Multiphase electroactive materials and associated articles, systems, and methods

Abstract

Generally described herein are multiphase electroactive materials and associated articles (e.g., electrodes), systems (e.g., electrochemical devices), and methods. In certain embodiments, an electroactive material comprises a first component and a second component that is softer than the first component. The first component and the second component may be intermixed such that the second component is dispersed throughout a bulk of the first component. In some embodiments, the first component comprises a first alkali metal (e.g., metallic lithium) and the second component comprises a second alkali metal (e.g., metallic sodium) and a third alkali metal (e.g., metallic potassium).

Description

[0001] MULTIPHASE ELECTROACTIVE MATERIALS AND ASSOCIATED ARTICLES, SYSTEMS, AND METHODS

[0002] RELATED APPLICATIONS

[0003] This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 63 / 649,185, filed May 17, 2024, and entitled “Multiphase Negative Electrode Material and Associated Methods and Systems,” and U.S. Provisional Patent Application No. 63 / 671,654, filed July 15, 2024, and entitled “Multiphase Negative Electrode Material and Associated Methods and Systems,” both of which are incorporated herein by reference in their entirety for all purposes.

[0004] GOVERNMENT SPONSORSHIP

[0005] This invention was made with Government support under HR0011-2-22-0032 awarded by the Defense Advanced Research Projects Agency. The Government has certain rights in the invention.

[0006] BACKGROUND

[0007] The use of alkali metal electrodes in solid state batteries would enable step-increases in energy density over today’s lithium-ion batteries, while averting issues associated with the use of liquid electrolytes. A failure mode that is known to limit the discharge power, cycle life, and reliability of metal-based solid-state batteries is void formation at the alkali metal-solid electrolyte interface upon lithium metal stripping. Void formation is manifested as impedance rise, power fade, and current-focusing at diminishing contact area at the alkali metal-solid electrolyte interface, which exacerbates dendrite formation in subsequent charge (lithium plating) cycles. High pressures (10-250 MPa) applied to the electrodes have been shown to suppress void formation.

[0008] TECHNICAL FIELD

[0009] Multiphase electroactive materials and associated articles (e.g., electrodes), systems (e.g., electrochemical devices), and methods are generally described herein.

[0010] SUMMARY

[0011] Generally described herein are multiphase electroactive materials and associated articles (e.g., electrodes), systems (e.g., electrochemical devices), and methods. The subject matter of the present disclosure involves, in some cases, interrelated products, alternative solutions to a particular problem, and / or a plurality of different uses of one or more systems and / or articles.

[0012] In some embodiments, an electroactive material is described, the electroactive material comprising: a first component comprising a first alkali metal; and a second component comprising a second alkali metal and a third alkali metal. In certain embodiments, the first alkali metal, the second alkali metal, and the third alkali metal are different alkali metals. In some embodiments, the first component and the second component are intermixed such that the second component is dispersed throughout a bulk of the first component. In certain embodiments, the first component and the second component are substantially immiscible at 20 °C.

[0013] According to some embodiments, a method of manufacturing an electroactive material is described, the method comprising: intermixing: (i) a mixture of a first alkali metal and a second alkali metal; and (ii) a third alkali metal. In certain embodiments, the first alkali metal, the second alkali metal, and the third alkali metal are different alkali metals. In some embodiments, the method further comprises forming the electroactive material comprising a composite structure including: (i) a first component comprising the first alkali metal; and (ii) a second component comprising the second alkali metal and the third alkali metal. In certain embodiments, the first component and the second component are intermixed such that the second component is dispersed throughout a bulk of the first component. In some embodiments, the first component and the second component are substantially immiscible at 20 °C.

[0014] According to certain embodiments, an electroactive material comprises: a first component comprising an alkali metal; and a second component. In some embodiments, the first component and the second component are intermixed such that the second component is dispersed throughout a bulk of the first component. In certain embodiments, the first component and the second component are immiscible at 20 °C.

[0015] In some embodiments, the alkali metal is a first alkali metal, and the second component comprises a second alkali metal.

[0016] According to certain embodiments, the second component comprises metallic sodium, metallic potassium, metallic rubidium, and / or combinations thereof.

[0017] In some embodiments, the second component comprises a transition metal.

[0018] According to some embodiments, the second component comprises metallic molybdenum.

[0019] In certain embodiments, the second component comprises a plurality of metallic molybdenum nanoparticles. According to certain embodiments, an electroactive material comprises: a first component comprising a first alkali metal; and a second component comprising a second alkali metal. In certain embodiments, the second alkali metal is softer than the first alkali metal. In some embodiments, the first component and the second component are intermixed such that the second component is dispersed throughout a bulk of the first component. In certain embodiments, the first component and the second component are immiscible at 20 °C.

[0020] According to some embodiments, an electroactive material comprises: a first component comprising metallic lithium; and a second component comprising metallic potassium. In certain embodiments, the first component and the second component are intermixed such that the second component is dispersed throughout a bulk of the first component. In some embodiments, the first component and the second component are immiscible at 20 °C.

[0021] According to certain embodiments, an electrochemical device is described. In certain embodiments, the electrochemical device comprises an electrode comprising an electroactive material as described herein.

[0022] In certain embodiments, the electrochemical device further comprises a solid electrolyte disposed on the electrode.

[0023] According to some embodiments, a method of manufacturing an electroactive material comprises intermixing a first component and a second component. In certain embodiments, the first component comprises an alkali metal. In some embodiments, the method further comprises forming a composite structure including the first component and the second component intermixed such that the second component is dispersed throughout a bulk of the first component. In certain embodiments, the first component and the second component are immiscible at 20 °C.

[0024] In certain embodiments, the alkali metal is a first alkali metal, and the second component comprises a second alkali metal.

[0025] In some embodiments, the second component comprises a transition metal.

[0026] According to certain embodiments, the second component comprises metallic molybdenum.

[0027] In some embodiments, the second component comprises a plurality of metallic molybdenum nanoparticles.

[0028] According to some embodiments, a method of manufacturing an electroactive material comprises intermixing a first component and a second component. In certain embodiments, the first component comprises a first alkali metal and the second component comprises a second alkali metal. In some embodiments, the method further comprises forming a composite structure including the first component and the second component intermixed such that the second component is dispersed throughout a bulk of the first component. In certain embodiments, the first component and the second component are immiscible at 20 °C.

[0029] In some embodiments, the method further comprises layering the first component and the second component, thereby forming a layered structure comprising a layer of the second component disposed over a layer of the first component.

[0030] In certain embodiments, the method further comprises deforming and folding the first component and the second component a plurality of times.

[0031] In some embodiments, the first alkali metal comprises metallic lithium and the second alkali metal comprises metallic potassium.

[0032] In certain embodiments, the electroactive material comprises the second component in an amount greater than or equal to 0.01 volume percent (vol.%) and less than or equal to 15 vol.% versus a total volume of the electroactive material.

[0033] According to some embodiments, the second component forms a plurality of discrete phase domains dispersed within the first component, wherein an average maximum characteristic dimension of the plurality of discrete phase domains is less than or equal to 1 micrometer.

[0034] According to certain embodiments, a grain size of the first component and / or the second component is less than or equal to 20 micrometers.

[0035] In some embodiments, the grain size of the first component and / or the second component is greater than or equal to 0.01 micrometers.

[0036] Other advantages and novel features of the present disclosure will become apparent from the following detailed description of various non-limiting embodiments of the disclosure when considered in conjunction with the accompanying figures. In cases where the present specification and a document incorporated by reference include conflicting and / or inconsistent disclosure, the present specification shall control.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Non-limiting embodiments of the present disclosure will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale unless otherwise indicated. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the disclosure shown where illustration is not necessary to allow those of ordinary skill in the art to understand the disclosure. In the figures:

[0039] FIG. 1 is a cross-sectional schematic illustration of an electroactive material, according to certain embodiments.

[0040] FIG. 2 is a schematic illustration representing a method of manufacturing an electroactive material, according to certain embodiments.

[0041] FIG. 3 is a cross-sectional schematic illustration of an electrochemical device, according to certain embodiments.

[0042] FIG. 4A is a cross-sectional schematic illustration of an electrochemical device during discharging, according to certain embodiments.

[0043] FIG. 4B is a cross-sectional schematic illustration of an electrochemical device during charging, according to certain embodiments.

[0044] FIG. 5 is a cross-sectional schematic diagram of an electrochemical cell, according to certain embodiments.

[0045] FIG. 6 is a box-and-whisker plot comparing Li-K (Li with 10 and 50 vol.% K) to a Li control for a stepped galvanostatic cycling test, according to certain embodiments.

[0046] FIG. 7 shows a typical test result for a unidirectional galvanostatic (constant current) test, according to certain embodiments.

[0047] FIG. 8 shows WE potential profiles displaying a self-regulatory oscillatory response of Li-K cells with 2 vol.% K at a variety of constant current densities, according to certain embodiments.

[0048] FIG. 9 is a magnification of the box shown in FIG. 8, according to certain embodiments.

[0049] FIG. 10 shows capacities for various metals added to Li, according to certain embodiments.

[0050] FIG. 11 shows reversibility (54 cycles) of symmetric cells including a Li-K anode, according to certain embodiments.

[0051] FIG. 12A shows a transverse cross section SEM image (Ga+-FIB milling under cryogenic conditions, 1.5 kV, 5 nA) of an electrode with 4 vol.% K added to low purity Li metal, according to certain embodiments.

[0052] FIG. 12B shows a transverse cross section from an X-ray microtomography reconstruction (70 kV, 8.5 W, 1601 projections each with 35 second exposure, room temperature) of an electrode with 10 vol.% K added to low purity Li metal, according to certain embodiments. FIG. 13 shows a log scale of critical capacity versus volume fraction of K added to low purity Li for unidirectional galvanostatic (constant current) tests at <0.2 MPa applied stack pressure and 20-30 °C (room temperature), according to certain embodiments.

[0053] FIG. 14 shows a log scale of critical capacity for different electrode purity levels and compositions under the same testing conditions as in FIG. 13, according to certain embodiments.

[0054] FIG. 15A shows a voltage profile of a high performing LP-Li-4K / LLZTO / LP-Li-4K symmetric cell under unidirectional galvanostatic conditions (0.7 mA cm'2) and low stack pressure (0.2 MPa) in comparison to three LP-Li cells, according to certain embodiments.

[0055] FIG. 15B is a magnification of the voltage / resistance profile box shown in FIG. 15 A, according to certain embodiments.

[0056] FIG. 15C shows Nyquist impedance plots corresponding to the highlighted DC resistance points in FIG. 15B, according to certain embodiments.

[0057] FIG. 15D shows that reversible changes in DC resistance and AC impedance correspond to changes in interfacial contact area, as illustrated in the schematic with the expansion and contraction of voids at the electrode / electrolyte interface, according to certain embodiments.

[0058] FIG. 16A shows voltage profiles of representative LP-Li-4K / LLZTO / LP-Li-4K symmetric cells under 4 applied stack pressures (0.2, 0.8, 1.4, 2.8 MPa) and unidirectional 0.7 mA cm'2nominal current density, according to certain embodiments.

[0059] FIG. 16B shows critical capacity versus applied stack pressure for N = 19 total cells under galvanostatic conditions and room temperature (0.7 mA cm'2, 20-30 °C), including cells in FIG. 16A, according to certain embodiments.

[0060] FIGS. 16C-16F show repeated resistance profiles for each pressure in FIG. 16A, according to certain embodiments.

[0061] FIG. 17A shows voltage and resistance profiles of a pristine cell (cell 1) and a cell paused during unidirectional galvanostatic stripping at 0.4 mA cm'2under 0.8 MPa stack pressure (cell 2) with LP-Li-4K / LLZTO / LP-Li-4K configurations, according to certain embodiments.

[0062] FIG. 17B shows transverse cross-sections of cells 1 and 2 prepared using ex-situ plasma focused ion beam (PFIB) milling with a Xe+source under cryogenic (liquid nitrogen) conditions, according to certain embodiments.

[0063] FIG. 17C shows a SEM image of cell 1, a pristine LP-Li-4K / LLZTO interface (scale bar, 10 pm), according to certain embodiments. FIG. 17D shows an interfacial void at the center, adjacent to Li / LLZTO interfaces (primarily on the right), and a build-up of Na-K / K (primarily on the left), according to certain embodiments.

[0064] FIG. 17E shows Li / LLZTO interfaces on the far left and right on either side of a Na-K / K layer enclosing a void, according to certain embodiments.

[0065] FIG. 17F shows smaller Na-K / K interfacial regions between Li / LLZTO interfaces, according to certain embodiments.

[0066] FIG. 18A shows a typical voltage or resistance profile versus time or capacity, with key timepoints numbered, according to certain embodiments.

[0067] FIG. 18B shows a proposed interfacial mechanism that describes the oscillatory profile by focusing on changes in local current density and local stress, according to certain embodiments.

[0068] FIG. 18C shows a void with low aspect ratio constrained horizontally between two layers of Na-K / K, according to certain embodiments.

[0069] FIG. 18D shows a void nucleating from an Li / LLZTO interface in the late stages of stripping, according to certain embodiments.

[0070] FIG. 18E shows an interface of a cell stripped at 0.2 MPa (lower pressure) with Na-K wetting the top Li surface of a void (left), and a void constrained by Na-K on one side and a small Li / LLZTO contact on the other side (center right), according to certain embodiments.

