Enabling > 4C Fast Charging of Li-Ion Batteries with Graphite/Hard Carbon Hybrid Anodes to Overcome Energy/Power Density Tradeoffs

Abstract

Li-ion batteries with high energy density and fast-charge capability are required to realize the widespread use of electric vehicles. However, current graphite-anode-based Li-ion batteries are unable to achieve fast charging without adversely impacting battery performance. This is because when graphite anodes are subjected to fast charging conditions, large cell polarizations reduce the accessible capacity and induce Li plating on the anode surface. The formation of metallic Li on the graphite anode surface results in irreversible loss of Li inventory, leading to significant capacity fade. In contrast to graphite, hard carbon is known to exhibit enhanced power performance. However, the high redox potential, low first-cycle efficiency, and low material density have prevented the adoption of hard carbon in high-energy-density battery systems. Thus, there is a unmet need to develop Li-ion technology to achieve both high energy density and power performance.In this work, we demonstrate hybrid anodes fabricated by mixing graphite and hard carbon to achieve fast-charging Li-ion batteries with high energy density, using industrially relevant multi-layer pouch cells (> 1Ah) and electrode loadings (3 mAh/cm2). Standard roll-to-roll slurry casting was performed to fabricate the hybrid anodes, demonstrating the compatibility with existing Li-ion manufacturing. By tuning the blend ratio of graphite and hard carbon, it is shown that the battery performance can be tailored to simultaneously achieve high energy density and power performance. As a result of the hybrid anode design, we demonstrate pouch cells with > 96% and > 93% capacity retention over 100 cycles of 4C and 6C fast-charge cycling respectively. Synchrotron tomography was employed to investigate the microstructure effects (porosity, tortuosity, and pore size) on the electrochemical performance. Electrochemical dynamic simulations were also performed to provide mechanistic insight into the origins of the improved fast-charging performance of the graphite/hard carbon hybrid anodes.