ZEBRA电池正极三维导电网络的构建及其电化学性能研究

The sodium nickel chloride battery (ZEBRA) has many merits,such as high voltage, high energy density,maintenance free operation and zero self-discharging,and is a safe battery system with on concerns of burning and explosion. The safety features of this storage technology make Na/NiCl2 batteries particularly suitable for electric and hybrid vehicles. The can also be used for stationary applications in support to renewable energy for load-leveling and emergency power supply. However, the formation of low-conducting NiCl2 and the grain growth of Ni and NaCl particles which is caused by the dissolution and recrystallisation of NiCl2 and NaCl in NaAlCl4 melt result in low energy density,low power density and capacity fade with time. In this project, Ni nanowires coated by carbon nano-tubes (Ni@CNT) are proposed as the active material instead of the nickel particles, which are beneficial to built a stable three-dimensional conductive network due to the stability of carbon in NaAlCl4 melt and no electrochemical reaction with other electrode materials. Moreover, the grain growth of Ni and NaCl particles and volume expansion during the charge-discharge process can be depressed significantly by the restriction effect of CNT channel, resulting in the improved performance of Na/NiCl2 batteries. The effects of aspect ratio of Ni nanowires, the wall thickness of CNT, the component and structure of positive electrode on the cell performance will be carried out systematically and the capacity degradation mechanisium will be focused in this project. It’s expected that the results of this project is helpful to the design and performance enhancement of Na/NiCl2 batteries and promotion of the practical application of this battery.

ZEBRA 电池具有开路电压高、比能量高、免维护、无自放电等优点，是为数不多的没有燃烧和爆炸危险的安全电池体系，广泛应用于电动汽车、可再生能源存储、应急电源等领域。然而，由于NiCl2的电绝缘性，及其与NaCl在NaAlCl4中的溶解析出造成晶粒长大，从而引起Ni的利用率低，电池的实际比能量低，倍率性能差、循环稳定性差等问题。本项目提出用表面包覆碳纳米管的Ni纳米线取代活性Ni粉作为正极活性物质以提高电池的性能。由于CNT不参与电化学反应，并在NaAlCl4熔盐中稳定，因此，可在电极中构建稳定的三维导电网络；同时，利用碳纳米管的孔道抑制Ni、NaCl颗粒的过分长大及充放电过程中的体积效应，进而提高电池的倍率性能和循环稳定性。项目将系统研究Ni纳米线的长径比、CNT的壁厚、正极的组成及微结构等因素对电池性能的影响，建立电池性能退化机制，为ZEBRA电池性能的改进提供新的思路和奠定理论基础。

ZEBRA电池具有开路电压高、比能量高、免维护、无自放电等优点，是为数不多的没有燃烧和爆炸危险的安全电池体系，可广泛应用于电动汽车、可再生能源存储、应急电源等领域。本项目针对ZEBRA电池正极结构不稳定，充电产物NiCl2不导电，造成电池内阻随充电深度的增加而增加，进而引起活性物质利用率低、电池倍率性能和循环稳定性差等问题，通过构建稳定的三维导电网络和低极化的负极/电解质界面，有效提升了ZEBRA电池的电化学性能，并对其性能衰减机制进行了深入分析，发现了有别于传统认知的新机制。系统研究了碳纳米管包覆Ni 纳米线（Ni@CNT）的可控制备技术，并建立了两种在Na/NiCl2电池复合正极中构建三维导电网络的新方法：一是采用Ni@CNT取代Ni颗粒，作为正极活性物质，利用Ni纳米线的交织作用，构建三维长程导电网络，增加电化学反应活性位点，从而提高电池的循环稳定性；二是弱化镍的导电功能，构建电化学惰性、轻质的碳材料（碳纳米管和碳纤维）三维交联导电网络，Ni颗粒仅作为正极活性物质分散在碳导电网络中，不仅降低了复合正极中镍的用量，而且有效提升了电池的比能量与循环稳定性。采用多孔导电涂层对beta-Al2O3电解质表面进行修饰，有效解决了熔融金属钠在beta-Al2O3电解质陶瓷管表面难以均匀润湿进而造成负极/电解质界面极化大的问题，界面阻抗降低了三个数量级，电池经历长时间的充放电循环后，界面层依然保持稳定。对复合正极在循环过程中显微结构的演变研究发现，Ni颗粒在长期循环过程中以及高温条件下会自发长大，导致其活性降低，进而造成电池容量衰减，提供了不同于传统认知的新视角，为Na/NiCl2电池性能的改进提升提供了新的方向。