To aid the development of cathode materials for use in Li-ion batteries, for the first time, LiCo 1−x Zr x O 2 single crystals were successfully grown via the traveling solvent floating zone (TSFZ) technique. The crystals grown using 0.50 at. % Zr-doped feeds were single crystals with very small (Co, Zr)O x inclusions (approximately 6 μm in size), whereas large size inclusions segregated in the crystals grown using the feed of 1.00 at. % Zr. The Zr concentrations of the LiCoO 2 phase in both crystals were determined to be 0.26 ± 0.01 and 0.26 ± 0.05 at. %, respectively, along the growth direction, which is almost identical to each other but lower than the actual Zr concentration in feeds (0.50 and 1.00 at. % Zr). The ionic conductivities of LiCo 0.9974 Zr 0.0026 O 2 single crystals along [100] and [001] were 8.3 × 10 −6 and 4.0 × 10 −8 S• cm −1 , respectively, which are approximately twice as large as those of LiCoO 2 (σ [100] = 4.0 × 10 −6 , σ [001] = 2.4 × 10 −8 S•cm −1 ). The anisotropy of ionic conductivity in LiCo 0.9974 Zr 0.0026 O 2 was found to be 207. The results obtained in this study have the potential to improve the energy density and safety of Li-ion batteries.

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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.