Coating of anode and cathode active materials with high-voltage stable solid electrolytes and an electron conductor in a multilayer system and lithium-ion battery cell

Abstract

Electrode active material for lithium-ion batteries, comprising active material particles (1) having a first coating (2) of a solid electrolyte, wherein the active material particles (1) have a second coating (3) of an electron-conducting material applied to the first coating (2), and wherein the first coating (2) is applied to the active material in such a thickness that electron transport through the first coating (2) can take place, characterized in that the electrode active material of the active material particles is LiNi0.5Mn1.5O4.

Description

The present invention relates to a coating of electrode active material for lithium-ion batteries, which are used in particular as traction batteries in electric vehicles, with the features of the preamble of claim 1. Coated electrode active materials are known in the prior art. For example, German patent application DE 34 43 455 A1 describes a galvanic cell with a polymer electrode. An aluminum substrate can be used as the electrode support, the surface of which comprises a conventional oxide protective layer. The aluminum substrate can then be further coated with an electron-conducting material such as graphite or a metal. An electrode material comprising lithium ions is not mentioned. The generic patent US 2018 / 0212233A1 describes an all-solid-state (ASS) secondary battery that uses a solid electrolyte, typically in a lithium-ion configuration, where lithium ions are transferred between the cathode and anode. Because both the cathode material and the electrolyte are solid, the electrolyte cannot penetrate deeply into the cathode material, thus limiting the interface and lithium-ion transport. To improve this, a mixed layer of cathode material and solid electrolyte is formed, increasing the interface area. However, when using sulfide electrolytes, reactions occur at the interface that generate resistance-forming components and increase resistance, especially under high load or high voltage.To prevent this, the cathode core particles are coated with two layers of lithium-containing compounds: the first layer protects the cathode, while the second layer protects the electrolyte. Both layers maintain the lithium-ion conductivity, thereby improving the interfacial properties, lithium-ion transport, and the battery's performance, lifespan, and cycle stability. The battery itself has a layered structure with a cathode layer, a solid electrolyte layer, and an anode layer. The cathode layer consists of a composite cathode material comprising the core particles and the two coating layers, as well as optional conductivity enhancers, binders, and fillers.The core particles consist of lithium-containing transition metal oxides such as LCO, NCA, NCM, or LiFePO4, but also of sulfides or vanadium oxides, and have a spherical or oval-spherical shape with a diameter of 0.1 to 50 micrometers. The first coating layer consists of lithium-containing oxides or phosphate oxides such as LZO or Li-Ti-PO4 and has a thickness of 1 to 50 nanometers, while the second layer contains lithium-containing oxides with germanium, niobium, or gallium, and the combined thickness of both layers is between 1 and 500 nanometers. These layers suppress reactions between the cathode and electrolyte, maintain lithium-ion conductivity, and thus improve the battery's properties. The anode layer contains anode particles, which are either lithium-alloyable or lithium-intercalable and have a lower charge and discharge potential than the cathode, as well as the solid electrolyte.The combination of coated cathode core particles, solid electrolyte and suitable additives such as conductors, binders and fillers ensures that the all-solid-state battery has a high energy and power density, improved lifespan and stable charging and cycle characteristics. Between the active material of the electrode material, particularly in the cathode, and the electrolyte in a lithium-ion battery cell, undesirable side reactions occur at high voltages (> 4.2 V) because these cathode active materials are in direct contact with the electrolyte. One consequence of these side reactions is the leaching of cations from the cathode active material, which leads to a degradation of the cathode active material's performance and thus to a reduction in the overall performance of the lithium-ion battery cell. Specifically, in the case of the cathode active material LiNi0.5Mn1.5O4, nickel and manganese ions are released from the structure at a voltage of 4.9 V, diffuse through the separator onto the anode, and are deposited there as metallic ions. Furthermore, the direct contact between the cathode active material and the electrolyte in a lithium-ion battery cell leads to electrolyte decomposition, especially at high voltages > 4.2 V. This results in reduced performance of the lithium-ion battery cell. Protecting the cathode active material and the electrolyte from the aforementioned degradation mechanisms or