Electrochemical impedance spectroscopy (EIS) is a powerful tool for investigating lithium metal anode stability, dendrite formation, and electrolyte interactions. By analyzing impedance spectra at different frequencies, EIS provides insights into interfacial reactions, charge transfer kinetics, and mass transport phenomena. This technique is particularly valuable for studying dynamic changes during cycling, enabling correlations between impedance trends and Coulombic efficiency.

Lithium metal anodes face challenges such as unstable solid electrolyte interphase (SEI) formation, dendritic growth, and electrolyte decomposition. EIS helps characterize these processes by decomposing the total impedance into contributions from bulk electrolyte resistance, SEI resistance, charge transfer resistance, and diffusion-related Warburg impedance. The evolution of these components during cycling reveals degradation mechanisms and their impact on performance.

A key application of EIS is monitoring SEI stability. The SEI layer forms during initial cycles and ideally acts as a protective barrier, preventing further electrolyte reduction. However, repeated cycling leads to SEI fracture and reformation, increasing interfacial resistance. EIS tracks this through the mid-frequency semicircle in Nyquist plots, which corresponds to SEI resistance. Studies show that electrolytes with optimized additives, such as fluoroethylene carbonate (FEC), produce more stable SEI layers, evidenced by smaller increases in resistance over time.

Dendrite formation is another critical issue. Dendrites not only pose safety risks but also increase impedance due to their high surface area and irregular morphology. EIS detects dendrite-related impedance changes through shifts in the low-frequency Warburg region, which reflects lithium ion diffusion. As dendrites grow, the diffusion path becomes more tortuous, altering the Warburg slope. Case studies demonstrate that electrolytes suppressing dendrite formation exhibit stable Warburg behavior, whereas uncontrolled growth leads to significant impedance spikes.

Electrolyte interactions are also probed by EIS. The high-frequency intercept in Nyquist plots represents bulk electrolyte resistance, which can change due to salt depletion or solvent decomposition. For instance, in concentrated electrolytes, reduced free solvent molecules lower ionic mobility, increasing bulk resistance. EIS helps identify optimal salt concentrations where trade-offs between conductivity and stability are balanced.

Case studies highlight correlations between impedance trends and Coulombic efficiency. In one study, a lithium metal cell with a conventional carbonate electrolyte showed a rapid rise in SEI resistance within 50 cycles, accompanied by a drop in Coulombic efficiency from 98% to 85%. In contrast, a cell with a dual-salt electrolyte maintained stable SEI resistance and Coulombic efficiency above 99% for 200 cycles. The EIS data revealed that the dual-salt formulation promoted a more conductive and mechanically robust SEI, reducing side reactions.

Another study tracked impedance during galvanostatic cycling of lithium-symmetric cells. Cells with unstable SEIs exhibited increasing charge transfer resistance and erratic Warburg slopes, indicating uneven lithium deposition. Post-mortem analysis confirmed dendritic growth and dead lithium formation. Cells with modified electrolytes, however, showed minimal impedance growth and smooth deposition morphologies, correlating with high Coulombic efficiency.

EIS also differentiates between charge transfer and diffusion limitations. At high current densities, charge transfer resistance often dominates, leading to voltage hysteresis and poor cycling. By analyzing the high-frequency semicircle, researchers can optimize electrode kinetics through surface coatings or electrolyte additives. For example, lithium nitrate additives reduce charge transfer resistance by facilitating ion transport across the interface.

Temperature effects on impedance are another area of interest. Low temperatures exacerbate SEI instability and dendrite growth, increasing overall impedance. EIS studies reveal that certain electrolytes maintain lower resistance at subzero temperatures, enabling better performance in extreme conditions.

In summary, EIS is indispensable for understanding lithium metal anode behavior. By dissecting impedance spectra, researchers identify failure modes, evaluate electrolyte formulations, and correlate electrochemical trends with cycling performance. Case studies demonstrate that stable SEI, suppressed dendrites, and optimized electrolytes lead to lower impedance and higher Coulombic efficiency. These insights guide the development of safer and more durable lithium metal batteries.

The following table summarizes key impedance components and their implications:

| Frequency Range | Impedance Component | Physical Meaning | Implications |
|----------------|---------------------|------------------|--------------|
| High | Bulk Resistance | Electrolyte conductivity | Salt concentration, solvent effects |
| Mid | SEI Resistance | Interfacial stability | SEI growth, fracture |
| Low | Charge Transfer | Reaction kinetics | Dendrites, surface coatings |
| Very Low | Warburg Diffusion | Ion transport | Porosity, dendrite morphology |

Quantitative EIS data from recent studies further illustrate these points. For example, cells with unstable SEIs exhibit SEI resistance increases of over 200% within 100 cycles, while stable systems show less than 50% growth. Similarly, charge transfer resistance in dendrite-prone cells can double after 50 cycles, whereas optimized systems remain within 20% of initial values.

These findings underscore the importance of EIS in diagnosing and mitigating lithium metal anode challenges. By linking impedance behavior to macroscopic performance metrics, researchers can accelerate the development of advanced battery systems.