Optical method and use of an optical method for determining the degree of lithiation of Li-ion batteries

Abstract

Optical method for determining the degree of lithiation of electrode layers (1,8) for Li-ion batteries, characterized in that the method comprises at least the following steps: a) Providing at least two electrode layers (1, 8) with different known degrees of lithiation, wherein each of the electrode layers (1, 8) comprises at least one silicon-containing and one further active material, wherein the weight fraction of the further active material is greater than or equal to 5 wt.% and less than or equal to 85 wt.% and the weight fraction of the silicon-containing active material is greater than or equal to 5 wt.% and less than or equal to 85 wt.% based on the total weight of the electrode layer (1, 8); b) Irradiating the electrode surface with light of one or more wavelengths, wherein the wavelength of the incident light is greater than or equal to 300 nm and less than or equal to 1000 nm; c) Determining the light intensity reflected from a surface of the electrode layer (1,8) as a function of wavelength; d) Correlate the reflected wavelength-dependent light intensity determined in step c) and the known degree of lithiation of the electrode layers (1,8); e) Determining an unknown degree of lithiation of an electrode layer (1,8) with a composition of the active material corresponding to the electrode layers (1,8) provided in process step a) by means of a reflection measurement according to process steps b) and c) and on the basis of the correlation obtained in step d).

Description

[0001] The present invention relates to an optical method for determining the degree of lithiation of electrode layers for Li-ion batteries, the method comprising at least the steps: a) providing at least two electrode layers each comprising a mixture of at least two active materials with different, known degrees of lithiation; b) irradiating the electrode surface with light of one or more wavelengths, wherein the wavelength of the irradiated light is greater than or equal to 300 nm and less than or equal to 1000 nm; c) determining the light intensity reflected from the surface of the electrode layer as a function of wavelength; d) correlating the reflected, wavelength-dependent light intensity determined in step c) and the known degree of lithiation of the electrode layers; and e) Determining an unknown degree of lithiation of an electrode layer with an active material composition corresponding to the electrode layers provided in process step a) via a reflection measurement according to process steps b) and c) and based on the correlation obtained in step d). Furthermore, the present invention relates to the use of the method for the in-line determination of degrees of lithiation and a system for determining these degrees.

[0002] The success of innovative electrical and electronic consumer goods is often closely linked to their application properties, with users particularly focusing on reliable and long-term self-sufficient 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. Lithium-ion-based cells are the most promising energy sources, as they can currently provide the highest energy densities commercially available.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.

[0003] In conventional lithium-ion battery production processes, the electrochemically active lithium is introduced into the cell exclusively via the positive electrode using lithiated metal oxides such as lithium cobalt oxide. However, to further increase service life and energy density, various negative electrodes are now also lithiated before cell assembly. This process is referred to as "pre-lithiation." Electrochemical or chemical-analytical methods are used to determine the amount of lithium stored in electrodes. In electrochemical determination, punched-out samples of the electrodes are installed in small electrochemical cells, and the amount of previously deposited lithium is determined using current and voltage measurements. Chemical-analytical determination can be performed using a variety of classic quantitative analytical methods.ICP (inductively coupled plasma) measurements are used as standard, for example. However, the electrochemical and chemical-analytical methods have in common that, due to the equipment required, they cannot be performed online, in-line, or at-line in large-scale industrial production. Consequently, to monitor industrial manufacturing processes for lithiated electrode materials, individual samples must always be taken from the electrode rolls, which are then separately analyzed for the amount of lithium incorporated. Direct process control in fully continuous production cannot be provided in this way, and furthermore, these complex individual measurements do not provide any statistically reliable information about possible fluctuations in the amount of lithium as a function of time or as a function of location in the electrode roll.

[0004] The patent literature also contains a wide variety of approaches to determining the degree of lithiation of Li electrodes.

[0005] For example, DE 102018 103 310 A1 describes a vehicle comprising: an electric motor; a battery for storing electrical energy for the electric motor; and a surface acoustic wave sensor for wirelessly detecting a magnetic field of the battery and the battery temperature, wherein the sensor includes a reactive magnetic field device for changing a surface wave in response to the magnetic field depending on the lithiation of an electrode of the battery.

[0006] Furthermore, DE 10 2015 117 816 A1 discloses a method for determining an amount of lithium in a lithium-ion battery electrode sample, the method comprising: determining powder X-ray peaks of the lithium-ion battery electrode sample; and comparing the powder X-ray peaks of the lithium-ion battery electrode sample with a set of lithium-containing samples with predetermined lithium concentrations to determine the amount of lithium in the lithium-ion battery electrode sample.

