Daniel A. Cogswell

Suppression of Phase Separation in LiFePO4 Nanoparticles During Battery Discharge

Using a novel electrochemical phase-field model, we question the common belief that Li(X)FePO(4) nanoparticles always separate into Li-rich and Li-poor phases during battery discharge. For small currents, spinodal decomposition or nucleation leads to moving phase boundaries. Above a critical current density (in the Tafel regime), the spinodal disappears, and particles fill homogeneously, which may explain the superior rate capability and long cycle life of nano-LiFePO(4) cathodes.

Suppression of Phase Separation in LiFePO 4 Nanoparticles During Battery Discharge

Using a novel electrochemical phase-field model, we question the common belief that Li(X)FePO(4) nanoparticles always separate into Li-rich and Li-poor phases during battery discharge. For small currents, spinodal decomposition or nucleation leads to moving phase boundaries. Above a critical current density (in the Tafel regime), the spinodal disappears, and particles fill homogeneously, which may explain the superior rate capability and long cycle life of nano-LiFePO(4) cathodes.

Coherency strain and the kinetics of phase separation in LiFePO4 nanoparticles.

A theoretical investigation of the effects of elastic coherency strain on the thermodynamics, kinetics, and morphology of intercalation in single LiFePO(4) nanoparticles yields new insights into this important battery material. Anisotropic elastic stiffness and misfit strains lead to the unexpected prediction that low-energy phase boundaries occur along {101} planes, while conflicting reports of phase boundary orientations are resolved by a partial loss of coherency in the [001] direction. Elastic relaxation near surfaces leads to the formation of a striped morphology with a characteristic length scale predicted by the model, yielding an estimate of the interfacial energy. The effects of coherency strain on solubility and galvanostatic discharge are studied with a reaction-limited phase-field model that quantitatively captures the influence of misfit strain, particle size, and temperature on solubility seen in experiments. Coherency strain strongly suppresses phase separation during discharge, which enhances rate capability and extends cycle life. The effects of elevated temperature and the feasibility of nucleation are considered in the context of multiparticle cathodes.

Current-induced transition from particle-by-particle to concurrent intercalation in phase-separating battery electrodes

Many battery electrodes contain ensembles of nanoparticles that phase-separate on (de)intercalation. In such electrodes, the fraction of actively intercalating particles directly impacts cycle life: a vanishing population concentrates the current in a small number of particles, leading to current hotspots. Reports of the active particle population in the phase-separating electrode lithium iron phosphate (LiFePO4; LFP) vary widely, ranging from near 0% (particle-by-particle) to 100% (concurrent intercalation). Using synchrotron-based X-ray microscopy, we probed the individual state-of-charge for over 3,000 LFP particles. We observed that the active population depends strongly on the cycling current, exhibiting particle-by-particle-like behaviour at low rates and increasingly concurrent behaviour at high rates, consistent with our phase-field porous electrode simulations. Contrary to intuition, the current density, or current per active internal surface area, is nearly invariant with the global electrode cycling rate. Rather, the electrode accommodates higher current by increasing the active particle population. This behaviour results from thermodynamic transformation barriers in LFP, and such a phenomenon probably extends to other phase-separating battery materials. We propose that modifying the transformation barrier and exchange current density can increase the active population and thus the current homogeneity. This could introduce new paradigms to enhance the cycle life of phase-separating battery electrodes.

Origin and hysteresis of lithium compositional spatiodynamics within battery primary particles

Watching batteries fail Rechargeable batteries lose capacity in part because of physical changes in the electrodes caused by electrochemical cycling. Lim et al. track the reaction dynamics of an electrode material, LiFePO4, by measuring the relative concentrations of Fe(II) and Fe(III) in it by means of high-resolution x-ray absorption spectrometry (see the Perspective by Schougaard). The exchange current density is then mapped for Li+ insertion and removal. At fast cycling rates, solid solutions form as Li+ is removed and inserted. However, at slow cycling rates, nanoscale phase separation occurs within battery particles, which eventually shortens battery life. Science, this issue p. 566; see also p. 543 X-ray microscopy shows the nanoscale evolution of the composition and reaction rate inside a Li-ion battery during cycling. The kinetics and uniformity of ion insertion reactions at the solid-liquid interface govern the rate capability and lifetime, respectively, of electrochemical devices such as Li-ion batteries. Using an operando x-ray microscopy platform that maps the dynamics of the Li composition and insertion rate in LixFePO4, we found that nanoscale spatial variations in rate and in composition control the lithiation pathway at the subparticle length scale. Specifically, spatial variations in the insertion rate constant lead to the formation of nonuniform domains, and the composition dependence of the rate constant amplifies nonuniformities during delithiation but suppresses them during lithiation, and moreover stabilizes the solid solution during lithiation. This coupling of lithium composition and surface reaction rates controls the kinetics and uniformity during electrochemical ion insertion.

Using Scanning Transmission X-ray Microscopy to Reveal the Origin of Lithium Compositional Spatiodynamics in Battery Materials

1. Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA. 2. Stanford Institute for Materials & Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA, USA 3. Hummingbird Scientific, Lacey, WA, USA. 4. Department of Aeronautics and Astronautics, Stanford University, Stanford, CA, USA. 5. Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA 6. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, USA 7. Department of Mathematics, Massachusetts Institute of Technology, Cambridge, MA, USA. 8. SUNCAT Interfacial Science and Catalysis, Stanford University, Stanford, CA, USA

Three Dimensional Analysis of High Volume Fraction Coarsened Dendritic Microstructures

Dendritic microstructures are very often present after the solidification of cast alloys. This makes their study of technological importance, since many properties of the cast materials are related to the dendritic structure. A coarsening or Ostwald ripening process is what determines the dendritic morphology. During the coarsening process, the mushy zone, namely a two-phased region consisting of the dendritic (solid) phase and the matrix (liquid) phase, evolves over time.