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

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

Cointercalation of Mg²⁺ Ions into Graphite for Magnesium-Ion Batteries

Cointercalation of Mg²⁺ Ions into Graphite for Magnesium-Ion Batterie

Enhanced Pseudocapacitance of Ionic Liquid/Cobalt Hydroxide Nanohybrids

Development of nanostructured materials with enhanced redox reaction capabilities is important for achieving high energy and power densities in energy storage systems. Here, we demonstrate that the nanohybridization of ionic liquids (ILs, 1-butyl-3-methylimidazolium tetrafluoroborate) and cobalt hydroxide (Co(OH)₂) through ionothermal synthesis leads to a rapid and reversible redox reaction. The as-synthesized IL-Co(OH)₂ has a favorable, tailored morphology with a large surface area of 400.4 m²/g and a mesopore size of 4.8 nm. In particular, the IL-Co(OH)₂-based electrode exhibits improvement in electrochemical characteristics compared with bare Co(OH)₂, showing a high specific capacitance of 859 F/g at 1 A/g, high-rate capability (∼95% retention at 30 A/g), and excellent cycling performance (∼96% retention over 1000 cycles). AC impedance analysis demonstrates that the introduction of ILs on Co(OH)₂ facilitates ion transport and charge transfer: IL-Co(OH)₂ shows a higher ion diffusion coefficient (1.06 × 10^–11 cm²/s) and lower charge transfer resistance (1.53 Ω) than those of bare Co(OH)₂ (2.55 × 10^–12 cm²/s and 2.59 Ω). Our density functional theory (DFT) calculations reveal that the IL molecules, consisting of anion and cation groups, enable easier hydrogen desorption/adsorption process, that is, a more favorable redox reaction on the Co(OH)₂ surface

Two-Dimensional Phosphorene-Derived Protective Layers on a Lithium Metal Anode for Lithium-Oxygen Batteries

Lithium-oxygen (Li-O₂) batteries are desirable for electric vehicles because of their high energy density. Li dendrite growth and severe electrolyte decomposition on Li metal are, however, challenging issues for the practical application of these batteries. In this connection, an electrochemically active two-dimensional phosphorene-derived lithium phosphide is introduced as a Li metal protective layer, where the nanosized protective layer on Li metal suppresses electrolyte decomposition and Li dendrite growth. This suppression is attributed to thermodynamic properties of the electrochemically active lithium phosphide protective layer. The electrolyte decomposition is suppressed on the protective layer because the redox potential of lithium phosphide layer is higher than that of electrolyte decomposition. Li plating is thermodynamically unfavorable on lithium phosphide layers, which hinders Li dendrite growth during cycling. As a result, the nanosized lithium phosphide protective layer improves the cycle performance of Li symmetric cells and Li-O₂ batteries with various electrolytes including lithium bis­(trifluoromethanesulfonyl)­imide in <i>N,N</i>-dimethylacetamide. A variety of <i>ex situ</i> analyses and theoretical calculations support these behaviors of the phosphorene-derived lithium phosphide protective layer