Solid-state microwave pathways to cation-disordered rock salt oxide and oxyfluoride cathodes

Abstract

Ionic solids useful as Li-ion cathodes (e.g., disordered rocksalt oxide / oxyfluoride (DRX) compounds) are synthesized via a microwave method that is two orders of magnitude faster than conventional solid-state and mechanochemical synthesis methods. Microwave synthesis can be performed in ambient atmosphere, resulting in significant reductions in synthesis time, energy consumption, and cost. In one exemplary embodiment of the method, a precursor mixture is mixed and compressed into a pellet. The pellet is then placed in a ceramic crucible surrounded by activated carbon. The crucible is then placed in a 1200 W microwave and heated for 5 to 20 minutes. Immediately after microwave radiation is stopped, the pellet is quickly removed from the crucible and quenched in water. The pellet is then dried and crushed into a powder, which is the final DRX product.

Description

[Technical Field]

[0001] This application claims priority to US 63 / 420,230, filed October 28, 2022, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to compositions of matter useful as electrodes and methods of forming electrodes. [Background technology]

[0003] The increasing demand for high-energy density portable power sources has motivated the development of Li-ion batteries, with the potential for expansion to sodium-ion (Na-ion) batteries. The cathode component is typically the most expensive and limited in terms of charge storage capacity. The transition metals used in current Li-ion cathodes consist primarily of Co and / or Ni, whose toxicity (Co), complex supply chain, and price fluctuations have prompted the development of more sustainable (e.g., Mn-based) cathode chemistries.

[0004] Cation-disordered rock salt oxides / oxyfluorides (DRX) have become promising options for high-energy density Mn-based and Co / Ni-free Li-ion (and possibly Na-ion) cathodes due to their high energy density and compositional flexibility. DRX materials are currently being developed through two routes: 1~5 They have been synthesized via: (I) solid-state synthesis, in which precursors are heated at high temperatures (>800 °C) under Ar flow for extended periods of time (typically 12 h), and (II) mechanochemical synthesis, in which precursors are ball-milled in a sealed, inert environment for minimal durations (>40 h).

[0005] These methods require an inert atmosphere to reduce impurity formation and are energy intensive. Furthermore, scaling up mechanochemical synthesis methods for industrial large-scale production is not straightforward. What is needed are improved methods for synthesizing rock salt structures. Summary of the Invention [Problem to be solved by the invention]

[0006] The present disclosure provides a method for the rapid synthesis of ionic compounds useful as Li-ion (or Na-ion) battery electrodes. The method comprises mixing precursors together to form a mixture; exposing the mixture to microwave radiation at a power of 100 to 1500 watts for a duration of less than one hour (e.g., 5 to 20 minutes) to produce an ionic compound having a rock-salt crystal structure and a disordered arrangement of cations; and terminating the microwave radiation and rapidly cooling the microwave-irradiated powder (within 15 seconds of terminating the microwave heating) to store the structure and the disordered cation arrangement at room temperature. The disordered rock-salt structure can be determined by powder X-ray diffraction (XRD), where peaks can be indexed to the cubic rock-salt space group Fm-3m (space group #225). Short-range order of cations within this long-range disordered structure can occur and is observed as a broad peak in the XRD pattern. The powder can then be further processed into a form useful as a Li-ion (or Na-ion) battery electrode.

[0007] Key components of the present method include: 1) the use of microwave radiation to rapidly heat the precursor and initiate a reaction that forms the product at high temperature; and 2) rapid quenching of the heated powder (e.g., in water) to ensure a pure final product. Conventional methods for synthesizing DRX include sintering / calcining the precursor at ≥1000 °C for ≥10 hours or ball milling the precursor at ∼500 rpm for 40 hours. While these conventional techniques are time- and energy-intensive, the present rapid microwave technique is two orders of magnitude faster and offers significant time and energy savings. Additionally, microwave synthesis (unlike solid-state and mechanochemical milling methods) does not require an inert atmosphere to produce pure DRX, adding to the scalability and cost-effectiveness of the present method for commercial / industrial manufacturing of electrodes. [Means for solving the problem]

[0008] Examples of DRX lithium metal oxides / oxyfluorides that can be produced by the inventive methods described herein include those with a cation-disordered rock salt structure and the general formula: Li x M' y M″ 2-x-y O 2-z F z wherein 1.05≦x≦1.35, 0.1≦y≦0.9, and 0≦z≦0.7; M′ is a low valent (e.g., ≦3+) transition metal; and M″ is a high valent (e.g., ≧4+) transition metal. M′ is selected from at least one of V, Cr, Mn, Fe, Co, Ni, Mg, Cu, and Al; and M″ is Ti. 4+ , Zr 4+ , Nb 5+ , V 5+ , Mo 6+ is selected from at least one of the following: [Effects of the Invention]

[0009] The DRX products obtained from the microwave synthesis of the present invention will exhibit favorable structural and electrochemical properties similar to those formed via conventional solid-state synthesis, but at a fraction of the typical synthesis time, energy, and cost. Moreover, the microwave synthesis of the present invention produces particles with improved particle morphology, including smaller and more uniform particle sizes, compared to solid-state and mechanochemical synthesis methods. [Brief explanation of the drawings]

