Active cathode material for secondary lithium cells and batteries

Abstract

Cathode material comprising particles of lithium metal oxide with a coating, wherein the coating consists of a solid lithium ion conductor with a garnet-like crystal structure and has been deposited onto the lithium metal oxide by a physical process, characterized in that the lithium metal oxide has the general formula xLiMO2(1-x)Li2M'O3 with 0 < x < 1, where M stands for at least one metal with the average oxidation state of three, which comprises at least nickel, and M' stands for at least one ion with the average oxidation state of four, which comprises at least manganese.

Description

[0001] The present invention relates to a cathode material for secondary lithium cells or batteries. The invention also relates to a positive electrode and an electrochemical device comprising the cathode material, as well as a method for producing the cathode material.

[0002] A battery consists of at least two interconnected cells. In this description, the terms "cell" and "battery" are used synonymously.

[0003] An example of secondary lithium batteries are lithium-ion batteries. In this battery system, electrical energy is stored using lithium ions (at the negative electrode) and (mostly) transition metal oxides (at the positive electrode) in a chemical process through intercalation. In lithium-ion batteries, lithium in ionized form can migrate back and forth through the electrolyte between the two electrodes. Unlike lithium ions, the transition metal ions present at the cathode are stationary and do not change their structure upon insertion and removal.

[0004] This lithium ion flow is necessary to balance the external current flow during charging and discharging so that the electrodes themselves remain (largely) electrically neutral. During discharging, lithium atoms at the negative electrode each give up an electron, which flows via the external circuit to the positive electrode. At the same time, the same number of lithium ions migrate through the electrolyte from the negative electrode (anode) to the positive electrode (cathode). At the positive electrode, however, it is not the lithium ions that absorb the electron, but the transition metal ions present there, which are highly ionized in the charged state. In lithium-ion systems, these can be cobalt, nickel, manganese, iron ions, etc. In the discharged state, the lithium is therefore still present in ionic form at the positive electrode.

[0005] The cathode materials currently used in secondary lithium batteries represent a bottleneck in lithium-ion technology in terms of battery cost and capacity. Against this background, the search for a new generation of cathode materials that enable cathodes with increased capacity, good rate capability, high operating voltage, and a long and safe cycle life, especially for operation in large-format cells, is essential.

[0006] CN 102738451 A discloses a cathode material for lithium batteries, wherein the active cathode material was coated with a fast lithium ion conductor having a garnet-like crystal structure by means of a sol-gel process followed by sintering.

[0007] Sébastien Patoux et al., "High voltage spinel oxides for Li-ion batteries: From the material research to the application", Journal of Power Sources - J POWER SOURCES, vol. 189 (2009), no. 1, pages 344-352, discloses high-voltage spinel oxides (HV spinels) for lithium-ion batteries with the general composition LiMn 2-x M x O4, where M is a transition metal element.

[0008] J. Liu, A. Manthiram, Journal of the Electrochemical Society 156, S13, 2009 and J. Liu and A. Manthiram, Chem. Mater. 21, 1695, 2009 disclose cathode materials made of Al2O3 coated HV spinels.

[0009] DE 102012203139 A1 relates to a solid-state cell (all-solid-state cell) comprising a lithium-containing anode, a cathode, and a lithium-ion-conducting solid electrolyte separator arranged between the anode and the cathode. To improve the safety and cycle stability of the cell, the cathode comprises a composite material comprising at least one lithium titanate and at least one lithium-ion-conducting solid electrolyte. Furthermore, the invention relates to a corresponding solid-state battery and a mobile or stationary system equipped therewith.

[0010] CHEN JM ET AL: “Electrochemical studies on LiCoO2 surface coated with Y3Al5O12 for lithium-ion cells”, JOURNAL OF POWER SOURCES, ELSEVIER SA, CH, vol. 189, no. 1, 1 April 2009 (2009-04-01), pages 279 - 287, XP025982697, ISSN: 0378-7753, DOI: 10.1016 / J.JPOWSOUR.2008.09.049 concerns spinel-type oxides coated with garnet-like ionic conductors.

