Surface and Electrochemical Studies on Silicon Diphosphide as Easy-to-Handle Anode Material for Lithium-Based Batteries-the Phosphorus Path

Isolation of cubic Si3P4 in the form of nanocrystals

This article describes an approach for synthesizing silicon phosphide nanoparticles with a defective zinc blende structure under mild conditions through thermal annealing of hydrogenated silicon nanoparticles with red phosphorus. The synthesized Si3P4 nanoparticles were analyzed using FTIR, XRD, electron diffraction, EDX, TEM, Raman spectroscopy, X-ray fluorescence spectrometry, and UV-vis spectrophotometry. For the isolated cubic Si3P4 phase, a cell parameter of a = 5.04 Å was determined, and the bandgap was estimated to be equal to 1.25 eV. Because of the nanoscale dimensions of the obtained Si3P4 nanoparticles, the product may exhibit several exceptional properties as a precursor for diffusion doping of wafers and as anode material for Li-ion batteries. A similar method with a hydrogenation step offers the possibility to obtain other compounds, such as silicon selenides, arsenides, and sulfides.

The millimeter-wave spectrum of the SiP radical (X2Πi): Rotational perturbations and hyperfine structure

The millimeter/submillimeter-wave spectrum of the SiP radical (X2Πi) has been recorded using direct absorption spectroscopy in the frequency range of 151–532 GHz. SiP was synthesized in an AC discharge from the reaction of SiH4 and gas-phase phosphorus, in argon carrier gas. Both spin–orbit ladders were observed. Fifteen rotational transitions were measured originating in the Ω = 3/2 ladder, and twelve in the Ω = 1/2 substate, each exhibiting lambda doubling and, at lower frequencies, hyperfine interactions from the phosphorus nuclear spin of I = 1/2. The lambda-doublets in the Ω = 1/2 levels appeared to be perturbed at higher J, with the f component deviating from the predicted pattern, likely due to interactions with the nearby excited A2Σ+ electronic state, where ΔEΠ-Σ ∼ 430 cm−1. The data were analyzed using a Hund’s case a β Hamiltonian and rotational, spin–orbit, lambda-doubling, and hyperfine parameters were determined. A 2Π/2Σ deperturbation analysis was also performed, considering spin–orbit, spin-electronic, and L-uncoupling interactions. Although SiP is clearly not a hydride, the deperturbed parameters derived suggest that the pure precession hypothesis may be useful in assessing the 2Π/2Σ interaction. Interpretation of the Fermi contact term, b F, the spin-dipolar constant, c, and the nuclear spin-orbital parameter, a, indicates that the orbital of the unpaired electron is chiefly p π in character. The bond length in the v = 0 level was found to be r0 = 2.076 Å, suggestive of a double bond between the silicon and phosphorus atoms.

Fe3+-Derived Boosted Charge Transfer in an FeSi4P4 Anode for Ultradurable Li-Ion Batteries

Ion and electron transportation determine the electrochemical performance of anodes in metal-ion batteries. This study demonstrates the advantage of charge transfer over mass transport in ensuring ultrastable electrochemical performance. Additionally, charge transfer governs the quality, composition, and morphology of a solid-electrolyte interphase (SEI) film. We develop FeSi₄P₄-carbon nanotube (FSPC) and reduced-FeSi₄P₄-carbon nanotube (R-FSPC) heterostructures. The FSPC contains abundant Fe³⁺ cations and negligible pore contents, whereas R-FSPC predominantly comprises Fe²⁺ and an abundance of nanopores and vacancies. The copious amount of Fe³⁺ ions in FSPC significantly improves charge transfer during Li-ion battery tests and leads to the formation of a thin monotonic SEI film. This prevents the formation of detrimental LiP and crystalline-Li_3.75Si phases and the aggregation of discharging/recharging products and guarantees the reformation of FeSi₄P₄ nanocrystals during delithiation. Thus, FSPC delivers a high initial Coulombic efficiency (>90%), exceptional rate capability (616 mAh g^-1 at 15 A g^-1), and ultrastable symmetric/asymmetric cycling performance (>1000 cycles at ultrahigh current densities). This study deepens our understanding of the effects of electron transport on regulating the structural and electrochemical properties of electrode materials in high-performance batteries.

