Doped zinc ferrite as active material for electrodes

Abstract

The present invention relates to a cathode for a rechargeable battery. The cathode has a current collector having a coating with at least one active material suitable for incorporating and removing electrochemically active species, and the active material comprises zinc ferrite doped with tetravalent ions, according to the formula ZnFe2-xZxO4, where x is greater than or equal to 0.01 and less than or equal to 0.25 and Z is selected from the group consisting of tetravalent ions of Ti, Ge, Si. The present invention furthermore relates to a battery comprising the cathode according to the invention.

Description

[0001] Doped zinc ferrite as active material for electrodes

[0002] The present invention relates to a cathode for a rechargeable battery. The cathode comprises a current collector with a coating containing at least one active material suitable for the incorporation and removal of electrochemically active species, and the active material comprises zinc ferrite doped with tetravalent ions according to the formula ZnFe 2.x Z x O4, where x is greater than or equal to 0.01 and less than or equal to 0.25, and Z is selected from the group consisting of tetravalent ions of Ti, Ge, and Si. Furthermore, the present invention relates to a battery with the cathode according to the invention.

[0003] Zinc ferrite with the composition ZnFe2O4 is a well-researched, versatile material with a spinel structure. It exhibits electrical conductivity in the range of 5 to 10 mS / cm, a relatively small electronic band gap of approximately 1.9 eV, and indirect and direct optical band gap values ​​in the range of 2.02 eV and 2.33 eV, respectively. Its low production cost, worldwide availability, high chemical stability toward atmospheric oxidation and moisture, as well as its health and environmental safety make it an ideal candidate for photocatalytic or energy harvesting applications. Furthermore, nanostructured zinc ferrite, in particular, has attracted scientific attention because, depending on the size of the nanostructure and the synthesis method, zinc and iron can partially exchange positions in the crystal structure.The degree of cation exchange is a parameter that can be used to adjust the band and spin structure of the material. This flexibility makes the material interesting for various technical applications, for example in the field of spintronics, microwave applications, and even sensor materials.

[0004] In the field of energy storage, zinc ferrite has already been discussed as a potential material for the negative electrode in lithium-ion batteries (LIBs). Despite theoretical advantages, such as a high theoretical capacity of 1072 Ah / kg and low toxicity, further studies have shown that the material's cycling stability is not ideal, as zinc ferrite decomposes into ZnO and Fe2O3 under repeated charging and discharging in a cell, and metal ions are released from the electrode. Furthermore, the energy yield is very low, resulting in the overall high inefficiency of this approach.

[0005] The patent literature also contains a wide variety of possibilities for the use of zinc ferrite in batteries.

[0006] For example, EP 2 226 869 B1 describes a battery pack comprising: a cell having first and second opposing surfaces and third and fourth opposing surfaces smaller than the first and second surfaces; and an absorber disposed on the first surface of the cell and comprising a ferrite sintered body; wherein the absorber comprises a plurality of separate sections, the sections being separated by channels extending across the center of the first surface.

[0007] US 2022 223 845A1 describes electrodes for alkaline iron batteries, wherein the battery electrode comprises an iron electrode body made of an active iron material and a zinc sulfide additive, wherein the zinc sulfide additive comprises crystalline cubic zinc sulfide.

[0008] CN 110 931 269 A discloses a capacitive iron-nickel battery iron electrode, wherein the electrochemically active material in the electrode material of the iron electrode is selected from iron ferrite, zinc ferrite, zinc stannate, tin ferrite, or iron stannate, wherein further additives of carbon materials and specific binders may be present.

[0009] Such solutions known from the prior art may offer further potential for improvement. This particularly relates to improved electrical properties of the active material and, in particular, improved cycling stability of the electrodes.

[0010] It is therefore the object of the present invention to at least partially overcome the disadvantages known from the prior art. In particular, the object of the present invention is to provide an electrode based on doped zinc ferrite for a rechargeable battery, which has improved electrical properties. Furthermore, the object of the present invention is to provide a battery with a cathode according to the invention, which, in particular, has improved cycle stability.

[0011] The problem is solved by the features of the independent claims, directed to the cathode according to the invention and the battery according to the invention. Preferred embodiments of the invention are specified in the subclaims, in the description, or in the figures. Further features described or shown in the subclaims, in the description, or in the figures may constitute a subject matter of the invention, individually or in any combination, unless the context clearly indicates otherwise.

