Research On High Performance Solid State Electrolyte And Its Application In All-solid-state Lithium Metal Batteries

The development of renewable energy becomes our society’s tendency.However,the stability of power grid is exposed to great pressure since most renewable energy is stochastic,intermittent,and fluctuant.Battery energy storage,which owns high-energy density,fast and precise response ability,has been supposed to be the development tendency for solving the issue of intermittently provides power of renewable energy and ensuring the stability and reliability of the power grid.Among the batteries with high-energy density,the solid-state electrolytes which applied to all-solid-state lithium metal batteries have better safety and higher specific capacity,when compared with the traditional organic liquid electrolytes.The Na-superionic conductor（NASICON）type solid-state electrolytes have high Li⁺conductivity,good chemical stability in the air,and low preparation cost.However,the Ti⁴⁺/Ge⁴⁺in commercialized Li_1.3Al_0.3Ti_1.7（PO₄）₃（LATP）and Li_1.5Al_0.5Ge_1.5（PO₄）₃（LAGP）solid state electrolyte will be reduced by lithium metal and the wettability between these electrolytes and electrodes is undesirable,which seriously impedes them applications.Therefore,this paper,on the one hand,replacing the Ti⁴⁺/Ge⁴⁺with more stable metal ions Zr⁴⁺/Hf⁴⁺,and using different valent ions doping to stabilize the material structure and improve the Li⁺conductivity.The NASICON-type electrolytes or（CF_x）_nas fillers,on the other hand,adding into the PEO-Li TFSI to prepare the composite solid electrolyte.The fillers can also increase the amorphous content of PEO and thus improve the Li⁺conductivity of the composite solid electrolyte.The results are summarized as follows:In Chapter 2,Li Zr₂（PO₄）₃was optimized by Y³⁺doping.It could retain the high Li⁺conductivity rhombohedral structure at room temperature.Y³⁺doping increases the free Li⁺concentration in Li_1.1Y_0.1Zr_1.9（PO₄）₃,leading to a high bulk Li⁺conductivity of 9×10^–5S cm^–1at room temperature,which is two orders of magnitude higher than Li Zr₂（PO₄）₃.A Li₃P layer is generated in situ at the interface between the electrolyte and lithium metal,which effectively improves the wettability.According,both the symmetric lithium battery and the all-solid-state lithium metal battery assembled with Li_1.1Y_0.1Zr_1.9（PO₄）₃electrolyte showed low voltage polarization and long cycle life.In Chapter 3,pure rhombohedral Li Hf₂（PO₄）₃can be synthesized by reduced sintering temperature.The Li⁺conductivity can be significantly improved by Sr²⁺doping.Li_1.4Sr_0.2Hf_1.8（PO₄）₃solid-state electrolyte synthesized by hot-pressing sintering has a Li⁺conductivity of 3.4×10^–5S cm^–1at room temperature.The electrolyte has been proved to have good chemical stability in the air,and the lithium metal battery with the electrolyte has a high discharge capacity and Coulombic efficiency at 50 ^oC,which can achieve a long-term stable charge and discharge cycle.In Chapter 4,Li_1.3Al_0.3Hf_1.7（PO₄）₃（LAHP）with a 7×10^–4S cm^–1bulk conductivity at room temperature was prepared and used as filler to prepare the composite solid electrolyte.The LAHP could not only improve the content of polymer amorphous phase,but also interact with PEO to improve the concentration of free-moving lithium ions.At 30 ^oC,the Li⁺conductivity of this composite electrolyte reaches 1.3×10^–4S cm^–1.The lithium metal battery employed the electrolyte had 82%capacity retention rate and 99%Coulombic efficiency after 200 cycles of charge and discharge.It is confirmed that increasing the mobile Li⁺concentration in the composite polymer electrolytes is a useful strategy to improve the total ionic conductivity of the polymer.In chapter 5,the PEO/Li_1.5Al_0.5Ge_1.5（PO₄）₃（LAGP）composite electrolyte was synthesized by introducing LAGP into PEO-Li TFSI.PEO/LAGP composite electrolyte has a Li⁺conductivity of 8×10^–5S cm^–1at room temperature,and its electro-chemical window exceeds 4.5 V.The lithium metal battery with PEO/LAGP composite solid electrolyte could realize charging and discharging cycle at room temperature.The interface resistance when applied a pressure of 1 MPa was reduced to 1/8 of that without pressure.After 5 days of heat preservation at 60 ^oC,the interface resistance at room temperature was reduced to 1/10 of the previous level.In chapter 6,（CF_x）_nfiller was added into PEO-Li TFSI.（CF_x）_neffectively reduced the crystallinity of PEO matrix and released more lithium ions that could move freely.The PEO/（CF_x）_ncomposite electrolyte had a Li⁺conductivity of 1×10^–4S cm^–1at 30 ^oC when 10 wt%（CF_x）_nfiller was added.The interface between PEO/（CF_x）_ncomposite solid electrolyte and lithium metal was stable.After cycled at 50μA cm^-2current density at 45 ^oC for 100 cycles,the lithium metal battery,use Li Fe PO₄as cathode which employed the electrolyte showed negligible capacity loss,and the Coulombic efficiency was maintained above 98%.Although graphite fluoride is an ionic insulator,it has a strong interaction with the polymer electrolytes,which significantly increases the mobile Li-ion concentration in the composite polymer electrolytes.