Method for pre-lithiating a silicon-containing negative electrode in a lithium-ion battery

Abstract

The present invention relates to a method for pre-lithiating a silicon-containing negative electrode in a lithium-ion battery, the lithium-ion battery comprising a positive electrode made of a lithium transition metal oxide, a negative electrode, a separator, and an organic electrolyte, wherein 1) during the charging process of the battery, the end voltage (U1) is between 4.35V and 4.80V, 2) during subsequent cycles of the battery, the end voltage (U2) when the battery is discharged does not drop below 3.01V, and 3) during subsequent cycles of the battery, the end voltage (U3) when the battery is charged is lower than the end voltage (U1) when the battery is charged. The present invention also relates to a lithium-ion battery that can be produced according to this method.

Description

Technical Field

[0001] The present invention relates to a method for pre-lithiating a silicon-containing negative electrode in a lithium-ion battery by applying a defined voltage during the charging process and during subsequent battery cycling, and also to a lithium-ion battery producible by this method. Background Art

[0002] Today, rechargeable lithium-ion batteries are practical electrochemical energy stores, characterized by a maximum gravimetric energy density of, for example, up to 250 Wh / kg. The most common active material for the negative electrode (anode) is graphite carbon. However, the electrochemical capacity of graphite is limited to a maximum of 372 mAh / g. Graphite-based anodes for high-energy lithium-ion batteries today have a maximum capacity of 650 mAh / cm 3 The recommended alternative negative electrode active material with higher electrochemical capacity is silicon. Silicon forms Li 4.4 A binary electrochemically active alloy of silicon, corresponding to a specific capacity of 4200 mAh per gram of silicon. Unfortunately, during the binding and extraction of lithium, silicon undergoes a volume change of up to 300%. Over the course of multiple charge and discharge cycles, this leads to a continuous, generally irreversible loss of battery capacity, also known as decay. A further problem lies in the reactivity of silicon. Therefore, when in contact with the electrolyte, a passivation layer (solid electrolyte interface; SEI) forms on the silicon surface to which lithium is immobilized, which reduces the capacity of the battery. The SEI forms during the first charge of a silicon-containing lithium-ion battery, which leads to an initial capacity loss. During the further operation of the lithium-ion battery, volume changes occur on some of the silicon particles during each charge and discharge cycle, which leads to the exposure of fresh silicon surfaces, which in turn react with components of the electrolyte and form further SEI in the process. This leads to the immobilization of further lithium and therefore to a continuous, irreversible capacity loss.

[0003] For example, EP 1 730 800 discloses anodes containing silicon particles. Typical additional components of such cathodes are binders, often graphite or conductive additives. Various methods have been described for reducing the continuous, irreversible capacity loss in lithium-ion batteries. For example, WO 2017 / 025346 proposes operating a lithium-ion battery so that, in the fully charged state of the battery, the silicon of the cathode is only partially lithiated and, therefore, the capacity of the silicon for lithium is not completely depleted. US 2005 / 0214646 charges the battery so that the lithium / silicon molar ratio in the cathode material is at most 4.0. In particular, a Li / Si ratio of 3.5 or higher is described.

[0004] Several specifications disclose the use of pre-lithiated silicon as an active negative electrode material for lithium-ion batteries. Pre-lithiation generally refers to a measure in which lithium is introduced into the active negative electrode material before the lithium-ion battery is operated, and this lithium is not extracted or at least not completely extracted from the negative electrode during the discharge of the battery. Pre-lithiation of the active silicon material can be achieved, for example, by grinding elemental lithium with silicon in a ball mill or in a melt, in which case a silicide phase can be formed, as described by Tang et al., J. Electrochem. Soc. 2013, 160, 1232-1240. DE 102013014627 describes a pre-lithiation method in which Si particles are reacted with an inorganic lithium compound (such as lithium oxide) or with an organic lithium compound (such as a lithium salt of a carboxylic acid). In US 2014212755, an inorganic lithium compound (such as an oxide, halide or sulfide) is introduced into the positive electrode. In that case, the active negative electrode material is pre-lithiated as part of the battery formation. A similar method is also described in US 10115998. DE 102015217809 describes the lithiation of active negative electrode materials by chemical vapor deposition (CVD process) using lithiated precursors (such as lithiated alkynes or lithiated aromatic hydrocarbons) and then coating them with carbon. According to WO 2018 / 112801, lithium peroxide is introduced into the positive electrode or electrolyte as a chemically reactive sacrificial salt and decomposes during battery formation, accompanied by pre-lithiation of the negative electrode. Similarly, in US 20150364795, an electrolyte comprising a lithium salt, such as lithium azide, lithium acetate, lithium-amine or lithium-acetylene, is used. Here again, pre-lithiation of the active negative electrode material occurs during battery formation. For example, formation can be completed at a voltage of 3.8 to 5 volts, more specifically 4.2 to 5 volts. WO 2016 / 089811 recommends various metals as active negative electrode materials, especially silicon alloys. Prelithiation of the active negative electrode material occurs in the half-cell with respect to lithium. US 2016141596 prelithiates the active negative electrode material by applying elemental lithium to the current collector in the form of a thin lithium foil. WO 2017 / 123443 A1 uses a stable lithium metal powder ( FMC lithium energy) pre-lithiated negative electrode. An example of SLMP is lithium metal particles coated with a lithium salt for passivation. Compressing these negative electrodes causes the passivation layer of the SLMP to rupture, allowing the lithium particles to participate in the redox process in the battery and pre-lithiate the active negative electrode material. However, SLMP is very expensive and sensitive to atmospheric humidity and is therefore incompatible with water-based processing of active negative electrode materials to electrodes. The lithium-ion battery of US 2018 / 0358616 also contains a negative electrode with pre-lithiated silicon. In US 2018 / 0358616, the battery is cycled with full utilization of the specific negative electrode capacity of the silicon-containing negative electrode. The active negative electrode material identified in that case is silicon particles with an average diameter of 30 to 500 nm. The amount of mobile lithium available for insertion and extraction processes (the sum of lithium from the positive electrode and lithium introduced by pre-lithiation) is established to be 1.1 to 2.0 times the amount of lithium in the positive electrode. The negative electrode coating of US 2018 / 0358616 contains 20 wt% silicon. However, in the case of a negative electrode with a relatively large silicon fraction, the loss of capacity occurs to an increased extent when the battery is cycled.

