氧化硅基电解液用于稳定水系锌锰电池

1 Introduction

Aqueous zinc-manganese batteries with the advantages of high safety, abundant resources and high energy density, are considered as one of the most promising large-scale energy storage batteries [1-3]. Nevertheless, their practical application still faces challenges, which is mainly due to the deterioration of electrochemical performance of manganese-based cathodes caused by aqueous electrolyte. Fundamentally, the electrochemical stability window (ESW) of aqueous electrolyte is limited by the thermodynamic potential difference between oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) of water. Although the ESW of the salt solution can be expanded relative to the pure water (1.23 V), it still cannot meet the requirements of the battery. On the one hand, the narrow ESW will greatly restrain the operating output voltage of aqueous zinc-manganese batteries and lead to insufficient energy density. On the other hand, the narrow ESW tends to lead to gas generation (e.g., O2 and H2) or electrolyte consumption, which will destroy the structure of the electrodes and result in inferior cycling stability.

To overcome these issues and achieve better electrochemical performance, it is crucial to suppress the water splitting and extend the ESW. At present, a lot of researches are devoted to electrolyte optimization, such as gel electrolyte [4], ionic liquid electrolyte [5], organic electrolyte [6-7], electrolyte additive [8], etc. Since many issues in aqueous electrolyte are caused by excess active water content, many of these strategies focus on reducing the content of water. It was reported that the gel electrolyte, ionic liquid electrolyte and organic electrolyte greatly inhibit the dendrite and corrosion of the zinc anode due to the decrease of active water and the zinc ion solvation structure dominated by anionic groups, then the cyclic stability and reversibility of the zinc anode are improved [9-10]. However, according to ZHANG et al’s report [11], the less water content in electrolyte is not conducive to the release of capacity and energy of Mn-based materials. Therefore, it is expected from the perspective of electrolyte by adjusting the active water content to realize the long-cycle-life and high-specific-capacity aqueous zinc-manganese batteries.

With this idea in mind, a promising strategy is to develop a mixed electrolyte that based on inert inorganic and zinc salt solution, in which the added inorganic components should be considered to inhibit the decomposition of water. This mixed electrolyte may inherit the high conductivity of water system electrolyte and also can overcome various adverse factors caused by the active water. Moreover, many inorganic substances, such as silicate and insoluble sulfates, are easy to form functionalized groups on the surface in aqueous solutions, which can give the electrolyte special electrochemical properties. In this work, we have developed a new electrolyte, which is made up of silica (SiOx) fibers and a ZnSO4+MnSO4 solution (named as Si-ZMSO electrolyte). The Si-ZMSO electrolyte can widen the ESW of electrolyte and restrain the side reaction of zinc anode, which can stabilize the cyclic performance of Zn-MnO2 batteries compared to pure ZnSO4 electrolyte.

2 Experimental

Synthesis of SiOx/ZnSO4+MnSO4 electrolyte: The synthesis of SiOx fibers was referred to the report of LIU’s group [12]. In a typical synthesis, appropriate amount of SiOx fibers were added to      10 mL 2 mol/L ZnSO4+0.1 mol/L MnSO4 aqueous solution (defined as ZMSO electrolyte), then the solution was placed under ultrasound for 30 min to obtain the viscous mixed electrolyte. The electrolyte containing 0.5 g and 1 g of SiOx fibers were named as Si-ZMSO and Si-ZMSO-10, respectively. Electrolyte preparation can be scaled up by adjusting the amount of raw materials.

Synthesis of MnO2 nanorods: The synthesis method is similar to the previous report [13]. The 15 mL 0.15 mol/L MnSO4 solution (solution A) and 15 mL 0.10 mol/L KMnO4 solution (solution B) were firstly prepared. Solution B was added to solution A drop by drop and stirred for 10 min. The solution was further transferred to a Teflon-lined autoclave and heated at 160 ℃ for 12 h to obtain MnO2 nanorods.

