Spinel materials with controlled atomic structure as electroactive materials

Abstract

The present invention refers an electroactive spinel material of formula LiyMn1.5-x-zFe0.5+xMzO4-d wherein: 5 i) -0.5<x≤1.5, 0≤y≤2, 0≤z≤0.5 and 0≤d<0.25, provided that 0≤(1.5-x-z) ≤2, ii) the material is characterized by having a spinel structure, iii) M is at least a metal selected from a transition metal and a main-group metal, and iv) less than 6% of the iron atoms contained in the spinel material are found in tetrahedral position as determined by Mössbauer spectroscopy. Such electroactive material has a spinel structure that has been found suitable for cathode materials, as the low to negligible amount of iron in tetrahedral position, i.e., low amount of Fe defects, leads to an improved electrochemical performance of the cell that comprises said cathode material in terms of discharge capacity and cycling stability in comparison with an electroactive material with 6% or more Fe defects. The present invention also refers to a method for producing the abovementioned electroactive material and the electroactive spinel material obtainable by said method.

Description

[0001] SPINEL MATERIALS WITH CONTROLLED ATOMIC STRUCTURE AS ELECTRO ACTIVE MATERIALS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to spinel materials with controlled atomic structure for use as electrochemical active materials, particularly as cathode materials. Such spinels are characterized by a low percentage of Fe defects (i.e. iron atoms in tetrahedral position are lower than 6%) which lead to substantial improvements in discharge capacity and rate capability.

[0004] BACKGROUND

[0005] In the coming decade, as applications continue to grow, a sharp rise in the demand for environmentally friendly rechargeable batteries is expected. Lithium-ion batteries (LIBs) are the most powerful high-capacity rechargeable batteries currently available, they will likely continue to hold this position in the near future. LiMn2O4 spinel has achieved success in industry thanks to its high power and cost-effectiveness when compared to other materials (F. Hase et al., Trends Chem. 2019, 1(3), 282-291; S. Choi et al., Adv. Mater. Technol. 2018, 3 (9), 1700376). In 1996, LiMmCL spinel (LMO) was first made available for sale by NEC corporation as a cathode material for LIBs. Since then, it was used as new generation for electric vehicles (EVs) and it is currently also used as a blend with layered oxides for EV batteries. LMO provides good ionic and electrical properties, low cost, high theoretical capacity (148 mAhg1), high potential (4V vs. Li), and environmental friendliness.

[0006] Spinel LiMn2O4 (LMO) has been considered as a promising cathode material due to its high thermal stability, low cost, abundance, and environmental affinity, however one of the major drawbacks of the LMO is that Mn3+is Jahn Teller active, and the capacity lowers over cycling thus leading to suboptimal electrochemical performance. This effect may be decreased to some extent by making partial substitution of Mn in the LMO structure by other transition metal divalent and trivalent ions. These substitutions have shown to improve the cycling stability in the 4 V region, as well as the electrochemical activity and performance at higher potentials vs Lithium. For example, LiCoo.5Mn1.5O4 operates at voltages up to 5.1 V vs Li (H. Shigemura et al., J. Mater. Chem. 2002, 12 (6), 1882-1891). Less expensive and abundant Fe andNi are very appealing for alternative spinel compositions. LiNio.5Mn1.5O4 spinel (LNMO) is characterized by a higher operating voltage than LMO that goes up to 4.7 V vs Li, keeping its theoretical capacity of 147 mAhg1(Y. Idemoto et al., Electrochemistry 2002, 70 (8), 587-589; T. Ohzuku et al., Electrochimica Acta 2001, 46 (15), 2327-2336). LNMO has been studied from an experimental and theoretical point of view and the voltage / composition curves are slightly different depending on its metallic Ni / Mn distribution as it can present ordered (P4332) and disordered (Fd-3m) structures owing to the difference in size between Ni+2and Mn+4.

[0007] In picking a suitable cathode material, consideration of price and environmental friendliness is certainly paramount. Iron is another excellent alternative to improve spinel materials due to low toxicity and high abundance in the earth’s crust. Iron- and nickel-containing spinels (LFMOs and LNMOs, respectively) contain less Mn3+atoms as compared to pure LMO therefore decreasing the Mn ions in solution and the Jahn Teller effect, maintaining high capacity (148mAh / g) and operating at higher voltages, particularly up to 5.3V vs Li for LFMOs and up to 4.7V vs Li for LNMOs, compared to only 4V vs Li for LMO.

[0008] Prior studies reported that the Fe and Mn in LFMOs occupy octahedral positions, and the Li occupies tetrahedral positions in the cubic structure (Fd-3mZ space group), that is the ideal structure of LFMO. However, when diffractograms of the LFMOs of the prior art are carefully analyzed, it is observed that the (220) diffraction peak is of variable but non-negligible intensity. Such diffraction peak is qualitatively indicative of having Fe located in tetrahedral sites. The change in the position of the atoms leads to a variable amount of Fe defects (see Figure 1, top).

[0009] Prior art documents fail to correlate the crystalline structure of iron-containing electroactive spinel materials with their electrochemical performance. For example, the reported literature on LiMn1.5Feo.5O4 spinel does not give an accurate mapping of the iron atoms in the spinel structure, let alone the quantification of iron defects or even a correlation structureelectrochemical activity.

[0010] Bhaskar et al. (J. Electrochem. Soc. 2010, 157 (6), A689) report the synthesis of LiM0.5Mn1.5O4 (M = Fe, Co, Ni) spinel oxides; however, they do not determine the amount tetrahedral iron in the samples and show a capacity of 120mAh / g in the first cycle at C / 2 at a 3.5-5.3 V voltage.

[0011] M. P. Pico et al. (Dalton Trans. 2014, 43 (39), 14787-14797) deals with cathode materials based on the lithium iron manganese oxide LiFeo.5Mn1.5O4 synthesized by different methods (sol- gel, in solution and hydrothermal) affording samples with distinct morphologies; tetrahedral iron is present in the spinel structure according to the X-ray studies.

[0012] Y. Mu et al. (Int. J. Electrochem. Sci., 2017, 12, 6045-6053) describe Fe-doped LiFexNio.s- xMni.504 (x=0-0.5) cathode materials which are increasingly better with increasing amounts of iron; though these samples contain Fe defects.

[0013] H. Shigemura et al. (J. Mater. Chem. 2002, 12 (6), 1882-189) report the electrochemical properties of Li(Fe, Co)xMn2-xO4 (0 < x < 0.5 for Fe, 0 < x < 1.0 for Co), however iron defects are clearly detected by X-ray diffraction. The same materials have been used by A. Eftekhari et al. (J. Power Sources 2003, 124 (1), 182-190)

[0014] K. Amine et al. (J. Power Sources 1997, 68 (2), 604-608) study the electrochemical performance of LiFeo.5Mn1.5O4 among other spinel materials, however a great amount of tetrahedral defects is present in the structure leading to a poor electrochemical performance.

[0015] CN110808357A mainly provide examples of LiNiPO4; one example refers to electrochemical data of a LiFeo.5Mn1.5O4 material, however the Fe defects are not described.

