Electrochemical performance and modeling of lithium‐sulfur batteries with varying carbon to sulfur ratios

Lithium‐sulfur batteries have attracted much research interest because of their high theoretical energy density and low‐cost raw materials. While the electrodes are composed of readily available materials, the processes that occur within the cell are complex, and the electrochemical performance of these batteries is very sensitive to a number of cell processing parameters. Herein, a simple electrochemical model will be used to predict, with quantitative agreement, the electrochemical properties of lithium‐sulfur cathodes with varying carbon to sulfur ratios. The discharge capacity and the polarization were very similar for the lowest sulfur loadings, while above 23.2 wt% sulfur the gravimetric capacity dropped significantly, and there was an increase in the cell polarization. In addition, a transition in the electrode morphology, from well dispersed to aggregated sulfur at the surface, will be reflected in the change in a critical model parameter demonstrating the sensitivity and functionality of even this simple model in predicting complex behavior in the lithium‐sulfur cells.     

β‐molybdenum carbide/carbon nanofibers as a shuttle inhibitor for lithium‐sulfur battery with high sulfur loading

We report the synthesis of β‐molybdenum carbide/carbon nanofibers (β‐Mo₂C/CNFs) by electrospinning and annealing process, when exploited as an interlayer in Li‐S batteries, demonstrating significantly improved electrochemical behaviors. The synthesized β‐Mo₂C/CNFs with 3D network structure and high surface area are not only conducive to ion transport and electrolyte penetration but also effectively intercept the shuttle of lithium polysulfide by polar surface interaction. Moreover, the reaction kinetics of the batteries enhanced is due to the presence of β‐Mo₂C, promoting the solid‐state polysulfide conversion reaction in the charge‐discharge process. Compared with the batteries with CNF interlayer and without interlayer, the batteries using a β‐Mo₂C/CNFs interlayer with a sulfur loading of 4.2 mg cm^‐2 delivered excellent electrochemical performance because of a facile redox reaction during cycling. The discharge capacity at the first cycle at 0.7 mA cm^?2 was 1360 mAh g^?1, maintaining a specific capacity of 974 mAh g^?1 after 160 cycles. Furthermore, it showed a high‐rate capacity of 700 mAh g^?1 at 14 mA cm^?2. This work demonstrates the β‐Mo₂C/CNFs as a promising interlayer to exploit Li‐S battery commercialization.     

Magnetic tubular carbon nanofibers as anode electrodes for high‐performance lithium‐ion batteries

Novel magnetic tubular carbon nanofibers (MTCFs) are prepared through the combination technique of hypercrosslinking, control extraction, and carbonization. The diameter of MTCFs is mainly concentrated between 90 and 120 nm, and the average tube diameter is about 30 nm. A trace amount of Fe₃O₄ exists inside the MTCFs with a particle size of 3 nm, which is formed by in situ conversion of the catalyst (FeCl₃) for the hypercrosslinking reaction. The MTCFs with high surface area (448.74 m² g^?1) and porous wall are used as anode material for lithium‐ion batteries. The electrochemical properties of MTCFs are compared, and tubular carbon nanofibers (TCFs) prepared by the complete extraction. Electrochemical analysis shows that the introduction of Fe₃O₄ nanoparticles makes MTCFs have higher reversible capacity and better rate performance. MTCFs exhibit high reversible specific capacity of 1011.7 mAh g^?1 after 150 cycles at current density of 100 mA g^?1. Even at high current density of 3000 mA g^?1, a remarkable reversible capacity of 270.0 mAh g^?1 is still delivered. Thus, the novel MTCFs show potential application value in anode material for high‐performance lithium‐ion battery.     

