Anisotropic Li intercalation in a Li(x)FePO4 nano-particle: a spectral smoothed boundary phase-field model

Anisotropic crystallite size distributions in LiFePO4 powders

The anisotropic crystallite sizes in high-performance LiFePO4 powders were measured by XRD and compared with the particle sizes found by TEM image analysis. Lognormal particle size distribution functions were determined for all three main crystallographic axes. A procedure was developed to determine the fraction of the composite particles which consists of several crystallites and contains small- and large-angle boundaries. In a sample with the most anisotropic crystallites (ratio of volume-weighted mean crystallite sizes L̄ V[001]/L̄ V[010] = 1.41) the number of the composite particles was at least 30%.

Kinetic Monte Carlo Study of Li Intercalation in LiFePO4

Even as a commercial cathode material, LiFePO₄ remains of tremendous research interest for understanding Li intercalation dynamics. The partially lithiated material spontaneously separates into Li-poor and Li-rich phases at equilibrium. Phase segregation is a surprising property of LiFePO₄ given its high measured rate capability. Previous theoretical studies, aiming to describe Li intercalation in LiFePO₄, include both atomic-scale density functional theory (DFT) calculations of static Li distributions and entire-particle-scale phase field models, based upon empirical parameters, studying the dynamics of the phase separation. Little effort has been made to bridge the gap between these two scales. In this work, DFT calculations are used to fit a cluster expansion for the basis of kinetic Monte Carlo calculations, which enables long time scale simulations with accurate atomic interactions. This atomistic model shows how the phases evolve in Li_xFePO₄ without parameters from experiments. Our simulations reveal that an ordered Li_0.5FePO4 phase with alternating Li-rich and Li-poor planes along the ac direction forms between the LiFePO₄ and FePO₄ phases, which is consistent with recent X-ray diffraction experiments showing peaks associated with an intermediate-Li phase. The calculations also help to explain a recent puzzling experiment showing that LiFePO₄ particles with high aspect ratios that are narrower along the [100] direction, perpendicular to the [010] Li diffusion channels, actually have better rate capabilities. Our calculations show that lateral surfaces parallel to the Li diffusion channels, as well as other preexisting sites that bind Li weakly, are important for phase nucleation and rapid cycling performance.

Design of a Porous Cathode for Ultrahigh Performance of a Li-ion Battery: An Overlooked Pore Distribution

Typical cathode materials of Li-ion battery suffer from a severe loss in specific capacity, and this problem is regarded as a major obstacle in the expansion of newer applications. To overcome this, porous cathodes are being extensively utilized. However, although it seems that the porosity in the cathode would be a panacea for high performance of LIBs, there is a blind point in the cathode consisting of porous structures, which makes the porous design to be a redundant. Here, we report the importance of designing the porosity of a cathode in obtaining ultrahigh performance with the porous design or a degraded performance even with increase of porosity. Numerical simulations show that the cathode with 40% porosity has 98% reduction in the loss of specific capacity when compared to the simple spherical cathode when the C-rate increases from 2.5 to 80 C. In addition, the loss over total cycles decreases from 30% to only about 1% for the cathode with 40% porosity under 40 C. Interestingly, however, the specific capacity could be decreased even with the increase in porosity unless the pores were evenly distributed in the cathode. The present analysis provides an important insight into the design of ultrahigh performance cathodes.

Probing the pseudo-1-D ion diffusion in lithium titanium niobate anode for Li-ion battery

Comprehensive understanding of the charge transport mechanism in the intrinsic structure of an electrode material is essential in accounting for its electrochemical performance.