Designing +/+ and -/- Symmetric Cells with Matching Full Cell Voltage As a Method to Simplify the Study of Cell Degradation
Authors: Liu Yulong , Dahn Jeff R. 
Published: December 2020

A lithium-ion cell is a complex electrochemical system with many degradation modes. To further improve the energy density and longevity of lithium-ion cells, various degradation modes have to be first understood before solutions can be devised and applied. To simplify the study of cell degradation, many devices and methods have been proposed1–3. Though these devices and methods are able to simplify degradation studies to some extent, the major drawback is the limited accessibility to many other researchers and scientist, not to mention industrial R&D enterprises.
Symmetric cells create an environment where only the positive electrode or only the negative electrode exists. In such an environment, the cell performance is only dictated by one type of electrode and its interaction with the electrolyte, which allows a significant simplification of the study of electrode and electrolyte of interest. In previous symmetric cell studies4,5, the operating voltage of symmetric cells was more or less arbitrarily chosen, that is, the operating voltage of each electrode vs. Li/Li+ in a symmetric cell (electrode voltage) was not purposely designed, but instead, was determined using dVdQ analysis4-6
after symmetric cells were made and cycled. However, it is much more desirable to think the opposite: having the electrode voltage of interest, at what voltage should a symmetric cell shall operate before cells are made and cycled. Knowing this allows a better control of experiments and avoids obtaining the desired electrode voltage by time-consuming trial and error on symmetric cell building and the following dVdQ analysis. More importantly, carefully-designed symmetric cells with electrode voltages of interest facilitate the study of full cell degradation by “deconvolution” of full cell performance into +/+ and -/- symmetric cells with matching voltage ranges, allowing cell degradation to be studied separately and electrodes interactions unraveled.
In this presentation, a detailed introduction of designing symmetric cells with desired electrode voltage vs. Li/Li+ that matches a full cell voltage profile will be discussed. Coated and uncoated single crystal NMC532 +/+ symmetric cells and graphite -/- symmetric cells were cycled together with the corresponding full cells. Using this method, two additives, vinylene carbonate (VC) and lithium difluorophosphate (LFO), were chosen to study their effect on positive and negative electrodes. It was found that +/+ cells degrade faster than -/- cells and full cells regardless of electrolyte and the presence of cathode coating. Symmetric cell results show that VC helps more on graphite by forming stable SEI whereas LFO helps more on the positive electrode but not much on graphite. More results will be discussed in the conference.

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Figure 1