of Li4Ti5O12 was confirmed at atomic scale through STEM analysis. As shown in Figure 14, for a partially sodiated Li4Ti5O12 phase, there are crystal domains ascribed to the Li4Ti5O12, Li7Ti5O12, and Na6LiTi5O12 phases separated by well-defined boundaries.210 It is not obvious that at the high NIB charging currents such a reaction would still be present, and more work is needed to understand the rate behavior. Full capacity (>170

Figure 13 shows several examples of nanostructured TiO2 for high-rate anode applications, including their microstructure and rate capability in a half-cell. Parts A−C of Figure 13 show graphene-wrapped anatase TiO2 nanofibers, employed as Na anodes. Parts D−F of Figure 13 show anatase TiO2 nanocubes also employed as Na anodes.229 Parts G−I of Figure 13 highlight a microstructure consisting of carbon dots supported upon N-doped TiO2 nanorods, employed as Na and Li anodes.223 For hybrid ion capacitor applications, the relevant charging rates would be roughly >10C. It may be seen that in parts A−C and D−F the working capacity at this rate is ∼100 mAh/g. When high levels of carbon are present, such as in parts G−I, the capacity reaches 200 mAh/g. A value of 100 mAh/g is significantly lower than what is possible with a nanostructured carbon at 10C (discussed in the next section), while 200 mAh/g is on par. Again there seems to be a tradeoff: Adding a substantial mass fraction of ion−active carbon significantly improves the rate capability and the overall O


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Figure 16. Examples of nanostructured Na2Ti3O7 for high-rate Na anode applications. (A−C) Na2Ti3O7 nanotube arrays synthesized through surface engineering for Na anodes.239 Reproduced with permission from ref 239. Copyright 2016 Wiley-VCH Verlag GmbH & Co. (D−F) Na2Ti3O7@N-doped carbon hollow spheres for Na anodes.240 Reproduced with permission from ref 240. Copyright 2017 Wiley-VCH Verlag GmbH & Co. (G−I) Na2Ti3O7 nanoplatelets and nanosheets derived from a modified exfoliation process for Na anodes.241 Reproduced with permission from ref 241. Copyright 2017 American Chemical Society.

mAhg−1) may be obtained at low current densities. However, capacity significantly decays as the rate increases, presumably due to the sluggish diffusion kinetics of Na+ within the 1D tunnel of the spinel phase. The diffusivity of Na+ in the spinel phase is several orders of magnitude lower than that of Li+.232,233 Li4Ti5O12 for NICs has been substantially improved within a short time span of 1−2 years, building on the transferrable knowledge base established for LICs.234 One of the most promising systems displayed ∼60 mAhg−1 at a current density of 8.75 Ag−1, which almost satisfies the rate-matching requirement with the adsorption carbon cathode in a NIC device.234,235 An unusually high capacity of >200 mAhg−1 has also been reported in studies.236,237 This extra capacity was ascribed to so-called interfacial capacitive Na storage, although it remains to be established whether the surface area of the electrodes is large enough to support such a contribution. A sodium-containing titanium oxide Na2Ti3O7 with welldefined layered structure has been shown to work for high-rate Na anodes.239−241 It is accepted that two phases are formed during sodiation, Na2Ti3O7 and Na4Ti3O7. As shown in Figure 15, as the Na ion intercalates into the lattice, the Ti−O slabs will shear. The coordination of Na ions also changes upon sodiation, going from 7,9-coordinated to 6-coordinated. The angle between neighboring Ti−O also changes in a relatively

large content.238 The crystallographic flexibility of the layered structure is unique and is expected to be beneficial for facile Na diffusion kinetics. By accommodating up to 3.5 Na+ per formula unit, Na2Ti3O7 will in theory deliver 310 mAhg−1 with a voltage near 0.3 V. This phase is popular of fast-rate applications, including for NICs, where the 0.3 V standoff from Na plating is a safety advantage. Figure 16 shows examples of nanostructured Na2Ti3O7 for high-rate Na anode applications. Parts A−C of Figure 16 show Na2Ti3O7 nanotube arrays synthesized through surface engineering.239 Parts D−F of Figure 16 show Na 2 Ti 3 O 7 @N-doped carbon hollow spheres.240 Parts G and H of Figure 16 show Na2Ti3O7 nanoplatelets and nanosheets derived from a modified exfoliation process. In all cases, the reversible capacity at 10C is in the 100 mAh/g range.241 After the proof-of-concept study of Tarascon’s group in 2011,242 this phase has been tremendously studied thereafter. The synthesis was diversified from the simple solid-state reaction route242,243 to various specially designed multisteps reaction strategies.239−241,244−247 Meanwhile, the phase morphology evolved from microsize particles to various nanoscale shapes, such as nanosheets,241,245,248 nanofibers,244 hollow spheres,240 nanotubes,239,249 etc. Although the low working voltage was maintained (ca. 0.3 V) for these nanomaterials, the nanosizing indeed greatly diminished the P


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Table 1. Summary of the Titanium Compounds-Based LIC and NIC Devices device configuration (anode//cathode) TiO2-based TiO2−CNT//active carbon268 TiO2 nanobelt arrays//graphene hydrogels269 TiO2@rGO//AC197 TiO2 hollow spheres@graphene// graphene270 TiO2@mesoporous carbon//AC271 TiO2@graphene//active carbon227 TiO2@CNT@C//bioderived active carbon272 Li4Ti5O12- or Na2Ti3O7-based graphene-wrapped Li4Ti5O12//active carbon273 spheres Li4Ti5O12//active carbon274 Li4Ti5O12//reduced graphene oxide105 nanocrystalline Li4Ti5O12//active carbon198 TiO2-coated Li4Ti5O12//active carbon195 Li4Ti5O12//N-doped porous carbon275 graphene-Li4Ti5O12//graphene-sucrose59 Na2Ti3O7@CNT//active carbon158 3D-Na2Ti3O7 sheets//graphene foam248

energy (Wh/kg) @ max power (W/kg)

capacity retention

59.6@120 82@570

31.2@13900 21@19000

73% over 600 cycles

1−3 V 0−3 V

42@800 72@303

8.9@8000 10@2000

65% over 1K cycles

LIC

0−3 V

67.4@75

27.5@5000

NIC NIC

1−3.8 V 1−4 V

64@56 81.2@126

26@1357 37.9@12400

80.5% over 10K cycles 90% over 10K cycles 84% over 5K cycles

LIC

1−2.5 V

50@16

15@2500

75% over 1K cycles

LIC LIC LIC

1−3.5 V 1−3 V 1.5−3 V

[email protected] 45@400 [email protected]

