Anion Hosting Cathodes in Dual-Ion Batteries - ACS Energy Letters

Anion Hosting Cathodes in Dual-Ion Batteries Ismael A. Rodríguez-Pérez and Xiulei Ji* Department of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003, United States ABSTRACT: Dual-ion batteries operate on two intercalants: anions for the cathode and cations for the anode. This battery was initially known as a dual-graphite battery, where both electrodes are graphitic carbon. The primary challenge of dual-graphite batteries is the very high operation potential of the cathode, often requiring an upper cutoff potential above 5 V vs Li+/Li. Such a potential readily oxidizes alkyl and alkylene carbonatebased electrolytes. The anode side, in fact, can employ any anode of most metal-ion batteries, although, to date, the focus has still been the Li− graphite anode. Recent progress has significantly advanced the technology readiness level for this battery. Additives or ionic liquid electrolytes help mitigate cathode irreversibility; nongraphite anodes, such as aluminum, allow new carbonate electrolytes that lack the necessity of ethylene carbonate; nongraphite cathodes, including metal−organic frameworks and polycyclic aromatic hydrocarbons have exhibited a remarkable potential. This Perspective highlights the challenges, summarizes the recent progress, and attempts to point out future directions in the field.

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of ions. Thus, the salt concentration of the electrolyte is a critical factor for the DIBs’ performance, where the electrolyte must be considered as a part of the active mass.6 For the anode side, there exist a plethora of chemistries that can use a variety of charge carriers, such as Li+, Na+, or K+. In principle, any anode material in most nonaqueous metal-ion batteries, such as LIBs,7 sodium-ion batteries (NIBs),8 and potassium-ion batteries (KIBs),9 would work as an anode for DIBs. Indeed, the intriguing properties of DIBs originate from the fact that the cathode operates on insertion/deinsertion of anions instead of cations.10,11 To date, most DIBs, often known as dual-graphite batteries,3 comprise a graphite cathode and a metal-ion insertion anode in a nonaqueous electrolyte. Anions that are readily solvated in nonaqueous electrolyte are typically polyatomic, such as ClO4−, PF6−, AsF6−, and SbF6−, where these ions are bulkier than monatomic metal ions in batteries. To host such anions reversibly, the structure of an electrode should contain large interstitial sites. It appears that such electrodes are preferably assembled by van der Waals forces at least along one crystallographic direction, thus exhibiting a layered structure. The first example is graphite that contains graphene layers stacked in an ABAB sequence with a large interlayer d spacing of 3.35 Å. As of now, the primary attention on the DIB cathode from the community has been focused on graphite. Graphite is known by its redox amphotericity, where its 2D zero-gap semimetallic electronic structure delocalizes and

enewable solar and wind power provide intermittent energy, which requires energy storage solutions.1 Largescale energy storage methods, such as pumped hydroelectric storage and compressed air storage, are geographically restricted, which cannot provide modular storage solutions. This is where batteries and electrochemical capacitors (ECs) could play a pivotal role to enhance energy efficiency and security.1,2 Despite enormous successes in portable electronics and electric vehicles (EVs), lithium-ion batteries (LIBs) face challenges of sustainability due to the rarity and high mining cost of lithium.1 In the search for alternatives of LIBs, dual-ion batteries (DIBs), which were first introduced by McCullough3 at Dow Chemical Company in the late 1980s, have caught much attention recently. Different from metal-ion batteries that have metal cations commuting between the anode and cathode during charge/discharge, thus resembling a rocking chair, DIBs, for their name’s sake, rely on both cations and anions, which are incorporated into the anode and cathode, respectively, during charge, where these ions are released from the electrodes into the electrolyte during discharge (Figure 1). A similar mechanism is that of hybrid ECs, for example, lithium-ion capacitors,4 where the anode as in DIBs and LIBs operates on the topochemistry of lithium intercalation in the anode lattice, while the cathode, that is, nanoporous carbon, takes in solvated anions onto the surface of its open structure in the fashion of electrical double layer capacitors (EDLCs).5 In a sense, such a device does operate on storage of both anions and cations in a “dual-ion” configuration; however, we deem that such a device is beyond the scope of this Perspective due to the different storage mechanisms on the cathode side. Note that, in DIBs, neither the anode nor cathode is a source of ionic charge carriers, where the electrolyte is the sole source © 2017 American Chemical Society

Received: April 14, 2017 Accepted: July 7, 2017 Published: July 7, 2017 1762

DOI: 10.1021/acsenergylett.7b00321 ACS Energy Lett. 2017, 2, 1762−1770

Perspective

http://pubs.acs.org/journal/aelccp

ACS Energy Letters

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Figure 1. Schematic of a DIB showing the use of both ions in the electrolyte.

