【Background】
Sodium-ion batteries have attracted extensive attention due to their abundant sodium resources and similar working mechanism to lithium-ion batteries. However, the relatively large ionic radius of Na+ leads to the problem of slow diffusion kinetics and significant changes in the volume of the electrode material, which leads to the unstable rate performance and cycling performance of sodium-ion batteries. The electrode-electrolyte interface plays a key role in the diffusion of Na+ and the stability of the electrode material. However, due to the high solubility of key components of the interface (such as NaF) in the electrolyte, the interface instability of sodium-ion batteries will be caused. Therefore, there is an urgent need to explore an effective strategy to construct a stable interface layer on the surface of both the cathode and anode to improve the electrochemical performance of sodium-ion batteries. Electrolyte design is an effective strategy for constructing a stable inorganic-rich interface, including high-concentration electrolyte, local high-concentration electrolyte, perfluorinated electrolyte, etc. However, the high cost of these electrolyte systems limits their practical application. Recently, sulfur-containing interfaces with good Na+ transmission rates and stability have been considered as a promising strategy for constructing high-performance sodium-ion batteries.
【Job Introduction】
Recently, Professor Shulei Yu and Lin Li Distinguished Professor of Wenzhou University and Professor Ronghua Zeng of South China Normal University published a research paper entitled "Regulation of Anion-Na+ Coordination Chemistry in Electrolyte Solvates for Low Temperature Sodium-Ion Batteries" in the journal ACS Energy Letters. In this paper, sulfolane (SL) with double S=O double bond was used as a sulfur-containing additive to adjust the solvation structure of 1.0 M NaPF6-EC/EMC electrolyte, and to construct an additive-derived sulfur-rich inorganic interface chemistry. SL has a strong coordination ability, with a coordination number as high as 0.79, which can effectively weaken the interaction between Na+ and solvents. This unique solvation structure constructs a sulfur-rich inorganic interface on the Prussian blue (PB) cathode and hard carbon (HC) anode, which also accelerates the desolvation process of Na+ at the electrode interface and ensures HC||Excellent electrochemical performance of PB whole cells and pouch cells. This work highlights how additive-induced solvation structures and inorganic-rich interfacial chemistry can be used to achieve advanced sodium-ion battery applications.
【Content Description】
Sulfur-containing additives mainly include sulfonates, sulfates, sulfites and sulfones. According to the literature, sulfur-containing additives (sulfonates, sulfates, sulfites) containing S-O single bonds are more likely to preferentially decompose into sulfur-containing organic compounds (Figure 1a). In this paper, we introduce the simplest and most representative sulfone additive, cyclobutanol (SL), into a commercial ester-based electrolyte (1.0 M NaPF6-EC/EMC, 1:1 by volume ratio, NEE electrolyte) to investigate its effect on the solvation structure and interface. Based on the front-line molecular orbital theory, density functional theory (DFT) predicts the redox stability of salts, solvents, and additives, as shown in Figure 1. Compared with commonly used carbonate solvents, SL exhibits higher energy levels of the highest occupied molecular orbital (HOMO, -7.79 eV) and lower lowest unoccupied molecular orbital (LUMO, 0.42 eV), indicating that it preferentially decomposes to form sulfur-containing interfaces. In addition, according to the electrostatic potential distribution (ESP) plot in Figure 1c, SL additive has a strong positive and negative potential and therefore easily binds to Na+ and PF6-. The intermolecular interactions between Na+-solvent/additive and PF6--solvent/additive were calculated by DFT simulation. As shown in Figure 1d, Na+-SL has the lowest electron affinity, highlighting the reducing power of SL additives. In Figure 1e, the redox potential of PF6--SL is lower than that of PF6--EC and PF6--EMC (3.76 V vs. 4.57 V and 4.07 V, respectively), indicating a preferential decomposition reaction of PF6--SL clusters.
图1. 电解液设计。 （a）含硫添加剂的界面化学示意图。 （b）NaPF6盐和常用溶剂/添加剂和SL的HOMO和LUMO能级。 （c）溶剂和添加剂的ESP密度分布。 （d）Na+-EC、Na+-EMC 和 Na+-SL 团簇的电子亲和能。 （e）PF6--EC、PF6--EMC 和 PF6--SL 团簇的氧化还原电位。
We further investigated the effect of SL additives on the electrolyte, especially in relation to the solvated shell within Na+. In Figure 2a, the proportion of coordination PF6- decreased dramatically, with contact ion pairing (CIPs, PF6- coordination with a single Na+) decreasing from 42.3% to 30.0%, and aggregates (AGGs, PF6- coordinating with two or more Na+) decreasing from 20.7% to 16.5%. In addition, the free PF6- increased from 37.0% to 53.5% and was dominant in the solvation configuration, indicating that the anion-cation interaction was weakened and the number of anions occupying the internal solvated shell was reduced. In Figure 2b, more free EC molecules and fewer coordination EC molecules were observed in the NEE+10S electrolyte, which corresponded to the reduced coordination capacity of the EC after the addition of SL supplement. The nuclear magnetic resonance (NMR) spectra in Figure 2c provide more information about the evolution of the Na+ solvation structure. Upon the addition of SL, the 23Na signal shifts slightly downward, which is due to the weakened anion binding or solvent binding, and the Na+ is shielded by the surrounding electron cloud. These have strongly demonstrated the important influence of SL additives on the composition of Na+ solvation structure.
