Separator for a battery cell, battery cell comprising a separator and method of producing a separator

Abstract

The present application relates to an improved separator (1) suitable for a battery cell. The separator (1) herein disclosed significantly improves battery heating, and consequently the performance of said battery cell. The separator (1) herein particularly suitable for a battery cell to operate in sub-zero degree conditions, without hindering the battery cell operation or performance. Herein is also disclosed said battery cell and a method of producing the separator (1).

Description

SEPARATOR FOR A BATTERY CELL, BATTERY CELL COMPRISING A SEPARATOR AND METHOD OF PRODUCING A SEPARATOR

[0001] The present application describes a separator for a battery cell, a battery cell comprising a separator and a method of producing a separator.

[0002] Sub-zero temperatures are found in many environments, such as cold climates, high altitudes, high latitudes on Earth and space. However, the performance of rechargeable batteries, such as lithium-ion batteries (LIBs), strongly decreases at sub-zero degree Celsius temperatures, hampering the use of these technologies at those extreme conditions.

[0003] LIB operation is optimal at the temperature range between 15-35ºC. Beyond this range, the Li-ion motion is disrupted, degrading the battery performance. Particularly, at low temperatures the performance is dramatically reduced (doi.org / 10.1038 / nenergy.2017.108). Several examples demonstrate how low temperatures affect key performance indicators of the battery, for example, at -40ºC the capacity of a LIB can drop to 12% of its room temperature (RT) capacity (doi.org / 10.1016 / S0013-4686(02)00620-5), at -10ºC the battery power density is reduced to about 70% of the RT power density (doi.org / 10.1016 / j.jpowsour.2010.08.070) and energy density can drop below 50% of the RT energy density (doi.org / 10.1149 / 2.1061810jes). This limits the deployment of LIBs in applications comprising airplanes, high-altitude aerial vehicles and military and aerospace applications.

[0004] At a fundamental level, battery operation is established by the microscale transport and storage of Li-ions within its electrolyte and electrodes. Particularly, temporary storage of Li-ions within negative (anode) / positive (cathode) electrodes is achieved during charge / discharge with corresponding transport of Li-ions between these electrodes. At low temperature conditions, the underlying LIB microscale processes are disrupted: 1) the Li-ion transport during charge / discharge operations is hampered by slow diffusion within the solid electrode material; 2) Li-ion electrochemical transfer from solid material to the electrolyte is hindered by a large charge-transfer resistance at the electrode / electrolyte interface; 3) Li-ion transport within the electrolyte is sluggish and slow. The lower the temperature, the more severe the impact on Li-ion storage and transport processes, which typically become seriously disrupted at temperatures below zero degree Celsius. As a result, LIBs at sub-zero temperatures are characterized by reduced battery capacity, poor cycling performance and decreased safety.

[0005] To address these, materials-based strategies such as development of electrolytes (doi.org / 10.1016 / S0378-7753(00)00578-4) or electrode materials (doi.org / 10.1038 / nenergy.2017.108) suitable for low temperatures have been performed. Yet, these do not alleviate the typical decrease in chemical activity at lower temperatures, and capacities at sub-zero degree Celsius are less than 50% of ambient temperature capacities.

[0006] As general practice in the battery field, batteries used at room temperature can be employed at low temperatures simply by heating them (via external or internal processes) to room temperature, hence going around the intrinsic temperature-dependent performance drop. In this context, both external and internal heating have been demonstrated. External heating comprises the heating of the battery via the surrounding medium (i.e. providing a hot surrounding fluid) or through the deployment of (solid) heating elements in contact with the battery.

[0007] Yet, external heating approaches present drawbacks such as low heating efficiency, long heating times, high energy consumption for heating purposes, and non-uniform heating which is detrimental for battery safety.

[0008] In turn, internal heating strategies incorporating metallic elements inside the battery cell have been demonstrated (doi.org / 10.1038 / nature16502). This comprised incorporation of a metallic (resistive) element inside the cell (e.g. a Ni plate inserted within the electrolyte) such that, during operation, the battery electric current is forced to pass through it and dissipate power into heat. This approachcan provide rapid self-heating and increase the temperature from -30 to 0ºC in ~30 seconds, providing better performance than non-heated cells at low temperatures while consuming ~6% of the overall capacity for heating purposes. However, as the heating element was inserted in the electrolyte at mid-distance from the electrodes, this method provided non-uniform heating which is detrimental for the cell safety. Several heating elements can be distributed in the battery electrolyte (doi.org / 10.1016 / j.jpowsour.2016.08.028) rendering the temperature more uniform and consequently facilitating the Li-ion transfer throughout the cell. This way, deploying multiple heating plates can effectively make the temperature more uniform, achieving 30% faster heating with 27% less energy consumption compared with a battery with a single internal heating element (single middle Ni plate). However, these approaches require taking out energy of the battery for heating and results in non-uniform heating profiles which can lead to dendrite formation, being detrimental for the cell safety. This is the case for example of asymmetric heating (doi.org / 10.1038 / s41467-019-09924-1) that can accelerate Li dendrite growth, potentially leading to catastrophic failure of the battery due to short-circuit.

