Identical-Location TEM Studies on Conductivity Vs. Structure in Metal Oxide Anodes

Metal oxides are a promising family of materials that could potentially replace traditionally-used graphite as the anode in lithium-ion batteries for future high capacity applications like (hybrid) electric vehicles or grid-scale energy storage [1-3].  Unlike graphite, metal oxides typically do not intercalate lithium during charge/discharge, but rather rely on chemical transformations to store and deliver energy.  Equation 1 shows a typical 2-electron charge/discharge reaction:

MO + 2Li⁺ + 2e^- ↔ M + Li₂O                                     (1)

Oftentimes metal oxides display a lack of structural reversibility which limits the cycle life and capacity retention over multiple charge/discharge cycles [4].  Understanding this structural degradation on both the nano- and macro-scales will be pivotal for future anode improvements.

Identical-location transmission electron microscopy (IL-TEM) is a unique and innovative technique that enables imaging of individual particles on the nanoscale both before and after electrochemical testing [5-7].  In this study, we synthesized ordered mesoporous nickel oxide (OM-NiO) as a representative metal oxide and used IL-TEM to observe structural changes [8].  Figure 1A-C shows IL-TEM images before and after multiple rounds of charge/discharge cycling.  Results were then correlated to bulk properties like capacity retention and rate capability using coin half cells.

In addition to structural changes, the effect of conductivity was investigated by adding varying amounts of carbon black to OM-NiO anodes, and Fig. 1D shows capacity results for anodes containing up to 40% carbon added.  Despite broad structural degradation, anodes with a large percentage of carbon demonstrated impressive capacity retention over repeated charge/discharge cycles.  These results are also consistent with literature reports for NiO anodes with either high carbon additive percentage or extremely low loading [9,10].  This suggests that conductivity may play the most crucial role in anode performance, and in this presentation the contributions from both structure and conductivity on metal oxide anodes in lithium-ion batteries will be discussed.

Figure 1 – IL-TEM images for OM-NiO (A) before cycling, (B) after 2 cycles, and (C) after 7 cycles; (D) shows capacity vs. cycle number for OM-NiO anode coin cells containing 0%, 10%, and 40% carbon black added.

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