Chun
Chen
abc,
Liansheng
Li
ab,
Zuxin
Long
ab,
Edison Huixiang
Ang
*d and
Qinghua
Liang
*abc
aKey Laboratory of Rare Earth, Chinese Academy of Sciences, Ganzhou, Jiangxi 341000, China. E-mail: qhliang@gia.cas.cn
bGanjiang Innovation Academy, Chinese Academy of Sciences, Ganzhou, Jiangxi 341000, China
cSchool of Rare Earth, University of Science and Technology of China, Hefei, Anhui 230026, China
dNatural Sciences and Science Education, National Institute of Education, Nanyang Technological University, Singapore 637616, Singapore. E-mail: edison.ang@nie.edu.sg
First published on 11th June 2024
The potential of zinc metal anodes (ZMAs) for use in emerging aqueous electrochemical devices like rechargeable zinc-ion batteries and hybrid capacitors is substantial, owing to their high theoretical capacity, low redox potential, non-toxicity, abundant availability, and cost-effectiveness. However, the practical application of ZMAs faces limitations due to issues such as uncontrolled zinc dendrite growth and side reactions. In this study, we demonstrate that simultaneously incorporating scandium ions (Sc3+) and acetate anions (Ac−) as electrolyte additives into a common ZnSO4 solution significantly enhances the cycling stability and reversibility of ZMAs. Our findings reveal that the Ac− acts as a pH regulator, dynamically buffering the electrolyte pH to be around 4.3, effectively suppressing water-induced side reactions. Additionally, the synergistic effect of Sc3+ and Ac− (Sc3+/Ac−) facilitates the desolation process of Zn2+ and lowers the energy barrier for electrochemical Zn plating, resulting in uniform Zn plating without noticeable zinc dendrite growth. Consequently, Zn‖Zn symmetric cells utilizing the Sc3+/Ac− electrolyte additive exhibit an ultra-long lifespan exceeding 1000 hours at 2.0 mA cm−2 and 1.0 mA h cm−2. Moreover, the Zn‖Cu cell demonstrates a high average coulombic efficiency of 99.4% after 400 plating/stripping cycles at 1.0 mA cm−2 and 1.0 mA h cm−2. Notably, when paired with an activated carbon (AC) cathode, Zn‖AC hybrid capacitors maintain a high-specific capacity of 62 mA h g−1 after 10000 cycles at 1.0 A g−1. The research outcomes indicate that Sc3+ in combination with Ac− are promising electrolyte additives for achieving highly stable aqueous ZMAs.
A range of effective strategies have been devised to tackle the above-mentioned interfacial challenges and bolster the stability and reversibility of ZMAs. These strategies encompass interface engineering,9–11 ZMA structure design,12,13 electrolyte optimization,14,15 and separator modification.16,17 Notably, electrolyte optimization, particularly through the incorporation of electrolyte additives, has emerged as a straightforward and efficient approach. This strategy offers distinct advantages, including simplicity in manufacturing, adaptability, and cost-effectiveness. A diverse array of electrolyte additives have been explored to optimize ZMA performance, spanning polymers, organic molecules, and ion compounds. Generally, electrolyte additives enhance ZMA reversibility in aqueous electrolytes by modulating the electric double layer (EDL),18,19 facilitating the formation of a solid electrolyte interface (SEI),20 or altering the solvation structure of Zn2+.21 Some organic electrolyte additives, such as trimethyl phosphate,22 gamma-butyrolactone,23 sulfolane,24 benzyl trimethylammonium,25etc., functioned by displacing water in the Zn2+ solvation sheath, restricting free water activity, and reshaping the hydrogen-bonding network, thereby mitigating the water-involved HER and diminishing zinc dendrite formation.26 Some organic electrolyte additives facilitate the in situ construction of polymeric-rich SEI layers with abundant functional groups and exceptional hydrophilicity, enabling dendrite-free zinc deposition by modulating Zn nucleation.27 Some ion compounds electrolyte additives, including thioacetamide,28 tetraethyl ammonium chloride,29 monosodium glutamate,30 glucose,31 trifluoro borate,32etc., have also been extensively investigated for their ability to modulate the solvation structure of Zn2+. In addition, certain ionic electrolyte additives, such as alkylammonium salt,33 methylammonium acetate,34 Zn(H2PO4)2,35 and glycine,36 can promote uniform Zn plating by facilitating the in situ formation of SEI layers. Despite notable advancements in electrolyte additive strategies, the cycle stability and reversibility of ZMAs remain heavily reliant on the specific electrolyte additive chosen. Moreover, considering the cost and toxicity issues associated with organic additives, there is a critical need to explore the development of economically feasible, environmentally sustainable, and efficacious additives to further enhance the electrochemical performance of ZMAs in aqueous electrolytes. Our recent study has revealed that the inclusion of a trace amount of Sc3+ additive (1.0 mol%) enhances the coulombic efficiency (CE) and cycle stability of ZMAs in aqueous ZnSO4 electrolyte by reducing nucleation overpotential and improving kinetics for Zn plating/stripping.37 However, the presence of even 1.0 mol% Sc3+ leads to a more acidic environment (pH = 2.3), accelerating unwanted HER side reactions and contributing to the persistently inadequate stability of ZMAs.
