With the severe spectrum shortage in conventional cellular bands, millimeter wave (mmW) frequencies between 30 and 300 GHz have been attracting growing attention as a possible candidate for next-generation micro-and picocellular wireless networks. The mmW bands offer orders of magnitude greater spectrum than current cellular allocations and enable very highdimensional antenna arrays for further gains via beamforming and spatial multiplexing. This paper uses recent real-world measurements at 28 and 73 GHz in New York City to derive detailed spatial statistical models of the channels and uses these models to provide a realistic assessment of mmW micro-and picocellular networks in a dense urban deployment. Statistical models are derived for key channel parameters including the path loss, number of spatial clusters, angular dispersion and outage. It is found that, even in highly non-line-of-sight environments, strong signals can be detected 100m to 200m from potential cell sites, potentially with multiple clusters to support spatial multiplexing. Moreover, a system simulation based on the models predicts that mmW systems can offer an order of magnitude increase in capacity over current state-of-the-art 4G cellular networks with no increase in cell density from current urban deployments.

Lithium ion batteries (LIBs) are one of the most potential energy storage devices among various rechargeable batteries due to their high energy/power density, long cycle life, and low self‐discharge properties. However, current LIBs fail to meet the ever‐increasing safety and fast charge/discharge demands. As one of the main components in LIBs, separator is of paramount importance for safety and rate performance of LIBs. Among the various separators, composite separators have been widely investigated for improving their thermal stability, mechanical strength, electrolyte uptake, and ionic conductivity. Herein, the challenges and limitations of commercial separators for LIBs are reviewed, and a systematic overview of the state‐of‐the‐art research progress in composite separators is provided for safe and high rate LIBs. Various combination types of composite separators including blending, layer, core–shell, and grafting types are covered. In addition, models and simulations based on the various types of composite separators are discussed to comprehend the composite mechanism for robust performances. At the end, future directions and perspectives for further advances in composite separators are presented to boost safety and rate capacity of LIBs.

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Lithium metal batteries (LMBs) have aroused extensive interest in the field of energy storage owing to the ultrahigh anode capacity. However, strong solvation of Li + and slow interfacial ion transfer associated with conventional electrolytes limit their long-cycle and high-rate capabilities. Herein an electrolyte system based on fluoroalkyl ether 2,2,2-trifluoroethyl-1,1,2,3,3,3-hexafluoropropyl ether (THE) and ether electrolytes is designed to effectively upgrade the long-cycle and high-rate performances of LMBs. THE owns large adsorption energy with ether-based solvents, thus reducing Li + interaction and solvation in ether electrolytes. With THE rich in fluoroalkyl groups adjacent to oxygen atoms, the electrolyte owns ultrahigh polarity, enabling solvation-free Li + transfer with a substantially decreased energy barrier and ten times enhancement in Li + transference at the electrolyte/anode interface. In addition, the uniform adsorption of fluorine-rich THE on the anode and subsequent LiF formation suppress dendrite formation and stabilize the solid electrolyte interphase layer. With the electrolyte, the lithium metal battery with a LiFePO 4 cathode delivers unprecedented cyclic performances with only 0.0012% capacity loss per cycle over 5000 cycles at 10 C. Such enhancement is consistently observed for LMBs with other mainstream electrodes including LiCoO 2 and LiNi 0.5 Mn 0.3 Co 0.2 O 2 , suggesting the generality of the electrolyte design for battery applications.