Origin of Cathode Potentials in DIBs. If we do not consider the entropy change during the charge process of an anion-insertion cathode//Li half-cell, we propose that the free energy change, ΔG, of the cathode//Li half-cell when transferring one electron can be described by the eq 1, where E(C+A−) is the energy of the charged cathode with an anion inserted, for example, forming an acceptor-type GIC, E(C) is the energy of the cathode before intercalation, E(Li) is the cohesive energy of Li metal for the addition of one lithium atom, and ΔH(desolv. of Li+) and ΔH(desolv. of A−) are the desolvation energies of Li+ cations and anions from the electrolyte, respectively. From this equation, it is suggested that the operation potential of the cathode is related to the identities of the cathode and the lattice energy of the C+A− “salt”, as well as the solvents used in the electrolyte. Equation 1 also provides a roadmap with respect to tuning the cathode potentials. In this Perspective, we will discuss the challenges of DIBs, which mainly arise from the high operation potentials of its cathode and the consequent incompatibility with aprotic solvents. We will then briefly summarize the known approaches to tackle these challenges.

consequently stabilizes either an excess of electrons in its antibonding π*-band by forming donor-type graphite intercalation compounds (GICs) or electron holes in its bonding πband by forming acceptor-type GICs. In acceptor-type GICs, first reported by Rüdorff and Hofmann, graphene is converted to a macrocarbocation when hosting inserted anions.12−14 In the 1970s, the electrochemical performance of acceptor-type GICs was systematically studied, which was reviewed by Armand and Touzain.15,16 Later attention on acceptor GIC electrodes was given to nonaqueous electrolytes that would be well-known by the LIB community later on. For example, Jobert et al. found that PF6− and SbF6− solvated by four propylene carbonate (PC) molecules can be inserted into graphite, forming a GIC of C24+A− 4PC at a potential of 5.2 V vs Li+/Li, and suggested such reactions for the use of highpotential batteries.10 Billaud et al. studied a similar C24+AsF6− CH3NO2, which is formed at a potential of 4.6 V vs Li+/Li.17 Therefore, hosting anions in graphite cathodes had been a known art before DIBs were invented. The donor-type GICs had been well-known before Libatteries were investigated; however, the donor GICs, such as Li-GICs, were not the most obvious of choices for anodes in LIBs because the more energetic lithium metal was contemplated as a commercially viable anode. Recently, however, lithium metal has received significant attention again, where this time it is the anode for Li−S and Li−O2 batteries.18 It is worth noting that during the late 1980s and early 1990s, due to safety concerns of the lithium metal anode, significant progress was made on Li-GICs as the anode for LIBs, particularly in terms of the electrolyte formula, which is able to form a stable solid electrolyte interphase (SEI) on the graphite anode surface.19 The SEI prevents co-intercalation of electrolyte solvents into graphite, thus facilitating superior cycling stability of LIBs.19 The concept of DIBs was proposed when both the graphite cathode and anode as well as the alkyl/ alkylene carbonate-based electrolytes were available for nonaqueous batteries.

ΔG = − eV ≈ ΔH = E(C+A−) − E(C) + E(Li) + ΔH(desolv. of Li+) + ΔH(desolv. of A−)

(1)

In order to decrease the cathode potentials in DIBs, one can turn ΔG in eq 1 to be less negative by lowering the absolute values of the negative terms or increasing the absolute values of the positive terms. Therefore, as the first route, if the term of E(C+A−) − E(C) becomes less negative, the resulting voltage of the cell may be lowered. Here, one can use different hosts other than graphite or employ different anions. The second route is related to the electrolyte solvents, where their solvation could affect cell potentials. This is vastly different from rocking-chair metal-ion batteries, in which the solvation term on one electrode and the desolvation term on the other electrode for the same ion would be canceled out. To lower the cell potentials, one can increase the desolvation enthalpy values ΔH(desolv. of Li+) and ΔH(desolv. of A−), where the fact that 1763


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Figure 2. (a) Voltage vs time (h profile of graphite in 3 M LiPF6/EC/DEC. (b) Voltage vs time profile of graphite in 2 M LiPF6/EMS. Reprinted with permission from ref 20. Copyright 2000, The Electrochemical Society.

