Transition metal (T_M) oxides are a fascinating class of materials, whose properties can be suitably tuned in a variety of ways; such as by selecting T_M-ions/dopants having preferred electronic configurations, engineering the crystallographic site occupancy by dopants, controlling/modifying the degree of covalence of T_M-O bonds, modifying lattice spacing(s), tuning phase assemblage etc. Such modifications done from the fundamental perspectives influence the performances of T_M-oxides for a variety of applications, including their widespread usage as cathode-active materials in alkali metal-ion batteries (i.e., the Li-ion and Na-ion battery systems).

In the context of Li-ion batteries, ‘cation disordering’ that takes place at deep states of delithiation of Ni-containing ‘layered’ Li-T_M-oxides leads to structural-cum-electrochemical instability during charging of Li-ion cells at potentials beyond 4.3 V (vs. Li/Li⁺). This forms a major bottleneck towards raising the operating voltage of Li-ion batteries and thus, leads to a compromise over the energy density. Nevertheless, our recent research results have indicated that optimal doping of such ‘layered’ Li-T_M- oxides with ion(s) having d¹⁰ electronic configuration (and, thus, no OSPE) suppresses Ni-migration from T_M-layer to Li-layer (viz., ‘cation disordering’) and, thus, preserves the structural integrity even upon deep states of delithiation (i.e., at high cell voltages) [recently, published as; Sharma et al., ACS Appl. Mater. Interfaces 13[22] (2021) 25836].

Sharma et al., ACS Appl. Mater. Interfaces 13[22] (2021) 25836

Figure: Effect of Tetrahedral site Zn-doping on the migration passage of Ni-containing Layered Transition Metal (T_M) oxide on High Voltage Structural and Electrochemical Stability.

In the context of the upcoming Na-ion battery system, O3-type ‘layered’ Na-T_M-oxides are promising as cathode-active materials due to their inherently high initial Na-content (as compared to the P2 counterparts); but suffer from instabilities caused due to multiple phase transformations during Na- removal/insertion and sensitivity to air/moisture. We have been able to tune the composition and structural features to suppress the phase transitions upon Na-removal/insertion and also improve the air/water-stability in significant terms; so much so that long-term cyclic stability has been achieved with health/environment-friendly ‘aqueous processed’ electrodes (sans, usage of toxic/hazardous/expensive chemicals like NMP and PVDF). The changes in structural features, which have led to such outstanding water-stability, include differential contraction/dilation of the Na-‘inter-slab’/T_M-‘slab’ spacing and partial occupancy of the dopant at tetrahedral sites of the structure [recently, published as; Kumar et al.; J. Mater. Chem. A 8 (2020) 18064].

Kumar et al.; J. Mater. Chem. A 8 (2020) 18064

Figure: Tuning T_M-ion ‘slab thickness’ and alkali metal-ion ‘inter-slab spacing’ of Na-T_M-oxides in opposite terms (and also suppressing T_M-dissolution in electrolyte) leads to outstanding air/water-stability and long-term electrochemical stability.

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