Electrolyte additives can help extend the life of electric vehicle (EV) batteries by stabilizing the electrode-electrolyte interfaces and mitigating the adverse side reactions that cause battery degradation over time.
This article focuses on how a few specific electrolyte additives affect the lifespan of EV batteries.
Improving battery lifespan with electrolyte additives
One primary way electrolyte additives help EV batteries last longer is by facilitating the formation of a stable solid electrolyte interphase (SEI) layer on the anode’s surface.
This layer is important because it keeps the anode safe from constant reactions with the electrolyte, which can use up Li-ion and cause the capacity to drop. A well-formed SEI layer is electrically insulating but allows for the efficient transport of Li-ions.
Similarly, electrolyte additives can contribute to the formation of a stable cathode electrolyte interface (CEI) layer on the cathode’s surface. The CEI layer helps to prevent the dissolution of transition metal ions from the cathode material. It reduces parasitic reactions between the cathode and the electrolyte, especially at high operating voltages.
Let’s now look at some specific examples of electrolyte additives and how they affect the lifespan of EV batteries, such as sodium-ion and Li-ion batteries.
VC for better capacitive contributions in sodium-ion batteries
Vinylene carbonate (VC) is a common electrolyte additive used in EV batteries, which forms a stable SEI layer on the anode. This layer protects the anode from continuous electrolyte decomposition and allows efficient Li-ion transport. VCs can be more flexible, which is beneficial for anodes that experience volume changes during charging and discharging.
Figure 1 illustrates the effect of VC on the electrochemical performance of sodium-ion TiO2 nanosheet anodes. It compares the performance of the electrolyte additive before and after cycling.
Figure 1. The impact of the VC additive on electrolyte-electrode interfacial stability in sodium-ion batteries. (Image: MDPI)
The bar graphs (a-d) illustrate how capacitive and diffusion-controlled contributions to the electrochemical processes change before and after cycling at different scan rates (from 0.1 to 10 mV/s).
Before cycling, at the lowest scan rate (0.1 mV/s), the capacitive contribution increases from 30% without VC to 51% with VC. In the same way, after cycling, the capacitive contribution goes from 8% without VC to 63% with VC at the slowest scan rate, showing a significant improvement in the stability and reactions on the electrode surface.
Graphs (e–f) show cyclic voltammograms (CV) with a scan rate of 1 mV/s. They clearly show the capacitive (shaded area) and total current contributions after cycling, highlighting the changes from adding VC. It visually shows the capacitive contribution, revealing a higher contribution with VC (87%) than without VC (37%). This difference highlights how VC stabilizes the SEI, reduces diffusion-controlled resistance, and enables superior electrochemical performance.
Therefore, VC improves the performance and stability of the electrolyte in EV batteries. It does this by helping to create a stable and strong SEI layer, which leads to higher capacitive contributions, better cycle stability, and lower internal resistance.
VC and FEC for capacity retention in Li-ion batteries
Fluoroethylene carbonate (FEC) is another common and significant electrolyte additive used in the Li-ion batteries found in EVs. FEC works so well as an electrolyte additive because it breaks down more quickly in the first few cycles. It forms protective interfacial layers that stabilize the electrodes and the electrolyte, ultimately leading to a longer and more reliable lifespan for EV batteries.
Figure 2 shows battery capacity retention under floating state-of-charge (SoC) conditions, highlighting how electrolyte additives, specifically VC and fluoroethylene carbonate (FEC), affect Li-ion battery lifespan over a shorter storage duration (six months).
Figure 2. The influence of electrolyte additives (VC and FEC) on Li-ion battery capacity retention under floating SoC storage conditions. (Image: MDPI)
At the lower SoC levels, capacity loss is relatively moderate, but differences emerge quickly between electrolytes. VC additive consistently shows superior performance with minimal capacity degradation. FEC additive generally performs better than the additive-free electrolyte (LP57) at low SoCs but is less effective than VC.
At higher SoC levels, accelerated capacity fade is observed due to the stress of maintaining batteries at high voltage levels. Batteries without additives experience the most significant degradation. VC additive improves capacity retention, clearly outperforming FEC and the additive-free electrolyte. The FEC additive also shows improvement over LP57 but performs worse than VC, which is particularly evident at 100% SoC.
Therefore, VC demonstrates clear potential to enhance EV battery lifespan under stressful operating conditions where batteries are kept at high states of charge. However, FEC’s effectiveness diminishes noticeably as SoC increases, highlighting a limitation in protecting batteries in high-voltage storage scenarios.
Effect of sulfur-containing compounds in Li-ion batteries
Sulfur-containing additives generally have lower lowest unoccupied molecular orbital (LUMO) energy levels, making them more prone to electrochemical reduction than organic carbonates. This preferential reduction leads to a stable SEI film on the negative electrode.
