Enhancing the polymer electrolyte-Li metal interface on high-voltage solid-state batteries with Li-based additives inspired by the surface chemistry of Li7La3Zr2O12

High-voltage Li metal solid-state batteries are in the spotlight as high energy and power density devices for the next generation of batteries. However, the lack of robust solid-electrolyte interfaces (SEIs) and the propagation of Li dendrites still need to be addressed for practical application with extended cyclability. In the present work, high-voltage Li metal cells with LiNi0.6Mn0.2Co0.2O2 active material were assembled with a polyethylene(oxide) based electrolyte mixed with lithium bis(fluorosulfonyl)imide (LiFSI) salt. The addition of Li7La3Zr2O12 garnet to form a composite electrolyte demonstrated a beneficial effect for cell cycling stability. Inspired by the improved interface of ceramic Li7La3Zr2O12 garnet and Li metal, as well as by previous knowledge of favorable SEI forming species, various additive candidates were selected to optimize its electrolyte composition. Among them, lithium hydroxide (LiOH) is a key favorable species that shows a relevant improvement in the cyclability of the cells. X-ray photoelectron spectroscopy showed that the SEI layer is composed mainly of chemical species arising from the reduction of the Li salt, with lithium fluoride (LiF) being the main product. In addition, solid-state nuclear magnetic resonance proved that LiOH induces the cleavage of the labile S-F bond, increasing the concentration of LiF. Herein, we highlight that SEI-forming additives need to be considered for the interfacial engineering design of stable SEIs to expand the performance boundary of SSBs.

Automated summary

In this study, high-voltage Li metal cells with LiNi0.6Mn0.2Co0.2O2 active material were assembled using a composite electrolyte containing Li7La3Zr2O12 garnet, which showed improved cycling stability. Additive lithium hydroxide (LiOH) played a key role in enhancing the cyclability of the cells by inducing the cleavage of the labile S-F bond and increasing the concentration of LiF in the solid-electrolyte interface layer. It was emphasized that SEI-forming additives should be considered for stable SEI interfacial engineering design to expand the performance boundary of solid-state batteries.