Structured metal electrode and combination thereof with non-liquid electrolytes

Abstract

The disclosure relates to a metal electrode or current collector for an energy storage device. The surface of the electrode or the current collector includes multiple blind hole-like recesses spaced apart from each other. The surface structured in this way is coated with a solid polymer electrolyte. The recesses are filled with the solid polymer electrolyte, as well as a primary or secondary energy storage device including the same.

Claims

1. Metal electrode or current collector for an energy storage device, wherein a surface of the metal electrode or the current collector comprises a plurality of blind-hole-like recesses spaced apart from each other, wherein the surface structured in this way is coated with a solid polymer electrolyte, wherein the blind-hole-like recesses are filled with the solid polymer electrolyte.

2. Metal electrode or current collector according to claim 1, wherein the structured surface of the metal electrode is enlarged in a range from ≥20% to ≤200% with respect to an area of the same dimension with a planar surface.

3. Metal electrode or current collector according to claim 1, wherein the blind-hole-like recesses have a length, width and/or depth in a range from ≥100 μm to ≤800 μm.

4. Metal electrode or current collector according to claim 1, wherein the solid polymer electrolyte is a polymer selected from the group comprising poly[bis((methoxyethoxy)ethoxy)phosphazene], poly((oligo)oxethylene)methacrylate-co-alkali metal methacrylate, poly[bis((methoxyethoxy)ethoxy)-co-(lithium-trifluoro-oxoborane)polyphosphazene], polyethylene oxide, polystyrene-b-poly(ethylene oxide), polyvinylidene fluoride, poly(vinylidene fluoride-co-hexafluoropropylene), polyacrylonitrile, polyester, polypropylene oxide, ethylene oxide/propylene oxide copolymer, polymethyl methacrylate, polymethylacrylonitrile, polysiloxane, poly(chlorotrifluoro-ethylene), poly(ethylene-chlorotrifluoro-ethylene) and mixtures thereof.

5. Metal electrode or current collector according to claim 1, wherein the solid polymer electrolyte forms a layer having a layer thickness in a range from ≥5 μm to ≤150 μm.

6. Metal electrode or current collector according to claim 1, wherein the metal is lithium, wherein the structured lithium surface has a chemical modification, selected from a lithium ion conductive layer containing lithium carbonate which is prepared by contact reactions of a lithium surface with carbon dioxide, 1-fluoroethylene carbonate (FEC), vinylene carbonate (VC) or lithium nitrate in 1,3-dioxolane.

7. Primary or secondary energy storage device comprising a current collector or, as a negative electrode (anode), a metal electrode according to claim 1, a non-liquid electrolyte and a counter electrode, as a positive electrode.

8. Energy storage device according to claim 7, wherein the non-liquid electrolyte comprises: a solid polymer electrolyte; a gel polymer electrolyte; or a composite electrolyte comprising a multilayer assembly of a lithium ion-conducting ceramic, vitreous or glass-ceramic solid electrolyte coated on opposite surfaces with a gel polymer electrolyte or a solid polymer electrolyte.

9. Energy storage device according to claim 8, wherein: the ceramic solid electrolyte is selected from the group comprising lithium lanthanum zirconate (LLZO) stabilised in a cubic crystal structure by substitution with Ta.sup.5+, Nb.sup.5+, Te.sup.5+0 or W.sup.6+ at the Zr.sup.4+ lattice site and/or Al.sup.3+ or Ga.sup.3+ at the Li.sup.+ lattice site, lithium lanthanum tantalum zirconate Li.sub.6.75La.sub.3Zr.sub.1.75Ta.sub.0.4O.sub.12 (LLZTO), lithium lanthanum titanate (La,Li)TiO.sub.3 (LLTO), and/or lithium aluminum germanium phosphate Li.sub.1+xAl.sub.yGe.sub.2-y(PO.sub.4).sub.3 (LAGP), wherein 0.3 x<0.6 and 0.3 y<0.5; the vitreous solid electrolyte is selected from the group comprising lithium phosphate (LIPON) and/or sulphide-based solid electrolytes selected from the group comprising Li.sub.2S—P.sub.2S.sub.5, Li.sub.3PS.sub.4 (LPS), Li.sub.2S—GeS.sub.2, Li.sub.2S—GeS.sub.2—P.sub.2S.sub.5, Li.sub.2S—GeS.sub.2—ZnS, Li.sub.2S—Ga.sub.2S.sub.3, Li.sub.2S—GeS.sub.2—Ga.sub.2S.sub.3, Li.sub.2S—GeS.sub.2—Sb.sub.2S.sub.5, Li.sub.2S—GeS.sub.2—Al.sub.2S.sub.3, Li.sub.2S—SiS.sub.2, Li.sub.2S—Al.sub.2S.sub.3, Li.sub.2S—SiS.sub.2—Al.sub.2S.sub.3, Li.sub.2S—SiS.sub.2—P.sub.2S.sub.5, Li.sub.2S—SiS.sub.2—LiI, Li.sub.2S—SiS.sub.2—Li.sub.4SiO.sub.4, Li.sub.2S—SiS.sub.2—Li.sub.3PO.sub.4, Li.sub.2SO.sub.4—Li.sub.2O—B.sub.2O.sub.3 and Li.sub.2S—GeS.sub.2—P.sub.2S.sub.5 (LGPS); and/or the glass-ceramic solid electrolyte is selected from the group comprising lithium compounds of the empirical formula Li.sub.1+x-yM.sup.V.sub.yM.sup.III.sub.xM.sup.IV.sub.2-x-y(PO.sub.4).sub.3 isostructural to NASICON, wherein 0≤x<1, 0≤y<1 and (1+ x-y)>1 and M.sup.III is a trivalent cation, M.sup.IV is a tetravalent cation and M.sup.V is a pentavalent cation (LATP, in particular Li.sub.11+xAl.sub.xTi.sub.2-x(PO.sub.4).sub.3), Li.sub.7P.sub.3S.sub.11 and/or Li.sub.7P.sub.2S.sub.8I.

