Very High Rate-Capable, Electrochemically Stable, Environmental Friendly and Safe Bi-Phase Na-Titanate Based Composite Anode for Na-Ion Batteries

Na-ion battery system lacks a safe, electrochemically stable, high ‘rate-capable’ and environmental/health friendly anode material. This is because the ‘workhorse’ anode material for Li-ion batteries, i.e. , graphitic carbon, cannot reversibly host Na-ions in its lattice and hard carbon which is the widely explored anode for Na-ion batteries, possesses serious safety concerns and operational hazards [1-3]. In this scenario, the present work reports bi-phase Na-titanate based anode material, which is safe and environmental/health friendly, and which has the potential to exhibit excellent rate-capability and long-term cyclic stability, even at very high current densities. ‘Bi-phase NTO’, having Na 2 Ti 3 O 7 and Na 2 Ti 6 O 13 as primary and secondary phases, respectively, shows contributions from both the phases towards electrochemical Na-storage, as confirmed from operando synchrotron XRD studies, and is more electrochemically stable than phase pure Na 2 Ti 3 O 7 . Additionally, ‘bi-phase NTO’ also has safe and favourable Na-insertion/removal potential in terms of operation as anode. The reinforcement with functionalised multi-walled carbon nanotubes (MWCNTs) further improves the electrochemical performance of ‘bi-phase NTO’, bestowing it with excellent stability, stable ‘charge-averaged’ discharge/charge voltages and negligible impedance build-up over multiple cycles. Due to the presence of O-containing functional groups (facilitating favorable surface interaction with titanate) and the high aspect ratio, the MWCNTs were observed to uniformly cover the surface of the Na-titanate particles, almost ‘wrapping’ the same. This suppresses the occurrence of deleterious reactions at the Na-titanate/electrolyte interface and leads to excellent connectivity across the active particles. Accordingly, even at a very high current density corresponding to 50C, a 1 st cycle reversible Na-storage capacity of >140 mAh g -1 ( viz. , negligible drop from capacities at lower C-rates) and capacity retentions of ~83%, ~65%, 58% after 100, 1000, 2000 cycles, respectively (see Fig. 1) were obtained with the as-developed ‘bi-phase NTO’/MWCNT-based electrodes; indicating the feasibility for long-term cycling even at very high current densities. A part of this work has been published as Pradeep et al ., Electrochim. Acta 362 (2020) 137122, i.e. , ref. [4] here. Keywords: Na-titanate; composite electrode; in-operando synchrotron X-ray diffraction; high rate performance; Carbon-based