Scalable Fracture-free SiOC Glass Coating for Robust Silicon Nanoparticle Anodes in Lithium Secondary Batteries

A variety of silicon (Si) nanostructures and their complex composites have been lately introduced in the lithium ion battery community to address the large volume changes of Si anodes during their repeated charge–discharge cycles. Nevertheless, for large-scale manufacturing it is more desirable to use commercial Si nanoparticles with simple surface coating. Most conductive coating materials, however, do not accommodate the volume expansion of the inner Si active phases and resultantly fracture during cycling. To overcome this chronic limitation, herein, we report silicon oxycarbide (SiOC) glass as a new coating material for Si nanoparticle anodes. The SiOC glass phase can expand to some extent due to its active nature in reacting with Li ions and can therefore accommodate the volume changes of the inner Si nanoparticles without disintegration or fracture. The SiOC glass also grows in the form of nanocluster to bridge Si nanoparticles, thereby contributing to the structural integrity of secondary particles during cycling. On the basis of these combined effects, the SiOC-coated Si nanoparticles reach a high reversible capacity of 2093 mAh g^–1 with 92% capacity retention after 200 cycles. Furthermore, the coating and subsequent secondary particle formation were produced by high-speed spray pyrolysis based on a single precursor solution

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

Tuning the Phase Stability of Sodium Metal Pyrophosphates for Synthesis of High Voltage Cathode Materials

Properties of the electrode materials are strongly influenced by their crystal structures, yet there is still a lack of design principles to control the polymorphism, showing multiple structures for a given composition with varying battery performance. Here, the underlying mechanism that governs the phase stability of Na₂CoP₂O₇, which has two polymorphs with different electrochemical properties, and a strategy to control it via transition metal substitution are investigated. It is found that the relative stability between the triclinic and orthorhombic polymorphs of Na₂MP₂O₇ (M = transition metals) is determined by two factors, the ionic size and crystal field stabilization energy. On the basis of this understanding, a computational strategy is devised for selecting the optimal substituents to produce a desired polymorph, from which the introduction of Ca, Ni, or Mn into Na₂CoP₂O₇ is identified to stabilize the preferred triclinic phase that has a higher voltage than the orthorhombic counterpart. This prediction of selective synthesis of a particular polymorph for improved battery performance is successfully verified by experimental syntheses, characterization, and electrochemical measurements. We expect that the current strategy can be generalized for other materials synthesis in which the functionalities of materials are sensitively dependent on the crystal polymorphs

Wisdom from the Human Eye: A Synthetic Melanin Radical Scavenger for Improved Cycle Life of Li–O₂ Battery

Li–O₂ batteries are attractive systems because they can deliver much higher energy densities than those of conventional lithium-ion batteries by engaging light gas-phase oxygen as a cathode active material. However, the inevitable generation of residual superoxide radicals gives rise to irreversible side reactions and consequently causes severe capacity degradation over cycling. To address this chronic issue, herein, we have taken a lesson from the human eye. Analogous to Li–O₂ batteries, the human eye is liable to attack by reactive oxygen species (ROS), from its lifetime exposure to sunlight. However, it protects itself from the ROS attack by using melanin as a radical scavenger. To mimic such a defense mechanism against radical attack, we included polydopamine (pD), which is one of the most common synthetic melanins, in the ether-based electrolyte. As an outcome of the superoxide radical scavenging by the pD additive, the irreversible side reaction products were alleviated significantly, resulting in superior cycling performance. The present investigation provides a message that simple treatments inspired by the human body or nature could be effective solutions to the problems in various energy devices

Computational Analysis of Pressure-Dependent Optimal Pore Size for CO₂ Capture with Graphitic Surfaces

There are a growing number of reports suggesting that the specific surface area in graphitic materials is not a critical parameter to determine the CO₂ capture capacity, but rather the pore size and its geometry are more relevant, yet a detailed theoretical and quantitative understanding that could facilitate further developments for the pore size effects is presently lacking. Using the thermodynamic continuum model combined with electronic structure calculations, we identify the critical size of pores in graphitic materials for enhanced carbon dioxide (CO₂) uptake as well as its selectivity relative to N₂. We find that there exists a value of pore size which is most optimal in the CO₂ capture capacity as well as CO₂/N₂ selectivity at a given pressure and temperature, supporting the previous experimental observations regarding critical parameters determining the CO₂ adsorption capacity of porous carbon materials. The calculated results emphasize the importance of graphitic pore size from 8 to10 Å in CO₂ capture and selectivity against N₂