[0010] Further features and advantages of the present invention may be seen from the following detailed description taken in conjunction with the drawings, in which like reference numerals represent corresponding parts throughout and in which: [Figure 1] 1 shows a flow chart illustrating a method according to the present invention for forming disordered rock salt oxide / oxyfluoride (DRX) ionic compounds. [Figure 2]Examples of rock-salt oxide crystal structures are shown, including (a) a disordered rock-salt structure in which all cation sites are equivalent (e.g., α-LiFeO), (b) a layered structure (e.g., α-NaFeO), (c) a low-temperature spinel structure (e.g., LT-LiCoO), and (d) a γ-LiFeO structure. Large open circles indicate anion (e.g., oxygen) sites, while small gray and black circles represent cation sites (e.g., alkali metal / lithium and transition metal sites, respectively). Gray / black semicircles represent cation sites that may be occupied by equivalent alkali metal or transition metal atoms. [Figure 3A] Synchrotron X-ray diffraction (SXRD), including λ = 0.24101 A, of as-synthesized DRX powder of Li-Mn-Ti-O (LMTO) DRX obtained via microwave heating (mw) for 5, 10, and 20 minutes and via solid-state (ss) synthesis with a 12-hour calcination step at >1000 °C. [Figure 3B] Li-Mn-Ti-O (LMTO) DRX obtained via solid-state synthesis via microwave heating (mw) for 5, 10, and 20 min and with a 12 h calcination step at >1000 °C, including scanning electron microscope (SEM) images of as-synthesized DRX particles. [Figure 3C] Li-Mn-Ti-O (LMTO) DRX obtained via solid-state synthesis via microwave heating (mw) for 5, 10, and 20 minutes and with a calcination step at >1000 °C for 12 hours, including SEM images of ball-milled DRX with Super C65 conductive additive. [Figure 4A] The characteristics of LMTO including 7Li pj-MATPASS are shown. [Figure 4B] Characterization of the LMTO is shown, including 19F spin-echo solid-state NMR spectra collected in a mw-LMTO with a short interscan delay (50 ms) and a T2 filter (15 rotor period delay) to suppress the 19F probe background signal. [Figure 4C]Characterization of LMTO is presented, including 19F spin-echo solid-state NMR spectra collected on mw-LMTO with long interscan delay to better quantify the LiF impurity signal. [Figure 5A] We present the characterization of LMTO, including Mn K-edge X-ray absorption near-edge spectroscopy (XANES) of mw- and ss-LMTO along with Mn2O3 and MnO2 standards. [Figure 5B] LMTO characterization, including X-ray PDF boxcar fits of mw-LMTO DRX. [Figure 5C] We present the properties of LMTO, including X-ray PDF boxcar fits of ss-LMTO DRX. Fits are shown for two space groups: the cubic 225 structure, corresponding to a completely disordered arrangement of cations in the DRX structure, and the tetragonal 141 structure, representing preferential cation ordering within the DRX structure. Fits were performed over a range of r, and the cubic DRX structure model best fits the experimental data over long correlation lengths r, while the tetragonal structure best fits the data over short correlation lengths r. The mw-LMTO sample was obtained with a 5-minute microwave heating step. [Figure 6A] 1 shows a galvanostatic cycling plot of synthesized LMTO, including LMTO synthesized via a solid-state method. [Figure 6B] 1 shows galvanostatic cycle plots of synthesized LMTO, including LMTO synthesized via microwave method. [Figure 6C] 1 shows galvanostatic cycling plots of the synthesized LMTO, including differential capacity plots obtained during the first five cycles. [Figure 6D] Figure 1 shows galvanostatic cycling plots of synthesized LMTO, including the evolution of discharge capacity and coulombic efficiency of ss- and mw-LMTO DRX cathodes over 50 cycles. [Figure 6E]Figure 1 shows a galvanostatic cycling plot of synthesized LMTO, including the rate capabilities of ss- and mw-LMTO DRX cathodes, with rate values ​​shown on the graph in units of mA / g. The mw-LMTO sample was obtained with a 5 min microwave heating step. [Figure 7A] Synchrotron XRD (λ = 0.24101 A) of Li-Mn-Nb-OF(LMNO / F) DRX synthesized via microwave heating (mw) for 5, 10, and 20 minutes and via solid-state (ss) synthesis with a 12-hour calcination step at >1000 °C. [Figure 7B] SEM images of as-synthesized DRX particles are shown. [Figure 7C] SEM images of DRX ball milled with Super C65 are shown. [Figure 8A] The 7Li pj-MATPASS NMR spectrum of LMNO / F is shown. [Figure 8B] 19F NMR spin-echo spectra of mw-LMNO / F and mw-LMTO are shown, collected with a short interscan delay (50 ms) and T2 filter (15 rotor period delay) to suppress the 19F probe background signal. [Figure 8C] 19F NMR spin-echo spectra of mw-LMNO / F and mw-LMTO collected with a long interscan delay (20 s) to better quantify the LiF impurity signal are shown. [Figure 9] Mn K-edge XANES of mw- and ss-LMNO / F are shown along with Mn2O3 and MnO2 standards. The mw-LMNO / F sample was obtained with a 5-minute microwave heating step. [Figure 10A] 1 shows the galvanostatic cycle plot of LMNO / F synthesized via the solid-state method. [Figure 10B] Figure 1 shows the galvanostatic cycle plot of LMNO / F synthesized via microwave method. [Figure 10C] Differential capacity plots obtained during the first five cycles are shown. [Figure 10D]Figure 1 shows the evolution of discharge capacity and coulombic efficiency of ss- and mw-LMNO / F DRX cathodes over 50 cycles. [Figure 10E] Figure 1 shows the rate capacity of ss- and mw-LMNO / F DRX cathodes, plotted as rate values ​​in mA / g. The mw-LMNO / F sample was obtained with a 5 min microwave heating step. [Figure 11A] Figure 1 shows 19F solid-state NMR spectra of mw-LMTO and LMTO / F synthesized with and without excess Li, respectively, at various microwave heating times, with a short recycle delay (50 ms). [Figure 11B] Figure 1 shows 19F solid-state NMR spectra of mw-LMTO and LMTO / F synthesized with and without excess Li, respectively, at various microwave heating times. The spectra were acquired with a long recycle delay (20 s). [Figure 11C] Figure 1 shows 19F solid-state NMR spectra of mw-LMTO and LMTO / F synthesized with and without excess Li, respectively, at various microwave heating times, with a short recycle delay (50 ms). [Figure 11D] 19F spectra of the 5 min mw-LMTO / F composition compared to LMNO / F samples obtained after various microwave heating times are shown, suggesting that the Nb-based DRX is still more highly fluorinated. [Figure 12A] 19F NMR with a short (50 ms) recycle delay is shown for Li1.2Mn0.5Ti0.3O1.8F0.2. [Figure 12B] 19F NMR with short (50 ms) recycle delays are shown for various LMTO / F and LMNO / F DRX compositions. [Figure 13A] FIG. 10 shows an SEM image of a LMTO sample synthesized via conventional solid-state synthesis, in which the precursor powder mixture was heated at high temperatures (>1000° C.) under Ar flow for 12 hours, according to the description of a second example. [Figure 13B] 1 shows an SEM image of a LMTO sample synthesized via the microwave synthesis method of the present invention, according to the description of the first example, after 5 minutes of microwave irradiation. [Figure 14A] A particle size plot for the LMTO particles in the SEM image of Figure 13A (conventional solid-state synthesis method) is shown, where the maximum diameter of each of the 814 particles in the image was measured using the ruler tool provided by the software package. No weighting factor was applied. [Figure 14B] A particle size plot for the LMTO particles in the SEM image of Figure 13B (microwave synthesis) is shown, where the maximum diameter of each of the 1500 particles in the image was measured using the ruler tool provided by the software package. No weighting factors were applied. [Figure 15] Particle size distribution data obtained with a PSD analyzer for solid and powder LMTO samples synthesized via microwave methods are shown, for which the SEM images in Figures 13A and 13B are representative. DETAILED DESCRIPTION OF THE INVENTION

[0011] In the following description, reference is made to the accompanying drawings, which form a part hereof, and which show, by way of illustration, one or more ways in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

[0012] The use of examples or exemplary language (e.g., "etc.") herein is intended merely to better elucidate the invention and does not pose a limitation on the scope of the invention unless otherwise asserted. No language in the specification should be construed as suggesting any non-claimed element as essential or otherwise required to the practice of the invention unless the context makes clear otherwise.

[0013] As used herein, the singular forms "a," "an," and "said" include plural references unless the context clearly dictates otherwise. The term "or" should be understood to be an inclusive "or" unless the context dictates otherwise. Terms such as "first," "second," and "third," when used to describe multiple devices or elements, are used only to convey the relative operation, positioning, and / or function of the separate devices and do not require a particular order for such devices or elements or any particular quantity or ranking of such devices or elements.

[0014] The term "essentially," as used herein with respect to any property or condition, refers to a degree of deviation that is small enough so as not to appreciably impair the identified property or condition. The exact degree of deviation that is acceptable in each situation will depend on the particular circumstances, as one of ordinary skill in the art would understand.

[0015] The use of the term "about" or "approximately" is intended to describe values ​​above and / or below a stated value or range, as would be understood by one of ordinary skill in the art in the respective context. In some cases, this may encompass values ​​within a range of approximately + / - 10%. In other cases, it may encompass values ​​within a range of approximately + / - 5%. In yet other cases, it may encompass values ​​within a range of approximately + / - 2%. In still yet other cases, it may encompass values ​​within a range of approximately + / - 1%.

[0016] The terms "comprises" and / or "comprising", when used herein, unless otherwise indicated herein or otherwise clearly contradicted by context, will be understood to specify the presence of stated features, integers, steps, operations, elements and / or components, but not to preclude the presence or addition of one or more features, integers, steps, operations, elements, components and / or groups thereof.

[0017] The term "transition metal," as used herein, is intended to describe metals in the d-block (Groups 3-12) of the periodic table, as well as other metals that play an equivalent role in lithium (transition) metal oxide cathode materials. These elements may play an active role in the electrochemical process by oxidation or reduction, or may remain in a constant oxidation state. Some representative examples include manganese and titanium. It will be understood that other metals that are not "transition metals" in the conventional context may be encompassed under this term to serve a similar purpose in the material as electrochemically inactive metals such as Ti; therefore, metals outside the d-block may be included in the DRX formulation and, in some cases, may be referred to as "transition metals."

[0018] The recitation of ranges of values ​​herein serves as a shorthand method of referring individually to each separate value falling within each range, including the endpoints of that range, each separate value within that range, and all intermediate ranges subsumed by that entire range, unless otherwise indicated, as if each incorporated herein were individually recited.

[0019] Unless otherwise indicated or clearly contradicted by context, the methods described herein may be performed with individual steps performed in any suitable order, including the exact order disclosed, without any intermediate steps or with one or more additional steps intervening between the disclosed steps, with the disclosed steps performed in an order other than the exact order disclosed, with one or more steps performed simultaneously, and with one or more disclosed steps being omitted.

[0020] This disclosure reports a method for the rapid synthesis of cation-disordered ionic compounds (e.g., rock salt oxides / oxyfluorides (DRX)) that can be used as high-energy density, durable, and inexpensive electrode (e.g., cathode) materials, for example, in Li-ion rechargeable batteries. As described herein, the method comprises microwave heating of precursors (e.g., for 5-20 minutes under ambient atmosphere) followed by rapid quench cooling. The method is scalable for commercial / industrial large-scale manufacturing of electrodes. Exemplary materials that may be produced via microwave synthesis include those of the general formula Li x M′ y M″ 2-x-y O 2-z F z (Wherein, 1.05≦x≦1.35, 0.1≦y≦0.9, and 0≦z≦0.7, M′ is a low-valent (e.g., ≦3+) transition metal selected from at least one of V, Cr, Mn, Fe, Co, Ni, Mg, Cu, and Al, and M″ is Ti 4+ , Zr 4+ , Nb 5+ , V 5+ , Mo 6+ The DRX oxides and oxyfluorides include, but are not limited to, DRX oxides and oxyfluorides having a high valence (eg, ≧4+) transition metal selected from at least one of:

[0021] The following Li synthesized by microwave (mw) and conventional solid-state (ss) methods: 1.2 Mn 0.4 Ti 0.4 O2(LMTO) and Li 1.3 Mn 0.4 Nb 0.3 O 2-x F x 1 is an example illustrating various embodiments of the present invention, including a comparative study of (LMNO / F). Fluorinated MnTi-based DRX (LMTO / F) compositions with various transition metal stoichiometries and fluorine contents have been synthesized and characterized.