[0009] Another internal strategy to heat the battery consists of using photo-thermal effects of materials. In this case, the use of external illumination will lead to a strong heating localized at the surface for opaque batteries, or within the battery volume if the battery is transparent / semi-transparent. The latter is particularly advantageous as it would allow to directly address the root cause of the decreased battery performance at the microscale, targeting the internal processes responsible for performance degradation at low temperatures, while providing uniform heating which is important to maintain safe battery operation.

[0010] Previous works have shown all-solid-state opaque battery electrodes incorporating Ruthenium nanoparticles (doi.org / 10.1039 / c9ee04039k), providing enough photo-heating for the battery to operate at -70ºC. However, in that case, the inclusion of the Ru nanoparticles in the electrode leads to a strong heat and temperature gradient across the cell, which might be detrimental for stability and safety as it can lead to accelerated dendrite growth (doi.org / 10.1038 / s41467-019-09924-1).

[0011] Recent works describe transparent batteries (doi.org / 10.1002 / smtd.202301572), which disclose current collectors made of fluorine-doped tin oxide (FTO), Indium tin oxide (ITO), polyethylene terephthalate (PET), gridded electrodes (as also shown in doi.org / 10.1073 / pnas.110287310), or glass, allowing light to go through the cell. These examples are oriented towards the growing field of photo-rechargeable batteries, where the effect of illumination on the cathode / anode electrodes provides an electrochemical driving force, resulting in battery charging. This can be conjugated with applied electrical potential (photo-assisted charging), with added benefits in terms of charging of batteries using solar illumination. Yet, these photo-rechargeable batteries would suffer from the same kind of difficulties at sub-zero temperatures, as the electrochemical processes, photo-driven or not, have decreased activities at lower temperatures.

[0012] The present invention relates to a separator (1) for heating a battery cell comprising:

[0013] - a substrate (1.1) and light-activated nanoparticles (1.2) distributed in said substrate (1.1);

[0014] wherein the substrate (1.1) material is selected from glass fibres, glass, an oxide material, or polymer; and

[0015] the light-activated nanoparticles (1.2) are selected from metal nanoparticles or carbon nanoparticles.

[0016] In one embodiment, the substrate (1.1) material is selected from SiO2, borosilicate, soda lime, CuO, AlO, Al2O3, TiO2, polypropylene, polyethylene, polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride or its copolymers such as PVDF-cotrifluoroethylene, PVDF-co-hexafluoropropene, polytetrafluoroethylene, polyvinyl chloride, polymethylmethacrylate, polyimide, cellulose, lignin, silk fibroin.

[0017] In one embodiment, the substrate (1.1) has a porosity between 40 and 60% and an average pore size between 0.03 and 3 µm in diameter.

[0018] In one embodiment, the light-activated nanoparticles (1.2) are selected from Au, Ag, Cu, Al, Ni, Pt, ZrN, Ta, TiN, Pd, Co, Cr, Ru, carbon nanotubes, multilayer or single graphene flakes, reduced graphene oxide flakes, carbon black nanoparticles.

[0019] In one embodiment, the light-activated nanoparticles (1.2) have an average size between 5 and 100 nm.

[0020] In one embodiment, the light-activated nanoparticles (1.2) are present in the separator (1) in a loading between 0.3e10 and 2e10 nanoparticles per cm2.

[0021] In one embodiment, the light-activated nanoparticles (1.2) are coated with a dielectric material selected from SiO2, TiO2, Si3N4.

[0022] In one embodiment, the light-activated nanoparticles (1.2) are alloyed with a metal selected from Ag, Ni, Cr, Pt, Pd.

[0023] The invention also relates to a battery cell (2) comprising:

[0024] - a cell enclosure (2.1), itself comprising:

[0025] - an anode (2.1.1);

[0026] - a cathode (2.1.4);

[0027] - the separator (1) comprising light-activated nanoparticles (1.2) described in any of the previous claims, arranged between the anode (2.1.1) and the cathode (2.1.4);

[0028] wherein the separator (1) is soaked with a liquid electrolyte (2.1.3).

[0029] In one embodiment, the cell enclosure (2.1) is made of a transparent material selected from silica, borosilicate glass, silicon nitride, or polymers such as Polyether ether ketone, polyethylene terephthalate, or polyimide.

[0030] In one embodiment, the cell enclosure (2.1) is made of a non-transparent, or a semi-transparent material, and comprises a transparent section (2.2), such as an optical window.

[0031] In one embodiment, the cathode (2.1.4) is provided as transparent electrode or a semi-transparent electrode.

[0032] In one embodiment, the cathode (2.1.4) is provided as a grid substrate coated with an active cathode material or as a polymer substrate coated with an active cathode material.

[0033] In one embodiment, the anode (2.1.1) is provided as counter-electrode and comprises an element selected from Li, Na, Al, or K, or combinations thereof, carbon-based composites, semiconducting materials or dielectric materials.

[0034] In one embodiment, it further comprises a light source selected from a laser, a lamp, a light-emitting diode, or other artificial light source with suitable wavelength in the range between 400 and 1200 nm.

[0035] The invention also relates to a method of producing the separator (1) comprising the following steps

[0036] Providing a colloidal solution of light-activated nanoparticles (1.2);

[0037] Providing a substrate (1.1);

[0038] Soaking the substrate (1.1) in the colloidal solution of light-activated nanoparticles (1.2) by drop-casting;

[0039] Heating the substrate (1.1) loaded with the light-activated nanoparticles (1.2) at a temperature between 50 and 75ºC in vacuum to evaporate the solution and obtain the separator (1).General description

[0040] In the context of the above identified problems related to the use of batteries in low temperatures or sub-zero degree Celsius conditions, the presently disclosed invention aims at providing a new passive, and hence more efficient, approach to photo-heating of said batteries.