[0005] WO 2020 / 233799 describes a lithium-ion battery in which the negative electrode comprises pre-lithiated silicon, and the negative electrode material of the fully charged lithium-ion battery is only partially lithiated, with the overall degree of lithiation α of the silicon being 10% to 75% based on the maximum lithiation capacity of the silicon.

[0006] For all of the aforementioned pre-lithiation methods, the materials and battery components and / or manufacturing operations must be significantly modified in order to introduce additional lithium for pre-lithiation of the silicon-containing anode coating. Furthermore, the additional mass introduced is detrimental to the energy content of the lithium-ion battery.

[0007] The aim is to provide a pre-lithiation method for batteries with Si-containing anode materials, which does not require any adaptation or changes to the battery production process but at the same time enables high reversible capacity and high cycle stability. Summary of the Invention

[0008] The subject of the present invention is a method for pre-lithiating a silicon-containing negative electrode in a lithium-ion battery comprising a positive electrode of a lithium transition metal oxide, a negative electrode, a separator and an organic electrolyte, wherein

[0009] 1) The termination voltage (U1) during the battery charging process is between 4.35V and 4.80V,

[0010] 2) During subsequent battery cycling, the termination voltage (U2) during battery discharge does not drop below 3.01V, and

[0011] 3) During subsequent battery cycling, the end voltage during battery charging (U3) is lower than the end voltage during battery charging (U1). Detailed implementation mode

[0012] Surprisingly, it has been found that in a lithium-ion battery having a silicon-containing negative electrode (partially lithiated), a positive electrode containing a lithium transition metal oxide, and an organic electrolyte, a combination of high cycle stability and high discharge capacity is achieved when the termination voltage defined according to the present invention is met. If the prelithiation method of the present invention is used, that is, if the termination voltage defined according to the present invention is met, the cycle stability is improved compared to a battery of the same configuration operated according to a conventional method. This also significantly reduces attenuation and continuous loss during cycling. A particular surprise is the effect of the termination voltage (U1) during battery charging, which is increased compared to the prior art, especially compared to WO 2020 / 233799.

[0013] During the first charging process in which the termination voltage (U1) of the battery (Q L，Z1 ) increases, the charging capacity is increased compared to a battery of the same configuration but operated according to a conventional method. Since the discharge capacity during battery (Q L，Z1 ) discharge is controlled by the termination voltage (U2) during battery discharge such that it lies within the conventional limits, a greater charge (prelithiation) remains in the negative electrode. During subsequent charging of the battery (Q L，Z2 ), the charging capacity is preferably lower than or the same as that of a battery of the same configuration but operated according to a conventional method.

[0014] It is self-evident that U2 is less than U1.

[0015] During charging of the battery in subsequent cycles, the termination voltage (U3) is preferably greater than U2 and less than U1. The difference between U1 and U3 is preferably between 0.05 V and 0.50 V.

[0016] U1 is preferably between 4.37 V and 4.70 V and particularly preferably between 4.40 V and 4.60 V.

[0017] U2 is preferably between 3.02 V and 3.30 V, and particularly preferably between 3.30 V and 3.10 V for a C rate < C / 5, and between 3.02 V and 3.20 V for a C rate > C / 5. The C rate refers to the charging or discharging current relative to the nominal capacity of the battery. 1C means that charging or discharging is completed within one hour, and C / 5 similarly means that charging or discharging is completed within five hours.

[0018] The method of the present invention is preferably part of a battery formation and is also preferably used during cycling. In particular, during subsequent battery cycling, the end voltage (U4) during the second discharge of the battery (after the end voltage (U1) during charging has increased) is lower than the end voltage (U2) during the first discharge of the battery. U4 is preferably between 3.00 V and 3.20 V and particularly preferably between 3.01 V and 3.15 V.

[0019] Lithiation of silicon generally refers to the introduction of lithium into silicon, typically forming a silicon-lithium alloy, also known as lithium silicide.

[0020] Pre-lithiation of silicon generally refers to the lithiation of silicon before or during the formation of a lithium-ion battery, wherein the amount of lithium introduced into the silicon is retained completely or at least partially in the silicon during cycling of the lithium-ion battery. In other words, pre-lithiation generally refers to the lithiation of silicon before the lithium-ion battery is cycled between voltage limits U3 and U2 or U4. Therefore, when the battery is cycled between the end voltages U3 and U2 or U4, the lithium introduced into the silicon by pre-lithiation is generally irreversible or at least not fully reversible.

[0021] A cycle generally refers to a complete cycle of charging and discharging a lithium-ion battery. During a complete cycle, the battery typically reaches its maximum state of charge during charging and its maximum state of discharge during discharging. As is known, during a battery's charge / discharge cycle, the battery's maximum current storage capacity is utilized once. For example, the maximum charge and discharge capacity of a battery can be adjusted by its upper or lower cutoff voltage. During the cycle, the battery continues to function as a current storage medium.

[0022] In a generally familiar manner, formation refers to the process by which a lithium-ion battery is converted into its ready-to-use form as a storage medium for electric current. Formation can include, for example, single or multiple charging and discharging of the battery with the effect of chemical modification of the battery components, in particular prelithiation of the active negative electrode material or formation of an initial solid electrolyte interface (SEI) on the active negative electrode material, or also storage with aging and possibly elevated temperatures, thereby converting the battery into its ready-to-use state as a storage medium for electric current. Generally speaking, a lithium-ion battery that has undergone formation therefore differs structurally from its unformed counterpart. Chronologically, formation generally occurs before cycling. As is known, formation does not include any cycling.

