Highly Ordered Mesoporous NiCo2O4 as a High Performance Anode Material for Li-Ion Batteries

The controlled synthesis of highly ordered mesoporous structure has attracted considerable attention in the field of electrochemistry because of its high specific surface area which can contribute the transportation of ions. Herein, a general nano-casting approach is proposed for synthesizing highly ordered mesoporous NiCo2O4 microspheres. The as-synthesized mesoporous NiCo2O4 microsphere materials with high Brunner-Emmett-Teller (BET) surface area (~97.77 m2g−1) and uniform pore size distribution around 4 nm exhibited a high initial discharge capacity of ~1,467 mAhg−1, a good rate capability as well as cycling stability. The superior electrochemical performance was mainly because of the highly porous nature of NiCo2O4, which rendered volume expansion during the process of cycling and shortened lithium-ions transport pathways. These properties showcase the inherent potential for use of highly ordered mesoporous NiCo2O4 microspheres as a potential anode material for lithium-ion batteries in the future.


INTRODUCTION
Lithium-ion batteries (Yoo et al., 2008;Pan et al., 2013) have been proven to be viable alternatives to traditional energy storage devices and are vital tools when coupled with emerging renewable energy sources (Lewis and Nocera, 2006;Song, 2006;Chheda et al., 2007) (i.e., wind, solar, etc.). However, current energy demands outpace what is commercial systems are capable of, leading to the development of next-generation lithium-ion batteries Lu et al., 2018;Shen et al., 2018). Particularly, the electrode is vital for optimal electrochemical performance (Toupin et al., 2004;Jiang et al., 2012;Wang et al., 2012;Yang et al., 2019a); however, conventional anode materials (Courtel et al., 2011;Chen et al., 2013;Chang et al., 2014) such as graphite have low theoretical specific capacities (372 mAh g −1 ) and fail to satisfy energy storage demands. Consequently, transition metal oxides (TMOs) (Yuan et al., 2014;Tabassum et al., 2018) have been proposed as viable alternatives, attributed to high energy densities, which result in capacities more than double of those observed with graphite. However, this capacity relies on the morphology and structure of the TMO, which can suffer from undesired volume expansion and collapse of the structure as the battery cycles, leading to catastrophic failure.
Recently, space designed nanostructures have been acknowledged as an effective strategy to remit the volume expansion as well as shorten the lithium-ion transport pathways by providing a larger specific surface area, including implementation of a porous network (Wen et al., 2007;Lou et al., 2008;Shen et al., 2012Shen et al., , 2014Yuan et al., 2012;Qie et al., 2013;Yang et al., 2019b), mesopores structures. For example, mesoporous Co 3 O 4 (Li et al., 2008), NiO (Yin et al., 2018), NiCo 2 O 4 , and SnO 2 (Han et al., 2019) have been fabricated and have shown good electrochemical performance. However, limited attention has been focused to study ternary systems such as NiCo 2 O 4 despite a higher electrical conductivity and specific capacity owing to its enhanced chemical kinetics. Inspired by ammonium hydrogen carbonate-assisted solvothermal route to prepare Ni 0.33 Co 0.67 CO 3 microspheres (Li et al., 2013), they led to a recent breakthrough technique to form mesoporous microspheres, we present a modified structure design containing ordered mesopores to further improve the electrochemical performance of NiCo 2 O 4.
Herein, we developed a template-assisted method to synthesize novel NiCo 2 O 4 microspheres containing highly ordered mesoporous structures and nanoparticles to effectively alleviate the huge volume expansion and enhance the electrical conductivity. The synthesis process is schematically shown in Figure 1. Firstly, mesoporous silica (KIT-6) is synthesized as a template by a typical approach which is illustrated in the experiment section in detail. Then the template is immersed in the mixed solution of Ni(NO 3 ) 2 and Co(NO 3 ) 2 to introduce the Ni 2+ and Co 2+ to be filled into KIT-6. After a subsequent calcination step at 450 • C under N 2 atmosphere, the NiCo 2 O 4 @KIT-6 is successfully prepared. Finally, NaOH solution is utilized to remove the template of KIT-6 in the NiCo 2 O 4 @KIT-6 and obtain mesoporous NiCo 2 O 4 microspheres. When acted as an anode material for Li-ion batteries, mesoporous NiCo 2 O 4 microsphere electrode exhibits the superior electrochemical performance, whose stable specific capacity was 430 mAhg −1 after 100 cycles, which is better than that of the non-porous NiCo 2 O 4 (270 mAhg −1 after 100 cycles). The improved lithium storage performance mainly benefits from the rationally designed mesoporous structures of NiCo 2 O 4 . We believe that this versatile strategy could be extended to more ternary TMO materials for the development of high property electrode in LIBs.

