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Effects of carbon additives on the performance of negative electrode of lead-carbon battery
Accepted Manuscript Title: Effects of carbon additives on the performance of negative electrode of lead-carbon battery Author: Xianping Zou Zongxuan Kang Dong Shu Yuqing Liao Yibin Gong Chun He Junnan Hao Yayun Zhong PII: DOI: Reference:
S0013-4686(14)02222-1 http://dx.doi.org/doi:10.1016/j.electacta.2014.11.027 EA 23699
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
6-8-2014 4-10-2014 5-11-2014
Please cite this article as: Xianping Zou, Zongxuan Kang, Dong Shu, Yuqing Liao, Yibin Gong, Chun He, Junnan Hao, Yayun Zhong, Effects of carbon additives on the performance of negative electrode of lead-carbon battery, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.11.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of carbon additives on the performance of negative electrode of lead-carbon battery Xianping Zou a, Zongxuan Kang a, Dong Shu a, c, d*, Yuqing Liao a, Yibin Gong b, Chun He b, Junnan Hao a, Yayun Zhong a a
School of Chemistry and Environment, South China Normal University, Guangzhou
510006, P. R. China b
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School of Environmental Science and Engineering, Sun Yat-sen University,
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Guangzhou 510275, P. R. China c
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Base of Production, Education & Research on Energy Storage and Power Battery of
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Guangdong Higher Education Institutes, Guangzhou 510006, P. R. China d
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Guangzhou Key Laboratory of Materials for Energy Conversion and Storage，
Corresponding author. E-mail address: [email protected] (Dong Shu)
The negative electrode sheets are prepared by simulating manufacture
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Highlights
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*
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Guangzhou 510006, P. R. China
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condition of negative plates.
The effect of carbon additives on negative electrode sheets is studied by
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electrochemical method.
Carbon additives in NAM enhance electrochemical properties of the negative sheets
The negative sheets with 0.5 wt% carbon additive exhibit better
electrochemical performance.
The charge-discharge mechanism is discussed in detail according to the experimental results.
Abstract In this study, carbon additives such as activated carbon (AC) and carbon black (CB) are introduced to negative electrode to improve its electrochemical performance,
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the negative electrode sheets are prepared by simulating the negative plate
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manufacturing process of lead-acid battery, the types and contents of carbon additives
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in the negative electrode sheets are investigated in detail for the application of
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chronopotentiometry,
galvanostatic
charge-discharge
and
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measured
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lead-carbon battery. The electrochemical performance of negative electrode sheets are
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electrochemical impedance spectroscopy, the crystal structure and morphology are
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characterized by X-ray diffraction and scanning electron microscopy, respectively.
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The experimental results indicate that the appropriate addition of AC or CB can
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enhance the discharge capacity and prolong the cycle life of negative electrode sheets
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under high-rate partial-state-of-charge conditions, AC additive exerts more obvious
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effect than CB additive, the optimum contents for the best electrochemical performance of the negative electrode sheets are determined as 0.5wt% for both AC
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and CB. The reaction mechanism of the electrochemical process is also discussed in this paper, the appropriate addition of AC or CB in negative electrode can promote the conversion of PbSO4 to Pb, suppress the sulfation of negative electrode sheets and reduce the electrochemical reaction resistance.
Keywords: Negative electrode sheet; Lead-carbon battery; Carbon additive; Discharge capacity; Cycle life
1. Introduction The environment problem will undoubtedly become one of huge challenges to human beings, thus the development of clean energy technology which was meeting
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the requirements of environmental protection was heavily pursued. As portable
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energy, batteries have become an important part of human life. Hybrid electric
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vehicles (HEVs) are developed to reduce vehicle emission due to environmental
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concerns. As a prospective candidate energy-storage system for HEVs applications,
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valve-regulated lead-acid (VRLA) battery has many advantages including low initial
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cost, well-established manufacturing base, distribution networks and high recycling
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efficiency [1,2] compared to other competitive technologies [3-5]. However, in HEVs,
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the battery is operated continuously in high-rate partial-state-of-charge (HRPSoC)
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cycling. In the HRPSoC mode, batteries experience short charge and discharge pulses
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with high currents, high charging currents and exhaustive discharges cause an
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inhomogeneous acid density profile (acid stratification) of sulfuric acid used in the battery [6]. An early loss of capacity can be observed during a partial-state-of-charge
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mode because of the insufficient utilization of active mass and the discharged product, PbSO4, which has not efficiently converted back to its original form, leading to progressive accumulation of irreversible PbSO4 on negative active material (NAM) [7-10]. The accumulation of PbSO4 reduces the effective reaction area of negative
plates, and makes the charge and discharge processes of the negative plates difficult, finally leading to battery breakdown. To address the abovementioned problems, many efforts have been made over the past few decades. Battery researchers and engineers have tried various innovative cell design solutions [11-13]. UltraBattery, a new technology, was developed by Commonwealth Scientific and Industrial Research Organization (CSIRO) Energy
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Technology. UltraBattery, which combines an asymmetric supercapacitor and a
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lead-acid battery in one unit cell, will reduce the cost and prolong the life of lead-acid
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battery in HEVs. These favorable parameters of the technology have been described
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in detail in the literatures [14,15]. Some studies also reported that the introduction of
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carbon materials to the negative plates of lead-acid battery can improve discharge
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capacity, charge acceptance ability and increase cycle life of the negative plates.
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Nakamura’s experiment [16,17] proved that the addition of carbon black to the NAM
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can improve the charge acceptance significantly and retard the sulfation of negative
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plates during the simulated HRPSoC test of HEV batteries. Calabek et al. [18]
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established that the presence of carbon in NAM hinders the continuous growth of
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PbSO4 crystals, sustaining the formation of small crystallites with high solubility leading to efficient charge process. Some researchers [19-22] found that addition of
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carbon nanotubes to the NAM may enhance the charge acceptance, reduce energy losses, reserve capacity and cold-cranking performance of the battery. Pavlov [23] reported that electrochemical active carbon in the negative paste improves the charge efficiency and slows down sulfation of the negative plates. Saravanan et al. [24]
reported that carbon from sugar into the NAM enhance the charge-discharge characteristic of the lead-acid cells. Moseley [25] reported that capacitive phenomena occur when Pb active mass contains considerable amounts of carbon. During charging with high currents, the electric double layer on carbon surface was charged first, and this process occurs at the potentials between the stationary electrode potential of Pb and the potential of hydrogen evolution. The electric double layer was discharged
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slowly at the expense of the reduction of PbSO4 to Pb. Thus, “the capacitive element
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can support charge and discharge events that occur at the highest rates, and the
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Faradaic part of the cell can cope with events that take place over a longer timescale.”
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Moseley et al. [26] summarized the hypotheses proposed in the literature for the effect
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of carbons on the HRPSoC performance of the batteries as follows: (a) carbon
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enhances the overall conductivity of NAM [16]; (b) carbon facilitates the formation of
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small isolated PbSO4 particles that are easy to dissolve and restricts the growth of
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PbSO4 crystals [17]; (c) some carbon forms contain impurities which hinder the
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hydrogen evolution, thus improving the efficiency of charge [27]; (d) carbon acts as
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an electroosmotic pump that facilitates acid diffusion in the inner NAM volume at
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high rates of charge and discharge; high-surface area carbon materials particles exhibit a supercapacitive effect on NAM [28,29]. The use of carbon minimizes the
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uneven distribution of PbSO4 in the cross-section of negative electrode sheet, reduces discharge and charge current densities, and provides a conductive network to facilitate subsequent charging process [17,30]. Pavlov and coworkers [23] established that during the cycling of cells under HRPSoC conditions, the electrochemical reactions of
charge at the negative plates occur not only on the PbSO4, but also on the surface of the carbon as well. They proposed a parallel electrochemical mechanism for charge and draw the conclusion that carbon should be incorporated and change the structure of NAM to enhance the battery performance under HRPSoC conditions [31]. Hollenkamp et al. [32] reported that the specific surface area (SSA) of NAM is an important parameter to the electrochemical performance of the negative plates. Pavlov
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et al. [33] reported that carbon added to NAM increase the SSA and decrease the
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median pore radius of NAM, which may change the electrochemical performance.
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The addition of carbon materials to NAM can improve the performance of the
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negative plates, indeed. However, most researchers studied the influence of carbon
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additive on the negative plates of lead-carbon battery by an actual battery. On one
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hand, the battery performance in an actual battery depends on not only negative plate
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but also positive plate, so the problem focused on carbon additive in the negative
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electrode becomes more complicated in an actual battery; On the other hand, the
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preparation of an actual battery consumes lots of electrode materials. Furthermore, the
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optimum type and quantity of carbon additives and the mechanism for improved
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HRPSoC cycling performance of lead-carbon battery are still unclear and need further
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investigation. In this paper, the negative electrode sheets were prepared by simulating the
negative plate manufacturing process of lead-acid battery, the active mass in the negative electrode sheets was only about 0.2 g for a three-electrode system and 1.0 g for simulated flooded test cells, two types of commercially available carbon materials
(activated carbon (AC) and carbon black (CB)) were selected as the additives for the negative electrode sheets. The discharge capacity and electrochemical impedance spectroscopy (EIS) of lead-carbon negative sheets were evaluated in a three-electrode cell, and the HRPSoC cycling performance was measured in the simulated test cells. The effect of AC or CB on negative plates and their reaction mechanism were discussed according to the experimental results.
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2. Experimental
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2.1. Carbon additives to the negative active materials
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Two types of carbon additives were selected: activated carbon (AC) and carbon
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black (CB). The characteristics of activated carbon and carbon black were
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summarized in Table 1.
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Three different contents of each kind of the carbon were added to the negative
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sheets to determine the optimum amounts of each carbon type that have the most
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beneficial effect on the discharge capacity and cycle life of the negative electrode
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sheet.
