New Perspective on the Development of Stable, High-Power-Density 5 V-Class All-Solid-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 LiCoPO₄ (LCP) or LiCoO₂ (redox potential ∼4.8 V and ∼3.8 V vs Li⁺/Li, respectively) thin-film cathode materials, Li_1+xAl_xTi_2-x(PO₄)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 Ti⁴⁺ ions attributed to electronic charge transfer from LCP to LATP without the involvement of the PO₄ polyanion in the process. Electrochemical activity of the ASSBs is primarily limited by the anode/electrolyte interface rather than the cathode/electrolyte interface. Postcycling XPS analysis of the Li|LATP interface indicates lateral and in-depth chemical inhomogeneity with a strong change in PO₄ 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 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.