[0016] KR100266074B1 discloses lithium secondary cells comprising LFMO materials with low, albeit fairly stable, discharge capacity.

[0017] JP2002260656A discloses LFMO materials as positive electrode active material of a lithium secondary batter; however, these materials feature Fe defects and the electrochemical cycling stability of the corresponding cells has not been proven.

[0018] Thus, there is a need in the art for improving the electrochemical performance of iron- containing spinel structures in a simple and predictable manner.

[0019] BRIEF DESCRIPTION OF THE INVENTION

[0020] The invention described herein provides new iron-containing spinel electroactive materials with low Fe defects that can be used as cathode material for lithium-ion batteries. As iron atoms can be in octahedral or tetrahedral positions, the inventors have found a synthetic protocol that considerably lowers the amount of Fe defects in such spinels, wherein the Fe defects are defined as the iron atoms which are found in tetrahedral positions according to Mossbauer measurements. Furthermore, the protocol ultimately aims to control the percentage and ratio of Fe in octahedral / tetrahedral position. Even more importantly, the inventors have found that the percentage of Fe defects is correlated with electrochemical performance, such as improved capacity and rate capability, being the best samples those with lower percentage of Fe defects, namely less than 6%.

[0021] Thus, the invention allows the design of iron-containing spinels with control over Fe defects, providing excellent electrochemical properties, such as long-lasting cycling stability at very high C-rates maintaining high discharge capacity. One advantage of the present invention is that the amount of Fe defects in a spinel material of the invention can be not only readily qualitatively checked by standard diffraction experiments, but even precisely quantified as per claim 1. Actually, the presence and precise quantification of Fe defects has been performed by Mossbauer measurements. Mossbauer experiments confirmed that there are Fe atoms in octahedral and tetrahedral positions in different proportions in samples obtained under different synthetic conditions.

[0022] Thus, in a first aspect, the invention refers to an electroactive material of formula

[0023] LiyMni.5-x-zFeo.5+xMz04-d wherein: i) -0.5<x<1.5, 0<y<2, 0<z<0.5 and 0<d<0.25, provided that 0<(1.5-x-z) <2, ii) the material is characterized by having a spinel structure, iii) M is at least a metal selected from a transition metal and a main-group metal, and iv) less than 6% of the iron atoms contained in the spinel material are found in tetrahedral position as determined by Mossbauer spectroscopy.

[0024] A second aspect of the invention refers to a cathode comprising the electroactive material of the first aspect of the invention.

[0025] Another aspect of the invention refers to an electrochemical cell comprising the cathode as defined in the second aspect of the invention.

[0026] Another aspect of the invention refers to a method for obtaining the LiyMni.5-x-zFeo.5+xMz04-d electroactive spinel material according to the first aspect of the invention comprising the steps of: a) providing: i) a water solution comprising at least a lithium source, ii) a water solution comprising at least an iron source, iii) a water solution comprising at least a manganese source, iv) a water solution comprising at least a source of M, provided that M is present in the spinel, and v) a water solution comprising at least an additive selected from a first chelating agent, a viscosity adjuster, and an organic fuel; b) mixing the water solutions of step a); c) adding at least a second chelating agent to the mixture of step b); and d) heating the solution from step c) in two steps: d.l.) a first heating step where the solution is first heated at a first temperature comprised between 50 and 100°C to form a gel network; and subsequently d.2.) a second heating step where the gel network is then heated at a second temperature comprised between 500 and 850 °C to form the electroactive spinel material.

[0027] Lastly, another aspect of the invention refers to a LiyMni.5-x-zFeo.5+xMz04-d electroactive material obtainable from the method of the invention, wherein: i) -0.5<x<1.5, 0<y<2, 0<z<0.5 and 0<d<0.25, provided that 0<(1.5-x-z) <2, ii) the material is characterized by having a spinel structure, iii) M is at least a metal selected from a transition metal and a main-group metal, and iv) less than 6% of the iron atoms contained in the material are found in tetrahedral position as determined by Mossbauer spectroscopy.

[0028] DESCRIPTION OF THE FIGURES

[0029] Figure 1. Diffractograms showing the growth of the (220) diffraction peak related to the Fe in tetrahedral position from Sample 1 (bottom, left) to Sample 2 (bottom, right) with the increasing amount of Fe defects.

[0030] Figure 2. Rietveld refinement of the XRD pattern of Samples 3 and 4.

[0031] Figure 3. Mossbauer spectra of the LFMO samples: a) Sample 1 (2% of the Fe atoms in tetrahedral position) and b) Sample 2 (26% of the Fe atoms in tetrahedral position). The doublets assigned to octahedral and tetrahedral Fe3+are indicated by arrows, while the experimental curve is the one with more noise compared to the calculated curve.

[0032] Figure 4. Mossbauer spectra of compositions: a) LiFeo.4Mn1.6O4 containing 4% of Fe defects (Sample 3) and, b) LiMn1.5Feo.25Nio.25O4 containing 2% of Fe defects (Sample 4). The doublets assigned to octahedral and tetrahedral Fe3+are indicated by arrows, while the experimental curve is the one with more noise compared to the calculated curve.

[0033] Figure 5. Rietveld refinement of the NPD patterns of Sample 1 and Sample 2.

[0034] Figure 6. First cycle at C / 10 for Sample 1 measured in a voltage window of 3.5-5.2V, and Sample 2 measured in a voltage window of 3-5V.

[0035] Figure 7. a) Discharge capacities vs. cycle number for the Rate Capability experiment on the Sample 1 containing 2% of Fe defects according to Mossbauer spectroscopy in a voltage window of 3.5-5. IN. Charge rates (C-rates) are maintained at C / 10 while discharge rates (D- rates) were increased each 5 cycles from D / 20 to ID. b) Discharge capacities vs cycle number for the Rate Capability experiment in a voltage window of 3-5 V on Sample 2 containing 26% of Fe defects according to Mossbauer spectroscopy. Charge-discharge-rates (C(D)-rates, applying the same rate to both charge and discharge processes. Five cycles were performed for each C-rate (C(D) / 20, C(D) / 10, C(D) / 5, C(D)2, C(D) and again C(D) / 20 to evaluate the reversibility).

[0036] DETAILED DESCRIPTION OF THE INVENTION

[0037] All terms as used herein in this application, unless otherwise stated, shall be understood in their ordinary meaning as known in the art. Other more specific definitions for certain terms as used in the present application are as set forth below and are intended to apply uniformly throughout the specification and claims unless an otherwise expressly set out definition provides a broader definition.

[0038] Throughout the description and claims the word “comprises" and variations of the word, are not intended to exclude other technical features, additives, components or steps. Furthermore, the word “comprise” encompasses the cases of “consist of’ and “consists essentially of’. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention.

[0039] For the purposes of the invention, any ranges given include both the lower and the upper endpoints of the range. Ranges or values given, such as temperatures, times, molar ratio, volume ratio and the like, should be considered approximate when they are defined by the term “about” (i.e. with a 5% margin of variation around indicated point).

[0040] In the context of the present invention, “low Fe defects” means that the percentage of iron atoms in tetrahedral position is lower than 6%.

