Method for improved preloading of battery electrodes with metal ions, and preloaded battery electrodes with improved electrical properties

Abstract

The present invention relates to a method for producing an electrode for an alkali-ion storage battery, the method comprising at least the method steps of: a) providing an electrode layer preloaded with alkali ions; and b) currentless contacting of the preloaded electrode layer provided in method step a) with a solution comprising an organic solvent and at least one additive dissolved therein, wherein the additive is selected from the group consisting of carbon dioxide, organic carbonates, organic silanes, derivatives thereof or mixtures of at least two additives from this group, and currentless deposition of at least part of the additive onto the preloaded electrode layer in the absence of an electrolyte salt. The present invention also relates to electrodes produced by means of the method according to the invention and to the use of the method for producing electrodes for alkali-ion storage batteries.

Description

[0001] Method for improved pre-loading of battery electrodes with metal ions and pre-loaded battery electrodes with improved electrical properties

[0002] The present invention relates to a method for producing an electrode for an alkali ion accumulator, wherein the method comprises at least the method steps: a) providing an electrode layer pre-charged with alkali ions and b) electroless contacting of the pre-charged electrode layer provided in method step a) with a solution comprising an organic solvent and at least one additive dissolved therein, wherein the additive is selected from the group consisting of carbon dioxide, organic carbonates, organic silanes, their derivatives or mixtures of at least two additives from this group and electroless deposition of at least a portion of the additive in the absence of an electrolyte salt onto the pre-charged electrode layer.Furthermore, the present invention relates to specially pre-charged electrodes produced by the process according to the invention and to the use of the process for producing electrodes for alkali-ion batteries.

[0003] The success of innovative electrical and electronic consumer goods is in many cases closely linked to their application properties, with users particularly focusing on reliable and long-term, autonomous use for things like smartphones, laptops, or electric vehicles. For this reason, industry and research are making significant efforts in the field of rechargeable batteries to increase their electrical performance, longevity, and cost-effectiveness. Alkaline and lithium-ion-based cells are the most promising energy sources, as they can currently provide the highest energy densities with practical service lives.The physical / chemical basis of the current output of Li-ion batteries is linked to the reversible storage and removal of Li-ions in the electrodes of the battery structure, whereby the charge status and the cell voltage are also a function of the total amount of Li stored.

[0004] In conventional lithium-ion battery production processes, the electrochemically active lithium is supplied to the cell exclusively via the positive electrode using lithiated metal oxides such as lithium cobalt oxide. To further increase service life and energy density, however, various negative electrodes are now also lithiated before cell assembly, or are made entirely of a lithium foil. This process is called "pre-lithiation" and can provide a predetermined portion of the maximum storable energy even before the battery is first charged by the end user.Due to the fact that pre-lithiation of electrodes is decoupled from the actual chemical environment of the battery, by selecting the chemical and physical environmental conditions during pre-lithiation, other electrode structures can be obtained that do not occur within the chemical environment of the actual battery. This decoupling during pre-lithiation can be exploited to create more advantageous electrode structures with improved electrical properties.

[0005] The patent literature also contains a wide variety of approaches to pre-loading electrodes with metal ions.

[0006] For example, EP 0 498 049 A1 describes an electrochemical secondary cell with a positive electrode, the active material of which comprises a lithium-intercalating chalcogen compound of a transition metal, a negative electrode, the active material of which comprises a lithium-intercalating carbon product with a disordered lattice structure formed from organic substances by a sustained coking process, and a non-aqueous electrolyte, characterized in that in the installed state of the cell the transition metal chalcogenide is loaded with lithium and that the carbon material is loaded by pre-lithiation only with such an amount of lithium as is irreversibly bound in the carbon skeleton by chemical reaction.

[0007] Furthermore, DE 11 2017 006 921 T5 discloses a lithium-ion cell comprising a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode contains an active material for the positive electrode and a pre-lithiation source from a group of LiVOs, LiVsOs, LisVCrf, Li2C2 and any combinations thereof, preferably Li2C2.

