The transition towards renewable energy systems has created an urgent and growing demand for efficient, scalable, and cost-effective energy storage technologies. While lithium-ion batteries (LIBs) have dominated this landscape, concerns regarding the scarcity and geopolitical concentration of lithium resources have spurred intensive research into alternative chemistries. Sodium-ion batteries (SIBs) have emerged as one of the most promising candidates due to the natural abundance and low cost of sodium, coupled with a working principle analogous to the “rocking-chair” mechanism of LIBs. Furthermore, the lower solvation energy of Na+ and a higher redox potential (by approximately 0.3 V versus Li+/Li) offer potential advantages in terms of safety and kinetics. However, the practical implementation of high-energy-density SIBs faces significant challenges, primarily stemming from unstable electrode/electrolyte interfaces.
The performance and longevity of a SIB are critically dependent on the formation and stability of the solid electrolyte interphase (SEI). This passivation layer, first conceptualized by Peled, forms spontaneously on the anode surface due to the electrochemical reduction of electrolyte components. A stable, ionically conductive but electronically insulating SEI is essential for preventing continuous electrolyte decomposition, facilitating reversible sodium ion (Na+) transport, and ensuring long-term cycle life. Unfortunately, the native SEI formed in conventional SIB electrolytes is often inhomogeneous, mechanically fragile, and excessively thick (>100 nm). This leads to low initial Coulombic efficiency (ICE), rapid capacity fade, and in the case of sodium metal anodes, dendritic growth that poses serious safety risks. The inherent defects of the native SEI vary with the anode material: sodium metal suffers from SEI fracture due to large volume changes during plating/stripping; hard carbon anodes exhibit low ICE due to high surface area and parasitic reactions; and alloying anodes (Sn, Sb, P) experience severe SEI instability triggered by massive volume expansion.
To overcome these limitations, the deliberate construction of an artificial SEI has become a cornerstone strategy in SIB anode engineering. Unlike the native SEI, an artificial SEI is pre-fabricated through in-situ or ex-situ methods, allowing for precise control over its chemical composition, mechanical properties, thickness, and morphology. The core objective is to engineer an interfacial layer that thermodynamically stabilizes the anode surface against electrolyte reduction and kinetically promotes uniform Na+ flux. An ideal artificial SEI should possess high mechanical strength to withstand volume changes and suppress dendrites, excellent ionic conductivity with electronic insulation, and superior chemical/electrochemical stability within the operational voltage window. This review provides a comprehensive analysis of the recent advances in artificial SEI design for SIB anodes, with a focus on mechanistic understanding, material strategies for different anodes (particularly sodium metal and hard carbon), and future research directions aimed at overcoming existing bottlenecks.
Fundamental Design Principles of Artificial SEI
The failure mechanisms of native SEI stem from its spontaneous and uncontrolled formation process. Its random composition, often rich in organic carbonates and alkoxides, lacks the necessary mechanical robustness. Its uneven topography leads to localized current hotspots, and its uncontrolled growth increases ionic transport resistance. The design of an artificial SEI is fundamentally guided by the need to rectify these flaws. Thermodynamically, the goal is to lower the interfacial free energy by incorporating components with high chemical stability (e.g., inorganic fluorides, oxides) that effectively shield the reactive anode from the electrolyte. Kinetically, the aim is to create “fast Na+ channels” by controlling film thickness (typically 5–50 nm) and pore structure, often utilizing inorganic phases for high conductivity or organic phases with sodophilic functional groups to guide uniform Na+ adsorption and diffusion.
The mechanical design philosophy often follows a “rigid-yet-flexible” paradigm. Pure inorganic layers (e.g., NaF, Al2O3) offer high shear modulus (e.g., ~31.4 GPa for NaF) to mechanically block dendrite penetration but are brittle and prone to cracking under strain. Pure organic/polymer layers (e.g., polyDOL, sodium formate) provide excellent toughness and strain tolerance (>200%) to accommodate volume changes but may lack the necessary mechanical strength and ionic conductivity. Composite or hybrid artificial SEI layers seek to synergistically combine these advantages, using a rigid inorganic framework for support and a soft organic matrix for stress dissipation and improved interfacial contact.
The performance of an artificial SEI can be described by several key parameters:
1. Ionic Conductivity ($\sigma_{Na^+}$): Governs the rate capability. It often follows an Arrhenius relationship: $$\sigma = A \exp\left(-\frac{E_a}{k_B T}\right)$$ where $E_a$ is the activation energy for ion migration, which should be minimized.
