Cathode active material and its use in rechargeable electrochemical cells

Abstract

The present invention relates to a cathode active material of the general formula (I) NaxNiyM1 aM2 bM3 cO2 (I) in which the variables are each defined as follows: M1 is Ti, Zr, Hf or a mixture thereof, M2 is Co, Mn, Al or a mixture thereof, M3 is Mg, Ca, Sr, Ba, Ce or a mixture thereof, x is in the range of from 0.6 to 1.0, y is in the range of from 0.6 to 0.95, a is in the range of from 0.05 to 0.4, b is in the range of from 0.00 to 0.35, c is in the range of from 0.00 to 0.35, wherein a + b + c = 1 - y. The present invention further relates to an electrode material comprising said cathode active material, to electrodes produced from or using said electrode material and to a rechargeable electrochemical cell comprising at least one electrode. The present invention further relates to a process for preparing said cathode active material of the general formula (I).

Description

[0001] Cathode active material and its use in rechargeable electrochemical cells

[0002] Description

[0003] The present invention relates to a cathode active material of the general formula (I)

[0004] NaxNiyM1aM2bM3cO2(I) in which the variables are each defined as follows:

[0005] M1is Ti, Zr, Hf or a mixture thereof,

[0006] M2is Co, Mn, Al or a mixture thereof,

[0007] M3is Mg, Ca, Sr, Ba, Ce or a mixture thereof, x is in the range of from 0.6 to 1.0, y is in the range of from 0.6 to 0.95, a is in the range of from 0.05 to 0.4, b is in the range of from 0.00 to 0.35, c is in the range of from 0.00 to 0.35, wherein a + b + c = 1 - y.

[0008] The present invention further relates to an electrode material comprising said cathode active material, to electrodes produced from or using said electrode material and to a rechargeable electrochemical cell comprising at least one electrode. The present invention further relates to a process for preparing said cathode active material of the general formula (I).

[0009] Secondary batteries, accumulators or rechargeable batteries are just some embodiments by which electrical energy can be stored after generation and used when required. Due to the significantly better power density, there has been a move in recent times away from the waterbased secondary batteries to development of batteries in which the charge transport in the electrical cell is accomplished by lithium ions.

[0010] Since the terrestrial abundance of lithium is several magnitudes lower than the abundance of sodium or potassium the development of sodium ion based rechargeable electrochemical cell has started.

[0011] US 2010 / 0015256 describes sodium ion secondary batteries, wherein the cathode active material is for example NaMn2O4, NaNiO2, NaCoO2, NaFeO2, NaNio.5Mno.5Ch or NaCrO2.

[0012] M. H. Han et al., J. Power Sources 258 (2014), 266-271 and L. Wang et al, Nano Energy 34, (2017), 215-223, both disclose structural changes of monoclinic NaNiO2during the charge and discharge process accompanied by a significant irreversible capacity loss.

[0013] The sodium-ion batteries known from the prior art and their components, in particular the cathode active material, have to be improved with respect to at least one of the following properties: operability at room temperature, discharge capacity, mechanical stability, rate-capability, thermal stability or lifetime of the electrochemical cells or batteries. In particular the discharge capacity and cycle performance of sodium based electrochemical cells have to be improved.

[0014] This object is achieved by a cathode active material of the general formula (I)

[0015] NaxNiyM1aM2bM3cO2(I) in which the variables are each defined as follows:

[0016] M1is Ti, Zr, Hf or a mixture thereof, in particular Ti.

[0017] In one embodiment of the present invention the cathode active material of the general formula (I) is characterized in that M1is Ti.

[0018] M2is Co, Mn, Al or a mixture thereof, in particular Co, Mn or a mixture thereof.

[0019] In one embodiment of the present invention the cathode active material of the general formula

[0020] (I) is characterized in that M2is Co, Mn or a mixture thereof.

[0021] M3is Mg, Ca, Sr, Ba, Ce or a mixture thereof, in particular Ce.

[0022] In one embodiment of the present invention the cathode active material of the general formula (I) is characterized in that M3is Ce. x is in the range of from 0.6 to 1.0, preferably in the range of from 0.7 to 1.0, more preferably in the range of from 0.8 to 1.0, y is in the range of from 0.6 to 0.95, preferably in the range of from 0.75 to 0.95, more preferably in the range of from 0.80 to 0.95, in particular in the range of from 0.85 to 0.95, a is in the range of from 0.05 to 0.4, preferably in the range of from 0.05 to 0.25, more preferably in the range of from 0.05 to 0.20, in particular in the range of from 0.05 to 0.15, b is in the range of from 0.00 to 0.35, preferably in the range of from 0.00 to 0.20, more preferably in the range of from 0.00 to 0.15, in particular in the range of from 0.00 to 0.10, c is in the range of from 0.00 to 0.35, preferably in the range of from 0.00 to 0.20, more preferably in the range of from 0.00 to 0.15, in particular in the range of from 0.00 to 0.10, wherein a + b + c = 1 - y.

[0023] In one embodiment of the present invention the cathode active material of the general formula (I) is characterized in that M1is Ti and M2is Co, Mn or a mixture thereof.

[0024] In another embodiment of the present invention the cathode active material of the general formula (I) is characterized in that M1is Ti and M3is Ce. In a further embodiment of the present invention the cathode active material of the general formula (I) is characterized in that M2is Co, Mn or a mixture thereof and M3is Ce.

[0025] In a further embodiment of the present invention the cathode active material of the general formula (I) is characterized in that M1is Ti, M2is Co, Mn or a mixture thereof and M3is Ce.

