SECONDARY CARBON BATTERY

Abstract

Secondary carbon batteries are attractive from an environmental perspective, as they have carbon-only electrodes and are therefore metal-free. Current invention refers to novel secondary carbon batteries with water-based brine electrolytes. These electrolytes have low toxicity, are not flammable, and allow for easy on-site battery recycling. The inventive carbon batteries feature graphite current collectors, activated carbon electrodes, and aqueous eutectic electrolytes comprising NaCl, KCl, MgCl.sub.2, or CaCl.sub.2. Further improvement of the batteries is performed by an initial repetitive charge-discharge cycling with subsequent replacement of the spent electrolyte. The improved secondary carbon batteries with the operating voltage of about 1.8 V can be used for electric storage utilities of renewable energy installations.

Claims

1. An improved secondary carbon battery comprising: i) two current collectors; ii) two electrodes comprising porous carbon; ii) a water-based eutectic brine electrolyte, wherein the improvement is achieved by an initial charge-discharge cycling followed by removal of the precipitate and replacement of the spent electrolyte with the fresh electrolyte.

2. The improved secondary carbon battery of claim 1, wherein the two current collectors are graphite current collectors.

3. The improved secondary carbon battery of claim 1, wherein the porous carbon is activated carbon with a surface area between 1000 and 2000 m.sup.2/g.

4. The improved secondary carbon battery of claim 1, wherein a weight ratio of the two porous carbon electrodes is between 2:3 and 3:2.

5. The improved secondary carbon battery of claim 1, wherein the water-based eutectic brine electrolyte is an aqueous solution of soluble chlorides selected from the group consisting of NaCl, KCl, MgCl.sub.2, or CaCl.sub.2, and mixtures thereof.

6. The improved secondary carbon battery of claim 1, wherein the eutectic water-based brine electrolyte is a binary eutectic aqueous solution of one soluble chloride selected from the group consisting of NaCl, KCl, MgCl.sub.2, or CaCl.sub.2.

7. The improved secondary carbon battery of claim 1, wherein the eutectic water-based brine electrolyte is a ternary eutectic aqueous solution of two soluble chlorides selected from the group consisting of NaCl, KCl, MgCl.sub.2, or CaCl.sub.2.

8. The improved secondary carbon battery of claim 1, wherein the initial charge-discharge cycling is repeated 3-7 times.

9. The improved secondary carbon battery of claim 1, wherein the initial charge-discharge cycling is performed using a 0.1-0.2 C constant current.

10. The improved secondary carbon battery of claim 1, wherein the initial charge-discharge cycling comprises charging at elevated voltages between 1.9 V and 2.1 V.

11. The improved secondary carbon battery of claim 1, wherein the initial charge-discharge cycling comprises discharging at lowered voltages between 0.2 and 0.4 V.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a schematic representation of the secondary carbon battery.

[0014] FIG. 2 shows separate positive (+) and negative () electrode contributions towards the total effective capacitance of the carbon battery with the eutectic NaCl electrolyte and equal-weight activated carbon electrodes.

[0015] FIG. 3 represents typical self-discharge curves of secondary carbon batteries with the eutectic NaCl electrolyte before (a) and after (b) the initial cycling improvement.

DETAILED DESCRIPTION OF THE INVENTION

[0016] It has been unexpectedly discovered in the present invention that secondary carbon batteries employing brine electrolytes can be charged above 2.0 V without evolution of chlorine at a positive electrode. Subsequently, it was further discovered that self-discharge rates of these carbon batteries could be substantially reduced by repeated cycling at elevated voltages with subsequent replacement of the spent electrolyte with the fresh electrolyte. The enhanced operating voltage of these improved carbon batteries allows for a substantial improvement of their performance as energy storage devices.

[0017] The composition of the brine electrolyte is the primary factor affecting the performance of carbon batteries. Among tested brine solutions of NaCl, KCl, MgCl.sub.2, or CaCl.sub.2, good results were achieved with concentrations between 15-25% for NaCl and KCl, 20-30% for MgCl.sub.2, and 20-40% for CaCl.sub.2. Mixed salt brine solutions, such as NaCl-KCl solutions with total salt concentrations between 15-25%, are also effective as electrolytes for carbon batteries.

