Electrochemical effect of lithium tungsten oxide modification on LiCoO₂ thin film electrode

We fabricated a lithium tungsten oxide (LWO)-modified LiCoO₂ (LCO) thin film electrode by pulsed laser deposition and investigated the reason for its lower resistance as compared with a bare LCO electrode. X-ray diffraction revealed that the LWO layer has a randomly oriented Li₂WO₄ structure with tetragonal symmetry. Scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDX) indicated that the LWO modification changes the LCO particle surface, and the electrochemical impedance spectroscopy demonstrated that the LWO modification on LCO decreases the lithium ion transfer resistance at the interface between the positive electrode and the liquid electrolyte and increases the frequency factor at the interface. X-ray photoemission spectroscopy, EDX, and electron energy loss spectroscopy (EELS) indicated the presence of phosphate on the surface of the unmodified LCO electrode after electrochemical tests, but EDX and EELS did not indicate the presence of phosphate in the LWO-modified LCO electrode. The absence of phosphates apparently alleviates the hindrance of Li⁺ ion diffusion and increases the frequency factor in LCO, resulting in lowered Li⁺ ion transfer resistance at the interface.