A method for increasing efficiency of an all-iron flow battery system

Abstract

The claimed invention relates to the energy industry, in particular, to electrochemical technologies for accumulating and storing energy, and it relates to a method for increasing efficiency of a redox all-iron flow battery that consists of an electrochemical cell that is divided by a membrane into compartments, in which a positive electrode and a negative electrode are placed. The compartments are filled with a negative electrolyte and a positive electrolyte with a possibility of their circulation through the corresponding compartments of the cell and external reservoirs for storing the electrolytes, and initiating the cyclic charging / discharging operation, and a metal coating is formed on the negative electrode, and to this end, a pre-charging of the system is carried out during circulation of the electrolytes. According to the invention, the pre-charging is carried out at a current density in a range from 20 to 60 mA / cm2 until a state of charge from 0.1 to 10% is achieved, then the cyclic operation is initiated without replacement of the electrolytes starting from charging, and the pre-charging is repeated after completion of the discharging upon increase of a charge voltage and / or decrease of a charge voltage and / or upon increase of a concentration of Fe2+ ions in the negative electrolyte. Said sequence of actions and conditions of performing thereof result in formation of a stable metallic coating of the negative electrode and a stable concentration of the electrolytes, which are indicators of stability of operation of the all-iron flow battery system during multiple repetition of the charge / discharge cycles and, accordingly, its high efficiency when used in power systems.

Description

[0001] IPC (2025.01)

[0002] H01M 8 / 18; HO1M 8 / 188 H01M4 / 88; H01M4 / 96

[0003] A METHOD FOR INCREASING EFFICIENCY OF AN ALL-IRON FLOW BATTERY SYSTEM

[0004] FIELD OF THE INVENTION

[0005] The claimed invention relates to the energy industry, in particular, to electrochemical technologies for accumulating and storing energy, and it relates to a method for increasing efficiency of a redox all-iron flow battery and achieving cyclic stability of its operation by applying an iron pre-coating on a negative electrode.

[0006] PRIOR ART

[0007] Currently, one of the most promising technologies for long-term energy storage is the use of redox flow batteries, especially since a demand for using renewable energy sources is increased. A general operation principle of the flow batteries implies use of redox reactions both for accumulating the energy in a form of a chemical potential in the liquid solutions of electrolytes and for converting the accumulated chemical energy back to the electrical energy.

[0008] Structurally, the flow batteries consist of a flow cell that is separated by a separator such as an ion-exchange separator into compartments having positive and negative electrodes arranged therein. The compartments are filled with positive and negative electrolytes, and each of the compartments is connected to a container for storing the corresponding electrolyte, while enabling a closed circulation of the electrolytes between the containers and the compartments of the cell. The formed closed electrochemical system is suitable for grid energy storage due to the icapability of independent scaling of power and / or capacity, as well as for charging and discharging over many cycles with reduced performance losses as compared to other battery technologies.

[0009] In the course of development of this technology, requirements for reliability, extension of service life, cost-effectiveness, and environmental compatibility of redox flow batteries also increase. Currently, the most promising option is to use iron-based flow batteries in view of affordability of materials, their potentially low price, as well as environmentally friendly production and usage.

[0010] Iron chlorides or sulphates solutions, e.g., FeCh, FeCh, FeSO4, Fe2(SO4)3, may be used as redox active compounds of the electrolytes of the all-iron battery, and these solutions are readily available and provide a consistency of the formulation of the electrolyte, since one and the same electrolyte may be used both for the negative electrolyte and for the positive electrolyte, thereby decreasing cross-contamination problems.

