Sodium plating on hard carbon anodes in sodium-ion batteries: mechanisms, detection methods, and mitigation strategies

Abstract

Due to sodium's abundance and cost advantages, sodium-ion batteries (SIBs) are promising alternatives to lithium-ion batteries. The commercial adoption of hard carbon (HC) as an anode material—attributed to its low sodiation potential, high Na⁺ storage capacity, and extensive availability—further reinforces the potential of SIBs. Nevertheless, the inherent thermodynamic instability of HC anodes predisposes them to irreversible Na plating during operation. This phenomenon not only poses considerable safety hazards due to dendrite-induced short circuits but also accelerates capacity degradation, thereby undermining the feasibility of large-scale SIB deployment. This review comprehensively delineates the mechanisms underlying Na plating on HC anodes by examining internal factors—such as the electrode structure, the N/P ratio, and the electrolyte composition—and external factors including the state of charge, low temperature, and fast charging conditions. It further details various detection methods, encompassing both electrochemical techniques and physical characterization techniques, and outlines mitigation strategies such as electrode structure design, surface engineering, and electrolyte regulation to suppress plating. By synthesizing current understanding, the review posits future directions for developing safer, high-performance SIB anodes. Addressing Na plating is thus critical for advancing SIB technology toward large-scale applications.

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1. Introduction

The pressing challenges associated with traditional fossil fuels—characterized by high carbon emissions and limited resource reserves—have catalyzed the transition toward renewable energy and the electrification of transportation.^1,2 Lithium-ion batteries (LIBs) have long served as the cornerstone of energy storage for renewable energy systems, owing to their high energy density, broad potential window, excellent cycle stability, and well-established market presence.^3,4 However, the high manufacturing costs and geographically uneven distribution of lithium reserves hinder further advancement of LIB technology (Fig. 1b).^5,6 In contrast, sodium-ion batteries (SIBs) have emerged as attractive alternatives due to the significantly greater natural abundance of sodium compared with lithium (Fig. 1a).⁷

Fig. 1 Overview of the LIBs and SIBs. (a) Elemental abundance in the curst of earth. Reproduced with permission from ref. 7. Copyright 2019, Elsevier. (b) The global distribution of Li resources. Reproduced with permission from ref. 5. Copyright 2017, Elsevier. (c) The compatibility of different carbon-based materials with SIBs. Reproduced with permission from ref. 24. Copyright 2021, Wiley-VCH. (d) TR induced by internal short circuits. Reproduced with permission from ref. 35. Copyright 2018, Elsevier.

In addition to exhibiting comparable electrochemical intercalation/deintercalation mechanisms to LIBs,⁸ SIBs offer further advantages. Notably, sodium avoids the electrochemical alloying reaction with aluminum, permitting the use of cost-effective aluminum foil as the negative current collector instead of copper.⁹ This substitution mitigates current collector oxidation caused by overdischarge,¹⁰ enhances battery safety, and reduces manufacturing costs.^11,12 Moreover, SIBs demonstrate superior cycling performance under both high-rate and cryogenic conditions.^13,14 Despite these merits, the development of SIBs faces significant challenges, particularly regarding the selection of anode materials. Although a wide range of candidates—including metal oxides, organic compounds, conversion-reaction materials, and carbon-based materials—have been explored, carbonaceous electrodes have attracted considerable interest because of their broad availability.^15–17 Conventional graphite, however, is limited by its inability to intercalate Na⁺ effectively due to sodium's larger ionic radius, weaker binding energy, and higher redox potential relative to lithium.^18,19 Consequently, research efforts have increasingly focused on soft carbon and hard carbon (HC). While soft carbon offers a high specific capacity, it operates at a consequentially elevated voltage.²⁰ In contrast, HC features a lower sodiation potential, higher Na⁺ storage capacity, larger interlamellar spacing, and a greater number of active sites, making it the commercial anode material of choice in SIBs (Fig. 1c).^21–24 Nonetheless, intrinsic heterogeneities in the distributions of active sites and pores—arising during material synthesis and electrode fabrication—lead to nonuniform Na⁺ distribution during cycling. Repetitive intercalation/deintercalation further exacerbates these imbalances by inducing expansion and collapse of the HC structure.²⁵ These intrinsic factors collectively contribute to the formation of concentration gradients and result in irreversible Na plating, which can, in turn, promote dendritic growth and raise severe safety concerns.^26–28

Safety hazards associated with battery systems extend beyond capacity degradation. In high-energy LIBs, thermal runaway (TR) events are often preceded by intense fires and explosions due to the emission of large quantities of flammable gases.²⁹ Research by Wang et al.³⁰ has demonstrated that gas evolution and exothermic reactions associated with lithium dendrites significantly compromise battery safety by lowering the temperature thresholds for both the initial reactions and the TR onset. Furthermore, an elevated energy density exacerbates the risk and severity of TR by fostering dendrite formation.³¹ Similarly, in SIBs, the aggregation of Na deposits can lead to dendritic growth that eventually punctures the separator, thereby establishing an internal short circuit.^32–34 This internal short circuit, in conjunction with ensuing parasitic electrolyte reactions, can trigger a rapid increase in battery temperature, ultimately resulting in catastrophic fires and explosions (Fig. 1d).^35–37 Given the global incidence of such accidents, battery safety has become a paramount public concern and a prerequisite for the widespread application of energy storage systems.^38,39 Despite these challenges, current research addressing Na plating remains notably insufficient.

This review aims to comprehensively delineate the underlying mechanisms associated with Na plating on hard carbon anodes by considering both internal factors—such as the electrode structure, the N/P ratio, and the electrolyte composition—and external factors including low temperature, fast charging, and state of charge conditions. It places particular emphasis on detection methods, spanning from advanced electrochemical techniques to detailed physical characterization techniques, and outlines mitigation strategies, including electrode structure design, surface engineering, and electrolyte regulation. In conclusion, by identifying existing research gaps and proposing future research directions, we anticipate that an in-depth understanding of Na plating mechanisms under various operational conditions will substantially mitigate latent safety concerns during battery cycling.

2. Na plating mechanisms

During the charging process, Na⁺ is released from the cathode, traverses through the electrolyte, and intercalates into the HC anode. Concurrently, electrons are conveyed via the external circuit in the same direction.^40,41 In line with Fick's law, Na⁺ and electrons tend to form a concentration gradient resulting from the difference in their diffusion coefficients during the process,^42,43 which can be fundamentally ascribed to the kinetic cause that leads to Na plating to a specific extent. In turn, in light of the Nernst equation, electrode polarization will be generated by the concentration gradient.⁴⁴ The electrode polarization refers to the phenomenon that the electrode potential deviates from its equilibrium potential during the electrochemical process, and it will occur at every stage: concentration polarization arises from the dual electrodes and the electrolyte, electrochemical polarization originates from the solid electrolyte interface (SEI), and ohmic polarization stems from the charge-transfer reaction. Consequently, electrode polarization tends to exacerbate the negativity of anodes. When the local potential at the anode drops below 0 V (vs. Na⁺/Na), the Na plating will form.⁴⁵ This mechanism is associated with the thermodynamic cause underlying Na plating and determines whether Na plating can occur. In brief, Na plating could be ascribed to the kinetic and thermodynamic causes. The slowest kinetic stage results in the maximal electrode polarization, which will become the rate-determining step to dominate the Na plating process and thereby affect the entire electrochemical performance. It is noteworthy that the internal and external factors governing Na plating are the specific manifestations of the kinetic and thermodynamic causes, which exert a decisive influence on the Na plating. Thus, a detailed analysis from these two aspects will be subsequently delineated.