Fabrication of ultrafine Gd2O3 nanoparticles/carbon aerogel composite as immobilization host for cathode for lithium‐sulfur batteries

Carbon aerogel (CA), possessing abundant pore structures and excellent electrical conductivity, have been utilized as conductive sulfur hosts for lithium‐sulfur (Li‐S) batteries. However, a serious shuttle effect resulted from polysulfide ions has not been effectively suppressed yet due to the weak absorption nature of CA, resulting in rapid decay of capacity as the cycle number increases. Herein, ultrafine (~3 nm) gadolinium oxide (Gd₂O₃) nanoparticles (with upper redox potential of ~ 1.58 V versus Li⁺/Li) are uniformly in‐situ integrated with CA through directly sol‐gel polymerization and high‐temperature carbonization. The Gd₂O₃ modified CA composites (named as Gdx‐CA, where x means molar ratio of Gd₂O₃ nanoparticles to carbon) are incorporated with S. Then, the products (S/Gdx‐CA) are acted as sulfur host materials for Li‐S batteries. The results demonstrate that adding ultrafine Gd₂O₃ nanoparticles can dramatically improve the electrochemical properties of the composite cathodes. The S/Gd2‐CA electrode (loading with 58.9 wt% of S) possesses the best electrochemical properties, including a high initial capacity of 1210 mAh g^?1 and a relatively high capacity of 555 mAh g^?1 after 50 cycles at 0.1 C. It is noteworthy that the performance of long‐term cycle (350 cycles) for the S/Gd2‐CA (317 mAh g^?1 after 100 cycles and 233 mAh g^?1 after 350 cycles at 1 C) is improved significantly than that of S/CA (150 mAh g^?1 after 150 cycles at 1 C). Overall, the enhancement of electrochemical performances can be due to the strong polar nature of the ultrafine Gd₂O₃ nanoparticles, which provide strong adsorption sites to immobilize S and polysulfide. Furthermore, the Gd₂O₃ nanoparticles present a catalytic effect. Our research suggests that adding Gd₂O₃ nanoparticles into S/CA composite cathode is an effective and novelty method for improving the electrochemical performances of Li‐S batteries.     

Cobalt oxide nanosheets anchored onto nitrogen‐doped carbon nanotubes as dual purpose electrodes for lithium‐ion batteries and oxygen evolution reaction

Transition metal oxides (TMOs) have been extensively explored as promising electrode materials for electrochemical energy storage and catalysis. However, TMOs intrinsically have low electronic conductivity and suffer severe volume change during electrochemical cycling. In this study, we develop an effective strategy to enhance conductivity and buffer volume changes of TMOs, in which networked nitrogen‐doped carbon nanotubes (N‐CNTs) are incorporated into Co₃O₄ nanosheets system. Based on the whole mass of Co₃O₄ and N‐CNT, the composites can maintain a stable discharge capacity of ~590 mAh g^?1 after 80 cycles at a current density of 0.5 A g^?1. Moreover, the composites also exhibit greatly enhanced catalysis ability towards oxygen evolution reaction (OER), ie, small Tafel slope of 84 mV dec^?1, low overpotential of 310 mV at a current density of 10 mA cm^?2, and almost no activity decay throughout 30‐hour continuous operation. This study lays a new route for smartly designing advanced electrode materials for energy storage and electrochemical catalysis.     

Modeling the effect of key cathode design parameters on the electrochemical performance of a lithium‐sulfur battery

A 1D model is developed for the Li‐S cell to predict the effect of critical cathode design parameters—carbon‐to‐sulfur (C/S) and electrolyte‐to‐sulfur (E/S) ratios in the cathode—on the electrochemical performance. Cell voltage at 60% depth of discharge corresponding to the lower voltage plateau is used as a metric for calculating the cell performance. The cathode kinetics in the lower voltage plateau is defined with a single electrochemical reaction; thus, the model has a single apparent kinetic model parameter, the cathode exchange current density (i_0,pe). The model predicts that cell voltage increases considerably with increasing carbon content until a C/S ratio of 1 is attained, whereas the enhancement in the cell voltage at higher ratios is less obvious. The model can capture the effect of the C/S ratio on the cathode kinetics by expressing the electrochemically active area in the cathode in carbon volume fraction; the C/S ratio in the cathode does not affect i_0,pe in the model. On the other hand, the electrolyte amount has a significant impact on the kinetic model parameter such that increasing electrolyte amount improves the cell voltage as a result of increasing i_0,pe. Therefore, in the model, i_0,pe needs to be defined as a function of the electrolyte volume fraction, which is known to have a crucial effect on reaction kinetics.     

