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Following renewable energy development, batteries are the most crucial way to establish an energy storage system. In the past hundred years, lead-acid batteries have been widely applied in industrial and vehicle applications. Their low cost, simple manufacturing, and good safety can compete with lithium-ion batteries. However, some challenges still exist, e.g., poor low-temperature performance and short cycle life; aqueous solution is the reason that oxygen evolves from positive electrodes, resulting in positive active material detached from the electrode. Thus, developing novel lead-ion batteries to replace lead-acid batteries is a significant issue. In metal-ion batteries research, deep eutectic solvent (DES) has been well developed in recent years due to its high thermal stability and wide electrochemical window. Herein, a new Al/Pb bimetallic eutectic electrolyte (BEE) composed of aluminium perchlorate (Al(ClO 4 ) 3 ·9H 2 O), lead perchlorate (Pb(ClO 4 ) 2 ·3H 2 O), and succinonitrile (SN) with acetonitrile (ACN) as an additive, mixing in molar ratio. To realize the medium effect in this electrolyte, the BEE is compared with concentrated Pb(ClO 4 ) 2 aqueous solution. The ion conductivity, viscosity, and thermal stability depend on the metallic salt/SN ratio, and we proposed that the optimized molar ratio is 1:1:12:5. It exhibits outstanding thermal stability within − 50 to 50 °C. From Fourier-transform infrared spectroscopy (FTIR) spectrum analysis in BEE, the O-H stretching decreased, and the ClO 4 - stretching band increased, compared to the concentrated Pb(ClO 4 ) 2 aqueous solution; it can be understood that O-H interaction between Pb 2+ and ClO 4 - is weakened; meanwhile, ClO 4 - ions participated in first solvation shell. This variation reduces the desolvation energy of Pb 2+ to facilitate smoother lead plating/stripping and enables better electrochemical performance. To validate the effect of the solvation structure variation, assembling the Pb/Pb symmetric cell and Pb/Platinum (Pt) asymmetric cell are essential methods. Pb/Pb cell maintains a stable voltage hysteresis of about 20 mV even cycling 2500 h at the current density of 0.5 mA cm -2 , Pb/Pt cell under galvanostatic conditions successfully performs highly reversible Pb plating/stripping, achieved average coulombic efficiency (ACE) >99.96% in BEE for more than 1000 h at current density 0.5 mA cm -2 . Furthermore, we utilize BEE in Na 3 V 2 (PO 4 ) 3 (NVP) and natural graphite (NG) as cathode material; the reversible Pb 2+ and ClO 4 - (de-)intercalation are respectively observed in Pb/NVP and Pb/NG full cell, Pb/NVP cell exhibited long-term cycling stability with 80.95% capacity retention and 98.21% ACE after 600 cycles at a current density of 100 mA g -1 , then Pb/NG cell remains high discharge voltage plateau (≈1.7V) with 72% capacity retention after 200 cycles at a current density of 100 mA g -1 . Electrochemical impedance spectroscopy (EIS) of Pb/NVP in BEE-1:1:12:0 and 1:1:12:5 shows that adding the ACN additive can decrease the solution resistance (R s ) and charge transfer resistance (R ct ) of the cell. Ex-situ X-ray photoelectron spectroscopy (XPS) was performed on the Pb surface at the charging state, observing that Al species are co-deposited, revealing the synergistic effect between Aluminium perchlorate and Lead perchlorate. From the ex-situ X-ray diffraction (XRD) patterns of cycled NVP and NG cathode at charge/discharge states, some irreversible variation of peak position occurred, indicating that ion (de-)intercalation after operation; these results demonstrate BEE performed bi-functional ion (Pb 2+ and ClO 4 - ) intercalation phenomenon. To further confirm the Pb 2+ dynamic in the NVP cathode at charge/discharge progress, the X-ray absorption spectroscopy (XAS) characterization was applied. The comparison of XANES spectra collected at the V K-edge, the XANES shift to lower energy during discharging, this exhibition is consistent with reduction reaction of V 4+ to V 3+ , the EXAFS spectra show the local structure change of V-P/Na/Pb bonds. The EXAFS spectra at the Pb L III -edge indicated that the Pb-O distance is shortened during discharging; the abovementioned results demonstrate the Pb 2+ (de-)intercalation process in the NVP structure. In conclusion, this work proposed a novel bimetallic eutectic electrolyte for potential lead-ion batteries that enables stable Pb plating/stripping. The cycle-life and capacity retention of Pb/NVP and Pb/NG full-cell with BEE is significantly improved by adding acetonitrile (ACN) additive. The XAS characterization further confirmed the (de-)intercalation of Pb 2+ in the NVP cathode. A stable, high-voltage, and high-capacity positive electrode is needed, allowing the reuse of secondary lead metal to be applied to next-generation energy storage. Figure 1
Huang et al. (Fri,) studied this question.