| Lithium-sulfur batteries present a high theoretical specific energy(2600 Wh Kg-1),which is much higher than the theoretical specific energy of current commercial lithiumion batteries.They are considered to be the most promising new generation lithium batteries.However,there are still many obstacles to the large-scale application of lithiumsulfur batteries,including the low conductivity of sulfur-containing active materials,the approximately 80%volume expansion of the positive electrode during discharge,and the shuttle effect of lithium polysulfides.The above problems lead to the problems of low utilization of active materials,poor cycle stability,low rate performance and coulombic efficiency etc.,hindering the commercial application of lithium-sulfur batteries.Based on these problems,this thesis focuses on modified interlayers and cathodes of lithium-sulfur batteries as the research object,and carries out related research work for the purpose of inhibiting the dissolution and shuttling of lithium polysulfides in the electrolyte.The main contents are as follows:(1)Carbon fiber cloths modified with Ni(OH)2 and Fe-doped Ni(OH)2 nanosheets(Ni(OH)2/CFC and Fe-Ni(OH)2/CFC)were prepared by a simple hydrothermal method.The composites were used as the cathode interlayers of lithium-sulfur batteries.The porous structure characteristics of Fe-Ni(OH)2 nanosheets grown on the surface of CFC prompt them to expose more adsorption active sites,increasing the contact with lithium polysulfide and maximizing the absorption of lithium polysulfide in the electrolyte.At the same time,the composites also act as secondary current collectors,accelerating the reaction processes of lithium polysulfides adsorbed on the surface of Fe-Ni(OH)2 nanosheets,finally improving the utilization of active materials.Fe doping in Fe-Ni(OH)2 nanosheets significantly improves their interaction with polysulfides.X-ray photoelectron spectroscopy(XPS)measurements revealed the interaction mechanism between Fe-Ni(OH)2 and lithium polysulfides.Fe-doped Ni(OH)2 improves the adsorption and catalysis of lithium polysulfide by enhancing the bonding and electron exchanges with Li2S6.Electrochemical tests suggested that the battery using FeNi(OH)2/CFC interlayers achieved higher specific capacity and more stable long-term cycling.The battery assembled with the Fe-Ni(OH)2/CFC intermediate layer delivered an initial discharge specific capacity of 1318 mAh g-1 at a rate of 0.5 C.After 500 cycles,the specific capacity remained to be 967 mAh g-1.(2)Based on the preparation method of Ni(OH)2/CFC and Fe-Ni(OH)2/CFC,activated carbon fiber cloths modified by Ni(OH)2 and Fe-doped Ni(OH)2 nanosheets(Ni(OH)2/ACC and Fe-Ni(OH)2/ACC)were prepared by the simple hydrothermal method.Following a high-temperature selenization process,NiSe2 and Fe-doped NiSe2 modified activated carbon fiber cloths(NiSe2/ACC and Fe-NiSe2/ACC)were obtained.The composites were employed as three-dimensional self-supporting lithium-sulfur battery positive cathodes with high sulfur loadings,and the effect of Fe-cation doping on the modification of NiSe2 was studied.Fe-doping in Fe-NiSe2 plays a key role in adjusting its electronic structure,further improving the charge transfers and interactions with the adsorbed lithium polysulfide molecules.XPS measurements revealed that Fe-doping enhances the interactions by promoting the transfer of electrons to the adsorbed Li2S6 molecules.DFT calculations suggested that Fe-doped NiSe2 enhances the charge exchange capacity with Li2S6 by increasing the electron density near the sulfur atoms of the newly formed S-Fe bonds,promoting the electrochemical reaction rate;Fe-doping improves the density of states near the Fermi level of Fe-NiSe2,elevating the anti-bonding orbital in the conduction band,leading to improved conductivity and facilitated conversion processes of the sulfur-containing active substances.Cyclic voltammetry(CV)measurements of the symmetric cells suggested that the Fe-NiSe2/ACC electrode presents sharp redox peaks,higher current densities,and larger peak areas,which proves that FeNiSe2 has an outstanding catalytic effect on the electrochemical conversion of lithium polysulfides.The unique three-dimensional structure of Fe-NiSe2/ACC promotes the transformation of the growth mode of Li2S from the conventional two-dimensional filmlike growth to the three-dimensional particle growth patterns.Li2S nucleation and deposition experiments intuitively proofed that Fe-NiSe2 significantly eliminates the obstacles to the nucleation and growth of liquid lithium polysulfides into solid Li2S.In addition,the unique three-dimensional Li2S deposition growth morphology improves the contact between Li2S and Fe-NiSe2/ACC cathode,which is beneficial to the electron and ion transmission processes.It also improves the utilization rate of sulfur,and effectively prevents the occurrence of "dead sulfur" phenomenon during charging and discharging.Even under a high sulfur loading,the material effectively promotes the sulfur redox kinetic processes and improves the utilization rate of active materials.The battery assembled based on the Fe-NiSe2/ACC cathode achieved a high areal capacity of 9.14 mAh cm-2 under a high sulfur loading of 9.9 mg cm-2.After 200 cycles at a rate of 0.1 C,the average capacity decay rate was only 0.11%.(3)Using activated carbon fiber cloths modified by Ni(OH)2 nanosheets(Ni(OH)2/ACC)as a precursor,"moss-like" P-doped NiSe2 grown on activated carbon fiber cloths(P-NiSe2/ACC)were prepared by a phosphorus selenization reaction.The composite was used as a three-dimensional self-supporting lithium-sulfur battery,and the effect of P-anion doping on the modification of NiSe2 was further studied.XPS measurements and DFT calculations together proofed that P-doping enhances the electron transfers between NiSe2 and lithium polysulfides,thereby improving the chemical adsorption and catalysis of the redox conversions of sulfur-containing active substances.P-doping in P-NiSe2 helps to increase the electron transfer rate of the material,and further promotes the redox rate of lithium polysulfides.P-doping also promotes the transfer of electrons from Li2S6 molecules to the P-NiSe2 matrix.P-NiSe2 and adsorbed Li2S6 form shorter chemical bonds as well as additional Li-P bonds,delivering higher chemical adsorption capacity;P-NiSe2 has a higher density of states at the Fermi level and the conductivity of the material is improved,benefiting higher catalytic activity.The strong interactions and catalytic effect of P-NiSe2 on lithium polysulfides promote the threedimensional growth of the discharge product Li2S on the P-NiSe2/ACC anode,which maximizes the utilization of sulfur and effectively inhibits the shuttling of lithium polysulfides.High specific capacities,outstanding high-rate performance and stable longterm cycling were achieved under high sulfur loads.The batteries assembled based on the P-NiSe2/ACC anodes exhibited discharge specific capacities of 1211,1096,941,806 and 679 mAh g-1 at 0.2,0.5,1,2 and 3 C,respectively.When the current density was switched back to 0.2 C,the discharge specific capacity restored to a high value of 1128 mAh g-1.Under the high sulfur load of 9.9 mg cm-2,a high areal capacity of 7.53 mAh cm-2 can be maintained after 100 charge/discharge cycles. |
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