EP2973777A1

EP2973777A1 - Process for obtaining mixtures of carbon nanotubes in solid or viscous matrices

Info

Publication number: EP2973777A1
Application number: EP14717486.6A
Authority: EP
Inventors: Beni CAHANA; Doron Aurbach; David Jacob
Original assignee: Bar Ilan University; Vulcan Automotive Ind Ltd
Current assignee: Bar Ilan University; Vulcan Automotive Ind Ltd
Priority date: 2013-03-14
Filing date: 2014-03-13
Publication date: 2016-01-20
Also published as: IL240697A0; CN105051930A; WO2014141279A1

Abstract

A process and system for obtaining homogenous distribution of carbon nanotubes in solid or viscous matrices and products produced thereby, typically carbon nanotubes distributed in lead-oxide matrices usable as electrode components in lead-acid batteries.

Description

PROCESS FOR OBTAINING MIXTURES OF CARBON NANOTUBES IN SOLID OR VISCOUS MATRICES

TECHNOLOGICAL FIELD

This invention relates to a process for obtaining homogenous distribution of carbon nanotubes in solid or viscous matrices and products produced thereby.

BACKGROUND

The lead-acid battery has been a successful secondary battery system for over a century. The advantages of these batteries are their low cost, stable voltage profile, high reliability, and safety. The main disadvantages of such flooded type configuration are a low specific energy and, subsequently, the poor utilization of the positive active- material (PAM). Both the cycle life and the capacity of these batteries are limited due to properties of the active material in the positive plate. Compared to the negative plate, the positive plate has low performance in deep discharge (high DOD), and therefore much research has been conducted to improve the performance of lead-acid batteries.

The low utilization of PAM stems from the associated sulfation and crumbling of the active material. The crumbling of the active material originates from the significant difference between the densities of the Pb0₂ and PbS0₄, leading to expansion of the active material that occurs during the discharge process. During discharging, non-conductive crystals of PbS0₄ are formed. When these electrically insulating crystals grow, as either plates or spatial crystals, they either prevent the lead ion oxidation back to the active Pb0₂ material or reach a stable crystal size (above 1-1.5 micron in diameter), which cannot be recharged via the common dissolution- precipitation mechanism.

Carbonaceous additives were studied mainly for negative active materials of valve-regulated lead acid (VRLA) cells. Amongst others, their contribution may be associated to the enhancement of the overall conductivity of the active material, facilitation of the formation of small isolated PbS0₄ particles which are easy to dissolve, and the ability of carbon to act as an electro-osmotic pump that facilitate acid diffusion within the inner volume of the active material, especially at high rates of charge and discharge. Pavlov et al. [1] suggested a mechanism for the role of the carbonaceous additives in the negative active material. According to the proposed mechanism, the lead sulfate dissolves and diffuses to conductive sites (i.e. pure lead, extremely thin lead sulfate layer or the carbonaceous additives surface), in which it may be reduced into metallic lead due to its sufficient electrical conductivity. At later stages, due to the mismatch in the crystal lattice parameters, the reduced lead diffuses from the carbon surface, releasing the electro-active sites on the carbonaceous surface to be available for further reduction.

Among the various carbon allotropes, carbon nanotubes (CNTs) seem to be a prominent additive due to their outstanding features, including high mechanical properties, and excellent electric and thermal conductivities. Many of these properties are best exploited by incorporation of CNTs into composites [2]. The extremely high aspect ratio of the CNT (up to 10⁶) turns the formation of dispersion into a challenge, as there is a need to overcome all of the local Van der Waals interactions which tend to hold the CNTs macro-scale bundles intact.

One of the major challenges in utilizing CNTs stems from the difficulty to obtain homogenous distribution of the CNTs within a given matrix. The common techniques currently used involve mainly the formation of suspensions by introducing ultrasonic waves into a liquid medium, thereby facilitating the separation of CNTs from the bundles. This technique, however, is difficult to employ when the distribution of CNTs is required within solids, solid mixtures or viscous liquid mediums. Attempts were made to incorporate CNTs into lead-oxide pastes for use in lead-acid batteries by various techniques such as functionalization of the CNTs to facilitate their distribution

[3] or by ball-milling CNTs with the lead-oxide particles [4] .

However, there still exists a need for a process enabling homogenous distribution of CNTs within such matrices which may be readily applied in-line in the production process of electrodes.

