Enhanced Anion‐Derived Inorganic‐Dominated Solid Electrolyte Interphases for High‐Rate and Stable Sodium Storage

It is highly desirable for the promising sodium storage possessing high rate and long stable capability, which are mainly hindered by the unstable yet conventional solvent‐derived organic‐rich solid electrolyte interphases. Herein, an electrolyte solvation chemistry is elaborately manipulated to produce an enhanced anion‐derived and inorganic components‐dominated solid electrolyte interphases by introducing a low permittivity (4.33) bis(2,2,2‐trifluoroethyl) ether diluent into the sodium bis(trifluoromethylsulfonyl)imide‐dimethoxyethane‐based high concentration electrolyte to obtain a localized high concentration electrolyte. The bis(2,2,2‐trifluoroethyl) ether breaks the balance of original cation solvation structure and tends to interact with Na+‐coordinated dimethoxyethane solvent rather than Na+ in high concentration electrolyte, leaving an enhanced Coulombic interaction between Na+ and (FSO2)2N−, and more (FSO2)2N− can enter the Na+ solvation shell, forming a further increased number of Na+‐(FSO2)2N−‐dimethoxyethane clusters (from 82.0% for high concentration electrolyte to 94.3% for localized high concentration electrolyte) at a low salt dosage. The preferential reduction of this (FSO2)2N−‐enriched clusters rather than the dimethoxyethane‐dominated Na+ solvation structure produces an enhanced anion‐derived and inorganic components‐dominated solid electrolyte interphases. The reversible charge storage process of Na is decoupled by operando Raman along with a shift of D and G peaks. Benefiting from the enhanced anion‐derived electrode‐electrolyte interface, the commercial hard carbon anode in localized high concentration electrolyte shows a well rate capability (5 A g−1, 70 mAh g−1), cycle performance and stability (85% of initial capacity after 700 cycles) in comparison to that of high concentration electrolyte (68%) and low concentration electrolyte (only 5% after 400 cycles), indicative of uniqueness and superiorities towards stable Na storage.

It is highly desirable for the promising sodium storage possessing high rate and long stable capability, which are mainly hindered by the unstable yet conventional solvent-derived organic-rich solid electrolyte interphases. Herein, an electrolyte solvation chemistry is elaborately manipulated to produce an enhanced anion-derived and inorganic components-dominated solid electrolyte interphases by introducing a low permittivity (4.33) bis(2,2,2-trifluoroethyl) ether diluent into the sodium bis(trifluoromethylsulfonyl)imidedimethoxyethane-based high concentration electrolyte to obtain a localized high concentration electrolyte. The bis(2,2,2-trifluoroethyl) ether breaks the balance of original cation solvation structure and tends to interact with Na +coordinated dimethoxyethane solvent rather than Na + in high concentration electrolyte, leaving an enhanced Coulombic interaction between Na + and (FSO 2 ) 2 N À , and more (FSO 2 ) 2 N À can enter the Na + solvation shell, forming a further increased number of Na + -(FSO 2 ) 2 N À -dimethoxyethane clusters (from 82.0% for high concentration electrolyte to 94.3% for localized high concentration electrolyte) at a low salt dosage. The preferential reduction of this (FSO 2 ) 2 N À -enriched clusters rather than the dimethoxyethane-dominated Na + solvation structure produces an enhanced anion-derived and inorganic components-dominated solid electrolyte interphases. The reversible charge storage process of Na is decoupled by operando Raman along with a shift of D and G peaks. Benefiting from the enhanced anion-derived electrode-electrolyte interface, the commercial hard carbon anode in localized high concentration electrolyte shows a well rate capability (5 A g À1 , 70 mAh g À1 ), cycle performance and stability (85% of initial capacity after 700 cycles) in comparison to that of high concentration electrolyte (68%) and low concentration electrolyte (only 5% after 400 cycles), indicative of uniqueness and superiorities towards stable Na storage.