2. Young’s Modulus (E) and Shear Modulus (G): Critical for mechanical stability. A modulus higher than that of sodium metal (~2-3 GPa) is generally required to suppress dendrite initiation.
3. Interfacial Energy ($\gamma$): A lower interfacial energy with sodium promotes better wettability and more uniform Na+ deposition, as described by Young’s equation for contact angle $\theta$: $$\cos\theta = \frac{\gamma_{sv} – \gamma_{sl}}{\gamma_{lv}}$$ where the subscripts denote solid-vapor (sv), solid-liquid (sl), and liquid-vapor (lv) interfaces.
4. Diffusion Coefficient ($D_{Na^+}$): Indicates the mobility of Na+ within the SEI layer. Higher $D_{Na^+}$ enables faster charge/discharge rates.
The following sections delve into the specific design strategies for the two most critical SIB anode classes: sodium metal and hard carbon.
Artificial SEI for Sodium Metal Anodes
Sodium metal is the ultimate anode for sodium-ion batteries, offering an ultra-high theoretical capacity of 1166 mAh g-1 and the lowest possible operating potential. However, its high reactivity and non-uniform Na+ plating/stripping lead to dendrite growth, continuous SEI breakdown, and poor cycling efficiency. Constructing an effective artificial SEI is paramount to harnessing its potential.
Inorganic Artificial SEI
Inorganic compounds, particularly halides and oxides, are favored for their high mechanical strength and electrochemical stability. Sodium fluoride (NaF) is a star component due to its high interface energy with Na metal, which thermodynamically discourages dendritic deposition. A common strategy involves using electrolyte additives that preferentially decompose to form NaF-rich SEI. For instance, additives like 4-acetylpyridine (4-APD) or perfluorobenzene (PFB) can facilitate the reduction of PF6– anions or themselves, leading to a SEI with significantly higher NaF content compared to additive-free electrolytes, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis. This NaF-dominated layer effectively stabilizes the interface, enabling improved cycling.
Beyond fluorides, other halides like NaI and NaBr have shown promise. An in-situ formed NaI-containing SEI can guide uniform Na+ deposition, while a NaBr-rich layer has been calculated to have an exceptionally low Na+ diffusion barrier (~0.02 eV), making it suitable for high-rate applications. Another innovative approach involves creating a hybrid interface containing both a sodiophilic alloy phase and a robust inorganic phase. This can be achieved via reactions between sodium metal and metal halide vapors (e.g., SnCl4, Sb2S3). The resulting composite SEI, containing phases like NaxSny/NaCl or Na3Sb/Na2S, leverages the alloy’s ability to homogenize Na+ flux and the inorganic’s strength to block dendrites, synergistically enhancing performance in both symmetric cells and full cells.
Organic Artificial SEI
Organic or polymeric artificial SEI layers offer superior flexibility and adhesion, which can better accommodate the volume fluctuations of the sodium metal anode during cycling. Their molecular structure can be tailored to include functional groups that interact favorably with Na+. Examples include in-situ polymerized 1,3-dioxolane (polyDOL) forming a smooth, protective film, and sodium formate (HCOONa) layers created via a solid-gas reaction, which exhibit high chemical stability and low Na+ diffusion barriers. Molecular design of specific monomers, such as quinone-based molecules, can create dense organic layers that prevent electrolyte penetration. Furthermore, electrolyte engineering itself can promote organic-rich SEI formation; for example, ultra-low concentration electrolytes (ULCE) have been shown to form more organic-dominated, stable SEIs that are less susceptible to etching, even at extreme temperatures.
Sulfur-containing organic compounds are particularly interesting. Due to sulfur’s low electronegativity and large atomic radius, S-containing SEI components can create spacious transport channels for Na+. Additives like tetramethylthiuram disulfide (TMTD) or purposely synthesized sodium benzenedithiolate (PhS2Na2) can decompose to form organic sulfide salts in the SEI. These layers have demonstrated excellent capability in stabilizing the sodium metal interface, maintaining a smooth surface after extended cycling, with theoretical calculations supporting their high ionic conductivity.
