Materials research has driven the development of modern nano-electronic devices. In particular, research in magnetic thin films has revolutionized the development of spintronic devices1,2 because identifying new magnetic materials is key to better device performance and design. Van der Waals crystals retain their chemical stability and structural integrity down to the monolayer and, being atomically thin, are readily tuned by various kinds of gate modulation3,4. Recent experiments have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr2Ge2Te6 (ref. 5) and CrI3 (ref. 6) at low temperatures. Here we develop a device fabrication technique and isolate monolayers from the layered metallic magnet Fe3GeTe2 to study magnetotransport. We find that the itinerant ferromagnetism persists in Fe3GeTe2 down to the monolayer with an out-of-plane magnetocrystalline anisotropy. The ferromagnetic transition temperature, Tc, is suppressed relative to the bulk Tc of 205 kelvin in pristine Fe3GeTe2 thin flakes. An ionic gate, however, raises Tc to room temperature, much higher than the bulk Tc. The gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2 opens up opportunities for potential voltage-controlled magnetoelectronics7-11 based on atomically thin van der Waals crystals.

Gate-tunable Room-temperature Ferromagnetism in Two-dimensional Fe 3 GeTe 2

As one important component of sulfur cathodes, the carbon host plays a key role in the electrochemical performance of lithium-sulfur (Li-S) batteries. In this paper, a mesoporous nitrogen-doped carbon (MPNC)-sulfur nanocomposite is reported as a novel cathode for advanced Li-S batteries. The nitrogen doping in the MPNC material can effectively promote chemical adsorption between sulfur atoms and oxygen functional groups on the carbon, as verifi ed by X-ray absorption near edge structure spectroscopy, and the mechanism by which nitrogen enables the behavior is further revealed by density functional theory calculations. Based on the advantages of the porous structure and nitrogen doping, the MPNC-sulfur cathodes show excellent cycling stability (95% retention within 100 cycles) at a high current density of 0.7 mAh cm −2 with a high sulfur loading (4.2 mg S cm −2 ) and a sulfur content (70 wt%). A high areal capacity (≈3.3 mAh cm −2 ) is demonstrated by using the novel cathode, which is crucial for the practical application of Li-S batteries. It is believed that the important role of nitrogen doping promoted chemical adsorption can be extended for development of other high performance carbon-sulfur composite cathodes for Li-S batteries.

Nitrogen-Doped Mesoporous Carbon Promoted Chemical Adsorption of Sulfur and Fabrication of High-Areal-Capacity Sulfur Cathode with Exceptional Cycling Stability for Lithium-Sulfur Batteries

Batteries based on multivalent metals have the potential to meet the future needs of large-scale energy storage, due to the relatively high abundance of elements such as magnesium, calcium, aluminium and zinc in the Earth’s crust. However, the complexity of multivalent metal-ion chemistries has led to rampant confusions, technical challenges, and eventually doubts and uncertainties about the future of these technologies. In this Review, we clarify the key strengths as well as common misconceptions of multivalent metal-based batteries. We then examine the growth behaviour of metal anodes, which is crucial for their safety promises but hitherto unestablished. We further discuss scrutiny of anode efficiency and cathode storage mechanism pertaining to complications arising from electrolyte solutions. Finally, we critically review existing cathode materials and discuss design strategies to enable genuine multivalent metal-ion-based energy storage materials with competitive performance. Batteries based on multivalent metal anodes hold great promise for large-scale energy storage but their development is still at an early stage. This Review surveys the main complexity arising from anodes, electrolytes and cathodes, and offers views on the progression path of these technologies.

Current status and future directions of multivalent metal-ion batteries

The electronic structure of the perovskite LaCoO$_3$ for different spin states of Co ions was calculated in the LDA+U approach. The ground state was found to be a nonmagnetic insulator with Co ions in a low-spin state. Somewhat higher in energy we found two intermediate-spin states followed by a high-spin state at significantly higher energy. The calculation results show that Co 3$d$ states of $t_{2g}$ symmetry form narrow bands which could easily localize whilst $e_g$ orbitals, due to their strong hybridization with the oxygen 2$p$ states, form a broad $\sigma^*$ band. With the increase of temperature which is simulated by the corresponding increase of the lattice parameter, the transition from the low- to intermediate-spin states occurs. This intermediate-spin (occupation $t_{2g}^5e_g^1$) can develop an orbital ordering which can account for the nonmetallic nature of LaCoO$_3$ at 90 K$<$T$<$500 K. Possible explanations of the magnetic behavior and gradual insulating-metal transition are suggested.