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2.2. Preparation of negative electrode sheets
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The negative electrode sheets were prepared using lead oxide power, acetylene black, short carbon fibers, barium sulfate, humic acids, H2SO4 (1.28 g cm3), and
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different amounts of each carbon type. The degree of oxidation of lead oxide was 76%. The contents of acetylene black, barium sulfate, humic acids and short carbon fibers were 0.2 wt%, 0.6 wt%, 0.16 wt% and 0.05 wt% (versus the lead oxide), respectively. The amount of AC was varied from 0% to 1.0 wt%, and the content of
CB was 0.2 wt%, 0.5 wt%, and 1.0 wt%. The preparation method of the negative electrode sheets was as follows: first, some lead oxide and carbon additives were ground using an agate mortar for few minutes; then, other negative additives were added to the agate mortar and ground in turn. Short fibers uniformly dispersed by deionized water were added to the mortar and stirred for a few minutes. Finally, a certain amount of H2SO4 was added to the mixture dropwise, and deionized water was
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used to adjust the consistency of the lead paste. A certain amount of lead paste was
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coated on the Pb-Ca grid with a geometric area of 1×1 cm2 to prepare the negative
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sheets. All as-prepared negative sheets need to be cured under carefully controlled
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conditions under 35 oC for 2 hours, 65 oC for 4 hours and proper humidity, and then
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these negative electrode sheets were formed in H2 SO4 (1.06 g cm3) solution.
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2.3. Electrochemical measurements
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The electrochemical performance of negative electrode sheets was tested using a
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three-electrode cell, and the mass of the lead paste of the negative electrode sheet was
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about ~0.2 g. The prepared negative electrode sheet with a geometric area of 1×1 cm2
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was used as the working electrode. A platinum electrode foil with relatively large area
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was used as the counter electrode and Hg/Hg2SO4 electrode as the reference electrode. The electrolyte was H2SO4 with a density of 1.28 g cm-3. Chronopotentiometric (CP)
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tests were performed in the potential range from -0.8 V to -1.2 V vs. Hg/Hg2SO4 under several discharge rates (1, 2, 3, and 5C). The currents under discharge rates (1, 2, 3, and 5C) were determined according to the theoretical capacity. Electrochemical impedance spectroscopy (EIS) measurements were carried out by applying an
alternating current voltage of 5 mV amplitude in the frequency range from 0.01 Hz to 100 kHz at -1.2 V and -0.8 V. The CP and EIS analyses were carried out using a CHI660E electrochemical workstation. 2.4. Simulated test cell A simulation flooded lead-acid test cell was assembled comprising a small negative plate and an oversized positive plate separated by 3 mm thick AGM
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separator. The preparation process of the negative plates were kept same as above
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negative electrode sheets, but the mass of the lead paste of the negative plate was
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about ~1.0 g, and these cells were flooded with 200 ml H2SO4 (1.28 g cm3). The
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influences of AC or CB on the cycle performance of lead-acid battery under simulated
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HRPSoC conditions were investigated. First, the cells were fully charged at 20 h
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discharge rate, and then the cells were discharged to 50% state-of-charge (SoC) at 1C
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rate in this cycling profile. Next, the charge/discharge cycle processes were carried
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out according to the following schedule: charge at 1C or 2C rate for 30 s, rest for 10 s,
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discharge at 1C or 2C rate for 30 s, rest for 10 s. The cell voltage was measured at the
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end of each charge and discharge processes, and the test process was stopped when
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the cut-off discharging voltage decreased to 1.70 V. All the tests were conducted at room temperature. The currents for 1 and 0.5 h discharge rates were 1C and 2C,
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respectively, as determined from the Peukert plot. 2.5. Physical characterizations The surface morphologies of negative electrode sheets were observed under a high-resolution scanning electron microscope (SEM, JSM-6380). The crystalline
phase of the negative electrode sheets was examined by powder X-ray diffraction (XRD) between 10-90° using Cu-Kα radiation on a Y-2000 X-ray generator. 3. Results and discussion 3.1. Effects of carbon additives on the discharge capacity of negative electrode sheets 3.1.1 Effects of AC Fig. 1 shows the discharge curves of the negative electrode sheets with different
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contents of AC at 3C and 5C discharge rates. The experimental results show that the
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discharge time is significantly affected by the amount of AC in NAM. It can be seen
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from Fig. 1 that, at 3C and 5C discharge rates, the discharge time of the negative
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electrode sheets with 0.2, 0.5, or 1.0 wt% AC are longer than those without AC. At a
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certain discharge current rate, with increasing amount of AC, the discharge time of the
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negative electrode sheets initially increases and then decreases. Clearly, the negative
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and 5C discharge rates.
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electrode sheets with 0.5 wt% AC in NAM obtain the longest discharge time at 3C
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Fig. 2 indicates the relationship between the discharge specific capacity of
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negative electrode sheets with various AC content and different discharge rates. It can
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be found from Fig. 2 that within the entire range of discharge rates, the discharge specific capacity of the negative electrode sheets with 0.2, 0.5, or 1.0 wt% AC are
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higher than those without AC, indicating that appropriate addition of AC contributes to the discharge specific capacity. It also can be seen from Fig. 2 that at a certain discharge rate, the discharge specific capacity of the negative electrode sheets first increases and then decreases with increasing AC content. The optimal amount of AC
is 0.5 wt%, at this content, the discharge specific capacity from 1C to 5C is 119, 89.2, 86.6, and 71.8 mAh g-1, which increases by 26.6%, 15.8%, 40.1%, and 61.7%, respectively, compared to the negative electrode sheets without AC addition. Therefore, it can be concluded that the addition of AC improves the utilization of active materials and increases the discharge specific capacity of the negative electrode sheets, particularly at a high discharge rate.
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3.1.2 Effects of CB
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Fig. 3 presents the discharge curves of the negative electrode sheets with
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different contents of CB at 3C and 5C discharge rates. The experimental results show
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that the amount of CB in NAM significantly affects discharge time. It can be seen
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from Fig. 3 that at 3C and 5C discharge rates, the discharge times of the negative
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electrode sheets with 0.2 and 0.5 wt% CB are longer than those without CB, whereas
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the discharge time is shorter at 1.0 wt% loading level. It also can be found from Fig. 3
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that with increasing content of CB, the discharge time of the negative electrode sheets
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initially increases and then decreases at a certain discharge current rate. Obviously,
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the negative electrode sheet with 0.5 wt% CB in NAM exhibits the longest discharge
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time at 3C and 5C discharge rates. Fig. 4 illustrates the correlations between the discharge specific capacity of the
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negative electrode sheets with three different content of CB in NAM and discharge rates. It can be seen from Fig.4 that within the entire range of discharge rates, the discharge specific capacity of the negative electrode sheets with 0.2 and 0.5 wt% CB are higher than those without CB, whereas the discharge specific capacity is lower at
1.0 wt% CB loading level, indicating that appropriate CB content contribute to the discharge specific capacity. Fig. 4 also shows that at a certain discharge rate, the discharge specific capacity of the negative electrode sheets initially increases and then decreases with increasing amount of CB. The optimal content of CB is 0.5 wt%, in this content, the specific capacity from 1C to 5C is 109.3, 89.0, 71.8, and 55.0 mAh g-1, which increases by 16.2%, 15.6%, 16.2%, and 23.8%, respectively, compared to
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the negative electrode sheets without CB. Based on the above results, it can be
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concluded that the addition of CB improves the utilization of active materials and
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increases the discharge capacity of the negative electrode sheet, especially at a high
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discharge rate.
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The data in the Figs. 2 and 4 evidences that the appropriate addition of AC or CB
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in the negative electrode sheets can increase the utilization of active mass, prolong
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discharge time, and increase the discharge capacity of negative electrode sheets,
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particularly at a high discharge rate. Furthermore, it also can be found that, within the
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entire range of discharge rates, the discharge time and specific capacity of the
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negative electrode sheets with AC are higher than those with CB at 0.5 wt% loading
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level. This can be mainly attributed to the characteristics of the two types of carbon materials, as described in Table 1. The SSA of AC (1800-2000 m2 g-1) is larger than
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that of CB (254 m2 g-1), the larger the SSA is, and the better the performance of capacitance is, thus improving the discharge capacity. Notably, the amount of AC or CB in NAM should be controlled in a reasonable level (below 0.5wt%), otherwise the amount of Pb active material in negative electrode sheets will decrease, and the
negative electrode sheets become loose due to high content of AC or CB with low density during charge-discharge process, finally leading to a shorter discharge time and a lower discharge capacity. 3.2 Effects of carbon additives on the cycle performance of negative plates under HRPSoC conditions The battery for HEVs is required to be charged and discharged continuously
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under HRPSoC conditions, leading to the progressive accumulation of PbSO4 on the
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negative plates. To investigate the effects of AC or CB on negative plates, a
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simulation flooded lead-acid test cell was assembled comprising a small negative
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plate and an oversized positive plate separated by 3 mm thick AGM separator. An
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oversized positive plate was used in this flooded lead-acid cell to reduce the effect of
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positive electrode plate on electrochemical performance.
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3.2.1 Effects of AC
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Fig. 5 presents the discharge curve of cells with various amounts of AC in NAM
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at different discharge rates (1C and 2C) under HRPSoC conditions. The experimental
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results show that the cycle life of negative plates is influenced by the addition of AC,
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apparently. It can be seen from Fig. 5 that at 1C discharge rate, with increasing amount of AC, the cycle numbers of the negative plates are 3010, 4520, 12057, and
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10817, which increased by 0%, 50.2%, 300.6%, and 259.4%, respectively, indicating that the addition of AC to NAM can prolong the cycle life of the negative electrode sheets. At 2C discharge rate, the cycle life increased by 0%, 68.2%, 366.6%, and 280.9%, respectively, indicating that appropriate AC significantly affects the cycle life
at a high discharge rate. It is obvious that the negative plates shows the best cycle performance at 0.5 wt% AC loading level. The addition of AC can increase electrochemically active surface, reduce electrode polarization, improve utilization of active material, and then enhance cycle performance. Thus, it can be concluded that the addition of AC can prolong the cycle life of negative plates, especially at a high discharge rate.
