Patent Application
20250023012
Coated particulate material for use in an electrode of an electrochemical cell Described are a coated particulate material for use in an electrode of an electrochemical cell and a process for preparing said coated particulate material, an electrode comprising said coated particulate material, an electrochemical cell comprising said coated particulate material, and a use of said coated particulate material for preparing an electrode for use in an electrochemical cell.
High energy density all-solid-state batteries may be realized through application of electrode active materials having a redox potential of 4 V or more vs. Li+/Li (cathode active material of the “4 V class”), so that a high cell voltage is obtainable. However, such cathode active materials may be incompatible with typical lithium ion-conducting solid electrolyte materials used for all-solid-state batteries, because the cathode active material may act as an oxidizing agent towards the solid electrolyte present in the cathode and/or in the separator layer.
To solve this problem, it was proposed to apply a coating to the cathode active material which serves as a protection layer protecting the solid electrolyte from being oxidized by the cathode active material, without inhibiting the transfer of lithium ions between the cathode active material and the solid electrolyte.
WO 2020/249659 A1 discloses a coated particulate material for use as electrode active material in an electrode and/or in a solid-state lithium-ion electrochemical cell, said coted particulate material comprising a plurality of core particles, each core particle comprising at least one nickel-containing complex layered oxide, and disposed on the surfaces of the core particles, a coating comprising carbonate ions, lithium and at least one further element. WO 2020/249659 A1 also discloses an electrode for use in a solid-state or lithium-ion electrochemical cell and a respective electrochemical cell, said electrode comprising said coated particulate material. Said coating comprises carbonate anions, lithium and at least one member of the group consisting of aluminium, boron, niobium, phosphorus, silicon, tantalum, titanium, zinc, zirconium and mixtures thereof. The at least one member of the group consisting of aluminum, boron, niobium, phosphorus, silicon, tantalum, titanium, zinc, zirconium and mixtures thereof may be present as a part of one or more compounds selected from the group consisting of LiNbO3, Li2ZrO3, LiTaO3, Li3PO4, Li3BO3, LiAlO2, Li6ZnNb4O14 and Zn3(PO4)2. No example is provided for a coating comprising Li6ZnNb4O14.
Moreover, no example is provided how to obtain a coating wherein both, zinc and niobium, are present. In contrast, all exemplary coatings disclosed in WO 2020/249659 A1 contain niobium, and no further member of the group consisting of aluminium, boron, phosphorus, silicon, tantalum, titanium, zinc, and zirconium is present.
Related art is also US 2020/0388841 A1.
Although application of a coating containing Li3NbO4 or LiNbO3 to the cathode active material was found to have an advantageous effect on the initial discharge capacity as well as on the cycling performance and stability, further improvement, i.e. increase of the initial capacity as well as reduction of the capacity loss during cycling, is desirable.
According to a first aspect, there is provided a coated particulate material, comprising or consisting of
Li1+t[CoxMnyNizMu]1−tO2 (I)
Compounds of formula (I) are capable of acting as a cathode active material in an electrochemical cell. In the context of the present disclosure, the electrode of an electrochemical cell where during discharging of the cell a net positive charge occurs is called the cathode, and the component of the cathode by reduction of which said net positive charge is generated is referred to as a “cathode active material”.
Preferably, each core particle C1) consists of at least one compound of formula (I).
Preferred cathode active materials are those having a redox potential of 4 V or more vs. Li+/Li (cathode active materials of the “4 V class”), which enable obtaining a high cell voltage. Many such cathode active materials are known in the art.
Suitable cathode active materials are oxides comprising lithium, and one or more members of the group consisting of nickel, cobalt and manganese.
According to the invention, the cathode active material present in the cores of the coated particulate material is selected from the group consisting of materials having a composition according to general formula (I)
Li1+t[CoxMnyNizMu]1−tO2 (I)
In certain cathode active materials according to formula (I), M may be one of Al, Mg, Ti, Mo, Nb, W and Zr. Exemplary cathode active materials of formula (I) are Li1+t[Ni0.88Co0.08Al0.04]1−tO2, Li1+t[Ni0.905Co0.0475Al0.0475]1−tO2, and Li1+t[Ni0.91Co0.045Al0.045]1−tO2, wherein in each case 0≤t≤0.2.
Suitable cathode active materials are e.g. oxides comprising lithium and one or more members of the group consisting of nickel, cobalt and manganese. Those cathode active materials have a composition according to general formula (Ia):
Li1+t[CoxMnyNiz]1−tO2 (Ia)
Preferably, the cathode active material according to general formula (Ia) is a mixed oxide of lithium and at least one of nickel and manganese. More preferably, the cathode active material is a mixed oxide of lithium, nickel and one or both members of the group consisting of cobalt and manganese.
Exemplary cathode active materials according to formula (Ia) are LiCoO2, Li1+t[Ni0.85Co0.10Mn0.05]1−tO2, Li1+t[Ni0.87Co0.05Mn0.08]1−tO2, Li1+t[Ni0.83Co0.12Mn0.05]1−tO2, and Li1+t[Ni0.6Co0.2Mn0.2]1−tO2, wherein in each case 0≤t≤0.2.