[email protected] 30@3300 28.8@10300

93% over 500 cycles 100% over 5K cycles

LIC LIC LIC NIC NIC

0.5−2.5 V 1−3 V 0−3 V 0−3 V 1−3 V

74.85@300 63@200 95@45 59@300 55@200

36@7500 16@5000 32@3000 22@3000 22@3000

83.3% over 5K cycles

type

voltage

LIC LIC

1−3 V 0−3.8 V

LIC LIC

max energy (Wh/kg) @ power (W/kg)

94% over 0.5K cycles 77% over 4K cycles 80% over 2.5K cycles

bulk of the carbon. A b-value of 1 indicates an interfacecontrolled process, although the interface may be solid−liquid or solid−solid (or even solid−gas, if it exists). For instance, nucleation-controlled growth of precipitate phases are a classic solid−solid process that follows linear time kinetics.254 For the case of Na in Na2Ti3O7, TiO2, etc., a b-value of 1 could also mean nucleation-controlled growth of Na metal on surfaces and interfaces, or Na intercalation control, or even control by some surface redox process. Without detailed combined analytical work and simulation, it is difficult to be conclusive apart from indicating that Na storage is not diffusioncontrolled! Per Figure 10, at a voltage of above ∼0.5 V, there are three other interesting options: K2Ti6O13, Nb2O5, and Na2Ti6O13. K2Ti6O13 is the most promising from a reversible capacity vantage, having the same structure as Na2Ti6O13 but with a dilated lattice.255−257 Nb2O5 is another promising high-power material for both lithium and sodium storage, exhibiting intercalation pseudocapacitive charge-storage behavior.258 A state-of-the-art NIC device based on a Nb2O5 anode will deliver >10 Wh kg−1 at a power as high as 7 000 W kg−1, which is competitive among the NIC devices reported.259 Some other compounds including alkali metal-containing titanium oxides have also been employed as anodes against sodium, including NaTiO2,260 Na4Ti5O12,153,261 Na2Ti2O4(OH)2,262 etc.263,264 Unfortunately these phases appear to have limitations in rate capability, although further optimization with carbon may address this.

plateau feature of the Na2Ti3O7, indicating the change of the intrinsic biphasic reaction mechanism. Researchers proposed a capacitive-like charge-storage contribution for these nano-Na2Ti3O7 materials, termed pseudocapacitance (although the term may not be exactly accurate; see prior discussion), which provided an explanation for the slopelike voltage profiles.158,239,248 A significant contribution by pseudocapacitance significantly improved the rate capability of a Na2Ti3O7-based anode, presumably because surface-controlled redox reactions are facile. A state-of-the-art anode has been tested up to a current density of 8 Ag−1 and delivered over 50 mAhg−1 of reversible capacity.240 This performance would enable in principle charge times of ∼1 min, assuming the cathode or the electrolyte diffusivity were not rate-limiting. The relative contribution of the capacitive charge storage may be determined with multirate cyclic voltammetry (CV) tests.93,250−253 The transition in the time dependence of peak current, i.e., the peak reaction rate, is a well-known approach at looking at the onset of diffusional limitations: Current changes with the sweep rate are expressed as i = avb, where a and b are adjustable constants. A b-value of 0.5 is a straightforward outcome of Fick’s law and signals a diffusion-limited process. A b-value of 1 is a standard expression for any activation (i.e., interface) polarization reaction. Researchers employ a version of such rate analysis to argue for bulk versus capacitive storage. We do want to provide some caution regarding potentially overinterpreting the above and similar sweep-rate electroanalysis: For systems with multiple charge-storage mechanisms, such as the disordered graphenes, carbon nanosheets, and perhaps titania, the above algebra applied without additional analysis may not be fully conclusive. For instance, at rates of 5C and above, Na storage process in a high-surfacearea carbon may be diffusion-limited. However, a b-value of 0.5 will not inform whether the diffusional limitations come from movement of Na within the electrolyte-filled pores or in the

9.3. Architectures Employing Titanium Compounds

Table 1 summarizes the titanium-based LICs and NICs that have been published to date. While there is a diverse range of anode architectures and counter cathodes, it appears that the maximum energy values are notably 81 Wh/kg and lower. Assuming a factor of 4 conversion from active-material-based to device-based energy values, the maximum device energy is 20 Wh/kg, i.e., roughly 3−4 times higher energy than a stateQ


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Table 2. Summary of the Carbonaceous Materials-Based LIC and NIC Devices max energy (Wh/kg) @ power (W/kg)


1.5−5 V

145.8@65

18@18000

LIC

2−4 V

82@100

14@20000

graphite//graphene92

LIC

2−4 V

135@50

105@1500

N-doped hard carbon// activated carbon278

LIC

2−4 V

28.5@348

13.1@6940

soft carbon//activated carbon279

LIC

0−4.4 V

115@25

16@15000

graphene//activated carbon142

LIC

2−4 V

95@27

[email protected]

graphite//activated carbon141

LIC

2−4 V

103

graphite//activated graphene145 graphite//functionalized graphene280

LIC LIC

2−4 V 2−4 V

147.8 106@84

85@4200

LIC

1.4−4.3 V

80@150

65@2350

hard carbon//bioderived mesoporous carbon

LIC

1.7−4.2 V

121@300

50@9000

reduced GO//resin-derived carbon combined with GO282 B&N-doped carbon nanofiber//B&N-doped carbon nanofiber178 microcrystalline graphite//mesoporous carbon nanospheres/graphene283 graphene//armored graphene284

LIC

0−4 V

148.3@150

45@6500

LIC

0−4.5 V

220@225

104@22500

LIC

2.2−4.2 V

80@152

32@11600

LIC

0−4.3 V

160@900

59@19000

N-doped carbon nanopipes//reduced graphene oxides285 hard carbon//activated carbon nanosheets107

LIC

0−4 V

262@450

78@9000

NIC

1.5−4.2 V

201@285

50@16500

PPy-derived disordered carbon//macroporous graphene345 hollow soft carbon spheres//activated carbon287

NIC

0−4.2 V

168@500

50@2800

NIC

0.4−4 V

110@245

43@10000

PPy-derived disordered carbon//activated carbon nanosheet286 bioderived active carbon//bioderived activated carbon109 disordered carbon//N-doped carbon hollow microsphere288

