Controllable Preparation And Sodium-ion Storage Of VS₄ Anode Materials

With the rapid development of lithium ion battery industry,the demand for lithium resources is increasing.Whereas,the reserves of lithium resources in the earth’s crust are not abundant.This imbalance between supply and demand increased the cost of lithium ion batteries and hampered it’s developments.Sodium ion batteries were widely observed because of the abundant sodium resources and lower costs.However,the ionic radius of Na⁺is larger than Li⁺,which will toughen the reaction between Na⁺and electrode materials.Therefore,higher requirements are put forward for sodium storage electrode materials.VS₄ has quasi-one-dimensional chains with interchain distance of 5.83（?）and weak interchain van der Waals force.This unique structure gives VS₄ sufficient active sites for sodium ion storage and improve the transportation of sodium ions.The theoretical capacity of VS₄ anode for sodium ion battery is calculated as much as 1197 m Ah g^-1.Thus,VS₄ shows a tremendous potential for sodium storage.Whereas,VS₄ features low electrical conductivity,which will hander the electron transportation.Meanwhile,VS₄ suffers from volume expansion problems when alkaline ion inserted into the crystal structure.What’s more,VS₄ has two-step transformation reaction process during cycling.The reaction mechanism is complex,and the detection results of transformation reaction products are easily affected by external factors,which makes the results from conventional ex-situ tests inaccurate and unreliable.To solve those problems,multiwalled carbon nanotube was introduced to enhance the electrical conductivity of VS₄/MWCNT composites;hollow sphere VS₄ was synthesized.The hollow sphere structure was used to provide a cavity to release the volume expansion during cycling and as a result,improve the cycle stability of VS₄ electrode;in-situ XRD was used to detect the dynamic change of VS₄ in the course of cycling,ex-situ XPS and TEM at different discharge/charge stage was also used to support the XRD results and revealed the reaction mechanism of VS₄during cycling.The detailed innovations as follows:（1）MWCNT was introduced into VS₄ and synthesized VS₄/MWCNT composites via a simple one step solvothermal synthesis.Compared with the VS₄,the VS₄ nanoparticles in VS₄/MWCNT composites are smaller,and no obvious aggregation effect.When used as electrode materials for sodium ion batteries,VS₄/MWCNT composite shows a reversible capacity of 680 m Ah g^-1 at 0.2 A g^-1 after 70 cycles.Whereas,the reversible capacity of VS₄is 521 m Ah g^-1.The VS₄/MWCNT electrode can still afford a high reversible capacity of 368m Ah g^-1 even at 5 A g^-1,much higher than the VS₄ electrode(204 m Ah g^-1).It can be concluded from the pseudocapacitance contribution and EIS results that the VS₄/MWCNT have higher pseudocapacitance contribution and lower charge transfer resistance and lower Warburg resistance.Thus,the addition of MWCNT can improve the electrical conductivity of VS₄/MWCNT composites,enhance the motivation of Na⁺in the electrode,and increase the pseudocapacitance contribution caused by the reduced particle size.Together,these increased the electrochemical properties of VS₄/MWCNT.（2）Hollow sphere VS₄ was synthesized through mixed solvothermal synthesis.The hollow structure will provide free volume to ease the volume expansion during cycling and improve the cycle stability of VS₄ electrode.When used as anode for sodium ion battery,the VS₄ delivered a reversible capacity of 710 m Ah g^-1 at 0.1 A g^-1 after 100 cycles.It can still afford a high reversible capacity of 475 m Ah g^-1 even a 10 A g^-1 after 1000 cycles,which shows excellent cycle stability.In-situ XRD was used to study the reaction mechanism of VS₄during cycling.Ex-situ XPS and ex-situ TEM tests are also introduced as supplementary methods to expose the reaction mechanism of VS₄.Based on these three test results,it can be concluded that during the discharge process,VS₄ firstly combined with sodium ions to form Na_xVS₄,and then convert into Na₂S and V with the discharge proceeds.During the subsequent charging process,Na₂S and V lose sodium to regenerate Na_xVS₄,and finally Na_xVS₄ transform into VS₄.VS₄ changed from crystalline structure to the amorphous structure,so the diffraction peak of VS₄ in the full state is not observed.The specific reaction equation is as follows.