https://arxiv.org/pdf/cond-mat/9709060.pdf

Intermediate-spin state and properties of LaCoO_3

Reversibly intercalating ions into host materials for electrochemical energy storage is the essence of the working principle of rocking-chair type batteries. The most relevant example is the graphite anode for rechargeable Li-ion batteries which has been commercialized in 1991 and still represents the benchmark anode in Li-ion batteries 30 years later. Learning from past lessons on alkali metal intercalation in graphite, recent breakthroughs in sodium and potassium intercalation in graphite have been demonstrated for Na-ion batteries and K-ion batteries. Interestingly, some significant differences proved to exist for the intercalation of Na+ and K+ into graphite compared with the Li+ case. Such different host-guest interactions are unique depending on the host materials and electrolytes, which greatly contribute to a deeper understanding of intercalation-type electrode materials for next generation alkali metal ion batteries. This review summarizes significant advances from both experimental and theoretical calculations with a focus on comparing the intercalation of three alkali metal ions (Li+, Na+, K+) into graphite and aims to clarify the intimate host-guest relationships and the underlying mechanisms. New approaches developed to achieve favorable intercalation coupled with the challenges in this field are also discussed. We also extrapolate alkali metal ion intercalation in graphite to mono-/multi-valent ions in layered electrode materials, which will deepen the understanding of intercalation chemistry and provide guidance to explore new guests and hosts.

Intercalation chemistry of graphite: alkali metal ions and beyond.

Aluminum-ion batteries (AIBs) are regarded as viable alternatives to lithium-ion technology because of their high volumetric capacity, their low cost, and the rich abundance of aluminum. However, several serious drawbacks of aqueous systems (passive film formation, hydrogen evolution, anode corrosion, etc.) hinder the large-scale application of these systems. Thus, nonaqueous AIBs show incomparable advantages for progress in large-scale electrical energy storage. However, nonaqueous aluminum battery systems are still nascent, and various technical and scientific obstacles to designing AIBs with high capacity and long cycling life have not been resolved until now. Moreover, the aluminum cell is a complex device whose energy density is determined by various parameters, most of which are often ignored, resulting in failure to achieve the maximum performance of the cell. The purpose here is to discuss how to further develop reliable nonaqueous AIBs. First, the current status of nonaqueous AIBs is reviewed based on statistical data from the literature. The influence of parameters on energy density is analyzed, and the current situation and existing problems are summarized. Furthermore, possible solutions and concerns regarding the construction of reliable nonaqueous AIBs are comprehensively discussed. Finally, future research directions and prospects in the aluminum battery field are proposed.

Emerging Nonaqueous Aluminum-Ion Batteries: Challenges, Status, and Perspectives.

A High-Energy Aqueous Aluminum-Manganese Battery

A low-cost deep eutectic solvent electrolyte for rechargeable aluminum-sulfur battery

The development of silicon-based anode materials is important for improving the energy density of current lithium ion batteries. However, there are still strong demands for these materials with better cycle stability and higher reversible capacity. Here, a kind of dual bond restricted MXene-Si-CNT composite anode materials with enhanced electrochemical performance is reported. These dual bonds have been clearly revealed by an X-ray photoelectron spectroscopy technique and also proven by theoretical calculations with spontaneous reaction energy values (-0.190 and -0.429 eV/atom for Ti-Si and C-Si bonds, respectively). The cycle stability of the composites, prepared by a facile ball-milling synthetic method, can obviously be improved because of the existence of these dual bonds and the multidimensional constructed architecture. The MXene-Si-CNT composite with 60 wt % silicon possesses the best overall performance, with ∼80% capacity retention after 200 cycles, and achieves 841 mAh g-1 at 2 A g-1. This approach demonstrates a promising strategy to exploit high-performance anode materials and lessens the immanent negative effect of silicon-based materials. Furthermore, it is significant to extend this method to other anode materials with serious volumetric change problems during the cycling process.

Dual Bond Enhanced Multidimensional Constructed Composite Silicon Anode for High-Performance Lithium Ion Batteries.

Two-dimensional (2D) atomic crystals are promising channel materials for next generation electronics due to its outstanding gate electrostatics and few dangling bonds. Recently, tellurene, a new experimentally accessible Group-VI 2D tellurium, has drawn attention due to its large on/off ratios, high mobility and significant air stability. Herein, for the first time, we comprehensively examine the interfacial characteristics of monolayer (ML) tellurene field-effect transistors with a series of common bulk metals and 2D graphene as electrodes by using ab initio electronic structure calculations and quantum transport simulations. Furthermore, a lateral n-type Schottky contact is formed when contacting with Au in the a direction and Sc in both directions, while a lateral p-type Schottky contact is formed with Au in the b direction, and Cu, Ni, Ag, Pt, and Pd in both directions as a result of strong Fermi level pinning (FLP). The obtained FLP factor is 0.15 in the a direction and 0.09 in the b direction. Remarkably, a highly desirable lateral p-type Ohmic contact is formed with graphene in both directions. This investigation gives insight into the interfacial properties and guidance in electrode selection for ML tellurene devices.

Shiqi Liu

Papers

Emerging Nonaqueous Aluminum-Ion Batteries: Challenges, Status, and Perspectives.

A High-Energy Aqueous Aluminum-Manganese Battery

A low-cost deep eutectic solvent electrolyte for rechargeable aluminum-sulfur battery

Dual Bond Enhanced Multidimensional Constructed Composite Silicon Anode for High-Performance Lithium Ion Batteries.

Monolayer tellurene–metal contacts