Polymer-based Solid Electrolytes for Rechargeable Zn-ion Batteries

Abstract

In the pursuit of achieving a balance between electrochemical performance, safety, and cost, aqueous Zn-based batteries are deemed a competitive candidate because the Zn metal anode is featured with a high theoretical gravimetric capacity of 820 mAh g^-1 and low redox potential of -0.76 V vs. the standard hydrogen electrode (SHE). However, the irreversible issues, such as the dendrites and corrosion of the Zn anode, are the Achilles heel of the renaissance of Zn-based batteries. In specific, the electrochemical performance and cyclic stability of Zn-based batteries would be degraded by the irreversible reactions of the Zn anode as hydrogen evolution reaction (HER) and the Zn dendrite issues due to the inevitable existence of H₂O molecules in the applied aqueous electrolytes. To solve the irreversible issues at the Zn metal anode, solid polymer electrolytes (SPEs) are the promising electrolyte candidates to fundamentally eliminate the H₂O-related HER and the dendrite issues of the Zn anode.

However, the low ionic conductivity of SPEs does not meet the performance demands for the high-power density of Zn-based batteries. Herein, we first regulated the high dielectric constant (εr) and the ferroelectric domain of the hosting polymer frameworks to promote salt dissociation and ion transportation, thereby enhancing the ionic conductivity and cation mobility of SPEs. Specifically, a high ionic conductivity of up to 1.07 mS cm^-1 was achieved by utilizing the SPE based on poly (vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene) [(PVDF-TrFE-CTFE)] (referred to as PVTF). The as-fabricated symmetric Zn||Zn batteries based on the PVTF SPE could deliver splendid cycling stability for more than 210 days (>5000 h). When paired with a cathode, the developed solid Zn hybrid batteries can operate at a high power up to 10 C-rate and exhibit stable cycling for 1000 cycles at 2 C.

Next, considering the charge transfer resistances for the bulk SPEs at high working currents would trigger enlarged working overpotentials to degrade the energy density and cycling stability, we incorporate Zn²⁺-insulating oxides (CeO₂) with a high concentration of oxygen vacancies into the bulk SPE (referred to as V-SPE) to increase the interaction between the surface of oxides and the anion of the Zn-salt in the polymer, promoting the mobility of Zn²⁺ ions and enhancing the Zn²⁺ conductivity of the composite electrolyte above 10^-3 S cm^-1 at room temperature. The developed solid Zn|V-SPE| pyrene-4,5,9,10-tetraone (PTO) batteries can operate at a high power up to 10 C-rate delivering a high specific capacity of 180 mAh g^-1. Excellent cycling performances at a high current density of 5 C are also achieved.

The electrical field in solid electrolytes is also of exceptional significance to regularize Zn deposition process and the distribution of Zn²⁺ flux. The low deformability and high interfacial resistance of SPEs usually show a poor critical current density, resulting in nucleation and growth anisotropic Zn filaments. Thus, we introduce piezoelectric CaBi₂Nb₂O₉ (CBN) nanosheets into the most widely used PVDF [poly (vinylidene fluoride)]-based SPEs to uniform the morphology of Zn deposits. Theoretical simulations and experimental results suggested that the piezoelectricity linked electrical field in SPEs would reduce the driving force of dendrite growth at high curvatures, and the ferroelectricity-linked electrical field reduces the overpotential. Finally, the Zn||Zn symmetric cells exhibit an ultralong cycle lifespan with a low polarization potential. Satisfactory electrochemical performances for Zn||PTO full cells are also obtained due to the even deposition of Zn²⁺ flux.

To conclude, research on SPEs for Zn-based batteries have been studied from multidimensions in this thesis. The mechanisms behind have been explored throughout and further improvement protocols have been proposed and validated. It’s believed that the studies in this thesis are significant in moving the commercialization of Zn-based batteries forward.

Date of Award	2 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chunyi ZHI (Supervisor)