Abstract
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 LiNi1/3Co1/3Mn1/3O2 (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.
Highlights
Li-ion batteries have driven the development of today’s portable electronics and electrical vehicles
The powder diffraction rings fall apart in single crystal reflections, each originating from a single electrode crystallite
Along operando experiments on two types of electrodes, (1) LiFePO4 and (2) LiNi1/3Co1/3Mn1/3O2, we demonstrate the principle, data analysis and interpretation, and the information that can be extracted from operando microbeam diffraction
Summary
Li-ion batteries have driven the development of today’s portable electronics and electrical vehicles. Next-generation electrochemical storage is expected to be a key technology for our renewable energy future, including large scale introduction of electrical mobility and lifting the difference between supply and demand of renewable energy sources This puts significant pressure on the development of battery chemistries, which requires comprehensive atomic scale understanding of electrochemical processes. A crucial aspect for many battery chemistries is the formation, decomposition and transformation of electrochemically active crystalline phases, which typically determine a battery’s cycle life, practical capacity and kinetic performance. To understand these structural processes, powder diffraction experiments are applied intensively (Harks et al, 2015; Ma et al, 2016). As a consequence there is a strong demand to do operando structural investigations, that is to perform diffraction during battery charging and discharging
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