NIC

0−4 V

111@67

38@14550

NIC

0−4 V

112@67

45@12000

NIC

0−4.4 V

157@620

25@2000

devices configuration (anode//cathode)

type

voltage

graphite//activated carbon145

LIC

hard carbon//activated carbon119

hard carbon//activated carbon152 281

capacity retention 65% over cycles 97% over cycles 97% over cycles 97% over cycles 63% over cycles 74% over cycles 77% over cycles

10K 600 3.5K 5K 15K 300 100

100% over 1K cycles 82% over 10K cycles 81% over 8K cycles 79% over 3K cycles 81% over 5K cycles 93% over 4K cycles 89% over 1K cycles 91% over 4K 66% over cycles 84% over cycles 71% over cycles 83% over cycles 80% over cycles 70% over cycles

10K 1.2K 1K 5K 3K 1K

charge−discharges. It is not known why cells with titania anodes degrade, as one may assume that higher cycle data would be presented otherwise. This is a topic that certainly warrants further in-depth study, as longer device lifetimes are absolutely essential for HICs to gain commercial footing.

of-the-art ultracapacitor. The factor of 4 denominator actually assumes near-commercial mass loading. A much lower mass loading of 0.5−1 mg/cm2 would further increase the conversion denominator, up to 10 or more.265,266 It should also be pointed out that commercial ultracapacitors typically possess 50−100 μm thick electrodes, which is often a practical thickness limit for fabrication. A commercial activated carbon such as Kuraray YP-50 gives an electrode (not skeletal) density of ∼650 mg/cm3 (in-house measurement), resulting in a mass loading of 3.25−6.5 mg/cm2. An electrode based on a highsurface-area, loosely packed nanocarbon such as graphene or an aerogel may give an electrode density that is substantially lower, for example, 250 mg/cm3.267 This is another reason why the energy density of a real device with a fluffy carbon may be significantly lower than what one would expect from a straightforward factor of 4 conversion, simply because commercial devices are not mass-loading but rather electrode-thickness limited. Looking at Table 1, one may also observe that, despite the higher terminal anode voltage and the zero expansion benefit of titanias, the full cells are not cycled above 10 000 full

9.4. Carbons for Li Electrodes

A survey of literature indicates that nanostructured carbons are an important class of HIC materials, being employed either separately or combined with a second Li active phase such as TiO2. Recently, Yao et al. published a focused summary of the carbons employed in LIBs, ultracapacitors, and their hybrids.276 Table 2 compiles a representative sample of various carbons employed for high-rate Li and Na storage. The table includes both LIC and NIC electrodes, with the NIC carbons being described in the next section. While some researchers prelithiate one or both of the electrodes (discussed), others rely on Li in the electrolyte such as from LiPF6.52,64,277 A key issue associated with employing carbon anodes is the SEI formation because the vast majority of carbons possess significant capacity down to 0 V vs Na/Na+. One would have to could keep SEI stable during cycling and avoid Li/Na R


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Figure 17. Examples of various carbon nanosheet structures employed for Na electrodes. (A) Thirty nm thick disordered undoped carbon nanosheet with micro- and mesoporosity employed for high-rate Na anodes.109 Reproduced with permission from ref 109. Copyright 2016 Royal Society of Chemistry. (B) Ordered (but not graphitic) undoped carbon nanosheet employed for high-rate Na anodes.107 Reproduced with permission from ref 107. Copyright 2015 Royal Society of Chemistry. (C) Dopamine-derived nitrogen-doped carbon sheets as anode materials for high-rate Na anodes.290 Reproduced with permission from ref 290. Copyright 2015 Elsevier Ltd. (D) Nitrogen-doped graphene foams employed for high-rate Na anodes.291 Reproduced with permission from ref 291. Copyright 2015 Wiley-VCH Verlag GmbH & Co.

continua in structure and chemistry between pristine graphite/ graphene and fully amorphous activated carbon. Any fundamental theoretical or experimental treatment of such materials must take account of these rich variations on the carbons’ chemical and structural defectiveness. Lithium storage in 2D nongraphitic carbons is poorly understood in general, even at relatively slow rates representative of battery carbons. It is much less understood at the charge rates necessitated by hybrid devices, making this a potentially exciting area to pursue further. Figure 18 compares well-understood Li staging reaction in commercial graphite to the proposed Li storage mechanism(s) in “hard carbons”, i.e., carbons that do not fully transform to graphite at 2800 °C. In the “fallen-cards” model for hard carbons, developed by Stevens and Dahn, the carbons’ structure consists of arrays of highly defective graphene planes that are stacked relatively randomly and contain sealed and open nanopores between them. There are also regions of higher alignment, with graphene planes being arranged graphite-like with their (0002) normals in parallel, but with translational and rotational randomness. Such partially ordered structure is termed “turbostratic”. The primary references for this qualitative model and the associated definitions are refs 292 and 293 and earlier related work being in refs 294−301. It is important to note that the fallen cards model is a low-rate− low-order model, with the vast majority of the experimental effort being made on carbons with minimal graphene order (Raman G-band to D-band intensity ratio IG/ID < 0.3) and a current density in the C/10 range. Many hard carbons

plating, which would necessitate having tight control of the negative electrode voltage. For optimum energy, a carbon− carbon architecture then requires asymmetric mass loading, with the mass of the lower-capacity adsorption carbon cathode being up to 5 times higher than that for the insertion anode. Table 2 provides a summary of the carbon−carbon architectures employed for LICs and NICs. As may be seen from Table 2, the vast majority of carbons employed are nongraphitic and highly nanostructured and may be nanoporous such as the aerogels and some of the carbon nanosheets. Both LIC and NIC carbons are listed. Overall we have not observed significant differences between the types of nongraphitic structures employed for Li versus Na. Examples of carbon nanosheets commonly employed for Na are shown in Figure 17. The carbons’ order, heteroatom content, and porosity may be controlled by selecting precursor, carbonization temperature, and activation process.267,289 Figure 17A shows a 30 nm thick disordered undoped carbon nanosheet with micro- and mesoporosity employed for highrate Na anodes.109 Figure 17B shows an ordered (but not graphitic) undoped carbon nanosheet employed for high-rate Na anodes.107 Figure 17C shows dopamine-derived nitrogendoped carbon sheets as anode materials for high-rate Na anodes.290 Figure 17D shows nitrogen-doped graphene foams employed for high-rate Na anodes.291 Moreover, the carbons are often heteroatom-rich (especially oxygen) and contain varying degrees of graphene ordering. Highly defective and/or heteroatom carbons do not fit into the classic taxonomy for carbon allotropes, rather representing S


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Figure 18. (Top) Schematic illustrating the well-known Li staging reactions in graphite, showing the orderly transition from stage 4 to stage 1. (Bottom) Illustration of the “falling cards” model of Li storage in disordered carbons, with ions adsorbed as a defect, locally intercalated, and plated out as metal in nanopores.

