Annotation of the results obtained in 2020
During 2020 (31-42 months of project implementation), the main work on the project was focused on the following areas: synthesis and detailed structural and electrochemical study of promising electrode materials (cathode and anode)for sodium-ion batteries (SIBs), upscaling of electrode materials with outstanding characteristics, assembly of full cells with developed electrode materials and optimization of the fabrication of functional layers of sodium-ion battery prototypes in a pouch cell with subsequent study of their electrochemical properties. Following the workplan for the project, the study of cathode materials with the NASICON Na3+xMnxV2-x(PO4)3, 0≤x≤1 structure was continued. In particular, the redox transitions that occur during the charging and discharging of Na4MnV(PO4)3 were first studied using operando X-ray absorption spectroscopy (XANES). The results obtained provide a reliable idea ofredox transitions and explain the charge compensation mechanism: the V3+/V4+couple is active at ~ 3.4 V, and Mn2+/Mn3+ at the second plateau ~ 3.65 V. For Na3+xMnxV2-x(PO4)3, the peculiarities of sodium ion transport, interphase charge transfer, and phase transition kinetics were also studied using electrochemical methods. It is shown that an increase in the manganese content leads to a significant expansion of the single-phase regions, which is reflected in the shape of the voltammetric responses of the electrodes. It is also worth noting an increase in the formal potential of the process from 3.370-3.375 V for Mn0V2 and Mn0.1V1.9 to 3.42 V for Mn0.5V1.5. For all materials, close capacity values are achieved at C/10 rate (105-110 mAh g-1), corresponding to a specific energy of about 375 Wh kg-1. The obtained results indicate a significant difference in the electrochemical properties of materials with different manganese content. So, in the materials Mn0V2 and Mn0.1V1.9, the diffusion coefficients are quite high: (20-60) 10-12 cm2s-1 in the sodium-rich phase and (3-20) 10-12 cm2s-1 in the sodium depleted phase. A characteristic feature is the pronounced difference in the diffusion coefficients in the potential regions that are more negative and positive than the phase transition point. In this case, in the sodium-depleted phase, the diffusion coefficients are 2-6 times lower than in the case of the sodium-rich phase. Thus, it has been demonstrated that Na3+xMnxV2-x(PO4)3materials with a high manganese content provide the highest energy density during cycling in the potential range of 2.5 - 3.8 V. Despite the observed increase in hysteresis with increasing manganese content, a more important factor is a significant expansion of single-phase regions, due to which the effect of the slowed motion of the phase boundary on the cyclability of materials is excluded. Studies of substitutions in Na3V2(PO4)3for an electrochemically inactive metal, in particular, scandium, have been carried out. When cycling in the "standard" potential range of 2.5-3.8 V vs. Na/Na+the capacity of Na3ScV (PO4)3reaches ~ 65 mAh/g, which corresponds to the reversible extraction/insertion of one Na+ cation with the V3+/V4+ redox transition. An increase in the anodic limit leads to the fact that at ~4 V one more redox process is observed, which can be interpreted as the V4+/V5+ transition. In this case, the discharge capacity in the first cycle is ~ 90 mAh/g, and in the second cycle it drops by ~ 10%. Thus, it has been shown that the structure of NASICON is quite stable with respect to the oxidation of vanadium cations to oxidation state +4 and +5. It should be especially noted that materials with the NASICON structure of the composition Na3V2(PO4)3 were also used as model objects for studying the low-temperature properties of SIBs. It can be concluded that the material retains about 70% of its capacity when charged at -20°C, which indicates good prospects for using Na3V2(PO4)3 as a cathode for low-temperature batteries. The reasons for the sharp degradation of capacity at lower temperatures, from -40°C, are most likely associated with the electrolyte and require additional research using various solvents. The chemical composition of O3-NaNi1-x-yFexMnyO2 layered oxides as cathode materials was varied. In this part of the work, it can be argued that a change in the composition of materials and the limits of the charge and discharge potential leads to noticeable changes in the electrochemical properties of O3-type oxides and makes it possible to effectively control the values of the reversible capacity and cycling stability. The most promising from the