聚环氧乙烷和聚碳酸丙烯酯共混聚合物电解质结构和性能的理论研究

Theoretical study on the structure and performance of polyethylene oxide and polypropylene carbonate blend polymer electrolyte

本文采用密度泛函理论,在B3LYP/6-31G(d,p)水平上对聚环氧乙烷(PEO)和聚碳酸丙烯酯(PPC)共混聚合物与锂离子的配位环境、电解质的电化学稳定性进行了理论研究.获得了锂离子的四配位稳定结构,PEO及PPC链中均有2个O原子参与配位.模拟的红外光谱显示,锂盐与PEO中的醚O原子和PPC中的羰基O原子均存在相互作用,表明2条聚合物链共同参与配位,该结构解释了共混体系中聚合物与锂盐的相互作用机制.体系的前线分子轨道显示,共混聚合物电解质较单一体系具有较好的氧化还原稳定性.锂盐阴离子(TFSI-)和溶剂化效应对配位结构和前线分子轨道有影响,在溶剂乙腈的作用下,锂盐与PPC中的1个羰基O原子以及PEO中的2个醚O原子相互作用.共混聚合物与锂盐的相互作用比单链体系更强,共混聚合物电解质可形成一系列能量相近的结构,它们之间的变换有利于锂离子在电解质中的移动,导致电解质具有更高的锂离子迁移数.PPC的加入促进了共混聚合物链的迁移,从而提高了电解质的离子电导率.共混体系的理论电化学窗口与实验报道的数据相一致.尽管在共混体系的还原过程中锂盐结构发生了变化,降低了电解质的还原稳定性,但仍获得了比PEO更宽的电化学窗口,共混体系比PEO更适合作为高电压电解质材料.该研究有助于了解共混聚合物电解质的结构及氧化还原性质,为进一步设计和制备其他共混聚合物电解质提供了理论基础.

The ionic conductivity and electrochemical stability of solid polymer electrolyte are important parameters to evaluate its performance.Many experimental studies show that the solid electrolyte materials with excellent performance can be obtained by mixing polyethylene oxide(PEO)and polypropylene carbonate(PPC)with lithium salts.In this work,we investigated the coordination environment of PEO and PPC blends with lithium ions and the electrochemical stability of the electrolyte by using density functional theory with the B3 LYP functional and 6-31 G(d,p)basis set.All computations have been performed using Gaussian 09 package.The four-coordinated stable structure of lithium ion was obtained,and the optimized structure is a local minimum point on the potential energy surface by the vibration frequency calculation.The two oxygen atoms in PEO and two oxygen atoms in PPC are involved in this four-coordinated structure.The simulated infrared spectrum show that the interactions are existed between lithium salt and ether oxygen atoms in PEO and carbonyl oxygen atoms in PPC,indicating that the two polymer chains are involved in coordination and this structure explains the interaction mechanism between polymer and lithium salt in the blending system.The frontier molecular orbital data indicate that the blended polymer electrolyte has better redox stability than that single system.The presence of salt anion(TFSI-)and solvation effect can affect the formation of the coordination structure and frontier molecular orbitals,lithium salts interact with one carbonyl oxygen atom in PPC and two ether oxygen atoms in PEO under the action of the recessive solvent acetonitrile.The interaction between the blended polymer and lithium salt is stronger than that of the single chain system,and the blended polymer electrolyte can form a series of local structures with similar energy.The transformation between them is conducive to the movement of lithium ions in the electrolyte,resulting in a higher lithium ion migration number.The addition of PPC promotes the migration of blended polymer chain,which improves the ionic conductivity of electrolyte.The theoretical electrochemical window of the blended system is consistent with the reported experimental data.Although the structure of lithium salt changes during the reduction process of the blended system and reduces the reduction stability of the electrolyte,the electrochemical window is wider than PEO,and the blending system is more suitable for high pressure electrolyte material.These studies are helpful to understand the structure and redox properties of the blended polymer electrolyte.Our work not only deeply analyze the experimental phenomenon in theory,but also provide a theoretical basis for the further design and preparation of other blended polymer electrolyte.