The imperative to achieve “carbon peak and carbon neutrality” has become a defining global challenge. Developing clean and efficient electrochemical energy storage systems is paramount for facilitating the green and low-carbon transformation of our energy infrastructure, enabling higher integration and storage capacity for renewable sources. The transportation sector, a major and rapidly growing contributor to global carbon emissions, represents a critical front in this battle. The widespread adoption of new energy vehicles is a vital pillar supporting the “dual-carbon” goals. However, the current market penetration of electric vehicles (EVs) falls short of expectations. While advancements in lithium-ion battery (LIB) materials and integration technologies have alleviated “range anxiety,” the challenge of fast charging remains largely unresolved. Compared to refueling a conventional vehicle, charging an EV is time-consuming, and fast charging often raises serious safety concerns. To address “charging anxiety,” organizations like the US Advanced Battery Consortium (USABC) have set targets for extreme fast charging (XFC), aiming for 80% state of charge within 15 minutes. Current commercial power batteries still have a significant gap to bridge to meet these aggressive power density goals. Therefore, developing a new generation of high-energy, fast-charging storage technology is key to unlocking the mass-market potential of electric mobility.
The fast-charging capability of alkali metal-ion batteries is largely dictated by the anode side. Commercial graphite anodes in lithium-ion batteries exhibit a distinct low-potential plateau due to the formation of stable intercalation compounds with lithium, granting LIBs a wide voltage window and high energy density. However, this plateau potential (< 0.1 V vs. Li/Li⁺) is perilously close to the lithium metal plating potential (0 V). Coupled with inherently sluggish kinetics in this plateau region, charging at high rates (> 1C) or low temperatures (< -20°C) leads to severe lithium plating on the graphite surface. This causes rapid capacity fade and poses significant safety risks. In contrast, sodium-ion batteries (SIBs) are widely recognized for their superior fast-charging capability and low-temperature performance. Furthermore, the abundance and low cost of sodium resources make SIBs a leading candidate for the next generation of resource-unconstrained, efficient energy storage systems. Crucially, unlike lithium resources, China’s sodium resources are self-sufficient, making the development of SIBs a strategic imperative for ensuring supply chain security and price stability in the new energy sector.
The excellent fast-charging performance of sodium-ion batteries is primarily attributed to the use of disordered carbon materials as the anode. Disordered carbons, broadly categorized as soft carbon and hard carbon, possess highly disordered microstructures featuring larger interlayer spacing, abundant surface defects, turbostratically stacked graphite-like microcrystals, and nanopores formed at the junctions of these microcrystals, providing numerous reactive sites for sodium storage. The charge-discharge profile of sodium storage in disordered carbons typically consists of a high-voltage slope region (0.1–1.0 V) and a low-voltage plateau region (< 0.1 V). The slope capacity originates mainly from the pseudocapacitive adsorption/desorption of sodium ions on carbon layer defects, a process endowed with excellent kinetics. Unlike graphite in LIBs, the slope region in SIB disordered carbon anodes often constitutes a significant portion of the total capacity. For instance, soft carbon derived from pitch can deliver a capacity of ~200 mAh g⁻¹ dominated by a long slope, while hard carbon from glucose hydrothermal carbonization can have a slope contribution close to 50% of its capacity. This high slope contribution significantly lowers the risk of sodium plating and contributes to excellent rate performance. However, a high proportion of slope capacity leads to a severe reduction in the full-cell voltage when paired with a cathode, resulting in lower energy density—a major shortcoming hindering the large-scale application of sodium-ion batteries. The average energy density of near-commercial SIBs is currently around 120-140 Wh kg⁻¹, significantly below the demand for power batteries (>160 Wh kg⁻¹).
Extending the low-potential plateau (< 0.1 V) of disordered carbons is a direct route to significantly boost the energy density of sodium-ion batteries. Consequently, the formation and extension mechanisms of this low plateau have been a focal point of research. With the gradual clarification of the plateau storage mechanism—increasingly attributed to sodium cluster formation within closed pores—many strategies to elongate the plateau have emerged successfully. Plateau capacities in disordered carbons can now readily exceed 300 mAh g⁻¹, even reaching 400 mAh g⁻¹. While this effectively addresses the low energy density issue, the pursuit of a long plateau has inadvertently compromised the innate fast-charging and high-safety advantages typically associated with sodium-ion batteries. Comparative studies in cylindrical cell configurations have shown that hard carbons with high slope capacity and short plateaus enable excellent fast-charging performance and cycle life without sodium plating. In contrast, hard carbons with extended plateaus, while delivering higher energy density, exhibit poor fast-charging performance, rapid capacity decay at high rates, and evident sodium metal plating upon post-mortem analysis. This reveals a critical dilemma: high plateau capacity and high safety (manifested as a higher plateau potential further from 0 V) appear mutually exclusive in current disordered carbons. Therefore, breaking this impasse to achieve a harmonious balance between high energy, high power, and high safety in sodium-ion batteries represents the most significant challenge in carbon anode research.
