Method for manufacturing an electrode paste

Abstract

A method for the manufacture of a paste composition suitable for the production of an electrode for lead-acid battery, including mixing a carbon nanofiller/lead oxide composite of a first particulate size with sulphuric acid, water and further lead oxide of a second particulate size. Also, the paste thus obtained, the composite used in its manufacture, and the electrode and lead-acid battery obtained from this paste.

Claims

1. Method for preparing a paste composition suitable for the production of an electrode of an electrode for lead-acid battery, comprising: (a) providing a composite material comprising carbon-based nanofillers and lead oxide of a first particulate size, and (b) mixing said composite material with sulphuric acid, water, lead oxide of a second particulate size and, optionally, at least one reinforcing filler, to thereby obtain the paste composition, wherein the first particulate size is a different average size than the second particulate size, wherein the composite material is obtained by grinding the carbon-based nanofillers and lead oxide to a first particulate size.

2. (Currently Amended Method according to claim 1, wherein the composite material is obtained by: grinding raw or oxidized carbon-based nanofillers, with lead oxide, in a weight ratio of carbon-based nanofillers relative to lead oxide ranging from 5:95 to 20:80, and optionally shaping the resulting ground particles, to thereby obtain said composite material comprising carbon-based nanofillers and lead oxide of a first particulate size.

3. Method according to claim 1, wherein said first particulate size is between 0.1 and 0.6 m.

4. Method according to claim 1, further comprising a preliminary step of preparing lead oxide prior to the manufacture of the composite material, by contacting, at high temperature, lead with air, to obtain a powder of lead oxide.

5. Method according to claim 1, further comprising a step of preparing lead oxide during the manufacture of the composite material, by contacting, at high temperature, lead with air, in a ball mill used for the preparation of the composite material.

6. Method according to claim 5, wherein lead is introduced in a solid form, in the presence of air, into the ball mill heated at a temperature ranging from 300 C. to 400 C.

7. Method according to claim 1, wherein the carbon-based nanofillers are carbon nanotubes or carbon nanofibers.

8. Method according to claim 1, wherein said second particulate size is between 0.5 and 3 m.

9. Method according to claim 1, wherein said first particulate size is between 0.1and 0.6 .m, and wherein said second particulate size is between 0.5 and 3 m.

10. Method according to claim 1, wherein said first particulate size is between 0.1and 0.4 .m, and wherein said second particulate size is between 0.5 and 3 m.

11. Method according to claim 1, wherein said first particulate size is between 0.2and 0.4 .m, and wherein said second particulate size is between 0.5 and 3 m.

12. Method according to claim 1, wherein the carbon-based nanofillers are carbon nanotubes.

Description

DETAILED DESCRIPTION

(1) This invention will now be described in further details. In the following description, the expression comprised between should be understood to designate the range of values identified, including the lower and upper bounds.

(2) Moreover, lead oxide, as used in this specification, refers to a mixture of lead oxides having formula PbO.sub.x with 1x2.

(3) As mentioned above, the process of this invention comprises a first step of providing a composite material comprising carbon-based nanofillers and lead oxide.

(4) The carbon-based nanofillers are preferably chosen from carbon nanotubes, carbon nanofibers and mixtures thereof.

(5) Carbon nanotubes are composed of one or more concentrically rolled graphene leaflets. Thus distinctions are made between single-wall nanotubes (or SWNT) and multi-wall nanotubes (MWNT). It is preferable according to the invention to use multi-walled CNTs which are prepared by a chemical vapour deposition (or CVD) process, by catalytic decomposition of a carbon source (preferably from renewable origin), such as described in EP 1 980 530.

(6) The carbon nanotubes used in this invention typically have an average diameter of from 0.1 to 100 nm, preferably from 0.4 to 50 nm, more preferably from 1 to 30 nm and even more preferably from 10 to 15 nm, and advantageously a length of 0.1 to 10 m. Their length/diameter ratio, i.e. aspect ratio, is advantageously greater than 10 and usually greater than 100. Their specific surface area is, for example, between 100 and 300 m.sup.2/g, preferably between 200 and 300 m.sup.2/g, and their apparent density may in particular be between 0.05 and 0.5 g/cm.sup.3 and more preferably between 0.1 and 0.2 g/cm.sup.3. The multi-walled carbon nanotubes may, for example, contain 5 to 15 leaflets and more preferably from 7 to 10 leaflets.

(7) One example of crude (raw) carbon nanotubes is in particular available commercially from ARKEMA under the trade name Graphistrength C100.

(8) The nanotubes may be purified and/or treated (especially oxidized) before being employed in the process according to the invention.

(9) The nanotubes may be purified by washing using a solution of sulphuric acid, or of another acid, in order to remove any residual metallic and mineral impurities from them, such as iron, originating from their production process. The weight ratio of the nanotubes to the sulphuric acid may in particular be between 1:2 and 1:3. The purifying operation may, furthermore, be carried out at a temperature of from 90 to 120 C., for example for a time of 5 to 10 hours. This operation may advantageously be followed by steps of rinsing with water and drying of the purified nanotubes. Another route to purification of the nanotubes, intended in particular for removing the iron and/or magnesium and/or alumina that they contain, involves subjecting them to a heat treatment at more than 1000 C.

