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Polymeric binders are a critical component to enhance mechanical integrity, maintain electronic conductivity, and achieve long durability of silicon (Si)-based electrodes. A fundamental understanding of the relationship between binder properties and mechanical degradation of Si electrodes is indispensable to developing durable Si-based electrodes. Using an environmental nanoindentation system, we measured the mechanical properties of Si composite electrodes made with different binders, including polyvinylidene fluoride (PVDF), Nafion, sodium-carboxymethyl cellulose (Na-CMC), and sodium-alginate (SA), as a function of the state-of-charge and cycle numbers under both dry and wet conditions. In contrast to electrodes made of Si alone, both elastic modulus (E) and hardness (H) of Si composite electrodes increase with lithium concentration within each cycle. E and H continuously decrease during long-term cycling. The mechanical property evolution of Si composite electrodes can be correlated with the porosity and irreversible thickness changes, which are largely determined by the mechanical properties of binders, instead of the adhesion between binders and Si. Electrodes under wet conditions have smaller E and H values than those under dry conditions because binders soften in the electrolyte. These findings not only provide useful mechanical parameters for battery modeling, but also may help design high performance and durable Si-based electrodes. • Elastic modulus and hardness of Si electrodes as a function of the state of charge. • Mechanical properties of binders and the adhesion strength between binders and Si. • Binder-dependent porosity and irreversible volume changes of Si composite electrodes. [ABSTRACT FROM AUTHOR]

Amorphous silicon is a promising high-capacity anode material for application in lithium-ion batteries. However, a huge drawback of the material is that the large capacity losses taking place during cycling lead to an unstable performance. In this study we investigate the capacity losses occurring during galvanostatic lithiation of amorphous silicon thin film electrodes by in-situ neutron reflectometry experiments for the first ten cycles. As determined from the analysis of the neutron scattering length density and of the film thickness, the capacity losses are due to irreversible storage of lithium in the electrode. The amount of stored lithium increases during cycling to 20% of the maximum theoretical capacity after the 10th cycle. Possible explanations are discussed. [ABSTRACT FROM AUTHOR]

Due to their high theoretical capacity compared to that of state-of-the-art graphite-based electrodes, silicon electrodes have gained much research focus for use in the development of next generation lithium-ion batteries. However, a major drawback of silicon as an electrode material is that it suffers from particle fracturing due to huge volume expansion during electrochemical cycling, thus limiting commercialization of such electrodes. Understanding the role of material microstructure in electrode degradation will be instrumental in the design of stable silicon electrodes. Here, we demonstrate the application of synchrotron-based X-ray tomographic microscopy to capture and track microstructural evolution, phase transformation and fracturing within a silicon-based electrode during electrochemical lithiation. [ABSTRACT FROM AUTHOR]

Silicon is a promising candidate to substitute or complement graphite as anode material in Li-ion batteries due, mainly, to its high energy density. However, the lithiation/delithiation processes of silicon particles are inherently related to drastic volume changes which, within a battery's physically constrained case, can induce significant deformation of the fundamental components of the battery that can eventually cause it to fail. In this work, we use non-destructive time-lapse X-ray imaging techniques to study the coupled electrochemo-mechanical phenomena in Li-ion batteries. We present X-ray computed tomography data acquired at different times during the first lithiation of custom-built silicon-lithium battery cells. Microstructural volume changes have been quantified using full 3D strain field measurements from digital volume correlation analysis. Furthermore, the extent of lithiation of silicon particles has been quantified in 3D from the grey-scale of the tomography images. Correlation of the volume expansion and grey-scale changes over the silicon-based electrode volume indicates that the process of lithiation is kinetically affected by the reaction at the Si/Li x Si interface. [ABSTRACT FROM AUTHOR]

Abstract: The understanding of volume change mechanism of silicon electrode is necessary to design a new negative electrode using silicon-based active materials. Here, the drastic volume expansion of one silicon secondary particle with μm-size was in-situ observed in order to find apparent volume expansion ratio during the first charging by using single particle measurement technique. The apparent volume expansion accompanied with the first lithiation is much larger than theoretical expectation due to the agglutination state and anisotropic property. The importance of direct observation with the single particle measurement has been affirmed for understanding the characteristics of silicon electrodes. [Copyright &y& Elsevier]

Abstract: The local composition of negative silicon electrodes is studied by ex situ electron energy-loss spectroscopy along the first delithiation in a lithium battery. By measuring dozens of sample areas for over a dozen compositions, the local and overall inhomogeneities in these practical electrodes are evaluated. The statistical treatment of the data highlights the existence of larger inhomogeneities at the beginning of the delithiation as well as at a 100nm scale. It is also shown that, even if incremental capacity curves are different, the compositional changes during delithiation are identical for nano- and micro-Si. Namely, an initial Li15Si4 phase is replaced by a Li2±0.3Si amorphous phase in a biphasic process, the latter compound being further delithiated to amorphous silicon in single phase process. Results also show that the electrochemical irreversibility associated with the liquid electrolyte reduction/degradation is generated during the lithiation process, not the delithiation process. [Copyright &y& Elsevier]

