Electrode structure

Abstract

The use of carboxymethyl cellulose within an interlayer of an electrode structure to improve the thermal stability of the electrode structure is described. The electrode structure is for use in a battery cell and comprises a current collector layer having a current collector surface, a polymer gel electrode layer having an electrode surface that faces the current collector surface, and the interlayer arranged between the current collector surface and the electrode surface. The interlayer comprises an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

Description

[0001] ELECTRODE STRUCTURE

[0002] The invention relates to an electrode structure for use in a battery cell, and to a method of making the electrode structure.

[0003] BACKGROUND

[0004] An electrode structure for a battery typically comprises an electrode and a current collector foil that minimises the path length for conduction of electrical current away from the electrode. In an assembled battery cell, two such electrode structures (one anode and one cathode) are arranged with an electrolyte between them.

[0005] Electrode structures of this type are typically made by forming the electrode directly onto the current collector, for example by slurry casting. In this case, the electrode is typically an oxide material. An electrode can also be formed on a current collector layer using a physical or chemical vapour deposition techniques (PVD and CVD), though such techniques are generally costly and are not compatible with all materials.

[0006] It is also important, especially for high-power applications such as EV batteries, that cells and electrode structures have high thermal stability, i.e. good performance (e.g. low resistance) at high temperature. It is therefore desirable to provide electrode structures which deliver such high thermal stability.

[0007] It is against this background that the invention has been devised.

[0008] SUMMARY

[0009] Against this background, the invention resides in the use of a specific polymer, carboxymethyl cellulose (CMC), within an interlayer of an electrode structure to improve the thermal stability.

[0010] The electrode structure comprises: a current collector layer having a current collector surface; a polymer gel electrode layer having an electrode surface that faces the current collector surface; and the interlayer arranged between the current collector surface and the electrode surface, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

[0011] By virtue of the electrically conducting interlayer, electrical contact between the electrode and the current collector layer is improved. The contact resistance is therefore reduced, and the performance of the cell is improved.

[0012] However it was found that the presence of CMC in a binder within the interlayer provided an electrode structure which exhibited surprising thermal stability. By “thermal stability” is meant the ability of the electrode structure or cell containing the electrode structure to perform well at higher temperatures, for example to exhibit lower than expected impedance. The contact resistance between solid interfaces within an electrode structure can be expected to increase as temperature increases, reducing the performance of a cell containing the electrode structure. However, when CMC is present, the observed increase in contact resistance as temperature rises is lower than expected and lower than observed for other polymers.

[0013] The interlayer and / or the electrode layer may be deformable. The deformability of one or both of these layers provides for particularly good contact between the layers.

[0014] In some embodiments, the electrode layer has a deformability greater than the interlayer. Where the electrode layer is deformable, the electrode layer may be compressible in a direction substantially orthogonal to the electrode surface. In this way, the electrode layer can deform to increase the contact area between the electrode surface and the interlayer. Without wishing to be bound by theory, the larger contact area may result in a lower contact resistance and hence a better cell performance. To achieve the deformability, the electrode layer may be made of a deformable material, and / or the electrode layer may have a deformable structure, such as a porous structure. The electrode layer may be elastically and / or plastically deformable. In some embodiments, the electrode layer is a cathode layer.

[0015] The electrode layer is a polymer gel electrode layer. The polymer gel electrode layer may comprise a matrix phase comprising a gelled polymer and a dispersed phase comprising an electrochemically active material. The dispersed phase may further comprise a conductive additive. The gelled polymer matrix phase may comprise a polymer binder and a liquid electrolyte.

[0016] The electrode may comprise an electrochemically active material, for example a positive active material or a negative active material. The electrochemically active material may comprise a lithium metal oxide. Lithium metal oxides are particularly effective electrochemically active materials. In some embodiments, the electrochemically active material comprises lithium nickel manganese cobalt oxide (NMC).

[0017] The electrode may be separable from the interlayer. In this way there is no need to adhere the electrode to the interlayer.

[0018] The polymer gel electrode layer may have a thickness of from 10 pm to 200 pm, for example from 10 pm to 100 pm or from 10 pm to 50 pm.

