Negative electrode

Abstract

A negative electrode for a solid-state battery, the negative electrode having a first surface and thickness, the negative electrode for a solid-state battery comprising: a first phase including a first metallic element; a second phase distributed in the first phase, wherein the second phase is an intermetallic phase including the first metallic element and at least a second element, and wherein a diffusion coefficient of the first metallic element in the second phase is greater than a diffusion coefficient of the first metallic element in the first phase, and wherein there are a plurality of continuous pathways of the second phase each through at least half the thickness of the negative electrode.

Description

Negative ElectrodeTechnical field

[0001] The present invention relates to a negative electrode for an electrochemical battery.Background Art

[0002] All-solid-state batteries (ASSB) are a promising development over conventional batteries which use a liquid electrolyte. For example, ASSB can solve many problems of liquid electrolyte batteries, such as Li-ion batteries, including flammability, limited voltage, unstable solid-electrolyte interface formation and poor cycling performance.

[0003] In batteries comprising a liquid electrolyte, the liquid electrolyte can permeate the electrodes and provide fast ionic transport. In a battery comprising a solid-state electrolyte, there may be a fixed planar boundary between the electrolyte and the electrode.

[0004] A battery typically comprises a negative electrode (anode) and a positive electrode (cathode) which are separated by an electrolyte. During discharge, the negative electrode (the anode) releases metal ions (such as lithium) to the cathode and generates a flow of electrons through an external circuit. The metal takes part in a redox reaction which enables conversion of chemical energy into electric energy during discharge and vice versa.

[0005] Transport of the metal across the negative electrode may be slow due to limited diffusivity of the metal through the electrode. This may limit the discharge rate of the battery.

[0006] The energy density of a battery may be affected by the energy density of the anode. It may be advantageous to provide a battery with an anode having high energy density, to improve the energy density of the battery. Metals such as lithium have a high energy density and are therefore desirable candidates for anodes. However, metals (such as lithium) may have low diffusivity in these known anode materials having high energy density. The energy density refers to the energy stored in the electrode per unit weight. For a given energy capacity, a heavier electrode has a lower energy density. The energy capacity of an electrode is the energy that can be delivered in the discharge process, i.e. how much energy can be supplied by the electrode.

[0007] The performance of known batteries having a high energy density anode, such as a pure lithium metal anode, is limited by slow lithium transport within the anode.

[0008] Wan, H., Wang, Z., Zhang, W. et al. “Interface design for all-solid-state lithium batteries” Nature 623, 739-744 (2023) (referred to as “Wan et al”) discloses a negativelithium electrode for a solid-state battery the electrode comprising an interlayer between the lithium anode and the solid electrolyte. The interlayer is a triple interlayer confined to a surface of the electrode.

[0009] Z. Li, X. Jiang, G. Lu, T. Deng, R. Wang, J. Wei, W. Zheng, Z. Yang, D. Tang, Q. Zhao, X. Hu, C. Xu, X. Zhou, “Composite lithium with high ionic conducting LisBi alloy enabled high-performance garnet-type solid-state lithium batteries”, Chemical Engineering Journal, Volume 465, 2023, 142895, ISSN 1385-8947, (referred to as “Li et al ”) discloses a composite lithium anode comprising a lithium matrix phase, discrete LisBi particles and LiF phases.

[0010] Liu, Y., Wang, C., Yoon, S.G. et al. “Aluminum foil negative electrodes with multiphase microstructure for all-solid-state Li-ion batteries”. Nat Commun 14, 3975 (2023), (referred to as “Liu et al.”) discloses an aluminium foil negative electrode with a multiphase microstructure for an all solid-state Li-ion battery.

[0011] Current implementations of negative electrodes in a solid-state battery result in a battery which has a low charge / discharge rate due to the low diffusivity of metal atoms / ions through the negative electrode.

[0012] The present invention aims to provide a negative electrode for a solid-state battery having high and preferably high capacity and high energy density.Summary of Invention

[0013] According to the present invention, there is provided a negative electrode for a solid- state battery, the negative electrode having a first surface and a thickness, the negative electrode for a solid-state battery comprising: a first phase including a first metallic element; a second phase distributed in the first phase, wherein the second phase is an intermetallic phase including the first metallic element and at least a second element, and wherein a diffusion coefficient of the first metallic element in the second phase is greater than a diffusion coefficient of the first metallic element in the first phase, and wherein there are a plurality of continuous pathways of the second phase each through at least half the thickness of the negative electrode.

[0014] The inventors have discovered that providing an intermetallic phase with continuous pathways throughout the first phase allows for higher extracted capacity at high current densities when the first element has a higher diffusivity in the intermetallic phase relative to the first phase. The continuous pathways act as a fast diffusion channels for atoms / ions of the first metallic element (acting as a fast-transporting phase), which leads to increaseddiffusivity of the metallic element in the electrode as a whole. High extracted capacity at high current densities may be observed without having to elevate the temperature, unlike what is known from the prior art. The diffusivity of the first metallic element in the intermetallic phase can be very high. This may lead to improved performance when the electrode is implemented in a battery, for example during discharge.

[0015] According to the present invention, there is also provided a negative electrode for a solid-state battery, the negative electrode having a first surface and a thickness, the negative electrode for a solid-state battery comprising: a first phase including a first metallic element; a plurality of precipitates of a second phase distributed in the first phase, wherein the precipitates have a mean aspect ratio of more than 1.1 and a mean of angles of the major axis of the precipitates to the thickness direction of the electrode (a) satisfies 0° < a < 87.5°; wherein second phase is an intermetallic phase including the first metallic element and at least a second element, wherein a diffusion coefficient of the first metallic element in the second phase is greater than a diffusion coefficient of the first metallic element in the first phase.

[0016] The present inventors have discovered that providing elongated precipitates of the intermetallic phase throughout the first phase provides fast diffusion channels for atoms / ions of the first metallic element (acting as a fast-transporting phase), which leads to increased diffusivity of the metallic element in the electrode as a whole. When the precipitates are oriented at least partially along the thickness direction of the electrode, the precipitates aid diffusion of the first metallic element across the thickness of the electrode. High extracted capacity at high current densities may be observed without having to elevate the temperature, unlike what is known from the prior art. The diffusivity of the first metallic element in the intermetallic phase can be very high. This may lead to improved performance when the electrode is implemented in a battery, for example during discharge.

[0017] In an embodiment, the mean aspect ratio is more than 1.2, preferably more than 1.3, preferably, more than 1.4, more preferably, more than 1.6, even more preferably more than 1.8, even more preferably at least 2.0.

[0018] In an embodiment, the second phase defines pores in a plane normal to the electrode thickness, the pores having a mean maximum internal tangential diameter of no more than 80 pm, preferably, no more than 40 pm, even more preferably, no more than 20 pm. In an electrode comprising this microstructure, any point in the slow-transporting first phase is sufficiently close to the fast-transporting second phase. The first metallic element may diffuse through the slow-transporting first phase over a relatively short distance to reach thefast-transporting second phase, in which it can diffuse at a greater speed. This increases the speed of transport of the metallic element through the whole electrode. The inventors have determined that pores of this size lead to a particularly high increase in the diffusion of the first metallic element through the electrode, while leaving ample space in the electrode for the first phase. The first phase is rich in the metal which takes part in a redox reaction of the battery, and may be relatively light compared to the second phase.

[0019] In an embodiment, the diffusion coefficient of the first metallic element in the second phase is at least 10 times greater, preferably 100 times greater, or even more preferably at least 1000 times greater, than the diffusion coefficient of the first metallic element in the first phase.

[0020] In an embodiment, the first phase is present within the pores defined by the second phase. The first metallic element of the first phase may take part in a redox reaction which may occur when the negative electrode is connected to a positive electrode. During discharge of a battery comprising the negative electrode, the first metallic element of the first phase may be reduced at the anode to form ions, which can then quickly diffuse through the electrode by the fast-transporting second phase.

[0021] In an embodiment, the continuous pathways of the second phase each have a length, and the length of each respective continuous pathway is 50-500% of the electrode thickness. The continuous pathway extends through the thickness of the electrode, thereby providing a fast-transporting channel for the first metallic element from one surface of the electrode to another surface of the electrode. The continuous pathway may be tortuous, meaning the length of the pathway exceeds the thickness of the electrode. The length of each respective continuous pathway being 50-500% of the electrode thickness may lead to a particularly high increase of the diffusivity of the first metallic element in the electrode.

[0022] In an embodiment, the second element is selected from one or more of: a metallic element, a metalloid element, or a semiconductor element. The first metallic element may exhibit unexpectedly high diffusivity in the intermetallic phase, when intermetallic phase comprises a metallic element, a metalloid element, or a semiconductor element. The diffusivity of lithium ions in this embodiment may be particularly high.

[0023] In an embodiment, the first phase consists of the first metallic element and optionally other elements in solid solution in the first metallic element. Some solid solution elements may exist in the first phase due to the manufacturing process. According to this embodiment, the capacity and energy density of the electrode may be particularly high.

[0024] In an embodiment, the first metallic element is lithium. A lithium anode is a particularly advantageous anode due to its high energy density and high energy capacity.

[0025] In an embodiment, the second phase is in thermodynamic equilibrium with the first phase. By providing a second phase which may exist in thermodynamic equilibrium with the first phase, high, near-perfect or perfect interfacial stability is achieved between the first and the second phase. This improves the longevity of the electrode.

[0026] In an embodiment, the intermetallic phase has a general formula AxBywhere A is the first metallic element and B is the second element. The values of x and y may be selected such that a thermodynamically stable compound of AxByis formed. The first metallic element may have particularly high diffusivity in an intermetallic phase according to this formula.

[0027] In an embodiment, the second element is bismuth. The first metallic element may have particularly high diffusivity in an intermetallic phase according to this formula, and the second phase may have particularly good stability with the first phase.

[0028] In an embodiment, the intermetallic phase makes a molar fraction of 0.1-50 wt%, more preferably 1-30 wt% of the negative electrode. The present inventors have discovered that this weight fraction leads to a particularly good balance between increased speed of transport of the first metal in the first electrode, while retaining high energy density and capacity of the electrode.

[0029] In an embodiment, the intermetallic phase makes a volume fraction of 1-40 vol. %, more preferably 5-20 vol. %, of the negative electrode. The present inventors have discovered that this volume fraction leads to a particularly good balance between improved lithium transport, while retaining high energy density and capacity of the electrode.

[0030] In an embodiment, an average of the minimum distance from a given point in the matrix to the nearest intermetallic phase is no greater than 40 pm, preferably no greater than 20 pm, more preferably no greater than 10 pm, and more preferably no greater than 5 pm, further preferably no greater than 2 pm, even further preferably no greater than 1 pm. A minimum distance from a given point in the matrix to the nearest intermetallic phase reduces the mean transport time of a lithium atom / ion through the electrode. The present inventors have discovered that this leads to a particularly good balance between improved conductivity, while retaining high energy density and capacity of the electrode.

[0031] In an embodiment, the second phase is an interconnected scaffold distributed in the first phase. By producing a microstructure with an interconnected network of intermetallic phase within a lithium metal matrix, a high effective electrode diffusivity can be achieved.

[0032] In an embodiment, the second phase comprises at least one 3-dimensional interconnected microstructure defining pores in which the first phase is located. By producing a microstructure with an interconnected network of the intermetallic phase within the first phase, a high effective electrode diffusivity can be achieved.

[0033] In an embodiment, the second phase comprises a scaffold structure, the scaffold structure defining pores, wherein the first phase is in the pores. This embodiment exhibits a particularly good balance between a high effective electrode diffusivity, while retaining high energy density and capacity of the electrode. This embodiment may be particularly convenient to form.

[0034] In an embodiment, the second phase defines needles which have an elongate dimension within 30 degrees of the direction normal to the first surface of the electrode. This is another advantageous embodiment of the micro structure which leads to good balance between a high effective electrode diffusivity, while retaining high energy density and capacity of the electrode.