high concentration electrolyte (HCE, >4 M), because of the rarity of solvent, the anions in electrolyte can be crushed into the cation solvation shell and further enhance the Columbic interaction between cation and anion, forming the solvation structure of contact ion pair (one cation coordinating one anion, CIP) and aggregate (two or more cation coordinating one anion, AGG, Scheme 1a). [21] This unique anionparticipated solvation structure in HCE enables an anion-dominated interfacial chemistry, producing an inorganic components-rich SEI (Scheme 1b) with high mechanical strength, thin and homogeneous features along with a good ionic conductivity, rapid and stable interfacial kinetics. [22] This manipulation of cation solvation structure in HCE identifies and reinforces the anion character in the construction of inorganic components-rich SEI. [23] However, there is a limit to the extent to which anion can be squeezed into the solvation shell of cation by increasing the concentration, which also brings about the high viscosity/cost, decreased wettability, and poor low-temperature application. [24] The coordination competition between the anion-cation and cationsolvent interactions exists and finally controls the electrolyte solvation structure. [24][25][26] Recently, a fluorinated ether as a weak solvated diluent (soluble with solvent and hardly dissolving salt) was introduced into the HCE. [27,28] The solvent originally coordinated in the solvation shell of the cation will interact with the diluent, so that the interaction of the anion and cation is further strengthened to form more compact ion pairs or AGG at a low concentration, leading to a reinforced anionderived SEI (Scheme 1c,d). [29] Meanwhile, this strategy to construct the localized HCE (LHCE) can decrease the viscosity/cost and enhance the wettability and low-temperature availability as well. [18,30] The localized solvation structure is sensitive to the salt, solvent, and diluent, and the compatibility between the enhanced anion-derived interface structure and promising hard carbon (HC) electrode for Na storage still remains unclear and needs to be further detected. [12,31] Herein, an enhanced anion-derived and inorganic components-dominated SEI is constructed on the model commercial HC anode surface to achieve a fast and steady Na storage through elaborately manipulating the solvation chemistry, where a weak solvated diluent bis(2,2,2-trifluoroethyl) ether (BTFE) with a low permittivity (4.33) is introduced into the 4.5 M sodium bis(trifluoromethylsulfonyl)imide (NaFSI)-dimethoxyethane (DME)-based HCE to prepare the 1.5 M LHCE made of BTFE and DME with a volume ratio of 2:1. The adopted BTFE breaks the balance of original cation solvation structure and tends to interact with Na +coordinated DME solvent rather than Na + in HCE. Raman spectroscopy technique confirms that the more FSI À anions in electrolyte can enter the solvation shell of Na + cations, leaving an enhanced Coulombic interaction between Na + and FSI À and increased number of Na + -FSI À -DME clusters at a reduced low salt dosage (53.4% of CIP and 28.6% of AGG for HCE; 52.6% of CIP and 41.7% of AGG for LHCE). This further promotes more anions to decompose and forms an enhanced anion-derived and inorganic components-rich SEI. The charge storage process of Na is decoupled by operando Raman. Benefiting from the enhanced anion-derived electrode-electrolyte interface, the LHCE enables the commercial HC anode to show a well rate capability (5 A g À1 , 70 mAh g À1 ), cycle performance, and stability (85% of initial capacity after 700 cycles) in comparison to that of HCE (68%) and LCE (only 5% after 400 cycles), indicative of uniqueness and superiorities toward stable Na storage.

Solvation Structure of LCE, HCE, and LHCE
Raman spectroscopy technique is first conducted to detect the signal of anion and solvent to well decouple the electrolyte solvation structure. Because of the relatively high solubility of NaFSI in DME, 1.5 M LCE and 4.5 M HCE are accessible (Table S1, Supporting Information). The 4.5 M HCE is then diluted by BTFE with a low permittivity (4.33) to prepare 1.5 M LHCE made of BTFE and DME with a volume ratio of 2:1. A broad Raman peak in a range of 690-770 cm À1 is presented in the three types of electrolytes, corresponding to the FSI À anions due to the different microenvironment (Figure 1a), which can be further subdivided into free FSI À (717.5 cm À1 ), CIP (730.6 cm À1 ), and AGG (743.4 cm À1 ), respectively. [20] The obvious differences for peak intensity are clearly observed, indicative of the changed solvation microstructure. Most of the FSI À anions are unbounded/free in LCE with a dominated proportion of free FSI À (55.9%), CIP (29.6%), and AGG (14.5%) (Figure 1c). An apparent blue shift of Raman peak is observed Scheme 1. Electrolyte solvation structure in a) conventional HCE and c) presented LHCE in the present work, and b) anion-derived SEI in HCE and d) proposed enhanced anion-derived and inorganic components-dominated SEI in LHCE on the surface of model HC electrode, respectively.