[0022] First Example: Microwave Synthesis of DRX Oxides / Oxyfluorides FIG. 1 shows a flow chart illustrating an example of a method for synthesizing rock salt compositions used for the comparative studies and characterization presented herein. Block 100 depicts the selection of precursor compounds and mixtures thereof in the amounts required for the desired stoichiometry. Li2CO3 and LiF were selected as the Li and F sources for the synthesis of LMTO and LMNO / F compounds. However, other Li and F-containing compounds may also be used, such as, but not limited to, Li2O, LiNO3, and polytetrafluoroethylene (PTFE). Mn2O3 was selected as the redox-active transition metal precursor. In general, oxides of the transition metal of interest may be selected. TiO2 and Nb2O5 were selected as the dopant sources for LMTO and LMNO / F. 0 The metal was chosen as the oxide. Again, oxides of the metal of interest may generally be selected.

[0023] For microwave synthesis of LMTO, Mn, Ti and F stoichiometry was calculated using the formula Li 1.2 Mn 0.4 Ti 0.4 O 1.9 F 0.1 Precursor powders of Li2CO3, LiF, Mn2O3, and TiO2 were weighed out to match the chemical composition of SiO2, with a 10% molar excess being weighed out to offset potential Li losses during heat treatment. LiF was added in an amount sufficient to provide approximately 5% fluorination as a sintering agent to allow for faster diffusion of the elements during synthesis.

[0024] For microwave synthesis of LMNO / F, the Li, Mn, Nb, and F stoichiometry was determined according to the formula Li 1.3 Mn 0.4 Nb 0.3 O 1.9 F 0.1 Precursor powders of Li2CO3, LiF, Mn2O3, and Nb2O5 were measured out to match the chemical composition of LMNO / F. Again, approximately 5% fluorine content was added as a sintering agent. No excess Li was added for LMNO / F.

[0025] For the microwave synthesis of fluorinated MnTi DRX, precursor powders of Li2CO3, LiF, Mn2O3, and TiO2 were measured out so that the Li, Mn, Ti, and F stoichiometries matched the target composition. No excess Li was added.

[0026] The precursor may also be prepared in other forms (e.g., pellet form). For the results presented herein, the precursor powder, once measured out, was hand-ground until sufficiently uniform and pressed into 200 mg pellets.

[0027] Block 102 indicates microwave heating of the precursor pellets. For the results presented herein, a double crucible setup was established, whereby a smaller (e.g., alumina) crucible was placed in the center (or inside) of a larger crucible, and a heat transfer medium (e.g., activated carbon) was added to the larger crucible to surround the smaller, inner crucible. A layer of sacrificial precursor powder was added to the small crucible, and the precursor (e.g., pellet) was carefully placed on top of the sacrificial powder in the smaller crucible. The setup was placed in a 1200-watt conventional microwave in ambient atmosphere and microwaved at a specified power level (6) and time (5-20 minutes). The activated carbon warms by resonating with the microwaves, and as a result, the pellets may initially heat via conductive processes (e.g., interdiffusion), where heat is transferred from the activated carbon to the pellets via thermal conduction. Activated carbon may be placed in physical contact with the pellets to facilitate this process. However, above a critical temperature, the pellet material becomes susceptible to microwave irradiation, resulting in heating via direct interaction of the pellets with the microwaves, leading to very high reaction temperatures (>1500°C, preferably >1600°C) in a short period of time (e.g., 5-20 minutes). During this time, the DRX phase forms.

[0028] Block 104 indicates quenching. For the results presented herein, the pellets were rapidly quenched in distilled water immediately after microwave heating. The quenching process allows the high-temperature DRX phase to stabilize at room temperature while preventing the formation of any layered / ordered oxide impurities during the cooling process. In some instances, quenching may be performed in a medium other than distilled water, such as ethanol or liquid nitrogen.

[0029] Block 106 indicates further processing. For the results presented herein, the pellets were dried on a hot plate and ground to obtain a fine powder of the final DRX product. Figures 2a-2d show examples of ionic lattices of materials compositions that may be made using the method of Figure 1, showing the disordered arrangement of cations at the lattice sites.

[0030] Second Example: Conventional Solid-State Synthesis of LMTO and LMNO / F Li 1.2 Mn 0.4 Ti 0.4 O2 composition was synthesized for ss-LMTO, and Li 1.3 Mn 0.4 Nb 0.3 The O2 composition was synthesized for ss-LMNO. Li2CO3, Mn2O3, and TiO2 (LMTO) or Nb2O5 (LMNO / F) were used as precursors for the solid-state synthesis. For LMTO, stoichiometric amounts of Mn and Ti and a 10% molar excess of Li were used. For LMNO / F, stoichiometric amounts of Mn, Nb, and Li were used, but no excess Li was added.

[0031] For solid-state synthesis, the precursor was ground and pressed into a ~200 mg pellet. The pellet was annealed at 1100 °C for 12 h under Ar flow, allowed to cool slowly to room temperature, and gently ground to obtain the final ss-DRX powder. This procedure is similar to that reported in the literature for the solid-state synthesis of similar disordered oxide compositions.

[0032] Third Example: Characterization of LMTO obtained using the synthesis methods described in the first and second examples (a) Structural characterization Figure 3 A shows synchrotron X-ray diffraction (SXRD) data demonstrating the formation of phase-pure LMTO DRX obtained via microwave (5, 10, and 20 min) and conventional solid-state (12 h) synthesis methods. The broad, low-angle (∼3° in 2θ) peak indicates the presence of cation short-range order. Although mw-LMTO DRX was obtained after rapid quenching, these SXRD results indicate a similar degree of cation short-range order in this material compared to the LMTO DRX compound obtained via conventional solid-state synthesis (sintering at 1000 °C followed by slow cooling).

[0033] Figure 3 B and Figure 3 C SEM micrographs show the presence of large, crystalline DRXed particles in all as-synthesized LMTO samples. The average particle size depends on the microwave heating time. After 5 minutes of heating, the particles are on the order of ~1 μm. After 10–20 minutes of heating time, the particles are on the order of ~3–5 μm. The mw-DRXed compounds have smaller, more uniform particle sizes compared to the ss-DRXed compounds.

[0034] [Table 1]

[0035] The ratios of Li, Mn, Ti, and F in the samples were determined by inductively coupled plasma (ICP). The presence of amorphous Li-containing impurity phases such as LiF and Li2CO3 prevents the determination of DRX stoichiometry using SXRD and ICP alone. 7 Li solid-state nuclear magnetic resonance (NMR) can detect the amorphous phase and provide quantitative information about the mole fraction of Li present in the LMTO DRXed phase versus amorphous and crystalline diamagnetic impurities (Figure 4).A The narrow peak at 0 ppm is associated with diamagnetic Li impurities (LiF, Li2CO3), while the broad, asymmetric signal is associated with Li in a range of local environments within the disordered and paramagnetic LMTO DRX phase.

[0036] Full relaxation spin echo 7 Li spectrum (fully relaxed spin-echo 7 Four paramagnetic Gaussian components were fitted to the Li spectra to model the disordered environment and a single diamagnetic 50% Lorentzian component. The integration of the model provides an upper bound on the amount of Li impurity present. This value slightly underestimates the amount of Li in DRX, because Li is coupled to the diamagnetic Mn 3+ This is because the temperature is close to , which leads to rapid NMR signal decay. However, this effect is relatively small.

[0037] Li impurity quantification shows that the fraction of Li in the diamagnetic impurity phase decreases as the microwave heating time increases. After 20 minutes of heating, the mole fraction of Li in the diamagnetic impurity in the mw-LMTO sample is equal to that present in the LMTO sample obtained via conventional solid-state synthesis (12 hours of sintering step). With longer reaction times, excess Li is volatilized, leading to a decrease in the impurity.

[0038] ICP and 7 By combining Li NMR, we can obtain the Li stoichiometry of the LMTO DRXed phase. As can be seen from Table 1, all DRXed samples have very similar Li stoichiometries of 1.14-1.15, regardless of synthesis method and time. Again, note that this is a lower bound for Li in DRXed samples because the amount of Li is 7 This is because Li NMR has led to an underestimation.

[0039] associated with the mw-LMTO phase obtained after heating for 5, 10, and 20 min. 7 The Li solid-state NMR signals were very similar to those associated with LMTO phases prepared via solid-state synthesis. 7 The Li signals are slightly different, suggesting a slight difference in the distribution of the local Li environment in the latter sample.

[0040] In one or more embodiments, LiF acts as a sintering agent in the microwave synthesis process.

