Multi-Functional Surface Engineering for Li-Excess Layered Cathode Material Targeting Excellent Electrochemical and Thermal Safety Properties

The Li­(Li_0.18Ni_0.15Co_0.15Mn_0.52)­O₂ cathode material is modified by a Li₄M₅O₁₂-like heterostructure and a BiOF surface layer. The interfacial heterostructure triggers the layered-to-Li₄M₅O₁₂ transformation of the material which is different from the layered-to-LiMn₂O₄ transformation of the pristine Li­(Li_0.18Ni_0.15Co_0.15Mn_0.52)­O₂. This Li₄M₅O₁₂-like transformation helps the material to keep high working voltage, long cycle life and excellent rate capability. Mass spectrometry, in situ X-ray diffraction and transmission electron microscope show that the Li₄M₅O₁₂-like phase prohibits oxygen release from the material bulk at elevated temperatures. In addition, the BiOF coating layer protects the material from harmful side reactions with the electrolyte. These advantages significantly improve the electrochemical performance of Li­(Li_0.18Ni_0.15Co_0.15Mn_0.52)­O₂. The material shows a discharge capacity of 292 mAh g^–1 at 0.2 C with capacity retention of 92% after 100 cycles. Moreover, a high discharge capacity of 78 mAh g^–1 could be obtained at 25 C. The exothermic temperature of the fully charged electrode is elevated from 203 to 261 °C with 50% reduction of the total thermal release, highlighting excellent thermal safety of the material.