Design, Preparation And Application Of Single Ion Conducting Polymer Electrolytes For Li-ion Battery

Lithium ion batteries, which take advantages of high gravimetric and volumetric energy densities, long cycle life, and improved reliability, have been widely used as power source. Electrolyte is an important component in the lithium ion battery. Among the developed electrolytes, single ion conducting polymer electrolytes, which act both as separator between anode and cathode, and medium to facilitate the flow of lithium ions between two electrodes, were rapidly developed and were further, carried out to overcome the serious safety concerns. In the present works, two classes of polymeric compounds with highly exposed lithium cations are presented. The first class of sp3boron based polymeric materials can be made in the form of one or three dimensional networks with rigid or soft morphology dictated by the choice of precursors. For the sp1boron based on polymeric materials, the four of lithium poly (hydroquinone borate)(LiPHB), lithium poly (phloroglucinol borate)(LiPPB), lithium poly (4,4’-biphenol borate)(LiPBB) and lithium poly (1,2,3,4-Butanetetracarboxylic acid borate)(LiPBAB) have been successfully synthesized and further characterized by FT-IR,11BMR, GPC, FT-SEM and XRD. Some significantly properties, for example, thermal stability, electrochemical stability and ionic conductivity have also been investigated. Among them, LiPHB, LiPPB and LiPBB diplays three dimensional networks and LiPBAB exhibits one dimensional network. Furthermore, the polymer electrolyte membrane has been successfully prepared by using solvent thermal method (M1) and solution casting method (M2), and their important performances have also been compared. The results of this investigation are summarized as follows:1. The FT-IR (1400cm-1-1000cm-1), nBNMR (<10ppm) and the number-average molecular weights (Mn) were in the range of16,400-27,800and polydispersity indices (Mw/Mn) were between1.32and2.13, suggest that sp3boron compounds with high molecular weight were successfully synthesized as expected2. Beside of PHB, the other sp3boron based polymeric materials displays regular shape as shown in SEM images. The XRD patterns of PBB and PBAB display obvious sharp diffraction peaks and PPB displays several week sharp diffraction peaks on the broad peak, suggesting that these materials have a long range order.3. These materials exhibits good thermal (160℃-260℃) are well suited for applications in Li-ion batteries as electrolytes as well as separators.4. These materials also exhibits electrochemical stability (4.3V-4.7V) are well suited for applications in Li-ion batteries as electrolytes as well as separators. 5. The considerable differences of interfacial resistances between M1membrane and M2membrane were observed. The M1membrane displays high and unstable interfacial resistance duo to the coarse and porous surface morphology. However, the M2membrane exhibits low and stable interfacial resistance which was attributed from the smooth and uniform surface morphology. So it can be concluded that the quality of M2membrane was better than M1membrane due to great uniform surface structure and stable interfacial and bulk resistance.6. The two kinds of single ion conductor polymeric electrolyte membranes before and after EIS test were measured by SEM. It was indicated that the somewhat more uniform of surface morphologies of two all single ion conductor polymeric electrolyte membranes were gained as the press between the stainless steel electrodes during EIS test. By comparing the decreasing of interface resistances as a function of storage time, we can see that the decreases of interfacial resistances of them were induced by the formation of their more uniform surface morphologies.7. On the other hand, the M1membrane exhibits high ionic conductivity in the same order of magnitude as the conventional liquid electrolytes due to larger pores and high solvent uptake. The M2membrane shows much lower ionic conductivity than M1membrane, which was caused by smaller pores and low solvent uptake.The second class of sulfonimide based polymeric materials was developed by designing the bis(4-carboxyl benzene sulphonyl) imide with high thermal stability and proton delocalization. The three of sulfonimide based polymeric materials, lithium poly(diacylbenzenesulfonylimide diphenylsulfoneamide)(LiPDSPA), lithium poly(diacylphenylsulfonylimide octylamide)(LiPDSOA) and Poly(isophthaloyl benzenesulfonic acid-2,4-diamide diphenylsulfone diamide) with pended lithium sulfonimide(LiPIBPSI), were designed, synthesized and further characterized by FT-IR,1HMR, GPC and FT-SEM. Thermal stability, electrochemical stability and ionic conductivity have also been investigated. The polymer electrolyte membrane has been successfully prepared by using solvent thermal method and solution casting method, and their important performances have also been compared. The results of this investigation are summarized as follows:1. The FT-IR and1HNMR and the number-average molecular weights (Mn) were in the range of9,500-44,600g/mol and polydispersity indices (Mw/Mn) were between1.87and3.86, suggest that polymeric compounds with high molecular weight were successfully synthesized as expected.2. These material exhibits moderate good thermal (>170℃) are well suited for applications in Li-ion batteries as electrolytes as well as separators.3. These material exhibits good electrochemical stability (>4.0V) are well suited for applications in Li-ion batteries as electrolytes as well as separators.4. SEM images revealed interesting morphologies of three polymer electrolytes. LiPDSPA shows a massive structure due to the high rigidity of the aromatic precursors and the characteristics of stronger ionic bonds existed in the polymer electrolyte structure, LiPDSOA powder displays a filiform shape due to the high the flexibility of the aliphatic precursor and the uniform-sized microsphere with the size of0.5μm for LiPIBPSI powder was obviously observed which was ascribed by the force of interattraction from stronger ionic bonds existed in the side chain of polymer electrolyte structure.5. For LiPDSOA and LiPIBPSI, both M1and M2membranes show the similar performances with that of the sp3boron based polymeric materials, which the M1membrane show high and unstable interfacial resistance and coarse and porous surface morphology and M2membranes display low and stable interfacial resistance and smooth surface morphology. However, LiPDSPA gave considerable performances of interfacial resistance. The extreme low interfacial resistances of around40Ω for M1membrane and75Ω for M2membrane were obtained. As discussed below, it should be noted that the abnormal interfacial of both two method composite polymer electrolyte membranes might be due to the special porous structures in the LiPDSPA polymer electrolyte.6. The two kinds of single ion conductor polymeric electrolyte membranes before and after EIS test were measured by SEM. It was indicated that the somewhat more uniform of surface morphologies of two all single ion conductor polymeric electrolyte membranes were gained as the press between the stainless steel electrodes during EIS test. By comparing the decreasing of interface resistances as a function of storage time, we can see that the decreases of interfacial resistances of them were induced by the formation of their more uniform surface morphologies.7. The M1membrane exhibits high ionic conductivity in the same order of magnitude as the conventional liquid electrolytes due to larger pores and high solvent uptake. The M2membrane shows much lower ionic conductivity than M1membrane, which was caused by smaller pores and low solvent uptake. It’s worthy to note that LiPIBPSI polymer electrolyte shows the better ionic conductivity of M2membrane than the LiPDSPA and LiPDSOA even although the content of lithium ion in the LiPIBPSI polymer electrolyte structure was large lower than that of LiPDSPA and LiPDSOA polymer electrolyte structures. It was concluded that this comb-like structure is helpful for enhancing the movement of lithium ion in the polymer electrolyte membrane and subsequently improving its ionic conductivity.With the charges on the framework anions well delocalized and the cations in the extra framework highly exposed, the polymeric compounds are both thermally and electrochemically stable with ionic conductivity of the fabricated membranes comparable to the values of the conventional liquid electrolytes for Li-ion batteries. The high conductivity coupled with the good mechanical strength of the membranes enables the materials to be used as single ion electrolytes as well as separators in the battery cells.