Figure 19. Li charge−discharge behavior of undoped carbon nanosheets with a Brunauer−Emmett−Teller (BET) surface area of 2200 m2/g and IG/ID = 0.9. (A, B) Effect of current density on the shape of the charge−discharge profiles, at cathode and anode potentials, respectively.

employed for NICs and high-rate NIBs are substantially more ordered with IG/ID ≈ 1 or higher. Moreover, the relevant charging rates are ∼4C (15 min charge rate) to upward of 60C (1 min charge rate). This creates issues in further overinterpreting the already qualitative falling cards model by extrapolating quasiequilibrium to fast-charge scenarios. Figure 19 shows the charge−discharge behavior of undoped carbon nanosheets with a BET surface area of 2200 m2/g and IG/ID = 0.9.11 As may be seen in Figure 19A, within the HIC cathode regime (4.5−3 V vs Li/Li+) the shape of the galvanostatic curve does not appear to vary significantly with the current density. This implies a similarity in the charge-

storage mechanisms with charge rate, likely being reversible ion surface adsorption. It should be noted that the initial voltage was 4 V vs Li/Li+, which is on par with state-of-the-art ceramic Li cathodes. The average voltage, however, is lower, being in the 3.5 V range, depending on the lower cutoff. Within the HIC anode regime, shown in Figure 19B, the galvanostatic profiles evolve substantially with current density, indicating one or a series of Li storage-mechanism transitions. Li charge-storage mechanisms in such highly defective carbons have recently been investigated through simulations. This is direct evidence for the necessity of graphene defects in achieving a reversible capacity with Li that far surpasses that of T


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Figure 20. Example of analysis of the sodiation charge−discharge behavior in undoped nongraphitic carbon nanosheets with IG/ID ≈ 1. (A) Per the XPS results, near 0 V vs Na/Na+ there is no evidence of Na metal plating but a clear change in Na−C bonding. Per the Raman results in (B), sodium insertion between the defective graphene planes causes ordering of the carbon. Per the XRD results in (C) and (D), sodium insertion causes reversible graphene layer dilation, qualitatively akin to Li and graphite.127 Reproduced with permission from ref 127. Copyright 2014 Royal Society of Chemistry.

reduced graphene oxide (RGO) do get employed for NIC and high-power NIB electrodes.111,291,310,321,325 RGO is a highly defective structure that is far from that of graphite or wellordered single-layer graphene. Rather, RGO is akin to carbon nanosheets in terms of Raman IG/ID ratio, heteroatom content, surface area, and porosity.107,109,286 For NICs, likewise to LICs, the charging rates that are employed for testing are far above the current densities normally employed for LIB graphite. With LIB graphite, the typical maximum is 5C (i.e., 12 min to full charge). Conversely, researchers test nanocarbon electrodes at rates of up to 100C. Carbon nanomaterials always possess high surface area and are often nanoporous. They may be heteroatom-rich and will contain varying degrees of graphene ordering. Such carbons do not neatly fit into the classic taxonomy of allotropes. Rather they represent a continuum in structure and chemistry, bordered by Na inert pristine graphite on one end and fully amorphous activated carbon on the other. Sodium storage in nanostructured nongraphitic carbons is poorly understood in general, even at slow rates where nearequilibrium Na arrangement may be achieved. This makes the already subjective fallen cards model developed for Li (really a fallen cards schematic) less applicable. While useful for obtaining near-equilibrium conditions, a charging current that takes 10 h to reach full capacity may not be representative for NICs or high-rate NIBs. Nevertheless, it is useful to understand the existing low-rate state of knowledge, so as to establish a baseline from which the high-rate mechanisms may be better understood. To

commercial graphite. At voltages above those for intercalation, charge storage in LIBs has been ascribed to the following mechanisms: chemisorption on surface heteroatoms285,302 and reversible adsorption at structural defect sites in the graphene.303−307 The sloping high-voltage charge-storage behavior is attributed to reversible binding of Na at graphene divacancies and Stone−Wales defects, as has been recently predicted by ab initio calculations.304 It has been shown that a divacancy is the thermodynamically most stable defect in graphene305,308,309 and will also act as a preferential Li adsorption site.305,310 9.5. Carbons for Na Electrodes

In our opinion, microstructurally designed carbons are perhaps the most economical and technically viable candidates for NIC applications, as Na kinetics in titania structures tend to be substantially more sluggish than corresponding Li kinetics.210−213 It is well-established that graphite will not intercalate Na to an appreciable extent, due to its larger ionic radius than Li (0.102 nm vs 0.076 nm) and weaker chemical interaction with graphene planes.293,311,312 Instead, a range of nongraphitic carbons has been employed to store Na, including N-doped carbons.68,281,313−323 Table 2 also compiles a representative sample of various 2D carbons employed for high-rate Na storage. Graphite or “graphite nanoflakes” with equilibrium graphite structure and layer spacing will not reversibly intercalate Na in conventional battery electrolytes. Because of this they have received minimal attention in the sodium literature, except for the unusual cases of Na cointercalation with a secondary ion.324 Some versions of U


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reversible increase in the Raman G/D ratio as 0 V vs Na/Na+ was approached. Such staging was not observed when these carbons were tested against Li. The partial staging phenomenon is caused by the intercalation of Na into the dilated graphene interlayers and had been confirmed recently in other partially ordered carbons.328 Importantly, the XRD pattern in the above references showed no evidence of metallic Na plating into the pores of the carbon at any of the voltages. The most intense Na metal reflection would correspond to the {110} family and would be located at 2θ = 29.3° in the pattern. Underpotential Na metal deposition into the carbons’ nanopores is expected to occur close to the half-cell potential of the metal in that electrolyte. For the case of Li plating into the nanopores of hard carbons, the phenomenon was confirmed by using analogous X-ray analysis.326 Conversely, Na metal pore filling did not occur, boding well for the potential long-term cyclability of highsurface-area carbon-based NIC anodes. Metal plating appears to be a key difference between lowrate charge-storage behaviors of Na versus Li nongraphitic carbons and may be an excellent reason to consider NIC devices. Ongoing plating−stripping of Li metal, even at the nanoscale, may be dangerous in terms of ultimately leading to dendrites. Even if that does not occur, fresh Li metal is catalytic to SEI formation, which means that the cycling CE would naturally become depressed. If this can be avoided by employing Na instead, there is a major safety and cyclability benefit. Certainly the field would benefit from more extensive research into the Na versus Li plating phenomena, as plating is the safety issue affecting and holding back LIC devices.

