Sodium metal oxide material for secondary batteries and method of preparation

Abstract

The present invention relates to a sodium metal oxide material for an electrode of a secondary battery. In particular, embodiments of the invention relate to a sodium metal oxide material in which more than 97 wt% of the crystalline part of the material has a hexagonal O3 crystal structure comprising a combination of Fe, Mn, Ni, Ti and Zn, which results in low cost, high capacity, high cycling stability and high stability in ambient air. Moreover, the present invention relates to a process for the preparation of such a material.

Description

[0001] SODIUM METAL OXIDE MATERIAL FOR SECONDARY BATTERIES AND METHOD OF PREPARATION FIELD OF THE INVENTION

[0002] The present invention relates to a sodium metal oxide material for an electrode of a secondary battery. In particular, embodiments of the invention relate to a sodium metal oxide material in which more than 97 wt% of the crystalline part of the material has a hexagonal 03 crystal structure comprising a combination of Fe, Mn, Ni, Ti and Zn, which results in low cost, high capacity, high cycling stability and high stability in ambient air. Moreover, the present invention relates to a process for the preparation of such a material.

[0003] BACKGROUND OF THE INVENTION

[0004] The depletion of fossil fuels and the escalating impact of environmental pollution have intensified the need for efficient and sustainable energy storage systems (ESS) to harness renewable energy sources such as wind, solar, and tidal power. While lithium-ion batteries (LIBs) remain the dominant electrochemical ESS due to their high energy density, long cycle life, and stable working voltage, the limited availability of lithium results in increasing and highly fluctuating cost of lithium have underscored the urgency for alternative battery technologies. Sodium-ion batteries (SIBs) have emerged as a viable solution for large-scale energy storage due to the abundance of sodium resources and the similarity in electrochemical behavior between Na+and Li+ions.

[0005] SIBs employ cathode materials that play a crucial role in determining their capacity, energy density, and structural stability. Among various cathode candidates, layered transition metal oxides (NaxTMOZ, where TM are transition metals like for instance Ni, Co, Mn, Fe, Cu, Ti) have garnered significant attention for their high theoretical capacity, wide operating voltage range, and favorable electrochemical kinetics "Research Development on Sodium-lon Batteries”. Chemical Reviews 2014 114 (23), 11636-11682. These layered oxides are classified into P2-type and 03-type structures, characterized by distinct Na+coordination environments (prismatic and octahedral, respectively) and specific oxygen stacking sequences. While 03-type layered oxides offer higher sodium content and theoretical capacity compared to P2-type, they are more prone to structural instability due to the largeionic radius of Na+(1.02 A). This leads to phase transitions (e.g. 03 -> 0'3 -> P3 -> P'3) during charge-discharge cycling, which can result in microcrack formation, surface degradation, and rapid capacity decay.

[0006] 03-type cathodes generally also suffer from poor air stability. This instability arises from spontaneous Na+extraction and the formation of surface residues such as Na2CO3and NaOH, which compromise interfacial ionic conductivity and impede practical application. This poor air stability significantly adds to the cost of producing these materials and processing them into electrodes for which dry room facilities will be required. Addressing these limitations is crucial for advancing SIB technology.

[0007] If very low reactivity towards liquid water can be achieved it might open the possibility to do water-based processing of the cathode materials into electrodes. Which is both more cost efficient and sustainable than the NMP (N-Methyl-2-pyrrolidone) based processing that is otherwise the preferred processing method.

[0008] One effective strategy is cation substitution with many different elements such as Li, Mg, Ca, Zn, Fe, Al, Cr, Sn, Cu, and Ti. These substitutions aim to stabilize the transition metal (TM) layers, suppress irreversible phase transitions, and improve both structural integrity and air stability.

