Preparation And Electrochemical Properties Of Magnesium Potassium Titanate Solid State Electrolyte

Potassium-ion batteries have emerged as promising candidates for large-scale energy storage due to the high earth abundance,low cast and environmental friendliness.However,the issues brought by using liquid electrolyte such as flammability,explosion,and volume expansion,still appear in K-ion batteries.In order to solve these problems,this thesis aimed to synthesize a magnesium potassium titanate solid K-ion super ionic conductor（KMTO）with high potassium-ion conductivity and environmental stability as a solid state electrolyte for K-ion batteries,and the physical,chemical properties and electrochemical performance of KMTO solid state electrolyte were studied.Furthermore,comprehensive research about the charging and discharging performance of the solid-state potassium-ion battery assembled with KMTO solid electrolyte,the improvement measure of the stability of the electrodes/electrolyte interface and the failure mechanism of the solid-state battery are also studied.It was hoped that the solid state electrolyte can be used to replace the liquid electrolyte,so as to effectively avoid safety risks and increase the energy density of K-ion battery.Firstly,magnesium potassium titanate solid state electrolyte（KMTO）was prepared by a ball milling and high temperature sinter method.By adjusting the sintering temperature,sintering time and pressing pressure,a KMTO solid state electrolyte pellet with uniform particle size,a flake microstructure and a density of about 90%was obtained.The X-ray diffraction refinement analysis suggests KMTO was crystallized in lepidocrocite structure type.And the chemical stability of KMTO pellet was also studied.The results showed that KMTO can be stored stably for 30 days in a dry environment at room temperature,and can also remain stable under extreme humidity.Secondly,the electrochemical performance of the KMTO electrolyte was studied by combining theoretical calculation and experimental test.The results showed that the path along the c-axis direction in the KMTO layered structure is the main migration channel for K⁺.The ionic conductivity of the KMTO electrolyte pellet with a density of about 90%is 1.14×10^-5 S·cm^-1 at room temperature,which is lower than the theoretically calculated value of 2.35×10^-4 S·cm^-1at room temperature.And the electrochemical stability window was 4.77 V,which can meet the working voltage requirements of almost all the cathode materials of potassium-ion batteries.In addition,The assembled K/KMTO/K_0.5Mn O₂ and K/KMTO/Bi OCl solid-state batteries exhibited a discharge capacities of 57and 218.6 m Ah·g^-1 with a current density of 100 m A·g^-1 after 50 cycles,which were nearly close to to the batteries with liquid electrolyte.Moreover,the assembled K/KMTO/K symmetric cell can operate up to 330 h without short-circuit under 0.05 m A·cm^-2 and 0.1 m A·cm^-2,respectively.However,the poor interfacial contact between electrodes and KMTO solid state electrolyte promoted dendrite growth,which is the main cause of battery failure.Lastly,the PEO coating layer was modified on the surface of KMTO solid state electrolyte pellet through solution casting method.The PEO interfacial layer not only enhance the contact between KMTO electrolyte and electrodes,but also resist the dendrite growth to reduce the interface impedance.Therefore,the assembled K/PEO-KMTO/K_0.5Mn O₂ cell exhibited higher capacity and cycle stability than the unmodified K/KMTO/K_0.5Mn O₂ cell.Additionally,the assembled K/PEO-KMTO/K symmetric cell can stably cycle about 450 h with a decreased polarization voltage,which shows that the interface stability has been improved.