水系离子电池(ZIB)因其具有清洁环保、生产成本低廉和高安全特性而受到广泛关注,被认为是一种极具潜力的储能设备。然而,AZIB的实际应用则是与该体系的能量密度相关,这在很大程度上取决于正极材料的优劣。目前针对阴极材料的改性策略包括缺陷改性、掺杂改性、表面包覆、复合结构等。
近年来,国内在水系电池的正极材料领域取得了显著进展。其中,基于普鲁士蓝正极材料的结构不稳定等特点,提出了一系列的改性策略,例如添加辅助剂、表面涂层保护技术、电解液改性等。Qin[20]等提出用柠檬酸钠调节成核和晶体生长速率,制备出高结晶度的 Na2Fe4[Fe(CN)6]。Wang[21]等人利用表面活性剂(SDS) 和调节总反应时间,来控制晶体表面的生长速率,还研究了蚀刻条件和蚀刻剂浓度对晶体形态的影响,并通过调整蚀刻剂的量、反应温度和时间实现了PBAs的合理组成和纳米结构设计。经过20小时的长期稳定性测试后,PBA-III-700仍保持95.8%的电流保持率。且与传统电极Pt/C 相比,在2000 次循环后几乎没有衰减。Liu[22]等实现聚苯胺(PANI)涂层应用于ZnHCF。PANI涂层能够抑制ZnHCF的溶解,并提供了150 mAh g-1的高放电容量,350次循环后保留率为75%。Zhang[23]等人使用一种新的球形ZnHCF作为阴极材料,展示了一种光纤柔性可充电AZIB,具有出色的电化学性能。Chen[24]等人开发了一种2.4 V高压柔性含水Zn/ZnHCF电池,该电池具有高倍率性能和出色的电化学性能。实现了120 Wh kg-1的高能量密度和37000 Wh kg-1的高功率密度,在2.5 C的电流下,经过260次循环,容量保持率仍在80%。Lu[25]等还构建了一种新型的可充电Na-Zn混合电池,以NiHCF为阴极,纳米结构锌为阳极。在放电/充电过程中含Na+的电解质与少量 ZnSO4+的掺入有利于改善插层过程和电极的可逆性,使水性电池的配置具有高比容量、高效率和可接受的循环稳定性(1000 次循环后容量保持率超过81%)。Ma[26]等人合成了一种高晶体单斜 NiHCF 表现出高比容量和优异的循环和倍率性能。组装的 AZSHB 表现出优异的电化学性能,平均工作电压高达 1.464 V,能量密度高达 99.1 Wh kg-1,循环稳定性好,1000 次循环后容量保持率为 91%。可以看出,国内的研究主要集中在水系锌离子正极材料的改进与创新研发上。
国外在水系离子电池的研究起步较早,形成了较为完善的理论体系和研究方法。比如V. D. Neff[27]于1978年报道了PB的电化学活性以来,PB和PBAs的电致变色性质在电催化中,在电位和电流传感器中,以及最近在电化学储能系统中作为电极材料得到了广泛研究。Renman[28]及其合作者对CuHCF结构中Zn2+的插入进行了详细研究,使用了同步X射线衍射。他们证实Zn2+占据了CuHCF立方晶胞内的Fe (CN)6空位和空腔,并且观察到隧道和空位Fe (CN)6之间的Zn2+交换。D.Kim[29]等人报告称,对于从1 mol dm-3和 3 mol dm-3Zn(NO3 )2 获得的电极,在浓电解液中的可逆充放电容量是稀电解液的两倍多,锌离子与水分子的摩尔比分别为1:50和1:13。这是浓缩电解质中锌离子的水合数和半径降低的结果。水合数较小的锌离子可以更大程度地嵌入和从 ZnHCF 电极中取出,这会导致在浓电解液中的充放电容量增加。Kjeldgaard[30]等采用不同的氧化剂,通过改变温度、流速等条件得到12个样品,在合成过程中, 通过使用柠檬酸螯合物可以降低成 核的速度,从而获得较少的空位和较高的钾含量。Trocoli[31]等人在锌基电解液中引入了钠盐,由于优先插入了钠盐而不是Zn2+,有效地抑制了电池的容量衰减。Silva[32]等人通过 X 射线衍射、扫描电子显微镜、Mossbaüer、拉曼光谱和红外光谱对所制备产物的晶体结构、形貌和微观结构进行了表征。结果表明,ZnHCF/多壁碳纳米管(MWCNTs)扩展了 ZnHCF 的电化学性质,并影响了六氰基铁酸盐颗粒的结构和形态。如果作为锌离子电池的正极材料进行测试,纳米复合薄膜的容量为 25.81 mA h g-1,远高于 ZnHCF 单组分薄膜的容量 (3.28 mA h g-1)。ZnHCF/MWCNT 薄膜实现的物理特性和特性允许对高电位差具有良好的可逆性,这表明半透明、柔性和轻型储能器件的制造有了新的发展,这些器件可以作为当前现有技术的低成本、更安全和环保的替代品。可以看出使用螯合剂、表面活性剂,覆盖涂层,改变阴极材料形态和结构等策略能更好的发挥出阴极材料的电化学性能。此外还可以对电解液进行浓度,引入其他离子等调试,致使阴极材料发挥出更佳的电化学性能。
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