The initial formulation window for electrochemical double layer capacitors operating above 2.5 V is largely defined by the solvent’s dielectric constant, viscosity, impurity sensitivity, and interfacial stability on porous carbon. Propylene carbonate exhibits a dielectric constant of 64.9 at 25 °C, a viscosity of approximately 2.5 mPa·s at 25 °C, a melting point of −49 °C, and a closed-cup flash point near 132 °C. These values permit quaternary ammonium tetrafluoroborate salts to dissociate sufficiently for practical double-layer capacitance, yet the viscosity imposes a conductivity penalty when compared with acetonitrile. Published datasheets for capacitor-grade electrolytes report conductivities of 11 mS cm⁻¹ to 14 mS cm⁻¹ for 1 M triethylmethylammonium tetrafluoroborate in propylene carbonate at 25 °C, whereas equivalent acetonitrile solutions commonly exceed 50 mS cm⁻¹. The wide liquid range of propylene carbonate supports storage and operation from −40 °C to 70 °C in sealed cells, although the useful low-temperature performance is restricted by resistance rise rather than freezing. Propylene carbonate is hygroscopic and equilibrates with ambient air at water concentrations above 1,000 mg kg⁻¹, so capacitor-grade material is transferred under dry nitrogen and stored in closed stainless-steel systems. Electrolyte preparation for high-voltage cells therefore requires drying to ≤20 mg kg⁻¹ water, with chloride and free acid limited to ≤5 mg kg⁻¹ and ≤10 mg kg⁻¹, respectively. The solvent’s high anodic stability on inert electrodes, reported above 5.0 V versus Li/Li⁺ for thoroughly dried material, is reduced on activated carbon electrodes by surface oxygen groups, edge-plane defects, and collector corrosion processes. Consequently, the practical rated voltage of propylene carbonate-based cells on production carbon is commonly specified between 2.7 V and 3.0 V, with the final value determined by endurance testing rather than solvent purity alone.

How Does Propylene Carbonate Maintain Electrochemical Stability Above 2.7 V?

