Composite anode and this comprehensive lithium-ion battery as well as methods for manufacturing the composite anode

Abstract

Composite anode comprising a current collector, active anode material, binder, solid inorganic lithium-ion conductor and liquid electrolyte, wherein the solid inorganic lithium-ion conductor in the composite anode is present in a higher volume and weight fraction than the liquid electrolyte, and the active electrode material consists of secondary particles and the grain size d50 of the secondary particles is more than 15 µm to 75 µm.

Description

[0001] The present invention relates to a composite anode, a lithium-ion battery comprising the composite anode and a method for producing the composite anode.

[0002] Publication US 2011 / 0045355A1 describes an electrode for lithium batteries, Publication US 2014 / 0011100A1 describes a solid electrolyte lithium battery, Publication US 2008 / 0268348A1 describes a lithium secondary battery, and Publication US 2010 / 0190061A1 describes a silicon anode for a rechargeable battery.

[0003] The problems of the invention are solved by the subject matter of claims 1 and 12 and by the method of claim 13.

[0004] In the following description, the terms "lithium-ion battery," "rechargeable lithium-ion battery," and "lithium-ion secondary battery" are used synonymously. These terms also include "lithium battery," "lithium-ion accumulator," and "lithium-ion cell," as well as all lithium and alloy batteries, in particular lithium-sulfur, lithium-air, and alloy systems. Thus, the term "lithium-ion battery" is used as a collective term for the aforementioned terms commonly used in the prior art. It encompasses both rechargeable batteries (secondary batteries) and non-rechargeable batteries (primary batteries). In particular, a "battery" within the meaning of the present invention also includes a single or individual "electrochemical cell."

[0005] In general terms, the operating principle of a lithium-ion battery can be described as follows: electrical energy is stored in lithium ions (at the negative electrode) and (mostly) transition metal oxides (at the positive electrode) in a chemical process involving a change in composition. Lithium ions can be stored in ionized form (Li + The lithium ions migrate back and forth between the two electrodes via the electrolyte, which usually contains lithium hexafluorophosphate (LiPF6) as the lithium conducting salt. In contrast to the lithium ions, the transition metal ions present at the cathode are stationary.

[0006] This lithium-ion flow is necessary to balance the external current flow during charging and discharging, ensuring the electrodes themselves remain electrically neutral. During discharge, the quasi-lithium atoms (or rather, the negative active material encompassing them) at the negative electrode each release an electron, which flows via the external circuit (load) to the positive electrode. Simultaneously, an equal number of lithium ions migrate through the electrolyte from the negative to the positive electrode. At the positive electrode, however, it is not the lithium ions that accept the electron, but rather the transition metal ions present there. Depending on the battery type, these can be cobalt, nickel, manganese, iron ions, etc. Thus, in the discharged state of the cell, the lithium at the positive electrode remains in ionic form (Li). + ) before.

[0007] Lithium-ion batteries are hermetically sealed, so no contents can enter or escape during normal operation. If the casing is mechanically damaged, which can happen, for example, in an accident involving an electric vehicle, contents can escape in vapor, gaseous, or liquid form. Gaseous leakage consists primarily of vaporized electrolyte (risk of explosion) and electrolyte decomposition products such as methane, ethane, hydrogen, propane, butane, and aldehydes. Liquid electrolyte, consisting of solvent and conducting salt, can also escape. The solvents are flammable and toxic. The conducting salt LiPF6 forms hydrogen fluoride (HF) when in contact with moisture. This is highly toxic and irritates the respiratory tract.

[0008] The present invention is based on the objective of providing a lithium-ion battery with increased safety.

[0009] This problem is solved according to the invention in a first aspect by a composite anode according to claim 1, in a second aspect by a lithium-ion battery according to claim 12, and in a third aspect by a method for producing the composite anode according to claim 13. Preferred embodiments are described in the dependent claims.

[0010] The following definitions apply to all aspects of the invention, insofar as they are applicable. Lithium-ion battery

[0011] According to the invention, the term "lithium-ion battery" has the meaning defined in the introduction. In particular, according to the invention, the term also includes a single or one "electrochemical cell". Preferably, two or more such electrochemical cells are connected together in a "battery", either in series (i.e., one after the other) or in parallel. Electrodes

[0012] The electrochemical cell according to the invention has at least two electrodes, i.e. a positive electrode (cathode) and a negative electrode (anode).

