High C-rate Research Articles

La-doped LiMnPO4/C cathode material for Lithium-ion battery

LiMnPO4 and LiMn1-xLaxPO4 (0.01 ≤ x ≤ 0.1) nanostructures with in-situ carbon coating are synthesized by low temperature solvothermal method using Glycerol: Water (2:1) ratio as solvent. Powder X-ray diffraction (XRD) studies confirmed the orthorhombic crystal structure of LiMnPO4 and LiMn0.99La0.01PO4. X-ray diffraction shows that up to 1% La3+ ion doping into manganese sites does not affect the crystal structure of LiMnPO4. Field emission scanning electron microscopy (FESEM) of LiMnPO4 and LiMn0.99La0.01PO4 depicts the formation of nanosized particles with spherical morphology. Electrochemical properties of LiMnPO4 and LiMn0.99La0.01PO4 are investigated by galvanostatic charge discharge tests. The present studies show that 1% La3+ ion doped LiMnPO4 could be a promising cathode material that can improve the performance of Lithium-ion cell by increasing the electrochemical stability at higher C-rate.

Critical roles of synthesis temperature and an effective solid electrolyte interphase in improving the electrochemical performance of carbon coated Fe3BO5 anode at high C-rates

Samples of core/yolk-shell structured, carbon-coated vonsenite (Fe3BO5) are synthesized by the solid-state method and utilized as conversion-type anode materials. The core consists of vonsenite nanoparticles, while the shell is composed of a defective (partially graphitized) carbon layer. A higher synthetic temperature leads to the formation of larger amounts of metallic impurities as a result of the carbothermal reduction. The electrochemical characteristics of the sample synthesized at a lower temperature (600 °C) are better than those of the samples synthesized at higher temperatures and without the carbon coating. This can be attributed to the smaller-sized crystallites having the least amount of metallic impurities and a better-formed solid electrolyte interphase (SEI). This optimized sample demonstrates an outstanding reversible specific discharge capacity of 976 mAh g-1 at a current density of 0.05 A g-1. Long cyclability test at an ultra-high C-rate of 5 A g-1 outlines the critical role of an effective SEI and carbon coating. Remarkably, this sample is able to sustain even further electrochemical abuse, and shows electrochemical activity up to a C-rate of 20 A g-1, thus validating the usefulness of this material as a safe and stable anode for fast charging-discharging Li-ion battery applications.

Safety assessment of thermal runaway behavior of lithium-ion cells with actual installed state

To quantitatively evaluate the overcharge-to-thermal runaway (OC-to-TR) behaviors of lithium-ion (Li-ion) cells in practice, here reported are overcharge tests on Li-ion pouch cells with NCM811 cathode under actual restricted condition. Such a restriction results from two splints during the overcharging process to simulate their actual installed state in the battery pack of electric vehicles (EVs). From the monitoring of the cell voltage, temperature, and expansion force, a unique feature that is discovered in this work is that the OC-to-TR process is strongly correlated to the evolution of expansion force, which can further be divided into five stages with distinctly different features. Moreover, this unique correlation depends critically on the current rate (C-rates) and preload. It is concluded that the Li-ion cells were more hazardous at higher C-rates; when a larger preload was applied to the cell, it exhibited better tolerance to overcharge, but the TR reactions became more hazardous (i.e., higher temperature and more severe TR reactions). These findings allow us to propose a safety evaluation method to evaluate the TR risks and TR hazards for Li-ion cells under different restricted conditions. In addition, a safety management method was proposed based on the evolution of several key parameters. The research results can provide some useful information to the safety design of Li-ion pouch cells and the enhancement of the safety function of battery management system.

Chemical synthesized nano-Li3V2O5 anode for high-rate rechargeable Li-ion batteries at low temperature

Lithium-ion batteries (LIBs) are widely used and considered as an ideal power supply for different applications. However, severe power/energy loss and lithium dendrite growth at low temperatures are still major problems for the graphite-based LIBs. Here we develop a highly stable nano ω-Li3V2O5 anode (named n-LVO-H) with a suitable lithium embedding potential via a chemical method. Benefiting from a short Li+ transport path, the product n-LVO-H anode accelerates the diffusion of Li+ in the particles. In addition, a robust solid-electrolyte interface layer built on the n-LVO-H particles using 1,3-dioxolane (DOL) based electrolyte with low desolvation energy facilitates Li+ transport at the interface. Such an unconventional combination of nano-size anode material with DOL electrolyte renders the n-LVO-H cell high C-rate capacity (207.1 mAh g−1 at 10C), while enjoying cycling stability (92.93 % retention at 10C after 1000 cycles) and low-temperature tolerance (248.1 mAh g−1 at -20 °C and 0.2C) concurrently.

