Silicon anodes represent a significant advancement in lithium-ion battery technology due to their high theoretical capacity, which is nearly ten times that of conventional graphite anodes. However, their commercialization faces challenges, including substantial volume expansion during lithiation, unstable solid-electrolyte interphase (SEI) formation, and rapid Coulombic efficiency loss. Electrolyte additives play a critical role in addressing these issues by modifying the electrolyte composition to enhance interfacial stability and electrochemical performance. Among the most effective additives for silicon anode electrolytes are fluoroethylene carbonate (FEC) and lithium bis(fluorosulfonyl)imide (LiFSI), which mitigate degradation mechanisms and improve cycle life.
Silicon undergoes a volume expansion of up to 300% during lithiation, leading to mechanical stress, particle cracking, and continuous SEI reformation. This repeated SEI breakdown and repair consume active lithium and electrolyte, reducing Coulombic efficiency and capacity retention. Conventional carbonate-based electrolytes, such as those with ethylene carbonate (EC) and dimethyl carbonate (DMC), are inadequate for silicon anodes because they form brittle, inorganic-rich SEI layers that fracture under strain. Additives like FEC and LiFSI address these limitations by promoting the formation of a flexible, organic-rich SEI that accommodates volume changes and reduces parasitic reactions.
Fluoroethylene carbonate is a widely studied additive that significantly improves the stability of silicon anodes. When added to the electrolyte, FEC decomposes preferentially over other solvents, forming a robust and elastic SEI layer rich in lithium fluoride (LiF) and polycarbonate species. The presence of LiF enhances mechanical strength and chemical inertness, while the polymeric components provide elasticity to withstand volume fluctuations. Research indicates that electrolytes containing 5-10% FEC by weight can double the cycle life of silicon anodes compared to FEC-free formulations. The fluorine atoms in FEC also reduce electrolyte reduction at the anode surface, minimizing gas evolution and improving Coulombic efficiency in the initial cycles.
Lithium bis(fluorosulfonyl)imide serves a dual role as both a conductive salt and an additive. Unlike traditional lithium hexafluorophosphate (LiPF6), LiFSI exhibits superior thermal and electrochemical stability, reducing salt decomposition and HF formation, which degrade the SEI. LiFSI-derived SEI layers contain sulfonyl-based compounds that enhance ionic conductivity and interfacial adhesion. Studies show that electrolytes with LiFSI reduce silicon anode impedance by up to 50% compared to LiPF6-based systems, leading to better rate capability and lower polarization. Additionally, LiFSI suppresses aluminum current collector corrosion at high voltages, making it compatible with full-cell configurations.
The synergy between FEC and LiFSI further enhances silicon anode performance. Combining these additives results in a hybrid SEI with balanced mechanical and transport properties. For instance, FEC contributes elasticity and LiF content, while LiFSI improves ionic conductivity and interfacial stability. Electrolytes with both additives demonstrate reduced lithium inventory loss, with Coulombic efficiency exceeding 99% after the first few cycles. This combination also mitigates silicon particle isolation caused by repeated expansion and contraction, maintaining electrical contact within the electrode.
Other additives, such as vinylene carbonate (VC) and lithium difluoro(oxalato)borate (LiDFOB), complement FEC and LiFSI by further stabilizing the SEI. VC polymerizes during cycling, forming a poly(VC) network that reinforces the SEI against mechanical stress. LiDFOB generates boron-containing compounds that passivate electrode surfaces and reduce electrolyte oxidation at high potentials. However, these additives are less effective than FEC and LiFSI in isolation, highlighting the importance of tailored formulations for silicon anodes.
The concentration of additives must be optimized to avoid detrimental effects. Excessive FEC can lead to excessive gas generation during formation cycles, while high LiFSI concentrations may increase electrolyte viscosity and reduce wettability. Typical formulations use 5-10% FEC and 0.5-1.0 M LiFSI, balancing SEI quality with electrolyte conductivity. Advanced electrolytes may also incorporate small amounts of film-forming co-additives, such as succinonitrile or cesium hexafluorophosphate, to further enhance interfacial properties.
Long-term cycling stability remains a challenge even with advanced additives. Silicon anodes still experience gradual capacity fade due to irreversible lithium loss and electrode porosity changes. However, additive-enhanced electrolytes can achieve 80% capacity retention after 500 cycles in half-cell configurations, a significant improvement over baseline electrolytes. Full-cell performance depends on the cathode compatibility of these additives, as some may oxidize at high voltages or form resistive layers on the cathode surface.
Future developments in additive chemistry will focus on multi-functional molecules that address multiple degradation mechanisms simultaneously. For example, additives that combine SEI-forming, lithium reservoir, and volume buffering properties could further improve silicon anode viability. Researchers are also exploring localized high-concentration electrolytes (LHCEs) that leverage additive-rich regions near the electrode surface to enhance stability without compromising bulk electrolyte properties.
In summary, electrolyte additives like FEC and LiFSI are indispensable for enabling silicon anodes in practical lithium-ion batteries. By forming stable, flexible SEI layers and improving interfacial kinetics, these additives mitigate volume expansion effects, reduce Coulombic efficiency loss, and extend cycle life. Continued optimization of additive formulations will be critical to overcoming remaining challenges and unlocking the full potential of silicon anode technology.