Disordered rocksalt material and method of forming it

Abstract

A disordered rocksalt comprising Fe and one or more of Nb, Ti, W, Sb, Zr, Hf and Ta having at least a portion of the Fe in a 2+ valence state is prepared by milling precursors under a non-oxidizing atmosphere and the precursors for Fe have an average valence of less than 3+.

Description

DISORDERED ROCKSALT MATERIAL AND METHOD OF FORMING ITCROSS-REFERENCE TO RELATED APPLICATION

[0001] This is a PCT International application which claims priority to U.S. Provisional Patent Application Number 63 / 621,736 filed on January 17, 2024, which is incorporated herein by reference in its entirety for all purposes.FIELD

[0002] The present invention is in the field of battery technology.BACKGROUND

[0003] The global shift towards electric vehicles and large-scale energy storage demands cost- effective alternatives to commercial Ni / Co-based layered cathodes (e.g., LiNio.6Mno.2Coo.2O2) in Li-ion batteries (LIBs). As these cathode chemistries rely upon multiple critical mineral supply chains, extensive efforts have focused on developing high-performance Fe-rich cathode materials, such as LiFePO4, EiFeSO4F, and Ei2FeSiO4. These alternatives hold promise for enhancing the affordability of LIBs, leveraging iron's status as the most affordable and abundant transition metal (TM). While the demand for iron based cathode chemistry (namely LiFePO4) has increased in recent years and is forecast to reach almost equal global share as Ni-based chemistry by 2035, a current challenge is the lower energy density of Fe-rich cathodes compared to Ni / Co-based layered cathodes. Ni / Co-based layered cathodes can reach 670-800 Wh / kg, for example, but EiFePCb and LiFcSCLF exhibit energy densities of -560 Wh / kg and -500 Wh / kg, respectively. This limitation hinders their application in EIBs, particularly in scenarios where certain energy density (Wh / kg; Wh / 1) is required.

[0004] Disordered rocksalt cathode’s may accommodate a variety of TMs and anions FIB cathodes with high energy density (-900 Wh / kg) Recently, attention has been focused on disordered rocksalt structures, such as those formed from particular lithium metal oxides. Compounds represented by the formula: xEi3NbO4*(l-x)EiMO2 (1)

[0005] where M is a divalent or trivalent cation, have been shown to be a promising class of transition metal oxides for use as cathodes in lithium ion batteries. The compounds of formula (1) are considered a disordered rocksalt in which a random atomic arrangement of lithium and transition metal ions are packed in a closely-packed cubic structure. These disordered rocksalt compositions offer the ability to contain up to 3 lithium atoms per formula unit, which is more than the conventional lithium-excess layered materials. Formula (1) can be transformed and represented as LixMyNz0w. Manganese DRs have displayed specific energy density exceeding 900 Wh / kg. (U.S. Pat. No. 10,280,092).

[0006] Compared to LiFcPCU Fe-rich disordered rocksalt (DR) compounds, potentially the most cost-effective among all DRs, have shown unimpressive performances. For instance, Lii.2iFe3+o.37Tio.4202 delivers only -138 mAh / g (-374 Wh / kg) at a very slow rate of -2 mA / g, with its voltage profile containing a significant voltage hysteresis. S. L. Glazier, et. al. Chemistry of Materials, 2015, 27, 7751] Fe-DRXs have been synthesized as Fe3+-containing compounds (Fe3+- DR), and their limited performance has been attributed to the prohibitively high Fe3+ / Fe4+redox potential. B. Li et. al., Nature Chemistry 2021, 13, 1070. While O redox is a promising contributor to overall capacities in DRX, it has complications, including O loss, irreversible cation migration, and electrolyte decomposition, resulting in capacity and voltage fading.

[0007] It would be desirable to provide an Fe based disordered rocksalt having improved properties.BRIEF SUMMARY

[0008] It has been discovered that certain DRs having sufficient Fe in the 2+valence realizes improved battery performance. Fe in the ground state may also be present. The DR may be made by mechanochemical synthesis starting with Fe precursors having an average valence state of less than 3 under a nonoxidizing atmosphere.

[0009] An illustration is a composition comprising a Fe disordered rocksalt (FDR) represented byLixM y zO2-(a+b)FaZb

[0010] where 1.0<x<1.75; 0<y<0.55; 0.1 <z< 1 ; 0<(a+b)<0.7; (b>0) M’ is comprised of one or more of Nb, Ti, W, Sb, Zr, Hf and Ta and M is Fe; and Z is one or more of P, N and S, wherein at least a portion of the Fe is in a 2+valence state as determined by deconvolution of the X-rayPhotoelectron Spectroscopy peak between 720 eV to 705 eV. M’ desirably has an average most stable valence state of of 5+.

[0011] Another illustration is a method comprising milling under an atmosphere one or more precursors of a disordered rocksalt represented byLixM yMzO2-(a+b)Fa Zb where 1.0<x<1.75; 0<y<0.55; 0.1 <z< 1 ; 0<(a+b)<0.7; (b>0) M’ is comprised of one or more of Nb, Ti, W, Sb, Zr, Hf and Ta and M is Fe; and Z is one or more of P, N and S and the precursor for Fe has an average valence of less than 3+.