2.1. Internal factors affecting Na plating

2.1.1. Electrode structure. From the perspective of kinetics, the diffusion coefficient is intrinsically correlated with the material properties, wherein a larger diffusion coefficient facilitates more rapid Na⁺ diffusion, to diminish the concentration gradient. In terms of thermodynamics, this can effectively suppress the Na plating. Therefore, the structure of HC exerts a direct impact on Na plating. In general, the structure of HC consists of amorphous carbon and graphite-like microcrystals, with abundant defects and pores (open pores and closed pores).⁴⁶ Primarily, the open pores with larger diffusion coefficients are advantageous for faster Na⁺ diffusion kinetics compared with the closed pores, which exhibit excellent inhibitory effects on the Na plating. Nevertheless, the reversible capacity associated with closed-pore storage systems surpasses that of their open-pore counterparts, primarily because of the better accessibility of open pores in HC resulting from their interconnected channels and high surface area, which promotes parasitic side reactions (e.g., SEI formation) at the electrode/electrolyte interface. Moreover, the interfacial reactions reduce the reversibility of plated Na, which significantly compromises the reversible Na⁺ storage capacity.^47–49 Therefore, the equilibrium between open pores and closed pores must be meticulously evaluated. Likewise, exposing more edges and defects in HC is conducive to the faster Na⁺ diffusion kinetics as well, thereby effectively suppressing the Na plating, but exhibiting a reduction in reversible capacity. Simultaneously, the unevenly distributed pores, edges and defects contribute to the local concentration gradient and further result in the electrode polarization, culminating in the Na plating. Furthermore, the impact of tortuosity and interlayer spacing of HC on the Na⁺ diffusion process cannot be ignored, because the large tortuosity and small interlayer spacing increase the diffusion distance and hence the resistance for Na⁺ transport to facilitate the Na plating. It was revealed that the hierarchical micro-mesoporous HC was prepared by Kong et al.⁵⁰ through a self-activation approach using lignin as a precursor. In the endeavor to attain high surface capacity in HC, the adoption of thick HC electrodes has been recognized as an effective approach. But the utilization of thick electrodes significantly prolongs the Na⁺ diffusion path, necessitating a greater consumption of energy, which is detrimental to the Na⁺ diffusion kinetics as well as electrode mechanical stability and consequently results in the development of a concentration gradient across the thickness of the HC anode, culminating in the Na plating. Zeng et al.⁵¹ put forward the question and successfully resolved it by employing a sophisticated chemical cross-linking approach. Undoubtedly, the size and morphology of HC particles also have a certain influence on the Na⁺ kinetics. Cao et al.⁵² investigated the relationship between the size of HC particles and the rate performance by using typical grinding and ball milling methods and revealed that the rate performance increases as the size decreases. In other words, a smaller particle size is more favorable to the rapid Na⁺ diffusion kinetics, thereby effectively mitigating the Na plating. Beda et al.⁵³ researched the effect of tannin-derived spherical HC on electrochemical performance and confirmed that the conversion from interconnected particles to individual particles caused by the formation of spherical particles is beneficial for the Na⁺ diffusion process and demonstrates remarkable inhibitory efficacy against Na plating. Notably, these structures could be determined by a certain precursor and carbonization temperature, of which the carbonization temperature has salient effects on the interlayer spacing, defects, the pore size and orderliness of the HC.^54,55 Generally, the Na⁺ storage mechanisms in the HC are governed by three key processes: (i) Na⁺ adsorption at the surfaces and defects (e.g., vacancies, heteroatoms, and edges), (ii) Na⁺ intercalation and Na⁺-solvent co-intercalation between graphite-like layers, and (iii) Na⁺ filling in the nanopores (Fig. 2a).⁵⁶ These specific microstructures of the HC determines which mechanism dominates, thereby influencing the resultant sodiation behaviour and subsequent Na plating.⁵⁷ Furthermore, the charge redistribution that takes place between intercalated Na⁺ and graphene layers during the electrochemical cycling process is of paramount importance in determining Na plating. This phenomenon was reported by Xia et al.,⁵⁶ who demonstrated that substantial charge transfer between intercalated Na⁺ and defects in the graphene is induced through progressive Na⁺ intercalation, leading to a consequential attenuation of their mutual interaction. Such a diminished interaction permits Na⁺ to sustain an elevated valence electron density, thereby fostering the nucleation of quasi-metallic Na clusters. The persistent aggregation and subsequent growth of these quasi-metallic Na clusters ultimately result in the formation of metallic Na.

Fig. 2 Effect of the HC structure and the N/P ratio on Na plating. (a) Schematic of the Na⁺ storage mechanism in HC. Reproduced with permission from ref. 56. Copyright 2025, Wiley-VCH. (b) Electrochemical performance and growth behaviour of Na dendrite in the diglyme (G2) electrolyte at different N/P ratios. Reproduced with permission from ref. 64. Copyright 2024, Wiley-VCH.

It is imperative to develop economically viable and industrially applicable approaches for enhancing the properties of HC. Among these, there is a trade-off between the Na⁺ diffusion kinetics and the reversible capacity. Given that numerous pores facilitate the Na⁺ diffusion kinetics and further contribute to mitigating the Na plating, their higher specific surface area can result in adverse reactions between the active material and the electrolyte.⁵⁸

2.1.2. N/P ratio. To inhibit the Na plating, the negative/positive areal capacity ratio (N/P ratio) stands as a pivotal regulatory parameter for designing a full battery.^59,60 Generally, an N/P ratio greater than 1 is required to effectively suppress Na plating, which ensures that active vacancies remain to store Na⁺ within the anode.^61–63 The resulting mismatch in active vacancies between the electrodes causes Na⁺ to plate on the anode surface. Liu et al.⁶⁴ discovered that an excessively low N/P ratio can result in electrode polarization and dendrite formation for HC, whereas a disproportionately high N/P ratio may lead to a diminished reversible sodium content (Fig. 2b). In line with the findings on SIBs, the research conducted by Kim et al.⁶⁵ revealed that Li plating can be effectively suppressed in LIBs when the N/P ratio surpasses 1.1, operating at a temperature of 25 °C and a charging rate of 0.85C. Therefore, the optimal N/P ratio is of paramount importance in the mitigation of adverse Na plating. It is claimed that the N/P ratio is a dynamic parameter that fluctuates throughout the charge–discharge cycles and its variations are markedly influenced by the cycling conditions as well as the differential degradation rates of the cathode and the anode.^66,67 Liu et al.⁶⁸ further corroborated this finding through their investigation of aging behaviour in SIBs during low-temperature cycling. Their study revealed that at low temperature, a marked decline in the capacity of HC anodes occurred, which, in turn, caused a reduced N/P ratio. Such an imbalance was found to initiate overcharging processes, culminating in the Na plating.

At present, there is a lack of research on the effect of the N/P ratio on the energy density of SIBs with pure HC as the anode, conclusions of which can be drawn from the study of LIBs. Teng et al.⁶⁹ showed that a low N/P ratio exerts a favourable influence on both the conceptualization and application of high energy density batteries. In brief, there is a trade-off between the energy density controlled by the N/P ratio and Na plating.

2.1.3. Electrolyte composition. The electrolyte consists primarily of a mixture of salts, solvents and additives,⁷⁰ which collectively governs the Na⁺ diffusion both within the electrolyte and at the interface, as well as the interfacial stability. Significantly, sluggish diffusion kinetics and inadequate interfacial stability are liable to cause the concentration gradient, which may induce the electrode polarization and consequently trigger Na plating. Therefore, the selection of the salts, solvents and additives will be discussed in the following sections.
2.1.3.1. Salt. During initial sodiation, a hybrid SEI layer forms on HC, comprising (i) flexible organic fragments (from solvent reduction) with low ionic conductivity, predominantly near the electrolyte interface, and (ii) rigid inorganic nanoparticles (from salt decomposition) with high ionic conductivity, concentrated near the electrode surface.^71–74 Generally, carbonate electrolytes tend to form an uneven, thick, and brittle SEI layer rich in inorganic nanoparticles (e.g., Na₂CO₃, NaF, and Na₂O), while ether-based electrolytes favour the formation of a thin, robust and multi-layered ordered organic–inorganic hybrid SEI layer (e.g., CH₃Ona, CH₃CH₂Ona, and NaF).^75–78 Moreover, under extreme temperature conditions, the SEI layer tends to develop a more uneven, thicker, and less compact structure under both low and high temperature conditions.^79,80 As reported by Cui et al.,⁸¹ at low temperature, a 0.5 M NaPF₆/G2 ether-based electrolyte tends to form a thick and uneven SEI layer on the HC anodes. Furthermore, Wan et al.⁸² revealed that a thick, uneven, and thermally unstable (highly soluble) SEI is inclined to develop on the HC anodes at high temperature by using carbonate (PC)-based electrolytes. During the electrochemical cycling process, high current density conditions typically result in the formation of non-uniform, loosely structured, porous, and thick SEI layers, whereas low current density conditions generally lead to the development of uniform, densely packed, and thin SEI layers.⁷⁴ Additionally, the thickness and composition of the SEI layer are also fundamentally influenced by anions from the salt. Li et al.⁸³ revealed that NaCF₃SO₃ (NaOTf) facilitates the formation of a thinner SEI layer on high-surface-area carbon (HSAC) during the initial cycle compared with NaPF₆ and exhibits superior efficacy in inhibiting Na plating (Fig. 3a). In terms of kinetics, the thinner SEI layer has the larger diffusion coefficient due to the shorter paths for the transfer of Na⁺, thereby diminishing the Na⁺ concentration gradient. From the thermodynamic perspective, this can reduce the occurrence of Na plating. However, due to the difference in ionic conductivity and mechanical properties among different compositions in SEI layers, it is equally crucial to precisely quantify their chemical composition, instead of merely focusing on their thickness. Eshetu et al.⁸⁴ discovered that NaPF₆ is conducive to the formation of the thinnest SEI layer but contains the highest proportion of organic components among the five distinct salts, namely, NaPF₆, sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium bis(fluorosulfonyl)imide (NaFSI), NaClO₄ and sodium fluorosulfonyl-(trifluoromethanesulfonyl)imide (NaFTFSI), which will decrease its diffusion coefficient and induce concentration gradients, ultimately resulting in the occurrence of Na plating (Fig. 3b). Additionally, the lattice energy of salt constitutes a crucial parameter that not only reflects the dissociation capacity of salt but also modulates the diffusion ability of Na⁺ by altering the content of contact-ion pairs (CIPs) within the solvation structure.^85,86 Meanwhile, the lower lattice energy facilitates the dissolution of salt in the solvent, consequently increasing the content of CIPs within the solvation shell during the solvation. The increase in CIPs is conducive to the formation of an inorganic SEI layer rich in anions, which enhances Na⁺ diffusion kinetics and effectively inhibits Na plating.⁸⁷ Generally, a lower viscosity of salt solution results in a larger diffusion coefficient, but it cannot inherently yield high ionic conductivity. It was demonstrated by Ekeren et al.⁸⁸ that NaBF₄ possessed the minimum viscosity among the five different salts composed of NaBF₄, NaClO₄, sodium difluoro(oxalate)borate (NaDFOB), NaFSI and NaPF₆ in the triethyl phosphate (TEP) solvent, yet it failed to exhibit the highest ionic conductivity (Fig. 3c and d). In other words, the assessment of ionic conductivity should not be conducted by focusing on merely one factor. Following the aforementioned analysis, it is imperative to undertake a holistic evaluation for the choice of the salt, taking into account factors such as the thickness and composition of the SEI layer as well as the lattice energy and viscosity of the dissolved salt.

