Forming an electrode

Abstract

Method for forming a negative electrode for a lithium-ion cell, comprising the steps of - first constant current charging with a first charging current until a first half-cell potential is reached against a reference electrode, - first constant voltage charging at the first half-cell potential against the reference electrode until a second charging current is reached, - AC excitation or AC excitation over a frequency period, - second constant current charging with a third charging current until a second half-cell potential is reached against the reference electrode, - second constant voltage charging at the second half-cell potential against the reference electrode until a final charging current is reached or until a maximum constant voltage charging time is reached.

Description

[0001] The invention relates to a method for forming a negative electrode for a lithium-ion cell.

[0002] The interface between the negative electrode and the electrolyte of a lithium-ion cell has a decisive influence on the properties and behavior of the cell. A layer forms on the negative electrode, the so-called SEl layer (solid electrolyte interphase). This covering layer is created when the cell is charged for the first time, a process also referred to as formation. During the initial charge, the SEl covering layer is formed in parallel with the initial intercalation of lithium ions in graphite (in the case of a graphite anode as the negative electrode). The precise forming conditions are important for the formation of the SEl covering layer. In addition to the choice of chemical components, such as the electrolyte, the properties of the covering layer depend on these conditions, such as the permeability for lithium ions or the stability of the covering layer, and thus the aging behavior of the entire cell.To achieve the most advantageous properties of the top layer, state-of-the-art forming is carried out with low current load. A time-efficient forming profile can be found, for example, in document US 2015 / 0 060 290 A1.

[0003] It is an object of the invention to provide an improved method for forming a negative electrode for a lithium-ion cell.

[0004] This object is achieved by a method according to claim 1. Advantageous embodiments and further developments of the invention emerge from the dependent claims.

[0005] According to the invention, a first constant current charge is carried out with a first charging current until a first half-cell potential is reached against a reference electrode, a first constant voltage charge is carried out at the first half-cell potential against the reference electrode until a second charging current is reached, an alternating voltage excitation or alternating current excitation over a frequency period is carried out, a second constant current charge is carried out with a third charging current until a second half-cell potential is reached against the reference electrode and a second constant voltage charge is carried out at the second half-cell potential against the reference electrode until a final charging current is reached or until a maximum constant voltage charging time is reached.

[0006] A forming process is therefore proposed that includes the described forming steps. The effects of these forming steps interact in such a way that advantageous effects are achieved in the formation of an SEI (solid electrolyte interphase) coating layer on the negative electrode. The process is applicable to a graphite electrode or a similar negative electrode (e.g., a Si-graphite composite).

[0007] The first constant current charging causes a first covering layer of preferred chemical composition to form.

[0008] The first constant-voltage charge ensures that the top layer components created during the first constant-current charge are fully formed until passivation. Only components whose upstream formation reactions are thermodynamically possible at the appropriate half-cell potential against a reference electrode are formed.

[0009] The alternating voltage or alternating current excitation causes the existing surface layer to undergo chemical and / or physical changes.

[0010] The second constant-current charge results in the formation of a second type of top layer with a different chemical composition, partly due to decomposition reactions of the first top layer. The lower half-cell potential against a reference electrode enables reactions that were thermodynamically impossible during the first constant-current and constant-voltage charge.

[0011] The second constant voltage charge ensures that the cover layer components created in the second constant current charge are fully formed until passivation.

[0012] The alternating voltage or alternating current excitation results in an SEI layer that is particularly elastic, durable, and highly permeable to lithium ions, i.e., it exhibits low penetration resistance for lithium ions despite its small thickness. This is explained by the reorientation and / or restructuring of the existing cap layer components during the alternating voltage or alternating current excitation. The cap layer formed in the second constant current and constant voltage charging is thus more advantageous in terms of the aforementioned properties.

[0013] The individual forming steps follow one another in the above-mentioned order, but do not necessarily have to be consecutive. In particular, additional steps, such as individual relaxation phases, can be performed between individual forming steps.

[0014] For example, it is advantageous if a rest voltage phase occurs over a relaxation period after the first constant voltage charging and before the AC voltage excitation or AC current excitation.

[0015] The resting voltage phase causes diffusive equilibration processes, i.e. local concentration equilibria of reactants involved in the formation, to occur.

