Investigation of the Lithium Solid Electrolyte Interphase in Vinylene Carbonate Electrolytes Using Cu||LiFePO\u3csub\u3e4\u3c/sub\u3e Cells

The influence of vinylene carbonate (VC) on the plating/stripping of lithium was investigated using Cu||LiFePO4 cells. These cells allow for easy fabrication and in-situ generation of lithium, with no excess lithium to influence performance. Addition of VC to the electrolyte improves both capacity retention and efficiency. IR and XPS spectroscopy of the surface of the plated lithium suggests the presence of a significant amount of poly(VC) when the electrolyte (1.2 M LiPF6 in ethylene carbonate (EC): ethyl methyl carbonate (EMC) (3:7, vol)) contains 5% of added VC. This suggests employing additives that generate polymeric species on the surface of lithium improves plating/stripping performance in carbonate electrolytes

Minimized Metal Dissolution from High-Energy Nickel Cobalt Manganese Oxide Cathodes with Al2O3 Coating and Its Effects on Electrolyte Decomposition on Graphite Anodes

High-energy nickel cobalt manganese oxides have been studied intensively as cathode materials for lithium-ion batteries. However, several hurdles need to be overcome to adopt these cathodes in commercial lithium-ion batteries. Herein, aluminum oxide (Al2O3) coating was applied to high-energy nickel cobalt manganese oxides (HE-NCM, Li1.33Ni0.27Co0.13Mn0.60O2+d) by atomic layer deposition (ALD) and its effects on HE-NCM/graphite full cells were investigated. HE-NCM/graphite full cells have better cycling performance and efficiency when HE-NCM is coated with Al2O3. ICP-MS measurements show that the Al2O3 coating can effectively prevent transition metal dissolution from HE-NCM. XPS and FT-IR analysis suggests that the surface film on HE-NCM cathodes does not change significantly with the Al2O3 coating even after 50 cycles, however the surface film on graphite anodes shows a significant change. The resistance of graphite electrodes cycled with the uncoated HE-NCM is higher than that of graphite electrodes cycled with the Al2O3-coated HE-NCM due to the increased SEI thickness. The improved cycling performance of HE-NCM/graphite cells with Al2O3 coating can be attributed to the minimized resistance increase on graphite as well as the suppression of cathode active material loss

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

Effect of electrolyte on the nanostructure of the solid electrolyte interphase (SEI) and performance of lithium metal anodes

Developing electrolytes that enable commercially viable lithium metal anodes for rechargeable lithium batteries remains challenging, despite recent exhaustive efforts. Electrolytes of similar composition, yet different structure, have been investigated to understand key mechanisms for improving the cycling performance of lithium metal anodes. Specifically, the electrolytes investigated include LiPF6, LiBF4, lithium bis(oxalato)borate (LiBOB), and lithium difluoro(oxalato)borate (LiDFOB) dissolved in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). There is a remarkable difference in the cycling performance of 1.2 M LiDFOB in EC:EMC (3:7) compared to 0.6 M LiBF4 + 0.6 M LiBOB in EC:EMC (3:7), despite the effectively equivalent chemical composition. The LiDFOB electrolyte has significantly better cycling performance. Furthermore, the chemical compositions of the SEI generated on the lithium metal electrode from the two electrolytes are very similar, especially after the 1st plating, suggesting that the chemical composition of the SEI may not be the primary source for the difference in cycling performance. Ex situ transmission electron microscopy (TEM) reveals that the difference in cycling performance can be traced to the presence of nanostructured LiF particles in the SEI from the LiDFOB electrolyte. It is proposed that the capping ability of the oxalate moiety from LiDFOB, in combination with simultaneous generation of LiF, leads to generation of uniform and evenly distributed nanostructured LiF particles. The presence of nanostructured LiF in the SEI results in uniform diffusion field gradients on the lithium electrode which leads to improved cycling performance. The proposed mechanism not only provides insight for improving lithium metal anodes for batteries, but also expands upon the understanding of the role of LiF in the SEI on graphite electrodes in commercial lithium ion batteries. A superior understanding of the structure and function of the SEI will facilitate the development of next-generation energy storage systems

In Situ Measurement of the Plane-Strain Modulus of the Solid Electrolyte Interphase on Lithium-Metal Anodes in Ionic Liquid Electrolytes

We present an experimental approach for in situ measurement of elastic modulus of the solid electrolyte interphase (SEI), which is formed from reactions between a lithium thin-film [on a polydimethylsiloxane (PDMS) substrate] and a room-temperature ionic liquid (RTIL) electrolyte. The SEI forms under a state of compressive stress, which causes buckling of the sample surface. In situ atomic force microscopy is used to measure the dominant wavelength of the wrinkled surface topography. A mechanics analysis of strain-induced elastic buckling instability of a stiff thin film on a soft substrate is used to determine the plane strain modulus of the SEI from the measured wavelength. The measurements are performed for three RTIL electrolytes: 1-butyl 1-methylpiperidinium bis(trifluoromethylsulfonyl)imide (P14 TFSI) without any lithium salt, 1.0 M lithium bis(trifluoromethylsulfonyl)imide (Li TFSI) in P14 TFSI, and 1.0 M lithium bis(fluorosulfonyl)imide (Li FSI) in P14 TFSI to investigate the influence of lithium salts on the plane strain modulus of the SEI. The measurements yield plane-strain moduli of approximately 1.3 GPa for no-salt P14 TFSI and approximately 1.6 GPa for 1.0 M Li TFSI in P14 TFSI and 1.0 M Li FSI in P14 TFSI. The experimental technique presented here eliminates some of the uncertainties associated with traditional SEI mechanical characterization approaches and offers a platform to engineer an SEI with desired mechanical properties by approaches that include altering the electrolyte composition

