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Progress on Designing Artificial Solid Electrolyte Interphases for Dendrite Free Sodium Metal Anodes

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22 May 2023

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23 May 2023

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Abstract
The nature abundant sodium metal is proposed as ideal anode materials for advanced batteries due to its high specific capacity of 1166mAh g-1 and low redox potential of -2.71V. However, the uncontrollable dendritic Na formation and low coulombic efficiency are still major obstacles for applications. Notably, the unstable and inhomogeneous solid electrolyte interphase (SEI) is recognized to be the root cause. As SEI layer plays a critical role in regulating uniform Na deposition and improving cycling stability, researches on SEI modification, especially the artificial SEI modifications has been extensively investigated recently. In this regard, we discussed the advances on artificial interface engineering from the aspects of inorganic, organic and hybrid inorganic/organic protective layers. Finally, we also highlighted the conclusions and key prospects for further investigations. We hope this review can provide a new insight for sodium metal protection.
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Subject: Chemistry and Materials Science  -   Electrochemistry

1. Introduction

To date, the sodium (Na) ion batteries have been commercialized as a supplement technology for lithium (Li) ion batteries due to the natural abundant Na resources and low price [1]. However, the energy density of Na ion batteries seems to be unsatisfied as compared with update commercial Li ion batteries [2,3]. To meet the rapidly growing demand markets on energy density of Na ion batteries, the development on advanced electrode materials with high capacity are highly desired [4].
Among various materials, the metal Na has been proposed as an ideal candidate due to its high specific capacity (1166mAh g-1) and low redox potential (-2.71V) [5,6,7]. In this regard, the investigations on metal Na based batteries such as Na-S; Na-O2 and Na-CO2 batteries have also been widely reported [5]. However, the cycling performances and safety issue of Na anode are still unsatisfactory. It is explored that the ultra-high reactivity of Na in electrolyte may be the root reason. The spontaneously reaction between Na and electrolyte can form a solid electrolyte interphase (SEI) on the surface, which is proved to be chemical/mechanical unstable and fragile [8,9]. During plating/striping, the thermodynamic SEI would be thickened, broken and collapsed. At the same time, the inhomogeneous SEI further induces uneven Na+ flux locally, as a result of mossy/dendritic Na formation [7,9]. Additionally, the volume change during Na plating/striping is very obvious, especially under industry conditions (more than 3.5 mAh cm-2 for each deposition cycle) [10]. Consequently, the thickness change can lead to great local stress, making the SEI to be much more unstable and more easily cracking [11]. The cracks then increases the local Na+ flux around, which in turn aggravates the growth of dendritic Na. In particular, the dendritic Na can penetrate through separator and detach from the matrix easily to form “dead” Na, leading battery short circuit and short cycle life [11,12]. This phenomenon also consumes electrolyte and active Na, as a consequence of low coulombic efficiency [13]. Therefore, effective efforts on modification Na metal anode are highly necessary.
Under this background, several approaches have been proposed to stable Na anodes. For instance, constructing 3D host to resolve infinite volume expansion [6,14,15]; coating separator to block Na dendrites [16,17] and employing Na alloy to build stable anodes [18,19,20]. Although these approaches have some positive effects in suppressing dendritic Na formation, the properties of SEI films are still unsatisfied and the irreversible side reactions can’t be totally suppressed. The electrolyte modification seems to be promising for increasing the stability of the SEI interphase. However, the additives, salts and solvents can’t hold for long-term cycling due to the continuous consumption [11,21]. According, the ideal SEI for Na metal should possess excellent chemical/electrochemical stability, good ionic conductivity, even Na+ flux/electric field distribution, sufficient Young’s module, good flexibility, and robustness [22]. In this regard, artificial interphase engineering is of vital important since the protective layer can be precisely designed and easily adjusted. More importantly, the artificial SEI can meet most of the above mentioned merits of ideal SEI. So far, extensive researches have been proposed for artificial interphase configuration to improve the stability of SEI [23,24]. Therefore, it is necessary to summarize the research progress on artificial SEI designing in recent years. Herein, in this review, we discussed the advances on artificial interface engineering from the aspects of inorganic, organic and hybrid inorganic/organic protective layers as shown in Figure 1. We also conclude by prospecting future direction of artificial interphase chemistry for advanced Na metal anodes. We hope this review can deep the understanding on artificial SEI layers, thus exploring stable and dendrite free Na anodes.

2. Challenges for Dendrite Free Na Metal Anodes

Like other alkali metals, Na is thermodynamic unstable, which is the root cause of uncontrollable parasitic reactions and the formation of chemical/mechanicl unstable SEI [23,24]. Figure 2 shows the main challenges of Na metal anodes. As compared with Li metal, Na metal is more prone to deposited in dendritic morphology and suffers from severer volume expansion, along with SEI breakage and low Coulombic efficiency during repeated plating/striping [25,26]. Overall the challenges for dendrite free Na metal anodes are discussed as following.

