Preparation And Performance Of Nanocomposite Polymer Electrolytes For Lithium-ion Batteries

The polymer lithium-ion batteries are considered as one of the best candidates for next generation power sources due to their high energy density, good cyclability, reliability and safety. poly(ethylene oxide) (PEO)-based polymer electrolytes have received extensive attentions for their potential capability to be used as alternative candidate materials for the traditional liquid electrolytes in all solid-state polymer lithium-ion batteries. However, there are many problems (e.g. low temperature conductivity of polymer electrolyte, the bad compatibility of the interface between polymer electrolyte and electrode material) that seriously affecting the cycle and safety performance of the batteries, must be solved. In this dissertation, several different ingredients, microstuctures of nanosized inorganic materials are prepared and PEO-based nanocomposite polymer electrolytes (NCPEs) have been developed prepared by solution casting method and hot-pressing method using these nanosized inorganic materials as the fillers. The inorganic filler and the NCPEs were characterized by means of XRD, BET, DSC, FT-IR, NMR, TEM, FESEM, and electrochemical test. The relationships among structure, composition of the inorganic filler and the electrochemical properties of the NCPEs were also investigated.1. Nanosized MgAlO was synthesized using MgAl-CO3-LDHs as a precursor prepared by the separate nucleation and aging steps method. A novel PEO-based NCPE by using nanosized MgAlO as the filler has been prepared. The experiment results showed that the nanosized MgAlO could improve the conductivity of the PEO-based NCPEs not only by suppressing the PEO crystalline phase but also producing new Li+conductive path through the interfacial between the polymer and MgAlO. The ionic conductivity reached at the maximum value 1.1×10-6 Scm-1 at 25℃by 8 wt.% loading content of MgAlO. It is found that the nanosized MgAlO can also improve the electrochemical stability of the PEO-based composite polymer electrolyte using Linar Sweep Voltammetry technique.2. Nanosized ZnAl2O4 with mesopore network was synthesized using ZnAl-CO3-LDHs as a precursor prepared by the separate nucleation and aging steps method. A novel PEO-based NCPE by using nanosized ZnAl2O4 with mesopore network as the filler has been prepared by solution-casting method and hot-pressing method. The experiment results showed that ZnAl2O4 could also improve the conductivity and lithium ion transference number of the PEO-based NCPEs not only by suppressing the PEO crystalline phase but also producing new Li+conductive path through the interfacial between the polymer and ZnAl2O4. The ionic conductivity and lithium ion transference number reach a maximum value of about 2.23×10-6 S cm-1 at room temperature and about 0.5 at 70℃respectively, for an 8 wt.% loading of ZnAl2O4. Compared with MgAlO, ZnAl2O4 can not only better suppress the PEO crystalline phase but also produce more new Li+conductive path through the interfacial between the polymer and ZnAl2O4 becasuse of the nanosized particle with mesopore network and the large specific surface area. Compared two NCPE films prepared by hot-pressing and solution-casting method, the thermal histories also can influence the PEO crystallization. The NCPE prepared by hot-pressing method has smoother surface, higher interface stability, better electrochemical stability than that prepared by solution-casting method. The conductivity and lithium ion transference number values of hot-pressed NCPE film were very close to that of solution cast film. The lithium polymer battery using the hot-pressed PE016-LiC104-8 wt.% ZnAl2O4 NCPE as electrolyte and lithium metal and LiFePO4 employed as anode and cathode, respectively, showed high discharge capacity and excellent cycling stability at different charge/discharge rates as revealed by galvanostatically charge/discharge cycling tests, which suggested that the hot-pressed PEO16-LiClO4-ZnAl2O4 NCPE could be used as candidate electrolyte materials for lithium polymer batteries.3. Nanosized Li0.1Ca0.9TiO3, as a lithium fast ionic conductor, was prepared by sol-gel method and PEO-based solid NCPE with nanosized Li0.1Ca0.9TiO3 as the filler has been developed. The effects of the content of nanosized Li0.1Ca0.9TiO3 on PEO12-LiClO4-Li0.1Ca0.9TiO3 NCPE performance were investigated. The ionic conductivity and lithium ion transference number of the NCPE reach at the maximum value by 15 wt.% loading content of Li0.1Cao.9TiO3. The ionic conductivity was about 1.02×10-5 S cm-1 at 30℃and the lithium ion transference number value was about 0.533 at 70℃. Meanwhile, the micronsized Li0.1Ca0.9TiO3 was prepared by solvothermal method and PEO-based solid composite polymer electrolyte (CPE) with micronsized Li0.1Ca0.9TiO3 as the filler has been developed. Compared with PEO12-LiClO4-Li0.1Ca0.9TiO3 NCPE, the ionic conductivity and lithium ion transference number of the CPE significantly declined. It is found that the mechanism that nanosized Li0.1Ca0.9TiO3 can improve the performance of NCPE is nanosized Li0.1Ca0.9TiO3 as a filler not only produced a stronger reduction of crystallinity but also can provide conductive Li+ions.4. Three different microstructures (massive, sheet, porous, respectively) of LiAlSiO4 were synthesized using three different ways (alcohol reflux method, sol-gel method and hydrothermal method, respectively). The PEO-based composite polymer electrolytes were prepared using these three different microstructures of LiAlSiO4 as the inorganic fillers. All the three different microstructures of LiAlSiO4 could suppress the PEO crystalline phase effectively. The ionic conductivity of the three compostie polymer electrolyte was enhanced than that of the polymer electrolyte PE012-LiC104. Both porous structure and sheet structure of LiAlSiO4 could better suppress the re-crystallization of PEO than the massive structure of LiAlSiO4, but the composite polymer electrolyte composed of sheet structure of LiAlSiO4 has the highest room temperature conductivity and lithium ion transference number. The results collectively show that the microstructures of the three type of LiAlSiO4 have a key effect on the properties of the composite polymer electrolytes when the effects of the inorganic fillers on the crystallinity of the polymer is little different.5. PEO12-LiClO4-MWCNTs NCPEs were prepared using different ultrasonic time acid-treated MWCNTs as fillers. The effects of different ultrasonic time acid-treated MWCNTs on the conductivity of the NCPEs were studied. It is found that the degree of cutted and end-modified MWCNTs depends on the ultrasonic acid-treated time. The longer ultrasonic acid-treated time, the more cutting of MWCNTs is severe and the more end-modification is adequate. Both the network structure formed by MWCNTs and the end-modified functional groups of MWCNTs can increase the conductivity of the NCPEs at room temperature. In general, the extent of cutting of MWCNTs plays an important role in enhancing the performance of NCPE.