Alumina modified sodium vanadate cathode for aqueous zinc-ion batteries

Abstract

Aqueous zinc-ion batteries (ZIBs) have great prospects for widespread application in massive scale energy storage. By virtue of the multivalent state, open frame structure and high theoretical specific capacity, vanadium (V)-based compounds are a kind of the most developmental potential cathode materials for ZIBs. However, the slow kinetics caused by low conductivity and the capacity degradation caused by material dissolution still need to be addressed for large-scale applications. Therefore, sodium vanadate Na₂V₆O₁₆·3H₂O (NVO) was chosen as a model material, and was modified with alumina coating through simple mixing and stirring methods. After Al₂O₃ coating modification, the rate capability and long-cycle stability of Zn//NVO@Al₂O₃ battery have been significantly improved. The discharge specific capacity of NVO@Al₂O₃ reach up to 228 mAh/g (at 4 A/g), with a capacity reservation rate of approximately 68% after 1000 cycles, and the Coulombic efficiency (CE) is close to 100%. As a comparison, the capacity reservation rate of Zn//NVO battery is only 27.7%. Its superior electrochemical performance is mainly attributed to the Al₂O₃ coating layer, which can increase zinc-ion conductivity of the material surface, and to some extent inhibit the dissolution of NVO, making the structure stable and improving the cyclic stability of the material. This paper offers new prospects for the development of cathode coating materials for ZIBs.

Graphical abstract

Keywords

cathodes / aqueous zinc-ion batteries / sodium vanadate / alumina / coating

1 Introduction

With the advent of the 21st century, modern economic society and industrial civilization have accelerated the progress of intelligent technology, and the demand for energy has rapidly increased [1–3]. Nevertheless, the total amount of energy available to humankind is not unlimited, so the transportation and storage of renewable energy is crucial for the effective utilization of energy [4–9]. However, renewable energies are intermittent, and consequently, it is very difficult to store and use them. Therefore, it is urgent to develop reliable, stable, and safe energy storage devices. Lithium-ion batteries (LIBs) have been widely used, whose poor security and shortage of its own resources requires exploration and development of new energy storage systems. In many options, rechargeable aqueous ZIBs stand out, benefited from their high safety, low cost, and environmental friendliness. Nevertheless, ZIBs are still far from practical application, and one of the biggest challenges hindering their application is the lack of cathode materials with a high reversible capacity and an excellent long-term cycling stability.

Vanadium based materials have the following advantages as cathode materials of aqueous ZIBs. First, it involves a multi-electron redox reaction accompanied by different oxidation states of V (i.e., V⁵⁺, V⁴⁺, V³⁺, and V²⁺) and therefore, with a high capacity (> 300 mAh/g). In addition, it has large ion transport channels, beneficial for rapid embedding/de-embedding of Zn²⁺ [10–16]. In various vanadium-based materials, sodium vanadate (such as Na₂V₆O₁₆·3H₂O (NVO)) has a three-dimensional tunneling structure, can effectively inhibit ion channel collapse, and achieve reversible embedding/de-embedding of Zn²⁺ [17]. Unfortunately, NVO, as a cathode material, still has many limitations, such as the instability of the layered structure, the susceptibility of phase transition after pre-embedding of metal ions, the dissolution of vanadium ions in weak acidic electrolytes, and the poor conductivity [18–22]. To overcome these challenges, several design strategies have been proposed, such as embedding of metal ions [15], regulation of structural water [21], addition of conductive polymers, and selection of electrolyte salts/additives [22]. Moreover, surface treatment of materials [23], especially coating, is a commonly used strategy to suppress material dissolution and improve material structural stability, which will be an effective strategy for modifying NVO. However, there is limited research in this area. As is known, the amorphous Al₂O₃ coatings can effectively enhance the electrochemical performance of the battery material [24,25]. Kim et al. [26] synthesized Al₂O₃@graphite to enhance the fast-charging capability of graphite. This is mainly attributed to the fact that the alumina coating on the surface can increase the electrolyte wettability of NVO and enhance the stability of the material structure.

