Performance Evaluation and Analysis of ARC-S38 Electrolyte Additive

Cell chemistry: NCM622/artificial graphite Cell type: 1.3 Ah pouch cell Voltage range: 2.75–4.4 V

Baseline formulation: EC:EMC:DEC = 3:5:2, 1.0 M LiPF₆ + 1.0% VC Experimental formulations: Baseline formulation + 0.5% S38 Baseline formulation + 1.0% S38 Baseline formulation + 1.5% S38

The differences in initial Coulombic efficiency among the groups are relatively small, with all groups achieving approximately 86.2%, while the 0.5%-S38 group is only about 0.2 percentage points lower. In terms of initial discharge capacity, all experimental groups containing ARC-S38 are approximately 12 mAh lower than the baseline group (BASE). This may be attributed to the preferential formation of a dense SEI film by ARC-S38 during formation, prior to the reduction of VC and solvent components. This process consumes a small amount of active lithium, resulting in a slight increase in initial irreversible capacity and consequently a slight decrease in initial discharge capacity. The essentially unchanged initial Coulombic efficiency indicates that, although the SEI film formed by ARC-S38 is relatively dense, it does not significantly compromise the structure of the cathode active material, and its overall impact remains controllable. The slightly lower initial Coulombic efficiency observed at the 0.5% addition level may be related to relatively less uniform film formation at this concentration.

The dQ/dV curves indicate that ARC-S38 preferentially undergoes reduction and forms a dense SEI film on the anode surface. This film initially results in higher interfacial impedance. EIS measurements show the following trend: 1.5%-S38 ≈ 1.0%-S38 > 0.5%-S38 > BASE The charge-transfer impedance also increases with increasing ARC-S38 concentration. However, a dense and stable SEI film provides an important foundation for subsequent improvements in cycling and storage performance. Therefore, the initial increase in impedance can be regarded as a consequence of effective film formation.

Low-temperature discharge performance: At temperatures of ≥ -10°C, the experimental groups show no significant difference from the baseline group. At -20°C, the baseline and 0.5%-S38 groups exhibit relatively better discharge performance, with an improvement of approximately 1.0%. One possible explanation is that the dense SEI film formed by ARC-S38 slightly affects low-temperature discharge performance by increasing interfacial resistance, resulting in a certain degree of limitation in low-temperature discharge capability.

For charging rates above 2C, capacity retention gradually improves with increasing ARC-S38 addition. At a 1.5% addition level, the improvement reaches approximately 0.6%. For discharge rates of ≥3C, the advantage in rate performance becomes increasingly pronounced as the ARC-S38 concentration increases. At 5C, the 1.5%-S38 group shows an improvement of approximately 4.1%, while all experimental groups demonstrate some degree of improvement compared with the baseline.

Under high-temperature storage conditions, the baseline electrolyte is more susceptible to parasitic reactions, resulting in capacity degradation and increased impedance. The dense SEI film formed by ARC-S38 effectively suppresses interfacial side reactions at elevated temperatures and helps maintain the structural stability of the electrodes. The DCR growth rate decreases as the ARC-S38 addition level increases. The 1.0% and 1.5% addition levels deliver similarly strong and optimal performance, significantly improving capacity retention from approximately 48.6% for BASE to above 66.0%, while also improving capacity recovery and substantially reducing DCR growth, thereby enhancing impedance stability.

The SEI film constructed by ARC-S38 effectively suppresses the continuous reductive decomposition of the electrolyte at the anode during repeated charge-discharge cycles, thereby reducing active lithium loss and maintaining high capacity retention. Both the 1.0% and 1.5% addition levels demonstrate excellent cycling stability, with capacity retention remaining above 92% after 320 cycles.

For the NCM622/artificial graphite system, an ARC-S38 addition level of 1.0%–1.5% is recommended. This concentration range provides the best overall performance in high-temperature storage and cycling tests while achieving a reasonable balance between initial capacity and impedance. Among the tested concentrations, 1.0% is recommended as the preferred starting point.

ARC-S38 is primarily suitable for cells requiring high-temperature durability and storage stability, including power batteries for high-temperature operating conditions and energy storage batteries. For applications requiring long cycle life and excellent high-temperature durability, ARC-S38 provides an effective electrolyte additive solution.

ARC-S38 can be used in combination with film-forming additives such as VC and FEC. However, given the preferential film-forming characteristics of ARC-S38, the VC concentration should be appropriately adjusted to avoid excessive film thickness. During the formation process, appropriate pre-charge voltage and aging procedures are recommended to fully leverage the advantages of the dense SEI film formed by ARC-S38.