Material Synthesis
Synthesis of Mesoporous SiO 2 (KIT-6) The typical synthetic process was as follows : 4.75 g of P123 were dissolved in a mixed solution containing 163 mL of deionized (DI) water and 7.5 ml of concentrated hydrochloric acid under stirring at 35 • C. Once the P123 was fully dissolved, 4.53 g of n-butanol were added. After 1 h, TEOS (9.675 g) were added into the above mixed solution. The solution was then stirred for an additional 24 h at 35 • C, followed by another 24 h incubation at 35 • C. Cooling the mixed solution to room temperature, the precipitates could be collected by centrifugation for 3 times and moved to a vacuum oven at 90 • C for 12 h. Then, after calcining the collected deposit at 550 • C for 6 h, white mesoporous SiO 2 was obtained.

Synthesis of Mesoporous NiCo 2 O 4 Microspheres
In a simple process, 0.4 g of KIT-6 were added to a solution containing 4 ml of 1 M Co(NO 3 ) 2 ·6H 2 O and 2 ml of 1 M Ni(NO 3 ) 2 ·6H 2 O in ethanol under stirring for 1.5 h at room temperature. Then, the solution was heated at 70 • C until the ethanol was completely evaporated, and the solid was calcined at 200 • C for 4 h. Immediately following, 2 mL of 1 M Co(NO 3 ) 2 ·6H 2 O and 1 mL of Ni(NO 3 ) 2 ·6H 2 O in ethanol were added following the previous steps and an additional calcination 450 • C for 6 h. To obtain mesoporous NiCo 2 O 4 microspheres, the powder was immersed in a 2 M NaOH solution to etch away the KIT-6 templates. Then, the samples were collected by centrifugation, washed for 3 times and moved to a vacuum oven at 60 • C for 12 h. As a control, conventional NiCo 2 O 4 microspheres were produced through the above-mentioned process without KIT-6 templates.

Materials Characterization
The mesoporous NiCo 2 O 4 microspheres were characterized by using a PANalytical X' Pert X-ray diffractometer (Holland), with Cu-Kα radiation at 40 kV and 40 mA, selected-area electron diffraction (SAED), scanning electron microscope (SEM, S-4800) and transmission electron microscope (TEM, JEM-2100F). The N 2 adsorption/desorption isotherms were used to calculate the specific surface area and Barrett-Joyner-Halenda (BJH) equation was used to calculate the pore size distribution and average pore diameter.