[0013] Both electrodes each contain at least one active material. This material is capable of absorbing or releasing lithium ions and simultaneously absorbing or releasing electrons.

[0014] The term "positive electrode" refers to the electrode that, when the battery is connected to a load, such as an electric motor, is capable of receiving electrons. In this nomenclature, it represents the cathode.

[0015] The term "negative electrode" refers to the electrode that is capable of emitting electrons during operation. In this nomenclature, it represents the anode.

[0016] The electrodes comprise inorganic material or inorganic compounds or substances that can be used for, in, or on an electrode, or as an electrode. Under the operating conditions of the lithium-ion battery, these compounds or substances, due to their chemical properties, can absorb (intercalate) and release lithium ions or metallic lithium. In this description, such a material is referred to as "active cathode material" or "active anode material," or more generally, "active material" or "active electrode material." For use in an electrochemical cell or battery, this active material is preferably applied to a support, preferably a metallic support, preferably aluminum for the cathode or copper for the anode. This support is also referred to as a "collector" or collector foil. Cathode (positive electrode)

[0017] All materials known from the prior art can be used as the active material for the positive electrode or active cathode material. These include, for example, LiCoO2, NCM, NCA, high-energy NCM (HE-NCM), lithium iron phosphate, or lithium manganese spinel (LiMn2O4). Therefore, there is no limitation with regard to the positive electrode as defined in the present invention.

[0018] In a preferred embodiment, the active cathode material can be selected from the group consisting of a lithium transition metal oxide (hereinafter also referred to as "lithium metal oxide"), layered oxides, spinels, olivine compounds, silicate compounds, and mixtures thereof. Such active cathode materials are described, for example, in Bo Xu et al., "Recent progress in cathode materials research for advanced lithium ion batteries," Materials Science and Engineering R 73 (2012) 51-65. Another preferred cathode material is HE-NCM. Layered oxides and HE-NCM are also described in Argonne National Laboratory patents US 6,677,082 B2, US 6,680,143 B2, and US 7,205,072 B2.

[0019] Examples of olivine compounds are lithium phosphates of the molecular formula LiXPO4 with X = Mn, Fe, Co or Ni, or combinations thereof.

[0020] Examples of lithium transition metal oxides, spinel compounds and layered transition metal oxides are lithium manganate, preferably LiMn2O4, lithium cobaltate, preferably LiCoO2, lithium nickelate, preferably LiNiO2, or mixtures of two or more of these oxides, or their mixed oxides.

[0021] The active material may also contain mixtures of two or more of the substances mentioned.

[0022] To increase electrical conductivity, the active material may contain additional compounds, preferably carbon-containing compounds, or carbon, preferably in the form of conductive carbon black or graphite. The carbon may also be introduced in the form of carbon nanotubes or graphene. Such additives are preferably applied in an amount of 0.1 to 6 wt.%, preferably 1 to 3 wt.%, based on the mass (excluding solvent) of the positive electrode applied to the support. Anode (negative electrode)

[0023] Any material known from the prior art can be used as the active material for the negative electrode or active anode material. Therefore, there is no restriction with regard to the negative electrode as defined in the present invention.

[0024] The active anode material can be selected from the group consisting of lithium metal oxides, such as lithium titanium oxide, metal oxides (e.g., Fe₂O₃, ZnO, ZnFe₂O₄), carbon-containing materials such as graphite (synthetic graphite, natural graphite), graphene, mesocarbon, doped carbon, hard carbon, soft carbon, fullerenes, silicon-carbon mixtures, silicon, tin, metallic lithium, and lithium-alloyable materials, and mixtures thereof. Niobium pentoxide, tin alloys, titanium dioxide, tin dioxide, and silicon can also be used as electrode materials for the negative electrode.

[0025] It is also possible to use a lithium-alloyable material for the active anode material. This can be metallic lithium, a lithium alloy, or a non-lithiated or partially lithiated precursor, from which a lithium alloy is formed during the reaction process.