[0041] As stated above, in a first aspect, the invention refers to an electroactive material of formula

[0042] LiyMni.5-x-zFeo.5+xMz04-d wherein: i) -0.5<x<1.5, 0<y<2, 0<z<0.5 and 0<d<0.25, provided that 0<(1.5-x-z) <2, ii) the material is characterized by having a spinel structure, iii) M is at least a metal selected from a transition metal and a main-group metal, and iv) less than 6% of the iron atoms contained in the spinel material are found in tetrahedral position as determined by Mossbauer spectroscopy.

[0043] The electroactive material of the invention is a neutral mixed-metal oxide, meaning that the negative charge of the oxygen is balanced by the positive charge of the metals (Li, Mn, Fe and M, wherein M is defined as above).

[0044] In a particular embodiment, M is at least a transition metal. More particularly, M is at least a transition metal selected from Ni, Co, Ti, Sc, V, Cr, Cu, Zn, Y, Zr, Nb, Mo, Lu, Hf, Ta and W. Even more particularly, M is a transition metal selected from Ni, Co, Ti, Sc, V, Cr, Cu, Zn, Y, Zr, Nb, Mo, Lu, Hf, Ta and W. Preferably M is a transition metal selected from Ni or Co.

[0045] In another particular embodiment, M is selected from at least a main-group metal. More particularly, M is at least one main-group metal selected from Mg, Ca, Sr, Al, Ga, In, Ge, Si, Sn and Sb. Even more particularly, M is a main-group metal selected from Mg, Ca, Sr, Al, Ga, In, Ge, Si, Sn and Sb.

[0046] In preferred embodiments, M is at least one metal selected from Co, Ni, Mg, Cr, Zn, Cu, Al, Ti or Sc. More preferably, M is selected from Ni, Co and Ti. In a particular embodiment, z is 0.

[0047] In another particular embodiment, 0<d<0.1, preferably d is 0.

[0048] In a more particular embodiment z is 0 and / or 0<d<0.1, preferably z and / or d subscripts are 0, thus in a more particular embodiment the general formula of the spinel of the first aspect of the invention is LiyMni.s-xFeo.s+xCL, wherein -0.5<x<1.5 and 0<y<2.

[0049] In another particular embodiment, y ranges from 0.75 to 1.25, more preferably y is 0.8, 0.9, 1, 1. lor 1.2, even more preferably y is 1.

[0050] In another particular embodiment, x ranges from -0.25 to 0.5, more preferably x is -0.1, 0, 0.1 or 0.25, even more preferably x is 0.

[0051] In a preferred embodiment, d is 0, z is 0, y is 1 and x is 0. Thus, in this preferred embodiment, the spinel of the first embodiment of the invention is L1Mn1.5Feo.5O4.

[0052] The electroactive spinel material of the invention is characterized by having low Fe defects, i.e., less than 6% of the iron atoms contained in the spinel material are found in tetrahedral position, 8a, in the F-3dm space group. This percentage is actually meant to refer to the number of iron atoms in tetrahedral position divided by the total number of iron atoms in the spinel structure, said ratio being then multiplied by 100.

[0053] The quantitative determination of the Fe defects or iron atoms in tetrahedral position has been made possible by Mossbaeur spectroscopy which is based on the so called Mossbauer effect, also known as recoil-free gamma-ray resonance absorption, a nuclear process permitting the resonance absorption of gamma rays. The primary characteristics looked at in Mossbauer spectra are isomer shift (IS), quadrupole splitting (QS), and magnetic splitting (MS or hyperfine splitting). These characteristics are effects caused by interactions of the absorbing nucleus with its environment. The oxidation state of an absorber nucleus, for example, is one characteristic that can be determined by the IS (mainly) and QS (secondarily) of the spectral component (e.g. doublet) associated with this absorber nucleus.

[0054] Each type of iron atom in the spinel material of the invention (tetrahedral and octahedral) correspond to a different doublet in the Mossbauer spectrum. However the overlapping of these doublets in the Mossbauer spectra when measuring a sample (the spinel material) requires fitting or processing to the deconvolution of the Mossbauer spectra in individual curves corresponding to tetrahedral and octahedral iron atoms. The deconvolution step can be performed by any methods known by a skilled person and that generally make use of a software which Mossbauer spectrometers are equipped with or commercial software such as Dev-C++, UnivemMS, CMspectra and the like. From the fitted curves it would be easy to determine the amount of tetrahedral iron. As the area of the curves is proportional to the amount of iron, the area ratios of the fitted curves indicates the ratio between the iron atoms in tetrahedral and octahedral positions, thus the percentage of tetrahedral Fe defects can be ultimately calculated.

[0055] Thus, the percentage of iron atoms in tetrahedral position, also referred to as the percentage of iron defects in the spinel material, can be measured by Mossbauer spectroscopy and from the following equation:

[0056] ATx 100 < 6

[0057] AQ + A’ wherein

[0058] AT is the area of the doublet assigned to tetrahedral iron;

[0059] Ao is the area of the doublet assigned to octahedral iron; and wherein AT and Ao results from deconvoluting the area of the doublet in the Mossbauer spectrum corresponding to the iron signal in the spinel structure LiyMni.5-x-zFeo.5+xMz04-d as defined above.

[0060] In a particular embodiment, the amount of iron atoms in tetrahedral position relative to the total iron atoms in the spinel material, i.e. the amount of Fe defects, is less than or equal to 5%, less than or equal to 4%, or less than or equal to 3% as determined by the Mossbauer spectroscopy.

[0061] In some embodiments, the amount of Fe defects is equal to or higher than 1% and less than 6% as determined by the Mossbauer spectroscopy. Preferably, the amount of Fe defects is equal to or higher than 1% and less than or equal to 5% as determined by the Mossbauer spectroscopy. More preferably, the amount of Fe defects is comprised between 1% and 4%, even more preferably between 2% and 4% (both upper and lower values being included) as determined by the Mossbauer spectroscopy.

[0062] In a second aspect, the invention refers to a cathode comprising the electroactive material of the first aspect of the invention; more particularly, the cathode consists of the electroactive material of the invention. Any of the above embodiments of the electroactive material described above apply to the cathode of the second aspect of the invention. The cathode may further comprise a carbonaceous additive. Carbon materials that can be used as a carbonaceous additive in cathodes are known in the art and include, not being limited to, activated carbon, carbon black carbon nanofibers, carbon nanoparticles, carbon nanotubes, graphene, fluorographene, graphene oxide, graphyne, graphdiyne. Many carbon materials can be found under a variety of nanostructures such as nanoplatelets, nanoribbons, nanomeshes, nanospheres, nanoonions and nanoflakes. The carbon material optionally comprised in the cathode is typically present in amounts varying from 0.1 to 20 wt.%, preferably from 1 to 18 wt.%, more preferably from 5 to 15 wt.%; even more preferably, the carbon material is present in an amount of about 10 wt.%.

