Abstract
The development of energy storage technology is an important topic for facilitating the employment of renewable energy in society. Therefore, current energy storage research is heavily focused on enabling rechargeable high-energy density lithium-based batteries. In particular, permitting reversible electrochemical plating and stripping of the lithium metal negative electrode (or lithium metal anode) in carbonate electrolytes can achieve this goal. Unfortunately, the performance of the lithium metal anode in carbonate electrolytes is plagued by unsafe dendrite formation and poor Coulombic efficiency upon cycling. This dissertation attempts to reveal the role of the composition and structure of the Solid Electrolyte Interphase (SEI) in relation to the performance of the lithium metal anode. Galvanostatic voltammetry was used to characterize the electrochemistry of the lithium metal anode, with Infrared Spectroscopy, X-ray Photoelectron Spectroscopy, and Transmission Electron Microscopy to investigate the surface of the lithium metal anode. In chapter 2, a method to electrochemically synthesize lithium metal such that a reliable SEI is generated is introduced, using Cu||LiFePO4 cells. Using this method, in conjunction with the analytical techniques described above, chapters 3 and 4 investigates electrolyte components that significantly improve the performance of the lithium metal anode, fluoroethylene carbonate (FEC) and lithium difluoro(oxalate) borate (LiDFOB), with an explanation proposed. Finally, chapter 5 shows how FEC and LiDFOB can work together to optimize the SEI composition and structure, hence optimizing the performance of the lithium metal anode in carbonate electrolytes.
Highlights
The plating and stripping of the lithium metal negative electrode in nonaqueous electrolytes has been investigated for decades.[1–3] In particular, carbonate solvents have relatively high voltage stability, making them desirable electrolytes for high-energy density lithium batteries.[3–6] the efficiency of plating/stripping lithium in carbonate electrolytes does not meet requirements for commercial application (> 99.9%).[7,8]It is common to measure the plating/stripping efficiency of lithium by assembling Li||Cu cells.[9–13] In this cell design, a small amount of Li is cycled, with an excess reservoir of lithium present
Since there is no excess lithium in the Cu||LiFePO4 cells, the reversible capacity of all cells decreases significantly over a short number of cycles as expected.[15]
The cells cycled with the LiBF4, lithium bis(oxalato)borate (LiBOB), and LiBF4 + LiBOB electrolytes have better initial capacity retention (Figure 4-2b) and cycling efficiency than cells cycled with the LiPF6 electrolyte, but retained capacity is insignificant after only 10 cycles
Summary
The plating and stripping of the lithium metal negative electrode in nonaqueous electrolytes has been investigated for decades.[1–3] In particular, carbonate solvents have relatively high voltage stability, making them desirable electrolytes for high-energy density lithium batteries.[3–6] the efficiency of plating/stripping lithium in carbonate electrolytes does not meet requirements for commercial application (> 99.9%).[7,8]It is common to measure the plating/stripping efficiency of lithium by assembling Li||Cu cells.[9–13] In this cell design, a small amount of Li is cycled, with an excess reservoir of lithium present. The in-situ formation of lithium metal and low reactivity of LiFePO4 ensures additives under investigation do not react with the electrode surface upon construction and are only reduced upon initial cycling This affords the possibility for controlled design and construction of the SEI on lithium metal since the reduction of the electrolyte can be controlled by current density, cell potential, and the quantity of lithium plated. FEC containing electrolytes have been reported to improve the performance of lithium metal electrodes via the generation of polymeric species similar to that reported for silicon anodes.[11]. Fluoroethylene carbonate (FEC) containing electrolytes have been reported to improve the performance of lithium metal electrodes via the generation of polymeric species within the Solid Electrolyte Intephase (SEI)[5] of lithium metal, similar to that reported for silicon anodes.[6,7]. It has been demonstrated that employing FEC in co-solvent amounts is optimal for achieving high performance lithium metal anodes.[6]
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