Certain suitable cathode active materials are mixed oxides comprising
Exemplary suitable cathode active materials have a composition according to general formula (Ib):
Li1+tA1−tO2 (Ib),
Suitable cathode active materials having a composition according to formula (Ib) are described e.g. in WO 2020/249659 A1.
The cathode active material having a composition according to general formula (I), especially according to general formula (Ib), may have a layered structure or a spinel structure.
Cathode active materials having a composition according to general formula (Ib) which have a layered structure as described in WO 2020/249659 A1 may be preferable in some cases.
Certain preferred cathode active materials have a composition according to general formula (Ic)
Li1+t[Ni1−u−v−wCouMnvMw]1−tO2 (Ic)
In formula (Ic), the variable “M” can stand for any individual member of the group of elements as defined above (e.g. “M” can stand for tungsten, i.e. “W”) or it can stand for two or more members of the group of elements as defined above (e.g. the “M” can stand for a group consisting of tungsten, zirconium and titanium). Where “M” stands for two or more members of the group of elements as defined above, the index (number) “w” accompanying the variable “M” applies to the total of elements represented by “M”, as defined above.
Exemplary cathode active materials of formula (Ic) are Li1+t[Ni0.85Co0.10Mn0.05]1−tO2, Li1+t[Ni0.87Co0.05Mn0.08]1−tO2, Li1+t[Ni0.83Co0.12Mn0.05]1−tO2, Li1+t[Ni0.6Co0.2Mn0.2]1−tO2, Li1+t[Ni0.88Co0.08Al0.04]1−tO2, Li1+t[Ni0.905Co0.0475Al0.0475]1−tO2, and Li1+t[Ni0.91Co0.045Al0.045]1−tO2, wherein in each case 0≤t≤0.2.
In the coated particulate material as described herein, the coating C2) comprises
Cubic Li3NbO4 has a rock-salt structure.
The coating comprises at least one crystalline phase comprising cubic Li3NbO4 having a crystallographic unit cell of space group Fm-3m, as determined by nano-beam electron diffraction.
Preferably, less than 10% of the niobium present in the coatings is present in the form of Li6ZnNb4O14, further preferably less than 5%, more preferably less than 1% of the niobium present in the coatings is present in the form of Li6ZnNb4O14. Most preferably, the monoclinic phase Li6ZnNb4O14 is not present in the coated particulate material.
In the coated particulate material described herein, the molar ratio Nb:Zn is in the range of from 3 to 5, preferably in the range of from 4.1 to 5.
In the coating C2), Zn is present in the form of Zn2+ cations or in the form of oxo- or hydroxoanions (zincate anions).
In the coating C2), at least a part of the zinc may be present in a compound having a composition derived from the parent composition Li3NbO4 or LiNbO3, wherein Li and/or Nb are partially substituted by Zn. Without wishing to be bound by any theory, it may presently be assumed that a compound having a composition derived from the parent composition Li3NbO4 wherein Li and/or Nb are partially substituted by Zn may be described by the following general formula (II):
Li(3−2a)Zn(a+b)Nb(1−0.4b)O4 (II)
A compound of formula (II) may be considered as a Zn-doped Li3NbO4.
In the coated particulate material described herein
Preferably, in the coated particulate material described herein
The mass fractions of Li, Zn and Nb may be determined by means of inductively coupled plasma-optical emission spectroscopy (ICP-OES). Said analytical technique is known in the art.
In cases where Nb is present not only in coating C2), but also the core particles C1), the amount of Nb present in the coating C2) is preferably determined by comparing the measuring results from (i) measuring (determining) the amount of Nb in the core particles C1) of a sample of said core particles C1) before coating and (ii) measuring (determining) the amount of Nb present in the same sample of after coating. The same approach is used for determining the amount of Li in the coating C2), since Li is also present in the core C1).
Further preferably, in the coated particulate material described herein
The molar amount of Nb in the coating C2) per g of the plurality of core particles is preferably in the range of from 6 to 540 μmol/g, preferably of from 6 to 270 μmol/g, more preferably of from 6 to 108 μmol/g, relative to the total mass of the plurality of core particles.
In the coated particulate material as described herein, the coating C2) is disposed on the surfaces of at least a part of the core particles C1), preferably the coating C2) is disposed on the surfaces of ≥50% of the total number of core particles C1), more preferably on the surfaces of ≥75% of the total number of core particles C1), even more preferably on the surfaces of ≥90% of the total number of core particles C1) and yet even more preferably on the surfaces of ≥95% of the total number of core particles C1) present in the coated particulate material. For the purposes of the present disclosure, the part of the core particles C1) on whose surfaces the coating C2) is disposed can be determined by electron microscopy performed on a (representative) sample of the coated particulate material.
In the coated particulate material as described herein, the coating C2) is disposed on at least a part of the surface of a (an individual) core particle C1), preferably the coating C2) is disposed on >50% of the total surface of a core particle C1), more preferably on 75% of the total surface of a core particle C1) and even more preferably on 90% of the total surface of a core particle C1). For the purposes of the present disclosure, the part of the surface of a core particle C1) on which the coating C2) is disposed can be determined by electron microscopy performed on a (representative) sample of an (individual) coated particle of the coated particulate material or a (representative) sample of the coated particulate material.