[0011] Electrochemical reactions involving iron ions which are dissolved in the corresponding electrolytes take place on the positive and negative electrodes of the all-iron flow battery. The positive electrode operates utilizing the Fe2+ / Fe3+redox couple, whereas the negative electrode operates utilizing the Fe2+ / Fe° redox couple. During system operation, the positive side reaction comprises the oxidation of Fe2+to Fe3+during the charging phase, and the reduction of Fe3+to Fe2+during the discharging phase. Respectively, the negative side reaction comprises the electrodeposition of metallic iron Fe° onto the negative electrode during charging, and the dissolution of metallic iron during discharging. During successive charging and discharging cycles of the all-iron flow battery, defective centers of iron crystallization are formed on the surface of the negative electrode due to nonideality of the dissolution process of the iron layer during battery discharge. When the number of charging and discharging cycles of the battery increases, the defectiveness of the coating with iron crystals will increase, thereby resulting in deterioration of technical characteristics of the battery, in particular, such as voltaicefficiency and cyclic stability. This is caused by an increase in the polarization of the iron deposition process, i.e., a difference between an equilibrium potential of the electrochemical reaction and an actual voltage required for its implementation, and in the case of the all-iron battery, this is an additional voltage required for the transition of Fe2+into metallic iron during charging of the battery. The greatest contribution thereto is made by phase overpotential, which arises due to slowed process of formation of a new iron crystal, because at initial stages of iron deposition it is necessary to overcome an energy barrier of nucleation of a new phase of crystals, while small crystal nuclei have a high surface energy, which prevents their growth. In turn, absence of any uniform distribution of deposition results in formation of an unstable structure of the coating that increases polarization. The prior art discloses an effect of reducing the polarization of iron deposition by using pre-charging of the negative electrode, which allows to create a preliminary thin layer of iron on its surface. Patent US11955678B2, dated April 9, 2024, teaches a method for providing a metallic coating on a negative electrode by performing pre-charging and controlling its degree and conditions, which allows to improve adhesion of iron layers on the negative electrode and improve technical characteristics of a battery.

[0012] According to this solution, in a method for increasing efficiency of a redox all-iron battery system, which consists of an electrochemical cell divided by a membrane into compartments in which positive and negative electrodes are placed, the compartments are filled with negative and positive electrolytes with a possibility of their circulation through the corresponding compartments of the cell and external reservoirs for storing electrolytes, and cyclic charging / discharging operation is initiated, and before initiation of the cyclic operation, a metallic coating is applied to the negative electrode, and to this end, during circulation of the electrolytes, pre-charging of the system is carried out.

[0013] This solution is based on a single application of pre-charging, which results in formation of the iron layer on the negative electrode, and then replacement ofthe electrolytes in the battery system is carried out and the cyclic operation is initiated. Although the method disclosed in this solution allows to improve adhesion of subsequent Fe° layers to the electrode surface during charging and to obtain an excess of Fe2+ions during battery discharge in the first cycles of its operation, it is not sufficient for effectively addressing the stated problem for the following reasons. At a current density of 10-30 mA / cm2, a minimum pre-charging time according to this solution is 8 hours, at which a state of charge (SOC) of up to 6% and a surface coating density of 0.016 g / cm2of Fe° are achieved, while a maximum pre-charging time is 16 hours, at which SOC = 16.7% and the coating density is 0.034 g / cm2of Fe°. In this case, a high value of overall efficiency RTE = 82.0% is achieved in 4-hour cycles during only 17 cycles, but the total capacity is 120 mA*h / cm, which does not fully indicate cyclic stability and high efficiency. In addition, this solution implies replacement of electrolytes, which is caused by the fact that the prolonged pre-charging results in a significant change in a concentration of ions in the electrolytes and affects their further conductivity and effective participation in redox reactions. Furthermore, this solution implies use of electrolytes with almost identical acidity on the negative and positive sides, which is pH = 0.80 and 1.75 for the positive and negative electrolytes, respectively, which is not applicable for large-capacity batteries, since it may result in increased polarization of deposition and / or dissolution of the metallic coating during charging and / or discharging, decrease in a current efficiency and, without replacement of the electrolyte, to a short circuit of the system.

[0014] Thus, a disadvantage of this solution is the excessive duration of the precharging process, the need for replacement of the electrolytes after the precharging cycle, and the achieved efficiency is short-term and does not solve the problem of balancing renewable energy sources.

[0015] SUMMARY OF THE INVENTIONAn objective of the claimed invention is to reduce the deterioration of electrochemical characteristics of an all-iron battery and to increase its voltaic efficiency by providing uniform coating of the negative electrode with a layer of metallic iron over a long time of cyclic operation of the battery. A technical result achieved thereby is a significant increase in the battery efficiency and an increase in a duration of its continuous use.

