Single-Phase Layered Compositions in the Li-Mn-Ni-O System Which Do Not Significantly Oxidize Electrolyte at 4.6 V Versus Li/Li+

Detailed phase diagrams of the Li-Mn-Ni-O system have been determined to provide a broader understanding of how synthesis conditions affect the phase composition of Li-Mn-Ni-O positive electrode materials.^1,2 Figure 1 shows the positive electrode region of the Li-Mn-Ni-O pseudoternary phase diagram for samples quenched from 900^oC. The phase diagram is labeled with various individual compositions, single-phase regions, tie-lines, and multi-phase co-existence regions all elucidated through extensive crystallographic analysis,³ introducing many opportunities for characterization of new positive electrode materials. In order to develop Li-ion batteries with high energy densities and long cycle lives, ideal novel materials would not react with carbonate-based electrolytes at high potentials (≥ 4.6 V vs. Li/Li⁺).

In this study, three single-phase layered compositions in the Li-Mn-Ni-O system, labeled as A, B and C in Figure 1, were studied by ICP-OES, XRD, and ultra high precision coulometry (UHPC), a technique which uses precise measurement of coulombic efficiency (CE) and charge endpoint capacity slippage to detect electrolyte oxidation. Sample A was determined to be Li[Li_0.157Ni_0.122Mn_0.650□_0.071]O₂, a Li-deficient, Mn-rich material containing 3.5% metal site vacancies.⁴ Sample B was Li[Li_0.117Ni_0.325Mn_0.558]O₂, an essentially Ni-rich member of the Li-rich oxide solid solution series (dashed orange line in Figure 1), while sample C was determined to be Li[Li_0.148Ni_0.480Mn_0.471]O₂, an approximate Li-rich analogue of Li[Ni_0.5Mn_0.5]O₂.

Figure 2 shows the CE, discharge capacity, and normalized charge endpoint capacity for cycle 10 onwards for samples A, B, and C cycled to 4.6 V and 4.8 V, and for Li[Ni_1/3Mn_1/3Co_1/3]O₂ cycled to 4.2 V, 4.4 V, and 4.6 V. In general, cycling to 4.6 V yielded better CE and lower slippage of the Li-Mn-Ni-O materials compared to cycling to 4.8 V, which produced more slippage due to electrolyte oxidation. The performance of  the Li-deficient Li[Li_0.157Ni_0.122Mn_0.650□_0.071]O₂  material cycled to 4.6 V was striking, as it maintained a substantially higher CE and a lower charge endpoint capacity slippage per cycle than Li[Li_0.117Ni_0.325Mn_0.558]O₂, Li[Li_0.148Ni_0.480Mn_0.471]O₂, and industry standard Li[Ni_1/3Mn_1/3Co_1/3]O₂ (cycled to only 4.2 V) while maintaining a reversible capacity of 225 mAh/g after 50 cycles. These results highlight the inherent “inertness” of Li[Li_0.157Ni_0.122Mn_0.650□_0.071]O₂ and its suitability as a thin protective shell in a core-shell particle configuration.

References

1. E. McCalla, A. W. Rowe, R. Shunmugasundaram, and J. R. Dahn, Chem. Mater., 25, 989–999 (2013).

2. E. McCalla, A. W. Rowe, C. R. Brown, L. R. P. Hacquebard, and J. R. Dahn, J. Electrochem. Soc., 160, A1134–A1138 (2013).

3. E. McCalla and J. R. Dahn, Solid State Ion., 242, 1–9 (2013).

4. E. McCalla, A. W. Rowe, J. Camardese, and J. R. Dahn, Chem. Mater., 25, 2716–2721 (2013).