Hydrogel Electrolytes in Aqueous Batteries for Enhanced Performance

Abstract

Hydrogel electrolytes for enhancing the performances of all kinds of aqueous batteries have attracted widespread attention and been regarded as one of the most promising electrolytes for various energy storage systems featured with high safety, low costs, environmental friendliness, and satisfactory electrochemical performances. Apart from the achievements of many functional hydrogel electrolytes for assembling aqueous batteries featured with self-healing, self-protecting, super-toughness, et.al., the electrochemical behaviors of hydrogel electrolytes, for example, expanding electrochemical stability windows (ESWs) while achieving high ionic conductivities, tuning carrier behavior for activating redox reactions at high potential as well as improving cycle lifespan, making full use of the thick cathodes, et al. with limited attention. Enormous effort has been devoted to suppressing the dendrites growth of zinc (Zn) anodes which greatly reducing the lifespan of Zn anode by hydrogel electrolytes. However, the performance enhances of aqueous batteries on the basis of hydrogel electrolytes requires a comprehensive exploring, which emphasizes not only Zn dendrites but the forms of ESWs of electrolytes with high ionic conductivities, cathode electrochemical behavior and so on. Consequently, it’s imperative to conduct muti-dimensional studies for the hydrogel electrolytes to better understand its electrochemical behaviors.

Hydrogel electrolytes with expanded ESWs usually exhibit low ionic conductivities and high cost based on “salts in polymer” strategy that is widely studied. Hydrogel electrolytes with high water content can integrate these all merits, namely, simultaneously demonstrating the expanded ESWs, high ionic conductivities and low costs. Herein, by profoundly investigating the states of water molecules in hydrogels, we designed supramolecular hydrogel electrolytes featuring much more nonfreezable bound water and much less free water than that found in conventional hydrogels. Specifically, two strategies are developed to achieve this goal. One strategy is adopting monomers with a variety of hydrophilic groups to enhance the hydrophilicity of polymer chains. The other strategy is incorporating zwitterionic polymers or polymers with counterions as superhydrophilic units. In particular, the nonfreezable bound water content increased from 0.129 in the conventional hydrogel to >0.4 mg mg^-1 in the fabricated hydrogels, while the free water content decreased from 1.232 to ∼0.15 mg mg^-1. As a result, a wide ESW of up to 3.25 V was obtained with the fabricated hydrogel electrolytes with low concentrations of incorporated salts and enhanced hydrophilic groups or superhydrophilic groups. The ionic conductivities achieved with our developed hydrogel electrolytes were much higher than those in the conventional highly concentrated salt electrolytes, and their cost is also much lower. The designed supramolecular hydrogel electrolytes endowed an aqueous K-ion battery (AKIB) system with a high voltage plateau of 1.9 V and contributed to steady cycling of the AKIB for over 3000 cycles. The developed supramolecular hydrogel electrolytes are also applicable to other batteries, such as aqueous lithium-ion batteries, hybrid sodium-ion batteries, and multivalent-ion aqueous batteries, and can achieve high voltage output.

Next, imposing the positive effect on the cathode based on hydrogel electrolytes is an effective way to improve the electrochemical performances of the cathode. Therefore, we developed an OH-rich hydrogel electrolyte (the evolution from the strategy of adopting monomers with a variety of hydrophilic groups, e.g., -OH to enhance the hydrophilicity of polymer chains) with extended ESWs can effectively activate the redox site of low-spin Fe of the K_xFe_yMn_1-y[Fe(CN)₆]_w·zH₂O (KFeMnHCF) cathode while tuning its structure. Additionally, the strong adhesion of the OH-rich hydrogel electrolyte inhibits KFeMnHCF particles from falling off the cathode and dissolving. The easy desolvation of metal ions in the developed OH-rich hydrogel electrolytes can lead to a fast and reversible intercalation/deintercalation of metal ions in the prussian blue analog (PBA) cathode. As a result, the Zn||KFeMnHCF hybrid batteries achieve the unprecedented characteristics of 14 500 cycles, a 1.7 V discharge plateau, and a 100 mAh g^-1 discharge capacity. The results of this study provide a new understanding of the development of zinc hybrid batteries with PBA cathode materials and present a promising new electrolyte material for this application.

Another dimension of study of tuning carrier behaviour based on the hydrogel electrolytes is the conversion cathode electrochemical behaviour. The mild amphiphilic hydrogel electrolytes (AHEs) with a wide ESW and high ionic activity are designed and fabricated based on the mechanism that trace amounts of hydrophobic moieties enhance the hydrogen bonding between hydrophilic groups and water molecules in the hydrogel electrolytes. Therefore, activating the multivalent conversion of MnO₂ (i.e., Mn⁴⁺↔Mn²⁺) in the mild electrolyte is achieved by tuning cation carrier behavior in the electrolytes. The developed AHE possesses an ESW up to ~3.0 V even at a high-water content of ~76 wt%. The assembled Zn||MnO₂ pouch cells using the hydrogel electrolytes demonstrated a large areal capacity of ~1.8 mAh cm^-2 at 1 mA cm^-2 due to taking full use of the thick cathode materials and a high-voltage and flat discharging plateau of ~1.75 V. This work highlights the rational design of wide-ESW AHE with high ionic activity as a promising approach to achieving aqueous high-voltage Zn||MnO₂ batteries. Apart from allowing the MnO₂/Mn²⁺ conversion reaction at a discharge plateau of >1.75 V in the mild environment, the AHEs also distinguish the discharge plateaus of ~1.33 V, previously assigned as the co-intercalation of Zn²⁺ and H⁺ ions in the MnO₂ cathode. In fact, the discharge plateaus of ~1.33 V is specified as the exclusive intercalation of Zn²⁺ ions due to an ultra-flat voltage plateau by tuning Zn²⁺/H⁺ behaviors. This fully distinguish different mechanisms at different potentials in aqueous Zn||MnO₂ batteries.

In summary, researches on hydrogel electrolytes have been conducted from multidimensions in this thesis. The mechanisms behind have been investigated throughout and further improvement methods have been proposed and validated. It’s believed that the studies in this thesis are significant in enhancing the performances of aqueous batteries on the basis of hydrogel electrolytes.

Date of Award	19 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chunyi ZHI (Supervisor)