Modeling the effect of electrolyte microstructure on conductivity and solid-state Li-ion battery performance

Maya Horii, Rebecca J. Christianson, Heena Mutha, John Christopher Bachman

Abstract: Solid-state batteries offer the potential to improve safety, cyclability, and energy density of Li-ion batteries. However, their conductivity, stability, and processing limit their use. To this end, the effects of solid electrolyte microstructure on overall battery performance are determined. The conductivities of Al-doped Li$_7$La$_3$Zr$_2$O$_1$$_2$ (Al-LLZO), with a range of microstructures, are found using a resistor network model. Varying electrolyte properties are combined with a Li-metal negative electrode and LiCoO$_2$ positive electrode in a 1-D continuum model to predict the effects on performance. Simulations suggest that the ratio of electrolyte conductivity to electrolyte thickness must remain above 10 S m$^{−2}$ to maintain high energy and power output, particularly at C-rates greater than 10. To maintain conductivity near the grain interior conductivity, a grain size at least 10,000 times larger than grain boundary thickness is needed. Further improvements in grain size, grain boundary thickness, and void fraction beyond typical Al-LLZO microstructures, with the goal of higher conductivities, are predicted to have diminishing returns in overall battery performance, even at more sensitive high C-rates up to 100.