Mengying
Ma‡
a,
Binbin
Chen‡
acd and
Huilin
Pan
*ab
aDepartment of Chemistry, Zhejiang University, Hangzhou, 310012, China. E-mail: panhuilin@zju.edu.cn
bState Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310012, China
cInstitute of Bioengineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027, China
dZJU-Hangzhou Global Scientific and Technological Innovation Center, Hangzhou, 311200, China
First published on 16th May 2023
Unlike solid materials, the molecular structure and chemical distribution in electrolyte solutions have been considered in isotropic states. Herein, we reveal controllable regulation of solution structures in electrolytes by manipulating solvent interactions for Na-ion batteries. Low-solvation fluorocarbons as diluents in concentrated phosphate electrolytes induce adjustable heterogeneity in electrolyte structures through variable intermolecular forces between high-solvation phosphate and diluents. An optimal trifluorotoluene (PhCF3) diluent weakens the solvation strength around Na+ and spontaneously leads to a locally enlarged Na+ concentration and global 3D continuous Na+ transport path thanks to the appropriate electrolyte heterogeneity. Besides, strong correlations between the solvation structure and the Na+ storage performance and interphases are demonstrated. PhCF3 diluted concentrated electrolyte enables superior operations of Na-ion batteries at both room temperature and a high temperature of 60 °C. A hard carbon anode exhibits a reversible capacity of 300 mA h g−1 at 0.2C and excellent life over 1200 cycles without decay.
The reversed salt/solvent ratios in HCEs and DHCEs bring dramatic changes in bulk liquid structures, ion transport, and interfacial and interphasial properties. Unusual ion transport phenomena, e.g., cation hopping and solvent/ligand-assisted cation transport have been reported in HCEs or DHCEs.17–19 Besides, the anion-involved solvation structures of HCE and DHCE might facilitate the formation of anion-derived solid electrolyte interphases (SEIs) to mitigate parasitic reactions in cells.20–22 Recent theoretical studies revealed that the diluents in DHCEs could regulate the solution network and generate structural heterogeneity in electrolytes.23,24 DHCEs even exhibit improved ionic conductivity and higher cation transfer numbers than “standard” electrolytes and HCEs.25–27 However, the general strategies for developing high-performance DHCEs still rely on empirical concepts (a typical mixture of low-solvation fluorinated solvents) instead of a rigorous molecular-level understanding of the interaction mechanisms of electrolyte components. Undoubtedly, a deep understanding of the roles of diluents and the correlations between specific electrolyte solvation structures and the physiochemical properties of electrolytes and cell performances should be dedicatedly explored to guide the electrolyte design.
Recent studies have indicated that diluents could influence the inner solvation structure through different types of weak interactions.28–31 It is found that diluent molecules could result in a low-dielectric environment or through solvent-diluent dipole–dipole interactions to regulate the cation–solvent and cation–anion interactions.28,30 Nevertheless, previous investigations have mainly focused on the internal solvation structure of cations in electrolytes while the external solvation structure and long-range electrolyte structure are rarely discussed. A previous study demonstrated that the solvent reorganization from optimal intermolecular interactions reinforced the long-range solution network and thus enabled the improved performance of Na-ion batteries.32 Currently, fundamental understandings of solvation structures are largely lacking for Na-ion batteries, which restricts the development of high-performance Na-ion batteries. In this work, we demonstrate the effective modification of internal and external solvation structures via distinctive solvent–solvent interactions between solvating solvents and diluent solvents. Such solvent–solvent interactions induced by diluents delicately regulate the global solution structures of electrolytes and consequently optimize global Na+ transport flux homogeneity. In addition, an optimal diluent weakens the coordination capability of solvating solvents and anions in the solvation structure, thus regulating the electrochemical stability of electrolytes and interphasial properties. The degree of influence depends on the type of diluents.