[0019] The interlayer comprises a binder. The binder may be a thermoplastic material: a thermoplastic material is particularly easy to handle and easily applied as a layer to the current collector. The binder comprises CMC. In some embodiments, the binder consists of CMC.

[0020] In some embodiments, the current collector surface comprises aluminium. In some embodiments, the current collector layer comprises or consists of aluminium.

[0021] In some embodiments, the electrode layer is a cathode layer and the current collector surface comprises aluminium. In some embodiments, the electrode layer is a cathode layer and the current collector layer comprises or consists of aluminium. The current collector may comprise a further current collector surface opposite the current collector surface, and the electrode structure comprises: a further polymer gel electrode layer having a further electrode surface that faces the further current collector surface; and a further interlayer arranged between the further current collector surface and the further electrode surface, the further interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose. In this way, a single current collector layer can act as a current collector for two electrodes, maximising efficiency, of the cell.

[0022] The invention also extends to a battery cell incorporating any electrode structure described above.

[0023] The electrode layer may be a free-standing electrode layer. In this way, the electrode layer can be made separately from the current collector, and applied to the current collector in a subsequent process. The electrode layer may for example be an extruded electrode, made by extrusion of a polymer gel. The polymer gel may be a compressible material.

[0024] The interlayer comprises a binder comprising CMC, and an electrically conducting material. The binder can act to adhere the interlayer to the electrode and to the current collector layer, while the electrically conducting material provide electrical conductivity. Adhering the electrode to the current collector secures the electrode structure together, and also provides a particularly effective improvement in the electrical contact, resulting in a particularly low contact resistance between the electrode and the current collector.

[0025] The binder comprises CMC which will not react readily with a carbonate electrolyte. In this way, structural integrity of the interlayer is generally maintained, and the interlayer maintains particularly good adhesion with the current collector layer. This has been found to be particularly effective in reducing contact resistance. However, unexpectedly a binder comprising CMC also increases the thermal stability of the electrode structure, allowing contact resistance to remain low even at elevated operation temperatures.

[0026] In some embodiments, the interlayer consists of the electrically conducting material and the binder, the binder comprising or consisting of CMC. In some embodiments, the interlayer comprises at least 12 wt% CMC, for example at least 13 wt%, at least 14 wt% or at least 15 wt%. In some embodiments, the interlayer comprises up to 80 wt% CMC, for example up to 75 wt%, up to 74 wt%, up to 73 wt% or up to 72 wt%. In some embodiments, the interlayer comprises from 12 to 80 wt% CMC, for example from 13 to 75 wt%, from 14 to 74 wt% or from 15 to 72 wt%. In some embodiments, the interlayer comprises from 30 to 80 wt% CMC, for example from 30 to 75 wt%, from 35 to 75 wt% or from 37 to 72 wt%. In some embodiments, alongside this amount of CMC, the balance of the weight of the interlayer is the electrically conducting material (with the exception of possible trace amounts of solvent remaining in the interlayer after drying steps).

[0027] Amounts of CMC within these ranges may help to provide optimal improvements in thermal stability of the electrode structure.

[0028] In some embodiments, the interlayer comprises at least 20 wt% electrically conducting material, for example at least 25 wt%, at least 26 wt% or at least 28 wt%. In some embodiments, the interlayer comprises up to 90 wt% electrically conducting material, for example up to 88 wt%, up to 87 wt%, up to 86 wt% or up to 85 wt%. In some embodiments, the interlayer comprises from 20 to 90 wt% electrically conducting material, for example from 25 to 90 wt%, from 26 to 88 wt% or from 27 to 85 wt%. In some embodiments, the interlayer comprises from 20 to 70 wt% electrically conducting material, for example from 25 to 65 wt%, from 26 to 64 wt% or from 28 to 63 wt%. In some embodiments, alongside this amount of electrically conducting material, the balance of the weight of the interlayer is CMC (with the exception of possible trace amounts of solvent remaining in the interlayer after drying steps). Amounts of electrically conducting material within these ranges may help to provide optimal improvements in thermal stability of the electrode structure.

