Optimizing Electronic Conductivity to Improve the Thick Battery Electrode Performance for Lithium-Ion Batteries

Abstract

Thick battery electrode designs have attracted broad interest from the lithium-ion battery industry because they represent a promising approach to significantly increase the battery energy density at the cell level and reduce the materials and manufacturing cost at the same time. However, increasing the electrode thickness also increases the ionic and electronic transport distance, leading to inferior rate performance. Previous studies on improving thick electrode performance have mainly focused on the design and fabrication of three-dimensional electrode architecture (e.g. electrodes with low tortuosity porous channels) to facilitate ionic transport. On the other hand, how the electronic conductivity should be optimized for thick electrodes has received less attention. Several existing studies report the effect of adding carbon nanotubes on the thick electrode performance, but it is not clear how the results could be generalized to other types of conductive additives. In this study, we ask the questions: how does the electrical conductivity affect the rate performance of thick electrodes, and are there general criteria for determining the optimal amount of conductive additives? Using LiFePO4 as a model system, we prepared a series of electrodes with different thickness and systematically varied electrical conductivity, which was achieved by adjusting the amount and ratios of two types of carbon additives, i.e. carbon black (C65) and vapor grown carbon nanofibers (VGCF). To obtain accurate readings of the intrinsic resistance of the LiFePO4 composite electrodes, a thickness extrapolation method was applied to remove the contact resistance at the Al/LiFePO4 and probe/LiFePO4 interfaces. We discovered that while 2 wt% C65 is sufficient for thin electrodes (<50 μm), at least 5 wt% C65 is required to maximize the rate performance of thicker electrodes (>100 μm), see Figure 1a&b. Further study reveals the existence of a critical electrical conductivity : the electrode’s rate capability increases with the conductivity at but saturates above (Figure 1c). The optimal amount of conductive additive is thus determined by . For electrodes thicker than 100 μm, we discovered that is independent of electrode thickness and comparable to the ionic conductivity of the electrodes. For thinner electrodes, however, increases monotonically with the electrode thickness L. We show that this phenomenon could be explained by the competition between three types of resistance present in the electrode: charge transfer (RCT), electrical (Relec) and ionic (Rion) resistances. Our prediction of the curve agrees with experiments, which could serve as a general guidance to the optimization of conductive additives for thick electrodes. Our study also reveals that the critical electrical conductivity could be most effectively achieved for thick electrodes via a combination of C65 and VGCF thanks to their complementary morphologies. While the fiber-shaped VGCF provides long-range pathways for electron conduction across the electrodes, the contact between particulate C65 and active materials facilitates the short-range electrical wiring. As a result, only 3 wt% of hybrid additives (2wt% C65 + 1wt% VGCF) is needed to reach , as opposed to 5 wt% of C65 only. Figure 1. Rate performance of electrode with different carbon amount (a) Thin electrode and (b) 150 μm electrodes. (c) Rate capability of electrode with different electrical conductivity. Acknowledgement This work is supported by Shell International Exploration and Production, Inc. Reference Kuang, Y.; Chen, C.; Kirsch, D.; Hu, L., Thick Electrode Batteries: Principles, Opportunities, and Challenges. Advanced Energy Materials 2019, 9 (33). Ju, Z.; Zhang, X.; King, S. T.; Quilty, C. D.; Zhu, Y.; Takeuchi, K. J.; Takeuchi, E. S.; Bock, D. C.; Wang, L.; Marschilok, A. C.; Yu, G., Unveiling the dimensionality effect of conductive fillers in thick battery electrodes for high-energy storage systems. Applied Physics Reviews 2020, 7 (4). Tian, R.; Alcala, N.; O’Neill, S. J. K.; Horvath, D. V.; Coelho, J.; Griffin, A. J.; Zhang, Y.; Nicolosi, V.; O’Dwyer, C.; Coleman, J. N., Quantifying the Effect of Electronic Conductivity on the Rate Performance of Nanocomposite Battery Electrodes. ACS Appl Energ Mater 2020, 3 (3), 2966-2974. Lee, B.-S.; Wu, Z.; Petrova, V.; Xing, X.; Lim, H.-D.; Liu, H.; Liu, P., Analysis of Rate-Limiting Factors in Thick Electrodes for Electric Vehicle Applications. J Electrochem Soc 2018, 165 (3), A525-A533. Figure 1