point of view of energy density is the composition NaNi1/3Fe1/3Mn1/3O2 (NFM111), and the best cyclability is demonstrated by oxides enriched with manganese. In 2020, the work continued on a promising new cathode material, first discovered by us,β-NaVP2O7. The possibilities of increasing the specific capacity and energy due to the iso- and heterovalent substitution of cations in the d-sublattice are considered. The Cr3+ and Al3+ cations were taken, since they can result in an increase in the average working potential either due to the activation of the V4+/V5+ redox transition, or due to the Cr3+/Cr4+ redox pair. In this case, the potential of the V4+/V5+couple was ~4.8 V vs. Na/Na+, being one of the highest operating potentials for SIB cathodes described in the literature. In general, it can be noted that the β-modification of NaVP2O7turned out to be extremely promising in terms of not only electrochemical properties, but also the possibilities for by replacing vanadium cations with other metals. From the anode materials side, a hydrothermal synthesis and study of the electrochemical properties of a newα-TiPO4 with the α-CrPO4 structure have been carried out. For the material, a side conversion reaction was found at potentials below 0.9 V with Ti and Li3PO4as the products of the conversion reaction. In the potential range from 2.85 to 0.45 V, the specific discharge capacity was 573 mAh/g, which approximately corresponds to the theoretically possible value for a 3-electron conversion reaction (558 mAh/g). In the potential range of 1.0-2.9 V, excluding the conversion reaction, the achieved capacity was 125/123 mAh/g (discharge-charge). Also, α-TiPO4 is characterized by an irreversible capacity reaching 69 mAh/g, which is 0.37 in terms of the molar equivalent of Li+. The irreversible capacity is probably associated with the formation of a stable phase with the Li1/3TiPO4composition. In a sodium system, the specific capacity corresponding to a potential range of 1.1-3.0 V is much lower than in a lithium system and does not exceed 30% of the theoretically possible one. The hard carbon materials exhibited significantly different electrochemical characteristics depending on the temperature and atmosphere of the dehydration step. It has been shown that the dehydration temperature of 200°C is optimal in terms of capacity and coulombic efficiency. The discharge capacity for the first cycle in the best material is about 340 mAh/g with a coulombic efficiency of more than 85% for the first cycle, which is associated with a small specific surface area of carbon materials <10 m2/g. An increase in the specific surface area leads to excessive degradation of the electrolyte with the formation of decomposition products in the first cycle, and, as a consequence, a decrease in efficiency . A non-graphitizable carbon material obtained by dehydration at 200°C was chosen as an anode material for electrochemical studies of full cells due to the highest coulombic efficiency. In addition, a number of experiments were carried out on the synthesis of anode materials using an initial precursor based on phenolic resins, both individually and in the form of hybrid compositions, with the addition of graphitized (carbon fiber, thermally expanded graphite) and pore-forming additives (carbon fiber, fluoropolypropylene). The use of a non-graphitized base with additives of graphitizable carbon can make it possible to intercalate sodium into finished graphite particles due to an inter-boundary layer with a non-graphitized carbon matrix. In the literature, such materials are practically not considered in this aspect. Within the framework of the current stage, the work was carried out in the field of scalable methods for the synthesis of an anode material based on hard carbon. The results obtained to date (capacities 200 ~ 350 mAh/g, coulombic efficiency of the first cycle 70-80%) indicate that this direction is highly promising. It should be noted, however, that hard carbon anodes, which exhibit excellent electrochemical performance at low current densities, perform just modestly with increasing charge rates. To solve this problem, the study of alternative methods of applying electrodes (including from aqueous suspensions) and the composition of the binder was carried out. It has been shown that a binder based on the poly (3,4-ethylenedioxythiophene): poly (4-styrene sulfonate) (PEDOT: PSS) complex provides a higher initial coulombicefficiency of composite anodes (85.7%) as compared to binders of other compositions. In this case, the capacity of the anode, with the binder PEDOT:PSS, does not change