In recent few decades, with the increasing prominence of non-renewable energy issues and promoting the realization of carbon neutrality, the search and utilization of clean energy resources are becoming the spotlight of worldwide research [1]. This has led to a fast growing demand for electrical energy, particularly in secondary batteries [2]. Lithium-ion batteries (LIBs) as a typical representative of secondary batteries with high energy density have been used as power sources in many fields. However, the limited resource of lithium and safety issues hindered its wide application [3,4]. Moreover, the evolution of sodium-ion batteries (SIBs) and potassium ion batteries (PIBs) was impeded by using volatile, flammable, and toxic organic electrolytes to some extent [5,6]. Based on these considerations, the interest in safe and resourceful aqueous rechargeable zinc-ion batteries (AZIBs) have re-emerged over the past years [7,8]. Besides, AZIBs possess advantages in their low redox potential and high ionic conductivity which is almost two orders of magnitude higher than those of non-aqueous electrolytes [9,10]. Nevertheless, the applications of AZIBs in energy storage field has bottlenecked by the insufficient energy density and limited life span. The dominating strategies for enhancing electrochemical performance involve the design of cathode materials [11], optimization of electrolytes [12], and modification of Zn anode [13,14]. In general, Zn²⁺ has higher electric charge and molecular weight, which makes ions more difficult to insert/extract into/from electrode materials [15]. Hence, the design and development of cathode material become the key aspect for the development of AZIBs.

The ZIB cathode materials developed so far range from Mn-oxides (MnO₂ with α-, β-, γ-, δ- and ZnMn₂O₄) [16-21], Prussian blue analogues [22], V-oxides (V₂O₅, VO₂, Zn_0.25V₂O₅ · nH₂O, H₂V₃O₈, NaCa_0.6V₆O₁₆ · 3H₂O, V₆O₁₅) [23-26] to organic compounds (conductive polymer polyaniline, PANI, carbonyl compounds, polypyrrole, quinone) [27-29]. Yet, the long-term commercial use of most cathode materials is restricted by low-storage, poor rate performance, rapid capacity fading, toxicity, and limited inorganic mineral resource. It is urgent for AZIBs to discover new cathode materials with excellent electrochemical properties and renewability [30]. Benefit from the absolute superiority that C, H, O, N and S elements are abundant on the earth and the increased concerns about the environment, rechargeable batteries based on organic electrodes, such as lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs), are burgeoning [31]. Indeed, many metal-organic frameworks (MOFs) have been explored as novel electrode materials for secondary energy storage devices, based on their properties of huge variety of structures, large surface areas and adjustable porosity. Such as, Armand and coworkers explored the performance of Li₂C₈H₄O₄ as the electrode of LIBs [32]. Xu's group has synthesized five kinds of MOFs materials, including Mn(BTC), Mn(BDC), Fe(BDC), Co(BDC), and V(BDC) (in which BDC is 1,4-dicarboxybenzene and BTC is 1,3,5-benzenetricarboxylic acid) and investigated their electrochemical behaviors as ZIB cathodes [33]. Nevertheless, some of these metal-organic electrodes are unstable in electrolytes, which may result in unstable electrochemical performance. Also, the reaction mechanism of those MOFs based energy storage devices is still elusive.

Purified terephthalic acid (PTA) is an optimized ligand used in the synthesis of MOFs due to its easy availability from cheap poly-ethylene terephthalate (PET) plastic products. Metal ions that possess higher ionization energy and larger ion radius coordinate with organic anions can increase the stability of electrodes in electrolytes [34]. In this work, a calcium-pure terephthalates acid metal-organic framework (Ca-PTA·3H₂O) is synthesized by facile hydrolysis and cationic exchange method and studied as an unexplored cathode for AZIBs. The Ca-PTA·3H₂O cathodes delivered a high-specific capacity of 431 mAh/g (0.51 mAh/cm²) at 50 mA/g, along with ~90% capacity retention (500 mA/g) after 2700 cycles. In addition, the storage mechanism of Zn²⁺ was studied by using ex-situ measurements. Results show that, on the one hand, Ca-PTA·3H₂O can render a voltage plateau of ~1.45 V as well as the CaSO₄·2H₂O generated from extraction Ca²⁺ of Ca-PTA·3H₂O facilitates the cycle stability, on the other hand, the generation of reversible discharging product Zn₄SO₄(OH)₆·4H₂O and intermediate compound Zn_xMnO(OH)_y co-contributed to the high reversible capacity of the battery at activation stage.

