Sodium ion cathode active materials for batteries

Abstract

The present invention relates to a cathode active material for rechargeable batteries comprising Na, M, and O, wherein M consists of Fe in a molar ratio a, wherein 0.05 ≤ a ≤ 0.40 relative to M; Mn in a molar ratio b, wherein 0.50 ≤ b ≤ 0.90 relative to M; and X in a molar ratio c, wherein 0.01 ≤ c ≤ 0.10 relative to M, and wherein X is at least one element selected from B, Si, K, Co, Ga, Rb, Rh, Cs, Re, Tl and Pb; wherein a+b+c is 1.00, the molar ratio of Na to M (Na / M) is between 0.40 and 1.10, and the content of Na, Fe, Mn and X is measured by ICP-OES and relates to a method for manufacturing the same.

Description

Description TitleSODIUM ION CATHODE ACTIVE MATERIALS FOR BATTERIESTECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a cathode active material for sodium-ionbatteries, a method for preparing such a cathode active material and sodium ionbatteries comprising such a cathode active material.BACKGROUND OF THE INVENTION

[0002] The growing global energy demand has propelled the widespread adoption oflithium batteries in electronic devices and transportation. However, concerns about the increasing demand and raw material crisis arising from overreliance on lithium batteries have intensified. As a result, sodium-ion batteries have emerged as aviable alternative for a technological revolution in energy storage, particularly when considering the need for large-scale energy storage solutions. Among components of sodium-ion batteries, layered oxides cathode active materials play a crucial role ,in particular, are promising due to their exceptional electrochemical performance,feasibility of synthesis, and versatility in elemental choices.

[0003] While Na is the most abundant alkali metal, Fe and Mn are the most widespreadtransition metals. Thus, Na-Fe-Mn-containing structures could be ideal as low-cost cathode active materials for Na+ electrodes. In 2012, Yabuuchi et al. demonstratedexcellent cycling performance for Na2 / 3Fe1 / 2Mn1 / 2O2[Nature Mater 11, 512–517 (2012)]. Nonetheless, this material encounters several challenges such as phasetransitions (Z-phase) at high voltages, inadequate air stability, and transition metal dissolution. Fe and / or Mn dissolution from cathode active materials is related toinstability in the cathode-electrolyte interphase and electrolyte corrosion, whichtranslates into poor battery performance. These materials usually undergo obviousdegradation of electrochemical performance due to the tendency of Mn dissolution and Fe migration during continuous sodium release and uptake.

[0004] The low stability in air results in great challenges in bringing this class of cathodeactive materials to the market.

[0005] NaxMn1 / 2Fe1 / 2O2 may be interesting due to the excellent capacity with the tworedox pairs of Mn3+ / Mn4+ and Fe3+ / Fe4+ and the low cost of Na, Fe, and Mn. Thepartial substitution of the transition metals with other metal elements can improve the stability, thus improving the capacity loss and rate capability. Partial substitutions of the transition metals can improve the structural stability and thus the capacityretention. The presence of an electrochemically inactive ion could stabilize the parent structure or facilitate desired phase transitions.

[0006] The aim of the present invention is to provide an optimized chemical compositionof a cathode active material with improved structural stability and air stability.

[0007] It is a further object of the present invention to provide a method formanufacturing said cathode active material.

[0008] It is a further object of the present invention to provide a sodium-ion batterycomprising said cathode active material.SUMMARY OF THE INVENTION

[0009] In a first aspect the aim of present invention is achieved by providing a sodium-ion cathode active material for rechargeable batteries comprising Na, M, and O, wherein M consists of: a. Fe in a molar ratio a, wherein 0.050.40 relative to M;b. Mn in a molar ratio b, wherein 0.500.90 relative to M; andc. X in a molar ratio c, wherein 0.010.10 relative to M,and wherein X is at least one element selected from B, Si, K, Co, Ga, Rb, Rh,Cs, Re, Tl and Pb; wherein a+b+c is 1.00, the molar ratio of Na to M (Na / M) is between 0.40 and 1.10, and the content of Na, Fe, Mn and X is measured by ICP-OES.

[0010] In one embodiment of the invention the cathode active material has acomposition according to a general formula (I): Nax2Fea2Mnb2Xc2O2, wherein 0.40 ≤ x2 ≤ 1.10, 0.05 ≤ a2 ≤ 0.40, 0.50 ≤ b2 ≤ 0.90, 0.01 ≤ c2 ≤ 0.10,and X is at least one element selected from B, Si, K, Co, Ga, Rb, Rh, Cs, Re, Tland Pb; wherein a2+b2+c2 is 1.00, and the content of Na, Fe, Mn and X ismeasured by ICP-OES.

