ZnO Nanocrystals as Anode Electrodes for Lithium-Ion Batteries

ZnO nanocrystals were synthesized via a thermal decomposition method. X-ray diffraction, transmission electron microscopy, and photoluminescence were used to investigate the composition and nanostructure of the material. Compared with commercial ZnO nanoparticles, ZnO nanocrystals showed higher lithium storage capacity and better cycling characteristics and exhibited a reversible discharge capacity of 500mAh g after 100 cycles at 200mAg.


Introduction
Transition metal oxides have gained more and more interest as electrode materials in lithium-ion batteries in the last decades because of their higher theoretical capacity and safety compared with the conventional carbon materials [1][2][3][4].Among them, ZnO, a wide-bandgap semiconductor of 3.37 eV at room temperature with large exciton binding energy of 60 meV, is a multifunctional material for a variety of practical applications such as piezoelectric devices [5], field emission [6], gas sensors [7], dye-sensitized solar cells [8], and photocatalysts [9] due to its excellent physical and chemical properties.ZnO has superior advantages such as low cost, facile preparation, morphologic diversity, and high chemical stability.ZnO nanocrystals with different morphologies have some special properties of physics, so intense interests have been devoted to the synthesis of ZnO with various morphologies such as nanowires [10], nanobelts [11], nanorods [12], and nanosheets [13].
As an anode material of lithium-ion batteries, ZnO has a theoretical capacity of 978 mAh g −1 [14].However, the poor electronic conductivity, large volume change during lithium/delithium process, and the resulting severe capacity fading hinder its practical application [15].Hitherto, some efforts have been made to improve its cycling performance including synthesis of nanostructures [15][16][17], doping and forming composite with metal [18,19], metal oxide [20][21][22], carbon [23,24], and graphene [25,26].For example, dandelion-like ZnO nanorod arrays showed higher lithium storage capacity and better cycling characteristics compared to powder-form ZnO [27].Mesoporous ZnO nanosheets exhibit a 50th charge capacity of 420 mAh g −1 , and the capacity and cycling performance are enhanced compared with those of common solid ZnO particles [17].More significant research efforts are needed to further improve the lithiumion battery performances of ZnO.
In the present work, ZnO nanocrystals have been prepared by a thermal decomposition method and their electrochemical performances as lithium anode materials are investigated.

Experimental Section
2.1.Preparation of ZnO Nanocrystals.0.316 g zinc stearate (Zn(St) 2 , 10-12% Zn basis, Sigma-Aldrich) and 10 g 1octadecene (ODE, technical grade, 90%, Sigma-Aldrich) were loaded in a 50 mL three-necked flask, degassed, and heated to 270 ∘ C under an argon flow.A separate solution of 2.5 g ODE containing 0.676 g 1-octadecanol (ODA, 95%, Sigma-Aldrich) at 200 ∘ C was rapidly injected into the reaction flask to generate nanocrystals.The reaction was allowed to proceed for 30 min.When the reaction was finished, the reaction mixture was cooled to room temperature.The resulting nanocrystals were precipitated out by adding ethyl acetate, carefully purified with hexane/ethanol mixtures, and dried at 70 ∘ C. Ethyl acetate (AR), hexane (AR), and ethanol (AR) were purchased from Sinopharm Chemical Reagent Co.Commercial ZnO nanoparticles (30±10 nm) were purchased from Aladdin Industrial Corporation for comparison.

Characterization of ZnO Nanocrystals.
Powder X-ray diffraction (XRD) analyses were performed on a Philips PW-1830 X-ray diffractometer with Cu K irradiation ( = 1.5406Å) at a scanning speed of 0.014 ∘ /sec over the 2 range of 30-60 ∘ .The electronic morphology of the samples was examined by a transmission electron microscope (TEM; FEI Tecnai G2 Spirit, USA).Photoluminescence (PL) spectra of the sample on Si substrate were recorded using Jobin-Yvon LabRAM high-resolution spectrometer with He-Cd laser ( = 325 nm).Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results were obtained with Zennium/IM6 electrochemical workstation (Zahner, Germany).

