Preferably, the compressible pores have a diameter that is less than 20% of a thickness of the compressible elastic composite material C the pores are comprised in. Providing the compressible elastic composite material C with pores with a small dimeter enables homogeneous distribution of the plurality of pores thought the layer they are comprised in. By dispersing distribution of the plurality of pores homogeneously compressive and tensile deformations may be accommodated for evenly. By using pores with a maximum diameter of 20% of the thickness of the layer they are comprised in, uneven pore distribution may be mitigated. A dimension of 20% was found to be a maximum at which plastic deformation may be avoided. Preferably, the combined volume of the pores in the compressible elastic composite material C is in the range of 30 to 150%, more preferably in the range of 100 to 120%, of the volume changes in the negative and/or positive electrode during charging and/or discharging of the rechargeable battery cell. The total pore volume is believed to correspond the maximum compressibility of the compressible elastic composite material C. By providing the compressible elastic composite material C with an increasing total pore volume may allow accommodating increasing volume changes in the electrodes. By providing the compressible elastic composite material C with a total pore volume that is similar to the expected volume changes of the electrode materials, the compressible elastic composite material C may completely counteract said volume changes within the cell. Providing the compressible elastic composite material. C with a total pore volume that exceeds the expected volume changes of the negative and/or positive electrode may not further contribute towards counteracting dais volume changes. The part of the total pore volume that exceeds the expected volume changes of the electrode materials may be considered as an unwanted dead volume, e.g. a volume that does not contribute the to the performance or integrity of the rechargeable battery cell 1. In typical battery applications, the above limitations to pore dimension correspond to pores having a dimension in a range between 10 micrometer and 50 nanometer, preferably between 5 micrometer and 100 nanometer, more preferably between 1 micrometer and 100 nm. It will be appreciated that the maximum thickness relates on the thickness of the compressible elastic composite material C layer the pores are comprised in. Thicker layers, e.g. a second current collector with a thickness of 50 micrometer allow for the use of larger pores than thin layers, e.g. a solid state electrolyte layer with a thickness of 1 micrometer.

In another or further embodiment, the compressible pores are at least in part provided by compressible hollow particles 10. Preferably, these particles comprise an elastic shell structure surrounding a hollow core. Particularly, for rechargeable battery cells wherein the compressible elastic composite material is to be provided in high aspect ratio structures, e.g. in between extending elements of a 3D battery, the pores may be preferably be provided by compressible hollow particles 20, as pore formation and pore distribution in foams may be hard to control, especially in 3D structures. Preferably, the compressible hollow particles 10 have a diameter in a range between 10 micrometer and 50 nanometer, preferably between 5 micrometer and 200 nanometer, more preferably between 1 micrometer and 300 nm. Using smaller compressible hollow particles 10 may allow the formation of thin compressible and elastic composite functional layers. Using thin compressible and elastic composite functional layers may allow manufacturing of high energy density rechargeable battery cells. It will be appreciated that, similar to the dimension of pores, the dimension of compressible hollow particles depends on the thickness of the layer they are to be incorporated. For example, if the compressible hollow particles are incorporated in a positive electrode layer with a thickness of one micrometer, the diameter of the compressible hollow particles should on the lower end of the range, e.g. in a range around 50 to 300 nm. If, for the compressible hollow particles are incorporated in a current collector with a thickness of five micrometer the diameter may be larger, e.g. in a range up to two micrometer. Preferably, the hollow core comprises a gas. By providing the hollow particles with a gas, the particle may be compressible and elastic. It is believed that by compressing a particle comprising a gas-filled core, the pressure of the gas in the particle may increase during compression. When the external force that compresses the particles is reduced the elevated gas pressure inside the hollow particle may cause the particle to re-expand, e.g. to an original volume. In addition, the elastic properties of the shell, may help in the recovery of the original volume.

In some preferred embodiments the hollow compressible particles 10 are hollow latex beads 11. Preferably, the hollow latex beads have a volumetric core to shell ratio (Vcore/Vshell) in a range from 5 to 0.05, and wherein the hollow latex beads have a diameter in a range between 50 nanometer and 5 micrometer, preferably in a range between 200 and 1000 nanometer. Beads with large pores may be compressible to a lager extent. Beads with thick shells may be more resilient and may be more elastic. It will be appreciated that, similar to the dimension of pores, beads with a small dimension relative to the thickness of the layer they are comprised in may be dispersed more evenly. Homogeneous of beads may lead to even compressible and elastic layers that allow for even accommodation of expansion/shrinkage of electrodes during operation of the rechargeable battery cell.

