Designing Positive/Positive and Negative/Negative Symmetric Cells with Electrodes Operating in the Same Potential Ranges as Electrodes in a Full Li-Ion Cell

This work shows how to design and build positive/positive (+/+) and negative/negative (-/-) symmetric cells with electrodes operating in the same potential ranges (vs Li/Li+) as those in a full Li-ion cell. When this is achieved, better understanding of full cell degradation can be obtained. This method uses only coin cells that are ubiquitous in lithium-ion cell research instead of novel electrochemical devices that are more or less unique to specific research groups and are difficult to access. Using this method, the capacity retention and impedance growth of single crystal LiNi0.5Mn0.3Co0.2O2 (SC532)/artificial graphite full coin cells were shown to lie between those of the +/+ and -/- symmetric cells, regardless of electrolyte additive and surface coating. Among all the cells, the +/+ cells have the worst capacity retention and highest impedance growth. Simultaneously cycled full coin cells and symmetric cells demonstrate the beneficial effect of cross-talk between the SC532 and the graphite electrodes to lower full cell impedance growth. Additionally, symmetric cell results show that 2% of vinylene carbonate (VC) increases the negative impedance more than 1% of lithium diflurophosphate (LFO), and that 1%LFO is also a better additive than VC to inhibit positive electrode impedance growth with coating.

Automated summary

This study demonstrates the design and construction of symmetric cells to gain insights into full Li-ion cell degradation. Results show that using coin cells, the capacity retention and impedance growth of electrodes lie between +/+ and -/- configurations.