The Effect of Ni Oxidation State on Li-Rich Layered Oxides
Authors: Knight James C. , Manthiram Arumugam 
Published: February 2020

Li-rich layered oxides (LLO) are being studied intensively as an upgrade over current layered oxide cathodes for Li-ion batteries due to their increased discharge capacity.1 There are several issues, however, that need to be overcome before LLO can be adopted for commercial use. One of the largest issues is the voltage decay and the subsequent energy decrease, which is caused by the slow transformation of the layered phase into a 3 V spinel-like phase, which occurs due to Mn4+ ions being reduced to Mn3+ during cycling.2 It has recently been reported that incorporating Ni3+ ions into LLO and reducing the Mn4+ content can suppress the voltage decay.3 Our study uses a systematic approach to investigate the effect of Ni3+ ions on LLO with the series Li1.2Mn0.6-x
Ni0.2+x
O2 with x = 0.00, 0.02, 0.05, 0.10, 0.15, and 0.20, which produce Ni oxidation states of, respectively, 2.00+, 2.18+, 2.40+, 2.67+, 2.86+, and 3.00+.
All the materials displayed the expected X-ray diffraction patterns, which consist of the R-3m structure with additional superstructure peaks due to Li/Mn ordering. The (003)/(104) peak ratio, which is a measure of cation mixing, generally decreases with increasing Ni oxidation state. This is because Ni3+ ions (0.56 Ǻ) are smaller than Ni2+ ions (0.69 Ǻ), so there is less Li/Ni cation mixing. Increasing the Ni oxidation state has several effects on the first charge-discharge cycle as well. It increases the sloping-voltage region because there are less insulating Mn4+ ions. It decreases the plateau-voltage region because a decrease in Mn4+ ions reduces the amount of Li2MnO3-like phase in the material and the decrease in Li/Ni cation mixing enhances this effect. Despite the decreased plateau-region capacity, the irreversible capacity loss (IRC) increases, which in turn causes the discharge capacity to decrease generally. The increased IRC may be caused by undesirable side reactions similar to those experienced by layered oxides with high nickel content. Figure 1(a) displays the first discharge cycle dQ/dV data, which show a large decrease in the size of the Mn4+ reduction peak at ~ 3.3 V and increase in the Ni reduction peaks at ~ 4.3 V and ~ 3.8 V, suggesting that increasing the Ni oxidation state could avoid the reduction of Mn4+.
Increasing the Ni oxidation state also generally increases the cyclability of these materials. Due to the decrease in the plateau-region capacity and Mn4+ reduction that a higher Ni oxidation state causes, the conversion to the 3 V spinel-like phase is suppressed, resulting in increased cyclability. Because of these effects, the voltage decay is also reduced with increasing Ni oxidation state. Figures 1(b) and (c) show, respectively, the voltage curves for the materials at their 2nd and 40thcycles. Besides the sample with a Ni oxidation state of 2.67+, the increased Ni oxidation state decreases the voltage decay upon extended cycling. The presentation will also focus on the synthesis of another series of materials with Co in order to increase the capacity of the materials.
1. H. Yu and H. Zhou, The Journal of Physical Chemistry Letters, 4, 1268 (2013).
2. E.-S. Lee and A. Manthiram, Journal of Materials Chemistry A, 2, 3932 (2014).
3. M. N. Ates, S. Mukerjee and K. M. Abraham, Journal of The Electrochemical Society, 161, A355 (2014).
Figure 1: Electrochemical data for materials with varying Ni oxidation states: (a) first discharge cycle dQ/dV data, (b) 2nd charge-discharge cycle voltage plots, and (c) 40th charge-discharge cycle voltage plots.

Figure 1