Consolidating the grain boundary of the garnet electrolyte LLZTO with Li3BO3 for high-performance LiNi0.8Co0.1Mn0.1O2/LiFePO4 hybrid solid batteries

Abstract

All solid-state batteries have received significant attention due to their excellent safety performance. As a key component, the garnet-type electrolyte is one of the best known electrolytes due to its air stability and good compatibility with metallic lithium. However, the total Li⁺ conductivity of this kind of electrolyte is usually lower than that of the bulk electrolyte primarily due to the grain boundary resistance. In this study, we focused on engineering the electrolyte Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) by introducing Li₃BO₃ (LBO) into it to form the electrolyte LLZTO/LBO with the aim to consolidate the grain boundary. Via characterization by both neutron and X-ray diffraction, the as-prepared LLZTO was indexed as a pure cubic phase, where Ta certainly substituted the Zr sites. LLZTO/LBO still maintained the cubic structure, and the B atoms did not occupy any cation sites in the unit cell. It was demonstrated that an amorphous phase of a boracic substance was trapped inside the cubic LLZTO phase. The amorphous boracic phase sutured the gaps among the LLZTO grains and then lowered the grain boundary resistance without introducing impurities, ultimately consolidating the solid-state electrolyte. Electrochemical impedance spectroscopy revealed that the total Li⁺ conductivity of LLZTO/LBO reached 5.47 × 10⁻⁴ S cm⁻¹, much higher than those of the as-prepared Li₇La₃Zr₂O₁₂ (LLZO) and LLZTO. Using LLZTO/LBO as an electrolyte, the LiNi_0.8Co_0.1Mn_0.1O₂/LiFePO₄ hybrid solid battery showed an excellent cycling performance with the reversible capacity of 147.8 mA h g⁻¹ at 0.2C for 100 cycles and the capacity retention of 93.8%. These results suggest that the consolidation of the grain boundary with LBO is a promising way to achieve an improved electrolyte, LLZO, with higher total Li⁺ conductivity.