锰离子破坏锂电池负极微观结构及性能的过程机制研究

Capacity fading is the bottleneck question limit the large-scale application of lithium manganese oxide batteries. The existing viewpoints think that manganese ions reduced to metal manganese and deposit on the surface of anode, which catalyzes decomposition of electrolyte, poisons solid electrolyte interface (SEI) film, results in capacity fading of lithium ion batteries. However, latest researches show that manganese ions immerges to the interior of anode particles, and the +2 valent manganese ion deposited on the surface of anode, which has not been reduced to metallic manganese. This project will study the following issues: path and process of manganese ions immerge to interior of anode particles; Mechanism of capacity fading and microstructure failure process in anode caused by deposition of manganese ions; find 2-3 methods to extend the service life or improve the high temperature performance of manganese lithium ion batteries. The advanced in situ detection technologies will be adopted in this project, which can intuitive characterize the microstructure, chemical composition, stress and strain, and evolution of electrochemical properties of anode during battery cycles. The project firstly explores the deposition mechanisms and kinetics of manganese ion immerge to interior of anode particles, and tries to reveal the new mechanism that the microstructure failure of anode caused by electrochemical and mechanical coupling damage of manganese ion, result in the capacity fading of lithium manganese oxide batteries. Try to improve cycle life or high temperature performance of manganese lithium ion batteries by restrain the immersion of transition metal ions and construct self-adaptive strain micro structures of anode.

锰酸锂电池容量衰减是限制其大规模应用的瓶颈问题。现有观点认为锰离子沉积在负极表面上，还原成金属锰，催化电解质分解、毒化固体电解质界面（SEI）膜，造成了锰系锂电池的容量衰减。最新研究表明锰离子浸入了负极颗粒材料内部、沉积在表面的二价锰并没有被还原成金属锰。本项目研究锰离子浸入负极颗粒内部的路径及过程；研究锰离子对负极的微观结构破坏过程及性能失效机制；找到2-3种延长锰系锂电池的使用寿命或提升高温性能的调控新方法。本项目采用先进的原位检测技术，直观表征负极材料在充放电循环过程的微观结构、应力应变、化学组分、电化学性能的演变过程。本项目将首次探索“过渡金属锰离子浸入负极颗粒内部的沉积机制和动力学”,并试图揭示“锰离子在电化学和力学耦合作用下，破坏负极微观结构，导致电池性能衰减”的过程机制，尝试通过抑制过渡金属离子浸入、构筑应变自适应负极等新方法，大幅提高锰系锂离子电池的使用寿命或高温性能。

锰基氧化物是锂离子电池最重要的正极材料，但其溶解的锰离子会对电池系统，尤其是负极，造成较大的破坏作用。本项目研究溶解锰离子沉积在负极颗粒表面的过程、及对负极破坏作用机制；找到了3种延长锰系锂电池使用寿命的调控新方法。锰沉积导致锰酸锂/石墨体系容量衰减的过程机制：Mn (II)与SEI中的活性锂离子进行离子交换，沉积在阳极上，堵塞了锂离子进出通道，导致容量衰减。锰沉积导致锰酸锂/硅体系容量衰减的过程机制：溶解锰离子会导致硅负极的容量衰减，锰离子会使硅负极SEI 膜变得脆弱，SEI生长量更多。发现 “二价锰离子可使硅负极的库伦效率降低、电化学容量减少”新现象，全电池库伦效率只有32%，而半电池库伦效率超过97%，活性锂离子源消耗过快也是全电池容量快速衰减的一个重要原因。抑制正极材料锰离子溶解的方法: 在尖晶石LiMn2O4表面中形成仅仅几纳米厚的阳离子掺杂LiMn2-xTixO4表面层，减少锰的溶解。浸泡15天后，LMO(锰酸锂)电解液的锰离子浓度为360µg/mL，LMO@TiO2/ (表面掺杂包覆)的锰离子浓度为160µg/mL。在55 ℃环境下0.1 C充放电循环，70次循环后LMO@TiO2的放电容量为53 mAh/g，比LMO的高出35%。NCM811/石墨体系电池性能的提升方法：B2O3作为优异的正极材料包覆层，减少过渡金属Mn, Ni等的溶解。在基础电解液中添加10wt% FEC保护石墨阳极SEI膜。NCM811采用2wt%B2O3表面包覆，电解液添加10wt% FEC， NCM811/石墨体系全电池100次循环后容量可达150mAh/g，其循环性能提升了50%。原位生长PANI改善硅负极循环性能：采用纳米硅颗粒表面原位生长导电高分子(聚苯胺PANi）制备硅负极，半电池以300 mA/g电流充放电100次之后, 电化学容量依然能保持在1000 mAh/g。