Process for producing a lithium-ion battery cell

Abstract

In a method according to the invention for producing a lithium-ion battery cell, a cell assembly with two complementary electrodes for the lithium-ion battery cell, separated by a separator, is provided, and a first electrolyte is introduced at least into the cell assembly, wherein the first electrolyte comprises ethyl methyl carbonate (EMC) as solvent and lithium hexafluorophosphate (LiPF6) as conductive salt and is free of ethylene carbonate (EC). The cell assembly with the introduced first electrolyte is then subjected to forming, and the cell assembly is subsequently degassed. Before cycling, a second electrolyte is introduced into the cell assembly, wherein the second electrolyte comprises ethylene carbonate (EC) as solvent and lithium hexafluorophosphate (LiPF6) as conductive salt.

Description

[0001] The invention relates to a method for producing a lithium-ion battery cell. Furthermore, the invention also relates to such a lithium-ion battery cell.

[0002] Lithium-ion battery cells exhibit the problem of the so-called "rollover" effect, a comparatively rapid reduction in capacity. This occurs particularly when a high energy density is to be achieved and voltages above 4 volts, especially above 4.3 volts, are used. The rollover effect results from the decomposition of transition metals in the cathode. These decomposition products are deposited on the anode and, together with lithium, lead to the formation of so-called dendrites (needle-like crystal structures). In an unfavorable case, these dendrites can pierce a separator arranged between the anode and cathode, which serves to electrically insulate the two electrodes from each other, thus causing a short circuit between the electrodes.This effect must be avoided or at least reduced, as it is known to lead to a reduction in the usability of the battery cell and also of the entire battery (which usually comprises several battery cells).

[0003] To this end, surface layers (functional layers) are generated to prevent this effect, particularly the decomposition of transition metals. One variant is the use of lithium difluorophosphate (LiPO2F2) as a conductive salt (an additive) in the electrolyte. This leads to suitable functional layers.

[0004] CN 1 17 638 269 A relates to the technical field of batteries, in particular to a liquid battery and a manufacturing method and application thereof. According to the preparation method of the liquid battery, the battery consists of a battery cell, and the preparation method includes the following steps: injecting a first electrolyte into the battery cell to form it; injecting a second electrolyte into the formed battery cell; the first electrolyte solvent and the second electrolyte solvent consist of cyclic carbonate and chain carbonate, the first electrolyte contains ethylene carbonate, and the second electrolyte does not contain ethylene carbonate. According to the manufacturing method of the invention, the problem of gas production of the battery can be inhibited, and the prepared liquid battery has excellent cycle performance and capacity.

[0005] The invention is based on the object of providing an alternative to the formation of functional layers on the electrodes of a lithium-ion battery cell.

[0006] This object is achieved according to the invention by the features of claim 1. Furthermore, this object is achieved according to the invention by the features of claim 12. Further advantageous and partly inventive embodiments and developments of the invention are set out in the subclaims and the following description.

[0007] The method according to the invention serves to produce a lithium-ion battery cell (also according to the invention). This lithium-ion battery cell (hereinafter referred to as "battery cell") is designed and intended in particular for use in a traction battery of a motor vehicle. The battery cell preferably has a high energy density (in particular greater than 150 up to 750 Wh / kg) at voltages of approximately 4.3 V or more.

[0008] To produce the battery cell, the process involves providing a cell structure (for the lithium-ion battery cell) which has two complementary electrodes (i.e., an anode and a cathode) separated by a separator. A first electrolyte is then introduced (in particular "injected") at least into the cell structure (i.e., at least between the electrodes; preferably into a housing in which the cell structure has been arranged, so that the electrodes are also wetted on the outside). This first electrolyte contains (preferably exclusively) ethyl methyl carbonate (EMC) as a solvent and lithium hexafluorophosphate (LiPF6) as a conductive salt. Preferably, the first electrolyte contains only (exclusively) lithium hexafluorophosphate as a conductive salt. The first electrolyte is in any case free of ethylene carbonate (EC).The cell assembly with the introduced first electrolyte undergoes a forming process, particularly to ensure that specific surface layers are formed on the surfaces of the anode and / or cathode. After forming, the cell assembly is degassed. This degassing process serves to remove gases formed during forming by the decomposition of components of the first electrolyte. Before cycling, and thus after degassing, a second electrolyte is introduced (injected) into the cell assembly. This second electrolyte contains (in particular at least) ethylene carbonate (EC) as a solvent and (in particular at least) lithium hexafluorophosphate (LiPF6) as a conductive salt.

