WO2022074369A1 - Method of forming an electrode - Google Patents

Abstract

A method for forming an electrode for an electrochemical cell is disclosed, the method involving providing a liquid mixture comprising an electrode material, a salt and a polymer precursor; partially, directionally structuring the liquid mixture to form a plurality of columnar structures; and at least partially curing the polymer precursor. Preferably, partially, directionally structuring the liquid mixture involves partially, directionally cooling the liquid mixture to preferentially form a plurality of columnar structures comprising the polymer precursor and optionally the ionic liquid and/or liquid electrolyte, and regions between the plurality of columnar structures comprising the electrode material. Also disclosed are lithium or sodium or manganese or aluminium or zinc batteries and solid electrolytes.

Description

METHOD OF FORMING AN ELECTRODE

FIELD OF THE INVENTION

The present invention relates to methods for forming an electrode, in particular for a metal (e.g. lithium) cell or metal (e.g. lithium) ion cell. The present invention also relates to cells and batteries having electrodes formed in such methods and to solid electrolytes.

BACKGROUND OF THE INVENTION

Rechargeable batteries, in particular lithium or lithium ion batteries, are of current interest and commercial importance. The batteries have a high energy density, generally no memory effects and low self-discharge.

In a lithium ion battery, lithium ions move from the negative electrode through an electrolyte to the positive electrode during discharge, and back when charging. Li-ion batteries may use an intercalated or conversion compound as the material at the positive electrode and often graphite at the negative electrode. In many batteries the electrolyte is liquid and may contain flammable materials and, if damaged or incorrectly charged, this can lead to explosions and fires.

Current lithium ion batteries (LIBs) have an energy density (-250 Wh kg'¹) that needs improvement for wider adoption of large energy storage applications such as electric vehicles (EVs) and large energy storage stations to store electrical energy generated from renewable sources such as solar and wind.

Solid-state lithium metal batteries (SSLMBs) that use a solid-state electrolyte (SSE) to replace the conventional flammable liquid electrolyte are currently being developed because SSLMBs are safer than LIBs. SSLMBs may comprise a lithium metal anode that can further increase battery energy density (with a theoretical energy density -500 Wh kg'¹). However, current SSLMBs can only reach -100 Wh kg'¹ energy density because the ionic conductivity of SSE is 1-2 orders of magnitude lower than a liquid electrolyte.

Current research focusing on developing SSE materials to increase their ionic conductivity has investigated inorganic materials such as NASICON (e.g. Lii.3Alo.3Tii.7(PC>4)3), garnet (e.g. LivLasZnOu), perovskite (e.g. Lio.34Lao.51TiO2.94), LISICON (e.g.

Li 10.42Si1.5P1.5Cl0.0sO11.92), sulfide (e.g. Li3.25Geo.25Po.75S4), argyrodite (e.g. LisPSsCl), glassy electrolytes (e.g. Li2S-P2Ss), and polymer-based electrolytes (e g polyethylene oxide, PEO).

The Li⁺ ion diffusivity in a conventional composite cathode - a combination of active material and solid-state electrolyte (SSE) - is at least an order of magnitude lower than that of the SSE alone because of the highly tortuous ion transport pathways in the cathode. This lowers the realizable capacity and mandates relatively thin (30-300 pm) cathodes, and hence low overall energy storage.

Most cathodes are currently made by coating a suspension of a mixture of electrode and SSE materials) followed by isostatic pressing and sintering. This method provides electrodes containing an isotropic, randomly mixed microstructure of the electrode material and the SSE material. The random microstructure of the electrode with tortuous ion transport pathways reduces its ionic conductivity.

Attempts have been made to improve the structure of separators and electrodes in cells. USA-2020/0303741 discloses improving conductive pathways within a battery cell by using a set of materials with a functional group and a set of bonding materials configured to bond together and thereby orient particles in a vertical direction. US-A-9 065 093 discloses porous electrodes in which the porosity has a low tortuosity.

However, the prior approaches have not be generally successful and would be difficult to use in large scale manufacturing.

There is a need, therefore, to address the problems of the prior art and to provide improved electrodes and improved cells.

It is an aim of the present invention to address this need.

SUMMARY OF INVENTION

The present invention accordingly provides in a first aspect a method for forming an electrode for an electrochemical cell, the method comprising: a) providing a liquid mixture comprising an electrode material, a salt and a polymer precursor; b) Partially, directionally structuring the liquid mixture to form a plurality of structures; and c) At least partially curing the polymer precursor.

The electrode material maybe any suitable material for use in electrochemical cells. Preferably, the electrode material may comprise a metal, a silicon and/or a carbon material or compound or a mixture of two or more such materials or compounds.

The structure is generally any directional structure (e.g. an elongate structure) with a directional axis generally transverse (in particular, generally orthogonal) to the plane of the electrode (e.g. in the plane of the current collector, electrode(s) and electrolyte in use). Examples of such structures include columnar, lamellar, or dendritic structures especially with an elongate component transverse (in particular, generally vertical or generally orthogonal to the plane of solid-state electrolyte).

The method is suitable for many types of electrochemical devices, including next generation batteries (e.g. sodium batteries), other batteries (e.g. redox flow battery) and other types of electrical energy generators (e.g. fuel cells). The method of the invention (since it results in anisotropic structured electrodes and films) is suitable for a wide range of other applications, e.g. sensor components to detect pressure changes for example for medical endoscopy cells.

The electrochemical cell maybe a cell selected from lithium metal/ion; sodium metal/ion; potassium metal/ion; manganese metal/ion; zinc metal/ion; aluminium metal/ion. In each case, the metal cell is such that the anode comprises the metal (usually a metal foil, e.g. lithium metal) and in ion cells the anode comprises a material that can accept ions, e.g. graphite. This present invention would be applicable to both metal and ion cells.

The electrode maybe an anode or cathode.

Preferably, the method may be for use in lithium-containing devices and the cell may be a lithium cell (or lithium ion cell) and the salt a lithium salt.

Thus, the first aspect preferably provides a method for forming an electrode for a lithium cell, the method comprising: providing a liquid mixture comprising an electrode material (e g. a metal compound and/or carbon-based compound and/or silicon-based compound), a lithium salt and a polymer precursor; partially, directionally structuring the liquid mixture to form a plurality of structures; and at least partially curing the polymer precursor.

Conveniently, partially, directionally structuring the liquid mixture may comprise partially, directionally cooling the liquid mixture to form a plurality of structures comprising the polymer precursor. Cooling may involve freezing of at least some components of the liquid mixture.

Preferably, the liquid mixture further comprises an ionic liquid and/or a liquid electrolyte. This is advantageous because an ionic liquid can improve the ionic conductivity of the mixture and appears to improve, in practice, the directional structuring (especially by a cooling method) of the liquid mixture.

An example of a liquid electrolyte maybe a lithium salt (e.g. lithium hexafluorophosphate) dissolved in an organic solvent (e.g. ethylene carbonate and/or diethyl carbonate and/or dimethyl carbonate).

Usually, the liquid mixture further comprises an initiator. This is advantageous to improve curing of the polymer. The initiator preferably comprises an electromagnetic radiation (optionally a UV) photo-initiator, and partially curing the polymer precursor preferably comprises partially electromagnetically (optionally UV) curing the polymer precursor.

The electrode material may comprise an intercalated material and/or a conversion material, optionally a lithium-containing metal oxide, and/or a lithium- containing conversion compound.

The electrode material may comprise an intercalated material or a conversion material (e g. lithium metal oxide, for example a lithium intercalated metal oxide). Examples of intercalated materials and/or conversion materials include (stoichiometric or non-stoichiometric) lithium cobalt oxide (e.g. LiCoCh), lithium iron phosphate (e.g. LiFePCk), lithium manganese oxide battery (e.g. LiM CE, Li2Mn0s, or LMO), lithium nickel manganese cobalt oxide (e.g. LiNiMnCoCh or NMC), graphite, silicon or combinations of two or more of these materials. The stoichiometry of these materials may vary during use and manufacture. The intercalated material and/or conversion material may comprise a material selected from a chalcogen, a metal chalcogenide (e g. FeSi) or a metal halide (e.g. FeF2) or a combination of two or more such materials. Generally, such materials may comprise at least one chalcogen anion and at least one metal component, or may comprise at least one halogen anion and at least one metal component. Other materials may be multicomponent and may contain chalcogenide, halide and/or pnictogen species.

The chalcogens maybe sulfur and/or selenium. The halides maybe F, Cl, Br, and/or I. The pnictogen maybe N, P, As, Sb, and/or Bi.

The polymer precursor may comprise any suitable precursor. Preferably the polymer precursor may comprise one or more precursors selected from precursors of poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) (PVA),poly(propylene) oxide (PPO), poly(ethylene glycol) methacrylate (PEGMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), Poly(vinylidene fluoride-co- hexafluoropropylene) (PVDF-HFP), polypropylene glycol (PPG), polydimethylsiloxane (PDMS), polyethylene carbonate (PEC), polypropylene carbonate (PPC), polycaprolactone (PCL), polyethyleneimine (PEI) and poly(trimethylene carbonate) (PTMC), or a mixture of two or more of these precursors; preferably comprising an acrylate monomer and/or a methaciylate monomer.