Composite (Organic-Inorganic) Artificial SEI
The most promising strategies often involve hybrid artificial SEI that combine inorganic and organic components to achieve a balance of mechanical strength, flexibility, and ionic transport. Polymers like polyvinylidene fluoride (PVDF) and its copolymers (e.g., PVDF-HFP) serve as excellent flexible matrices. When composited with inorganic fillers like NaF or Al2O3, the resulting layer benefits from the polymer’s strain accommodation and the filler’s mechanical reinforcement and enhanced ionic pathways.
Other creative composite designs include: grinding sodium-potassium alloy with polytetrafluoroethylene (PTFE) to form a 3D framework SEI containing KF, NaF, and PTFE derivatives; and reacting sodium with a mixture of SnCl2 and an organic phenol to generate a hybrid layer comprising NaCl, Sn nanocrystals, and an aromatic polymer. These composite SEIs are engineered with specific phase ratios (e.g., 50/50 or 60/40 inorganic/organic) to optimize properties such as mechanical modulus, ionic conductivity, and Na+ diffusion energy, leading to exceptional stability in symmetric cell tests over thousands of hours. Electrolyte additives can also be designed to tune the SEI composition towards an optimal organic-inorganic balance, for instance, by promoting the formation of alkyl sulfonates alongside NaF.
| SEI Type | Typical Components / Method | Key Properties / Function | Reported Performance (Example) | Advantages | Challenges |
|---|---|---|---|---|---|
| Inorganic | NaF (from additives: PFB, 4-APD); NaI; NaBr; Alloy/Inorganic hybrids (NaxSny/NaCl) | High mechanical strength, High interfacial energy, Low Na+ diffusion barrier for some halides. | High CE (~98.9%), Stable long-term plating/stripping in symmetric cells. | Excellent dendrite suppression, Good electrochemical stability. | Can be brittle, May require complex additive formulations. |
| Organic | PolyDOL; Sodium Formate (HCOONa); Organic sulfide salts (PhS2Na2) | High flexibility and adhesion, Tunable functional groups, Good compatibility. | Stable cycling, Low overpotential, High CE (>99.5%). | Accommodates volume change, Can enable uniform deposition. | Generally lower mechanical strength and ionic conductivity than top-tier inorganic SEI. |
| Composite | PVDF/NaF; PVDF-HFP/Al2O3; PTFE-derived/KF/NaF; Polymer/Sn/NaCl hybrids | Synergy of strength (inorganic) and toughness (organic), Tunable properties. | Ultra-long symmetric cell cycling (>2000 h), High-rate capability, Wide temperature operation. | Balanced performance, Most promising for practical applications. | Synthesis/processing can be more complex, Optimal composition needs careful design. |
Artificial SEI for Hard Carbon Anodes
Hard carbon (HC) is the leading practical anode material for sodium-ion batteries, offering a good balance of capacity (~300-400 mAh g-1), low working potential (~0.1 V vs. Na+/Na), and low cost. Its primary challenges are a low initial Coulombic efficiency (ICE) and poor rate capability, largely due to its high specific surface area and defective structure which promote excessive, irreversible SEI formation during the first cycle. Therefore, artificial SEI design for HC focuses on creating a thin, uniform, and stable layer that passivates surface defects without hindering Na+ intercalation.
Inorganic Artificial SEI (Coatings)
Applying thin, conformal inorganic coatings is a direct method to modify the HC surface. Atomic layer deposition (ALD) excels at this, enabling the deposition of ultra-thin (e.g., 2 nm) layers of TiO2 or Al2O3. These coatings act as a physical barrier, reducing direct contact between the carbon surface and the electrolyte, thereby minimizing parasitic side reactions and stabilizing the interface. ALD-Al2O3 on biomass-derived HC has been shown to significantly improve surface smoothness and cycling stability. However, ALD’s high cost and low throughput limit scalability.
More scalable solution-based coating methods, such as simple liquid-phase coating or chemical immersion, have been developed to deposit amorphous Al2O3 or other metal oxides (e.g., ZrO2, V2O5) on HC. These coatings effectively shield surface active sites, leading to enhanced ICE and long-term cycle life. For instance, a ZrO2-coated HC anode maintained 82.6% capacity after 2000 cycles. Electrolyte engineering is another powerful tool. Using functional additives like methyl 2,2-difluoro-2-(fluorosulfonyl)acetate (MDFA) or 1,3-propane sulfone (1,3-PS) can promote the formation of inorganic-rich (NaF-dominated) SEI on HC, improving rate performance and cycling stability. High-concentration or locally high-concentration electrolyte systems can also alter the solvation structure and decomposition pathway, favoring the formation of robust, inorganic-dominated SEI layers on HC.