10. Method of producing a metal electrode or a current collector for an energy storage device according to claim 1, wherein the structuring of the metal surface with recesses is carried out by a roll-to-roll process.

11. Metal electrode or current collector according to claim 1 wherein the structured surface of the metal electrode is enlarged in a range from ≥30% to ≤150% with respect to an area of the same dimension with a planar surface.

12. Metal electrode or current collector according to claim 1, wherein the structured surface of the metal electrode is enlarged in a range from ≥50% to ≤100%, with respect to an area of the same dimension with a planar surface.

13. Metal electrode or current collector according to claim 1, wherein the blind-hole-like recesses have a length, width and/or depth in a range from ≥200 μm to ≤500 μm.

14. Metal electrode or current collector according to claim 1, wherein the blind-hole-like recesses have a length, width and/or depth in a range from ≥300 μm to ≤400 μm.

15. Metal electrode or current collector according to claim 1, wherein the solid polymer electrolyte forms a layer having a layer thickness in a range from ≥15 μm to ≤100 μm.

16. Metal electrode or current collector according to claim 1, wherein the solid polymer electrolyte forms a layer having a layer thickness in a range from ≥20 μm to ≤50 μm.

Description

[0062] In the figures:

[0063] FIG. 1 is a schematic representation of the production or a process for producing a structured, chemically modified metal electrode with a functional coating according to one embodiment of the invention;

[0064] FIG. 2 shows mechanically modified lithium metal electrodes, wherein in FIG. 2a) a punch with cuboids in a distance of 1000 μm and in FIG. 2b) a punch with cuboids in a distance of 500 μm was used;

[0065] FIG. 3 shows in FIG. 3a) an SEM image of a structured and coated lithium electrode, and in FIG. 3b) an EDS analysis for carbon;

[0066] FIG. 4 shows cell structures of energy storage devices comprising a lithium electrode with a structured surface coated with a solid polymer electrolyte as well as non-liquid electrolytes according to various embodiments of the invention as Li/Li symmetrical cells;

[0067] FIG. 5 shows in FIG. 5a) and c) a Nyquist plot for a structured lithium electrode comprising a solid polymer electrolyte (“modified (56%)”, squares) and a reference cell comprising non-structured lithium electrodes (“untreated”, asterisks) and in FIGS. 5b) and d) the corresponding potential profiles measured at 20° C. or 60° C.;

[0068] FIG. 6 shows in FIG. 6a) a Nyquist plot for a structured lithium electrode comprising a gel-polymer electrolyte (“modified (56%)” squares) and a reference cell comprising non-structured lithium electrodes (“untreated”, asterisks) and in FIG. 6b) the corresponding potential profiles;

[0069] FIG. 7 shows in FIG. 7a) a Nyquist plot for a structured lithium electrode comprising a hybrid electrolyte (“modified (56%)” squares) and a reference cell comprising non-structured lithium electrodes (“untreated”, asterisks) and in FIG. 7b) the corresponding potential profiles;

[0070] FIG. 8 shows in FIG. 8a) results of the lithium deposition-dissolution experiment and in FIG. 8b) the time evolution of the complex cell resistance in a no-load condition for a symmetrical button cell assembly comprising structured lithium electrodes which were stored in a CO.sub.2 atmosphere and for reference cells;

[0071] FIG. 9 shows in FIG. 9a) results of the lithium deposition-dissolution experiment and in FIG. 9b) the time evolution of the complex cell resistance in a no-load condition for a symmetrical button cell assembly comprising structured lithium electrodes which were stored in FEC and for reference cells;