Preferably, the (meth)acrylate monomer may comprise poly(ethylene glycol) (meth)acrylate.

The ionic liquid preferably comprises a salt having a melting point of below 100 °C or below 80 °C.

Preferably the ionic liquid comprises a salt having a melting point of above -60 °C, preferably above - 55 °C, more preferably above - 50 °C and most preferably above -45 °C.

The ionic liquid may be selected from a salt having an ion selected from ammonium, imidazolium, phosphonium, pyridinium, pyrrolidinium, piperidinium, sulfonium and/or bis(trifluoromethanesulfonyl)imide. The ionic liquid may comprise a dialkylpyrrolidinium salt, preferably dialkylpyrrolidinium bis(trifluoromethanesulfonyl)imide.

In the method of the first aspect, partially, directionally structuring the liquid mixture to form a plurality of structures preferably comprises contacting the liquid mixture with a structured mould and cooling the liquid mixture, thereby partially, directionally freezing the liquid mixture to form a plurality of structures comprising the polymer precursor.

Surprisingly, during this cooling step the electrode material particles and polymer precursors (with optional ionic liquid) are each self-assembled directly into an anisotropic structure, with no need for any subsequent pressing, heating, SSE infiltration or template removal steps.

Thus, preferably partially, directionally structuring the liquid mixture comprises partially, directionally cooling the liquid mixture to preferentially form a plurality of structures comprising the polymer precursor and optionally the ionic liquid and/or liquid electrolyte, and regions between the plurality of structures comprising the electrode material.

Usually, the volume ratio of the ionic liquid and/or liquid electrolyte to the polymer precursor in the liquid mixture is in the range 0.1: I to 1:0.1, preferably 0.2: I to 1.5: 1; more preferably 0.3: 1 to 1.2:1 and most preferably 0.4:1 to 1: 1.

At least partially curing the polymer precursor may be preferably conducted at a temperature below the melting point of the polymer precursor or the ionic liquid, preferably below -5 °C, more preferably below -10 °C and most preferably at a temperature below -20 °C, preferably below -30 °C, preferably below -35 °C, below -40 °C, below -45 °C, below -50 °C, below -55 °C and most preferably at a temperature below -60 °C.

Overall, electrodes formed according to the first aspect of the invention provide a safer electrochemical cell (especially a safer battery) that may overcome the specific energy density limit of current lithium ion batteries and enable a higher specific energy density to be achieved.

The present invention accordingly provides in a second aspect a method for forming an electrochemical cell (e g. a lithium cell or lithium ion cell), the method comprising: a) providing a first electrode formed by a method according to the first aspect; b) coating the first electrode with a solid electrolyte precursor mixture comprising a salt, an ionic liquid and/or liquid electrolyte and a polymer precursor; c) curing the solid electrolyte precursor mixture to form a solid electrolyte membrane on the first electrode, and d) contacting the solid electrolyte membrane with a second electrode.

The second electrode may comprise a source of metal ions selected from lithium, sodium, manganese, zinc and aluminium ions, optionally comprising a metal selected from lithium, sodium, manganese, zinc, and aluminium, and may optionally comprise graphite.

Curing the solid electrolyte precursor mixture may comprise electromagnetically radiation (e.g. UV) curing the solid electrolyte precursor mixture.

One of the advantages of the present invention is that directionally structured electrodes may be formed.

Thus, in a third aspect the present invention according provides an electrode formed as according to the first aspect and having a tortuosity r in the range the range 1 to 2.0, preferably 1 to 1.9, more preferably 1 to 1.8, most preferably 1 to 1.5. Other ranges of tortuosity T may be 1.05 to 2.0, 1.07 to 1.9, 1.1 to 1.8, 1.1 to 1.5.

Such improved electrode structure provides for improved capacity in electrochemical (e.g. lithium) cells. Furthermore, the electrode may be made thicker further improving capacity. The electrode may be of thickness 5 nm to 1000 pm, optionally 10 nm to 1000 pm, optionally 25 nm to 1000 pm, optionally 50 nm to 1000 pm, optionally 100 nm to 1000 pm, optionally 500 nm to 1000 pm, optionally 1 pm to 1000 pm, optionally 10 pm to 1000 pm, optionally 20 pm to 1000 pm, optionally 30 pm to 1000 pm, optionally 40 pm to 900 pm, optionally 50 pm to 800 pm, optionally 60 pm to 700 pm or optionally 60 pm to 650 pm.

Thus, in a fourth aspect the present invention according provides a lithium battery comprising an electrode formed according to the first aspect and having a discharge capacity in the range too to 2000 mAhg^-1, preferably a discharge capacity in the range 120 to 2000 mAhg^-1, more preferably a discharge capacity in the range 150 to 2000 mAhg ¹, most preferably a discharge capacity in the range 170 to 2000 mAhg ¹. In a fifth aspect the present invention according provides a solid electrolyte comprising a lithium salt, an ionic liquid and/or a liquid electrolyte having a melting point between 0 °C and -60 °C and a polymer precursor, wherein the ratio of the ionic liquid and/or a liquid electrolyte to the polymer precursor in the electrolyte is in the range 0.1 : 1 to 1 :0.1.

In a sixth aspect the mass ratio of the electrode material to the liquid mixture is in the range 0.1: 50 to 50:0.1.

In a seventh aspect the gap between the plurality of structures comprising the electrode material is in the range 5 nm to 1000 pm, preferably 7 nm to 900 pm, more preferably 11 nm to 800 pm, most preferably 15 nm to 700 pm.

The polymer precursor may comprise one or more precursors of poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) (PVA),poly(propylene) oxide (PPO), poly(ethylene glycol) methacrylate (PEGMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), Poly(vinylidene fluoride-co- hexafluoropropylene) (PVDF-HFP), polypropylene glycol (PPG), polydimethylsiloxane (PDMS), polyethylene carbonate (PEC), polypropylene carbonate (PPC), polycaprolactone (PCL), polyethyleneimine (PEI) and poly(trimethylene carbonate) (PTMC), or a mixture of two or more of these precursors; preferably comprising an acrylate monomer and/or a methacrylate monomer.

The mass ratio of the electrode material to the other components of the liquid mixture may be in the range 0.1 : 50 to 50:0.1.

The structure of the electrode maybe such that, the distance between the structures (and/or the distance between the regions of electrode materials) is in the range 5 nm to 1000 pm, preferably 7 nm to 900 pm, more preferably 11 nm to 800 pm, most preferably 15 nm to 700 pm.

Those skilled in the art will appreciate that though specific examples and conceptions of the application have been provided herein, these disclosures may be readily modified and utilised as a basis for undertaking the same purposes as the present invention. Those skilled in the art will additionally appreciate that these modifications do not depart from the spirit and scope of the invention.

The ionic liquid may be formed by a combination of any suitable cation and suitable anion. Examples of cations and anions that may be combined in the ionic liquid include:

)

wherein each Ri, R2, R3, R4 is independently selected from Ci to C10 hydrocarbyl, preferably Ci to Ce alkyl. In this specification the terms metal (e.g. lithium) cell and metal ion (e.g. lithium ion) cell are used interchangeably unless the context otherwise requires.

In this specification “(meth)acrylate” and similar uses indicate and encompass an acrylate, a methacrylate or a mixture of acrylate and methacrylate.

BRIEF DESCRIPTION OF FIGURES

Embodiments of the present invention will be described in more detail with reference to the accompanying Figures in which:

Figure 1 shows schematic diagrams of (a) a solid-state Li metal battery (SSLMB) design that uses a polymer-based electrolyte with a randomly mixed cathode structure ( 300 r/m thick) involving tortuous percolative Li⁺ ion transport pathways through the inter-connected SSE; (b) the SSLMB design with an anisotropic cathode structure (600 mi thick) comprising vertically aligned NMC811 -rich pillars surrounded by a polymer-based electrolyte; and (c) the steps of the directional freezing and polymerization (DFP) process to fabricate the anisotropic cathode structure, showing the micro- and nano-scale Li⁺ ion transport channels. Figure 2 shows in (a) X-ray diffraction (XRD) patterns of NMC811 feedstock powder, a directional freezing and polymerization with polymer only [DFP+P] cathode and a directional freezing and polymer- ization with ionic polymer [DFP+IP] cathode; scanning electron microscopy (SEM) images of (b) NMC811 particles, (c) the upper surface and (d) a crosssection of the as-fabricated [DFP+IP] cathode.

Figure 3 shows cross-sectional SEM images of (a) an [IFP+IP] cathode, and (b) a [DFP+IP] cathode, (c,d) magnified cross-sectional SEM images of a pillar from the [DFP+IP] cathode, and (e-g) energy dispersive X-ray spectroscopy (EDS) maps for Ni, Co and Mn from the [DFP+IP] cathode. All electrodes were prepared after Ar⁺ ion etching.