effects is essential to maintain or extend the lifespan of a lithium-ion battery cell. Furthermore, European patent EP 2 472 662 B1 discloses an electrolyte for lithium secondary batteries comprising a lithium salt, a non-aqueous organic solvent, and an additive from the group of vitamins G, B4, B5, H, M, D2, Bx, D3, and K1. According to the aforementioned patent, the cathode can have a thin film on its surface, which is produced from the additive by oxidation upon initial application of a voltage. Furthermore, electrode materials for lithium-ion batteries are known which provide a polymer coating to protect the active material, especially from oxidation, or in which the cathode active material is coated with a solid electrolyte, as disclosed, for example, in the American patent application US 2017 / 0018760A1 or in the German patent application DE 102015217749A1. In all previously known coated electrode materials, the coating serves either as protection for the sensitive active material or simultaneously as a lithium-ion conductor. A disadvantage of this approach, however, is that electron transport is impaired because the known coatings cannot conduct electrons. Solid electrolytes are purely ionic conductors and not electron conductors. Additionally, due to the direct contact between the electrode material and the electrolyte, especially at high voltages, electrolyte decomposition occurs, leading to reduced performance of the lithium-ion battery. The object of the present invention is therefore to provide an improved electrode active material that offers good protection of the active material while simultaneously exhibiting good electron conductivity. This should enable improved performance, high high-voltage capability, and an extended lifespan of the lithium-ion battery. This problem is solved according to the invention by an electrode active material with the features of claim 1. The invention comprises an electrode active material for lithium-ion batteries, comprising active material particles which have a first coating of a solid electrolyte. According to the invention, the active material particles have a second coating made of an electron-conducting material, which is applied to the first coating, and wherein the first coating is applied to the active material in such a thickness that electron transport through the first coating can take place. A coating using only a conventional solid electrolyte would electronically isolate the electrode particles from each other, because solid electrolytes are purely ionic conductors and are not conductive for electrons. Advantageously, with the electrode active material according to the invention, which can be both a cathode material and an anode material, it can be achieved that anode and cathode active material particles are in ionic and electronic contact with each other in order to conduct lithium ions and simultaneously transport electrons to the current collector. In a first aspect of the invention, this is achieved by making the solid electrolyte layer of the first coating very thin, but thick enough to protect the active material particle. The solid electrolyte layer can be, for example, one to a few atomic layers thick. The transmission coefficient for electronic communication through the solid electrolyte should be very high, that is, as close as possible to 1. The second aspect of the invention consists in the deposition of a second layer onto this first layer, which is a very good electron conductor. It should have a low Fermi energy and a low work function. As a result of the at least double coating of the anode and cathode active material in the multilayer system with a high-voltage stable solid electrolyte as the first coating and an electron-conducting layer as the second coating, utilizing the tunnel effect, the performance of the anode and cathode active materials is increased at voltages > 4.2 V, especially at 4.9 V. The cause is the prevention of material degradation and electrolyte breakdown. As a result, the performance of the lithium-ion battery cell is increased. Improved performance of a lithium-ion battery cell refers in particular to a longer lifespan and higher current capability. In one embodiment of the invention, the first coating comprises a solid electrolyte comprising a solid electrolyte material from the group of NASICON solid electrolytes, in particular LATP or LAGP, and (anti-)perovskites, in particular LLTO or Li3OCl. Deposition can be carried out using physical, wet-chemical, or mechanical processes. These deposition and coating processes are generally known to those skilled in the art. Examples include ALD, MLD, CVD, PVD, electron beam deposition, laser deposition, plasma deposition, radio frequency sputtering, microemulsion deposition, successive ionic layer deposition, aqueous deposition, solid-phase diffusion, sputter coating, sol-gel coating, and powder coating. By selecting the aforementioned solid electrolytes as the first coating, it can be ensured that, depending on the electrode type, cathode or anode material, very good ionic conductivity and, at the same