[0012] According to the invention, a cathode for a rechargeable battery is provided, wherein the cathode comprises a current collector with a coating comprising at least one active material suitable for the storage and removal of electrochemically active species, wherein the active material is zinc ferrite doped with tetravalent ions according to the formula ZnFe 2.x Z xO4, where x is greater than or equal to 0.01 and less than or equal to 0.25 and Z is selected from the group consisting of tetravalent ions of Ti, Ge, Si or mixtures of at least two elements thereof.

[0013] Surprisingly, it was found that doping zinc ferrite in the above-mentioned ranges with tetravalent ions results in a particularly suitable active material for electrodes, which is suitable for the incorporation and removal of electrochemical substances. The zinc ferrite doped according to the invention exhibits an unchanged spinel structure, and the electronic conductivity is only insignificantly reduced by the introduced dopants, so that overall the high electronic conductivity of the zinc ferrite is retained even in the doped state. Likewise, the addition of the tetravalent ions according to the invention appears to have little or no effect on the electronic band structure of the base material. Without being bound by theory, the incorporation of tetravalent ions with a specific ion size as dopants into the zinc ferrite crystal structure also appears to lead, in particular, to a stabilization of the structure.The latter is reflected in improved cycling stability of the electrodes in battery applications. It is assumed that the inventive group of metallic dopants, in the charged state, has a suitable ion size, which can be incorporated into the zinc ferrite particularly well and without significant disruption of the prevailing crystal lattice. A further advantage may arise from the fact that the incorporation of ions from the claimed group has a very small and negligible influence on the magnetic properties of the zinc ferrite, so that even the active material can be picked up with a magnet. This property is interesting for battery recycling applications, as it significantly facilitates the separation of the cathode material from the remaining battery components.

[0014] The cathode according to the invention is a cathode for a rechargeable battery. Rechargeable batteries are electrochemical energy storage devices that contain two electrodes and an electrolyte and store electrical energy on an electrochemical basis. Rechargeable means that, during the charging process, a reversal of the electrolytic processes occurring during the discharging process is induced by applying an electrical voltage. With regard to the definition of the individual electrodes, when current passes, the cations migrate to the cathode and the anions to the anode. Depending on the electrical polarity between the electrodes, the cathode corresponds to either the positive (+) or the negative pole. For spontaneous redox reactions, such as during the discharging process, the cathode is the positive electrode. In a redox reaction forced by an applied voltage, such as during charging, the cathode is the negatively polarized electrode.The definition of cathode in this application is based on a situation in the context of spontaneous redox reactions during the discharge of a battery containing a cathode loaded with active material.

[0015] The cathode comprises a current collector with a coating containing at least one active material suitable for the storage and discharge of electrochemically active species. The structure of the cathode corresponds in principle to conventional electrode structures, with the differences in the chemical composition of the active material arising from the coating of the electrode. A current collector, usually metallic, is arranged inside the electrode. This can, for example, be in the form of a more or less thin sheet or rod. The material of the current collector can be copper, for example. The actual electrode coating is applied to the current collector itself or via an adhesive layer. The chemical composition of this electrode coating changes during the electrical charging and discharging processes at the electrode. In this respect, the electrical potential of the electrode also changes as a function of the composition of this layer.The electrode coating can have a complex chemical composition. The basis of the electrode coating is the active material, which is capable of reversibly absorbing elemental metal, non-metal, or charged species, such as metal ions, into its structure. The active material can therefore store and release metal or metal ions. In addition to the active material, the electrode can also contain other substances. These include, for example, binders that can influence the mechanical strength and adhesion of the electrode coating. Furthermore, the electrode coating can also contain conductivity additives that can, for example, increase the electrical conductivity of the electrode layer.

[0016] The active material comprises zinc ferrite doped with tetravalent ions according to the formula ZnFe2. X Z XO4. The active material is based on zinc ferrite with the formula ZnFe2O4, into whose structure another substance is incorporated. This further incorporation into the crystal structure leads to a change in the chemical and electrical properties of the zinc ferrite. The incorporation of the dopant occurs at iron sites in the zinc ferrite crystal lattice. This relationship is expressed by the fact that the dopant Z, in combination with the iron, still fulfills the basic stoichiometry of the zinc ferrite.