[0011] It was surprisingly found that such compositions comprising B, Si, K, Co, Ga, Rb,Rh, Cs, Re, Tl or Pb show an improved air stability and / or reduced transition metaldissolution which indicates improved stability in the cathode-electrolyte interphaseand reduces electrolyte corrosion.

[0012] In another aspect the invention provides a battery comprising a cathode activematerial as described in the first aspect of the invention or any embodiment orcombination of embodiments thereof.

[0013] In a further aspect, the invention provides a method for manufacturing thecathode active material according to the first aspect of the invention or anyembodiment or combination of embodiments thereof. DETAILED DESCRIPTION

[0014] In the following detailed description, preferred embodiments are described indetail to enable practice of the invention. Although the invention is described with reference to these specific preferred embodiments, it will be understood that the invention is not limited to these preferred embodiments. To the contrary, the invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description and accompanying drawings.

[0015] The term “comprising”, as used herein and in the claims, should not beinterpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It needs to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a composition comprising components A and B” should not be limited to compositions consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the composition are A and B. Accordingly, the terms “comprising” and “including” encompass the more restrictive terms “consisting essentially of” and “consisting of”.

[0016] The term “a cathode active material” (also known as positive electrode activematerial) as used herein and in the claims is defined as a material which is electrochemically active in a positive electrode or cathode. By active material, it must be understood to be a material capable to capture and release Li ions when subjected to a voltage change over a predetermined period of time.

[0017] In the framework of the present invention, at% signifies atomic percentage. Theat% or “atomic percent” of a given element expression of a concentration means how many percent of all atoms in the concerned compound are atoms of said element. The designation at% is equivalent to mol% or “molar percent”.

[0018] The term "about" as used herein referring to a measurable value such as aparameter, an amount, a temporal duration, and the like, is meant to encompass variations of + / -20% or less, preferably + / -10% or less, more preferably + / -5% or less, even more preferably + / -1% or less, and still more preferably + / -0.1% or less of and from the specified value, in so far such variations are appropriate to perform inthe present disclosure. However, it is to be understood that the value to which the modifier "about" refers is itself also specifically disclosed.

[0019] As used herein, a range of values “from X to Y” and “between X and Y” includesthe endpoints X and Y. Cathode active material

[0020] In a first aspect, the object of the invention is achieved by providing a cathodeactive material for rechargeable batteries comprising Na, M, and O, wherein M comprises: a. Fe in a molar ratio a, wherein 0.050.40 relative to M;b. Mn in a molar ratio b, wherein 0.500.90 relative to M; andc. X in a molar ratio c, wherein 0.010.10 relative to M,and wherein X is at least one element selected from B, Si, K, Co, Ga, Rb, Rh,Cs, Re, Tl and Pb; d. wherein a+b+c is 1.00, the molar ratio of Na to M (Na / M) is between 0.40and 1.10, and the content of Na, Fe, Mn and X is measured by ICP-OES.

[0021] In a further embodiment the molar ratio of Na to M (Na / M) is between 0.50 and0.75, preferably between 0.60 and 0.70, more preferably between 0.65 and 0.70, even more preferably about 0.66; 0.050.30, preferably 0.080.13,more preferably a is about 0.08 or 0.13; 0.600.90, preferably 0.770.87, more preferably b is about 0.77, about 0.82 or about 0.87; and 0.050.10, preferably c is about 0.05 or about 0.10.

[0022] In a further embodiment the molar ratio of Na to M (Na / M) is between 0.50 and0.75, preferably between 0.60 and 0.70, more preferably between 0.65 and 0.70, even more preferably about 0.66; 0.050.30, preferably 0.080.13,more preferably a is about 0.08 or about 0.13; 0.600.90, preferably 0.77 ≤ bc0.10, preferably cis about 0.05 or about 0.10; wherein a+b+c is 1.00.

[0023] In one embodiment present invention provides a cathode active material having acomposition according to a general formula (I): Nax2Fea2Mnb2Xc2O2, (I)wherein 0.401.10, 0.050.40, 0.500.90,0.01 0.10, and X is at least one element selected from B, Si, K, Co, Ga, Rb,Rh, Cs, Re, Tl and Pb; wherein a2+b2+c2 is 1.00, and the content of Na, Fe, Mn andX is measured by ICP-OES.

[0024] In a further embodiment the cathode active material is according to formula (I),wherein a. x2 is between 0.50 and 0.90;b. a2 is between 0.08 and 0.13;c. b2 is between 0.77 and 0.87;d. c2 is between 0.05 and 0.10,wherein a2+b2+c2 is 1.00.

