Abstract
Directional, micron-scale honeycomb pores in Li-ion battery electrodes were fabricated using a layer-by-layer, self-assembly approach based on spray-printing of carbon nanofibers. By controlling the drying behavior of each printed electrode layer through optimization of (i) the volume ratio of fugitive bisolvent carriers in the suspension and (ii) the substrate temperature during printing, self-assembled, honeycomb pore channels through the electrode were created spontaneously and reliably on current collector areas larger than 20 cm × 15 cm. The honeycomb pore structure promoted efficient Li-ion dynamics at high charge/discharge current densities. Incorporating an optimum fraction (2.5 wt %) of high-energy-density Si particulate into the honeycomb electrodes provided a 4-fold increase in deliverable discharge capacity at 8000 mA/g. The spray-printed, honeycomb pore electrodes were then investigated as negative electrodes coupled with similar spray-printed LiFePO4 positive electrodes in a full Li-ion cell configuration, providing an approximately 50% improvement in rate capacity retention over half-cell configurations of identical electrodes at 4000 mA/g.
Highlights
Porous or hollow frameworks and particulate materials are useful in a range of applications, including photovoltaics,[1,2] electronics,[3,4] sensing,[5,6] biologics,[7,8] electrocatalysis,[9,10] and energy storage,[11,12] where maximizing mass transport behavior and active surface area is desired
Porous and hollow structures have been studied extensively for rechargeable Li-ion battery (LIB) technologies where interconnected pores within a LIB anode or cathode are essential for effective ion diffusion and “deep” penetration to all active sites within the electrode;[13,14] these relatively open structures may help to mitigate the mechanical strain of repeated lithiation/delithiation processes.[15,16]
We show that the principal electrochemical benefit of the bottom-up spontaneous self-assembly of the active materials into honeycomb electrodes is to sustain capacity into the high current density region
Summary
Porous or hollow frameworks and particulate materials are useful in a range of applications, including photovoltaics,[1,2] electronics,[3,4] sensing,[5,6] biologics,[7,8] electrocatalysis,[9,10] and energy storage,[11,12] where maximizing mass transport behavior (e.g., ion mobility) and active surface area is desired. A wide range of high surface area, hollow particles and tube structures at the nanoscale have been proposed and successfully demonstrated in LIB electrodes based on hydrothermal, sol−gel, and sacrificial template methods.[17−20] Recently, yolk−shell designs for various electrode materials, including TiO2, Si and others have been developed with the intention of improving both high-rate capability and charge/ discharge cycle stability in a LIB configuration.[21−24] At the electrode manufacture stage, these high-specific-surface area materials are formed into an LIB electrode using slurry casting, which is the dominant and most productive electrode fabrication method.[25−27] A composite slurry, including carbon-based conductivity enhancers and polymeric binders, in an aqueous or nonaqueous solvent, is cast onto a current collector via a doctor blade or slot-die process and sequentially dried to produce a robust, well-adhered composite coating over large areas, including using semicontinuous or near-continuous roll-to-roll operations.[28,29]. The honeycomb structure allows the intrinsic electrochemical performance of well-known but difficult-to-process materials to be more readily realized
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