Fig. 1

(a) Relationships between battery components and electrochemical performance; (b) Comparison of energy densities of lithium-ion, sodium-ion, and potassium-ion batteries[22]; (c) The distribution of Li, Na, and K in the Earth[23]; (d) Proportion of research on organic liquid electrolytes, aqueous electrolytes, ionic liquid electrolytes, and solid-state electrolytes[24]; (e) Problems in electrolyte of potassium ion batteries[25]"

Fig. 2

(a) Crystal structure of common organic solvents and salts of potassium ion batteries[25]; (b) HOMO and LUMO energy levels of solvent molecules and potassium salts: FSI- and DME in DME electrolyte. FSI-, EC, and DEC in EC/DEC electrolyte[28]; (c) The LUMO and HOMO energy levels of commonly used solvents and potassium salts in PIBs"

Fig. 3

(a) An illustration that compares the hazardous properties of traditional (carbonate and ether-based) and non-flammable (phosphate ester-based) electrolytes; (b) Flame tests of various electrolytes[30]; (c) Schematic illustration of the proposed stabilization effects of electrolyte on the SEI layer of Bi/rGO electrode[32]; (d) Cycling curves of K|K symmetric cells with different electrolytes at a current density of 1 mA/cm2; (e) 0.9 mol/L KFSI TEP passivation; (f) 2 mol/L KFSI TEP passivation[33]; (g) In-situ optical observations of K metal plating on copper substrate in 0.8 mol/L KPF6 EC/DEC, 1 mol/L KFSI EC/DEC, and 3∶8 (KFSI∶TMP)[34]"

Fig. 4

(a) Schematic illustration of di?erent SEI layers formed on the surface of graphite and their evolution after many cycles; (b) Comparing galvanostatic K plating and stripping on the Cu substrate among different electrolyte formulations at the rate of 0.05 mA/cm2[47]; (c) Schematic illustration for graphite response in different electrolytes[52]; (d) Stage structure evolutions during K intercalation[51]; (e) Schematic illustration of the potassiation/depotassiation process in SnSb/C electrode[56]"

Fig. 5

(a) Linear voltammetry curves recorded at 1 mV/s in 1 mol/L, 10 mol/L, and 30 mol/L KAc electrolytes[63]; (b) The electrochemical stability window of the 1?mol/L and 22?mol/L KCF3SO3 electrolytes respectively[60];(c) A possible redox mechanism of potassium storage in PAQS"

Fig. 6

(a) All-solid-state potassium battery of PPCB-SPE[76]; SEM images of 3 h Ni3S2@Ni electrode decomposed from solid PIBs after (b) 1 cycle and (c) 10 cycles[77]"

Fig. 7

K+ migration maps as Voronoi graphs (hereafter shown by green lines) of voids ZA obtained within the geometrical-topological approach in the K2CoSiO4 (a), K4Fe2O5 (b) and K6V2P4O16 (c) crystal structures[80]; (d) Nyquist plots of V0T0 (left) and V150T10 (right) at 100 ℃ and 150 ℃, test with a stainless steel ||KNH2|| stainless steel cell; (e) ionic conductivities and activation energies of V0T0 and V150T10[81]"

Fig. 8

(a) High voltage cell configuration by coupling a high voltage honeycomb layered framework with a suitable anode and an ionic liquid based on potassium 1-methyl-1-propylpyrrolidine bis (trifluoromethylsulfonyl) amide (KTFSA) salt; (b) Cyclic Voltammetric Curves of 5 mV/s K, Na, Li[62]; (c) long-term cycling performance at 50 mA/g (up); Electrolyte: Pyr14TFSI + 0.3 mol/L KTFSI+2% ES, Potential profiles of graphite/K metal dual-ion cells for selected cycles between 3.4 V and 5.0 V vs. K/K+ at a charge/discharge current of 50 mA/g (down)[84]"

Table 1

Electrochemical performance of K-ion batteries with various electrolytes"

Fig. 9

Classification, advantages and disadvantages of electrolyte for potassium ion battery"