[0007] DE 10 2019 007 509 A1 discloses a method for determining a state of charge of an electrical energy storage device of an at least partially electrically operated motor vehicle by means of an electronic computing device, in which a plurality of open circuit voltage curves of the electrical energy storage device are generated by means of a stored state of charge model for the state of charge and the plurality of open circuit voltage curves are evaluated to determine the state of charge, wherein a current anode overhang effect of an anode of the electrical energy storage device is determined by means of the electronic computing device and the current anode overhang effect is taken into account in determining the state of charge by the stored state of charge model.

[0008] Further methods for investigating galvanic cells are described, for example, in DE 10 2009 018 079 A1, DE 10 2013 217 039 A1, DE 10 2019 005 317 A1, US 2016 / 0 322 677 A1, JP 2000- 131 405 A, JP 2011-100 565 A, US 2019 / 0 181 440 A1, US 2021 / 0 159 500 A1, Yue Feng et al: Lithiation of pure and methylated amorphous silicon: Monitoring by operando optical microscopy and ex situ atomic force microscopy. Electrochimica Acta, 2019, 302, pp.249-258 and Shellikeri A. et al.: Investigation of Pre-lithiation in Graphite and Hard-Carbon Anodes Using Different Lithium Source Structures, In: Journal of The Electrochemical Society, Vol. 164, 2017 A3914-A3924.

[0009] Such state-of-the-art solutions generally relate to electrodes already installed in battery cells and may offer further potential for improvement. This particularly applies to the applicability of methods for determining the degree of lithiation in industrial settings, as well as the flexibility and reproducibility of the measurements.

[0010] It is therefore the object of the present invention to at least partially overcome the disadvantages known from the prior art and to expand the application possibilities to include electrodes that are not incorporated into battery cells. In particular, the object of the present invention is to provide an optical method that can be used continuously to determine the lithium content of large electrode surfaces at the site of electrode material production. Furthermore, the object of the present invention is to provide a sensitive method that can quantitatively and very accurately determine the amounts of lithium incorporated in electrodes.

[0011] The problem is solved by the features of the independent claims, directed to the method according to the invention, the system according to the invention, and the use of the method according to the invention. 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, an optical method for determining the degree of lithiation of electrode layers for Li-ion batteries is according to the invention, wherein the method comprises at least the steps: a) providing at least two electrode layers with different, known degrees of lithiation, wherein each of the electrode layers comprises at least one silicon-containing and one further active material, wherein the weight fraction of the further active material is greater than or equal to 5 wt.% and less than or equal to 85 wt.% and the weight fraction of the silicon-containing active material is greater than or equal to 5 wt.% and less than or equal to 85 wt.% based on the total weight of the electrode layer; b) irradiating the electrode surface with light of one or more wavelengths, wherein the wavelength of the irradiated light is greater than or equal to 300 nm and less than or equal to 1000 nm; c) determining the light intensity reflected from the surface of the electrode layer as a function of wavelength; d) correlating the reflected, wavelength-dependent light intensity determined in step c) and the known degree of lithiation of the electrode layers; e) Determining an unknown degree of lithiation of an electrode layer having a composition of the active material corresponding to the electrode layers provided in process step a) via a reflection measurement according to process steps b) and c) and based on the correlation obtained in step d).