Fig. 3 The correlation between salt species and Na plating. (a) Comparison of the impact of NaPF6 and NaOTf on the HSAC anode. Reproduced with permission from ref. 83. Copyright 2021, American Chemical Society. (b) Comparative analysis of the impact of various salts on the thickness and organic/inorganic composition of the SEI layer. Reproduced with permission from ref. 84. Copyright 2019, Elsevier. (c) Comparison of ionic conductivity and (d) viscosity among five salts in TEP solvent. Reproduced with permission from ref. 88. Copyright 2024, Wiley-VCH.

2.1.3.2. Solvent. The solvent component comprises the predominant fraction of the electrolyte, making it also an indispensable factor influencing Na plating. Conventional carbonate solvents like ethylene carbonate (EC) exhibit exceptionally high dielectric constants and exceptional solvation capabilities, effectively weakening interactions among solute molecules or ions and thereby enhancing the dissolution rates of Na salts. Given that the inorganic SEI enriched with anions facilitates the Na⁺ diffusion, this phenomenon establishes a theoretical foundation for the selection of appropriate salts capable of forming such an SEI on the anode during subsequent processes, thereby effectively inhibiting Na plating (Fig. 4a).⁸⁹ However, because of its high melting point (36.4 °C), it maintains the solid state at ambient temperature, which limits its utility as a standalone solvent. Moreover, its considerable viscosity poses an obstacle to the migration of Na⁺, necessitating blending with solvents of lower viscosity to optimize ionic conductivity like diethyl carbonate (DEC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC). It is worth noting that the decrease in temperature drastically increases the viscosity of the electrolyte, which severely hampers ion diffusion kinetics and promotes the formation of concentration gradients, thereby resulting in Na plating. Additionally, when the temperature falls below the melting point of solvent, localized solidification of the electrolyte occurs, resulting in severe heterogeneous ion diffusion. Interestingly, the effect of low temperature on the dielectric constant is negligible. Therefore, the solvent selection for low-temperature electrolytes prioritizes a low melting point and a low viscosity, aiming to ensure the optimal electrolyte fluidity and enhance the ionic mobility within the electrolyte at low temperature and thereby effectively mitigating Na plating.⁹⁰ In contrast to other carbonate electrolyte solvents, propylene carbonate (PC) exhibits a lower melting point of −48.8 °C and a higher dielectric constant, which facilitates its utilization under low temperature conditions. However, its higher viscosity and the suboptimal compatibility with the electrode lead to elevated SEI resistance.⁹¹ Also, it can be embedded within the electrode framework, resulting in structure collapse of the electrodes. This will generate the concentration gradient, culminating in the Na plating. Alternatively, the integration of supplementary cosolvents can efficiently tackle the challenge. EC as a solvent demonstrates inadequate conductivity at low temperature and a significant inclination towards crystallization. Nevertheless, blending PC with EC allows for the leveraging of their individual merits, thus ensuring superior conductivity and electrode stability. Consequently, a thin and stable SEI can be formed at low temperature, thereby effectively mitigating Na plating. In contrast, the ether solvent exhibits a lower viscosity and melting point, facilitating the migration of Na⁺ and its deployment at low temperature.⁹¹ In addition, the ether solvent exhibits better reductive stability, and a thin but robust inorganic-rich SEI is formed on almost all anodes in SIBs (Fig. 4b).⁹² Unfortunately, its oxidative stability is insufficient and its dielectric constant is extremely low, which facilitates the decomposition of the electrolyte and hinders the dissolution of Na salts, respectively.^93,94 This will not be compatible with high-voltage cathodes and severely limits salt selection. In general, ether solvents can be categorized into cyclic and linear derivatives, among which linear dimethoxyethane (DME) and diethylene glycol dimethyl ether (DEGDME) are more commonly utilized due to their robust chemical stability.^95,96 However, the strong solvation affinity of DME and DEGDME with Na⁺ at low temperature results in sluggish Na⁺ diffusion kinetics, which is conducive to the occurrence of Na plating (Fig. 4c).⁹⁷ Conversely, cyclic ethers exhibit better inhibitory efficacy on Na plating at low temperature, owing to their relatively low solvation energy with Na⁺, which facilitates anion ingress from the electrolyte salt into the inner solvation shell of Na⁺ and increases the proportion of CIPs.⁹⁸ Notably, the de-solvation of Na⁺ at the SEI is the primary cause of energy loss throughout the entire Na⁺ transfer process. Therefore, maintaining a weakly solvating structure between Na⁺ and anions is essential to address this challenge effectively. Fang et al.⁹⁹ advocated for the utilization of a cosolvent comprising 2-methyltetrahydrofuran (2MeTHF) with low solvating energy and tetrahydrofuran (THF), which effectively facilitates the attenuation of weak ion–dipole interactions and the formation of an inorganic-rich SEI primarily derived from anion decomposition, thereby effectively suppressing the Na plating on the HC at low temperature (Fig. 4d). Equally critical is the flash point of solvents, which is directly related to battery safety, as solvents with lower flash points are more flammable. This finding was robustly substantiated by Hess et al.¹⁰⁰ through comprehensive measurement of flash points and self-extinguishing times across 25 solvents (including carbonates, ethers, esters, etc.), 3 solvent mixtures, and 15 electrolytes. In conclusion, both the Na plating and the resultant battery safety hazards are significantly influenced by the solvent.

Fig. 4 The influence of the solvent composition on Na plating. (a) Comparative analysis of bonding energies and dielectric constants across various solvents. Reproduced with permission from ref. 89. Copyright 2025, Royal Society of Chemistry. (b) Schematic diagram of the LUMO and HOMO energy levels in various solvents. Reproduced with permission from ref. 92. Copyright 2021, Wiley-VCH. (c) The comparison of the desolvation process for ions in DME and CPME solvents, respectively. Reproduced with permission from ref. 97. Copyright 2023, American Chemical Society. (d) The impact of Na⁺-dipole regulation on the formation mechanism of inorganic-rich SEI layers. Reproduced with permission from ref. 99. Copyright 2024, Wiley-VCH.

