Solid-State Lithium Metal Batteries Promoted by Nanotechnology

May 5, 2017 - In this Perspective, we will show our views on improving this emerging battery system by nanoscience. Discussions will be placed, from b...
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Solid-State Lithium Metal Batteries Promoted by Nanotechnology: Progress and Prospects Sen Xin,†,§ Ya You,†,§ Shaofei Wang,#,§ Hong-Cai Gao,# Ya-Xia Yin,†,‡ and Yu-Guo Guo*,†,‡ †

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, People’s Republic of China ‡ School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China # Materials Science and Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States ABSTRACT: Driven by an increasing demand on storage devices with higher energy outputs and better safety, solid-state lithium metal batteries have shown their potential to replace the traditional liquid-based Li-ion batteries and power the future storage market. In this Perspective, we will show our views on improving this emerging battery system by nanoscience. Discussions will be placed, from both scientific and engineering points of view, on the fundamentals and problems of the battery and its key components. The corresponding “nano” strategies will also be addressed, as well as recent progress in related fields including materials synthesis, battery design, and characterization techniques. With these efforts, we want to provide insights on rational design of the solid-state Li metal battery for optimized performance.

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insulating nature, and unstable electrochemistry vs Li challenge the short-term realization of practical batteries.6,13,20

onventional lithium-ion batteries (LIBs) have been serving consumer electronics for over 2 decades, and they still continue to serve emerging applications such as automobiles and stationary storage.1,2 With an increasing demand proposed on the energy output of batteries, LIBs based on traditional intercalation electrochemistry are incapable of satisfying these raising industries, and new battery systems with higher energy densities are desired.3−7 The theoretical energy density of a battery is largely determined by the capacities delivered by electrochemical reactive materials on both electrodes, namely, the cathode and the anode, and the electrochemical potential difference between the two electrodes.1,2 For cathode materials, they are classified into two types based on their Li storage electrochemistry: the intercalation type (e.g., LiCoO2 and LiFePO4) and the conversion type (e.g., O2, S).8 Intercalation cathodes (ICs) are provided with high electrochemical potentials (normally >3.3 V vs Li+/Li) and stable electrochemistry during Li insertion/extraction yet limited theoretical specific capacities (a commercially available maximum of 160−200 mA h g−1 obtained on the LiNi0.80Co0.15Al0.05O2 cathode material).9 The conversion-type cathodes, in contrast, show high theoretical specific capacities (S: 1675 mA h g−1 with Li2S as the discharge product; O2: 3351 mA h g−1 with LiOH as the discharge product in aqueous electrolyte or 1675 mA h g−1 with Li2O2 as the discharge product in organic electrolyte).6,8,10−21 However, their relatively low potentials (normally 5 V (vs Li+/Li).51,58 However, a recent study by Han et al. challenges this perspective by claiming that the electrochemical window measurements are performed based on the semiblocking electrodes, which overestimate the true electrochemical window governed by the intrinsic thermodynamics of the material.58 On the basis of the theoretical calculation results, the actual electrochemical window of the Li7La3Zr2O12 is much narrower (0.05−2.91 V vs Li+/Li), which reasonably explains the poor electrochemical performance of the bulk-type solid-state batteries assembled with oxide ceramic electrolytes as the high-voltage decomposition products of the SSE inevitably contribute to resistance at the electrode|SSE interface.58 The sulfide glass−ceramics, as represented by Li10GeP2S12, also claim to be stable within 0−5 V vs Li+/Li,59

Figure 1. Comparisons between the conventional LIB and the nextgeneration SSLMB of the battery structure and energy output.

configuration of LA|SSE|IC, the use of a Li incompatible, flammable liquid electrolyte is avoided, which significantly improves the safety of the battery.37,49 Second, the shift from liquid-based LIBs to SSLMBs is expected to significantly improve the energy output, and the estimated values could be 40 and 70% increments in practical specific energy and volumetric density, respectively.37 Moreover, because some of the SSEs support a high operation voltage (e.g., some ceramic/glass SSEs support a voltage higher than 5 V vs Li+/Li), the use of a LA to pair with some high-voltage ICs is expected to further raise the energy output of the battery (Figure 1).51 Meanwhile, expectations are also given to dendrite suppression (especially when the discharge/charge process of the batteries is performed at a low current density) and SEI stabilization by the SSE, which may bring a higher efficiency for the anode and a longer cycle life for the battery.37 However, as an emerging technique, the 1386

