Main Findings
The composition of the cathode material significantly affects the initial charging behavior of commercial layered oxide/hard carbon sodium-ion batteries. Compared to the P2 cathode, the O3 cathode induces more severe electrolyte decomposition, accompanied by significant heat generation and gas evolution at high state of charge. Although the additives succinonitrile (SN) and propylene-1,3-sultone (PES) increase the total heat generation, they effectively suppress the decomposition of solvents and salts. Specifically, these additives reduce the decomposition of ethylene carbonate (EC) and alleviate gas release (especially CO₂), with more pronounced protective effects observed in O3/HC batteries. These findings are confirmed through online electrochemical mass spectrometry analysis and X-ray photoelectron spectroscopy analysis.
Main Conclusions
The Cathode Composition Significantly Affects Initial Charging Behavior and Electrolyte Decomposition
The study shows that the crystal structure of the layered oxide cathode (especially the P2 versus O3 phases) plays a decisive role in the extent of electrolyte decomposition during the initial charging process. Compared to the P2/HC battery (113 J g⁻¹ and 2 bar), the O3/HC battery exhibits significantly higher heat generation (242 J g⁻¹) and pressure increase (6 bar). This difference is attributed to the higher reactivity of the O3 cathode, which triggers substantial solvent and salt decomposition, particularly at high state of charge (SOC), with operational infrared fiber attenuation total reflection spectroscopy (IR-FEWS) showing a marked decrease in the absorbance of EC and NaPF₆ spectral bands.
Electrolyte Additives Suppress Solvent and Salt Decomposition Despite Increased Heat Generation
The addition of succinonitrile (SN) and propylene-1,3-sultone (PES) to the electrolyte effectively mitigates electrolyte decomposition, as confirmed by IR-FEWS and online electrochemical mass spectrometry (OEMS). Although these additives increase the total heat generation of P2/HC and O3/HC batteries by 1.9 times and 1.8 times, respectively, they significantly reduce gas evolution—particularly decreasing C₂H₄ release by over 90% and suppressing CO₂ generation. The additives weaken the decomposition of solvents and salts, especially reducing EC consumption and Na⁺ coordination changes, indicating their protective role at the electrode interface.
Operational Optical Sensing Technology Analyzes Thermodynamics and Pressure Dynamics During SEI/CEI Formation
Optical calorimetry and pressure sensing using fiber Bragg grating (FBG) sensors enabled real-time monitoring of thermal evolution and gas evolution events during the first charging process. Different heat generation rate regions were identified: Region II (approximately 20-40% SOC) corresponds to the formation of the solid electrolyte interphase (SEI) at the hard carbon anode, while Regions IV and V (above 50% SOC) are associated with the formation of the cathode-electrolyte interphase (CEI) and phase changes. The O3/HC battery exhibits additional thermal pressure peaks at high SOC (e.g., 4.1 V and 4.3 V), which are related to CO₂ release and structural changes in the cathode, verified through OEMS and operational X-ray diffraction.
IR-FEWS Provides Direct Chemical Evidence of Solvent Consumption and Decomposition Product Formation
Operational IR-FEWS analysis directly tracked the evolution of electrolyte components, revealing key decomposition pathways. In batteries using the baseline electrolyte, the absorbance of EC (at 1777 cm⁻¹) significantly decreased during SEI formation (Region II), accompanied by the decomposition of NaPF₆ (decreased νPF spectral band intensity) and the generation of ester exchange products (dimethoxy-2,5-dioxahexanoic acid ester, DMDOHC identified at 1255 cm⁻¹). The O3/HC battery showed accelerated solvent decomposition at high SOC (Region V), while the electrolyte with additives mitigated these changes, confirming the reduction of EC and salt degradation.
Additives Alter SEI/CEI Composition and Thickness to Enhance Electrode Protection
X-ray photoelectron spectroscopy (XPS) analysis of cycled electrodes confirmed that SN and PES additives altered the interfacial chemical composition and morphology. The additives resulted in thicker SEI/CEI layers, as evidenced by the attenuation of polyvinylidene fluoride (PVDF) and carbon black peaks in the C 1s spectrum. The SEI formed with additives contained fewer carbonyl species and increased C-H content, while the S 2p spectrum showed RSO₃ and Na₂SO₃ species generated from PES decomposition. This compositional change is associated with improved electrolyte stability and reduced gas evolution, particularly protecting the O3 cathode from oxidative decomposition at high voltages.
Experimental Methods
Electrolyte Preparation
The baseline electrolyte consists of 1 M NaPF₆ (Stella Chemica) dissolved in a mixed solvent of ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC) in a volume ratio of 1:1:2. The electrolyte with additives was prepared by adding 2 wt.% succinonitrile (SN, Sigma-Aldrich, 99%) and 2 wt.% propylene-1,3-sultone (PES, TCI, >99%) to the baseline electrolyte.
Electrochemical Testing
Cycling tests for 18650 batteries were conducted using a BCS-198 or MPG2 potentiostat (Bio-logic). The rate calculations were based on the mass of active material in the cathode (P2 or O3), with 1C rates of 85 mA g⁻¹ for P2 and 180 mA g⁻¹ for O3. The batteries were cycled at a constant current (CC) at a C/10 rate within a voltage window of 2.0-4.3 V.
FBG Sensors
Fiber Bragg grating (FBG) sensors inscribed on single-mode fiber (SMF) were purchased from SAMYON (China). Each fiber has a polyimide coating except for a 10 mm segment containing the grating. The microstructured optical fiber (MOF) used for inscribing the FBG was provided by Professor Tam’s research group and was fabricated according to reported methods.
