Extended Cycle Life Pb-Ion Batteries with High Coulombic Efficiency Exceeding 99.96% from Al/Pb Bimetallic Eutectic Electrolyte
Authors: Huang Ren-Jie , Pan Chun-Jern , Lee Chun-I , Hwang Bing-Joe 
Published: August 2024

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(ClO4)3·9H2O), lead perchlorate (Pb(ClO4)2·3H2O), 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(ClO4)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 ClO4
- stretching band increased, compared to the concentrated Pb(ClO4)2 aqueous solution; it can be understood that O-H interaction between Pb2+ and ClO4
- is weakened; meanwhile, ClO4
- ions participated in first solvation shell. This variation reduces the desolvation energy of Pb2+ 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 Na3V2(PO4)3(NVP) and natural graphite (NG) as cathode material; the reversible Pb2+ and ClO4
- (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 (Rs) and charge transfer resistance (Rct) 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 (Pb2+ and ClO4
-) intercalation phenomenon. To further confirm the Pb2+ 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 V4+ to V3+, the EXAFS spectra show the local structure change of V-P/Na/Pb bonds. The EXAFS spectra at the Pb LIII-edge indicated that the Pb-O distance is shortened during discharging; the abovementioned results demonstrate the Pb2+ (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 Pb2+ 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