[0029] In some embodiments, the interlayer consists of from 10 to 80 wt% CMC, from 20 to 90 wt% electrically conducting material, and optional trace amounts of water (e.g. less than 1000 ppm water, less than 500 ppm water or less than 100 ppm water). In some embodiments, the interlayer consists of from 15 to 73 wt% CMC, from 27 to 85 wt% electrically conducting material, and optional trace amounts of water (e.g. less than 1000 ppm water, less than 500 ppm water or less than 100 ppm water).

[0030] Amounts of CMC and electrically conducting material within these ranges may help to provide optimal improvements in thermal stability of the electrode structure.

[0031] In some embodiments, the interlayer consists of from 10 to 80 wt% CMC, from 20 to 90 wt% SWCNTs, and optional trace amounts of water (e.g. less than 1000 ppm water, less than 500 ppm water or less than 100 ppm water). In some embodiments, the interlayer consists of from 15 to 73 wt% CMC, from 27 to 85 wt% SWCNTs, and optional trace amounts of water (e.g. less than 1000 ppm water, less than 500 ppm water or less than 100 ppm water).

[0032] In some embodiments, the interlayer consists of from 12 to 80 wt% CMC, from 20 to 90 wt% MWCNTs, and optional trace amounts of water (e.g. less than 1000 ppm water, less than 500 ppm water or less than 100 ppm water). In some embodiments, the interlayer consists of from 15 to 72 wt% CMC, from 27 to 85 wt% MWCNTs, and optional trace amounts of water (e.g. less than 1000 ppm water, less than 500 ppm water or less than 100 ppm water).

[0033] In some embodiments, the interlayer is prepared from a precursor dispersion comprising the electrically conducting material and CMC dispersed in a solvent, wherein the precursor dispersion comprises from 0.5 to 10 wt% CMC and from 0.2 to 10 wt% electrically conducting material, based on the total weight of the dispersion. In some embodiments, the precursor dispersion consists of from 0.5 to 10 wt% CMC, from 0.2 to 10 wt% electrically conducting material, and balance solvent, based on the total weight of the dispersion. In some embodiments, the precursor dispersion consists of from 0.6 to 5 wt% CMC, from 0.4 to 5 wt% electrically conducting material, and balance solvent, based on the total weight of the dispersion. In some embodiments, the solvent is water.

[0034] The binder may comprise a thermoplastic material. The binder may alternatively comprise a thermoset material.

[0035] The interlayer may comprise an electrically conducting material. The electrically conducting material may comprise or consist of a carbon additive. The interlayer may comprise carbon, preferably a compressible carbon, such as graphite. Carbon, and particularly graphite, is an inexpensive electrically conducting material that can easily be formed in a layer on the current collector. Carbon can be easily formed in a deformable structure, for example a porous structure, so that the interlayer can be made as a deformable layer.

[0036] The electrically conducting material may comprises metal or carbon. Both are convenient electrically conducting materials. Preferably the electrically conducting material comprises carbon nanotubes, which offer particularly good conductivity. Carbon nanotubes can also be used for particularly thin material layers, meaning the overall volume of material required is relatively low. In some embodiments therefore the electrically conducting material comprises or consists of carbon nanotubes.

[0037] In some embodiments the carbon nanotubes comprise or consist of single-walled carbon nanotubes (SWCNTs).

[0038] In some embodiments the carbon nanotubes comprise or consist of multi-walled carbon nanotubes (MWCNTs).

[0039] In some embodiments therefore the electrically conducting material comprises graphene. To further improve adhesion, the interlayer may comprise a plasticiser. The plasticiser may comprise propylene carbonate, which is particularly suitable in combination with polyvinylidene fluoride.

[0040] The interlayer may comprise a salt. The salt may be configured to passivate the current collector surface: passivation improves the performance of the current collector layer. For example, the salt may comprise a lithium-based salt.

[0041] The current collector may comprise a further current collector surface opposite the current collector surface. In this case, the electrode structure may comprise: a further polymer gel electrode layer having a further electrode surface that faces the further current collector surface; and a further interlayer arranged between the further current collector surface and the further electrode surface, the further interlayer comprising a conducting material. In this way, a single current collector layer can act as a current collector for two electrodes, maximising efficiency, of the cell.