[0016] The stated objective is achieved in a method for increasing efficiency of a redox all-iron battery system, which consists of an electrochemical cell divided by a membrane into compartments in which a positive electrode and a negative electrode are placed, and comprises the following steps. The compartments are filled with a negative electrolyte and a positive electrolyte, allowing circulation of electrolytes through the corresponding compartments of the cell and external reservoirs for storing the electrolytes, and initiating a cyclic charging / discharging operation. Therewith, prior to the step of initiating the cyclic operation, a metal coating is formed on the negative electrode, and to this end, a pre-charging of the system is carried out during circulation of the electrolytes. According to the invention, the pre-charging is carried out at a current density in a range from 20 to 60 mA / cm2until a state of charge from 0.1 to 10% is achieved, then the cyclic operation is initiated without replacement of the electrolytes starting from charging, and the pre-charging is repeated after completion of the discharging upon increase of a charge voltage and / or decrease of a discharge voltage and / or upon increase of a concentration of Fe2+ions in the negative electrolyte.

[0017] Use of the charging current density in the range from 20 to 60 mA / cm2allows to decrease the pre-charging duration down to 10-45 mins upon achievement of a low state of charge, which, as per conducted tests, is sufficient for formation of small iron crystallization centers throughout the entire volume and area of the negative electrode, which become a basic coating layer for its subsequent uniform application during initiation of the cyclic operation of the system. In addition, the short-term pre-charging process does not result indegradation of a formulation of the electrolytes and allows their use for further cyclic operation starting from charging, which, in turn, allows to decrease a phase overvoltage in the system, according to which, the overall polarization of the processes decreases and it results in stabilization of voltages during subsequent discharging and charging of the system. Repetition of the pre-charging upon change of stabilized voltages, respectively, towards increase during charging and towards decrease during discharging, or upon significant increase of the concentration of Fe2+ions in the negative electrolyte, allows to restore iron crystallization centers throughout the entire volume and area of the negative electrode.

[0018] In a preferable exemplary embodiment of the invention, an initial acidity of the electrolytes is pH = 0-1.0 for the positive electrolyte and pH = 2.0-5.0 for the negative electrolyte, thereby additionally affecting the decrease of the deposition polarization and / or dissolving the metal iron during charging and / or discharging. The selected initial range of acidity of the electrolytes, at which the low initial acidity of the negative electrolyte that is increased during cross-transition of active substances due to use of the high charging current, allows to ensure, during numerous cycles, uniform deposition during charging and avoid quick loss and corrosion of the metal coating upon discharge of the battery system, while maintaining a maximum balance of the concentration of Fe2+ions in both electrolytes.

[0019] This results in a long-term stabilization of values of charge and discharge voltages at a level from 1.19 to 1.25 V and from 0.80 to 1.00 V, and the concentration of Fe2+ions at a level of first-second cycle with a change being within ±0.05 M.

[0020] In another exemplary embodiment of the invention, the pre-charging is repeated upon increase of the charge voltage greater than 1.25-1.30 V and / or decrease of the discharge voltage lower than 0.5-0.95 V and / or upon increase ofthe concentration of Fe2+ions in the negative electrolyte by more than 0.05-0.30 M of the initial concentration.

[0021] The change in voltages or the increase in the concentration of Fe2+ions in the negative electrolyte within said ranges of values, or simultaneous achievement of all these indicators, may indicate degradation / corrosion of previously deposited iron crystallization centers. The repetition of the pre-charging step upon achievement of said indicators allows to timely stabilize the formulation of the electrolytes and restore formation of iron crystallization centers on the negative electrode.

[0022] In a possible exemplary embodiment of the invention, a pause with a duration of up to 24 hours may be applied between pre-charging and initiation of charge / discharge, and it does not affect the state of the system, and allows to coordinate its operating mode depending on usage needs.

[0023] In a possible exemplary embodiment of the invention, the temperature during the cyclic operation or at least during the pre-charging of the system should be from 30 to 60°C, which, in combination with other conditions of the claimed method, may further prevent an increase in polarization of deposition / dissolution of the metallic coating.

[0024] When implementing the claimed method with the use of all essential operating conditions, after use of the pre-charging in the battery system, a value of the average overall efficiency of the all-iron flow battery at a level of or greater than 70% is achieved from the first charge-discharge cycle, which is maintained for at least 50 successive cycles, until a need to perform the next pre-charging arises, which may be applied repeatedly.

[0025] Thus, said sequence of actions and conditions of their performing result in formation of a stable metallic coating of the negative electrode and a stable concentration of the electrolytes, which are indicators of stability of operation of the all-iron flow battery system during multiple repetition of the charge / dischargecycles and, accordingly, its high efficiency when used in power systems for smoothing production peaks and consumption of electric energy.