the ionic liquid (IL)-based DIBs could function well by only charging to 5.0 V, (all potentials in this Perspective are versus Li+/Li, unless otherwise noted) whereas it requires 5.2 V for nonaqueous electrolytes generally supports this view because the ionic bonding in ILs may involve higher “desolvation” energy values than the ion−dipole interaction. Challenges of Aprotic Electrolyte in DIBs. In 2000, Seel and Dahn investigated a conventional LIB electrolyteLiPF6 solvated in ethylene carbonate/diethyl carbonate (EC/DEC) for the use in DIBs.20 When the concentration was 1.0 M, no reversible deinsertion behavior was recorded before a cutoff potential of 5.25 V; however, beyond 5.25 V, the electrolyte decomposition took over the primary charging current. The authors conducted in situ X-ray diffraction (XRD) on a graphite cathode in 3 M LiPF6 in EC/DEC, which showed a lack of reversible PF6− storage behavior as well (Figure 2a). They also studied a more anodically stable ethyl methyl sulfone (EMS) electrolyte (2 M LiPF6/EMS), which could be charged to 5.45 V with good Coulombic efficiencies (Figure 2b). With the EMS electrolyte, they observed the reversible formation of the stage2 GICs, which is suggested to be (PF6)0.5C8, where the theoretical specific capacity can reach 140 mAh g−1. However, reversible charging must employ a relatively high current rate, that is, C/7, in order to surpass the current of electrolyte decomposition, whereas at a slow charging current rate, that is, C/40, the electrolyte decomposition current dominates.20 At C/7, a reversible (discharge) capacity of 95 mAh g−1 was observed for the first cycle, lower than the theoretical value. Note that as a drawback, the EMS-based electrolyte may not be compatible with the Li−graphite anode due to the possible lack of robust protective SEI formation. This work touched on the major challenge of DIBs that the average operation potentials for DIB cathodes are above 4.5 V with a large capacity contribution gained from above 5 V. A potential at 5 V represents an electrochemical potential of the cathode being more negative than that of the highest occupied molecular orbitals (HOMOs) of alkyl/alkylene carbonate-based electrolytes, at which potentials the electrolyte solvent molecules are oxidized. Such oxidation of the carbonate electrolytes was widely observed.20−23 The ill compatibility between the graphite cathode and alkyl/alkylene carbonate electrolytes calls for either more anodically stable electrolytes or nongraphite cathodes that could operate at lower potentials.

The ill compatibility between the graphite cathode and alkyl/alkylene carbonate electrolytes calls for either more anodically stable electrolytes or nongraphite cathodes that could operate at lower potentials. This Perspective will be devoted to the progress made along this line. Impacts of Graphitic Carbon’s Structure. The graphitic structure of carbon cathodes has a significant impact on their capacity of anion intercalation. Ishihara et al. reported that a more graphitized carbon exhibits a higher reversible capacity of PF6− intercalation, where the largest reversible capacity was achieved on the graphite with a d(002) of 3.35 Å and a particle size of 6 μm.24 They found that amorphous carbon could deliver a large intercalation capacity but with extremely poor reversibility during deintercalation. However, this graphitization−capacity correlation is challenged if other factors come into play, for example, when electrolyte solvents’ co-intercalation could cause graphite exfoliation or when the particle size of the graphitic carbon is vastly different. Märkle et al. reported that graphitic carbon with smaller particles exhibits better capacity than the one consisting of large particles, where this trend was more obvious when PC/ EMC (ethyl methyl carbonate) was used as the electrolyte than when EC/DMC (dimethyl carbonate) was employed.25 They showed that the heat-treated (3000 °C for 2 days) highly crystalline graphite with a large particle size showed no capacity with PC/EMC electrolyte and nearly nil capacity after four cycles with EC/DMC electrolyte, where this graphite exfoliates at only 4.8 V with either of the above electrolytes. The graphite of low graphitic crystallinity with smaller particle sizes (still on the scale of 10 μm), however, presents certain reversibility in PC and amenable cycling up to 11 cycles in EC/DMC electrolyte. The failure of higher-degree crystalline graphite of large particle sizes was attributed to PC-caused exfoliation of the flaky graphite, where such exfoliation is less destructive to smaller and less graphitic carbon electrodes. Thus, Märkle et al. concluded that graphite particles that exhibit a smaller particle size and lower degrees of crystallinity are more suitable for anion intercalation, where the integrity of the graphite structure 1764


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Figure 3. (a) Cycling performance of graphite//Li half-cells in 1.7 M LiPF6 solvated in FEC-EMC (4:6 w/w) + 5 mM (HFIP), operating in a voltage range of 4.0−5.2 V. The half-cells showed much improvement over the 1.2 M LiPF6 EC-EMC electrolyte (red/bottom data points). (b) Overlapped charge/discharge profiles of cycles 21−25. (ref 28), Reproduced by permission of The Royal Society of Chemistry.

