Preparation Of Graphene-Like Composites And Its Application In Lithium-Sulfur Batteries

Faced with the depletion of fossil fuels and the growing problem of environmental contamination,the development of new renewable energy sources has become an inevitable trend.However,the fluctuating,intermittent and decentralized characteristics of new energy sources such as solar and wind power seriously restrict the development of renewable energy,and the development of clean and efficient energy storage systems is an important means to solve the non-stationary characteristics of renewable energy generation.Lithium-sulfur（Li-S）batteries are regarded as one of the most promising rechargeable energy storage devices of the new generation(2600 Wh kg^-1).Sulfur has the advantage of low pollution,large reserves,low prices and a very high theoretical capacity of 1675 m Ah g^-1,which is in line with the new clean energy.However,there are multiple problems within Li-S batteries that hinder their large-scale commercial application.For example,the dissolution shuttle of soluble lithium polysulfides（Li PSs）,the expansion of the electrode volume,the insulation of the discharge end product,and the growth of lithium dendrites severely reduce the cycle stability performance and rate performance of Li-S batteries.Reducing the shuttle of sulfur cathode Li PSs has become a hot research topic in the field of electrochemical energy storage today.As an important component of Li-S batteries,properly design separators can effectively improve the electrochemical performance of Li-S batteries.Graphene-like materials are ideal electrode materials for Li-S batteries because of their excellent mechanical and electrical properties as well as their specific surface groups.However,the disadvantages of some graphene-like materials,such as poor electrical conductivity and few active sites of Li PSs,also lead to poor electrochemical performance of Li-S batteries.As a result,in this thesis,constructed separators containing various types of graphene-like composites are built and applied to Li-S batteries to alleviate the shuttle effect,improve the sulfur utilization rate and polysulfide ion conversion rate,and therefore improve the overall performance of the Li-S battery.（1）Graphitic phase carbon nitride（UC₃N₄）was obtained by a simple secondary thermal treatment method using urea as a precursor and compounded with graphene（G@UC₃N₄）by solution mixing method as a Li PSs cathode side barrier layer on polypropylene（PP）separators for Li-S batteries.The UC₃N₄ obtained by two oxygen etching heat treatment methods has a larger specific surface area and a greater abundance of active sites.This procedure was advantageous since it is simple,convenient,and economical.The combination of graphene with the UC₃N₄ sheet layer both prevents self-stacking of the material and compensates for the low electrical conductivity of UC₃N₄ itself.The composites have a large specific surface area of217.72 m² g^-1 and a pore size of 12.03 nm on average.The high conductive network of graphene enhances the transport rate of polysulfide ions and Li⁺within electrode,while the high specific surface area of UC₃N₄ provides a richer trapping site for Li PSs to inhibit their shuttling back and forth between cathode and anode.Through their respective advantages,they accelerate the"solid-liquid-solid"transition of polysulfide ions in electrochemical reactions,improving the electrochemical reaction kinetics.The Li-S battery with the G@UC₃N₄ modified separator exhibited an average capacity decay rate of 0.047%over 1000 cycles at 2 C,demonstrating excellent long cycle stability.（2）A simple van der Waals self-assembly method was used to decorate nitrogen-deficient graphitic phase carbon nitride（NDCN）on the surface of Ti₃C₂T_x nanosheets to obtain Ti₃C₂T_x/NDCN composites,which were introduced into Li-S batteries as modified separator coating materials to obtain high rate and long-life Li-S batteries.The N concentration was lowered to 46.37%by the NDCN created using the molten salt assisted method,and a significant amount of N vacancies were created.The Ti₃C₂T_x nanosheets obtained by Li F/HCl etching have good mechanical properties as well as abundant terminal groups.After the two materials were composited,the insertion of NDCN between the Ti₃C₂T_x nanosheets prevented self-stacking of the Ti₃C₂T_x nanosheets and exposed a high number of active adsorption sites on the material’s surface for enhanced Li PSs adsorption.The density functional theory（DFT）adsorption energy theoretical calculations showed that both Ti₃C₂T_x and NDCN can adsorb Li PSs by chemical bonding,and this is demonstrated in Li₂S₆ adsorption experiments,and electrolytic cell visualization shuttle experiments.In addition,the contact angle between the PP@Ti₃C₂T_x/NDCN separator prepared by the surface coating method and the electrolyte was 8.8°,indicating that the Ti₃C₂T_x/NDCN material has good wettability with the electrolyte and was more conducive to ion transport inside the electrode.The PP@Ti₃C₂T_x/NDCN cell has distinct charge/discharge plateaus at a high current rate of 4 C and is capable of delivering a capacity of 655.8 m Ah g^-1,demonstrating superb rate performance,with an average capacity decay rate as low as 0.025%per cycle for 1300 charges and discharges at 2 C,verifying that the PP@Ti₃C₂T_x/NDCN cell possesses long cycling capability at high current densities and high sulfur utilization.Ti₃C₂T_x/NDCN significantly decreased the overpotential for Li₂S nucleation.Due to the dynamic adsorption of NDCN,liquid Li PSs were more readily trapped on the Ti₃C₂T_x/NDCN surface to induce nucleation and have the same facilitative effect on the oxidation of Li₂S,implying that Ti₃C₂T_x/NDCN is a bifunctional reversible catalyst.