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3.2.2 Effects of CB
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Fig. 6 shows the discharge curve of the cells with different CB content at
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different discharge rates (1C and 2C) under HPRSoC conditions. A similar behavior is
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also observed with the cells with CB, which implies that the addition amount of CB
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exerts an influence on the cycle life of negative plates. It can be seen from Fig. 6 that
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at 1C discharge rate, with increasing amount of CB, the cycle numbers of the negative
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plates are 3010, 9050, 10701, and 8400, which increased by 0%, 200.7%, 255.5%,
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and 179.1%, respectively, indicating that CB added into the NAM can prolong the
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cycle life of negative plates. The influence is more obviously at 2C discharge rate (the
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cycle life increased by 0%, 240.6%, 295.3%, and 207.9%, respectively), which
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indicates that appropriate CB significantly affects the cycle life at a high discharge rate. It is obvious that the cycle number reaches the maximum value when the amount
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of CB was 0.5 wt%. The addition of CB can enhance the conductivity of negative sheets, reduce electrode polarization, and then improve the utilization of active material and prolong the cycle life of negative plates. Therefore, it can be concluded that the addition of CB to the NAM can prolong the cycle life of negative plates, and
the effect is more apparent at a high discharge rate. From the above experiment results, we conclude that the appropriate AC or CB can prolong the cycle life of negative electrode sheet, except for the difference that the influences of AC on cycle performance are better than that of CB when the amount of carbon additive is 0.5 wt%, the reasons may probably because the SSA of AC (1800-2000 m2 g-1) is larger than that of CB (254 m2 g-1). A higher SSA enhances
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electrochemically active surface, decreases electrode polarization, thus improving the
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charge efficiency and cycle performance of negative plates. M. Fernandez et al. [28]
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reported that the addition of medium/high SSA carbon to NAM affected the
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performance negative plates. Thus, the characteristics of carbon additives have a great
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effect on battery cycle performance.
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3.3 Effects of carbon additives on the electrochemical impedance of negative
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electrode sheet
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Electrochemical impedance measurements were performed to gain deeper insight
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into the effect of carbon additives on negative electrode sheet. EIS measurements
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were carried out by applying an alternating current voltage of 5 mV amplitude in the
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frequency range from 0.01 Hz to 100 kHz at -1.2 V and -0.8 V. The impedance of the as-prepared electrode sheets was investigated at -0.8 V (discharge state) and -1.2 V
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(charge states). In general, the x-intercept on real axis (Z) represents a combined resistance (Rs) including ionic electrolyte resistance and contact resistance; the diameter semicircle is associated with the electrochemical reaction resistance (Rct) caused by Faradic reactions and electrical double-layer capacitor at the
electrode/electrolyte interface. Figs. 7 and 8 show the EIS of negative electrode sheets with different AC and CB contents in different potentials, respectively. As shown in the Figs. 7 and 8, AC and CB have a great effect on the Rct of negative electrode sheets. All the measured impedance plots have similar semicircle shape, and the Rct is calculated from the diameter of the semicircle. It can be seen from Fig. 7 that in the discharge and charge
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states, with increasing amount of AC, the diameter of semicircle initially decreases
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and then increases, indicating that the Rct initially decreases and then increases. The
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Rct reaches the minimum value at 0.5 wt% AC loading level. Fig. 8 also shows that,
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with the increase of CB content, the Rct first decreases and then increases, when the
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amount of CB is increased to 0.5 wt%, the Rct reaches the minimum value. It is
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obvious from Figs. 7 and 8 that the Rct in the charge state is clearly smaller than that
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in the discharge state. In the discharge state, the main composition of NAM is PbSO4;
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the presence of carbon particles among PbSO4 particles can form a conductive
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network, facilitate the formation of numerous small-sized and uniform PbSO4 crystals
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with higher solubility, and then reduce the Rct. Moseley [25] reported that carbon
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enhances the overall conductivity of NAM. After discharging, the negative electrode sheet was recharged. During charging, the addition of AC or CB increases the
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electrochemically active surface of the NAM, decreases electrode polarization and provides many sites for H2SO4 accommodation which promotes the transformation between PbSO4 and Pb, then reduces the Rct. Notably, the amount of AC or CB in NAM should be controlled in a reasonable level (below 0.5wt%), otherwise the
negative plate becomes loose due to high content of carbon with low density during charge-discharge process, so the Rct of negative electrode sheets increases. The Rct in the discharge state is clearly larger than that in the charge state, this is because the main component is PbSO4 in the discharge state and Pb in the charge state, the conductivity of PbSO4 is far less than Pb. Table 2 summarizes the Rct of the negative electrode sheets with different
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contents of AC and CB in different potentials. It can be found from Table 2 that there
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are some differences between AC and CB on Rct, especially in discharge state The Rct
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of negative sheets with CB is smaller than that with AC in discharge state, this is
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probably due to the conductivity of CB is better than that of AC.
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Therefore, it can be concluded that the appropriate addition of AC or CB to the
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NAM can improve the conductivity of negative electrode sheets and reduce the Rct,
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thus leading to higher discharge specific capacity and better cycle performance.
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Furthermore, the type of carbon additives affects the Rct.
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3.4 Effect of carbon additive on the microstructure of NAM
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The experimental results indicate that AC and CB significantly affect the
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discharge capacity and cycle performance of negative electrode sheets. To better understand the influences of AC or CB on the discharge capacity and cycle
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performance of negative electrode sheets, the surface morphologies of the negative electrode sheets with different contents of AC or CB were investigated by SEM. Because the negative electrode sheets were maintained in the discharge state after 2000 cycling, the main component of the negative electrode sheets was PbSO4.
Figs. 9 and 10 show the SEM images of negative electrode sheets with different contents of AC and CB after 2000 cycling, respectively. It can be found from Fig. 9 that the negative electrode sheets without AC contains some relatively uneven PbSO4 crystals including large and small PbSO4 particles, and small PbSO4 particles are present in the gap among the large PbSO4 particles. When AC was added to negative electrode sheet, PbSO4 particles become smaller and well distributed; the pores
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among PbSO4 particles increase. Xiang et al. [34] reported that carbon additives can
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optimize NAM microstructure. At 0.5 wt% AC loading level, the PbSO4 crystals
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distribute more homogeneously and generate smaller PbSO4 particles which dissolve
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more readily in the subsequent electrochemical reaction for Pb formation. It also can
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be found from Fig. 10 that the addition of CB to the NAM changes the shape and size
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of PbSO4 crystals. With increasing content of CB, it affects the growth of PbSO4
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crystals. At 0.5 wt% CB loading level, the PbSO4 crystals have well shaped crystal
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faces and edges and become well distributed. These PbSO4 crystals are more easily
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dissolved, which is helpful for transforming PbSO4 to Pb. Therefore, it can be
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concluded that appropriate addition of AC or CB can increase the active surface of
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NAM, make PbSO4 well distributed, hence reduces the size of PbSO4 crystals and the Rct of the NAM. Finally, the negative electrode sheet with AC or CB obtains a greater
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discharge specific capacity and possessed a better cycle performance 3.5 Composition analysis of NAM with carbon additives under simulated HRPSoC conditions Many researchers reported that the NAMs suffer from the sulfation phenomenon
after several thousand cycles, thus producing a portion of irreversible PbSO4 particles in the NAM. In order to explore the sulfation of NAMs with different contents of AC or CB, the active materials in the negative electrode sheets were characterized by XRD analysis in fully charged state after HRPSoC cycle tests. Fig. 11 shows the XRD patterns of the NAMs with different contents of AC (Fig. 11a) and CB (Fig. 11b), respectively. As evident from Fig. 11, almost same XRD
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patterns are observed for the several negative electrode sheets, the main characteristic
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peaks of Pb and PbSO4 and their intensities (IPb and IPbSO4) are marked. It can be seen
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from Fig. 11 that the intensities of the main characteristic varied with addition amount
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of AC or CB. The intensity of the characteristic peaks of Pb varies faster than that of
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PbSO4 with increasing content of AC or CB. In order to determine the relative content
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of Pb and PbSO4 in the active materials, the ratio of IPb/IPbSO4 is calculated, as shown
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in Table 3. It also can be found from Table 3 that the IPb/IPbSO4 value initially
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increases and then decreases with increasing content of AC or CB, and the IPb/IPbSO4
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value reaches the maximum value when the content of AC or CB is 0.5 wt%. A higher
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IPb/IPbSO4 value indicates that less PbSO4 particles are present in the active materials,
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and the IPb/IPbSO4 value of the active materials with AC or CB is higher than that without AC or CB. The phenomenon shows that appropriate AC or CB can promote
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the transformation of PbSO4 into Pb and reduce the formation of irreversible PbSO4. Therefore, it can be concluded that the addition of appropriate amount of AC or CB to NAM can improve the utilization of active mass, promote conversion of PbSO4 to Pb, and suppress the sulfation in negative electrode especially in 0.5wt% carbon content,
Finally, the negative plates with AC or CB addition obtains higher discharge specific capacity and better cycle performance. The experimental results indicate that the addition of AC or CB can reduce the formation of irreversible PbSO4 and improve the utilization of active mass. Fig. 12 illustrates the schematic diagram for the effects of AC or CB on negative electrode sheets under HPRSoC conditions [34]. Under high-rate discharge conditions, some
PT
large and uneven PbSO4 particles and a compact PbSO4 layer on the surface are
RI
formed, and some Pb particles surrounded by irreversible PbSO4 particles (marked by
SC
red circle) are difficult to oxidize. During high-rate charging, the compact PbSO4
U
layer on the surface prevents the electrolyte to enter the interior of negative plates
N
during charging, which leads to many PbSO4 particles reduced difficulty. When AC
A
or CB are added to NAM, under high-rate discharge conditions, the addition of AC or
M
CB results in much electrochemically active surface inside the NAM during the
D
discharging process, which can provide extra nucleation sites for PbSO4 crystals and
TE
facilitate the formation of numerous small-sized and uniform PbSO4 crystals with
EP
higher solubility. In the subsequent high-rate charging process, the carbon particles
CC
can provide much active site for H2SO4 accommodation and form a conductive network among PbSO4 crystals; on the other hand, small and uniform PbSO4 particles
A
dissolve readily during the electrochemical reaction of Pb, thus promoting the transformation between Pb and PbSO4.and suppressing the sulfation. Thus, appropriate addition of AC or CB in the NAMs can suppress the accumulation of PbSO4, improve the utilization of active mass and obtain a higher specific capacity
and longer cycle life [15, 35-39]. 4. Conclusions In this study, two kinds of carbon materials (AC and CB) are used as additives in the negative electrode, negative electrode sheets are prepared by simulating the negative plate manufacturing process of lead-acid battery, and the effects of AC or CB on the electrochemical performance of negative electrode sheets are investigated in
PT
detail. The electrochemical experimental results indicate that an appropriate amount
RI
of AC or CB can increase the discharge specific capacity and prolong the cycle life of
SC
negative electrode sheets under HRPSoC conditions. The optimum amount of AC or
U
CB is selected as 0.5 wt%, and the addition of AC exerts more obvious effect than CB
N
on the electrochemical performance. The addition of AC or CB in NAM can reduce
A
the size of PbSO4 crystals by SEM observation and reduce the electrochemical
M
reaction resistance of the negative electrode sheets by EIS analysis, the XRD results
D
indicate that the appropriate addition of AC or CB in the NAMs can promote the
TE
conversion between PbSO4 and Pb, then suppress the sulfation of negative electrode
EP
sheet. Therefore, the discharge specific capacity and cycle life of the negative
CC
electrode sheet of lead-carbon battery can be improved, and the longer cycle life may be developed for HEV battery market and other fields.