Herein, alumina coating was chosen to modify NVO. NVO@Al₂O₃ was synthesized by simply mixing and stirring. Various physical and electrochemical characterization techniques were used to assess the composition and electrochemical performance of the material. The cyclic voltammetry (CV) tests at different scan rates and electrochemical impedance spectroscopy (EIS) analysis show that the Al₂O₃ coating can promote uniform distribution of charge in electrode and accelerate electron transfer during redox reaction of V⁵⁺/V⁴⁺ and V⁴⁺/V³⁺. Meanwhile, the Al₂O₃ coating can also stabilize the structure of NVO and optimize its cycling performance. Compared to NVO, the prepared NVO@Al₂O₃ exhibits an excellent Zn²⁺ storage performance, with a higher specific capacity (up to 420 mAh/g at 100 mA/g), better rate capability (with capacity of 228 mAh/g at 4 A/g) and cycling stability (with the capacity reservation rates of 68% at 4 A/g after 1000 cycles). These results indicate that the Al₂O₃ coating can effectively optimize the electrochemical performance of V-based oxides.

2 Results and discussion

In this paper, NVO is prepared by a simple ion-exchange method, with a morphology of nanowire, as shown in Fig.1(a). After coating with alumina, its shape is still nanowire (Fig.1(d)). However, the transmission electron microscope (TEM) images (Fig.1(b) and Fig.1(c), Fig.1(e) and Fig.1(f)) of the two materials are not significantly different and cannot distinguish the alumina coating layer. Energy dispersive spectrometry (EDS) mappings of NVO@Al₂O₃ collected by TEM (Fig.1(g)) reveal a uniform distribution of O, V, Na, and Al elements. In addition, the inductively coupled plasma (ICP) testing indicates that the sample element content of Al is approximately 0.943% (mass fraction). As shown in Fig.2(a), the peaks of NVO and NVO@Al₂O₃ are in good agreement with monoclinic NVO (JCPDS-16-0601). The characteristic peaks of the tow materials are located at 10.84°, 26°, 28.1°, and 50.98°, and their peaks are not sharp, indicating the poor crystallinity of the materials. The diffraction peak of NVO@Al₂O₃ is similar to that of NVO, and there is no characteristic peak of alumina. This suggests that the crystal structure of NVO has not changed after coating, and the alumina on NVO exists in an amorphous form, explaining the reason why the alumina coating layer cannot be observed in TEM images (Fig.1(e) and Fig.1(f)), which is consistent with that reported in Ref. [27]. The above analysis demonstrates that Al₂O₃ uniformly coated NVO has been successfully prepared.

To identify the existence of Al₂O₃ in the as-prepared NVO@Al₂O₃, the FTIR spectrum (Fig.2(b)), the X-ray photoelectron spectroscopy (XPS) spectrum are used to analysis NVO and NVO@Al₂O₃. The peak at 1402 cm⁻¹ is associated with the stretching vibration of the Al–O bonds, which proves the existence of alumina in NVO@Al₂O₃ [28]. The XPS of NVO and NVO@Al₂O₃ are shown in Fig.2(c), from which, it can be observed that the XPS spectrum of NVO@Al₂O₃ is around 72 eV, with a peak of Al 2p. The Al 2p XPS spectra of NVO@Al₂O₃ is shown in Fig.2(c), indicating the existence of aluminum on the surface of NVO, the successful introduction of Al and O after coating, and confirming the formation of Al₂O₃ shell [29].

