In this contribution, we combined electrochemical cycling and X-ray photoelectron spectroscopy (XPS) to understand the nonpassivating behavior of the solid electrolyte interphase (SEI) on Si anodes during the first cycles. Based on galvanostatic measurements, we show that the irreversible capacity loss is reduced after the first cycle, and it stays almost constant from the second cycle onwards. XPS was used to determine the root causes of the Coulombic inefficiency, showing that the rate of decomposition of the organic solvents strongly decreased after the first cycle, whereas the rate of salt decomposition is almost unchanged between cycles. We determine that the inhibition of the decomposition reaction of the organic solvent is responsible for the lower Coulombic loss during the second electrochemical cycle in comparison to the first, whereas the nonpassivating behavior toward the salt decomposition is one of the main causes of capacity loss upon cycling. We further revisit the role of cracking in contributing to capacity loss. Whereas high volumetric expansion remains an issue plaguing the performance of Si anodes, our chronoamperometry studies reveal that the SEI formed on Si anodes does not passivate even when the electrode is fully expanded, and no additional surface is exposed. Overall, our work establishes the need to address the chemical and electrochemical instability of the SEI on the Si anode in addition to the more notorious issue of cracking.

Protective Polymer Coatings (PPCs) have been proposed to protect lithium metal anodes in rechargeable batteries to stabilize the Li/electrolyte interface and to extend the cycle life by reducing parasitic reactions and improving the lithium deposition morphology. However, the ion transport mechanism in PPCs remains unclear. Specifically, the degree of polymer swelling in the electrolyte and the influence of polymer/solvent/ion interactions are never quantified. Here we use poly(acrylonitrile-co-butadiene) (PAN-PBD) with controlled cross-link densities to quantify how the swelling ratio of the PPC affects conductivity, Li⁺ ion selectivity, activation energy, and rheological properties. The large difference in polarities between PAN (polar) and PBD (non-polar) segments allows the comparison of PPC properties when swollen in carbonate (high polarity) and ether (low polarity) electrolytes, which are the two most common classes of electrolytes. We find that a low swelling ratio of the PPC increases the transference number of Li⁺ ions while decreasing the conductivity. The activation energy only increases when the PPC is swollen in the carbonate electrolyte because of the strong ion-dipole interaction in the PAN phase, which is absent in the non-polar PBD phase. Theoretical models using Hansen solubility parameters and a percolation model have been shown to be effective in predicting the swelling behavior of PPCs in organic solvents and to estimate the conductivity. The trade-off between conductivity and the transference number is the primary challenge for PPCs. Our study provides general guidelines for PPC design, which favors the use of non-polar polymers with low polarity organic electrolytes.

Alloying anodes such as silicon are of great interest for lithium-ion batteries due to their high lithium-ion storage capacities, but have only seen minimal commercial deployment due to their limited calendar life. This has been attributed to an intrinsically unstable solid electrolyte interphase (SEI) that is aggravated by mechanical failure. An amorphous structure can mitigate lithiation strains, and amorphous alloys, or metallic glasses, often exhibit exceptional fracture toughness. Additional elements can be added to metallic glasses to improve passivation. Splat quenching was utilized to prepare an amorphous Al64Si25Mn11 Li-ion anode with a specific capacity >900 mAh g-1 that remains amorphous upon cycling. On this metallic glass electrode, parasitic electrolyte reduction is found to be much reduced in comparison to pure Si or Al, and comparable to that on Cu. The SEI is much thinner, more stable, and richer in fluorinated inorganic phases than the SEI formed on Si, while organic carbonate compounds such as lithium ethylene decarbonate (LiEDC) are notably absent. This study indicates that metallic glasses can become a viable new class of Li-ion anode materials with improved surface passivity.

This work focuses on the mechanisms of interfacial processes at the surface of amorphous silicon thin-film electrodes in organic carbonate electrolytes to unveil the origins of the inherent nonpassivating behavior of silicon anodes in Li-ion batteries. Attenuated total reflection Fourier-transform infrared spectroscopy, X-ray absorption spectroscopy, and infrared near-field scanning optical microscopy were used to investigate the formation, evolution, and chemical composition of the surface layer formed on Si upon cycling. We found that the chemical composition and thickness of the solid/electrolyte interphase (SEI) layer continuously change during the charging/discharging cycles. This SEI layer "breathing" effect is directly related to the formation of lithium ethylene dicarbonate (LiEDC) and LiPF₆ salt decomposition products during silicon lithiation and their subsequent disappearance upon delithiation. The detected appearance and disappearance of LiEDC and LiPF₆ decomposition compounds in the SEI layer are directly linked with the observed interfacial instability and poor passivating behavior of the silicon anode.

We report experimental studies to understand the reaction mechanism of the intermetallic anode Cu2Sb with Na and demonstrate that it is capable of retaining about 250 mAh g-1 over 200 cycles when using fluoroethylene carbonate additive. X-ray diffraction data indicate during the first discharge the reaction leads to the formation of crystalline Na 3Sb via an intermediate amorphous phase. Upon desodiation the Na 3Sb reverts to an amorphous phase, which then recrystallizes into Cu2Sb at full charge, indicating a high degree of structural reversibility. The structure after charging to 1 V is different from that of Cu2Sb, as indicated by X-ray absorption spectroscopy and 121Sb Mössbauer spectroscopy, and is due to the formation of an amorphous Na-Cu-Sb phase. At full discharge, an isomer shift of -8.10 mm s -1 is measured, which is close to that of a Na3Sb reference powder (-7.95 mm s-1) and in agreement with the formation of Na3Sb domains. During charge, the isomer shift at 1 V (-9.29 mm s-1) is closer to that of the pristine material (-9.67 mm s -1), but the lower value is consistent with the lack of full desodiation, as expected from the potential profile and the XAS data. © 2014 American Chemical Society.