[0017] The parameter x is greater than or equal to 0.01 and less than or equal to 0.25. According to the invention, the dopant concentration in the zinc ferrite cannot assume arbitrary values. The stoichiometry of the usable doped zinc ferrite is limited to the specified range. Smaller dopant concentrations lead to only an insufficient improvement in the electronic behavior. Higher concentrations can be disadvantageous, as in these cases the electrical properties of the zinc ferrite are altered excessively. The dopant concentration and the zinc ferrite concentration can be determined in the electrode layer using X-ray measurements, for example, by ED-RFX.

[0018] The dopant Z is selected from the group consisting of tetravalent ions of Ti, Ge, Si, or mixtures of at least two elements thereof. The doping of the zinc ferrite is achieved by incorporating tetravalent metal cations into the zinc ferrite lattice. The group is homogeneous in that the metal cations have a very similar ionic radius. Furthermore, the individual ions from this group have a comparably high amount of energy required to convert the tetravalent ions into trivalent ions. Under the given boundary conditions, these tetravalent ions are considered very stable, which can particularly contribute to improved cycling stability of an electrode.

[0019] In a preferred embodiment of the cathode, Z can be Ti. The use of tetravalent titanium ions to dope zinc ferrite has proven particularly suitable for improving the electrical properties of zinc ferrite as an active material, particularly when used as an electrode. Titanium ions as dopants lead to only an insignificant change in the (good) electrical conductivity of zinc ferrite, and the titanium ions are particularly stable in this chemical environment against reduction to trivalent ions. The titanium ions only insignificantly disrupt the cubic crystal lattice of the zinc ferrite, thus resulting in a homogeneous phase, even with the incorporation of higher dopant quantities.

[0020] In a further preferred embodiment of the cathode, the titanium-doped zinc ferrite can be present in particulate form in the coating, wherein the titanium-doped zinc ferrite particles have an average particle size, determined by scanning electron microscopy, of greater than or equal to 0.01 pm and less than or equal to 50 pm. To obtain the most efficient active material possible, it has proven particularly suitable for the active material to be introduced into the electrode layer in particulate form. In this case, particulate means that the doped zinc ferrite is present in more or less irregularly shaped, possibly rather round particles and is processed with the other components of the electrode layer, for example by mechanical pressing. Due to the mechanical processing, the particulate shape of the active material remains virtually unchanged. The size distribution of the active material can be influenced during production.Smaller particle sizes can be disadvantageous because the individual particles become too unstable during the electrochemical conversion. Larger particle sizes can be disadvantageous because in these cases an unfavorable surface-to-volume ratio can arise. The average particle size can preferably be greater than or equal to 0.1 pm and less than or equal to 30 pm, further preferably greater than or equal to 0.2 pm and less than or equal to 10.0 pm. To determine the average size, the largest dimension is determined for each particle and this is arithmetically averaged. An average is taken over at least 100 particles.

[0021] Within a further preferred aspect of the cathode, x can be greater than or equal to 0.05 and less than or equal to 0.10. Without being bound by theory, the suitability of the doped zinc ferrite as an active material also appears to depend on the phase stability of the zinc ferrite. Within the specified range, very stable and homogeneous doped cubic phases can be obtained, which remain stable even upon incorporation of larger amounts of electrochemical substances. The cubic phases also appear to allow a higher proportion of reversible incorporation and removal. Thus, the use of these amounts of dopants results in electrochemically very stable electrodes.

[0022] According to a preferred characteristic of the cathode, the titanium-doped zinc ferrite can have a total conductivity at 20°C of greater than or equal to 0.01 mS / cm and less than or equal to 0.2 mS / cm. The conductivity range specified above has proven particularly suitable for obtaining an electrode material that is as conductive as possible but stable. Lower conductivities of the active material only lead to insufficient electrical properties of the electrode. Higher conductivities can negatively influence the cycling stability of electrodes. Preferably, the conductivity can also be greater than or equal to 0.05 mS / cm and less than or equal to 0.175 mS / cm, and more preferably greater than or equal to 0.075 mS / cm and less than or equal to 0.155 mS / cm. The conductivity of the doped zinc ferrite can be obtained, for example, on a pressed piece of the doped material using known methods.The conductivity is determined on the dry pellet at 20 °C and normal pressure.