[0041] This invention addresses the shortcomings of the previous known, external and internal heating methods such as high energy consumption for heating purpose and non-uniform heating, by utilizing a completely new approach for battery heating based on passive and uniform (i.e. spatially homogeneous) photothermal heating in ensembles of light-activated nanoparticles (NPs) introduced within the battery cell volume. These light-activated nanoparticles were physically realized by employing certain types of nanoparticles, such as plasmonic nanoparticles, due to their ability to controllably interact with light, their efficient light-to-heat transduction properties, and low interaction with battery electrochemistry.

[0042] In general, the features of the present invention are:

[0043] 1. For the separator: light-activated nanoparticles with a suitable shape, material, or coating, and arrangement chosen in accordance with the desired temperature for the wavelength of illumination;

[0044] 2. For the battery cell: transparent or semi-transparent battery electrodes, suitable to allow light to access the separator, and consequently the nanoparticles.

[0045] Coating the nanoparticles may improve their stability in different conditions, such as acidic electrolytes with hydrofluoric acid (HF) acid or that generate HF acid via parasitic reactions.

[0046] The presence of light-activated nanoparticles in the separator, and consequently within the battery cell comprising said separator, is a passive internal heating strategy which does not consume energy of the battery cell for heating purposes and allows for an efficient and fast conversion of light energy into heat in a uniform manner.

[0047] Additionally, there is potential for this technology in other application scenarios to locally control the temperature at the micro / nanoscale in photo-electrochemical reactions in the field of solar fuels.

[0048] To physically demonstrate this invention, plasmonic nanoparticles of noble metals (e.g. Au) as proof of concept were chosen due to their efficient photothermal properties in the near-infrared and visible ranges, their typical chemical stability and low chemical reactivity allows to preserve the electrolyte properties.

[0049] As an example, the mass of Au makes up for way less than 1% of the battery mass, which strongly reduces heater mass when compared to current external and internal heating strategies.

[0050] For radiation with wavelength of ~500 nm (approximately corresponding to the spectral maximum of solar irradiation at Earth’s surface), the calculated size of spherical particles to maximize heating is ~50nm, which was found to provide an increase in temperature of ~10 K in experimental tests under 1 sun illumination for particular particle concentrations.

[0051] Moreover, provided that the battery cell is transparent to the excitation wavelength range, e.g. for solar applications these are in the visible – a desired property for applications merging solar cells and transparent energy storage systems. This approach can be applied to any type of metal-ion battery chemistry with liquid electrolyte, such as LIBs.

[0052] The crucial innovative points reside in how to materialize this concept. In particular, the two points above (features of the invention) materialize in the use of:

[0053] 1. Separator: purposely prepared and loaded with dispersed nanoparticles which area posteriorisoaked by electrolyte;

[0054] 2. Battery cell: comprising transparent or semi-transparent electrodes based on grid and / or semi-transparent substrates.

[0055] The currently disclosed invention development targets the use of batteries in applications under very low temperatures. This comprises high added-value niche applications such as, but not limited to, deep space probes operating at ultra-low temperatures (e.g. Mars polar probes) or high altitude unmanned aerial vehicles operating at low temperatures under solar illumination.

[0056] In the given examples, the final users of the invention, like space agencies or aerospace companies, will benefit from the current development. Additional applications for this invention can be electric mobility, such as electric mobility companies.

[0057] Known devices in these applications utilize part of the power stored in the batteries for heating. However, in the current invention, a passive solution is used, hence being more energy efficient than previous solutions, and could ideally employ solar illumination. To ensure that the battery cell is illuminated, the battery electrodes are transparent or semi-transparent.

[0058] Furthermore, current heating routes for known devices are slow (external heating), and non-optimal position of the macro / micro heaters causes the heating to be non-homogeneous (internal heating).

[0059] In this invention, the heating route here proposed is expected to be passive and efficient, i.e. the received and absorbed optical energy is directly converted in heat, fast, i.e. the heating time is in sub-minute range, and spatially homogeneous for a given, properly engineered distribution / dispersion of the heating elements and illumination.

[0060] The use of light-activated nanoparticles in the separator to provide heating is truly unique. This is probably because it is counter-intuitive, i.e. the use of metallic elements within the battery cell could decrease the performance of said battery. In fact, preliminary data showed a low concentration of well-dispersed metallic elements within the battery separator / electrolyte had no effect on battery operation and performance, other than the desired heating effect under illumination.

