Probing the Effect of Titanium Substitution on the Sodium Storage in Na 3 Ni 2 BiO 6 Honeycomb-Type Structure

Na 3 Ni 2 BiO 6 with Honeycomb structure suffers from poor cycle stability when applied as cathode material for sodium-ion batteries. Herein, the strategy to improve the stability is to substitute Ni and Bi with inactive Ti. Monoclinic Na 3 Ni 2-x Bi 1-y Ti x+y O 6 powders with different Ti content were successfully synthesized via sol gel method, and 0.3 mol of Ti was determined as a maximum concentration to obtain a phase-pure compound. A solid-solution in the system of O3-NaNi 0.5 Ti 0.5 O 2 and O3-Na 3 Ni 2 BiO 6 is obtained when this critical concentration is not exceeded. The capacity of the first desodiation process at 0.1 C of Na 3 Ni 2 BiO 6 (~93 mAh g −1 ) decreases with the increasing Ti concentration to ~77 mAh g −1 for Na 3 Ni 2 Bi 0.9 Ti 0.1 O 6 and to ~82 mAh g −1 for Na 3 Ni 0.9 Bi 0.8 Ti 0.3 O 6 , respectively. After 100 cycles at 1 C, a better electrochemical kinetics is obtained for the Ti-containing structures, where a fast diffusion effect of Na + -ions is more pronounced. As a result of in operando synchrotron radiation diffraction, during the first sodiation (O1-P3-O’3-O3) the O’3 phase, which is formed in the Na 3 Ni 2 BiO 6 is fully or partly replaced by P’3 phase in the Ti substituted compounds. This leads to an improvement in the kinetics of the electrochemical process. The pathway through prismatic sites of Na + -ions in the P’3 phase seems to be more favourable than through octahedral sites of O’3 phase. Additionally, at high potential, a partial suppression of the reversible phase transition P3-O1-P3 is revealed.