2.1. Unstable SEI

Because of the highly reactive nature and very negative voltage potential, the electrolyte can be reduced spontaneously on Na metal anode to form an SEI layer. It is expected that such a passivation layer is dense and inert so as to effectively isolate electron transfer and prevent further parasitic reactions [24,27,28,29]. Nevertheless, the SEI layer is demonstrated to be structure porous and fragile in common electrolytes.
As it is recognized, the SEI layer formed on Na metal is mainly composed of inorganic species (e.g., NaF, Na2O and Na2CO3) and organic species (e.g., RONa, ROCO2Na, and RCOONa; where R is the alkyl group) [25]. This kind hybrid organic/inorganic SEI layer are proved to be dissolved in electrolyte more easily than those of Li [30,31]. Due to the non-uniform distribution of SEI compositions, the SEI layer formed in common electrolytes is spatial variation in ionic conductivity, resulting in uneven distribution of Na+ flux. Meanwhile, with a “host-less” nature of Na matrix, this SEI cracks easily during repeated Na+ plating/striping, which then in turn accelerates dendrite growth due to increased Na+ flux and preferential Na+ plating around the cracks. Furthermore, the repeated breakage of SEI also leads to uncontrollable electrolyte consumption, followed by low coulombic efficiency and high SEI impedance [32,33]. As a result, Na metal with unsatisfied SEI properties inevitably suffers from poor performances.
Based on previous understanding [34,35], further progresses on building ideal SEI for dendrite free Na metal should be with following characteristics: firstly, highly Na+ conductive so as to facilitate uniform Na+ deposition and regulate preferential Na plating; secondly, electrochemically stable and electronically insulated to prevent further side reactions; thirdly, robust enough to long-time hold huge volume expansion and dendrite propagation, and finally homogeneity in composition to decentralize the Na+ flux.

2.2. Uncontrollable Dendritic Na formation

The notorious dendrite is also a serious problem for achieving safe and stable Na anode. The dendrite growth can penetrate the separator, leading to battery short circuit and poor cycling stability. Different from conventional needle-like Li dendrites, the morphology of Na dendrites is reported to be mossy or flocculent like with severe porous structures [36,37].
Based on previous understanding [38], it is widely accepted that the concentration of Na+ will decrease to zero near the surface in Sand’s time. Due to the spatial variation in ionic conductivity and localized electric field, the rough surface will induce uneven Na+ plating/striping, resulting in dendrite formation. Subsequently, the tips of dendrites would be hot sites for further dendritic Na nucleation and growth due to its larger electric field and ionic concentration gradients. Once the dendrite nucleated, the growth rate of dendrites is a key parameter to determine the lifetime of Na anode. According to the Sand’s law, the speed of dendrite formation is inversely proportional to square of the deposition current [32,37,39,40].
The dendrite growth can expose fresh Na surface with depletion of electrolyte and active Na. Meanwhile, the structure unstable dendrites would detach from the matrix to form “dead” Na. Through microscopy observation, it is proved that the porous Na dendrites can break away from the bulk Na matrix easily, as compared with Li dendrites. The dendrites intrinsically exhibit much higher chemically reactivity and weaker mechanical stability [39].

2.3. Severe Volume Expansion

The severe volume expansion can be regard as the root cause of the continuous side reactions. Theoretically, the thickness would increase 8.86µm under 1 mAh cm-2 deposition of Na. To satisfy industrial requirements, the deposited capacity would be above 3.5 mAh cm-2 [34,41]. Due to uneven deposition, the practical volume variation would be more obvious than theoretical expected. In addition, due to the host-less nature, the volume expansion is considered to be relatively infinite. It is recognized this volume expansion would further induce inner stress and local expansion/contraction, resulting fracture of the SEI layer and pulverization of active Na. This phenomenon in turn accelerates side reactions and dendrite growth [42]. To alleviated the volume expansion and mitigate the inner strain, nanostructured host such as Cu foam [43,44,45], carbon matrix [42,46,47] and Mxene [48,49] are proposed to accommodate Na. Nevertheless, these host increases the total weight and volume of the Na anode at the expense of total energy density. The recent development of hosts for dendrite free Na metal has been discussed in several reviews [26,50].

3. Advances in Artificial SEI Interphase

Stable SEI is the ultimate pursuit for achieving dendrite free Na metal anodes. With deep understanding of plating/stripping mechanism, several strategies (e.g., chemical pretreatment, building protective film by advanced deposition technologies and free-standing protective layers) for building artificial interphase have been proposed [36,37,39]. Typically, the chemical composition, structure, and thickness of artificial SEI layers can be precisely controlled by optimizing the reagent species, concentration and reaction temperature, time, etc [36]. As reported, the artificial SEI can be classified into inorganic rich, organic rich or their hybrids [24,51,52]. The characteristic of the inorganic rich and organic rich SEI are schematic presented in Figure 3a-b. In this section, we will discuss the recent advances in constructing artificial SEI for stable Na metal anodes.

