Revealing salt-expedited reduction mechanism for hollow silicon microsphere formation in bi-functional halide melts

The thermochemical reduction of silica to silicon using chemical reductants requires high temperature and has a high activation energy, which depends on the melting temperature of the reductant. The addition of bi-functional molten salts with a low melting temperature may reduce the required energy, and several examples using molten salts have been demonstrated. Here we study the mechanism of reduction of silica in the presence of aluminum metal reductant and aluminum chloride as bi-functional molten salts. An aluminum-aluminum chloride complex plays a key role in the reduction mechanism, reacting with the oxygen of the silica surfaces to lower the heat of reaction and subsequently survives a recycling step in the reaction. This experimentally and theoretically validated reaction mechanism may open a new pathway using bi-functional molten salts. Furthermore, the as-synthesized hollow porous silicon microsphere anodes show structural durability on cycling in both half/full cell tests, attributed to the high volume-accommodating ability

Synthesis of Ultrathin Si Nanosheets from Natural Clays for Lithium-Ion Battery Anodes

Two-dimensional Si nanosheets have been studied as a promising candidate for lithium-ion battery anode materials. However, Si nanosheets reported so far showed poor cycling performances and required further improvements. In this work, we utilize inexpensive natural clays for preparing high quality Si nanosheets via a one-step simultaneous molten salt-induced exfoliation and chemical reduction process. This approach produces high purity mesoporous Si nanosheets in high yield. As a control experiment, two-step process (pre-exfoliated silicate sheets and subsequent chemical reduction) cannot sustain their original two-dimensional structure. In contrast, one-step method results in a production of 5 nm-thick highly porous Si nanosheets. Carbon-coated Si nanosheet anodes exhibit a high reversible capacity of 865 mAh g(-1) at 1.0 A g(-1)-with an outstanding capacity retention of 92.3% after 500 cycles. It also delivers high rate capability, corresponding to a capacity of 60% at 20 A g(-1) compared to that of 2.0 A g(-1). Furthermore, the Si nanosheet electrodes show volume expansion of only 42% after 200 cyclesclose

Multiscale Hyperporous Silicon Flake Anodes for High Initial Coulombic Efficiency and Cycle Stability

Three-dimensional (3D) hyperporous silicon flakes (HPSFs) are prepared via the chemical reduction of natural clay minerals bearing metal oxides. Natural clays generally have 2D flake-like structures with broad size distributions in the lateral dimension and varied thicknesses depending on the first processing condition from nature. They have repeating layers of silicate and metal oxides in various ratios. When the clay mineral is Subjected to a reduction reaction, metal oxide layers can perform a negative catalyst for absorbing large amounts of exothermic heat from the reduction reaction of the silicate layers with metal reductant. Selectively etching out metal oxides shows hyperporous nanoflake structure containing 100 rim macropores and meso-/micropores on its framework. The resultant HPSFs are demonstrated as anode materials for lithium-ion batteries. Compared to conventional micro-Si anodes, HPSFs exhibit exceptionally high initial Coulombic efficiency over 92%. Furthermore, HPSF anodes show outstanding cycling performance (reversible capacity of 1619 mAh g(-1) at a rate of 0.5 C after 200 cycles, 95.2% retention) and rate performance (similar to 580 mAh g(-1) at a rate of 10 C) owing to their distinctive structure.close0

Intertwining porous silicon with conducting polymer for high-efficiency stable Li-ion battery anodes

Porous silicon anodes have been extensively investigated for the high-performance lithium-ion battery, owing to their high capacity and structural robustness as dominantly incorporated with conductive carbon sheath. However, the typical high-temperature annealing process for carbon coating induces the collapse of pre-engineered pores and limits the full utilization of porous structures. In this work, porous silicon flake intertwined with doped polyaniline was prepared via redox-transmetalation reaction followed by a wet coating process of conducting polymer. The proposed method eliminates the risk of pore collapse and utilizes the conductive network without compromising the porous structure. As a result, the prepared composite consisting of porous silicon flake and doped polyaniline shows an extended battery cycle life, reduced electrode swelling, and increased efficiency

Nanotubular structured Si-based multicomponent anodes for high-performance lithium-ion batteries with controllable pore size via coaxial electro-spinning

