Process for producing a lithium-ion battery cell

Abstract

In a process according to the invention for producing a lithium-ion battery cell, a cell structure having 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 structure, the first electrolyte comprising ethyl methyl carbonate (EMC) as solvent and lithium hexafluorophosphate (LiPF6) as conductive salt and being free of ethylene carbonate (EC). The cell structure with the first electrolyte introduced is then subjected to forming and then the cell structure is degassed. Before cycling, a second electrolyte is introduced into the cell structure, the second electrolyte comprising ethylene carbonate (EC) as solvent and lithium hexafluorophosphate (LiPF6) as conductive salt.

Description

[0001] Description

[0002] Method for manufacturing a lithium-ion battery cell

[0003] The invention relates to a method for manufacturing a lithium-ion battery cell. Furthermore, the invention also relates to such a lithium-ion battery cell.

[0004] Lithium-ion battery cells exhibit the problem of the so-called "rollover" effect, a comparatively rapid reduction in capacity. This occurs particularly when high energy density is desired and voltages above 4 volts, especially above 4.3 volts, are used. The rollover effect stems from the decomposition of transition metals on the cathode. These decomposition products are deposited on the anode and, together with lithium, lead to the formation of dendrites (needle-like crystal structures). In unfavorable conditions, these dendrites can penetrate the separator located 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 should 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).

[0005] This involves, among other things, generating surface layers (functional layers) designed to prevent this effect, particularly the decomposition of transition metals. One approach is the use of lithium difluorophosphate (UPO₂F₂) as a conducting salt (an additive) in the electrolyte. This results in suitable functional layers.

[0006] The invention is based on the objective of providing an alternative to the formation of functional layers on the electrodes of a lithium-ion battery cell.

[0007] This problem is solved according to the invention by the features of claim 1. Furthermore, this problem is solved according to the invention by the features of claim 12. Further advantageous and partly inventive embodiments and further developments of the invention are set out in the dependent claims and the following description. The method according to the invention serves to produce a (also according to the invention) lithium-ion battery cell. This lithium-ion battery cell (hereinafter referred to as: battery cell) is designed and intended for use in a traction battery of a motor vehicle. Preferably, the battery cell has a high energy density (in particular greater than 150 up to 750 Wh / kg) at voltages of about 4.3 V or more.

[0008] To manufacture the battery cell, a cell assembly (for the lithium-ion battery cell) is provided according to the process, comprising two complementary electrodes (i.e., an anode and a cathode) separated by a separator. Subsequently, a first electrolyte is introduced (in particular, "injected") at least into the cell assembly (i.e., at least between the electrodes; preferably into a housing in which the cell assembly is arranged so that the electrodes are also wetted externally). This first electrolyte comprises (preferably exclusively) ethyl methyl carbonate (EMC) as a solvent and lithium hexafluorophosphate (LiPFe) as a conducting salt. Preferably, the first electrolyte comprises (exclusively) lithium hexafluorophosphate as the conducting salt. The first electrolyte is, in any case, free of ethylene carbonate (EC).The cell assembly, containing the first electrolyte, undergoes a formation process, specifically to ensure the targeted formation of surface layers on the anode and / or cathode. Following formation, the cell assembly is degassed. This degassing serves to remove gases generated during formation through 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 (at least) ethylene carbonate (EC) as a solvent and (at least) lithium hexafluorophosphate (LiPF6) as a conducting 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 electrolyte formulation described here and below, these SEI and CEI are such that transition metal decomposition (especially of manganese, cobalt, and / or nickel) on the cathode can be avoided during operation and / or cycling of the battery cell, particularly at voltages above 4.3 V, thereby increasing the battery cell's lifespan. Furthermore, this eliminates the need for lithium difluorophosphate (UPO₂F₂) in the first—and advantageously also in the second—electrolyte, while still achieving the same or at least equally effective (i.e.,SEI and CEI can be generated (with comparable properties regarding the prevention of transition metal decomposition). 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 cycling.

[0010] 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.

