The Effects of Boron Doping in a Garnet-like Solid Electrolyte

Abstract

Li-ion batteries have been widely used as a power source in many areas such as IT mobiles, xEV, and ESS. However, the safety problem of the Li-ion battery became critical issue ranging from fire accident in Boeing 787 airplane to some electric vehicles. For the reason, cell manufacturers adopt ceramic coated separators or organic/inorganic additives in liquid electrolyte to improve safety problems. One of possible route is to substitute conventional organic liquid electrolyte with non-flammable solid state electrolyte. If the solid electrolytes are applied in a battery system, it might satisfy to accomplish below the EUCAR Hazard level L1., and energy densities will be most likely double when the Li-metal is used as an anode materials(theoretical capacity ~4,000 mAh/g). In addition, the amounts of Al pouch can be decreased by skipping a degassing process in a lithium ion pouch cell manufacturing. Solid electrolytes consist of three areas: sulfides, oxides and polymers. Although sulfides show the high ionic conductivities and easy cell fabrication, it requires high cost equipment to prevent reaction with moisture or air. However, although oxides based electrolytes show a critical disadvantage, which is low ionic conductivity, they are currently widely studied because of their stability in air and easy preparation. Therefore, oxide-based electrolytes could be promising one for the commercial products. Since Professor Weppner group reported the garnet-like LLZO solid electrolyte in 2007, which shows the relatively high ionic conductivity(~ 10-4 S/cm) and wide stability window as well as no reaction with Li-metal, the LLZO solid electrolyte has been widely studied by a lot of researchers. Although the cubic structured-LLZO which shows high ionic conductivities can be easily synthesized by doping elements such as Nb, Ta, and Al, it is difficult to fabricate all-solid-state batteries due to high sintering temperatures. For the fabrication of all-solid-state batteries, the co-sintering process of the solid electrolyte with cathode materials is required. However, the co-sintering process at high temperatures could give rise to serious troubles such as Li-volatilization of cathodes and reactions between cathodes and solid electrolytes. For example, if the LLZO solid electrolyte performs a co-sintering process with LCO cathode materials at over 1,000oC, reactions between cathode and electrolytes can be easily occurred. Thus, the sintering temperatures should be kept under 900 oC. In order to maintain cubic structure of the LLZO and decrease sintering temperature, appropriate sintering-agents are essential. In this research, boron materials as a sintering agent were used to decrease sintering temperature of LLZO. Because boron materials can increase particle size of cathode materials or prevent to increase residual lithium, boron materials can be applied effectively in fabrication of all solid state batteries. Cubic structured-LLZO was kept at over 1,000 oC by Ta doping and show high ionic conductivities(~10-4 S/cm). Therefore, Ta-doped LLZO materials used as a starting material. In addition, pellet densities were evaluated to determine total ionic conductivities with various B doping amounts. By using B dopants as a sintering agent, the possibility of all-solid-state batteries by co-sintering processes will be increased.