Perfluoroalkyl sulfonimide anions are a family of ions containing sulfonimide anionic center [–SO₂–N⁽⁻⁾–SO₂–] and fluoroalkyl chains (R_F, e.g., C _x F_2x+1, C _x F _y H _z ), in which the former conjugated unit allows sufficient delocalization of negative charges over four oxygen atoms and one nitrogen atom, while the strong electronegativities of the terminal fluorinated groups further decrease the negative charge densities on nitrogen/oxygen atoms (Linert et al., 2002; Zhang et al., 2017; Zhang et al., 2023). The extremely weak coordinating ability of these perfluoroalkyl sulfonimide anions allows ready access to a variety of inorganic (M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, etc.) and organic (e.g., NR₁R₂R₃R₄ ⁺) salts with low melting points and superb chemical/thermal stabilities, being of great interest for electrochemical applications (e.g., batteries, capacitors, electrochromic devices, etc.) (Bonhôte et al., 1996; MacFarlane et al., 1999; Tokuda et al., 2006; Tsunashima and Sugiya, 2007; Armand et al., 2009; Eshetu et al., 2016; Oteo et al., 2019).

The bis(trifluoromethanesulfonyl)imide anion {[N(SO₂CF₃)₂]⁻, TFSI⁻}, first suggested as countercharge of electrolyte salts by Armand and Moursli (1983); Zhang and Armand (2021), has aroused significant attention in battery community, owing to their high thermal and electrochemical stabilities (>5.0 V vs. Li/Li⁺) and high ionic conductivities in aprotic solvents. Presently, lithium bis(trifluoromethanesulfonyl)imide has been widely utilized as main conducting salt for building highly Li-ion conductive polymer electrolytes, leading to the commercialization of solid-state lithium metal batteries by Bolloré group (Zhang et al., 2020; Zhang and Armand, 2021). Analogously, the TFSI-based ionic liquids are also highly relevant in electrochemical energy storage applications due to their low viscosities, high ionic conductivities, low melting points and high stability, both thermal, as compared to conventional liquid electrolytes, and electrochemical, e.g., high anodic stability (Armand et al., 2009; Eshetu et al., 2016). Unfortunately, the application of LiTFSI in non-aqueous liquid electrolyte is severely hindered by its corrosive nature towards aluminum current collector above ca. 3.8 V vs. Li/Li⁺ (Krause et al., 1997; Oteo et al., 2019; Zhang et al., 2019; Qiao et al., 2022).

Recently a novel hydrogen-containing sulfonimide salt, (difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide {Li [N(SO₂CF₂H)(SO₂CF₃)], LiDFTFSI} was reported by our group (Oteo et al., 2019; Zhang et al., 2019; Qiao et al., 2022). This salt could be readily obtained with cost-effective synthetic approaches, and shows high anodic stability and good compatibility with aluminum current collector (>4.2 V vs. Li/Li⁺) while it presents high chemical stability. The hydrogen atom plays a critical role as it is believed to form LiH, with Li-ion conductive nature, which regulates the properties of the solid-electrolyte-interphase (SEI) layer formed on the lithium metal (Li°) electrode (Qiao et al., 2022). In the case of solid polymer electrolytes, the use of LiDFTFSI lead to an increase of the lithium-ion conductivity by anion-anion and anion-polymer interactions via H-bonding, limiting the migration of anionic species (Lee et al., 1988; Oteo et al., 2019; Zhang et al., 2019). Thus, LiDFTSI stands as a safe option for both liquid and solid electrolytes and circumvents the problems that classic LiTFSI exhibits (Oteo et al., 2019; Zhang et al., 2019; Qiao et al., 2022).

The effect of C-H···F like hydrogen bond interactions on packing of crystalline structures has been previously identified (Becke, 1993). Those, in addition to other weak interactions such as fluorine-metal and fluorine-fluorine interactions, may have a significant contribution in sulfonimide salts with a extensive delocalization of the negative charge. In this sense, crystal structures of LiTFSI, NaTFSI, KTFSI and CsTFSI (Matsumoto et al., 2015) have been reported describing their layered structured with prevalent metal-oxygen and metal-nitrogen interactions. Detailed structural data obtained by X-ray diffraction may serve to identify possible hydrogen bonding interactions on DFTFSI salts as anticipated in electrolyte analysis studies.