time, good protection of the electrode active material are achieved. Decomposition of the solid electrolyte material due to high voltages can be suppressed by utilizing the tunneling effect and the resulting electronic conductivity. In one embodiment of the invention, the second coating is made of an electron-conducting material selected from the group consisting of graphene, titanium, zirconium, boron, vanadium oxide, titanium oxide, niobium oxide, lithium metal alloy, in particular Zn-Li, Sn-Li, Al-Li, lithium metal oxides, in particular Li2ZrO3, Li3.5Al2O3, Li4Ti5O12, and lithium metal fluorides, in particular Li3AlF6, Li2AlF4, Li3VF6. These materials are excellent electron conductors and possess a low Fermi energy. Additionally, they exhibit a low work function (WA). Deposition can be achieved using physical, wet-chemical, or mechanical methods. According to the invention, the electrode active material of the active material particles is LiNi0.5Mn1.5O4. This design allows for a significant improvement in high-voltage stability, particularly for the aforementioned cathode active material, and thus extends the service life of a battery containing this material. In a further preferred embodiment of the invention, the first coating has a layer thickness of 0.05 nm to 100 nm, in particular a layer thickness of 0.1 nm to 80 nm, and preferably a layer thickness of 0.5 nm to 50 nm. In this way, the goal of utilizing the tunnel effect can be achieved, meaning that the electrons bridge the path from the cathode active material to the second layer, which is electron-conducting, even though there is an electronically insulating solid electrolyte layer in between. In other words, the first coating preferably has a layer thickness of one to several atomic layers and is applied using physical, wet-chemical or mechanical methods. The second coating can also be applied using physical, wet-chemical or mechanical methods. Depending on the manufacturing process, the method for coating the electrode active material particles can be adapted. Suitable methods include, for example, physical or chemical vapor deposition (ALD), chemical vapor deposition (MLD), chemical vapor deposition (CVD), chemical vapor deposition (PVD), electron beam deposition (EVD), laser deposition, plasma deposition, radio frequency sputtering, microemulsion deposition, successive ionic layer deposition (SID), aqueous deposition, solid-phase diffusion, sputter coating, sol-gel coating, or powder coating. All substances known for lithium-ion batteries are suitable as electrode active materials. The following are listed as examples, but are not limited to them: Suitable oxide electrode active materials for the cathode include, for example: LiNiCoAlO2, LiNiCoMnO2(NMC), LiMn2-xMxO4 with M = Ni, Fe, Co or Ru and with x = 0 to 0.5, and LiCoO2(LCO). Suitable anode active materials include, for example, the following materials: V2O5, LiVO3, Li3VO4 and Li4Ti5O12(LTP). Compounds comprising phosphates are also suitable as electrode materials, such as Li3V2(PO4)3 or LiMPO4 with M = ¼ (Fe, Co, Ni, Mn) for a cathode or LiM2(PO4)3 with M = Zr, Ti, Hf or a mixture thereof for an anode. In a further embodiment of the invention, a third coating made of a solid electrolyte is applied to the second coating, and optionally a fourth coating made of an electron-conducting material is applied to this third coating. The sequence of the two layers thus remains the same; only more layers are applied. This allows for further improved high-voltage stability and long-term durability of the electrode material according to the present invention. The invention is also not limited to a two-layer or four-layer multilayer coating. It is equally within the scope of the present invention to apply further coatings in the same sequence, whereby a solid electrolyte layer or an electron-conducting layer should always be optionally provided as the outer layer. Another object of the present invention is a secondary battery comprising an electrode active material according to the invention. It is equally an object of the present invention that an electrode active material according to the invention, as described above, finds use in the manufacture of battery cells, in particular for traction batteries of vehicles. There are numerous possibilities for designing and further developing the electrode active material. For this purpose, reference may first be made to the claims subordinate to claim 1. In the following, a preferred embodiment of the invention will be explained in more detail with reference to the drawings and the accompanying descriptions. In the drawings: Fig. 1 shows, in a highly schematic representation, a cross-section of an electrode material and, as a detailed view, a section of such a particle of the electrode material according to the invention; Fig. 2 shows, in a highly schematic representation, an energy diagram of the coated active material according to the invention under applied voltage; Fig. 3 shows, in a highly schematic representation as a sectional view, an electrode material particle with a multilayer coating according