The Hydrolyzed Mil-88B(Fe) With Improved Surface Area for High-Capacity Lithium Ion Battery

In this work, Mil-88B(Fe) is modified by a facile hydrolysis method for high-performance lithium ion battery (LIB). The hydrolyzed Mil-88B(Fe) [H-Mil-88B(Fe)] heritages the spindle-like shape of Mil-88B(Fe) and forms a porous structure, which possesses relatively high specific surface area (427.86 m2 g−1). It is 15 times higher than that of pristine Mil-88B(Fe). As anode for LIB, it reaches to high specific capacity of 600.1 mAh g−1 after 100 cycles at 100 mA g−1, while it is 312.5 mAh g−1 for pure Mil-88B(Fe). Furthermore, the kinetic analysis on i = a v b reveals that the b value of H-Mil-88B(Fe) is 0.888, which suggests the mixed contribution from the diffusion and capacity reactions. Furthermore, the capacitance contribution fractions of H-Mil-88B(Fe) are 47.6%, 53.28%, 56.88%, 74.68%, and 69.14% at the sweep rate of 0.2, 0.4, 0.6, 0.8, 1.0 mV s−1, respectively, demonstrating a capacitance-dominated charge storage process at fast charging rates.


INTRODUCTION
With the increasing popularity of portable electronic products and grid energy storage systems, the LIB with high energy density and power density is of great importance (Du et al., 2020;Wei et al., 2020;Zhang et al., 2021a;Chang et al., 2021). The natural graphite as the commercial LIB anode has shown advantages of well stability and abundant resource Li et al., 2018;Ming et al., 2018;Huang et al., 2020). However, the low theory specific capacity (372 mAh g −1 ) becomes the biggest obstacle Xu et al., 2017;Xi et al., 2019;Liu et al., 2020).
The metal-organic frameworks (MOFs), a self-assembled organic-inorganic hybrid Wang et al., 2019a;Wang et al., 2019b), are regarded as promising materials that are applied in the domains of catalysis (Rao and Mandal, 2019;Wang et al., 2020), energy storage (Xu et al., 2018a;Dou et al., 2019), and gas storage (Xu et al., 2018b) due to high chemical stability (Chen et al., 2015), huge specific surface areas (Luo et al., 2016), controllable structure , and abundant resource (Du et al., 2011). Among them, the Mil-88B(Fe) with Fe as the core metal ion and terephthalic acid as the organic ligand is an important member of the Fe-MOF family (Shen et al., 2021;Wang et al., 2021;Yang et al., 2021). Remarkably, the Mil-88B(Fe) derivatives have emerged as environmentally benign materials for energy storage device (Pan et al., 2018;Zhang et al., 2021b). As an LIB anode, the homogenous structure of Mil-88B(Fe) ensures the uniform distribution of the accessible transition metal (Fe 3 O) clusters as active sites throughout the electrode reaction, leading to a prominent performance of LIB (Férey et al., 2007). In the charge/discharge process, both the Fe 3 O cluster and the organic ligands contribute to the capacity. The Fe 3+ is reduced to Fe 2+ , and meanwhile, the organic ligands can accommodate partial Li + at the discharge process . In 2007, Férey et al. first reported the synthesis of MIL-53 as the positive electrode in LIB. Unfortunately, the specific capacity (75 mAh g −1 ) and cycling stability were below the expectation (Wang and Cao, 2008). Actually, the low conductivity restricts the LIB performance of pure Mil-88B(Fe) electrode . To solve this issue,  reduced graphene oxide composite for LIB anode. It exhibited high specific capacity of 550 mAh g −1 at 100 mA g −1 after 100 cycles. The improved performance is ascribed to the presence of graphene, which acts as a conductive bridge to enhance the bulk conductivity of the whole electrode (Jin et al., 2017). Besides the graphene, Jin et al. prepared Fe 3 O 4 /C composite by sintering the Mil-88B(Fe) under Ar atmosphere. The Fe 3 O 4 /C manifests the high specific capacity of 928 mAh g −1 at 0.5°C after 200 cycles, which are attributed to the improved electrical conductivity and electrochemical behavior originating from the carbon coatings . Interestingly, Shen et al. developed a polyhedral nanorod structure, which was derived from Mil-88B(Fe) for LIB anode. It exhibited reversible capacity of 744.5 mAh g −1 at 60 mA g −1 after 400 cycles. The nanorods shorten the Li + diffusion length, which promotes the reaction kinetics (Guo and Li, 2021).
In this work, we attempt to trigger the water hydrolysis on Mil-88B(Fe) toward the enhanced specific surface area (SSA). When employed as anode for LIB, the huge SSA of the hydrolyzed Mil-88B(Fe) [H-Mil-88B(Fe)] promise to provide abundant redox sites, and meanwhile, the spindle-like nanostructure benefits to relive the volume changes. As a result, the H-Mil-88B(Fe) verifies the superior LIB performance including high reversible capacity, moderate rate capability, and cyclic stability.

