Challenges and Strategies toward Cathode Materials for Rechargeable Potassium‐Ion Batteries

Abstract

Li resources, together with its growing depletion, has seriously hindered the sustainable development of LIBs. [6] In recent years, enormous effort has been devoted to developing alternative EES technologies, especially sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs), owing to the similar chemical properties of Na, K, and Li, [6] and the high abundance and accessibility of Na and K. Figure 1a shows the physical properties and economic indicators of Li, Na, and K carrier ions for rechargeable batteries. The ranking of Li, Na, and K according to their natural abundance in the crust is 27th (0.0017 wt%), 6th (2.36 wt%), and 7th (2.09 wt%), [7] respectively. In particular, PIBs are more attractive because the redox potential versus the standard hydrogen electrode of K + /K (−2.93 V) is lower than that of Na + /Na (−2.71 V) and closer to that of Li + /Li (−3.04 V) in aqueous electrolytes. [8,9] Moreover, both theoretical and experimental studies have revealed that the K/K + couple exhibits a low reduction potential compared with Na/Na + and even Li/Li + in nonaqueous electrolytes (e.g., KPF 6 -ethylene carbonate [EC]/propylene carbonate [PC]). [10,11] The weaker Lewis acidity of K + relative to Li + or Na + favors a smaller Stokes radius of solvated ions, [12] which facilitates faster ion mobility and higher ion conductivity, thereby improving the rate performance of PIBs. [13] As illustrated in Figure 1b, potassium plating takes place at a lower potential for PIBs compared with LIBs and SIBs, thus providing a greater possibility to achieve a wider electrochemical voltage window. [14] These findings, in principle, give rise to a higher energy density for PIBs. [15] More strikingly, PIBs benefit from the fact that K + can be electrochemically inserted into graphite (like Li + ) to form graphite intercalation compounds (KC 8 ) with a theoretical capacity of ≈279 mAh g −1 ; [16][17][18] thus, PIBs have great potential for commercial applications. These characteristics make PIBs an exciting alternative or complementary energy storage candidate to the present LIBs.Nevertheless, although the pioneering study on the K-intercalation reaction can be traced back to 2004, [19] the development of PIBs is less satisfactory owing to the difficulty in finding suitable host materials with acceptable electrochemical performance. This difficulty arises mostly from the large atomic radius (1.38 Å) of K + , [8] which considerably reduces With increasing demand for grid-scale energy storage, potassium-ion batteries (PIBs) have emerged as promising complements or alternatives to commercial lithium-ion batteries owing to the low cost, natural abundance of potassium resources, the low standard reduction potential of potassium, and fascinating K + transport kinetics in the electrolyte. However, the low energy density and unstable cycle life of cathode materials hamper their practical application. Therefore, cathode materials with high capacities, high redox potentials, and good structural stability are required with the advance...

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