Classical molecular dynamics (MD) simulations reveal the solvation configuration of the electrolyte at the molecular level. Figures 2d and 2e show the radial distribution function (RDF) and the corresponding coordination numbers for anions, solvents, and additives. The distance between Na+ and SL was the smallest, 0.242 nm, while the distance between Na+-anion and Na+- solvent was 0.254 nm and 0.244 nm, respectively, which verified that SL additives were widely used in the inner solvated shell. Figures 2f and 2g show structural snapshots of the NEE and NEE+10S electrolytes. More importantly, as shown in Figure 2h, the coordination numbers of anions and solvents with Na+ showed a significant downward trend after the introduction of SL additives, with the coordination numbers of PF6-, EC and EMC decreasing from 0.60 to 0.48, 2.78 to 2.55 and 2.01 to 1.66, respectively. This is consistent with the analytical results of Raman spectroscopy and NMR spectroscopy. The coordination number of SL with Na+ is as high as 0.79, which indicates that SL is more likely to occupy the internal solvated shell structure.
图2. 溶剂化结构。 （a）自由PF6-，CIP和AGG的占比。 NEE和NEE+10S电解液的（b）Raman图谱和（c）23Na-NMR图谱。 （d）NEE和（e）NEE+10S电解液的径向分布函数和配位数。 通过 MD 模拟计算得出的 (f) NEE和(g) NEE+10S电解液的溶剂化结构快照。 (h) Na+ 内溶剂化壳层中阴离子/溶剂/添加剂的配位数。
The solvation configuration has a very important influence on the interface properties, such as the composition of the interface layer, the desolvation energy, and the interfacial kinetics. On the basis of MD simulations, representative solvation structures, namely Na(EC)3(EMC)2 and Na(EC)5(EMC)1 from NEE electrolyte and Na(EC)3(EMC)1(SL)1 and Na(EC)4(EMC)1(SL)1 from NEE+10S electrolyte, were selected for further theoretical calculations. As shown in Figure 3a, a lower LUMO energy level for the solvation configuration was achieved by replacing the EMC/EC solvent with SL. This suggests that SL enters the internal solvated shell making it easier to reduce to a solid electrolyte interface (SEI). According to the ESP distribution shown in Figure 3b, the SL-involved solute exhibits a tendency to be more positively charged, which greatly accelerates the migration rate of Na+. In addition, Figure 3c compares the desolvation energies of separating Na+ from various solvations. The desolvation energy was lower when the coordinated EMC or EC was replaced with SL, indicating that the desolvation process was accelerated with the introduction of SL additives. In Figure 3e, the activation energy in the electrolyte with SL additive is relatively small, at 1.98 kJ mol-1, compared to the 2.87 kJ mol-1 in the NEE electrolyte. Also, the Tafel plot of Figure 3f. The exchange current density in the NEE+10S electrolyte is larger, indicating that the interfacial dynamics of the electrode-electrolyte interface are faster. These results demonstrate the superiority of SL in reducing the desolvation energy barrier and enhancing the interfacial dynamics.
Figure 3. Interfacial dynamics. (a) HOMO/LUMO energy levels, (b) ESP density profiles, and (c) desolvation energy of representative solvation configurations. (d) Temperature-dependent impedance between 263-343 K and (e) desolvation energy. (f) Tafel diagram.
The large difference between the solvation structure and interface dynamics of the electrolyte has inspired us to further investigate the mechanism of the electrode-electrolyte interface, which is highly correlated with the solvation structure and electrochemical performance. Using transmission electron microscopy (TEM) images, the surface of the PB electrode after circulating with NEE electrolyte appears a thick and non-uniform interface layer in Figure 4a. In contrast, in the electrolyte containing SL additives, the interfacial layer is thin and homogeneous, with a thickness of about 19.1 nm, which confirms that the designed NEE+10S electrolyte inhibits electrolyte decomposition (Fig. 4b). X-ray photoelectron spectroscopy (XPS) further investigated the surface chemistry of the PB cathode. In Figures 4c and 4d, with the introduction of SL additives, the S content increased from 0.37% to 0.69%, and the circulating PB cathode interface layer in the NEE+10S electrolyte contained a large amount of sulfides, such as Na2SO4 (169.3 eV), Na2SO3 (168.2 eV), and Na2S (164.1 eV). From Figures 4c, 4e, and 4f, it is observed that the NEE+10S electrolyte contains more phosphide and fluoride in the interfacial layer, which is related to the deep decomposition of PF6-anion. Due to the presence of NaF, Na2S and Na2SOx in the cathode interface layer, the increase in ionophore concentration enables accelerated Na+ transport. These results suggest that the chemical structure of the sulfur-rich inorganic interfacial structure was constructed by introducing SL as an additive.
