Electrochemical Cell and Electrochemical System

Abstract

In an embodiment an electrochemical cell includes a first electrode having a first surface area A1, a second electrode having a second surface area A2, an electrolyte arranged between the first electrode and the second electrode, wherein the electrochemical cell is configured to provide a first electrochemical half-cell reaction at the first electrode and provide a second electrochemical half-cell reaction at the second electrode, and wherein a surface area ratio A1/A2 is larger than a stoichiometric ratio of the first half-cell reaction and the second half-cell reaction.

Claims

1.-18. (canceled)

19. An electrochemical cell comprising: a first electrode having a first surface area A1; a second electrode having a second surface area A2; an electrolyte arranged between the first electrode and the second electrode, wherein the electrochemical cell is configured to: provide a first electrochemical half-cell reaction at the first electrode, and provide a second electrochemical half-cell reaction at the second electrode, and wherein a surface area ratio A1/A2 is larger than a stoichiometric ratio of the first half-cell reaction and the second half-cell reaction.

20. The electrochemical cell according to claim 19, wherein the electrochemical cell is configured to provide the first electrochemical half-cell reaction with slower reaction kinetics than the second electrochemical half-cell reaction.

21. The electrochemical cell according to claim 20, wherein the larger, over-stoichiometric first surface area A1 is configured to compensate for the slower reaction kinetics.

22. The electrochemical cell according to claim 19, wherein the electrochemical cell is configured to provide a first theoretical maximum specific current density j1 of the first half-cell reaction that is smaller than a second theoretical maximum specific current density j2 of the second half-cell reaction.

23. The electrochemical cell according to claim 22, wherein the surface area ratio A1/A2 equals a theoretical maximum specific current density ratio j2/j1.

24. The electrochemical cell according to claim 19, wherein the electrochemical cell is configured to provide a theoretical maximum specific rated capacity C1 of the first half-cell reaction that is smaller than a theoretical maximum specific rated capacity C2 of the second half-cell reaction.

25. The electrochemical cell according to claim 24, wherein the surface area ratio A1/A2 equals a theoretical maximum specific rated capacity ratio C2/C1.

26. The electrochemical cell according to claim 19, wherein the first electrode comprises a first red/ox active compound configured to participate in the first electrochemical half-cell reaction, wherein the second electrode comprises a second red/ox active compound configured to participate in the second electrochemical half-cell reaction, wherein a normalized concentration of the first red/ox active compound in the first electrode equals the normalized concentration of the second red/ox active compound in the second electrode, and wherein the normalized concentration of a red/ox active compound in an electrode is a molar concentration of the red/ox active compound in an associated electrode normalized to a number of electrons exchanged in an associated half-cell reaction.

27. The electrochemical cell according to claim 19, wherein the first electrode has the same surface morphology as the second electrode.

28. The electrochemical cell according to claim 19, wherein the first electrode has the same thickness as the second electrode.

29. The electrochemical cell according to claim 26, wherein the first electrode consists of a first sub-electrode and a second sub-electrode, wherein the second electrode, the first sub-electrode and the second sub-electrode have a flat shape and are assembled in parallel with regard to an electrode plane, wherein the second electrode is arranged in a height different to the first sub-electrode, and wherein the second sub-electrode is arranged on the same height next to the second electrode.

30. The electrochemical cell according to claim 26, wherein the first red/ox active compound and the second red/ox active compound are identical.

31. The electrochemical cell according to claim 19, wherein the electrochemical cell is an all-solid-state electrochemical cell.

32. The electrochemical cell according to claim 19, wherein the first electrode and the second electrode are lithium vanadium phosphate electrodes on a charge collector material, wherein the electrolyte is a Li-conducting solid electrolyte, wherein the first electrode is an anode comprising Li4V2(PO4)3, oxidizable in the first half-cell reaction, and wherein the second electrode is an cathode comprising Li2V2(PO4)3, reduceable in the second half-cell reaction.

33. The electrochemical cell according to claim 32, wherein the first surface area A1 is twice the second surface area A2.