[0071] FIG. 19 shows the steady-state creep strain rate at low stack pressure comparing LP-Li- 4K to HP-Li.

[0072] FIG. 20 shows the hardness and associated indentation strain rates from Vickers hardness tests comparing LP-Li-4K to HP-Li.

[0073] DETAILED DESCRIPTION

[0074] Performance of alkali metal electrodes, such as Li metal anodes, is tied to their ability to cycle with high coulombic efficiency at high current densities and capacities. In conventional batteries (e.g., solid-state batteries) that include an alkali metal electrode, coulombic efficiency degrades due to the formation of voids at an interface between the alkali metal electrode and the electrolyte (e.g., solid-state electrolyte) during discharge. For example, alkali metal vacancies form in conventional batteries due to a flux imbalance between the rate of electrode dissolution during stripping and the rate of alkali metal ion transport to the electrode / electrolyte interface. The alkali metal vacancies coalesce into voids and subsequently grow during cycling, resulting in a morphological instability. Interfacial contact between the alkali metal electrode and the electrolyte is diminished as the voids extend laterally across the alkali metal electrode / electrolyte interface, resulting in increases in local current density at the remaining interfacial contact areas. The diminished contact area at the alkali metal electrode / electrolyte interface further exacerbates dendrite formation during charge. Such void and dendrite formation during battery cycling (e.g., discharge and charge) may lead to a shortened battery life and / or battery failure.

[0075] Stack pressure may be applied to electrochemical devices to avoid or otherwise reduce the formation of voids at the electrode / electrolyte interface. The inventors have recognized that conventional devices, however, are subject to commercially impractical high pressures above 1 megapascal (MPa) to achieve void suppression. Furthermore, conventional methods of pressure application to electrochemical devices, such as applying compression pads to Li-ion pouch cells, detract from pack- and module-level energy density of the devise.

[0076] Alloying strategies in conventional Li-ion batteries (e.g., using Li-In, Li-Mg, Li- Al, and Li-Sn electrodes) have been employed to improve low stack pressure performance of Li ion batteries, but such strategies have not reached practically useful capacity and current density targets, for example, 3 mAh cm'2at currents exceeding 1 mA cm'2. Other strategies such as reducing the grain size of Li metal to enhance Li transport show decreased performance rather than increased performance. Use of interlayers (e.g., an Ag / Ag-C composite layer or a Na-K liquid alloy layer) has been shown to achieve practically useful capacity and current density targets with no external pressure, but the use of such interlayers presents scalability challenges.

[0077] The inventors have realized and appreciated that multiphase electroactive materials comprising a first component and a second component that is softer than the first component can help to reduce void formation and improve battery performance, for example, by achieving high capacities at high current densities, low stack pressures, and room temperature. In some embodiments, by tailoring the composition and microstructure of the electroactive material using such compositions, void formation at an interface between an electrode comprising the electroactive material and an electrolyte (e.g., solid electrolyte) disposed on the electrode is advantageously mitigated, reduced, and / or avoided. The generally disclosed multiphase electroactive materials may be used separately and / or in associated articles (e.g., electrodes), systems (e.g., electrochemical devices), and / or methods as detailed herein.

[0078] According to certain embodiments, an electroactive material comprises a composite structure (e.g., a composite alloy). For example, in certain embodiments, the first component and the second component are intermixed such that the second component is dispersed throughout a bulk of the first component. In some embodiments, the second component forms a plurality of discrete phase domains dispersed within the first component. According to some embodiments, the second component is a comparatively softer phase than the first component and is capable of being deformed (e.g., plastically deformed or otherwise undergoing a permanent deformation) under an applied pressure within a desired pressure range. The first component may not be substantially deformable in the same pressure range. For example, the second component composition may be selected such that it is deformable under relatively low stack pressures (e.g., less than or equal to 1 MPa). By including this softer deformable second component that is dispersed throughout a bulk of the first component, it may be possible to advantageously mitigate, reduce, and / or avoid the formation of voids in the electroactive material when appropriate stack pressures are applied to the material. For example, void formation at one or more grain boundaries of the first component and / or the second component and / or at the electrode / electrolyte interface may be reduced. In some embodiments, electrochemical devices comprising the electroactive materials described herein therefore may advantageously avoid the use of large stack pressures (e.g., greater than 1 MPa) that are associated with conventional devices and which result in comparatively lower energy densities. That said, while the disclosed materials are capable of being used and avoiding void formation at these lower stack pressures, other stack pressures greater than this may also be used as the disclosure is not so limited.

[0079] In certain embodiments, the first component comprises a first alkali metal and the second component comprises a second alkali metal and a third alkali metal (e.g., the second component comprises an alloy). In some embodiments, the first alkali metal, the second alkali metal, and the third alkali metal are different alkali metals. In certain non-limiting embodiments, for example, the first alkali metal comprises metallic lithium, the second alkali metal comprises metallic sodium, and the third alkali metal comprises metallic potassium. Other combinations of materials and alkali metals for the first component and the second component are possible and are described herein in greater detail.

[0080] According to some embodiments, the first component and the second component are substantially immiscible at 20 °C. As used herein, “substantially immiscible,” with respect to the first component and the second component, refers to the first component and the second component being largely but not wholly incapable of being mixed and / or becoming a homogenous mixture. In some embodiments, for example, less than or equal to 10 wt.%, less than or equal to 5 wt.%, less than or equal to 2 wt.%, less than or equal to 1 wt.%, less than or equal to 0.1 wt.%, or less, of one component based on a total weight of the one component is capable of being mixed and / or becoming a homogeneous mixture with the other component at 20 °C. In some embodiments, the first component and the second component have a mutual solubility less than 1 wt.%. In certain embodiments, using a first component and a second component that are substantially immiscible at 20 °C advantageously maintains the soft and deformable second component as a phase that is separate from the first component so that the second component may suppress void formation in the electroactive material (e.g., at one or more grain boundaries of the first component and / or the second component, at an electrode / electrolyte interface) at practical battery operating temperatures (e.g., room temperature) without dissolving and / or otherwise forming a homogeneous mixture with the first component.

[0081] In certain embodiments, the second component is a eutectic mixture (e.g., a eutectic alloy). As used herein, “eutectic mixture” is given its ordinary meaning in the field of chemistry and refers to a mixture of substances that has a melting point lower than those of its constituents. In certain embodiments, the second component is a liquid eutectic mixture at 20 °C. For example, in certain non-limiting embodiments, the second component is liquid eutectic mixture of metallic sodium and metallic potassium at 20 °C.

[0082] According to some embodiments, the multiphase electroactive materials are produced by mixing (e.g., intermixing) a first component and a second component. In certain embodiments, the multiphase electroactive materials may be produced by mixing: (i) a mixture (e.g., an alloy) of the first alkali metal and a small amount of the second alkali metal; with (ii) a small amount of the third alkali metal. For example, according to certain embodiments, the electroactive material is manufactured by mixing (e.g., intermixing): (i) a mixture (e.g., an alloy) of the first alkali metal (e.g., metallic lithium) and the second alkali metal (e.g., metallic sodium); and (ii) a third alkali metal (e.g., metallic potassium). In certain non-limiting embodiments, the mixture of the first alkali metal and the second alkali is low purity metallic lithium (e.g., 99 wt.% metallic lithium) with high levels of a metallic sodium impurity (e.g., 1 wt.% metallic sodium). According to some embodiments, the electroactive materials described herein therefore take advantage of the impurities (e.g., metallic sodium impurity) present in low cost, low purity metallic lithium.

[0083] In some embodiments, a method of manufacturing an electroactive material comprises layering one or more layers of each of: (i) the mixture of the first alkali metal and the second alkali metal; and (ii) the third alkali metal. In some embodiments, the layering forms a layered structure comprising a layer of the third alkali metal disposed over a layer of the mixture of the first alkali metal and the second alkali metal. In some embodiments, the layered structure may include alternatingly stacked layers of the mixture of the first and second alkali metals with the third alkali metal. The method may further comprise deforming and folding: (i) the mixture of - li the first alkali metal and the second alkali metal; and (ii) the third alkali metal. For example, in some embodiments, the method comprises deforming and folding the layered structure comprising the layer of the third alkali metal disposed over the layer of the mixture of the first alkali metal and the second alkali metal, though other methods for mixing materials including ball milling, melt quenching, electrowinning, chemical vapor deposition (CVD), physical vapor deposition (PVD), and / or any other appropriate method for mixing the materials may also be used as the disclosure is not so limited. In certain embodiments, the intermixing (e.g., the layering, the deforming, and the folding) causes the second alkali metal and the third alkali metal to combine, thereby forming a composite structure comprising the second component (e.g., comprising the second alkali metal and the third alkali metal) dispersed throughout a bulk of the first component (e.g., comprising the first alkali metal).

[0084] Electrochemical devices (e.g., electrochemical cells) comprising an electrode comprising the electroactive material, and related methods of operating the electrochemical devices, are also described. In some embodiments, an electrochemical device comprises an electrolyte (e.g., a solid electrolyte) disposed on the electrode. Suitable electrolytes are described herein in greater detail. In certain embodiments, the second component advantageously enhances diffusional transport of an ion of the first alkali metal, for example, to an interface between an electrode comprising the electroactive material and an electrolyte disposed on the electrode.

[0085] Methods of operating an electrochemical device comprise discharging, charging, and / or cycling (e.g., discharging and charging) the electrochemical device. In some embodiments, during discharging, the first alkali metal of the electroactive material (e.g., metallic lithium) is oxidized to a first alkali metal ion (e.g., a lithium ion), which decreases the volume fraction of the first component in the electroactive material and increases the volume fraction of the second component in the electroactive material. One or more vacancies formed by discharging the electrochemical device and the first alkali may coalesce to form voids in the electroactive material, in accordance with certain embodiments. In certain embodiments, deformation of the second component (e.g., the soft and deformable second component) advantageously mitigates, reduces, and / or avoids the formation of voids in the electroactive material, for example, at one or more grain boundaries of the first component and / or the second component and / or at the electrode / electrolyte interface.

[0086] According to some embodiments, an advantageously low stack pressure (e.g., less than or equal to 1 MPa) may be applied to the electrochemical device during charging and / or discharging. In certain embodiments, the electrochemical device advantageously provides an increase in specific capacity at the advantageously low stack pressure as compared to an electrochemical device that is otherwise equivalent but does not include the electroactive material. In accordance with some embodiments, high flux of the ions of the first alkali metal through the second component (e.g., at edges of voids at the electrode / electrolyte interface) allows sufficient time for creep deformation of the alkali metal to re-establish contact with the electrolyte at the interface as the low stack pressure is applied. In certain embodiments, the second component at the void edges further mitigates the intensification of local current on the interfacial contact between the electrode and the electrolyte and avoids dendrite formation during charging. The composite structure therefore advantageously enables the passage of high capacities at high current densities, low pressure, and room temperature without the use of impractically high stack pressures.

[0087] The electroactive materials disclosed herein may comprise the first component in any of a variety of suitable amounts. In some embodiments, for example, the electroactive material comprises the first component in an amount greater than or equal to 85 wt.%, greater than or equal to 90 wt.%, greater than or equal to 95 wt.%, greater than or equal to 99 wt.%, or greater than or equal to 99.9 wt.% based on a total weight of the electroactive material. In certain embodiments, the electroactive material comprises the first component in an amount less than or equal to 99.99 wt.%, less than or equal to 99.9 wt.%, less than or equal to 99 wt.%, less than or equal to 95 wt.%, or less than or equal to 90 wt.% based on a total weight of the electroactive material. Combinations of the above recited ranges are possible (e.g., the electroactive material comprises the first component in an amount greater than or equal to 85 wt.% and less than or equal to 99.99 wt.% based on a total weight of the electroactive material, the electroactive material comprises the first component in an amount greater than or equal to 95 wt.% and less than or equal to 99.9 wt.% based on a total weight of the electroactive material). Other ranges are also possible.

[0088] In some embodiments, the first component is or comprises a first alkali metal. The first alkali metal may be any of a variety of suitable alkali metals. In some embodiments, for example, the first alkali metal is selected from the group consisting of metallic lithium (Li), metallic sodium (Na), metallic potassium (K), metallic rubidium (Rb), metallic cesium (Cs), and metallic francium (Fr). In certain non-limiting embodiments, the first alkali metal is metallic lithium.

[0089] The first component may comprise the first alkali metal in any of a variety of suitable amounts. In some embodiments, for example, the first component comprises the first alkali metal in an amount greater than or equal to 90 wt.%, greater than or equal to 95 wt.%, greater than or equal to 99 wt.%, greater than or equal to 99.9 wt.%, or greater than or equal to 99.99 wt.% based on a total weight of the first component. In certain embodiments, the first component comprises the first alkali metal in an amount less than or equal to 100 wt.%, less than or equal to 99.99 wt.%, less than or equal to 99.9 wt.%, less than or equal to 99 wt.%, or less than or equal to 95 wt.% based on a total weight of the first component. Combinations of the above recited ranges are possible (e.g., the first component comprises the first alkali metal in an amount greater than or equal to 90 wt.% and less than or equal to 100 wt.% based on a total weight of the first component, the first component comprises the first alkali metal in an amount greater than or equal to 99 wt.% and less than or equal to 99.9 wt.% based on a total weight of the first component). Other ranges are also possible. In certain non-limiting embodiments, the first component comprises the first alkali metal in an amount of 100 wt.% based on a total weight of the first component.

