Sodium-ion cell
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2026-02-04
- Publication Date
- 2026-08-13
Smart Images
Figure GB2026050150_13082026_PF_FP_ABST
Abstract
Description
[0001] Sodium-ion Cell
[0002] The present invention relates to a sodium-ion cell, and to a battery comprised of such cells.
[0003] Sodium-ion cells are being developed as an alternative technology to lithium-ion cells. They are seen as being cost-effective for a range of different applications such as large-scale energy storage, for example to store energy generated by intermittent renewable generators (such as wind power or solar power). Although a sodium-ion cell gives a lower average voltage than a lithium-ion cell, a sodium-ion cell has advantages as sodium is less expensive than lithium, and much more widely available; and a sodium-ion cell is significantly safer in use and assembly, maintenance and transportation.
[0004] In a typical sodium-ion cell, the anode compartment contains 'hard' carbon into which sodium ions reversibly intercalate during charge and discharge at a low electrical potential, while the cathode compartment contains a material into which sodium ions can intercalate but at a high electrical potential. A wide range of materials may be suitable for this purpose, such as layered transition metal oxides (LTMO) of the general formula NaxMO2, x ≤ 1 and M = Co, Ni, Mn, Cr, Cu, Fe and V. Such LTMO materials are comparatively low-cost, and provide high energy density. These layered cathode materials can be subdivided into P2, P3, 02 and 03, based on the sites occupied by sodium ions, P and O signifying that the sodium ions tend to locate at prismatic and octahedral sites respectively, and the number (2 or 3) indicating the number of transition metal oxide layers occupied by a unit cell; other structures are also found, and structures may change depending in the amount of intercalated sodium. Other materials that have been proposed for this purpose include polyanionic compounds, and Prussian blue analogues including Prussian white which is a fully sodiated Prussian blue structure.
[0005] According to the present invention there is provided a sodium-ion cell comprising an anode compartment and a cathode compartment on either side of an electrolyte element, wherein the cathode compartment contains a material into which sodium ions can reversibly intercalate, and wherein the electrolyte element comprises a perforated sheet of a metal, and a non-permeable layer of sodium-ion-conducting ceramic bonded to one face of the perforated sheet by a porous and permeable ceramic sub-layer.The layer of sodium-ion-conducting ceramic is non-permeable to gases or liquids, but it is a conductor of the sodium ions that must pass between the anode and cathode compartments during operation. The electrolyte element is not an electronic conductor, because the sodium-ion-conducting ceramic is an electronic insulator. The ion-conducting ceramic may be β''-alumina; other options are Na1+xZr2SixP3-xO12(Nasicon), or combinations such as Na3PO4 – Na2SO4, or glass ceramics such as Na3PS4.
[0006] Another potential material is the glass-ceramic NasRSiziO^, where R is a rare-earth element; in this document the term glass-ceramic means a polycrystalline ceramic material produced through crystallisation of a glass, so producing a uniform dispersion of fine crystals. A glass is an amorphous material that has been molten and has then hardened and become rigid without crystallising. The rare earth element R may be Sc, Er, Y, Ho, Dy, Gd, Eu, or Sm. It has been established that ionic conductivity increases with increasing radii of trivalent cations from Sc3+ to Sm3+, ranging from 3 × 10-3S / cm (Sc3+) to 0.1 S / cm (Sm3+) at 200°C. It has also been found that the higher the ionic conductivity, the lower the sintering temperature required to achieve a fully dense layer (see for example Materials 2022, 15, 1104). In this document the term " N5-ceramic" means a glass-ceramic that comprises Na5RSi4O12, where R may be Sc, Er, Y, Ho, Dy, Gd, Eu, or Sm.
[0007] The porous and permeable sub-layer may also be of a sodium-ion-conducting ceramic. For example the impermeable layer may be made of a glass-ceramic and the porous sub-layer also of such a glass-ceramic, in particular of an N5-ceramic, but the impermeable layer and the permeable sub-layer are made from different precursors. For example the precursors may both be of the same chemical composition, but differ in the particle sizes; or may be of different chemical compositions. For example the porous layer may be of the version of Na5RSi4O12 in which R is Y (yttrium), whereas the dense and impermeable layer may be the version of Na5RSi4O12 in which R is Sm (samarium) or Gd (gadolinium). As mentioned above, the higher the ionic conductivity, the lower the sintering temperature needed to achieve a fully dense layer. So the sintering can be carried out at a temperature that ensures the impermeable layer is fully dense while the other layer remains porous; and this also optimises the ionic conductivity of the impermeable layer.
