Shrinking annuli mechanism and stage-dependent rate capability of thin-layer graphite electrodes for lithium-ion batteries

Abstract

The kinetic performance of graphite particles is difficult to deconvolute from half-cell experiments, where the influences of the working electrode porosity and the counter electrode contribute nonlinearly to the electrochemical response. Therefore, thin-layer electrodes of circa 1 µm thickness were prepared with standard, highly crystalline graphite particles to evaluate their rate capability. The performance was evaluated based on the different stage transitions. We found that the transitions towards the dense stages 1 and 2 with LiC 6 in-plane density are one of the main rate limitations for charge and discharge. But surprisingly, the transitions towards the dilute stages 2L, 3L, 4L, and 1L progress very fast and can even compensate for the initial diffusion limitations of the dense stage transitions during discharge. We show the existence of a substantial difference between the diffusion coefficients of the liquid-like stages and the dense stages. We also demonstrate that graphite can be charged at a rate of ~6C (10 min) and discharged at 600C (6 s) while maintaining 80 % of the total specific charge for particles of 3.3 µm median diameter. Based on these findings, we propose a shrinking annuli mechanism which describes the propagation of the different stages in the particle at medium and high rates.Besides the limited applicable overpotential during charge, this mechanism can explain the long-known but as yet unexplained asymmetry between the charge and discharge rate performance of lithium intercalation in graphite.

Examination of Graphite Particle Cracking as a Failure Mode in Lithium-Ion Batteries: A Model-Experimental Study

“…35,36 We confirmed this in our ideal electrode (19 μm) using the porous electrode model (see Appendix A). In order to determine the diffusion coefficient, key to accurately predicting the mechanical failure, we first examined the rate capability of the graphite.…”

“…This asymmetry in terms of graphite rate capabilities between lithiation and delithiation is consistent with the work reported by Heß and Novák. 36 The rate limitation on the lithiation process also supports the fact that rapid charging in practical lithium-ion batteries, using a graphite anode, is challenging because one needs to keep the potential above 0 V to prevent lithium plating.…”

Li Intercalation into Graphite: Direct Optical Imaging and Cahn–Hilliard Reaction Dynamics

“…In addition to Li transport within the graphite, we find that Li injection kinetics also affected the overall dynamics. We relate the Li injection flux to the electrochemical free energy change with a Computer simulation ( Figure 5 and a movie in supplemental) shows each intralayer region spontaneously separating into Li-rich and Li-poor phases, forming a "checkerboard" pattern, much like the schematic originally proposed by Daumas and Hérold 30 and supported by some modeling [31][32][33] and experimental [34][35][36] studies. The simulation closely matches the observed kinetics.…”

Pressure Characteristics and Chemical Potentials of Constrained LiFePO4/C6Cells

“…10,[25][26][27][28] The hexagonal crystal structure of the C 6 -bond expands its volume up to V /V 0 ≤ 12.8 % 29 during lithium intercalation. Thereby it passes from a solidsolution stage called 1L (see Figure 2b) through different liquid-like (L) stage phases 1L, 4L, 3L, 2L and the stage 2 with coexisting regimes between the phases finally to the solid-solution stage 1.…”