Currently, extensive efforts have been devoted to a deeper understanding or evaluation of the properties of TFSI-based electrolytes while little is known about DFTFSI and its full potential. Indeed, it has been observed that cesium (difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide {Cs [N(SO₂CF₂H)(SO₂CF₃)], CsDFTFSI} has a very low melting point (72°C) (Oteo et al., 2019), becoming a potential candidate for a low-temperature molten salt electrolytes. In this work, the crystal structure of KDFTFSI determined by single-crystal X-ray diffraction is reported and the results are discussed and compared with previously reported structure of KTFSI (Xue et al., 2002). It is important to highlight that potassium salts are much less sensitive toward humidity, allowing the successful preparation of single crystals without any contamination of solvents, as generally observed for lithium and sodium salts. Coordination environment of potassium atoms and relative disposition of fluoroalkyl terminals are comprehensively studied. Computational studies of the bare anion, lithium and potassium salts are also included, to shed some light on the preferential conformation of the DFTFSI⁻ anion. The knowledge accumulated with the hydrogenated sulfonimide salts is likely to be transferable among different electrolytes ranging from lithium/sodium to potassium-based systems (Zhang et al., 2023).

Crystallographic data on measured monocrystal of KDFTFSI is presented. Details on the conditions and refinement are given in Table 1. Further details can be found in Supplementary Tables S1‒S6.

Bis(perfluoroalkylsulfonyl)imides may present both trans and cis conformations according to their relative positions of the perfluoroalkyl groups with respect to the S‒N‒S plane. While trans conformation is typically thermodynamically favoured (Johansson et al., 1998) cis conformation is commonly observed in crystalline structure of alkali metal salts (Matsumoto et al., 2013). The cation-anion interactions diminish the energy barrier between cisoid and transoid forms as the chelation of the anion becomes stronger. The potassium salt of (difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide (KDFTFSI) also presents a cisoid conformation. As seen in Table 1, both KDFTFSI and KTFSI compounds present an orthorhombic structure and similar cell dimensions.

The ORTEP diagram of the asymmetric unit is shown in Figure 1 and comprises two ion pairs. It should be noted that the representative diagram shows the structure with the highest occupancy as there is certain positional disorders in the C‒H and C‒F of the CF₂H group. While one of the C‒F in this group is fixed, the other two positions have a similar population distribution. The atomic occupancy for each H atom is included in the (Supplementary Table S5). Larger displacement parameters are observed for the F atoms in the terminal CF₃ and CF₂H groups than those of the other atoms (see, Supplementary Table S3), this may relate to some rotational freedom of the CF₃ and CF₂H group around the S‒C bond. Certainly, this phenomenon also identified in KTFSI (Matsumoto et al., 2015) is more clearly observed in KDFTFSI due to the substitution of a fluorine atom in the CF₃ group.

As seen in Figure 1, the diagram shows two ion pairs with the potassium atoms located in the center of the asymmetric unit, the hydrophilic domain of the anion points toward the cations while the hydrophobic fluoroalkylated groups stand away from the potassium atoms.

Attending to bond lengths and angles, the intramolecular geometry of KDFTFSI is similar to that of KTFSI. Table 2 includes a list of bonding parameters for KDFTFSI, and the data of KTFSI is provided for comparison. The complete list of bonding parameters for KDFTFSI is included in Supplementary Table S1. Thus, bond length range: N‒S, 1.562(6)‒1.581(6) Å; S‒O, 1.426(5)-1.439(5) Å; S‒C, 1.719(7)‒1.826(7) Å; C‒F, 1.278(13)‒1.3888(10) Å; and a bond angle range: S‒N‒S, 125.7(3)‒128.0(4)°; N‒S‒O, 106.4(3)‒117.0(3)°; N‒S‒C, 103.7(3)‒106.0(3)°; O‒S‒O, 116.1(3)‒117.3(3)°; O‒S‒C, 103.5(3)‒105.4(3)°; S‒C‒F, 107.8(5)‒118.6(5)°; F‒C‒F, 98.8(7)‒111.2(7)°. The DFTFSI⁻ anions present a cis conformation with a C‒S···S‒C torsion angle of 20° (B) and 17° (A), respectively.

