Electrolyte Design for Aqueous Zn-ion Batteries with Enhanced Stability and Durability

Abstract

The high intrinsic safety and low cost of rechargeable aqueous Zn-ion batteries (ZIBs) make them promising candidates for large-scale energy storage. Recent advancements have witnessed the remarkable prosperity of ZIBs, primarily attributed to the development of high-performance cathode materials capable of enduring thousands of cycles. However, the lifespan and practical application of ZIBs are constrained by water-induced side reactions and the formation of dendrites. Regulation of the Zn²⁺ solvation structure by organic molecules has been demonstrated effective in prolonging the lifespan of the Zn anode by suppressing the H₂O-induce reactions. However, the underlying mechanism that an alteration of solvation structure can enable a dendrite-suppressed plating morphology has not been well elucidated.

The main reason for metal dendrite growth lies in the relatively slow ion migration (mass transport) compared to the fast electrochemical reduction (charge transfer). The contradiction between the fast electrochemical kinetics and slow mass transfer may create significant concentration gradients on the electrode surface, which in turn causes uneven growth of the metal Zn and short circuits in batteries. Therefore, controlling the electrochemical reduction to achieve a slower reaction rate may help to homogenize the Zn deposition in ZIBs.

In our first work, we introduced tributyl phosphate (TBP) with steric hindrance effect into the electrolyte, which is proposed to moderate the fast electrochemical kinetics and responsible for uniform Zn deposition. The electrochemical analysis reveals a more moderate reduction rate, and the SEM observation demonstrates the smooth and compact morphologies of deposited Zn. As a result, the Zn||Cu half-cell has demonstrated stable Zn plating/stripping with an average Coulombic efficiency of ~99.5% and cumulative capacity of 3000 mAh cm^-2, even under harsh cycling conditions of 10 mA cm^-2 and 10 mAh cm^-2. When coupling with the Mn²⁺ expanded hydrated V₂O₅ cathode (MnVO), the full cells also exhibit high areal capacity (3.97 mAh cm^-2), high capacity retention (91.4%), and long cycling life (650 cycles).

Next, we employed electrolyte additives containing urea derivatives with different alkyl chain lengths to gain a deeper insight into the influence of steric hindrance on the electrodeposition of a zinc anode. By modifying the alkyl chains of these organic additive molecules, we aimed to introduce varying degrees of steric hindrance effects that could potentially affect the dynamics of mass transfer and charge transfer during the electrodeposition process. Among these derivatives, the electrolyte containing N-dimethylurea (DMU) additives exhibited the highest ratio between the activation energy for charge transfer and mass transfer, effectively regulating and balancing the kinetics during the electrodeposition process. As a result, the half-cells utilizing the DMU electrolyte exhibited remarkable stability, with a large cumulative capacity of 11000 mAh cm^-2 for Zn||Zn cells and 7500 mAh cm^-2 for Zn||Cu cells at a high current density of 10 mA cm^-2. Furthermore, when incorporating four stacks of cathodes, the Zn||Zn_0.25V₂O₅·nH₂O pouch cell demonstrated a significant capacity of 6948 mAh.

In addition to the stability of aqueous ZIBs, durability represents another significant challenge in their practical application. Despite its crucial significance in real battery usage scenarios, the durability investigation is largely overlooked in the context of ZIBs. Our next work investigates the durability of two typical aqueous ZIBs under overcharge conditions (MnVO and manganese dioxide (MnO₂) as cathode materials. Experimental findings highlight the detrimental effects of overcharging on ZIBs, leading to rapid battery failure primarily attributed to electrolyte decomposition and subsequent deterioration of interfacial contact. Subsequently, self-sacrificial electrolytes are developed by introducing bromine-based additives into the electrolyte (tetrabutylammonium and benzyl trimethylammonium bromine). These additives undergo oxidation before the electrolyte decomposition, introducing an additional Br^-/Br₂ redox couple. Consequently, this approach effectively stabilizes the electrolyte environment. It provides efficient overcharge protection for extended periods, with the capability to sustain Zn||MnVO and Zn||MnO₂ batteries for over 650 hours and 550 hours, even at harsh 200% state-of-charge conditions, respectively.

In conclusion, this thesis thoroughly investigates the stability and durability of ZIBs by developing novel electrolytes with carefully selected additives. Comprehensive investigations were conducted to gain a deeper understanding of the underlying mechanisms, and novel approaches were proposed and experimentally validated to enhance the stability and durability of ZIBs. The findings of this thesis have significant implications for the advancement and commercialization of aqueous ZIBs.

Date of Award	15 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chunyi ZHI (Supervisor)