[0035] In an embodiment, a mean maximum internal tangential diameter of the pores is half the thickness of the negative electrode or less, preferably one third of the thickness of the negative electrode or less, more preferably one quarter the thickness of the negative electrode or less, and even more preferably one tenth the thickness of the negative electrode or less. According to this embodiment, the fast-transporting second phase leads to a particularly high increase in the speed of diffusion of the first metallic element across the thickness of the electrode.

[0036] In an embodiment, the negative electrode consists of the first phase, the second phase and up to 10 vol. % of an additional phase and incidental impurities. According to this embodiment, high diffusivity, high energy density and capacity of the electrode are achieved, and other additional phases may be incorporated which further improve the performance of the electrode.

[0037] The negative electrode may be directly obtainable by quenching an alloy. Such an electrode may have a particularly convenient method of manufacturing which is relatively quick.

[0038] The negative electrode may be directly obtainable by directionally solidifying an alloy. Such an electrode may have a particularly convenient method of manufacturing which is relatively quick.

[0039] According to the present invention, there is provided a method of manufacturing a negative electrode. In an embodiment, the method comprises: forming a scaffold of thesecond element, wherein the scaffold defines pores, treating the scaffold with the first element or a compound thereof to form a scaffold of the intermetallic, at least partially filling the pores of the scaffold with the first element. This results in a homogenous distribution of the second phase in the electrode advantageously meaning that every part of the first phase is close to a part of the second phase and so to a quick route through the electrode towards the electrolyte.

[0040] In an embodiment, the treating the scaffold with the first element or a compound thereof to form a scaffold of the intermetallic comprises: forming an electrical circuit comprising the scaffold and a positive electrode comprising the first metallic element, and applying a voltage across the scaffold and the positive electrode. When a voltage is applied, ions of the first metallic element are transported from the positive electrode to the scaffold. The first metallic element ions react with the scaffold to form a scaffold of the intermetallic.

[0041] In an embodiment, the filling the pores of the scaffold with the first element comprises forming an electrical circuit with the negative electrode, a cathode comprising the first element and a source of energy, and charging the negative electrode. Upon a voltage being applied across the electrodes, the cell formed by the negative and positive electrodes charges. Upon charging by the voltage, the first element fills in the pores.

[0042] According to the present invention, there is provided a method of manufacturing a negative electrode according to the present invention, the method comprising: providing an alloy of the first metallic element and the second element at a temperature above the liquidus temperature; and cooling the molten alloy to form an ingot comprising the first phase and the second phase. This method may be a particularly convenient way of obtaining an electrode according to the present invention, as the second phase may precipitate within the first phase to form the fast-diffusion channels for atoms / ions of the first metallic element (acting as a fast-transporting phase).

[0043] In an embodiment, the providing an alloy comprises providing a mixture of the first metallic element in granule form and the second element in granule form; and heating the mixture above the higher of the liquidus temperature of the first metallic element and the liquidus temperature of the second element. Upon cooling, a plurality of continuous pathways of the second phase distributed within the first phase may be formed. Instead of or in addition to the continuous pathways, the microstructure of the formed anode may comprise a plurality of precipitates of the second phase distributed within the first phase, wherein the second phase is an intermetallic phase including the first metallic element and at least asecond element, the elongated precipitates at least partially oriented along the thickness direction of the electrode and having a mean aspect ratio of more than 1.1 .

[0044] In an embodiment, the step of providing an alloy may comprise providing a mixture of the first metallic element in granule form and an intermetallic phase including the first metallic element and at least a second element in granule form; and heating the mixture above the higher of the liquidus temperature of the first metallic element and the liquidus temperature of the intermetallic.

[0045] In an embodiment, the alloy comprises lithium, bismuth, and optionally, magnesium.

[0046] In an embodiment, the alloy has a chemical composition of, in atomic %: 1 at.% to 5 at.% Bi, 0.0 at.% to 1.5 at.% Mg, the balance being Li and incidental impurities. Preferably, the alloy has a chemical composition of, in atomic %: 1.0 at.% to 3.0 at.% Bi, 0.0 at.% to 1.5 at.% Mg, the balance being Li and incidental impurities. An alloy having this composition strikes a particularly good balance between providing enough lithium to form an anode having high capacity, while providing enough of the second phase which increases the effective diffusion coefficient of the anode.

[0047] In an embodiment, the cooling the molten alloy comprises quenching the molten alloy, optionally at a rate of at least 100 K / s. This may lead to a fine distribution of precipitates, meaning that a fast-diffusion channel is easily accessible from any point in the first phase.

[0048] According to the present invention, there is provided a battery comprising the negative electrode. The battery may comprise a positive electrode, wherein the positive electrode comprises the first metallic element. The battery may comprise a solid-state electrolyte. A battery comprising a solid-state electrolyte may overcome safety problems associated with liquid electrolyte batteries.

[0049] In an embodiment, the method comprises providing an alloy of the first metallic element and the second metallic element at a temperature above the liquidus temperature; and cooling the molten alloy to form the matrix phase (the first phase) and the intermetallic phase (the second phase).According to the invention, there is provided a negative electrode obtained by a method according to the present invention. According to the invention, there is provided a battery comprising the electrode, and optionally, a positive electrode. The positive electrode may comprise the first metallic element. A solid-state electrolyte may be sandwiched between the negative electrode and positive electrode.

[0050] The term “consisting of’ is used herein to indicate that 100% of the composition is being referred to and the presence of additional components is excluded so that percentages add up to 100 volume percent. Unless stated otherwise, all amounts are given in volume percent (vol%). The term comprising is used in a non-exhaustive way, indicating that other components may be present.Figures

[0051] The invention will be more fully described by way of example only, with reference to the accompanying drawings in which:

[0052] Figures 1 A and IB are schematic diagrams of a negative electrode according to the present invention.

[0053] Figures 2A and 2B shows examples of a microstructure of the second phase of an electrode according to a method of the present invention.

[0054] Figure 3 shows the discharge behaviour of lithium negative electrode, according to a comparative example at 30°C, 5 MPa stack pressure.

[0055] Figure 4 part A shows a schematic of a simulated example of a microstructure of the electrode according to the present invention. Figure 4 part B shows the effect of intermetallic spacing (may be referred to as mean distance to nearest neighbour) on the extracted capacity of Li according to the microstructure shown on Figure 4 part A.

[0056] Figure 5 A is an Arrhenius plot of diffusivity with temperature of Li ion diffusivity in LisBi. Figure 5B shows data fit to give values in Figure 5A.

[0057] Figures 6A, 6B, 6C show an example of part of the method for measuring the mean maximum internal tangential diameter according to the present invention.

[0058] Figure 7A shows a microstructure of an as-cast sample of Li+2.75%Bi. Figure 7B shows the distribution of pore size in the micrograph shown in Figure 7A. Figure 7C shows a microstructure of the Li+2.75%Bi sample after cold-rolling. Figure 7D shows the distribution of pore size in the micrograph shown in Figure 7C.

[0059] Figures 8A, 8B and 8C show the electrochemical performance of anodes according to the invention as well as comparative examples.

[0060] Figure 9 is a schematic illustrating how rolling an as-cast sample may lead to reduction in the distance between precipitates of the second phase, forming a network of fastdiffusion channels throughout the electrode.

[0061] Figure 10A is a schematic of a preferable microstructure the anode, comprising lamellae of intermetallic dispersed within the lithium matrix. Figure 10B shows the predictedelectrochemical performance (accessible capacity vs current density) of a microstructure of Figure 10A, for different spacing of the intermetallic lamellae. Figure 10C predicted shows the electrochemical performance (accessible capacity vs current density) of a microstructure of Figure 10A with a spacing of 20 pm, for intermetallic having different diffusivities.

[0062] Figure 11 illustrates the major and minor axes of a precipitate.Detailed Description

[0063] An electrochemical cell (or a “battery”) typically comprises an anode and a cathode separated by an electrolyte. During discharge, oxidation of a metallic element occurs at the anode; this releases electrons into the external circuit and positive ions of the metallic element are transferred to a cathode via an electrolyte. During discharge, reduction of the metallic element ions occurs at the cathode; electrons from the external circuit combine with ions in the cathode. The process is reversible. The anode may be referred as “a negative electrode” and the cathode may be referred to as “a positive electrode”.

[0064] The cathode and the anode are separated by an electrolyte: a substance that allows ions to move between the anode and the cathode. In known liquid electrolyte batteries, the electrolyte is a liquid.

[0065] In an example, the anode comprises a metallic element such as lithium. During discharge, the anode releases lithium ions to the cathode via the electrolyte, which may be represented by the following half-reaction: Li —> Li++ e'. During charging, lithium ions are released by the cathode to the anode, which may be represented by the following halfreaction: Li++ e' — Li. The anode may comprise another metallic element X, in which case ions of X are transported during charge and discharge.

[0066] The battery may comprise current collectors disposed on the anode and on the cathode, which collect electrons from the electrodes providing a path for the electrical current to exit the battery.

[0067] There are a number of key parameters which may be considered in assessing the performance of a battery. For example, the battery capacity (electricity generated, which may be measured in Ampere hours, Ah), the energy density (how much energy a battery contains in proportion to its weight, which may be measured in measured Watt-hours per kilogram, Wh / kg). Transport of the metallic element (which may be referred to as the first metallic element) in the electrode affects the rate of charge and discharge. The rate of diffusion of ions of the metallic element during charge and discharge through the electrode(s) may affect the rate of battery charge and / or discharge. It is noted that some ions may diffuse through theelectrode to some degree during charge / discharge, so diffusion behaviour of the metallic element and / or ions of the metallic element may affect the charge / discharge kinetics of the battery. These parameters may depend on the choice of anode and cathode.

[0068] In a battery comprising a solid-state electrolyte, there may be a fixed planar boundary between the electrolyte and the electrode. If transport of ions of the metallic element is slow within the electrode, the charge rate and discharge rate of the battery may be diffusion limited. A battery having a lithium anode may exhibit slow lithium transport through the anode, meaning that the charge rate and discharge rate of the battery may be limited by the rate of lithium diffusion across the electrode.

[0069] For example, a lithium metal electrode is known to have high energy density. However, lithium exhibits relatively slow self-diffusion, meaning that a battery comprising a lithium electrode (e.g. lithium anode) may have slow charge and discharge rates. Lithium anodes may also exhibit low energy density.

[0070] Pure lithium metal anodes do not meet the necessary discharge performance due to slow transport of metallic ions. For example, diffusion of lithium in a lithium anode is relatively low. Specifically, the extracted capacity during discharge is limited by slow selfdiffusion in lithium metal. While there are some reports of lithium metal anodes having a high accessible capacity, these results are typically reported for lithium batteries which are subject to a high applied pressure or elevated temperature, which is not realistic for practical implementations. Other anodes comprising a metallic element other than lithium may exhibit the same problem. While some of the problem associated with anodes are discussed in relation to lithium anodes, anodes comprising a different metallic element may exhibit some or all of the problems discussed herein.

[0071] It may be preferable to incorporate a lithium metal anode into ASSBs. The solid electrolyte may be a high ionic conductivity stiff ceramic. To be commercially viable, ASSBs must be capable of fast discharge, optionally, at room temperature. Whereas in liquid electrolyte batteries, the electrode can be made porous, such that the electrolyte can infuse the electrode and provide fast ionic transport; in solid state batteries there is a fixed planar electrolyte - electrode interface. The anode must therefore have fast ionic transport kinetics for fast discharge to be possible. Ionic transport kinetic refer to the transport kinetics of the ions which move between the anode and cathode during charge and discharge.

[0072] The present inventors have realised that the introduction of a second phase into the (lithium) metal anode, with fast metallic element diffusion kinetics has the potential to increase the effective diffusivity of a metallic element in the anode as a whole; enabling fulldischarge at a higher current density. The second phase should ideally be stable against the metal of the anode, else interfacial reaction products will impede transport.

