Robust interfaces enabled by chemically stable Li₃PO₄ buffer layer toward high-performance thin-film all-solid-state...

Lithium phosphate (Li₃PO₄) is a promising buffer layer for stabilizing electrolyte-electrode interface in thin-film all-solid-state supercapacitors (ASSSCs). However, the chemical stability of Li₃PO₄ is limited by the dissociation of weak P-O-P bonds under operational conditions, leading to interfacial degradation. Here, we systematically investigate the role of oxygen partial pressure (P_O) during sputtering deposition in tailoring the chemical bonding configuration of Li₃PO₄ films. By precisely controlling P_O, we effectively convert unstable P-O-P bonds into robust PO and Li-O-P configurations, significantly enhancing both the stability and ionic conductivity of the Li₃PO₄ layer. Consequently, the optimized Li₃PO₄-coated ASSSC displays much better electrochemical performance than the no-coated one in terms of its higher specific capacitance (∼22.5 vs. 9.0 mF cm^-2 at 0.5 μA cm^-2), superior rate capability (∼11.2 vs. 0.9 mF cm^-2 at 30.0 μA cm^-2) and better cycling stability (∼92.9% vs. 77.8% of the retained capacity after 20,000 cycles). Furthermore, the device exhibits superior electrochromic functionality, showcasing its dual potential for energy storage and smart window applications.