Research On Electrode Process And Strategy To Enhance Performance Of Heavily S-loaded Cathode

Elemental sulfur(S)exhibits the advantages of high theoretical specific capacity(1675 mAh g-1),abundant reserves,low cost and environmental friendliness.Lithium sulfur(Li-S)battery composed of sulfur cathode and lithium anode is considered as a prospective candidate in next-generation energy storage systems.In the past decade,the developments of sulfur cathode have attracted wide attention and great progress has been made in improving the specific capacity and lifespan of low S-loaded cathode.However,considering that the average lithiated potential(2.2 V)of sulfur is much lower than that of the cathode material of traditional lithium-ion(Li-ion)battery(such as 3.8 V of LixCoO2),it is an inseparable subject to increase the S-loading of cathode to make up for the disadvantage of low lithiated potential of sulfur.The mass transfer of traditional Li-ion battery is relatively simple and single,with only the migration of Li+between cathode and anode.The reversibility of the mass transfer is fully guaranteed due to the fixed active materials.Nevertheless,the dissolution and deposition behavior of different active materials in sulfur cathode and the difference of phase transition process during charging(Li2S→Li2Sx→Ss)and discharging(S8→Li2Sx→Li2S)lead to the deterioration of the reversibility of mass transfer process.To deeply investigate the key issue of the reversibility of the mass transfer of sulfur cathode,an in-situ monitoring system for soluble polysulfides based on optical fiber UV-vis spectrophotometer was creatively constructed.Based on the research of sulfur electrode process,this thesis developes a nitrogen-doped porous carbon with high conductivity and uniform pore,which effectively improve the reversibility of electrode process.In addition,the construction of self-supporting electrode effectively regulates the mass transfer at the mesoscopic level and inhibits the segregation of active materials,thus obtain the heavily S-loaded cathodes with sable cycle performance.The specific research results are listed below:(1)Combining the electrochemical analysis with the ex-site SEM observation of active material deposition(S8 and Li2S),it is found that the irreversible deposition of Li2S in the discharge process is the key factor causing the segregation of active material,the instability of electrode structure and the acceleration of capacity decay of heavily S-loaded cathode.The more sulfur load,the more irreversible Li2S is formed,and the faster the capacity of sulfur electrode declines.(2)We have developed a method for in-site analysis of the composition and evolution of polysulfides in electrolyte during the charge and discharge process.An optical fiber UV-vis spectrum analysis technology is designed to realize the operando monitoring for polysulfides,and the evolution and migration process of polysulfides during charge and discharge process of Li-S battery are observed.The UV-vis spectrum results show that the migration of lithium polysulfides during the cycling induced the segregation of active substances,which led to the deterioration of the cycling stability of the sulfur cathode.(3)In order to inhibit the segregation of active materials,improve the utilization rate of sulfur and reduce the irreversible deposition of Li2S,this thesis developed a unique macroporous carbon material using NaCl as template through synthesis process of flash-freezing,freezing-dry and carbonization,which is used as a sulfur host to inhibit the active substances segregation and improve the consistency of mass transfer process at the micro-level.The agar gel containing NaCl can refine the NaCl crystal after flash-freezing and form a microcapsule of NaCl crystal coated with agar layer during freeze-drying process,effectively preventing agar and NaCl aggregation during calcination.Agar was completely carbonized at the melting temperature of NaCl to obtain macroporous carbon with high conductivity.(4)Nitrogen doping modification effectively improves the electrolyte wettability and Li2S affinity of macroporous carbon,promotes the uniform nucleation and growth of S8 and Li2S,strengthens the micro regulation of mass transfer process,improves the reversibility of phase change process during charge and discharge process,reduces the capacity decline rate of heavily S-loaded electrode,and delays the termination of poly sulfides delithiation.The sulfur cathode based on N-MPC delivers excellent rate performance and long cycle stability.After 1000 cycles at 5 C,it still retains a reversible capacity of 517 mAh g-1,and the capacity attenuation rate of a single cycle is only 0.028%(5)In order to eliminate the influence of active material segregation on the structural stability of heavily S-loaded cathode,a self-supporting sulfur electrode with good mechanical properties is constructed by rolling method with polytetrafluoroetylene(PTFE)as binder.C/S particles are firmly bound together by fibrotic PTFE,which effectively improves the structural stability of electrode,ensures the rapid and unobstructed electron/Li+transfer and inhibits the segregation of active substances.Benefit from the improvement of the reversibility of mass transfer at the mesoscopic level,the sulfur cathode with high sulfur loading of 36.5 mg cm-2 delivers an area capacity of 41.6 mAh cm-2(corresponding to an area energy density of 87.4 mWh cm-2)and maintains at 24.4 mAh cm-2(corresponding to 51.2 mWh cm-2)after 100 cycles at 0.1 C,which was much higher than those of the conventional cathode of Li-ion battery.(6)Heavily S-loaded cathode can easily induce the uneven deposition of lithium and the rapid development of lithium dendrites on anode surface,resulting in the rapid occurrence of "micro short circuit".Here,using an Ag2S-modified carbon cloth(Ag2S@CC)to prestore Li via molten infusion has been testified to effectively weaken the tendency of lithium dendrite and improve the reversibility of electrode process of anode.Furthermore,the Li-S full battery assembled with self-supporting lithium electrode and sulfur electrode(18.0 mgs cm-2)with a capacity ratio of 1.2:1 exhibits an area capacity of 23.3 mAh cm-2(corresponding to an area energy density of 46.8 mWh cm-2)and maintain a stable performance over 500 cycles without "micro short circuit".