Photoluminescence Properties and Electrochemical Performance of Fe:ZnO Nanoparticles in Li-ion Batteries

The pure and Fe doped ZnO nanoparticles with varying iron mole percentages of 3%, 4.5% and 6% were synthesized by using co-precipitation. Structural and morphological properties as well as electrochemical properties, serving as anode in Li-ion batteries, were studied. All samples have hexzagonal wurtzite ZnO crystal structure and a slightly shift from x-ray difraction patterns of Fe:ZnO samples have shown that Fe 3+ ions substituted by Zn 2+ ions. As the percentage of Fe mol increases from 3% to 4.5%, the size of the particles decreases, and as the percentage of Fe mol decreases from 4.5% to 6%, it also increases. The emission bands originated due to energy levels generated by ZnO intrinsic defects in all samples. In half cell configuration, 3% Fe:ZnO and 6 % Fe:ZnO anode exhibit rapid capacity fading. ZnO anode with Fe mol percentage of 4.5 % results a substantially enhances sepecific capacity during 50 cycle.


Introduction
With the growing demand for portable electronic devices and electric vehicles, achieving high-performance Li-ion batteries have been become crucial. Therefore, the challenges associated with energy density and power density have been investigated further with development of components of Li-ion batteries. Although graphite, which has a theoretical specific capacity of 372 mAh g -1 , is the most commonly used anode material in commercial Liion batteries, electronic and ionic transport limitations have led to the search for different anode materials [1]. To improve specific capacity and charge rate capability of electrodes many materials have been investigated such as transition metal sulfides [2], silisium [3], germanium oxides [4] and metal oxides [5] etc.. Among them conversion reaction-based transition metal oxides (Fe2O3, NiO, ZnO, TiO2 etc.) have attracted particular attention due to their high theoretical capacity (> 600 mAh g -1 ), large amount of specific surface areas, abundant in nature and low toxicity [6]. The main drawbacks in their using are their poor cycling stability, large volume expansion and pulverization of active particles in the practical Li-ion battery applications [7,8].
As a promising high capacity anode material, ZnO is so attractive due to high theoretical capacity of 978 mAh g -1 , high chemical and mechanical stability, easy synthesis and low cost [9]. In the first cycle, Li + ion insertion /extraction reactions of ZnO can be represented as follows; ZnO + 2 Li + + 2 e -→ LiZn + Li2O (1)

Experimental
All chemicals were analytical grade and used without any further purification.  (7); Microstrain (ε) of the samples is calculated by Equation (8); In addition, dislocation density (δ) which is a measure of the amount of defects in the crystal is measured by Equation (9); where is the wavelength of x-ray radiation, is the full width at half maximum (FWHM) and is diffraction angle and ε is the strain. The average crystallite size of varies from 9 nm. to 12 nm. depending on the creation of new nucleation hub and rate of particle nucleation. The reduction of grain size may cause an increase microstrain and dislocation density. This can be interpreted as the inclusion of the iron additive in the ZnO overall crystal structure.

Surface Morphology
Surface morphology of the pure ZnO and Fe doped ZnO nanoparticles was shown in Fig.2 (a-d) under high magnification ratio of x30000. Spherical-like and their agglomerative forms exhibits for all samples. Surface of the samples has composed particles in nano-dimension range. As can be observed, their dimesions changes with varying iron contribution in ZnO host due to the synthesis process that takes place in aqueous solutions reveals more hydroxyl groups [22]. These dimensions are consistent with the average crystalline sizes obtained using the Debye-Scherrer formula, as shown in Table 1. The surface of the 4.5 % ZnO samples appears to be more porous and homogeneous, allowing the facility to shorten the transport length for Li + ions upon insertion/deinsertion. Fe alloy nanocluster forms have been observed in 6% Fe:ZnO samples with granular ZnO forms have become more irregular due to less nuclei formation with lower nuclei rates [23].

Elemental Analysis
The presence of Zn, O and Fe as a elemental ratio of w% in the synthesized particles are confirmed by EDX measurements, as shown in Fig 3 (a-d). No other impurity element is detected. The reason why the difference in elemental ratio of w% differs from the amounts placed is due to ZnO crystal growth process.

Photoluminescence Spectrum
To determine emission bands and defect levels in the synthesized nanoparticles, photoluminescence (PL) spectra was used with using excitation wavelength 245 nm. in the 350-700 nm. range. All samples exhibit distinctive band at 489 nm. and weak emission bands at 587 nm. and 609 nm. Although no major intensity difference is detected at 587 nm. and 609 nm.,