[0023] According to a preferred characteristic of the cathode, at least 90 mol% of the doped zinc ferrite can exhibit cubic symmetry, determined by X-ray diffraction. The improved electrical properties of the active material arise particularly in cases where the cubic symmetry of the zinc ferrite is retained as much as possible despite the incorporation of the dopant. The result is an isotropic crystal, which has homogeneous properties due to the presence of only one phase. The quantitative determination of the individual phase components can be carried out using X-ray structural measurements and a Rietveld analysis. The basic procedure for such an analysis is known to the person skilled in the art.

[0024] In a further preferred embodiment of the cathode, the coating can comprise at least one conductivity additive in addition to the active material, wherein the conductivity additive consists of carbon and is present in the coating in a weight proportion of greater than or equal to 1 wt.% and less than or equal to 10 wt.%. An advantage of the active material doped according to the invention is that the conductivity of the electrode coating can be achieved even without excessively high proportions of conductivity additives. In this respect, sufficiently high conductivities can be achieved with very small additives. The proportion can preferably be greater than or equal to 2 wt.% and less than or equal to 8 wt.%, further preferably greater than or equal to 2.5 wt.% and less than or equal to 7 wt.%.

[0025] Furthermore, according to the invention, a battery cell comprises at least an anode, a cathode, and an electrolyte, wherein the cathode is a cathode according to the invention. The electrodes according to the invention are particularly suitable as cathodes in battery cells. Due to the improved electrical properties, systems based in particular on zinc ferrite as the active material can be obtained, which exhibit improved cycle stability due to the doping. For further advantages of a battery cell with an electrode according to the invention as the cathode, explicit reference is made to the advantages of the electrode according to the invention.

[0026] In a further preferred embodiment of the battery cell, the anode can comprise, as active material, a metal or ion selected from the group consisting of Zn, Sn, or mixtures thereof. The electrode material according to the invention is particularly suitable for batteries whose electrochemical potential changes are based on the migration of zinc ions. With the appropriate electrode pairs and the species to be incorporated and removed, batteries can be obtained that exhibit improved electrical properties through the inventive doping of the zinc ferrite.

[0027] In a further embodiment of the battery cell, the electrolyte can comprise a conductive salt and a solvent, wherein the solvent is a non-aqueous solvent. The use of non-aqueous solvents as solvents for the actual electrolyte has proven particularly advantageous in conjunction with an active material made of the zinc ferrite doped according to the invention. No substances are leached or released from the active material, even during long periods of use. Furthermore, the electrical potentials are not negatively affected, or are only slightly negatively affected, during cycling. A non-aqueous solvent has a water content of less than 10 wt. %, more preferably less than 5 wt. %, and more preferably less than 0.5 wt. %. The water content can be determined using known methods, for example, via Karl Fischer titration. The solvent content relates to the solvent composition of the electrolyte.

[0028] In a further preferred embodiment of the battery cell, the solvent can be selected from the group consisting of nitriles, carbonates, or mixtures of at least two solvents from this group. This group of non-aqueous electrolyte solvents, in particular, can contribute to improved electrical behavior of the electrode and thus also of the entire battery cell. In particular, this can result in improved cycle stability of the battery.

[0029] Examples

[0030] I) Doping of zinc ferrite with tetravalent cations ZnFe 2.x Ti x O4 samples with and without Ti doping were prepared using the Pechini method. The starting materials used for the synthesis are listed in the following table:

[0031] Since titanium nitrate is not readily soluble in water, a soluble titanium nitrate solution (TiO(NO3)2) was prepared using titanium isopropoxide and nitric acid as reactants. Titanium isopropoxide was mixed with water, the resulting fine titanium hydrate was thoroughly washed with water, and then redissolved with HNO3 to form a TiO(NO3)2 solution, which was stabilized with citric acid. The titanium content of the solution was monitored by TGA (Discovery TGA 5500, Waters, USA) before it was used for further synthesis steps.