[0025] In a certain embodiment the cathode active material is according to formula (I),wherein x2 = 0.66, a2 = 0.08 or 0.13, b2 = 0.77, 0.82 or 0.87 and c2 is 0.05 or 0.10,wherein a+b+c is 1.00.

[0026] In one embodiment of the present invention, the cathode active material has acomposition according to a formula selected from formulae IIa, IIb and IIc:a. Formula IIa: Na0.66Fe0.13Mn0.77X0.1O2b. Formula IIb: Na0.66Fe0.13Mn0.82X0.05O2c. Formula IIc: Na0.66Fe0.08Mn0.87X0.05O2wherein X is at least one element selected from B, Si, K, Co, Ga, Rb, Rh, Cs,Re, Tl and Pb, wherein the content of Na, Fe, Mn and X is measured by ICP- OES.

[0027] In one embodiment of the cathode active material X is at least one elementselected from B, Si, Ga, Rb, Rh, Cs, Re, Tl, and Pb, preferably X is selected from B, Si, Tl, and Ga, more preferably X is selected from B and Si.

[0028] In a particular embodiment of the present invention, the cathode active material isselected from: Na0.66Fe0.13Mn0.77B0.1O2, Na0.66Fe0.13Mn0.77Si0.1O2; Na0.66Fe0.13Mn0.77K0.1O2;; Na0.66Fe0.13Mn0.77Ga0.1O2; Na0.66Fe0.13Mn0.77Rb0.1O2; Na0.66Fe0.13Mn0.77Rh0.1O2; Na0.66Fe0.13Mn0.77Cs0.1O2; Na0.66Fe0.13Mn0.77Re0.1O2; Na0.66Fe0.13Mn0.77Tl0.1O2; Na0.66Fe0.13Mn0.77Pb0.1O2; Na0.66Fe0.13Mn0.77Co0.1O2; Na0.66Fe0.13Mn0.82B0.05O2; Na0.66Fe0.13Mn0.82Si0.05O2; Na0.66Fe0.13Mn0.82K0.05O2; Na0.66Fe0.13Mn0.82Ga0.05O2; Na0.66Fe0.13Mn0.82Rb0.05O2; Na0.66Fe0.13Mn0.82Rh0.05O2; Na0.66Fe0.13Mn0.82Cs0.05O2; Na0.66Fe0.13Mn0.82Re0.05O2; Na0.66Fe0.13Mn0.82Tl0.05O2; Na0.66Fe0.13Mn0.82Pb0.05O2; Na0.66Fe0.13Mn0.82Co0.05O2; Na0.66Fe0.08Mn0.87B0.05O2; Na0.66Fe0.08Mn0.87Si0.05O2; Na0.66Fe0.08Mn0.87K0.05O2;; Na0.66Fe0.08Mn0.87Ga0.05O2; Na0.66Fe0.08Mn0.87Rb0.05O2; Na0.66Fe0.08Mn0.87Rh0.05O2; Na0.66Fe0.08Mn0.87Cs0.05O2; Na0.66Fe0.08Mn0.87Re0.05O2;Na0.66Fe0.08Mn0.87Tl0.05O2; Na0.66Fe0.08Mn0.87Pb0.05O2; Na0.66Fe0.08Mn0.87Co0.05O2;Na0.50Fe0.13Mn0.82Si0.05O2; Na0.58Fe0.13Mn0.82Si0.05O2; Na0.66Fe0.13Mn0.82Si0.05O2;Na0.75Fe0.13Mn0.82Si0.05O2, Na0.50Fe0.13Mn0.82B0.05O2, Na0.58Fe0.13Mn0.82B0.05O2;Na0.66Fe0.13Mn0.82B0.05O2; Na0.75Fe0.13Mn0.82B0.05O2; Na0.66Fe0.23Mn0.72Si0.05O2;Na0.66Fe0.33Mn0.62Si0.05O2; Na0.66Fe0.43Mn0.52Si0.05O2; Na0.66Fe0.23Mn0.72B0.05O2;Na0.66Fe0.43Mn0.52B0.05O2; Na0.66Fe0.13Mn0.77B0.10O2; Na0.66Fe0.13Mn0.77Si0.10O2;Na0.66Fe0.4Mn0.5B0.1O2; Na0.66Fe0.4Mn0.5Si0.1O2; Na0.66Fe0.4Mn0.5K0.1O2; Na0.66Fe0.4Mn0.5Co0.1O2; Na0.66Fe0.4Mn0.5Ga0.1O2; Na0.66Fe0.4Mn0.5Rb0.1O2; Na0.66Fe0.4Mn0.5Rh0.1O2; Na0.66Fe0.4Mn0.5Cs0.1O2; Na0.66Fe0.4Mn0.5Re0.1O2; Na0.66Fe0.4Mn0.5Tl0.1O2; and Na0.66Fe0.4Mn0.5Pb0.1O2.

