Modified Solid State Electrolyte Membrane For Lithium/Sulfur Batteries

Of the high-energy density chemical power systems currently being considered, lithium/sulfur (Li/S) batteries which use elemental sulfur as the cathode material, are attracting increased attention. These batteries have a high theoretical energy density, consist of abundant raw materials, are low cost and environmentally friendly. Central to the operation of these Li/S batteries are polysulfide anions (S_n^2-, 8≥n≥3) which are intermediate products generated during the electrochemical reduction process. These anions have high solubility in organic electrolytes. They typically diffuse to the lithium anode according to a concentration gradient and directly react with the anode. This diffusion causes an internal shuttle phenomenon and significantly corrodes the anode, which decreases active material utilization during the discharge progress and reduces its cycle life. These drawbacks have seriously retarded industrial production of Li/S batteries.Two novel barrier systems have been proposed. One uses a PP/LiPON composite membrane. The other uses a poly(ethylene glycol) methacrylate (PEGMA) gel polymer electrolyte (GPE) membrane. The purpose of introducing a barrier system is to allow only the lithium ion though while preventing the active material of the cathode from migrating to the lithium anode. This improves the active material utilization and cycle life of the Li/S battery. The main work of the paper includes:Research on preparation of LiPON thin films using low-temperature radio frequency magnetron sputtering. LiPON thin films were deposited using radio frequency magnetron sputtering from a Li₃PO₄ target in an ambient nitrogen atmosphere at a low temperature (10¹5οC) sample stage obtained by using circulating cooling water. The following sputtering conditions: nitrogen pressure; nitrogen flow rate; and, sputtering power on the ionic conductivity of LiPON thin films, have been investigated. Optimized LiPON thin film deposition conditions were: nitrogen pressure of 1.5 Pa; nitrogen flow rate of 70 sccm; and, sputtering power of 120 W. This resulted in a LiPON thin film with a maximum ionic conductivity of 1.5×10^-6 S/cm.Research on the factors affecting the ionic conductivity of LiPON thin films. The ionic conductivity, composition and bonding structure of LiPON thin films were obtained using scanning electron microscope (SEM), electrochemical impedance spectra (EIS), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). It was found that increasing lithium ion mobility contributes most to improving ionic conductivity and reducing transfer activation energy of LiPON thin films. Lithium ion mobility is influenced by the bonding structure change of LiPON thin films. Incorporating nitrogen atoms into the γ-Li₃PO₄ tetrahedral structure, by substituting Nd and N_t for Ob and Od respectively, provided a cross-linking structure in the glass network. Situation PO₂N₂ tetrahedron, which has two N_t structural units, within LiPON thin films provides a higher cross-linking density in the glass network with lower electrostatic energy than other nitride phosphate tetrahedrons.Research on preparation conditions and barrier performance of the PP/LiPON composite membranes. SEM and atomic force microscope (AFM) results indicated that three PP/LiPON composite membranes coating types ("partial coating"(PC),"growth coating"(GC) and"complete coating"(CC)) can be prepared by controlling sputtering time. The degree of coverage of the PP/LiPON composite membrane has been quantitatively described through the diffusion experiment of S8(l) and S_n^2- (8≥n≥3) particles. It was found that with a LiPON thin film apparent thickness of 1.0μm, CC type composite membrane coverage reached 94%. It was also found that the concentration of elemental sulfur diffusing through the PP/LiPON composite membrane to the negative half-cell was only 1/12th of that through the PP separator under the same experimental conditions.Research on the electrochemical performance of PP/LiPON composite membrane systems with three coating types. The ionic conductivity and electrochemical stable window of the PP/LiPON composite membrane systems with three coating types have been investigated using EIS and linear sweep voltammetry (LSV). The results indicated that ionic conductivity (10-4 S/cm) in the three cases were one order of magnitude lower than the electrolyte system with PP separator yet still fulfilled the demand of the Li/S batteries for the ionic conductivity at room temperature. In addition, the electrolyte system with GC or CC type composite membrane displayed outstanding electrochemical stability.Research on the relationship between the coating type and electrical performances at conventional charge/discharge current density (100 mA/g). The results indicated that sulfur utilization, cycle performance and coulombic efficiency of the Li/S batteries improved with LiPON thin film increased coverage. Discharge capacity loss arising from the shuttle phenomenon and lithium anode corrosion has been reduced due to blocking the diffusion channel of the S_n^2- (8≥n≥3) by the PP/LiPON composite membrane with a high coverage LiPON thin film. The Li/S battery using a CC type composite membrane with coverage of 94% maintained 56% of the initial discharge capacity (483 mAh/g) after 50 cycles. Its coulombic efficiency was almost 100% in the first cycle and remained stable during subsequent cycles.Research feasibility on PGEMA based GPE membrane applied in Li/S batteries, evaluating GEP membrane influence on sulfur utilization, and cycle performance for Li/S batteries at the conventional charge/discharge current density (100 mA/g). The results indicated that, due to the steric effect and the Lewis acid characteristic of B-PEG and Al-PEG, PEGMA based PEG membranes significantly inhibit active material diffusion, and led to significant sulfur utilization and cycle performance of the Li/S batteries improvements. Initial discharge capacity achieved 1267 mAh/g with a coulombic efficiency of 97% which maintained at 790 mAh/g of discharge capacity after 20 cycles.Comparison of Li/S battery cycle performance using the PP/LiPON composite membrane and PEGMA based GPE membrane. The results indicated that at conventional charge/discharge current density (100 mA/g), the Li/S battery cycle process with the CC type PP/LiPON composite membrane gradually fell to a low capacity fade rate (0.9%). The cycle process of the battery with the PGEMA based GPE membrane was stable with a capacity fade rate of only 0.4% after the first cycle. This indicated that the physical-chemical barrier system-a combination of the steric effect and the Lewis acid characteristic-was more efficient at suppressing the active material diffusion than a pure physical barrier system.Research on Li/S battery sulfur utilization and cycle performance using the PEGMA based GPE membrane. The results indicated that the PEGMA based GPE membrane effectively suppresses the shuttle phenomenon and retards the lithium anode corrosion, while maintaining the active material of the interior cathode with a uniform distribution, the sulfur utilization and cyclic performance of the battery have been remarkably improved within the discharge current density range of 50⁵00 mA/g. Specifically, when ID = 50 mA/g, the initial discharge capacity of the Li/S battery using the PEGMA based GPE membrane, can provide 1404 mAh/g. This was doubled more than that of the organic electrolyte based batteries. This effect explained, from one aspect, that the GPE membrane acted as an outstanding barrier to S_n^2- (8≥n≥3). When I_D = 500 mA/g, the Li/S battery using a PEGMA based GPE membrane, maintained a discharge capacity of 585 mAh/g after 20 cycles. The EIS results showed that the change in Rt (charge transfer resistance at the conductive agent interface of the Li/S battery with the PEGMA based GPE membrane) was small. Its rate of change was only 1/6th or 1/8th of that of the organic electrolyte battery regardless of the discharge current density is 100 mA/g or 500 mA/g.