[0023] Formation and cycling also typically differ in that the loss of mobile silicon, or the capacity loss of the lithium-ion battery, is greater during formation than during cycling. Formation of a lithium-ion battery is accompanied by, for example, a capacity loss of ≥1% or ≥5%. In two consecutive cycling steps, more specifically in two consecutive cycling steps within the first ten cycling steps after formation, there is preferably a capacity loss of ≤1%, more preferably ≤0.5%, and even more preferably ≤0.1%.

[0024] The volume capacity of the negative electrode coating can be determined by dividing the delithiation capacity per unit area β described in the examples by the thickness of the negative electrode coating. The thickness of the negative electrode coating can be determined using a Mitutoyo digital dial gauge (1 μm to 12.7 mm) with a precision measuring table.

[0025] Lithiation capacity generally refers to the maximum amount of lithium that can be accommodated by the active negative electrode material. For silicon, this amount is generally given by the formula Li 4.4 The maximum specific capacity of silicon for lithium (in other words, the maximum lithiation capacity of silicon) generally corresponds to 4200 mAh per gram of silicon.

[0026] The total degree of lithiation α generally represents the fraction of the lithiation capacity of silicon that is maximally occupied during cycling of a lithium-ion battery. Thus, the total degree of lithiation α generally includes the fraction of the lithiation capacity of silicon occupied by pre-lithiation of silicon (pre-lithiation degree α1) and also the fraction of the lithiation capacity of silicon occupied by partial lithiation of the negative electrode material during charging of the lithium-ion battery, more specifically during full charging (lithiation degree α2). The total degree of lithiation α is generally given by the sum of the pre-lithiation degree α1 and the lithiation degree α2. The total degree of lithiation α is preferably based on a fully charged lithium-ion battery.

[0027] The total degree of lithiation of silicon, a, is 10% to 75%, preferably 20% to 65%, more preferably 25% to 55%, and most preferably 30% to 50% of the maximum lithiation capacity of silicon.

[0028] In the partially lithiated negative electrode material of a fully charged lithium-ion battery, the ratio of lithium atoms to silicon atoms preferably corresponds to the formula Li 0.45 Si to Li 3.30 Si, more preferably Li 0.90 Si to Li 2.90 Si, very preferably Li 1.10 Si to Li 2.40 Si, and most preferably Li 1.30 Si to Li 2.20 Si. These data can be based on the lithiation degree α and the formula Li 4.4 Si to confirm.

[0029] In the case of partially lithiated negative electrode materials for fully charged lithium-ion batteries, the silicon capacity is preferably 400 to 3200 mAh per gram of silicon, more preferably 850 to 2700 mAh per gram of silicon, very preferably 1000 to 2300 mAh per gram of silicon, and most preferably 1250 to 2100 mAh per gram of silicon. These data are given by the degree of lithiation α and the maximum lithiation capacity of silicon (4200 mAh per gram of silicon).

[0030] In maximizing the lithiation capacity of the silicon used in the lithium ion battery of the present invention, more particularly, a total degree of lithiation α of preferably 50% to 90%, more preferably 60% to 85% and most preferably 70% to 80% is reversibly used for cycling or charging and / or discharging of the lithium ion battery.

[0031] The degree of pre-lithiation α1 of silicon is preferably 5% to 50%, more preferably 7% to 46%, very preferably 8% to 30% or 10% to 44%, and most preferably 10% to 20% or alternatively 20% to 40% of the lithiation capacity of silicon. The degree of pre-lithiation α1 generally represents the fraction of the lithiation capacity of silicon occupied by pre-lithiation. A method for determining the degree of pre-lithiation α1 is described later in the Examples below.

[0032] The amount of lithium introduced into silicon by prelithiation preferably corresponds to the formula Li 0.20 Si to Li 2.20 Si, more preferably Li 0.25 Si to Li 1.80 Si, very preferably Li 0.35 Si to Li 1.30 Si, and most preferably Li 0.45 Si to Li 0.90 Si. These data can be obtained using the degree of prelithiation α1 and the formula Li 4.4 Si to confirm.

[0033] The amount of lithium introduced into the silicon by pre-lithiation corresponds to a lithiation capacity of preferably 200 to 2100 mAh per gram of silicon, more preferably 250 to 1700 mAh per gram of silicon, very preferably 340 to 1300 mAh per gram of silicon, and most preferably 400 to 850 mAh per gram of silicon. These data are given by the degree of pre-lithiation α1 and by the maximum lithiation capacity of silicon (4200 mAh per gram of silicon).

[0034] During electrochemical prelithiation, the negative electrode is charged at preferably 800 to 1500 mAh / g, more preferably 900 to 1200 mAh / g, based in each case on the mass of the negative electrode coating, and after complete discharge, preferably ≤1500 mAh / g, more preferably 150 to 1000 mAh / g.

[0035] The formation preferably does not include pre-doping. Pre-lithiation typically does not include pre-doping. During pre-doping of silicon (more specifically, silicon containing silicon oxide or low-silicon oxide), lithium silicate is often formed. Conversely, during pre-lithiation, lithium silicide is typically formed.

[0036] Lithium-ion batteries are typically constructed, configured, and / or operated in such a manner that, in a fully charged battery, the negative electrode (negative electrode material), more specifically silicon, is only partially lithiated. "Fully charged" refers to a battery state in which the negative electrode material, more specifically silicon, of the battery exhibits its highest level of lithiation. Partial lithiation of the negative electrode material means that the lithiation capacity or maximum lithium uptake capacity of the active negative electrode material, more specifically silicon, has not been exhausted.

