Interface Engineering of Thin‐Film Lithium Phosphorus Oxynitride Electrolyte by Appropriate Oxygen Plasma Treatment for Flexible All‐Solid‐State Supercapacitor

The interfacial incompatibility between lithium phosphorus oxynitride (LiPON) and anode greatly deteriorates the performance of thin-film all-solid-state supercapacitors (ASSSCs). This article investigates oxygen plasma treatment to improve the interface. Through appropriate plasma treatment, a Li₂O/Li₃PO₄ composite layer is formed by replacing nitrogen with oxygen at the LiPON surface owing to strong reactivity of oxygen plasma. This composite layer inherits the merits of both Li₂O (including good mechanical strength and ultralow electrical conductivity) and Li₃PO₄ (including good chemical stability and relatively high ionic conductivity), and thus is quite desirable for service as a LiPON/anode buffer layer with excellent chemical and mechanical stability, high ionic conductivity and low electrical conductivity. Consequently, the corresponding plasma-treated ASSSC displays much better electrochemical performance than the no-treated one in terms of its higher specific capacitance (≈15.4 mF cm^-2 at 0.5 µA cm^-2), better cycling stability (≈95.1% of the retained capacity after 15 000 cycles) and lower self-discharge rate (66.4% of the retained voltage after 20 h). The plasma-treated one also shows both excellent flexible and electrochromic characteristics and demonstrates the ability of self-adaptive temperature adjustment for smart window applications.