Herein, we observed that the addition of acetate anions (Ac−) synergistically enhances the positive impact of Sc3+ additives, leading to long-term and highly reversible zinc plating/stripping of ZMAs. The hydrolysis reaction of Ac− ions within the standard ZnSO4 electrolyte dynamically regulates the concentration of H+, maintaining an optimal pH range (∼4.3). This adjustment effectively reduces the occurrence of the HER, corrosion and side reactions. Additionally, the synergistic effect of Sc3+ and Ac− lowers the energy barrier for Zn plating, thus modulating the plating behaviour of Zn2+ and achieving uniform and dense zinc plating. Consequently, the HER, side reactions and zinc dendrite growth are notably suppressed. As a result, after introducing a small amount of Sc3+ and Ac− additives into 2.0 M ZnSO4 (ZSO) electrolyte, the lifespan of Zn‖Zn symmetric cells demonstrates more than a ten-fold increase in cycle life compared to those employing ZSO electrolyte alone at 2.0 mA cm−2 and 1.0 mA h cm−2. Moreover, the Zn‖Cu asymmetric cell achieves a high average CE of 98.45% within 400 plating/stripping cycles. Notably, the Zn‖activated carbon (AC) hybrid capacitor exhibits stable cycling performance at a current density of 1.0 A g−1, maintaining a specific capacity of 62 mA h g−1 after 10000 cycles.
To assess the corrosion rate of ZMAs in various electrolytes, Tafel plots were initially obtained. The corrosion current densities were determined for Zn plates in the ZSO, ZSO–Sc/Ac-1, ZSO–Sc/Ac-3, ZSO–Sc/Ac-5, and ZSO–Sc/Ac-10 electrolytes (Fig. S2†), yielding values of 4.58, 4.15, 1.29, 1.81, and 2.37 mA cm−2, respectively. Notably, the optimal amount of Ac− additive exhibits a mitigating effect on the corrosion rate, whereas excessive Ac− shows a reduced inhibitory effect on ZMA corrosion.38 Particularly in the ZSO–Sc/Ac-3 electrolyte, the lowest corrosion current density was observed, indicating enhanced corrosion resistance of ZMAs. Furthermore, Zn‖Cu batteries were employed to assess the reversibility of Zn plating/stripping across different electrolytes. As the concentration of Ac− in the electrolyte rises from 1.0 mol% to 5.0 mol%, the CE of the corresponding Zn‖Cu cell increases from 95.07% to 97.73% (Fig. S3†), indicating an improved reversibility of Zn plating/stripping within the concentration range of 1.0 mol% to 5.0 mol%. However, with excessive Ac− concentration (≥10 mol%), the polarization voltage of the Zn‖Cu cell notably elevates during operation. This phenomenon may be attributed to the accumulation of “dead Zn” or by-products on the ZMA surface, leading to increased diffusion and reduction resistance of Zn2+.35 We subsequently conducted a comparison of the cycling stability of ZMAs in ZSO–Sc/Ac-3 and ZSO–Sc/Ac-5 electrolytes. The Zn‖Zn symmetric cell utilizing the ZSO–Sc/Ac-3 electrolyte exhibits an extended lifespan of up to 1000 hours at 2.0 mA cm−2 under 1.0 mA h cm−2, while the Zn‖Zn symmetric cell employing the ZSO–Sc/Ac-5 electrolyte only cycles for 200 hours (Fig. S4†). Likewise, the Zn‖Zn symmetric cell utilizing the ZSO–Sc/Ac-3 electrolyte also demonstrates a prolonged lifespan of up to 500 hours, whereas the Zn‖Zn battery with the ZSO–Sc/Ac-5 electrolyte cycles for only 100 hours at 5.0 mA cm−2 under 2.0 mA h cm−2. These experimental findings underscore that the ZMAs in ZSO electrolytes with 1.0 mol% Sc3+ and 3.0 mol% Ac− ions exhibit superior corrosion resistance, cycling stability, and reversibility. We also studied the effect of only 1.0 mol% Sc3+ or 3.0 mol% Ac− in ZSO electrolyte on the performances of ZMAs. The lifespan of Zn‖Zn cells assembled with ZSO–Sc-1 electrolyte or ZSO–Ac-3 electrolyte is 358 hours and 306 hours, respectively (Fig. S5†). The cycle life of Zn‖Zn cells with both ZSO–Sc-1 electrolyte and ZSO–Ac-3 electrolyte is much shorter than that of the Zn‖Zn cell with ZSO–Sc/Ac-3 electrolyte, indicating the synergistic effect of Sc3+ and Ac− additives.
