锰离子破坏锂离子电池硅基负极的过程机制及应对措施研究

Silicon is the most important negative electrode material with high energy densityfor lithium ion batteries. Besides a well-known problem of dissolving divalent manganese ions in manganese based positive materials, a new phenomenon of the Coulombic efficiency and capacity of the silicon negative electrode reduced by the divalent manganese ions is found. A novel silicon based composite material is prepared based on exploring the failure mechanism of manganese ions on the electrochemical performance of silicon negative electrode and searching 2 to 3 ways to slow down the destruction of manganese ions. By the advanced in situ detection technologies, the microstructure, chemical composition and evolution of electrochemical properties of silicon anode during battery cycles is intuitively characterized, and the deposition path of manganese ions on silicon particles surface and SEI film of silicon anode more easily broken and thicken by manganese ions are revealed. Finally, the fundamental reason of decreasing of Coulombic efficiency and capacity recession of silicon electrode is explained. A novel functional structured silicon material with hindered deposition of manganese ions, high Coulombic efficiency and high volume energy density is prepared, where a stable electrochemical reaction interface is kept and the Coulombic efficiency is improved by the hollow structure; volume energy density is raised by the filled stannic an hydridein cavity; the manganese ion deposition and the excessive growth of SEI film is hindered by the filled conductive polymer electrolyte in cavity.

硅是锂离子电池最重要的高能量密度负极材料。锰基正极材料都存在着二价锰离子溶解的问题,而我们发现“二价锰离子可使硅负极库伦效率及容量降低”新现象。本项目以硅基负极为研究对象，探究锰离子对硅负极电化学性能的破坏机制；找到2-3种调控方法减缓锰离子的破坏作用，设计合成一种新的硅基复合材料。本项目采用先进的原位检测技术，直观表征硅负极在循环过程的微观结构、化学组分、电化学性能的演变过程，揭示锰离子在硅基负极颗粒表面的沉积路径，揭示锰离子导致硅负极SEI膜变得更容易破碎和增厚,从而导致硅负极库伦效率降低和容量衰退的根本原因。为硅基材料设计一种功能型结构，并制备出具有阻碍锰离子沉积、高库伦效率、高体积能量密度等功能的新硅基复合材料：(1)采用中空结构稳定电化学反应界面，提高库伦效率；(2)空腔填充二氧化锡，提高电极的体积能量密度；(3)空腔填充导电聚合物电解质，阻碍锰离子沉积，抑制SEI膜过度生长。

硅具有高比容量，表面不会析出锂金属，具有低成本大规模生产的潜力，是下一代锂离子电池高容量负极。硅负极锰系锂离子电池容量衰减的过程机制：溶解锰离子使硅负极SEI 膜变得脆弱，SEI生长量更多，减少了活性锂离子是导致硅负极电化学容量衰减的主要机制。在LMO/Si全电池循环次数试验中，经过16、26、36和50次循环后，硅阳极的总质量增长（SEI）分别为0.94mg、1.13mg、1.40mg和1.52mg。相应的锰离子总量分别为39.67ug、42.79ug、51.22ug和53.48ug。而在静置130h后，Mn2+在电极中的质量仅为4.39ug，电化学循环是Mn2+溶解的主要原因。抑制正极材料锰离子溶解的方法:表面中形成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，放电容量可高出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。NCM应用研究：高截止电压获得更高的比容量。在相同截止电压（4.4V）下，薄电极比容量高10%。FEC有助于石墨半电池的循环稳定性和初始容量的发挥，但不同比例的FEC添加也会对循环性能有一定的影响。