Energy Environ. Mater. 2023, 6, e12602 2 of 8 in 4.5 M HCE, indicative of an enhanced interaction between FSI À and Na + due to the decreased DME solvent. Also, the FSI À anions enter the Na + solvation shell, leading to a sharp decrease of free FSI À (18%) and increase of CIP (53.4%) and AGG (28.6%). [32] A further blue shift of Raman peak is presented for the LHCE with BTFE diluent, demonstrating a further strengthened Na + -FSI À interaction and more FSI À entering the inner solvation shell of Na + . This is mainly due to the fact that the BTFE tends to interact with the DME originally coordinated in the solvation shell of the Na + rather than Na + , leaving an increased proportion of Na + -FSI À -DME clusters from 82% (CIP and AGG) for HCE to 94.3% for LHCE in the form of CIP (52.6%) and AGG (41.7%) and decreased free FSI À (5.7%). [33] The DME presents two Raman peaks at 820.4 and 847.8 cm À1 (Figure 1b), indexed to the stretching vibration of CH 2 -O-CH 3 . [29] In LCE, the NaFSI tends to be dissociated and solvated in DME; therefore, two extra Raman peaks at 834.7 and 862.8 cm À1 are indexed to Na +coordinated DME with a proportion of 45.2%, and the rest 54.8% is the unbounded/free DME (Figure 1d). Similarly, in HCE, due to the increased salt concentration effects, the DME is mainly bounded in the solvation shell of Na + with a sharp increase for Na + -coordinated DME (79.7%) and decrease of free DME (20.3%). Furthermore, in LHCE, the used BTFE diluent does not participate in the solvation shell of Na + and can easily interact with DME originally coordinated with Na + and enables to reduce the number of the Na + -coordinated DME to 61%, thus increase the number of free DME that is uncoordinated with Na + to 39% in turn, which confirms the further enhanced interaction between FSI À and Na + in the LHCE system.

The Effects of Electrolyte Solvation Structure on the Electrochemical Performance of Na Storage
An available commercial hard carbon (HC), with an amorphous structure and typical mesoporous pore (specific surface area of 1176 m 2 g À1 , main pore size of 2-12 nm, Figure S1, Supporting Information), is chosen as a model active material of electrode to assemble half-cell. There are evident reduction peaks in the first-time cyclic voltammetry (CV) curves, mainly corresponding to the electrolyte decomposition and SEI formation (Figure 2a). [34] The obvious difference for initial reduction potential exists, and the potential follows the order: LHCE > HCE > LCE in the potential range of 1.25-1.75 V. This is mainly attributed to the formed Na + -FSI À -DME clusters in LHCE and HCE systems, where the FSI À anions are more preferentially reduced and decomposed compared with the DME-dominated reduction and composition in LCE because the Na + -FSI À -DME clusters present a lower LUMO energy level. [29] Also, the relatively higher proportion of Na + -FSI À -DME clusters in LHCE (52.6% of CIP and 41.7% of AGG) than that of the HCE (53.4% of CIP and 28.6% of AGG) is presented, leading to more FSI À anions to be preferentially reduced. [20] There is no obvious polarization of HC electrode in LCE, HCE ( Figure S2, Supporting Information), and LHCE systems (Figure 2b) as the scan rates increase. At 0.2 A g À1 , the first galvanostatic charge-discharge (GCD) curve delivers a discharge capacity up to 1084, 1246, and 799 mAh g À1 along with a charge capacity of 530, 269, and 275 mAh g À1 for LCE, HCE, and LHCE, respectively, thus an initial Coulombic efficiency (ICE) of 48.9%, 21.7%, and 34.5% Figure 1. a, b) Raman spectra of the solvents (DME and BTFE) and electrolytes (LCE, HCE, and LHCE). Proportion of c) free FSI À , CIP, and AGG, d) free and Na + -coordinated DME. These data are obtained from the area of fitting curves on the basis of the corresponding Raman spectra.