[0013] Surprisingly, it was found that the amount of lithium ions stored in electrode layers can be determined very quickly and accurately using the optical method described above. This is surprising given that the prior art typically assumes that purely optical methods for determining the amount of lithium stored in electrode active material only lead to unsatisfactory results. Determining the amount of lithium by calibrating known samples with the same electrode material is highly accurate, and the measurement method, by determining the amount using a reflectance measurement, also enables the method to be used in an industrial environment in the production of pre-lithiated electrode materials. Thus, the method according to the invention can be used for non-destructive at-line, online, or in-line monitoring of the production process on any sample size.No special environmental conditions are required to determine the amount of lithium in the electrodes, meaning that there is no need for a protective gas atmosphere or for temperature control of the electrode layers for Li determination. The sample sizes can be determined very easily using the size of the optical measuring points, with the degree of lithiation then being obtained as an average over the sample size in question. If very precise information as a function of location is required, small measuring points can be selected. If a more statistically averaged statement over a larger electrode area is desired, larger optical measuring ranges can be selected. What is particularly surprising is that the optical method according to the invention can also be used to examine electrode layers with a relatively complex chemical structure.It has been shown that the method is particularly suitable for electrode layers which have at least two different active materials, i.e. materials in which lithium ions are actively incorporated and released. In contrast to the presence of only a single active material in the electrode layer, the presence of two different active materials complicates the optical reflection properties of the electrode layer and it was therefore not expected that a relatively complex structure with different active materials could be detected using a "simple" reflection measurement. It has been shown that the total lithium ions incorporated in at least silicon and one other active material can be quantitatively detected using the reflection measurements. Quantitative detection is also possible in cases where the electrode layer contains other components, such as a binder, an electrolyte or a solvent.These additional components can be quantitatively calculated using appropriate calibration. Another advantage is that the specified method can also yield exponential dependencies of light reflection as a function of the degree of lithiation, which, mathematically speaking, can contribute to a better and more precise determination of the degree of lithiation compared to purely linear dependencies. Overall, this results in a very precise method that is easily adaptable to different sample sizes and is particularly suitable for electrode layers with mixed Si / X active materials.

[0014] The method according to the invention is a method for determining the degree of lithiation of electrode layers for lithium-ion batteries. In lithium-ion batteries, the electrical work is performed via the transport of lithium ions between the two electrodes and the incorporation or removal of the lithium ions into the electrode layers themselves. The quantitative amount of lithium incorporated in an electrode layer therefore determines the electrical properties of the electrode. The degree of lithiation can advantageously be specified as a fraction of the maximum possible amount of incorporation into the electrode layer, whereby mathematically speaking, this lithium proportion corresponds to the degree of lithiation. The maximum possible degree of incorporation can be determined, for example, using electrical methods as described later. The electrode layers therefore necessarily comprise (active) materials which enable the reversible incorporation of lithium ions orsuitable for mixed metal formation with lithium. According to the invention, the electrode layers comprise a mixture of at least one silicon-containing and one further active material. The silicon can be present as such or in the form of chemical compounds, such as silicon suboxide (SiO). x) can be used. A method which is directed to measuring the degree of lithiation of electrode layers which only have an active material for the storage of lithium is not according to the invention. In addition to the active materials, the electrode layers can of course also have other conventional components which themselves are not actively capable of the reversible storage of lithium. These other materials can include, for example, binders which contribute to the physical cohesion of the active materials but not to the absorption of lithium. The method according to the invention is particularly suitable for determining the Li content of pre-lithiated electrode layers which are able to provide a voltage and a current in batteries without a prior charging cycle.

[0015] In process step a), at least two electrode layers with different, known degrees of lithiation are provided, each of the electrode layers comprising at least one silicon-containing and one further active material. To calibrate the optical method, samples with a known degree of lithiation are required, whereby the number of samples can be freely determined. For a linear dependency, at least two different samples must be provided. If dependencies other than linear are expected, at least three different samples should be provided. The influence of the number of calibration samples on the statistics of the expected results is known to the person skilled in the art. The electrode layers used for calibration in process step a) have, in addition to a different amount of lithium incorporated, the same chemical composition of the electrode layers and, in particular, the same composition of the active material.The calibration samples considered here comprise at least two different active materials, with one of the active materials in the electrode layer comprising a silicon element and the other comprising an element chemically different from Si. In contrast to the other components of the electrode layer, the active materials are capable of reversibly incorporating and removing lithium ions. Silicon, for example, or an active material containing silicon (di)oxide can be used as the silicon-containing active material. Other usable silicon-based active materials capable of incorporating lithium or forming a mixed metal with lithium are known to those skilled in the art. Coated silicon materials, for example, can also be used.The additional active materials can be selected, for example, from the group consisting of carbon (in a wide variety of modifications) or metals or their compounds such as Al, Sb, Sn, Bi, TiO2, mixed metal compounds (alloys) consisting of e.g. Si / FeO or mixtures of at least two materials from this list. The electrode layers of the different samples have the same composition if, apart from the amount of lithium used, the weight proportions of the individual components of the electrode layer differ by a maximum of + / - 5 wt. %, further preferably by a maximum of + / - 2.5 wt. %, further preferably by a maximum of + / - 1 wt. %. For calibration purposes, the degree of lithiation of the different samples can preferably differ by greater than or equal to five percent and less than or equal to 50 percent.