As show in Fig.3(a), in the CV curves of NVO@Al₂O₃ at 0.2 mV/s, there are a pair of reduction peaks and oxidation peaks at 0.76/0.538 V and 1.039/0.85 V. Compared with NVO, NVO@Al₂O₃ shows a smaller potential difference, indicating that it has a smaller polarization, which is caused by faster ion diffusion [29–31]. To study the changes in the valence state of V in NVO@Al₂O₃ during charging and discharging, XPS testing is conducted on NVO@Al₂O₃ in different charging and discharging states, and the V 2p3/2 spectra obtained are de-convoluted for analysis (Fig. S1). When discharge to 0.4 V, the V³⁺ in the material increases, which means that V⁴⁺ and V⁵⁺ are reduced, corresponding to the embedding of Zn²⁺. When charging to 1.4 V, V³⁺ in the material disappears, indicating that most V³⁺ and V⁴⁺ are re-oxidized, corresponding to the de-embedding of Zn²⁺. Fig.3(b) show the charge/discharge curves of the first two cycles of Zn//NVO@Al₂O₃ battery and Zn//NVO battery at 100 mA/g. Both profiles show an average discharge voltage of approximately 0.82 V (versus Zn²⁺/Zn). The reversible capacity of the first cycle of Zn//NVO@Al₂O₃ battery can reach 420 mAh/g, while the first cycle capacity of Zn//NVO battery is only about 380 mAh/g. The cycling performance tests of the two materials at 0.1, 0.5, and 4 A/g are manifested in Fig.3(d)–Fig.3(f), respectively. The Zn//NVO battery exhibits a poor cycling stability with capacity reservation rates of only 15.1%, 38.4%, and 26.6% after 60, 100, and 1000 cycles at 0.1, 0.5, and 4 A/g, respectively. This is mainly attributed to the rapid dissolution of NVO in electrolyte [32]. Compared to it, the Zn//NVO@Al₂O₃ battery shows a higher capacity and capacity reservation rate at the same current density, especially at a high current density (4 A/g). At 0.1, 0.5, and 4 A/g, the capacity reservation rates after 60, 100, and 1000 cycles are 40.2%, 53.3%, and 68%, respectively. Moreover, the CE is significantly better than that of the Zn//NVO battery (94.14%), approaching 100% (Fig.3(f)). The above is attributed to the Al₂O₃ coating layer, inhibiting the dissolution of NVO in the ZnSO₄ solution and improving the stability of their layered structure during cycling. As shown in Fig.3(g), the Zn//NVO@Al₂O₃ battery have a better rate performance. When the current densities are 0.1, 0.2, 0.5, 1, 2, and 4 A/g, its discharge specific capacities are 410, 342, 292, 255, 214, and 155 mAh/g, respectively. When the current density returns to 0.1 A/g, its capacity is approximately 77% of the initial value. In comparison, the discharge specific capacities of the Zn//NVO battery are 325, 225, 210, 195, 155, and 102 mAh/g at 0.1, 0.2, 0.5, 1, 2, and 4 A/g, respectively. When the current density returns to 100 mA/g, its capacity is only 64% of the initial value. Figure S3 shows that the contact angle of the NVO electrode (86°) is much larger than the NVO@Al₂O₃ electrode (45°), suggesting a great improvement in hydrophilicity in the presence of Al₂O₃ coating. Hence, the excellent rate performance of NVO@Al₂O₃ is mainly attributed to the alumina coating layer which can improve the wettability between NVO and electrolytes, increase the utilization rate of the material, accelerate the exchange rate between interface electrons and Zn²⁺, and thus improve the rate performance of NVO. It can be observed from Table S1 that the performance of the Zn//NVO@Al₂O₃ battery is better than that of the most advanced Zn//vanadate-cathode battery published in Refs. [13,22,28–32].

To further explain the working mechanism of NVO with the alumina coating layer, the change of charge transfer resistance (R_ct) during the cycle of the battery is monitored by EIS analysis, which analyzes ion diffusion and R_ct (Fig.3(h) and Fig.3(i)). The R_ct of the Zn//NVO@Al₂O₃ battery and the Zn//NVO battery decreases in the first few cycles, which corresponds to the gradual activation process of the battery. As the cycle proceeds, the R_ct of the battery gradually increases, which is caused by the dissolution and the destruction of the structure of cathodes during the long cycling process. Compared with the Zn//NVO battery, the Zn//NVO@Al₂O₃ battery has a significantly smaller impedance, indicating that the Zn//NVO@Al₂O₃ battery has a better rapid kinetic performance and a better structural stability than the Zn//NVO battery.

