Surface Modification And Properties Of Lithium Sulfide Cathode Materials

By virtue of its high theoretical capacity,low cost and environmental friendliness,lithium-sulfur batteries have drummed up extensive attention and are considered one of the most promising next-generation rechargeable power systems.With the rise of new energy vehicles,requirements for the performance of power battery become higher,especially for its safety performance.The safety performance of traditional lithium-sulfur battery assembled with organic electrolyter is not satisfying because of its use of lithium anode with high activity and flammable organic electrolyte.In this connection,using solid electrolyte instead of organic electrolyte to assemble all-solid-state lithium-sulfur batteries with high safety performance has become a popular choice for many researchers.However,the use of solid electrolyte brings new problems.The most prominent one is the interface among the active material,conductive agent and solid electrolyte in the cathode.The large interface impedance among them is not conducive to the transport of electrons and Li⁺.Faced with the problems brought by the interface among solid electrolyte,this thesis aims to improve the electrochemical performance of all solid-state lithium sulfur battery.It studies the the cathode of all solid-state lithium-sulfur batteries and the interface among the active material,solid electrolyte and conductive agent was improved by in-situ reaction.This paper also investigates the effects of the content of each component and preparation process on the distribution of conductive and ion conducting network in the cathode and its electrochemical performance.（1）In order to improve the interface between active material and solid electrolyte in cathode of all-solid-state lithium-sulfur batteries,Li₃PS₄ was generated on surface of Li₂S by ball milling with Li₂S and P₂S₅ as starting materials.The results show that chemical bonding occurs at the interface between Li₃PS₄ grown by in-situ reation and Li₂S,thus reducing the impedance of Li₂S/Li₃PS₄ interface and facilitates the transport of Li⁺.The 45Li₂S@Li₃PS₄-300 obtained by heat treatment at 300℃for 1h shows the best electrochemical performance,and the ionic conductivity is as high as 2.67×10^-4 S cm^-1 at 30℃。The Li₂S@Li₃PS₄+AB was prepared by mixing Li₂S@Li₃PS₄ prepared by ball milling with acetylene black（AB）by ball milling.After 30 times of cycling at a current density of 0.05m A cm^-2,the specific discharge capacity of Li₂S@Li₃PS₄+AB remained at 624.5 m Ah g^-1.Under the same current density,the discharge capacity of Li₂S+Li₃PS₄+AB prepared by direct mixing was only 213.6 m Ah g^-1 after 30 times cycling.This proves that the electrochemical performance of the cathode can be largely improved by modifying the interface between the active material and the solid electrolyte by in-situ growth of Li₃PS₄.In addition,the influence of the content and type of the conductive agent on the electrochemical performance of the cathode was also studied.The results suggests that when the content of AB in the cathode is too high,the ion transport network in the cathode will be blocked,thus reducing the electrochemical performance of cathode.On the contrary,if the AB content is too low,an effective electron transport network can not be constructed in the cathode.Li₂S@+Li₃PS₄+10AB with an AB mass fraction of 10 wt%delivered the highest specific discharge capacity,and the discharge specific capacity at the 30^th cycle was 637.6 m Ah g^-1.With an appropriate amount of one-dimensional conductive agent carbon nano tube（CNT）add in the cathode to assist the construction of electron transport network,the electrochemical performance of the cathodes have been largely improved.Li₂S@+Li₃PS₄+9.2AB+0.8CNT with 0.8 wt%CNT shows the best electrochemical performance,and the discharge specific capacities at current densities of 0.10,0.20,0.40 and 0.80 m A cm^-2 are 1273.2,1131.8,951.6 and 598.5 m Ah g^-1,respectively.Li₂S@Li₃PS₄+AB with high Li₂S content was prepared by changing the mass ratio of Li₂S and Li₃PS₄ in cathode.The specific discharge capacity of the positive