Dr. Dee Strand, Dr. Bin Li
Wildcat Discovery Technologies
The performance of a lithium ion battery – energy density, power/rate, durability will depend upon the specific components within as well as the battery design. Today’s state of the art lithium ion batteries used for automotive applications use high nickel containing cathodes, graphite dominant anodes, carbonate-based electrolyte formulations, and ceramic coated separators. However, even within the boundaries of a single component – high nickel cathodes – there are myriad commercial options. Yet, selection of the “best” cathode material for your application is critical to maximize performance and minimize cost. How can we do this better?
A simple way to compare cell performance with different materials might be to process and formulate the materials all in the same fashion. The resulting electrodes are then tested in cells containing identical electrolytes, separators, and anodes. The test results will then show which cathode material performs best for each protocol. But was the best material truly identified? It was only identified within the constraints of the formulation and components used within the experimental tests. A much better means to compare materials is to optimize the performance of each cathode material – by varying the slurry/coating process, the electrode recipe, and the other components within the cell to determine the “best” performance of each cathode material. Only then can a fair comparison be achieved between all the cathode materials. Figure 1 shows an example of electrode optimization for two cathode materials, in which each cathode material is evaluated with several different binders and conductive carbons using several different processing methods. The best quality electrodes at the target loadings and densities are then tested. The tests of best electrodes for each cathode are performed with several different anodes, anode formulations, and electrolytes.
Figure 1. Each cathode formulation is optimized before comparing performance
Results of a comparison of nine high nickel cathode materials are shown in Figure 2, where each material is represented by a different color. The cycle 1 capacity and coulombic (left) and capacity retention at cycle 150 (right) are shown for each cathode (color) tested with different binders, carbons, anodes, etc. Therefore, each cathode gives a range of results. The formulations and components for the best performance for each cathode can then be identified.
Figure 2. Range of performance of cathode materials for all variables
These results are used for the ultimate comparison between cathodes, as shown in Figure 3. In this analysis, cathodes are ranked by different performance metrics. However, not all cathodes results use the same binders or carbons or anodes. For example, Cathodes 1 and may have used the same binder but different carbons, while Cathode 3 may have used both a different binder and carbon. The ranked results are the “best” formulation for each cathode. Metrics can be weighted depending upon the application requirements, to select the best materials.
Figure 3. Ranked cathode results for optimized formulations
This type of true comparison requires more experiments than simply formulating and testing every material with the same recipe and process, but ultimately results in better performance and potentially lower cost materials. High throughput experimental workflows for battery research is a cost effective and fast means for material selection from the broad range of industry options.