Metal-induced crystallization of highly corrugated silicon thick films as potential anodes for Li-ion batteries

Building Organic-Inorganic Robust Interphases from Deep Eutectic Solution for Highly Stable Mg Metal Anode in Conventional Electrolyte

Magnesium metal batteries (MMBs) currently face challenges suffering from severe Mg metal passivation and extremely high overpotential in conventional electrolytes. Herein, a strategy of using a low-cost deep eutectic solution (DES) is proposed to modify Mg anode with the monolithic and compact coating of a MgCl2-Al-MgCl2 sandwich structure, enabling the stable and reversible Mg plating-stripping behavior. An organic/nanocrystal hybrid interphase is in-situ built through a facile Mg-Al displacement reaction between aluminum-chloro clusters and Mg in AlCl3/Et3NHCl solution, and it can effectively minimize the adverse interfacial passivation reaction and surface diffusion barrier, affording the high ion-conduction and electronic insulation. This DES-assisted method guarantees a highly reversible cycling of Mg metal anode (over 5000 h at 0.1 mA cm-2 and 400 h at 2.0 mAh cm-2) in Mg(TFSI)2/DME electrolyte with the improved interfacial kinetics and low overpotential. Even at a much higher current density of 1 mA cm-2, the overpotential only undergoes a slight increase from 0.2 V (at 0.1 mA cm-2) to 0.23 V. The corresponding full cells with CuS and phenanthraquinone cathodes deliver satisfactory cyclic performance. The DES modification strategy provides a new solution to the design of robust and conductive solid electrolyte interphase for achieving high-voltage and durable MMBs.

Formation of Type II Silicon Clathrate with Lithium Guests through Thermal Diffusion

At low guest atom concentrations, Si clathrates can be viewed as semiconductors, with the guest atoms acting as dopants, potentially creating alternatives to diamond Si with exciting optoelectronic and spin properties. Studying Si clathrates with different guest atoms would not only provide insights into the electronic structure of the Si clathrates but also give insights into the unique properties that each guest can bring to the Si clathrate structure. However, the synthesis of Si clathrates with guests other than Na is challenging. In this study, we have developed an alternative approach, using thermal diffusion into type II Si clathrate with an extremely low Na concentration, to create Si clathrate with Li guests. Using time-of-flight secondary-ion mass spectroscopy, X-ray diffraction, and Raman scattering, thermal diffusion of Li into the nearly empty Si clathrate framework is detected and characterized as a function of the diffusion temperature and time. Interestingly, the Si clathrate exhibits reduced structural stability in the presence of Li, converting to polycrystalline or disordered phases for anneals at temperatures where the starting Na guest Si clathrate is quite stable. The Li atoms inserted into the Si clathrate lattice contribute free carriers, which can be detected in Raman scattering through their effect on the strength of Si-Si bonds in the framework. These carriers can also be observed in electron paramagnetic resonance (EPR). EPR shows, however, that Li guests are not simple analogues of Na guests. In particular, our results suggest that Li atoms, with their smaller size, tend to doubly occupy cages, forming "molecular-like" pairs with other Li or Na atoms. Results of this work provide a deeper insight into Li guest atoms in Si clathrate. These findings are also relevant to understanding how Li moves through and interacts with Si clathrate anodes in Li-ion batteries. Additionally, techniques presented in this work demonstrate a new method for filling the Si clathrate cages, enabling studies of a broad range of other guests in Si clathrates.

Thickness Dependency of Battery Anode Properties in Multilayer Graphene

With the development of practical thin-film batteries, multilayer graphene (MLG) is being actively investigated as an anode material. Therefore, research on determining a technique to fabricate thick MLG on arbitrary substrates at low temperatures is essential. In this study, we formed an MLG with controlled thickness at low temperatures using a layer exchange (LE) technique and evaluated its anode properties. The LE technique enabled the formation of a uniform MLG with a wide range of thicknesses (25-500 nm) on Ta foil. The charge/discharge characterization using coin-type cells revealed that the total capacity, which corresponded to Li intercalation into the MLG interlayer, increased with increasing MLG thickness. In contrast, cross-sectional transmission electron microscopy showed a metal oxide formed at the MLG/Ta interface during annealing, which had small Li capacity. MLG with sufficient thickness (500 nm) exhibited an excellent Coulombic efficiency and capacity retention compared to bulk graphite formed at high temperatures. These results have led to the development of inexpensive and reliable rechargeable thin-film batteries.

