Study on the Homogenization of Hetero-phase Conversion-type Electrodes

Abstract

In addition to highly stable batteries without hetero-phase conflicts based on diffusion mechanisms in the homo-solid phase, and conversion mechanisms in the homo-solution phase, metastable batteries based on hetero-solid-phase conversion mechanisms are also hot research topics. To obtain the advantages of conversion-type batteries, such as cost, safety, and energy density, it is urgent to improve the stability level, namely, further reversibility and stability of electrodes beyond the chemical species. The hetero-phase conflict limits the capability of electrodes to maintain initial states. Therefore, the research on homogenization of hetero-phase conversion-type electrodes has progressive significance.

We studied hetero-phase conflicts and contradictions through three representative electrodes: bromine electrode (Br₂/Br^-), anthraquinone electrode (AQ/AQH), and sulfide electrode (LiPS^-/Li₂S), which broke away the solubility dependence to reconcile the contradiction between electrode stability and main reaction kinetics through the homogenized electrode phases.

For the bromine electrode, we reconstructed a novel quasi-homogeneous bromine phase that renders both Br-Br (charged state) and Br- species (discharged state) into a non-typical-solid-phase in the electrode phase to solve the contradiction between self-discharge and impedance, achieved ZBBs with superior energy efficiency (EE) based on a large areal capacity and tight assembly state. Our optimized approach has realized ZBBs with superior charge-discharge characteristics, a remarkable EE of 92.7% based on an areal capacity of 12 mA h cm^-2 in a period duration of 13 h, an energy density of whole battery (EDB) of ~80 W h L^-1 with average EE of 92.5% for an extended cycle life of approximately 500 cycles, and a maximum EDB of 186 W h L^-1 without pre-added zinc metal.

For the anthraquinone electrode, we explored an anthraquinone-based solid-solution-like phase EtAQSiS to reconcile the hetero-phase conflicts. The purified electrode processes based on the homogenized phase possess perfect symmetry and the rate-determined step based on single diffusion species. The EtAQSiS electrode can maintain superior plateau characteristics up to a high current density of 40 mA cm^-2, and a high-capacity utilization of ~1.5 e- equivalent. An EtAQSiS-AC pouch cell with 2 mmol EtAQ per 20 cm² and 4 mA cm^-2 achieved an ultra-high CE of 99.8% over 1000 cycles, higher utilization and retention rate due to the suppressed recrystallization failure mode.

For the sulfur electrode, we investigated an intermediate phase matrix with covalency Bi-S species ASBC. It can facilitate the spontaneous degradation of S₈ and high-order lithium-ionized polysulfides (HLiPS^-), and further form stabilized low-order sulfide chain species (LSCs). The ASBC-LSCs phase with a higher apparent diffusion coefficient dominants the electrode processes toward homogenization to eliminate the dissolution and dissipation of S₈/HLiPS^- and the high-impedance conversion between hetero-solid Li₂S₂/Li₂S phases, and obtained purified and enhanced electrode processes: the outstanding single-plateau characteristics of ~2.1 V plateau with discharge capacity of more than 1200 mA h g^-1_S8, and a total capacity of ~1372 mA h g^-1_S8. After formation, the sulfur electrodes can maintain good characteristics up to constant current charge (CCC) of 4 mA cm^-2 and discharge (CCD) of 1 mA cm^-2. Based on an areal capacity of 2 mA h cm^-2, the coin cells can maintain stable cycling with an average Coulombic efficiency (CE) of 98.47% and an average energy efficiency (EE) of 91.89% within ~700 cycles and ~4,200 h.

The hetero-phase conflicts and contradictions for conventional conversion-type electrodes have been fully expounded and highlighted. Three successfully stabilized electrodes have displayed the effectiveness of reconstructed homogenized phase-state conditions. The advanced hetero-phase conversion-type electrodes should not be limited in the conventional dissolution-deposition mechanism and should possess characteristic bulk-phase processes accompanied by general interfacial processes.

Date of Award	10 Mar 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chunyi ZHI (Supervisor)