The Ca-PTA·3H₂O was synthesized as follows: briefly, 2 mmol NaOH (Aladdin, analytically pure) and 1 mmol Pure Terephthalic acid (H₂TP, Aladdin, 99%) were dissolved in 30 mL of de-ionized water in a beaker. The white suspension liquid was vigorously stirred for 10 h, after this, the solution became transparent. Then, 2 mmol CaCl₂ (Aladdin, analytically pure) was dissolved in 10 mL of de-ionized water and added into the above clear solution drop by drop. After about 10 min, the white precipitate appeared. Afterward, the beaker was covered with plastic wrap and heated to 80 ℃ in water bath, stirring constantly for 12 h to complete the exchange of Na⁺ by Ca²⁺. The precipitate was centrifuged and washed with de-ionized water several times. Finally, the product was dried at 60 ℃ overnight under vacuum.

The X-ray diffraction (XRD) pattern of Ca-PTA·3H₂O was detected by a Rigaku Ⅱ X-ray diffraction spectrometer (Japan Science Co., Tokyo, Japan) using Cu-Kα radiation (λ = 0.15418 nm). 2-theta degree was determined by range of 5° to 70°. Scanning electron microscopy (SEM) images were taken by a JEOL JSM-7800F microscope (Japan Electronics Co., Tokyo, Japan). Morphology of Ca-PTA·3H₂O and its particles after circulation were measured by SEM and ex-situ SEM. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific ESCALab 250Xi+ X (Thermo Fisher Scientific Co., Waltham, America), the valence states were measured by XPS. Fourier transform-infrared (FT-IR) spectroscopy was performed using a Frontier NIR Std spectrophotometer (Thermo Fisher Scientific Co., Waltham, America). The thermal gravimetric analyses were conducted on Ltd STA409PC, DSC404F3 from the Setaram Instrumentation (France).

The CR2032 corn-type cells (Canrd Co., Guangdong, China) were used for AZIB electrochemical performance tests. The cell consisted of a Ca-PTA·3H₂O cathode, a zinc anode with a diameter of 12 mm (Canrd Co., Guangdong, China), a glass fiber separator (diameter-16 mm Whatman Co., Maidstone, England) and 2 mol/L ZnSO₄ with 0.2 mol/L MnSO₄ aqueous electrolyte. A conventional slurry-coating process on titanium foils was used to fabricate the electrodes. The slurry was made of 70 wt% Ca-PTA·3H₂O powder, 20 wt% acetylene black (AB) and 10 wt% polyvinylidene fluoride (PVDF, Macklin, M_w = 1,000,000) in an agate mortar. After slurry-coating, titanium foils were dried out at 60 ℃ for 12 h in a vacuum oven. The loading of Ca-PTA·3H₂O electrode is about 1.8 mg/cm², corresponding to the active materials of around 1.3 mg/cm². Galvanostatic charge-discharge (GCD) was performed on a LAND battery test system CT3001A (Wuhan LAND Electronic Co., Ltd., Wuhan, China) in a voltage range of 0.8-1.85. Cyclic voltammetry (CV) measurements were carried out on a CHI760E electrochemical workstation (Shanghai Chenhua Instrument Co., Shanghai, China) at scan rate (0.3 mV/s).

