Li-ion Selective Polymer Electrolytes For Lithium/Sulfur Cells

Lithium Sulfur cell(Li/S cell) is attracting increased attention, for its high energy density, low costs and environmentally friendly. Central to the operation of Li/S cells are polysulfide anions(Sn2-, 4 ≤ n ≤ 8), which are intermediate products generated during the discharge process. These anions have high solubility in organic electrolytes. Once dissolving in electrolyte, the anions diffuse to the lithium anode by concentration gradient and react with Li anode directly. This diffusion causes an internal shuttle phenomenon and corrodes Li anode significantly, which decreases active material utilization and reduces cycle life. These drawbacks have seriously retarded industrial production of Li/S cells.Li-ion selective polymer electrolyte had been proposed for Li/S cells to improve the cycle life and coulombic efficiency in this paper. The key of the Li-ion selective polymer electrolyte is the high Li-ion transference number of the polymer electrolyte, which allows Li-ion transferring though while prevents polysulfide anions from migrating to Li anode.This paper increased the Li-ion transference number of polymer electrolyte by adding organic Lewis acid PEGn-B in CPL electrolyte firstly, and then used the perfluorinated Li-ion ioinomer as the electrolyte directly. Researches were carried out on the electrochemical properties of the two type electrolytes, and the improvement of Li/S cells by the electrolytes had been studied, respectively. The main work of the paper includes:The influence on ionic conductivity and of CPL electrolyte by the side chain length and contains of PEGn-B in the polymerization system were investigated, and the optimized polymerization system was confirmed. Combined with the outcomes and molecule simulation, the Li-ion conductive mechanism and the enhanced mechanism by PEGn-B were comfirmed. Results showed that the optimized polymerization system to form CPL electrolyte membrane was PME400: PDE600: PEG3-B at weight ratio of 1:1:5, and the corresponding CPL electrolyte’s ionic conductivity was 3.3×10-4 S/cm,was 0.54. The Li-ion conduction of CPL electrolyte was depended on the relaxed motion of EO chain, and the enhanced effect ofwas relied on Lewis aicd-alkali reaction between the central B atom in PEGn-B and the anions in electrolyte.The influence on the cycle performance of Li/S cells by CPL electrolyte had been studied. The Li/S cell with CPL electrolyte had a capacity retentive ratio of 87% after 50 cycles, and coulombic efficiencies were above 90% along with cycles when cycled at 0.1C rate. Compared with the battery with liqud electrolyte, the cycle performance of the cell with CPL electrolyte had improved. However, limited by the concentration and the distribution of PEGn-B in CPL electrolyte, the of CPL electrolyte was less than 1, causing the polysulfide anions transport through the electrolyte, resulting the slowly fading capacities in the cell with CPL electrolyte.The Li-PFSA electrolyte had been prepared, and the influence on the cycle performance and rate performance of Li/S cells had been studied. The cell with Li-PFSA electrolyte had a capacity retentive ratio of 78% after 100 cycles, and coulombic efficiencies were above 95% along with cycles when cycled at 0.1C rate, which verified that the perfluorinated ionomer electrolyte which had the unity transference number could improve the cycle performance of Li/S cells. However, due to the low conductivity of the electrolyte, the rate performance of the cell with Li-PFSA electrolyte was unsatisfactory. When the cell cycled at 1C, the capcity was only 130 m Ah/g.Li-PFSD ionomer and Li-PFSI ionomer had been synthetized, and the ionomer membranes had been prepared by solution-casting method. The relationship of the solution-casting condition, the crystallization of the membranes, and the electrochemical properties of the corresponding electrolytes had been revealed. Reults showed that, with the increasing casting temperature, the Li-PFSD and PFSI membranes’ crystallinity decreased, and the crystallite size increased. And with the increasing annealing temperature or annealing time, the membranes’ crystallinity decreased firstly and then increased, and the crystallite size increased continuously. At the meanwhile, the low crystallinity of the membranes achieved high ionic conductivity of the corresponding electrolyte, and the high crystallite size of the membranes achieved high of the corresponding electrolyte.The influence on the cycle performance and rate performance of Li/S cells by Li-PFSD electrolyte and Li-PFSI electrolyte had been studied. The cell with Li-PFSD electrolyte had a capacity retentive ratio was 82.9% after 100 cycles, and coulombic efficiencies were above 97% along with cycles when cycled at 0.1C rate, and the stable capcity of 530 m Ah/g at 1C rate, indicating an improved cycle performance and rate performance. The cell with Li-PFSI also had good cycle performance and rate performance. The cell with Li-PFSI electrolyte had a capacity retentive ratio was 88.1% after 100 cycles, and coulombic efficiencies were above 97% along with cycles when cycled at 0.1C rate, the stable capcity was 700 m Ah/g at 1C rate, and the stable capcity was 620 m Ah/g at 2C rate. The long cycle performance test showed that, the capacity retentive ratio was 80% after 210 cycles, and 68% after 500 cycles when cycled at 0.2C.The reason of the perfluorinated ionomer electrolytes enhancing the cycle performance and rate performance of Li/S cell had been analysised. By means of analyzing the changes of cathodes, anodes and electrolytes before and after cycles, the causes of which enhancing the cycle performance and rate performance of Li/S cells by the perfluorinated ionomer electrolytes had cleared up. The perfluorinated ionomer electrolytes had unity, which could be considered as the Li-ion selective polymer electrolytes. These type of electrolytes would prevent polysulfide anions from migrating to lithium anode, which decrease both the wasting of active sulfur by the reaction of polysulfide anions and Li and the shuttle phenomenon.The changes of swelling ratio and the ionic conductivity of the perfluorinated ionomer electrolytes in different solvents had been analyzed, and the influence on the ionic conductivity of the perfluorinated ionomer electrolytes by both the properties of solvents and the structure of the end ionic groups had been investigated. Results showed that, for the same perfluorinated ionomer electrolyte, the higher dielectric constant and doner numbers, and the lower viscosity of the solvent leaded to higher ionic conductivity of the perfluorinated ionomer electrolyte. And for the different perfluorinated ionomer electrolyte, the higher delocalization tendevcy of the end ionic group leaded to higher ionic conductivity of the perfluorinated ionomer electrolyte.