Triple Conductive Wiring by Electron Doping, Chelation Coating and Electrochemical Conversion in Fluffy Nb2 O5 Anodes for Fast-Charging Li-Ion Batteries

High-rate anode material is the kernel of developing fast-charging lithium ion batteries (LIBs). T-Nb₂ O₅ , well-known for its "room and pillar" structure and bulk pseudocapacitive effect, is expected to enable the fast lithium (de)intercalation. But this property is still limited by the low electronic conductivity or insufficient wiring manner. Herein, a strategy of triple conductive wiring through electron doping, chelation coating, and electrochemical conversion inside the microsized porous spheres consisting of dendrite-like T-Nb₂ O₅ primary particles is proposed to achieve the fast-charging and durable anodes for LIBs. The penetrative implanting of conformal carbon coating (derivative from polydopamine chelate) and NbO domains (induced by excess discharging) reinforces the global supply of electronically conductive wires, apart from those from Co/Mn heteroatom or O vacancy doping. The polydopamine etching on T-Nb₂ O₅ spheres promotes their evolution into fluffy morphology with better electrolyte infiltration. The synergic electron and ion wiring at different scales endow the modified T-Nb₂ O₅ anode with ultralong cycling life (143 mAh g^-1 at 1 A g^-1 after 8500 cycles) and high-rate performance (144.1 mAh g^-1 at 10.0 A g^-1 ). The permeation of multiple electron wires also enables a high mass loading of T-Nb₂ O₅ (4.5 mg cm^-2 ) with a high areal capacity of 0.668 mAh cm^-2 even after 150 cycles.

Porous Amorphous Silicon Hollow Nanoboxes Coated with Reduced Graphene Oxide as Stable Anodes for Sodium-Ion Batteries

Amorphous silicon (a-Si), due to its satisfactory theoretical capacity, moderate discharge potential, and abundant reserves, is treated as one of the most prospective materials for the anode of sodium-ion batteries (SIBs). However, the slow Na⁺ diffusion kinetics, poor electrical conductivity, and rupture-prone structures of a-Si restrict its further development. In this work, a composite (a-Si@rGO) consisting of porous amorphous silicon hollow nanoboxes (a-Si HNBs) and reduced graphene oxide (rGO) is prepared. The a-Si HNBs are synthesized through "sodiothermic reduction" of silica hollow nanoboxes at a relatively low temperature, and the rGO is covered on the surface of the a-Si HNBs by electrostatic interaction. The as-synthesized composite anode applying in SIBs exhibits a high initial discharge capacity of 681.6 mAh g^-1 at 100 mA g^-1, great stability over 2000 cycles at 800 mA g^-1, and superior rate performance (261.2, 176.8, 130.3, 98.4, and 73.3 mAh g^-1 at 100, 400, 800, 1500, and 3000 mA g^-1, respectively). The excellent electrochemical properties are ascribed to synergistic action of the porous hollow nanostructure of a-Si and the rGO coating. This research not only offers an innovative synthetic means for the development of a-Si in various fields but also provides a practicable idea for the design of other alloy-type anodes.

Si1-xGex anode synthesis on plastic films for flexible rechargeable batteries

SiGe is a promising anode material for replacing graphite in next generation thin-film batteries owing to its high theoretical charge/discharge capacity. Metal-induced layer exchange (LE) is a unique technique used for the low-temperature synthesis of SiGe layers on arbitrary substrates. Here, we demonstrate the synthesis of Si_1-xGe_x (x = 0-1) layers on plastic films using Al-induced LE. The resulting SiGe layers exhibited high electrical conductivity (up to 1200 S cm^-1), reflecting the self-organized doping effect of LE. Moreover, the Si_1-xGe_x layer synthesized by the same process was adopted as the anode for the lithium-ion battery. All Si_1-xGe_x anodes showed clear charge/discharge operation and high coulombic efficiency (≥ 97%) after 100 cycles. While the discharge capacities almost reflected the theoretical values at each x at 0.1 C, the capacity degradation with increasing current rate strongly depended on x. Si-rich samples exhibited high initial capacity and low capacity retention, while Ge-rich samples showed contrasting characteristics. In particular, the Si_1-xGe_x layers with x ≥ 0.8 showed excellent current rate performance owing to their high electrical conductivity and low volume expansion, maintaining a high capacity (> 500 mAh g^-1) even at a high current rate (10 C). Thus, we revealed the relationship between SiGe composition and anode characteristics for the SiGe layers formed by LE at low temperatures. These results will pave the way for the next generation of flexible batteries based on SiGe anodes.