(10) The nanotubes are advantageously oxidized by contacting them with a solution of sodium hypochlorite containing from 0.5% to 15% by weight of NaOCl and preferably from 1% to 10% by weight of NaOCl, in a weight ratio, for example, of the nanotubes to the sodium hypochlorite of from 1:0.1 to 1:1. The oxidation is advantageously performed at a temperature less than 60 C., and preferably at room temperature, for a time of from a few minutes to 24 hours. This oxidizing operation may advantageously be followed by steps of filtration and/or centrifugation, washing and drying of the oxidized nanotubes.

(11) Oxidized nanotubes may also be obtained by introducing air at elevated temperatures during the milling process or during the paste composition preparation process, as will be further discussed below.

(12) It is preferred, however, for the nanotubes to be used in the process according to the invention in the crude state.

(13) Carbon nanofibres are, like carbon nanotubes, nanofilaments produced by chemical vapour deposition (or CVD) from a carbon source which is decomposed on a catalyst comprising a transition metal (Fe, Ni, Co, Cu), in the presence of hydrogen, at temperatures of from 500 to 1200 C. However, these two nanofillers differ from each other by their structure (I. MARTIN-GULLON et al., Carbon 44 (2006) 1572-1580). Specifically, carbon nanotubes are made of one or several graphene sheets concentrically rolled around the longitudinal axis of the fibre so as to form a cylinder having a diameter of 10 to 100 nm. In comparison, carbon nanofibres consist in more or less organized graphitic areas (also called turbostratic stacking), the planes of which are inclined at variable angles relative to the fibre's longitudinal axis. These stackings may take the form of platelets, of fishbones, or of cups which are stacked to form structures having a diameter generally comprised between 100 nm and 500 nm or even more.

(14) The composite material of this invention may advantageously be obtained by: grinding (i.e., milling), preferably in a ball mill, raw or oxidized carbon-based nanofillers, with lead oxide, in a weight ratio of carbon-based nanofillers relative to lead oxide ranging from 5:95 to 20:80, and optionally shaping the resulting ground (milled) particles,
to thereby obtain said composite material comprising carbon-based nanofillers and lead oxide of a first particulate size.

(15) This grinding step may be performed in any ball mill, such as a planetary centrifugal mixer, at a speed of for instance below 150 rpm, preferably from 60 to 120 rpm, or even below 60 rpm, and for a period of time from 4 hours to 16 hours, preferably from 6 to 8 hours. Moreover, grinding is preferably performed in the absence of a polymer. The grinding means may include from 100 to 150 balls, such as ceramic balls, having for instance a diameter from 1 cm to 5 cm. As a result of this grinding step, the median diameter (D50) of carbon nanotubes is usually less than 100 m, starting from carbon nanotubes which D50 is of about 400 m, as measured by laser diffraction using a Malvern particle size analyzer.

(16) According to an embodiment of this invention, this method may further comprise a preliminary step of preparing lead oxide prior to the manufacture of the composite material, by contacting, at high temperature, lead with air, to obtain a powder of lead oxide.

(17) Alternatively, and according to a preferred embodiment, the method of this invention further comprises a step of preparing lead oxide during the manufacture of the composite material, by contacting, at high temperature, lead with air, in the ball mill used for the preparation of the composite material. In this embodiment, lead is generally introduced in the solid form, in the presence of air, in the ball mill heated at a temperature ranging from 300 C. to 400 C., preferably from 330 C. to 370 C. Typical ball mills that may be used in this embodiment are cylindrical ball mills and conical ball mills. The resulting lead oxide particles have a first particulate size in the range of from 0.1 to 0.6 m, preferably from 0.2 to 0.4 m, as measured by scanning-electron microscopy (SEM) particle size analysis. Part of the lead introduced into the ball mill may remain un-oxidized after grinding. This un-oxidized lead may be present, for instance, in a weight ratio of free lead to lead oxide ranging from 1:4 to 1:3. The mixture of oxidized and un-oxidized lead will be designated hereafter as lead oxide.

(18) In the second step of the method according to this invention, said composite material is mixed with sulphuric acid, water, and lead oxide of a second particulate size, and optionally with at least one reinforcing filler. This second particulate size is typically in the range of from 0.5 to 3 m, as measured by SEM particle size analysis. The weight ratio of lead oxide of first particulate size (contained in the composite) to lead oxide of second particulate size to which the composite is added may represent from 0.5:10 to 1.5:10 for instance about 1:10.

(19) The final amount of carbon-based nanofillers in the paste composition may range from 0.2 to 2 wt. %, for instance from 0.5 to 1.5 wt. % and preferably from 0.8 to 1.2 wt. %. Moreover, the lead oxide of first particulate size and lead oxide of second particulate size may together represent from 75% to 85% by weight, relative to the weight of the paste composition. The sulphuric acid may be in a concentration of from 1 to 20 mol/l and preferably from 3 to 5 mol/l. It may represent from 1 to 10 wt. % and preferably from 2 to 7 wt. % of the total weight of the paste composition. Moreover, the total amount of water in the paste composition, including that provided by diluted sulphuric acid, generally ranges from 7 wt. % to 20 wt. %, such as 10 to 15 wt. %.