[0026] In one embodiment of the present invention the cathode active material of the general formula (I) is characterized in that M1is Ti, M2is Co, Mn or a mixture thereof, M3is Ce, x is in the range of from 0.6 to 1 , y is in the range of from 0.75 to 0.95, a is in the range of from 0.05 to 0.25, b is in the range of from 0.00 to 0.20 and c is in the range of from 0.00 to 0.20.

[0027] In another embodiment of the present invention the cathode active material of the general formula (I) is characterized in that M1is Ti, M2is Co, Mn or a mixture thereof, M3is Ce, x is in the range of from 0.6 to 1 , y is in the range of from 0.85 to 0.95, a is in the range of from 0.05 to 0.15, b is in the range of from 0.00 to 0.10 and c is in the range of from 0.00 to 0.10.

[0028] The inventive cathode active material of the general formula (I) NaxNiyM1aM2bM3cO2, also called cathode active material (A) for short hereafter, preferably has an 03 structure comprising an R3-m phase in the range of from 70 to 100 wt. %, preferably in the range of from 70 to 99 wt. %, and a C2 / m phase in the range of from 0 to 30 wt. %, preferably in the range of from 0 to 25 wt. %, wherein the sum of the content of the R3-m phase and the C2 / m phase is in the range of from 70 to 100 wt. %, preferably in the range of from 95 to 99 wt. % based on the total mass of the cathode active material. The structure type, the phase types and their weight proportions can be determined by X-ray diffraction.

[0029] In one embodiment of the present invention the cathode active material of the general formula (I) is characterized in that the material has an 03 structure comprising an R3-m phase in the range of from 70 to 100 wt. % and a C2 / m phase in the range of from 0 to 30 wt. %, wherein the sum of the content of the R3-m phase and the C2 / m phase is in the range of from 70 to 100 wt. % based on the total mass of the cathode active material determined by X-ray diffraction.

[0030] In addition to the R3-m phase and the C2 / m phase the inventive cathode active material of the general formula (I) having 03 structure preferably comprises at least one further phase, wherein the sum of the content of the R3-m phase and the C2 / m phase is in the range of from 95 to 99 wt. % based on the total mass of the cathode active material determined by X-ray diffraction.

[0031] In one embodiment of the present invention the cathode active material of the general formula (I) is characterized in that the material having an 03 structure comprises in addition to the R3-m phase and the C2 / m phase at least one further phase, wherein the sum of the content of the R3-m phase and the C2 / m phase is in the range of from 95 to 99 wt. % based on the total mass of the cathode active material determined by X-ray diffraction.

[0032] The inventive cathode active material of the general formula (I) NaxNiyM1aM2bM3cO2 comprises preferably secondary particles, which are spherical agglomerates, which consist of multiple primary particles, wherein these primary particles have a particle size in the range of from 100 nm to 800 nm, preferably from 200 nm to 400 nm. The size of the primary particles can be determined by scanning electron microscopy. In one embodiment of the present invention the cathode active material of the general formula (I) is characterized in that the cathode active material comprises secondary particles which are spherical agglomerates consisting of multiple primary particles having a particle size in the range of from 200 nm to 400 nm determined by scanning electron microscopy.

[0033] The present invention further also provides a process for preparing a cathode active material of the general formula (I)

[0034] NaxNiyM1aM2bM3cO2(I) as described above, comprising the process steps of

[0035] (a) preparation of a mixture of oxides of Na, Ni, M1, M2and M3or compounds of said metals forming oxides during calcination wherein in said mixture the metals are available in the following molar ratio:

[0036] 0.6 to 1.0 molar equivalents of Na,

[0037] 0.6 to 0.95 molar equivalents of Ni,

[0038] 0.05 to 0.4 molar equivalents of M1,

[0039] 0.00 to 0.35 molar equivalents of M2, and

[0040] 0.00 to 0.35 molar equivalents of M3,

[0041] (b) optionally pelletizing the mixture formed in process step (a),

[0042] (c) precalcination of the mixture formed in process step (a) or process step (b) in a temperature range of from 200 to 400 °C, preferably in a temperature range of from 290 to 310 °C, preferably for 2 to 48 h, more preferably for 4 to 24 h, in particular for 8 to 12 h, for example for 10 h.

[0043] (d) optionally mixing and homogenizing the pre-calcined mixture obtained in process step (c), and

[0044] (e) calcination of the mixture formed in process step (c) or process step (d) in a temperature range from 600 °C to 1200 °C, preferably for 8 to 16 h, more preferably in a temperature range from 800 °C to 1000 °C for 10 to 14 h.

[0045] In this process, M1, M2and M3are each as defined above, especially also with regard to preferred embodiments thereof.

[0046] Oxides of Na, Ni, M1, M2and M3or compounds of said metals forming oxides during calcination are in principle known to the person skilled in the art. Suitable compounds of said metals forming oxides during calcination are for example the corresponding hydroxides, carbonates, acetates, nitrates, sulfates, halides, citrates, or oxalates.

[0047] Preferred sodium compounds are NaOH, Na2COs, NaHCCh or Na2O2, in particular NaOH. Preferred nickel compounds are Ni(OH)2, NiOOH, Ni(NOa)2, NiO, Ni(acetate)2, NiSC>4 or Ni(oxalate), in particular Ni(0H)2. Preferred manganese compounds are Mn(OH)2, MnCCh, Mn2O3, MnC>2, or Mn(NC>3)2, in particular Mn(OH)2. Preferred cobalt compounds are Co(OH)2, CoOOH, CoCOa, CO2O3, COO2, or CO(NO3)2, in particular Co(OH)2. Preferably mixed metal hydroxides, oxides or oxyhydroxides, in particular hydroxides, comprising Ni and at least one element selected from the list comprising Co, Mn and mixtures thereof, in the molar ratio according to general formula (I).

