WO2024009109A1

WO2024009109A1 - Solid electrolyte

Info

Publication number: WO2024009109A1
Application number: PCT/GB2023/051799
Authority: WO
Inventors: Richard Todd; Leonhard Damian Indra Adiguna TANNESIA
Original assignee: Oxford University Innovation Limited
Priority date: 2022-07-08
Filing date: 2023-07-07
Publication date: 2024-01-11
Also published as: GB202210054D0

Abstract

The present invention relates to a solid electrolyte for use in a lithium ion electrochemical cell. The electrolyte comprises a first layer comprising a first electrolyte material which forms a surface of the electrolyte, such that the first layer is in compressive stress. Methods of making the electrolyte are also described.

Description

SOLID ELECTROLYTE

Field of the Invention

The present invention relates to a solid electrolyte for use in a lithium ion electrochemical cell. The electrolyte comprises a first layer comprising a first electrolyte material which forms a surface of the electrolyte, such that the first layer is in (or under) compressive stress.

Background to the Invention

Lithium ion batteries are composed of cells in which lithium ions move from the negative electrode (anode) through an electrolyte to the positive electrode (cathode) during discharge. During re-charging, the flow is reversed and the lithium ions are re-deposited at the anode. They have been widely used for over 30 years on account of their high energy and/or power density and low self-discharge.

Traditional lithium ion batteries use a liquid electrolyte with a separator that allows lithium ions to move back and forth. However, liquid electrolytes represent a safety hazard since they are flammable, and if damaged or incorrectly charged can lead to explosions and fires. The energy density provided by traditional lithium ion batteries is also insufficient to be used in high energy applications such as batteries for electric vehicles.

Solid state electrolytes are being pursued as an alternative to liquid-based lithium ion batteries. Solid electrolytes, which are typically made of ceramics, are non-flammable and have excellent high temperature stability. Furthermore, the use of solid state electrolytes facilitates the use of a Li metal anode, instead of the more traditional graphite or silicon based anode. Li metal anodes are crucial to meet the high energy requirements of batteries for electric vehicles.

However, over time and over many re-charging cycles, some lithium is not replated onto the anode uniformly and Li dendrites grow into the electrolyte from the anode surface. Such dendrites can also be associated with the development of cracks in the solid electrolyte. These dendrites therefore compromise the efficiency of the cell and, if they reach the cathode, can create a short-circuit.

Previous attempts to limit the formation of dendrites in Li ion batteries have focussed on making the re-plating of lithium onto the anode more uniform, and avoiding areas of charge build up. For example, US 2020/0411916 describes the application of pressure normal to the face of the anode/cathode to improve the uniformity of Li distribution. However, this approach requires a precise external structure to be applied to the cell, which increases manufacturing complexity.

There is therefore a need to develop Li ion batteries which have good resistance to dendrite growth, and are easy to manufacture.

Summary of the invention

The present inventors have found that a multi-layered solid electrolyte, in which a layer that forms a surface of the electrolyte is under compressive stress, can be used to reduce and/or prevent dendrite growth and cracking of the solid electrolyte in a lithium ion electrochemical cell. In particular, dendrite growth is inhibited and/or prevented in a cell when the surface under compressive stress is placed in contact with the anode, for example a Li metal anode.

Accordingly, the present invention provides a solid electrolyte for use in a lithium ion electrochemical cell, the electrolyte comprising: one or more first layers each comprising a first electrolyte material, and one or more second layers each comprising a second electrolyte material, wherein at least one first layer forms a surface of the solid electrolyte; and each first layer is under compressive stress and each second layer is under tensile stress.

The present invention also provides a method of manufacturing a solid electrolyte for use in a lithium ion electrochemical cell, the method comprising: a) providing a layered structure having one or more first layers each comprising a first electrolyte material and one or more second layers each comprising a second electrolyte material, such that at least one first layer forms a surface of the solid electrolyte, wherein the layered structure is at a temperature of at least Tl; and b) cooling the layered structure to a temperature T2, wherein the cooling step generates compressive stress in each first layer and tensile stress in each second layer.

The present invention further provides a solid electrolyte for use in a lithium ion electrochemical cell, obtainable by a method as defined herein.

The present invention yet further provides a lithium ion electrochemical cell comprising a solid electrolyte as defined herein, an anode and a cathode, wherein the anode is in contact with a first layer of the solid electrolyte. The invention also provides the use of a solid electrolyte as defined herein in a lithium ion electrochemical cell. The present invention yet further provides a battery comprising one or more lithium ion electrochemical cells as defined herein.

Description of Figures

Figure 1: A representative solid electrolyte of the present invention, which is formed from a single second layer sandwiched between two first layers, such that the two first layers form the surfaces of the electrolyte.

Figure 2: Schematic showing how compressive stress in the first layer of the electrolyte which forms a surface of the electrolyte inhibits crack formation.

Figure 3: A representative solid electrolyte of the present invention, which is formed from a single second layer adjacent to a single first layer, such that the first and second layers each form a surface of the electrolyte.

Figure 4: A representative solid electrolyte of the present invention, wherein the first layer comprises sublayers.

Figure 5: A representative solid electrolyte of the present invention, wherein the first and second layers each have graduated changes in stress.

Figure 6: A representative lithium ion electrochemical cell according to the invention.

Figure 7: X-ray diffraction spectrum from the surface of the first layer of an Al- LLZO/Ta-LLZO sandwiched electrolyte according to the invention.

Figure 8: SEM image of Vickers indentation cracking in an Al-LLZO/Ta-LLZO sandwiched electrolyte according to the invention.

Figure 9: SEM image of Vickers indentation cracking in a monolithic, unstressed Al-LLZO ceramic piece.

Figure 10: Galvanostatic profile plotting a) voltage vs time and b) current vs time for an Al-LLZO/Ta-LLZO sandwiched electrolyte according to the invention.

Figure 11 : Galvanostatic profile plotting a) voltage vs time and b) current vs time for a monolithic, unstressed Al-LLZO ceramic piece.

Figure 12: SEM image of Vickers indentation cracking in an Al-LLZO/Ta-LLZO sandwiched electrolyte according to the invention.

Figure 13: SEM image of Vickers indentation cracking in a monolithic, unstressed Al-LLZO ceramic piece. Detailed Description

Layered solid electrolyte

The electrolyte of the present invention is further described with reference to the figures. Figure 1 schematically depicts a solid electrolyte (3) comprising one or more first layers (la, lb) and one or more second layers (2), wherein at least one first layer forms a surface of the solid electrolyte.

According to the invention, each first layer is under compressive stress and each second layer is under tensile stress. The compressive stress in the first layers, and in particular in the at least one first layer which forms a surface of the electrolyte (i.e. the outermost first layer), serves to reduce and/or prevent the formation of cracks in the electrolyte, within which Li metal can reside. Thus, the solid electrolyte of the invention may assist in the reduction of dendrite formation when the electrolyte is used in an electrochemical cell, such as a lithium ion electrochemical cell, as shown in Figure 2.