[0008] Furthermore, DE 10 2021 105 975 A1 describes a method for producing a pre-lithiated electrode for a lithium-ion battery cell, the method comprising: electrochemically bonding a magnesium-lithium alloy to an electrode; pre-lithiating the electrode by transferring lithium ions from the magnesium-lithium alloy to the electrode; and electrochemically separating the magnesium-lithium alloy from the electrode.

[0009] Such solutions known from the state of the art may offer further potential for improvement. This applies overall to the electrical properties and, in particular, to the cycle stability and specific capacity of precharged electrode layers in alkali-ion cells.

[0010] It is therefore the object of the present invention to at least partially overcome the disadvantages known from the prior art and to provide a method for producing and improving precharged electrode layers and electrodes for alkaline-ion batteries. In particular, the object of the present invention is to provide a method for treating precharged electrode layers that can be carried out highly reproducibly and cost-effectively, functions independently of the actual precharge process, and yields modified precharged electrodes characterized by improved electrical properties.

[0011] The object is achieved by the features of the independent claims, directed to the method according to the invention, electrodes produced by the method according to the invention, and the inventive use of the method for producing pre-charged electrodes. Preferred embodiments of the invention are specified in the subclaims, in the description, or in the figures. Further features described or shown in the subclaims, in the description, or in the figures may constitute a subject matter of the invention, individually or in any combination, unless the context clearly indicates otherwise.

[0012] Accordingly, according to the invention, an electrode for an alkali ion accumulator is produced, characterized in that the method comprises at least the method steps: a) providing an electrode layer pre-charged with alkali ions and b) electroless contacting of the pre-charged electrode layer provided in method step a) with a solution comprising an organic solvent and at least one additive dissolved therein, wherein the additive is selected from the group consisting of carbon dioxide, organic carbonates, organic silanes, their derivatives or mixtures of at least two additives from this group and electroless deposition of at least part of the additive in the absence of an electrolyte salt onto the pre-charged electrode layer.

[0013] Surprisingly, it was discovered that treating a pre-charged electrode in a de-energized state, and especially in a chemical environment outside of conventional battery cell arrangements without electrolyte, with the above-mentioned specific additives leads to improved electrical behavior of the electrodes during battery or accumulator operation. The electrode layers and electrodes preconditioned by additional deposition of the specified additives can, in particular, exhibit improved electrical properties, for example, in the form of improved cycle stability during battery operation.Without being bound by theory, this effect can probably be attributed to the fact that by depositing or segregating the additives onto an already at least partially pre-charged electrode, without the influence of electrical voltages or electrical currents, and especially in the absence of an electrolyte, a particularly efficient deposition structure of the additive on the electrode material and on the already partially embedded alkali ions is obtained, which leads to particularly cycle-stable electrode layers during actual battery operation. The latter can probably also result, in particular, from the fact that the deposition of the additives occurs without simultaneous further or additional deposition of alkali ions onto the electrode, so that a particularly well-defined additive layer can be obtained on the electrode material itself or on the already embedded alkali ions.

[0014] The method according to the invention is a method for producing an electrode for an alkali-ion rechargeable battery. The electrical properties of alkali-ion rechargeable batteries are based on the transfer of alkali ions, such as lithium, sodium or potassium, between two battery electrodes. Typical lithium-ion batteries or rechargeable batteries consist of a first and a second electrode and, arranged between these, an electrolyte material and a separator. Often, several lithium-ion battery cells are electrically connected to one another in the form of stacks to increase the overall performance. Conventional lithium-ion rechargeable batteries function by lithium ions being reversibly transferred between the negative and positive electrodes and being incorporated into or removed from them. The electrolyte present in the batteries as such is suitable for conducting lithium ions and can be in solid or liquid form.The electrodes for these batteries typically comprise a metallic conductor and, mounted on this conductor, an active material suitable for the reversible absorption and release of lithium. The process presented here involves the further conditioning of electrode materials already pre-charged with alkali ions.

