A Facile Synthesis of ZnCo2O4 Nanocluster Particles and the Performance as Anode Materials for Lithium Ion Batteries

Abstract ZnCo2O4 nanocluster particles (NCPs) were prepared through a designed hydrothermal method, with the assistance of a surfactant, sodium dodecyl benzene sulfonate. The crystalline structure and surface morphology of ZnCo2O4 were investigated by XRD, XPS, SEM, TEM, and BET analyses. The results of SEM and TEM suggest a clear nanocluster particle structure of cubic ZnCo2O4 (~100 nm in diameter), which consists of aggregated primary nanoparticles (~10 nm in diameter), is achieved. The electrochemical behavior of synthesized ZnCo2O4 NCPs was investigated by galvanostatic discharge/charge measurements and cyclic voltammetry. The ZnCo2O4 NCPs exhibit a high reversible capacity of 700 mAh g−1 over 100 cycles under a current density of 100 mA g−1 with an excellent coulombic efficiency of 98.9% and a considerable cycling stability. This work demonstrates a facile technique designed to synthesize ZnCo2O4 NCPs which show great potential as anode materials for lithium ion batteries. Graphical Abstract


Introduction
It is well known that novel renewable energy sources and energy storage materials are two major challenges in electrochemical technology. Rechargeable lithium ion batteries (LIBs), which have been recognized as vitally important devices of power sources, have attracted widespread attention. LIBs with high energy and power density, low cost, and short charging time are needed urgently to meet the rapid development of hybrid and electric vehicles. In principle, the electrochemical performance of safe LIBs depends largely on the electrode materials for lithium storage.
Among the array of promising anode materials for LIBs, transition metal oxides have been widely studied due to their higher specific capacities compared to traditional graphite with a specific capacity of 372 mAh g -1 . Ternary oxides, AB 2 O 4 (A=Mg, Mn, Fe, Co, Ni, Cu, or Zn; B=Mn, Fe, Co, Ni, or Cu; A=B), with a variety of crystal structures (spinel, scheelite, brannerite, etc.) have been investigated as anode materials for LIBs [1][2][3][4]. This class of materials contains at least one transition metal ion and one or more electrochemically active/inactive ions. AB 2 O 4 in previous electrochemical studies were synthesized via molten salt method [5][6][7][8], oxalate decomposition method [9,10], combustion method [11,12], solvothermal method [13], etc. And they were found to show good Li cyclability with relatively high specific capacities.
The typical ternary oxide, zinc cobaltite (ZnCo 2 O 4 ), possesses a spinel structure, where the Zn 2? occupies the tetrahedral sites and the Co 3? occupies the octahedral sites. ZnCo 2 O 4 has been demonstrated to be a promising candidate as anode materials for LIBs because of the outstanding electrochemical performance (the theoretical specific capacity of 975 mAh g -1 ) and the abundant source, low cost, and low toxicity of zinc. Generally, the electrochemical performance of electrode materials depends on the preparation technique, the size and shape of particles and the morphology. The strategies deployed to prepare ZnCo 2 O 4 are similar to those designed to synthesize AB 2 O 4 mentioned above [1,14].
Hao [15] reported porous ZnCo 2 O 4 microspheres synthesized by a solvothermal method, with a high reversible capacity of 940 mAh g -1 at 0.1°C. In Huang's work [16], core-shell ZnCo 2 O 4 microspheres were fabricated by a hydrothermal method. They showed an initial discharge capacity of 1280 mAh g -1 at 200 mA g -1 , and only 3.9% capacity was lost between the 2nd and the 5th cycles at 400 mA g -1 . According to Zhao's study [17], highly ordered mesoporous spinel ZnCo 2 O 4 was prepared with SBA-15 as templates. It displayed a high reversible capacity of 1623 mAh g -1 at 2.0 A g -1 . The capacity still remained at 1470 mAh g -1 with a high current density of 8.0 A g -1 . Wang's group [18] prepared hierarchical porous ZnCo 2 O 4 microspheres by simply decomposing PBA followed by sintering at 550°C, which showed an initial lithiation and delithiation capacity of 1737.1 and 1051.6 mAh g -1 , respectively, after 100 cycles at 100 mA g -1 . In general, nanosized ZnCo 2 O 4 with uniquely designed structures showed promising results in enhancing the electrochemical performance due to the high surface-to-volume ratio and the excellent electronic transport property. However, the limitation for the industrial application of this anode material is the control in preparation of the active material.