REFERENCES

[1] Pavlov, D.; Rogachev, T.; Nikolov, P.; Petkova, G., J. Power Sources 2009, 191, 58-75

[2] Vaisman, L.; Wagner, H.; Marom, G., Adv. Colloid and Interf. Sci. 2006, 128- 130, 37-46 [4] PCT Application no. PCT/IB2013/000161

SUMMARY OF THE INVENTION

Dispersion of CNTs in active matrix has been a great challenge and much work has been done in scientific world in developing a process for reproducible dispersion of CNTs. One of the methods known from the art is the alteration of the surface energy of the CNTs by using surfactant or chemical functionalization in order to improve the wetting properties of the highly hydrophobic CNTs within the matrix. Another approach is the formation of mechanical dispersion by aggressive processes, such as high shear mixing, which reduces the entanglements within the CNTs bundles. Both processes are disadvantageous, as they require the use of undesired chemicals which need to be eliminated from the final product by complex and costly cleaning processes. Additional costs are incurred by utilizing high-energy consumption dispersion processes (such as high shear mixing), that may also damage the CNTs, i.e. break or fragmentation.

Thus, the invention aims at providing a process (and products produced thereby) for efficient dispersion of CNTs in matrices, a process which is cost-effective and does not utilize chemical species which may be regarded as undesired impurities in the final product.

Lead acid batteries comprising electrodes made from the composition manufactured by a process of the invention exhibit superior characteristics in comparison to batteries containing conventional electrodes, typically devoid of CNTs or produced by CNT mixing process as known in the art. These superior characteristics being at least in the dispersion quality of the CNTs within the matrix, e.g. lead oxide, and are reflected in battery operation, in at least enhanced electrical cycle ability and delayed electrodes (cathode, anode, or both) failure. The homogeneity of CNTs dispersion within the matrix allows for stable current transfer throughout the electrode, resulting in a battery showing a longer life-span. The CNTs are in a concentration around the percolation concentration, thereby allowing maintaining electrical conductivity without the need to form a continuous carbonaceous grid within the matrix.

Thus, the inventors of the present invention generally provide a process and a system for effectively distributing carbon nanotubes (CNTs) within a matrix. In an aspect of the invention, there is provided a process for manufacturing a composition comprising a matrix material and carbon nanotubes (CNTs) distributed within said matrix material, the process comprising applying droplets of a CNT suspension onto the matrix material. In some embodiments, the droplets are of a predetermined size (or volume).

As will be further explained hereinbelow, the CNT may be provided in a liquid medium, e.g., as a suspension, which may be prepared in advance to the adding of the CNT into the matrix; alternatively, the suspension may be obtained as such from a commercial source.

Thus, in some embodiments, the process for manufacturing a composition of a matrix material and carbon nanotubes (CNTs), comprises:

(a) providing a liquid suspension of CNTs in a liquid medium;

(b) forming the liquid suspension into droplets of a predetermined size; and

(c) applying said droplets onto the matrix material, e.g., for obtaining a homogenous distribution of CNTs within the matrix.

The process of the invention provides a matrix material in which the CNTs are homogenously distributed. In some embodiments, the matrix is in a form selected from a particulate solid (i.e. particles of matter), a viscous liquid (homogenous or heterogeneous, namely comprising a single component or a plurality of components) and a paste.

The matrix may be made of any material known to a person of skill in the art. In some embodiments, the matrix comprises at least one metal oxide, such as lead oxide of the formula PbO_x, wherein 1 < x < 2.

When in a particulate form, the lead oxide (LO) particles may be of any shape and of random or preselected particle size. The "particle size" typically refers to the average diameter of the particles. When the particles are of non-spheroid shape, the term refers to the average equivalent diameter of the particle, namely the diameter of an equivalent spherical particle based on the longest dimension of the particle. In some embodiments, the lead oxide has a typical average particle size of between about 0.5 μιη and 5 μιη (micrometers).

In other embodiments, the matrix may further comprise water. Such matrices are typically in the form of a paste. The "CNTs" employed in the process and products of the invention are carbon nanowires selected in a non-limited fashion from single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), double-walled carbon nanotubes (DWCNTs) and few-walled carbon nanotubes (FWCNTs), each may or may not be further functionalized (substituted by one or more organic or inorganic non- carbon atom or group).

In some embodiments, the CNTs encompass multi-walled carbon nanotubes (MWNTs), double-walled carbon nanotubes, buckytubes, fullerene tubes, tubular fullerenes, graphite fibrils, and combinations thereof.

