While various high-energy-density electrode materials for lithium-ion (Li-ion) batteries<!–[if supportFields]> XE “battery” <![endif]–><!–[if supportFields]><![endif]–> exist, Lu et al. (2011) suggest that other engineering approaches, such as electrode optimization, be considered to meet the energy requirements for plug-in hybrid<!–[if supportFields]> XE “hybrid” <![endif]–><!–[if supportFields]><![endif]–> electric vehicles (PHEVs). The authors investigate the impact of the electrode thickness on the energy density of Li-ion batteries, although their findings show that the electrode thickness would affect the battery’s “integrity, electrochemical performance, and cycleability.” That is why it is important to find both an appropriate and practical thickness of the electrode for high-energy batteries. The authors also found that the hybrid pulse power characterization (HPPC) test indicates that the electrode resistance is “inversely proportional” to the electrode thickness. This allows for the use of thicker electrodes, practical thickness measuring around 100 µm, in Li-ion batteries to meet standard PHEV power requirements. The authors also find that cycle performance shows that cells with a “higher loading density have a similar capacity retention to cells with a lower loading density.” —Laura Silverberg
Lu et al. assess high-energy electrode density levels for the purpose of meeting the energy requirements for PHEVs. In fiscal year 2009, the Department of Energy (DOE) began its pledge to enable the development of PHEVs with a 40-mile all-electric range. However, along with other challenges regarding Li-ion batteries<!–[if supportFields]>XE “battery” <![endif]–><!–[if supportFields]><![endif]–> in the HEV application, the issue of cycle life, as well as providing sufficient energy within the weight and volume requirements of PHEVs became apparent. Currently, there is no commercially available high-energy material to meet the needs of the proposed 40-mile PHEV. However, this issue can be addressed by engineering approaches. Thus far, various possibilities have been investigated for experimental and modeling techniques including electrode porosity and inactive additives. Both of these possibilities can be utilized to improve the energy density.
Lu et al. examine a series of cathode and anode electrodes with varying thicknesses and porosities that were prepared using a small laboratory coater. Both lithium half-cells and fuel cells were created in an “inert atmosphere glove box with an oxygen level of less than 5 ppm.” Additionally, the results of the electrochemical performance of coin cells, investigated using the HPPC profile, were also used in the study to determine thickness effects.
While no commercially available electrode materials currently exist to meet the weight and volume requirements of a 40-mile PHEV, it is important to consider battery<!–[if supportFields]> XE “battery” <![endif]–><!–[if supportFields]><![endif]–> structure optimization to increase battery energy density, for example, by increasing electrode thickness. Using a battery design model based on PHEV energy requirements, the authors experiment with various cathode and anode electrode thicknesses. They found that the cathode electrode should be thicker than 130 µm if only 70% of the total battery energy will be utilized and that the “battery weight reduction rate levels off when the electrode thickness is greater than 100 µm.” This is due to less weight and volume from the current collector and separator. The authors also evaluate the relationship between the electrode thickness and coating weight before and after calendering. Electrode integrity gets worse when the electrode thickness reaches above 100 µm. Thicker electrodes, however, do not affect the useable capacity of the NCA cathode material at a low cycling rate, although it is affected by the electrode thickness at a higher discharge rate. Due to this, better binder strength must be developed which will allow for thicker electrodes and volume expansion. The authors determine that within the relationship between the deliverable capacity and discharge rate of NCA half-cells, the usable capacity will drop significantly when the discharge rate increases.
Power performance of the NCA half-cells with varying electrode thicknesses also utilized the HPPC test. Area specific impedance (ASI) of the half-cells decreases as the electrode thickness increases and levels off according to the results from the half-cell and full-cell. Lu et al. clarify that “[t]his does not necessarily mean that the cell with the thicker electrode always has a higher power capability,” thus confirming that thicker electrode cells must be discharged at a higher current density. This process can offset the lower impedance. However, this combination can cause tension on the transport of lithium ions in the electrolyte, limiting its rate capability. The last test regards the assessment of capacity fading of cells with various electrode-loading densities. No obvious difference was observed for the full-cells under this condition.
The authors conclude that while the HPPC test results indicate that the electrode impedance is “inversely proportional” to the electrode thickness, this is will allow for good power capability of the Li-ion battery<!–[if supportFields]>XE “battery” <![endif]–><!–[if supportFields]><![endif]–> with a higher loading density in the future. Higher loading densities have similar capacity retention as lower loading densities.