Study On High-voltage Lini_0.5Mn_1.5O₄ Material For High-energy Density And Reliable PEO-based Solid-state Lithium Batteries For Room Temperature

Lithium-ion batteries（LIBs）have been considered as one of the most promising choices in the new-generation green energy storage devices,due to their high operating voltage,high specific energy,long cycle life and design flexibility.LIBs have been widely used in portable electronic devices,and become a competitive candidate for storage systems of other application fields.With the development of electric vehicle,the energy density and safety performance of LIBs are facing new challenges.To improve the energy density,utilization of both high-voltage cathode and Li-metal anode is a straightforward and effective strategy.Due to the lowest potential of Li⁺/Li and the highest theoretic capacity of Li metal anode,the energy density of LIBs will be increased significantly.With regard to the safety issues,solid-state electrolytes（SSEs）are adopted to replace the flammable organic electrolytes.Because of the stable physical and chemical properties of SSEs,safety problems of traditional liquid electrolytes,relating to leakage,combustion,gas expansion and so on,can be eliminated.The main topic of this thesis is to study on key materials for high-energy-density and high safety solid-state lithium batteries—including the preparation and modification of high-voltage cathode materials,and the synthesis and research of solid polymer electrolytes.Owing to its high operating voltage of 4.7 V and high specific capacity of 146.7mAh g^-1,as well as the superior structural stability and safety,spinel LiNi_0.5Mn_1.5O₄（LNMO）is considered as one of the most attractive cathode materials for high-energy-density LIBs.However,impurities are often formed during the calcination of LNMO at high temperature,which would deteriorate the electrochemical properties of LNMO.In addition,the intrinsic low electric and ionic conductivity of LNMO need to be improved.Furthermore,the dissolution of transition metal cations into liquid electrolytes and the lattice distortion during cycling will lead to the capacity fading.Thus,several strategies are adopted here to improve the electrochemical performance of LNMO.The main contents of this part are listed as follows:A composite cathode CNTs/MnO₂-LiNi_0.45Cr_0.05Mn_1.5O₄（Cr-LNMO）was prepared by solid-state method with optimized conditions.The ordering of cations in the lattice was tuned by Cr³⁺doping,thus a Fd3 m-type spinel Cr-LNMO with better electrochemical properties were obtained.CNTs/MnO₂ composite coating was chosen to modify the surface of Cr-LNMO,which can build a bridge for electron and ion conduction between the particles,and also cover the surface of particles to prevent the direct contact between Cr-LNMO and electrolyte,thus effectively inhibiting the Jahn-Teller distortion and the dissolution of transition metal cations into electrolyte.The cell performances indicate that the modified material exhibits a high discharge capacity of 146.4 mAh g^-1 at 0.2 C,with a coulombic efficiency of 83.3%,for the first cycle under room temperature.The 1 C cycling performance shows a remarkable capacity retention of 93.1%after 500 cycles.Electrochemical characterizations were conducted to study the reaction mechanism.It is found that the excellent properties of cells are attributed to the higher ion diffusion coefficient of modified material and the lower electrochemical impedance of cells.For the study of solid polymer electrolytes（SPEs）,polyethylene oxide（PEO）was selected because of its high dielectric constant and dissociation ability.In order to reduce the semi-crystallinity of PEO,various methods have been investigated,and SPEs of high ionic conductivity at ambient temperature were successfully obtained.The main contents are listed as follows:（1）To design the end group of PEO molecule,an active group（—O—（CS）—NH—（CH₂CH₂O）₂—CH₂CH₂NH₂）with large volume was introduced through organic synthesis.The large end group and the disordered hydrogen-bonds can hinder the regular arrangement of polymer chains.Moreover,the plasticization effect of TFSI^-on PEO polymer can effectively increase the free volume and mobility of polymer segments.As a result,the room temperature ionic conductivity of modified SPE is increased to 1.2×10^-4 S cm^-1.The cell performances were conducted by LiFePO₄//Li cells,and the discharge capacity of LFP is close to 140 mAh g^-1 at 0.1 C rate for 200 cycles,suggesting excellent cycle stability.（2）A flexible crosslinker tetraglyme（TEGDME）and a rigid monomer triethylene glycol dimethacrylate（TEGDMA）were introduced into the PEO polymer electrolytes through free radical polymerization reaction under UV light irradiation.Crystallinity of modified PEO was significantly reduced through cross-linking of polymer chains and breaking-down of orderly arrangement in PEO chains.At the same time,TEGDMA itself formed a rigid long-chain by self-polymerization,which can enhance the mechanical strength of electrolyte film.As a result,the modified PEO-based SPE processes both high ionic conductivity(2.7×10^-4 S cm^-1)and good mechanical strength.Moreover,a decent lithium transference number of 0.56 and a wide electrochemical stability window of 5.38 V vs.Li⁺/Li were obtained for modified PEO SPE.The LiFePO₄//Li cells were tested at room temperature,it shows an average discharge capacity of 148.7 mAh g^-1 at 0.05 C within 100 cycles.Combining the polarization tests in symmetrical Li//Li cells,the SEM tests of LFP//Li cells after cycling with the EIS tests,both the stripping/plating behavior of Li⁺at interface and the changes of cathode/electrolyte interface resistance are well investigated in order to understand the origin of capacity fading.