Fabrication of Ruthenium-Based Cathode Material/Solid Electrolyte Composites

Introduction Oxide-based all-solid-state batteries (ASSBs) are considered safe due to their chemical stability and are attracting attention as a power source for electric vehicles (EVs). Because the driving range of EVs is not sufficient compared to conventional gasoline-powered vehicles, ASSBs with higher capacities are required. To overcome the above issue, it is necessary to adopt the electrode material having high energy density such as Li-rich layered oxides, or to increase the amount of active material in the cell. The positive electrode of an ASSB needs to be a composite of electrode material and solid electrolyte to ensure ionic conductive pathways. Therefore, it is necessary to increase the ratio of active material in the composite cathode to improve energy density in the battery. Previously, the ASSB with positive electrode composed of only active material without any conductive additive was reported for Ru containing oxide, Li2Ru0.8S0.2O3.2 [1], having high electronic and ionic conductivity. Therefore, we focused on the Ru containing Li-rich layered oxide, Li2RuO3 and Li2Mn0.4Ru0.6O3. The garnet-type solid electrolyte was selected as an oxide solid electrolyte due to its high ionic conductivity and the reductive tolerance to Li metal having a large capacity of 3860 mAh g–1. The high stiffness of the oxide solid electrolytes leads to a decrease in the contact area and prevents the formation of a superior electrode/electrolyte interface only by mixing and applying pressure, which hinders the operation of ASSBs. To lower the resistance of the interface, the co-firing of cathode material and solid electrolyte is required. However, side reactions associated with co-firing are an issue. Li2TiO3 as a buffer layer was introduced between the cathode material and the solid electrolyte to suppress side reactions. Li2TiO3 has a similar structure to Li2RuO3, which is expected to improve sinterability by a partial substitution. The co-firing of the cathode materials and the garnet-type solid electrolyte and the effect in the suppression of side reactions using buffer layers will be presented. Experimental Li2RuO3 and LMRO were used as cathode materials, Li6.25Ga0.25La3Zr2O12 (LLZ-Ga) as a solid electrolyte, and Li2TiO3 as a buffer layer. The buffer layer was introduced into the cathode surface by solid-phase and liquid-phase methods. LiOH-H2O and TiO2 were mixed and sintered with the cathode material in the solid-phase method. LiOH-H2O and tetra n-butyl titanate monomer were weighed and dissolved in ethanol separately in the liquid-phase method. Each solution and the cathode material were mixed and stirred overnight, then dried at 60 ºC in a rotary evaporator. The resultant powder was pelletized by uniaxial press and then heated at 800 ºC for 5 hours in an oxygen atmosphere. The electrochemical property of the cathode material with the buffer layer was confirmed by a constant-current charge-discharge test using a coin cell. The cathode composite composed of the cathode material (with buffer layer) and the solid electrolyte was fabricated by co-firing at 600-800ºC after the mixing by hand or ball mill. The obtained samples were subjected to phase identification by X-ray diffraction. The reaction process was observed by SEM observation and EDX composition analysis. All-solid-state cells were fabricated by a uniaxial press using the obtained sample as the positive electrode, Li5.5PS4.5Cl1.5 as the solid electrolyte, which has excellent ion conductivity and can easily form a good interface simply by applying pressure, and Li-In alloy as the negative electrode. Constant current charge-discharge tests were performed on the fabricated all-solid-state cells. Results and discussion X-ray diffraction and EDX analysis confirmed that the impurity phase of La2Li(RuO6) formed by co-firing the cathode material and LLZ-Ga. Therefore, the buffer layer was introduced by the solid-phase and liquid-phase methods. In the solid-phase method, Li2TiO3 was not distributed uniformly on the particle surface of the cathode material. In the liquid-phase method, Li2TiO3 covered the cathode surface more uniformly and thinner than that formed in the solid-phase method. Both Li2RuO3 and LMRO retained their original structure, while the lattice parameter of LMRO changed, indicating that LMRO formed the solid solution with Li2TiO3. No inter-diffusion of lanthanum and ruthenium between the cathode material with buffer layer and LLZ-Ga was observed in the EDX analysis. It suggests that the introduction of a buffer layer suppressed the inter-diffusion. The cathode composite co-fired did not exhibit the discharge capacity in the charge-discharge test using the all-solid-state cell. It is attributed to the low conductivity derived from the low sinter ability of the cathode composite. [1] K. Nagao et al., Sci. Adv. 2020; 6 : eaax7236 (2020). Acknowledgements This work was supported by JSPS KAKENHI Grant Number JP22H04615.