Scanning electron microscopy (SEM) and X-ray diffraction (XRD) further evaluated the superiority of the addition of SL in inhibiting structural changes in PB materials. After 100 cycles in the NEE electrolyte, a wide range of microcracks appeared on the surface of the PB cathode, indicating that the electrode material has undergone severe structural changes under the action of a corrosive decomposition reaction (Fig. 4g). In stark contrast, the surface of the PB cathode was smooth and crack-free with the addition of SL additive (Fig. 4h). At the same time, the XRD pattern in Figure 4i characterizes the structural stability of the PB after cycling. In the NEE+10S electrolyte, the characteristic peaks of PB were well maintained, while there was a significant shift in the blank electrolyte, which verified the advantages of SL additives in inhibiting the volume change of the electrode material.
Figure 4. Electrode-electrolyte interface. Transmission electron microscopy images of PB electrodes circulating in (a) NEE and (b) NEE+10S electrolytes. (c) Proportion of elements at the CEI interface on the PB cathode surface after 100 cycles. XPS spectra of (d) S2p, (e) P2p, and (f) F1s. SEM images of the PB cathode after circulating in (g) NEE and (h) NEE+10S electrolytes. (i) Post-cycling XRD spectra.
To demonstrate the effect of SL additive-induced solvation structure and stable inorganic-rich interface layer on electrochemical performance, we assembled a full cell consisting of an HC anode and a PB cathode, as shown in Figure 5a. As shown in Figure 5b, HC||| using NEE+10S electrolyteThe rate performance of the PB battery is better, with a capacity of 123.4 to 99.8 mAh g-1 when the current density is increased from 0.1 C to 2.0 C, while the battery capacity with NEE electrolyte is 114.2 and 80.2 mAh g-1, respectively. When the current density is restored to 0.1 C, the capacity retention rate after the introduction of SL additive is 91.4%, and the capacity is up to 112.8 mAh g-1, which is much higher than the 86.1% in the NEE electrolyte (98.3 mAh g-1) In Figures 5c and 5d, the whole cell using the NEE+10S electrolyte can achieve stable long-cycle performance. Added HC||for SLThe PB full battery shows a high discharge capacity of 133.0 mAh g-1 with an initial Coulombic Efficiency (CE) of 80.3% and can still maintain a capacity of 108.5 mAh g-1 after 100 cycles, with a capacity retention rate of 88.6%. In stark contrast, the blank NEE electrolyte has an initial capacity of 130.4 mAh g-1 with an initial CE value as low as 79.3% and a volume retention rate of 75.0% with a capacity of 88.4 mAh g-1 after 100 cycles. This is attributed to the construction of a stable sulfur-rich inorganic interface layer on the cathode and anode sides with the help of SL, which is beneficial for inhibiting solvent decomposition and speeding up Na+ transport.
To further validate the functionality of the additive-induced interface layer, Figure 5e shows a cell impedance map analysis using different electrolytes during cycling. The charge transfer impedance (Rf) value of the battery containing SL additives is smaller, such as 746.9 Ω NEE after 5 cycles, while NEE+10S is 378.1 Ω, reflecting that the products of SL decomposition have a great influence on the resistance. It is worth noting that even after 100 cycles, the Rf value in the NEE+10S electrolyte is small at 145.3 Ω, which is due to the construction of a sulfur-rich inorganic interface layer during long-term cycling. At the same time, the HC||, NEE+10S electrolyte is usedThe PB pouch cell maintained an ideal capacity of 309.5 mAh and a capacity retention rate of 78.3% at 500 cycles, demonstrating the potential of optimized electrolytes for large-scale production development (Figure 5f). These results highlight the importance of sulfur-rich inorganic interface chemicals formed by SL to improve the performance of sodium-ion whole batteries.
Figure 5. Electrochemical properties. （a）HC|||Schematic diagram of a PB sodium-ion battery. (b) Rate performance. （c）HC||Charge-discharge voltage curve and (d) long-term cycling performance of a PB coin cell battery. (e) Impedance diagram. （f）HC||Long cycle performance of PB coin cell batteries.
【Conclusion】
In conclusion, we propose a sulfur-containing additive SL with double S=O double bonds and no S-O single bonds for the construction of stable inorganic-rich interfaces in commonly used ester-based electrolytes. SL is largely involved in the internal solvation shell of Na+, weakening the coordination of Na+ with the solvent. The sulfur-rich interface derived from the additives facilitated the migration of Na+ and enhanced the interfacial kinetic rate. As expected, HC||, containing 10wt% SLPB full battery exhibits stable cycling performance. This work highlights the impact of molecular design on the interface layer and provides forward-looking guidance for sulfur-rich inorganic interface chemistry in advanced sodium-ion batteries.
Wenxi Kuang, Xunzhu Zhou, Ziqiang Fan, Xiaomin Chen, Zhuo Yang, Jian Chen, Xiaoyan Shi, Lin Li*, Ronghua Zeng*, Jia-Zhao Wang, Shulei Chou*. Sulfur-Containing Inorganic-Rich Interfacial Chemistry Empowers Advanced Sodium-Ion Full Batteries. ACS Energy Lett. 2024.
https://doi.org/10.1021/acsenergylett.4c01445
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