Separator-Wetted, Acid- and Water-Scavenged Electrolyte with Optimized Li-Ion Solvation to Form Dual Efficient Electrode Electrolyte Interphases via Hexa-Functional Additive

The performance of lithium metal batteries (LMBs) is determined by many factors from the bulk electrolyte to the electrode-electrolyte interphases, which are crucially affected by electrolyte additives. Herein, the authors develop the heptafluorobutyrylimidazole (HFBMZ) as a hexa-functional additive to inhibit the dendrite growth on the surface of lithium (Li) anode, and then improve the cycling performance and rate capabilities of Li||LiNi_0.6 Co_0.2 Mn_0.2 O₂ (NCM622). The HFBMZ can remove the trace H₂ O and HF from the electrolyte, reducing the by-products on the surface of solid electrolyte interphase (SEI) and inhibiting the dissolution of metal ions from NCM622. Also, the HFBMZ can enhance the wettability of the separator to promote uniform Li deposition. HFBMZ can make Li⁺ easy to be desolvated, resulting in the increase of Li⁺ flux on Li anode surface. Moreover, the HFBMZ can optimize the composition and structure of SEI. Therefore, the Li||Li symmetrical cells with 1 wt% HFBMZ-contained electrolyte can achieve stable cycling for more than 1200 h at 0.5 mA cm^-2 . In addition, the capacity retention rate of the Li||NCM622 can reach 92% after 150 cycles at 100 mA g^-1 .

First-Principles Insights into Lithium-Rich Ternary Phosphide Superionic Conductors: Solid Electrolytes or Active Electrodes

Lithium-rich ternary phosphides are recently found to possess high ionic conductivity and are proposed as promising solid electrolytes (SEs) for solid-state batteries. While lithium ions can facilely transport within these materials, their electrochemical and interfacial stability in complex battery setups remain largely uncharacterized. We study the phase stability and electrochemical stability of phosphide-type SEs via first-principles calculations and thermodynamic analysis. Our results indicate that these SEs have intrinsic electrochemical stability windows narrower than 0.5 V. The SEs exhibit low anodic limits of about 1 V vs Li/Li⁺ due to the oxidation of the electrolytes to form various P binary compounds, indicating the poor electrochemical stability in contact with the cathode. In particular, we find that the thermodynamic driving force of such electrochemical decomposition is critically dependent on the new phases formed at the interfaces. Therefore, these phosphides might not be suitable as electrolytes. Despite the electrochemical instability, further calculations of Li diffusion kinetics show that the Li conduction is highly efficient through face-sharing octahedral and tetrahedral sites with low energy barriers, in spite of the possible variation of the local environments. In addition, an analysis of the terminal decomposition products shows impressive Li storage capacity as high as 2547 mAh·g^-1 based on the conversion mechanism, indicating they are capable as high-rate and energy-dense anode materials for battery applications.

Bimetallic phosphide AlP/SiP2 composite as an anode material for lithium-ion batteries with long cycle life

AlP and SiP₂ are promising alloy-type anode materials for lithium-ion batteries (LIBs), owing to their good conductivity, high storage capacity and appropriate working potential. However, they still suffer from rapid capacity decay due to the huge volume expansion and the resultant pulverization. Carbon modification can not only relieve volume changes but also provide a conducting matrix for the active material. Moreover, the charge transfer of the multi-phase composite can be accelerated owing to its electric field at the heterointerface. Hence, a bimetallic phosphide AlP/SiP₂@C composite was synthesized for the first time via a facile and scalable high energy ball milling method and applied as an anode material for LIBs. Benefitting from the above combined advantages of the heterostructure and carbon layer protection, the AlP/SiP₂@C electrode delivered a high reversible capacity (1482 mA h g^-1 at the current density of 0.3 A g^-1) and durable lifespan (516 mA h g^-1 after 4000 cycles at a current density of 3 A g^-1), which are superior to those of the binary AlP@C and SiP₂@C composites.