[0063] The cathode may additionally comprise at least one binder. In a particular embodiment, the at least one binder may comprise aqueous and non-aqueous based polymeric binder. In a preferred embodiment, the at least one binder is selected from carboxymethyl cellulose (CMC), polyacrylonitrile (PAN), polyacrylic latex (LA132), styrene-butadiene rubber (SBR), poly(vinylidenedifluoride) (PVDF), polyurethane (PU), polytetrafluoroethylene (PTFE), poly(vinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyethylene oxide (PEO) and poly(acrylic acid) (PAA) or a combination thereof. More preferably, the at least one binder consists of one binder. In a most preferred embodiment, the one binder are is polyvinylidene fluoride (PVdF) binder. Said at least one binder is present in at least 0.1 wt.%, at least 1 wt.%, at least 2 wt.%, at least 3 wt.%, at least 4 wt.%, at least 5 wt.% with respect to the total weight of the cathode. In a preferred embodiment, the at least one binder is present between 5-25 wt.% with respect to the total weight of the cathode. In a more preferred embodiment, the at least one binder is present between 7.5-15 wt.% with respect to the total weight of the cathode. In a most preferred embodiment, the at least one binder is present in about 10 wt.% with respect to the total weight of the cathode.

[0064] In preferred embodiments, the cathode of the invention comprises a LiyM .s-x-zFeo.s+xMzC d electroactive material according to any of the above embodiments, carbon black and polyvinylidene fluoride (PVdF). In even more preferred embodiments, the cathode of the invention consists of a LiyMni.5-x-zFeo.5+xMz04-d electroactive material according to any of the above embodiments, carbon black and polyvinylidene fluoride (PVdF). A preferred weight ratio between the electroactive material, the carbonaceous additive and the binder is about 80 / 10 / 10, respectively. Another aspect of the invention refers to an electrochemical cell comprising a cathode as defined in the second aspect of the invention.

[0065] In a particular embodiment, the electrochemical cell comprises an anode, an electrolyte, a cathode as defined in the second aspect of the invention and, optionally, a separator.

[0066] In an embodiment, the negative electrode (or anode) is selected from a metal anode, preferably an alkali metal anode, and more preferably a lithium metal anode (even more preferably in the form of lithium foil). In another embodiment, the anode comprises a metal or metalloid suitable for reversibly forming an alloy with metal cations, preferably alkali metal cations, and more preferably lithium cations. The skilled person knows how to select appropriate metals or metalloids suitable for forming the alloy. For instance, examples of metals or metalloids suitable for reversibly forming an alloy with lithium cations are Mg, Al, Zn, Bi, Cd, Sb, Ag, Si, Pb, Sn, or In which in particular can form alloys such as LiMg, LiAl, LiZn, Li3Bi, Li3Cd, Li3Sb, Li4Ag, Li44Si, Li44Pb or Li- Sn. In an embodiment, the anode comprises such alloys.

[0067] In an embodiment, the electrolyte is a liquid or a gel electrolyte, preferably a liquid electrolyte which comprises an organic solvent. Suitable liquid electrolytes include, but are not limited to, cyclic and / or linear ethers and carbonates, such as diethylether, methylbutylether, dimethoxy ethane (DME), 1,2-diethoxyethane (DEE), 1,3-dioxolane (DOL), bis(2 -methoxy ethyl)ether (DEGDME), triethylene glycol dimethyl ether (G3), tetraethylene glycol dimethyl ether (TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME), tetrahydropyran (THP), tetrahydrofiiran (THF), dimethyl carbonate (DMC), propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), and any mixtures thereof. In preferred embodiments, the electrolyte comprises one or more carbonates. In more preferred embodiments, the electrolyte comprises ethylene carbonate (EC) and dimethyl carbonate (DMC), preferably in a 3 to 7 weight ratio. The electrolyte may additionally comprise a lithium salt (dissolved in the organic solvent matrix) which increases the ionic conductivity in the electrochemical cell. The lithium salt may be an organic lithium salt, an inorganic lithium salt, or any combination thereof. Examples of organic lithium salts include, but are not limited to, LiN(SO2CF3)2, LiN(SO2F)2, LiN(SO2CF3)(SO2F), LiN(C2F5SO2)(SO2F) LiB(C2O4)2, LiBF2(C2O4), LiC(SO2CF3)3, LiPF3(C2F5)3, and LiCF3SO3). Examples of inorganic lithium salts include, but are not limited to, LiC104, LiNO3, LiBF4, LiAsFe, LiPFe, LiBF3Cl, and LiF. In an embodiment, the at least one lithium salt is selected from LiC104, LiNO3, LiBF4, LiAsF6, LiPF6, LiBF3Cl, LiF, LiN(SO2CF3)2(LiTFSI), LiN(SO2F)2(LiFSI), LiN(SO2CF3)(SO2F), LiN(C2F5SO2)(SO2F) LiB(C2O4)2, LiBF2(C2O4), LiC(SO2CF3)3, LiPF3(C2Fs)3, LiCF3SO3and any combination thereof. In preferred embodiments, the lithium salt is an inorganic lithium salt; preferably the inorganic lithium salt is LiBF4. The concentration of the lithium may vary depending on the desired ionic conductivity. Typically, the concentration of the lithium salt is comprised between 0.1M and 2M, preferably between 0.5M and 1.5M, more preferably is about IM.

[0068] In a particular embodiment, the electrochemical cell of the present invention may further comprises a separator. The separator is a medium between the two electrodes of an electrochemical cell that has to fulfill at least two functions. One function is to store and accommodate the electrolyte and, simultaneously, to assure ionic conductivity within the electrodes and between the anode and cathode. The further function of the separator is to electrically insulate the two electrodes from one another, in order to avoid short circuits. The separator is a separating means, usually a plate, positioned between the anode and cathode in the electrochemical cell to avoid the electrical contact between them. Moreover, the separator is in contact with the electrolyte, particularly it is partially or completely contacted with the electrolyte, which favors the flow of ions from one electrode to the other one. Typical separators include, but are not limited to, polymeric membrane mainly based on a polyolefin, such as polypropylene, (PP) polyethylene (PE), or any combination thereof (PP-PE). The polypropylene (or polyethylene or PP-PE) membrane may comprise one or more layer of polypropylene (or polyethylene or PP-PE). In some embodiments, the separator of the electrochemical cell comprises inorganic materials such a glass fiber. In a particular embodiment, the separator comprises polypropylene, (PP) polyethylene (PE), any combination thereof (PP-PE) or a glass fiber. In preferred embodiments, the separator of the electrochemical cell essentially consists of a glass fiber.

[0069] Method for obtaining the electroactive spinel material of the invention

[0070] As stated above, another aspect of the invention refers to a method for obtaining the LiyMni.s- x-zFeo.5+xMz04-d electroactive spinel material of the first aspect of the invention. Said process comprises the steps of: a) providing: i) a water solution comprising at least a lithium source, ii) a water solution comprising at least an iron source, iii) a water solution comprising at least a manganese source, iv) a water solution comprising at least a source of M, provided that M is present in the spinel, and v) a water solution comprising at least an additive selected from a first chelating agent, a viscosity adjuster, and an organic fuel; b) mixing the water solutions of step a); c) adding at least a second chelating agent to the mixture of step b); and d) heating the resulting solution from step c) in two steps: d.l.) a first heating step where the solution is first heated at a first temperature comprised between 50 and 100°C to form a gel network; and subsequently d.2.) a second heating step where the gel network is then heated at a second temperature comprised between 500 and 850 °C to form the electroactive spinel material.