[0048] Preferred titanium compounds are nanosized (<100 nm in size) TiC>2 or TiOSCU, in particular TiO2.

[0049] The listed starting compounds can comprise water, in certain cases well defined amount of crystallization water.

[0050] In process step (a) a mixture of the starting compounds is prepared. Usually the molar ratio of Na, Ni, M1, M2and M3in the mixture is close to or almost identical with the sought ratio of these metals in the final cathode active material of general formula (I). The starting compounds can be mixed together in pulverous form or together with certain amounts of a liquid dispersion medium. The mixture can be prepared in typical industrial mixers or blenders, like a ball mill, high- shear mixer, a V-type mixer, or a planetary mixer. Preferably the starting compounds are not only mixed together for homogenization but also grinded in order to obtain a very homogenous mixture of these compounds as very fine powder.

[0051] In the optional process step (b) the mixture prepared in process step (a) is pelletized in order to simplify the handling of said mixture.

[0052] In process step (c) the mixture formed in process step (a) or (b) is pre-calcined in a temperature range of from 200 °C to 400 °C, preferably in a temperature range of from 290 to 310 °C. The time of pre-calcination can be varied in a wide range. Preferably the mixture is pre-calcined for 2 to 48 h, more preferably for 4 to 24 h, in particular for 8 to 12 h, for example for 10 h. The precalcination step can be performed in an air atmosphere, an inert atmosphere, a reducing atmosphere, or an oxidizing atmosphere, depending on the nature of the starting compounds.

[0053] In the optional process step (d) the pre-calcined mixture obtained in process step (c) is mixed and homogenized. Applicable methods and machines for mixing and homogenization have been described above.

[0054] In process step (e) the mixture formed in process step (c) or process step (d) is calcined in a temperature range from 600 °C to 1200 °C, preferably in a temperature range from 800 °C to 1000 °C. The time of calcination can be varied in a wide range. Preferably the mixture is precalcined for 8 to 16 h, more preferably for 10 to 14 h.

[0055] In a preferred embodiment of process step (e) the mixture formed in process step (c) or process step (d) is calcined in a temperature range from 600 °C to 1200 °C for 8 to 12 h, preferably in a temperature range from 800 °C to 1000 °C for 10 to 14 h.

[0056] The calcination step can be performed in an air atmosphere, an inert atmosphere, a reducing atmosphere, or an oxidizing atmosphere, depending on the nature of the starting compounds. A preferred form of the above-described process for preparing a cathode active material of the general formula (I) is characterized in that in process step (a) a mixture of hydroxides of Na and Ni and of oxides of M1, M2and M3or compounds of said metals M1, M2and M3forming oxides during calcination is prepared, wherein in said mixture the metals are available in the following molar ratio:

[0057] 0.6 to 1.0 molar equivalents of Na,

[0058] 0.6 to 0.95 molar equivalents of Ni,

[0059] 0.05 to 0.4 molar equivalents of M1,

[0060] 0.00 to 0.35 molar equivalents of M2, and

[0061] 0.00 to 0.35 molar equivalents of M3, and wherein the process step (b), (c), (d) and (e) are done as described above.

[0062] In one embodiment of the present invention the process for preparing the cathode active material of the general formula (I)

[0063] NaxNiyM1aM2bM3cO2(I) as described above, is characterized in that in process step (a) a mixture of hydroxides of Na and Ni and of oxides of M1, M2and M3or compounds of said metals M1, M2and M3forming oxides during calcination is prepared.

[0064] In one embodiment of the present invention a process for preparing a cathode active material of the general formula (I)

[0065] NaxNiyM1aM2bM3cO2(I) as described above, comprising the process steps of

[0066] (a) preparation of a mixture of hydroxides of Na and Ni and of oxides of M1, M2and M3or compounds of said metals M1, M2and M3forming oxides during calcination wherein in said mixture the metals are available in the following molar ratio:

[0067] 0.6 to 1.0 molar equivalents of Na,

[0068] 0.6 to 0.95 molar equivalents of Ni,

[0069] 0.05 to 0.4 molar equivalents of M1,

[0070] 0.00 to 0.35 molar equivalents of M2, and

[0071] 0.00 to 0.35 molar equivalents of M3,

[0072] (b) optionally pelletizing the mixture formed in process step (a),

[0073] (c) precalcination of the mixture formed in process step (a) or process step (b) in a temperature range of from 200 to 400 °C, preferably in a temperature range of from 290 to 310 °C, preferably for 2 to 48 h, more preferably for 4 to 24 h, in particular for 8 to 12 h, for example for 10 h, (d) optionally mixing and homogenizing the pre-calcined mixture obtained in process step (c), and

[0074] (e) calcination of the mixture formed in process step (c) or process step (d) in a temperature range from 600 °C to 1200 °C, preferably for 8 to 16 h, more preferably in a temperature range from 800 °C to 1000 °C for 10 to 14 h.

[0075] In this process, M1, M2and M3are each as defined above, especially also with regard to preferred embodiments thereof.

[0076] Oxides of M1, M2and M3or compounds of said metals forming oxides during calcination are in principle known to the person skilled in the art. Suitable compounds of said metals forming oxides during calcination are for example the corresponding hydroxides, carbonates, acetates, nitrates, sulfates, halides, citrates, or oxalates.

[0077] Preferred manganese compounds are Mn(OH)2, MnCCh, M^Ch, MnC>2, or Mn(NOs)2, in particular Mn(OH)2. Preferred cobalt compounds are Co(OH)2, CoCOa, CO2O3, COO2, or Co(NC>3)2, in particular Co(OH)2. Preferably mixed metal hydroxides, oxides or oxyhydroxides, in particular hydroxides, comprising at least one element selected from the list comprising Co, Mn and mixtures thereof, further comprise Ni, wherein the molar ratio of Ni and Co and / or Mn corresponds to general formula (I).