Herein, a facile approach is designed to synthesize uniform ZnCo 2 O 4 NCPs. The cycling stability study of our ZnCo 2 O 4 NCPs shows a delithiation capacity of 700 mAh g -1 over 100 cycles under a current density of 100 mA g -1 . Excellent electrochemical performance of ZnCo 2 O 4 NCPs demonstrates that it is promising to employ this material in high-energy storage devices. The morphology and structure of ZnCo 2 O 4 NCPs were examined by a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Thermal analysis of the pink precursor power was characterized by thermogravimetrydifferential thermal analysis (TG-DTA). The specific surface area of pure ZnCo 2 O 4 NCPs powder was measured on Micromeritics Instrument Corporation TriStar II 3020 using N 2 adsorption-desorption isotherms at -196°C.

Electrochemical Characterization
CR2032 coin cell was used to carry out the electrochemical experiments with Li foil serving as a reference and a counter electrode. Slurries of the active material (ZnCo 2 O 4 NCPs), carbon black, and poly (vinyl difluoride) (PVDF; weight ratio of 70:20:10) in N-methyl-2-pyrrolidone were pasted on pure Cu foil with a thickness of 150 lm and dried under vacuum at 95°C overnight to make working electrodes. The active material loading was 1.0-1.5 mg cm -2 . 1.0 mol L -1 LiPF 6 dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC; volume ratio of 1:1:1) was used as the electrolyte. The cells were assembled in an Ar-filled glove box, with one microporous polypropene film (Celgard 2400) and one glass fiber as separator. An electrochemical workstation (VMP3/ Z, Bio-logic, France) and a battery test system (CT-3008-5 V/5 mA, Neware Technology Ltd., Shenzhen, China) were used to test the electrochemical performance of all the cells under different current densities from 0.005 to 3.0 V vs. Li ? /Li.

Structure and Morphology of ZnCo 2 O 4
To determine a suitable calcination temperature to prepare ZnCo 2 O 4 powder, TG-DTA was used and the result is shown in Fig. 1a. A small peak occurs at 367°C, and two main exothermal peaks are at *324 and *402°C, respectively. They are corresponding to the conversion of intermediates (metal glycoalates or alkoxides derivatives from the reaction of ethylene glycol with the metal ions) into ZnCo 2 O 4 [19,20]. Meanwhile, these peaks were accompanied by a drastic mass lose of about 29% in the temperature range 300-405°C. In order to ensure that the precursor can be completely decomposed, the calcination temperature was finally set at 500°C to prepare ZnCo 2 O 4 NCPs. To investigate the composition and surface electronic state of ZnCo 2 O 4 NCPs, XPS analysis was conducted. In the O 1s spectrum (Fig. 2), there are two main peaks at 529.2 and 530.8 eV, which should be attributed to the lattice oxygen from ZnCo 2 O 4 NCPs and the oxygen from hydroxide ions, respectively. The two minor O 1s peaks around 532.2 and 533.7 eV are believed to be generated from surface bound water or adsorbed oxygen [21][22][23]. There are two major peaks at binding energies of 1044.4 and 1021.3 eV in the Zn 2p spectrum, attributed to Zn 2p1/ 2 and Zn 2p3/2 of Zn 2? [24]. The binding energy values of the two major peaks are 780.4 and 795.2 eV in the Co 2p spectrum, associated with Co 2p3/2 and Co 2p1/2, respectively. Additionally, the spineorbit splitting of the  mentioned two peaks is 14.8 eV. Two accompanied weak satellite peaks are also visible at 790.0 and 805.0 eV and the energy gap between the main peak and the satellite peaks is around 9.8 eV. This suggests that Co cation can be assigned a value of ?3 [15]. The results are quite close to those reported about MCo 2 O 4 (M=Mg, Cu, Zn) [8]. In addition, the survey spectrum shows the presence of Zn, Co, and O as well as C.
