Surface Modification Of Lithium-Rich Layered Oxide Cathode Materials

The flourishing development of lithium-ion batteries?LIBs? has accelerated the manufacturing of potable electric devices, electric vehicles?EVs? and hybrid electric vehicles?HEVs?. However, the unsatisfied performance of LIBs, such as poor cycle life, low energy density, et al., limits its extensive application in EVs and HEVs in the future. Lithium-rich layered oxide?LLMO? cathode materials are receiving international attention because they can deliver an exceptionally high rechargeable capacity of 250 m Ah/g between 2.0V-4.8V. However, this high capacity cathode material still faces challenges, such as transition metal?TM? dissolution occurs at high electrode potentials?>4.5V vs. Li/Li⁺?, and the evolution from layered structure to spinel-like upon cyling leads to the poor cycle life. Therefore, we have tried to synthesis LLMO cathode materials by sol-gel method and modify them via surface coating approach in this paper.Sol-gel is a very promising method to synthesize the LLMO cathode materials with more stable structures, better electrochemical properties compared to the solid-state and co-precipitation method. The mechanisms and effects of three typical chelating agents, namely glucose, citric acid and sucrose on the sol-gel synthesis process, electrochemical degradation and structural evolution LLMO cathode materials are systematically compared for the first time. Electrochemical tests further prove that the LLMO material obtained from sucrose maintains 258.4mAh/g with 94.8% capacity retention after 100^th cycle at 0.2C. The superior electrochemical performance can be ascribed to the exceptional complexing mechanism of sucrose,compared to those of the glucose and citric acid. X-ray diffraction?XRD?, X-ray photoelectron spectroscopy?XPS? and high-resolution transmission electron microscopy?HRTEM? analysis indicate m that the sample synthesized from sucrose owns well structure, homogenous distribution, low Ni³⁺ concentration and good surface structural stability during cycling, respectively. This discovery is an important step towards understanding the selection criterion of chelating agents for sol-gel method, that chelating agent with excellent complexing capability is beneficial to the distribution, structural stability and electrochemical properties of advanced lithium-rich layered materials.Surface coating has been considered as the most promising method to protect The flourishing development of lithium-ion batteries?LIBs? has accelerated thecathode materials from the damage by decomposition of electrolyte. Herein,highly-ordered Al₂O₃ coatings from the hydrolysis of aluminium isopropoxide are coated on LLMO cathode material with controlling the growth of Al₂O₃ crystals. The coin cell with bare cathode material delivers a high discharge capacity over 268.2 mAh/g between 2.0V-4.8V, while the Al₂O₃ coated cathode material shows the excellent cycling stability corresponding to 98% capacity retention after 100^th cycle at 1C.More importantly, the highly-ordered Al₂O₃ coated cathode materials exhibit a signi?cantly lower discharge voltage decay compared to the bare cathode materials,which could be ascribed to the suppression of the layered-to-spinel transformation by compact Al₂O₃ layer. The results here will shed light on developing cathode materials with special structures and superior electrochemical properties for high-performance lithium ion batteries.In this paper, a double-shelled architecture consisting of the inner conductive polyacene?PAS? layer and the outer mesoporous Al₂O₃ layer is constructed. Although the cycle life of LLMO cathode material can be enhanced by highly crystalline Al₂O₃ coating, the capacity is decreased by this insulating layer. A PAS layer with high electron conductivity is first coated on the surface of LLMO cathode material.However, this single PAS layer can not effectively suppress the errosion effect from the electrolyte, while the double-shelled architecture has protected the PAS layer and the bulk of LLMO cathode material. The results show that electrochemical capacity is greatly improved, reaching to 280 m Ah/g?2.0V-4.8V at 0.1C? and the transition from layered phase to spinel is delayed, leading to the superior capacity retention of 98% after 100^th cycle.