[0072] FIG. 10 shows in FIG. 10a) results of the lithium deposition-dissolution experiment and in FIG. 10b) the time evolution of the complex cell resistance in a no-load condition for a symmetrical button cell assembly comprising structured lithium electrodes stored in LiNO.sub.3 in DOL and for reference cells;

[0073] FIG. 11 shows in FIG. 11a) results of the lithium deposition-dissolution experiment and in FIG. 11b) the time evolution of the complex cell resistance in a no-load condition for a symmetrical button cell assembly comprising structured lithium electrodes stored in VC and for reference cells;

[0074] FIG. 12 shows cell structures of energy storage devices comprising a lithium electrode with a structured surface coated with a solid polymer electrolyte with or without an additional polymer electrolyte membrane as Li/Li symmetrical cells;

[0075] FIG. 13 shows a Nyquist plot for structured lithium electrodes comprising a solid polymer electrolyte with or without an additional polymer electrolyte membrane;

[0076] FIG. 14 shows a Nyquist plot of symmetrical cells comprising a solid polymer electrolyte for different electrode thicknesses and depths of recesses against unstructured references;

[0077] FIG. 15 shows a Nyquist plot of symmetrical cells comprising a solid polymer electrolyte for different electrode thicknesses and depths of the recesses;

[0078] FIG. 16 shows potential profiles at 60° C. and at a current density of 0.075 mA/cm.sup.2 for structured lithium electrodes comprising a solid polymer electrolyte and for non-structured reference cells; and

[0079] FIG. 17 shows in FIG. 17a) an SEM image of a recess of a depth of 150 μm in a 300 μm thick lithium metal electrode, and in FIG. 17b) an enlarged section.

[0080] FIG. 1 shows a schematic view of the process steps in the production of a structured, chemically modified metal electrode comprising a functional coating according to one embodiment of the invention. A metal film 2, for example lithium, is provided in a first step by means of a roller 4, for example by a roll-to-roll process, with a plurality of blind-hole-like recesses 6 spaced apart from each other. In a subsequent step, the surface structured in this way is subjected to a chemical modification 8, wherein by means of contact reactions of the lithium surface for example with carbon dioxide or 1-fluoroethylene carbonate a lithium ion-conducting lithium carbonate layer 10 is formed on the structured surface. In a subsequent step, a coating solution 14 comprising a polymer, a lithium salt and a crosslinking additive activatable by ultraviolet radiation (UV) in a solvent, for example MEEP, LiBOB and benzophenone in THF, is applied onto the structured lithium surface by means of spray coating 12. In a subsequent step, the solvent is removed by drying 16. The dried coating is then irradiated with UV light 18, whereby the polymer cross-links. As a result, the structured surface is coated with a solid polymer electrolyte 20, wherein the recesses 6 are filled with the solid polymer electrolyte 20.

EXAMPLE 1

[0081] Production of Structured Lithium Electrodes with a Functional Coating

[0082] 1.1 Structuring

[0083] A lithium film was structured by pressing a punch onto the lithium surface. Since lithium has a high reactivity with water, the process was carried out in a glovebox under argon atmosphere or in a drying room with dehydrated air. 19 mm wide and 500 μm thick lithium metal strips (Albemarle, thickness 500 μm, purity Battery Grade) were used for the manufacturing of the electrodes. These were machined with punches made of polyoxymethylene (POM), which were provided with regularly arranged small cuboids with the dimensions 300 μm×300 μm×300 μm. The punch was placed on the lithium strip so that the cuboids pointed in the direction of the lithium. A hydraulic press was then used to apply a pressure of 15 bar from above for four to five seconds so that the cuboids left impressions in the soft metal. Two punches were used which differed in their distance between the individual cuboids of 1000 μm (punch 1) and 500 μm (punch 2), so that depending on the punch used, the density of the defects on the lithium was different and thus the surface was enlarged differently. Circular electrodes with a diameter of 12 mm were then punched out of the modified lithium metal strips.

[0084] FIG. 2a) shows a lithium metal electrode which was mechanically structured with a punch with cuboids at a distance of 1000 μm (punch 1), FIG. 2b) shows a lithium metal electrode which was mechanically structured with a punch with cuboids at a distance of 500 μm (punch 2). Due to the structuring the entire surface area of the lithium electrode was enlarged by 20% at a distance of 1000 μm and by 56% at a distance of 500 μm relative to the plane surface.

[0085] 1.2 Coating

[0086] The structured lithium surface was then coated with a solid polymer electrolyte. For the coating, a solution of poly[bis((methoxyethoxy)ethoxy)-phosphazene] (MEEP), lithium bis(oxalato)borate (LiBOB) and the UV light-active crosslinking additive benzophenone in tetrahydrofuran (THF) was prepared in a weight ratio of 50:2:3. MEEP, LiBOB and benzophenone were dissolved in tetrahydrofuran (THF) for better homogenisation, which was evaporated under reduced pressure after stirring for one hour. The obtained yellowish, highly viscous solution was stored at 20° C. for further use.

