Fabrication And Modification Of High Voltage LiCoO₂ Thin Films Cathode

With the development of integrated circuit and Micro-Electro-Mechanical System(MEMS),there is an urgent need for energy storage devices featuring with ultra-thin thickness,lightweight,and high energy density.The rechargeable thin film Li-ion battery is one of the best candidates for powering micro electronic system.It is well known that film play a key role on the properties of thin film battery.LiCoO₂ successfully commercialized as the cathode material due to its high theoretical specific capacity(274m Ah/g),high discharge platform(?3.9 V)and so on.However,the structure collapse and interface problems raised by the higher charging cutoff voltage prevent the practical application of high voltage LiCoO₂ thin film electrode.It is believed that,the surface two coating layers Al₂O₃ are sputtered on LiCoO₂ thin film electrode at high/low temperatures for the promoted high voltage performance.The main content of this thesis is listed as follows:(1)The thickness of Al₂O₃ coating layers on the in-situ annealed LiCoO₂ films is optimized at high or low temperatures via varying the RF magnetron sputtering time.The cyclic performance and rate performance of the Al₂O₃ coated LiCoO₂ films are investigated at 4.5/4.6V charging cutoff voltage.It is found that the modified LiCoO₂films with the combined two layers fabricated at high and low temperatures shows the best performance between 3 and 4.5V.The specific capacity of 10 nm thick Al₂O₃ coated film is 75?Ah·cm^-2·?m^-1 in the first cycle at 1 C,and the capacity retention rate is 74%after 200 cycles.The formation of the solid solution structure at high temperature can stabilize the crystal structure of LiCoO₂ and the oxide layer coated at low temperature can effectively inhibit side reactions between the electrode and the electrolyte.(2)The multi-target magnetron sputtering is applied to coat Al₂O₃ bilayers before and after high temperature annealing treating LiCoO₂ cathode electrode film.The LiCoO₂/Li Al_xCo_1-xO₂/Li Al O₂/Al₂O₃ multilayer composites are in situ formed on the surface of the LiCoO₂ electrode.The in-situ annealing temperature of 550?can aid Al element diffuse into the LiCoO₂ lattice to form a Li-Al-Co-O solid solution layer,which stabilizes the interface structure and increases the kinetic properties.After the annealing is down to 150?,Al₂O₃ is again sputtered to the annealed LiCoO₂ film electrode where the coated species can alleviate the negative effect of the side reactions.The capacity retention rate of the multilayer protected LiCoO₂ cathode film is 73%at 3?4.5 V for 200cycles with respect to 54%of the pristine LiCoO₂ film.In addition,the rate performance is also significantly improved.The preliminary results on all solid-state thin-film batteries indicates that the composite multilayer coating can increase the contact area between the cathode and the solid-state electrolyte,reducing the interface impedance and so on.