Safer Lithium-Ion Batteries from the Separator Aspect:Development and Future Perspectives

With the rapid development of lithium-ion batteries(LIBs),safety problems are the great obstacles that restrict large-scale applications of LIBs,especially for the high-energy-density electric vehicle industry.Developing component materials(e.g.,cathode,anode,electrolyte,and separator)with high thermal stability and intrinsic safety is the ultimate solution to improve the safety of LIBs.Separators are crucial components that do not directly participate in electrochemical reactions during charging/discharging processes,but play a vital role in determining the electrochemical performance and safety of LIBs.In this review,the recent advances on traditional separators modified with ceramic materials and multifunctional separators ranging from the prevention of the thermal runaway to the flame retardant are summarized.The component–structure–performance relationship of separators and their effect on the comprehensive performance of LIBs are discussed in detail.Furthermore,the research challenges and future directions toward the advancement in separators for high-safety LIBs are also proposed.

HIGH-ORDER NUMERICAL METHOD FOR SOLVING A SPACE DISTRIBUTED-ORDER TIME-FRACTIONAL DIFFUSION EQUATION

This article proposes a high-order numerical method for a space distributed-order time-fractional diffusion equation.First,we use the mid-point quadrature rule to transform the space distributed-order term into multi-term fractional derivatives.Second,based on the piecewise-quadratic polynomials,we construct the nodal basis functions,and then discretize the multi-term fractional equation by the finite volume method.For the time-fractional derivative,the finite difference method is used.Finally,the iterative scheme is proved to be unconditionally stable and convergent with the accuracy O(σ^(2)+τ^(2-β)+h^(3)),whereτand h are the time step size and the space step size,respectively.A numerical example is presented to verify the effectiveness of the proposed method.

Highly efficient H-bonding charge-transfer complex for microsupercapacitors under extreme conditions of low temperatures

Owing to sluggish ionic mobility at low temperatures, supercapacitors, as well as other energy-storage devices, always suffer from severe capacity decay and even failure under extreme low-temperature circumstances. Solar-thermal-enabled self-heating promises an attractive approach to overcome this issue.Here, we report a unique H-bonding charge-transfer complex with a high photothermal conversion efficiency of 79.5% at 405 nm based on chloranilic acid and albendazole. Integrated with a microsupercapacitor, the chloranilic acid-albendazole complex(CAC) film prompts an apparent temperature increase of 22.7 °C under 1 sun illumination at-32.6 °C, effectively elevating the working temperature of devices.As a result, the rate capability of the microsupercapacitor has been significantly improved with a 17-fold increase in capacitance at a current density of 60 μA cm^(-2), leading to outstanding low-temperature performances. Importantly, the integrated device is capable of working at a low temperature of-30 °C in the open air, which demonstrates the potential of CAC in practical applications for low-temperature ultracapacitive energy-storage devices.

Fast thermal responsive separators toward long-life and safe lithium metal batteries

Lithium metal batteries(LMBs)have been extensively investigated during the past decades because of their ultrahigh energy densities.With the increasing demand for energy density,however,the safety issue of LMBs has become a significant challenge.In particular,localized areas of increased temperature(namely,hotspots)may be induced and even exacerbated within LMBs by uneven current distribution,internal short circuits,or inadequate heat dissipation,which significantly sacrifices battery safety and cycle life.Here,we report the rational design and fabrication of a fast thermal responsive separator capable of inhibiting the growth of lithium dendrites and mitigating thermal propagation,thereby reducing the risk of thermal runaway.The as-achieved separator comprises both an electrospun membrane using a phase change material with superior thermal-storage ability and a thermally conductive modification layer of hexagonal boron nitride nanosheets with a fast heat-transfer feature.It is demonstrated that such a unique integration of heat conduction and heat storage enables the functional separator with attractive abilities to mitigate hotspots and inhibit the growth of lithium dendrites upon the cycling of LMBs.Moreover,pouch cells with the thermal-responsive separator,as well as numerical simulations,verify much enhanced safety and cycle life of LMBs.This work may offer a new conceptual design of intelligent separators that acts as a functional unit encapsulated within a single cell to boost in-situ thermal management,which will help to develop high-safety and energy-dense LMBs.

钠离子电容器负极的双功能补钠研究

电极的首次库伦效率低是限制快充钠离子电容器规模化使用的原因之一,这主要源于电解液的不可逆分解以及电极材料造成的不可逆损失.我们选取高倍率特性的Nb_(2)O_(5)负极材料作为研究对象,采用钠萘/2-甲基-四氢呋喃作为预钠化剂,发展了一种简单且可精确调控的双功能预钠化剂,实现了预活化,获得了高稳定循环的NaxNb_(2)O_(5)电极,从而补偿了钠在材料中的不可逆吸收.研究发现,预钠化过程同时抵消了电解液分解所产生的损失,有利于在电极/电解液界面形成坚固的富含无机物的固体电解质界面膜.通过补偿Nb_(2)O_(5)负极的体相与界面的钠损失,构建了稳定且具有高能量密度的钠离子电容器.本研究为调控储钠电极的首次库伦效率及其在下一代钠离子电容器中的应用提供了新方法.