[0016] The weight fraction of the further active material in the electrode layer is greater than or equal to 5 wt.% and less than or equal to 85 wt.% and the silicon weight fraction is greater than or equal to 5 wt.% and less than or equal to 85 wt.% based on the total weight of the electrode layer. It has surprisingly been found that, by means of optical reflection measurements, electrode layers in particular can be measured which each have a significant proportion of silicon and a further active material. Smaller proportions of one of the two active materials are not according to the invention. The weight fractions of the individual active materials are stated for the respective compound used. If, for example, silicon dioxide is selected as the silicon-containing active material, the weight fraction stated above applies to the entire silicon dioxide compound and not just to the silicon contained therein as such.If only two active materials are used, the weight proportions add up to 100 percent of the total active material used. However, in this case, the weight proportions are based on the total weight of the electrode layer. Therefore, the total weight of the layer also includes the other materials in the layer that are not actively capable of reversibly incorporating lithium ions or forming a mixed metal with lithium. Based on this information, the silicon content and the proportion of the other active material do not necessarily add up to 100 percent.

[0017] In process step b), the electrode surface is irradiated with light of one or more wavelengths, whereby the wavelength of the irradiated light is greater than or equal to 300 nm and less than or equal to 1000 nm. To calibrate the samples provided in process step a), the electrode surface is irradiated with light that contains at least a portion of a previously defined wavelength. To determine the reflected intensity, monochromatic light of only one wavelength or polychromatic light of two or more wavelengths can be used. It is also possible to use a larger wavelength range, which extends over several wavelengths, for example 20 nm or 50 nm, to determine the degree of lithiation. The area over which the irradiated light falls on the electrode layer can expediently be selected as a function of the total electrode layer size.For example, in industrial production the area can be at least 1 cm. 2 , furthermore preferably 1.5 cm 2 and still preferably 5 cm 2 However, if the degree of lithiation of a smaller electrode layer is to be determined, the area can be, for example, 0.1 cm 2 or for example 0.25 cm 2 Within this area, sufficient statistics can be achieved to determine the degree of lithiation of the electrode layer.

[0018] In process step c), the light intensity reflected by the surface of the electrode layer is determined as a function of wavelength. The light irradiated in process step b) interacts with the surface components of the electrode layer and is either absorbed or reflected by them, particularly by the lithium. In this step, only the portion of the irradiated light of one or more defined wavelengths reflected by the electrode layer is detected. The measurement of the reflected light component can, in principle, be performed at different angles, as long as the angular relationship – direction of light incidence – detection of the reflected light – remains constant for the calibrated samples and the samples to be measured later.If only one wavelength is used to determine the degree of lithiation, the reflected light intensity is determined by dividing the reflected intensity by the incident intensity of that wavelength. If two or more wavelength ranges are used to determine the degree of lithiation, the mathematically corresponding ratios are determined by the integral over the wavelengths considered. To determine the reflected light intensity, it is also possible to perform a noise correction for both the incident and reflected light. The latter can contribute to more reproducible measurement results, especially when measuring in a non-dedicated laboratory environment.

[0019] In process step d), the reflected, wavelength-dependent light intensity determined in step c) is correlated with the known degree of lithiation of the electrode layers. Based on the provided calibration samples and the determination of the wavelength-dependent light reflectance in specified wavelength ranges, and knowing the degree of lithiation of the samples, a correlation between the reflected light quantity and the degree of lithiation for the provided electrode layers can be calculated. The correlation can be based on various mathematical models, such as linear regression. However, it is also possible to use fitting models other than linear ones to determine the degree of lithiation for the evaluation. If the measurement data permit, exponential fittings, for example, can also be used to calculate the degree of lithiation.

[0020] In process step e), an unknown degree of lithiation of an electrode layer with an active material composition corresponding to the electrode layers provided in process step a) is determined using a reflectance measurement according to process steps b) and c) and based on the correlation obtained in step d). After establishing a suitable calibration using the calibration samples, measurements on samples with an unknown degree of lithiation but otherwise identical electrode layer composition can also be evaluated using the mathematical model created in step d). For this purpose, the unknown samples are subjected to the corresponding measurement conditions of the calibration samples. The measurement of the unknown samples does not have to be carried out in any temporal or spatial connection with the measurement of the calibration samples.