Organic Artificial SEI and Prelithiation/Sodiation
Organic interfacial layers can be formed via molecular engineering or pre-sodiation techniques. Chemically bonding organic molecules with sodophilic groups (e.g., carboxylate -COONa) to the HC surface can create a dense, thin SEI that pre-passivates the surface. For example, a disodium phthalate (DP) layer (~7.4 nm thick) on HC can provide a Na+ reservoir to compensate for initial SEI formation loss, resulting in an ICE exceeding 96% and stable cycling. Direct pre-sodiation using reagents like sodium naphthalenide (Naph-Na) solution chemically forms “-ONa” and “-COONa” groups on the HC surface, effectively increasing the initial sodium inventory and boosting ICE.
Structural engineering is another approach. Co-carbonizing HC with polymers like polystyrene can create a closed-pore structure with a very low surface area, intrinsically reducing sites for SEI formation while maintaining Na+ storage capacity. Furthermore, electrolyte additives can be designed to undergo in-situ electropolymerization on the HC surface during the first charge, forming a robust organic polymer network that constitutes a major part of the SEI, enabling excellent long-term cycling stability at high rates.
Composite Artificial SEI for Hard Carbon
As with sodium metal, composite strategies are highly effective for HC. A straightforward method involves soaking HC in a warm electrolyte containing film-forming additives like fluoroethylene carbonate (FEC) before cell assembly, creating a pre-formed “artificial SEI” rich in both organic fluorinated compounds and inorganic NaF. Additives like sodium difluorophosphate (NaDFP) in the electrolyte can induce the formation of a more compact and uniform SEI with a balanced composition, enhancing performance across a wide temperature range. Electrolytes based on tetraglyme (TEGDME) can facilitate the stepwise reduction of Na+ complexes, leading to the construction of a sophisticated SEI alongside a Na+-storing “pseudo-SEI,” which contributes to exceptional long-cycle stability.
The optimal ratio of inorganic to organic components in the composite SEI is crucial. For instance, a coating of AlF3 on HC via a co-precipitation method was found to optimize the SEI composition to approximately 43% inorganic and 57% organic phases, balancing mechanical and electrochemical properties. Similarly, using trans-difluoroethylene carbonate (DFEC) as an additive can promote the concurrent and uniform consumption of inorganic and organic SEI precursors, leading to a homogeneous composite structure that stabilizes the HC interface effectively.
| SEI Strategy | Typical Method / Component | Primary Function / Mechanism | Reported Performance Gains | Advantages | Challenges |
|---|---|---|---|---|---|
| Inorganic Coating | ALD (Al2O3, TiO2); Solution coating (ZrO2, Al2O3); Electrolyte additives (MDFA, NaDFP) | Physical barrier, Defect passivation, Promotes inorganic-rich SEI formation. | Increased ICE (up to ~98.9%), Enhanced cycle life (e.g., >2000 cycles), Improved rate capability. | Effective surface shielding, Good stability. | ALD is expensive; Solution coating uniformity can be an issue; Additive optimization required. |
| Organic/ Molecular Engineering | Surface bonding (e.g., DP); Chemical pre-sodiation (e.g., Naph-Na); In-situ electropolymerization additives. | Pre-forms stable passivation layer, Provides Na+ reservoir, Creates polymer-rich SEI matrix. | High ICE (>96%), Stable long-term cycling, Good compatibility. | Can be simple and scalable (pre-sodiation), Targets SEI chemistry directly. | Pre-sodiation control is critical; Specificity to monomer/ molecule. |
| Composite / Electrolyte Engineering | Pre-formation with FEC; NaDFP additive; Glyme-based electrolytes (TEGDME); Balanced additive (DFEC). | Creates hybrid organic-inorganic SEI, Balances flexibility and strength, Enables stable interface dynamics. | Superior long-cycle stability (>1000 cycles), Wide temperature performance, High CE retention. | Holistic approach, Often leverages existing manufacturing, Tunable properties. | Requires careful formulation; Mechanism can be complex; Compatibility with full cell needed. |
Artificial SEI for Other Promising Anode Materials
While sodium metal and hard carbon are focal points, other anode materials for sodium-ion batteries present unique challenges that also benefit from artificial SEI design. Titanium-based oxides (e.g., TiO2) are attractive for their “zero-strain” characteristics but suffer from low ICE and poor rate capability due to interfacial resistance. Strategies like pre-potassiation to form a KF/organic composite SEI or surface modification with Al2O3 can significantly improve Na+ kinetics and cycling stability. Alloying anodes like tin (Sn) and antimony (Sb) offer high theoretical capacities but undergo enormous volume changes (>300%). Artificial SEI design here is crucial to maintain electrical contact and prevent pulverization. For Sn anodes, ether-based electrolytes (e.g., diglyme, THF) have proven highly effective in forming flexible, organic-dominated SEI layers that can accommodate volume swings, enabling stable cycling of micro-sized Sn particles. For Sb anodes, constructing carbon-based composite structures or using ball-milling to disperse NaF on the Sb surface can guide the formation of a stable, inorganic-rich SEI that protects the electrode during repeated (de)sodiation cycles.