The Ca-PTA·3H₂O was synthesized by facile hydrolysis and cationic exchange method. To identify the phase composition of the as-prepared samples, XRD measurements were performed and shown in Fig. 1a. Collected patterns display sharp peaks and are in line with the simulated patterns from the single crystal data (JCPDS No. 46-1873 CaC₆H₄O₄·3H₂O, space group P21/c, lattice parameters a = 7.11Å, b = 21.66 Å, c = 6.60 Å, β = 92.29°), indicating the high crystallinity and purity of as-synthesized material [32]. Fig. 1c is a view of the crystalline framework of Ca-PTA·3H₂O along a and c axis. Briefly, the Ca-carboxylate layer structure with 8-coordinated Ca²⁺ (four H₂O, four O each calcium ion) forms the dodecahedral. This layer structure is advantageous for the diffusion of Zn²⁺. The molecular structure of CaTPA·3H₂O powder and the properties of bonds are exhibited in Fig. S1 (Supporting information). The Ca²⁺ is connected to -COO(Ⅰ) and shares the delocalized electrons of two oxygen atoms. The -COO(Ⅱ) is coordinated with the H₂O molecules by hydrogen bonds. Fig. 1b displays the morphologies of as-synthesized Ca-TPA·3H₂O which were investigated using scanning electron microscopy (SEM), this micro-particle with smooth surface is composed of layers. And the EDS mapping (Fig. 1d) demonstrates a well distribution of C, O and Ca elements of particles.

Fig. 2a shows the FT-IR spectrum of the as-prepared Ca-PTA·3H₂O. The broad bands with the peak at 3462 and 3280 cm⁻¹ are due to the coordinated H₂O. Compare to PTA ligands (Fig. S2 in Supporting information), the vanish of the typical C=O stretching vibration at 1674 cm⁻¹ in addition to the δOH⁻ and νCO coupling vibration of carboxylic acid (-COOH) groups observed at 1286 and 924 cm⁻¹ demonstrates a complete reaction in our synthesis procedure. After coordination with Ca²⁺, there are two new bands located at 1544 and 1432 cm⁻¹ attributed to asymmetric and symmetric stretching of carboxylic acid. The peak at 510 cm⁻¹ is the bending vibration of δ(Ca-O). The above change of characteristic functional group clearly proves that coordination with Ca²⁺ occurs and results in the formation of Ca-PTA·3H₂O.

Thermogravimetric analysis (TGA) curve of as-prepared Ca-PTA·3H₂O (Fig. 2b) from room temperature to 800 ℃ in air shows the first weight change step with the mass loss (20.5%) from 100 ℃ to 150 ℃. According to the chemical formula CaC₆H₄O₄·3H₂O, this is due to the removal of three coordinated water and agrees well with the value (20.9%) calculated according to its chemical formula. The weight loss in the range of 600-750 ℃ is attributed to the further thermal decomposition of CaC₆H₄O₄ to the mixture of Ca(OH)₂, CaO and CaCO₃ (Fig. S3 in Supporting information). The TGA curve also indicates the successful synthesis of Ca-PTA·3H₂O.

To identify composition and valence state of Ca-PTA·3H₂O, X-ray photoelectron spectroscopy (XPS) was taken and presented in Fig. 3. The sharp peaks observed in the survey spectrum are O 1s, Ca 2p and C 1s, respectively (Fig. 3a). In Fig. 3b, two peaks existing at 347.3 and 350.9 eV displays doublets for Ca 2p_3/2 and Ca 2p_1/2 and spin-orbit splitting for the Ca 2p_3/2 and Ca 2p_1/2 is 3.6 eV. This is derived from the calcium atoms that are successfully bounded with a carboxyl group [35]. The emission peaks of the C 1s are displayed in Fig. 3c. The large peak at low energy is assigned to Acetylene black. The other two peaks centered at 284.9 and 288.4 eV are attributed to phenyl carbons (-C₆H₄) and carboxyl carbons (-COOH) [36,37]. Fig. 3d exhibits emission peaks of O 1s. In accordance with previous observations and the molecular structure of CaTPA·3H₂O in Fig. S1, the two contributions located at 532.1 eV and 533.2 eV are assigned to oxygen in the hydroxyl (C-OH) and carbonyl (C=O) groups of carboxyl(Ⅱ) which is coordinated to the water molecules. The peak positioned at 531.1 eV is identified as two chemically identical oxygen in carboxyl(Ⅰ) which is coordinated to calcium that Ca²⁺ shares the delocalized electrons from two O atoms [38]. Consequently, this layered structured composites Ca-PTA·3H₂O was successfully synthesized.

