A method for re-lithiation of spent lithium iron phosphate

Abstract

A method for re-lithiation of spent lithium iron phosphate is provided The method allows recovery of lithium iron phosphate (LiFePO4) from spent lithium iron phosphate (FePO4) by an electrochemical process. The method is carried out in an electrochemical flow system where a redox mediator and spent lithium iron phosphate are placed in a catholytic tank, and a lithium ion solution is placed in the anolytic tank. When the flow system is electrically charged, lithium ions generated in the anolytic tank migrate to the catholytic tank and react with spent lithium iron phosphate (FePO4) to recover or regenerate lithium iron phosphate (LiFePO4). A system for carrying out the re-lithiation or recovery process is also provided.

Description

A METHOD FOR RE-LITHIATION OF SPENT LITHIUM IRON PHOSPHATEFIELD OF THE INVENTION

[0001] The present invention relates to regeneration of spent lithium iron phosphate in a flow battery rejuvenation process. More specifically, the invention describes a method of recovering lithium iron phosphate (LiFePO4) from spent iron phosphate (FePO4).BACKGROUND OF THE INVENTION

[0002] A flow battery, or redox flow battery (after reduction-oxidation), is a type of electrochemical cell where chemical energy is provided by two chemical components dissolved in liquids that are pumped through the system on separate sides and in opposite direction of a membrane. Ion transfer inside the cell (accompanied by flow of electric current through an external circuit) occurs through the membrane while both liquids circulate in their own respective space. It’s a rechargeable fuel cell in which an electrolyte containing one or more dissolved electroactive elements flows through an electrochemical cell that reversibly converts chemical energy to electricity. Similarly, a lithium-ion flow battery stores energy separately from its system for discharging. The amount of energy it can store is determined by its tank size and the size of its reaction chamber determines its power density.

[0003] The technical merits make redox flow batteries a well-suited option for large-scale energy storage. Flow batteries are normally considered for relatively large (1 kWh - 10 MWh) stationary applications with multi-hour charge -discharge cycles. Soe applications include grid energy storage, load balancing during peak and off-peak hours, power conversion, and electric vehicles.

[0004] With the remarkable development of the flow battery industry, the number of spent batteries has dramatically increased. To reduce environmental pollution and resource depletion, several technologies for recycling and regenerating spent materials have been developed, especially for valuable metals, such as lithium.

[0005] Current methodology, as described in Zhang, L., Xu, Z. & He, Z. Electrochemical Relithiation for Direct Regeneration of LiCoO2 Materials from Spent Lithium-Ion Battery Electrodes. ACS Sustain. Chem. Eng. 8, 11596-11605 (2020), or Zhou, S. et al. Direct recovery of scrapped LiFePO4 by a green and low-cost electrochemical re-lithiation method. Green Chem. 24, 6278-6286 (2022), utilizes a directelectrochemical method to regenerate spent lithium. The materials are coated directly onto the electrode. As such, the rate of recovery and batch size is limited by the electrochemical reaction on the electrode surface.BRIEF DESCRIPTION OF DRAWINGS

[0006] The above and other aspects, features and advantages of the invention will become more readily apparent from the following description, reference being made to the accompanying drawings in which:

[0007] Figure 1 shows a schematic representation of the recovery of spent LiFePO4 through redox- mediated reaction to recover LiFePO4 from Fcl’CU

[0008] Figure 2(a) shows cyclic voltammograms of AQDS at various pH and that of FePO4 powder in IM lithium acetate solution (pH 7).

[0009] Figure 2(b) shows voltage profile of the recovery process by AQDS in an electrolytic flow cell paired with OER reaction in the counter electrode compartment at a constant current density of 10 mA cm .

[0010] Figure 3(a) shows FTIR spectra of FP powder before and after the recovery process (regenerated LFP) and its comparison with the standard LFP.

[0011] Figure 3(b) shows XRD patterns of FP powder before and after the recovery process (regenerated LFP) and its comparison with the standard LFP.

[0012] Figure 4 shows SEM images of (a) standard LFP (b) FP powder before (c) FP powder after the recovery process.

[0013] Figure 5 shows coin cell cycling performance of the regenerated LFP.SUMMARY OF THE INVENTION

[0014] The present invention discloses a novel process based on a flow battery electrochemical system to rejuvenate spent lithium iron phosphate (LFP). A close-loop redox-mediated re-lithiation process is described to realize the recovery of the electrode materials.

[0015] The invention describes a method of recovering / regenerating lithium iron phosphate (LiFePO4) from spent lithium iron phosphate (FePO4) in an electrochemical flow system. The method involves a step of electrically charging the electrochemical flow system for a predetermined period to generate lithium (Li+) ions from a lithium source placed in an anolytic tank of the electrochemical flow system, to allow reduction of spent lithium iron phosphate (FePO4) by a redox mediator in a catholytic tank of the electrochemical flow system and to allow the lithium (Li+) ions to react with the spent lithium iron phosphate (FePO4) in the presence of the redox mediator to regenerate lithium iron phosphate (LiFePO4).

