Designing high capacity cation-disordered cathode materials for lithium ion batteries

Li-excess disordered rock-slat transition metal oxides (LEX-RS), have received considerable attention as high-capacity cathode materials. The excess Li reduces the transition metal (TM) content and increases the average TM oxidation state, leading to a decrease in TM-based redox capacity. Hence, the high capacity of such materials relies on oxygen redox processes wherein delivery of high capacity cold trigger oxygen loss and formation of high-impedance layers that limit the performance. Researchers at University of California, Berkeley and Lawrence Berkeley National Laboratory, USA have demonstrated that fluorine substitution is a viable option to overcome this limitation. Since partial substitution of fluorine in place of oxygen lowers the average anion valance, more Ni²⁺ ions could be incorporated. This strategy helps to increase the Ni redox reservoir, limits oxygen redox process and prevents oxygen loss.

Li-Ni-Ti-Mo based metal oxides with suitable stoichiometric ratios were synthesized by a solid-state method using Li₂CO₃, NiCO₃, TiO₂, MoO₂, and LiF as precursors. The precursors (suitable stoichiometric ratios) were dispersed in acetone and ball milled for 15 h, dried overnight in an oven, pelletized, calcined at 700-750 °C for 2-10 h in air, furnace cooled and ground to form fine powders. Li_1.15Ni_0.375Ti_0.375Mo_0.1O₂ (LN15), Li_1.2Ni_0.333Ti_0.333Mo_0.133O₂ (LN20) and Li_1.15Ni_0.45Ti_0.3Mo_0.1O_1.85F_0.15 (LNF15) were evaluated.

The voltage profile of LN15, recorded during galvanostatic cycling between 1.5 and 4.6 V, exhibits a large hysteresis (voltage gap, polarization) between charge and discharge cycles with a discharge plateau at ~2.2 V (Fig. 1(a)). In contrast, LNF15 exhibits a much reduced polarization, delivering high discharge capacities, in which this discharge plateau is hardly noticed (Fig. 1(b)).

Fig. 1 Comparison of voltage profiles of (a) LN15; and (b) LNF15 when cycled between 1.5 and 4.6 V at 20 mA/g (Inset: capacity retention – first 20 cycles)

Differential electrochemical mass spectrometry (DEMS) measurements performed on LN15 and LNF15 indicate that LNF15 has experienced a lower oxygen loss than LN15. Upon first charge to 4.8 V, O₂ gas could be detected from ~4.35 V (~185 mAh/g) for LN15 (Fig. 2(a)) whereas detection of O₂ gas is delayed up to ~4.5 V (~220 mAh/g) for LNF15 (Fig. 2(b)). Upon charging, reaction between the oxygen radicals generated at the cathode and the carbonate-based electrolytes could lead to the formation of CO₂ gas. Irrespective of the type of cathode materials, the evolution of  CO₂ gas occurs > ~4.4 V for both LN15 and LNF15.  The total amount of O₂ evolved for LN15 and LNF15 is 0.30 and 0.09 μmol/mg, respectively. Similar to O₂, the amount of CO₂ gas evolved is also low for LNF15 (0.05 μmol/mg) than for LN15 (0.14 μmol/mg). If all of the O₂ is presumed to be originated from the cathode, then the amount of O₂ evolved corresponds to a loss of 2.3 and 0.7% of the total oxygen content for LN15 and LNF15, respectively.

Fig. 2 Comparison of the DEMS study of LN15 and LNF15 when charged to 4.8 V and discharged to 1.5 V at 20 mA/g

The combined effect of fluorine substitution along with an increase in nickel content has enabled LNF15 to achieve a decrease in O₂ loss from the cathode, better capacity retention and improved performance. Fluorine substitution opens up new avenues for designing high-capacity cathode materials with transition metal ions.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: Jinhyuk Lee et al., Mitigating oxygen loss to improve the cycling performance of high capacity cation-disordered cathode materials, Nature Communications, 8: 981  DOI: 10.1038/s41467-017-01115-0