[0021] In a further embodiment of the process, the additional active material can be a carbon-containing active material. The process according to the invention has proven particularly suitable for the combination of silicon and carbon. The degrees of lithiation can be determined extremely precisely and over a wide range of active material mixing ratios. Carbon-containing active materials suitable for the reversible incorporation and removal of lithium ions include, for example, carbon / graphite, carbon nanotubes, graphene, carbon fullerenes, etc., or mixtures thereof.

[0022] In a preferred embodiment of the method, in step b), light of one or more wavelengths selected from a range of greater than or equal to 300 nm and less than or equal to 400 nm or a range of greater than or equal to 850 nm and less than or equal to 1000 nm or a combination of both ranges can be irradiated. The degree of lithiation for electrode layers with two different active materials can be determined in particular via a reflection measurement with light from the wavelength ranges specified above. It has been shown that particularly low-noise results are obtained within these wavelength ranges, which can ultimately contribute to a more reproducible and significantly more accurate determination of the degree of lithiation of the electrode layers.For reflection measurement, for example, a wavelength from one of these two ranges, a wavelength from each of the two different ranges, or even wavelength ranges comprising several wavelengths from one or both ranges can be used. If multiple wavelengths are used to determine the reflection properties, the integral of the incident light quantity and the integral of the reflected light quantity are expediently used to determine the degree of lithiation.

[0023] Within a further preferred embodiment of the method, in step b), monochromatic light of a wavelength selected from a range of greater than or equal to 340 nm and less than or equal to 360 nm or selected from a range of greater than or equal to 840 nm and less than or equal to 860 nm can be irradiated. For a particularly accurate and reproducible determination of the degree of lithiation, it has proven particularly suitable to use only one wavelength from the above-mentioned wavelength ranges. The measurements are highly reproducible, low-noise, and suitable for a wide variety of electrode layer compositions. Furthermore, within these ranges, there is the advantage that an exponential dependence of the reflected light intensity on the degree of lithiation is obtained.Due to the exponential dependence, very large changes in reflected light intensity result from relatively small changes in the degree of lithiation. This can help determine the degree of lithiation with particular precision.

[0024] Within a further preferred aspect of the method, the carbon weight fraction 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 fraction 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. It has been shown that the method according to the invention can be used, in particular, to measure electrode layers that comprise two different active materials, and in particular silicon and carbon in significant weight fractions. By using two different active materials with different optical properties, one would actually assume that a reflection measurement would lead to inaccurate or non-reproducible values. Surprisingly, this is not the case.In particular, for the active materials specified above in the concentration ranges specified above, the degrees of lithiation can be determined very precisely.

[0025] According to a preferred characteristic of the method, in method step c), the reflected light intensity can be determined at an angle of greater than or equal to 160° and less than or equal to 200° relative to the angle of incidence of the light irradiated in method step b). For precise and low-noise determination of the reflected intensity, it has proven particularly suitable to measure the light intensity reflected from the surface of the electrode material approximately antiparallel to the incident light intensity. This measuring arrangement can lead to significantly better results than arrangements in which the reflected light intensity is recorded at a shallower angle to the surface of the electrode material. The number of degrees is specified in relation to the incident light beam.If the angle is, for example, 180°, the reflected light intensity is measured, for example via semi-transparent mirror arrangements, along the incident light beam, but in the opposite direction away from the surface of the electrode material.

[0026] In a further preferred embodiment of the method, the degree of lithiation of the active material can be greater than or equal to 5% and less than or equal to 35%. The method according to the invention is particularly suitable for quantifying small to medium lithium loadings of the electron material. In this range, the degree of lithiation of the electrode material can be determined particularly accurately and reproducibly. The degree of lithiation of the active material is specified in relation to the maximum loading of the electrode layer. The maximum loading can be calculated from the maximum loadings of the pure active materials and the percentage composition in the electrode layer. The maximum lithium loading of the active materials can also be obtained from the literature or the manufacturer's specifications.One possible direct experimental determination method involves the use of battery cell arrays with a lithium metal electrode and the corresponding electrode by integrating the current over time. The electrode is charged 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.

[0027] Within a preferred aspect of the method, the electrode layer can have a porosity of greater than or equal to 10% and less than or equal to 50%. Surprisingly, it has been shown that even porous electrode layers can be measured using optical reflectance measurement. This is unexpected because it must be assumed that the porosity of the electrode material also influences the obtainable reflectivity. The pores in the electrode layer can lead to a higher proportion of scattered light, which should hinder an accurate and reproducible determination of the degree of lithiation. However, this is surprisingly not the case, and even with a relatively high pore content, the degrees of lithiation of the electrode layers can be determined very precisely.The pore content of the electrode layers can be determined, for example, using mercury porosimetry or theoretically based on the volume, weight and densities of the individual components of the electrode layer.