The electrochemical performance of Ca-PTA·3H₂O as the cathode of AZIBs with a voltage window between 0.80-1.85 V is explored. Using classic ZnSO₄ (2 mol/L)+MnSO₄ (0.2 mol/L) as electrolyte, the cyclic voltammetry (CV) curves at a scan rate of 0.3 mV/s and the galvanostatic charge and discharge (GCD) profiles at 50 mA/g were displayed in Figs. 4a and b, respectively. The curve of the first charge process is different from the following cycles, so we studied the phase change of the battery's first charge process. From ex-situ XRD pattern (Fig. S4 in Supporting information) we can see the emergence of CaSO₄·2H₂O, which indicates that some extracted Ca²⁺ of Ca-PTA·3H₂O diffuse into to the electrolyte and form insoluble CaSO₄·2H₂O on the electrode surface. Therefore, those oxidation peaks can be mainly ascribed to the extraction of Ca²⁺ and phase change process of cathode material. The reduction peaks in the first discharge process are attributed to the zinc ion multi-insertion [39]. After the first cycle, the wide oxidation peaks at ~1.57 V and the corresponding reduction peaks at around 1.30 V are attributed to sedimentation/resolution of manganese and Zn₄SO₄(OH)₆·4H₂O on electrode surface. It also could be observed that those new redox peaks have a mildly rising in the following cycles which is consistent with the rising specific capacity observed in Figs. 4c and e. According to some recent studies [40], the assisted deposition-dissolution of Zn₄SO₄(OH)₆·4H₂O can co-contribute to a high capacity, which is common in Mn²⁺-containing AZIB systems. The mildly rising peak intensity is meant for the gradual battery activation. Another pair of redox peaks suited at ~1.8 V/~1.45 V remain stable. This strong and stable couple of redox peaks mainly derives from the reversible insertion/extraction of Zn²⁺ into/from Ca-PTA·3H₂O. In keeping with the CV curves, a repeatable discharge voltage plateau at about 1.45 V could be observed in the voltage profiles (Fig. 4b). After the activation process, the specific capacity stabilizes at 431 mAh/g (0.51 mAh/cm² at 50 mA/g). Fig. 4d show the discharge-charge curves at different current density. The reversible capacity summarized as 121, 80, 59, 39, 35 mAh/g, at 100, 300, 500, 800, 1000 mA/g, respectively. All the pseudo-plateau at larger current density matched those at 50 mA/g. And the prolonged cycle performance testing at 500 mA/g is shown in Fig. 4e. After 2700 cycles, the specific capacity retention is ~90%. Both the GCD curves at low (Fig. 4c) and high (Fig. 4e) current density are highlighting the excellent cycle stability. In addition, Table S1 (Supporting information) compares the electrochemical performances of many reported cathode materials used in AZIBs with that of Ca-PTA·3H₂O, clearly indicating its competitive performance.

To analyze the Zn²⁺ storage mechanism of Ca-PTA·3H₂O, some detailed characterizations were performed. The data of ex-situ XRD, ex-situ XPS and ex-situ SEM are recorded to probe the reversible changes of cathode.

The phase and morphology evolution of the Ca-PTA·3H₂O cathode during a GCD cycle of battery activation stage is shown in Fig. 5. The ex-situ XRD and SEM patterns of the cathode are taken at different states (1.85, 1.40, 1.25, 0.80, 1.60 and 1.85 V). The prominent peaks of MOF-73 disappear, indicating that the structure of Ca-PTA·3H₂O gradually converted along with each charge and discharge process. The peak intensity that in/decreases during the discharge/charge process from 1.85 V to 1.40 V/1.60 V to 1.85 V belongs to CaSO₄·2H₂O, corresponding to the insertion/extraction of Zn²⁺ into/from cathode material and the formation of CaSO₄·2H₂O. The "rod-like" particles are shown in the surface morphology evolution of the electrode at 1.4 and 1.25 V in Fig. 5b. Also, EDS mapping (Fig. 5c) shows that the Ca, O and S elements are uniformly distributed in the "rod-like" particles. Unifies the ex-situ XRD date, this "rod-like" particle is CaSO₄·2H₂O. Ca²⁺ species extract from Ca-PTA·3H₂O and form insoluble CaSO₄·2H₂O which facilitates the stability of electrochemical performance [39]. The emergence (from 1.4 V to 0.8 V) and vanishment (from 0.8 V to 1.6 V) of Zn₄SO₄(OH)₆·4H₂O characteristic peaks also can be observed in the patterns. It is because the insertion/extraction of partial Zn²⁺ and H⁺ was accompanied by the deposition-dissolution of the Zn₄SO₄(OH)₆·4H₂O [41]. As a consequence, the Zn₄SO₄(OH)₆·4H₂O nanoplates can be observed at the voltage states of 1.25 and 0.80 V (Fig. 5b). When discharged to 1.40 V and charge to 1.60 V, many "nanoflower-like" particles appear, which we assumed as intermediate compound Zn_xMnO(OH)_y. The EDS mapping (Fig. S5a in Supporting information) shows that these particles contain O, Zn and Mn elements and absence of S elements that also matches well with previous reports [42,43].