34. A electrochemical system comprising: a plurality of electrochemical cells, each being the electrochemical cell according to claim 19, wherein the electrochemical cells are stacked.

35. The electrochemical system according to claim 34, wherein the electrochemical cells are stacked with the same orientation, and wherein the electrolyte is arranged between two neighboring electrochemical cells of the same orientation.

36. A method for manufacturing an electrochemical system, wherein the electrochemical system comprises multiple first electrodes each having a first surface area A1, consisting of a first sub-electrode and a second sub-electrode, and the same number of second electrodes as the first electrodes, wherein each second electrode has a second surface area A2, wherein each first sub-electrode, each second sub-electrode and each second electrode comprises an electrochemically active layer of an electrochemically active material and a charge collector layer, wherein the multiple first and second electrodes are embedded into a solid electrolyte, wherein the charge collector layers of all first sub-electrodes and of all second sub-electrodes are in electrical contact with a first external electrode on a surface of the electrochemical system, and the charge collector layers of all second electrodes are in electrical contact with a second external electrode on a surface of the electrochemical system opposite to the first external electrode, wherein a first electrochemical half-cell reaction is able to take place at the first electrodes, and a second electrochemical half-cell reaction is able to take place at the second electrodes, and wherein a surface area ratio A1/A2 is larger than a stoichiometric ratio of the first and the second half-cell reaction, the method comprising: providing a ceramic electrolyte slurry from a ceramic electrolyte powder, an organic solvent, a binder, a dispersive agent and a plasticizer; forming of a preliminary solid electrolyte tape from the ceramic electrolyte slurry; forming a preliminary electrode layer comprising a preliminary charge collector layer and a preliminary electrochemically active layer on the preliminary solid electrolyte tape; cutting of the preliminary solid electrolyte tape with the preliminary electrode layer into sheets; forming a sheet stack from multiple sheets and by arranging an solid electrolyte sheet without the preliminary electrode layer on a top and a bottom of the sheet stack; cutting green chips from the sheet stack; removing the binder by heating; sintering the green chips; and forming the first external electrode and the second external electrode on opposing surfaces of a chip.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0072] In the following the invention is explained in greater detail on the basis of exemplary embodiments and the associated figures. The figures serve solely to elucidate the invention and are therefore only illustrated schematically and not in a manner true to scale. Individual parts may be illustrated in an enlarged manner or in a distorted manner in terms of their dimensions. Therefore neither absolute nor relative dimensions nor specifications can be inferred from the figures. Identical or identically acting parts are provided with identical reference signs.

[0073] FIG. 1 shows a first embodiment of an electrochemical cell in schematic cross-section;

[0074] FIG. 2 shows a second embodiment of an electrochemical cell in schematic cross-section;

[0075] FIG. 3 shows a third embodiment of an electrochemical cell in schematic cross-section;

[0076] FIG. 4 shows a fourth embodiment of an electrochemical cell in schematic cross-section;

[0077] FIG. 5 shows a fifth embodiment of an electrochemical cell in three orthogonal schematic cross-sections in FIGS. 5A and, B and a top view in FIG. 5C, wherein the electrolyte is omitted; and

[0078] FIG. 6 shows an embodiment of an electrochemical system.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0079] A first embodiment of an electrochemical cell 1 is displayed in a schematic cross-sectional view in FIG. 1. The electrochemical cell 1 comprises a first electrode 2 and a second electrode 3 facing each other and an electrolyte 4 arranged between both electrodes.

[0080] The shape of the electrochemical cell is not restricted. It can be for example cylindrical, or, preferentially, block shaped.

[0081] A first electrochemical half-cell reaction takes place at the first electrode 2 and a second electrochemical half-cell reaction takes at the second electrode 3.

[0082] Both electrodes 2 and 3 are plane electrodes.

[0083] The shape of the electrodes 2 and 3 is not limited in particular. For example, the electrodes may be of a circular shape, may be square-shaped or be of a rectangular shape.