The solid electrolyte (3) may comprise an equal number of first and second layers, or a different number of first and second layers. Typically, the solid electrolyte is formed of one, two or three first layers and one, two or three second layers. Typically, each first layer is adjacent to at least one second layer and each second layer is adjacent to at least one first layer. Preferably, the solid electrolyte is formed of one or two first layers and one second layer. Typically, the solid electrolyte is symmetrical about a line of symmetry which runs through the centre of the electrolyte in a direction parallel to the boundaries between the first and second layers. Preferably, the solid electrolyte comprises two first layers (la, lb), such that each first layer (la, lb) forms a surface of the electrolyte (3). More preferably, the solid electrolyte (3) is formed of a single second layer (2) sandwiched between two first layers (la, lb), such that each first layer (la, lb) forms a surface of the solid electrolyte, as shown in Figure 1. Yet further preferably, the solid electrolyte (3) is formed of a single second layer (2) sandwiched between two first layers (la, lb) such that each first layer (la, lb) forms a surface of the solid electrolyte and the electrolyte is symmetrical about a line of symmetry which runs through the centre of the electrolyte in a direction parallel to the boundaries between the first (la, lb) and second (2) layers. A solid electrolyte having two first layers each forming a surface of the solid electrolyte, for example a solid electrolyte having two first layers each forming a surface of the solid electrolyte and which is symmetrical about a line of symmetry which runs through the centre of the electrolyte parallel to the boundaries between the first and second layers, has a reduced tendency to warp and/or bend during manufacture. Furthermore, such a solid electrolyte has greater compressive stress in each first layer, which results in more effective prevention of dendrite formation. Alternatively, the solid electrolyte (3) may be formed of a single first layer (1) adjacent to a single second layer (2), such that the first (1) and second (2) layers each form a surface of the solid electrolyte, as shown in Figure 3.

The magnitude of the compressive stress in each first layer, and in particular the at least one first layer which forms a surface of the electrolyte, may be maximised in order to provide effective inhibition of dendrite formation and/or crack growth. Typically, the magnitude of the compressive stress in each first layer is greater than 50 MPa, for example greater than 60 MPa, 70 MPa, 80 MPa, 90 MPa, 100 MPa, 110 MPa, 120 MPa, 130 MPa, 140 MPa, or 150 MPa, preferably greater than 80 MPa, 90 MPa, 100 MPa, 110 MPa, or 120 MPa, more preferably greater than 80 MPa, 90 MPa, or 100 MPa, most preferably greater than 100 MPa. Typically, the magnitude of the compressive stress in each first layer is less than 2000 MPa, for example less than 1900 MPa, 1800 MPa, 1700 MPa, 1600 MPa, 1500 MPa, 1400 MPa, 1300 MPa, 1200 MPa, 1100 MPa, 1000 MPa, 900 MPa, 800 MPa, 700 MPa, 600 MPa, 500 MPa, 400 MPa, 300 MPa, or 200 MPa, preferably less than 1500 MPa, 1400 MPa, 1300 MPa, 1200 MPa, 1100 MPa, 1000 MPa, 900 MPa, 800 MPa, 700 MPa, 600 MPa, 500 MPa, 400 MPa, 300 MPa, or 200 MPa, more preferably less than 1000 MPa, 900 MPa, 800 MPa, 700 MPa, 600 MPa, or 500 MPa, most preferably less than 500 MPa. Typically, the magnitude of the compressive stress in each first layer is in the range of from 50 to 2000 MPa, for example from 50 MPa to 1500 MPa, from 50 MPa to 1200 MPa, from 50 MPa to 1000 MPa, from 50 MPa to 800 MPa, from 50 MPa to 600 MPa, from 50 MPa to 500 MPa or from 100 MPa to 500 MPa, preferably 50 MPa to 1500 MPa, from 50 MPa to 1200 MPa, from 50 MPa to 1000 MPa, from 50 MPa to 800 MPa, from 50 MPa to 600 MPa, from 50 MPa to 500 MPa or from 100 MPa to 500 MPa, more preferably from 50 MPa to 1000 MPa, from 50 MPa to 800 MPa, from 50 MPa to 600 MPa, from 50 MPa to 500 MPa or from 100 MPa to 500 MPa, most preferably from 100 MPa to 500 MPa. Compressive stress may be defined as a negative stress value to indicate the compaction, instead of by magnitude only. The compressive stresses defined herein may therefore also be defined as negative values rather than by magnitude, e.g. the compressive stress in each first layer may be in the range of from (-) 50 MPa to (-) 2000 MPa.

The magnitude of the compressive stress in each first layer may be the same or different. Preferably, the magnitude of the compressive stress in each first layer is the same. Typically, the magnitude of the compressive stress in the at least one first layer which forms a surface of the solid electrolyte is greater than 50 MPa, for example greater than 60 MPa, 70 MPa, 80 MPa, 90 MPa, 100 MPa, 110 MPa, 120 MPa, 130 MPa, 140 MPa, or 150 MPa, preferably greater than 80 MPa, 90 MPa, 100 MPa, 110 MPa, or 120 MPa, more preferably greater than 80 MPa, 90 MPa, or 100 MPa, most preferably greater than 100 MPa. Typically, the magnitude of the compressive stress in the at least one first layer which forms a surface of the solid electrolyte, is less than 2000 MPa, for example less than 1900 MPa, 1800 MPa, 1700 MPa, 1600 MPa, 1500 MPa, 1400 MPa, 1300 MPa, 1200 MPa, 1100 MPa, 1000 MPa, 900 MPa, 800 MPa, 700 MPa, 600 MPa, 500 MPa, 400 MPa, 300 MPa, or 200 MPa, preferably less than 1500 MPa, 1400 MPa, 1300 MPa, 1200 MPa, 1100 MPa, 1000 MPa, 900 MPa, 800 MPa, 700 MPa, 600 MPa, 500 MPa, 400 MPa, 300 MPa, or 200 MPa, more preferably less than 1000 MPa, 900 MPa, 800 MPa, 700 MPa, 600 MPa, or 500 MPa, most preferably less than 500 MPa. Typically, the magnitude of the compressive stress in the at least one first layer which forms a surface of the solid electrolyte is in the range of from 50 to 2000 MPa, for example from 50 MPa to 1500 MPa, from 50 MPa to 1200 MPa, from 50 MPa to 1000 MPa, from 50 MPa to 800 MPa, from 50 MPa to 600 MPa, from 50 MPa to 500 MPa or from 100 MPa to 500 MPa, preferably from 50 MPa to 1500 MPa, from 50 MPa to 1200 MPa, from 50 MPa to 1000 MPa, from 50 MPa to 800 MPa, from 50 MPa to 600 MPa, from 50 MPa to 500 MPa or from 100 MPa to 500 MPa, more preferably from 50 MPa to 1000 MPa, from 50 MPa to 800 MPa, from 50 MPa to 600 MPa, from 50 MPa to 500 MPa or from 100 MPa to 500 MPa, most preferably from 100 MPa to 500 MPa.

The magnitude of the tensile stress in each second layer is preferably minimised in order to prevent cracking of the second layer(s), since ceramic materials are stronger in compression than in tension. Typically, the magnitude of the tensile stress in each second layer is greater than 5 MPa, for example greater than 10 MPa, 25MPa, 50 MPa, 75 MPa, 100 MPa, 125 MPa, 150 MPa, 175 MPa, 200 MPa, 225 MPa or 250 MPa, preferably greater than 5 MPa, 10 MPa, 25MPa, 50 MPa, 75 MPa, or 100 MPa, more preferably greater than 10 MPa, 25MPa, 50 MPa, most preferably greater than 50 MPa. Typically, the magnitude of the tensile stress in each second layer is less than 500 MPa, for example less than 450 MPa, 400 MPa, 350 MPa, 300 MPa, 250 MPa, 200 MPa, or 150 MPa, preferably less than 400 MPa, 350 MPa, 300 MPa, 250 MPa, 200 MPa, or 150 MPa, more preferably less than 300 MPa, 250 MPa, 200 MPa, or 150 MPa, most preferably less than 200 MPa. Typically, the magnitude of the tensile stress in each second layer is in the range of from 5 to 500 MPa, for example from 10 MPa to 450 MPa, from 25 MPa to 400 MPa, from 25 MPa to 350 MPa, from 25 MPa to 300 MPa, from 50 MPa to 250 MPa, or from 50 MPa to 200 MPa, preferably from 25 MPa to 400 MPa, from 25 MPa to 350 MPa, from 25 MPa to 300 MPa, from 50 MPa to 250 MPa, or from 50 MPa to 200 MPa, more preferably from 25 MPa to 300 MPa, from 50 MPa to 250 MPa, or from 50 MPa to 200 MPa, most preferably from 50 MPa to 200 MPa. Tensile stress may be defined as a positive stress value to indicate the tension, instead of by magnitude only. The tensile stresses defined herein may therefore also be defined as positive values rather than by magnitude, e.g. the tensile stress in each first layer may be in the range of from (+) 5 MPa to (+) 500 MPa.

