Incremental Impedance Technique for Determining Optimal Capacity Control Limits in C-Si Composite Batteries

Abstract

It is well known that limiting the cycling SOC of a battery material leads to improved cycle life.1 Determining optimal cycling limits, however, can be an expensive trial-and-error process. Candidate voltage or capacity limits must be proposed and tested, with tests consisting of capacity face measurements over large numbers of cycles, perhaps even at various cycling rates. A more efficient method is to identify a quantifiable phenomenon that manifests early in a cell’s cycle life, and which accurately indicates an SOC range for optimal cycling. In the current work, we use an Incremental Impedance technique to identify critical impedance jumps that serve as markers for optimal SOC limits, detectable from a cell’s very first post-break-in cycle. To determine an optimal SOC (“Capacity Control”) range for cycling, a five-cycle Incremental Impedance scan was conducted on in-house C-Si anodes, formulation A (PVDF binder, 5 mg/cm2 loading). Figure 1 shows a distinct jump in impedance was observed at 70% SOC during lithiation, and indicates 0-70% as a promising SOC range for cycling. Based on the Incremental Impedance analysis, cycling was done with SOC cutoffs of 60, 80, and 100%, to probe around the 70% point. Figure 2 shows a 60% SOC cutoff improves cycle life over 80%, while 80% and 100% (full cycling) display similar fade characteristics. 70% SOC is thus a key “point of no return” for SOC limiting. The technique was repeated for formulation B (CMC/SBR binder, 2 mg/cm2 loading). Results in Figure 3 show that both produce the same sharp increase in impedance at 70%, independent of binder type and loading. ______________________________________________________ 1Mohan Karulkar, Rachel Blaser, Bob Kudla, Automotive assessment of carbon–silicon composite anodes and methods of fabrication, Journal of Power Sources, Volume 273, 1 January 2015, Pages 1194-1201 Figure 1