[0084] Both electrode 2 and 3 comprise a charge collector layer 21 and 31 respectively. The charge collector layers 21 and 31 consist of a charge collector material which can be any suitable conductive material, for example Al, Cu, Pt, or preferentially copper.

[0085] Both electrodes 2 and 3 comprise an electrochemical active layer 22 and 32 of the first electrode 2 and the second electrode 3, respectively.

[0086] The electrochemically active layers 22 and 32 each comprise an electrochemically active material, of which they, preferentially, solely consist of.

[0087] The electrochemically active layers 22 and 32 are formed preferably as coating layers on the charge collector layers 21 and 31, respectively. In the present embodiment the electrochemically active layers 22 and 32 cover only one side of the charge collector layers 21 and 31. Due to this and due to the arrangement on the rim of the electrochemical cell 1, the charge collector layers 21 and 31 may be also employed as external electrodes in the present embodiment.

[0088] The electrochemically active materials of each of the electrochemically active layers 22 and 32 can be any material suitable of participating in an electrochemical reaction.

[0089] For example in the case of a wet electrochemical cell 1, the electrochemically active materials may be electro catalysts promoting the electrochemical reaction of the red/ox active compounds dissolved in a liquid electrolyte 4.

[0090] However, preferentially the electrochemical cell 1 is an all-solid-state electrochemical cell, with a solid electrolyte 4 and solid electrochemically active materials at the first and the second electrode 2 and 3.

[0091] In this case it is preferred that the red/ox active compounds are embedded in the electrochemically active layers 22 and 32. The degree of loading with the red/ox active compound in the electrochemically active layers 22 and 32 is identically.

[0092] A preferred example for such an all-solid-state electrochemical cell 1 is a lithium-vanadium-phosphate battery cell, with a Li-conducting solid electrolyte, such as Li.sub.1.3Al.sub.0.3Ti.sub.1.7(PO.sub.4).sub.3. There, the electrochemically active layers 22 and 32 consist of Li.sub.3V.sub.2(PO.sub.4).sub.3 as electrochemically active material. In charged state, Li.sub.4V.sub.2(PO.sub.4).sub.3 is present in the electrochemically active material of the first electrode 2, which is the anode of the discharge reaction and Li.sub.2V.sub.2(PO.sub.4).sub.3 is present in the electrochemically active material of the second electrode 3, which is the cathode of the discharge reaction.

[0093] The discharge reaction at the anode (first half-cell reaction) is represented by:

Li.sub.4V.sub.2(PO.sub.4).sub.3.fwdarw.Li.sub.3V.sub.2(PO.sub.4).sub.3+Li.sup.++e.sup.−

[0094] The discharge reaction at the cathode (second half-cell reaction) is represented by:

Li.sub.2V.sub.2(PO.sub.4).sub.3+Li.sup.++e.sup.−.fwdarw.Li.sub.3V.sub.2(PO.sub.4).sub.3

[0095] The electrochemical sum-reaction, which is a comproportionation reaction, is thus represented by:

Li.sub.4V.sub.2(PO.sub.4).sub.3+Li.sub.2V.sub.2(PO.sub.4).sub.3.fwdarw.2 Li.sub.3V.sub.2(PO.sub.4).sub.3

[0096] As is depicted in FIG. 1, the surface area A.sub.1 of the first electrode is larger than the surface area A.sub.2 of the second electrode. In particular, as depicted the surface area ratio A.sub.1/A.sub.2 can be 2/1 if both electrodes are formed, for example as stripes, with the same width.

[0097] This means for the case of a lithium-vanadium-phosphate battery cell as described above that the surface area surface area ratio A.sub.1/A.sub.2 exceeds the stoichiometric ratio of the electrodes which would be a 1:1 ratio, which corresponds to the conventional symmetric battery setup.

[0098] By having a surface area ratio of A.sub.1/A.sub.2=2/1, a theoretical maximum specific current density ratio j.sub.2/j.sub.1 of 2/1 can be fully compensated.