[0061] For better understanding of the present application, figures representing preferred embodiments are herein attached which, however, are not intended to limit the technique disclosed herein.Fig.1

[0062] (left) CBS-SEM image of the NP-loaded separator with an NP loading ~1.14e10 Au NPs / cm2, corresponding to ~25 µg Au per cm2; (right) Thermal imaging of the NP-loaded separator shown in the CBS-SEM image in the left under ~1 sun illumination intensity.Fig.2

[0063] (left) CBS-SEM image of the NP-loaded separators with an NP loading ~2e10 Au NPs / cm2, corresponding to ~50 µg Au per cm2; (right) Thermal imaging of the NP-loaded separator shown in the CBS-SEM image in the left, under ~1 sun illumination intensity.Fig.3

[0064] Thermal imaging for NP-loaded separator with NP loading of ~2e10 Au NPs / cm2, corresponding to ~50 µg Au per cm2under ~2 sun illumination intensity.Fig.4

[0065] (left) Galvanostatic Charge-Discharge cycling of the semi-transparent LFP / Li cells with conventional separator (Whatman GF / A); (right) Galvanostatic Charge-Discharge cycling of the semi-transparent LFP / Li cells with NP-loaded separator with an NP loading ~1.14e10 Au NPs / cm2, corresponding to ~25 µg Au per cm2.Fig.5

[0066] Rate capacity of semi-transparent LFP / Li cells with NP-loaded separators with an NP loading ~1.14e10 Au NPs / cm2, corresponding to ~25 µg Au per cm2. Applied currents correspond to 0.1C, 0.2C, 0.5C, 0.8C, 1C, 2C and 0.2C, where C = 170*0.8=136 mAh / g, shown both in not normalized (left) and normalized representations (right) with respect to 0.1C current data.

[0067] With reference to the drawings, some embodiments are now described in more detail, which are however not intended to limit the scope of the present application. While individual features may be present in various versions, they can potentially be combined in other configurations. The presence of features in different versions does not imply that combining them is unfeasible. Additionally, singular references do not exclude the possibility of plurality. In the context of this invention, the terms "a" and "an" should be understood to include multiple instances.

[0068] In the context of the present application, the term “light-activated nanoparticles” is understood as a type of nanoparticles that undergo a change in their physical or chemical state upon exposure to specific wavelengths of light. These changes enable the nanoparticles to perform targeted functions. Furthermore, the light-activated nanoparticles for the context of this invention are nanoparticles suitable to produce heat when subjected to light.

[0069] In the context of the present application, the term “battery cell” is understood as a single electrochemical unit that comprises an anode (negative) electrode and a cathode (positive) electrode, kept physically and electrically isolated by a separator which can be soaked by or constituted by an electrolyte medium for metal-ion transport between these electrodes.

[0070] In the context of the present application, the term “glass fibre substrate” refers to a non-woven material made up of glass fibres that are randomly interlaced. Although the glass fibres substrate can also comprise a woven material embedded with glass fibres, non-woven glass fibre materials are generally more mechanically robust. This increased strength is due to the randomly arranged fibres that form the primary structure of the material.

[0071] In the context of the present invention, the term “soaked” refers to a state in which an object or material (e.g., the separator (1)) is thoroughly or partially saturated or permeated with a liquid (e.g., electrolyte (2.1.3)), to the extent that the liquid has penetrated the structure of said object or material.

[0072] In the context of the present invention, the term “transparent” is understood as the property of a material that allows light to pass through it with minimal absorption and scattering, such that elements behind the material can be clearly seen. This property is characterized by the material's ability to transmit light without significant distortion or loss of intensity.

[0073] In the context of the present invention, the term “non-transparent” is understood as a property of a material that does not allow light to pass through it. This property is characterized by the material's ability to absorb or reflect all incident light, preventing any visibility of elements behind it.

[0074] In the context of the present invention, the term “semi-transparent” is understood as a property of a material that allows some light to pass through it, but not enough to see elements clearly on the other side. This property is characterized by partial light transmission, where the material scatters or diffuses light, resulting in a blurred or obscured view of elements behind it.

[0075] Separator (1)

[0076] In a first aspect, the present application provides an improved separator (1) suitable for a battery cell. The separator (1) herein disclosed significantly improves the photothermal heating of a battery cell, and consequently the performance of said battery. The separator (1) herein disclosed is particularly suitable for a battery cell operating in sub-zero degree conditions, without hindering the battery operation or performance.

[0077] In one embodiment, the separator (1) is suitable for a battery cell to operate at an external temperature below 0ºC.

[0078] In one embodiment, the separator (1) is suitable to provide an increase of temperature up to 40ºC to a battery cell.

[0079] In one embodiment, the separator (1) is suitable for a battery cell to operate when exposed to a light source. In another embodiment, the light source is an artificial or natural light source. In another embodiment, the light source is selected from, but not limited to, a laser, a lamp, or the sun. In another embodiment, the light source is in the visible to near-infrared range, corresponding to a wavelength between 400 and 1200 nm. In another embodiment, the light source has a wavelength between 500 and 1100 nm.

[0080] In one embodiment, the separator (1) is suitable to provide photothermal heating to a wide range of battery cells, to electrically separate an anode from a cathode, act as a barrier to prevent short-circuits, and, when soaked with electrolyte, provide ionic transfer between anode and cathode.

[0081] In one embodiment, the separator (1) is suitable for rechargeable metal-ion batteries.

[0082] In one embodiment, the separator (1) is capable of uptaking liquid electrolyte, thus allowing ion transport.

[0083] In one embodiment, the separator (1) for heating a battery cell comprises:

[0084] - a substrate (1.1); and

[0085] - light-activated nanoparticles (1.2) distributed in the substrate (1.1).

[0086] In one embodiment, the substrate (1.1) material is selected from glass fibres, such as but not limited SiO2; or glass, such as but not limited from borosilicate or soda lime; or an oxide material, such as but not limited from CuO, AlO, Al2O3, TiO2; or polymer.