The CNTs may be commercially attained or may be prepared by any known method including, such as those detailed in the following publications Ebbesen, Annu. Rev. Mater. Sci. 1994, 24, 235-264; Thess et al., Science 1996, 273, 483-487; Vander Wal et al., Chem. Phys. Lett. 2001, 349, 178-184; US Patent No. 5,374,415; Hafner et al., Chem. Phys. Lett. 1998, 296, 195- 202; Cheng et al., Chem. Phys. Lett. 1998, 289, 602-610; and Nikolaev et al., Chem. Phys. Lett. 1999, 313, 91-97.

In some embodiments, the CNTs are selected from native CNTs and functionalized CNTs, which bear one or more functional groups associated with the CNT external carbon surface. The native CNTs are those prepared without further fictionalization of their external surface. Functionalized CNTs are CNTs having at least one functional group decorating the CNTs sidewalls, external surface, or the CNTs ends. The functional groups may be selected from aliphatic groups, hydroxyl groups, amine groups, thiol groups, nitro groups, silyl groups, halide atoms (e.g., fluorinated CNTs), sulfo groups, carboxylic acid groups, ester groups, epoxy groups, and others as known in the field.

In some embodiments, the functionalized CNTs are oxygen-functionalized CNTs (CNT having a functional group including oxygen atom(s) on a surface thereof).

In some embodiments, the functionalized CNTs bare one or more functional groups which are associated with the CNT external carbon surface. These functional groups are oxygen- containing groups such as carboxyl and hydroxyl groups. The treatment of the CNTs to afford such functionalization may be achieved by wet chemical methods, photo-oxidation, oxygen plasma or gas phase treatment, as known in the art. In some embodiments, the functionalized CNT is oxidized CNT. In other embodiments, the oxidized CNTs are further functionalized via ester bonds where the acid moiety is carried by the oxidized CNT or by the functionalizing moiety.

In some embodiments, the functionalized CNTs are CNTs associated with at least one oligomer or polymer.

In some embodiments, the CNTs are selected to have a diameter of between about lOnm and about lOOnm, and independently a length of between about 1 and about 50 μηι. In other embodiments, the length of the CNTs is between about 1 and about 5 μιη.

In other embodiments, the CNTs are premixed with a dispersing additive. A non-limiting example of such a dispersing additive is carboxy methyl cellulose (CMC). Thus, in some embodiments, the CNT suspension is provided with at least one dispersing agent.

In the composition of the invention, the CNTs are substantially orderly in contact with each other, yet are "homogenously distributed" within the matrix. On average, each CNT is separable from another by a plurality of lead oxide particles coating and protecting each CNT. Thus, the CNTs form a discontinuous net or a fragmented grid, wherein on average each volume portion of the composition comprises approximately the same number of CNTs as in different volume portion within the same composition batch.

The process of the invention involves first providing a suspension of CNTs within a liquid medium. Such a suspension may be commercially attained, if available. The "suspension" is a prepared as a homogenous colloidal mixture; in other words, the CNTs are homogenously dispersed or suspended in the continuous liquid phase, i.e., the liquid medium, in which the suspension is prepared. In some embodiments, the liquid medium comprises water. In other embodiments, the liquid medium may comprise volatile solvents, which may be removed by evaporation from the composition after mixing with the matrix material. In some embodiments, the suspension comprises a mixture of volatiles and water, or a mixture or water-soluble organic solvents and water.

The suspension, according to some embodiments, may be obtained by mixing CNTs into said liquid medium. In such embodiments, the mixing is carried out, for example, by mechanical means or by introducing ultrasonic waves into the suspension. In some embodiments, the CNTs are present in the suspension in a concentration ranging between about 0.005 and about 0.1 % by weight (wt%). In other embodiments, the concentration of CNTs in the suspension ranges between about 0.01 and about 0.1 wt%, between about 0.025 and about 0.1 wt%, between about 0.05 and about 0.1 wt% or between about 0.075 and about 0.1 wt%. In some other embodiments, the concentration of CNTs in the suspension ranges between about 0.005 and about 0.08 wt%, between about 0.005 and about 0.075 wt%, between about 0.005 and about 0.05 wt% or between about 0.005 and about 0.025 wt%.

In additional embodiments, the CNTs are present in the suspension in a concentration of between 0.1 % and 0.2 % by weight. In other embodiments, the CNTs are present in the suspension in a concentration of about 0.1 % by weight.

In the process of the invention, the suspension is applied into the matrix material by further formed into droplets of a predetermined size. The droplets size may range from about 0.02 mm to about 1.5 mm. According to some embodiments, the droplets are formed by spraying or atomizing the liquid suspension. In such embodiments, the spraying or atomizing is carried out by forcing the suspension under pressure through a spraying assembly comprising at least one nozzle of a predetermined size.