[0029] In one embodiment the cathode active material according to the inventioncomprises a layered structure.

[0030] In one embodiment the cathode active material is according to the inventionhaving a Fe dissolution value of less than 950 ^g / g, preferably less than 900 ^g / g,more preferably less than 800 ^g / g, even more preferably less than 700 ^g / g, most preferably less than 600 ^g / g.

[0031] In one embodiment the cathode active material is according to the inventionhaving a Mn dissolution value of less than 150 ^g / g, preferably less than 100 ^g / g, more preferably less than 75 ^g / g, even more preferably less than 50 ^g / g.

[0032] The Fe dissolution value and Mn dissolution value are determined via thetransition metal dissolution analysis as described herein.

[0033] In a further embodiment the cathode active material is according to the inventionhaving a structure retention of at least 5%, preferably at least 10%, more preferably at least 15%, even more preferably at least 20%, even more preferably at least 50%, most preferably at least 80%, as determined via the air stability analysis as described herein.Method for manufacturing a cathode active material

[0034] In a further object, the present invention provides a method for manufacturing acathode active material according to the invention, or any embodiment orcombination of embodiments thereof, wherein the method comprises the followingsteps: Step 1) dissolving one or more salts comprising Na, Fe, Mn and X in stoichiometric molar ratios into an alcohol, water or a mixture thereof; andstirring while heating at a temperature in the range of 30 to 100 °C to obtain a mixture, Step 2) drying said mixture at a temperature in the range of 300 to 500 °C to obtain a dried mixture, and Step 3) heating the dried mixture at a temperature in the range of 800 to 1200 °C to obtain the cathode active material.

[0035] In particular, the mixture obtained by Step 1) may be a sol-gel.

[0036] In an embodiment of the method of the present invention in Step 1) citric acid isadded to the mixture in an equimolar amount with the total amount of Na, Fe, Mn and X.

[0037] In an embodiment of the method of the present invention, in Step 1) the heatingis at a temperature in a range of 50 to 80 °C, preferably at about 65 °C.

[0038] In an embodiment of the method of the present invention, in Step 2) the mixtureis dried at a temperature in a range of 350 to 450 °C, preferably at about 400 °C.

[0039] In an embodiment of the method of the present invention, in Step 3) the driedmixture is heated at a temperature in a range of 800 to 900 °C, preferably at about 850 °C.

[0040] In an embodiment nitrate salts were chosen for the sol-gel synthesis. Thus, in anembodiment of the method of the present invention, the salts comprising Na, Fe, Mn and X are nitrate salts of Na, Fe, Mn and X.

[0041] In an embodiment, the method of the present invention is a sol-gelautocombustion method to prepare any one of the different compositions accordingto the invention. Sol–gel autocombustion may be enabled by the exothermic reactionbetween oxidants, herein the metal salts, in particular metal nitrates, and fuel, suchas organic amines, urea and acids, in particular citric acid. The autocombustion mayin particular take place upon heating of a sol-gel, in an initial drying step and / or in a subsequent heating step.

[0042] In an embodiment of the method of the present invention, the method comprisesin Step 1) preparing 2.4 M solutions of sodium nitrate (NaNO3), manganese nitrate(Mn(NO3)2), and iron nitrate (Fe(NO3)3) and dispensing into an 8×8 well plateaccording to the desired ratios listed below. In a preferred embodiment nitrate salts were chosen for the sol-gel synthesis. The method further comprises adding aqueous solutions (1M) of the corresponding element X to the precursor solutions at 5mol% or 10 mol% relative to the total molar content of Na, Fe, and X, wherein X isat least one element selected from B, Si, Ga, Rb, Rh, Cs, Re, Tl, and Pb. Themethod comprises further adding citric acid (3M) in an equal molar ratio to the metalcations as a chelating agent to stabilize the metal ions. In a more preferredembodiment the gelation process was carried out at 65 °C for 2 days to formhomogeneous gels via strong carboxylic-metal bonding. Then the resulting gels were pulverized and transferred onto an alumina plate covered with an 8×8 aluminum smokestack, with the gels separated into different combustion chambers to prevent cross-contamination. Preferably combustion was performed at 400 °C for 2 hours to remove the citric acid and nitrates (heating rate: 2 °C per min). In a further step after removing the smokestack, the preheated samples were further heated at 850 °C for 12 hours in ambient air (heating rate: 5 °C per min), followed by a cooling rate of 5 °C per min to room temperature. Battery

[0043] In a further object, the present invention provides a sodium-ion batterycomprising said cathode active material, in particular the cathode active materialaccording to the first aspect of the invention, or according to any embodiment orcombination of embodiments thereof. EXAMPLES Description of methodsGeneral protocol for the synthesis of cathode active material

[0044] A sol-gel autocombustion method was employed to achieve exampledcompositions according to the invention.

