The Effect of Ni Oxidation State on Li-Rich Layered Oxides

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 Li_1.2Mn_0.6-xNi_0.2+xO₂ 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 2^nd and 40^thcycles.  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) 2^nd charge-discharge cycle voltage plots, and (c) 40^th charge-discharge cycle voltage plots.