[0078] Preferred titanium compounds are TiC>2 or TiOSCU, in particular TiC>2.

[0079] The listed starting compounds can comprise water, in certain cases well defined amount of crystallization water.

[0080] In process step (a) a mixture of the starting compounds is prepared. Usually the molar ratio of Na, Ni, M1, M2and M3in the mixture is close to or almost identical with the sought ratio of these metals in the final cathode active material of general formula (I). The starting compounds can be mixed together in pulverous form or together with certain amounts of a liquid dispersion medium. The mixture can be prepared in typical industrial mixers or blenders, like a ball mill, a V- type mixer, or a planetary mixer. Preferably the starting compounds are not only mixed together for homogenization but also grinded in order to obtain a very homogenous mixture of these compounds as very fine powder.

[0081] In the optional process step (b) the mixture prepared in process step (a) is pelletized in order to simplify the handling of said mixture.

[0082] In process step (c) the mixture formed in process step (a) or (b) is pre-calcined in a temperature range of from 200 °C to 400 °C, preferably in a temperature range of from 290 to 310 °C. The time of pre-calcination can be varied in a wide range. Preferably the mixture is pre-calcined for 2 to 48 h, more preferably for 4 to 24 h, in particular for 8 to 12 h, for example for 10 h. The precalcination step can be performed in an air atmosphere, an inert atmosphere, a reducing atmosphere, or an oxidizing atmosphere, depending on the nature of the starting compounds.

[0083] In the optional process step (d) the pre-calcined mixture obtained in process step (c) is mixed and homogenized. Applicable methods and machines for mixing and homogenization have been described above. In process step (e) the mixture formed in process step (c) or process step (d) is calcined in a temperature range from 600 °C to 1200 °C, preferably in a temperature range from 800 °C to 1000 °C. The time of calcination can be varied in a wide range. Preferably the mixture is precalcined for 8 to 16 h, more preferably for 10 to 14 h.

[0084] In a preferred embodiment of process step (e) the mixture formed in process step (c) or process step (d) is calcined in a temperature range from 600 °C to 1200 °C for 8 to 12 h, preferably in a temperature range from 800 °C to 1000 °C for 10 to 14 h.

[0085] The calcination step can be performed in an air atmosphere, an inert atmosphere, a reducing atmosphere, or an oxidizing atmosphere, depending on the nature of the starting compounds.

[0086] The inventive cathode active material of general formula (I) (A) as described above is particularly suitable as component of an electrode material for a rechargeable electrochemical cell. In addition to the cathode active material (A) the electrode material for a rechargeable electrochemical cell comprises carbon in a polymorph comprising at least 60% sp2-hybridized carbon atoms and optionally at least one polymer as a binder.

[0087] The present invention further provides an electrode material for a rechargeable electrochemical cell comprising

[0088] (A) a cathode active material as described above,

[0089] (B) carbon in a polymorph comprising at least 60% sp2-hybridized carbon atoms, and

[0090] (C) optionally at least one polymer as a binder.

[0091] The inventive electrode material for a rechargeable electrochemical cell comprises, as well as the inventive cathode active material (A), carbon in a polymorph comprising at least 60% sp2- hybridized carbon atoms, preferably from 75% to 100% sp2- hybridized carbon atoms. In the context of the present invention, this carbon is also called carbon (B) for short, and is known as such. The carbon (B) is an electrically conductive polymorph of carbon. Carbon (B) can be selected, for example, from graphite, carbon black, carbon nanotubes, graphene, or mixtures of at least two of the aforementioned substances.

[0092] In one embodiment of the present invention, carbon (B) is carbon black. Carbon black may, for example, be selected from lamp black, furnace black, flame black, thermal black, acetylene black and industrial black. Carbon black may comprise impurities, for example hydrocarbons, especially aromatic hydrocarbons, or oxygen-containing compounds or oxygen-containing groups, for example OH groups. In addition, sulfur- or iron-containing impurities are possible in carbon black.

[0093] In one variant, carbon (B) is partially oxidized carbon black.

[0094] In one embodiment of the present invention, carbon (B) comprises carbon nanotubes. Carbon nanotubes (CNTs for short), for example single-wall carbon nanotubes (SW CNTs) and preferably multiwall carbon nanotubes (MW CNTs), are known per se. A process for preparation thereof and some properties are described, for example, by A. Jess et al. in Chemie Ingenieur Technik 2006, 78, 94 - 100. In the context of the present invention, graphene is understood to mean almost ideally or ideally two-dimensional hexagonal carbon crystals of analogous structure to single graphite layers.

[0095] In a preferred embodiment of the present invention, carbon (B) is selected from graphite, graphene, activated carbon and especially carbon black.

[0096] Carbon (B) may, for example, be in the form of particles having a diameter in the range from 0.005 to 50 pm, preferably 0.01 to 10 pm. The particle diameter is understood to mean the mean diameter of the secondary particles, determined as the volume average.

[0097] In one embodiment of the present invention, carbon (B) and especially carbon black has a BET surface area in the range from 20 to 1500 m2 / g, measured to ISO 9277.

[0098] In one embodiment of the present invention, at least two, for example two or three, different kinds of carbon (B) are mixed. Different kinds of carbon (B) may differ, for example, with regard to particle diameter or BET surface area or extent of contamination.

[0099] In one embodiment of the present invention, the carbon (B) selected is a combination of two different carbon blacks.