[0016] According to a further variant of the invention, the first charging current is in a range from one hundredth to twice the one-hour discharge current.

[0017] The initial charging current therefore assumes a constant value, ranging between one-hundredth and twice the one-hour current. The one-hour current, 1C, represents the current at which the cell's nominal capacity is drawn in charge within one hour under nominal conditions. Multiples of this one-hour current are referred to as the C-rate. The initial charging current is therefore between 0.01C and 2C.

[0018] Furthermore, it is advantageous if the first half-cell potential is in a range of 500 mV to 1200 mV against a Li / Li + -reference electrode. The first constant current charging takes place until this potential value is reached against the Li / Li + -Reference electrode, i.e., until the voltage drops to this potential to maintain the first charging current. Potentiostatic charging continues at this potential until the second charging current is reached.

[0019] The reference electrode is the Li / Li +-electrode. At an electrical potential of approximately 500 mV to approximately 1200 mV against this electrode, a first reaction step begins in the formation of the SEI layer. This step continues until the second charging current is reached at the same voltage, ie, with decreasing current.

[0020] The second charging current is therefore smaller than the first charging current.

[0021] A relaxation period of at least one minute is also advantageous. During this time, diffusion processes equalize concentration gradients of the components involved in the reaction to provide sufficient reactants for the alternating voltage or current excitation.

[0022] According to a particularly preferred embodiment of the invention, the alternating voltage excitation or alternating current excitation occurs at a constant frequency. This constant frequency assumes a value within the range of 0.001 kHz to 1000 kHz.

[0023] The amplitude of the AC excitation is approximately 0.1 mV to 50 mV around the value of the no-load potential, i.e., the open-circuit potential (OCP). The excitation is oscillating, preferably sinusoidal or a simple superposition of circular functions.

[0024] According to a further variant of the invention, the alternating voltage excitation or alternating current excitation consists of a successive or superimposed excitation with at least two frequencies or in an excitation by a frequency sweep.

[0025] Alternatively, multiple frequencies can be used consecutively or superimposed for excitation, or a frequency sweep can be performed. The multiple frequencies and the sweep each lie in the frequency band from 0.001 kHz to 1000 kHz.

[0026] The alternating voltage excitation takes place over the frequency period, which is in the range of 0.01 seconds to 120 minutes, but is at least of such a duration that at least 10 oscillations are achieved even at low excitation frequency.

[0027] After the frequency excitation, the second constant current charge preferably takes place, with the third charging current ranging from one-hundredth to twice the one-hour current. This charging step preferably takes place until the second half-cell potential is reached, at a level of 5 mV to 300 mV against a Li / Li + -Reference electrode. Once the potential has dropped to this value, the second constant-voltage charge is initiated until the charging current has also dropped to the final charging current, which is again lower than the third charging current. However, the second constant-voltage charge is initiated over a period of at least one minute.

[0028] Thus, the half cell, ie the negative electrode, is charged for the first time and the SEI layer is advantageously formed.

[0029] The invention is based on the following considerations: In addition to the active materials (at the negative and positive electrodes, also known as the anode and cathode), lithium-ion batteries (LIBs) contain an electrolyte consisting of conductive salt(s), solvents, and additives. Electrolytes used in the state of the art typically consist of a mixture of ethylene carbonate (EC) and linear carbonates such as diethyl carbonate (DEC), dimethyl carbonate (DMC), and / or ethyl methyl carbonate (EMC). Further additives such as vinylene carbonate (VC) in low concentrations and the conductive salt lithium hexafluorophosphate (LiPF6) are then added.

[0030] To achieve the highest possible total voltage of the LIB, materials with a very low thermodynamic electrochemical potential (< 1 V vs. Li / Li+) are used as the negative electrode. The potential range in which the negative electrode is operated (charged and discharged) lies significantly below the electrochemical stability window of the electrolyte, which is why the electrolyte is electrochemically reduced at the negative electrode.

