‘Aqueous Processed’ O3-Type Transition Metal Oxide Cathodes Enabling Long-Term Cyclic Stability for Na-Ion Batteries
Authors: Kumar Bachu Sravan , Pradeep Anagha , Dutta Animesh , Mukhopadhyay Amartya 
Published: November 2022

Among the potential cathode material classes for Na-ion batteries, O3-type layered NaxTMO2s (TM => transition metal ion) are of importance due to their high starting Na-content (of ~1 per formula unit; x). However, the O3-type NaxTMO2s suffer from multiple structural phase transformations during electrochemical charge/discharge cycles, TM-dissolution into electrolyte [1-2] and, more importantly, inherent sensitivity to moisture [3]. The moisture sensitivity of these ‘layered’ NaxTMO2s 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(Li0.05Mn0.5-xTixNi0.30Cu0.10Mg0.05)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 NaxTMO2s. The Mn-containing Na-TM-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-TM-oxides. The excellent stability of the optimized O3-type NaTMO2 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-TM-oxide based cathode materials for Na-ion batteries.

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

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Figure 1