Conclusions and Future Perspectives
The strategic design of artificial solid electrolyte interphases represents a pivotal avenue for unlocking the full potential of sodium-ion battery anodes. By moving beyond the limitations of spontaneously formed native SEI, researchers have developed sophisticated inorganic, organic, and hybrid interfacial layers that address the specific degradation mechanisms of different anode materials. For sodium metal, the focus is on suppressing dendrites and stabilizing plating/stripping, achieved through robust inorganic barriers, flexible organic layers, or optimally designed composites. For hard carbon, the goal shifts towards minimizing irreversible sodium loss and creating thin, uniform passivation layers via coatings, molecular engineering, and advanced electrolyte formulations.
Despite remarkable progress, several key challenges and opportunities lie ahead for the field:
- Process Optimization and Scalability: Bridging the gap between lab-scale precision and industrial-scale manufacturing is critical. While techniques like ALD offer exquisite control, their cost is prohibitive. Future work must refine scalable, cost-effective methods like advanced liquid-phase coating, spray deposition, or roll-to-roll processes that can produce uniform, thin artificial SEI layers with high reproducibility. Integrating in-line monitoring and feedback control could ensure consistent quality.
- Deepening Mechanistic Understanding: A more fundamental and predictive understanding of structure-property-performance relationships is needed. This requires the integration of advanced in-situ/operando characterization techniques (e.g., electrochemical atomic force microscopy, synchrotron X-ray microscopy, neutron depth profiling) with multi-scale computational modeling (density functional theory, molecular dynamics). Such studies should aim to elucidate:
- The precise nucleation and growth mechanisms of artificial and subsequent SEI layers under operating conditions.
- The Na+ transport dynamics and diffusion barriers within complex composite SEI structures, described by equations like: $$J = -D \frac{\partial c}{\partial x}$$ where $J$ is the flux, $D$ the diffusion coefficient, and $\partial c/\partial x$ the concentration gradient.
- The evolution of mechanical properties (stress, strain, modulus) during cycling and their correlation with electrochemical performance.
- Performance Under Practical Conditions: Research must increasingly evaluate artificial SEI strategies under conditions mirroring real-world applications. This includes testing in lean-electrolyte configurations, with limited sodium excess (or anode-free designs), under high current densities and fast-charging protocols, and across an extended temperature range (-40°C to 60°C). Understanding and mitigating failure modes like SEI dry-out, cracking under high stress, or chemical evolution at high voltages is essential.
- System-Level Compatibility and Integration: The artificial SEI must be compatible not only with the anode but also with the chosen cathode, electrolyte, and cell format (e.g., pouch cell). The interplay between the artificial SEI and electrolyte additives needs clearer guidelines to avoid antagonistic effects. Furthermore, the potential impact of the artificial SEI formation process on the overall energy density, cost, and environmental footprint of the sodium-ion battery must be assessed holistically.
In conclusion, the engineering of artificial SEI layers has matured from a conceptual approach to a diverse and powerful toolbox for stabilizing SIB anodes. The future of this field lies in the intelligent, targeted design of multifunctional interfaces based on deep mechanistic insights, coupled with the development of scalable and economical fabrication processes. By addressing these challenges, artificial SEI technology will play a central role in accelerating the commercialization of high-performance, safe, and durable sodium-ion batteries for large-scale energy storage.