1. Electrochemical Characteristics of Al2O3-Doped ZnO Films by Magnetron Sputtering
He-Qun Dai1,2, Hao Xu1,2, Yong-Ning Zhou2, Fang Lu1, and Zheng-Wen Fu*,2
1Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, State Key Laboratory of Surface Physics & Department of Physics, Fudan University, Shanghai , China
2Department of Chemistry and Laser Chemistry Institute, Fudan University, Shanghai , China
Introduction
Al2O3-doped ZnO (AZO) is an interesting transparent conductive oxide (TCO) material. It is widely used as a transparent electrode in thin-film solar cells, flat panel displays, and optoelectronic devices. Since AZO film has a much higher electrical conductivity than pure ZnO film, it can be used as an anode material for lithium ion batteries. As far as we know, there is no available report on the electrochemical behavior of AZO film with lithium.
Experiment
Thin films were deposited on stainless steel substrates with the same sputtering condition. The sputtering targets used in this study were homemade from the mixture of high purity ZnO and Al2O3 with different weight fractions of Al2O3 (0, 1, 2, 3, 5, 7, and 9 wt %) for depositing AZO films with different compositions.
Fig. 6 Ex situ (a) HRTEM image and (b) SAED patterns of AZO2 films after the first discharging to 0.01 V; ex situ (c) HRTEM image and (d) SAED patterns of AZO2 films after the recharging to 4.0 V. The d-spacing was measured to be Å, which can be attributed to the (220) plane of the LiZn. The corresponding d-spacing value is Å which can be attributed to (113) plane of Al2O3. This result suggests that Al2O3 is formed after recharging to 4.0 V.
Discussion
The electrochemical reaction mechanism of AZO film (< 3 wt %) with lithium is proposed as follows:
Discharge process
AZO + 2Li+ + 2e- → Zn + Al + Li2O (1)
Zn + Li+ + e- → LiZn (2-1)
Al + Li+ + e- → LiAl (2-2)
Charge process
LiZn → Zn + Li+ + e (3-1)
LiAl → Al + Li+ + e (3-2)
Zn + Li2O → ZnO + 2Li+ + 2e (4-1)
2Al + 3 Li2O → Al2O3 + 6Li+ + 6e (4-2)
Fig1. The two-electrode system which was employed in the electrochemical experiments. The as-deposited AZO thin film was served as the working electrode and a sheet of high-purity metallic lithium were used as both reference and counter electrodes, respectively. The electrolyte consisted of 1M LiPF6 in a nonaqueous solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 (Merck).
Fig. 4 shows the discharge/charge curves of AZO films with different compositions (0, 1, 2, 3, 5, 7, and 9 wt % Al2O3) and the discharge capacity versus cycle number plotted up to 40 cycles for AZO films with different compositions. All of the AZO/Li cells were cycled between 0.01 and 4.0 V at a constant current density of 2 μA/cm2. It can be seen that the 2 wt % AZO/Li cell exhibits the best capacity retention (about 590 mAh/g after 40 cycles).
Results
Fig 2. XRD patterns of as-deposited AZO films with different compositions (0, 1, 2, 3, 5, 7, and 9 wt % Al2O3). Along with the increase of the Al2O3 content, the peak of ZnO (002) shifted gradually to large angle. When the mass fraction of Al2O3 is higher than 7 wt%, a peak at 25.57º (Al2O3 crystal phase) can be observed.
After doping a certain amount of Al2O3 (1, 2, and 3 wt %) into pure ZnO, their capacities and cyclabilities are much better than pure ZnO (shown in Fig. 4). This indicates that Al2O3 formed after the first charging should play an important role in the enhancement of the electrochemical performance of AZO films with lithium. It has no electrochemical activity but may act as a buffer to alleviate the stress caused by the volume changes during the formation of lithium-zinc alloys.
Fig. 5 Ex situ XRD patterns of the as-deposited AZO films, after the discharging to 0.01 V and after the recharging to 4.0 V for (a) pure ZnO; (b) AZO2; (c) AZO9.
Conclusions
Al2O3-doped ZnO (AZO) films have been fabricated by Rf magnetron sputtering. XRD results confirm that Al atoms in AZO films are substituted in the Zn site when the content of Al2O3 is lower than 5 wt %. Hall measurements showed that doping with Al2O3 (<3 wt %) can improve the conductivity of ZnO. AZO2 film exhibited the best behavior with a large reversible special capacity around 590 mAh/g and excellent capacity retention. After the first discharge and charge processes, LiAl alloy and nanosized Al2O3 were observed, respectively. Nano-sized Al2O3 formed after the first charge in AZO films plays an important role in their good electrochemical performance. Our results enrich the lithium electrochemistry of metal oxides and show the possibility of using AZO as anode materials for rechargeable lithium-ion batteries.
J. Phys. Chem. C 2012, 116, 1519–1525