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Figure 2. (a) Schematic illustration showing the mechanisms of classic bulk chemical diffusion and job-sharing diffusion. (b) Ultrafast performance of an all-solid-state battery using RbAg4I5/graphite, as demonstrated by the battery capacity at various C rates. Reproduced with permission from ref 72 (Copyright 2016, Nature Publishing Group).

and finally brought the concept of nanoionics.69,70 According to the concept, the nanocomposites may show a remarkably improved conductivity by taking advantage of fast ion diffusion at the interface and a large volume ratio of the interface among nanoparticles. On the basis of the theory of nanoionics, various types of nanocomposite SSEs were built, which included ceramic−ceramic composites, ceramic−glass composites, ceramic−polymer composites, and glass−polymer composites. With their high flexibility and easy integration into battery systems, the glass−polymer and ceramic−polymer nanocomposite materials are considered promising electrolytes for nextgeneration SSLMBs. However, because of difficult in situ/ ex situ characterizations and unavoidable formation of plasticizer residues (e.g., ethylene carbonate and diethyl carbonate), so far no solid evidence has been found to directly demonstrate enhanced interfacial ion transport between the inorganic and polymer SSE (PSSE). The improved conductivity of the polymer-based composite electrolytes was thus ascribed to a decreased glass phase transition temperature of the polymer after introducing the inorganic nanoparticles.71 Besides the low ionic conductivity of the solid electrolyte, the huge ionic transfer resistance in the composite electrode is also a challenge for solid-state batteries because of the low wetting ability of the solid electrolyte materials, a large volume ratio of the nonionic conductive component, and the large charge transfer reaction barrier at the solid electrolyte−solid electrode interfaces. Recently, Maier et al. showed that very fast ionic transport could be achieved at the interface of the superionic conductor RbAg4I5 and electronic conductor graphite heterogeneous system through a job-sharing transport mechanism (Figure 2a).72 Benefiting from a high diffusion efficiency, an extremely high rate performance was achieved in solid-state batteries with Ag as the anode and a RbAg4I5−graphite composite as the cathode (Figure 2b).72 An extremely high rate performance was also observed in the solid battery using sulfide electrolyte, Li4Ti5O12 as the anode, and LiCoO2 as the cathode by Kanno et al.73 However, a similar phenomenon has not yet been observed in normal bulk materials, which is mainly ascribed to a limited interfacial volume ratio. Therefore, one can come to a conclusion that only the nanocomposites can provide a sufficiently

yet they have been found to be unstable against reduction by Li at low voltage or extraction of Li with decomposition at high voltage.58,60 As a result, the Li10GeP2S12 shows an even narrower electrochemical window of 1.71−2.14 V vs Li+/Li.58 The polymer electrolytes are provided with an improved electrode|SSE interface, low cost, and better machinability so that they are easily integrated into batteries; however, their highest occupied molecular orbitals are too low to be compatible with high-voltage cathode materials (with voltage outputs > 4.0 V vs Li+/Li). The thin-film SSEs show low ASR and improved electrochemical performance yet are too high in cost and poor in scalability. The combined use of different types of electrolytes is expected to integrate their advantages; nevertheless, improvements in some properties are always at the cost of others. Hence, it is crucial to explore new strategies to trade off all of the properties that enable commercial solid-state batteries. Interfacial ion transport is a major issue to be addressed for SSEs.61,62 It is worth noting that the interfaces in solid-state batteries include not only the grain boundaries inside of the ceramics but also the multiphase interfaces among ceramics, glasses, and polymers in the composite electrolyte and, more importantly, the LA|SSE and SSE|IC interfaces in the battery. The ion transports at these interfaces proceed along two directions, that is, jumping across the interface and diffusion along the interface. Normally, jumping across the interface is kinetically more sluggish because of the large diffusion barrier at the interfaces between different structures. However, fast transport may happen along the interface. In 1973, Liang observed faster Li+ conduction when the LiI and Al2O3 were mixed at appropriate mass ratios.63 The following research confirmed fast ion transport along the LiI|Al2O3 interface.64,65 A percolation model was then used to simulate the conductivity value in the two-phase composite electrolyte.66,67 In 2000, Maier et al. found that the parallel ionic conductivity of multiphase, heterolayered CaF2/BaF2 films increased proportionally with the number of heterojunctions, which was increased by simultaneously decreasing the thickness of CaF2 and BaF2 layers.68 The above finding started the research on interfacial ion transport inside of multiphase nanocomposites 1387

