Emerging Opportunities for Two-Dimensional Materials in Lithium-Ion

Aug 4, 2017 - The unique advantages of 2D materials and their novel processing strategies will be further assessed in the context of next-generation L...
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Emerging Opportunities for Two-Dimensional Materials in Lithium-Ion Batteries Kan-Sheng Chen,† Itamar Balla,† Norman S. Luu,† and Mark C. Hersam*,†,§,∥ †

Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ∥ Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, United States §

ABSTRACT: Lithium-ion batteries (LIBs) have achieved widespread utilization as primary rechargeable energy storage devices. In recent years, significant advances have been made in two-dimensional (2D) materials that have the potential to bring unprecedented functionality to next-generation LIBs. While many 2D materials can serve as a new class of active materials that exhibit superlative energy and power densities, they can also be employed as versatile additives that improve the kinetics and stability of LIBs. Here, we present a Perspective on how 2D materials can impact each of the primary components of a LIB including the anode, cathode, conductive additive, electrode−electrolyte interface, separator, and electrolyte. In this manner, emerging opportunities and challenges for 2D materials are identified that can inform future research on highperformance LIBs.

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common van der Waals bonding motif of 2D materials also allow them to be seamlessly integrated with pre-existing active materials for improved LIB cell performance. With wideranging properties, 2D materials have the potential to impact nearly every element of a LIB including the anode, cathode, conductive additive, electrode−electrolyte interface, separator, and electrolyte. In an effort to assess this potential and identify future challenges and opportunities, this Perspective provides an overview of recent research that has explored the utilization of 2D materials in each of the aforementioned components of LIBs. The unique advantages of 2D materials and their novel processing strategies will be further assessed in the context of next-generation LIB technology. Operating Principles of Lithium-Ion Batteries. LIBs are composed of two lithium-hosting electrodes (positive and negative) and a nonaqueous electrolyte. During cycling, lithium ions migrate back and forth between the electrodes, as illustrated in Figure 1. When a LIB is discharged, lithium ions (cations) in the negative electrode (graphite) are extracted, diffuse across the electrolyte, and are inserted into the positive electrode (LiCoO2), while external electrons move inward to the positive electrode. During charge, the electrons and ions flow in the reverse direction. Because lithium ions flow inversely during charge and discharge, by convention, the terms “cathode” and “anode” that will be used in this Perspective designate positive and negative electrodes in the discharge cycle, respectively. A separator that is ionically transparent and

he emergence of lithium-ion battery (LIB) technology has profoundly impacted society by providing energy storage for portable electronics, medical devices, and green energy applications such as electric vehicles and renewable energy sources.1,2 The widespread growth of LIBs has been primarily driven by the introduction of innovative materials since rechargeable lithium batteries were first proposed in 1978 by Whittingham at Exxon. In the early days, lithium metal was used as an anode material because of its high energy density. However, these initial lithium battery cells were plagued by safety issues that result from lithium dendrite formation during cycling. To address this issue, lithium intercalation anode materials (e.g., graphite) were proposed in 1981. This scheme was refined by the Sony Corporation to produce the first commercial LIB cells in 1991. The charge capacity of cylindrical 18650 LIB cells then proceeded to improve from 800 mAh to 2.6 Ah in 2005 due largely to the development of electrode materials and cell designs. Further electrolyte development has allowed high-voltage operation and facilitated the formation of a stable solid−electrolyte interphase (SEI) on graphite surfaces.3 More recently, an additional increase of cell capacity to 3.5 Ah was achieved by introducing LiNixMnyCozO2 (x + y + z = 1) as a class of high-capacity cathode materials. Despite the success of current LIB designs, the rapid growth of global energy demand necessitates continual advances in this field.2 To meet these needs, further LIB material breakthroughs are imperative. Recently, a class of two-dimensional (2D) materials has shown substantial promise as active materials in LIBs due to their high specific surface areas that offer electrochemically active sites for ion storage and open 2D channels for fast ion transport.4 The intrinsic mechanical flexibility and © 2017 American Chemical Society

Received: June 2, 2017 Accepted: August 4, 2017 Published: August 4, 2017 2026

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Figure 1. Schematic illustration of the operation of a LIB with a LiCoO2 cathode and graphite anode.

