功能化粘结剂稳定 4.6 V 高电压钴酸锂正极材料的研究

       随着第五代移动通信技术的快速发展，对高容量锂离子电池的需求日益增加。钴酸锂正极材料由于其超高的压实密度和体积比能量，目前仍主导着便携式电子市场。进一步提高钴酸锂的充电电压可以显著提高比容量，但会引起结构和表面不稳定，使得钴酸锂的循环性能迅速衰减。目前稳定高电压钴酸锂的常用策略是结构掺杂和表面包覆，本论文提出通过功能化粘结剂稳定4.6 V高电压钴酸锂正极材料的新策略。
本文采用具有丰富的硫酸基团的葡聚糖硫酸锂（DSL）粘结剂替代聚偏氟乙烯（PVDF）粘结剂，能够与钴酸锂颗粒表面含氧官能团氢键作用来形成均匀的包覆层，有效抑制界面降解和电解液分解。更重要的是，DSL粘结剂极大提高了钴酸锂表面Co-O键的稳定性，进一步抑制了高电压下有害的相变发生。这些优点共同作用确保了钴酸锂在4.6 V电压下的稳定循环，在0.5 C的倍率下100次循环后仍有93.4 %的高容量保持率。
       本文制备了一系列的水溶性海藻酸盐粘结剂，来抑制钴酸锂在高电压下的容量衰减。其中海藻酸镁粘结剂由于羧基官能团的氢键作用和Mg离子的交联作用，能在钴酸锂颗粒表面形成均匀的包覆层，以起到人工界面的作用来减轻电解液的分解和有害的相变发生。最终显著提高高压钴酸锂的电化学性能，在0.5 C的倍率下300次循环仍有的80.1 %的容量保持率。
       本论文的研究成果证明水溶性功能化粘结剂可有效提升钴酸锂正极材料在4.6 V高截至电压下的循环稳定性。揭示粘结剂特定官能团、阳离子对高电压钴酸锂的稳定机理。这项工作为实现钴酸锂在高电压下的稳定长循环提供了一种简单方案，为其他高电压正极材料的研究开阔了思路。

        With the rapid development of 5G technology, the demand of lithium-ion batteries with high capacity is ever-growing. Lithium cobalt oxides (LiCoO₂), a promising cathode with high compact density and volumetric energy density still dominates the portable electronic market. Further elevating the charging voltage of LiCoO₂ could greatly enhance its practical capacity, but will unavoidably bring about structure and surface instability issues, which greatly deteriorate the cyclability of LiCoO₂. At present, bulk doping and surface coating are commonly used to stabilize high-voltage LiCoO₂. In this paper, we propose a new strategy for stabilizing 4.6 V high-voltage LiCoO₂ via functionalized binders.
        Herein, we propose water-soluble dextran lithium sulfate (DSL) with abundant sulfate acid groups as the binder to replace polyvinylidene fluoride (PVDF). DSL binder can form hydrogen bonds with the oxygen-containing groups on the surface of LiCoO₂ to form a uniform coating layer, significantly preventing interface degradation and electrolyte decomposition. More importantly, the DSL binder greatly enhances the stability of superficial Co-O bonds of LiCoO₂, further suppressing the detrimental phase transition at high voltage. All these benefits contribute to the stable cycling of LiCoO₂ at 4.6 V， with a high capacity retention of 93.4 % at 0.5 C over 100 cycles.
        In addition, A series of water-soluble alginate-based binders were prepared to suppress the capacity fading of high-voltage LiCoO₂. Among them, we show that Magnesium alginate (MA) binder owns the hydrogen bonding effect of the rich carboxyl functional groups and the cross-linking effect of Mg ions, which can form a uniform coating layer on the surface of the primary LiCoO₂ particles as the artificial interface to alleviate decomposition of electrolyte and harmful phase transition. Consequently, the MA binder can significantly enhance the electrochemical performance of high-voltage LiCoO₂, with a high capacity retention of 80.1 % at 0.5 C after 300 cycles.
        In summary, water-soluble functionalized binders have been prepared to enhance the stability of LiCoO₂ for lithium-ion batteries at a high cut-off voltage of 4.6 V. After that, the stabilization mechanism of high-voltage LiCoO₂ through functional groups and cations of binders have been revealed. More importantly, this work provides a unique method to stabilize the cyclability of high-voltage LiCoO₂, and opens up ideas for the research of other high-voltage cathode materials.

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