Significantly improved high-rate partial-state-of-charge performance of lead-acid batteries induced by trace amount of graphene oxide nanosheets

Highlights

• Trace amount of GONs is incorporated into the NAMs of LABs through a novel and simple way.

• The NAMs containing 2 ppm GONs presents a spongy-like structure composed of porous Pb sticks.

• Sulfation is suppressed due to the acceleration of the redox reaction between Pb and PbSO₄.

• The HRPSoC cycling life of the simulated cell increased by more than 1.4 times to 29,971 cycles.

• The hydrogen generation almost has not any improvement because of the trace GONs content.

Abstract

In this work, trace amount of graphene oxide nanosheets (GONs) is incorporated into the negative active materials (NAMs) of lead-acid batteries (LABs) using an innovative and simple way. The effect of GONs on the morphologies, structures and compositions of the synthesized GONs-containing NAMs are investigated. It is observed that after formation, the NAMs containing 2 ppm GONs presents a spongy-like structure comprised of one-dimensional (1D) porous Pb sticks. Such a 1D structure of Pb sticks provides fast electron transport channel, while the pores on Pb sticks and the voids among Pb sticks facilitate electrolyte transportation. Therefore, accumulation of PbSO₄ crystals is greatly suppressed. Several electrochemical methods such as cyclic voltammetry and electrochemical impedance spectroscopy are used to understand the enhancement mechanism. It is found that the high-rate partial-state-of-charge (HRPSoC) cycling life of the simulated cell is increased by more than 1.4 times from 21,305 to 29,971 cycles. Although the electrochemical performance of NAMs is significantly improved, the hydrogen generation does not have any promotion because of the trace GONs content. The results demonstrate that our method is of great convenient and ultra-low cost with feasibility in large-scale application in LABs industry.

Introduction

In the worldwide efforts to explore clean and sustainable energy sources including solar, wind, water-fall, and etc., development of the corresponding electricity energy storage technologies such as electrochemical batteries, fuel cells and water electrolysis have been recognized as one kind of the most efficient and practical options. Among different technologies, lead-acid batteries (LABs), which have been playing a significant role for human progress and industrial development in a long history of more than 150 years [1]. LABs have several advantages over other types of batteries, such as the lowest cost, the highest reliability, the highest safety, and the most efficient recyclability of the spent batteries [2]. Nowadays, LABs is still the most commonly used batteries in a variety of applications, and the demand for LABs is still rising [3]. However, with the fast development of electric vehicles (EV) and the wind/solar energy storage, LABs is facing new challenges because in both circumstances, batteries have to be operated under partial-state-of-charge (PSoC) condition and experience short charge/discharge events with high currents, i.e., high-rate PSoC cycling duty (HRPSoC) [4]. The failure mode of LABs is no longer the softening of positive active materials (PAMs) and corrosion of positive grids. Instead, sulfation of negative plates becomes the main reason leading to the failure of LABs [5]. Under the HRPSoC conditions during the charge process, LABs is subject to high charging current for a very short duration, and PbSO₄ crystals cannot be effectively reduced to Pb within such a short time, resulting in the gradual growth of PbSO₄ crystals [6]. As the solubility of PbSO₄ crystals decreases with increasing crystal size, large PbSO₄ crystals cannot be effectively reduced to Pb, leading to an accumulation of PbSO₄ crystals and then sulfation of negative plates [7].