[0045] Sodium nitrate (NaNO3), manganese nitrate (Mn(NO3)2), and iron nitrate(Fe(NO3)3) from Sigma-Aldrich were prepared as 2.4 M solutions and dispensed into an 8×8 well plate according to the desired ratios listed below. Nitrate salts were chosen for the sol-gel synthesis, considering their potential influence on the final products. Aqueous solutions (1M) of elements selected from B, Si, Ga, Rb, Rh, Cs, Re, Tl or Pb, were added to the precursor solutions at 5 mol% or 10 mol% relative tothe total molar content of Fe, Mn, and X. To stabilize the metal ions, citric acid (3M)was added in an equal molar ratio to the metal cations as a chelating agent. The gelation process was carried out at 65 °C for 2 days to form homogeneous gels via strong carboxylic-metal bonding. To prevent cross-contamination, the resulting gels were pulverized and transferred onto an alumina plate covered with an 8×8 aluminum smokestack, with the gels separated into different combustion chambers. Combustion was performed at 400 °C for 2 hours to remove the citric acid and nitrates (heating rate: 2 °C per min). After removing the smokestack, the preheatedsamples were further heated at 850 °C for 12 hours in ambient air (heating rate: 5 °C per min), followed by a cooling rate of 5 °C per min to room temperature. Analysis methods

[0046] The following analysis methods are used to analyze the examples according tothe invention and comparative examples:Inductively coupled plasma – optical emission analysis (ICP-OES)

[0047] The inductively coupled plasma (ICP) method is used to quantify elements byusing an Agilent ICP 5110.1 g of powder sample is dissolved in 50 mL high purity hydrochloric acid in an Erlenmeyer flask. The flask is covered by glass and heated on a hot plate for complete dissolution of the material. After being cooled to room temperature, the solution is moved to a 500 mL volumetric flask that has been thoroughly cleaned and rinsed with distilled (DI) water. After filling the flask with the solution, the volumetric flask is filled with DI water up to the 500 mL mark, followed by complete homogenization.5 mL solution is taken out with a 5 mL pipette and transferred into a 50 mL volumetric flask along with an internal standard for a second dilution, where the volumetric flask is filled with 10% hydrochloric acid up to the 50 mL mark and then homogenized. Finally, this 50 mL solution is used in the IPC measurement. X-ray diffraction (XRD)

[0048] The X-ray diffraction measurements were conducted using a PanalyticalEmpyrean diffractometer equipped with a Mo target (60 kV, 40 mA) and PIXcel3D detector in high throughput measurement mode. The samples were mounted onto a transparent mylar film with 3D-printed 96 sample slots without cross-contamination. A scattering angle range (l = 0.70926 Å for Mo Ka1) of 4–30° was initially investigated and then converted into 10-70° for Cu radiation. The major peak intensity with over 3000 counts was achieved in under 10 min per sample assuring the high-quality XRD for further Rietveld refinement. The phase identification and refinement of XRD patterns were performed on Panalytical's HighScore Plus software combined with manual and batch modes. Transition metal dissolution analysis

[0049] Samples were assembled into Swagelok-type cells and subjected to cycling from1.5 V to 4.6 V under galvanostatic conditions at a rate of 10 mAh / g. After 10 fullcycles, the cells were disassembled in an argon-filled glovebox. The separators and Na anodes were collected for further analysis using inductively coupled plasma- optical emission spectrometry (ICP-OES) analysis, performed by Agilent Technologies 5100 ICP-OES with the autosampler.

[0050] X-ray photoelectron spectroscopy (XPS) from Thermo Fisher-Scientific Nexsa G2was utilized to analyze the surface elemental compositions of the cycled cathode materials. MAGCIS ion gun with an ion energy of 500 eV and medium current intensity was utilized to perform depth profiling through Ar gun etching. The sputter rate on oxide material was estimated at 0.09 nm / sec.

[0051] To evaluate the battery performance of the cathode active materials according tothe invention, inventors applied electrochemical analysis to the different samples inhalf cells.