[0100] In one embodiment of the present invention, the carbon (B) selected is a combination of carbon black and graphite.

[0101] In addition, the inventive electrode material for a rechargeable electrochemical cell optionally comprises, as well as the inventive cathode active material (A) and the carbon (B), at least one further polymer as a binder, which is also referred to in the context of the present invention as binder (C) for short. Binder (C) serves principally for mechanical stabilization of inventive electrode material.

[0102] In one embodiment of the present invention, binder (C) is selected from organic (co)polymers. Examples of suitable organic (co)polymers may be halogenated or halogen-free. Examples are polyethylene oxide (PEO), cellulose, carboxymethylcellulose, polyvinyl alcohol, polyethylene, polypropylene, polytetrafluoroethylene, polyacrylonitrile-methyl methacrylate copolymers, sty- rene-butadiene copolymers, tetrafluoroethylene-hexafluoropropylene copolymers, vinylidene flu- oride-hexafluoropropylene copolymers (PVdF-HFP), vinylidene fluoride-tetrafluoroethylene copolymers, perfluoroalkyl vinyl ether copolymers, ethylene-tetrafluoroethylene copolymers, vinylidene fluoride-chlorotrifluoroethylene copolymers, ethylene-chlorofluoroethylene copolymers, ethylene-acrylic acid copolymers, optionally at least partially neutralized with alkali metal salt or ammonia, ethylene-methacrylic acid copolymers, optionally at least partially neutralized with alkali metal salt or ammonia, ethylene-(meth)acrylic ester copolymers, polyimides and polyisobutene.

[0103] Suitable binders are especially polyvinyl alcohol and halogenated (co)polymers, for example polyvinyl chloride or polyvinylidene chloride, especially fluorinated (co)polymers such as polyvinyl fluoride and especially polyvinylidene fluoride and polytetrafluoroethylene. The mean molecular weight Mwof binder (C) may be selected within wide limits, suitable examples being 20 000 g / mol to 1 000 000 g / mol.

[0104] In one embodiment of the present invention, the inventive electrode material comprises in the range from 0.1 to 15% by weight of binder, preferably 1 to 8% by weight and more preferably 3 to 6% by weight, based on the total mass of components (A), (B) and (C).

[0105] Binder (C) can be incorporated into inventive electrode material by various processes. For example, it is possible to dissolve soluble binders (C) such as polyvinyl alcohol in a suitable solvent or solvent mixture, water / isopropanol for example being suitable for polyvinyl alcohol, and to prepare a suspension with the further constituents of the electrode material. After application to a suitable substrate, the solvent or solvent mixture is removed, for example evaporated, to obtain an electrode composed of the inventive electrode material. A suitable solvent for polyvinylidene fluoride is NMP.

[0106] If it is desired to use sparingly soluble polymers as the binder (C), for example polytetrafluoroethylene or tetrafluoroethylene-hexafluoropropylene copolymers, a suspension of particles of the binder (C) in question and of the further constituents of the electrode material is prepared and pressed together while being heated.

[0107] Inventive cathode active materials (A) and inventive electrode materials as described above are particularly suitable as or for production of electrodes, especially for production of cathodes of sodium-containing batteries. The present invention provides for the use of inventive cathode active materials (A) or inventive electrode materials as or for production of electrodes for rechargeable electrochemical cells.

[0108] The present invention further provides an electrode which has been produced from or using the inventive electrode material as described above.

[0109] In addition, the inventive electrode may have further constituents customary per se, for example an output conductor, which may be configured in the form of a metal wire, metal grid, metal mesh, expanded metal, metal sheet or metal foil, stainless steel being particularly suitable as the metal.

[0110] In the context of the present invention, that electrode which has reducing action in the course of discharging (work) is referred to as the cathode.

[0111] In one embodiment of the present invention, inventive cathode active material (A) or inventive electrode material is processed to cathodes, for example in the form of continuous belts which are processed by the battery manufacturer.

[0112] Cathodes produced from inventive cathode active material (A) or inventive electrode material may have, for example, thicknesses in the range from 20 to 500 pm, preferably 40 to 200 pm. They may, for example, be in the form of rods, in the form of round, elliptical or square columns or in cuboidal form, or in the form of flat cathodes. The present invention further provides a rechargeable electrochemical cell comprising at least one inventive electrode as described above.

[0113] In one embodiment of the present invention, inventive rechargeable electrochemical cells comprise, as well as inventive cathode active material (A) or inventive electrode material, at least one anode, which comprises an alkali metal, preferably lithium or sodium, in particular sodium. The alkali metal, in particular sodium, may be present in the form of pure alkali metal or in the form of an alloy of an alkali metal with at least another metal or in the form of an alkali metal carbon compound.

[0114] In a further embodiment of the present invention, above-described inventive rechargeable electrochemical cells comprise, as well as inventive cathode active material (A) or inventive electrode material, a liquid electrolyte comprising a sodium-containing conductive salt.

[0115] In one embodiment of the present invention, inventive rechargeable electrochemical cells comprise, as well as inventive cathode active material (A) or inventive electrode material and a further electrode, especially an electrode comprising sodium, at least one nonaqueous solvent which may be liquid or solid at room temperature and is preferably liquid at room temperature, and which is preferably selected from polymers, cyclic or noncyclic ethers, cyclic or noncyclic acetals, cyclic or noncyclic organic carbonates and ionic liquids.

[0116] Examples of suitable polymers are especially polyalkylene glycols, preferably poly-Ci-C4-al- kylene glycols and especially polyethylene glycols. Polyethylene glycols may comprise up to 20 mol% of one or more Ci-C4-alkylene glycols in copolymerized form. Polyalkylene glycols are preferably doubly methyl- or ethyl-capped polyalkylene glycols.

