Synthesis of Lithium-Rich Manganese-Based Layered Cathode Materials and Study on Its Structural Evolution of First Cycle Overcharge

Abstract

Lithium-rich manganese-based cathode materials have become one of promising cathode materials due to their low cost and large discharge specific capacity exceeding 250 mAh·g^-1. However, their problems such as low coulombic efficiency of first cycle and apparent voltage decay influence commercialization process. The high charging voltage will cause instability of structure and increase the hidden danger of the battery. Therefore, structural evolution of first cycle at higher voltage needs to be further studied. In this work, the precursor was synthesized by the co-precipitation method, and the lithium-rich manganese-based layered cathode materials were prepared by lithium-mixed and high-temperature sintering, and the effects of coulombic efficiency and cycle performance were studied at different charge cut-off voltages. Results have shown that high charging voltage would increase the capacity, but reduce the coulombic efficiency greatly in the first cycle, leading to the decayed specific capacity of long cycle. Cyclic voltammetric investigation proves that when the charge cut-off voltage was 5.0 V, part of the bulk lattice oxygen underwent a reversible oxidation reaction, which lead to the increase of capacity. TEM, XRD and SEM characterization results show that the electrode not only went deep into the bulk phase structural changes, including a large number of stacking faults and spinel phases MnO_x and NiO_x, and other irreversible phase changes, but also reacted with the electrolyte. Mapping and XPS results show that when the charging voltage became higher, more bulk lattice oxygen participated during redox reaction, which causes stronger oxidizing peroxygen and superoxide ions to undergo side reactions with the electrolyte and accelerates the structural collapse of the electrode, ultimately, becomes not conducive to long cycle performance of the battery accompanied by the dissolution of the transition metal.

Keywords

lithium-ion batteries / lithium-rich manganese-based cathode material / initial coulombic efficiency / high-voltage / anion redox

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