Solvation structure fine-tuning enables high stability sodium metal batteries†

Xiaotong Gao, Jiyuan You, Liwei Deng, Bo Zhang, Yuqian Li* and Wenju Wang*
School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China. E-mail: wangwenju@njust.edu.cn; liyuqian@njust.edu.cn

First published on 28th August 2024

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1. Introduction

Lithium-ion batteries (LIBs) have successfully propelled the development of emerging fields such as electric vehicles and smartphones over more than three decades of commercialization.^1–3 However, due to the scarcity and uneven distribution of lithium resources, as well as the high cost per kilowatt-hour of LIBs, sodium-ion batteries (SIBs) are emerging as a promising alternative.^4–8 Nonetheless, the energy density of carbon-based anode materials in SIBs is relatively low and approaching theoretical limits, necessitating the urgent development of rechargeable batteries with higher energy densities to meet societal demands.^9,10 Sodium metal exhibits a high theoretical capacity of up to 1166 mA h g⁻¹ and a lower electrochemical potential (−2.71 V vs. standard hydrogen electrode). Sodium metal batteries (SMBs), comprising sodium metal anodes and high-capacity cathode materials, exhibit a high theoretical energy density, which has quickly attracted considerable attention.^11–14

In SMB systems, in addition to anode and cathode electrode materials, the electrolyte plays a crucial role as a decisive component.¹⁵ It essentially determines the electrochemical window of batteries based on the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital energy (HOMO) for the thermodynamic stability of reactions. Furthermore, the electrolyte directly participates in constructing the solid electrolyte interface (SEI) film on the sodium metal anode.^16–18 A suitable SEI film is crucial for enhancing battery rate capability and cycle performance.^19–21 However, in actual operation, SEI films on sodium metal surface are unstable and prone to fracture and regeneration during the plating/stripping process of sodium.^13,22 This phenomenon can lead to the eventual depletion of electrolytes and uneven deposition of Na⁺ on electrode surface, resulting in the reproduction of a large number of sodium dendrites. The growth of dendrites will accelerate the depletion of electrolytes, reduce coulombic efficiency, and even puncture separators, causing battery circuits and safety problems.^23–25 Moreover, due to the “hostless” characteristics of metal sodium, sodium metal anodes will undergo a huge volume expansion during cycling. Thus, it is necessary to find appropriate strategies to construct an electrode–electrolyte interface film with high ionic conductivity and strong mechanical properties.

Electrolyte additives, although used in small quantities, play significant roles in stabilizing SEI films, adjusting solvation structures, managing high voltages, and providing flame retardancy, offering significant cost advantages and making them a focal point of recent research.^26–29 Traditionally, researchers typically determine suitable electrolyte additives based on the redox potential window, theoretically ensuring the orderly formation of films by additives, solvents, and electrolyte salts.^30,31 While the aforementioned method theoretically confirms the preferential film formation of ideal additives, their impact on the solvation shell structure of Na⁺ in electrolytes cannot be ignored.³² By strategically fine-tuning or restructuring the solvation structure, not only can solvent molecule degradation be avoided, but SEI films with an appropriate inorganic-to-organic component ratio can be achieved. In addition, additives exhibit varying intrinsic mechanisms in stabilizing liquid electrolytes, depending on the types of functional groups they contain. Currently, due to widespread acceptance of SEI films containing nitrides and fluorides, research on additives containing fluorine and nitrogen is highly active.^33–37 Moreover, an increasing number of researchers are focusing on promoting the effects of sulfur-containing substances on SEI films.³⁸ Hence, there is a need to delve deeper into the effects of specific functional groups in SMBs, including their synergistic interactions with solvation structures and among multiple functional groups.

Herein, a stable SEI film with good ionic conductivity is constructed by introducing a 3 vol% PMS additive into a commercial carbonate electrolyte (abbreviated as EDF), comprising 1 M NaPF₆, ethylene carbonate (EC), diethyl carbonate (DEC), and fluoroethylene carbonate (FEC) (1:1:0.05 vol%). Molecular dynamics (MD) simulations are utilized to investigate how the PMS additive influences the solvation shell structure of Na⁺. In situ optical microscopy experiments demonstrate the significant inhibitory effect of the PMS additive on sodium dendrite formation. Subsequent characterization using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) elucidate the promoting role of enriched inorganic components on SEI films. Furthermore, the PMS additive considerably enhances the rate capability and cycling stability of Na‖NVP full cells, promising favorable applications.