[0012] Herein, when a majority is specified of a component, it means more than 50% by mole or (readily understood from the context used) to essentially all of that component (99% or less). That is, the majority specified constituent of a component is present in an amount greater than 50% to 99%, 90, 80%, 70% or 60% of that component. When a minority of a component is a specified constituent, it is present in an amount less than 50% to about 1% with the balance being the majority specified constituent.

[0013] The composition’s FDR may be used to form a cathode in primary and secondary batteries such as lithium ion batteries. The composition’s DR may be used with any suitable electrolyte, separator and anode such as those known in the art.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is scanning electron micrograph of a disordered rocksalt not of this invention.

[0015] Figure 2 is a scanning electron micrograph of a disordered rocksalt of this invention.

[0016] Figure 3 is an X-ray diffractogram of a disordered rocksalt not of this invention.

[0017] Figure 4 is an X-ray diffractogram of a disordered rocksalt of this invention.

[0018] Figure 5 is an X-ray photoelectron spectroscopy intensity v. binding energy of a disordered rocksalt not of this invention.

[0019] Figure 6 is an X-ray photoelectron spectroscopy intensity v. binding energy graph of a disordered rocksalt of this invention.

[0020] Figure 7 displays the charging and discharging and capacity as function of charging and discharging cycling of a battery with a disordered rocksalt cathode not of this invention.

[0021] Figure 8 displays the charging and discharging and capacity as function of charging and discharging cycling of a battery with a disordered rocksalt cathode of this invention.

[0022] Figure 9 is the second cycle voltage profiles of disordered rocksalt cathodes of and not of this invention.

[0023] Figure 10 is the differential capacity (dQ / dV) of a disordered rocksalt cathode not of this invention in the 1st, 10th, and 20thcycle.

[0024] Figure 11 is the differential capacity (dQ / dV) of a disordered rocksalt cathode of this invention in the 1st, 10th, and 20thcycle.

[0025] Figure 12 is average discharge voltage during discharge v. cycle number of a disordered rocksalt of and not of this invention.

[0026] Figure 13 is the initial three-cycle voltage profiles of a disordered rocksalt cathode not of this invention when cycled at 40 mA / g between 1.3-4.6 V.

[0027] Figure 14 is the initial three-cycle voltage profiles of a disordered rocksalt cathode not of this invention cycled at 40 mA / g between 1.3-4.4 V.

[0028] Figure 15 is the initial three-cycle voltage profiles of a disordered rocksalt cathode of this invention when cycled at 40 mA / g between 1.3-4.6 V.

[0029] Figure 16 is the initial three-cycle voltage profiles of a disordered rocksalt cathode not of this invention cycled at 40 mA / g between 1.3-4.4 V.

[0030] Figure 17 is the average discharge voltage evolution of a disordered rocksalt cathode of and not of this invention obtained from the data of Figures 13 to 16.

[0031] Figure 18 is first discharge specific energy and average discharge voltage of a disordered rocksalt cathode of and not of this invention when cycled at 40 mA / g and different voltage windows (1.3-4.8 / 4.7 / 4.6 / 4.5 / 4.4 V).

[0032] Figure 19 is the Fe 2p_3 / 2 XPS spectra of a disordered rocksalt of and not of this invention at differing states of charge in the first cycle.

[0033] Figure 20 is the percentage of Fe and O species from the data of XPS spectra of Figure 19 as calculated from fitting the XPS peak areas.

[0034] Figure 21 is the electron paramagnetic resonance (EPR) spectra of the evolution of Fe species of a disordered rocksalt of and not of this invention at differing states of charge.

[0035] Figure 22 shows the X-ray diffraction patterns of a rocksalt cathode of and not of this invention at differing states of charge.

[0036] Figure 23 is the lattice parameter of a disordered rocksalt cathode of and not of this invention at differing states of charge based on fitting of the X-ray diffractograms of Figure 22.DETAILED DESCRIPTION

[0037] The following definitions apply to some of the aspects described with respect to some embodiments of the invention. These definitions may likewise be expanded upon herein. Each term is further explained and exemplified throughout the description, figures, and examples. Any interpretation of the terms in this description should take into account the full description, figures, and examples presented herein.

[0038] The singular terms “a,” “an,” and “the” include the plural unless the context clearly dictates otherwise. Thus, for example, reference to an object can include multiple objects unless the context clearly dictates otherwise.

[0039] A rate “C” refers to either (depending on context) the discharge current as a fraction or multiple relative to a “1 C” current value under which a battery (in a substantially fully charged state) would substantially fully discharge in one hour, or the charge current as a fraction or multiple relative to a “1 C” current value under which the battery (in a substantially fully discharged state) would substantially fully charge in one hour.

[0040] To the extent certain battery characteristics can vary with temperature, such characteristics are specified at 25 degrees C, unless the context clearly dictates otherwise.

[0041] Ranges presented herein are inclusive of their endpoints. Thus, for example, the range 1 to 3 includes the values 1 and 3 as well as the intermediate values.

[0042] The Fe disordered rocksalts (FDRs) are useful in formulating electrodes of electrochemical cells. More specifically, the FDRs may be used to form the cathode. The lithium ion battery includes an electrolyte formulation with a lithium salt present at a concentration suitable for conducting the lithium ions through the electrolyte formulation between the cathode and an anode during the discharge and recharge operations.

