Coating LixNbOy on the Surface of Cathode Materials for All Solid State Batteries Using ALD (Atomic Layer Deposition)

1. Introduction

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.^[1] 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 Li_xNbO_y coating on the surface of LiCoO₂ using ALD.

2. Experimental

Li_xNbO_y 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. ^[4] 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 Li_xNbO_y. LiCoO₂ powder was coated with Li_xNbO_y through above step using ALD equipment for powder. LiCoO₂ coated with Li_xNbO_y formed by 15 and 50 ALD-cycles was calcined. Li_xNbO_y coating layer was characterized by X-ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM). Composition of Li_xNbO_y 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 Li_xNbO_ywith 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 Li_xNbO_y formed by 50 ALD-cycles. It was confirmed that LiCoO₂ coated with Li_xNbO_y 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 Li_xNbO_y coating layer. Fig.2 shows charge and discharge curves of the batteries with LiCoO₂ without and with Li_xNbO_y formed by 15 ALD-cycles and 50 ALD-cycles of deposition. Reversible capacity was greatly improved with Li_xNbO_y coating. Fig. 3 shows the results of resistance measurements of the batteries using LiCoO₂ coated by Li_xNbO_y with different thickness. The resistance was drastically reduced with Li_xNbO_y coating. Li_xNbO_y coating layer could suppress the side reaction between LiCoO₂ and sulfide solid electrolyte. Analysis of Li_xNbO_y coating layer by TEM, composition of Li_xNbO_y by RBS-NRA and electrochemical properties of the batteries with LiCoO₂ coated with Li_xNbO_y formed through different deposition cycles using ALD will be presented in the conference.

Acknowledgements

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.

References

[1] N. Ohta, K. Takada, Electrochem. Commun., 9 (2007) 1486

[2] J. Woo, S. Lee, J. Electrochem. Soc., 159 (2012) A1120

[3] Y. Jung, S. Lee, J. Electrochem. Soc., 157 (2010) A75

[4] E. Østreng, H. Fjellvag, J. Mater. Chem., C, 1, (2013) 4283