The compressible elastic composite material C may be comprised in any one or more of the functional layers within a battery stack.

FIG. 2B depicts schematic cross-section views of exemplary embodiments of a compressible and elastic first current collector 2, a compressible and elastic solid state electrolyte 4, a compressible and elastic positive electrode 3, and a compressible and elastic second current collector 2 that is provided with an electrically conductive capping layer 8.

As stated before, it is an aspect of the present disclosure to provide a rechargeable battery cell wherein one or more of the functional layers in the stack forming the cell is provided with a compressible elastic composite material C to, at least in part, counter act compressive and/or tensile forces within the stack resulting from volume changes in the negative and/or positive electrode during charging and/or discharging of the rechargeable battery cell.

Accordingly, the present disclosure relates to a rechargeable battery cell wherein one or more of the first and second current collector 2,6 is provided with the compressible elastic composite material C and an electrically conductive material to form a compressible and elastic current collector. In one embodiment, e.g. as shown in FIG. 2B, the compressible elastic composite material C comprises hollow compressible particles 10. The conductive material comprises one or more of: carbon black powder, carbon nanotubes, graphene, carbon fibers, graphite, conductive particles, metal particles, conductive nanowires, and conductive polymers, e.g. polyaniline and polythiophene, and/or conductive copolymer. Alternatively or in addition, the compressible elastic composite material C may comprise a conductive polymer. In some preferred embodiments, the compressible elastic composite material C comprises hollow latex beads 11. In some of these embodiments, the elastic properties may be provided by hollow latex beads. Alternatively or in addition, the hollow latex beads 11 are electrically conductive.

The present disclosure further relates to a rechargeable battery cell, wherein the solid state electrolyte 4 comprises the compressible elastic composite material C formed of: d a solid state electrolyte material to form a compressible and elastic solid state electrolyte. In one embodiment, e.g. as shown, the compressible elastic composite material C, comprises hollow compressible particles. The solid state electrolyte comprises an ion conductive polymer and one or more salts Typically the ion conductive polymer may be polyether polymer such as one or more of polyethylene glycol, polypropylene glycol and glycol comprising copolymers. Preferably, the salt comprises a non-coordinating anion and a metal cation that corresponds to the metal used in the negative electrode. For example, in case of a lithium ion battery, the solid state electrolyte may comprise a lithium salt and polyethylene glycol. In some preferred embodiments, the compressible elastic composite material C comprises hollow latex beads 11. In some of these embodiments, the elastic properties may be provided by hollow latex beads. Alternatively or in addition, the hollow latex beads are provided with an ion conductive coating. Providing the hollow latex beads with an ion conductive coating may improve ion conductivity throughout the compressible and elastic solid state electrolyte and/or improve compatibility between coated hollow latex beads and the solid state electrolyte material. Alternatively or in addition, the solid state electrolyte layer may be provided with a layer of a lithiophilic metal oxide for improving wettability at the interface between the electrolyte layer and of the lithium metal electrode. Alternatively or in addition, the compressible and elastic solid state electrolyte layer current collector layer may at the interface to a negative electrode be provided with a layer of a lithiophilic metal oxide for improving wettability at the interface between the current collector layer and the lithium metal electrode to mitigate non uniform electrode material plating and further mitigate dendrite formation. Preferably the lithiophilic metal oxide is selected from a group consisting of ZnO, Al2O3, Fe2O3, CoO2, MnO2, V2O5, and TiO2.

The present disclosure further relates to a rechargeable battery cell, wherein the positive electrode 3 comprises the compressible elastic composite material C formed of: a positive electrode material, an ionically conductive material, and an electronic conductive material dispersed throughout the compressible elastic composite material C, to form a compressible and elastic positive electrode. In one embodiment, e.g. as shown, the positive electrode comprises hollow compressible particles 10. The conductive material comprises one or more of: carbon black powder, carbon nanotubes, graphene, carbon fibers, graphite, conductive particles, metal particles, conductive nanowires, and conductive polymers, e.g. polyaniline and polythiophene. Dispersing additives might be added to form homogenous mixtures binder materials may be used for the electrode fabrication. In some preferred embodiments, the compressible elastic composite material C comprises hollow latex beads 11. In some of these embodiments, the elastic properties may be provided by hollow latex beads. Alternatively or in addition, the hollow latex beads are provided with an electrically conductive coating. Providing the hollow latex beads with an electrically conductive coating may improve conductivity throughout the compressible and elastic positive electrode. Alternatively or in addition, the hollow latex beads are provided with an ion conductive coating. Providing the hollow latex beads with an ion conductive coating may improve ion conductivity throughout the compressible and elastic positive electrode.

