Lithium-ion batteries (LIBs) play a pivotal role in achieving the “zero-carbon emission” objective as countries agreed to reach a 1.5^oC global warming target according to the Paris agreement. Nowadays, due to the tremendous mobile and stationary consumption, the demand and consequently the price for small/large-format LIBs is constantly soaring up. The aforementioned challenges originate from the shrinkage of the major applied critical materials in these batteries, such as cobalt (Co), nickel (Ni), Lithium (Li), graphite (G), and manganese (Mn)¹. Therefore, it is imperative to consider alternative elements to address issues corresponding to the limitation of resources globally. Potassium (K) ion batteries (PIBs) are envisaged as an effective alternative to LIBs since K is a more abundant element, has a higher operating potential, a faster diffusion rate, and the lowest stokes radius in comparison to the closest neighbors in the periodic table (Li and Na).² Among all reported materials for metal-ion batteries, those possessing the general formula AMXO₄L [A = Li, Na, K; M = Fe, Ti, V; X = P, S, Si; L= O, F, OH] can be applied both as anode and cathode to enable the full symmetric battery format.³ KTiPO₄F (KTiOPO₄ (KTP) structure-type material) has been previously reported by our group as a promising cathode for PIBs with decent electronic properties.⁴

In addition to the above-mentioned fascinating bifunctional application of such compounds, the pre-alkaliation of electrode materials either at an electrode or powder level is considered as an effective strategy to provide extra alkali ions in order to boost the capacity of metal-ion batteries.⁵ Researchers have reported pre-alkaliation of some materials for negative electrode materials such as Hard carbon (HC), Phosphorous (P), Silicon (Si), Graphite (G), and so on.⁶ However, there is no report on materials with a polyhedral framework structure.

Herein, we report a comparison of crystal structure, chemical composition, morphology, and K-ion storage properties of KTP-type KTiPO₄F and P-KTiPO₄F (P = pre-potassiated) anode materials for PIBs. Energy dispersive X-ray (EDX) analysis shows that there is an extra amount of K on the surface of materials, presenting the ratio of K: Ti as 1.24:1. On the other hand, the careful synchrotron X-ray powder diffraction data refinement demonstrates that just a small portion of non-stoichiometric K resides in the crystal structure giving rise to the “K_1.05TiPO₄F” formula. Our investigation reveals both pristine and K-rich composite electrodes, delivering the identical discharge capacity > 150 mAh g^-1 at 26.6 mA g^-1 (C/5 rate) in the potential window of 0.001-3 V vs. K⁺/K. More interestingly, we achieved to assemble full symmetric batteries where carbon-coated K_1.05TiPO₄F serves as both negative and positive electrodes, delivering >70 mAh g^-1 in the voltage range of 0.001-4.2V.

References

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