[0026] Preferred materials for lithium alloying are lithium alloys selected from the group consisting of silicon-based, tin-based, and antimony-based alloys. Such alloys are described, for example, in the review article by W.-J. Zhang, Journal of Power Sources 196 (2011) 13-24. Electrode binder

[0027] The materials used for the positive or negative electrode, such as the active materials, are held together by one or more binders that hold these materials on the electrode or on the current collector.

[0028] The binder(s) may be selected from the group consisting of polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), polyethylene oxide (PEO), polytetrafluoroethylene, polyacrylate, styrene-butadiene rubber, and carboxymethylcellulose (CMC), and mixtures and copolymers thereof. The styrene-butadiene rubber and optionally the carboxymethylcellulose and / or the other binders, such as PVdF, are preferably present in an amount of 0.5–8% by weight, based on the total amount of active material used in the positive or negative electrode. separator

[0029] The electrochemical cell according to the invention comprises a material that separates the positive and negative electrodes. This material is permeable to lithium ions, i.e., it conducts lithium ions, but is a non-conductor to electrons. Such materials used in lithium-ion batteries are also referred to as separators.

[0030] In a preferred embodiment according to the present invention, polymers are used as separators. In one embodiment, the polymers are selected from the group consisting of: polyester, preferably polyethylene terephthalate; polyolefin, preferably polyethylene or polypropylene; polyacrylonitrile; polyvinylidene fluoride; polyvinylidene hexafluoropropylene; polyetherimide; polyimide, polyamide, or polyether; polyetherketone; or mixtures thereof. The separator is porous, allowing it to pass through to lithium ions. In a preferred embodiment according to the present invention, the separator consists of at least one polymer. electrolyte

[0031] The term "electrolyte" preferably means a liquid in which a lithium conducting salt is dissolved. Preferably, the liquid is a solvent for the conducting salt. Preferably, the lithium conducting salt is then present in dissociated form.

[0032] Suitable solvents are preferably chemically and electrochemically inert. Suitable solvents are preferably organic solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, sulfolanes, 2-methyltetrahydrofuran, and 1,3-dioxolane. Organic carbonates are preferred.

[0033] In one embodiment, ionic liquids can also be used as solvents. Such "ionic liquids" contain exclusively ions. Preferred cations, which may be alkylated, are imidazolium, pyridinium, pyrrolidinium, guanidinium, uronium, thiuronium, piperidinium, morpholinium, sulfonium, ammonium, and phosphonium cations. Examples of usable anions are halide, tetrafluoroborate, trifluoroacetate, triflate, hexafluorophosphate, phosphinate, and tosylate anions.

[0034] Examples of ionic liquids include: N-methyl-N-propyl-piperidinium-bis(trifluoromethylsulfonyl)imide, N-methyl-N-butyl-pyrrolidinium-bis(trifluoromethylsulfonyl)imide, N-butyl-N-trimethyl-ammonium-bis(trifluoromethylsulfonyl)imide, triethylsulfonium-bis(trifluoromethylsulfonyl)imide and N,N-diethyl-N-methyl-N-(2-methoxyethyl)-ammonium-bis(trifluoromethylsulfonyl)imide.

[0035] Preferably, two or more of the aforementioned liquids are used. Preferred conducting salts are lithium salts containing inert anions and preferably non-toxic. Suitable lithium salts are preferably lithium hexafluorophosphate (LiPF6) or lithium tetrafluoroborate (LiBF4) and mixtures of one or more of these salts. In one embodiment, the separator is impregnated or wetted with the lithium salt electrolyte.

[0036] The various aspects of the present invention will be described in more detail below.

[0037] In a first aspect of the invention, the invention relates to a composite anode.

[0038] The composite anode according to the invention comprises a current collector, active anode material, binder, solid inorganic lithium-ion conductor, and liquid electrolyte, wherein the solid inorganic lithium-ion conductor is present in the composite anode in a higher volume and weight fraction than the liquid electrolyte. The coating of the current collector, consisting of active anode material, binder, solid inorganic lithium-ion conductor, and liquid electrolyte, is preferably porous and preferably homogeneous.