[0042] The invention also extends to a battery cell incorporating the electrode structure of any preceding claim.

[0043] The invention also extends to the use of an interlayer arranged between a current collector surface and an electrode surface within an electrode structure to improve the thermal stability of the electrode structure; the electrode structure comprising: a current collector layer having the current collector surface; a polymer gel electrode layer having the electrode surface that faces the current collector surface; and the interlayer arranged between the current collector surface and the electrode surface, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

[0044] The invention also extends to the use of carboxymethyl cellulose within an interlayer of an electrode structure to achieve one or more of the following: (a) improve the thermal stability of the electrode structure;

[0045] (b) reduce the electrical impedance of an electrochemical cell comprising the electrode structure at high temperature;

[0046] (c) reduce the electrical resistance of an electrochemical cell comprising the electrode structure at high temperature;

[0047] (d) improve the rate performance of a cell comprising the electrode structure operating at high temperature; or

[0048] (e) improve the long-term stability of a cell comprising the electrode structure operating at high temperature; wherein the electrode structure is for use in a battery cell and comprises: a current collector layer having a current collector surface; a polymer gel electrode layer having an electrode surface that faces the current collector surface; and the interlayer arranged between the current collector surface and the electrode surface, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

[0049] The electrode structure may be manufactured by a method comprising: providing a current collector layer having a current collector surface; providing a gel polymer electrode having an electrode surface; preparing an aqueous dispersion of an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose; applying the aqueous dispersion to the current collector surface to form a coated current collector surface, and applying the gel polymer electrode to the coated current collector surface, thereby arranging an electrically-conducting interlayer between the current collector surface and the electrode surface. The electrically-conducting interlayer improves electrical contact between the electrode and the current collector layer, as described above.

[0050] For particular ease of manufacture, the method may comprise arranging the electrically- conducting interlayer on the current collector surface, and arranging the gel polymer electrode on the electrically-conducting interlayer. The method may comprise forming the interlayer by extrusion and arranging the interlayer on the current collector surface. Extrusion is a particularly simple method of forming a gel- polymer electrode, and can provide a relatively smooth electrode surface, which assists in obtaining good electrical contact.

[0051] The method may comprise casting the interlayer onto the current collector surface. Casting is a simple method of providing the interlayer, that can advantageously be implemented as a continuous process.

[0052] The method may comprise casting the aqueous dispersion of electrically conducting material and binder onto the current collector surface.

[0053] The method may comprise adhering the electrode surface to the current collector surface with the interlayer. Adhering the electrode secures the electrode in place, and provides particularly good electrical contact.

[0054] To facilitate adhesion, the method may include applying pressure to the electrode layer in a direction substantially perpendicular to the electrode surface, optionally using a roller, for example by calendaring.

[0055] Also to facilitate adhesion, the method may include heating the electrode layer during or after the step of arranging the interlayer between the current collector surface and the electrode surface. Where pressure is also applied using a roller, heating may be implemented by heating the roller.

[0056] The current collector may comprises a further current collector surface opposite the current collector surface, and the method may further comprise: providing a further gel polymer electrode having a further electrode surface; and arranging a further electrically-conducting interlayer between the further current collector surface and the further electrode surface, the further interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose. In all of the above aspects and embodiments, the electrode may be an anode or a cathode. In some embodiments, the electrode layer is a cathode, and the current collector surface comprises aluminium.

[0057] In all of the above embodiments the electrode may be capable of receiving and / or supplying alkali metal ions such that the electrode structure can form part of an alkali metal cell. In particular, the electrode may be capable of receiving and / or supplying lithium and / or sodium metal ions. Lithium and sodium ions are particularly preferred because they are light but highly reactive and hence provide a high energy density cell. Sodium and lithium also advantageously insert. In some circumstances, lithium may be particularly preferred for its particularly high energy density.

[0058] Preferred and / or optional features of one aspect or embodiment may be used alone, or in appropriate combination, with other aspects also.