In the coated particulate material according to the first aspect described herein, the coating C2) may comprise carbonate anions in a total amount of ≥0.08 mass-% relative to the total mass of the plurality of (uncoated) core particles C1). More specifically, the coating C2) may comprise carbonate anions in a total amount in the range of from 0.08 mass-% to 1.62 mass-%, preferably of from 0.122 mass-% to 1.22 mass-%, more preferably of from 0.162 mass-% to 0.812 mass-%, relative to the total mass of the plurality of (uncoated) core particles C1). If the content of lithium carbonate in the coating C2) is too high, the lithium ion conductivity may be decreased.
Without wishing to be bound by any theory, it is presently assumed that the carbonate present on the surface of the core particles C1) originates from unavoidable impurities of the cathode active material which may be formed when the cathode active material is prepared or stored in the presence of traces of carbon dioxide and humidity, and/or in certain cases from using lithium carbonate as a precursor for the synthesis of the cathode active material, and/or from the decomposition of the organic solvent of the liquid reaction mixture used in preparing the coated particulate material (for details see below) in air resp. oxygen and reactivity with residual lithium on the particle surface of the cathode active material.
In the coated particulate material according to the first aspect described herein, at least a part of the carbonate ions present in the coating C2) may be present as part of an ionic compound, e.g. as part of a salt. Herein, at least a part of the carbonate ions present in the coating C2), preferably the total amount of carbonate ions present in the coating C2), is present as lithium carbonate.
For the purposes of the present disclosure, the amount of carbonate ions present in the coating C2) may be determined by acid titration, coupled with mass spectroscopy, more preferably according to the method as defined in the examples section of WO 2020/249659 A1 performed on a (representative) sample of the coated particulate material.
In the coated particulate material, the lithium present in the coating C2) is preferably present as part
The molar ratio of Li:Nb in the coating C2) is preferably in the range of from 0.5 to 5.0, preferably of from 0.75 to 4.5, more preferably of from 1.0 to 4.0, more preferably 1.1 to 1.6.
In the coating C2), nanocrystals of cubic Li3NbO4 may be embedded in a matrix formed of lithium carbonate.
Lithium carbonate may be present in a crystalline phase and in an amorphous phase. At least one of a crystalline and an amorphous phase comprising lithium carbonate is present in the coatings C2) of the coated particles.
The coating C2) may comprise lithium carbonate in a total amount in the range of from 0.1 mass-% to 2.0 mass-%, more preferably of from 0.15 mass-% to 1.5 mass-%, yet more preferably of from 0.15 mass-% to 1.0 mass-%, relative to the total mass of the plurality of core particles. The total mass (or molar amount) of lithium carbonate in the coating C2) is preferably determined (calculated) as follows: first, the molar amount (or mass) of carbonate anions present in the coating C2) is determined by acid titration, coupled with mass spectroscopy, as explained above. It is next assumed for calculation purposes of the present invention that the total molar amount (or mass) of carbonate ions found in the coating C2) is solely present as Li2CO3.
According to a second aspect, there is provided a process for preparing a coated particulate material according to the first aspect as described above. Said process comprises the steps
Methods for preparing core particles C1) comprising at least one cathode active material (step P1)), preferably consisting of at least one cathode active material, are known in the art. Core particles C1) comprising or consisting of at least one cathode active material are commercially available. Regarding preferred and specific cathode active materials, reference is made to the disclosure provided above in the context of the coated particulate material according to the first aspect. For the sake of reducing the amount of residual surface carbonates, step P1) may include a heat treatment of the core particles C1). Heat treatment of the core particles C1) may be performed at a temperature in the range of from 700 to 800° C. Heat treatment of the core particles C1) may be performed in the presence of an oxygen flow.
The liquid compositions provided in steps P2) and P3) comprise precursors for the coating C2).
In step P2) the liquid composition comprising lithium ions may be prepared by dissolving lithium metal in a solvent selected from the group consisting of water, ethanol, methanol, isopropanol, butanol, acetone, tetrahydrofuran (THF) and N,N-dimethylformamid (DMF) and mixtures thereof. The preferred solvent is anhydrous ethanol.
In step P3)
The molar ratio Nb:Zn as provided in step P3) is preferably in the range of from 3 to 5, more preferably in the range of from 4.1 to 5.
The molar ratio of Li provided in step P2) relative to Nb provided in step P3) is preferably in the range of from 0.5 to 5.0, preferably of from 0.75 to 4.5, more preferably of from 1.0 to 4.0.
