Type of presentation: Poster

MS-14-P-2600 Single-crystallization of LiMn0.4Fe0.6PO4 nanowires via oriented attachments

Kikkawa J.1, Hosono E.2, Okubo M.2, Kagesawa K.2, Zhou H.2, Nagai T.1, Kimoto K.1
1National Institute for Materials Science, 2National Institute of Advanced Industrial Science and Technology
KIKKAWA.Jun@nims.go.jp

Electrospinning, i.e., ejecting a jet of a polymer solution from the tip of a needle using an electric field, is a versatile method for obtaining nanowires (NWs) of various materials [1]. Nevertheless, few attempts to date have successfully fabricated single-crystalline NWs. Recently, advanced single-crystalline NWs of an olivine-structured LiMn0.4Fe0.6PO4 covered with amorphous carbon shells were fabricated using electrospinning [2]. Carbon-coated LiMn0.4Fe0.6PO4 NWs are attractive electrode materials for a high-rate lithium ion battery. However, the single-crystalline NW formation mechanism related to electrospinning remains unclear. For this study, we performed transmission electron microscopy (TEM) to investigate the formation mechanism of single-crystalline NWs from electrospun NWs by heating.

TEM observations were performed using an electron microscope (HF-3000S; Hitachi Ltd.) operated at 300 kV. Results showed that the dried NWs were amorphous (Fig. 1(a)). After heating at 600 °C for 30 min in ambient Ar, crystallized NWs were observed (Fig. 1(b)). NWs were covered with amorphous carbon shells. The selected-area electron diffraction (SAED) pattern (Fig. 1(b)) revealed single-crystalline characteristics of the olivine structure for the NW core, although the outlines of grains are discernible. We also observed polycrystalline NWs composed of crystallographically-orientated and coalesced grains, which indicates the oriented attachment is a key mechanism for single-crystallization [3]. After heating at 800 °C for 10 h in ambient Ar, amorphous gaps left between grains in the NW core (presented in Fig. 1(b)) disappeared, forming a complete crystal core. SAED revealed that single crystallization occurred almost completely beyond the range of approximately 7 μm. We think self-forming amorphous carbon shells play a key role in confining grains and maintaining NW geometry to achieve single-crystalline NW. Details of the single-crystallization mechanism is discussed.

References

[1] E. Hosono, Y. Wang, N. Kida, M. Enomoto, N. Kojima, M. Okubo, H. Matsuda, Y. Saito, T. Kudo, I. Honma, H. Zhou, ACS Appl. Mater. Interfaces, 2, 212 (2010).

[2] K. Kagesawa, E. Hosono, M. Okubo, J. Kikkawa, D. Nishio-Hamane, T. Kudo, and H. Zhou, CrystEngComm, 15, 6638 (2013).

[3] J. Kikkawa, E. Hosono, M. Okubo, K. Kagesawa, H. Zhou, T. Nagai, and K. Kimoto, J. Phys. Chem. C  (in press).

This work was partly supported by "Nanotechnology Platform"(project No. A-13-NM-0060) of MEXT, Japan.

Fig. 1: TEM images and the inset SAED patterns for dried NW after electrospinning, (a) and NW after heating at 600 °C for 30 min, (b).