Sodium-ion batteries have emerged as a promising alternative to lithium-ion systems due to the abundance of sodium and potential cost advantages. However, several failure modes specific to their chemistry and materials can degrade performance and limit cycle life. Understanding these mechanisms is critical for improving battery durability and safety.

Metal plating, particularly sodium deposition, is a significant failure mode in sodium-ion batteries. Unlike lithium, sodium has a lower reduction potential, which increases the risk of metallic plating on the anode during fast charging or low-temperature operation. Plating occurs when sodium ions accumulate faster than they can intercalate into the anode material, forming dendrites that penetrate the separator and create internal short circuits. Hard carbon, a common anode material, is susceptible to plating due to its sloping voltage profile, which reduces the thermodynamic overpotential for sodium nucleation. In-situ X-ray diffraction (XRD) can detect early-stage plating by identifying crystalline sodium metal peaks during cycling. Post-mortem analysis using scanning electron microscopy reveals dendritic structures and their penetration depth into the separator.

Electrode cracking is another critical failure mechanism, driven by the larger ionic radius of sodium compared to lithium. The repeated insertion and extraction of sodium ions induce mechanical stress in electrode materials, leading to particle fracture and loss of electrical contact. Layered oxide cathodes, such as NaₓMnO₂, experience phase transitions during cycling that exacerbate structural degradation. In-situ XRD tracks these phase changes in real time, correlating them with capacity fade. Cross-sectional analysis of cycled electrodes shows microcracks propagating through active material particles, increasing internal resistance. Binders and conductive additives play a crucial role in mitigating cracking by maintaining electrode integrity, but their effectiveness diminishes over extended cycling.

Electrolyte depletion is a gradual but impactful failure mode in sodium-ion batteries. The solid-electrolyte interphase (SEI) formed on anode surfaces consumes active sodium and electrolyte components, reducing available charge carriers. Unlike lithium systems, sodium SEI layers are less stable and more prone to dissolution, leading to continuous electrolyte breakdown. Fluorine-containing salts, such as NaPF₆, decompose into corrosive byproducts that accelerate electrolyte depletion. Gas chromatography-mass spectrometry of aged electrolytes identifies decomposition products, while electrochemical impedance spectroscopy quantifies increasing resistance due to SEI growth. The loss of electrolyte conductivity directly impacts rate capability and energy efficiency.

Diagnostic techniques for these failure modes require specialized approaches beyond standard safety testing. Post-mortem analysis involves disassembling cells in an inert atmosphere to prevent air exposure from altering degradation products. X-ray photoelectron spectroscopy characterizes SEI composition, revealing differences between sodium and lithium systems. In-situ pressure measurements detect gas evolution from electrolyte decomposition, which correlates with capacity loss. Transmission electron microscopy provides nanoscale insights into electrode cracking and interface degradation.

Quantitative studies show that sodium plating onset occurs at current densities above 1 mA/cm² in hard carbon anodes, with dendritic growth accelerating beyond this threshold. Electrode cracking severity depends on the cathode material, with polyanionic compounds exhibiting better structural stability than layered oxides. Electrolyte depletion rates vary with solvent composition, reaching up to 20% loss after 500 cycles in carbonate-based systems.

Mitigation strategies for these failure modes include anode surface modifications to homogenize sodium deposition, strain-tolerant cathode architectures to resist cracking, and electrolyte additives to stabilize the SEI. Advanced diagnostics like in-situ XRD and spatially resolved spectroscopy enable targeted improvements by linking specific degradation mechanisms to performance loss. Continued research into sodium-ion battery failure modes will be essential for achieving commercial viability and matching the reliability of established lithium-ion technologies.