Sodium-ion and potassium-ion batteries

Beyond lithium on one page: a sodium-ion cell with aluminium foil on both electrodes, hard carbon and where it stores sodium, a P2 layered oxide, a Prussian blue analogue and a NASICON phosphate cathode, and potassium entering graphite in stages. Every caption and number is quoted from the cited papers.

1. A sodium-ion cell: Sodium is widely available and its chemistry is like lithium’s; layered transition-metal oxides, phosphates and organic compounds serve as sodium-ion cathodes, carbonaceous materials among the anodes (Hwang 2017). Sodium does not alloy with aluminium, so cheaper aluminium replaces copper on the anode; one current-collector metal eases recycling and lets the cells be stored at 0 V (Joy 2024). 2. Hard carbon: Hard carbon stores sodium two ways: a sloping region (2.0–0.1 V vs Na/Na⁺) from adsorption at surfaces and defects, and a plateau below 0.1 V from closed-pore filling and pseudo-graphitic intercalation; hard carbons that deliver 300–400 mAh/g need pseudo-graphitic domains with d = 0.36–0.40 nm and closed pores under 1 nm (Zhou 2025). 3. P2 layered oxide: O3 layered oxides stack their oxygen layers ABCABC with Na⁺ in octahedral sites; their high sodium content (x > 0.7) gives high capacity, but Na⁺ diffuses slowly and multi-step phase transitions follow. P2 oxides stack ABBA with Na⁺ in trigonal prismatic sites, a larger interlayer spacing and faster ion transport; less sodium (x ≈ 2/3) limits capacity, and P2 → O2 or P2 → Z transitions occur at high voltage (Dong 2026). 4. Prussian blue analogue: Prussian blue analogues have 3D open frameworks with large interstitial space for fast Na⁺ insertion and extraction (Yuan 2024); Fe sits on the carbon end of each cyanide, Mn, Ni or Co on the nitrogen end. Fe(CN)₆ vacancies and coordinated water distort the lattice, promote electrolyte decomposition and lower conductivity; controlling them improves the cathodes (Joy 2024; Ma 2017). 5. NASICON phosphate: NASICON NaₓM₂(PO₄)₃ (M = V, Ti) has a stable 3D framework: each MO₆ octahedron shares corners with three PO₄ tetrahedra in [M₂(PO₄)₃] lantern units, leaving Na1 and Na2 sites joined by conduction paths. Phosphates are thermally stable thanks to the P–O bond, but polyanionic cathodes conduct electrons and ions poorly; coatings, doping and nanostructuring help (Joy 2024). 6. Potassium in graphite: Potassium inserts electrochemically into graphite from a nonaqueous electrolyte with a reversible capacity of 273 mAh/g; ex situ XRD showed KC₃₆, KC₂₄ and KC₈ forming in turn, and a nongraphitic soft carbon cycled and ran at high rates much better than graphite (Jian 2015). From experiments and calculations a three-step staging was proposed, C → KC₂₄ (stage III) → KC₁₆ (stage II) → KC₈ (stage I), with 207 mAh/g in graphite (Luo 2015). K-ion cells need no scarce or toxic elements, the low K/K⁺ potential gives voltages competitive with Li-ion, and K⁺ diffuses faster in electrolytes (Hosaka 2020).

Hwang et al., “Sodium-ion batteries: present and future”, Chem. Soc. Rev. 2017 (doi:10.1039/c6cs00776g); Joy et al., “A comprehensive review on strategies for enhancing the performance of polyanionic-based sodium-ion battery cathodes”, ACS Omega 2024 (doi:10.1021/acsomega.4c02709); Zhou & Shen, “From sodium storage mechanism to design of high-capacity carbon-based anode: a review”, Materials 2025 (doi:10.3390/ma18102248); Dong et al., “Toward general design of Mn-based layered oxide cathodes for sodium-ion batteries: from thermodynamic principles to entropy engineering”, Molecules 2026 (doi:10.3390/molecules31050836); Yuan et al., “Research progress of Prussian blue and its analogs as high-performance cathode nanomaterials for sodium-ion batteries”, Small Methods 2024 (doi:10.1002/smtd.202301372); Ma et al., “Energy storage materials derived from Prussian blue analogues”, Sci. Bull. 2017 (doi:10.1016/j.scib.2017.01.030); Jian et al., “Carbon electrodes for K-ion batteries”, J. Am. Chem. Soc. 2015 (doi:10.1021/jacs.5b06809); Luo et al., “Potassium ion batteries with graphitic materials”, Nano Lett. 2015 (doi:10.1021/acs.nanolett.5b03667); Hosaka et al., “Research development on K-ion batteries”, Chem. Rev. 2020 (doi:10.1021/acs.chemrev.9b00463).

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