Investigation Of The LiCoO₂/LLTO Composite Cathodes And Interfaces For Li-ion Batteries Of High Specific Energy

To satisfy the requirement of the new applications such as micro-electro-mechanical system,micro medical devices,wireless communications,sensors,and electric vehicles,the energy and power density of lithium-ion batteries need to be improved urgently.To increase the energy density of the lithium battery,it is typical to increase the specific capacity of the electrode materials or/and the battery voltage level,and reduce the weight of the inactive materials at the same time.For cathode materials,the specific capacity and redox voltage directly affect the energy density,and the low electron/ion conductivity will increase the internal resistance of the batteries,resulting in a decrease in the capacity release efficiency and rate performance of the batteries.In this dissertation,the interfacial and the bulk conductivity of LiCoO₂ cathode were studied by the addition of Li_0.35La_0.56TiO₃（LLTO）solid electrolyte films.For conventional LiCoO₂ electrodes,the LLTO solid electrolyte film was deposited as an artificial cathode-electrolyte interface（CEI）layer to improve the ionic conductivity of the interface between the LiCoO₂ cathode and the electrolyte,thereby improving the cycling stability and rate performance of the LiCoO₂ electrodes.Based on the problem that the ionic conductivity of LiCoO₂ thin films is much lower than the electronic conductivity(the ionic conductivity:¹0^-8 S/cm,and the electronic conductivity:¹0^-3S/cm),LLTO/LiCoO₂ composite multilayers were prepared through the LLTO electrolyte film and LiCoO₂ thin film multilayer deposition design.This results that the ionic conductivity is improved,the internal resistance is reduced,and the polarization is decreased,indicating the significant improvment in the specific capacity and power density of the LiCoO₂ thin film per unit area.The main research and results are as follows:（1）The influences of oxygen partial pressure and annealing temperature on the structure,morphology,and ionic conductivity of sputter-deposited LLTO thin films were systematically studied.Firstly,oxygen partial pressure should be high enough to compensate for the oxygen loss,but not too high to guard against the abusive oxygen scattering effect on lithium precursors resulting in lithium-poor composition.The oxygen partial pressure of 30%leads to a comprised and optimized balance of oxygen compensation and lithium loss.Secondly,a proper annealing temperature should be exquisitely chosen to facilitate the formation of Li⁺-conductive short-range structures and maintain the LLTO thin films in an amorphous state.The annealing temperature of 300?C can achieve the best homogenization of amorphous LLTO clusters.Combined with the optimized oxygen partial pressure（30%）and annealing temperature（300?C）,an amorphous LLTO thin film with an ionic conductivity of 5.32×10^-5 S/cm at room temperature and activation energy of 0.26 eV was prepared.（2）The stability issue of LiCoO₂ cycled at high voltages is one of the burning questions for the development of lithium ion batteries with high energy density and long cycling life.Herein,amorphous Li_0.33La_0.56TiO₃,one of the most successful solid electrolytes,was directly deposited on the surface of made-up LiCoO₂ electrodes through magnetron sputtering.Not only the inherent conductive network in the made-up LiCoO₂electrodes was retained,but also the Li⁺transport in bulk and across the cathode-electrolyte interface was enhanced.In addition,the surface chemical analysis of the cycled LiCoO₂ electrodes suggests that most of the stability issues can be addressed via the deposition of amorphous Li_0.33La_0.56TiO₃.With an optimized deposition time,the LiCoO₂ electrodes modified by Li_0.33La_0.56TiO₃ performed a steady reversible capacity of150 mAh/g at 0.2 C with the cut-off voltage from 2.75 to 4.5 V vs.Li⁺/Li,and an 84.6%capacity gain at 5 C compared with the pristine one.The ionic diffusion coefficient of the LiCoO₂ electrodes modified by Li_0.33La_0.56TiO₃ is determined to be 7.52?10^-12 cm²/s,which is much higher than that of the pristine one(2.32?10^-12 cm²/s).The surface modification strategy for LiCoO₂ presented here provides an encouraging avenue for improving the energy density and cycle life of Lithium-ion batteries.（3）LiCoO₂/LLTO LiCoO₂ multilayer films with laminated structure were prepared by deposition of LLTO electrolyte nano-layers using magnetron sputtering.The interfacial impedance caused by the lattice mismatch between polycrystalline LiCoO₂ can be effectively reduced by the utilization of amorphous LLTO,resulting in an enhanced lithium ion diffusion ability in LiCoO₂ thin films.The multilayered LCO-LLTO composite thin film enhances the rate performance and cycle stability of the LiCoO₂ film.The lithium ion diffusion coefficient of multilayered LCO-LLTO composite films is increased by an order of magnitude than that of LiCoO₂ single-layered film.Multilayered LCO-LLTO composite films after 500?C annealing display the best rate performance with a high capacity of 74.8 mAh/g at 6.4 C,and the best cycling performance with a high capacity of 133.64 mAh/g after 70 cycles.Multilayered LCO-LLTO composite films can be applied in all-solid-state thin-film lithium batteries to increase the thickness of the thin film electrode and thereby improve the energy density.