[0071] This aspect will be also referred to as “method of the invention”.

[0072] Within this application the term “water solution” may be used interchangeably with “aqueous solutions”. In the step a), any inorganic salt, optionally hydrated, of lithium, iron and manganese can be used as a lithium source, iron source and manganese source, respectively. Particularly suitable are chlorides, bromides, iodides, nitrates, sulfates, tetrafluoroborates, hexafluorophosphates and perchlorates of said lithium, iron and manganese, more preferably nitrates.

[0073] In a particular embodiment, the lithium source is LiNCfr.

[0074] In another particular embodiment, the iron source is Fe(NO3)3, preferably in hydrated form Fe(NO3)3-9H2O.

[0075] In another particular embodiment, the manganese source is MniNOs)?, preferably in hydrated form Mn(NO3)2'H2O.

[0076] In another particular embodiment, the source of M, provided that M is present in the spinel, is selected from chlorides, bromides, iodides, nitrates, sulfates, tetrafluoroborates, hexafluorophosphates and perchlorates of a metal M selected from a transition metal and a main-group metal as defined in the first aspect of the invention. In a more particular embodiment, the source of M is selected from nitrates of Co, Ni, Mg, Cr, Zn, Cu, Al, Ti or Sc; preferably from nitrates of Ni, Co, or Ti. Said metal sources are dissolved in water and mixed in stoichiometric amounts depending on the spinel to be obtained. It will be apparent to those skilled in the art how to calculate the amounts of the corresponding lithium, iron, manganese and M sources, provided that M is present in the spinel, to be weighed and dissolved in water in order to obtain a spinel material with the desired proportion of each metal.

[0077] In a preferred embodiment, the molar ratio between lithium, iron, and manganese sources is about 2: 1:3. A slight excess of lithium source can be used with respect to the desired stoichiometry favoring the formation of a spinel structure with lower Fe defects. Thus, in an embodiment, the amount of lithium source is used in excess amount up to 0.2 mol, preferably up to 0.12 mol, more preferably up to 0.08 mol with respect to the formula unit of the final spinel product. In another embodiment, the amount of lithium source is used in excess amount comprised between 0.005 and 0.2 mol, preferably between 0.01 and 0.12 mol, more preferably about 0.08 mol with respect to the formula unit of the final spinel product.

[0078] A water solution (also referred to as “aqueous solution” throughout the text) comprising at least an additive selected from a first chelating agent, a viscosity adjuster, and an organic fuel is also mixed in stoichiometric amount with respect to the composition of the final spinel material. The amount of the at least an additive and the at least a lithium source is based on formula unit of spinel product and ranges from 1: 1 to 5: 1, preferably from 2: 1 to 4: 1, more preferably is about 3: 1, relative to the formula unit of final spinel product. For example, if the final product is LiMn1.5Feo.5O4, in the most preferred embodiment, 3 moles of at least an additive will be used per mole of LiMn1.5Feo.5O4.

[0079] In an embodiment the first chelating agent is a (poly)carboxyl ic acid with two or more -COOH groups, preferably the polycarboxylic acid is selected from tartaric acid, citric acid, EDTA (ethylenediaminetetraacetic acid), NTA (nitrilotriacetic acid), DCTA (trans- 1,2- diaminocyclohexanetetraacetic acid), DTPA (diethylene-triamine pentaacetic acid), EGTA (ethylene glycol bis(2-aminoethyl ether)-N,N,N',N' -tetraacetic acid) and combinations thereof. In a preferred embodiment, the first chelating agent is a tri-acid (i.e. has 3 -COOH groups), more preferably is citric acid.

[0080] In an embodiment, the viscosity adjuster is selected from a (poly)carboxylic acid with two or more -COOH groups, hydroxypropyl cellulose, carboxymethyl cellulose sodium, carboxymethyl cellulose calcium, cellulose derivatives or combinations thereof; preferably the viscosity adjuster is a (poly)carboxylic acid with two or more -COOH groups, more preferably is citric acid.

[0081] In an embodiment, the organic fuel is selected from fuel selected from the group consisting of urea, glycine, carbohydrazide, oxalyldihydrazide, hexamethylenetetramine citric acid, tartaric acid, oxalic acid, sucrose, glucose, acetylacetone, and combinations thereof; preferably the organic fuel is citric acid, tartaric acid, or, oxalic acid; more preferably the organic fuel is citric acid.

[0082] In an embodiment, the same additive can be at the same time a first chelating agent and viscosity enhancer, a first chelating agent and a fuel, a viscosity enhancer and a fuel, or, simultaneously a first chelating agent, a fuel, and a viscosity adjuster. In a preferred embodiment, the additive is simultaneously a first chelating agent, a fuel, and a viscosity adjuster as defined in the above embodiments.

[0083] The at least an additive allows a better control of the reactions of the lithium, iron, manganese and M (when M is present) precursors during the heating step d) of the method for preparing the spinel material. This is based on well-known principles of sol-gel synthesis implying hydrolysis and polycondensation reactions to form a gel network such as that obtained in step d.l. This gelation step is beneficial towards the final spinel structure with less than 6% iron atoms in tetrahedral sites.

[0084] The water solutions from step a) can be mixed in a conventional manner by combining them in the same container and letting them homogenize under stirring.

[0085] Once the mixture of water solutions from step b) is homogeneous, a second chelating agent is added. Typically, the second chelating agent is added as a water solution. In a preferred embodiment, the second chelating agent is a diol, more preferably is ethylene glycol. In an embodiment, the molar ratio between the second chelating agent and the formula of the final spinel product ranges from 1: 1 to 5:1, preferably from 2: 1 to 4: 1 , more preferably is about 3: 1. For example, if the final product is LiMn1.5Feo.5O4, in the most preferred embodiment, 3 moles of the second chelating agent will be used per mole of LiMn1.5Feo.5O4.

[0086] The solution resulting from step c) is then heated in two steps: d.l.) a first heating step where the solution is first heated at a first temperature comprised between 50 and 100°C to form a gel network; and subsequently d.2.) a second heating step where the solution is then heated at a second temperature comprised between 500 and 850 °C to form the electroactive spinel material.

[0087] The first heating step d.l is not only suitable to remove the volatiles, wherein water and other possible low-boiling compounds will evaporate, but is also responsible for the formation of a gel network compressing the anionic species. The first temperature of the first heating step is comprised between 50 and 100 °C, preferably between 70 °C and 100, more preferably is about 80 °C. This first heating step is performed for at least 1 hour, preferably for at least 2 hours, more preferably between 3 hours and 10 hours, even more preferably between 3 hours and 5 hours.

[0088] The gel network from step d.1 is next heated at a second temperature comprised between 500 and 850 °C, preferably between 550 and 750 °C, more preferably between 600 and 725 °C, even more preferably between 650 and 725 °C, even more preferably at about 700 °C.

