Li-ion batteries that can simultaneously achieve high-energy density and fast charging are essential for electric vehicles. Graphite anodes enable a high-energy density, but suffer from an inhomogeneous reaction current and irreversible Li plating during fast charging. In contrast, hard carbon exhibits superior rate performance but lower energy density owing to its lower initial coulombic efficiency and higher average voltage. In this work, these tradeoffs are overcome by fabricating hybrid anodes with uniform mixtures of graphite and hard carbon, using industrially-relevant multi-layer pouch cells (>1 Ah) and electrode loadings (3 mAh cm−2 ). By controlling the graphite/hard carbon ratio, this study shows that battery performance can be systematically tuned to achieve both high-energy density and efficient fast charging. Pouch cells with optimized hybrid anodes retain 87% and 82% of their initial specific energy after 500 cycles of 4C and 6C fast-charge cycling, respectively.
This is significantly higher than the 61% and 48% specific energy retention with graphite anodes under the same conditions. The enhanced performance is attributed to improved homogeneity of the reaction current throughout the hybrid anode, which is supported by continuum-scale modeling. This process is directly compatible with existing roll-to-roll battery manufacturing, representing a scalable pathway to fast charging. 1. Introduction Lithium-ion (Li-ion) batteries with both high-energy density and fast-charge capability are needed to accelerate the widespread use of electric vehicles (EVs). However, current highenergy density EV batteries are unable to achieve fast chargingwithout adversely impacting battery performance and safety. When Li-ion batteries are charged at high rates, increased cell polarization results in limited energy utilization, increased capacity fade, excessive heat generation, and other deleterious effects.[1–3]These consequences limit the charging time of the state-of-the-art EV batteries. Therefore, there is an unmet need to develop Li-ion technology that can simultaneously achieve high-energy density and efficient fast charging. To address these technological challenges, the U.S. Department of Energy (DOE) has identified performance targets for extreme fast charging of batteries with >180 Wh kg−1specific energy within a 10 min charging time and <20% energy fade over 500 cycles.Graphite has been predominantly used as the anode material in state-of-the-art Li-ion batteries due to its stable electrochemical performance and high specific capacity.During charge/discharge processes, graphite displays well-defined voltage plateaus at low potentials (<0.2 V versus Li/Li+) as well as relatively-high first-cycle (initial) and subsequent cycling coulombic efficiencies. These properties make graphite an attractive material for achieving high-energy density Li-ion batteries. As a result, graphite anodes have been commercialized since the beginning of the Li-ion industry and continue to play a critical role in EV battery technology.
While the low redox potential of graphite promotes higher cell energy density, it also raises concerns when graphite anodes are subjected to fast-charge conditions. During fast charging, high current densities induce large anode polarizations as a consequence of transport and kinetic limitations.[8–10]These limitations can spatially vary throughout the anode thickness/ volume, leading to a spatially inhomogeneous charging current.
As a result, poor electrode utilization and a nonuniform state-of-charge (SOC) occur during fast charging, with large portions of the anode near the current collector not being utilized (Scheme 1).[9,13]Moreover, the graphite anode can reach electrochemical potential values more negative than the thermodynamicpotential of Li metal (<0 V versus Li/Li+), making Li plating a favorable process. The formation of metallic Li on graphite anode surface has been shown to cause irreversible loss of Li inventory, leading to significant cell capacity fading. The high anode polarizations during fast charging have been attributed to several physicochemical processes, including 1) ionic transport in the electrolyte phase, 2) reaction kinetics at the graphite/electrolyte interface, and 3) solid-state Li diffusion in graphite particles.
Consequently, previous research efforts have focused on enhancing ionic transport in porous electrodes by reducing electrode tortuosity,improving interfacial and transport kinetics through new electrolyte and/ or additive design,[ increasing the cell temperature during charging, and applying coatings to the anode surface. While improved electrochemical performances have been demonstrated in these approaches, achieving long-term cycling (≥500 cycles) of graphite-based Li-ion batteries with high capacity loading (≥3 mAh cm−2 ) under ≥4C fast-charge conditions at room temperature still remains challenging. In addition, developing scalable processing approaches that are directly compatible with existing Li-ion manufacturing with minimal additional cost and implementation time is also important to accelerate commercialization.
