VC or FEC? How to select electrolyte additives for graphite and silicon-carbon anodes

VC (vinylene carbonate) and FEC (fluoroethylene carbonate) are both widely used film-forming electrolyte additives, but they are not prioritized in the same applications. For graphite anodes, VC is often one of the first additives to evaluate; for silicon-carbon anodes, FEC is commonly a primary candidate. For mixed graphite/silicon-carbon anodes, VC/FEC combinations can also be included in screening. Their selection should also take into account the lithium salt, solvent system, cathode material, and formation conditions.

The first question is the common target of both additives: the solid electrolyte interphase (SEI) on the anode surface. During the first charge, the anode operates at low potential, causing electrolyte reduction and SEI formation. An effective SEI should allow Li+ transport while limiting further electron transfer into the electrolyte and thereby suppressing continued side reactions [1]. Graphite undergoes relatively limited volume change, whereas silicon-carbon anodes repeatedly expand and contract, making the interphase more prone to cracking and exposing fresh surface. Repeated film rupture and re-formation consume active lithium, increase gas generation, and raise impedance. This difference in interfacial failure mode is a key reason why VC and FEC are prioritized differently. The central question is whether each additive can provide suitable protection for the target anode [1][4].

VC contains a carbon-carbon double bond that can undergo reduction relatively readily and participate in polymerization or film formation at the anode surface. In conventional graphite systems, VC-derived interphases can help reduce solvent decomposition and improve cycling stability. Studies have shown that adding VC to a baseline electrolyte can provide benefits relative to formulations without a film-forming additive [2]. This makes VC a common first-screening additive for graphite systems, but it does not mean that more VC is always better. Excessive VC may increase initial impedance, promote gas generation, or impair low-temperature performance. Its effect also depends on the lithium salt, solvent system, graphite surface, and formation protocol.

When the anode shifts from graphite to silicon or silicon-carbon, the evaluation focus also shifts from simply limiting ongoing side reactions to whether the interphase can tolerate repeated volume change. FEC introduces fluorine into the EC structure and can alter the reduction pathway and interphase composition, often producing an SEI containing both fluorinated inorganic species and polymeric components [4][5]. For silicon and silicon-carbon anodes, this type of interphase can help reduce soluble decomposition products and improve adhesion to the silicon surface, which is why FEC is often used to address interfacial degradation associated with repeated silicon expansion [3][5]. However, excessive FEC or an unfavorable combination with a high-voltage cathode or lithium salt may increase gas generation, acidic by-products, or impedance [6]. Its suitability therefore still needs to be verified in the specific electrolyte and cell system.

These different interfacial behaviors provide a practical basis for setting the screening order. In graphite-dominant systems focused on first-cycle efficiency, room-temperature cycling, and interfacial impedance, VC can be used as an initial benchmark. If low-temperature operation, fast charging, or high-voltage conditions are also important, FEC should be evaluated at the same time for its effects on gas generation, impedance, and the cathode interface. In anodes with higher silicon content, repeated volume change, interphase cracking, and continued lithium loss become more pronounced, so FEC can be prioritized first, followed by evaluation of whether VC or other additives should be combined based on cycling and storage results. For mixed graphite/silicon-carbon anodes, VC-only, FEC-only, and combined formulations should be compared while also considering silicon content, particle size, binder, and formation conditions.

However, setting a VC-versus-FEC screening priority based on anode behavior is only the starting point. In a full cell, the base electrolyte and cathode conditions must also be included because additive performance is shaped jointly by the solvents, lithium salt, and cathode reactions. Solvents such as EC, PC, DMC, EMC, and DEC can alter solvation structure and reduction sequence, while salts such as LiPF6 and LiFSI can affect interphase composition and acidic side reactions. Cathode voltage and surface chemistry can also change additive behavior [1][6]. Accordingly, VC or FEC performance in an anode half-cell should be treated only as one screening input; final confirmation still needs to be carried out in a full cell matched to the target cathode.

Because the final suitability of VC and FEC must be verified in a full cell, development and procurement evaluation should also consider additive quality consistency and test results generated under standardized conditions. In addition to nominal purity, VC and FEC should be checked for water content, acidity, metal impurities, color, storage stability, and batch-to-batch consistency. A useful validation matrix is to compare a blank electrolyte, VC, FEC, and VC/FEC combinations under the same solvent, lithium salt, electrodes, and formation protocol, and evaluate first-cycle coulombic efficiency, gas generation, impedance, rate capability, cycling, storage, and high- and low-temperature performance. For silicon-carbon systems, electrode-thickness change or cell swelling should also be monitored.

In summary, VC and FEC selection can be organized as a continuous decision process: first identify the anode material and its main interfacial failure mode to set screening priorities; then return the candidate additives to the complete electrolyte formulation and target cathode for validation; finally, combine the performance results with quality consistency and manufacturing cost to support procurement decisions. Conventional graphite systems often begin with VC, while silicon-carbon anodes often justify prioritizing FEC. Mixed-anode and high-voltage systems should also compare combined use and possible side effects. Ultimately, additive selection should be based on complete test results from the target cell system.