[0016] The invention also describes a method of recovering / regenerating lithium iron phosphate (LiFePO4) from spent lithium iron phosphate (FePO4) in an electrochemical flow system or a method of re-lithiation of spent lithium iron phosphate in a lithium ion flow system. The method involves prefilling an anolytic tank of the electrochemical flow system with an anolyte, preferably a lithium-ion solution. In some cases, the anolyte is lithium hydroxide solution (LiOH). In some cases, the anolyte is a mixture of a lithium ion solution and one or more other anolytes. The method further involves prefilling a catholytic tank of the electrochemical flow system with spent lithium iron phosphate, optionally in a solid form, and a catholyte. The catholyte is a redox mediator, preferably a redox mediator with a lower redox potential than the spent lithium iron phosphate (FePO4). In some case, the catholyte is a mixture of a redox mediator and other known catholytes. The method further involves providing an electrical charge to the electrochemical flow system or the lithium ion flow system for a predetermined period, which causes migration of lithium ions (Li+) from the anolytic tank to the catholytic tank, where lithium ions (Li+) are generated by disintegrating the anolyte when the flow system is electrically charged, preferably, by disintegrating the lithium-ion solution, reduction of the redox mediator. When the flow system is charged, it also causes release of spent lithium iron phosphate (FePO4). The release may be caused when the solid mass of spent LFP reacts with the reduced redox mediator. Once spent LPF is releases, it reacts with lithium ions (Li+) to generate lithium iron phosphate (LiFePO4).

[0017] The invention also teaches an electrochemical flow system for recovering / regenerating lithium iron phosphate (LiFePO4) from spent lithium iron phosphate (FePO4). The system has a cathode, an anode, and a permeable membrane between the cathode and the anode. It further comprises an anolytic tank filled with a lithium-ion source, preferably, a lithium-ion solution such as lithium hydroxide (LiOH) solution. Additionally, a catholytic tank filled with spent lithium iron phosphate (FePO4), optionally in the form of a solid mass, and a redox mediator solution. In some cases, the redox mediator has a lowerredox potential than the spent lithium iron phosphate (FePO4). The system also has a means for providing electrical charge to the electrochemical flow system.

[0018] The electrochemical system or the flow system is charged or connected to an electrical source for a predetermined period, which can range from one to five horns for the chemical reactions to occur.

[0019] The redox mediator generally has a different or lower redox potential than the spent lithium iron phosphate (FePCE). For instance, anthraquinone-2,7-disulfonate (AQDS), 1,8-dihydroxyanthraquinone- 2,7-disulfonic acid (1,8-DHAQDS), ferrocene derivatives, or viologen derivatives could be used. The electrode materials may be made of any standard electrode materials, e.g. carbon felt, carbon paper, carbon cloth or graphite foil. The permeable membrane may be made of any standard membrane material, e.g. NAFION, SPEEK or PBI.DETAILED DESCRIPTION

[0020] The following description is of preferred embodiments by way of example only and without limitation to the combination of features necessary for carrying the invention into effect.

[0021] All terms are intended to be understood as they would be understood by a person skilled in the art. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure pertains. The section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0022] Although various features of the present disclosure can be described in the context of a single embodiment, the features can also be provided separately or in any suitable combination. Conversely, although the present disclosure can be described herein in the context of separate embodiments for clarity, the present disclosure can also be implemented in a single embodiment.

[0023] The following definitions supplement those in the art and are directed to the current application. Accordingly, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

[0024] Tn this application, the use of the singular includes the plural unless specifically stated otherwise. It must be noted that, as used in the specification, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise.

[0025] In this application, the use of "or" means "and / or" unless stated otherwise. The terms "and / or" and "any combination thereof and their grammatical equivalents as used herein, can be used interchangeably. These terms can convey that any and all combinations are specifically contemplated. The term "or" can be used conjunctively or disjunctively, unless the context specifically refers to a disjunctive use.

[0026] Furthermore, use of the term "including" as well as other forms, such as "include", "includes," and "included," is not limiting.

[0027] Reference in the specification to "some embodiments," "an embodiment," "one embodiment" “alternate embodiment”, or "other embodiments" means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the present disclosures.

[0028] As used in this specification and claim(s), the words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes" and "include") or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps. It is contemplated that any embodiment discussed in this specification can be implemented with respect to any method or composition of the present disclosure, and vice versa. Furthermore, compositions of the present disclosure can be used to achieve methods of the present disclosure.