[0089] This second heating step allows the annealing of the final spinel structure which is a solid characterized by less than 6% of Fe defects, more uniform chemical composition, and fine grains.

[0090] The duration of the second heating step is at least 1 hour, at least 2 hours, at least 4 hours, at least 6 hours, at least 8 hours, at least 10 hours; alternatively, the duration of the second heating step is comprised between 1 hours and 72 hours, preferably between 5 hours and 48 hours, more preferably between 8 hours and 24 hours, even more preferably between 8 hours and 12 hours, even more preferably is of about 10 hours. In most preferred embodiments, the second heating step is performed at a temperature between 600 and 725 °C and for a time between 5 hours and 15 hours. In some embodiments, the heating can be performed at a given heating ramp, preferably this is set at 1°C / min, 2°C / min, 3°C / min, 5°C / min, 7°C / min, 10°C / min; more preferably the heating ramp is set at a range of between 1°C / min and 10°C / min, between 2°C / min and 7°C / min; even more preferably the heating ramp is set at 5°C / min. The above embodiments for heating ramps also apply to embodiments for cooling ramps (i.e. after the second heating step is concluded).

[0091] In a particular embodiment, the process of the invention comprises: a) providing: i) a water solution of a lithium source, ii) a water solution of an iron source, iii) a water solution of a manganese source, iv) optionally, a water solution of a source of M wherein M is selected from Ni, Co, or Ti, and v) a water solution comprising (poly)carboxylic acid with two or more -COOH groups, preferably a tricarboxylic acid, more preferably, citric acid; optionally, wherein the amount of lithium source is used in excess amount up to 0.2 mol with respect to the formula unit of the final spinel product; b) mixing the water solutions of step a); c) adding at least a diol, preferably ethylene glycol, to the mixture of step b); and d) heating the solution resulting from step c) in two steps: d.l.) a first heating step where the solution is first heated at a first temperature comprised between 70 and 100°C and for an amount of time between 3 hours and 10 hours to form a gel network; and subsequently d.2.) a second heating step where the solution is then heated at a second temperature comprised between 600 and 725 °C to form the electroactive spinel material.

[0092] Lastly, another aspect of the invention refers to an electroactive material obtainable by the method of the invention, and wherein said electroactive spinel material has formula:

[0093] LiyMni.5-x-zF eo.s+xMzCL-d wherein i) -0.5<x<1.5, 0<y<2, 0<z<0.5 and 0<d<0.25, provided that 0<(1.5-x-z) <2, ii) the material is characterized by having a spinel structure, iii) M is at least a metal selected from a transition metal and a main-group metal, and iv) less than 6% of the iron atoms contained in the spinel material are found in tetrahedral position as determined by Mossbauer spectroscopy.

[0094] In an embodiment, said electroactive spinel material is characterized by less than 6% of the iron atoms in tetrahedral position, compared to the total iron atoms, as determined by Mossbauer spectroscopy, preferably less than 5%. In a particular embodiment, the amount of Fe defects is equal to or higher than 1% and less then 6% as determined by the Mossbauer spectroscopy. Preferably, the amount of Fe defects is equal to or higher than 1% and less than or equal to 5% as determined by the Mossbauer spectroscopy. More preferably, the amount of Fe defects is comprised between 1% and 4%, even more preferably between 2% and 4% (the upper and lower values being included) as determined by the Mossbauer spectroscopy. In a particular embodiment, M is at least a transition metal. More particularly, M is at least a transition metal selected from Ni, Co, Ti, Sc, V, Cr, Cu, Zn, Y, Zr, Nb, Mo, Lu, Hf, Ta and W. Even more particularly, M is a transition metal selected from Ni, Co, Ti, Sc, V, Cr, Cu, Zn, Y, Zr, Nb, Mo, Lu, Hf, Ta and W.

[0095] In another particular embodiment, M is selected from at least a main-group metal. More particularly, M is at least one main group metal selected from Mg, Ca, Sr, Al, Ga, In, Si, Ge, Sr and Sb. Even more particularly, M is a main-group metal selected from Mg, Ca, Sr, Al, Ga, In, Ge, Si, Sr and Sb.

[0096] In preferred embodiments, M is at least one metal selected from Co, Ni, Mg, Cr, Zn, Cu, Al, Ti or Sc. In a more preferred embodiment, M is selected from Ni, Co, and Ti. More preferably, M is a metal selected from Ni, Co and Ti.

[0097] In a particular embodiment, z is 0. In another particular embodiment, 0<d<0.1, preferably d is 0. In a more particular embodiment, z is 0 and 0<d<0.1, preferably z and / or d subscripts are 0, thus in this particular embodiment the general formula of the spinel of the first aspect of the invention is LiyMn1. -xFeo. xO4, wherein -0.5<x<1.5 and 0<y<2.

[0098] In another particular embodiment, y ranges from 0.75 to 1.25, more preferably y is 0.8, 0.9, 1.0, 1.1 or 1.2, even more preferably y is 1.

[0099] In another particular embodiment, x ranges from -0.25 to 0.5, more preferably x is -0.2, 0.1, 0, 0.1 or 0.25, even more preferably x is 0.

[0100] In a preferred embodiment, d is 0, z is 0, y is 1 and x is 0. Thus, in a most preferred embodiment, the spinel material is obtainable by the method of the invention and has formula LiMn1.5Feo.5O4.

[0101] EXAMPLES

[0102] The following examples are intended to illustrate but not to limit the disclosed embodiments.

[0103] Methods

[0104] 1. X-Ray diffraction

[0105] XRD patterns were recorded on a Bruker D8 Discover instrument using monochromatic copper radiation (Cu Kai X. = 1.54053 A). Patterns were measured in a 20 range of 5-80° with a step size of 0.02° and a step time of 0.45 s. X-ray patterns were refined using FullProf Representations and analyses of the crystal structures were done using the program VESTA.

[0106] 2. Mdssbauer spectroscopy

[0107] Mossbauer spectroscopy was performed at room temperature in transmission geometry using a conventional constant-acceleration spectrometer with a57Co-Rh source.

[0108] 3. NPD measurements

[0109] Neutron Powder Diffraction (NPD) was used for the characterization and quantification of Fe defects in the LFMO samples. NPD of pristine LFMO samples have been carried out at D2B diffractometer at the Institute Laue-Langevin (ILL) (Grenoble, France). A wavelength of 1.5948 A was used for the measurements. The samples were mounted on a 6mm diameter vanadium sample holder. The neutron diffraction patterns were collected at RT with high statistics to enhance the data accuracy. Rietveld analysis was carried out using the FullProf suite.

[0110] 4. ICP measurements

[0111] An inductively coupled plasma optical emission spectroscopy - ICP-OES. Agilent 5800 was used.

[0112] Sample Preparation: The starting material was weighed 20-25 mg (not less) with a precision of 0.1 mg, in a 25ml beaker. Hereafter, the following was added in the following order: 2.15 mL of water type I, 2.15 ml of HNO3 and 0.3 ml of H2O2. The beaker was sonicated for 10 minutes (if needed), after which the content was quantitatively transferred to a 100 ml volumetric flask and made up to the mark with type I distilled water. After this, a stirring bar was introduced in the flask, which was stirred for 15 hours. If something was not fully dissolved, then filtering might be carried out (not the case). The ICP-OES measurement was carried out the same day the samples were prepared.