The observed stability above 2.7 V in propylene carbonate-based electrolytes is an interfacial condition produced by low moisture content, controlled carbon surface chemistry, and passivated aluminum current collectors. In three-electrode cyclic voltammetry on glassy carbon, 1 M triethylmethylammonium tetrafluoroborate in dry propylene carbonate shows an anodic onset beyond 3.0 V versus Ag/Ag⁺, but porous activated carbon electrodes introduce quinone, lactone, and phenol groups that shift the practical oxidation limit downward. Differential electrochemical mass spectrometry on cells held at 3.0 V and 60 °C has identified carbon dioxide, propylene, and hydrogen as the dominant volatile products when water contamination exceeds 50 mg kg⁻¹. The degradation pathway begins with oxidation of residual water at the positive carbon surface, generating protons that catalyze ring-opening of propylene carbonate and produce hydroxypropyl carbonate intermediates. These intermediates undergo further oxidation to carbon dioxide and oligomeric deposits that increase equivalent series resistance. High-voltage stability is therefore maintained by limiting post-fill water to 15 mg kg⁻¹ to 30 mg kg⁻¹, selecting activated carbon with oxygen content below 1.0 wt% by X-ray photoelectron spectroscopy, and using carbon-coated aluminum current collectors. Practical cells rated at 2.7 V continuous may permit short-term excursions to 2.85 V only when leakage current remains below 0.5 mA F⁻¹ after 72 h of voltage hold. The aluminum current collector is a critical weak point: unetched aluminum in propylene carbonate can pit above 2.5 V versus activated carbon when chloride exceeds 5 mg kg⁻¹, and the native oxide layer can be dissolved by fluoroboric acid generated from salt hydrolysis. Tests under IEC 62391-1 define rated voltage through endurance at the upper category temperature, typically requiring capacitance retention above 80% of initial value after 1,000 h at 65 °C. Cells with propylene carbonate electrolytes often require a voltage derating from 3.0 V to 2.7 V when internal gas generation exceeds 10% of initial cell volume, because gas evolution signals solvent oxidation even when capacitance remains temporarily stable. The rated voltage therefore reflects the stability of the entire electrode–collector–electrolyte interface, not the intrinsic electrochemical window of propylene carbonate on an inert substrate. In contrast to acetonitrile-based formulations, propylene carbonate electrolytes impose a lower ionic conductivity that becomes the dominant performance limit below −10 °C. The conductivity of 1 M triethylmethylammonium tetrafluoroborate in propylene carbonate decreases from approximately 12.8 mS cm⁻¹ at 25 °C to 2.0 mS cm⁻¹ to 3.5 mS cm⁻¹ at −30 °C, while acetonitrile solutions retain values above 25 mS cm⁻¹ under the same conditions. This decrease follows Vogel-Tammann-Fulcher behavior rather than simple Arrhenius activation, because ion transport is coupled to solvent segmental motion and free volume. The consequence in a 3,000 F cylindrical cell is an increase in equivalent series resistance from approximately 0.25 mΩ at 25 °C to 1.8 mΩ at −30 °C when propylene carbonate is the sole solvent. Manufacturers compensate by blending linear carbonates or esters that reduce viscosity while retaining the high flash point of the mixture. Ethyl methyl carbonate lowers viscosity but reduces flash point to approximately 23 °C when present above 30 vol%, creating a flammability penalty that must be managed in module certification. Gamma-butyrolactone has a higher boiling point but introduces a lactone ring that can hydrolyze to gamma-hydroxybutyric acid under acidic conditions, and this hydrolysis product may accelerate current collector corrosion. The blending window is therefore constrained by viscosity, flash point, hydrolytic stability, and conductivity. Conductivity measurements are performed by electrochemical impedance spectroscopy in platinized conductivity cells calibrated with 0.1 M potassium chloride according to ASTM D1125-23, and the temperature coefficient is recorded over −30 °C to 70 °C. In a 48 V module with 10 A pulse current, the voltage sag at −20 °C can exceed 3 V when cells use propylene carbonate-only electrolytes, because the internal resistance of each cell rises rapidly with decreasing temperature. This behavior forces system designers to specify minimum operating temperature based on the conductivity threshold at which internal resistance doubles, not on electrolyte freezing or melting point.

Solvent-Co-Solvent Blends That Extend the Operational Temperature Floor

Blending propylene carbonate with ethylene carbonate, ethyl methyl carbonate, or propyl propionate modifies the low-temperature conductivity and high-voltage endurance of the electrolyte. Ethylene carbonate is solid at room temperature but dissolves in propylene carbonate to raise dielectric constant and improve salt dissociation; however, ethylene carbonate content above 30 wt% raises the eutectic melting point and can cause precipitation at −20 °C. A ternary blend of propylene carbonate, ethylene carbonate, and ethyl methyl carbonate at 40:20:40 vol% retains a conductivity above 8 mS cm⁻¹ at −20 °C and supports operation down to −30 °C, but the flash point drops below 30 °C, which may trigger additional transport classification requirements under UN 38.3. Propyl propionate lowers viscosity to approximately 0.7 mPa·s at 25 °C but has lower anodic stability, and gas evolution at 2.85 V increases by a factor of three when propyl propionate exceeds 20 vol%. The property cliffs occur at specific volume fractions: ethylene carbonate above 25 vol% causes precipitation; linear esters above 30 vol% compromise flash point; and propylene carbonate below 50 vol% reduces the high-voltage advantage. Industrial formulations for high-voltage modules typically maintain propylene carbonate at 60 vol% to 80 vol%, with ethylene carbonate as a minority additive and linear carbonate limited to 10 vol% to 20 vol%. Rheological and electrochemical data for these blends are generated with cone-and-plate viscometers at shear rates from 10 s⁻¹ to 1,000 s⁻¹, and conductivity is measured under dry nitrogen in sealed cells with ±0.1 °C temperature control. The viscosity of propylene carbonate-rich blends at −20 °C rises to 30 mPa·s to 60 mPa·s, which extends electrode wetting time during production and increases the required vacuum soak from 4 h to 12 h for a 3,000 F cell depending on separator thickness. The comparative data below summarizes typical ranges reported in capacitor-grade electrolyte supplier literature for different solvent systems.