The morphology of ZnCo 2 O 4 NCPs was characterized by SEM and TEM, as shown in Fig. 3. The SEM images of the precursor and ZnCo 2 O 4 are exhibited, respectively, in Fig. 3a, b. A uniform powder has been synthesized by the hydrothermal method with the assistance of SDBS. The sizes of precursor and ZnCo 2 O 4 particles are approximately the same, and the fluffy surface turns to be tighter with the process of calcinations. From Fig. 3c, the ZnCo 2 O 4 NCPs comprise small primary nanoparticles with a diameter around 10 nm. Figure 3d shows a typical HRTEM image of the ZnCo 2 O 4 particles, revealing a structurally uniform lattice spacing of about 0.47 nm, which corresponds to the (111) lattice plane of the ZnCo 2 O 4 . Meanwhile, the crystallization of ZnCo 2 O 4 is also well confirmed. The BET surface area of ZnCo 2 O 4 NCPs is 30.0 m 2 g -1 , which is similar to those of other metal oxides or oxysalts as electrode materials for LIBs reported recently [25][26][27][28][29][30][31].

Electrochemical Performance of ZnCo 2 O 4 NCPs
The electrochemical performance of the ZnCo 2 O 4 NCPs as the anode materials for LIBs was evaluated by galvanostatic discharge/charge experiments. The curves were measured at room temperature with a current density of 100 mA g -1 ranging from 0.005 to 3 V(vs. Li ? /Li). Figure 4 shows the voltage-capacity profile of prepared ZnCo 2 O 4 NCPs electrode for the first three lithiation/ delithiation cycles. There is a stable potential plateau around 0.85 V during the first discharge process and the long discharging plateau becomes steeper and moves upward, consequently forming a long slope between 1.25 and 0.60 V in the following two cycles. The first lithiation capacity reaches as high as 1110(±5) mAh g -1 with a coulombic efficiency of 84.7% in the 1st cycle. The irreversible capacity may be attributed to the kinetic limitations of reactions [32], the formation of solid electrolyte interphase (SEI), the polymeric layer formation on the metal and nanoparticles (active material) under the deep discharge conditions (0.005 V vs. Li) [33], and the reduction of active metal to metal with Li 2 O formation, which is commonly observed for several types of electrode materials [34][35][36]. The lithiation and delithiation capacities in the 2nd cycle are 932 and 912(±5) mAh g -1 , respectively.  Fig. 5. The ZnCo 2 O 4 electrode reveals a large capacity fading during the initial 16 cycles. For the 16th cycle, the retention of delithiation capacity is 85.2%. In the following cycles, the reversible capacities decrease at a slower rate and a retention value about 81.0% is maintained in the 50th cycle. The decay rate of delithiation capacities increases until the 76th cycle, where it has a slight rebound with the continuous cycling. After 100 cycles, a high delithiation capacity of 700(±5) mAh g -1 is still retained with a retention of 74.4%, demonstrating the high specific capacity and superior cyclability of ZnCo 2 O 4 NCPs. The coulombic efficiencies are ranging from 99.3% to 98.4% except in the first two cycles. The outstanding electrochemical behavior of ZnCo 2 O 4 NCPs could be attributed to the unique structure (shown in Fig. 2c), which has possessed high specific surface area and empty space among  the aggregated nanosized primary ZnCo 2 O 4 particles. In this way, ZnCo 2 O 4 NCPs can provide a short pathway for Li ? diffusion and a large electrode-electrolyte contact area for Li ? migration across the interface. More importantly, the empty space between adjacent particles can significantly improve the structural integrity caused by the volume change associated with the repeated lithiation and delithiation processes. The rate performance of ZnCo 2 O 4 NCP electrode is provided in Fig. 6. The reversible delithiation capacity is 884(±5) mAh g -1 at the 10th cycle under 100 mA g -1 , and this value decreases to 731, 506, 345, 188(±5) mAh g -1 with a continuously increasing current density from 500 to 3000 mA g -1 . More importantly, as the current density gradually decreases from 3000 back to 100 mA g -1 , the ZnCo 2 O 4 NCP electrode also shows good performance with slight decay. A reversible delithiation capacity of 698(±5) mAh g -1 could be resumed and maintained at the last cycle when the current is back to 100 mA g -1 . This result demonstrates the excellent performance of ZnCo 2 O 4 NCPs.