[0011] Preferably, the formation process takes place over 1 to 3 cycles at the lower and upper discharge voltages specified for the selected cell chemistry. During formation, the aforementioned surface layers (SEI and CEI) are formed by applying an undervoltage to the electrodes. Preferably, the SEI and CEI are formed by decomposition products of ethyl methyl carbonate (EMC) and / or lithium hexafluorophosphate (LiPFe). In particular, during or after formation, in a "precharge step," preferably at 40 °C, the cell assembly or the lithium-ion battery cell is charged for 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.

[0012] Preferably, a vacuum of 1 Torr is applied to the (preferably pre-charged) cell assembly, in particular the battery cell, for degassing for up to 30 or 20 seconds. The casing is then preferably filled completely with the second electrolyte.

[0013] Especially for cycling, after filling the housing, and 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 (especially at 4.3 V or higher) and then discharged to the lower termination voltage (especially at 2.7 V or lower, e.g. down to 2 or 1.5 V).

[0014] According to a convenient variant of the process, after the introduction of the first electrolyte and before formation, a resting phase (also referred to as "aging") of at least approximately 24 hours, preferably approximately 48 hours, is observed ("approximately" in this context is understood as a tolerance range of + / - 5 hours). Optionally, the battery cell is stored at an elevated temperature (i.e., higher than room temperature) during this resting phase (e.g., between 30 and 60 degrees Celsius). Preferably, however, the resting phase is carried out at room temperature. This resting phase (aging) enables uniform wetting of the electrodes with the (first) electrolyte. In particular, this wets the porous structures of the electrode active materials and thus enables lithium-ion transport between the active materials on the anode and cathode sides.

[0015] According to a preferred method variant, the anode of the two complementary electrodes is formed as a graphite anode and / or the cathode of the two complementary electrodes as a nickel-manganese-cobalt oxide cathode (“NMC cathode”). The graphite of the anode is formed in particular by synthetic graphite (also: “artificial graphite”).

[0016] 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).

[0017] According to a suitable process variant, the second electrolyte additionally contains (besides ethylene carbonate and lithium hexafluorophosphate) ethyl methyl carbonate (EMC) as a further solvent and / or further additives. In particular, the second electrolyte is also free of lithium difluorophosphate (UPO₂F₂).

[0018] According to another advantageous method variant, the concentration of lithium hexafluorophosphate (LiPFe) 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 has, for example, a concentration of 1.1 M (in this case, the first electrolyte preferably contains at least 1.2 M or more).

[0019] According to another expedient embodiment, the surface layer (CEI) formed by the decomposition products at the cathode (especially its surface) has a Li concentration of 0 to 20 at%. 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.

[0020] Furthermore, the surface layer (SEI) formed by the decomposition products at the anode (especially its surface) contains 0 to 10% Li compounds. 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.

[0021] The battery cell according to the invention exhibits both the physical characteristics and the advantages resulting from the method described above.

[0022] The conjunction “and / or” is to be understood here and in the following in particular as meaning that the features linked by means of this conjunction can be formed both jointly and as alternatives to each other.

[0023] An embodiment of the invention is explained in more detail below with reference to a drawing. Figure 1, the only figure in the drawing, shows a schematic flowchart for the production of a lithium-ion battery cell.

[0024] In the first process step S1, two complementary electrodes, namely an anode and a cathode, are provided. The anode is a graphite anode, and the cathode is an NMC cathode. The two electrodes are stacked on top of each other, separated by a separator, to form a cell assembly. This cell assembly can also contain multiple pairs of electrodes (each separated from the others and from each other by a separator). The cell assembly is then placed in a housing, forming a (lithium-ion) battery cell.

[0025] Subsequently, in a second process step S2, a first electrolyte is introduced—injected—into the cell assembly (specifically, 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 conducting 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 or the like.

[0026] In a subsequent process step S3, the cell assembly filled with the first electrolyte undergoes an aging phase. During this phase, the cell assembly is stored for 48 hours at approximately 25 degrees Celsius.

