Effect of Vinylene Carbonate and Fluoroethylene Carbonate on SEI Formation on Graphitic Anodes in Li-Ion Batteries

Binder free (BF) graphite electrodes were utilized to investigate the effect of electrolyte additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) on the structure of the solid electrolyte interface (SEI). The structure of the SEI has been investigated via ex-situ surface analysis including X-ray Photoelectron spectroscopy (XPS), Hard XPS (HAXPES), Infrared spectroscopy (IR) and transmission electron microscopy (TEM). The components of the SEI have been further investigated via nuclear magnetic resonance (NMR) spectroscopy of D2O extractions. The SEI generated on the BF-graphite anode with a standard electrolyte (1.2 M LiPF6 in ethylene carbonate (EC) / ethyl methyl carbonate (EMC), 3/7 (v/v)) is composed primarily of lithium alkyl carbonates (LAC) and LiF. Incorporation of VC (3% wt) results in the generation of a thinner SEI composed of Li2CO3, poly(VC), LAC, and LiF. Incorporation of VC inhibits the generation of LAC and LiF. Incorporation of FEC (3% wt) also results in the generation of a thinner SEI composed of Li2CO3, poly(FEC), LAC, and LiF. The concentration of poly(FEC) is lower than the concentration of poly(VC) and the generation of LAC is inhibited in the presence of FEC. The SEI appears to be a homogeneous film for all electrolytes investigated

Role of lithium salt on solid electrolyte interface (SEI) formation and structure in lithium ion batteries

A comparative investigation of the different lithium salts on formation of the solid electrolyte interface (SEI) on binder free graphite anodes for lithium ion batteries has been conducted. The electrolytes investigated include 1MLiPF6, LiBF4, LiTFSI, LiFSI, LiDFOB or LiBOB dissolved in ethylene carbonate (EC). The SEI has been investigated via a combination of spectroscopic and microscopic techniques. Transmission electron microscopy (TEM) allows direct observation of the SEI formed from the different electrolytes. Nuclear magnetic resonance (NMR) spectroscopy of D2O extracts are utilized to characterize the soluble species of SEI. XPS and FTIR provide additional elemental and functional group information for the SEI components. The SEI for all electrolytes contains lithium ethylene dicarbonate (LEDC), the primary reduction product of EC. In addition, the SEI for all electrolytes contain LiF except for the SEI generated from the LiBOB electrolyte. The SEI generated in the presence of LiBOB or LiDFOB electrolytes contain multiple oxalate containing species, including lithium oxalate (Li2C2O4), and borates. © 2014 The Electrochemical Society

Silicon solid electrolyte interphase (SEI) of lithium ion battery characterized by microscopy and spectroscopy

The surface reactions of electrolytes with a silicon anode in lithium ion cells have been investigated. The investigation utilizes two novel techniques that are enabled by the use of binder-free silicon (BF-Si) nanoparticle anodes. The first method, transmission electron microscopy with energy dispersive X-ray spectroscopy, allows straightforward analysis of the BF-Si solid electrolyte interphase (SEI). The second method utilizes multi-nuclear magnetic resonance 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 LiPF 6 salt and ethylene carbonate or fluoroethylene carbonate solvent. Capacity retention was significantly better for cells cycled with LiPF 6/FEC electrolyte than for cells cycled with LiPF6/EC electrolyte. Our unique combination of techniques establishes that for LiPF 6/EC electrolyte the BF-Si SEI continuously grows during the first 20 cycles and the SEI becomes integrated with the BF-Si nanoparticles. The SEI predominantly contains lithium ethylene dicarbonate, LiF, and Li xSiOy. BF-Si electrodes cycled with LiPF6/FEC electrolyte have a different behavior; the BF-Si nanoparticles remain relatively distinct from the SEI. The SEI predominantly contains LiF, Li xSiOy, and an insoluble polymeric species. © 2013 American Chemical Society

Stability of inactive components of cathode laminates for lithium ion batteries at high potential

The stability of inactive components in LIB (lithium ion batteries) electrodes upon exposure to high potentials can affect cell performance. A series of Li/ inactive component cells with aluminum, conductive carbon, and graphite as the inactive component were prepared and stored at high potential for one week. Electrochemical measurements and ex-situ surface analysis, including TEM (transmission electron microscopy), XPS (X-ray photoelectron spectroscopy), and FTIR (Fourier transform infrared spectroscopy), were conducted to investigate the stability of inactive components in the presence of LiPF6 in 3:7 ethylene carbonate (EC) and ethyl methyl carbonate (EMC) electrolyte at different potentials. The results show that all components are stable upon storage at 4.3 V. Storage at 4.6 or 4.9 V results in no aluminum corrosion, but limited decomposition on conductive carbon and greater decomposition on graphite. Storage at 5.3 V results in significant electrolyte oxidation to generate poly(ethylene carbonate) on the surface of all inactive electrodes and aluminum corrosion

High capacity, stable silicon/carbon anodes for lithium-ion batteries prepared using emulsion-templated directed assembly

Silicon (Si) is a promising candidate for lithium ion battery anodes because of its high theoretical capacity. However, the large volume changes during lithiation/delithiation cycles result in pulverization of Si, leading to rapid fading of capacity. Here, we report a simple fabrication technique that is designed to overcome many of the limitations that deter more widespread adoption of Si based anodes. We confine Si nanoparticles in the oil phase of an oil-in-water emulsion stabilized by carbon black (CB). These CB nanoparticles are both oil- and water-wettable. The hydrophilic/hydrophobic balance for the CB nanoparticles also causes them to form a network in the continuous aqueous phase. Upon drying this emulsion on a current collector, the CB particles located at the surfaces of the emulsion droplets form mesoporous cages that loosely encapsulate the Si particles that were in the oil. The CB particles that were in the aqueous phase form a conducting network connected to the CB cages. The space within the cages allows for Si particle expansion without transmitting stresses to the surrounding carbon network. Half-cell experiments using this Si/CB anode architecture show a specific capacity of ∼1300 mAh/g Si + C and a Coulombic efficiency of 97.4% after 50 cycles. Emulsion-templating is a simple, inexpensive processing strategy that directs Si and conducts CB particles to desired spatial locations for superior performance of anodes in lithium ion batteries. © 2014 American Chemical Society

Lithium ion battery graphite solid electrolyte interphase revealed by microscopy and spectroscopy

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. © 2013 American Chemical Society

Role of solution structure in solid electrolyte interphase formation on graphite with LiPF6 in propylene carbonate

An 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. © 2013 American Chemical Society

Reduction reactions of carbonate solvents for lithium ion batteries

Lithium naphthalenide has been investigated as a one electron reducing agent for organic carbonates solvents used in lithium ion battery electrolytes. The reaction precipitates have been analyzed by IR-ATR and solution NMR spectroscopy and the evolved gases have been analyzed by GC-MS. The reduction products of ethylene carbonate and propylene carbonate are lithium ethylene dicarbonate and ethylene and lithium propylene dicarbonate and propylene, respectively. The reduction products of diethyl and dimethyl carbonate are lithium ethyl carbonate and ethane and lithium methyl carbonate and methane, respectively. Lithium carbonate is not observed as a reduction product. © 2014 The Electrochemical Society