[0009] For instance, substituting Li in the TM layer, as disclosed in "Improving Energy Density and Structural Stability of Manganese Oxide Cathodes for Na-Ion Batteries by Structural Lithium Substitution" Chemistry of Materials 2016 28 (24), 9064-9076; can stabilize the host structure and enhance Na+diffusion by facilitating reversible hopping between the TM layer and the Na+layer during charge and discharge cycles. Similarly, incorporating Cu and Ti into NaNio.5Mno.5O2 (Designing Air-Stable O3-Type Cathode Materials by Combined Structure Modulation for Na-Ion Batteries, Journal of the American Chemical Society 2017 139 (25), 8440-8443) has significantly improved capacity retention and resistance to moisture and CO2exposure. In parallel, the patent application US2022 / 0263085 Al describes an 03 material formed by incorporating Ti and Zn into NaNio.5Mno.5O2, which shows good moisture stability and electrochemical performance, particularly in terms of long-life cycles and energy retention at voltages exceeding 4.0 V.Similar advancements have been achieved with Nao.95Nio.4oFeo.15Mno.3Tio.15O2, an 03 material enhanced by Fe and Ti substitution (O3-Type Na0.95Ni0.40Fe0.15Mn0.3Ti0.1502 Cathode Materials with Enhanced Storage Stability for High-Energy Na-Ion Batteries -PubMed, Journal of the American Chemical Society 2017 139 (25), 8440-8443) and this 03-type material offers high cycling stability and reduced reactivity to ambient air, highlighting the growing importance of multi-component compounds in achieving improved material and electrochemical performance.

[0010] Recently, a high-entropy substitution strategy has emerged as a new approach to improve the performance of layered oxide Na-ion cathode active materials. This involves incorporating five or more transition metal elements in equimolar or near-equimolar ratios into the host structure, resulting in high-entropy oxides (HEOs). The entropy-stabilization effect in HEOs helps delay and mitigate complex phase transitions, thereby supporting reversible O3-to-P3 phase transitions and enhancing both cycling stability and rate capability. For example, Nao.94Nio.29Cuo.1Feo.i6Mno.3Tio.15O2 (Pentanary transition-metals Na-ion layered oxide cathode with highly reversible O3-P3 phase transition - ScienceDirect Chemical Engineering Journal, volume 412, 2021) demonstrates good capacity retention. This performance is attributed to the combined roles of Ni and Mn in charge compensation and Cu, Fe, and Ti in stabilizing the host structure, thereby suppressing intermediate phase transitions.

[0011] In our previous patent, EP17701048.5, we disclosed an 03-type Na-ion material, Nao.83Nio.i6Mno.35Feo.26Tio.i6Lio.o7O2, containing Ni, Fe Mn as the host structure, with Li, and Ti stabilizing the host structure. It also fulfills the criteria to be a HEO material by containing 5 elements in almost equimolar ratios. This material exhibited good cycling stability, though its stability in ambient air and water remained unexplored.

[0012] Despite the development of many promising materials using the strategies mentioned above, significant challenges persist with 03-type oxides as cathode active Na-ion materials. The materials still need to be improved to unleash their full potential and make Na-ion batteries a real alternative to low-cost and long-cycling Li-ion batteries for energy storageapplication. In this respect it is important to achieve lower cost, higher capacity, better cycling stability at elevated voltages (>4.0 V), and better air stability. The latter significantly improves the ease and cost of preparing battery cells from the materials.

[0013] BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1. Graph showing discharge capacity as function of charge and discharge cycle number for cells made from Nao.90Nio.30Mno.35Tio.15Feo.10Zno.10O2 (Example 15) and Nao.90Nio.30Mno.35Tio.15Feo.13Lio.03Zno.05O2 (Example 16). Black dots = Example 15, Grey dots = Example 16.

[0015] Figure 2. Graph showing the mole fraction of Na extracted from selected cathode active materials as a function of time when stirring a slurry of 1 g of the cathode active material and 30 mL demineralized water.

[0016] SUMMARY OF THE INVENTION

[0017] The current invention provides an 03 type layered oxide sodium ion battery cathode active material and a preparation method thereof, wherein the layered oxide sodium ion battery anode material has the characteristics of relatively high capacity, relatively high cycling stability, relatively enhanced storage stability in ambient air (Relative slow gain in weight when exposed to ambient air indicating that the material reacts relatively slowly with moisture and CO2 in ambient air) and low cost (due to the use of relatively cheap raw materials like Fe); the preparation process is easy to industrialize and is based on forming a precursor by crushing and mixing of simple raw material of the constituting elements which is followed by solid state reaction at high temperature to form the desired 03 crystal structure.

[0018] The material Na-ion cathode active material with hexagonal 03 crystal structure is more than 97% phase pure. The cathode material is characterized by the following desirable features when comparing to state-of-the-art 03 materials:

[0019] • It contains Fe for low cost.• It is characterized by relatively high capacity.