[0039] According to the invention, solid inorganic lithium-ion conductors comprise crystalline, composite, and amorphous solid lithium-ion conductors. Crystalline lithium-ion conductors include, in particular, perovskite-type lithium-lanthanum titanates, NASICON-type, LiSICON-type, and thiolisicon-type lithium-ion conductors, as well as garnet-type lithium-ion conducting oxides. The composite lithium-ion conductors include, in particular, materials containing oxides and mesoporous oxides. Such solid inorganic lithium-ion conductors are described, for example, in the review article by Philippe Knauth, “Inorganic solid Li-ion conductors: An overview,” Solid State Ionics, Volume 180, Issues 14–16, June 25, 2009, Pages 911–916. According to the invention, all solid lithium-ion conductors described in Cao C, Li ZB, Wang XL, Zhao XB and Han WQ (2014) “Recent advances in inorganic solid electrolytes for lithium batteries”, Front. Energy Res., 2:25 are also included.In particular, the grenades described in EP1723080 B1 are also included according to the invention.

[0040] The composite anode according to the invention thus has a composition in which a solid inorganic lithium-ion conductor is predominantly used as the inorganic solid electrolyte. A liquid electrolyte is also present as an auxiliary electrolyte in a smaller proportion by weight and volume.

[0041] The inventors have recognized that the amount of liquid electrolyte in the composite anode can be reduced using the inventive design. This makes it possible to significantly reduce the total amount of liquid electrolyte in a lithium-ion battery comprising the composite anode. In this way, both the amount of solvents and the amount of conducting salt, in particular LiPF6, can be reduced, thus minimizing both the risk of ignition from escaping liquids or gases and the health hazard posed by the formation of hydrogen fluoride (HF) when LiPF6 reacts with moisture.

[0042] In a preferred embodiment according to the invention, the composite anode has interconnected pores comprising solid inorganic lithium-ion conductors and liquid electrolyte. By arranging the solid inorganic lithium-ion conductor and the liquid electrolyte in interconnected pores, the contact resistance between the particles of the solid inorganic lithium-ion conductor can be reduced.

[0043] In a preferred embodiment of the invention, the composite anode has a porosity of 10 to 25% based on the volume without liquid electrolyte, and the porosity is filled with the liquid electrolyte to more than 90%, more preferably to more than 95%, and particularly preferably completely. By filling the porosity with liquid electrolyte as completely as possible, the contact resistance between the particles of the solid inorganic lithium-ion conductor can be improved.

[0044] In a preferred embodiment of the invention, the active anode material and the solid inorganic lithium-ion conductor each consist of particles or secondary particles, if present, and the particles of the active anode material have a larger mean grain size d50, preferably 5 to 1000 times larger, and more preferably 10 to 100 times larger, than the particles of the solid inorganic lithium-ion conductor. The measured values ​​are determined by scanning electron microscopy (SEM). Such a measurement method is described, for example, in US 5872358 A. By using particles or secondary particles of the solid inorganic lithium-ion conductor that have a larger grain size d50 than that of the solid inorganic lithium-ion conductor, the volume-specific energy density of the composite anode can be increased.

[0045] In a preferred embodiment, the active anode material consists of secondary particles, and the grain size d50 of the secondary particles is greater than 3 µm to 75 µm, preferably 5 µm to 35 µm. The measured values ​​are determined as described above.

[0046] In a preferred embodiment, the solid inorganic lithium-ion conductor consists of particles, and the particle size d50 is greater than 0.05 µm to 5 µm, preferably 0.1 µm to 2 µm. The measured values ​​are determined as described above.

[0047] In a preferred embodiment, the solid inorganic lithium-ion conductor is present in the composite anode at a weight of 10 to 50 wt.%, preferably 20 to 40 wt.%, relative to the active anode material.

[0048] In a preferred embodiment, the active anode material is selected from a group consisting of synthetic graphite, natural graphite, carbon, lithium titanate and mixtures thereof.

[0049] In a preferred embodiment, the solid inorganic lithium-ion conductor has a lithium-ion conductivity of at least 10 -5 S / cm at room temperature (20°C). The measured values ​​are determined according to the GITT (galvanostatic intermittent titration technique), as described, for example, in W. Weppner and RA Huggins, J. Electrochem. Soc., 124 1569-1578 (1977).

[0050] In a preferred embodiment, the solid inorganic lithium-ion conductor is selected from a group consisting of perovskite, glass formers, garnet, and mixtures thereof. The garnets described in EP1723080 B1 are particularly preferred because they are chemically and electrochemically especially stable in the 3-5 V potential range of the cathode (positive electrode).

