Tris(trimethylsilyl) Phosphite as an Efficient Electrolyte Additive To Improve the Surface Stability of Graphite Anodes

Tris­(trimethylsilyl) phosphite (TMSP) has received considerable attention as a functional additive for various cathode materials in lithium-ion batteries, but the effect of TMSP on the surface stability of a graphite anode has not been studied. Herein, we demonstrate that TMSP serves as an effective solid electrolyte interphase (SEI)-forming additive for graphite anodes in lithium-ion batteries (LIBs). TMSP forms SEI layers by chemical reactions between TMSP and a reductively decomposed ethylene carbonate (EC) anion, which is strikingly different from the widely known mechanism of the SEI-forming additives. TMSP is stable under cathodic polarization, but it reacts chemically with radical anion intermediates derived from the electrochemical reduction of the carbonate solvents to generate a stable SEI layer. These TMSP-derived SEI layers improve the interfacial stability of the graphite anode, resulting in a retention of 96.8% and a high Coulombic efficiency of 95.2%. We suggest the use of TMSP as a functional additive that effectively stabilizes solid electrolyte interfaces of both the anode and cathode in lithium-ion batteries

Fast Magnesium Ion Transport in the Bi/Mg₃Bi₂ Two-Phase Electrode

Bi has recently attracted much interest as a promising Mg battery anode to replace the Mg metal, which is ideal but impractical because of instability issues associated with the electrolyte. Our first-principles molecular dynamics study addresses why the alloying reaction of Bi with Mg creates only two crystals of Bi and Mg₃Bi₂ and how Mg ions move in the two crystals. The formation of crystalline Mg₃Bi₂ is energetically much more favorable than that of amorphous Mg_<i>x</i>Bi (0.0 ≤ <i>x</i> ≤ 2.5). This high thermodynamic stability of the Mg₃Bi₂ crystal serves as a driving force for the complete crystalline Bi/crystalline Mg₃Bi₂ two-phase reaction, without allowing the formation of amorphous phases typically observed in alloy-type anode materials. The Mg ions diffuse preferentially along the [001] direction in both Bi and Mg₃Bi₂ and diffuse about 6000 times faster in Mg₃Bi₂ than in Bi. Despite the relatively slow Mg transport in Bi, the absolute kinetics of Mg ions in Bi is reasonable in terms of fast battery operation. Interestingly, the calculated Mg ion diffusivity in Bi is comparable to that for a Li ion in Bi, contrary to the general perspective that multivalent ions are very sluggish compared with monovalent ions. Our study suggests that fast multivalent-ion transport can be achieved when the multivalent ion has weak bonds with the nearest host elements and exhibits small fluctuations in its coordination number and bond length during ion transport

Thermodynamic and Kinetic Origins of Lithiation-Induced Amorphous-to-Crystalline Phase Transition of Phosphorus

Despite its fundamental importance, real-time observation of atomic motions during phase transition is challenging because the transition processes usually occur on ultrafast time scales. Herein, we directly monitored a fleeting and spontaneous crystallization of Li₃P from amorphous Li_<i>x</i>P phases with <i>x</i> ∼ 3 at room temperature via first-principles molecular dynamics simulations. The crystallization is a collective atomic ordering process continued for 0.4 ps and it is driven by the following key impetuses: (1) the crystalline Li₃P phase is more stable than its amorphous counterpart, (2) the amorphous Li_<i>x</i>P phase corresponds thermodynamically to the local minimum energy state at <i>x</i> ∼ 3, which enables its crystallization under an electrochemical equilibrium condition without net flux of lithium ions in the host material, (3) the crystalline and amorphous structures of Li₃P are so similar that the average displacement of the mobile Li atoms during crystallization is only 0.56 Å, and (4) highly lithiated materials with all isolated host elements, such as the amorphous Li₃P phase, are advantageous for crystallization because the isolation induces a kinetically favorable low-barrier transition without complicated multistep P–P bond breaking/forming processes

How Do Li Atoms Pass through the Al₂O₃ Coating Layer during Lithiation in Li-ion Batteries?