This article focuses on the kinetic properties of the plateau region sodium storage in disordered carbons. We begin by deconstructing the reaction pathway of plateau sodium storage and discussing the influence of each elementary step on the overall kinetics. Subsequently, we review research progress in accelerating plateau kinetics from two key perspectives: optimizing the electrode-electrolyte interface and tuning the solid-phase structure of the carbon. Finally, we provide a brief outlook on future development directions and key challenges for disordered carbon anodes.
Deconstructing the Plateau Sodium Storage Pathway and Identifying Rate-Limiting Steps
Similar to LIBs, sodium-ion batteries are often described as “rocking-chair” batteries. By analogy with the reaction pathway in graphite, the kinetic process of plateau sodium storage in disordered carbon can be decoupled into four elementary steps, as illustrated below and summarized in Table 1.
| Elementary Step | Process Description | Key Influencing Factors | Typical Metrics/Values | Potential Optimization Strategies |
|---|---|---|---|---|
| 1. Bulk Liquid Diffusion | Solvated Na⁺ ions diffuse through tortuous liquid channels in the electrode towards the anode surface. | Electrolyte viscosity, ion conductivity, electrode porosity & tortuosity. | Ionic conductivity in PC: ~7.98 mS cm⁻¹ for 1M NaPF₆ (vs. ~5.80 for LiPF₆). | Optimize electrolyte salt concentration, solvent composition, electrode architecture. |
| 2. Interfacial Transport | Na⁺ desolvation and diffusion through the Solid Electrolyte Interphase (SEI) layer. | SEI composition, thickness, homogeneity, ionic conductivity; desolvation energy. | Activation energy: ~9-23 kJ mol⁻¹ (interface). RSEI ~ tens of Ω. | Electrolyte formulation, SEI engineering, surface functionalization, artificial SEI. |
| 3. Solid-State Diffusion | Na⁺ (or Na clusters) diffuse within the carbon bulk via layer gaps, pore channels, and along defects. | Interlayer spacing, microcrystal size, defect chemistry, pore connectivity. | Apparent DNa+ ~ 10⁻¹¹ to 10⁻¹⁵ cm² s⁻¹ (GITT). Energy barrier ~0.6-1.1 eV. | Expand d-spacing, introduce ordered diffusion channels, single-atom doping. |
| 4. Charge Transfer | Electron transfer to Na ions leading to cluster formation within nanopores. | Pore size/shape, carbon electronic structure, Na-C interaction energy. | Activation energy: ~20-44 kJ mol⁻¹ (charge transfer). Rct ~ tens of Ω. | Design “small and numerous” closed pores, tailor defect concentration. |
- Bulk Liquid Diffusion: Solvated sodium ions, surrounded by solvent molecules, diffuse through the electrolyte-filled pores of the electrode towards the anode surface. Sodium ions generally exhibit faster diffusion rates in the liquid phase compared to lithium ions due to their weaker Lewis acidity, leading to a lower desolvation energy (15-20% lower) and a smaller Stokes diameter of the solvated ion.
- Interfacial Transport: Upon reaching the electrode surface, sodium ions must undergo desolvation. The stripped solvent molecules and salt anions participate in the formation of the solid electrolyte interphase (SEI) during the initial cycles. The naked Na⁺ ion then migrates through this SEI layer. The SEI’s composition, thickness, and structure critically determine the ionic conductivity and stability of this interface.
- Solid-State Diffusion: After entering the carbon bulk, sodium species diffuse towards the final storage sites. This occurs through pathways such as interlayer gaps, along microcrystalline edges, and through internal nanopores.
- Charge Transfer: This step involves the reduction of sodium ions and the eventual formation of sodium clusters within nanopores, a process accompanied by electron transfer from the carbon matrix.
