Coating LixNbOy on the Surface of Cathode Materials for All Solid State Batteries Using ALD (Atomic Layer Deposition)
All solid state batteries with Li+ as carrier ion are one of candidate of next generation batteries with high energy. Low resistance and high cycle performance are demanded to all solid state batteries for cars, and the interface between cathode material and sulfide solid electrolyte Li2S-P2S5 strongly affects these battery performances. It was reported that battery performance could be improved by coating LiNbO3 as protective layer on the surface of LiCoO2 to suppress interfacial reaction between LiCoO2 and sulfide solid electrolyte. ALD (Atomic Layer Deposition) is used as a method to make very thin coating layer of Al2O3, ZnO, etc., on the surface of electrode materials for Li-ion batteries.[2-3] In this study, we tried to improve battery performance of all solid state batteries by thin and uniform LixNbOy coating on the surface of LiCoO2 using ALD.
LixNbOy was deposited using NbOEt5 (Niobium(V) ethoxide), LiOtBu (Lithium tert-butoxide) and H2O as precursors of ALD at 235°C, and formed NbOEt5–H2O and LiOtBu–H2O successively.  LiOtBu–H2O was deposited at 1 time after NbOEt5–H2O was deposited at 4 times. This ALD-cycle was repeated at 15 and 50 cycles for different thickness of LixNbOy. LiCoO2 powder was coated with LixNbOy through above step using ALD equipment for powder. LiCoO2 coated with LixNbOy formed by 15 and 50 ALD-cycles was calcined. LixNbOy coating layer was characterized by X-ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM). Composition of LixNbOy film was characterized by Rutherford Backscattering Spectrometry - Nuclear Reaction Analysis (RBS-NRA). The electrochemical property was measured with all solid state batteries composed of LiCoO2 coated by LixNbOywith different thickness as cathode, graphite as anode and sulfide solid electrolyte.
3. Results and discussion
Fig.1 shows XPS Nb3d and Co2p spectra of LiCoO2 without and with LixNbOy formed by 50 ALD-cycles. It was confirmed that LiCoO2 coated with LixNbOy has high intensity of Nb3d peaks and very low intensity of Co2P peaks. These results indicate that almost all surface of LiCoO2 was uniformly covered with LixNbOy coating layer. Fig.2 shows charge and discharge curves of the batteries with LiCoO2 without and with LixNbOy formed by 15 ALD-cycles and 50 ALD-cycles of deposition. Reversible capacity was greatly improved with LixNbOy coating. Fig. 3 shows the results of resistance measurements of the batteries using LiCoO2 coated by LixNbOy with different thickness. The resistance was drastically reduced with LixNbOy coating. LixNbOy coating layer could suppress the side reaction between LiCoO2 and sulfide solid electrolyte. Analysis of LixNbOy coating layer by TEM, composition of LixNbOy by RBS-NRA and electrochemical properties of the batteries with LiCoO2 coated with LixNbOy formed through different deposition cycles using ALD will be presented in the conference.
This study was partly supported by the “Applied and Practical LiB Development for Automobile and Multiple Application” project of the New Energy and Industrial Technology Development Organization (NEDO), Japan.
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