[0032] For the sol-gel synthesis, the nitrates and the TiO(NO3)2 solution were combined with deionized water in a flask in the desired molar ratio and stirred until completely dissolved. The solution was then heated to 50 °C, and citric acid monohydrate (amount of citric acid = 2 x amount of cations) and, 30 minutes later, ethane-1,2-diol (amount of diol = 4 x amount of cations) were added as a chelating agent. The solution was then gelled at 100 °C for ~24 h and subsequently at 400 °C for 2 h and then at 600 °C for 1 h at a heating / cooling rate of 5 K min -1 calcined. In selected experiments, the ethane-1,2-diol in the Pechini synthesis was replaced with 1,4-butanediol. Significantly better calcination behavior was observed, resulting in a single-phase material before sintering. Furthermore, the dried material before the calcination step was significantly less sticky and therefore easier to handle.

[0033] All calcined materials were thoroughly ground in an agate mortar and then pressed into pellets. The pellets were heated at 1200 °C for 6 hours at a heating / cooling rate of 5 K / min. 1 fired in AI2O3 sinter boxes.

[0034] Previous sintering tests were conducted at temperatures between 1000 and 1400 °C with the same heating / cooling rate and holding time. At temperatures above 1200 °C, clear reflections of a secondary phase (Fe2O3) were observed. Below 1200 °C, additional reflections appeared that could not be precisely identified. They likely originate from an orthorhombic phase, with the probable formation of pseudobrookite (Fe2TiOs, PDF 00-041-1432).

[0035] The chemical composition of the sintered samples was monitored using micro-X-ray fluorescence (pXRF, M4 Tornado, Bruker, USA). The measurements were performed on the sintered samples without further preparation in air. Detailed information on the cation ratios and the corresponding abbreviations are shown in the following table:

[0036] The error of the pXRF measurements is estimated to be in the range of 0.01. ZFTO25 (marked with an asterisk) contained secondary phases, which may lead to inhomogeneities affecting the cation distribution.

[0037] II) Production of electrodes

[0038] For testing as an active material in battery test cells, the sintered pellets were crushed in an agate mortar and mixed with PVDF and conductive carbon black in a ratio of 80:10:10 to produce an electrode. The electrode paste was stirred overnight in NMP and coated onto a titanium foil as a current collector using the doctor blade method. After drying for 6 hours at 80 °C, the electrode coating was calendered to 70% of its previous thickness, dried, and baked for 12 hours at 110 °C in a vacuum. The mass loading of the fabricated electrodes with sintered ZFO and ZFTO13 as the active material was approximately 3.5 mg cm³. -2 . In parallel, the calcined powders with the composition ZFO and ZFTO9 were also used for the production of electrodes, since in the case of ZnMn2O4 the capacity of the materials sintered at high temperatures was very low.

[0039] III) Analysis of samples

[0040] Illa) Structural analysis and microstructure characterization of the electrode layers

[0041] X-ray powder diffraction was measured using a D8 Advance instrument (Bruker Corporation, USA) with CuKa radiation and a 20°C range of 10-80°. The samples were thoroughly ground in an agate mortar prior to measurements. SEM measurements to examine the grain sizes of the tablets were performed using a JEOL JSM 6510 (Jeol, Japan). To ensure good conductivity of the sample, the samples were previously sputtered with a thin Au layer. Grain size analysis was performed using Mountains SPIP Starter 8.0 (DigitalSurf, France). To obtain the grain sizes, the largest diameter of the grains was measured for 100 grains of each sample.

[0042] IIIb) Electrochemical analysis

[0043] For temperature-dependent impedance spectroscopy, ceramic pellets were sputtered on both sides with an (ion-blocking) Au layer to ensure good electrode-sample contact. Measurements were performed using an Autolab M204 potentiostat / galvanostat with an FRA32M module (Metrohm AG, Switzerland) at temperatures between 0 and 80 °C in a drying oven (Binder, Germany). All compositions were measured two to six times over the entire temperature range to ensure reproducibility of the results.

[0044] Kelvin probe atomic force microscopy (KPFM) measurements were performed in an Ar atmosphere using a Cypher ES (Asylum Research / Oxford Instruments, UK) with PPP-NCSTPt tips (Nanosensors, Switzerland). The samples were examined without further surface modification and dried in an Ar stream within the instrument before measurements began. KPFM measurements provide data on the local surface potential of a sample, which, under ideal conditions, allows direct conclusions to be drawn about the local voltaic potential. The surface potential analysis was performed using Gwyddion 2.55 (GNU General Public License): the average surface potential for each sample was determined from multiple images. Before and after measurements, the tip was calibrated on a freshly cleaved reference surface of highly ordered pyrolytic graphite. The KPFM analysis for the potential difference between the grain boundaries was performed using the Mountains SPIP Starter 8 software.0 (DigitalSurf, France).