[0113] The ICP-OES operating conditions were as follows: 1.20 kW of RF power, 12.0 l / min of a plasma-gas flow rate and 1.00 l / min of auxiliar-gas flow rate. Same conditions were set for axial and radial modes. Solutions were introduced into the plasma torch a concentric glass nebulizer and a double pass spray chamber at a flow rate of 0.70 L / min. Different wavelengths (nm) were checked for every single element to discard possible spectral interference, results were given at the following wavelengths: 238.204 nm for Fe, 670.783 nm for Li, 260.568 nm for Mn and 231.604 nm for Ni determination.

[0114] Example 1: Synthesis of LiMn1.5Feo.5O4 samples (Sample 1 and Sample 2)

[0115] For the synthesis of the LiMn1.5Feo.5O4 spinels, a sol-gel synthesis approach was used. Stoichiometric volumes of the reagent solutions (LiNOi IM; Fe(NO3)3.9H2O 0.3M; Mn(NO3)2.H2O 0.5M; citric acid2.5M) have been pumped automatically to different crucibles. After adding the solutions into the crucibles, these were left under stirring overnight. Once the solutions were well mixed altogether, an ethylene glycol solution (6M) was added to the crucibles and the resulting solutions were heated at 80°C to remove the volatiles and left under stirring for about 4 hours. The crucibles were subjected to annealing in an oven a) at 700°C for lOh (Sample 1) and b) at 900°C for lOh (Sample 2). Both the heating and the cooling ramps were set at 5°C / min.

[0116] Example 2: Synthesis of a LiMn1.6Feo.4O4 spinel material (Sample 3 .

[0117] For the synthesis of the LiMn1.6Feo.4O4 spinel, a sol-gel synthesis approach was used according to example 1. In this case, stoichiometric volumes of the reagent solutions (LiNOi IM; Fe(NO3)3.9H2O 0.3M; Mn(NO3)2.H2O 0.5M; citric acid 2.5M) were adjusted to obtain the desired stoichiometry. The conditions for the annealing step are shown below.

[0118] Example 3: Synthesis of a LiMn1.5Feo.25Nio.25O4 spinel material (Sample 4)

[0119] The same procedure for the synthesis of Samples 1 and 3 was followed using as nickel reagent solution Ni(NO3)2’6H2O (at a 0.5M concentration) as a source of Ni in the final spinel material and by adjusting the volumes of reagent solutions (LiNCh IM; Fe(NO3)3.9H2O 0.3M; Mn(NO3)2.H2O 0.5M; Ni(NO3)2’6H2O, 0.5M; citric acid 2.5M) to obtain the desired stoichiometry. Also the sol-gel and annealing conditions were also identical to those used for Samples 1 and 3.

[0120] Example 4: Characterization of the samples

[0121] Samples 1-4, as obtained in examples 1-3, were characterized by X-Ray diffraction (figures 1 and 2) to compare the crystalline structures of the materials synthesized by varying the synthetic parameters, depending on which differences in the XRD patterns were detected (different intensity distributions). Rietveld refinements of the diffraction results (Rietveld, H. M. A Profile Refinement Method for Nuclear and Magnetic Structures. J Appl Cryst 1969, 2 (2), 65-71) were performed using FullProf suite (Rodriguez-Carvajal, J. Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction. Physica B: Condensed Matter 1993, 192 (1), 55-69) to better understand the occupancy of iron in the structure. In particular, a non-detectable (220) diffraction peak means that Fe atoms are predominantly in octahedral sites, whereas (220) diffraction peak of detectable intensity means that a significant amount of Fe defects are present in the spinel (Figure 1). The data obtained seem to suggest that crucibles which were subjected to annealing at 700°C for 10 h result in spinels (such as Sample 1, 3 and 4) with low amount of Fe defects, whereas the crucible subjected to annealing at 900°C for 10 h provide a spinel structure (Sample 2) which may contain higher amount of Fe defects.

[0122] In addition to XRD characterization, Samples 1-4 were also characterized by Mossbauer spectroscopy (figures 3 and 4) for a more precise and reliable quantification of Fe defects. Deconvoluted spectra refer to one doublet assigned to Fe in tetrahedral position (alpha) while the other doublet is assigned to Fe (beta) in octahedral position. Mossbauer parameters of isomer shift (IS) and quadrupole splitting (QS) could be calculated from the peak positions of the fitted doublets. Parameter WID (line width) is a measure in mm / s of the full width at half maximum peak-height. The percentage of each kind of iron is calculated by the ratio between the area of the doublets. Considering the isomer shift of both doublets, the oxidation state of iron in all cases corresponds to Fe3+. Considering the Mossbauer parameters of isomer shift (IS) and quadrupole splitting (QS) on the one hand, the doublets with the highest isomer shifts, 0.34-0.35 mm / s, were assigned to Fe in octahedral position. On the other hand, the doublets with the smallest isomer shifts, 0.11 and 0.22 mm / s, were assigned to Fe in tetrahedral position for both samples. The percentage of each kind of iron is calculated by the ratio between the surface of both doublets. Mossbauer results are shown in Table 1 (the numbers between brackets indicate the error on the percentage of Fe in octahedral and tetrahedral positions).

[0123] Table 1.

[0124] Mossbauer spectroscopy indicated that the amount of Fe defects in Samples 1-3 is less than 6% compared to the total number of Fe atoms. Samples were also characterized by neutron powder diffraction (NPD) which showed agreement with the results obtained by Mossbauer spectroscopy.

[0125] The Rietveld refinement of the NPD led to a percentage of iron in the tetrahedral position of 24(2)% in the case of Sample 2 and 4(2)% in the case of Sample 1, which are in line with the Mossbauer measurements. Elemental concentrations for Fe, Li, Mn and Ni were measured by ICP and quantified using four-point external calibration curves (working range). Three independent replicates were done per sample and unless otherwise specified, results are calculated considering all of them. The results are summarized in Table 2. Table 2. ICP results relative to Samples 3 and 4.

[0126] Example 3: Electrochemistry

[0127] All the electrochemical measurements were carried out in half-cell configuration using Swagelok type cells and EL PAT-cells assembled in an argon-filled glovebox. Positive electrodes were prepared by mixing samples 1-2 with black carbon and polyvinylidene fluoride (PVdF) binder (80: 10: 10 weight ratio) mixed for 3 hours with a magnet. A glass fiber separator was soaked with the electrolyte: a solution of IM LiBF4 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (3:7 by wt) or an electrolyte solution LP30 IM LiPFe EC:DMC (50:50 wt%) with 2% FEC (fluoroethylene carbonate) as additive. Lithium foil was used as counter electrode.

[0128] All the electrochemical tests were carried out at room temperature.

[0129] Electrical characterization of the cells was done employing both, a Basytec CTS-XL multichannel battery tester and a Biologic VMP3 multichannel potentiostat.