Ultrafast Na Transport into Crystalline Sn via Dislocation-Pipe Diffusion

The charging process of secondary batteries is always associated with a large volume expansion of the alloying anodes, which in many cases, develops high compressive residual stresses near the propagating interface. This phenomenon causes a significant reduction in the rate performance of the anodes and is detrimental to the development of fast-charging batteries. However, for the Na-Sn battery system, the residual stresses that develop near the interface are not stored, but are relieved by the generation of high-density dislocations in crystalline Sn. Direct-contact diffusion experiments show that these dislocations facilitate the preferential transport of Na and accelerate the Na diffusion into crystalline Sn at ultrafast rates via "dislocation-pipe diffusion". Advanced analyses are performed to observe the evolution of atomic-scale structures while measuring the distribution and magnitude of residual stresses near the interface. In addition, multi-scale simulations that combined classical molecular dynamics and first-principles calculations are performed to explain the structural origins of the ultrafast diffusion rates observed in the Na-Sn system. These findings not only address the knowledge gaps regarding the relationship between pipe diffusion and the diffusivity of carrier ions but also provide guidelines for the appropriate selection of anode materials for use in fast-charging batteries.

A conductive self healing polymeric binder using hydrogen bonding for Si anodes in lithium ion batteries

A ureido-pyrimidinone (UPy)-functionalized poly(acrylic acid) grafted with poly(ethylene glycol)(PEG), designated PAU-g-PEG, was developed as a high performance polymer binder for Si anodes in lithium-ion batteries. By introducing both a ureido-pyrimidinone (UPy) unit, which is capable of self-healing through dynamic hydrogen bonding within molecules as well as with Si, and an ion-conducting PEG onto the side chain of the poly(acrylic acid), this water-based self-healable and conductive polymer binder can effectively accommodate the volume changes of Si, while maintaining electronic integrity, in an electrode during repeated charge/discharge cycles. The Si@PAU-g-PEG electrode retained a high capacity of 1,450.2 mAh g-1 and a Coulombic efficiency of 99.4% even after 350 cycles under a C-rate of 0.5 C. Under a high C-rate of 3 C, an outstanding capacity of 2,500 mAh g-1 was also achieved, thus demonstrating its potential for improving the electrochemical performance of Si anodes.

Heightened Integration of POM-based Metal-Organic Frameworks with Functionalized Single-Walled Carbon Nanotubes for Superior Energy Storage

To increase the conductivity of polyoxometalate-based metal-organic frameworks (POMOFs) and promote their applications in the field of energy storage, herein, a simple approach was employed to improve their overall electrochemical performances by introducing a functionalized single-walled carbon nanotubes (SWNT-COOH). A new POMOF compound, [Cu₁₈ (trz)₁₂ Cl₃ (H₂ O)₂ ][PW₁₂ O₄₀ ] (CuPW), was successfully synthesized, then the size-matched functionalized SWNT-COOH was introduced to fabricate CuPW/SWNT-COOH composite (PMNT-COOH) by employing a simple sonication-driven periodic functionalization strategy. When the PMNT-COOH nanocomposite was used as the anode material for Lithium-ion batteries (LIBs), PMNT-COOH(3) (CuPWNC:SWNT-COOH=3:1) showed superior behavior of energy storage, a high reversible capacity of 885 mA h g^-1 up to a cycle life of 170 cycles. The electrochemical results indicate that the uniform packing of SWNT-COOH provided a favored contact between the electrolyte and the electrode, resulting in enhanced specific capacity during lithium insertion/extraction process. This fabrication of PMNT-COOH nanocomposite opens new avenues for the design and synthesis of new generation electrode materials for LIBs.

Beyond dealloying: development of nanoporous gold via metal-induced crystallization and its electrochemical properties

Nanoporous metals (NPMs) possess a number of intriguing properties that result in NPMs being an important family of nanomaterials for many advanced applications. However, the methods of preparing NPMs are relatively complicated and have many limitations, which have hindered the commercial application of NPMs thus far. By introducing metal-induced crystallization, a solid-phase reaction method for preparing NPMs was developed in this study, which is highly efficient and environmentally friendly. The microstructure of the prepared nanoporous gold (NPG) was characterized on an atomic scale by scanning electron microscopy and high-resolution transmission electron microscopy. The results confirmed that the solid-phase reaction method is an effective alternative means of preparing highly pure NPG. The results of electrochemical tests demonstrated that thus-prepared NPG possesses higher electrocatalytic activity than other types of gold electrodes toward oxygen reduction in alkaline media. The combination of a simple preparation process and higher activity suggests that the developed method may promote the future use of NPG in new energy applications, such as fuel cells and metal-air batteries.