[0099] Alternatively a theoretical maximum specific rated capacity ratio C.sub.2/C.sub.1 of 2/1 can be fully compensated.

[0100] Generally for electrochemical systems the ratios j.sub.2/j.sub.1 and C.sub.2/C.sub.1 are not necessarily identical, though they often have the same tendency.

[0101] Often, if j.sub.2>j.sub.1 then also C.sub.2>C.sub.1. This means by fully compensating for either one of j.sub.2/j.sub.1 or C.sub.2/C.sub.1, the other preferentially may also become at least partly compensated.

[0102] This, in particular, may be found in the case of the above described type of lithium-vanadium-phosphate battery cell to have either a theoretical maximum specific current density ratio j.sub.2/j.sub.1 of 2/1 or a maximum specific rated capacity ratio C.sub.2/C.sub.1 of 2/1, depending on the parameter details for the cell and the discharge conditions.

[0103] This is due to the rate limiting step being the diffusion of Li.sup.+ from the anode into the solid electrolyte 4 during the discharge reaction.

[0104] However, in many cases j.sub.2/j.sub.1 at least roughly equals C.sub.2/C.sub.1 for lithium-vanadium-phosphate battery cells of the above type.

[0105] As depicted in FIG. 1 both electrodes 2 and 3 have the thickness and the same surface morphology, meaning that the surface morphology of electrochemically active layers 22 and 32 facing the electrolyte is mainly identical. Preferentially both electrodes have no structuring on the nanometer scale and can be considered as merely flat.

[0106] FIG. 2 shows a second embodiment of an electrochemical cell 1 in schematic cross section.

[0107] This second embodiment can be identical to the first embodiment as discussed above, except or the following details.

[0108] The first electrode 2 and the second electrode 3 are assembled not at the rim of the electrochemical cell 1, but are fully embedded in the electrolyte, and are thus internal electrodes.

[0109] Consequently, the charge conductor layers 21 and 31 of the first and the second internal electrode 2 and 3 is covered on both sides by the electrochemically active layers 22 and 32, respectively.

[0110] Thus the electrochemically active area of the electrodes can be increased as compared to a case of electrodes with same size, of which only one side is coated.

[0111] As the internal electrodes 2 and 3 are basically flat, the surface area is basically unaffected by the coating of the edges of the charge collector layers 21 and 31 by the electrochemically active layers 22 and 32, respectively.

[0112] To electrically contact the electrodes a first external electrode 5 leads to the charge collector layer 21 of the first internal electrode 2 and a second external electrode 6 leads to the charge collector layer 31 of the second internal electrode 3.

[0113] The external electrodes 5 and 6 can be of any suitable conductive material and are insulated against the electrolyte 4.

[0114] FIG. 3 shows a third embodiment of an electrochemical cell 1 in schematic cross section.

[0115] This third embodiment of an electrochemical cell 1 is block-shaped and is a lithium-vanadium-phosphate battery, similar to the one described with regard to the first exemplary embodiment, except for the following details.

[0116] A first external electrode 5 and a second external electrode 6 are arranged on opposing sides of the electrochemical cell 1. Preferably, the external electrodes 5 and 6 fully cover the sides of the electrochemical cell 1 they are arranged on.

[0117] The external electrodes 5 and 6 can be of any suitable electrically conducting material, for example they are Cr/Ni/Ag triple layers.

[0118] From the side of the first external electrode 5 the first internal electrode 2 extends into the Li-ion conducting solid electrolyte 4.

[0119] The second internal electrode 3 extends from the side of the second external electrode 6 into the electrolyte 4.

[0120] Both internal electrodes 2 and 3 are flat rectangular platelets of the same thickness, which face each other.

[0121] The charge collector layers 21 and 31 are in electrical contact with the external electrodes 5 and 6, respectively.

[0122] Forming the interface to the electrolyte, the charge collector layers 21 and 31 are entirely coated by the electrochemically active layers 22 and 32, respectively.

