Towards a Greener and Scalable Synthesis of Na2_{2}Ti6_{6}O13_{13} 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 Na$_{2}$Ti$_{6}$O$_{13}$ 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 Na$_{2}$Ti$_{6}$O$_{13}$ tunnel structure indicated by electron energy‐loss and Raman spectroscopy. The suitability of pristine Na$_{2}$Ti$_{6}$O$_{13}$ 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

Polymer-derived-SiCN ceramic/graphite composite as anode material with enhanced rate capability for lithium ion batteries

We report on a new composite material in view of its application as a negative electrode in lithium-ion batteries. A commercial preceramic polysilazane mixed with graphite in 1:1 weight ratio was transformed into a SiCN/graphite composite material through a pyrolytic polymer-to-ceramic conversion at three different temperatures, namely 950 °C, 1100 °C and 1300 °C. By means of Raman spectroscopy we found successive ordering of carbon clusters into nano-crystalline graphitic regions with increasing pyrolysis temperature. The reversible capacity of about 350 mAh g−1 was measured with constant current charging/discharging for the composite prepared at 1300 °C. For comparison pure graphite and pure polysilazane-derived SiCN ceramic were examined as reference materials. During fast charging and discharging the composite material demonstrates enhanced capacity and stability. Charging and discharging in half an hour lead to about 200 and 10 mAh g−1, for the composite annealed at 1300 °C and pure graphite, respectively. A clear dependence between the final material capacity and pyrolysis temperature is found and discussed with respect to possible application in batteries, i.e. practical discharging potential limit. The best results in terms of capacity recovered under 1 V and high rate capability were also obtained for samples synthesized at 1300 °C

Novel 3D Si/C/SiOC nanocomposites: Toward electrochemically stable lithium storage in silicon

In this work, we present an easy and environmentally friendly approach to stabilize nanostructured, porous crystalline (Sitc) and amorphous (Sisa) silicon synthesized via magnesiothermic reduction. As matrix, fructose-derived carbon, polymer-derived SiOC ceramic or both, carbon and SiOC, are used. By means of X-ray diffraction and Raman spectroscopy it is found that the crystallinity of Sitc as well as the amorphous character of Sisa is preserved in the final composites. Embedding of crystalline silicon into carbon leads to high initial capacities of ~ 600–650 mAh·g− 1, but only the matrix consisting of carbon and SiOC results in a stable cycling behaviour over 50 cycles with a final capacity of 575 mAh·g− 1. All composites derived from amorphous Sisa show a stable cycling behaviour; the highest, stable capacity of ~ 500 mAh·g− 1 is observed when silicon is covered with carbon and SiOC

Carbon-rich SiOC anodes for lithium-ion batteries: Part I. Influence of material UV-pre-treatment on high power properties

Polymer-derived carbon-rich SiOC ceramics were studied in view of its application as anode material for lithium ion batteries. The samples were prepared at 1100 and 1300 °C by direct precursor pyrolysis or by a UV-radiation supported crosslinking applied before the thermal treatment. By means of various characterization techniques (Raman spectroscopy, X-ray diffraction, and elemental analysis) we found out that the UV-crosslinking procedure preceding the thermal treatment significantly influences the amount as well as the structural and electrochemical properties of the resulting carbon phase. The strong impact of those properties on the lithium insertion capacity and high current capability was analysed. The crosslinked sample prepared at 1100 °C revealed a stable reversible capacity of more than 650 mAh g− 1 for lower currents and more than 80 mAh g− 1 when a charge/discharge current of 4000 mAg− 1 is used. The extended cycling with high current does not lead to sample damage and the initial capacity is recovered when lower currents are applied after the high current charging/discharging procedure. We attribute these excellent properties of the crosslinked sample prepared at 1100 °C to the particular disorder of the carbon phase portioned in situ from the SiOC matrix under these conditions

Synthesis of 3D silicon with tailored nanostructure: Influence of morphology on the electrochemical properties

Within this work, monodisperse porous silicon nanospheres have been derived from monodisperse silica nanospheres via two different magnesiothermic reduction routes, namely (i) magnesiothermic reduction using a two-chamber set-up, and (ii) magnesiothermic reduction using NaCl as heat scavenger. Both methods allow a size- and shape-preserving preparation of mesoporous silicon. Crystalline silicon with a particle size of 56 nm and a specific surface area of 198 m2 g− 1 and amorphous silicon with a particle size of 35 nm and a specific surface area of 623 m2 g− 1 are synthesized using the two chamber and salt assisted routes, respectively. TEM micrographs confirm enhanced porous character of silicon from NaCl assisted route. An unstable electrochemical performance of the crystalline silicon is found, whereas the amorphous Si presents a stable electrochemical behavior