[0011] One object of the present invention is to provide a cathode material for lithium-ion batteries with improved lifetime, energy density, stability, and performance. A further object is to provide an electrode and an electrochemical device comprising the cathode material, as well as a method for producing the cathode material.

[0012] The above object is achieved by a cathode material according to claim 1, an electrode according to claim 7 and an electrochemical device according to claim 9.

[0013] Preferred embodiments are presented in the dependent claims. In the present description, the term lithium metal oxide refers to all compounds suitable for active cathode materials which, in addition to lithium, comprise at least one other metal selected from the group of transition metals, as well as oxygen. A coating produced in this way differs structurally from coatings deposited by sol-gel processes and subsequently sintered in that the coating is less rough and more tightly closed. This difference is detectable by electron transmission microscopy (see, for example, electron transmission microscopy images for LiCoO2 coated with ZrO2 by sol-gel processes in: Chen, ZH and Dahn JR, Solid-State Lett., 2002, 5, A213-A216).

[0014] Suitable fast lithium-ion conductors with a garnet-like crystal structure are those described in DE 102007030604 A1 and DE 102004010892 B3. For example, Li5La3M2O can be used as a fast lithium-ion conductor with a garnet-like crystal structure. 12 (M = Ta, Nb) or Li6ALa2M2O 12 (A = Ca, Sr, Ba; M = Ta, Nb) can be used.

[0015] By using the cathode material according to the invention in a lithium-ion battery, the decomposition of liquid electrolytes (for example 1M lithium hexafluorophosphate (LiPF6) in a mixture of the organic solvents ethylene carbonate (EC) and ethyl methyl carbonate (EMC)) in the potential range 4.2 V to 4.3 V can be significantly reduced and thus the service life of the lithium battery can be increased.

[0016] Preferably, the physical deposition method is selected from the group consisting of atomic layer deposition (ALD), plasma-enhanced chemical vapor deposition (PECVD), and pulsed laser deposition (PLD). Further preferred are laser evaporation and atomic layer deposition. Atomic layer deposition is particularly preferred.

[0017] Plasma-enhanced chemical vapor deposition (CVD) is a special form of chemical vapor deposition (CVD) in which the chemical deposition is assisted by a plasma. The plasma can burn directly on the substrate to be coated (direct plasma method) or in a separate chamber (remote plasma method). While in CVD the dissociation (breaking up) of the molecules of the reaction gas occurs through the external application of heat and the energy released by the subsequent chemical reactions, in PECVD this task is performed by accelerated electrons in the plasma. In addition to the radicals formed in this way, ions are also generated in the plasma, which, together with the radicals, cause the layer to be deposited on the substrate. The gas temperature in the plasma usually only increases by a few hundred degrees Celsius, which means that, in contrast to CVD, even more temperature-sensitive materials can be coated.

[0018] In the direct plasma method, a strong electric field is applied between the substrate to be coated and a counter electrode, igniting a plasma. In the remote plasma method, the plasma is positioned so that it does not have direct contact with the substrate. This provides advantages in terms of selective excitation of individual components of a process gas mixture and reduces the possibility of plasma damage to the substrate surface by the ions. Disadvantages include the possible loss of radicals along the path between the remote plasma and the substrate and the possibility of gas-phase reactions before the reactive gas molecules reach the substrate surface.

[0019] The plasmas can also be generated inductively / capacitively by irradiating an alternating electromagnetic field, making electrodes unnecessary.

[0020] Laser evaporation is a physical vapor deposition (PVD) process and closely related to thermal evaporation. It involves the deposition of layers by laser ablation. For this process, both the layer material to be deposited (target) and the substrate on which the layer is to be deposited (substrate) are placed in a vacuum container (receiver).

