TSFZ Growth and Anisotropic Ionic Conductivity of Mg-Doped LiCoO₂ Single Crystals

Abstract

We have successfully grown LiCo1–x Mg x O2 (x = 0.01 ∼ 0.04) single crystals using the traveling solvent floating zone (TSFZ) technique for the first time. Inclusion-free single crystals were obtained from 1.0 atom % Mg-doped feeds, whereas (Co, Mg)O x inclusions segregated in the crystals were grown using feeds more than 2.0–4.0 atom % Mg. The Mg concentration of the LiCoO2 phase in the crystal grown with 1.0 atom % Mg-doped feed was determined to be 0.87 ± 0.02 atom % Mg, which suggests that the distribution coefficient of Mg into LiCoO2 is slightly lower than unity. However, the Mg concentration in the LiCoO2 phase was saturated toward about 2.1 atom % due to the solubility limit of Mg into LiCoO2, even though the Mg concentration in feeds increased up to 4.0 atom % Mg. The room temperature ionic conductivities of 1.0 atom % Mg-doped LiCoO2 single crystals along [100] and [001] were 7.2 × 10–5 and 1.1 × 10–8 S·cm–1, respectively, indicating significantly high anisotropy of about 6545. The ionic conductivity along [100] is about 18 times higher than that of undoped LiCoO2 single crystals, whereas the ionic conductivity along [001] is slightly lower than that of undoped LiCoO2 crystals. Mg doping into LiCoO2 is significantly effective in the enhancement of ionic conductivity along [100] rather than along [001]. Thus, high ionic conductivity and proper orientation of crystallographic planes may assist in increasing capacity as well as upper cutoff voltage in the next-generation solid-state Li-ion batteries.

Recent Progress of Floating-Zone Techniques for Bulk Single-Crystal Growth

“…Since the successful growth of silicon in the middle of the last century [27,28], the floating-zone technique has been widely applied to various categories of materials: oxides [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45], chalcogenides [46][47][48][49], silicides [50][51][52][53][54][55], borides [56][57][58][59][60][61][62][63], carbides [64][65][66][67][68][69][70], and intermetallic alloys [71][72][73][74][75][76]…”