Mn based P2-type layered oxides, Na_2/3Me_yMn_1-yO₂ (Me=a variety of metals), have been widely studied as positive electrode materials for Na-ion batteries, especially consisting only of the abundant elements in the Earth’s crust, such as P2-Na_2/3[Fe_1/2Mn_1/2]O₂. [1] The end member, P2-type Na_2/3MnO₂ is known to have crystal polymorphism, P2- and P’2- Na_2/3MnO₂ with hexagonal and orthorhombic lattice, respectively. The orthorhombic system is stabilized by co-operative Jahn-Teller distortion (CJTD) derived from six-coordinated high spin Mn^III (t_2g³-e_g¹). Very recently, our group reported that the single-phase samples of these polymorphs, P2-Na_0.59Mn_0.9O₂ and P’2-Na_0.64MnO₂, possess different stoichiometry and demonstrate the superior electrochemical performance of distorted P’2 sample [2], though the distortion in insertion materials is generally considered to be disadvantageous for battery application. Therefore, quantitative understanding how CJTD affects electrochemical properties such as reversibility, kinetics of sodium diffusion, and long-cycle stability is of great importance to realize high capacity performance of Mn^III based oxide materials for high energy density NIBs. In this study, we demonstrate the systematic synthesis of differently distorted P’2-Na_2/3MnO₂samples by partial replacement of Mn with different metal elements to correlate their electrochemical properties and lattice distortion.

Single phases of P’2-Na_2/3Mn_0.9Me_0.1O₂ (Me = Mg, Ti, Co, Ni, Cu, and Zn) are synthesized when we optimized facile solid-state reaction. The distortion calculated from lattice constant is highly correlated with average oxidation state of manganese varied by divalent metals (Mg^II, Ni^II, Cu^II, Zn^II), trivalent metal (Co^III) and tetravalent metal (Ti^IV). Because of Jahn-Teller active Cu^II (t_2g⁶-e_g³), P’2-Na_2/3Mn_0.9Cu_0.1O₂ possesses anomalous distortion compared to other samples with divalent-metal dopants as shown in Figure 1a. All samples show high reversible capacity about 200 mAh g^-1 and have smooth charge/discharge profiles. Fig. 1b compares charge/discharge curves of divalent-metal doped samples. The P’2-Na_2/3Mn_0.9Ni_0.1O₂ delivers the reversible capacity of 227 mAh g^-1 and the Na_2/3Mn_0.9Cu_0.1O₂shows stable electrochemical cycling, 95% capacity retention after 25 cycles. In this presentation, we will further discuss the effect of metal-substitution and lattice distortion on electrochemical properties and demonstrate successful improvement of both cycle and rate performance by Cu doping.

References:

1. N. Yabuuchi, S. Komaba, et al., Nat. Mater., 11, 512 (2012).

2. S. Kumakura, Y. Tahara, K. Kubota, S. Komaba, et al., Angew. Chem. Int. Ed. 55, 12760 (2016).