With respect to this issue, over the past a few years, many efforts have been devoted to improve the HRPSoC performance of LABs by adding various carbon additives into negative active materials (NAMs) [8], [9], [10], [11], [12]. The carbon additives can increase the surface area of NAMs, which not only improves the specific capacitance of negative plates, but also provides extra nucleation sites to form small-sized PbSO₄ crystals for increasing the solubility PbSO₄ and accelerating the electrochemical reduction of PbSO₄ to Pb [13]. Meanwhile, the carbon additives themselves play the role of spacer, sterically hindering the growth of PbSO₄ crystals [14]. On the other hand, carbon additives can increase the porosity of NAMs, facilitating the diffusion of sulfuric acid into the interior of negative plates. At the same time, the pores in NAMs can also serve as the reservoirs for sulfuric acid, promoting the redox reaction to proceed in the interior of negative plates [15]. In this way, the small-sized PbSO₄ crystals rather disperse throughout the entire plates than just accumulate on the surface, greatly inhibiting the sulfation of negative plates [16]. Due to their high specific capacitance, carbon additive can act as a buffer to share the charge/discharge currents, preventing the formation of the irreversible PbSO₄ crystals [17,18]. It is also confirmed that the high electrical conductivity of carbon additives can improve the overall electrical conductivity of NAMs and promote the electrochemical redox reactions between PbSO₄ and Pb [19].

Graphene, a type of carbon materials, which is consisted of sp² hybridized carbon atoms arranged in a honeycomb crystal, has attracted much attention due to its outstanding intrinsic properties. Graphene has been widely used in electrochemical energy storage systems (EES), including supercapacitors [20,21], lithium-ion batteries [22,23], metal-air batteries [24], and so on. Researchers have also tried to introduce graphene into NAMs to improve the cycling life of LABs under HRPSoC condition. For example, Yeung et al. [25] reported that after an addition of 0.2 wt.% graphene, the PSoC cycling life of LABs could be improved by more than 140% from 7078 to 17,157 cycles. Long et al. [26] prepared a three-dimensional reduced graphene oxide (3D-RGO) material, and found that the addition of 1.0 wt% 3D-RGO into NAMs could greatly increase the discharge capacity of LABs as well as their HRPSoC cycling life. Yang et al. [3] incorporated the composite comprised of polypyrrole and graphene oxides (GO) into NAMs, and found that the HRPSoC cycling life of the simulated cell was greatly increased. Although graphene can greatly improve the HRPSoC performance of LABs, the high cost is supposed to impede its large-scale applications. Another issue, which may hinder the application of graphene in LABs, is the accelerated hydrogen generation. Graphene, as well as some other carbon additives, can lower the overpotential of hydrogen evolution reaction and promote hydrogen evolution, which will induce the increased water loss, leading to dry-out and capacity loss [27]. Many strategies have been proposed to suppress the hydrogen evolution originated from the carbon additives, such as doping heteroatoms into carbon additives, covering carbon additives [28], [29], [30] with nano lead [31], [32], [33], and incorporating PPy with carbon additives [3]. These strategies, however, are complex, which are not favorable for scalable manufacturing.

In this work, the negative lead paste of LABs is prepared with the simply modified formula and procedure to introduce only trace amount of graphene oxide nanosheets (GONs). In the preparation, lead oxide powder is first mixed with barium sulfate, vanisperse A and carbon black with the mass ratio of 100:0.8:0.2:0.2. Then trace amount of GONs is dispersed in water, which is added into the mixture quickly followed by an addition of H₂SO₄ drop by drop. It is found that the incorporation of trace amount of GONs has great effect on the chemical reactions that occur during plate making, curing and forming processes. When 2 ppm GONs is incorporated, after formation process, the NAMs doesn't show a spongy-like structure comprised of stacked Pb particles, but a spongy-like structure constructed by porous Pb sticks is formed. The one-dimensional (1D) structure of Pb sticks provides fast electron transfer channels, while the pores on the Pb sticks and the voids among the Pb sticks can promote electrolyte diffusion. Such two features can both accelerate the rapid electrochemical transformation between PbSO₄ and Pb, and suppress the sulfation of negative plate. The HRPSoC cycling life of the simulated cell increases by more than 140% from 21,305 to 29,971 cycles within the first cycle-set. The simulated cell succeeds to sustain four cycle-sets, and the cycling life in the fourth cycle-set still reaches 6033 cycles. Despite that the HRPSoC performance of the negative plate has shown such a great improvement, the amount of GONs used in NAMs is too low to affect the situation of hydrogen evolution. Since our method is of great convenience and ultra-low cost, it is expected to have a great value in large-scale application of LABs industry.