[0018] Of particular importance are eutectic electrolyte compositions for low temperature operations. The preferred embodiments include the eutectic water-based electrolytes containing 23.3% of NaCl with the freezing point of about 21.1 C.; or 21.6% of MgCl.sub.2 with the freezing point of about 33 C.; or 30.3% of CaCl.sub.2 with the freezing point of about 50 C. The eutectic solution of KCl in water is less desirable due to a relatively high freezing point (about 10.6 C. for 19.7% solution). In this case, the preferred embodiments also include the ternary eutectic electrolyte containing 5.8% of KCl and 20.2% NaCl with the freezing point of about 22.9 C.

[0019] The composition of material for current collectors also affects the battery performance. Among other materials tested, graphite current collectors provide the best results. Similar observations were noted in the US patent U.S. Pat. No. 6,706,079. In our experiments, metal current collectors, while providing a lower resistance, substantially increase self-discharge rates of carbon batteries. Various shapes of graphite current collectors can be used, but the best results were achieved with the plate electrodes, as depicted in FIG. 1. The thickness of the graphite current collectors can be between 2 and 7 mm, preferably 3-4 mm to achieve the best balance between the weight and mechanical strength.

[0020] The quality of porous carbon is another parameter affecting the overall effectiveness of carbon batteries. Our experiments demonstrated that inexpensive and widely available activated carbon can be successfully used for the construction of porous carbon electrodes. Good results were achieved with activated carbons having a surface area of about 1500 m.sup.2/g. In general, commercial activated carbons with a surface area of 1000-2000 m.sup.2/g are acceptable as electrode materials without any pre-treatment. A negative effect of minor quantities of impurities found in these commercial samples can be mitigated during the battery improvement process outlined further below.

[0021] Utilization of porous carbon electrodes with different weights provides additional benefits. Positive and negative electrodes perform differently in carbon batteries. The positive electrode is more efficient and has a higher effective capacitance compared to the negative electrode, other parameters being equal. At the same time, the maximum faradic contribution towards the capacitance of the negative electrode occurs at a higher voltage compared to the positive electrode (FIG. 2). Another factor is the self-discharge rate, which is relatively higher for the negative electrode than for the positive electrode. Yet another factor is overcharge stability of the electrodes, which is higher for the negative electrode and lower for the positive electrode. As a result, carbon batteries with larger negative electrodes and smaller positive electrodes have slightly higher maximum energy densities, and lower self-discharge rates. At the same time, carbon batteries with larger positive electrodes and smaller negative electrodes have better electrode stability. The weight ratio between the positive and negative electrodes for the unsymmetrical carbon batteries can be between 7:3 to 3:7. In the preferred embodiments, this ratio is between 2:3 to 3:2.

[0022] The improvement of carbon batteries, which allows for attaining a substantially higher operating voltage and increases their energy density, is achieved through special steps in their fabrication. These steps include repetitive initial charge-discharge cycles followed by subsequent removal of the contaminated electrolyte with the precipitate and re-filling of the battery with the fresh electrolyte. During these initial charge-discharge cycles, small amounts of impurities from the porous carbon electrodes are leached into the electrolyte. Subsequent removal of the electrolyte and precipitate yields a clean carbon battery with improved performance. The best results were achieved with 2-7 initial charge-discharge cycles between lowered 0.2-0.4 V and elevated 1.9-2.1 V voltages employing a 0.05-0.25 C constant current. In the preferred embodiments, the improvements are made by employing five charge-discharge cycles between 0.3 V and 2.0 V using a 0.1-0.2 C constant current. The maximum operating voltage of the improved carbon batteries can reach 1.75-1.85 V compared to 1.45-1.55 V for non-improved batteries, measured at the self-discharge rate of less than 1% per day (see FIG. 3).

[0023] In the preferred embodiments (see examples 1-6), the inventive secondary carbon batteries comprise graphite current collectors, activated carbon electrodes, and binary or ternary aqueous eutectic solutions of NaCl, KCl, MgCl.sub.2, and CaCl.sub.2 as electrolytes. The improvement of the carbon batteries is achieved by the repetitive charger-discharge cycling followed by removal of the spent electrolyte together with leached impurities and the precipitate, and replacement of the spent electrolyte with the fresh one. The carbon batteries performance was tested at room temperatures, 19-21 C.