A microporous carbon derived from metal-organic frameworks for long-life lithium sulfur batteries

A polyhedral microporous carbon derived from metal-organic frameworks (ZIF-8) could present good property for sulfur loading and trapping. A melting-evaporation route was adopted to synthesize two sulfur/microporous carbon (S/MC) composites, of which sulfur content is controllable, and ether-based or ester-based electrolytes were used to evaluate the synthesized composites for the lithium sulfur batteries. According to electrochemical results, the S/MC composite with 65.2 wt% S in the ether-based electrolyte exhibited optimized performance as compared with the composite with 65.2 wt% S in the ester-based electrolyte, as well as the composite with 58.6 wt% S in the two kinds of electrolytes. For the S/MC composite with 65.2 wt% S in ether-based electrolyte, the initial discharge capacity could reach up to 1505.9 mAh g⁻¹ and the reversible capacity could be 833.3 mAh g⁻¹ after 40 cycles at 0.1 C. Furthermore, while being respectively evaluated at 0.5, 1.0, and 2.0 C, the discharge capacities could still maintain at 544, 493 and 354 mAh g⁻¹ after 300, 500, and 800 cycles, demonstrating appreciable cyclic reversibility and rate capability.     

Improving lithium‐sulfur battery performance with lignin reinforced MWCNT protection layer

Multi‐walled carbon nanotube (MWCNT) protection layers have previously been used to trap polysulfides and suppress the shuttle effect in lithium sulfur (Li‐S) batteries, leading to significant performance improvement. While the MWCNT is inherently highly conductive and mechanically strong, the cost can be significant and in turn hampered wider application of MWCNT protection layers. Here, we employed lignin, a byproduct during high‐quality bleached paper manufacturing, to replace a portion of MWCNT in the protection layer to reduce cost and enhance surface properties of pristine MWCNT protection layers. We found that the protection layer with 25 wt% lignin leads to the best overall electrochemical performance of Li‐S batteries during charging/discharging at 0.5°C and 1C rate (1C = 1,675 mA g^?1) among various weight‐ratios of lignin/MWCNT, and a low decay rate (0.20% per cycle) and high initial capacity (1342 mA g^?1 and 1437 mA g^?1 for 1C and 0.5C, respectively) are demonstrated. Besides, Li‐S cells with 25 wt% lignin/MWCNT composite protection layer also exhibited great rate capability, of which the specific capacities at 0.1C, 0.5C, 1C, and 2C were 1150, 913, 824, and 637 mAh g^?1, respectively. The enhanced electrochemical stability and performance of Li‐S batteries can be attributed to strengthened polysulfide trapping and improved lithium ion transport with lignin reinforced MWCNT protection layers. We showcased an economic approach to extend cycle life and improve rate capability of Li‐S batteries.     

Morphology‐dependent electrochemical performance of nitrogen‐doped carbon dots@polyaniline hybrids for supercapacitors

In this work, the binary N‐CDs@PANI hybrids were fabricated by introducing zero‐dimensional nitrogen‐doped carbon dots (N‐CDs) into reticulated PANI. Firstly, N‐CDs were prepared by one‐pot microwave method, and then, the N‐CDs were introduced into in situ oxidative polymerization of aniline (ANI) monomer. The N‐CDs with abundant functional groups and high electronic cloud density played a significant role in guiding the polyaniline‐ordered growth into intriguing morphologies. Moreover, morphology‐dependent electrochemical performances of N‐CDs@PANI hybrids were investigated and N‐CDs improve static interaction and enhance the special capacitances in the N‐CDs@PANI hybrids. Especially, the specific capacitance of PC4 hybrid can reach 785 F g^?1, which exceed that of pure PANI (274 F g^?1) at current density of 0.5 A g^?1 according to three‐electrode measurement. And the capacitance retention of the PC4 hybrid still keeps 70% after 2000 cycles of charge and discharge. The N‐CDs@PANI hybrids can have potential applications in electrode materials, supercapacitors, nonlinear optics, and microwave absorption.     

