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Vital need to reduce battery cost inspired intensive research in the field of sodium-storage systems, which required extending the range of perspective electrode materials. Herein, we present a new member of the electrochemically active NASICON material family, rhombohedral NaMo2(PO4)3, which demonstrates both Mo+4/Mo+5 and Mo+4/Mo+3 redox activities towards sodium (de)intercalation. Desodiation of the initial NaMo2(PO4)3 material with subsequent discharge within the 1.2–4.0 V range leads to a multi-electron redox transition with reversible capacity of 130 mAh g−1. However, the single Mo+4/Mo+3 redox transition at 2.45 V (vs. Na/Na+) with capacity of 95 mAh g−1 was found to be more stable during cycling, and operando X-ray diffraction confirmed its two-phase mechanism. Sodiation of the initial phase results in Na2.5Mo2(PO4)3, which maintains the parent NASICON structure (R-3c, a = 8.90532(7) A, c = 22.2379(3) A, V = 1527.30(3) A3), with two Na-positions being almost equal in occupancy and energy.

In this work we analyze the applicability limits of the rotating ring-disk electrode (RRDE) technique for quantifying the amount of oxygen produced during the oxygen evolution reaction (OER). We utilize a state-of-the art IrO2 oxide as a carbon-free OER catalyst and La1−xSrxMn0.5Co0.5O3−δ, x = 0.25 and 0.5 perovskites, which are studied in the presence of carbon. RRDE experiments are performed at different IrO2 loadings under both potentiodynamic and potentiostatic modes. The experimental data allow us to formulate the requirements to the experimental conditions necessary to avoid underestimation of the oxygen yield.

High-voltage stability of LiBF4 - propylene carbonate solutions in intermediate concentration range is studied by means of cyclic voltammetry, galvanostatic cycling and X-ray photoelectron spectroscopy using LiCoPO4 and LiNi0.5Mn1.5O4 cathode materials. Coulombic efficiency improves with increasing salt-to-solvent molar ratio from 1:12 to 1:4 (∼0.8 m–2.5 m solutions), reaching 98% at 1C charge/discharge rate for 1:4 electrolyte upon cycling of LiNi0.5Mn1.5O4 up to 5 V vs. Li/Li+. The same positive trend is observed for discharge capacities, cycling stability and capacity scattering for both high-voltage cathode materials. X-ray photoelectron spectroscopy of the electrodes studied after cycling in solutions of different concentrations does not reveal any drastic difference in surface composition. Interface pre-formation experiment shows that the presence of the interface layer formed at semi-concentrated 1:4 electrolyte does not have a major impact on the electrochemical properties of the dilute solution. We assume that the enhanced oxidation stability of the concentrated solutions itself is a reason of improving the electrochemical performance rather than the cathode-electrolyte interface properties.

Theory of the convective-diffusion transport for the steady-state electroreduction of bromate-anion at rotating disk electrode has been developed for the first time for the compositions of the bulk solution where the bromate concentration is much higher than the proton one. Because of the non-electroactivity of the bromate anions and protons the process proceeds via a redox cycle composed of the electrochemical reduction of bromine (always present in acidic bromate solutions) to bromide and of the comproportionation reaction between bromate, bromide and protons regenerating bromine species. The autocatalytic feature of the cycle can lead under certain conditions on the convection intensity to very strong accumulation of the redox-couple components, bromine and bromide, in the kinetic layer of the solution near the electrode surface, resulting in a very high rate of the overall process, which is controlled by transport of principal components of the bulk solution, namely by that of protons for systems with bromate excess. Theoretical description of such a situation is based on transport equations for all components of the system taking into account of both the convective and diffusional contributions to their fluxes, the bromate concentration being approximately considered as constant across the diffusion layer. Analytical formulas for the concentration distributions have been derived for all current densities that can pass through the RDE system for any rotation frequency. Principal results of this analysis for the bromate excess in the bulk solution are similar to those obtained within the framework of the previous approaches, in particular existence of an “anomalous frequency range” where the maximal current (for a fixed rotation frequency) rises drastically if the rotation frequency (i.e. the solution agitation intensity) decreases, as well as approach of the maximal current for the lower frequency range to the one limited by the convective-diffusion transport of protons. At the same time quantitative predictions of this novel theory and of conventional Nernst model of a stagnant layer has shown significant differences among them that cannot be avoided for any choice of the diffusion layer thickness. On the contrary, the recently proposed theoretical approach of the “Generalized Nernst Layer” model has turned out to be able to reproduce adequately both the concentration distributions inside the kinetic layer (in particular, the values of the concentrations at the electrode surface) for any passing current, as well as the dependence of the maximal current density on the disk rotation frequency (including the frequency ranges of the anomalous behavior and of very strong currents). As it has been expected intuitively, no model replacing the convective-diffusion transport outside the (thin) kinetic layer by a convection-free layer is able to describe properly the concentration profiles there.

