Initiating Jahn–Teller Effect in Vanadium Diselenide for High Performance Magnesium‐Based Batteries Operated at −40 °C

Poor electronic and ionic conductivity of electrode materials at low temperatures of −20 °C and below has significantly impeded development of batteries for cold conditions. However, for the first time, layer‐structured metallic vanadium diselenide (1T‐VSe2) is reported as a cathode material for low‐temperature Mg2+/Li+ hybrid batteries. A high electronic conductivity and fast ion diffusion kinetics for 1T‐VSe2 are demonstrated at selected temperatures, and a very safe 1T‐VSe2/Mg battery for operation at temperatures to −40 °C. The battery exhibits 97% capacity retention over 500 cycles, which is better performance than reported Mg‐based batteries. The Jahn–Teller effect in compressed configuration is initiated in 1T‐VSe2 with the change of electronic state occurring on electrochemical intercalation of alkali metal ions. Using combined experiment and theory via operando synchrotron X‐ray diffraction, ex situ X‐ray absorption spectroscopy and DFT computation, it is confirmed that the weak Jahn–Teller distortion contributes significantly to fast‐overall kinetics, structural stability, and high electronic conductivity of the electrode. Understanding at an atomic level of the mechanism is demonstrated, that provides valuable guidance in designing high‐performance electrode materials for low‐temperature batteries.


Introduction
Batteries are significantly important in 'green energy' as a power source for electronic devices. Highly significant capacity loss at sub-zero temperature limits application. This is a result of poor electronic and ionic conductivity because of sluggish charge transfer and slow metal-ion mobility in the bulk of Li + . [20] The complexes of V 4+ (V 3+ ) with d 1 (d 2 ) electronic configuration exhibit a weak J-T distortion. However, unlike the strong distortion in the complexes of Mn 3+ and Ni 3+ , the weak J-T distortion in 1T-VSe 2 remains poorly understood, and, significantly, 1T-VSe 2 as a cathode at low temperature is rarely reported.
Here, we report for the first time 1T-VSe 2 as the cathode for low-temp Mg-based hybrid (Mg 2+ /Li + ) batteries. The Mg-based hybrid battery combines the tetrahydrofuran (THF)-containing electrolyte which has a low melting point (−108 °C) and viscosity with a Mg metal anode. Mg-foil is used as the pure metal anode, that can be readily recycled and is dendrite-free. [ 21 ] It is acknowledged that lithium metal batteries usually exhibit poor electrochemical capabilities and safety risks due to the formation of Li dendrites at low temperature. Mg-based hybrid batteries outperform conventional lithium metal batteries with better safety and low-temp performance. The designed 1T-VSe 2 / Mg hybrid battery in this work exhibits long-term cyclability with capacity retention of 97% over 500 cycles and 92.2% of its room temperature (RT, 25 °C) capacity at −20 °C. The 1T-VSe 2 electrode without special modification can withstand low temperature to −40 °C. Significantly, this is the reported 'best' cycling and low-temp capabilities with Mg-based batteries. Findings from combined operando synchrotron X-ray powder diffraction (XRD), ex situ X-ray absorption spectroscopy (XAS), density functional theory (DFT) computations and electrochemical spectroscopies confirm, 1) occurrence of J-T compression and 2) an ultra-stable lattice structure in 1T-VSe 2 on electrochemical intercalation of Li + , 3) high electronic and ionic conductivity of the electrode, and 4) excellent low-temp performance of the newly designed battery.

Synthesis and Structural Characterization of 1T-VSe 2
VSe 2 was synthesized via direct reaction of vanadium and selenium powders at 550 °C in a vacuum-sealed, borosilicate glass tube ( Figure S1, Supporting Information). XRD patterns presented in Figure 1a are indexed to the singlephase of hexagonal VSe 2 with a space group of P-3m1 (JCPDS card No. 89-1641). The lattice parameters, a(b) = 3.356 Å and c = 6.108 Å were obtained through the Rietveld refinement using TOPAS software. VSe 2 as a typical layered TMDs material has a single, sandwich repeat-unit with V atoms in an octahedral configuration. The sandwiched Se-V-Se layers are perpendicular to the c-axis, whilst V-V chains along the a and b axis form a network structure of the ab-plane ( Figure S2, Supporting Information). Scanning electron microscopy (SEM) images of VSe 2 are presented in Figure 1b (and Figure  S3a-c, Supporting Information). The layered structure is observed from the cross-section of the bulk material. The transmission electron microscopy (TEM) images, Figure 1c (and Figure S3d, Supporting Information) show a lattice fringe of 0.26 nm, assigned to the (0 1 1) crystallographic plane of VSe 2 . The configuration of the material is verified by the Raman spectrum, Figure 1d. A distinct Raman peak at ≈205 cm −1 corresponds to the out-of-plane active A 1g mode for 1T-VSe 2 . [18,22] These findings evidence that the as-synthesized bulk VSe 2 is of high quality and an 1T (octahedral) phase with metallic character. Following electrode fabrication, the XRD pattern of the prepared 1T-VSe 2 electrode exhibited an enhanced intensity of (0 0 l) peaks, Figure 1e, confirming the formation of a layer-by-layer assembled 1T-VSe 2 thin film with preferred crystalline orientation along [0 0 1] axis.

