No abstract

The effect of the cation nature is explored for the reaction of alkali metal ions intercalation into the AVPO 4 F material. Application of electrochemical methods allowed determining the key diffusional and kinetic parameters for Li + , Na + and K + intercalation reactions. The obtained formal redox potential values, apparent diffusion coefficients and charge transfer resistance values are contrasted, providing the possibility to assess the variation in the reaction energetics for metal ion insertion/extraction. The observed differences in reaction rates are rationalized in terms of different contributions of ion desolvation and transition through adsorbate layer/electrode interface for various ions. The mechanism of ion intercalation reactions is still poorly understood in spite of the obvious importance of intercalating systems in metal-ion batteries development for applications in energy conversion and storage. The research in the field of establishing ion intercalation kinetic patterns is mainly focused on evaluating charge transfer resistance parameters and diffusion coefficients for materials with presumably higher energy density and rate capability, while the fundamental aspects of the reaction mechanisms are typically overlooked. However, the research aimed at finding the limiting step of intercalation reaction might provide additional non-empirical approaches to control the rate of the intercalation reaction. In this regard, fundamental studies exploring the effects of solvent and cation nature on the overall reaction rate are of primary importance.In a series of recent publications, the intercalation reaction rate was shown to be highly dependent on the solvent nature, providing an implication to consider ion transfer rather than electron transfer or ion insertion into the host matrix step to be rate limiting. [1][2][3][4] It was also shown that intercalation of solvated cations is kinetically more facile as compared to the reaction of ions without the solvation shell. 5,6 These findings imply that the contribution of ion desolvation to the reaction activation barrier could be dominant. On the other hand, the reaction rate was shown to be dependent on the structure of electrode/electrolyte interface (EEI) especially in cases when adsorbate layers are formed at high potentials at the electrode surface (solid electrolyte interface, SEI, on anode, or cathode/electrolyte interface, CEI).2 All these results point to the complex interplay between the energetics of ion desolvation and the transition of the ion through the EEI. Variation of the solvent nature cannot be applied to distinguish between these two contributions, as changing the solvent results in simultaneous changes in the EEI structure, desolvation energy and the preexponential factor, which is determined by the solvent effective frequency. Variation of the intercalating ion provides another possibility to assess the mechanism of the intercalation reaction. The desolvation energy contributions should be very different for ions of different charge...

In this paper, we report on a novel α-VPO4 phosphate adopting the α-CrPO4 type structure as a promising anode material for rechargeable metal-ion batteries. Obtained by heat treatment of a structurally related hydrothermally prepared KTiOPO4-type NH4VOPO4 precursor under reducing conditions, the α-VPO4 material appears stable in a wide temperature range and possesses an interesting “sponged” needle-like particle morphology. The electrochemical performance of α-VPO4 as the anode material was examined in Li-, Na-, and K-based cells. The carbon-coated α-VPO4/C composite exhibits 185, 110, and 37 mA h/g specific capacities respectively at the first discharge and around 120, 80, and 30 mA h/g at consecutive cycles at a C/10 rate. The considerable capacity drop after the first cycle in Li and Na cells is presumably due to irreversible alkali ion consumption taking place upon alkali-ion de/insertion. The EDX analysis of the recovered electrodes revealed an uptake of ∼23% of Na after the first discharge with significant cell parameter alteration validated by operando XRD measurements. In contrast to the known β-VPO4 anode materials, both Li and Na de/insertion into the new α-VPO4 proceed via an intercalation mechanism with the parent structural framework preserved but not via a conversion mechanism. The dimensionality of alkali-ion migration pathways and diffusion energy barriers was analyzed by the BVEL approach. Na-ion diffusion coefficients measured by the potentiostatic intermittent titration technique are in the range of (0.3–1.0)·10–10 cm2/s, anticipating α-VPO4 as a prospective high-power anode material for Na-ion batteries.

No abstract

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.