Data_Sheet_1_Revealing Operando Transformation Dynamics in Individual Li-ion Electrode Crystallites Using X-Ray Microbeam Diffraction.pdf

<p>For the development of next-generation batteries it is important to understand the structural changes in electrodes under realistic non-equilibrium conditions. With microbeam X-ray diffraction it is possible to probe many individual electrode grains concurrently under non-equilibrium conditions in realistic battery systems. This makes it possible to capture phase transformation behavior that is difficult or even impossible with powder diffraction. By decreasing the X-ray beam size, the diffraction powder rings fall apart in the (hkl) reflections belonging to individual electrode crystallites. Monitoring these reflections during (dis)charging provides direct insight in the transformation mechanism and kinetics of individual crystallite grains. Here operando microbeam diffraction is applied on two different cathode materials, LiFePO₄ (LFP) displaying a first-order phase transformation and LiNi_1/3Co_1/3Mn_1/3O₂ (NCM) displaying a solid solution transformation. For LFP four different phase transformation mechanisms are distinguished within a single crystallite: (1) A first-order phase transformation without phase coexistence, (2) with phase coexistence, (3) a homogeneous solid solution phase transformation and (4) an inhomogeneous solid solution crystal transformation, whereas for NCM only type (3) is observed. From the phase transformation times of individual crystallites, the local current density is determined as well as the active particle fractions during (dis)charge. For LFP the active particle fraction increases with higher cycling rates. At low cycling rates the active particle fraction in NCM is much larger compared to LFP which appears to be related to the nature of the phase transition. In particular for LFP the grains are observed to rotate during (dis)charging, which can be quantified by microbeam diffraction. It brings forward the mechanical working of the electrodes due to the volumetric changes of the electrode material possibly affecting electronic contacts to the carbon black conducting matrix. These results demonstrate the structural information that can be obtained under realistic non-equilibrium conditions, combining local information on single electrode crystallites, as well as global information through the observation in many crystallites concurrently. This provides new and complementary possibilities in operando battery research, which can contribute to fundamental understanding as well as the development of electrodes and electrode materials.</p

Rate-Induced Solubility and Suppression of the First-Order Phase Transition in Olivine LiFePO₄

The impact of ultrahigh (dis)­charge rates on the phase transition mechanism in LiFePO₄ Li-ion electrodes is revealed by in situ synchrotron diffraction. At high rates the solubility limits in both phases increase dramatically, causing a fraction of the electrode to bypass the first-order phase transition. The small transforming fraction demonstrates that nucleation rates are consequently not limiting the transformation rate. In combination with the small fraction of the electrode that transforms at high rates, this indicates that higher performances may be achieved by further optimizing the ionic/electronic transport in LiFePO₄ electrodes

<i>Operando</i> Nanobeam Diffraction to Follow the Decomposition of Individual Li₂O₂ Grains in a Nonaqueous Li–O₂ Battery

Intense interest in the Li–O₂ battery system over the past 5 years has led to a much better understanding of the various chemical processes involved in the functioning of this battery system. However, detailed decomposition of the nanostructured Li₂O₂ product, held at least partially responsible for the limited reversibility and poor rate performance, is hard to measure <i>operando</i> under realistic electrochemical conditions. Here, we report <i>operando</i> nanobeam X-ray diffraction experiments that enable monitoring of the decomposition of individual Li₂O₂ grains in a working Li–O₂ battery. Platelet-shaped crystallites with aspect ratios between 2.2 and 5.5 decompose preferentially via the more reactive (001) facets. The slow and concurrent decomposition of individual Li₂O₂ crystallites indicates that the Li₂O₂ decomposition rate limits the charge time of these Li–O₂ batteries, highlighting the importance of using redox mediators in solution to charge Li–O₂ batteries