[0015] In process step a), an electrode layer pre-charged with alkali ions is provided. The process according to the invention is carried out on electrodes which are outside the chemical environment of a battery or an accumulator. The electrode layer can comprise either only the electrode active material or, for example, also an electrical conductor in the form of a metal foil and the actual electrode material. The electrode layer is suitable for the reversible incorporation and removal of alkali ions and, in particular, lithium ions. As a function of the electrical potential, alkali ions can be incorporated into and removed from the active material of the electrode. The electrode material of the electrode layer is already pre-charged, i.e. the electrode material of the electrode layer has at least a partial load of alkali ions which are incorporated into the electrode structure.The pre-charging rate can, for example, range from 5% to 100% of the theoretically possible charge. The actual pre-charging of the electrode with alkali ions can be achieved, for example, in a suitable electrolyte solution via a current flow.

[0016] In process step b), the pre-charged electrode layer prepared in process step a) is contacted with a solution without current. The pre-charged electrode is further processed outside the chemical environment of the pre-charge bath. For this purpose, the pre-charged electrode layer or electrode is transferred into another liquid process solution. In this process solution, the electrode layer is treated without applying a voltage or allowing an electric current to flow. This can be achieved, for example, by immersing the electrode layer in a bath containing the process solution for a certain period of time. Immersion in the solution can take place over a period of time, for example, from 30 minutes to several hours. The solution can have a temperature of, for example, 0°C to 60°C. Immersion or rinsing of the electrode can also take place at room temperature, for example.The amount of solution is not critical as long as the entire electrode surface is wetted by the solution. The weight ratio of electrode to solution can advantageously be more than 1:100, more preferably more than 1:500, and further preferably more than 1:1000.

[0017] The solution comprises an organic solvent and at least one additive dissolved therein. The organic solvent can, for example, be a solvent used in the field of alkali-ion batteries, and in particular lithium-ion batteries. Common solvents in this field include, for example, cyclic organic carbonates such as ethylene carbonate or propylene carbonate, acyclic carbonates such as dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate, aliphatic carboxylic acid esters such as methyl formate, methyl acetate, or methyl propionate, gamma-lactones such as gamma-butyrolactone or gamma-valerolactone, acyclic esters such as 1,2-dimethoxyethane, 1,2-diethoxyethane, or ethoxymethoxyethane, or cyclic esters such as tetrahydrofuran or 2-methyltetrahydrofuran. However, solvents such as tetrahydrofuran (THF), alcohols, organic esters, and ethers can also be used.The concentration of the solvents is not critical, but may preferably be greater than 70 wt.%, further preferably greater than 80 wt.% and further preferably greater than 90 wt.%.

[0018] The additive is selected from the group consisting of carbon dioxide, organic carbonates, organic silanes, their derivatives, or mixtures of at least two additives from this group. In addition to the actual solvent, the solution contains at least one other substance which is at least partially deposited on the electrode surface during the treatment or immersion of the pre-charged electrode. Without being bound by theory, the additive interacts with the electrode surface or the pre-charged electrode surface and thus contributes to stabilizing the electrical performance of the electrode structure. The additives can be used, for example, in a concentration of greater than or equal to 0.5 wt.% and less than or equal to 30%, further preferably greater than or equal to 1.0 wt.% and less than or equal to 25%, and further preferably greater than or equal to 2.5 wt.% and less than or equal to 20%.

[0019] Organic carbonates contain more than two carbon atoms and thus differ from inorganic carbonates. Examples of this group include substituted or unsubstituted cyclic or acyclic C2-C8 carbonates.

[0020] The organic silanes can, for example, be selected from the group consisting of unsubstituted or substituted silanes or mixtures of at least two members thereof. Possible substituted silanes include cyanosilanes. For example, the silanes can have the formula (RO)x-Si-[(CH2) y -Z](4- X), where Z can be hydrogen or a monovalent substituent, for example -CN. R in the formula can represent a linear, branched, or cyclic C1-C7 alkyl group. The alkyl group can also bear one or two additional substituents. The index x can be 1-3, and the index y can be 1-5. Possible representatives of this group include, for example, (CH3CH2O)3Si(CH2)2CN or (CH3CH2O)3Si(CH2)3CN.

