Performance-Enhancing Asymmetric Separator for Lithium–Sulfur Batteries

Asymmetric separators with polysulfide barrier properties consisting of porous polypropylene grafted with styrenesulfonate (PP<i>-<i>g</i>-</i>PLiSS) were characterized in lithium–sulfur cells to assess their practical applicability. Galvanostatic cycling at different C-rates with and without an electrolyte additive and cyclic voltammetry were used to probe the electrochemical performance of the cells with the PP<i>-<i>g</i>-</i>PLiSS separators and to compare it with the performance of the cells utilizing state-of-the-art separator, Celgard 2400. Overall, it was found that regardless of the applied cycling rate, the use of the grafted separators greatly enhances the Coulombic efficiency of the cell. An appropriate Li-exchange-site (−SO₃^–) concentration at and near the surface of the separator was found to be essential to effectively suppress the polysulfide shuttle without sacrificing the Li-ion mobility through the separator and to improve the practical specific charge of the cell

Cycling Behavior of Silicon-Containing Graphite Electrodes, Part A: Effect of the Lithiation Protocol

Silicon (Si) is a promising additive for enhancing the specific charge of graphite negative electrodes in Li-ion batteries. However, Si alloying with lithium leads to an extreme volume expansion and in turn to rapid performance decline. Here we present how controlling the lithiation depth affects the performance of graphite/Si electrodes when different lithiation cutoff potentials are applied. The relationship between Si particle size and cutoff potential was investigated to clarify the interdependence of these two parameters and their impact on the performance of Si-containing graphite electrodes. For Si with a particle size of 30–50 nm, Li₁₅Si₄ is only formed for the potential cutoff of 5 mV vs Li⁺/Li, whereas using a higher cutoff of 50 mV has no impact on the performance. For larger Si nanoparticles, 70–130 nm in size, Li₁₅Si₄ is already formed at 50 mV. However, in these larger particles only 70% of the Si initially participates in the lithiation, independent of the cutoff potential (5 or 50 mV), and the performance fades rapidly. For the highest tested cutoff potential of 120 mV, the contribution of larger Si particles to the specific charge of the electrodes was negligible, but for the smaller particles a stable and still significant Si specific charge was obtained

One-step grown carbonaceous germanium nanowires and their application as highly efficient lithium-ion battery anodes

Developing a simple, cheap, and scalable synthetic method for the fabrication of functional nanomaterials is crucial. Carbon-based nanowire nanocomposites could play a key role in integrating group IV semiconducting nanomaterials as anodes into Li-ion batteries. Here, we report a very simple, one-pot solvothermal-like growth of carbonaceous germanium (C-Ge) nanowires in a supercritical solvent. C-Ge nanowires are grown just by heating (380−490 °C) a commercially sourced Ge precursor, diphenylgermane (DPG), in supercritical toluene, without any external catalysts or surfactants. The self-seeded nanowires are highly crystalline and very thin, with an average diameter between 11 and 19 nm. The amorphous carbonaceous layer coating on Ge nanowires is formed from the polymerization and condensation of light carbon compounds generated from the decomposition of DPG during the growth process. These carbonaceous Ge nanowires demonstrate impressive electrochemical performance as an anode material for Li-ion batteries with high specific charge values (>1200 mAh g−1 after 500 cycles), greater than most of the previously reported for other “binder-free” Ge nanowire anode materials, and exceptionally stable capacity retention. The high specific charge values and impressively stable capacity are due to the unique morphology and composition of the nanowires

Cycling Behavior of Silicon-Containing Graphite Electrodes, Part B: Effect of the Silicon Source

Silicon (Si) is a promising candidate to enhance the specific charge of graphite electrode, but there is no consensus in the literature on its cycling mechanism. Our aim in this study was to understand Si electrochemical behavior in commercially viable graphite/Si electrodes. From the comparison of three types of commercial Si particles with a producer-declared particle sizes of 30–50 nm, 70–130, and 100 nm, respectively, we identified the presence of micrometric Si agglomerates and the Si micro- and mesoporosity as the main physical properties affecting the cycling performance. Moreover, ex situ SEM, XRD, and Raman investigations allowed us to understand the lithiation/delithiation mechanism for each type of Si particles. For nanoscale Si particles, the entire Si particle is utilized, resulting in high specific charge, and the stress induced by the formation of Li₁₅Si₄ alloy upon deep lithiation is well managed within the Si mesoporosity. This leads to reversible cycling behavior and, thus, to good cycling stability. On the other hand, micrometric Si aggregates undergo a two-phase lithiation mechanism with early Li₁₅Si₄ formation in the particle shell. This leads to stress-induced core disconnection during the first lithiation, and shell pulverization during the following delithiation, resulting in overall low specific charge and rapid performance fading