New Electrode Materials In All-solid-state Thin Film Lithium-ion Battery Research

Micro-power sources are increasingly required with the development of microelectromechanical systems (MEMS).The advantages of all-solid-state thin film lithium ion battery (TFLB),high power densities, low self-discharge rate and long cycle life and easy to be fabricated in specific shape and size, make them be one of the most promising micro-power sources.Obviously, the battery performance is mostly influenced by its electrode properties,such as capacity, potential, etc. Because it is very difficult to enlarge the capacities of current lithium-inserting electrode of TFLB,for example, LiCoO2 and graphite, great efforts should be made to develop new thin film electrodes with higher capacities to satisfy the increasing requirements.Four kinds of thin film electrodes available for lithium batteries, CuF2, MnF2, CoP3 and InP, were successfully obtained by the pulsed laser deposition method (PLD).Their composition and structure were characterized by XRD, SEM, EDX, TEM, SAED and XPS,and their electrochemical porformance were also investigated by constant current charge/discharge method and cyclic voltammograms (CV), respectively. The results are listed as following:(1)There are two reversible discharge plateaus located on 2.8 and 2.0 V for CuF2 film, which respectively correspond to two processes:the inset of Li ions into CuF2 lattice and the appearance of nano-particles of Cu after their displacement by Li, and a reversible transformation between LiF and CuF2 activated by the nano-particles of Cu. Much loss of the capacity after the first discharge may be resulted from the phase change and partial Cu atoms not back to the lattice to reform CuF2. However, the reversible capacity of 544 mAh/g of CuF2 film is obtained during the first 45 cycles between the scope of [1.0,4.0V],in which the capacity decreases 0.6% per cycle with a stable discharge plateau of 1.8-2.0V. It suggests the CuF2 thin film fabricated by PLD could be used as the cathode film of the TFLB because of its high capacity and little fading. It is noticeable that a pair of reversible redox peaks above 2.8 V appears on its CV curve, and a discharge plateau located on 3.1V and an average capacity 125mAh/g in first 20 cycles during [2.8,4.2V].It indicates that the nano film of CuF2 fabricated by PLD shows good electrochemical reversibility when voltage is above 2.8V.(2) The MnF2 film fabricated by PLD shows good capacity retentivity, high reversible capacity, and lower polarization. The polycrystalline film of MnF2 has a reversible capacity of 530 mAh/g, implying one mol MnF2 reacts with 1.84 mol Li. There is a pair of redox peaks sited on 1.0 and 0.5 V on its CV curve, respectively corresponding to the reversible decomposition and formation of LiF activated by nano particles of metal Mn, which is confirmed by SAED results.Nano metal Mn appears after the first discharge and drives the reactions of LiF subsequently, which proves again the metal reaction center mechanism of fluoride.It shows there is the least difference in charge/discharge plateaus among all fluorides in experiments, suggesting good properties available for anode film. At the same time, it is an attractive way to deposit MnF2 at room temperature, which just satisfies the temperature required in IC technology.(3)A Li electrochemical reaction in CoP3 film takes place after the first metal reduction, which does not take part in the subsequent reactions. This result agrees with the previous research on powdery CoP3.Differently, the highly diffused nano metal in the film obtained by PLD can activate the reversible reaction between Li3P and LiP, in which there is an anion center mechanism.(4) InP film deposited by PLD was firstly applied as a electrode material for lithium battery with good electrochemical reversibility. The reaction of InP under the condition of 0 to 2 V includes:in discharge, Li is firstly inserted into LiP lattice and then In is displaced with the product of L13P, and successively, In reacts with Li to form InLi alloy below 0.3 V; in charge, InLi alloy decomposes.It is In which is the reaction center in the decomposition and the formation of the alloy. Afterwards, there are transformations between Li3P and InP, where P3" is the reaction center.(5)To complement the experiments, the structures and properties of Li/CuF2 system was theoretically investigated using the first principle method, including the electronic structure changes after Li lithiation, lithiation pathway and electrochemical parameters. The calculated results are comparable to the experimental data. That is, the conductivity of the system enhances with the increasing of Li atoms, and the calculated average potentials are close to the experimental.Summarily, this work shows that PLD is an important approach to excellent film electrode materials in lithium battery. There is an attractive perspective for novel nano film of fluoride and phosphide due to their large capacities resulted from their unique reaction mechanism and nano effect.