Preparation of Mil-88B(Fe)
The uniform Mil-88B(Fe) was prepared by a facile solvothermal method. Briefly, 0.1661 g of terephthalic acid (TPA) and 0.2703 g FeCl 3 ·6H 2 O was added into 10 ml of N,Ndimethylformamide (DMF) under vigorous stirring at room temperature to obtain solutions A and B, respectively. After that, solutions A and B were mixed thoroughly, and then transferred into 15-ml Teflon-lined stainless autoclaves and followed by heating at 150°C for 150 min. After natural cooling, the sediments were washed with DMF and ethanol. Finally, it was collected by centrifugation at 8,000 rpm/min for 5 min, and then dried at 80°C in a vacuum oven overnight. The brick-red Mil-88B(Fe) powder was obtained successfully.

Preparation of H-Mil-88B(Fe)
Mil-88B(Fe) (40 mg) was dissolved in 14 ml of ethanol under ultrasound for 20 min. Then 6 ml of deionized water (DI) was added into the solution. After extra magnetic stirring for 10 min, the mixed solution was heated in an oil bath under gentle stirring at 90°C for 1 h. Subsequently, the precipitate was collected by centrifugation and washed with ethanol three times. Finally, the product was dried overnight in a vacuum oven at 80°C.

Electrochemical Measurements
The working electrode is prepared by a typical method that was described in our previous work (Come et al., 2011). In brief, the Mil-88B(Fe)/H-Mil-88B(Fe) is employed as an active material, which is completely mixed with acetylene black and polyvinylidene fluoride (PVDF) to prepare the slurry. Most importantly, the mass ratio of active material, acetylene black, and PVDF was set as 8:1:1. After that, the slurry was uniformly coated on the nickel foam (Ø16 mm) and sent to an oven to get rid of impurities. The coin-cell batteries (CR 2032) were assembled in an argon-filled glove box where the as-prepared electrodes and lithium metal was utilized as the anode and counter electrode, respectively. Furthermore, the LiPF 6 / ethylene carbonate (EC)/dimethyl carbonate (DMC) mixed solution and the Celgard membrane (Celgard 2,500, Ø19 mm) was used as electrolyte and separator, respectively. The electrochemical performance [cyclic voltammograms (CVs) and electrochemical impedance spectra (EIS)] was explored by an electrochemical workstation (Princeton Applied Research PARSTAT 3000A-DX, United States). The galvanostatic charge/discharge (GCD) test was examined on a battery test system (LANHE CT 2001A, China) in a working voltage of 0.001-3 V (vs. Li/Li + ). Moreover, the capacitance contribution was calculated by a classical method. During the charge/discharge processes, the Li + extraction/insertion leads to a response of the redox peak extracted from the CV curve at different scanning rates (Shen et al., 2018). The relationship between the current (i) and sweep rates (v) follows Eq. 1 (Ma et al., 2013): where a and b are adjustable values. Furthermore, the total current can be divided into two parts. One is considered to vary with the sweep rate (k 1 v), and another relates to the square root of the sweep rate (k 2 v 1/2 ), as shown in Eq. 2 (Ma et al., 2013): where k 1 and k 2 are constants at a particular voltage, and the value of k 1 v represent the capacitance contribution. Figure 1A shows the X-ray diffraction (XRD, Rigaku Smart Lab, Japan) patterns of Mil-88B(Fe) and H-Mil-88B(Fe). Essentially, the XRD pattern of Mil-88B(Fe) displays the standard characteristic peaks, which are indexed in the hexagonal space group for p62c . Among these peaks, the peak at 9. 51, 9.72, 12.67, 14.1, 16.29, 16.68, 17.12, 18.97, and 22.15°r epresents the (101)

