Insights for the New Function of N,N-Dimethylpyrrolidone in Preparation of a High-Voltage Spinel LiNi0.5Mn1.5O4 Cathode

The solid-state method is extensively applied to the synthesis of electrode materials for its simplicity and low cost. However, particles obtained using the traditional solid-state method exhibited a large, uneven particle size and a severe aggregation phenomenon, leading to an unsatisfactory electrochemical performance. Here, spinel LiNi0.5Mn1.5O4 (LNMO) with good dispersion was synthesized using the solid-state method with the addition of N,N-dimethylpyrrolidone (NMP). During the LNMO preparation process, NMP is effective in refining and optimizing the particle size and suppressing the aggregation phenomenon. Meanwhile, the N element migration phenomenon was also observed in the bulk of LNMO, and it was beneficial for extending solid-solute reactions as demonstrated by in situ X-ray diffraction. LNMO prepared with NMP (LNMO-N-x) exhibited a higher discharge voltage and capacity (115.3 mA h g–1 at 2 C) compared with LNMO (105.8 mA h g–1). These results reveal the function of NMP in the preparation of LNMO and the effect of the physical characteristic changes on structure and phase transition in a working battery, and it can be easily incorporated into other electrode materials; if well engineered, it will contribute a lot to the further applications of lithium ion batteries.