High electronic conductivity as the origin of lithium dendrite formation within solid electrolytes

Solid electrolytes (SEs) are widely considered as an 'enabler' of lithium anodes for high-energy batteries. However, recent reports demonstrate that the Li dendrite formation in Li₇La₃Zr₂O₁₂ (LLZO) and Li₂S–P₂S₅ is actually much easier than that in liquid electrolytes of lithium batteries, by mechanisms that remain elusive. Here we illustrate the origin of the dendrite formation by monitoring the dynamic evolution of Li concentration profiles in three popular but representative SEs (LiPON, LLZO and amorphous Li₃PS₄) during lithium plating using time-resolved operando neutron depth profiling. Although no apparent changes in the lithium concentration in LiPON can be observed, we visualize the direct deposition of Li inside the bulk LLZO and Li₃PS₄. Our findings suggest the high electronic conductivity of LLZO and Li₃PS₄ is mostly responsible for dendrite formation in these SEs. Lowering the electronic conductivity, rather than further increasing the ionic conductivity of SEs, is therefore critical for the success of all-solid-state Li batteries.