Figure 4: (a) The molecular structure of poly(ethylene glycol) methacrylate (PEGMA) cross-linked with a 1 -methyl- 1 -propylpyrrolidinium cation ([MePrPyl]⁺) and a bis(trifluoromethanesulfonyl)imide anion ([TFSI]-); calculated (b) highest occupied molecular orbital (HOMO) and (c) lowest unoccupied molecular orbital (LUMO) of the cross-linked molecule; (d) a summary of the energy differences for the materials in this work and for some of the other common polymer electrolytes and ionic liquids (ILs) estimated by computational density functional theory (DFT) calculations; and (e) experimental results of ionic conductivity of the two types of solid-state electrolyte (SSE): [LiTFSI+PEGMA] and [LiTFSI+PEGMA+MePrPyl TFSI], Inset: Nyquist plot measured by electrochemical impedance spectroscopy (EIS) for [LiTFSI+PEGMA+MePrPyl TFSI] in a stainless steel (SS)/SSE/SS cell configuration.

Figure 5: (a) A grey-scale 2D slice of a [DFP+IP] cathode from X-ray micro-computed tomography (microCT), (b) a segmented 3D volume rendering of a magnified region in (a), (c) simulated Li⁺ ion total flux, flux through electrode thickness and flux along electrode plane of the [DFP+IP] electrode, and (d) Nyquist plot of the Li/SSE/cathode full cell using the [DFP+IP], [IFP+IP] and [DFP+P] cathodes.

Figure 6: (a) Galvanostatic charge and discharge profiles of the [IFP+IP] and [DFP+IP] hybrid cathodes in SSLMB full cells at 0.05 C, (b) galvanostatic charge and discharge profiles of the [DFP+IP] cathode in a SSLMB full cell at increasing charge and discharge rates, (c) rate capability of the [IFP+IP] and [DFP+IP] cathodes in SSLMB full cells, and (d) cycling performance and coloumbic efficiency of the [DFP+IP] cathode at 0.5 C, all at 25 °C. Figure 7: shows a schematic illustration of the directional freezing and polymerization (DFP) apparatus.

Figure 8 shows XRD patterns of (a) the NMC811 feedstock powder, (b) a [DFP+P] cathode, and (c) a [DFP+IP] cathode.

Figure 9: DSC traces (at 50 °C min'¹) between -85 and 300 °C for NMC811 feedstock powder, [DFP+P] cathode and [DFP+IP] cathode.

Figure 10: SEM image of NMC811 feedstock powder particles.

Figure 11 : TEM image of the edge of a NMC811 feedstock particle.

Figure 12: Large area plan view SEM image of a [DFP+IP] cathode comprising NMC811 and [LiTFSI+PEGMA+ MePrPyl TFSI] SSE.

Figure 13: SEM image of a [DFP+P] cathode comprising NMC811 and [LiTFSI+PEGMA] SSE.

Figure 14: Cross-sectional SEM image of a [DFP+IP] cathode comprising NMC811 and [LiTFSI+PEGMA+ MePrPyl TFSI] SSE after battery cycling.

Figure 15: Linear sweep voltammogram (LSV) of the polymer composite electrolyte in a SS/SSE/Li cell configuration.

Figure 16: In |V - Voo| vs t graphs showing the depolarization processes in a Li/cathode/Li cell configuration using (a) [DPF+IP], (b) [IFP+IP] and (c) [DFP+P] cathodes. The black curves were obtained experimentally while the red lines are linear best-fit when the cell potential V approached equilibrium (dV/dt < 0.1 mV h'¹).

Figure 17: Cyclic voltammogram of a SSLMB cell utilizing a [DFP+IP] NMC811-based hybrid cathode at a scan rate of 0.1 mV s'¹.

Figure 18: Galvanostatic charge and discharge profiles of the [IFP+IP] and [DFP+IP] NMC811-based hybrid cathodes in SSLMB full cells at 0.05 C, showing areal capacities. Figure 19: Galvanostatic charge and discharge profiles of a [DFP+IP] NMC811-based hybrid cathode in a SSLMB full cell at increasing charge and discharge rates, as a function of areal capacity.

Figure 20: Areal capacity of the [IFP+IP] and [DFP+IP] NMC811-based hybrid cathodes in SSLMB full cells as a function of cycles and C-rate.

DETAILED DESCRIPTION

Rechargeable batteries that provide increased specific energy and improved safety over commercial Li ion batteries (LIBs) are in demand for applications such as electric vehicles (EVs), all-electric aircraft and the grid-scale storage of electricity from renewable but intermittent electrical generation [1], Many commercial LIBs use a graphite anode (theoretical capacity 372 mAh g^-1, electrochemical potential -0.43 V vs standard hydrogen electrode) [2], Although graphite is relatively low cost and easy to process into electrodes at large scale, a switch to a Li metal anode would provide a theoretical capacity of 3860 mAh g^-1 and a lower electrochemical potential (-3.04 V vs standard hydrogen electrode) [3], When a Li metal anode is coupled with a high capacity cathode (e.g. LiNio.8Mno.1Coo.1O2 (NMC811)), the resulting battery would approximately double specific energy from 250-300 Wh kg^-1 for commercial LIBs to ~500 Wh kg^-1 [4-6], However, a Li metal anode is unstable with conventional liquid electrolytes (which are also flammable) and Li dendrites formed during cycling may readily penetrate through standard porous olefin separators and cause short circuits, rapid discharge and a range of subsequent safety hazards [7],

Solid-state Li metal batteries (SLMBs) use a solid-state electrolyte (SSE) to replace the liquid electrolyte and separator, and alongside capacity improvements, also have the potential to enable safer cycling, although achieving practical current densities remains a significant challenge [8], Compared with the high ionic conductivity of liquid electrolytes (10^-3 to IO^-2 S cm^-1 at room temperature, RT), SSEs usually have much lower intrinsic ionic conductivities at RT [9], and most SSLMB research has therefore focused on increasing the ionic conductivity of SSEs. The SSEs divide into two main families: inorganic electrolytes and polymer electrolytes [10], Inorganic electrolyte types include NASICON [11-13], garnet [14-16], perovskite [17-19], LISICON [21], sulfide [20,22-24], argyrodite [25-27], glassy [9], etc. Inorganic electrolytes typically have ionic conductivities of 2 x 10^-5 to 2 x 10^-3 S cm^-1 at RT. Practical applications have been limited by manufacturing difficulties (fragility over large areas), poor electrode/SSE interfacial contact and risk of Li dendrite growth along grain boundaries, although steady progress is being made [10],

In most polymer electrolytes, Li⁺ ionic conductivity is achieved by the solvation of a Li salt such as lithium hexafluorophosphate (LiPFe) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a polymer matrix. Polymer electrolyte types include solid polymer only [28] and ionic polymers [29], Poly(ethylene oxide) (PEO) [30], poly(acrylonitrile) (PAN) [31], poly(methyl methacrylate) (PMMA) [32], poly(vinyl alcohol) (PVA) [33] and poly(propylene) oxide (PPO) [34] are commonly used as solid polymer-only electrolytes, but their ionic conductivity (2 x 10^-5 to 2 x 10^-4 S cm^-1 at RT [35, 36]) is relatively low. Ionic polymers that solidify a cation-anion pair into a polymer matrix are attractive because ILs have negligible vapor pressure, high electrochemical stability and excellent thermal resistance [37, 38], ILs may increase the ionic conductivity to 2 to 4 x 10^-4 S cm^-1 at RT because the number of Li⁺ conduction sites along the polymer backbone is increased and polymer crystallization is suppressed. The resulting ionic polymers have a much higher entropy and free volume for polymer segmental motion of the Li⁺ conduction sites. Other advantages of ionic polymer electrolytes include compatibility with large-scale manufacturing processes, good electrode/SSE interfacial contact and high toughness that can either help to slow Li dendrite formation [5] or buffer the elastic deformation caused by the Li front movement to prevent dendrite penetration [42],

Alongside research to increase the intrinsic ionic conductivity of SSEs, there is a growing interest in tailoring the electrode and cell structure at a range of length-scales. For example, at the sub-particle scale through particle grading and coating [43, 45] and at the electrode scale through grading, layering, etc. [13, 44], In the case of SSLMB anodes, this has been explored to improve the uniformity of Li plating on charging and to buffer Li volume changes, e.g. using Ag-C composite interlayers [6], ID Cu/C pillars [46], 2D graphene/MXene [47], 3D Cu [48] and 3D C frameworks [49],

SSLMB cathodes usually comprise a randomly mixed microstructure of electrochemically active and SSE materials in which the SSE forms a highly tortuous percolating network for Li⁺ transport (Figure 1(a)). Combined with the low intrinsic ionic conductivity of most SSEs, this tortuous ion transport network leads to limited ion transport [50-54], a steep Li⁺ ion concentration gradient across the cathode thickness and ion starvation in some regions [55— 57], significantly hindering overall cell performance [58, 59], Further, in order for the cathode capacity to match the high capacity of the Li metal anode (5-10 /m), modeling suggests that the cathode thickness should increase from typically <300 z/m to 600 /zm [8], Unfortunately, the increased thicknesses only amplify the Li⁺ transport restrictions and reduce even further the achievable cell capacity.