[0130] Different experiments have been performed to study how the Fe defects affect the electrochemical behavior of the electroactive material of the invention. Sample 1 and Sample 2 were selected for electrochemical characterization. These samples were cycled vs. Li / Li+at a C-rate of C / 10 in a voltage window from 3 to 5V using IM LiBF4 in EC:DMC (3:7 weight ratio) as electrolyte. Two plateaus were observed during the measurements. A first plateau related to the Fe3+ / 4+redox activity (contribution of the Fe3+ / 4+redox couple to the discharge capacity between -4.5V and 5.2V, indicated with an arrow in the upper part of the graph for Samplel in Figure 7) and, a second one related to the Mn3+ / 4+redox activity (contribution of the Mn3+ / 4+redox couple to the discharge capacity between ~4.5 V and 3.5V, indicated with an arrow in the lower part of the graph for Samplel in Figure 7) were observed. A remarkable effect of the Fe defect on electrochemistry is seen during the cycling. Sample 2 featuring a higher amount of defects resulted the absence of the plateau corresponding to Fe3+ / 4+redox activity due to the high amount of Fe in tetrahedral position that is not electrochemically active. Only the redox activity of Mn3+ / 4+is observed in discharge (as indicated with the arrow in the graph for Sample 2 in Figure 7). It was also observed that the discharge capacity in Sample 2 was also lower.

[0131] Rate capability experiments for Sample 1 were performed during 5 cycles at each C-rate, charging the cell at a C / 10 rate and making the discharges at rates from D / 20 to ID in a voltage window of 3.5-5.2V (Figure 7a). Rate capability experiments were performed also for Sample 2 during five cycles for each C-rate (C(D) / 20, C(D) / 10, C(D) / 5, C(D)2, C(D) and again C(D) / 20 to evaluate the reversibility (Figure 7b).

[0132] When comparing the rate capability results performed on Sample 1 and Sample 2 it can be observed that when discharging at low rates (D / 20), Sample 1 resulted in a discharge capacity of 122mAh / g while Sample 2 shows 67 mAh / g (45% less than Sample 1). After 24 cycles, increasing the rate each 5 cycles, Sample 1 resulted in 81 mAh / g of discharge capacity charging and discharging at 1C(D), while Sample 2 resulted in 8 mAh / g (90% less than Sample 1) discharging at the same rate even if during the 24 cycles it was charge at a low C-rate of C / 10. Therefore, Sample 1, with low Fe defects, demonstrated an outstanding rate capability as compared to Sample 2, which has a higher amount of Fe defects. Moreover, the profile of the capacity decay observed for Sample 1 in the rate capability test did not show the usual staircase steps of a rate capability test when increasing the rate (as observed for Sample 2); the decay rather corresponds to the capacity decay observed for cycling at a given rate, which is related to the degradation of the electrolyte at high voltage. Therefore, the spinel material of the invention with low amount of Fe defects (Samplel) behaves as it is not affected by the cycling rate (even for high rates), which was entirely unexpected based on previous knowledge in the field.

Claims

CLAIMS1. An electroactive material of formulaLiyMni.5-x-zFeo.5+xMz04-d wherein: i) -0.5<x<1.5, 0<y<2, 0<z<0.5 and 0<d<0.25, provided that 0<(1.5-x-z) <2, ii) the material is characterized by having a spinel structure, iii) M is at least a metal selected from a transition metal and a main-group metal, and iv) less than 6% of the iron atoms contained in the spinel material are found in tetrahedral position as determined by Mossbauer spectroscopy.

2. The electroactive material according to claim 1, wherein M is at least one metal selected from Co, Ni, Mg, Cr, Zn, Cu, Al, Ti or Sc.

3. The electroactive material according to claims 1, wherein z and / or d are 0.

4. The electroactive material according to any one of the preceding claims, which is LiMn1.5Feo.5O4.

5. The electroactive material according to any one of the preceding claims, wherein the amount of iron atoms contained in the spinel material in tetrahedral position is equal to or higher than 1% and less than or equal to 5%.

6. A cathode comprising the electroactive material according to any one of claims 1 to 5.

7. An electrochemical cell comprising a cathode as defined in claim 6.

8. The electrochemical cell according to claim 7 further comprising an anode, an electrolyte and, optionally, a separator.

9. The electrochemical cell according to claim 8, wherein the anode is lithium metal.

10. The electrochemical cell according to any one of claims 8 and 9, wherein the electrolyte comprises at least an organic solvent and a lithium salt.

11. The electrochemical cell according to any one of claims 7 to 10, wherein:- the anode is lithium metal;- the electrolyte comprises i) an organic solvent selected from a cyclic ether, a linear ether, a carbonate, or mixture thereof, and ii) a lithium salt selected from LiC104, LiNO3, LiBF4, LiAsF6, LiPF6, LiBF3Cl, LiF, LiN(SO2CF3)2(LiTFSI), LiN(SO2F)2(LiFSI), LiN(SO2CF3)(SO2F), LiN(C2F5SO2)(SO2F) LiB(C2O4)2, LiBF2(C2O4), LiC(SO2CF3)3, LiPF3(C2Fs)3, LiCF3SO3or a combination thereof;- the cathode comprises a LiyMni.5-x-zFeo.5+xMz04-d electroactive spinel material as defined in any one of claims 1 to 5; and- optionally, a glass fiber separator.

12. A method for obtaining the LiyMni.5-x-zFeo.5+xMz04-d electroactive material as defined in any one of claims 1 to 5 comprising the steps of: a) providing: i) a water solution comprising at least a lithium source, ii) a water solution comprising at least an iron source, iii) a water solution comprising at least a manganese source, iv) a water solution comprising at least a source of M provided that M is present in the spinel, and v) a water solution comprising at least an additive selected from a first chelating agent, a viscosity adjuster, and an organic fuel; b) mixing the water solutions of step a); c) adding at least a second chelating agent to the mixture of step b) to form a solution; and d) heating the resulting solution from step c) in two steps: d.l.) a first heating step where the solution is first heated at a first temperature comprised between 50 and 100°C to form a gel network; and subsequently d.2.) a second heating step where the gel network is then heated at a second temperature comprised between 500 and 850 °C to form the electroactive spinel material.

13. The method according to claim 12, wherein the solution resulting from step c) is heated in two steps:d.l.) a first heating step where the solution is first heated at a first temperature comprised between 70 and 100°C for an amount of time between 3 hours and 10 hours; and d.2.) a second heating step where the solution is then heated at a second temperature comprised between 600 and 725 °C for at least 2 h.

14. An electroactive material obtainable by the method as defined in any one of claims 12 and 13, which has a formula:LiyMni.5-x-zFeo.5+xMz04-d wherein: i) -0.5<x<1.5, 0<y<2, 0<z<0.5 and 0<d<0.25, provided that 0<(1.5-x-z) <2, and ii) the material is characterized by having a spinel structure, iii) M is at least a metal selected from a transition metal and a main-group metal, and iv) less than 6% of the iron atoms contained in the material are found in tetrahedral position as determined by Mbssbauer spectroscopy.

15. The electroactive material according to claim 14 wherein z and / or d are 0.

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