DISCUSSION
, and (211) planes of Mil-88B(Fe), respectively. In terms of the XRD pattern of H-Mil-88B(Fe), the peak at around 9.51°b ecomes weak. Significantly, two new sharp characteristic peaks appear at 9.08°and 10.66°, which are ascribed to the variation of the lattice . Accordingly, the crystal structure diagrams of Mil-88B(Fe) and H-Mil-88B(Fe) are given in Figure 1E. Figure 1B  and H-Mil-88B(Fe) exhibit a typical type-IV isotherm with a hysteresis loop arising at medium pressure range, which verifies the existence of considerable mesopores. As a result, the SSA is 27.761 and 427.862 m 2 g −1 , corresponding to Mil-88B(Fe) and H-Mil-88B(Fe), respectively. By comparing with Mil-88B(Fe), a dominant peak focusing at 2.5 nm is examined from the pore size distribution curve of H-Mil-88B(Fe) ( Figure 1D). The significantly improved SSA is ascribed to the hydrolysis, which benefits to create pores on Mil-88B(Fe) by breaking the Fe-O bonds. Furthermore, the huge SSA is deductive to bring more active sites to accommodate Li + , causing high specific capacity.
The morphology of Mil-88B(Fe) and H-Mil-88B(Fe) was investigated by scanning electron microscope (SEM, Hitachi SU5000, Japan) and transmission electron microscopy (TEM, FEI Talos 200s, United States). As shown in Figures 2A, C relating to the SEM and TEM image of Mil-88B(Fe), respectively, Mil-88B(Fe) displays a uniform spindle-like structure with an average size of 1 μm. Moreover, the surface of Mil-88B(Fe) is quite smooth. After hydrolysis, H-Mil-88B(Fe) (Figures 2B,D) basically exhibits a hollowed spindle-like structure with relatively rough surface. Furthermore, plenty of pores can be clearly captured in the inset of Figure 2D, which is favorable for promoting ion diffusion. Thus, the hollowed spindlelike structure provides a significant basis toward the high SSA. Figures 3A, B illustrate the first five successive CV curves of Mil-88B(Fe) and H-Mil-88B(Fe) within the voltage window ranging from 0.001 to 3 V at 0.1 mV s −1 . Essentially, they both have one pair of redox peaks relating to the conversion between Fe 2+ and Fe 3+ , in which the oxidation peak and reduction peak locate at 1.11 and 0.71 V, respectively (Jin et al., 2017;Guo and Li, 2021). Upon the increase in cycle number, two peaks are going to stabilize, indicating the reversibility of the electrode. On the other hand, the CV curve of the first cycle is, to some extent, deforming due to the activation of the electrode and the formation of the solid electrolyte interface (SEI) film. Moreover, the area extracted from the CV curve of H-Mil-88B(Fe) is higher than that of Mil-88B(Fe) at each scan rate, realizing the higher specific capacity. Figure 4A shows the GCD curves of H-Mil-88B(Fe) for the 1st, 2nd, 20th, and 100th cycles at a current density of 100 mA g −1 . In the first cycle, the H-Mil-88B(Fe) electrode delivers a discharge capacity of 1,615.7 mAh g −1 and charge capacity of 967.9 mAh g −1 , suggesting that the Coulombic efficiency (CE) is 59.9%. The low CE and irreversible capacity loss are originated from the generation of SEI film and irreversible redox reaction. On the other hand, the first discharge capacity is much higher than the theoretical capacity of Mil-88B(Fe) (900.4 mAh g −1 ), which is presumably due to the movable ions in both of the electrolyte and electrodes for Li ion storage (Shang et al., 2021). Moreover, two voltage platforms are observed at ～0.7 and ～ 1.2 V from the charge and discharge curve, respectively, which are consistent with the redox peaks of the CV curve. Importantly, the GCD curves of the 20th and 100th cycles are almost a coincidence, ensuring the enhanced