to a further aspect of the present invention; and Fig.4 in a highly schematic representation as a sectional view an electrode material particle with a multilayer coating according to a further aspect of the present invention. Figure 1 schematically shows a cross-section of an electrode material according to the invention, as well as an enlarged section thereof. The electrode active material for lithium-ion batteries comprises active material particles 1, which have a first coating 2 made of a solid electrolyte. According to the invention, the active material particles 1 have a second coating 3 made of an electron-conducting material, which is applied to the first coating 2. The first coating 2 is applied to the active material 1 with such a thickness that electron transport through the first coating 2 can take place. Suitable oxide electrode active materials for the cathode include, for example: LiNiCoAlO2, LiNiCoMnO2(NMC), LiMn2-xMxO4 with M = Ni, Fe, Co or Ru and with x = 0 to 0.5 as well as LiCoO2(LCO). Suitable anode active materials include, for example, the following materials: V2O5, LiVO3, Li3VO4 and Li4Ti5O12(LTP). Compounds comprising phosphates are also suitable as electrode materials, such as Li3V2(PO4)3 or LiMPO4 with M = ¼ (Fe, Co, Ni, Mn) for a cathode or LiM2(PO4)3 with M = Zr, Ti, Hf or a mixture thereof for an anode. According to the invention, the electrode active material of the active material particles is LiNi0.5Mn1.5O4. In other words, the coating of anode and cathode active material particles is thus carried out with a first layer 2, which is a very thin, ion-conducting solid electrolyte layer on the order of one to several atomic layers. Since the solid electrolyte layer cannot conduct electrons itself, this layer 2 is applied so thinly that the tunneling effect can be utilized. High-voltage stable doped or undoped solid electrolytes of the NASICON type such as LATP, LAGP or (anti-)perovskites such as LLTO or Li3OCl are deposited onto anode and cathode active material particles 1. The following are some examples of specific compounds suitable as solid electrolytes in the aforementioned sense, without being considered an exhaustive list: • - Oxides, such as Li7-xLa3Zr2AlxO12 with x = 0 to 0.5 or Li7La3Zr2-xTaxO12 with x = 0 to 0.5, • - Lithium aluminum titanium phosphates, such as Li4+xMxTi2-x(PO4)3 with x = 0 to 7 and M = Al (LATP), Fe, Y or Ge, • - Lithium lanthanum zirconates, where additional dopants of tantalum, aluminum and iron can be used, • - Lithium phosphorus sulfides, where germanium and selenium can be added, such as Li7P3S11, Li10P3S12, Li10MxP3-xS12 with M = Ge, Se and x = 0 to 1, with M = AyBz, where A = Si, Ge and B = Sn, Si and with y = 0 to 0.5 and z = 1 - y. The first coating 2 can preferably have a layer thickness of 0.05 nm to 100 nm, in particular a layer thickness of 0.1 nm to 80 nm, and preferably a layer thickness of 0.5 nm to 50 nm. The second important aspect of the invention is that a layer 3 is deposited onto this first layer 2, which can conduct electrons very well and has a low Fermi energy (E3 in Fig. 2) as well as a low work function (WA(3) in Fig. 2), such as graphene and metals (titanium, boron, zirconium etc.), metal oxides (VO2, TiO, NbO2, etc.), lithium metal alloys (Zn-Li, Sn-Li, Al-Li etc.), lithium metal oxides (Li2ZrO3, Li3.5Al2O3, Li4Ti5O12 etc.) or lithium metal fluorides (Li3AlF6, Li2AlF4, Li3VF6 etc.). Furthermore, the lithium ions bridge the path from the active material or solid electrolyte across the electronically conductive material by means of tunnels. As a result of coating the anode and cathode active material in the multilayer system with a high-voltage stable solid electrolyte and an electron-conducting layer, utilizing the tunnel effect, the performance of the anode and cathode active materials is increased at voltages > 4.2 V, especially at 4.9 V. The cause is the prevention of material degradation and electrolyte breakdown. As a result, the performance of the lithium-ion battery cell is increased. Improved performance of a lithium-ion battery cell refers in particular to a longer lifespan and higher current capability. The multilayer coating of solid electrolyte 2 and electron-conducting material 3 on the anode and cathode active material 1 can conduct electrons as well as lithium ions and is stable at high voltages. This results in one or more of the following advantages: - Protection of the electrolyte from decomposition due to high voltages; - Protection of the cathode active material, especially LiNi0.5Mn1.5O4, from material dissolution at high temperatures and high voltages; - Protection against structural changes of the cathode active material during battery cell cycling; - Utilization of the tunneling effect of the electronically non-conductive solid electrolyte; - Increased performance of the cathode active material, the electrode, and the resulting lithium-ion cell. Both coatings 2 and 3 are applied to the active material particles 1 using suitable processes such as vapor deposition or similar methods known to those skilled in the art from the manufacture of