[0087] In another embodiment, the polymer is selected from, but not limited to polypropylene (PP), polyethylene (PE), polyethylene oxide (PEO), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF) or its copolymers such as PVDF-cotrifluoroethylene (PVDF-TrFE), PVDF-co-hexafluoropropene (PVDF-HFP), polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), polymethylmethacrylate (PMMA), polyimide, cellulose, lignin, silk fibroin.

[0088] In one embodiment, the substrate (1.1) comprises a thickness between 25 and 800 µm. In another embodiment, the substrate (1.1) comprises a thickness between 25 and 50 µm. In another embodiment, the substrate (1.1) comprises a thickness between 80 and 800 µm.

[0089] In one embodiment, the substrate (1.1) has a porosity between 40 and 60%.

[0090] For the light-activated nanoparticles (1.2) to successfully load into the separator (1), the average size (diameter) of the substrate (1.1) pores must be larger than the average size (diameter) of the nanoparticles. In one embodiment, the average pore size of the substrate (1.1) is between 0.03 and 3 µm in diameter. In another embodiment, the average pore size of the glass fibre substrate is between 0.7 and 3 µm in diameter. In another embodiment, the average pore size of the polymer substrate is 0.03 and 0.5 µm in diameter.

[0091] The glass fibre substrate is typically mouldable, allowing it to be produced in various shapes. In one embodiment, the glass fibre substrate can be a commercially available product.

[0092] In the glass fibre substrate, the light-activated nanoparticles (1.2) are arranged on the surface of the fibre material, i.e. they do not penetrate inside, or intercalate with, the fibre material itself. The light-activated nanoparticles (1.2) penetrate inside the separator (1) as a whole, due to its porous nature, but always remain in the surface of the fibres.

[0093] In one embodiment, the polymer substrate can be a commercially available product.

[0094] The role of the light-activated nanoparticles (1.2) in the presently disclosed separator (1) is to efficiently heat up resulting in an increase of temperature of the separator (1) and consequently the battery cell, with a spatially uniform photo-induced temperature across the separator (1) and battery cell, without altering the electrochemistry of the battery itself.

[0095] In one embodiment, the light-activated nanoparticles (1.2) are selected from metal nanoparticles or carbon nanoparticles.

[0096] In one embodiment, the light-activated nanoparticles (1.2) are selected from, but not limited to, Au, Ag, Cu, Al, Ni, Pt, ZrN, Ta, TiN, Pd, Co, Cr, or Ru.

[0097] In another embodiment, the light-activated nanoparticles (1.2) are selected from carbon nanoparticles, such as, but not limited to, carbon nanotubes, multilayer or single graphene flakes, reduced graphene oxide flakes, carbon black nanoparticles. These types of carbon-based nanoparticles are typically cheap, commercially available, easy to manufacture and have strong photothermal effect required for the context of the present invention.

[0098] In one embodiment, the light-activated nanoparticles (1.2) have an average size (diameter) between 5 and 100 nm. In another embodiment, light-activated nanoparticles (1.2) have an average size (diameter) between 20 and 100 nm.

[0099] In one embodiment, the light-activated nanoparticles (1.2) are present in the separator (1) in a loading between 0.3e10 and 2e10 nanoparticles per cm2, corresponding to between 7 and 50 µg nanoparticle material per cm2of separator (1) (i.e., per cross-sectional (top) area). Although these values were observed for Au nanoparticles, the similar values are expected for other types of NPs as well.

[0100] In one embodiment, the shape of the light-activated nanoparticles (1.2) is selected from, but not limited to, spheres, rods, cubes, prisms, dodecahedrons, or nanostars.

[0101] The amount of heating provided can be controlled by the nanoparticle’s material, size and shape, as well as the nanoparticle arrangement and their interparticle distance. Typically, for uniform and efficient photo-induced heating, arrangements of well separated particles (i.e. low agglomeration) are sought.

[0102] From the experimental data, a volumetric distribution was observed between 4 and 30 nanoparticles per µm3of separator (1), which translates into interparticle distances between 0.1 to 1 µm. Although these values were observed for Au nanoparticles, the similar values are expected for other types of NPs as well.

[0103] In one embodiment, the light-activated nanoparticles (1.2) are coated with a dielectric material such as, but not limited to, SiO2, TiO2, Si3N4.

[0104] In another embodiment, the light-activated nanoparticles (1.2) are alloyed with other metal, selected from, but not limited to, Ag, Ni, Cr, Pt, Pd, for providing stability and / or targeted electrochemical properties.

[0105] Battery cell (2)

[0106] In a second aspect, the present application provides an improved battery cell (2) comprising the separator (1).

[0107] In one embodiment, a battery cell (2) comprises:

[0108] - a cell enclosure (2.1), itself comprising:

[0109] - an anode (2.1.1);

[0110] - a cathode (2.1.4);

[0111] - a separator (1) comprising light-activated nanoparticles (1.2), arranged between the anode (2.1.1) and the cathode (2.1.4);

[0112] wherein the separator (1) is soaked with a liquid electrolyte (2.1.3).