The crystal packing of KDFTFSI and KTFSI is illustrated in Figure 2. Both salts show a layered structure alternating cationic and anionic layers stack along the b axis. The potassium cations are sandwiched by the oxygen atoms in the sulfonyl groups, this structure is promoted by the favoured K···O interactions that push aside the fluorine rich groups (CF₃, CF₂H) that adopt a cis-conformation leading to the fluorine rich layer. This stacking pattern is present in the crystalline structure of those fluorinated sulfonimide salts (NaTFSI (Matsumoto et al., 2015), KTFSI (Xue et al., 2002), CsTFSI (Xue et al., 2005), Sr(TFSI)₂ (Zak et al., 1998) with only minor differences in the repeating distance of those layers being attributed to the cation size (Matsumoto et al., 2015) and no major differences between similar sulfonimide anions (TFSI⁻, FSI⁻, and DFTFSI⁻) (Matsumoto et al., 2013). This distance corresponds in orthorhombic structures as KTFSI and KDFTFSI, to approximately half of the value of a cell parameter. Thus, a value of 11.276 Å is obtained for KDFTFSI while a value of 11.173 Å is inferred from the cell parameters in KTFSI (Matsumoto et al., 2015). Alternatively, sulfonimide salts with a transoid disposition in the crystalline phase, such as LiTFSI (Nowinski et al., 1994) and NaTFSI (Matsumoto et al., 2013) present a relatively simpler packing pattern where cations are simple bridging anion layers.

The coordination environment of the cations in KDFTFSI is also investigated and compared with the reference KTFSI salt. Figure 3 represents the coordination of each of the potassium atoms identified in both KDFTFSI and KTFSI salts. Potassium cation atoms are mainly coordinated with oxygen atoms from the two sulfonyl groups. Thus, one of the potassium atoms presents primary interactions with four O atoms and secondary interactions with four O atoms and 2 N atoms. The second potassium atom is exclusively coordinated to oxygen atoms through seven primary interactions and one secondary interaction (see Table 3). The relative dispositions adopted by potassium and the O and/or N atoms correspond to pentagonal antiprism and triangular dodecahedron coordination geometries, respectively. K1 atom in KTFSI presents a distorted square antiprism disposition according to previous reports (Xue et al., 2002).

As it can be observed in Figure 4, a total of six anions are interacting with K1 cation, four of those coordinate as bidentate ligands forming the corresponding chelates. It is worth mentioning that when anions act as monodentate ligands, the anion is exclusively interacting through oxygen atoms while there is no N-coordinating monodentate anion. K2 cation is also coordinated by six anions, in this case only two of those sulfonimide anions interact in a bidentate fashion. v = exp R 0 − R B (1)

In order to better understand the strength of each of those interactions, the bond valence (ν) and the summatory, defined as bond valence sum (BVS) were determined and therefore quantify their contribution to the coordination state. BVS was calculated for each cation (K1, K2) and bond valence was calculated according to Equation 1: where R₀ and B are empirically determined parameters that serve to correlate the bond distance and the bond valence for a certain interaction. Those parameters have been tabulated for a variety of atom-atom interactions and are obtained from a systematic analysis of the Inorganic Crystal Structure Database. R₀ for K···O and K···N atoms is defined as 2.132 and 2.26 Å respectively, B is equal to 0.37, and R is the experimental distance (Brown and Altermatt, 1985). The results of bond valence calculation are summarized in Table 3. Clearly, the values obtained for KDFTFSI show the higher contribution of O over N and also the stronger interaction of oxygen in contact with K1 as compared to those in contact with K2. In addition, it can be observed that the contribution of the so-called secondary interactions cannot be neglected (0.05 vs. 0.19).

In comparison with KTFSI (Matsumoto et al., 2015), potassium atoms in KDFTFSI present a similar coordination environment. Both structures present two types of potassium cations, a K⁺ cation that coordinates with six anions only through K···O interactions and a second K⁺ cation that is also interacting with six anions but exhibits two additional K···N interactions. Thus, despite the differences in their coordination geometries, both KDFTFSI and KTFSI coordinate in a similar way to the anions in terms of the number of anions involved in the coordination as well as in the anion atoms involved in the coordination (Figure 4 and Matsumoto et al., 2015). Different from sodium salts reported in the literature (Matsumoto et al., 2015) the bond valence sums of potassium atoms within the same structure (i.e., K1 vs. K2) in KDFTFSI (1.09, 1.47) and KTFSI (1.35, 0.86) differ significantly. This difference is slightly higher in KTFSI (0.49). Indeed, KDFTFSI presents in general higher bond valence for K···O contacts which may relate to stronger interactions. This phenomenon could be ascribed to the lower dissociation energy/higher delocalization in the TFSI⁻ anion, which could lead to several lower-level interactions of the cation that are not considered within this list of primary and secondary interactions.