[0052] The electrochemical analysis was evaluated using lab-developed combinatorialcells consisting of 64 parallel channels. A printed circuit board (PCB, Optima Tech) with 64 parallel gold pads served as cathode current collectors and was coated with aluminum foil. The cathodes were prepared by combining 2 mg of active materials with 20 wt% carbon black, followed by drop-casted onto the contact pads via 20 wt% polyvinylidene fluoride (PVDF) as a binder in N-methyl-2-pyrrolidone (NMP). NMP was then removed and dried for 12 hours at 80 °C, resulting in a cathode loading of 20 mg / cm2. In an argon-filled glove box, the assembly of the combinatorial cell was carried out using the electrolyte composed of 1M sodium perchlorate in propylene carbonate (PC) with 2 wt% fluoroethylene carbonate (FEC). Sodium metal foil served as the anode, and a GF / D glass microfiber prefilter was used as a separator. Cyclic voltammetry (CV) measurements were conducted using a lab-built high- throughput electrochemical system, equipped with a quad voltage source (Keithley 213) and a multimeter with a multiplexer (Keithley 2750). The voltage range was set between 1.5 and 4.3 V versus Na / Na+ at a scan rate of 0.1 V h-1. The CV curves were further processed by integrating the current-time product (Idt) to obtaincapacity as a function of voltage. The high throughput measurements exhibited excellent reproducibility for the same cathode materials, with a relative standard deviation (RSD) of 7% for specific capacities. The CV was performed from 1.5V to4.3V at 0.1V / h to analyze the electrochemical properties of samples as cathode materials. Air stability analysis

[0053] To investigate the air stability, samples were carefully stored under identicalconditions. XRD measurements were conducted at different time intervals and under varying humidity conditions. Initially, the samples were stored in dry air (RH < 10%) for 2 months, followed by exposure to ambient air (RH = 45%) for one week, and finally subjected to humid air (RH > 90%) for another week. An enclosed systemwas assured to provide stable relative humidity (RH) levels without interference, where the temperature was maintained within 23 ± 1 °C. The XRD scans obtained from these storage conditions were first analyzed by identifying the different phases present in the sample and subsequently subjected to quantitative refinement using the method described in the X-ray diffraction section.

[0054] Using Rietveld refinement, inventors correlate the stability of a sample byquantifying the % of structure retention after being exposed to humid conditions. Accordingly, the higher the % of structure retention in a sample, the higher the air stability.Exemplified compositions according to the invention

[0055] The invention will be described below in greater detail with reference toexamples, but the invention is not limited in any way by these examples, as long as it does not exceed the scope and spirit of the present invention.

[0056] Some preferred examples of mixed metal compounds according to the generalformula (I) are selected from the group:Na0.66Fe0.13Mn0.77B0.1O2, Na0.66Fe0.13Mn0.77Si0.1O2; Na0.66Fe0.13Mn0.77K0.1O2; Na0.66Fe0.13Mn0.77Ga0.1O2; Na0.66Fe0.13Mn0.77Rb0.1O2; Na0.66Fe0.13Mn0.77Rh0.1O2; Na0.66Fe0.13Mn0.77Cs0.1O2; Na0.66Fe0.13Mn0.77Re0.1O2; Na0.66Fe0.13Mn0.77Tl0.1O2; Na0.66Fe0.13Mn0.77Pb0.1O2; Na0.66Fe0.13Mn0.77Co0.1O2; Na0.66Fe0.13Mn0.82B0.05O2; Na0.66Fe0.13Mn0.82Si0.05O2; Na0.66Fe0.13Mn0.82K0.05O2; Na0.66Fe0.13Mn0.82Ga0.05O2; Na0.66Fe0.13Mn0.82Rb0.05O2; Na0.66Fe0.13Mn0.82Rh0.05O2; Na0.66Fe0.13Mn0.82Cs0.05O2; Na0.66Fe0.13Mn0.82Re0.05O2; Na0.66Fe0.13Mn0.82Tl0.05O2; Na0.66Fe0.13Mn0.82Pb0.05O2; Na0.66Fe0.13Mn0.82Co0.05O2; Na0.66Fe0.08Mn0.87B0.05O2; Na0.66Fe0.08Mn0.87Si0.05O2; Na0.66Fe0.08Mn0.87K0.05O2;; Na0.66Fe0.08Mn0.87Ga0.05O2; Na0.66Fe0.08Mn0.87Rb0.05O2; Na0.66Fe0.08Mn0.87Rh0.05O2; Na0.66Fe0.08Mn0.87Cs0.05O2; Na0.66Fe0.08Mn0.87Re0.05O2;Na0.66Fe0.08Mn0.87Tl0.05O2; Na0.66Fe0.08Mn0.87Pb0.05O2; Na0.66Fe0.08Mn0.87Co0.05O2;Na0.50Fe0.13Mn0.82Si0.05O2; Na0.58Fe0.13Mn0.82Si0.05O2; Na0.66Fe0.13Mn0.82Si0.05O2;Na0.75Fe0.13Mn0.82Si0.05O2, Na0.50Fe0.13Mn0.82B0.05O2, Na0.58Fe0.13Mn0.82B0.05O2;Na0.66Fe0.13Mn0.82B0.05O2; Na0.75Fe0.13Mn0.82B0.05O2; Na0.66Fe0.23Mn0.72Si0.05O2;Na0.66Fe0.33Mn0.62Si0.05O2; Na0.66Fe0.43Mn0.52Si0.05O2; Na0.66Fe0.23Mn0.72B0.05O2;Na0.66Fe0.43Mn0.52B0.05O2; Na0.66Fe0.13Mn0.77B0.10O2; Na0.66Fe0.13Mn0.77Si0.10O2;Na0.66Fe0.4Mn0.5B0.1O2; Na0.66Fe0.4Mn0.5Si0.1O2; Na0.66Fe0.4Mn0.5K0.1O2; Na0.66Fe0.4Mn0.5Co0.1O2; Na0.66Fe0.4Mn0.5Ga0.1O2; Na0.66Fe0.4Mn0.5Rb0.1O2; Na0.66Fe0.4Mn0.5Rh0.1O2; Na0.66Fe0.4Mn0.5Cs0.1O2; Na0.66Fe0.4Mn0.5Re0.1O2; Na0.66Fe0.4Mn0.5Tl0.1O2; and Na0.66Fe0.4Mn0.5Pb0.1O2. Results of the transition metal dissolution analysis