[0041] [Table 2]

[0042] In one or more instances, LiF acts as a sintering agent in the microwave synthesis process. Fluoride ion-selective electrode (F-ISE) results indicate that even for a fast 5-minute reaction time, roughly half of the fluorine initially present in the precursor is lost. This suggests that minimal fluorination occurs for mw-LMTO. Furthermore, like ICP, F-ISE cannot distinguish between fluorine present in the DRX and that present as a LiF impurity, which may further reduce the DRX fluorination level. X-ray and neutron diffraction techniques cannot distinguish F from O due to the similar scattering cross sections of the two elements. On the other hand, 19 F NMR is an effective probe of F species present in DRX and potential LiF phases / domains. 19 F The ss-NMR signals correspond to the LiF impurity phases / regions, whereas the F in the DRX structure is Fig. 4 B and Figure 4 C This leads to a broad and asymmetric signal in

[0043] Obtained using fast (50 ms) and slow (20 s) recycle delays 19 The F NMR spectrum is shown in Figure 4. B and Figure 4 C The spectra obtained with a longer recycle delay of 20 seconds provide more quantitative information about the amount of LiF phase / domain in the sample.

[0044] collected here 19 The F spectrum is only semi-quantitative as a fraction of the F incorporated in the DRX cathode (F directly bound to the paramagnetic Mn) and cannot be observed experimentally. However, a fast recycling delay (Figure 4) B ) obtained 19 The poor signal-to-noise ratio observed for the F spectrum, even after 48k scans, provides a further indication that very little F is incorporated into the DRXed phase.

[0045] Obtained with a slow recycle delay (20 seconds) and Fig. 4 C Shown in 19 The F spectrum shows that the diamagnetic LiF impurity peak at -204 ppm is quite prominent. 19 The fit of the F spectrum indicates that the very small amount of fluorine present in this sample is mostly incorporated into the LiF impurity phase (39 mol % fluorine). Note that this value is an overestimate (upper bound) for the small fraction of LiF in the sample. This is due to the presence of a small amount of LiF in the sample. 19This is because some of the F NMR signal is not visible experimentally. Increasing the microwave heating time to 10 and 20 min reduces the amount of LiF impurity, but also reduces the total amount of F in the DRXed sample, as seen by the decrease in the intensity of the broad paramagnetic signal.

[0046] Overall, very little F is incorporated into the DRX phase, and the F from the LiF precursor is better explained as a sintering agent rather than as a precursor in the synthesis of LMTO.

[0047] (c) Analysis of the local and electronic structure of LMTO (i) Mn K-edge XAS Mn K-edge X-ray absorption spectroscopy (XAS) data were acquired to probe the differences in the local coordination and oxidation states of Mn species in the mw- and ss-LMTO DRX compounds of interest. XANES edge energy shifts indicate that mw-LMTO has a slightly higher average Mn oxidation state than ss-LMTO. The slightly higher average Mn oxidation state observed for the microwave sample is most likely due to the more oxidizing ambient air environment used for microwave synthesis compared to the inert Ar atmosphere used for solid-state synthesis.

[0048] (ii) X-ray PDF analysis X-ray pair distribution function (PDF) analysis provides information about short-range order, which significantly influences Li-ion transport and, therefore, the electrochemical performance of DRX cathodes. PDF analysis was performed on SXRD data collected for mw- and ss-LMTO compounds using two different structural models: a cubic rocksalt model (space group Fm-3m, #225), corresponding to a disordered distribution of cations, and a tetragonal model (space group I41 / amd, #141), representing an ordered cation arrangement. The results are shown in Figure 5. A to Figure 5C Shown below.

[0049] For both models, fits were performed over a range of correlation lengths to see how the DRX structure develops from short to long range. Over long correlation lengths, the cubic 225 space group provides the best fit to the data, consistent with the SXRD results and with the depiction of these compounds as, on average, disordered cations.

[0050] However, over shorter correlation lengths (1.6–3.2 A), Fig. 5 A to Figure 5C clearly shows that a cubic structure results in a poor fit. Rather, the tetragonal 141 space group gives a much better fit, suggesting the presence of short-range ordering of the cations.

[0051] In both mw- and ss-LMTO DRX structures, the transition from short-range cation order to long-range disordered cation arrangement occurs at similar correlation lengths r, suggesting that there is not much difference in the degree of cation short-range order when DRXs are synthesized using these two methods.

[0052] (d) Electrochemistry of ss- and mw-LMTO The DRX samples synthesized via microwave (with a 5 min heating step) and solid-state methods were post-treated in the same manner: To reduce the particle size and carbon-coat the active material to improve its ionic and electronic conductivity, the DRX powder was mixed with Super C65 in a 70:20 mass ratio in a planetary ball mill at 400 rpm for 6 h.

[0053] SEM image (Fig. 3 C ) show that both the solid and microwave post-treated powders are of similar size, and consequently, the difference in electrochemical performance can be attributed to the composition and bulk structure of the active material or the presence of impurity phases.

[0054] The DRX + Super C65 powder was further mixed with PTFE to yield a weight ratio of raw material:carbon:binder of 70:20:10. The cathode film was coated with 4-5 mg / cm2 A CR2023-type coin cell was assembled using a cathode film, a Li metal anode, a Celgard 2325 trilayer membrane, and a commercially available 1 M LiPF salt in a 1:1 EC:DMC solvent as the electrolyte.

[0055] Figure 6 A and Figure 6 B The galvanostatic profiles of ss- and mw-LMTO shown in Fig. 6 are very similar. Both showed a 35% capacity fade (Fig. 6 D The initial and 50-cycle discharge capacities of 200 mAh / g and 130 mAh / g, respectively, correspond to those of the 50-cycle AA battery (see reference).

[0056] The mw-DRX has a higher open circuit voltage (OCV) than the ss-cathode (3.0 V vs. 2.7 V), consistent with the higher Mn oxidation state observed in the XANES data.

[0057] The coulombic efficiency is also the same (Fig. 6 D ) and remains near ~99% throughout the first 50 cycles. C The differential capacity analysis shown in Figure 1 suggests that the redox processes for ss- and mw-DRX are very similar, with the lower voltage oxidation and reduction peaks corresponding to Mn redox and the higher voltage peak corresponding to O redox. For mw-LMTO, the O oxidation peak occurs at a slightly higher voltage than for the solid sample.

[0058] The operating voltage, energy density, voltage retention, and voltage hysteresis are also identical for the ss- and mw-LMTO cathodes. E shows the rate performance of the LMTO cathode, suggesting that ss-DRX works better at higher rates. This may be due to slightly higher amounts of LiF and Li2CO3 impurities in the mw-DRX sample, which will likely be resolved by further fine-tuning of the microwave synthesis conditions.

[0059] Fourth Example: Characterization of LMNO / F obtained using the synthesis methods described in the first and second examples Microwave synthesis of Nb-based Li 1.3 Mn 0.4 Nb 0.3 O 2-x F x This is applicable to other DRX compositions such as (LMNO / F). Here, no Li excess was used during microwave synthesis. This contrasts with the 10% molar Li excess used in the LMTO-based synthesis discussed in the third example.

[0060] LMNO compositions were also prepared by conventional solid-state synthesis for comparison. Again, phase-pure DRX (based on synchrotron XRD) was obtained through microwave heating (and through conventional solid-state synthesis), and particle size could be tuned by adjusting the microwave heating time between 5 and 20 min.

[0061] [Table 3]

[0062] (a) solid 7 Li / 19 F NMR ICP analysis is not typically attempted for DRX compounds such as LMNO / F because acid digestion is notoriously difficult for Nb-containing compounds. 19 F NMR still provides valuable information about the amount and extent of fluorination of Li-containing impurity phases / domains in DRX.

[0063] The higher average Mn oxidation state in the LMNO / F compound (see XANES analysis below) and the different d 0 As expected from the presence of transition metals (Nb vs. Ti) and the type and / or extent of cation short-range order, the LMNO / F 7The Li NMR spectrum shows slightly different lineshapes than those obtained for LMTO.

[0064] The results in Table 3 suggest that longer microwave heating times result in fewer diamagnetic Li impurities, and that overall LMNO / F has more Li impurities than LMNO.

[0065] Figure 8 B and Figure 8 C Shown in 19 The F NMR spectrum suggests the absence of LiF impurities in mw-LMNO / F and the presence of significantly more fluorine in the DRX-like local environment in this sample compared to mw-LMTO. Thus, mw-LMNO / F is indeed an oxyfluoride with a low F content (the upper limit of fluorination is 5% based on the amount of fluorine precursor).

[0066] (b) Mn K-edge XAS and PDF analysis The Mn K-edge XANES spectrum shows that the average Mn oxidation state of mw-LMNO / F is slightly higher than that of ss-LMNO / F, even though F is present in the microwave sample and not in the solid sample (incorporation of F reduces the average Mn oxidation state), suggesting that Li vacancies exist in the microwave sample.

[0067] PDF analysis also suggests that there are no significant differences in the nature and extent of cation short-range order between the LMNO / F DRX phases synthesized via microwave heating and via conventional solid-state synthesis.