Lithium insertion into dense and porous carbon-rich polymer-derived SiOC ceramics

Two polymer-derived SiOC ceramics with different amount of carbon were synthesized either as dense or porous SiOC powders. The dense materials were produced up to a maximum temperature of 1400 °C and show a phase separated nanostructure consisting of SiO2-rich clusters, nanocrystalline SiC and nanocrystalline carbon phase. The corresponding porous materials were obtained by etching the silica phase of the dense SiOC with 20% HF solution. The electrochemical properties of the dense and porous SiOC ceramics in terms of lithium insertion/extraction were studied. Accordingly, the SiOC materials show a first lithium insertion capacity between 380 and 648 mAh g−1 followed by significantly lower extraction capacities between 102 and 272 mAh g−1. We consider the free carbon phase present in the ceramic as the major lithium intercalating agent. The porous samples show a stable electrochemical behavior up to 30 cycles while for the dense materials the efficiency drops to almost zero after 10 cycles

High Rate Capability of SiOC Ceramic Aerogels with Tailored Porosity as Anode Materials for Li-ion Batteries

Porous carbon-rich SiOC ceramic aerogels have been synthesized from a linear polysiloxane cross-linked with divinylbenzene (DVB) via hydrosilylation reaction in presence of a Pt catalyst and acetone as a solvent. The obtained wet gels are aged in solvent followed by drying under supercritical conditions using liquid carbon dioxide. The resulting pre-ceramic aerogels are subjected to pyrolysis at 1000 °C under controlled argon atmosphere to form the desired SiOC aerogel. The synthesized SiOC ceramics contain 43 wt% of free carbon, which is segregated within amorphous SiOC matrix. The high BET surface area up to 230 m2g−1 of preceramic aerogels is only slightly diminished to 180 m2g−1 after pyrolysis at 1000 °C. The electrochemical characterization reveals a high specific capacity of more than 600 mAh g−1 at a charging rate of C (360 mA g−1) along with a good cycling stability. At a rate of 10C (3600 mA g−1) the specific capacities as high as 200 mAh g−1 are recovered. The excellent properties of the materials are discussed with respect to their structural features. The porous nature of the carbon rich ceramics allows for fast ionic transport and helps to accommodate the structural changes which in turn allow a stable performance during repeated lithiation/delithiation

Influence of external compressive stress on the ionic conductivity of melt-quenched lithium silicate (15Li2O-85SiO2) glass

This paper reports measurements of the electrical resistances of bulk and milled amorphous lithium silicate (15Li2O-85SiO2) under uniaxial load at 200 °C. With imposed load up to 700 MPa, the resistance of the melt-quenched glass increases from 1.56 to 1.95 MΩ cm. This change is reversible, namely after releasing the load the initial electrical resistance is recovered. Milled and compacted glass consists of the bulk and particle contacts resistances. Both of them increase when the external compression up to 200 MPa. For the bulk element, when compression is released, the initial resistance is recovered, whereas the resistance changes within the particles contacts are irreversible

New insights on lithium storage in silicon oxycarbide/carbon composites: Impact of microstructure on electrochemical properties

In this work, we study the impact of the preceramic precursor vinyltriethoxysilane (VTES) on the electrochemical performance of silicon oxycarbide (SiOC) glass/graphite composites. We apply an innovative approach based on high-power ultrasounds in order to obtain highly homogenous composites with a uniform distribution of small graphitic flakes. This procedure enhances gelation and drying of VTES-based preceramic polymer/graphite blends. The SiOC/graphite composites reveal stable capacities (up to 520 mAh g-1 after 270 cycles), which are much higher than the sum derived from the ratio of the components. Additionally, the first cycle Coulombic efficiencies obtained for the composites are 15% higher than that of the pristine VTES-based SiOC ceramic. These properties are identified as the synergistic effect, originated from the addition of graphite to VTES-based SiOCs. Interestingly, such improvement in electrochemical performance is not noticed in the case of analogous SiOC/ graphite composites based on phenyltriethoxysilane (PhTES) precursor. The microstructural investigation of the composites based on two different preceramic precursors using solid-state 29Si NMR and Raman Spectroscopy unveils the reason for such discrepancy in their electrochemical behaviour. Keyword

Towards a Greener and Scalable Synthesis of Na 2 Ti 6 O 13 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 degrees C in a yield >= 80 wt%. It is fast, cost-efficient, and suitable for industrial upscaling. Electrochemical tests reveal stable performance providing capacities of approximate to 100 mA h g(-1) (Li) and 40 mA h g(-1) (Na). Increasing the synthesis temperature to 1100 degrees 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