LiNi0.6Co0.2Mn0.2O2 Cathodes Coated with Dual-Conductive Polymers for High-Rate and Long-Life Solid-State Lithium Batteries

Effect of TiO2 Coating on Structure and Electrochemical Performance of LiNi0.6Co0.2Mn0.2O2 Cathode Material for Lithium-Ion Batteries

High-nickel ternary LiNi0.6Co0.2Mn0.2O2 (NCM622) is a promising cathode material for lithium-ion batteries due to its high discharge-specific capacity and energy density. However, problems of NCM622 materials, such as unstable surface structure, lithium-nickel co-segregation, and intergranular cracking, led to a decrease in the cycling performance of the material and an inability to fully utilize high specific capacity. Surface coating was the primary approach to address these problems. The effect of TiO2 coating prepared by the sol-gel method on the performance of LiNi0.6Co0.2Mn0.2O2 was studied, mainly including the morphology, cell structure, and electrochemical properties. LiNi0.6Co0.2Mn0.2O2 was coated by TiO2 with a thickness of about 5 nm. Compared with the pristine NCM622 electrode, the electrochemical performance of the TiO2-coated NCM622 electrodes is improved. Among all TiO2-coated NCM622, the NCM622 cathode with TiO2 coating content of 0.5% demonstrates the highest capacity retention of 89.3% and a discharge capacity of 163.9 mAh g-1, in contrast to 80.9% and145 mAh g-1 for the pristine NCM622 electrode, after 100 cycles at 0.3 C between 3 and 4.3 V. The cycle life of the 5 wt% TiO2-coated NCM622 electrode is significantly improved at a high cutoff voltage of 4.6 V. The significantly enhanced cycling performance of TiO2-coated NCM622 materials could be attributed to the TiO2 coating layer that could block the contact between the material surface and the electrolyte, reducing the interface side reaction and inhibiting the transition metal dissolution. At the same time, the coating layer maintained the stability of layered structures, thus reducing the polarization phenomenon of the electrode and alleviating the irreversible capacity loss in the cycle process.

Additive-Derived Surface Modification of Cathodes in All-Solid-State Batteries: The Effect of Lithium Difluorophosphate- and Lithium Difluoro(oxalato)borate-Derived Coating Layers

Sulfide-based electrolytes, with their high conductivity and formability, enable the construction of high-performance, all-solid-state batteries (ASSBs). However, the instability of the cathode-sulfide electrolyte interface limits the commercialization of these ASSBs. Surface modification of cathodes using the coating technique has been explored as an efficient approach to stabilize these interfaces. In this study, the additives lithium difluorophosphate (LiDFP) and lithium difluoro(oxalato)borate (LiDFOB) are used to fabricate stable cathode coatings via heat treatment. The low melting points of LiDFP and LiDFOB enable the formation of thin and uniform coating layers by a low-temperature heat treatment. All-solid-state cells containing LiDFP- and LiDFOB-coated cathodes show electrochemical performances significantly better than those comprising uncoated cathodes. Among all of the as-prepared coated cathodes, LiDFP-coated cathodes fabricated using a slightly lower temperature than the phase-transition temperature of LiDFP (320 °C) show the best discharge capacity, rate capability, and cyclic performance. Furthermore, cells comprising LiDFP-coated cathodes showed significantly low impedance. X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy confirm the effectiveness of the LiDFP coating. LiDFP-coated cathodes minimized side-reactions during cycling, resulting in a significantly low cathode-surface degradation. Hence, this study highlights the efficiency of the proposed coating method and its potential to facilitate the commercialization of ASSBs. Overall, this study reports an effective technique to stabilize the cathode-electrolyte interface in sulfide-based ASSBs, which could expedite the practical implementation of these advanced energy-storage devices.