In step P4) of the process according to the second aspect described herein, the components prepared or provided in steps P1) to P3) can be contacted with each other by means of any suitable technique, e.g. by mixing the components, and/or by spraying the liquid compositions prepared or provided in steps P2) and P3) or a mixture thereof onto the core particles C1) prepared or provided in step P1). For enhanced or completed contact, e.g. for finalizing the preparation of a reaction mixture e.g. in the form of a dispersion or a gel, sonicating may be used, preferably at a temperature in the range of from 15° C. to 30° C. and for a time period in the range of from 15 min to 60 min. Thus, in step P4) contacting may comprise preparation of a dispersion comprising the particles prepared or provided in step P1) and a mixture of the liquid compositions prepared or provided in steps P2) and P3), followed by ultrasonic treatment of said dispersion. Before adding the core particles C1) prepared or provided in step P1), a mixture of the solutions prepared or provided in steps P2) and P3) may be formed.
In the reaction mixture resulting in step P4), the molar ratio Nb:Zn as provided in step P3) is preferably in the range of from 3 to 5, more preferably in the range of from 4.1 to 5, and the molar ratio of Li:Nb is preferably in the range of from 0.5 to 5.0, preferably of from 0.75 to 4.5, more preferably of from 1.0 to 4.0.
Preferably, steps P1) to P4) are carried out under protective gas atmosphere (e.g. argon or nitrogen).
In step P5) removal of the solvents of the liquid reaction mixture (as prepared in step (i)) is preferably achieved by subjecting the solution to a reduced pressure (relative to standard pressure 101.325 kPa) at a temperature in the range of from 0° C. to 100° C., preferably of from 20° C. to 40° C.
In step P6) heat treating the solid residue may comprise calcining the solid residue. Heat treatment in step P6) may be carried out in the presence of oxygen flow. Most preferably, heat treatment is carried out in a temperature range of from 450° C. to 550° C.
In step P6), the solid residue may be ground prior to the heat treatment.
In step P6), after removal of the solvents, heat treatment is performed for a duration of 1 to 12 hours, preferably 1 to 6 hours, more preferably 2 to 4 hours, at a temperature in the range of from 200° C. up to 600° C., preferably 250° C. to 550° C., most preferably in the range of from 450° C. to 550° C.
Surprisingly it has been found that the monoclinic phase Li6ZnNb4O14 is substantially not obtained by the process according to the second aspect, despite the molar ratio Nb:Zn may be close to 4:1.
According to a third aspect, there is provided an electrode for use in an electrochemical cell, especially in an all-solid-state lithium-ion electrochemical cell. Said electrode comprises
In the electrode according to the third aspect, a coated particulate material E1) according to the first aspect as disclosed above or provided by a process according to the second aspect as disclosed above and a solid electrolyte material E2) and optionally further constituents E3) and E4) as defined above may be admixed with each other.
The disclosure regarding coated particulate materials provided above in the context of the first and second aspect applies mutatis mutandis to the electrode according to the third aspect. Regarding preferred and specific coated particulate materials, reference is made to the disclosure provided above in the context of the coated particulate material according to the first aspect.
Typically, an electrode according to the third aspect as described herein is a cathode, i.e. the electrode of an electrochemical cell where during discharging of the cell a net positive charge occurs.
In the electrode according to the third aspect described herein, the coating C2) serves the purpose of facilitating the transfer of lithium ions between (i) the cathode active material (which is present in the cores C1) of the coated particulate material) and (ii) the solid electrolyte E2). Moreover, in case that the cathode active material has a redox potential of 4 V or more vs. Li+/Li (cathode active material of the “4 V class”) and the solid electrolyte does not have electrochemical oxidation stability up to 4 V vs Li+/Li, the coating C2) serves as a protection layer protecting the solid electrolyte E2) from being oxidized by the cathode active material.
Suitable solid electrolyte materials which are capable of conducting lithium ions are known in the art. For instance, the solid electrolyte E2) may be selected from the group consisting of lithium-containing sulfides, lithium-containing oxysulfides, lithium-containing oxy-phosphates, lithium-containing thiophosphates, lithium argyrodites, lithium transition metal halides, and lithium-containing oxyphosphonitrides. Herein, the term “lithium-containing” means that lithium cations are present in the electrolyte, but cations of other metals than lithium may also be present in the chemical compound forming the solid electrolyte.
Such solid electrolytes have superior lithium ion conductivity but in some cases may be prone for being oxidized by the cathode active material, especially in case of cathode active materials of the 4 V class. In an electrode according to the third aspect, the coating C2) serves as a protection layer protecting the solid electrolyte E2) from being oxidized by the cathode active material present in the cores C1) of the coated particulate material E1).
Alternatively, in an electrode according to the third aspect described herein, the solid electrolyte material admixed to the coated particulate material according to the first aspect as disclosed above or provided by a process according to the second aspect as disclosed above may be a solid material having a composition which is also present in the coating C2) of the coated particulate material of the electrode. Applying a solid material having a composition which is also present in the coating C2) as the solid electrolyte in the electrode E2) reduces the diversity of materials present in the electrode, resulting in reduced complexity of the electrode and omission of undesirable interactions between the different materials present in the electrode. Moreover, presence of the same material in the coating C2) of the coated particulate and in the solid electrolyte E2) creates favorable conditions for the transfer of lithium ions between the electrode active material (which is present in the cores C1) of the coated particulate material) and the solid electrolyte.
