Abstract
Higher-capacity lithium-ion batteries are required for applications of electronic vehicles. Lithium cobalt oxides (LiCoO2, LCO) with a layered rock salt-type structure is a popular cathode material. LCOs are typically operated below 4.2 V (vs. Li/Li+) when combined with organic electrolyte. In this voltage range, there is no significant decrease in capacity after charge-discharge cycles. Operating at higher voltages can further increase the battery capacity because the amount of deintercalated Li-ion increases. However, high voltage operation leads to the capacity fade after cycling. This is suggested to be due to irreversible changes in the LCO structure.
The capacity fades due to high voltage cycling can be reduced by coating the cathode surface with metal oxides [1]. Since most metal oxides have low electron and ion conductivity, excessive coating seems to lead to suppress the Li (de)intercalation. The understanding of the coating effect has been desired toward optimizing the coating. Typical cathode is a mixture of polycrystalline LCO, conductive additives, and binders. This results in various interfaces, such as LCO−LCO with different orientations, LCO−additives, and LCO−binders. The complicated interfaces make difficult it to interpret the coating effect of metal oxides to LCO.
In this study, we investigated the effect of metal oxides coating on the structural change of LCO after high-voltage operation. Epitaxial growth was used to construct a model battery with almost a single crystalline LCO. Zirconium (Zr) oxides were chosen as a typical coating material. Scanning transmission electron microscopy (STEM) was used to examine the local structural changes in the LCO films with and without Zr oxides coating by high voltage cycling.
LCO cathode was epitaxially grown on a substrate by pulsed laser deposition. The cathode surface was coated with Zr oxide by further deposition of Li2ZrO3 (LZO/LCO). STEM observation of the LZO/LCO showed island growth of Zr oxides on the cathode surface (Fig.1). Two type of batteries were constructed with the bare-LCO or LZO/LCO cathode, organic electrolyte, and lithium metal anode. The batteries were operated for 25 charge-discharge cycles at 4.5 V. Although the discharge capacity of the bare LCO decreases continuously, that of LZO/LCO was maintained almost constantly.
After a further 100 cycles, the cathodes were processed for STEM observations. Bare-LCO showed the irreversible structural change into spinel-type Co3O4 at the surface. In contrast, irreversible structural changes were partially suppressed in the vicinity of the Zr oxides islands on LZO/LCO. The interface of LZO and LCO showed a disarranged structure. This suggests the interphase of LZO-LCO, which may realize both the ionic conductivity and protection of LCO.
