织造碳纤维负载硫化锂复合正极的原位构筑及其循环稳定机制的研究

Unlike elemental sulfur, lithium sulfide (Li2S) is in a fully-lithiated state and can be paired with lithium metal-free anodes, such as silicon or tin, thus obviating the safety concerns and dendrite formation associated with lithium metal. However, there are several significant issues that currently limit the utilization of Li2S. Li2S is expensive and facile synthesis routes are not yet readily available. Additionally, problems related to the insulating nature and polysulfides shuttling still exist as in elemental sulfur cathodes. Furthermore, Li2S is highly moisture- and oxygen-sensitive, necessitating a controlled atmosphere whenever it is handled. To solve these shortcomings, herein, we propose a new method for one-pot synthesis of the textile carbon fibers supported Li2S (TCFs-Li2S) composite cathode through direct pyrolysis of a mixture of textile carbon fibers and lithium sulfate. In this process, the textile carbon fiber matrixes are oxidized and etched in situ by lithium sulfate, meanwhile, the generated Li2S is uniformly distributed in the etched textile carbon fiber matrixes, and finally forms the TCFs-Li2S composite cathode. Then, by theoretical analysis and experimental exploration of the mechanism for the in-situ synthesis of TCFs-Li2S composite cathodes, an effective process is developed to regulate and control the properties and structures of the TCFs-Li2S composite cathode. Based on these progresses, an in-situ visual-electrochemical test in combination with theoretical calculation will be conducted to further study the mechanism of ion diffusion and electrochemical reaction belonging to the TCFs-Li2S composite cathode, and then clarify the involved correlations among the microstructures, ion diffusion behaviors, properties of interfaces and electrochemical performance. In addition, the kinetics of the ion diffusion, the mechanism for the capacity fading and cycle stability of TCFs-Li2S composite cathodes will be addressed. It is expected that the researches in this project will provide both new ideas and theoretical guidance for the resolution of the problems of Li2S and Li2S-based cathodes, and promotion of the research and development of high stability and light weigh lithium-sulfur batteries.

采用硫化锂（Li2S）作为锂硫电池的正极，负极就能够使用硅或锡等代替金属锂，可彻底解决金属锂带来的安全隐患和不稳定问题。但价格高、难制备、导电性差以及在空气中不稳定等因素，严重阻碍Li2S的实际应用。本项目提出一种直接制备Li2S电极的新方法，拟采用硫酸锂在惰性气氛中高温下氧化织造碳纤维载体，刻蚀织造碳纤维的同时将还原生成的Li2S原位负载在其中，一步制得织造碳纤维-Li2S复合电极。通过揭示其原位构筑的机制和织造碳纤维载体同步刻蚀的机理，实现织造碳纤维-Li2S复合电极的可控制备和结构调控。在此基础上，再通过深入研究织造碳纤维-Li2S复合电极中的离子扩散机制和电化学反应本质，揭示其中离子扩散的动力学，阐明其微观结构、界面特性、离子扩散规律与电化学极化、电化学性能之间的关系及调控机制，明确其电化学性能的衰减机理和循环稳定机制，为高稳定性、轻量化锂硫电池的研究提供新的思路和理论指导。

常规锂硫电池通常由硫正极和锂金属负极构成，在充放电过程中，锂负极上不可控的锂枝晶生长将会导致短路和安全问题。硫化锂（Li2S）是完全锂化的硫，可与石墨、硅或锡等非锂负极匹配，从而避免金属锂造成的内部短路和安全隐患，因此，硫化锂是一种非常有前途的锂硫电池正极材料。然而，与单质硫类似，Li2S仍存在低电子导电性和多硫化物中间体高溶解性的问题，再加上成本高、制备和加工困难等问题严重阻碍了其在锂硫电池中的广泛应用。本研究提出一种新颖、低成本且可批量直接制备锂硫电池Li2S电极的方法。该方法克服了传统的手套箱内制备Li2S电极的缺点，直接将原位生成的超细Li2S纳米粒子（5–10 纳米）均匀地生长在织造碳纤维（WCF）基底上，形成一种空间导电网络结构的Li2S/WCF和Li2S-FC/WCF复合电极。由于氟（F）掺杂碳对锂硫电池的电化学氧化还原反应具有一定的催化作用，因此，与Li2S/WCF复合电极相比，Li2S-FC/WCF复合电极的循环伏安曲线具有较低的氧化起始电位和更高的还原起始电位，在0.2C下，其第二次循环的放电比容量为573 mA h g−1，在1C下经过200次循环后，其放电比容量仍能保持75%，在整个循环过程中，其库仑效率高达98%。此外，为了进一步提高Li2S-FC/WCF电极的导电性、抑制多硫化锂中间产物的溶解，将Li2S纳米颗粒封装在氮/氟（N/F）共掺杂的碳骨架中，直接制备出具有立体结构的Li2S-纳米晶体@掺杂的碳/WCF（Li2S@DC/WCF）复合电极，N/F共掺杂碳对锂硫电池的电化学氧化还原反应具有一定的催化作用，此外，密度泛函理论计算的结果发现含N基团与Li2S/多硫化锂之间产生强烈的化学作用，最终有效改善了电化学反应动力学并抑制了多硫化物的穿梭。Li2S@DC/WCF复合电极在0.2C下表现出913 mA h g−1的高放电比容量，即使在1C下经过200次循环后，其放电比容量仍保持98%，相当于平均每次循环容量衰减为0.01％。综上所述，本项目为解决目前硫化锂以及硫化锂电极存在的问题和面临的挑战提供新的研究思路和科学依据，为高稳定性锂硫电池的研究和发展提供实验和理论指导。