Recently, urgent concerns over lithium resources have focused attention on Na-ion batteries owing to the abundance and low cost of sodium. However, the presently-considered negative-electrode materials for Na-ion batteries are Na-ion intercalation (carbon compounds, metal oxides and metal sulfides), alloying (phosphorus and tin) or conversion materials (metal oxides and metal sulfides), the energy and power densities of which are not satisfactory if the Na-ion batteries were to outperform the state-of-the-art Li-ion batteries. Hence, exploration for novel negative electrode materials of sodium system is primarily important.

 We have targeted MXenes as potential electrodes for sodium system. MXenes have been developed as a novel family of nanosheet compounds, which are chemically derived from layered M_n+1AX_n or MAX phases (M: early transition metal, A: A-group element, and X: C and/or N). As the MXene has both a high electrical conductivity and interlayer porous space, its application to Na-ion battery negative electrodes holds great promise.

 In this presentation, we report on Ti_n+1C_nT_x as negative electrode materials for Na-ion batteries. Ti_n+1C_nT_x consists of abundant and low-cost elements, enabling fabrication of sustainable energy sources. We demonstrate that Ti₂CT_x exhibits a reversible capacity of 175 mAh/g at the average potential of 1.3 V vs. Na/Na⁺. Furthermore, the prototype full cell consisting of alluaudite Na₂Fe₂(SO₄)₃-Ti₂CT_x delivers a high specific energy of 260 Wh/kg at a high specific power of 1.4 kW/kg based on the weight of Ti₂CT_x. Using various experimental and theoretical analyses, we clarify that the Ti_n+1C_nT_x electrodes allow Na-ion batteries to be liberated from the trade-off between high energy and high power.

[Reference]

X. Wang, et al., M. Okubo, & A. Yamada, Nat. Commun., 2015, 6, 6544.