In an electrode according to the third aspect as defined herein, a coated particulate material E1) according to the first aspect as disclosed above or provided by a process according to the second aspect as disclosed above and a solid electrolyte material E2) may be admixed with each other and with one or more binding agents E4) and/or with one or more electron-conducting materials E3). Typical electron-conducting materials E3) are those comprising or consisting of elemental carbon, e.g. carbon nanofibers, carbon nanotubes, graphene, carbon black, acetylene black, coke, and graphene oxide. Typical binding agents are poly(vinylidenefluroride) (PVDF), styrene-butadiene rubber (SBR), polyisobutene, poly(ethylene vinyl acetate) and poly(acrylonitrile butadiene).
An electrode according to the third aspect as defined herein may comprise a coated particulate material according to the first aspect as disclosed above or provided by a process according to the second aspect as disclosed above in a total amount of from 50% to 99%, more preferably of from 70% to 97%, relative to the total mass of the electrode (without current collector, here and hereafter).
An electrode according to the third aspect as defined herein may comprise a solid electrolyte in total amount of from 1% to 50%, more preferably of from 3% to 30%, relative to the total mass of the electrode.
Optionally, an electrode according to the third aspect as defined herein may comprise electron-conducting material comprising or consisting of elemental carbon in total amount from 0% to 5%, more preferably from 0% to 1%, relative to the total mass of the electrode.
Optionally, an electrode according to the third aspect as defined herein may comprise binding agents in a total amount of from 0.1% to 3%, relative to the total mass of the electrode.
Preferred electrodes according to the third aspect as defined herein are those having one or more of the specific and preferred features disclosed herein.
The present invention also pertains to a method of making an electrode according to the third aspect as described above. Said method comprises the steps
The disclosure regarding coated particulate materials provided above in the context of the first and second aspect and the disclosure regarding the electrode according to the third aspect apply mutatis mutandis. Regarding preferred and specific coated particulate materials, reference is made to the disclosure provided above in the context of the coated particulate material according to the first aspect. Regarding preferred and specific electrode constituents E2), E3) and E4), reference is made to the disclosure provided above in the context of the electrode according to the third aspect.
In step M4), a composite comprising electrode constituents E1) and E2, and optionally one or both of E3) and E4) and optionally a solvent may be prepared by mechanical mixing, e.g. by means of a planetary mill or ball mill.
In step M5) of the method of making an electrode, the pressure above atmospheric pressure is preferably a pressure in the range of from 1 to 450 MPa, more preferably of from 50 to 450 MPa and yet more preferably of from 75 to 400 MPa.
The present invention also pertains to an electrode, obtainable by the method according to the fourth aspect as described above.
The present invention also pertains to the use of a coated particulate material according to the first aspect or prepared by the process according to the second aspect for preparing an electrode according to the third aspect as defined above. The disclosure regarding coated particulate materials provided above in the context of the first and second aspect and the disclosure regarding the electrode according to the third aspect apply mutatis mutandis. Regarding preferred and specific coated particulate materials, reference is made to the disclosure provided above in the context of the coated particulate material according to the first aspect.
According to a further aspect, there is provided an electrochemical cell comprising a coated particulate material according to the first aspect as disclosed above or provided by a process according to the second aspect as disclosed above.
In said cell, preferably the coated particulate material may be present in an electrode according to the third aspect as disclosed above, especially in a cathode.
The above-defined electrochemical cell may be a rechargeable electrochemical cell comprising the following constituents
Electrochemical cells as described herein may be alkali metal containing cells, especially lithium-ion containing cells. In lithium-ion containing cells, the charge transport is effected by Li+ ions. In the separator, such electrochemical cell may comprise a solid electrolyte selected from the group consisting of lithium-containing sulfides, lithium-containing oxysulfides, lithium-containing oxy-phosphates, lithium-containing thiophosphates, lithium argyrodites, lithium transition metal halides, and lithium-containing oxyphosphonitrides. Preferably, the solid electrolyte of the separator has the same composition like the solid electrolyte in the electrode E2), so that the diversity of materials present in the cell is reduced, resulting in reduced complexity of the cell and omission of undesirable interactions between the different materials present in the cell. Moreover, presence of the same material in the solid electrolyte E2) and the separator creates favorable conditions for the transfer of lithium ions between the electrode and the solid electrolyte in the separator.
Suitable separator materials, electrochemically active cathode materials (cathode active materials) and suitable electrochemically active anode materials (anode active materials) are known in the art. Exemplary cathode active materials are disclosed above in the context of the first aspect. Anode active materials are capable of reversibly plating and stripping lithium metal resp. de-intercalating and intercalating lithium ions. In an electrochemical cell as described herein the anode a) may comprise graphitic carbon, lithium metal or a metal alloy comprising lithium as the anode active material.
The electrochemical cell may be an all-solid-state electrochemical cell.