Examples of sulfur-containing compounds used as SEI formers include 1,3-propane sultone (PS), 1,3-propanediol cyclic sulfate (PCS), prop-1-ene-1,3-sultone (PES), 1,3,2-dioxathiolane-2,2-dioxide (DTD), and ethylene sulfite (ES).
Figure 3 illustrates the influence of different electrolyte additives—specifically DTD, 1,2-PS, and 1,3-PCS (each at 1 wt%)—on battery cells’ performance and structural stability, focusing on four critical parameters.
Figure 3. The effect of electrolyte additives (DTD, 1,2-PS, 1,3-PCS) on capacity retention, recovery, internal resistance, and thickness swelling in battery cells. (Image: Wiley)
It’s observed that without additives, it led to a high internal resistance change and noticeable thickness swelling, suggesting significant structural stress and degradation. It also had slightly lower capacity retention and recovery compared to additive-enhanced electrolytes.
With additives (DTD, 1,2-PS, 1,3-PCS), improved capacity retention and recovery, nearly approaching or reaching 100%, demonstrating enhanced battery longevity and cycling stability. Significantly reduced internal resistance changes, indicating better electrode-electrolyte interfaces, lower degradation, and improved efficiency. Reduced thickness swelling, implying enhanced structural integrity and less internal stress.
Therefore, specific electrolyte additives (DTD, 1,2-PS, 1,3-PCS) enhance battery performance, longevity, and safety by stabilizing battery internal structures, minimizing degradation, and maintaining battery capacity.
The effect of other electrolyte additives
Phosphorus atoms in polyphosphonates act as trapping agents for hydrogen radicals, key components in combustion processes. This naturally flame-retardant quality makes it much less likely that something will catch fire because of electrolyte leakage or thermal runaway. This makes the product safer and could last longer by stopping catastrophic failures.
Copolymers with flame-retardant phosphonate units show ionic conductivity around 10⁻⁵ S cm⁻¹ at room temperature and stability in a broad electrochemical window (0.5–4.5 V vs. Li⁺/Li) and at temperatures higher than 120° C.
Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is chemically and thermally stable and has a high thermal decomposition temperature. This can help batteries last longer by keeping the electrolyte from breaking down at high temperatures. A LiTFSI-LiODFB dual-salt electrolyte showed better thermal stability than a LiPF6 electrolyte.
However, when used alone, LiTFSI can cause severe corrosion to the aluminum cathode current collector at voltages above 3.7 V, which can negatively impact battery lifespan.
LiPO₂F₂ is an effective electrolyte additive that significantly improves the cycling stability of Li-rich cathode materials by promoting a stable CEI film. This CEI layer stops the breakdown of electrolytes at high voltage and lowers the dissolution of transition metals, two main ways that these cathodes break down. This makes the battery last longer.
Disadvantages of selecting electrolyte additives
While electrolyte additives improve the lifespan of EV batteries, there are also trade-offs due to various disadvantages such as instability, thermal, and impedance concerns, to name a few.
VC is prone to degradation during storage, compromising its reliability as a long-term additive in battery systems. While widely used, ethylene carbonate (EC) contributes to considerable gas evolution during formation and storage. Additionally, being solid at room temperature limits its flexibility in low-temperature applications.
PS is highly sensitive to moisture as it is both toxic and known to generate harmful byproducts. Its instability poses safety and handling concerns during battery manufacturing. PES is thought to help with interphase formation, but it causes high interfacial impedance after being cycled many times in high-voltage systems.
DTD exhibits poor shelf-life stability and releases significant gas during battery formation, raising performance and safety issues. ES additive suffers from low oxidative stability and weak storage performance. It also increases interfacial resistance during battery operation.
Summary
Electrolyte additives are an ongoing research effort to improve the performance of EV batteries. However, VC, FEC, and sulfur-containing compounds are critical electrolyte additives that enhance Li-ion and sodium-ion battery performance and lifespan. They do this by forming protective interfacial layers on electrodes and mitigating detrimental side reactions. Due to the rich chemistry of sulfur, sulfur-containing compounds offer a particularly diverse range of functionalities.
References
- Thermal Stability Analysis of Lithium-Ion Battery Electrolytes Based on Lithium Bis(trifluoromethanesulfonyl)imide
- Lithium Difluoro(oxalato)Borate Dual-Salt, Polymers, MDPI
- Effect of Vinylene Carbonate Electrolyte Additive on the Surface Chemistry and Pseudocapacitive Sodium-Ion Storage of TiO₂ Nanosheet Anodes, Batteries, MDPI.
- Influence of Vinylene Carbonate and Fluoroethylene Carbonate on Open Circuit and Floating SoC Calendar Aging of
- Lithium-Ion Batteries, Batteries, MDPI.
- LiPO2F2 electrolyte additive for high-performance Li-rich cathode material, Journal of Energy Chemistry, ScienceDirect
- Sulfur‐containing compounds as electrolyte additives for lithium‐ion batteries, Wiley.
- Phosphorus-Containing Polymer Electrolytes for Li Batteries, Batteries, MDPI