[0028] Furthermore, the invention relates to the use of the method according to the invention for the in-line determination of the degree of lithiation of an electrode layer for a Li-ion battery in a roll-to-roll process. The advantages of the method according to the invention arise in particular in its application as process control in large-scale manufacturing processes. The method can therefore be directly coupled to production, allowing continuous quality control of the degree of lithiation of the electrode layers. This is significantly more advantageous than the methods known in the prior art, which are based on separating individual samples and then examining them in a laboratory environment. Continuous monitoring of the process can therefore be carried out in-line and directly at the production site.By selecting the number of measuring points and the respective measured area, it is possible to measure either individual points or the entire manufactured electrode material without significant equipment expenditure. This approach also provides additional information regarding the uniformity of the electrode layers during production. It is best used on individual electrode sheets or in a roll-to-roll process, which means that the electrode layers are supplied as rolls, and the rolls are unwound and rewound during this process.

[0029] Furthermore, the invention relates to a system for the in-line determination of the degree of lithiation of an electrode layer for a Li-ion battery, wherein the system comprises at least one lithiation unit for an electrode layer present as a roll, a processing unit for processing the electrode layer and a computing unit, characterized in that one or more light sources and one or more light detectors are arranged before or after the processing unit, wherein the light sources are configured to emit monochromatic light in a wavelength range of greater than or equal to 240 nm and less than or equal to 360 nm or of greater than or equal to 840 nm and less than or equal to 860 nm and the light detectors are configured to measure light intensities in reflection. The system according to the invention allows the degree of lithiation of electrode layers to be determined simply and precisely under production conditions.The system is therefore capable of ensuring continuous process control of the pre-lithiation of electrode layers. The system also makes it very easy to configure whether individual points on the electrode layers are only sporadically checked or the entire roll material is continuously measured for the degree of lithiation. The system according to the invention can therefore also provide information about the range of fluctuation within the lithiation process. This process control is not possible with state-of-the-art methods, as the samples are destroyed or mechanically altered during these processes. Furthermore, the processes are so complex that processing a correspondingly large number of samples is not economically feasible.

[0030] The lithiation unit can be configured to perform an electrochemical, mechanical, physical vapor deposition (PVD), or chemical vapor deposition (CVD) process for lithium. Furthermore, the lithiation unit can also be a continuous or discontinuous chemical reactor in which lithium is chemically reacted with the electrode in solution.

[0031] The assembly unit can, for example, be a mechanical or laser-assisted cutting or punching device that divides the rolled material into smaller units in order to prepare the electrodes for installation in battery cells.

[0032] Suitable light sources for the system can be selected, for example, from the group consisting of monochromatic or polychromatic light sources such as mercury vapor lamps, lasers, halogen lamps, IR emitters, deuterium-tungsten lamps, or combinations of at least two light sources from this group. A deuterium-tungsten source can be conveniently selected as the light source, whose emitted wavelength can, for example, be in the range of 210–2500 nm. The emitted light from the light source can be directed to the corresponding surface areas of the electrode layer, for example, via optical fibers or a purely optical beam path with lenses and beam splitters. The measurement of the reflected light can also be conducted, for example, via optical fibers or, optionally, via the same lens and beam splitter system as the incident light beam.

[0033] The reflected light can then be directed, for example, to a light detector in the form of a spectrometer. The spectrometer determines the wavelength-dependent (reflected) light intensity and can correlate this with the incident light intensity either as such or using a computing unit. Other suitable light detectors for wavelength-specific detection of light intensities are known to those skilled in the art.

[0034] Further advantages and advantageous embodiments of the inventive objects are illustrated by the drawings and explained in the following description. It should be noted that the drawings are for descriptive purposes only and are not intended to limit the invention.

