Lithium Intercalation and Structural Changes at the LiCoO2 Surface Under High Voltage Battery Operation

Abstract

LiCoO2 electrode exhibits severe capacity fading when charged to 4.4 V, although it delivers a higher capacity of over 150 mAh g-1 at the initial cycles [1]. Surface coating with metal oxides and phosphates has been one of the effective methods to improve cycle performance [1,2]. The coating layer with very small thickness of ca. 0.5−1 nm improves the stability of LiCoO2 more efficiently than the thick coating layer [2], although a small region of coverage by coating species on LiCoO2. These experimental evidences bring up a question about the role of the surface coating, because the surface coating is widely considered to stabilize the electrode by suppressing interfacial reactions between electrodes and the electrolyte species. In this study, the origin of reversible intercalation into the surface-coated LiCoO2 is investigated using uncoated and Li3PO4-coated epitaxial LiCoO2 films which have intercalation-active (104) plane. The role of the surface coating is discussed based on the surface structural changes characterized using in situsurface X-ray diffraction (XRD). An epitaxial-film model electrode of LiCoO2(104) was fabricated on SrRuO3(100)/Nb:SrTiO3(100) using pulsed laser deposition. The LiCoO2 surface was modified by the deposition of amorphous Li3PO4 using radio-frequency (RF) magnetic sputtering with a target of g-Li3PO4. Charge-discharge measurements were performed with the 2032-type coin cell composed of a Li anode and 1 mol dm-3 LiPF6 in a 3:7 mixture of ethylene carbonate and diethyl carbonate as the electrolyte. To clarify the effects of the Li3PO4 coating on the crystal structure at the LiCoO2 surface, in situsurface XRD measurements were performed using grazing angle synchrotron X-rays at the bending-magnet BL14B1 beamline at SPring-8. The 50 nm thick LiCoO2(104) film exhibited lithium (de-)intercalation activity with a first discharge capacity of 119 mAh g-1 between 3.0 and 4.4 V, followed by a gradual capacity fading with subsequent charge-discharge cycles. In contrast, a 3.2 nm thick Li3PO4-coated film exhibited a higher intercalation capacity of 148 mAh g-1 with superior cycle retention than the uncoated film. In situ surface X-ray diffraction measurements revealed a small lattice change at the coated surface during the (de-)intercalation processes compared to the uncoated surface. The surface modification of LiCoO2 by the Li3PO4 coating could lead to improvement of the structural stability at the surface region during lithium (de-)intercalation at high voltage. The surface structure changes during the electrochemical reactions is one of the factors for the reversible lithium (de-)intercalation reaction [3].