The application of sodium-ion batteries (SIBs) requires a suitable cathode material with low cost, nontoxic, high safety, and high energy density, which is still a big challenge; thus, a basic research on exploring new types of materials is imperative. In this work, a manganic pyrophosphate and carbon compound Na_3.12Mn_2.44(P₂O₇)₂/C has been synthesized through a feasible sol–gel method. Rietveld refinement reveals that Na_3.12Mn_2.44(P₂O₇)₂ adopts a triclinic structure (P1̅ space group), which possesses spacious ion diffusion channels for facile sodium migration. The off-stoichiometric phase is able to offer more reversible Na⁺, delivering an enhanced reversible capacity of 114 mA h g^–1 at 0.1 C, and because of the strong “inductive effect” that (P₂O₇)^4– groups imposing on the Mn³⁺/Mn²⁺ redox couple, Na_3.12Mn_2.44(P₂O₇)₂/C presents high platforms above 3.6 V, contributing a remarkable energy density of 376 W h kg^–1, which is among the highest Fe-/Mn-based polyanion-type cathode materials. Furthermore, the off-stoichiometric compound also presents satisfactory rate capability and long-cycle stability, with a capacity retention of 75% after 500 cycles at 5 C. Ex situ X-ray diffraction demonstrates a single-phase reaction mechanism, and the density functional theory calculations display two one-dimensional sodium migration paths with low energy barriers in Na_3.12Mn_2.44(P₂O₇)₂, which is vital for the facile sodium storage. We believe that this compound will be a competitive cathode material for large-scale SIBs.

中文翻译：

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用于高能/功率钠离子电池的三斜非化学计量Na _3.12 Mn _2.44（P ₂ O ₇）₂ / C阴极材料

钠离子电池（SIB）的应用需要低成本，无毒，高安全性和高能量密度的合适阴极材料，这仍然是一个很大的挑战。因此，探索新型材料的基础研究势在必行。在这项工作中，通过可行的溶胶-凝胶法合成了焦磷酸锰和碳化合物Na _3.12 Mn _2.44（P ₂ O ₇）₂ / C。Rietveld精炼显示Na _3.12 Mn _2.44（P ₂ O ₇）₂采用三斜晶结构（P1̅空间群），具有宽敞的离子扩散通道，可轻松进行钠迁移。非化学计量比相能够提供更多的可逆Na ⁺，在0.1 C时可提供114 mA hg ^–1的增强可逆容量，并且由于（P ₂ O ₇）^4–基团强加于其上的“感应效应” Mn ³⁺ / Mn ²⁺氧化还原对，Na _3.12 Mn _2.44（P ₂ O ₇）₂ / C表现出高于3.6 V的高平台，贡献了376 W h kg ^–1的显着能量密度，这是最高的基于Fe- / Mn的聚阴离子型阴极材料。此外，非化学计量化合物还具有令人满意的速率能力和长周期稳定性，在5 C下500次循环后的容量保持率为75％。异位X射线衍射显示出单相反应机理，并且密度函数理论计算显示出在Na _3.12 Mn _2.44（P ₂ O ₇）_2中具有低能垒的两个一维钠迁移路径，这对于方便的钠存储至关重要。我们相信，该化合物将成为大规模SIB的竞争性阴极材料。