Date of Award

1-24-2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Ian Hosein

Keywords

anode;battery;calcium-ion;cathode;electrolyte;solid-electrolyte interface

Subject Categories

Chemical Engineering | Engineering

Abstract

The development of electric vehicles and energy storage devices are part of larger international efforts to create a sustainable future. Lithium-ion batteries have become crucial components to powering these technologies. However, the growing dependence of this singular battery chemistry to power an ever expanding landscape of technologies raises sustainability issues around lithium. Exploration of alternate battery chemistries have become part of research efforts to meet global energy demands. Calcium has emerged as one such battery system that could help alleviate stresses placed on lithium with its energy density and cost. Battery chemistries are unique, and each battery possesses its own set of challenges. Regarding calcium, one such challenge is the identification of suitable cathodes. Open framework oxides, based on previous work, have been identified as the most promising cathode structures for calcium intercalation chemistry. However, many oxides have yet to be experimentally evaluated. In addition to the issues surrounding cathodes for calcium batteries, another challenge surrounding calcium chemistry is the use of suitable electrolytes that would allow for calcium metal to be used as anodes. Calcium electrolytes form passivation layers on calcium metal electrodes that quickly prevent any calcium diffusion from occurring in the battery. Recent efforts on calcium electrolytes have focused on engineering the solid electrolyte interphase to form phases that are more accommodating with calcium diffusion. This dissertation describes my efforts at experimentally exploring two cathodes for calcium-ion batteries along with an effort at engineering the solid electrolyte interphase for calcium batteries by using an electrolyte that is not native to calcium. The electrochemical activity within a cathode, when using intercalation chemistry, produces structural changes to the electrode that can be tracked with x-ray diffraction (XRD). My first project with this dissertation was focused on the design and fabrication of an in situ XRD cell that could track the structural changes to a cathode as it was cycled. I implemented a glassy carbon window that would be transparent to x-rays and validated the functionality of the cell with an established battery system for intercalation chemistry. Lithium Cobalt Oxide (LiCoO2) was cycled against a lithium metal anode within the in situ cell. The cell was cycled at a rate of C/40 and achieved reversible capacities of 32 mAh/g. Structural changes to the (003), (101), (009), (107) and (018) lattice planes were tracked with the in situ cell and validated its functionality. My following efforts were aimed at evaluating the electrochemical activity of calcium manganese oxide (CaMn2O4) post-spinel and its capacity to function as a cathode for calcium-ion batteries. The CaMn2O4 post-spinel cathode was synthesized using a solid-state synthesis method and verified with XRD. The electrochemical activity of the cathode was first analyzed with cyclic voltammetry and galvanostatic cycling. Oxidation potentials of the cathode were identified at 0.2 and 0.5 V while broad insertion potentials were identified at -1.5 V. A maximum charge of the cathode was performed at a rate of C/200, yielding a maximum capacity of 100 mAh/g. Structural characterization of the electrode was performed and compared to theoretical models, confirming the redox activity of the cathode. Cycling capabilities of the cathode were also performed in coin cell configurations. Using a c-rate of C/33, the cathode was measured to reversibly cycle 52 mAh/g and was further verified with Energy-Dispersive X-ray Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS). The results from the evaluation revealed that CaMn2O4 is a promising cathode for calcium-ion batteries. Beyond the successful cycling of CaMn2O4, the next goal of my dissertation was focused on a similar line of testing with calcium iron oxide (CaFe2O4). The CaFe2O4 post-spinel was synthesized through an auto-combustion route and its phase was also verified with XRD. Cyclic voltammetry studies were performed on the cathode and reported limited oxidative behavior at approximately 0.3 V and was not reproduced on subsequent CV cycles. There was no reinsertion activity observed with the post-spinel. Galvanostatic cycling of the cathode was also performed on CaFe2O4 at a c-rate of C/100, cycling a capacity of 50 mAh/g. Postmortem XRD of the cathode revealed one structural development within the crystal structure of CaFe2O4 that aligned with theoretical calculations of the post-spinel. The data collected on CaFe2O4 concluded that this cathode had limited electrochemical activity and would not be a feasible candidate for calcium-ion batteries. The last efforts of my dissertation were focused on addressing SEI issues that occur when using calcium metal anodes with calcium electrolytes. Previous research on designing SEIs for calcium batteries have used alternate electrolytes to produce phases that are more ionically conductive. My project was aimed at using a similar strategy, immersing calcium metal electrodes in a potassium electrolyte and cycle charge. Using potassium hexafluorophosphate (KPF6) in a ternary mixture of carbonate solvents, calcium symmetrical cells were cycled with a charge density of 0.025 mA/cm2 with a capacity of 0.15 mAh/cm2 for over 200 hours. Throughout plating and stripping, overpotentials were maintained below 1.8 V. The phases formed in the SEI were a combination of permanent and transient phases that were verified with XRD, EDS and Fourier Transform Infrared (FTIR) spectroscopy. Increased cycling of calcium in the symmetrical cells was further verified with in situ Raman spectroscopy and the calcium content was measured with inductively coupled plasma mass spectrometry (ICP-MS). The results confirm the effectiveness of potassium electrolytes being used to tailor a hybrid SEI for calcium plating and maintaining cycling stability throughout testing. In summary, the contributions in this dissertation offer an experimental evaluation of cathodes for calcium-ion batteries along with identifying a new electrolyte to be used for engineering interphases. These findings can offer better insights for future strategies with calcium batteries.

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