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and reduced the resistance at the grain boundary, resulting in a high ionic conductivity. Randall et al. developed a lowtemperature sintering technology named “cold sintering” for the preparation of ceramic and composite electrolytes.81,82 By controlling the dissolution and precipitation of particles in a transient solvent, this approach could achieve an effective densification of ceramics at a temperature below 200 °C. Moreover, nanosized particles with a large specific surface area and high chemical activity would benefit the process. The method is also applicable to polymer−ceramic composite electrolytes, which usually show high flexibility but suffer low Li+ conductivity due to poor connection between ceramic particles. Benefiting from the low densification temperature, the polymer−ceramic composite can be effectively densified, which brings high conductivity and high flexibility as well.

large interface to enable such fast mass transportation. Moreover, the above results also indicate that a high electrochemical reaction rate also could be achieved for the nanocomposite electrode materials. Therefore, achievement of low inner resistance for solid-state batteries is possible by reasonable design of the composite electrode structure. However, with advancement in battery-related characterization techniques, the interfacial transport mechanism may be figured out to shed light on the rational design of nanocomposite SSEs. Nanocoating is another way to combine the electrochemical advantages of different SSEs. For example, in order to reduce the large solid−solid interfacial resistance for Li+ transport of the ceramic SSE (CSSE), Goodenough et al. have introduced a polymer coating on both sides of ceramics to form a polymer/ ceramics/polymer sandwiched electrolyte.50 The PSSE coated the outside of the ceramics and improved the wetting on both electrodes and resulted in a significantly reduced resistance at the electrode|SSE interface. Nanolayer surface coating is another effective way to enhance the chemical stability and electrochemical compatibility with both electrodes without consuming other properties of the electrolyte. Due to their favorable electrochemical stability with Li, poly(ethylene oxide) and thin-film lithium phosphorus oxynitride are the most engaged coating layers.74,75 Some other nanolayers, such as Au, Si, ZnO, and Al2O3, are also subjected to being coated on the surface of ceramics to improve the electrochemical performance.50,52,56,76 The mass production of large-area thin films at an affordable cost constitutes another challenge for battery application of ceramic or glass SSEs. Realizing that nanoparticles with a large specific surface area and high sintering activity could lower the sintering temperature and enable a complex sintering process, some new fabrication technologies based on nanoengineering have been developed for flexible thin-film CSSEs and polymer− ceramic composite SSEs. For example, Laine et al. successfully synthesized flexible Li−garnet CSSE films with a thickness of 0.85 and a high ionic conductivity of 1.3 × 10−5 S cm−1 at the same time.86 On the other hand, the stable structure of the polymer also contributed to an improved mechanical strength (10 MPa) and an enlarged electrochemical stability window (up to 5 V vs Li+/Li).86 However, such a SICP can only be operated at a temperature of 60 °C and above, which limits its practical battery use.86 To overcome the above drawback, Luca et al. reported a novel SICP gel electrolyte by incorporating the anion into a network of poly(ethylene glycol) methyl ether methacrylate and bifunctional poly(ethylene glycol) methyl ether dimethacrylate with propylene carbonate as a plasticizer.87 At room temperature, the electrolyte showed a tLi+ approaching unity and an ionic conductivity of 10−4 S cm−1 while also maintaining good mechanical robustness and stable electrochemistry against the IC and LA.87 Further, by employing an in situ polymerization technique, the above gel electrolyte could form a nanocoating layer on the cathode, so that it could be directly paired with the LA to enable a solid-state battery with excellent performance.87 In addition, growth of Li dendrites is still possible when plating/stripping occurs at a higher current density, though the PSSEs are reported to inhibit the dendrites due to their favorable wettability on the LA. Once the dendrites form, they can penetrate through the cross-linked network structure of the polymer to cause short circuiting of the battery. It is found that by applying a nanocoating with high mechanical strength (normally characterized by Young’s modulus) on the anode surface, the growth of Li dendrite is effectively restrained and the side reactions at the LA|polymer interface are mitigated.24 Inspired by this point, Guo et al. realized a bifunctional PSSE with an interpenetrating network of poly(ether-acrylate) (ipn-PEA) by photopolymerization of Li+-conductive PEO and branched acrylate (Figure 4a).88 By combining the plasticity of PEO and the structural rigidity of PEA, the composite PSSE maintained its flexibility for processing and showed a high Young’s modulus of 12 GPa to effectively suppress the dendrite growth (Figure 4b).88 Meanwhile, the ipn formed by PEO and PEA brought high room-temperature ionic conductivity (0.22 mS cm−1) and a Li+ transfer number (tLi+ = 0.65) exceeding that of the conventional liquid electrolyte (tLi+ = 0.2−0.5), which were helpful at inhibiting the side reactions on the anode.88 As the bifunctional PSSE reshaped the Li plating/stripping behavior, it also reshaped the feasibility of realizing the SSLMBs. There are two major challenges for the IC. The first one comes with a limited contact area at the SSE|IC interface