[0090] The first component may have any of a variety of suitable grain sizes. In some embodiments, it may be advantageous for the first component to have a substantially small grain size (e.g., less than or equal to 50 micrometers). For example, in certain embodiments, a first component having a substantially small grain size increases the yield stress of the first component as compared to a material that is otherwise equivalent but has a larger grain size (e.g., greater than 50 micrometers). In some embodiments, the first component has a grain size less than or equal to 50 micrometers, less than or equal to 40 micrometers, less than or equal to 30 micrometers, less than or equal to 20 micrometers, less than or equal to 10 micrometers, less than or equal to 1 micrometer, or less than or equal to 0.1 micrometers. In certain embodiments, the first component has a grain size greater than or equal to 0.01 micrometers, greater than or equal to 0.1 micrometers, greater than or equal to 1 micrometer, greater than or equal to 10 micrometers, greater than or equal to 20 micrometers, greater than or equal to 30 micrometers, or greater than or equal to 40 micrometers. Combinations of the above recited ranges are possible (e.g., the first component has a grain size less than or equal to 50 micrometers and greater than or equal to 0.01 micrometers, the first component has a grain size less than or equal to 20 micrometers and greater than or equal to 10 micrometers). Other ranges are also possible.

[0091] According to some embodiments, the grain size of the first component is measured by scanning electron microscopy (SEM) and electron channeling contrast imaging (ECO).

[0092] The electroactive material may comprise the second component in any of a variety of suitable amounts. In some embodiments, for example, the electroactive material comprises the second component in an amount greater than or equal to 0.01 wt.%, greater than or equal to 0.1 wt.%, greater than or equal to 1 wt.%, greater than or equal to 5 wt.%, or greater than or equal to 10 wt.% based on a total weight of the electroactive material. In certain embodiments, the electroactive material comprises the second component in an amount less than or equal to 15 wt.%, less than or equal to 10 wt.%, less than or equal to 5 wt.%, less than or equal to 1 wt.%, or less than or equal to 0.1 wt.% based on a total weight of the electroactive material.

[0093] Combinations of the above recited ranges are possible (e.g., the electroactive material comprises the second component in an amount greater than or equal to 0.01 wt.% and less than or equal to 15 wt.% based on total weight of the electroactive materials, the electroactive material comprises the second component in an amount greater than or equal to 0.1 wt.% and less than or equal to 5 wt.% based on a total weight of the electroactive material). Other ranges are also possible.

[0094] In certain embodiments, the second component comprises a second alkali metal (e.g., an alkali metal that is different than the first alkali metal). In some embodiments, the second component also comprises a third alkali metal (e.g., an alkali metal that is different than the first alkali metal and the second alkali metal). The second alkali metal and the third alkali metal may be any of a variety of suitable alkali metals that are different from the first alkali metal and each other. In some embodiments, the second alkali metal and the third alkali metal are separately selected from the group consisting of metallic lithium (Li), metallic sodium (Na), metallic potassium (K), metallic rubidium (Rb), metallic cesium (Cs), and metallic francium (Fr). In certain embodiments wherein the first alkali metal is metallic lithium (Li), the second alkali metal and the third alkali metal are separately selected from the group consisting of metallic sodium (Na), metallic potassium (K), metallic rubidium (Rb), metallic cesium (Cs), and metallic francium (Fr). In certain non-limiting embodiments, the second alkali metal is metallic sodium. In some non-limiting embodiments, the third alkali metal is metallic potassium.

[0095] In certain embodiments, the second component is an alloy. In some embodiments, the alloy comprises at least two alkali metals. For example, in certain embodiments the second component is a metallic sodium-bearing alloy. In some non-limiting embodiments, the second component is an alloy of metallic sodium and metallic potassium.

[0096] The second component may comprise the second alkali metal in any of a variety of suitable amounts. In certain embodiments, for example, the second component comprises the second alkali metal in an amount greater than or equal to 1 wt.%, greater than or equal to 10 wt.%, greater than or equal to 20 wt.%, greater than or equal to 30 wt.%, greater than or equal to 40 wt.%, greater than or equal to 50 wt.%, greater than or equal to 60 wt.%, greater than or equal to 70 wt.%, greater than or equal to 80 wt.%, or greater than or equal to 90 wt.% based on a total weight of the second component. In some embodiments, the second component comprises the second alkali metal in an amount less than or equal to 99 wt.%, less than or equal to 90 wt.%, less than or equal to 80 wt.%, less than or equal to 70 wt.%, less than or equal to 60 wt.%, less than or equal to 50 wt.%, less than or equal to 40 wt.%, less than or equal to 30 wt.%, less than or equal to 20 wt.%, or less than or equal to 10 wt.% based on a total weight of the second component. Combinations of the above recited ranges are possible (the second component comprises the second alkali metal in an amount greater than or equal to 1 wt.% and less than or equal to 99 wt.% based on a total weight of the second component, the second component comprises the second alkali metal in an amount greater than or equal to 40 wt.% and less than or equal to 60 wt.% based on a total weight of the second component). Other ranges are also possible.

[0097] The second component may comprise the third alkali metal in any of a variety of suitable amounts. In certain embodiments, for example, the second component comprises the third alkali metal in an amount greater than or equal to 1 wt.%, greater than or equal to 10 wt.%, greater than or equal to 20 wt.%, greater than or equal to 30 wt.%, greater than or equal to 40 wt.%, greater than or equal to 50 wt.%, greater than or equal to 60 wt.%, greater than or equal to 70 wt.%, greater than or equal to 80 wt.%, or greater than or equal to 90 wt.% based on a total weight of the second component. In some embodiments, the second component comprises the third alkali metal in an amount less than or equal to 99 wt.%, less than or equal to 90 wt.%, less than or equal to 80 wt.%, less than or equal to 70 wt.%, less than or equal to 60 wt.%, less than or equal to 50 wt.%, less than or equal to 40 wt.%, less than or equal to 30 wt.%, less than or equal to 20 wt.%, or less than or equal to 10 wt.% based on a total weight of the second component. Combinations of the above recited ranges are possible (e.g., the second component comprises the third alkali metal in an amount greater than or equal to 1 wt.% and less than or equal to 99 wt.% based on total weight of the second component, the second component comprises the third alkali metal in an amount greater than or equal to 40 wt.% and less than or equal to 60 wt.% based on a total weight of the second component). Other ranges are also possible.

[0098] According to some embodiments, the second component comprises one or more additional alkali metals. For example, in some embodiments, the second component comprises a fourth alkali metal, a fifth alkali metal, etc. The one or more additional alkali metals may be any of a variety of suitable alkali metals that are different from the first alkali metal, the second alkali metal, and the third alkali metal. In certain embodiments, the one or more additional alkali metals are selected from the group consisting of metallic sodium (Na), metallic potassium (K), metallic rubidium (Rb), metallic cesium (Cs), and metallic francium (Fr). In some nonlimiting embodiments, the one or more additional alkali metals comprise metallic rubidium and / or metallic cesium. In certain non-limiting embodiments wherein the second component comprises the one or more additional alkali metals, the second component comprises the second alkali metal (e.g., metallic sodium), the third alkali metal (e.g., metallic potassium), the fourth alkali metal (e.g., metallic rubidium), and the fifth alkali metal (e.g., metallic cesium).

[0099] The second component may comprise the one or more additional alkali metals in any of a variety of suitable amounts. In certain embodiments, for example, the second component comprises the one or more additional alkali metals in any of the amounts described herein with respect to the second alkali metal and / or the third alkali metal.

[0100] In some embodiments, the second component comprises a transition metal and / or a lanthanide. The second component may comprise any of a variety of suitable transition metals and / or lanthanides. In some embodiments, for example, the second component comprises metallic molybdenum, metallic niobium, metallic tantalum, metallic nickel, metallic cerium, metallic lanthanum, and / or combinations thereof. In certain embodiments, for example, the second component comprises a plurality of metallic molybdenum nanoparticles, a plurality of metallic niobium nanoparticles, a plurality of metallic tantalum nanoparticles, a plurality of metallic nickel nanoparticles, a plurality of cerium nanoparticles, a plurality of lanthanum nanoparticles, and / or combinations thereof. Other transition metals and / or lanthanides are also possible, as the disclosure is not meant to be limiting in this regard.

[0101] The second component may comprise the transition metal and / or the lanthanide in any of a variety of suitable amounts. In certain embodiments, for example, the second component comprises the transition metal and / or the lanthanide in an amount greater than or equal to 1 wt.%, greater than or equal to 10 wt.%, greater than or equal to 20 wt.%, greater than or equal to 30 wt.%, greater than or equal to 40 wt.%, greater than or equal to 50 wt.%, greater than or equal to 60 wt.%, greater than or equal to 70 wt.%, greater than or equal to 80 wt.%, greater than or equal to 90 wt.%, or greater than or equal to 99 wt.% based on a total weight of the second component. In some embodiments, the second component comprises the transition metal and / or the lanthanide in an amount less than or equal to 99.9 wt.%, less than or equal to 99 wt.%, less than or equal to 90 wt.%, less than or equal to 80 wt.%, less than or equal to 70 wt.%, less than or equal to 60 wt.%, less than or equal to 50 wt.%, less than or equal to 40 wt.%, less than or equal to 30 wt.%, less than or equal to 20 wt.%, or less than or equal to 10 wt.% based on a total weight of the second component. Combinations of the above recited ranges are possible (e.g., the second component comprises the transition metal and / or the lanthanide in an amount greater than or equal to 1 wt.% and less than or equal to 99.9 wt.% based on total weight of the second component, the second component comprises the transition metal and / or the lanthanide in an amount greater than or equal to 40 wt.% and less than or equal to 60 wt.% based on a total weight of the second component). Other ranges are also possible.

[0102] The second component may have any of a variety of suitable grain sizes. In some embodiments, it may be advantageous for the second component to have a substantially small grain size (e.g., less than or equal to 50 micrometers). For example, in certain embodiments, a second component having a substantially small grain size increases the yield stress of the second component as compared to a material that is otherwise equivalent but has a larger grain size (e.g., greater than 50 micrometers). In some embodiments, the second component has a grain size less than or equal to 50 micrometers, less than or equal to 40 micrometers, less than or equal to 30 micrometers, less than or equal to 20 micrometers, less than or equal to 10 micrometers, less than or equal to 1 micrometer, or less than or equal to 0.1 micrometers. In certain embodiments, the second component has a grain size greater than or equal to 0.01 micrometers, greater than or equal to 0.1 micrometers, greater than or equal to 1 micrometer, greater than or equal to 10 micrometers, greater than or equal to 20 micrometers, greater than or equal to 30 micrometers, or greater than or equal to 40 micrometers. Combinations of the above recited ranges are possible (e.g., the second component has a grain size less than or equal to 50 micrometers and greater than or equal to 0.01 micrometers, the second component has a grain size less than or equal to 20 micrometers and greater than or equal to 10 micrometers). Other ranges are also possible.

[0103] According to some embodiments, the grain size of the second component is measured by SEM and ECCI.

[0104] As described herein in greater detail, the second component may form a plurality of discrete phase domains dispersed within a matrix of the first component, in accordance with certain embodiments. The plurality of discrete phase domains may have any of a variety of suitable maximum characteristic dimensions. In certain embodiments, for example, the plurality of discrete phase domains has an average maximum characteristic dimension less than or equal to 50 micrometers, less than or equal to 40 micrometers, less than or equal to 30 micrometers, less than or equal to 20 micrometers, less than or equal to 10 micrometers, less than or equal to 5 micrometers, less than or equal to 1 micrometer, or less than or equal to 0.1 micrometers. In some embodiments, the plurality of discrete phase domains has an average maximum characteristic dimension greater than or equal to 0.01 micrometers, greater than or equal to 0.1 micrometers, greater than or equal to 1 micrometer, greater than or equal to 5 micrometers, greater than or equal to 10 micrometers, greater than or equal to 20 micrometers, or greater than or equal to 30 micrometers. Combinations of the above recited ranges are possible (e.g. the plurality of discrete phase domains has an average maximum characteristic dimension less than or equal to 50 micrometers and greater than or equal to 0.01 micrometers, the plurality of discrete phase domains has an average maximum characteristic dimension less than or equal to 20 micrometers and greater than or equal to 10 micrometers).

[0105] Turning now to the figures, specific non-limiting embodiments are described in more detail. It should be understood that various features of the separately described embodiments may be used together as the current disclosure is not limited to the specific embodiments depicted in the figures and described below.

[0106] FIG. 1 is a cross-sectional schematic illustration of electroactive material 100, according to certain embodiments. In some embodiments, as described herein in greater detail, electroactive material 100 comprises first component 102 (e.g., comprising a first alkali metal) and second component 104 (e.g., comprising a second alkali metal and a third alkali metal). According to certain embodiments, first component 102 and second component 104 are intermixed such that second component 104 is dispersed throughout a bulk of first component 102. For example, in some embodiments, second component 104 forms a plurality of discrete phase domains dispersed within a matrix of the first component 102.

