High-power durability of LiCoO₂ thin film electrode modified with amorphous lithium tungsten oxide

To investigate electrochemical performances of an amorphous lithium tungsten oxide (LWO) layer, an amorphous LWO-modified LiCoO₂ (LCO) thin film electrode is fabricated by pulsed laser deposition and is exposed under a humid environment. The amorphous LWO-modified LCO exhibits high capacity retention of 80% at a rapid charge-discharge rate of 20 C. Conversely, the bare LCO exhibits capacity retention of 0% at the rates of 20 C. Electrochemical impedance spectroscopy demonstrates that the LWO-modified LCO maintains a low interfacial resistance after the cycling test compared with the bare LCO. X-ray photoemission spectroscopy (XPS), scanning transmission microscopy (STEM), and electron energy loss spectroscopy (EELS) indicate the presence of Li₂CO₃ on the surface of the bare LCO electrode and a thick degraded surface layer of CoO structure on the surface of LCO primary particle after electrochemical tests. XPS, STEM, and EELS indicate the presence of low amounts of Li₂CO₃ on the surface of the LWO-modified LCO, the LCO layer remains in a normal state, and LWO layer maintains the amorphous LWO state after the tests. Thus, the amorphous LWO protective layer contributes to suppressing the degradation of LCO and maintaining an amorphous LWO state with a lithium ion conductor, resulting in high-power durability.