[0068] (c) Electrochemistry of ss- and mw-LMNO / F Similar to the LMTO system, the electrochemical properties of the solid-state and microwave LMNO / F (with a 5 min heating step) are very similar ( Figure 10 A and Figure 10 B). The capacity for both the mw- and ss-DRX samples evolves from 270 mAh / g during the first discharge to 130 mAh / g after 50 charge-discharge cycles. This corresponds to a 50% capacity fade, which is more severe than for the LMTO system. The energy density and voltage fade are also comparable for the mw- and ss-LMNO / F.

[0069] The coulombic efficiency is stable at ~99%. Similar to LMTO, the oxidation and reduction peaks appear at lower voltages in the differential capacity plot (Figure 10). C ) corresponds to Mn redox, and the higher voltage peak corresponds to O redox. In this case, mw-LMNO / F shows a slightly higher voltage O redox peak than ss-LMNO / F.

[0070] The OCV of mw-LMNO / F is higher (3.0 V) than that of ss-LMNO (2.7 V), consistent with the higher average Mn oxidation state observed by XAS.

[0071] Again, the most notable difference between the two LMNO / F cathode samples is their rate performance, as shown in Figure 10. E As shown in Fig. 1, ss-DRX performs better. Again, this is probably due to the presence of small amounts of impurity phases in the microwave sample.

[0072] Fifth Example: Fluorination of MnTi DRX system via microwave synthesis ICP and LMTO 7 The Li NMR results suggested that Li excess was not necessary to obtain phase-pure DRX by microwave synthesis. This is presumably because Li evaporation does not occur to a significant extent in such a short (5 min) reaction time. Therefore, we investigated whether F-containing MnTi compositions could be obtained without Li excess in the precursor mixture. This was because it was expected that Li excess would lead to more volatile F and may affect its incorporation into the DRX structure. Li without Li excess 1.2 Mn0.4 Ti 0.4 O 1.9 F 0.1 The (LMTO / F) composition was prepared as the target. 11A to 11D Shown in 19 F NMR results suggest significant F incorporation into the bulk DRXed structure. Again, to quantify the absolute amount of F present in the DRXed phase, 19 It is noted that F NMR is not available.

[0073] The longer the reaction time, the less LiF impurity there is (Figure 11 C ), but at the cost of reduced fluorination of DRX (consistent with LiF volatilization). Therefore, fast reaction times are preferred to fluorinate the DRX compounds.

[0074] 19 Comparison of F NMR signal intensities indicates that the mw-LMTO / F phase is still in the mw-LMNO / F composition (Li 1.3 Mn 0.4 Nb 0.3 O 1.9 F 0.1 ) contains less fluorine than (Figure 11 D ) suggests that

[0075] By adjusting the target DRX composition, more fluorine can be incorporated into the DRX structure (Figure 12 A ). Li 1.2 Mn 0.5 Ti 0.3 O 1.8 F 0.2 The composition is LMNO / F(Li 1.3 Mn 0.4 Nb 0.3 O 1.9 F 0.1 ) sample (Figure 12 B ). The DRX phase (Li) with a higher F content 1.2 Mn 0.6 Ti 0.2 O 1.8 F 0.2 and Li 1.2 Mn 0.7 Ti 0.1 O 1.7 F0.3 ) synthesis can be optimized.

[0076] Sixth Example: Particle Size Measurement Figure 13 A and Figure 13 B are SEM images of LMTO samples synthesized via solid-state and microwave methods, respectively, and have the general formula: Li x Mn′ y Ti″ 2-x-y O 2-z F z The resulting powder 1300 comprises a lithium metal oxide / oxyfluoride compound for rechargeable batteries with the formula: where 1.05≦x≦1.35, 0.1≦y≦0.9, and 0≦z≦0.7 (as measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) in Table 1). The specific formulation synthesized for these samples was Li 1.2 Mn 0.4 Ti 0.4 O2 (ss-LMT44 and mw-LMT44). Figure 14 A and Figure 14 B shows the particle size distribution for the particles in the powder measured in the SEM images.

[0077] Figure 14 B The particle size distribution data in Figure 1 indicates that powders produced using microwave synthesis as described herein have a median particle size of at least 1.00 μm to 3 μm, with a standard deviation of only 1.5 μm. The median particle size of particles 1302 was obtained by measuring the size of particles 1302 in SEM images along their longest dimension 1304 using the protocol described in the next section. Additionally, the median particle size is measured for particles obtained after synthesis without further processing to reduce the particle size.

[0078] Seventh Example: Particle Size Measurement Figure 15 provides additional particle size distribution data for particles in further powdered LMTO samples synthesized via solid-state and microwave methods, which are also shown in Figure 13. A and Figure 13 BThe SEM images in the figure are again representative. The various LMTO samples are listed below in Table 4a, followed by their descriptions, and particle size distribution data in Table 4. b and the discussion follows.

[0079] [Table 4a]

[0080] The samples in Table 4a above were prepared by microwave and solid-state synthesis as presented in Examples 1 and 2 above. All microwave-synthesized samples were prepared with a microwave heating step having a duration of 5 minutes. Solid-state synthesized samples ss-LMT53 and ss-LMT62 were both prepared with a calcination step having a duration of 12 hours.

[0081] [Table 4b]

[0082] This particle size distribution data confirms that microwave synthesis of a given composition typically produces a smaller average particle size (e.g., Dx(50)) and a more uniform particle size distribution (e.g., smaller span) than that achieved by solid-state synthesis of the same given composition. According to this data, powders produced using microwave synthesis have a median particle size in the range of about 5.0 to about 6.0 μm (e.g., 5.3 to 5.68 μm), with 90% of the powder particles (e.g., Dx(90)) having a maximum diameter in the range of about 10 μm to about 13 μm (e.g., 10.4 μm to 12.5 μm), and a particle size span ((Dx(90)-Dx(10)) / Dx(50)) of about 1.4 to about 1.8 (e.g., 1.394 μm to 1.804 μm). These particle distributions were found to be significantly smaller relative to those produced by solid-state synthesis, with median particle sizes ranging from about 9.0 to about 28.0 μm (e.g., 8.95-28.4 μm), with 90% of the powder particles (e.g., Dx(90)) having a maximum diameter ranging from about 18 μm to about 62 μm (e.g., 18 μm to 61.5 μm), and a particle size span ((Dx(90)-Dx(10)) / Dx(50)) of about 1.7 to about 1.9 (e.g., 1.733-1.93). It was further observed that the particle distributions of microwave-synthesized powders typically had a greater proportion of particles with a maximum diameter less than 3 μm (e.g., "%<3 μm").

[0083] Experimental methods and protocols used to obtain the data in the examples X-ray diffraction (Fig. 3 A ) and pair distribution function measurements (Figure 5 A to Figure 5C) was performed on ≈40 mg samples at beamline 17-BM of the Advanced Synchrotron Radiation Facility at Argonne National Laboratory. All samples were measured at room temperature (≈303 K). Scattering intensities were measured using a Perkin-Elmer amorphous silicon flat-panel detector. The wavelength for measurements was 0.24117 A. The TOPAS software suite was used for Pawley refinement treatment of these datasets. Pair distribution function g(r) datasets were obtained by reducing image files acquired from the area detector at 17-BM to .chi files using GSAS II. The .chi files were converted to g(r) data files using the xPDF suite software package with a Qmax of 16.9 A. Least-squares refinement of the PDF data was performed using PDFGUI.

[0084] Scanning electron microscope (SEM) images ( Figure 3 ) were taken using a Thermo Fisher Apreo C LoVac SEM instrument at an accelerating voltage of 5 keV and a current of 0.4 nA. B ,Figure 3 C ,Figure 7 B~Figure 7C ,Figure 13 A ,Figure 13 B ) was obtained.

[0085] Bulk chemical composition was determined using an Agilent 5800 ICP-OES, and F-ISE measurements were determined using a Cole-Parmer system. DRX samples were digested in a mixture of nitric acid and hydrochloric acid. For ICP-OES, the digested solution was diluted with distilled water. For F-ISE measurements, the solution was diluted using 23 sodium acetate buffer and fluoride ionic strength adjustment solution (TISAB, Cole-Parmer).

[0086] The amounts of Li, Mn, Ti, and Nb (listed in Table 1) in the positive electrode active material powder were measured by ICP method using an Agilent 5800 ICP-OES instrument. 10 mg of powder sample was dissolved in a mixture of 4 mL of concentrated nitric acid and 1 mL of concentrated hydrochloric acid. A 1 mL aliquot of the dissolved sample solution was then pipetted into a Falcon tube, followed by the addition of 13 mL of distilled water, resulting in a 14x dilution by volume. The diluted solution was used for ICP-OES measurement. The measured Li, Mn, and Ti are expressed as mol% of their total content.