Solvent system	Conductivity at 25 °C (mS cm⁻¹)	Conductivity at −20 °C (mS cm⁻¹)	Flash point (°C)	Practical voltage window (V)
100 vol% propylene carbonate, 1 M TEABF₄	11–14	3–5	132	2.7–3.0
Propylene carbonate/ethylene carbonate/ethyl methyl carbonate 40:20:40 vol%, 1 M TEABF₄	16–20	8–10	25–30	2.7–2.8
Propylene carbonate/gamma-butyrolactone 70:30 vol%, 1 M TEABF₄	14–18	4–6	90–110	2.7

When Propylene Carbonate Replaces Acetonitrile in High-Voltage Modules

When propylene carbonate replaces acetonitrile in high-voltage modules, the design must absorb lower conductivity, higher viscosity, and a different gas-generation profile under abuse conditions. Acetonitrile has a closed-cup flash point of 2 °C and a boiling point of 81 °C, while propylene carbonate has a flash point near 132 °C and a boiling point above 240 °C. This difference reduces the probability of solvent vapor ignition in vented cells but does not eliminate thermal risk because propylene carbonate decomposes exothermically above 200 °C and can produce carbon dioxide, propylene oxide, and allyl alcohol. Module manufacturers replacing acetonitrile with propylene carbonate often measure a 10% to 20% increase in equivalent series resistance at 25 °C and a larger increase at −20 °C, requiring a higher cell count or a reduction in maximum continuous current. The change also affects electrolyte filling: propylene carbonate wets polyolefin separators more slowly than acetonitrile due to higher surface tension and viscosity, so vacuum pressure during filling is reduced from −95 kPa to −99 kPa and the wetting hold time is extended. The sealed cell design must tolerate slightly higher internal pressure from carbon dioxide formation if voltage exceeds 2.85 V at 65 °C. Cylindrical cells with propylene carbonate-based electrolytes are tested under IEC 62391-2 for endurance cycling, which requires 1,000 h at rated voltage and upper category temperature with capacitance loss below 20% and equivalent series resistance increase below 200%. In practice, propylene carbonate-based cells at 2.7 V and 65 °C may exhibit equivalent series resistance increases of 50% to 100% after 1,500 h, depending on electrode purity and electrolyte additives. The replacement decision is therefore driven by safety classification, transport regulations, and end-use exposure rather than electrical performance. Under UN 38.3 transport testing, cells must pass altitude simulation, thermal cycling, vibration, and shock; propylene carbonate-based electrolytes can reduce the severity of thermal runaway propagation but do not exempt a module from the rechargeable cell test series. Uncontrolled moisture ingress during electrolyte production creates a cascade of degradation reactions that shorten high-voltage double-layer capacitor life. Propylene carbonate is hygroscopic and reaches equilibrium with ambient air at water contents above 1,000 mg kg⁻¹; capacitor-grade electrolytes are therefore blended and packaged in closed stainless-steel systems purged with nitrogen having a dew point below −50 °C. Incoming propylene carbonate is dried over molecular sieves with pore diameters of 0.3 nm to 0.4 nm or by vacuum distillation at pressures below 1 mbar. The specification for water in the final electrolyte is typically ≤20 mg kg⁻¹, and production lines use coulometric Karl Fischer titration according to ASTM E1064-24 to verify each batch. Chloride and acid impurities are controlled to ≤5 mg kg⁻¹ and ≤10 mg kg⁻¹, respectively, because chloride can pit aluminum current collectors and free acid can protonate carbonate solvents. Electrolyte batches are also analyzed for ethylene glycol and propylene glycol by gas chromatography with flame ionization detection; these diol impurities form during storage and indicate hydrolysis of propylene carbonate. The acceptable total glycol content in capacitor-grade propylene carbonate is below 50 mg kg⁻¹. In production, lot-to-lot variation in water concentration can shift leakage current at 2.7 V from 0.2 mA to 1.0 mA for a 3,000 F cell, and this directly affects end-of-line aging yield. To control this, filling is performed in dry rooms with dew point below −40 °C, and the electrolyte is dispensed through stainless-steel lines with volumetric dosing accuracy of ±2%. After filling, cells are rested for 24 h to 48 h before first charge to allow wetting and to avoid localized dry spots where voltage gradients can drive solvent oxidation. The aging protocol for propylene carbonate-based cells typically includes a constant-voltage hold at 2.7 V and 60 °C for 12 h to 24 h, during which leakage current decays as the carbon surface passivates. Cells that exceed the leakage current threshold after aging are either rejected or re-aged depending on batch history and voltage hold results.

