Meaning
The undesirable deposition of metallic sodium on the surface of the anode in a sodium-ion battery during high-rate or low-temperature charging. This phenomenon occurs when the anode potential drops below the reduction potential of sodium ions, causing them to form metallic deposits instead of inserting into the active material. It is measured by differential capacity analysis, low-temperature cell tear-downs, and cycling efficiency calculations.
The boundary of this reaction is defined by the electrochemical potential of the anode, which must remain above zero volts against the sodium reference potential to prevent plating.
Plating Mechanism
This metallic deposition is driven by kinetic limitations that prevent the rapid insertion of sodium ions into the hard carbon anode. When the charging current is high, the concentration of sodium ions at the anode surface exceeds the rate of diffusion into the carbon structure. This creates a high localized polarization that drives the anode potential below the threshold for metallic sodium reduction.
Low temperatures exacerbate this issue by reducing the diffusion coefficient of sodium within the carbon lattice and increasing the viscosity of the electrolyte. Consequently, the ions accumulate at the interface and deposit as a metallic layer on the electrode surface.
Safety Risk
The presence of metallic sodium on the anode poses severe reliability and safety hazards for the battery system. This plated metal is highly reactive and continuously decomposes the electrolyte, leading to the rapid growth of resistive surface layers and the consumption of active sodium. Over time, the metallic deposits can grow into branch-like structures called dendrites that penetrate the separator and cause an internal short circuit.
This short circuit triggers rapid localized heating, which can lead to thermal runaway and fire. Managing this risk is essential for cells used in electric vehicles where fast-charging protocols are frequently applied.
Detection Method
Identifying this metallic deposition without dismantling the cell involves analyzing the voltage relaxation curve immediately after charging is completed. When plated sodium re-dissolves into the anode, it creates a distinct voltage plateau during the relaxation phase, which can be detected by sensitive monitoring systems. Additionally, continuous cycling under conditions that trigger plating results in a sudden drop in coulombic efficiency and an increase in internal resistance.
Battery management systems utilize these diagnostic signatures to adjust the charging current dynamically. This proactive control prevents the onset of plating and ensures the safe operation of the battery pack.
