Mesoscale simulations conducted by Partha Mukherjee’s research group at Purdue University have revealed the role of the morphology of the inactive or secondary phase material in lithium ion battery electrodes. The results, published in a recent issue of ACS Applied Materials and Interfaces, could be used to optimize battery performance to target application-specific energy and power requirements.

Many battery chemistries rely on composite electrodes that consist of particles of an active or primary phase, responsible for either the reduction or oxidation, and an inactive or secondary phase, which binds the primary phase particles together. Within the microstructure created by primary and secondary phases are pores or voids through which the electrolyte can diffuse. Using particles of the primary phase in the electrode increases its surface area, so particle-based electrodes often outperform a solid electrode made of the same active material. “Many commercial batteries, lithium ion batteries in particular, use a particle-based electrode: the electrochemically active part is made of particles that are coated as a thin film on a current collector,” says Anne M. Grillet of Sandia National Laboratories in Albuquerque, New Mexico. Grillet also studies lithium ion batteries, but was not connected with Mukherjee’s recent work.

“Even though the secondary phase is essential for lithium ion battery electrode performance, the role of this secondary phase, resulting from the underlying spatial arrangement and microstructural stochasticity, still remains poorly understood,” Mukherjee says. Different types of secondary phase materials or fabrication methods can yield different coverage of the active phase particles by the secondary phase. Different morphologies of the secondary phase impact the pore structure that the electrolyte can diffuse through, which impacts the performance of the battery. But understanding the connection between the morphology of the secondary phase and battery performance has been challenging experimentally. Within the electrode structure, the active phase comprises most of the electrode and is relatively dense, which makes the secondary phase less visible using scanning electron microscopy or x-ray imaging.  

Mukherjee’s group simulated hundreds of microstructures of the composite battery electrode by slowly varying the morphology of the secondary phase. Composite electrodes with different morphologies were generated by tuning the secondary phase particles’ relative affinity for themselves versus the primary phase particles. The computed structures were also varied by changing the percentage of active material as well as concentration of pores for diffusion of the electrolyte solution. Statistical analysis of a comprehensive set of unique structures allowed the research group to identify combinations of active material, secondary phase morphology and porosity that could lead to optimized battery performance.  

“This work attempts to quantitatively correlate microstructural changes—emphasizing the role of the secondary phase—with the measurable response and thus identifies the directions for further improvement,” Mukherjee says.

“Understanding the complex transport in these systems is widely applicable to [many] batteries,” Grillet says. By changing the shape or size of both the primary and secondary phase or the interactions between the two phases, researchers could investigate and optimize a particular battery’s electrochemistry or physics. To mimic a battery using a graphite-based active material, for example, the simulation could use platelet-shaped primary phase particles instead of spheres when generating the composite electrode.

In the future, new simulations of different composite electrode microstructures could be generated and analyzed to understand and control other battery properties. Grillet mentions using similar simulations to correlate mechanical strength to secondary phase morphology, because commercial batteries must be able to withstand changes in electrode size during charging and discharging. Both Grillet and Mukherjee see the possibility of tailoring aspects of battery performance—such as batteries that can slowly release energy over a long period of time or ones that supply a large burst of power—by optimizing the battery morphology.

“Based on our study, we anticipate battery electrodes can be tuned to become more application-specific and in turn more efficient,” Mukherjee says.

Read the abstract in ACS Applied Materials and Interfaces.