Mitigating Water Ingress through Sealed Cell Design and Electrolyte Filling

Mitigating water ingress in propylene carbonate-based cells requires welded seals, low-permeability gaskets, and closed-loop electrolyte delivery. Aluminum-laminate pouch cells use a three-layer film with an aluminum barrier thickness of 30 μm to 50 μm; the water vapor transmission rate of the laminate is specified below 0.1 g m⁻² day⁻¹ at 40 °C and 90% relative humidity according to ISO 15106-3. Cylindrical cells rely on laser-welded seams with a weld penetration of 80% to 95% of the case thickness to minimize leak paths. The electrolyte fill port is sealed with a vent that opens at 0.8 MPa to 1.2 MPa; this pressure is selected above normal gas generation at 2.7 V and 65 °C but below the case burst pressure. In production, electrolyte filling is performed under a nitrogen atmosphere with oxygen concentration below 100 ppm and dew point below −40 °C. The fill amount is determined by separator porosity and electrode pore volume, with a target electrolyte excess of 5% to 15% by volume to account for consumption during formation. A typical 3,000 F cylindrical cell with 40 mm diameter and 150 mm height contains 90 mL to 110 mL of electrolyte. After filling, the cell is subjected to vacuum cycles between −95 kPa and atmospheric pressure to remove trapped gas from electrode pores. In propylene carbonate-based cells, incomplete wetting is detected by electrochemical impedance spectroscopy as a depressed semicircle at frequencies between 0.1 Hz and 10 Hz, corresponding to distributed ionic resistance in the electrode depth. The manufacturing limit for this charge-transfer resistance after wetting is typically 0.15 mΩ to 0.35 mΩ at 1 kHz for a 3,000 F cell, but values above 0.5 mΩ indicate insufficient vacuum soak or separator misalignment. Water ingress after sealing is monitored by destructive cell teardown and Karl Fischer titration of the electrolyte every 200 production cycles or when the end-of-line leakage current distribution shifts upward.

