Research Progress on Coating Structure of Silicon Anode Materials for Lithium-Ion Batteries

Silicon, which has been widely studied by virtue of its extremely high theoretical capacity and abundance, is recognized as one of the most promising anode materials for the next generation of lithium-ion batteries. However, silicon undergoes tremendous volume change during cycling, which leads to the destruction of the electrode structure and irreversible capacity loss, so the promotion of silicon materials in commercial applications is greatly hampered. In recent years, many strategies have been proposed to address these shortcomings of silicon. This Review focused on different coatings materials (e. g., carbon-based materials, metals, oxides, conducting polymers, etc.) for silicon materials. The role of different types of materials in the modification of silicon-based material encapsulation structure was reviewed to confirm the feasibility of the protective layer strategy. Finally, the future research direction of the silicon-based material coating structure design for the next-generation lithium-ion battery was summarized.

Molecular Understanding of Electrochemical-Mechanical Responses in Carbon-Coated Silicon Nanotubes during Lithiation

Carbon-coated silicon nanotube (SiNT@CNT) anodes show tremendous potential in high-performance lithium ion batteries (LIBs). Unfortunately, to realize the commercial application, it is still required to further optimize the structural design for better durability and safety. Here, the electrochemical and mechanical evolution in lithiated SiNT@CNT nanohybrids are investigated using large-scale atomistic simulations. More importantly, the lithiation responses of SiNW@CNT nanohybrids are also investigated in the same simulation conditions as references. The simulations quantitatively reveal that the inner hole of the SiNT alleviates the compressive stress concentration between a-Li_xSi and C phases, resulting in the SiNT@CNT having a higher Li capacity and faster lithiation rate than SiNW@CNT. The contact mode significantly regulates the stress distribution at the inner hole surface, further affecting the morphological evolution and structural stability. The inner hole of bare SiNT shows good structural stability due to no stress concentration, while that of concentric SiNT@CNT undergoes dramatic shrinkage due to compressive stress concentration, and that of eccentric SiNT@CNT is deformed due to the mismatch of stress distribution. These findings not only enrich the atomic understanding of the electrochemical–mechanical coupled mechanism in lithiated SiNT@CNT nanohybrids but also provide feasible solutions to optimize the charging strategy and tune the nanostructure of SiNT-based electrode materials.

Application of Reaction Force Field Molecular Dynamics in Lithium Batteries

Lithium batteries are widely used in portable electronic products. Although the performance of the batteries has been greatly improved in the past few decades, limited understanding of the working mechanisms at an atomic scale has become a major factor for further improvement. In the past 10 years, a reaction force field (ReaxFF) has been developed within the molecular dynamics framework. The ReaxFF has been demonstrated to correctly describe both physical processes and chemical reactions for a system significantly larger than the one simulated by quantum chemistry, and therefore in turn has been broadly applied in lithium batteries. In this article, we review the ReaxFF studies on the sulfur cathode, various anodes, and electrolytes of lithium batteries and put particular focus on the ability of the ReaxFF to reveal atomic-scale working mechanisms. A brief prospect is also given.

Two-dimensional porous silicon nanosheets as anode materials for high performance lithium-ion batteries

In this paper, silicon nanosheets (Si-NSs) are chemically synthesized by using graphene oxide nanosheets as the template. The obtained Si-NSs, which are aggregations of silicon nanocrystals with a size of ∼10 nm, are applied directly as the anode material for lithium ion batteries, delivering a reversible capacity of 800 mA h g-1 after 900 cycles at a rate as high as 8400 mA g-1. Ex situ measurements and in situ observations show the positive effect of the mesoporous structure on the structural stability of Si-NSs. The evolution and survivability of the porous structures during lithiation and delithiation processes are investigated by molecular dynamics simulations, demonstrating that the porous structure can enhance the amount of "active" Li atoms during the stable stage of cycling and therefore promote mass capacity. The longer the survival of the porous structure, the longer the high mass capacity can be retained.