We report the voltammetric and soft X-ray absorption spectroscopy (XAS) data for two series of perovskite oxides, namely La1−ySryMn0.5Co0.5O3−δ with a constant Mn:Co ratio and variable extent of Sr doping, and Sr0.5Gd0.5MnxCo1−xO3−δ perovskite oxides with a constant Sr content but variable extent of the Co doping. The essential differences in the values and scan rate dependences of the total charges observed between the studied materials in alkaline solutions in the potential regions of their stability are interpreted on the basis of separation of the ‘fast’ and ‘slow’ recharging components. Our analysis of the total charge is based on two approaches, the first proposed by Trasatti, the second by Conway. The ‘slow’ charge is assigned to the oxygen intercalation in sub-surface oxide layers. We demonstrate correlations of the ‘fast’ and the ‘slow’ recharging components with the electronic state of Mn, Co and O determined from spectroscopic data. We also present the arguments in favor of possible contribution of the interfacial oxygen vacancies to the ‘fast’ interfacial recharging. These findings may be useful for understanding correlations of the composition and stoichiometry of the oxides on the one hand, and their electrocatalytic activity on the other.

Potassium and sodium-ion transfer kinetics were compared for the intercalation reactions into the KVPO4F positive electrode material in acetonitrile- and ethylene carbonate-based electrolytes, which implied the formation of different electrode/electrolyte interface structures. The presence of surface layers was found to result in a significantly more pronounced effect on the ion transfer kinetics for K+ compared with Na+, while the barrier layers in K+ electrolytes were demonstrated to be less resistive. Difficulties associated with the stabilization of the electrode material/potassium electrolyte interface under high operating potentials require the application of higher voltage electrolytes. The kinetic trends in three high voltage electrolytes were compared for the potassium (de)intercalation reaction, and the general obstacles to developing a high-voltage potassium-ion battery were identified.

A novel polymorph of lithium vanadium pyrophosphate has been obtained using electrochemical Na+ for Li+ exchange in β-NaVP2O7. β-LiVP2O7 has a triclinic KAlP2O7-type structure with large tunnels along the [100] direction and demonstrates electrochemical capacity of 100 mAh g−1 within a high-voltage region of 4.0-4.2 V vs. Li/Li+ with excellent C-rate capability: increasing discharge current density from C/10 to 10C results in a capacity drop of only 7%. The (de)intercalation process is characterized by a solid solution-like behavior with the diffusion coefficient amounting to 10−12 – 10−11 cm2 s−1. A redox process at ~ 1.8 V vs. Li/Li+ leads to 200 mAh g−1 of the overall intercalation capacity within the extended voltage range that corresponds to “β-VP2O7 ↔ β-Li2VP2O7” two-electron process.

NASICON-type lithium-vanadium phosphate (Li 3 V 2 (PO 4 ) 3 ) based electrodes capable to provide extremely fast lithium transport properties were studied by a combination of structure, morphology and surface characterization methods: X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), laser diffraction particle size distribution (PSD), as well as electrochemical methods: potentiostatic (PITT) and galvanostatic (GITT) intermittent titration techniques, electrochemical impedance spectroscopy (EIS), and constant current chronopotentiometry. Significant differences in the kinetics of reversible lithium intercalation depending on the lithiation stage were found: lithium diffusion coefficients, found from the PITT, GITT and EIS data, demonstrate an abrupt drop by 2–3 orders of magnitude (from 10 −9 to 10 −12 cm 2 s −1 ) in the 4.3–4.4 V potential range vs Li/Li + , which is attributed to the LiV 2 (PO 4 ) 3 ↔ V 2 (PO 4 ) 3 phase transition. The electrochemical extraction/insertion of two lithium equivalents can occur at ultra-high rates (up to 320C) from/into structurally more accessible Li2 and Li3 sites, while the de/intercalation of the third lithium equivalent from/into the Li1 position is supposedly hindered kinetically. To analyze the electrochemical data, specially developed theoretical models were used, which take into account geometry and phase configuration of the diffusion space, as well as the properties of the phase boundary interfaces. Morphology and size distribution parameters, essential for mathematical processing of the electrochemical data, were obtained by SEM, PSD and BET methods.