3.1. Inorganic Interphase

By adopt the experiences and knowledge of LiX (X=F, Cl, Br, I) for dendrite suppression in Li metal batteries, NaX are proposed for inorganic interphase configuration through chemical pretreatment methods [22,36,53]. In the early stage, Wang et al. proposed a facile chemical pretreatment of Na with SbF3/DMC solution. Through an exchange reaction, an inorganic SEI rich in NaF and Na3Sb alloy is formed. Take advantages of the synergistic effect of NaF and Na3Sb, the hybrid NaF/Na3Sb interphase greatly reduces the surface reactivity and interfacial impedance [54,55]. Lately, the NaF rich interphase was also reported by reaction with 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BdmimBF4) [56], CoF2 [57], AlF3 coated solid-state electrolyte [58] and triethylamine trihydrofluoride [59]. The role of NaF for dendrite suppression is benefit from the high shear modulus of 31.4 GPa, which is much higher than that of metal Na (3.3 GPa) [57,60]. Inspired by these works, NaCl rich interphase is also investigated. For instance, Huang et al. adopted SnCl2 to treat Na with the formation of NaCl/Na-Sn alloy interphase [61]. As expected, both rapid ion transportation and suppressed parasitic reactions is obtained, which jointly achieve a nondendritic morphology over 500h in Na||Na batteries. Similar treat methods have also been reported by ZnCl2 and SnCl4 [62,63,64]. Analogous to NaF and NaCl, the NaI and NaBr rich interphase was reported by reaction with 1-iodopropane and 1-bromopropane, respectively [65,66]. In Na||Na cells, the NaI coated Na was stable for 500h under 0.25 mA cm-2, 0.75 mAh cm-2, while the NaBr coated Na was stable for 250h under 1.0 mA cm-2, 1.0 mAh cm-2. According to the density functional theory calculations in Figure 4a, the energy barriers for interface ion diffusion is decreased in following order: NaF>NaCl≈NaI>NaBr [66]. The lower energy barrier is more favorable for nondendritic deposition.
The S containing protective layer is also attractive for nondendritic Na plating /stripping due to its high ionic conductivity. Sun et al. synthesized Na3PS4 as an artificial protective layer by reacting Na with P4S16 in diethylene glycol dimethyl ether. By controlling the concentrations of P4S16 and reacting time, the thickness and composition of the Na3PS4 can be optimized. The thin Na3PS4 layer can prevent unwanted side reactions and uniform the Na+ flux during plating/striping [67]. The Mo6S8 and MoS2 were also used for building NaxMo6S8 and Na2S protective layers as shown in Figure 4b-c, respectively [68,69]. Besides NaX and the S containing protective layer, the Na3N layer is also attractive due to its high ionic conductivity. In 2021, Sun et al. directly embedded NaNO3 into Na matrix with the formation of Na3N and NaNxOy in Figure 4d-e. The Na3N and NaNxOy provides good SEI stability and Na+ conductivity, while the remained NaNO3 remain as a SEI stabilized for long term cycling [70].
Recently, the Yu’s group built a Na3P protective layer to stable Na metal via treating with red phosphorus. As shown in Figure 4f, the Na3P layer can provide high ionic conductivity of ~0.12 mS cm-1 and high Young’s modulus of 8.6 GPa, which regulates uniform Na+ flux and prevents the dendrite growth. As proved by cryo-TEM, the Na3P phase can still be remained after repeated plating/striping, which is highly attractive for achieving stable Na anode. Benefiting from these advantages, the Na||Na cells with the Na3P artificial layer present a nondendritic morphology for 780h at 1.0 mA cm-2, 1.0 mAh cm-2. In addition, the artificial phosphorus derived protection layer was also applied for dendrite free potassium metal with satisfied performances [71]. More recently, the Yu’s group also proved that the Na2Te artificial interfacial layer showed similar advantages [72]. At 1.0 mA cm-2, 1.0 mAh cm-2, the Na@Na2Te provides excellent cyclic stability for 700h. As the interface with a single component can hardly meet all the requirements of ideal SEI, the Wu et al. and Ji et al. further developed a hybrid Na3P/NaBr interphase with faster ionic conductivity compared with Na3P as shown in Figure 4g [73,74]. The Rui et al. also adopted V2S3 [75], VN [13], VSe [76], VP2 [77] and BiOCl [78] as precursors to build artificial heterogeneous interphase layer. With vanadium and Na3Bi, a more uniform deposition of Na+ is promoted and the better cycling performance is achieved.
