Controlling Structure And Chemical Composition Of Cathode Materials For Lithium Sulfur Batteries

Lithium sulfur battery with the advantages of high energy density and low price is a promising energy storage system.Constructing high performance sulfur cathodes is crucial to Li-S batteries.Sublimed sulfur(S₈)and sulfurized polyacrylonitrile(S@pPAN)are two promising sulfur materials for commercial application.S₈ based cathodes face the problems of poor conductivity,serious volume expansion and shutting effect of lithium polysulfide(LiPS),causing fast capacity degradation and inferior C-rate performance.S@pPAN exhibits high cycling performance in ester electrolyte,while the slugglish reaction kinetics,controversial molecular structure,reaction mechanism,low sulfur content and shuttle effect in ether and polymer electrolyte are still vital problems.Herein,based on S₈ and S@pPAN cathodes,the materials structure and chemical composition regulated are used to optimize the electrochemical performance of sulfur based cathodes at various electrolytes,and the melolecular structure and(dis-)charge reaction mechanism are further uncovered via electrochemical and physical characterization.To solve the slugglish reaction kinetics and shuttle effect of LiPS in S₈ cathodes,a Pd@HCS sulfur host has been synthesized using a hard tempelate method,and the electrochemial performance and reaction mechanism of materials in ether electrolyte have been investigated.The hollow carbon sturcture with high pore volume and conductivity,can suppress the dissolving of LiPS from cathode.According to the electrochemial testing,the effect of Pd nanoparticles for enhancing LiPS covervision kinetics and suppressing shuttle effect are explored.By means of LiPS adsorption test,density functional theory(DFT)calculation and X-ray adsoption spectra,the Pd nanoparticles can adsorb LiPS via Pd-S bond,and keep reversible structure during(dis-)charge process.The Pd@HCS/S displays an initial discharge capacity of 805 mAh g^-1 at 1 C,and capacity degradation rate of 0.068%/per cycle during 400 cycles.At a high sulfur area loading of 5.88 mg cm^-2,the Pd@HCS/S exhibits discharge capacity of 587 mAh g^-1 after 100 cycles at 0.2 C.To address the poor conductivity and obscure reaction mechanism of the S@pPAN,iodine-doped sulfurized polyacrylonitrile(I-S@pPAN)is synthesized by a one-step co-heating method,and electrochemical performance and reaction mechanism in ester electrolyte are explored.The short-chain sulfur oligomers(-S_x-)are interacted with the cyclized and dehydrogenated six-membered ring PAN backbones via the C–S bonds,and iodine is covalently interacted with carbon backbones via C-I bond.The elctrochemical measurements and DFT calculation reveal that iodine-doping can effectively faciliate the electron and Li⁺migration of S@pPAN.Ex-situ solid state NMR and XPS results demonstrate both short-chain sulfur oligomers(-S_x-)and double bonds(C=N)in the composites can react with Li⁺ions to provide capacity,which is the reason of exceeding the theoretical value of elemental sulfur(1675 mAh g^-1).In-situ EIS spectra reveals the generated cathode electrolyte interface(CEI)layer,containing Li F and Li I,is benefcial to enhance the cycling performance and reaction kinetics of the cahtode.In ester electrolyte,the I-S@pPAN delivers a high initial reversible capacity of 1267 mAh g^-1 and1077 mAh g^-1 after 1000 cycles at 2 C,with capacity retention of 85%,and displays an initial capacity of 1085 mAh g^-1 at high C-rate of 8 C.It exhibits an unprecedented capacity of 994 mAh g^-1 at 2 C for the room temperature Na/S battery,whilst a reversible capacity of 947 mAh g^-1 at 0.1 C for the room temperature K/S battery.To investigate the slugglish reaction kinetics and LiPS shuttle effect of S@pPAN in ether and polymer solid-state electrolyte,the black phosphorus modified sulfurized polyacrylonitrile(BP-S@pPAN)and selenium-doped high sulfur content sulfurized polyacrylonitrile(Se/S-S@pPAN)are prepared and applied to ether and PEO polymer solid-state electrolyte,respectively.The elctrochemical measurements and characterization reveal the amorphous P₂S_5+x in BP-S@pPAN is crucial to suppress the shuttling effect of lithium polysulfides and enhance the reaction kinetics.In ether electrolyte,the BP-S@pPAN delivers reversible discharge capacity of 1266 mA h g^-1 at0.1 C and capacity retention of 91.1%after 100 cycles,and displays discharge capacity of 1086 mAh g^-1 at 2 C.The Se/S-S@pPAN with high sulfur content,contains short-chain sulfur oligomers(-S_x-)and sublimed sulfur(S₈),and the Se-S eutectic structure is beneficial to enhancing the conductivity of cathode.The Se/S-S@pPAN displays high reversible(dis-)charge capacity,C-rate and cycling performance in PEO polymer based all solid state batteries.The Se/S-S@pPAN displays high reversible capacity,C-rate and cycling performance in PEO polymer all solid state batteries.When tested in PEO-LLZTO polymer solid-state electrolyte at 60 ^oC,the Se/S-S@pPAN delivers a reversible capacity of 1042 and 445 mAh g^-1 at 0.1 C and 2 C.