1- Introduction

2- Experimental

3- Results and discussion

4- Conclusion

References

Abstract

A new cathode material, K2V3O8, is reported reports as a new active material for potassium storage. The K2V3O8 is stabilized into tetragonal structure with P4bm space group, and we verify the unique crystal structure that is coordinated with four hendecahedra of KO11, VO5, and VO4, in which one VO5 shares its four oxygens with four VO4 tetrahedra to produce four penta-shaped voids that allows insertion and extraction of potassium ions. The K2V3O8 modified with carbon exhibits a reversible capacity of 75 mAh (g-oxide)−1, and its electrochemical activity originates from the reversible de-/intercalation of potassium ions into/out of the hendecahedrally coordinated K2V3O8–carbon composite cathode, accompanied by a reversible V5+/4+ redox pair, which is retained for 200 cycles with 80% retention of its initial capacity.

Introduction

Various potassium insertion materials have been introduced and evaluated for their electrochemical performance as cathodes in potassium-ion batteries (KIBs) [1–4]. K-deficient P2- and P3-type KxMO2 (x � 0.7, M ¼ transition metal) layered oxides have been extensively investigated as analogues of sodium-ion batteries (SIBs) [5–12]. They exhibit good electrode performances in terms of specific capacities, although they suffer from detrimental phase transitions resulting from the insertion (and extraction) of the large Kþ ions (1.38 Å) into (and out of) the crystal structure, which, in turn, limit their cycling stability. Vanadium vanadates including oxides have been extensively investigated as cathode materials for rechargeable batteries activated by monovalent and divalent charge carriers (Li, Na, K and Zn ions) because of their diverse structures and outstanding electrochemical activities resulting from the multiple valence states of vanadium [13–16]. Recently, Lu et al. introduced K2V3O8 as an anode material for KIBs [17], presumably activated by conversion process. This unique K2V3O8 structure, arranged with alternating K–O layers and V–O layers along the c-axis, inspires us to adopt it as a cathode material for KIBs. It is thought that de-/intercalation of potassium ions into/out of this crystal structure is possible because hendecahedrally coordinated Kþ-ion layers are located between the edge-sharing VO5 and VO4 assemblies along the c-axis. In this study, we firstly introduce the physical and electrochemical properties of K2V3O8. Due to its poor electrical conductivity, the active material, K2V3O8, is modified by electro-conducting carbon (K2V3O8/C) (Fig. 1a). As per designated, this treatment increases the resulting electrical conductivity to 10 4 S cm 1 at 25 �C, which enables the present K2V3O8 active for Kþ extraction and insertion for 200 cycles, delivering a reversible capacity of 75 mAh (goxide) 1 .