[0008] The porous ceramic sub-layer, and the non-permeable layer of sodium-ion-conducting ceramic too, extend over the perforations, but do not occupy the perforations. In this electrolyte element the strength is provided by the perforated metal sheet, so theimpermeable layer of sodium-ion-conducting ceramic can be thin, for example between 0.01 and 0.05mm. This results in a cell that can perform adequately at significantly lower temperatures, for example less than 200°C, less than 100°C, or even at ambient temperatures. As it is a sodium ion cell, in the charged state there is sodium in the anode compartment; the sodium may be in metallic form, and either solid or liquid depending on the operating temperature. This leads to a cell with high energy density. If the sodium were instead be intercalated into hard carbon, as in the prior art, this would not benefit from the advantages that arise from the solid-state impermeable layer of sodium-ion-conducting ceramic with the supporting perforated metal sheet; in particular the solid-state impermeable ceramic layer is effective at preventing the growth of sodium dendrites.
[0009] It will be appreciated that the provision of the perforated metal sheet makes possible a robust and somewhat flexible electrolyte element. The thermal expansivity of the metal is at least slightly greater than that of the ceramic, and consequently after sintering to form the ceramic layer the ceramic layer is under a residual compressive stress, which inhibits cracking; this also enhances the resistance to dendrites that might otherwise open paths between crystals or grains in the ceramic layer. Indeed, sodium is less prone to form dendrites than is lithium, but rather tends to spread laterally across the surface of the electrolyte structure. The use of the robust solid state electrolyte allows for the use of a metallic anode rather than the standard hard carbon intercalation anode. This improves the energy density and avoids the stability problems and material consistency problems associated with hard carbons.
[0010] The cathode compartment contains a suitable intercalation material such as an LTMO (layer transition metal oxide) e.g. P2-Na2 / 3Fe1 / 2Mn1 / 2O2 or O3-Na0.9Cu0.22Fe0.3Mn0.48O2), ora polyanion material such as Na3V2(PO4)2F3, (which may be called NVPF), or an analogue of Prussian blue (Fe4[Fe(CN)6]3), such as Prussian white in which much of the iron is replaced by sodium e.g. Na1.92Fe2(CN)6. It generally also contains carbon, to enhance electronic conduction, a liquid catholyte, and a binder. One such binder is PVdF (polyvinylidene fluoride). The liquid catholyte is an electrolyte that enables sodium ions to move through the cathode compartment, and consists of a sodium salt dissolved in a non-aqueous solvent. Some options would be NaPF6 or NaClO4 dissolved in EC / DMC / PC or DEC / FEC (i.e. a mixture of ethylene carbonate, dimethyl carbonate, and propylene carbonate, or a mixture of diethyl carbonate and fluoroethylene carbonate). Other potential solvents for the catholyte are Di-and Tri-glyme (diethylene glycol dimethyl ether and triethylene glycol dimethyl ether). If thecell is operated at a moderately elevated temperature for example in the range 100°C up to 130°C, an alternative catholyte solvent is beneficial. This may be based on a glycol such as ethylene or propylene glycol with NaPF6 or NaClO4 dissolved in it.
[0011] In both the anode compartment and the cathode compartment there may be a current collector, for example of aluminium foil. The anode compartment and the cathode compartment are enclosed in part by an anodic wall and a cathodic wall respectively, and the anodic wall and / or the cathodic wall may act as the respective current collector, rather than there being a separate component.
[0012] In one option, the cell may operate at ambient temperatures, for example in the range 10° to 30°C. At such a temperature the sodium in the anode compartment is solid, and the electrolyte structure may be arranged with the layer of sodium-ion-conducting ceramic facing the anode compartment. It may be beneficial to provide a compression force to push the sodium against the electrolyte element; this implies that the anode compartment has a wall that is flexible so that external pressure can push the sodium against the electrolyte element.