[0043] The composition comprises a Fe disordered rocksalt (FDR) represented byLixM y zO2-(a+b)Fa Zb

[0044] where 1.0<x<1.75; 0<y<0.55; 0.1 <z< 1 ; 0<(a+b)<0.9; (b>0); M’ is comprised of one or more of Nb, Ti, W, Sb, Zr, Hf and Ta and M is Fe; and Z is one or more of P, N and S, wherein at least a portion of the Fe is in a 2+valence state. Further dopants may be included such as those substituting for Li such as Na and Mg, which may be at any useful amount, but generally are at most about 10% or 5% to 0.01 % by mole of the lithium and such dopants present in the FDR. The M / M’ by moles may be any useful to make the FDR, but desirably, M / M’ is 0.5, 1 to 5, 4 3, 2 or 1.5 (e.g., z > y, z > 1.5(y) or z > 2(y)).

[0045] The amount of Fe2+present in the FDR may be any sufficient to realize the improved battery performance. The amount of Fe2+desirably is at least 20 %, 25%, 35%, or 50% by mole of the Fe present in the FDR as measured before charging or after end of discharge (below 1.3V). There may be Fe in the ground state as well. This Fe as evidenced by XPS participates in the energy storage, which is surprising.

[0046] The amount of F and Z, when present, is a minority of the anion (i.e. , O, F and one or more of P, S and N). Illustratively (a+b) is 0.05 to 0.9, 0.8, 0.65, 0.5. It may be desirable for a to be 0.05 to 0.5, 0.7 or 0.9. Z may be any combination of P, N and S, or may be just one of them. The ratio between P, N and S when two or more are present may be any useful ratio depending on the attributes sought. For example, it may be desirable to have S present when a reduced redox potential is desired. It may also be desirable for S or P to be the majority of the P, S and N present in the composition.

[0047] The composition may have any desirable Li of 1 or above, but it may be desirable for Li as represented by x to be at least 1.1, 1.15, 1.2 to 1.65, 1.5 or 1.4.

[0048] M’ is comprised of one or more of Nb, Ti, W, Sb, Zr, Hf and Ta. Desirably M’ is comprised of Nb or is solely Nb. When other transition metals other than Fe and Nb are present their most stable oxide valence is desirably greater than 4+ to realize the FDR where Fe2+participates in the energy storage. F is preferably present, with the composition displaying greater amounts of Fe2+with surprising improved batter performance (i.e., 0.1 < a < 0.9), which may be in the absence of Z (i.e., b = 0).

[0049] The DRS may be made by any suitable method such as those known in the art to make disordered rocksalts. Illustrative methods are described in U.S. Pat. Nos. 10,280,092, 10,978,706 and ACS Appl Mater Interfaces. 2019 Oct 2;11(39):35777-35787, each incorporated herein by reference. Preferably, the FDR is made by a mechanochemical synthesis.

[0050] Illustratively, to form the FDR, the precursors are comprised of Fe precursors having an average valence of less than 3+, 2.5+ to 1.5+. Illustratively, the Fe precursor may include Fe in the ground state and Fe compounds having a valence state equal to or greater than Fe3+(e.g., Fe and Fe2O3) or the Fe precursor may be comprised of Fe compound having a valence state below 3+ (e.g., FeO). The precursors may include metal compounds and are selected based on the desired composition of the FDR. One or more of the precursors may be compounds (e.g., metal compound) comprised of oxygen, fluorine and one or more P, S and N, such as oxides, hydroxides, oxynitrides, nitrides, nitrates, sulfides, sulfates, sulfites, phosphates, phosphites, fluorides and combinations thereof. Examples of precursors may include, LiOH, Nb2Os, LiF, NbFs, and / or the like. For doping the oxygen site with one or more of P, S and N at least one precursor includes one of these elements. Possible P, S and N precursors may include elemental P, S or N, metal nitrides (e.g. lithium nitride), metal nitrates, metal nitrites, metal phosphates, metal phosphides, metal phosphites, metal sulfites, metal sulfates, metal sulfides (e.g., lithium sulfide), wherein the metal is one that is desired in the FDR and combination thereof.

[0051] The precursors are desirably milled under an atmosphere for a sufficient milling time to form the FDR. The milling may be performed by any method useful to realize the desired particle size and composition with examples being a micromedia mill, ball mill, planetary mill or attrition mill. The primary particles may have an average particle size that is at most 2 micrometer,1 micrometer, 400 nanometers (nm), 200 nm, or 100 nm. For example, the average particle size could be no greater than 50 nm, 40 nm, 30 nm, 20 nm, or the like. An example of a suitable mill is a Fritsch Planetary Micro Mill (PULVERISETTE 7) employed in the absence of a liquid such as a non-reactive solvent such as an alkane.

[0052] The milling is performed under an atmosphere that realizes the desired FDR where there is sufficient amounts of Fe2+to realize the surprising battery performance. Illustratively, the atmosphere desirably is nonoxidizing such as nitrogen, noble gas (e.g., argon) or combination thereof. In some instance it may be desirable for the atmosphere to be reducing such as those comprised of CO or bF. The milling may be for any useful time to form the FDR, but typically is at least 30 minutes to 50 hours. The use of a heating step depending on the composition desired may also be utilized such as described above in combination with the milling.