investigate the Na charge-storage mechanisms in nongraphitic carbons, Mitlin and co-workers were the first to perform X-ray studies during different stages of sodiation, discovering and quantifying reversible Na intercalation at low voltages.107,127,326,327 It was demonstrated that with progressive sodiation at voltages below 0.2 V vs Na/Na+ there is a reversible shift of the (002) graphene peak to reduced 2θ values. The authors proposed that such reversible dilation− contraction of the graphene layer spacing is qualitatively analogous to Li intercalation into equilibrium graphite, which occurs in a series of staging reactions. It was demonstrated that such intercalation only occurs for well-ordered, albeit amorphous, carbons with a Raman IG/ID integrated intensity ratio ∼1 or greater. Sodium intercalation was shown not to occur with highly disordered carbons, such as conventional activated carbon where IG/ID is 220 Wh/ kg for an optimized commercial LIB but also much higher than ∼7 Wh/kg for an optimized commercial ultracapacitor. For instance, Ruoff’s group employed active graphene to replace the activated carbon and achieved 148 Wh kg−1 with a 2−4 V testing window.60 Typical device anodes include hard carbons, 119,152 soft carbons, 279 graphene-based carbons,142,280,282 and other forms of nongraphitic carbon materials.177,278 Mitlin’s group was the first to fabricate a hard carbonactivated carbon NIC device.107 The anode design approach W


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Table 3. Summary of the Pseudocapacitive Oxides/Nitrides and MXenes-Based LIC and NIC Devices device configuration (anode//cathode) Nb2O5-based Nb2O5−CNT//activated carbon346 T-Nb2O5@C//MSP-20 activated carbon347 Nb2O5-carbide-derived carbon//YP-50F AC348 T-Nb2O5-graphene//activated carbon349 mesoporous Nb2O5−C//activated carbon350 Nb2O5@rGO//MSP-20 activated carbon259 Nb2O5 nanosheets// activated carbon258 V2O5-based PPy@V2O5 nanoribbons//AC351 V2O5 nanosheets//carbon fibers352 V2O5@CNT//activated carbon353 V2O5@CNT//activated carbon354 MXene-based CTAB-Sn@Ti3C2 MXene//activated carbon355 Ti2C MXene//YP17 active carbon356 TiC MXene//N-doped porous carbon177 Ti2CTx MXene//Na2Fe2(SO4)3357 hard carbon//V2CTx MXene108 Ti3C2 MXene-CNT//Na0.44MnO2358



type

voltage

capacity retention

LIC LIC

0.5−3 V 1−3.5 V

33.5@82 63@70

4@4000 10@6500

LIC

1−2.8 V

30@220

18@5000

LIC LIC

0.8−3 V 1−3.5 V

47@393 74@120

15@18000 20@12137

93% over 2K cycles

NIC

1−4.3 V

76@80

12@8000

66% over 3K cycles

NIC

1−3 V

43.2@160

24@5760

80% over 3K cycles

aq. aq. LIC NIC

0−1.8 V 0−2 V 0−2.7 V 0−2.8 V

42@120 22.3@320 40@210 38@140

24@2500 8@1500 6.9@6300 7.5@5000

95% over 10K cycles 90% over 10K cycles 78% over 10K cycles 80% over 0.9K cycles

LIC

1−4 V

105.6@495

45.3@10800

70% over 4K cycles

LIC LIC NIC

1−3.5 V 0−4.5 V 0.1−3.8 V

50@190 101.5@450 64@72

15@600 23.4@67500 52@288

NIC NIC

0−3.5 V 0−4 V

85% over 1K cycles 82% over 5K cycles 100% over 100 cycles 89% over 300 cycles 90% over 60 cycles

75% over 1K cycles

extremely facile, giving “capacitor-like” charging behavior. These materials were termed pseudocapacitive intercalation compounds because, while the Li storage process was not EDLC per se, it displayed EDLC-like triangular galvanostatic curves and box-shaped CVs.251 Parts A and B of Figure 22 show the galvanostatic and potentiodynamic of this material.251 From both the CV and the galvanostatic curves it may be observed that the useful voltage range for T-Nb2O5 with graphene is 1−1.75 V, putting it in the same category as the titanium oxide compounds. The T-Nb2O5 has been employed for anodes in LICs.346−349,359−361 Figure 22C shows a schematic of the T-Nb2O5 structure. As shown in Table 3, these LICs will deliver a maximum of 76 Wh kg−1 at relatively low power,259 making their overall specific energy about a factor of 2 lower than that for carbonbased devices. Issues reducing the overall device energy are the low electrical conductivity of T-Nb2O5 and its intrinsically sloping profile, which as discussed reduces the overall cell energy. This is another illustration of what we believe is a key guiding heuristic for future NIC and LIC material design: Create electrodes that possess facile and highly reversible charge−discharge kinetics but which demonstrate flat batterylike profiles. Of course this is extremely difficult to achieve and, hence, should be a focus of future research activity. Vanadium oxide V2O5 undergoes bulk ion intercalation reaction during reversible charging, also producing a capacitorlike sloping profile when employed as an anode. In aqueous systems, bulk V2O5 will accommodate cations (e.g., H+, Li+, and K+) from the electrolyte.351−353 Authors have combined elemental analysis (ICP) and X-ray diffraction to investigate the charge-storage mechanism of V2O5 in aqueous system. They discovered that K+ ions insert in the interlayer spaces between the (00l) planes.351 Sodium intercalation becomes

sloping and with a higher hysteresis, both effects leading to an energy reduction in the device. On the cathode side at high voltage, the Na and Li storage capacities of ion-adsorption electrodes appear to be comparable.60,107,111,141,280,282,286 More work is needed to establish the electrolyte transport and solid-state transport differences (in carbon) for Li versus Na. Also more work is needed to understand the high-voltage charge-storage mechanisms in high-surface-area carbons, which may also involve anions because the material is positively polarized. 9.7. Emerging Architectures: Pseudocapacitive Oxides