Material characterization: X-ray diffraction (XRD) data were collected using a Rigaku D/max2500 powder diffractometer (Cu Kα, λ=0.15405 nm). The scanning electron microscope (SEM) images were collected on a Sirion 200 operating at 10 kV. The transmission electron microscopy (TEM) images were collected using a Titan G2 60-300 transmission electron microscopy.

Electrochemical measurement: To prepare the cathode electrode, a slurry mixed with 70% MnO2, 20% acetylene black, and 10% polyvinylidene fluoride was coated onto a stainless steel wire mesh disk and dried at 80 ℃ in vacuum overnight. The zinc metal foil was used as anode. Glass fiber filter paper was used as a separator. ZMSO or Si-ZMSO electrolyte was used as electrolyte. The cycling stability and rate capability were studied using a multichannel battery testing system (Land CT 2001A).

3 Results and discussion

Figure 1(a) shows that the morphology of SiOx sample is interwoven nanofibers. It can be clearly seen in the enlarged SEM image (Figure 1(b)) that there are a lot of interspaces between the nanofibers. This feature makes it easy to mix evenly with aqueous electrolyte. The TEM image (Figure 1(c)) further confirms the characteristic of alternating intervals of SiOx samples. Each nanofiber is uniform and has a diameter of 50 nm, as shown in         Figure 1(d). Si-ZMSO electrolyte was prepared via adding 0.5 g of SiOx nanofibers into 10 mL 2 mol/L ZMSO electrolyte, which forms a mushy mixed electrolyte. Optical photograph of Si-ZMSO electrolyte in an inverted transparent bottle (Figure 2(a)) demonstrates that this electrolyte is viscous and has weak fluidity. We all know that high ionic conductivity is the unique advantage of aqueous electrolyte, and the influence of filler on ionic conductivity needs to be considered. It is gratifying that the ionic conductivity of Si-ZMSO electrolyte is 6.0 mS/cm, which is comparable to that of aqueous ZMSO electrolyte (6.4 mS/cm) in our tests, as shown in Figure 2(b). In the Si-ZMSO electrolyte, the interwoven SiOx nanofibers can construct a network skeleton for liquid electrolyte in which ions can move freely. This maximizes the ionic conductivity of the electrolyte. Compared to the aqueous counterpart, Si-ZMSO electrolyte shows a wider stable voltage window (Figure 2(c)). The addition of SiOx nanofibers can reduce the content of active water in electrolyte. In addition, previous reports have demonstrated that the surface of silica nanowires has many hydrophilic functional groups [14], which can further limit the activity of water. Such hybrid electrolyte is suitable for use in zinc-manganese batteries because the reduced water activity is conducive to the stability and reversibility of the zinc anode (Figure 2(d)). At the same time, the electrolyte retains the appropriate water content, which can match the manganese-based cathode materials to play a high capacity and stable performance.

Figure 1  Characterization of SiOx nanofibers: (a, b) SEM images; (c, d) TEM images

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Figure 2  Characterization of Si-ZMSO and ZnSO4 + 0.1 mol/L MnSO4 electrolyte: (a) Optical photograph of Si-ZMSO electrolyte; (b) Ionic conductivity; (c) LSV curves of Si-ZMSO and ZnSO4 + 0.1 mol/L MnSO4 electrolyte; (d) Diagram of Zn/MnO2 battery with Si-ZMSO electrolyte