[0053] The FDR may be used to form a cathode by any suitable method such as those known in the art. For example, the DRS powder may be mixed with a binder such a polymer useful to make cathodes (e.g., polyfluoropolymer such as polyvinylidene fluoride and polytetrafluoroethylene) and one or more solvents to form a slurry. Non-limiting examples of the one or more solvents may be an aprotic polar solvent such as methyl-2-pyrrolidinone (NMP). The slurry may then be deposited on a metal current collector (e.g., stainless steel, copper, or any suitable conductive metal thin) and the solvent removed to form the cathode.

[0054] Desirably the DRS of the cathode has an average secondary particle size of 1 to 20 micrometers. Each of the secondary particles is an agglomeration of primary particles. The DRS primary particles desirably have an average particle size as described herein and may contain other particles that may be useful such as increasing the electrical conductivity (e.g., carbon or other inorganic high ionic conductive particles).

[0055] The DRS cathode may be used in a rechargeable lithium ion battery cell. The battery cell includes the cathode, an anode, separator and electrolyte. The battery or battery cell may be formed in any suitable atmosphere such as common in the art. For example, a high purity argon atmosphere may be used to limit any undesirable contamination from species present in atmospheric air.ILLUSTRATIONS

[0056] Illustration 1. A composition comprising a disordered rocksalt represented byLixM’yMzO2-(a+b)Fa Zb where 1.0<x<1.75; 0<y<0.55; 0.1<z<l; 0<(a+b)<0.7; (b>0) M’ is comprised of one or more of Nb, Ti, W, Sb, Zr, Hf and Ta and M is Fe; and Z is one or more of P, N and S, wherein at least a portion of the Fe is in a 2+valence state.

[0057] Illustration 2. The composition of illustration 1, wherein M’ is comprised of Nb.

[0058] Illustration 3. The composition of either illustration 1 or 2, wherein a > 0.

[0059] Illustration 4. The composition of illustration 3, wherein 0.1 < a < 0.7.

[0060] Illustration 5. The composition of any of the preceding illustrations where b = 0.

[0061] Illustration 6. The composition of any of the preceding illustrations where a cathode comprising the composition exhibits a low V charging plateau and high V charging plateau.

[0062] Illustration 7. The composition of illustration 6, where low V charging plateau is longer than the high V charging plateau.

[0063] Illustration 8. The composition of illustration 6 or 7, wherein the low V charging plateau is between 1.5 V to 3 V and the high V charging plateau is above 4 V.

[0064] Illustration 9. The composition of any of the preceding illustrations wherein z > y.

[0065] Illustration 10. The composition of illustration 9, wherein z > 1.5(y).

[0066] Illustration 11. The composition of illustration 10, wherein z > 2(y)

[0067] Illustration 12. The composition of any one of the preceding illustrations, wherein Fe2+ is at least 10% by mole of the Fe present.

[0068] Illustration 13. The composition of illustration 12, wherein the Fe2+ is at least 20% by mole of the Fe present.

[0069] Illustration 14. A cathode comprising the composition of any one of the preceding illustrations

[0070] Illustration 15. A method comprising milling under an atmosphere one or more precursors of a disordered rocksalt represented byLixM yMzO2-(a+b)Fa Zb where 1.0<x<1.75; 0<y<0.55; 0.1<z<l; 0<(a+b)<0.7; (b>0) M’ is comprised of one or more of Nb, Ti, W, Sb, Zr, Hf and Ta and M is Fe; and Z is one or more of P, N and S and the precursor for Fe has an average valence of less than 3+.

[0071] Illustration 16. The method of illustration 15, wherein the milling atmosphere is comprised of a noble gas.

[0072] Illustration 17. The method of either illustration 15 or 16, wherein the milling is comprised of planetary milling.

[0073] Illustration 18. The method of any one of illustrations 15 to 17, wherein the milling is performed in the absence of a liquid.

[0074] Illustration 19. The method of any one of illustrations 15 to 17, wherein the method is further comprised of heating.ExamplesExamples:

[0075] A control Li1.2Feo.6Nbo.2O2 (LFNO) DR is synthesized by milling a stoichiometric amount of Li2O (Alfa Aesar, 99.5%), FC2O3 (Sigma-Aldrich, 96%), and Nb2Os (Alfa Aesar, 99.5%) were used as precursors. A Lii.2Feo.6Nbo.20i.4Fo.6 (LFNOF), a stoichiometric amount of Li2O (Alfa Aesar, 99.5%), Fe (Sigma- Aldrich, 99%), FC2O3 (Sigma-Aldrich, 96%), LiF (Sigma- Aldrich, 99.99%), and Nb2Os (Alfa Aesar, 99.5%) were used as precursors with the average valence of the Fe being about 2+. A total amount of 2 g precursors were mixed with a Fritsch Planetary Micro Mill (PULVERISETTE 7) at a rate of 500 rpm for 40 h in stainless steel vials assembled in an Argon-filled glovebox. The grinding media are fifteen 10 mm (diameter) stainlessballs and ten 5 mm (diameter) balls. After the mixing, the synthesized powders are collected in an Argon-filled glovebox.

