(Invited) Solution-Processable Li and Na Superionic Conductors for All-Solid-State Batteries
Authors: Park Kern Ho , Banerjee Abhik , Kim Dong Hyeon , Hong Seung-Tae , Oh Seung Mo , Jung Yoon Seok 
Published: February 2020

Recently, rechargeable batteries have been emerging as the key to large-scale energy storage devices. For electric vehicles (EVs), lithium-ion batteries (LIBs) are considered to be the most competitive because of their high operating voltage and light weight of electrode materials. For larger-scale applications than electric vehicles, energy grid or energy storage systems, the use of Na rather than Li may be more reasonable to satisfy the requirements of cost and environmental effects. This has leaded to the explosive research efforts on Na-ion batteries (NIBs). The success of LIBs and the promising results for NIBs are primarily indebted to development of novel electrode materials. However, their operation becomes possible by employment of liquid electrolytes in which the charge carriers, Li or Na ions, can move between negative and positive electrodes. Due to the high operating voltages of LIBs and NIBs, the electrolytes for them are based on organic liquid solvents. However, their intrinsic properties give rise to serious safety concerns such as flammability and leakage.

Development of all-solid-state batteries employing inorganic solid electrolytes (SEs) is an ultimate solution to tackle the safety issues.1 Thin film all-solid-state lithium batteries (ASLBs) based on LiPON as SE is commercially available type of batteries, and exhibit excellent electrochemical performances. However, they are not suitable for large-scale applications for EVs or energy storage systems due to the high fabrication cost. In contrast, bulk-type all-solid-state batteries in which particulate mixtures of active materials, SEs, and conductive additives may compete with the LIBs because the powder-based manufacturing process similar to that for LIBs might be adaptable.1

To date, promising electrochemical performances of bulk-type ASLBs have been demonstrated by the use of state-of-the-art sulfide SE materials. The sulfide SE show not only extremely high conductivities which is comparable to that for liquid electrolytes but also excellent deformability which makes it possible to achieve two-dimensional contacts with active materials by pressing at room temperature. Despite the intrinsically good deformability of sulfide SEs, achieving full surface coverage of SEs on active materials has been hindered by restriction of synthesis protocols for SEs.2,3

A proof-of-concept that direct coating of SEs on active materials can effectively enhance the electrochemical performance of bulk-type ASLBs was demonstrated by using laser vapor deposition.4 The next step inspired by this approach should be developments of commercially meaningful protocols for SE coating. In this context, solution-process of SEs is of prime importance. However, only a few works on the wet chemistry of SEs, overlooking the possibility of its application to the SE coating, has been reported so far. This might be because of unavailability of solvents, considering the severe reactivity of sulfide materials with common solvents.3

By the aforementioned backgrounds, recently our group have succeeded to develop new, highly conductive (>10-4 S cm-1) Li-ion and Na-ion conductive solid electrolytes by scalable solution process using cheap and environmentally-benign solvents.5 The solution-process was directly applied for the SE-coating on active materials for ASLBs and ASNBs, demonstrating its excellence in enhancement of electrochemical performances. In this presentation, our recent results on solution-processable Li- and Na-ion SEs and their applications to all-solid-state Li- and Na-ion batteries will be presented.

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