[0087] For the drop coating process for coating the electrodes, 100 mg of the viscous non-crosslinked polymer mixture was dissolved in about 300 μl THF, and 60 μl per electrode was dropped evenly onto the lithium electrodes obtained in step 1.1 by use of an Eppendorf pipette. The electrodes were dried overnight in an oven at 65° C. so that the solvent evaporated, and the polymer layer on the structured lithium surface was then crosslinked for 18 minutes under UV light.

[0088] The coated lithium electrodes were examined by scanning electron microscopy (SEM, ZEISS Auriga® electron microscope) and EDX analysis (Oxford instruments). FIG. 3a) shows an SEM image of the lithium electrodes with a functional coating mechanically structured with a punch with cuboids at a distance of 500 μm (punch 2). As can be seen in FIG. 3a), the polymer electrolyte coating followed the structure of the lithium surface and covered the entire surface. The polymer layers obtained had a thickness of about 450 μm (±20 μm) including the depth of the recesses. Here, the layer thickness above the filled recesses was about 150 μm. This was also the case for the less structured surface. FIG. 3b) shows the EDX analysis for carbon. As can be seen from FIG. 3b), the carbon was distributed consistently and uniformly in the recesses and the layer, with the exception of voids caused by the embossing. The EDX analysis moreover showed that the elements oxygen and nitrogen as well as phosphorus from the MEEP polymer and boron from the LiBOB salt were respectively present consistently and evenly distributed.

[0089] This result shows that by means of the applied method a thin layer of about 150 μm of polymer electrolytes can be deposited on a structured lithium electrode so that the layer follows the surface pattern and thus wets the entire surface.

EXAMPLE 2

[0090] Production of Electrochemical Cells

[0091] Cells with three different types of electrolyte were produced: solid polymer electrolyte, gel polymer electrolyte and a hybrid electrolyte formed from a ceramic electrolyte coated on both sides with a gel polymer electrolyte. In all cases, the lithium structuring was carried out by means of the method described in Example 1.1 by use of punch 2 with a 500 μm spacing of the recesses, the coating with solid polymer electrolyte was carried out according to Example 1.2. As reference cells, cells of identical structure with non-structured lithium electrodes, which were identically coated, were respectively produced.

[0092] 2032 button cells were built for the electrochemical analysis. The cells were built as symmetrical lithium cells. To this end, two coated electrodes were respectively placed on top of each other with the coated side facing each other. In order to avoid a contact between the two electrodes and thus a possible short-circuit of the cell, a non-liquid electrolyte was respectively placed between the electrodes. Other components of the cell structure were spacer plates and a spring washer, which were disposed between the electrodes and the cell housing. These were intended to press the electrodes together sufficiently so that no contact problems between the coatings of the electrodes arised. Depending on the thickness of the combination of electrodes and electrolyte, spacers and springs were adapted.

[0093] 2.1 Preparation of a Solid Polymer Membrane

[0094] The non-crosslinked, highly viscous polymer mixture prepared in Example 1.2 was placed on a siliconized polyester film (Mylar®). A second piece of film was placed on top and the still liquid polymer mixture was spread to form a layer with a thickness of about 150 μm (±20 μm). For crosslinking, the polymer mixture in this form was exposed to UV light for 18 minutes. Subsequently, one of the films could be easily peeled off the crosslinked polymer membrane and small, round membranes of the desired size of 13 mm could be punched out by means of a hole punch. These could be handled with tweezers without any problems.

[0095] 2.2 Production of a Cell Comprising a Solid Polymer Electrolyte

[0096] For cells comprising solid polymer electrolytes, structured lithium electrodes and as a reference non-structured lithium electrodes were first coated according to the procedure described in Example 1.2. In the cells, an additional solid polymer electrolyte membrane made of a crosslinked mixture of MEEP polymer and LiBOB salt according to Example 2.1 was inserted between the lithium electrodes.

[0097] FIG. 4a) shows schematically the cell structure, wherein a structured lithium electrode 2 comprising a solid polymer electrolyte coating 20 was respectively used as anode and cathode and a solid polymer electrolyte membrane 22 was used as non-liquid electrolyte.

[0098] 2.3 Production of a Cell Comprising a Gel-Polymer Electrolyte

[0099] For cells comprising gel-polymer electrolytes, structured lithium electrodes and as a reference non-structured lithium electrodes were first coated according to the process described in Example 1.2. In the cells, an additional solid polymer electrolyte membrane made of a crosslinked mixture of MEEP polymer and LiBOB salt according to Example 2.1 was inserted between the lithium electrodes. In order to produce a gel polymer electrolyte therefrom, liquid electrolyte made from 0.7 M LiBOB in EC:DMC (1:1 wt.-%) was added to the solid polymer electrolyte in a mass ratio of 1:1 during cell construction. Wetting of the solid polymer with the liquid electrolyte caused gelation of the polymer and resulted in a leakage-free gel electrolyte. This is thus defined as non-liquid.

