Operando Observation of Lithiation and Delithiation Reactions of a LiCoO₂-Li₃BO₃ Composite Electrode Formed on a Li_6.6La₃Zr_1.6Ta_0.4O_{12 Solid Electrolyte Sheet by Laboratory-based Hard X-ray Photoelectron Spectroscopy}

A positive electrode composed of LiCoO2 (LCO) and Li3BO3 (LBO) was formed on one side of a Li6.6La3Zr1.6Ta0.4O12 (LLZT) solid electrolyte sheet by applying LCO fine powder to LLZT sheet precoated with a Nb thin layer, placing a droplet of aqueous solution of LiOH and H3BO3, and being annealed at similar to 700 degrees C in an oxygen atmosphere. After binding a negative electrode, a Li metal foil, on the other side of LLZT sheet precoated with a Li thin layer, electrochemical reaction at the LiCoO2-Li3BO3 composite positive electrode was observed in an all-solid-state battery configuration, i.e., a LCO-LBO/Nb/LLZT/Li cell, by a newly-developed laboratory-based hard x-ray photoelectron spectroscopy (HAXPES) apparatus equipped with a Cr-K alpha source (5414.9 eV) and bias application system. A sharp main peak and a broad satellite peak characteristic to LCO were observed in the Co 2p(3/2) region at the pristine state. During charging, i.e., delithiation from LCO, the main peak was asymmetrically broadened to a higher binding energy due to the partial oxidation of Co3+ ions at 780 eV to Co4+ ions at similar to 781 eV. In addition, the full width half maximum (FWHM) of the Co4+ peak increased with increasing the amount of lithium insertion, while that of the Co3+ peak remained unchanged. The decrease of satellite peak further confirms the oxidation of Co3+ ions. During the subsequent discharging, i.e., lithiation of LCO, those recovered to the original states, confirming the reversible reduction of Co4+ ions to Co3+ ions. When all the peaks were calibrated with respect to B 1s peak corresponding to LBO as a bulk electrolyte, the Co3+ peaks shifted consistently with the change in cell voltage during charge/discharge cycles, due to the shift of Fermi level of LCO.