Freestanding graphitic carbon nitride-based carbon nanotubes hybrid membrane as electrode for lithium/polysulfides batteries

Lithium sulfur batteries have drawled worldwide attention in recent years, which benefit of its high-density energetic, low cost, and environmental benignity. Nevertheless, the shuttle effect of polysulfides and resulting self-discharge lead to capacity fade loss and poor electrochemical performance. Herein, graphitic-carbon nitride/carbon nanotubes (g-C₃N₄/CNTs) hybrid membrane is fabricated by the flow-direct vacuum filtration process. The as-prepared 3-D freestanding g-C₃N₄/CNTs membrane employed as positive current collector containing Li₂S₆ catholyte solution for lithium/polysulfides batteries. The fabricated g-C₃N₄/CNTs provide a physical barriers and chemisorption resist polysulfide shuttling. Moreover, the conductive network constructed by CNTs can empower sulfur to be evenly distributed in the cathode and accelerates electron transport. Thus, to further prove the cooperative effect of g-C₃N₄ and CNTs, the freestanding g-C₃N₄/CNTs/Li₂S₆ electrode exhibits more stable electrochemical performance than CNTs/Li₂S₆ electrode, deliver the first discharge capacity of 876 mAh g⁻¹ at 0.5 C and maintained at 633 mAh g⁻¹ after 300 cycles. The sulfur mass in electrode was increased to 7.11 mg, and the g-C₃N₄/CNTs/Li₂S₆ electrode also possess a high capacity retention of 75.5%. Meanwhile, g-C₃N₄ modified CNTs can not only trap polysulfides by strong adsorption but also effectively inhibit the self-discharge behavior of lithium/polysulfides batteries. As a consequence, the g-C₃N₄/CNTs composites for lithium/polysulfides batteries are indicating an excellent electrochemical stability with a long-term storage without obvious capacity degradation.     

Synergistic effect of sulfur on electrochemical performances of carbon‐coated vanadium pentoxide cathode materials with polyvinyl alcohol as carbon source for lithium‐ion batteries

Vanadium pentoxide (V₂O₅) is a common cathode material for lithium‐ion battery, but its low electronic and ionic conductivity seriously affect its electrochemical performances. In this paper, a type of carbon‐coated V₂O₅ and S composite cathode material with PVA as the carbon source is utilized to lithium‐ion batteries. X‐ray diffraction and Raman test results illustrate that sulfur can make the V₂O₅ lose part of oxygen atoms and become nonstoichiometric vanadium oxide (V₂O_5‐x). Electrochemical test results show that sulfur can provide a considerable proportion of the specific capacity of the whole cathode. This illustrates that the synergistic effect of sulfur can optimize the structure of vanadium pentoxide in order to increase more electron transfer channels, and at the same time, it also can provide additional specific capacity for the whole cathode. When the ratio of V₂O₅ and sulfur is 1:3, the discharge specific capacity can reach 923.02, 688.37, and 592.70 mAh g^?1 at 80, 160, and 320‐mA g^?1 current density, respectively, and after 100 times charge and discharge cycles at 320‐mA g^?1 current density, the capacity retention rate can achieve to more than 60%.     

Enhanced performance of alpha‐Fe2O3 nanoparticles with optimized graphene coated layer as anodes for lithium‐ion batteries

A series of different α‐Fe₂O₃ nanoparticles composites containing different amounts of graphene coatings have been successfully prepared using a simple electrostatic self‐assembly (ESA) method. The structure and electrochemical properties of these α‐Fe₂O₃@graphene composites have been investigated. The α‐Fe₂O₃ nanoparticles composite containing 40 wt% graphene coating exhibits the highest specific capacity (385 mAh g^?1) under 1000 mA g^?1, resulting in superior cycle stability with no downward trend after 500 cycles. These results demonstrate that graphene coatings can be used to enhance the electrochemical properties and morphological stability of α‐Fe₂O₃ nanoparticles as anodic materials for high performance lithium‐ion batteries (LIBs). The low‐energy self‐assembly method employed in the paper has good potential for the broad‐scale preparation of other graphene‐modified materials because of its simplicity and the relatively low temperature conditions.     

