Efficient Stabilization of Na Storage Reversibility by Ti Integration into O′3-Type NaMnO2

Abstract

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 NaMnO2 in a binary system of x NaMnO2–(1–x) TiO2 (0.5≤x≤1) is systematically examined for rechargeable Na battery applications. Stoichiometric NaMnO2, 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, Na4Mn4Ti5O18 with a tunnel-type structure, is observed for Ti-rich phases. Among the samples in this binary system, Na0.8Mn0.8Ti0.2O2 (x=0.8), which is a mixture of a partially Ti-substituted O′3-type layered oxide (Na0.88Mn0.88Ti0.12O2) and tunnel-type Na4Mn4Ti5O18 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.