Study Of The Impact Of BaTiO₃ Surface Coating/Interlayer Modification On Li₂MnO₃ Cathode Material In Liquid And All-solid-state Thin Film Batteries

Lithium-rich manganese-based（LMR）cathode materials with high capacity（>250 m Ah/g）,high voltage（>4.5V）,low cost and environmentally friendly features have drawn people’s attention and are widely considered as next-generation cathodes of lithium-ion batteries.However,a series of problems appear in the electrochemical reaction process,including oxygen release,transition metal ion migration,irreversible structural transformation,which finally manifest as unceasing loss of capacity,continuous reduction of voltage and low first cycle coulombic efficiency.In order to fundamentally study and solve the problems existing in Li₂MnO₃ cathode material,it is necessary to conduct in-depth research on Li₂MnO₃ cathode material.Li₂MnO₃,the core component of LMR cathode materials,plays an important role in the electrochemical performance and reaction mechanism of LMR cathode materials.In this paper,different proportions of Ba Ti O₃ piezoelectric materials were coated on the surface of Li₂MnO₃ powder by sol-gel method to modify the electrochemical performance of Li₂MnO₃ cathode material in liquid half-cell.The existence of Ba Ti O₃ nanocoating layer was observed by XRD,XPS,TEM and other characterization methods,and a series of electrochemical tests showed that Ba Ti O₃ coating material had greatly improved the rate performance of Li₂MnO₃ cathode material.When the atomic ratio of Ba Ti O₃ was 0.5%,Li₂MnO₃ cathode material had the highest specific discharge capacity,and exhibited smaller charge transfer impedance and larger lithium ion diffusion coefficient by EIS and GITT tests respectively.Ba Ti O₃,served as a piezoelectric material coating layer,provided an electric field at the interface between Li₂MnO₃ powder and electrolyte which enhanced with the increase of current density and was conducive to lithium ion transmission,thus improving the electrochemical reaction kinetics,and greatly improving the rate performance of Li₂MnO₃cathode material.In addition,due to the existence of coating layer,the direct contact between Li₂MnO₃ powder and electrolyte was inhibited,which could slow down the irreversible phase transformation of the material from the surface to a certain extent,so that the material had good cycling and structural stability.This paper also carried out the construction of Li₂MnO₃ all-solid-state thin film battery and focused on the charge-discharge mechanism of Li₂MnO₃ film in the all-solid-state battery system.Firstly,Li₂MnO₃ thin films were prepared by magnetron sputtering,and the crystallinity of Li₂MnO₃ thin films was controlled by changing the annealing temperature.Then,an Li₂MnO₃/Li PON/Li all-solid-state thin film battery was constructed based on Li₂MnO₃ thin film,and its electrochemical performance was tested systematically.It was found that the Li₂MnO₃ thin film cathode changed from amorphous to well-crystallized thin film gradually with the increase of annealing temperature.The Li₂MnO₃ thin film annealed at 800℃showed the highest capacity and good crystallinity.Subsequently,the electrochemical properties of Li₂MnO₃ thin film cathode prepared by magnetron sputtering and Li₂MnO₃powder cathode prepared by sintering were proved to be the same.Then a series of electrochemical tests demonstrated that Li₂MnO₃ all-solid-state thin film battery had the extremely good cycle stability,with its capacity remained unchanged after 1000 cycles.Although the phase transition from layered to spinel still occurred during the sputtering process of Li PON solid electrolyte and cycling,where the new phase was very stable and had an excellent rechargeability.Finally,the modification of Li₂MnO₃ thin film by Ba Ti O₃piezoelectric material served as the transition layer was studied.It was found that the modification mechanism was similar to that in Li₂MnO₃ powder in liquid half-cell.Ba Ti O₃piezoelectric material as transition layer effectively improved the electrochemical reaction kinetics of Li₂MnO₃ thin film at the interface and the rate performance of Li₂MnO₃ film was greatly improved.In this paper,the electrochemical behavior of Li₂MnO₃ thin film in all-solid-state battery and the Ba Ti O₃ piezoelectric material served as coating layer/transition layer on the electrochemical behavior of Li₂MnO₃ cathode were studied,which verified the feasibility of Ba Ti O₃ as a new modified material applied to Li₂MnO₃ and LMR cathode materials.And it paved the way to the application of LMR cathode materials with high energy density,high rate and long cycle performance in all-solid-state thin film lithium batteries in the future.