For further understanding the effect of Al₂O₃ coating on the electrochemical storage kinetics of electrodes, the CV of the two batteries at different scanning rates is tested and analyzed (Fig.4(a) and Fig.4(b)). The main factors controlling the electrochemical process are analyzed by Eq. (1) [33,34]:

(1)i=a{v}^b,

in which v is the scan rate, i is the peak current, and parameters a and b are constants. When the value of b approaches 0.5, it indicates that the semi-infinite linear diffusion dominates, but when the value of b approaches 1, it indicates that the surface pseudocapacitance behavior dominates. As scan rates increases from 0.2 to 0.8 mV/s, the b values of peaks 1–4 in the Zn//NVO@Al₂O₃ battery are 0.78, 0.92, 0.83, and 0.88, whereas those in the Zn//NVO battery are 0.63, 0.81, 0.71, and 0.77, respectively (Fig.4(c)). This indicates that the electrochemical reactions of the Zn//NVO@Al₂O₃ battery is controlled by surface-controlled capacitive reactions, while those of the Zn//NVO battery is controlled by semi-infinite linear diffusion, which also reveals that the Zn//NVO@Al₂O₃ battery has a better rate performance. Figure S4 provides a visualization of surface-controlled capacity contribution in the Zn//NVO@Al₂O₃ battery, obtained at 0.2 mV/s. The surface-controlled capacity contribution in the capacity of the NVO@Al₂O₃ battery is about 54.74%. By virtue of the enhanced interfacial compatibility of the aluminum oxide coating with the ZnSO₄ electrolyte, more Faradaic redox reactions are initiated on the surface of the NVO, thereby ensuring a higher incidence of pseudocapacitive reactions [35–37].

The galvanostatic intermittent titration technique (GITT) analysis (Fig.4(d)–Fig.4(f)) is used to assess the diffusion of Zn²⁺ (D_Zn²⁺) in the NVO and NVO@Al₂O₃. During the Zn²⁺ extraction/insertion process, the Zn//NVO battery shows a higher I–R drop at the same relaxation and pulse time. The I–R drop detected are transformed into Zn²⁺ diffusion coefficients. As shown in Fig.4(e), during the Zn²⁺ insertion process, the average D_Zn²⁺ in the Zn//NVO@Al₂O₃ battery is 6.9 × 10⁻⁹ cm²/s. By contrast, for the Zn//NVO battery, the average D_Zn²⁺ is 2.6 × 10⁻¹⁰ cm²/s. During the Zn²⁺ extraction process (Fig.4(f)), the Zn²⁺ diffusion coefficient in the Zn//NVO@Al₂O₃ battery is 3.6 × 10⁻⁸ cm²/s, but is only 1.4 × 10⁻⁹ cm²/s in the Zn//NVO battery. The GITT test results indicate that the aluminum oxide coating improves the Zn²⁺ diffusion coefficient of NVO, resulting in a better rate performance of the material [38–40].

Due to the V-based compounds dissolved in the electrolyte, the Zn anode is prone to dendrite problems [32,41]. Therefore, detecting the content of V on the zinc surface after cycling through EDS can demonstrate the inhibitory effect of Al₂O₃ coating on V dissolution in NVO. As shown in Fig.5, the V content of the Zn surface reaches 7.35% in the Zn//NVO battery after 10 cycles at 0.1 A/g, and the surface is uneven. The reason for this is that NVO dissolves into the electrolyte and undergoes some side reactions with the Zn negative electrode, leading to the formation of dendrites. On the contrary, the V content of the Zn anode surface is only 1.37% in the Zn//NVO@Al₂O₃ battery after 10 cycles at 0.1 A/g, indicating that the alumina coating can inhibit NVO dissolution and avoid side reactions on the surface of Zn and the growth of zinc dendrites.

3 Conclusions

In conclusion, the cycling performance of the Zn//NVO@Al₂O₃ battery has been significantly improved. Especially at a higher current density of 4 A/g, the specific capacity of the Zn//NVO@Al₂O₃ battery reaches 228 mAh/g, and after 1000 cycles, its capacity reservation rate is about 68%. The CE in the course of the cycle is significantly better than that of the Zn//NVO battery, approaching 100%. Additionally, the rate performance of the Zn//NVO@Al₂O₃ battery has also been significantly improved. The evaluations of the electrochemical performance and the examination of the morphological transformations and surface elemental distribution of the zinc negative electrode post-cycling substantiate that the aluminum oxide coating can effectively mitigate the dissolution of NVO. This coating stabilizes the structure, enhances the surface hydrophilicity, diminishes the interface impedance, and bolsters the rate performance of the electrode. This is beneficial for the development and application of high-performance aqueous ZIBs, thus will promote the industrialization of aqueous ZIBs.

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