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In order to demonstrate the advantages of Si-ZMSO electrolyte, cycling stability of Zn||Zn symmetrical batteries in various electrolytes was conducted. In the 2 mol/L ZnSO4 electrolyte, the Zn||Zn symmetrical battery quickly shorted out after a 70-hour cycle at the current density of 1 mA/cm2 and area capacity of 1 mA·h/cm2, as shown in Figure 3(a), which is consistent with most reported results [15-17]. Although the ZMSO electrolyte with MnSO4 additive, which is a typical electrolyte for zinc-manganese batteries, can reduce the electrochemical polarization, the cycle life of Zn||Zn symmetrical battery is only extended to 110 h (Figure 3(b)). HUANG et al [18] and LI et al [19] proved that the Mn(OH)2 sphere particles formed in the electrolyte aggregated, and the Zn2+ flow was mechanically adjusted to make the ion field uniform. In addition, the nucleation overpotential of the ZnSO4/MnSO4 mixture electrolyte is smaller than that of the ZnSO4 electrolyte, which is also beneficial to retard the growth of zinc dendrites and the uniform deposition of zinc. Therefore, Mn2+ additive in ZnSO4 electrolyte does suppress the formation of dendrite-like zinc, and improves electrochemical stability of Zn||Zn symmetrical battery with ZnSO4/MnSO4 mixture electrolyte [18]. Moreover, Mn2+ in the form of MnSO4 was often added into ZnSO4 aqueous electrolyte to suppress the dissolution of MnO2 cathode and provide extra capacity by deposition on the cathode [19]. Compared to the ZMSO electrolyte, electrochemical polarization increases in the Si-ZMSO electrolyte. Generally, the increased electrochemical polarization was not advantageous for the long-term stable cycle. However, it was found that the Zn||Zn symmetrical battery with Si-ZMSO electrolyte exhibited more excellent cycle stability over 400 h at the current density of 1 mA/cm2 with an area capacity of 1 mA·h/cm2 (Figure 3(c)). The smooth deposition at the anode interface may be the key factor for the stable cycle of zinc anode. When the content of SiOx nanofibers was increased, such as in Si-ZMSO-10 electrolyte, the Zn||Zn symmetrical battery showed a similar cyclic behavior with that of Si-ZMSO electrolyte at 200 h before. But after that, the overpotential in Si-ZMSO-10 electrolyte rapidly increased, and then it expired at 300 h. This may be due to the fact that the increased interfacial impedance resulted from the addition of excessive solids. As the current density increases to 5 mA/cm2, Zn||Zn symmetrical battery with Si-ZMSO electrolyte also demonstrated a good cyclic stability over 400 h, while the failure is more pronounced in Si-ZMSO-10 electrolyte (Figure 3(d)). The results indicate that the appropriate amount of SiOx is beneficial to the improvement of cycle stability.

Figure 3  Cycling stability of Zn||Zn symmetrical batteries: (a) 2 mol/L ZnSO4 at 1 mA/cm2; (b) 2 mol/L ZnSO4+0.1 mol/L MnSO4 at 1 mA/cm2; (c) Si-ZMSO and Si-ZMSO-10 at 1 mA/cm2; (d) Si-ZMSO and Si-ZMSO-10 at 5 mA/cm2

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The excellent cyclic stability of Zn anode is ascribed to the uniform deposition, which is also reflected in other mixed state electrolytes [20-22]. To understand the distinction of Zn depositing behavior between Si-ZMSO and ZMSO electrolytes, ex-situ analysis of Zn anode after 1st discharged state was conducted. As shown in Figure 4(a), after discharging in Si-ZMSO electrolyte, SEM image shows a smooth flat surface of Zn anode. The high-magnification SEM image (Figure 4(b)) indicates that the morphology of depositing production is connected particles. In ZMSO electrolyte, however, it is clear that the surface of the zinc anode has accumulated by-products (Figure 4(c)). SEM image at high- magnification shows that these by-products exhibit the shape of prominences, as shown in Figure 4(d). A large number of by-products will cause other problems such as corrosion, hydrogen evolution, [23] and will not be conducive to the stability of the zinc anode. Such side reactions in aqueous electrolyte would be detrimental for large-scale practical application of ZIBs [24-25]. The XRD patterns of zinc anode after discharged state in ZMSO electrolyte also confirmed the aggregation of by-products. After deposition, the zinc anode in ZMSO electrolyte shows several obvious new peaks, except for the diffraction peak of Zn phase (Figure 4(e)). These new peaks match well with the Zn4SO4(OH)6·xH2O (ZSH) phase (e.g., PDF#44-0673 and PDF#39-0689). On the contrary, zinc anode in Si-ZMSO electrolyte shows weaker diffraction peak of the by-product compared to that in ZMSO electrolyte, which is calibrated by the intensity of the diffraction peak of Zn phase. These results show that the addition of silica nanofibers (denoted as Si) additive to ZMSO electrolyte can effectively alleviate the side reaction between ZMSO and Zn electrode, thus maintaining the structural stability of the electrode.

