高容量锂离子电池正极材料的研发

Commercial lithium-ion batteries (LIBs) built with Ni- and Co-based “intercalation-type” cathode materials currently suffer from low specific energy density, high cost, and toxicity to human beings and environment, which cannot meet the growing demands of energy storage in emerging areas, such as electric vehicles. It is crucial to develop novel cathode materials with high energy density, low cost, and environmental benignity. Compared with “intercalation-type” materials, “conversion-type” materials, such as metal fluorides and metal sulfides, generally possess higher theoretical capacities and energy densities. However, these materials face substantial challenges towards practical applications, including low intrinsic electrical conductivity, large volume changes during charge and discharge, dissolution of metals into electrolyte during cycling, unfavorable side reactions between active materials and electrolyte, absence of built-in lithium source etc., all of which result in very poor electrochemical performances, limiting the application of “conversion-type” materials. To solve these issues, this project proposes to use a comprehensive fabrication method, which covers doping, nanoengineering, and structure/stoichiometry manipulation etc., to prepare ternary composite materials containing porous carbon, built-in lithium sources, and metals. The electrochemical performance, electron and ion transport principle, and charge storage mechanism of these composite materials will be thoroughly investigated by various electroanalytical techniques and in situ methods. The goal of this project is to develop high performance “conversion-type” cathode materials for LIBs, which break the energy density limit of current LIBs and meet the urgent demands for high energy density energy storage devices in the fields of consuming electronics and electric vehicles.

商业化的锂离子电池主要采用含镍和钴的“嵌入型”正极材料，其能量密度低、成本高、对环境和人体有害等问题已无法满足我国电动汽车等新领域快速发展的需要，因此亟待研发高能量密度、价格低廉、环境友好的新型正极材料。与“嵌入型”材料相比，“转换型”材料，例如氟化物和硫化物等，具有较高的理论比容量和能量密度。目前，“转换型”材料主要存在的问题有：材料本征电导率低、充放电过程体积变化大、循环过程中材料和电解质发生副反应，材料本身不含锂等，限制了这类型材料的实际应用。针对这些问题，本项目提出综合运用结构调制、相复合、元素掺杂、纳米合成等技术制备预锂化的“多孔碳-锂源-金属”三元复合材料，并运用多种电化学方法和原位技术对材料的电子、离子传输机制和电化学储能机理进行深入研究，开发出高性能的“转换型”正极材料，打破现有锂离子电池能量密度的瓶颈，从而缓解消费电子、电动交通等领域对高能量密度储能器件的迫切需求。

基于转换型反应的锂离子电池电极材料受到广泛关注，该类材料资源丰富、对环境友好，适合大规模生产和应用。然而，其面临的主要问题包括：材料本征电导率低、充放电过程中体积变化大、材料和电解液发生副反应等，导致这类材料循环寿命短、动力学性能差、库仑效率偏低等，严重制约了其发展。针对以上问题，本项目以廉价的铁和铜基硫化物为研究对象，制备了两种高性能的转换型锂离子电池正极材料，并探究了其电化学储能机理。1）我们采用一步水热法制备了一种新型的白铁矿(M-FeS2)和黄铁矿(P-FeS2)共生的混合相P/M-FeS2。将M-FeS2引入到P-FeS2后，FeS2实现了四电子的氧化还原反应，提供894 mAh/g的储锂比容量。随后，采用铜(Cu)集流体对可溶性多硫化锂进行化学固定，从根本上改变了反应途径。与Fe相比，Li2S更倾向于与Cu集流体反应生成Cu2S，这得益于Cu2S与Li2S相似的晶格结构，因此反应可以通过Li+和Cu+简单的置换来完成，不需要经历晶格的破坏和重组，使得FeS2电极表现出优异的速率性能和长循环稳定性：在2 A g-1的电流密度下，比容量高达730 mAh g-1，并且循环3200个周期后，容量保持率高达89.7%。2）基于上述研究结果，本项目进一步制备了一种高活性、低成本的纳米级Li2S-Cu高性能复合正极材料来解决Li2S的高活化能垒、低倍率性能以及差循环稳定性等问题。实验表征结合第一性原理计算表明，金属Cu对Li2S解离的催化作用有效降低了Li2S的解离能垒及氧化电位。另外，Li2S在热力学和动力学上更倾向于与Cu发生置换反应，使氧化还原电对由Li2S/S转变为Cu/Cu2S，从根本上避免了可溶性多硫化锂的生成，使得复合材料获得优异的电化学性能。以Li2S-Cu复合材料为正极，商业化石墨为负极构建的全电池亦表现出良好的稳定性。