According to some embodiments, the droplets are formed under a pressure in the range of between about 1.5 and 100 bars, between about 1.5 and 50 bars, or between about 1.5 and 20 bars. In other embodiments, the pressure is in the range of between about 2 and 20 bars, between about 5 and 20 bars, between about 10 and 20 bars, or between about 15 and 20 bars. According to some other embodiments, the pressure is in the range of between about 1.5 and 15 bars, between about 1.5 and 10 bars, between about 1.5 and 5 bars, or between about 1.5 and 2 bars.

In some embodiments, the nozzle is of a diameter ranging between about 0.2 mm and 5 mm.

The combination of pressure and nozzle size allows for obtaining a full cone spray pattern to afford improved distribution of the droplets onto the matrix.

The droplets (in the form of spray, mist) are applied onto the matrix, for obtaining a homogenous distribution of CNTs within the matrix. While the droplets are applied, the matrix receiving the droplets may be mechanically stirred under conditions which assist in the homogenous distribution of the droplets. In some embodiments, the mechanical mixing or stirring is carried out at a temperature of up to 65°C, at about 10- 60 Hz.

In another aspect, the invention provides a composition comprising a matrix and CNTs homogenously distributed therein, the composition obtained by a process as herein described.

In some embodiments, the CNTs are present in said matrix in a concentration ranging between about 0.01 and 0.1 wt%.

In another aspect, the invention provides a composition comprising lead oxide and CNTs homogenously distributed therein, wherein said CNTs are present in said matrix material being between about 0.01 and 0.1 % by weight.

The process of the invention allows for the manufacture of a composition comprising a matrix and CNTs distributed homogeneously therein, the composition being typically in a paste form. The process of the invention allows for obtaining said paste in a viscosity suitable for further processing of the composition, said viscosity being obtained without the need for adding excess amount of diluents. Thus, the composition comprising CNTs, according to the invention, has a viscosity comparable (i.e. similar) to the viscosity of a composition devoid of CNTs.

Another aspect of the invention provides a composition as herein described for use in preparing a product selected from a paste and an electrode.

In a further aspect, there is provided an electrode comprising the composition as described herein. In some embodiments, the electrode is a cathode or an anode.

Yet a further aspect of the invention provides an energy storage device comprising at least one electrode of the invention.

In some embodiments, the energy storage device may comprise an electrode assembly, a busbar, an electrolyte solution and housing, at least one of the electrodes in said electrode assembly being at least one electrode as herein described.

In other embodiments, the electrode assembly comprises one or more positive electrode (i.e. cathode) and one or more negative electrode (i.e. anode).

In some other embodiments, the energy storage device is a lead-acid battery.

The energy storage device (e.g. battery) according to the present invention may be utilized in a variety of applications. In an aspect of the invention, there is provided a lead acid battery system comprising one or more positive and negative electrodes, at least one of said electrodes being an electrode as herein described; separator means disposed between said electrodes; an electrolyte solution being substantially in contact with said electrodes and separators; and housing.

As noted above, the composition of the present invention may be processed into components of lead acid batteries, e.g., electrodes (cathode and/or anode). Thus, the invention also provides a processing method for application of a composition of the invention as an electrode in order to prevent or minimize creep deformation, inter- granular corrosion and cracking in lead acid batteries. For such applications and others, the composition may be provided in the form of a paste.

A composite of the invention, as an electrode active material, may be provided as a paste and applied on an electrode substrate. The paste may be applied or otherwise provided onto at least a portion of an electrode substrate and allowed to dry or cure to form an electrode plate. The lead acid battery may then be constructed.

The surface of the electrode may be the entire surface or a region thereof. The region of the substrate's surface to be coated may be of any size and structure, the region may be a single continuous region or comprise of several non-continuous sub-regions which are spaced apart. The electrode "substrate" may be a flexible or rigid structure, which may be substantially two-dimensional (a thin flat substrate) or a three- dimensional, e.g., curved (non-flat) surface. The substrate can be of any smoothness. In most general terms, the substrate may be of a solid material.

While traditional lead acid batteries have proven to be dependable, they have a limited life span and energy density. The life span is typically limited by creep (growth), corrosion and cracking of the electrodes, as well as the formation of an insulating lead-sulfide layer resulting from successive charge-discharge cycles, each of which disadvantages being minimized or resolved in a lead acid battery according to the present invention.

