Electrolyte Requirements for Sustained 4.6V Lithium Cobalt Oxide Cycling

Lithium cobalt oxide (LiCoO₂, LCO) charged to a potential of 4.6 V versus Li/Li⁺ confronts every conventional electrolyte component with a thermodynamic instability exceeding 1.5 eV above the highest occupied molecular orbital of typical carbonate solvents. The sustained cycling of such a cathode demands an electrolyte formulation that simultaneously suppresses solvent oxidation, passivates the delithiated oxide surface, scavenges protic impurities, and maintains ionic conductivity across a temperature window that in production cells frequently extends from -20 °C to 60 °C. Without a robust interphase on the cathode, the lattice oxygen released from LCO at states of charge above 80 % reacts exothermically with ethylene carbonate (EC), generating CO₂, oligocarbonates, and water—the latter hydrolyzing LiPF₆ to HF and POF₃ in a self-amplifying cascade. Production data from 18650 cylindrical cells assembled with baseline 1.0 M LiPF₆ in EC:ethyl methyl carbonate (EMC) (3:7 by weight) shows capacity retention falling below 70 % after 200 cycles at a 1C rate at 45 °C when the upper cutoff is raised from 4.45 V to 4.60 V. The rollover is not gradual; post-mortem gas chromatography of the headspace reveals a tenfold increase in CO₂ and methane when the charge voltage exceeds 4.55 V, a threshold that aligns with the onset of lattice oxygen oxidation characterized by differential electrochemical mass spectrometry (DEMS) equipped with a quadrupole residual gas analyzer sampling directly from a Swagelok-type cell. Consequently, the formulation of a 4.6 V-grade electrolyte must be approached as an exercise in kinetic passivation: the intrinsic thermodynamic instability is unavoidable, and only a correctly engineered cathode electrolyte interphase (CEI) can decouple the electron-transfer rate at the oxide surface from the bulk electrolyte decomposition rate. When the cathode potential is raised beyond that which a given solvent molecule can withstand, the oxidation current measured on an inert glassy-carbon electrode becomes predictive only in the absence of surface films. Linear sweep voltammetry using a 3 mm diameter glassy-carbon working electrode, a lithium metal counter electrode, and a Li/Li⁺ reference in a three-electrode flooded cell (EL-Cell PAT-Cell) yields an anodic onset for 1.0 M LiPF₆ in EC:dimethyl carbonate (DMC) (1:1 by volume) at 5.2–5.4 V versus Li/Li⁺ at a scan rate of 0.1 mV s⁻¹. On an LCO composite electrode, however, the same electrolyte shows a pronounced oxidation wave beginning near 4.55 V, because the delithiated LCO surface, with its partially occupied oxygen 2p band, acts as an electrocatalyst. X-ray photoelectron spectroscopy (XPS) analysis of LCO electrodes retrieved after 50 formation cycles at 4.6 V identifies a CEI composed primarily of polycarbonates, LiF, LiₓPOᵧF₂, and cobalt oxide decomposition products, yet the film thickness—measured by argon-ion sputter depth profiling calibrated against a Ta₂O₅ standard—remains below 5 nm unless specific film-forming additives are present. Without additives, the CEI is porous and continuously repaired, consuming electrolyte and increasing cell impedance at a rate of 0.8–1.2 Ω cm² per 100 cycles as tracked by electrochemical impedance spectroscopy (EIS) at 50 % state of charge applying a 10 mV AC perturbation from 200 kHz to 10 mHz on a Biologic VSP potentiostat.