Identifying the rate-limiting step (RLS) is crucial for enhancing plateau kinetics, yet it remains a subject of debate and is highly system-dependent. Electrochemical impedance spectroscopy (EIS) often reveals two semicircles, corresponding to interfacial resistance (RSEI, high frequency) and charge transfer resistance (Rct, medium frequency). In the plateau region, these resistances can be comparable (e.g., both around 60 Ω), suggesting neither step is overwhelmingly dominant. Activation energy analysis via the Arrhenius equation sometimes indicates a higher value for charge transfer (e.g., ~44 kJ mol⁻¹) compared to interfacial transport (~23 kJ mol⁻¹) in ester electrolytes. However, a higher activation energy does not automatically designate a step as rate-limiting, as the pre-exponential factor also determines the rate constant. The apparent solid-state diffusion coefficient (DNa+) calculated by the galvanostatic intermittent titration technique (GITT) typically ranges from 10⁻¹¹ to 10⁻¹⁵ cm² s⁻¹. While this is not exceptionally fast, it is comparable to values in some cathode materials like NASICON-type Na₃V₂(PO₄)₃. It’s important to note that GITT-derived values are apparent, based on simplified one-dimensional Fickian diffusion models that may not capture the complex three-dimensional diffusion and phase transition occurring in disordered carbons.
In essence, the rates of these four steps are often within the same order of magnitude, and the RLS can shift depending on the specific conditions (current rate, temperature, electrode thickness, carbon microstructure, electrolyte). For instance, charge transfer with its higher activation energy may become limiting at low temperatures, while solid-state diffusion could dominate in thick electrodes. The fundamental reason poor fast-charging performance accompanies high-plateau-capacity carbons is thermodynamic: strategies that extend the plateau often significantly lower the plateau operating potential, bringing it closer to the sodium plating potential (0 V). This leaves minimal “polarization headroom” during fast charging, drastically increasing plating risk. Therefore, to realize high-energy, fast-charging sodium-ion batteries, strategies must aim to both accelerate kinetics (reducing polarization) and raise the plateau potential thermodynamically.
Optimizing the Carbon-Electrolyte Interface for Rapid Ion Transport
A stable electrode-electrolyte interface is the cornerstone of reversible operation for “rocking-chair” batteries. The success of LIBs is inseparable from the discovery of ethylene carbonate (EC) as a solvent, which forms a stable SEI on graphite. Early cells using propylene carbonate (PC) failed due to solvent co-intercalation and graphite exfoliation. The interfacial chemistry in sodium-ion batteries shares similar principles. The SEI formation involves multiple steps and phase changes, and slow ion transport across this interface can severely limit fast-charging performance. Constructing an ideal SEI with high ionic conductivity and stability is thus a major research focus.
The SEI’s composition is complex, deriving from the reductive decomposition of electrolyte components (solvent, salt, additives) at potentials above the sodium storage plateau. Its characteristics—composition, morphology, thickness, and homogeneity—profoundly impact interfacial ion transport kinetics. Comparative studies between ether-based and ester-based electrolytes reveal that ethers tend to form thinner, denser, and more homogeneous SEI layers. The SEI in ethers often features a thin organic outer layer that passivates the surface, with uniformly distributed inorganic components (like NaF) underneath, facilitating rapid Na⁺ transport. In contrast, ester-based electrolytes often yield thicker, more heterogeneous SEI with longer and more tortuous ion diffusion paths. An ideal SEI for fast kinetics should possess: (1) minimal thickness to shorten diffusion length; (2) a dense and homogeneous structure with continuous ion channels; and (3) a high content of inorganic components (e.g., NaF, Na₂O) which typically have lower ionic migration barriers.
Current interface optimization strategies can be broadly classified into three directions:
1. Optimizing Sodium Ion Solvation Structure: The solvation sheath structure of Na⁺ in the electrolyte influences the desolvation energy barrier at the interface. Weakly coordinated solvation structures generally lower this barrier. Furthermore, the solvents shed during desolvation participate in SEI formation. Designing electrolytes with weak-binding solvents (e.g., tetrahydrofuran – THF) or additives that modify the solvation shell (e.g., increasing PF₆⁻ coordination) can promote the formation of inorganic-rich, high-quality SEI, thereby accelerating interfacial transport.
2. Surface Chemical Modification of Disordered Carbon: Electrolyte reduction reactions are often catalyzed by specific sites on the carbon surface. Modifying the surface chemistry can therefore dictate SEI formation. For example, grafting carboxyl-rich molecules onto the carbon surface can create preferential sites that catalyze the reduction of salt anions, leading to a dense, uniform, and inorganic-rich SEI that lowers interfacial impedance.
3. Constructing Artificial SEI Layers: Coating the carbon surface with an artificial layer (e.g., Al₂O₃ via atomic layer deposition, or a 3Å molecular sieve film) can act as a physical barrier to suppress parasitic electrolyte reduction. More ingeniously, some artificial layers can enable “stepwise desolvation.” For instance, a 3Å molecular sieve film can partially desolvate Na⁺ before they reach the carbon surface, significantly reducing the desolvation energy barrier and fostering the formation of a superior native SEI, all contributing to ultrafast interfacial kinetics.