[0059] Brief Description of the Figures

[0060] By way of non-limiting example, embodiments of the invention will now be described in relation to the accompanying drawings, in which:

[0061] Figure 1 is a perspective view of an electrode structure comprising a current collector, an electrode, and an electrically conducting interlayer between them;

[0062] Figures 2 to 4 are steps in the process of assembling the electrode structure of Figure 1;

[0063] Figure 5 is a further embodiment of an electrode structure, comprising a further interlayer and a further electrode;

[0064] Figure 6 shows Nyquist plots from EIS testing (after 2, 8 and 16 hours), for foil-only symmetric cells containing aluminium foil current collectors carrying a coating composition containing SWCNTs along with (a) PVP binder tested at 30 °C; (b) PVP binder tested at 45 °C; (c) CMC binder tested at 30 °C; and (d) CMC binder tested at 45

[0065] Figure 7 shows Nyquist plots from EIS testing (after 2, 8 and 16 hours), for foil-only symmetric cells containing aluminium foil current collectors carrying a coating composition containing MWCNTs along with (a) CMC binder tested at 30 °C; (b) CMC binder tested at 45 °C; (c) PVP binder tested at 30 °C; (d) PVP binder tested at 45 °C; (e) PAA binder tested at 30 °C; and (f) PAA binder tested at 45 °C.

[0066] Figure 8 shows Nyquist plots from EIS testing at 30 °C (after 2, 8 and 16 hours), for cathode-laminated symmetric cells containing aluminium foil current collectors which were either (a) bare (uncoated) aluminium foil; or (b) primed aluminium foil carrying a coating composition containing SWCNTs along with CMC binder.

[0067] Detailed Description of Embodiments of the Invention

[0068] Figure 1 illustrates an embodiment of an electrode structure 10. This electrode structure includes a current collector layer 12 having a current collector surface 13, an electrode layer 16 having an electrode surface 17 that faces the current collector surface 13, and an electrically-conducting interlayer 14 provided between the current collector surface 13 and the electrode surface 17. The interlayer 14 is electrically conducting, so as to conduct current between the electrode layer 16 and the current collector layer 12.

[0069] The current collector layer 12 may be made of any material that is suitable for conducting current. Preferably, the current collector layer is a metal foil, and the material is selected depending on the electrode. Transition metals including Al, Cu, Pt, Ni, Mo, and W are particularly effective. For example, aluminium may be a preferred material where the electrode is a cathode, and copper may be a preferred material where the electrode is an anode. The current collector layer may be any suitable thickness, for example between approximately 5 microns and 20 microns. In this embodiment, the electrode 16 is a gel polymer electrode. The gel polymer electrode 16 may also be a free-standing electrode, though embodiments are also envisaged in which the gel polymer electrode is not freestanding. The gel polymer electrode 16 may be an extruded electrode.

[0070] Freestanding in this sense means that the electrode layer has initially been made separately from the current collector layer, without a current collector layer to support it. A freestanding electrode layer is therefore of sufficient integrity to be self-supporting without a current collector layer. When initially provided, the electrode layer 16 comprises two electrode surfaces 17 that are free surfaces.

[0071] In this embodiment, the interlayer 14 acts as a binder or an adhesion layer that adheres the electrode layer 16 to the current collector layer 12. To this end, the interlayer 14 comprises a binder comprising CMC, and a conducting material, to perform the functions of adhesion and electrical conduction.

[0072] The gel polymer electrode 16 comprises a gel matrix formed from a polymer and a solvent. One or more electrochemically active materials are loaded into the gel matrix, typically in the form of solid particles. The electrochemically active material is capable of releasing or receiving an ion species, preferably an alkali metal ion, and most preferably lithium and / or sodium. The solvent of the gel matrix will typically be an electrolyte material, for example a carbonate electrolyte. In this particular example, the electrochemically active material is a lithium-containing metal oxide material, and preferably a lithium transition metal oxide such as a lithium cobalt oxide.

[0073] Typically the polymer gel electrode layer 16 has a thickness of approximately 10 pm to approximately 200 pm.