[0020] • It has relatively high cycling stability.

[0021] • Is shows relatively high stability in ambient air (Relatively slow gain in weight when exposed to ambient air)

[0022] • It reacts relatively slowly with water.

[0023] DETAILED DESCRIPTION OF THE INVENTION

[0024] The invention relates to a sodium metal oxide material for an electrode of a secondary battery, in which more than 97 wt% of the crystalline part of the material has a hexagonal 03 crystal structure comprising the formula:

[0025] NaaNixI InyFezTivLiwZnuO2

[0026] wherein

[0027] 0.85 < a < 0.95,

[0028] 0.2 < x < 0.35,

[0029] 0.25 < y < 0.45,

[0030] 0.05 < z < 0.20,

[0031] 0.05 < v < 0.20,

[0032] 0 < w < 0.05,

[0033] 0.02 < u < 0.10,

[0034] wherein

[0035] x + y + z + v + w + u = l,

[0036] For economic reasons it is preferred that Ni is mainly present in oxidation state +2 in the materials. Furthermore, to avoid the instabilities that can be induced by the presence of significant amounts of Mn in oxidation state 3+ it is preferred that Mn is mainly present in oxidation state +4 in the materials. To allow for both the following must preferably be true:

[0037] 3.9 < a < 4.1, where a = a + w + 2(x + u) + 3z + 4(y + v)

[0038] To enable sufficient theoretical redox capacity of the cathode active material (oxidation of Ni2+ to Ni4+, Fe3+ to Fe4+ and Mn3+ to Mn4 is possible during charging of Na-ion batteries based on layered oxides) the following must also be true:P > 4.6, where |3 = a + 4(x + y + v +z) + w + 2u

[0039] In the examples of the present invention the sodium layered oxide active material is synthesized through a solid-state process where a precursor is heated to high temperatures in an oxygen containing atmosphere, e.g. air.

[0040] • To obtain a homogeneous material with high phase purity it is important that the raw materials making up the precursor are well mixed and grinded down to a volume based average particle size of 0.1- 3 pm. This is achieved by crushing and mixing raw materials in appropriate proportions, for example by paint shaking the raw materials together with ceramic beads and a liquid. This could also be done using other methods like ball milling, , bead milling, dry milling etc.

[0041] • The calcination process, where the precursor is transformed into the 03 phase, takes place at a temperature above 800°C, preferably at 900°C or higher, in an oxygen containing atmosphere, followed by a cooling period. The high temperature is maintained between 1 to 24 hours, with 5-15 hours being optimal. The heating rate used to reach the high temperature is between 1°C and 10°C per minute, preferably 2-6°C per minute. Cooling rate is also between 1°C and 10°C per minute. In the last part of the cooling period (when the temperature is below 500°C), it is particularly important that the atmosphere is low in moisture, e.g. this can be accomplished by a flow of dried air in the calcination furnace. Optionally the CO2 level can also be controlled to a level below that found in ambient air (420 ppm) in the last part of the cooling period.

[0042] • The sodium layered oxide compound exits the calcination process as a slightly agglomerated powder. This powder can advantageously be deagglomerated and sieved before it is used for making battery electrodes. In small scale this is accomplished by using grinding equipment like a mortar or a ball mill followed by sieving. Industrially this is achieved by impact or jet milling followed by classification in an air sieve. The mill and the air sieve must be operated under moisture and CO2 free conditions.Oher methods can also be used to obtain an appropriate precursor. It is for instance possible to ensure good mixing of some of the elements by co-precipitating these as hydroxides or carbonates prior to mixing with the remaining elements.

[0043] Precursors are preferably made up from oxides, hydroxides, bicarbonates and / or carbonates of the metal elements constituting the final 03 material. Alternatively, other raw materials that decompose in such a way in the solid-state process that all elements apart from the constituting metals and oxygen are decomposed into gasses during the solid-state process can be used.

[0044] Another object of the invention is making the conductive positive electrode that includes a 03-type layered oxide compound (active material) and an electronically conductive additive. The electronically conductive additive can be carbon black, such as super-P™, C-45™, C-65™, acetylene black, Ketjen Black™, or similar types, typically available in powder form. A carbon black variant containing a small amount of graphitized carbon is preferred. The concentration of the conductive additive is ideally between 1% and 10% by weight in respect to the weight of the total layered oxide compound, with 2 - 4% by weight being particularly preferred.