The present disclosure further relates to a rechargeable battery cell wherein, the negative electrode 5 comprises the compressible elastic composite material C formed of a negative electrode material, an ionically conductive material, and an electronically conductive material to form the compressible and elastic negative electrode 5. The conductive material comprises one or more of: carbon black powder, carbon nanotubes, graphene, carbon fibers, graphite, conductive particles, metal particles, conductive nanowires, and conductive polymers, e.g. polyaniline and polythiophene. Dispersing additives might be added to form homogenous mixtures binder materials may be used for the electrode fabrication. In some preferred embodiments, the compressible elastic composite material C comprises hollow latex beads 11. In some of these embodiments, the elastic properties may be provided by hollow latex beads. Alternatively or in addition, the hollow latex beads are provided with an electrically conductive coating. Providing the hollow latex beads with an electrically conductive coating may improve conductivity throughout the compressible and elastic negative electrode.

In other or further embodiments a rechargeable battery cell is provided, wherein the one or more of the first and second compressible and elastic current collectors is provided with an electrically conductive capping at an interface to connect to the negative or positive electrode. Preferably, the capping layer has an elastic modulus that is higher that the elastic modulus of the electrode it is in contact with. Having a capping layer that is harder than the electrode it is in contact may allow for more uniform pressure distribution, e.g. uniform compression along the layer, and accordingly mitigate mechanical degradation of the compressible layer. In embodiments wherein the capping layer is in contact with a lithium metal electrode the modulus of the capping layer is preferably larger than 4.9 GPa. Having a capping layer with a modulus>4.9 GPa may further prevent lithium dendrite formation. Alternatively or in addition, the elastic current collector layer for collecting electrons from a negative lithium electrode may be provided with a layer of a lithiophilic metal oxide for improving wettability at the interface between the current collector layer and the lithium metal electrode to mitigate non uniform electrode material plating and further mitigate dendrite formation. Preferably the lithiophilic metal oxide is selected from a group consisting of ZnO, Al2O3, Fe2O3, CoO2, MnO2, V2O5, and TiO2.

In other or further embodiments, a rechargeable battery cell is provided, wherein the one or more of the first and second compressible and elastic current collectors is at least in part provided with a conductive metal layer to improve spatial conductivity of the compressible and elastic current collectors. The metal layer may be provided as a foil or as a mesh and can be either embedded in the current collector, or provided at a surface. Alternatively or in addition, the compressible and elastic current collector can be sandwiched between two metal foils, metal meshes, or a combination thereof. Providing a good spatial high conductivity is particularly important at the interface for connecting to a negative electrode. Fox example in a rechargeable lithium metal battery, inhomogeneous spatial conductivity may, during a charging cycle, lead to inhomogeneous electric field formation and may thus lead to inhomogeneous electrode material deposition and/or plating and may lead to formation of dendrites. Accordingly, the present disclosure relates to a compressible and elastic current collector wherein the compressible and elastic current collector is provided with a metal film at the interface for connecting to a negative electrode material. Compressible and elastic current collectors for connecting to a positive electrode material are preferably provided with a metal mesh, as homogeneous spatial conductivity is less critical at these interfaces. By providing the compressible and elastic current collector for connecting to a positive electrode material with a metal mesh may allow manufacturing of rechargeable battery cells with improved gravimetric energy density, e.g. an increased energy output of the cell per total mass of the cell compared to a similar cell that is provided with a metal film instead of a mesh). Alternatively of in addition, the compressible elastic current collector for collecting electrons from a negative lithium electrode may be a hybrid laminate formed with a metal grid inside the current collector layer and wherein the elastic current collector comprises a capping layer facing the lithium electrode to prevent formation of dendrites, wherein the capping layer also comprises electrically conductive material dispersed throughout the capping layer to form an electrically conductive network across the capping layer.