2.1.3.3. Additive. Another indispensable element integral to the development of an effective electrolyte solution is the selection of suitable additives. Additives are integrated into electrolytes to facilitate the stabilization of the SEI layer and decrease its electrochemical resistance. Furthermore, the incorporation of additives with a low-energy-level lowest unoccupied molecular orbital (LUMO) and a high-energy-level highest occupied molecular orbital (HOMO) can preferentially result in redox reactions on the electrode,^101,102 consequently facilitating the formation of the desired SEI composition. Therefore, it can effectively inhibit the Na plating. Liao et al.¹⁰³ investigated pentafluoro(phenoxy)cyclotriphosphazene (FPPN) as a functional electrolyte additive for stabilizing the interface on HC anodes via its preferential reduction process at the anode, culminating in the formation of a uniform, ultrathin, and inorganic-rich SEI, thereby exhibiting an excellent inhibitory effect on Na plating (Fig. 5a). Zhang et al.¹⁰⁴ opted for N-phenyl-bis(trifluoromethanesulfonimide) (PTFSI) as the anodic film-forming electrolyte additive, which effectively enhances the long-term cyclic stability of HC anodes in SIBs. This is because PTFSI exhibits preferential decomposition, polymerization, and subsequent deposition on the surface of HC, thereby facilitating the formation of a thin and stable SEI layer. In addition, additives cannot be ignored when regulating the solvation process of Na⁺, owing to the fact that the low consumption of solvation energy during the process is conducive to faster Na⁺ diffusion, which can effectively inhibit the Na plating. Ye et al.¹⁰⁵ revealed that 1,3,2-dioxathiolane-2,2-dioxide (DTD) demonstrates the capability to produce more efficacious high-valence sulfur-containing compounds, resulting in a thinner SEI layer and a solvation structure that requires lower solvation energy compared with ethylene sulfite (ES) (Fig. 5b). Thereby, the Na plating is suppressed effectively. Notably, the efficacy of the fluoroethylene carbonate (FEC) additive in facilitating the formation of a more robust interface layer is widely recognized, yet the stability and Na⁺ conductivity of the interface layer are significantly influenced by the fluoride content within the interface (Fig. 5c).^106,107 In other words, the inhibitory effects of FEC on Na plating vary with its different contents. It is claimed that the primary inorganic species NaF derived from FEC displays limited ionic conductivity, necessitating the combination with additional inorganic compounds (Na₂O and Na₂CO₃), but achieving precise regulation exclusively through FEC presents a huge challenge.^108,109 In view of the fact that each additive has its own deficiencies, it is readily conceivable that the amalgamation of diverse additives represents an efficacious approach towards the construction of a stable SEI and further effectively suppresses the Na plating. Desai et al.¹¹⁰ demonstrated that the synergistic combination of NaDFOB and vinylene carbonate (VC) contributes to the formation of a robust SEI layer. Meanwhile, the inclusion of SN and TMSPi facilitates the fine-tuning of the SEI layer by introducing four distinct additives, namely, NaDFOB, VC, succinonitrile (SN) and tris-trimethylsilyl phosphite (TMSPi), into an electrolyte mixture of EC/PC/DMC with 1 M NaPF₆. Thereby, the Na plating is effectively inhibited. Undoubtedly, the structural integrity of the SEI layer can be effectively maintained by additives through their inhibitory effect on component dissolution, thereby manifesting a pronounced capacity to suppress the Na plating. NaF and Na₂CO₃ were successfully employed by Ma et al.¹¹¹ as electrolyte additives to inhibit the dissolution of inorganic constituents such as NaF and Na₂CO₃ within the typical SEI through the saturation of the electrolyte and effectively suppress the Na plating (Fig. 5d).

Fig. 5 The effects of additives on Na plating. (a) Theoretical energy level of various molecular and ionic clusters. Reproduced with permission from ref. 103. Copyright 2024, Wiley-VCH. (b) Illustration of interfacial characteristics of NFM‖HC pouch batteries in the electrolyte comprising ES and DTD, respectively. Reproduced with permission from ref. 105. Copyright 2024, Elsevier. (c) The influence of FEC on the stability of the SEI film. Reproduced with permission from ref. 107. Copyright 2023, American Chemical Society. (d) Effective suppression of SEI dissolution in SIBs achieved by the saturation of the electrolyte. Reproduced with permission from ref. 111. Copyright 2021, Wiley-VCH.

Apart from the internal structure of the battery itself having a significant influence on Na plating, external conditions like low temperature, fast charging and the state of charge (SOC) also have conspicuous effects on it. A detailed analysis of these external factors will be presented in the next section.

2.2. External factors affecting Na plating

2.2.1. Low temperature. By comprehensively considering both kinetic and thermodynamic causes, the low temperature exerts a significant influence on the occurrence of Na plating by altering the Na⁺ diffusion coefficients in the electrolyte and the electrode. A detailed analysis is presented as follows.

The influence of temperature on the migration of Na⁺ in the electrolyte can be related to the viscosity of the electrolyte using the Stokes–Einstein equation,¹¹²

where k is the Boltzmann constant, T is the temperature, r is the Stokes radius, α is a constant from 4 to 6 under the perfect slipping conditions, and μ is the viscosity of the electrolyte.

The variation in the viscosity of the electrolyte as a function of temperature can be determined using the Vogel–Tammann–Fulcher (VTF) model,^113,114

where μ₀ refers to the prefactor (materials dependent constant), B represents the activation energy parameter, which is associated with the potential energy of molecular motion, and T₀ denotes the optimal glass transition temperature.

Another factor that affects the low-temperature performance of the anode material is the diffusion rate of Na⁺ in the solid material, which is usually expressed by the diffusion coefficient D_s, representing the diffusion flux under the unit concentration gradient. The diffusion process of Na⁺ the follows Arrhenius equation,^115,116

where D₀ is the prefactor evaluated empirically, ΔG is the activation energy, k is the Boltzmann constant, and T is the temperature.

Considering the described diffusion behaviour, it is noted that upon experiencing the lower temperature, the viscosity of the electrolyte experiences an upsurge, which, in turn, results in the reduction of diffusion coefficients and hampers the migration rate of Na⁺ within the electrolyte. Additionally, the migration rate within the anode is also diminished, and the convergence of these two factors acts synergistically to block the Na⁺ diffusion, thereby causing the Na plating to take place. In terms of electrolytes, the salt precipitation constitutes a consequential phenomenon arising from the elevated viscosity of the electrolyte at the low temperature, which will induce the Na⁺ concentration gradient, culminating in the Na plating. It was effectively addressed by Yang et al.¹¹⁷ through the manipulation of solvation entropy in the mixture of strong-solvation solvents and weak-solvation solvents. However, the low Na⁺ diffusion kinetics caused by the elevated electrolyte viscosity is not the primary cause of the Na plating at low temperature. Analogous to LIBs, the limited Na⁺ diffusion kinetics within the electrode potentially constitutes a primary factor contributing to the Na plating at low temperature, where Na⁺ accumulates at the interface between the HC electrode and the electrolyte.⁶⁸ The Na plating inevitably occurs when the surface concentration of Na⁺ in the HC electrode is saturated.¹¹⁸ Therefore, Hou et al.¹¹⁹ implemented an advanced multi-scale modification technique, which entailed the manipulation of the microstructure of HC at the granular level and simultaneously replenished the electrode with additional Na inventories using a uniform pre-sodiation method. A comprehensive kinetic analysis has elucidated that the diffusion rate of Na⁺ within the low potential domain (<0.1 V vs. Na⁺/Na) is elevated by two orders of magnitude, and furthermore, the Na deposition behaviour on the HC electrode has been significantly mitigated at low temperature.

2.2.2. Fast charging. In general, fast charging is accomplished by augmenting the current density i (high C-rate), which is prone to Na plating. The kinetic and thermodynamic reasons are as follows.

The electrochemical reaction governed by charge transfer is optimally elucidated using the Butler–Vollmer equation,¹²⁰

where i is the net current density, i₀ = k₀FA is the exchange current density (k₀ is the reaction rate constant of the electrode reaction, F is the Faraday constant, and A is the activity of the reactants), which is indicative of the current density at the point where the electrode reaction attains equilibrium, α is the transfer coefficient, η is the overpotential, R is the gas constant, and T is the temperature.

It is evident from the equation that as the current density i escalates, to maintain the equilibrium of the equation, an increase in the overpotential is imperative. This necessity arises because overpotential serves as the modulator of the electrochemical reaction rate, enabling the reaction rate to vary by several orders of magnitude through manipulation of the overpotential. In addition, in conformity with the Tafel equation, it is observed that at elevated overpotentials, the current density i is exponentially proportional to the overpotential, and the equation is presented as follows:¹²¹

where a and b are constants. This also indicates that overpotential will increase with the increase of current density i. Thus, the high C-rate charging will produce a larger overpotential, thereby resulting in Na precipitation.

From the kinetic perspective, the charging rate is more prone to exceeding the insertion rate in the HC electrode during fast charging, resulting from the fact that the amount of Na⁺ migrating from the cathode to the anode per unit time rises.¹²² With the high-rate charging continuing, there is an accumulation of Na⁺, which brings about a high concentration on the surface of the HC. In terms of thermodynamics, once the surface concentration in the HC electrode is saturated, Na plating will occur.¹²³ Similar to LIBs, Paul et al.¹²⁴ observed the heterogeneous and irreversible Li plating on the graphite anode at a millimeter scale under extreme fast charging conditions ranging from 4C to 9C, and the extent of Li plating increased proportionally with the C-rate. It was further demonstrated by Tanim et al.¹²⁵ that under the extremely fast-charging condition, the local inhomogeneity of electrodes leads to exacerbated inhomogeneous Li deposition.

