What is the CEI film of sodium-ion batteries?

CEI 膜即正极电解质界面膜，对应负极 SEI，生长在正极颗粒表面，由电解液在高压正极氧化分解，结合正极溶出金属离子形成固态钝化薄膜，主要在首次化成与前期循环成型。钠离子半径更大、正极晶格易析出金属，使 CEI 成分、生成机理比锂电更复杂，普鲁士蓝、聚阴离子、层状氧化物三类正极生成的 CE 形貌、稳定性差异巨大。CE 厚度、致密程度影响正极耐高压能力、金属溶出速度与电解液氧化损耗，是长循环、高能量密度正极关键瓶颈。目前行业对 CE 研究成熟度不及 SE，主流优化方案使用高压稳定电解液添加剂、正极表面包覆，形成薄层稳定 CE，降低界面副反应损耗。

CEI, short for Cathode Electrolyte Interphase film, corresponds to anode SEI and grows on cathode particles. It is a solid passivation film formed by oxidative decomposition of electrolytes on high-voltage cathodes combined with dissolved metal ions from cathodes, mainly generated during initial formation and early cycles. Larger sodium ions and easy metal elution from cathode lattices make CEI composition and formation more complex than lithium batteries. CEI morphology and stability differ drastically among Prussian blue, polyanion and layered oxide cathodes. The thickness and compactness of CEI affect cathode high-voltage tolerance, metal dissolution rate and electrolyte oxidative loss, serving as a key bottleneck for long-cycle, high-energy-density cathodes. Current industrial research on CEI lags behind SEI. Main optimizations adopt high-voltage stable electrolyte additives and cathode surface coating to form thin stable CEI and reduce interfacial side reactions.