[0057] The results for each example are presented in Table I as a dissolved mass (µg)of Fe or Mn deposited into the anode divided by cathode active material mass (g). The comparative example (Com Ex 1) relates to Na0.66Fe0.5Mn0.5O2.

[0058] Table IExample # Composition Fe (µg / g) Mn (µg / g)Com Ex 1 Na0.66Fe0.5Mn0.5O2 969 188Ex 1 Na0.66Fe0.4Mn0.5B0.1O2 500 31Ex 2 Na0.66Fe0.4Mn0.5Si0.1O2 1250 125Ex 3 Na0.66Fe0.4Mn0.5K0.1O2 586 138Ex 4 Na0.66Fe0.4Mn0.5Co0.1O2 655 241Ex 5 Na0.66Fe0.4Mn0.5Ga0.1O2 875 156Ex 6 Na0.66Fe0.4Mn0.5Rb0.1O2 679 107Ex 7 Na0.66Fe0.4Mn0.5Rh0.1O2 931 172Ex 8 Na0.66Fe0.4Mn0.5Cs0.1O2 1080 320Ex 9 Na0.66Fe0.4Mn0.5Re0.1O2 1097 129Ex 10 Na0.66Fe0.4Mn0.5Tl0.1O2 767 100Ex 11 Na0.66Fe0.4Mn0.5Pb0.1O2 581 65

[0059] The comparative example Na0.66Fe0.5Mn0.5O2 presents a higher level of Feand / or Mn dissolution after 10 full charging and discharging cycles in a battery whencompared with the exemplified compositions according to the invention (Ex 1-8, and Ex 10-11), see Table I.

[0060] A cathode active material according to the invention comprising B, Si, K, Co, Ga,Rb, Rh, Re, Tl or Pb shown a positive effect suppressing the dissolution of either one or both of Fe and Mn when compared with the comparative example 1.

[0061] Accordingly, a cathode active material according to the invention comprising B,Si, K, Co, Ga, Rb, Rh, Re, Tl or Pb, shows an improved stability in the cathode-electrolyte interphase and / or reduced electrolyte corrosion. Results of the air stability analysis

[0062] Compositions were tested under the conditions described above, and the resultsare shown on Table II. The comparative example 2 (Com Ex 2) corresponds to Na0.66Mn0.13Fe0.87O2.

[0063] Table IIExample # composition % of structure retentionCom Ex 2 Na0.66Fe0.13Mn0.87O2 3.8Com Ex 3 Na0.66Fe0.13Mn0.77Sc0.1O2 0.3Ex 12 Na0.66Fe0.13Mn0.77B0.1O2 80.2Ex 13 Na0.66Fe0.13Mn0.77Si0.1O2 64.4Ex 14 Na0.66Fe0.13Mn0.77K0.1O2 12.6Ex 15 Na0.66Fe0.13Mn0.77Co0.1O2 19.5Ex 16 Na0.66Fe0.13Mn0.77Ga0.1O2 63.8Ex 19 Na0.66Fe0.13Mn0.77Rb0.1O2 32.6Ex 20 Na0.66Fe0.13Mn0.77Cs0.1O2 52.3Ex 21 Na0.66Fe0.13Mn0.77Tl0.1O2 23.0Ex 22 Na0.66Fe0.13Mn0.77Pb0.1O2 86.8

[0064] According to the results shown on Table II, the comparative examples 2 and 3present a lower retention of the structure when compared with the exemplified compositions according to the invention (Ex 12-22), see Table II.