is better preserved.25 This study again featured the difficulty of using carbonate electrolytes in terms of reversibility and longterm cycling, where EC/DMC offered limited anodic stability; PC causes rapid oxidation rates, resulting in graphite exfoliation, which should certainly be avoided in the DIBs’ electrolyte. Placke et al. employed an IL electrolyte and revealed a similar relationship between particle size/surface areas and the reversible capacity in DIBs. At 20 °C, when the particle size is increased, the reversible capacity decreases, whereas when the surface area is increased, the reversible capacity is increased.23,26 No observable trend was detected at 60 °C, which was attributed to the increased ion mobility at higher temperatures, leaving the graphite kinetic characteristics to play a smaller role.26 Here, we hope to point out that caution should be taken to consider the results of the structure−capacity correlations above because the electrolyte may play the major role instead of the carbon structures. On the other hand, there may exist a threshold of graphitic crystallinity, below which anions could not be intercalated into such “amorphous” structures due to the tortuosity issues. Additives for Carbonate-Based Electrolytes. Forming a stable SEI is indispensable for long-term cycling of high-potential cathode materials, such as LiNi0.5Mn1.5O4 and LiCoPO4, if alkyl carbonates are in the electrolyte, where such a SEI is expected to curtail further solvent oxidation on the cathode surface.19 Among additives, fluorinated compounds, such as fluoroethylene carbonate (FEC), have been widely employed, for example, to enhance the cathode materials for the emerging nonaqueous KIBs.27 In 2014, Read et al.28 employed an electrolyte formula with 1.7 M LiPF6 solvated in FEC-EMC (4:6 w/w) + 5 mM tris(hexafluoro-isopropyl)phosphate (HFIP), where the graphite//Li half-cell operates in a voltage range of 4.0−5.2 V.28 The half-cells could achieve a maximum reversible capacity of 85 mA h g−1, comparable to the performance with EMS electrolyte, and stable cycling for 200 cycles with a slowly improving Coulombic efficiency (CE) (Figure 3a). Their charge/discharge profiles indicate the staging mechanism of PF6− intercalation in graphite (Figure 3b). The full cells demonstrate the maximum reversible capacities of 60 mA h g−1 and ∼70% capacity retention after 50 cycles. The less impressive full-cell results are attributed to the possible electroactive species in the electrolyte and the associated shuttling upon cycling.28 Benef its of Lacking EC in an Electrolyte by Using an Aluminum Anode. Recently, good performance of DIBs was achieved by employing aluminum as the anode, instead of graphite.29,30 The

use of an aluminum anode, instead of graphite, eliminates the necessity of having EC in the electrolyte that functions to form a protective SEI on graphite. Zhang et al. reported a DIB that employed 4.0 M LiPF6 in EMC, where there are two primary novel points: (1) the lack of EC helps reversible PF6− intercalation into graphite, where, previously, Wang et al. have suggested that the presence of EC inhibits intercalation of PF6− into graphite;31 (2) a pure EMC electrolyte allows for a high concentration of LiPF 6, 4.0 M, where a higher concentration of electrolyte could lower the anion insertion potential to graphite.29 Plus, aluminum as the negative electrode acts as a current collector simultaneously, which decreases the inert mass of batteries. One of the advantages of this system rests on the fact that the aluminum anode allows for much greater theoretical capacities of 993 or 2235 mA h g−1 for the formation of LiAl or Li9Al4.32 One should note that Al has been one of the anode candidates contemplated since early on for LIBs and other batteries.32,33 However, Al has yet to be selected as the anode for commercial LIBs despite its various advantages, including low cost, high conductivity, and a theoretical capacity at least nearly three fold that of graphite. The challenge of Li−Al alloying anodes is the significant volume “breathing” of the Al anode during cycling, which is sufficiently dramatic to cause the electrode’s pulverization. To address this challenge, Zhang et al. added 2 wt % vinylene carbonate (VC); the cycling life was improved significantly compared to the case without this additive. They further demonstrated 200 cycles with a capacity retention of 88% (104 to 92 mA h g−1) at a current rate of 2 C. It was rationalized that VC additive induces the formation of a SEI layer on the aluminum electrode, protecting it from cracking and pulverization. Following this work, the same group reported a carbon-coated porous aluminum foil as a DIB anode to ease the mechanical strain caused by volume expansion upon cycling.30 With the same electrolyte and 5 wt % VC additive, their optimized system offered good rate performance, 85 mA h g−1 at 20 C, and allowed for 1000 cycles at a current rate of 2 C with capacity retention of 89.4% (from 104 to 93 mA h g−1).30 Instead of using Al as a Li-host, Li et al. employed electrochemical deposition and dissolution of aluminum as the anode and a porous graphite foam as the cathode.32 The porous graphite foam is formed by chemical vapor deposition on nickel foam as the template, which exhibited much better rate capability than pyrolytic graphite. Interestingly, in the electrolyte of AlCl3/1-ethyl-3-methylimidazolium chloride, AlCl4− and Al2Cl7− anions are present, where these ions are 1765


