Two-Dimensional Layered Materials As Potassium Ion Battery Electrode Mateaials

Due to their high energy density and excellent energy storage performance,Li-ion batteries have been widely used in various portable devices.However,the lack of lithium resources on the earth leads to their high price and un-sustainability,which makes it hard for lithium ion batteries to apply to large-scale storage power stations.So,it is very necessary to look for the alternatives of lithium.Potassium shows similar properties of lithium.Besides,the abundance and lower cost implicit potassium ion battery a promising prospect,which attracts more and more researchers exploring in this field.Otherwise,the major problem of potassium ion batteries lies in the huge radius decelerating the move of potassium ion,which makes it hard to achieve a fast charge/discharge process.In addition,the tremendous volumetric effect might crack the material structure,which leads to fast capacity decay.To avoid the problems mentioned above,we reported a creative work using TiS₂ with large interlayer gap as the cathode material.Started with lithium ion batteries,we confirmed the excellent performance of TiS₂cathode material.The reversible capacity at 0.2 C was 220 mAh·g^-1,which equals to 92%of the theoretical capacity(240 mAh·g^-1),and decreased by only 3%after 100 cycles.What's more,TiS₂ were able to release a capacity of 145 mAh·g^-1 at the current rate of20 C,which displayed excellent fast charge/discharge performance.Afterwards,the performance of TiS₂ in potassium ion batteries was studied,which failed to achieve a decent performance in KPF₆·EC/DMC electrolyte.The capacity under 1.5-3.0 V were 98 mAh·g^-1 and 12 mAh·g^-1 at the current of 0.2 C and 10 C,respectively.Subsituting KPF₆ for KFSI still remains a mediocre performance.However,the electrochemical performance increased a lot when we used ether-based KPF₆·DME as electrolyte,which achived a capacity retention of 93%after 100 cycles under the current of 2 C(the original capacity was 105 mAh·g^-1).Besides,the fast charge/discharge performance was excellent that the capacity remained 75 mAh·g^-1 at a high current of 20 C.Likewise,we explored the performance of KPF₆·DEGDME,another ether-based electrolyte,which showed a lower capacity and a worse capacity retention property compared with KPF₆·DME.The results implied that only some kinds of electrolytes can exert the potential preformance of TiS_2.Additionally,the performance under different working potentials was tested.It was found that expanding working potential window?1.0-3.0 V?would promote the reversible capacity to a large extent.However,a faster capacity fade velocity occurred.As a result,the proper working potential should be 1.5-3.0 V.Huge performance difference was observed between KPF₆·EC/DMC and KPF₆·DME electrolyte,but GITT and CV results strongly suggest that the potassium insertion mechanisms are the same.Subsequently,the potassium insertion/extration mechanism was estabilshed via ex-situ XRD and high-resolution TEM.XRD refinement results implied:It obeyed a stage model when potassium ions inserted into TiS₂.A structure Stage II was observed when potassium content was between 0-0.19.Stage II turned into Stage I gradually when the potassium content reached 0.27-0.56.The Ti-S layer slided 1.97?towards[11 0]direction when potassium ions were inserted.The K-ions were located at the center of triangular prism constituted by S atoms.High-resolution TEM confirmed the Ti-S layers'slide phenomenon during potassium insertion prosess.Also,the defects were observed.Thus the Daumas-Hérold model is more suitable than the Rüdorff model to explain the potassium insertion/extration behaviors.