[0107] The electroactive material may have any of a variety of suitable thicknesses. According to some embodiments, referring to FIG. 1, electroactive material has thickness 110. In certain embodiments, the thickness of the electroactive material is measured from a first surface of the electroactive material that is configured to be in contact with an electrolyte (e.g., as described herein in greater detail and shown in FIG. 3) to a second surface of the electroactive material that is substantially opposite the first surface. In certain embodiments, the electroactive material has a thickness greater than or equal to 1 micrometer, greater than or equal to 5 micrometers, greater than or equal to 10 micrometers, greater than or equal to 20 micrometers, greater than or equal to 50 micrometers, greater than or equal to 100 micrometers, or greater than or equal to 200 micrometers. In some embodiments, the electroactive material has a thickness less than or equal to 300 micrometers, less than or equal to 200 micrometers, less than or equal to 100 micrometers, less than or equal to 50 micrometers, less than or equal to 20 micrometers, less than or equal to 10 micrometers, or less than or equal to 5 micrometers. Combinations of the above recited ranges are possible (e.g., the electroactive material has a thickness greater than or equal to 1 micrometer and less than or equal to 300 micrometers, the electroactive material has a thickness greater than or equal to 50 micrometers and less than or equal to 100 micrometers). Other ranges are also possible. In certain embodiments, the electroactive material is manufactured by mixing (e.g., intermixing) the first component and the second component. FIG. 2 is a schematic illustration representing a method of manufacturing electroactive material 100, according to certain embodiments. In some embodiments, the method comprises intermixing: (i) mixture 202 of first alkali metal 204 and second alkali metal 206; and (ii) third alkali metal 208, wherein first alkali metal 204, second alkali metal 206, and third alkali metal 208 are different alkali metals. For example, in certain non-limiting embodiments, the first alkali metal is metallic lithium, the second alkali metal is metallic sodium, and the third alkali metal is metallic potassium. In some non-limiting embodiments, the mixture is low purity metallic lithium (e.g., 99 wt.% metallic lithium) with high levels of a metallic sodium impurity (e.g., 1 wt.% metallic sodium), and the third alkali metal is metallic potassium, such as low purity metallic potassium (e.g., 98 wt.% metallic potassium).

[0108] The mixture (e.g., mixture 202) of the first alkali metal (e.g., first alkali metal 204) and the second alkali metal (e.g., second alkali metal 206) may comprise the first alkali metal in any of a variety of suitable amounts. In certain embodiments, for example, the mixture of the first alkali metal and the second alkali metal comprises the first alkali metal in an amount greater than or equal to 90 wt.%, greater than or equal to 95 wt.%, greater than or equal to 99 wt.%, greater than or equal to 99.9 wt.% based on a total weight of the mixture. In some embodiments, the mixture of the first alkali metal and the second alkali metal comprises the first alkali metal in an amount less than or equal to 99.99 wt.%, less than or equal to 99.9 wt.%, less than or equal to 99 wt.%, or less than or equal to 95 wt.% based on a total weight of the mixture. Combinations of the above recited ranges are possible (e.g., the mixture of the first alkali metal and the second alkali metal comprises the first alkali metal in an amount greater than or equal to 90 wt.% and less than or equal to 99.99 wt.% based on a total weight of the mixture, the mixture of the first alkali metal and the second alkali metal comprises the first alkali metal in an amount greater than or equal to 99 wt.% and less than or equal to 99.9 wt.% based on a total weight of the mixture). Other ranges are also possible.

[0109] The mixture (e.g., mixture 202) of the first alkali metal (e.g., first alkali metal 204) and the second alkali metal (e.g., second alkali metal 206) may comprise the second alkali metal in any of a variety of suitable amounts. In certain embodiments, for example, the mixture of the first alkali metal and the second alkali metal comprises the second alkali metal in an amount greater than or equal to 0.01 wt.%, greater than or equal to 0.1 wt.%, greater than or equal to 1 wt.%, or greater than or equal to 5 wt.% based on a total weight of the mixture. In some embodiments, the mixture of the first alkali metal and the second alkali metal comprises the second alkali metal in an amount less than or equal to 10 wt.%, less than or equal to 5 wt.%, less than or equal to 1 wt.%, or less than or equal to 0.1 wt.% based on a total weight of the mixture. Combinations of the above recited ranges are possible (e.g., the mixture of the first alkali metal and the second alkali metal comprises the second alkali metal in an amount greater than or equal to 0.01 wt.% and less than or equal to 10 wt.% based on total weight of the mixture, the mixture of the first alkali metal and the second alkali metal comprises the second alkali metal in an amount greater than or equal to 0.1 wt.% and less than or equal to 1 wt.% based on a total weight of the mixture).

[0110] According to some embodiments, the method comprises layering: (i) mixture 202 of first alkali metal 204 and second alkali metal 206; and (ii) third alkali metal 208. In some embodiments, the layering forms layered structure 210 comprising layer 212a of third alkali metal 208 disposed over layer 212b of the mixture (e.g., mixture 202) of first alkali metal 204 and second alkali metal 206. This may include either single layers of the separate materials or altematingly stacked layers of the different materials.

[0111] According to certain embodiments, the method comprises deforming and folding: (i) the mixture (e.g., mixture 202) of first alkali metal 204 and second alkali metal 206; and (ii) third alkali metal 208. In some embodiments, for example, the method comprises deforming and folding layered structure 210 comprising layer 212a of the third alkali metal 208 disposed over layer 212b of the mixture (e.g., mixture 202) of first alkali metal 204 and second alkali metal 206. The mixture (e.g. of the first alkali metal and the second alkali metal) and the third alkali metal may be repeatedly deformed and folded (e.g., at least 2 times, at least 5 times, at least 10 times, at least 20 times, at least 50 times, or more). Of course, while folding is disclosed other methods for mixing the materials may also be used including, but not limited to, ball milling, melt quenching, electrowinning, CVD, PVD, and / or combinations thereof. In certain embodiments, melt quenching comprises melting the components (e.g., the first alkali metal, the second alkali metal, and the third alkali metal) to form a melt, and allowing the melt to phase separate at a lower temperature. Melt quenching may optionally include mechanical deformation (e.g., of the melt). According to certain embodiments, electrowinning is performed from one or more solutions comprising ions of the components (e.g., ions of the first alkali metal, the second alkali metal, and the third alkali metal).

[0112] In certain embodiments, the deforming and folding comprises calendering: (i) the mixture (e.g., mixture 202) of first alkali metal 204 and second alkali metal 206; and (ii) third alkali metal 208. For example, in some embodiments, layered structure 210 comprising layer 212a of third alkali metal 208 disposed over layer 212b of the mixture (e.g., mixture 202) of first alkali metal 204 and second alkali metal 206 is passed between calender rollers at any of a variety of suitable temperatures and / or pressures. In certain embodiments, during the calendering, layer 212a of third alkali metal 208 becomes increasingly thin and breaks up into particulates. According to some embodiments, as third alkali metal 208 breaks up into particulates, it combines with second alkali metal 206 to form second component 104.

[0113] The temperature of the calendering process may be any of a variety of suitable temperatures. In some embodiments, for example, the temperature is greater than or equal to - 196 °C, greater than or equal to -150 °C, greater than or equal to -100 °C, greater than or equal to -50 °C, greater than or equal to 0 °C, greater than or equal to 50 °C, greater than or equal to 100 °C, or greater than or equal to 150 °C. In certain embodiments, the temperature is less than or equal to 180 °C, less than or equal to 150 °C, less than or equal to 100 °C, less than or equal to 50 °C, less than or equal to 0 °C, less than or equal to -50 °C, less than or equal to -100 °C, or less than or equal to -150 °C. Combinations of the above recited ranges are possible (e.g., the temperature is greater than or equal to -196 °C and less than or equal to 180 °C, the temperature is greater than or equal to 0 °C and less than or equal to 50 °C). Other ranges are also possible.

[0114] According to some embodiments, the temperature of the calendering process is slightly below the melting point of the first alkali metal (e.g., the temperature is 5 °C less, 2 °C less, 1 °C less, 0.5 °C less, or 0.1 °C less than the melting point of the first alkali metal).

[0115] The pressure of the calendering process may be any of a variety of suitable pressures. In certain embodiments, for example, the pressure is greater than or equal to 0.1 MPa, greater than or equal to 0.5 MPa, greater than or equal to 1 MPa, greater than or equal to 5 MPa, greater than or equal to 10 MPa, or greater than or equal to 15 MPa. In some embodiments, the pressure is less than or equal to 20 MPa, less than or equal to 15 MPa, less than or equal to 10 MPa, less than or equal to 5 MPa, less than or equal to 1 MPa, or less than or equal to 0.5 MPa. Combinations of the above recited ranges are possible (e.g., the pressure is greater than or equal to 0.1 MPa and less than or equal to 20 MPa, the pressure is greater than or equal to 5 MPa and less than or equal to 10 MPa). Other ranges are also possible.

[0116] In certain embodiments, the method comprises forming electroactive material 100 comprising a composite structure. As described herein in greater detail, the composite structure includes first component 102 (e.g., comprising the first alkali metal) and second component 104 (e.g., comprising the second alkali metal and the third alkali metal), in accordance with certain embodiments.

[0117] FIG. 3 is a cross-sectional schematic illustration of electrochemical device 300, according to certain embodiments. Electrochemical device 300 may, in certain embodiments, be a battery (e.g., a primary battery, a secondary or rechargeable battery), a capacitor, a supercapacitor, or any other appropriate electrochemical device. In certain embodiments, electrochemical device 300 is used in transportation (e.g., electric automobiles, electric trucks, electric trains, electric aviation, and / or electric shipping), stationary electrical storage for an electricity grid and / or commercial applications (e.g., data centers, factories, hospitals, and / or other buildings), portable electronic devices (e.g., consumer electronics), and / or any other appropriate application.

[0118] In some embodiments, electrochemical device 300 comprises an electrode (e.g., first electrode 302). In certain embodiments, the electrode (e.g., first electrode 302) comprises an electroactive material (e.g., electroactive material 100 shown in FIG. 1 and described herein in greater detail). In certain embodiments, first electrode 302 is an anode or negative electrode during nominal usage of electrochemical device 300 (e.g., during discharge).

[0119] In certain embodiments, electrochemical device 300 comprises electrolyte 304. In some embodiments, electrolyte 304 is disposed on first electrode 302. For example, in some embodiments, a surface of electrolyte 304 is in contact with a surface of first electrode 302 at interface 308a.

[0120] In some embodiments, the electrolyte (e.g., electrolyte 304) is a solid electrolyte. Any of a variety of suitable solid electrolytes are possible. According to some embodiments, the solid electrolyte comprises an inorganic compound, such as a ceramic and / or crystalline material. In certain embodiments, for example, the solid electrolyte comprises lithium lanthanum zirconium oxide (LLZO) (LiyLaaZ^On), Li6.6La3Zr1.6Tao.4O12 (LLZTO), lithium phosphorus sulfur chloride (LiePSsCl), lithium phosphorus oxynitride (LixPOyNz), Na-P”-A12O3 (NASICON) (Nai+xZr2SixP3-xOi2, 0 < x < 3), K-P”-A12O3, poly(ethylene oxide) (PEO), and / or combinations thereof. Other solid electrolytes are also possible.

[0121] According to certain embodiments, the electrolyte (e.g., electrolyte 304) is configured to transport an ion of the first alkali metal. For example, in certain embodiments wherein the first alkali metal comprises metallic lithium, the electrolyte may comprise LLZO, LLZTO, LiePSsCl, and / or LixPOyNz. In some embodiments wherein the first alkali metal comprises metallic sodium, the electrolyte may comprise NASICON. In certain embodiments wherein the first alkali metal comprises metallic potassium, the electrolyte may comprise K-P”-A12O3. In some embodiments, the electrolyte (e.g., electrolyte 304) is not configured to transport an ion of the second alkali metal and / or an ion of the third alkali metal.

[0122] According to certain embodiments, electrochemical device 300 comprises a second electrode (e.g., second electrode 306). In some embodiments, second electrode 305 is a cathode or positive electrode during nominal usage of electrochemical device 300 (e.g., during discharge).

[0123] The second electrode may comprise any of a variety of suitable electroactive materials. In some embodiments, the electrochemical device is a symmetric cell and the second electrode comprises the same electroactive material as the first electrode. In other embodiments, the electrochemical device is an asymmetric cell and the second electrode comprises a different electroactive material than the first electrode. In certain embodiments, for example, the second electrode is an intercalation cathode configured to intercalate alkali metal ions. In certain embodiments, the second electrode comprises a metal oxide, a metal sulfide, a metal halide, and / or a redox-active organic component. In certain embodiments, the second electrode comprises a lithium transition metal oxide or a lithium transition metal phosphate. In some embodiments, the second electrode comprises lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminum oxide (NCA), lithium cobalt oxide, lithium manganese oxide, lithium nickel manganese spinel, lithium iron phosphate, and / or combinations thereof. Other materials for the second electrode are also possible.

[0124] According to some embodiments, electrolyte 304 is disposed on second electrode 306. For example, in certain embodiments, a surface of electrolyte 304 is in contact with a surface of second electrode 306 at interface 308b. In some embodiments, the surface of electrolyte 304 that is in contact with the surface of second electrode 306 is opposite the surface of electrolyte 304 that is in contact with the surface of first electrode 302.

[0125] Although not shown in the figures, the electrochemical device may, in some embodiments, comprise other appropriate components such as current collectors (e.g., wire, foil, grid, etc.) that are in electrical contact with first electrode 302 and / or second electrode 306.

