Understanding SEI Formation on Conversion-Based Metal Oxide Anodes
Authors: Ng Benjamin , Mustain William E. 
Published: February 2020

Li ion batteries have pushed the boundaries of portable electronics, grid storage, and also allowed people to re-envision the design of motor vehicles. A typical modern Li ion battery consists of a graphite anode, lithium metal oxide cathode, and an organic carbonate electrolyte with dissolved LiPF6. These cells are able to achieve a gravimetric energy density of ca. 150 Wh/kg and volumetric energy density of 250 Wh/L.1 However, the demand for faster processors, cell phones with more capabilities, and electric vehicles (EV) with longer driving ranges means that the cell energy and power density must improve significantly. The target gravimetric and volumetric energy densities for sustained EV discharge are 235 Wh/kg and 500 Wh/L, respectively.2 Therefore, researchers have called for new anode and cathode materials with higher theoretical and achievable capacity. At the anode, graphite has a relatively low capacity of 372 mAh/g. On the other hand, conversion metal oxide anodes have a capacity range of 700 – 1200 mAh/g. Additionally, metal oxide anodes have a reversible potential slightly positive of Li/Li+, unlike graphite, meaning that they tend not to experience Li dendrite formation during high rate charge3, which can also improve safety.
In recent years, researchers have shown that metal oxide anodes can have excellent capacity retention (many 100’s of cycles between 0 and 100% SOC) and rate capability. However, the fundamental understanding of the conversion reaction and the growth of the solid electrolyte interphase (SEI), and the role of these two mechanisms on the capacity retention and Coulombic efficiency is very limited. The conversion reaction involves phase transition and volumetric expansion, which can lead to instability in the SEI. Also, unlike graphite, there are no organic tethers between the surface and SEI, suggesting that SEI adhesion might be low.4 This gap in fundamental knowledge about the growth of the SEI on metal oxides presents a real practical challenge in the deployment of these materials in realistic battery systems.
Therefore, this study investigates the chemical and physical evolution of the solid electrolyte interphase on conversion-based metal oxides. It’s been found that there is a correlation between the uniformity of the SEI distribution and the electronic conductivity of these materials. The relatively poor electronic conductivity of metal oxides causes non-uniform SEI deposition.5 The formation of holes and clusters lead to high reactive zones and high mass transfer resistances, respectively. Lastly, this poster will show that the primary mechanisms for capacity loss and Coulombic efficiency with metal oxides are due to metal trapping and the evolution of higher oxidation state materials throughout battery life.

References

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