Au quantum dots engineered room temperature crystallization and magnetic anisotropy in CoFe2O4 thin films

For the first time, this work presents a novel room temperature time-effective concept to manipulate the crystallization kinetics and magnetic responses of thin films grown on amorphous substrates. Conventionally, metal-induced crystallization is adopted to minimize the crystallization temperature of the upper-layer thin film. However, due to the limited surface area of the continuous metal under-layer, the degree of crystallization is insufficient and post-annealing is required. To expose a large surface area of the metal under-layer, we propose a simple and novel approach of using an Au nanodots array instead of a continuous metallic under-layer to obtain crystallization of upper-layer thin films. Spinel cobalt ferrite (CFO) thin film as a 'model' was deposited on an Au nano-dots array to realize this methodology. Our findings revealed that the addition of quantum-sized Au nano-dots as a metal under-layer dramatically enhanced the crystallization of the cobalt ferrite upper layer at room temperature. The appearance of major X-ray diffraction peaks with high intensity and well-defined crystallized lattice planes observed via transmission electron microscopy confirmed the crystallization of the CFO thin film deposited at room temperature on 4 nm-sized Au nano-dots. This crystallized CFO thin film exhibits 18-fold higher coercivity (H_c = 4150 Oe) and 4-fold higher saturation magnetization (M_s = 262 emu cm^-3) compared to CFO deposited without the Au under-layer. The development of this novel concept of room-temperature crystallization without the aid of additives and solvents represents a crucial breakthrough that is highly significant for exploring the green and energy-efficient synthesis of a variety of oxide and metal thin films.

Supernormal Conversion Anode Consisting of High-Density MoS2 Bubbles Wrapped in Thin Carbon Network by Self-Sulfuration of Polyoxometalate Complex

Large-capacity conversion electrodes are highly required to raise the energy density of batteries. However, their undesired phase segregation and volume expansion during cycling lead to the motivation for nanofabrication and nanochemistry of active species in order to decrease "dead mass" and promote better construction of conductive networks. However, the inactivity of the conductive skeleton and loose nanostructure would compromise the energy density of the electrode. The integration of large-sized (high-density) grains into an electroactive conductive network in a compact way is still a big challenge. Here we report a proof-of-concept prototype consisting of unusual MoS₂ solid bubbles of hundreds of nanometers in size, which are conformally encapsulated by thin-layer carbon. The seamless welding between this carbon coating and the surrounding broader carbon substrate can effectively avoid the peel-off of active species and breakage of the conductive network. This MoS₂-C composite is achieved by simultaneous self-sulfuration and self-carbonization of a polyoxometalate (POM)-based chelate, and its Li-storage performance is superior to most MoS₂-based anodes even with ultrathin 2D nanosheets. Partially benefiting from the electroactivity of a dithiooxamide (DTO)-derivate carbon network, the reversible capacity of MoS₂-C by pyrolyzing the POM-DTO chelate can reach 1500-2000 mAh/g at 0.5-1 A/g even after 700 cycles and be maintained around 1000 mAh/g under as high as 10-20 A/g.

High-Capacity Molecular Scale Conversion Anode Enabled by Hybridizing Cluster-Type Framework of High Loading with Amino-Functionalized Graphene

Exploring high-capacity anodes with multielectron reaction, sufficient charge/mass transfer, and suppressed volume expansion is highly desired. The open frameworks consisting of independent structure units, which possess conversion reaction potentiality, can meet these demands and show advantages over routine insertion-type open frameworks with at most one-electron transfer or conversion materials with compact ligand linkage. Here, we report a class of electrochemically stable cluster-like polyoxometalates (POMs) as such open framework anodes. Their high loading and low solubility are enabled by Al- or Si-driven polymerization and hybridization with positively charged graphene, which immobilizes polyanions of POMs and improves their electric contact. Al-based POM composite (NAM-EDAG) for Li-storage achieves a high reversible capacity above 1000 mAh g(-1) and tolerates a long-term cycling with more than 1100 cycles and a current density up to 20 A g(-1). A six-electron conversion reaction occurring at molecular scale and the consequent optimized distribution of products benefiting from original open framework are also responsible for the high electroactivity. POM-based open frameworks give inspiration for exploring advanced, less soluble (or insoluble) framework materials made up of electroactive molecule or cluster moieties for Li- and Na-storage.

Eutectic nano-droplet template injection into bulk silicon to construct porous frameworks with concomitant conformal coating as anodes for Li-ion batteries

Building porosity in monolithic materials is highly desired to design 3D electrodes, however ex-situ introduction or in-situ generation of nano-scale sacrificial template is still a great challenge. Here Al-Si eutectic droplet templates are uniformly injected into bulk Si through Al-induced solid-solid convection to construct a highly porous Si framework. This process is concomitant with process-inherent conformal coating of ion-conductive oxide. Such an all-in-one method has generated a (continuously processed) high-capacity Si anode integrating longevity and stable electrolyte-anode diaphragm for Li-ion batteries (e.g. a reversible capacity as large as ~1800 mAh/g or ~350 μAh/cm²-μm with a CE of ~99% at 0.1 C after long-term 400 cycles).