The Riddle of Dark LLZO: Cobalt Diffusion in Garnet Separators of Solid_State Lithium Batteries

Abstract

Abstract Solid_state batteries (SSBs) with a Li 7 La 3 Zr 2 O 12 (LLZO) garnet electrolyte are attracting much attention as robust and safe alternative to conventional lithium_ion batteries. Technical challenges in the practical implementation of garnet SSBs are related to the need for high_temperature sintering, which often leads to undesirable chemical reactions with the cathode material. While these reactions are well understood for composite cathodes, very little is known about similar processes between cathode and separator during battery fabrication. This work focuses on understanding the processes between the composite LiCoO 2 _LLZO cathode and the LLZO separator and how they affect the battery performance. The extensive diffusion of Co_ions within LLZO, which leads to the often_observed LLZO darkening, is shown to have a significant impact on ionic conductivity, electronic conductivity, and dendrite stability of the separator. Experimental data coupled with large_scale molecular dynamics simulations uncover the diffusion mechanism for Co_ions and identify secondary phases that form during these interactions. In addition to extensive Co_ion diffusion within the grains, a non_uniform segregation of Co_ions at grain boundaries is found leading to the formation of three distinct Co_containing phases. This work offers a general approach to studying the fundamental ion diffusion processes that occur during the fabrication of oxide SSBs.