According to some embodiments, the energy storage device is a lead-acid battery comprising an electrode composed of the composition of the invention, and an electrolyte solution, being typically a dilute aqueous sulfuric acid solution, comprising 3 to 5M sulfuric acid; the solution provides the sulfate ions necessary for the discharge reactions.

The invention also provides, in another aspect, a system for homogenously distributing CNTs within a matrix, the system comprising:

(a) a container for holding a CNTs suspension; (b) a droplet-forming unit, e.g., having at least one nozzle of at least one predetermined size; and

(c) a matrix container positioned in communication with said droplet forming unit.

In some embodiments, the droplet-forming unit is in liquid communication with the CNTs suspension container to allow transfer of the suspension from the CNTs suspension container to the droplet-forming unit.

The suspension container may be of any shape and size, and may additionally contain internal flow-diverting ribs or baffles (removable or as structural elements of the container).

In some embodiments, the CNTs suspension container comprises a mixing peddle. Either (or both) the flow-diverting ribs or the mixing peddle assist in the homogenous dispersion of the CNTs within the liquid medium in order to form the suspension. In embodiments where the container comprises the mixing peddle, the mixing peddle may comprise a central shaft and a plurality of discs, a plurality of fins (e.g. at least 2, 3, 4 or more) or any combination thereof, the discs and fins being positioned in at least one plane perpendicular to the shaft.

According to some embodiments, the fins may assume at least one angle relative to said plane.

In additional embodiments, the discs and the fins may include a plurality (e.g. at least 1, 2, 3 or more; at times, at least 10, 20, 30 or more) of through-holes of at least one predetermined size, such as between 1 mm and 10 mm.

The droplet-forming unit may be of any configuration known to person of skill in the art, and typically comprises at least one, usually a plurality, of nozzles of a predetermined size. In some embodiments, the droplet-forming unit is adapted for operation under pressure ranging between 1.5 and 20 bars.

In other embodiments, said at least one nozzle having a diameter of between about 0.2 and about 5 mm.

According to further embodiments, the droplet-forming unit is selected from a sprayer, an atomizer, a drizzling unit, a liquid injector, a diffuser, and an aerosolizing unit. The suspension droplets formed by the droplet-forming unit are applied onto the matrix, which is contained with a matrix container. In some embodiments, the matrix container is adapted for continuous or batch mixing of the matrix.

The matrix container may be of any shape and size suitable for carrying out the invention. In some embodiments, the mixing is carried out by a rotating drum, a screw- mixer, and a planetary centrifugal mixer.

According to some embodiments, the matrix is in a form selected from a particulate solid, a viscous liquid and a paste.

In other embodiments, the matrix comprises at least one metal oxide. In such embodiments, the at least one metal oxide may be lead oxide.

In some other embodiments, the matrix further comprises water.

According to some embodiments, the CNTs used in a system of the invention may be selected from native CNTs and functionalized CNTs.

In another aspect, the invention provides a system as herein described for use in a process for preparing a composition comprising CNTs homogenously distributed within a matrix. In some embodiments, the CNTs are present in said matrix in a concentration ranging between about 0.05 and 0.1 % by weight.

Within the scope of the invention, there is contemplated a lead acid battery system comprising one or more (or a plurality of alternating) positive and negative electrodes, at least one of said electrodes being an electrode according to the present invention; separator means disposed between said electrodes; an electrolyte solution being substantially in contact with said electrodes and separators, and a housing.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 is an SEM image of a drop of suspension comprising CNTs produced in the method of the invention.

Fig. 2 is a schematic diagram of a system of the invention.

Fig. 3A is a schematic representation of a mixing peddle used the system of the invention. Fig. 3B is a cross-sectional view of the mixing paddle of Fig. 3A along longitudinal axis I-I.

Fig. 3C is a cross-sectional view of the mixing paddle of Fig. 3A across plane II-II, with the fins tilted at an angle relative to plane II-II.

Fig. 3D is a cross-sectional view of the mixing paddle of Fig. 3A along longitudinal axis I-I, with the fins at an angle relative to both axis I-I and plane II-II.

DETAILED DESCRIPTION OF THE INVENTION

Quality of suspension

Fig. 1 is a scanning electron microscopy (SEM) image of a suspension comprising CNTs and water prepared by the process of the invention. lOOOppm of non- functionalized (i.e. raw) CNTs were suspended in 3 liters of water by using the system of the invention. Then 1 drop of the suspension was placed over a carbon tape to form the SEM imaging specimen.

As can be seen from Fig. 1, the CNTs appear to be well dispersed, without apparent bundling or damage to the CNTs.