[0031] By using suitable solvent components and, in particular, by adding certain additives mentioned above, the electrolyte reduction can be passivated, as the reduction products themselves are electrically insulating, thus preventing electron transfer from the electrode to the electrolyte. If the reduction products nevertheless exhibit ionic conductivity with respect to lithium ion migration and diffusion, the desired reversible reaction of lithium at the electrode can still occur. Since the reduction products form a non-negligibly thin layer on the negative electrode, this is referred to as a solid electrolyte interphase (SEI).

[0032] Ideally, the SEI is a long-lasting, cycle-resistant electrical insulator and ionic conductor. In reality, the SEI's limited ionic conductivity causes ohmic resistance due to the restricted movement of lithium ions. Resistances of this type reduce both the power density and the energy efficiency of LIBs (lithium-ion batteries composed of lithium-ion cells).

[0033] To minimize these effects, in addition to the use of electrolyte additives, the reduction reactions must also be controlled. This allows the chemical and physical properties of the SEI to be controlled. The controlled reduction of the electrolyte is called formation. According to current technology, LIBs are formed at a defined temperature with low current load, while relaxation periods are incorporated in between.

[0034] The SEI passivates the electrolyte reduction even at higher potentials, but at lower potentials, the first-type reduction products also react at the electrode, which is why the SEI then grows from within. The physical morphology of the first-type reaction products at the electrode significantly determines the homogeneity and chemical composition of the second-type SEI, which then remains or continues to grow on the electrode throughout its entire life cycle. The formation protocols used to date cannot influence the morphology of the first-type SEI and cannot reduce the impedance of the second-type SEI.

[0035] One proposed measure is to influence the morphology of the first-type SEI, which consists primarily of organic species, by using high-frequency electrochemical excitation of the LIB at appropriate times during formation, thus reducing the (particularly real) impedance. To do this, it is necessary to determine the voltage or potential at which the first-type SEI are finally formed and passivated. Then, using high-frequency excitation, such as that used in electrochemical impedance spectroscopy (EIS), the polar organic layer can be physically manipulated to selectively promote further reduction to inorganic species (e.g., carbonate, fluoride, oxide; Li2CO3, LiF, Li2O) and thus exploit their different ionic conductivities.

[0036] By forming an ionically highly conductive and elastic SEI, impedances in the LIB can be reduced and long-lasting stability of the SEI can be achieved. As a result, the power density and energy efficiency directly increase, and indirectly the energy density and lifetime of a LIB.

[0037] The potential profile of a carbon anode during galvanostatic (constant current) lithiation followed by potentiostatic (constant voltage) lithiation is characterized by several lithiation stages. If such a cell is subjected to 50 EIS measurements at a suitable location during formation, this leads to a decreasing total impedance (both real and imaginary) compared to an anode without EIS measurement. During EIS, frequencies from 500 kHz to 1 Hz are passed through. For an anode treated with high-frequency EIS (EIS anode), the potential during the subsequent further formation is against the reference electrode Li / Li. +higher than for a non-high-frequency ICE-treated anode (standard anode). This shows that for the same current, i.e., in the galavanostatic range, a lower overpotential (toward lower potentials) is necessary at the negative electrode after ICE treatment. Lithium is ionized at potentials of approximately <0.5 volts versus Li / Li. +embedded in the microcrystalline graphite. Thus, after the EIS, the potential drop in the EIS anode requires less of a reduction in the EIS potential to maintain the same charging current as the standard anode in a comparable formation stage. Since the electrochemical reaction remains unchanged, the smaller potential drop is caused by a lower ohmic resistance. The earlier attainment of the final potential during lithiation is a logical consequence of this. A higher capacity can be drawn from the EIS anode, and this can occur in a shorter time, which underscores the cell's higher performance.

[0038] In the following, a preferred embodiment of the invention is described with reference to the accompanying drawings. Further details, preferred embodiments, and developments of the invention will emerge from these. In detail, schematically Fig. 1 Formation profile according to the invention: potential and current profile of the anodic half-cell Fig. 2 Potential profile during delithiation of two differently formed anodes

[0039] A graphite electrode is used as the negative electrode. The Li / Li electrode serves as the reference electrode. + -Electrode.

[0040] It shows the Fig. 1 shows an embodiment of the forming process according to the invention. It shows the potential of the graphite electrode against the Li / Li + The graph is shown as a function of the reference electrode and plotted against time on the x-axis (dashed on the left high-value axis). Additionally, the current is plotted as a solid line on the right high-value axis, with a negative current indicating a charging current, i.e., the formation or deposition of lithium.