Jahn-Teller Compression Captured via Extended X-Ray Absorption Fine Structure (EXAFS) Spectroscopy
J-T effect is typically observed in octahedral complexes, where two z (axial) bonds can be either longer (J-T elongation) or shorter (J-T compression) than those of the x and y (equatorial) bonds ( Figure S4, Supporting Information). An atomic structural analysis via ex situ XAS was used to determine the J-T effect. Figure 2a shows the X-ray absorption near edge structure (XANES) of the V K-edge for electrodes at different cut-off potentials, namely L0, L1, L2, and L3, as marked on the inset of the discharge-charge curve. Pristine 1T-VSe 2 electrode (L0) exhibited a weak pre-edge, evidencing a slightly distorted octahedral symmetry of V atoms in the prepared sample. [23] The spectrum changed highly significantly following Li + intercalation. As the depth of discharge increased, the intensity of the pre-edge peak from L0, L1 to fully discharged L2, progressively increased, evidencing a growing distortion of V-Se octahedra and promoting a dipole-allowed absorption. Figure 2b and Figure S5 (Supporting Information) present the enlarged image of the pre-edge area for L0 and L2, and vanadium oxide references. The increased pre-edge of 1T-VSe 2 on electrochemical intercalation of Li + is similar to the vanadium oxide because of decreased coordinate symmetry of center V atoms. [24] The electrochemical intercalation of alkali metal cations accompanied by the electron transfer, induces a local structural distortion of 1T-VSe 2 in which V coordination symmetry decreases. Figure 2c shows the Fourier-transformed (FT) EXAFS spectra for V K-edge for L0 and L2. The pristine electrode exhibits a dominant feature at 2.13 Å that corresponds to the V-Se distance, while the peak at 2.74 Å is because of scattering from the nearest V atom. The least-square fit for the FT-EXAFS spectra for L0 are given in Figure S6 (Supporting Information), and the phase-corrected structural parameters obtained are listed in Table S1 (Supporting Information) to show that the original VSe bond length in pristine 1T-VSe 2 is ≈2.47 Å. The actual distance is 0.3-0.5 Å longer than the observed distance in FT-EXAFS spectra. [25] When discharged to 0.4 V (L2), the spectrum changes significantly and a new dominant peak at 1.55 Å appears. Both peaks at 1.55 and 2.08 Å for the L2 spectrum are assigned to V-Se scattering. To boost the sensitivity of EXAFS analyses, the wavelet transform (WT) was applied to provide a high resolution of the signal features in both k and R spaces. The backscattering amplitude factors F(k) in k space are significantly sensitive to the scattering coordinate atomic number (Z). [26][27][28] As is illustrated in Figure 2d, the A, B and C lobs in WT-EXAFS contour plots locate almost at the same k value, which is assigned to Se atoms. These findings all evidence that the original VSe bond splits into two different VSe bond lengths following electrochemical intercalation of Li + , as is Figure 2. Local structure analysis for V via XAS spectra. a) XANES spectra for V K-edge at differing potential states during first cycle. Inset is dischargecharge curve. b) Enlarged pre-edge spectra with reference to vanadium oxides. c) FT-EXAFS spectra. (Inset: Schematic for 1T-VSe 2 lattice distortion before and following Li + intercalation). d) Wavelet transforms for k 2 -weighted EXAFS signals for pristine (L0) and fully discharged (L2) 1T-VSe 2 .
depicted in the inset of Figure 2c. The newly appeared, shorter VSe bonds correspond to the shorter z bonds, appearing as a compressed octahedral complex ( Figure S4, Supporting Information). Therefore, we conclude the electrochemical intercalation of Li + initiates J-T compression in 1T-VSe 2 .