2.2.3. SOC. The SOC is defined as the proportion of residual capacity of the battery relative to its full-charge capacity, typically denoted as a percentage. Notably, the inadequate thermal stability of the electrode is prone to induce TR, thereby generating the localized temperature gradient.^126,127 From the kinetic standpoint, it can result in the heterogeneous distribution of Na⁺ diffusion coefficients, subsequently generating the concentration gradient of Na⁺. In terms of thermodynamics, this will result in the occurrence of Na plating. Generally, the thermal stability of electrode fluctuates in relation to its different SOC. Mohsin et al.¹²⁸ investigated the thermal stability of Na₃V₂(PO₄)₃/C (NVP/C) and HC across varying SOC, ranging from 0 to 100%, and concluded that the thermal stability of the NVP/C electrode diminished in correlation with the escalation of the SOC. Conversely, the thermal stability of the HC electrode was observed to augment in conjunction with the SOC increment, which could be linked to the complexation between the polymer binder and Na⁺. As shown in Fig. 6a,¹²⁹ it is especially noteworthy that during the charging process, if the charging voltage or current surpasses the rated capacity of the battery, this situation is deemed overcharging, which can accelerate TR and further exacerbate the Na plating behaviour. Xu et al.¹³⁰ studied the degradation behaviour of SIBs utilizing NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) as the cathode across a variety of overcharge scenarios and revealed that due to the excessive design of anodic capacity, the battery experiences the negligible Na plating before reaching the 120% SOC. As the SOC enters the range of 120% to 140%, the progressive thickening of the SEI coupled with the degradation of the active material of the electrode will result in substantial Na plating, which tends to exacerbate progressively as the SOC surpasses the 140% threshold. Notably, different cathode materials demonstrate varying degrees of susceptibility to overcharging conditions. Gui et al.¹³¹ discovered that the NFM batteries exhibit a greater accumulation of metallic Na within their anodes at every stage of overcharging and an increasing rate that outpaces that of Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) batteries. Additionally, they concluded that the primary factor accelerating the premature onset of the SOC and expediting the TR is the Na plating resultant from overcharging, and the scale of Na plating on the anode is positively correlated with the degree of overcharge (Fig. 6b). Simultaneously, as the charging C-rate rises, there is a corresponding progression in the SOC at each interval, which may trigger the transition of Na dendrites from a mossy appearance to a needle-like structure (Fig. 6c).

Fig. 6 The impact of the SOC on Na plating. (a) Na intercalation and plating process under varying SOC. Reproduced with permission from ref. 129. Copyright 2023, Elsevier. (b) Comparative analysis of the thermal runaway process in NFM cells induced by overcharging at a rate of 1C and (c) morphological evolution of dendrites in NFPP batteries under varying current densities (0.2 mA cm⁻² and 0.05 mA cm⁻²) during pre- and post-overcharge conditions. Reproduced with permission from ref. 131. Copyright 2024, Elsevier.

Based on a mechanism analysis of the internal and external factors contributing to Na plating as mentioned above, it can offer valuable insights for subsequent research on the inhibition of Na plating. Meanwhile, the detection methods are equally crucial for verifying the occurrence and morphological evolution of Na plating, which will be thoroughly investigated in the subsequent section.

3. Na plating detection methods

3.1. Electrochemical techniques

Given the unique Na⁺ storage mechanism in HC for SIBs, where certain sodium states within the electrode are metastable, and considering that the materials in batteries tend to be highly sensitive to environmental changes during post-mortem analysis after disassembly, it is imperative to develop non-invasive real-time monitoring techniques to better elucidate the Na plating mechanism.

3.1.1. Electrochemical impedance spectroscopy (EIS). EIS operates as a precise measurement technique that enables the comprehensive evaluation of electrochemical interface attributes, kinetic parameters of reactions and intrinsic material properties. This is achieved by introducing controlled alternating current signals of minimal amplitudes to an electrochemical system and subsequently quantifying its impedance characteristics across a broad frequency domain. Therein, EIS facilitates the acquisition of resistance parameters pertaining to processes operating across varying temporal scales, which include electron transport in solid phases, ion transfer in electrolyte solutions and interfacial charge transfer reactions.^132,133 The measured resistance can serve as an indicator for deposited Na. Nevertheless, the chief challenge associated with employing EIS as a Na detection technique lies in verifying that the observed resistance variation is indeed attributable to deposited Na within the battery, rather than being influenced by other electrochemical processes. To effectively identify the Na plating behaviour, Mandl et al.¹³⁴ employed relaxation time distribution analysis to replace the traditional equivalent circuit model for the analysis of complex impedance spectra (Fig. 7b). Owing to the fact that the deposited Na metal on the surface of the HC anode exhibits faster charge transfer kinetics compared to the intercalation of Na⁺ within the HC bulk phase, Dai et al.¹³⁵ further employed EIS combined with relaxation time distribution analysis to systematically detect the Na plating behaviour at different charging rates and demonstrated that the resistance associated with the mid-frequency arc (including SEI impedance and charge transfer process impedance) exhibited a progressive decline with the increase in the SOC at all investigated charging rates.

Fig. 7 Electrochemical techniques for monitoring Na plating behaviour. (a) Differential charging voltage analysis. Reproduced with permission from ref. 143. Copyright 2021, Elsevier. (b) EIS. Reproduced with permission from ref. 134. Copyright 2020, Elsevier. (c) DRT analysis. Reproduced with permission from ref. 135. Copyright 2024, Elsevier. (d) Voltage relaxing analysis. Reproduced with permission from ref. 138. Copyright 2016, Elsevier. (e) CE. Reproduced with permission from ref. 140. Copyright 2015, Electrochemical Society.

3.1.2. Voltage relaxation analysis. The voltage shoulder phenomenon serves as a critical indicator for monitoring Li plating, manifesting distinctly on the voltage–time curve during the relaxation phase following the charge completion. To achieve more precise observation and analysis of Li plating from the voltage profile, it is typically analysed in conjunction with the time derivative of the voltage curve, which concurrently facilitates the monitoring of Na plating.¹³⁶ In other words, this process entails the reversible intercalation of precipitated Na into the HC anode. This innovative method eliminates the necessity for supplementary battery components, thereby establishing itself as a highly adaptable solution that can be seamlessly integrated with other methods. Employing voltage relaxation analysis, Epding et al.¹³⁷ revealed that the Li deposition behaviour triggered by elevated current density constitutes the fundamental cause of capacity augmentation. Additionally, Schindler et al.¹³⁸ further integrated EIS to investigate the Li plating behaviour in commercial graphite anodes of LIBs and demonstrated that the decrease in high-frequency intersection resistance (R₀), along with the reduction of the arc representing charge transfer impedance of the anode, can serve as a reliable indicator of Li plating (Fig. 7d). It is noteworthy that the technique is a qualitative analysis and can only monitor the presence of Na plating on the anode surface, while it lacks the capability to quantify its amount.

3.1.3. Coulombic efficiency (CE). The CE measurement serves as a valuable analytical tool for characterizing the onset and progression of Na deposition in both half cells and full cells, as evaluated through capacity retention during one and more cycles. Similar to LIBs, the CE measurement can be conducted on a set of nominally identical batteries operating under varying charging conditions to ascertain the minimum charge threshold for Na plating on the HC electrode.¹³⁹ Nevertheless, when employing the technique, it is crucial to recognize that numerous processes may contribute to capacity loss, including parasitic side reactions that occur during the cycling process. To clarify this matter, Burns et al.¹⁴⁰ proposed a novel metric of calculated coulombic inefficiency per hour (CIE per h) to normalize the time-dependent parasitic side reactions associated with Li plating in LIBs and discovered that higher CIE per h values than the baseline coincided with the onset of Li plating (Fig. 7e), which is equally applicable to SIBs. However, this technique lacks precise quantifiability resulting from the multiple capacity degradation mechanisms, even though the upper bound for the quantity of dead Na can be determined by evaluating the total capacity lost in a specific cycle.

3.1.4. Differential charging voltage analysis. Differential charging voltage analysis (dV/dQ vs. Q) represents an in situ, non-invasive, and time-resolved analytical technique, which enables the detection of the onset of Na plating in SIBs and has been effectively applied for Li deposition monitoring in LIBs.^141,142 In general, the onset of Na precipitation can be accurately identified by integrating the peak value of the dV/dQ curve with the abrupt decrease point observed in the d²V/dQ² curve (Fig. 7a).¹⁴³ Akin to voltage relaxation analysis, the implementation necessitates no supplementary battery components. This analytical technique is frequently integrated with CE analysis to derive more robust conclusions regarding the extent of irreversible deposition, thereby enabling the quantification of Na plating.^141,144

3.1.5. Distribution of relaxation times (DRT) analysis. Real-time monitoring of deposited Na on the HC electrode is made possible by the DRT analysis, which precisely determines the time constant τ associated with the charge transfer process. Moreover, the monitoring of Na plating is achieved through the identification of an inflection point, which manifests as a pronounced deceleration in the charge transfer process during the charging cycle. As Na precipitation initiates, the interfacial charge transfer process increasingly becomes governed by deposited Na, resulting in the manifestation of a nearly constant, smaller τ value. Dai et al.¹³⁵ integrated EIS to validate the feasibility of this technique and took into account the significantly faster charge transfer kinetics exhibited by deposited Na on the HC surface relative to Na⁺ insertion within the HC matrix (Fig. 7c). Furthermore, Xu et al.¹⁴⁵ also documented a comparable phenomenon in their investigation of Li plating onsets in LIBs through the application of this technique.