The solely Na alloy interphases are also attractive. Due to the low reduction potential of Na, the metal cations dissolved in solvents can spontaneously alloy with Na. For Li metals, Li et al. immersed Li in Mg(TFSI)2 containing electrolyte with the formation of Li-Mg alloy [79]. The pre-alloying with Mg avoids the nucleation of Li at the hot points for dendrite growth and prevents the electrolyte corrosion. As similar system, this approach also apples for Na metal anodes. In Figure 4h, by taking advantage of Sn(TFSI)2, the Na-Sn alloy interphase rich in Na9Sn4 and Na14.7Sn4 can be obtained. Under 0.25 mAh cm-2, the Na||Na cells can physically mitigated dendrite growth for 1700h due to its fast ion transport property. Despite surface alloy, some cations can be reduced as metals, which usually act as nucleation seeds for dendrite suppression [80]. As shown in Figure 4i, Chen’s group use Bi(SO3CF3)3 to treat Na with the formation of Bi. Under 0.5 mA cm-2, the Na-Bi anode can cycle for 1000h without overpotential increase [81]. Analogous AgTFSI, AgCF3SO3 were also achieved with the formation of Ag seeds [82,83]. Notably, these species are also powerful for using as additives for electrolyte modification [39]. More recently, the Yu et al. also alloy Na surface with Ga liquid metal and Sn foil via in suit rolling as shown in Figure 4j-k [84,85,86].
Despite chemical pretreatment methods for inorganic SEI configuration, the physical deposition method is also proposed. Among them, the atomic layer deposition (ALD) technology is most attractive since it has long been used for building advanced protective layers for batteries. Early in 2017, a thin Al2O3 protective layer was achieved on Na through low-temperature plasma-enhanced ALD as shown in Figure 5a-b [87,88,89]. The low temperature ALD can avoid the melting of Na (98 oC) due to its low working temperature of 75 oC. Based on the growth rate of Al2O3, the thickness of 10, 25, and 50 cycles of ALD Al2O3 is confirmed to be 1.4, 3.5, and 7 nm, respectively. Attractively, the Al2O3 can convert into highly conductive NaAlOx during cycling. The Na@Al2O3 displayed an island-like morphology up to 500 cycles even at 3 mA cm-2. Analogue to ALD, the molecular layer deposition (MLD) technology is also proposed for building hybrid inorganic/organic protective layers [90]. The MLD will be discussed in the following hybrid interphase section.
Another type inorganic SEI is designed by using prefabricated free standing films. Typically, these free standing films can improve the surface sodiophilicity with function groups. Peng et al. presented an oxygen-functionalized 3D carbon nanotube film (Of-CNT) in Figure 5c [91]. According to DFT calculation, the oxygen function group has strong interaction with Na+ as shown in Figure 5d, which provides a robust sodiophilic interphase. Benefit from the sodiophilic nature, the Of-CNT offers preferential Na nucleation with a reduced overpotential and improve the reactions kinetics. Similar free standing films also proposed with O, N functioned 3D nanofibers (ONCNFs) [92]. Despite 3D carbon nanofibers, the 2D materials such as MXenes, graphene, silicene, germanene, phosphorene, h-BN, SnS, SnSe and g-C3N4 film have also attracted tremendous attentions [93,94,95]. In order to accelerate surface Na+ transfer and improve the ionic conductivity of the protective layer, the introduction of defect, the increase in bond length, the proximity effect should be serious considered as confirmed by first principles calculations. Meanwhile, their balance with surface stiffness against dendrite suppression is also a critical factor. In this regard, Chen et al. used MXene and carbon nanotubes (CNTs) to construct a 3D MXene/CNTs sodiumphilic layer for rapid Na+ diffusion and dendrite suppression [96]. Li et al. also prepared Sn2+ pillared Ti3C2 MXene [97]. The Sn2+ can act as sodiophilic seeds and form highly conductivity Na15Sn4 alloy to balance the electric field. Tian et al. also reported Mg2+ decorated Ti3C2 MXene as protective layer for Na metal [98]. In addition to these, Wang et al. prepared 3D sodiophilic Ti3C2 MXene@g-C3N4 hetero-interphase in Figure 5e, in which MXene act as the highly conductive substrate and the g-C3N4 act as an interfacial modulation layer to regulate Na+ deposition. As shown in Figure 5f, the Ti3C2 MXene@g-C3N4 hetero-interphase show largest adsorption energy, contributing to form sodiophilic surface [99]. In conclusion, these freestanding protective layers possess tuned electronic properties, strong sodiophilicity and structural robustness.

