Direct view on the phase evolution in individual LiFePO4 nanoparticles during Li-ion battery cycling

Abstract

Phase transitions in Li-ion electrode materials during (dis)charge are decisive for battery performance, limiting high-rate capabilities and playing a crucial role in the cycle life of Li-ion batteries. However, the difficulty to probe the phase nucleation and growth in individual grains is hindering fundamental understanding and progress. Here we use synchrotron microbeam diffraction to disclose the cycling rate-dependent phase transition mechanism within individual particles of LiFePO4, a key Li-ion electrode material. At low (dis)charge rates well-defined nanometer thin plate-shaped domains co-exist and transform much slower and concurrent as compared with the commonly assumed mosaic transformation mechanism. As the (dis)charge rate increases phase boundaries become diffuse speeding up the transformation rates of individual grains. Direct observation of the transformation of individual grains reveals that local current densities significantly differ from what has previously been assumed, giving new insights in the working of Li-ion battery electrodes and their potential improvements.

Origin and hysteresis of lithium compositional spatiodynamics within battery primary particles

“…Recent studies using in situ X-ray diffraction observed a continuous distribution of lattice constants at high rates of (de)lithiation (6)(7)(8)(9). This finding supports the hypothesis that phase separation is suppressed during (de)lithiation and replaced with a solid solution crystallographic insertion pathway (10), consistent with theoretical predictions (11,12).…”

“…Precise quantification of the Li composition (X) is difficult because the lattice constant change convolves information from both Li composition and mechanical strain (8). Whereas heterogeneous current distributions between particles have been studied (7, 13, 14), there exists little understanding of how compositional nonuniformities evolve within individual particles.…”

“…Precise quantification of the Li composition (X) is difficult because the lattice constant change convolves information from both Li composition and mechanical strain (8). Whereas heterogeneous current distributions between particles have been studied (7, 13, 14), there exists little understanding of how compositional nonuniformities evolve within individual particles.Diffuse interfaces have been proposed from diffraction patterns (7), but it is unclear where they occur or how they develop over time. Even less understood is the effect of interfacial reactivity on the single-particle lithiation pathway, which has been explored using models (11, 15) but not probed experimentally.…”

“…Diffuse interfaces have been proposed from diffraction patterns (7), but it is unclear where they occur or how they develop over time. Even less understood is the effect of interfacial reactivity on the single-particle lithiation pathway, which has been explored using models (11,15) but not probed experimentally.…”

Comprehensive analysis of TEM methods for LiFePO4/FePO4 phase mapping: spectroscopic techniques (EFTEM, STEM-EELS) and STEM diffraction techniques (ACOM-TEM)

“…Afterwards, we could confirm many individual nanoparticles with uniform single-crystal like orientation exhibiting both, LFP and FP phases, in agreement with published experimental observations [4,5,14,16] and theoretical modellings [25,26], especially a recent in-situ X-ray study confirmed the coexistence of both phases (LFP+FP) in individual single crystalline particles [27]. Some of the particles in figure 1a containing both phases are shown in detail in figure 2.…”