[0117] The molecular weight Mwof suitable polyalkylene glycols and especially of suitable polyethylene glycols may be at least 400 g / mol.

[0118] The molecular weight Mwof suitable polyalkylene glycols and especially of suitable polyethylene glycols may be up to 5 000 000 g / mol, preferably up to 2 000 000 g / mol.

[0119] Examples of suitable noncyclic ethers are, for example, diisopropyl ether, di-n-butyl ether, 1 ,2- dimethoxyethane, 1 ,2-diethoxyethane, preference being given to 1 ,2-dimethoxyethane.

[0120] Examples of suitable cyclic ethers are tetrahydrofuran and 1 ,4-dioxane.

[0121] Examples of suitable noncyclic acetals are, for example, dimethoxymethane, diethoxymethane, 1 ,1 -dimethoxyethane and 1 ,1 -diethoxyethane.

[0122] Examples of suitable cyclic acetals are 1 ,3-dioxane and especially 1 ,3-dioxolane.

[0123] Examples of suitable noncyclic organic carbonates are dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate. Examples of suitable cyclic organic carbonates are compounds of the general formulae (X) and (XI) in which R1, R2and R3may be the same or different and are each selected from hydrogen and Ci-C4-alkyl, for example methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, and tertbutyl, where R2and R3are preferably not both tert-butyl.

[0124] In particularly preferred embodiments, R1is methyl and R2and R3are each hydrogen, or R1, R2and R3are each hydrogen. Particularly preferred are propylene carbonate and ethylene carbonate.

[0125] Another preferred cyclic organic carbonate is vinylene carbonate, formula (XII).

[0126] Preference is given to using the solvent(s) in what is called the anhydrous state, i.e. , with a water content in the range from 1 ppm to 0.1% by weight, determinable, for example, by Karl Fischer titration.

[0127] Examples of suitable ionic liquids are known to the person skilled in the art. Several ionic liquids, which are liquid salts with a melting point below 100 °C, in particular below room temperature, are commercially available or can be prepared according to known protocols. Preferred ionic liquids are salts comprising a cation selected from the group of cations consisting of substituted imidazolium, substituted pyrrolidinium, substituted piperidinium, substituted pyridinium, substituted phosphonium and substituted ammonium, preferably consisting of substituted imidazolium and substituted pyrrolidinium, wherein substituted means the presence of at least one organic radical, and an anion selected from the group of anions consisting of (FSCh^N-, (CF3SO2)2N’ (TFSF), CF3SO3" (TFO-), ROSO3", RSO3" (R= e.g. Me or Et), tosylate, acetate, dialkylphosphates and hydrogensulfate, preferably consisting of (FSCh^N-, (CFaSCh^N-, CF3SO3", ROSO3" and RSO3" with R = Me or Et. Preferred examples of suitable ionic liquids are N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide, -butyl-1-methylpyrrolidinium bis(trifluo- romethylsulfonyl) imide (BMP-TFSI) or 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (EMIm-TFSI). In one embodiment of the present invention, inventive rechargeable electrochemical cells comprise one or more conductive salts, preference being given to sodium salts. Examples of suitable sodium salts are NaPFe, NaBF4, NaCICU, NaAsFe, NaCFsSCh, NaC(CnF2n+iSO2)3, sodium imides such as NaN(CnF2n+iSO2)2, where n is an integer in the range from 1 to 20, NaN(SC>2F)2, Na2SiFe, NaSbFe, NaAICU, and salts of the general formula (CnF2n+iSO2)mXNa, where m is defined as follows: m = 1 when X is selected from oxygen and sulfur, m = 2 when X is selected from nitrogen and phosphorus, and m = 3 when X is selected from carbon and silicon.

[0128] Preferred conducting salts are selected from NaCFsSCh, NaC(CF3SO2)3, NaN(CF3SC>2)2, NaPFe, NaBF4, NaCICk, and particular preference is given to NaPFe and NaCFsSCh.

[0129] In one embodiment of the present invention, inventive rechargeable electrochemical cells comprise one or more separators by which the electrodes are mechanically separated from one another. Suitable separators are polymer films, especially porous polymer films, which are unreac- tive toward metallic alkali metal, in particular metallic sodium, and toward the electrolyte in the inventive rechargeable electrochemical cells.

[0130] In one embodiment of the present invention, inventive rechargeable electrochemical cells comprise one or more separators by which the electrodes are mechanically separated from one another. Suitable separators are polymer films, especially porous polymer films, which are unreac- tive toward metallic alkali metal, in particular metallic sodium, and toward the electrolyte in the inventive rechargeable electrochemical cells.

[0131] Polyolefin separators, especially of polyethylene or polypropylene, may have a porosity in the range from 35 to 45%. Suitable pore diameters are, for example, in the range from 30 to 500 nm.

[0132] In another embodiment of the present invention, the separators selected may be separators composed of PET nonwovens filled with inorganic particles. Such separators may have a porosity in the range from 40 to 55%. Suitable pore diameters are, for example, in the range from 80 to 750 nm.

[0133] The inventive rechargeable electrochemical cells can be assembled to rechargeable batteries, preferably rechargeable alkali metal ion batteries, in particular rechargeable sodium ion batteries.

[0134] Accordingly, the present invention also further provides for the use of inventive rechargeable electrochemical cells as described above in rechargeable batteries, especially rechargeable sodium ion batteries.

