Visualizing non-equilibrium lithiation of spinel oxide via in situ transmission electron microscopy

Abstract

Spinel transition metal oxides are important electrode materials for lithium-ion batteries, whose lithiation undergoes a two-step reaction, whereby intercalation and conversion occur in a sequential manner. These two reactions are known to have distinct reaction dynamics, but it is unclear how their kinetics affects the overall electrochemical response. Here we explore the lithiation of nanosized magnetite by employing a strain-sensitive, bright-field scanning transmission electron microscopy approach. This method allows direct, real-time, high-resolution visualization of how lithiation proceeds along specific reaction pathways. We find that the initial intercalation process follows a two-phase reaction sequence, whereas further lithiation leads to the coexistence of three distinct phases within single nanoparticles, which has not been previously reported to the best of our knowledge. We use phase-field theory to model and describe these non-equilibrium reaction pathways, and to directly correlate the observed phase evolution with the battery's discharge performance.

Atomic‐Scale Monitoring of Electrode Materials in Lithium‐Ion Batteries using In Situ Transmission Electron Microscopy

“…HRTEM) Reproduced with permission. [81] Copyright 2016, Nature Publishing Group. STEM) Reproduced with permission.…”

“…For example, strainsensitive bright-field (BF) STEM imaging has confirmed that a two-step reaction mechanism occurs with Fe 3 O 4 nanoparticles and those three distinct phases coexist within the individual nanoparticles. [81] Because the product of the first step of the intercalation reaction is an intermediate phase, the main capacity contribution is from the second displacement reaction. The two-step reaction is also called a conversion reaction.…”

“…Further lithiation is also accompanied by the extrusion of ultrafine Fe nanoparticles, which leads to the formation of a composite with amorphous Li 2 O (shown in bright contrast). [81,89] Thus, it is clear that the intercalation and conversion reactions both occur in the shrinking-core mode and proceed from the outer surface to the inner region. [79] As an approximate measure of the reaction kinetics, the projected areas of the three phases are shown as functions of lithiation time in Figure 5b.…”

“…The intercalation process occurs through a two-phase mode, as expressed in Equation (4) Figure 6b,d. [81] The Li intercalation process overlaps with the subsequent conversion reaction within individual nanoparticles, and as a consequence, the discharge profile is not distinct. This observation also explains the mechanism of metallic Fe extrusion during the entire lithiation process.…”

“…[89] Thus, these findings elucidate the ion occupancy at the atomic level and reveal how the rate-dependent kinetics can influence the reaction pathway. [81] Huang et al developed a nanoscale electrochemical cell for imaging a single SnO 2 nanowire electrode inside a TEM system. They observed that a reaction front containing a high density of dislocations propagates along the nanowire upon charging, and that this front serves as a structural precursor to electrochemically driven solid-state amorphization for the lithiation reaction, causing the nanowire to swell, elongate, and spiral, resulting in a volume change of 250%.…”