Most SSLMB cathodes are manufactured by doctor-blade coating of a mixture of active, SSE and sometimes C electrically conductive additives and binder, usually followed by heating and/or pressing [60, 61], Some approaches dispense with C electrical conductive additives, for example if the active material is C coated, but can still exhibit adequate electrochemical performance (e.g. 107 to 152 mAh g^-1 at 0.1 C) [8, 62-64] because in these configurations Li⁺ transport rather than electron percolation is the limiting factor on SSLMB performance [65, 66], Doctor-blade coating of a porous cathode followed by infiltrating of a SSE (“backfilling”) is also used [67], but the cathode tortuous pore network cannot be easily infiltrated (only >2 /m pores can be effectively infiltrated with SSE [68]) and leads to poor active material/SSE contact and a high interfacial resistance.

To reduce the randomness of the ion pathways, anisotropic transport networks have been contrived using sacrificial pore formers, or templates. Most of these template methods have focused on the SSE membrane only (not the composite cathode), e.g. 3D printing of an insulating sacrificial polymer template followed by infiltration of a Lii.4Alo.4Gei.6(P04)3 (LAGP) SSE then removal of the sacrificial template through heating, and finally back-filling the LAGP framework with polypropylene (PP) to pro- vide structural stabilization [69], These templating approaches require several discrete processing steps, including removal of the sacrificial template using high temperature (600-900 °C) or chemical reagents that may either damage the active materials or place severe constraints on the choice of materials [70],

In the present work, in some embodiments of the invention we synthesize an ionic polymer electrolyte composed of the conducting salt LiTFSI and IL 1 -methyl- 1-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (MePrPyl TFSI) solidified in a poly(ethylene glycol) methacrylate (PEGMA) matrix. We then fabricate a 600 /m thick cathode comprising vertically aligned NMC811-rich pillars surrounded by the ionic polymer electrolyte in a single step, with no templating required (Figure 1(b)). These hybrid cathodes are fabricated using an innovative directional freezing and polymerization (DFP) process in which active cathode particles and ionic polymer are self-assembled directly into a preferred anisotropic and dense cathode structure, with no need for any subsequent pressing, heating, SSE infiltration or template removal steps. The [LiTFSI + PEGMA + MePrPyl TFSI] after polymerization has a competitive ionic conductivity of 4.2 x IO^-4 S cm^-1 at 25 °C. Critically, the DFP method produces an intimate active/SSE interfacial contact.

We combine X-ray micro-computed tomography (microCT) and Li⁺ ion transport simulations to show that ion conduction pathway tortuosity is reduced from 3.3 to 4.9 for standard SSLMB composite cathodes [71] to 1.2 for the cathode made by DFP. DC depolarization shows that the effective ion diffusivity in the same cathode is increased from 4.4 x 10”⁹ to 1.4 x 10”⁷ cm² s’¹. A SSLMB using the cathode exhibits a near theoretical gravimetric capacity of 199 mAh g^-1 and an ultra-high areal capacity of 16.7 mAh cm^-2 at 0.05 C, and 120 mAh g”¹ (10.1 mAh cm”²) at 1 C at RT, which are amongst the highest reported at the same testing conditions.

A thick (600 pm) hybrid cathode comprising vertically aligned LiNio.8Mno.1Coo.1O2 (NMC811)-rich channels filled with a [LiTFSI+PEGMA+MePrPyl TFSI] polymer composite electrolyte was fabricated by a directional freezing and polymerization (DFP) method. X-ray micro- computed tomography, ion mobility simulations and DC depolarization show that the cathode structure using the polymer composite electrolyte and made by DFP improves Li⁺ ion diffusivity in the cathode from 4.4 x 10”⁹ to 1.4 x 10”⁷ cm² s”¹. In a SSLMB full cell at 25 °C, the cathode provides gravimetric capacities of 199 and 120 mAh g”¹, and ultra-high areal capacities of 16.7 and 10.1 mAh cm”² at 0.05 and 1 °C, respectively. The work demonstrates a scalable approach to realizing composite cathode structures with kinetically favourable ion transport characteristics in SSLMBs.

Figure 1(c) shows a schematic diagram of the DFP process. A suspension was first prepared containing NMC811 particles, LiTFSI conducting salt, PEGMA monomer solution, MePrPyl TFSI IL and 1 -hydroxy cyclohexyl phenyl ketone-based Irgacure photoinitiator. The suspension was then directionally frozen in the vertical (z) direction, “bottom to top” at a controlled freezing rate of 2 mm min^-1. Supercooled clusters of PEGMA monomer and MePrPyl TFSI molecules crystallized [72], Because heterogeneous crystal nucleation at a foreign surface has a lower free energy barrier than homogeneous nucleation directly from solution, the PEGMA and MePrPyl TFSI crystals were first formed on the surface of the cold finger at -30 °C [73] (Figure 7 shows a schematic of the DFP apparatus). As the solution of [LiTFSI + PEGMA + MePrPyl TFSI] continued to freeze, the crystals grew parallel to the strong vertical temperature gradient, and self-assembled into a vertically aligned structure spanning hundreds of micrometers that pushed the NMC811 particles sideways into the regions in-between the growing crystal pillars. On subsequent in-situ UV cryopolymerization at -25 °C, the [LiTFSI + PEGMA + MePrPyl TFSI] solidified with the PEGMA acting as the host polymer matrix for cross-linking with the MePrPyl TFSI [74],

UV polymerization is usually associated with thinner layers (50-100 pm) but UV polymerization of thicker layers (100 pm - 2.5 mm) can be achieved by increasing irradiance intensity and exposure duration [75], e.g. 1.4 mm thick methacrylate films [76] and 800 pm thick polyethylene films containing LiTFSI, ethoxylated trimethylolpropane triacrylate (ETPTA) and succinonitrile (SN) [77] were cured at 300-800 mW cm^-2 for 120 s - 60 min. Here, an irradiance intensity of 400 mW cm^-2 and exposure duration of 40 min were used, which as shown later, fully cured the 600 pm thick cathode structures into vertically aligned NMC811-rich pillars with the interstices between the pillars filled with dense [LiTFSI + PEGMA + MePrPyl TFSI]. All the functional materials in the suspension were directly incorporated into the final cathode structure and no subsequent pressing, heating, template material removal or SSE back-filling steps of any type were required.

The anisotropic cathode structural design was intended to provide micro-scale, low tortuosity SSE pathways in-between the vertical NMC-rich pillars and fast Li⁺ ion transport through the hybrid cathode thickness, while SSE incorporated within the NMC-rich pillars themselves would facilitate nano-scale transport of ions to each NMC particle. It was also intended that the C-coated NMC-rich pillars would provide sufficient electrical conductivity along the pillars that the addition of electrical conductive additives could be avoided, which has already been shown for anisotropic LiCoCh cathodes made by co-extrusion [84] and by magnetic templating [85], and for LiNio.sCoo.15Alo.05O2 cathodes by freeze casting [86], Note that the polymer-based SSE also provided mechanical stabilization of the cathode structure and there was no need for any additional inactive component such as a binder.

The resulting hybrid cathode has excellent structural integrity that enabled easy handling.

Other polymer electrolyte compositions were also investigated and compared. The mechanical strength of a polymer composite containing the same MePrPyl TFSI used here at an IL : polymer volume ratio of 1 : 1 was investigated by tensile testing at a speed of 1 cm min^-1, giving a tensile strength of 6.7 MPa and an elongation of 178% at room temperature

[78], Mechanical strength was improved on reducing the IL : polymer volume ratio to 0.5 : 1

[79], Although the addition of IL decreased the glass transition temperature of the composite, the strong ion-dipole interaction between the dissociated Li⁺ cation from LiTFSI and ether oxygen from PEGMA contributed to mechanical stability, even up to 40 °C. Further improvement in mechanical stability at elevated temperatures >60 °C has been investigated through (i) a higher degree of cross linking (e.g. by adding a polymerizable IL 1,4- di(vinylimidazolium)butane bis-bromide [81]) and (ii) adding nanoparticles such as AI2O3 to promote physical entanglements between the nanoparticles and polymer backbone [82],

To make a SSLMB full cell, a mixed solution of LiTFSI, MePrPyl TFSI, PEGMA monomer and photoinitiator was directly drop cast onto the composite cathode followed by the same in- situ UV cryopolymerization process to make a ~150 pm thick SSE membrane. A Li metal foil as the anode was then added onto the SSE membrane. The arrangement was assembled into a coin cell that provided moderate compression to ensure sustained and reproducible contact of the electrode/SSE interfaces.