electrically insulating is sandwiched between the two electrodes to prevent electrical shorting while maintaining sufficient lithium ion diffusion. Traditionally, conductive additives such as carbon black (i.e., carbon particles comprised of small grains of graphite) are employed in cathode electrodes to alleviate the poor electrical conductivity of traditional lithium transition metal oxide cathode materials.5 In addition, during cycling, a SEI forms on the surfaces of the active materials (both cathode and anode) as a result of irreversible chemical reactions between the electrodes and electrolyte. Because the operation of LIBs relies on the distinct functions and interplay among all of these elements, LIB performance can be substantially enhanced if one or more of their components is improved via new materials or processing strategies. 2D Materials in Anodes. Graphite has been the dominant anode material in LIBs since the 1990s due to its greatly improved safety relative to lithium metal, low lithium intercalation voltage, and well-defined SEI for highly reversible lithium insertion/extraction. However, its theoretical capacity of 370 mAhg−1 limits the energy density of a LIB full cell to approximately 250 Whkg−1, which is inadequate for future electric vehicles that target a driving range of 300−400 miles on a single charge.3 One strategy to significantly increase the specific capacity of graphite is to reduce its dimensionality down to the 2D limit (i.e., graphene).6 The exposed graphene edges are strong reactive sites for lithium-ion storage with binding energies (1.7−2.3 eV) that are up to 50% higher than that of the graphene basal plane (1.55 eV).7 In addition, the edges of graphene can accommodate more lithium per carbon atom relative to bulk graphite. For example, Hassoun et al.8 demonstrated that graphene nanoflakes produced via liquid phase exfoliation (Figure 2a) exhibit higher lithium ion uptake (LiC2) than those normally obtained with graphite (LiC6). Consequently, LIB anodes containing exfoliated graphene nanoflakes achieve a reversible specific capacity as high as 1500 mAhg−1 at a current rate of 100 mAg−1. When the graphene anode is incorporated in a full cell using LiFePO4 as a positive electrode, as schematically depicted in Figure 2b, the voltage profiles of the full cell (Figure 2d) show less pronounced plateaus with respect to that of the LiFePO4 half cell (blue curves in Figure 2c) due to the plateau-free charge/discharge curves of the graphene half cell (black curves in Figure 2c). Despite these nonideal voltage curves, the graphene/LiFePO4 full cell has an estimated practical specific energy density of 190 Whkg−1 with excellent cycling stability, as indicated in Figure 2e.

Figure 2. (a) Schematic that shows the liquid phase exfoliation of graphite via ultrasonication and subsequent ultracentrifugation to obtain the graphene ink used for the anode. (b) Electrochemical testing schematic of a graphene nanoflake (anode)/LiFePO4 (cathode) LIB. (c) Charge/discharge voltage profiles of the graphene nanoflake anode (black curves) and LiFePO4 cathode (blue curves). (d) Voltage profiles of the graphene/LiFePO4 LIB full cell. (e) Specific capacity versus cycle number of the graphene/ LiFePO4 LIB full cell. Adapted from ref 8. Copyright 2014 by the American Chemical Society.

Defects in the basal plane of graphene can also serve as electrochemically active sites for reversible lithium-ion storage.9 When graphene oxide (GO) prepared with the Hummers method10 is subsequently reduced via low-temperature pyrolysis or electron-beam irradiation to yield reduced graphene oxide (rGO), small sp2-bonded carbon domains surrounded by defects are formed.9 These abundant defect sites exhibit similar reversible lithium-ion storage properties as the edges of graphene nanoflakes and thereby contribute to a large specific capacity in excess of 1000 mAhg−1. While rGO anodes have a lower reversible capacity compared to the graphene nanoflakes reported by Hassoun et al.,8 the first cycle Coulombic efficiency of the rGO anodes reported by Pan et al.9 is about 50%, which is 5 times higher than that of graphene nanoflakes. This difference can likely be attributed to the majority of reactive edges on the graphene nanoflakes that are directly exposed to the electrolyte, inducing a large amount of defect-mediated irreversible reactions during the first discharge cycle. In contrast, many of the defect sites in multilayered rGO are within the basal planes and are not in direct contact with the electrolyte,9 which limits irreversible reactions to only those defects on the outer rGO layers. An alternative strategy to controllably introduce defects is to dope graphene with other species. For example, it has been demonstrated that nitrogen-doped or boron-doped graphene can simultaneously enhance the electrochemical activity and electrical conductivity of graphene, giving rise to not only high energy capacity but also exceptional rate capability.11 Specifically, with 0.88% boron doping achieved following annealing of pristine graphene at 800 °C in BCl3 and Ar (1:4 v/v), the reversible specific capacity exceeds 1500 mAhg−1. The specific 2027