[0035] They show: Fig. 1 schematically shows the measurement setup for the optical determination of the degree of lithiation of electrode layers; Fig. 2 the dependence of the reflected light intensity as a function of wavelength for a 0 wt.% pre-lithiated electrode layer in a multiple measurement; Fig. 3 the dependence of the reflected light intensity as a function of wavelength and as a function of the degree of lithiation; Fig. 4 the dependence of the reflected light intensity at 350 and 850 nm as a function of the degree of lithiation; Fig. 5 the dependence of the reflected light intensity as a function of the degree of lithiation in the measurement of different surface points of the same sample; Fig. 6 a schematic structure of a system for in-line determination of the degree of lithiation of roll-to-roll electrode layers. Production of the electrode layers

[0036] The electrode layers measured using the methods according to the invention were produced as follows: A mixer is used to produce a homogeneous powder mixture of graphite, silicon, carbon black, and the 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, dried onto the metallic conductor, is then calendered to a target porosity of approximately 30%.

[0037] 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.%, 2% conductivity additive.

[0038] The physical properties of the manufactured anode 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 amount 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.

[0039] For the controlled lithiation of the calibration samples, 1.0 M LiPF6 in ethylene carbonate:ethyl methyl carbonate EC:EMC (3:7 wt%) with a 10 wt% addition of fluoroethylene carbonate (FEC) is used. The differently 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 for different times. 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 electrical target capacities (10% lithiation degree - 0.76 mAh capacity, 20% lithiation degree - 1.52 mAh capacity, etc.), the anodes are removed from the button cells, washed with EMC for 5 minutes, and then the reflectance measurements are performed on the dried, lithiated electrode layers.

[0040] The Fig. Figure 1 schematically shows the measurement setup for the optical determination of the degree of lithiation of electrode layers 1. The electrode layer 1 is irradiated with light of a specific wavelength range using a light source 2. The light is focused from the light source 2 onto the sample via one or more beam splitters 4 and one or more light-guiding fibers 5 or lenses 6. The light reflected from the sample is also guided via one or more light-guiding fibers 5 or lenses 6 via the beam splitters 4 to a spectrometer 3, which measures the intensity of the reflected radiation as a function of wavelength and relates it to the intensity of the incident radiation of this wavelength. The latter can be done within the spectrometer 3 or via a separate computing unit (not shown). The beam path of the reflected radiation can, but does not have to, fully or partially coincide with the beam path of the incident light.

[0041] The Fig. Figure 2 shows the wavelength-dependent reflection spectrum of a 0 wt.% pre-lithiated electrode layer in a 25-fold superposition of individual measurements. The individual measurements were performed with an integration time of 26 ms, and each measurement represents an average of 60 scans. The measurements were performed in an environment unconditioned except for the humidity of the air. This diagram shows that the wavelength ranges between 300-400 nm and 850-1000 nm, in particular, exhibit a very low noise component. These ranges are suitable for particularly precise and low-noise determination of the degree of lithiation.

[0042] The Fig. Figure 3 shows the dependence of reflected light intensity as a function of wavelength and as a function of the degree of lithiation. The top line with the highest reflected intensity corresponds to the light reflection of the non-lithiated electrode layer and serves as a reference. The reflectivities of the pre-lithiated electrode layers are lower, with generally lower reflectivities being obtained with increasing degree of lithiation. The dotted line corresponds to a degree of lithiation of 10%, and the bottom solid line corresponds to a degree of lithiation of 50%. The lines between the 10% and 50% lines correspond to the degrees of lithiation of 20%, 30%, and 40%.

[0043] The Fig. Figure 4 shows the dependence of the reflected light intensity at 350 and 850 nm as a function of the degree of lithiation. The lower line (squares) results from the measurement at a wavelength of 350 nm, and the upper line (dots) results from the measurement at a wavelength of 850 nm. The further line (crosses) represents the open cell voltage (OCV) as a function of the degree of lithiation. It is clearly visible that the reflectivity is a function of the degree of lithiation, and an exponential dependence of the measured reflected light intensity on the degree of lithiation is obtained at the wavelengths specified here. This exponential dependence means that the degree of lithiation can be determined very precisely using the method according to the invention.

[0044] The Fig. Figure 5 shows the dependence of reflected light intensity as a function of the degree of lithiation when measuring different surface points on the same sample. From the wavelength-dependent determination of the reflected light intensity of different surface areas of samples with different degrees of lithiation, it can be seen that a highly reproducible measurement is possible in the specified wavelength range. In particular, the signals from the lithiated samples are very stable and uniform.

