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Single-Wall Carbon Nanotubes Doping in Lead-Acid Batteries: A New Horizon Anjan Banerjee, Baruch Ziv, Yuliya Shilina, Elena Levi, Shalom Luski, and Doron Aurbach ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13377 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 13, 2017
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Single-Wall Carbon Nanotube Doping in Lead-Acid Batteries: A New Horizon Anjan Banerjee, Baruch Ziv, Yuliya Shilina, Elena Levi, Shalom Luski and Doron Aurbach* Department of Chemistry, Bar-Ilan University, Ramat-Gan 590002, Israel Email:
[email protected] Abstract The addition of single-wall carbon nanotubes (SWCNT) to lead-acid battery electrodes is the most efficient suppresser of uncontrolled sulfation processes. Due to the cost of SWCNT, we studied the optimization loading of SWCNT in lead-acid batteries electrodes. We optimized the SWCNT loading concentrations in both the positive and negative plates, separately. Loadings of 0.01% and 0.001% in the positive and negative active masses were studied, respectively. Two volts of lead-acid laboratory cells with sulfuric acid, containing silica gel-type electrolytes, were cycled in a 25% and 50% depth-ofdischarge (DOD) cycling with a charging rate of C and 2C, respectively, and discharge rates of C/2 and C, respectively. All tests successfully demonstrated an excellent service life up to about 1,700 and 1,400 cycles for 25% and 50% DOD operations, respectively, at a low loading level of SWCNT. This performance was compared with CNT-free cells and cells with a multi-wall carbon nanotube (MWCNT) additive. The outstanding performance of the lead-acid cells with the SWCNT additive is due to the oxidative stability of the positive plates during charging and the efficient reduction in sulfation in both plates while forming conducting active-material matrices. Keywords Lead-acid batteries, Gel electrolyte, SWCNT, MWCNT, Sulfation
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1. Introduction During more than 150-years, since the work of Gaston Planté, lead-acid batteries are the most widely used electrochemical storage technology until now.1 The range of application of lead-acid batteries starts from a few watt-hours (SLI, house-hold UPS, etc.) and can reach several megawatt-hour (load-leveling, submarine power, etc.) operations. The lead-acid battery technology is still popular worldwide, although the Li-based batteries are widely spread in the global battery market.2 Lead-acid batteries have several advantages, such as the high abundance (16 ppm) of lead in the Earth’s crust,3 the relatively low-cost manufacturing process, the highest cell voltage among all aqueous electrolyte systems, the ability to operate over a wide range of temperatures, over 80% energy efficiency, low self-discharge and very good safety features.4,5 The environmental pollution from lead poisoning is significantly reduced by the recycling of battery components, which can reach up to 99% of the active battery materials.1 Energy densities and operational cycle numbers of lead-acid batteries are limited compared to the advanced Liion batteries.1 Further research on the enhancement of energy densities of lead-acid batteries is not worthwhile due to the thermodynamic constraint, but research towards an improved lifecycle is highly relevant in lead-acid battery science and technology fields, mainly for load-leveling purposes. The limited lifecycle of lead-acid batteries is mainly attributed to the formation of large uncontrolled nonconducting PbSO4 crystals in both the positive and negative plates, which do not participate in reversible cell reactions.6 Such a sulfation phenomenon is more pronounced in the case of a high charge-discharge rates or high depth-of-discharge (DOD) operations. The accumulation of non-conducting large-sized PbSO4 crystals reduce the electrical conductivity of the active masses and also reduce the amounts of accessible active masses in both plates. Acid stratification, the drying of electrolytes, active material shedding from the plates and grid corrosion are also responsible for premature capacity loss of lead-acid batteries.1 Valve regulated lead acid (VRLA) battery technology with gelled electrolytes is capable of reducing the acid stratification and drying the electrolyte to a great extent.1 Tubular plate technology prevents active-material shedding and utilizes more electrochemically active surface areas from active 2 ACS Paragon Plus Environment
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masses.1 Proper additives and alloying control the corrosion by increasing the oxidative over-potential of grid alloys during battery operation.1 However, the most important method for enhancing battery life is by reducing the sulfation in both plates, especially in the negative plate. The addition of carbonaceous materials, such as graphitic powder,7,8 carbon black,7,9 various activated carbons10,11 and various carbon-based nano-materials4,9,12-15 to the active masses is beneficial for reducing the sulfation phenomenon. At present, graphitic and various amorphous carbons are extensively used in negative plates as sulfation-suppressing additives, because the sulfation is more prominent in negative plates due to slower kinetics in the negative plate reaction.16,17 However, the present lead-acid batteries are not completely sustainable for long-life operations such as load-leveling, renewable energy storage, etc. Carbon nano-materials such as single-wall carbon nanotubes (SWCNT), multi-wall carbon nanotubes (MWCNT) and graphenes may be very promising due to