| Energy plays a pivotal role in the production activities of human beings.Traditional fossil energy has been facing the dilemma of depletion,and its massive use has also caused serious environmental pollution and ecological damage.Therefore,the development and utilization of green renewable energy to meet the growing demand of human for energy is one of the hot spots of concern in the world today.However,most renewable energy sources,such as solar energy,wind energy,tidal energy,etc.,are unstable and intermittent,and require energy storage systems to obtain a stable energy supply.Therefore,the development of high-efficiency energy storage equipment is of great significance in the utilization of new energy sources and the realization of the“double carbon”goal.Lithium-ion batteries dominate the secondary battery market today and are widely used in people’s daily life.However,the theoretical energy density of conventional Li-ion is low,and the actual energy density of current commercial Li-ion batteries is close to its theoretical achievable limit,making it difficult to meet the growing demand for applications.Research on new electrochemical energy storage systems with high energy density is imperative.As a new type of lithium secondary battery,lithium-sulfur（Li-S）batteries not only have a high theoretical energy density(2567 Wh kg-1)and theoretical specific capacity(1675 m Ah g-1),but also have the advantages of low cost,non-toxicity,environmental friendliness and compatibility with existing battery manufacturing technology,etc.,which is the next generation battery that is closest to practicalization and most promising.It is one of the most promising next-generation battery systems.Despite the advantages of Li-S batteries,there are still many problems to be solved in order to achieve their commercial application:（1）the conductivity of sulfur and lithium sulfide is extremely poor.This will hinder the electron transport,which is not conducive to electrochemical reactions;（2）serious volume expansion.Before and after lithiation sulfur has up to 80%volume change can lead to the destruction of the electrode structure or even pulverization;（3）shuttle effect.Lithium sulfide battery intermediate discharge product lithium polysulfide easily dissolved in the electrolyte and diffused to the negative electrode area,resulting in a large loss of active material;（4）lithium dendrite growth.With lithium metal as the negative electrode,the uneven deposition of lithium during the charging and discharging process of the battery will form lithium dendrites,which may cause safety accidents.In response to the above problems,solution strategies such as preparing the composite structure of sulfur cathode,selecting electrolyte additives,modifying the diaphragm to achieve multifunctionality,and designing the lithium cathode interface have been proposed to improve the electrochemical performance of lithium-sulfur batteries.This dissertation focuses on the functionalization of Li-S battery separators,from the simple physical barrier of pure conductive carbon materials,strong chemical adsorption and capacity contribution of polar oxides,to the introduction of catalytic sulfides to accelerate the conversion of polysulfides,and finally the synergistic effect of conductivity,adsorption and catalysis are combined to explore the design and preparation of multifunctional Li-S battery separators,investigating the effects of different separators on the performance of lithium-sulfur batteries,achieving the effective suppression of the shuttle effect and the construction of a high-performance Li-S battery are investigated.The main contents are as follows:1.Carbon micron tubes modified functional separator for Li-S batteries.The hierarchical hollow carbon microtubes（CMTs）are simply prepared by carbonization of willow catkins and coated on a commercialized separator for high-performance Li-S batteries.The CMTs modified separator can inhibit the diffusion of polysulfides through physical blocking,which alleviates the shuttle effect effectively.In addition,the CMTs interlayer acts as an upper current collector to provide continuous highways for electron and ion transport to enhance the utilization of active materials.As a result,the capacity of cells with CMTs modified separators was significantly increased.2.CeO2 decorated graphene as a separator modifier for Li-S batteries.We have designed and developed a multifunctional separator modified by CeO2 decorated graphene（CeO2@G）to enhance the performance of Li-S batteries.The CeO2nanoparticles not only immobilize polysulfides by strong chemisorption,but also act as catalytic agent to accelerate polysulfides redox reaction.Moreover,the highly conductive graphene sheets functioned as an upper current collector to improve the electron/ion conductivity and facilitate the reutilization of sulfur species.As a result,the Li-S battery with the CeO2@G modified separator delivers high specific capacity,excellent rate performance,and long cycle life.3.Constructing V2O5/graphene interlayer to improve the electrochemical performance of Li-S batteries.We developed a new strategy to construct the V2O5 microspheres/graphene nanosheets（V2O5/G）layer on commercial polypropylene separator as a vice-electrode.On one hand,the V2O5/G interlayer can act as a barrier to block polysulfides diffusion and then suppress the troublesome shuttle effect.On the other hand,lithium ions can insert in and extract from V2O5 during charge/discharge processes,and thus contribute extra cathode capacity and large voltage window to the batteries.As expected,the Li-S batteries with a V2O5/G interlayer deliver a large voltage window of 2.0 V（3.6-1.6 V）and a significantly improved electrochemical performance.4.Co9S8/RGO-functionalized separator for high-performance Li-S batteries.An integrated structure consisting of reduced graphene oxide（RGO）nanosheets modified with Co9S8 nanoparticles has been synthesized,which is expected to be used as a mediator for Li-S batteries to improve the anchoring and catalyzing of polysulfides.The Co9S8 nanoparticles not only have excellent chemisorption capability to capture polysulfides,but also can be used as catalysts to accelerate the conversion of polysulfides.Highly conductive reduced graphene oxide nanosheets can provide appropriate ion/electron diffusion paths to promote interfacial reaction kinetics.Accordingly,the as-assembled Li-S battery has excellent cycling stability and enables the device to operate over a wide temperature range.5.Efficient regulation of polysulfides by MoS2/MoO3 heterostructures for high-performance Li-S batteries.A novel architecture of MoS2/MoO3 heterostructure uniformly distributed on carbon nanotubes（MoS2/MoO3@CNT）has been designed and introduced into Li-S batteries via decorating commercial separator to regulate the redox reactions of polysulfides.Systematic experiments and theoretical calculation showed that the heterostructure not only provides sufficient surface affinity to capture polysulfides and acts as an active catalyst to promote the conversion of polysulfides,but also the highly conductive CNT enable the rapid electron/ion migration.As a result,Li-S batteries with the MoS2/MoO3@CNT-PP separator deliver a high reversible capacity,excellent rate capacity,and low self-discharge capacity loss.Moreover,even at an elevated temperature of 70°C,it still exhibits high capacity retention.When the sulfur load is increased to 8.7 mg cm-2,the high reversible areal capacity of 6.61 m Ah cm-2 can be stably maintained,indicating a high potential for practical application. |