[0021] The target material is illuminated in a vacuum chamber with high intensity pulsed laser radiation (≈ 10 MW / cm 2) and thereby evaporates. The evaporation process of the target material occurs via the absorption of the energy of the laser beam by the material to be evaporated. From a certain (sufficient) amount of energy, a plasma forms on the target, from which atoms can detach from the target. Using high process gas pressures (> 1 mbar), condensation of the material vapor into clusters (groups of atoms) is possible in the gas phase. This material vapor moves through the vacuum chamber away from the target towards the substrate, where it condenses into a thin layer. To produce crystalline layers, the substrate is additionally heated to enable diffusion processes and thus the rearrangement of the atoms. In this way, other particles can also be incorporated into the crystal, either to produce more complex materials or to create doping.

[0022] Particularly good results are achieved with UV lasers (e.g., XeCl or KrF excimer lasers), as their radiation has high photon energy, which is absorbed by a wide variety of materials because it lies above the plasma frequency. Other pulsed lasers for PLD include transversely excited CO2 lasers, Q-switched Nd:YAG lasers, and increasingly also pulsed femtosecond lasers. The pulse length is typically in the range of 10–50 ns with a repetition frequency of a few hertz.

[0023] For the deposition of fast lithium-ion conductors with a garnet-like crystal structure, an experimental setup can be used, for example, as described in Katherine A. Sloyan et al., “Growth of crystalline garnet mixed films, superlattices and multilayers for optical applications via shuttered Combinatorial Pulsed Laser Deposition”, Optics Express, Vol. 18, Issue 24, pages 24679-24687 (2010).

[0024] Atomic layer deposition is a significantly modified CVD process for depositing thin films through two or more cyclic, self-limiting surface reactions. As with other CVD processes, ALD also achieves film formation through a chemical reaction between at least two starting materials (so-called precursors). Unlike conventional CVD processes, ALD introduces the starting materials into the reaction chamber cyclically, one after the other. Between the gas inlets of the starting materials, the reaction chamber is typically purged with an inert gas (e.g., argon). This is intended to clearly separate the partial reactions from one another and limit them to the surface.An essential feature of ALD is the self-limiting nature of the partial reactions, i.e. the starting material of a partial reaction does not react with itself or its own ligands, which limits the layer growth of a partial reaction to a maximum of one monolayer per cycle for any length of time and gas quantity.

[0025] The cycle must be repeated several times during the coating process to achieve the desired layer thickness. Ideally, each exposure step is completed, meaning the precursor molecules chemisorb or react with the surface groups until the surface is completely covered. After that, no further adsorption occurs (self-limitation). Under these reaction conditions, the layer growth is self-controlling or self-limiting, meaning the amount of layer material deposited in each reaction cycle remains constant.

[0026] Depending on the process and reactor, one cycle lasts between 0.5 and several seconds, with 0.1 to 3 Å of film material being produced per cycle (highly dependent on the material system and process parameters). In reality, however, the spatial expansion of the starting substrates (steric hindrance) and incomplete partial reactions mean that a continuous layer of the target material cannot be achieved in one cycle.

[0027] Despite the non-ideal growth in real-world processes, the deposition of thin films using atomic layer deposition offers several advantages. A key point is the excellent layer thickness control of ultra-thin films of less than 10 nm. This is because the self-limiting reaction mentioned above ensures that the film grows only by a definable amount per cycle, which, in the saturation range, is independent of the cycle duration. The film grows proportionally to the number of reaction cycles, enabling precise control of the film thickness. Furthermore, the separate dosing of the precursor substances prevents unwanted gas-phase reactions in the sample chamber and also enables the use of highly reactive precursors. The fixed dosing allows sufficient time for each reaction step to complete, enabling highly pure films even at relatively low temperatures.

[0028] The molar ratio of the coating to the lithium metal oxide is preferably at most 0.01. In this way, compared to a conventional coating, the energy density, specific energy and high current load capacity of the cell (since the coating is an electrical insulator) can be improved and costs can be reduced at the same time. In addition, a proportion greater than 0.1 leads to a deterioration in electrical conductivity, i.e. the lithium metal oxide particle is electrically insulated because the coating is only ionically conductive and not electrically conductive; this reduces the performance of the electrode or cell. The coating preferably has a thickness of 10 to 100 nm, more preferably 20-50 nm.