We consider compositional and structural factors which can affect the activity of bixbyite α-Mn2O3 towards the oxygen reduction reaction (ORR) and the stability of this oxide in alkaline solution. We compare electrochemistry of undoped, Fe and Al-doped α-Mn2O3 with bixbyite structure and braunite Mn7SiO12 having bixbyite-related crystal structure, using the rotating disk electrode (RDE), the rotating ring-disk electrode (RRDE), and cyclic voltammetry (CV) techniques. All manganese oxides under study are stable in the potential range between the ORR onset and ca. 0.7 V vs. Reversible Hydrogen Electrode (RHE). It is found that any changes introduced in the bixbyite structure and/or composition of α-Mn2O3 lead to an activity drop in both the oxygen reduction and hydrogen peroxide reactions in this potential interval. For the hydrogen peroxide reduction reaction these modifications also result in a change in the nature of the rate-determining step. The obtained results confirm that due to its unique crystalline structure undoped α-Mn2O3 is the most ORR active (among currently available) Mn oxide catalyst and favor the assumption of the key role of the (111) surface of α-Mn2O3 in the very high activity of this material towards the ORR.

Hard carbon is the most prospective anode material for sodium-ion batteries with outstanding electrochemical performance. However, the type of reaction associated with charge storage is a subject of contentious deliberations within the scientific society. We examined the pseudocapacitive behavior of glucose-derived hard carbon with different porosity by detailed linear sweep voltammetric analysis which demonstrated the presence of three distinct processes occurring during Na+-deinsertion. A combination of pseudocapacitive-intercalation-pseudocapacitive behavior for all synthesized hard carbons is proposed. Obtained results differ drastically from the behavior demonstrated by graphite/Li cell, soft carbon/Na cell and pure Na metal plating.

NASICON-type Na3V2(PO4)3 cathode materials can be regarded as promising candidates for high-power Na-ion batteries due to the observed facile kinetics of Na-ion de/intercalation. Substitution of V for Mn provides additional advantages related to the increase in the average operating potential and reduced cost of the active material. In this work, we explore the kinetics of Na+ intercalation into Mn-substituted Na3+xMnxV2-x(PO4)3 (x = 0, 0.1, 0.5, 1) materials with a primary focus on the impact of Mn content on the rate capability of the materials. We demonstrate that Mn substitution results in quite subtle changes in bulk ionic diffusivity and charge transfer rates, while more significant impact is observed on the nucleation kinetics, which induces large hysteresis between charge and discharge curves for Mn-rich materials. The increase in hysteresis between charge and discharge curves does not limit the specific energy retention at high C-rates significantly, yet the performance losses are mainly related to the slow phase boundary propagation for biphasic processes. The Mn-rich materials, which demonstrate wider single-phase regions, are shown to outperform the unsubstituted materials in terms of rate-capability and should be preferred for high-power applications.

Efficient polyacrylonitrile-based gel-polymer electrolyte for sodium-ion batteries was prepared using commercially available copolymer poly(acrylonitrile-co-methyl acrylate), propylene carbonate as plasticizer, and NaClO4 or NaPF6 as electrolyte salts. Specific and molar conductivities were determined at different salt concentrations using the ac impedance technique with reversible rather than blocking electrodes. The cation transference numbers were measured using the potentiostatic polarization technique. The electrolytes showed high room-temperature ionic conductivities up to 1.8 × 10−3 S cm−1 and high cation transference numbers up to 0.89. The conductivity followed the Arrhenius behavior with the activation energy of 12–15 kJ mol−1. The polymer electrolyte was able to sustain over 700 charge-discharge cycles in full sodium-ion cells at room temperature and at −20 °C. The results indicate that polyacrylonitrile-based gel-polymer electrolytes are promising systems to replace liquid electrolytes in future sodium-ion batteries.

