Operando Gravimetric and Energy Loss Analysis of Na₃V₂(PO₄)₂F₃ Composite Films by Electrochemical Quartz Microbalance with Dissipation Monitoring

The rising demand for energy storage calls for technological advancements to address the growing needs. In this context, sodium-ion (Na-ion) batteries have emerged as a potential complementary technology to lithium-ion batteries (Li-ion). Among other materials, Na₃V₂(PO₄)₂F₃ (NVPF) is a promising cathode for Na-ion batteries due to its high operating voltage and good energy density. In order to further characterize the (dis)charge behavior of NVPF, the electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) was employed to track both the frequency and dissipation loss changes at the electrode/electrolyte interface. The electrode composite preparation proved to be crucial for extending the potential window to both Na₃V₂(PO₄)₂F₃/Na₂V₂(PO₄)₂F₃ and Na₂V₂(PO₄)₂F₃/Na₁V₂(PO₄)₂F₃ domains. Composites prepared with rawNVPF powder (1-20 μm particles) and polyvinylidene fluoride (PVDF) binder (raw-NVPF:PVDF) exhibited large dissipation changes during (dis)charging, attributed to the soft viscoelastic nature of the binder and substantial hydrodynamic interaction caused by the large particles. On the other hand, composites prepared by sieved NVPF particles (<1 μm particles) with sodium carboxymethyl cellulose (NaCMC) binder (sieved-NVPF:NaCMC) showed rigid properties, enabling an extended and more accurate gravimetric analysis. This allowed for the determination of a linear charge-to-mass relationship for the full potential window of NVPF, reflecting the potential independent insertion/deinsertion of bare Na ions (23 g·mol^-1). Additionally, reversible dissipative changes were observed for the Na₃V₂(PO₄)₂F₃/Na₂V₂(PO₄)₂F₃ transition, with no further dissipative changes observed during the Na₂V₂(PO₄)₂F₃/Na₁V₂(PO₄)₂F₃ process.