[0029] Preferably, the coating is circumferential and continuous. Particularly preferably, the coating is free of pinholes. This prevents direct contact of the electrolyte with the active cathode material, i.e., the lithium metal oxide, thus reducing undesirable decomposition of the electrolyte during operation of the electrochemical cell and thus extending the service life of the electrochemical cell.

[0030] In a reference example, the lithium metal oxide has a spinel crystal structure. For example, lithium manganese spinel (LiMn2O4) of the spinel structure type can be used. Doped or undoped HV spinels are preferably used. HV spinels with the general composition LiMn are particularly preferred. 2-x M xO4, where M is a transition metal element and x can take different values ​​between 0 and 2 depending on the transition metal element. For example, the HV spinel LiMn 1,5 No 0,5 O4 can be used. Such materials are disclosed, for example, in Sébastien Patoux et al., "High voltage spinel oxides for Li-ion batteries: From the material research to the application", Journal of Power Sources - J Power Sources , Vol. 189 (2009), No. 1, pages 344-352.

[0031] According to the invention, the layered lithium metal oxide has the general formula xLiMO2(1-x)Li2M'O3 with 0 < x < 1, where M represents at least one metal with an average oxidation state of three, which comprises at least nickel, and M' represents at least one ion with an average oxidation state of four, which comprises at least manganese. Such materials are disclosed, for example, in Michael M. Thackeray et al., Journal of Materials Chemistry, J MATER CHEM, 2007, 17, 3112-3125.

[0032] In a reference example, the lithium metal oxide is a layered Ni oxide with an alpha-NaCrO2 structure with at least 30% Ni content. Such materials are disclosed, for example, in EP 0017400B1 (Goodenough, JB et al.).

[0033] In a reference example, the lithium metal oxide is LiMSiO4, where M is a metal selected from the group consisting of Fe, Mn, Ni, Co, and a mixture thereof. Such materials are described, for example, in Zhou F, Cococcioni M, Kang K, Ceder G.; "The Li intercalation potential of LiMPO4 and LiMSiO4 olivines with M = Fe, Mn, Co, Ni"; [J]. Electrochemistry Communications, 2004, 6: 1144-1148.

[0034] In a reference example, the lithium metal oxide has an olivine structure. In this reference example, a material having the general formula LiMPO4, where M is a divalent metal selected from the group consisting of Fe 2+ , Mn 2+, Co 2+ and a mixture thereof. LiMnO4 is particularly preferred. These materials are described, for example, in Zhumabay Bakenov and Izumi Taniguchi, "LiMnPO4 Olivine as a Cathode for Lithium Batteries," The Open Materials Science Journal, 2011, 5, (Suppl 1: M4) 222-227.

[0035] Preferably, the average weight-related particle size d50 of the lithium metal oxide particles is 0.1 - 30 µm, preferably 0.5 - 20 µm.

[0036] In a second aspect, the present invention relates to an electrode comprising the above cathode material and a current collector. For example, rolled aluminum foil can be used as the current collector. Preferably, the electrode further comprises a binder and an electrically conductive additive. The electrically conductive additive may comprise carbon. Preferably, carbon fibers, carbon black, or a mixture thereof are used. Particularly preferred is conductive carbon black, e.g., Super P from Timcal.

[0037] In a third aspect, the present invention relates to an electrochemical device comprising the above-described electrode as a positive electrode, an ion-conducting medium, and a negative electrode. The device is preferably configured as a battery.

[0038] In one reference example, the present invention relates to a method for producing the cathode material, wherein particles of lithium metal oxide with a coating of a solid lithium ion conductor with a garnet-like crystal structure are deposited onto the lithium metal oxide by a physical method. Preferably, the physical deposition method is selected from the group consisting of atomic layer deposition (ALD), plasma-enhanced chemical vapor deposition (PECVD), and laser beam evaporation (PLD). Atomic layer deposition is particularly preferred. Fig. Figure 1 shows a schematic drawing of a particle of lithium metal oxide (1) with a coating of a fast lithium ion conductor of the garnet-like crystal structure type (2), wherein the coating was deposited by a sol-gel process (state of the art) and subsequently sintered. Fig.Figure 2 shows a schematic drawing of a particle of lithium metal oxide (1) with a coating of a fast lithium ion conductor of the garnet-like crystal structure type (2), the coating being deposited by a physical process.