[0029] The term "about" in relation to a reference numerical value and its grammatical equivalents as used herein can include the numerical value itself and a range of values plus or minus 10% from that numerical value. The term "about" or "approximately" means within an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, i.e., the limitations of the measurement system. For example, "about" can mean within 1 or more than 1 standard deviation, per the practice in the art. Alternatively, "about"can mean a range of up to 20%, up to 10%, up to 5%, or up to 1 % of a given value. In another example, the amount "about 10" includes 10 and any amounts from 9 to 11.

[0030] Embodiments

[0031] A lithium-ion electrochemical flow battery comprises lithium iron phosphate (LiFePO4 or LFP) along with other components. LFP gets spent over time which reduces the shelf life of the battery. When the battery is charged or used, spent lithium iron phosphate ( FePO4 or spent LFP) gets absorbed on the surface of the cathode which can be collected as a solid mass. Accordingly, in an embodiment of the invention, there’s provided a method of recovering / regenerating LFP from spent LFP, preferably from the solid mass of spent LFP. This method descried herein is merely an exemplary embodiment, and a person skilled in the art would understand that many variations of the method are encompassed within the scope of the disclosure.

[0032] In an exemplary embodiment, a method of recovering LFP from spent LFP in an electrochemical flow cell system is provided. The electrochemical flow system may be a lithium-ion flow battery, for instance a lithium iron phosphate flow battery. The method involves placing a lithium source, preferably a lithium-ion solution, more preferably, a lithium hydroxide (LiOH) solution in an anolytic tank of the electrochemical system. In some embodiments, Lithium hydroxide (LiOH) is used either alone, or in combination with other chemical solutions as an anolyte.

[0033] A redox mediator is placed in the catholytic tank of the electrochemical system as the catholyte. In certain embodiments, the redox mediator is placed along with the spent LFP in the catholytic tank. Spent LFP may be placed as a solid mass in the catholytic tank. Alternatively, cathodes containing spent LFP on its surface may be used as electrodes of the electrochemical system.

[0034] When the electrochemical flow system or flow battery is charged by electrical means, it initiates various chemical reactions in the reaction chambers (in the catholytic / anolytic tanks of the flow system). For instance, in an embodiment of the invention, lithium ions (Li+) are generated by disintegrating the lithium-ion source (the lithium-ion solution or LiOH solution) in the anolytic tank, and the spent LFP is reduced by the redox mediator. Upon reduction the spent LFP (FePO4) enters the catholytic tank of the electrochemical flow cell system and reacts with lithium (Li+) ions to regenerate lithium iron phosphate (LiFePO4) in the presence of the redox mediator. Optionally, the redox mediator has a different redoxpotential compared to the spent LFP (FePO4). In an embodiment or the invention, the redox potential of the redox mediator is lower than the redox potential of spent LFP.

[0035] In an alternate embodiment, where spent LFP (FePO4) is placed in the catholytic tank along with the redox mediator, optionally in the form of a solid mass, when the electrochemical flow system or flow battery is charged by electrical means, it initiates various chemical reactions in the reaction chambers (in the catholytic / anolytic tanks of the flow system). For instance, in an embodiment of the invention, lithium ions (Li+) are generated from the lithium ion source (a lithium ion solution or LiOH solution) in the anolytic tank. Optionally, oxygen is generated in the anolytic tank by an oxygen evolution reaction. In some embodiments, the oxygen evolution reaction provides a continuous feed or supply of oxygen. Furthermore, when the flow system is charged, the redox mediator gets reduced on the surface of the cathode. After reduction, the redox mediator re-enters the catholytic tank to react with the spent LFP (FePO4). After reacting with the redox mediator, spent LFP gets liberated / releases into the catholytic tank. If spent LFP is a solid mass, upon reacting with the reduced redox mediator, spent LFP ions are liberated from the solid mass and enter the catholytic tank. Spent LFP reacts with the migrated Li+ions, to generate lithium iron phosphate (LiFePO4). The redox mediator reduces the spent LFP (FePO4) which liberates it from the solid mass, thereby making it available to react with the migrated Li+ions. In some embodiments, spent LFP reacts with lithium (Li+) ions to regenerate lithium iron phosphate in the presence of the redox mediator. Optionally, the redox mediator used has a different redox potential compared to the spent LFP. In an embodiment or the invention, the redox potential of the redox mediator is lower than the redox potential of spent LFP (FePO4).