In contrast to graphite, which has long-range order in its crystallographic structure, hard carbon is defined as nongraphitizable carbon consisting of highly disordered carbon layers (Scheme 1). During the lithiation process, Li can be inserted in between the disordered carbon sheet domains as well as in the micro-pores of the hard carbon structure.When used as the anode material for batteries, hard carbon has the following characteristics: 1) low material density (1.6 g cm−3 ) compared to graphite (2.2 g cm−3 ), 2) sloping charge/discharge voltage profile between 0–1.2 V versus Li/Li+, 3) low initial coulombic efficiency (ICE, <80%), and 4) enhanced power performance.The low ICE of the hard carbon has been attributed to solid electrolyte interphase (SEI) formation, trapped Li in the micro-pore structures, and reactions between Li and surface functional groups. This large first-cycle irreversible capacity loss translates into a significant energy density penalty. Therefore, while the improved rate performance of hard carbon is attractive for high-power applications, poor ICE, along with a high redox potential and low density, have limited its adoption in high-energy density battery systems.
Therefore, a tradeoff between the energy density and power performance is present (Scheme 1). In this study, we introduce a strategy to overcome this energy/power density tradeoff by fabricating uniform mixtures of graphite/hard carbon active material particles into a bulk hybrid anode. By doing so, it is possible to balance the desirable characteristics of both materials and rationally tune the electrode properties in a synergistic manner to improve current homogeneity and reduce Li plating during fast charging, while maintaining sufficiently-high cell energy densities (Scheme 1). Previous studies on the concept of graphite/hard carbon hybrid anodes have been largely limited to surface modifications of the active materials prior to electrode fabrication, such as coating hard carbon onto graphite particle surfaces to improve rate capability[33,34] or applying graphite microcrystallites onto hard carbon particles to improve ICE and reversible capacity. While mixing of varying carbonaceous materials have been studied for battery systems,[36–38] the charge rates in these studies have not addressed the DOE and industry fast-charge targets (10 min charging time). In this work, we demonstrate hybrid anodes fabricated by mixing graphite and hard carbon to achieve fast charging Li-ion batteries with an energy density of >180 Wh kg−1 , using industrially relevant multi-layer pouch cells (>1 Ah) and electrode capacity loadings (3 mAh cm−2 ). Standard roll-to-roll slurry casting was performed to fabricate the hybrid anodes, demonstrating compatibility with existing Li-ion manufacturing. By tuning the blend ratio of the graphite and hard carbon, it is shown that the battery performance can be systematically tuned to simultaneously achieve high-energy density and fast charging. As a result of the optimized hybrid anode design, we demonstrate pouch cells with 87% and 82% specific energy retention after 500 cycles of 4C and 6C fast-charge cycling using hybrid anodes, compared to 61% and 48% for cells using graphite anodes under the same conditions. In addition, while the optimized hybrid cells show 10% lower initial specific energy compared to the graphite cells, the remaining specific energy after 500 cycles of fast charging is 27% larger at 4C and 53% larger at 6C. Systematic electrochemical analysis was performed to demonstrate the efficacy of the hybrid anode design, and synchrotron tomography was employed to analyze the electrode microstructures. Continuum-scale electrochemical simulations were further performed to provide insights into the enhanced fast-charge performance, which is attributed to the improved homogeneity in reaction current distribution throughout the hybrid anode. The cell performance presented in this work addresses the DOE goal for fast charging high-energy density Li-ion batteries.
2. Results and Discussion
2.1. Fabrication of Hybrid Anodes Graphite/hard carbon hybrid anodes were prepared using a pilot-scale roll-to-roll processing facility at the University of Michigan Battery Lab (further details are in Supporting Information).