during 10 consecutive charge/discharge cycles at a rate of C/10. Thus, the binder PEDOT:PSS combines high electrical conductivity and adhesive strength, which allows for a high specific capacity, coulombic efficiency, and cyclability of the composite anode. Special attention should be paid to highlighting the results of upscaling electrode materials. The mass of materials obtained ranged from 100 to 500 g. Subsequently, they were used to optimize the technology of applying active layers and assemblefull cells and prototypes. In addition, it was shown that for electrode materials it is possible to scale up synthesis methods and to replace expensive imported laboratory reagents with more cost-effective local analogs without affecting the electrochemical properties of the materials obtained. Summing up this part of the work, it should be noted that the obtained data are a clear demonstration of the prospects of sodium ion technology in general and the materials we are developing in particular. The coulombicefficiencies of the first cycle of 70-80%, obtained by us for three different types of cathode materials in full cells with hard carbon as the anode, are close to those for lithium-ion batteries (85-90%). A further increase in the coulombic efficiency and cyclability should be carried out by more fine tuning the capacities balance between the anode and cathode, as well as further optimization of the electrolyte composition. The most important result of the project was the construction of efficient full-format SIB pouch-cell prototypes. For their manufacture, the laboratory technique ofcasting functional layers was optimized. The best results were demonstrated by the method of mixing the slurry in a planetary mill with preliminary drying of all components in a vacuum drying oven. The ratio "active material: carbon black: PVDF", as well as the amount of solvent (N-methylpyrrolidone) were selected individually for each electrode material. As a result, it was possible to obtain active layers of all four basic materials with a specific surface capacity of 2-3 mAh/cm2, which is comparable with industrial LIB electrodes. We used three cathode materials (Na3V2(PO4)3, Na(Ni1/3Fe1/3Mn1/3)O2, and β-NaVP2O7) and one anode material (“hard carbon”). The electrolytes solutions used were optimized. A three-layer (PP-PE-PP) Celgard 30 μm thick served as a separator. The capacity of prototypes in a laminated foil case was 400-3300 mAh. The prototypes were demonstrated at the Chemistry-2020 exhibition (Expocenter, Moscow, October 26-30, 2020).

 

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Annotation of the results obtained in 2017
For the first four months, the project was primarily focused on the design of new electrode materials for metal-ion batteries, search and optimization of methods and synthesis conditions. It comprised the development of new electrolytes resistant to redox processes, and gel-polymer membranes for sodium-, potassium-ion and redox-flow batteries, as well as the development of infrastructure for modern in situ and operando diffraction and spectroscopy (XANES, Mössbauer spectroscopy) studies. Within the framework of this project, an original electrochemical cell was designed and manufactured for conducting modern in situ and operando diffraction measurements of electrode materials on a synchrotron accelerator. An important feature of the cell design was the use of single crystal sapphire x-ray windows resistant to electrolyte solutions at high (~ 5 V Li/Li+) and low (~ 0 V rel. Li/Li+) potentials and providing sufficient sealing and stable contact between components of the electrochemical system, which significantly improved the quality and reduced the accumulation time of the signal. The cell successfully passed the tests and was actively used in this project to study structural transformations and intercalation mechanisms, understanding of which allowed us to propose rational ways to optimize the composition, structure and morphology of materials in order to achieve improved electrochemical performance. Operando studies of layered Na2FePO4F cathode materials in the lithium-ion system revealed the competitive processes of Na+ extraction from the structure and chemical exchange of residual Na+ on Li+, which resulted in fundamentally different phase transformations suggesting the possibility of extracting more than 1 cation of alkali metal per d-element due to the realization of the two-electron M2+/M4+redox transition. In a sodium-ion cell, the Na+ de/intercalation occurs via a two-phase mechanism with the formation of an intermediate Na1.5FePO4F