Phase Change with Ultra-Stable Structure Identified via Operando Synchrotron XRD
For the hybrid system, the superior Li + mobility takes place in the 1T-VSe 2 cathode, whereas Mg stripping /plating proceeds in the anode ( Figure S7, Supporting Information). The structural evolution of the 1T-VSe 2 electrode was monitored via operando synchrotron XRD, Figure 3a and Figure S8 (Supporting Information). The corresponding discharge-charge curve is shown in the right of Figure 3a, in which an apparent flat plateau is exhibited. The XRD peaks for the pristine electrode (L0) are assigned to 1T-VSe 2 with a space group of P-3m1. The extra peaks, as marked with an asterisk, are from Mg-foil. The Rietveld refinement for the synchrotron data using GSAS II package [29] is based mainly on three major peaks. The peaks from Mg-foil and current collector are excluded. The diffraction pattern for the fully discharged state (L2) is indexed to the LiVSe 2 phase that is captured in situ during cycling. The 1T-VSe 2 host lattice undergoes a first-order phase transition on Li + intercalation which corresponds to the flat plateau of the discharge-charge curve. The newly formed LiVSe 2 has almost the same hexagonal structure as for 1T-VSe 2 , but exhibits increased lattice parameters in a, b, and c, as all peaks shift toward lower angles following Li + intercalation. Figure 3b and Figure S9 (Supporting Information) show selected individual XRD patterns including, pristine, fully discharged and fully charged states of the electrode. The peaks corresponding to (0 1 1), (1 0 2), and (1 0 3) planes are shifted back to the original position following a full cycle, demonstrating a highly reversible process. The interlayer distance change with 1T-VSe 2 was calculated from Bragg's formula (d = 0.5λ/sin(θ)). The calculated distance for pristine 1T-VSe 2 is 6.10 Å, a value that is in line with the Rietveld refinement as c = 6.10 Å ( Figure S10, Supporting Information). With electrochemical intercalation of Li + , the calculated interlayer distance has increased to 6.38 Å, corresponding to the refinement result of c = 6.38 Å for a fully discharged state, Figure 3c. The parameter a(b) corresponding to the basal dimension, also increases with Li + intercalation, as depicted in Figure S11 (Supporting Information). The increased a(b) is attributed to the elongated V-V distance because of the electron transfer from intercalated Li + . [30] The unit cell parameters from the Rietveld refinement for pristine and fully discharged electrodes are summarized in Table S2 (Supporting  Information).
It is noteworthy that the percentage expansion value for parameter c following intercalation of Li + is less than parameter a(b) in 1T-VSe 2 .  lattice parameters for 1T-VSe 2 before and following Li + intercalation and a summary of lattice parameters for other TMDs. The c/a ratio for 1T-VSe 2 decreases following Li + intercalation, which is different from those for other dichalcogenides in groups IV and V with increased c/a ratio. The Δc (difference in c value following and before Li + intercalation) for other dichalcogenides, except 2H-MoS 2 , [31] is >0.42 Å (vs 0.28 Å for 1T-VSe 2 ). As is acknowledged the structure degradation of layered electrode materials is a major obstacle to long-term cyclability. The highly significant difference in Δc can cause irreversible structural damage and capacity fading with foreign ion intercalation. The lattice stability in 1T-VSe 2 with small Δc is achieved via the weak J-T (compressed) distortions resulting in the rearrangement of atoms in V-Se complexes with the intercalation of alkali Li + . The small expansion of the host lattice in the c direction suppresses possible structural collapse and determines the long-term cycling performance of the 1T-VSe 2 electrode.