Performance evaluation of lead-acid cells

Suspensions having the concentrations of 0.1-0.2 %wt of CNTs in water were prepared by using a system according to the invention.

The suspensions were then used for the preparation of composition comprising lead oxide (containing 26% free lead) and CNTs homogeneously disperse therein. Each of the suspensions was sprayed onto a lead oxide, 0.1 %wt fiberglass binder and water paste by pressure-feeding (approximately 2 bars) the suspension through an atomizer having a nozzle having a diameter of 0.2-5mm. The droplets of suspension were mixed into the paste by using a Freudenthale mixer, cycles at 50 Hz. The temperature of paste was kept below 64°C during the mixing process.

The final concentrations of the CNTs within the composition were 0.01 and 0.02 %wt, respectively.

The compositions thus formed were applied onto 1.3 mm thick lead antimony grids, and allowed to cure for 24 hours at 90% relative humidity at 40°C, followed by 24 hours of drying at 60°C, to result in electrodes. The electrodes were used as cathodes in lead-acid cells for testing the life-span and functionality compared to an electrode devoid of CNTs. The test conditions were loading cycles of charging at 14V and discharging at 10.5V, at 25%+0.3% Depth of Discharge (DOD) of the initial capacity of the cell. The number of loading cycles was counted until failure of the cell was obtained. Table 1 summarized comparative test results obtained from the cells.

Table 1: performance test results of lead-acid cells (^reference cathode comprising pure lead oxide and Dynel^® fibers)

It is noted that Test 1 in Table 1 was carried out on a dispersion of CNTs added to the paste of PbO by simple mixing, i.e. without the use of a process of the invention. It is evident from the results that the simple addition of the CNTs dispersion into the PbO paste, NOT in accordance with the invention, resulted in inferior capacitance.

The reproducibility of the obtained capacity was assessed by comparing the capacity of 7 cells including an electrode of the invention comprising 0.01%wt CNTs in the cathode composition. The capacity results are provided in Table 2.

ea ox e an Dyne ers; ot cat o e an ano e conta ne . wt NTs)

It is evident from Tables 1-2 that both capacity and the number of cycles to failure of the cell improved significantly when using a cathode produced by the process of the invention as compared to standard cathodes.

Performance evaluation of lead-acid batteries

Electrodes (cathode and anode) comprising 0.01 %wt CNT where used in 66Ah lead-acid batteries for testing the life-span and functionality compared to batteries containing standard electrodes (i.e. devoid of CNTs). The test conditions were loading cycles of charging for 2 hours at C/2 at 14V and discharging for 1 hour at C/4 (25% Depth of Charge) of the initial capacity of the battery. The number of loading cycles was counted until failure of the battery was obtained. Table 3 summarized the comparative test results obtained from the batteries.

Tab e 3: performance test results of lead-acid batteries (^reference electrodes comprising pure lead oxide and Dynel fibers)

Life-span for shallow cycling (ignition profile) of 66 Ah lead-acid batteries with electrodes (cathode and anode) comprising 0.01 wt% CNTs was evaluated by using a 2- step discharge profile. The batteries were cycled according to the following protocol: charge for 60 seconds at 100 Ah, and 2-step discharge - 300Ah for 1 second followed by 40 Ah for 60 seconds. The number of cycles was counted until failure of the battery was obtained. Table 4 summarized comparative test results obtained from the batteries. Test No. of cycles to failure

Ref * 37,000

1 78,000

2 89,300

3 102,900

4 114,00<

Tab e 4: performance test results of lead-acid batteries (^reference electrodes comprising pure lead oxide and Dynel fibers)

As can be seen from Tables 3-4, the number of cycles to failure of batteries comprising electrodes of the invention is significantly higher than the cycles to failure of batteries comprising standard electrodes, both in deep and shallow cycling.

Lead-acid batteries with electrodes comprising 0.01 %wt CNT were tested for cold cranking amperage (CCA test) for 46Ah, 55Ah, 60Ah, 62Ah, 65Ah, 72Ah, 95Ah, and 120Ah NATO batteries. The tests were carried out using a Midtronics electronic battery tester MDX-600 utilizing the EN standard. The results were compared with standard batteries, i.e. standard electrodes lacking CNTs. The results are provided in Table 5. Values are provided as average results for 50 tested batteries of each type (except for the 60Ah batteries which shows average results for 10 tested batteries of each type).

Table 5: CCA test results It is evident from Table 5 that significant improvement is obtained in CCA for all battery types tests. As the CCA test is typically carried out at the customer's end, the improved CCA values can be used to characterize batteries comprising electrodes (both anodes and cathodes) produced by the process of the invention and distinguish them from standard batteries.