[0113] As shown in the embodiment of, the functional elements of the battery cell (2) are provided inside the cell enclosure (2.1). These elements are arranged as a sequential stack of anode (2.1.1) layer, separator (1) layer and cathode (2.1.4) layer. These elements are aligned and compacted to ensure uniform contact and minimize internal resistance. In this arrangement, the battery cell (2) is also filled with a liquid electrolyte (2.1.3).

[0114] In this sense, when arranged in a battery cell (2) the separator (1) comprises the liquid electrolyte (2.1.3).

[0115] In one embodiment, the cell enclosure (2.1) is made of a transparent material, selected from, but not limited to, silica, borosilicate glass, silicon nitride, or polymers such as Polyether ether ketone (PEEK), polyethylene terephthalate (PET), or polyimide (PI). The material must be suitable to allow light to penetrate the battery cell (2) and illuminate the nanoparticles loaded in the separator (1).

[0116] In another embodiment, the cell enclosure (2.1) can be made of a non-transparent, or a semi-transparent material, and comprise a transparent section (2.2), such as an optical window, suitable to allow light to penetrate the battery cell (2) and illuminate the nanoparticles loaded in the separator (1).

[0117] Thus, the cell enclosure (2.1) can be transparent or semi-transparent for a pre-determined light wavelength range of interest, such as between 400 and 1200 nm, according to the desired temperature increase.

[0118] In another embodiment, the cathode (2.1.4) is provided as transparent electrode. In another embodiment, the cathode (2.1.4) is provided as a semi-transparent electrode.

[0119] In one embodiment, the cathode (2.1.4) is provided as a grid substrate coated with an active cathode material. In one embodiment, the grid substrate of the cathode (2.1.4) is made of a stainless-steel mesh comprising holes with an average diameter between 75 and 500 µm.

[0120] In one embodiment, the cathode (2.1.4) is a polymer substrate coated with an active cathode material. In one embodiment, the polymer is selected from but, not limited to, polyimide.

[0121] In another embodiment, other substrate materials for the cathode (2.1.4) include fluorine-doped tin oxide (FTO), tin oxide (TO), silicon nitride, glass (i.e., silica), or borosilicate glass.

[0122] In one embodiment, the cathode (2.1.4) comprises pores with an average diameter between 8 and 50 µm.

[0123] In one embodiment, the active cathode material of the cathode (2.1.4) is selected from, but not limited to, metal oxides such as lithium iron phosphate (LiFePO4), lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4) or lithium nickel manganese cobalt oxide (LiNixMnyCozO2).

[0124] In one embodiment, the cathode (2.1.4) further comprises a conductive additive, such as carbon black. In one embodiment, the cathode (2.1.4) further comprises a polymeric binder, such as polyvinylidene fluoride (PVDF).

[0125] In one embodiment, the anode (2.1.1) is provided as counter-electrode and comprises an element selected from, but not limited to, Li, Na, Al, or K, or combinations thereof. In another embodiment, anode materials also extent towards carbon-based composites, such as widely used graphite, semiconducting materials, such as Silicon, or dielectric materials such as lithium titanate oxides.

[0126] In one embodiment, the anode (2.1.1) comprises a thickness between 8 and 500 µm.

[0127] While the invention described herein applies to any electrode thickness, thin electrodes (<8 µm) could provide more transparency, which is beneficial for the photo-heating mechanism, although these thicknesses are more difficult to control in industrial battery manufacturing processes, and thicker electrodes are preferred.

[0128] In one embodiment, the electrolyte (2.1.3) is a lithium salt with a concentration between 1.0 and 1.6 M, dissolved in at least one organic solvent. In one embodiment, the lithium salt is selected from lithium hexafluorophosphate (LiPF6), Lithium Bis(trifluoromethanesulfonyl)imide (LiTFS), among others, and the organic solvent can be selected from ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), or mixtures thereof.Examples

[0129] In one embodiment, the battery cell (2) further comprises a light source selected from, but not limited to, a laser, a lamp, a light-emitting diode (LED), or other artificial light source with suitable wavelength in the range between 400 and 1200 nm.

[0130] In one embodiment, the battery cell (2) further comprises other elements known in the art as suitable for its function.

[0131] Method of producing the separator (1)

[0132] In a third aspect, the present application provides a method of producing the separator (1) for a battery cell (2).

[0133] In one embodiment, the method of producing the separator (1) for a battery cell (2) comprises the following steps:

[0134] Providing a colloidal solution of light-activated nanoparticles (1.2);

[0135] Providing a substrate (1.1);

[0136] Soaking the substrate (1.1) in the colloidal solution of light-activated nanoparticles (1.2) by drop-casting;

[0137] Heating the substrate (1.1) loaded with the light-activated nanoparticles (1.2) at a temperature between 50 and 75ºC in vacuum to evaporate the solution and obtain the separator (1).

[0138] In one embodiment, the light-activated nanoparticles (1.2) are selected from, but not limited to, Au, Ag, Cu, Al, Ni, Pt, ZrN, Ta, TiN, Pd, Co, Cr, or Ru.

[0139] In another embodiment, the light-activated nanoparticles (1.2) are selected from carbon nanoparticles, such as, but not limited to, carbon nanotubes, multilayer or single graphene flakes, reduced graphene oxide flakes, carbon black nanoparticles.

[0140] In one embodiment, the substrate (1.1) is selected from, but not limited to, glass fibres, glass or a polymer.

