Switching from Lithium to Sodium—an Operando Investigation of an FePO$_{4}$ Electrode by Mechanical Measurements and Electron Microscopy

Many physical and chemical properties of Na$^{+}$ are very similar to those of Li$^{+}$, and therefore, some electrode materials for lithium-ion batteries can also work with sodium ions. As the Na$^{+}$ ion is larger than Li$^{+}$, the strains in the host lattice are larger, which can cause deviations in the electrochemical reactions. Herein, mechanical stresses are compared, which are measured by the in situ substrate curvature method during (de)lithiation/(de)sodiation of an FePO$_{4}$ electrode. The (de)lithiation and (de)sodiation experiments are performed on the same electrode. According to the change of the lattice parameters, during electrode operation, NaxFePO$_{4}$ particles experience a volume change that is 2.6 times larger than that of LixFePO$_{4}$. In the measurements, the composite electrode exhibits a change of the stress amplitude between operation with Li and Na by roughly one order of magnitude for 0 < x < 1. Compared with Li$^{+}$, the mechanical stress evolution during extraction and insertion of Na$^{+}$ is highly asymmetric. The observed asymmetry in the electrochemical and the mechanical data may be explained by the different energies that are required to move an intermediary amorphous phase away from or toward the crystalline sodium-rich regions during the (de)sodiation of NaFePO$_{4}$.