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

Mussel- and Diatom-Inspired Silica Coating on Separators Yields Improved Power and Safety in Li-Ion Batteries

In this study, we developed an integrative bioinspired approach that improves the power and safety of Li-ion batteries (LIBs) by the surface modification of polyethylene (PE) separators. The approach involves the synthesis of a diatom-inspired silica layer on the surface of the PE separator, and the adhesion of the silica layer was inspired by mussels. The mussel- and diatom-inspired silica coating increased the electrolyte wettability of the separator, resulting in enhanced power and improved thermal shrinkage, resulting safer LIBs. Furthermore, the overall processes are environmentally friendly and cost-effective. The process described herein is the first example of the use of diatom-inspired silica in practically important energy storage applications. The improved wetting and thermal properties are critical, particularly for large-scale battery applications

Spray Drying Method for Large-Scale and High-Performance Silicon Negative Electrodes in Li-Ion Batteries

Nanostructured silicon electrodes have shown great potential as lithium ion battery anodes because they can address capacity fading mechanisms originating from large volume changes of silicon alloys while delivering extraordinarily large gravimetric capacities. Nonetheless, synthesis of well-defined silicon nanostructures in an industrially adaptable scale still remains as a challenge. Herein, we adopt an industrially established spray drying process to enable scalable synthesis of silicon–carbon composite particles in which silicon nanoparticles are embedded in porous carbon particles. The void space existing in the porous carbon accommodates the volume expansion of silicon and thus addresses the chronic fading mechanisms of silicon anodes. The composite electrodes exhibit excellent electrochemical performance, such as 1956 mAh/g at 0.05C rate and 91% capacity retention after 150 cycles. Moreover, the spray drying method requires only 2 s for the formation of each particle and allows a production capability of ∼10 g/h even with an ultrasonic-based lab-scale equipment. This investigation suggests that established industrial processes could be adaptable to the production of battery active materials that require sophisticated nanostructures as well as large quantity syntheses

Controlled Lithium Dendrite Growth by a Synergistic Effect of Multilayered Graphene Coating and an Electrolyte Additive

Lithium (Li) metal is the most ideal anode material in lithium ion batteries due to its large theoretical capacity (3860 mAh g^–1) and low redox potential (−3.04 V vs standard hydrogen potential, H₂/H⁺). Nevertheless, surface dendrite formation during repeated charge–discharge cycles limits the cycle life and thus its practical use. The research efforts engaging polymer/ceramic coating or electrolyte additives have made noticeable progress, but further improvement is still desirable. Here, we report significantly improved performance by a synergistic effect of multilayered graphene (MLG) coating and Cs⁺ additive in the electrolyte. MLG separates solid-electrolyte-interphase (SEI) formation from Li dendrites and thus stabilizes Coulombic efficiency in each cycle. Cs ions facilitate efficient interlayer diffusion of Li ions by enlarging the interlayer distance of MLG and also assists further for suppression of Li dendrite growth by electrostatic repulsion against Li ions. When paired with a stable sulfur–carbon composite electrode as a high capacity cathode, the Li–sulfur cell delivers an areal capacity of 4.0 mAh cm^–2, a value comparable to those of current commercial lithium ion batteries, with 81.0% capacity retention after 200 cycles

Anisotropic Lithiation Onset in Silicon Nanoparticle Anode Revealed by <i>in Situ</i> Graphene Liquid Cell Electron Microscopy

Recent real-time analyses have provided invaluable information on the volume expansion of silicon (Si) nanomaterials during their electrochemical reactions with lithium ions and have thus served as useful bases for robust design of high capacity Si anodes in lithium ion batteries (LIBs). In an effort to deepen the understanding on the critical first lithiation of Si, especially in realistic liquid environments, herein, we have engaged <i>in situ</i> graphene liquid cell transmission electron microscopy (GLC-TEM). In this technique, chemical lithiation is stimulated by electron-beam irradiation, while the lithiation process is being monitored by TEM in real time. The real-time analyses informing of the changes in the dimensions and diffraction intensity indicate that the very first lithiation of Si nanoparticle shows anisotropic volume expansion favoring the ⟨110⟩ directions due to the smaller Li diffusion energy barrier at the Si–electrolyte interface along such directions. Once passing this initial volume expansion stage, however, Li diffusion rate becomes isotropic in the inner region of the Si nanoparticle. The current study suggests that the <i>in situ</i> GLC-TEM technique can be a useful tool in understanding battery reactions of various active materials, particularly those whose initial lithiation plays a pivotal role in overall electrochemical performance and structural stability of the active materials