Fluid-enhanced surface diffusion controls intraparticle phase transformations

Abstract

Phase transformations driven by compositional change require mass flux across a phase boundary. In some anisotropic solids, however, the phase boundary moves along a non-conductive crystallographic direction. One such material is LiFePO, an electrode for lithium-ion batteries. With poor bulk ionic transport along the direction of phase separation, it is unclear how lithium migrates during phase transformations. Here, we show that lithium migrates along the solid/liquid interface without leaving the particle, whereby charge carriers do not cross the double layer. X-ray diffraction and microscopy experiments as well as ab initio molecular dynamics simulations show that organic solvent and water molecules promote this surface ion diffusion, effectively rendering LiFePO a three-dimensional lithium-ion conductor. Phase-field simulations capture the effects of surface diffusion on phase transformation. Lowering surface diffusivity is crucial towards supressing phase separation. This work establishes fluid-enhanced surface diffusion as a key dial for tuning phase transformation in anisotropic solids.

Toward High‐Performance Hybrid Zn‐Based Batteries via Deeply Understanding Their Mechanism and Using Electrolyte Additive

“…As the simulation proceeds, the Li + coordinates to water molecules, and then the water molecules lift the Li + ion from its original position, allowing it to migrate into the bulk of electrolyte. Thus, water molecules are involved into the Li + ion migration process rather than acting as a simple adsorbent . When the additive was added into electrolyte, the LFP surface adsorbs the tail groups of the surfactants molecules (Figure d), which helps to reduce the wetting free energy.…”

Size-dependent phase morphologies in LiFePO4 battery particles

“…Another neglected effect is surface diffusion, recently identified as an important phenomenon in LiFePO 4 [21], which counters the effects of driven reactions on thermodynamic stability [30]. We previously acknowledged that something like surface diffusion must occur by noting that stripes could only form during chemical delithiation if a particle was somehow able to exchange lithium with itself, but not with neighboring particles [24].…”

“…Such a dramatic reversal of fortune has drawn attention to this system as a model for studying the role of phaseseparation in electrochemical systems, and spurred the development of sophisticated imaging techniques designed to image the phase state of single crystals. Observations of the morphology include stripes [4][5][6][7], lithiated cores [8][9][10][11][12][13][14][15], delithiated cores [16], both lithiated and delithiated cores [17], complex nonequilibrium morphologies [18][19][20][21], and mosaic patterns of lithiated and delithiated particles (i.e. only single-phase particles) [22,23].…”

Interphases in Electroactive Suspension Systems: Where Chemistry Meets Mesoscale Physics

“…It was soon demonstrated that decreasing the particle size had a more significant impact on the electrode resistance than introducing a carbon coating . Interfacial energy, surface energy, coherency strains, free energy of mixing, solubility limits of lithium, lithium transport in bulk and on the surface, and lithium insertion kinetics were shown to impact the miscibility gap; determining whether solid solution or phase separation mechanism take place, and induce complex multi‐particle effects like the domino cascade effect . Given the importance of surface transport mechanisms in phase changes, SEI and its evolving nature along cycling could also play a role.…”

“… Schematic showing a selection of generalized aspects that introduce complexity in batteries with reference to a) active material and b) SEI on the surface of active material. Feedback loops between categories shown here are difficult to visually demonstrate but are intended implicitly …”