Li+ concentration and morphological changes at the anode and cathode interphases inside solid-state lithium metal batteries

Abstract Irregular Li heterostructure growth at the interphase between solid electrolyte and anode reduces solid-state Li metal battery (SSLMB) performance, but the fundamental cause is still elusive. Measuring and imaging Li + ion diffusion in operando inside an SSLMB using a commercially standard cell configuration are extremely challenging because the ultra-light Li element exhibits a minute signal-to-noise ratio using most X-ray-related characterisation methods, and the weak X-ray signals of Li + are buried by strong signals of other heavy transition metal elements in the cathode and battery enclosure. Here, we pioneer novel operando correlative imaging of coupling X-ray Compton scattering with computed tomography (XCS-CT) that is able to quantify the interplay between spatially resolved Li + ion diffusion kinetics and Li 0 metal structures growth at the interphases of both anode and cathode sides inside a full cell SSLMB using solid polymer electrolyte (SPE) and commercially standard cell configuration during (dis)charging. We show a 61% increase in the efficiency of extracting Li + ions from the cathode LiNi 0.6 Mn 0.2 Co 0.2 O 2 to the anode during charging at 0.1 C than at 1 C due to restricted Li + ion diffusion at the higher rate inside SSLMB. However, this led to the formation of more irregular interfacial morphology, not only Li 0 dendrites, but also sub-surface pore formation at the anode/SPE interphase. We find the irregular Li 0 structure initiation and growth are accelerated during the first Li stripping step, not the Li plating step, and the root cause is the onset imbalance of Li + ion diffusion and redox reactions between the anode and cathode. These insights highlight the benefits of asymmetric charging and discharging rates as a promising solution to improving SSLMB performance with SPEs. The operando correlative XCS-CT imaging technique has the potential for studying the relationship between active ion concentrations and buried morphological changes for a variety of battery chemistries.