their ordered structure, high chemical stability, high aspect ratio and high intrinsic electrical conductivity. Various research groups, including ours, have published studies on the effects of MWCNT and graphene additives on lead-acid batteries and the enhanced performance in terms of operational cycle numbers.13-15,18,19 In contrast, we reported the effect of SWCNT on lead-acid batteries for the first time in literature,20 and we elucidated the reasons that SWCNT is a better additive than MWCNT by electrochemical (cell performance), spectroscopic (Raman), microscopic (scanning electron microscopy, SEM) and structural (X-ray diffraction, XRD) studies. Here, we studied the optimized loading level of SWCNT due to their cost. We optimize the SWCNT loading levels in both the positive and negative plates, separately. Two-volt lead-acid laboratory cells with a gelled electrolyte were tested at different depth of discharge (DOD). The advantage in using SWCNT as an additive in lead acid batteries electrodes was clearly demonstrated. 2. Experimental Details 2.1 Plates preparation and cells assembly
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A standard positive paste contained 84.11% leady oxide (25% Pb + 75% PbO), 0.08% Dynel fiber (type A), 8.24% sulfuric acid and 7.57% water. A standard negative paste comprised 83.56% leady oxide, 0.18% Dynel fiber (type B), 0.63% expander, 7.44% sulfuric acid and 8.19% water. The expander was a mixture of lignin, BaSO4 and carbon black. The leady oxide and Dynel fiber for the positive paste, and the leady oxide, expander and Dynel fiber for the negative paste, were dry-mixed homogeneously, and then mixed for ten more minutes with water, followed by, sulfuric acid solution (specific gravity: ~1.3) was slowly added to the paste mixtures and mixed thoroughly. For the CNT-modified pastes, aqueous dispersions of SWCNT and MWCNT were added instead of water, maintaining the same masses of water as added to the standard pastes. Different CNT-modified positive and negative plates were prepared with different CNT loadings, such as 0.001, 0.005 and 0.010% for SWCNT and 0.010, 0.025 and 0.050% for MWCNT. A much lower SWCNT doping than 0.001% is practically unrealistic, and a doping higher than 0.010% is not economically viable due to the cost. At the same time, a doping higher than 0.050% in the case of MWCNT is also not viable because of cost considerations, and a doping lower than 0.010% is insignificant in terms of cell performance. The SWCNT (diameter: ~2 nm, length: ~12 µm) and MWCNT (diameter: ~50 nm, length: ~15 µm) suspensions in water with Polyvinylpyrrolidone (PVP) and carboxy methylcellulose (CMC) surfactants were received from OCSiAl Group (Luxembourg) and Hongwu Nanometer Inc. (China), respectively. The pastes were spread onto lead-antimony (positive plate) and lead-calcium (negative plate) grids by a custom-made pasting device. After the pasting of the grids, the plates were cured at 50 °C in 90% relative humidity for 24 hours, and then dried at 60 °C for 24 hours. The cured plates predominately contained tri-basic lead sulfate, a 3PbO· PbSO4· H2O (3BS) phase, as confirmed by X-ray diffraction analysis. Two-volt lead-acid laboratory cells with one positive and one negative plate (geometric area: 1 cm2) were assembled in a custom-made Plexiglas container with a sulfuric acid (specific gravity ~1.2) solution. Different flooded cells were assembled using various positive and negative plates with different CNT loadings, in order to select the optimized CNT-loading values for the SWCNT and MWCNT in the
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positive and negative plates, separately. Based on the optimization study, two-volt lead-acid cells in sulfuric acid containing silica gel electrolyte21-25 were assembled with optimized CNT-doped positive and negative plates. The various types of cells assembled in this study are summarized in Table 1, along with the cell description. Each cell was formed before measuring the initial capacity and then characterized by cycling experiments. All the electrochemical tests were carried out at room temperature (~25 °C) by an Arbin Model BT2000 multichannel battery cycler. A 125 ml silica gel was prepared by thoroughly mixing 9 g of fumed silica (CAB-O-SIL® M-5) with 50 ml of deionized water. Next, 70 ml of sulfuric acid (specific gravity ~1.4) was added to the as-prepared suspension. The total mixture was continuously stirred by a mechanical stirrer at a speed of 800 RPM for 3 h. The prepared silica gel solution was injected into the cells under vacuum. 2.2 Characterization of active materials Raman spectroscopic measurements were conducted on the positive and negative active materials at different stages: after curing, after formation and after cycling, to determine the stability of CNT in real cell-operating conditions. The Raman spectroscopic data were collected in back-scattered configuration, by using a micro-Raman HR-800 spectrometer (Jobin Yvon Horiba) with a holographic grating of 1,800 grooves mm-1 and a He-Ne laser (excitation line 632.8 nm). The cycled positive and negative active materials were also characterized by SEM images and XRD measurements. The SEM images were carried out using a Quanta E-SEM, FEG scanning electron microscope. The XRD measurements were performed using a Bruker Inc. (Germany) AXS D8 ADVANCE diffractometer (reflection θ–θ geometry, Cu Kα radiation, receiving slit 0.2 mm, high-resolution energy-dispersive detector). Due to the air sensitivity of the samples, the diffraction data for the Rietveld refinement were collected in a relatively short angular range of 10°
