Fabrication Of LiCoO₂ Based Solid-state Thin Film Battery And Its High Voltage Performance

With the development of microelectronics technology towards nanometerization and integration,micro power devices are urgently required for micro devices and system.As a kind of micro battery,all solid-state thin-film lithium batteries have a thickness of only micrometers and the advantages of high energy density,long cycle life,high safety,and flexible structure,filling the gap of lithium batteries in the microscale.These advantages endow the thin film battery with a broad application in micro-power systems.As one of the famous cathode materials,lithium cobalt oxide（LiCoO₂）is also the best choice for thin film battery cathode materials.The preparation of LiCoO₂ thin film cathode is the key step in all solid-state thin film batteries.The factors determining the quality of LiCoO₂ thin film were systematically investigated in this thesis.Following that,the solid-state batteries were built with the optimized LiCoO₂ film.In addition,the surface of the LiCoO₂ thin film was modified,and the performance of the LiCoO₂ thin film cathode at high cut-off voltage was initially studied.The principle contents of this thesis are listed as follow:（1）The influence of sputtering parameters such as substrate temperature,radio power,sputtering gas pressure,and other factors such as the sputtering target and the substrate on the properties of LiCoO₂ thin film were studied.The qualified LiCoO₂ cathode films with high crystallinity and good electrochemical performance were successfully prepared.Results show that the temperature has a great impact on the crystallinity and operating voltage of the thin film.When the substrate temperature is above 350 ^oC,the discharge voltage plateau is close to 3.9 V;as the substrate temperature is increased to 500 ^oC,the LiCoO₂ thin film demonstrates typical hexagonal structure.The radio power has a significant effect on the transport of Li⁺ions in the film:the film prepared at low radio power has a larger bulk impedance and poorer cycle stability;the resistance is lower and the cyclic stability of the thin film are much better for the LiCoO₂ thin film prepared at high radio power.The composition and depositing rate of the LiCoO₂ thin film are greatly influenced by the sputtering gas pressure.The Li/Co ratio of the film deposited at 1.3 Pa reaches 0.96,while the value is 0.8 at 2.5 Pa.A suitable buffer layer on the substrate is also crucial to the cycling stability of the LiCoO₂ thin film,especially for the all solid-state batteries.The sputtering targets from different suppliers can produce LiCoO₂ thin films with changeable morphology even under the same sputtering condition.Furthermore,the sputtering history of the target also affects the morphology of the deposited film since the target gradually becomes rough and uneven in surface.（2）All solid-state thin film battery on an Au coated quartz substrate was prepared with the sputtered electrolyte using a Li-rich lithium phosphate target and metallic Li as the anode,which has a specific capacity of 60?Ah cm^-2?m^-1.76%of the initial capacity is retained after 130 cycles,where the internal impedance of the cell is comparable to that with liquid electrolyte.Besides,the capacity of the all solid-state thin film battery prepared at 350 ^oC is stabilized at about 40?Ah cm^-2μm^-1 after 50 cycles at 0.5 C without significant degradation.Furthermore,a flexible thin film Li battery was successfully prepared on the polyimide sheet,which can be cycled for multiple cycles.（3）The charge-discharge properties of LiCoO₂ thin film cathode at high cut-off voltage were studied.The cycling behaviors in the range of 3-4.5 V is same as that in 3-4.2 V.However,the capacity fading is greatly accelerated in the range of 3-4.6 V and 3-4.7 V.Additionally,the surface modification with sputtered ZrO₂ can enhance the cycle stability of LiCoO₂ thin film cathode.