One-Pot, One-Step Direct Fabrication of Spinel/Layered Li excess cathode materials for High Performance Rechargeable Lithium Ion Batteries

摘要

To a greater extent, development of new-generation lithium ion batteries is limited by the low energy density, low operating voltage and poor rate capability of cathode materials. Recently, the emerging Li-excess layered oxides have attracted a great deal of research efforts due to their high capacities. These cathode materials can be cycled over a broad voltage range between 2.0 and 4.8 V vs. Li/Li~+ and deliver specific capacities higher than 250 mAhg~(-1). Compared to the widely used cathode materials such as LiCoO2 (140 mA h g~(-1)), these cathode materials also offer many other advantages such as low cost, environmental benignity, and safety. Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (marked as LMNCO) is one of the most promising Li excess layered cathode materials, which has an impressive theoretical capacity of 321 mAh/g with operating voltage up to 4.8 V us. Li/Li~+. However, the unavoidable layered-to-spinel phase transformation of LMNCO during in-situ electrochemical cycling causes drastic capacity degradation and short cycle life, which limits its practical applications. Recently, we developed an ex-situ activation approach for complete phase conversion of Li-excess layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 to a Li4Mn5O_(12)-type spinel phase. More impressively, the newly-formed spinel material delivers an initial discharge capacity of 313.6 mAh/g at 0.1C (1C = 250 mA/g), much higher than 211.3 mAh/g from pristine Li-excess layered cathode. However, this process is very time consuming and energy intensive. The requirement for long time, high reaction temperature is against the original idea of energy saving and sustainability. In this work, we report our efforts to directly and rapidly fabricate lithium excess cathode materials with a mixture of layered and spinel structure without requirement of high temperature annealing. Using microwave irradiation instead of traditional convection heating, the approach not only dramatically shortens the fabrication time from hours to minutes, but also tremendously improves the quality. It exhibits excellent performance with a discharge capacity of 444.8mAh/g at 1C, almost double the discharge capacity of the spinel structures transformed from LMNCO. Further, adding a small amount carbon source into the metal salt mixtures before microwave irradiation improve the recycle stability of the materials even though with a slight decrease of the initial discharge capacity of 340.9mAh/g at 1C. The as obtained product does not need further annealing, making the reaction time for the synthesis of the materials very short. This facilitates the use of the as prepared material for the next generation lithium ion batteries.