Figure 4  Ex-situ analysis of Zn anode in ZMSO and Si-ZMSO electrolytes: (a) Low-magnification and (b) high-magnification SEM images after discharged state in Si-ZMSO electrolyte; (c) Low-magnification and (d) high-magnification SEM images after discharged state in ZMSO electrolyte; (e) XRD patterns of Zn anode after discharged state in ZMSO and Si-ZMSO electrolytes

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Manganese-based materials are considered as the most promising cathode materials for aqueous zinc metal batteries [26-28]. However, most manganese-based materials exhibit poor cyclic life, specially under the low current density. To explore the benefits of the Si-ZMSO electrolyte, the electrochemical storage capability of Zn-MnO2 batteries was tested at a low current density of     100 mA·h/g. Figure 5(a) shows the different discharge/charge profiles of the Zn-MnO2 batteries with Si-ZMSO and ZMSO electrolytes. Both samples exhibit two distinct discharge voltage platforms between 1.2 V and 1.4 V. We note that the Zn-MnO2 battery delivers a high second specific capacity of more than 350 mA·h/g in the ZMSO electrolyte, but unfortunately it decayed by 62% when it reached 60 cycles. Remarkablely, the discharge plateaus of MnO2 cathode exhibited a gradual disappearance during cycling, as shown in purple dotted line. In the ZMSO electrolyte, the high capacity is mainly due to the large amount of active water in the electrolyte, which stimulates the capacity of manganese-based materials [11]. But the problems caused by the aqueous electrolyte lead to its poor cycling stability, and even destroy the original material system. While in Si-ZMSO electrolyte, Zn-MnO2 battery delivers only a specific capacity of 249 mA·h/g at second cycles, but it can maintain a considerable capacity of 209 mA·h/g after 60 cycles. More importantly, the discharge plateaus maintained well during cycling. As shown in Figure 5(b), we can see the difference between the two systems even more clearly from the cyclic curve. To better illustrate its prospects, we increased the loading of MnO2 to ~5 mg/cm2, which is rarely involved in previous reports. As shown in Figure 5(c), in spite of the lower second specific capacity (172 mA·h/g) in Si-ZMSO electrolyte system, but it has a higher capacity retention rate of ~63% after 200 cycles, which is much higher than that in ZMSO electrolyte (~23%). The above results demonstrate that the introduction of silica naofibers in Si-ZMSO electrolyte effectively improves the stability of Zn-MnO2 battery system.

Figure 5  Cycling performance of Zn-MnO2 batteries with ZMSO and Si-ZMSO electrolytes: (a) The charging/discharging curves; (b) Cyclic performance at 100 mA/g; (c) Cyclic performance at 100 mA/g with high cathode loading

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4 Conclusions

In this work, a novel and practical hybrid Si-ZMSO electrolyte was proposed by adding SiOx nanofibers into ZMSO electrolyte, which obviously improved the cycle stability of Zn-MnO2 battery under low current density. The Si-ZMSO electrolyte can widen the ESW of electrolyte and restrain the side reaction of zinc anode, which can achieve a good cyclic stability over 400 h for Zn||Zn symmetrical battery. This novel hybrid electrolyte can also stabilize the cyclic performance of            Zn-MnO2 batteries compared to ZMSO electrolyte. In particularly, it exhibits a good capacity retention after 200 cycles under the high cathode loading and low current density.