Solution-Based Self-Assembly and Stability of Ruthenium(II) Tris-bipyridyl Monolayers on Gold

Ruthenium(II) polypyridyl complexes are commonly applied as photosensitizers in the fields of artificial photosynthesis and light harvesting. Their immobilization on gold surfaces is also of interest for sensing and biological applications. Here, we report the self-assembly of [Ru(dmbpy)₂(dcbpy)](PF₆)₂ complexes on gold substrates from solution (dmbpy: 4,4'-dimethyl-2,2'-bipyridine; dcbpy: 2,2'-bipyridine-4,4'-dicarboxylic acid). Applying X-ray photoelectron spectroscopy, we demonstrate the formation of self-assembled monolayers (SAMs) of the Ru(II) complexes upon loss of counterions with carboxylate groups oriented toward the gold surface. We investigate the stability of the formed SAMs toward the substitution in solvents with competing aliphatic and aromatic thiols such as 4'-nitro[1,1'-biphenyl]-4-thiol, [1,1'-biphenyl]-4-thiol, and 1-hexadecanethiol. We show that the exchange reactions may lead to both complete replacement of the Ru(II) complexes and controlled formation of mixed SAMs. Moreover, we demonstrate that thiol-based SAMs can also be replaced completely from gold via their immersion into solutions of [Ru(dmbpy)₂(dcbpy)](PF₆)₂, indicating a relatively high stability for the Ru(II) complex SAMs. Our findings open up a variety of opportunities for applications of carboxylate-based SAMs on gold in nanotechnology.

Stable Cycle Performance of a Phosphorus Negative Electrode in Lithium-Ion Batteries Derived from Ionic Liquid Electrolytes

Although high-capacity negative electrode materials are seen as a propitious strategy for improving the performance of lithium-ion batteries (LIBs), their advancement is curbed by issues such as pulverization during the charge/discharge process and the formation of an unstable solid electrolyte interphase (SEI). In particular, electrolytes play a vital role in determining the properties of an SEI layer. Thus, in this study, we investigate the performance of a red phosphorus/acetylene black composite (P/AB) prepared by high-energy ball milling as a negative electrode material for LIBs using organic and ionic liquid (IL) electrolytes. Galvanostatic tests performed on half cells demonstrate high discharge capacities in the 1386-1700 mAh (g-P/AB)^-1 range along with high Coulombic efficiencies of 85.3-88.2% in the first cycle, irrespective of the electrolyte used. Upon cycling, the Li[FSA]-[C₂C₁im][FSA] (FSA^- = bis(fluorosulfonyl)amide and C₂C₁im⁺ = 1-ethyl-3-methylimidazolium) IL electrolyte (2:8 in mol) demonstrates a high capacity retention of 78.8% after 350 cycles, whereas significant capacity fading is observed in the Li[PF₆] and Li[FSA] organic electrolytes. Electrochemical impedance spectroscopy conducted with cycling revealed lower interfacial resistance in the IL electrolyte than in the organic electrolytes. Scanning electron microscopy and X-ray photoelectron spectroscopy after cycling in different electrolytes evinced that the IL electrolyte facilitates the formation of a robust SEI layer comprising multiple layers of sulfur species resulting from FSA^- decomposition. A P/AB|LiFePO₄ full cell using the IL electrolyte showed superior capacity retention than organic electrolytes and a high energy density under ambient conditions. This work not only illuminates the improved performance of a phosphorous-based negative electrode alongside ionic liquid electrolytes but also displays a viable strategy for the development of high-performance LIBs, especially for large-scale applications.

Silicon monophosphides with controlled size and crystallinity for enhanced lithium anodic performance

New electrode materials are crucial to high-performance lithium-ion batteries (LIBs). Silicon monophosphides (SiPs), composed of silicon and phosphorus, have a very high theoretical capacity (3060 mA h g^-1), which is more than 8 times that of graphite (372 mA h g^-1). The two-dimensional structure of SiPs also benefits ion transportation and diffusion. In this work, the chemical vapor transport (CVT) method is employed to synthesize SiPs for LIB anodes, and the lithium storage capacity co-affected by size and crystallinity is investigated using controllably synthesized thin belts and bulk crystals. The SiPs prepared by the high-temperature iodine-assisted CVT method have a belt-like morphology about 72 nm thick. After 200 cycles, the stable capacity is about 615 mA h g^-1 at 100 mA g^-1, and a reversible capacity of ∼320 mA h g^-1 is achieved at a high current density of 5.0 A g^-1. In contrast, the micrometer-thick bulk SiP crystals cannot provide efficient lithium ion extraction. Moreover, the smaller and thinner SiPs obtained at a lower temperature show abnormally high mass transport resistance and low lithium ion diffusivity. These results demonstrate that SiPs are promising LIB anode materials, and the size and crystallinity are closely related to the anodic performance. This new knowledge is valuable for the development of high-performance LIBs.