具有自激发热保护功能的锂离子电池用正温度系数电极材料

The safety issue has been a major obstacle retarding the commercial applications of high-capacity lithium ion batteries (LIBs) in electric vehicles and renewable power stations. It is now well recognized that thermal runaway, which is usually induced by abusive conditions such as electrical overcharge, internal or external short-circuit, or high thermal impact, is a major cause for the hazardous behaviors of LIBs. For building thermally stable and safer LIBs, this project is proposed to develop novel temperature-sensitive electrode materials with self-activating thermal protection mechanism by coating a thin layer of conducting polymer with the required positive-temperature-coefficient (PTC) effect on the surface of the conventional electrode-active particles. Since the outer PTC skin can sensitively respond to the temperature rise in the microenvironment of the electrode and transform promptly at risky temperature from an electrical conducting state to a highly resistive state with several orderers of magnitude increase in the electric resistance, all the electrochemical reactions taking place on the surface of the electrode-active particles would be shut down, so as to prevent the thermal runaway from happening, thus ensuring the safety of lithium ion batteries at dangerous thermal abuse. In order to realize this concept, this project is planned to selectively synthesize a number of electro-active polymers and to extensively investigate their PTC behaviors at a wide temperature region, as well as their temperature response dynamics, so as to develop target polymers with the required high conductivities at ambient temperature, good processability, strong PTC effect, and especially, appropriate PTC transition temperature. These properties ensure that, as an outer coating layer, these polymers not only can function as a thermal switch to block off the electrode reactions at elevated temerature, but also can act as conductive matrix to facilitate the normal charge-discherge reactions at ambient temperature. Subsequently, this work is planned to use these polymers as PTC coating layer to develop several typic temerature-sensitive electrode materials and to investigate the thermal stability and thermal protection actions of the resulting electrode materials in the battery environment. As a whole, the aim of this project is to provide technical and material supports for building safer LIBs and also to promote the applied development of the high capacity LIBs in new energy technologies.

安全性问题严重制约了大容量锂离子电池在电动汽车、储能电站等新能源技术领域的商业化应用。研究表明，热失控是导致电池发生不安全行为的根本原因。为此，项目提出采用具有正温度系数特征（PTC）的导电聚合物修饰常规电极材料表面，构建温度敏感电极材料，发展高安全性锂离子电池的新思路。由于温度敏感电极材料的表面聚合物PTC层能够实时、灵敏地感知电池内部的温度变化，并在敏感温度下迅速地从常规的导电态转变为绝缘态，自发地关闭发生在活性材料表面的所有电化学反应，因此，可以有效地防止电池热失控。项目拟在前期工作基础上，通过广泛研究各类电活性聚合物的PTC效应及温度响应动力学特征，设计合成出具有合适阻变温度（即居里温度）的聚合物PTC材料。在此基础上，制备温度敏感电极材料，并考察其在实时电池环境中的热稳定性及过热保护功效。项目成果可为发展高安全性锂离子电池提供技术原理和基础材料的支持，具有重大的理论和应用意义。

针对锂离子电池的安全性问题，项目提出利用导电聚合物PTC材料，发展温度敏感性电极，防止电池热失控的技术思路。为此，项目开展了以下四个方面的研究工作：（1）、导电聚合物PTC材料的设计与合成。设计合成了系列电活性聚合物分子，通过系统考察它们的温度敏感特性及电化学反应行为，筛选出五种具有良好PTC响应行为的导电聚合物材料，包括聚苯胺（PAni）、聚3-辛基噻吩（P3OT）、聚3-癸基噻吩（P3DT）、聚吡咯、聚3-己基吡咯（P3HPy）。研究证实，它们的转变温度为90-120˚C、升阻比达到3个数量级，并具有在电解液中不溶和电化学掺杂-脱杂可逆性高等特性，非常适合用作电极材料的表面涂层，构筑温度敏感电极；（2）、导电聚合物PTC材料结构及其性能的关联性研究。研究发现，烷基取代基的链越长，聚合物的熔点越低，可溶性增加；而取代烷基链的链长越长，聚合物的PTC转变温度越低。可能的原因是：烷基链的热运动随链长增长而加剧，使得聚合物的阻变温度降低。热响应机制的研究结果表明：导电聚合物的PTC行为来自于其高温下的热脱杂，研究结果为优化设计聚合物PTC材料的结构提供了理论指导；（3）、开展了温度敏感电极材料的制备技术研究。重点选用聚苯胺（PAni）和聚3，4-乙烯二氧噻吩（PEDOT）为PTC材料、商品化LiCoO2和石墨为电极活性物质，分别采用搅拌包覆、喷雾干燥和现场电氧化聚合三种方式，制备出LiCoO2@PAni、石墨@PEDOT:PSS和LiCoO2@PEDOT三种温度敏感电极材料，优化了其制备技术；（4）、考察了温度敏感电极材料在实验扣式电池和实际软包电池中的电化学性能以及温度敏感特性。研究结果显示，三种复合材料不仅在110-120 ˚C的高温下展现出良好的自激发热关闭功能，有效提高电池在滥用条件下的安全性，而且对电池的常规性能不产生任何不利影响，展示出良好的应用前景。