[0073] The present inventors have discovered that incorporation of the second phase as a plurality of continuous pathways into a first phase comprising the metallic element provides fast-diffusion pathways for the metal ion / atom during charge and discharge of a battery.

[0074] A problem with incorporating additional phases into the electrode is that the capacity and the energy density of the electrode are reduced. Incorporation of an additional phase into the electrode (for example, Li et al. discloses a composite lithium anode comprising a lithium matrix phase, LisBi and LiF phases) results in areas of the electrode being replaced with the additional phase, which reduces the total volume occupied by the energy-storing material. Also, additional phases may be relatively heavy, which leads to increased mass of the electrode and further reduces energy density. Furthermore, Li et al. discloses an electrode comprising large globules of additional phases incorporated therein. This reduces the total capacity and energy density of the electrode.

[0075] Li et al. focus on the advantages of including LiF in an electrode to improve wetting and so the interface with the electrolyte. Li et al. note that to compensate for lithiophobic LiF the LisBi can quickly transport lithium ions from the bulk to the solid-state interface and compensate for lithium depletion during stripping. However, Li et al. only discloses microstructures having isolated particles of the LisBi dispersed in the lithium matrix. Li et al. discloses that an SEM image of an example on Supplementary Figure S10. This figure shows discrete Bi-rich regions in a lithium matrix. These discrete regions are not interconnected in that they are isolated from each other. A schematic diagram of a lithium anode structure is shown on Figure 4 of Li et al. Figure 4 of Li et al. is a cartoon of an interface between the LLTZO electrolyte and the anode. Figure 4 of Li et al. does not correspond to a real microstructure and the microstructure of the example is shown in Figure S10 of Li et al. Furthermore, Figure 4 of Li et al. is merely a schematic illustration that does not correspond to any real examples of the electrode.

[0076] Wan et al. discloses a lithium electrode and an electrolyte with an interlayer disposed therein, wherein the interlayer is confined to the interface between the electrode and electrolyte. The interlayer is a triple interlayer confined to a surface of the electrode, comprising three layers: a layer of Li and Mg in solid solution, a layer comprising an intermetallic of Li BL and a LiMgSx interphase layer. The central layer comprising the intermetallic is separated from the lithium electrode by the Li / Mg solid solution. Wan et al. postulates that this triple interlayer may suppress lithium dendrite growth. However,introduction of this interlayer does not affect lithium transport through the electrode, because lithium must travel through the bulk lithium electrode to reach the current collector.

[0077] Liu et al. discloses a multi-phase electrode for use in a lithium ion battery, namely isolated islands of lithium-indium dispersed in an aluminium-lithium matrix. In other words, the electrode of Liu et al. comprises two different intermetallic phases, rather than a metallic phase and an intermetallic phase. A lithium-aluminium matrix may be disadvantageous, due to the high content of non-lithium elements. This is disadvantageous because incorporation of non-lithium elements into the anode of a battery may result in a battery having poor energy density. In addition, an electrode consisting of (or comprising) two intermetallic phases, such as the electrode of Liu et al., may result in an unstable electrode due to the chemical incompatibility between the two different intermetallic phases. Furthermore, incorporation of additional phases comprising elements other than the matrix may cause local stress fields in the matrix and / or interfacial instability, leading to defects which may degrade battery performance. In Liu et al., the lithium indium phase is present as distinct particles which are not interconnected, and extend parallel to the anode surface. These particles offer limited or no improvement in the diffusivity of lithium ions within the electrode. This is in contrast to the present invention in which there are a plurality of continuous pathways of the second phase which extend through the thickness of the anode.

[0078] The microstructure of the electrode described herein is the microstructure which may be observed when the electrode is in a charged state. The charged state is that which may be observed if the negative electrode is configured to be part of a battery, wherein the battery is charged. In the discharged state, the matrix phase (i.e. the first phase) may be partially or fully removed from the structure. The electrode may be manufactured and incorporated into a battery in a discharged state. The first phase may be introduced into the negative electrode structure upon charging of a battery comprising the electrode. Upon discharge, some of the first metallic element may be removed from the intermetallic phase, when ions of the first metallic element move to the cathode. Upon charging, the first phase the anode is restored as the first metallic element diffuses from the cathode to the anode.

[0079] When the first phase comprises lithium as the first metallic element, the intermetallic phase is a lithium intermetallic. In the charged state, the electrode comprises regions of metallic lithium (i.e. the first phase) and regions of the intermetallic (i.e. the second phase). Upon discharge, lithium ions may diffuse from the first phase towards a cathode of the battery, leaving empty space in the electrode. The intermetallic phase may be partially of fully delithiated when the negative electrode is discharged. Upon charging, lithium may betransported back into the anode from the cathode, filling or partially filling the empty spaces left behind upon discharge.

[0080] The present inventors have discovered an anode (a negative electrode) having improved conductivity, which overcomes the problems mentioned above. According to the present invention, there is provided a negative electrode having a matrix phase of a first metallic element (such as lithium) and an interconnected three-dimensional network of intermetallic phase distributed in the matrix phase, wherein lithium has a diffusion coefficient in the intermetallic phase which is greater than a diffusion coefficient of lithium in the matrix phase. Although the present invention is described with reference to increase lithium diffusivity in the intermetallic phase compared to the matrix phase, the present invention is applicable to other type of battery where the ion being transported is other than lithium.

[0081] According to the present invention there is provided a negative electrode for a solid- state battery, the negative electrode having a first surface and a thickness, the negative electrode for a solid-state battery comprising: a first phase including a first metallic element; a second phase distributed in the first phase, wherein the second phase is an intermetallic phase including the first metallic element and at least a second element, and wherein a diffusion coefficient of the first metallic element in the second phase is greater than a diffusion coefficient of the first metallic element in the first phase, and wherein there are a plurality of continuous pathways of the second phase each through at least half the thickness of the negative electrode.

[0082] According to another embodiment of the present invention there is provided a negative electrode for a solid-state battery, the negative electrode having a first surface and a thickness, the negative electrode for a solid-state battery comprising: a first phase including a first metallic element; a second phase distributed in the first phase, wherein the second phase is an intermetallic phase including the first metallic element and at least a second element, and wherein a diffusion coefficient of lithium in the second phase is greater than a diffusion coefficient of lithium in the first phase, and wherein there are a plurality of continuous pathways of the second phase each through at least half the thickness of the negative electrode.

[0083] According to the present invention, the intermetallic phase is distributed in a network of continuous pathways such that it provides pathways for the diffusion of lithium from the bulk of the anode to the electrolyte interface. A schematic of the negative electrode according to the present invention adjacent to a solid-state electrolyte layer (SSE) is shown in Figure 1A. Figure 1A is illustrative only, and the relative and absolute dimensions of Figure1 A do not necessarily reflect the relative and absolute dimensions in a negative electrode according to the present invention. Figure 1 A shows an embodiment comprising a lithium matrix (i.e. the first phase comprising lithium as the first metallic element) and a LisBi intermetallic matrix (i.e. the second phase comprising LisBi as the intermetallic phase). This is an example, and other materials are possible.

[0084] The thickness direction (d) of the electrode is the direction of charge flow when the electrode is configured to be part of a battery. The thickness is perpendicular or substantially perpendicular to the first surface of the electrode. The electrode thickness may be up to 300 pm, or up to 100 pm, or preferably up to 50 pm, or more preferably up to 30 pm.

[0085] Figure IB shows three examples of continuous pathways of the second phase (Al, A2 and A3) through the electrode shown in Figure 1 A. The continuous pathway may extend substantially perpendicular to a first surface of the electrode. When the negative electrode is configured to be part of a battery, the first surface of the negative electrode may be a surface which is in contact with a SSE (as shown in the example in Figure IB). Alternatively, the first surface may be in contact with a current collector.

[0086] The continuous pathway follows along the second phase. A continuous pathway may be defined as the shortest pathway in the second phase across the electrode thickness direction (or substantially parallel to the thickness direction) from a first point in the second phase to a second point in the second phase. The first and second points are separated in the thickness direction by a distance of at least half of the thickness of the electrode from each other. A continuous pathway may be identified by a 3D reconstruction of the electrode via computed tomography.

[0087] The continuous pathway may not extend in a straight line. The continuous pathways may have a tortuosity. For example, Al in Figure IB is a continuous path in the second phase having low tortuosity. Continuous path A2 has a higher tortuosity than Al, and continuous path A3 has a higher tortuosity than A2.

[0088] As the continuous path may have a tortuosity, its length may be longer than a thickness of the electrode. The continuous pathway does not necessarily extend from one surface of the electrode to another surface of the substrate.

[0089] A continuous pathway has a tortuosity factor (TF), which is defined as the actual path length divided by direct path length. The actual path length corresponds to the length of the continuous pathway as defined above. The direct path length is the shortest straight line from the first point to the second point.

[0090] The effective diffusivity Deff of the first metallic element (e.g. lithium) though the electrode may then be approximated to:Deff = D * (volume fraction of the second phase in the electrode) (TF) where TF is the tortuosity factor and D is the diffusivity of the first metallic element in the second phase (in an example, D may be the diffusivity of lithium in the second phase). In an embodiment Deff is greater than the diffusivity in the first phase, preferably at least 1.1 times the diffusivity in the first phase. In the example given below where the first element is lithium and the second element is bismuth, Deff will be several hundred times greater than the self-diffusivity of lithium.

[0091] In an embodiment, the tortuosity factor of the continuous pathways may be up to 100, preferably up to 50, more preferably up to 10, even more preferable up to 5. For example, up to 90% of the continuous pathways may have a tortuosity factor of up to 100, preferably up to 50, more preferably up to 10, even more preferable up to 5. Additionally or alternatively, the mean average tortuosity factor of the continuous pathways may be up to 100, preferably up to 50, more preferably up to 10, even more preferable up to 5.

[0092] A lower tortuosity factor of the continuous pathways may lead to greater improvement in the effective first metallic element diffusivity (e.g. lithium diffusivity) through the electrode because more direct fast-diffusion pathways are available.

[0093] The diffusivity of the first metallic element in the second phase may be orders of magnitude higher than diffusivity of the first metallic element in the first phase. For example, the diffusivity of lithium atoms in LisBi is 4500 times higher than diffusivity of lithium atoms in metal lithium. Therefore, even if the tortuosity of the continuous pathways is high and / or the porosity of the electrode is high, significant improvements in performance of the electrode can be achieved.

[0094] If D in the intermetallic is lOOOx as high as Li (LisBi is 4500x higher), then you can have very high tortuosity and porosity, and still achieve performance improvement.Preferably, the tortuosity factor is less than the Dsecond phase / Dfirst phase, wherein Dsecond phase is the diffusivity of the first metallic element in the second phase and Dfirst phase is the diffusivity of the first metallic element in the first phase. In an embodiment, the tortuosity factor is less than half the Dsecond phase / Dfirst phase. In an emboidment, Dsecond phase is the diffusivity of the first metallic element (e.g. lithium element) in the second phase and Dfirst phase is the diffusivity of the first metallic element (e.g. lithium) in the first phase. Improvements in performance may be achieved even if the difference in diffusivity is less than lOOOx, as described with reference to Figure 10C. For example, improvements in performance may be achieved if thediffusivity of the first metallic element in second phase is at least 10 times higher than the diffusivity in the first phase, or at least 100 times higher than the diffusivity in the first phase. The term “diffusivity” is used as short-hand to refer to the diffusion coefficient of the first metallic element in the relevant phase. It is noted that the diffusion coefficient may vary with temperature, and the relevant diffusion coefficient is the diffusion coefficient under typical operating conditions (temperature of 20 °C).

[0095] The provision of continuous pathways through at least half the thickness of the negative electrode may be enough to achieve an improvement in the diffusivity of metal ions / atoms across the electrode. The continuous pathway may be longer, and it may extend from the first surface of the electrode to an opposing surface of the electrode, i.e. it may be provided through the entire thickness of the negative electrode. Alternatively, the continuous pathways may be provided through a portion of the electrode which is greater than half the thickness of the negative electrode but smaller than the full the thickness of the negative electrode.