2. Results and discussion

Low cycling efficiency and dendritic deposition of sodium metal are key issues of the short cycle life and poor safety of SMBs.¹⁶ Fig. 1 shows a visual schematic illustrating how electrolyte additive solvation adjustment facilitates uniform Na⁺ deposition. 2-Propyn-1-ol methanesulfonate (PMS) with one alkyne group (CC) and one sulfonyl group (R–S(O)₂–) is considered as a potential film forming stabilizer for sodium metal anodes. Among them, electron delocalization on the sulfonyl group confers inherent oxidation resistance and hydrolysis resistance to the PMS additive, while the alkyne group, acting as a strong electron-withdrawing group, has a potential of being preferentially decomposed into films on the sodium metal anode side.^39,40

To determine the optimal amount of PMS additive, as shown in Fig. 2a, the mean square displacement of electrolytes with different additive concentrations was analyzed by classical MD simulation.⁴¹ One-sixth of the slope of the mean square displacement curve represents the diffusion coefficient of a system. Based on this calculation, diffusion coefficients for EDF, 2 vol% PMS, 3 vol% PMS, and 4 vol% PMS electrolytes are 0.0046, 0.011, 0.022, and 0.0098, respectively. Although an optimal additive concentration obtained by simulation was 3 vol%, the idea of combining theory and experiment was adopted to assess the long cycling performance of Na‖NVP full cells with different additive concentrations in Fig. S1.† Observationally, at 3 vol% PMS concentration, Na‖NVP full cell shows the best specific capacity and capacity retention rate, validating the feasibility of experimental simulation. Furthermore, experiments and simulations indicate that an appropriate amount of additives can significantly enhance battery performance. However, an excess of additives may increase viscosity,⁴² thereby reducing the diffusion coefficient and even affecting the cycling stability of batteries. Above all, an optimal concentration of PMS additive can be determined for further analysis and characterization. Linear sweep voltammetry (LSV) was performed to investigate the effect of PMS additive on the oxidation stability of electrolytes (Fig. 2b). For the EDF electrolyte, the oxidation current increases significantly after 3.8 V, which is attributed to the oxidative decomposition of EDF electrolyte. For the EDF/PMS electrolyte, adding PMS effectively extends the electrochemical window to 4 V. Looking further at the illustration in Fig. 2b, EDF/PMS electrolyte has an additional oxidation peak at 4.17 V and a stable plateau (4.17–4.38 V). LSV results indicate that PMS additives can effectively inhibit electrolyte decomposition at high voltage.⁴³ As shown in Fig. 2c, Na‖Cu cells were assembled to study the effect of PMS additive on sodium deposition/stripping behaviour on Cu current collectors.⁴⁴ The average coulombic efficiency of Na‖Cu cells in EDF and EDF/PMS electrolytes is 69% and 89.55%, respectively. In particular, the eighth-cycle coulombic efficiency of Na‖Cu cell in EDF electrolyte is dramatically reduced, only 29.69%, which is attributed to rapid growth and shedding of sodium dendrites due to non-uniform deposition of Na⁺ on Cu foil. Sodium stripping “dead sodium” causes considerably irreversible capacity loss and reduces plating/stripping efficiency. However, increased CE suggests PMS additive can considerably improve sodium plating/stripping behaviour, effectively suppressing sodium dendrite growth. To validate the aforementioned rational prediction that PMS additive can serve as a preferred anode film-forming additive, GaussView 5.0 was used to calculate frontier molecular orbital energies of solvent molecules by DFT.⁴⁵ Fig. 2d displays HOMO and LUMO energies of solvents and additive, where PMS has the lowest LUMO energy (−0.37 eV). According to frontier molecular orbital theory, the stability of a solvent in an electrolyte is primarily determined by its ability to gain and lose electrons. A higher HOMO energy indicates poorer electron stability and lower antioxidant capacity in the system. A higher LUMO energy suggests weaker electron affinity, indicating better resistance to reduction.⁴⁶ Thus, it can be theorized that PMS additive can preferentially react on the sodium metal anode surface to form SEI films, which may be related to its strong electron-withdrawing alkyne group. MD simulations elucidate changes in the solvation structure of electrolytes upon the addition of the PMS additive. Fig. 2e and f present final state model diagrams from simulations of EDF and EDF/PMS electrolyte systems, showing more uniform molecular distribution after the addition of the PMS additive. Radial distribution function of molecular distribution near Na⁺ was analyzed to examine the first solvation shell structures in two electrolytes (Fig. 2g and h). Both solvent molecules (EC, DEC, FEC) and anions participate in the solvation shell structure of Na⁺; anions exist at 2.125 Å away from Na⁺, and solvent molecules mainly exist at 2.375 Å away from Na⁺. Furthermore, it is necessary to study the coordination number as a key parameter to describe the geometric arrangement of solvent molecules.⁴⁷ In EDF electrolyte, the coordination numbers of Na⁺-O=(EC), Na⁺-O=(DEC), Na⁺–O(FEC) and Na⁺–F(PF₆⁻) are 1.68, 1.03, 0.17, and 1.51, respectively, indicating the solvation structure dominated by EC and PF₆⁻ (Fig. 2g). In EDF/PMS electrolyte, it can be observed that the addition of PMS additive did not substantially alter the original solvation structure of EDF electrolyte (Fig. 2h). Interestingly, the corresponding coordination numbers of Na⁺–O(EC), Na⁺–O(DEC), Na⁺–O(FEC), and Na⁺–F(PF₆⁻) changed to 1.69, 0.98, 0.22, and 1.50. Significant changes in the coordination numbers of FEC and DEC indicate that while PMS additive does not directly participate in the solvation shell structure of Na⁺, it can induce more FEC to participate and hinder DEC from participating. Fluorinated functional groups (C–F) in FEC contribute to forming SEI films composed of NaF while synergistically enhancing interfacial stability of SEI films with PMS anode-preferred film-forming additive.