[0076] For the preparation of a cathode film, 140 mg of the synthesized powders and 40 mg of carbon black (TIMCAL, Super C65) are mixed in the PULVERISETTE 7 for 1 h at 300 rpm, with 20 g of 3 mm (diameter) grinding media in stainless-steel vials assembled in an Argon-filled glovebox. Upon collection in an Argon-filled glovebox, 90 mg of the as-mixed powders and 10 mg of polytetrafluoroethylene (PTFE) were manually mixed with a mortar and pestle. The mixture was then rolled into a thin film inside an Argon-filled glovebox. The weight ratio between the active material, carbon black, and PTFE is 70:20:10 in the cathode film. Coin cells (CR2032) were assembled with the cathode film, the Li-counter electrode, a polypropylene separator (Celgard 2400), and 1 M solution of Li PFe in a mixture of ethyl carbonate / dimethyl carbonate (EC / DMC, 1:1 v / v) electrolyte in an Argon-filled glove box. The galvanostatic charge / discharge and ratecapability tests are performed using a Landt CT3002A battery testing system at room temperature (20 °C to 25 °C). The specific capacity was calculated based on the amount of active materials (LFNO, LFNOF) in the cathode film.

[0077] X-ray diffraction (XRD) patterns are collected on a Malvern PANalytical Empyrean X-ray diffractometer (Cu source) in the 20 range of 10-90°. To perform XRD on the as-cycled electrodes, coin cells are disassembled in an Ar-filled glovebox and washed with DMC for 30 s. Then, the cathode film is placed on an airtight polycarbonate-domed sample holder with a zerobackground plate. The Rietveld refinement on the collected XRD patterns is performed using the PANalytical X’pert HighScore Plus software. Scanning electron microscopy (SEM) is performed with the Hitachi SU-8000 SEM. X-ray Photoelectron Spectroscopy (XPS) of the as-synthesized powders and as-cycled cathode films was performed on a Thermo-Scientific K- Alpha with Al K- alpha radiation as the X-ray source for excitation. The cycled cathode films are collected from the disassembled coin cells, washed with DMC for 30 seconds, and dried under vacuum in the Argon- filled glovebox overnight. The dried, washed, cycled cathode films are then transferred into the XPS spectrometer with a vacuum transfer module (Thermo Scientific) to prevent any air exposure. The post-mortem XPS measurements on the as-cycled electrodes are conducted after 30 s Argon sputtering with 0.5 keV ion energy. The binding energy scale was charge-corrected using the C Is peak at 284.8 eV from the hydrocarbon contamination. The peak positions and areas are optimizedusing 70% Gaussian and 30% Lorentzian line shapes using Av antage (Thermo Scientific) software. XPS Quantification was performed based on Scofield’s relative sensitivity factors.

[0078] Fig. 1 is a scanning electron micrograph (SEM) of the as-synthesized LFNO. Fig. 2 is a SEM of the LFNOF as synthesized. Both of these display polycrystalline Fe-DR nanoparticles that aggregate into loosely -packed secondary particles (100 nm < d < 200 nm.

[0079] The X-ray diffraction (XRD) patterns of the as- synthesized LFNO and LFNOF powders are shown in Figures 3 and 4 respectively. All peaks can be indexed to a targeted disordered rock-salt structure (space group: Fm3m). The broad diffraction peaks indicate the nanoparticle character of the as-synthesized materials, consistent with the SEM images. Rietveld XRD refinement reveals that LFNO has a lattice parameter of 4.1807 A while that of LFNOF is 4.2031 A. The larger lattice parameter of LFNOF may be due to LFNOF containing Fe as Fe , a bigger cation than Fe3+in LFNO.

[0080] X-ray photoelectron spectroscopy (XPS) on the as- synthesized LFNO and LFNOF powders reveals that Fe in LFNO is predominately Fe3+or Fe4+(much greater than the majority of the Fe) whereas the LFNOF is primarily comprised of Fe2+with a lesser amount Fe3+and Fe (Figs. 6 and 7 respectively). Thus, overall, the average Fe oxidation state is lower for LFNOF than LNFO and is close to Fe2+for LFNOF and Fe3+for LFNO reflective of the starting precursors.

[0081] Figures 7 and 8 display the voltage profiles of LFNO and LFNOF when cycled between 1.3-4.8 V at 40 mA / g at room temperature. LFNOF has a lower open-circuit voltage (OCV) of -2.2 V than LFNO (-2.5 V) before cycling, indicating a more reduced chemical state (~Fe2+) in LFNOF than LFNO (~Fe3+). LFNO delivers the 1stdischarge capacity and specific energy of 243 mAh / g and 599 Wh / kg, whereas LFNOF achieves 292 mAh / g and 704 Wh / kg, respectively. 704 Wh / kg from LFNOF is impressive and among the highest specific energies achieved in Fe-based Li-ion cathode materials (vs. -560, -500, and -440 Wh / kg for LiFePO4, LiFeSO4E and Li FeSKX, respectively). For both materials, the first charging includes a low voltage plateau at -2.7 V and a high voltage plateau at -4.2 V. The -4.2 V plateau is predominant for LFNO, whereas LFNOF exhibits a notable -130 mAh / g charge capacity at -2.7 V before the -4.3 V charging. Previously Dahn et al. attributed the -4.2 V charging plateau in Li1.2Feo.4Tio.4O2 (Fe3+-based DRX) to be primarily originating from O oxidation with partial Fe3+ / Fe4+oxidation. The prolonged -4.2 V charging plateau in LFNO is believed, without being limiting, to be extensive O oxidation withlimited Fe3+ / Fe4+oxidation. In contrast, more readily accessible Fe2+ / Fe3+oxidation (prior to Fe3+ / Fe4+and 0 oxidation) appears to contribute to the notable -2.7 V charging capacity observed for LFNOF.