[0123] To compensate for the slower reaction kinetics of the first half-cell reaction at the first electrode compared to the second half cell reaction at the second electrode, the first electrode 1 area A.sub.1 of the first electrode is larger than the second electrode area A.sub.2 of the second electrode 3.

[0124] In particular, the area ratio A.sub.1/A.sub.2 is chosen to be 2/1, to compensate for the theoretical maximum specific current density j.sub.2/j.sub.1 or the theoretical maximum specific rated capacity ratio C.sub.2/C.sub.1, of which at least one, preferably is 2/1.

[0125] In the present embodiment the width of the internal electrodes 2 and 3 is identical. The internal electrodes 2 and 3 however differ in length, to generate the difference in surface area.

[0126] The third exemplary embodiment can be formed by any suitable process, but can be preferably formed by a conventional multilayer ceramic process.

[0127] To this end first a ceramic a homogeneous slurry is prepared from an electrolyte powder which is mixed with an organic solvent, a binder, a dispersive agent, and a plasticizer. The slurry is casted on a carrier tape to form a uniformly thick preliminary solid electrolyte tape.

[0128] Onto this preliminary solid electrolyte tape preliminary electrode layers are printed layer by layer, i.e. first a lithium-vanadium-phosphate layer, as a lower part of a preliminary electrochemically active layer, then a preliminary charge collector layer from a metal paste and again a lithium vanadium phosphate layer as an upper part of a preliminary electrochemically active layer.

[0129] The layers are cut and assembled such that the first and the second electrode are formed with the appropriate arrangement. At the top and the bottom unprinted preliminary electrolyte sheets are arranged.

[0130] Green chips are cut from the as described stacks. They undergo a debinding treatment and a subsequent sintering procedure. Both are carried out under reduced atmosphere or inert atmosphere to avoid oxidation.

[0131] Finally the external electrodes are formed on the side surfaces of the chips, for example by sputtering deposition of the Cr/Ni/Ag triple layers.

[0132] FIG. 4 shows a fourth embodiment of an electrochemical cell 1 in schematic cross section.

[0133] The fourth embodiment is identical to the third embodiment except for the arrangement of the internal electrodes 2 and 3.

[0134] The second electrode 3 is basically identical to the second electrode 3 of the third embodiment.

[0135] However, the first electrode 2 consists of a first sub-electrode 210 and a second sub-electrode 220.

[0136] Each of the first and the second sub electrode have a similar structure as the first electrode 2 of the third embodiment. Both extend from the side of the first external electrode 5 into the electrolyte and comprise each a charge collector layer 211 and 221, which is covered by an electrochemically active layer 212 and 222, respectively. All of the second electrode 3, the first sub electrode 210 and the second sub electrode 220 have the same thickness, surface morphology and loading with the redox active compound.

[0137] The second sub electrode 220 is the smaller of the sub-electrodes and is arranged on the same level in the electrochemical cell 1 as the second electrode 3. Both have the same orientation and face towards the first sub-electrode 210 which is arranged below in the electrochemical cell 1.

[0138] The electrode area of the first and the second sub-electrode together form the first electrode area A.sub.1 of the second electrode 2, which underlies the same conditions in the third exemplary embodiment.

[0139] Preferentially, the internal electrodes 2 and 3 (or 210, 220 and 3) are arranged such within the electrochemical cell 1 that the cross sectional cutting plane of FIG. 4 forms a mirror plane, to which they are symmetric.

[0140] Having a second sub-electrode arranged on the same height as the second electrode allows for a denser packing of the electrodes in the electrochemical cell as compared to the third embodiment. In particular, by this stacking of electrodes the same packing density with electrodes and with the electrochemically active material as in a conventional symmetric cell can be achieved.

[0141] The fourth embodiment can be formed analogously to the third embodiment.

[0142] FIG. 5 shows a fifth embodiment of an electrochemical cell 1 in different schematic cross sectional views.

[0143] The fifth embodiment is identical to the fourth embodiment, except for the following details. In particular, the arrangement of the internal electrodes 2 and 3 (or 210, 220 and 3) differs as compared to the third embodiment.