[0096] Provision of a continuous pathway through at least half the thickness of the negative electrode ensures that there is a fast-transporting phase provided through a significant portion of the electrode. Thus, metal can diffuse though a substantial part of the electrode via the fast-transporting second phase, which decreases the mean travel time of the metal through the electrode. Providing a continuous pathway of the second phase through less than half of the electrode may not achieve a significant decrease in the mean travel time of the metal through the electrode, because most of the path taken by a metal when diffusing across the electrode would be taken through the slow-transporting first phase. Provision of a continuous pathway of the fast-conducting second phase through at least half the thickness of the negative electrode leads to a statistically significant decrease in the mean travel time of the metal through the electrode, even if some of the metal happens to take a path through the slow- transporting first phase.

[0097] For example, the length of each respective continuous pathway may be 50-500% of the electrode thickness. Preferably, the length of each respective continuous pathway may be 60-400% of the electrode thickness. More preferably, the length of each respective continuous pathway may be 70-300% of the electrode thickness. Even more preferably, the length of each respective continuous pathway may be 80-200% of the electrode thickness. Pathways having a length above that range may be too long to offer a significant improvement in the time taken for a metal atom to diffuse through the second phase.Pathways that are shorter are not long enough to provide fast-diffusion channels through the electrode.

[0098] In an embodiment, like illustrated in figure 2B, the plurality of pathways may extend all the way from the first surface to the opposing surface of the electrode.

[0099] By providing an intermetallic phase which is distributed in the first phase, transport of the metallic element in the electrode is increased.

[0100] The intermetallic phase may be distributed throughout the electrode substantially evenly. For example, the intermetallic phase is not provided in a layer, or concentrated in a particular area it is spread out throughout the electrode. The first metallic element exhibits higher diffusivity in the intermetallic phase than in the first phase. Thus the fact that the intermetallic phase is evenly distributed means that transport of Li ions from the first phase to the electrolyte is increased due to the average distance of each part of the first phase to a part of the intermetallic phase is low. The second phase may be referred to as the “fasttransporting” phase, and the first phase may be referred to as the “slow-transporting phase”.

[0101] By providing a homogeneous distribution of the second phase in the electrode, fasttransporting areas are spread out throughout the electrode so that the average time take of a Li ion in the metallic phase to diffuse to the interface with the electrolyte is reduced compared to the situation where the intermetallic phase is not present or is not homogenously distributed. Where homogeneity exists, it is present in a plane parallel to the surface of the electrode. In some embodiments homogeneity is also present in the planes perpendicular to the surface (e.g. in the scaffold embodiment). Homogenous may be different for different morphologies of intermetallic phase. For example, in the case of the intermetallic phase being a scaffold, homogeneity may be measured over a distance of a few tens of microns, for the needle morphology, homogeneity would be more likely apparent over a distance of 100 microns or more. The intermetallic network provides fast diffusion channels for the metal atoms or ions, such as lithium. Even if some metal diffusion occurs through the slow-transporting phase, the overall time taken for an metal atom / ion to pass across the electrode is reduced.

[0102] The intermetallic particles provide a path for quick ionic transport (when compared to the matrix), so longer pathways in the direction of charge flow (perpendicular to the electrode / electrolyte interface) may be particularly advantageous. For example, a microstructure having a plurality of low-tortuosity paths (such as Al) may be advantageous over a microstructure which has a plurality of high -tortuosity paths (such as A3).

[0103] Providing a continuous network of intermetallic provides fast diffusion channels for ion diffusion across the electrode (for example, from the electrolyte to a current collector, ifthe battery has a current collector). For example, lithium ions in the first phase can diffuse quickly from the current collector to the electrolyte interface, by travelling at least part of the way within intermetallic phase where the diffusion rate is faster. Thus, although a path through intermetallic phase may be longer than a direct path mailing through matrix, the time take for the journey through intermetallic phase will be significantly shorter.

[0104] In an embodiment, the negative electrode has a first surface and a thickness, the negative electrode for a solid-state battery comprising: a first phase including a first metallic element; a plurality of precipitates of a second phase distributed in the first phase, wherein the precipitates have a mean aspect ratio of more than 1.1 and are oriented at least partially along the thickness direction of the electrode; wherein second phase is an intermetallic phase including the first metallic element and at least a second element, wherein a diffusion coefficient of the first metallic element in the second phase is greater than a diffusion coefficient of the first metallic element in the first phase.

[0105] The thickness direction (d) of the electrode is the direction of charge flow when the electrode is configured to be part of a battery. The thickness is perpendicular or substantially perpendicular to the first surface of the electrode. The electrode thickness may be up to 300 pm, or up to 100 pm, or preferably up to 50 pm, or more preferably up to 30 pm.

[0106] The precipitates have a mean aspect ratio of more than 1.1, in other words, they are elongated. Elongated means that the precipitate is longer along a first direction, compared to its length in two orthogonal directions. For example, in a Cartesian plane, an elongated precipitate may have a length in the x-direction which is longer than the respective lengths along the y-direction and z-direction. In an embodiment, the mean aspect ratio is greater than 1.2, preferably, more than 1.4, more preferably, more than 1.6, even more preferably more than 1.8, even more preferably at least 2.0. Even higher aspect ratios such as 3.0 or higher, 4.0 or higher, 5.0 or higher, 10 or higher, 20 or higher, 30 or higher, 40 or higher or even 50 and higher may be preferable as these may lead to even faster diffusion of the first metallic element (e.g. lithium) through the electrode. Higher aspect ratios are advantageous because more elongate precipitates provide longer fast-diffusion channels through the electrode thickness, without occupying too high a volume of the negative electrode. While higher aspect ratios may be preferable, improvements in conductivity through the electrode are seen at relatively low aspect ratios. For example, the Li+2.75%Bi rolled sample made via a method according to the invention (shown in Figure 7C) has a mean aspect ratio of 2.05 and it exhibits significantly improved electrochemical performance when compared to a pure lithium anode, as discussed with reference to Figure 8A. Thus, precipitates of the secondphase that are at least partially elongate contribute to increasing the diffusion of the first metallic element through the electrode, leading to the increase of the effective electrode diffusivity.

[0107] The aspect ratio of an elongated precipitate is defined as [the Feret diameter of the precipitate along its major axis, FDmax] divided by the [the Feret diameter of the precipitate along its minor axis, FDmin]. The major axis of a particle is defined by the maximum Feret diameter of the precipitate. The minor axis is perpendicular to the major axis. The Feret diameter is the distance between the two parallel planes restricting the object perpendicular to that direction. The Feret diameter could be thought of as the calliper diameter, referring to the measurement of the object size if it were to be measured with a calliper. The aspect ratio of a precipitate in of the negative electrode is measured by measuring performing X-ray tomography of the negative electrode and calculating the value of FDmax / FDmin for that precipitate. The mean aspect ratio is obtained by taking the mean average of the respective aspect ratios of 10 precipitates with a minimum minimum ferret diameter greater than 20 nm chosen randomly. The Feret diameter of the precipitate along its major axis, FDmax, and the the Feret diameter of the precipitate along its minor axis, FDmin are illustrates schematically in Figure 11.

[0108] In a preferable embodiment, FDmax is the thickness of the electrode (i.e. the precipitates span the whole thickness of the electrode).

[0109] The precipitates are oriented, meaning that the direction along which they are elongated (i.e. their respective major axes) are at least partially aligned with the thickness direction of the negative electrode. In other words, the major axes of the precipitates are at least partially aligned with the thickness direction of the electrode. An angle defined by the major axis of the precipitate and the thickness direction of the electrode (a) is preferably 0°, i.e. preferably the major axis of the precipitate is parallel to the thickness direction.However, the major axes may be inclined with respect to the thickness direction. The respective major axes of the precipitates may not be parallel to each other, i.e. the precipitates may be oriented along different directions. It is possible that some precipitates may not be oriented along the thickness direction, while others are (i.e. the value of a differs between different precipitates). The mean angle defined by the major axes of the precipitates and the thickness direction of the electrode (a) satisfies 0° < a < 87.5°, preferably 0° < a < 85°, more preferably 0° < a < 80°. For example, the Li+2.75at.%Bi and Li+lat.%Mg+2.75at.%Bi anodes whose electrochemical performance is shown in Figures 8A-8C have precipitates which are aligned with the thickness direction of the electrode, so that the their respectivevalues of a fall within this range. The value of a for the Li+lat.%Mg+2.75at.%Bi anode is 74 °, and the value of a for the Li+2.75at.%Bi anode is 71.5 °. Improvements in performance are expected when the precipitates are even more aligned with the electrode thickness direction, as greater alignment with the discharge direction provides a more direct fastdiffusion path through the electrode thickness. In embodiments of the invention, the value of a may fall within 0° < a < 30°, or even 0° < a < 15°, or even 0° < a < 5°. The value a is measured by taking the mean average of 10 values of a for the 10 randomly chosen precipitates for measuring the average aspect ratio.

[0110] When the elongated precipitates of the second phase are at least partially aligned with the thickness of the negative electrode they provide fast-diffusion pathways for electrons and / or ions of the first metallic element. For example, precipitates of LhBi which are elongated along the electrode thickness (i.e. its charging direction) act as fast-diffusion pathways for Li+ions and / or electrons during charge and discharge, leading to faster charge / discharge of a battery comprising the anode. In preferable embodiment, the precipitates are aligned with the thickness direction of the electrode (e.g. the precipitates are lamellae aligned parallel to the electrode thickness, as shown in the schematic in Figure 10A). However, an improvement in effective diffusivity is observed even when the precipitates are only partially aligned with the electrode thickness. As shown in Figure 9, precipitates may be only partially aligned with the electrode thickness, but this still provides the benefit of providing a fast-diffusion pathway through the electrode thickness.[OHl] The higher the mean aspect ratio and / or the more highly aligned the precipitates, the greater the increase in effective diffusivity of the negative electrode. The relatively slow- transporting first phase may be disposed in these gaps, meaning that at least some diffusion of the metal ions (e.g. Li+) occurs through the first phase. However, the overall time taken for the metal ions (e.g. Li+ions) to diffuse through the electrode thickness is reduced, leading to an improvement in the discharge rate of a battery.

[0112] In an embodiment, the second phase defines pores in a plane normal to the electrode thickness, the pores having a mean maximum internal tangential diameter of no more than 40 pm. In an embodiment, the second phase defines pores in a plane normal to the electrode thickness, the pores having a mean maximum internal tangential diameter of no more than 30 pm. In an embodiment, the second phase defines pores in a plane normal to the electrode thickness, the pores having a mean maximum internal tangential diameter of no more than 20 pm. The mean maximum internal tangential diameter may be measured as described belowin the sub-section Mean Maximum Internal Tangential Diameter Measurement. The first phase may be present within the pores defined by the second phase. Thus, a metal atom / ion in the first phase may diffuse through the slow-transporting first phase over a relatively short distance to reach the fast-transporting second phase, in which it can diffuse at a greater speed. This increases the speed of transport of the metallic element through the whole electrode.

[0113] The mean maximum internal tangential diameter of the pores may be half of the thickness of the negative electrode or less, preferably one third of the thickness of the negative electrode or less and more preferably one quarter the thickness of the negative electrode or less. Accordingly, the pores are small enough to ensure that the fast-transporting intermetallic phase is distributed throughout the electrode, while leaving enough space for the first metallic element to maintain energy density and battery capacity.