We studied the lithiation of Al₂O₃ and found the energetically most favorable composition of Li_3.4Al₂O₃ using ab initio molecular dynamics simulations. The calculated Li/Al ratio and corresponding volume expansion ratio agree well with reported experimental observations. The Al atoms accept electrons from the incoming Li atoms during lithiation, leading to the formation of various Al structures, that is, isolated atoms, dimers, trimers, and ring- and chain-type clusters. The Li atoms in the optimal concentration diffuse faster by four (five) orders of magnitude than the Al (O) atoms, and they also diffuse faster by four orders of magnitude than the Li atoms in a dilute Li concentration. We suggest that in Li-ion batteries the lithiation of the Al₂O₃ coating layer proceeds until a thermodynamically stable phase is reached; then, extra Li atoms overflow into the electrode by passing through the coating layer

Reductive Decomposition Mechanism of Prop-1-ene-1,3-sultone in the Formation of a Solid–Electrolyte Interphase on the Anode of a Lithium-Ion Battery

A novel electrolyte additive, prop-1-ene-1,3-sultone (PES), has recently attracted great attention due to its formation of effective solid–electrolyte interphase (SEI) films and remarkable cell performance in lithium-ion batteries. Herein, the reductive decomposition of PES is investigated through density functional calculations combined with a self-consistent reaction field method, in which the bulk solvent effect is accounted for by the geometry optimization and transition-state search. We examine three ring-opening pathways, namely, O–C, S–C, and S–O bond-breaking processes. Our calculations reveal that the Li⁺ ion plays a pivotal role in the reductive decomposition of PES. While the most kinetically favored processthe S–O bond breakingis effectively blocked via the formation of an intermediate structure, namely, the Li⁺-participated seven-membered ring, the other decomposition processes via O–C and S–C bond breaking lead to stable decomposition products. The constituents of SEI observed in previous experimental studies, such as RSO₃Li and ROSO₂Li, can be reasonably understood as the decomposition products resulting from O–C and S–C bond breaking, respectively

Anisotropic Volume Expansion of Crystalline Silicon during Electrochemical Lithium Insertion: An Atomic Level Rationale

The volume expansion of silicon is the most important feature for electrochemical operations of high capacity Si anodes in lithium ion batteries. Recently, the unexpected anisotropic volume expansion of Si during lithiation has been experimentally observed, but its atomic-level origin is still unclear. By employing first-principles molecular dynamics simulations, herein, we report that the interfacial energy at the phase boundary of amorphous Li_<i>x</i>Si/crystalline Si plays a very critical role in lithium diffusion and thus volume expansion. While the interface formation turns out to be favorable at <i>x</i> = 3.4 for all of the (100), (110), and (111) orientations, the interfacial energy for the (110) interface is the smallest, which is indeed linked to the preferential volume expansion along the ⟨110⟩ direction because the preferred (110) interface would promote lithiation behind the interface. Utilizing the structural characteristic of the Si(110) surface, local Li density at the (110) interface is especially high reaching Li_5.5Si. Our atomic-level calculations enlighten the importance of the interfacial energy in the volume expansion of Si and offer an explanation for the previously unsolved perspective

Important Role of Functional Groups for Sodium Ion Intercalation in Expanded Graphite

Expanded graphite oxide (GO) has recently received a great deal of attention as a sodium ion battery anode due to its superior characteristics for sodium ion storage. Here, we report that the sodium ion intercalation behavior of expanded GO strongly depends on the amounts and ratios of different functional groups. The epoxide-rich GO shows significantly higher specific capacities than those of the hydroxyl-rich counterpart utilizing strong sodium–epoxide attractions and appropriately enlarged interlayer spacing during sodiation. The epoxide-rich GO also enables fast sodium ion transport on account of the diminishment of interlayer hydrogen bonds that could reduce the free volume. Our calculations suggest that the theoretical capacity of epoxide-only GO with a stoichiometry of Na_2.5C₆O₃ can reach 930 mAh g^–1, which is far higher than recent experimental results as well as even those of conventional graphite materials in lithium ion batteries

