ИСТИНА

The rapid progress in mass-market applications of metal-ion batteries intensifies the research of electrode materials for Na-ion and K-ion batteries as a possible alternative to the already matured Li-ion technology. Similar to the Li-ion intercalation systems, the Na and K-based mixed oxides and polyanion materials are extensively scrutinized as potential cathodes with the aim to enhance the specific energy, durability and rate capability. Whereas the layered oxides are characterized by greater volumetric energy density, the polyanion materials usually exhibit better cycling and thermal stability and higher C-rate capability due to covalently bonded structural frameworks in these compounds. The polyanion compounds also demonstrate an extra dimension in their crystal chemistry which significantly expands the search space for the materials with better electrochemical performance. Further advantages are expected from combining different anions (such as (XO4)p- and F-) in the anion sublattice. The fluoride-phosphates AMPO4F (A=Li, Na, K; M=V and Ti) with the KTP-type structure are considered now as perspective cathode materials for the metal-ion batteries. The “VPO4F” framework featured outstanding rate capability and capacity retention. Another quality of this framework is supporting reversible Na+, K+ and even Rb+ ions de/insertion sustaining the host structure. The ion diffusion coefficients obtained by PITT were the lowest for Li+ and the highest for K+, with the latter anticipating high-power applications of KVPO4F in K-ion batteries. Moreover, the titanium redox activity traditionally considered as “reducing” can be upshifted to near-4V electrode potentials in the “TiPO4F” framework thus providing a playground to design sustainable and cost-effective titanium-containing positive electrode materials with promising electrochemical characteristics. An overview of the research on these fluoride-phosphates will be presented with a special emphasis on the interrelation between chemical compositi