Through the comprehensive analysis of the aforementioned electrochemical techniques, it is evident that the techniques enable the in situ, non-destructive and real-time monitoring of Na plating behaviour. This achievement paves the way for the development of future engineering applications in Na plating monitoring. Notwithstanding, the occurrence of multiple parasitic side reactions within the battery presents considerable obstacles to the quality and quantity of Na plating. Therefore, the physical characterization techniques also need to be considered.

3.2. Physical characterization techniques

3.2.1. Optical imaging/microscopy. Considering that optical imaging serves as a widely adopted post-mortem qualitative analytical technique for evaluating the extent and heterogeneity of metallic deposition on the anode surface, it can further be extended to investigate Na plating in SIBs.^146–148 Moreover, owing to distinctive metallic Na lusters, which stand in stark contrast to deep black HC, coupled with the unique sodiated properties of HC species, optical imaging will demonstrate exceptional efficacy in the employment of SIBs.¹³⁵ In general, application of optical imaging at the length scale enables systematic qualitative investigation of the heterogeneity of irreversible Na plating on the anode surface. However, such a post-mortem optical imaging technique is typically constrained by a spatial resolution threshold ranging from submillimeter to micro scales, and its performance is significantly influenced by ambient illumination conditions, thereby complicating the qualitative comparison across different experimental datasets. Furthermore, to address the constraints associated with the spatial resolution of optical imaging, advanced optical microscopies with a superior spatial resolution (10s of micrometers) have been extensively employed for the morphological characterization of deposited Li,¹⁴⁹ which is equally applicable to SIBs.

3.2.2. Scanning electron microscopy (SEM). SEM stands out in materials characterization through its sub-micrometer resolution capability, along with its adaptable field of view and magnification parameters. Deposited Na is visualized by secondary electron microscopic imaging in SEM. Zhou et al.¹⁵⁰ employed SEM to study the impact of varying electrolyte compositions on the morphological characteristics of Na plating on copper foil substrates under fast charging (Fig. 8c). Dai et al.¹³⁵ observed the morphological evolution of HC anodes under varying charging conditions and concurrently noted the high sensitivity of deposited Na to air in the SEM chamber. Moreover, Jin et al.³³ used SEM to demonstrate that the enhancement of the local tip effect is positively correlated with the increase of the charging state, consequently exacerbating the heterogeneity of Na plating and ultimately inducing internal short circuits. Notably, the SEM analysis conducted by Yao et al.¹⁵¹ showed that Na plating on the HC anodes predominantly exhibits a spherical morphology, with its diameter demonstrating a positive correlation with current density, which is in contrast to morphologies of Li plating on the graphite anodes. Energy dispersive spectroscopy (EDS) is commonly employed in conjunction with SEM to provide comprehensive elemental composition analysis.¹⁵²

Fig. 8 Physical characterization techniques for detecting Na plating behaviour. (a) AFM image. Reproduced with permission from ref. 146. Copyright 2015, Elsevier. (b) Cryo-TEM image. Reproduced with permission from ref. 154. Copyright 2021, Elsevier. (c) SEM image. Reproduced with permission from ref. 150. Copyright 2020, American Chemical Society. (d) X-ray tomography. Reproduced with permission from ref. 157. Copyright 2021, Elsevier. (e) ²³Na NMR spectra. Reproduced with permission from ref. 161. Copyright 2016, Royal Society of Chemistry. (f) EPR. Reproduced with permission from ref. 164. Copyright 2023, Wiley-VCH. (g) XRD. Reproduced with permission from ref. 160. Copyright 2021, American Chemical Society.

3.2.3. Transmission electron microscopy (TEM/STEM). To investigate the nanoscale morphology of Na and Na-based compounds, TEM has been utilized as it provides a superior spatial resolution compared to SEM. However, since TEM relies on electron transmission through the sample and offers an atomic-scale spatial resolution, it necessitates more meticulous sample preparation. Moncayo et al.¹⁵³ observed the foam-like morphology of Na deposition using ex situ STEM as strong evidence of gas evolution arising from electrolyte degradation. To prevent incomplete imaging of Na plating caused by electron beam damage during characterization, cryo-TEM has been extensively employed. Zhu et al.¹⁵⁴ conducted a study utilizing in situ cryo-TEM to capture the initial growth of Na dendrite nuclei on the Na metal anode within a high-concentration organic phosphate-based electrolyte and revealed the absence of inorganic crystalline compositions during the early stage of Na dendrite growth (Fig. 8b).

3.2.4. Atomic force microscopy (AFM). AFM represents one of the most advanced analytical techniques for characterizing electrode surface morphologies at the nanoscale. This technique precisely measures changes in the substrate surface by detecting mechanical/electrical/topological variations induced through the interaction between the nanoscale cantilever tip and the substrate surface (Fig. 8a).¹⁴⁶ Furthermore, the three-dimensional morphological resolution achieved by this technique surpasses that achieved by both optical microscopy and SEM. Generally, it has integrated optical cameras to study the Na plating behaviour. Han et al.¹⁵⁵ conducted a comprehensive investigation into the morphological evolution of Na dendrites by utilizing in situ AFM in conjunction with optical imaging, which effectively captured the dynamic processes of nucleation and subsequent growth. Liu et al.¹⁵⁶ further integrated the comprehensive analytical capabilities of environmental transmission electron microscopy (ETEM) for the real-time characterization of Na dendrite growth, which was measured through simultaneous mechanical property analysis. Their findings revealed that Na dendrites deposited in situ via electrochemical processes exhibited enhanced stability, attributable to the presence of Na₂CO₃ on their surfaces. Additionally, they observed that these Na dendrites possessed characteristic dimensions of several hundred nanometers and manifested diverse morphologies, including nanorods, polyhedral nanocrystals and nanospheres. Overall, the AFM as a sophisticated analytical tool has demonstrated remarkable precision in characterizing the morphological features of SIBs, yet maintaining an inert atmosphere throughout the sample manipulation and transfer operations within AFM systems represents a primary technological barrier.

3.2.5. X-ray tomography. X-ray tomography is a pivotal technique for achieving three-dimensional reconstruction of battery structures from the microscale to the nanoscale. In the context of conventional absorption-based X-ray tomography, the difficulty of Na characterization in SIBs primarily stems from the challenge of distinguishing HC anodes, electrolytes, and Na plating, resulting from their similar X-ray mass attenuation coefficients. Moreover, this analytical complexity is further exacerbated by imaging artifacts arising from high X-ray attenuation materials, such as copper current collectors. Notably, phase-contrast X-ray tomography demonstrates significant potential in differentiating battery components with comparable mass attenuation coefficients. Through the integration of diverse phase-contrast X-ray tomography techniques, including propagation-based imaging and differential phase-contrast imaging, both X-ray absorption attenuation data and phase shift information are simultaneously acquired. Subsequently, a high-resolution three-dimensional spatial distribution image of the battery components is reconstructed. Liu et al.¹⁵⁷ employed a non-destructive three-dimensional synchrotron X-ray tomography technique to intuitively observe the internal short-circuit phenomenon induced by Na dendrites in the cycled Na symmetric battery (Fig. 8d). Moreover, Qian et al.¹⁵⁸ demonstrated that X-ray computed tomography enables in situ visualization of the three-dimensional dendritic morphology and its dynamic evolution during several varying battery operations. The TR resulting from Na dendrites was demonstrated by Robinson et al.¹⁵⁹ by employing X-ray tomography with a high X-ray attenuation coefficient to reconstruct the volumetric structure of the SIBs experiencing thermal failure.

3.2.6. X-ray diffraction (XRD). XRD constitutes a non-destructive analytical technique that identifies crystalline peaks of metallic Na and other Na-containing species (such as Na_xC). The non-destructive nature of X-rays enables XRD scans to be performed on most battery geometries under in situ conditions. Furthermore, through the utilization of a micro-focused X-ray beam integrated with spatial scanning protocols to cover the entire battery, XRD offers both global and localized structural information (Fig. 8g). Consequently, XRD is an effective technique for detecting Na plating. Paul et al.¹²⁴ utilized spatially resolved XRD to examine the localized Li plating in industrial-grade pouch batteries after hundreds of extreme fast-charging cycles (4C–9C), revealing that irreversible Li plating on the anodes manifests substantial spatial heterogeneities. Building on this, their team employed in situ, spatially resolved high-energy XRD to quantify the total Li plating on the anodes.¹⁶⁰ Notably, XRD characterization of deposited Na on the anodes is significantly hindered by its intrinsic poor crystallinity, extreme air sensitivity, and weak diffraction signals that are obscured by competing battery components.