The magnitude of the tensile stress in each second layer may be the same or different. Preferably, the magnitude of the tensile stress in each second layer is the same.

The magnitude of the compressive stress may be substantially constant throughout each first layer or it may vary through the layer. The magnitude of the tensile stress may be substantially constant throughout each second layer or it may vary through the layer. Alternatively, the magnitude of the compressive stress may decrease within each first layer towards the centre of the electrolyte, i.e. in a direction perpendicular to and away from the surface of the electrolyte. Thus, as depicted in Figures 3 and 5, a first layer may have a first face 1 la which forms the surface of the electrolyte and a second opposing face 21a. The compressive stress may decrease in a direction from face 1 la to face 21a such that, for example, the highest compressive stress is at the surface of the electrolyte and the lowest compressive stress is at the interface between the first and second layers.

The magnitude of the tensile stress may increase within each second layer towards the centre of the electrolyte, i.e. in a direction perpendicular to and away from the surface of the electrolyte. As depicted in Figures 3 and 5, the second layer may have a face 22 at the interface with a first layer. The tensile stress may increase moving away from face 22 perpendicularly to the surface such that the tensile stress in the centre part of the second layer is greater than that at the face 22.

A solid electrolyte in which the magnitude of the stress in one or more of the layers varies in this way may show reduced tendency to fracture, on account of a more gradual change in stress from compressive to tensile stress and reduced shear stresses at the edges.

Each first layer may comprise one or more sublayers, wherein each sublayer has a different magnitude of stress compared to its adjacent sublayer(s), such that the magnitude of the compressive stress decreases stepwise in a direction perpendicular to and away from the surface of the electrolyte. Each second layer may comprise one or more sublayers, wherein each sublayer has a different magnitude of stress compared to its adjacent sublayer(s), such that the magnitude of the tensile stress increases stepwise in a direction perpendicular to and away from the surface of the electrolyte. For example, the solid electrolyte (3) may be formed of a single second layer (2) sandwiched between two first layers (la, lb), wherein each first layer comprises one or more sublayers (1 a(i), la(ii), la(iii), lb(i), lb(ii), Ib(iii)), as shown in Figure 4.

Alternatively, the decrease in the magnitude of the compressive stress in each first layer may be graduated, for example such that there is a smooth decrease in compressive stress away from the surface of the electrolyte. The increase in the magnitude of the tensile stress in each second layer may be graduated, for example such that there is a smooth increase in tensile stress away from the surface of the electrolyte. For example, the solid electrolyte (3) may be formed of a single second layer (2) sandwiched between two first layers (la, lb), wherein each first layer (la, lb) comprises a decreasing magnitude of compressive stress in a direction perpendicular to and away from the surface of the electrolyte, and each second layer (2) comprises an increasing magnitude of tensile stress in a direction perpendicular to and away from the surface of the electrolyte, as shown in Figure 5. Thus, the compressive stress at face 1 la may be greater than the compressive stress at opposing face 21a, and the tensile stress at face 22 may be lower than the tensile stress in the central part of the second layer, at 32.

The magnitude of the maximum compressive stress in each first layer is typically greater than the magnitude of the maximum tensile stress in each second layer. However, the magnitude of the maximum compressive stress in each first layer may be the same as the magnitude of the maximum tensile stress in each second layer.

Where there is a graduated change in stress throughout a layer, the edge of the layer may be defined as the point in which the stress is zero.

The magnitude of the compressive stress in the first layer which forms the surface of the solid electrolyte may be indirectly measured using the Vickers indentation method. This method is well-known to the person skilled in the art. In the method, the surface of the electrolyte is indented with a Vickers indenter (a sharp, diamond pyramid) at a specified load for a specific time (for example, a 0.5kg load for 10 seconds or a 0.3 kg load for 15 seconds). Local expansion of the indent causes cracks to extend from the comers of the indent. The presence of compressive stress in the surface layer opposes the crack propagation, so the cracks are shorter than in an unstressed ceramic. Therefore, the

Vickers indentation crack length of the solid electrolyte of the invention may be less than the Vickers indentation crack length of an unstressed monolithic layer of first electrolyte material, under the same conditions.

The magnitude of the compressive stress in the first layer which forms the surface of the solid electrolyte can also be measured using X-ray diffraction, by the sine squared psi method. The method is routine to the person skilled in the art. In the sine squared psi method using X-ray diffraction, the electrolyte is tilted relative to the X-rays in order to measure interplanar spacings at different orientations relative to the specimen surface. Planes normal to the surface are compressed by the residual compressive stresses and the spacing of planes parallel to the surface are slightly expanded by the Poisson reaction to compressive stress. The stress can be deduced from the variation of d spacing with angle and the elastic constants of the material.

The magnitude of the stresses in each first and/or second layer can be measured using neutron diffraction. Compared to X-ray diffraction, neutrons penetrate further into the solid electrolyte. Collimation of the ingoing and detected beams allow for a gauge volume in the interior to be selected, so the stresses of internal layers (i.e. layers that do not form a surface of the electrolyte) can be measured. The sine squared psi method can be used as for X-ray diffraction to quantify the stresses, or alternatively a simple comparison of interplanar spacing between the electrolyte of the invention and an unstressed, monolithic specimen can be used.

The magnitude of the stresses in each first and/or second layer can also be measured by measuring the strain at the surface of the electrolyte using a strain gauge or with reference to the curvature of the surface, whilst layers are removed one by one from one surface. The removal of layers changes the strain/curvature of the surface, and the magnitude of the stresses can be back-calculated from the results.

Typically, the magnitude of the compressive stress in the first layer which forms the surface of the solid electrolyte is indirectly measured by the Vickers indentation method, using a Vickers indenter with a 0.3 kg load for 15 seconds.

In a solid electrolyte formed of a single second layer sandwiched between two first layers of the same thickness, such that the two first layers each form a surface of the electrolyte and the stress in each layer is constant, the residual compressive stress in each first layer can be estimated with the assumption of purely elastic deformation and equal elastic constants in the different layers as follows:

where: oi is the in-plane stress in the first layer;

012 is the coefficient of thermal expansion (CTE) of the second layer and on is the CTE of the first layer;

AT is the temperature difference between the stress-free temperature and the temperature it is cooled to; v is Poisson’s ratio; hi is the thickness of each first layer, and I12 is the thickness of the second layer; E is Young’s modulus.

