Classical electrodes for Li-ion technology operate by either single-phase or two-phase Li insertion/de-insertion processes, with single-phase mechanisms presenting some intrinsic advantages with respect to various storage applications. We report the feasibility to drive the well-established two-phase room-temperature insertion process in LiFePO4 electrodes into a single-phase one by modifying the material's particle size and ion ordering. Electrodes made of LiFePO4 nanoparticles (40 nm) formed by a low-temperature precipitation process exhibit sloping voltage charge/discharge curves, characteristic of a single-phase behaviour. The presence of defects and cation vacancies, as deduced by chemical/physical analytical techniques, is crucial in accounting for our results. Whereas the interdependency of particle size, composition and structure complicate the theorists' attempts to model phase stability in nanoscale materials, it provides new opportunities for chemists and electrochemists because numerous electrode materials could exhibit a similar behaviour at the nanoscale once their syntheses have been correctly worked out.
The intriguingly fast electrochemical response of the insulating LiFePO4 insertion electrode toward Li
is of both fundamental and practical importance. Here we present a comprehensive study of its deinsertion/insertion mechanism by high-resolution electron energy loss spectroscopy on thin platelet-type particles
of Li
x
FePO4 (b
Pnma
axis normal to the surface). We find that the lithium deinsertion/insertion process is
not well-described by the classical shrinking core model. Compositions of the same x value obtained by
both deinsertion and insertion gave the same results, namely that the Li
x
FePO4 so formed consists of a
core of FePO4 surrounded by a shell of LiFePO4 with respective ratios dependent on x. We suggest that
lattice mismatch between the two end members may be at the origin of the peculiar microstructure observed.
Furthermore, because of the appearance of isosbestic points on the overlaid EELS spectra, we provide
direct experimental evidence that the nanometer interface between single-phase areas composed of LiFePO4
or FePO4 is the juxtaposition of the two end members and not a solid solution. One future prospect of
such knowledge is to determine strategies on how to control, on a large scale, the synthesis of nanometer-sized thin platelet-type particles to prepare high-rate LiFePO4 electrodes for future energy storage devices.
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