In the presence of literatures [1-4], high-voltage (> 4.8V; 5V class) Li-rich layer compounds (Li[Ni_xLi_(1-2x)/3Mn_(2-x)/3]O₂) have been reported to the next candidate of cathode materials in order to increase the power and energy density instead of LiCoO₂ or LiNi_xMn_yCo_zO₂ conventional materials. These high-voltage Li-rich layer materials dedicate their high reversible capacities more than 270 mAh g^-1, which is almost twice as higher as LiCoO₂. However, this material suffers dramatically challenge due to the irreversible phase change in the first charge. Several studies had discussed the failure mechanism of this high voltage material; however, those comments are different. Mantia et al. [1] have demonstrated direct evidence of oxygen evolution from the Li-excess material at high potentials by in situ differential electrochemical mass spectrometry (DEMS) as well as Hy et al. [2] shown an oxygen activation occurs following electrochemical reaction and forms an irreversible formation of Li₂O by surface enhanced Raman spectroscopy (SERS). In Jiang’s result [3], they are unable to detect any oxygen in their ex situ observation. Their XRD pattern and ⁶Li MAS NMR spectrum comment O^2- loss is accompanied with the electrolyte to form CO₂.

In our novel in-situ gas evolution experiment, we showed that there are two reaction mechanisms in the first charge process in Figure 1. In the first reaction mechanism, the CO₂ (m/z 44) and CO (m/z 28) generate drastically at the potential of 4.25V. In comparison with cyclic voltammogram (CV) (not shown in here), the potential of 4.25V is indicated to the reaction of Ni³⁺/Ni⁴⁺; therefore, the gas evolution at this stage can be concluded the electrolyte is partially decomposed with the catalyst of Ni³⁺/Ni⁴⁺ and the oxygen evolution is not correlated. Second reaction mechanism starts at 4.65V, the CO₂forms solely in accompanied with oxygen evolution (m/z 32) from cathode material. Above result indicates that the second reaction mechanism is lead by the oxygen evolution and further reacts with electrolyte.

In this research, we establish a novel in-situ observation for evaluating gas evolution and define the reaction mechanisms of high voltage cathode material.

Reference

[1] F. La Mantia, F. Rosciano, N. Tran, P. Novak, J. Appl. Electrochem. 38 (2008) 893

[2] Sunny Hy, Felix, John Rick, Wei-Nien Su, Bing Joe Hwang, J. Am. Chem. Soc. 136 (2014) 999

[3] Meng Jiang, Baris Key, Ying S. Meng, and Clare P. Grey,
Chem. Mater. 21 (2009) 2733