Tuning Disordered Carbon Microstructure to Enhance Plateau Storage Kinetics
The highly complex and diverse microstructure of disordered carbons—comprising curved graphene-like sheets, turbostratic microcrystals, defects, and a hierarchical pore system—directly influences the solid-state diffusion and charge transfer processes during plateau storage. Understanding the structure-property relationships is key to designing carbons with fast kinetics and higher plateau potentials.
Solid-State Diffusion Process
Parameters like interlayer spacing (d002), microcrystal size (Lc, La), pore architecture (especially mesopores as ionic highways), and defect chemistry (e.g., heteroatom doping) can significantly affect Na⁺ diffusion rates within the carbon bulk. Experimental studies and density functional theory (DFT) calculations suggest that:
- An optimal interlayer spacing (e.g., > 0.39 nm) coupled with moderate microcrystal size can enhance the apparent diffusion coefficient (DNa+).
- Mesopores can serve as rapid transport channels, increasing their volume fraction benefits bulk diffusion.
- Single-atom doping (e.g., Zn) can create a local electric field within the carbon matrix, effectively lowering the Na⁺ diffusion energy barrier from ~1.10 eV to ~0.60 eV.
- The “diffuse region” at the interface between graphitic microcrystals and amorphous domains is critical; sodium accumulates here before pore filling, and its structure dictates diffusion rates.
A generalized expression for the solid-state diffusion flux (Jss) considering these factors can be conceptualized as:
$$J_{ss} = -D_{eff} \nabla C \approx – (f(d_{002}, L_c, \Phi_{meso}, E_{a,defect}) ) \nabla C$$
where \(D_{eff}\) is the effective diffusion coefficient, a function of interlayer spacing \(d_{002}\), microcrystal size \(L_c\), mesopore volume fraction \(\Phi_{meso}\), and the activation energy \(E_{a,defect}\) influenced by defect engineering. \(\nabla C\) is the concentration gradient.
Despite these insights, a comprehensive design principle is lacking due to the difficulty in decoupling and precisely controlling these structural features. Furthermore, the fundamental diffusion mechanism—whether Na⁺ hops between adjacent sites, flows through interlayer galleries, or uses pores as the main channel, and whether it diffuses as single ions or correlated dipoles—remains unclear. Advanced in situ characterization and high-throughput computational modeling (molecular dynamics, phase-field simulations) on realistic carbon models are urgently needed to elucidate these mechanisms and establish clear structural guidelines.
Charge Transfer Process and Plateau Potential
The “pore-filling” mechanism, where sodium ions are reduced to form quasi-metallic clusters within closed nanopores, is now widely accepted for the plateau region. The structure of these closed pores—their size distribution, shape, and connectivity—is the primary factor governing this process. It affects the physicochemical nature of the sodium clusters, the thermodynamics of cluster formation, and the kinetics of charge transfer.
Key Finding 1: Pore Size Dictates Cluster Metallicity and Plateau Potential. In situ solid-state NMR and ex-situ studies consistently show that the average pore size is inversely correlated with the plateau potential. Smaller pores lead to sodium clusters with less metallic character (evidenced by NMR chemical shift), which are stabilized at a higher potential relative to Na/Na⁺. Larger pores foster more metallic, bulk-like sodium clusters that form at potentials very close to 0 V, increasing plating risk. This can be linked to an “underpotential deposition” (UPD)-like phenomenon, where the carbon pore wall acts as a substrate that wets the sodium cluster. The interfacial energy (\(\gamma_{\text{interface}}\)) between the sodium cluster and the carbon wall determines the stability, and thus the potential, of the cluster. Smaller pores increase the Na-C contact area, enhancing wetting and reducing the effective interfacial energy, thereby raising the equilibrium potential. The change in free energy for cluster formation in a pore can be related to the interfacial energy:
$$\Delta G_{\text{cluster}} \propto \gamma_{\text{interface}} A_{\text{interface}}$$
where \(A_{\text{interface}}\) is the Na-C contact area. Minimizing \(\gamma_{\text{interface}} A_{\text{interface}}\) by designing small, well-wetted pores raises the plateau potential.
Key Finding 2: Defects and Carbon Layer Structure Also Influence the Plateau. Beyond pore size, the carbon matrix itself plays a role. DFT calculations on model systems (e.g., single-layer graphene with defects) indicate that increasing defect concentration (topological defects, vacancies) strengthens the Na-C interaction relative to the Na-Na interaction. This may reduce the average charge transferred to each sodium atom, making the cluster less metallic and potentially raising the plateau potential. Furthermore, introducing ordered carbon layers (e.g., via chemical vapor deposition) within the pores of a host carbon has been shown to effectively elevate the plateau working potential.