[0135] The present invention further provides a rechargeable battery comprising at least one inventive rechargeable electrochemical cell as described above. Inventive rechargeable electrochemical cells can be combined with one another in inventive rechargeable batteries, for example in series connection or in parallel connection. Series connection is preferred. Inventive electrochemical cells are notable for particularly high capacities, high performances even after repeated charging and greatly retarded cell death. Inventive electrochemical cells comprising inventive cathode active material as described above show an improved capacity retention combined with high energy density in comparison to electrochemical cells comprising known cathode active materials. Inventive rechargeable electrochemical cells are very suitable for use in motor vehicles, bicycles operated by electric motor, for example pedelecs, aircraft, ships, or stationary energy stores. Such uses form a further part of the subject matter of the present invention.

[0136] The present invention further provides for the use of inventive rechargeable electrochemical cells as described above in motor vehicles, bicycles operated by electric motor, aircraft, ships, or stationary energy stores.

[0137] The use of inventive rechargeable electrochemical cells in devices gives the advantage of prolonged run time before recharging and a smaller loss of capacity in the course of prolonged run time. If the intention were to achieve an equal run time with electrochemical cells with lower energy density, a higher weight for electrochemical cells would have to be accepted.

[0138] The present invention therefore also further provides for the use of inventive rechargeable electrochemical cells in devices, especially in mobile devices. Examples of mobile devices 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.

[0139] The present invention further provides a device comprising at least one rechargeable electrochemical cell as described above.

[0140] The invention is illustrated by the examples which follow but do not restrict the invention.

[0141] Figures in percent are each based on % by weight, unless explicitly stated otherwise.

[0142] Cathode active materials were characterized by X-ray diffraction and scanning electron microscopy. The structural refinement of cathode active materials was carried out using the diffraction patterns obtained by using an X-ray diffractometer (MultiFlex, Rigaku Co.) with Mo Ka radiation without air exposure by using a laboratory made attachment. The morphological features of samples of cathode active material were observed by using a scanning electron microscope (Carl Zeiss Inc., SUPRA40, Germany). I. Preparation of cathode active materials

[0143] C-1.1 Synthesis of NaNiCh

[0144] NaNiCh was prepared by solid state reaction from the stoichiometric amounts of NaOH and Ni(OH)2. The precursors were mixed in a lab mixer for 10 min. The resulting mixture was then heated at 300 °C for 10 h under an O2 atmosphere. The obtained mixture was then mixed again using the lab mixer. The resulting pre-calcinated mixture was then heated at 700 °C for 12 h under an O2 atmosphere. The heating and cooling rates were 3°C / min and 2°C / min, respectively.

[0145] 1.2 Synthesis of NaNio.9Tio.1O2

[0146] NaNio.9Tio.1O2 was prepared by solid state reaction from the stoichiometric amounts of NaOH , Ni(OH)2 and nanosized TiO2. The precursors were mixed using a lab mixer for 10 min. The resulting mixture was then heated at 300 °C for 10 h under an O2 atmosphere. The obtained mixture was then mixed again with lab mixer. The resulting pre-calcinated mixture was then heated at 800 °C for 12 h under an O2 atmosphere. The heating and cooling rates were 3°C / min and 2°C / min, respectively.

[0147] 1.3 Synthesis of NaNi0.89Ti0.1Ce0.01O2

[0148] NaNi0.89Ti0.1Ce0.01O2 was prepared by solid state reaction from the stoichiometric amounts of NaOH, Ni(OH)2, TiO2 and CeO2. The precursors were mixed using a lab mixer (10 min). The resulting mixture was then heated at 300 °C for 10 h under an O2 atmosphere. The obtained mixture then mixed again with lab mixer. The resulting pre-calcinated mixture was then heated at 800 °C for 12 h under an O2 atmosphere. The heating and cooling rates were 3°C / min and 2°C / min, respectively.

[0149] 1.4 — 1.6 Synthesis of Na0.99Ni0.9Ti0.1Ca0.01O2, Na0.97Ni0.9Ti0.1Ca0.03O2, Na0.95Ni0.9Ti0.1Ca0.05O2

[0150] Na0.99Ni0.9Ti0.1Ca0.01O2, Na0.97Ni0.9Ti0.1Ca0.03O2, Na0.95Ni0.9Ti0.1Ca0.05O2 were prepared by solid state reaction from the stoichiometric amounts of NaOH, Ni(OH)2, TiO2 and Ca(OH)2. The precursors were mixed using a lab mixer (10 min). The resulting mixture was then heated at 300 °C for 10 h under an O2 atmosphere. The obtained mixture then mixed again with lab mixer. The resulting pre-calcinated mixture was then heated at 800 °C for 12 h under an O2 atmosphere. The heating and cooling rates were 3°C / min and 2°C / min, respectively.

[0151] C-I.7 Synthesis of NaNi0.9Co0.05Mn0.05O2

[0152] NaNi0.9Co0.05Mn0.05O2 was prepared by solid state reaction from the stoichiometric amounts of NaOH and Nio.9Coo.o5Mno.o5(OH)2. The precursors were mixed using a lab mixer (10 min). The resulting mixture was then heated at 400°C for 4 h under an O2 atmosphere. The obtained mixture then mixed again with lab mixer. The resulting pre-calcinated mixture was then heated at 850°C for 12 h under an O2 atmosphere. The heating and cooling rates were 3°C / min and 1 °C / min, respectively. II. Characterization of cathode active materials

[0153] Cathode active materials were characterized by X-ray diffraction All of Bragg diffraction lines of were assigned into 03 type layered structure (space group: R-3m). Note: impurities of monoclinic (C2 / m) NaNiO2 and cubic (Fm / 3m) NiO phases were observed in NaNi0.89Ti0.1Ce0.01O2 and NaNi0.9Co0.05Mn0.05O2.

