The rapid progress in mass-market applications of metal-ion batteries intensifies the development of economically feasible electrode materials based on earth-abundant elements. Here, we report on a record-breaking titanium-based positive electrode material, KTiPO 4 F, exhibiting a superior electrode potential of 3.6 V in a potassium-ion cell, which is extraordinarily high for titanium redox transitions. We hypothesize that such an unexpectedly major boost of the electrode potential benefits from the synergy of the cumulative inductive effect of two anions and charge/vacancy ordering. Carbon-coated electrode materials display no capacity fading when cycled at 5C rate for 100 cycles, which coupled with extremely low energy barriers for potassium-ion migration of 0.2 eV anticipates high-power applications. Our contribution shows that the titanium redox activity traditionally considered as "reducing" can be upshifted to near-4V electrode potentials thus providing a playground to design sustainable and cost-effective titanium-containing positive electrode materials with promising electrochemical characteristics.

Polyanion compounds offer a playground for designing prospective electrode active materials for sodium-ion storage due to their structural diversity and chemical variety. Here, by combining a NaVPO4F composition and KTiOPO4-type framework via a low-temperature (e.g., 190 °C) ion-exchange synthesis approach, we develop a high-capacity and high-voltage positive electrode active material. When tested in a coin cell configuration in combination with a Na metal negative electrode and a NaPF6-based non-aqueous electrolyte solution, this cathode active material enables a discharge capacity of 136 mAh g−1 at 14.3 mA g−1 with an average cell discharge voltage of about 4.0 V. Furthermore, a specific discharge capacity of 123 mAh g−1 at 5.7 A g−1 is also reported for the same cell configuration. Through ex situ and operando structural characterizations, we also demonstrate that the reversible Na-ion storage at the positive electrode occurs mostly via a solid-solution de/insertion mechanism.

The effect of salt concentration in diglyme-based electrolytes on cycling performance of promising KVOPO 4 and K 1.69 Mn[Fe(CN) 6 ] 0.85 •0.4H 2 O positive electrodes (cathodes) and a hard carbon negative electrode (anode) for next-generation potassium-ion (K-ion) batteries is investigated. A decrease in free solvent molecule number with increasing electrolyte concentration is found, which results in a better aluminum current collector stability, formation of thinner solid electrolyte interface (SEI) passivation layers, and further inhibition of solvent degradation redox processes occurring at the electrode surface upon cycling. The KVOPO 4 and K 1.69 Mn[Fe(CN) 6 ] 0.85 •0.4H 2 O cathodes exhibit an enhanced specific discharge capacity (54 and 105 mA•h•g −1 , respectively) in K-ion cells at the highest electrolyte concentrations (2 and 2.5 M KPF 6 in diglyme, respectively) at a 0.1 C rate. However, the behavior of the hard carbon anode is noticeably affected by the salt concentration over the first few cycles, a phenomenon tentatively attributed to the SEI layer formation and the presence of irreversible intercalation sites for K + ions in the hard carbon framework. Finally, electrochemical tests on K-ion full cells consisting of the K 1.69 Mn[Fe(CN) 6 ] 0.85 •0.4H 2 O cathode, a hard carbon anode, and an ether-based electrolyte show capacity retention of 86% over 300 cycles at a 0.6 C rate.

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To realize high-power performance, lithium-ion batteries require stable, environmentally benign, and economically viable noncarbonaceous anode materials capable of operating at high rates with low strain during charge–discharge. In this paper, we report the synthesis, crystal structure, and electrochemical properties of a new titanium-based member of the MPO4 phosphate series adopting the α-CrPO4 structure type. α-TiPO4 has been obtained by thermal decomposition of a novel hydrothermally prepared fluoride phosphate, NH4TiPO4F, at 600 °C under a hydrogen atmosphere. The crystal structure of α-TiPO4 is refined from powder X-ray diffraction data using a Rietveld method and verified by electron diffraction and high-resolution scanning transmission electron microscopy, whereas the chemical composition is confirmed by IR, energy-dispersive X-ray, electron paramagnetic resonance, and electron energy loss spectroscopies. Carbon-coated α-TiPO4/C demonstrates reversible electrochemical activity ascribed to the Ti3+/Ti2+ redox transition delivering 125 mAh g–1 specific capacity at C/10 in the 1.0–3.1 V versus Li+/Li potential range with an average potential of ∼1.5 V, exhibiting good rate capability and stable cycling with volume variation not exceeding 0.5%. Below 0.8 V, the material undergoes a conversion reaction, further revealing capacitive reversible electrochemical behavior with an average specific capacity of 270 mAh g–1 at 1C in the 0.7–2.9 V versus Li+/Li potential range. This work suggests a new synthesis route to metastable titanium-containing phosphates holding prospective to be used as negative electrode materials for metal-ion batteries.