Abstract
The layered structure of LiCoO2 implies anisotropic ionic conduction; however, experimental data have never demonstrated this. The anisotropy can be observed clearly in epitaxial films with controlled orientations. Our previous study had reported that LiCoO2 grows epitaxially on Nb-doped SrTiO3 (100) and (110) substrates with complete (104) and (018) orientations, respectively. On the other hand, the growth on SrTiO3 (111) substrates with (001) orientation was accompanied by the inclusion of (012)-oriented domains, although the (012) orientation is higher in the energy state than the (001). The present study reveals that lower laser energy density (fluence) and lower substrate temperature decrease the amount of inclusions; that is, the occurrence of the (012) orientation in spite of its higher energy is governed by these factors. Higher fluence leading to higher deposition rates does not provide sufficient time for the cations to be rearranged into the (001) orientation, and the higher substrate temperature increases the nucleation frequency for the (012) orientation. A micrograph of the final (001)-oriented film reveals that the LiCoO2 film grows in an island growth mode.
Highlights
Layered rock-salt type LiCoO2 is the most important and most widely studied positive electrode material for Li-ion batteries
Iriyama et al reported relatively higher Li diffusivity in a LiCoO2 thin film with random orientation, than in a preferentially (001)-oriented thin film prepared by pulsed laser deposition (PLD).[1]
Our previous study revealed that LiCoO2 films grow epitaxially on Nb-doped SrTiO3 (100) and (110) substrates with (104) and (018) orientations, respectively, without other concomitant orientations.[6,7,8]
Summary
Layered rock-salt type LiCoO2 is the most important and most widely studied positive electrode material for Li-ion batteries. Iriyama et al reported relatively higher Li diffusivity in a LiCoO2 thin film with random orientation, than in a preferentially (001)-oriented thin film prepared by pulsed laser deposition (PLD).[1] Bouwman et al reported an obvious crystallographic dependence for the Li diffusivity in LiCoO2, where the Li diffusivity in a (110)-oriented LiCoO2 film was several orders of magnitude higher than that in a (001)-oriented film.[2] Xia et al reported diffusivity in a (104)-oriented film, which was one order of magnitude higher than that in a (001)-oriented film.[3] In addition, Xie et al reported that the Li diffusion coefficient of a LiCoO2 thin film with a preferred (104) orientation was larger than that of a LiCoO2 thin film with a preferred (001) orientation.[4] On the other hand, Bates et al reported comparable electrochemical properties for the (001), (101), and (104) orientations in films prepared by sputtering.[5] That is, previous studies showed inconsistencies in the influence of crystallographic orientation on the electrochemical properties This may have arisen from the differences in the incompleteness in orientation or film quality among the different films.
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