[0154] III. Electrochemical testing of cathode active materials

[0155] Assembly and operation of an electrochemical cell comprising an electrode comprising a cathode active material

[0156] Coin-type cells (2032 type) were assembled to evaluate the electrode performance of synthesized materials. Positive electrodes consisted of 94 wt. % active materials, 3 wt. % super C65, and 3 wt. % poly(vinylidene fluoride), which were mixed with NMP and pasted on Al foil, and then dried at 120 °C in vacuum. Metallic sodium was used as a negative electrode. Electrolyte solution used was 1.0 mol / l NaCIC>4 dissolved in propylene carbonate, ethylene carbonate and dimethyl carbonate in ratio 1 :1 :1 by volume with 5 % fluoroethylene carbonate. A glass fiber filter (GF / D, Whatman) was used as a separator. The cells were electrochemically cycled at a current density of C / 30 (2 cycles), C / 10 (10 cycles) and 1C (10 cycles) in a voltage range between 2.0 and 4.2 V at 25°C.

[0157] Table 1 shows the comparison of inventive (NaNio.9Tio.1O2, NaNi0.89Ti0.1Ce0.01O2,) and non-in- ventive (NaNiO2, NaNi0.9Co0.05Mn0.05O2) cathode active materials.

[0158] Table 1 : Comparison of cathode active materials

[0159] Figure 1 . SEM micrograph of the comparative NaNiO2 cathode active material.

[0160] Conditions: EHT= 10.00 kV; WD= 3.4 mm; Mag= 5.00 K X

[0161] Figure 2. SEM micrograph of the inventive NaNio.9Tio.1O2 cathode active material.

[0162] Conditions: EHT= 10.00 kV; WD= 3.7 mm; Mag= 10.00 K X

Claims

Claims1 . A cathode active material of the general formula (I)NaxNiyM1aM2bM3cO2(I) in which the variables are each defined as follows:M1is Ti, Zr, Hf or a mixture thereof,M2is Co, Mn, Al or a mixture thereof,M3is Mg, Ca, Sr, Ba, Ce or a mixture thereof, x is in the range of from 0.6 to 1.0, y is in the range of from 0.6 to 0.95, a is in the range of from 0.05 to 0.4, b is in the range of from 0.00 to 0.35, c is in the range of from 0.00 to 0.35, wherein a + b + c = 1 - y.

2. The cathode active material according to claim 1 , wherein M1is Ti.

3. The cathode active material according to claim 1 or 2, wherein M2is Co, Mn or a mixture thereof.

4. The cathode active material according to any of claims 1 to 3, wherein M3is Ce.

5. The cathode active material according to any of claims 1 to 4, wherein M1is Ti, M2is Co,Mn or a mixture thereof, M3is Ce, x is in the range of from 0.6 to 1 , y is in the range of from 0.85 to 0.95, a is in the range of from 0.05 to 0.15, b is in the range of from 0.00 to 0.10 and c is in the range of from 0.00 to 0.10.

6. The cathode active material according to any of claims 1 to 5, wherein the material has an 03 structure comprising an R3-m phase in the range of from 70 to 100 wt. % and a C2 / m phase in the range of from 0 to 30 wt. %, wherein the sum of the content of the R3-m phase and the C2 / m phase is in the range of from 70 to 100 wt. % based on the total mass of the cathode active material determined by X-ray diffraction.

7. The cathode active material according to claim 6, wherein the material having an 03 structure comprises in addition to the R3-m phase and the C2 / m phase at least one further phase, wherein the sum of the content of the R3-m phase and the C2 / m phase is in the range of from 95 to 99 wt. % based on the total mass of the cathode active material determined by X-ray diffraction.

8. The cathode active material according to any of claims 1 to 7, wherein the cathode active material comprises secondary particles which are spherical agglomerates consisting of multiple primary particles having a particle size in the range of from 200 nm to 400 nm determined by scanning electron microscopy.

9. An electrode material for a rechargeable electrochemical cell comprising(A) a cathode active material according to any of claims 1 to 8,(B) carbon in a polymorph comprising at least 60% sp2-hybridized carbon atoms, and(C) optionally at least one polymer as a binder.

10. An electrode which has been produced from or using the electrode material according to claim 9.

11. A rechargeable electrochemical cell comprising at least one electrode according to claim 10.

12. The use of the rechargeable electrochemical cell according to claim 11 in motor vehicles, bicycles operated by electric motor, aircraft, ships, or stationary energy stores.

13. A device comprising at least one rechargeable electrochemical cell according to claim 11.

14. A process for preparing a cathode active material of the general formula (I)NaxNiyM1aM2bM3cO2(I) according to any of claims 1 to 8 comprising the process steps of(a) preparation of a mixture of oxides of Na, Ni, M1, M2and M3or compounds of said metals forming oxides during calcination wherein in said mixture the metals are available in the following molar ratio:0.6 to 1.0 molar equivalents of Na,0.6 to 0.95 molar equivalents of Ni,0.05 to 0.4 molar equivalents of M1,0.00 to 0.35 molar equivalents of M2, and0.00 to 0.35 molar equivalents of M3,(b) optionally pelletizing the mixture formed in process step (a),(c) pre-calcination of the mixture formed in process step (a) or process step (b) in a temperature range of from 200 to 400 °C(d) optionally mixing and homogenizing the pre-calcined mixture obtained in process step (c), and(e) calcination of the mixture formed in process step (c) or process step (d) in a temperature range from 600 °C to 1200 °C.

15. The process according to claim 14, wherein in process step (a) a mixture of hydroxides of Na and Ni and of oxides of M1, M2and M3or compounds of said metals M1, M2and M3forming oxides during calcination is prepared.

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