A
Acknowledgement The authors wish to acknowledge the following financial supporters of this work: the National Natural Science Foundation of China (Grant No. 21273085), the Guangdong Province Science and Technology Bureau (Grant No. 2010B090400552),
the Natural Science Foundation of Guangdong Province, China (Grant No. S2011010003416 and S2013010012927), the Project of Guangzhou Science and Information Technology Bureau (Grant No. 2012J4300147) and the Fundamental Research Funds for the Central Universities (No. 13lgjc10). References [1] D. Pech, T. Brousse, D. Bélanger, D. Guay, Electrochim. Acta 54 (2009) 7382-
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Atanassova , Jesús. Valenciano, J. Power Sources 239 (2013) 483-489. [38] D. Pavlov, P. Nikolov, J. Electrochemical Society 159 (2012) 1215-1225.
A
CC
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D
M
A
N
U
SC
RI
PT
[39] P. Krivik, P. Baca, K. Tonar, P. Toser, ECS Transactions 40 (2012), 145-152.
Figures captions Fig.1 The discharge time results for negative electrode sheets with different contents of activated carbon at 3C and 5C discharge rates. Fig.2 Discharge specific capacity of negative electrode sheets with different contents of activated carbon in different discharge rates. Fig.3 The discharge time results for negative electrode sheets with different contents
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of carbon black at 3C and 5C discharge rates. Fig.4 Discharge specific capacity of negative electrode sheets with different contents
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of carbon black in different discharge rates.
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Fig.5 Discharge curve of cells with different amounts of activated carbon in different
U
discharge rates (a: 1C, b: 2C) under HRPSoC conditions.
A
N
Fig.6 Discharge curve of cells with different amounts of carbon black in different
M
discharge rates (a: 1C, b: 2C) under HRPSoC conditions.
D
Fig.7 Electrochemical impedance spectroscopy of negative electrode sheets with
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different contents of activated carbon in different state.
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Fig.8 Electrochemical impedance spectroscopy of negative electrode sheets with different contents of carbon black in different state.
CC
Fig.9 SEM images of negative electrode sheets with different concentrations (a: 0 %,
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b: 0.2 %, c: 0.5 %) of activated carbon after 2000 cycling. Fig.10 SEM images of negative electrode sheets with different concentrations (a: 0 %, b: 0.2 %, c: 0.5 %) of carbon black after 2000 cycling. Fig.11 XRD patterns of negative active materials with different contents of carbon additive ( a: AC, b: CB) under HRPSoC conditions.
Fig.12 Schematic diagram for the effects of carbon additives on negative electrode
A
CC
EP
TE
D
M
A
N
U
SC
RI
PT
sheets under HPRSoC conditions.
Fig .1
-1.000
-0.950 -0.925 -0.900 -0.875 -0.850 -0.825
PT
Potential / V vs. Hg/Hg2SO4
0% 0.2% 0.5% 1.0%
3C
-0.975
-0.800
-0.750 50
100
150
200
250
300
400
450
N
U
Discharge time / s
350
SC
0
RI
-0.775
A
-1.000
5C
D
-0.925
-0.850
TE
-0.900
EP
Potential / V vs. Hg/Hg2SO4
-0.950
-0.875
0% 0.2% 0.5% 1.0%
M
-0.975
-0.825
CC
-0.800 -0.775
A
-0.750 0
20
40
60
80
100
120
140
Discharge time / s
160
180
200
220
Fig.2
0% 0.2% 0.5% 1.0%
120
100 90 80 70 60
PT
Specific capacity / mAh g
-1
110
40 2
3
4
A
CC
EP
TE
D
M
A
N
U
Discharge rate / C
5
SC
1
RI
50
Fig .3
-1.000
-0.950 -0.925 -0.900 -0.875 -0.850 -0.825
PT
Potential / V vs. Hg/Hg2SO4
0% 0.2% 0.5% 1.0%
3C
-0.975
-0.800
-0.750 50
100
150
200
250
350
M
A
N
U
Discharge time / s
300
SC
0
RI
-0.775
-1.000
5C
D
0% 0.2% 0.5% 1.0%
TE
-0.950 -0.925 -0.900
EP
Potential / V vs. Hg/Hg2SO4
-0.975
-0.875
A
CC
-0.850 -0.825 -0.800 -0.775 -0.750
0
20
40
60
80
100
Discharge time / s
120
140
160
Fig.4
110
0% 0.2% 0.5% 1.0%
90 80 70 60 50 40
PT
Specific capacity / mAh g
-1
100
30
10 0 2
3
4
A
CC
EP
TE
D
M
A
N
U
Discharge rate / C
5
SC
1
RI
20
Fig.5
2.2
0% 0.2% 0.5% 1.0%
a 2.1
Voltage / V
2.0
1.9
PT
1.8
1.6 2000
4000
6000
8000
10000
12000
14000
N
U
Cycle Number
SC
0
RI
1.7
A
2.2
D
2.1
TE
EP
Voltage / V
2.0
1.9
0% 0.2% 0.5% 1.0%
M
b
A
CC
1.8
1.7
1.6 0
2000
4000
6000
Cycle Number
8000
10000
Fig.6
2.2
0% 0.2% 0.5% 1.0%
a 2.1
Voltage / V
2.0
1.9
PT
1.8
1.6 0
2000
4000
6000
8000
10000
12000
N
U
SC
Cycle Number
RI
1.7
2.2
A
b
M
2.1
1.8
D TE
1.9
EP
Voltage / V
2.0
0% 0.2% 0.5% 1.0%
A
CC
1.7
1.6 0
2000
4000
Cycle Number
6000
8000
Fig.7 (a) Discharge state (-0.8 V)
300
a
0% 0.2% 0.5% 1.0%
250
150
100
PT
-Z'' / Ohm
200
0 50
100
150
200
250
300
SC
0
RI
50
U
Z' / Ohm
A
N
(b) Charge state (-1.2 V)
16
b
TE
8
EP
-Z'' / Ohm
10
D
12
0% 0.2% 0.5% 1.0%
M
14
6
A
CC
4 2 0
0
2
4
6
8
Z' / Ohm
10
12
14
16
Fig.8 (a) Discharge state (-0.8 V)
300
a
0% 0.2% 0.5% 1.0%
250
-Z'' / Ohm
200
150
PT
100
0 50
100
150
200
250
300
SC
0
RI
50
N
U
Z' / Ohm
M
A
(b) Charge state (-1.2 V)
16
b
A
CC
EP
-Z'' / Ohm
8
TE
12 10
0% 0.2% 0.5% 1.0%
D
14
6 4 2 0
0
2
4
6
8
Z' / Ohm
10
12
14
16
TE
EP
CC
A D
PT
RI
SC
U
N
A
M
Fig.9
a b
c
TE
EP
CC
A D
PT
RI
SC
U
N
A
M
Fig.10
a b
c
Fig.11
a
450
Pb PbSO4 1.0%
720
Intensity /a.u.
608 1000 0.5%
0.2% 696
RI
540
PT
576 815
0% 20
40
60
80
100
485
750
Pb PbSO4 1.0%
TE
D
b
M
A
N
U
2theta / deg
SC
0
EP
Intensity / a.u.
569
601
950 0.5% 720
CC
0.2% 696
540
A
0%
0
20
40
60
2theta / deg
80
100
Fig.12
Discharge
Charge
SC
RI
PT
(a)The negative electrode sheets without carbon additives
Charge
M
A
N
U
Discharge
TE
H+ SO42C
D
(b)The negative electrode sheets with carbon additives
CC
EP
Pb PbSO4
A
Fig. 12
Table 1 Characteristics of the carbon additives added into NAM. Symbol
Type of material
AC CB
Activated carbon Carbon black
Conductivity (S cm)
BET m2/g
0.35 70
surface
Particle size
1800-2000 254
5-10μm 30 nm
Table 2 The electrochemical reaction resistance of the negative electrode sheets with
Charge AC
CB 485.0 322.5 205.0 260.0
15.5 6.2 3.9 13.1
SC
485.0 388.5 225.0 330.6
state
CB
15.5 9.6 2.1 5.5
A
N
U
0% 0.2% 0.5% 1.0%
state
RI
Discharge AC
PT
different contents of activated carbon (AC) or carbon black (CB) in different state.
M
Table 3 The intensities (IPb and IPbSO4) of the main characteristic peaks of NAM with
D
different activated carbon (AC) or carbon black (CB) content and the ratio of
TE
IPb/IPbSO4 in charge state.