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g−1, but mitigated the kinetic hindrance effect. However, at higher temperatures, they observed a slightly lower capacity retention after 500 cycles, 97 and 93% at 40 and 60 °C, respectively, which is attributed to slight TFSI− anion decomposition at higher temperatures.21,26 Self-discharge seemed to be more prominent at higher temperatures as well, where 1.3 and 5.0% h−1 self-discharge was observed at 40 and 60 °C, respectively. Because IL-based electrolytes without additives typically cannot generate a protective SEI on a graphite anode, they also discussed the use of a nongraphite anode, that is, Li4Ti5O12 (LTO), which provides DIBs a cell voltage of ∼3.5 V. Similarly, stable cycling was realized on this dual-ion system as well. To cultivate a SEI on graphite in IL electrolytes, Rothermel et al.22,38 sought to incorporate additives, that is, 2 wt % ethylene sulfite (ES) in Pyr14TFSI-LiTFSI. In their studies, a similar “activation” process is observed, where the first cycle gives a low CE of only 55.9%, which increases to 91.5 and 94.7% by the second and third cycles, respectively. The system exhibits a highly stable cycling life for 500 cycles in a voltage range of 3.0−5.0 V, where the capacity values are 78, 91, and 99 mA h g−1 in the first, second, and third cycles, respectively, at a current rate of 10 mA g−1. The capacity retains 74 mA h g−1 on the 500th cycle at a current rate of 50 mA g−1. Interestingly, the use of ES additive not only nearly doubled the reversible capacity (in a graphite//Li half cell22) but also reduced the onset potential for anion intercalation, which the authors attributed to the reduced coordination of cations to anions in the presence of the ES additive.22 Metal−Organic Frameworks. Aubrey and Long recently studied a new type of DIB cathode material for anion insertion.39 They proposed that a redox-active metal−organic framework (MOF) would have the ability to incorporate weakly coordinated anions into its spacious structure. This work demonstrated the redox-active MOF Fe2(dobpdc)(dobpdc−4 = 4,4′-dioxidobiphenyl-3,3′dicarboxylate) as a functioning cathode material for DIBs. This MOF could undergo oxidative insertion reactions, where it weakly coordinates the anions, that is, BF4− and PF6−.39 During insertion, the MOF cathode retains its one-phase structure by exhibiting a sloping potential profile. A MOF//Na half-cell was tested in a potential range of 2.00 to 3.65 V vs Na+/Na using an electrolyte of NaPF6 in EC/DMC (30:70 v/v). After the initial 10 cycles, the cell delivered reversible capacity values of approximately 90 mA h g−1 for 50 cycles with a CE greater than 99%.39 These results pave the way toward employing a MOF cathode for DIBs. The operation potential of this MOF cathode is fairly low, which is quite intriguing. Stressed by the authors about the weak bonds between this MOF and the inserted anions, one can tentatively use eq 1 to rationalize it, where the term E(C+A−) for the MOF may be much smaller than that of graphite in its absolute value (Figure 5). Organic Solid Cathodes. Only recently has attention started to deviate from graphite cathodes to other cathode materials for DIBs. Organic crystalline solids comprising aromatic molecules

instrumental for the function of the graphite//Al Al-ion batteries by the following reactions (discharge): Al + 7AlCl4 − → 4Al 2Cl 7− + 3e−

(2)

Cn[AlCl4] + e− → Cn + AlCl4 −

(3)