[0126] While a solid electrolyte has been described and shown in the figures, any appropriate component capable of separating the two electrodes from one another may be used (in place of or in addition to the solid electrolyte), as the disclosure is not meant to be limiting in this regard. In some embodiments, for example, the electrolyte comprises a liquid electrolyte. In certain embodiments, the electrochemical device comprises an appropriately constructed separator. The separator may be positioned in the electrolyte, in accordance with certain embodiments.

[0127] According to some embodiments, a method of operating an electrochemical device is described. In some embodiments, the method comprises discharging the electrochemical device. For example, FIG. 4A is a cross-sectional schematic illustration of electrochemical device 300 during discharging, according to certain embodiments. According to certain embodiments, during discharging, the first alkali metal of first component 102 of electroactive material 100 is oxidized to first alkali metal ion 402 at first electrode 302 via transfer of electrons 404a from first electrode 302 to second electrode 306. In certain embodiments, as described herein in greater detail, second component 104 enhances diffusional transport of first alkali metal ion 402 (e.g., through electroactive material 100 to interface 308a between first electrode 302 and electrolyte 304). In some embodiments, during discharging, first alkali metal ion 402 is transported from electroactive material 100, to interface 308a between first electrode 302 and electrolyte 304, through electrolyte 304, to interface 308b between second electrode 306 and electrolyte 304, and to second electrode 306.

[0128] According to certain embodiments, the oxidation of the first alkali metal to first alkali metal ion 402 decreases the volume fraction of first component 102 in electroactive material 100 and increases the volume fraction of second component 104 in electroactive material 100. In some embodiments, for example, the oxidation of the first alkali metal to the first alkali metal ion 402 forms one or more vacancies in electroactive material 100. The one or more vacancies may, in some embodiments, coalesce to form voids in electroactive material 100. In certain embodiments, as described herein in greater detail, second component 104 is configured to mitigate, reduce, and / or avoid the formation of voids in electroactive material 100, for example, at one or more grain boundaries of first component 102 and / or second component 104 and / or at interface 308a between first electrode 302 and electrolyte 304.

[0129] In certain embodiments, the method comprises charging the electrochemical device. For example, FIG. 4B is a cross-sectional schematic illustration of electrochemical device 300 during charging, according to certain embodiments. In certain embodiments, during charging, alkali metal ion 402 is transported from second electrode 306, to interface 308b between second electrode 306 and electrolyte 304, through electrolyte 304, to interface 308a between first electrode 302 and electrolyte 304, and to first electrode 302. In some embodiments, during charging, first alkali metal ion is reduced to the first alkali metal at first electrode 302 via transfer of electrons 404b from second electrode 306 to first electrode 302.

[0130] According to certain embodiments, the method comprises cycling (e.g., discharging and charging) the electrochemical device. The electrochemical device may be cycled any of a variety of suitable number of times. In certain embodiments, for example, the electrochemical device is cycled at least 1 time, at least 2 times, at least 5 times, at least 10 times, at least 20 times, at least 50 times, at least 100 times, at least 200 times, at least 500 times, at least 1,000 times, at least 2,000 times, at least 5,000 times, or more. In some embodiments, the electrochemical device is cycles less than or equal to 10,000 times, less than or equal to 5,000 times, less than or equal to 2,000 times, less than or equal to 1,000 times, less than or equal to 500 times, less than or equal to 200 times, less than or equal to 100 times, less than or equal to 50 times, less than or equal to 20 times, less than or equal to 10 times, less than or equal to 5 times, or less than or equal to 2 times. Combinations of the above recited ranges are possible (e.g., the electrochemical device is cycled at least 1 time and less than or equal to 10,000 times, the electrochemical device is cycles at least 100 times and less than or equal to 200 times). Other ranges are also possible.

[0131] According to some embodiments, the method comprises applying a stack pressure to the electrochemical device. In certain embodiments, the stack pressure is applied during the discharging and / or charging (e.g., during discharging, during charging, during cycling). For example, referring to FIGS. 4A-4B stack pressure 406 is applied to electrochemical device 300 during charging and / or discharging. In certain embodiments, as described herein in greater detail, stack pressure 406 applied to electrochemical device 300 causes second component 104 (e.g., the soft and deformable second component) dispersed throughout a bulk of first component 102 to deform and mitigate, reduce, and / or avoid the formation of voids in electroactive material 100, for example, at one or more ground boundaries of first component 102 and / or second component 104 and / or at interface 308a between first electrode 302 and electrolyte 304.

[0132] According to certain embodiments, the reduction of voids in electroactive material 100, for example, at interface 308a between first electrode 302 and electrolyte 304, may advantageously increase the surface area contact between first electrode 302 and electrolyte 304 as compared to an electrochemical device that is otherwise equivalent but does not include electroactive material 100. In some embodiments, if voids form at interface 308a between first electrode 302 and electrolyte 304, stack pressure 406 applied to electrochemical device 300 causes electroactive material 100 to establish surface area contact with electrolyte 304. According to certain embodiments, the increase in surface area contact between first electrode 302 and electrolyte 304 advantageously decreases dendrite formation (e.g., during charging), for example, as compared to an electrochemical device that is otherwise equivalent but does not include electroactive material 100.

[0133] The stack pressure may be applied via any of a variety of suitable techniques. In some embodiments, for example, the stack pressure is applied using a mechanical fixture. In certain embodiments, the mechanical fixture is fitted with a spring that applies a substantially constant spring force to the electrochemical device. In other embodiments, the stack pressure is applied by setting an initial pressure and fixturing to constrain one or more electrochemical devices to a set gap / displacement. In some such embodiments, the one or more electrochemical devices is under a non-constant pressure when the one or more electrochemical devices is discharged and / or charged (e.g., as the volume of the one or more electrochemical devices expands and / or contracts). In certain embodiments, the stack pressure is applied using elastomeric pads. The elastomeric pads may be placed between electrochemical devices positioned in series, in accordance with certain embodiments. In some embodiments, the elastomeric pads comprise silicone and / or polyurethane.

[0134] The stack pressure applied to the electrochemical device may be any of a variety of suitable pressures. In some embodiments, for example, the stack pressure is less than or equal to 5 MPa, less than or equal to 2 MPa, less than or equal to 1 MPa, less than or equal to 0.8 MPa, less than or equal to 0.6 MPa, less than or equal to 0.4 MPa, less than or equal to 0.2 MPa, less than or equal to 0.1 MPa, or less. In certain embodiments, the stack pressure is greater than or equal to 0.1 MPa, greater than or equal to 0.2 MPa, greater than or equal to 0.4 MPa, greater than or equal to 0.6 MPa, greater than or equal to 0.8 MPa, greater than or equal to 1 MPa, or greater than or equal to 2 MPa. Combinations of the above recited ranges are possible (e.g., the stack pressure is less than or equal to 5 MPa and greater than or equal to 0.1 MPa, the stack pressure is less than or equal to 0.6 MPa and greater than or equal to 0.4 MPa). Other ranges are also possible.

[0135] According to certain embodiments, no stack pressure is applied to the electrochemical device during the discharging and / or charging (e.g., during discharging, during charging, during cycling).

[0136] U.S. Provisional Patent Application No. 63 / 649,185, filed May 17, 2024, and entitled “Multiphase Negative Electrode Material and Associated Methods and Systems,” and U.S. Provisional Patent Application No. 63 / 671,654, filed July 15, 2024, and entitled “Multiphase Negative Electrode Material and Associated Methods and Systems,” are incorporated herein by reference in their entirety for all purposes.

[0137] EXAMPLE 1

[0138] In some embodiments, constituents of a negative electrode are combined by physical mixing of solid constituents. In one particular embodiment, two or more constituents of said negative electrode are combined and mixed by repeated folding and compression. Take as a non-limiting example a negative electrode which comprises a majority phase of lithium metal (Li) with a minority secondary phase of potassium (K). The two phases are highly immiscible. The two alkali metals are mixed at room temperature to form a layered nano / microstructured sheet which is subsequently attached to a solid-state ceramic electrolyte. The processing involves initially layering three metal sheets to form a Li-K-Li sandwich, which is then placed - 1 - between two sheets of polypropylene-coated aluminum film and rolled in a calender, or roller press, to a thickness set by the spacing of the rollers. Subsequent steps of folding the layered metal assembly in half and rolling to a set thickness results in an increasing number of layers given by 2", where n is the number of folds. Initially, the lithium and potassium metal may each form continuous sheets, and the negative electrode is a laminate. However, with continued folding, the layers of potassium become increasingly thin, and may no longer form continuous sheets, instead undergoing breakup so that the potassium is dispersed as particulates, forming a composite of potassium particle distributed throughout the lithium. After initial Li-K-Li layers are rolled to a set thickness, they are folded in half and calendered at a set gap repeatedly to decrease the size of the K inclusions. The resulting film can be reduced to a thickness desired for the high energy density battery cells, including thicknesses below 30 pm.

[0139] In comparison to the incumbent Li-ion battery (LIB), solid-state batteries (SSBs) with lithium metal negative electrodes promise safer and more energy dense storage. However, large resistances in the battery develop due to pores that form at interfaces between the solid-state electrolyte (SSE) and electrode active materials. As a result, state-of-the-art cells require substantial and commercially impractical stack pressures during operation to maintain electrode / electrolyte contact. The multiphase electroactive materials and associated electrodes described herein enable SSB operation at low stack pressure.

[0140] At the solid-solid interface between Li metal and the SSE, voids at the interface nucleate and grow, increasing interfacial resistance and overpotential. Because batteries are discharged to a designated lower cutoff voltage to avoid undesired reduction of the SSE, these larger contact loss overpotentials lead to a loss in discharge capacity with each cycle and decrease cycle life. Furthermore, the charging ionic current intensification at this interface, which magnifies with increased contact loss, can increase the driving force to nucleate dendrites which grow until a short circuit causes cell failure.

[0141] Common workarounds in the SSB community for these contact loss issues include using high uniaxial stack pressures (>1 MPa) applied normal to the planar anode interface, and processing interfaces or operating cells under elevated temperatures to increase the influence of thermally-activated creep deformation mechanisms which creates more conformal interfaces. These approaches detract from the proposed benefits of SSBs: high stack pressures require additional mechanical fixturing components which reduce the specific energy, and elevated processing or operating temperatures reduce energy efficiency. Silicone and polyurethane compressive pads are commonly used in LIBs between planar pouch cells to improve pressure uniformity and apply pressures of approximately constant 50 kPa. While currently optimized for LIBs over their lifetimes, engineering these elastomer pads could achieve relatively constant pressures of approximately 0.1 MPa and potentially up to 1 MPa given the 10-30% volume changes expected in SSBs. Despite the importance of this practical consideration, there are exceedingly few demonstrations of SSBs operating at these low applied stack pressures.

[0142] The multiphase electroactive materials described herein improve stability of the anode / SSE interface by limiting the increase of contact loss overpotentials and thus increases the accessible capacity during battery discharge, which increases the specific energy of the battery cell. This has been demonstrated using stack pressures as low as 0.15 MPa. In addition, preparation of the anode / SSE interface typically involves an annealing step just below the melting temperature of Li (180.5 °C) under applied pressure after the two layers are placed in contact. This step decreases interfacial resistance by eliminating voids that exist at the interface upon assembly; at higher homologous temperatures (T (K) / Tmeiting (K)) just below 1, deformation via creep is enhanced. The addition of distributed small K pockets within the Li metal matrix enables enhanced deformability at room temperature, and improved performance is achieved without the need for heating / annealing during the pressure formation step. K is a softer, more deformable metal compared to Li, and without being bound by any particular scientific interpretation, this fact may play a role in the improved performance and ease of processing at room temperature.

[0143] In addition to improved capacity during cell discharge, the disclosed material also displays resistance to short circuit failure via dendritic growth during charging (electroplating). Open circuit failure (excessive voiding) is a preferred failure mode over short / closed circuit failure (dendrites) because the only voiding is recoverable. In a stepped galvanostatic cycling experiment using an Li6.6La3Zr1.6Tao.4O12 (LLZTO) solid electrolyte, Li-K symmetric cells (see EIG. 5) with 10 vol.% K added achieved higher cumulative capacities before cell failure in comparison to a Li control group (see EIG. 6). 0% of the cells with added K (N = 9) displayed failure via short circuit (dendrites) compared to 36% for the Li cells (N = 11). This is despite the K cells reaching higher current densities and capacities, which is known to increase the propensity for dendritic failure. At 50 vol.% K, cells all reached the voltage cutoff (failure via open circuit) within the first stage of cycling. It is possible that an interface enriched in K decreased the concentration of active Li sites to strip from the interface and resulted in a rapid rise in interfacial resistance.

[0144] The multiphase electroactive materials were tested with improved performance between

[0145] 2 and 15 vol.% K. Extensive unidirectional galvanostatic (constant current) tests were carried out on compositions of Li foils with 2, 6, 8, 10, and 12 vol.% K at room temperature and under low applied stack pressure (0.15 MPa). These tests applied a constant current of 0.1, 0.4, 0.7, or 1.0 mA cm'2to strip Li metal from the working electrode (WE, oxidation) and plate Li metal on a Li-K counter electrode (CE, reduction) with 10 vol.% K. This CE was used because at high current densities and capacities, there was a lower short circuit risk, so the terminal voiding behavior of the WE was more likely to be observed. In this configuration, a Li reference electrode (RE) was used to deconvolute the contributions to total cell potential between the WE and CE. LLZTO was used as an SSE and is selective to Li+ions, so K metal is not transported across the anode / SSE interface. A common potential profile for tests on this cell design is shown as an example in FIG. 7, with terminal voiding behavior denoted by a potential above 5 V vs. Li / Li+. At this point of failure, a cell maximum capacity is determined.