[0045] The positive electrode can further include a binder that provides structure by binding the active material and the conductive additive together. The binder may be a polymer, such as sodium carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF) or its derivatives. The binder can be dissolved in water or non-aqueous solvents like N-methyl pyrrolidone (NMP), to obtain a slurry where active material, conductive additive, and binder is well dispersed. The ratio by weight of the active material, conductive additive, and binder can be, for example, 92:4:4.

[0046] The slurry is coated onto a substate. The substrate which is the current collector for the electrode can be made from aluminum, copper, nickel, or similar materials.

[0047] According to another aspect, this invention features an electrochemical cell that includes:

[0048] • A negative electrode configured to reversibly accept sodium ions from an electrolyte during charging of the cell and to reversibly release sodium ions to an electrolyte during discharge, the negative electrode having at least one current collector;• A positive electrode containing the sodium layered oxide compound described in the invention, also capable of accepting and releasing sodium ions, with at least one current collector.

[0049] • A separator soaked with electrolyte and in contact with both electrodes, where the electrolyte comprises sodium ions.

[0050] The negative electrode may contain an active material such as hard carbon, antimony, tin, phosphorus, or a combination of these. This material is well-suited for use in full-cell configurations. More specifically, the active material can be a hard carbon powder characterized by Raman spectra bands at approximately 1350 cm-1(D-band) and 1580 cm-1(G-band). The hard carbon particles typically range in size from 1 to 20 micrometers, with a specific surface area of 1 to 10 m2 / g- The active material can be mixed with a conductive additive like carbon black, where the additive concentration ranges from 1% to 8% by weight, typically around 4%.

[0051] The electrolyte contains a suitable salt, such as sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaCI04), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium bis(pentafluoroethanesulfonyl)imide (NaBETI), sodium tetrafluoroborate (NaBF4), or a combination thereof. The electrolyte also includes a non-aqueous solvent like dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), propylene carbonate (PC), ethylene carbonate (EC), ethyl acetate (EA), ethyl propionate (EP), methyl propionate (MP), or diglyme. The solvent concentration typically falls between 60% and 98% by weight of the electrolyte.

[0052] The salt concentration within the solvent can range from 0.1 mol / L to 3 mol / L, advantageously from 0.5 mol / L to 2 mol / L.

[0053] Electrolyte additives can be included in the electrolyte to enhance high-temperature performance and minimize self-discharge.

[0054] The separator used in the electrochemical cell is a porous membrane, which can be made from materials like glass fiber, polypropylene (PP), polyethylene (PE), cellulose, or combinations thereof. It can also consist of multiple layers, particularly of PP and / or PE.Another aspect of the invention is a battery that consists of one or more of the described electrochemical cells. The battery can include external connectors to power devices. It may be configured as a coin cell, pouch cell, cylindrical cell (such as 18650, 21700, 36500), or a prismatic cell.

[0055] In Figure 1, the discharge capacity as a function of charge and discharge cycle number for cells made from Nao.90Nio.30Mno.35Tio.15Feo.10Zno.10O2 (Example 15) and Nao.90Nio.30Mno.35Tio.15Feo.13Lio.03Zno.05O2 (Example 16) is shown. It shows that the two materials that both are examples of the current invention have good cycling stability and that the material that contains Li has the best cycling life.

[0056] Figure 2 shows the mole fraction of Na extracted from the selected cathode active materials as function of time when stirring a slurry of 1 g of the cathode active material and 30 mL demineralized water. From the figure it is seen that there is a large difference in stability (high stability means that it is difficult to extract Na from the materials) of the tested samples. Materials containing both Zn and Fe together with Ni, Mn and Ti show the best performance and adding Li improves the performance further.

[0057] As a definition for clarity of the invention, a secondary battery, also known as a rechargeable battery, is an electrochemical energy storage device that can be charged, discharged, and recharged multiple times. Unlike primary batteries, which are designed for single-use and disposal after their energy is depleted, secondary batteries can undergo numerous charge and discharge cycles, making them suitable for applications requiring long-term energy storage and repeated use. Secondary batteries are commonly used in a variety of applications, including portable electronics, electric vehicles, and renewable energy storage systems.