3.2. Organic Interphase

Apart from the inorganic interphase, organic interphase is also attractive since the precursor can be precisely designed and optimized from molecular level [24,100]. The organic SEI layer is capable of alleviating the volume expansion and preventing dendrite growth due to its excellent flexibility.
Previously, the polar polymers (poly(dimethylsiloxane)(PDMS), polyacrylic acid(PAA), etc.) were proved to be strongly interacted with Li+, which would be effective for regulating uniform distribution of ion flux [102,103,104]. Inspired by these works, Ma’s group prepared a fibrillar poly(1,1-difluoroethylene) (PVDF) fiber film (f-PVDF) with non-through pores by electro-spun. By working as a blocking interlayer for dendrite suppression, the f-PVDF film is superior to the conventional compact PVDF film, PVDF film with through pores, polyethylene oxide (PEO), and polytetrafluoroethylene (PTFE) film. It is noticed that the polar C-F group affinity to Na+ more stronger than C-O groups in PEO, which provides a better environment for uniform Na+ deposition. Meanwhile, the f-PVDF show better electrolyte uptake for faster ion conductivity. More recently, Lu et al. protected Na metal anode by soaking in 1,3-dioxolane (DOL) in Figure 6a. The polar C-O of DOL can break with the formation of poly(DOL), which enables a faster interfacial transport and a lower interfacial resistance. In detail, the polymerization of DOL forms Na alkoxides (CH3OCH2CH2ONa and CH3CH2OCH2ONa) and HCOONa. Then the Na alkoxides transformed into RONa by further reacting with Na. Finally, the RONa and HCOONa in turn reacting with DOL continuously. With protected poly(DOL), a cycling life over 2800h at 1mA cm-2 can be obtained in symmetric cells. Attractively, Lu et al. also proposed spraying DOL for large scale manufacturing [105]. Meanwhile, as shown in Figure 6b, Wei et. al also using imidazolium ionic liquid monomers to prepare ionic membrane through in suit electro-polymerization. The obtained ionic membrane (about 50 nm thick) can regulate the electric filed and stable Na anode [106].
Despite polymer based SEI, the organic Na benzenedithiolate (PhS2Na2) and HCOONa were also reported as shown in Figure 6c-d. In 2020, Wu et al. reported a PhS2Na2-rich protection layer for Na metal. They first chemical treat Na metal with S8 and para-dichlorobenzene (p-DB) in tetrahydrofuran solution, along with the formation of poly(phenylene sulfides) (PPS), NaCl, and Na2Sy. Then it was converted into PhS2Na2 upon cycling. By DFT calculations, they concluded that the function of PhS2Na2 species. Since the binding energy of Na+ in PhS2Na2 (-2.3eV), Ph-S-Na (-2.13eV) are much lower than that of CH3ONa (-2.49eV), CH3OCO2Na (-2.497eV), and Na2CO3 (-3.5eV), a higher ionic conductivity is proved in the PhS2Na2 based SEI [107]. More recently, Zheng et al. treated Na with formic acid vapor via a solid-gas reaction strategy. After 10s, the original silvery white Na surface turned into dark red HCOONa, as confirmed by X-ray diffraction and Raman. Then the organic HCOONa layer can work as a robust interfacial layer with low Na+ diffussion barrier. Additionally, the HCOONa interface can also extend to anode free batteries with format modified collectors [108].
Recently, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have been reported to sever as ionic sieves to control uniform Na+ plating. In 2019, the Chen et al. prepared MOF-199 and ZIF-8 as a coating layer on Cu substrate [109]. By acting as a compact and robust shield, the MOF-199 layer can physically prevent dendrite growth, thus regulating dense Na deposition and less excess SEI formation in Figure 6e. Similar Mg based MOF-74 has also been proposed by Yang et al. They first prove that the main group II metal (Be, Mg, and Ba) can act as nucleation seeds for homogeneous Na deposition. Benefit from these merits, the Mg-based MOF-74 was used to control Na deposition. With eliminated nucleation barriers, a uniform morphology can be obtained [110]. The liquid MOFs of ZIF-62 and have also been proposed to building protective layers for solid batteries [111]. The ZIF-62 interlayer is synthesized from high-temperature monophasic of liquid MOFs. The uniform ZIF-62 layer can increase interfacial sodiophilicity and improve e-/Na+ transport kinetics. More recently, the sp2 carbon COF (sp2c-COF) functioned separator is also built to induce a robust SEI in Figure 6f [16]. The high-polarity architecture shows a good affinity toward Na+, which helps to achieve a uniform ion flux and a nondendritic morphology during plating/stripping [112,113]. So far, reports on applying MOFs and COFs to preventing dendrite growth of Na metal are still limit.