[0144] FIG. 5c shows a schematic top view on the electrochemical cell, in which the electrolyte 4 is omitted in this view in FIG. 5c to show the other internal electrodes.

[0145] The second electrode 3 expands from the second external electrode 6 into the electrolyte 4, as can be also seen in the cross sectional view of FIG. 5b.

[0146] Both the first sub-electrode 210 and the second sub-electrode 220, which together form the first electrode 2 extend from the side of the first external electrode into the electrolyte.

[0147] As can be seen in FIGS. 5a-5c, the second electrode 3, the first sub-electrode 210 and the second sub-electrode 220 each have the same length, but differ in width.

[0148] The second sub-electrode 220 is arranged on the same level next to the second electrode 2. Both face the first sub-electrode 210 arranged below.

[0149] The advantages of this arrangement are mainly the same as for the fourth embodiment. However, this fifth arrangement allows for a slightly increased overlap between the second electrode and the first sub-electrode, which makes ion transport in the electrolyte more efficient.

[0150] The fifth embodiment can be formed analogously to the third embodiment.

[0151] By arranging electrodes as according to the third, fourth, or fifth embodiment the maximum current or the capacity which can be received may be up to 33% higher as in the case of a symmetrically designed conventional all-solid-state battery with same combined surface area of first and second internal electrodes.

[0152] FIG. 6 shows an embodiment of an electrochemical system according to the present invention.

[0153] This embodiment consists of four electrochemical cells 1 according to the fourth embodiment stacked one upon another with the same orientation to form the electrochemical system.

[0154] Thus, the electrochemical system is an all-solid-state multilayer Li-ion battery.

[0155] One of the electrochemical cells 1 is marked in FIG. 6 by dashed lines depicting upper and lower interface to the respective neighboring cell.

[0156] Between the stacked electrochemical cells 1 additional electrolyte 4 is arranged so that the distance of the first sub-electrodes 210 to all directly neighboring second electrodes 3 (and to all second sub-electrodes 220 on the same level) is identical.

[0157] Facing electrodes of opposing charge on both sides of one electrode increases the capacity of the system as compared to the sum of the individual electrochemical cells by optimizing the ion-transport distances through the electrolyte.

[0158] The external electrodes are formed on the entire surface of two opposing side surfaces of the electrochemical system.

[0159] By the same principle electrochemical systems of any desired size can be constructed by stacking of the necessary number of electrochemical cells 1.

[0160] Of course, instead of forming an electrochemical system from electrochemical cells of the fourth embodiment, also electrochemical cells of the third or the fifth embodiment can be stacked analogously.

[0161] The electrochemical system can be formed by a modified procedure similar to the one described for the third embodiment of an electrochemical cell.

[0162] First, a preliminary solid electrolyte tape is formed, on which a preliminary electrode layer of a certain pattern is formed. Therefore, first lower part of the preliminary electrochemically active layer is screen-printed. Subsequently a preliminary charge collector layer is screen printed, an upper part of the preliminary electrochemically active layer is screen-printed. Thus the preliminary charge collector layer is embedded in the preliminary electrochemically active layer. Then the as printed tape may be cut into sheets. From this a sheet stack can be formed by stacking the sheets one upon another, and arranging a sheet of unprinted preliminary electrolyte tape on top and bottom.

[0163] Subsequently, after an isostatic hot press procedure green chips can be cut from the stack. The green chips then undergo a treatment to remove the binder, preferably by heating to 700° C. and under protective gas, to prevent oxidation. Subsequently the green chips are sintered at elevated temperature, for example at 850° C., under reduced atmosphere. On two opposing surface of the sintered chips, a first and as second external electrode are formed, for example by sputter-deposition of metallic layers such as Cr/Ni/Ag triple layers.

[0164] Although the invention has been illustrated and described in detail by means of the preferred embodiment examples, the present invention is not restricted by the disclosed examples and other variations may be derived by the skilled person without exceeding the scope of protection of the invention.