[0082] Comparing the 2ndcycle voltage profile of LFNO and LFNOF, a significantly larger voltage hysteresis / polarization is observed in LFNO (Fig. 9). The O-redox process in Li-excess layered or DRX materials is known to contribute to voltage hysteresis due to unequal redox mechanisms during charging and discharging, involving the formation of 0 dimers (e.g., peroxide, superoxide, trapped O2) at the top of charging. These dimers get reduced and dissociated only after deep discharging to a low voltage. In this context, the substantial voltage hysteresis in LFNO is indicative of a higher involvement of 0 redox in the material compared to LFNOF. That is, LFNOF has an extended low voltage charging plateau as described in the illustrations.

[0083] The dQ / dV plots of LFNO and LFNOF reveal a more pronounced change for LFNO than LFNOF (Figs. 10 and 11). Notably, the high-voltage charging peaks above 4 V (associated with the -4.2 V plateau) rapidly disappear after 10 and 20 cycles for LFNO, and LFNO’s 1stdischarging dQ / dV peak at -2.7 V is lost after cycling. This shift of dQ / dV charging and discharging peaks to lower voltages for LFNO indicates voltage fading, a phenomenon attributed to irreversible O loss from various Li-excess cathode materials. Conversely, the changes in the dQ / dV plot are less pronounced for LFNOF, aligning with the slower discharge voltage fading for LFNOF compared to LFNO (Fig. 12): the average discharge voltage decreases by 254 mV for LFNO but only by 57 mV for LFNOF after 10 cycles.

[0084] We cycled LFNO and LFNOF under 1.3-4.6 V and 1.3-4.4 V at 40 mA / g to compare their behaviors further (Figs. 13-16). The discharge capacity of LFNO (LFNOF) decreases by 75 (65) mAh / g from 238 (239) mAh / g to 163 (174) mAh / g after 25 cycles between 1.3-4.6 V and by 49 (32) mAh / g from 213 (217) mAh / g to 164 (185) mAh / g between 1.3-4.4 V. The average discharge voltage of LFNO (LFNOF) decreases by 0.32 (0.17) V from 2.48 (2.41) V to 2.16 (2.24) V after 20 cycles between 1.3-4.6 V, whereas a smaller voltage loss of 0.26 (0.04) V from 2.53 (2.29) V to 2.27 (2.25) V is observed between 1.3-4.6 V (Fig. 17). The capacity and voltage retention of the compounds improve with a lowered upper cut-off voltage, and LFNOF shows more stable performance than LFNO in all voltage windows.

[0085] LFNO’s specific discharge energy being less sensitive to the upper cut-off voltage is attributed primarily due to its average discharge voltage barely changing with the upper cut-off voltage (Fig. 18). In fact, LFNO’s average discharge voltage “increases” from 2.47 V to 2.53 Vwith a “decreasing” charge cut-off voltage from 4.8 V to 4.4 V, contrasting the typical behavior of a cathode material showing a higher discharge voltage with a higher cut-off voltage. This result is likely due to oxygen loss from LFNO upon charging to a high voltage, making the LFNO a more reduced compound (rather than a more oxidized compound) after a high-voltage charging, leading to a decrease in the operating voltage rather than an increase. This observation is also consistent with its fast voltage fading observed during 1.3-4.8 V cycling. In this regard, LFNO shows a clear -3.7 V discharge plateau when the charge cut-off is limited to 4.4 V, which was barely seen after a 4.8 V charge, leading to more symmetric charging and discharging voltage profiles. On the other hand, LFNOF shows more typical behavior of operating voltage lowered with decreasing upper charge cut-off voltage. The -540 Wh / kg achieved by LFNO between 1.3-4.4 V and -700 Wh / kg achieved by LFNOF between 1.3-4.8 V shows the value of having greater amount of Fe in the 2+valence.

[0086] LFNO and LFNOF demonstrate comparable rate capabilities, with LFNOF exhibiting a slight advantage over LFNO. When charged at 20 mA / g up to 4.8 V and discharged at different rates of 10, 20, 40, 100, 100, 200, 1000 mA / g to 1.3 V, LFNO’s discharge capacity decreases from -272 mAh / g (at 10 mA / g) to -131 mAh / g (at 1000 mA / g). Similarly, for LFNOF, the capacity changes from -314 mAh / g (at 10 mA / g) to -152 mAh / g (at 1000 mA / g) for LFNOF.