[0039] In one embodiment, the cathode protection layer is deposited by PLD on HV spinel (LiMn 1,5 No 0.5 O4) particles with a weight-average particle size d50 of 10 µm are deposited. A garnet-like compound prepared using standard sol-gel methods serves as the target. The synthesis conditions during the deposition process are in an O2 atmosphere with an oxygen pressure between 1 and 10 Pa.

[0040] The coating is examined using imaging techniques to rule out the possibility of a "rough" coating, where the active material's surface is not completely covered. SEM (scanning electron microscopy), for example, is suitable for this purpose. Elemental surface analysis (XPS) is performed to analyze the composition of the protective layer. Alternatively, other structural analysis methods, such as X-ray powder diffraction, can also be used.

[0041] XRR analyses (X-ray reflectometry) can be used to analyze the thickness.

[0042] Laboratory cells with a nominal capacity of 40 mAh for long-term cycling are assembled using the following construction: aluminum composite foil as packaging material (Showa, JP); Hitachi SMG A3 synthetic graphite; Celgard 25 µm PP / PE / PP separator (type 2335) with the cathode-facing side coated with 3 µm Al2O3 / PVdF-HFP (80:20 w / w); PVdF (cathode binder); CMC / SBR (anode binder). Liquid electrolyte: 1 M LiPF6 in EC:DEC (3 / 7, v / v).

[0043] Variants: a) Reference cell with HV spinel (LiMn 1,5 No 0.5 O4) without garnet solid coating. b) Cell with HV spinel (LiMn 1,5 No 0.5 O4) with Al2O3 coating according to the state of the art. c) Cell with HV spinel (LiMn 1,5 No 0.5 O4) with garnet solid coating according to the invention. Table 1 : Results of room temperature long-term cycling (1C charge, 1C discharge) Cell variant Cycles until 80% of the nominal capacity is reached. 1C cycling remark a: without any protective layer. 300 Particle surface without protection b: Protective layer SdT 400 Surface-surface with protection, but without Li-conductor function c: protective layer according to the invention 450 Surface-surface with protection, but with Li-conductor function

Claims

[1] Cathode material comprising particles of lithium metal oxide with a coating, wherein the coating consists of a solid lithium ion conductor with a garnet-like crystal structure and has been deposited onto the lithium metal oxide by a physical process, characterized by that the lithium metal oxide has the general formula xLiMO2(1-x)Li2M'O3 with 0 < x < 1, where M stands for at least one metal with the average oxidation state of three, which comprises at least nickel, and M' stands for at least one ion with the average oxidation state of four, which comprises at least manganese. [2] The cathode material of claim 1, wherein the physical deposition method is selected from the group consisting of atomic layer deposition (ALD), plasma enhanced chemical vapor deposition (PECVD), and laser beam evaporation (PLD). [3] Cathode material according to claim 1 or 2, wherein the molar ratio of the coating to the lithium metal oxide is at most 0.

01. [4] Cathode material according to one of the preceding claims, wherein the coating has a thickness of 10 to 100 nm, preferably 20-50 nm. [5] Cathode material according to one of the preceding claims, wherein the coating is circumferential and closed. [6] Cathode material according to one of the preceding claims, wherein the size of the particles of lithium metal oxide is 0.1 - 30 µm, preferably 0.5 - 20 µm. [7] An electrode comprising the cathode material according to any one of claims 1 to 6 and a current collector. [8] The electrode of claim 7, wherein the electrode further comprises binder and a conductive additive. [9] An electrochemical device comprising the electrode according to claim 7 or 8 as a positive electrode, an ion-conducting medium and a negative electrode. [10] An electrochemical device according to claim 9, wherein the device is configured as a battery.

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