[0114] The microstructure of the electrode may also be described in terms of the average of the minimum distance from a given point in the matrix to the nearest intermetallic phase. In an embodiment, the average of the minimum distance from a given point in the matrix to the nearest intermetallic phase is no greater than 40 pm, preferably no greater than 20 pm, preferably no greater than 10 pm, and more preferably no greater than 5 pm, further preferably no greater than 2 pm. even further preferably no greater than 1 pm. The lower the minimum distance, the higher the overall diffusivity of lithium through the matrix because of the reduced diffusion distance through the matrix before entering the intermetallic phase where diffusion is faster. This distance may be measured as described below in the subsection average of the minimum distance from a given point in the matrix to the nearest intermetallic phase.

[0115] An example of a microstructure of the second phase is shown in Figure 2A. Figure 2A is a micrograph of a LisBi scaffold defining pores, which are subsequently filled with the first phase by suitable means, as discussed below. The scaffold defines multiple continuous pathways extending from top to bottom (corresponding to the thickness direction of the electrode). An example of a continuous path A4 is shown in Figure 2B

[0116] The second element may be selected from one or more of a metallic element, a metalloid element, or a semiconductor element. The second element may be one or more of the following: bismuth, boron, silicon. When the first element is lithium and the second element is bismuth, the intermetallic formed is LisBi, which exhibits particularly high diffusivity towards lithium. Bismuth may be particularly advantageous.

[0117] The first phase may consist of the first metallic element and optionally other elements in solid solution in the first metallic element. This may be preferable for retaining high energy density of the electrode.

[0118] The electrode may consist of the first phase, the second phase and up to 10 vol. % of an additional phase and incidental impurities. Preferably, only up to 5 vol. % of the electrode is occupied with additional phases. Keeping the presence of additional phases relatively low may be advantageous for retaining high energy density of the electrode. Additional phases may be incorporate into the structure for a number of different reasons, for example, to improve lithium plating morphology within the pores. Additional phases may be included to coat or decorate the surface of the second phase.

[0119] The first metallic element may be lithium. A lithium anode may have particularly high energy density and capacity. Pure lithium metal anodes may not meet the necessary discharge performance due to slow lithium transport. Specifically, the extracted capacity during discharge may be limited by slow self-diffusion in lithium metal and insufficient plastic flow if practical pressures are applied.

[0120] Figure 3 shows the discharge behaviour of lithium negative electrode, according to a comparative example. Where large overpotentials are seen at practical currents, the extractable capacity of the anode is limited. As shown in the below examples, the present invention addresses this deficiency of pure lithium anodes.

[0121] Optionally, the intermetallic phase may be in thermodynamic equilibrium with the matrix phase. Accordingly the long-term stability of the electrode may be improved.

[0122] The intermetallic phase may have general formula AxBywhere A is the first metallic element and, B is the second metallic element and preferably wherein x is greater than y. The value of x may be between 0.5 and 7, preferably between 0.8 and 4. The value of y may be between 0.5 and 7, preferably between 0.8 and 4 The values of x and y may be determined based on the oxidation states of A and B, so that the intermetallic compound is neutral.Element B may be bismuth (Bi).

[0123] The intermetallic phase may make up a weight fraction of 0.1 to 50 wt.% of the negative electrode. Preferably, the intermetallic phase makes up 1 to 30 wt.% of the negative electrode. Even more preferably, the intermetallic phase makes up 5-15 vol.% of the negative electrode. Further even more preferably, the intermetallic phase makes up 7-13 vol.% of the negative electrode. If the weight fraction of the intermetallic phase is too high, the electrode becomes heavy and energy density is reduced. When the weight fraction of theintermetallic phase is too low, transport of the first metallic element in the electrode becomes too slow.

[0124] The intermetallic phase may make up a volume fraction of 1-40 vol. % of the negative electrode. Preferably, the intermetallic phase makes up 5-20 vol.% of the negative electrode. Even more preferably, the intermetallic phase makes up 10-15 vol.% of the negative electrode. Further even more preferably, the intermetallic phase makes up 11-14 vol.% of the negative electrode. If the volume fraction of the intermetallic phase is too high, the energy density of the electrode may be compromised even though the lithium diffusivity will be very high. The above preferred volume fractions strike the best balance between increased diffusivity and reduced energy density.

[0125] In an embodiment, the second phase comprises an interconnected scaffold distributed in the first phase. The first phase may fill the pores defined by the scaffold phase fully, or at least partially.

[0126] In another embodiment, the second phase comprises at least one 3-dimensional interconnected microstructure defining pores in which the first phase is located.

[0127] An example of a preferable microstructure to maximise transport whilst minimising energy density would be thin needles or lamellar of the intermetallic. Preferably such needles or intermetallic would be spaced out substantially equidistantly. In an optimal embodiment, the needles would be perpendicular to the solid electrolyte surface such that the diffusion length is optimised based on the required sustained current. Thus, a lithium ion in the first phase can diffuse a short distance through the first phase perpendicularly to the interface with the electrolyte into the intermetallic phase and then along the intermetallic phase to the interface. However advantage can be taken of the present invention even in the case where the needles and / or lamella have an elongate direction within 30 degrees of the normal to the first surface. A reducing of the angle to 20 degrees or less or even 15 degrees or less results in improved performance.

[0128] An example schematic of this microstructure is shown on Figure 4 part A. Figure 4 part B shows the effect of intermetallic spacing (may be referred to as mean distance to nearest neighbour) on the extracted capacity of Li.

[0129] For full utilisation of the lithium in the matrix at higher discharge rates a closer spacing of the intermetallic phase may be preferable. Closer spacing may lead to a greater extracted lithium fraction from the electrode at a higher rate (lithium will be extracted from close to the intermetallic phase first, before lithium from regions of the matrix further fromthe intermetallic are depleted in lithium). The intermetallic phase can act as a scaffold in the discharged state, reducing the volume change on a cell level.

[0130] The microstructure may differ from this example, but this example may be helpful for understanding the invention. In an example, the microstructure may comprise an intermetallic scaffold which penetrates the matrix. The intermetallic scaffold may act as the fast diffusion channel.

[0131] A relatively low volume fraction, e.g. 1-40 vol%, or 5-20 vol.%, or 10-15 vol. %, or 11-14 vol. %, of Li3Bi is enough to significantly enhance lithium transport.

[0132] Lithium bismuthide (Li3Bi) has high diffusivity and exists in thermodynamic equilibrium with lithium metal.

[0133] Li3Bi has a cubic Fm3m lattice structure. Figures 5A and 5B show the diffusivity of Li ions in Li3Bi intermetallic measured by7Li PFG-AIAIR.

[0134] Fitting the results on Figures 5A and 5B with the Arrhenius expression (D=Doexp(- Ea / RT)) gives a Do value of 3.340 ± 0.174 * 10-5 cm2s_1and an activation energy of 10.48 kJ / mol (0.1086 eV). Extrapolating to the 303 K temperature used for electrochemical tests, gives a diffusivity of 5.21 ± 0.61 * 10'7cm2s , equivalent to an ionic conductivity of 125.5 ± 15.4 mS cm . This room temperature diffusivity is approximately 4000 times higher than lithium metal 1.31 * 10'10cm2s .

[0135] The average of the minimum distance from a given point in the matrix to the nearest intermetallic phase is no greater than 40 pm, preferably no greater than 20 pm, preferably no greater than 10 pm, and more preferably no greater than 5 pm, further preferably no greater than 2 pm. even further preferably no greater than 1 pm. The lower the minimum distance, the higher the overall diffusivity of lithium through the matrix because of the reduced diffusion distance through the matrix before entering the intermetallic phase where diffusion is faster.

[0136] The average minimum distance from a given point in the matrix to the nearest intermetallic phase may be measured as follows:

[0137] The intermetallic phase may be an interconnected scaffold distributed in the matrix phase. A scaffold may be an interconnected network of intermetallic precipitates which are connected by portions of the intermetallic. For example, intermetallic phase may extend all the way across the electrode in a direction parallel to the plane of the first surface, like illustrated in figure 2B.

[0138] The intermetallic phase may comprise at least one 3 -dimensional interconnected microstructure defining pores in which the matrix phase is located. In the charged state, all ofthe pores, or a substantial proportion of the pores may be filled with the matrix phase (such as lithium). In the discharged state, some of the pores may be empty.

[0139] The intermetallic phase may comprise a scaffold structure, the scaffold structure defining pores, wherein the matrix phase is in the pores.

[0140] The mean maximum internal tangential diameter of the pores in the plane normal to the electrode thickness (parallel to the surface) is 80 pm or less, preferably 40 pm or less, even more preferably 20 pm or less, or preferably 10 pm or less, or more preferably 5 pm or less. This is advantageous for the same reason that a low minimum distance to the nearest intermetallic phase is advantageous.

[0141] A mean minimum distance separating adjacent particles of intermetallic phase in a plane perpendicular to the thickness direction of the negative electrode and in the centre of the negative electrode may be half the thickness of the negative electrode or less, preferably one third of the thickness of the negative electrode or less and more preferably one quarter the thickness of the negative electrode or less. The term “particle” in a plane perpendicular to the thickness direction of the negative electrode means a region of the intermetallic phase which appears in the cross-section. The mean minimum distance may be 80 pm or less, preferably 70 pm or less, preferably 60 pm or less, preferably 50 pm or less, preferably 40 pm, preferably 30pm or less, preferably 10pm or less, most preferably 5pm or less, even more preferably 2pm or less or less. The mean minimum distance may be referred to as intermetallic spacing or mean distance to nearest neighbour. The lower the mean minimum distance separating adjacent particles of intermetallic phase, the greater the diffusivity is likely to be. This is because any given position in the first phase of a first metallic element (e.g. lithium) will be closer to intermetallic phase for a lower mean minimum distance.

[0142] The mean minimum distance may be defined by the mean maximum internal tangential circle diameter.

[0143] A mean minimum distance separating adjacent particles in the plane of the thickness direction of the negative electrode and in the centre of the negative electrode may be half the thickness of the negative electrode or less, preferably one third of the thickness of the negative electrode or less, more preferably one quarter the thickness of the negative electrode or less, and even more preferably one tenth the thickness of the negative electrode or less. The lower this spacing, the lower the distance that a lithium ion needs to travel through the matrix phase as opposed to through the intermetallic phase. Thus a lower mean minimum distance likely results in faster diffusion through the electrode as a whole.

[0144] The negative electrode may be directly obtainable by quenching an alloy. This may be the case if the alloy an alloy of the first metallic element and the second element at the eutectic composition of the first metallic element and the second element. This may be a particularly convenient way of making the electrode. The alloy may be one of lithium and calcium, lithium and gallium or lithium and zinc, but other lithium alloys are possible.

[0145] The negative electrode may be directly obtainable by directionally solidifying an alloy.

[0146] According to an embodiment, there is provided a method of manufacturing a negative electrode, the method comprising: forming a scaffold of the first element, wherein the scaffold defines pores, treating the scaffold with the second element or a compound thereof to form a scaffold of the intermetallic, applying the first element in a molten state to the intermetallic scaffold so that the first element fills in the pores. An example of this method is provided in Method 1 below.

[0147] The LisBi scaffold may be formed as described in Lai, W.; Liu, Y.; Zeng, M.; Han, D.; Xiao, M.; Wang, S.; Ren, S.; Meng, Y. One-Step Electrochemical Dealloying of 3D Bi- Continuous Micro-Nanoporous Bismuth Electrodes and CO2RR Performance. Nanomaterials 2023, 13, 1767. 1.

