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 D 2 O 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 LiPF 6 /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 x SiO y . BF-Si electrodes cycled with LiPF 6 /FEC electrolyte have a different behavior; the BF-Si nanoparticles remain relatively distinct from the SEI. The SEI predominantly contains LiF, Li x SiO y , and an insoluble polymeric species.

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 D 2 O 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 LiPF 6 and ethylene carbonate (EC), ethyl methyl carbonate (EMC), and EC/EMC blends. This unique combination of techniques establishes that for EC/LiPF 6 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/LiPF 6 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/LiPF 6 electrolytes, the graphite SEI is ∼50 nm thick, and predominantly contains LEDC, LMC, and LiF.

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 LiPF 6 in propylene carbonate (PC) with a binderfree (BF) graphite electrode. Varying the concentration of LiPF 6 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 LiPF 6 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 // PF 6 − ), and the primary reduction product of the electrolyte is lithium propylene dicarbonate (LPDC). At high concentrations of LiPF 6 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) 3 PF 6 −), and the primary reduction product of the electrolyte is LiF.

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 1 M LiPF 6 , LiBF 4 , 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 D 2 O 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 (Li 2 C 2 O 4 ), and borates.

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 D 2 O extractions. The SEI generated on the BF-graphite anode with a standard electrolyte (1.2 M LiPF 6 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 Li 2 CO 3 , 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 Li 2 CO 3 , 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. Lithium ion batteries have been used to power portable electronic devices for decades. Interest in lithium ion batteries has expanded to include electric vehicles (EV) due to their high energy density. [1][2][3] However, the calendar life of many lithium ion batteries is insufficient for the >10 year life expectancy of an EV.1 Thus there have been many recent investigations on methods to improve the calendar life of lithium ion batteries. Graphite is the most widely used anode material in lithium ion batteries. 4,5 During the initial charging cycles of the lithium ion battery a Solid Electrolyte Interphase (SEI) is generated on the graphite surface.6,7 The SEI acts as a passivation layer to inhibit further electrolyte reduction. 8 The SEI generated from standard ethylene carbonate based electrolytes has moderate thermal stability which leads to moderate calendar life. 9 In an effort to improve the stability of the SEI many film forming additives have been investigated. 10,11 Vinylene Carbonate (VC) and Fluoroethylene Carbonate (FEC) are among the most widely investigated electrolyte additives.12 VC has been used in many lithium ion batteries to increase first cycle efficiency, improve the high temperature stability, and improve the calendar life.13-16 FEC has largely been used in silicon-based anode materials to improve capacity retention, but has also been investigated with graphite anodes.15,17 However, there have been limited direct comparisons of the effects from FEC and VC on graphite electrodes especially related to differences in the structure of the anode SEI.The investigations of the components and morphology changes on graphite anode surfaces upon incorporation of small quantities of additives in a standard electrolyte 1....