We particularly select ZSO–Sc/Ac-3 electrolytes for further electrochemical testing. To assess the HER in various electrolytes, linear sweep voltammetry (LSV) was conducted. As depicted in Fig. 1a, to deliver a current density of 40 mA cm−2, the potential of the HER in the electrolyte with Sc3+ (−1.21 V vs. Ag/AgCl) is close to that of the ZSO electrolyte (−1.23 V vs. Ag/AgCl). This indicates a negligible inhibition effect on the HER owing to an increased H+ concentration caused by the hydrolysis of Sc3+ in the electrolyte. However, the potential increases from −1.21 to −1.25 V (vs. Ag/AgCl) at 40 mA cm−2 upon the further introduction of the Ac− additive, indicating the inhibited HER in the presence of Sc3+ and Ac−. All the results mentioned above are associated with the regulatory effect of Ac− on the concentration of H+ in the electrolyte. This can be illustrated by the changes in the pH values of various electrolytes (Fig. S6† and 1b). As mentioned earlier, corrosion and the HER intensely occur in the ZSO–Sc-1 electrolyte during AZB cycling due to a significantly decreased pH value (Fig. 1c). As hydrogen gas continues to evolve, the concentration of OH− at the ZMA interface increases. OH− reacts with Zn2+ and SO42− to form pits and the inactive side product Zn4SO4(OH)6·4H2O.39 The formation of side products increases the ZMA surface roughness and accelerates dendrite growth, consequently weakening the inhibitory effect of the Sc3+ additive on Zn dendrite growth. In sharp contrast, with the introduction of the Ac− additive, the intrinsic hydrolysis reaction of Ac− (Ac− + H+ ↔ HAc) dynamically regulates the pH of the bulk electrolyte to a stable value of 4.3, thereby significantly mitigating the HER, corrosion, and side reactions on ZMAs, while effectively amplifying the inhibitory effect of the Sc3+ additive on Zn dendrites (Fig. 1d).
To investigate the influence of Sc3+/Ac− additives on the solvation structure of Zn2+, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and Raman spectroscopy were conducted. As depicted in Fig. 1e, the peaks originating from ν-Zn–O, ν-SO42–, and ν-O–H can be observed for ZSO, ZSO–Sc-1, and ZSO–Sc/Ac-3 electrolytes. Notably, there is no obvious difference among the ATR-FTIR spectra, suggesting that the Sc3+/Ac− additive does not significantly affect the Zn2+ solvation structure.40 Moreover, the Sc3+/Ac− additive does not induce any significant change in the Raman spectra of the ν-Zn–O, ν-SO42–, and ν-O–H peaks at 350–550, 926–998, and 3000–3800 cm−1, respectively, further indicating that trace Sc3+/Ac− additive does not significantly alter the hydration structure of Zn2+ or the water structure in the electrolyte (Fig. 1f–h).41,42 Furthermore, the contact angle was measured to assess the wettability of the electrolyte on Zn foil (Fig. S7†). With the Sc3+/Ac− additive, the contact angle on the Zn foil slightly decreases from 100.3° in the ZSO electrolyte to 98.4° in the ZSO–Sc/Ac-3 electrolyte. The negligible change indicates that the Sc3+/Ac− additive has a minor effect on electrolyte wettability.43
To further examine the Zn plating process, a constant current discharge test was conducted on Zn‖Zn cells using ZSO and ZSO–Sc/Ac-3 electrolytes at an applied current density of 2.0 mA cm−2. As illustrated in Fig. 2e, the Zn‖Zn symmetric cell assembled with ZSO electrolyte experiences a short circuit when the plating capacity reaches 32.4 mA h, attributed to Zn dendrite growth. Conversely, the Zn‖Zn symmetric cell with ZSO–Sc/Ac-3 electrolyte does not experience a short circuit until the counter Zn metal anode is entirely consumed, indicating that the Sc3+/Ac− additive promotes uniform Zn2+ distribution and smooth Zn plating. Similar outcomes were also obtained when the applied current density was 0.5 or 1.0 mA cm−2 (Fig. S9†). Furthermore, the regulated Zn2+ distribution and plating behaviour in ZSO–Sc/Ac-3 electrolyte are confirmed through chronoamperometry (CA) measurements (Fig. 2f). Following a brief 25 s test, the Zn‖Zn symmetric cell with ZSO–Sc/Ac-3 electrolyte exhibits stable current density for up to 230 s after applying an overpotential of −150 mV, indicating significant restriction of rampant 2D diffusion of Zn2+ responsible for dendrite formation.45
