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In the quest for high-capacity Li-ion batteries, moving from classical intercalation reactions such as those occurring at graphite-based electrodes to alloying reactions is a promising alternative. Among active materials which form alloys upon lithiation, silicon is a good candidate thanks to its high theoretical capacity, although it shows limited cyclability due to significant aging effects. In comparison, germanium presents improved Li-ion conduction and mechanical properties. Mixing silicon and germanium, as in Si1−xGex alloys, is an attractive strategy for combining the best advantages of both elements. In this study, we report a combined operando X-ray diffraction (XRD) and electrochemical investigation of the influence of the germanium content on the (de)lithiation processes in crystalline Si1−xGex nanoparticle-based anodes during the first charge/discharge cycle. The alloyed particles, which show pronounced heterogeneities in composition, evidence a sequential amorphization of the different c-Si1−xGex phases depending on their Ge content, where the lithiation potential decreases upon increasing the silicon content, following Vegard's law-type of behavior. Operando XRD and galvanostatic cycling investigation of the highly lithiated crystalline phase Li15(Si1−xGex)4 evidence a narrow domain of existence with a composition close to x = 1. This study brings essential knowledge on the (de)lithiation mechanisms at play in Si1−xGex alloys, which is critical for mastering these promising materials that combine the best properties of silicon and germanium, with the possibility to tune their composition to tailor (de)lithiation properties and trade off performance and cycle life. [ABSTRACT FROM AUTHOR]

Using Si as anode materials for Li-ion batteries remain challenging due to its morphological evolution and SEI modification upon cycling. The present work aims at developing a composite consisting of carbon-coated Si nanoparticles (Si@C NPs) intimately embedded in a three-dimensional (3D) graphene hydrogel (GHG) architecture to stabilize Si inside LiB electrodes. Instead of simply mixing both components, the novelty of the synthesis procedure lies in the in situ hydrothermal process, which was shown to successfully yield graphene oxide reduction, 3D graphene assembly production, and homogeneous distribution of Si@C NPs in the GHG matrix. Electrochemical characterizations in half-cells, on electrodes not containing additional conductive additive, revealed the importance of the protective C shell to achieve high specific capacity (up to 2200 mAh.g−1), along with good stability (200 cycles with an average Ceff > 99%). These performances are far superior to that of electrodes made with non-C-coated Si NPs or prepared by mixing both components. These observations highlight the synergetic effects of C shell on Si NPs, and of the single-step in situ preparation that enables the yield of a Si@C-GHG hybrid composite with physicochemical, structural, and morphological properties promoting sample conductivity and Li-ion diffusion pathways. [ABSTRACT FROM AUTHOR]

Carbon coatings can help to stabilize the electrochemical performance of high-energy anodes using silicon nanoparticles as the active material. In this work, the comparison of the behavior and chemical composition of the Solid Electrolyte Interphase (SEI) was carried out between Si nanoparticles and carbon-coated Si nanoparticles (Si@C). A combination of two complementary analytical techniques, Electrochemical Impedance Spectroscopy and X-ray Photoelectron Spectroscopy (XPS), was used to determine the intrinsic characteristics of the SEI. It was demonstrated that the SEI on Si particles is more resistive than the SEI on the Si@C particles. XPS demonstrated that the interface on the Si particles contains more oxygen when not covered with carbon, which shows that a protective layer of carbon helps to reduce the number of inorganic components, leading to more resistive SEI. The combination of those two analytical techniques is implemented to highlight the features and evolution of interfaces in different battery technologies. [ABSTRACT FROM AUTHOR]

Silicon nanowires are appealing structures to enhance the capacity of anodes in lithium-ion batteries. However, to attain industrial relevance, their synthesis requires a reduced cost. An important part of the cost is devoted to the silicon growth catalyst, usually gold. Here, we replace gold with tin, introduced as low-cost tin oxide nanoparticles, to produce a graphite–silicon nanowire composite as a long-standing anode active material. It is equally important to control the silicon size, as this determines the rate of decay of the anode performance. In this work, we demonstrate how to control the silicon nanowire diameter from 10 to 40 nm by optimizing growth parameters such as the tin loading and the atmosphere in the growth reactor. The best composites, with a rich content of Si close to 30% wt., show a remarkably high initial Coulombic efficiency of 82% for SiNWs 37 nm in diameter. [ABSTRACT FROM AUTHOR]