compound with the Fe2+/Fe3+ ordering coupled with the formation of vacancies in the positions of Na+ cations. At the same time, de/intercalation of alkali metal ions in a lithium-ion cell proceeds by a quasi-solid-solution mechanism.The results of the operando experiments are confirmed by density functional theory (DFT) calculations. It was also shown by calculations that the electrochemical deintercalation potentials of alkali metal cations from various positions in the structure are largely determined by the number of neighboring "semi-labile" oxygen anions that are associated only with phosphorus and alkali metal cations. Another significant result of the project was the development and optimization of the solvothermal method of synthesis for cathode materials adopting the NASICON structure for sodium-ion batteries demonstrating high-rate capabilities. For instance, Na3V2(PO4)3 cathode materials are capable of retaining ~ 70% of the theo. specific capacity at 10C rates, which corresponds to a discharge time of 6 minutes. It is shown that substitution in the cation sublattice of Na3+δV2-xMx(PO4)3, M = Cr, Mn leads to increased specific energy of the material compared to the pure material due to the presence of an additional high-voltage plateau in the region of 4 V vs. Na/Na+. During project implementation, new electrode materials were synthesized, as well as the high-voltage electrolytes were prepared for actively studied potassium-ion batteries. Original methods of hydro (solvo) thermal, sol-gel and solid-phase synthesis of both cathode (phosphates and fluoride phosphates) and anode (oxides and oxochlorides) materials were developed. The proposed high-voltage electrolyte based on fluoroethylene carbonate demonstrated significant stability to oxidation during cycling to 4.8 V vs. K/K+ due to the formation of a stable cathode-electrolyte interface, which opens prospects for the creation of high-voltage potassium-ion batteries. A technique for manufacturing gel-polymer electrolytes and membranes based on polyacrylonitrile for sodium-ion batteries has also been developed, which makes it possible to achieve, at low current densities (C / 20), attractive values of specific capacity close to theoretical for the cathodes under tests (~ 110 mAh/g for Na2FePO4F). Owing to a specially designed cell, it became possible not only to measure the crossover value for vanadium ions, but also to determine the diffusion coefficient of V4+ ions through such membranes for redox-flow batteries.

 

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Annotation of the results obtained in 2018
A second year of project implementation was focused on the following main areas: a detailed study of electrode materials for metal-ion batteries, a design of brand new electrode materials and optimization of synthesis conditions of previously obtained materials, including methods and techniques for upscaling with subsequent manufacture of prototypes, conductive carbon coating and the development of new electrolytes for sodium-, potassium-ion batteries and the design of gel-polymer membranes for redox flow batteries. Special attention was paid to the synthesis of new representatives of promising oxide and polyanion electrode materials, including the synthesis of anode material based on double Ti and Nb oxide with extremely high reversible specific capacity (201 mAh / g) at an average operating potential of 1.5 V vs. Li/Li+. The conditions for the synthesis of VOCl and the preparation of electrodes based on it with a reversible capacity of ~ 100 mAh/g during cycling in a limited range of potentials (1-3 V vs M/M+) have been optimized. The effect of annealing temperature, synthesis time, and quenching conditions on the phase formation of NaxMO2 samples was established. For the О3 modification of NaNi0.5Mn0.5O2, a reversible capacity of ~ 140 mAh / g was achieved with an average operating potential of ~ 3 V vs. Na/Na+. A new modification of vanadium phosphate (hereinafter referred to as α-VPO4) adopting the α-CrPO4 structure was obtained using the hydrothermal method and examined as an anode material for metal-ion batteries. The material exhibits a reversible capacity of 115-120 mAh/g in a Li-ion cell, 80 mAh/g Na-ion cell. The conditions for the synthesis of isostructural α-CrPO4 and α-TiPO4 were determined. A method for the synthesis of cathode material based on Na4MnV(PO4)3 with a reversible capacity of 101 mAh/g during cycling in the range of 2.5-3.8 V vs. Na/Na+ and 114 mAh/g - 2.5-4.0 V has been developed. A cathode material based on Na2CoPO4F was successfully obtained demonstrating