[0036] In another embodiment of the invention, the method of regenerating or recovering LFP from spent LFP is provided. An electrochemical system with depleted LFP that needs recharging is used. The method comprises pre-filling the reaction chambers i.e. the catholytic and anolytic tanks of an electrochemical flow system or an electrochemical flow battery with reactive solutions. For instance, the anolytic tank may be filled with a lithium ion source, preferably a lithium hydroxide solution or any solution capable of releasing lithium ions by disintegration. The catholytic tank may be filled with a redox mediator along with other electrolytes if necessary. In some embodiments, the catholytic tank is pre-filled with spent LFP, optionally in the form of a solid mass, along with the redox mediator. When the flow battery or electrochemical system is charged or electrified, by means of chemical reactions, the spent LFP / depleted LFP (FePO4) that is either absorbed on the cathode or comprised within the solid mass, gets liberated and enters the catholytic tank. On the anolytic side, lithium ions (Li+) are generatedfrom LiOH which migrate from the anolytic reaction chamber to the catholytic reaction chamber. The migrated lithium ions (Li+) react with the depleted LFP (FePO4) to regenerate lithium iron phosphate (LiFePO4). Optionally, the redox mediator has a lower redox potential than the spent LFP to allow the chemical reactions to occur. Optionally, oxygen is generated in the anolytic tank, in some cases, a continuous feed or supply of oxygen, by oxygen evolution reaction.

[0037] In another embodiment, a method of recharging a lithium-ion electrochemical flow battery or flow system is provided. The method comprises placing a redox mediator with a lower redox potential than spent lithium-iron phosphate in the catholytic chamber. The redox mediator may be placed with one or more electrolytes. A lithium-ion electrolyte such as lithium hydroxide is placed in the anolytic chamber of the flow battery. Electrical charge is then provided to the flow battery to initiate chemical reactions in the reaction chambers. On the anolytic side, the lithium hydroxide solution is disintegrated to generate lithium ions, thereby providing a continuous feed / supply of lithium ions. Optionally, by oxygen evolution reaction a continuous feed / supply of oxygen is provided. Lithium ions migrate through the permeable membrane between the anode / cathode and enter the catholytic chamber. On the catholytic side, when the battery is electrified, the redox mediator is reduced on the surface of the cathode. If electrodes with spent LFP are used, the redox mediator reacts with the spent LFP when it gets reduced on the cathode. Upon reaction with the reduced redox mediator, spent LFP gets liberated either from the solid mass or from the cathode and enters the catholytic chamber. In the presence of redox mediator, lithium ions react with the spent LFP, thereby recovering LFP from spent LFP.

[0038] In another embodiment, a method of re-lithiation of spent lithium iron phosphate (FePO4) in a lithium ion flow system is provided. The method involves prefilling an anolytic tank of the flow system with an anolyte, preferably a lithium-ion solution, more preferably a lithium hydroxide (LiOH) solution, prefilling a catholytic tank of the flow system with a catholyte, preferably a redox mediator, more preferably a redox mediator with a lower redox potential than the spent lithium iron phosphate (FePO4), along with spent lithium iron phosphate, optionally in the form of a solid mass. The method further involves providing an electrical charge to the electrochemical flow system for a predetermined period. This allows chemical reactions to occur in the reaction chambers i.e. in the catholytic and anolytic tanks / chambers of the flow system. Specifically, the flow system when charged causes disintegration of the anolyte, preferably lithium hydroxide. Optionally, an oxygen evolution reaction also occurs to generate oxygen in the anolytic chamber. The lithium ions then migrate from the anolytic tank to the catholytic tank. On the catholytic side, the redox mediator is reduced on the cathode of the flow systemwhen charged. The reduced redox mediator then reacts with the spent LFP that’s either on the surface of the cathode or may be placed as a solid mass in the catholytic tank. This reaction between the redox mediator and the spent LFP, preferably a reduction reaction, causes spent LFP to liberate and enter the catholytic tank. When spent LFP enters the catholytic tank it reacts with lithium ions in the presence of the redox mediator to generate lithium-ion phosphate. In some embodiments, spent LFP releases into the catholytic tank as charged FePO4 which readily reacts with the charged Li+ions.[0039J In any of the above embodiments, the redox mediator generally causes reduction of the spent LFP that’s either absorbed on the cathode or placed as a solid mass. The reduction of spent LFP by the redox mediator causes it’s release into the catholytic tank. The reduction reaction causes spent LFP to detach / release from the cathode / solid mass and enter the catholytic tank. Once in the tank, the redox mediator may promote or facilitate reaction of the migrated lithium ions with the spent LFP to generate highly pure LFP.

[0040] In any of the above proposed embodiments, the predetermined period may be more than one hour, or between one to five hours, i.e. between 60 minutes to 300 minutes. Accordingly, the chemical reactions occur for a period ranging from 1-5 hours i.e. the reaction time ranges from 60-miutes to 300 minutes.