lithium-ion cells. Examples of such processes include ALD, MLD, CVD, PVD, electron beam deposition, laser deposition, plasma deposition, radio frequency sputtering, microemulsion deposition, successive ionic layer deposition, aqueous deposition, solid-phase diffusion, sputter coating, sol-gel coating, and powder coating. Fig. 2 schematically shows an energy diagram of the anode or cathode active material particles 1, the solid electrolyte 2 and the electron conductor 3. One effect of the invention is to utilize the tunnel effect, i.e., the electrons bridge the path from the cathode active material 1 to the second layer 3, which is electron-conducting, even though there is an electronically insulating solid electrolyte layer 2 in between. When a voltage is applied to both sides of the barrier, as is the case with a powered battery, the Fermi levels E1 and E3 shift relative to each other by eV because 3 electrons are pulled out on the right side. As a result, there are 3 free states on the right side for electrons from the left side 1, thus facilitating tunneling, i.e., the transport of electrons through the non-electron-conducting layer 2. Fig. 3 schematically shows a cross-sectional view of the anode or cathode active material particle 1 according to the invention with a multilayer system of layers of solid electrolyte 2 and layers of electronic conductor 3, wherein the outermost layer is always a solid electrolyte layer 2. Fig. 4 schematically shows a cross-sectional view of the anode or cathode active material particle 1 according to the invention, comprising a multilayer system of layers of solid electrolyte 2 and layers of electron conductor 3, wherein the outermost layer is always an electron conductor layer. The sequence of solid electrolyte layer 2, electron-conducting layer 3 can be extended as often as desired until optimal protection against degradation of the cathode active material and the electrolyte is achieved, but no insulating effects occur. As a result of coating the anode and cathode active material 1 in the multilayer system with a high-voltage stable solid electrolyte 2 and an electron-conducting layer 3, utilizing the tunnel effect, the performance of the anode and cathode active materials is increased at voltages > 4.2 V, especially at 4.9 V. The cause is the prevention of material degradation and electrolyte breakdown. As a result, the performance of the lithium-ion battery cell is increased. Improved performance of a lithium-ion battery cell refers in particular to a longer lifespan and higher current capability. Reference symbol list 1 Electrode active material particles 2 First coating 3 Second coating

Claims

Electrode active material for lithium-ion batteries, comprising active material particles (1) having a first coating (2) of a solid electrolyte, wherein the active material particles (1) have a second coating (3) of an electron-conducting material applied to the first coating (2), and wherein the first coating (2) is applied to the active material in such a thickness that electron transport through the first coating (2) can take place, characterized in that the electrode active material of the active material particles is LiNi0.5Mn1.5O4. Electrode active material according to claim 1, characterized in that the first coating (2) is made of a solid electrolyte comprising a solid electrolyte material from the group of NASICON solid electrolytes, in particular LATP or LAGP, and (anti-)perovskites, in particular LLTO or Li3OCl. Electrode active material according to claim 1 or 2, characterized in that the second coating (3) is made of an electron-conducting material selected from the group consisting of graphene, titanium, zirconium, boron, vanadium oxide, titanium oxide, niobium oxide, lithium metal alloy, in particular Zn-Li, Sn-Li, Al-Li, lithium metal oxides, in particular Li2ZrO3, Li3.5Al2O3, Li4Ti5O12, and lithium metal fluorides, in particular Li3AlF6, Li2AlF4, Li3VF6. Electrode active material according to one of the preceding claims, characterized in that the first coating (2) has a layer thickness of 0.05 nm to 100 nm, in particular a layer thickness of 0.1 nm to 80 nm, and preferably a layer thickness of 0.5 nm to 50 nm. Electrode active material according to one of the preceding claims, characterized in that the first coating (2) has a layer thickness of one to several atomic layers and is applied by means of physical, wet-chemical or mechanical methods. Electrode active material according to one of the preceding claims, characterized in that the second coating (3) is applied by physical, wet-chemical or mechanical methods. Electrode active material according to one of the preceding claims, characterized in that a third coating of a solid electrolyte (2) is applied to the second coating (3). Secondary battery comprising an electrode active material according to one of the preceding claims. Use of an electrode active material according to any of the preceding claims 1 to 7 in the manufacture of battery cells, in particular for traction batteries of vehicles.

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