To further investigate the Zn²⁺ storage mechanism during the charge-discharge process at battery stable stage, the ex-situ XRD and SEM are characterized and displayed in Figs. 6a and b. Different from the reversible phase change at battery activation stage, the ex-situ XRD patterns of battery stable stage remain constant during the GCD cycle. Particularly, the peaks can fit well with CaSO₄·2H₂O and ZnMn₂O₄. The CaSO₄·2H₂O is derived from the extracted Ca²⁺ from Ca-PTA·3H₂O and embedded in the electrode. This result is verified by the ex-situ SEM (Fig. 6b). The ZnMn₂O₄ is a byproduct generated from Mn²⁺-contained electrolyte proceed along the electrochemical reaction displayed in Eq. S1 (Supporting information) [44].

Based on the above discussion, a proposed conversion Zn²⁺ storage mechanism at battery activation stage of Ca-PTA·3H₂O cathode is illustrated in Fig. 6c. (Ⅰ) and (Ⅱ) electrochemical redox paths correspond to their platforms in the GCD curves, respectively. When battery goes to stable stage, there is no phase evolution but Zn²⁺ insertion/extraction. According to the electrochemical mechanism of this kind of MOF electrode material in LIBs and SIBs, the reaction of Ca-PTA·3H₂O is a two-electron transfer reaction and offers a theoretical capacity of 207.57 mAh/g relying on the reversible reaction of two –C=O groups. Meanwhile, We observe that the practical capacity, especially at low current densities, is higher than the theoretical capacity, which probably comes from the sedimentation/resolution of manganese and Zn₄SO₄(OH)₆·4H₂O on the electrode surface.

In summary, we have fabricated an ultra-stable aqueous Zinc-ion batteries system using an unexplored AZIBs cathode material Ca-PTA·3H₂O. As a result, a high specific capacity of 431 mAh/g (0.51 mAh/cm²) and outstanding cycling stability is achieved by the co-contribution of Zn²⁺ insertion/extraction into/from Ca-PTA·3H₂O and sedimentation/resolution of manganese and Zn₄SO₄(OH)₆·4H₂O on the electrode surface. The ex-situ measurements reveal that after the activation stage, Ca²⁺ species extract from Ca-PTA·3H₂O and form insoluble CaSO₄·2H₂O which facilitates the stability of electrochemical performance (after 2700 cycles, the specific capacity retention is ~90% at 500 mA/g). The unchanged phase and morphology deliver the stable electrochemical behaviors of cathode material. This work provides a new way for the development of high-performance aqueous Zinc-ion batteries based on the MOFs.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This work was supported by the discipline construction funds from Qingdao Municipal Science and Technology Commission and Qingdao University (Nos. DC1900013623 and DC2000003363) as well as by Youth Project of Natural Science Foundation of Shandong Provincial (No. ZR2021QB175). This work is also supported by Natural Science Foundation of Shandong and National Natural Science Foundation of China (No. 51877045), the Foundation from State Key Laboratory of Materials Oriented Chemical Engineering (No. KL19-09) the Fundamental Research Funds for the Central Universities.

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.107760.