Electrode design

• US 7,227,737 B2
• Filed: 10/27/2004
• Issued: 06/05/2007
• Est. Priority Date: 04/02/2004
• Status: Active Grant

• Current Assignee: Tesla Inc.
• Original Assignee: Maxwell Technologies Incorporated (Tesla Inc.)
• Inventors: Mitchell, Porter, Xi, Xiaomei, Zhong, Linda, Zou, Bin
• Primary Examiner(s): Thomas; Eric W.
• Application Number:
  US10/974,091
• Publication Number:
  US 20060148112A1
• Time in Patent Office: 951 Days
• Field of Search: 361/502, 361/503
• US Class Current: 361/502
• CPC Class Codes: H01G 11/26 characterised by their structure, e.g. multi-layered, porosity or surface features
• CPC Class Codes: H01G 11/32 Carbon-based
• CPC Class Codes: H01M 10/0431 Cells with wound or folded electrodes H01M10/045 takes precedence
• CPC Class Codes: H01M 10/052 Li-accumulators
• CPC Class Codes: H01M 10/0564 the electrolyte being constituted of organic materials only
• CPC Class Codes: H01M 2300/0025 Organic electrolyte
• CPC Class Codes: H01M 4/02 Electrodes composed of, or comprising, active material
• CPC Class Codes: H01M 4/0404 by coating on electrode collectors
• CPC Class Codes: H01M 4/0409 by a doctor blade method, slip-casting or roller coating
• CPC Class Codes: H01M 4/0435 Rolling or calendering
• CPC Class Codes: H01M 4/50 of manganese
• CPC Class Codes: H01M 4/58 of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
• CPC Class Codes: H01M 4/62 Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
• CPC Class Codes: H01M 4/621 Binders
• CPC Class Codes: H01M 4/624 Electric conductive fillers
• CPC Class Codes: H01M 4/625 Carbon or graphite
• CPC Class Codes: H01M 4/661 Metal or alloys, e.g. alloy coatings H01M4/669 take precedence
• CPC Class Codes: H01M 4/8642 Gradient in composition
• CPC Class Codes: H01M 4/8896 Pressing, rolling, calendering membrane electrode assemblies H01M8/1004
• CPC Class Codes: H01M 4/96 Carbon-based electrodes
• CPC Class Codes: H01M 6/16 : with organic electrolyte H01M6/18 takes precedence
• CPC Class Codes: Y02E 60/10 : Energy storage using batteries
• CPC Class Codes: Y02E 60/13 : Energy storage using capacitors
• CPC Class Codes: Y02E 60/50 : Fuel cells

Abstract

Electrodes of a double-layer capacitor are designed so that sub-capacitors formed at each electrode are stressed substantially equally at the rated voltage of the double-layer capacitor. In an exemplary embodiment, each electrode includes a current collector and an active electrode layer, such as a layer of activated carbon. The electrodes are held apart by a porous separator, and the assembly is immersed in an electrolyte. The thicknesses of the active electrode layers differ, resulting in asymmetrical construction of the capacitor. Different thicknesses cause the sub-capacitors to have different capacitances. When voltage is applied to the double-layer capacitor, the voltage is divided unequally between the unequal sub-capacitors. Properly selected thicknesses allow the voltages at the sub-capacitors to stress equally each sub-capacitor. The rated voltage of the double-layer capacitor can then be increased without overstressing the sub-capacitors.

Claims

1. A double-layer capacitor, comprising:

an electrolyte;

a positive electrode layer of a first thickness, the positive electrode layer being immersed in the electrolyte; and

a negative electrode layer of a second thickness, the negative electrode layer being immersed in the electrolyte;

wherein;

a first sub-capacitor is formed at an interface between the positive electrode layer and the electrolyte;

a second sub-capacitor is formed at an interface between the negative electrode layer and the electrolyte; and

the first and second thicknesses are selected by;

determining a positive voltage limit for the first sub-capacitor and a negative voltage limit for the second sub-capacitor;

dividing the positive voltage limit by the negative voltage limit to obtain a first ratio of the second sub-capacitor to the first sub-capacitor; and

setting relative thicknesses of the positive electrode layer and the negative electrode layer so that capacitance of the second sub-capacitor is substantially equal to a product of the first ratio and the capacitance of the first sub-capacitor.

2. A double-layer capacitor in accordance with claim 1, wherein the relative thicknesses are set by:

determining a first normalized sub-capacitance of the first sub-capacitor;

determining a second normalized sub-capacitance of the second sub-capacitor;

dividing the first normalized sub-capacitance by the second normalized sub-capacitance to obtain a specific proportionality constant; and

choosing the relative thicknesses so that a ratio of thickness of the negative electrode layer to thickness of the positive electrode layer is substantially equal to a product of the first ratio end the specific proportionality constant.

3. A double-layer capacitor in accordance with claim 2, wherein the positive voltage limit and the negative voltage limit are determined by using an equidistant selection method.

4. A double-layer capacitor in accordance with claim 2, wherein the positive voltage limit and the negative voltage limit are determined by using an equal reliability selection method.

5. A double-layer capacitor in accordance with claim 2, wherein the positive voltage limit and the negative voltage limit are determined by using cyclic voltammetry measurements.

6. A double-layer capacitor in accordance with claim 2, wherein the first and second normalized sub-capacitances are determined by measuring.

7. A double-layer capacitor in accordance with claim 2, wherein:

the first and second normalized sub-capacitances are determined by applying an analytical capacitance model to physical properties of the electrolyte and of material of the positive and negative electrode layers.

8. A double-layer capacitor in accordance with claim 7, wherein the physical properties used in the analytical model comprise sizes of cations and anions of the electrolyte.

9. A double-layer capacitor in accordance with claim 7, wherein the electrolyte comprises an aqueous electrolytic solution, and the physical properties used in the analytical model comprise sizes of cations and anions of the aqueous electrolytic solution.

10. A double-layer capacitor in accordance with claim 2, further comprising a positive current collector attached to the positive electrode layer, a negative current collector attached to the negative electrode layer, and a porous separator interposed between the positive electrode layer and the negative electrode layer to prevent direct electronic current flow between the positive electrode layer and the negative electrode layer, and to allow ionic current flow between the positive electrode layer and the negative electrode layer.

11. A double layer capacitor in accordance with claim 1, wherein the first ratio is less than or equal to about 0.47.

12. A double layer capacitor in accordance with claim 1, wherein the first ratio is less than or equal to about 0.47 and more than or equal to about 0.35.

13. A double layer capacitor in accordance with claim 1, wherein the electrode layers comprise dry fibrillized particles.

14. A double layer capacitor in accordance with claim 1, wherein a sum of the absolute values of the positive and the negative voltage limits is equal to or greater than about 2.5 volts.