[0045] The Fig.Figure 6 shows a schematic structure of a system for in-line determination of the degree of lithiation of electrode layers processed using a roll-to-roll process. In this case, the electrode layers or entire electrode assemblies 8 are present as strips and are pre-lithiated within a process step not shown. The pre-lithiated strip 8 can be guided over the roller pairs 9 as such or, if appropriate, after a drying step (not shown). One or more optical detectors 13 can be arranged above or below the roller conveyor of the electrode layer 8, which direct an incident light beam onto a specific surface area 12 of the electrode layer 8. An arrangement of the detectors 13 on both sides of the electrode layer 8 can be used for differential measurements, which provides a further parameter for quality control.The incident light 12 is reflected by the electrode surface and passes via the detector 13 and, if necessary, via a fiber optic cable 11 to a spectrometer 10 with an evaluation unit. In this embodiment, the incident light beam can also be guided via this beam path onto the electrode layer 8. Efficient process control for the lithiation step can be established from the intensity ratio of incident to reflected light. Advantageously, the measurements can be performed continuously and on any desired surface area of ​​the electrode layer. This not only provides a snapshot from a single measurement, as with prior art methods, but also enables continuous process monitoring.

Claims

[1] Optical method for determining the degree of lithiation of electrode layers (1,8) for Li-ion batteries, characterized by that the procedure comprises at least the following steps: a) providing at least two electrode layers (1, 8) with different, known degrees of lithiation, wherein each of the electrode layers (1, 8) comprises at least one silicon-containing and one further active material, wherein the weight fraction of the further active material is greater than or equal to 5 wt.% and less than or equal to 85 wt.% and the weight fraction of the silicon-containing active material is greater than or equal to 5 wt.% and less than or equal to 85 wt.% based on the total weight of the electrode layer (1, 8); b) irradiating the electrode surface with light of one or more wavelengths, wherein the wavelength of the irradiated light is greater than or equal to 300 nm and less than or equal to 1000 nm; c) determining the light intensity reflected from a surface of the electrode layer (1,8) as a function of wavelength; d) correlating the reflected, wavelength-dependent light intensity determined in step c) and the known degree of lithiation of the electrode layers (1,8); e) determining an unknown degree of lithiation of an electrode layer (1,8) having a composition of the active material corresponding to the electrode layers (1,8) provided in process step a) via a reflection measurement according to process steps b) and c) and based on the correlation obtained in step d). [2] The method according to claim 1, wherein the further active material is a carbon-containing active material. [3] Method according to claim 1 or 2, wherein in step b) light of one or more wavelengths selected from a range of greater than or equal to 300 nm and less than or equal to 400 nm or a range of greater than or equal to 850 nm and less than or equal to 1000 nm or a combination of both ranges is irradiated. [4] Method according to one of the preceding claims, wherein in step b) monochromatic light of a wavelength selected from a range of greater than or equal to 340 nm and less than or equal to 360 nm or selected from a range of greater than or equal to 840 nm and less than or equal to 860 nm is irradiated. [5] Method according to claim 2, wherein the carbon weight fraction of the electrode layer (1,8) is greater than or equal to 30 wt.% and less than or equal to 60 wt.% and the silicon weight fraction of the electrode layer (1,8) is greater than or equal to 10 wt.% and less than or equal to 60 wt.% based on the total weight of the electrode layer (1,8). [6] Method according to one of the preceding claims, wherein in method step c) the reflected light intensity is determined at an angle of greater than or equal to 160° and less than or equal to 200° relative to the direction of incidence of the light irradiated in method step b). [7] Method according to one of the preceding claims, wherein the degree of lithiation of the active material is greater than or equal to 5% and less than or equal to 35%. [8] Method according to one of the preceding claims, wherein the electrode layer (1,8) has a porosity of greater than or equal to 10% and less than or equal to 50%. [9] Use of a method according to one of claims 1 to 8 for in-line determination of the degree of lithiation of an electrode layer (1, 8) for a Li-ion battery in a roll-to-roll process. [10] System for in-line determination of the degree of lithiation of an electrode layer (8) for a Li-ion battery, wherein the system comprises at least one lithiation unit for an electrode layer (8) present as a roll, a finishing unit for finishing the electrode layer (8) and a computing unit, characterized bythat one or more light sources and one or more light detectors (13) are arranged before or after the assembly unit, wherein the one or more light sources are configured to emit monochromatic light in a wavelength range of greater than or equal to 240 nm and less than or equal to 360 nm or of greater than or equal to 840 nm and less than or equal to 860 nm and the one or more light detectors (13) are configured to measure light intensities in reflection.

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