After lithium-ion batteries complete formation and aging, inspection and analysis help identify defects and evaluate electrochemical performance. The following overview covers common visual, electrical, cycle-life, rate and safety tests used in cell assessment.
I. Visual Appearance Inspection
Pouch Cells
Inspect the overall integrity of pouch cells, focusing on swelling, electrolyte leakage, surface corrosion and other defects. A preliminary assessment of cell condition and abnormal operating conditions can be made based on surface abnormalities.
Prismatic Aluminum-shell Cells
Check for electrolyte leakage, deformation or dents in the aluminum shell, asymmetric terminal installation and other defects.
Battery Swelling Analysis
Determine whether swelling occurs at room temperature or high temperature, and check the consistency of the battery pack (single-cell swelling or batch swelling).
Batch swelling: Generally attributed to flaws in electrode material systems or overall production processes; titanium lithium oxide batteries are prone to obvious swelling, and unreasonable structural design & assembly techniques will also cause batch expansion.
Single-cell swelling: Usually caused by severe capacity degradation, internal gas generation and other individual cell failures.
Electrolyte Leakage Analysis
Locate leakage points (tab area or liquid injection port), then confirm root causes: shell design defects, incomplete aluminum plastic film sealing, poor welding on liquid injection holes, etc.
Surface Corrosion Analysis
Determine corrosion triggers: electrolyte splashing during liquid injection, long-term leakage, collision-induced short circuits, or electrochemical corrosion. Defective cells shall be eliminated promptly once detected.
II. Basic Electrochemical Performance Tests
1. OCV, Internal Resistance & Thickness Measurement
For new battery batches, open-circuit voltage (OCV), internal resistance (IR) and cell thickness are measured first.
OCV measurement: Pinpoints abnormal initial cell status.
Internal resistance test: Quickly screens defective cells, providing warning of potential degradation and micro-short circuits.
Thickness monitoring: Tracks swelling problems generated during cycling and operation.
2. Capacity Verification
2 to 3 rounds of capacity tests are adopted instead of a single test to guarantee accuracy while avoiding redundant testing.
Conventional test standard: Charging and discharging at 1C in accordance with national standards; 0.5C discharging will yield slightly higher measured capacity than 1C discharging.
Low-current charge-discharge test (0.1C or lower current): Eliminates polarization interference, accurately reflecting actual cell capacity, median voltage and complete cycling curves.
Advanced precise testing: With external high-precision voltage collectors, voltage-capacity curves can be derived and differentiated to analyze the degree of cell degradation and the failure modes by observing the shift and shape of characteristic peaks.
3. Three-Electrode Testing
Current flowing inside batteries produces an ohmic voltage drop and electrode polarization, making it impossible to accurately measure the working electrode potential directly. A reference electrode is therefore introduced in three-electrode systems. The reference electrode has a stable potential and carries no current, so it is free from polarization. The potential of the working electrode is measured against the reference electrode, while current flows through the working electrode and counter electrode loop. Three-electrode testing is widely used in lithium battery research to identify potential changes in cathodes and anodes separately and locate problematic electrodes. Lithium metal is commonly used as the reference electrode, placed between the cathode and anode rather than randomly inside the cell. During three-electrode assembly, measures must be taken to preserve the original cell state and avoid short circuits or abnormal contact interference.
4. Electrochemical Impedance Spectroscopy (EIS)
Electrochemical Impedance Spectroscopy (EIS) is a core analytical method for electrochemistry; precise fitting and analysis of impedance spectra lay the foundation for accurate battery diagnosis. Impedance reflects the energy consumption inside electrochemical systems. Nyquist plots and Bode plots intuitively characterize electrochemical reactions, interfacial impedance and ion diffusion behavior inside cells. Impedance analysis relies heavily on comparative testing between qualified reference samples and defective cells, which helps identify advantages, defects and optimization directions. Combining three-electrode technology with EIS enables separate impedance measurement of cathodes and anodes, clarifying internal impedance distribution. It provides detailed cell information without destructive disassembly.
III. Advanced Lithium Battery Performance Tests
After basic electrochemical screening, cells with excellent consistency and low impedance are selected for comprehensive performance evaluation.
1. Cycle Life Test
Cycle life refers to the maximum number of repeated charge-discharge cycles of batteries. Cycle tests are conducted separately at low, room, and high temperatures according to the intended application.
Power batteries (EVs, electric forklifts): The end-of-life standard is defined as capacity retention falling to 80% of initial capacity.
Energy storage batteries: The end-of-life standard is set at 60% capacity retention. Cells with capacity retention below 60% are generally considered unqualified for practical use due to severe performance attenuation.
2. Rate Performance Test
Lithium batteries are widely used in consumer electronics and new energy vehicles. Dynamic driving conditions and fast-charging demands require outstanding rate capability, which is tested following national standards for power batteries. High-rate batteries can be optimized from multiple dimensions: active material selection, electrode areal density, compaction density, tab design, welding technology and assembly process.
3. Safety Performance Test
Safety is the top priority for battery users. Mandatory safety tests include overcharging, overdischarging, short circuit, drop, heating, vibration, extrusion, nail penetration and other abuse tests. These are passive destructive tests that simulate external damage. Although standardized abuse tests can verify cell safety, actual collisions and accidents are far more complex and irregular, while full simulations of real-world scenarios are prohibitively expensive, so standardized testing remains the mainstream evaluation method. Currently, ternary lithium and lithium iron phosphate (LFP) dominate the market. Ternary materials suffer structural collapse under high temperatures, leading to poorer thermal stability than LFP batteries, while ternary lithium possesses higher energy density. The two technical routes will coexist and develop in parallel.
Additional conventional tests include low-temperature discharge performance and high-temperature discharge performance characterization.