To push the energy density of modern batteries even further, several approaches are in recent development. The higher thicknesses of electrodes and the application of silicon are especially interesting but lead to significantly more volume changes of the battery system, which affect the overall performance. In the past, the expansion of commercialized battery chemistries was evaluated based on anode materials alone, as they show higher macroscopic volumetric changes.
In the BMBF project KaSiLi, electrodes for a silicon/nickel manganese cobalt oxide system are optimized and balanced from three angles: electrochemical preparation, microstructure and mechanical behavior. This work is focused on the mechanical behavior, as NMC cathodes show compression when anodes are expanding in these battery systems. So, these cathodes may be able to compensate the expansion of anodes, leading to systems with higher energy density.
Understanding the dilation of cathodes benefits the battery systems in many ways: Several results in literature suggest that pressure may be one way to prevent mechanical failure and prolong battery life (Nature Energy, Vol 4, July 2019, 551-559). Too much Pressure is mechanical stress leading to damage inside the cathode, so careful balance is needed. These effects can be observed in operando by dilatometry with high resolution (up to 5 nm) at every point of the electrochemical cycle. Supported by in situ XRD measurements, the behavior on an atomic scale can be connected to the behavior on a microscopic scale.
The focus of this work is on the NMC622 cathode. Thicker, more porous electrodes are compared to thinner, less porous electrodes. For thicker electrodes, the behavior is inconsistent with the crystallographic data available: The pores inside the cathodes absorb the dilation detected by the dilatometer. For thinner electrodes with low porosity, the behavior is consistent with crystallographic data: The expansional behavior is inverse in comparison to the thicker electrode: The highest expansion is observed at the lowest voltage in the cycle. Lower porosity electrodes maintain high capacity in comparison to thicker electrodes while showing better ability to compensate the anode pressure.
Future experiments will alter other cathode properties, like binder systems, electrolyte and distribution of particle sizes to further control the expansional behavior of NMC cathodes.