Despite the interaction dynamics becoming quite complex within a mixture of multiple groups, the effects of additives containing unique atoms can still be identified.⁴⁸ Fig. S2† calculates adsorption energies and bond length of Na⁺ with solvent molecules and anions by DFT. Calculation results indicate that Na⁺ and PF₆⁻ exhibit significant adsorption properties. Weaker binding energy and longer bond length between Na⁺ and PMS further support the above phenomenon that Na⁺ and PMS form a weak solvation structure. To further explore the theoretical feasibility of the aforementioned phenomenon, adsorption energies between PMS and EC, DEC, FEC, and PF₆⁻ are calculated by DFT. As shown in Fig. 2i, compared to EC and FEC, PMS exhibits stronger adsorption with DEC and PF₆⁻. It can be inferred that during the process of solvent molecules and anions participating in solvation, DEC and PF₆⁻ are most hindered by the molecular attraction of PMS outside the solvation layer, thereby promoting solvation behavior of FEC. More FEC contributes to formation of a flat and dense SEI film and synergizes with preferential film-forming additives (PMS) to enhance the electrochemical performance of SMBs.

Visualization of sodium dendrites can be realized by means of in situ optical experiments.⁴⁹ To visually assess the regulatory effect of electrolytes on sodium deposition, as shown in Fig. 3, Na‖Na symmetric cells were subjected to in situ optical tests at 10 mA cm⁻², 20 mA cm⁻², and 50 mA cm⁻² for 60 seconds each. Removal of separator hindrance between two metal sodium electrodes significantly amplified the growth/stripping phenomenon of sodium dendrites.

Fig. 3a and b illustrate the dendrite growth/stripping behavior of sodium metal surface in symmetric cells with EDF and EDF/PMS electrolytes, respectively. See attached Videos 1 and 2† for corresponding video files. In EDF electrolyte, significant sodium plating is observed when the symmetrical cell is charged to 60 s at a constant current density of 10 mA cm⁻². As current density continues to increase, irregular deposition of sodium ions in electrolyte leads to onset of sodium dendrites, accompanied by generation of “dead sodium.” Additionally, due to “tip effect,” sodium dendrites tend to grow increasingly localized. Notably, when the symmetrical cell is charged to 100 s at a constant current density of 50 mA cm⁻², “dead sodium” produced by sodium dendrite stripping rapidly diffuses into the whole electrolyte (Fig. S3†). In contrast, in EDF/PMS electrolyte, sodium metal anode maintained a mostly smooth surface at different current densities, with no observable sodium dendrites (Fig. 3b). Only a very thin and indistinct sodium plating was present, indicating PMS additive contributes to the formation of an electrostatic protective layer, facilitating uniform sodium deposition.