Fluorinated Carbonates and the Anodic Stability Window

The substitution of hydrogen by fluorine at the β-position of a cyclic carbonate raises its oxidative stability by withdrawing electron density from the carbonate oxygen atoms. Fluoroethylene carbonate (FEC), when incorporated at 5–10 wt% of the total solvent mass, shifts the anodic decomposition onset on LCO to approximately 4.75 V as measured by potentiostatic hold experiments in which the leakage current is recorded after 24 h at 4.6 V under 40 °C storage. Yet FEC alone cannot sustain prolonged cycling because its reduction potential on graphite or lithium metal lies near 1.2–1.5 V, leading to excessive anode-side film growth if used as the primary solvent. The compromise adopted in production is a dual-solvent system: a baseline linear carbonate such as EMC or methyl propyl carbonate (MPC) combined with 15–25 vol% fluoroethylene carbonate and, in higher-performance formulations, 2–5 vol% of a fully fluorinated ether such as 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE). TTE behaves as a non-solvating diluent, reducing the free-solvent concentration at the cathode surface and thereby kinetically suppressing oxidative dehydrogenation of the alkyl carbonate. Cells containing 1.2 M LiPF₆ in FEC:EMC:TTE (20:70:10 by volume) have demonstrated 83 % capacity retention after 500 cycles at 4.6 V and 25 °C when cycled between 3.0 V and 4.6 V at a 0.5C charge / 1C discharge rate in 2032 coin cells with a 15 µL electrolyte fill volume and a 25 µm Celgard 2500 separator. The critical metric, however, is the coulombic inefficiency per cycle, which must remain below 0.02 % to achieve thousand-cycle lifetimes; in the fluorinated formulation it averages 0.015 % between cycle 50 and 500, versus 0.08–0.12 % for the baseline EC:EMC electrolyte. Manufacturing-scale electrolyte blending of FEC-rich formulations introduces distinct moisture-handling challenges. FEC is hygroscopic and susceptible to ring-opening hydrolysis that generates HF and fluorohydrins even before the first formation charge. Online Karl Fischer titration per ASTM E1064-22 must confirm moisture below 10 ppm in the blended electrolyte; any excursion above 20 ppm correlates with a 5–8 % increase in initial irreversible capacity loss and a doubling of the post-formation HF content, which can be quantified by ion chromatography after hexane extraction of the electrolyte from an opened jelly-roll.

Sulfone-Based Co-Solvents—Are They a Viable Passivation Strategy?

Ethyl methyl sulfone (EMS) and tetramethylene sulfone (sulfolane, TMS) possess anodic stability windows extending beyond 5.8 V versus Li/Li⁺, as determined by linear sweep voltammetry on platinum microelectrodes in a 0.1 M LiTFSI supporting electrolyte. When a 1.0 M LiPF₆ solution in EMS:fluoroethylene carbonate (80:20 by weight) is cycled against LCO at 4.6 V at 60 °C, the discharge capacity retention after 300 cycles at 1C reaches 88 %—a marked improvement over EC-containing electrolytes—yet the rate capability at 2C suffers a 22 % penalty because the viscosity of EMS at 20 °C is approximately 4.5 cP compared to 0.6 cP for EMC. This trade-off confines sulfone adoption to applications where charge-rate acceleration above 1C is not mandated, such as stationary storage or high-temperature downhole batteries. The sulfone class also introduces a lithium plating risk on graphite anodes during fast-charge pulses because the lithium-ion transference number in sulfone-rich electrolytes drops to 0.25–0.30, compared to 0.35–0.40 in carbonate systems, raising the salt concentration gradient across the separator under a current density of 2 mA cm⁻². Thus, the design space for sulfone co-solvents is narrow: the EMS or TMS fraction cannot exceed 30 vol% without triggering dendritic lithium deposition during the constant-current portion of the charge protocol unless the cycling temperature is maintained above 45 °C. Electrolyte oxidation at high potentials is accompanied by acid generation. The combination of HF and reactive oxygen species dissolves cobalt from the LCO surface, and dissolved cobalt ions can migrate to the anode, where they are reduced and form metallic deposits that catalyze electrolyte reduction, increasing the anode SEI thickness and consuming lithium inventory. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis of anodes extracted from cells cycled at 4.6 V for 500 cycles shows cobalt deposition densities exceeding 0.5 µg cm⁻² for baseline electrolytes, which correlates with a linear fade rate of 0.08 % per cycle after the 200th cycle. An effective CEI must therefore be dense enough to impede cobalt dissolution as well as solvent oxidation.