In certain cases, an electrochemical cell according to the present invention comprises
In said electrochemical cell, the coating C2) present in the coated particulate material according to the above-defined first aspect resp. obtained by the process according to the above-defined second aspect separates the cathode active material from the lithium ion-conducting separator layer. Since the cathode active material in the cores C1) is coated with a coating C2) as described in the above-defined first aspect resp. obtained by the process according to the above-defined second aspect, direct contact between the cathode active material and the lithium ion-conducting separator layer is prevented. Thus, coating the cathode active material in the cores C1) with a coating C2) as described in the above-defined first aspect resp. obtained by the process according to the above-defined second aspect, allows for implementing electrochemical cells, especially all-solid-state lithium batteries, wherein a cathode active material having a redox potential of 4 V or more vs. Li+/Li is combined with a solid electrolyte E2) resp. with a separator layer comprising or consisting of a lithium ion-conducting material which as such does not exhibit oxidation stability at a redox potential of 4 V or more vs. Li+/Li, e.g. a sulfide-based, thiophosphate-based or oxy-sulfide-based solid electrolyte. Such lithium ion-conducting materials which do not have high oxidation stability often exhibit one or more favorable properties like stability in the presence of lithium metal or of a metal alloy comprising lithium, easy processability, superior ionic conductivity and low cost which render them suitable for forming a solid electrolyte layer resp. a separator layer. Thus, the solid electrolyte for the separator layer may be suitably selected according to the criteria of stability in the presence of lithium metal or a metal alloy comprising lithium, ionic conductivity, processability and costs, while oxidation stability is not an issue.
Thus, coating the cathode active material with a coating C2) as described above in the context of the first aspect allows for implementing electrochemical cells, especially all-solid state lithium batteries, wherein a cathode active material having a redox potential of 4 V or more vs. Li+/Li is combined with a solid electrolyte E2) and/or a separator layer comprising or consisting of a lithium ion-conducting material which as such does not exhibit oxidation stability at a redox potential of 4 V or more vs. Li+/Li, e.g. sulfide-based solid electrolytes, but exhibits one or more favorable properties like stability in the presence of lithium metal or of a metal alloy comprising lithium and superior ionic conductivity, easy processability, which render said solid electrolyte suitable for forming a solid electrolyte layer resp. a separator layer.
The electrochemical cell may have a disc-like or a prismatic shape. The electrochemical cell can include a housing that can be made of steel or aluminum.
A plurality of electrochemical cells as described above may be combined to an all-solid-state battery, which has both solid electrodes and solid electrolytes. A further aspect of the present disclosure refers to batteries, more specifically to an alkali metal ion battery, in particular to a lithium ion battery comprising at least one electrochemical cell as described above, for example two or more electrochemical cells as described above. Electrochemical cells as described above can be combined with one another in alkali metal ion batteries, for example in series connection or in parallel connection. Series connection is preferred.
The electrochemical cells resp. batteries described herein can be used for making or operating cars, computers, personal digital assistants, mobile telephones, watches, camcorders, digital cameras, thermometers, calculators, laptop BIOS, communication equipment or remote car locks, and stationary applications such as energy storage devices for power plants. A further aspect of the present invention is a method of making or operating cars, computers, personal digital assistants, mobile telephones, watches, camcorders, digital cameras, thermometers, calculators, laptop BIOS, communication equipment, remote car locks, and stationary applications such as energy storage devices for power plants by employing at least one inventive battery or at least one inventive electrochemical cell.
A further aspect of the present disclosure is the use of the electrochemical cell as described above in motor vehicles, bicycles operated by electric motor, robots, aircraft (for example unmanned aerial vehicles including drones), ships or stationary energy stores.
The present disclosure further provides a device comprising at least one inventive electrochemical cell as described above. Preferred are mobile devices such as are vehicles, for example automobiles, bicycles, aircraft, or water vehicles such as boats or ships. Other examples of mobile devices are those which are portable, for example computers, especially laptops, telephones or electrical power tools, for example from the construction sector, especially drills, battery-driven screwdrivers or battery-driven tackers.
The invention is illustrated further by the following examples which are not limiting.
Core particles C1) comprising LiNi0.85Co0.10Mn0.05O2 (NCM-851005; BASF SE, herein also referred to as NCM) as the cathode active material (CAM) were heated in 02 flow at 750° C. for 3 h to reduce the amount of residual surface carbonates. Heating and cooling rates were set to 5° C./min.
A portion of the core particles remained uncoated and was used for comparison experiments (herein referred to as “uncoated”).
1 M lithium ethoxide solution was prepared by the reaction of absolute ethanol (Sigma-Aldrich; 99.8%) with Li metal (Albemarle Germany GmbH).
To prepare 0.5 M niobium ethoxide and 0.01 M zinc acetate solutions, Nb(OCH2CH3)5 (Sigma-Aldrich; 99.95%) and Zn(O2CCH3)2 (Sigma-Aldrich) were dissolved in absolute ethanol.
