By Kent Griffith
July 25, 2019 | Decreasing the time required to recharge a lithium-ion battery is a crucial target in the effort to stimulate electric vehicle ownership and transition to zero-emissions vehicles. High-efficiency rapid charging also improves the utility of regenerative braking. It would enable electric buses and autonomous vehicles to minimize operational downtime and save costs. Beyond transportation applications, faster charging improves the user experience in portable electronics, household appliances, and power tools. At the grid scale, high power input capability is necessary for frequency balancing and smoothing power spikes from intermittent renewables.
At the Advanced Automotive Battery Conference in San Diego, Daniel Abraham of Argonne National Laboratory presented a detailed analysis of Electrode Behavior during Fast Charging of Lithium-ion Cells. Efforts are being undertaken to move from 1C (60 min) charging toward 6C (10 min) charging. There are inherent challenges to this including particle fracture and crystal structure changes at the oxide cathode as well as fracture, degradation of the solid–electrolyte interphase (SEI), and dendrite formation at the graphite anode. These phenomena are well known but they are considerably harder to understand quantitatively; additionally, Abraham and his team made some counterintuitive observations. In their NMC532/graphite cell (nickel manganese cobalt oxide (Li1.03[Ni0.5Mn0.3Co0.2]0.97O2)), the lithium plating condition was met at a rate of 3C. However, no lithium plating was observed onto the graphite anode at rates above 10C. Using a Li-plated copper wire reference electrode, the Argonne team was able to separate contributions to the overpotential from the cathode and anode. The three-electrode work revealed that the cathode overpotentials are significantly higher, meaning that the upper cut-off voltage (4.39 V in this case) can be met at high rates even before the anode reaches a lithium plating potential. The implication is that, in these cells, polarization of the cathode is the capacity-limiting factor. Furthermore, with slow rate cycling after the high rate cycles, the capacity is not affected because the plated lithium is able to intercalate into the graphite and/or plated lithium can be stripped off the graphite. The electrolyte composition is an additional factor to consider for high-rate cycling as LiBOB was observed to double the cell impedance vs. LiPF6 due to the formation of a different SEI.
To add a spatial dimension to their understanding of fast charging on ~100 mm-thick electrode structures, Abraham et al. performed depth-resolved energy-dispersive X-ray diffraction (EDXRD) at the Advanced Photon Source (beamline 6BM-A) at Argonne National Laboratory. This measurement can be performed directly through a coin cell, pouch cell, or cylindrical 18650 cell without modification. The team observed that the lithiation and delithiation reactions are inhomogeneous even at moderate 1C cycling, with the region near the separator reacting first.
Combining the Argonne studies, Abraham offered some advice for approaches to enable fast charging with conventional NMC/graphite cell chemistries. From the electrolyte side, it is important maximize lithium ionic conductivity and minimize cell impedance through the formation of a low resistance SEI. Electrodes could be engineered to have aligned pores with minimal tortuosity and porosity gradients with higher porosity near the separator. Particle morphology must be considered to minimize diffusion path lengths, while not forgetting the inter-relationship between particle size/morphology and SEI formation. Finally, there are external controls such as temperature, which improves ionic transport, and charging protocols. Pulsed or intermittent charging enables Li-ion diffusion and may mitigate dendrite growth by providing time for ions to intercalate into graphite and thus improve rate capability.
Materials strategies to achieve fast charging by moving to advanced anodes beyond graphite were presented by several companies. Dai Yamamoto of Toshiba described the advantages of an LTO (lithium titanium oxide (Li4Ti5O12)) oxide anode. As it operates at 1.55 V vs. Li metal, Li dendrites cannot stably form on the surface of LTO even under high current densities. As a result, their batteries have been incorporated into over three million vehicles worldwide. Both Zenlabs Energy and Enevate Corporation are targeting fast charging batteries through silicon alloying electrode technologies, albeit through considerably different chemistries. Zenlabs has developed an electrode architecture utilizing micrometer SiOx particles while Enevate is pursuing a silicon-dominant anode that is neither a graphite composite nor an oxide. Certainly, the insights provided by the detailed studies at Argonne are applicable beyond graphite and provide lessons for next-generation materials for fast charging lithium-ion batteries.
Further information on the Argonne studies can be found in the following publications:
Rodrigues et al. Journal of The Electrochemical Society 2019, 166, A996–A1003.
Yao et al. Energy and Environmental Science 2019, 12, 656–665.