Phosphate Additives: Radical Scavenging and HF Neutralization

Tris(trimethylsilyl) phosphite (TMSPi) and tris(2,2,2-trifluoroethyl) phosphate (TFEP) represent two distinct electrode-protection mechanisms. TMSPi, used at 0.5–1.0 wt%, functions primarily as a sacrificial HF scavenger and water-binding agent, forming trimethylsilyl fluoride and phosphoric acid derivatives that incorporate into the nascent CEI. The consumption rate is measurable: in an electrolyte spiked with 100 ppm H₂O and stored at 45 °C for 7 days, the TMSPi concentration decays following first-order kinetics with a half-life of approximately 48 h, as tracked by ³¹P nuclear magnetic resonance spectroscopy using a 500 MHz Bruker spectrometer and an external D₂O lock. TFEP, by contrast, acts as a high-voltage film-former; its reduction potential lies below 1.0 V, but it oxidizes on the LCO surface above 4.5 V to yield a polyphosphate-rich CEI that inhibits electron tunneling. In a full-cell configuration, a combination of 0.5 wt% TMSPi and 2 wt% TFEP reduces the post-formation cobalt dissolution by 65 % relative to an additive-free baseline, as determined by ICP-OES of the electrolyte after 10 formation cycles at C/10. The protective layer, however, adds 3–5 Ω to the charge-transfer resistance (R_ct) as fitted from EIS data to an equivalent circuit consisting of an ohmic resistance, two R-CPE elements, and a finite-length Warburg diffusion element; this penalty is acceptable only if the long-term stability gain compensates for the initial impedance increase. Boron-containing additives, including lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFOB), contribute a complementary passivation action on both electrodes. LiDFOB at 1–2 wt% in a standard carbonate electrolyte forms an oxalate-rich SEI on graphite and simultaneously generates a boron-oxide-containing CEI on LCO. At 4.6 V, the boronaceous CEI is particularly effective at suppressing the lattice oxygen release that triggers solvent combustion; differential scanning calorimetry (DSC) of delithiated LCO‑electrolyte mixtures shows that the exothermic onset temperature increases from 210 °C to 248 °C when LiDFOB is present. Yet LiDFOB is sparingly soluble in linear carbonates; at concentrations above 2.5 wt% in EC:EMC (30:70), precipitation occurs upon storage at -10 °C, forming crystals that can clog separator pores. The solubility limit mandates pre-dissolution in a small volume of EC or γ‑butyrolactone before blending into the main solvent mixture, a step that complicates large-scale batch preparation in 2000 L stirred reactors. The internal pressure evolution in a 4.6 V pouch cell during formation is a direct probe of electrolyte stability. Cells fabricated with a 2 Ah nominal capacity, using a stack of 12 double-sided coated electrodes and a 12 µm Al₂O₃-coated polyethylene separator, are instrumented with a strain gauge and a pressure transducer during the first charge at C/20 to 4.6 V. In additive-free electrolytes, the differential pressure (ΔP) rises by 0.8–1.5 bar between 4.4 V and 4.6 V, attributable to CO₂ and C₂H₄ evolution from solvent oxidation and lithium alkoxide decomposition. With a fully formulated electrolyte containing 10 % FEC, 2 % LiDFOB, and 0.5 % TMSPi, the ΔP remains below 0.2 bar across the same voltage window. This pressure signature is now used as an incoming quality-control gate; any electrolyte lot that produces a ΔP exceeding 0.3 bar in a standardized 200 mAh single-layer pouch cell test is rejected before use in production cylindrical cells, per an internal specification protocol aligned with IEC 62660‑3:2022 secondary lithium-ion cell testing.

