Revealing the critical effect of solid electrolyte interphase on the deposition and detriment of Co(Ⅱ) ions to graphite anode

"Dissolution,migration,and deposition"of transition metal ions (TMIs) result in capacity degradation of lithium-ion batteries (LIBs).Understanding such detrimental mechanism of TMIs is critical to the development of LIBs with long cycle life.In most previous works,TMIs were directly introduced into the electrolyte to investigate such a detrimental mechanism.In these cases,the TMIs are deposited directly on the fresh anode surface.However,in the practical battery system,the TMIs are deposited on the anode covered with solid electrolyte interphase (SEI) film.Whether the pre-presence of SEI film on anode surface influences the deposition and detriment of TMIs is unclear.In this work,the deposition of Co element on graphite anode with and without SEI film were systematically studied.The results clearly show that,in comparison with that of fresh graphite (SEI-free),the presence of SEI film aggravates the deposition of Co ions due to the Li^(+)–Co^(2+) ion exchange between the SEI and Co^(2+)-containing electrolyte without the driving of the electric field,leading to faster capacity fading of graphite anode.Therefore,how to regulate electrolytes and film-forming additives to design the components of SEI and prevent its exchange with TMIs,is a crucial way to inhibit the deposition and detriment of TMIs on graphite anode.