K‐Ion Batteries Based on a P2‐Type K_0.6CoO₂ Cathode

Abstract

perspective, K-ion cells would be a more reasonable choice as a low-cost alternative to Li-ion since K has high natural abundance and a lower standard redox potential than Na (K/K + : −2.9 V and Na/ Na + : −2.7 V vs saturated hydrogen electrode in aqueous media). [1] Indeed, the potential of K/K + was theoretically calculated to be even more negative than that of Li/Li + in propylene carbonate, a well-known battery electrolyte solvent. [2] Experimental demonstration that the standard redox potential of K/K + is lower than that of Li/Li + by ≈0.15 V in ethylene carbonate/diethyl carbonate (EC/DEC) electrolyte is consistent with this prediction. [3] This interesting feature suggests that KIBs might potentially deliver a higher cell voltage than Na-ion batteries (NIBs) and even LIBs. In addition, K-ion technology would benefit from the fact that graphite can be used on the anode side [3,4] (as in Li ion), which is not the case for Na ion and has seriously limited application of Na-ion technology.Only a few cathode compounds have been reported to date for KIBs, [5][6][7][8][9] with most studies focusing on the development of anodes. [3,4,[10][11][12][13][14][15][16][17][18][19][20] For example, Eftekhari demonstrated that Prussian blue can reversibly store K ions in a nonaqueous electrolyte system. [5] Later, the amorphous phase of FePO 4 as well as organic materials were also suggested as cathodes for KIBs. [6][7][8][9] Recently, K-ion insertion was shown to be possible in K 0.3 MnO 2 , [21] but the low K content of that material requires the use of K metal or pre-potassiated anodes.In this paper, we demonstrate highly reversible cycling of a P2-type K 0.6 CoO 2 cathode, and show implementation of a full cell KIB using a graphite anode. Alkali transition metal oxides with an ordered rock salt structure have been widely studied as promising cathodes for LIBs and NIBs because their layered framework allows topotactic de/intercalation of alkali ions, leading to excellent energy storage properties. [22][23][24][25][26][27][28] Cyclability and rate capability of K x CoO 2 are shown to be very good, indicating that with further improvements K ion may be an effective technology for large-scale energy storage. To the best of our knowledge, this is the first demonstration of reversible K de/intercalation in P2-type layered K x CoO 2 in the range 0.33 < x < 0.68. This work creates new research opportunities for the development of KIBs as a particularly exciting and promising technology for large-scale energy storage applications. K-ion batteries are a potentially exciting and new energy storage technology that can combine high specific energy, cycle life, and good power capability, all while using abundant potassium resources. The discovery of novel cathodes is a critical step toward realizing K-ion batteries (KIBs). In this work, a layered P2-type K 0.6 CoO 2 cathode is developed and highly reversible K ion intercalation is demonstrated. In situ X-ray diffraction combined with electrochemical titration reveals that P2-type ...

Recent Progress and Perspective in Electrode Materials for K‐Ion Batteries

“…It is notable that even though the research on KIBs has just begun, some positive electrodes already exhibit good performance, comparable to that of NIB technology, leading to their demonstration in practical rocking-chair KIBs. [24,57,88,95,101] Nevertheless, more focused research on cathode materials should be conducted for such a nascent chemistry to become practical.…”

“…A similarly high slope for K was observed in layered cathode materials. [57,58] In Figure 4c, we also compare the volume expansions computed for the K and Na alloying anodes. The volume expansion for the K system is substantially larger than that for the Na system because of the larger size of K. To make the K alloying anodes competitive, morphology optimization (the introduction of high porosity or nanostructuring) should be considered to prevent particle pulverization upon cycling.…”

“…Kim et al also demonstrated the practical feasibility of a P2-type K 0.6 CoO 2 by pairing with a graphite anode in a full cell, delivering a reversible capacity of ≈53 mA h per g of cathode. [57] In general, the voltage curves of K layered compounds have steeper slopes than those of Li and Na systems. This feature is likely attributable to the size of K + : The K + creates a much larger slab spacing in K x MO 2 compounds compared to the Na x MO 2 and Li x MO 2 analogues.…”

“…Recently, Hironaka et al [58] and Kim et al [57] reported P2-type K x CoO 2 (x ≈ 0.4 and 0.6, respectively) cathodes for KIBs. According to Hironaka et al, P2-type K 0.4 CoO 2 exhibits a reversible capacity of ≈60 mA h g −1 between 2.0 and 3.9 V. [58] However, Kim et al demonstrated that P2-K 0.6 CoO 2 delivers a reversible capacity of ≈80 mA h g −1 between 1.7 and 4.0 V. [57] The different K activities can likely be attributed to different cut-off Adv.…”

“…[96][97][98] In addition, polyanionic frameworks are expected to better screen the K-K repulsion in the structure, unlike the layered structure where the strong K-K repulsion limits the K storage capability. [57] In this respect, polyanion materials may be particularly relevant for KIBs.…”