On the Role of Additive Chemistries in Passivating Carbon Electrodes

Additive chemistries in propylene carbonate-based double-layer capacitor electrolytes serve to passivate carbon electrode surfaces, complex trace moisture, and protect aluminum current collectors at high voltage. Vinylene carbonate is widely used in lithium-ion carbonate electrolytes, but its use in double-layer capacitors is limited because it can polymerize on carbon and reduce accessible surface area. Published data for propylene carbonate-based double-layer capacitor additives is more limited than for lithium-ion systems, and additive packages are often proprietary. Known additives include propane sultone, which forms a sulfur-containing passivation film on carbon and aluminum, and adiponitrile, which adsorbs on high-potential carbon sites and suppresses solvent oxidation. In accelerated testing at 2.85 V and 60 °C, cells with 0.5 wt% propane sultone have shown lower gas generation than additive-free cells, but the exact capacitance retention figures are not consistently reproducible across carbon sources. The effectiveness of an additive is evaluated by leakage current decay, equivalent series resistance growth, gas volume, and capacitance retention over 1,000 h at rated voltage. A suitable additive for propylene carbonate electrolytes must be electrochemically stable at the positive electrode up to 3.0 V, thermally stable to 70 °C, soluble in propylene carbonate at −30 °C, and non-reactive with tetrafluoroborate salts. It must also avoid introducing mobile ions that reduce double-layer capacitance or increase self-discharge. Fluoroethylene carbonate is sometimes considered for high-voltage stability, but its reduction at the negative electrode can generate fluoride species and gas, and in double-layer capacitors with quaternary ammonium salts the absence of lithium ions changes the decomposition pathway. Nitrile additives such as succinonitrile and glutaronitrile have been reported to improve anodic stability of carbonate electrolytes, but their viscosity-raising effect in propylene carbonate can further impair low-temperature performance. The addition of 2 wt% succinonitrile to 1 M triethylmethylammonium tetrafluoroborate in propylene carbonate is reported to shift the anodic oxidation onset by 0.1 V to 0.3 V on inert electrodes, but the corresponding reduction in conductivity at −20 °C may exceed 15%. Because additive effects are electrode-specific, electrolyte qualification for high-voltage double-layer capacitors requires full cell testing on the exact carbon used in production rather than reliance on glassy carbon half-cell data. Activated carbon electrodes with a high mesopore fraction present a heterogeneous interface where propylene carbonate oxidation and current collector corrosion proceed at different rates depending on local potential, carbon surface chemistry, and electrolyte purity. The positive electrode in a double-layer capacitor operates near the anodic stability limit, and the aluminum current collector is separated from the electrolyte only by the carbon coating and any conductive adhesive. Unprotected aluminum in propylene carbonate-based electrolytes exhibits pitting above 2.5 V versus activated carbon when chloride contamination exceeds 5 mg kg⁻¹; below this concentration, the native aluminum oxide layer remains passivated but can be compromised by fluoroboric acid generated from hydrolysis of tetrafluoroborate salts. Carbon-coated aluminum current collectors with a coating thickness of 1 μm to 3 μm reduce corrosion by increasing the potential barrier at the collector interface. The carbon coating is applied by roll-to-roll gravure or slot-die methods with an adhesion strength above 0.3 N mm⁻¹ as measured by a 180° peel test according to ASTM D903-98. Electrode formulations for propylene carbonate-based high-voltage cells typically contain 90 wt% to 95 wt% activated carbon, 3 wt% to 7 wt% conductive carbon black, and 2 wt% to 5 wt% binder such as polytetrafluoroethylene or carboxymethyl cellulose-styrene butadiene rubber. The binder must be electrochemically stable at 3.0 V and must not swell excessively in propylene carbonate. Polytetrafluoroethylene is preferred for high-voltage cells because it contains no oxygen-functional groups that could oxidize, but its fibrillation during dry electrode processing can close pores and increase ionic resistance. Electrode porosity is maintained between 40% and 60% to balance electrolyte accessibility and mechanical integrity. The electrochemical surface area of the activated carbon is measured by nitrogen adsorption at 77 K and calculated using the Brunauer-Emmett-Teller method according to ISO 9277:2022; values for double-layer capacitor carbons range from 1,500 m² g⁻¹ to 2,500 m² g⁻¹. Capacitance per electrode in propylene carbonate-based cells is typically 90 F g⁻¹ to 120 F g⁻¹ based on the carbon mass of one electrode, and the mismatch between positive and negative electrode capacitance must be controlled within ±3% to avoid local overvoltage during charging. This mismatch is measured by three-electrode reference cells and adjusted through electrode coating thickness.