As shown in Table 2, both KTFSI and KDFTFSI present a distorted cis conformation. Those torsion angles formed by the fluorinated terminals are slightly higher in KDFTFSI. To gain further insights into the preferential disposition of those anions and the effect of the anion and the cation, we performed a series of DFT calculations to compute the energy difference between the cis and trans conformations in TFSI⁻ and DFTFSI⁻anions, as well as for the corresponding potassium salts (Figure 5). Our DFT calculations reveal that, in the gas phase, the trans conformation is the most stable in both TFSI⁻ and DFTFSI⁻ anions. Specifically, the trans-TFSI⁻ (DFTFSI⁻) is 2.1 (12.2) kJ mol⁻¹ more stable than the cis conformer. This is because of internal atomic distortions of the trans conformer that enable an additional H···O interaction (2.3 Å) between the hydrogen atom from the CF₂H group and the oxygen atom from the opposed sulfonyl group. (Figure 5B). Interestingly, the inclusion of a screening dielectric medium by means of an MPE implicit solvent model flips the stability of cis and trans cations (Figure 5E), where the cis already becomes the preferred conformation at relatively low values of dielectric constants for both TFSI⁻ and DFTFSI⁻ anions. The interactions with alkaline cations in the corresponding salts have a major impact on the stability of the conformers. Previous works have showed that the LiTFSI salt has an increased energy gap, where the trans conformer was 5.0 kJ mol⁻¹ more stable than its cis counterpart (Zhang et al., 2018). In fact, a preferential disposition for the trans conformer is also observed in the crystalline structure of LiTFSI (Nowinski et al., 1994; Matsumoto et al., 2015). We note that this gap observed for LiTFSI is larger than the one considering the TFSI⁻ anion alone. Interestingly, the contrary happens when considering the potassium salt in the gas phase, where the trans-cis energy gap is again flipped to 3.3 kJ mol⁻¹ for KTFSI and 2.1 kJ mol⁻¹ for KDFTFSI, this time favoring the cis conformer. Similar to the case of the isolated anions, the cis conformer gets further stabilized by the inclusion of an implicit solvent (Figure 5F), where the trans-cis energy gap increases to 13.1 kJ mol⁻¹ for KTFSI and 13.6 kJ mol⁻¹ for KDFTFSI. This is a consequence of the larger size and polarizability of K⁺ in comparison with Li⁺, which enables a K···O tridentate coordination (Figures 5C, D) in contrast to the bidentate coordination established in the lithium systems. These results reveal two effects that tip the balance between cis and trans conformations of the potassium salts. On the one hand, the inclusion of an isotropic dielectric medium stabilizes the cis conformers of both isolated anions and salts. On the other hand, the inclusion of the K⁺ cation itself further stabilizes the cis conformer. This suggests that KTFSI and KDFTFSI ion-pairs will prefer to adopt the cis conformation when precipitated from a saturated solution, these preliminary nucleation points would then dictate the patterning for further crystal growth.

Still, the final cis or trans conformations of the TFSI⁻ and DFTFSI⁻ anions in the crystal structure are subject to additional considerations that are not fully captured by isolated molecular models. For example, the ion-pairs are not screened in an isotropic medium, but rather a crystal field. Entropic contributions might also play an important role in the conformation of the anions within the crystal. Indeed, TFSI⁻ is found in both cis and trans conformations depending on the experimental temperature, indicating solid-solid phase transitions (Henderson et al., 2005; Herstedt et al., 2006; Zhou et al., 2011). While the latter is out of the scope of the work and is generally challenging for DFT methods, we addressed the former by constructing additional crystalline models for KTFSI and KDFTFSI, where the anions appear in both conformations. Figure 6 shows the relaxed crystal structures of KTFSI and KDFTFSI salts with cis and trans conforming anionic units. The calculations reveal that the crystal structures with cis conformations are 57.8 (52.1) kJ mol⁻¹ more stable than the trans counterparts for KTFSI (KDFTFSI). While such large observed energy difference could be due to the fact that the trans crystal structures are optimized to a less stable local minimum, we argue that the preference of K⁺ for a tridentate K···O coordination in the isolated models, allow for a more efficient crystal packing with cis conforming anions. In fact, the original LiTFSI crystal structure reported (Nowinski et al., 1994) showed that each trans TFSI⁻ anion forms a bidentate Li···O coordination, such that each Li⁺ only interacts with the TFSI⁻ anions located along the crystallographic a-axis. Our calculations reveal that the structure undergoes significant distortions after substituting Li⁺ by the larger K⁺, where the latter tries to coordinate with other O ions along the b-axis (Figure 6), but this coordination is incomplete when compared to that of the cis structure.

