Construction And Electrochemical Properties Of Transition Metal Nitride Nanomaterials For Lithium Sulfur Batteries

The lithium-sulfur batteries are considered as one of the most promising next-generation lightweight and high-energy-density batteries due to their ultra-high theoretical energy density(2600 Wh kg^-1).However,the practical applications of lithium-sulfur batteries still face many challenges,mainly including the shuttle effect of polysulfides（Li PSs）and the uncontrollable growth of lithium dendrites.In response to these issues,the polar transition metal nitrides were chosen as the research object,and nano-functional materials of nitrides were constructed in the separator and sulfur host to systematically study their suppression rules on Li PSs shuttle effect and lithium dendrite growth.The analysis revealed the electrochemical mechanisms that enhance the performance of lithium-sulfur batteries.The main research contents and results are as follows:1.Construction of nitride interface layers on separator and their performance study in lithium-sulfur batteries:（1）By employing a rapid microwave-assisted hydrothermal combined with ammoniation method,the niobium nitride（Nb N）nanodots uniformly anchored nitrogen-doped graphene interface layer（Nb N@NG）was prepared.Studies have shown that Nb N@NG not only can adsorb and catalyze the conversion of Li PSs but also promote the uniform nucleation and growth of Li₂S.Lithium-sulfur batteries using Nb N@NG separator exhibited an initial reversible capacity as high as 1284 mAh g^-1 at 0.2 C,with a decay rate of only 0.036%per cycle after 500 cycles at 2 C.（2）A solvent-thermal reaction combined with ammoniation treatment was used to prepare tungsten nitride(WN_0.67)nanoneedles uniformly anchored on nitrogen-doped graphene interface layer(WN_0.67@NG).Studies have shown that the high conductivity of WN_0.67@NG can facilitate electron transfer,and its strong adsorption and catalytic conversion of Li PSs improve the performance of lithium-sulfur batteries:Lithium-sulfur batteries using WN_0.67@NG separator showed a capacity decay rate of only 0.07%per cycle after 200 cycles at 0.2 C.2.Construction of dual-functional nitride interface layers on separator and their performance study in lithium-sulfur batteries:（1）Using reflux and high-temperature calcination methods,a dual-functional interface layer of molybdenum nitride（Mo₂N）quantum dots uniformly anchored on nitrogen-doped graphene nanosheets in the separator（Mo₂N@NG）was prepared.The study showed that Mo₂N@NG exhibits excellent sulfur affinity and lithium affinity.The synergistic electron coupling between NG and Mo₂N is conducive to the adsorption and catalytic conversion of Li PSs in the sulfur reaction kinetics process.Mo₂N@NG possesses strong chemical lithium affinity and high electron conductivity,which helps alleviate and inhibit dendrite formation.Lithium-sulfur batteries using Mo₂N@NG separator demonstrated excellent electrochemical performance:a specific capacity of860.2 mAh g^-1 at 4 C,a capacity decay of only 0.039%per cycle after 800 cycles at 2 C,and the lithium anode remained stable even after cycling for 1500 hours at 5 m A cm^-2.（2）A simple two-step pyrolysis strategy was proposed to prepare vanadium nitride（VN）quantum dots uniformly anchored on nitrogen-doped carbon nanosheets in the separator as a dual-functional interface layer（VN@NC）.The study demonstrated that VN@NC exhibits excellent lithiophilicity and sulfiphilicity.VN@NC could chemically anchor Li PSs rapidly and efficiently catalyze their conversion,thereby inhibiting the shuttle effect of Li PSs.The VN quantum dots,as lithium-affinitive sites,could effectively regulate the lithium deposition and stripping processes to achieve uniform lithium deposition.Lithium-sulfur batteries with the VN@NC-modified separator layer achieved an ultra-long cycle life（decay rate of only 0.036%per cycle after 1000 cycles at 2 C）.3.Construction of dual-functional nitride hosts and their performance study in lithium-sulfur full batteries:（1）Using a ball milling combined with high-temperature pyrolysis strategy,a dual-functional host of niobium nitride quantum dots with both lithiophilicity and sulfiphilicity was constructed on nitrogen-doped hollow carbon nanorods（Nb N@NHCR）.The study showed that the uniformly dispersed sulfiphilic Nb N quantum dots exhibits excellent chemical adsorption and catalytic conversion of Li PSs,effectively suppressing their shuttle effect.The lithiophilic Nb N quantum dots could redistribute lithium ions uniformly,achieving even lithium deposition and effectively inhibiting the growth of lithium dendrites.These advantages enabled the Nb N@NHCR pouch full battery to maintain a capacity of 5.0 mAh cm^-2 after 50 cycles under the conditions of a sulfur loading of 5.8 mg cm^-2 and an electrolyte/sulfur ratio of 5.2μL mg^-1.（2）By employing a two-step process of microwave-assisted hydrothermal and high-temperature calcination,uniformly sulfiphilic and lithiophilic WC-WN_0.67 heterojunction quantum dots were constructed on nitrogen-doped graphene nanosheets(WC-WN_0.67@NG).The research demonstrates that this host can not only accelerate the kinetics of sulfur redox reactions but also efficiently chemically adsorb and catalytically convert Li PSs.Its lithium-affinitive properties promote the uniformity of lithium-ion concentration and the dispersion of local charge density,leading to uniform lithium deposition/stripping.These synergistic interactions enable the lithium-sulfur pouch full battery to maintain a high reversible areal capacity of 5.79 mAh cm^-2 after 50 cycles under the conditions of 0.1 C and a sulfur loading of 6.2 mg cm^-2.