Thermal Ageing Signatures of PC-Based Electrolytes in Cylindrical Cells

Thermal ageing of propylene carbonate-based electrolytes in cylindrical cells produces characteristic signatures in capacitance, equivalent series resistance, gas composition, and electrolyte color that are used for end-of-line reliability screening. At 65 °C and 2.7 V, the dominant degradation path is oxidation of residual water and carbonate solvent at the positive electrode, producing carbon dioxide, propylene, and trace acetaldehyde. The gas composition after 1,000 h typically shows carbon dioxide as the major component above 70 mol%, with hydrogen and propylene as minor components; the exact proportions depend on water concentration and carbon oxygen content. Internal cell pressure during aging can rise from 0.1 MPa to 0.4 MPa without activating the vent, but pressure above 0.6 MPa indicates accelerated solvent decomposition. An equivalent series resistance increase at 1 kHz from 0.25 mΩ to 0.35 mΩ after 1,000 h is considered acceptable for 2.7 V rated cells, whereas an increase above 0.5 mΩ is associated with loss of electrolyte conductivity and passivation film growth. Capacitance retention of 80% to 90% after 1,000 h at 65 °C is commonly reported for propylene carbonate-based double-layer capacitor cells, but published data for specific cell formats and carbon sources can vary widely. The color of the electrolyte after ageing is also a diagnostic: fresh propylene carbonate electrolyte is water-white, and a shift to amber or brown after 1,000 h indicates oxidative oligomerization. Ultraviolet-visible absorbance at 400 nm is used to quantify color, with an absorbance below 0.05 considered acceptable for capacitor-grade electrolyte. In accelerated ageing at 70 °C, the rate of capacitance loss follows a first-order kinetic model with reported activation energies of 40 kJ mol⁻¹ to 60 kJ mol⁻¹ for propylene carbonate oxidation on activated carbon; this range is derived from lifetime extrapolation studies but is sensitive to electrode surface chemistry. The upper category temperature for long-term operation is therefore limited by the thermal stability of the electrolyte–carbon interface, not by the boiling point of propylene carbonate. Cells stored at 85 °C without voltage show lower gas generation than cells aged at 65 °C under voltage, confirming that electrochemical oxidation, not thermolysis, is the primary degradation pathway in sealed propylene carbonate-based cells. Automated winding lines for cylindrical double-layer capacitor cells encounter distinct failure modes when propylene carbonate-based electrolytes are introduced because the higher viscosity and surface tension alter wetting, bubble entrapment, and separator handling. In a winding machine with a central mandrel, the separator must be tensioned within 0.5 N to 2.0 N to prevent telescoping; excessive tension compresses the electrode and reduces electrolyte uptake, while insufficient tension creates gaps where propylene carbonate cannot penetrate uniformly. Electrodes are fed from rolls with thickness tolerances of ±5 μm to maintain cell diameter and capacitance, and batch-to-batch variation in electrode porosity changes the electrolyte fill volume required for full wetting. On production lines with a nominal 3,000 F cell, the fill volume is adjusted by ±5 mL based on in-line gravimetric measurements of electrode and separator mass. Propylene carbonate-based electrolytes have a higher contact angle on polypropylene separators than acetonitrile-based electrolytes; contact angle values above 60° are observed on untreated separators, leading to localized dry regions that develop into high-resistance spots during formation. To mitigate this, manufacturers use separator grades with plasma or surfactant treatment to lower the contact angle below 45°, but surfactant residues can increase leakage current if not properly rinsed. The vacuum filling station must achieve a chamber pressure below 5 kPa and maintain it for 10 min to 20 min depending on cell size; interruption of vacuum cycles results in incomplete wetting and higher equivalent series resistance scatter. Batch variance in electrolyte filling is monitored by weighing each cell before and after filling with a resolution of 0.1 g, and the fill weight standard deviation is maintained below 2% of the target. In high-voltage ageing, cells with fill volumes below 95% of the target show increased gas evolution and capacitance instability. A common equipment failure is the gradual accumulation of propylene carbonate vapor in the dry room exhaust system, which condenses in ductwork and creates a flammable residue if the dry room humidity control intermittently fails. For this reason, electrolyte handling systems are equipped with hydrocarbon sensors that alarm at 10% of the lower flammability limit and shut down the filling line. These production-scale observations are based on typical double-layer capacitor manufacturing lines with winding speeds of 10 m min⁻¹ to 30 m min⁻¹ and cell assembly capacities of 200 to 500 cells h⁻¹.