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capacity also remains reasonably high (235 mAhg−1) at fast charge/discharge current rates of 25 Ahg−1. Black phosphorus (BP) is another promising elemental 2D material that has an even higher theoretical capacity of 2596 mAhg−1.12 However, due to the large intake of lithium from P to Li3P, BP undergoes an almost 300% volume change during electrochemical cycling. Therefore, anodes produced from bulk BP inevitably suffer from poor cycling stability due to mechanical fracturing and the subsequent loss of electrical contact to electrodes. To address this issue, Sun et al.13 developed a strategy to produce thin BP flakes that form covalent phosphorus−carbon (P−C) bonds with graphene flakes using high-energy ball milling in a pressurized Ar atmosphere (1.2 MPa). The resulting BP−graphene composite possesses exceptionally high specific capacity (2786 mAhg−1) and greatly improved capacity retention (80% after 100 cycles) due to the formation of stable P−C bonds. In contrast, a control anode that simply mixes BP and graphene without forming P−C bonds showed substantially reduced capacity retention (close to 0% after 35 cycles).

by HF etching of Al from the Ti2AlC MAX precursor only exhibits a specific capacity of 160 mAhg−1 at 0.1C.19 To realize a more effective MXene-based anode, Ren et al.20 developed a chemical etching strategy to introduce pores into Ti3C2 MXene flakes. By combining porous Ti3C2 MXene flakes with carbon nanotubes, the resulting composite anode produced a high specific capacity of 1250 mAhg−1 at 0.1C and exhibited excellent rate capability (∼400 mAhg−1 at 5C). Luo et al.21 further showed that Sn4+ can be electrostatically attached to the surfaces of Ti3C2 when they are negatively charged with the presence of −F and −OH surface groups. The resulting Sn(IV) @Ti3C2 composite anode fabricated with the assistance of polyvinylpyrrolidone surfactant achieved a uniform distribution of Sn(IV) nanoscale aggregates within the Ti3C2 matrix and delivered high volumetric and gravimetric capacities of 1365 mAh cm−3 and 635 mAhg−1 at 100 mAg−1, respectively. The 2D anode materials discussed thus far exhibit significantly higher specific charge capacities than graphitic anode materials. However, similar to graphene derivative anode materials, most 2D anode materials have qualitatively similar voltage profiles that are plateau-free and higher than that of graphite. Therefore, to assess their performance with respect to that of a benchmark graphite anode, the specific energy in a fullcell geometry should be used as a general practice in the field. The beneficial effects of 2D nanostructure are not limited to layered materials. When spinel Li4Ti5O12 (LTO) is synthesized to obtain a 2D nanostructure, this structural morphology gives rise to a high surface area and short lithium diffusion distance, leading to significantly improved kinetics. For example, Chen et al.22 demonstrated the fabrication of well-aligned LTO nanosheets directly grown on a conductive Ti substrate as a binder-free anode. This LTO nanosheet anode showed a specific capacity of 163 mAhg−1 at 20C (93% retention) and 78 mAhg−1 at 200C (44% retention). In contrast, the control anode that consisted of randomly dispersed LTO nanosheets mixed with carbon conducting agents and polymer binders exhibited significantly reduced rate performance. This result indicates that both fast ion and electron transport are required to achieve optimal rate performance because the direct growth of LTO nanosheets on Ti foil ensures high lithium-ion transport and a well-defined electrical current path between the current collector and the active materials. 2D Materials in Cathodes. To date, most van der Waals 2D materials have only been explored for anode applications because of their low lithium intercalation voltages versus Li/Li+. Therefore, the realization of 2D nanostructured cathode materials, which are either layered with strong interlayer bonds or not layered at all, requires different processing strategies than those used to exfoliate traditional van der Waals 2D materials. For instance, Zhao et al.23 demonstrated single-crystal (010)-oriented LiFePO4 (LFP) nanosheets via solvothermal synthesis in diethylene glycol solvent. Because diethylene glycol molecules have a binding energy of −1.5 eV mol−1 to the (010) facet of LFP, the growth of (010) LFP facets is more thermodynamically favorable than the growth of (001) and (100) facets due to the minimization of the total surface free energy. Bulk LFP has 1D lithium diffusion channels along the [010] direction, which have been widely acknowledged as a major obstacle to fast kinetics in polycrystalline LFP micropowders. Therefore, LFP nanoparticles are typically used to both shorten the lithium diffusion path length and increase the surface area per unit mass. The (010)-oriented LFP nanosheets reported by Zhao et al.23 further increase the ratio of (010)