EP
0%
A
CC
IPb IPbSO4 IPb/IPbSO4
540 696 0.78
Activated 0.2%
Carbon 0.5%
1.0%
815 576 1.41
1000 608 1.64
720 450 1.60
Carbon 0.2% 720 601 1.20
Black 0.5%
1.0%
950 569 1.70
750 485 1.55
S0013-4686(14)02222-1 http://dx.doi.org/doi:10.1016/j.electacta.2014.11.027 EA 23699
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
6-8-2014 4-10-2014 5-11-2014
Please cite this article as: Xianping Zou, Zongxuan Kang, Dong Shu, Yuqing Liao, Yibin Gong, Chun He, Junnan Hao, Yayun Zhong, Effects of carbon additives on the performance of negative electrode of lead-carbon battery, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.11.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of carbon additives on the performance of negative electrode of lead-carbon battery Xianping Zou a, Zongxuan Kang a, Dong Shu a, c, d*, Yuqing Liao a, Yibin Gong b, Chun He b, Junnan Hao a, Yayun Zhong a a
School of Chemistry and Environment, South China Normal University, Guangzhou
510006, P. R. China b
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School of Environmental Science and Engineering, Sun Yat-sen University,
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Guangzhou 510275, P. R. China c
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Base of Production, Education & Research on Energy Storage and Power Battery of
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Guangdong Higher Education Institutes, Guangzhou 510006, P. R. China d
N
Guangzhou Key Laboratory of Materials for Energy Conversion and Storage，
Corresponding author. E-mail address: [email protected] (Dong Shu)
The negative electrode sheets are prepared by simulating manufacture
EP
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Highlights
D
M
*
A
Guangzhou 510006, P. R. China
CC
condition of negative plates.
The effect of carbon additives on negative electrode sheets is studied by
A
electrochemical method.
Carbon additives in NAM enhance electrochemical properties of the negative sheets
The negative sheets with 0.5 wt% carbon additive exhibit better
electrochemical performance.
The charge-discharge mechanism is discussed in detail according to the experimental results.
Abstract In this study, carbon additives such as activated carbon (AC) and carbon black (CB) are introduced to negative electrode to improve its electrochemical performance,
PT
the negative electrode sheets are prepared by simulating the negative plate
RI
manufacturing process of lead-acid battery, the types and contents of carbon additives
SC
in the negative electrode sheets are investigated in detail for the application of
by
chronopotentiometry,
galvanostatic
charge-discharge
and
N
measured
U
lead-carbon battery. The electrochemical performance of negative electrode sheets are
A
electrochemical impedance spectroscopy, the crystal structure and morphology are
M
characterized by X-ray diffraction and scanning electron microscopy, respectively.
D
The experimental results indicate that the appropriate addition of AC or CB can
TE
enhance the discharge capacity and prolong the cycle life of negative electrode sheets
EP
under high-rate partial-state-of-charge conditions, AC additive exerts more obvious
CC
effect than CB additive, the optimum contents for the best electrochemical performance of the negative electrode sheets are determined as 0.5wt% for both AC
A
and CB. The reaction mechanism of the electrochemical process is also discussed in this paper, the appropriate addition of AC or CB in negative electrode can promote the conversion of PbSO4 to Pb, suppress the sulfation of negative electrode sheets and reduce the electrochemical reaction resistance.
Keywords: Negative electrode sheet; Lead-carbon battery; Carbon additive; Discharge capacity; Cycle life
1. Introduction The environment problem will undoubtedly become one of huge challenges to human beings, thus the development of clean energy technology which was meeting
PT
the requirements of environmental protection was heavily pursued. As portable
RI
energy, batteries have become an important part of human life. Hybrid electric
SC
vehicles (HEVs) are developed to reduce vehicle emission due to environmental
U
concerns. As a prospective candidate energy-storage system for HEVs applications,
N
valve-regulated lead-acid (VRLA) battery has many advantages including low initial
A
cost, well-established manufacturing base, distribution networks and high recycling
M
efficiency [1,2] compared to other competitive technologies [3-5]. However, in HEVs,
D
the battery is operated continuously in high-rate partial-state-of-charge (HRPSoC)
TE
cycling. In the HRPSoC mode, batteries experience short charge and discharge pulses
EP
with high currents, high charging currents and exhaustive discharges cause an
CC
inhomogeneous acid density profile (acid stratification) of sulfuric acid used in the battery [6]. An early loss of capacity can be observed during a partial-state-of-charge
A
mode because of the insufficient utilization of active mass and the discharged product, PbSO4, which has not efficiently converted back to its original form, leading to progressive accumulation of irreversible PbSO4 on negative active material (NAM) [7-10]. The accumulation of PbSO4 reduces the effective reaction area of negative
plates, and makes the charge and discharge processes of the negative plates difficult, finally leading to battery breakdown. To address the abovementioned problems, many efforts have been made over the past few decades. Battery researchers and engineers have tried various innovative cell design solutions [11-13]. UltraBattery, a new technology, was developed by Commonwealth Scientific and Industrial Research Organization (CSIRO) Energy
PT
Technology. UltraBattery, which combines an asymmetric supercapacitor and a
RI
lead-acid battery in one unit cell, will reduce the cost and prolong the life of lead-acid
SC
battery in HEVs. These favorable parameters of the technology have been described
U
in detail in the literatures [14,15]. Some studies also reported that the introduction of
N
carbon materials to the negative plates of lead-acid battery can improve discharge
A
capacity, charge acceptance ability and increase cycle life of the negative plates.
M
Nakamura’s experiment [16,17] proved that the addition of carbon black to the NAM
D
can improve the charge acceptance significantly and retard the sulfation of negative
TE
plates during the simulated HRPSoC test of HEV batteries. Calabek et al. [18]
EP
established that the presence of carbon in NAM hinders the continuous growth of
CC
PbSO4 crystals, sustaining the formation of small crystallites with high solubility leading to efficient charge process. Some researchers [19-22] found that addition of
A
carbon nanotubes to the NAM may enhance the charge acceptance, reduce energy losses, reserve capacity and cold-cranking performance of the battery. Pavlov [23] reported that electrochemical active carbon in the negative paste improves the charge efficiency and slows down sulfation of the negative plates. Saravanan et al. [24]
reported that carbon from sugar into the NAM enhance the charge-discharge characteristic of the lead-acid cells. Moseley [25] reported that capacitive phenomena occur when Pb active mass contains considerable amounts of carbon. During charging with high currents, the electric double layer on carbon surface was charged first, and this process occurs at the potentials between the stationary electrode potential of Pb and the potential of hydrogen evolution. The electric double layer was discharged
PT
slowly at the expense of the reduction of PbSO4 to Pb. Thus, “the capacitive element
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can support charge and discharge events that occur at the highest rates, and the
SC
Faradaic part of the cell can cope with events that take place over a longer timescale.”
U
Moseley et al. [26] summarized the hypotheses proposed in the literature for the effect
N
of carbons on the HRPSoC performance of the batteries as follows: (a) carbon
A
enhances the overall conductivity of NAM [16]; (b) carbon facilitates the formation of
M
small isolated PbSO4 particles that are easy to dissolve and restricts the growth of
D
PbSO4 crystals [17]; (c) some carbon forms contain impurities which hinder the
TE
hydrogen evolution, thus improving the efficiency of charge [27]; (d) carbon acts as
EP
an electroosmotic pump that facilitates acid diffusion in the inner NAM volume at
CC
high rates of charge and discharge; high-surface area carbon materials particles exhibit a supercapacitive effect on NAM [28,29]. The use of carbon minimizes the
A
uneven distribution of PbSO4 in the cross-section of negative electrode sheet, reduces discharge and charge current densities, and provides a conductive network to facilitate subsequent charging process [17,30]. Pavlov and coworkers [23] established that during the cycling of cells under HRPSoC conditions, the electrochemical reactions of
charge at the negative plates occur not only on the PbSO4, but also on the surface of the carbon as well. They proposed a parallel electrochemical mechanism for charge and draw the conclusion that carbon should be incorporated and change the structure of NAM to enhance the battery performance under HRPSoC conditions [31]. Hollenkamp et al. [32] reported that the specific surface area (SSA) of NAM is an important parameter to the electrochemical performance of the negative plates. Pavlov
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et al. [33] reported that carbon added to NAM increase the SSA and decrease the
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median pore radius of NAM, which may change the electrochemical performance.
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The addition of carbon materials to NAM can improve the performance of the
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negative plates, indeed. However, most researchers studied the influence of carbon
N
additive on the negative plates of lead-carbon battery by an actual battery. On one
A
hand, the battery performance in an actual battery depends on not only negative plate
M
but also positive plate, so the problem focused on carbon additive in the negative
D
electrode becomes more complicated in an actual battery; On the other hand, the
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preparation of an actual battery consumes lots of electrode materials. Furthermore, the
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optimum type and quantity of carbon additives and the mechanism for improved
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HRPSoC cycling performance of lead-carbon battery are still unclear and need further
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investigation. In this paper, the negative electrode sheets were prepared by simulating the
negative plate manufacturing process of lead-acid battery, the active mass in the negative electrode sheets was only about 0.2 g for a three-electrode system and 1.0 g for simulated flooded test cells, two types of commercially available carbon materials
(activated carbon (AC) and carbon black (CB)) were selected as the additives for the negative electrode sheets. The discharge capacity and electrochemical impedance spectroscopy (EIS) of lead-carbon negative sheets were evaluated in a three-electrode cell, and the HRPSoC cycling performance was measured in the simulated test cells. The effect of AC or CB on negative plates and their reaction mechanism were discussed according to the experimental results.
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2. Experimental
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2.1. Carbon additives to the negative active materials
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Two types of carbon additives were selected: activated carbon (AC) and carbon
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black (CB). The characteristics of activated carbon and carbon black were
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summarized in Table 1.
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Three different contents of each kind of the carbon were added to the negative
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sheets to determine the optimum amounts of each carbon type that have the most
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beneficial effect on the discharge capacity and cycle life of the negative electrode
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sheet.
