In Situ Atom Probe Deintercalation of Lithium-Manganese-Oxide 319
extend to the specimen apex region (not shown). As the diffusion coefficient of Li in LMO can vary up to several orders of magnitude depending on the Li content (Bach et al., 1998; Yang et al., 1999; Ouyang et al., 2004), Li diffusion is expected to be strongly localized in the inhomogeneous microstructure. However, there is no direct experimental evidence to differentiate whether Li transport predominantly occurs in one of the observed most likely metastable Li-depleted phases, or along the interface of different phases. As in Figure 5, the Li-rich and Li-depleted regions are
Figure 6. Ratio of Li to Mn (complex ions split) perpendicular to the alternating regions of different Li density for the part of the reconstruction marked in Figure 5a.
After a transition region, from 120 to 160nm with an increased slope, the ratio of Li to Mn exhibits no dependence
on the reconstruction depth and appears constant with some fluctuations. The ratio of Li to Mn perpendicular to the alternating
regions of different Li density, for the part of the recon- struction marked in Figure 5a, is displayed in Figure 6. The ratio is alternating between three different ranges of values, with relatively sharp transitions between them. A ratio between 0.45 and 0.5 is typically found for not or just slightly deintercalated LxMO specimens (cf. Maier et al., 2016). The other ratios from 0.35 to 0.3 and from 0.15 to 0.1 do not fitto the equilibrium phases of LMO reported in Ohzuku et al. (1990). However, they are similar to those reported for the two nonequilibrium phases observed in chemical lithiation experiments of λ-MnO2 (Li et al., 2000). As similar compo- sitions were observed for all deintercalated specimens, this indicates that the in situ atom probe deintercalation yields an early nonequilibrium stage of deintercalated LxMO. Amicroscopic model for the deintercalation mechanism
can be developed by relating the two regions in Figure 5a to the two stages of deintercalation introduced above. The first part in Figure 5, up to a depth of 120 nm, can be associated with the homogeneous field evaporation behavior at stage 2. The spatially and temporally homogeneous field evaporation and the absence of specific structures in the reconstruction, indicate temporally and spatially homogeneous volume diffusion of Li as deintercalation mechanism. Furthermore, comparing the Li content in the first 120 nm, of Figure 5 with the inhomogeneous microstructure behind, shows that the amount of missing Li approximately equals the amount of ions detected at 19.3amu (cf. Fig. 3). On the other hand the inhomogeneous microstructure
in Figure 5a most likely is related to the inhomogeneous field evaporation behavior of stage 1. This interpretation is
supported by deintercalation experiments stopped at stage 1, where the inhomogeneous microstructure was observed to
often nearly parallel and usually oriented such that their surface normal is not perpendicular to the specimens axis of symmetry. Thus, preferential Li transport along the Li-depleted regions causes an azimuthally inhomogeneous Li distribution, in accordance with the observation that Li is typically detected on one side of the detector during stage 1 (cf. Fig. 4). In this process, Li-ions do not necessarily have to be field evaporated once they reach the specimen surface. It is also conceivable that first surface diffusion takes place and localized field evaporation occurs at specific sites. Hence, the spatially and temporally inhomogeneous
in situ deintercalation at stage 1 is expected to be correlated to the microstructural evolution and phase transition in the LxMO. Nevertheless, also during stage 1 the deintercalation mechanism of stage 2 is expected to be active, although at very low rates.
CONCLUSION
In this work, the influence of Li-ion mobility in LiMn2O4 on atom probe analysis of the material was investigated. It was demonstrated that for conventional atom probe analysis at 30 K, Li-ion mobility is suppressed below the detection limit, so that the materials microstructure can be characterized reliably. At 298 K, a substantial increase in mobility can be achieved, enabling in situ deintercalation of the material in the atom probe without affecting the Mn–O host structure significantly. Two different modes of deintercalation have been observed: spatially and temporally inhomogeneous deintercalation at high rates at the beginning, followed by homogeneous deintercalation at low rates. Characterization by conventional atom probe analysis revealed a complex nonequilibrium microstructure, with alternating Li-rich and Li-depleted regions subsequent to deintercalation. A model for the deintercalation mechanism based on homogeneous volume transport and localized transport in the Li-depleted regions was proposed. The methodological approach presented here allows
in situ preparation and subsequent characterization of non- equilibrium microstructures in an ionic conductor. This way, insights into the microstructural evolution accompanying deintercalation processes can be obtained. Particularly for nanowire Li-ion batteries, new insights into the mechanisms related to electrochemical energy conversion are expected, that are currently not accessible with other in situ techniques (Lee et al., 2013, 2015).
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