3.3. Hybrid Interphase

To combine the advantages of artificial inorganic SEI and organic SEI, researchers have proposed hybrid organic/inorganic SEI, in which the inorganic components offers sufficient mechanical strength to suppress dendrites and the organic components gives provides a certain flexibility to alleviate the volume expansion. In 2017, Kim et al. presented a free standing inorganic/organic protective layer composed of mechanically robust Al2O3 and flexible PVDF polymer (FCPL). They FPCL has high shear modulus, which is critical for dendrite suppression. Nevertheless, the FCPLs with low ionic conductivity could not enhance cycling stability [114]. In order to further improve the ionic conductivity, Jiao et al. using NaF and PVDF prepared a similar free standing and implantable artificial film (FIAPL) to protect Na [115]. In FIAPL, the organic PVDF film could accommodate the volume expansion and thereby maintain the integrity of the interface, while the inorganic NaF particles can improve ionic conductivity and mechanical strength, resulting in uniform Na nucleation and deposition. The same PVDF/NaF layer was also coated on Cu substrate for Na deposition [116]. Inspired by the PVDF/NaF layer, the Yu et al. further treated Na with PTFE via in suit rolling with the formation of NaF/organic carbon species, which function with C=C and C-F groups, as shown in Figure 7a [117]. They experimentally verified the high mechanical strength, fast ionic kinetics and good sodiophilicity of this protective layer [117,118,119,120]. As reported by Tao et al. the PTFE derived NaF/carbon layer can be rapidly induced by pressure and diglyme-induced defluorination reaction in Figure 7b. It is explained that the diglyme can bonds with Na easily to form chains of O-Na-O, which reacting with PTFE film rapidly. Benefit from these merits, the NaF/organic carbon protective layer shows a long life of 1800h under 3mAh cm-2. They also confirmed similar HnC-O-HnC chain can be obtained by using other solvents [121].
The polymer/metal interphases are also proposed. In 2020, Huang et al. reported a well-designed artificial protective layer consisting of PVDF and Sn by coating on Cu collector. The PVDF film is flexible to accommodate surface expansion, whereas the sodiophilic Sn metal can offer sufficient Na+ ions and high mechanical modulus for dendrite free plating/striping [122]. With the PVDF-Sn protective layer, a high average CE of 99.73% can be obtained for 2800h at 2 mA cm-2. Li et al. also proposed a polyacrylonitrile (PAN) film with thin Sn layer coated on the down side. As shown in Figure 7c, benefit from the low nucleation barrier of Sn seeds, the PAN-Sn protective layer can regulate Na deposition with a controlled location and orientation [123]. More recently in 2022, Li et al. constructed a similar polymer PVDF and metal Bi layer on Cu substrate (PB@Cu). The cyclic voltammetry and galvanostatic discharge curves in Figure 7d-e confirmed the alloying/dealloying of Bi. With Bi metal, the deposition kinetics of Na is increased. At 1 mA cm-2 and 1 mAh cm-2, the PVDF-Bi layer provides a high utilization of Na and a long life time of 2500h in Figure 7f. The superior electrochemical performance of PVDF-Bi layer is revealed to originate from flexible PVDF, which could accommodate severe volume change induced by Na+ plating/stripping. Meanwhile the Bi and/or sodiated Na3Bi can offer high ionic conductivity and sufficient mechanical strength [124].
Different from stiff and dense inorganic ALD coatings, the MLD coatings are confirmed to release volume expansion due to the reduced density and increased flexibility of hybrid organic-inorganic layers [90]. Meanwhile, the hybrid layers provide higher tune ability since the integration of organic bonds in MLD coatings provides attractive chemical/electrochemical, mechanical and electrical performances. As expected, the MLD technologies show great improvements for stabilizing Na metal without dendrite growth. As shown in Figure 8a, in 2017, Zhao et al. used trimethylaluminum and ethylene glycol (Alucone) to introducing an organic-inorganic composite layer on Na anode via MLD at 85 °C. During experimental testing, thickness of 10, 25 and 40 MLD cycles were performed. It is proved that the 25 MLD cycles of AIEG (Na@25Alucone) was optimized for the best. As reported, the SEI on Na@25Alucone showed higher contents of beneficial NaF and Na2O [125]. The MLD technology is also benefit for solid Na batteries. Lately in 2020, Sun et al. also coating the same Alucone via MLD between Na and solid Na3SbS3 and Na3PS4 electrolytes as presented in Figure 8b, in which the Alucone layer work as an interfacial stabilize [126]. As it is confirmed, the type of artificial SEI layer is dependent on the ALD and MLD depositions cycles. If the deposition cycles of ALD and MLD are small, it will form a nano alloy interface; if the deposition cycles of ALD and MLD are large, it will form full monolayers. More recently in 2023, Sun et al. formed nano hybrid interfaces with nano-alloy and nano-laminated structures (from Al2O3 to alucone) through ALD deposited inorganic Al2O3 and MLD deposited organic alucone for alkali metal anodes in Figure 8c [127]. By time-of-flight secondary ion mass spectrometry (TOF-SIMS) in Figure 8d-f, the Na-, Al-, CAL- and AlO2- are probed on the Na surface, which is realized to be robust and chemical/electrochemical stable upon plating/striping. In this study, three types of nano hybrid interfaces are investigated: 1 layer of Al2O3 with 1 layer alucone (1ALD-1MLD); 2 layer of Al2O3 with 2 layer alucone (2ALD-2MLD) and 5 layer of Al2O3 with 5 layer alucone (5ALD-5MLD). At the same time, the total thickness of the nano hybrid interfaces can be controlled by deposited ALD/MLD cycles, mainly including 5, 10 and 25 cycles. For instance, the corresponding samples are donated as (1ALD-1MLD)5; (1ALD-1MLD)10 and (1ALD-1MLD)25. Among all samples, the (1ALD-1MLD)10 alloy interfaces show the best performances at 3 mA cm-2, 1 mAh cm-2 for Na metal. The mossy/dendritic Na growth and “dead” Na formation are effectively suppressed, which would account for the improved performance. Finally, the optimal thickness of the Al2O3-alucone alloy interface for Na metal is 4 nm.

4. Conclusions and Perspectives

In this review, we have summarized recent progress on artificial SEI designing for Na metal anodes. The configuration of advanced artificial SEI layers have been proposed by several approaches, including chemical coating, physical deposition, ex-suit conversion reactions and free standing films. On the basis of experimental understanding, the artificial SEI layers can be precisely designed by optimization the composition, thickness and morphology. The protective layer is either highly conductive or mechanical stiffness/flexible. Benefit from these merits, the protective layers are highly effective in regulating uniform Na+ deposition and suppression dendritic Na formation.
Despite the advantages mentioned above, some deeper challenges still needs to be explored for Na metal: (1) The effect of physical structure, chemical composition, optimization method and Na+ diffusion mechanism on dendrite suppression should be further investigated; (2) the evolution and failure of artificial SEI during plating/striping need to be studied; (3) Advanced characterization technologies are desired to reveal the inner relationship between SEI and Na metal; (4) The design of SEI should meet the practical conditions especially with limit Na excess and lean electrolyte; (5) the formation/growth of dendritic Na and the dynamic evolution of interphase layer need to be fully understand for better SEI configuration.
In a word, artificial SEI layers with high ionic conductivity, high Young’s modulus and mechanical flexible are effective for suppression dendritic Na formation. However, only building artificial SEI is insufficient to solve all the existing issues of Na metal anodes. For these reason, multi-approaches with specific objectives are necessary for promoting the realization of metal Na. We expect this review will promote deeper understanding on the SEI of Na.