[0051] In a preferred embodiment, the binder is selected from the group consisting of polyvinylidene fluoride, copolymer of polyvinylidene fluoride and hexafluoropropylene, copolymer of styrene and butadiene, cellulose, cellulose derivatives and mixtures thereof.

[0052] In a preferred embodiment, the liquid electrolyte comprises organic carbonates and a conducting salt, preferably LiPF6 or LiBF4.

[0053] The thickness of the composite electrode is generally 5 µm to 250 µm, preferably 20 µm to 100 µm. The measured values ​​are determined by optical methods as specified in US 4008523 A.

[0054] In a second aspect of the invention, the present invention relates to a lithium-ion battery comprising electrodes, separator and electrolyte, wherein one of the electrodes is a composite anode according to the first aspect of the invention.

[0055] In a third aspect of the invention, the present invention relates to a method for producing the composite anode according to the invention. The method comprises the following steps: -Mixing at least active anode material, a binder dissolved in a solvent, an inorganic ionic conductor and preferably an electrical conductive additive to form a homogeneous slurry. -Applying the slurry to a drain -Withdrawal of the solvent under reduced pressure and / or increased temperature, whereby porosity forms in the slurry. -Adjusting the porosity by calendering - Filling the free porosity of the composite anode with a liquid electrolyte. This can be done by impregnation, possibly supported by vacuum and / or tempering.

[0056] The lithium-ion battery according to the invention is suitable for both stationary and mobile applications. Due to the reduction in the amount of liquid electrolyte combined with the lower risk to occupants, the lithium-ion battery according to the invention is particularly suitable for applications in motor vehicles.

[0057] The invention is described below using examples. Examples of anode design: Reference anode:

[0058] 1.0 g of cellulose binder (Wollf Cellulose) is dissolved in 90 ml of demineralized water at room temperature. Then, 1.0 g of conductive carbon black (Super C65, Timcal) is introduced using a dissolver disc. Next, 96.0 g of synthetic graphite (MAG D20, Hitachi) is dispersed, followed by 2.0 g of SBR binder (ZEON, Japan). This creates a homogeneous suspension, which is applied to a copper carrier foil (10 µm rolled copper foil, Schlenk) using a semi-automatic film-drawing machine. After removing the water, a composite anode film is formed. Calendering (compression) of the anode film results in a porosity of 34% (by volume), corresponding to an anode thickness (excluding the collector) of 50 µm. Inventive anode:

[0059] 1.0 g of cellulose binder (Wolff Cellulose) is dissolved in 90 ml of demineralized water at room temperature. Then, 1.0 g of conductive carbon black (Super C65, Timcal) is introduced using a dissolver disc. Next, 64.0 g of LLZ garnet (average grain diameter 1 µm) and 96.0 g of synthetic graphite (MAG D20, Hitachi) are dispersed, followed by 2.0 g of SBR binder (ZEON, Japan). This results in a homogeneous suspension, which is applied to a copper support film (10 µm rolled copper foil, Schlenk) using a semi-automatic film-drawing device. After removing the water, a composite anode film is formed. After calendering (pressing) the anode film according to the invention with ceramic Li-ion conductors, a porosity of 16% (based on volume) results, corresponding to an anode thickness (without collector) of 50 µm. Examples of cell designs

[0060] For further cell construction, a cathode with an areal weight of 14.0 mg / cm² (4.5 g PVdF (Solvay), 4.5% Super C65, 91% lithium nickel cobalt manganese oxide (NCM111; BASF)) is used, coated onto a 15 µm aluminum foil (Hydro-Aluminium). A 25 µm thick polyolefin separator with the sequence PP / PE / PP is used. A 1.1 M solution of LiPF6 in EC:DEC (3:7 v / v) is used as the liquid electrolyte, penetrating the free volume (pores) of the anode, cathode, and separator. A lithium-ion cell with a nominal capacity of 2.0 Ah is constructed from the respective electrode / separator assemblies in a stacked design.

