Electrolyte Design and Interface Regulation Enable High-performance Zinc Metal Batteries

Abstract

Rechargeable zinc metal batteries (ZMBs) have been scrutinized as a promising energy storage technology that is cost-effective and safe. The fulfillment of the ZMBs' full potential largely relies on stable Zn anodes. Currently, the Zn anode is severely restricted by the inferior reversibility and short-circuit hurdles, represented by parasitic reactions (e.g., electrolyte decomposition, passivation) and notorious dendrite issues during the Zn stripping/plating reactions, respectively. These critical drawbacks are primarily caused by the incompatible properties/reactivities of the Zn anode and electrolyte on their interfaces, which aggravate particularly upon high anode utilization ratios. Interfacial chemistry regulations via electrolyte engineering and/or Zn surface modification are crucial to circumventing these problems. In this regard, solid-electrolyte interphases (SEIs)-based modification is deemed the most successful strategy that has been vibrantly investigated and leveraged in the last few years. Efficient SEI can simultaneously enable high stability and high utilization ratios of the Zn anode during battery cycling, which is vital for the ultimate construction of energy-dense and reliable rechargeable ZMBs.

Firstly, we justify that the rate-determining step (RDS) of in-cell Zn²⁺ charge transfer kinetics in typical intercalation-type ZMBs is the cation transport within the SEIs. This conclusion was established by comparing the Zn²⁺ transfer kinetics in different electrolyte-derived SEIs with those in other essential processes. As an exemplification study, we meticulously fine-tuned the SEIs in an acyclic amide-based deep eutectic electrolyte (DEE) with screened cyclic amide additives and unraveled the corresponding SEI properties and their impacts on the Zn anode's reversibility and stability. Remarkably, a highly Zn²⁺-conductive Zn₃N₂ species, uniquely formed within the SEI in the adjusted DEE, was demonstrated for the first time to outperform the state-of-the-art ZnF₂ moiety in facilitating Zn²⁺ transfer and mitigating Zn dendrite growth. Configurated with this enhanced in-cell Zn²⁺ transfer kinetics from the SEI chemistries, the Zn||Mn-doped V₂O₅ (MnVO) pouch cells delivered significantly improved rate capability (surpassing those with mostly used aqueous electrolytes) and higher cycling stability (over 1300 cycles with a capacity fading of only 0.007% per cycle). Moreover, a high Zn anode utilization ratio of 72% and device-level specific energy of 118.6 Wh kg⁻¹ (based on the cathode and anode (including its de facto current collector moiety)) was realized in the practical pouch cells.

Next, we propose a dual construction of superior SEI and cathode-electrolyte interphases (CEI) in ZMBs using a localized high-concentration fluorine-rich electrolyte. By employing a highly fluorinated alcohol, hexafluoroisopropanol (HFIP), as a co-solvent/diluent, a high F/O atomic ratio electrolyte system was established to effeciently reducing side reactions at the interface. Moreover, the substantial retention of contact ion pair (CIP) structure and free HFIP molecules promote the formation of highly fluorinated electrode-electrolyte interfaces (EEIs). Leveraging the commendable contributions of both the ZnF₂-rich SEI and C-F bond-rich CEI, the scale-up Zn||MnVO soft-packed battery and anode-free zinc metal battery demonstrates reliably cycling stability with an admirable capacity retention These findings highlight the great potential of the localized high-concentration fluorine-rich electrolyte with proposed dual F-rich SEI/CEI construction in achieving superior performance in zinc-based batteries.

Furthermore, we developed a grain boundary strengthening reverse micelle electrolyte (GBSRME) through microstructural design adjustments. The refining hydrodynamic size of reverse micelle with numerous reverse micelle barriers effectively inhibited the transport of H⁺ or OH^- in the hydrogen-bond network of water molecule via the Grotthuss mechanism. Meanwhile, the anions and cosolvent within the barriers of the reverse micelles form a configuration akin to the chain structure found in solid polymer electrolytes, promoting high-rate zinc ion transport kinetics and transport number of Zn²⁺. The reduced water content in the solvation shell and the increased proportion of fluorine in the electrical double layer (EDL) at the zinc interface regulated the formation of a fluorine-rich SEI. Merit form the GBSRME, the zinc platting/striping behavior exhibited high coulombic efficiency (99.8%) at a high current density of 5 mA cm^-2 and an areal loading capacity of 10 mAh cm^-2 for 1200 hours, demonstrating high dendrite-free performance and reversibility. Benefiting from the extensive electrochemical stability window and robust stability of the GRMRME, the high-voltage Zn||graphite battery exhibited 92.5 % capacity retention for 310 cycles between 0.8 V and 2.4 V after initial activation. Ampere-hour-scale double-layer pouch cells can deliver stable cycles for 450 cycles with 96.5% capacity retention and 65% depth of discharge (DOD) of Zn, making it as viable cell systems for practical applications.

In summary, studies on micro-structure in electrolyte and SEI optimization for ZMBs have been studied 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 moving the commercialization of ZMBs forward.

Date of Award	5 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chunyi ZHI (Supervisor)