[0087] The amount of F in this positive electrode active material powder (listed in Table 1) was measured by F-ISE measurement. A 0.5 g aliquot of the dissolved sample solution prepared above was weighed and added to a plastic HDPE bottle. 2 g of distilled water was added to the bottle. Finally, 25 mL of a 10:1 mass ratio mixture of 15% aqueous sodium acetate and Tisab buffer (obtained from Cole-Parmer) was added to adjust the pH of the final solution to 5–8 and provide an ionic strength adjuster for fluoride. This final solution was used for F-ISE measurement. Since the masses of all components in the final solution were measured, the amount of F in the original dissolved sample solution could be back-calculated, and the mol% of F relative to the Li, Mn, and Ti from the ICP results was obtained. The measured Li, Mn, Ti, and F contents are expressed as mol% of the sum of their contents.

[0088] As provided in Example 6, the particle size distribution of the positive electrode active material powder was measured by SEM using ImageJ software according to the following steps: SEM measurements are performed on a Thermo Fisher Apreo C LoVac SEM instrument at 25°C under a high vacuum environment of <1e-5 Torr. Then, the file (Figure 13 A and Figure 13 B) were loaded into ImageJ software at magnifications ranging from 800x to 1500x. The SEM must have adequate contrast and brightness so that the edges of the particles can be clearly observed. The scale is set according to the magnification of the SEM. Particles were manually sized across their longest axis. For ss-LMTO, Figure 13 A 814 particles were measured in the image. For 5 min mw-LMTO, Fig. 13 B 1500 particles in the image were measured. For each sample, the mean, median, and standard deviation were calculated for the set particle sizes measured. The results of this statistical analysis, showing the mean, median, and standard deviation, are shown in Figure 14. A and Figure 14 B is shown in.

[0089] Data on particle size distribution (PSD), such as settling percentiles (Dx(10), Dx(50), Dx(99)) and particle size in span, were obtained by laser PSD measurement, as provided in Example 7. Laser PSD was measured using a Malvern Mastersizer 3000 with a Hydro 3000MV wet dispersion accessory. First, 5 drops of surfactant (Dolapix) were added to a beaker. Then, 0.5 grams of cathode was added using a spatula. The beaker was then filled with 40 mL of demineralized (DI) water. The solution was then sonicated for 30 seconds using successive pulses to improve dispersion. The solution was then dispersed in a Mastersizer Hydro Can using a 5 mL pipette until 7% opacity was reached. Two measurement methods were used to measure the scattering of both red and blue light sources, and the average of both measurements was used for reporting. Note that narrow span (in addition to particle size uniformity) is an indicator of the significant sphericity of the particles, and the span value is used in the examples to measure sphericity.

[0090] Advantages and Improvements Cation-disordered rock salt oxides / oxyfluorides (DRX) have become an industrially relevant class of Li-ion cathode materials because they are formed with earth-abundant transition metals and enable the deployment of more sustainable, lower-cost, and higher-energy-density secondary batteries. A battery's cathode material is the component that limits the cell's overall energy density and is typically the most expensive. Currently, the majority of Li-ion batteries use Co and Ni-based cathode materials, which suffer from complex supply chains, large price fluctuations, and high raw material costs. Therefore, the development of cathode materials relying on more sustainable elements, such as Mn and Fe, is desirable. DRX materials can accommodate a variety of different transition metals, and Mn-based DRX cathodes have shown good electrochemical performance and are promising for the commercialization of low-cost, high-energy-density Co / Ni-free batteries.

[0091] However, current methods for synthesizing DRX cathodes, whether sintering precursor powders at ≥800 °C for ≥10 hours or mechanochemical ball milling of precursor powders at ∼500 rpm for extended periods (40 hours), involve time-consuming and energy-intensive steps, typically performed in a sealed, inert argon (Ar) environment. In contrast, the microwave synthesis process disclosed herein is a rapid, energy-efficient method for synthesizing DRX compounds, which can be performed in 5–20 minutes (more than two orders of magnitude faster than conventional methods). Furthermore, microwave synthesis does not require an inert Ar atmosphere, as is required for conventional solid-state and mechanochemical techniques. Therefore, this microwave synthesis method should be extremely attractive to battery cathode manufacturers as a significantly more scalable and cost-effective way to produce DRX compounds.

[0092] The microwave synthesis method of the present invention is advantageous in that it produces similar DRX compositions and atomic structures available from conventional solid-state synthesis, but with much shorter processing times and commensurate energy savings.1.2 Mn 0.4 -Ti 0.4 The results show that the amount of Li in LMTO (O2) is the same when the microwave heating time is increased from 5 to 10-20 min and during the solid-state synthesis after the 12-h calcination step. 1.3 Mn 0.4 Nb 0.3 O 2-x F x There is no significant difference in the cation short-range order between mw- and ss-DRX, as evidenced by X-ray PDF analysis for the (LMNO / F) compound. Long-term galvanostatic cycling shows very similar electrochemical performance between mw- and ss-DRX for both LMTO and LMNO / F.

[0093] The higher amount of Li impurity at shorter microwave heating times suggests that Li excess is not necessary to synthesize phase-pure DRX via this novel microwave synthesis method, providing the possibility to produce suitable DRX materials with reduced importance of Li content. Experiments have shown that the incorporation of F into the bulk DRX structure is dependent on the amount of Li excess used during microwave synthesis and the d 0 It has also been shown to depend on the nature of the transition metal. For example, very little F is incorporated into LMTO prepared with a 10% Li excess. Without Li excess, more significant fluorination is achieved in the MnTi DRX system, especially at short reaction times. Compared to LMTO, a larger amount of F can be incorporated into the Li-Mn-Nb-O DRX structure prepared without excess Li, and no LiF is present in the as-synthesized microwave cathode sample. Thus, the microwave synthesis method of the present invention offers additional possibilities for LMTO compositions where more benefit can be derived from the incorporation of F in the presence of relatively less Li content.

[0094] Slightly different Mn oxidation states are accessible in the DRX compounds obtained via microwave synthesis and conventional solid-state synthesis. XANES reveals that the oxidation states of Mn in mw-LMTO and mw-LMNO / F DRX are slightly higher than in ss-LMTO and ss-LMNO / F DRX due to the more oxidizing environment used in the former methods (ambient air vs. Ar). Slightly different average redox potentials are also observed for the DRX compounds obtained via microwave synthesis and conventional solid-state synthesis. Differential capacity analysis indicates that oxygen redox occurs at slightly higher potentials for the mw-DRX compounds.

[0095] Surprisingly and unexpectedly, the microwave synthesis of the present invention can produce powders of lithium metal oxide or oxyfluoride compounds having particle size distributions suitable for use in rechargeable batteries and with a reduced number of post-synthesis processing steps (e.g., without or with a reduced number of additional milling, grinding, or other processing steps to reduce the size of the particles).

