Research On Modification Of PEO Electrolyte And Interface Engineering For All-Solid Lithium-Sulfur Battery

In the past two decades,lithium-ion batteries have been widely used in mobile electronic device,electric vehicles,and smart grids with their excellent performance,and have promoted the progress of our society and daily life.However,due to the limited energy density of conventional lithium-ion batteries,the development of new battery system is necessary.The theoretical specific energy of lithium-sulfur battery can reach 2600 Wh kg^-1,which shows a highest energy density in lithium batteries.At present,its commercial application is limited by the problems such as the combustion,leakage of the ether electrolyte,and the shuttle effect caused by the dissolution of the cell reaction intermediates.All-solid-state lithium-sulfur battery design using solid electrolytes instead of liquid electrolytes has been proved as a very effective way to solve these problems.Poly（ethylene oxide）（PEO）-based solid polymer electrolyte not only guarantees high specific energy and safety of the cell,but also exhibits the advantages of high flexibility and simple packaging.The current research on PEO-based all-solid-state lithium-sulfur batteries still faces some challenges.Among them,the low conductivity of the electrolyte and the irreversible shuttle effect are the main factors.Ether-oxygen bonds of PEO can interact with Li⁺to promote its motion,and the polymer is usually in a semi-crystalline state in which amorphous segment facilitates the migration of ions.However,its ionic conductivity is still only 10^-7-10^-6 S cm^-1 at room temperature,so this dissertation focuses on increasing the ionic conductivity of PEO-based polymer electrolytes and suppressing the shuttle effect.We try different strategies to increase the amorphous phase in PEO and increase the ionic conductivity of the polymer electrolyte.In addition,the lithium polysulfide partially dissolves in PEO,resulting in low Coulombic efficiency and utilization of sulfur.Therefore,we suppress the shuttle effect by interface modification.The microstructure and electrolyte interface are also optimized through composite porous fillers,molecular brush grafting,introduction of inorganic solid electrolyte.Characterization and analyses were carried out in this work:（1）Improving the electrochemical performance of PEO electrolyte through nanoporous adsorption effect:A CPE was obtained by adding a nanoporous molecular sieve SSZ-13 with different charges on the surface to PEO.The strong adsorption and interaction of the SSZ-13 with anions and cations in lithium salts effectively promote the dissociation of lithium salt.The ionic conductivity of the PEO electrolyte reachs 1.96×10^-3 S cm^-1at70℃,and 4.43×10^-5 S cm^-1 at 20℃.The electrochemical performances of assembled symmetrical lithium metal batteries,lithium ion batteries,and lithium sulfur batteries were tested.Lithium-ion battery shows a specific capacity of 155 m Ah g^-1 after 160cycles at 60°C and 0.1C,the capacity remains 920 m Ah g^-1 after 40 cycles in lithium-sulfur batteries,which are superior in the current researches.（2）Molecular brushes on the surface of LLZTO to achieve high-performance all-solid-state Li-S batteries and exploration of the mechanism:Li_6.5La₃Zr_1.5Ta_0.5O₁₂（LLZTO）electrolyte particles were grafted with imidazole-based organic segments to obtain LLZTO modified with molecular brush structure and compounded with PEO electrolyte.The ionic conductivity and mechanical properties of the composite electrolyte have been greatly improved(4.5×10^-4 S cm^-1 at 40℃).The lithium ion transport mechanism at the MB-LLZTO interface was explored by ^6,7Li solid-state NMR test,and the rapid migration of ions at the inorganic-organic electrolyte interface was verified.The cycle performance of Li-S batteries was explored.The discharge capacity of all solid-state Li-S batteries reaches 1010 m Ah g^-1 after 50 cycles.It suppresses the dissolution of lithium polysulfide,thereby suppressing the shuttle effect,confirming the positive effect of this molecular brush structure on the research of all-solid-state Li-S batteries.（3）Construct interface of all-solid-state Li-S batteries by the electronic and ionic conductor:Although the all-solid-state Li-S battery based on Li_1.5Al_0.5Ge_1.5（PO₄）₃（LAGP）electrolyte can eliminate the shuttle effect,it is limited by poor solid-solid contact at the interface and side reactions between the electrolyte and lithium metal.This dissertation proposes a novel and effective modification method to build a high-performance all-solid-state Li-S battery.We introduce a conductive graphite layer at the LAGP positive interface and a polymer buffer layer at the CPE negative interface.The interface impedance is obviously reduced,meanwhile,the introduction of a redox reaction site and a second current collector enables the sulfur active material to be fully utilized,and at the same time effectively reduces the reduction reaction between LAGP and lithium metal.The all-solid-state Li-S battery based on this structure shows good cycle stability with a specific capacity of 1080 m Ah g^-1 after 150 cycles and a Coulombic efficiency reaching 100%.This modification strategy is also effective for the enlarged Li-S batteries.（4）Structural design of honeycomb porous LAGP/LLZTO electrolyte and research on solid-state batteries:Because a dense inorganic all-solid-state electrolyte cannot effectively solve the slow kinetic problem of solid-solid interface between the electrode and the electrolyte,and the lack of a lithium ion conductor in the positive electrode also limits the active material load.In this disserttion,an array of LLZTO/LAGP porous ceramic is prepared using the ice template method.A porous ceramic skeleton with a honeycomb structure is introduced as a lithium ion conductor inside the positive electrode,and a dense electrolyte layer is obtained through impregnation and sintering to the honeycomb porous-dense electrolyte.The precursor slurry impregnated with C on the porous side was further carbonized to obtain an integrated battery structure.Rich reaction sites around the honeycomb ceramic skeleton holes is obtained,and a lithium metal battery with high current density and reactivity is realzied.