[0148] [Mean Maximum Internal Tangential Diameter]

[0149] The mean maximum internal tangential diameter is measured according to the following method.The mean maximum internal tangential circle diameter of pores is obtained from a crosssection of the electrode, as defined in the following steps. The cross-section of the electrode is obtained in a plane perpendicular to the charging direction (i.e. normal to the plane of the electrode). The cross-section is obtained as defined in steps 1-2. Care is taken through this procedure to avoid oxygen, nitrogen, or water exposure of the cross-section electrode surface.The FIB-SEM measurements of step 1 are carried out under cryogenic conditions.1. Focused ion beam (FIB) tomography is performed on a sample of the electrode in a combined focused ion beam-scanning electron microscope (FIB-SEM) setup, for example using a Helios FIB-SEM, supplied by Thermo Fischer™. Using a FIB-SEM setup, 3Dreconstruction of a sample volume is achieved by serial sectioning using the FIB in combination with high-resolution SEM imaging of each cross-section.A sampled volume of an electrode is obtained having the following volume: height = 50 pm, width = 50 pm, depth = electrode thickness.A slice thickness of 15 nm is used. For example, the following milling conditions: 30 kV, 15 nA may be suitable. It may be needed for highly porous sample to partially fill pores with platinum between each layer cut to ensure effective imaging. The FIB-SEM is used to create a reconstruction (a “FIB-SEM tomograph”) of the entire sampled volume.An SEM beam voltage (e.g. 10-30 kV) is selected to provide good atomic number contrast between phases, whilst avoiding significant artefacts due to large electron beam penetration depth. Once the images are captured, the 3D microstructure is reconstructed using the machines associated reconstruction software, for example Avizo, with settings selected by an operator skilled in the technique.2. From the reconstructed FIB-SEM tomograph, five cross sections at varying depths of the electrode perpendicular to the charging direction (in other words, substantially parallel to the first surface of the electrode, or substantially perpendicular to the thickness direction) are analysed. The cross sections are taken at 5 pm intervals.3. The pore size is measured by fitting circles within the spaces between the intermetallic phase, in accordance with steps 5a-5d. In the discharged state of the electrode, these spaces may primarily be empty. In the charged state these spaces may primarily be lithium metal or a solid solution of lithium metal, optionally with up to 10 vol. % of additional phases and / or incidental impurities.4. A perimeter of each intermetallic precipitate in each of the cross-sections is identified, wherein the perimeter is the boundary between the intermetallic phase (i.e. the second phase) and the adjacent phase (e.g. the first phase or the empty space which is primarily empty). The cross-section of the intermetallic phases will be referred to as “precipitate” in the following explanation for the sake of clarity. Precipitates with a size of 20nm circle area equivalent diameter or less are effectively ignored in this analysis. The circle area equivalent diameter is that of the precipitate as it appears in the cross-section (rather than the circle area equivalent diameter of the same precipitate in a different cross-section).5. The mean maximum internal tangential circle diameter of pores is obtained by taking an arithmetic mean of the diameters of the maximum internal tangential circles fitted 30 times to the 3 different cross-sections of the electrode (i.e. 90 measurements in total). The maximum internal tangential circle is measured according to the following the method, as defined in steps a-d. a. A starting position is selected at random within the cross-section. b. If the starting position is located within an area of an intermetallic precipitate bound by a perimeter of the intermetallic precipitate with a size of 20nm circle area equivalent diameter or greater, the starting position is discarded and step a is repeated. In other words, a determination is made whether the starting position is located within an intermetallic precipitate. If the starting position is located not in the second phase (the intermetallic phase), the method proceeds to step c. c. An imaginary circle having the maximum diameter possible and fulfilling the following properties is drawn: i. The circle does not include any intermetallic phase with a size of 20nm or greater, ii. The circle does not cross the perimeter of any intermetallic phase with a size of 20nm or greater, and iii. The circle contains the starting position of step a. d. The diameter of the imaginary circle of step c is the maximum internal tangential circle diameter.An example of part of this method is shown schematically in Figures 6A-6C. Figures 6A-6C are a schematic of a cross-section of the electrode. The number of precipitates shown in Figures 6A-6C is less than 100 precipitates for improved clarity of the figures. According to the method of the present invention, the cross-section should be selected to have a minimum of 100 identifiable precipitates.Figures 6A-6C show random points xi, X2, X3 being selected. Following the method defined in steps a-d above with random points xi, X2, X3 selected in step a, maximum internal tangential circles having respective diameters di, d2, 3 are drawn. The process is repeated until 20 maximum internal tangential circles are obtained for this cross section. Steps 5a-5dare repeated 20 times for each of the further cross-sections. An arithmetic mean of the obtained results is taken to calculate the mean maximum internal tangential circle diameter of the electrode.

[0150] [Method i]

[0151] Lead-Bismuth alloy (8 at% used) is prepared in a furnace within an argon glovebox at 450 °C before quenching. The ingot is subsequently annealed for 24 hours at 130°C. The ingot is then rolled to form a foil (30-100 pm thickness). This foil is attached to a stainless- steel foil substrate using Kapton tape (poly (4,4'-oxydiphenylene-pyromellitimide)) before electrochemical removal of the lead at 25 °C in a 0.5M nitric acid electrolyte with a platinum counter electrode and calibrated MSE reference electrode, at a constant working potential of - 0.55 V. After full dealloying the material was rinsed in pure deionised water before drying in an oven at 70 °C. The thickness of the porous material is approximately half that of the pristine foil. The microporous bismuth is immersed in a IM Lithium / Biphenyl in Tetrahydrofuran (THF) solution prepared by adding lithium metal to a IM solution of Biphenyl in THF, before washing three times in THF. After immersion in this solution, the porous Bi reacts to form Li3Bi. On charge, lithium fills the pores.

[0152] An example of a Bi scaffold obtained by Method 1 is shown in Figures 2A and 2B.

[0153] To incorporate the first element into the scaffold of the second element and / or to incorporate the first phase into the pores of the scaffold, the scaffold may be incorporated into a battery comprising a cathode comprising the first element. Upon charging by applying a voltage across the battery, the first element diffuses to the negative electrode. The first element may be incorporated into to scaffold to form and intermetallic and / or it may fill the pores of the scaffold to form the first phase.

[0154] When the first metallic element is lithium, the electrode may be manufactured to comprise the second phase scaffold only. Upon incorporation into a battery and charging, the microstructure according to the present invention may be achieved.

[0155] An alternative method of manufacturing a negative electrode comprises providing an alloy of the first metallic element and the second element at a temperature above the liquidus temperature; and cooling the molten alloy to form the matrix phase and the intermetallic phase. Upon cooling, the first and second phases may form such that the second phase is distributed in the intermetallic. After cooling, the solidified alloy may be referred to as an ingot. The alloy may be homogenised prior to cooling. The ingot may be used as thenegative electrode, or it may undergo further processing before being used as a negative electrode.

[0156] The alloy may be a mixture of the first metallic element in granule form and the second element in granule form which is heated to at least a temperature which is the higher of the liquidus temperature of the first metallic element and the liquidus temperature of the second element. Granule form includes granules of the metal, as well as powders of the metal. Accordingly, complete melting of the mixture is achieved and the microstructure of the present invention is formed upon cooling. The second element may be present in the form of an intermetallic of the first element and the second element, rather than in its elemental form. When the second element is present in the form of an intermetallic, the alloy is heated to at least the higher of the liquidus temperature of the first metallic element and the liquidus temperature of the intermetallic.

[0157] The alloy may comprise the first metallic element (e.g. lithium) and the second element (e.g. bismuth), and optionally additional elements such as magnesium. In an embodiment, the alloy may comprise 0.5 at.% to 5.0 at.% bismuth (Bi), optionally, 0.0 at.% to 5 at.% magnesium (Mg), the balance being lithium (Li) and incidental impurities. The alloy may consist of 0.5 at.% to 5.0 at.% bismuth (Bi), optionally, 0.0 at.% to 5 at.% magnesium (Mg), the balance being lithium (Li) and incidental impurities. In an embodiment, the alloy may comprise 1.0 at.% to 3.0 at.% bismuth (Bi), optionally, 0.0 at.% to 1.5 at.% magnesium (Mg), the balance being lithium (Li) and incidental impurities. The alloy may consist of 1.0 at.% to 3.0 at.% bismuth (Bi), optionally, 0.0 at.% to 1.5 at.% magnesium (Mg), the balance being lithium (Li) and incidental impurities. According to an embodiment, the alloy composition may be Li+2.75at.%Bi or Li+lat.%Mg+2.75at.%Bi. The alloy may be a eutectic composition, e.g. a eutectic composition of lithium and bismuth. The eutectic composition is preferably that which forms a metallic phase and an intermetallic phase upon solidification (e.g. for Li-Bi, lithium metal and LisBi are formed upon cooling through the eutectic temperature). The alloy may be heated as high as 850 °C so that the alloy is completely molten before cooling, but lower temperatures may be suitable depending on the composition of the alloy. When the alloy is a eutectic composition, the alloy is cooled through the eutectic temperature. During cooling, the alloy may be quenched, e.g. in oil.

[0158] Upon cooling through the liquidus, precipitates of the second phase may begin to form in the alloy. Upon cooling through the solidus, the first phase is formed.

[0159] The alloy may be directionally cooled. In other words, there may be a temperature gradient along a particular direction during cooling. There may be an interface between themolten alloy and solidified alloy which moves along the particular direction during directional cooling. Directional cooling may lead to precipitates of the second phase being aligned with the direction of cooling. For example, the elongated precipitates may be rodlike. Performing this method comprising continuous pathways of the second phase or a plurality of precipitates of a second phase distributed in the first phase, wherein at least some of the precipitates are elongated along the thickness of the negative electrode.

[0160] The ingot may undergo further processing steps before being disposed in a negative electrode. For example, the ingot may be rolled. Rolling the ingot may increase the aspect ratio of the precipitates along the rolling direction (i.e. the precipitates may become more elongated along the rolling direction). These elongate precipitates may act as fast-diffusion channels, leading to an increase in high effective electrode diffusivity, while retaining high energy density and capacity of the electrode. Rolling the ingot may result in a film wherein the precipitates respective major axes become less aligned with the film thickness. Even if the precipitates become less aligned with the thickness direction of the film, they may still provide an increase in the effective diffusivity of the electrode by providing a network of fast-diffusion channels. Thus, the film may be used as the negative electrode. The effects of cold rolling are illustrated schematically in Figure 9. Rolling modifies the microstructure to bring the precipitates closed together, thereby reducing the average distance from a point in the first phase (e.g. Li metal) to a point in the second phase (e.g. LisBi).

[0161] Figure 7A shows the microstructure of an as-cast sample of Li+ 2.75at. %Bi. The lighter regions correspond to LisBi (the second phase) and the darker regions correspond to Li metal (the first phase). A composition of Li+ 2.75at. %Bi results in 10 vol.% LisBi and 90 vol. % Li metal, meaning that 10vol.% of the sample contains the fast-diffusion LisBi phase with the remainder being lithium metal. The as-cast sample of Li+ 2.75at. %Bi was formed by heating lithium and bismuth pellets to 850 °C in a crucible so that the sample was fully molten. The alloy was allowed to homogenise for two hours before cooling. The alloy was oil quenched - the crucible was removed from the furnace at 850 °C using steel tongs and plunged into mineral oil, rapidly solidifying the metal (cooling rate ~100 K / s).

[0162] The as-cast microstructure shown in Figure 7A shows a fine distribution of the second phase (LisBi) in the first phase (Li metal). These act as fast diffusion channels for lithium ions, meaning the effective conductivity is increased. Figure 7B shows a histogram of the maximum internal tangential circle diameter of the pores of the sample shown in Figure 7A, measured as described with reference to Figures 6A-6C. The mean maximum internal tangential circle diameter of the sample is 26 pm.

[0163] Figure 7C shows a micrograph of the Li+ 2.75at. %Bi sample described with reference to Figure 7A after cold rolling. Figure 7D shows a histogram of the maximum internal tangential circle diameter of the pores of the sample shown in Figure 7C, measured as described with reference to Figures 6A-6C. The mean maximum internal tangential circle diameter of the sample is 33 pm. The micrograph shown in Figure 7C is taken perpendicular to the cold rolling plane and perpendicular to the electrode thickness.