Layered transition metal sulfides such as MoS2 possess weak interplanar bonding based on van der Waals interactions that can accommodate Li insertion/extraction with less severe volume expansion, resulting in improved cycling stability. Transition metal sulfides, MSx (M = Mo, Fe, Co, Ni, etc.), are another class of high-capacity anode materials that undergo conversion or alloying reactions upon lithium insertion/ extraction.4 Although their theoretical capacities are much higher than that of graphite and are similar to those of other alloying types of anode materials (e.g., Sn and Ge), many bulk transition metal sulfides (e.g., FeS,14 FeS2,15 and CoS216) have poor cycling stabilities due to volume expansion during lithiation. In contrast, layered transition metal sulfides such as MoS2 possess weak interplanar bonding based on van der Waals interactions that can accommodate Li insertion/extraction with less severe volume expansion, resulting in improved cycling stability. Furthermore, nanostructured layered transition metal sulfides can yield additional improvements in electrochemical performance. In particular, Hwang et al.17 reported MoS2 nanoplate anodes that have a stable reversible capacity of 1060 mAhg−1 for 50 cycles at 1C and extraordinary rate capability with a specific capacity of 700 mAhg−1 at 50C. The authors attributed this outstanding electrochemical performance to the large interlayer distance of MoS2 that provides ample space for lithium-ion intercalation. Recently, a new family of exfoliated transition metal carbides and carbonitrides, known as MXenes, has been explored as a class of promising LIB anode materials due to their high electrical conductivity and low lithium diffusion barrier.18 MXenes are generally synthesized via selective etching of the A group element from MAX phase precursors, where “M” is an early transition metal, “A” is an A-group element (mainly group 13 or 14), and “X” is carbon or nitrogen. Following etching, MXene surfaces are usually terminated with OH and F groups, which undesirably block lithium transport and result in decreased lithium storage capacity.18 For example, a Ti2C anode synthesized 2028

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to the bulk LFP/C control. More significantly, the ultrathin LFP nanosheets deliver more than 120 mAhg−1 at 20C and 70 mAhg−1 at 80C, whereas the bulk LFP/C control has only 30 mAhg−1 at 20C and zero capacity at 50C and higher current rates. Layered LiCoO2 (LCO) has also been exfoliated using an oxidation−reduction method.25 Raw LCO is first treated by electrochemical oxidation to form Li1−xCoO2, which is subsequently intercalated by tetraethylammonium hydroxide (TEA) and exfoliated via ultrasonication agitation. The obtained nanosheets are collected and reacted with LiOH to reassemble nanostructured LCO. Due to the increased surface area, the resulting nanostructured LCO is typically passivated by atomic layer deposition of Al2O3 to obtain stable electrochemical behavior. While this work demonstrated a novel process to exfoliate layered LCO, which has strong interlayer electrostatic interactions, the resulting 2D LCO did not exhibit improved rate performance in contrast to 2D LFP. This lack of improved kinetics is likely due to the exfoliation of LCO along its layered planes, which does not expose more channels for lithium diffusion from the electrolyte into the active material. In addition, the Al2O3 passivating layer that is necessary for stabilizing the surface of 2D LCO presents a barrier for lithium ion and electron transport to the LCO surface.26 2D Materials as Conductive Additives. The most prevalent LIB conductive additives used in industrial applications and academic research are carbon particles composed of randomly oriented graphite sheets (i.e., carbon black). However, recent developments in functional conductive additives have focused on 2D graphene, which has a significantly higher aspect ratio and electrical conductivity than polycrystalline carbon particles. One of the most effective strategies has been to exploit the high aspect ratio of graphene to encapsulate active materials, resulting in improved electrochemical properties and stability.27−29 For example, Li et al.29 demonstrated direct growth of conformal

facets that maximize lithium transport from the electrolyte to the active material. Overall, these LFP nanosheets of 30−60 nm in thickness outperform nonplanar LFP nanoparticle control samples in terms of rate capability, with the nanosheets retaining a specific capacity of 80 mAhg−1 at 20C compared to the nanoparticle control only retaining 40 mAhg−1 at the same current rate.