[0058] A positive electrode, also known as cathode in a discharge cycle, is a crucial component of an electrochemical cell or battery. It is the electrode where the reduction reaction occurs during discharge, meaning it gains electrons from the external circuit. In rechargeable batteries, such as lithium-ion batteries, the positive electrode typically consists of a lithium metal oxide, such as lithium cobalt oxide (LiCoO2), lithium iron phosphate (LiFePO4), or lithium nickel manganese cobalt oxide (LiNiMnCoOZ). The positive electrode plays a vital role in determining the battery's voltage, capacity, and overall performance. During thecharging process, the positive electrode acts as the anode, where oxidation occurs, releasing electrons to the external circuit.

[0059] EXPERIMENTAL SECTION

[0060] Materials synthesis

[0061] The raw materials Na2COs, basic nickel carbonate, Mn3O4, FeOOH, TiO2, U2CO3 and ZnO are weighed off according to the desired stoichiometry. The raw materials, which have a total mass of 140 - 150 g, are transferred to a 500 mL PE bottle together with 150 g absolute ethanol and 800 g 2 mm ZrO2 beads. The container is shaken for 40 - 50 min in a paint shaker (Red Devil).

[0062] The ZrO2 beads are removed from the slurry by sieving. Subsequently, the ethanol is removed by placing the slurry in a porcelain bowl in a ventilated drying cabinet at 80 °C. 20 - 25 g of the dried precursor powder is loaded into an alumina crucible and heat treated in air with a dew point of -40 °C. The temperature is ramped with a rate of 3 K / min to 900 or 950°C and kept there for 10 hours. Then, it is cooled down to 300°C where the atmosphere is changed to nitrogen to avoid reaction with the low content of moisture and COZ in the air during the remainder of the cooling.

[0063] After the heat treatment the material is immediately vacuum packed.

[0064] Examples of materials

[0065] Example 1 (comparative)

[0066] Asa comparative example inspired by the publication (L. Yu et al, ACS Appl. Mater. Interfaces 2023, 15, 23236-23245) "O3-Type Na0.95Ni0.40Fe0.15Mn0.3Ti0.1502 Cathode Materials with Enhanced Storage Stability for High-Energy Na-Ion Batteries" (O3-Type Na0.95Ni0.40Fe0.15Mn0.3Ti0.1502 Cathode Materials with Enhanced Storage Stability for High-Energy Na-Ion Batteries - PubMed, Journal of the American Chemical Society 2017 139 (25), 8440-8443), a material with the following composition Nao.93Nio.41Mno.31Feo.15Tio.13O2 was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0067] Example 2 (comparative)A material with the following composition Nao.98Nio.31Mno.31Feo.15Tio.13Zno.10O2 was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0068]

[0069] A material with the following composition Nao.83Nio.i6Mno.35Feo.26Tio.i6Lio.07O2 which is representative of a material from (Our previous patent EP17701048.5) was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0070]

[0071] A material with the following composition Nao.90Nio.32Mno.39Feo.12Tio.13Lio.04O2 which is representative of a material from (Our previous patent EP17701048.5) was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0072]

[0073] A material with the following composition Nao.85Nio.38Mno.48Tio.10Zno.04O2 which is representative of a material from patent application "US2022 / 023085 Al" was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0074]

[0075] A material with the following composition Nao.83Nio.i6Mno.37Feo.26Tio.13Lio.04Zno.04O2 was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0076]

[0077] A material with the following composition Nao.98Nio.31Mno.31Feo.15Tio.13Cuo.10O2 was synthesized using the raw materials Na2COs, basic nickel carbonate, MnsO4, FeOOH, TiO2, and CU2(OH)2CO3. The heat treatment temperature was 950°C.

[0078]

[0079] A material with the following composition Nao.98Nio.31Mno.33Feo.10Tio.13Mgo.13O2 was synthesized using the raw materials Na2COs, basic nickel carbonate, MnsO4, FeOOH, TiO2, and Mg(OH)2. The heat treatment temperature was 950°C.