[0096] Methods, Compositions, and Electrode Embodiments Exemplary compositions, methods and electrodes according to the present invention include, but are not limited to: 1. A method for forming a cathode active material useful as an electrode, comprising: mixing ionic compound precursors together to form a precursor mixture; microwaving the precursor mixture, said step comprising exposing the precursor mixture to microwave radiation having a power and duration to form a microwave-irradiated powder comprising ionic compounds having a rock salt-type crystal structure with a disordered arrangement of cations, wherein the rock salt-type crystal structure may be indexed to the cubic Fm-3m space group (#225); ceasing the microwave irradiation and rapidly cooling (within 15 seconds of cessation) the microwaved powder to form a cathode active material that retains a rock salt crystal structure at room temperature. 2. The method of Example 1, further comprising grinding the cathode active material into a ground powder and further processing the ground powder into a form useful as an electrode comprising a Li-ion battery electrode. 3. The method of any one of Examples 1 to 2, wherein the ionic compound comprises a cation comprising a mobile alkali metal ion and at least one of a transition metal ion or a major element ion, and an anion comprising at least one of an oxygen ion or a fluorine ion. 4. The method of example 3, wherein the mobile alkali metal ions comprise lithium or sodium. 5. The method of any one of Examples 3 and 4, wherein the ionic compound precursor does not contain excess of said mobile alkali metal ions to offset loss of said corresponding element during exposure to microwave radiation. 6. The method of any one of Examples 3 and 4, wherein the ionic compound precursor contains an excess of the mobile alkali metal ion to offset the loss of the corresponding element during exposure to microwave radiation. 7. The method of any of Examples 1-6, wherein the rock salt crystal structure comprises a disordered rock salt (DRX) oxide structure. 8. Ionic compounds are with a cubic rock-salt crystal structure corresponding to a disordered arrangement with a random distribution of cations at longer correlation lengths, and The method according to any of Examples 1 to 7, wherein a shorter correlation length is described by a more ordered arrangement of the cations. 9. The method of any of Examples 1-7, further comprising selecting a specific quenching rate (°C / min) for quenching to a temperature in the range of 77-323 K to prevent the formation of layered / ordered oxide impurities during quenching. 10. The method of any one of Examples 1-9, wherein the cathode active material comprises an oxyfluoride. 11. The ionic compound precursor is a compound of formula A x M′ y M″ 2-x-y O 2-z F zwherein 1.05≦x≦1.35, 0.1≦y≦0.9, and 0≦z≦0.7; A is a mobile alkali metal ion; M′ is a transition metal ion or a main element ion; and M″ is a transition metal ion having a higher valence than M′. 12. A is for Li + or Na + M' is at least one of V, Cr, Mn, Fe, Co, Ni, Mg, Cu, and Al, and M" is Ti. 4+ , Zr 4+ , Nb 5+ , V 5+ , Mo 6+ 12. The method of Example 11, wherein the method is at least one of: 13. The method of any one of Examples 1-12, wherein the step of microwave irradiation is carried out in an inert atmosphere. 14. The method of any of Examples 1-12, wherein the step of microwaving is carried out in an atmosphere consisting essentially of air at ambient (atmospheric) pressure. 15. The method further comprising contacting the precursor mixture with a heat transfer medium having an absorption spectrum resonantly tuned to the frequency of the microwave radiation; the step of microwaving comprises exposing the precursor mixture to microwave radiation while in thermal contact with a heat transfer medium; in a first stage, the heat transfer medium warms by resonantly absorbing microwave radiation, and the heat transfer medium transfers at least some thermal energy to the precursor mixture via conduction through thermal contact, thereby heating the precursor mixture above a critical temperature at which the precursor material becomes more susceptible to absorbing microwave radiation; The method of any of Examples 1-14, wherein in a second stage, the precursor mixture is above the critical temperature and microwave radiation warms the precursor mixture to a higher reaction temperature (above 1500°C, preferably above 1600°C) that initiates the formation of ionic compounds with disordered arrangement of cations. 16. The method of example 15, wherein the heat transfer medium comprises activated carbon. 17. The step of microwaving comprises microwaving a precursor mixture disposed in a crucible having a heat transfer medium; The quenching step comprises: placing the microwave irradiated powder in a quenching medium after removing the microwave irradiated powder from the crucible; and 17. The method of example 15 or 16, comprising drying the cathode active material after quenching. 18. The method of any of Examples 15-17, wherein the quenching step comprises immersing the microwave-irradiated powder in a non-reactive liquid comprising water (e.g., distilled water), ethanol, or liquid nitrogen. 19. The method of example 2, wherein the further processing step does not require ball milling or mechanochemical milling of the ionic compound precursor or precursor mixture. 20. The method of any of Examples 1-19, wherein a fluorinated compound is added to the mixture of ionic compound precursors to act as a sintering agent, allowing faster diffusion of cations and anions to form a rock salt crystal structure during the microwave irradiation step. 21. Multiple particles formed by quenching microwave-irradiated precursors (e.g., Figure 3 B or Figure 13 B 2. A composition of material useful as a cathode active material in an electrode comprising: 22. Each of the particles has the formula: A x M′ y M″ 2-x-y O 2-z F z (In the formula, 1.05 ≦ x ≦ 1.35, 0.1 ≦ y≦0.9 and 0≦z≦0.7, A is a mobile alkali metal ion (e.g., Li or Na), M′ is a transition metal ion, and M″ is a transition metal ion having a higher valence than M′. 23. The composition of matter of Examples 21 or 22, comprising a powder comprising particles comprising a lithium metal oxide or oxyfluoride compound for use in a rechargeable battery. 24. The lithium metal oxide or oxide fluoride has the general formula: Li x Mn′ y Ti″ 2-x-y O 2-z F z where 1.05≦x≦1.35, 0.1≦y≦0.9, and 0≦z≦0.7 as measured by ICP-OES, and the powder comprises particles having a median particle size of about 5 μm to about 6 μm, with a span in the range of about 1.4 to about 1.8. 25. Mixing ionic compound precursors together to form a precursor mixture; microwaving the precursor mixture, said step comprising exposing the precursor mixture to microwave radiation having a power and duration to form a microwave-irradiated powder comprising ionic compounds having a rock salt-type crystal structure with a disordered arrangement of cations, wherein the rock salt-type crystal structure may be indexed to the cubic Fm-3m space group (#225); and ceasing the microwave irradiated powder (within 15 seconds of cessation) to form a cathode active material that retains a rock salt crystal structure at room temperature. 26. A is Li + or Na + wherein M′ is Mn and M″ is Ti, and 80% by number of the particles have a maximum diameter in the range of 0.5 to 5 micrometers as synthesized by quenching of the microwave irradiated precursor and without further processing to reduce the size of the particles (e.g., grinding or milling). 27. The composition of the material of Example 26, wherein the maximum diameter of the particles is as measured using a software tool applied to scanning electron microscope images of the particles. 28. The composition of the material of Example 26 or 27, wherein the maximum diameter has a standard deviation of only 1.13 micrometers. 29. The composition of the material of any of Examples 26-28, wherein 50% of the particles have a maximum diameter less than 2.5 microns. 30. Transition metal ions have oxidation states that correlate with the elemental composition of compounds formed by microwave irradiation of precursors in air; The composition of matter of any of Examples 22-29, wherein the lithium content in the precursor prior to microwave irradiation does not contain excess lithium that would cause lithium fluctuations during microwave irradiation, and reduces fluorine loss during microwave irradiation. 31. A is at least one of Li and Na, M' is at least one of V, Cr, Mn, Fe, Co, Ni, Mg, and Al, and M" is Ti. 4+ , Zr 4+ , Nb 5+ , Mo 6+ The composition of matter of any of Examples 22-30, wherein the composition is at least one of: 32. The composition of matter of any of Examples 22-31, further comprising fluorine substituting for oxygen. 33. The composition of matter of any of Examples 22-32, wherein the fluorine content is greater than 0.1 or 0.2. 34. The composition of matter of any of the Examples, wherein the microwave irradiation is for a duration in the range of 2 minutes to 60 minutes. 35. Measured by ICP, formula: A x M′ y M″ 2-x-y O 2-z F z (wherein 1.05≦x≦1.35, 0.1≦y≦0.9, and 0≦z≦0.7; A is Li + or Na + wherein M′ is Mn and M″ is Ti; the particles have a median particle size of about 5 μm to about 6 μm with a span in the range of about 1.4 to about 1.8; and / or A composition of matter useful as a cathode active material in an electrode, wherein, as synthesized without grinding, polishing, grinding, or any other further processing to reduce the size of the particles, 90% of the particles by number have a maximum diameter in the range of from about 10.4 μm to about 12.5 μm, and 50% of the particles by number have a maximum diameter smaller than about 5.3 μm to about 5.7 μm. 36. General formula Li x Mn′ y Ti″ 2-x-y O 2-z F z wherein 1.05≦x≦1.35, 0.1 ≦y≦0.9, and 0 ≦z≦0.7 as measured by ICP-OES, the powder having a median particle size of about 5 μm to about 6 μm, with a span in the range of about 1.4 to about 1.8. 37. The powder of example 36, wherein the lithium metal oxide or oxyfluoride has a cation-disordered rock salt (DRX) structure. 38. The powder described in Example 36 or 37, wherein the median particle size is obtained for the intermediate product without further processing to reduce the size of the particles. 39. The powder of any of Examples 36-38, produced using the method of any of Examples 1-20 or 34. 40. The powder of any of Examples 36-39, wherein the median particle size is as measured by sizing the particles in a scanning electron microscope image along their longest dimension. 41. The powder of any of Examples 36-40, wherein the median particle size is as measured for particles obtained without further processing to reduce the size of the particles. 42. An electrochemical cell (e.g., a lithium or sodium ion battery), comprising: an anode; Electrolytes, 10. An electrochemical cell comprising: a cathode coupled to an anode via an electrolyte, the cathode comprising the composition of matter of any of Examples 21-35, and wherein mobile alkali ions (Li or Na) are inserted into or extracted from the cathode during operation of the cell.

[0097] While the present invention has been described with reference to particular embodiments, it will be understood by those skilled in the art that the above disclosure deals only with exemplary embodiments, that the scope of the present invention is not limited to the disclosed embodiments, and that the scope of the present invention may encompass additional embodiments, including any combination of the disclosed embodiments, in whole or in part, as well as various changes and modifications to the examples disclosed herein, without departing from the scope of the invention as defined in the appended claims and equivalents thereof.

[0098] To the extent necessary to understand or complete the disclosure of the present invention, all publications, patents, and patent applications mentioned herein are expressly incorporated by reference to the same extent as if each were individually incorporated as such.

[0099] The present invention is not limited to the exemplary embodiments described herein, but rather is characterized by the appended claims, which are not intended to limit the scope of the disclosure in any way.