Optical Calorimetry and Pressure Sensing
A 0.8 mm diameter hole was drilled at the center of the 18650 battery anode for electrolyte filling and FBG sensor insertion. The central cavity of the roll core allows this operation without damaging the electrode. The battery was vacuum dried overnight at 80°C in a Buchi oven and then transferred to an argon glove box for the injection of 5.2 mL of electrolyte.
Following the protocol reported by Huang et al., the MOF and SMF inscribed with FBG were inserted into the central cavity of the battery. This combination allows for the decoupling of pressure and temperature changes. The battery was placed in an insulated box, with two additional SMF-FBG sensors monitoring temperature changes on the internal wall and the battery surface. The HYPERION si255 demodulator (LUNA, Micron Optics) served as the FBG signal source and detector. Due to the lower signal amplitude of the MOF-FBG, the internal FBG used an enhanced visibility (EV) demodulator, while the surface and environmental FBGs used the standard si255 demodulator.
Thermal calibration was performed before formation, including maintaining constant temperatures of 35, 30, 25, 20, and 15°C for 4 hours each in a temperature-controlled oven (IPP110ECOPLUS, Memmert) to determine the temperature sensitivity coefficients (k_T) of each FBG sensor. The battery was cycled in a temperature-controlled oven at 25°C. A 3-hour, 2 Hz, 1C current pulse test was conducted to determine the thermal equivalent circuit parameters of each battery.
Online Electrochemical Mass Spectrometry (OEMS)
Self-supporting electrodes of hard carbon (HC), P2, and O3 layered oxides were prepared in an argon glove box. The mass ratio was 80 wt.% active material, 10 wt.% super conductive carbon black (CSP), and 10 wt.% polytetrafluoroethylene (PTFE). The components were mixed in a mortar with four drops of DMC, pressed into sheets, and cut to the desired mass and size after 15 minutes of vacuum drying in the glove box front chamber. The positive and negative electrode capacity ratio (P/N) of the OEMS full battery was consistent with that of the 18650 battery.
The OEMS tests used a custom-made battery containing self-supporting positive electrode material, HC negative electrode, two layers of GF/A glass fiber separator, and 120 μL of electrolyte (with or without additives). The battery was allowed to rest for 4 hours before cycling. The gas evolution rates of H₂ (m/z 2), C₂H₄ (m/z 26), O₂ (m/z 32), and CO₂ (m/z 44) were monitored and calculated.
TAS Fiber
Te₂As₃Se₅ (TAS) fiber was drawn from a synthesized preform. The fiber was cut into 50 cm segments, and both ends were polished sequentially with 3 μm, 1 μm, and 0.3 μm grinding discs (Thorlabs). IR-FEWS testing utilized a Fourier-transform infrared (FTIR) spectrometer (Invenio S, Bruker) and auxiliary devices to focus the infrared beam onto the fiber end face. A liquid nitrogen-cooled mercury cadmium telluride detector (spectral range: 12,000-600 cm⁻¹) recorded the transmitted infrared signal.
IR-FEWS Operational Measurements Inside 18650 Batteries
Each 18650 battery had one 0.8 mm hole drilled at the center of the positive cap and two 0.8 mm holes drilled at the center of the negative cap. After vacuum drying the battery overnight at 80°C in a Buchi oven, it was transferred to an argon glove box. Following the protocol by Gervillé-Mouravieff et al., the TAS fiber was inserted into the battery, and the positive cap hole was sealed with epoxy resin (EpoxiCure 2, Buehler). 5.2 mL of electrolyte was injected through the free hole in the negative cap, and both negative cap holes were sealed with epoxy resin. After removing from the glove box, the fiber end was connected to the spectrometer. During cycling, infrared spectra were recorded every 2 minutes, with each recording being the average of 128 scans within 30 seconds.
X-ray Photoelectron Spectroscopy (XPS)
Button cells (CR2032 type) of P2/HC and O3/HC were assembled using single-sided coated electrodes (provided by TIAMAT), two layers of glass fiber separator, and 100 μL of electrolyte. Four sets of batteries were prepared for each layered oxide (P2 and O3): two sets with baseline electrolyte and two sets with electrolyte containing additives. One set of batteries for each electrolyte condition was stopped at 50% SOC during the first charge, while the other set was stopped at the end of the first charge (100% SOC). Cycling was conducted in a temperature-controlled oven at 25°C. After cycling, the batteries were immediately transferred to an argon glove box, disassembled, and the electrodes were rinsed three times with DMC, followed by 15 minutes of vacuum drying in the glove box front chamber.
XPS testing utilized a THERMO Escalab 250Xi spectrometer equipped with a focused monochromatic Al Kα radiation source (hν = 1486.6 eV). Samples were analyzed under conditions of no air exposure, as the spectrometer was connected to the argon glove box. High-energy resolution spectra were recorded at a constant pass energy of 20 eV and charge neutralization was applied. The analysis chamber pressure was approximately 2×10⁻⁷ mbar. Short acquisition time spectra were recorded before each experiment to check for X-ray irradiation degradation. Each sample was analyzed at two different points to assess homogeneity and reproducibility. The binding energy scale was calibrated using the C 1s peak at 290.9 eV (CF₂ of the PVDF binder). Core peak curves were fitted with the minimum number of components.
DOI
This public account content is a summary of the key points of the paper by a large language model for reference only. If you are interested in the details of the research or review, please refer to the original literature (10.1002/aenm.202503527).
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