a reversible capacity of ~90 mAh/g, and an average operating potential ~ 4.5 V vs. Na/Na+. An almost complete (> 90%) ion exchange of Na for Li in Na2CoPO4F was carried out with the formation of a new layered modification of Li2CoPO4F, which was not previously described in the literature. A method was developed for the synthesis of Na2+2xM2–x(SO4)3 (M = Fe, Co) cathode materials with the allaudite structure characterized by a solid-solution (de) intercalation mechanism, including the mixed allaudite Na4+2x(Fe1-2yCoy)2–x(SO4)3 with a nominal y = 0.5. The polyacrylamide method of synthesis enabled obtaining samples of the compositions Na2Ni2Cr(PO4)3, Na2Ni2Al(PO4)3, Na2Mg2Cr(PO4)3 with the α-CrPO4 structural type. “Hard carbon” anode material samples obtained in our work demonstrate > 200 mAh/g reversible capacity in Na-ion cell. Modern in situ and operando diffraction and spectroscopic methods (Mössbauer spectroscopy) were broadly used in this project. They allow studying the processes occurring during electrochemical cycling, in particular, structural transformations, phase transitions, charge-discharge mechanisms, etc. With the operando experiments using synchrotron radiation for Na4MnV(PO4)3, it was found for the first time, that the first stage of deintercalation (~ 3.3 V) proceeds via the solid solution mechanism, and then the next stage (~ 3.5 V) revealed a two-phase mechanism with the formation of a rhombohedral phase of the Na2MnV(PO4)3 approximate composition. When charging up to 3.8 V, the reverse process (discharge) takes place symmetrically, and when the charge potential increases to 4.0 V, the additional charge plateau is characterized by the solid-solution type of Na+ deintercalation. The Na1.7(1)MnV(PO4)3 phase is formed, and the occupation of the Na1 position, previously considered electrochemically inactive, decreases. Reverse intercalation of Na+ takes place in the solid-solution mechanism, as well as the subsequent charge-discharge cycles. The reversible capacity increases by 13%. For the Na2CoPO4F cathode material, a new Na1.42CoPO4F phase with an ordering of sodium vacancies, sodium cations and Co2+ and Co3+ cations, isostructural to Na1.55FePO4F was found. At the maximum cell voltage (4.8 V vs. Na/Na+), the main phase was the NaCoPO4F charged form, isostructural to NaFePO4F. For the first time, the features of phase transformations of the VOCl material were revealed during cycling in the lithium-ion and sodium-ion cells using operando experiments. Different experiments were performed using the Mössbauer spectroscopy method in the operando mode at low (C/20) current densities. It is shown that the data obtained at the laboratory source can be used to determine the parameters of the hyperfine interaction - quadrupole splitting and isomeric shift - iron cations. The behavior of the components of the “defective” iron in the olivine structure during galvanostatic charge/discharge not previously described has been revealed. During the project, significant work has been done to develop the methodology and optimize the conditions for coating cathode materials with conductive polymers based on poly (3,4-ethylenedioxythiophene), PEDOT. A significant stability during cycling of such composites and high specific capacity (up to 159 mAh/g at C/10 and 115 mAh/g at 3C) were achieved. The use of a binder based on PEDOT: PSS/SPFO, characterized by low porosity, provides up to one and a half times higher density of the cathode material compared to traditional cathodes, which leads to an increase in specific volumetric capacity from ~ 150 to 250 mAh/cm3 at a discharge rate of C/10. The effect of dopamine polymerization conditions on the production of carbon coatings of various thickness and degree of homogeneity was also established. A crucial part of the project was the design and development of new promising electrolytes for metal-ion batteries and membranes for redox flow batteries. Electrochemical properties of electrolytes for sodium-ion batteries based on NaClO4 and NaPF6 salts in various carbonate solvents and sulfolane were studied. The effect of fluoroethylene carbonate (FEC) as a functional additive that has a positive effect on the anodic stability of the electrolyte is considered in detail. It was shown that electrolytes with NaPF6 are more stable than those with NaClO4. A method for preparing gel-polymer electrolytes based on acrylonitrile-methacrylate copolymer has been developed. Propylene carbonate was used as a plasticizer, and NaClO4 and NaPF6 were used as source salts of sodium ions. The best results were demonstrated by NaClO4 based