A wide variety of studies are being conducted on lithium-ion batteries with high capacity, high energy density, and safety as more medium- and large-sized batteries are used in mobile devices, electric vehicles (EV), and energy storage devices. Curren...
A wide variety of studies are being conducted on lithium-ion batteries with high capacity, high energy density, and safety as more medium- and large-sized batteries are used in mobile devices, electric vehicles (EV), and energy storage devices. Currently, lithium-ion batteries mainly use liquid electrolytes. These liquid electrolytes serve as the medium for moving lithium ions between the cathode and anode with a 10-2S/cm ionic conductivity, and they affect the reaction rate of batteries. However, these liquid electrolytes contain flammable organic solvents, and raise safety issues due to leaks and explosions. So, their application to emerging technologies, including electric vehicles and smart grids, is extremely limited. Therefore, research is being performed on Perovskite-type, NASICON-type, Garnet-type, LISICON-type, and Sulfide-type inorganic solid electrolytes with excellent electrochemical and thermal safety at room temperature to resolve these safety issues. Among them, NASICON-type solid electrolytes are promising materials because they are environmentally friendly, stable in the air, have low raw material costs, and exhibit high ionic conductivity. NASICON-type, Garnet-type, LISICON-type, and Sulfide-type inorganic solid electrolytes with excellent electrochemical and thermal safety at room temperature to resolve these safety issues. Among them, NASICON-type solid electrolytes are promising materials because they are environmentally friendly, stable in the air, have low raw material costs, and exhibit high ionic conductivity.
LiTi2(PO4)3(LTP), a NASICON-type solid electrolyte, has a rhombohedral crystal structure and forms a three-dimensional network in which octahedral TiO6 share their corners with tetrahedral PO4. Although this material has a high grain ionic conductivity of 10-4S/cm, , the resistance is very high due to the grain boundary. As a result, the total ionic conductivity is very low of 10-8S/cm ~10 -6 S/cm at room temperature.
To solve this problem, this study substituted trivalent elements in the Ti4+site to improve the densification and ionic conductivity of the solid electrolytes. These solid electrolytes have been mainly synthesized by solid-state reaction and melt-quenching methods due to simple manufacturing and mass production, but these methods are time-consuming (12h or more) and require high temperatures (1,200℃ or higher). This leads to problems, such as high energy, impurities, different particle sizes, and lithium volatilization. Therefore, this study fabricated solid electrolytes using the sol-gel methode, which is advantageous for low-temperature synthesis of multi-component systems due to benefits such as efficiency and lowering the possibility of lithium volatilization.
This study synthesized NASICON-type Li1+XMXTi2-X(PO4)3(M=Al,Ga,Fe) (x=0.1,0.3,0.4) solid electrolytes using the sol-gel method. Also, the Ti4+site(0.61Å)was partially substituted with Ga3+(0.62Å) and Fe3+(0.64Å), which are similar to ionic radius and Al3+(0.53Å) with a small ionic radius, to examine the effect on ionic conductivity according to the effect on additives, optimization of manufacturing process conditions, and the densification of the sintered body. The synthesized powder and sintered body were analyzed by TG-DTA, XRD, XPS, and FE-SEM, and the ion conduction properties as solid electrolytes were evaluated using AC impedance. The results are as follows.
- The sol-gel methode was applied to fabricate the precursor of the NASICON-type Li1+XMXTi2-X(PO4)3(M=Al, Ga, Fe) (x=0.1, 0.3, 0.4), and it was heat-treated at 450℃ to obtain the amorphous precursor. The powder was sintered at various temperatures (800℃~100℃) to synthesize solid electrolytes with a rhombohedral crystal structure.
- The solid electrolytes were obtained by partially substituting trivalent elements (Al3+,Ga3+,Fe3+)with different ionic radii in the Ti4+site. The solid electrolytes were formed a secondary phase by the addition of above above 0.5 of Al3+,and above 0.4 of Ga3+and Fe3+as the x content of the additive increased regardless of the sintering temperature.
- All of the sintered solid electrolytes turned into micron-sized cubic-shaped particles, and the particles tended to grow and densify as the x-content of the additive and the sintering temperature increased. However, at a sintering temperature of 1,000℃, grain coarsening occurred due to rapid grain growth, resulting in low densification caused by the multiple grain boundaries.
- The ionic conductivity properties of the fabricated solid electrolytes were evaluated using AC impedance. The results showed that the ionic conductivity improved due to densification as the x-content of the additive and the sintering temperature increased. The overall ionic conductivity of each solid electrolyte measured by AC impedance at room temperature is as follows. The LATP(x=0.3), LGTP(x=0.3), and LFTP(x=0.3) sintered at 900℃, 800℃, and 1000℃, showed high ionic conductivities of 3.36×10-4S/cm, 8.27×10-5S/cm and 1.51×10-4S/cm, respectively.