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2.2. Preparation of negative electrode sheets
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The negative electrode sheets were prepared using lead oxide power, acetylene black, short carbon fibers, barium sulfate, humic acids, H2SO4 (1.28 g cm3), and
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different amounts of each carbon type. The degree of oxidation of lead oxide was 76%. The contents of acetylene black, barium sulfate, humic acids and short carbon fibers were 0.2 wt%, 0.6 wt%, 0.16 wt% and 0.05 wt% (versus the lead oxide), respectively. The amount of AC was varied from 0% to 1.0 wt%, and the content of
CB was 0.2 wt%, 0.5 wt%, and 1.0 wt%. The preparation method of the negative electrode sheets was as follows: first, some lead oxide and carbon additives were ground using an agate mortar for few minutes; then, other negative additives were added to the agate mortar and ground in turn. Short fibers uniformly dispersed by deionized water were added to the mortar and stirred for a few minutes. Finally, a certain amount of H2SO4 was added to the mixture dropwise, and deionized water was
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used to adjust the consistency of the lead paste. A certain amount of lead paste was
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coated on the Pb-Ca grid with a geometric area of 1×1 cm2 to prepare the negative
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sheets. All as-prepared negative sheets need to be cured under carefully controlled
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conditions under 35 oC for 2 hours, 65 oC for 4 hours and proper humidity, and then
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these negative electrode sheets were formed in H2 SO4 (1.06 g cm3) solution.
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2.3. Electrochemical measurements
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The electrochemical performance of negative electrode sheets was tested using a
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three-electrode cell, and the mass of the lead paste of the negative electrode sheet was
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about ~0.2 g. The prepared negative electrode sheet with a geometric area of 1×1 cm2
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was used as the working electrode. A platinum electrode foil with relatively large area
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was used as the counter electrode and Hg/Hg2SO4 electrode as the reference electrode. The electrolyte was H2SO4 with a density of 1.28 g cm-3. Chronopotentiometric (CP)
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tests were performed in the potential range from -0.8 V to -1.2 V vs. Hg/Hg2SO4 under several discharge rates (1, 2, 3, and 5C). The currents under discharge rates (1, 2, 3, and 5C) were determined according to the theoretical capacity. Electrochemical impedance spectroscopy (EIS) measurements were carried out by applying an
alternating current voltage of 5 mV amplitude in the frequency range from 0.01 Hz to 100 kHz at -1.2 V and -0.8 V. The CP and EIS analyses were carried out using a CHI660E electrochemical workstation. 2.4. Simulated test cell A simulation flooded lead-acid test cell was assembled comprising a small negative plate and an oversized positive plate separated by 3 mm thick AGM
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separator. The preparation process of the negative plates were kept same as above
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negative electrode sheets, but the mass of the lead paste of the negative plate was
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about ~1.0 g, and these cells were flooded with 200 ml H2SO4 (1.28 g cm3). The
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influences of AC or CB on the cycle performance of lead-acid battery under simulated
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HRPSoC conditions were investigated. First, the cells were fully charged at 20 h
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discharge rate, and then the cells were discharged to 50% state-of-charge (SoC) at 1C
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rate in this cycling profile. Next, the charge/discharge cycle processes were carried
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out according to the following schedule: charge at 1C or 2C rate for 30 s, rest for 10 s,
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discharge at 1C or 2C rate for 30 s, rest for 10 s. The cell voltage was measured at the
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end of each charge and discharge processes, and the test process was stopped when
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the cut-off discharging voltage decreased to 1.70 V. All the tests were conducted at room temperature. The currents for 1 and 0.5 h discharge rates were 1C and 2C,
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respectively, as determined from the Peukert plot. 2.5. Physical characterizations The surface morphologies of negative electrode sheets were observed under a high-resolution scanning electron microscope (SEM, JSM-6380). The crystalline
phase of the negative electrode sheets was examined by powder X-ray diffraction (XRD) between 10-90° using Cu-Kα radiation on a Y-2000 X-ray generator. 3. Results and discussion 3.1. Effects of carbon additives on the discharge capacity of negative electrode sheets 3.1.1 Effects of AC Fig. 1 shows the discharge curves of the negative electrode sheets with different
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contents of AC at 3C and 5C discharge rates. The experimental results show that the
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discharge time is significantly affected by the amount of AC in NAM. It can be seen
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from Fig. 1 that, at 3C and 5C discharge rates, the discharge time of the negative
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electrode sheets with 0.2, 0.5, or 1.0 wt% AC are longer than those without AC. At a
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certain discharge current rate, with increasing amount of AC, the discharge time of the
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negative electrode sheets initially increases and then decreases. Clearly, the negative
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and 5C discharge rates.
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electrode sheets with 0.5 wt% AC in NAM obtain the longest discharge time at 3C
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Fig. 2 indicates the relationship between the discharge specific capacity of
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negative electrode sheets with various AC content and different discharge rates. It can
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be found from Fig. 2 that within the entire range of discharge rates, the discharge specific capacity of the negative electrode sheets with 0.2, 0.5, or 1.0 wt% AC are
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higher than those without AC, indicating that appropriate addition of AC contributes to the discharge specific capacity. It also can be seen from Fig. 2 that at a certain discharge rate, the discharge specific capacity of the negative electrode sheets first increases and then decreases with increasing AC content. The optimal amount of AC
is 0.5 wt%, at this content, the discharge specific capacity from 1C to 5C is 119, 89.2, 86.6, and 71.8 mAh g-1, which increases by 26.6%, 15.8%, 40.1%, and 61.7%, respectively, compared to the negative electrode sheets without AC addition. Therefore, it can be concluded that the addition of AC improves the utilization of active materials and increases the discharge specific capacity of the negative electrode sheets, particularly at a high discharge rate.
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3.1.2 Effects of CB
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Fig. 3 presents the discharge curves of the negative electrode sheets with
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different contents of CB at 3C and 5C discharge rates. The experimental results show
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that the amount of CB in NAM significantly affects discharge time. It can be seen
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from Fig. 3 that at 3C and 5C discharge rates, the discharge times of the negative
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electrode sheets with 0.2 and 0.5 wt% CB are longer than those without CB, whereas
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the discharge time is shorter at 1.0 wt% loading level. It also can be found from Fig. 3
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that with increasing content of CB, the discharge time of the negative electrode sheets
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initially increases and then decreases at a certain discharge current rate. Obviously,
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the negative electrode sheet with 0.5 wt% CB in NAM exhibits the longest discharge
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time at 3C and 5C discharge rates. Fig. 4 illustrates the correlations between the discharge specific capacity of the
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negative electrode sheets with three different content of CB in NAM and discharge rates. It can be seen from Fig.4 that within the entire range of discharge rates, the discharge specific capacity of the negative electrode sheets with 0.2 and 0.5 wt% CB are higher than those without CB, whereas the discharge specific capacity is lower at
1.0 wt% CB loading level, indicating that appropriate CB content contribute to the discharge specific capacity. Fig. 4 also shows that at a certain discharge rate, the discharge specific capacity of the negative electrode sheets initially increases and then decreases with increasing amount of CB. The optimal content of CB is 0.5 wt%, in this content, the specific capacity from 1C to 5C is 109.3, 89.0, 71.8, and 55.0 mAh g-1, which increases by 16.2%, 15.6%, 16.2%, and 23.8%, respectively, compared to
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the negative electrode sheets without CB. Based on the above results, it can be
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concluded that the addition of CB improves the utilization of active materials and
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increases the discharge capacity of the negative electrode sheet, especially at a high
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discharge rate.
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The data in the Figs. 2 and 4 evidences that the appropriate addition of AC or CB
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in the negative electrode sheets can increase the utilization of active mass, prolong
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discharge time, and increase the discharge capacity of negative electrode sheets,
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particularly at a high discharge rate. Furthermore, it also can be found that, within the
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entire range of discharge rates, the discharge time and specific capacity of the
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negative electrode sheets with AC are higher than those with CB at 0.5 wt% loading
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level. This can be mainly attributed to the characteristics of the two types of carbon materials, as described in Table 1. The SSA of AC (1800-2000 m2 g-1) is larger than
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that of CB (254 m2 g-1), the larger the SSA is, and the better the performance of capacitance is, thus improving the discharge capacity. Notably, the amount of AC or CB in NAM should be controlled in a reasonable level (below 0.5wt%), otherwise the amount of Pb active material in negative electrode sheets will decrease, and the
negative electrode sheets become loose due to high content of AC or CB with low density during charge-discharge process, finally leading to a shorter discharge time and a lower discharge capacity. 3.2 Effects of carbon additives on the cycle performance of negative plates under HRPSoC conditions The battery for HEVs is required to be charged and discharged continuously
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under HRPSoC conditions, leading to the progressive accumulation of PbSO4 on the
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negative plates. To investigate the effects of AC or CB on negative plates, a
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simulation flooded lead-acid test cell was assembled comprising a small negative
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plate and an oversized positive plate separated by 3 mm thick AGM separator. An
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oversized positive plate was used in this flooded lead-acid cell to reduce the effect of
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positive electrode plate on electrochemical performance.
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3.2.1 Effects of AC
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Fig. 5 presents the discharge curve of cells with various amounts of AC in NAM
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at different discharge rates (1C and 2C) under HRPSoC conditions. The experimental
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results show that the cycle life of negative plates is influenced by the addition of AC,
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apparently. It can be seen from Fig. 5 that at 1C discharge rate, with increasing amount of AC, the cycle numbers of the negative plates are 3010, 4520, 12057, and
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10817, which increased by 0%, 50.2%, 300.6%, and 259.4%, respectively, indicating that the addition of AC to NAM can prolong the cycle life of the negative electrode sheets. At 2C discharge rate, the cycle life increased by 0%, 68.2%, 366.6%, and 280.9%, respectively, indicating that appropriate AC significantly affects the cycle life
at a high discharge rate. It is obvious that the negative plates shows the best cycle performance at 0.5 wt% AC loading level. The addition of AC can increase electrochemically active surface, reduce electrode polarization, improve utilization of active material, and then enhance cycle performance. Thus, it can be concluded that the addition of AC can prolong the cycle life of negative plates, especially at a high discharge rate.