1T-VSe 2 /Mg Batteries Operated at Ultra-Low Temperature
1T-VSe 2 was assessed as a cathode for Mg 2+ /Li + hybrid batteries at selected temperatures. The magnesium-aluminium chloride complex (MACC) with the addition of LiCl in THF was used as the hybrid electrolyte. 1T-VSe 2 exhibits a negligible capacity in electrolyte without Li-salt ( Figure S12, Supporting Information). It confirms that the cathode is dominated by Li + intercalation/ de-intercalation. The electrochemical performance for 1T-VSe 2 with hybrid electrolyte at RT is shown in Figures S13 and S14 (Supporting Information). The galvanostatic discharge-charge curves exhibit a stable capacity output, little polarization together with a flat discharge plateau at high current density 500 mA g −1 . The flat curves correspond to a two-phase behavior that is significantly different from other members of TMDs, [32] ( Figure S15, Supporting Information). With 1T-TiSe 2 as the control sample, its detailed structural information and electrochemical properties at RT are given in Figures S16 and S17 (Supporting Information). Because of the low melting point of THF (−108 °C, Table S4, Supporting Information), the hybrid electrolyte remains fluid, even as low as −80 °C ( Figure S18, Supporting Information). Figure 4a shows the temperaturedependent rate performance for 1T-VSe 2 and 1T-TiSe 2 . 1T-VSe 2 is operational at temperature to −35 °C. In contrast, with temperature decreasing the capacity of 1T-TiSe 2 decreases significantly at −20 °C. At low temperature −20 °C, Figure 4b, (and Figure S19, Supporting Information), 1T-VSe 2 exhibited a high initial discharge capacity of 126 mAh g −1 and a low capacity decay rate of 0.007% per cycle (101-97.5 mAh g −1 over 500 cycles) at 100 mA g −1 . The battery exhibited 92.2% of the RT capacity at 64 mA g −1 (0.5 C, 1 C as 128 mA g −1 ) ( Figure S20, Supporting Information). At an ultra-low temperature of −40 °C, 1T-VSe 2 retained a discharge capacity of 68.5 mAh g −1 , inset Figure 4b. The discharge-charge curves for 1T-VSe 2 , Figure 4c (and Figure S21, Supporting Information) exhibit a flat discharge plateau even at −30 °C, together with a significantly low polarization of 183 mV. The high rate performance for 1T-VSe 2 at −20 °C is confirmed as is presented in Figure 4d. The electrode material without modification is capable of operation at a high current density 500 mA g −1 (≈4 C) at −20 °C, evidencing excellent low-temp kinetics. The designed 1T-VSe 2 /Mg battery exhibited best electrochemical performance in rechargeable Mg batteries for stability and low-temp rate performance. Importantly, it outperforms most reported lithium and sodium batteries at −20 °C, Figure 4e (Table S5, Supporting Information).
To establish the origin of the electrochemical performance of 1T-VSe 2 at low temperature, the electronic conductivity and ion diffusion kinetics of the electrode was assessed via electrochemical spectroscopies at selected temperatures. Electrochemical impedance spectroscopy (EIS) was used to monitor battery impedance from 40 to −30 °C, Figure 5a. Fitted results with corresponding equivalent circuit for 1T-VSe 2 are presented in Figure S22 and Table S6 (Supporting Information). R s is the series ohmic resistance of the battery, involving resistance of the electrolyte, and other ohmic contacts and elements in the battery. R ct is the resistance of charge transfer corresponding to the semicircle of the high-frequency portion in EIS. As is presented in Figure 5a (I), with temperature decreasing, R ct for 1T-VSe 2 undergoes an increase from 45 Ω (20 °C) to 136 Ω (−30 °C). However, R ct for 1T-TiSe 2 highly significantly increases, to some thousands of ohms, and R s increases significantly to 230.2 Ω (vs 24.6 Ω for 1T-VSe 2 ) at −30 °C (Figure 5a (II) and Figure S23, Supporting Information). Although 1T-TiSe 2 shares the character of 1T phase and layered structure of 1T-VSe 2 , it exhibits a semi-metallic-character. [33,34] 1T-TiSe 2 as a semi-metal exhibits increasing resistance with decreasing temperature in the range 300-165 K. [35] At low temperature, the increased resistance of 1T-TiSe 2 significantly limits charge transfer of the reaction, resulting in a significantly increased value for R ct . When temperature is switched back to 20 °C, the electrical resistance of both samples returns to a low value ( Figure S24, Supporting Information). 1T-TiSe 2 is more susceptible to the temperature change than 1T-VSe 2 . The inferior low-temp performance for 1T-TiSe 2 is caused by rapidly increased overall resistance with temperature decrease. Whilst 1T-VSe 2 as metallic material reduces resistivity when cooled. [17][18][19]36] And the varied electronic states perform high electronic conductivity on cycling with the initiation of J-T effect.
