Study On All-solid-state Lithium-sulfur Batteries With Sulfide Solid Electrolyte

With the increase in energy storage demand in recent years,the development of new battery systems has received extensive attention worldwide.All-solid-state Li-S batteries(ASSLSBs)have attracted great attention due to their ability to resolve the two key issues(polysulfides shuttle effect and safety)of traditional liquid electrolyte based Li-S batteries.However,it remains a huge challenge for ASSLSBs to achieve high areal active mass loading and high utilization efficiency of the active materials simultaneously due to the insulating nature of sulfur and Li2S,and the large volume change during cycling.The construction of sulfur composite electrode with uniform ion/electron conductive network and low strain/stress play significant roles in the preparation of high performance ASSLSBs.In this thesis,sulfur composite cathodes with different S,solid electrolyte and C compositions and Li2S@C nanocomposites were prepared.And they electrochemical performance as cathode materials for ASSLSBs is investigated with the use of Li7P3S11(LPS)as solid electrolyte.Sulfide solid electrolyte Li7P3S11 with high ionic conductivity is synthesized by ball milling and two-step heat treatment using lithium sulfide and phosphorus pentasulfide as raw materials.XRD and SEM analysis showed that the pure phase Li7P3S11 is successfully synthesized,and highly dense solid electrolyte pellets can be prepared under cold pressure.The electrolyte pellet prepared by cold pressing at room temperature shows a room temperature ionic conductivity of 1.7× 10-3 S cm-2.Using element sulfur as the active material,the effect of composition of the sulfur composite cathode on electrochemical performance of ASSLSBs is systematically investigated.It shows that compared with Super P and Ketjen Black,ASSLSBs using acetylene black(AB)as conductive additive show optimal electrochemical performance.When using acetylene black as a conductive additive,sulfur composite cathode with mass ratio of S:LPS:AB=4:4:2 presents best electrochemical performance.Li2S@C nanocomposit with Li2S nanocrystals uniformly embedded in conductive carbon matrix,is in-situ generated by the combustion of lithium metal with CS2.The in-situ formed conductive carbon network not only significantly improve the electronic conductivity but also effectively inhibit the aggregation of Li2S and accommodate the large volume change of Li2S during cycling.Meanwhile,the in situ formed nanoscle percolation network offer predominant pathways for both electrons and ions,and mitigates the stress/strain during lithiation/delithiation.Benefiting from its unique structure,the Li2S@C nanocomposit exhibits exceptional electrochemical performance as cathode for ASSLBs,with both ultrahigh areal Li2S loading(8 mg cm-2),high Li2S content(42%)and 81%of Li2S utilization(corresponding to a reversible capacity of 945 mAh g-1 and areal capacity of 7.56 mA h cm-2)at 60 ℃.It delivered a high reversible capacity of 1050 mAh g-1 at 0.2 mA cm-2 with a Li2S loading of 2 mg cm-2 at 60 ℃.Moreover,the Li2S@C nanocomposit also possesses outstanding rate capability and cycle stability.It exhibited high reversible capacity of 613 mAh g-1 and superior long-term cycling performance with high capacity retention of 93%after 700 cycles even at a high current density of 2 mA cm-2 at 60 ℃.The Li2S@C nanocomposite also exhibits outstanding electrochemical performance at room temperature.With a Li2S loading of 4 mg cm-2,the cell delivers high specific capacity of 900,760 and 620 mAh g-1 at current density 0.2,0.5 and 1 mA cm-2,respectively.