硫酸铁钠正极材料的改性制备与性能研究

  在电化学储能技术中，锂离子电池已得到了广泛的应用，但资源紧张和成本问题限制了其进一步发展。钠作为锂同主族、相邻周期的元素，与锂的电化学性质相似，储量丰富，成本较低，钠离子电池的开发具有广阔的前景。对于钠离子电池来说，目前研究的关键是寻找合适的电极材料。

  聚阴离子型化合物Na₂Fe₂(SO₄)₃是一种极具应用前景的钠离子电池正极材料，具有工作电压较高、钠离子脱嵌过程体积变化小、原材料资源丰富等优点；然而材料较低的电子导电率和比容量限制了其实际应用。近年来，石墨材料也发展成为一种新型正极材料，其较高的导电性以及在高电压下良好的阴离子存储能力使基于该类正极材料的电池通常具有更高的能量密度和动力学性能。因此，本论文拟将膨胀石墨与Na₂Fe₂(SO₄)₃复合，以提高材料导电性并同时利用阴阳离子进行储能，实现高性能复合正极材料的开发。具体研究内容如下：

  （1）在硫酸铁钠的制备过程中加入膨胀石墨，通过低温固相法合成得到硫酸铁钠/膨胀石墨（NFS/EG）复合材料，得到分布在层状膨胀石墨基底上的Na₂Fe₂(SO₄)₃颗粒网络结构，提高了材料的电子导电性；通过对导电剂、 集流体、负载量以及电解液组分进行筛选适配，以提高材料导电性和循环稳定性，优化了复合材料的电极/电池制备条件：在导电剂为科琴黑，集流体为涂碳铝箔，提高极片厚度增加负载量时，电解液为 1M NaPF₆/EC DEC+2 vol% FEC 时，得到 NFS材料的初始容量为 71.8 mAh g^-1，经过200 圈循环后，容量保持率为80.3%。

  （2）对硫酸铁钠和膨胀石墨进行组分优化，确认在膨胀石墨/硫酸铁钠比例为 40:60 时，电化学性能最佳：NFS/EG-40%在钠离子半电池中能够 提供79 mAh g^-1的放电容量，在24 mA g^-1的电流密度下经 200 圈循环后容 量几乎不存在衰减，在20 C（1 C=120 mA g^-1）的电流密度下，仍具有 60.7 mAh g^-1的放电容量，而原始的NFS材料容量仅为23 mAh g^-1；然后通过电化学表征及非原位拉曼测试，验证了高压下材料的阴阳离子共存储储能机制以及更佳的电化学性能。

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