Study On Electrode Materials,Process And Properties Of LiCoO₂/LiPON/Li All-Solid-State Thin Film Batteries

With the development of miniaturization and intellectualization of electronic devices,especially the rapid progress of highly integrated energy devices,human society puts forward higher requirements for the miniaturization of batteries.It is an effective solution to develop thin film batteries with high volume and energy density.All-solid-state thin film batteries（TFBs）have become the focus of micro energydevices due to their high energy density,long life,safety and micro volume.Physical vapor deposition technology is suitable for the preparation of solid state thin filmbatteries because of its high film density,good technology adjustment and preferable technique repetition.In this paper,Li Co O₂ cathode thin films and Li PON electrolyte thin films areprepared by adjusting the process parameters,and metal Li thin films are prepared by vacuum thermal evaporation.Finally,Li Co O₂/Li PON/Li TFBs are obtained,and its performance is characterized.Exploring different types of TFBs for differentscenarios further enriches the diversity of TFBs applications.（1）The effects of power,pressure and oxygen content on the growth of Li Co O₂thin films by magnetron sputtering are studied.The thickness of the film is directly proportional to the power and inversely proportional to the amount of oxygen added.It is affected by the energy of sputtering particles and the obstruction of gas molecules per unit volume when the pressure increases.The films prepared at 400W exhibit（003）texture,and the energy between Li Co O₂ and substrate is minimized spontaneously which promotes the formation of（003）surface with the lowest surface energy.The discharge capacity of the first cycle is 56.10μAh cm^-2μm^-1 and 56.84μAh cm^-2μm^-1respectively.The capacity retention rate is 94.3%and 97.36%after 10 cycles.The（003）peak disappeared gradually with the addition of oxygen.The films prepared at20%oxygen ratio show（110）texture and compact surface.The discharge capacity of the 1st cycle is 60.26μAh cm^-2μm^-1,and the capacity retention rate is 94.99%after10 cycles.The growth of（003）texture can be inhibited by increasing the power and changing the oxygen ratio.The Li Co O₂ films grown along the ab surface have no obvious columnar grains and are more compact.The results show that the growth of Co₃O₄ grains is inhibited and the transition from Co₃O₄ to Co O₂ is induced in the oxygen-rich environment.The lowest volume stress-strain energy（110）plane is preferred in the post-annealing process.Secondly,the buffer effect of oxygen reduces the deposition rate and makes the film smoother and denser.（2）The target substrate distance plays a decisive role in the deposition rate of Li PON.Sputtering power and Ar/N₂ ratio are in the second and third place.The order of influencing factors of ionic conductivity is target substrate distance>temperature>Ar/N₂ ratio>power.The optimal parameters are 320 W,2 Pa,65 mm and 100%N₂.The highest ionic conductivity is 5.44×10^-6 S cm^-1 at room temperature.With the addition of CO₂,the surface of Li CPON electrolyte film becomes rough and its impedance increases,which indicates that it is not feasible to introduce C into Li PON by this method.（3）The transport direction of Li⁺in Li Co O₂（110）/Li PON/Li cells is perpendicular to the substrate surface,which shortening the diffusion distance of Li⁺and has the minimum polarization potential difference of 104 m V.The charge and discharge capacities of the second cycle at 0.2 C are 60.00μAh cm^-2μm^-1 and 57.88μAh cm^-2μm^-1,respectively.The discharge capacity of 47μAh cm^-2μm^-1 can still be maintained at 10 C rate,both discharge capacity and coulomb efficiency verify the excellent orientation of（110）crystal plane.The（003）plane with low surface energy and the anisotropic Li Co O₂ film have poor high rate characteristics.The full potential impedance test of the battery under the optimal parameters shows that the reaction of Li/Li PON interface makes the thickness of Li PON decrease and the impedance of R₁//CPE₁ decrease.Li₂O,Li₃N,Li₃P,and Li₃PO₄ are generated at the Li/Li PON interface during the first charging process,which makes the R₂//CPE₂ impedance increase,and the interface layer remains stable after being completely formed by the cycle.In the fully charged state,Li Co O₂ structure changes,the ion diffusion mechanism caused by grain boundary and the"insulator metallicity"transition caused by lithium intercalation interact with each other,which makes the resistance capacitance effect prominent and Li⁺transport more complex.R₃//CPE₃ increases with the number of cycles and occupies a dominant position in the total impedance of the battery.Under the optimal parameters,the discharge capacity of TFB is 40.73μAh cm^-2μm^-1 and the capacity retention rate is 84.15%after 350 cycles at room temperature of 10 C.It can work in a wide temperature range from-20℃to 120℃,and the discharge capacity of-20℃and 120℃is 31.10 and 57.70μAh cm^-2μm^-1at 2 C rate,respectively.The stability of the battery is confirmed by the 100 C current pulse during the discharge.The 5 cm×5 cm TFB has a clear cross-section stratification,a discharge capacity of 38.73μAh cm^-2μm^-1 and a capacity retention rate of 73.63%after 100 cycles.The TFB prepared on glass substrate showed good cycle stability.After 100 cycles,the discharge capacity is 47.76μAh cm^-2μm^-1 and the capacity is maintained 90.62%.The adhesion of the battery is improved by post-annealing the positive collector on the flexible mica sheet.The discharge capacity of the first cycle is 51.83μAh cm^-2μm^-1.The capacity retention rate is 70.62%after 70 cycles.The capacity of the film is greatly reduced due to the film stress during the cycle.