The long-term performance of lithium-ion batteries is crucial for applications in new markets such as energy storage in electric vehicles and large-scale electronic storage devices. A vital requirement of these applications is excellent long-term and cycling performance.
For studying complex aging processes in batteries, the process heavily depends on minuscule changes in cell chemistry and material composition and ideally requires monitoring the structural evolutions of both anode and cathode electrodes at the same time.
"High-quality in situ and operando X-ray diffraction analysis of pouch bag lithium-ion batteries"
In situ and operando XRD
X-ray diffraction (XRD) is a powerful tool for the characterization of battery materials. XRD is a laboratory technique using monochromatic X-rays to probe atoms in a material to reveal structural information, such as crystal form, lattice dimensions, crystallite size, defects in the crystal structure, and preferred orientation of crystalline material.
XRD instruments, like the Empyrean diffractometer as shown in Figure 1, can be used to conduct in-depth structural analysis and makes it possible to identify the different phases present in battery materials.
In situ XRD analysis allows the structural analysis of electrode materials within the electrochemical cell at specific states of charge. The operando (Latin word for “operating”) technique enables the characterization of the structural evolution of the crystalline phases contained in battery cells, simultaneous with the operation of the reaction.
XRD transmission measurements using hard radiation (e.g., from a Mo or Ag anode X-ray tube) provide the ability to penetrate sufficiently through working batteries during charge and discharge cycling and follow the Li uptake of cathode and anode. Coin cells, prismatic batteries such as those found in a cell phone, or pouch cells are all suitable for XRD in situ / operando analysis.
Using hard X-rays in transmission geometry to perform in situ and operando studies during charge/discharge cycles and for aging studies makes it possible to correlate variations in the crystallographic structure of the elements in the cell directly with the amount of Li incorporated in the electrodes.
A battery pouch cell was mounted in the XRD system and cycled twice between discharges and charges states following a sequential CCCV (constant current-constant voltage) procedure. Diffraction scans were collected continuously, and the data was processed using the Rietveld method to accurately determine the lattice parameters of the Li bearing cathode phase LiNi0.33Mn0.33Co0.33O2(LiNMC). When the sample mass and thus the absolute number of moles of active LiNMC material is known, it is possible to calculate the amount of x(Li) in the formula for the charge-discharge process:Li1- xNi0.33Mn0.33Co0.33O2, as every transferred electron equals one lithium atom. One can then plot the lattice parameters or the parameter ratio c/a against the calculated amount of x(Li), as seen in Figure 2.
During the charging process, which corresponds to the removal of Li ions from the structure of the cathode, the cell parameter first shows a decrease followed by a shorter region where it is nearly constant. On the other hand, the c-axis first indicates a sharp increase and then switches in a reduction just before the cell is fully charged. This hysteresis between the first and successive charge/discharge cycles regarding the lattice parameters is an effect known as the irreversible capacity loss (ICL). ICL is the result of the very slow uptake of Li ions upon re-insertion into the host Li1-xNMCstructure when reaching x-values close to 1. The active Li in the cathode can then be compared to the electrically observable capacity. Deviations in the values from each other offer valuable insights for the understanding of aging processes in Li-ion batteries, as changes in lattice parameters relate directly to the structural evolution of the electrode materials.
In conclusion, research and development for battery materials can benefit greatly from in situ and operando XRD to shed insight into mechanisms for battery life/performance that will propel energy storage technology into the future. More details on this study can be found in the application note High-quality in operando X-ray diffraction analysis of pouch bag lithium-ion batteries on the Malvern Panalytical website.
We want to acknowledge the data and method contributions of Armin Kriele, Helmholtz-ZentrumGeesthacht, Centre for Materials and Coastal Research, Branch MLZ Garching, Lichtenbergstr. 1, 85748 Garching, Germany. The Heinz Maier- Leibnitz Zentrum (MLZ) is a neutron research facility open for user experiments in science and industrial applications.
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