Unstable SEI film, growth of sodium dendrites and relatively large volume expansion are huge bottlenecks in the safety and stability of SMBs.^50–52 To further visualize the surface morphology of SEI films, scanning electron microscopy (SEM) images of Na metal anodes after 80 cycles in Na‖NVP full cells with two different electrolytes are shown in Fig. 4a–c and 4d–f, respectively. Under same test conditions, the surface morphology of SEI film is significantly flatter and denser with PMS additive. As depicted in Fig. 4a–c, the sodium metal surface in EDF electrolyte appears to have numerous pores, noticeable cracks and electrode powdering. These phenomena are closely related to weak SEI films, uneven deposition of Na⁺, and volume expansion of sodium metal. Specifically, during sodium plating and stripping, SEI films are prone to breaking and regenerating, causing uneven Na⁺ deposition and intensifying the growth and proliferation of sodium dendrites through “tip effect.” The rapid growth of dendrites will accelerate electrolyte depletion, reduce coulombic efficiency, and even puncture separators, resulting in battery short circuit and causing safety problems.^53,54 Interestingly, the introduction of PMS additive significantly mitigated the occurrence of aforementioned issues (Fig. 4d–f). A dense and flat surface morphology contributes to achieving uniform electric field distribution on the electrode surface, promoting uniform deposition of Na⁺ and reducing side reactions between electrolytes and sodium metal, thereby significantly enhancing the electrochemical performance of batteries.⁵⁵ Additionally, energy dispersive spectrometer (EDS) mapping of SEI films from EDF-80r and EDF/PMS-80r were used to analyze the distribution of Na, P, F, and S elements (Fig. 4g, h, and S4†). In comparison, the introduction of PMS additive was found to enhance the uniform distribution of Na, F, P, and S elements on SEI film surface. Based on surface morphology and elemental distribution analysis, it can be concluded that PMS additive effectively participates in the film formation process of sodium metal anode, leading to a smoother and flatter surface, thereby facilitating uniform deposition of Na⁺.

To further investigate the relationship between chemical composition of SEI films and sodium metal plating/stripping, Fig. 5 presents X-ray photoelectron spectroscopy (XPS) of sodium metal surface after 80 cycles in EDF and EDF/PMS electrolytes. The XPS full spectrum is shown in Fig. S5.† Compared to EDF electrolyte, the presence of S element was detected in SEI film in EDF/PMS electrolyte, indicating PMS additive was successfully involved in the construction of SEI film. As displayed in Fig. S6,† SEI films formed by EDF and EDF/PMS electrolytes have similar C 1s spectrum, in which the binding energy peaks of 284.8 eV (calibration peak) belong to C–C, and the binding energy peaks of ∼286.4 eV, ∼288.4 eV, and ∼289.2 eV belong to C–O, ROCO₂Na, and Na₂CO₃, respectively.⁵⁶ High-resolution P 2p spectrum in Fig. 5a detected the presence of Na_xPF_y and Na_xPF_yO_z in two electrolytes, indicating decomposition of sodium salts (NaPF₆).⁵⁷ For S 2P spectrum in Fig. 5b, due to the introduction of sulfonyl-containing PMS additive, five sulfur-containing substances were detected in EDF/PMS electrolyte, specifically labeled as Na₂S (∼161.7 eV), Na₂SO₃ (∼166.4 eV), Na₂SO₄ (∼168.6 eV), C–S (∼169.4 eV), and S–F (∼173.2 eV).⁵⁸ In contrast to organic components, a richer content of inorganic components not only enhances the ionic conductivity of SEI films but also boosts its mechanical strength to withstand unstable volume changes of sodium metal.⁵⁹ In Na 1s spectrum of EDF/PMS electrolyte, a higher intensity NaF peak (∼1071.5 eV) compared to EDF electrolyte was observed in Fig. 5c, primarily due to decomposition of more FEC involved in the solvation shell structure. NaF can synergize with sulfides to enhance the strength and stability of SEI films, further inhibiting side reactions between solvent molecules and sodium metal.⁶⁰ In O 1s spectrum (Fig. 5d), Na₂SO₄ were detected, further confirming that PMS participated in forming SEI film on sodium metal surface. Additionally, an O 1s spectrum dominated by Na₂SO₄ peak (∼532.3 eV) was observed in EDF/PMS electrolyte.⁶¹ Compared to Na₂CO₃, Na₂SO₄ exhibits higher thermal stability, which is more conducive to uniform deposition of sodium on SEI film surface.