IL Electrolytes. With the major challenge of DIBs being the limited anodic stability of the aprotic electrolytes, one solution is to employ IL-based electrolytes on the basis of two considerations. First, ILs typically allow for a wider electrochemical stability window with much better anodic stability;34 second, IL-based electrolytes comprising only ions may overcome the solvent cointercalation challenge if we assume that the ion pairs of ILs are too large to be co-inserted into graphite. Plus, IL-based electrolytes possess high thermal stability and the consequent better safety.34 As early as in 1994, Carlin et al. investigated a series of ILs as the electrolyte for DIBs, where the molten salt electrolytes consist of bulky cations, that is, 1-ethyl-3-methylimidazolium (EMI+) or 1,2dimethyl-3-propylimidazolium (DMPI+), and anions, that is, AlCl4−, BF4−, PF6−, CF3SO3−, or C6H5CO2−.35 They reported good cycling efficiency; however, long-term cycling was not reported. Placke et al. employed 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14-TFSI)/LiTFSI as the electrolyte for DIBs.21,26,36 They systematically investigated the graphite//Li DIBs, where Li metal serves as the counter electrode. Different upper cutoff voltages were compared, where a higher voltage led to a higher capacity, that is, from 50 mAh g−1 at 5.0 V to 96 mAh g−1 at 5.3 V (Figure 4); however, the CE decreases from 99.4 to 90.3% (the 50th cycle).21

Figure 4. Voltage profiles of the graphite cathode (vs metallic Li) in (Pyr14-TFSI)/LiTFSI at several different cutoff voltages. Reprinted with permission from ref 21. Copyright 2012, The Electrochemical Society.

At an optimized potential of 5.00 V, the graphite cathode achieved a reversible capacity of ∼50 mA h g−1 for 500 cycles (99% capacity retention).21 It is important to note that when using ILs, an initial “activation” process is observed, which may involve a large energy penalty when opening up the graphite layers for insertion of large anions, that is, 20% expansion for hosting TFSI−.37 Moreover, due to the viscosity of ILs, this activation process may relate to the wetting of the electrodes as well.21,26,36 The authors also considered the temperature dependence, where increasing the cell temperature to 60 °C not only raised the reversible capacity to be above 100 mA h

Organic crystalline solids comprising aromatic molecules as constituents with lower density values than graphite may be highly amenable for the storage of anions. 1766


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been cycling for over 2000 cycles with no signs of capacity fading at the time of preparation of this Perspective. Rather, capacity is slowly increasing (Figure 6c). The reversible anionstorage properties of the PAH coronene may serve as a stepping stone toward new DIBs based on PAHs as electrodes. Note that PAHs are one of the major pollutants from coal-fired power plants, where the collection of such PAHs that may not necessarily have to be a pure compound for battery purposes can provide sustainable and cost-effective supplies of electrode materials. Nitrogen-Containing Organics. Deunf et al. reported the use of a p-type organic host lattice in a DIB setup for anion intercalation.41 In this work, they used a nonpolymeric organic crystalline electrode having a layered crystal arrangement, namely, dilithium 2,5-(dianilino) terephthalate (Li2DAnT), where they tested a variety of electrolyte compositions using PC and EC/DMC as solvents and 1 M salts, including LiClO4, LiPF6, and LiTFSI. The Li2DAnT cathode has the ability to form cationic radicals; therefore, all of the anions studied showed reversible capacities in the potential range of 2.8−3.5 V for 20 cycles in a half-cell setup using lithium metal as the anode. The electrolyte of 1 M LiClO4/PC showed the highest reversible capacity of nearly 80 mA h g−1 and the best capacity retention after 20 cycles, where the authors suggested that PC is co-intercalated into the electrode’s solid structure.41 They further tested this system without using any carbon additive, which decreased the reversible capacity to ∼60 mA h g−1 and showed relatively good rate capability and cycling performance for 100 cycles; after 400 cycles, a low capacity retention of 33% was observed.41 Deunf et al. further investigated another organic crystalline solid, this time using 5,12-diaminorubicene (DARb) as the nonpolymeric anion-insertion cathode, again employing the electrolyte compositions with PC or EC/DMC as solvents and 1 M salts of LiClO4, LiPF6, or LiTFSI. Like in their previous work, the DARb would be expected to form a radical cation with a following second oxidation, therefore creating a dication species, which would theoretically contain a fully delocalized π system (Scheme 1).42 Using 1 M LiPF6 in EC/DMC (1:1 vol %) as the electrolyte, they were able to achieve a reversible capacity of 115 mA h g−1 (75% of the theoretical capacity) and a relatively good capacity retention after 60 cycles with an upper cutoff potential of 4 V.42 The authors emphasized that the facile operation of this electrode pertains to its solid-state structure. Future Perspectives. Seel and Dahn pointed out the limits of the energy density and specific energy for dual-graphite batteries as a function of the electrolyte concentration. According to the trajectory of the energy metric of dualgraphite batteries, it was concluded that it would be challenging for the DIB technology to be able to compete against LIBs unless high-molarity electrolytes could operate at high potentials (Figure 7).6 One should recognize that with the technological progress of LIBs in the last 17 years the mass/ volume percentage of current collectors, cell cases, and other inert parts has been significantly reduced. The values on the yaxis in Figure 7 would be much larger at the time this article was written. In order to put DIBs into the large context, we compare the possible ranges of the specific density and energy density of two types of DIBs, dual-graphite batteries and graphite//Al batteries, to other types of batteries, as shown in Figure 8. The energy performance of dual-graphite batteries is estimated based on the range provided in Figure 7. This underestimates