[0141] Embodiments of the materials and conditions to carry out the method are also referred in the paragraphs above related to the separator (1).

[0142] In one embodiment, the battery cell (2) is obtained via conventional battery cell assembly techniques. A cell enclosure (2.1) is provided, and the separator (1) is arranged between the anode (2.1.1) and cathode (2.1.4), and the cell enclosure (2.1) is filled with liquid electrolyte (2.1.3).Examples

[0143] Experimental tests were performed with in-house developed NP-loaded separators.

[0144] Particularly, commercial Whatman GF / A glass microfibers were employed as separator substrates. These microfibers are constituted by woven cylinder-shaped silicon dioxide structures with diameters from around 0.1 to about 3 μm, as determined from Scanning Electron Microscopy (SEM) images,Prior to NP loading, the glass microfiber separator substrates are cut into circles of ~1.3 cm diameter and arranged within a boat container, without physical overlap.

[0145] NP loading was obtained from drop-casting a mono-dispersed colloidal solution of ~60 nm Au nanospheres in stabilized suspension of citrate buffer with a concentration of 1.9e10 nanoparticles / mL (CytoDiagnostics, acquired from Merck, Portugal) into the cut glass fiber substrates using a pipette. Each substrate circle is soaked with variable amount of colloidal Au nanoparticle solution, ranging from 200 to 600 µL, depending on desired NP loading mass amount. The buffer solution is evaporated by heating overnight at 65ºC in a vacuum oven, leaving well dispersed plasmonic NPs distributed at the surface of the woven glass microfibers disposed throughout the whole porous volume of the separator.

[0146] Battery cells employing these NP loaded separators were assembled in transparent and semi-transparent embodiments. The fully transparent embodiment comprised a glass vial with two copper wires connections drilled through the vial cap and sealed with epoxy, where each wire connects to anode (Li foil) and cathode (stainless-steel gridded cathode coated with a LFP:carbon black:PVDF slurry with mass ratio 8:1:1). The semi-transparent embodiment was realized with a CR2032 coin cell where one of the cell caps has a circular hole window with ~5 mm diameter, allowing light to access the cell internal constituents, where the cathode (stainless-steel gridded cathode coated with a slurry with LFP:carbon black:PVDF, materials acquired from Shenzhen Dynanonic, China, and Merck, Portugal, with mass ratio 8:1:1) is electrically connected to the cell cap in contact with a polyimide window which is sealed on the circular hole cap by an epoxy; the other cap of the CR2032 cell is in contact with a Li-foil counter-electrode. Assembly of the battery cell embodiments is performed inside an Ar-filed glovebox with less than 1 ppm of O2and H2O. During the assembly, the in-house developed NP-loaded separators are positioned between the anode and cathode utilizing typical battery cell assembly procedures, being then soaked with liquid electrolyte (1.0 M of LiPF6 in EC:DEC:DMC in 1:1:1 mix ration, acquired from Merck, Portugal).

[0147] The light-activated nanoparticles-loaded separators produced as proof of concept were characterized via Scanning Electron Microscope (SEM) using a concentric back-scattered (CBS) detector, allowing to discern between silica (glass fiber) and Au nanoparticle based on the imaging contrast, as the Au NPs will show white contrast against a gray / dark-gray contrast of the silica glass fibre.

[0148] The separators were tested for their photo-thermal heating ability under LED illumination with several intensities. For this purpose, an infrared camera was used to measure the temperature reached by the separators under illumination.

[0149] An Odepro KL41 PLUS white LED was used as lighting source, mounted within a vertical setup which allow to maintain a constant distance of 7.5 cm between LED and illuminated sample, while monitoring the temperature in time with a Noyafa NF-586s compact thermal imager camera sensor. During illumination the thermal image of the samples are recorded. The temperature initially rises as the sample is illuminated, reaching a steady state typically within 30 seconds. The steady state temperature after one minute is recorded and considered to be the reachable temperature for that illumination intensity value. The tests were performed with increasing LED illumination intensities, and with a 10-minute rest period between illumination tests, determined to be sufficient to reach ambient temperature after illumination. A HWPLM-Mini photometer was used to measure the illumination power at 7.5 cm distance, previous to each photothermal test. The intensity is calculated from the power measured in the photometer over the circular area of the separator which is positioned within the LED beam constant illumination area.

[0150] Results

[0151] With low mass loading, the presence of the nanoparticles within the surface of the nanofibers is scarce, as seen inHowever, already at this concentration, the temperature reached by the separator sample showed an increase of ~4~5 ºC which is approximately spatially uniform over the area of the separator, when illuminated by LED white light with measured intensity comparable to 1 sun (=1000 W / m2). Although this setup is not a “solar simulator” due to the difference in the power distribution along the illumination wavelengths, the reference to 1 sun intensities will be employed to refer to the intensity amount measured in the photometer (~1000 W / m2).

[0152] Increasing the mass loading towards ~2e10 Au NPs / cm2, corresponding to about 50 µg Au / cm2, resulted in the NP loaded separator ofAt this loading, the temperature reached by the separator sample showed an increase of ~10 ºC under a LED white light with 1000 W / m2 intensity (comparable to 1 sun).

[0153] Further increase in the light power to reach 2 sun intensity allows to increase the temperature at this mass loading, as shown in the

[0154] A control run with conventional separators without any loading was also performed under 1 sun and 2 sun illuminations, being verified that no temperature increase is reached under that situation, which highlights the efficient photo-thermal properties of the NP loaded separators.