[0037] During the cycling or charging and / or discharging of the lithium-ion battery, the ratio of lithium atoms to silicon atoms (Li / Si ratio) in the negative electrode material changes by preferably ≤2.2, more preferably ≤1.3, and most preferably ≤0.9 according to the partial lithiation of the present invention. The above-mentioned Li / Si ratio changes by preferably ≥0.2, more preferably ≥0.4, and most preferably ≥0.6.

[0038] The degree of lithiation α2 generally refers to the fraction of the lithiation capacity of silicon that is maximized for cycling in a lithium-ion battery. In other words, the degree of lithiation α2 is a measure of the extent to which the lithiation capacity of silicon that is maximized for cycling in a battery is maximized. The degree of lithiation α2 of silicon is preferably 5% to 50%, more preferably 10% to 45%, and most preferably 25% to 40% of the lithiation capacity of silicon. A method for determining the degree of lithiation α2 is described later in the Examples below.

[0039] During cycling of a lithium-ion battery, the capacity of the silicon anode material is preferably used at ≤50%, more preferably at ≤45% and most preferably at ≤40%, based on a capacity of 4200 mAh per gram of silicon.

[0040] The ratio of lithium atoms to silicon atoms (Li / Si ratio) in the negative electrode of a lithium-ion battery can be adjusted, for example, by the charge flow during charging and discharging of the lithium-ion battery. The degree of lithiation α2 of the active negative electrode material (more specifically silicon) generally changes proportionally with the charge flow. With this variation, the lithiation capacity of the active negative electrode material is generally not fully exhausted during charging of the lithium-ion battery, and not all lithium is extracted from the active negative electrode material during discharging of the lithium-ion battery. This can be established, for example, by a corresponding cutoff voltage during charging or discharging of the lithium-ion battery, or alternatively, by limiting the charge flow. In this way, the overall degree of lithiation α and, therefore, also the degree of pre-lithiation α1 can be established.

[0041] In an alternative preferred variant, the Li / Si ratio of the lithium-ion battery is adjusted via the ratio of the negative electrode to the positive electrode (cell balancing). In this case, the lithium-ion battery is designed such that the lithium absorption capacity of the negative electrode is preferably greater than the lithium release capacity of the positive electrode. As a result, the lithium absorption capacity of the negative electrode is not completely depleted in a fully charged battery. Similarly, in this way, the degree of lithiation α2, the total degree of lithiation α, and therefore also the degree of pre-lithiation α1 can be adjusted.

[0042] The silicon-containing negative electrode preferably comprises silicon-containing particles, very preferably silicon particles, as active negative electrode material.

[0043] The volume-weighted particle size distribution of the silicon particles is preferably located at d 10 ≥0.2μm to d 90 ≤20.0μm, more preferably between d 10 ≥0.2μm to d 90 ≤10.0 μm and most preferably between d 10 ≥0.2μm to d 90 Percentile of diameters between ≤3.0 μm.

[0044] Silicon particles have a diameter percentile d 10 The silicon particles have a volume weighted particle size distribution of preferably ≤ 10 μm, more preferably ≤ 5 μm, still more preferably ≤ 3 μm and most preferably ≤ 1 μm. 90 The volume weighted particle size distribution is preferably ≥ 0.5 μm. In one embodiment of the present invention, the above d 90 Preferably ≥5 μm.

[0045] The volume-weighted particle size distribution of the silicon particles has a diameter percentile d preferably from 0.5 to 10.0 μm, more preferably from 0.6 to 7.0 μm, still more preferably from 2.0 to 6.0 μm and most preferably from 0.7 to 3.0 μm. 50 Alternatively, it is also preferred that its volume-weighted particle size distribution has a diameter percentile d of 10 to 500 nm, more preferably 20 to 300 nm, still more preferably 30 to 200 nm and most preferably 40 to 100 nm. 50 of silicon particles.

[0046] The volume-weighted particle size distribution of the silicon particles can be determined by static laser light scattering using the Mie model with a Horiba LA 950 instrument and ethanol as the dispersion medium for the silicon particles.

[0047] The silicon particles are preferably non-aggregated, preferably non-agglomerated, and / or preferably not nanostructured. "Aggregated" means that, for example, during the production of silicon particles by a gas-phase operation, many initially formed spherical or largely spherical primary particles grow together, fuse together, or sinter together to form aggregates. Thus, an aggregate is a particle containing multiple primary particles. Aggregates can form agglomerates. An agglomerate is a union of loose aggregates. Agglomerates can usually be easily broken back into aggregates using kneading or dispersion methods. Aggregates cannot be completely broken into primary particles using such methods. As a result of the way they are formed, aggregates and agglomerates inevitably have a sphericity and particle morphology that are completely different from those of the silicon particles of the present invention. The presence of silicon particles in the form of aggregates or agglomerates can be made visible, for example, by means of conventional scanning electron microscopy (SEM). In contrast, static light scattering methods for determining the particle size distribution or particle diameter of silicon particles cannot distinguish aggregates or agglomerates.

[0048] Non-nanostructured silicon particles typically have a characteristic BET surface area. The BET surface area of the silicon particles is preferably from 0.01 to 30.0 m 2 / g, more preferably from 0.1 to 25.0 m 2 / g, very preferably from 0.2 to 20.0 m 2 / g, and most preferably from 0.2 to 18.0 m 2 / g. The BET surface area is determined in accordance with DIN 66131 (using nitrogen).

[0049] The silicon particles have a sphericity of preferably 0.3 ≤ ψ ≤ 0.9, more preferably 0.5 ≤ ψ ≤ 0.85, and most preferably 0.65 ≤ ψ ≤ 0.85. Silicon particles having such sphericity can be obtained in particular by production by means of a grinding operation. The sphericity ψ is the ratio of the surface area of a sphere of equal volume to the actual surface area of the body (defined by Wadell). For example, the sphericity can be determined from a conventional SEM image.

