Yifei Sun1
Cornell University1
The battery stability is related to nanoscale processes such as the structural evolution during the (de)intercalation. The two-phase reaction - one type of structural evolution - is generally considered a slow kinetic process. Yet many high-rate materials such as LiFePO<sub>4 </sub>experience phase separation during (dis)charge. Thus, nanoscale characterization techniques are needed to understand the (de)lithiation mechanism and its interplay with local defects in phase-changing materials.<br/><br/>Disordered spinel LiNi<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>4</sub> (LNMO) is a promising high-voltage cathode material that experiences an extended period of two-phase reaction when cycling. Operando Bragg coherent diffractive imaging (BCDI) is a powerful technique that maps nanoscale heterogeneity inside the materials. In this study, we measure consecutive diffraction patterns of a single LNMO nanocrystal operando. With the improved phase retrieval algorithm, we map the three-dimensional strain field of the nanocrystal while it undergoes a structural two-phase reaction. The strain mapping confirms two-phase behavior at the single grain level and reveals different mechanisms upon charge and discharge. With its unique ability to track buried single defects, BCDI also elucidates the critical role of local defects in relieving the coherency strain at the interphase during the phase transformation. This nanoscale understanding of reaction kinetics provides new insights into the development of next-generation durable batteries with faster kinetics.