For the future progress of lithium ion batteries, identifying and understanding degradation mechanisms that affect the cycle-life are of major interest. Beyond identification, it is necessary to assess the relative influence of individual mechanisms to the totality of capacity fade ^[1-3]. Specifically, comparisons between the relative impact of several mechanisms—primary active material loss (Li⁺ inventory), secondary active material loss (LiCoO₂ and/or Graphite), and increased internal cell resistance (caused by surface films)—remain the topic of continued investigation ^[1].  Such effort is complicated by the dependence of these mechanisms to several parameters as it has been shown that their relative significance can vary based upon: the physical and chemical nature of the system, cycle rate ^[1], and testing temperature ^[2].  

Owing to this complexity, multiple means of characterization are necessary to link global performance to the local mechanisms belying such performance. Most often electrochemical techniques are employed to link capacity fade to the aforementioned mechanisms ^[2-4]. Such techniques are unable, however, to specify local, crystal structure, alterations to this fade.  In response, ex-situ Raman and electrochemical techniques are combined here to link local structural information of active particles to capacity loss. The Raman spectrum of LiCoO₂ is sensitive to the state of lithiation and can therefore be leveraged to quantify the state of lithiation. Specifically, we investigate the Raman response of LiCoO₂ cathode particles in full cells subject up to 80 cycles at slow rates (C/10), see Fig. 1. Cycle dependent changes in Raman spectra are compared to bulk electrochemical behavior – and hence capacity loss- demonstrating that such loss is linked to reductions in Li⁺ inventory commensurate with SEI formation and growth.

Acknowledgment

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

References

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2. M. Safari, and C. Delacourt, Journal of The Electrochemical Society, 158 (10), p. A1123-A1135, 2011
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