Measurement of mechanical and fracture properties of solid electrolyte interphase on lithium metal anodes in lithium ion batteries

Mechanical integrity of the solid electrolyte interphase (SEI) plays an essential role in determining the life and performance of lithium-ion batteries. Fracture and continued formation of the SEI contribute to consumption of lithium, drying of electrolyte, increase in impedance, and growth of dendrites resulting in capacity fade and premature failure. Electrolyte additives such as fluoroethylene carbonate (FEC) have been known to improve performance, but the underlying reasons have been elusive. Despite its importance, reliable methods for mechanical characterization of SEI have been lacking. Here, we present a new experimental technique that combines atomic force microscopy and membrane-bulge configuration to accurately measure the stress-strain behavior of SEI, including the onset of inelastic response and evolution of fracture. We characterize the SEI formed with two ethylene carbonate-based electrolytes, without and with fluoroethylene carbonate (FEC) additive. The measurements show a striking contrast; SEI with FEC additive has 80% higher elastic modulus and a vastly higher resistance to fracture. These findings offer a mechanical-behavior based rationale to understand how SEI controls battery performance. Moreover, the experimental technique offers a robust diagnostic tool to design electrolytes that can form SEI with the desired mechanical properties for optimal battery performance

Effect of Fluoroethylene Carbonate Electrolytes on the Nanostructure of the Solid Electrolyte Interphase and Performance of Lithium Metal Anodes

The mechanism for the performance enhancement of lithium metal electrodes by fluoroethylene carbonate (FEC) is revealed. Electrolytes containing FEC, 1.2 M LiPF₆ in ethylene carbonate (EC):ethyl methyl carbonate (EMC) (3:7, vol) with 10% FEC (mass %) and 1.2 M LiPF₆ in FEC, improve the electrochemical performance of both Li∥Li and Cu∥LiFePO₄ cells compared to the baseline electrolyte, 1.2 M LiPF₆ in EC:EMC (3:7, vol). Ex situ surface analysis of lithium metal electrodes after the initial plating demonstrates that the solid electrolyte interphase (SEI) generated from FEC containing electrolytes is similar to the SEI generated from the baseline electrolyte, yet the corresponding Coulombic efficiencies are markedly different. Electron microscopy investigations reveal the presence of a unique SEI containing nanostructured LiF particles for the lithium electrode plated from the 1.2 M LiPF₆ in FEC electrolyte. The presence of the nanostructured LiF particles correlate with the improved cycling performance, suggesting that the morphology of the SEI is as important as the composition of the SEI

The electrochemical behavior of poly 1-pyrenemethyl methacrylate binder and its effect on the interfacial chemistry of a silicon electrode

The physico-chemical properties of poly (1-pyrenemethyl methacrylate) (PPy) are presented with respect to its use as a binder in a Si composite anode for Li-ion batteries. PPy thin-films on Si(100) wafer and Cu model electrodes are shown to exhibit superior adhesion as compared to conventional polyvinylidene difluoride (PVdF) binder. Electrochemical testing of the model bi-layer PPy/Si(100) electrodes in a standard organic carbonate electrolyte reveal higher electrolyte reduction current and an overall irreversible cathodic charge consumption during initial cycling versus the uncoated Si electrode. The PPy thin-film is also shown to impede lithiation of the underlying Si. XAS, AFM, TGA and ATR-FTIR analysis indicated that PPy binder is both chemically and electrochemically stable in the cycling potential range however significant swelling is observed due to a selective uptake of diethyl carbonate (DEC) from the electrolyte. The increased concentration of DEC and depletion of ethylene carbonate (EC) at the Si/PPy interface leads to continuous decomposition of the electrolyte and results in non-passivating behavior of the Si(100)/PPy electrode as compared to pristine silicon. Consequently, PPy binder improves the mechanical integrity of composite Si anodes but it influences mass transport at the Si(100)/PPy interface and alters electrochemical response of silicon during cycling in an adverse manner

The feasibility of a pyrrolidinium-based ionic liquid solvent for non-graphitic carbon electrodes

The feasibility of a pyrrolidinium-based room-temperature ionic liquid (RTIL) as the solvent for lithium-ion batteries is tested by analyzing its intercalation behavior and thermal stability. The RTIL-cations are intercalated into a graphitic carbon and a part of them are irreversibly trapped inside the graphene layers. These trapped cations block Li+ intercalation to give only a marginal capacity. In contrast, such a cation insertion/trapping is absent in two non-graphitic carbons; hard carbon and soft carbon. A stable cycle performance with a Li+ insertion capacity of about 200 mAh g - 1 is attained. The absence of RTIL-cation insertion is evidenced by the cyclic voltammograms and Raman spectra. A calorimetric study reveals that this RTIL has a higher thermal stability and less reactivity with lithiated carbons as compared with the carbonate-based solvent. The use of this RTIL solvent for the non-graphitic carbons seems to be feasible.close3