Tokyo University of Science , 1-3 Kagurazaka , Shinjuku, Tokyo 162-8601 , Japan.
Elements Strategy Initiative for Catalysts & Batteries (ESICB) , Kyoto University , Nishikyo-ku, Kyoto 615-8245 , Japan.
ACS Appl Mater Interfaces. 2018 Apr 4;10(13):10986-10997. doi: 10.1021/acsami.8b01362. Epub 2018 Mar 22.
P2-NaNiMnO (P2-NiMn) is one of the promising positive electrode materials for high-energy Na-ion batteries because of large reversible capacity and high working voltage by charging up to 4.5 V versus Na/Na. However, the capacity rapidly decays during charge/discharge cycles, which is caused by the large volume shrinkage of ca. 23% by sodium deintercalation and following electric isolation of P2-NiMn particles in the composite electrode. Serious electrolyte decomposition at the higher voltage region than 4.1 V also brings deterioration of the particle surface and capacity decay during cycles. To solve these drawbacks, we apply water-soluble sodium poly-γ-glutamate (PGluNa) as an efficient binder to P2-NiMn instead of conventional poly(vinylidene difluoride) (PVdF) and examined the electrode performances of P2-NiMn composite electrode with PGluNa binder for the first time. The PGluNa electrode shows Coulombic efficiency of 95% at the first cycle and capacity retention of 89% after 50 cycles, whereas the PVdF electrode exhibits only 80 and 71%, respectively. The alternating current impedance measurements reveal that the PGluNa electrode shows a much lower resistance during the cycles compared with the PVdF one. From the surface analysis and peeling test of the electrodes, the PGluNa binder was found to cover the surface of the P2-NiMn particles and suppresses the electrolyte decomposition and surface degradation. The PGluNa binder further enhance the mechanical strength of the electrodes and suppresses the electrical isolation of the P2-NiMn particles during sodium extraction/insertion. The efficient binder with noticeable adhesion strength and surface coverage of active materials and carbon has paved a new way to enhance the electrochemical performances of high-voltage positive electrode materials for Na-ion batteries.
P2-NaNiMnO (P2-NiMn) 是一种很有前途的高能钠离子电池正极材料，因为它可以在充电至 4.5 V 相对于 Na/Na 时提供较大的可逆容量和较高的工作电压。然而，在充放电循环过程中，容量会迅速衰减，这是由于 P2-NiMn 颗粒在复合电极中的脱钠和随后的电隔离导致约 23%的体积收缩所致。在高于 4.1 V 的较高电压区域，严重的电解质分解也会导致颗粒表面恶化和循环过程中容量衰减。为了解决这些缺点，我们首次应用水溶性聚γ-谷氨酸钠 (PGluNa) 作为高效粘结剂替代传统的聚偏二氟乙烯 (PVdF) 应用于 P2-NiMn，并对 P2-NiMn 复合电极的电极性能进行了研究。PGluNa 电极在第一次循环中表现出 95%的库仑效率，50 次循环后容量保持率为 89%，而 PVdF 电极分别仅为 80%和 71%。交流阻抗测量表明，PGluNa 电极在循环过程中的电阻明显低于 PVdF 电极。从电极的表面分析和剥离试验可知，PGluNa 粘结剂覆盖在 P2-NiMn 颗粒的表面，抑制了电解质的分解和表面的降解。PGluNa 粘结剂进一步增强了电极的机械强度，抑制了 P2-NiMn 颗粒在钠提取/插入过程中的电隔离。这种具有显著粘附强度和对活性材料和碳的表面覆盖的高效粘结剂为增强钠离子电池高压正极材料的电化学性能开辟了新途径。