Exemplary system

A non-limiting exemplary system according to the invention is schematically presented in Fig. 2. It will be clearly understood by any person of skill in the art that each of the components of the exemplary system may be substituted with an equivalent component providing the same functionality.

The system of Fig. 2, generally designated 100, comprises a CNTs suspension container 102, having at least one inlet, at least one outlet, and a mixing peddle 104 (the schematic presentation thereof is specifically shown in Fig. 3, as will be explained below). The container 102 may be fed with CNTs and water in a batch- wise or a consecutive process from raw materials containers (not shown). In order to facilitate dispersion of the CNTs, a circulation pump may be used during mixing, drawing suspension from the bottom of container 102 and re-feeding it to the container through circulation line 124.

The outlet of the suspension container 102 is configured to be in fluid- communication with an inlet of a droplet-forming unit 108 via, for example, pipe line 106, thereby allowing the suspension to be fed into the droplet-forming unit. The droplet-forming unit 108 comprises a plurality of nozzles (commonly designated 110), having a typical size of between 0.2 and 5 mm.

The unit 108 is coupled to a gas-pressure supply line 112, which may be supplied with gas (such as air, argon, nitrogen, etc.) from a pressurized container 114. Typical gas pressures which may be utilized in the system range from about 1.5 to about 20 bars.

Below the droplet-forming unit 108 there is positioned a matrix container 116, enabling application of the droplets thus formed onto the matrix 118. In the application of a lead-acid battery, the matrix typically comprises lead oxide particles and water, and optionally additives, such as fiberglass binder. Within the matrix container 116, the matrix is mixed with the suspension droplet to form the composition of the invention by using a mechanical mixing mechanism 120, such as a screw-mixer.

The composition may then be fed into a dosing funnel 122, for dosing amounts of the composition to be applies onto the electrode grids (not shown).

Turning now to Figs. 3A-3D, schematic representations of the mixing peddle 104 which may be used in a system of the invention are shown. The mixing paddle typically comprises a central shaft 130, a plurality of fins, commonly designated 132. The fins may be perpendicular to the longitudinal axis I-I of the shaft (and parallel to the plane II-II) as can be seen in Figs. 3A-3B. Alternatively, the fins may assume various angles relative to the axis I-I and/or to the plane II-II, as can be seen in Figs. 3C-3D. The fins may be positioned at a single position along the shaft's longitudinal axis. In some configurations, the fins may be distributed along the longitudinal axis, as is also shown in Fig. 3A.

At least part of the fins, at times all of the fins, may include a plurality of through-holes, commonly designated 134, of at least one predetermined size. The various arrangements of fins and through-holes allow for controlled modifications of turbulent flow in the dispersion container, thereby impacting the quality of mixing of the CNTs into the liquid medium.

Claims

CLAIMS:

1. A process for manufacturing a composition comprising a matrix material and carbon nanotubes (CNTs), wherein said CNTs being distributed within said matrix material, the process comprising applying droplets of a CNT suspension onto the matrix material.

2. The process according to claim 1, wherein said droplets being of a predetermined size.

3. The process according to claim 1, wherein the CNT suspension is provided in a liquid medium.

4. The process according to any one of claims 1 to 3, the process comprising:

(a) providing a liquid suspension of CNTs in a liquid medium;

(b) forming the liquid suspension into droplets of a predetermined size; and

(c) applying said droplets onto the matrix material.

5. The process according to claim 1, wherein said matrix material is in a form selected from a particulate solid, a viscous liquid and a paste.

6. The process according to any one of claims 1 to 5, wherein the matrix material comprises at least one metal oxide.

7. The process according to claim 6, wherein the at least one metal oxide is lead oxide.

8. The process according to claim 6 or 7, wherein said matrix material further comprises water.

9. The process according to any one of claims 1 to 8, wherein the CNTs are selected from native CNTs and functionalized CNTs.

10. The process according to any one of claims 1 to 9, wherein the liquid medium comprises water.

11. The process according to any one of claims 3 to 10, wherein said suspension is obtained by mixing CNTs into said liquid medium.

12. The process according to claim 11, wherein said mixing is carried out by mechanical means or by introducing ultrasonic waves into the suspension.

13. The process according to any one of claims 1 to 12, wherein the CNTs are present in the suspension in a concentration ranging between about 0.005 and about 0.1 by weight. 43

14. The process according to claim 13, wherein the CNTs are present in the suspension in a concentration of about 0.1% by weight.

