Li-ion batteries with thick electrodes could aid in the development of low-cost energy storage solutions because of the battery’s reduced inactive cell components. However, such thick electrodes offer high resistance to ion transport through the solution phase of the electrode. Particularly graphite, which is a promising electrode material, has high resistance to ion transport though the thickness, but the electrode material offers less resistance in the direction parallel to the graphite sheets [Ebner et al., Adv. Energy Mater., 4, 1301278 (2014)]. The anisotropic nature of graphite material is exploited to provide alternate pathways for the Li-ions to flow through thus decreasing the effective tortuosity of the electrode [Bae et al., Adv. Mat., 25, 1254 (2013)]. For this, we propose a bi-tortuous electrode structure where electrolyte-rich macro-pores are embedded at regular intervals in the micro-porous electrode. We have modelled a full-cell battery (LiCoO2 cathode/graphite anode) using a modified two-dimensional version of the porous-electrode theory that includes the anisotropic nature of the ion transport through the electrode.

Figure 1(a) represents the material distribution in a bi-tortuous electrode structure. In this analysis the areal loading of active material was held constant among all structures considered. This constraint would therefore increase the loading of the active material in the reactive electrode domain (thus reducing the porosity) with increase in the size of the macro-pore. The macro-pores of width g are regularly placed at a distance s. The important geometrical parameters that effect the performance are (1) the macro pore coverage (ν_mp=g/s) and (2) the spacing to thickness ratio (s/w). The impact these two parameters have on the capacity of the cell is extensively studied to analyze the underlying mechanism for the ion transport and some design guidelines are proposed for ensuring optimal performance of the structured electrodes.

Our simulation results showed that closely spaced macro-pores (low s/w) provide considerable enhancement to the battery’s cycling capacity at 0.5C rate. Charge storage capacity almost doubles when macro-pore coverage is about 20% spaced at intervals equivalent to half the thickness of the electrode (s/w=0.5). For low s/w values, capacity increases with increase in macro-pore coverage until about 20%, then declines because porosity becomes extremely small, which inhibits ion transport through the dense electrode. Fig.1 (b) delineates the effects of macro-pore design on the capacity, showing an optimum macro-pore coverage for each s/w. This could be used as a guide for designing the bi-tortuous electrodes.

The charge-discharge voltage curves are shown in Fig. 1(c) for three cases: (i) traditional homogeneous electrode, (ii) a bi-tortuous electrode with s/w=2.0, and (iii) a bi-tortuous electrode with s/w=0.5 and ν_mp=20%. Intercalated Li fraction thumbnails are also shown at five different strategic points during the charge-discharge process and the ion current pathways are embedded on two thumbnails to give a qualitative understanding of the underlying mechanism. Case (iii) is an example of a properly designed macro pore where the intercalated-Li is more uniformly distributed through the thickness of the electrode. The ion current plots indicate that the ion flux is dense at the entry of the macro-pore and ions prefer transport in the direction parallel to the graphite sheets in the anode (grahite) region. This establishes our hypothesis of providing an alternate less tortuous pathway for fast ion transport.

To establish the benefits of having the bi-tortuous structure, multiple simulations were performed at various C-rates, for different electrode thicknesses and average loading of the active material. In all cases the presence of macro-pores provided significant improvement in ion transport. This establishes in principle that a properly designed bi-tortuous structure provides capacity enhancement through enhanced ion transport. Macro-pores, which are spaced at high frequency (low s/w), give the maximum benefit, but one has to evaluate the trade-off between how low s/w values can be manufactured against the performance benefit.

Keywords: tortuosity, anisotropy, ion transport, structured electrodes