Interfaces in Solid-State Lithium Batteries

Abstract

The influence of interfaces represents a critical factor affecting the use of solid-state batteries (SSBs) in a wide range of practical industrial applications. However, our current understanding of this key issue remains somewhat limited. Therefore, this review presents the mechanisms and advanced characterization techniques associated with interfaces in SSBs. First, we compare liquid- and solid-state batteries and emphasize the challenges in solid-solid interfaces. Second, we discuss different aspects of interfaces including interphase formation, cathode-electrolyte interface, anode-electrolyte interface, and interparticle interface, which contain a detailed description of the formation mechanisms and current research. Third, the characterization strategies for effective interfacial observation and analysis are summarized and discussed. In particular, two unique characterization techniques, vibrational sum-frequency generation spectroscopy and on-chip single-nanowire battery characterization, are highlighted. Lastly, we summarize the scientific issues associated with solid-solid interfaces and provide our perspectives to better understand the fundamental issues and improve the performance of SSBs. Lithium-ion batteries (LIBs) are the promising power sources for portable electronics, electric vehicles, and smart grids. The recent LIBs with organic liquid electrolytes still suffer from safety issues and insufficient lifetime. Solid-state batteries (SSBs) are expected to address these issues. In principle, the nonflammable solid electrolytes could prevent battery combustion and explosion, and only Li ions are mobile in solid electrolytes, which could suppress side reactions. Some solid electrolytes, such as sulfides, have sufficiently high ionic conductivity, which is comparable to that of with organic liquid electrolytes. Thus, solid-solid interfaces appear to be the key to push SSBs toward practical applications. In this review, we start by introducing the challenges in solid-solid interfaces versus liquid-solid interfaces. We then discuss different interfaces in SSBs, including cathode-electrolyte interface, anode-electrolyte interface, and interparticle interface. Lastly, we present the advanced characterization techniques to help deepen understanding of the composition and structure evolution at the interfaces during battery cycling. The on-chip single-nanowire electrochemical devices developed by our group are highlighted as a unique platform for in situ characterization. We suggest and emphasize some future directions for SSBs. First, different in situ or operando characterization techniques should be developed and combined to track the real-time composition and structure changes at the interfaces in SSBs. Second, in addition to metal ions, metal-air and metal-sulfur systems with much higher energy density should also receive sufficient attention for SSBs. Lastly, a unique advantage of SSBs over liquid-electrolyte batteries is that SSBs could be flexible, stretchable, and shrunk on a chip. Thus, SSBs are promising for integration with microelectronic circuits to fabricate self-powered wearable or implantable micro-/nanoscale devices. SSB with a nonflammable solid electrolyte is a promising approach to address the safety issues of rechargeable batteries with flammable liquid organic electrolyte. However, the high impedance and/or instability of the solid-solid interfaces limit the practical applications of SSBs. This review focuses on the mechanisms and advanced characterization techniques associated with interfaces in SSBs, as well as provides our perspectives on future directions to better understand the fundamental issues and improve the performance of SSBs.