A new perspective on the development of stable, high-power-density 5 V-class allsolid-state lithium-ion batteries

The correlation between the electrochemical performance and the electronic structure and chemical composition of the cathode/electrolyte-and anode/electrolyte-interfaces in all-solid-state batteries (ASSB) is systematically studied. ASSBs are composed of LiCoPO 4 (LCP) or LiCoO 2 (redox potential ~4.8 V and ~3.8 V vs Li + /Li, respectively) thin-film cathode materials, Li 1+x Al x Ti 2-x (PO 4 ) 3 (LATP) as the solid-state electrolyte and lithium metal as the anode. X-ray photoelectron spectroscopy (XPS) interface experiments on LCP thin-film deposition on the LATP surface reveal a partial reduction of Ti 4+ ions attributed to electronic charge transfer from LCP to LATP without the involvement of the PO 4 polyanion in the process. Electrochemical activity of the ASSBs is primarily limited by the anode/electrolyte interface rather than the cathode/electrolyte interface. Post-cycling XPS analysis of the Li∣LATP interface indicates lateral and in-depth chemical inhomogeneity with a strong change in PO 4 polyanionic chemical environment and Li + accumulation at the areas with a better electrolyte/anode contact. Engineering the Li∣LATP interface via coating of the LATP surface with lithium oxynitride (LiPON) or a LiTFSI-PEO polymer electrolyte, or by wetting the LATP with the liquid electrolytes, significantly improves battery cycling stability even without artificial interface modification at the cathode side. The optimized Li∣LiTFSI-PEO∣LATP∣LCP cells showed excellent cycling performance between 3.0 V and 5.0 V at a charging time of 12 min (≈ 5C rate). Stable cycling was sustained over 10 cycles, followed by 30 and 50 additional cycles at reduced charging rates (≈ 4C and 1C, respectively). Good electrochemical cyclability was further demonstrated with an upper cutoff voltage of 5.3 V vs. Li + /Li.