[0061] Twenty reference cells with reference anode and 20 cells according to the invention with an anode according to the invention are installed. Results of long-term cycling

[0062] During long-term RT cycling (voltage range 2.8 V to 4.2 V (1C, CCCV charge, 1C CC discharge) identical behavior is observed with a batch of 5 reference and inventive cells: After 500 cycles, 80% of the initial capacity (2 Ah) is reached. Safety tests

[0063] Ten cells each (reference and invention) are subjected to a so-called Sandia nail test ("penetration test", SANDIA REPORT, SAND2005-3123, Unlimited Release Printed August 2006 on page 18f; see http: / / prod.sandia.gov / techlib / access-control.cgi / 2005 / 053123.pdf) in a fully charged state (4.2V). The cells are pierced with a 3 mm thick nail.

[0064] The test results were evaluated using the "EUCAR Hazard Levels" in Table 2 on page 15f. of the Sandia Report. Safety Level 3 means a leak of less than 50% by weight of liquid electrolyte without ignition or explosion. Safety Level 4 corresponds to the previous Safety Level, but with a leak of more than 50% by weight of liquid electrolyte. At Safety Level 5, the cells also ignite. Table 1: Results of the security tests Cell type Observed cells of Safety Level 3 Observed cells of Safety Level 4 Observed cells of Safety Level 5 Reference cell 0 9 1 Inventive cell 10 0 0

[0065] Result: the cells according to the invention show improved safety behavior.

Claims

[1] Composite anode comprising a current collector, active anode material, binder, solid inorganic lithium-ion conductor and liquid electrolyte, wherein the solid inorganic lithium-ion conductor in the composite anode is present in a higher volume and weight fraction than the liquid electrolyte, and the active electrode material consists of secondary particles and the grain size d50 of the secondary particles is more than 15 µm to 75 µm. [2] Composite anode according to claim 1, wherein the composite anode has interconnected pores and the pores comprise solid inorganic lithium-ion conductor and liquid electrolyte. [3] Composite anode according to claim 1 or 2, wherein the composite anode has a porosity of 5 to 25% based on the volume without liquid electrolyte and the porosity is filled with the liquid electrolyte to more than 90%, more preferably to more than 95%, and in particular preferably completely. [4] Composite anode according to one of the preceding claims, wherein the active anode material and the solid inorganic lithium-ion conductor each consist of particles and the particles of the active anode material have a larger mean grain size d50, preferably a grain size d50 5 to 1000 times larger, than the particles of the solid inorganic lithium-ion conductor. [5] Composite anode according to one of the preceding claims, wherein the solid inorganic lithium-ion conductor consists of particles and wherein the particle size d50 of the particles is more than 0.05 µm to 5 µm. [6] Composite anode according to one of the preceding claims, wherein the solid inorganic lithium-ion conductor is present in the composite anode to a weight of 10 to 80 wt.%, preferably 20 to 60 wt.%, in relation to the active anode material. [7] Composite anode according to any of the preceding claims, wherein the active anode material is selected from the group consisting of synthetic graphite, natural graphite, carbon, lithium titanate and mixtures thereof. [8] Composite anode according to one of the preceding claims, wherein the solid inorganic lithium-ion conductor has a lithium-ion conductivity of at least 10 at room temperature. -5 possesses S / cm. [9] Composite anode according to any of the preceding claims, wherein the solid inorganic lithium-ion conductor is selected from the group consisting of perovskite, glass formers, garnet and mixtures thereof. [10] Composite anode according to any of the preceding claims, wherein the binder is selected from the group consisting of polyvinylidene fluoride, copolymer of polyvinylidene fluoride and hexafluoropropylene, copolymer of styrene and butadiene, cellulose, cellulose derivatives and mixtures thereof. [11] Composite anode according to one of the preceding claims, wherein the liquid electrolyte comprises organic carbonates and a lithium conducting salt, preferably LiPF6 or LiBF4. [12] Lithium-ion battery comprising electrodes, separator and electrolyte, wherein one of the electrodes is a composite anode according to any one of claims 1 to 11. [13] Method for producing a composite anode according to any one of claims 1 to 11 comprising: -Mixing at least active anode material, a binder dissolved in a solvent, and a solid inorganic lithium-ion conductor to form a homogeneous slurry. -Applying the slurry to a drain -Withdrawal of the solvent under reduced pressure and / or increased temperature, whereby porosity forms in the slurry. -Adjusting the porosity by calendering -Replenishment of the porosity with a liquid electrolyte.