Efficient Stabilization of Na Storage Reversibility by Ti Integration into O′3-Type NaMnO 2

The development of an energy storage system with abundant elements is a key challenge for a sustainable society, and the interest of Na intercalation chemistry is extending throughout the research community. Herein, the impact of Ti integration into NaMnO 2 in a binary system of x NaMnO 2 –( 1 – x ) TiO 2 ( 0.5 ≤ x ≤ 1 ) is systematically examined for rechargeable Na battery applications. Stoichiometric NaMnO 2 , which is classified as an in-plane distorted O ′ 3-type layered structure, delivers a large initial discharge capacity of approximately 200 mAh g -1 , but insufficient capacity retention is observed, most probably associated with dissolution of Mn ions on electrochemical cycles. Ti-substituted samples show highly improved electrode performance as electrode materials. However, the appearance of a sodium-deficient phase, Na 4 Mn 4 Ti 5 O 18 with a tunnel-type structure, is observed for Ti-rich phases. Among the samples in this binary system, Na 0.8 Mn 0.8 Ti 0.2 O 2 ( x = 0.8 ), which is a mixture of a partially Ti-substituted O ′ 3-type layered oxide (Na 0.88 Mn 0.88 Ti 0.12 O 2 ) and tunnel-type Na 4 Mn 4 Ti 5 O 18 as a minor phase elucidated by Rietveld analysis on both neutron and X-ray diffraction patterns, shows good electrode performance on the basis of energy density and cyclability. Both phases are electrochemically active as evidenced by in situ X-ray diffraction study, and the improvement of reversibility originates from the suppression of Mn dissolution on electrochemical cycles. From these results, the feasibility of Mn-based electrode materials for high-energy rechargeable Na batteries made from only abundant elements is discussed in detail.