The realization of 2D nanostructured cathode materials, which are either layered with strong interlayer bonds or not layered at all, requires different processing strategies than those used to exfoliate traditional van der Waals 2D materials. By decreasing the LFP sheet thickness, diffusion lengths can be minimized, thereby further improving power performance. For example, Rui et al.24 developed a liquid phase exfoliation approach combined with a solvothermal lithiation process in high-pressure, high-temperature (HPHT) supercritical fluids. This strategy yielded ultrathin LFP nanosheets with thicknesses of ∼4 nm, as schematically illustrated in Figure 3a. The starting material was layered bulk ammonium iron phosphate NH4FePO4·H2O, which swelled by intercalation of formamide molecules to weaken the interlayer attraction and was subsequently exfoliated via ultrasonication agitation. The resulting NH4FePO4·H2O nanosheets were functionalized with polyvinylpyrrolidone as a carbon source and finally converted into LFP/C ultrathin nanosheets using a solvothermal lithiation process. The voltage profile of LFP/C nanosheets as shown in Figure 3b has a characteristic plateau at 3.4 V and exhibits smaller electrode polarization and larger specific capacity relative

Figure 3. (a) Schematic illustration of the liquid phase exfoliation of bulk NH4FePO4·H2O into nanosheets followed by the solvothermal lithiation process to prepare carbon-coated lithium iron phosphate nanosheets. (b) Initial galvanostatic charge−discharge voltage profiles and (c) rate performance of LiFePO4/C compared to a bulk control sample. Adapted from ref 24. Copyright 2013 by the American Chemical Society. 2029

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graphene cages on micrometer-sized Si particles. Si is one of the most promising anode materials due to its high theoretical capacity of 4200 mAhg−1. However, due to a 300% volume expansion during electrochemical cycling, Si particles suffer from mechanical fracturing, leading to uncontrolled SEI formation and loss of electrical contact, as schematically depicted in Figure 4a. In contrast, when a conformal coating of graphene

Recent developments in functional conductive additives have focused on 2D graphene, which has a significantly higher aspect ratio and electrical conductivity than polycrystalline carbon particles. reduced in a hydrothermal reaction or microwave irradiation process. In this manner, nanoparticles can be grown uniformly on dangling bond seeding sites without aggregation and be electrically connected to highly conductive rGO surfaces. Cathode materials such as spinel LiMn2O4 (LMO)30 and LFP31 and anode materials such as Mn3O4,28 TiO2,32 Fe2O3,33 Co3O4,34 and SnO235 show enhanced electrochemical performance when they form composites with rGO. Composites based on 2D materials as active materials and graphene as conductive additives also show substantial electrochemical improvements. BP anodes, as discussed in the anode section, rely on robust P−C bonds between BP and graphene nanosheets to achieve excellent cycling stability.12 MoS2/graphene composites (where the MoS2 synthesis and reduction of GO are carried out concurrently) also demonstrate outstanding specific capacity and cycling stability.36 When the graphene additives are utilized at both cathode and anode electrodes, full cells with fast charge and discharge rates can be realized. For instance, Li et al.37 have shown that when LFP and LTO are directly grown on graphene foam (GF) via hydrothermal reactions, a full-cell battery consisting of the resulting LFP/GF cathode and LTO/GF anode can deliver exceptionally high power with a specific capacity of 125 mAhg−1 being retained at 200C. The desirable fast kinetics afforded by graphene can also translate to low-temperature performance of LIB electrodes. Chen et al.38 have utilized LMO/graphene dispersions that result in a graphene/LMO composite cathode with a conformal graphene coating of the LMO nanoparticles. Due to efficient charge transfer enabled by the conformal contact between the graphene and LMO surfaces, cycling efficiency is retained at high rates and low temperatures. Specifically, the graphene/LMO composite exhibits 96% capacity retention at −20 °C at 0.2C and excellent power performance at 0 °C (98 and 86% retention at 4 and 10C, respectively). Controlling the Electrode−Electrolyte Interface with 2D Materials. The electrode−electrolyte interface is critical to the operation of LIBs because it directly controls the transport of lithium ions in and out of the battery active materials. Historically, the electrode−electrolyte interface at the anode side is defined as the SEI. The formation of a well-defined SEI on the graphite anode is essential to maximize lithiation capacity and ensure prolonged cycling stability. One holy grail for LIB technology is to use lithium metal directly as an anode due to its high specific capacity and its lowest electrochemical potential versus a hydrogen electrode (−3.04 V). However, lithium metal electrodes experience unsafe dendritic growth during lithium plating. In addition, the unstable surface morphology of the dendritic lithium metal surface leads to uncontrolled SEI growth that detrimentally affects Coulombic efficiency. To address these issues, Yan et al.39 have proposed a solution to suppress dendritic growth and uncontrolled SEI formation by using ultrathin hexagonal boron nitride (hBN) or graphene as a stable interfacial layer. When a hBN or graphenecovered Cu surface is lithiated, instead of showing dendritic