In an Ar-filled glovebox, a reaction mixture in the form of a dispersion was obtained by adding an amount of 5.94 g of the core particles provided in step P1) to a mixture of the lithium ethoxide solution (512 μL) prepared in step P2), and the niobium ethoxide solution (683 μL) and the zinc acetate solution (8500 μL) prepared in step P3), followed by 30 min ultrasonication of the dispersion (Step P4)). Subsequently, the reaction mixture was dried in a vacuum overnight (Step P5)). The resultant powder was ground using a mortar and pestle and heated in oxygen flow at 300 or 500° C. for 2 h in each case (5° C./min heating rate) (Step P6)). Based on the applied amounts of Nb and Zn, the coated particles according to the invention obtained in this way shall have a nominal composition of Li6ZnNb4O14 although it was found that the monoclinic phase Li6ZnNb4O14 was not present (see below).
Referring to the temperature applied in step P6), the coated particulate material according to the invention are referred to as “coated-inv300”, and “coated-inv500”.
For comparison, coated particulate materials were prepared using a reaction mixture not comprising zinc acetate (not according to the invention).
A first coated particulate material not according to the invention (“coated-noninv1”) was prepared as follows:
A second coated particulate material not according to the invention (“coated-noninv2”) was prepared as follows:
A third coated particulate material not according to the invention (“coated-noninv3”) was prepared as follows:
Scanning electron microscopy analysis was carried out at an accelerating voltage of 10 kV using a LEO-1530 electron microscope (Carl Zeiss AG) with a field emission source. Scanning electron microscopy (SEM) images (not shown) of uncoated cathode active material prepared in step P1) and coated particulate material according to the invention as obtained after step P6) exhibit amounts of agglomerated coating material randomly distributed over the core particles of the coated particulate material according to the invention. The morphology of the core particles remained substantially unaltered. Both of these observations were made irrespective of the temperature of the heat treatment carried out in step P6).
Attenuated total reflection-infrared spectroscopy was carried out using an ALPHA FT-IR spectrometer (Bruker) equipped with a Ge crystal. By means of ATR-IR spectroscopy the presence of Li2CO3 in the uncoated core particles and the coated particulate material was verified with minor differences between the spectra (not shown) of the uncoated core particles and of the coated particulate material, irrespective of the temperature of the heat treatment carried out in step P6). This result indicates that some surface impurities from the synthesis remained and the carbonate content did not change much upon coating.
X-ray diffraction (XRD) was used to assess possible changes in the lattice structure of the coated particulate material relative to the uncoated core particles. XRD data were collected using a Stadi-P diffractometer (STOE) with a Mo anode (λ=0.70926 Å) and a MYTHEN 1 K strip detector (Dectris) in Debye-Scherrer geometry. The instrumental contribution to the reflection broadening was obtained by measuring a NIST 640f Si standard reference material. Rietveld refinement was performed using GSAS-II. During refinement the scale factor, zero shift and crystallite size broadening parameters were allowed to vary. Sample absorption was calculated based on the capillary diameter of 0.3 mm and a powder packing density of 1.44 g/cm3. A Chebyshev polynomial function with 17 terms was used to fit a fixed background to the data. Unit cell parameters, oxygen site position and atomic displace parameters (isotropic, uiso) for each site were refined. Atoms occupying the same site were constrained to have the same atomic parameters, and site occupancy factors were constrained such that each site remained fully occupied. All reflections in the patterns (not shown) of the different samples irrespective of the temperature of the heat treatment carried out in step P6) could be indexed within the R-3m space group (α-NaFeO2-type structure), as expected for the NCM material present in the cores C1).
Transmission electron microscopy (TEM) was used to gain insights into the microstructure of the coating (not shown). TEM characterization was done using a double aberration corrected Themis-Z microscope (ThermoFisher Scientific) at an accelerating voltage of 300 kV equipped with an Oneview IS camera (Ametek), a Super-X EDX detector (ThermoFisher Scientific) and a high-resolution GIF Continuum 970 (Ametek) electron energy-loss spectrometer. 4D-STEM datasets were collected using the OneView IS camera with a screen current of ˜10 pA, a small convergence semi-angle of 0.47 mrad and a camera length of 580 mm. Virtual imaging of the nano-beam electron diffraction (NBED) patterns was done using DigitalMicrograph (version 3.42). Sample cross-sections were prepared using a dualbeam focused Ga ion beam (FIB) in a Strata 400 (ThermoFisher Scientific). Carbon layers were deposited by ion beam-induced deposition to protect the coating during sample preparation and processing. Initial thinning was performed at 30 kV. Final thinning and cleaning of the samples were done at lower voltages of 5 kV/2 kV.
In a high-resolution TEM (HRTEM) image (not shown), a shell on the outer surface of an NCM-851005 particle can be recognized. The thickness of this shell ranges from a few nanometres to a few tens of nanometres and appears to be larger at the outward facing grain boundaries. The presence of uncoated surfaces is also conceivable.
In high-angle annular dark-field scanning TEM (HAADF-STEM) mode (not shown) a clear separation of core C1) and coating C2) was observed. Elemental maps of Nb and Ni obtained by energy-dispersive X-ray spectroscopy (EDS) of the surface region of a focused ion beam (FIB)-prepared particle cross-section coincide with the expected location of Ni in the core C1) and of Nb in the coating C2). Diffusion of Nb into the core C1) was not apparent, in agreement with expectations considering the size and charge of the Nb ions. The elemental map of Zn suggests its presence both in the coating C2) and in the core C1). However, given the minor Zn content, the EDS results are not unambiguous. This issue could not to be solved by electron energy-loss spectroscopy (EELS) either because of the overlap of the Ni L-edge (1008 eV) and the Zn L-edge (1020 eV) or/and the small amount of Zn relative to that of Ni. Consequently, elucidating the specific role of Zn in the coating requires further study.