[0080] A material with the following composition Nao.93Nio.31Mno.35Feo.14Tio.13Zno.05Lio.02O2 was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0081]

[0082] A material with the following composition Nao.93Nio.305Mno.50Feo.125Tio.05Lio.01Zno.01O2 was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0083]

[0084] A material with the following composition Nao.93Nio.315Mno.50Feo.05Tio.075Lio.01Zno.05O2 was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0085]

[0086] A material with the following composition Nao.93Nio.305Mno.485Feo.05Tio.05Lio.01Zno.10O2 was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0087]

[0088] A material with the following composition Nao.93Nio.315Mno.325Feo.20Tio.05Lio.01Zno.10O2 was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0089]

[0090] A material with the following composition Nao.93Nio.305Mno.32Feo.2oTio.o5Lio.o5Zno.o7502 was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0091]

[0092] A material with the following composition Nao.90Nio.30Mno.35Feo.10Tio.15Zno.10O2 was synthesized as described under materials synthesis. The heat treatment temperature was 900°C.

[0093]

[0094] A material with the following composition Nao.90Nio.30Mno.35Feo.13Tio.15Lio.03Zno.05O2 was synthesized as described under materials synthesis. The heat treatment temperature was 900°C.

[0095]

[0096] A material with the following composition Nao.93Nio.315Mno.325Feo.05Tio.20Lio.01Zno.10O2 was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0097]

[0098] A material with the following composition Nao.93Nio.305Mno.32Feo.o5Tio.2oLio.o5Zno.o7502 was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0099]

[0100] A material with the following composition Nao.93Nio.305Mno.275Feo.20Tio.20Lio.01Zno.01O2 was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0101]

[0102] A material with the following composition Nao.93Nio.315Mno.25Feo.20Tio.175Lio.05Zno.01O2 was synthesized as described under materials synthesis. The heat treatment temperature was 950°C.

[0103] Table 1: a, x, y, z, v, w, u, a, 6 for the materials described in Example I to 6 and 9 to 20. A grey coloured cell indicates how the comparative materials are outside the present invention.

[0104]

[0105] In Table 1, it can be seen in what way the comparative materials in Example 1 - 6 and Example 10 - 14 are outside the current invention, it also shows that the materials in Example 9 and Example 15 to 20 are inside the current invention. The comparative materials in Example 7 and 8 are outside the current invention because they contain Cu and Mg as divalent doping element instead of Zn. Due to the difference in divalent doping elements, these two examples are not included in Table 1.

[0106] Electrochemical test - Table 2 and 3

[0107] The cathode active material was crushed in a mortar and sieved through a 45 pm sieve inside a glove box with low moisture content, < lppm.

[0108] A NMP based slurry was made from 5 g material, 0.218 g conductive carbon (Super C65 from Imerys), 3.63 g PVDF binder solution (6 wt% Kynar HSV1810 from Arkema in NMP) to obtain a weight ratio of 92:4:4 between active material, conductive carbon and binder. First, NMP, binder solution and conductive carbon were mixed for 10 minutes at 2000 RPM using a Thinkymixer (model ARE-250). Then, the active material was added, and the mixing is continued for 3 minutes.The slurry was cast onto a carbon coated aluminum foil (12 ^m aluminum, 1 ^m carbon coat) using a coating bar with a 200 ^m gap. Subsequently the foil was dried at 80°C for 2 hours. The resulting electrodes had a loading of 11-12 mg active material / cm2.

[0109] Circular 14 mm diameter cathode electrodes were cut. The electrodes were dried under vacuum for 10 h at 120°C before they were used for making coin cell batteries. 15 mm hard carbon electrodes (cut from electrode sheets obtained from Xiamen AOT) were used as anode. A 16 mm glass fiber separator was used and 1 M NaPF6 in EMC:EC (50:50 by wt%) was used as electrolyte. Two batches of electrolyte were used, one contained 2 wt% 1,3,2-dioxathiolane 2,2-dioxide (DTD) as additive and the other without any additive. The coin cells were assembled in an argon filled glove box.

[0110] Electrochemical cycling tests were performed on a Maccor battery tester at 25°C. The voltage interval was 2.0 - 4.1 V. The test consisted of the following charge and discharge cycles:

[0111] • Cycles 1-2: Both charge and discharge done at constant current of 16.5 mA / g cathode active material.

[0112] • Cycles 3-51: Charging at constant current of 55 mA / g cathode active material followed by a constant voltage step at 4.1 V until the current reach 16.5 mA / g. Discharge at constant current of 55 mA / h.

[0113] • Cycle 52 is a repeat of cycles 1 and 2.