of the SSE) may occur, which accounts for a low Coulombic efficiency and brings safety concerns to hamper use with the LA (Figure 3). To address the above challenges, a feasible way is to artificially introduce into the LA|SSE interface a Li+ conductive nanointerlayer that wets both the LA and the SSE, which improves the Li+ conduction and homogenizes the Li+ flux at the LA|SSE interface (Figure 3). Using the nanoengineering technique, it is now possible to realize such an improvement. For example, Hu et al. used a plasma-enhanced chemical vapor deposition method to deposit an ultrathin (ca. 10 nm) amorphous Si layer on the surface of the garnet electrolyte.53 Due to the high reactivity of Si with Li, the in situ formed, lithiated Si nanolayer created perfect contacts with both the garnet and the LA, and the surface of garnet became lithiophilic to enable rapid wetting by molten Li.53 The results were much reduced impedance at the LA|SSE interface and improved Li plating/stripping stability compared to those using the bare garnet.53 Recently, Hu et al. further employed an atomic layer deposition technique to deposit an alumina (Al2O3) nanocoating layer on the garnet electrolyte and observed negated impedance at the LA|garnet interface by 3 orders of magnitude at room temperature.56 Further experiments and computational results confirmed that the Al2O3 coating enabled wetting of metallic Li on the garnet surface and generated a lithiated alumina interface to allow fast Li+ transport.56 Other materials, such as ZnO, LiF, and Li+ conductive polymers, were also employed to build nanointerlayers to improve the Li wettability and electrochemical performance of the CSSE.50,52,54 In a recent work by Goodenough et al., they synthesized a rhombohedral structured LiZr2(PO4)3 electrolyte and utilized the reaction between the LA|SSE interface to form a Li+-conducting passivation nanolayer.55 As the major components of the passivation layer, that is, Li3P and Li8ZrO6 wetted well both the LA and the SSE, the mass/charge transfer through the interface was regulated, and the passivation layer further stopped penetration of Li dendrites, bringing stable cycling performance.55 For the sulfide ceramic−glass electrolytes, their reaction with Li metal is too significant to be neglected.58,84 Hence, proper protection strategies, such as artificial SEI layers, should be applied at the interface.58 However, some sulfide electrolytes, such as Li10GeP2S12, can be reduced by Li to generate a Li-conducting passivation nanolayer (i.e., amorphous Li2S) at the LA|SSE interface, which would prevent further decomposition of the electrolyte so that the electrochemical performance does not degrade.60 For the PSSEs (especially those based on nitrile or ether), the major challenge lies in unfavorable side reactions at the LA|SSE interface. As most of the PSSEs hold relatively low Li+ transfer numbers (tLi+ = 0.2−0.5), rapid anion depletion on the anode side inevitably occurs during charging of the battery, which creates a large electric field across the LA|polymer interface to initiate dendrite nucleation and electrolyte decomposition to form SEI (Figure 3).50 In case the charge/discharge process proceeds with higher current densities applied, the PSSE shows poorer electrochemical stability, and the battery performance is thus deteriorated. Hence, it is required to inhibit these side reactions at the nanoscale. Recently, Goodenough et al. inserted a thin layer of CSSE into two layers of PSSE to build a sandwich-type composite electrolyte and observed almost completely blocked anion migration by ceramics (Figure 3).55 The result was a significantly raised Li+ transfer number (tLi+ ≈ 1) and much reduced electric field intensity due to anion accumulation at the LA|SSE interface, which inhibited Li dendrite 1389