The MD simulation results show that the participation of fluorocarbons regulates the solvation structures. The fluorocarbon diluents exclude substantial TEP solvents and FSI− from different solvation shells with decreased participation ratios of TEP and FSI− (Fig. 1d). Moreover, it is found that fluorocarbon diluents change the coordination number of Na+ within a distance of 1 nm from centered Na+ (Fig. 1e). In the first (0–0.3 nm) and second (0.3–0.5 nm) solvation sheaths, the solvated fluorocarbon diluents induce a lower solvation environment around Na+ and exhibit impacts on reducing the coordination numbers of Na+ in these two sheaths. This suggests that low-solvation fluorocarbon diluents play subtle roles in weakening the inner coordination capability of TEP solvents and FSI− (Table S1†). It could be found that the participation ratio of low-solvation diluents in the inner coordination sheaths is limited because the high-solvation TEP solvent and FSI− still dominate the coordination with Na+ in this region (Fig. 1d), while the strong interactions between the diluent molecule and TEP solvent (Fig. S1†) allow the diluent to tailor the outer coordination sheath. The 1,4-Ph(CF3)2 diluent with more F atoms is intensively involved in the third coordination shell (0.5–1 nm), resulting in DHCE-1,4-Ph(CF3)2 showing a much higher coordination number of Na+ than the other two electrolytes (Fig. 1e). The intensive participation of 1,4-Ph(CF3)2 in the third coordination of Na+ leads to a strengthened external solvation structure. In contrast, the PhCF3 diluent with fewer F atoms exhibits the lowest coordination numbers of Na+ from inner to outer coordination sheaths in the DHCE-PhCF3 electrolyte, suggesting a loose solvation network generated by PhCF3. The weakened solvation network could promote easy Na+ transport from one solvation cluster to the adjacent.
The different global solution structures and Na+ distribution would lead to differential Na+ transport kinetics in the electrolytes. Fig. 2d exhibits the ionic conductivity of HCE, DHCE-PhCF3, and DHCE-1,4-Ph(CF3)2 in a temperature range of 0–50 °C. Though with a practical lower overall salt molarity, DHCE-PhCF3 exhibits higher ionic conductivity than the HCE below 40 °C (purple and cyan curves in Fig. 2d). In contrast, DHCE-1,4-Ph(CF3)2 presents the lowest ionic conductivity at all temperatures, which agrees well with the unfavorable global distribution of Na+ shown in Fig. 2c. The temperature dependence of ionic conductivity obviously deviates from the Arrhenius feature for both HCEs and DHCEs, suggesting obvious deviation in ion transport phenomena from conventional dilute solution. Empirical Vogel–Tammann–Fulcher (VTF) correlation is employed to fit the temperature-dependent ionic conductivity as shown in Fig. S4.† The HCE, DHCE-PhCF3, and DHCE-1,4-Ph(CF3)2 exhibit activation energies of 0.062, 0.058, and 0.082 eV, and DHCE-PhCF3 exhibits the easiest diffusion of Na+. Besides, the transference number (τ) provides informative Na+ transport properties in electrolytes.33Fig. 2e presents the Na+ transference number in different electrolytes, showing a sequence of DHCE-PhCF3 (0.62) > DHCE-1,4-Ph(CF3)2 (0.37) > HCE (0.32). The details for the measurements of the Na+ transference number are shown in Fig. S5.† It can be found that low-solvation fluorocarbon diluents increase the Na+ transference number of DHCEs. The PhCF3 diluent has appropriate interactions with solvating solvents, which effectively loosens the internal solvation structure of Na+ and builds locally high Na+ concentrations with continuous Na+ flux for the fastest Na+ transport (Fig. 2b). Nevertheless, the 1,4-Ph(CF3)2 diluent exhibits too strong interactions with solvating solvent and cannot effectively enhance the transport properties of Na+ due to less-mobile Na+ ions being trapped in the strong solvation network and blocked by the “disconnected” Na+ transport pathway (Fig. 2c).
Besides, the different global solution networks in HCE and DHCEs are expected to cause varied thermal stability of the electrolytes. Differential scanning calorimetry (DSC) measurements are performed as shown in Fig. 2f and S6.† Both HCE and DHCE-PhCF3 could retain a liquid state until −50 °C during the cooling stage, showing a glass transition temperature of about −50 °C, while DHCE-1,4-Ph(CF3)2 exhibits an exotherm crystallization peak at −20.16 °C, which could be correlated to the strong solvation network induced by TEP-1,4-Ph(CF3)2 interaction, thus bringing forward early liquid–solid phase transition during the cooling stage. Besides, during the following heating process from −70 to 20 °C, only DHCE-PhCF3 could keep the liquid phase (Fig. S6†). The wide liquid temperature range should be related to the appropriate TEP-PhCF3 interaction and delicate solution structure of DHCE-PhCF3.