[0114] • For some cells cycling was continued by repeating the 50-cycle sequence as described for cycles 3-52 until a total of 202 cycles.

[0115] From the electrochemical tests the following data are extracted:

[0116] First charge capacity in mAh / g cathode active material.

[0117] First discharge capacity in mAh / g cathode active material. Both first charge and first discharge capacity are extracted from the first cycle. The discharge capacity is significantly lower than the first charge capacity mostly because the first cycle inefficiency of the hard carbon anode.Capacity fade is determined form the discharge capacities measured in cycle 2 and cycle 52 (202 for the tests that continued that long) and normalized to 100 cycles by assuming that the absolute capacity drop pr cycle is constant.

[0118] The electrochemical results forthe samples described in Examples I to 9 can be seen in Table 2 and 3.

[0119] Table 2 Results from electrochemical testing with electrolyte with the DTD additive. Grey colored cells indicate best performing materials in each category.

[0120]

[0121] Table 3: Results from electrochemical testing with electrolyte without additive Grey colored cells indicate best performing materials in each category.

[0122]

[0123] From the results in Table 2 and 3 it is seen that the materials of the invention (Example 9 and 15-20) are the only materials that displays the relative best performance with respect to both capacities and capacity fade. The material of example 1 which closely resembles the composition disclosed in O3-Type Na0.95Ni0.40Fe0.15IVIn0.3Ti0.1502 Cathode Materials with Enhanced Storage Stability for High-Energy Na-Ion Batteries - PubMed, Journal of theAmerican Chemical Society 2017139 (25), 8440-844, shows good capacity but inferior cycle life. The material of Example 5 which is covered by the patent application US2022 / 0263085 is inferior both with respect to capacities and cycle life. The materials of Example 3 and 4, covered by our previous patent, are inferior with respect to cycle life (high fade observed) and capacity. The data for example 2 shows that too high concentration of Na (a > 0.95) leads to high fade compared to the materials of the invention, and the data for example 6 shows that a material with low Na, which should lead to low fade, shows high fade if the Ni content is too low and the Fe content is too high. The capacity is also relatively low.

[0124] Weight gain stability test in ambient air - Table 4

[0125] For each of the samples, approximately 0.5 g material is placed in an open container. Precise mass of container and material is noted down. Then, the samples are placed in ambient air (22 °C and 40% relative humidity) in a fume hood ensuring a flow of air over the samples. After 118 h the mass of the samples is determined and the relative mass increase of each of the samples is calculated. The results are given in Table 4. A low mass gain shows that the material reacts only slowly with moisture and CO2 present in ambient air.

[0126] Na extraction from material in water - Table 4

[0127] 1.0 g material is added to 30 g demineralized water. The slurry is stirred with a magnetic stirrer and the pH is followed as a function of time with a calibrated pH meter.

[0128] Some of the Na+ in the material will ion-exchange with protons in the water. This will produce equivalent amounts of OH-in the water. Therefore, from the measured pH the extent of Na ion exchange can be calculated according to the following formula:

[0129] Na+exchanged (formular units) = 10’(14’pH)mol / L * 30 mL * M (g / mol) / 1000 mg

[0130] The results obtained after 8 min reaction are given in Table 4. The time dependency for some of the samples can be seen in Figure 1. A slow extraction rate indicates that the material is relatively stable in the presence of water and could open the possibility to do water-based processing when making electrodes from the material.

[0131] Table 4: Results from the reaction of materials with ambient air and water. Grey colored cells indicate best performing materials in each category.

[0132]

[0133] The data presented in Table 4 shows that there is a big difference in the reactivity of different materials towards moisture and CO2 in ambient air as well as in the Na extraction taking place when the materials are placed in liquid water. It is not only the materials of the invention (Example 9 and Example 17 - 20) that have low reactivity in ambient air; however, the other samples with low reactivity (Examples 2, 3, 4, 6 and 13) have inferior electrochemical performance compared to the materials of the invention (see Table 2 and 3). It is interesting to compare the reactivity of the materials described in examples 2, 7 and 8, which shows that when introducing the divalent metals Mg, Cu and Zn in the materials, it is only Zn that leads to low reactivity with ambient air and liquid water. Not all the materials of the invention show low reactivity with liquid water even though they show good stability in ambient air. This only means that such samples are very far from allowing water-based processing in the battery manufacturing.