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Li+-conductive polymer (LCP) to replace the conventional binder (CB) such as polyvinylidene fluoride (Figure 5).50 Benefiting from its structural flexibility, the polymer formed a Li+-conductive nanocoating layer outside of the IC particles to form a conductive network (Figure 5).50 In this way, the IC particles were easily accessible by Li ions, ensuring favorable cathode performance (Figure 5). Another challenge lies in unstable chemistry at the SSE|IC interface. Some PSSEs, as instanced by the PEO, decompose when the voltage goes above 4.0 V vs Li+/Li. By integrating PEO into the rigid network of PEA with improved electrochemical stability, Guo et al. successfully raised the decomposition voltage of the PSSE to 4.5 V vs Li+/Li, so that it might be compatible with high-voltage ICs.88 Apart from high-voltage decomposition during the Li extraction process, the sulfide electrolytes easily reacted with TM ions (e.g., Mn2+, Ni2+, Co2+, etc.) during cycling (Figure 5).58,60,89 Surface modification on IC particles may be an effective method to address the problem. For instance, by coating an Al2O3, AlPO4, or Zr2O3 nanolayer on the IC particles, the transitional metal dissolution was effectively inhibited, and the side reactions with the sulfide electrolytes might be alleviated (Figure 5).90−93 Also, direct contact between the IC and sulfide electrolyte would form a highly resistive layer, which has been speculated to be a spacecharge region with Li segregation into the cathode side to deplete Li ions at the SSE|IC interface and decrease the Li+ conductivity.94 Employment of a Li+ conductive, nanometerthick buffer layer (e.g., LiNbO3) between the IC and the sulfide electrolyte may help to suppress the formation and growth of the space-charge layer, so that it can benefit from a smooth Li+ transport path and contribute to a much reduced interfacial resistance for stable operation of the battery.95,96 However, if these “nano” strategies are used improperly, they will be detrimental to the interface stability. For example, when active nanomaterials with a large specific surface area are employed on the cathode, they may aggravate the unfavorable side reactions between the electrolyte and the cathode and lead to a poor chemical stability in between.

Figure 4. Schematic illustrations showing (a) preparation of the ipn-PEA electrolyte and (b) the Li deposition electrochemistry using liquid and ipn-PEA electrolytes. Reproduced with permission from ref 88 (Copyright 2016, American Chemical Society).

With surface modification at the nanoscale, the Li+ accessibility and electrochemical stability at the interface between the solid-state electrolyte and intercalation cathode could be improved, which is equally important for realizing optimal performance of solid-state batteries.

Nanotechnology may not be a panacea for every challenge in the realization of solid-state Li metal batteries, but, in the case of proper use, it will offer opportunities to tackle some key problems and provide a sustainable future for the emerging technique.

(Figure 5). Due to the rigid nature of a glass−ceramic electrolyte, it merely contacts with the particles on the surface of the cathode, leading to poor Li+ accessibility inside of the cathode (Figure 5). To address this issue, people used a

Figure 5. Main challenges and corresponding solutions at the SSE|IC interface of a SSLMB. 1390

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Table 1. Problems and Strategies for Key Components of SSLMB component

problem

SSE

sluggish ion transport

difficult integration into batteries

electrode

bulk electrode LA|SSE interface

SSE|IC interface

difficult manufacture (sintering) process at high cost large ionic transfer resistance in the composite electrode “lithiophobic” nature of ceramic oxide SSE

strategy nanocomposite electrolyte based on nanoionics nanocoating on surface of solid electrolyte glass−polymer and ceramic−polymer nanocomposite electrolyte nanoengineering nanocomposite electrode with jobsharing transport artificially introduced, Li+ conductive and lithiophilic nanointerlayer

reduction of sulfide SSE by Li

in situ formed, Li+ conductive and lithiophilic passivation nanolayer artificial SEI nanolayer

decomposition of PSSE due to a low Li+ transfer number

Li+ conductive passivation nanolayer formed in situ polymer−ceramic−polymer sandwiched composite SSE

growth and penetrate of Li dendrites through the PSSE poor physical contact between IC and glass/CSSE decomposition of PEO-based SSE at the cathode voltage unfavorable side reaction between sulfide SSE and IC formation of Li+ resistive spacecharge region

SICP PSSE with high Young’s modulus use of Li+ conductive polymer as binder integration of PEO into a matrix with a rigid nanostructure protection of IC particles by surface nanocoating introduction of Li+ conductive nanobuffer layer

effect improved bulk conductivity reduced solid−solid interfacial resistance high flexibility and easy integration into batteries lowered sintering temperature and new sintering methods improved ionic/electronic conductivity of the electrode reduced impedance at the LA|SSE interface and improved Li plating/ stripping stability

improved chemical stability of sulfide electrolyte against Li

suppressed anion depletion at the anode side and enhanced electrochemical stability of PSSE dentrite-proof PSSE improved Li+ accessibility to IC particles improved high-voltage durability suppressed interfacial side reactions suppressed formation of space-charge layer and improved Li+ conduction

of the chemical similarities among the alkali metals, these strategies may show their significance in building substitutional types of rechargeable solid-state batteries, for example, rechargeable solid-state sodium batteries.97 We hope that our Perspective can shed light on the reasonable design of keyenabling materials for SSLMBs and contribute to a helpful discussion of “post-lithium” technology.