To delve deeper into the influence of the Sc3+/Ac− additive on Zn plating behaviour, we employed in situ optical microscopy to scrutinize the morphological changes during Zn plating in both ZSO and ZSO–Sc/Ac-3 electrolytes on Zn foil (Fig. 3a and b). In ZSO electrolytes, the tendency of Zn2+ to aggregate at the Zn protrusions, known as the “tip effect”, leads to localized plating at specific sites. Within 5 minutes of electroplating, noticeable Zn protrusions emerge on the surface, gradually aggregating into large dendritic crystals over the subsequent 10 minutes. This phenomenon is known to cause internal short circuits, decreased CE, and compromised battery performance over time. In contrast, in the ZSO–Sc/Ac-3 electrolyte, the Zn electrode displays a consistently dense and uniform plating morphology throughout the 15-minute electroplating process. This can be attributed to the enhanced nucleation kinetics, reduced nucleation barrier, and an electrostatic shielding effect of adsorbed Sc3+ provided by the ZSO–Sc/Ac-3 electrolyte, leading to a more uniform Zn plating process. Scanning electron microscopy (SEM) images of the plated Zn confirm these observations. At different current densities, the Zn electrode cycled in the ZSO electrolyte exhibits disordered dendrites and irregular plate-like by-products, disrupting electric/ion field distribution and accelerating battery degradation (Fig. 3c, d, g and h). Conversely, in the ZSO–Sc/Ac-3 electrolyte, the Zn electrode maintains a smooth, dendrite-free surface, indicating homogeneous ion distribution throughout plating (Fig. 3e, f, i and j). Furthermore, X-ray diffraction (XRD) analysis reveals unique diffraction peaks corresponding to the by-product Zn4SO4(OH)6·4H2O on the ZMA electrode in the ZSO electrolyte, confirming the accumulation of by-products (Fig. S10†). Conversely, the inclusion of the Sc3+/Ac− additive allows a more even distribution of Zn2+ across the ZMA surface, leading to a near elimination of the by-product peaks. This underscores the notable suppression of side reactions enabled by the ZSO–Sc/Ac-3 electrolyte.
Building upon the preceding analysis, a proposed operational mechanism illustrating the role of the Sc3+/Ac− additive in guiding the Zn plating process at the Zn metal anode/electrolyte interface is presented in Fig. 3k. In the ZSO electrolyte, the Zn metal anode experiences severe HER and corrosion, which are primarily induced by the presence of a water-rich interface and the high desolvation energy barrier of Zn(H2O)62+.46 Continuous hydrogen evolution leads to an elevated concentration of OH− at the interface, resulting in the formation of Zn4SO4(OH)6·xH2O by-products. The increased surface roughness due to the scattered corrosion areas disrupts the uniform distribution of the electrical field on the ZMA surface, culminating in an irregular dispersion of Zn2+ at the interface and subsequent dendrite formation. In contrast, in the ZSO–Sc/Ac-3 electrolyte, the Sc3+/Ac− additive can reduce the energy barrier for Zn plating and enhance Zn plating kinetics, promoting uniform Zn2+ flux and deposition. Simultaneously, Sc3+ tends to be absorbed on the Zn anode surface, forming an electrostatic shielding layer.47,48 This shielding layer helps mitigate the “tip effect”, facilitating a uniform Zn-plated layer. Moreover, the hydrolysis reaction of Ac− dynamically regulates the concentration of H+ at the Zn anode/electrolyte interface, maintaining the pH of the electrolyte at approximately 4.3, thereby inhibiting the HER and related side reactions. Consequently, the synergistic effect of Sc3+ and Ac− enables highly stable and reversible ZMAs.