Energy Environ. Mater. 2023, 6, e12602 3 of 8 presented for LCE, HCE, and LHCE, respectively (Figure 2c). The presented low ICE in three systems originates from the irreversible side reactions that are mainly the electrolyte decomposition and formation of SEI. [35] Further, for LCE, a relatively higher values of capacity and ICE are presented but with a poor cycling stability (proved in the following part) mainly due to the unstable solvent-derived SEI. As for HCE, an improved discharge capacity is presented, nevertheless, accompanied by a low ICE. While for the presented LHCE with the BTFE diluent, an increased ICE value and better stability are observed because of the enhanced anion-derived SEI.
The ion transport behaviors of configured system made of prepared electrolytes and HC electrode are detected by the electrochemical impedance spectroscopy (EIS) technique. A linear feature is presented in the low frequency (Warburg diffusion resistance, Figure 2d), and a relatively big slope of the straight line is displayed in the LHCE than that of the HCE and LCE, indicating a faster ion transport in the LHCE system. [36] Meanwhile, a smaller overall resistance is also presented in the LHCE system than that of the HCE and LCE, which are mainly due to the enhanced anion-derived SEI in LHCE system that can efficiently reduce the resistance mainly including the Na + desolvation and transport through the SEI. [37] The impact of different electrolytes on the rate performance of HC is measured. A relatively low discharge capacity is presented for the HCE (226 mAh g À1 @0.2 A g À1 , 10th cycle) than the LCE (456 mAh g À1 , Figure 2e), mainly because of the reduced conductivity and increased viscosity of electrolyte due to the increased concentration. A relatively improved discharge capacity is presented for the LHCE (283 mAh g À1 ) compared with that of the HCE due to the decreased viscosity with the BTFE diluent. The smaller capacity drops are presented for the HCE and LHCE than that of the LCE system as the current density increases to 5 A g À1 , where 88 and 70 mAh g À1 of capacities remained for the HCE and LHCE, respectively ( Figure S3, Supporting Information), which mainly benefit from the smaller interface resistance of the anion-derived SEI, indicative of superior rate performance. Furthermore, the Coulombic efficiency (CE) and specific capacity during the long-time cycling stability test in different electrolytes are also compared and are shown in Figure 2f. There is a fast and distinct capacity attenuation in LCE after 200 cycles, and only 5% of initial capacity can be kept after 400 cycles, accompanied by a huge fluctuation of CE derived from the big difference of charge and discharge capacity. The apparently separated GCD curves at different cycles also indicate a poor cycling stability in LCE (Figure 2g), which are mainly the fact that the unstable solvent-derived SEI film cannot efficiently restrain the continuous electrolyte decomposition and the possible electrode structure collapse/destruction due to the co-intercalation of Na + Figure 2. Electrochemical performance of half-cell using HC anode in different electrolytes. The comparison of a) CV (@0.1 mV s À1 ), c) GCD (@0.2 A g À1 , first time) curves, d) Nyquist plots, e) rate performance, and f) stability test (@1 A g À1 ) in LCE, HCE, and LHCE, respectively. b) The CV curves at different scanning rates in LHCE. The curves for selected GCD cycles at 1 A g À1 in different electrolytes including g) LCE, h) HCE, and i) LHCE.
