Fig. 1

Critical challenges for low-temperature operation of sodium-ion batteries"

Fig. 2

(a) Conductivity (left hand side y axis) and viscosity (right hand side y axis) values of electrolytes based on 1 mol/L NaClO4 dissolved in various solvents and solvent mixtures; (b) DSC heating curves of electrolytes after cooling the sample down to -120 ℃. Electrolytes based on 1 mol/L NaClO4 dissolved in various solvent mixtures (top) and PC based electrolytes with 1 mol/L of various Na salts (bottom)[41]; (c) Rate performance of the NVP@C at different temperatures; (d) charge/discharge profiles of the NVP@C electrodes with different rates at -20 ℃ to -30 ℃[42]"

Fig. 3

(a) Concentration dependence of ionic conductivity and viscosity at 25 and 0 ℃ for NaPF6 in EC/PC electrolyte[43]; (b), (c) The solvation structure computed by Ab initio molecular dynamics (AIMD) simulations, which are displayed as snapshots based on NaClO4 dissolved in a EC-PC-FEC system; Most probable solvation structure extracted from AIMD simulations together with their binding energy values (B.E.): (d) the weakly-solvating 0.3 mol/L electrolyte (labeled as WSE), (e) the normal-used standard 1.0 mol/L electrolyte (labeled as NUE); (f), (g) TEM images of the NVPF cathodes after cycling; (h) Long-term cycling performance of the NVPF cathodes at 1C at -25 ℃[44]"

Fig. 4

(a) Representative solvation sheath determined by MD simulations, (b) Viscosity and (c) ionic conductivity test of the carbonate, F-carbonate and WT electrolytes under different temperatures; (d) The binding energy between Na+ ions and corresponding solvents; (e) Impedance spectra the Na||Na symmetric cells with the WT and carbonate (inset image) electrolytes at -20~60 ℃ after 3 formation cycles; (f) Long-term cycling performance of Na||NVP cells operated in the corresponding electrolytes at 0.5C, -20 ℃[51]"

Fig. 5

(a) Galvanostatic charge and discharge curves of HCP at 50 mA/g at the temperature range from -25 to 25 ℃; (b) Temperature-dependent capacity retention at various current densities[52]; (c) DSC thermograms of different electrolytes from 0 °C to -150 ℃; (d) Temperature-dependent ionic conductivity of the electrolyte solutions in the range between 20 ℃ and -80 ℃; (e) Contents of C 1s, O 1s, F 1s, S 2p and Na 1s elements in the SEI at -80 ℃; (f) Contents of the main SEI inorganic components, including NaF, Na2SO4 and Na2SO3, at -80 ℃; (g) Long-term cycling performance of cells at 22 mA/g at -20 ℃, -40 ℃ and -60 ℃[27]; (h) Calculated electron density of BCP for Na+ —O bond in three electrolytes. (I) O in N-THF, (II) O of THF in N-mixTHF, (III) O in N-2MeTHF, and (IV) O of 2MeTHF in N-mixTHF; (i) Long cycling performance in N-mixTHF at different temperatures (100 mA/g beyond -20 ℃, 50 mA/g at -40 ℃, and -60 ℃)[54]"

Fig. 6

(a) Ionic conductivity of the CSE, BLTE and ES6-BLTE electrolytes at -40 ℃; (b) The numbers of FSI- around Na+ in the solvated structure obtained from the MD simulations in BLTE and ES6-BLTE (A represents the number of FSI- coordinated with per Na+. In the SSIPs structure, Na+ is only coordinated with solvent in the first solvation shell; in the CIPs structure, Na+ is coordinated with one FSI-; in the AGGs structure, Na+ is coordinated with more than one FSI-); Most probable solvation structure extracted from MD simulation for (c) BLTE and (d) ES6-BLTE electrolytes; (e) The desolvation energy of BLTE and ES6-BLTE electrolytes obtained from DFT calculations[61]; (f) Donor number of commonly used anions; (g) Binding energy of Na+ with anions; (h) Proportion of free G2 and solvated G2 in different electrolytes; (i) Coordination number from MD simulation of electrolytes without and with NaTFA; (j) Desolvation energy of four representative solvation configurations[33]"

Fig. 7

(a) Diagram of the synergistic regulatory mechanisms driven by distinct intermediates for interfacial modification and HF consumption with the role played by TMSPi and FEC; (b) Cycling performances with the NPFT electrolyte at -25 ℃[26]; XPS spectra of the cathode in the battery with the electrolyte containing 0% ADN after the first charge-discharge cycle: (c) F 1s and (d) N 1s; XPS spectra of the cathode in the battery with the electrolyte containing 3% ADN after the first charge-discharge cycle: (e) F 1s and (f) N 1s[67]; (g) Calculated lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels for the solvent molecules, sodium salts, additives; (h) The capacity retention of Na||NVP cells using varied electrolytes at 0.1C under -45 ℃[35]"

Fig. 8

The solvation structure variations with temperature driven by entropy change: (a) non adaptive electrolyte; (b) temperature-adaptive electrolyte. ΔS here, is the difference between the solvation entropy at room temperature (S1) and low temperature (S2), i.e., ΔS = S2-S1. RT: room temperature; LT: low temperature; (c) The calculated entropy change values along with solvation structure variations. The ∆ S of solvation structure change from 25 ℃ to -40 ℃ shows a positive value at NDT system, while it keeps a negative entropy change for ND and NT; (d) Photographs of NDT, ND, and NT electrolytes stored at different temperatures[72]"