(Invited) In Situ XPS and Aes Characterization of Cathode Surface Chemistry Evolution and Its Effect on Cycle Life
Authors: Dillon Shen J. , Feng Lin , Ma Yonghui 
Published: February 2020

Lithium ion battery cycle life varies significantly with cathode chemistry, structure, and properties. However, it is well known that deconstructing cycled and faded full cells and reassembling them as half cells can retrieve much of the lost capacity. Therefore, cathode chemistry must play a role in exacerbating other side reactions that induce capacity fade without significantly degrading themselves.(1, 2) A well known example in the literature is the effect of Mn(II) dissolution from the cathode in degrading the anode.(3) However, analogous fade mechanisms resulting from interactions between cathode and anode side reactions are not as well understood in other systems. Since such ‘cross-talk’ interactions can have multiple complex steps and inherently have surface reaction contributions, they can be difficult to characterize and identify. The complex nature of degradation reactions motivates the development of in situ and operando characterization techniques. Of particular value for understanding associated mechanisms are surface sensitive in situ characterization techniques such as x-ray photoelectron spectroscopy (XPS), auger electron spectroscopy (AES), etc. Such methods require specialized electrochemical cells that function in vacuum. Ideally, we seek to relate such information to associated changes occurring in the bulk of the electrode particles, in the physical properties of the underlying material, and in the commercially relevant battery environments.
We will describe the development of novel Li ion cells suitable for in situ and operando characterization via a variety of electron, optical, and x-ray based methods.(4-7) In situ surface spectroscopy is revealing new insights into the importance of cathode solid electrolyte interphase (SEI) stability during cycling in affecting cycle life. We compare cells based on LiCoO2, LiMn2O4, LiNiO2, Li[Ni1/3Mn1/3Co1/3]O2, and LiFePO4. In particular, we find that gaseous products produced during SEI formation and SEI decomposition drive degradation at the counter electrode. This has motivated ongoing work to investigate the role of gas in affecting capacity fade in traditional battery configurations, as well as model studies employing in situ characterization during cycling in gaseous environments. We find that the presence of different gases commonly evolved during cycling have varying effects on the rate of capacity fade. The effect is non-linear and the gases have different effects in full cells based on different cathode chemistries. This presentation will highlight the relationships between surface reactions observed in situ during model experiments, gas phase evolution in full cells, and the effects of subsequent reactions on capacity fade. The results provide a reasonable basis for understanding capacity fade associated with several commercially relevant cathode systems and can be extended to provide insights into designing next-generation intercalation cathodes.

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