Electrolyte Salt Selection Beyond Lithium Hexafluorophosphate

LiPF₆, despite its thermal lability and sensitivity to hydrolysis, remains the dominant salt because it passivates aluminum current collectors at high potentials. The corrosion current of aluminum at 4.6 V in 1.0 M LiPF₆ EC:EMC at 60 °C is below 0.5 µA cm⁻², measured by chronoamperometry on a 1 cm² aluminum foil electrode. Replacement candidates such as lithium bis(fluorosulfonyl)imide (LiFSI) offer superior conductivity and thermal stability but corrode aluminum above 3.8 V unless the electrolyte contains a passivating co-salt or a high concentration of LiPF₆. Dual-salt systems containing 0.6 M LiPF₆ and 0.4 M LiFSI exhibit acceptable aluminum stability up to 4.7 V at 25 °C, yet at 55 °C the pitting potential drops to 4.5 V, as revealed by cyclic polarization curves recorded at a scan rate of 1 mV s⁻¹ according to the methodology of ASTM G61-86(2023). This temperature constraint restricts LiFSI-rich electrolytes to moderate-temperature applications unless additional aluminum-corrosion inhibitors such as lithium tetrafluoroborate (LiBF₄) at 0.05–0.1 M are added. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is prohibited in cells with aluminum current collectors unless used in highly concentrated electrolytes (> 3 M) where the scarcity of free solvent molecules suppresses the dissolution of the aluminum-TFSI complex. Such “water-in-salt” or “solvent-in-salt” concepts have been demonstrated in laboratory coin cells to sustain LCO cycling at 4.6 V for 200 cycles, but their viscosity at room temperature exceeds 60 cP, making electrode wetting and electrolyte filling in production jelly-rolls a bottleneck. A 18650 cell with a 15 µm base-film separator required a vacuum filling cycle extended from the standard 2 min to 15 min to achieve complete wetting, as verified by electrochemical impedance mapping across the electrode length using a multichannel frequency response analyzer during the first rest period. The selection of lithium salt also governs the lithium-ion transference number and the concentration polarization across the separator at high currents. Pulsed-field gradient nuclear magnetic resonance (PFG-NMR) measurements using a 7T magnet with a diffusion probe equipped with a 5 mm ¹H/¹⁹F coil yield lithium self-diffusion coefficients and transference numbers that predict a salt depletion region of approximately 15 µm into the cathode at a discharge rate of 3C for a 1 M LiPF₆ EC:EMC electrolyte. This region widens to 25 µm in a 0.8 M LiPF₆ + 0.2 M LiDFOB formulation, amplifying polarization and causing premature voltage cutoff if the cathode thickness exceeds 70 µm per side. Thus, the electrolyte salt mixture must be co-optimized with electrode loading—a parameter typically held fixed at 3.0–3.5 mAh cm⁻² for high-energy LCO cells—to prevent diffusion-limited capacity loss at discharge rates above 1C. When cycling at 4.6 V is extended to thousands of cycles, the accumulation of decomposition products in the electrolyte eventually changes the solvation environment. The evolution of lithium alkoxides, oligoethers, and phosphate esters increases the electrolyte viscosity by 30–50 % after 800 cycles at 45 °C, as measured by capillary viscometry at 25 °C. This viscosity increase reduces the ionic conductivity from an initial value of 8.5 mS cm⁻¹ to 5.2 mS cm⁻¹, elevating the ohmic drop by 40 mV at a 1C discharge rate. Consequently, a sustained 4.6 V electrolyte must incorporate a sacrificial species that decomposes preferentially to form stable, thin films without generating soluble products that accumulate in the bulk. Sulfone and fluorinated ether co-solvents partially serve this role, but their effectiveness fades as the solvent fraction is continuously consumed.

Monitoring Impedance Growth as an Electrolyte Quality Indicator

A standardized protocol for assessing electrolyte suitability for 4.6 V LCO cycling involves a sequence of formation, rate capability testing, and long-term cycling with periodic EIS interruptions, all conducted in 2032 coin cells with a 16 mm diameter lithium anode and a 14 mm LCO cathode. The acceptance criteria derived from a round-robin among three independent laboratories using a Maccor Series 4000 battery tester and a Solartron 1455A frequency response analyzer include: (i) a first-cycle coulombic efficiency not less than 92.5 % at C/20; (ii) an R_ct increase of no more than 1.5 Ω cm² after 100 cycles at 1C charge/1C discharge, referenced to the value after the third cycle; (iii) a capacity retention above 85 % after 400 cycles at 25 °C; and (iv) a ΔP in a 200 mAh pouch cell formation test below 0.3 bar as described earlier. These metrics are embedded in a material qualification specification following IEC 63218:2021 for secondary cells and batteries for portable applications, clause 7.3.2, with additional internal requirements. Surface-enhanced Raman spectroscopy (SERS) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) provide complementary fingerprints of electrolyte degradation. The emergence of a vibrational band at ~1020 cm⁻¹, assigned to lithium alkoxide stretching, serves as an early warning of solvent oxidation that precedes bulk gas generation. A Raman spectrometer equipped with a 785 nm laser and an in-situ electrochemical cell allows detection of this band after as few as 10 cycles at 4.6 V in unstable electrolytes. This real-time diagnostic capability is being transferred to production lines where a fiber-optic Raman probe inserted into a formation chamber monitors the exhaust gas or electrolyte condensate for the 1020 cm⁻¹ marker, enabling early rejection of defective jellyroll batches before they proceed to aging.