To delve deeper into the impact of the Sc3+/Ac− additive on the reversibility of Zn plating/stripping on ZMAs, Zn‖Cu cells were assembled and evaluated. As depicted in Fig. 4g, the Zn‖Cu cell using the ZSO electrolyte exhibits significant fluctuations in CE after 66 cycles at a current density of 1.0 mA cm−2 and a capacity of 1.0 mA h cm−2. These fluctuations imply insufficient reversibility, likely due to water-induced side reactions and dendrite formation. The Zn‖Cu cell with ZSO–Sc-1.0 exhibits an average CE of 99.11% and a lifespan of 265 cycles. The Zn‖Cu cell assembled with ZSO–Ac-3 shows an average CE of 98.56% and a short life of 160 cycles (Fig. S13†). In contrast, the Zn‖Cu cell employing ZSO–Sc/Ac-3 electrolyte can endure 400 charge/discharge cycles with an average CE of 99.40%, indicating that the Sc3+/Ac− additive contributed to enhancing the reversibility of Zn plating/stripping (Fig. 4h). Notably, at 5.0 mA cm−2 and 2.0 mA h cm−2, Zn‖Cu cells with the Sc3+/Ac− additive also exhibit remarkable stability and reversibility, with an average CE of 99.44% in 200 cycles. In contrast, the Zn‖Cu cell without additives displays noticeable CE fluctuations, which are relatively lower at 99.19% (Fig. S14†). These findings further suggest that the Sc3+/Ac− additive plays a significant role in enhancing the stability and reversibility of Zn plating/stripping.
To further assess the efficacy of Sc3+/Ac− additives in a full battery, Zn ion hybrid capacitors (ZHCs) were constructed utilizing commercially available activated carbon (AC), followed by the evaluation of their electrochemical performance. As illustrated in Fig. 5a, the cyclic voltammetry (CV) curve of ZHCs remains stable under a scan rate of 0.5 mV s−1, exhibiting reversible oxidation/reduction peaks associated with the reversible ion adsorption/desorption on the AC cathode and Zn2+ plating/stripping on the ZMA.52Fig. 5b illustrates the constant current charge–discharge profiles of ZHCs across various current densities, ranging from 0.1 to 5.0 A g−1. The charge–discharge behaviour of ZHCs demonstrates a nearly symmetric pattern, characterized by a linear voltage change over time, indicative of a typical capacitive storage mechanism consistent with the CV curve. Moreover, as the current density escalates from 0.1 to 5.0 A g−1, the ZHC utilizing ZSO–Sc/Ac-3 electrolyte exhibits a heightened specific capacity compared to that with ZSO electrolyte (Fig. 5c). Despite a fifty-fold increase in current density, the former displays superior rate capability, retaining 71.4% of its capacitance (from 97 to 69 mA h g−1), while the ZHC with ZSO electrolytes shows diminished specific capacity and capacity retention (69.5%, from 81 to 56 mA h g−1). Furthermore, the ZHC employing ZSO electrolyte demonstrates capacity fluctuations and a noticeable decrease in CE after 1880 cycles at a current density of 1.0 A g−1, likely attributed to potential side reactions at the ZMA interface and dendrite formation (Fig. 5d). In contrast, the ZHC incorporating ZSO–Sc/Ac-3 electrolytes exhibits the capability to undergo continuous 10000 cycles while retaining a specific capacity of 62 mA h g−1, highlighting the promising practical applications of ZSO–Sc/Ac-3 electrolyte. Notably, a significant increase in capacity was observed in the ZHC using the ZSO–Sc/Ac-3 electrolyte. This improvement can be attributed to the ongoing activation of the porous carbon electrode during cycling, the inhibition of side reactions, and the enhanced stability and reversibility of the electrode due to the presence of the Sc3+/Ac− additive.53
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta02133a |
This journal is © The Royal Society of Chemistry 2024 |