Epitaxial Synthesis Of Flexible Li₄Ti₅O₁₂ Thin Films And Their Electrochemical Properties

With the development of miniaturization and miniaturization of electronic devices,all solid state flexible thin film batteries have attracted more and more attention as a suitable power source for microdevices.Thin film batteries could be mainly used in wearable devices,implantable medical devices,microsensors and micro-electro-mechanical systems（MEMS）,etc.They have great advantages in improving battery energy density,broadening operating temperature range,and prolonging service life.Compared with metal lithium anodes,silicon anodes,and graphite anodes that have been studied more at present,Li₄Ti₅O₁₂（LTO）anode has the advantages of high safety,long cycle life,and little structural change during charge and discharge,making it suitable for use as an anode material for all-solid-state thin film lithium-ion batteries.In this paper,mica with high temperature resistance and atomic smooth surface is selected as the flexible substrate for thin film growth,which realizes the epitaxial preparation of LTO thin film and makes the thin film electrode have the characteristics of flexibility and transparency.The main work of this paper is as follows:（1）LTO ceramic targets with different Li contents were prepared by high temperature solid state reaction method,and LTO thin film electrodes were epitaxial grown on a flexible transparent mica substrate by pulsed laser deposition（PLD）.The optimum growth conditions of LTO thin films on mica substrates were determined,including target ratio Li:Ti of 4.8:5,substrate temperature of 700℃,laser energy of100 m J and oxygen pressure of 15 Pa.（2）The electrochemical performance of LTO thin film electrodes with different temperatures and thicknesses was studied by using Au thin film as the current collector.The results show that 700℃is the best substrate deposition temperature for LTO thin films,where the discharge specific capacity reaches 145 m Ah/g,and the specific capacity is maintained at 133 m Ah/g after 60 cycles at a rate of 0.2 C,the charge transfer resistance is 152.4Ω.The study found that with the increase of the film thickness,the specific capacity of the film electrode gradually increased.The discharge specific capacity of the film with a thickness of 187.5 nm reached 156 m Ah/g,which remained at 153 m Ah/g after 60 cycles,reaching 98.1%of the initial capacity.It is mainly attributed to the stability of the material itself and the sufficiently small lattice mismatch between the LTO thin film and the Au thin film.The LTO thin film electrode also exhibits excellent visible light transmittance and bending resistance.（3）The electrochemical performance of LTO thin film electrodes prepared under different conditions studied with Sr Ru O₃（SRO）thin films as current collectors.The results show that the films prepared at 650℃have regular and tidy surface morphology and a root mean square roughness of 7.44 nm,and the initial capacity reaches the highest 148 m Ah/g,the specific capacity remains at 121 m Ah/g after 60 cycles.The discharge specific capacity of the thin film electrode with a thickness of 187.5 nm is162 m Ah/g,which is 92.6%of the theoretical capacity.However,only the specific capacity of 138 m Ah/g was retained after 60 cycles at a rate of 0.2 C.This may be because there is a large lattice mismatch between the SRO thin film and the LTO thin film,resulting in more defects,affecting the growth quality of the thin film,resulting in a decrease in its electrochemical performance,but the LTO thin film electrode still has high visible light transmittance and excellent bending resistance.