Performance comparison of electrolyte additive packages for 4.6 V LCO cycling at 25 °C (coin cell data, 1C/1C, 3.0–4.6 V)

Formulation composition	Initial discharge capacity (mAh g⁻¹)	Capacity retention at cycle 400 (%)	Coulombic efficiency at cycle 400 (%)	Post-400 cycle Rct increase (Ω cm²)
1.0 M LiPF₆ EC:EMC 3:7 (baseline)	185	62	99.85	+4.8
Baseline + 10 wt% FEC	186	78	99.92	+2.9
Baseline + 10 wt% FEC + 2 wt% LiDFOB	184	88	99.96	+1.7
Baseline + 10 wt% FEC + 2 wt% TFEP + 0.5 wt% TMSPi	183	91	99.97	+1.4
1.2 M LiPF₆ FEC:EMC:TTE 20:70:10 + 1 wt% LiDFOB	182	89	99.95	+1.8

The manufacture of electrolyte for 4.6 V application must exclude any impurity that catalyzes decomposition. Transition metal ions, particularly iron and copper, can originate from piping and storage vessels and promote Fenton-type reactions. Glow discharge mass spectrometry of raw solvents must confirm transition-metal content below 0.1 ppb, a specification enforced by sampling each incoming tanker of EMC and FEC. The blending facility itself is constructed from electropolished 316L stainless steel with a surface roughness (Ra) below 0.4 µm and hermetically sealed under a dry-air atmosphere with a dew point below -60 °C. The completed electrolyte is filtered through a 0.05 µm PTFE membrane into 200 L fluorinated HDPE drums that are stored under nitrogen pad pressure of 0.5 bar. Further complicating the electrolyte design is the 4.6 V charging protocol itself. Constant-current charging to 4.6 V followed by a constant-voltage (CV) hold until the current drops below C/20 can extend the time the cathode spends at the highest potential, accelerating oxidation. When the CV step is shortened to a C/10 cutoff, the capacity retention improves by 3–5 % over 300 cycles because the average cathode exposure time at > 4.55 V is reduced by approximately 20 %. This fact is incorporated into battery management system algorithms that adjust the charge termination based on accumulated cycles and measured internal resistance, but the electrolyte itself must be robust enough to survive occasional CV holds that result from cell balancing in series-connected packs, where weaker cells may be held at 4.6 V for several hours.

