Investigation of Rate Capability and Mass Transfer Dynamics in Lithium-Ion Batteries
Authors: Ezeigwe Ejikeme Raphael , Dunne Ronan H , Bua Tone G , Solberg Simon Birger B. , Lamb Jacob Joseph , Wind Julia , Burheim Odne Stokke 
Published: December 2024

Lithium-ion batteries are poised to play a pivotal role in achieving net-zero emissions amidst the ongoing global green transition. The quest for new and sustainable battery chemistries is imperative to meet the anticipated increase in demand. Consequently, the development of cost-effective electrochemical techniques holds significant importance in advancing battery research. Ongoing efforts aim to surpass conventional energy storage solutions, focusing on enhancing energy density and capacity. Thicker electrodes offer a promising pathway for enhancing the electrochemical performance of lithium-ion batteries (LIBs); however, their utilization presents challenges stemming from issues such as increased tortuosity, cracking, and delamination. These challenges ultimately compromise electrode mechanical integrity and reduce energy density, particularly at higher C rates. Addressing these challenges requires exploring the mass transport properties within the electrodes to enhance diffusion. Investigating the rate capabilities and the phenomenon of electrode mass transport presents an opportunity to gain comprehensive insights into the diffusion processes, which is essential for optimizing LIB performance [1], [2].

In this study, cathodes were fabricated using lithium nickel manganese cobalt oxide (NMC111) as the active material, employing two different binders and corresponding solvents: the conventional binder polyvinylidene fluoride (PVDF) with N-methyl-2-pyrrolidone (NMP), and a more sustainable alternative utilizing lignin as the binder and water as the solvent. These cathodes were integrated into half-cells, and their electrochemical performance was assessed through galvanostatic cycling. The rate performance analysis revealed an improvement in performance correlating with electrode thickness, reaching an optimal point where capacity retention begins to decline. The PVDF/NMP cathodes utilized for assessing rate performance exhibited active mass loadings ranging from 3.4 ± 0.2 to 19.8 ± 0.5 mg/cm2. Notably, a notable decline in specific discharge capacity was observed at higher C-rates as the active mass loadings decreased for PVDF/NMP half-cells. Among these, electrodes with an active mass loading of 3.4 ± 0.2 mg/cm2 demonstrated superior performance, achieving a specific discharge capacity of 95 mAh/g at 5.0 C.

The main overpotentials (ohmic, reaction, and concentration) were investigated by plotting cell potentials against current densities in lignin/water and PVDF/NMP half-cells. This facilitated the calculation of the effective mass transfer coefficient using two definitions of state of charge (SoC): nominal, referencing the initial capacity, and dynamic, based on the capacity achieved in each cycle. This analysis aimed to explore cathode characterization possibilities and evaluate the influence of the binder. Mass transport analysis indicated that cathodes fabricated with lignin and utilizing nominal SoC yielded more anticipated curves and reliable outcomes than those using PVDF/NMP. Polarization curves revealed that lignin/water cathodes exhibited the highest concentration overpotentials. However, challenges were observed in estimating ohmic resistance. Additionally, the results demonstrated that the effective mass transfer coefficient declines with increasing current indicative of a larger diffusion layer at lower SoC.

References

[1] S. N. Bryntesen, I. Tolstorebrov, A. M. Svensson, P. Shearing, J. J. Lamb, and O. S. Burheim, “Introducing lignin as a binder material for the aqueous production of NMC111 cathodes for Li-ion batteries,” Mater. Adv., vol. 4, no. 2, pp. 523–541, 2023, doi: 10.1039/D2MA00850E.

[2] S. N. Bryntesen et al., “Structured aqueous processed lignin-based NMC cathodes for energy-dense LIBs with improved rate capability,” J. Mater. Chem. A, vol. 11, no. 12, pp. 6483–6502, 2023, doi: 10.1039/D2TA08606A.