[0045] Cyclic voltammetry (CV) was performed on an Autolab M204 potentiostat (Metrohm AG, Switzerland) to determine the oxidation and reduction points of the synthesized materials. The measurements were performed in a three-electrode arrangement in Swagelok® cells, with the positive electrode serving as the working electrode, a circular area of ​​1.13 mm 2 Zinc metal was used as counter (area 1.13 mm 2 ) and reference electrode (area ~1 mm 2 ) was used. The electrolyte used was 0.5 M zinc triflate in acetonitrile, which was soaked in Whatman GF-D Separator. The CV measurements were carried out at a scan rate of 0.2 mV-s for three cycles starting from the OCP value in a range between 0.7 and 1.9 V vs. Zn|Zn 2+ The electrolyte stability measurements were carried out in a setup with platinum as the working electrode (area 0.785 mm 2), stainless steel as counter electrode (area 8.635 mm 2 ) and Ag|AgCl reference electrode (area ~1 mm 2 ). The CV measurements were performed with a sampling rate of 1 mV-s 1 from the open-circuit potential to 4 V versus Ag|AgCl. The amount of electrolyte used in the CV cell was 250 μl. Galvanostatic cycling experiments were performed using a Maccor® Series 4000 battery tester (Maccor Inc., Oklahoma, USA). Symmetric cell setups were performed in a button cell (CR2032) using the two positive or negative electrodes (0 = 12 mm) and a Whatman GF-A separator (0 = 13 mm, single-layer) impregnated with 0.5 M zinc triflate in acetonitrile. In a symmetric setup with two cathodes, Zn was previously deposited in one of the electrodes. 2+in a cathode || Zn cell, the cell was disassembled, the electrode was washed with diethyl ether and then reassembled in a symmetrical setup. A current density of either 0.1 mA cm -2 or a current density of 0.1, 0.15 to 0.2 mA cm -2 For full-cell tests, Swagelok® cells were constructed with sintered ZF0|| Sn metal and calcined ZFTO9|| Sn metal using a Whatman GF-D separator, using 0.5 M zinc triflate in acetonitrile as the electrolyte. The cells were galvanostatically cycled at 0.1C in the voltage range between 0.7 and 1.9 V. All electrochemical cells with organic electrolytes were assembled in an argon-filled glove box (M. Braun, Germany). Measurements were performed at 20 °C.

[0046] IIIc) Optical analysis

[0047] Diffuse reflectance spectra were recorded on powders prepared by grinding ceramic pellets in a mortar. An Edinburgh Instruments FS920 spectrometer equipped with a PTFE-coated integrating sphere was used. The samples were irradiated in the 250–800 nm range with a 450 W xenon arc lamp. The excitation and emission wavelengths were adjusted using two TMS300 monochromators with 1800 grooves / mm gratings. A Peltier-cooled (-20 °C) R928 detector from Hamamatsu was used for detection. BaSO4 (99.99%, Sigma-Aldrich) was also used as a white reflectance standard material.

[0048] IV) Results

[0049] IVa) Cyclic voltammetry

[0050] The results of a selected cyclic voltammetry measurement on a doped zinc ferrite sample (ZFTO9 | Sn) are shown in Figure 1. The cyclic voltammetry measurements show an oxidation peak during the anodic sweep for ZFTO9 between 1.55 and 1.65 V vs. Zn|Zn 2+ , which is in a similar range to the experimental determination for ZnMn2O4. This oxidation peak can be attributed to the deposition of Zn 2+ and the oxidation of the ZFTO9 material. Therefore, the value is significantly lower than the value of 2 V postulated for pure ZFO. After the oxidation of ZFTO9 in the first cycle, no reduction peak is observed. In the following cycle, the oxidation peak becomes smaller and smaller because the Zn 2+ is removed from the cathode material, but cannot be reinserted. The galvanostatic cycling in symmetric cells with two ZFO or ZFTO9 show that commuting of Zn 2+Ions between the two electrodes is possible for about 800 cycles (see Figure 2). However, the overpotential in ZFTO9 is quite high at 1.8 V, compared to the overpotential of about 0.1 V during zinc deposition and zinc dissolution in the symmetrical Zn||Zn cell. In a ZFTO || Zn cell, this leads to a more difficult de- / insertion of Zn 2+ ions and thus to a low specific capacity during galvanostatic cycling.

