Date of Award
Doctor of Philosophy in Chemistry
The surface reactions of electrolytes with the graphitic anode of lithium ion batteries have been investigated. The investigation utilizes two novel techniques, which are enabled by the use of binder-free graphite anodes. The first method, transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy, allows straightforward analysis of the graphite solid electrolyte interphase (SEI). The second method utilizes multi-nuclear magnetic resonance (NMR) spectroscopy of D2O extracts from the cycled anodes. The TEM and NMR data are complemented by XPS and FTIR data, which are routinely used for SEI studies. Cells were cycled with LiPF6 and ethylene carbonate (EC), ethyl methyl carbonate (EMC), and EC/EMC blends. This unique combination of techniques establishes that for EC/LiPF6 electrolytes, the graphite SEI is ∼50 nm thick after the first full lithiation cycle, and predominantly contains lithium ethylene dicarbonate (LEDC) and LiF. In cells containing EMC/LiPF6 electrolytes, the graphite SEI is nonuniform, ∼10−20 nm thick, and contains lithium ethyl carbonate (LEC), lithium methyl carbonate (LMC), and LiF. In cells containing EC/EMC/LiPF6 electrolytes, the graphite SEI is ∼50 nm thick, and predominantly contains LEDC, LMC, and LiF. The novel techniques as combinations of binder free graphite electrodes with Transmission Electron Microscopy (TEM) grids and D2O extraction allowed us to analyze SEI species efficiently and also can be applied in other systems. The investigation utilizes thoes novel techniques which are enabled by the use of binder free silicon (BF-Si) nano-particle anodes. The first method, Transmission Electron Microscopy (TEM) with Energy Dispersive X-ray Spectroscopy (EDX), allows straightforward analysis of the BF-Si solid electrolyte interphase (SEI). The second method, utilizes Multi-Nuclear Magnetic Resonance (NMR) spectroscopy of D2O extracts from the cycled anodes. The TEM and NMR data are complemented by XPS and FTIR data, which are routinely used for SEI studies. Coin cells (BF-Si/Li) were cycled in electrolytes containing LiPF6 salt and ethylene carbonate (EC) or fluoroethylene carbonate (FEC) solvent. Capacity retention was significantly better for cells cycled with LiPF6/FEC electrolyte than for cells cycled with LiPF6/EC electrolyte. Our unique combination of techniques establishes that for LiPF6/EC electrolyte, the BF-Si SEI continuously grows during the first 20 cycles and the SEI becomes integrated with the BF-Si nano-particles. The SEI predominantly contains lithium ethylene dicarbonate (LEDC), LiF, and LixSiOy. BF-Si electrodes cycled with LiPF6/FEC electrolyte have a different behavior; the BF-Si nano-particles remain relatively distinct from the SEI. The SEI predominantly contains LiF, LixSiOy and an insoluble polymeric species.
The investigation of the interrelationship of cycling performance, solution structure, and electrode surface film structure has been conducted for electrolytes composed of different concentrations of LiPF6 in propylene carbonate (PC) with a binder free (BF) graphite electrode. Varying the concentration of LiPF6 changes the solution structure altering the predominant mechanism of electrolyte reduction at the electrode interface. The change in mechanism results in a change in the structure of the solid electrolyte interface (SEI) and the reversible cycling of the cell. At low concentrations of LiPF6 in PC (1.2 M), electrochemical cycling and cyclic voltammetry (CV) of BF graphite electrodes reveal continuous electrolyte reduction and no lithiation/delithiation of the graphite. The solution structure is dominated by solvent separated ion pairs (Li+(PC)4//PF6-) and the primary reduction product of the electrolyte is lithium propylene dicarbonate (LPDC). At high concentrations of LiPF6 in PC (3.0 - 3.5 M), electrochemical cycling and CV reveal reversible lithiation/delithiation of the graphite electrode. The solution structure is dominated by contact ion pairs (Li+(PC)3PF6-) and the primary reduction product of the electrolyte is LiF.
Nie, Mengyun, "ANODE SOLID ELECTROLYTE INTERPHASE (SEI) OF LITHIUM ION BATTERY CHARACTERIZED BY MICROSCOPY AND SPECTROSCOPY" (2014). Open Access Dissertations. Paper 202.