[0013] In another option the cell may operate at a moderately elevated temperature above the melting point of sodium (98°C), for example in the range 100°C up to 130°C. At these temperatures the sodium is liquid, and the electrolyte element may be arranged with the perforated metal sheet facing the anode compartment. To ensure the molten sodium wets the surface of the perforated metal sheet, that surface may be painted with a solution such as lead acetate in aqueous solution, or manganese nitrate or iron chloride or tin chloride in solution in ethanol, so the solution soaks through the perforations and into the porous sublayer. Drying and baking the electrolyte element produces very small particles of oxide, metal or chloride on the surfaces of the perforations and the pores, which enhance weting by molten sodium in the operating cell. There are benefits from using lead acetate, with the baking being in an oxygen-free atmosphere, as the resulting Pb / Pb(IV) oxide mixture forms an electronically conductive structure that extends into the pores of the porous sub-layer of ceramic.
[0014] Operating at such a moderately elevated temperature provides a cell that has lower internal resistance, because the conductivity of the sodium-ion-conducting ceramic isgreater at higher temperature; and avoids the requirement for pressure to ensure contact between the sodium and the surface of the electrolyte element.
[0015] The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which:
[0016] Figure 1 shows a cross-sectional view of an electrolyte element of the invention;
[0017] Figure 2 shows a cross-sectional view of a cell incorporating the electrolyte element of figure 1; and
[0018] Figure 3 shows a cross-sectional view of an alternative cell incorporating the electrolyte element of figure 1.
[0019] Referring now to figure 1 there is shown an electrolyte element 10 of the invention partly broken away: only a region near an edge is shown. The electrolyte element 10 comprises a metal sheet 12 of a metal such as nickel, or aluminium-bearing ferritic steel (such as the type known as Fecralloy (trade mark)), or a steel that forms an electronically-conductive and adherent scale, for example a CrMn oxide scale, when heated in air. Most of the sheet 12 is perforated to produce a very large number of through holes 13, as shown schematically, the holes 13 being of mean diameter 30 pm, potentially produced by a laser drilling process, or of mean diameter between 50 pm and 100 pm, and may for example be made by chemical etching. A margin 14 around the periphery of the metal sheet 12, typically of width 5 mm, is not perforated. The perforated portion of the sheet 12 is covered by a porous and permeable ceramic sub-layer 16a which is itself covered by a non-permeable ceramic layer 16b, the ceramic layer 16b being of a sodium-ion-conducting ceramic. The non-permeable ceramic layer 16b may for example comprise beta alumina or Nasicon, but in addition it may contain a material that forms a glass during the sintering process. Thus although it is referred to as a ceramic layer, the term "ceramic" in this context includes combinations of ceramic and glass, as long as the layer is conductive to sodium ions during operation. The non-permeable ceramic layer 16b must not be permeable, that is to say it would be impermeable to gases, and consequently impermeable to liquids during operation. The non-permeable layer 16b also covers the edges of the sub-layer 16a.
[0020] The porous sub-layer 16a may be of the same sodium-ion-conducting ceramic as the non-permeable ceramic layer 16b, or may be of a different sodium-ion-conducting ceramic, but would typically be formed from a slurry containing somewhat larger particles. Each layer 16a and 16b is formed by depositing a slurry of ceramic particles, drying the slurry, andsintering so the ceramic forms a coherent layer: the porous sub-layer 16a bonds to oxides at the surface of the metal sheet 12; and then the non-permeable ceramic layer 16b coheres to the surface of the porous layer 16a. The porous and permeable ceramic sub-layer 16a may be of thickness between 10 pm and 100 pm, while the non-permeable layer 16b may be of thickness in the range 5 pm to 50 pm, for example 20 pm, 30 pm or 40 pm. (Figure 1 is not to scale.)
[0021] By way of example both layers 16a and 16b may be of a glass-ceramic that comprises NasRSi40i2, where R is a rare-earth element. For example the porous sub-layer 16a may be formed of Na5RSi4O12 in which R is Y (yttrium), whereas the dense and impermeable layer 16b may be formed of Na5RSi4O12 in which R is Sm (samarium) or Gd (gadolinium). These materials have a thermal expansivity that is closer to that of the metal sheet 12 than for example Nasicon, so there is less residual stress in the layers 16a and 16b after sintering.
