LiFSI in Lithium Battery Electrolyte Design: Disadvantages and Improvement Strategies
| Jerry Huang
Summary
Lithium bis(fluorosulfonyl)imide (LiFSI) is widely regarded as the next-generation lithium salt to replace lithium hexafluorophosphate (LiPF₆) in lithium-ion and lithium-metal battery electrolytes. Its superior thermal stability, high ionic conductivity, and excellent hydrolysis resistance make it particularly attractive for fast-charging and extreme-temperature applications. However, several critical challenges — most notably aluminum current collector corrosion, high production costs, cathode–electrolyte interphase (CEI) instability, and thermal safety concerns — hinder its widespread adoption. This article provides a comprehensive analysis of these disadvantages and reviews the latest improvement strategies reported in the literature through 2026.
1. Introduction
The global transition toward high-energy-density lithium batteries — targeting >300 Wh kg⁻¹ for electric vehicles — has placed unprecedented demands on electrolyte performance. Conventional LiPF₆-based electrolytes, while dominant in commercial cells, suffer from poor thermal stability (decomposing above ~80 °C), severe hydrolysis sensitivity (generating corrosive HF), and inadequate low-temperature conductivity. These shortcomings become particularly acute in fast-charging scenarios, where large currents generate significant ohmic heating that pushes the electrolyte beyond its stability window.
LiFSI (LiN(SO₂F)₂) has emerged as a compelling alternative. The FSI⁻ anion features a highly delocalized negative charge across two sulfonyl groups, resulting in low lattice energy, high dissociation, and consequently superior ionic conductivity — typically 15–30% higher than LiPF₆ at 25 °C (~10.2 mS cm⁻¹). Its thermal decomposition temperature exceeds 200 °C, and its resistance to hydrolysis is markedly better than that of LiPF₆. These properties have made LiFSI a standard additive — and increasingly a primary salt — in fast-charging electrolyte formulations.
Yet beneath the promise lie significant engineering challenges that must be resolved before LiFSI can fully replace LiPF₆ at scale.
2. Key Disadvantages of LiFSI
2.1 Aluminum Current Collector Corrosion
The most critical and widely studied limitation of LiFSI is its incompatibility with the aluminum (Al) foil current collector used on the cathode side of lithium-ion cells.
Mechanism. In conventional LiPF₆ electrolytes, the PF₆⁻ anion decomposes at high potentials (>4.0 V vs. Li/Li⁺) to form a thin, passivating layer of aluminum fluoride (AlF₃) on the Al surface. This film is insoluble in organic solvents and effectively prevents further corrosion. In contrast, the FSI⁻ anion does not generate a stable passivation layer. Instead, FSI⁻ attacks the native Al₂O₃ film, leading to pitting corrosion and, under prolonged cycling, a so-called "osteoporosis-like" dissolution of the Al substrate.
Voltage and temperature dependence. The corrosion is strongly exacerbated at elevated potentials (>4.3 V) and temperatures (>55 °C). Dissolved Al³⁺ ions can migrate through the electrolyte to the anode side, where they participate in parasitic reactions — a phenomenon known as "crosstalk degradation" that accelerates capacity fade and impedance growth.
Stainless steel vulnerability. Beyond aluminum, recent work by Yan et al. (Nature Communications, 2026) has demonstrated that FSI⁻ can also destabilize the passive oxide layer on stainless steel (SUS) current collectors, causing severe metal dissolution under high-voltage cycling conditions.
2.2 High Production Cost
LiFSI remains significantly more expensive than LiPF₆. As of late 2025, the market price of LiFSI was approximately 250,000 RMB/ton (~25 USD/kg equivalent in bulk), roughly twice that of LiPF₆ (~130,000 RMB/ton). The production cost ranges from 120,000 to 200,000 RMB/ton, driven by:
• Complex multi-step synthesis involving fluorination, sulfonation, and imidation reactions with stringent purity requirements.
• Low overall yield due to side reactions and the difficulty of removing trace impurities (e.g., residual chloride, moisture).