reversibility of the H-Mil-88B(Fe) electrode. Figure 4B illuminates the cycling behavior of the Mil-88B(Fe) and H-Mil-88B(Fe) electrodes at the current density of 100 mA g −1 . By activation, both electrodes exhibit relatively good cycling stability after 10 cycles. Remarkably, the specific capacity of  the H-Mil-88B(Fe) electrode is almost twice higher than that of the Mil-88B(Fe) electrode. The reversible capacity of the H-Mil-88B(Fe) electrode is 600.1 mAh g −1 after 100 cycles, while Mil-88B(Fe) remains to have a capacity of 312.5 mAh g −1 , suggesting that the high SSA can effectively enhance the specific capacity, and the novel 3D structure is beneficial to improve the cycling stability. Figure 4C explores the rate performance of the Mil-88B(Fe) and H-Mil-88B(Fe) electrodes at the current densities of 50, 100, 200, 500, and 1,000 mA g −1 . The reversible capacity of the H-Mil-88B(Fe) electrode is 640.5, 446.3, 267.5, 142.8, and 66.4 mAh g −1 , respectively, while the Mil-88B(Fe) electrode delivers the capacities of 273.4, 245.4, 212, 142.6, and 83.6 mAh g −1 , respectively. When the current density recovered to 50 mA g −1 , the specific capacity can reach to 615.7 and 382.4 mAh g −1 , respectively. Figure 4D illustrates the Nyquist plots of EIS for Mil-88B(Fe) and H-Mil-88B(Fe). Basically, the semicircle and the straight line represent the contact impedance and charge transfer resistance, and the spread resistance, respectively (Tang et al., 2018). Based on the simulation, the charge transfer resistance of the H-Mil-88B(Fe) electrode is 212.8 Ω, which is much lower than that of the Mil-88B(Fe) electrode (261.5 Ω), indicating the higher activity and improved connectivity of the H-Mil-88B(Fe) electrode. To illuminate the prominent LIBs' performance of H-Mil-88B(Fe), the capacitive and diffusion-controlled contribution was explored by employing Eqs. 1, 2. The CV curves of the Mil-88B(Fe) and H-Mil-88B(Fe) electrodes at different scan rates are presented in Figures 5A, B, respectively. The cathodic and anodic peaks are preserved well under different sweep rates. As shown in Figure 5C, the b value for the Mil-88B(Fe) electrode is 0.535, which suggests a diffusion-controlled mechanism. On the other hand, the b-value is 0.888 for the H-Mil-88B(Fe) electrode, demonstrating that the majority charge storage process is relating to capacitive characteristics (Ma et al., 2013;Shen et al., 2018), The total capacitive contribution to the current response can be calculated by Eq. 2. In Figure 5D, 69.14% of the total capacity is symbolized as the capacitive contribution for the H-Mil-88B(Fe) electrode. Figures 5E, F   indicating that it can afford the high current densities and thereby achieves a better rate capability.

CONCLUSION
In summary, we have prepared the modified Mil-88B(Fe) [H-Mil-88B(Fe)] material by employing a hydrolysis method. H-Mil-88 exhibits a hollowed spindle-like structure, which facilitates the Li + diffusion, electron transfer, and provides a large specific surface area for Li + storage. Benefiting from the unique structure, the H-Mil-88 electrode delivers a high reversible specific capacity of 600.1 mAh g −1 after 100 cycles at a current density of 100 mA g −1 . Furthermore, it also displays moderate rate capability. The mechanism analysis demonstrates that the H-Mil-88 electrode realizes the enhanced capacitive behavior. Thus, this study proposes the novel strategy for the morphological engineering of Mil-88, which can be spanned to other MOFs for a highperformance energy storage device.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article. Further inquiries can be directed to the corresponding author.