Energy Environ. Mater. 2023, 6, e12602 4 of 8 and DME. [12,38] While an obviously enhanced stability is manifested in the HCE and LHCE counterparts. As for HCE, 68% of initial capacity is maintained after 700 cycles, mainly due to the improved interface stability originating from the anion-derived SEI (Figure 2h). [39] Furthermore, a capacity retention rate as high as 85% is obtained for LHCE with nearly coincident GCD curves at different cycles (Figure 2i), demonstrating a superior stability on account of the further strengthened interface stability of the enhanced anion-derived SEI. [29] 2.3. Chemical Components and Structure of SEI Compared with the pristine HC ( Figure S4a,e, Supporting Information), after five cycles at 0.1 A g À1 , there is a relatively compact/thick SEI in LCE ( Figure S4b,f, Supporting Information), and a relatively porous and thin SEI in HCE ( Figure S4c,g, Supporting Information) and LHCE ( Figure S4d,h, Supporting Information), which is in favor of the process of ion transport and desolvation. The detailed SEI chemical component of HC in different electrolytes is detected through the X-ray photoelectron spectroscopy (XPS). There are similar elements (Na, O, F, C, N, and S, etc., Figure S5, Supporting Information) on the electrode surface in the LCE, HCE, and LHCE systems. The S, N, and F atomic contents in LHCE are obviously higher than those of the HCE and LCE, which are mainly from the FSI À decomposition. Also, a peak corresponding to NaF species (683.9 eV) is dominated for LHCE in the F 1s XPS spectra (Figure 3a), which is beneficial to the Na + diffusion and enhances the stability of electrode interface, [40] indicative of the enhanced inorganic-enriched characteristics of SEI compared with that in HCE and LCE. This can be also confirmed in the N 1s ( Figure 3b) and O 1s spectra ( Figure S6, Supporting Information) for the LHCE with the high peak area of Na 3 N (398.6 eV) and Na 2 O (531.5 eV) species, respectively. A reduced peak area indexed to S-F (687.3 eV, F 1s XPS spectra) and N-SO x (400.0 eV, N 1s XPS spectra) species derived from FSI À in LHCE indicates the more complete decomposition of FSI À . [20] In the S 2p spectrum of LHCE, there are also peaks indexed to Na 2 S (161.9 eV) and Na oxysulfide species from the further decomposition of FSI À (Figure 3d). In the Na 1s XPS spectrum of LHCE (Figure 3e), the about-mentioned inorganic species are also presented. Meanwhile, in the C 1s spectrum of LHCE, the relatively low C-O (286.4 eV) intensity is presented (Figure 3c), indicating the relatively low content of organic species and the less decomposition of DME in LHCE system. [25] The main reason is that the more FSI À can be squeezed into the Na + solvation shell due to the further strengthened interaction between Na + and FSI À in the presence of BTFE with a low permittivity, leading to a reinforced Na + -FSI À -DME clusters in the diluted LHCE, which rapidly migrates to the electrode surface and involves in the inorganic-rich SEI formation. These results strongly confirm that an enhanced anion-induced inorganic-rich SEI is presented in the LHCE system.
To further identify the SEI chemical components and structure in the LHCE system along with its depth, after five cycles at 0.1 A g À1 , the HC anode is further exposed in the continuous Argon sputtering for 0, 15, and 30 s, respectively. The C and O atomic content decreases, and F and Na increases from 0 to 15 s, corresponding to the decreased organic species and increased inorganic species, respectively, and then nearly stabilizes from 15 to 30 s (Figure 4a and Figure S7, Supporting Information). The variation of element content with depth is mainly due to the step-by-step decomposition of FSI À with high intensity of Fcontaining species in the inner layer near the electrode and S/N counterpart in the out layer. [29] Furthermore, this structure of SEI can also be confirmed by the decreased S-F, N-SO x , and RSO 2 R 0 (169.8 eV) in the S 2p spectra (Figure 4e) and increased NaF (Figure 4b), Na 2 O, S-O x (Figure 4c), NaNO 2 (403.9 eV), Na 3 N (Figure 4d), and Na 2 S (Figure 4e and Figure S8, Supporting Information) from 0 to 15 s, accompanying with the more complete decomposition of FSI À , and the proportion of these species basically remains steady from 15 to 30 s.

Operando Raman Spectroscopy Revealing Na Storage Behavior
Operando Raman spectroscopy technique is employed to detect the nanoscaled structure of electrode in the LHCE system along with Na storage behavior during the first sodiation (from 3 to 0.01 V)-desodiation (from 0.01 to 3 V) behavior in real time (Figure 5a). There are typical Raman D and G peaks of the HC at about 1350 and 1600 cm À1 ( Figure S1, Supporting Information), corresponding to the structure of amorphous carbon or defective graphitic structure and sp 2 -hybridized   6 of 8 C in the graphitic layers, respectively. [36] The Na + adsorption on the carbon defective site and the Na + intercalation into the carbon layer do have an impact on the position of Raman D and G peaks. As shown in the Figure 5b,c, the D peak shifts from about 1350 (@3 V) to 1332 cm À1 (@0.01 V) after the full sodiation in the discharge process, mainly due to the vibration and rotation of defective carbon that is affected by the Na + preferential adsorption on the defects, along with the change of localized charge concentration. [11] Meanwhile, there is also a red shift for the G peak, from about 1600 (@3 V) to 1565 cm À1 (@0.01 V), primarily because of the electron doping originating from the charge transfer between the Na + intercalated into the stacked carbon sheets and the carbon p* antibonding orbital, [41] which was already reported in that of the graphite intercalation compounds of alkali metal. [42] The I D /I G value, reflecting the defect/disorder to a certain extent, slightly increases during the sodiation, which can be relevant to the SEI formation on the electrode surface (Figure 5d). After the full desodiation, the D and G peaks move back to the initial peak position, with a negligible fluctuation of I D /I G value, indicating the excellent stability and reversibility of Na storage in the LHCE system.