[0155] The NP loading could be further increased, which would result in higher temperature. However, care must be taken to keep the concentration at a minimum to avoid jeopardizing the stability of the electrochemical phenomena within a battery assembled with the NP loaded separators. In this regard, battery cells comprising the developed separators soaked with 1.0 M of LiPF6 in EC:DEC:DMC (1:1:1) electrolyte were assembled between a stainless-steel gridded cathode coated with a LFP:carbon black (Super-P):PVDF slurry with mass ratio 8:1:1, and a Li metal foil counter-electrode.

[0156] To test the electrochemical working of the assembled battery cell, Galvanostatic Charge-Discharge (GCD) cycling was performed in battery cells with conventional separators and NP loaded separators with loading of ~1.14 Au NPs / cm2, at different constant currents to understand the influence of the Au NPs in the rate capability of the cells.

[0157] shows the charge and discharge cycling data, where it can be observed the presence of the Au NPs has negligible effect on the cycling ability, at these concentrations.

[0158] The rate capacity of the cells was further investigated via GCD, as shown inIt becomes clear that this small Au NP loading has little effect on the battery cell charge-discharge processes, and that these additives are effective only for increasing temperature while allowing the battery to operate normally.

Claims

A separator (1) for heating a battery cell comprising:- a substrate (1.1) and light-activated nanoparticles (1.2) distributed in said substrate (1.1);wherein the substrate (1.1) material is selected from glass fibres, glass, an oxide material, or polymer; andthe light-activated nanoparticles (1.2) are selected from metal nanoparticles or carbon nanoparticles.The separator (1) according to the previous claim, wherein the substrate (1.1) material is selected from SiO2, borosilicate, soda lime, CuO, AlO, Al2O3, TiO2, polypropylene, polyethylene, polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride or its copolymers such as PVDF-cotrifluoroethylene, PVDF-co-hexafluoropropene, polytetrafluoroethylene, polyvinyl chloride, polymethylmethacrylate, polyimide, cellulose, lignin, silk fibroin.The separator (1) according to any of the previous claims, wherein the substrate (1.1) has a porosity between 40 and 60% and an average pore size between 0.03 and 3 µm in diameter.The separator (1) according to any of the previous claims, wherein the light-activated nanoparticles (1.2) are selected from Au, Ag, Cu, Al, Ni, Pt, ZrN, Ta, TiN, Pd, Co, Cr, Ru, carbon nanotubes, multilayer or single graphene flakes, reduced graphene oxide flakes, carbon black nanoparticles.The separator (1) according to any of the previous claims, wherein the light-activated nanoparticles (1.2) have an average size between 5 and 100 nm.The separator (1) according to any of the previous claims, wherein the light-activated nanoparticles (1.2) are present in the separator (1) in a loading between 0.3e10 and 2e10 nanoparticles per cm2.The separator (1) according to any of the previous claims, wherein the light-activated nanoparticles (1.2) are coated with a dielectric material selected from SiO2, TiO2, Si3N4.The separator (1) according to any of the previous claims, wherein the light-activated nanoparticles (1.2) are alloyed with a metal selected from Ag, Ni, Cr, Pt, Pd.A battery cell (2) comprising:- a cell enclosure (2.1), itself comprising:- an anode (2.1.1);- a cathode (2.1.4);- the separator (1) comprising light-activated nanoparticles (1.2) described in any of the previous claims, arranged between the anode (2.1.1) and the cathode (2.1.4);wherein the separator (1) is soaked with a liquid electrolyte (2.1.3).The battery cell (2) according to the previous claim, wherein the cell enclosure (2.1) is made of a transparent material selected from silica, borosilicate glass, silicon nitride, or polymers such as Polyether ether ketone, polyethylene terephthalate, or polyimide.The battery cell (2) according to claims 9, wherein the cell enclosure (2.1) is made of a non-transparent, or a semi-transparent material, and comprises a transparent section (2.2), such as an optical window.The battery cell (2) according to any of the claims 9 to 11, wherein the cathode (2.1.4) is provided as transparent electrode or a semi-transparent electrode.The battery cell (2) according to any of the claims 9 to 13, wherein the cathode (2.1.4) is provided as a grid substrate coated with an active cathode material or as a polymer substrate coated with an active cathode material.The battery cell (2) according to any of the claims 9 to 13, wherein the anode (2.1.1) is provided as counter-electrode and comprises an element selected from Li, Na, Al, or K, or combinations thereof, carbon-based composites, semiconducting materials or dielectric materials.The battery cell (2) according to any of claims 9 to 14, wherein it further comprises a light source selected from a laser, a lamp, a light-emitting diode, or other artificial light source with suitable wavelength in the range between 400 and 1200 nm.Method of producing the separator (1) described in any of the claims 1 to 8 for a battery cell (2), comprising the following steps:Providing a colloidal solution of light-activated nanoparticles (1.2);Providing a substrate (1.1);Soaking the substrate (1.1) in the colloidal solution of light-activated nanoparticles (1.2) by drop-casting;Heating the substrate (1.1) loaded with the light-activated nanoparticles (1.2) at a temperature between 50 and 75ºC in vacuum to evaporate the solution and obtain the separator (1).

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