(Invited) How Does Intercalation Ion Species Determine the Electrochemical Properties of Cathode Materials for Rechargeable Batteries?
Authors: Kim Haegyeom , Ceder Gerbrand 
Published: December 2020

Alkali-ion intercalation compounds are widely used as cathode materials for rechargeable batteries, including Li-, Na-, and K-ion batteries. For many years, researchers have developed layered oxide cathode materials for Na- and K-ion batteries given that the layered oxides show high reversible capacity and high working voltage in Li-ion technology. However, our works have demonstrated that the layered oxide compounds may not be good cathode candidates for Na and K systems. There are two main issues:[1-4] (i) Na and K transition metal oxide compounds form non-stoichiometric composition (x<1.0 in A
x
MO2, A= Na and K). It leads practical difficulty of realizing Na- and K-ion batteries because all the Na and K ions should come from the cathode in rocking-chair batteries. (ii) As the insertion ion size increases, the voltage curve becomes much sloped. The sloped voltage curves result in low specific capacity and average voltage. Both the problems are attributable to much stronger Na+-Na+ and K+-K+ interaction than Li+-Li+ in the layered oxide structure. In this respect, polyanion compounds that have 3 dimensional arrangements of Na and K ions, resulting in longer Na+-Na+ and K+-K+ distance, will be better candidates. We proposed KVPO4F[5] as an example of polyanion compounds and it has stoichiometric composition and provides high average voltage of ~4.3 V (vs. K/K+) with a reversible capacity of ~105 mAh/g.
We further investigated the effect of intercalation ion species in K
x
VPO4F (x~0).[6] Our work demonstrates the voltage for Na intercalation is even higher than that for Li insertion unlike the common belief that Li insertion voltage is always higher than Na insertion. The lower Li intercalation voltage is likely attributed to unstable Li site in large cavity, making less stable discharged product upon Li insertion vs. Na insertion. In addition, we found that Li intercalation is more sluggish than Na and K intercalation in K
x
VPO4F (x~0). Since Li ion is too small compared to cavity in the cathode, Li ions are undercoordinated in the transition state, in which a Li ion is coordinated by two anions only. Therefore, Li ion migration barrier is much higher than Na and K migration. This finding teaches us that large cavity size (or channel size) is not always good for fast alkali ion migration and we need to finely tune the cavity size suitable for each intercalating ion species.

References

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[6] H. Kim, Y. Ishado, Y. Tian, G. Ceder. Investigation of Alkali‐Ion (Li, Na, and K) Intercalation in K
x
VPO4F (x ∼ 0) Cathode. Adv. Funct. Mater. 29 (2019) 1902392.