Magnesium batteries represent an emerging energy storage technology with potential advantages in safety, cost, and sustainability. Evaluating their performance requires specific metrics that account for the unique electrochemical behavior of magnesium. Key performance indicators focus on voltage characteristics, capacity retention, and efficiency, while testing protocols must address activation overpotential and conditioning requirements inherent to magnesium electrochemistry.

Voltage characteristics in magnesium batteries differ significantly from lithium-ion systems. The theoretical redox potential of Mg/Mg²⁺ is -2.37 V versus the standard hydrogen electrode, suggesting high energy density potential. However, practical discharge voltages typically range between 1.0-1.8 V due to overpotential at both electrodes. The voltage profile shows distinct plateaus corresponding to magnesium insertion or alloying reactions in the cathode material. Voltage hysteresis between charge and discharge cycles is more pronounced than in lithium systems, often exceeding 0.5 V, which directly impacts energy efficiency. During initial cycles, the formation of a passivation layer on the magnesium anode creates an additional voltage drop of 0.2-0.4 V that must be accounted for in performance assessments.

Capacity metrics require careful interpretation due to magnesium's divalent nature. Gravimetric capacity calculations must consider the two-electron transfer per magnesium ion, with theoretical values reaching 2,205 mAh/g for magnesium metal anodes. Practical anode capacities typically achieve 300-800 mAh/g depending on the electrolyte system and cycling conditions. Cathode capacities vary widely by material class, with Chevrel phase compounds demonstrating 80-120 mAh/g and sulfur-based cathodes reaching 400-600 mAh/g in experimental cells. Areal capacity loading is particularly important for magnesium systems, with optimal performance observed between 1.0-2.5 mAh/cm². Higher loadings exacerbate polarization effects due to magnesium's slow solid-state diffusion kinetics.

Efficiency measurements focus on three key parameters: Coulombic efficiency, energy efficiency, and cycling efficiency. Initial Coulombic efficiency in magnesium batteries often falls below 80% due to irreversible electrolyte decomposition and anode passivation. Stable cycling typically achieves 95-99% Coulombic efficiency after conditioning cycles. Energy efficiency, calculated as the ratio of discharge energy to charge energy, ranges from 75-85% for optimized systems, significantly lower than lithium-ion's 90-95% due to higher overpotentials. Cycling efficiency accounts for capacity fade over time, with magnesium systems typically losing 0.5-1.5% capacity per cycle compared to 0.1-0.3% for commercial lithium-ion.

Testing protocols for magnesium batteries must incorporate specific conditioning procedures. A standard protocol includes:
1. Formation cycling at C/20 rate for 2-5 cycles to establish stable interfaces
2. Activation cycles at progressively increasing rates from C/10 to C/3
3. Polarization tests with voltage hold steps to characterize overpotential components
4. Reference electrode measurements to separate anode and cathode contributions

The activation overpotential in magnesium systems requires extended stabilization periods. Testing should include at least 20 preconditioning cycles before collecting performance data, compared to 3-5 cycles for lithium-ion. Temperature control is critical, with recommended testing at 25±2°C due to magnesium's strong temperature dependence. Electrochemical impedance spectroscopy should be performed at multiple states of charge to track interface evolution.

Comparative performance data shows magnesium batteries currently lag behind lithium-ion in most metrics but offer potential advantages:

Energy Density:
Magnesium prototype cells: 100-150 Wh/kg
Commercial Li-ion: 250-300 Wh/kg

Power Density:
Magnesium: 50-200 W/kg
Li-ion: 300-500 W/kg

Cycle Life:
Magnesium: 200-500 cycles (80% capacity)
Li-ion: 1,000-2,000 cycles

Cost Projections:
Magnesium materials: $5-10/kWh
Lithium materials: $15-25/kWh

Safety testing reveals magnesium's advantages in thermal stability. Magnesium cells withstand nail penetration tests without thermal runaway up to 150°C, compared to lithium-ion failure at 80-120°C. Accelerated rate calorimetry shows peak heat generation rates of 5-10°C/min for magnesium versus 50-100°C/min for lithium-ion under short-circuit conditions.

The divalent nature of magnesium ions creates unique challenges in rate capability testing. Standard C-rate definitions require adjustment since each magnesium ion carries twice the charge of lithium. A proposed modified C-rate calculation divides current by twice the theoretical capacity to enable direct comparison with lithium systems. Even with this adjustment, magnesium batteries typically show poorer rate performance, with capacity retention dropping to 60-70% at 1C compared to lithium-ion's 85-95%.

Long-term degradation mechanisms differ substantially from lithium systems. Magnesium batteries experience three primary failure modes:
1. Anode passivation layer growth (0.5-1 nm/cycle)
2. Cathode structural collapse from Mg²⁺ insertion strain
3. Electrolyte depletion through side reactions

Post-mortem analysis protocols require special handling due to magnesium's air sensitivity. Sample preparation should occur in argon gloveboxes with oxygen levels below 0.1 ppm. X-ray diffraction analysis must account for magnesium's tendency to form amorphous discharge products that lack crystalline signatures.

Performance validation requires statistical sampling across multiple cell formats. Coin cells (CR2032) provide initial screening but overestimate performance due to limited current collection issues. Pouch cell testing with capacities of 50-100 mAh provides more reliable data for energy density calculations. Three-electrode configurations are strongly recommended for mechanistic studies.

Standardized testing protocols are still evolving for magnesium batteries. Current best practices recommend:
- Minimum three-cell replicates for each test condition
- Calendar aging tests at multiple states of charge (0%, 50%, 100%)
- Electrochemical windows verified by cyclic voltammetry before full-cell testing
- Electrolyte conductivity measurements at multiple concentrations

Future development of magnesium battery technology will require improved testing standards that account for the chemistry's unique characteristics while enabling meaningful comparison with established battery systems. The current performance gap with lithium-ion reflects the early stage of development rather than fundamental limitations, with significant improvements expected through electrolyte engineering and cathode materials development.