锂硫电池用氧化物/硫复合正极构筑及电化学性能研究

Since the 1990s, Lithium-ion batteries (LIBs) have dominated the market of energy storage of portable electronic devices and inspired practical application in electric vehicles (EVs) or hybrid electric vehicles (HEVs). However, the low specific capacity of the cathode used in the commercial products, impedes their further development. Lithium sulfur (Li-S) batteries have shown promise as the successor to the highly successful lithium-ion batteries because sulfur is a high capacity, low cost and environmentally benign cathode which can support a very high theoretical capacity (~1675 Ah kg-1) and energy density (~2600 Wh kg-1). Two main issues however have significantly undermined the practicality of Li-S batteries: 1) the low electronic conductivity of sulfur and Li2S, 2) and sulfur dissolution (as polysulfides) during battery operation, which permeated across the separator, reacted with the lithium anode, and deposited on the surface, thus resulted in loss of active materials. This project will design and synthesize the metal oxide/S composites considering the good conductivity of the former and the interaction between the transition metal atom on the surface and the polysulfide ion, in order to improve the performance of the S cathode. The use of in situ XRD, TEM, CV, and combining with density functional theory (DFT) are expected to reveal the interaction between different metal atoms and polysulfide ion and its influence into the electrochemical behaviors. Finally we will summarize the regularity and aim to make such metal oxide/S composites to be applied to the high performance Li-S batteries.

自上世纪90年代以来，锂离子电池在便携电子设备能量存储中占据了重要的位置，并应用于电动或者混合动力汽车中。商业化锂电池中正极材料的低容量制约了进一步的发展。锂硫电池因具有高的理论容量（1672 mA h g-1）和能量密度（2600 W h kg-1）有望成为下一代二次电池。但其目前表现出来的问题也阻碍了商业化发展：（1）无论是荷电态单质硫还是放电态硫化锂都是绝缘体，对传递电荷造成较大困扰；（2）中间产物聚硫化物易溶解在电解质中，造成活性物质损失和能量损耗。本项目拟通过对氧化物/S复合正极的构筑，利用氧化物优良的导电性及表面金属原子对聚硫离子的吸附作用，实现对锂硫电池用正极材料的改进；并通过利用原位XRD 、TEM等表征手段，结合密度泛函理论和第一性原理等理论计算揭示不同金属原子与聚硫离子的键合作用对锂硫电池性能的影响，总结规律，最终使氧化物/硫复合正极应用到高性能锂硫电池当中。

锂硫电池因其较高的理论容量（1672 mAh g-1）和能量密度（2600 Wh kg-1）被认为是最有可能取代锂离子电池的下一代二次电池。但多硫化锂的穿梭效应和后续Li2S沉积过程中的迟滞动力学，也降低了锂硫电池的容量、倍率性能和循环寿命。针对上述问题，本研究分别设计开发了超薄TiO2二维结构、Mn3(PO4)2片状结构与石墨烯的复合结构，以及生物质炭作为正极S的载体，系统考察了构筑的复合S正极的电化学特性，围绕高性能锂硫电池正极载体材料的设计和开发，提出了几种提高锂硫电池正极材料电化学活性的方案。1、采用TiO2纳米薄片和石墨烯为基体的S@TiO2@r-GO复合材料，TiO2纳米薄片抑制多硫化物溶解，同时还原氧化石墨烯(r-GO)薄片基体为电解质和锂离子扩散提供额外的电子传导途径。通过验证TiO2和r-GO片的协同效应，证实他们可以抑制多硫化物的溶解并且增强电池内部的导电率。2、采用花生壳为原料制备的生物分级多孔炭作为载体实现对锂硫电池导电性差、多硫溶解以及体积膨胀等问题的解决。实验证明，花生壳为原料制备的活性炭价格低廉、较大的比表面积，而且对多硫离子具有优异的吸附性能，从而提高了这种C/S复合正极材料的电化学性能。由三维分级生物炭由丰富的中/微孔组成，能显著提高电子导电性，有效抑制多硫化物的溶解。3、选用Mn3(PO4)2纳米片作为多硫离子转化的催化剂，结合石墨烯较强的电子传导性，为此我们设计了Mn3(PO4)2/rGO的复合材料作为S的载体来解决正极所面临的问题，这种“面-面”复合结构可以加速电子的传导，实现锂硫电池中S正极性能的提升。