• High environmental treatment costs — approximately 10,000 RMB per ton of waste — owing to fluorine-containing byproducts and acidic effluents.
In electrolyte formulations where LiFSI constitutes a growing fraction of the salt content (with some fast-charging electrolytes using LiFSI as the primary salt), the salt cost can account for over 60% of total electrolyte manufacturing cost, making cost reduction a critical barrier to large-scale adoption.
2.3 Cathode–Electrolyte Interphase (CEI) Instability and Cathode Cracking
The decomposition pathway of FSI⁻ differs fundamentally from that of TFSI⁻ or PF₆⁻. Under oxidative conditions at the cathode surface, the S–F bonds in FSI⁻ can undergo cleavage, generating species such as SO₂ and various fluorosulfonate fragments. The resulting CEI film tends to be mechanically fragile and insufficiently protective at high voltages.
Tong et al. (Materials Today, 2025) reported that LiFSI-based electrolytes can induce micro-cracking in cathode electrode sheets, particularly in high-nickel layered oxides (NMC811, NCA). These cracks expose fresh cathode surface to the electrolyte, triggering continuous electrolyte decomposition, transition metal dissolution, and impedance growth — a self-accelerating degradation loop.
2.4 Thermal Runaway Behavior in Fast-Charging Systems
Although LiFSI itself is thermally stable (decomposition onset >200 °C), the thermal runaway behavior of LiFSI-based electrolyte systems differs from conventional LiPF₆ formulations. Research published in RSC Energy & Environmental Science (2025) revealed that after extended fast-charging cycling, LiFSI-based cells exhibit altered heat generation profiles and gas evolution patterns. The decomposition products — including SO₂ and HF — follow different release kinetics, and the thermal safety boundaries of the cell must be re-evaluated rather than simply extrapolated from LiPF₆-based systems.
2.5 Compatibility Challenges with Silicon-Based Anodes
While LiFSI contributes to the formation of an SEI rich in inorganic LiF — which is generally beneficial for mechanical robustness — the enormous volume expansion of silicon anodes (~300%) during lithiation causes repeated SEI fracture and reformation. This leads to continuous electrolyte consumption, low initial Coulombic efficiency, and rapid capacity fade over extended cycling. The interaction between LiFSI decomposition products and silicon surfaces requires careful electrolyte engineering to achieve long-term stability.
3. Improvement Strategies
3.1 Mitigating Aluminum Corrosion
3.1.1 Dual-Salt and Multi-Salt Electrolyte Systems
The most industrially mature approach combines LiFSI with LiPF₆ in a dual-salt formulation. The PF₆⁻ anion provides the essential AlF₃ passivation layer on the aluminum current collector, while LiFSI contributes its superior conductivity and thermal stability. Kautz et al. (Journal of The Electrochemical Society, 2026) demonstrated an optimized LiFSI–LiPF₆ dual-salt electrolyte with controlled solvation structure that exhibited excellent cycling stability across a wide temperature range, from sub-zero to elevated temperatures.
3.1.2 Borate-Based Additives
Lithium borate salts — particularly lithium difluoro(oxalato)borate (LiDFOB) and lithium bis(oxalato)borate (LiBOB) — have proven highly effective as corrosion-inhibiting additives. These compounds oxidize preferentially at the Al surface, forming a protective film that prevents FSI⁻ attack. Notably, Yan et al. (Nature Communications, 2026) showed that LiDFOB addition simultaneously suppresses both aluminum and stainless steel dissolution in LiFSI-based electrolytes, offering a dual-protection mechanism.
3.1.3 Solvation Structure Engineering
A more fundamental approach involves restructuring the Li⁺ solvation sheath to limit FSI⁻ access to the Al surface. Strategies include:
• High-concentration electrolytes (HCEs): At very high salt concentrations (>3 M), the number of free solvent molecules is drastically reduced, and FSI⁻ anions become tightly coordinated to Li⁺, limiting their activity toward Al corrosion.