Dual surface/bulk engineering of Nb_(2)O_(5) for high-rate sodium storage

Orthorhombic Nb_(2)O_(5) is a highly promising fast-charging anode material for sodium-ion capacitors.However,its poor intrinsic electronic/ionic conductivity limits its performance.Here,we developed a one-step heat treatment method to create an N-doped carbon coating on the outside and S-doped Nb_(2)O_(5) on the inside(CN-SCN).Ionic liquids are used as the source of C/N/S,which synergistically enhance the surface and bulk electronic/ionic conductivity.The N-doped carbon coating on the surface exhibits excellent electronic conductivity and a low ion-diffusion barrier,thanks to the high nitrogen ratio and extremely low content(<2 wt%).Auger electron spectroscopy analysis confirms that S atoms detach from the carbon chain of the ionic liquids and enter the bulk Nb_(2)O_(5),resulting in S-doped Nb_(2)O_(5),significantly facilitating reaction kinetics.The CN-SCN electrodes exhibit outstanding rate capability,achieving a capacity of up to 94 mAh g^(−1) even at a high current rate of 50 C.When paired with activated carbon as the positive electrode,the sodium-ion capacitor with the CN-SCN anode exhibits a high-energy density of up to 59 Wh kg^(−1) and a long cycle life with 73%capacity retention after 10,000 cycles.This work opens up possibilities for low-cost and large-scale production of high-rate Nb_(2)O_(5) for sodium-storage applications.

A monotone finite volume method for time fractional Fokker-Planck equations

We develop a monotone finite volume method for the time fractional Fokker-Planck equations and theoretically prove its unconditional stability. We show that the convergence rate of this method is of order 1 in the space and if the space grid becomes sufficiently fine, the convergence rate can be improved to order 2.Numerical results are given to support our theoretical findings. One characteristic of our method is that it has monotone property such that it keeps the nonnegativity of some physical variables such as density, concentration,etc.

ON A MOVING MESH METHOD FOR SOLVING PARTIAL INTEGRO-DIFFERENTIAL EQUATIONS

This paper develops and analyzes a moving mesh finite difference method for solving partial integro-differential equations. First, the time-dependent mapping of the coordinate transformation is approximated by a a piecewise linear function in time. Then, piecewise quadratic polynomial in space and an efficient method to discretize the memory term of the equation is designed using the moving mesh approach. In each time slice, a simple piecewise constant approximation of the integrand is used, and thus a quadrature is constructed for the memory term. The central finite difference scheme for space and the backward Euler scheme for time are used. The paper proves that the accumulation of the quadrature error is uniformly bounded and that the convergence of the method is second order in space and first order in time. Numerical experiments are carried out to confirm the theoretical predictions.

Lithium-ion insertion kinetics of Na-doped Li2TiSiO5 as anode materials for lithium-ion batteries

Li2TiSiO5 receives much interest recently in lithium-ion battery anodes because of its attractive Liinsertion/extraction potential at 0.28 V(vs. Li+/Li), which bridges the potential gap between graphite and Li4 Ti5 O12. However, Li2TiSiO5 suffers from the low intrinsic electronic conductivity and sluggish Liion transfer kinetics. In this work, we report lithium-ion insertion kinetics of Li2TiSiO5 by Na doping,achieving high-rate capability. Rietveld refinement of X-ray diffraction results reveals that Na doping can enlarge the space of Li slabs, thus reducing the Li-ion transfer barrier and enhancing the Li-ion diffusion kinetics. According to first-principles calculations, Na doping can tune the band structure of Li2TiSiO5 from indirect to direct band, leading to improved electronic conductivity and electrochemical performance. In particular, the Na-doped Li2TiSiO5(Li1.95 Na(0.05)TiSiO5) electrode exhibits outstanding rate capability with a high capacity of 101 m A h g^(-1) at 5 A g^(-1) and superior cyclability with a reversible capacity of 137 m A h g^(-1) under 0.5 A g^(-1) over 150 cycles.

A numerical test method of California bearing ratio on graded crushed rocks using particle flow modeling

In order to better understand the mechanical properties of graded crushed rocks （GCRs） and to optimize the relevant design, a numerical test method based on the particle flow modeling technique PFC2D is developed for the California bearing ratio （CBR） test on GGRs. The effects of different testing conditions and micro-mechanical parameters used in the model on the CBR numerical results have been systematically studied. The reliability of the numerical technique is verified. The numerical results suggest that the influences of the loading rate and Poisson＇s ratio on the CBR numerical test results are not significant. As such, a loading rate of 1.0-3.0 mm/min, a piston diameter of 5 cm, a specimen height of 15 cm and a specimen diameter of 15 cm are adopted for the CBR numerical test. The numerical results reveal that the GBR values increase with the friction coefficient at the contact and shear modulus of the rocks, while the influence of Poisson＇s ratio on the GBR values is insignificant. The close agreement between the CBR numerical results and experimental results suggests that the numerical simulation of the CBR values is promising to help assess the mechanical properties of GGRs and to optimize the grading design. Be- sides, the numerical study can provide useful insights on the mesoscopic mechanism.