[0024] EXAMPLE 1. The secondary carbon battery (FIG. 1), equipped with two 58 cm graphite current collectors (3 mm thickness) and two 58 cm carbon electrodes (1.5 cm thickness) made of activated carbon tablets (45 g each), was filled with 150 ml of the eutectic solution of NaCl in distilled water. The battery was charged using a constant 100 mA current to 1.6 V at 20 C. The charged battery showed the following characteristics: internal resistance0.42 , capacity907 mAh. The maximum working voltage of the battery with less than 1% per day self-discharge rate was 1.52 V.

[0025] EXAMPLE 2. The same as the example 1, but the eutectic solution of MgCl.sub.2 in distilled water was used as the electrolyte. The battery was charged using a constant 90 mA current to 1.6 V at 21 C. The charged battery showed the following characteristics: internal resistance0.61 , capacity863 mAh. The maximum working voltage of the battery with less than 1% per day self-discharge rate was 1.41 V.

[0026] EXAMPLE 3. The same as the example 1, but the eutectic solution of CaCl.sub.2 in distilled water was used as the electrolyte. The battery was charged using a constant 100 mA current to 1.6 V at 20 C. The charged battery showed the following characteristics: internal resistance0.57 , capacity886 mAh. The maximum working voltage with less than 1% per day self-discharge rate was 1.47 V.

[0027] EXAMPLE 4. The same as the example 1, but the eutectic solution of KCl in distilled water was used as the electrolyte. The battery was charged using a constant 100 mA current to 1.6 V at 20 C. The charged battery showed the following characteristics: internal resistance0.36 , capacity912 mAh. The maximum working voltage with less than 1% per day self-discharge rate was 1.51 V.

[0028] EXAMPLE 5. The same as the example 1, but the ternary eutectic solution of KCl-NaCl in distilled water was used as the electrolyte. The battery was charged using a constant 100 mA current to 1.6 V at 20 C. The charged battery showed the following characteristics: internal resistance0.38 , capacity908 mAh. The maximum working voltage with less than 1% per day self-discharge rate was 1.51 V.

[0029] EXAMPLE 5. The same as the example 1, but the weight of the positive electrode was reduced to 40 g (approximately 1.4 cm thickness) and the weight of the negative electrode was increased to 50 g (approximately 1.6 cm thickness). The battery was charged using a constant 100 mA current to 1.6 V at 20 C. The charged battery showed the following characteristics: internal resistance0.41 , capacity923 mAh. The maximum working voltage with less than 1% per day self-discharge rate was 1.56 V.

[0030] EXAMPLE 6. The same as the example 1, but the battery was cycled five times between 2.0 V and 0.3 V using a constant 120 mA current for charging and a constant 120 mA current for discharging. The spent electrolyte and the precipitate were removed, and then were replaced with the fresh electrolyte (the eutectic solution of NaCl in distilled water). The battery was charged using a constant 120 mA current to 1.9 V at 19 C. The charged battery showed the following characteristics: internal resistance0.44 , capacity1036 mAh. The maximum working voltage with less than 1% per day self-discharge rate was 1.83 V.

[0031] Thus, the inventive carbon batteries have several similar elements outlined below.

[0032] All disclosed carbon batteries comprise: [0033] i) two current collectors comprising graphite; [0034] ii) two electrodes comprising porous carbon wherein in the preferred embodiments: [0035] porous carbon is activated carbon; [0036] the weight ratio of porous carbon in the positive electrode versus porous carbon in the negative electrode is between 3:2 to 2:3; [0037] iii) water-based brine electrolytes wherein in the preferred embodiments: [0038] the water-based brine electrolytes comprising soluble inorganic chlorides selected from the group consisting of NaCl, KCl, MgCl.sub.2, or CaCl.sub.2, and mixtures thereof; [0039] the concentrations of salts in the brine electrolytes correspond to the binary and tertiary eutectic solutions of NaCl, KCl, MgCl.sub.2, or CaCl.sub.2 in water.

[0040] The performance of the disclosed carbon batteries can be improved by a repetitive initial charge-discharge cycling followed by removal of the spent electrolyte and precipitate, and re-filling of the batteries with the fresh electrolyte wherein in the preferred embodiments: [0041] i) the initial charge-discharge cycle is repeated 3-7 times; [0042] ii) the maximum charge voltage of the battery is set to 1.9-2.1 V, and the minimum discharge voltage of the battery is set to 0.2-0.4 V; [0043] iii) an initial charge-discharge constant current is set to 0.1-0.2 C.