[0050] Polycrystalline silicon particles are preferred. The silicon particles are preferably based on elemental silicon. The elemental silicon can be high purity silicon or silicon from metallurgical processing, which can, for example, have elemental contaminants such as Fe, Al, Ca, Cu, Zr, C. The silicon particles can optionally be doped with foreign atoms (for example, such as B, P, As). Such foreign atoms are usually present only to a small extent.

[0051] The silicon particles can contain silicon oxide, especially on the surface of the silicon particles. If the silicon particles contain silicon oxide, the stoichiometry of the oxide SiO x is preferably in the range 0 < x < 1.3. The layer thickness of the silicon oxide on the surface of the silicon particles is preferably less than 10 nm.

[0052] The surface of the silicon particles may optionally be covered with an oxide layer or other inorganic and organic groups. Particularly preferred silicon particles carry Si-OH- or Si-H- groups or covalently attached organic groups, such as alcohols or olefins, for example on the surface.

[0053] The silicon particles have a silicon content of ≥90 wt%, preferably ≥95 wt%, more preferably 97 wt% and most preferably 99 wt%, based on the total weight of the silicon particles.

[0054] Silicon particles can be produced, for example, by a grinding operation. Contemplated grinding operations include, for example, wet grinding operations or preferably dry grinding operations, as described, for example, in DE-A 102015215415.

[0055] The silicon particles may also optionally be coated with carbon (C-coated Si particles) or be present in the form of silicon / carbon composite particles (Si / C composite particles). The C-coated Si particles contain preferably 1 to 10 wt % of carbon and preferably 90 to 99 wt % of silicon particles, based in each case on the total weight of the C-coated Si particles. In the Si / C composite particles, the silicon particles are preferably incorporated into a porous carbon matrix. Alternatively, the pores of the porous carbon matrix may be coated with silicon, for example in the form of a silicon film or in the form of silicon particles. The silicon-containing porous carbon matrix is ​​preferably coated with non-porous carbon. The carbon coating of the C-coated Si particles or Si / C composite particles has an average layer thickness preferably in the range of 1 to 50 nm (determination method: scanning electron microscopy (SEM)). The C-coated Si particles or Si / C composite particles have an average particle size d preferably in the range of 1 to 15 μm. 50 The BET surface area of ​​the above particles is preferably from 0.5 to 5 m 2 / g (determined using nitrogen according to DIN ISO 9277:2003-05). Further information on C-coated Si particles or Si / C composite particles and methods for producing them can be found in WO 2018 / 082880, WO 2017 / 140642 or WO 2018 / 145732.

[0056] The negative electrode material preferably comprises silicon particles, one or more binders, optionally graphite, optionally one or more further conductive components and optionally one or more adjuvants.

[0057] The silicon fraction in the negative electrode material is preferably 40 to 95 wt %, more preferably 50 to 90 wt %, and most preferably 60 to 80 wt %, based on the total weight of the negative electrode material.

[0058] Preferred binders are polyacrylic acid or its alkali metal salts, more particularly the lithium or sodium salts, polyvinyl alcohol, cellulose or cellulose derivatives, polyvinylidene fluoride, polytetrafluoroethylene, polyolefins, polyimides, more particularly polyamide-imides, or thermoplastic elastomers, more particularly ethylene-propylene-diene terpolymers. Particularly preferred are polyacrylic acid, polymethacrylic acid, or cellulose derivatives, more particularly carboxymethylcellulose. Also particularly preferred are alkali metal salts of the above binders, more particularly the lithium or sodium salts. The binder preferably has a molar mass of 100,000 to 1,000,000 g / mol.

[0059] The graphite used can generally be natural or synthetic graphite. The graphite particles preferably have a diameter percentile of d 10 >0.2μm to d 90 Volume-weighted particle size distribution <200 μm.

[0060] Preferred additional conductive components are conductive carbon black, carbon nanotubes, or metal particles, such as copper. Amorphous carbon, more particularly hard carbon or soft carbon, is also preferred. As is known, amorphous carbon is not graphitic. The negative electrode material preferably comprises 0 to 40 wt %, more preferably 0 to 30 wt %, and most preferably 0 to 20 wt %, of the additional conductive component, based on the total weight of the negative electrode material.

[0061] Examples of adjuvants for negative electrode materials are pore formers, dispersants, leveling agents or dopants, such as elemental lithium.

[0062] A preferred formulation of a negative electrode material for lithium-ion batteries comprises preferably 5 to 95 wt %, more particularly 60 to 85 wt % of silicon particles; 0 to 40 wt %, more particularly 0 to 20 wt % of other conductive components; 0 to 80 wt %, more particularly 5 to 30 wt % of graphite; 0 to 25 wt %, more particularly 1 to 15 wt % of binder; and optionally 0 to 80 wt %, more particularly 0.1 to 5 wt % of adjuvants; the data in wt % are based on the total weight of the negative electrode material, and the fractions of all constituents of the negative electrode material add up to 100 wt %.

[0063] In a preferred formulation for the negative electrode material, the fraction of graphite particles and other conductive components is in total at least 10 wt%, based on the total weight of the negative electrode material.

[0064] The components of the negative electrode material can be processed into anode ink or negative electrode paste in, for example, a solvent such as water, hexane, toluene, tetrahydrofuran, N-methylpyrrolidone, N-ethylpyrrolidone, acetone, ethyl acetate, dimethyl sulfoxide, dimethylacetamide or ethanol, or in a solvent mixture, preferably using a rotor-stator machine, a high-energy mill, a planetary kneader, a stirred ball mill, a shaker or an ultrasonic device.

[0065] The negative electrode ink or paste preferably has a pH of 2 to 7.5, more preferably ≤ 7.0 (measured at 20° C. using, for example, a WTW pH 340i pH meter with a SenTix RJD probe).