[0100] FIG. 4b) shows schematically the cell structure, wherein a structured lithium electrode 2 comprising a solid polymer electrolyte coating 20 was respectively used as anode and cathode, and a gel polymer electrolyte resulted from the application of a solid polymer electrolyte membrane 22 made of a crosslinked mixture of MEEP polymer and LiBOB salt and a liquid electrolyte 24.

[0101] 2.4 Production of a Cell Comprising a Hybrid Electrolyte

[0102] For the cells comprising a hybrid electrolyte, lithium electrodes were coated as described above. A polymer-coated solid electrolyte compact consisting of a 400 μm thick layer of LLZO material (Al-doped Li.sub.6.6La.sub.3Zr.sub.1.6Ta.sub.0.4O.sub.12, Jülich Research Centre) coated on both sides with an approximately 100 μm thick layer of the gel-polymer electrolyte was placed between the electrodes. The gel-polymer coated solid electrolyte was prepared analogous to the coating of the electrodes by drop-coating the LLZO material on both sides with the non-crosslinked polymer mixture of MEEP polymer, LiBOB salt and benzophenone dissolved in THE as a crosslinker and crosslinking under UV light after drying. In order to produce a gel polymer from the solid polymer layer on the LLZO material, liquid electrolyte consisting of 0.7 M LiBOB in a mixture of ethylene carbonate and dimethyl carbonate (EC:DMC, 1:1 wt.-%) was respectively added in a mass ratio of 1:1 to the total solid polymer electrolyte amount between the polymer solid electrolyte pellet and the lithium electrode during cell construction.

[0103] FIG. 4c) schematically shows the cell structure, wherein a structured lithium electrode 2 comprising a solid polymer electrolyte coating 20 was respectively used as anode and cathode. A pellet of LLZO material 26 coated on both sides with a solid polymer electrolyte membrane 22 made of a crosslinked mixture of MEEP polymer and LiBOB salt was impregnated with a liquid electrolyte 24, resulting in a double-sided coating with a gel polymer electrolyte.

EXAMPLE 3

[0104] Electrochemical Characterisation of a Cell Comprising a Solid Polymer Electrolyte

[0105] The cells comprising structured and coated lithium electrodes comprising a solid polymer electrolyte produced according to example 2.1 and the reference cell comprising non-structured lithium electrodes were compared with each other by means of impedance measurements and cycling experiments (lithium dissolution/deposition).

[0106] The electrochemical investigations were carried out in 2032 button cells. The cells were assembled in a glovebox (MBraun) filled with an inert gas atmosphere of argon. A constant current density of 0.01 mA/cm.sup.2 was applied during the cycling, wherein a lithium deposition occurred on the one electrode and a lithium dissolution on the other. The subsequent polarisation on the electrodes was measured as an overvoltage. The current direction was reversed after one hour. The measurement was made between −1.5 V and 1.5 V (termination criterion of the cell voltage) at 20° C. and 60° C. The results at 20° C. are shown in FIGS. 5a) and 5b), the results at 60° C. in FIGS. 5c) and 5d).

[0107] FIG. 5a) shows the Nyquist plot of the structured and coated lithium electrode comprising a solid polymer electrolyte (“modified (56%)” squares) and the reference cell comprising non-structured lithium electrodes (“untreated”, asterisks). As can be seen in FIG. 5a), the impedance measurement showed a significant reduction in resistance with the lithium structuring for the cells comprising the solid polymer electrolyte. Further tests using lithium electrodes the surface of which was formed with recesses at a distance of 1000 μm and thus had a surface area enlarged by 20%, also showed a significant reduction in resistance, which was slightly lower in comparison. This shows that the higher the structure density and thus the larger the lithium surface area, the more the resistance decreased. It is assumed that the larger surface area of the electrode facilitates the electrochemical reactions at the surface and the overvoltage reduces.

[0108] FIG. 5b) shows the potential profiles of the structured lithium electrode comprising a solid polymer electrolyte (“modified (56%)”) and the reference cell comprising non-structured lithium electrodes (“untreated”). The deposition/dissolution experiments also showed a reduction in the overvoltage due to the lithium structuring. Again, further comparative experiments with the less structured surface showed that the denser the structuring and thus the larger the surface area, the greater the effect.

[0109] FIGS. 5c) and d) show that the surface resistance and the overpotentials are lower than at 20° C., which means a better ionic conductivity.

EXAMPLE 4

[0110] Electrochemical Characterisation of a Cell Comprising a Gel-Polymer Electrolyte

[0111] The cell comprising structured and coated lithium electrodes and gel polymer electrolyte produced according to Example 2.2 and the reference cell comprising non-structured lithium electrodes were investigated by means of impedance measurements and cycling experiments (lithium dissolution/deposition) as described in Example 3, wherein the measurement was carried out at 20° C.