Figure 4. (a) Fracturing of Si microparticles due to volume expansion upon lithiation results in loss of electrical contact and uncontrolled SEI growth. (b) Si microparticles encapsulated within a mechanically flexible graphene cage maintain their electrical contact to the graphene cage upon fracturing. The SEI thus forms only on the graphene surface, giving rise to high initial-cycle and subsequent-cycle Coulombic efficiency. (c) In situ TEM observation of a graphene-encapsulated Si microparticle before, during, and after lithiation. The expanded and fractured Si microparticle is well confined in the graphene cage, allowing the particle−graphene contact to remain intact. Adapted from ref 29. Reproduced with permission from the Nature Publishing Group.

sheets is employed to encapsulate the Si particles (Figure 4b), direct contact between the Si surfaces and electrolyte is minimized. Therefore, even after the Si particles have mechanically fractured during cycling, the graphene sheets effectively suppress uncontrolled SEI growth and maintain electrical contact with the current collector. In situ TEM, as shown in Figure 4c, reveals that a mechanically robust graphene cage is flexible enough to accommodate the expansion of lithiated Si particles. Consequently, graphene-encapsulated Si anodes exhibit high first-cycle and subsequent-cycle Coulombic efficiency (93.2 and 99.9%, respectively) in addition to stable cycling behavior (90% retention after 100 cycles). Graphene has also been employed in composite electrodes with both cathode and anode materials to achieve significantly improved battery performance. The high aspect ratio of graphene is most useful when incorporated with other nanostructured battery materials as its high surface area offers sufficient electron conduction pathways for fast charge transfer. rGO is one of the most utilized forms of graphene in graphene composite electrodes because the synthesis of nanostructured active materials can be performed concurrently when GO is 2030

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passage of lithium ions while blocking manganese ions; (2) graphene defects strongly react with dissolution-prone surface Mn3+ species, converting them into stable Mn4+. 2D Materials as Separators and Electrolytes. The primary purpose of separators in LIBs is to prevent electrical contact between the cathode and anode without compromising lithiumion diffusion. Therefore, mechanical robustness and lithium-ion diffusivity are important criteria when assessing the performance of a LIB separator. Luo et al.41 reported improved lithium metal anode performance via hBN-coated separators. When lithium ions were stripped and plated many times on a Cu working electrode, a cell with hBN-coated separators showed improved electrochemical performance (Coulombic efficiency of 92% for 100 cycles at a current rate of 0.5 mA cm−2) compared to an uncoated control. SEM characterization of cycled Li/Cu surfaces also reveals that the lithium dendrites were larger in diameter for hBN-coated separators compared to those for uncoated separators. Larger-diameter dendrites contain reduced total surface area compared to thinner dendrites for the same amount of lithium plating, which implies that less electrolyte is consumed during lithium plating/ stripping processes for cells with hBN-coated separators. Because dendritic growth is facilitated by the exothermal nature of electrochemical reactions, the desirable morphology of large-diameter dendrite growth was attributed to the more uniform temperature distribution at the Li/Cu surface due to the high thermal conductivity of the hBN separator. The electrolyte is the medium through which lithium ions transport between cathodes and anodes during electrochemical cycling. An ideal electrolyte should have high ionic conductivity, low electrical conductivity, and high stability over a wide electrochemical window and temperature range. To date, organic electrolytes are most commonly employed in LIBs because they meet almost all of the requirements except high stability at elevated temperatures. To address this weakness of organic electrolytes, Rodrigues et al.42 developed a hBN-based electrolyte composite that allows stable LIB operation from room temperature to 150 °C. The composite electrolyte consists of hBN and lithium-containing solutions in ionic liquids that are formed into a thick paste. Because of its mechanical robustness and the electrically insulating nature of hBN, this composite electrolyte also serves as the separator in a LIB cell. Moreover, the presence of hBN broadens the electrochemical window from 4.7 to 5 V relative to the control without hBN. A LTO half-cell based on the hBN composite electrolyte/ separator was shown to have stable electrochemical cycling up to 150 °C, including minimal capacity fading in galvanostatic measurements and the absence of extra peaks in cyclic voltammetry characterization. Future Outlook. While 2D materials have shown promise in nearly all components of LIBs, additional advances are required before they are widely employed in practical devices. For example, one key issue associated with nanostructured electrodes is poor tap density that results in low volumetric energy density.43,44 Similarly, it remains a challenge to simultaneously achieve high gravimetric and volumetric energy density in electrodes based on 2D materials because of highly porous electrode morphologies, as schematically illustrated in Figure 6a (top). In this regard, future work should focus on developing processing strategies that align 2D materials into a more compact electrode, as shown in Figure 6a (bottom). Moreover, while many aspects of battery performance can be improved by utilizing 2D materials as active or additive