Electrochemical Performance. The coin cells (CR2025)
were assembled to test the electrochemical performance of the as-prepared electrodes.The as-prepared samples were mixed with acetylene black and carboxymethyl cellulose, in a weight ratio of 60 : 20 : 20 in an aqueous solution to form homogeneous slurry.The slurry was spread onto 10 m thick copper foil and dried at 60 ∘ C for 12 h in a vacuum oven and then pressed to obtain the electrode sheet with a 9-10 m coating thickness and a loading level of about 1.4 mg cm −2 .The electrolyte of 1 M LiPF 6 and 5% fluoroethylene carbonate (FEC) in ethylene carbonate (EC, >99.9%)/diethylene carbonate (DEC, >99.9%)/dimethyl carbonate (DMC, >99.9%) (v : v : v = 1 : 1 : 1, water content <20 ppm) was purchased from Zhangjiagang Guotai-Huarong New Chemical Materials Company (China).The cells were assembled in an Ar filled glove-box.The cells were charged and discharged galvanostatically in the fixed voltage window from 0.01 V to 3 V on a Shenzhen Neware battery cycler (China) at 25 ∘ C.
EIS was measured by applying an alternating voltage of 5 mV over the frequency ranging from 10 −2 to 10 5 Hz.In this work, unless otherwise specified, all impedance measurements were carried out after one cycle of the prepared electrode.

