聚碳酸丙烯酯和丁二腈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 polypropylene carbonate and butanedinitrile blended polymer electrolyte

聚碳酸酯基聚合物作为常见的电解质材料因其结构具有强极性而被广泛应用.为制备更高性能的固态聚合物电解质材料,对典型的聚碳酸丙烯酯(PPC)和丁二腈(SN)共混体系进行了理论研究,希望通过该研究可以得到共混改性优势的理论依据.利用密度泛函理论方法,在B3LYP/6-31G(d, p)水平上对PPC和SN共混电解质在乙腈隐式溶剂化模型下的结构、相互作用及电化学稳定性进行了理论研究.通过模拟PPC/Li^(+)复合物体系与PPC/SN/Li^(+)共混体系的配位构型,对比得到锂离子的五配位稳定结构,配位原子由PPC提供的4个羰基O原子与SN提供的1个N原子组成.此外,通过分析PPC/SN/Li^(+)共混体系的配位原子间距、前线分子轨道能级与锂离子上电荷等数据,证明该结构具有良好的结构稳定性与电化学稳定性.向得到的四配位结构中引入盐阴离子(TFSI-),模拟得到五配位的共混聚合物电解质结构,其中,TFSI-与PPC链各提供2个O原子参与配位,而SN则提供1个N原子参与配位.模拟得到的电解质红外光谱进一步证明PPC和SN均与锂盐存在相互作用.分析PPCn/SN/Li+与PPCn/SN/LiTFSI体系的lI^(+)…O=C距离可以发现,PPCn/SN/LiTFSI体系的结构较为松散,有助于锂离子的迁移.计算得到PPCn/SN/LiTFSI体系的电化学窗口为5.72 V,有较好的电化学稳定性.此外,在PPC3/SN,PPCn/SN/Li^(+)和PPCn/SN/LiTFSI三种体系的前线分子轨道和自旋密度对比分析中还发现,PPC与LiTFSI共同影响电解质的氧化还原稳定性,而SN则与电解质的还原稳定性有关.以上研究结果证明了共混体系的结构和电化学优势,有望满足目前对具有更高能量密度锂离子电池的需求,有助于人们深入理解PPC和SN共混电解质性能,并为以后的相关研究提供了一定的理论基础.

As a common electrolyte material,polycar-bonate-based polymer has drawn the great interest of numerous researchers because of its strong polarity.In order to manufacture a higher-performing solid polymer electrolyte material,a mixing system comprising the typical polypropylene carbonate(PPC)and succinoni-trile(SN)was designed in this research,with the goal of obtaining theoretical proof of the benefits of blending modification.The structure,interaction,and electro-chemical stability of PPC and SN blends with lithium ion in the acetonitrile implicit solvation model are theoretically investigated by using density functional theory at the B3LYP/6-31G(d,p)level in this study.The five-coordinated structure of lithium ion was finally derived by carefully comparing the coordination structures of the PPC/Li^(+)composite system and the PPC/SN/Li^(+)blending system.From the results we can see the coordination atoms are consists of four O atoms supplied by PPC and a N atom supplied by SN.Furthermore,it is demonstrated that the complex has better structural stability and electrochemical performance than other blending complex in PPC/SN/Li^(+)blending systems by analyzing coordination atomic spacing,frontier molecular orbital energy level,and the NBO charges on Li^(+).On this basis,salt anion(TFSI^(-))was added into the four-coordinated stable complex,yielding a five-coordination polymer electrolyte structure with TFSI~-and PPC chains each providing two O atoms for coordination and SN providing one N atom to participate in the coordination.The simulated infrared spectrum further show that both PPC and SN interact with lithium salts.In addition,after the structure of the Li^(+)…O=C distance between PPCn/SN/Li^(+)and PPCn/SN/LiTFSI system was compared,it is discovered that the structure of the PPCn/SN/LiTFSI blending electrolyte system is looser than PPCn/SN/Li^(+)system,which aids in the transport of lithium ions very much.What's more,the electrochemical window for the five-coordination polymer electrolyte structure(PPCn/SN/LiTFSI)is calculated to be 5.72 V,indicating good electrochemical stability.Besides,after a comparison of the frontier molecular orbitals and spin densities of PPC3/SN,PPCn/SN/Li^(+)and PPCn/SN/LiTFSI,it is also found that the PPC and LiTFSI jointly affect the redox stability of electrolytes,whereas the SN affects the reduction stability of electrolytes.The results of the study above demonstrate that the structural and electrochemical advantages of the blending system,which is expected to meet the current demand for the lithium-ion batteries with higher energy density.In a word,these conclusions may not only assist individuals better understand the PPC and SN blending electrolyte properties,but they may also give a theoretical foundation for future research.