[0087] The Fe and O oxidation states in the materials at different state-of-charges (SOCs) using ex-situ XPS is shown in Fig. 19. The Fe 2p3 / 2 X-ray photoelectron spectroscopy (XPS) spectra is obtained after 30 s Ar sputtering of (a) LFNO and (b) LFNOF at progressive 1st cycling stages at 40 mA / g: before cycle (BC), charged to 80 and 160 mAh / g (C80, Cl 60), top-of-charge (ToC at 4.8 V), discharge to 80 and 160 mAh / g (D80, D160), and at the end-of-discharge (EoD at 1.3 V). Nb is barely redox active in the compounds. Additionally, the fraction (%) of Fe and O species at each SOC was estimated through XPS fitting (Fig. 20). The LFNO and LFNOF cathode films were cycled at 40 mA / g, with an increment of 80 mAh / g until the top-of-charge (ToC) at 4.8 V, and then to the end-of-discharge (EoD) at 1.3 V. The XPS measurement was conducted using an air-tight XPS holder to prevent air exposure. Due to the near-random ionic distribution in the DRX structure, various local environments exist for Fe and O ions, leading to a broad distribution of electron energies (and thus Binding Energy, BE, in XPS) for ions with the same nominal charge (e.g., Fe3+). The nanoparticle morphology of LFNO and LFNOF, with a characteristic size of d~100 nm, implies that the -5 nm surface (and consequently the XPS signal) represents asignificant portion (~30 %) of the particles’ total volume and thus is a reasonable indicator of the redox occurring.For LFNO, a subtle increase in the average Fe oxidation states is observed from ~Fe3+toward ~Fe4+after charging to 80 mAh / g (C80). This is evidenced by the growth of a peak at -716 eV (assigned as Fe4+), accompanied by a reduction in the peak at -712 eV (assigned as Fe3+). However, a significant portion of Fe remains in the Fe3+state (or possibly Fe2+). Further charging to 160 mAh / g (Cl 60) and reaching the TOC results in a decrease in the average Fe oxidation state back toward ~Fe3+. This is supported by the increase prominence of the -712 peak over the -716 eV peak, likely attributed to the reductive coupling of Fe with oxygen during charging. The 0 15 XPS spectra reveal that O2' ions undergo oxidation as early as 80 mAh / g charge (C80), and the population of oxidized O species increases upon further charging (Fig. 20). These findings suggest limited Fe3+ / Fe4+oxidation in LFNO during the 1stcharging and O oxidation, which concurrently occurs with Fe oxidation during the early charge but becomes the predominant oxidation process at the later stage of charging for LFNO, possibly in conjunction with Fe-0 reductive coupling. Upon discharging (D80, D160, EoD), the Fe 2p3 / 2 spectrum shifts to lower binding energies, signifying Fe reduction. Simultaneously, the population of oxidized O species decreases, i.e., O reduction (Figs. 19 and 20). Notably, the intensity of the Fe2+peak (at -710 eV) at the EoD is greater than BC, likely due to O loss upon the 1stcharging, allowing Fe to be reduced more toward Fe2+in discharging compared to BC. i.e., LFNO becomes a more reduced compound after the first cycle, as reflected in the Fe lp3 / 2 spectrum appearing at lower binding energies after the 1stdischarge than before cycling (BC).Compared to LFNO, we observe much clearer participation of Fe redox in LFNOF from the Fe lp3 / 2 XPS spectra, mainly involving the Fe2+ / Fe3+redox couple (Figs. 19 and 20). Upon charging to 80 and 160 mAh / g (C80, C160), the peak at -710 eV (assigned as Fe2+) gradually decreases in intensity, replaced by growing peaks at -712 eV (assigned as Fe3+) and -716 eV (assigned as Fe4+). At the ToC, the Fe3+peak shows the strongest intensity and dominates the Fe Zp3 / 2 XPS spectra instead of the Fe4+peak, indicating that Fe3+ / Fe4+oxidation is also limited in LFNOF. The Ols XPS spectra show concurrent O oxidation with Fe upon charging, but the detection of oxidized O species is less compared to the case of LFNO (Fig. 20). These results suggest that Fe2+ / Fe3+oxidation occurs readily in LFNOF to reduce the O oxidation upon charging, explaining the long -2.7 V 1stcharging plateau for LFNOF that is barely seen for LFNO. Upondischarging, the Fe2+peak recovers its intensity over Fe3+or Fe4+peaks, and oxidized O species are detected less in XPS spectra, indicating Fe and O reduction

[0088] To further confirm the Fe redox activity in the LFNO and LFNOF, we performed electron paramagnetic resonance (EPR) spectroscopy, characterizing the Fe oxidation state before the cycle (BC), at the top-of-charge (ToC) to 4.8 V, and at the end of discharge (EoD) during the 1stcycle (Fig. 21). We focused on the Fe3+EPR signal since Fe3+is easily distinguishable due to its strong EPR activity, while Fe2+and Fe4+can be EPR-silent under certain low-spin conditions or exhibit weak and broadened signals in high-spin states. For LFNO, the Fe3+EPR signal at -3330 G is prominently observed in both BC, ToC, and EoD samples, underscoring the predominant presence of Fe3+in LFNO throughout the 1stcycle (Fig. 21). In the case of LFNOF, the EPR signal is silent for the BC sample (Fe2+is EPR silent). Subsequently, the Fe3+EPR signal prominently appears at the ToC, and the EPR signal becomes silent again at the EoD. This finding aligns with our XPS observation that Fe2+ / Fe3+oxidation is readily accessible in LFNOF, whereas Fe3+ / Fe4+oxidation is limited for both LFNO and LFNOF.