The change in charge transfer resistance with temperature is largely determined by the activation energy (E a ). According to the Arrhenius relationship (Equation (S1)), the E a for the charge transfer of 1T-VSe 2 derives from the slope of the ln(1/R ct ) versus the inverse of temperature (1/T). As is shown in Figure 5b, the calculated value for E a from charge transfer resistance is 0.12 eV. Via another method, by fitting the peak current (I p ), which is measured from cyclic voltammetry (CV) curves (Figure 5c) at varying temperature into the Arrhenius equation (Equation S2, Supporting Information), the same value for E a of 0.12 eV is obtained, Figure 5d. Importantly, the calculated value for E a in this work is significantly less than reported with Li + intercalation. [2,37] The low threshold energy barrier for the charge transfer for 1T-VSe 2 evidences the fast reaction kinetics of the electrode that significantly boosts lowtemp performance. The excellent cyclability and rate capability of the electrode at different temperatures is expected from high ion diffusion kinetics. The ion diffusion coefficients for 1T-VSe 2 are calculated based on the Randles-Sevcik equation (Equation S3, Supporting Information). CV measurements, for scanning rates from 0.08 to 0.4 mV s −1 were conducted at both RT ( Figure S25, Supporting Information) and −20 °C ( Figure S26, Supporting Information). The calculated diffusion coefficient at RT is 1.67 × 10 −9 cm 2 s −1 which is ≈two orders of magnitude Figure 5. Electronic conductivity and ion diffusion kinetics for 1T-VSe 2 at low temperature. a) Electrochemical impedance spectroscopy for I) 1T-VSe 2 and II) 1T-TiSe 2 at selected temperatures. b) Arrhenius plot for charge transfer resistance of 1T-VSe 2 . c) CV curves at differing temperature, and d) Arrhenius plot for peak current for 1T-VSe 2 . e) Linear relationship between peak current and square root of scan rate based on CV curves for 1T-VSe 2 at −20 °C. greater than reported. [38][39][40][41] At a low temperature of −20 °C (Figure 5e), 1T-VSe 2 continues to exhibit a high diffusion coefficient of 1.7 × 10 −10 cm 2 s −1 . The fast ion diffusion kinetics and high electronic conductivity of 1T-VSe 2 are directly associated with the J-T effect because of the change in electronic states and local atomic structures to give the excellent low-temp performance of the battery. It is concluded therefore that 1) 1T-VSe 2 is a practically promising low-temp electrode material, and 2) weak J-T compression with a change in local atomic structures, exhibits a highly significant impact on physical properties of the complex to boost stability, overall kinetics and electronic conductivity of the electrode for excellent performance.

Elucidating Electronic States and Jahn-Teller Effect via Theoretical Computation
Theoretical computations were performed to confirm kinetics and electronic properties of 1T-VSe 2 on the electrochemical intercalation of Li + . Li atoms prefer to occupy the octahedral sites in the crystal structure. [20] The computed lowest diffusion energy barriers for both 1T-VSe 2 and 1T-TiSe 2 are plotted as Figure 6a, and corresponding migration pathways shown in Figure 6b. 1T-VSe 2 exhibits a lower migration barrier energy of 0.27 eV compared with 1T-TiSe 2 0.35 eV, evidencing a more ready migration of Li + in 1T-VSe 2 . Figure 6c presents the computed total density of states (TDOS) for 1T-VSe 2 at varying Li-containing states, together with the partial density of states (PDOS) projected on V and Se atoms. The electrochemical intercalation for each Li + is accompanied by one electron transfer to the host, that significantly changes the electronic states of the host, resulting in a weak J-T compression in 1T-VSe 2 . Importantly, the product gets more metallic on Li + intercalation with significantly increased electronic states around the Fermi level. The improved electronic conductivity of the electrode from 1T-VSe 2 to LiVSe 2 results the fast electron transfer and as consequence, boosts two-phase transition with high rate capability at varying temperature. The weak J-T distortion with the change of electronic state boosts electronic conductivity, produces fast overall kinetics, and maintains ultra-stable lattice structures in 1T-VSe 2 to result in excellent low-temp performance of our designed battery.

Conclusions
1T-VSe 2 has been demonstrated for the first time as a low-temp cathode in a Mg 2+ /Li + hybrid cell configuration. A 1T-VSe 2 /Mg battery is operational at an ultra-low temperature of −40 °C. It exhibits a long-term cycling performance with a low capacity decay rate of 0.007% and performs over 500 cycles. This performance is better than reported Mg-based batteries and many lithium and sodium batteries. Combined operando synchrotron X-ray diffraction, ex situ X-ray absorption spectroscopy and DFT computations confirm that weak J-T compression occurs and contributes significantly to fast-overall kinetics, structural stability, and high electronic conductivity of 1T-VSe 2 on intercalation/de-intercalation of Li + . Therefore, electrochemical performance at low temperature is boosted. Using 1T-VSe 2 as a model, evidenced understanding at an atomic level of the J-T effect inspires rational design of new, and optimization of existing electrode materials for low-temp batteries.