• Fluorinated ether solvents: Fluorinated diluents such as TTE (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether) and newer alternatives like HFTEP reduce the oxidative decomposition of the electrolyte while modifying the solvation environment to protect the current collector.
• Dual-anion solvation structures: Introducing co-solvents such as tetraethylene glycol dimethyl ether (TEGDME) alongside LiNO₃ has been shown to construct a stable dual-anion Li⁺ solvation structure. The TEGDME assists in coordinating FSI⁻ away from the Al surface, while LiNO₃ enhances Li⁺–solvent ion–dipole interactions and generates a nitrogen-rich passivation film on the lithium metal surface. This dual strategy achieved 89.6% capacity retention after 400 cycles at 55 °C.
3.1.4 Ionic Liquid Additives
Ionic liquids containing anions such as DFOB⁻ and PO₂F₂⁻ have been explored to simultaneously address lithium inventory loss and Al corrosion. In anode-free lithium metal cells operating at high voltages, these additives facilitate the formation of protective interphases on both the cathode and current collector surfaces.
3.2 Reducing Cost
• Process optimization: Continuous-flow synthesis, improved purification methods, and catalyst-assisted routes are being developed to increase yield and reduce waste generation.
• Scale-up and supply chain maturation: As production capacity expands — particularly in China, where major electrolyte manufacturers are investing in LiFSI production lines — economies of scale are expected to narrow the price gap with LiPF₆.
• Blended salt formulations: Using LiFSI as a partial replacement (e.g., 20–50% molar fraction) rather than a full substitute reduces per-cell salt cost while retaining most of the performance benefits.
3.3 Stabilizing the Cathode–Electrolyte Interphase
3.3.1 Fluorinated Solvent Systems
Fluorinated solvents have shown exceptional promise in enhancing CEI stability. Liu et al. (Nature Energy, 2026) demonstrated a hybrid fluorinated ionic liquid electrolyte in which a highly fluorinated cation interacts synergistically with anions and diluents to boost oxidative stability, enabling reliable cycling at voltages exceeding 4.5 V.
3.3.2 Sulfonamide-Based Solvent Systems
A breakthrough approach by Xue et al. (Xi'an Jiaotong University, published in Advanced Materials, 2025) introduced a mixed sulfonamide electrolyte composed of LiFSI salt and two sulfonamide solvents — requiring no additional additives. This electrolyte forms a dense, inorganic-rich, self-limiting interphase on silicon anodes during the first lithiation. Under aggressive conditions (4.5 V cutoff, 5 mAh cm⁻² areal capacity), NMC811||high-Si pouch cells achieved 80% capacity retention after 1,150 cycles — compared to just 383 cycles with conventional industrial electrolyte.
3.4 Addressing Thermal Safety
• System-level thermal management: Battery thermal management systems must be re-calibrated for LiFSI-based electrolytes, accounting for their distinct thermal runaway onset temperatures and gas evolution profiles.
• Electrolyte formulation tuning: Adjusting solvent ratios and incorporating flame-retardant additives (e.g., phosphorus-containing compounds) can modulate the thermal behavior without compromising ionic conductivity.
• Accelerating rate calorimetry (ARC) characterization: Comprehensive thermal runaway testing specific to each LiFSI formulation is essential, rather than relying on extrapolation from LiPF₆ benchmarks.
3.5 Improving Silicon Anode Compatibility
• Electrolyte engineering for robust SEI: Combining LiFSI with co-salts and tailored solvent systems to form an SEI that accommodates volume expansion while maintaining ionic transport.
• Pre-lithiation and artificial SEI strategies: Supplementing the lithium inventory and pre-forming a mechanically compliant interphase layer can offset the initial SEI instability associated with LiFSI on silicon surfaces.
• Electrolyte engineering for SEI regulation on Si anodes: Recent work (OAE Publishing, EnergyZ, 2026) has shown that combining ethylene glycol diglycidyl ether (EGDE) with LiFSI increases anion accessibility by tailoring Li⁺ coordination, thereby enhancing SEI stability on micrometer-scale silicon particles.