[0100] References The following references are incorporated herein in their entirety: (1) Lee, J.; Kitchaev, D. A.; Kwon, D. H.; Lee, C. W.; Papp, J. K.; Liu, Y. S.; Lun, Z.; Clement, R. J.; Shi, T.; McCloskey, B. D.; Guo, J.; Balasubramanian, M.; Ceder, G. Reversible Mn2+ / Mn4+ Double Redox in Lithium-Excess Cathode Materials. Nature 2018, 556 (7700), 185-190. (2) Ji, H.; Urban, A.; Kitchaev, D. A.; Kwon, D.-H.; Artrith, N.; Ophus, C.; Huang, W.; Cai, Z.; Shi, T.; Kim, J. C.; Kim, H.; Ceder, G. Hidden Structural and Chemical Order Controls Lithium Transport in Cation-Disordered Oxides for Rechargeable Batteries. Nat. Commun. 2019, 10 (1), 592. (3) Hyeseung Chung; Zachary Lebens-Higgins; Baharak Sayahpour; Carlos Mejia; Antonin Grenier; E. Kamm, G.; Yixuan Li; Ricky Huang; J. Piper, L. F.; W. Chapman, K.; Jean-Marie Doux; Shirley Meng, Y. Experimental Considerations to Study Li-Excess Disordered Rock Salt Cathode Materials. J. Mater. Chem. A 2021, 9 (3), 1720-1732. (4) Baur, C.; Kallquist, I.; Chable, J.; Chang, J. H.; Johnsen, R. E.; Ruiz-Zepeda, F.; Ateba Mba, J. M.; Naylor, A. J.; Garcia-Lastra, J. M.; Vegge, T.; Klein, F.; Schur, A. R.; Norby, P.; Edstrom, K.; Hahlin, M.; Fichtner, M. Improved Cycling Stability in High-Capacity Li-Rich Vanadium Containing Disordered Rock Salt Oxyfluoride Cathodes. J. Mater. Chem. A 2019, 7 (37), 21244-21253. (5) Crafton, M. J.; Yue, Y.; Huang, T.-Y.; Tong, W.; McCloskey, B. D.; Crafton, M. J.; Huang, T.; McCloskey, B. D.; Yue, Y.; Tong, W. Anion Reactivity in Cation-Disordered Rocksalt Cathode Materials: The Influence of Fluorine Substitution. Adv. Energy Mater. 2020, 10 (35), 2001500. (6) Ji, H.; Urban, A.; Kitchaev, D. A.; Kwon, D.-H.; Artrith, N.; Ophus, C.; Huang, W.; Cai, Z.; Shi, T.; Kim, J. C.; Kim, H.; Ceder, G. Hidden structural and chemical order controls lithium transport in cation-disordered oxides for rechargeable batteries. Nature Communications 2019, 10, 592. (7) Clement, R. J.; Lun, Z.; Ceder, G. Cation-disordered rocksalt transition metal oxides and oxyfluorides for high energy lithium-ion cathodes. Energy Environ. Sci. 2020, 13, 345. (8) Wu, V. C., Evans, H. A., Giovine, R., Preefer, M. B., Ong, J., Yoshida, E., Cabelguen, P.-E., Clement, R. J., Rapid and Energy-Efficient Synthesis of Disordered Rocksalt Cathodes. Adv. Energy Mater. 2023, 13, 2203860.

Claims

1. 1. A method of forming an active material useful as an electrode, comprising: mixing ionic compound precursors together to form a precursor mixture; exposing the precursor mixture to microwave radiation of sufficient power and duration to form a microwave irradiated powder comprising ionic compounds having a rock salt crystal structure with a disordered arrangement of cations, the rock salt crystal structure being in the cubic Fm-3m space group; and quenching the microwave irradiated powder to form the active material that retains the rock salt crystal structure at room temperature.

2. 10. The method of claim 1, further comprising grinding the active material into a ground powder and processing the ground powder into a form useful as an electrode comprising a Li-ion battery electrode.

3. 3. The method of claim 1 or 2, wherein the ionic compound comprises a cation comprising a mobile alkali metal ion and at least one of a transition metal ion or a major element ion, and an anion comprising at least one of an oxygen ion or a fluorine ion.

4. 4. The method of claim 3, wherein the mobile alkali metal ions comprise lithium or sodium.

5. 5. The method of claim 3 or 4, wherein the ionic compound precursor does not contain excess of the mobile alkali metal ions to offset loss of the corresponding elements during exposure to microwave radiation.

6. 5. The method of claim 3 or 4, wherein the ionic compound precursor contains an excess of the mobile alkali metal ions to offset the loss of the corresponding element during exposure to microwave radiation.

7. 7. The method of any one of claims 1 to 6, wherein the rock salt crystal structure comprises a disordered rock salt (DRX) oxide structure.

8. The ionic compound with the cubic rock salt crystal structure corresponding to the disordered arrangement with a random distribution of the cations at longer correlation lengths, and 8. The method of claim 1, wherein a shorter correlation length is described by a more ordered arrangement of the cations.

9. 8. The method of any one of claims 1 to 7, further comprising the step of selecting a specific quenching rate (°C / min) for said quenching to a temperature in said range of 77 to 323 K to prevent the formation of layered / ordered oxide impurities during said quenching.

10. The method of any one of claims 1 to 9, wherein the cathode active material comprises an oxyfluoride.

11. The ionic compound precursor is a compound of Formula A x M′ y M″ 2-x-y O 2-z F z 11. The method of claim 1, wherein 1.05≦x≦1.35, 0.1≦y≦0.9, and 0≦z≦0.7, A is a mobile alkali metal ion, M′ is a transition metal ion or a main element ion, and M″ is a transition metal ion having a higher valence than M′.

12. A is for Li - or Na + M' is at least one of V, Cr, Mn, Fe, Co, Ni, Mg, and Al, and M" is Ti. 4+ , Zr 4+ , Nb 5+ , Mo 6+ 12. The method of claim 11, wherein the at least one of

13. The method of any one of claims 1 to 12, wherein the step of microwaving is carried out in an inert atmosphere.

14. 13. The method of any one of claims 1 to 12, wherein the step of microwaving is carried out in an atmosphere consisting essentially of air at ambient (atmospheric) pressure.

15. contacting the precursor mixture with a heat transfer medium having an absorption spectrum resonantly tuned to the frequency of the microwave radiation; the step of microwaving comprises exposing the precursor mixture to the microwave radiation while in thermal contact with the heat transfer medium; in a first stage, the heat transfer medium warms by resonantly absorbing the microwave radiation, and the heat transfer medium transfers at least some thermal energy to the precursor mixture via conduction through thermal contact, thereby heating the precursor mixture above a critical temperature at which the precursor material becomes more susceptible to absorbing the microwave radiation; 15. The method of any one of claims 1 to 14, wherein in a second stage, the precursor mixture is above the critical temperature and the microwave radiation warms the precursor to a higher reaction temperature that initiates the formation of the ionic compounds with the disordered arrangement of the cations.

16. The method of claim 15 , wherein the heat transfer medium comprises activated carbon.

17. the step of microwaving comprises microwaving the precursor mixture disposed in a crucible containing the heat transfer medium; 17. The method of claim 15 or 16, wherein the quenching step comprises placing the microwave irradiated powder in a quenching medium after removing the microwave irradiated powder from the crucible.

18. 18. The method of any one of claims 15 to 17, wherein the quenching step comprises immersing the microwave irradiated powder in a non-reactive liquid comprising water, ethanol, or liquid nitrogen.

19. 3. The method of claim 2, wherein the further processing step does not require ball milling or mechanochemical milling.

20. 20. The method of any one of claims 1 to 19, wherein a fluorinated compound is added to the mixture of ionic compound precursors to act as a sintering agent and allow faster diffusion of the cations and anions to form the rock salt crystal structure during the microwave irradiating step.

21. General formula Li x Mn′ y Ti″ 2-x-y O 2-z F z 1. A powder of lithium metal oxide or oxyfluoride compounds for rechargeable batteries, comprising: 1.05≦x≦1.35, 0.1≦y≦0.9, and 0≦z≦0.7 as measured by inductively coupled plasma optical emission spectroscopy (ICP-OES); The powder comprises particles having a median particle size of about 5 μm to about 6 μm, with the span being within the range of about 1.4 to about 1.

8.

22. 22. The powder of claim 21, wherein the lithium metal oxide or oxyfluoride has a cation-disordered rock salt (DRX) structure.

23. 23. The powder of claim 21 or 22, wherein the median particle size is as determined by sizing the particles in a scanning electron microscope image along their longest dimension.

24. 24. The powder of any one of claims 21 to 23, wherein the median particle size is as measured for the particles obtained without further processing to reduce the size of the particles.

25. 25. The powder of any one of claims 21 to 24, as synthesized by the method of any one of claims 1 to 20.

26. 1. An electrochemical cell comprising: an anode; Electrolytes, a cathode coupled to the anode via the electrolyte, the cathode comprising the powder of any one of claims 21 to 25, and wherein the Li is inserted into or extracted from the cathode during operation of the cell.

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