嵌段共聚物协同自组装构筑高比能有序介孔金属磷化物负极材料

In virtue of their high capacity, low polarization and suitable working potential, metal phosphide anode materials such as CoP, Sn4P3 are holding great promise for building high energy density lithium-ion batteries. Nevertheless, the poor cycling and rate performances of metal phosphides caused by the huge volume change during charge-discharge and slow ion diffusion kinetics have severely hindered their practical use. In this program, we will synthesize ordered mesoporous metal phosphides (CoP, Sn4P3) based on the solvent evaporation induced co-assembly of PEO-b-PS and Co/Sn-containing precursors through a preformation- carbonization- phosphorization route. The PS core of the micelles can be in situ converted into rigid carbon skeletons with high yields and act as hard support to well preserve the parental ordered mesostructure upon phosphorization reaction with hypophosphite. Further, we will adjust the porosity of the prepared mesoporous metal phosphides by tuning the chain length of the hydrophobic PS blocks of PEO-b-PS. The introduce of mesoporous structure could significantly alleviate the mechanical stress generated by the volume change, and hence maintain the structure and solid electrical contact. The open and interconnected ordered mesopores and thin pore walls are highly favorable for the rapid transport of ions and electrons during electrode reactions. Meanwhile, the electrode wettability and the utilization of the active material will be remarkably enhanced. This work will pave a new way for fabricating various ordered mesoporous metal phosphides, and disclose the correlations between their mesostructural dimensions and electrochemical performances, thereby providing new idea and guidance for the future design of high-performance electrode materials.

CoP、Sn4P3等金属磷化物负极具有比容量高、极化小及低电位适度等优势，极具潜力发展高能量密度的锂离子电池，但其充放电时巨大的体积变化和较低的离子扩散速率导致循环及倍率性能较差。本项目拟利用溶剂挥发诱导聚环氧乙烷-b-聚苯乙烯(PEO-b-PS)与含Co或Sn前驱体的协同自组装，再借助两嵌段选择性碳化，得到有序碳骨架支撑金属氧化物，继而以次磷酸盐为磷源制备有序介孔CoP和Sn4P3，并通过改变疏水PS链段的长度，调节有序介孔金属磷化物的孔结构。介孔结构有助于缓释金属磷化物体积变化产生的应力；有序排列的开放孔道网络及薄的孔壁，则促进电极反应过程中电子传导和离子传输，同时提升电极的浸润性和活性物质的利用率。本项目建立预组装、再碳化、后磷化的介孔材料合成策略，揭示不同储锂类型磷化物的介孔结构与电化学性能之间的构效关系及其调控规律，为发展比容量高、循环及倍率性能优异的电池材料提供新的思路和依据。

鉴于金属磷化物负极比容量高、极化小、工作电压适度低，有望用于发展高比能锂离子电池，但其充放电时体积变化巨大且离子扩散速率较低导致循环及倍率性能较差，本项目首先以两亲性嵌段共聚物PEO-b-PS为软模板剂，利用溶剂挥发诱导其与含钴前驱体协同自组装，再借助嵌段选择性碳化玻璃化温度高且富含sp2杂化碳原子（苯环）的PS链段，得到有序介孔碳骨架支撑金属氧化物；继而以“次亚磷酸盐原位分解法”磷化得到较为有序的介孔碳负载磷化钴复合电极材料（CoP/Co2P@MC）。作为锂离子电池负极，CoP/Co2P@MC展现出较高的可逆比容量、改善的循环稳定性和增强的倍率性能，显示出一定的应用潜能。其次，通过改变嵌段共聚物PEO-b-PS疏水链段（PS）的链段数调节有序介孔结构的孔性。通过可逆加成-断裂链转移聚合(RAFT)法自制更高分子量的PEO114-b-PS250，以市售的较低分子量的PEO114-b-PS180为参照组，对比研究不同PS链段数的嵌段共聚物制得的有序介孔碳负载磷化钴纳米晶的孔结构。最后，在前期建立的“前驱体预组装-碳化-磷化”策略的基础之上，初步制备了有序介孔碳负载磷化锡/二氧化锡复合材料（Sn4P3/SnO2@MC）并对表征其结构，一定程度上印证了本项目提出的合成方法的可拓展性，为设计有序介孔碳结构复合金属 化合物应用于高能量密度的电化学储能器件提供了思路。