[0074] The gel polymer electrode 16 is deformable to help ensure good contact between the gel polymer electrode 16 and the interlayer 14. The interlayer 14 also has some deformability, but the deformability of the gel polymer electrode 16 in the present embodiment is greater. Considering the interlayer 14 in more detail, as noted above, the interlayer comprises a binder and a conducting material. The binder of the interlayer 14 is CMC.

[0075] Because the CMC binder reacts to a limited extent with the electrode material, the binder remains more structurally stable after incorporation into the electrode structure, and hence maintains a particularly good adhesion to the current collector layer.

[0076] Furthermore, it was found that the electrode structure has surprising thermal stability when CMC is used as binder.

[0077] The conducting material may be any suitable material capable of conducting current, with any suitable physical form. For example, the conducting material may take the form of carbon nanotubes, though it is also envisaged that the conducting material may be particles or flakes of metal, or other carbon allotropes such as graphite or graphene.

[0078] The interlayer 14 may optionally include a plasticiser to increase the adhesive properties of the interlayer even further. Any suitable plasticiser may be used, but in one particular example the plasticiser is propylene carbonate.

[0079] The interlayer may also optionally include a salt additive, particularly in combination with a plasticiser. The salt additive may be selected so as to act to passivate the current collector material. To this end the salt additive preferably contains ions of the species that will be exchanged between the anode and the cathode. For example, where the battery is a lithium battery, the salt additive may be a lithium-based salt.

[0080] The interlayer 14 may be any suitable thickness, but a thickness of between approximately 0.01 pm and approximately 0.5 pm is preferred.

[0081] To form the electrode structure 10, the current collector layer 12 is first provided. The interlayer 14 is then arranged on the current collector layer 12, and the electrode layer 16 is arranged on the interlayer 14. To form the interlayer 14 on the current collector, the binder comprising CMC and the conducting material (and optionally the plasticiser and salt additive) are mixed with a sacrificial solvent. The solvent may be selected for compatibility with the binder and the electrode material. For example, the solvent may be water, which has good compatibility with CMC. Where the plasticiser is used, the plasticiser and sacrificial solvent are selected such that a boiling point and vapour pressure of the solvent is lower than a boiling point and vapour pressure of the plasticiser.

[0082] The mixture is coated onto the current collector surface 13, and the electrode 16 is then arranged over the mixture. The structure 10 is pressed together and heated to a temperature sufficient to soften the polymer gel electrode layer such that it adheres to the interlayer, before being brought back to room temperature.

[0083] To form the electrode structure 10, the current collector layer 12 is first provided as shown in Figure 2. The interlayer 14 is then arranged on the current collector layer 12, as shown in Figure 3 and the electrode layer 16 is arranged on the interlayer 14 as shown in Figure 4.

[0084] Figure 5 illustrates an alternative electrode structure 210, which may encompass the gel polymer electrode and associated binder-based interlayer.

[0085] The alternative electrode structure 210 is substantially the same as the electrode structure 10 of Figure 1, except that both surfaces 213, 213f of the current collector layer 212 are provided with corresponding interlayers 214, 214f and electrodes 216, 216f. To this end, the current collector 212 comprises a further current collector surface 213f, with a further interlayer 214f arranged thereon. A further electrode 216f is arranged over the further interlayer 214f, such that a further electrode surface 217f contacts the further interlayer 214f. The alternative electrode structure 210 may be made using the same methods already described above.

[0086] Both interlayers 214, 214f comprise a binder comprising CMC. Any of the methods described above may be implemented as continuous methods. For example a continuous roll of current collector may be supplied to an interlayer station, where the interlayer is formed continuously on the current collector to ‘prime’ the current collector. A continuous roll of free-standing electrode may then be supplied to the primed current collector to arrange the electrode on top. The assembled structure may then be pressurised and / or heated. Pressure may be supplied by rollers, for example at a calendaring station. Where heat is also applied, the rollers may be heated rollers.

[0087] The completed structure may be fed onwards to a battery assembly station, to be assembled with other components into a battery.

