Lithium Deposition in Single-Ion Conducting Polymer Electrolytes

Abstract

Lithium metal is considered as potential anode material due to its superior energy density compared to conventional graphitic anodes.[1] However, so far its application is restricted due to inhomogeneous Li deposition during cycling, potentially causing contact losses between bulk and deposited Li or short circuit of the cell related to formation of ‘dendritic’ deposits. Theoretical models propose that single-ion conducting electrolytes afford safe and ‘dendrite-free’ cycling of lithium metal batteries (LMBs) due to immobilization of anions and thus, prevention of cell polarization.[2] In this contribution, a blend type quasi-solid single-ion conducting polymer electrolyte (SIPE), a polymer class that has attracted much attention recently, is consulted for analysis of the Li deposition behaviour. Combined scanning electron microscopy (SEM) and X-ray tomography are applied illustrating the occurrence of inhomogeneous, rather dense Li deposits agglomerating and eventually penetrating the polymer membrane independent of the applied current densities. Raman mapping, nano-mechanical (QNM) and electrochemical strain microscopy (ESM) mapping further reveal that membrane constituents and consequently membrane properties such as (mechanical) stiffness and ionic conductivity are non-homogeneously distributed within the membrane related to the phased separated morphology of the blend. This implies that the intrinsic nature of the polymer membrane is responsible for the formation of conduction channels, also governing the homogeneity of Li deposition. Beyond that, it is demonstrated how characteristics of the Li│polymer interface in salt- free SIPEs limit the achievable cell performance, particularly since limited Li+ ion diffusivity through the interfaces substantially impair efficient charge carrier transport within the cell system.