With sodium-ion batteries (SIBs) finding widespread application, the demand grows for hard carbon, the most popular anode material for SIBs. Hydrothermal carbonization facilitates the production of hard carbon with desired characteristics from various sources. Despite the considerable volume of literature addressing this subject, there is a notable absence of investigations elucidating the relationship between synthesis conditions and the electrochemical characteristics of the product. Here we study systematically the influence of hydrothermal carbonization parameters on hard carbon characteristics and emphasize the potential of hard carbon as an anode material for SIBs. The initial Coulombic efficiency (ICE) is significantly affected by the particle size of the glucose-derived hard carbon, which, in turn, depends on glucose concentration in the initial solution, pH, and stirring regime. By optimizing the hydrothermal carbonization parameters, the ICE up to 91% and a good reversible capacity of ∼300 mAh g⁻¹ in a half cell are achieved. Full cells with Na₃(VO)₂(PO₄)₂F cathode material demonstrate ICE of about 80% and reversible capacity of up to 100 mAh g⁻¹ _cath. Considering the effective performance of pouch-cell SIB prototypes based on Na₃(VO)₂(PO₄)₂F and hard carbon, hydrothermal carbonization of glucose yields hard carbon with the necessary characteristics required for its successful application in SIBs.

The need for an alternative to lithium-ion batteries motivates the development of sodium-ion batteries, specifically focusing on biomass-derived hard carbon (HC) as anode material. However, the compositional variability in biomass leads to a lack of correlation between the microstructure and properties of HC. In this study, we used four simple carbohydrates present in all biomass-derived precursors as model objects to establish a reliable relationship between pretreatment temperature, the carbonization mechanism, defectiveness, and the electrochemical characteristics of HCs. The statistical analysis of Raman spectra from 25 HC samples enabled us not only to determine the degree of defectiveness in HCs but also, for the first time, to compare materials based on their inhomogeneity—that is, the variation in defectiveness within each HC material. The initial Coulombic efficiency of hard carbons was improved by 40%, reaching 89%, through the simultaneous reduction of surface area, defectiveness, and inhomogeneity. Additionally, the full cell with the HC and Na₃V₂(PO₄)₃/С cathode material demonstrated 80% capacity retention after 300 cycles.

Powders with a phase composition including syngenite (K₂Ca(SO₄)₂·H₂O) and/or calcium sulfate dihydrate (gypsum, CaSO₄·2H₂O) were synthesized from the powder of calcium carbonate (CaCO₃) and water solutions of potassium hydrosulfate (KHSO₄) of various concentrations (0.5 M, 1 M, and 2 M). A molar ratio of starting salts, KHSO₄/CaCO₃ = 2, was used to provide the formation of syngenite (K₂Ca(SO₄)₂·H₂O). But when using a 0.5 M water solution of potassium hydrosulfate (KHSO₄), the phase composition of the synthesized powder was presented by calcium sulfate dihydrate (gypsum, CaSO₄·2H₂O). When using 1 M and 2 M water solutions of potassium hydrosulfate (KHSO₄), the syngenite (K₂Ca(SO₄)₂·H₂O) was found as the predominant phase in synthesized powders. According to estimations made from thermal analysis data, powders synthesized using 1.0 M and 2.0 M water solutions of potassium hydrosulfate (KHSO₄) contained no more than 7.9 and 1.9 mass % of calcium sulfate dihydrate (gypsum, CaSO₄·2H₂O), respectively. The phase composition of products isolated from mother liquors via water evaporation consisted of syngenite (K₂Ca(SO₄)₂·H₂O) and potassium sulfate (arcanite, K₂SO₄). Synthesized powders can be used in preparation of biocompatible bioresorbable materials with phase compositions in the K₂O-CaO-SO₃-H₂O system; as matrix of thermo- or photo-luminescent materials; as components reducing the setting time and increasing the strength of sulfate cements; in the fertilizing industry; and also as components of Martian regolith simulants.

Powders with phase composition including syngenite K2Ca(SO4)2·H2O and/or calcium sulfate di-hydrate (gypsum) CaSO4·2H2O were synthesized from powder of calcium carbonate CaCO3 and water solutions of potassium hydrosulfate KHSO4 of various (0.5M, 1M and 2M) concentrations. Molar ratio of starting salt KHSO4/CaCO3=2 were used to provide formation of syngenite K2Ca(SO4)2·H2O. But when using 0.5M water solution of potassium hydrosulfate KHSO4 the phase composition of synthesized powder was presented by calcium sulfate dihydrate (gypsum) CaSO4·2H2O. When using 1M and 2M water solution of potassium hydrosulfate KHSO4 the syn-genite K2Ca(SO4)2·H2O was found as predominant phase in synthesized powders. According es-timations made from thermal analysis data when 1.0M and 2.0M water solutions of potassium hydrosulfate KHSO4 were used the content of calcium sulfate dihydrate (gypsum) CaSO4·2H2O in these powders were not higher then as 7.9 and 1.9 % respectively. Phase composition of products isolated from mother liquors via water evaporation consisted of syngenite K2Ca(SO4)2·H2O and potassium sulfate (arcanite) K2SO4. Synthesized powders can be used in preparation of biocom-patible bioresorbable materials with phase composition in the K2O‑CaO‑SO3‑H2O system; as matrix of thermo- or photo-luminescent materials; as components reducing the setting time and increasing strength of sulfate cements; in the fertilizing industry and also as a components of Martian regolith simulant.