Other than the hybrid cathode made by DFP with the ionic polymer already described, termed [DFP+IP], to investigate the effect of SSE and differences in cathode structure on electrochemical properties, two other types of hybrid cathode were fabricated: (i) using [LiTFSI + PEGMA] SSE without MePrPyl TFSI made by directional freezing and polymerization, termed [DFP+P], and (ii) again using the [LiTFSI + PEGMA + MePrPyl TFSI] SSE but now with isotropic freezing and polymerization, termed [IFP + IP], All the cathodes contained the same proportion (62±5 vol%) of NMC811 active material, and similar to 40-60 vol% active material used in the majority of cathodes for SSLMBs [8, 87, 88] A summary of the materials, average cathode thickness and active material mass loading for the three types of cathode is given in Table 1.

P10752GB

Table 1.

Vertically aligned cathode structure

Figure 2(a) shows X-ray diffraction (XRD) patterns of the NMC811 feedstock powder and the [DFP+P] and [DFP+IP] cathodes. The split peaks of (006)/(102) and (108)/(l 10) reflections (magnified in Figure 8 (a-c)) in all three patterns was the typical structure of a close-packed oxygen lattice with alternating layers of Li⁺ and transition metal ions in an octahedral coordination configuration [83, 89], All of the reflections readily indexed to LiNio.8Mno.1Coo.1O2 with a R3m space group [90] i.e. there were no additional chemical or material phase changes caused by fabrication. Figure 9 shows differential scanning calorimetry (DSC) traces of the NMC811 feedstock powder and the [DFP+P] and [DFP+IP] cathodes. There were no sharp endothermic (melting) peaks suggesting good thermal stability and no significant phase separation in the range -85 to 300 °C [91], Figure 2(b) shows a scanning electron microscopy (SEM) image of the NMC811 feedstock particles (and a magnified individual NMC811 particle in Figure 11), with typical 4-17 urn spherical secondary particles consisting of 200 - 700 nm primary particles [92], Figure 11 is a transmission electron microscopy (TEM) image of the edge of a NMC811 particle showing a ~8 nm thin C coating because the NMC811 was synthesized in the presence of oxygen at high calcination temperature (800 °C) where organic reagents reacted to leave a carbonaceous film with useful electrical conductivity [93-95],

Figure 2(c) shows a plan view SEM image of the dense upper free surface of the [DFP+IP] cathode after polymerization (a larger area electrode plan view SEM image is given in Figure 12). Figure 2(d) shows a corresponding cross-sectional SEM image of the [DFP+IP] cathode after polymerization. Both SEM images show full densification of the polymeric matrix across the entire 600 uni thickness, and the cured [LiTFSI+PEGMA+MePrPyl TFSI] SSE readily wetted onto the NMC811 particles and filled the inter-particulate interstices within the NMC-rich pillars. High density (>95%) is considered crucial for useful rates of Li⁺ ion transport in the cathodes of solid-state batteries [96], In contrast, a similar SEM image of the [DFP+P] cathode after polymerization in Figure 13 shows 800 nm - 4 mi pores, indicating that polymerization shrinkage was developed after curing [97], These results indicate that the addition of MePrPyl TFSI to the [DFP+IP] cathode suppressed PEGMA crystallinity [98] and eliminated the tendency for shrinkage of pores [74, 99], To investigate the effects of directional freezing on the cathode structure, crosssections of the [IFP+IP] and [DFP+IP] hybrid cathodes were prepared for SEM by Ar⁺ ion etching to gently and preferentially remove some of the SSE. Figure 3(a) is a resulting cross-sectional SEM image of the [IFP+IP] cathode after ion etching, showing a pattern of similar-sized (—0.8 «m) circular “dimples” with no obvious long range alignment or directionality. In contrast in Figure 3(b), and consistent with Figure 2(d), the [DFP+IP] cathode showed a distinct vertically aligned structure with larger NMC811 secondary particles exposed on the top of the majority of the NMC-rich pillars, suggesting that the pillar growth (during freezing) was stopped when encountering a particularly large NMC secondary particle. Figure 3(c,d) shows magnified cross-sectional SEM images of a pillar with embedded smaller (200 - 700 nm) NMC811 primary particles. Figure 3(e-g) shows energy dispersive X-ray spectroscopy (EDS) maps for Ni, Co and Mn respectively, confirming that the NMC811 particles were incorporated and concentrated within the pillar-like structures. Furthermore, Figure 14 shows the cross-sectional SEM image of the [DFP+IP] cathode after 200 charge and discharge cycles (cycling performance described later) and after the battery was dissembled, showing that the aligned anisotropic electrode structure was maintained and confirming the mechanical stability of the cathode structure.

Electrochemical stability and ionic conductivity of the polymer-based SSE A wide battery operating voltage window is advantageous for increasing the specific energy and is often determined by the electrochemical stability of the SSE for a SSLMB [8], To investigate the relative stability of the SSE computational density functional theory (DFT) calculations were used. The geometric structure of the PEGMA molecular unit cross-linked with a [MePrPyl]+ cation and a [TFSI]- anion was first obtained (Figure 4(a)), and the electron density distribution during redox reactions then investigated. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the molecule were calculated. In general, the stability window between upper and lower potentials for oxidation and reduction processes has a linear correlation with the energy difference between HOMO and LUMO [43, 100, 101], Figure 4(b) shows the calculated HOMO band of the cross-linked molecule. The contours indicate the extent to which regions have a tendency to be oxidized, in this case located principally on the [TFSI]- anion due to its electron-rich regions around the oxygen and sulfur atoms in the sulfonyl groups [102], Hence, the resistance of [TFSI]- to oxidation determines the upper potential limit of the cross-linked molecule. Figure 4(c) shows the calculated LUMO band of the same molecule, with contours now indicating the extent to which the region has a tendency to be reduced, located on the PEGMA unit due to its reactive hydroxyl group that gains an extra electron [103], Hence, the resistance of the PEGMA unit to reduction determines the lower potential limit of the cross-linked molecule.

The HOMO and LUMO of some of the other common polymer units and IL molecules were also calculated and are summarized in Figure 4(d). The energy difference for PEGMA is 6.34 eV and higher than some of the other common polymer electrolytes such as PAN (6.18 eV) and PPO (6.22 eV). The energy difference for MePrPyl TFSI was 5.90 eV and higher than some of the other common ILs such as BuMelm TFSI (5.72 eV) and N, N-Di ethyl -2-methoxy-N-m ethyl ethanamonium tetrafluoroborate (DeMe BF4, 5.74 eV) [99], Further, the energy difference for the cross-linked [PEGMA + MePrPyl TFSI] molecule was 6.43 eV and higher than either of PEGMA or MePrPyl

TFSI on its own, confirming that cross-linking increased electrochemical stability due to the strong electrostatic interactions between the [MePrPyl]⁺ cation, [TFSI]- anion and the OH- group of PEGMA [100, 101], To investigate the potential stability of the SSE against a Li metal anode, Figure 15 is a linear sweep voltammogram (LSV) between -3.2 and 5.6 V in a stainless steel (SS)/SSE/Li cell configuration showing anodic and cathodic stability limits of 4.7 and - 2.9 V respectively and an overall potential stability window of 7.6 V. This window is larger than some of the other SSEs such as PEO (—4.5 V) [102] and other high voltage cathode materials such as NMC811 (upper voltage —4.2 V) [78, 103],

To investigate the effects of adding MePrPyl TFSI on ionic conductivity, electrochemical impedance spectroscopy (EIS) was performed in a SS/SSE/SS cell configuration at 25-125 °C (inset of Figure 4(e)). The ionic conductivity a was estimated from the equivalent series resistance (R?) at the intercept of the Nyquist curve with the real Z -axis at the highest frequency and using <J = 11 Rs A where t is the thickness of the SSE and A is the contact area [109,110], Figure 4(e) summarizes the ionic conductivities showing <7 = 4.2 x 10 ⁴ S cm for [LiTFSI+PEGMA+MePrPyl TFSI] at 25 °C, more than one order of magnitude higher than o = 2.1 x 10^-5 S cm^-1 for [LiTFSI+PEGMA] at the same temperature. The ionic conductivity of 4.2 x 10^-4 S cm^-1 at 25 °C was comparable or slightly higher than similar ionic polymers [29,39— 41,112, 113], The higher ionic conductivity of [LiTFSI+PEGMA+MePrPyl TFSI] than [LiTFSI+PEGMA] was because Li⁺ transport mechanism in [LiTFSI+PEGMA] is through association and dissociation of Li⁺ with the ether oxygen atoms and the negatively charged [OH]- dipole from PEGMA along the molecular chain using electrostatic interactions [111] whereas the Li⁺ transport mechanism in [LiTFSI+PEGMA+MePrPyl TFSI] involves the additional association and dissociation of Li⁺ with the [TFSI]- anion from MePrPyl TFSI. The good miscibility of MePrPyl TFSI and PEGMA (shown by the DSC results) ensured sufficiently uniform dispersion of [TFSI]- in the polymer matrix, and MePrPyl TFSI provided PEGMA with a higher degree of local segmental movement [98], Li+ ion transport in the cathode