According to previous studies [14,15,37], the electrochemical reactions of ZnCo 2 O 4 involved in the lithium insertion and extraction reactions can be illustrated as follows: Zn þ Li þ þ e À $ LiZn ð2Þ Cyclic voltammetry can provide additional detail on the electrochemical reactions of ZnCo 2 O 4 NCPs with the electrolyte. Figure 7 presents the first three cyclic voltammograms of ZnCo 2 O 4 NCPs electrode in the voltage of 0.005-3.0 V at a scan rate of 0.1 mV s -1 . The initial cathodic process observed on the electrode starts at *0.8 V and a sharp peak occurs at *0.6 V versus Li, which should be resulted from the intercalation reaction of Li x ZnCo 2 O 4 , the reduction of Zn 2? and Co 3? to Zn 0 and Co 0 (Eq. 1), the formation of Li-Zn alloys (Eq. 2), and an irreversible reaction related to the decomposition of the electrolyte [14,38]. In the anodic sweep, two main oxidation peaks are observed at 1.7 and 2.0 V characteristic of the oxidation process of Zn and Co to Zn 2? and Co 3? (Eqs. [3][4][5] [39]. The second CV scan contains a cathodic peak *1.0 V, distinguishing the reduction mechanism from that in the 1st cycle [40] and two anodic peaks at 1.7 and 2.0 V. Similar CV scan is observed in the 3rd cycle although the intensity of all peaks decreases slightly, typical for reversible lithium ion intercalation/deintercalation and reversible cycling of the cells above.
In order to investigate the morphology changes after continuous discharge and charge cycles, the cell of ZnCo 2 O 4 NCPs after 100 cycles at 100 mA g -1 was disassembled and monitored by TEM. As revealed in Fig. 8, ZnCo 2 O 4 NCPs anode after 100 cycles still shows well spherical morphology with a diameter * 100 nm. However, the primary clustered structure is not obvious after cycling test, which can be attributed to the irreversible structure destruction during the cycling process. In sum, these results strongly explain the reasons for the excellent electrochemical properties of ZnCo 2 O 4 NCPs.
Nyquist plots of ZnCo 2 O 4 NCPs electrode at open-circuit voltage (OCV, 2.8-3.0 V) after different cycles at 0.1°C are shown in Fig. 9a, to investigate how the impedance changes with continuous cycling. The impedance spectra were fitted to an equivalent circuit, consisting of the resistances for electrolyte, cell components, surface film (sf), and charge-transfer(ct); a constant phase element (CPE i ); Warburg impedance (W s ) and intercalation capacitance (C i ) [32,33]. The circuit is shown in Fig. 9b. The fitted impedance data values are listed in Table 1. The R b values were relatively stable (*4.0 X) and R (sf?ct) values were found to decrease with continuous cycle. The decrease of R ct may be related to the wetting process between the ZnCo 2 O 4 NCPs (active material) and electrolyte, as well as the lower polarization and higher reactivity of ZnCo 2 O 4 NCPs. The CPE (sf?ct) values increased from 12 lF (fresh cell) to 160 lF (after 10 cycles), corresponding to the formation of SEI film. After the 50th cycle, the values are almost stable. As cycling, the electrolyte can soak into the ZnCo 2 O 4 particles, and the active ZnCo 2 O 4 is converted to lower oxidation state, cobalt oxide, zinc oxide, and Li 2 O. This result is consistent with the cycling performance (Fig. 4).

Conclusions
In summary, ZnCo 2 O 4 NCPs are synthesized successfully by a designed hydrothermal method with the assistance of SDBS. The characterizations by XRD, SEM, and TEM show uniform ZnCo 2 O 4 NCPs around 100 nm in diameter, comprising aggregated primary ZnCo 2 O 4 nanoparticles (*10 nm in diameter). The electrochemical measurements reveal that the first lithiation and delithiation capacities of ZnCo 2 O 4 NCPs are 1110 and 941 mAh g -1 , respectively. After 100 cycles, a high reversible delithiation capacity of 700 mAh g -1 is retained. The high capacities and good