Application Of Polypyrrole And Its Hybrid Materials In The Cathode Of Lithium-sulfur Batteries

Although lithium-sulfur?Li-S?battery is a promising energy storage device with high energy density,environmental friendliness and low cost,many obstacles such as poor conductivity,volume expansion and“shuttle effect”still limit its large-scale commercialization.To solve these problems,this dissertation focuses on constructing polypyrrole?PPy?conductive network,followed by structural design and shape control of the cathode to improve conductivity and bond active materials.On the above basis,preparation of PPy and inorganic material hybrids could further inhibit the dissolution of lithium polysulfides?Li PSs?.In addition,the moderate catalysis function and efficient true-sulfur-confinement effect of?-ZrP are investigated by experimental characterizations as well as theoretical computations,which provides the basis for rational design of cathode materials to achieve high capacity and long cycle life in Li-S batteries.The main research contents are as follows:To suppress the shuttle effect,reduce the capacity fade,and improve coulombic efficiency of Li-S battery,polypyrrole hollow nanosphere?PHNS?with ultra-thin wrinkled shell,which is expected to synergistically trap sulfur?S?with excellent elasticity and buffer capability,is fabricated by an in situ polymerization method.When S is encapsulated in PHNS-3?S@PHNS-3?with an optimum shell structure and used as the cathode,surprisingly,a specific capacity of 1238.4 m A·h·g^-1(sulfur loading:1.2–1.5 mg·cm^-2)is obtained in the first discharge at a current rate of 0.1 C with high coulombic efficiency,and a stable reversible capacity of 842.1 m A·h·g^-1 could still be achieved after 200cycles.Also,a reversible capacity as high as 542.9 m A·h·g^-1 is obtained even at a high current rate of 5 C.Such a PHNS with the ultra-thin wrinkled shell has good conductivity,short ion and electron transport channel,and fast kinetics characteristics.Especially during the charge-discharge cycles,it is conducive to the access of electrolyte and effective accommodation of the volume change.This PHNS should be a promising alternative cathode material for Li-S batteries.A Sulfur@hollow-V₂O₅/PPy?S@H-V₂O₅/PPy?cathode with dual core-shell structure is designed and synthesized,which is supposed to chemically trap polysulfide by introducing polar V₂O₅,and improve the cycling stability.The middle layer polar H-V₂O₅ spheres can not only provide enough inner space for alleviating the volume expansion for sulfur,but also effectively moderate the dissolution of polysulfides by synergistic effect of structural restriction and chemical adsorption.Additionally,outside PPy shell serves as conductive frameworks and generates sufficient electrical conduction paths.The initial capacities of S@H-V₂O₅/PPy cathode are up to 812.2,723.5,577.7 and446.3 m A·h·g^-1 at 0.5,1,2 and 4 C,respectively.Surprisingly,the discharge capacities are 825.6,723.5,577.7 and 446.3 m A·h·g^-1 respectively even after300 cycles.In this dissertation,two-dimensional?2D?sandwich-like?-ZrP/polypyrrole??-ZrP/PPy?nanosheets are designed as the sulfur host material.It is found that transfer and thus catalytically promote Li PSs conversion.Moreover,the?-ZrP nanosheet can provide a true-chemical anchor for sulfur species while PPy helps to build a physical barrier and form a conductive network for accelerating electron transport and finally promoting the electrochemical performance.In addition,the moderate catalysis function and efficient true-sulfur-confinement effect of ?-ZrP are confirmed by experimental characterizations.As a result,?-ZrP/PPy/S cathode exhibits a highly reversible capacity of 727.2 m A h g^-1even at a high current density of 5 C,offering a novel cathode material for high performance Li-S batteries.In order to prevent the dissolution and“shuttle effect”of Li PSs,we systematically investigate the adsorption of sulfur species on?-ZrP and study the detailed interaction and electronic structure,including adsorption strength,bonding length,charge transfer,lithium ion transfer,transformation energy and decomposition energy of sulfur species by using the first-principles approach.We demonstrate that?-ZrP possesses strong affinity toward Li PSs through S-O bond and delivers an ultralow lithium ion diffusion barrier.The strong interaction not only helps?-ZrP to confine S from dissolution but also catalytically promotes the conversion of Li PSs to Li₂S₂/Li₂S,indicating a promising way to finally suppress the Li PSs shuttling.In this dissertation,PPy is designed with different structures to solve the problems of volumetric expansion,poor conductivity and polysulfide diffusion,and further improve the cycle stability of Li-S batteries.Herein,PPy combined with polar inorganic materials for dual-confinement of sulfur species catalytically promotes conversion of Li PSs to control the polysulfide shuttle effect,enabling the improvement of capacity,cyclic durability and rate performance,which provides a novel design strategy for the preparation of cathode materials for high performance lithium sulfur batteries.