Reduction Reactions of Electrolyte Salts for Lithium Ion Batteries: LiPF6, LiBF4, LiDFOB, LiBOB, and LiTFSI

The reduction products of common lithium salts for lithium ion battery electrolytes, LiPF6, LiBF4, lithium bisoxalato borate (LiBOB), lithium difluorooxalato borate (LiDFOB), and lithium trifluorosulfonylimide (LiTFSI), have been investigated. The solution phase reduction of different lithium salts via reaction with the one electron reducing agent, lithium naphthalenide, results in near quantitative reactions. Analysis of the solution phase and head space gasses suggests that all of the reduction products are precipitated as insoluble solids. The solids obtained through reduction were analyzed with solution NMR, IR-ATR and XPS. All fluorine containing salts generate LiF upon reduction while all oxalate containing salts generate lithium oxalate. In addition, depending upon the salt other species including, LixPFyOz, LixBFy, oligomeric borates, and lithium bis[N-(trifluoromethylsulfonylimino)] trifluoromethanesulfonate are observed

Citric Acid Based Pre-SEI for Improvement of Silicon Electrodes in Lithium Ion Batteries

Silicon electrodes are of interest to the lithium ion battery industry due to high gravimetric capacity (∼3580 mAh/g), natural abundance, and low toxicity. However, the process of alloying and dealloying during cell cycling, causes the silicon particles to undergo a dramatic volume change of approximately 280% which leads to electrolyte consumption, pulverization of the electrode, and poor cycling. In this study, the formation of an ex-situ artificial SEI on the silicon nanoparticles with citric acid has been investigated. Citric acid (CA) which was previously used as a binder for silicon electrodes was used to modify the surface of the nanoparticles to generate an artificial SEI, which could inhibit electrolyte decomposition on the surface of the silicon nanoparticles. The results suggest improved capacity retention of ∼60% after 50 cycles for the surface modified silicon electrodes compared to 45% with the surface unmodified electrode. Similar improvements in capacity retention are observed upon citric acid surface modification for silicon graphite composite/ LiCoO2 cells

Decomposition Reactions of Anode Solid Electrolyte Interphase (SEI) Components with LiPF6

The anode solid electrolyte interface (SEI) on the anode of lithium ion batteries contains lithium carbonate (Li2CO3), lithium methyl carbonate (LMC), and lithium ethylene dicarbonate (LEDC). The development of a strong physical understanding of the properties of the SEI requires a strong understanding of the evolution of the SEI composition over extended timeframes. The thermal stability of Li2CO3, LMC, and LEDC in the presence of LiPF6 in dimethyl carbonate (DMC), a common salt and solvent, respectively, in lithium ion battery electrolytes, has been investigated to afford a better understanding of the evolution of the SEI. The residual solids from the reaction mixtures have been characterized by a combination of X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy with attenuated total reflectance (IR-ATR), while the solution and evolved gases have been investigated by nuclear magnetic resonance (NMR) spectroscopy and gas chromatography with mass selective detection (GC-MS). The thermal decomposition of Li2CO3 and LiPF6 in DMC yields CO2, LiF, and F2PO2Li. The thermal decomposition of LMC and LEDC with LiPF6 in DMC results in the generation of a complicated mixture including CO2, LiF, ethers, phosphates, and fluorophosphates. (Figure Presented)

Thermal Decomposition of the Solid Electrolyte Interphase (SEI) on Silicon Electrodes for Lithium Ion Batteries

Thermal behavior of the solid electrolyte interphase (SEI) on a silicon electrode for lithium ion batteries has been investigated by TGA. In order to provide a better understanding of the thermal decomposition of the SEI on silicon, the thermal decomposition behavior of independently synthesized lithium ethylene dicarbonate (LEDC) was investigated as a model SEI. The model SEI (LEDC) has three stages of thermal decomposition. Over the temperature range of 50–300 °C, LEDC decomposes to evolve CO₂ and C₂H₄ gases leaving lithium propionate (CH₃CH₂CO₂Li) and Li₂CO₃ as solid residues. The lithium propionate decomposes over the temperature range of 300–600 °C to evolve pentanone leaving Li₂CO₃ as a residual solid. Finally, the Li₂CO₃ decomposes over 600 °C to evolve CO₂ leaving Li₂O as a residual solid. A very similar thermal decomposition process is observed for the SEI generated on cycled silicon electrodes. However, two additional thermal decomposition reactions were observed characteristic of Li_<i>x</i>PO_<i>y</i>F_<i>z</i> at 300 °C and the polyimide binder at 550 °C. TGA measurements of Si electrodes after various numbers of cycles suggest that the LEDC on Si electrodes thermally decomposes during cycling to form lithium propionate and Li₂CO₃, resulting in increased complexity of the SEI

Systematic Investigation of Alkali Metal Ions as Additives for Graphite Anode in Propylene Carbonate Based Electrolytes

Propylene carbonate (PC) is an electrolyte co-solvent with a wide working temperature range, which can improve the performance of lithium ion batteries (LIBs). Unfortunately, PC co-intercalates into graphite with lithium ions leading to exfoliation and rapid capacity decay. Incorporation of low concentrations of Cs+ or K+ ions as additives improves the performance by inhibiting graphite exfoliation and leading to better first cycle efficiency. The electrochemical behavior of graphite anodes with a series of electrolytes containing added alkaline metal acetate salts, Li, Na, K, and Cs, has been investigated. Cells containing K and Cs acetate have the highest first cycle efficiency and reversible cycling capacity. In an effort to better understand the role of the cation on performance, the solid electrolyte interphase (SEI) on the graphitic anodes cycled with the different electrolytes has been investigated via a combination of X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Infrared Spectroscopy (ATR-IR), Transmission electron microscopy (TEM) and Inductive coupled plasma mass spectrometry (ICP-MS). The presence of the heavier cations (K and Cs) leads to a thinner SEI with higher LiF content which is likely responsible for the performance enhancement

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

Investigation of the solid electrolyte interphase on hard carbon electrode for sodium ion batteries

The electrochemical performance of hard carbon (HC)/Na cells with NaPF6 in a mixture of EC/DEC (1:2, v/v) has been investigated in the voltage range of 0.05–2 V. An initial reversible capacity of 162 mAh g− 1 is observed. With continuous cycling, the reversible capacities fluctuate slightly and obtain a value of 183 mAh g− 1 on the 25th cycle. Ex-situ surface analysis of cycled HC electrodes has been conducted by a combination of scanning electron microscopy (SEM), infrared spectroscopy with attenuated total reflectance (IR-ATR) and X-ray photoelectron spectroscopy (XPS). The ex-situ surface analysis suggests that the major composition of solid electrolyte interphase (SEI) formed on the surface of HC electrode is sodium ethylene dicarbonate (SEDC) and NaF with lower concentrations of sodium alkyl carbonates and Na2CO3 which is similar with that reported in lithium ion batteries