Using nano-beam electron diffraction (NBED), the presence of crystalline species in an amorphous matrix was verified in the coatings C2) (not shown). Virtual dark-field images (not shown) exhibit a large number of nanoparticles having a cubic crystal lattice, unlike the monoclinic phase Li6ZnNb4O14. The measured lattice spacings match well with those of rocksalt-type (Fm-3m space group) lithium niobium oxides, such as Li3NbO4 and related compounds. In addition, a broad reflection observed at low diffraction angles not indexable in the Fm-3m space group is most likely resulting from the presence of crystalline Li2CO3, which was observed by ATR-IR.
For elemental analysis, a sample of the coated particulate material was dissolved in acid using a graphite furnace. The Zn and Nb contents were determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES) using a Thermo Fisher Scientific iCAP 7600 DUO. The carbon content was probed using a CS analyzer. Mass fractions represent the mean of at least three independent measurements. The Zn and Nb mass fractions were determined to be 0.077(1) and 0.50(1) wt %, respectively, slightly below the targeted values of 0.093 and 0.53 wt % (corresponding to 1 wt % of the compound Li6ZnNb4O14). All mass fraction data are based on to the total mass of the plurality of core particles. The molar ratio of Nb:Zn≈4.57 also deviates from the molar ratio of 4 in the compound Li6ZnNb4O14.
A composite for the cathode was prepared by mixing
A composite for the anode was prepared by mixing carbon-coated Li4Ti5O12 (LTO; NEI Corp.), Li6PS5Cl and Super C65 carbon black in a ratio of 30:65:5 (
All-solid-state cells having different cathodes were tested using a customized setup comprising a polyether ether ketone sleeve and two stainless steel dies. Cells in the form of 10 mm diameter pellet stacks were prepared starting from the separator layer that was made by cold pressing 100 mg Li6PS5Cl at a uniaxial pressure of 62 MPa. The cells were finished by adding 65 mg anode composite (see above) and 12 mg cathode composite (see above, ˜2.9 mAh/cm2, qth=274 mAh/gCAM) at the opposite surfaces of the cold-pressed solid electrolyte and pressing the stack at 437 MPa.
The cells were galvanostatically cycled at 45° C. in a voltage range between 2.9 and 4.3 V versus Li+/Li while maintaining a uniaxial pressure of 81 MPa. Cycling stability tests were performed at a rate of 1C (1C=190 mA/gCAM) for 200 cycles. Rate performance tests were done at 0.1C, 0.2C, 0.5C and 1C, with two cycles at each C-rate. Results are averaged from at least three cells.
Electrochemical impedance spectroscopy was conducted on all solid state-cells after 200 cycles at frequencies from 7 MHz to 100 mHz, with an amplitude of 10 mV, using a VMP3 impedance analyzer (Bio-Logic Science Instruments Ltd.).
The cells according to the invention delivered a reversible specific discharge capacity of 210 mAh/gCAM (approx. 2.2 mAh/cm2), compared to 180 mAh/gCAM for the comparison cells. It is assumed that the superior reversible specific discharge capacity of the cells according to the invention is enabled by superior kinetics and Coulombic efficiency. The voltage profile reveals improved delithiation of coated cathode active material “coated inv500”, resulting in a larger capacity than for the uncoated counterpart. The difference in Coulombic efficiency (88.6% vs 78.0% for the cells according to the invention and the comparison cells, respectively) suggests that more severe interfacial side reactions (oxidation of the solid electrolyte material) occurred in the case of uncoated cathode active material. This may explain the strongly reduced overpotential observed for the cell according to the invention, since it is known in the art that build-up of resistance may result from detrimental reactions between the electrode constituents during cycling.
Finally, after 200 cycles a representative cell according to the invention and a representative comparison cell as defined above were examined by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM).
The cells are cycled at different charge/discharge rates (as denoted in the graphs): 1C, 0.2C, 0.5C, 1C (2 charge/discharge cycles each); followed by 0.2C from the 9th cycle onwards.
Best results were obtained with cells according to the invention containing the cathode active material “coated inv500”, while the cells according to the invention containing the cathode active material “coated inv300” delivered slightly lower specific discharge capacities. While the comparison cells with the uncoated cathode active material showed rapid capacity fading over the first 10 cycles, the capacity degradation of the cells according to the invention was found to occur more gradually. The comparison cells having a cathode comprising any of the three coated particulate materials “coated noninv1”, “coated noninv2” and coated noninv3” are still favorable relative to the comparison cell with the uncoated cathode active material, but nevertheless exhibit lower specific discharge capacities and a faster capacity degradation than the cells according to the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21212091.9 | Dec 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/084000 | 12/1/2022 | WO |