[0088] EXAMPLES

[0089] To further illustrate the invention, the following examples are provided.

[0090] Example 1 - Coated foil preparation

[0091] The following dispersions were prepared:

[0092] Table 1

[0093] Primed aluminium foils were prepared by casting each of the Dispersions from Table 1 onto aluminium foil (99.9% purity, 12 pm thickness). The Dispersion was cast using a sacrificial method. Aluminium foil was placed onto a drawdown coater, and a small amount of the Dispersion is poured on top. The Dispersion was drawn down using a doctor blade with a width of -120 mm and a blade gap set to achieve a desired wet thickness. The coated foils were left to dry at temperatures from room temperature to 70 °C.

[0094] Individual sheets were dried as a bundle from 120-160 °C overnight under constant vacuum.

[0095] After drying, the dry coatings were measured to have a loading of 1.00±0.99 GSM, and a thickness of 0.63±0.62 microns.

[0096] Coated Foils 1, 2, A and B were prepared by coating Dispersions 1, 2, A and B respectively onto aluminium foils. In addition, a pre-coated primed aluminium foil was obtained, which carried 2 pm thick coatings on each side prepared using a water-based dispersion containing MWCNTs (2.0-6.0 wt.%) and PAA dispersant (<1.0 wt.%); this was denoted Coated Foil C.

[0097] Example 2 - Cathode preparation

[0098] Electrode Structure 1 was prepared by laminating a polymer gel cathode layer onto Coated Foils 1.

[0099] A further Electrode Structure D was prepared by laminating a polymer gel cathode layer onto an unprimed aluminium foil layer, i.e. a bare aluminium foil layer which had not been subject to any coating step.

[0100] Polymer gel cathodes were prepared by a method which included first mixing bulk powdered components (NMC electrochemically active material; PvDF binder and a conductive carbon component) with liquid electrolyte in a ‘premix’ stage. The premix slurry was then batch-injected into a twin-screw extruder. Granules were then produced and allowed to freefall from the end of the twin screw. The granules were then sandwiched between two pieces of release film and hot-rolled down to a target thickness of 50 pm to form the polymer gel cathode.

[0101] Coated Foil 1 was cut into strips and redried at 120 °C overnight under vacuum. The polymer gel cathode was placed onto the foil and covered with release film. The resultant layered structure was placed into a laminator (110 °C, 30 kg load on both sides of the laminator, with a set gap of 50 pm). The laminated cathode structure containing a polymer gel cathode layer laminated onto foil was then removed from the laminator and cut into usable electrodes for processing.

[0102] Example 3 - Foil-only cell preparation

[0103] Symmetrical cells were prepared containing the primed aluminium foil current collectors prepared in Example 1. The cells were “foil only”, i.e. they did not contain any polymer gel electrode layers, such that EIS testing could be performed to investigate impedance effects attributable to the coated foil layers specifically.

[0104] Samples of each of Coated Foils 1, 2, A, B and C were punched.

[0105] Symmetric cells were assembled using two identical electrodes (foil-only) with a glass fibre layer sandwiched between the two.

[0106] The electrode area was 14.1 cm2for both the positive and negative electrode.

[0107] The symmetrical cells were filled with electrolyte and sealed.

[0108] The symmetric cells containing Coated Foils 1, 2, A, B and C were denoted Cells 1, 2, A, B and C respectively.

[0109] Example 4 - Cathode structure-containing cell preparation

[0110] For cathode laminated symmetric cells, Electrode Structures 1 and D were punched.

[0111] Symmetric cells were assembled using two identical electrode structures (cathode laminated) with a glass fibre layer sandwiched between the two.

[0112] The electrode area was 14.1 cm2for both the positive and negative electrode. The symmetrical cells were filled with electrolyte and sealed.

[0113] The symmetric cells containing Electrode Structures 1 and D were denoted Cells 11 and D respectively.

[0114] Example 5 - Cell testing

[0115] Symmetrical cells from Examples 3 and 4 were tested using a frequency response analyser. Cells were tested using potentiostatic electrochemical impedance spectroscopy (PEIS) between 1 MHz and 0.1 Hz with an amplitude of 10 mV every hour for 16 h total. Time points at 2, 8 and 16 hours were selected to display on Nyquist plots.