3.2.7. ²³Na nuclear magnetic resonance (²³Na NMR) spectroscopy. ²³Na NMR spectroscopy serves as a non-destructive and real-time analytical technique for quantifying Na plating during electrochemical processes in batteries. Owing to the metallic characteristics of deposited Na, it exhibits a distinct knight shift relative to the electrolyte peak in ²³Na NMR spectra, thereby facilitating clear differentiation. Notably, ²³Na NMR spectroscopy is predominantly sensitive to deposited Na within the skin depth of approximately 10 mm, consequently positioning it as an effective surface analysis technique for the quantification of deposited Na as well. Previously, ²³Na NMR spectroscopy has been effectively employed by Stratford et al.¹⁶¹ to quantitatively analyze Na deposition on the Na metal anode (Fig. 8e). However, this method lacks visualization of the morphology of deposited Na. This issue was addressed by Bray et al.²¹ through incorporating ¹H magnetic resonance imaging (¹H MRI) techniques, but this structure is derived from the three-dimensional surface morphology of the ¹H signal of the electrolyte within the separator.

3.2.8. Electron paramagnetic resonance (EPR) spectroscopy. The local environment and the number of unpaired electron spins, which are parameters that conventional analytical methods usually cannot access, can be sensitively detected by employing EPR spectroscopy, an advanced magnetic resonance technique.¹⁶² This detection delivers unique insights into the electronic structure and electrochemical reactions of electrode materials (Fig. 8f).^163,164 Consequently, EPR spectroscopy stands out as a potent analytical technique for revealing Na plating behaviour. Unfortunately, the exceptional sensitivity of EPR spectroscopy to chemical and electronic structural changes also renders measurements vulnerable to interference from the defects (e.g., magnetic impurity phases) or compositional inhomogeneities in electrode materials. Specifically, cycled HC anodes routinely exhibit a multi-component EPR signal consisting of three distinct features: a dominant broad component (C1, >97% signal contribution), accompanied by two narrow components (C2 and C3). Among these, C1 originates from unpaired electron spins in sodiated HC—the intercalation of Na⁺ between graphene layers alters the electronic structure of the HC, rendering it paramagnetic with metallic-like characteristics. C2 is attributed to the formation of sub-/nanoscale Na species exhibiting quasi-metallic/metallic electronic properties. Finally, C3 stems from relatively isolated localized radicals which do not undergo significant electron–electron exchange interactions. This has been effectively demonstrated by Xia et al.⁵⁶ through the integration of in situ Raman spectroscopy, in situ synchrotron X-ray diffraction, and density functional theory calculations.

Through the thorough analysis of the aforementioned physical characterization techniques, it is evident that the majority of these techniques can intuitively visualize the morphology of deposited Na, but lack quantification. Fortunately, this limitation can be effectively addressed via the integration of electrochemical techniques. Consequently, the synergistic combination of these two methods holds significant potential for the development of an engineering-feasible real-time monitoring technique in future applications.

4. Mitigation strategies

Drawing from the established mechanistic understanding for the determinants of Na plating, the mitigation strategies can be systematically categorized into three core components: electrode structure design, surface engineering and electrolyte regulation. This section will provide a detailed analysis focusing on these three aspects.

4.1. Electrode structure design

Based on the previous mechanistic analysis of the influence of the electrode structure on Na plating, the Na plating can be effectively inhibited by optimizing the pores, defects, tortuosity, interlayer spacing and the morphology and size of particles in the electrode structure.

The larger interlayer spacing with faster Na⁺ diffusion kinetics contributes to the mitigation of Na plating. It was demonstrated by Wang et al.¹⁶⁵ that doping HC with P induces structural changes in the graphite domains, including the expansion of interlayer spacing and an increase in the number of internal closed micropores. Consequently, it enhances the Na⁺ diffusion kinetics and delivers superior rate performance. Adjusting the pore structure is conducive to suppressing the Na plating. It can be achieved by employing HC prepared with bamboo powder refined through an acid-washing process reported by Wu et al.¹⁶⁶ (Fig. 9a). Notably, HC materials with abundant mesopores can effectively inhibit Na plating, as the mesopores within HC materials facilitate Na⁺ intercalation, whereas the micropores are incapable of accommodating Na⁺. It was successfully obtained by Yang et al.¹⁶⁷ through treating carbonized walnut shells with cetyltrimethylammonium bromide or potassium hydroxide under high-temperature conditions, endowing the HC materials with exceptional cycling performance (Fig. 9b). Shortening the diffusion paths of Na⁺ and electrons can significantly suppress Na plating by reducing the tortuosity of the electrode. It has been realized through a unique rod-shaped HC proposed by Liang et al.,¹⁶⁸ which involves the thermal decomposition of bimetallic organic framework (MOF) nanorods combined with the deposition of Bi nanospheres on the three-dimensional framework, endowing the material with excellent rate performance. Given that irreversible active sites typically induce the concentration gradient during cycling processes, a decrease in their amounts contributes to the suppression of Na plating. It was successfully accomplished by Qiu et al.¹⁶⁹ via the synthesis of N and P co-doped HC modified with Ni-single atoms, and the Ni-single atoms function as catalysts to regulate the active site to accelerate the Na⁺ diffusion. The synthesis of spherical HC particles with a low specific surface area and a high closed micropore volume was achieved by Gao et al.¹⁷⁰ through the utilization of longan kernel starch under the low-temperature pre-oxidation crosslinking condition, and the resultant HC anodes contribute to the inhibition of Na plating, further exhibiting high reversible capacity and superior rate performance. The HC particles with a smaller size are more conducive to suppressing Na plating. Yang et al.¹⁷¹ successfully processed lignite-derived HC into particles with a size distribution of 1–15 μm through ball milling and ultrasonic vibration classification, which demonstrated remarkable electrochemical performance. Biomass precursors necessitate carbonization at an optimal temperature to fabricate the electrode structure that effectively mitigates Na plating. Damodar et al.¹⁷² synthesized palm mesocarp-derived HC through carbonization at 700 °C, which exhibits a highly porous and interconnected network structure, enabling the material to effectively suppress Na plating and have superior cycling performance.

Fig. 9 Mitigated Na plating with electrode structure design (a and b), surface engineering (c and d), and electrolyte regulation (e and f). (a) Schematic illustration of the synthesis process for HC materials. Reproduced with permission from ref. 166. Copyright 2024, Wiley-VCH. (b) Mechanistic diagram of the influence of micropores and mesopores on Na⁺ diffusion. Reproduced with permission from ref. 167. Copyright 2020, Elsevier. (c) Schematic diagram of the modulation of the SEI composition through surface treatment with short-chain ligands. Reproduced with permission from ref. 174. Copyright 2024, Elsevier. (d) The inhibitory effect of nano coating on Na plating on the HC surface. Reproduced with permission from ref. 135. Copyright 2024, Elsevier. (e) The synergistic effect on the electrolyte and the interface caused by TFBB and NaDFP. Reproduced with permission from ref. 180. Copyright 2025, Elsevier. (f) Schematic illustration of the synergistic mechanism of film formation and interfacial property enhancement in the NPFD electrolyte via additive combinations. Reproduced with permission from ref. 182. Copyright 2024, Elsevier.

4.2. Surface engineering

Fundamentally, the interfacial properties of the electrode are intrinsically related to the Na plating reaction occurring on its surface. For example, functional surface treatment that introduces specific functional groups can significantly enhance electrolyte wettability. Similarly, applying a protective surface coating reduces the direct electrode–electrolyte contact, thereby effectively mitigating adverse reactions. Together, these approaches simultaneously regulate the mechanisms and kinetics of interfacial reactions, leading to improved overall electrochemical performance. In conclusion, a suitable surface treatment and surface coating can efficiently regulate interfacial reactions and suppress Na plating on the electrode surface.

4.2.1. Surface treatment. Oxygen plasma treatment was adopted by Xie et al.¹⁷³ to introduce oxygen functional groups onto the HC surface. It was demonstrated that the wettability of HC towards the electrolyte was significantly enhanced due to the increased content of oxygen functional groups. Moreover, the overpotential was reduced by stabilizing the SEI film during the charge/discharge cycles, and the Na plating was suppressed effectively. Additionally, Aina et al.¹⁷⁴ introduced an innovative approach to surface modification treatment at room temperature, employing short-chain 3-mercaptopropionic acid (MPA), 1,2-ethanedithiol (EDT), and oxalic acid (OxA). The study revealed that the application of sulfur-containing molecules (MPA/EDT) substantially improved the micropore blockage in HC, enabling efficient Na⁺ reversible intercalation. Moreover, the integration of thiol functional groups was observed to stimulate the development of a more compact SEI abundant in NaF and Na₂O, leading to enhanced Na⁺ diffusion kinetics in the plateau region (Fig. 9c). As is expected, the Na plating was remarkably inhibited. Beyond surface functionalization, precise tailoring of surface pores in HC also effectively suppresses Na plating. Utilizing a hydrogen peroxide-assisted hydrothermal synthesis strategy, Huang et al.¹⁷⁵ successfully fabricated a surface-porous HC anode material. This unique structural characteristic not only provides abundant active sites for Na⁺ storage but also facilitates ion diffusion kinetics. Furthermore, the porous structure effectively accommodates volume expansion during cycling and promotes the formation of a stable SEI layer, thereby effectively suppressing Na plating.

