Understanding Fundamental Ionic Storage Mechanisms in Atomic Layers of Transition Metal Oxides for Next-Generation Zn-ion Batteries
Abstract
Energy density and safety are two most critical factors to be considered for battery use. Lithium-ion batteries (LIBs) are the current benchmark for rechargeable batteries, However, their widespread use in military space has been limited by their poor safety. In contrast, traditional aqueous batteries are safe, but their energy density is rather low. Recently, aqueous Zn-ion batteries (ZIBs) emerge as a front runner to achieve high energy density and excellent safety. However, a further development of ZIBs into a robust product for military use requires a solid fundamental understanding on several critical, but poorly understood fundamental issues, viz. 1) co-Zn2+/H+ storage mechanisms; 2) unique pseudocapacitive phenomenon; 3) Zn2+ solvation and desolvation mechanisms; and 4) cathode dissolution mechanisms. The proposed work aims to achieve a fundamental understanding on the above rudimentary mechanisms through a combined experimental and theoretical study. First, atomic layers of two model ZIB cathodes, i.e. MnO2 and V2O5, will be adopted as the studying platform to investigate the ionic storage mechanisms. In this way, the interference from the binder and solvent in the conventional slurry form of cathode can be avoided and intrinsic ionic storage chemistry can be best revealed. The PIÕs lab is equipped with a state-of-the-art atomic layer deposition (ALD) reactor. The plan is to deposit few-nanometer atomic layers of MnO2 and V2O5 on a conducting and transparent glass (e.g. ITO) substrate by ALD for in-operando spectroelectrochemical study of a live ZIB cell. When Zn2+ and/or H+ are intercalated into MnO2 or V2O5 cathode during discharge, the colors of ALD-MnO2 and ALD-V2O5 thin films are expected to change because of altered bandgap by ionic intercalation. The frequency change associated with bandgap is determined by a real-time UV-Vis spectrometer and can then be compared with density function theory (DFT) calculations based on intercalation mechanisms. The agreement and disagreement between the two will provide the theoretical insights for the ionic storage mechanisms. A combined DFT and 3D reference-interaction-site-model (3D-RISM) approach will be applied to understand the pseudocapacitive (a redox capacitor) phenomenon, which plays a substantial role in fast charging capability. The hypothesis is that the intercalated, fully solvated Zn2+/H+ would only lead to electric double-layer capacitive behavior, while partially solvated or bare Zn2+/H+ would result in pseudocapacitive behavior because of orbital coupling of the intercalated Zn2+/H+ with O2- in MnO2ánH2O or V2O5ánH2O, leading to charge transfer. Solvation and desolvation of Zn2+ and H+ in aqueous electrolytes play a decisive role in charge transfer resistance. A systematic study on the Zn2+/H+ solvation and desolvation mechanisms in a variety of aqueous Zn-salt electrolytes with and without solvation shell breakers will be carried out using Fourier Transform Infrared (FTIR) spectroscopy. Once a proper solvation shell breaker is identified, it will be incorporated in a ZIB cell for performance evaluation. Cathode dissolution is a major degradation mechanism for ZIB cells. Within a certain range of pH, bulkier anion tends to exhibit a lower solvation penalty on Zn2+, thus yielding better performance. While the pH is easy to control, Zn-salts with bulkier anions are often expensive. For example, the bulkier Zn(OTf)2 salt is 28 times more expensive than the slender ZnSO4. The work will focus on identifying the ways in which the low-cost ZnSO4 electrolyte can be used to yield a performance equivalent to Zn (OTf)2. At the completion of the project, the gained fundamental knowledge is expected to provide crucial comprehension for an in-depth understanding of cathode ionic storage mechanisms, materials advancement, novel battery design, and ultimately accelerating the commercialization of ZIBs.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jun 25, 2021
- Source ID
- W911NF2110308
Entities
People
- Kevin Huang
Organizations
- Army Contracting Command
- United States Army
- University of South Carolina