All-solid-state silicon/LiPON/lithium model batteries are assembled via magnetron sputtering followed by a lithium evaporation process. The cells were characterized via cyclic voltammetry, linear sweep voltammetry and impedance spectroscopy. Cyclic voltammetry confirms a long-term stability and high coulombic efficiency of the solid cells. Also, the assembled batteries were tested at extraordinary high scan rates, showing remarkably high output currents and an exceptional fast-cycling capability. Further rate-dependent investigations via linear sweep voltammetry indicate clearly two different ion-transport mechanisms in the silicon electrode at low and high scan rates. While a diffusion-controlled mechanism is suggested at high scan rates, a homogenous concentration profile is expected at extremely low rates. As a decisive advantage of solid-state cells, electrochemical cycling can be performed at elevated temperatures. This is used to determine the temperature dependence of the diffusion coefficients of lithium in the silicon electrode from room temperature to 165 °C. The effective activation energy of the lithium-ion migration in silicon is obtained to 0.1 eV, which is remarkably lower than that of inside LiPON, i.e. 0.55 eV. Complementary impedance data reveal that the kinetics of transport in the solid-state setup at very high output currents and low temperatures is controlled by lithium-ion migration in the LiPON film. • High rate capability and temperature durability of solid-state battery are shown. • Long-term stability and high coulombic efficiency are demonstrated. • Lithiation of silicon exhibits different kinetic regimes at low and high scan rates. • Temperature dependence of diffusion coefficients of lithium in Si was determined. [ABSTRACT FROM AUTHOR]

Coin half-cells containing 80 wt% silicon electrodes are assembled and cycled at the ~C/10 rate in the temperature range of 25–55 °C. To the best of our knowledge, this is the first time that the effect of temperature is reported for such high-silicon-containing cells. Two different electrolytes are used in this study, a baseline electrolyte and the baseline electrolyte +10 wt% fluoroethylene carbonate (FEC). Analysis of the capacity vs. cycle count data by curve fitting reveals that the addition of FEC markedly affects the capacity loss mechanism. Without FEC, the kinetic rate law for the capacity loss mechanism can be described as the sum of two logistic growth models. With the addition of FEC, the rate law depends on ln(t). Clearly, the addition of FEC has a profound effect on the mechanism of capacity loss. Interestingly, X-ray photoelectron spectroscopy (XPS) shows that the composition of the solid electrolyte interphase (SEI) layer changes markedly from mostly organic to mostly inorganic in the presence of FEC and how it varies at the different temperatures tested, especially in the absence of FEC. • Without FEC, a double logistic model is followed during silicon (de)lithiation. • With FEC, capacity loss follows ln(t) kinetics at most temperatures, except at 25 °C. • The double logistic model may imply two processes: particle cracking and diffusion. • FEC changes the nature of the SEI layer from mostly organic to mostly inorganic. • Temperature affects the SEI composition and above 45 °C changes are more drastic. [ABSTRACT FROM AUTHOR]

Mechanical degradation caused by lithiation/delithiation-induced stress and large volume change is the primary cause of fast capacity fading of silicon (Si)-based electrodes. Although intensive efforts have been devoted to understanding electromechanically induced fractures of electrodes made of Si alone (e.g., Si particles, Si thin films, and Si wafers), the cracking behavior of Si/polymeric binders/carbon black composite electrodes is unclear and poorly understood. Here, we investigate, by in situ and ex situ techniques, the cracking behavior of Si composite electrodes made with different binders, including polyvinylidene fluoride (PVDF), sodium-alginate (SA), sodium-carboxymethyl cellulose (Na-CMC), and Nafion. We found that extensive cracks form during the 1st delithiation process, which periodically open and close during subsequent lithiation/delithiation cycles at the same locations in the Si composite electrodes made with SA, Na-CMC, and Nafion. In contrast, a significantly fewer number of cracks form in the Si/PVDF electrodes after electrochemical cycling. A possible mechanism is proposed to help understand the effects of binders on the cracking behavior (e.g. , crack spacing and island size) of Si composite electrodes. We also suggest possible approaches, including reducing the electrode thickness, patterning electrodes, and using highly recoverable binders, to inhibit cracks and improve the mechanical integrity of Si composite electrodes. • In situ observations of the cracking behavior of Si composite electrodes during cycling. • Cracks open/close at the same locations during delithiation/lithiation. • Understanding the effects of binders on the cracking behavior of Si composite electrodes. • Strategies to inhibit cracks in Si composite electrodes. [ABSTRACT FROM AUTHOR]