A carbon-coated pyrosilicate, Na₂Mn₂Si₂O₇/C, was synthesized and characterized for use as a new positive-electrode material for sodium ion batteries. The material consists of 20–80 nm primary particles embedded in a ≈10 nm-thick conductive carbon matrix. Reversible insertion of Na⁺ ions is clearly demonstrated with ≈25% of its theoretical capacity (165 mA h g⁻¹) being accessible at room temperature at a low cycling rate. The material yields an average potential of 3.3 V vs. Na⁺/Na on charge and 2.2 V on discharge. DFT calculations predict an equilibrium potential for Na₂Mn₂Si₂O₇ in the range of 2.8–3.0 V vs. Na⁺/Na, with a possibility of a complete flip in the connectivity of neighboring Mn-polyhedra – from edge-sharing to disconnected and vice versa. This significant rearrangement in Mn coordination (≈2 Å) and large volume contraction (>10%) could explain our inability to fully desodiate the material, and illustrates well the need for a new electrode design strategy beyond the conventional “down-sizing/coating” procedure.

中文翻译：

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焦硅酸锰作为Na离子电池的新型正极材料†

合成了碳包覆的焦硅酸盐Na ₂ Mn ₂ Si ₂ O ₇ / C，并表征其用作钠离子电池的新型正极材料。该材料由嵌入约10 nm厚的导电碳基质中的20–80 nm初级粒子组成。Na ⁺离子的可逆插入得到了明显证明，其理论容量的约25％（165 mA hg ^-1）可以在室温下以低循环速率获得。与充电时的Na ⁺ / Na和放电时的2.2 V相比，该材料产生的平均电势为3.3V 。DFT计算可预测Na ₂ Mn ₂ Si的平衡势₂ O ₇的相对于Na ⁺ / Na的范围为2.8–3.0 V，相邻Mn-多面体的连通性可能会发生完全翻转，即从边缘共享变为断开，反之亦然。Mn配位（≈2Å）和大体积收缩（> 10％）的显着重排可以解释我们无法对材料进行完全脱粉，并很好地说明了除常规的“缩小尺寸/涂层”之外还需要一种新的电极设计策略“ 程序。