[0027] After aging, in a fourth process step S4, the cell structure, particularly the electrode surfaces, is "formed," i.e., subjected to a formation process. During this step, the electrodes are subjected to voltage, causing the ethyl methyl carbonate and the conducting salt to decompose. The decomposition products are deposited 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 (UPO₂F₂) can be omitted (in fact, lithium difluorophosphate is not used) while still generating equivalent or identical surface layers. These surface layers 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.

[0028] After formation, in a fifth process step S5, the cell assembly, specifically the battery cell, is degassed to remove gaseous decomposition products of the first electrolyte that may still be present in the cell assembly 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 performed beforehand (optionally as part of the formation process) in which the lithium-ion battery cell is charged at a C-rate of 0.05 C for a period of 3 hours at a temperature of preferably 40 °C. This is followed by a "rest step" of 10 minutes and then a further charge of the battery cell at a C-rate of 0.1 C for 90 minutes.

[0029] Subsequently, in a sixth process step S6, a second electrolyte is injected into the cell assembly. This second electrolyte uses ethylene carbonate as a solvent, in which lithium hexafluorophosphate (LiPF6) is dissolved as the conducting salt. Optionally, the second electrolyte can also contain ethyl methyl carbonate and / or additional additives. Optionally, the aforementioned housing is closed (sealed) after the injection of the second electrolyte.

[0030] In a seventh process step, S7, the cell assembly injected with the second electrolyte undergoes a cycling process. Here, the battery cell is charged to the upper termination voltage (at least 4.3 V, optionally higher) and then discharged to the lower termination voltage (at 2.7 V, optionally lower). The solvent ethylene carbonate ensures that the battery cell formed from the cell assembly exhibits high electrical conductivity.

[0031] The battery cell is then used in a traction battery of a motor vehicle. The subject matter of the invention is not limited to the embodiment described above. Rather, further embodiments of the invention can be derived by a person skilled in the art from the above description.

[0032] Reference symbol list

[0033] S1 - S7 Process step

Claims

Patent claims 1. Method for manufacturing 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 the solvent and lithium hexafluorophosphate (LiPF6) as the conducting salt and is free of ethylene carbonate (EC), - the cell structure undergoes formation with the introduced first electrolyte, - the cell structure is degassed after formation, and - prior to cycling, a second electrolyte is introduced into the cell structure, the second electrolyte comprising ethylene carbonate (EC) as the solvent and lithium hexafluorophosphate (LiPFe) as the conducting salt.

2. The method according to claim 1, wherein, after the introduction of the first electrolyte and before formation, a resting period of at least about 24, preferably about 48 hours, is observed.

3. Method according to claim 1 or 2, wherein one anode of the two complementary electrodes is designed as a graphite anode and / or one cathode of the two complementary electrodes is designed as a nickel-manganese-cobalt oxide cathode.

4. Method according to any one of claims 1 to 3, wherein the first electrolyte is free of further additives and / or is free of lithium difluorophosphate (UPO2F2).

5. A method according to any 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 (UPO2F2).

6. Method according to any one of claims 1 to 5, wherein, prior to the introduction of the second electrolyte, a surface layer (SEI, CEI) is formed on the complementary electrodes by decomposition products of ethyl methyl carbonate (EMC) and / or lithium hexafluorophosphate (LiPFe).

7. A method according to any one of claims 1 to 6, wherein the concentration of lithium hexafluorophosphate (LiPF6) in the first electrolyte is higher than in the second electrolyte.

8. Method according to any one of claims 1 to 7, wherein the first electrolyte comprises 0.8-1.4 M lithium hexafluorophosphate (LiPFe).

9. A method according to any one of claims 1 to 8, wherein the second electrolyte comprises 1.1 M lithium hexafluorophosphate (LiPFe).

10. Method according to any one of claims 6 to 9, wherein the surface layer formed by the decomposition products at the cathode contains 0 to 20 at% of Li compounds x PO y F z with a comparatively high proportion of fluorine, 0 to 5 at% of Li compoundsx 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. Method according to any one of claims 6 to 9, wherein the surface layer formed by the decomposition products at the anode contains 0 to 10% of Li compounds. 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 content 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, manufactured according to a method according to any one of claims 1 to 11.

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