[0051] IVb) Electrochemical and optical analysis

[0052] The electronic conductivity of all samples was measured using impedance spectroscopy. For the analysis of the impedance spectra, an equivalent circuit was used, consisting of a resistor (R_GI and R_GB) and a constant-phase element (CPE GI and CPE GB), connected in parallel for the internal grain (Gl) and grain boundary resistance (GB).

[0053] The impedance spectroscopic measurements demonstrate that all samples exhibit a comparatively high electronic conductivity, which decreases with the addition of Ti. The decrease in electronic conductivity is not linear; rather, there is a drop in conductivity followed by a local maximum around x = 0.1. The comparatively low electronic conductivity of ZTFO9 can possibly be explained, at least in part, by the increased Zn concentration in this sample. The conductivity of the two pure ZFO samples, prepared with ethane-1,2-diol and 1,2-butanediol, did not differ within the measurement error.

[0054] The activation energies, calculated from measurements of total, bulk and grain boundary conductivities, measured by impedance spectroscopy, are as follows for the different samples:

[0055] Impedance spectroscopic measurements show that the grain boundaries exhibit a higher electronic conductivity than the grain interior, but also a higher activation energy for charge transport. All activation energies were in the range between 0.31 and 0.21 eV, with a minimum for ZFTO13. The low activation energies indicate that the dominant process for electronic conduction in the samples is small polaron hopping across the Fe 3+ cations.

[0056] Kelvin probe atomic force microscopy measurements of ZFO, ZFTO7, and ZFTO13 showed a slight decrease in the work function with increasing Ti content. Furthermore, it was observed that for all measured samples, the grain boundaries exhibited a lower surface potential than the grain interior. The difference was evaluated for 25 different grain boundaries in the three samples, but the potential difference for the samples did not depend on the Ti concentration and was approximately in the range of 25 mV in all cases, although there was a large variation between individual grain boundaries. It can therefore be assumed that this difference is merely due to band bending, which is typically found at grain boundaries due to the different crystallographic orientation of neighboring grains. The optical band gap energy was calculated for all materials from reflectance measurements. It ranges between 2.1 and 1.9 eV.The value of 2.09 ± 0.10 eV for pure ZnFe2O4 is in good agreement with the literature value of 2.02 eV.

[0057] IVc) Microstructure analysis, microstructure characterization and magnetic properties

[0058] XRD measurements of the powders after calcination revealed a low content of additional oxides in the samples prepared with ethane-1,2-diol (ZFTO4, ZFTO7, and ZFTO10). The additional reflections were mainly attributed to Fe2O3 and ZnO. In contrast, the samples prepared with 1,2-butanediol (ZFTO9, ZFTO13, and ZFTO25) exhibited a significantly lower content of secondary phases.

[0059] Compositions with a Ti content of x < 0.1 were phase-pure after sintering, whereas ZFTO25 and, to a very small extent, ZFTO13 exhibited additional reflections from a phase that could not be readily identified. The best match was orthorhombic pseudobrookite (Fe2TiO5). Ti concentration had no effect on the reflection positions, meaning the unit cell parameters correspond to those of undoped zinc ferrite.

[0060] SEM measurements of the tablets showed slightly larger grains on average in Ti-doped samples compared to untreated ZFO, but the standard deviation for all samples was relatively high. The grain size distribution of ZnFe2O4 prepared with ethane-1,2-diol or 1,2-butanediol did not differ significantly. The results of the grain size distribution as a function of the doping level are shown in Figure 3.

[0061] All samples obtained could be picked up using a standard magnet. This is an advantage for later recovery of the material for recycling when used in composite materials. IVd) Cell tests

[0062] Three samples were selected for the battery tests: i) sintered, undoped ZFO, ii) sintered ZFTO13 and iii) calcined ZFTO9, as this material has slightly higher Zn concentrations than the other samples and is considered to be well below the onset of secondary phase formation.