The calculated structural descriptors of the isolated TFSI⁻, DFTFSI⁻, KTFSI, and KDFTFSI trans (cis) models are shown in Table 4. All cases show values that compare well with the experiments shown in Table 2. Additionally, DFTFSI⁻ shows H‒C distances of 1.095 (1.095) Å, and angles S‒C‒H 108.9 (109.2)°; F‒C‒H 110.6 (110.0)°. Our calculations show that very slight differences exist between cis and trans conformers in terms of bond distances and angles, both for TFSI⁻ and DFTFSI⁻ anions. The inclusion of K⁺ does not lead to significant differences in this regard either. However, noticeable differences can be seen in the C‒S···S‒C dihedral angles. In the case of TFSI⁻, we observe an angle of 173.3° and 39.4° for the trans and cis conformers, respectively, while the dihedral angles of DFTFSI⁻ are 149.7° and 28.8°. Indeed, these differences in the dihedral angles could be ascribed to the above-mentioned increased stability of the trans conformers of the DFTFSI⁻ anions, where an additional H···O interaction is observed. Thus, in DFTFSI⁻, the CFX groups do not present a fully eclipsed relative disposition in the minimum energy state of the cis conformer. In the case of potassium salts, KTFSI and KDFTFSI, trans (cis) dihedral angles are 173.4 (77.7)°, and 176.6 (78.7)°, respectively. The inclusion of the K⁺ cation results in larger dihedral angles of the trans conformers of KDFTFSI, while the contrary is true or KTFSI. The relaxation of the trans conformer of KDFTFSI results in a reconfiguration of the CF₂H group, where the H···O interaction is no longer present (Figure 5D), which could explain the increment on the value of the dihedral angle. Looking at the cis conformer, the inclusion of the K⁺ cation results in larger dihedral angles for both KTFSI and KDFTFSI. Additionally, the structural descriptors of the trans (cis) models for the KTFSI and KDFTFSI crystals are shown in Table 5. Once again, all bond distances and angles correlate well with the experimental values shown in Table 2, with small differences between the cis and trans conformers. However, we can see that their bond distances are slightly overestimated probably due to the use of the PBE functional. The dihedral angles for KTFSI and KDFTFSI with trans (cis) configurations are 157.6 (17.8)°, and 149.3 (28.1)°, respectively. As we expected, these values are closer to experimental ones when compared to those of the isolated ion-pair calculations and show the importance of the crystalline environment in the geometry and conformation of molecular solids. Lastly, we note that the dihedral angles of the cis conformers are slightly larger for KDFTFSI than for KTFSI, in agreement with the experimental observations (Table 2).

In summary, the crystal structure and cation-anion interactions of the potassium salt with a hydrogen-containing sulfonimide anion, (difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide {[N(SO₂ CF₂H)(SO₂CF₃)]⁻, DFTFSI⁻}, have been thoroughly examined, and comparatively studied with the conventional perfluorinated sulfonimide salt, potassium bis(trifluoromethanesulfonyl)imide {K[N(SO₂CF₃)₂], KTFSI}. From the single-crystal X-ray diffraction analyses, the KDFTFSI salt shows a layered structure alternating cationic and anionic layers, in which K⁺ cations are sandwiched by the oxygen atoms in the sulfonyl groups. Despite a similar stacking pattern, KDFTFSI presents a higher bond valence for K···O contacts as compared to KTFSI, which is a clear sign for the increased cation-anion interactions by replacing highly electronegative fluorine with hydrogen atom in the former salt. The DFT calculations further reveal that the cis conformation is the most stable geometries for both TFSI⁻ and DFTFSI⁻ anions, especially when the effects of dielectric screening and interaction with K ions are accounted for in the calculations. Moreover, the presence of H-bonding in the DFTFSI⁻ anion brings a higher energy gap for the conversion between trans and cis conformers. The structural information of KDFTFSI provided in this work is of great benefit to getting a deeper understanding on the unique properties of the DFTFSI-based liquid and solid polymer electrolytes, thereby promoting the development of high-performance rechargeable batteries.