4.2.2. Surface coating. Dai et al.¹³⁵ successfully employed nano-sized ZIF-8, Al₂O₃ and MgO to form a nanoporous coating on the HC surface, effectively suppressing underpotential Na plating while enabling uniform Na⁺ migration at the interface. Among these, ZIF-8 demonstrated the most remarkable improvement, which is likely attributed to its distinctive molecular structure (Fig. 10d). Notably, the thickness of the surface coating is also a critical factor influencing its efficacy in suppressing Na plating. Lu et al.¹⁷⁶ investigated the impact of Al₂O₃ coating and its thickness on electrochemical performance by the atomic layer deposition (ALD) technique, revealing that the interfacial resistance and electrode overpotential can be effectively reduced by the Al₂O₃ coating with a critical thickness of 2 nm, originating from an optimal balance between electronic and ionic conduction at the electrolyte–HC interface, which, in turn, significantly inhibits Na plating. Compared with the relatively complex ALD technique, the direct in situ electrochemical polymerization coating is undoubtedly a simplified method. It was utilized by Yu et al.¹⁷⁷ to achieve the synthesis of the HC-DEBA material in a 2,2-dimethylvinyl boric acid (DEBA) aqueous solution. Their study demonstrated that the C=C bonds of DEBA molecules undergo an in situ electro-polymerization during the cycling process, resulting in the formation of a polymeric network. This network functions as a passive protective layer, which can effectively curb the irreversible decomposition of the electrolyte. As a result, it leads to the formation of a thinner SEI film with lower impedance, thereby exhibiting an excellent inhibitory effect on Na plating. Notably, the associated production cost is a critical factor that needs to be carefully considered to facilitate the transition towards industrialization. Ji et al.¹⁷⁸ successfully grafted chemically active glucose molecules onto the surface defects of pitch-based hard carbon (PHC) at a low cost via esterification in a solution, followed by carbonization to form a dense carbon layer. Their research results indicated that the carbonized glucose formed a thin carbon layer with fewer defects, which results in a decrease in the specific surface area, an optimization of the pore structure and an improvement in the migration rate of Na⁺, thereby effectively suppressing the Na plating.

Fig. 10 Schematic illustration of future perspectives for suppressing Na plating.

4.3. Electrolyte regulation

Based on the preceding mechanistic analysis, rational formulation of electrolytes (including salts, solvents, and additives) significantly improves Na⁺ diffusion kinetics and enhances SEI stability, thereby effectively suppressing Na plating.

As proposed by Cai et al.,¹⁷⁹ sodium difluorophosphate (NaDFP), as an additive in carbonate-based electrolytes, enhances the SEI stability and reduces the desolvation energy of Na⁺, effectively mitigating the gas evolution and Na plating. Thereby, it enables the SIBs to exhibit better cycling stability within a wide temperature range of −30 to 60 °C. Considering the suboptimal solubility of NaDFP, Cai et al.¹⁸⁰ further implemented an innovative “anion–anion receptor synergy” approach, in which tris(pentafluorophenyl)borane (TFBB) was utilized as an anion receptor to promote the in situ dissolution of NaDFP, and successfully achieved 550 stable cycles of pouch NFM/HC at a high voltage of 4.0 V, with no observable gas evolution throughout the cycling process (Fig. 9e). EMC and ethoxy(pentafluoro)cyclotriphosphazene (PFPN) with different solvation abilities can reconstruct the first and second solvation shells of Na⁺ and regulate its solvation processes. They were incorporated into the PC-based electrolyte by Li et al.¹⁸¹ and the electrolyte exhibits high ionic conductivity, a low desolvation energy barrier, and the ability to form thin and stable SEI/CEI layers, which enables accelerated Na⁺ kinetics at low temperature and superior thermal stability at high temperature. DTD with a (p–d) π bond can precisely regulate the decomposition process of FEC through a tunable redox reaction and introduce composite organic and inorganic components. It was proposed by Liang et al.¹⁸² as an additive in the carbonate electrolyte to construct a stable SEI and enable fast Na⁺ transport (Fig. 9f). As expected, the high-voltage Na₃V₂(PO₄)₂O₂F (NVPF) cathode maintained exceptional cycling stability, enduring over 8000 cycles with a capacity retention exceeding 90%. And the HC‖NVPF full cell exhibited a capacity retention of 80% after 3000 cycles. Sodium pentadecafluorooctanoate (SPFO) with preferential adsorption and oxidation can form the CEI rich in C–F components and NaF, which greatly enhances the antioxidant capacity of the electrolyte and facilitates faster Na⁺ kinetics. It was employed by Hou et al.¹⁸³ as an additive in ether-based electrolytes (DEGDME), achieved outstanding high-voltage durability up to 4.5 V vs. Na⁺/Na and improved the thermal stability of the electrolyte up to 60 °C in NVP-based SIBs. Nanodiamonds (NDs) with strong affinity for Na⁺ can construct a robust SEI abundant in NaF and Na₂CO₃ and diminish the activation energy barrier required for charge transfer. They were integrated into a diglyme electrolyte by Zhang et al.¹⁸⁴ to enhance the diffusion of Na⁺, effectively suppress dendrite formation and bolster anodic stability at sub-zero temperature. 2-Methyltetrahydrofuran (2-MTHF) with low solvation resulted from the weak ion–dipole interaction, which can enhance the anion-reinforced solvation structure across a temperature range from ambient to sub-zero conditions and promote the formation of the inorganic-rich SEI. It was incorporated into THF by Fang et al.⁹⁹ and exhibits a reduced freezing point of −83.3 °C as well as accelerates the desolvation kinetics, enabling the NVP/HC full cell to maintain an exceptional capacity retention of 100% after 250 charge–discharge cycles at −40 °C. Acetonitrile-based electrolytes (AN), which exhibit low viscosity and desolvation energy, a high dielectric constant, an enhanced Na⁺ diffusion coefficient and transference number, as well as a wide electrochemical stability window, are proposed for dissolving the Na salt (NaFSI)⁹³ and display superior high-voltage tolerance, fast ionic conductivity, the anion (FSI-)-rich solvation structure and a wide liquid-phase temperature range, culminating in the effective suppression of Na plating. As a result, the electrolyte achieves the stable cycling of HC‖NVP full cells for 2500 cycles with a capacity retention of 79.1%. And the HC‖NCM full cells maintain 82% of their capacity after 1400 cycles at a high cut-off voltage of 4.15 V and a high rate of 5C.

5. Conclusions and outlook

The battery safety has become a critical concern in the contemporary society, as its widespread adoption extends from personal electronics to electric vehicles and grid-scale energy storage systems, directly affecting both device performance and personal safety. Specifically, Na plating on the surface of HC anodes is one of the primary factors affecting the safety of SIBs, based on the fact that the propensity of Na plating to develop clusters/dendrites may induce internal short circuits and, in severe cases, trigger thermal runaway events including fires and explosions. Notwithstanding the low sodiation potential, high Na⁺ storage capacity, and wide availability of HC, the Na plating behaviour remains ubiquitous. Consequently, it is imperative to systematically investigate and elucidate the underlying mechanisms of Na plating. In this review, the Na plating mechanisms considering both internal factors (electrode structure, N/P ratio and electrolyte composition) and external factors (low temperature, fast charging and SOC) are firstly summarized. And then, the existing detection methods are concluded. Finally, the effective mitigation strategies are proposed. The outlook is presented as follows (Fig. 10):

(1) The mechanism can be further elucidated through comprehensive investigations into the correlation between the threshold of Na plating and internal factors, including the morphological characteristics of HC materials (particle size and shape), electrode structural parameters (open pores, closed pores and tortuosity), and the N/P ratio. Meanwhile, the evolution of these internal factors under external conditions such as low temperature, fast charging, and the SOC should also be taken into account.

(2) The mechanistic understanding of Na plating serves as a fundamental basis for guiding both the synthesis of HC materials and the optimization of electrode structure design, ensuring effective suppression of Na plating while maintaining energy density, which is essential for meeting the demanding specifications of next-generation high-energy-density batteries.

(3) A real-time and quantitative monitoring method can be developed with viable engineering applications. The deposited Na exhibits partial reversibility and a substantial portion of the reversible Na plating may dissipate during sample transfer, which poses significant challenges to the ex situ study of Na plating. Furthermore, the currently available electrochemical methods are not truly real-time quantitative monitoring techniques, owing to their inability to precisely differentiate between complex side reactions and Na plating reactions occurring within batteries.

(4) A sophisticated and engineering-feasible Na plating model can be integrated into the battery management system (BMS). By utilizing the feedback derived from the Na plating model to optimize the charging protocol, it is possible to effectively mitigate Na plating and significantly prolong the operational lifespan of batteries. Currently, the machine learning algorithm presents a promising approach for establishing the sophisticated and engineering-feasible Na plating model.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Acknowledgements

L. F. acknowledges the financial support from the National Natural Science Foundation of China (Grant no. 22569003) and the Guizhou Provincial Basic Research Program (Natural Science; grant no. QKHJC-ZK YB046).

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