Nanoscale Protection Layers To Mitigate Degradation in High-Energy Electrochemical Energy Storage Systems

In the pursuit of energy storage devices with higher energy and power, new ion storage materials and high-voltage battery chemistries are of paramount importance. However, they invite-and often enhance-degradation mechanisms, which are reflected in capacity loss with charge/discharge cycling and sometimes in safety problems. Degradation mechanisms are often driven by fundamentals such as chemical and electrochemical reactions at electrode-electrolyte interfaces, volume expansion and stress associated with ion insertion and extraction, and profound inhomogeneity of electrochemical behavior. While it is important to identify and understand these mechanisms at some reasonable level, it is even more critical to design strategies to mitigate these degradation pathways and to develop means to implement and validate the strategies. A growing set of research highlights the mitigation benefits achievable by forming thin protection layers (PLs) intentionally created as artificial interphase regions at the electrode-electrolyte interface. These advances illustrate a promising-perhaps even generic-pathway for enabling higher-energy and higher-voltage battery configurations. In this Account, we summarize examples of such PLs that serve as mitigation strategies to avoid degradation in lithium metal anodes, conversion-type electrode materials, and alloy-type electrodes. Examples are chosen from a larger body of electrochemical degradation research carried out in Nanostructures for Electrical Energy Storage (NEES), our DOE Energy Frontier Research Center. Overall, we argue on the basis of experimental and theoretical evidence that PLs effectively stabilize the electrochemical interfaces to prevent parasitic chemical and electrochemical reactions and mitigate the structural, mechanical, and compositional degradation of the electrode materials at the electrode-electrolyte interfaces. The evidenced improvement in performance metrics is accomplished by (1) establishing a homogeneous interface for ion insertion and extraction, (2) providing mechanical constraints to maintain structural integrity and robust electronic and ionic conduction pathways, and (3) introducing spatial confinements on the electrode material matrix to alter the phase transformation (delaying the occurrence of the conversion reaction) upon Li insertion, which results in superior electrode performance, excellent capacity retention, and improved reversibility. Taken together, these examples portray a valuable role for thin protection layers synthesized over electrode surfaces, both for their benefit to cycle stability and for revealing insights into degradation and mitigation mechanisms. Furthermore, they underscore the impact of complex electrochemical behavior at nanoscale materials and nanostructure interfaces in modulating the behavior of energy storage devices at the mesoscale and macroscale.

Molecular dynamic simulation for thermal decomposition of RDX with nano-AlH3 particles

Reactive molecular dynamic simulation of a high explosive, RDX, mixed with AlH₃ nanoparticles was performed by a newly parameterized ReaxFF force field.

ReaxFF Reactive Force-Field Study of Molybdenum Disulfide (MoS2)

Two-dimensional layers of molybdenum disulfide, MoS₂, have been recognized as promising materials for nanoelectronics due to their exceptional electronic and optical properties. Here we develop a new ReaxFF reactive potential that can accurately describe the thermodynamic and structural properties of MoS₂ sheets, guided by extensive density functional theory simulations. This potential is then applied to the formation energies of five different types of vacancies, various vacancy migration barriers, and the transition barrier between the semiconducting 2H and metallic 1T phases. The energetics of ripplocations, a recently observed defect in van der Waals layers, is examined, and the interplay between these defects and sulfur vacancies is studied. As strain engineering of MoS₂ sheets is an effective way to manipulate the sheets' electronic and optical properties, the new ReaxFF description can provide valuable insights into morphological changes that occur under various loading conditions and defect distributions, thus allowing one to tailor the electronic properties of these 2D crystals.

Atomistics of the lithiation of oxidized silicon (SiOx) nanowires in reactive molecular dynamics simulations

The atomistic lithiation mechanism of silicon oxides (SiO_x) is clarified using the ReaxFF reactive molecular dynamics simulation.