In combination with in situ optical experiments and SEM surface morphology analysis, as well as XPS elemental composition analysis of sodium metal anodes after cycling, further verification was provided regarding the significant inhibitory effect of SEI films rich in NaF and sulfides on sodium dendrites during cycling. Furthermore, additional observations were made on the surface morphology of SEI film after 1000 cycles in Na‖NVP full cell with EDF/PMS electrolyte, aiming to investigate the stability of SEI film rich in abundant inorganic components during ultra-long cycling processes (Fig. 5e–g). After adding PMS additive, SEI films maintained a smooth and dense surface morphology even after 1000 cycles (Fig. S7 and S8†), indicating that incorporating NaF, which has good flexibility and mechanical stability, and “sodiophilic” sulfur-containing substances such as Na₂S into SEI films can better accommodate the volume expansion of sodium metal and promote the initial nucleation of sodium, significantly inhibiting dendrite growth.⁶²

A comprehensive comparison of the battery performance of Na‖Na symmetric cells and Na‖NVP full cells in two different electrolytes provides a more intuitive evaluation of the film-forming effectiveness of PMS additive. Sodium plating/stripping stability is a critical factor that influences battery electrochemical performance. The reversibility of sodium plating/stripping was studied using Na‖Na symmetric cells in two different electrolytes, as depicted in Fig. 6a. Results show that in EDF/PMS electrolyte, the cycling stability of Na‖Na symmetric cells improved significantly to 100 hours, indicating a pronounced promotion effect of PMS on sodium plating/stripping. A comparison of voltage interpolation plots from 30 to 42 hours showed the rapid occurrence of soft short circuiting in cells with EDF electrolyte. Further examination of plots from 83 to 88 hours demonstrated a lower polarization in cells using EDF/PMS electrolyte, consistent with in situ experimental findings. Fig. 6b–d depict the rate capability of Na‖NVP full cells with EDF and EDF/PMS electrolytes. Na‖NVP full cells with EDF/PMS electrolyte exhibit markedly improved rate stability when comparing the charge and discharge curves for the first three cycles at different rates (Fig. 6b and c). At a low rate, both exhibit similar specific capacities (∼110 mA h g⁻¹). However, with increasing rate, EDF electrolyte demonstrates greater capacity decay. In contrast, even at a high rate of 15C, EDF/PMS electrolyte still maintains a specific capacity of 107 mA h g⁻¹, attributed to its SEI film rich in inorganic components, ensuring excellent ion conductivity.

To further verify the effect of PMS additive on the cycling stability of Na‖NVP full cells, long-term charge and discharge tests in the voltage range of 2.6 V to 3.8 V were conducted, as illustrated in Fig. 6e. Test results indicate that the coulombic efficiency of full cells with EDF electrolyte sharply decreases at the 300th cycle, with specific capacity decaying to 89% at the 450th cycle, attributed to the rapid growth and proliferation of sodium dendrites during cycling, which leads to formation of “dead sodium” and causes severe irreversible capacity loss and reduced coulombic efficiency. In contrast, Na‖NVP full cells with EDF/PMS electrolyte maintained a high-capacity retention of 92% even after 1000 cycles, with an exceptionally high average CE of 99.9%, attributed to the formation of a stable SEI film through PMS decomposition. Full cell charge–discharge curves at different cycles further visually confirm a significant improvement in cycling stability due to PMS (Fig. S9†). Comprehensive electrochemical characterizations and tests indicate that anode preferential film forming stabilizer (PMS) actively induces FEC solvent behavior, significantly enhancing the rate capability and cycling stability of Na‖NVP full cells, promising favorable applications.

3. Conclusion

In summary, this work successfully constructed a uniform and dense SEI film on sodium metal anode surface by introducing a small amount of PMS additive. Analysis based on density functional theory of reaction barriers and solvation structures revealed that PMS preferentially undergoes reduction decomposition on sodium metal anodes and actively induces FEC solvation behavior, thereby forming a stable SEI film enriched with sulfide compounds and NaF. In situ optical microscopy experiments and SEM observations further confirmed that PMS additive promotes the uniform deposition of Na⁺ and significantly inhibits the growth of sodium dendrites. Ultimately, Na‖NVP full cells exhibited an ultra-high average CE of 99.9% and a capacity retention of 92% after 1000 cycles at 2C. This cost-effective and minimally additive strategy notably improves the electrochemical performance of batteries, presenting a strong option for advancing battery technology.

Data availability

Data will be made available on request.

Author contributions

Xiaotong Gao: writing – review & editing, writing – original draft, visualization, validation, software, resources, methodology, investigation, formal analysis, data curation, conceptualization. Jiyuan You: writing – review & editing. Liwei Deng: methodology. Bo Zhang: writing – review & editing. Yuqian Li: methodology, funding acquisition. Wenju Wang: supervision, resources, funding acquisition.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (No. 21676148), the Fundamental Research Funds for the Central Universities (No. 30918012202), and the National Natural Science Foundation of Jiangsu Province (No. SBK2023044488).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta04755a

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