[0134] Structural characterization byXRD - Ta b le 5

[0135] All samples were characterized by XRD within a few hours after calcination, to avoid changes caused by reaction with ambient air. Rough Rietveld refinements were performed to obtain the following information: impurities in the samples, weight fractions of 03 and P2 phases which are the only layered phases observed. The XRD results for the samples described in examples 1 to 11 can be seen in Table 5. Rocksalt which is also observed in many samples inthe concentration is also given. Sometimes other phases are also identified but always in very small quantities (1 % or less), so they are not given in the table.

[0136] Table 5: Phase composition of materials as determined by XRD.

[0137]

[0138] It is seen in Table 5 that all the materials of the invention described in Examples 9 and 15-20 fulfil the requirement to be more than 97% phase pure 03. Many of the materials outside the invention also fulfil the requirement but not all of them.

[0139] The present invention relates to a sodium metal oxide material for an electrode of a secondary battery. In particular, the invention relates to a sodium metal oxide material in which more than 97 wt% of the crystalline part of the material has a hexagonal 03 crystal structure comprising a combination of at least Fe, Mn, Ni, Ti and Zn and optionally Li. By examples it is shown that materials in which the 5-6 metals and Na are in the limited compositional range of the present invention result in low cost, high capacity, high cycling stability and high stability in ambient air. Some of the materials also show good stability in liquid water which could pave the way for water-based processing in the battery manufacturing. Known materials close to but clearly outside this compositional range fail to deliver all the combined positive properties. The compositional range of the present invention is a small sub-range of the total compositional space of all sodium metal oxidematerials that has the hexagonal 03 crystal structure, and it is surprising that the materials perform so much better than other combinations that have been claimed to be excellent cathode active materials.

Claims

CLAIMS1. A sodium metal oxide material for a positive electrode of a secondary battery, in which more than 97 wt% of the crystalline part of the material has a hexagonal 03 crystal structure of the formula:NaaNixMnyFezTivLiwZnu02wherein0.85 < a < 0.95,0.2 <x< 0.35,0.25 <y< 0.45,0.05 < z < 0.20,0.05 < v < 0.20,0 < w < 0.05,0.02 < u < 0.10,whereinx + y + z + v + w + u = l,and preferably wherein3.9 < a + w + 2(x + u) + 3z + 4(y + v) < 4.1 and;4.

62. Sodium ion battery material, according to claim 1 , wherein 0.01 w 0.05.

3. Sodium ion battery material, according to claims 1 or 2, wherein 0.25 < y < 0.4.

4. Sodium ion battery material, according to any of the claim 1 to 3, wherein 0.10 < v < 0.20.

5. Sodium ion battery material, according to claim 1, wherein the material has a stability in ambient air of less than 1.5% weight gain / 100 hours in a weight gain stability test, wherein a sample of the material is exposed to ambient air having a relative humidity of 40% at a temperature of 22°C for 100 hours.

6. A sodium ion battery comprising an electrolyte layer interposed between a positive electrode and a negative electrode, wherein the positive electrode is made of a material as described in any of the claims 1 to 5.

7. A process for preparation of a sodium metal oxide material as described in the claims 1 to 5, wherein the process is a synthesis method comprising:a) preparing an homogeneous precursor mix containing Na, Ni, Mn, Fe,Ti and Zn, and optionally Li in the desired stoichiometry;b) calcining the mixture at a temperature above 800°C, in an oxygen containing atmosphere;c) cooling the mixture under low moisture atmosphere;d) deagglomerating the obtained powder from step c).8 . Process for preparation of a sodium metal oxide material, according to claim 7, wherein in the step a) the amount of the combined elements in the precursor are the following:0.85 < Na < 0.95,0.2 < Ni < 0.35,0.25 < Mn < 0.45,0.05 < Fe < 0.20,0.05 < Ti < 0.20,0 < Li < 0.05,0.02 < Zn < 0.10.

9. Process for preparation of a sodium metal oxide material, according to claim 7, wherein in the step a) the precursor is mixed and grinded down to a volume based average particle size between 0.1 - 3 pm.

10. Process for preparation of a sodium metal oxide material, according to claim 8, wherein in the step b), the mixture is calcined at a temperature around 900°C or above, in an oxygen containing atmosphere.

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