The relative thicknesses of each first and second layer can be varied to control the magnitude of the stress in each layer. Where the first and/or second layers contain sublayers, the thickness of each sublayer can also be varied to control the magnitude of the stress in each sublayer. Typically, the thickness of each second layer is greater than the thickness of each first layer to prevent the second layers fracturing, because solid state electrolyte materials are stronger in compression rather than tension. The ratio of the thickness of each first layer to the thickness of each second layer may be greater than 0.01:1, for example greater than 0.05:1, 0.1:1, 0.2:1, 0.3;l, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, or 0.9:1, preferably greater than 0.1:1, 0.2:1, 0.3:1, 0.4:1, or 0.5:1. The ratio of the thickness of each first layer to the thickness of each second layer may be less than 1 : 1 , for example less than 0.9:1, 0.8:1, 0.7:1, 0.6:1, 0.5:1, 0.4:1, 0.3:1, or 0.2:1, preferably less than 1:1, 0.9:1, 0.8:1, 0.7:1, 0.6:1, or 0.1:1. The ratio of the thickness of each first layer to the thickness of each second layer may be in the range of from 0.01:1 to 1:1, for example from 0.1:1 to 1:1, from 0.2:1 to 0.9:1, from 0.3:1 to 0.8:1, or from 0.4:1 to 0.7:1.

The above relative thicknesses also apply to the ratio of the total thickness of the first layers to the total thickness of the second layers.

Where the solid electrolyte comprises more than one first layer, the thickness of each first layer may be the same or different. Preferably, the thickness of each first layer is the same. Where the solid electrolyte comprises more than one second layer, the thickness of each second layer may be the same or different. Preferably, the thickness of each second layer is the same. Where the solid electrolyte comprises one or more sublayers, the thickness of each sublayer may be the same or different. Preferably, the thickness of each sublayer is the same.

According to the invention, each first layer comprises a first electrolyte material and each second layer comprises a second electrolyte material. Additional materials (for example, a binder or other additive) may also be present in each layer, in addition to the electrolyte material.

Lamination

In one embodiment, the compressive stress in each first layer and the tensile stress in each second layer is achieved by using electrolyte materials with different coefficients of thermal expansion (CTE), i.e. using lamination.

Where two adjacent layers with different CTE are joined together and/or formed at elevated temperature, and then allowed to cool, the thermal expansion mismatch between said adjacent layers during cooling causes compressive in-plane residual stresses to be formed in the layer with the lower CTE, and tensile in-plane residual stresses to be formed in the layer with the higher CTE. Therefore, where the CTE of the first electrolyte material in the first layer (1, la, lb) which forms a surface of the solid electrolyte (i.e. the outermost first layer) is less than the CTE of the second electrolyte material in the second layer (2), an in-plane compressive stress is produced in said first layer (1, la, lb). Where the first (la, lb) and/or second (2) layers comprise sublayers, the CTE of each sublayer (1 a(i), la(ii), la(iii), lb(i), lb(ii), Ib(iii)) increases in a direction perpendicular to and away from the surface of the electrolyte. This provides stepwise decreasing compressive stresses in the first layer in a direction perpendicular to and away from the surface of the electrolyte.

The exact difference in CTE between the electrolyte materials of adjacent layers, and specifically between the CTE of the first electrolyte material in the at least one first layer which forms a surface of the solid electrolyte and the CTE of the second electrolyte material in second layer, will depend on the desired stress in each layer and the strength of each layer when under compression/tension. If the difference in CTE is too low, then the stresses formed in each layer may be too small to inhibit dendrite/crack formation. If the difference in CTE is too great, then the electrolyte material may fracture.

Typically, the difference in CTE between adjacent layers (for example, between a first and second layer) is in the range of from 1 x 10'⁷ to 2 x 10^-5 K^-1. For example, the difference in CTE between adjacent layers may be from 5 x 10'⁷ to 2 x 10^-5 K^-1, or from 9 x 10^-7 to 2 x 10^-5 K^-1, or from 1 x 10^-6 to 2 x 10^-5 K^-1, or from 1 x 10^-6 to 1 x 10^-5 K^-1, or from 1 x 10^-6 to 9 x 10^-6 K^-1, or from 1 x 10^-6 to 6 x 10^-6 K^-1, or from 2 x 10^-6 to 6 x 10^-6 K^-1. The difference in CTE between adjacent layers may be from 1 x 10^-6 to 5 x 10^-6 K^-1, or from 2 x 10^-6 to 5 x 10^-6 K^-1. The difference in CTE between adjacent layers may be from 1 x 10^-6 to 3 x 10^-6 K^-1. Preferably, the difference in CTE between adjacent layers is from 1 x 10^-6 to 9 x 10^-6 K^-1, more preferably from 1 x 10^-6 to 5 x 10^-6 K^-1, most preferably from 1 x 10^-6 to 3 x 10^-6 K^-1. Alternatively, preferably the difference in CTE between adjacent layers is from 2 x 10^-6 to 6 x 10^-6 K^-1 or from 2 x 10^-6 to 5 x 10^-6 K^-1, more preferably 2 x 10^-6 to 5 x 10^-6 K^-1.

Methods of measuring CTE are readily known in the art and the skilled person will be able to measure the CTE of any given material using such well-established methods without any technical difficulty. Such routine methods include (i) carrying out X-ray diffraction experiments at varying temperatures to measure the variation of interplanar spacing/unit cell dimensions with temperature; or (ii) using a dilatometer to measure the length of a specimen of material as it is heated and/or cooled. The skilled person will also readily be aware of CTE values of suitable electrolyte materials, and/or be able to easily locate them through routine literature searches (see, for example, Cai et al., Materials Today Energy, 20 (2021) 100669).

The Young’s modulus of the electrolyte materials is a measure of the stiffness of the material. It depends on the bonding and crystal structure of the material. Typically, the Young’s modulus of each electrolyte material in the solid electrolyte is greater than 40 GPa, for example greater than 50 GPa, 75 GPa, 100 GPa, 110 GPa, 120 GPa, 130 GPa, 140 GPa, 150 GPa, 160 GPa, 170 GPa, 180 GPa, 190 GPa or 200 GPa, preferably greater than 100 GPa, 110 GPa, 120 GPa, 130 GPa, or 140 GPa, more preferably greater than 120 GPa or 130 GPa, most preferably greater than 120 GPa. Typically, the Young’s modulus of each electrolyte material in the solid electrolyte is less than 300 GPa, for example less than 275 GPa, 250 GPa, 225 GPa, 200 GPa, 175 GPa, or 150 GPa, preferably less than 250 GPa, 225 GPa, 200 GPa, 175 GPa, or 150 GPa, more preferably less than 200 GPa. Typically, the Young’s modulus of each electrolyte material in the solid electrolyte is in the range of from 40 GPa to 300 GPa, for example from 50 GPa to 250 GPa, from 50 GPa to 200 GPa, or from 100 GPa to 200 GPa, preferably from 100 GPa to 200 GPa.

The electrolyte materials of adjacent layers are chemically compatible. In particular, the electrolyte materials of adjacent layers must not react so as to prevent the transport of lithium ions through the solid electrolyte.

Each electrolyte material in the solid electrolyte has sufficiently high conductivity to lithium ions to be useful in lithium ion electrochemical cells at a typical operating temperature, preferably room temperature, for example, a conductivity of at least 0.1 mS/cm, preferably at least 1 mS/cm at room temperature. The final solid electrolyte also has sufficiently high conductivity to lithium ions to be useful in lithium ion electrochemical cells, for example, a conductivity of at least 0.1 mS/cm, preferably at least 1 mS/cm at room temperature. Suitable solid electrolyte materials are well-known to the person skilled in the art.