Raman spectra are used to further elucidate the effects of diluents on solvation structures. As shown in Fig. 3b, the Raman peak at 729.6 cm−1 corresponds to the stretching vibration of P–O–C in free TEP solvent.34 A new Raman peak at 741.6 cm−1 from solvated TEP emerges after adding NaFSI salt (1NaFSI-6TEP). As shown in Fig. 3d, the increased salt concentration in HCE (1NaFSI-1.5TEP) significantly improves the ratio of solvated TEP:
free TEP (92.9
:
7.1). Moreover, fluorocarbon diluents reduce the overall salt molarity but still retain high ratios of solvated TEP
:
free TEP in DHCEs (86.7
:
13.3 and 83.3
:
16.7 for DHCE-PhCF3 and DHCE-1,4-Ph(CF3)2, respectively). Besides, the Raman peaks for the stretching vibration of S
O (FSI−) exhibit enhanced peaks for contact ion pairs (CIPs) and aggregated ion pairs (AGG) in HCE and DHCEs compared to the diluted electrolyte (Fig. 3c and e). According to the detailed analysis of Raman spectra, the addition of diluent solvents into HCE subtly regulates the first solvation sheath of Na+ in the following ways: (1) the ratios of AGG reduce as the CIP ratios increase in DHCEs; (2) the free TEP solvent and FSI− are slightly increased with introducing diluents (please see detailed fitting ratios in Fig. 3d and e). Besides, the Raman results indicate that the diluent with more F atoms (1,4-Ph(CF3)2) is more likely to release TEP and FSI− from the inner coordination sheath of Na+. This is consistent with MD simulations which show that the ratios of the diluent solvent with more F atoms increase and the solvated TEP and FSI− decrease in the primary solvation sheath as discussed above.
Based on the above discussion, the spectroscopic characterization and theoretical results agree well, which confirms that the chemical environment formed by the fluorocarbon diluents is not “ideally inert”. Instead, the diluents intensively regulate the local and global solution structures and thus remarkably tailor the Na+ transport phenomenon in electrolytes.
The fluorocarbon diluents play substantial roles in regulating the global and local electrolyte solution structure and the TEP–Na+–FSI− solvation strength. Therefore, it is expected that the diluent would modulate the reduction and oxidation voltages and kinetics of TEP solvent and FSI− during the SEI/CEI layer formation. Fig. 4a, b and S8† show the initial and following cyclic voltammetry (CV) curves of hard carbon (HC) anodes in different electrolytes. It can be found that HCE and DHCEs effectively limit the reduction of TEP solvents at low voltage. SEI layer formation of HCE and DHCEs is mainly contributed by the decomposition of NaFSI salt, which is confirmed by X-ray photoelectron spectroscopy (XPS) characterization of HC anodes as shown in Fig. 4c–e. Limited P 2p signals from the decomposition of TEP and obvious N 1s and S 2p peaks from the decomposition of NaFSI are observed in both HCE and DHCEs.