[0026] DESCRIPTION OF THE FIGURES

[0027] In order to provide a more complete understanding of the invention and advantages thereof, the following description provides an explanation of possible exemplary embodiments on the invention with a reference to the appended figures, wherein identical designations denote identical parts:

[0028] Fig. 1 illustrates a schematic view of the all-iron battery system;

[0029] Fig. 2 (Fig. 2.1 - Fig. 2.3) illustrates an enlarged image of a substrate of the negative electrode without deposited iron, with deposited iron without precharging during 4 hours of operation, and with pre-charging during 4 hours;

[0030] Fig. 3 (Fig. 3.1 - Fig. 3.3) illustrates an enlarged image of a substrate of the negative electrode after 6 hours of charging and discharging, with pre-charging of 45 min, 6-hour charging and discharging, and with pre-charging of 1 hour in the same period;

[0031] Fig. 4 (Fig. 4.1 - Fig. 4.3) illustrates an image of an opposite side of the electrodes after a 6-hour cycle of charging and discharging, with pre-charging of 45 min, 6-hour charging and discharging, and with pre-charging of 1 hour in the same period;

[0032] Fig. 5 illustrates a plot of voltages of cyclic operation of the all-iron flow battery over 25 6-hour cycles, with the use of the claimed method and in a conventional mode;

[0033] Fig. 6 illustrates a plot of values of overall efficiency of the cyclic operation of the all-iron flow battery over 25 6-hour cycles, with the use of the claimed method and in a conventional mode.

[0034] Main designations:1. cell of the all-iron flow battery,

[0035] 2. membrane of the cell of the all-iron flow battery,

[0036] 3. positive compartment,

[0037] 4. negative compartment,

[0038] 5. positive electrode,

[0039] 6. negative electrode,

[0040] 7. positive electrolyte circulation circuit,

[0041] 8. negative electrolyte circulation circuit,

[0042] 9. positive electrolyte reservoir,

[0043] 10. negative electrolyte reservoir,

[0044] 11. positive electrolyte pump,

[0045] 12. negative electrolyte pump,

[0046] 13. positive chamber current lead,

[0047] 14. negative chamber current lead.

[0048] INVENTION IMPLEMENTATION POSSIBILITY

[0049] In order to increase the efficiency of a redox all-iron battery system that is illustrated in Fig. 1 and that consists of an electrochemical cell (1) divided by a membrane (2) into compartments (3, 4) in which a positive electrode and a negative electrode (5, 6) are placed, the compartments are filled with a negative electrolyte and a positive electrolyte, while providing a possibility of their circulation, through closed circuits, of negative electrolyte (7) and positive electrolyte (8) through corresponding compartments (3, 4) of the cell (1) and external reservoirs (9, 10) for storing electrolytes by means of circulation pumps (11, 12), and cyclic charging / discharging operation is initiated, and to this end, the compartments (3, 4) are equipped with current leads (13, 14). An acid-resistant cation-exchange membrane Nafion may be used as the membrane of the system, while a conductive porous material placed between the membrane and the currentlead is used as the positive electrode, and a combination of a conductive material on the side of the current lead and a non-conductive material on the side of the membrane is used as the negative electrode. A carbon felt of commercially available grades CFD 2.5 EA (Sigracet), GFD 2.5 EA, or GFD 4.6 EA may be used as the conductive material for formation of the electrodes, and a polymer felt, which may be made of polyester or polypropylene, may be used as the non-conductive material.

[0050] According to the claimed invention, before initiating the cyclic operation, a metal coating is formed on the negative electrode (5), and to this end, during circulation of electrolytes, pre-charging is carried out by applying a current having a density in a range from 20 to 60 mA / cm2to the current lead (13) until a state of charge from 0.1 to 10% is reached. Therewith, initial acidity of the positive electrolyte before the pre-charging step and initiating the cyclic operation is adjusted to values pH = 0-1.0, and initial acidity of the positive electrolyte is adjusted to values pH = 2.0-5.0 respectively. Iron chlorides or sulfates solutions, such as FeCh, FeCh, FeSC ,

[0051]

[0052] Fe2(SC>4)3, may be used as the electrolytes. After performing pre-charging, cyclic operation is initiated, starting with charging and, during operation of the battery, monitoring of charge / discharge voltages is carried out. In case of increase in the charging voltage above 1.25-1.30 and decrease in the discharging voltage below 0.5-0.95 V, and in case of increase in the concentration of Fe2+ions in the negative electrolyte by more than 0.05-0.30 M from the initial concentration, a technical cycle of pre-charging is again carried out for 10-45 minutes at a current density of 20-60 mA / cm2, followed by initiation of cyclic operation of the battery. According to one of exemplary embodiments, temperature during the cyclic operation or at least during pre-charging of the system must be 30-60°C in order to decrease polarization of formation of the metal coating, and in order to maintain stable temperature of the electrolytes in the battery cell, heaters (not illustrated in the drawings) may be introduced into the system, the heaters may be located in tanks with the electrolytes or connected to a shell of the cell.