RESULTS AND DISCUSSION
The mesoporous NiCo 2 O 4 microspheres were prepared via nano-casting, with the crystalline structure and phase purity    (Figure S1), is significantly rougher with highly ordered, uniform pores. This suggests that the polymeric precursor was fully injected within the KIT-6 microspheres, allowing the continuity of the mesoporous structure in subsequent deposition reactions. The highly ordered pores of the NiCo 2 O 4 microspheres are clearly revealed with a pore size of ∼4 nm as shown by high-resolution TEM in Figures 2d,e. Additionally, in Figure 2f, the measured interplanar distance was found to be 0.29 nm, which aligns well with the (220) planes of spinel NiCo 2 O 4 . It is worth noting that well-defined diffraction rings were presented by the SAED pattern (Inset in Figure 2f), which correspond to the (440), (422), (400), (311), and (200) planes. The polycrystalline diffraction rings are in accordance with the result from the XRD pattern.
The pore diameter distribution and specific surface area of mesoporous NiCo 2 O 4 samples were determined via N 2 adsorption-desorption measurements. The result of specific surface area was calculated from the isotherms (Figure 3) was 97.77 m 2 g −1 for mesoporous NiCo 2 O 4 microspheres, while the non-porous NiCo 2 O 4 microspheres were 26.63 m 2 g −1 . Additionally, the mesoporous structure was further analyzed by pore diameter distribution in the inset of Figure 3. NiCo 2 O 4 microspheres displayed a pore volume (0.494 cm 3 g −1 ) with an average pore diameter of 3.416 nm. Ascribed to this special microsphere structure, the mesoporous structure could shorten the diffusion paths for lithium ions and provided buffering space to adapt FIGURE 4 | First three consecutive CV curves of mesoporous NiCo 2 O 4 (a), galvanostatic discharge and charge profiles for 1st, 2nd,10th, 100th cycles of mesoporous NiCo 2 O 4 (b) at the current densities of 100 mA.g −1 , cycling performance of ordinary NiCo 2 O 4 and mesoporous NiCo 2 O 4 at the current densities of 100 mAh.g −1 (c), rate performances for ordinary NiCo 2 O 4 and mesoporous NiCo 2 O 4 at various current densities (d). EIS pattern of ordinary NiCo 2 O 4 and mesoporous NiCo 2 O 4 (e). The typical plots for Z' vs. ω −0.5 for the mesoporous NiCo 2 O 4 (f). the volume expansion during the process of Li + insertion and extraction.
To confirm whether the mesoporous NiCo 2 O 4 microspheres would be applicable as the anode materials in lithium-ion batteries, the as-prepared products were further investigated using cyclic voltammetry (CV), where Figure 4a exhibits the first three cycles of the mesoporous NiCo 2 O 4 microspheres. In the 1st cycle, one dominant peak at 0.8 V could be assigned to the decomposition and reduction of Ni and Co ions. Meanwhile, the anodic peaks at approximately 2.1 V could be owing to the oxidation reaction of metallic Ni and Co to NiO and CoO. In subsequent tests, the cathodic peak broadened and shifted to 1.0 V. According to the analysis of the CV curves and previous literature reports, the Li reactions for mesoporous NiCo 2 O 4 materials are as follows: Galvanostatic tests were executed to ensure the influence of highly ordered mesoporous NiCo 2 O 4 samples on the specific capacity values and capacity retention. The 1st, 2nd, 10th, and 100th cycles of the galvanostatic charge and discharge curves for mesoporous samples, at a current of 100 mA.g −1 , are shown in Figure 4b. A long voltage plateau was showed in the initial discharge curve between 0.7 and 1.2 V, corresponding to the strong reduction peak that appeared during the first cathodic CV scan. The initial charge capacity for the mesoporous NiCo 2 O 4 microspheres was ∼1467 mAhg −1 , which is markedly higher than the non-porous NiCo 2 O 4 microspheres (∼820 mAhg −1 , Figure S2); however, this decreased to 1,060 mAhg −1 after the 2nd discharge. The irreversible capacity increase during the 1st charge may be due to the generation of a solid electrolyte interface (SEI) layer. A comparison of the specific capacity values obtained for non-porous NiCo 2 O 4 and mesoporous NiCo 2 O 4 over 100 cycles plotted is shown in Figure 4c. The non-porous NiCo 2 O 4 suffers the most severe capacity fading with capacity values decreasing to 270 mAhg −1 after 100 cycles, while the