membranes. Electrochemical cells with gel-polymer electrolyte and various anode (solid carbon and metallic sodium) and cathode (Na2FePO4F and Na3V2(PO4)3) materials showed stable cycling (200 cycles) in a wide range of charge-discharge rates. The capacity normalized to the cathode material was 80-100 mAh/g. A comparative study of permeability for protons and vanadium cations of membranes based on polyacrylonitrile and polysulfone for redox flow cells has been carried out. An increase in the porosity of the polysulfone membranes, obtained by incorporating a hydrophilic (water soluble) polymer into the continuous film being formed, allowed the electrochemical cell to be slowly cycled at a low current density (about 6 mA/cm2). In this case, the Coulomb efficiency of such a cell is more than 95% characterized by practically no self-discharge. According to the project workflow, the second year of implementation was an important stage in the transition to upscaling metal-ion battery electrode materials for subsequent prototyping and production of a working device. In particular, translation of the laboratory procedure of the hydrothermal synthesis of LiFePO4 to the “pilot” level was carried out using a 10-liter Parr Instruments reactor (yield is about 700 g in one synthesis), annealing of relatively large (50–200 g) batches of material was developed to create carbon coating, cathode materials with a capacity of ~ 150 mAh/g at low current density and maintaining up to 70% of the original capacity during charge/discharge at a rate of 10C were obtained. A set-up of a 5-cell vanadium battery with an electrode area of 12 cm2 and an output power of 18 W was designed and manufactured. The possibility of using the electricity generated by it to power its own pumps and the load layout is shown. A prototype of a battery management system was manufactured and tested, which allows charging, discharging and indication of battery parameters. The software that controls its work system has been developed.

 

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Annotation of the results obtained in 2019
During 2019 (18-30 months of the project) the main work was focused on the following areas: synthesis and detailed structural and electrochemical research of promising electrode materials and the development of gel-polymer electrolytes for sodium-ion batteries, search and testing of methods for upscaling electrode materials with outstanding characteristics, design and manufacture of the prototype sodium-ion battery and the prototype of the vanadium redox-flow battery pack of 100 watts. Due to optimization and increasing the efficiency of the team, certain areas that were considered unpromising were excluded. At the same time, the report for 2019 was supplemented by paragraphs that were not announced earlier, but which already allowed obtaining the most important patentable results, namely, the development of a new electrode material based on β-NaVP2O7 for sodium-ion batteries (patent application filed, May 2019). NASICON-structured cathode materials are considered as possible candidates for high-performance Na-ion batteries. Further increase of energy density of the Na3V2(PO4)3 may be achieved by substitution of the V cations by other transition metals. Here, we show that a family of Na3+xMnxV2-x(PO4)3(0≤x≤1, Δx=0.2) cathode materials demonstrates remarkable diversity of the electrochemical properties and phase transformations depending on degree of substitution and cut-off voltage. An intermediate “Na2M2(PO4)3” phase was found for all compounds studied by means of operando powder X-ray diffraction. When Mn content is low (x~0-0.4), it coexists with Na3+xMnxV2-x(PO4)3 or Na1+xMnxV2-x(PO4)3 phases. Increase in Mn content extends the length of the solid solution region corresponding to sodiated, intermediate and desodiated phases. All Mn-substituted samples are characterized by additional high-voltage plateau (~3.9 V) at charge-discharge curves. Cathode material with Na3.8Mn0.8V1.2(PO4)3 composition exhibits 10% energy density gain in comparison to Na3V2(PO4)3, but Na3.2Mn0.2V1.8(PO4)3 and Na3.4Mn0.4V1.6(PO4)3 are most preferable in terms of cycling stability An experimental study was made of the kinetics of intercalation of sodium ions in the structures of Na3V2(PO4)3 and Na4MnV(PO4)3. Phosphates are characterized by nucleation-limited de/intercalation process, which is expressed in hysteresis on the charge-discharge curve. The effective diffusion coefficients (D) of sodium cations in the Na3V2(PO4)3 and NaV2(PO4)3 phases reach 6•10-11 cm2 / s in the sodium-enriched Na3V2(PO4)3 