Mechanical Characterization of Battery Nanomaterials
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This dissertation delivers a measurement methodology based on three-point bending tests in AFM for intrinsic material properties at the single particle level of three different nanomaterial systems. Sodium manganese oxide NWs, which are promising cathodic nanomaterials for Na ion batteries, represent the first material system investigated within this thesis. YM of pristine Na0.44MnO2 and acid leached Na0.44-yMnO2 (y~0.23) NWs was determined through AFM based three-point bending tests. It was observed that acid leaching modulates the Na-content within these materials and is reflective of the material microstructure changes during electrochemical de-sodiation. SOC dependent YM affects the computation of stresses generated during reversible intercalation in any rechargeable battery. Therefore, the information gained in this investigation is important in modeling stress fields and predicting fracture in these Na-ion battery materials. Next, Ge NWs, which represent a high capacity Li-alloying anode material, were investigated. YM of these single crystalline  Ge NWs was measured using AFM based three-point bending tests. This study revealed the significance of extremely thin surface layers on the elastic properties of NW electrodes. Finally, alpha-phase MnO2 NWs, which are another tunnel crystal structured cathodic nanomaterial candidates for rechargeable battery systems, were studied. These NWs were mechanically tested at two different SOCs: (i) lithiation at 3.4V, and (ii) one full electrochemical cycle of lithiation and delithiation at 3.4V and 4.6V respectively. While this material showed relatively no change in YM at the levels of lithium loading induced in these experiments, it exhibited plastic recovery at low loading rates as opposed to the brittle fracture observed in past reports at much higher loading rates. This is attributed to dynamic recrystallization induced by shear distortions within the crystalline tunnels and points to an important avenue for alleviating mechanical fracture / degradation within ceramic battery nanomaterials. In summary, these studies present new information, which would be essential inputs to predicting the mechanical stability of these battery material systems. The capabilities discussed within this dissertation, together with capabilities to engineering the crystal structure of electrode nanomaterials offer a pathway to enhance the mechanical stability of next-generation, rechargeable battery systems.