Results and Discussion
The crystal structure of the sample was characterized by XRD, as shown in Figure 1.All diffraction peaks can be indexed as the hexagonal phase of ZnO, which is in good agreement with JCPDS number 36-1451.
The typical morphology of ZnO nanocrystals and commercial ZnO nanoparticles, as observed by TEM, is presented in Figure 2. It can be seen that the reaction generated trigonal ZnO nanocrystals with the size of 10-15 nm (side-edge length).The diameter of commercial ZnO nanoparticles was 30 ± 10 nm.PL spectrum of the sample is shown in Figure 3. Strong excitonic emission at 375 nm and a broad but weak spectral band ranging from 450 to 600 nm were observed.The near-UV peak is due to the recombination of electron and hole in an exciton, while the visible emission is due to the presence of various point defects, such as interstitial oxygen and oxygen vacancies.
The electrochemical properties of ZnO nanocrystal and commercial ZnO nanoparticle electrode for lithium-ion batteries were investigated.Figure 4 reveals the CV of the electrodes at a scan rate of 0.5 mV s −1 in the voltage ranging from 0.01 to 3 V.Both of the first discharge process (negative scan) of ZnO nanocrystal and the commercial ZnO electrodes showed broad peaks around 1.2-1.0V and 0.5-0.7 V and a sharp reduction peak below 0.3 V, corresponding to the irreversible reactions (the reaction of FEC decomposition and formation of primal solid electrolyte interface (SEI) film on the surface of composite electrode) and the insertion of Li ions into ZnO, respectively.The relative peak at 0.5 V originated from the reduction of ZnO into Zn and the formation of amorphous Li 2 O, while the strong peak near 0.25 V was caused by the generation of Li-Zn alloy together with the decomposition of electrolyte.These peaks disappeared in further cycles, indicating the irreversible reduction of ZnO with a large irreversible capacity in the first cycle.New reduction peaks at about 0.80 and 0.30 V appeared after the first cycle and shifted to a lower voltage during further cycles.In the subsequent delithium process for ZnO nanocrystals (Figure 4(a)), six weak oxidation peaks centred at 0.27, 0.52, 0.63, 1.48, 1.78, and 2.20 V could be carefully discerned which could be attributed to multistep dealloying of Li-Zn alloy.However, for commercial ZnO electrode (Figure 4(b)), only two oxidation peaks of 0.65 and 1.52 V could be found in the CV curves.Besides, the ZnO nanocrystal electrode displayed a higher peak current in both oxidation-reduction reaction processes, indicating that a better and more active electrode reaction happened.
The cycling performances of ZnO nanocrystal and commercial ZnO nanoparticle electrode at a current density electrode showed a first charge/discharge capacity of 570/ 1272 mAh g −1 with a coulombic efficiency of 44.8%.At a higher current density of 500, 1000, 2000, 3000, and 4000 mA g −1 , the capacity dropped to about 309, 187, 114, 75, and 51 mAh g −1 , respectively.
To investigate the difference in electrochemical performance, EIS was employed to characterize the impedance properties of electrodes.The Nyquist complex plane impedance plots of the electrodes after the first two charge/discharge cyclic processes are presented in Figure 7.Both Nyquist plots which consisted of a depressed semicircle at high frequency range correlated with the electron transfer resistance on the interface of electrode/electrolyte.These Nyquist plots were fitted with the equivalent circuit (inset), as shown in Figure 7.This equivalent circuit consisted of a series of four resistors elements, three constant-phase elements (CPE), and a Warburg diffusion element.In the equivalent circuit,  1 was composed of the electrolyte resistance (  ) and the electrode resistance (  );  2 represented the SEI film resistances;  3 represented the interphase electronic contacts    resistance;  4 was the charge-transfer resistance across electrode/electrolyte interface; CPE 1 and CPE 2 were attributed to the Li + diffusion in the SEI film and pore channel of the electrode materials, respectively; CPE 3 represented the electric double-layer capacitance of electrode/solution interface;   which represented Warburg impedance was related to the semi-infinite diffusion of lithium ions into the bulk electrode [37,38].By fitting the impedance data, the typical parameters were obtained and summarized in Table 1.It was seen that the smaller  3 was also observed in ZnO nanocrystal electrode,  which usually favored the fast transport of Li + ions and the electrons across the interface.It also indicated that the carbon layer supplied fast charge-transfer channels on the interface of ZnO nanocrystal. 0,1 ,  0,2 , and  0,3 of the ZnO nanocrystal electrode were higher than those of commercial ZnO electrode.The increase of  0,3 representing the electric double-layer capacitance favored the charge transfer for the electrode reaction.And the increase of  0,1 and  0,2 favored the diffusion of Li + in the SEI film and within the pore channels in the electrode, respectively.ZnO nanocrystal electrode showed a smaller chargetransfer resistance and higher electric double-layer capacitance as compared with the commercial ZnO electrode, indicating an improved kinetic character of electrode reactions (i.e., charge transfer and polarization) which could be ascribed to the better availability of electrons and perhaps also Li + .The smaller particle size and better crystal type constituted a fast pathway for mass transport and electron transfer and hence improved Li storage capacity.The excellent capacity of the ZnO nanocrystals is highly attractive when compared with other reported ZnO-based anode materials (Table 2).

Conclusion
In summary, ZnO nanocrystals with side-edge length of 10-15 nm were synthesized via the thermal decomposition method.A high reversible discharge capacity of 500 mAh g −1 of ZnO nanocrystals was observed after 100 cycles at 200 mA g −1 .Compared with commercial ZnO nanoparticles, ZnO nanocrystals showed higher lithium storage capacity.These results are attributed to the structural difference of ZnO nanocrystals resulting in different cell impedance, which affects the Li-ion diffusion.

Figure 7 :
Figure 7: Nyquist plots for ZnO nanocrystal and commercial ZnO nanoparticle electrodes.The spots correspond to the experimental data, and the solid lines stand for the calculated data from the equivalent circuits of inset.

Table 1
Y 0 and  are two parameters of the constant-phase element.

Table 2 :
Electrochemical performance comparison of ZnO-based anodes for lithium-ion batteries.