锂离子电池单晶薄膜层状正极材料表面包覆层作用的研究

LiCoO2 is a representative of the layered cathode materials including Li[Ni1-x-yCoxMny]O2 and Li[Ni1-x-yCoxAly]O2, which are dominated in the commercial market of lithium ion batteries. To coat electro-active particles is one of the best ways for improving the energy density and cyclic lifetime. However, the role of the coating layer has not been well understood due to the limitation of the following three issues. Firstly, the powders of the electro-active materials are polymorphs, and the surface facets might be different which lead to the variation in the electrochemical performances and the corresponding analysis results. Secondly, porous electrode composed of the conducting additive and binder renders the electrode interface very complex. Thirdly, the interface reaction in liquid electrolyte system is very complicated including the concurrence of chemical reaction and electrochemical reaction and phase transition. Herein, single crystal thin films are applied as working electrodes. We will investigate the effect of the crystal orientation on the electrochemical performance of the thin film electrodes, and then make sure if the effect of the coating is sensitive to the crystal orientation of the thin film electrodes. Following that, high throughput method is adopted to fabricate and test the coating layer so as to clarify the roles of its composition, thickness, and crystallinity. Finally, the performance in liquid electrolyte system and solid electrolyte system is to be compared. As a result, the correlation among the electro-active material, the coating layer, and the electrolyte will be outlined for constructing a model of electrochemical interface, which will accelerate the development of the coating technique in the field of electrochemical energy storage.

LiCoO2为代表的层状正极材料（包括Li[Ni1-x-yCoxMny]O2和Li[Ni1-x-yCoxAly]O2）是当前商业市场的主流，通过对活性物质颗粒进行包覆是提高其能量密度和循环性能的重要手段之一。目前对包覆改性的机理仍然缺乏深入认识。其原因有三，一是粉体颗粒一般呈现多晶结构，暴露晶面的比例具有不可确定性，导致性能与机理研究均有一定的偏差；二是多孔电极含有导电添加剂、粘结剂等，导致电极界面异常复杂；三是液体电解质体系中化学/电化学反应和相变过程同时发生，界面反应复杂。本项目采用单晶薄膜作为电极，首先研究不同取向薄膜的电化学行为，然后确认包覆作用对晶面取向的选择性，随后采用高通量法制备包覆层和高通量测试，快速确定包覆层成分、厚度和结晶度的影响，最后比较在液体电解质和固体电解质两个体系中的异同，理解活性材料-包覆层-电解质三者之间的关系，构建电极界面模型，指导包覆的发展方向。

LiCoO2由于压实密度高、工作电压高、理论比容量高是高能量密度正极材料的首选。在截止电压为4.2V时其实际比容量仅理论值的一半，通过提高截止电压是释放更高容量的理想手段。针对层状正极在高电压下循环性能差、容量衰减快的困境，本项目系统的研究了包覆方法、包覆材料和正极材料的形态对高电压下电化学性能的影响。首先通过优化磁控溅射条件，优化衬底种类、溅射气压、氩氧比、原位退火温度、沉积时间、靶材种类等因素，实现了（003）取向占优的高结晶性LiCoO2准单晶薄膜的制备；然后，溅射LiF薄膜层或者Al2O3包覆层，提高了截止电压在4.5、4.6及4.7V的电化学性能；最后采用高低温顺序溅射法，制备了Al2O3双层膜，并发现在高温条件形成了LiAlO2和掺Al的LiCoO2层，实现了表面掺杂和表面包覆层共存的多层修饰层。在高电压条件下，无论在液态电解质还是固态电解质中均体现了较好性能。在此基础上，将磁控溅射包覆法应用于多孔复合LiCoO2电极片，并同时引入高通量磁控溅射的概念，快速筛选出合适的包覆条件和包覆材料的种类，发现AZO是一种较理想的包覆介质。为了探索包覆层的工作机理，研究了液相法，包括水溶液及低温熔融液体包覆LiCoO2正极粉体材料，然后高温退火处理获得氧化物保护膜，所得材料在高电压下的电化学性能改善明显。证实了表面掺杂与表面包覆相结合的思路是将LiCoO2正极工作电压提高到4.5V以上的关键，在固态电池体系中的衰减速度要弱于在液态电解液体系，该研究成果为后续高性能高电压正极材料的修饰改性指明了方向。