The electrode has first cycle charge capacity of 702 mAh g⁻¹_electrode and ~470 mAh cm⁻³_electrode at 100 mA g⁻¹_electrode and stable charge capacity of 543 mAh g⁻¹_electrode (~363 mAh cm⁻³_electrode) at charge current density of 2,400 mA g⁻¹_electrode. The capacity is ~200 mAh g⁻¹_electrode when cycled at ~-15 °C.

Further, the composite electrode has exceptionally high strain-to-failure (exceeds 2%) as measured in a uniaxial tensile test and the mode of failure differ significantly from pristine rGO papers.

The elimination of inactive ingredients such as metal current collector and polymeric binder reduces the total electrode weight and may provide the means to produce efficient lightweight batteries, the researchers suggest. An open-access paper on their work is published in the journal Nature Communications.

Concentrated efforts are currently employed to discover a practical replacement for traditional Li-ion battery electrodes that is, graphite anode and LiCoO₂ cathode with materials that continuously deliver high power and energy densities at high cycling efficiencies without damage. Alloying reaction electrodes such as silicon that can deliver as much as 5–10 times higher discharge capacity than traditional graphite, are at the forefront of this research. High capacity electrodes, however, are prone to enormous volume changes (~300%) that generally lead to structural collapse and capacity fading during successive lithiation/delithiation.

Recent work has shown that decreasing particle size or electrode nanostructuring allows the electrode to withstand high volumetric strains associated with repeated Li alloying and de-alloying. Pomegranate-inspired carbon-coated Si nanoparticles, yoke shell-structured SiC nanocomposites and Si/C core/shell composites (prepared at low mass loading) have proven to survive several hundred cycles without damage. Yet, electrode nanostructuring has lead to new fundamental challenges such as low volumetric capacity (low tap density), increased electrical resistance between the nanoparticles, increased manufacturing costs and lower Coulombic efficiency due to side reactions with the electrolyte. These challenges have not been fully addressed. What’s more, a particle-based electrode’s long-term cyclability hinges on the inter-particle electrical connection and particle adhesion to the metallic substrate, which decreases rapidly with increasing charge/discharge cycles, particularly for thick high-loading electrodes.

—David et al.

Graphene-based multicomponent composite anodes have attracted interest as solutions to these issues—mainly because of graphene’s superior electronic conductivity, mechanical strength and ability to be interfaced with Li active redox components, such as silicon. However, it has been difficult to incorporate graphene and silicon into practical batteries because of challenges that arise at high mass loadings—e.g., low capacity per volume, poor cycling efficiency and chemical-mechanical instability.

The team led by Gurpreet Singh has addressed these challenges by manufacturing a self-supporting and ready-to-go electrode that consists of a glassy ceramic called silicon oxycarbide sandwiched between large platelets of chemically modified graphene, or CMG. The paper-like design is made of 20% chemically modified graphene platelets.

Electrochemical characteristics and proposed lithium storage mechanism. (a) Charge capacity and cycling efficiency of various paper electrodes when cycled asymmetrically at increasing charge current densities. (b) Extended cycling behavior of rGO and 60SiOC electrodes cycled symmetrically at 1,600 mA g−1electrode. After 970 cycles, the electrodes showed good recovery when the current density was lowered back to 100 mA g−1electrode. Insets show the post-cycling digital and SEM images of the dissembled rGO and 60SiOC electrodes. Scale bar is 10 μm. (c) Voltage profile of 60SiOC electrode and corresponding (d) differential capacity curves for 1st, 2nd and 1,010th cycle. (e) Cycling behavior of 60SiOC at sub-zero temperature. After cooling down to ~-15 °C, the cell demonstrated a stable charge capacity of ~200 mAh g−1electrode at 100 mA g−1electrode. The cell regained ~86% of its initial capacity when returned to cycling at room temperature (~25 °C). (f) Schematic representing the mechanism of lithiation/delithiation in SiOC particles. Majority of lithiation occurs via adsorption at disordered carbon phase, which is uniformly distributed in the SiOC amorphous matrix. Large rGO sheets serve as an efficient electron conductor and elastic support. David et al. Click to enlarge.

The design enables an approximate 10% weight reduction in the cell. The lightweight electrode is capable of near 100% cycling efficiency. The low temperature performance suggests that rechargeable batteries from silicon-glass and graphene electrodes may also be suitable for unmanned aerial vehicles flying at high altitudes, or maybe even space applications, Singh said.

The silicon oxycarbide material is prepared by heating a liquid resin to the point where it decomposes and transforms into sharp glasslike particles. The silicon, carbon and oxygen atoms are rearranged into random 3-D structure and any excess carbon precipitates out into cellular regions.

Such an open 3-D structure creates large sites for reversible lithium storage and smooth channels for lithium-ion transportation. This structure and mechanism of lithium storage is different than crystalline silicon electrodes. Silicon oxycarbide electrodes are expected to be low cost because the raw material—liquid resin—is a byproduct of the silicone industry.

Moving forward, Singh and his team want to address practical challenges. Singh’s goal is to produce this electrode material at even larger dimensions. The team also would like to perform mechanical bending tests to see how they affect performance parameters.

Ultimately, we would like to work with industry to explore production of lithium-ion battery full-cells. Silicon oxycarbide can also be prepared by 3-D printing, which is another area of interest to us.

—Gurpreet Singh

The research received funding from the National Science Foundation.

Resources

• Lamuel David, Romil Bhandavat, Uriel Barrera & Gurpreet Singh (2016) “Silicon oxycarbide glass-graphene composite paper electrode for long-cycle lithium-ion batteries” Nature Communications 7, Article number: 10998 doi: 10.1038/ncomms10998