[0146] The cell maximum capacity is shown for cells as a function composition (vol.% K) in FIG. 13. Test data for a Li control group (0% K) is shown. On average and across the range of current densities, LP-Li-K, or low-purity Li-K (i.e., Li with significant Na impurities and intermixed K) cells achieve 10 times the capacity as Li cells before open circuit failure, which is a significant improvement. Conventional cells can achieve a capacity of 26.8 mAh cm'2at 0.1 mA cm'2current density, 6.4 mAh cm'2at 0.4 mA cm'2, 2.2 mAh cm'2at 0.7 mA cm'2, and 1.8 mAh cm'2at 1.0 mA cm'2.

[0147] While typical Li WEs in cells exhibit a monotonically increasing potential profile versus capacity (or time) in constant current tests, LP-Li-K WEs in cells can exhibit an oscillatory profile, which implies that following local maxima in the profile, an internal response limits resistance as it decreases to more desirable levels. WE potential profiles of 4 cells displaying this behavior (all 2 vol.% K) are shown in FIGS. 8-9. This phenomenon was unexpected but is a desirable feature. Control of this behavior could achieve even higher current densities and capacities in SSBs with Li metal anodes at low stack pressure.

[0148] X-ray microtomography is a transmission-based microscopy technique which assembles transmission images of a sample rotated 180° or 360° into a 3D reconstruction. Planar slices from this 3D reconstruction can be used to visualize sample characteristics. Cross-sectional slices through samples of LP-Li-K with 2 and 10 vol.% K were obtained. These samples showed good contrast, due to the different mass absorption coefficients of Li and Na-K / K, with bright layers corresponding to Na-K / K domains. Darker, less numerous pores were more pronounced in the 10 vol.% K sample, and tended to exist near the Li and Na-K / K interfaces. In both samples, there was strong horizontal preference, as a result of cold-rolling the foils with a calender. In both samples, K domains were a maximum of ~5 pm thick and had varying widths. Images of the flat surface of the LP-Li-K foil were carried out using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM / EDX). SEM / EDX images under various magnifications confirmed the distribution of Na-K / K inclusions throughout the LP-Li-K foil. Li is not detectable using EDX, so pixels not containing K were assumed to have significant Li content. The magnified images showed the existence of K inclusions (e.g., Na- K / K, or a coexistence of Na-K and K phases) with sizes below 1 pm, so a spread exists for the K particle size distribution.

[0149] X-ray diffraction (XRD) tests on the LP-Li-K foil with 10 vol.% K confirmed the presence of texturing. The absence of the standard Li (110) peak implied texture in the sample. In comparison to a Li standard diffraction pattern, the (110) peak was absent in the as-rolled foil, indicating a preference of grain orientation. The different relative intensities of the K (200) and (211) peaks relative to the K standard also indicated a preferred K orientation. The XRD profile at -150 °C showed the presence of a Na2K structure, which confirmed that Na-K is present in the material at room temperature.

[0150] K metal is very soft and sticky, and it was challenging to find a suitable material with which to roll the material. It was determined that if the K was initially sandwiched between LP- Li metal, which does not stick to polypropylene, the composite foil would stick minimally to the polypropylene. K metal can also be rolled to an initial thickness -200-250 pm when surrounded on both sides by either nitrile gloves or wax paper. Cycling results using cells with two negative electrodes were obtained. Good stability of this structure over the course of extended battery lifetime was shown, where the K addition benefits remained over the course of at least 2 weeks of a continuous cycling. Short term (several hours to days) microstructural evolution of the LP- Li-K foil could also impact performance, and cells tested displayed a large range of maximum achieved capacity. However, even the worst performing LP-Li-K cells still outperformed the Li control samples.

[0151] Referring to EIG. 13, it was found that LP-Li + 4 vol.% K (LP-Li-4K) had optimal performance in the range studied.

[0152] Transverse cryo-LIB SEM images of 4 vol.% K added to LP-Li were obtained. A plan view SEM image of 4 vol.% K added to LP-Li was also obtained. The images showed small (-5-10 micrometer, with less than 5 micrometer grains) grain size and submicron K inclusions in transverse and plan view LP-Li + 4 vol.% K foil samples, with evidence of Na-K / K particles pinning Li grain boundaries.

[0153] EIG. 10 shows capacities for various metals added to Li (e.g., 6 vol.% Mo added to Li and 6 vol.% Rb added to Li). It was shown that additional additives also show improved performance vs. pure Li. Li + 6 vol.% of Na, Rb, and inert (not alkali metal) Mo nanoparticles can also improve the attainable capacity under at least one current density (0.1 mA cm'2) and stack pressures less than 0.2 MPa with similar processing steps.

[0154] FIG. 11 shows reversibility of symmetric cells including a LP-Li-K anode (LP-Li-lOK) at 0.8 MPa, with 0.7 mA / cm2current density. It was found that LP-Li + 10 vol.% K and LP-Li + 4 vol.% K symmetric cells (LP-Li-K / LLZTO / LP-Li-K) displayed some level of reversible cyclability.

[0155] EXAMPLE 2

[0156] Lithium alloy composition and micro structure were tested and shown to suppress interfacial voiding-related failure at low stack pressures (0.2, 0.8 MPa) using low-cost industrial grade lithium metal in which sodium is the most common impurity. The addition of potassium (<5 vol.%) was shown to getter impurity sodium in the starting lithium, forming immiscible Na- K and K phases which enhance diffusional transport of Li+to the interface. Symmetric cell tests with an LLZTO solid electrolyte showed a greater than tenfold increase in the stripping areaspecific capacity (up to 25 mAh cm'2at 0.7 mA cm'2at <0.2 MPa pressure) before voiding- related failure, compared to undoped and high purity lithium. Galvanostatic electrochemical stripping experiments with doped lithium electrodes exhibited an oscillatory profile which was attributed to reversible changes in interfacial contact. Through cryo-FIB-SEM and pressure- resolved analysis of the oscillatory behavior, a mechanism driven by creep of Li, where Na-K at void edges mitigates local current intensification on remaining Li / SSE interfaces, is proposed.

[0157] Design and performance of multiphase Li alloy anode at low stack pressure

[0158] As described herein, small amounts of K (<5 vol.%) were added to low cost, low purity Li metal with high levels of Na impurities (0.3 vol.%, “LP-Li”) to create a bulk multiphase foil with small amounts of Na-K and K in a Li matrix (“LP-Li-K”). Performance was shown using symmetric cells with an LLZTO electrolyte at low 0.2 MPa stack pressure, stripping 24.7 mAh cm'2at 0.7 mA cm'2and 3.1 mAh cm'2at 1.0 mA cm'2. Galvanostatic stripping experiments with LP-Li-K electrodes exhibited an oscillatory voltage profile which is attributed to reversible changes in interfacial contact. Through cryo-FIB-SEM and pressure-resolved analysis of the oscillatory behavior, it is suggested that Na-K at void edges mitigates local current intensification at Li / SSE interfaces that remain after void nucleation and growth. After reaching a critical contact area, interfacial contact is re-established through creep of Li enhanced by the Na-K phase, until voids nucleate and grow again, leading to periodicity in the voltage profile. As described herein, the presence of Na (0.3 vol.% in the starting Li) was utilized in low cost, low purity Li metal by adding K metal to fabricate a foil with Li, Na-K, and K phases. Electrodes of thickness 20-250 pm were achieved with this process. Repeated folding and calendering of a Li-K-Li stack at a set gap 15 times enabled room temperature mixing of the Na impurity and K phases, which combined to form Na-K. Other processing techniques such as melt-quench may produce equivalent foils, as Li, Na, and K all have melting points below 181 °C. The Li-Na-K ternary phase diagram was used to predict the phases present at thermodynamic equilibrium based on the starting materials and compositions. Na, K, and Na-K phases were all highly immiscible with Li at ambient temperatures. For all compositions explored (2-12 vol.% K added), the K was also low purity (98% trace metals basis) and was present in excess, therefore both Na-K and K were present in the foils. Cryogenic XRD confirmed the presence of the solid Na2K phase which was formed from the cooling of liquid Na-K in the structure. Fabricated EP-Ei-K foils were calendered at 250 pm nominal thickness, but thicknesses as low as 20 pm were achieved. Despite having a small amount of liquid Na-K present, the LP-Li-K foils had a higher equivalent yield stress than HP-Li (high purity Li) that has not undergone the repeated folding and calendering process.

[0159] The process of repeated folding and calendering refined the grain size to approximately 5-10 pm, as is shown in FIG. 12A, which increased yield stress. This SEM image of a transverse cross section (cryo-Ga+-FIB milling) of LP-Li-4K (4 vol.% K) showed Li grains in shades of gray and bright regions of Na-K and K primarily located at Li grain boundaries. EDS mapping confirmed the presence of Na and K within these regions in a similar sample. Electron channeling contrast imaging (ECCI) using an energy selective backscattered (EsB) detector distinguished between grain orientations to reveal the Li grain structure without the need for the less readily accessible and potentially damaging high-energy electron backscatter diffraction (EBSD) technique. It is believed that there are no other studies on Li metal using this technique to visualize contrast between grains. While preferential Ga+-FIB milling under cryogenic conditions may lead to some darker, low attenuation regions adjacent to domains of Na and K in SEM images, X-ray microtomography (XCT) imaging in FIG. 12B provided evidence that pores were stable within the structure under ambient conditions. Bright domains of high photoelectric absorption were assumed to be Na-K and K interspersed within Li metal. Darker, low attenuation regions were expected to be pores within the structure. XCT imaging on this LP-Li- 10K (10 vol .% K) sample was conducted at room temperature over 16 hours, and no evidence of significant movement of the pore boundary over this timeframe was observed. Pores in contact with Na and K regions also displayed low contact angles, which indicated favorable wettability of Na-K towards Li.

[0160] FIG. 13 shows the low stack pressure performance of LP-Li-K foils as a function of volume fraction of K added to LP-Li and current density. Data series transitioning represents increasing current densities, from 0.1 to 1.0 mA cm'2. Error bars represent the standard error in critical capacity, with N > 3 for each set of composition and current density. N = 119 cells were included in this dataset. An industry target capacity of 3 mAh cm'2is displayed for reference. Performance was quantified as critical capacity (shown in log scale) for unidirectional galvanostatic (constant current) tests on N = 119 cells with LLZTO SSEs at <0.2 MPa and room temperature (20-30 °C). Critical capacity is defined as the maximum capacity achieved before open circuit or excessive contact loss, defined as exceeding 1 kQ cm2or equivalently, 0.5 V at a constant current density (based on the electrode diameter) of 0.5 mA cm'2. Reporting normalized resistance instead of voltage thresholds to determine critical capacities could improve comparisons across studies that use different cell geometries and current densities. At all current densities, the maximum critical capacity is reached at 4 vol.% K in LP-Li, with lower currents generally achieving higher critical capacities. The highest performing cells at 0.1, 0.4, 0.7, and 1.0 mA cm'2current density reached critical capacities of 39.4, 26.1, 24.7, and 3.1 mAh cm'2, respectively. These capacities are among the highest reported in the Li metal SSB literature at elevated current densities and low stack pressure.

[0161] The ability of LP-Li-4K / LLZTO / LP-Li-4K cells to cycle commercial capacities with current densities up to 1.5 mA cm'2was also shown. This also highlighted the ability of the cells to resist dendrite formation and short circuit failure at high current densities. As K content was increased from 0 (control) to 4 vol.% in LP-Li, more Na-K may be produced, as more K may enable improved mixing. More K addition beyond the maximum at 4 vol.% K may substitute and decrease Li active sites at the LP-Li-K / LLZTO interface, increasing resistance, because LLZTO does not transport K. An immediate rise in impedance for cells with 50 vol.% K was shown. EIG. 14 compares the critical capacity across different anode purity levels with and without K addition. In EIG. 14, pure Li (99.9%, battery grade) is HP-Li; low purity Li (99%, industrial grade) is LP-Li; pure Li with 4 vol.% K added is HP-LL4K; and low purity Li with 4 vol.% K is LP-LL4K. At the same 0.4 mA cm'2current density, LP-LL4K achieved more than a tenfold increase in average critical capacity compared to the other groups. Error bars represent the standard error, with N > 3 for each group, and N = 38 total cells. HP-Li and LP-Li perform equally poorly under moderate currents and low stack pressure regardless of processing. HP-Li- 4K performed equivalently if not worse than HP-Li and LP-Li, which demonstrated the importance of the Na impurity in creating the Na-K phase.