[0065] Accordingly, the examples comprising B, Si, K, Co, Ga, Rb, Cs, Tl or Pb presenta higher % of retention of the structure when compared with the comparative example 2. Therefore, a cathode active material according to the inventioncomprising B, Si, K, Co, Ga, Rb, Cs, Tl or Pb shows an improved air stability.

Claims

Claims 1. A cathode active material for rechargeable batteries comprising Na, M, and O, wherein M consists of: Fe in a molar ratio a, wherein 0.05 ≤ a ≤ 0.40 relative to M;Mn in a molar ratio b, wherein 0.50 ≤ b ≤ 0.90 relative to M; andX in a molar ratio c, wherein 0.01 ≤ c ≤ 0.10 relative to M, and wherein X is at leastone element selected from B, Si, K, Co, Ga, Rb, Rh, Cs, Re, Tl and Pb;wherein a+b+c is 1.00, the molar ratio of Na to M (Na / M) is between 0.40 and 1.10, and thecontent of Na, Fe, Mn and X is measured by ICP-OES.

2. The cathode active material according to claim 1, having a composition according to ageneral formula (I): Nax2Fea2Mnb2Xc2O2,wherein 0.40 ≤ x2 ≤ 1.10, 0.05 ≤ a2 ≤ 0.40, 0.50 ≤ b2 ≤ 0.90, 0.01 ≤ c2 ≤ 0.10, and X isat least one element selected from B, Si, K,Co, Ga, Rb, Rh, Cs, Re, Tl and Pb;wherein a2+b2+c2 is 1.00, and the content of Na, Fe, Mn and X is measured by ICP-OES.

3. The cathode active material according to claim 1 or 2 comprising a layered structure.

4. The cathode active material according to any one of the claims 1-3, wherein the molar ratioof Na to M (Na / M) is between 0.50 and 0.75, preferably between 0.60 and 0.70, more preferably between 0.65 and 0.70, even more preferably about 0.66.

5. The cathode active material according to any one of the claims 1-4, wherein X is at leastone element selected from B, Si, Ga, Rb, Rh, Cs, Re, Tl, and Pb, preferably X is selected fromB, Si, Tl, and Ga, more preferably X is selected from B and Si.

6. The cathode active material according to any one of the claims 1-5, wherein a or a2 is suchthat 0.05 ≤ a or a2 ≤ 0.30, preferably 0.08 ≤ a or a2 ≤ 0.13, more preferably a or a2 isabout 0.08 or about 0.13.

7. The cathode active material according to any one of the claims 1-6, wherein b or b2 is suchthat 0.60 ≤ b or b2 ≤ 0.90, preferably 0.77 ≤ b or b2 ≤ 0.87, more preferably b or b2 isabout 0.77, about 0.82 or about 0.87.

8. The cathode active material according to any one of the claims 1-7, wherein c or c2 is suchthat 0.05 ≤ c or c2 ≤ 0.10, preferably c or c2 is about 0.05 or about 0.10.

9. The cathode active material according to any one of the claims 1-8, having a compositionaccording to a general formula Na0.66Fe0.13Mn0.77X0.1O2, Na0.66Fe0.13Mn0.82X0.05O2 orNa0.66Fe0.08Mn0.87X0.05O2.

10. A method for manufacturing a cathode active material according to any of the claims 1-9, wherein the method comprises the following steps: oStep 1) dissolving one or more salts comprising Na, Fe, Mn and X instoichiometric molar ratio into an alcohol, water or a mixture thereof; andstirring while heating at a temperature in the range of 30 to 100 °C to obtain amixture, oStep 2) drying said mixture at a temperature in the range of 300 to 500 °C toobtain a dried mixture, ando Step 3) heating the dried mixture at a temperature in the range of 800 to1200 °C to obtain the cathode active material.

11. The method according to claim 10, wherein in Step 1) citric acid is added to the mixture in an equimolar amount with the total amount of Na, Fe, Mn and X.

12. The method according to claim 10 or 11, wherein in Step 1) the heating is at a temperature in a range of 50 to 80 °C, preferably at about 65 °C.

13. The method according to any one of the claims 10-12, wherein in Step 2) the mixture is dried at a temperature in a range of 350 to 450 °C, preferably at about 400 °C.

14. The method according to any one of the claims 10-13, wherein in Step 3) the dried mixture is heated at a temperature in a range of 800 to 900 °C, preferably at about 850 °C.

15. A sodium-ion battery comprising the cathode active material according to any one of the claims 1-9.

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