Apart from carbons and titanium compounds, there is a range of emerging anode materials that are either being developed explicitly for HICs or are being applied to hybrid devices that had original applications in LIBs or NIBs. Overwhelmingly, these nanomaterials are utilized for anodes, not cathodes. Conversely, even the best adsorption cathodes, which are based on high-surface-area carbons, offer a fraction of the capacity of existing HIC anodes. There is little evidence of extended cyclability (10 000 cycles or more) when noncarbon cathodes are employed, per the data provided in Table 3. There is much more need for HIC cathode research in the field as LIB and NIB cathodes are not directly transferable. It is the lack of comparable high-power and high-cyclability hybrid device cathode that is a key outstanding problem for the field, limiting the potential technological applications of devices. The pseudocapacitive materials are emerging as an important materials subset for HICs due to their rapid charging−discharging kinetics, combined with a higher gravimetric and volumetric capacity as compared to true EDLC electrodes.13 For instance, Dunn and co-workers demonstrated that the Li+ intercalation into the ordered channels of bulk orthorhombic Nb2O5 (i.e., T-Nb2O5) was X


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Figure 22. (A) Cyclic voltammograms of thick T-Nb2O5 electrodes obtained at 100−500 mV s−1. (B) Galvanostatic cycling of T-Nb2O5 electrode, obtained at 10C current density. A schematic illustration of the structure of T-Nb2O5 stacked along the c axis. The layered arrangement of oxygen (red) and niobium (inside polyhedra) atoms along a−b plane is demonstrated.251 (A−C) Reproduced with permission from ref 251. Copyright 2013 Springer Nature.

Figure 23. Dilation of the interlayer space of bilayered V2O5 upon sodiation. Reproduced with permission from ref 362. Copyright 2017 American Chemical Society.

energy values of the LIC and NIC cells are in the 40 Wh kg−1 range, akin to the aqueous electrolyte systems.

significantly more facile in the bilayered V2O5 phase due to the dilated layer spacing (∼11.53 Å) in the 001 direction, as compared to the orthorhombic V2O5 phase. (Figure 23).362 Per Table 3, a maximum energy of 42 Wh kg−1 can be obtained with 50%) in the initial ∼10 cycles.390,402,403 Of course, this prohibits its utilization in high-rate−high-cycling applications.

alloy + Li2O/Na2O system, which is a classic conversion compound with a sloping voltage profile for both Li and Na. 93,98 While it is popular to label such systems pseudocapacitive, in our opinion this label is not entirely appropriate: The sloping charge−discharge profiles for almost all conversion compounds are far from perfectly triangular and contain a large hysteresis. Importantly, a large number of these systems are actually diffusion-limited (rather than reactionlimited), even at intermediate charge−discharge rates. Examples of systems to date employed for HICs include MoS2,366 NbN,367 VN,368 MnO,11,369,370 Fe2O3/Fe3O4,371−373 NiCo2O4,374 various alloys,375−378 etc.379−384 For instance, while MoS2 does undergo Li intercalation reaction down to 1.1 V (arguably only during cycle 1), the majority of the reversible capacity stems from the reversible conversion reaction to Mo and Li2S down to 0 V.385 The reversible capacity of MoS2 has been reported to be as high as 1 000 mAhg−1, which is actually higher than the theoretical. The discrepancy may be due to a capacitive contribution and a contribution from reversible SEI growth.94,386 Looking ahead, it should be possible to employ almost any fast-reacting conversion electrode material for LIC or NIC anodes, as there is not a strict requirement for orderly ion intercalation as there is in T-Nb2O5, etc. Authors have employed MoS2-graphene composites in LICs, taking advantage of sloping charge−discharge profiles.366 The final LIC delivered energy density as high as 188 Wh kg−1 at 200 W kg−1 and 45.3 Wh kg−1at 40 000 W kg−1, which is quite outstanding. The practical capacity of MoS2 was ∼600 mAh/g and helps to offset the sloping voltage plateau in delivering high energy. The outstanding rate capability and cycling capacity retention likely originated from optimized carbonbased secondary phase in the anode. The same group also employed similar materials in a NIC.387 Interestingly in the sodium system, they used a bottom cutoff voltage of 0.5 V vs Na/Na+, which largely limits MoS2 primarily to intercalation. Therefore, ∼200 mAhg−1 capacity was obtained in the anode side. With a PANI-derived porous carbon cathode, the final AA


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Table 5. Summary of the Ceramic Intercalation Compounds-Based LIC and NIC Devices type

voltage



capacity retention

aq. LIC

0−1.8 V (aq.) 0−3 V

37@125 49@900

18@850 19@3000

40% over 3K cycles 100% over 3K cyles

LIC LIC NIC

0.7−3 V 1.5−3.25 V 0−2.7 V

45@60 19@120 26@76

10@800 8@3500 13@750

81% over 1K cycles

LIC

2−4 V

69

activated carbon// NaMn1/3Co1/3Ni1/3PO4417 NASICON-based Li3V2(PO4)3//activated carbon419

NIC

0−3 V

50@180

LIC

0−4 V

125@300

65@6000

activated carbon//Li3V2(PO4)3419 activated carbon//Li3V2(PO4)3418 activated carbon//Na3V2(PO4)3420

LIC LIC NIC

0−2.5 V 0.5−2.7 V 0−1.5 V (aq.)

28@35 25@88 16@490

14@1500 13@320 11.6@750

Na3V2(PO4)3//bioinspired activated carbon405 NaTi2(PO4)3//graphene nanosheets406

NIC

0−3 V

118@95

60@850

NIC

0−3 V

80@150

33@8000

NaTi2(PO4)3-rGO//activated carbon422 NaTi2(PO4)3 mesocrystals//activated carbon421 other architectures Li2MnSiO4//activated carbon423 LiMnBO3//PANI425

NIC NIC

0−2.7 V 0−2.5 V

53@334 56@39

31@6680 31@4096

LIC LIC

0−3 V 0−3 V

54@150 42@1500

37@1500 15@5350

device configuration (anode//cathode) layered/spinel oxides-based activated carbon//LiCoO2407 activated carbon//LiMn1/3Ni1/3Fe1/3O2− PANI61 activated carbon//LiMn2O448 LiNi0.5Mn1.5O4//activated carbon411 activated carbon//Na0.67Mn0.75Al0.25O2408 olivine-based mesocarbon microbeads//LiFePO4416

