Charge and discharge mechanism

A charger applies a voltage above the cell’s instantaneous open-circuit voltage and drives electrons out of the cathode current collector, through the charger, and into the anode. To maintain charge neutrality, transition metals or other charge-compensation centres in the cathode are oxidized and sodium ions leave the host lattice. Solvent molecules coordinate the ions as they move through cathode pores, separator, and anode pores. At the anode surface the ions desolvate and enter hard-carbon layers, defects, or nanopores. Electrons cannot cross the separator; if they do, the cell has an internal short circuit. Charging does not put electricity into a container. It uses electrical work to change the chemical states of two electrodes.

When a load is connected, sodium stored in the anode tends to leave. Sodium is oxidized to sodium ions on the anode side, while electrons pass through the anode collector, tab, and external load toward the cathode. Sodium ions migrate through the electrolyte in the opposite direction and re-enter the cathode as cathode species are reduced. The external device receives energy because the free energy of the two electrode reactions decreases. Electrode composition and potential difference change throughout discharge, so terminal voltage is rarely a perfectly flat line. Continued discharge below the lower cutoff can damage active material, collectors, or interphases, which is why the BMS disconnects the load.

From inside a cathode particle to a storage site in the anode, a sodium ion normally crosses solid-state diffusion, surface charge transfer, solvation and migration in electrolyte, separator pores, desolvation at the other electrode, and re-entry into a solid. At high rate the slowest stage magnifies concentration gradients and overpotential. At low temperature, electrolyte viscosity, desolvation, and interfacial charge transfer often slow sharply. In a thick electrode, ionic and electronic paths lengthen and different regions can react unevenly. Fast-charge design therefore has to improve the entire transport chain rather than only the diffusion coefficient of an active-material powder.

Layered oxides and many polyanionic cathodes mainly use insertion and extraction. Structural transitions, sodium-vacancy ordering, and multiple redox centres produce slopes or plateaux. Prussian blue analogues host sodium at defined sites in open frameworks, with water and vacancies altering transport. Hard-carbon anodes commonly show both a higher-voltage slope and a low-voltage plateau, associated with combinations of surface adsorption, interlayer insertion, and nanopore filling. Alloying or conrevisions anodes undergo still larger structural changes. The familiar arrow from cathode to anode describes only the overall direction; it cannot replace material-specific reaction mechanisms.

A real charge-discharge profile includes changing equilibrium potential, ohmic drop, charge-transfer overpotential, and concentration polarization. At higher current the charge voltage rises and discharge voltage falls, causing earlier contact with cutoff and reducing usable capacity. When current stops, voltage partly relaxes. If a plateau shortens or charge-discharge separation grows with cycling, possible causes include loss of cyclable sodium, increasing impedance, or structural change. Diagnosis cannot rely on curve shape alone; temperature, current, rest periods, impedance, and capacity retention are needed together. These cell-level effects then appear in a pack as power, heat, and consistency problems.