The lack of fundamental understanding of the oxygen reduction and oxygen evolution in nonaqueous electrolytes significantly hinders the development of rechargeable lithium-air batteries. Here we employ a solid-state Li(4+x)Ti(5)O(12)/LiPON/Li(x)V(2)O(5) cell and examine in situ the chemistry of Li-O(2) reaction products on Li(x)V(2)O(5) as a function of applied voltage under ultra high vacuum (UHV) and at 500 mtorr of oxygen pressure using ambient pressure X-ray photoelectron spectroscopy (APXPS). Under UHV, lithium intercalated into Li(x)V(2)O(5) while molecular oxygen was reduced to form lithium peroxide on Li(x)V(2)O(5) in the presence of oxygen upon discharge. Interestingly, the oxidation of Li(2)O(2) began at much lower overpotentials (~240 mV) than the charge overpotentials of conventional Li-O(2) cells with aprotic electrolytes (~1000 mV). Our study provides the first evidence of reversible lithium peroxide formation and decomposition in situ on an oxide surface using a solid-state cell, and new insights into the reaction mechanism of Li-O(2) chemistry.

During Li-ion battery operation, (electro)chemical side reactions occur within the cell that can promote or degrade performance. These complex reactions produce byproducts in the solid, liquid, and gas phases. Studying byproducts in these three phases can help optimize battery lifetimes. To relate the measured gas-phase byproducts to species dissolved in the liquid-phase, equilibrium proprieties such as the Henry's law constants are required. The present work implements a pressure decay experiment to determine the thermodynamic equilibrium concentrations between the gas and liquid phases for ethylene (C₂H₄) and carbon dioxide (CO₂), which are two gases commonly produced in Li-ion batteries, with an electrolyte of 1.2 M LiPF₆ in 3:7 wt/wt ethylene carbonate/ethyl methyl carbonate and 3 wt % fluoroethylene carbonate (15:25:57:3 wt % total composition). The experimentally measured pressure decay curve is fit to an analytical dissolution model and extrapolated to predict the final pressure at equilibrium. The relationship between the partial pressures and concentration of dissolved gas in electrolyte at equilibrium is then used to determine Henry's law constants of 2.0 × 10⁴ kPa for C₂H₄ and k _CO2 = 1.1 × 10⁴ kPa for CO₂. These values are compared to Henry's law constants predicted from density functional theory and show good agreement within a factor of 3.

Single crystals of (Nd1−xCex)2Fe14B, some reaching ∼6×8×8mm3 in volume, are grown out of Fe-(Nd, Ce) flux. This crystal growth method allows for large (Nd1−xCex)2Fe14B single crystals to be synthesized using a simple flux growth procedure. Chemical and structural analyses of the crystals indicate that (Nd1−xCex)2Fe14B forms a solid solution until at least x=0.38 with a Vegard-like variation of the lattice constants with x. Refinements of single crystal neutron diffraction data indicate that Ce has a slight site preference (7:3) for the 4g rare earth site over the 4f site. Magnetization measurements at 300 K show only small decreases with increasing Ce content in saturation magnetization (Ms) and anisotropy field (HA), and Curie temperature (TC). First principles calculations are carried out to understand the effect of Ce substitution on the electronic and magnetic properties. For a multitude of applications, it is expected that the advantage of incorporating lower-cost and more abundant Ce will outweigh the small adverse effects on magnetic properties. Ce-substituted Nd2Fe14B is therefore a potential high-performance permanent magnet material with substantially reduced Nd content.

In this work we report the solid reaction products from the chemical reaction of aprotic battery electrolyte and three purported components of the Si-based anode SEI : SiO2 nanoparticles (NPs), lithium silicate (LixSiOy) powders, and Si NPs. We use FTIR and classical molecular dynamics/density functional perturbation theory to assess the solid products remaining with these model materials after exposure to electrolyte. The absence of electrochemical bias provides a view of the chemical speciation resulting from early-stage chemical reactivity during battery assembly as well as under open circuit storage conditions. We believe these species represent the initial stages of SEI growth and predict they likely drive subsequent chemical and electrochemical reactions by controlling molecular interactons at the Si active material interface. We find that nominally equivalent materials react differently even before any electrochemistry is performed (e.g., acidic SiO2 dissolves whereas alkaline SiO2 is relatively robust), and derive new understanding of the chemical species that could and could not form stable SEI components in Si-based anodes. These results can be used to inform how to passivate Si anode surfaces and potentially generate an artificially engineered SEI that would be stable and enable next-generation battery anodes.

Li-ion batteries will lose both capacity and power over time due to calendar aging caused by slow parasitic processes that consume Li+ ions. Studying and mitigating these processes is traditionally an equally slow venture, which is especially taxing for the validation of new active materials and electrolyte additives. Here, we evaluate whether potentiostatic holds can be used to accelerate the diagnosis of Li+ loss during calendar aging. The technique is based on the idea that, under the right conditions, the current measured as the cell voltage is held constant can be correlated with the instantaneous rate of side reactions. Thus, in principle, these measurements could capture the rate of capacity fade in real time. In practice, we show that this method is incapable of quantitatively forecasting calendar aging trends. Instead, our study demonstrates that potentiostatic holds can be applied for initial qualitative screening of systems that exhibit promising long-term stability, which can be useful to shrink the parameter space for calendar aging studies. By facilitating the identification of improved formulations, this approach can help accelerate innovation in the battery industry.