Author Contributions

P.C. prepared and revised the raw manuscript. X. W. edited the figures. Y. J. and X. L. revised this manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (Nos.52002083) and the start-up grants from Anhui University (No. S020318031/001).

Data Availability Statement

There is no data support for this review.

Conflicts of Interest

The authors declare no competing financial interest.

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Figure 1. Schematic illustration of artificial interface engineering from the aspects of inorganic, organic and hybrid inorganic/organic protective layers.
Figure 1. Schematic illustration of artificial interface engineering from the aspects of inorganic, organic and hybrid inorganic/organic protective layers.
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Figure 2. Schematic illustration of challenges for Na metal anodes [25]. Copyright 2018, Elsevier.
Figure 2. Schematic illustration of challenges for Na metal anodes [25]. Copyright 2018, Elsevier.
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Figure 3. Schematic illustration of (a) the inorganic-enriched SEI and (b) organic-enriched SEI on Na metal [51]. Copyright 2021, Wiely-VCH.
Figure 3. Schematic illustration of (a) the inorganic-enriched SEI and (b) organic-enriched SEI on Na metal [51]. Copyright 2021, Wiely-VCH.
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Figure 4. (a) Calculated energy barriers for Mg, Na and Li atoms diffusion on the surface with noted chemistry [66]. Copyright 2017, Nature. (b) The function of Mo6S8 formed NaxMo6S8 protective layer [68]. Copyright 2019, American Chemistry Society. (c) The fabrication of Mo2S based protective layer and the corresponding conversion reaction [69]. Copyright 2017, American Chemistry Society. (d) Mechanical fabricated NaNO3 derived Na3N/NaNxOy protective layer. (e) Image of the Na anodes with and without NaNO3 [70]. Copyright 2021, American Chemistry Society. (f) Schematic illustration of red phosphorus formed Na3P layer and the dendrite suppression mechanism [71]. Copyright 2021, Wiely-VCH. (g) The Na3P/NaBr hybrid layer formed form PBr3 [73]. Copyright 2022, Elsevier. (h) Sn(TFSI)2 solution formed Na-Sn alloy protected layer [80]. Copyright 2018, Nature. (i) Photo of Bi(SO3CF3)3 treated Na anode [81]. Copyright 2019, Wiely-VCH. Rolling of Na with (j) liquid Ga [85] and (k) Sn foil [86]. Copyright 2022, Wiely-VCH. Copyright 2021, Elsevier.
Figure 4. (a) Calculated energy barriers for Mg, Na and Li atoms diffusion on the surface with noted chemistry [66]. Copyright 2017, Nature. (b) The function of Mo6S8 formed NaxMo6S8 protective layer [68]. Copyright 2019, American Chemistry Society. (c) The fabrication of Mo2S based protective layer and the corresponding conversion reaction [69]. Copyright 2017, American Chemistry Society. (d) Mechanical fabricated NaNO3 derived Na3N/NaNxOy protective layer. (e) Image of the Na anodes with and without NaNO3 [70]. Copyright 2021, American Chemistry Society. (f) Schematic illustration of red phosphorus formed Na3P layer and the dendrite suppression mechanism [71]. Copyright 2021, Wiely-VCH. (g) The Na3P/NaBr hybrid layer formed form PBr3 [73]. Copyright 2022, Elsevier. (h) Sn(TFSI)2 solution formed Na-Sn alloy protected layer [80]. Copyright 2018, Nature. (i) Photo of Bi(SO3CF3)3 treated Na anode [81]. Copyright 2019, Wiely-VCH. Rolling of Na with (j) liquid Ga [85] and (k) Sn foil [86]. Copyright 2022, Wiely-VCH. Copyright 2021, Elsevier.
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Figure 5. (a) Schematic of the ALD deposition of Al2O3 with TMA and O2 plasma [87]. Copyright 2017, Wiely-VCH. (b) Plating of Na without and with ALD deposited Al2O3 layer [88]. Copyright 2017, Wiely-VCH. (c) Function of Of-CNT network on homogeneous nucleation and smooth Na deposition. (d) Binding energies of Na with various function groups [91]. Copyright 2019, Wiely-VCH. (e) Fabrication process of Ti3C2 MXene@g-C3N4 hetero-interphase for dendrite suppression. (f) Binding energies of Na with Cu, Ti3C2 MXene, g-C3N4 and Ti3C2 MXene@g-C3N4, respectively [99]. Copyright 2022, American Chemistry Society.
