Understanding the impact of calcination time of high-voltage spinel Li1+xNi0.5Mn1.5O4 on structure and electrochemical behavior

Abstract

The high-voltage spinel cathode material LiNi0.5Mn1.5O4 (LNMO) with its high energy per mass and volume is widely considered as promising alternative to the cobalt-containing state-of-the-art layered cathode materials. LNMO is usually charged/discharged at high voltage to make use of Ni-redox at the 4.7 V plateau, while after insertion of additional lithium the utilization of excess Li at the low voltage region of LNMO (<3.0 V) may compensate active lithium losses upon operation. Such a Li-excess stoichiometry of the spinel cathode can be achieved for instance through chemical pre-lithiation. Besides the compensation of Li losses, the additional Li also may also lead to an increase of the specific capacity. This study aims for a better understanding of the impact of the calcination time of LNMO particles on the low voltage region and an in-depth discussion of the results in the context of previous literature reports. Therefore, LNMO is synthesized by a ball-mill assisted solid-state synthesis and the calcination time is systematically changed. The subsequent structural and electrochemical analysis of the material shows for the first time that the material properties, including particle size and lattice parameter, as well as the charge/discharge current rate in general have a strong impact on the capacity ratio of the 2.7 V/2.1 V plateaus and that discharging (=lithiating) the cathode material to 1.5 V leads to irreversible changes within the material, which are however beneficial for its specific energy. Furthermore, the cathode material is cycled in the intermediate voltage range (2.4–4.95 V), demonstrating high specific capacities and excellent long-term stability over 100 cycles (≈179.5 ± 0.1 mAh g−1 and 96.4% ± 0.3%, respectively).