phase, while in the sodium-depleted phase NaV2(PO4)3 are about 1•10-11 cm2 / s. Moreover, the effective diffusion coefficients in the structure of Na4MnV(PO4)3 in single-phase regions are lower than in the structure of Na3V2(PO4)3: 10-11 - 10- 12 cm2/s. Nucleation rate constants were determined at various potentials. High values of nucleation rate constants (10-3 - 10-2 s-1) correspond to low activation energies of critical formation nucleus in processes of the phase transformations in Na3V2(PO4)3. Introduction of Mn into Na3V2(PO4)3 structure results in a significant increase in nucleation barrier (reduction rate (de) intercalation of Na+) Novel layered (2D) modification of LiNaCoPO4F fluoride phosphate, isostructural LiNaNiPO4F was obtained. This result shows that the synthesis temperature remains an “unexplored dimension” in the chemistry of A2MPO4F fluoride phosphates and its effect requires careful study. Galvanostatic measurements of 2D-LiNaCoPO4F showed the presence of reversible electrochemical activity with low Coulomb efficiency, which is associated with the decomposition of the electrolyte at high potentials. In the sodium cell, the specific capacity is 48 mAh/g during the first cycle; during further cycling, its noticeable decrease is observed. The average potential in the Na-ion cell is ~ 4.2 V vs. Na/Na +, which is in good agreement with the theoretical value calculated by the DFT method. In our opinion, the main task that needs to be solved for the further development of the topic of 2D-LiNaCoPO4F is the creation of a carbon coating in order to improve the electrochemical characteristics. The methods of ionothermal synthesis using ionic liquid EMI-TFSI (1-ethyl-3-methylimidazolium (trifluoromethanesulfonyl) imide) for the preparation of single-phase solid solutions of alluoudite Na2+2xM2-x(SO4)3, M = Fe, Mn, Co, Fe/Mn, Fe/Co, Co/Mn. The average charge-discharge potential (≈3.8 V vs. Na/Na+) for Na2.6(Fe0.5Mn0.5)1.7(SO4)3 indicates the activity of the Fe2+/Fe3+ redox pair. The absence of peaks in the CV or plateau curves of the charge – discharge curves at higher potentials suggests that the manganese cations in Na2.6(Fe0.5Mn0.5)1.7(SO4)3 do not exhibit electrochemical activity. According to galvanostatic cycling, the sample capacity of Na2.6 (Fe0.5Mn0.5)1.7(SO4) 3 is about 50 mAh/g, which confirms the assumption that only the Fe2+/Fe3+ transition is realized and corresponds to a reversible extraction of ~ 0.8Na at . To activate the Mn2+/Mn3+ transition, it is planned to conduct electrochemical measurements at elevated temperatures, optimize the electrode microstructure (for example, by grinding with soot), and also obtain samples with a different Fe/Mn ratio. Promising samples of the cathode material based on the mixed oxide NaNi1/3Fe1/3Mn1/3O2 (layered O3 structure) were obtained. The material exhibits capacity of 130-140 mAh/g when charged to 3.9 V and > 150 mAh/g with an increase in the charging potential limit to 4.2–4.3 V, however, the material’s cyclability worsens. Observed degradation, according to Mössbauer spectroscopy in operando mode, may appear due to appearance of an additional component Fe3+z, which probably corresponds to iron cations migrating into the tetrahedral void into the interlayer space in the structure, as was suggested for NaFeO2. It should be noted that at potentials above 4.2 V, Fe cations cease to participate in redox processes: it is likely that further sodium extraction is associated with the Ni3+/Ni4+ transition. Special attention should be paid to the coverage of the results of the first obtained new modification of sodium-vanadium pyrophosphate, for which the functional properties of the electrode material for Na-ion batteries were studied. The achieved capacity at a current density of 10 mA/g at the discharge was 104 mAh/g, which is 96% of the theoretical (108 mAh/g). The oxidation of V3+ occurs at a potential of ≈ 4.15 V vs. Na/Na+. Despite the relatively large particle sizes, an increase in current density leads to a slight decrease in capacity: at a current density of 50C, the electrode is discharged by 77 mAh/g. According to the operando of X-ray diffraction, at the very beginning of Na+ deintercalation, a two-phase mechanism is observed, accompanied by the appearance of reflexes of a new phase with triclinic symmetry. The second stage of deintercalation proceeds via solid-solution mechanism, which follows from the weak shift of the triclinic phase reflexes and the final disappearance of reflections of the monoclinic phase. It was found that the change in the unit cell volume is only 0.4% during the