X-ray micro-computed tomography (microCT) was used to investigate the long-range alignment of the [DFP+IP] cathode structure in 3D. The 3D data volume consisted of a stack of 2D grey-scale image slices and Figure 5(a) shows one 2D image slice with preferential vertical ordering of white and grey phases along the z -axis (through the electrode thickness). The 3D data volume was segmented into the different phases based on the different attenuated X-ray intensities that were directly proportional to the phase density (segmentation details in Methods and [114, 115]). The highest attenuating phase with a density of ~2.2 g cm^-3 (red) was the larger (4-17 /mi) NMC811 secondary particles. The intermediate attenuating phase with a density of ~ 1.7 g cm^-3 (green) was smaller (2-8 /m) NMC811 particles mixed with a relatively low fraction of SSE (the

NMC-rich pillars). The lowest attenuating phase with a density of ~1.2 g cm^-3 (blue) was the SSE alone. Not all of the smaller NMC811 primary particles could be resolved by the microCT instrument (voxel size 700³ nm³) [116], nevertheless, Figure 5(b) shows a magnified 3D rendered volume that mostly comprises a NMC-rich pillar, showing some smaller NMC811 particles encapsulated by the SSE and corroborating the cross- sectional SEM images in Figure 3(c,d). To investigate electrode microstructural effects on long-range Li⁺ ion transport, finite difference method- based ion transport simulations were performed using the segmented microCT data to quantify the orthogonal x-y-z Li⁺ ion transport fluxes. Li⁺ ions were simulated moving from one side of the cathode to the other in the x-z plane (i.e. in the direction through the cathode thickness), and then along the x-y plane (i.e. horizontally within the cathode plane) according to a difference in ion concentration imposed on opposite faces of the volume. This approach does not simulate ion mobility during actual charge/discharge but allows quantification of the relative ion mobility in orthogonal directions in a real 3D microstructure [117]. Figure 5(c) shows the corresponding simulated ion fluxes, showing an 81% increase in the ion flux through the electrode thickness (the preferable ion transport direction during (dis)charge) compared with the ion flux in the electrode plane.

Tortuosity r was used to quantify the resistance to the Li⁺ ion flux in orthogonal directions and was obtained by comparing the effective Li⁺ ion flux through the cathode (Fp) with the idealized Li⁺ ion flux through a volume of the same size of the cathode (Fc according to:

where is volume fraction of SSE, D is diffusivity of Li⁺ ions, AC is the imposed, starting Li⁺ ion concentration difference across an electrode thickness L with cross- sectional area A . Hence, T was estimated from the ratio between F_P and Fcv (r = 1 indicates an idealized straight path [118] .) For the through cathode thickness direction, previous simulations have shown that for T _Z >3, Li⁺ ion transport in the cathode is significantly impeded and restricts the realizable capacity of SSLMBs [52], In practice, r_z = 3.3-4.9 is obtained experimentally for most cathodes (<300 mi thick) comprising a conventional randomly mixed microstructure of cathode and SSE materials [71, 118, 119], Even for an idealized cathode comprising a randomly mixed microstructure of

NMC and LiioGeP₂S 12 SSE with no microvoids, computational simulations have yielded r_z = 2.3 [59], In this study, T _Z = 5.6 was estimated for the 600 /zm thick [IFP+IP] cathode that reduced significantly to T _Z = 1.2 for the 600 /m thick [DFP+IP] cathode which was comparable with r_z = 1.2-1.9 for cathodes containing anisotropic pore structures for liquid electrolyte-based batteries made by multi-step pore templating methods [116, 120, 121],

To explore the microstructural effects on electrode resistance, EIS was used to investigate the [IFP+IP], [DFP+P] and [DFP+IP] cathodes in a cathode/SSE/Li full cell configuration at 25 °C and Figure 5(d) shows the corresponding Nyquist plots. The intercept of the Nyquist curves with the real Z -axis at the highest frequency represents the equivalent series resistance (Rs) of the cathode and the SSE, while the diameter of a best-fit semi-circle to the data represents the charge resistance (Rcr ) of the cathode and the interfacial resistance between the cathode and SSE [110], Rs was estimated at 8.9, 21.2 and 31.4 Q cm², and Rcr at 37.7, 85.5 and 148.6 Q cm² for the cells using

[DFP+IP], [IFP+IP] and [DFP+P] cathodes, respectively. The Rs and Rcz for the [DFP+IP] and [IFP+IP] cathodes were lower than for the [DFP+P] cathode due to the better NMC811 particle/SSE contact and higher intrinsic ionic conductivity of [LiTFSI+PEGMA+MePrPyl TFSI] that enabled relatively fast Li⁺ ion diffusion through the cathode. The [DFP+IP] cathode had both the lowest Rs and RCT because it had the lowest through thickness SSE tortuosity and the C-coated NMC particles were contrived into pillars that improved electrical conductivity. Nonetheless, resistances remained relatively high, as is often the case for SSLMBs, and contriving electron conductivity additives into the NMC -rich pillars might be beneficial, although the active material fraction would have to be proportionally reduced. The effective Li ion diffusivity (Deff) of the [DFP+IP], [IFP+IP] and [DFP+P] cathodes was estimated via DC depolarization experiments in a Li/cathode/Li cell configuration [121], The cell was first polarized at a low current of 10 /A for 2 h to produce a Li⁺ ion concentration gradient that was then allowed to relax (depolarize) until the cell potential V approached equilibrium (dV/dt < 0.1 mV h ') [122], Figure S13(a-c) shows graphs of ln|V(t)-V(t=oo)| vs time t for the three types of cathode. The linear best-fit (red line) to the data suggested that Li⁺ transport in the electrode was diffusion-limited [123] and so the relaxation time t_re was determined from the linear region according to: t_re = L¹/^Deff') (3) where £ is the thickness of the electrode, and De^= 1.4 ± 0.06 x 10^-7, 8.9 ± 0.08 x 10^-8 and 4.4 ± 1.01 x 10^-9 cm² s^-1 was estimated for the [DFP+IP], [IFP+IP] and [DFP+P] cathodes, respectively. The ion diffusivity in the [IFP+IP] cathode was consistent with previous reports of 3.1 - 9.1 x 10^-8 cm² s^-1 for composite cathodes with a randomly mixed structure of active and an ionic polymer PEO-IL SSE [124], Overall, X-ray microCT, simulations, EIS and DC depolarization studies consistently showed that the anisotropic structure improved ion mobility kinetics in the critical through-thickness direction of the [DFP+IP] cathode.

Electrochemical properties of a SSLMB full cell

Figure 17 shows a cyclic voltammogram of a cathode/SSE/Li full cell with a [DFP+IP] hybrid cathode between 2.4 and 4.7 V at a scan rate of 0.1 mV s^-1. The maxima of the three largest peaks during charge were at 3.8, 4.0 and 4.2 V, and the three corresponding peaks during discharge were at 3.7, 4.0 and 4.2 V. These peaks relate to changes in Li⁺ layer spacing and Li⁺ mobility in the NMC811 crystal structure and consistent with previous work [83], The EIS results in Figure 5(d) show the high resistance of the [DFP+P] cathode and no useful capacity was obtained. Figure 6(a) compares the galvanostatic charge and discharge profiles of the SSLMB full cells using the [IFP+IP] and [DFP+IP] cathodes at 0.05 C (assuming the theoretical capacity of NMC811 is 200 mAh g^-1 [83]), at the first and tenth cycle and 25 °C. The [IFP+IP] cathode delivered discharge capacities of 152 and 149 mAh g^-1 at the first and tenth cycles respectively, corresponding to areal capacities of 12.5 and 12.2 mAh cm^-2. The [DFP+IP] cathode delivered higher discharge capacities of 199 and 196 mAh g^-1 at the first and tenth cycles respectively, corresponding to areal capacities of 16.7 and 16.4 mAh cm^-2. Figure 18 shows the corresponding charge and discharge profiles as a function of areal capacity. The gravimetric capacity of the [DFP+IP] cathode was higher than other SSLMB cathodes, e.g. 110-120 mAh g ¹ for LiM C [125], 120-160 mAh g^-1 for LiCoO₂ [120,126], 116-162 mAh g"¹ for LiFePCU [116,127], 140-170 mAh g"¹ for LiMnCh [128, 129], 160-170 mAh g"¹ for LiNio.33Mno.33Coo.33O2 [130], and 162-180 mAh g^-1 forNMC811 [131,132] at the same measurement conditions. The high capacity arose from the intrinsic higher capacity of NMC811, the higher ionic conductivity of the [LiTFSI+PEGMA+MePrPyl TFSI] SSE and the higher effective Li⁺ ion diffusivity in the vertically aligned cathode structure.