capacity retention was improved for mesoporous NiCo 2 O 4 microspheres, obtaining capacity values of 430 mAhg −1 after 100 cycles. The significant increase in the capacities for mesoporous NiCo 2 O 4 compared to ordinary NiCo 2 O 4 is attributed to the inherent properties of the mesoporous structure. Nevertheless, the final capacity is relatively low compared to the initial capacity, which may be attributed to the aperture being too small to relieve the expansion of the active material completely (Bhaway et al., 2017). For conversion-mode materials, the mesoporous NiCo 2 O 4 provides particularly high buffering space, to adapt the large volume expansion during the process of Li + insertion/extraction and shorten the diffusion paths for lithium ions due to the interconnected architectures ( Figure S3). Moreover, the coulombic efficiency for mesoporous NiCo 2 O 4 over 100 cycles is also shown in Figure 4c. The initial coulombic efficiency is quite low (∼70%); however, the efficiency remains >95% after the 10th cycle. The rate performance of ordinary NiCo 2 O 4 microspheres and mesoporous NiCo 2 O 4 microspheres at various current densities were then compared in Figure 4d. The mesoporous NiCo 2 O 4 microsphere electrode exhibits high initial discharge capacity and the capacity was recession with the increase of current density. However, the mesoporous NiCo 2 O 4 microsphere electrode exhibited a higher capacity and rate of lithium-ion storage than the non-porous NiCo 2 O 4 microsphere electrode.
To further investigate the mechanism for the improved electrochemical performance of the mesoporous NiCo 2 O 4 microsphere, electrochemical impedance measurements (Figure 4e) and a fitting process ( Figure S4) were measured containing the mesoporous NiCo 2 O 4 electrode and ordinary NiCo 2 O 4 . As shown in Figure 4e, a semicircle and a straight line were acquired in the high frequency part and low frequency part, respectively. The intersection point from the curve in the high frequency part is on behalf of the electrolyte resistance. Moreover, the high frequency semicircle corresponds to the charge transfer resistance and the low frequency region with an inclined line represents the process of Li-ion diffusion. The initial value for charge transfer resistance is 66 , which is far less than the ordinary NiCo 2 O 4 (134 , Table S1), which could be related to the more effective ion and electron transfer in the interface of electrolyte and active material, so that the cell has improved electrode reaction kinetics and a better cell cycling results. Moreover, the diffusion coefficient of the lithium ions (D + Li ) can be confirmed in accordance with the following equation: Where R = gas constant, T = absolute temperature, A = surface area of the electrode, F = Faraday constant, c = concentration of Li+ in the material, and σ = Warburg factor obeying the following relationship: Where Z Re is the real part of impedance, thus ω is the angular frequency at low frequency. The linear relationship of Z Re and ω −0.5 is shown in Figure 4f, with the slope of the fitted straight line indicating the value of σ.

CONCLUSION
Highly ordered mesoporous NiCo 2 O 4 microspheres with a honeycomb-like structure, were synthesized via a nano-casting method. The mesoporous NiCo 2 O 4 electrode possesses a high initial discharge capacity of ∼1,467 mAh.g −1 at 100 mAg −1 and it exhibited both a high surface area and good rate capability. Such high electrochemical performance is due to its excellent surface area of mesoporous NiCo 2 O 4 and the rapid ion transport in the electrolyte/electrode interface. This work may open a new sight in the synthesis of excellent Li-storage electrode materials and show an application in a promising candidate for Li-ion batteries.

DATA AVAILABILITY
The raw data supporting the conclusions of this manuscript will be made available by the authors, without undue reservation, to any qualified researcher. Requests to access the datasets should be directed to hu.junqing@dhu.edu.cn.

AUTHOR CONTRIBUTIONS
GW, BL, and JHu designed the project. QR, WX, JHa, PL, JC, SW, and RZ carried out the experiment and performed the experimental data analysis. QR, GW, and WX wrote the paper. BL revised the manuscript. All authors contributed to discussion of the results.