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3.2.2 Effects of CB
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Fig. 6 shows the discharge curve of the cells with different CB content at
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different discharge rates (1C and 2C) under HPRSoC conditions. A similar behavior is
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also observed with the cells with CB, which implies that the addition amount of CB
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exerts an influence on the cycle life of negative plates. It can be seen from Fig. 6 that
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at 1C discharge rate, with increasing amount of CB, the cycle numbers of the negative
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plates are 3010, 9050, 10701, and 8400, which increased by 0%, 200.7%, 255.5%,
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and 179.1%, respectively, indicating that CB added into the NAM can prolong the
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cycle life of negative plates. The influence is more obviously at 2C discharge rate (the
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cycle life increased by 0%, 240.6%, 295.3%, and 207.9%, respectively), which
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indicates that appropriate CB significantly affects the cycle life at a high discharge rate. It is obvious that the cycle number reaches the maximum value when the amount
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of CB was 0.5 wt%. The addition of CB can enhance the conductivity of negative sheets, reduce electrode polarization, and then improve the utilization of active material and prolong the cycle life of negative plates. Therefore, it can be concluded that the addition of CB to the NAM can prolong the cycle life of negative plates, and
the effect is more apparent at a high discharge rate. From the above experiment results, we conclude that the appropriate AC or CB can prolong the cycle life of negative electrode sheet, except for the difference that the influences of AC on cycle performance are better than that of CB when the amount of carbon additive is 0.5 wt%, the reasons may probably because the SSA of AC (1800-2000 m2 g-1) is larger than that of CB (254 m2 g-1). A higher SSA enhances
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electrochemically active surface, decreases electrode polarization, thus improving the
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charge efficiency and cycle performance of negative plates. M. Fernandez et al. [28]
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reported that the addition of medium/high SSA carbon to NAM affected the
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performance negative plates. Thus, the characteristics of carbon additives have a great
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effect on battery cycle performance.
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3.3 Effects of carbon additives on the electrochemical impedance of negative
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electrode sheet
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Electrochemical impedance measurements were performed to gain deeper insight
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into the effect of carbon additives on negative electrode sheet. EIS measurements
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were carried out by applying an alternating current voltage of 5 mV amplitude in the
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frequency range from 0.01 Hz to 100 kHz at -1.2 V and -0.8 V. The impedance of the as-prepared electrode sheets was investigated at -0.8 V (discharge state) and -1.2 V
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(charge states). In general, the x-intercept on real axis (Z) represents a combined resistance (Rs) including ionic electrolyte resistance and contact resistance; the diameter semicircle is associated with the electrochemical reaction resistance (Rct) caused by Faradic reactions and electrical double-layer capacitor at the
electrode/electrolyte interface. Figs. 7 and 8 show the EIS of negative electrode sheets with different AC and CB contents in different potentials, respectively. As shown in the Figs. 7 and 8, AC and CB have a great effect on the Rct of negative electrode sheets. All the measured impedance plots have similar semicircle shape, and the Rct is calculated from the diameter of the semicircle. It can be seen from Fig. 7 that in the discharge and charge
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states, with increasing amount of AC, the diameter of semicircle initially decreases
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and then increases, indicating that the Rct initially decreases and then increases. The
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Rct reaches the minimum value at 0.5 wt% AC loading level. Fig. 8 also shows that,
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with the increase of CB content, the Rct first decreases and then increases, when the
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amount of CB is increased to 0.5 wt%, the Rct reaches the minimum value. It is
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obvious from Figs. 7 and 8 that the Rct in the charge state is clearly smaller than that
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in the discharge state. In the discharge state, the main composition of NAM is PbSO4;
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the presence of carbon particles among PbSO4 particles can form a conductive
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network, facilitate the formation of numerous small-sized and uniform PbSO4 crystals
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with higher solubility, and then reduce the Rct. Moseley [25] reported that carbon
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enhances the overall conductivity of NAM. After discharging, the negative electrode sheet was recharged. During charging, the addition of AC or CB increases the
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electrochemically active surface of the NAM, decreases electrode polarization and provides many sites for H2SO4 accommodation which promotes the transformation between PbSO4 and Pb, then reduces the Rct. Notably, the amount of AC or CB in NAM should be controlled in a reasonable level (below 0.5wt%), otherwise the
negative plate becomes loose due to high content of carbon with low density during charge-discharge process, so the Rct of negative electrode sheets increases. The Rct in the discharge state is clearly larger than that in the charge state, this is because the main component is PbSO4 in the discharge state and Pb in the charge state, the conductivity of PbSO4 is far less than Pb. Table 2 summarizes the Rct of the negative electrode sheets with different
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contents of AC and CB in different potentials. It can be found from Table 2 that there
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are some differences between AC and CB on Rct, especially in discharge state The Rct
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of negative sheets with CB is smaller than that with AC in discharge state, this is
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probably due to the conductivity of CB is better than that of AC.
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Therefore, it can be concluded that the appropriate addition of AC or CB to the
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NAM can improve the conductivity of negative electrode sheets and reduce the Rct,
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thus leading to higher discharge specific capacity and better cycle performance.
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Furthermore, the type of carbon additives affects the Rct.
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3.4 Effect of carbon additive on the microstructure of NAM
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The experimental results indicate that AC and CB significantly affect the
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discharge capacity and cycle performance of negative electrode sheets. To better understand the influences of AC or CB on the discharge capacity and cycle
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performance of negative electrode sheets, the surface morphologies of the negative electrode sheets with different contents of AC or CB were investigated by SEM. Because the negative electrode sheets were maintained in the discharge state after 2000 cycling, the main component of the negative electrode sheets was PbSO4.
Figs. 9 and 10 show the SEM images of negative electrode sheets with different contents of AC and CB after 2000 cycling, respectively. It can be found from Fig. 9 that the negative electrode sheets without AC contains some relatively uneven PbSO4 crystals including large and small PbSO4 particles, and small PbSO4 particles are present in the gap among the large PbSO4 particles. When AC was added to negative electrode sheet, PbSO4 particles become smaller and well distributed; the pores
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among PbSO4 particles increase. Xiang et al. [34] reported that carbon additives can
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optimize NAM microstructure. At 0.5 wt% AC loading level, the PbSO4 crystals
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distribute more homogeneously and generate smaller PbSO4 particles which dissolve
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more readily in the subsequent electrochemical reaction for Pb formation. It also can
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be found from Fig. 10 that the addition of CB to the NAM changes the shape and size
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of PbSO4 crystals. With increasing content of CB, it affects the growth of PbSO4
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crystals. At 0.5 wt% CB loading level, the PbSO4 crystals have well shaped crystal
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faces and edges and become well distributed. These PbSO4 crystals are more easily
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dissolved, which is helpful for transforming PbSO4 to Pb. Therefore, it can be
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concluded that appropriate addition of AC or CB can increase the active surface of
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NAM, make PbSO4 well distributed, hence reduces the size of PbSO4 crystals and the Rct of the NAM. Finally, the negative electrode sheet with AC or CB obtains a greater
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discharge specific capacity and possessed a better cycle performance 3.5 Composition analysis of NAM with carbon additives under simulated HRPSoC conditions Many researchers reported that the NAMs suffer from the sulfation phenomenon
after several thousand cycles, thus producing a portion of irreversible PbSO4 particles in the NAM. In order to explore the sulfation of NAMs with different contents of AC or CB, the active materials in the negative electrode sheets were characterized by XRD analysis in fully charged state after HRPSoC cycle tests. Fig. 11 shows the XRD patterns of the NAMs with different contents of AC (Fig. 11a) and CB (Fig. 11b), respectively. As evident from Fig. 11, almost same XRD
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patterns are observed for the several negative electrode sheets, the main characteristic
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peaks of Pb and PbSO4 and their intensities (IPb and IPbSO4) are marked. It can be seen
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from Fig. 11 that the intensities of the main characteristic varied with addition amount
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of AC or CB. The intensity of the characteristic peaks of Pb varies faster than that of
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PbSO4 with increasing content of AC or CB. In order to determine the relative content
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of Pb and PbSO4 in the active materials, the ratio of IPb/IPbSO4 is calculated, as shown
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in Table 3. It also can be found from Table 3 that the IPb/IPbSO4 value initially
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increases and then decreases with increasing content of AC or CB, and the IPb/IPbSO4
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value reaches the maximum value when the content of AC or CB is 0.5 wt%. A higher
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IPb/IPbSO4 value indicates that less PbSO4 particles are present in the active materials,
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and the IPb/IPbSO4 value of the active materials with AC or CB is higher than that without AC or CB. The phenomenon shows that appropriate AC or CB can promote
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the transformation of PbSO4 into Pb and reduce the formation of irreversible PbSO4. Therefore, it can be concluded that the addition of appropriate amount of AC or CB to NAM can improve the utilization of active mass, promote conversion of PbSO4 to Pb, and suppress the sulfation in negative electrode especially in 0.5wt% carbon content,
Finally, the negative plates with AC or CB addition obtains higher discharge specific capacity and better cycle performance. The experimental results indicate that the addition of AC or CB can reduce the formation of irreversible PbSO4 and improve the utilization of active mass. Fig. 12 illustrates the schematic diagram for the effects of AC or CB on negative electrode sheets under HPRSoC conditions [34]. Under high-rate discharge conditions, some
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large and uneven PbSO4 particles and a compact PbSO4 layer on the surface are
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formed, and some Pb particles surrounded by irreversible PbSO4 particles (marked by
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red circle) are difficult to oxidize. During high-rate charging, the compact PbSO4
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layer on the surface prevents the electrolyte to enter the interior of negative plates
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during charging, which leads to many PbSO4 particles reduced difficulty. When AC
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or CB are added to NAM, under high-rate discharge conditions, the addition of AC or
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CB results in much electrochemically active surface inside the NAM during the
D
discharging process, which can provide extra nucleation sites for PbSO4 crystals and
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facilitate the formation of numerous small-sized and uniform PbSO4 crystals with
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higher solubility. In the subsequent high-rate charging process, the carbon particles
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can provide much active site for H2SO4 accommodation and form a conductive network among PbSO4 crystals; on the other hand, small and uniform PbSO4 particles
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dissolve readily during the electrochemical reaction of Pb, thus promoting the transformation between Pb and PbSO4.and suppressing the sulfation. Thus, appropriate addition of AC or CB in the NAMs can suppress the accumulation of PbSO4, improve the utilization of active mass and obtain a higher specific capacity
and longer cycle life [15, 35-39]. 4. Conclusions In this study, two kinds of carbon materials (AC and CB) are used as additives in the negative electrode, negative electrode sheets are prepared by simulating the negative plate manufacturing process of lead-acid battery, and the effects of AC or CB on the electrochemical performance of negative electrode sheets are investigated in
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detail. The electrochemical experimental results indicate that an appropriate amount
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of AC or CB can increase the discharge specific capacity and prolong the cycle life of
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negative electrode sheets under HRPSoC conditions. The optimum amount of AC or
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CB is selected as 0.5 wt%, and the addition of AC exerts more obvious effect than CB
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on the electrochemical performance. The addition of AC or CB in NAM can reduce
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the size of PbSO4 crystals by SEM observation and reduce the electrochemical
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reaction resistance of the negative electrode sheets by EIS analysis, the XRD results
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indicate that the appropriate addition of AC or CB in the NAMs can promote the
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conversion between PbSO4 and Pb, then suppress the sulfation of negative electrode
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sheet. Therefore, the discharge specific capacity and cycle life of the negative
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electrode sheet of lead-carbon battery can be improved, and the longer cycle life may be developed for HEV battery market and other fields.