[0089] We observe “lattice breathing” behavior in DRX cathodes from LFNOF, where the volume (lattice parameter) contracts (decreases) upon charging and expands (increases) upon discharging, reflecting ionic radius changes. In contrast, LFNO exhibits a non-intuitive volume expansion (lattice parameter decrease) during the 1stcharging. Fig. 22 depicts the (002) XRD peak of LFNO and LFNOF during the 1stcycle at various initial cycling stages at 40 mA / g: ToC and EoD as well as non-cycled (pristine). For LFNOF, the (002) peak gradually shifts to a higher angle with charging, returning to a position similar to the pre-cycling state after the 1stdischarge. The XRD refinement indicates that this peak shift corresponds to LFNOF’ s lattice parameter changing from -4.215 A (before cycle) to -4.174 A (ToC) and then to -4.220 A (EoD) (Fig. 23), showcasing a typical structural evolution observed in other DRXs. Meanwhile, for LFNO, the (002) peak shifts to a lower angle after the 1stcharge, corresponding to the lattice parameter increase from -4.191 A to -4.197 A (Figs. 22 and 23). The lattice parameters are calculated from the XRD refinement. Subsequently, the (002) peak further shifts to a lower angle, (-4.222 A) after the 1 discharge (EoD). The increase in LFNO’ s lattice parameter (volume) during the 1stcharging differs from the typical behavior of DRXs, where the volume usually decreases upon charging due to ions (TM or O) getting smaller upon oxidation. We currently speculate that LFNO’s volume expansion upon 1stcharging is related to dominant O oxidation in the structure, leading to excessive O dimerformation. This process may cut some Fe-0 bonds, ultimately increasing the average TM-0 distance in LFNO.

[0090] The performance of LFNOF, achieving ~704 Wh / kg and ~290 mAh / g (1.3-4.8 V, 40 mA / g), stands out and is surprising. This is believed to represent the highest energy density and capacity reported for Fe-redox-based disordered rocksalt cathodes. Also, it ranks among the highest energy densities achieved by any Fe-redox-based cathode, surpassing benchmarks such as LiFePC (-560 Wh / kg) and LiFeSC F (-500 Wh / kg). Interestingly, when considering more practical voltage windows, LFNO (Fe3+-DRX) demonstrates superior energy density compared to LFNOF. Specifically, LFNO achieves -590 Wh / kg in the range of 1.3-4.6 V and -540 Wh / kg in 1.3-4.4 V. This observation suggests potential benefits in combining Fe3+ / Fe4+and O-redox with Fe2+ / Fe3+for enhanced performance. While Fe2+ / Fe3+redox provides better reversibility, the less reversible Fe3+ / Fe4+and O-redox, with a higher operating voltage than Fe2+ / Fe3+, may offer an option for increasing the energy density of FDR within a more practical voltage window.

Claims

What is claimed is,1. A composition comprising a disordered rocksalt represented byLixM yMzO2-(a+b)Fa Zb where 1.0<x<1.75; 0<y<0.55; 0.1<z<l; 0<(a+b)<0.7; (b>0) M’ is comprised of one or more of Nb, Ti, W, Sb, Zr, Hf and Ta and M is Fe; and Z is one or more of P, N and S, wherein at least a portion of the Fe is in a 2+valence state.

2. The composition of claim 1, wherein M’ is comprised of Nb.

3. The composition of either claim 1, wherein a > 0.

4. The composition of claim 3, wherein 0.1 < a < 0.7.

5. The composition of claim 1 where b = 0.

6. The composition of claim 1 where a cathode comprising the composition exhibits a low V charging plateau and high V charging plateau.

7. The composition of claim 6, where low V charging plateau is longer than the high V charging plateau.

8. The composition of claim 6, wherein the low V charging plateau is between 1.5 V to 3.5 V and the high V charging plateau is above 4 V.

9. The composition of claim 1, wherein z > y.

10. The composition of claim 9, wherein z > 1.5(y).

11. The composition of claim 10, wherein z > 2(y)12. The composition of claim 1, wherein Fe2+is at least 20% by mole of the Fe present.

13. The composition of claim 12, wherein the Fe2+is at least 30% by mole of the Fe present.

14. A cathode comprising the composition of any one of the preceding claims15. A method comprising milling under an atmosphere one or more precursors of a disordered rocksalt represented byLixM yMzO2-(a+b)Fa Zb where 1.0<x<1.75; 0<y<0.55; 0.1<z< 1; 0<(a+b)<0.7; (b>0) M’ is comprised of one or more of Nb, Ti, W, Sb, Zr, Hf and Ta and M is Fe; and Z is one or more of P, N and S and the precursor for Fe has an average valence of less than 3+.

16. The method of claim 15, wherein the milling atmosphere is comprised of a noble gas.

17. The method of claim 15, wherein the milling is comprised of planetary milling.

18. The method of claim 15, wherein the milling is performed in the absence of a liquid.

19. The method of any one of claims 15 to 18, wherein the method is further comprised of heating.

Images