Real-Time Detection and Speciation of Mn Dissolution in a Li-Ion Battery Cathode

Abstract

Rechargeable lithium-ion batteries (LIB) are widely used in portable electronic devices, electric vehicles, medical devices, etc. thanks to their improved performance in recent decades. The properties of the cathode still largely limit the overall performance, as the cathode usually has lower theoretical capacity and suffers from more serious degradation effects.1 Among these, dissolution of transition metals is an important but poorly understood side reaction. On the one hand, it dominates the cathode capacity fading by causing irreversible loss of active redox sites, phase transformation, cation mixing, grain fracture, etc. On the other hand, the dissolved transition metal ions can exacerbate the formation of the solid electrolyte interphase (SEI) on the anode through a dissolution-migration-deposition (DMD) process.2 Here, we focus on the dissolution of manganese from LiMn2O4 (LMO), a benchmark cathode material, through real-time analysis of the electrolyte by combining online inductively coupled plasma–optical emission spectroscopy (online ICP-OES) and rotating ring-disc electrode (RRDE) collection, Figure 1. Both methods offer real-time dissolution information as functions of time and potential/state of charge, while the former shows superior detection limits and the latter provides oxidation state information of the dissolving species.According to online ICP-OES, we find that Mn dissolves readily from fully lithiated LMO when brought in contact with aqueous 0.2 M Li2SO4, but that a dissolution-passivation process stops further detectable dissolution until well past fully delithiated λ-MnO2. The dissolution becomes detectable again on the returning scan to the potential of pristine LMO, and grows exponentially in the overlithiated region.3 Our RRDE results indicate that Mn2+ is the dominating dissolved Mn species only in overlithiation region, whereas it only accounts for a minor proportion in the dissolved species from delithiation region. The data from both techniques suggest that the dissolution of Mn is related to the surface state of the electrode, rather than its degree of lithiation, as widely believed in the past.4 Our results provide a better understanding of the mechanism of transition metal dissolution in cathode materials, as well as an entry point for screening next-generation cathodes and metal ion batteries with superior degradation characteristics.5