电解液添加剂对碱金属电池中SEI作用的冷冻透射电镜研究

冷冻电镜作为一种有效的研究手段，在电池界面等敏感材料研究领域发挥了重要作用。冷冻电镜已经在锂金属负极研究中广泛应用，但在其他碱金属负极研究中还鲜有涉及。

为了减少燃烧化石燃料对环境造成的不利影响，我们迫切需要探索可再生能源技术，并开发下一代储能系统。其中锂离子电池是目前最成功的电化学储能器件。但锂的高成本和稀缺性使锂离子电池不适用于规模储，而钠和钾在地球上的含量较为丰富以及优异的物理化学性能被认为是目前最有潜力替代商用锂离子电池的研究方向，其中钾在某些电解液中的离子电导率比钠和钾都要高的多，因此在研究钠金属电池之后，钾金属电池也具有较强的研究价值。然而，在充放电过程中碱金属负极表面形成的金属枝晶会导致严重的安全以及循环性能下降等问题，这对电池的发展造成了极大的阻碍。而负极材料表面的固态电解质层（SEI）是抑制电化学循环过程中金属枝晶生长的关键。由于钠、钾金属以及生成的SEI十分敏感，目前常规手段并不能对其进行直接观测表征，因此使用冷冻电镜对此类样品进行表征是最为可行的一种方法。冷冻电镜可以使样品在液氮中转移，防止空气对样品的侵蚀，同时可以使样品保持在液氮温度下进行观测，配合超低剂量技术，可以有效降低电子束对样品的损伤，保证观测数据的真实性。所以通过冷冻电镜可以研究此类电池的真实失效机制并建立相应模型，从而针对性提出相关问题的解决办法。

在上述基础上，本文采用超低剂量冷冻电镜研究电池负极表面SEI的成分及其分布，探究不同电解液添加剂对于SEI成分结构的改变情况。

（1）在钠金属电池中，向电解液中添加的氟代碳酸乙烯酯（FEC）添加剂可以在钠金属负极表面首先形成一层均匀的氟化钠（NaF）膜，继而形成最外层为包含NaF的非晶层和内部为Na3PO4的层状结构，有效阻止电解液与负极的进一步反应，防止SEI增厚，最终提高电池性能。而不含FEC中生成的SEI极不稳定，并且电解液与钠金属负极会持续反应，厚度达百纳米，且内部的Na2CO3和Na3PO4分布杂乱无序。

（2）在钾电池中，首次使用超低剂量冷冻电镜成像技术得到了钾金属负极表面SEI的原子级图像，图像显示有机磷酸盐电解液生成的SEI薄且稳定，主要成分为KPO3、K2SO4和K2(SO2)3，电池性能也得到增强。而碳酸酯类电解液生成的SEI中含有大量如KHCO3和K4CO4的亚稳定物质，最终会耗尽电池中的钾源，导致电池性能下降。

通过超低剂量冷冻电镜，首次得到钠、钾电池内部SEI的原子级图像，解释了钠金属电池中FEC提高电池性能的原因，以及钾金属电池中不同电解液体系电池的失效机制。这为钠、钾电池中电解液的发展提供了新的信息，并有力地促进此领域未来走向实际应用的发展。

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