A sandwich‐structured separator based on in situ coated polyaniline on polypropylene membrane for improving the electrolyte wettability in lithium‐ion batteries

Designing separators with excellent electrolyte wettability is of great significance to improve the electrochemical behavior of lithium‐ion batteries (LIBs). Herein, we develop a sandwich‐structured separator by facile in situ polymerization of polyaniline (PANI) on the both sides of polypropylene (PP) separator. The introduction of PANI coating improves the electrolyte wettability of the PP separators. Consequently, the cells equipped with this sandwich‐structured separator demonstrate superior electrochemical performance including initial capacity, rate performance, and cyclic stability compared with the PP separator. This work may provide a facile approach for separator modification to improve the performance of LIBs.     

Thermal management of lithium‐ion batteries for electric vehicles

Thermal issues associated with electric vehicle battery packs can significantly affect performance and life cycle. Fundamental heat transfer principles and performance characteristics of commercial lithium‐ion battery are used to predict the temperature distributions in a typical battery pack under a range of discharge conditions. Various cooling strategies are implemented to examine the relationship between battery thermal behavior and design parameters. By studying the effect of cooling conditions and pack configuration on battery temperature, information is obtained as to how to maintain operating temperature by designing proper battery configuration and choosing proper cooling systems. It was found that a cooling strategy based on distributed forced convection is an efficient, cost‐effective method which can provide uniform temperature and voltage distributions within the battery pack at various discharge rates. Copyright © 2012 John Wiley & Sons, Ltd.     

Computational study about the effect of electrode morphology on the performance of lithium‐ion batteries

Electrode morphology has significant influence on the performance of lithium‐ion batteries in that it controls electrical conductivity and electrode utilization by establishing electrical connectivity in the electrode. The present study investigates the effect of the electrode morphology on battery performance by combining two different mathematical models. First, a two‐dimensional, direct numerical simulation (DNS) model is introduced, which stochastically generates electrode morphology and calculates electrical conduction and electrode utilization. Various simulations are conducted to evaluate the effect of the active particle coating, conductive agent loading, particle size, and electrode compression by using the DNS model. Second, data acquired from the DNS model are applied to the blended‐electrode model to evaluate battery performance. Calculation result confirms that electrode morphologies have significant effects on both capacity and power of lithium‐ion batteries. Copyright © 2016 John Wiley & Sons, Ltd.     

A freestanding MoO2‐decorated carbon nanofibers interlayer for rechargeable lithium sulfur battery

Lithium‐sulfur (Li‐S) battery based on sulfur cathodes is of great interest because of high capacity and abundant sulfur source. But the shuttling effect of polysulfides caused by charge‐discharge process results in low sulfur utilization and poor reversibility. Here, we demonstrate a good approach to improve the utility of sulfur and cycle life by synthesizing carbon nanofibers decorated with MoO₂ nanoparticles (MoO₂‐CNFs membrane), which plays a role of multiinterlayer inserting between the separator and the cathode for Li‐S battery. The S/MoO₂‐CNFs/Li battery showed a discharge capacity of 6.93 mAh cm^?2 (1366 mAh g^?1) in the first cycle at a current density of 0.42 mA cm^?2 and 1006 mAh g^?1 over 150 cycles. Moreover, even at the highest current density (8.4 mA cm^?2), the battery achieved 865 mAh g^?1. The stable electrochemical behaviors of the battery has achieved because of the mesoporous and interconnecting structure of MoO₂‐CNFs, proving high effect for ion transfer and electron conductive. Furthermore, this MoO₂‐CNFs interlayer could trap the polysulfides through strong polar surface interaction and increases the utilization of sulfur by confining the redox reaction to the cathode.     