[0066] For example, the negative electrode ink or paste may be applied to a copper foil or another current collector, for example as described in WO 2015 / 117838.

[0067] Regarding the dry film thickness of the negative electrode coating, the layer thickness is preferably 2 μm to 500 μm, more preferably 10 μm to 300 μm.

[0068] The negative electrode of a lithium-ion battery generally comprises a negative electrode coating and a current collector. The negative electrode coating is generally based on a negative electrode material. The process of the present invention also advantageously enables the negative electrode coating to have a high volumetric capacity. The negative electrode coating preferably has a capacity of ≥660 mAh / cm 3 The volumetric capacity of the negative electrode coating can be determined by dividing the delithiation capacity per unit area β described below by the thickness of the negative electrode coating. The thickness of the negative electrode coating can be determined using a Mitutoyo digital micrometer (1 μm to 12.7 mm) with a precision measuring table.

[0069] The positive electrode material in the positive electrode preferably comprises lithium cobalt oxide, lithium nickel oxide, lithium nickel cobalt oxide (doped or undoped), lithium manganese oxide (spinel), lithium nickel cobalt manganese oxide, lithium nickel manganese oxide (LiCoO2, Li(Ni x Mn y Co 1-x-y )O2 (x is greater than or equal to 0 and less than or equal to 1, y is greater than or equal to 0 and less than or equal to 1), Li(Ni x Co y Al 1-x-y )O2 (x is greater than or equal to 0 and less than or equal to 1, y is greater than or equal to 0 and less than or equal to 1), Li2MnO4, LiNi 0.5 Mn 0.5 O2、LiNi 0.5 Mn 1.5 O4 and aLi2MnO3(1-a)Li(Ni x Mn y Co 1-x-y )O2 (x is greater than or equal to 0 and less than or equal to 1, and y is greater than or equal to 0 and less than or equal to 1)) or lithium vanadium oxide.

[0070] The separator is generally an electrically insulating membrane that is permeable to ions, as is customary in battery production. As is known, the separator separates the negative electrode from the positive electrode and thus prevents an electrically conductive connection (short circuit) between the electrodes.

[0071] The electrolyte is typically a solution of a lithium salt (i.e., a conductive salt) in an aprotic solvent. Examples of conductive salts are lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, LiCF3SO3, LiN(CF3SO2), or lithium borate. Based on the solvent, the concentration of the conductive salt is preferably between 0.5 mol / l and the solubility limit of the corresponding salt. Particularly preferably, it is between 0.8 mol / l and 1.2 mol / l.

[0072] The solvent used may be cyclic carbonate, propylene carbonate, ethylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethoxyethane, diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, dioxolane, acetonitrile, organic carbonate or nitrile, alone or as a mixture thereof.

[0073] The electrolyte preferably comprises a film former, such as vinylene carbonate or fluoroethylene carbonate.The fraction of the film former in the electrolyte is preferably between 0.1 wt% and 20.0 wt%, more preferably between 0.5 wt% and 10 wt%.

[0074] The electrolyte is preferably mixed with one or more fluorinated additives, such as fluoroacetates, fluorocarbamates, fluoronitriles, fluorosulfones and / or fluorocarbamates, such as methyl (2,2,2-trifluoroethyl) carbonate (FEMC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (F2EC), trifluoroethylene carbonate (F3EC), ethyl (1-fluoroethyl) carbonate (FDEC), di(1-fluoroethyl) carbonate F2DEC, 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate (F4DEC). The additive content is between 0 wt% and 50 wt%. The fluorinated additives increase the high voltage stability of the electrolyte.

[0075] As mentioned above, all substances and materials for producing the lithium-ion battery of the invention are known. The production of the components of the battery of the invention and their assembly to form the battery of the invention is carried out according to methods known from the field of battery production.

[0076] Example:

[0077] The methods and compositions of the present invention are described in the following examples. All percentage data are based on weight. Unless otherwise specified, all operations are carried out at a room temperature of 23°C and under atmospheric pressure (1.013 bar). Unless otherwise specified, all data related to product performance descriptions are valid at a room temperature of 23°C and under atmospheric pressure (1.013 bar).

[0078] The equipment includes standard commercial laboratory instruments of the type commercially available from a number of equipment manufacturers.

[0079] Experimental determination of the total degree of lithiation α: The degree of lithiation α of the active material can be determined using the following formula I:

[0080] in

[0081] β: delithiation capacity per unit area of ​​the negative electrode containing the active material at the corresponding end-of-charge voltage of a lithium-ion battery that has been delithiated in a half-cell measurement on lithium;

[0082] γ: Maximum capacity of active material for lithium (corresponding to 4200 mAh / g for silicon, stoichiometric Li 4, 4Si);

[0083] FG: g / cm 2 The surface weight of the negative electrode coating measured;

[0084] ω AM : The percentage weight fraction of active material in the negative electrode coating.

[0085] Experimental determination of the delithiation capacity β per unit area:

[0086] The lithium ion battery is charged by the cc (constant current) method with a constant current of 5 mA / g (corresponding to C / 25) until the corresponding charge termination voltage, more particularly the voltage limit of 4.2V, is reached. The negative electrode is lithiated here. The lithium ion battery thus charged is opened, the negative electrode is removed and used to construct a button half-cell (CR2032 type, Hohsen Corp.) with a lithium counter electrode (Rockwood lithium, thickness 0.5 mm, diameter = 15 mm). Glass fiber filter paper (Whatman, GD type D) impregnated with 120 μl of electrolyte can be used as a diaphragm (diameter = 16 mm). The electrolyte used is a 1.0 molar lithium hexafluorophosphate solution in a 1:4 (v / v) mixture of fluoroethylene carbonate and diethyl carbonate. The battery is typically constructed in a glove box (<1 ppm of H2O and O2). The water content of the dry mass of all components is preferably less than 20 ppm. The delithiation capacity β per unit area of ​​the active material-containing negative electrode coating was determined by charging the resulting coin cell half-cell at C / 25 until a voltage limit of 1.5 V was reached (working electrolyte = positive electrode = active material negative electrode; counter electrode = negative electrode = lithium). The Si negative electrode was delithiated. Electrochemical measurements of the full and half cells were performed at 20°C. The constant current is based on the weight of the positive electrode coating.