[0112] FIG. 6a) shows the Nyquist plot of the structured and coated lithium electrode comprising a gel polymer electrolyte (“modified (56%)” squares) and the reference cell comprising non-structured lithium electrodes (“untreated”, asterisks). As can be seen in FIG. 6a), a significant reduction in resistance was also achieved with the lithium structuring for the cells comprising the gel polymer electrolyte. Accordingly, a significant reduction in overvoltage was also obtained in the deposition/dissolution experiments shown in FIG. 6b).

EXAMPLE 5

[0113] Electrochemical Characterisation of a Cell Comprising a Hybrid Electrolyte

[0114] The cell comprising structured and coated lithium electrodes comprising a hybrid electrolyte consisting of LLZO solid electrolyte coated on both sides with a 100 μm thick layer of gel polymer electrolyte, produced according to Example 2.3, and the reference cell comprising non-structured lithium electrodes were investigated by means of impedance measurements and cycling experiments (lithium dissolution/deposition) as described in Example 3. The measurements were carried out at 60° C.

[0115] FIG. 7a) shows the Nyquist plot for the cell comprising a hybrid electrolyte and structured and coated lithium electrodes (“modified (56%)” squares) and the reference cell comprising non-structured lithium electrodes (“untreated”, asterisks). As can be seen in FIG. 7a), similar to the solid and gel-polymer electrolytes, a significant reduction of the interfacial resistance was also obtained for the cells comprising the hybrid electrolyte. Accordingly, a reduction in overvoltage was also achieved in the deposition/dissolution experiments shown in FIG. 7b).

EXAMPLE 6

[0116] Chemical Coating of the Structured Lithium Electrode by Treating with CO.sub.2 Gas

[0117] Furthermore, the effect of a chemical protective layer, a so-called “artificial SEI” on the structured lithium electrodes was investigated. These support the longevity of the batteries and potentially increase safety. For this purpose, lithium was mechanically processed as described in Example 1.1, wherein recesses at a distance of 500 μm and thus an enlargement of the surface area by 56% was used. The structured electrode was then stored in pure CO.sub.2 for three weeks. The electrode was then coated as described in Example 1.2.

[0118] For an electrochemical investigation of the effects of the formed carbonate protective layer, symmetrical button cells comprising a solid polymer electrolyte membrane were prepared from a crosslinked mixture of MEEP polymer and LiBOB salt between the lithium electrodes as described in Example 2.2, and impedance measurements and cycling experiments were performed. As reference cells, structured lithium electrodes stored for three weeks in a drying room under water exclusion were used.

[0119] Cyclings were performed at constant current densities of 0.01 mA/cm.sup.2, 0.025 mA/cm.sup.2 and 0.05 mA/cm.sup.2. The subsequent polarisation on the electrodes was measured as overvoltage. The current direction was reversed after one hour. The measurement was made in a range between −1.5 V and 1.5 V (termination criterion of the cell voltage) at 60° C. The results are shown in FIGS. 8a) and 8b).

[0120] FIG. 8a) shows the results of the lithium deposition-dissolution experiment and FIG. 8b) the time evolution of the complex cell resistance in a no-load condition for the symmetrical button cell assembly comprising structured lithium electrodes with 56% surface enlargement, functional coating with MEEP polymer and solid polymer membrane, for structured electrodes which were stored in a CO.sub.2 atmosphere and for reference cells comprising electrodes without enriched carbonate layer. The comparison shows that in the case of a treatment with CO.sub.2 gas, a reduced overvoltage occurred, as can be seen in FIG. 8a), and reduced surface resistances were found, as can be seen in FIG. 8b).

[0121] These results show that by a treatment with CO.sub.2 gas the overvoltages as well as the surface resistance were further reduced. Thus, the protective layer formed by contact with CO.sub.2 shows a further improvement.

EXAMPLE 7

[0122] Chemical Coating of the Structured Lithium Electrode by Layer Formation Additives

[0123] Furthermore, the effect of a chemical protective layer formed from the known additives vinylene carbonate (VC) and 1-fluoroethylene carbonate (FEC) as well as lithium nitrate was investigated. To this end, structured lithium electrodes were stored for two days in vinylene carbonate, 1-fluoroethylene carbonate or 10 wt.-% lithium nitrate dissolved in 1,3-dioxolane. For reference cells, structured lithium electrodes were stored for two days in a drying room under exclusion of water. Subsequently, symmetrical button cells comprising a solid polymer electrolyte membrane were prepared as described in Example 6 and impedance measurements and cycling experiments were carried out.