electrode with Li₂S content of 40.5 wt%,56.3 wt%and 64.4 wt%are 1131.8,714.8 and 381.3m Ah g^-1,respectively.This reveals that when the content of conductive agent is fixed,the increase of Li₂S and the decrease of Li₃PS₄ will increase the polarization of positive electrode,resulting in the decline of discharge specific capacity of cathode.40.5Li₂S@Li₃PS₄+AB+CNT was used to assemble the all-solid-state lithium-sulfur battery with a Li₂S area density of 6.19 mg cm^-2.The results show that the specific discharge capacities of battery with a Li₂S areal density of 6.19 mg cm^-2 at 0.4,0.8,1.6and 3.2 m A cm^-2 are 1068.4,975.0,818.1 and 369.9 m Ah g^-1,respectively.（2）In this paper,the preparation of solid electrolyte Li₃PS₄ on the surface of Li₂S by liquid-phase shaking with tetrahydrofuran（THF）as solvent was explored.The results of NMR show that there is also a chemical bonding in interface between Li₂S and Li₃PS₄ in the Li₂S@Li₃PS₄ prepared by the liquid-phase shaking.The ionic conductivity of the product obtained at a constant temperature of 320℃for 1h was the highest,which was 5.67×10^-5 S cm^-1 at 30℃.The Li₂S@Li₃PS₄+AB was prepared by mixing Li₂S@Li₃PS₄ prepared by liquid-phase shaking with AB by ball milling.After 40 times of cycling with a constant current of 0.1 m A cm^-2,the specific discharge capacity of Li₂S@Li₃PS₄+20AB remained at 535.1 m Ah g^-1.After 30 times of cycling with a constant current of 0.2 m A cm^-2,the discharge specific capacity of Li₂S@Li₃PS₄+19.2AB+0.8CNT obtained by the adding of 0.8 wt%CNT remained at 651.0 m Ah g^-1.In order to further improve the electrochemical performance of Li₂S@Li₃PS₄+19.2AB+0.8CNT prepared by the liquid-phase shaking,the particle size of Li₂S was reduced by ball milling pretreatment.The results show that as the particle size of Li₂S decreased,the contact area among the electrolyte and conductive carbon increases and it gets bigger,and the electrochemical performance of the obtained cathode appears to be better.The specific discharge capacity of the positive electrode prepared with Li₂S with a particle size of 0.33 um at current densities of 0.1,0.2,0.4and 0.8 m A cm^-2 are 859.9,779.6,702.5 and 583.9 m Ah g^-1,respectively.Based on the above research,the all-solid-state lithium-sulfur battery with an area density of 5.50 mg cm^-2 was assembled with 36.0Li₂S@Li₃PS₄+AB+CNT.The specific discharge capacities of the batteries were 689.8,586.1,413.3 and 5.9 m Ah g^-1 at current densities of 0.4,0.8,1.6 and 3.2 m A cm^-2,respectively.（3）In this thesis,Li₂S@AB was prepared by carbothermic reduction method.Then,Li₃PS₄ was in situ formed at the active material Li₂S of Li₂S@AB by ball milling.The electron and ion transport networks were built for Li₂S through this two step in-situ reactions.The specific discharge capacities of Li₂S@（AB-Li₃PS₄）+CNT with 38.8 wt%Li₂S at 0.1,0.2,0.4 and 0.8 m A cm^-2 are 1047.9,978.2,892.7 and 768.9m Ah g^-1,respectively,which are much higher than those of Li₂S+Li₃PS₄+AB+CNT prepared by direct ball milling.The results show that the electrochemical performance of Li₂S@（AB-Li₃PS₄）+CNT has been effectively improved thanks to the improvement of the interface among Li₂S,Li₃PS₄ and AB in the cathode.Besides,after the Li₂S load quadrupled to 5.93 mg cm^-2,the specific discharge capacity of Li₂S@（AB-Li₃PS₄）+CNT prepared by ball milling at 0.4,0.8,1.6 and 3.2 m A cm^-2 was1044.3,955.2,787.2,494.6 m Ah g^-1.In addition,Li₂S@（AB-Li₃PS₄）+CNT was prepared by the liquid-phase shaking with Li₂S@AB.38.8Li₂S@（AB-Li₃PS₄）+CNT prepared with carbothermal reduction of3.0Li₂SO₄-1AB showed the best electrochemical performance,and the discharge specific capacities were 943.7,804.0,553.6 and 309.8 m Ah g^-1 at 0.1,0.2,0.4 and 0.8m A cm^-2,respectively.When the surface density of Li₂S was increased to 5.93 mg cm^-2,the area specific capacities of the cells at 0.06c,0.12c,0.24c and 0.48c were 4.13,3.7,2.57 and 1.2 m Ah cm^-2,respectively.Compared with Li₂S@Li₃PS₄+AB+CNT prepared by liquid-phase shaking using Li₂S as raw material,the electrochemical performance of Li₂S@（AB-Li₃PS₄）+CNT prepared by two step in-situ method has been significantly improved.