Evaluating Gas Generation with Online Electrochemical Mass Spectrometry

A direct technique for screening electrolyte stability at 4.6 V is online electrochemical mass spectrometry (OEMS) using a specially designed cell with a porous current collector that permits gas transport to the mass spectrometer inlet. In a typical OEMS experiment, conducted on an apparatus incorporating a Pfeiffer Vacuum OmniStar GSD 320 mass spectrometer with a capillary inlet and a gold-coated stainless steel cell body, the CO₂ ion current (m/z = 44) is monitored during a slow potentiodynamic scan at 0.05 mV s⁻¹. For an additive-free EC:EMC electrolyte on LCO, the CO₂ signal begins to rise above baseline at 4.45 V and increases by a factor of 15 by 4.6 V. With a fully formulated electrolyte containing FEC, LiDFOB, and a fluorinated ether, the CO₂ signal remains within 3× the baseline up to 4.7 V. The OEMS cell also detects POF₃ (m/z = 85, 104), whose appearance corresponds to the hydrolysis of LiPF₆ by water released from LCO oxidation; this signal is virtually absent in dry, additive-protected electrolytes, confirming the intertwined nature of solvent oxidation and salt hydrolysis. The interfacial chemistry that sustains 4.6 V cycling relies on an inner CEI layer rich in inorganic species—LiF, Li₂O, LixPOyFz—and an outer organic layer formed by oxidized carbonate and additive fragments. Time-of-flight secondary ion mass spectrometry (ToF‑SIMS) depth profiles, obtained using a Bi₃⁺ primary ion beam and a Cs⁺ sputtering beam, reveal that the inorganic inner layer reaches a thickness of approximately 2 nm in an effective electrolyte, while the organic outer layer extends to 8–10 nm. The organic layer remains partially permeable to lithium ions but blocks electron tunneling, as evidenced by scanning electrochemical microscopy (SECM) approach curves showing a decrease in the heterogeneous electron-transfer rate constant for ferrocene oxidation from 1.2×10⁻³ cm s⁻¹ on pristine LCO to 4.5×10⁻⁵ cm s⁻¹ on the CEI-covered surface. This four-orders-of-magnitude suppression is necessary to interrupt the catalytic cycle; any electrolyte additive that cannot achieve at least a hundredfold reduction in the electron-transfer rate constant will fail to deliver thousand-cycle life at 4.6 V. Operational boundaries define the processing window. Electrolytes containing high concentrations of fluorinated additives exhibit reduced ionic conductivity at low temperature; at -20 °C, the conductivity of the 20 % FEC-containing formulation drops to 2.1 mS cm⁻¹, compared to 4.3 mS cm⁻¹ for the baseline. This restricts the cold-cranking performance of batteries intended for automotive stop-start functions, where a 5C pulse at -18 °C must maintain a voltage above 2.5 V per cell according to ISO 16750‑2:2023 clause 4.1.2. Therefore, fluorinated electrolytes for 4.6 V LCO are preferentially deployed in consumer electronics and portable medical devices, where operating temperature rarely falls below 0 °C, rather than in automotive traction batteries that demand full functionality from -20 °C to 60 °C. A parallel development focuses on nitrile-functionalized additives such as succinonitrile and glutaronitrile, which coordinate to the cobalt ions on the LCO surface and suppress their dissolution. Electrolytes containing 3 wt% succinonitrile show cobalt dissolution reduced by 70 % after 200 cycles at 4.6 V, but the nitrile group is susceptible to anodic decomposition above 4.8 V, generating cyanide-containing species that can plate on the anode and impair the SEI. The safe operating limit is therefore 4.65 V when nitriles are used, imposing a margin that process engineers must monitor through voltage-sensing accuracy better than ±10 mV.

Specification limits for raw electrolyte components used in 4.6 V LCO formulations

Component	Purity (minimum)	Moisture (max, ppm)	Acidity as HF (max, ppm)	Test method
Ethylene carbonate	99.99 %	10	5	GC‑MS, ASTM E1064
Ethyl methyl carbonate	99.95 %	10	3	GC‑MS, ASTM E1064
Fluoroethylene carbonate	99.9 %	15	10	GC‑MS, ASTM E1064
LiPF₆ salt	99.99 %	5	50 (as HF equivalent)	Ion chromatography, Karl Fischer
TTE co-solvent	99.8 %	10	2	GC‑MS, ASTM E1064

The interplay of all these requirements forces electrolyte formulators to operate within a narrow compositional corridor. FEC content must be kept above 10 wt% to provide anodic stability but below 25 wt% to avoid excessive anode SEI growth and low-temperature conductivity loss. LiDFOB or LiBOB must be present at a level sufficient to scavenge oxalate-forming moieties and passivate aluminum, yet below their solubility thresholds at low temperature. TMSPi must be dosed to exactly consume the expected moisture ingress during cell assembly without leaving unreacted phosphite that can be oxidized at the cathode, generating species that increase the cathode impedance. The balance is verified through accelerated aging tests at 60 °C with a 4.6 V float for 7 days, after which the capacity recovery is measured and the gas volume is quantified by Archimedes’ principle. A passing formulation recovers more than 92 % of its initial capacity at the subsequent C/5 discharge. Finally, the rinsing and drying of electrode sheets prior to cell assembly removes residual N‑methyl‑2‑pyrrolidone (NMP) that interferes with electrolyte stability. Residual NMP concentrations above 50 ppm, quantified by headspace gas chromatography, are associated with a 15 % reduction in columbic efficiency during the first five cycles due to NMP oxidation at 4.6 V. The drying protocol in a continuous convection oven at 120 °C with an air turnover rate of 0.3 m s⁻¹ must reduce residual solvent to below 30 ppm, a value confirmed by extracting a 10 g electrode sample with methanol and analyzing the extract by GC‑FID according to an in-house standard derived from ISO 16017‑1:2000. This procedural detail, often overlooked at the research scale, becomes a critical production checkpoint when battery lifetime warranty periods extend to three years or beyond.