[0021] Within process step b), at least a portion of the additive is electrolessly deposited onto the pre-charged electrode layer in the absence of an electrolyte salt. During the immersion treatment of the electrode in the solvent bath, a portion of the additive is deposited onto the pre-charged electrode layer by pure diffusion. There, the additives can react with the electrode material or the pre-charged electrode material. Preferably, more than 0.01 wt. %, further preferably more than 0.05 wt. %, and further preferably more than 0.1 wt. % of additive, based on the weight of the pre-charged electrode, is applied by the electroless deposition through pure diffusion processes in the solution without any additional electrical driving forces.In a preferred embodiment of the process, the solvent can be selected from the group consisting of substituted or unsubstituted C3-C8 carbonates, gamma-butyrolactone, acetonitrile, or mixtures of at least two solvents from this group. This group of solvents appears particularly suitable for obtaining particularly stable precharged electrodes during the electroless deposition of the additives, which can exhibit an improved number of cycles in charge / discharge processes. Without being bound by theory, this may be due to the solvent itself being partially incorporated into the precharged electrode structure and causing a change in the electrical properties of the precharged electrode layer.A further effect may be that these solvents provide favorable solubility for the additives and contribute to a very efficient deposition of the additives on the pre-loaded electrode layer.

[0022] In a further preferred embodiment of the process, the solvent can be selected from the group consisting of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, propylene carbonate, gamma-butyrolactone, or mixtures of at least two solvents from this group. This group of solvents appears particularly suitable for obtaining particularly stable precharged electrodes during the electroless deposition of the additives, which can exhibit an improved number of cycles in charge / discharge processes. Furthermore, the precharged electrode layers treated in this way can exhibit an improved initial capacity. Without being bound by theory, this may be due to the solvent itself being partially incorporated into the precharged electrode structure and causing a change in the electrical properties of the precharged electrode.

[0023] Within a further preferred aspect of the process, the additive can be selected from the group of fluorinated organic C2-C5 carbonates. This group of additives can be cyclic or non-cyclic compounds, whereby the term "organic carbonates" indicates that they are not inorganic, salt-like carbonates. This group of organic carbonates, in particular, can lead to particularly efficient stabilization of the electrode layer, resulting in improved cycle stability as a function of the number of charge / discharge cycles. In addition, the pre-charged electrodes treated in this way can exhibit improved initial capacity. Possible compounds from this group are, for example, mono- or di-fluorinated vinyl carbonate or, for example, mono-, di-, or trifluoropropylene carbonate.

[0024] In a further preferred embodiment of the process, the additive can be carbon dioxide. In particular, the use of carbon dioxide as an additive can surprisingly lead to batteries with particularly long-term stability, whose specific discharge capacity decreases to a significantly lesser extent compared to untreated pre-charged, for example, pre-lithiated electrodes or samples pre-charged with other additives. It is evident from the examples that the pre-charged electrode layers treated with carbon dioxide in this way result in a particularly stable specific discharge capacity over the number of cycles.

[0025] According to a preferred characteristic of the process, in process step a), the electrode can be precharged to a precharge level of greater than or equal to 5% and less than or equal to 70%. Surprisingly, it has been found that the degree of precharge prior to the additional treatment according to the invention also influences the electrical performance parameters of accumulators or batteries produced with these electrodes. Within these precharge ranges, further treatment can result in particularly long-term stable electrode layers, electrodes, and battery arrangements. More preferably, the degree of precharge can be greater than or equal to 10% and less than or equal to 50%, further preferably greater than or equal to 12.5% ​​and less than or equal to 35%. The precharge of the active material is specified in relation to the maximum charge capacity of the electrode layer.The maximum loading can be calculated from the maximum loading of the pure active materials and the percentage composition in the electrode layer. The maximum loading of the active materials with alkali ions can also be obtained from the literature or the manufacturer's specifications. A possible direct experimental determination method involves the use of battery cell arrangements with, for example, a lithium metal electrode and the corresponding electrode via the integration of the current over time. The electrode is discharged with lithium from the initial voltage after installation to a final voltage of 0.02 V, and the charge is determined accordingly via the time integral of the current.

[0026] Within a preferred aspect of the process, the concentration of additives in process step b) can be greater than or equal to 2 wt.% and less than or equal to 20 wt.%. These additive concentration ranges have proven particularly suitable for obtaining a pre-charged electrode surface that is treated as uniformly as possible. Uniform loadings can be achieved within very short process times, which has a positive effect on the electrical properties of the electrode layer and thus of the entire electrode.