[0041] In any of the above proposed embodiments, the redox mediator is chosen such that it has a lower redox potential than the spent lithium iron phosphate (FePO4). For instance, anthraquinone-2,7- disulfonate (AQDS), l,8-dihydroxyanthraquinone-2,7-disulfonic acid (1,8-DHAQDS), ferrocene derivatives, or viologen derivatives. The redox mediator is reusable, and therefore, can be reused or repeated for next recharge / regeneration cycle. In any of the above embodiments, the electrode material may be made of any known electrode material in the art that could be employed for electrochemical systems. In an exemplary embodiment, the electrode material may be carbon felt, carbon paper, carbon cloth or graphite foil. In any of the above embodiments, the permeable membrane may be made of any known membrane material in the art that could be employed for electrochemical systems. In an exemplary embodiment, the permeable membrane may be NAFION, SPEEK or PBI.

[0042] When the flow battery / electrochemical flow system is charged, lithium hydroxide (LiOH) on the anolytic side undergoes disintegration, thereby generating Lithium (Li+). Optionally, via an oxygen evolution reaction, a continuous feed of water and oxygen is generated in the anolytic chamber. The charged lithium ions generated on the anolytic chamber / tank, migrate towards the catholytic chamber / tank by passing through the permeable membrane that separates the two reaction chambers / tanks.

[0043] When the flow battery / electrochemical flow system is charged, the redox mediator undergoes a reduction reaction by means of which it gets reduced on the electrode i.e. cathode of the flow battery. Optionally, the cathode has spent LFP absorbed on its surface, the reduced redox mediator reacts with the spent LFP causing it to release from the cathode. Alternatively, spent LFP may be placed in the catholytic tank, optionally as a solid mass, and when it reacts with the redox mediator, spent LFP is liberated from the solid mass into the catholytic tank. The liberated spent LFP enters the catholytic tank. In some embodiments, spent LFP is reduced by the redox mediator which makes it available to react with other ions in the catholytic tank. Once liberated, the spent LFP enters the catholytic tank of the flow battery. The redox mediator then promotes a reaction between spent LFP and the charged migrated lithium ions to regenerate highly pure lithium iron phosphate.

[0044] The regenerated lithium iron phosphate (LiFePO4) exhibits the same properties as standard lithium iron phosphate (LiFePO4), and therefore, can be used as an electrolyte for flow batteries, or in coin cells or any other applications known in the art. The regenerated LFP can be used directly without any further processing steps such as purification or re-synthesis.

[0045] In another embodiment an electrochemical flow system for recovering / regenerating lithium iron phosphate (LiFePO4) is envisioned. The electrochemical flow system may comprise a cathode, an anode and a permeable membrane. The permeable membrane may be an ion selective membrane and is operationally connected to the anode and the cathode and generally placed between the two electrodes. In case of an electrochemical system that needs recharge or regeneration of spent electrolyte material, the cathode may already have spent LFP on its surface. In case of a custom designed electrochemical system for recovery of spent LFP, the system may be fitted with a cathode derived from other depleted electrochemical systems. Alternatively, spent LFP may be placed as a solid mass in the catholytic chamber / tank. The electrode material may be made of any known electrode material in the art that could be employed for electrochemical systems. Tn an exemplary embodiment, the electrode material may be carbon felt, carbon paper, carbon cloth or graphite foil. The permeable membrane may be made of any known membrane material in the art that could be employed for electrochemical systems. In an exemplary embodiment, the permeable membrane may be NAFION, SPEEK or PBI.

[0046] The flow system may additionally comprise a catholytic tank or chamber and an anolytic tank or chamber, also referred to as reaction chambers. The catholytic tank is filled with a redox mediator that has a different redox potential, preferably a lower redox potential than the spent LFP. The catholyticchamber may have additional catholytes along with the redox mediator if needed. In some embodiments, spent LFP is placed in the catholytic chamber as a solid mass. The anolytic chamber or tank are filed with a lithium-ion source / solution that can undergo a disintegration reaction to generate lithium ions, preferably a lithium hydroxide solution. Lithium ion solution may be mixed with other anolytes if needed. The flow system may additionally comprise a means for providing electrical charge to the flow system, i.e. a means for electrifying or charging the flow battery / system. The flow system may additionally comprise pumps to pump the electrolytes between the reaction chambers.

[0047] Technical Discussion

[0048] A flow battery or an electrochemical flow system is formed by two liquids with opposite charge (electrolytes) that can convert chemical energy into electricity. This is achieved by exchanging ions through a membrane which is generally a permeable membrane and can be an ion-selective membrane. The electrolytes are generally stored in two external tanks, also called reaction chambers, wherein the tank on the anodic side is referred to as an anolytic tank, and the tank on the cathode side is referred to as the catholytic tank.

