Potassium-Ion Storage Mechanism And Performance Of Metal Sulfide Anodes

With the advantages of green,convenient and high energy conversion efficiency,rechargeable battery technology is considered as an important solution to achieve the utilization of new clean energy and the popularization of electric vehicles,smart grid and other large-scale energy storage technology.Based on this,lithium-ion batteries have been widely used in portable electronic equipments and new-energy vehicles.However,the low reserves and uneven distribution of lithium resources in the earth's crust are becoming an important factor limiting the further development of lithium-ion batteries.Additionally,with the increasing demand for hybrid vehicles and smart grid,the development of large-scale low-cost energy storage system is urgent.As a new type of electrochemical energy storage system,potassium-ion batteries have shown great potential to surpass lithium-ion battery technology in the field of large-scale energy storage due to the abundant and low-cost potassium resources.However,the?de?insertion of the large-size potassium ions often causes structural degradation and thus short lifespan to the electrode materials.Therefore,it is important to develop low-cost electrode materials with high structural stability and long cycle-life.Due to the advantages of high theoretical specific capacity,abundant resources and low cost,we proposed two different metal sulfides as anode materials for potassium-ion batteries.Firstly,layered tungsten disulfide?WS₂?,whose interlayer spacing is 6.18 angstrom,was selected to relieve volume expansion upon potassium-ions insertion.Thus it can promote the formation of stable solid electrolyte interphase and the achievement of high structural stability.Secondly,stannous sulfide?SnS?was employed as anode materials where an"ion conduction matrix"composing of K₂S₄ was formed.This K₂S₄ matrix not only enhances the ion conductivity of the anode but also serves as a buffer to accommodate the volume change caused by?de?insertion of potassium ions,which enables fast ion transport,high structural integrity and long lifespan.The research contents are described as follows:?1?Tungsten disulfide?WS₂?was used as a new electrochemical K⁺-intercalation host,which can store 0.62 K⁺per formula unit with a reversible capacity of 67 mAh/g and well-defined voltage plateaus at an intrinsically safe operating potential of 0.72 V versus K/K⁺.The capacity retention is 89.2%after 600 cycles at a rate of 0.3 C.The specific capacity retention and average Coulombic efficiency are up to 96.3%and 99.9%,respectively,after 1000 cycles at 2.2 C.The in-situ X-ray diffraction and ex-situ electron microscopy further revealed the underlying K⁺?de?intercalation mechanism and confirmed the electrochemical reversibility and a relatively small volume expansion?37.81%?.Such characteristics are responsible for the stable structure and remarkable potential of WS₂ for long-cycle-life potassium-ion batteries.The results show that this new potassium-ion intercalation electrochemistry will lead to the further exploration of anode materials for potassium-ion batteries with long cycle life and high safety.?2?Tin sulfide/carbon composites with two distinctive morphologies of micron flower and irregular bulk were obtained by using a simple solvent thermal reaction with different tin sources.It was found that the micron flower-like SnS@C?FSnS-50?could achieve a maximum reversible specific capacity of 272.6 mAh/g at a current density of20 mA/g and high cycling stability.At the current density of 200 mA/g,the specific capacity retention was 83.51%after 200 cycles which is significantly higher than that of the bulk material?69.13%after 200 cycles?.Ex situ X-ray diffraction revealed the underlying mechanism.In the FSnS-50 electrode,the flower-like SnS firstly transformed into Sn and K₂S₄ though conversion reaction and Sn subsequently transformed into K₄Sn₂₃3 and K₄Sn₄ via alloying reactions.Additionally,due to the ion conductive matrix of K₂S₄,the synergistic effect between petal nanosheets and carbon materials,a fast potassium ion migration rate and an effective buffer for the volume change were achieved,resulting in fast potassium-ion transport kinetics and high structural integrity.