[0053] ioThe effect of the claimed method on formation of the metallic coating is demonstrated in SEM images of a surface of the conductive carbon felt of the negative electrode at 100-fold magnification during operation of the battery with a 10 cm2cell.

[0054] Fig. 2 illustrates images of the surface of the electrodes at the initial step of operation of the battery system in the following order:

[0055] Fig. 2.1 - the electrode without the deposited iron;

[0056] Fig. 2.2 - the electrode with the deposited iron during 4 hours of the charging cycle in a conventional mode;

[0057] Fig. 2.3 - the electrode with the deposited iron as a result of pre-charging during 45 min that carried out at the current density of 60 mA / cm2and T=60°C in the 10 cm2cell and 4-hour charging cycle.

[0058] This image demonstrates improvement of the quality of primary iron deposition and formation of iron crystals on the electrode substrate when applying pre-charging as compared to a conventional operating cycle of the system already at the initial operation step of the all-iron flow battery system.

[0059] Fig. 3 illustrates images of the surface of the electrodes at the first cycle of operation of the system in the following order:

[0060] Fig. 3.1 - the electrode after 6-hour charging and discharging cycle without any pre-charging;

[0061] Fig. 3.2 - the electrode with pre-charging according to the claimed invention at the current density of 60 mA / cm2and T=30°C during 30 min and 6-hour charging / discharging cycle;

[0062] Fig. 3.3 - the electrode with pre-charging according to the claimed invention at the current density of 60 mA / cm2, T=60°C during 45 min and 6-hour charging / discharging cycle;

[0063] In this image, a residue of iron crystals on the electrode substrate after discharging, that is, after completion of the cycle, is observed, which indicates that, when applying the claimed method during pre-charging within 30-45 minutes, a iinumber of grains of iron crystals increases, which allows to perform subsequent charging not from the electrode surface but from its coating, thereby improving the structure of the metallic iron deposit and reduces the amount of evolved hydrogen.

[0064] At the same time, the dependence of crystallization on application of the sequence and conditions according to the claimed invention is also valid for the reverse side of the electrode, which is illustrated in Fig. 4 in the following order:

[0065] Fig. 4.1 - after 6-hour charging and discharging cycle without any pre¬ charging;

[0066] Fig. 4.2 - with pre-charging according to the claimed invention at the density during 30 min and 6-hour charging / discharging cycle;

[0067] Fig. 4.3 - with pre-charging according to the claimed invention at the density during 45 min and 6-hour charging / discharging cycle.

[0068] According to the results of tests, the density of the coating at the first charging after application was 0.021 g / cm2of Fe° for SOC of 0.1% and 0.183 g / cm2of Fe° for SOC of 15% that is 5.4 times greater than results of the solution according to the closest analog.

[0069] The above-mentioned information confirms that use of pre-charging allows to eliminate unstable crystal nuclei that could grow chaotically and increase polarization, to form favorable conditions for uniform deposition, since after precharging, iron is deposited on already more stable grains, to reduce the energy barrier of nucleation of a new phase of the metallic coating, which lowers the required voltage for the deposition process, and overall to improve the morphology of the deposit, reducing internal defects and non-uniformities.

[0070] Fig. 5 illustrates plots of voltage changes of the 25th 6-hour cycle, where black color denotes the voltage change after application of the pre-charging before the 1st and 20th cycle, while the dashed line denotes the voltage change without application of the pre-charging.The plots indicate a significant difference in the charging voltage, at the same time, the discharging voltage, upon application of pre-charging, changes more uniformly without a sharp drop.

[0071] Fig. 6 illustrates studies of the change in the value of round trip efficiency (RTE) during 25 6-hour cycles. The dashed line indicates values of the round trip efficiency without application of pre-charging, and the plot of application of the method according to the claimed invention is indicated in black color.

