Synergistic Effects of Stabilizing the Surface Structure and Lowering the Interface Resistance in Improving the Low-Temperature Performances of Layered Lithium-Rich Materials.

The layered lithium-rich cathode material, Li1.2Ni0.2Mn0.6O2, was successfully synthesized by a sol-gel method followed by coating with different amounts of Li2O-2B2O (LBO, 1, 3, and 5 wt %). The effects of LBO-coating layer on the structure, morphology, and low-temperature (-30 °C) electrochemical properties of these materials are investigated systematically. The morphology, crystal structure, and grain size of the Li-rich layered oxide are not essentially changed after surface modification; according to the TEM results, the Li-B-O coating layer exists as an amorphous layer with a thickness of 5-8 nm when the amount is 3 wt %. Electrochemistry tests reveal that 3 wt % LBO-coated samples present the best electrochemical capability at low temperature. At -20 °C, the 3 wt % LBO-coated sample could retain 45.7% of the initial discharge capacity (131.7/288.0 mAh g-1) of that at 30 °C, while the pristine material could only retain 22.5% (57.5/256.0 mAh g-1). XPS spectra and EIS results reveal that such an enhancement of low-temperature discharge capacity should be attributed to the proper LBO-coating layer, which not only endows the modified materials with more stable surface structure but also lowers the interface resistance of Li+ diffusion through the interface and charge transfer reaction.