[0041] According to this embodiment, the forming process begins with charging at a constant current (CC) of 0.1C, where 1C corresponds to the one-hour current of the half-cell. The termination criterion is a drop in voltage to 780 mV. Potentiostatic charging (CV) then begins at this potential until the current drops to 0.02C. This is followed by a 60-minute rest period, during which the electrode's open-circuit potential (OCP) rises to an equilibrium value that is not fully reached during the 60 minutes.

[0042] The electrode is then excited for a period of 4400 seconds with an alternating voltage excitation ("EIS, electrochemical impedance spectrum) with an amplitude of 10 mV around the open-circuit voltage. Fifty-five frequencies, each distributed logarithmically in the frequency band from 500 kHz to 1 Hz, are applied for 1.6 seconds each. Such a sweep, lasting 88 seconds, is repeated 50 times.

[0043] This is followed by another galvanostatic charge ("CC") at 0.1C until the decreasing potential reaches 20 mV. Finally, potentiostatic charging at 20 mV is performed for 60 minutes. All steps refer to room temperature conditions.

[0044] It shows Fig.2 shows the discharge curve of a negative electrode formed according to this embodiment (dashed line) versus the specified weight-specific discharge quantity. The potential curve of a negative electrode formed according to the prior art is shown in solid lines. It can be seen that the potential versus the Li / Li + -Reference electrode for a negative electrode formed according to this exemplary embodiment rises less rapidly at the same discharge rate than for the negative electrode formed according to the prior art. When such an electrode (anode) is used in a lithium-ion battery, a larger potential difference to the positive electrode (cathode) is maintained against the discharge rate. Consequently, a higher electrical discharge power is achieved. Due to the lower voltage losses, the amount of charge drawn and the energy efficiency are higher.

Claims

[1] A method for forming a negative electrode for a lithium-ion cell, comprising the steps of - first constant current charging with a first charging current until a first half-cell potential is reached against a reference electrode, - first constant voltage charging at the first half-cell potential against the reference electrode until a second charging current is reached, - AC voltage excitation or AC current excitation over a frequency period, - second constant current charging with a third charging current until a second half-cell potential is reached against the reference electrode, - second constant voltage charging at the second half-cell potential against the reference electrode until a final charging current is reached or until a maximum constant voltage charging time is reached. [2] A method according to claim 1, wherein a rest voltage phase occurs over a relaxation period after the first constant voltage charging and before the AC voltage excitation or AC current excitation. [3] Method according to claim 1 or 2, wherein - the first charging current is in a range of one hundredth to twice the one-hour current. [4] Method according to one of the preceding claims, wherein - the first half-cell potential in a range of 500 mV to 1200 mV against the Li / Li + -reference electrode. [5] Method according to one of the preceding claims, wherein - the second charging current is smaller than the first charging current. [6] Method according to one of the preceding claims, wherein - the relaxation period is at least one minute. [7] Method according to one of the preceding claims, wherein characterized by , that - the alternating voltage excitation or alternating current excitation occurs at a constant frequency. [8] Method according to claim 7, characterized by , that - the constant frequency is in a range from 0.001 kHz to 1000 kHz. [9] Method according to one of claims 1 to 6, characterized by , that - the AC voltage excitation or AC current excitation consists of a successive excitation with at least two frequencies or of an excitation by a frequency sweep. [10] Method according to claim 8, characterized by , that - each frequency of the at least two frequencies or the frequency sweep lies in a range of 0.001 kHz to 1000 kHz. [11] Method according to one of claims 7 to 10, characterized by , that - the frequency period is in a range from 0.01 seconds to 120 minutes. [12] Method according to one of the preceding claims, wherein - the third charging current is in a range from one hundredth to twice the one-hour current. [13] Method according to one of the preceding claims, wherein - the second half-cell potential in a range of 5 mV to 300 mV against the Li / Li + -reference electrode. [14] Method according to one of the preceding claims, wherein - the final charging current is less than the third charging current. [15] Method according to one of the preceding claims, wherein - the maximum constant voltage charging time is at least one minute.

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