[0009] The formulation of the first electrolyte advantageously enables the formation of a “solid electrolyte interphase” (hereinafter referred to as “SEI”) on the anode and a “cathode electrolyte interphase” (hereinafter referred to as “CEI”) on the cathode. Due to the formulation of the electrolyte described here and below, these SEI and CEI are such that during operation and / or cycling of the battery cell, transition metal decomposition (in particular of manganese, cobalt and / or nickel) on the cathode can be avoided, especially at voltages greater than 4.3 V, which in turn increases the service life of the battery cell. In addition, the use of lithium difluorophosphate LiPO2F2 in the first - and advantageously also in the second - electrolyte can be avoided, while still achieving the same or at least equally “effective” (i.e.with comparable properties regarding the avoidance of transition metal decomposition) SEI and CEI can be generated.

[0010] In particular, the first electrolyte serves to generate these SEI and CEI, while the second electrolyte preferably serves to achieve the highest possible conductivity - the solvent ethylene carbonate contributes to this - during the subsequent cyclization.

[0011] Preferably, the cell structure together with the housing, which surrounds the cell structure, i.e. the complementary electrodes, the separator and, after cycling, also the second electrolyte, forms the (lithium-ion) battery cell.

[0012] Preferably, the formation takes place over 1 to 3 cycles at the lower and upper discharge voltages specified for the selected cell chemistry. During the formation, the aforementioned surface layers (SEI and CEI) are formed by applying voltage to the electrodes. The SEI and CEI are preferably formed by decomposition products of ethyl methyl carbonate (EMC) and / or lithium hexafluorophosphate (LiPF6). In particular, during or after the formation, in a "precharge step", preferably at 40 °C, the cell assembly or the lithium-ion battery cell is charged for a period of 3 hours at a C rate of 0.05 C. This is followed by a "rest step" of 10 minutes and then a further charge of the cell assembly or the battery cell at a C rate of 0.1 C for 90 minutes.

[0013] For degassing, a vacuum of 1 Torr is preferably applied to the (preferably precharged) cell assembly, in particular the battery cell, for up to 30 or 20 seconds. The housing is then preferably completely filled with the second electrolyte.

[0014] In particular for cycling, after filling the housing, in particular the battery cell, with the second electrolyte, the cell structure or the battery cell is charged to a so-called upper termination voltage (in particular at 4.3 V or higher) and then discharged to the lower termination voltage (in particular at 2.7 V or lower, e.g. down to 2 or 1.5 V).

[0015] According to an expedient process variant, after the introduction of the first electrolyte and before forming, a rest phase (also referred to as "aging") of at least approximately 24 hours, preferably approximately 48 hours ("approximately" in this context is understood as a tolerance range of + / - 5 hours) is waited for. Optionally, the battery cell is stored at an elevated temperature (i.e. higher than room temperature) (e.g. between 30 and 60 degrees Celsius) during this rest phase. Preferably, however, the rest phase is carried out at room temperature. This rest phase (aging) enables uniform wetting of the electrodes with the (first) electrolyte. In particular, this wets porous structures of the electrode active materials and thus enables lithium ion transport between the active materials on the anode and cathode sides.

[0016] According to a preferred process variant, the anode of the two complementary electrodes is formed as a graphite anode and / or the cathode of the two complementary electrodes is formed as a nickel-manganese-cobalt oxide cathode ("NMC cathode"). The graphite of the anode is formed, in particular, from synthetic graphite (also known as "artificial graphite").

[0017] According to an optional process variant, the first electrolyte is free of further additives. In particular, the first electrolyte is at least free of lithium difluorophosphate (LiPO2F2).

[0018] According to a suitable process variant, the second electrolyte contains ethyl methyl carbonate (EMC) as an additional solvent and / or other additives (in addition to ethylene carbonate and lithium hexafluorophosphate). In particular, the second electrolyte is also free of lithium difluorophosphate (LiPO2F2).

[0019] According to another advantageous process variant, the concentration of lithium hexafluorophosphate (LiPF6) in the first electrolyte is higher than in the second electrolyte. For example, the first electrolyte has a concentration of 0.8 to 1.4 M (molar concentration, mol / l) of lithium hexafluorophosphate, while the second electrolyte, for example, has a concentration of 1.1 M (in this case, the first electrolyte preferably contains at least 1.2 M or more).