Examples of suitable electrolyte materials for use in the first and second layers include Li_3/8Sr_7/16Ta_3/4Zr_1/4O₃ (LSTZ) and Li₇La₃Zr₂O₁₂ (LLZO). LLZO is typically doped with one or more dopants selected from Al, Ta and Ga. Preferably, the LLZO is doped with one or two dopants selected from Al, Ta and Ga, more preferably one or two dopants selected from Al and Ta. Preferably, the LLZO is doped with a single dopant selected from Al and Ta, most preferably the LLZO is doped with Ta only. The total amount of dopant in the LLZO is typically present in a relative amount of from 0.1 to 1, for example from 0.1 to 0.75 or from 0.1 to 0.5 (wherein the relative amounts refer to the relative number of moles of each compound in the formula unit). The amount of each dopant in the LLZO may be present in a relative amount of from 0.1 to 1 , for example from 0.1 to 0.75 or from 0.1 to 0.5. Typically, LLZO doped with Al is of the formula Al_0.25Li_6.25La₃Zr₂O₁₂. Typically, LLZO doped with Ta is of the formula Ta_0.5Li_6.5La₃Zr_1.5O₁₂. Typically, LLZO doped with Ga is of the formula Ga_0.2Li6.4La₃Zr₂O₁₂.

In one embodiment, the first electrolyte material in each first layer (for example, in the at least one first layer which forms a surface of the solid electrolyte) is Li_3/8Sr_7/16Ta_3/4Zr_1/4O₃ (LSTZ) and the second electrolyte material in each second layer is Li₇La₃Zr₂O₁₂ (LLZO), wherein said LLZO is doped with one or more dopants selected from Al, Ta and Ga. In one embodiment, the first electrolyte material in each first layer (for example, in the at least one first layer which forms a surface of the solid electrolyte) is Li₇La₃Zr₂O₁₂ (LLZO), and the second electrolyte material in each second layer is Li_3/8Sr_7/16Ta_3/4Zr_1/4O₃ (LSTZ), wherein said LLZO is doped with one or more dopants selected from Al, Ta and Ga. In one embodiment, the first electrolyte material in each first layer (for example, in the at least one first layer which forms a surface of the solid electrolyte) is Li₇La₃Zr₂O₁₂ (LLZO), and the second electrolyte material in each second layer is Li₇La₃Zr₂O₁₂ (LLZO), wherein each LLZO is doped with one or more dopants selected from Al, Ta and Ga and the electrolyte material in each layer is different.

Preferably, the electrolyte is formed from a single first layer and a single second layer, such that the first and second layers each form a surface of the electrolyte, wherein either: a) the first electrolyte material in the first layer is Li_3/8Sr_7/16Ta_3/4Zr_1/4O₃ (LSTZ), and the second electrolyte material in the second layer is Li₇La₃Zr₂O₁₂ (LLZO), wherein said LLZO is doped with one or more dopants selected from Al, Ta and Ga; or b) the first electrolyte material in the first layer is Li₇La₃Zr₂O₁₂ (LLZO), and the second electrolyte material in the second layer is Li₇La₃Zr₂O₁₂ (LLZO), wherein each LLZO is doped with one or more dopants selected from Al, Ta and Ga and the electrolyte material in each layer is different.

More preferably, the electrolyte is formed of a single second layer sandwiched between two first layers such that each first layer forms a surface of the electrolyte, and wherein the two first layers comprise the same electrolyte material, wherein either a) the first electrolyte material in the first layers is Li_3/8Sr_7/16Ta_3/4Zr_1/4O₃ (LSTZ), and the second electrolyte material in the second layer is Li₇La₃Zr₂O₁₂ (LLZO), wherein said LLZO is doped with one or more dopants selected from Al, Ta and Ga; or b) the first electrolyte material in the first layers is Li₇La₃Zr₂O₁₂ (LLZO), and the second electrolyte material in the second layer is Li₇La₃Zr₂O₁₂ (LLZO), wherein each LLZO is doped with one or more dopants selected from Al, Ta and Ga and the electrolyte material in each layer is different.

Preferably, the first electrolyte material is Al_0.25Li_6.25La₃Zr₂O₁₂ and the second electrolyte material is Ta_0.5Li_6.5La₃Zr_1.5O₁₂. More preferably, the solid electrolyte is formed of a single first layer adjacent to a single second layer, such that the first and second layers each form a surface of the solid electrolyte, and the first electrolyte material is Al_0.25Li_6.25La₃Zr₂O₁₂ and the second electrolyte material is Ta_0.5Li_6.5La₃Zr_1.5O₁₂. Most preferably, the solid electrolyte is formed of a single second layer sandwiched between two first layers, such that each first layer forms a surface of the solid electrolyte, wherein the first electrolyte material in each first layer is Al_0.25Li_6.25La₃Zr₂O₁₂ and the second electrolyte material is Ta_0.5Li_6.5La₃Zr_1.5O₁₂.

Thermal tempering

In one embodiment, the compressive stress in each first layer and the tensile stress in each second layer is achieved by providing temperature-induced stresses in a single ceramic material, i.e. by thermal tempering.

In this embodiment, each first electrolyte material is the same as each second electrolyte material, such that the solid electrolyte (3) is formed of a single electrolyte material. In this embodiment, each layer (la, lb, 2) is integral with each other layer (la, lb, 2), so that there is a graduated variation in stress between compressive and tensile stress in the relevant layers. Where there is a graduated change in stress throughout a layer, the edge of the layer may be defined as the point in which the stress is zero, or in other words where there is no residual stress.

The electrolyte material in the solid electrolyte has sufficiently high conductivity to lithium ions to be useful in lithium ion electrochemical cells at a typical operating temperature, preferably room temperature, for example, a conductivity of at least 0.1 mS/cm, preferably at least 1 mS/cm at room temperature. The final solid electrolyte also has sufficiently high conductivity to lithium ions to be useful in lithium ion electrochemical cells, for example, a conductivity of at least 0.1 mS/cm, preferably at least 1 mS/cm at room temperature. Suitable solid electrolyte materials are well-known to the person skilled in the art.

Suitable electrolyte materials include: Lithium Lanthanum Titanium Oxide (LLTO; Li_0.34La_0.56TiO₃), LSTZ (Li_3/8Sr_7/16Ta_3/4Zr_1/4O₃), LLZO (Li₇La₃Zr₂O₁₂), Lithium Aluminium Titanium Phosphate (LATP; Li_1.4Al_0.4Ti_1.6(PO₄)₃), Lithium Aluminium Germanium Phosphate (LAGP; Li_1.5Al_0.5Ge_1.5(PO₄)₃); Lithium Tin Phosphorus Sulfide (LSPS; Li₁₀SnP₂S₁₂), Lithium Phosphorus Sulfide (β-Li₃PS₄); Lithium Phosphorus Sulfur Chloride Iodide (Li₆PS₅Cl_0.9l_0.1) and Lithium Phosphorus Sulfur Chloride (Li₆PS₅Cl), wherein the LLZO is typically doped with one or more dopants selected from Al, Ta and Ga as defined above.

Methods of manufacture

The solid electrolyte (3) of the present invention can be made by providing a layered structure having one or more first layers (la, lb) each comprising a first electrolyte material and one or more second layers (2) each comprising a second electrolyte material, such that at least one first layer forms a surface of the solid electrolyte. The layered structure is formed at an elevated temperature, in which the layered structure is stress-free. The layered structure is then cooled to a lower temperature, which generates compressive stress in each first layer (la, lb) and tensile stress in each second layer (2).

