Elsevier

Carbon

Volume 196, 30 August 2022, Pages 525-531
Carbon

Bilayer hybrid graphite anodes via freeze tape casting for extreme fast charging applications

https://doi.org/10.1016/j.carbon.2022.05.023Get rights and content

Abstract

Extreme fast charging (XFC) of lithium-ion batteries is critical for continuous market adoption of electric vehicles. However, mass transport limitations and sluggish kinetics lead to lithium plating in graphite anodes under fast charging conditions. One approach to address the mass transport limitation in graphite is to design low-tortuosity electrodes to enhance ionic transport. In this study, a bilayer hybrid structured electrode with directionally aligned channels was developed via freeze tape casting, which enables faster lithium ion diffusion through the graphite electrode. A scalable roll-to-roll process was designed in which slurry can be cast on any substrates for simple processing. Electrochemical impedance spectroscopy measurements indicated that the bilayer hybrid coating had both low tortuosity and short diffusion pathways, enabling XFC charging. Rate testing for the bilayer hybrid electrode indicated superior performance over the other coatings, exhibiting a ∼20% improvement in the charge capacity compared with the conventional coating at 5C and 10 min total charging time. The bilayer hybrid electrode also demonstrated a 10% improvement in capacity retention compared with the conventional electrode after 1000 cycles at XFC conditions. This study demonstrates freeze tape casting as a scalable way to fabricate low-tortuosity electrodes for XFC applications.

Introduction

The growing demand for electrification of the transportation sector has brought about significant development in lithium-ion batteries (LIBs). There are continuous research efforts in increasing the energy density of LIBs and achieving fast charging [1,2]. To achieve the widescale adoption of electric vehicles, two major criteria must be met: (1) reduce the total charging time to be comparable with refueling a gas tank of a combustion engine counterpart; and (2) increase the range of the electric vehicle to >300 mi on a single charge. Extensive research has focused on developing high-energy cathodes and next-generation anodes, such as titanium niobium oxide (387 mAh g−1), silicon (3580 mAh g−1), and lithium-metal (3862 mAh g−1) [[3], [4], [5], [6], [7], [8], [9], [10]]. However, degradation of silicon particles during cycling and poor cycle life of lithium-metal batteries have limited their application [6,11,12].

Graphite is the most commonly used negative electrode in commercial LIBs because of its low cost and good cycling performance. However, there are some major issues with graphite during fast charging, including lithium plating and underutilization of active material when increasing the areal loading. The primary reasons for lithium plating are the limitations in charge transfer at the electrode–electrolyte interface, and mass transport in the electrolyte phase at the anode [[13], [14], [15]]. Furthermore, slow lithium-ion kinetics at the interface during the desolvation process, a high activation energy barrier during diffusion through the solid electrolyte interphase layer, and slow ion transport through the electrode structure owing to high-tortuosity pathways lead to lithium plating and dendrite formation in graphite-based chemistries under extreme fast charging (XFC) conditions [[16], [17], [18]].

One approach to improve the high rate capabilities and enhance active material utilization is via tailoring the electrode architecture to reduce tortuosity. Various methods have been deployed to improve the ion transport by changing the electrode architecture. Sander et al. used sacrificial features to create directional pores in graphite via magnetic alignment, enabling faster charge transport kinetics [19,20]. Chen et al. developed highly ordered laser-patterned electrodes with vertical pores that enabled rapid ion transport through graphite electrodes [21]. A co-extrusion method was used by Bae et al. to fabricate dual-scale structures in LiCoO2 [22]. Layered electrode structures have also been developed, which demonstrated improved rate performance [22,23]. In addition, freeze casting has emerged as an alternative to fabricate thick electrodes with high energy density and improved rate capability owing to short diffusion pathways enabled by low-tortuosity electrodes with highly oriented channels [[23], [24], [25], [26], [27], [28]]. However, all the previously deployed techniques (ingot-type electrode casting and freeze drying) are batch processes that require further machining before assembly into coin cells.

