Among the potential cathode material classes for Na-ion batteries, O3-type layered Na_xT_MO₂s (T_M => transition metal ion) are of importance due to their high starting Na-content (of ~1 per formula unit; x). However, the O3-type Na_xT_MO₂s suffer from multiple structural phase transformations during electrochemical charge/discharge cycles, T_M-dissolution into electrolyte [1-2] and, more importantly, inherent sensitivity to moisture [3]. The moisture sensitivity of these ‘layered’ Na_xT_MO₂s necessitates the usage of toxic/hazardous non-aqueous solvents like N-Methyl-2-pyrrolidone (NMP) during electrode preparation. Against this backdrop, a carefully designed composition has been developed in this work, which addresses the aforementioned problems, in particular, the air/water-instability. Partial/complete substitution of Ti-ion for Mn-ion in Na(Li_0.05Mn_0.5-xTi_xNi_0.30Cu_0.10Mg_0.05)O₂ eliminated the presence of Mn³⁺ (which dissolves in electrolyte) at the particle surface, supressed increment in impedance and voltage hysteresis during electrochemical cycling and, thus, significantly improved cyclic stability of Ti-substituted O3-type layered Na_xT_MO₂s. The Mn-containing Na-T_M-oxides were found to be extremely unstable in terms of phase/structure retention upon exposure to air and water; progressively evolving O’3 and P3 phases due to spontaneous Na-loss and thereby forming undesired NaOH and Na₂CO₃ phases on the particle surface (see Fig. 1a), causing increase in electrochemical impedance. By contrast, no phase/structural change occurred upon partial/complete Ti-substitution (for Mn-ion), even after 40 days of air-exposure and 12 h of soaking, as well as stirring, in water (viz., very stringent hydration condition) (see Fig. 1b). Such excellent stability against hydration, which was partly due to reduced Na-ion ‘inter-slab spacing’ in the presence of Ti-ion, was not reported earlier for O3-type Na-T_M-oxides. The excellent stability of the optimized O3-type NaT_MO₂ enables the usage of environment/health-friendly and economical ‘aqueous-binder’ (viz., Na-alginate) and water (as solvent) for electrode preparation. Overall, the ‘aqueous-processed’ cathode exhibits first cycle capacity of ~125 mAh/g (between 2-4 V; vs. Na/Na⁺), with smooth electrochemical cycling profiles (see Fig. 2a) and excellent long-term cyclic stability, with a capacity retention of ~56% after 750 cycles at C/5 (see Fig. 2b). Overall, the present work, as published in ref. [4], has established important correlations between the composition, structure (viz., reduction in ‘inter-slab spacing’), stability against hydration (viz., in air and water), feasibility for health/environmental-friendly ‘aqueous processing’ of electrodes, electrochemical impedance, stability of average voltages and cyclic stability of O3-type Na-T_M-oxide based cathode materials for Na-ion batteries.

Keywords: Na-ion battery; layered transition metal oxide cathode; air/water-stability; aqueous processing; electrochemical behaviour

References:

[1] S. Komaba, N. Yabuuchi, T. Nakayama, A.Ogata and T. Ishikawa, Inorg.chem, 51, 6211–6220 (2012).

[2] P. F. Wang, Y. You, Y. X. Yin and Y. G. Guo, J. Mater. Chem. A, 4, 17660–17664 (2016).

[3] H. R. Yao, P. F. Wang, Y. Gong, J. Zhang, X. Yu, L. Gu, C. Ouyang, Y. X. Yin, E. Hu, X. Q. Yang, E. Stavitski, Y. G. Guo and L. J. Wan, J. Am. Chem. Soc., 139, 8440–8443 (2017).

[4] B. S. Kumar, A. Pradeep, A. Dutta, A. Mukhopadhyay., J. Mater. Chem. A, 8, 18064-18078 (2020).