Preparation And Modification Of The Cathode Lithium Cobalt Oxide For Thin Film Lithium Batteries

Lithium-ion batteries have been widely used in the 3C field since the commercialization in 1991.Nowadays,with the rapid development of MEMS and wearable devices,the integratable power supply unit becomes more and more important.Therefore,thin film has become one of the inevitable trends in the development of lithium batteries.Thin-film lithium battery is the ideal energy supply unit in active microelectronic devices due to its advantages of high energy density,long cycle life,safety and sustainability.Thin film cathode is a core component of all-solid-state thin-film lithium batteries,which will directly affect the performance of thin-film batteries.Therefore,fabrication of advances cathode film with excellent performance is the focus of all-solid-state thin-film battery.Among the well-known cathode materials,LiCoO₂ is one of the most suitable candidates for thin-film cathodes,and has the following advantages:?1?LiCoO₂ has high voltage platform and specific discharge capacity,ensuring high energy density of all-solid-state thin film lithium batteries;?2?The volume change of Li CoO₂during charging and discharging is small?about 1.9%?,resulting in a small stress,which avoids the electrode cracking and cell swelling in expanded lifetime test;?3?LiCoO₂has relatively good conductivity and can satisfy the electron transport without conductive additives.In this thesis,Li CoO₂ cathode for thin film lithium battery was fabricated and modified by magnetron sputtering.The achievements are listed as below:?1?The effects of in situ heating temperature,sputtering power,oxygen partial pressure and film thickness on the performance of Li CoO₂ were investigated during magnetron sputtering.It was found that,the?003?diffraction peak intensity of the LiCoO₂ film gradually increased and the crystallinity was improved while the heating temperature of the substrate was promoted from 580?to 780?.Subsequently,the voltage platform of the charging curve was more pronounced and the specific capacity retention ratio was also better with the higher heating temperature;the sample fabricated with sputtering power 160W and 20%oxygen atmosphere showed the best performance,the XRD diffraction peak was strong,the surface was smooth,and the electrochemical performance was the best.As the film thickness increased,the specific capacity of the film decreased,with the increased capacity retention capability.In order to enhance the cycling performance of LiCoO₂ under high voltage,the LiCoO₂ thin film was modified by Li F surface coating or Al co-sputtering.?2?Based on the successful preparation of LiCoO₂ films,a series of all-solid-state lithium batteries were designed and prepared.Firstly,directly attaching Li metal foil as the anode on Li PON/LiCoO₂ double layer generated a solid-state quasi-thin-film battery,which unfortunately,a large polarization due to poor interfacial contact.Secondly,Li metal thin films were prepared in sequence and the produced solid-state battery demonstrated no decay with in 50 weeks cycles.Finally,all-solid-state thin-film lithium batteries were repeatable prepared on quartz substrates or stainless steel flexible substrates with LiCoO₂ as the positive electrode,LiPON as the solid electrolyte and Li metal film as the negative electrode.