[0162] Oscillatory interface response of the LP-Li-K multiphase Li alloy anode

[0163] This sustained Li stripping at high current densities resulted in high critical capacities and warranted a closer examination of the underlying interfacial changes. FIG. 15A shows voltage profiles under unidirectional galvanostatic (constant current) conditions of a LP-Li- 4K / LLZTO / LP-LL4K symmetric cell versus a LP-Li / LLZTO / LP-Li control (3 cells) with no added K at low stack pressure and high current (0.2 MPa, 0.7 mA cm'2). Total resistance normalized by the electrode / electrolyte interfacial area (Q cm2) is represented on the primary y- axis as the voltage divided by the current density (secondary y-axis, mA cm'2). After a sharp initial increase, the resistance stabilized and oscillated with changing amplitude and periodicity. This cell reached a 17.8 mAh cm'2critical capacity before resistance increased to above 1 k cm2, indicating excessive loss of interfacial contact. LP-LL4K electrodes were 8 mm in diameter with thickness 250 pm. Kapton tape masks (51 pm thickness) defined an interfacial contact area of 0.5 cm2that was used for resistance and current density calculations. The LLZTO pellet had a 12.7 mm diameter and a thickness between 1 and 2 mm. Cu current collectors of diameter 8 mm were placed on top of the LP-LL4K electrodes. The cell was radially symmetric. Referring to FIG. 15A, resistance is shown as the primary y-axis in an attempt to normalize results across different current densities. While the cells with LP-Li rapidly increased in resistance, the LP-LL4K cell reached a local maximum in resistance and oscillated with time (capacity) at relatively low frequency (0.1 to 1.5 mAh cm'2, or 10 to 150 minutes) until it ultimately failed in a similar fashion almost 18 mAh cm'2later. It is believed that this is the first report of such an oscillatory response in voltage or resistance for a Li metal solid state cell under constant current.

[0164] It was shown that these oscillations persisted for galvanostatically cycled cells. A region of low frequency oscillations in the middle of the test is magnified in FIG. 15B. In this region of the profile, the period was approximately 1.5 mAh cm'2, or every 2 hours. Three each of local maximums (peaks) and minimums (valleys) are highlighted on the profile. For this cell, impedance was measured by superimposing a current perturbation (GEIS) on top of the applied constant current every 0.02 mAh cm'2, or 2 minutes. FIG. 15C shows Nyquist impedance plots corresponding to the highlighted DC resistance points in FIG. 15B. Frequencies from 100 kHz to 1 Hz are shown. Impedances measured at resistance valleys were smaller than impedances measured at resistance peaks. Discrepancies in impedance between resistance peaks and valleys occurred at low frequencies, or <5 kHz, indicating interfacial changes with no changes in bulk electrolyte transport properties (high frequency, >5 kHz). Further evidence of bulk impedance staying constant throughout the test as well as initial and final cell impedance was shown. These changes in interfacial impedance were expected to be associated with the LP-Li-4K / LLZT0 interface associated with stripping; Li plating should not change electrode / electrolyte contact area at that interface and the constant bulk impedance suggests there is no electrolyte degradation. Oscillations for a cell with a LP-Li plating (counter) electrode were also shown, strengthening the case that this behavior is specific to changes at the stripping interface. To ensure that the current perturbation was not responsible for the oscillations, it was shown that oscillations occurred in cells with no overlaid current perturbation. It is noted that a quantitative comparison between DC resistance and AC impedance values was challenging due to additional interfacial capacitance, which contributed to reactance at different frequencies. These reversible changes in DC resistance and AC impedance corresponded to changes in interfacial contact area, as illustrated in the schematic in FIG. 15D with the expansion and contraction of voids at the electrode / electrolyte interface.

[0165] Having identified oscillations as indicators of interfacial changes at the stripping electrode, it was next examined how increased stack pressure influenced their amplitude and frequency. FIG. 16A shows four representative voltage and resistance profiles for LP-Li- 4K / LLZTO / LP-Li-4K symmetric cells under varying pressures (0.2, 0.8, 1.4, and 2.8 MPa) for unidirectional galvanostatic tests at 0.7 mA cm'2and room temperature. Total resistance normalized by the electrode / electrolyte interfacial area (Q cm2) is represented on the primary y- axis and current density (mA cm'2) on the secondary y-axis. Despite displaying varying extents of oscillatory behavior, all profiles followed the general trend of initially rising to a local maximum in resistance, decreasing to a stable minimum, and a rapid increase in resistance at the critical capacity. The electrode thickness (250 pm) and composition (4 vol.% K, 0.3 vol.% Na) enforced an upper bound on the maximum achievable capacity, with a Li content of 49.3 mAh cm'2available for stripping. Many oscillations were observed in the profiles of cells under lower 0.2 and 0.8 MPa applied stack pressure, and few oscillations in the profiles of cells under higher 1.4 and 2.8 MPa pressure. The critical capacities of the lower pressure cells were similar and are less than the higher pressure cells. This is shown for an expanded dataset with N = 19 cells in FIG. 16B. Error bars represent the standard error, with N > 3 for each group. There was a clear transition in critical capacity between 0.8 and 1.4 MPa. In both plots, a dotted line represents the maximum achievable capacity before all Li is stripped from the 250 pm thick LP-LL4K electrodes (49.3 mAh cm'2), assuming perfectly planar stripping and electrode alignment. Using thicker electrodes could increase critical capacities far beyond this value for the higher pressure cells (1.4, 2.8 MPa), as resistances only increase considerably when approaching this threshold.

[0166] FIGS. 16C-16F plot the first 18 mAh cm'2of each of the cells in FIG. 16A. In each subfigure, a region of 15 mAh cm'2was selected for further analysis of the oscillations. The beginning of this region was selected to follow the initial local maximum in resistance which typically occurred within the first 1 mAh cm'2. The data was treated by fitting a black trendline to remove low frequency contributions. The trendline is fitted from a seasonal-trend decomposition using LOESS (locally estimated scatterplot smoothing), or STL, regression. The only input parameter (common across all datasets) for the STL regression is a period of 1 mAh cm'2as the seasonal smoother. STL is used to fit trends to seasonal or periodic time-series data, and is a combination of a moving average and local polynomial regression. The detrended data, calculated as the difference between the raw resistance data and the fitted STL regression, was also obtained and highlighted the oscillation characteristics at medium to high frequencies. The y-axis scaling of the resistance was common across all detrended data plots, highlighting the decrease in amplitude as the applied pressure increased. There appeared to be damping of the oscillation amplitude as capacity (and time) increased. The oscillation amplitude was very small, less than 5 cm2, for cells under 1.4 and 2.8 MPa pressure.

[0167] Pourier analysis was carried out on the detrended data via a continuous wavelet transform (CWT). CWT is a time- or capacity-resolved Pourier transform that can be used for data with changing oscillation characteristics (amplitude and frequency). At a given capacity or time point, the detrended data had a characteristic period (inverse frequency) and amplitude. Imperfect sinusoidal shape of the oscillations led to energy dispersion, which appeared as a spread in amplitude and period. However, there were local maximums and minimums in amplitude, and the corresponding periods elucidated the dominant modes. The dominant periods generally occurred between 0.1 and 1 mAh cm'2. While these cells showed damping of oscillations (decrease in amplitude) as capacity increased, this was not always the case, as shown in PIG. 15A. In addition to showing that amplitude decreased as pressure increased, the quantitative CWT analysis revealed that the 0.8 MPa cell oscillated with a higher frequency (shorter period) than the 0.2 MPa cell. This increase in oscillation frequency and decrease in oscillation amplitude from 0.2 to 0.8 MPa was shown for other cells.

[0168] To further investigate the cause of oscillations, the LP-Li-4K / LLZTO interface was imaged. Resistance profiles of two LP-Li-4K / LLZTO / LP-Li-4K symmetric cells used for imaging are shown in PIG. 17A. Cell 1 was pristine and no charge was passed. The electrode thickness of cell 1 was 42 pm. The plotted resistance was estimated from the measured cell impedance. Cell 2 was subject to unidirectional constant current at 0.4 mA cm'2(0.8 MPa pressure) before it was paused after 1.75 mAh cm'2capacity was passed. The oscillation amplitude appeared to be dampened as time (capacity) progressed. It is noted that cell 2 had a 30 pm thick stripping electrode, demonstrating that oscillations persisted and commercially relevant capacities could be passed even for thin electrodes at <1 MPa pressure with LP-Li-4K. As shown in FIG. 17B, transverse cross-sections of cells 1 and 2 were prepared using ex-situ plasma focused ion beam (PFIB) milling with a Xe+source under cryogenic (liquid nitrogen) conditions. Subsequent SEM images were taken of the milled trenches.

[0169] FIG. 17C shows an SEM image of the interface, with key features labeled. The electrode is in good contact with the LLZTO solid electrolyte. Bright regions within the electrode are domains of Na-K and K based on Time-of-Flight Secondary Ion Mass Spectrometry (ToF- SIMS) heatmaps of Na and K. Na-K / K domains were interspersed within a Li matrix for the bulk electrode, with sizes from the submicron level to 10 pm. The LLZTO surface had roughness on the order of 1 pm, in agreement with the 1 pm particle size of the diamond suspension used for polishing. While the majority of the interface post-assembly was composed of Li / LLZTO interfaces, there were several Na-K / K domains at or near the interface, and in some cases small interfacial voids which may cause contact loss to accelerate immediately upon current application. Other examples of the LP-Li-4K interface in a different pristine cell were shown. In the ToE-SIMS heatmaps of Na and K, high concentration pixels and low concentration pixels were observed. Most regions of high Na concentration coincided with regions of high K concentration, but regions of high K concentration with low Na concentration were also observed. This indicated significant mixing of Na impurities with excess K. Under cryogenic conditions, two solid phases were expected: Na2K and K, which correspond to singlephase liquid NaK and solid phase K at room temperature, based on the phase diagram.

[0170] In EIGS. 17D-17E, SEM images at different transverse cross sections in cell 2 showed different interfacial configurations, but the same components were present: Li, Na-K / K, and LLZTO. In EIGS. 17D-17E (scale bars, 5 pm), 1.75 mAh cm'2(~8.5 pm Li) has been electrochemically stripped from the LP-Li-4K / LLZTO interface according to the resistance profile in EIG. 17A. The direction of Li and Li+transport during unidirectional stripping is shown in EIG. 17D. The brighter film that builds up at the LP-Li-4K / LLZTO interface was composed of Na and K, and was a mixture of Na-K and K phases at ambient temperature. Several voids with height of approximately 1 pm were adjacent to or were enclosed by Na-K / K films with similar thickness. In this cell, there was a large area of Li / LLZTO interfaces. However, a similar cell tested at the same current density but lower 0.2 MPa stack pressure was evaluated, and most of the interface was composed of voids and Na-K / K, with minimal Li in contact with LLZTO. Across all images, the Na-K / K layer wets Li and LLZTO well.

[0171] Hypothesized mechanism for reversible contact loss and gain in LP-Li-K

[0172] The previous sections have established that oscillations in the voltage and resistance profiles correspond to interfacial changes at the stripping electrode interface, oscillation amplitude decreases and frequency increases as pressure increases from 0.2 to 0.8 MPa, and that the LP-Li-4K / LLZTO interface changes from mostly Li / LLZTO to a complex structure of voids, Na-K / K, and Li at the LLZTO interface as Li is stripped. It has also been established through modeling and experimental results that Na-K wets Li. A comprehensive mechanism that follows the typical resistance profile is shown in FIG. 18 A, with selected time or capacity points corresponding to schematics of the interfacial structure in FIG. 18B. The profiles, as well as others in conventional SSBs, initially increased rapidly and followed steps (1) through (3). This coincides with the nucleation and growth of voids. Based on the SEM images of transverse cross sections of the interface, the void nucleation events may occur at Li / LLZTO interfaces, but that void nucleates may also be present before current is applied due to nonzero surface roughness and assembly procedures. The nucleates shown in (1) and (2) grow laterally, in accordance with the low aspect ratios seen in SEM cross sections. Cells with HP-Li and LP-Li follow this trend, and normally reach the resistance or voltage threshold under aggressive conditions (low stack pressure and moderate to high currents). However, under mild conditions (high stack pressure and low currents) these Li cells can reach a local maximum, as void growth slows. The average lateral velocity of void edges diminish to zero as the few remaining Li / LLZTO contact points have enough local stress to deform at a faster rate than the Li is stripped at the void edge. For cells with LP-Li-4K electrodes, this local maximum was often reached well below the resistance threshold, even under aggressive conditions such as 1.5 mA cm'2at 0.2 MPa and room temperature.

[0173] Based on images of voids adjacent to Na-K films at the interface in FIGS. 17D-17F and FIG. 18C, the Na-K at the interface arrested lateral void growth as shown in (3). A limiting current experiment was conducted on a model cell with a thick Na-K layer separating HP-Li and LLZTO which showed high diffusivity of Li in Na-K, estimated to be approximately 7 x 10'5cm2s'1. This suggests that even with limited Li / LLZTO interfacial area, small regions of Na-K at the interface could keep cell resistance low, accommodating high Li flux as local current intensification occurs at remaining contact points. At stages (1) and (2), the locations of void nucleates and Na-K rich regions at the interface were uncorrelated, so in (3) some voids may stop expanding before reaching Na-K domains, as local stresses may be high enough to counteract current intensification at the void edges, much like HP-Li and LP-Li cells under mild conditions. Rearrangement could occur at the void edges in contact with Na-K, as high local currents will quickly dissolve Li adjacent to the void as the Na-K moves to wet the void edge.

