Dendrite growth and inhibition in all-solid-state lithium metal batteries: in situ optical observation

Abstract

Dendrite growth behavior in a thin lithium phosphorus sulfide (LPSC) solid electrolyte has not been well revealed due to the lack of a suitable characterization method. This work introduces a unique yet simple method to monitor dendrite growth inside a pressurized all-solid-state cell in real time. Once a lithium metal dendrite penetrates through a thin LPSC layer and enters a LPSC-PTFE (polytetrafluoroethylene) indicator layer, it will react with PTFE and form a black-colored electron-conductive substance that traces the dendrite growth pathways, which can be in situ or ex situ observed using an optical microscope. It is shown that dendrites tend to grow through a very thin LPSC layer even at a very low current density, probably along the pre-existing chains of defects, revealing a potential challenge for practical application of all-solid-state lithium metal batteries. To address this challenge, we added an appropriate amount of LiF in LPSC electrolyte to increase its interface energy with lithium and effectively inhibit dendrite growth. Using the LPSC-LiF composite electrolyte, a Li|Li symmetric cell can achieve 1800 h ultralong cycling life at a current density of 0.5 mA cm⁻² and an all-solid-state Li|LPSC-LiF|LiCoO₂ full cell can operate over 200 cycles without short-circuiting, showing great promise for its application in future all-solid-state lithium metal batteries.