Figure 5. Comparison of materials showing charge storage via the Fe2+/3+ redox couple vs C20(TFSI), where MOF Fe2(dobpdc) is clearly at a lower potential, rationalized by eq 1. The starred curves did not report reversible behavior, and discharge curves measured vs Li/Li+ were not rescaled. Reprinted (adapted) with permission from ref 39. Copyright 2015, American Chemical Society.

as constituents with lower density values than graphite may be highly amenable for the storage of anions. The primary d spacing in such molecular solids would be larger than the d(002) inside of graphite, which would be particularly friendly for hosting bulky anions. According to eq 1, it is reasonable to consider that the anion-insertion potentials for such molecular solids would be lower than that of graphite. Hydrocarbons. Similar to graphene, polycyclic aromatic hydrocarbon (PAH) molecules are fused C6 rings, which

Similar to graphene, polycyclic aromatic hydrocarbon (PAH) molecules are fused C6 rings, which facilitates their capability of hosting delocalized extra electrons or electron holes on the lowest unoccupied molecular orbitals (LUMO) or HOMO, respectively. facilitates their capability of hosting delocalized extra electrons or electron holes on the LUMO or HOMO, respectively. Furthermore, PAHs are ampler regarding the space between molecules; the structures are long-range ordered 3D stacking of “graphene nanosheets”, held together by van der Waals forces in all directions. Rodrı ́guez-Peŕ ez et al. first demonstrated the use of a PAH electrode, coronene (density: 1.47 vs 2.23 g/cm3 for graphite), as a DIB cathode in a 1 M LiPF6 EC/DEC electrolyte.40 The coronene cathode demonstrated highly reversible PF6− storage with a capacity of ∼40 mA h g−1 and flat plateaus at potentials of 4.2 V (charge) and 4.0 V (discharge) (Figure 6a). Note that such a potential eliminates the necessity of IL-based electrolytes, which is of a large cost advantage. The much lower operation potential is compatible with the general prediction of eq 1, where PAH+A− would be of a less negative free energy than graphite+A−. Of course, this will need to be confirmed by computational studies. The authors performed ex situ characterization, revealing that coronene retains its crystalline structure (Figure 6b) and chemical bonding upon PF6− incorporation. Complete amorphization is evident after approximately 50 cycles; however, the coronene electrode exhibits impressive cycling stability, which is unexpected of an amorphous materialthe electrode has 1767


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Figure 6. (a) Coronene cathode potential profiles of the 1st, 2nd, 3rd, and 10th cycles. (b) Ex situ XRD patterns of the first cycle at different state-of-charge (shown in the inset). (c) Capacity vs cycle number of the coronene electrode. Reprinted (adapted) with permission from ref 40. Copyright 2016, American Chemical Society.

Scheme 1. Scheme of the Electrochemical Process for the DARb Electrode Showing the Reversible Single-Electron Exchange Forming a Radical Cation Followed by a Reversible Second Electron Exchange (Oxidation) Creating the Dication Species with a Fully Delocalized π Systema

a

Figure 8. Comparison of estimated performances between conventional batteries and DIBs. Energy characteristics of dual-graphite DIBs (DG-DIBs) come from the prediction of Dahn and Seel.6 The energy values of graphite//Al (Al-G) are estimated based on the article of Zhang et al.29

Reprinted with permission of ref 42. Copyright 2016, Elsevier.

the energy characteristics of dual-graphite batteries as the inert components of the current batteries have become much lighter and smaller over years of relentless development. The reported

Figure 7. Prediction of (a) volumetric energy and (b) gravimetric energy density as a function of electrolyte concentration for DIBs. These predictions include both the (bottom) electrodes and electrolyte only, as well as including the (top) current collectors and cell cases (20% of the total cell volume and 30% of the total cell mass). Reprinted with permission from ref 6. Copyright 2000, The Electrochemical Society. 1768