[0087] Experimental determination of pre-lithiation degree α1:

[0088] The lithium-ion battery is converted to an uncharged state by discharging it by the cc (constant current) method with a constant current of 5 mA / g (corresponding to C / 25) until the corresponding discharge end voltage, more particularly a voltage limit of 3.2 V, is reached. Here, the negative electrode is delithiated. The lithium-ion battery thus discharged is opened, the negative electrode is removed and used to construct a button half-cell (CR2032 type, Hohsen Corp.) with a lithium counter electrode (Rockwood lithium, thickness 0.5 mm, diameter = 15 mm). Glass fiber filter paper (Whatman, GD type D) impregnated with 120 μl of electrolyte can be used as a separator (diameter = 16 mm). The electrolyte used is a 1.0 molar lithium hexafluorophosphate solution in a 1:4 (v / v) mixture of fluoroethylene carbonate and diethyl carbonate. The battery is usually constructed in a glove box (<1 ppm of H2O and O2). The water content of the dry mass of all components is preferably less than 20 ppm. The degree of prelithiation α1 caused by prelithiation was determined by charging the resulting coin cell half-cell at C / 25 until a voltage limit of 1.5 V was reached (working electrode = positive electrode = active material negative electrode; counter electrode = negative electrode = lithium). The Si negative electrode was further delithiated. Electrochemical measurements of the full and half cells were performed at 20°C. The constant current is based on the weight of the positive electrode coating.

[0089] The degree of prelithiation α1 is then calculated using the following formula II:

[0090] in

[0091] δ: delithiation capacity per unit area of ​​the negative electrode containing the active material at the corresponding discharge end voltage of a lithium-ion battery that has been further delithiated in a half-cell measurement on lithium;

[0092] γ: Maximum capacity of active material for lithium (corresponding to 4200 mAh / g for silicon, stoichiometric Li 4, 4Si);

[0093] FG: g / cm 2 The surface weight of the negative electrode coating measured;

[0094] ω AM : The percentage weight fraction of active material in the negative electrode coating.

[0095] Determination of lithiation degree α2:

[0096] The degree of lithiation α2 is calculated from the difference between the total degree of lithiation α and the pre-lithiation degree α1, as shown in the following formula:

[0097] Degree of lithiation α2 = (total degree of lithiation α) - (pre-lithiation degree α1).

[0098] Example 1:

[0099] Production of unaggregated, split-shaped silicon particles by grinding:

[0100] According to WO 2018 / 041339, a fluidized bed jet mill (Netzsch-Condux CGS16, 90 m 3 Silicon powder is produced by grinding coarsely ground Si from the production of solar silicon in a mixture of nitrogen (100 % N2 / h as grinding gas).

[0101] The obtained product consisted of individual, unaggregated, fragmented particles (SEM) and had a particle distribution of d10 = 2.23 μm, d50 = 4.48 μm and d90 = 7.78 μm and also a span (d90-d10) of 5.5 μm (determined by static laser scattering using the Mie model, Horiba LA950 instrument, in a suspension in highly diluted ethanol).

[0102] Example 2:

[0103] Negative electrode comprising silicon particles from Example 1:

[0104] 29.709 g of polyacrylic acid (Sigma-Aldrich, Mw 450000 g / mol) and 751.60 g of deionized water dried to constant weight at 85° C. were shaken on an oscillator (290 1 / min) for 2.5 h until the polyacrylic acid was completely dissolved. Lithium hydroxide monohydrate (Sigma-Aldrich) was added portionwise to the solution until the pH was 7.0 (measured using a WTWpH 340i pH meter with a SenTix RJD probe). The solution was then mixed on an oscillator for another 4 hours.

[0105] 7.00 g of the silicon particles from Example 1 were then dispersed in 12.50 g of a neutralized polyacrylic acid solution (4 wt % concentration) and 5.10 g of deionized water using a dissolver at a peripheral speed of 4.5 m / s for 5 minutes and at 12 m / s for 30 minutes, with cooling at 20° C. 2.50 g of graphite (Imerys, KS6L C) were then added, followed by stirring at a peripheral speed of 12 m / s for 30 minutes. After degassing, the dispersion was applied to a 0.030 mm thick copper foil (Schlenk Metallfolien, SE-Cu58) using a film drawing frame (Erichsen, Model 360) with a gap height of 0.10 mm. The negative electrode coating produced in this manner was subsequently dried at 80° C. and 1 bar atmospheric pressure for 60 minutes.

[0106] The negative electrode coating thus dried has a surface area of ​​2.85 mg / cm 2 and a layer thickness of 32 μm.

[0107] Example 3:

[0108] Lithium-ion battery with electrode coating from Example 2:

[0109] The electrode coating from Example 2 was used as the counter or negative electrode (Dm=15 mm), and a coating based on 6:2:2 lithium nickel manganese cobalt oxide (available from SEI Corp.) with a content of 94.0 wt% and a carbon content of 15.9 mg / cm was used as the working or positive electrode (Dm=15 mm). 2 The average surface weight of the electrolyte was 200 μg / min. A glass fiber filter paper (Whatman, GD type A / E) impregnated with 60 μl of electrolyte was used as a separator (Dm=16 mm). The electrolyte used consisted of a 1.0 molar lithium hexafluorophosphate solution in a 1:4 (v / v) mixture of fluoroethylene carbonate and diethyl carbonate. The battery (button cell, 2-electrode arrangement, CR2032 type, Hohsen Corp.) was constructed in a glove box (<1 ppm of H2O, O2) and the water content in the dry mass of all components used was less than 20 ppm.

