Thickness Variation of Conductive Polymer Coatings on Si Anodes for the Improved Cycling Stability in Full Pouch Cells.

Si-dominant anodes for Li-ion batteries provide very high gravimetric and volumetric capacity but suffer from low cycling stability due to an unstable solid electrolyte interphase (SEI). In this work, we improved the cycling performance of Si/NCM pouch cells by coating the Si anodes with the conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) prior to cell assembly via an electropolymerization process. The thicknesses of the PEDOT coatings could be adjusted by a facile process parameter variation. Glow-discharge optical emission spectroscopy was used to determine the coating thicknesses on the electrodes prior to the cell assembly. During electrochemical testing, improvements were observed closely linked to the PEDOT coating thickness. Specifically, thinner PEDOT coatings exhibited a higher capacity retention and lower internal resistance in the corresponding pouch cells. For the thinnest coatings, the cell lifetime was 18% higher compared to that of uncoated Si anodes. Postmortem analyses via X-ray photoelectron spectroscopy and cross-sectional scanning electron microscopy revealed a better-maintained microstructure and a chemically different SEI for the PEDOT-coated anodes.

The Optimal Amount of Lithium Difluorophosphate as an Additive for Si-Dominant Anodes in an Application-Oriented Setup.

Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are considered the most effective electrolyte additives for improving the solid electrolyte interphase (SEI) of Si-containing anodes while lithium difluorophosphate (LiDFP) is known to improve the interphases of cathode materials and graphite. Here, we combine VC, FEC, and different amounts of LiDFP in a highly-concentrated electrolyte to investigate the effect on Si-dominant anodes in detail. Cycle life tests, electrochemical impedance spectroscopy and rate tests with anode potential monitoring were conducted in Si/NCM pouch cells. The results reveal that adding LiDFP to the electrolyte improves all performance criteria of the full cells, with a concentration of 1 wt% being the optimal value for most cases. Post-mortem analyses using scanning electron microscopy and x-ray photoelectron spectroscopy showed that a more beneficial SEI film was formed for higher LiDFP concentrations, which led to less decomposition of electrolyte components and a better-maintained anode microstructure.

Influence of the Silicon-Carbon Interface on the Structure and Electrochemical Performance of a Phenolic Resin-Derived Si@C Core-Shell Nanocomposite-Based Anode.

Silicon is one of the most promising materials when it comes to lithium-ion battery anodes because of its high theoretical capacity and the low working potential versus Li/Li+. However, the drastic volume change during lithiation and delithiation leads to a rapid failure of the electrode. In order to accommodate the large volume change, Si@C core-shell nanocomposites have been investigated, as they efficiently protect the Si surface from being exposed to the electrolyte and thus limit side reactions and improve the cycling stability through a stable solid electrolyte interface layer. In recent years, phenolic resins have been investigated as the carbon source due to their facile synthesis and the possibility of scale-up. Here, the influence of the chemical structure of the Si-C interface on electrochemical performance has been analyzed by comparing pristine, silanol-rich and epoxide-functionalized Si/phenolic resin-derived nanocomposites. Whereas pristine Si@C exhibits the highest initial specific capacity of around 2000 mA h/gSi, introduction of silanol groups to the native surface leads to a more homogeneous carbon shell around the Si and thus to an overall higher Coulombic efficiency and a more stable cycling behavior. Additional epoxide functionalization, however, leads to a drastic decrease in initial capacity due to an overall increased resistance and prolongs the activation process. Nevertheless, in the long term, the additional layer leads to more stable cycling, especially at high current rates. For all nanocomposites, the electrochemical performance, characterized by cyclic voltammetry, cycling experiments, and electrochemical impedance spectroscopy, is correlated with the structure of the Si-C interface, determined by transition electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Raman, scanning electron microscopy, and IR-spectroscopy. To the best of our knowledge, the influence of the Si-C interface of a core-shell nanocomposite on structure and electrochemistry by chemically modifying the silicon surface is analyzed and reported for the first time.

Towards a Greener and Scalable Synthesis of Na2Ti6O13 Nanorods and Their Application as Anodes in Batteries for Grid-Level Energy Storage.

Grid applications require high power density (for frequency regulation, load leveling, and renewable energy integration), achievable by combining multiple batteries in a system without strict high capacity requirements. For these applications however, safety, cost efficiency, and the lifespan of electrode materials are crucial. Titanates, safe and longevous anode materials providing much lower energy density than graphite, are excellent candidates for this application. The innovative molten salt synthesis approach proposed in this work provides exceptionally pure Na2Ti6O13 nanorods generated at 900-1100 °C in a yield ≥80 wt%. It is fast, cost-efficient, and suitable for industrial upscaling. Electrochemical tests reveal stable performance providing capacities of ≈100 mA h g-1 (Li) and 40 mA h g-1 (Na). Increasing the synthesis temperature to 1100 °C leads to a capacity decrease, most likely resulting from 1) the morphology/volume change with the synthesis temperature and 2) distortion of the Na2Ti6O13 tunnel structure indicated by electron energy-loss and Raman spectroscopy. The suitability of pristine Na2Ti6O13 as the anode for grid-level energy storage systems has been proven a priori, without any performance-boosting treatment, indicating considerable application potential especially due to the high yield and low cost of the synthesis route.

Highly Porous Silicon Embedded in a Ceramic Matrix: A Stable High-Capacity Electrode for Li-Ion Batteries.

We demonstrate a cost-effective synthesis route that provides Si-based anode materials with capacities between 2000 and 3000 mAh·gSi-1 (400 and 600 mAh·gcomposite-1), Coulombic efficiencies above 99.5%, and almost 100% capacity retention over more than 100 cycles. The Si-based composite is prepared from highly porous silicon (obtained by reduction of silica) by encapsulation in an organic carbon and polymer-derived silicon oxycarbide (C/SiOC) matrix. Molecular dynamics simulations show that the highly porous silicon morphology delivers free volume for the accommodation of strain leading to no macroscopic changes during initial Li-Si alloying. In addition, a carbon layer provides an electrical contact, whereas the SiOC matrix significantly diminishes the interface between the electrolyte and the electrode material and thus suppresses the formation of a solid-electrolyte interphase on Si. Electrochemical tests of the micrometer-sized, glass-fiber-derived silicon demonstrate the up-scaling potential of the presented approach.