Ionic Conductivity And Electrochemical Performance Of K₂Fe₄O₇ Solid Electrolyte

In recent years,with the depletion of resources such as oil and natural gas,renewable energy such as solar and wind energy has become the focus of attention,besides our country is in desperate need of energy storage equipment.Therefore,it is very necessary to develop a fast charge and discharge battery that can be used over a wide temperature range and has high safety and high energy density.Now most of the commercial ion batteries are using liquid electrolyte and gel electrolyte containing carbonate organic solvents.These electrolytes have high ionic conductivity at room temperature,they can effectively infiltrate electrode particles,and form stable solid electrolyte films on the surface of electrode materials.However the ionic conductivity decreased significantly at a low temperature if the liquid-solid conversion of the organic liquid electrolyte occurs.When the external or the internal temperature originating from high current charging,discharging or short circuit of the battery is rising,there will be a violent chemical reaction between the electrolyte and the electrode.This will aggravates rise of the internal temperature,generates a certain gas,and finally causes the expansion of the battery in volume and seal failure.What's more,the combustible gas and organic solvent encounter can cause an explosion when they encounter the oxygen.Comparing with the liquid electrolyte,solid electrolyte has significant advantages,for example:?1?Non-volatile,non-flammable;?2?Maintains chemical stability in a wide temperature range;?3?maintain good chemical stability in the air and water for a long time;?4?some solid electrolytes have a wide electrochemical window;?5?have high strength and hardness.Therefore,the solid-state batteries with solid electrolyte can have excellent safety characteristics,cycle characteristics,high energy density and low cost,if a suitable solid electrolyte was synthesized.Now,a large number of solid electrolytes have been reported.For instance,extensive and in-depth research by scientists including chemistry,physics and materials,such as Li-garnet solid electrolyte,NASICON solid electrolyte et al.However,the ionic conductivity of these solid electrolytes(10^-4-10^-55 Scm^-1)at room temperature are much lower than the commercial liquid electrolyte(10^-22 S cm^-1),which seriously affects the performance of batteries and limit the development of the battery.In recent years,due to its low cost and abundant reserves,potassium-ion batteries have attracted much attention.However,there are few reports on solid-state potassium ion electrolyte materials,which makes potassium ion batteries unable to develop as well as lithium-ion batteries and sodium-ion batteries.K₂Fe₄O₇ has a 3 D open-channel containing a large amount of mobile potassium ions,it should be a potential solid electrolyte material,according to the existing related theory.This paper aims to synthesize the large single crystals of K₂Fe₄O₇ by hydrothermal method and study the ionic conductivity and the electrochemical performance.We have carried out a series of studies about synthesis of the large single crystal of K₂Fe₄O₇,the improvement of the density of the ceramic K₂Fe₄O₇,the increase of the ionic conductivity,electronic conductivity and the electrochemical properties.We also studied the capacity,rate performance and the cycle stability of the all-solid potassium battery based on K₂Fe₄O₇.The detailed research content are as follows:In the first chapter,we introduced the history of solid electrolyte and described the development of the solid electrolyte.Several common types of electrolyte and solid electrode are described,we introduced the importance of solid electrolyte for the improvement of the all-solid-state battery's properties and assemblyIn the second chapter,we synthesize K₂Fe₄O₇ single crystal of different size under different conditions of alkali,temperature and reaction time by hydrothermal synthesis.The synthesis can only obtain K₂Fe₄O₇ under the condition of KOH as the mineralizer.We can obtain the best K₂Fe₄O₇ crystal which has the shape of uniform hexagonal pieces and there are no twin crystals on the surface.The K₂Fe₄O₇ was synthesized under the conditions:Fe?NO₃?₃?4.0 g?and KOH?72 g?were dissolved in distilled water?32 ml?and then heated at 240?for 48 h.We test the ionic conductivity of the K₂Fe₄O₇ single crystal along different channel,according to the open channel of K₂Fe₄O₇.As a result,it proves that the K₂Fe₄O₇ is a kind solid electrolyte which has high ionic conductivity and low electronic conductivity.In the third chapter,K₂Fe₄O₇ powder is pressed into wafers and sintered at high temperature to obtain compact ceramic electrolyte sheets.We studied the effects of binder,sintering temperature and sintering time on the density of electrode ceramic sheets.We can obtain the ceramic sheets with density of 80%,with the follow steps:the K₂Fe₄O₇ with 5%PVA was calcined at 600?for 5 h and then increase the temperature to 750?,and keep the temperature for 24 h.We conducted the AC impedance on samples with different density,concluding that the ionic conductivity increasing with the increasing of density.The ionic conductivity of the sample with the density of 80%is only 6.8×10^-22 S cm^-1.In the fourth chapter,we assembled the all-solid-state battery with the polycrystalline ceramic K₂Fe₄O₇ as electrolyte,the metal potassium as anode and a potassium prussian blue as cathode,respectively.First,we tested the electrochemical stability of K₂Fe₄O₇ by cyclic voltammetry,and the electrochemical window voltage can reach 5 V.At a 1 C rate,the specific capacity of the solid state battery is 75.6mAh/g,which is 87%of the theoretical specific capacity.After 50 cycles of testing,its capacity can still maintain 78%of the initial capacity.When at a rate of 10 C,the battery was also able to maintain 76%of the initial specific capacity after 50 cycles of testing.Thus,the K₂Fe₄O₇ shows an excellent cycle performance.Even under the high rate of 250 C,the battery structure still maintained well.These confirmed that the K₂Fe₄O₇ is a good solid electrolyte material and with the ability of rapid charge and discharge when used in the all-solid battery.We used the Prussian white?K₂FeFe?CN?₆?material which has a relatively high theoretical specific capacity to assemble an all-solid potassium ion battery K/K₂Fe₄O₇/K₂FeFe?CN?₆ according to the previous section,and tested the specific capacity,rate performance and cycle stability of the battery.It shows that although the specific capacity of the battery has improved,the cycle stability of the battery is lower than that of the solid state battery using potassium Prussian blue as the cathode material.The possible cause of this result is that there are bound water and Fe?CN?₆defects in the synthetic Prussian white,which causes the crystal lattice of Prussian white to be distorted during charge and discharge,and the ability to embed or extract potassium ions decreases,As a result,the circle stability performance of the battery deteriorates.