growth, lithium metal is deposited between the protective 2D crystal layer and the Cu surface, resulting in smoother lithium plating with minimal deleterious side reactions. The hBN/Cu anode possesses relatively high Coulombic efficiency of 95−97% over 50 cycles with a practical areal capacity of 1.0 mAh cm−2. This work suggests a pathway toward practical lithium metal anodes and highlights the importance and potential of electrode−electrolyte interfacial engineering. Recent studies have also explored engineering of the cathode−electrolyte interface through cathode surface modification. One of the most prominent issues associated with lithium transition metal oxide cathodes is the dissolution of metal ions into the electrolyte, causing capacity fading during cycling. This issue can be potentially addressed by directly modifying the cathode surface to isolate it from the electrolyte. Toward this end, Jaber-Ansari et al.40 conducted a systematic study that explored how single-layer graphene transferred on a planar LMO thin film influences the interfacial interactions between the LMO cathode and electrolyte. Figure 5a,b

Figure 5. (a,b) Bright-field cross-sectional STEM images of graphene-coated LMO and uncoated LMO samples after cycling, respectively. The cathode−electrolyte interfacial layer in the graphene-coated LMO case is more uniform and thinner than that in the uncoated LMO case. (c,d) XPS depth profiling of graphene-coated LMO and uncoated LMO after cycling, respectively. The intensity of Mn 2P peaks is much higher in the graphene-coated LMO case than that in the uncoated LMO case, indicating suppression of Mn dissolution by the graphene coating. Adapted from ref 40. Reproduced with permission from John Wiley & Sons, Inc.

illustrates a marked contrast between the interfacial layers formed on graphene/LMO and bare LMO surfaces. In particular, a minimal interfacial layer is formed on the graphene/ LMO surface (Figure 5a), whereas the bare LMO surface forms an uneven SEI layer with a thickness ranging from 40 to 100 nm (Figure 5b). Furthermore, after the cathodes are electrochemically cycled, X-ray photoelectron spectroscopy depth profiling reveals that the graphene/LMO cathode retains significantly more Mn ions in the LMO thin film compared to the unmodified LMO cathode. These surface characterization methods are well-correlated with the improved capacity retention of the graphene/LMO cathode. Density functional theory calculations propose two plausible mechanisms that underlie the graphene-induced suppression of Mn dissolution: (1) graphene serves as a selective ionic filter that allows the 2031

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Figure 6. (a) Processes that can control the orientation of 2D material assembly have the potential to reduce electrode packing density and thus increase volumetric capacity. (b) Coatings based on 2D materials may enable efficient passivation that is capable of stabilizing the electrolyte−electrode interface without compromising electrochemical activity. (c) 2D materials are also promising candidates for interfacial engineering. (d) 2D material coatings on separators could potentially be used as filters to block dissolved transition metal ions (M+) from migrating between cathode and anode electrodes. (e) High surface area graphene anodes enable reduced local current density that gives rise to smooth lithium plating and reduced dendrite formation for lithium metal anode applications. (f) 2D materials may possess advantages in all-solid-state LIBs as electrolyte additives (yellow) and active cathode materials (red).

