Mesoscopic Phase Transition Kinetics in Secondary Particles of Electrode-Active Materials in Lithium-Ion Batteries

Abstract

Many compounds used as battery storage electrodes undergo large composition changes during use that are accompanied by a first-order phase transition. Most studies of these phase transitions have focused on the unit cell to single-crystallite scale, whereas real battery electrodes are typically composed of mesoscopic assemblies of nanocrystallites, for which phase transformation mechanisms are poorly understood. In this work, a systematic study is conducted of the potentiostatic (constant driving force) kinetics of phase transition in secondary particles of representative intercalation compounds: LiFePO 4 , LiMn 1−x Fe x PO 4 , and Li 4 Ti 5 O 7 . Storage kinetics are studied as a function of overpotential, material composition, primary particle size, and temperature. We find that in regimes where phase transformation occurs, the results can be self-consistently explained as nucleation and growth kinetics within the framework of the Johnson−Mehl−Avrami−Kolmogorov model. This implies that despite the common secondary particle topology, the electrochemically driven phase transformations occur by nucleation and growth with little apparent resistance to phase propagation across the grain boundaries. Growth appears to be one-dimensional in nature, consistent with a hybrid growth model in which rapid surface propagation is followed by slower growth into particles.

Dynamics of Lithium Insertion in Electrochromic Titanium Dioxide Nanocrystal Ensembles

“…Phasefield simulations have been applied to LiFePO 4 and TiO 2 particles with excellent agreement to experiment. 53,123,124 However, the effects of surface energy on Li-insertion reactions has received less attention, 123 and the dependence of surface energy on Li concentration in TiO 2 is poorly understood. Square prism particles of various sizes were tested with similar morphology to the nanoplatelets shown in Figure 1 (i.e.…”

“…The JMAK model, described in detail in the Supporting Information, has been effectively used to model transformation kinetics of nanocrystalline electrodes that undergo two-phase Li-insertion, such as LiFePO4. 54,124,131 The model accounts for both nucleation and growth rates, and is described by an adjusted exponential:…”

Strategy to control magnetic coercivity by elucidating crystallization pathway-dependent microstructural evolution of magnetite mesocrystals

“…It is hypothesized that a diffusion-controlled process (c = 0.5) determines the growth rate when primary crystallites approach each other. Here, a is deduced to be 1.3-1.4, indicating the accelerated and autocatalytic nucleation rate 35 . This is in good agreement with the TEM results as the Fe 3 O 4 primary crystallites formed from lepidocrocite gradually increase over time (Fig.…”

“…Synthesis of Fe 3 O 4 mesocrystals. We use a modified polyol method that has been widely employed to synthesize metal/metal oxide mesocrystals with a controlled diameter in a wide range 7,35 . This method is effective for investigating the prenucleation attachment model of Fe 3 O 4 mesocrystal formation and explaining changes in the geometric and microstructural sizes.…”

“…It is expressed as f ¼ 1 À exp½À kt ð Þ n , where f is the volume fraction of the transformed volume, t is reflux time, k is a constant, and n is the Avrami exponent. The Avrami exponent is written as n ¼ a þ b c ð Þ, where a is the time-dependent nucleation rate (a > 0), b is the dimensionality of the grown phase (0 < b < 3), and c is the growth rate (c = 0.5 or 1) [33][34][35] . Figure 4a shows the timeline of the reaction and the JMAK model.…”