[0049] The flow battery or a redox flow battery technology has emerged to be a promising alternative for flexible, long life and safe energy storage system. Unlike static batteries, flow batteries allow spatial separation of the reaction chamber and the storage chamber (i.e., catholyte / anolyte tanks). The power of a flow battery is determined by the surface area of the electrodes. Tire stability of the catholyte, anolyte, electrodes, and membrane determine the cycle life of the flow battery. The concentration and redox potentials of the catholyte and the anolyte determine the capacity and voltage of the flow battery. Moreover, the flow battery’s capacity is determined by the concentration of electrolytes and the sizing / volume of storage tanks. This separation ensures that the battery power and the battery capacity are independent of each other. Moreover, the flow battery provides room for electrochemical redox reactions but does not trigger their initiation until electrically charged.

[0050] The proposed invention describes a novel and efficient rejuvenation process for lithium-ion flow batteries to recover the spent lithium iron phosphate (spent LFP or FePO4) by electrical means. The electrified method allows recovery of lithium iron phosphate (LFP or LiFePO4) materials that may be compositionally compromised without consuming additional reducing agent.

[0051] The schematic in Figure 1 illustrates the recovery process of spent lithium battery material (spent LFP) i.e. the recovery of I .iFePO4 from FePO4 through redox-mediated reaction. Throughout the flow cell-based electrochemical system (flow battery), a redox mediator (RM) with a lower redox potential than Iron Phosphate (i.e. spent LFP or FePO4) is present on one side (catholytic side) and Lithium Hydroxide (LiOH) solution on the other side (anolytic side). When the electrolyzer / flow battery is charged, the RM is reduced on the cathode. This is supported by the oxygen evolution reaction (OER) occurring at the anode side.

[0052] Meanwhile, Li+ions migrate from the anodic compartment to the cathodic compartment through the permeable membrane. The reduced RM flows into the tank and reacts with the spent LFP (FePO4) that’s pre-loaded in the catholytic tank. As a result, the RM reduces FePO4(FP) presented inside the spent LFP black mass loaded inside the tank and Li+ions are inserted into FP. The spent LFP mass is separately loaded in the tank. In some embodiments, the spent LFP mass is loaded in the tank at intermittent intervals.

[0053] Accordingly, re-lithiation of spent LFP or regeneration of spent LFP is realized as shown below:Anodic reaction: 4Cathodic reactionRecovery of spentOverall reaction:

[0054] As shown above, the recovery of used LFP is recovered, the reduction of RM in the presence of Li+and the production of oxygen by OER can be accomplished continuously by simply applying voltage to the electrodes. Apart from consuming LiOH, the overall recovery process is highly efficient and cost- effective.

[0055] The recovered LFP could be used for any known applications of LFP in the art. For instance, it can be used as Li-ion battery cathode material or in coin cell batteries. Most importantly, the recovered LFP can be used with / without post-treatment. It does not require any post-treatment steps such as purification or re-synthesis.

[0056] Any stable redox mediator with a potential lower than the redox potential of lithium iron phosphate can be used. For recovery of LiFePO4 from FePOr. in an exemplary embodiment, anthraquinone-2,7-disulfonate (AQDS) was used as the RM, merely for experimental demonstration and analysis of results. A person skilled in the art would understand that any know redox mediator with a lower redox potential than FcPOr can be used.

[0057] In order to test the capability of AQDS detailed experimental results were carried out by the inventors. The redox potential of AQDS was measured in different concentrations of acetic acid (pH from 2 to 6.7) by cyclic voltammetry (CV) method. As shown in Figure 2a, the redox potential of AQDS was found dependent on pH, which is lower than that of I .iFePO4 / FePO4 (0.39 V vs. SHE). This makes it suitable to be used as an RM for the recovery of FePO4 Cyclic voltammograms of AQDS at various pH and that of FePC>4 powder in 1 M lithium acetate solution (pH 7) is shown in Figure 2(a). Voltage profile of the recovery process by AQDS in an electrolytic flow cell paired with OER reaction in the counter electrode compartment at a constant current density of 10 mA cm'2is shown in Figure 2(b).

[0058] In an exemplary embodiment, the re-lithiation of spent LFP was carried out in a reactor tank filled with 20 mL of 50 mM AQDS in 1 M lithium acetate solution. Spent LFP with lithium deficiency was loaded into the catholytic tank. The reaction time was around 200 minutes. During the reaction, the electrolytic cell (flow battery, flow cell or electrochemical cell) was charged with constant current, and the voltage profile and the corresponding capacity were recorded in Figure 2b. A very rapid rise in voltage at the end of the electrolytic process indicated that all the redox mediators were fully charged, i.e., re-lithiation of all FP was completed. The average voltage plateau was around 1.15 V at a current density of 10 mA cm-2.