In this work, a scalable freeze tape cast (FTC) process was employed to develop graphite anodes with highly oriented channels to enable XFC capabilities. FTC (single-layer and bilayer) and conventional coating electrodes were compared to understand the high rate capabilities of different electrode architectures. For the single-layer FTC coating, the electrode slurry was cast on the copper current collector, followed by freeze casting. Vertically aligned channels were formed owing to ice crystal formation during the freezing process, followed by ice sublimation during the vacuum drying process. The bottom layer of the bilayer hybrid FTC electrode was developed with a thin conventional coating with N-Methyl-pyrrolidone (NMP) as the solvent and polyvinylidene fluoride (PVDF) as the binder; the top layer was developed with an FTC coating. Electrode tortuosity and diffusion length were determined via impedance spectroscopy in a symmetric cell configuration. Optical microscopy images were used to image the well-defined channels on the FTC electrodes. The XFC capabilities of graphite electrodes were investigated. The bilayer hybrid FTC electrodes outperformed the other electrodes, showing an improvement of ∼20% compared with the conventionally coated electrodes at a 5C rate. The bilayer hybrid coating also demonstrated 1000 fast charge cycles with superior performance than the other anode coatings.

Section snippets

Experimental

Slurry preparation: As-received SLC1520T graphite (Superior Graphite), PVDF (Kureha 9300), and carbon black (C45; Imerys Graphite & Carbon) were used to fabricate the conventional anodes using NMP as the solvent and following the procedures reported by Li et al. [29]. The slurry for the conventionally coated graphite consisted of 92 wt % graphite, 6 wt % PVDF, and 2 wt % C45. For anodes fabricated via freeze tape casting, SLC1520T graphite was mixed with styrene butadiene rubber (SBR; 40%

Results and discussion

Fig. 2 shows the rate performance of the conventionally coated graphite in symmetric cells with different porosities in terms of the specific charge capacity under XFC conditions. Three porosity conditions (25%, 35%, and 50%) were evaluated, with 8.5 mg cm−2 mass loading and triplicate cells for each condition. All the cells performed well at a low rate (0.1C) and had almost identical charge capacities for the different porosity conditions. As the charge rates increased to 5C and beyond, a

Conclusions

The performance of different graphite electrodes was evaluated under various XFC testing protocols with different anode architectures: conventional coating, single-layer FTC coating, and bilayer hybrid FTC coating. Symmetric cells were tested to study the rate performance of the electrodes under XFC conditions. This research demonstrated the following:

  • One of the best ways to develop a low-tortuosity electrode with high structural stability is via hybrid freeze tape casting. The bilayer hybrid

CRediT authorship contribution statement

Jianlin Li: contributed on conceptualization, Data curation, data, Formal analysis, Writing – review & editing, and Funding acquisition. Dhrupad Parikh: Experimentation, Data analysis, Manuscript Writing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research at Oak Ridge National Laboratory (ORNL), managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO) (Deputy Director: David Howell; Applied Battery Research (ABR) Program Manager: Peter Faguy).

References (39)

  • D. Parikh et al.

    Elucidation of separator effect on energy density of Li-ion batteries

    J. Electrochem. Soc.

    (2019)
  • G. Liang et al.

    Developing high-voltage spinel LiNi0.5Mn1.5O4 cathodes for high-energy-density lithium-ion batteries: current achievements and future prospects

    J. Mater. Chem. A.

    (2020)
  • D. Parikh et al.

    Operando analysis of gas evolution in TiNb2O7 (TNO)-Based anodes for advanced high-energy lithium-ion batteries under fast charging

    ACS Appl. Mater. Interfaces

    (2021)
  • M. Ashuri et al.

    Silicon as a potential anode material for Li-ion batteries: where size, geometry and structure matter

    Nanoscale

    (2016)
  • S. Chae et al.

    Integration of graphite and silicon anodes for the commercialization of high-energy lithium-ion batteries

    Angew. Chem. Int. Ed.

    (2020)
  • K.N. Wood et al.

    Lithium metal anodes: toward an improved understanding of coupled morphological, electrochemical, and mechanical behavior

    ACS Energy Lett.

    (2017)
  • R.E. Ruther et al.

    Chemical evolution in Silicon−Graphite composite anodes investigated by vibrational spectroscopy

    ACS Appl. Mater. Interfaces

    (2018)
  • Z. Li et al.

    Silicon anode with high initial coulombic efficiency by modulated trifunctional binder for high‐areal‐capacity lithium‐ion batteries

    Adv. Energy Mater.

    (2020)
  • X.G. Yang et al.

    Fast charging of lithium-ion batteries at all temperatures

    Proc. Natl. Acad. Sci. U. S. A

    (2018)
  • Cited by (18)

    View all citing articles on Scopus

    This manuscript has been authored in part by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

    View full text