Atomic-Level Understanding toward a High-Capacity and High-Power Silicon Oxide (SiO) Material

Silicon oxide (SiO) has attracted much attention as a promising anode material for Li-ion batteries. The lithiation of SiO results in the formation of active Li–Si alloy cores embedded in an inactive matrix consisting of Li-silicates (Li₂Si₂O₅, Li₆Si₂O₇, and Li₄SiO₄) and Li₂O. The maximum Li content in lithiated SiO (Li_<i>x</i>SiO) is known to be <i>x</i> = 4.4 based on experiments. Our calculations reveal that Li-silicates are dominant over Li₂O among matrix components of the experimental Li_4.4SiO phase. We show that Li_<i>x</i>SiO can become thermodynamically more stable and thus accommodate more Li ions up to <i>x</i> = 5.2 when Li₂O dominates over Li-silicates. The minor portion of Li₂O in the experimental phase is attributed to kinetically difficult transformations of Li-silicates into Li₂O during electrochemical lithiation. The Li₂O subphase can act as a major transport channel for Li ions because the Li diffusivity in Li₂O is calculated to be faster by at least 2 orders of magnitude than in Li-silicates. We suggest that Li₂O is a critical matrix component of lithiated SiO because it maximizes the performance of SiO in terms of both capacity and rate capability

Flexible Few-Layered Graphene for the Ultrafast Rechargeable Aluminum-Ion Battery

Fast ion transport is essential for high rate capability in rechargeable battery operation. Recently, an ultrafast rechargeable aluminum-ion battery was experimentally demonstrated through the reversible intercalation/deintercalation of chloroaluminate anions (AlCl₄^–) in graphitic-foam cathodes. Using first-principles calculations, herein, we report that the unique structural characteristic of graphitic foam, i.e., mechanical flexibility of few-layered graphene nanomaterials, plays a key role for the ultrafast aluminum-ion battery. We found that AlCl₄^– is stored by forming doubly stacked ionic layers in the interlayer space between graphene sheets, and their diffusivity increases dramatically once graphene film is less than five layers thick; the diffusivity begins to increase when the film thickness reduces below five layers in such a way that the film thickness of four, three, and two graphene layers enables 48, 153, and 225 times enhanced diffusivity than that of the bulk graphite, respectively, and this nanoscale thickness is mainly responsible for the observed ultrafast rate capability of graphitic foam. The faster anion conductivity with the reduced film thickness is attributed to high elasticity of few-layered graphene, providing more space for facile AlCl₄^– diffusion. This study indicates that even bulky polyanions can be adopted as carrier ions for ultrahigh rate operation if highly elastic few-layered graphene is used as an active material

Two-Dimensional Superstructure Formation of Fluorinated Fullerene on Au(111): A Scanning Tunneling Microscopy Study

A two-dimensional fluorinated fullerene (C₆₀F₃₆) superstructure has been successfully formed on Au(111) and was investigated using scanning tunneling microscopy (STM) and density functional theory calculations. Although there exist three isomers (<i>C</i>₃, <i>C</i>₁, and <i>T</i>) in our molecular source, STM images of the molecules in the well-ordered region all appear identical, with 3-fold symmetry. This observation together with the differences in the calculated lowest unoccupied molecular orbital (LUMO) distribution among the three isomers suggests that a well-ordered monolayer consists of only the <i>C</i>₃ isomer. Because of the strong electron-accepting ability of C₆₀F₃₆, the adsorption orientation can be explained by localized distribution of its LUMO, where partial electron transfer from Au(111) occurs. Intermolecular C–F···π electrostatic interactions are the other important factor in the formation of the superstructure, which determines the lateral orientation of C₆₀F₃₆ molecules on Au(111). On the basis of scanning tunneling spectra obtained inside the superstructure, we found that the LUMO is located at 1.0 eV above the Fermi level (<i>E</i>_F), while the highest occupied molecular orbital (HOMO) is at 4.6 eV below the <i>E</i>_F. This large energy gap with the very deep HOMO as well as uniform electronic structure in the molecular layer implies a potential for application of C₆₀F₃₆ to an electron transport layer in organic electronic devices