Syntheses And Characterization Of Solid Electrolyte And Cathode Materials For All-Solid-State Lithium-Ion Batteries

Lithium-ion batteries are widely used in portable power equipment, electric vehicles, energy storage power plants and other large areas because of their high energy density, better cycle performance and environmental friendiness. However, the liquid organic electrolyte of traditional lithium-ion batteries may suffer from issues like leakage, fire or explosion. Moreover, the electrochemical window and the operating temperature range of organic electrolyte are narrow, and the organic electrolyte is easy to decompose. In recent years, many researchers pay more attention on the solid-state lithium batteries because they have the longer cycle life, higher safety performance. Although the solid-state lithium-ion batteries have many advantages, there are still some difficulties to prepare the all-solid cells. The key issues are the fabrication of solid electrolyte materials with high lithium ion conductivity and the interface integration between the solid electrolytes and the electrode materials. Hence many researchers focus on how to prepare solid electrolytes with high lithium ionic conductivities and how to match them with electrodes. This thesis mainly studies the preparations and modification of two kinds of solid electrolyte and one electrode material.In Chapter 1, the author briefly introduces the research progress of the traditional lithium-ion batteries and the all-solid-state lithium-ion batteries. This chapter also gives an introduction of several types of electrolytes and some electrode materials. The importance of the solid electrolyte for solid-state ion batteries is summarized in detail.In Chapter 2, the authors gives a brief introduction of the experimental reagents, equipment used in this thesis. This chapter also describes the methods used in materials characterization, the assembly of the coin cell and the testing methods.In Chapter 3, the author first uses the thermal polymerization method to prepare a uniform Li₇La₃Zr₂O₁₂ precursor powder. Compared with the solid state reaction method, a thermal polymerization method enables the product powder with more uniform particle size distribution. It can also reduce the sintering temperature and shorten the sintering time. We finally obtain the pure cubic garnet phase Li7La3Zr2O₁₂ by sintering for 12 hours at a temperature of 1150℃. The lithium ion conductivity can reach 1.73×10-4 S cm-1 at room temperature.Chapter 4. the author attempts to improve the lithium ion conductivity of )O₄ by doping Mg²⁺, Al3+, Ca2+, Sr2* and Ba2+ via a asolid state reaction method. .so discusses the effects of different amounts of Mg and Al. But for Ca, Sr. Ni and a as the doping elements, we fixed the doping level at 0.1. We finally obtain a Li_2.8NbMg_0.1O₄ sample with a relatively high lithium ion conductivity; it can reach 1.2×10-5 S cm-1 at 300℃ and 1.6×10-4S cm-1 at 500℃.In Chapter 5, the author synthesizes uniform powders of LiNi_0.5Co_0.2Mno_0.3O₂ by a thermal polymerization method in the temperature range from 750℃ to 950℃ in air and oxygen. It is found that the LiNio 5Co_0.2Mno0.O₂ powder sintered in oxygen has significantly decreased degree of cationic mixing. The sample sintered at 900℃ in oxygen has the highest ratio of I003/I104 and the highest initial discharge capacities of 200 mAh g-1 at a rate of 0.1 C in the voltage range of 2.8-4.5V. The capacity retention of the LiNi_0.5Co_0.2Mn0.3O₂ powder sintered in oxygen at 900℃ is 83.33% after 100 cycles.In Chapter 6, Zr⁴⁺ and Ti⁴⁺ are used to partially substitute the Mn⁴⁺ in LiNi_0.5Co_0.2Mno_0.3O₂ to improve the electrochemical properties. A series of Zr-doped LiNi_0.5Co_0.2Mn_0.3O₂ powders (LiNi_0.5Co_0.2Mn_0.3-xZr_xO₂, x=0.01,0.02,0.03,0.04,0.05) and (LiNi_0.5Co_0.2Mn_0.3-xTi_xO₂, x=0.01,0.025,0.05,0.1) are prepared. Both less amount of Zr⁴⁺ and Ti⁴⁺ doping can improve the electrochemical properties. Among all the doped samples, LiNi_0.5Co_0.2Mn_0.29Zr_0.01O₂ electrode exhibits the best electrochemical performance with a capacity retention of 93.92% after 100 cycles at 0.2C in the voltage range of 2.8-4.5 V and a capacity of 129 mAh g-1 at 10C rate. For the Ti-doped samples, LiNi_0.5Co_0.2Mn_0.29Ti_0.01O₂ has the best electrochemical performance with a capacity retention of 86.88% after 100 cycles at 0.5C rate and a capacity of 116 mAh g-1 at 10C rate.In Chapter 7, Mg²⁺, Fe³⁺and Cr³⁺ are used to partially substitute Mn in LiNi_0.5Co_0.2Mn_0.3O₂ to compare their electrochemical properties. All of the prepared samples have a hexagonal α-NaFeO₂ structure except for a small amount of second phase Li₂CrO₄ in LiNi_0.5Co_0.2Mn_0.3-xCr_xO₂（x=0.05,0.1） samples. A small amount of Mg and Fe substitutions can improve the cycling performance of LiNi_0.5Co_0.2Mn_0.3O₂. However, the Cr-doping is harmful for the electrochemical performance. Moreover, among all the doped samples, LiNi_0.5Co_0.2Mn_0.29Ti_0.01O₂ has the best electrochemical performance with a capacity retention of 85.30% after 100 cycles at 0.5C rate and a capacity of 112 mAh g-1 at 10C rate.In Chapter 8, the author gives a summary of the innovation and deficiencies of this thesis. Some prospects and suggestion for the future research are also pointed out.