[0027] In a further preferred embodiment of the method, the electrode can be a graphite / silicon composite electrode. It has been shown that the method according to the invention can be used to improve, in particular, the electrical properties of electrode layers which comprise two different active materials, and in particular silicon and carbon in significant weight proportions. In particular, the carbon weight proportion of the electrode layer can be greater than or equal to 30 wt.% and less than or equal to 60 wt.%, and the silicon weight proportion of the electrode layer can be greater than or equal to 10 wt.% and less than or equal to 60 wt.%, based on the total weight of the electrode layer. This embodiment can be particularly suitable for Li-ion batteries. In a further embodiment of the method, the electrode layer can have a porosity determined via mercury porometry of greater than or equal to 10% and less than or equal to 55%.The use of porous electrode active materials with the porosity specified above has proven particularly suitable for the inventive treatment of pre-lithiated electrodes. Within this porosity range, particularly improved and particularly long-term stable electrodes can be obtained, which also exhibit a particularly high specific discharge capacity. More preferably, the porosity can be greater than or equal to 15% and less than or equal to 45%, and further preferably greater than or equal to 20% and less than or equal to 40%.

[0028] Also according to the invention are precharged electrodes obtained by the process according to the invention. The additionally modified electrode layers obtainable by the process according to the invention can have a particularly stable surface whose electrochemical properties remain very stable even after many charge / discharge cycles. In addition to increased long-term stability, the electrodes produced in this way can exhibit improved electrical storage capacity for alkali ions. Without being bound by theory, the additives present in the solvent appear to form special complexes with the materials of the precharged electrode during the additional treatment, which improve the electrical properties.Surprisingly, the advantages arise from a purely diffusion-driven deposition without further electrochemical forces, which can also change the charge status of the pre-charged electrode surface or, if applicable, the additives.

[0029] Furthermore, the invention relates to the use of the method according to the invention for producing a precharged electrode for use in an alkaline-ion accumulator. The treated, precharged electrodes according to the invention can be used in particular in a battery assembly of an alkaline-ion cell. In this battery assembly, the electrodes produced according to the method according to the invention lead to an improvement in the long-term electrical stability of the assembly and, in particular, also an increase in the specific discharge capacity. Overall, therefore, a higher electrical power can be provided by the assembly over a longer period of time.

[0030] Examples

[0031] 1. Production of a graphite / silicon electrode

[0032] A mixer is used to produce a homogeneous powder mixture of graphite, silicon, carbon black, and binders. The mixture is dispersed in deionized water and, if necessary, neutralized with LiOH. The dispersion is applied to a Cu substrate (0.01 mm thick) using a coating device and dried. The electrode layer, coated onto the metallic conductor and dried, is then calendered to a target porosity of approximately 30%. The electrode layer used in the following measurements has the following composition: graphite 70 wt.%, silicon in the form of SiO x 20 wt.%, binder (polyacrylic acid, carboxymethylcellulose, styrene-butadiene rubber) approx. 8 wt.%, and 2 wt.% conductivity additive.

[0033] The physical properties of the anode produced in this way are as follows: thickness of the electrode layer 0.044 mm, weight of the electrode layer 0.03 g, weight of the active material 0.012 g, application quantity 6.051 mg / cm 2 ; Density of the electrode layer 1.375 g / cm 3 , Capacity 7,568 mAh, Area capacity 3,764 mAh / cm 2 , specific capacity 622,018 mAh / g.

[0034] 2. Pre-lithiation of the anode

[0035] For controlled pre-lithiation of the electrodes, 0.8 M LiCl in gamma butyrolactone (GBL) is used as the electrolyte solution. The lithiated anodes are fabricated using a button cell setup with a lithium metal counter electrode. In this setup, the anodes are lithiated at a constant rate of 0.2 C. The lithiation rate is set as a function of the achieved electrical capacity, with the maximum anode capacity being 7.6 mAh. After reaching the specified target electrical capacity of 18%, the anodes were removed from the button cells.