[0022] Referring now to figure 2 there is shown a sodium ion cell 20 that incorporates an electrolyte element 10 as described above, consisting of a perforated metal sheet 12 with a ceramic coating 16; the ceramic coating 16 consists of the porous sub-layer 16a and the non-permeable ceramic layer 16b, but these are not shown separately in figure 2. By way of example the electrolyte element 10 may be square, with sides for example of length 10 cm.
[0023] The sodium ion cell 20 is operable at ambient temperatures, and at such temperatures sodium is a solid. The sodium ion cell 20 includes an aluminium foil cathode 22 that is fixed by a laser weld 23 to the peripheral margin 14 of the metal sheet 12, the foil cathode 22 being spaced away from the perforated portion of the metal sheet 12 so as to define a cathode compartment 24, and this is filled with a cathode material 25. The cathode material 25 is a mixture of an intercalation material such as an LTMO (layer transition metal oxide) with carbon, to enhance electronic conduction, a liquid catholyte, and a binder. One such binder is PVdF (polyvinylidene fluoride). The liquid catholyte is a sodium salt NaPFe dissolved in EC / DMC / PC. This composition for the cathode material 25 is given by way of example; other options were discussed previously.
[0024] The cell 20 also contains sodium metal 26 in contact with the ceramic coating 16, and the opposite face of the sodium metal 26 is covered by an aluminium foil anode 28. A sealant 30 such as silicone surrounds the sodium metal 26, sealing between the peripheral margin 14 of the metal sheet 12 and the aluminium foil anode 28. Along one side of the cell20 the aluminium foil cathode 22 and the aluminium foil anode 28 project beyond the edge of the electrolyte element 10 to define cathode and anode terminals (shown by + and -symbols) for the cell 20. The cell 20 is enclosed within a flexible pouch 32, from which the terminals project. During operation pressure can be provided to the pouch 32, to compress the cell 20 and so to ensure good contact between the solid sodium metal 26 and the ceramic coating 16.
[0025] During discharge, sodium ionises at the interface with the ceramic coating, and the sodium ions migrate through the ceramic coating 16 and the perforations 13 to dissolve in the catholyte and to intercalate in the intercalation material that forms part of the cathode material 25. The formation of voids at the interface between the sodium metal 26 and the ceramic coating 16 is prevented by the applied pressure, assodium metal is soft, and diffusion of metal atoms is rapid within the metal. It will be appreciated that the provision of the seal 30 and the laser weld 23 ensures there can be no direct contact between the sodium metal 26 and the cathode material 25. During charging, sodium ions migrate from the intercalation material and through the perforations 13 and through the ceramic coating 16 to deposit as metal on the sodium metal 26.
[0026] In a modification, if the flexible pouch 32 is sealed and contains no residual gas, the laser weld 23 would not be needed; but the anode seal 30 to prevent the sodium metal 26 creeping round the edge and shorting on the cathode 22 is still required.
[0027] Referring now to figure 3 there is shown a sodium ion cell 35 that incorporates an electrolyte element 10 as described in relation to figure 1, consisting of a perforated metal sheet 12 with a ceramic coating 16; the ceramic coating 16 consists of the porous sub- layer 16a and the non-permeable ceramic layer 16b, but in figure 3 again these are not shown separately. By way of example the electrolyte element 10 may be square, with sides for example of length 10 cm.
[0028] The sodium ion cell 35 is operable at an elevated temperature above the melting point of sodium, which is 98°C, for example it may operate in the range between 100°C and 110°C or 120°C, and at such temperatures sodium is a liquid. The sodium ion cell 35 includes a stainless-steel dish-shaped anode plate 36 that, during operation, contains molten sodium 37, and the anode plate 36 has a rim that is fixed by a laser weld 38 to the peripheral margin 14 of the metal sheet 12. The electrolyte element 10 is arranged so the molten sodium 37 contacts the perforated metal sheet 12. The sodium ion cell 35 also includes astainless steel dish-shaped cathode plate 40 which in combination with the ceramic layer 16 defines a cathode compartment 41 that contains a suitable cathode material 42 that includes an intercalation material, carbon, and a binder, and a catholyte; the rim of the cathode plate 40 is sealed by a sealant 44 to the edge of the ceramic layer 16 and to the peripheral margin 14 of the metal sheet 12. For example the cathode material 42 may comprise an intercalation material such as an LTMO (layered transition metal oxide) with carbon, to enhance electronic conduction, a liquid catholyte, and a binder such as PVdF (polyvinylidene fluoride). The liquid catholyte is a sodium salt such as NaPFe or NaCIC dissolved in ethylene or propylene glycol.
