Optimizing Areal Capacities through Understanding the Limitations of Lithium-Ion Electrodes

Abstract

Increasing the areal capacity or electrode thickness in lithium ion batteries is one possible means to increase pack level energy density while simultaneously lowering cost. The physics that limit use of high areal capacity as a function of battery power to energy ratio are poorly understood and thus most currently produced automotive lithium ion cells utilize modest loadings to ensure long life over the vehicle battery operation. Here we show electrolyte transport limits the utilization of the positive electrode at critical C-rates during discharge; whereas, a combination of electrolyte transport and polarization lead to lithium plating in the graphite electrode during charge. Experimental measurements are compared with theoretical predictions based on concentrated solution and porous electrode theories. An analytical expression is derived to provide design criteria for long lived operation based on the physical properties of the electrode and electrolyte. Finally, a guideline is proposed that graphite cells should avoid charge current densities near or above 4 mA/cm 2 unless additional precautions have been made to avoid deleterious side reaction. Lithium-ion (Li-ion) batteries are currently being used as the primary energy storage device in hybrid, plug-in, and all electric vehicles. This commercialization has been possible only by leveraging decades of previous scientific and engineering advances on materials, electrodes, and cell development. However, interactions in this complex system are still not fully understood. Automotive grade battery cells are required to fulfill a variety of optimization criteria in order to meet customer expectations and enable highly functional, robust and competitive products. In Fig. 1, key cell level criteria are shown for available technology as well as future development goals. Many of these values are highly influential on each other. In order to optimize one of the criteria it is always necessary to critically evaluate the impact on others. Key goals are to increase vehicle range and decrease cost at the same time. Minimizing the fraction of non-active material is an intuitive path to achieve these goals; however, the cell power and rate capability must simultaneously be maintained. [1][2][3] To derive clear development goals, the high level targets can be broken down to specific component requirements on the different levels of a storage system. 1,4 In Fig. 2, an analysis is shown for a state of the art prismatic hard-case automotive cell format. A battery level specific energy of ∼225 Wh/kg is widely accepted to be a critical value for sustainable implementation of long range electric vehicles. It represents a useful ratio between vehicle weight and range. In order to achieve this high specific energy, all subcomponents of the storage system have to meet demanding requirements as well. On the cell level, large format cells are favorable as they reduce the amount of cell housing needed per cell volume. Prismatic cell formats have a positive influence on the packing densi...

Toward Low-Cost, High-Energy Density, and High-Power Density Lithium-Ion Batteries

“…63,64 LiFePO 4 and NMC thick electrodes have been optimized by a combined experimental and simulation approach. 65,66 Efforts have been put forth in a variable porosity electrode, but this has only led to marginal improvements in energy density compared with well-designed constant-porosity electrodes, suggesting it is more important to decrease the tortuosity. 67 Numerical simulation has also been applied to investigate the limiting factors of the energy-power density relationship in LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA)/ graphite cells with thick electrodes.…”

Influence of the Binder on Lithium Ion Battery Electrode Tortuosity and Performance

“…Thus, the accessible lithiation capacities at high C-rate (1 1/h) differ strongly amongst these electrodes, although their porosity, thickness, and areal capacities are comparable (see Table II), clearly showing the significant effect of electrode tortuosity on the charging potential vs. SOC behavior, as would be expected based on theoretical models. 20,21 In addition to the difference in lithiation capacities at 1 1/h, Figure 4b also illustrates two other distinctive features in the lithiation potential vs. SOC profiles for increasing tortuosities, namely an increasing overpotential and a smearing out or complete loss of the typical potential steps in the graphite lithiation process at high C-rates. At the slow charging rate of 0.1 1/h, the graphite lithiation plateaus (around SOC values of 10%, 20%, and 55%) are clearly distinguishable and similarly well-defined for all graphite electrodes despite their vastly different tortuosities (compare Figure 4a).…”

“…14 in Ref. 20). Applied to our measurements this would mean that the product of the critical C-rate R crit.…”

“…To verify the expected behavior of better performance for low tortuosity electrodes, 20,21 we picked three electrode compositions with different tortuosities but similar areal capacity, thickness, and porosity in order to compare their electrochemical performance and their rate capability using three-electrode cells with a lithium counter and a lithium reference electrode. As the limiting operation mode for graphite anodes is the charging step, due to the proximity of the graphite intercalation potential to the lithium plating potential, we focus our analysis on the charging (i.e., lithiation) behavior of graphite anodes.…”

Nanostructured Networks for Energy Storage: Vertically Aligned Carbon Nanotubes (VACNT) as Current Collectors for High-Power Li4Ti5O12(LTO)//LiMn2O4(LMO) Lithium-Ion Batteries

“…This clearly demonstrates the limitation of the C-rate with increasing nanoarray height. The reason for this limitation can be found in the uneven distribution of current density in the nanoarray, which is due to limited Li + -ion diffusivity within the nanoarray [58,59]. On discharging, the current density is shifted from the separator/electrode boundary towards the current collector.…”