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.

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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 feasibility of sodium-ion batteries as an alternative to lithium-ion batteries in large-scale storage systems largely depends on the availability of advanced electrode materials leading to enhanced energy density and operational sustainability. Herein, we present a novel βpolymorph of sodium vanadium pyrophosphate NaVP 2 O 7 with the KAlP 2 O 7 -type structure obtained via hydrothermal synthesis and further thermal dehydration of a hydrophosphate intermediate. β-NaVP 2 O 7 demonstrates attractive electrochemical behavior as a Na-ion positive electrode (cathode) material with practically achieved a reversible capacity of 104 mAh/g at C/10 current density, an average operating voltage of 3.9 V vs Na/Na + , and only 0.5% volume change between the charged and the discharged states. Electrode material exhibits excellent C-rate capability and cycling stability, providing a capacity of 90 mAh/g at 20C discharge rate and <1% capacity loss after 100 charge−discharge cycles. In the low-voltage region (≈1.5 V vs Na/Na + ), β-NaVP 2 O 7 reversibly intercalates additional sodium cations, leading to extraordinary overall Na-ion storage ability exceeding 200 mAh/g within the 1.5−4.4 V vs Na/Na + voltage region. This material is one of only a few materials that exhibit reversible sodium-ion storage over such a large potential window.

In this paper, we report on a novel RbVPO4F fluoride phosphate, which adopts the KTiOPO4 (KTP) type structure and complements the AVPO4F (A = alkali metal) family of positive electrode (cathode) materials for metal-ion batteries.