The Design Principle: To achieve high-capacity, safe, and fast plateau storage, the goal is to construct a microstructure with “small but numerous” closed pores. This maximizes the number of cluster storage sites (high capacity) while ensuring each cluster forms at a higher, safer potential (low plating risk). Concurrently, the small pore size and tailored carbon wall chemistry should facilitate rapid charge transfer kinetics. The target plateau capacity (\(Q_{\text{plateau}}\)) can be expressed as a function of the total volume of suitably sized closed pores:
$$Q_{\text{plateau}} \propto \int_{0}^{r_{crit}} V_{pore}(r) \, dr \times \rho_{\text{Na,cluster}}$$
where \(V_{pore}(r)\) is the pore volume distribution, \(r_{crit}\) is the critical pore radius for safe operation (~1.0 nm as suggested by some studies), and \(\rho_{\text{Na,cluster}}\) is the effective density of sodium in the cluster state.
However, traditional synthesis methods (high-temperature carbonization of oxygen-rich precursors) or common pore-engineering strategies (template methods, CO₂ etching, pore-forming agents) often struggle to create this ideal “small and numerous” pore structure. They tend to either produce few small micropores or create larger mesopores (>2 nm) during the process of increasing pore volume. Novel, precise pore-engineering strategies are therefore a critical need for the future of sodium-ion battery anodes.
Summary and Future Perspectives
Sodium-ion batteries hold immense promise as a sustainable and cost-effective energy storage solution. However, the current trade-off between energy density and the innate fast-charging/safety advantages presents a major bottleneck. The root causes lie in the difficulty of achieving both a high plateau capacity and a high (safe) plateau potential in disordered carbon anodes, compounded by sluggish plateau storage kinetics. As reviewed, progress is being made by engineering ideal SEI for fast interfacial transport, tailoring carbon microstructure for rapid solid-state diffusion, and designing “small and numerous” closed pores to optimize the thermodynamics and kinetics of sodium cluster formation.
To systematically overcome these challenges and realize the full potential of high-energy, fast-charging sodium-ion batteries, future efforts should focus on several key areas:
1. Development of Advanced Electrochemical and Characterization Methodologies. The widespread adoption of three-electrode full-cell configurations is essential to accurately deconvolute and assess the intrinsic kinetic behavior of the carbon anode without interference from the cathode. Advanced operando and in situ techniques—such as in situ solid-state NMR, Raman spectroscopy, and small-angle X-ray scattering (SAXS)—are crucial for directly probing the dynamic evolution of sodium species, cluster formation, and pore filling mechanisms in real time. Furthermore, new electrochemical methods to precisely quantify the polarization contribution and activation energy of each elementary step under various operating conditions (rate, temperature) are needed to unambiguously identify the rate-limiting step in specific systems.
2. Pursuit of Precise, Customized Microstructural Control. The complex and intertwined nature of microcrystalline structure, pore architecture, and surface chemistry in disordered carbons makes it difficult to establish definitive structure-property relationships using traditionally synthesized materials. There is an urgent need for novel synthesis strategies that can decouple and precisely control these structural elements independently, creating model materials for fundamental study. Concurrently, advanced characterization techniques specifically for closed pores (beyond gas sorption, e.g., advanced SAXS fitting models) and for quantifying defect types/concentrations within the bulk are required to fully characterize these tailored structures.
3. Integration of High-Throughput Computation and Machine Learning. Computational approaches are indispensable for bridging the gap between atomic-scale structure and macroscopic performance. High-throughput density functional theory (DFT) calculations can screen for optimal defect types, pore geometries, and heteroatom dopants that maximize Na-C interaction and raise the plateau potential. Molecular dynamics (MD) and phase-field simulations on increasingly realistic carbon models can elucidate the complex solid-state diffusion mechanisms of sodium. Finally, machine learning models trained on large datasets of synthetic conditions, characterized structures, and electrochemical performance can accelerate the discovery of novel carbon anodes and establish predictive design principles for the next generation of sodium-ion battery materials.
By addressing these interconnected challenges—through a combination of precise material synthesis, advanced multi-scale characterization, and powerful computational guidance—the development of disordered carbon anodes that truly harmonize high energy density, exceptional fast-charging capability, and inherent safety can be accelerated, paving the way for the broad commercialization of sodium-ion batteries.