9.10. Emerging Architectures: Battery-Related Intercalation Ceramics

100% over 100 cycles 90% over 1K cycles

80% over 200 cycles 87% over 1K cycle 80% over 100 cycles 95% over 10K cycles 90% over 75K cycles 98% over 5K cycles 100% over 20K cycles 85% over 1K cycles 91% over 1K cycles

reported energies are on-par with conventional asymmetric ultracapacitors. For instance, an AC//LiCoO2 aqueous hybrid cell worked between 0 and 1.8 V and delivered maximum 37 Wh kg−1 energy at a power of 125 W kg−1.407 Authors prepared a layered LiCo1/3Ni1/3Fe1/3O2 based on the parent LiCo1/3Ni1/3Mn1/3O2 phase.61 They coated the compounds with conductive polymers (e.g., PANI, PPy) to further improve the kinetics of the intercalation cathode. The best AC// LiCo1/3Ni1/3Fe1/3O2−PANI setup delivered maximum energy of 49 Wh kg−1 with negligible decay upon 3000 cycles.61 For the sodium system, authors prepared Na0.67[Mn0.75Al0.25]O2 compound by Al substituting from the layered P2− Na0.67MnO2 phase. The Na0.67[Mn0.75Al0.25]O2 had a sloping capacity profile between 2 and 4 V and was coupled to a carbon anode.408 In an organic electrolyte, this NIC operated between 0 and 2.7 V and delivered maximum energy of 26 Wh kg−1. Converted to device specific energy, such a NIC is o- par with commercial EDLC systems. In general, intercalation cathodes give worse performance for Na than they do for Li. The lower energy of the NIC devices are in part due to the lower voltage of the layered oxides; for instance, the voltage of LiCoO2 is ∼4 V, while for Na0.67MnO2 the average voltage is only ca. 2.5 V. Olivine LiFePO4 is a well-established commercial LIB cathode that has seen applications in hybrid devices as well. It undergoes a two-phase reaction during lithium intercalation/ extraction and exhibits a flat plateau at 3.4 V. Authors designed a hybrid AC + LiFePO4 composite as the cathode and mesocarbon microbeads as the anode.416 The device was cycled between 2 and 4 V with a maximum energy density of 69 Wh kg−1 and a lifetime of 100 cycles with negligible decay. Authors synthesized and tested NaMn1/3 Co 1/3 Ni 1/3 PO 4

Researchers have also utilized ceramic battery cathode materials coupled against carbon counter electrodes, in effect designing a high-power battery with a more sloping voltage profile. In 2014, Aravindan et al. offered an excellent overview of intercalation-type materials for hybrid lithium ion configurations.404 Advances in such architectures continue to accelerate, now including the sodium-based systems as well. Table 5 summarizes the published configurations. The vast majority of these architectures possess battery-like cyclability, surviving several thousands of cycles. The two very notable and hence uniquely promising exceptions are the Na3V2(PO4)3// bioinspired carbon405 and NaTi2(PO4)3//graphene nanosheets,406 which survive 10 000 and 75 000 cycles, respectively. These architectures clearly warrant further investigation. Because the anode in each case appears to be a fairly straightforward, albeit well optimized, carbon, the secret of enhanced cyclability may lie in the unusual stability of the cathodes. As detailed in Table 5, there are various types of ceramic battery cathodes being used in both LICs and NICs, for instance, layered oxides,61,407−409 spinel oxide,410−414 olivine,415−417 NASICON,405,406,418−422 and silicates.423,424 In 2006, Xia’s group developed an aqueous Li 2 SO 4 electrolyte hybrid system based on a layered oxide cathode (i.e., LiCoO2, LiCo1/3Ni1/3Mn1/3O2) and an activated carbon anode.407 The authors argued that the SO4− ions do not participate in charge storage and that only Li+ is relevant. This is different from symmetric carbon ultracapacitors, where it is assumed that both the cations and the anions are relevant to the total reversible capacity.267 With aqueous HICs the AB


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Figure 25. Possible crystal structures of Na1, Na3, Na5-V2(PO4)3, and the voltage profiles corresponding to the phase transformation in sodium cells. (Top and Bottom Right) Reproduced with permission from ref 434. Copyright 2015 Royal Society of Chemistry. (Bottom Left) Reprinted with permission from ref 435. Copyright 2014 Elsevier Ltd.

LIC device, obtaining 25 Wh kg−1.418 An energy density of 28 Wh kg−1 was reported for a AC//LVP configuration.419 However, when the activated carbon is employed as a cathode while LVP is the anode, the energy density is markedly improved. For instance, 125 Wh kg−1 can be obtained at 300 W kg−1.419 An AC anode−NVP cathode-based NIC delivered maximum 16 Wh kg−1 in an aqueous electrolyte.420 However, a device based on an opposite configuration, i.e., a NVP anode− bioderived carbon cathode, delivered 118 Wh kg−1.405 Given this data, it may be argued that NASICON possesses primary utility as an anode material only.

compound referring to the parent NaFePO4 phase.417 Similar to the above-mentioned LiFePO4 LIC configuration, the sodium-based olivine phase was also used as the cathode and commercially available carbon was employed as the anode. The NIC delivered 50 Wh kg−1 energy at the power of 180 W kg−1, which is comparable to the olivine-based LICs.417 To us, it is somewhat surprising that better performance has not been achieved with LiFePO4, as this is a well-known system that has undergone many optimization routes for high-power applications.125,426−433 Recently NASICON cathodes have received attention for hybrid device applications. The most often studied NASICONs are Li3V2(PO4)3 (LVP) and Na3V2(PO4)3 (NVP) phases, with vanadium as the active transition metal element. Due to the multivalence change of vanadium, these compounds can reversibly intercalate multiple alkali ions at different potentials. Taking Na3V2(PO4)3 as an example, as shown in Figure 25, there are 6b, 18e, and 6a sodium sites in the polyhedral framework in the NVP structure. Phase transformation happens during sodiation from Na1V2(PO4)3 to Na3V2(PO4)3 and further to Na5V2(PO4)3, being accompanied with the reduction of V4+ to V3+ and V2+. The V4+/V3+ redox relates to the high-voltage plateau at 3.4 V and the V3+/ V2+ redox related to the low voltage below 2 V.419,434,435 On the basis of this kind of multielectron reaction of vanadium, the LVP and NVP can act as both a cathode and an anode, depending on which voltage plateaus are employed. Authors have incorporated a LVP-C cathode with an AC as anode in a

