Bi-Tortuous Anisotropic Graphite Electrodes for Fast Ion Transport in Li-Ion Batteries

The amount of cost and weight contributed by inactive cell components in Li-ion batteries decreases with electrode thickness, and, therefore, thick electrodes are desirable in many energy-storage applications. However such thick electrodes are limited by the transport of ions in the electrolyte and this effect leads to underutilization of charge/discharge capacity when cycled at substantial rates.  Previous work¹has shown that homogeneous graphite electrodes possess higher thru-plane tortuosity than the in-plane value because of alignment of graphite particles with the current collector.  Here, we propose a bi-tortuous structured electrode (containing electrolyte-rich macro-pores embedded in micro-porous graphite) which provides a path of least resistance for Li-ion transport in the electrolyte that by-passes transport through the thickness of a porous, graphite electrode.  This enhancement in ion transport enables substantial improvement in charge/discharge capacity over conventional graphite electrodes that have microporosity alone.

In the present work,² a full-cell battery (LiCoO₂ cathode/ graphite anode) was simulated using a new two-dimensional version of porous-electrode theory that accounts for the anisotropic nature of ion transport in the electrolyte phase of the electrode.  A systematic study was conducted to determine design guidelines for such electrodes. The material distribution and topology of the electrodes is shown in Fig. 1a.  Here, the average porosity and loading (volume fraction) of active material were held constant among all cases considered.  The macro-pores of width g are regularly spaced at a distance s.  The principal parameters that affect the performance of a given design are (1) macro-pore coverage (defined as ν_mp=g/s) and (2) the ratio of spacing-to-thickness (s/w). The variation of discharge capacity with respect to these parameters is shown in Fig. 1b.  When macro-pores are frequently spaced along the electrode (low s/w) discharge capacity is greatest. For s/w=0.5, capacity nearly doubles that of a conventional electrode when macro-pore coverage is approximately 20%. For a 200-micron thick electrode, these conditions translate to 20-micron wide macro-pores spaced at 100 microns, revealing the fine level of microstructural control necessary to produce an enhancement in capacity with macro-pores.  For electrodes with low s/w an optimum macro-pore coverage of approximately 20% maximizes discharge capacity (Fig. 1b). When a macro-pore (of certain coverage level) is introduced the constraints on average porosity and loading cause porosity to reduce and loading to increase locally inside the micro-porous region of the electrode.  When macro-pore coverage increases further, micro-porosity becomes extremely small.  This reduction in micro-porosity decreases effective ionic conductivity, and capacity declines as a result.

Figure 1c shows the voltage curves of full-cells with three types of graphite anodes: (i) a homogeneous anode, (ii) a bi-tortuous anode with s/w=2.0 and ν_mp=20%, and (iii) a bi-tortuous anode with s/w=0.5 and ν_mp=20%. The fraction of intercalated-Li is shown at five instances in time for each type over the charge/discharge process.  A properly designed macro-pore (case iii in Fig. 1c) leads to a more uniform distribution of intercalated-Li through the thickness of the electrode.  Current-density lines overlaid on these distributions reveal that ions conduct preferentially through the macro-pore and transversely into the micro-porosity where electrochemical reactions take place.  These simulations predict that properly designed macro-pores can perform significantly better than their conventional electrode counterparts over a range of C-rates. Also the sensitivity of performance to electrode thickness and average porosity/electro-active material is investigated.