[0036] 3. Electroless pretreatment of the prelithiated electrodes according to the invention

[0037] The pre-lithiated anodes described in step 2 are immersed for 10 minutes at room temperature in the gamma-butyrolactone solutions specified in the table below, containing the specified amounts of the corresponding additives. The solutions contain no other substances.

[0038] The immersed and removed electrodes were combined in a coin cell assembly with a positive counter electrode (cathode), a separator to prevent electrical short circuits, and an electrolyte solution. The cathode consists of 95.5 wt% of a mixed oxide (nickel (60 mol%), manganese (20 mol%), cobalt (20 mol%)), 2 wt% SuperP, and 2.5 wt% PVDF. The electrolyte of the cell assembly consists of IM LiPFe in ethylene carbonate / ethylene methylene carbonate (ratio 3:7 (LP57)) with 10% fluoroethylene carbonate. A glass fiber membrane was used as the separator. The ratio of the capacities of the negative and positive electrodes is 1.07 in all experiments and refers to the capacity of the negative electrode after pre-lithiation in GBL as described above. The electrical parameters of the charge-discharge cycles to determine the battery properties of the manufactured electrodes are listed in the table below;

[0039] The capacity curve as a function of the number of charge / discharge cycles is shown for the variously treated electrodes in Figure 1. Figure 3 shows the ratio of the amount of charge between a charge and discharge process as a function of the number of charge / discharge cycles. In contrast to the untreated reference electrode (PORO), a significant improvement in the long-term stability of the specific discharge capacity can be observed for the electrodes according to the invention, which were immersed in a solvent containing the additives specified above after pre-lithiation. Furthermore, it is clearly evident that the initial capacity for the treated electrodes increases significantly through the incorporation of the additives. In summary, the pretreatment of the anodes in a currentless state in the absence of an electrolyte results in anodes with improved electrical properties.

[0040] Figure 2 shows the discharge capacity and Coulombic efficiency results for non-prelithiated electrodes, prelithiated electrodes without the inventive preloading with additives, and a prelithiated and loaded electrode according to the invention. These results clearly show that the Coulombic efficiency can be increased by approximately 2% compared to conventionally prelithiated electrodes.

Claims

Patent claims 1. A method for producing an electrode for an alkali-ion battery, characterized in that the method comprises at least the following steps: a) providing an electrode layer pre-charged with alkali ions and b) contacting the pre-charged electrode layer provided in step a) without current with a solution comprising an organic solvent and at least one additive dissolved therein, wherein the additive is selected from the group consisting of carbon dioxide, organic carbonates, organic silanes, their derivatives or mixtures of at least two additives from this group and depositing at least part of the additive onto the pre-charged electrode layer without current in the absence of an electrolyte salt.

2. The method of claim 1, wherein the solvent is selected from the group consisting of substituted or unsubstituted C3-C8 carbonates, gamma-butyrolactone, acetonitrile or mixtures of at least two solvents from this group.

3. A method according to any of the preceding claims, wherein the solvent is selected from the group consisting of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, propylene carbonate, gamma-butyrolactone or mixtures of at least two solvents from this group.

4. Method according to any of the preceding claims, wherein the additive is selected from the group of fluorinated organic C2-C5 carbonates.

5. Method according to any one of claims 1 to 3, wherein the additive is carbon dioxide.

6. Method according to one of the preceding claims, wherein in method step a) the electrode is preloaded to a preload level of greater than or equal to 5% and less than or equal to 70%.

7. A method according to any of the preceding claims, wherein the concentration of the additives in process step b) is greater than or equal to 2 wt.% and less than or equal to 20 wt.%.

8. Method according to any of the preceding claims, wherein the electrode is a graphite / silicon composite electrode and the alkali ion is lithium.

9. Method according to claim 8, wherein the electrode layer has a porosity determined by mercury porometry of greater than or equal to 10% and less than or equal to 55%.

10. An electrode pre-loaded with alkali metal ions obtained by a method according to one of claims 1-9.

11. Use of a method according to any one of claims 1 to 9 for the production of a pre-charged electrode for use in an alkaline-ion battery.

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