(20) The paste formulation may also include reinforcing agents such as polyester (for example polyethylene terephthalate, PET), polyacrylonitrile, glass or carbon fibres. They may have a thickness of 1 to 30 m and a length of 0.05 to 4.0 mm. These fibres, preferably glass fibres, may represent from 0.1 to 1 wt. % and preferably from 0.1 to 1 wt. % of the total weight of the paste composition, with the proviso that the total content of the constituents of the paste amounts to 100%.

(21) The invention also pertains to a process for manufacturing an electrode for lead-acid battery based on the above-described paste composition. Said process comprises the steps of: (a) impregnating a grid with the paste composition; (b) pressing the impregnated grid to obtain a plate; and (c) drying said plate to cure the paste composition.

(22) The grid may be flexible or rigid. It may be flat or two-dimensional or alternatively curved and thus three-dimensional. It is generally made of lead or an alloy thereof. After applying the electrode paste onto the grid, curing is generally performed at, for instance, from 30 to 65 C. under at least 80%, and preferably from 90 to 95% relative humidity, for more than 18 hours, such as 24 hours. Maturing is then preferably performed, for instance at from 55 to 80 C. under ambient relative humidity, for one to three days.

(23) The electrode may be the positive electrode of a lead-acid battery. In this case, the negative electrode may comprise any electroactive material chosen from the group consisting of cadmium, metal hydrides, lead and zinc and preferably spongy lead.

(24) This lead-acid battery generally includes a separator between each pair of positive and negative electrodes. This separator may be any porous non-conductive material, such as a sheet of polypropylene or polyethylene. Its thickness may range from 0.01 to 0.1 mm. One pair of electrodes together with a separator define a cell. The lead battery of this invention may include from 1 to 12 cells, which may provide each for a voltage of 1.5 to 2.5 volts. It also includes a first conductor for directly connecting the positive electrodes and a second conductor for directly connecting the negative electrodes.

(25) The lead-acid battery obtained according to the process of this invention is operational for at least 170 cycles of operation, preferably at least 180 cycles of operation, and more preferably at least 200 cycles of operation, between charging at 14V and discharging at 10.5 V, with 250.3% Depth of Discharge (DOD) of the initial capacity of the battery.

(26) This invention will be further understood in light of the following non-limiting examples which are given for illustration purposes only.

EXAMPLES

Example 1

Preparation of a Paste According to the Invention

(27) 6 kg of ceramic balls were introduced into a 10 l jar, to which 505 g of carbon nanotubes (Graphistrength C100 from ARKEMA) and 4545 g of lead oxide were added. The jar was then placed in a ball mill which was rotated at 60 rpm for 8 hours. 5001.5 g of a black powder was recovered. This product contained 10% of carbon nanotubes and 90% of lead oxide. From the SEM analysis it arises that the ground LO has an average diameter of 0.2-0.4 m.

(28) 1.6 g of this product was introduced into a beaker with 1.84 g of water and 14.4 g of lead oxide. A pasty mixture was obtained, to which 2.66 g of 43% H.sub.2SO.sub.4 were added dropwise. A paste containing carbon nanotubes and lead oxide in a weight ratio of 1:10 was thus obtained.

Example 2:

Preparation of a Comparative Paste

(29) 12 g of the co-milled product obtained as described in example 1 were introduced into a beaker with 1.38 g of water, so as to obtain a pasty mixture, to which 2 g of 43% H.sub.2SO.sub.4 were added dropwise. A paste containing carbon nanotubes and lead oxide in a weight ratio of 1:10 was thus obtained.

Example 3

Battery Performances

(30) The influence of the paste of Examples 1 and 2 on the performance of lead-acid cells was investigated using 2V polypropylene cells containing approximately 200 g negative and 100 g positive electrodes, a glass separator, and a H.sub.2SO.sub.4 electrolyte solution (SG=1.205).

(31) The positive electrodes were made from the paste of Example 1 or 2:

(32) 500 grams of the lead oxide powder from examples 1 and 2 was mixed with 0.5 grams of glass fiber for 3 minutes, then 45 ml water was added during the mixing for another 7 minutes, followed by adition of 49 ml of sulphuric acid 1.325 S.g, the acid was added graduly during another 15 minutes. The paste was then pasted onto lead antimony (1.5) grids (1.3 mm thick). These plates were then cured for at least 24 hours at 40 c at 90% RH followed by drying at 8% RH 60 C.

(33) Standard negative electrodes were used in the above cells.

(34) All electrochemical procedures were conducted on a DTI channel potentiat and included complete discharge at a rate C/20 to determine the effective capacity.

(35) The paste compositions of Examples 1 and 2 allowed performing the same number of cycles, i.e. 234. This experiment shows that the paste prepared according to this invention had the same performances than the comparative paste, although its preparation process is less expensive.