[0059] In order to verify the composition of the regenerated material after the restoration process, Fourier Infrared Spectroscopy (FT1R) spectroscopy and X-ray diffraction (XRD) were used to characterize the materials. From the FTIR spectra and XRD patterns shown in Figure 3, it was noticed that the regenerated LiFePO4 exhibited in addition to the same characteristics as the standard LFP, a significant difference from the FePO4 that did not undergo the restoration process before the reaction.

[0060] FTIR spectra of FP powder before and after the recovery process (regenerated LFP) and its comparison with the standard LFP are shown in Figure 3(a) and XRD patterns of FP powder before and after the recovery process (regenerated LFP) and its comparison with the standard LFP in Figure 3(a). Scanning electron microscopy (SEM) was also used. SEM images of three different samples (a) standardLFP (b) FP powder before (c) FP powder after the recovery process is shown in Figure 4. It's evident from the images that there is no observable change in morphology.

[0061] The spent battery material LFP with lithium deficiency (FP) after the recovery process, i.e., the regenerated LFP, was prepared into electrodes and tested with coin cells. The battery cycling results were obtained and are shown in Figure 5, which indicates that the regenerated LFP has considerable electrochemical performance and could be put into use again.

[0062] In this experiment, the total mass loading of the regenerated LFP electrode was 12.5 mg, while excess amount of lithium metal was used as the anode and 1 M lithium hexafluorophosphate (LiPBp dissolved in EC:DMC (1 :1 ) was used as the electrolyte. Galvanostatic charging and discharging were performed at 1 mA. As can be seen from the cycling results, the regenerated LFP exhibited good specific capacity and capacity retention, which indicated that the recovery method, and the recovery system disclosed in this invention can well recover spent LiFePO4, directly regenerate it, with which it can be directly put into use for making new batteries. Coin cell cycling performance of the regenerated LFP is shown in Figure 5.

[0063] It is pertinent to note that the proposed method and system are more energy efficient than other methods of recycling spent LiFePO4 cathode and does not consume additional chemicals other than a lithium source. Most importantly, the recovery steps are simple and easy to perform, and the recovered product (recovered LFP) can be used directly as a new cathode material rather than requiring additional steps such as purification or re-synthesis.

[0064] In comparison to the existing technology, the method disclosed in the invention disclosure, utilizes a redox mediator to liberate LFP from the electrode surface into a reactor tank. As such, we could benefit from an electrochemical-chemical process to recover LFP at a higher rate and efficiency, and more importantly a better scalability. Another advantage of the proposed invention compared with other direct recycling methods, is that it discloses an electrified method for the recovery of spent LiFePO4 materials (compositionally compromised) without consuming additional reducing agent.

[0065] While preferred embodiments of the present invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. It is not intended that the invention be limited by the specific examples provided within the specification. While the invention has been described with reference to the aforementioned specification, thedescriptions and illustrations of the embodiments herein are not meant to be construed in a limiting sense. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. Furthermore, it shall be understood that all aspects of the invention are not limited to the specific depictions, configurations or relative proportions set forth herein which depend upon a variety of conditions and variables. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is therefore contemplated that the invention shall also cover any such alternatives, modifications, variations or equivalents. It is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.

Claims

Claims1. A method of recovering / regenerating lithium iron phosphate (LiFePO4) from spent lithium iron phosphate (FePO4) in an electrochemical flow system comprising: electrically charging the electrochemical flow system for a predetermined period: to generate lithium (Li+) ions from a lithium source placed in an anolytic tank of the electrochemical flow system; to allow reduction of spent lithium iron phosphate (FePO4) by a redox mediator in a catholytic tank of the electrochemical flow system; to allow the lithium (Li+) ions to react with the spent lithium iron phosphate (FePO4) in the presence of the redox mediator to regenerate lithium iron phosphate (LiFePO4).

2. The method of claim 1, wherein the method involves placing the spent lithium iron phosphate (FePO4), optionally in the form of a solid mass, along with the redox mediator in the catholytic tank.

3. The method of claim 1 or 2, wherein the redox mediator has a lower redox potential than the spent lithium iron phosphate (FePO4).

4. The method of claim 1-3, wherein lithium (Li+) ions are generated in the anolytic tank by disintegrating the lithium source when the flow system is electrically charged.

5. The method of any one of claims 1-4, wherein the lithium source is lithium hydroxide (LiOH) is disintegrated by an oxygen evolution reaction to generate lithium (Li+) ions and oxygen.

6. The method of any one pf claim 1 -5, wherein the method additionally involves an oxygen evolution reaction in the anolytic tank to generate oxygen, optionally, providing a continuous flow of oxygen.

7. The method of any one of claims 1-6, wherein lithium (Li+) ions migrate from the anolytic tank to the catholytic tank to react with the spent lithium iron phosphate (FePO4).