ideally stores five times more energy per mass (1675 mAh g −1 ) than intercalationtype cathodes by multielectron reactions of sulfur, namely S 8 + 16 e − + 16 Li + ⇌ 8 Li 2 S, [8,9] and leads to extensive researches on Li-S batteries.Nevertheless, the development of Li-S batteries is plagued by three issues: (1) the sluggish electrical conductivities of sulfur (σ = 5 × 10 −30 S cm −1 ) and its end products (Li 2 S, σ = 1 × 10 −13 S cm −1 ) lead to slow conversion from soluble lithium polysulfides (LiPSs, namely Li 2 S x , 4 ≤ x ≤ 8) to solid Li 2 S 2 /Li 2 S; (2) the hydrophilic LiPS species are inclined to shuttle through porous separator and deposit at Li anode as Li 2 S which is difficult to be reused owing to the high activation energy [10][11][12][13] ; (3) the shuttling effect of polysulfides causes severe self-discharge, continuous energy loss and unsatisfactory energy density. [14][15][16] Reconstructing separators architecture is an effective strategy to ameliorate the aforementioned issues. [17][18][19] Carbon materials with a high specific surface area are introduced to the separator surface to physically block LiPS and accelerate its conversion due to the high conductivity (σ = 9 × 10 1 -5 × 10 3 S m −1 ), [20] but porous carbon shows limited capability to confine LiPS owing to the feeble van der Waals adsorption. [21,22] Polar compounds, such as metal compounds (MA, where M is metal, and A is oxygen, nitrogen, or sulfur), are promising materials to bond to LiPS through surface M-S or A-Li bonding, which prevents the shuttling effect and changes the reduction pathway of LiPS with decreased the redox energy barrier. [23,24] However, the strong A-Li bonding of ≈2 eV impedes Li + ion transport which delays the reaction kinetics of LiPSs. [25] In addition, most of them own complex design processes. [26] Recent studies show that the low cost and natural abundance lamellar clays own much lower Li-ion diffusion barrier (such as lithium-montmorillonite (0.15 eV), MA materials (ZnS (0.494 eV), MgO (0.45 eV), Al 2 O 3 (1.22 eV), and CeO 2 (0.66eV)), [27,28] which allows free lithium-ion diffusion in the sulfur cathode and gives rise to improved electrochemical performances of Li-S batteries. Unfortunately, these lamellar clays modified structures still show unsatisfactory rate performances for practical application. [29] Hence, it is necessary to further develop optimized lamellar clays structure for high-rate Li-S batteries.

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In this paper we focus on the studies of graphene wrinkling, from its formation to collapse, and its dependence on aspect ratio and temperature using molecule dynamics simulation. Based on our results, the first wrinkle is not formed on the edge but in the interior of graphene. The fluctuations of edge slack warps drive the wrinkling evolution in graphene which is distinguished from the bifurcation in continuum film. There are several obvious stages in wrinkling progress, including incubation, infancy, youth, maturity and gerontism periods which are identified by the atomic displacement difference due to the occurrences of new wrinkles. The wrinkling progress is over when the C-C bonds in highly stretched corners are broken which contributes to the wrinkling collapse. The critical wrinkling strain, the wrinkling pattern and extent depend on the aspect ratio of graphene, the wrinkling level and collapsed strains do not. Only the collapsed strain is sensitive to the temperature, the other wrinkling parameters are independent of the temperature. Our results would benefit the understanding of the physics of graphene wrinkling and the design of nanomechanical devices by tuning the wrinkles.

With the severe spectrum shortage in conventional cellular bands, millimeter wave (mmW) frequencies between 30 and 300 GHz have been attracting considerable attention as a possible candidate for next-generation micro-and picocellular wireless networks. The mmW frequency bands offer orders of magnitude greater spectrum than current cellular microwave frequencies currently deployed below 3 GHz. However, even with typical microcellular radii of 100m to 200m, the propagation of mmW signals in outdoor non line-of-sight (NLOS) links remains challenging and the feasibility of such mmW networks is far from clear. This paper uses recent real-world measurements at 28 GHz to provide the first systematic assessment of mmW picocellular networks. It is found that, even with its limited propagation characteristics, mmW systems can offer an order of magnitude increase in capacity over current state-of-the-art 4G cellular networks with similar cell density. However, it is also shown that such mmW networks will operate in an extremely powerlimited regime where the full spatial and bandwidth degrees of freedom are not fully utilized. This power-limited regime contrasts significantly with current bandwidth-limited cellular systems, requiring alternate technologies for mmW systems that may unlock further gains that mmW frequency bands offer.