Conclusion
In summary, an enhanced anion-derived and inorganic componentsdominated SEI is constructed by using the BTFE diluent with a low permittivity. The unique solvation structure is decoupled in detail by Raman spectroscopy technique where the BTFE breaks the balance of original Na + solvation structure and tends to interact with Na +coordinated DME solvent in solvation shell rather than Na + of HCE, leaving an enhanced Coulombic interaction between Na + and FSI À , and more FSI À anions can enter the Na + solvation shell, leaving a further increased number of Na + -FSI À -DME clusters at a reduced low salt dosage (53.4% of CIP and 28.6% of AGG for HCE; 52.6% of CIP and 41.7% of AGG for LHCE). The localized and FSI À -enriched clusters at interface are characterized by the preferential reduction compared with the solvent-dominated Na + solvation structure, and to further produce an enhanced anion-derived and inorganic components-dominated SEI instead of organic species. The reversible charge storage process of Na is decoupled by operando Raman along with a shift of D and G peaks. Benefiting from the stable anion-derived electrode-electrolyte interface, the commercial HC anode with the LHCE shows a well rate capability (5 A g À1 , 70 mAh g À1 ), cycle performance, and stability (85% of initial capacity after 700 cycles) in comparison to that of HCE (68%) and LCE (only 5% after 400 cycles), indicative of uniqueness and superiorities toward stable Na storage. This contribution will offer a good reference through the manipulation of electrolyte solvation chemistry to obtain an enhanced anion-derived and inorganic componentsdominated SEI for rapid and stable Na storage.

Experimental Section
Electrolyte acquisition: The NaFSI (0.305 g, battery grade; Dodochem) is mixed with DME (1 mL, battery grade; Dodochem) in the glove box to prepare the 1.5 M LCE. The NaFSI (0.914 g) and DME (1 mL) are mixed in the glove box to prepare 4.5 M HCE. The NaFSI (0.229 g), DME (0.25 mL), and BTFE (0.5 mL, AR, Aladdin) are mixed in the glove box to prepare the 1.5 M LHCE.
Materials characterization: The morphology, structure, and BET surface area of commercial HC (Macklin) are tested by cold field emission SEM (SU8220), XRD (Miniflex600; Rigaku), and Micromeritics ASAP 2020. The electrolyte solvation structure is characterized by Raman spectroscopy (LabRAM HR Evolution Raman Microscope). The proportion of free FSI À , CIP, and AGG, free and Na +coordinated DME is based on the ratio of the corresponding area of individual fitting Raman curve to the total area of fitting Raman curves. (In-depth) XPS (ESCALAB250Xi; Thermo) is used to detect the SEI component.
Electrochemical measurement: With a mass ratio of 8:1:1, the HC, acetylene black, and polyvinylidene fluoride (PVDF) binder are well blended in N-methyl-2pyrrolidone (NMP) to yield a slurry, which is coated on the Cu foil, and dried (vacuum, 80°C) to prepare the electrode. The electrochemical performance is evaluated in 2016-type coin cell with the Na foil, glass fiber (Whatman, GF/D), and as-prepared HC electrode serving as counter, separator, and working electrode, respectively. The prepared LCE, HCE, and LHCE are used as the electrolytes. The electrochemical workstation (Bio-logic, VSP) is used to test the CV (0.01-3.0 V) and EIS (amplitude: 5 mV, Hz: 10 5 to 10 À2 ). The GCD (0.01-3.0 V) is tested in Land battery testing system.