[0063] The particular advantage of zinc-metal batteries is that water with a conducting salt can be used as the electrolyte, as the conductivities are incomparably higher than with organic electrolytes. Furthermore, the desolvation disadvantage during the transfer of the cation from the electrolyte to the cathode material is significantly lower in water than in organic electrolytes. Nevertheless, the use of aqueous electrolytes with ZnFe2O4 proved problematic. The exclusive use of 3 M zinc triflate in water as the electrolyte in sintered ZF0||Zn-metal and ZFTO13||Zn-metal cells led to leaching of Fe 3+from the ferrite. This caused the pH of the electrolyte to drop from an initial pH of 4 to 0, leading to severe gas formation and dissolution of the stainless steel current collector on the cathode side, as well as corrosion of the Zn metal anode and the reference during the cell's first charging cycle. To prevent this, Fe was added to the electrolyte, similar to cells with ZnMn2O4 active material. 3+ -ions are added to prevent the leaching of Fe 3+Iron salts are often poorly soluble in water. The addition of the highly soluble Fe(NO3)3 to the 3 M zinc triflate solution results in an unfavorably low pH (approximately pH 1), and the addition of FeSO4 leads to the immediate formation and precipitation of Fe(OH)3, which could only be made soluble again by adding HNO3. For this reason, ferrous gluconate dihydrate (CnH^FeOu - 2H2O) was tested as an environmentally friendly iron source for the electrolyte, but the pH of the electrolyte was lowered to pH 2, and cycling was only slightly improved compared to pure zinc triflate solution. Due to the unsatisfactory cycling results for the cells with aqueous electrolyte, further measurements were carried out with an organic electrolyte, 0.5 M zinc triflate in acetonitrile. Compared to aqueous zinc triflate solution, zinc triflate in acetonitrile exhibits a significantly wider electrochemical stability window.The determined oxidative stability of this electrolyte is up to 3.6 V versus Zn|Zn. 2+ , resulting in an electrochemical stability window of 3.6 V with a Zn anode. Acetonitrile was chosen because the interaction between Zn 2+ The interaction between the cations and the solvent is comparatively low, and the solvent itself has a broad electrochemical stability window. Very high cycle numbers could be achieved with a non-aqueous electrolyte (see Figure 2).

Claims

Patent claims 1. Cathode for a rechargeable battery, wherein the cathode comprises a current collector with a coating comprising at least one active material suitable for the storage and removal of electrochemically active species, characterized in that the active material is zinc ferrite doped with tetravalent ions according to the formula ZnFe 2.x Z x O4, where x is greater than or equal to 0.01 and less than or equal to 0.25 and Z is selected from the group consisting of tetravalent ions of Ti, Ge, Si or mixtures of at least two elements thereof.

2. Cathode according to claim 1, wherein Z = Ti.

3. Cathode according to claim 2, wherein the titanium-doped zinc ferrite is present in particulate form in the coating, wherein the titanium-doped zinc ferrite particles have an average particle size, determined by scanning electron microscopy, of greater than or equal to 0.01 pm and less than or equal to 50 pm.

4. Cathode according to one of the preceding claims, wherein x is greater than or equal to 0.05 and less than or equal to 0.

10.

5. Cathode according to one of claims 2 to 4, wherein the titanium-doped zinc ferrite has a total conductivity at 20°C of greater than or equal to 0.01 mS / cm and less than or equal to 0.2 mS / cm.

6. Cathode according to one of the preceding claims, wherein at least 90 mol% of the doped zinc ferrite has a cubic symmetry, determined by X-ray diffraction.

7. Cathode according to one of the preceding claims, wherein the coating comprises, in addition to the active material, at least one conductivity additive, wherein the conductivity additive consists of carbon and is present in the coating in a weight proportion of greater than or equal to 1 wt.% and less than or equal to 10 wt.%.

8. Battery cell comprising at least an anode, a cathode and an electrolyte, characterized in that the cathode is a cathode according to one of claims 1 to 7.

9. Battery cell according to claim 8, wherein the anode comprises as active material metal or ions selected from the group consisting of Zn, Sn or mixtures thereof.

10. Battery cell according to one of claims 8 or 9, wherein the electrolyte comprises a conductive salt and a solvent, wherein the solvent is a non-aqueous solvent.

11. Battery cell according to claim 10, wherein the solvent is selected from the group consisting of nitriles, carbonates or mixtures of at least two solvents from this group.

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