[0164] Figures 8A, 8B, 8C show the electrochemical performance of cells made with anodes of the present invention, as well as a comparative anode made from pure Li and Li+lat.%Mg. The anodes were made via casting. Figure 8A shows the chronopotentiometric discharge of samples of pure Li metal, Li+lat. %Mg, Li+2.75at.%Bi and Li+lat.%Mg+2.75at.%Bi alloys with a InLi / In cathode at a discharge rate of 0.3m Acm2. As it can be seen, Li+2.75at.%Bi and Li+lat.%Mg+2.75at.%Bi alloys show significant improvements over pure Li and Li+lat.%Mg . This improvement in performance is attributed to the present of elongate precipitates of the high-diffusion intermetallic phase LisBi. The mean aspect ratio of the Li+2.75at.%Bi is 2.05. Figure 8B shows the chronopotentiometric discharge of samples of Li+lat. %Mg, Li+2.75at.%Bi and Li+lat.%Mg+2.75at.%Bi alloys with a InLi / In cathode at a discharge rate of 1 mAcm2. Figure 8C shows the chronopotentiometric discharge of samples of Li+lat. %Mg, Li+2.75at.%Bi and Li+lat.%Mg+2.75at.%Bi alloys with a InLi / In cathode at a discharge rate of 3 mAcm2. The cells were discharged unidirectionally at the specified low current density in conditions of 2.5 MPa stack pressure and 30 °C. The anodes were cold rolled to 300 pm thick.

[0165] The addition of l%Mg to lithium substantially improves accessible areal capacity. The areal capacity represents the amount of charge that can be extracted from the anode per unit area of the anode. The Li+2.75at%Bi and Li+l%Mg+2.75at%Bi cells both deliver substantially more capacity than both pure lithium and Li+l%Mg. This demonstrates that the addition of a second phase with high lithium transport properties can substantially improve accessible capacity.

[0166] The overpotential observed for the two-phase alloys is greater than for the solid solution Li+l%Mg cells, indicating that the electrochemically active area on the discharging electrode is decreased, and charge transfer is primarily occurring between the intermetallic phase and the solid electrolyte. This matches the modelling which shows with increasing stripping time, the interface between matrix and solid electrolyte became less active for Li transport, leading to more Li transport through the LnBi intermetallic lamellar.

[0167] Method 2 is an example of a method according to the present invention, which was used to obtain a negative electrode according to the present invention. The samples shown in Figures 7A and 7C were obtained using Method 2.

[0168] [Method 2]

[0169] Pure alloy components of lithium (Sigma Aldrich foil 99.9%), magnesium (Alfa Aesar, turnings 99.9%), and bismuth (Goodfellow, granules 99.9%) were weighed (MTI PCB-200) and placed in a stainless steel crucible, lined with molybdenum foil. For large batches, a molybdenum crucible was used. The furnace (MTI KSL-1200X-J-UL), contained within an argon glovebox, was used to heat the crucible. For bismuth containing alloys, the furnace was heated 850 °C, where the sample was fully molten. Alloys were allowed to homogenise for two hours before cooling. Li-lat.% Mg alloy was furnace cooling was at a rate of 5 K / min. Bismuth containing alloys were oil quenched - the crucible was removed from the furnace at 850 °C using steel tongs and plunged into mineral oil, rapidly solidifying the metal (cooling rate ~100 K / s). The solid metal was removed from the crucible with pliers. To prepare electrodes for electrochemical testing, a piece of the cast ingot was rolled to a foil with thickness of 300 micron.

[0170] All electrochemical measurements were performed within a constant temperature chamber at 30°C. For low current tests (0.3 mA cm-2), a LiePSsCl layer of electrolyte is prepared by pressing and densifying 100 mg LiePSsCl powder (Ampcera, 10 pm) at a pressure of 400 MPa for 300 seconds within custom Macor cell cylinders (diameter = 10 mm) with stainless steel plungers. For high current tests (>1 mA cm-2), a Li4.7PS5Clo.65Bro.65 layer of electrolyte is prepared by pressing and densifying 100 mg of Li4.7PS5Clo.65Bro.65 (1 pm particle size) powder.

[0171] Lithium or lithium alloy foil was hole punched to form 10 mm diameter anodes. A counter electrode of indium foil of >300 pm thickness (Alfa Aesar) was applied to the other side. Stainless-steel foil current collectors (10 mm diameter, 30 pm thickness (Advent Research Materials) was added after each electrode. The electrodes were attached to the solid electrolyte at a pressure of 80 MPa for 30 seconds. When fully assembled, the cell testing assembly has a compressed spring that maintains a constant stack pressure of 2.5 MPa throughout cycling. All cells were rested for 12 hours inside the environmental chamber before any chronopotentiometry test.

[0172] According to the invention, the diffusion coefficient of the first metallic element in the second phase may be at least 100 times greater than the diffusion coefficient of the firstmetallic element in the first phase, optionally, at least 1000 times greater. The advantageous effects of this difference are explained with reference to Figures 10A, 10B and 10C.

[0173] Figure 10A is a schematic of a preferable microstructure the anode, comprising lamellae of intermetallic dispersed within the lithium matrix. Figure 10B shows the electrochemical performance (accessible capacity vs current density) of a microstructure of Figure 10A, for different spacing of the intermetallic lamellae. The lines on the graph correspond to different spacing of the intermetallic lamellae as indicated in the legend in Figure 10B, with the top line corresponding to the 2 pm spacing and the lines below following the order in the legend. Figure 10C shows the electrochemical performance (accessible capacity vs current density) of a microstructure of Figure 10A with a spacing of 20 pm, for intermetallic having different diffusivities.

[0174] The influence of microstructural configuration on the stripping capacity is shown in Figure 10B, where the accessible capacities under various discharging rates are provided for a pure Lilat.%Mg matrix alloy anode and electrodes with varying intermetallic (LisBi) lamellar spacing from 80 to 2 pm. As the discharging current density approaches zero, the accessible capacity approaches the theoretical limit of 5.7 mAhcm2. A small performance improvement is achieved by the introduction of the intermetallic lamellar with an 80 pm spacing, with greater capacity accessed for all current densities than the pure matrix anode. As the intermetallic lamellar spacing is further reduced, the accessible capacity is significantly increased approaching the theoretical limits when the lamellar is distributed at a sufficiently small spacing. With a 10 pm spacing, 77% of the theoretical capacity is accessed at the high current density of 5.0 mAcm-2. The improvement of accessible capacity with reducing lamellar spacing, especially at high discharging rates, is attributed to the fact that the diffusion distance becomes comparable to the depth of the Li concentration gradient.

[0175] The impacts of material properties on the accessible capacity are demonstrated in Figure 10C, where the stripping capacity as a function of diffusivity of the intermetallic phase is obtained from a parametric study while keeping other parameters unchanged. For a given diffusivity of the intermetallic phase, there exists a critical current density beyond which the accessible capacity sharply decreases.

[0176] An intermetallic diffusivity of 1*105cm2s'1is sufficiently high to be nonlimiting for realistic discharge rates. The impact of lower intermetallic diffusivity on the accessible capacity is demonstrated in Figure 10C, where the stripping capacity as a function of diffusivity of the intermetallic phase is obtained from a parametric study in which theintermetallic spacing was kept constant at 10 pm and the intermetallic diffusivity (Dint) was varied. With an intermetallic diffusivity below 106cm2s'1the accessible capacity sharply decreases. At high current densities, most or all of the lithium flux must go through the intermetallic phase. The effective current density in this phase is therefore i / Af (where Af is area fraction of intermetallic). As the resultant flux is very high, a meaningful concentration gradient evolves within the intermetallic phase despite a very high diffusion coefficient. Over time the concentration of this phase at the interface with the solid electrolyte decreases until the lithium-depleted limit of the intermetallic phase solid solubility window is reached, defining a limit on accessible capacity. The diffusion coefficient of lithium in lithium metal is approximately 10'10cm2s'1.

[0177] An electrode according to the present invention may be incorporated into a battery. The battery may comprise a positive electrode, wherein the positive electrode comprises the first metallic element. The battery may comprise a solid-state electrolyte.

[0178] This work provides a rational strategy to improve room temperature discharge rates of solid state batteries using two-phase alloys, such as lithium metal alloys. The introduction of a high diffusivity intermetallic phase in equilibrium with lithium metal can significantly improve the discharge rate capability of the electrode. The LisBi intermetallic is a suitable intermetallic, which combines fast lithium diffusion kinetics, a reasonable solid solubility window of 1.3% and low cost. Alternative intermetallic phases may be used.

[0179] Having described the invention it will be appreciated that variations may be made on the above described embodiments which are not intended to be limiting. The invention is defined in the appended claims and their equivalents.

[0180] This application claims priority from GB 2402753.4 filed on 27 February 2024, the contents of which are hereby incorporated by reference.

[0181] The invention may be defined by one or more of the following clauses:

[0182] 1. A negative electrode for a solid-state battery, the negative electrode having a first surface and a thickness, the negative electrode for a solid-state battery comprising: a first phase including a first metallic element; a plurality of precipitates of a second phase distributed in the first phase, wherein the precipitates have a mean aspect ratio of more than 1.1 and are oriented at least partially along the thickness direction of the electrode; wherein second phase is an intermetallic phase including the first metallic element and at least a second element,wherein a diffusion coefficient of the first metallic element in the second phase is greater than a diffusion coefficient of the first metallic element in the first phase.2. The negative electrode according to clause 1, wherein the mean aspect ratio is more than 1.2, preferably more than 1.3, preferably more than 1.4, more preferably more than 1.6, even more preferably more than 1.8, even more preferably at least 2.0, preferably more than 3.0, preferably more than 4.0, preferably more than 5.0, preferably more than 10, preferably more than 20, preferably more than 30, preferably more than 40, preferably more than 50.3. A negative electrode for a solid-state battery, the negative electrode having a first surface and a thickness, the negative electrode for a solid-state battery comprising: a first phase including a first metallic element; a second phase distributed in the first phase, wherein the second phase is an intermetallic phase including the first metallic element and at least a second element, and wherein a diffusion coefficient of the first metallic element in the second phase is greater than a diffusion coefficient of the first metallic element in the first phase, and wherein there are a plurality of continuous pathways of the second phase each through at least half the thickness of the negative electrode.4. The negative electrode of clause 3, wherein the continuous pathways of the second phase each have a length, and the length of each respective continuous pathway is 50-500% of the electrode thickness.5. The negative electrode of any one of clauses 1 to 4, wherein the second phase defines pores in a plane normal to the electrode thickness, the pores having a mean maximum internal tangential diameter of no more than 80 pm, preferably, no more than 40 pm, even more preferably, no more than 20 pm.6. The negative electrode of any one of clauses 1 to 5, wherein the diffusion coefficient of the first metallic element in the second phase is at least 10 times greater than the diffusion coefficient of the first metallic element in the first phase, preferably at least 100 times greater, more preferably, at least 1000 times greater.7. The negative electrode according to any one of clauses 1 to 6, wherein the first phase is present within the pores defined by the second phase.8. The negative electrode of any of clauses 1 to 7, wherein the second element is selected from one or more of: a metallic element, a metalloid element, or a semiconductor element.9. The negative electrode of any of clauses 1 to 8, wherein the first phase consists of the first metallic element and optionally other elements in solid solution in the first metallic element.10. The negative electrode of any of clauses 1 to 9, wherein the first metallic element is lithium.11. The negative electrode of any of clauses 1 to 10, wherein the second phase is in thermodynamic equilibrium with the first phase.12. The negative electrode of any of clauses 1 to 11, wherein the second phase has a general formula AxBy where A is the first metallic element and B is the second element.13. The negative electrode of any of clauses 1 to 12, wherein the second element is bismuth.14. The negative electrode of any of clauses 1 to 13, wherein the second phase makes a molar fraction of 0.1 to 50 wt% of the negative electrode, or preferably, 1 to 30 wt.% of the negative electrode.15. The negative electrode of any of clauses 1 to 14, wherein the intermetallic phase makes a volume fraction of 1-40 vol. % of the negative electrode, or preferably, 5-20 vol. % of the negative electrode.16. The negative electrode of any of clauses 1 to 15, wherein an average of the minimum distance from a given point in the first phase to a nearest point in the second phase is no greater than 40 pm, preferably no greater than 20 pm, preferably no greater than 10 pm, and more preferably no greater than 5 pm, further preferably no greater than 2 pm and even further preferably no greater than 1 pm.17. The negative electrode of any of clauses 1 to 16, wherein the second phase is an interconnected scaffold distributed in the first phase.18. The negative electrode of any of clauses 1 to 17, wherein the second phase comprises at least one 3-dimensional interconnected microstructure defining pores in which the first phase is located.19. The negative electrode of any of clauses 1 to 18, wherein the second phase comprises scaffold structure, the scaffold structure defining the pores, wherein the first metallic phase is in the pores.20. The negative electrode of any of clauses 1 to 19, wherein the second phase defines needles which have an elongate dimension within 30 degrees of the direction normal to the first surface of the electrode.21. The negative electrode of any of clauses 1 to 20, wherein the mean maximum internal tangential diameter of pores is half the thickness of the negative electrode or less, preferably one third of the thickness of the negative electrode or less and more preferably one quarter the thickness of the negative electrode or less, and even more preferably one tenth the thickness of the negative electrode or less.22. The negative electrode of any of clauses 1 to 21, wherein the negative electrode consists of the first phase, the second phase and up to 10 vol. % of an additional phase and incidental impurities.23. The negative electrode of any of clauses 1 to 22, wherein the negative electrode is directly obtainable by quenching an alloy.24. The negative electrode of any of clauses 1 to 23, wherein the negative electrode is directly obtainable by directionally solidifying an alloy.25. A method of manufacturing a negative electrode, the method comprising: forming a scaffold of the second element, wherein the scaffold defines pores, treating the scaffold with the first metallic element or a compound thereof to form a scaffold of the intermetallic, at least partially filling the pores of the scaffold with the first metallic element.26. The method of clause 25, wherein the treating the scaffold with the first metallic element or a compound thereof to form a scaffold of the intermetallic comprises: forming an electrical circuit comprising the scaffold and a positive electrode comprising the first element, and applying a voltage across the scaffold and the positive electrode.27. The method of clause 25 or 26, wherein the filling the pores of the scaffold with the first element comprises forming an electrical circuit with the scaffold and a positive electrode comprising the first element, and applying a voltage across the scaffold and the positive electrode.28. A method of manufacturing a negative electrode according to any of clauses 1 to 24, the method comprising: providing an alloy of the first metallic element and the second element at a temperature above the liquidus temperature; and cooling the molten alloy to form an ingot comprising the first phase and the second phase.29. The method of manufacturing a negative electrode according to clause 28, wherein the providing an alloy comprises:providing a mixture of the first metallic element in granule form and the second element in granule form; and heating the mixture above the higher of the liquidus temperature of the first metallic element and the liquidus temperature of the second element.30. The method of manufacturing a negative electrode according to clause 28 or clause 29, wherein the providing an alloy comprises: providing a mixture of the first metallic element in granule form and an intermetallic phase including the first metallic element and at least a second element in granule form; and heating the mixture above the higher of the liquidus temperature of the first metallic element and the liquidus temperature of the intermetallic.31. The method according to any one of clauses 28 to 30, wherein the alloy comprises lithium, bismuth, and optionally, magnesium.32. The method according to any one of clauses 28 to 31, wherein the alloy has a chemical composition of, in atomic %:0.5 at.% to 5.0 at.% Bi,0.0 at.% to 5.0 at.% Mg, the balance being Li and incidental impurities.33. The method according to any one of clauses 28 to 32, wherein the alloy has a chemical composition of, in atomic %:1.0 at.% to 3.0 at.% Bi,0.0 at.% to 1.5 at.% Mg, the balance being Li and incidental impurities.34. The method according to any one of clauses 28 to 33, wherein the cooling the molten alloy comprises quenching the molten alloy, optionally at a rate of at least 100 K / s.35. A negative electrode obtained by the method of any one of clauses 25 to 34.36. A battery comprising the negative electrode of any one of clauses 1 to 24 or a negative electrode obtained by the method of any one of clauses 25 to 35.37. The battery of clause 36, comprising a comprising a positive electrode.38. The battery of clause 37, wherein the positive electrode comprises the first metallic element.39. A battery according to any one of clauses 36 to 38, comprising a solid-state electrolyte sandwiched between the negative electrode and positive electrode.

Claims

CLAIMS1. A negative electrode for a solid-state battery, the negative electrode having a first surface and a thickness, the negative electrode for a solid-state battery comprising: a first phase including a first metallic element; a plurality of precipitates of a second phase distributed in the first phase, wherein the precipitates have a mean aspect ratio of more than 1.1 and a mean of angles of the major axis of the precipitates to the thickness direction of the electrode (a) satisfies 0° < a < 87.5°; wherein second phase is an intermetallic phase including the first metallic element and at least a second element, wherein a diffusion coefficient of the first metallic element in the second phase is greater than a diffusion coefficient of the first metallic element in the first phase.

2. The negative electrode according to claim 1, wherein the mean aspect ratio is more than 1.2, preferably more than 1.3, preferably, more than 1.4, more preferably, more than 1.6, even more preferably more than 1.8, even more preferably at least 2.0.

3. A negative electrode for a solid-state battery, the negative electrode having a first surface and a thickness, the negative electrode for a solid-state battery comprising: a first phase including a first metallic element; a second phase distributed in the first phase, wherein the second phase is an intermetallic phase including the first metallic element and at least a second element, and wherein a diffusion coefficient of the first metallic element in the second phase is greater than a diffusion coefficient of the first metallic element in the first phase, and wherein there are a plurality of continuous pathways of the second phase each through at least half the thickness of the negative electrode.

4. The negative electrode of claim 3, wherein the continuous pathways of the second phase each have a length, and the length of each respective continuous pathway is 50-500% of the electrode thickness.

5. The negative electrode of any one of claims 1 to 4, wherein the second phase defines pores in a plane normal to the electrode thickness, the pores having a mean maximum internaltangential diameter of no more than 80 pm, preferably, no more than 40 pm, even more preferably, no more than 20 pm.

6. The negative electrode of any one of claims 1 to 5, wherein the diffusion coefficient of the first metallic element in the second phase is at least 10 times greater than the diffusion coefficient of the first metallic element in the first phase, preferably at least 100 times greater, more preferably, at least 1000 times greater.

7. The negative electrode according to any one of claims 1 to 6, wherein the first phase is present within the pores defined by the second phase.

8. The negative electrode of any of claims 1 to 7, wherein the second element is selected from one or more of: a metallic element, a metalloid element, or a semiconductor element.

9. The negative electrode of any of claims 1 to 8, wherein the first phase consists of the first metallic element and optionally other elements in solid solution in the first metallic element.

10. The negative electrode of any of claims 1 to 9, wherein the first metallic element is lithium.

11. The negative electrode of any of claims 1 to 10, wherein the second phase is in thermodynamic equilibrium with the first phase.

12. The negative electrode of any of claims 1 to 11, wherein the second phase has a general formula AxBywhere A is the first metallic element and B is the second element.

13. The negative electrode of any of claims 1 to 12, wherein the second element is bismuth.

14. The negative electrode of any of claims 1 to 13, wherein the second phase makes a molar fraction of 0.1 to 50 wt% of the negative electrode, or preferably, 1 to 30 wt.% of the negative electrode.

15. The negative electrode of any of claims 1 to 14, wherein the intermetallic phase makes a volume fraction of 1-40 vol. % of the negative electrode, or preferably, 5-20 vol. % of the negative electrode.

16. The negative electrode of any of claims 1 to 15, wherein an average of the minimum distance from a given point in the first phase to a nearest point in the second phase is no greater than 40 pm, preferably no greater than 20 pm, preferably no greater than 10 pm, and more preferably no greater than 5 pm, further preferably no greater than 2 pm and even further preferably no greater than 1 pm.

17. The negative electrode of any of claims 1 to 16, wherein the second phase is an interconnected scaffold distributed in the first phase.

18. The negative electrode of any of claims 1 to 17, wherein the second phase comprises at least one 3-dimensional interconnected microstructure defining pores in which the first phase is located.

19. The negative electrode of any of claims 1 to 18, wherein the second phase comprises scaffold structure, the scaffold structure defining the pores, wherein the first metallic phase is in the pores.

20. The negative electrode of any of claims 1 to 19, wherein the second phase defines needles which have an elongate dimension within 30 degrees of the direction normal to the first surface of the electrode.

21. The negative electrode of any of claims 1 to 20, wherein the mean maximum internal tangential diameter of pores is half the thickness of the negative electrode or less, preferably one third of the thickness of the negative electrode or less and more preferably one quarter the thickness of the negative electrode or less, and even more preferably one tenth the thickness of the negative electrode or less.

22. The negative electrode of any of claims 1 to 21, wherein the negative electrode consists of the first phase, the second phase and up to 10 vol. % of an additional phase and incidental impurities.

23. The negative electrode of any of claims 1 to 22, wherein the negative electrode is directly obtainable by quenching an alloy.

24. The negative electrode of any of claims 1 to 23, wherein the negative electrode is directly obtainable by directionally solidifying an alloy.

25. A method of manufacturing a negative electrode, the method comprising: forming a scaffold of the second element, wherein the scaffold defines pores, treating the scaffold with the first metallic element or a compound thereof to form a scaffold of the intermetallic, at least partially filling the pores of the scaffold with the first metallic element.

26. The method of claim 25, wherein the treating the scaffold with the first metallic element or a compound thereof to form a scaffold of the intermetallic comprises: forming an electrical circuit comprising the scaffold and a positive electrode comprising the first element, and applying a voltage across the scaffold and the positive electrode.

27. The method of claim 25 or 26, wherein the filling the pores of the scaffold with the first element comprises forming an electrical circuit with the scaffold and a positive electrode comprising the first element, and applying a voltage across the scaffold and the positive electrode.

28. A method of manufacturing a negative electrode according to any of claims 1 to 24, the method comprising: providing an alloy of the first metallic element and the second element at a temperature above the liquidus temperature; and cooling the molten alloy to form an ingot comprising the first phase and the second phase.

29. The method of manufacturing a negative electrode according to claim 28, wherein the providing an alloy comprises:providing a mixture of the first metallic element in granule form and the second element in granule form; and heating the mixture above the higher of the liquidus temperature of the first metallic element and the liquidus temperature of the second element.

30. The method of manufacturing a negative electrode according to claim 28 or claim 29, wherein the providing an alloy comprises: providing a mixture of the first metallic element in granule form and an intermetallic phase including the first metallic element and at least a second element in granule form; and heating the mixture above the higher of the liquidus temperature of the first metallic element and the liquidus temperature of the intermetallic.

31. The method according to any one of claims 28 to 30, wherein the alloy comprises lithium, bismuth, and optionally, magnesium.

32. The method according to any one of claims 28 to 31, wherein the alloy has a chemical composition of, in atomic %:0.5 at.% to 5.0 at.% Bi,0.0 at.% to 5.0 at.% Mg, the balance being Li and incidental impurities.

33. The method according to any one of claims 28 to 32, wherein the alloy has a chemical composition of, in atomic %:1.0 at.% to 3.0 at.% Bi,0.0 at.% to 1.5 at.% Mg, the balance being Li and incidental impurities.

34. The method according to any one of claims 28 to 33, wherein the cooling the molten alloy comprises quenching the molten alloy, optionally at a rate of at least 100 K / s.

35. A negative electrode obtained by the method of any one of claims 25 to 34.

36. A battery comprising the negative electrode of any one of claims 1 to 24 or a negative electrode obtained by the method of any one of claims 25 to 35.

37. The battery of claim 36, comprising a comprising a positive electrode.

38. The battery of claim 37, wherein the positive electrode comprises the first metallic element.

39. A battery according to any one of claims 36 to 38, comprising a solid-state electrolyte sandwiched between the negative electrode and positive electrode.

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