addition to identifying electrolyte additives that can control the local growth of lithium metal and SEI.48−50 Moving beyond LIBs based on organic liquid electrolytes, high surface area 2D material solid-state electrolytes may facilitate the development of all-solid-state LIBs, which have traditionally been limited by slow kinetics in the bulk electrolyte and at the electrolyte/ electrode interfaces. Because lithium ions can diffuse rapidly on the basal planes of some 2D materials,51,52 they may improve the ionic conductivity of standard solid-state electrolytes when employed in a composite architecture (Figure 6f). Furthermore, when utilized as cathode materials in all-solid-state LIBs (Figure 6f), the high surface area of 2D materials should have desirable electrode/electrolyte morphology when interfacing with solid-state electrolytes to achieve improved kinetics. In conclusion, the early application of 2D materials in LIBs has shown significant promise, including improvements in specific capacity, capacity retention, rate capability, and electrode and interface stability. Ongoing efforts are expanding not only the library of 2D materials in LIB applications but also the architectures, morphologies, and methods in which these materials can be used. In order to enable next-generation LIBs based on 2D materials, further research needs to address the inherent issues associated with undesirable surface chemical reactions, thereby allowing their high surface areas to lead to overall improvements in LIB performance, such as enhanced electrochemical kinetics. Many of the advances in LIBs that have been facilitated by 2D materials may also be translatable to other electrochemical storage technologies such as lithium− sulfur batteries, lithium−air batteries, and supercapacitors.

materials, the drastically increased surface area of the resulting electrodes often promotes irreversible side reactions at much higher rates, which limits electrochemical performance and creates potential safety hazards (e.g., gas evolution and thermal runaway). Therefore, additional effort should be dedicated to both studying the interfacial chemistry of 2D materials and developing passivation strategies that can stabilize their surfaces (Figure 6b) while maintaining desirable electrochemical performance and safety. In parallel, other strategies to alleviate the capacity losses due to irreversible side reactions should be actively pursued. One notable approach is to employ prelithiated cathode additives to release extra lithium during the first charge cycle to compensate for first-cycle lithium loss,45 which is a phenomenon commonly associated with high-capacity 2D material anodes. Building off of the effectiveness of graphene to modify the interfacial chemistry of LMO, additional combinations of 2D material coatings and active electrode materials may yield improved electrochemical stability, as illustrated in Figure 6c. The chemical selectivity of 2D materials may also be useful for ionic filtering, as has been shown for graphene derivatives in lithium−sulfur batteries.46 Analogous strategies (Figure 6d) using other 2D materials may enable the functionalization of traditional separators to allow filtering of dissolved transition metal ions from LIB cathode electrodes. This approach would prevent transition metal ion migration from the cathode to the anode, which has been shown to degrade the anode SEI and thus compromise electrochemical stability. The high surface area of 2D materials also presents opportunities for LIBs in certain contexts. For instance, a highly porous graphene 3D scaffold has recently been utilized as a template for lithium plating in a lithium metal anode application,47 resulting in reduced local current during plating due to the high surface area of the scaffold. Because lithium dendrite formation is suppressed at lower current density, a smoother lithium metal layer forms on the porous graphene scaffolds compared to planar Cu surfaces (Figure 6e). Despite these promising results, further efforts are required to optimize the porosity and morphology of the graphene scaffold in

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kan-Sheng Chen: 0000-0002-6094-2396 Itamar Balla: 0000-0002-9358-5743 Mark C. Hersam: 0000-0003-4120-1426 Notes

The authors declare no competing financial interest. 2032

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Biographies

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Kan-Sheng Chen is a postdoctoral researcher in the Department of Materials Science and Engineering at Northwestern University. His primary research interests are high-performance composite lithium-ion battery electrodes based on 2D materials and the development and application of novel scanning probe microscopy characterization methods for 2D materials. Itamar Balla is a Ph.D. candidate in the Department of Materials Science and Engineering at Northwestern University. He obtained a B.Sc. in Materials Science and Engineering in 2011 from the Technion − Israel Institute of Technology. His research interests include the synthesis and characterization of 2D materials for energy applications. Noman S. Luu is currently pursuing a M.S. in Materials Science and Engineering at Northwestern University. He received his B.S. in Materials Science and Engineering from Northwestern University in 2016. His research interests lie at the intersection between nanomaterials and next-generation energy storage technologies, especially lithium-ion battery cathode and anode materials. Mark C. Hersam is the Walter P. Murphy Professor of Materials Science and Engineering and Director of the Materials Research Center at Northwestern University. He earned his B.S. in Electrical Engineering from the University of Illinois at Urbana−Champaign (UIUC) in 1996, his M.Phil. in Physics from the University of Cambridge in 1997, and his Ph.D. in Electrical Engineering from UIUC in 2000. His research interests include nanofabrication, scanning probe microscopy, semiconductor surfaces, and nanoelectronic materials.

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ACKNOWLEDGMENTS This work was primarily supported by the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (DE-AC02-06CH11357).

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