In Situ Atom Probe Deintercalation of Lithium-Manganese-Oxide 315
32e positions, Li at tetrahedral 8a, and Mn at octahedral 16d sites (Shimakawa et al., 1997; Rodrìguez-Carvajal et al., 1998; Huang et al., 2011). Deintercalating LixMn2O4 in the range 0.5<x<1, leads to a homogeneous single phase material with lattice parameter decreasing to ~8.15Å for x=0.5 (Ohzuku et al., 1990; Kanamura et al., 1996). At x=0.5, Li ordering in the cubic spinel structure takes place (Ohzuku et al., 1990; Kucza, 1999; Zhang et al., 2015). For 0<x<0.5 two phases are formed: a Li-rich phase with x=0.5 and a cubic spinel-related phase of MnO2 with small solubility of Li. After deintercalation of LiMn2O4 all resulting phases exhibit the space group Fd3m (Ohzuku et al., 1990). In Li et al. (2000), the deintercalated system is addi-
values in the range from 10−11 to 10−8cm2/s are reported with qualitatively different and even contradictory depen- dencies on the Li content x. Yang et al. (1999) showed a decreasing diffusion coefficient with increasing x, whereas Bach et al. (1998) measured a maximum for the diffusion coefficient at x=0.55. Two maxima at x=0.3 and 0.7 for DLi(x), and similarly for the conductivity are reported by Ouyang et al. (2004). The reason for these differences are not clear. Although Bach et al. (1998) discuss a possible influence of the preparation method of LiMn2O4 on the Li mobility, a detailed study on the influences of defects and the micro- structure on the mobility of Li in LiMn2O4 is still missing. For specimen preparation lithium-manganese(III,IV)-
tionally observed in a nonequilibrium state, with two-phase regions at around 0.25<x<0.55 and 0.60<x<0.95. After several days, equilibrium is reached and the phases reported in Ohzuku et al. (1990) are observed. As structural infor- mation on LixMn2O4 was mostly obtained using X-ray diffraction, it represents averages over relatively large sample volumes and gives only indirect information on the local distribution of the occurring phases in the material. For the diffusion coefficient DLi(x)ofLiinLixMn2O4
Figure 2. a: Logarithmically plotted normalized detection rate for an atom probe analysis at a constant base voltage of 5.8kV and a laser pulse energy of 27.5 nJ. The automatic voltage adjustment of the preceding conventional atom probe analysis was stopped at t = 0. The maximum detection rate before normalization was 3.5 million/hour. b: Content of Li, Mn, or O for the detected ions displayed in (a) plotted with dotted lines. The solid lines show the content after subtraction of a constant offset of 4,500 detected O atoms/hour to consider O supply from the residual gas.
EXPERIMENTAL
oxide (LiMn2O4) from Sigma Aldrich with a particle size <5μmis used (cf. Supplementary Fig. 1). The facetted particles
are expected to be monocrystalline and transmission electron microscopic (TEM) diffraction analysis after specimen preparation reveals a single crystallinity for the specimens analyzed in this work. Selected area diffraction (SAD) patterns of the specimen apex regions fit to the expected cubic spinel structure, with a lattice parameter of around 8.24Å (cf. Supplementary Fig. 2).
Specimen preparation was carried out using an FEI Nova NanoLab 600 focused ion beam (FIB; FEI, Eindhoven, The Netherlands) with integrated scanning electron microscope. Lift out of the LiMn2O4 particles and the following connection to a tungsten support tip was done with a micromanipulator and deposition of standard platinum precursor trimethyl (methyl- cyclopentadienyl)platinum(IV). The specimens were sharpened by side cuts of the Ga-ion beam at 30keV beam energy to a full shank angle of around 12° or by annular milling (Larson et al., 1999). The last step in preparation, was irradiation of the specimen in axial direction with the ion beam at 5keV energy to reduce the volume implanted with Ga (Miller et al., 2007). This way, apex radii of as-prepared specimens between 10 and 30nm were achieved. The morphology and crystal structure of the LMO specimens before and after atom probe analysis was investigated with a Philips CM12 or CM30 TEM (Philips, Eindhoven, The Netherlands) using a single tilt holder. Laser-assisted atom probe analysis was performed with
Figure 1. Logarithmically plotted normalized mass spectrum of an atom probe analysis of lithium-manganese-oxide at 30 K. Only major peaks are labeled. The spectrum is truncated at 210amu as no further peaks are observed for higher mass-to-charge ratios.
an in house constructed system described in Maier et al. (2016). The specimen base temperature between 30 and 298K was controlled by a combination of closed cycle He cryostat and resistive heating. The background pressure was 6×10−8 Pa at 298K and 1×10−8 Pa at 30 K, respectively. The laser, with a pulse duration of 15ps, a focus diameter of
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