[0124] FIG. 9a) shows the results of the lithium deposition-dissolution experiment and FIG. 9b) the time evolution of the complex cell resistance in a no-load condition for the symmetrical button cell assembly comprising structured lithium electrodes with 56% surface enlargement, functional coating with MEEP polymer and solid polymer membrane which were stored in FEC and for the reference cell comprising electrodes without FEC layer. The comparison shows that in the case of the FEC coating, unchanged overvoltages occurred, as can be seen in FIG. 9a), but reduced surface resistances could be found, as can be seen in FIG. 9b). Although the overvoltages for FEC storage were identical to those for the reference electrode, the effect is most noticeable in the surface tension. After 24 h, this was half that of the mechanically structured cell without FEC coating. This can be explained by a layer with the characteristic of an increased lithium transport number.

[0125] FIG. 10a) shows the results of the lithium deposition-dissolution experiment and FIG. 10b) the time evolution of the complex cell resistance in a no-load condition for the symmetrical button cell assembly comprising structured lithium electrodes stored in LiNO.sub.3:DOL and for the reference cell. The comparison shows that, as can be seen in FIG. 10a), slightly reduced overvoltages occurred in the case of the LiNO.sub.3:DOL storage and reduced surface resistances could be found, as can be seen in FIG. 10b).

[0126] The combination of LiNO.sub.3 and 1,3-dioxolane thus also shows an improved behaviour compared to the non-chemically modified structured electrode. In particular, there was a difference in the case of the surface resistance, which was already constant after the construction of the cell. This indicates an already well passivated lithium surface.

[0127] In contrast, as is shown in FIGS. 11a) and 11b), the cell exhibited increased overvoltages and increased surface resistances after storage of the lithium electrodes in vinylene carbonate. Vinylene carbonate thus produced a layer on lithium that exhibited an increased resistance. This can be explained by the formation of a thicker layer or a layer of more polymeric components. Vinylene carbonate can thus be classified as less advantageous in terms of resistance and overvoltages compared to the other substances, but may be favoured in the case of a flexible protective layer.

[0128] Altogether, by the combination with a chemical coating of the structured, in particular microstructured, electrode a lithium surface with further modified properties can thus be obtained. Among the examples shown, CO.sub.2 could be identified as the best chemical modification.

EXAMPLE 8

[0129] Comparison of Cells Comprising a Solid Polymer Electrolyte of Different Thicknesses

[0130] A cell comprising a solid polymer electrolyte was prepared as described in Example 2.2 by coating structured lithium electrodes by use of a stamp 2 (cuboid 300 μm×300 μm×300 μm, 500 μm spacing, resulting in 56% enlargement in surface area) according to the procedure described in Example 1.2 and inserting a solid polymer electrolyte membrane made of a crosslinked mixture of MEEP polymer and LiBOB salt according to Example 2.1 between the electrodes.

[0131] Another cell with a smaller amount of solid polymer electrolyte was prepared by omitting the use of the additional polymer electrolyte membrane between the electrodes.

[0132] Thus, the thickness of the polymer electrolyte resulted exclusively from the two electrode coatings (drop coating).

[0133] FIG. 12a) schematically shows the cell structure 1, which corresponds to that shown in FIG. 4a), wherein respectively a structured lithium electrode 2 comprising a solid polymer electrolyte coating 20 was used as anode and cathode and a solid polymer electrolyte membrane 22 was used as non-liquid electrolyte. FIG. 12b) shows a corresponding cell structure II without an additional solid polymer electrolyte membrane.

[0134] The electrochemical tests were carried out in 2032 button cells as described in example 3. FIG. 13 shows the Nyquist plot for the structured lithium electrodes comprising a solid polymer electrolyte in the form of a membrane in addition to the electrode coating (“membrane+drop coating”, asterisks, scheme 1) as well as for the cell structure only comprising an electrode coating (“drop coating”, squares, scheme 1l) measured at 60° C. As can be seen in FIG. 13, the lower amount of electrolyte resulted in a reduction of the charge transfer resistance.

[0135] This shows that also embodiments in which the solid polymer electrolyte is formed between the structured electrodes only by combining the two thin layers on the structured lithium electrode show good results.

EXAMPLE 9

[0136] Comparison of Different Depths of the Recesses in the Lithium Electrodes

[0137] The change in the depth of the recesses in the lithium electrodes and the associated change in the surface enlargement of the metal electrodes was investigated. Here, the electrode material, which was modified by structurings, was also reduced in the form of thinner lithium metal electrodes. To this end, the originally 500 μm thick lithium film (Albemarle) was rolled out to 300 μm and 150 μm, respectively, in a press process (roll pressing).