Figure 5. (a) Schematic of the ALD deposition of Al2O3 with TMA and O2 plasma [87]. Copyright 2017, Wiely-VCH. (b) Plating of Na without and with ALD deposited Al2O3 layer [88]. Copyright 2017, Wiely-VCH. (c) Function of Of-CNT network on homogeneous nucleation and smooth Na deposition. (d) Binding energies of Na with various function groups [91]. Copyright 2019, Wiely-VCH. (e) Fabrication process of Ti3C2 MXene@g-C3N4 hetero-interphase for dendrite suppression. (f) Binding energies of Na with Cu, Ti3C2 MXene, g-C3N4 and Ti3C2 MXene@g-C3N4, respectively [99]. Copyright 2022, American Chemistry Society.
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Figure 6. (a) Process of preparing poly(DOL) protected Na metal [105]. Copyright 2021, Royal Society of Chemistry. (b) In suit polymerization of imidazolium ionic liquid monomers on Na [106]. Copyright 2017, Wiely-VCH. (c) The fabrication of PhS2Na2-rich protection layer on Na [107]. Copyright 2019, Wiely-VCH. (d) The HCOONa protective layer on Na metal and Cu foil for Na||Na3V2(PO4)3 and HCOONa-Cu|| Na3V2(PO4)3 batteries [108]. Copyright 2023, Wiely-VCH. (e) Deposition of Na morphology on Cu foil without and with MOFs layer [109]. Copyright 2019, Elsevier. (f) Schematic illustration of COF derived artificial layer for dendrite Na suppression [16]. Copyright 2023, Wiely-VCH. .
Figure 6. (a) Process of preparing poly(DOL) protected Na metal [105]. Copyright 2021, Royal Society of Chemistry. (b) In suit polymerization of imidazolium ionic liquid monomers on Na [106]. Copyright 2017, Wiely-VCH. (c) The fabrication of PhS2Na2-rich protection layer on Na [107]. Copyright 2019, Wiely-VCH. (d) The HCOONa protective layer on Na metal and Cu foil for Na||Na3V2(PO4)3 and HCOONa-Cu|| Na3V2(PO4)3 batteries [108]. Copyright 2023, Wiely-VCH. (e) Deposition of Na morphology on Cu foil without and with MOFs layer [109]. Copyright 2019, Elsevier. (f) Schematic illustration of COF derived artificial layer for dendrite Na suppression [16]. Copyright 2023, Wiely-VCH. .
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Figure 7. (a) Preparation of PTFE derived hybrid inorganic NaF and organic carbon layers [117]. Copyright 2023, Wiely-VCH. (b) Schematic of pressure and diglyme-induced defluorination reaction for preparing PTFE derived NaF/carbon layer [121]. Copyright 2023, Wiely-VCH. (c) Schematic illustration of PAN-Sn guiding Na deposition with a controlled location and orientation [123]. Copyright 2020, Wiely-VCH. (d) CV curves and (e) the 1st discharge curves of Na||Cu batteries with bare Cu and PB@Cu. (f) Voltage-time curve of Cu and PB@Cu with Na anode at 1 mA cm-2 and 1 mAh cm-2 [124]. Copyright 2022, Elsevier.
Figure 7. (a) Preparation of PTFE derived hybrid inorganic NaF and organic carbon layers [117]. Copyright 2023, Wiely-VCH. (b) Schematic of pressure and diglyme-induced defluorination reaction for preparing PTFE derived NaF/carbon layer [121]. Copyright 2023, Wiely-VCH. (c) Schematic illustration of PAN-Sn guiding Na deposition with a controlled location and orientation [123]. Copyright 2020, Wiely-VCH. (d) CV curves and (e) the 1st discharge curves of Na||Cu batteries with bare Cu and PB@Cu. (f) Voltage-time curve of Cu and PB@Cu with Na anode at 1 mA cm-2 and 1 mAh cm-2 [124]. Copyright 2022, Elsevier.
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Figure 8. (a) Na plating/striping on bare Na and MLD coated alucone Na [125]. Copyright 2017, American Chemistry Society. (b) Schematic diagram of the interfaces stability of Na and Na@alucone coupled with solid sulfide electrolytes [126]. Copyright 2020, Wiely-VCH. (c) Schematic illustration of the fabrication of the nano-alloy and nano laminated interfacial structures by ALD and MLD deposition. (d) The TOF-SIMS images and depth profiles of Na-, Al-, CAL- and AlO2- ions. (e) The Rutherford backscattering spectrometry and the (f) calculated depth profiles of (1ALD-1MLD)10 alloy interface [127]. Copyright 2023, Wiely-VCH.
Figure 8. (a) Na plating/striping on bare Na and MLD coated alucone Na [125]. Copyright 2017, American Chemistry Society. (b) Schematic diagram of the interfaces stability of Na and Na@alucone coupled with solid sulfide electrolytes [126]. Copyright 2020, Wiely-VCH. (c) Schematic illustration of the fabrication of the nano-alloy and nano laminated interfacial structures by ALD and MLD deposition. (d) The TOF-SIMS images and depth profiles of Na-, Al-, CAL- and AlO2- ions. (e) The Rutherford backscattering spectrometry and the (f) calculated depth profiles of (1ALD-1MLD)10 alloy interface [127]. Copyright 2023, Wiely-VCH.
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