Typically, the layered structure is provided at a temperature of at least T1 and is cooled to a temperature T2. The choice of temperature T1 will vary depending on the electrolyte materials in each layer, and on the method of providing the layered structure that is used. The temperature at which the layered structure is provided may vary throughout the structure, such that a different temperature exists in different layers, or in different sublayers. In this case, the layered structure is provided at a temperature which may be Tl, or higher than Tl, in other words T1 is the minimum temperature throughout the layered structure. The skilled person will readily understand which temperatures should be selected, based on said materials and method.

Typically, Tl is at least 200 °C greater than T2, for example at least 300 °C, at least 400 °C, at least 500 °C, at least 600 °C, at least 700 °C, at least 800 °C, at least 900 °C, at least 1000 °C, at least 1100 °C, at least 1200 °C, at least 1300 °C, at least 1400 °C, at least 1500 °C or at least 1600 °C greater than T2, preferably at least 600 °C, at least 700 °C, at least 800 °C, at least 900 °C, at least 1000 °C, at least 1100 °C, or at least 1200 °C greater than T2, more preferably at least 700 °C, at least 800 °C, at least 900 °C, or at least 1000 °C greater than T2, most preferably at least 900 °C greater than T2. Preferably, T2 is about room temperature i.e. in the range of from 20 to 25 °C. Typically, Tl is in the range of from 200 to 1600 °C, for example from 300 to 1600 °C, from 200 to 1500 °C, from 300 to 1500 °C, from 400 to 1500 °C, from 500 to 1500 °C, from 600 to 1400 °C, from 700 to 1300 °C, from 800 to 1300 °C, or from 900 to 1200 °C, preferably from 500 to 1500 °C, from 600 to 1400 °C, from 700 to 1300 °C, from 800 to 1300 °C, or from 900 to 1200 °C, more preferably from 700 to 1300 °C, from 800 to 1300 °C, or from 900 to 1200 °C, most preferably from 900 to 1200 °C.

The configuration of the layers may be any configuration as defined herein, such that at least one first layer forms a surface of the solid electrolyte. In one embodiment, the layered structure is formed of a single first layer and a single second layer. More preferably, the layered structure is formed of a single second layer sandwiched between two first layers.

The electrolyte materials in the first and second layers may be chosen such that the coefficient of thermal expansion of the first electrolyte material in the first layer which forms a surface of the solid electrolyte (la, lb) is less than the coefficient of thermal expansion of the electrolyte material in the second layer (2), as described above for laminated electrolytes.

Methods of making laminated ceramic materials are well-known in the art, and can be routinely applied to the manufacture of laminated electrolytes.

In one embodiment, providing the layered structure comprises co-sintering precursor material of the first and second electrolyte materials at a temperature Tl, to form the layered structure. Typically, the precursor material is powdered electrolyte material or a gel of the electrolyte material, preferably powdered electrolyte material. Co-sintering typically comprises providing layers of powdered ceramic material, and heating the powders to a sufficiently high temperature to fuse the powder particles together. The process is driven thermodynamically by the lowering of surface energy. Typically, an external pressure is also applied. The powdered material is typically formed by mixing particles of the desired material with water and a dispersing agent to form a slurry, and then spray- or freeze-drying the slurry. The powdered material is then layered into moulds before co-sintering. For co-sintering, T1 is typically in the range of from 300 to 1600 °C, for example from 300 to 1500 °C, from 400 to 1500 °C, from 500 to 1500 °C, from 600 to 1400 °C, from 700 to 1300 °C, from 800 to 1300 °C, or from 900 to 1200 °C, preferably from 500 to 1500 °C, from 600 to 1400 °C, from 700 to 1300 °C, from 800 to 1300 °C, or from 900 to 1200 °C, more preferably from 700 to 1300 °C, from 800 to 1300 °C, or from 900 to 1200 °C, most preferably from 900 to 1200 °C. For example, where the first and second electrolyte materials are LSTZ and/or doped LLZO as described herein, T1 is typically in the range of from 900 to 1200 °C, preferably from 1000 to 1100 °C.

In one embodiment, providing the layered structure comprises diffusion bonding the first and second layers at a temperature Tl, to form the layered structure. Diffusion bonding typically comprises heating layers of material under pressure such that the solid state diffusion of atoms bonds the layers together. The technique is usually performed at 50-70% of the melting temperature of the materials. Preferably, the surface of each layer which is being diffusion bonded is smooth and free from residues and chemical impurities. For diffusion bonding, Tl is typically in the range of from 200 to 1500 °C, for example from 300 to 1500 °C, from 400 to 1500 °C, from 500 to 1500 °C, from 600 to 1400 °C, from 700 to 1300 °C, from 800 to 1300 °C, or from 900 to 1200 °C, preferably from 500 to 1500 °C, from 600 to 1400 °C, from 700 to 1300 °C, from 800 to 1300 °C, or from 900 to 1200 °C, more preferably from 700 to 1300 °C, from 800 to 1200 °C, or from 800 to 1100 °C, most preferably from 800 to 1100 °C. For example, where the first and second electrolyte materials are LSTZ and/or doped LLZO as described herein, Tl is typically in the range of from 800 to 1100 °C, preferably from 900 to 1000 °C.

Alternatively, each first electrolyte material may be the same as each second electrolyte material such that each layer is integral with each other layer and the electrolyte is formed of a single ceramic material, as described above for thermally tempered electrolytes.

In one embodiment, thermal tempering is achieved by cooling the first layer (la, lb) which forms the surface of the electrolyte (i.e. the outermost first layer) more rapidly than the second layer (2). This produces compressive stress in the first layer which forms the surface of the electrolyte. Typically, the outermost first layer (la, lb) is rapidly cooled using forced air drafts, whilst the inner layers (2) are not exposed to the air drafts and therefore cool more slowly.

In one embodiment, thermal tempering is achieved by heating the first layer which forms the surface of the electrolyte (la, lb) to a temperature Tla and heating the second layer (2) to a temperature Tib, wherein Tla is lower than Tib. Both layers are then allowed to cool to T2 at the same rate. The temperatures Tla and Tib are typically within the range described above for T1. The difference between Tla and Tib may be in the range of from 100 to 500 °C, for example from 200 to 400 °C.

Lithium ion electrochemical cells and batteries

The solid electrolyte of the present invention can be used in a lithium ion electrochemical cell. The present invention therefore provides a lithium ion electrochemical cell comprising the electrolyte of the invention as well as the use of a solid electrolyte as defined herein in a lithium ion electrochemical cell.

The lithium ion electrochemical cell (7) comprises the solid electrolyte (3) of the present invention, an anode (4), a cathode (5) and preferably a current collector (6), as shown in Figure 6. Dendrite formation typically occurs at the interface between the anode and the electrolyte. Therefore, in order to inhibit and/or prevent dendrite formation, or to prevent crack formation associated with dendrite formation, the anode (4) is typically in contact with a first layer of the solid electrolyte (i.e. a first layer which forms a surface of the electrolyte) (la, lb, 1). In particular, the compressive stress in the first layer which forms the surface of the electrolyte (la, lb, 1) inhibits and/or prevents dendrite and/or crack formation at the interface between the anode and the electrolyte.