15. The process according to any one of claims 1 to 11, wherein said applying of droplets being by spraying or atomizing said suspension.

16. The process according to claim 15, wherein said spraying or atomizing is by forcing the suspension under pressure through at least one nozzle of a predetermined size.

17. The process according to claim 16, wherein said pressure is in the range of between about 1.5 and 20 bars.

18. The process according to claim 16 or 17, wherein said nozzle having a diameter ranging between about 0.2 mm and 5 mm.

19. The process according to any one of claims 1 to 18, wherein said application of droplets being carried out under mechanical mixing.

20. A composition comprising a matrix material and CNTs, said CNTs being homogenously distributed within said matrix material, the composition being obtained by a process according to any one of claims 1 to 19.

21. The composition according to claim 20, wherein said CNTs are present in said matrix material in a concentration ranging between about 0.01 and 0.1 % by weight.

22. The composition according to claim 20 or 21 for use in preparing a product selected from a paste and an electrode.

23. The composition according to claim 20, wherein said matrix material is lead oxide, or lead acid, and the concentration of said CNTs are present in said matrix material being between about 0.01 and 0.1 % by weight.

24. An electrode comprising the composition according to claim 20, claim 22 or claim 23.

25. The electrode according to claim 24, being a cathode or an anode.

26. The electrode according to claim 25, comprising a composition comprising lead oxide or lead acid and CNTs.

27. The electrode according to claim 26, wherein said CNTs being in a concentration ranging between about 0.01 and 0.1 % by weight.

28. An energy storage device comprising at least one electrode according to claim 24 or 27.

29. The energy storage device according to claim 28, comprising an electrode assembly, a busbar, an electrolyte solution and housing, at least one of the electrodes in said electrode assembly being at least one electrode according to claim 27 or 28.

30. The energy storage device according to claim 29, wherein the electrode assembly comprises one or more positive electrode and one or more negative electrode.

31. The energy storage device according to any one of claims 28 to 30, being a lead-acid battery.

32. A lead acid battery system comprising one or more positive and negative electrodes, at least one of said electrodes being an electrode according to any one of claims 24 to 27; separator means disposed between said electrodes; an electrolyte solution being substantially in contact with said electrodes and separators; and housing.

33. A system for distributing CNTs within a matrix, the system comprising:

(a) a container for holding a CNT suspension;

(b) a droplet-forming unit, having at least one nozzle of at least one predetermined size; and

(c) a matrix container positioned in communication with said at least one nozzle.

34. The system according to claim 33, wherein said droplet-forming unit being in liquid communication with the CNTs suspension container to allow transfer of the suspension from the CNTs suspension container to the droplet-forming unit.

35. The system according to claim 33 or 34, wherein said CNTs suspension container comprises a mixing peddle.

36. The system according to claim 35, wherein said mixing peddle comprises fins and a central shaft, the fins being positioned in at least one plane perpendicular to the shaft.

37. The system according to claim 36, wherein said fins assume at least one angle relative to said plane.

38. The system according to claim 36 or 37, wherein said fins include a plurality of through-holes of at least one predetermined size.

39. The system according to any one of claims 33 to 38, wherein said droplet- forming unit is adapted for operation under pressure ranging between 1.5 and 20 bars.

40. The system according to claim 39, wherein said droplet-forming unit is selected from a sprayer, an atomizer, a drizzling unit, a liquid injector, a diffuser, and an aerosolizing unit.

41. The system according to any one of claims 33 to 40, wherein said at least one nozzle having a diameter of between about 0.2 and about 5 mm.

42. The system according to any one of claims 33 to 41, wherein said matrix container is adapted for continuous or batch mixing of the matrix.

43. The system according to claim 42, wherein said mixing is carried out by a rotating drum, a screw-mixer, and a planetary centrifugal mixer.

44. The system according to any one of claims 33 to 43, wherein said matrix is in a form selected from a particulate solid, a viscous liquid and a paste.

45. The system according to any one of claims 33 to 44, wherein the matrix comprises at least one metal oxide.

46. The system according to claim 45, wherein the at least one metal oxide is lead oxide.

47. The system according to claim 45 or 46, wherein said matrix further comprises water.

48. The system according to any one of claims 33 to 47, wherein the CNTs are selected from native CNTs and functionalized CNTs.

49. The system according to any one of claims 33 to 48 for use in a process for preparing a composition comprising CNTs homogenously distributed within a matrix material.

50. The system according to claim 49, wherein said CNTs are present in said matrix material in a concentration ranging between about 0.01 and 0.1 % by weight.