The [DFP+IP] cathode areal capacity was also higher than other SSLMB cathodes, e.g. 0.2 mAh cm^-2 for LiNi0.8Co0.15Al0.05O2 cathode with Li2S-P₂S5 SSE [133], 1 mAh cm^-2 for LiFePO4 cathode with poly(styrene trifluoromethanesulphonylimide of Li) P(STFSILi)-PEO-P(STFSILi) polyanionic block copolymer SSE [134] and 10 mAh cm^-2 for LiCoO₂ cathode with polystyrene-b-poly(4-vinylpyridine) block copolymer and Li?La3Zr₂0i2 composite SSE [64], Figure 6(b) shows the galvanostatic charge and discharge profiles of the [DFP+IP] cathode in a SSLMB full cell with increasing charge and discharge rates from 0.05 to 1 C (1 C ~ 1 hr charge or discharge) at 25 °C. The cell was charged and discharged for 5 cycles at each increasing C rate, followed by another 5 cycles at 0.05 C. Rates of <1 C are normally used for relatively thick electrodes in SSLMBs that require current densities 2-20 times higher than used for conventional thickness electrodes (30-300 irn) [31, 135-137],

Figure 19 shows the corresponding galvanostatic charge and discharge profiles at increasing C rate as a function of areal capacity. Figure 6(c) summarizes the rate- dependent discharge gravimetric specific capacities of the [DFP+IP] and [IFP+IP] cathodes, with the [DFP+IP] cathode maintaining a higher capacity at all C rates, e.g.

120 and 71 mAh g^-1 (10.1 and 5.8 mAh cm^-2) for the [DFP+IP] and [IFP+IP] cathodes respectively at 1 C (corresponding discharge areal capacity summary in Figure 20). The capacity retention was 93% ± 2.1% at 0.1 C, 78% ± 2.0% at 0.5 C and 63% ± 1.7% at 1 C for the [DFP+IP] cathode, and 89% ± 2.3% at 0.1 C, 68% ± 2.2% at 0.5 C and 46% ± 1.8% at 1 C for the [IFP+IP] cathode. The composite cathode capacity and performance determines the maximum energy density possible in a SSLMB full cell using Li metal as the anode [8], The energy density of the full cell E was estimated using E=VCm/M where V is the average voltage of discharge, C is the specific capacity of the cathode, m is the mass of the cathode and M is the total mass of the full cell including the mass of solid-state electrolyte and Li metal anode [8], The SSLMB using the [DFP+IP] cathode yielded E = 301 Wh kg^-1 at room temperature, higher than E = 227 Wh kg^-1 using the [IFP+IP] cathode, and also than higher than E = 232 Wh kg^-1 for a solid-state battery comprising a LiCoCh-based cathode, a Ta-substituted LnLaiZriOii solid electrolyte and a Li metal anode [63] and E = 166 Wh kg^-1 for a solid-state battery using a Li3V2(PO4)3 cathode, a Lii.3Alo.3Tii.₇(P04)3 solid electrolyte and a LiTi2(PC>4)3 anode [138], In principle, the energy density of the [DFP+IP] cathode could be further increased by increasing the active material NMC811 : SSE ratio but it anticipated that only marginal gains are achievable before the continuity of the ionic pathways would be undermined. Figure 6(d) shows the discharge capacity and coulombic efficiency of the [DFP+IP] cathode in a SSLMB full cell during cycling at 0.5 C, showing 94% capacity retention and 97% coloumbic efficiency after 200 cycles. The relatively high cycling performance and coloumbic efficiency were due to the high electrochemical stability of the SSE against the Li metal anode and the mechanical stability conferred by the UV cured ionic polymer, corroborating the LSV results in Figure 16.

Conclusion

We have developed a directional freezing and polymerization (DFP) processing method to fabricate 600 «m thick solid-state lithium metal battery (SSLMB) cathodes with an anisotropic structure containing vertically aligned LiNio.8Mno.1Coo.1O2 (NMC81 l)-rich pillars surrounded by a [PEGMA+MePrPyl TFSI+LiTFSI] polymer composite electrolyte. The DFP method does not require any post-processing steps such as template removal and infiltration that are typically used to make anisotropic structured electrodes. Instead, the anisotropic cathode structure was formed in-situ during selfassembly under directional freezing followed by polymerization that provided excellent NMC/SSE contact. DFT calculations, DSC and EIS results showed the PEGMA+MePrPyl TFSI+LiTFSI] SSE had high electrochemical and thermal stability and a competitive intrinsic ionic conductivity of 4.2 x 10 ⁴ S cm ¹ at 25 °C. X-ray micro-computed tomography, Li⁺ ion transport simulations and DC depolarization results demonstrated that the vertically aligned cathode structure reduced Li⁺ ion transport pathway tortuosity from 3.3-4.9 for standard SSLMB composite cathodes to 1.2, and increased effective ion diffusivity from 4.4 x 10^-9 to 1.4 x 10^-7 cm² s^-1. SSLMB full cells using the [DFP+IP] hybrid cathode exhibited a near theoretical gravimetric capacity of 199 mAh g^-1 and a superb areal capacity of 16.7 mAh cm^-2 at 0.05 C at 25 °C, as well as delivering 120 mAh g^-1 (10.1 mAh cm^-2) at 1 C. The vertically aligned structure allowed efficient use of active material in a practical thick electrode format. The relatively straightforward DFP processing method, and the many potential variants that are readily suggested, may provide new opportunities for scalable SSLMB fabrication routes that more effectively realize the high capacity of thick cathodes in SSLMBs.

Methods Electrode and SSE fabrication

LiNio.8Mno 1Coo 1O2 (NMC811) powder was provided by Targray, UK. A viscous electrode suspension was prepared by mixing the NMC811 powder, lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salt (Sigma Aldrich, UK) and a photoinitiator Irgacure (Sigma Aldrich, UK) at a weight ratio of 2.5:2:0.5 in a mixture of 1 -methyl- 1-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (MePrPyl TFSI) and a monomer solution of polyethylene glycol) methacrylate (PEGMA, Mn=500) at a volume ratio of 0.5: 1. For the directional freezing and polymerization (DFP) process, the suspension was directionally frozen in a custom-made 3D printed acrylonitrile butadiene styrene (ABS) mould at a freezing rate of 2 mm min^-1 that was controlled by a heating coil around a copper cold finger, one end of which was immersed in liquid nitrogen. Free-standing frozen electrodes were extracted from the mould and then directly underwent a UV-initiated cryopolymerization (average UV light intensity of 400 mW cm^-2 at 365 nm) at -25 °C. For comparison, electrodes using the same materials were also fabricated by isotropic freezing and polymerization (IFP) method by placing the electrode suspension in a temperature controlled freezer. To form an electrically insulating SSE membrane on the NMC811 -based cathode, a viscous suspension of LiTFSI and Irgacure in a mixture of MePrPyl TFSI and PEGMA was drop cast on the cured cathode, and UV cured at -25 °C. Finally, a Li metal foil was placed on the SSE membrane and the battery was sealed in a CR2032 coin cell in an Ar filled glovebox (H2O < 0.1 ppm, O2 < 0.1 ppm).

Characterization The NMC811 powder and the resulting electrodes were investigated by X-ray diffraction (XRD, D5000, Siemens) with Cu_a radiation 2=1.5 A. Thermal stability and any phase separation in the cathodes was evaluated using differential scanning calorimetry (DSC, Diamond Hyper, Perkin Elmer) by heating samples at 50 °C min^-1 from -85 to 300 °C. Cross-sections of the electrodes were prepared using a precision etching coating system (PECS 685, Gatan) at an energy of 4 keV Ar⁺ beam in vacuum for 2 hrs. The electrode cross-sections were examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis (Merlin Analytical, Zeiss). The electrode particle was examined by transmission electron microscopy (TEM) (Tecnai G²F20, FEI). X-ray micro-computed tomography (microCT) was performed on the cathode (Xradia Versa530, Zeiss); each scan comprised 3,142 2D radiographic projections. The cells were galvanostatically charged and discharged between 2.5 and 4.2 V at different C rates using a battery cycler (BT-G-25, Arbin) at room temperature. Gravimetric capacity was calculated based on the weight of actives with a variance of ±3% over 50 samples of each type. Cyclic voltammetry was performed between 2.3 and 4.7 V at 0.1 mV s^-1 and electrochemical impedance spectroscopy (EIS) was performed by applying a voltage amplitude of 100 mV at open circuit voltage in the 10⁶ - 0.01 Hz frequency range using a potentiostat/galvanostat (Reference 600/EIS300, Gamry). Modelling and image quantification