The transferring research interests from liquid-based LIBs to SSLMBs have reflected a persistent demand on battery devices with better economic sustainability. With potentially improved battery safety and energy output, the future of solid-state batteries is also solid. However, in view of a practical battery, other parameters, such as the mass/volume ratios of the electrode active materials and the ionic conductivity of the SSE, will also affect the final energy output and should be taken into consideration. Also, there are still many challenges to overcome before we can practically take advantage of the technology. With substantial progress made on improving the bulk conductivity of the SSE, the major concern for the SSLMB has now been placed on the two electrochemical interfaces, that is, the LA|SSE interface and the SSE|IC interface. Aiming at improving the Li+ diffusion kinetics and thermodynamic stability at these interfaces, various “nano” strategies, such as surface nanoengineering and nanocomposite SSE, have been developed to show their promise in reducing the resistance at solid−solid interfaces, inhibiting dendrite formation and avoiding any unfavorable side reactions between the electrolyte and the electrode (Table 1). Positive effects have been demonstrated by much extended operational lives of SSLMBs, which are now able to last for hundreds of discharge−charge cycles to meet the requirement of consumer electronics.50,54 Though the reality is still a long way off, one can be optimistic about solving the fundamental problems of the SSLMB at the nanoscale because there is always plenty of room at the bottom. For the long-term goal, a transition from ICs to conversion cathodes such as S or O2 is expected to further elevate the energy density of the battery, and nanotechnology can help more to accelerate the transition.6,8,10−21 Moreover, in view

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sen Xin: 0000-0002-0546-0626 Yu-Guo Guo: 0000-0003-0322-8476 Author Contributions §

S.X., Y.Y., and S.W. contributed equally to this paper.

Notes

The authors declare no competing financial interest. Biographies Sen Xin received his Ph.D. degree in 2013 from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) under the supervision of Prof. Yu-Guo Guo. He then worked as a lecturer at Hefei University of Technology. In 2015, he joined the group of Prof. John B. Goodenough at the University of Texas at Austin (UT Austin) as a postdoctoral research fellow. His research focuses on advanced materials for high-energy rechargeable lithium/sodium batteries. Ya You received her Ph.D. degree in 2015 from ICCAS under the supervision of Prof. Yu-Guo Guo. After that, she worked as a postdoctoral research fellow with Prof. Arumugam Manthiram at UT 1391

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Austin. Her research focuses on electrochemistry and key materials of room-temperature rechargeable sodium-ion batteries. Shaofei Wang received his Ph.D. degree in 2014 from the Institute of Physics, Chinese Academy of Sciences. After that, he started his postdoctoral research at UT Austin under the supervision of Prof. Arumugam Manthiram. His research focuses on solid-state electrolytes for rechargeable Li batteries. Hongcai Gao obtained his Ph.D. degree in 2013 from Nanyang Technological University. After that, he joined UT Austin and started his postdoctoral research under the supervision of Prof. John B. Goodenough. His research is currently focused on the development of polymer electrolytes for rechargeable lithium and sodium batteries. Ya-Xia Yin received her Ph.D. degree in Materials Science and Technology from Beijing University of Chemical Technology in 2012. Afterwards, she worked as an Associate Professor at ICCAS. Her research focuses on electrode materials for rechargeable lithium-ion batteries and lithium−sulfur batteries. Yu-Guo Guo received his Ph.D. degree in Physical Chemistry from ICCAS in 2004. He worked at the Max Planck Institute for Solid State Research at Stuttgart (Germany) from 2004 to 2007. He joined ICCAS as a full professor in 2007. His research focuses on electrochemical energy storage and nanostructured energy materials.

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ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (Grant 2016YFA0202500), the National Natural Science Foundation of China (Grants 51225204, U1301244, and 21127901), and the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant XDA09010300).

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