Preparation Of NiO Nanocomposite Films By Pulsed Laser Deposition And Its Lithium Storage Properties

Energy shortage and environmental pollution are important topics in the21st century. It is urgent to develop clean energy and new energy storage system. Rechargeable lithium ion batteries are considered to be the most promising energy storage system due to its significant advantages such as high voltage, large capacity, high energy density, long cyclic life, safety and reliability. It has been widely used in moble phone, computer and other portable electronic products. With the expansion of the energy storage requirements, many researchers are devoted to fabricate new anode materials with higher electrochemical performance for lithium ion battery. In2000, Tarascon et al. revealed the reversible conversion reaction based on the size effect of nanoparticles in metal oxide system, whose theoretical capacity is up to1000mAh·g-1. However, except large capacity, metal oxide as anode material has its own drawbacks. For example, the electronic conductivity of metal oxides is usually poor, and some of them will form alloy with lithium during the reaction process which cause huge volume change. In order to overcome these obstacles, lots of methods have been presented. Mainly includes the following ways:1) Fabricating thin film materials,2) Synthesizing nanostructured materials,3) Doping or preparing composite materials.In this thesis, NiO-NiS and NiO-NiSe nanocomposite films were fabricated by plused laser deposition. Charge/discharge measurements, cyclic voltammograms and AC impedance tests were used to investigate cyclic performance and electrochemical properties of the materials. X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were employed to detect the structure, morphology and composition of these nanocomposite films in certain electrochemical states. In addition, the electrochemical reaction mechanism of the films versus Li were discussed. The following two aspects were mainly studied:1. Fabrication of NiO, NiS and NiO-NiS nanocomposite films with different targets by PLD successfully. Their electrochemical performance were examined. The results indicated that:Compared with the single system NiO or NiS, the transfer resistance of lithium ion in NiO-NiS nanocomposite thin film is smaller. NiO-NiS electrode also exhibited a little higher discharge-charge capacity and better cycle performance (with reversible capacity around500mAh·g-1at the initial30cycles). However, during the reaction process, NiO and NiS separated from each other, the volume and inner stress of the film changed correspondingly. Besides, sulfides tend to form lithium polysulfides during the cycling, which can be dissolved in liquid electrolytes. These obstacles cause rapid capacity fading of NiO-NiS composite film after30cycles. The electrochemical reaction mechanism of NiO-NiS nanocomposite with lithium was revealed that NiO and NiS in the mixture films reacted with lithium separately and could be reversibly reproduced.2. Fabrication of another nanocomposite film (NiO-NiSe) by PLD and carrying out study on its Li-storage performance. Compared with NiO-NiS nanocomposite film, the discharged product of NiO-NiSe reacted with lithium does not dissolve in liquid electrolytes, which makes it has better cycle stability. After50cycles, a reversible capacity of495mAh.g-1can still be obtained. NiO-NiSe nanocomposite film also has good rate capability and is more suitable for lithium-ion battery anode material. The electrochemical reaction mechanism of NiO-NiSe is similar to NiO-NiS nanocompo-site film, NiO and NiSe in the electrode reacted with lithium separately and could be reversibly reproduced.In addition, appendix I introduced some other related work during the master: fabricating seven types of Al2O3doped ZnO (AZO) thin films with different ratios (0、1、2、3、5、7、9wt%Al2O3), followed by the research and analysis in their electrical and electrochemical properties. The results of XRD and electrical test indicated that: impurity Al atoms will cause the variation in ZnO structure’s lattice constant, the lowest resistivity of4.94×10-4Ω·cm was achieved in2wt%doping ratio. Secondly, the electrochemical performance of different AZO/Li systems demonstrated that: doping with a small amount of aluminum (<3wt%) can improve the electrochemical performance of ZnO significantly. Among all the AZO films, AZO2(2wt%Al2O3) film shows the best behavior with a large reversible specific capacity around590mAh·g-1and excellent capacity retention. However, excessive doping will produce independent inactive Al2O3crystal phase, which caused the increase of resistivity and blocked the reversible oxidation/reduction reaction. Surprisingly, LiAl and nanosized Al2O3formed during the first discharge and charge processes of AZO2film, respectively. It is believed that the nanosized Al2O3formed after the charge process in AZO films plays an important role in the improvement of their electrochemical performance. The in-depth investigation of electrochemical properties and reaction mechanism on NiO nanocomposite films and AZO films in this thesis will contribution to the preparation and research of secondary lithium ion battery anode material. These results provide directive significance for developing new high-energy lithium storage materials.