The components of a lithium ion electrochemical cell are well-known to the person skilled in the art. Typically, the anode (4) is formed of graphite, Li metal, lithium titanate (Li₄Ti₅O₁₂), or tin/cobalt alloys, although any anode suitable for use in a lithium ion electrochemical cell may be used. Preferably, the anode (4) is a Li metal anode. Typically, the cathode (5) is an intercalated lithium compound such as lithium cobalt oxide (LiCoO₂), lithium manganese oxide (LiMn₂O₄), lithium nickel cobalt aluminium oxide (LiNiCoAlO₂), lithium nickel manganese cobalt oxide (LiNi_xMn_yCo_zO₂) or lithium iron phosphate (LiFePO₄ ). The lithium ion electrochemical cell (7) as defined herein can be used in a battery. The present invention therefore provides a battery comprising one or more lithium ion electrochemical cells (7) as defined herein. Typically, the battery comprises one, two, three, four, five, six, seven, eight, nine or ten electrochemical cells.

Example 1

A solid electrolyte was formed according to the structure of Figure 1, in which the first electrolyte material was Al_0.25Li_6.25La₃Zr₂O₁₂ (Al-LLZO) and the second electrolyte material was Ta_0.5Li_6.5La₃Zr_1.5O₁₂ (Ta-LLZO). The CTE of Ta-LLZO is reported as 9.2 x 10^-6 K^-1 and thr CTE Al-LLZO is reported as 7.9 x 10^-6 K^-1.

1 μm sized Ta_0.50Li_6.50La₃Zr_1.5O₁₂ and Al_0.25Li_6.25La₃Zr₂O₁₂ powders were synthesized via a solid-state synthesis route with 12.5 % lithium excess to compensate for Li loss. The powders were uniaxially pressed in a 10mm die with 100 MPa pressure, followed by cold isostatic pressing (CIP) at 300 MPa. The resulting green bodies were sintered with mother powder at 1200 °C for 6 hours. Laminates were subjected to 12.5 kPa pressure during sintering to prevent delamination. SEM imaging of the laminate crosssection suggests good interlayer bonding. X-ray diffraction spectrum of the laminate surface shows that AL-LLZO is the dominant phase, with a small amount of a secondary phase of Li₂ZrO₃ (Figure 7).

Potentiostatic electron impedance spectroscopy (PEIS) measurements were carried out on Au|sample|Au cells with Au as blocking electrodes. Conductivity of the laminate was calculated to be 8.6 x 10^-4 S-cm^-1 at room temperature, higher than the constituents with similar treatments (Ta-LLZO: 4.5 x 10^-4 S-cm-1; Al-LLZO:) 3.9 x 10^-4 S-cm-1). An activation energy of 0.29 eV was calculated for the laminate conductivity, well in line with reports in the literature.

The residual compressive stress in the first Al-LLZO layer was measured using a Vickers indenter, using 0.5 kg load for 10 seconds. The resulting indents were observed under an optical microscope (see Figure 8). The same test was also applied to a monolithic ceramic piece of Al-LLZO (see Figure 9). As can be seen from Figures 8 and 9, the crack length in the sandwiched laminate is shorter than in the monolithic Al-LLZO piece. The crack length in the sandwiched laminate was measured as 45.9 +/- 1.7 microns, compared to 52.7 +/- 3.7 microns in the monolithic piece. This indicates that compressive stress was present in the Al-LLZO layer. Example 2

Synthesis

A solid electrolyte was formed according to the structure of Figure 1, in which the first electrolyte material was Al_0.25Li_6.25La₃Zr₂O₁₂ (Al-LLZO) and the second electrolyte material was Ta_0.5Li_6.5La₃Zr_1.5O₁₂ (Ta-LLZO).

Laminated green body pellets were first prepared by uniaxial pressing. 0.3 g of Al- LLZO powder mixed with 2 wt% PVB binder was lightly pressed into a 10 mm steel die. A second layer of 0.9 g Ta-LLZO with 2 wt% PVB was put in and lightly pressed onto the first layer. A final layer of 0.3 g Al-LLZO - 2 wt% PVB was then carefully put into the die, and the pellet was subjected to 120 MPa uniaxial pressure to form the laminate pellet. The formed pellet was then cold isostatically pressed at 300 MPa.

After the laminated green bodies were formed, they were put inside an alumina crucible with a bed of Al-LLZO mother powder. More mother powder was put on top of the pellets to fully cover them. Alumina weights were then balanced on top, such that 12.5 kPa of pressure was applied to the pellets. An alumina lid was then put on top of the weights, before transferring the crucibles into a conventional tube furnace. The samples were then sintered with 5 °C/min heating rate, with a 1 h debinding step at 600 °C, up to a temperature of 1200 °C for 6 h dwell time.

Basic characterisation and properties

CTE measurements were taken in air using a vertical dilatometer (L75/1550, Linseis) on Ta-LLZO and Al-LLZO pellets stood on their sides with the aid of diametrically opposing parallel flats ground onto the curved surfaces. Since the thermal shrinkage of the samples during the cooldown is of most importance, the CTEs were taken from the cooling part of the thermal cycle. Values of 1.40 x 10^-5 K^-1 for Ta-LLZO and 1.14 x 10^-5 K^-1 for Al-LLZO were obtained.

For mechanical and microstructural characterization, laminates were sectioned vertically using a slow speed saw with a diamond tipped wafering blade (Isomet, Buehler) to expose the cross-sectional face. Cross-sections and surfaces of pellets were ground using SiC pads (400, 800, 1600, 2400, 4000) and then polished using oil-based diamond suspensions of decreasing diamond particle size (OP-3, OM-1, OM-1/4, Kemet). SEM- EDX imaging of the laminate cross-section suggested good interlayer bonding. X-ray diffraction (XRD; Rigaku Miniflex) with Cu K_α radiation in an argon glovebox was used to determine the crystalline phases in the laminate surface. Cubic Al- LLZO was the dominant phase, with a small amount of a secondary phase of Li₂ZrO₃.

Electrochemical characterisation was performed by coating polished samples with gold on both sides, making sure no channel was formed on the pellet’s side connecting the two gold electrodes. The samples were mounted onto an ElS-specific sample holder with through-plane electrode configuration (CESH, Biologic) which was connected to an impedance analyzer (MTZ-35, Biologic). Electron impedance spectroscopy (EIS) measurement sweeps from 35 MHz to 5 Hz with 10 points per decade were conducted at 30 °C. The resulting responses were recorded and plotted as Nyquist plots in EC-Lab (Biologic). By fitting the equivalent circuit to the Nyquist plot, the resistance of the electrochemical cell with the pellet as electrolyte was calculated. In order to calculate conductivity, the inverse of resistance multiplied by contact area over thickness was taken. Taking into consideration sample thickness of 2.7 mm, the resulting total ionic conductivity of the laminate was measured as 7.78 x 10^-4 S cm^-1. This is almost twice the conductivity of either conventionally sintered Al-LLZO, conventionally sintered Ta- LLZO, or the control monolithic pellets measured.

Vickers indentation residual stress measurement

A series of Vickers indents was made on the Al-LLZO - Ta-LLZO - Al-LLZO laminate, with a load of 0.3 kg for 15 s. Sites where porosity could be avoided were chosen as porosity can affect the fracture behaviour.

The resulting indents were observed under an optical microscope (see Figure 8). The same test was also applied to a monolithic ceramic piece of Al-LLZO (see Figure 9). As can be seen from Figures 8 and 9, the crack length in the sandwiched laminate was shorter than in the monolithic Al-LLZO piece. The crack radius in the sandwiched laminate was measured as 28 +/- 4 microns, compared to 43 +/- 6 microns in the monolithic piece. This indicates that compressive stress was present in the Al-LLZO layer. Using indentation mechanics, the compressive residual stress can be estimated to be 126 MPa. Galvanostatic cycling

Samples were polished in an Ar-atmosphere glovebox to remove any Li₂CO₃ surface layer that might be present due to the pellet’s contact with air. The samples were then dipped into 1 M HC1 for 10 s to react any remaining Li₂CO₃ surface impurity.

