Preparation And Research Of High Energy Density Sulfurized Polyacrylonitrile-based Lithium-sulfur Battery Positive Electrod

With increasing time-of-use requirements in areas such as portable electronic devices,drones and electric vehicles,it is becoming increasingly difficult for traditional lithium-ion batteries to meet the energy storage needs of emerging markets.Lithium-sulfur（Li-S）batteries hold great promise for new high-specific energy storage systems due to their high energy density and low cost of raw material.Sulfurized poly（acrylonitrile）（SPAN）is one of the most promising cathode materials for lithium-sulfur batteries with good cycle stability,high sulfur utilization,and low self-discharge rate.However,the sulfur content of SPAN is insufficient,the molecular structure and electrochemical reaction mechanism are not clear,and the electrochemical performance,especially the high-rate capacity,volumetric energy density and areal capacity still have room to improve.Hence,this paper synthesized SPAN composites with high sulfur content,then clarified its molecular structure and electrochemical reaction mechanism,and adopted various strategies to further improve its electrochemical performance.The main contents are summarized as follows:（1）Polyacrylonitrile（PAN）nanofibers were prepared by an electrospinning method,and then SPAN composites with high sulfur content were synthesized by optimizing the sulfuration process.When the reaction temperature is 330°C and the reaction time is 6 h,the sulfur content of SPAN reaches 51.73%,which is higher than the value reported in most literatures.Based on the whole composite material,the SPAN cathode delivers a reversible discharge capacity of 737 m Ah g^-1 at 0.2 C,corresponding to an active materials utilization of 85.1%.In addition,the SPAN cathode maintains stable cycling of over 500 cycles at 1 C,showing excellent cycle stability.The electrochemical reaction mechanism of SPAN was studied by ex-situ test.During the first discharge,all the C-S and S-S bonds in SPAN are broken and reduced to form Li₂S₂ and Li₂S.At the same time,the C=C and C=N double bonds in the conjugated carbon skeleton react with lithium ions and contribute capacity.During subsequent charging,Li₂S₂ and Li₂S are oxidized to reform C-S and S-S bonds,while the lithium ions in the conjugated carbon skeleton no longer participate in the delithiation reaction,thus leading to irreversible capacity loss in the first discharge.The lithium ions in the conjugated carbon skeleton help to improve the conductivity of the electrode and reduce the polarization,so the voltage platform of SPAN increases significantly during subsequent discharge.（2）Eutectic accelerator-selenium（Se）and highly conductive carbon material-multi-walled carbon nanotubes（MWCNTs）were introduced into the SPAN nanofibers at the same time,which effectively improved the discharge capacity and rate performance of the SPAN cathode.Consequently,the Se_0.03SPAN/MWCNT-3 cathode delivers a high discharge capacity of 791 m Ah g^-1at 0.2 C,corresponding to an active materials utilization rate of 94.8%.At 4 C high rate,the Se_0.03SPAN/MWCNT-3 cathode can still release a specific capacity of 638 m Ah g^-1,which is much higher than that of185 m Ah g^-1 for SPAN and 422 m Ah g^-1 for Se_0.03SPAN.Moreover,the Se_0.03SPAN/MWCNT-3 cathode maintains high reversible specific capacity of 733 m Ah g^-1（0.1 C）and 672 m Ah g^-1（0.2 C）with stable cycling of over 60 cycles.（3）To solve the problem of insufficient volumetric energy density of Li-S batteries,Se_0.4SPAN was applied as the active material to construct compact cathode for lithium storage.The addition of Se not only elevates the content of active materials in the composite,but also effectively facilitate the transport of lithium ions and electrons,thus improving the active materials utilization of the electrode.Consequently,the Se_0.4SPAN composite presents a high volumetric specific capacity of 1185 m Ah cm^-3 at 0.1 C,about1.4 times that of SPAN.Under high mass loading(7.5 mg cm^-2)and lean electrolyte(6.4μL mg^-1)conditions,the calendered Se_0.4SPAN cathode still maintains good electrochemical performance.After treatment at 10 MPa,the density of Se_0.4SPAN cathode can reach 1.51 g cm^-3,and the corresponding volumetric energy density can reach 1537 Wh L^-1（0.1 C）and 1454 Wh L^-1（0.2 C）.Even with low electrolyte amount to 1.5μL mg^-1,the calendered Se_0.4SPAN cathode still reveals normal electrochemical behavior and delivers a high specific capacity of 608 m Ah g^-1 in the second cycle,corresponding to a benign active materials utilization of 81.2%.（4）Thick Se_0.1SPAN cathodes with high mass loading were prepared by a facile coating-lyophilization method.Freeze-dried cathode（FD-C）can avoid the electrode film cracking due to surface tension in the process of thermal drying.In the meantime,the micro-cracks caused by in-situ sublimation of“ice crystal”during the freeze-drying process is conducive to the infiltration of electrolyte and lithium ions transport,which effectively improves the electrochemical reaction kinetics of thick electrodes.When the mass loading is 5.8 mg cm^-2,the FD-C delivers a high discharge capacity of 719 m Ah g^-1at 0.2 C,corresponding to an active materials utilization of 89.1%.In addition,the FD-C shows excellent rate performance,even at 2 C high rate,it can release a discharge capacity of 526 m Ah g^-1,which is much higher than that of 323 m Ah g^-1 for the thermal drying electrode（TD-C）.More impressively,the FD-C maintains excellent electrochemical performance even under ultra-high mass loading(17.0 mg cm^-2)and lean electrolyte conditions.The capacity retention of FD-C under 5 and 4μL mg^-1 electrolyte after 100 cycles are 84.7%and 80.3%,respectively,demonstrating great potential for practical applications.In summary,aiming to fabricate high-performance SPAN-based cathode,this paper systematically studied the molecular structure,electrochemical performance,and electrochemical mechanism of SPAN composite.This systematic investigation would play an important role in promoting the development and application of high-performance cathode materials for Li-S batteries.