Furthermore, it can be found that all initial CV curves exhibit reduction peaks (mainly from NaFSI) in a broad voltage range in both HCE and DHCEs (Fig. 4b). Two reduction peaks centered at 0.68 V and 0.40 V are observed in HCE. For the DHCEs, the two reduction peaks both shift to more negative voltage sides (0.64/0.37 V for DHCE-PhCF3 and 0.54/0.33 V for DHCE-1,4-Ph(CF3)2). The different reduction voltages for NaFSI salt in different electrolytes confirm variable decomposition pathways and kinetics of electrolytes, which is closely associated with electrolyte structures in different DHCEs. The loosened solvation structure in DHCE-PhCF3 allows a much more complete decomposition of NaFSI salt with higher contents of NaxN, NaxSOy, and Na2S2O3 observed in the SEI (Fig. 4d and e). While the strengthened external solvation structure in DHCE-1,4-Ph(CF3)2 postpones the reduction of FSI− in the solvation sheaths and causes incomplete reduction products of FSI− which could not effectively suppress the subsequent electrolyte decomposition. This is evidenced by the irreversible intercalation/deintercalation of Na+ in the HC anode from the CV curves (yellow curve in Fig. 4a and S8d†) of this electrolyte. The complete decomposition of NaFSI in DHCE-PhCF3 benefits the formation of an effective SEI on the HC anode and allows reversible Na+ storage in the HC anode. This is evidenced by the well-overlapped following CV curves in these two electrolytes (Fig. S8b and c†). Besides, a similar trend is observed with NaCu1/9Ni2/9Fe1/3Mn1/3O2 (CNFM) cathodes. Fig. S9† shows the CNFM cathodes exhibiting well-overlapped CV curves in HCE and DHCE-PhCF3 while the dilute electrolyte and DHCE-1,4-Ph(CF3)2 could not allow reversible Na+ storage in the CNFM cathode. This notes that DHCE-PhCF3 shows greatly enhanced reduction/oxidation peaks for the CNFM cathode, suggesting faster Na+ storage kinetics, which is consistent with the improved Na+ transport properties discussed above. Therefore, the solvation structure of Na+ could be used to engineer the properties of the SEI/CEI layers35 and furthermore, modify the performance of Na-ion batteries (which will be further discussed later).
CNFM‖HC full cells are further evaluated for these electrolytes. As shown in Fig. 5e, the CNFM‖HC full cell could demonstrate a reversible capacity of 105.6 mA h g−1 at 0.2C and a capacity retention of 77% over 300 cycles in the HCE. Noticeably, the introduction of PhCF3 and 1,4-Ph(CF3)2 diluents in the HCE brings a significant difference in the electrochemical performance of the CNFM‖HC full cells. With DHCE-1,4-Ph(CF3)2, the CNFM‖HC full cell exhibits the lowest initial capacity of 37.6 mA h g−1 and significantly reduced coulombic efficiency (∼86% after 30 cycles at 0.2C) (Fig. 5e and S12†). In sharp contrast, the CNFM‖HC full cell using DHCE-PhCF3 presents a greatly improved reversible capacity of 113.1 mA h g−1 at 0.2C and capacity retention of 82.7% after 300 cycles (Fig. 5e). The charge–discharge curves of the CNFM‖HC full cell with DHCE-PhCF3 also exhibit minimal changes over 300 cycles (Fig. 5f). Additionally, the high-temperature performances of the DHCE-PhCF3 are further evaluated. As shown in Fig. S13,† the HC‖Na, CNFM‖Na, and CNFM‖HC cells all demonstrate robust cycling stability at a high temperature of 60 °C. These results indicate that the electrochemical performances of cells are highly correlated with solvation structures at the molecular level of the electrolytes. Appropriate optimization of the internal and external solvation structures via diluents could provide a potential approach for future goal-oriented electrolyte design for different battery chemistries.
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Fig. 6 HRTEM images of the HC anodes cycled in (a) HCE, (b) DHCE-PhCF3, and (c and d) DHCE-1,4-Ph(CF3)2. The evolution of (e) Rct and (f) Rinterphase during cycling in the studied electrolytes. |
Though 1,4-Ph(CF3)2 has a similar molecular structure to PhCF3, DHCE-1,4-Ph(CF3)2 fails to form a stable interphase on the HC anode and CNFM cathode due to the less stable interfacial layers obtained on electrodes (Fig. 6c). The HRTEM image further demonstrates that the thick SEI layer is filled with many electrolyte by-products, which consist of nanocrystals such as nano-NaF and Na2S particles imbedded in the amorphous base (Fig. 6d). A severe phase change of CNFM from a layered to a rock salt phase and dendritic CEI products are observed on CNFM (Fig. S14c†). Operando-electrochemical impedance spectroscopy (operando-EIS) is further used to study the charge transport kinetics across the interphase in the Na-ion batteries. As shown in Fig. 6e, f and S17,† DHCE-PhCF3 could retain greatly reduced charge transfer resistance (Rct) and interphase resistance (Rinterphase) and present excellent interphasial stability during its long cycle life.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3sc01453c |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2023 |