4. Summary and Outlook
Challenge 1: Al current collector corrosion Cause: FSI⁻ fails to passivate Al surface Current Best Mitigation: LiFSI + LiPF₆ dual-salt + LiDFOB additive Maturity: Industrially adopted
Challenge 2: High production cost Cause: Complex synthesis, low yield, waste treatment Current Best Mitigation: Scale-up + process optimization + blended salts Maturity: In progress
Challenge 3: CEI instability / cathode cracking Cause: S–F bond cleavage at high voltage Current Best Mitigation: Fluorinated solvents, sulfonamide electrolytes Maturity: Lab validation
Challenge 4: Thermal runaway risk Cause: Altered decomposition pathways under fast charge Current Best Mitigation: System-level thermal management + ARC characterization Maturity: Early stage
Challenge 5: Si anode incompatibility Cause: SEI fracture under volume expansion Current Best Mitigation: Mixed sulfonamide electrolytes, EGDE additives Maturity: Lab validation
The trajectory of LiFSI research is shifting from passive mitigation (additive-based protection) toward active molecular design — engineering the solvation structure, anion coordination, and interfacial chemistry to fundamentally eliminate the conditions that give rise to corrosion and instability. With continued advances in electrolyte formulation and manufacturing scale-up, LiFSI is transitioning from a high-end additive for fast-charging systems toward a viable next-generation primary lithium salt.
The coming years will likely see the convergence of three trends: (1) the maturation of dual-salt and multi-salt formulations that balance cost and performance, (2) the emergence of novel solvent systems (sulfonamides, fluorinated ethers, ionic liquids) that address CEI and corrosion challenges at their molecular origins, and (3) the continued reduction in LiFSI production costs through process innovation and capacity expansion. Together, these developments position LiFSI at the center of the next major evolution in lithium battery electrolyte technology.
References
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[E2] Liu, Q. et al. "Hybrid fluorinated ionic liquid electrolyte for high-voltage lithium metal batteries." Nature Energy, 2026.
[E3] Kautz, D.J. et al. "Advanced LiFSI–LiPF₆ electrolyte for wide-temperature and fast-charging applications." Journal of The Electrochemical Society, 2026.
[E4] Tong, L. et al. "The rise of lithium bis(fluorosulfonyl)imide: An efficient lithium salt for next-generation batteries." Materials Today, 2025.
[E5] Zhang, M. et al. "Challenges, strategies and prospects in interfaces between solid-state electrolytes and electrodes." EnergyZ (OAE Publishing), 2026.
[E6] Chen, Y. et al. "Aluminum corrosion chemistry in high-voltage lithium metal batteries with LiFSI-based ether electrolytes." ACS Applied Materials & Interfaces, 2024.
[E7] Jang, T. et al. "Effects of Li⁺ solvation structures on aluminum corrosion in fluorinated ether electrolytes." PubMed, 2025.
[E8] Scheer, K.M. et al. "Anodic dissolution of the aluminum current collector in lithium-ion cells with LiFSI, LiPF₆, and LiBF₄." Journal of The Electrochemical Society, 2025.
[E9] Xue, W. et al. "Mixed sulfonamide electrolyte for long-cycle-life lithium-ion batteries." Advanced Materials, 2025.
[E10] "The thermal runaway mechanism of fast-charging lithium-ion batteries using LiFSI-based electrolytes." RSC Energy & Environmental Science, 2025.
[E11] Zhang, S.S. "Unveiling the mystery of LiFSI as a single salt in low-to-moderate concentration electrolytes." Journal of Energy Chemistry, 2022.
[E12] "Electrolyte evolution: A roadmap from solvation structure to next-generation batteries." Nano-Micro Letters (Springer), 2026.
[E13] "Electrolyte engineering for SEI regulation on Si anodes in Li batteries." EnergyZ (OAE Publishing), 2026.