Research On Key Materials Of Lithium Sulfur Battery

High-energy-density lithium-sulfur(Li/S)batteries are promising candidates for next-generation energy storage systems.However,there are many problems existed in the system.For sulfur cathodes,conventional granular electrodes suffer from low-efficient preparation processes.Herein,an electrochemical method and an organic solvent method were used to assemble self-supporting sulfur cathodes,respectively,to improve the efficiency.For the anode,Owing to the lithium dendrite growth and the soft property,lithium metal has to be overused to support itself,resulting in extremely low lithium utilization.In view of this,we introduced mechanically robust frameworks to extend the liftetime and reduce the amount of lithium.As for low conductivity and poor cycling performance for polymer electrolytes,we developed two kinds of inorganic/organic polymer films with excellent electrochemical performance,enhanced mechanical strength and safety.In the meantime,a new type of functional binder was developed to suppress the dissolution and shuttle of polysulfides.The details are as follows:1)An electrochemical method was used to prepare sulfur/carbon felt(CF)composites.By investigating different electrodepositing ways,the mechanism of sulfur depositing on CF was analyzed.The effects of sulfide concentration,current density,deposition time,and the spatial distribution of sulfur at CF were investigated on composite electrodes.The S/CF-8 after electrodepositing 8 h was optimized as sulfur cathodes,which displayed a capacity of 2.3 m Ah cm^-2 after 90 cycles at 0.5 C.A preliminary study was conducted on sulfur-containing solutions(0.35 mol L^-1Na₂S/0.5 mol L^-1 Na₂CO₃),which verified the potential of treating sulfur-containing wastewater via electrochemical methods.2)The preparation of polysulfide(Li PS)/carbon fiber fabric(CFFs)electrode via an organic solvent method was studied.The effects of sizing agent,oxidation temperature,and oxidation time on the electrochemical performance and mechanical strength of CFFs were investigated,and the optimum condition at 500?for 2 h were comprehensively obtained.The corresponding Li PS/OCFFs electrode showed an ultimate tensile strength and Young's modulus of 213.9±15.2 MPa and 9.2±1.2 GPa.The electrode delivered an initial discharge specific capacity of 1159 m Ah g^-1 at 0.2 C and maintained 91.9%after 150 cycles with average coulombic efficiency of 99.2%.Since high mechanical strength of the CFFs without using any carbon powder or current collector,the electrode shows great prospect in the field of structural batteries.3)The mechanical extrusion method and hot-dip method were used to fabricate titanium mesh/lithium and lithium/oxidized carbon fiber fabric(Li/OCFFs),respectively.The electrochemical stability and mechanical strength were compared with lithium anode,and Li/OCFFs were preferred.The effects of different modifiers,oxidation temperature,and oxidation time on the the electrochemical performance of the corresponding electrodes were investigated.Compared with lithium anode,Li/OCFFs 400?15 min(CFFs heated at 400?in air for 15 minutes)with the same thickness while using only half of the lithium showed 20 times higher at the ultimate tensile strength.At current density of 1 m A cm^-2,the polarization potential for sysmetrical cells was much smaller than the former after cycling 800 h,effectively extending the cycle life of the anode.4)Two different inorganic/organic polymer films,GF/LLZO/PEO and BN/PVd F were prepared.Compared with Celgard 2325,the Young's modulus of BN/PVd F42 increased by 2.3 times(processing direction)and 3.6 times(transverse direction).The average Coulombic efficiency of the structural Li/S battery cycled 100times at 0.2 C reached up to 99.4%,which was higher than 98.6%for the latter.Owing to the excellent thermal stability of BN,the quasi polymeric electrolyte(BN/PVd F42 wetted by liquid electrolyte)was flame-retardant with enhanced safety.5)The mechanism of the anti self-discharge for PEt Ox was explored by comparing with PVd F and CMCNa-based electrodes.The amide functional groups were confirmed to effectively anchor Li PS and cause gelation of the liquid electrolyte,thereby delaying Li PS shuttle.In order to stabilize the binder,PEt Ox-PAA composites were constructed via hydrogen bonding.The mechanical strength and the electrochemical performance of the composites with different ratios were investigated.It was concluded that PEt Px-PAA(4:1)has the most suitable hydrogen bond crosslink density to ensure the best stretch state of the segments.Even when the sulfur load was2.5 mg cm^-2,the discharge specific capacity could maintain at 632 m Ah g^-1 after 120cycles at 0.2 C,and the capacity decay rate per cycle was 0.25%.