[0138] For the mechanical modification of the lithium electrodes, new block press punches with adapted dimensions were used. While the punch 2 with the block dimensions of 300 μm×300 μm×300 μm and a block spacing of 500 μm was used for the 500 μm thick lithium film, a punch 3 with the block dimensions of 150 μm (height)×300 μm×300 μm and a punch 4 with the block dimensions of 75 μm (height)×300 μm×300 μm were used for the 300 μm thick lithium film. The following table 1 summarizes the parameters of the structuring:

TABLE-US-00001 TABLE 1 Dimensions of the punches, corresponding surface enlargements and thickness of the lithium film Punch 2 Punch 3 Punch 4 Block hight 300 μm 150 μm  75 μm Block spacing 500 μm 500 μm 500 μm Surface enlargement 56% 28% 14% Lithium thickness 500 μm 300 μm 150 μm

[0139] The correspondingly manufactured cells comprising structured and coated lithium electrodes comprising a solid polymer electrolyte and a reference cell comprising a non-structured lithium electrode of corresponding thickness were compared with each other by means of impedance measurements as described in example 3 at 60° C.

[0140] The Nyquist plot in FIG. 14 shows the symmetrical cells comprising a solid polymer electrolyte for an electrode thickness of 500 μm and recesses of a depth of 300 μm in FIG. 14a), 300 μm and a depth of 150 μm in FIG. 14b) and 150 μm and a depth of 75 μm in FIG. 14c) respectively against the unstructured reference cell. The Nyquist plots shown in FIG. 14 show a reduction in resistance for the structured cells (squares) compared to the unstructured reference cells for each of the structurings. However, this effect becomes weaker the less deep the recess was, since in this case the enlargement in surface area was smaller. This is also shown by the direct comparison of the different modified electrodes in FIG. 15. The 150 μm thick electrode structured with 75 μm deep recesses and 14% enlarged surface area exhibited a significantly higher resistance than the other two, while the 500 μm thick electrode with 300 μm deep recesses exhibited the lowest resistance value.

[0141] Furthermore, cycling experiments (electrochemical lithium dissolution/deposition) were performed with symmetrical cells comprising modified and unmodified lithium metal electrodes. FIG. 16 shows the potential profiles at 60° C. and a current density of 0.075 mA/cm.sup.2 for the respective structured lithium electrodes comprising a solid polymer electrolyte (“modified”, solid line) and the respective reference cell comprising unstructured lithium electrodes (“untreated”, dashed line) for an electrode thickness of 500 μm and recesses of a depth of 300 μm in FIG. 16a), 300 μm and a depth of 150 μm in FIG. 16b) and 150 μm and a depth of 75 μm in FIG. 16c) over a period of 100 hours. FIGS. 16d), e) and f) respectively show the overvoltage for the first 20 hours for the electrode thickness of 500 μm and recesses of a depth of 300 μm, 300 μm and a depth of 150 μm and 150 μm and a depth of 75 μm.

[0142] As can be seen in FIG. 16, the cells comprising unmodified electrodes partially directly reached the specified overvoltage limit of 1.5 V at a current density of 0.075 mA/cm.sup.2 and a temperature of 60° C., irrespective of their thickness, which immediately resulted in the termination of the charging or discharging process. In comparison, the modified electrodes showed a constantly lower overpotential of a maximum of approx. 0.6 V for the electrodes with 300 μm lithium and 300 μm recesses and approx. 0.3 V for the 150 μm lithium film with 75 μm recesses over the first 50 cycles. Thus, the mechanical modification also seems to increase the cycling stability and the lifetime of the cell.

[0143] The cycled electrodes were then analyzed by use of a scanning electron microscope. FIG. 17a) shows the cross-section through the recess of a depth of 150 μm in a 300 μm thick lithium metal electrode. Here, the polymer-coated side faces upwards. FIG. 17b) shows an enlarged detail. FIG. 17a) shows that the recess has been filled very well with the polymer. Thus, the drop coating process can also be applied to thinner lithium electrodes. Above the recess, moreover a bulge can be seen in the polymer. It is assumed that the polymer was pressed there upwards by lithium deposited in the recess. Overall, it can be seen that the lithium has been deposited uniformly predominantly in the recess. In the enlarged view of FIG. 17b) it can be seen that the lithium was nevertheless not only deposited homogeneously in the recess. Lithium deposits with a higher surface area, which protrude into the polymer, were also formed on the wall and bottom of the recess.

[0144] These results show that the amount of polymer electrolyte applied by drop coating alone was sufficient to wet the entire electrode. An additional membrane can extend the lifetime of the cell by acting as a barrier, but at the same time can increase the electrolyte resistance in the cell. Furthermore, it could be shown that the drop-coating and block-press process can also be applied to thinner lithium electrodes. Recesses of 75 μm, 150 μm and 300 μm showed good wetting by the polymer and by means of the modification impedances and overpotentials could be reduced. The polymer electrolyte ensured a largely uniform deposition of the lithium.

[0145] The invention on which this patent application is based was developed in a project supported by the BMBF under the promotional references 03XP0084A and 03XP0084C.