O3 type NaNi1/2Mn1/2O2 materials with Mg and Ti co-substitution demonstrate better capacity capability with an initial discharge capacity of 200 mA h g−1 in non-aqueous Na cells without any capacity loss due to substitution.

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We believe that Na-ion battery is a possible next-generation alternative to Li-ion battery because sodium, the most Earth-abundant alkali, is providing comparable performance of insertion materials to lithium counterpart. To realize comparable energy storage to Li-ion, development of high capacity sodium insertion materials is of a key issue because of inevitable low-voltage operation of Na-ion. Since the demonstration of 3-volt Na-ion battery [1], our group has studied non-graphitizable carbon (hard carbon) and sodium-containing layered 3d transition-metal oxides, Na x MeO2 (Me = 3d transition metal), as promising electrode materials for Na-ion battery. In this study, we present our achievement and recent progress on high capacity sodium insertion materials. Among Na x MeO2 materials, Mn-based layered oxides deliver high reversible capacities of > 200 mAh g-1 [2]. For example, orthorhombic Na2/3MnO2 delivers 216 mAh g-1 with an average discharge voltage of 2.74 V, leading to 590 Wh (kg oxide)-1. Our group also studied influence of the crystal polymorphism from a fundamental aspect and found insufficient electrochemical properties of undistorted Na2/3MnO2 [3]. For improving electrochemical performances of Mn-based layered oxide, partial substitution for Mn site was efficient for NaNi1/2Mn1/2O2. Mg and Ti co-substituted NaNi1/2Mn1/2O2 exhibits better cycle stability than that of non-substituted one. Hard carbon is broadly known as one of the most promising negative electrode materials for Na-ion batteries because of the low-operation voltage and high capacities. Generally, hard carbon includes small graphitic domains and micropores which are randomly distributed in the structure, and their microstructures influence electrochemical properties. Thus, much effort has been devoted to optimizing microstructures suitable for Na storage by changing synthesis conditions such as carbon sources and heat-treatment temperatures [4-6]. Recently, we successfully synthesized high-capacity hard carbon via MgO-templated carbon technique [7]; pyrolyzing a mixture of Mg gluconate and glucose and heat-treatment above 1300 ºC under our optimized condition. The hard carbon electrode delivered an extremely high capacity, ca. 480 mAh g-1, corresponding to apparent composition of NaC4.6 with a high initial coulombic efficiency of 88% [8]. The capacity was mainly achieved in the low potential plateau region at ca. 0.1 V, in which Na+ ions are inserted/extracted into/from internal pores. We believe further optimization of the synthesis conditions provides higher capacity with low-voltage operation; i.e. higher energy density comparable to alloy electrode materials. Reference s : [1] S. Komaba, et al., Adv. Funct. Mater., 21, 3859 (2011). [2] K. Kubota, S. Komaba, et al., Adv. Energy Mater., 8, 1703415 (2018). [3] S. Kumakura, S. Komaba, et al., Angew. Chem. Int. Ed., 55, 12760 (2016). [4] H. Yamamoto, S. Komaba, et al., J. Mater. Chem. A., 6, 16844 (2018). [5] A. Kano et al., 228th ECS Meeting, MA2016–02, 668 (2016). [6] A. Kamiyama, S. Komaba, et al., ACS Appl. Energy Mater., 3, 135 (2020). [7] T. Morishita, I. Michio, et al., Tanso, 242, 60 (2010). [8] A. Kamiyama, S. Komaba, et al., Abstract of 6th Int’l Conf. on Na Batteries, Chicago, Nov-6 2019.