The equivalent circuit battery model parameter sensitivity analysis for lithium‐ion batteries by Monte Carlo simulation

To enhance the estimation accuracy of battery's state of charge, it is imperative to estimate the battery model parameter. To reduce the calculation efforts, the number of the battery model parameter to be estimated should be less while ensuring the state of charge estimation accuracy. Especially in engineering applications, the calculating ability is usually limited. So, it needs to choose the critical battery model parameter to be estimated. This paper's contributions are as follows: The global sensitivity analysis of the battery model parameter is achieved by the Monte Carlo simulation method. The results show that the open circuit voltage and the ohmic resistance are the high sensitivity parameters. Guided by the results of parameter sensitivity analysis, a dual extended Kalman filters method is utilized to achieve online battery model parameter estimation. The experiments prove that the state of charge estimation accuracy is improved by the online parameter estimation. Estimating high sensitivity parameters can reduce running time. And the SOC estimation accuracy can be guaranteed.     

Synthesis and characterization of PVDF‐coated cotton‐derived hard carbon for anode of Li‐ion batteries

A carbonized cotton that is coated by pyrocarbon of Polyvinylidene Fluoride (PVDF) (HC/P) was synthesized by a facile method. Its microstructure and electrochemical performances as anode for Li‐ion batteries were characterized by X‐Ray Diffraction (XRD), Raman, X‐ray photoelectron spectrometer (XPS), Scanning Electron Microscope (SEM), and other means. It is found that coating process does not affect the phase structure but influences the morphology, specific area, and electrochemical performances. HC/P series have abundant pores with diameter of 2 to 5 nm and small‐size particles; thus, they have much more lithium storage position or charge/discharge capacity than pristine hard carbon (HC). Besides, Cyclic Voltammetry (CV) curves of HC/P‐2 proved that the irreversible side reactions of Li⁺ with surface functional groups were reduced, which can explain the advancement of initial coulomb efficiency. Among three HC/P samples, HC/P‐2 owns the best electrochemical performances. It delivered 520 mAh/g under current density of 20 mA/g and kept 95.9% retention rates of capacity after 100 cycles under 200 mA/g of current density.     

Highly efficient poly(fluorene phenylene) copolymer as a new class of binder for high‐capacity silicon anode in lithium‐ion batteries

In this research, a novel approach involving the use of a fluorescent and ductile polymer for the high capacity Li‐ion battery application is reported. Poly(fluorene phenylene) copolymer as a conjugated polymer containing with lateral substituents, poly(ethylene glycol) (PEG) units, as a latent building unit for conjugation and electrolyte uptake was prepared and characterized. The synthesis process was carried out via Suzuki coupling reaction with Pd‐based catalyst by using separately obtained PEG functionalized dibromobenzene in combination with dioctylfluorene‐diboronic acid bis(1,3‐propanediol) ester. A flexible and conductive polymer was synthesized and utilized as a binder for high performance Si‐anode. The observed full capacity of cycling of silicon particles, ie, at C/3 with the capacity of 605 mAh/g after 1000^th cycle, confirms the good performance without any supplementary conductive additive. The designed and prepared binder polymer with multi‐functionality exhibits better features such as better electronic conductivity, high polarity, good mechanical strength, and adhesion.     

Surface modification of LiNi0.5Co0.2Mn0.3O2 cathode materials with Li2O‐B2O3‐LiBr for lithium‐ion batteries

LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) cathode material suffers from phase transformation and electrochemical performance degradation as its main drawbacks, which are strongly dependent on the surface state of NCM523. Herein, an effective surface modification approach was demonstrated; namely, the fast lithium‐ion conductor (Li₂O‐B₂O₃‐LiBr) was coated on NCM523. The Li₂O‐B₂O₃‐LiBr coating layer as a protecting shell can prevent NCM523 particles from corrosion by the acidic electrolyte, leading to a superior discharge capacity, rate capability, and cycling stability. At room temperature, the Li₂O‐B₂O₃‐LiBr–coated NCM523 exhibited an excellent capacity retention of 87.7% after 100 cycles at the rate of 1 C, which is remarkably better than that (29.8%) without the uncoated layer. Furthermore, the coating layer also increased the discharge capacity of NCM523 cathode material from 68.7 to 117.0 mAh g^?1 at 5 C. Those can be attributed to the reduction in the electrode polarization and improvement in the electrode conductivity, which was supported by electrochemical impedance spectroscopy and cyclic voltammetry measurements.