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ions from the electrolyte. Therefore, a need for electrolyte optimization is also imperative for new electrodes. The unique configuration of DIBs provides a powerful research platform, where we expect that a booming number of combinations of cathodes and anodes will be proposed and studied. It is our opinion that such studies should be aligned with goals on certain desirable performance metrics, such as cost, safety, cycling life, power, and energy density. Although such combinations are most likely only limited to imagination, ultimately, their successes should be evaluated by new in-depth understanding or technical progress toward commercial viability.

specific energy of graphite//Al cells is on par with that of LIBs, which is promising. However, the volumetric energy density of the graphite//Al cells was not given in references, where our estimation is based on the consideration of the lower density of the graphite cathode compared to that of metal oxide cathodes in LIBs and the larger volume of electrolyte needed in DIBs. Without generally estimated values, we attempt to point out that the volumetric energy density of DIBs, even in the graphite//Al configuration, may be difficult to compete against LIBs. For future development of DIBs, we propose that there may be two primary directions. The first is to pursue higher energy density so that DIBs may be nearly competitive compared to LIBs in the transportation market, where high-energy anodes such as Al, Sn, Si, or simply plating Li-metal will be promising. With new high-capacity anodes adopted, the energy characteristics of DIBs should be reconsidered. The cationic charge carriers for the anode here are most likely Li-ions, which could facilitate high capacity and redox reversibility. However, this does not completely rule out the possibility of other charge carriers, such as Na+, K+, or Mg2+. For example, a red or black phosphorus electrode can exhibit a very large capacity as the anode when hosting Na-ions.43 For this front, it is a must that the impact of concentrations of electrolyte on the full-cell performance should be studied in detail, and the possible tradeoff between the electrolyte mass and full-cell energy density should be investigated. Furthermore, it is an open question whether it is absolutely necessary to employ graphite as the anion cathode for such high-energy DIBs. The benefits of high voltage and the consequently high energy density may be canceled by the disadvantage of shorter cycling life and the higher expense of IL electrolytes. Another direction is toward large-scale stationary applications, where the performance metric is simply the levelized energy costthe cost of stored electricity over the lifetime of a storage device. This requires the acquisition cost to be minimized, where earth-rare elements, such as Li, should be avoided, and the operation cost needs to be lowered, where cycling life should be maximized. Along this line, it can be expected that future research will explore anodes that operate upon insertion/deinsertion of earth-abundant elements, such as Na+, K+, Mg2+, and Al3+. In terms of the long cycle life of the devices, to avoid electrolyte decomposition, it is acceptable to lower the cathode’s operation potentials. On the basis of eq 1, we believe that the value of E(C+A−) − E(C) can be tuned when suitable electrode materials, such as PAH molecular solids, are selected as the electrodes. Thus, it is possible to finely tune the cathode operation potentials to be below the threshold voltage, at which aprotic electrolytes get oxidized. Of course, the energy density of such devices will most likely be much lower than that of LIBs or even NIBs and KIBs. Additionally, we should note that solid PAHs exhibit a lower density than graphite. For example, coronene is 1.47 g cm−3 vs graphite at 2.23 g cm−3. However, this would not be a primary issue for stationary energy storage, and its energy density will still be competitive against supercapacitors or pseudocapacitors. To position such DIBs to be commercially competitive, their cycling life is the key metric. DIBs have a unique advantage of cycling life, where the CE is independent of individual electrodes. The main consequence is that a low CE of one electrode would not affect the capacity of the other electrode, and both electrodes can maintain their capacity by inquiring

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiulei Ji: 0000-0002-4649-9594 Notes

The authors declare no competing financial interest. Biographies Mr. Ismael A. Rodrı ́guez-Peŕ ez immigrated to the U.S. at the age of 9 from Mexico. He obtained his B.Sc. degree in Chemistry from Gonzaga University in 2014. Currently, he is a Ph.D. candidate at Oregon State University working with Dr. Xiulei Ji investigating organic crystals as battery electrodes. Dr. Xiulei Ji is an Assistant Professor of Chemistry at Oregon State University. He obtained his B.Sc. from Jilin University and Ph.D. from the University of Waterloo in 2009. He did postdoctoral research at the University of California, Santa Barbara. His group focuses on carbon chemistry for new energy storage. http://jigroup.chem. oregonstate.edu/

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ACKNOWLEDGMENTS X.J. thanks the American Chemical Society Petroleum Research Fund (Grant Number: PRF 55708-DNI10) for financial support.

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REFERENCES

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Anion Hosting Cathodes in Dual-Ion Batteries - ACS Energy Letters

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