8. The method of any one of claims 1-7, wherein the redox mediator gets reduced on the cathode when the flow system is electrically charged.

9. The method of any one of claims 1-8, wherein the reduced redox mediator enters the catholytic tank and reacts with the spent lithium iron phosphate (FePO4), optionally, the solid mass of the spent lithium iron phosphate (FePO4), placed in the catholytic tank of the flow system.

10. The method of any one of claims 1-9, wherein the spent lithium iron phosphate (FePO4) is liberated, optionally from the solid mass, into the catholytic tank after reacting with the reduced redox mediator.

11. The method of any one of claims 1-10, wherein spent lithium iron phosphate (FePO4) reacts with lithium (Li+) ions in the presence of the redox mediator to generate lithium iron phosphate (LiFePO4).

12. The method of any one of claims 1-11, wherein the redox mediator is reusable.

13. The method of any one of claims 1 -12, wherein the redox mediator is anthraquinone-2,7- disulfonate (AQDS), l,8-dihydroxyanthraquinone-2,7-disulfonic acid (1,8-DHAQDS), ferrocene derivatives, or viologen derivatives.

14. The method of any one of claims 1-13, wherein the regenerated lithium iron phosphate (LiFePO4) exhibits the same properties as standard lithium iron phosphate (LiFePO4).

15. The method of any one of claims 1 -14, wherein the regenerated lithium iron phosphate (LiFePO4) can be used without any purification or re-synthesis.

16. A method of recovering / regenerating lithium iron phosphate (LiFePO4) from spent lithium iron phosphate (FePO4) in an electrochemical flow system comprising: prefilling an anolytic tank of the electrochemical flow system with an anolyte, preferably a lithium-ion solution, more preferably a lithium hydroxide solution (LiOH); prefilling a catholytic tank of the electrochemical flow system with spent lithium iron phosphate, optionally in a solid form, and a catholyte, preferably a redox mediator, more preferably a redox mediator with a lower redox potential than the spent lithium iron phosphate (FePO4); providing electrical charge to the electrochemical flow system for a predetermined period, causing migration of lithium ions (Li+) from the anolytic tank to the catholytic tank, wherein the lithium ions (Li+) are generated by disintegrating the anolyte, preferably, by disintegrating the lithium-ion solution; reduction of the redox mediator; release of spent lithium iron phosphate (FePO4), optionally from the solid mass, upon reacting with the reduced redox mediator; andreaction of spent lithium iron phosphate (FePO4) with lithium ions (Li+) to generate lithium iron phosphate (LiFePO4).

17. A method of re-lithiation of spent lithium iron phosphate in a lithium ion flow system comprising: prefilling an anolytic tank of the electrochemical flow system with an anolyte, preferably a lithium-ion solution, more preferably a lithium hydroxide solution (LiOH); prefilling a catholytic tank of the electrochemical flow system with spent lithium iron phosphate, optionally in a solid form, and a catholyte, preferably a redox mediator, more preferably a redox mediator with a lower redox potential than the spent lithium iron phosphate (FePO4); providing electrical charge to the electrochemical flow system for a predetermined period, causing migration of lithium ions (Li+) from the anolytic tank to the catholytic tank, wherein the lithium ions (Li+) are generated by disintegrating the anolyte, preferably, by disintegrating the lithium-ion solution; reduction of the redox mediator; release of spent lithium iron phosphate (FePO4), optionally from the solid mass, upon reacting with the reduced redox mediator; and reaction of spent lithium iron phosphate (FePO4) with lithium ions (Li+) to generate lithium iron phosphate (LiFePO4).

18. The method of any one of claims 1-17, wherein the predetermined period may range from 60 minutes - 300 minutes.

19. Use of the regenerated lithium iron phosphate from any one of claims 1 -16 in the construction of a lithium ion battery or a coin battery.

20. An electrochemical flow system for recovering / regenerating lithium iron phosphate (LiFePO4) from spent lithium iron phosphate ( FePO4) comprising: a cathode, an anode and a permeable membrane between the cathode and the anode, an anolytic tank filled with a lithium-ion source, preferably, a lithium-ion solution, more preferably, a lithium hydroxide (LiOH) solution; a catholytic tank filled with spent lithium iron phosphate (FePO4), optionally in the form of a solid mass, and a redox mediator solution, optionally, wherein the redox mediator has a lower redox potential than the spent lithium iron phosphate (FePO4); and a means for providing electrical charge to the electrochemical flow system.

21. The electrochemical flow system of claim 20, wherein the cathode and the anode are made of carbon felt, carbon paper, carbon cloth or graphite foil.

22. The electrochemical flow system of claim 21 or 22, wherein the permeable membrane is made of NAFION, SPEEK or PBI.

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