[0110] Example 4-9:

[0111] Electrochemical testing of lithium-ion cells with and without pre-lithiation from Example 3:

[0112] Electrochemical tests were performed on the lithium-ion cells from Example 3.

[0113] The electrochemical tests were carried out at 20°C. The cells were charged by the cc / cv (constant current / constant voltage) method at a constant current of 12.5 mA / g (corresponding to C / 10) to the point where the voltage limit was reached in each case in the first cycle (forming), and in subsequent cycles (cycling) until 60 mA / g (corresponding to C / 2) was reached. After reaching the voltage limit in each case, they were charged at a constant voltage until the current dropped below 1.2 mA / g (corresponding to C / 100) or 15 mA / g (corresponding to C / 8). The cells were discharged by the cc (constant current) method at a constant current of 12.5 mA / g (corresponding to C / 10) to the point where the corresponding effective voltage limit was reached in the first cycle, and in subsequent cycles at a constant current of 60 mA / g (corresponding to C / 2) until the corresponding effective voltage limit was reached. The specific currents selected were based on the weight of the coating of the positive electrode.

[0114] Depending on the formulation, lithium-ion batteries operate by cell balancing utilizing partial lithiation of the negative electrode.

[0115] The test conditions for Examples 4 and 9 and the non-inventive comparative examples, Examples 5 to 8, can be found in Table 1.

[0116] Table 1. Test conditions for Examples 4 and 9 and Comparative Examples 5 to 8

[0117]

[0118] *Not invented here

[0119] Table 2 contains the test results of the examples. Compared to the lithium-ion batteries from non-inventive comparative examples 5 to 8, the lithium-ion batteries from inventive examples 4 and 9 unexpectedly showed more stable electrochemical behavior (>20 cycles) while having a high discharge capacity after cycle 2.

[0120] Table 2. Test results of Examples 4 and 9 and Comparative Examples 5 to 8 not according to the present invention:

[0121]

[0122] *Not invented here

[0123] Table 3 lists the lithiation degrees α, α1 and α2 of the examples:

[0124]

[0125] *Not of the present invention.

Claims

1. A method for a silicon-containing negative electrode in a prelithiated lithium-ion battery, the lithium-ion battery comprising a lithium transition metal oxide positive electrode, a negative electrode, a separator, and an organic electrolyte, wherein 1) The termination voltage U1 during battery charging is between 4.35 V and 4.80 V, 2) During subsequent battery cycles, the termination voltage U2 during battery discharging does not drop below 3.01 V, and 3) During subsequent battery cycles, the termination voltage U3 during battery charging is lower than the termination voltage U1 during battery charging, where the battery is charged by the constant current / constant voltage method, It is characterized by: The termination voltage U4 during subsequent battery discharging is lower than the termination voltage U2 during battery discharging and does not drop below 3.01 V.

2. The method according to claim 1, wherein U1 is between 4.37 V and 4.70 V.

3. The method according to claim 1 or 2, wherein: For a C-rate < C / 5, U2 is between 3.30 V and 3.10 V, and for a C-rate > C / 5, U2 is between 3.02 V and 3.20 V.

4. The method according to claim 1 or 2, the method being part of a battery formation process.

5. The method according to claim 1 or 2, wherein: In the partially lithiated anode material of a fully charged lithium-ion battery, the ratio of lithium atoms to silicon atoms corresponds to the formula Li 0.45 Si to Li 3.30 Si.

6. The method according to claim 1 or 2, wherein: In the partially lithiated negative electrode material of a fully charged lithium-ion battery, the capacity of silicon is used at 400 to 3200 mAh per gram of silicon, where the capacity of the silicon is given by the degree of lithiation α and the maximum lithiation capacity of silicon (4200 mAh per gram of silicon), and the degree of lithiation α of the active material is calculated using the following formula (I): in β: The delithiation capacity per unit area of the negative electrode containing the active material at the corresponding charging termination voltage of the delithiated lithium-ion battery in a half-cell measurement against lithium; γ: Maximum capacity of the active material for lithium, corresponding to 4200 mAh / g for silicon, stoichiometrically Li 4,4 Si; FG: g / cm 2 The surface weight of the negative electrode coating measured; ω AM : The percentage weight fraction of the active material in the negative electrode coating.

7. The method according to claim 1 or 2, wherein: The amount of lithium introduced into silicon by prelithiation corresponds to the formula Li 0.20 Si to Li 2.20 Si.

8. The method according to claim 1 or 2, wherein: The amount of lithium introduced into silicon by prelithiation corresponds to a lithiation capacity of 200 to 2100 mAh per gram of silicon, where the lithiation capacity is given by the degree of prelithiation α1 and the maximum lithiation capacity of silicon at 4200 mAh per gram of silicon, and the degree of prelithiation α1 is calculated using the following formula (II): in δ: The delithiation capacity per unit area of the negative electrode containing the active material at the corresponding discharging termination voltage of the further delithiated lithium-ion battery in a half-cell measurement against lithium; γ: Maximum capacity of the active material for lithium, corresponding to 4200 mAh / g for silicon, stoichiometrically Li 4,4 Si; FG: g / cm 2 The surface weight of the negative electrode coating measured; ω AM : The percentage weight fraction of active material in the negative electrode coating.

9. The method according to claim 1 or 2, wherein: The silicon-containing negative electrode comprises silicon particles as the active negative electrode material.

10. The method according to claim 9, wherein: The volume-weighted particle size distribution of the silicon particles is at the diameter percentile d 10 ≥0.2μm to d 90 ≤20.0μm.

11. A lithium-ion battery producible by the method according to any one of claims 1 to 10.

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