[0174] At (4), voids reached a maximum threshold in area coverage and thus the cell resistance was at a local maximum. The process by which resistance decreased and interfacial contact was regained is shown in the transition from (4) to (5). From the earlier CWT analysis, the timescale of the oscillations was typically between 10 and 150 minutes across different stack pressures and currents. It is unlikely that the time and length scales relate to diffusion within Na-K. Instead, it is hypothesized that the rate of change in resistance with respect to time aligns with expectations for creep deformation of Li. Notably, examples of HP-Li and LP-Li cells show oscillations of similar timescale under the mildest conditions (0.1 mA cm'2, 0.8 MPa). Mechanical properties of LP-LL4K are similar to HP-Li and LP-Li. The bulk yield stress, approximated using a Vickers microhardness test at a high 1 x 10’2s'1strain rate, is 2.4, 2.7, and 3.1 MPa for LP-Li- 4K, LP-Li, and HP-Li (see FIGS. 19-20). In FIG. 19, cylindrical samples with a 2:1 aspect ratio were used to measure the bulk properties of the materials. It was shown that for an approximation of void geometry based on the SEM images, the von Mises stress at void edges was generally between 0.5 and 1 MPa throughout the oscillatory region of a typical resistance profile for a cell at 0.2 MPa nominal stack pressure, which was well below the measured flow stress. This suggested that creep deformation (below the flow stress) was responsible for timedependent plasticity of Li at the void edges. It is expected that power law creep dominates in the 0.5 to 1 MPa range. Creep may also be enhanced at the void edges by the Na-K phase which has high Li diffusivity. Li dissolution from the top of the void surface and precipitation at the void edge may occur to reduce the curvature of the low aspect ratio voids. From these conditions, strain rates were roughly estimated to be between 10’2and 10’5s’1. If stress was the only force on Li / LLZTO contacts, the oscillation time scale of 10 to 150 minutes would correspond to strain rates of approximately 10’3to 10’4s’1, which was within this broad range.

[0175] The trajectory of interfacial resistance with time in a SSB was determined by the aggregate of local flux balances across the interface at the remaining interfacial contact points (non- voided regions). This flux balance compared the rate of electrochemical stripping from Li active sites to the rate of transport of Li metal atoms from the anode at the void edges. In (5) it was deliberately shown that the velocity of the void edge without Na-K was less than the velocity of the void edges with wetted Na-K. The current intensification factor within the electrolyte at the edge of a void compared to an interface in contact has been modeled and it was shown that for a local current density of 0.4 mA cm’2, stripping flux intensifies by a factor of 5 at the void edge, which was further magnified by the increased current density across the entire interface due to driving a constant total current over a smaller contact area. The Na-K absorbed this elevated Li flux localized at the void edges which enabled an increased total velocity of the Li / LLZTO void edge, shrinking the void laterally, when compared to Li with no Na-K.

[0176] As shown in (6), the local stress in the middle of the Li / LLZTO interfacial contact is low enough to eventually allow void nucleation, and in (7) through (9), interfacial contact area gained from voids with Na-K at the edges contracting was outpaced by the growth of voids from Li / LLZTO interfaces until they were stopped either by void edges reaching a critical local stress or contacting a Na-K domain. As the oscillation cycle continued, more Na-K was transported to the interface from the bulk (FIGS. 17D-17F) which may have resulted in oscillation damping, as is seen in some resistance profiles, such as in FIGS. 16C-16F. Eventually, ionically blocking K that was also transported to the interface from the bulk may eventually limit enough Li active sites that resistance rapidly increases and the cell dies at the critical capacity. The aforementioned processes may occur on numerous length and time scales, so the macro-level resistance that was measured likely represents these processes on the aggregate. Lower amplitude, higher frequency oscillations when increasing stack pressure from 0.2 to 0.8 MPa are explained by this mechanism, as thresholds for deformation of the void edge are lower, and larger strain rates promote faster deformation. Above 1 MPa, the local stresses were large enough to resist void nucleation and growth, as seen by the monotonically decreasing resistance profiles for cells under 1.4 and 2.8 MPa pressure in FIG. 16A. Observed interfacial features supporting the proposed mechanism are shown in FIGS. 18C-18E (SEM images of interfaces from LP-Li-4K / LLZTO / LP-Li-4K cells with at least 1.75 mAh cm'2of Li stripped. In FIG. 18C, a void was constrained by Na-K / K on both sides. In FIG. 18D, a void nucleated from an Li / LLZTO interface in the late stages of stripping. In FIG. 18E, an interface of a cell stripped at 0.2 MPa (lower pressure) with Na-K wetting the top Li surface of a void (left) is shown with a void constrained by Na-K on one side and a small Li / LLZTO contact on the other side (center right). Further examples of interfacial features supporting the proposed oscillation schematic can be seen in FIGS. 17D-17F.

[0177] In FIG. 18B, initially, the interface consists of mostly Li / LLZTO with some Na- K / LLZTO domains. Voids nucleate and grow stochastically at Li / LLZTO interfaces until they either reach Na-K domains or the rate of creep deformation (caused by high local stress, decreases void size) becomes equivalent to the rate of electrochemical contact loss (increasing void size). From (3) to (4), the void edge that reaches Na-K restructures itself to minimize surface energy, as Na-K shows favorable wettability towards Li and provides a high Li flux transport path at the radius of curvature. A decrease in resistance from the local maximum in (4) to (5) is due to the rate of creep deformation at the void edges exceeding the stripping rate. Arrows of larger magnitude in (5) are shown at the void edges with Na-K compared to ones without. This is because stripping current density is intensified at void edges, and high Li diffusivity Na-K can reduce intensification on the adjacent Li / LLZTO interface, reducing the stripping rate while keeping the creep deformation rate the same. The balance of these rates determines the change in position of the void edge with time. The resistance reaches a local minimum in (6), as the rate of void contraction balances with interfacial contact loss due to void nucleation. It is hypothesized that there is a local stress threshold, below which the probability of void nucleation at the Li / LLZTO interface increases drastically. These voids are likely to nucleate in the center of the Li / LLZTO interface between voids where the stress is lowest. In (7), the contracting voids get even smaller and at a critical size the Na-K wets the Li surface of the void. Meanwhile, contact loss accelerates as the voids grow until they reach the Na-K domains again and the Na-K redistributes to wet the void edge in (8) and (9), and the cycle continues. It is hypothesized that the aforementioned processes are occurring on numerous length and time scales, so the macro-level resistance that we measure likely represents these processes on the aggregate. Bulk Na-K / K and interfacial K effects are not considered in this sequence, but as time progresses, it is expected Na-K and K to build-up at the surface.

[0178] Competing effects are expected. Large amounts of ionically blocking K may decrease Li active sites and lead to eventual cell failure due to excessive resistance increase. Additional Na-K with high Li diffusivity may infiltrate voids, re-establishing contact and decreasing resistance with time.

[0179] Performance Observations

[0180] The disclosed and tested electroactive material compositions were observed to limit void growth at <1 MPa by designing an electrode processed by mixing small amounts (<5 vol.%) of K metal into low cost, low purity Li metal with Na impurities. Formation of small amounts of Na-K with high Li diffusivity enabled the passage of large capacities at high current densities, low pressure, and room temperature. Low-frequency oscillatory behavior (minutes to hours) in the voltage profile under galvanostatic conditions is linked to reversible interfacial impedance changes, which were assessed through cryo-PFIB-SEM and pressure-dependent analysis. These findings suggest a mechanism driven by creep of Li, while Na-K at void edges mitigates local current intensification on remaining Li / SSE interfaces. While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and / or structures for performing the functions and / or obtaining the results and / or one or more of the advantages described herein, and each of such variations and / or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and / or configurations will depend upon the specific application or applications for which the teachings of the present invention is / are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, and / or method described herein. In addition, any combination of two or more such features, systems, articles, materials, and / or methods, if such features, systems, articles, materials, and / or methods are not mutually inconsistent, is included within the scope of the present invention.

[0181] The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.”

[0182] The phrase “and / or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Other elements may optionally be present other than the elements specifically identified by the “and / or” clause, whether related or unrelated to those elements specifically identified unless clearly indicated to the contrary. Thus, as a non-limiting example, a reference to “A and / or B,” when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A without B (optionally including elements other than B); in another embodiment, to B without A (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

[0183] As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and / or” as defined above. For example, when separating items in a list, “or” or “and / or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of’ or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law.

[0184] As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and / or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

[0185] As used herein, “wt.%” is an abbreviation of weight percentage. As used herein, “vol.%” is an abbreviation of volume percentage. As used herein, “at.%” is an abbreviation of atomic percentage.

[0186] Some embodiments may be embodied as a method, of which various examples have been described. The acts performed as part of the methods may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include different (e.g., more or less) acts than those that are described, and / or that may involve performing some acts simultaneously, even though the acts are shown as being performed sequentially in the embodiments specifically described above.

[0187] Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements.

[0188] In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of’ and “consisting essentially of’ shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.

Claims

1. CLAIMSWhat is claimed is:

1. An electroactive material comprising: a first component comprising a first alkali metal; and a second component comprising a second alkali metal and a third alkali metal, wherein the first alkali metal, the second alkali metal, and the third alkali metal are different alkali metals, wherein the first component and the second component are intermixed such that the second component is dispersed throughout a bulk of the first component, and wherein the first component and the second component are substantially immiscible at 20 °C.

2. The electroactive material of claim 1, wherein the second component comprises the second alkali metal in an amount greater than or equal to 1 wt.% and less than or equal to 99 wt.% based on a total weight of the second component.

3. The electroactive material of any one of the preceding claims, wherein the second component comprises the third alkali metal in an amount greater than or equal to 1 wt.% and less than or equal to 99 wt.% based on a total weight of the second component.

4. An electrochemical device comprising: an electrode comprising the electroactive material of any one of the preceding claims.

5. The electrochemical device of claim 4, further comprising a solid electrolyte disposed on the electrode.

6. The electrochemical device of any one of the preceding claims, wherein the solid electrolyte comprises lithium lanthanum zirconium oxide (LLZO) (LiyLaaZ^On), lithium phosphorus sulfur chloride (LiePSsCl), lithium phosphorus oxynitride (LixPOyNz), poly(ethylene oxide) (PEO), and / or combinations thereof.

7. A method of operating the electrochemical device of any one of claims 4-6, the method comprising: charging and / or discharging the electrochemical device.

8. The method of claim 7, furthering comprising: applying a stack pressure to the electrochemical device during the charging and / or discharging, wherein the stack pressure is less than or equal to 1 megapascal.

9. A method of manufacturing an electroactive material, the method comprising: intermixing: (i) a mixture of a first alkali metal and a second alkali metal; and (ii) a third alkali metal, wherein the first alkali metal, the second alkali metal, and the third alkali metal are different alkali metals; and forming the electroactive material comprising a composite structure including: (i) a first component comprising the first alkali metal; and (ii) a second component comprising the second alkali metal and the third alkali metal, wherein the first component and the second component are intermixed such that the second component is dispersed throughout a bulk of the first component, and wherein the first component and the second component are substantially immiscible at 20 °C.

10. The method of claim 9, wherein the mixture of the first alkali metal and the second alkali metal comprises the first alkali metal in an amount greater than or equal to 90 wt.% and less than or equal to 99.99 wt.% based on a total weight of the mixture.

11. The method of any one of claims 9-10, wherein the mixture of the first alkali metal and the second alkali metal comprises the second alkali metal in an amount greater than or equal to 0.01 wt.% and less than or equal to 10 wt.% based on a total weight of the mixture.

12. The method of any one of claims 9-11, further comprising: layering: (i) the mixture of the first alkali metal and the second alkali metal; and (ii) the third alkali metal, thereby forming a layered structure comprising a layer of the third alkali metal disposed over a layer of the mixture of the first alkali metal and the second alkali metal.

13. The method of any one of claims 9-12, further comprising: deforming and folding: (i) the mixture of the first alkali metal and the second alkali metal; and (ii) the third alkali metal, thereby forming the composite structure.

14. The electroactive material, electrochemical device, or method of any one of the preceding claims, wherein the first alkali metal is metallic lithium.

15. The electroactive material, electrochemical device, or method of any one of the preceding claims, wherein the second alkali metal is metallic sodium.

16. The electroactive material, electrochemical device, or method of any one of the preceding claims, wherein the third alkali metal is selected from the group consisting of metallic potassium, metallic rubidium, and metallic cesium.

17. The electroactive material, electrochemical device, or method of any one of the preceding claims, wherein the third alkali metal is metallic potassium.

18. The electroactive material, electrochemical device, or method of claim 17, wherein the second component is a liquid eutectic mixture of the metallic sodium and the metallic potassium at 20 °C.

19. The electroactive material, electrochemical device, or method of any one of the preceding claims, wherein the electroactive material comprises the first component in an amount greater than or equal to 85 wt.% and less than or equal to 99.99 wt.% based on a total weight of the electroactive material.

20. The electroactive material, electrochemical device, or method of any one of the preceding claims, wherein the electroactive material comprises the second component in an amount greater than or equal to 0.01 wt.% and less than or equal to 15 wt.% based on a total weight of the electroactive material.

21. The electroactive material, electrochemical device, or method of any one of the preceding claims, wherein the second component forms a plurality of discrete phase domains dispersed within the first component.

22. The electroactive material, electrochemical device, or method of claim 21, wherein an average maximum characteristic dimension of the plurality of discrete phase domains is less than or equal to 50 micrometers.

23. The electroactive material, electrochemical device, or method of any one of the preceding claims, wherein a grain size of the first component and / or the second component is less than or equal to 50 micrometers.

24. The electroactive material, electrochemical device, or method of claim 23, wherein the grain size of the first component and / or the second component is greater than or equal to 0.01 micrometers.

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