[0116] The tested cells are set out in Table 2.

[0117] Table 2 - Symmetric cell properties

[0118] The results are shown in Figures 6-8.

[0119] Figure 6 shows the EIS results (Nyquist plots) for foil-only cells containing SWCNT carbon additives, i.e. Cell A (top two plots) and Cell 1 (bottom two plots). Each cell was tested at 30 °C and 45 °C, with the left-hand plots showing the results at 30 °C and the right-hand plots showing the results at 45 °C.

[0120] Despite the stability of the SWCNT / P VP-coated foil during 30 °C testing, there is a clear increase in the high-frequency portion of the spectra after 16 hours at 45 °C, which indicates an increase in electronic resistance.

[0121] It is evident that the CMC polymer enables minimal contact resistance, which is stable at both 30 °C and 45 °C for 16 hours.

[0122] Figure 7 shows the EIS results (Nyquist plots) for foil-only cells containing MWCNT carbon additives, i.e. Cell 2 (top two plots), Cell B (middle two plots) and Cell C (bottom two plots). Each cell was tested at 30 °C and 45 °C, with the left-hand plots showing the results at 30 °C and the right-hand plots showing the results at 45 °C.

[0123] Both the PVP and PAA-containing MWCNTs exhibit a very large RC process at the start of impedance collection, whereas the CMC variant exhibits no such response. It is clear that the polymer choice is having an impact upon the resultant spectra, with very large resistance up to 100 Q cm2observed. These responses are undesirable for high power applications, as any large initial resistances will detrimentally affect rate performance. Additionally, any process growth at high temperature will ultimately cause instability in the cell and again cause poorer long-term performance as cells are cycled.

[0124] Figure 8 shows the EIS results (Nyquist plots) for cathode-laminated cells, both with unprimed foil layer and foil primed with an interlayer containing SWCNT and CMC, i.e. Cell 11 and Cell D. Each cell was tested at 30 °C.

[0125] The results show how the impedance response is considerably reduced when comparing symmetric cathode cells. By coating the foil prior to applying the cathode, it is possible to reduce the RC component to effectively zero, and finer responses are visible such as electrode porosity which are masked by such a large response in the initial case.

Claims

CLAIMS1. Use of carboxymethyl cellulose within an interlayer of an electrode structure to improve the thermal stability of the electrode structure; wherein the electrode structure is for use in a battery cell and comprises: a current collector layer having a current collector surface; a polymer gel electrode layer having an electrode surface that faces the current collector surface; and the interlayer arranged between the current collector surface and the electrode surface, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

2. The use of claim 1, wherein the electrode layer is a cathode layer.

3. The use of claim 1 or 2, wherein the current collector surface comprises aluminium.

4. The use of any one of the preceding claims, wherein the electrode layer is a freestanding electrode layer.

5. The use of any one of the preceding claims, wherein the electrically conducting material comprises metal or carbon.

6. The use of any one of the preceding claims, wherein the electrically conducting material comprises carbon nanotubes.

7. The use of claim 6, wherein the carbon nanotubes comprise or consist of singlewalled carbon nanotubes (SWCNTs).

8. The use of claim 6 or 7, wherein the carbon nanotubes comprise or consist of multiwalled carbon nanotubes (MWCNTs).

9. The electrode structure of any one of the preceding claims, wherein the current collector layer comprises a further current collector surface opposite the current collector surface, and the electrode structure comprises: a further polymer gel electrode layer having a further electrode surface that faces the further current collector surface; and a further interlayer arranged between the further current collector surface and the further electrode surface, the further interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

10. A battery cell incorporating the electrode structure of any preceding claim.

11. Use of an interlayer arranged between a current collector surface and an electrode surface within an electrode structure to improve the thermal stability of the electrode structure; the electrode structure comprising: a current collector layer having the current collector surface; a polymer gel electrode layer having the electrode surface that faces the current collector surface; and the interlayer arranged between the current collector surface and the electrode surface, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

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