Electrochemically stable and very high ‘Rate-capable’ bi-phase Na-titanate based composite anodes for Na-ion batteries

Abstract

The upcoming Na-ion battery system still lacks in terms of a safe, electrochemically stable and high ‘rate-capable’ anode material. Against this backdrop, we report here the development of bi-phase Na-titanate based composite, which satisfies the above requirements and displays exceptionally high rate-capability, along with long-term cyclic stability even at very high current densities. ‘Bi-phase NTO’, having Na2Ti3O7 and Na2Ti6O13 in ~2:1 ratio (as formed in-situ during synthesis), shows contributions from both the phases towards electrochemical Na-storage (as cross-confirmed by operando synchrotron XRD), with reversible sodiation of Na2Ti6O13 component being > 1 Na-ion per-formula-unit. ‘Bi-phase NTO’ also has favorable Na-insertion/removal potential w.r.t. operation as anode, as well as safety aspects, and is electrochemically more stable than Na2Ti3O7. Furthermore, addition of functionalized multi-walled carbon nanotubes bestows ‘bi-phase NTO’ with reversible Na-storage capacity of ~162 mAh g−1at C/5, excellent stability of ‘charge-averaged’ discharge/charge voltages and negligible impedance build-up over multiple cycles. More importantly, even at 50C, 1st cycle reversible capacity of > 140 mAh g−1 is obtained, with stable capacity retention (up to 2000 cycles), including negligible fade from the 300th cycle onward; indicating feasibility for long-term cycling even at very high current densities. Overall, this development will allow addressing the major concerns associated with the Na-ion battery system.