During End-of-Line Aging, Voltage and Leakage Current Determine Rated Performance

During end-of-line aging, the interaction between voltage, leakage current, and temperature establishes the rated performance of propylene carbonate-based double-layer capacitors. The aging procedure applies a near-rated voltage, typically 2.7 V, at an elevated temperature between 55 °C and 65 °C for 12 h to 48 h depending on cell size and electrode carbon. Leakage current is logged continuously with a sampling interval of 1 s to 10 s, and the acceptance threshold for a 3,000 F cell is commonly 0.5 mA to 1.0 mA after the hold period. Cells that show non-decaying leakage current or abrupt current spikes are quarantined because these patterns indicate localized shunt paths, separator damage, or excessive water in the electrolyte. Capacitance and equivalent series resistance are measured before and after aging by galvanostatic cycling and impedance spectroscopy according to IEC 62391-1 and IEC 62391-2. The capacitance on discharge between 2.7 V and 1.35 V is normalized to the mass or volume of the cell and compared with the specification. The equivalent series resistance is recorded at 1 kHz, and the initial and aged values are used to calculate the percentage increase that must remain below the endurance limit. Gas evolution during aging is inferred from cell swelling in pouch cells or from vent activation in cylindrical cells; destructive gas analysis is performed on sample cells from each lot. The electrolyte fill and wetting quality are further verified by electrochemical impedance spectroscopy at low frequency, where the distributed pore resistance appears as a 45° Warburg-like region. A shift of this region after aging indicates increased ionic resistance in the electrode depth from oligomer deposition or binder swelling. The end-of-line leakage current distribution across a production lot is also used as a statistical control parameter; a shift in the median leakage current above 0.3 mA for a 3,000 F cell after aging may signal an electrolyte batch with higher water content or a change in carbon surface chemistry. Because propylene carbonate-based high-voltage cells are sensitive to trace impurities, the final rated voltage is assigned only after the lot demonstrates stable capacitance, leakage current, and internal resistance under endurance conditions. Cells outside these limits are quarantined and subjected to root cause analysis.

Standard or method	Relevant clause or designation	Test condition	Typical acceptance criterion
IEC 62391-1	Fixed electric double-layer capacitors for use in electric and electronic equipment, general performance	1,000 h at rated voltage and 65 °C	Capacitance ≥ 80% initial; ESR ≤ 200% initial
IEC 62391-2	Fixed electric double-layer capacitors for power application, endurance cycling	Rated voltage cycling at upper category temperature	Capacitance loss ≤ 20%; ESR increase ≤ 200%
UN 38.3	Transport testing for rechargeable cells and batteries	Altitude simulation, thermal test, vibration, shock	No leakage, venting, fire, or rupture
ISO 15106-3	Plastics film and sheeting, water vapor transmission rate	40 °C, 90% RH	Laminate WVTR ≤ 0.1 g m⁻² day⁻¹
ASTM E1064-24	Karl Fischer water determination	Electrolyte sample under dry nitrogen	Water ≤ 20 mg kg⁻¹
ASTM D1125-23	Electrical conductivity of water and industrial water	Calibrated conductivity cell, 25 °C	Report conductivity in mS cm⁻¹