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Acknowledgement The authors wish to acknowledge the following financial supporters of this work: the National Natural Science Foundation of China (Grant No. 21273085), the Guangdong Province Science and Technology Bureau (Grant No. 2010B090400552),
the Natural Science Foundation of Guangdong Province, China (Grant No. S2011010003416 and S2013010012927), the Project of Guangzhou Science and Information Technology Bureau (Grant No. 2012J4300147) and the Fundamental Research Funds for the Central Universities (No. 13lgjc10). References [1] D. Pech, T. Brousse, D. Bélanger, D. Guay, Electrochim. Acta 54 (2009) 7382-
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[35] D.P. Boden, D.V. Loosemore, M.A. Spence, T.D. Wojicinski, J. Power Sources 195 (2010) 4470-4493. [36] L. Zhao, B.S. Chen, D.L. Wang, J. Power Sources 231 (2013) 34-38. [37] Ellen. Ebner, Daniel. Burow, Alexander. Börger, Michael. Wark, Paolina.
Atanassova , Jesús. Valenciano, J. Power Sources 239 (2013) 483-489. [38] D. Pavlov, P. Nikolov, J. Electrochemical Society 159 (2012) 1215-1225.
A
CC
EP
TE
D
M
A
N
U
SC
RI
PT
[39] P. Krivik, P. Baca, K. Tonar, P. Toser, ECS Transactions 40 (2012), 145-152.
Figures captions Fig.1 The discharge time results for negative electrode sheets with different contents of activated carbon at 3C and 5C discharge rates. Fig.2 Discharge specific capacity of negative electrode sheets with different contents of activated carbon in different discharge rates. Fig.3 The discharge time results for negative electrode sheets with different contents
PT
of carbon black at 3C and 5C discharge rates. Fig.4 Discharge specific capacity of negative electrode sheets with different contents
RI
of carbon black in different discharge rates.
SC
Fig.5 Discharge curve of cells with different amounts of activated carbon in different
U
discharge rates (a: 1C, b: 2C) under HRPSoC conditions.
A
N
Fig.6 Discharge curve of cells with different amounts of carbon black in different
M
discharge rates (a: 1C, b: 2C) under HRPSoC conditions.
D
Fig.7 Electrochemical impedance spectroscopy of negative electrode sheets with
TE
different contents of activated carbon in different state.
EP
Fig.8 Electrochemical impedance spectroscopy of negative electrode sheets with different contents of carbon black in different state.
CC
Fig.9 SEM images of negative electrode sheets with different concentrations (a: 0 %,
A
b: 0.2 %, c: 0.5 %) of activated carbon after 2000 cycling. Fig.10 SEM images of negative electrode sheets with different concentrations (a: 0 %, b: 0.2 %, c: 0.5 %) of carbon black after 2000 cycling. Fig.11 XRD patterns of negative active materials with different contents of carbon additive ( a: AC, b: CB) under HRPSoC conditions.
Fig.12 Schematic diagram for the effects of carbon additives on negative electrode
A
CC
EP
TE
D
M
A
N
U
SC
RI
PT
sheets under HPRSoC conditions.
Fig .1
-1.000
-0.950 -0.925 -0.900 -0.875 -0.850 -0.825
PT
Potential / V vs. Hg/Hg2SO4
0% 0.2% 0.5% 1.0%
3C
-0.975
-0.800
-0.750 50
100
150
200
250
300
400
450
N
U
Discharge time / s
350
SC
0
RI
-0.775
A
-1.000
5C
D
-0.925
-0.850
TE
-0.900
EP
Potential / V vs. Hg/Hg2SO4
-0.950
-0.875
0% 0.2% 0.5% 1.0%
M
-0.975
-0.825
CC
-0.800 -0.775
A
-0.750 0
20
40
60
80
100
120
140
Discharge time / s
160
180
200
220
Fig.2
0% 0.2% 0.5% 1.0%
120
100 90 80 70 60
PT
Specific capacity / mAh g
-1
110
40 2
3
4
A
CC
EP
TE
D
M
A
N
U
Discharge rate / C
5
SC
1
RI
50
Fig .3
-1.000
-0.950 -0.925 -0.900 -0.875 -0.850 -0.825
PT
Potential / V vs. Hg/Hg2SO4
0% 0.2% 0.5% 1.0%
3C
-0.975
-0.800
-0.750 50
100
150
200
250
350
M
A
N
U
Discharge time / s
300
SC
0
RI
-0.775
-1.000
5C
D
0% 0.2% 0.5% 1.0%
TE
-0.950 -0.925 -0.900
EP
Potential / V vs. Hg/Hg2SO4
-0.975
-0.875
A
CC
-0.850 -0.825 -0.800 -0.775 -0.750
0
20
40
60
80
100
Discharge time / s
120
140
160
Fig.4
110
0% 0.2% 0.5% 1.0%
90 80 70 60 50 40
PT
Specific capacity / mAh g
-1
100
30
10 0 2
3
4
A
CC
EP
TE
D
M
A
N
U
Discharge rate / C
5
SC
1
RI
20
Fig.5
2.2
0% 0.2% 0.5% 1.0%
a 2.1
Voltage / V
2.0
1.9
PT
1.8
1.6 2000
4000
6000
8000
10000
12000
14000
N
U
Cycle Number
SC
0
RI
1.7
A
2.2
D
2.1
TE
EP
Voltage / V
2.0
1.9
0% 0.2% 0.5% 1.0%
M
b
A
CC
1.8
1.7
1.6 0
2000
4000
6000
Cycle Number
8000
10000
Fig.6
2.2
0% 0.2% 0.5% 1.0%
a 2.1
Voltage / V
2.0
1.9
PT
1.8
1.6 0
2000
4000
6000
8000
10000
12000
N
U
SC
Cycle Number
RI
1.7
2.2
A
b
M
2.1
1.8
D TE
1.9
EP
Voltage / V
2.0
0% 0.2% 0.5% 1.0%
A
CC
1.7
1.6 0
2000
4000
Cycle Number
6000
8000
Fig.7 (a) Discharge state (-0.8 V)
300
a
0% 0.2% 0.5% 1.0%
250
150
100
PT
-Z'' / Ohm
200
0 50
100
150
200
250
300
SC
0
RI
50
U
Z' / Ohm
A
N
(b) Charge state (-1.2 V)
16
b
TE
8
EP
-Z'' / Ohm
10
D
12
0% 0.2% 0.5% 1.0%
M
14
6
A
CC
4 2 0
0
2
4
6
8
Z' / Ohm
10
12
14
16
Fig.8 (a) Discharge state (-0.8 V)
300
a
0% 0.2% 0.5% 1.0%
250
-Z'' / Ohm
200
150
PT
100
0 50
100
150
200
250
300
SC
0
RI
50
N
U
Z' / Ohm
M
A
(b) Charge state (-1.2 V)
16
b
A
CC
EP
-Z'' / Ohm
8
TE
12 10
0% 0.2% 0.5% 1.0%
D
14
6 4 2 0
0
2
4
6
8
Z' / Ohm
10
12
14
16
TE
EP
CC
A D
PT
RI
SC
U
N
A
M
Fig.9
a b
c
TE
EP
CC
A D
PT
RI
SC
U
N
A
M
Fig.10
a b
c
Fig.11
a
450
Pb PbSO4 1.0%
720
Intensity /a.u.
608 1000 0.5%
0.2% 696
RI
540
PT
576 815
0% 20
40
60
80
100
485
750
Pb PbSO4 1.0%
TE
D
b
M
A
N
U
2theta / deg
SC
0
EP
Intensity / a.u.
569
601
950 0.5% 720
CC
0.2% 696
540
A
0%
0
20
40
60
2theta / deg
80
100
Fig.12
Discharge
Charge
SC
RI
PT
(a)The negative electrode sheets without carbon additives
Charge
M
A
N
U
Discharge
TE
H+ SO42C
D
(b)The negative electrode sheets with carbon additives
CC
EP
Pb PbSO4
A
Fig. 12
Table 1 Characteristics of the carbon additives added into NAM. Symbol
Type of material
AC CB
Activated carbon Carbon black
Conductivity (S cm)
BET m2/g
0.35 70
surface
Particle size
1800-2000 254
5-10μm 30 nm
Table 2 The electrochemical reaction resistance of the negative electrode sheets with
Charge AC
CB 485.0 322.5 205.0 260.0
15.5 6.2 3.9 13.1
SC
485.0 388.5 225.0 330.6
state
CB
15.5 9.6 2.1 5.5
A
N
U
0% 0.2% 0.5% 1.0%
state
RI
Discharge AC
PT
different contents of activated carbon (AC) or carbon black (CB) in different state.
M
Table 3 The intensities (IPb and IPbSO4) of the main characteristic peaks of NAM with
D
different activated carbon (AC) or carbon black (CB) content and the ratio of
TE
IPb/IPbSO4 in charge state.
EP
0%
A
CC
IPb IPbSO4 IPb/IPbSO4
540 696 0.78
Activated 0.2%
Carbon 0.5%
1.0%
815 576 1.41
1000 608 1.64
720 450 1.60
Carbon 0.2% 720 601 1.20
Black 0.5%
1.0%
950 569 1.70
750 485 1.55