[0029] As mentioned above, the weting of the electrolyte element 10 by the molten sodium 37 may be enhanced by suitable treatment of that surface of the electrolyte element. For example that surface may be painted with a solution such as lead acetate in aqueous solution, or manganese nitrate or iron chloride or tin chloride in solution in ethanol, so the solution soaks through the perforations 13 and into the porous sub-layer 16a. Drying and then baking the electrolyte element 10 leads to the formation of very small particles of oxide, metal or chloride on all the treated surfaces, which enhance weting by molten sodium in the operating cell.
[0030] The sodium ion cell 35 operates in substantially the same way as the cell 20, but there is no need to apply external pressure, as the sodium is molten. It will be appreciated that the catholyte must be stable at the operating temperature. As compared to the cell 20, the cell 35 can be expected to provide greater output power, because the higher temperature means that the conductivity of the sodium-ion-conducting ceramic is higher. The cell 35 is substantially rigid, unlike the cell 20. In a modification the cathode plate 40 and the anode plate 36 may be of aluminium rather than stainless steel.
Claims
Claims1. A sodium-ion cell comprising an anode compartment and a cathode compartment on either side of an electrolyte element, wherein the cathode compartment contains a material into which sodium ions can reversibly intercalate, and wherein the electrolyte element comprises a perforated sheet of a metal, and a non-permeable layer of sodium-ion-conducting ceramic bonded to one face of the perforated sheet by a porous and permeable ceramic sub-layer.
2. A cell as claimed in claim 1 wherein the ion-conducting ceramic is selected from: β''-alumina; Na1+xZr2SixP3-xO12(Nasicon); Na3PO4 – Na2SO4; or glass ceramics such as Na3PS4 or the glass-ceramic Na5RSi4O12, where R is a rare-earth element.
3. A cell as claimed in claim 1 or claim 2 wherein the porous and permeable sub-layer is also of a sodium-ion-conducting ceramic.
4. A cell as claimed in any one of the preceding claims wherein the cathode compartment contains an intercalation material selected from LTMO (layered transition metal oxide), ora polyanion material, or an analogue of Prussian blue.
5. A cell as claimed in any one of the preceding claims wherein the cathode compartment also contains carbon, to enhance electronic conduction, a liquid catholyte, and a binder.
6. A cell as claimed in claim 5 wherein the liquid catholyte consists of a sodium salt dissolved in a non-aqueous solvent.
7. A cell as claimed in claim 6 wherein the sodium salt is NaPF6 or NaClO4.
8. A cell as claimed in claim 6 or claim 7 wherein the non-aqueous solvent is a mixture of ethylene carbonate, dimethyl carbonate, and propylene carbonate, or a mixture of diethyl carbonate and fluoroethylene carbonate); or comprises ethylene or propylene glycol; or diethylene glycol dimethyl ether, or triethylene glycol dimethyl ether.
9. A cell as claimed in any one of the preceding claims adapted to operate at ambient temperatures at which sodium in the anode compartment is solid, and the electrolyte structure is arranged with the layer of sodium-ion-conducting ceramic facing the anode compartment.
10. A cell as claimed in claim 9 wherein at least the anode compartment has a wall that is flexible so that external pressure can push the sodium against the electrolyte element.
11. A cell as claimed in any one of claims 1 to 8 wherein the cell is adapted to operate at a moderately elevated temperature above the melting point of sodium (98°C), for example in the range 100°C up to 130°C.
12. A cell as claimed in claim 11 wherein the electrolyte element is arranged with the perforated metal sheet facing the anode compartment.
13. A cell as claimed in claim 12 wherein the surface of the perforated metal sheet has been treated to provide very small particles of metal oxide, metal or metal chloride on the surfaces of the perforations and the pores, which enhance weting by molten sodium in the operating cell.