High Energy Polyanion Cathode for K-Ion Batteries: KVPO4F
Authors: Kim Haegyeom , Seo Dong-Hwa , Bianchini Matteo , Clément Raphaële J. , Ceder Gerbrand 
Published: May 2020

The development of high-energy cathode materials is crucial for the commercialization of emerging K-ion batteries (KIBs). While layered potassium-transition-metal-oxides (K
x
MO2, M=transition metals) have been investigated as potential cathodes, they have two intrinsic challenges. First, most of the K-layered oxides have K-poor composition (x<1.0 in K
x
MO2) and it leads practical difficulty of realizing KIBs because all the K ions should come from the cathode in a rocking-chair KIB.[1-6] Second, K-layered oxides have too high a voltage slope, resulting in low specific capacity and average voltage. Both the problems are attributable to much stronger K+-K+ interaction than Na+-Na+ and Li+-Li+ in the layered oxide structure.[2, 4] In contrast, polyanion cathodes can be better alternatives because 3-dimensional arrangement of K ions in their frameworks can significantly reduce the strength of effective interaction between K ions. As a result, polyanion cathodes can have K-rich (or K-stoichiometric) composition and high working voltage.
In this work, we demonstrate that KVPO4F, a polyanion compound, is a high-energy cathode of KIBs. The KVPO4F cathode delivers a reversible capacity of ~105 mAh g−1 with an average voltage of ~4.3 V (vs. K/K+) (Figure 1), which is the highest among the K-cathodes developed up to date.[7] The specific energy (a gravimetric energy) of KVPO4F reaches ~450 Wh kg−1. Ex-situ X-ray diffraction characterization and ab-initio calculations demonstrate reversible multiple phase transitions of K
x
VPO4F during electrochemical cycling. K
x
VPO4F goes through various intermediate phases at x = 0.75, 0.625, and 0.5 upon K extraction and reinsertion. We further explain the role of oxygen substitution in KVPO4+xF1−x: the oxygenation of KVPO4F leads to an anion-disordered structure which prevents the formation of K+/vacancy orderings without electrochemical plateaus and hence to a smoother voltage profile.

References

[1] H. Kim, J. C. Kim, S.-H. Bo, T. Shi, D. –H. Kwon, G. Ceder. K‐Ion Batteries Based on a P2‐Type K0.6CoO2 Cathode. Adv. Energy Mater. 7, (2017) 1700098
[2] H. Kim, D. –H. Seo, A. Urban, J. Lee, D. –H. Kwon, S. –H. Bo, T. Shi, J. Papp, B. McCloskey, G. Ceder. Stoichiometric Layered Potassium Transition Metal Oxide for Rechargeable Potassium Batteries. Chem. Mater. 30 (2018) 6532-6539.
[3] H. Kim, D. –H. Seo, J. C. Kim, S. –H. Bo, L. Liu, T. Shi, G. Ceder. Investigation of Potassium Storage in Layered P3‐Type K0.5MnO2 Cathode. Adv. Mater. 29 (2017) 1702480.
[4] H. Kim, H. Ji, J. Wang, and G. Ceder. Trands in Chem. 1 (2019) 682.
[5] C. Liu, S. Lu, H. Huang, Z. Wang, A. Hao, Y. Zhai, Z. Wang. K0.67Ni0.17Co0.17Mn0.66O2: A cathode material for potassium-ion battery.Electrochem. Commun. 82 (2017) 150.
[6] X. Wang, X. Xu, C. Niu, J. Meng, M. Huang, X. Lium Z. Liu, L. Mai. Earth abundant Fe/Mn-based layered oxide interconnected nanowires for advanced K-ion full batteries. Nano Lett. 17 (2017) 544.
[7] H. Kim, D. –H. Seo, M. Bianchini, R. Clement, H. Kim, J. C. Kim, Y. Tian, T. Shi, W. –S. Yoon, G. Ceder. A New Strategy for High‐Voltage Cathodes for K‐Ion Batteries: Stoichiometric KVPO4F. Adv. Energy Mater. 8, (2018) 1801591.

Figure 1