Bilayering Enables Faster Charging and High Areal Capacity of Na-Ion Electrodes
Authors: Brown Galen , Zankowski Stanislaw , Grant Patrick S. 
Published: December 2024

Sodium-ion batteries (NIBs) are a promising alternative to lithium-ion batteries (LIBs), which currently dominate electric vehicle and portable device applications. But while NIBs have potential cost and safety advantages, they still lag behind LIBs in capacity, power output and longevity. Considerable research and development effort is now being expended to develop more energy-dense electrodes in order for NIBs to become commercially viable.
A possible route to increasing the energy density of NIBs is to increase electrode thickness (and therefore areal capacity), from presently used ~50 μm to 100 - 150 μm. However, achieving this in practice is limited by multiple factors related to manufacturing and rate performance of thicker electrodes. Slot-die coating is the dominant manufacturing process for electrodes, which involves mixing of active material, conductive additive, binder and solvent, coating the resulting slurry onto current collecting foil, and drying. The drying step places a practical limit on the dried thickness, as it is accompanied by binder migration away from the current collector, potentially reducing the adhesion and mechanical strength of the electrode. As a result, the maximum thickness of electrodes in industry is typically constrained to 50 - 90 µm. Furthermore, given the mobility of Na ions in the electrolyte in the torturous electrode pore network is normally restricted, thicker electrodes undergo greater polarisation and capacity loss as C-rate (current density) is increased. This means thicker electrodes cannot be charged/discharged quickly without detriment to energy density.
Here we present a novel bilayer architecture of Na-ion positive electrodes obtained through slurry casting two successive sub-layers of different active materials: sodium manganese oxide (Na0.7MnO2, NMO) and sodium vanadium phosphate (Na3V2(PO4)3, NVP). The bilayer electrodes are 100 µm thick with high areal loading of ≈17 mg/cm2, which might normally be expected to inhibit high-rate performance. As expected, for low current densities < 1C ≡ 45 mA/gcm2, the charge capacity of the bilayer cathodes is similar and comparable to a simple, homogeneous (i.e. random) mixture of the two active materials, regardless of how the sub-layers are arranged. However, at 2C and 3C, overall electrode capacity is revealed to be sensitive to the microstructural arrangement, such as the relative position of NMO and NVP sub-layers. There is a significant increase in retained capacity when the NMO sub-layer is on the current collector. In order to understand the sensitivity of overall capacity to structural arrangement, simulations were performed that are formulated in a way to capture the designed microstructural heterogeneity of the electrodes. The simulations show the complex interaction between the voltage-capacity profile of the active material, the relative position of the sub-layer, and the evolution of Na ion gradients through the thickness of the electrode. In the most favourable arrangement, the dispersion of local overpotential due to local concentration polarization effects is reduced, which in turn enables greater utilization of the active materials. The sensitivity of sub-layer position is shown to relate to the significant difference in voltage-capacity profiles of NMO and NVP. Differences in charge-transfer resistance, electrical conductivity, and ionic mobility within the two materials also have a role in how the critical Na ion concentration profile evolves through the charge/discharge cycle.
We discuss how the concept of layered or other types of structured electrode might be applied to Na ion batteries, and to what extent these effects might increase the competitiveness of Na ion batteries compared with their more established LIB counterparts.

Figure 1