10. RAGONE CHART COMPARISONS AND CONCLUDING THOUGHTS As shown in Figure 26, the final energy and power of the LIC and NIC devices largely depends on the configuration strategies and the charge-storage nature of each electrode. The panels are separated along a similar taxonomy as the discussion: (A) carbonaceous materials based;107,109,119,145,286,345 (B) titanium oxides and compounds based;158,195,227,248,269,271,272,275 (C) pseudocapacitive oxides/ nitrides259,350,353,354,374,436 and MXenes;355,357 and (D) ceramic intercalation compounds.405,407,408,416,417,419,420 In general, architectures with the flattest anode and cathode voltage versus capacity characteristics and largest total voltage window come out on top. The energy−power advantage that is observed with conventional lithium versus sodium ion batteries is not obvious with LIC versus NIC devices. In fact, for allAC


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Figure 26. Ragone plot of representative LIC and NIC devices based on different categories of electrode materials. (A) Carbonaceous materials based. (B) Titanium oxides and compounds based. (C) Pseudocapacitive oxides/nitrides and MXenes based. (D) Ceramic intercalation compounds based.

carbon-based devices, Na systems appear superior. Because Na is a slower solid-state diffuser in most materials, further investigation for this interesting and nonintuitive finding is needed. One implication may be that, at least for the carbon− carbon systems, solid-state diffusivity is not the rate-limiting phenomena in either electrode, hence showing a fundamental difference versus LIBs and NIBs and greater similarly to ultracapacitors. While the current HIC architectures are fairly straightforward in terms of materials sets, the associated highrate charge-storage mechanisms in the cathode and especially in the anode are poorly understood and clearly warrant more research. To summarize, in our opinion, the core outstanding questions for the HIC field include the following: electrode design, because hybrid high-surface-area positive electrodes do not intrinsically contain a solid source of shuttling ions while ion-containing ceramic battery cathodes do not provide the necessary cycling life; the energy−power limitations associated with the triangular capacitor-like charge−discharge profiles of most HICs; the fast charge-storage mechanisms in the negative electrodes that are not understood and may fundamentally differ from the low-rate charge-storage behavior in LIBs; the HIC cycling life that is superior to lithium ion batteries but lacks far behind classic EDLCs; and a potential performance (and cost) advantage of transitioning from the dominant lithium ion capacitor (LIC) to the relatively unexplored sodium ion capacitor (NIC). However, we should caution against over interpreting the performance comparisons: In our opinion, a substantial part of “good” versus “OK” performance of various devices is strongly linked to electrode and device design factors rather than just

materials sets per se. These include quality/integrity of the fabricated electrodes in terms of material slurry and dispersion, proper capacity balancing of the anode versus cathode, the use of additives, the charging−discharging depths/rates employed, electrode-preconditioning and prelithiating/sodiating, etc. These extrinsic parameters are distinct from intrinsic properties limiting performance such as the volume expansion of a given material upon lithiation/sodiation or the reversibility of certain conversion reactions. We feel that at this point there are still not enough NIC publications so as to allow for a completely rigorous comparison of the intrinsic performance of device architectures based on materials sets. The NIC topic is just too new, which also makes it a very exciting area in which to work. In principle, it should be the negative electrode that is problematic for cycling life due to formation of SEI, possibility of low-voltage metal plating, and volume expansion associated with deep discharge. While an activated carbon or other highsurface-area cathode would operate at a voltage where CEI is formed, the process should, at least in principle, be less damaging due to minimal volume expansion: CEI may not accumulate over cycling due to the low expansion of the carbon that does not create new catalytic surfaces with every charge−discharge. However, CEI formation is almost entirely unexplored for LICs and NICs. From an energy density vantage, the situation is reversed: There does not seem to be a satisfactory cathode material that could operate at comparable capacity as the anode while maintaining cycling stability. Activated carbon is stable,but offers down to one-fifth of the reversible capacity of a high-quality hard carbon anode, as well as the undesirable sloping voltage profile. More work is needed AD


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AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

David Mitlin: 0000-0002-7556-3575 Notes

The authors declare no competing financial interest. Biographies Jia Ding is a postdoc in State University of New York at Binghamton. Prior to that, Jia Ding received his Doctorate from University of Alberta, Canada. He also holds a M.S. degree in Materials Science from Chinese Academy of Science, received in 2012, and a B.S. degree from Huazhong University of Sciences and Technology, received in 2009. Dr. Ding is passionate in design and fabrication of advanced materials for various energy-storage systems including Li/Na ion batteries and capacitors. Wenbin Hu is a Professor and Dean of the School of Materials Science and Engineering at Tianjin University. Prior to joining the faculty at Tianjin University, he worked as a Professor in Department of Materials Science and Engineering at Shanghai Jiao Tong University. He graduated from Central-South University with a B.Sc. in 1988 and received his M.Sc. from Tianjin University in 1991. He received his Ph.D. from Central-South University in 1994. Dr. Hu’s research interests focus on design, synthesis, and characterization of advanced micro-/nanomaterials for energy-storage and -conversion applications. Eunsu Paek is an assistant professor in the Department of Chemical & Biomolecular Engineering at Clarkson University. Dr. Paek’s research focuses on developing theoretical foundations for guiding the rational design and synthesis of novel nanomaterials for energy and environmental applications. She has published over 25 publications in peer-reviewed journals and has given over 30 invited and contributed talks. Dr. Paek received a Ph.D. in the Chemical Engineering Department at the University of Texas at Austin in 2012, and M.S. and B.S. from Seoul National University in 2006 and 2004. David Mitlin is a Professor and General Electric Chair at Clarkson University, in the Department of Chemical & Biomolecular Engineering. Prior to that, Dr. Mitlin was an Assistant, Associate, and full Professor at the University of Alberta, Canada. Dr. Mitlin has published over 135 peer-reviewed journal articles on various aspects of energy-storage and -conversion materials. He holds five granted patents, four of which are licensed, and has presented over 100 invited, keynote, or plenary talks. Dr. Mitlin is an Associate Editor for Sustainable Energy and Fuels, a Royal Society of Chemistry Journal focused on renewables. Dave received a Doctorate in Materials Science from U.C. Berkeley in 2000, M.S. from Penn State in 1996, and B.S. from RPI in 1995.

ACKNOWLEDGMENTS This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award no. DE-SC0018074. AE


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