Lithium-ion batteries are a class of rechargeable batteries often used in consumer electronics such as computers and cell phones. In a lithium-ion battery lithium ions migrate from the negative to the positive electrode when discharging and back to the negative electrode when charging. Since intercalation compounds are used as the electrode material instead of metallic lithium, the molecular structure of the electrode and the position and amount of interstitial Li incorporated into the electrodes molecular lattice is critical in understanding battery operation and performance. X-ray diffraction is one of the few techniques that can identify the position and occupancy of lithium ions incorporated into the electrodes molecular lattice.
Pictured below in Figure 1 is a specialized battery cell available for most Rigaku X-ray diffraction systems mounted on the Rigaku Ultima IV multipurpose diffraction system. This cell allows the measurement of X-ray diffraction data collected "in-situ" as the electrode is cycled through discharged and charged states.
The x-ray diffraction patterns and phase identification analysis results of an LiFePO4 electrode material are shown below in Figure 2. The results show that the main phase in the electrode changes from LiFePO4 to FePO4 as it is charged and goes back to LiFePO4 as it is discharged. This indicates that the lithium ions migrating back and forth during the discharging and charging processes are the intercalated lithium in the LiFePO4 molecular lattice and the lattice framework does not change as the lithium ions migrate in and out of the framework.
Figure 3 shows the molecular lattice structures of both the FePO4 and intercalated LiFePO4 compounds.