1211.0027 (Michael Heß et al.)

Shrinking annuli model and stage-dependent rate capability of thin-layer
graphite electrodes for lithium-ion batteries    [PDF]

Michael Heß, Petr Novák

The kinetic performance of graphite particles is difficult to deconvolute from half-cell experiments where the influence of both, working electrode porosity and the counter electrode contribute nonlinearly to the electrochemical response. Therefore, thin-layer electrodes of a 1 \mu m 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 LiC6 in-plane density are the main rate limitation for charging and also for 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 be-tween the diffusion coefficients of the liquid-like stages com-pared to the dense stages. We also demonstrate that graphite can be charged with a rate limited to circa 6C (10 min) but dis-charged at 600C (6 s) with regard to 80 % total specific charge for particles of 3.3 \mu m median diameter. Based on these findings, we propose a shrinking annuli model in which the different stages propagate into the particle at medium and high rates. This model can explain a long time known but unexplained asymmetry between the charge and discharge rate performance of lithium intercalation in graphite.

View original: http://arxiv.org/abs/1211.0027