We demonstrate a simple but straightforward process for the synthesis of nanotube-type Si-based multicomponents by combining a coaxial electrospinning technique and subsequent metallothermic reduction reaction. Si-based multicomponent anodes consisting of Si, alumina and titanium silicide show several advantages for high-performance lithium-ion batteries. Alumina and titanium silicide, which have high mechanical properties, act as an effective buffer layer for the large volume change of Si, resulting in outstanding volume suppression behavior (volume expansion of only 14%). Moreover, electrically conductive titanium silicide layers located at the inner and outer layers of a Si nanotube exhibit a high initial coulombic efficiency of 88.5% and an extraordinary rate capability. Nanotubular structured Si-based multicomponents with mechanically and electrically improved components can be used as a promising alternative to conventional graphite anode materials. This synthetic route can be extended to other high capacity lithium-ion battery anode materialsclose

A polymeric separator membrane with chemoresistance and high Li-ion flux for high-energy-density lithium metal batteries

Lithium (Li) metal batteries are limited by the unstable deposition structure of Li metal, triggering aggressive elec-trolyte consumption and presenting safety concerns. A fundamental solution regulating the Li metal deposition structure is contingent on the homogeneity of Li-ion flux at the Li-electrolyte interface and electrolyte-infiltrated separator matrix. Here, we report a rational design for a separator coupling with two functional polymers, i.e., ferroelectric terpolymer-polydopamine in a core-shell structure. A conformal polydopamine layer prevents elec-trolyte dissolution of the terpolymer, improves electrolyte affinity, and suppresses detrimental chemical crossover. Incorporating the high ferroelectricity of the terpolymer increases the Li-ion transference number and ionic con-ductivity, ensuring the homogeneity of Li-ion transport through the separator's pore network. With this bicom-ponent separator, stable cycling of a pouch-type full cell is achieved, comprising a thin Li metal (20 mu m) anode and a layered oxide cathode under the limited electrolyte condition

A multi-stacked hyperporous silicon flake for highly active solar hydrogen production

3D multi-stacked hyperporous silicon flakes (MHSFs) are prepared via a selective chemical reduction of natural clay minerals bearing MgO negative catalyst layers. The resultant MHSFs are used as a photocatalyst for solar-driven hydrogen evolution and exhibit the highest photocatalytic acitivty (1031 mu mol H-2 h(-1) g(-1) Si) coupled with a Pt cocatalyst.yclos

Vinyl-Integrated In Situ Cross-Linked Composite Gel Electrolytes for Stable Lithium Metal Anodes

Achieving high energy/power densities, longer operating cycles, and ensured battery safety of rechargeable batteries become the primary mission to satisfy the requirements of electrified applications. Lithium metal batteries (LMBs) have been spotlighted because of the potential high energy density with the high theoretical specific capacity (3860 mA h g(-1)) and low redox potential. However, a safety hazard and the resulting short cycle life of batteries from uncontrollable dendritic growth of Li during the electrochemical cycles are a deterrence to their their commercialization. Here, we report in situ formation of a covalent network of composite gel polymer electrolytes (CPEs) integrating the vinyl-functionalized silica (VSNP) with polymer structures. A small addition (1 wt %) of VSNP into the CPE greatly enhances the mechanical modulus and Li-ion transference number of the CPE while maintaining high ionic conductivity. The VSNP-CPE assembled LiCoO2/Li and Li(Ni0.8Co0.1Mn0.1)O-2/Li metal batteries show durable battery operation with a capacity retention of 88% after 500 cycles and 80% after 200 cycles, respectively, compared to liquid electrolytes, which show a gradual decay. The functionalized nanofiller with in situ formed CPE provides a feasible platform to improve mechanical properties and ionic conduction of CPEs as well as the processability of fabrication of high energy density LMBs

Electrolyte-mediated nanograin intermetallic formation enables superionic conduction and electrode stability in rechargeable batteries

Toward realizing the high-energy-density rechargeable batteries, self-supporting aluminum (Al) foil has been explored as an emerging anode to replace the graphite anode. However, the implementation of Al foil anodes into the rechargeable batteries has been plagued by limited charge-carrier kinetics, substantial volume variation, and poor electrochemical reversibility. Herein, we introduced an electrolyte-mediated mechanical prelithiation method at relatively low pressure, resulting in a gradient and nanograins intermetallic LiAl layer onto the Al under the consideration of matrix hardness to circumvent the large volume change. The designed electrode can provide superionic conduction, structural integrity, as well as high Coulombic efficiency compared with those of bare Al anode, as evidenced by theoretical calculations and battery experiments. This electrode showed fast-charging (112.3 mAh g(-1) at 5 C), ultrastable capacity retention (similar to 100.0% at after 600 cycles), and high Coulombic efficiency > 99.7% at 10 C under the high-capacity loading condition in the dual-ion battery. When paired with LiFePO4 cathode, the gradient and nanograins intermetallic electrode render conventional lithium-ion battery long-lasting for 200 cycles, demonstrating the decent interfacial and architectural design for the foil-type electrodes