In view of applied prospects of the bromate electroreduction in acidic medium this process has been analyzed for the solution flow through a channel filled in with porous conducting material playing the role of 3D electrode. The process passes via mediator redox cycle where the bromine is reduced at the electrode surface to bromide which is subject to the comproportionation reaction with bromate inside the solution phase to regenerate bromine. Inside the entering solution protons are in excess compared to bromate ions while the bromine concentration is very low. Ohmic losses across the channel are disregarded. Perpendicular size of pores is assumed to be sufficiently small so that the diffusion of solute components suppresses concentration gradients in the transversal direction. Solute species are transported along the channel by the convective mechanism. Numerical and approximate analytical methods of solving coupled transport equations have provided distributions of solute components inside the porous medium and of the current density along the flow coordinate, y. Quite unusual features of the process have been discovered, in particular non-monotonous variation of the bromine concentration and of the current density as functions of y, with narrow maxima at an intermediate distance from the entrance, dependent on the flow velocity and rate constants of the electrochemical and chemical steps of the redox cycle. All these surprising features of the bromate reduction process originate directly from the autocatalytic character of this cycle where each passage of the cycle increases the amount of the catalytic bromine/bromide couple in solution, thus accelerating the bromate transformation. As a result one can reach simultaneously both high efficiency of the bromate transformation and strong current for an optimal value of the channel length-to-flow velocity ratio, even for extremely small ratio of the incoming bromine and bromate concentrations.

Knowledge on the mechanisms of oxygen reduction reaction (ORR) and descriptors linking the catalytic activity to the structural and electronic properties of transition metal oxides enable rational design of more efficient catalysts. In this work ORR electrocatalysis was studied on a set of single and complex Mn (III) oxides with a rotating disc electrode method and cyclic voltammetry. We discovered an exponential increase of the specific electrocatalytic activity with the potential of the surface Mn(IV)/Mn(III) red-ox couple, suggesting the latter as a new descriptor for the ORR electrocatalysis. The observed dependence is rationalized using a simple mean-field kinetic model considering availability of the Mn( III) centers and adsorbate-adsorbate interactions. We demonstrate an unprecedented activity of Mn2O3, ca. 40 times exceeding that of MnOOH and correlate the catalytic activity of Mn oxides to their crystal structure. (C) 2015 Elsevier Ltd. All rights reserved.

A new model of lithium-ion transport processes in the LiFePO 4 electrode is proposed. This model takes into account the phase transition LiFePO 4 ↔ FePO 4 accompanying reversible lithium intercalation into the electrode during potential or current steps. The diffusion coefficient of Li + ion and its dependence on the LiFePO 4 /FePO 4 phase ratio have been determined by means of processing of experimental potential and current transients in accordance with the model's equations. The results of galvanostatic and potentiostatic intermittent titration techniques are in good agreement. The value of diffusion coefficient varies within 10 −10 –10 −16 cm 2 s −1 depending on the lithium content in solid solution Li X FePO 4 and Li 1− X FePO 4 ( X 4 /FePO 4 phase ratio.

Wadsley–Roth phase Li x Nb 9 PO 25 has been studied as a potential candidate for anode material of Li-ion batteries. Its crystal structure, which consists of ReO 3 -type blocks of NbO 6 octahedra connected with PO 4 tetrahedra, provides a good stability and performance during Li + insertion/removal. Li-ion chemical diffusion coefficient ( D chem ) in Li x Nb 9 PO 25 was determined by means of potentiostatic intermittent titration technique and electrochemical impedance spectroscopy. Different data treatments (classical Warburg equation or the model of an electrode system with ohmic potential drop and/or slow kinetics of the interfacial Li + ion transfer across the electrode/electrolyte interface) were used for calculation of D chem of the Li ion inside this material; their applicability is discussed in the article. D chem changes with the Li-ion doping degree, x, in Li x Nb 9 PO 25 and has a sharp minimum near the two-phase region at appr. 1.7 V vs. Li + /Li. These values of D chem in Li x Nb 9 PO 25 (∼10 −9 –10 −11 cm 2 s −1 ) were found to be in average noticeably higher than in the widely studied anode material, Li 4 Ti 5 O 12 .