[0072] In the plots it can be seen that during conventional cyclic operation of the battery, a gradual decrease in the value of round trip efficiency occurs, whereas periodic application of pre-charging in the mode of 30 minutes of charging at the beginning of cyclic studies and after 10 and 20 cycles with a duration of 30 min each allows not only to increase the value of the round trip efficiency in the first 20-30 cycles, but also to stabilize it.

[0073] This approach allows to reduce polarization of deposition / dissolution of metallic iron during charging / discharging, to maintain optimal values of charging and discharging voltages and concentration of Fe2+ions at the level of the first-second cycle ±0.05 M.

[0074] Achievement of the technical effect underlying the invention is also confirmed by the following examples of tests of operation of the all-iron flow battery within the ranges of the specified degree of pre-charging and current density, with different formulations of the electrolytes and pH of the electrolytes, and in the sequence that forms the essence of the invention.

[0075] Example 1.

[0076] Testing of an all-iron flow battery system containing 1.65 M of FeCh in the negative and 0.75 M in the positive electrolytes was carried out.

[0077] In order to test this system, positive and negative electrolytes were prepared. pH of the negative electrolyte was adjusted to a value of 3.80-4.55 using 5-10%solution of NH3 H2O. pH of the positive electrolyte was adjusted to a value of 0.1-0.7 using 25-35% solution of HC1.

[0078] Operation of the battery was tested on a 10 cm2cell at a current density of 60 mA / cm2and T = 30-60°C during 50 6-hour cycles. Pre-charging was not performed. The results of efficiency determination are included in the table.

[0079] Example 2.

[0080] Testing of an all-iron flow battery system containing 1.65 M of FeCh in the negative and 0.75 M in the positive electrolytes was carried out.

[0081] In order to test the system, the positive and negative electrolytes were prepared as follows:

[0082] The negative electrolyte - 40 ml (FeCh 1.65 M; NH4CI 2 M, H3BO30.4 M), pH was adjusted to value of 2.0 by means of 5-10% solution of NH3 H2O.

[0083] The positive electrolyte - 80 ml (FeCh 0.6 M; NH4CI 2 M; FeCE 0.15 M), pH was adjusted to value of 0 by means of 25-35% solution of HC1.

[0084] Operation of the battery was tested on a 10 cm2cell at a current density of 20 mA / cm2and T=30-60°C during 506-hour cycles until the state of charge of the pre-charging of 0.1% was reached. The results of efficiency determination are provided in the table.

[0085] Example 3.

[0086] In order to test the all-iron flow battery system, the positive electrolyte and the negative electrolyte were prepared:

[0087] The negative electrolyte - 40 ml (FeCE 1.65 M; NH4CI 2 M, H3BO30.4 M), pH of the negative electrolyte was adjusted to value of 5.0 by means of 5-10% solution of NH3 H2O.

[0088] The positive electrolyte - 80 ml (FeCE 0.6 M; NH4CI 2 M; FeCE 0.15 M), pH of the negative electrolyte was adjusted to value of 1 by means of 25-35% solution of HC1.Operation of the battery was tested on a 10 cm2cell at a current density of 60 mA / cm2and T=30-60°C during 506-hour cycles until the state of charge of the pre-charging of 10% was reached. The results of efficiency determination are provided in the table.

[0089] Example 4.

[0090] In order to test the system, the positive electrolyte and the negative electrolyte were prepared as follows:

[0091] The negative electrolyte - 40 ml (FeCh 1.65 M; NH4CI 2 M, H3BO30.4 M), pH of the negative electrolyte was adjusted to value of 2.5 by means of 5-10% solution ofNHs ^O.

[0092] The positive electrolyte - 80 ml (FeCl20.6 M; NH4C12 M; FeCl30.15 M )). pH of the negative electrolyte was adjusted to value of 0.45 by means of 25-35% solution of HC1.

[0093] Operation of the battery was tested on a 10 cm2cell at a current density of 45 mA / cm2and T=30-60°C during 506-hour cycles until the state of charge of the pre-charging of 5% was reached. The results of efficiency determination are provided in the table.

[0094] Table Results of operation of the all-iron flow battery with different electrolyte formulations and different states of pre-charge, within the ranges of the claimed pH of the electrolytes and operating current density.