[0020] According to a further advantageous embodiment, the surface layer (CEI) formed by the decomposition products on the cathode (in particular its surface) contains 0 to 20 at.% (atomic concentration) of compounds Li x PO y F z with a comparatively high proportion of fluorine, 0 to 5 at.% of Li compounds x PO y F zwith a comparatively low proportion of fluorine, 0 to 5 at.% lithium hexafluorophosphate, 0 to 15 at.% carbonates, 0 to 15% carbon monoxide, 0 to 20 at.% phosphorus oxide species and 0 to 52 at.% lithium fluoride.

[0021] Furthermore, the surface layer (SEI) formed by the decomposition products on the anode (especially its surface) contains 0 to 10% of compounds Li x PO y F z with a comparatively high proportion of fluorine, 0 to 5 at.% of Li compounds x PO y F z with a comparatively low proportion of fluorine, 0 to 2 at.% lithium hexafluorophosphate, 0 to 10 at.% carbonates, 0 to 15% carbon monoxide, 0 to 15 at.% phosphorus oxide species and 0 to 60 at.% lithium fluoride.

[0022] The battery cell according to the invention has the physical features resulting from the method described above and the advantages resulting therefrom.

[0023] The conjunction “and / or” is to be understood here and in the following in particular in such a way that the features linked by means of this conjunction can be formed both together and as alternatives to one another.

[0024] An embodiment of the invention is explained in more detail below with reference to a drawing, in which the only Fig. 1 a schematic flow diagram of the production of a lithium-ion battery cell.

[0025] In a first process step S1, two complementary electrodes, namely an anode and a cathode, are provided. The anode is designed as a graphite anode, and the cathode as an NMC cathode. The two electrodes are stacked on top of each other, separated by a separator, thus forming a cell structure. This cell structure can also comprise multiple pairs of electrodes (separated from each other and also from each other by a separator). The cell structure is, in particular, introduced into a housing and, together with it, forms a (lithium-ion) battery cell.

[0026] Subsequently, in a second process step S2, a first electrolyte is introduced—injected—into the cell structure (specifically, into the housing) so that the electrodes and, in particular, the separator are wetted with the first electrolyte. This first electrolyte is formed by a solution of lithium hexafluorophosphate (LiPF6), a conductive salt, in ethyl methyl carbonate (EMC) as a solvent. No other solvent—in particular, ethyl carbonate (EC)—is used in the first electrolyte. In an optional embodiment, the first electrolyte also contains no other ingredients such as additives and the like.

[0027] In a subsequent process step S3, the cell assembly filled with the first electrolyte undergoes an aging step. The cell assembly is stored for 48 hours at approximately 25 degrees Celsius.

[0028] After aging, the cell structure, in particular the surfaces of the electrodes, is "formed" in a fourth process step S4. The electrodes are subjected to a voltage, causing the ethyl methyl carbonate and the conductive salt to decompose. The decomposition products, in turn, settle on the anode and cathode, forming surface layers known as SEI (anode side) and CEI (cathode side). These surface layers serve to reduce or prevent the rollover effect. The advantage of the first electrolyte is that lithium difluorophosphate (LiPO2F2) can be omitted (in concrete terms, lithium difluorophosphate is not used either) and yet equivalent or identical surface layers can be generated. These surface layers formed in this way contain, in particular, Li compounds. x PO y F zwith a comparatively high proportion of fluorine, compounds Li x PO y F z with a comparatively low content of fluorine, lithium hexafluorophosphate, carbonates, carbon monoxide, phosphorus oxide and lithium fluoride.

[0029] After forming, the cell structure, specifically the battery cell, is degassed in a fifth process step S5 to remove any gaseous decomposition products of the first electrolyte still remaining in the cell structure or battery cell. For this purpose, a pressure of 1 Torr is applied to the battery cell for 20 seconds. Preferably, a "precharge step" is carried out beforehand (optionally as part of the forming process), in which the lithium-ion battery cell is charged at a C-rate of 0.05 C for a period of 3 hours, preferably at 40 °C. This is followed by a "rest step" of 10 minutes, followed by a further charge of the battery cell at a C-rate of 0.1 C for 90 minutes.