The density functional theory (DFT) geometries were optimized for each of the functionals with the 6-31+G(d,p) basis set that considers p-type polarization for hydrogen atoms and d-type polarization and diffuse functions for all other elements, which are important in improving the accuracy of the calculation for soft molecular systems [139], All microCT scans were reconstructed into a 3D volume using filtered back projection and beam hardening correction algorithms embedded in Scout-and-Scan Control System Reconstructor software. The reconstructed image volumes were post- processed and quantified by Avizo 9.7.0 software. A 3D median filter with a kernel of 3x3x3 was applied to all image volumes for noise removal [114], The filtered image volumes were then segmented using the Otsu threshold [140], resulting in binarized image volumes for analysis. We determined individual phases from the microCT scans based on -ln(I/Io)=pt where I is the intensity of attenuated X-rays, Io is the intensity of incoming X-rays, p is the mass attenuation coefficient of X-ray and t is the effective thickness of the phase. Directional tortuosity was estimated using the TauFactor code in MatLab [117],

For the choice of ionic liquid in the suspension, since UV curing generates heat even at cryogenic temperatures, care was taken to ensure the IL component in the SSE remained frozen during the polymerization process to maintain the anisotropic structure. For example, the widely used l-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide (BuMelm TFSI) has a melting point of -30 °C [SI], We investigated the cathode structure stability during cryopolymerization using two suspensions: one suspension contained NMC811, LiTFSI, PEGMA monomer, the widely studied BuMelm TFSI and photoinitiator, and the other suspension contained the same components at the same concentrations except for using MePrPyl TFSI instead of BuMelm TFSI. The DFP process was used for both suspensions under the same conditions. The cathode using BuMelm TFSI could not maintain an aligned structure (see X-ray microCT 3D rendering image below), which was attributed to partial melting of the BuMelm TFSI during UV curing due to local temperature increase. This led to horizontal coalescence of the aligned regions. In contrast, when using MePrPyl TFSI, the aligned structure remained stable during UV curing because MePrPyl TFSI had a higher melting point (-18 °C). References

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All publications mentioned in the above specification are herein incorporated by reference. Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be performed therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

Claims

1. A method for forming an electrode for an electrochemical cell, the method comprising: a) Providing a liquid mixture comprising an electrode material, a salt and a polymer precursor; b) Partially, directionally structuring the liquid mixture to form a plurality of structures; and c) At least partially curing the polymer precursor.

2. A method as claimed in claim 1, wherein partially, directionally structuring the liquid mixture comprises partially, directionally cooling the liquid mixture to form a plurality of structures comprising the polymer precursor.

3. A method as claimed in either claim 1 or claim 2, wherein the liquid mixture further comprises an ionic liquid or a liquid electrolyte.

4. A method as claimed in any one of the preceding claims, wherein the liquid mixture further comprises an initiator.

5. A method as claimed in claim 4 wherein the initiator comprises an electromagnetic radiation photoinitiator, optionally a UV photoiniator.

6. A method as claimed in any one of the preceding claims, wherein partially curing the polymer precursor comprises partially electromagnetically radiation curing the polymer precursor.

7. A method as claimed in any one of the preceding claims wherein the electrode material comprises an intercalated material and/or a conversion material, optionally a lithium-containing metal oxide, and/or a conversion compound.

8. A method as claimed in claim 7, wherein the intercalated material and/or conversion material comprises a material selected from lithium cobalt oxide, lithium iron phosphate, lithium manganese oxide, lithium nickel manganese cobalt oxide or a combination of two or more of these materials.

46

9. A method as claimed in claim 7 or claim 8, wherein the intercalated material and/or conversion material comprises a material selected from a chalcogen, a metal chalcogenide, a metal halide or a combination of two or more such materials.

10. A method as claimed in any one of the preceding claims, wherein the polymer precursor comprises one or more precursors selected from precursors of poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) (PVA),poly(propylene) oxide (PPO), polyethylene glycol) methacrylate (PEGMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), polypropylene glycol (PPG), poly dimethyl siloxane (PDMS), polyethylene carbonate (PEC), polypropylene carbonate (PPC), polycaprolactone (PCL), polyethyleneimine (PEI) and poly(trimethylene carbonate) (PTMC), or a mixture of two or more of these precursors, preferably comprising an acrylate monomer and/or a methacrylate monomer.

11. A method as claimed in claim 10, wherein the (meth)acrylate monomer comprises poly(ethylene glycol) (meth)acrylate.

12. A method as claimed in any one of the preceding claims 3 to 11, wherein the ionic liquid or liquid electrolyte has a melting point of below 100 °C.

13. A method as claimed in any one of the preceding claims 3 to 12, wherein the ionic liquid or liquid electrolyte comprises a salt having a melting point of above -60 °C, preferably above - 55 °C, more preferably above - 50 °C and most preferably above -45 °C.

14. A method as claimed in any one of the preceding claims 3 to 13, wherein the ionic liquid is selected from a salt having an ion selected from ammonium, imidazolium, phosphonium, pyridinium, pyrrolidinium, piperidinium, sulfonium and bis(trifluoromethanesulfonyl)imide.

15. A method as claimed in claim 14, wherein the ionic liquid comprises a dialkylpyrrolidinium salt, preferably dialkylpyrrolidinium bis(trifluoromethanesulfonyl)imide.

16. A method as claimed in any one of the preceding claims, wherein partially,

47 directionally structuring the liquid mixture to form a plurality of structures comprises contacting the liquid mixture with a structured mould and cooling the liquid mixture, thereby partially, directionally freezing the liquid mixture to form a plurality of structures comprising the polymer precursor.

17. A method as claimed in any one of the preceding claims 3 to 16, wherein the volume ratio of the ionic liquid or liquid electrolyte to the polymer precursor in the liquid mixture is in the range 0.1: 1 to 1:0.1, preferably 0.2: 1 to 1.5:1; more preferably 0.3: 1 to 1.2:1 and most preferably 0.4:1 to 1:1.

18. A method as claimed in any one of the preceding claims, wherein at least partially curing the polymer precursor is conducted at a temperature below the melting point of the polymer precursor or the ionic liquid, preferably below -5 °C, more preferably below -10 °C and most preferably at a temperature below -20 °C, preferably below -30 °C, preferably below -35 °C, below -40 °C, below -45 °C, below -50 °C, below

-55 °C and most preferably at a temperature below -60 °C.

19. A method as claimed in any one of the preceding claims, wherein partially, directionally structuring the liquid mixture preferentially results in partially, directionally cooling the liquid mixture to form a plurality of structures comprising the polymer precursor and optionally the ionic liquid, and regions between the plurality of structures comprising the electrode material.

20. A method for forming an electrochemical cell, the method comprising: a) providing a first electrode formed by a method according to any one of the preceding claims; b) coating the first electrode with a solid electrolyte precursor mixture comprising a salt, an ionic liquid or liquid electrolyte and a polymer precursor; c) curing the solid electrolyte precursor mixture to form a solid electrolyte membrane on the first electrode, and d) contacting the solid electrolyte membrane with a second electrode.

21. A method as claimed in claim 20, wherein the second electrode comprises a source of metal ions selected from lithium, sodium, manganese, zinc and aluminium ions, optionally comprising a metal selected from lithium, sodium, manganese, zinc, and aluminium, and optionally comprises graphite.

48

22. A method as claimed in either claim 20 or claim 21, wherein curing the solid electrolyte precursor mixture comprises electromagnetic radiation curing the solid electrolyte precursor mixture.

23. A lithium battery comprising an electrode formed as claimed in any one of claims 1 to 19 and having a tortuosity T in the range 1 to 2.0, preferably 1 to 1.9, more preferably 1 to 1.8, most preferably 1 to 1.5.

24. A lithium battery comprising an electrode formed as claimed in any one of claims 1 to 19 and having a discharge capacity in the range 100 to 2000 mAhg ¹, preferably a discharge capacity in the range 120 to 2000 mAhg ¹, more preferably a discharge capacity in the range 150 to 2000 mAhg ¹, most preferably a discharge capacity in the range 170 to 2000 mAhg ¹.

25. A solid electrolyte comprising a lithium salt, an ionic liquid having a melting point between o °C and -60 °C and a polymer precursor, wherein the ratio of the ionic liquid to the polymer precursor in the electrolyte is in the range 0.1: 1 to 1:0.1.

26. A solid electrolyte as claimed in claim 25, wherein the polymer precursor comprises one or more precursors of poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) (PVA),poly(propylene) oxide (PPO), poly(ethylene glycol) methacrylate (PEGMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), Poly(vinylidene fluoride- co-hexafluoropropylene) (PVDF-HFP), polypropylene glycol (PPG), polydimethylsiloxane (PDMS), polyethylene carbonate (PEC), polypropylene carbonate (PPC), polycaprolactone (PCL), polyethyleneimine (PEI) and poly(trimethylene carbonate) (PTMC), or a mixture of two or more of these precursors, preferably comprising an acrylate monomer and/or a methacrylate monomer.

27. A method as claimed in any one of claims 1 to 19, wherein the mass ratio of the electrode material to the liquid mixture is in the range 0.1: 50 to 50:0 1.

28. A method as claimed in any one of claims 1 to 19, wherein the gap between the plurality of structures comprising the electrode material is in the range 5 nm to 1000 pm, preferably 7 nm to 900 pm, more preferably 11 nm to 800 pm, most preferably 15 nm to 700 pm.