operation of the material. This parameter, apparently, directly affects the stability of material cycling: the loss of material capacity after 90 cycles at a current density of 1C was less than 1%. In addition to the electrochemical activity at high potentials, corresponding to the transition "NaVP2O7↔VP2O7", a process at low potentials is also observed. During 2019, the study of anode materials for sodium-ion batteries was also continued. The 2018 report presented the results of work on the new VPO4 material, crystallizing in the α-CrPO4 structural type. As part of the project, this year the possibility of obtaining phosphates of trivalent 3d metals: Ti, Cr, V and solid solutions based on them was considered. A two-stage synthesis technique has been developed for a continuous series of compounds (for any “x”) of CrxV1-xPO4 composition in the α-CrPO4 modification. Non-graphitizable carbon materials (“hard carbon”) were synthesized using various precursor processing techniques with a variation in the final annealing temperature from 1100 to 1400°C. The discharge capacities of non-graphitizable carbon materials at a current density of 25 mA/g amounted to more than 290 mAh/g with a Coulomb efficiency above 70% in the first cycle. It was found that an increase in the final annealing temperature leads to decrease in the capacitance responsible for the inclined section, which may be associated with a decrease in the number of channels for the penetration of sodium cations into the partially preserved interlayer space, as well as an increase in the capacitance responsible for the plateau section. To scale the synthesis of non-graphitizable carbon material, preliminary dehydration of D-glucose in air and subsequent annealing in an argon stream at 1300 ° С were chosen, after which the sample was ball-milled in the planetary mill and dried in vacuo. Within the framework of this project, functional polymer binders based on PEDOT were synthesized: PSS – cPFO, cPFO – ONT and PFO – MNT, which provide higher adhesion of the cathode composite to the current collector compared to the traditional PVDF – S binder, which leads to an improvement in capacity stability during cycling. It is also shown that a gel-polymer electrolyte based on a polymer of acrylonitrile, propylene carbonate and sodium salt (perchlorate, hexafluorophosphate) have characteristics sufficient for use in sodium-ion batteries. For the manufacture of real devices based on it, it is necessary to obtain such an electrolyte in the form of thin films. An important step in the implementation of the project is certainly the upscaling of the synthesis and prototyping of sodium-ion batteries. Full cells (with hard carbon as the anode) were fabricated for three “basic” cathode materials - Na3V2(PO4)3, O3-NaFe1/3Mn1/3Ni1/3O2 and β-NaVP2O7. Using these cathode materials and hard carbon as the anode, several prototypes of sodium-ion batteries in a soft case with a capacity of ≈ 500, 500, 200, 100 mAh were manufactured and presented at the exhibition held at the Fundamental Library of Moscow State University during the celebration of the 90th anniversary of the Chemical Faculty of Moscow State University and the closing of the year of the Periodic Table of Chemical Elements in Russia. In addition to the sodium-ion full cells, a prototype of a 100 W vanadium battery pack has been assembled. Optimization of the design of the smaller electrochemical unit (membrane area 12 cm2 (3x4cm), both single-cell with an operating voltage of 1.2 V and five-cell with an operating voltage of 6 V, was preliminarily carried out. Creating a large battery (100 cm2 of membrane area) required careful analysis and modeling of the process mass transfer between the electrolyte in the cell and the flowing electrolyte, as this process determines the efficiency of the battery The effective internal resistance of the battery was 18 mOhm. In addition to the manufacture of the prototype, the electrochemical characteristics of a flowing vanadium battery assembled using MF-4SK membranes were studied: membrane stability in a medium simulating the operating conditions of a flowing battery, membrane permeability with respect to vanadium ions, proton conductivity, swelling in various media, ion-exchange capacity and performance model flowing redox battery. The listed characteristics were compared with those for the standard Nafion 112 membrane. Preliminary tests showed that all provided membranes are stable under conditions simulating the operation of a flowing redox battery based on vanadium compounds.

 

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