Activated Carbon For Li-S Batteries And The Effect Of Pore Structure On The Performance

Lithium-sulfur batteries are considered as a promising energy storage system due to the high theoretical energy density(2600 Wh·kg^-1)and specific capacity(1675m Ah·g^-1).And the sulfur has the advantages of cheap,environmental protection,abundant and easy to obtain,which make its large-scale practical application possible.Although lithium-sulfur batteries have many advantages,the commercialization of them still faces many challenges,including the actual capacity of lithium-sulfur batteries is far from the theoretical value,the long cycle stability and the scaling performance can not meet the practical application requirements,especially with the increase of the sulfur loading,the above problems become more serious.The results show that these problems are mainly related to the following factors:the poor conductivity between the sulfur and the final discharge product Li₂S₂/Li₂S;the volume expansion of sulfur occurs during the lithium process;the sulfur in the discharge process produce soluble lithium polysulfides（Li₂S_x,4≤x≤8）,dissolved in electrolyte,resulting the shuttle effect.In order to overcome these shortcomings,many methods have been proposed to improve the electrochemical performance of the sulfur cathode.One of the most widely used is using activated carbon as electrode material and sulfur encapsulation materials,conductive materials as activated carbon material is qualitative light,large specific surface area,pore structure,high electrical conductivity,etc.These advantages make it more widely in the field of lithium-sulfur batteries,and have gained excellent performance.However,the specific effect of pore structure on the electrochemical performance of lithium-sulfur batteries is seldom studied.It is well known that the suitable microstructure of activated carbon can significantly affect the migration resistance of lithium ions and electrons in S/C composite cathode materials,thus affecting the macroscopic reaction kinetics of sulfur redox process of active compounds and the charge/discharge performance of batteries.Therefore,in this paper,we use walnut shell and sodium citrate as raw materials,by changing the weight ratio of KOH to carbonized precursor（KOH/carbonized precursor ratio）,and the pore structure of the activated carbon sample is micro-regulated.The effects of activator dosage on the specific surface area,pore volume and other pore structures of activated carbon are investigated.At last,the relationship between these factors and the electrochemical performance of lithium-sulfur batteries is also discussed.This thesis mainly content:（1）We use walnut shell（WS）as carbon source and KOH as activator.A series of activated carbon with different pore structures are produced by changing the weight ratio of KOH to carbonized precursor from walnut shell（KOH/carbonized precursor ratio）,and investigating the relationship between the pore structure and the electrochemical performance.The result shows that the specific surface area,total pore volume and percentage of mesoporous volume of activated carbon increased obviously with the rising KOH/carbonized precursor ratio.Both the initial discharge specific capacity and the scaling performance of the composite cathode are firstly increased and then decreased slightly with the increase of the specific surface area,total pore volume and percentage of mesoporous volume.When the KOH/carbonized precursor ratio is 6:1,WSAC-6/S with large specific surface area(3913 m²·g^-1),total pore volume(2.26 cm³·g^-1)and percentage of mesoporous volume（59.4%）shows high initial specific capacity(1397 m Ah·g^-1)and stable charge/discharge performance(869 m Ah·g^-1 after 100 cycles at 0.2 C).Especially at 0.5 C,it still has excellent electrochemical performance,and the specific discharge capacity can be maintained at707 m Ah·g^-1 after 200 cycles.（2）We use sodium citrate（SC）as carbon source and KOH as activator.A series of activated carbon with different pore structures are produced by changing the weight ratio of KOH to carbonized precursor from sodium citrate（KOH/carbonized precursor ratio）,and investigating the relationship between the pore structure and the electrochemical performance.The result shows that the specific surface area and total pore volume of activated carbon increased obviously with the rising KOH/carbonized precursor ratio.However,the micro specific surface area and micropore volume increased and then decreased.And the long cyclic stability and the scaling performance of the composite anode are positively correlated with the increase of the micro specific surface area and micropore volume.When the KOH/carbonized precursor ratio is 4:1,the ASC-4/S sample with the largest micropore specific surface area(623 m²·g^-1)and micropore volume(0.26 cm³·g^-1)shows high initial specific capacity(1506 m Ah·g^-1)and stable charge/discharge performance(907 m Ah·g^-1 after100 cycles at 0.2 C).Especially at other high rates,it still has excellent electrochemical performance.The specific discharge capacity can be maintained at869 m Ah·g^-1 after 200 cycles at 0.5 C and 685 m Ah·g^-1 after 300 cycles at 1 C.