[0095] Designation State of preNumber of CE* (%) VE* (%) RTE* (%) of example charge (SoC) cycles

[0096] Example 1 0% 93.08 73.16 68.10 Example 2 0.1 % 93.11 73.63 70.01

[0097] 50

[0098] Example 3 10 % 94.76 76.94 72.91

[0099]

[0100] Example 4 5 % 94.12 74.36 71.99*CE (Coulombic efficiency) - efficiency of current that is calculated according to formula CE=Q_(discharge) / Q_(charge) 100%, wherein Q is an amount of electricity passed during the discharging and charging process (A*h);

[0101] VE (Voltage efficiency): - voltaic efficiency that is calculated according to the formula VE=U_(discharge) / U_(charge) 100%, where U is an average discharge and charge voltage (V);

[0102] RTE (Round trip efficiency) - overall efficiency that is calculated according to the formula RTE =(P_(discharge-) r_(discharge)) / (P_(charge-) r_(charge-) )• 100%, wherein P is power (W), and r is time (h).

[0103] According to the conducted tests, during performance of the experiment during 50 6-hour cycles, a significant improvement in overall efficiency by 2-4% was obtained when using pre-charging even upon reaching the minimum state of charge. The highest efficiency value was obtained when using periodic precharging to the SoC of 10% with initial concentrations of 1.65 M of FeCh in the negative electrolyte and 0.75 M in the positive electrolyte (example 3). When testing different combinations of compositions and volumes of the electrolytes and SoC of 0.1-10%, in all cases an increase in voltaic efficiency was confirmed due to decrease of polarization of deposition and / or dissolution of metallic iron during charging and / or discharging, an increase in the average discharging voltage above 0.80 V with stabilization of the current efficiency within 93-95%, and the round trip efficiency value greater than 70% was obtained. Indicators of testing in long 6- hour cycles indicate a high specific capacity of 360 mA-h / cm2of the battery system, which is 2.5 times greater than in the solution according to the closest analog, and indicate stability of concentrations of the electrolytes, which is also confirmed by periodic control of concentration of Fe2+ions and indicates its maintenance at the level of the first-second cycle ±0.05 M.

[0104] Ultimately, efficiency and stability of concentrations of the electrolytes are indicators of cyclic stability of operation of the all-iron flow battery system forstorage of electrical energy and confirm that the claimed invention can be used from the first charge-discharge cycle, which is maintained for at least 50 consecutive 6-hour cycles and applied at least ten times, that is, the total continuous operation of the system may amount to up to 500 cycles, while preserving high operating efficiency.

[0105] Thus, the method according to the claimed invention, in terms of the combination of essential features, results in formation of iron crystallization centers throughout the volume and area of the negative electrode, promotes reduction of polarization of deposition / dissolution of metallic iron during charging / discharging of the battery, whereby the battery operates with lower energy losses, and iron deposition occurs in a more controlled manner, which results in increase of the efficiency of the redox all-iron battery system and duration of its continuous use.

Claims

CLAIMS1. A method for increasing efficiency of a redox all -iron flow battery system that consists of an electrochemical cell divided by a membrane into compartments in which positive and negative electrodes are placed, the compartments are filled with a negative electrolyte and a positive electrolyte with a possibility of their circulation through corresponding compartments of the cell and external reservoirs for storing the electrolytes, and a cyclic charging / discharging operation is initiated, and, prior to the step of initiating the cyclic operation, a metal coating is formed on the negative electrode, and to this end, a pre-charging of the system is carried out during circulation of the electrolytes, wherein the pre-charging is carried out at a current density in a range from 20 to 60 mA / cm2until a state of charge from 0.1 to 10% is achieved, then the cyclic operation is initiated without replacement of the electrolytes starting from charging, and the pre-charging is repeated after completion of the discharging upon increase of a charge voltage and / or decrease of a discharge voltage and / or upon increase of a concentration of Fe2+ions in the negative electrolyte.

2. The method according to claim 1, wherein an initial acidity of the electrolytes is pH = 0-1.0 for the positive electrolyte and pH = 2.0-5.0 for the negative electrolyte.

3. The method according to claim 1 or claim 2, wherein the pre-charging is repeated upon increase of the charge voltage greater than 1.25-1.30 V and / or decrease of the discharge voltage lower than 0.5-0.95 V and / or upon increase of the concentration of Fe2+ions in the negative electrolyte by more than 0.05-0.30 M of the initial concentration.

4. The method according to claims 1-3, wherein at least the pre-charging is carried out while maintaining a temperature of 30-60°C in the cell.

5. The method according to claims 1-4, wherein a pause with a duration of up to 24 hours is applied between pre-charging, initiation of the cyclic operation and / or further pre-charging.

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