[0030] Subsequently, in a sixth process step S6, a second electrolyte is injected into the cell structure. The second electrolyte contains ethylene carbonate as the solvent, in which lithium hexafluorophosphate (LiPF6) is dissolved as the conductive salt. Optionally, the second electrolyte can also contain ethyl methyl carbonate and / or additional additives. Further optionally, the aforementioned housing is closed (sealed) after the injection of the second electrolyte.

[0031] In a seventh process step S7, the cell assembly injected with the second electrolyte is then cycled. The battery cell is charged to the upper limit voltage (at least 4.3 V, optionally higher) and then discharged to the lower limit voltage (at 2.7 V, optionally lower). The solvent ethylene carbonate enables the battery cell formed from the cell assembly to exhibit high electrical conductivity.

[0032] The battery cell is then used in a motor vehicle’s traction battery.

[0033] The subject matter of the invention is not limited to the exemplary embodiment described above. Rather, further embodiments of the invention can be derived by those skilled in the art from the above description. List of reference symbols S1 - S7 process step

Claims

[1] Method for producing a lithium-ion battery cell, in particular for a traction battery of a motor vehicle, wherein according to the method - a cell structure with two complementary electrodes for the lithium-ion battery cell, separated by a separator, is provided, - a first electrolyte is introduced at least into the cell structure, wherein the first electrolyte has ethyl methyl carbonate (EMC) as solvent and lithium hexafluorophosphate (LiPF6) as conductive salt and is free of ethylene carbonate (EC), - the cell structure is subjected to formation with the introduced first electrolyte, - the cell structure is degassed after formation, and - before cycling, a second electrolyte is introduced into the cell structure, wherein the second electrolyte has ethylene carbonate (EC) as solvent and lithium hexafluorophosphate (LiPF6) as conductive salt. [2] Method according to claim 1, wherein after the introduction of the first electrolyte and before the forming, a rest period of at least about 24, preferably about 48 hours, is waited for. [3] Method according to claim 1 or 2, wherein one anode of the two complementary electrodes is formed as a graphite anode and / or one cathode of the two complementary electrodes is formed as a nickel-manganese-cobalt oxide cathode. [4] Method according to one of claims 1 to 3, wherein the first electrolyte is free from further additives and / or is free from lithium difluorophosphate (LiPO2F2). [5] Method according to one of claims 1 to 4, wherein the second electrolyte additionally contains ethyl methyl carbonate (EMC) and / or further additives, but in particular is free of lithium difluorophosphate (LiPO2F2). [6] Method according to one of claims 1 to 5, wherein before the introduction of the second electrolyte, a surface layer (SEI, CEI) is formed on each of the complementary electrodes by decomposition products of ethyl methyl carbonate (EMC) and / or lithium hexafluorophosphate (LiPF6). [7] The method according to any one of claims 1 to 6, wherein a concentration of lithium hexafluorophosphate (LiPF6) in the first electrolyte is higher than in the second electrolyte. [8] A method according to any one of claims 1 to 7, wherein the first electrolyte comprises 0.8-1.4 M lithium hexafluorophosphate (LiPF6). [9] A method according to any one of claims 1 to 8, wherein the second electrolyte comprises 1.1 M lithium hexafluorophosphate (LiPF6). [10] A process according to any one of claims 6 to 9, wherein the surface layer formed by the decomposition products on the cathode contains 0 to 20 at.% of compounds Li x PO y F zwith a comparatively high proportion of fluorine, 0 to 5 at.% of Li compounds x PO y F z with a comparatively low proportion of fluorine, 0 to 5 at.% lithium hexafluorophosphate, 0 to 15 at.% carbonates, 0 to 15% carbon monoxide, 0 to 20 at.% phosphorus oxide species and 0 to 52% lithium fluoride. [11] A process according to any one of claims 6 to 9, wherein the surface layer formed by the decomposition products on the anode contains 0 to 10% of compounds Li x PO y F z with a comparatively high proportion of fluorine, 0 to 5 at.% of Li compounds x PO y F z with a comparatively low proportion of fluorine, 0 to 2 at.% lithium hexafluorophosphate, 0 to 10 at.% carbonates, 0 to 15% carbon monoxide, 0 to 15 at.% phosphorus oxide species and 0 to 60% lithium fluoride. [12] Lithium-ion battery cell, in particular for a traction battery of a motor vehicle, produced according to a method according to one of claims 1 to 11.

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