Acid treated samples were then assembled into metallic Li|sample|metallic Li cells with 3 mm diameter contacts. The assembled cells were then subjected to a 170 °C heat treatment for 1 h on each side under 200 g weight, to give better contact of the metallic Li with the laminate.

GCPL (Galvanostatic Cycling with Potential Limitation) protocol was conducted in a potentiostat (VMP-3, Biologic) to investigate the critical current density before cell failure occurred. The cell was charged and discharged to 0.1 mA h cm^-2 every half cycle starting from 0.01 mA cm^-2 and increasing by 0.01 mA cm^-2 for every cycle until failure.

The results are shown in Figures 10 and 11. The laminate was shown to endure higher current and more cycles, and failed at around 0.023 mA (Figure 10). On the contrary, the control monolithic Al-LLZO was shown to fail at around 0.004 mA (Figure 11). This demonstrates that the laminate of the invention can survive more cycles, and a higher current, than a monolithic electrolyte.

Claims

1. A solid electrolyte for use in a lithium ion electrochemical cell, the electrolyte comprising: one or more first layers each comprising a first electrolyte material, and one or more second layers each comprising a second electrolyte material, wherein at least one first layer forms a surface of the solid electrolyte; and each first layer is under compressive stress and each second layer is under tensile stress.

2. A solid electrolyte as defined in claim 1, wherein the magnitude of the compressive stress in a first layer which forms a surface of the solid electrolyte is greater than 100 MPa.

3. A solid electrolyte as defined in claim 1 or claim 2, wherein the ratio of the total thickness of the first layers to the total thickness of the second layers is in the range of from 0.01:1 to 1:1.

4. A solid electrolyte as defined in any one of the preceding claims, wherein the solid electrolyte comprises two first layers, such that each first layer forms a surface of the solid electrolyte.

5. A solid electrolyte as defined in any one of the preceding claims, wherein the solid electrolyte is formed of a single second layer sandwiched between two first layers, such that each first layer forms a surface of the solid electrolyte.

6. A solid electrolyte as defined in claim 4 or claim 5, wherein the first electrolyte material in each first layer is the same.

7. A solid electrolyte as defined in any one of the preceding claims, wherein the magnitude of the compressive stress in each first layer decreases in a direction perpendicular to and away from the surface of the electrolyte and/or the magnitude of the tensile stress in each second layer increases in a direction perpendicular to and away from the surface of the electrolyte, wherein in each case the decrease is (a) a stepwise decrease or (b) a graduated decrease.

8. A solid electrolyte as defined in any one of the preceding claims, wherein each first electrolyte material has a first coefficient of thermal expansion and each second electrolyte material has a second coefficient of thermal expansion, and wherein the coefficient of thermal expansion of the first electrolyte material in the first layer which forms a surface of the solid electrolyte is less than the coefficient of thermal expansion of each second electrolyte material in each second layer.

9. A solid electrolyte as defined in claim 8, wherein the difference between the coefficient of thermal expansion of the first electrolyte material in the first layer which forms a surface of the solid electrolyte and the coefficient of thermal expansion of each second electrolyte material in each second layer is from 1 x 10^-6 to 2 x 10^-5 K^-1.

10. A solid electrolyte as defined in claim 8 or claim 9, wherein the first electrolyte material in the first layer which forms a surface of the solid electrolyte is Li_3/8Sr_7/16Ta_3/4Zr_1/4O₃ (LSTZ), and the second electrolyte material in at least one second layer is Li₇La₃Zr₂O₁₂ (LLZO), wherein said LLZO is doped with one or more dopants selected from Al, Ta and Ga.

11. A solid electrolyte as defined in claim 8 or claim 9, wherein the first electrolyte material in the first layer which forms a surface of the solid electrolyte is Li₇La₃Zr₂O₁₂ (LLZO), and the second electrolyte material in at least one second layer is Li₇La₃Zr₂O₁₂ (LLZO), wherein each LLZO is doped with one or more dopants selected from Al, Ta and Ga and the electrolyte material in each layer is different.

12. A solid electrolyte as defined in any one of the preceding claims, wherein the first electrolyte material is Al_0.25Li_6.25La₃Zr₂O₁₂ and the second electrolyte material is Ta_0.5Li_6.5La₃Zr_1.5O₁₂.

13. A solid electrolyte as defined in any one of claims 1 to 7, wherein each first electrolyte material is the same as each second electrolyte material and each layer is integral with each other layer.

14. A method of manufacturing a solid electrolyte for use in a lithium ion electrochemical cell, the method comprising: a) providing a layered structure having one or more first layers each comprising a first electrolyte material and one or more second layers each comprising a second electrolyte material, such that at least one first layer forms a surface of the solid electrolyte, wherein the layered structure is at a temperature of at least Tl; and b) cooling the layered structure to a temperature T2, wherein the cooling step generates compressive stress in each first layer and tensile stress in each second layer.

15. A method as defined in claim 14, wherein Tl is at least 200 °C greater than T2.

16. A method as defined in claim 14 or claim 15, wherein Tl is in the range of 200 °C to 1600 °C and T2 is about room temperature.

17. A method according to any one of claims 14 to 16, wherein each first electrolyte material has a first coefficient of thermal expansion and each second electrolyte material has a second coefficient of thermal expansion, and wherein the coefficient of thermal expansion of the first electrolyte material in a first layer which forms a surface of the solid electrolyte is less than the coefficient of thermal expansion of each second electrolyte material in each second layer.

18. A method according to claim 17, wherein the difference between the coefficient of thermal expansion of the first electrolyte material in the first layer which forms a surface of the solid electrolyte and the coefficient of thermal expansion of each second electrolyte material in each second layer is from 1 x 10^-6 to 2 x 10^-5 K^-1.

19. A method as defined in claim 14, wherein step a) comprises co-sintering precursor material of the first and second electrolyte materials at a temperature Tl, to form the layered structure.

20. A method as defined in claim 14, wherein step a) comprises diffusion bonding the first and second layers at a temperature Tl, to form the layered structure.

21. A method as defined in any one of claims 14 to 16, wherein each first electrolyte material is the same as each second electrolyte material and each layer is integral with each other layer, and wherein step (b) comprises cooling a first layer which forms the surface of the electrolyte more rapidly than each second layer.

22. A method as defined in any one of claims 14 to 16 or 21, wherein each first electrolyte material is the same as each second electrolyte material and each layer is integral with each other layer, and wherein step (a) comprises heating a first layer which forms the surface of the electrolyte to a temperature Tla and heating each second layer to a temperature Tib, wherein Tla is lower than Tib.

23. A method as defined in any one of claims 14 to 22, wherein step (a) comprises providing a layered structure comprising two first layers, such that each first layer forms a surface of the electrolyte.

24. A method as defined in any one of claims 14 to 23, wherein step (a) comprises providing a layered structure having a single second layer sandwiched between two first layers, such that the solid electrolyte is formed of a single second layer sandwiched between two first layers.

25. A solid electrolyte for use in a lithium ion electrochemical cell, obtainable by a method as defined in any one of claims 14 to 24.

26. A lithium ion electrochemical cell comprising a solid electrolyte as defined in any one of claims 1 to 13 or 25, an anode and a cathode, wherein the anode is in contact with a first layer of the solid electrolyte.

27. A battery comprising one or more electrochemical cells as defined in claim 26.

28. Use of a solid electrolyte as defined in any one of claims 1 to 13 or 25 in a lithium ion electrochemical cell.