Scalable Interlayer Nanostructure Design for High-Rate (10C) Submicron Silicon-Film Electrode by Incorporating Silver Nanoparticles

High C-rate capability at 10C is a key performance indicator for the commercialization of the next-generation high-charging lithium microbattery. However, silicon (Si) anode satisfying the prerequisite high specific capacity suffers from poor electron/ionic conductivity, seriously limiting the 10C rate capability. Accordingly, we propose the strategy of inserting highly conductive silver nanoparticles (AgNPs) as an interlayer between two RF-sputtered amorphous Si thin films to form an Si/Ag/Si multilayered anode, with the density and spatial distribution of the AgNPs well-controlled by thermal evaporation. This strategy is exclusively beneficial to scale up film thickness for higher capacity. Without AgNPs, the 10C rate performance of the double-layer Si (D_Si) is worse than the single layer (S_Si) in the same total thickness, suggesting the adverse effect of the interface. However, this situation is progressively improved with the AgNPs density incorporated at the interface, where the densest AgNPs anode (D_SiAg3) demonstrated a noticeable improvement reaching 1250 mAh/g at 10 C with a 46% capacity retention rate. By scaling up to triple layers, T_SiAg3 performed the superior 10C rate capability to T_Si, testifying to the scalable potential of the unique design for boosting high-power batteries. Finally, with electrochemical impedance spectroscopy results, a possible mechanism to explain the enhancement in rate capability is subject to where Li-ion diffusion is accelerated by the charge-induced electric field condensing around the AgNPs. This design for a multilayered nanocomposite can contribute to the design and fabrication of high-charging batteries and battery-on-chip.

Enhanced Performance of KVPO4F0.5O0.5 in Potassium Batteries by Carbon Coating Interfaces

Potassium vanadium oxyfluoride phosphate of composition KVPO4F0.5O0.5 was modified by a carbon coating to enhance its electrochemical performance. Two distinct methods were used, first, chemical vapor deposition (CVD) using acetylene gas as a carbon precursor and second, an aqueous route using an abundant, cheap, and green precursor (chitosan) followed by a pyrolysis step. The formation of a 5 to 7 nm-thick carbon coating was confirmed by transmission electron microscopy and it was found to be more homogeneous in the case of CVD using acetylene gas. Indeed, an increase of the specific surface area of one order of magnitude, low content of C sp2, and residual oxygen surface functionalities were observed when the coating was obtained using chitosan. Pristine and carbon-coated materials were compared as positive electrode materials in potassium half-cells cycled at a C/5 (C = 26.5 mA g-1) rate within a potential window of 3 to 5 V vs K+/K. The formation by CVD of a uniform carbon coating with the limited presence of surface functions was shown to improve the initial coulombic efficiency up to 87% for KVPFO4F0.5O0.5-C2H2 and to mitigate electrolyte decomposition. Thus, performance at high C-rates such as 10 C was significantly improved, with ∼50% of the initial capacity maintained after 10 cycles, whereas a fast capacity loss is observed for the pristine material.

Discharge Behavior within Lithium–Sulfur Batteries Using Li–Glyme Solvate Ionic Liquids

Practical use of lithium–sulfur batteries can be realized by using “sparingly solvating electrolytes” such as [Li(G4)][TFSA] (G4, tetraglyme; TFSA, bis(trifluoromethanesulfonyl)amide) solvate ionic liquid, superconcentrated electrolyte solutions, and their hydrofluoroether-diluted electrolytes. On the other hand, the battery performance such as C-rate characteristics of lithium–sulfur batteries is different depending on the electrolyte used. In order to investigate the relationship between the discharge reaction and the battery performance for lithium–sulfur batteries with 1,1,2,2-tertafluoroethyl 2,2,3,3-tetrafluoropropyl ether-diluted [Li(G4)][TFSA], in situ electrochemical impedance spectroscopy has been conducted. During discharge at a high C-rate, a voltage drop was observed in the discharge curve at DOD = 20–35%. The charge transfer resistance of the lithium negative electrode remarkably increased when the voltage drop occurred. Moreover, this increase was only observed for in situ measurements. The charge transfer resistance of the negative electrode is related to the resistance of the Li+ ion dissolution/deposition reaction. During discharging at a high C-rate, the Li+ ion is abundant at the negative electrode interface owing to the changes in the Li+ ion solvation structure, suppressing the Li+ ion dissolution reaction.

Modeling the Impacts of Structural Heterogeneities on the Reaction-Transport Coupling in Porous Electrodes of Lithium-Ion Batteries Using Lattice Boltzmann Method

Structural optimization of porous electrodes has been a practical route to improve the performance of lithium-ion batteries (LIBs). Currently, this relies mainly on the structure-featureless pseudo-two-dimension (P2D) model in the theoretical aspect, which is inherently contradictory to the emphasis on the importance of electrode structure. Herein, a 2D pore-scale LIBs model based on the lattice Boltzmann method (LBM) and the galvanostatic simulation scheme are established. The model is used to investigate the effects of physical structures on the coupling between ions transport and electrochemical reactions in porous electrodes, and the results are compared with a P2D model on the same electrode. The results show that for battery systems composed of homogeneously distributed structures, the LBM model gives nearly identical results to that of the P2D model. However, for battery systems with heterogeneously structured electrodes, obvious difference from the prediction of P2D model are obtained, especially at high C-rates. The P2D model significantly underestimates the structure-sensitive transport-reaction coupling and the non-uniform utilization of active materials, even when using the physical tortuosity based on electrode structure. These results emphasize the significance of developing a pore-scale model of LIBs based on realistic physical structure for the design of LIBs with satisfactory performance.

Lithium-Ion Conductivity Epitaxial Layer Contributing to the Structure and Cycling Stability of LiMn2O4 Cathodes

A critical challenge in the commercialization of spinel lithium manganate materials is the bulk structure damage and capacity degradation due to manganese dissolution during battery operation. Hence, Li3PO4 with good ionic conductivity was homogeneously deposited on the surface of spinel lithium manganate by the sol–gel method, aiming to improve the stability and cycling of the spinel structure by an epitaxial layer. The structural and electrochemical characterizations reveal that the presence of the Li3PO4 epitaxial coating prevents the direct contact of the cathode surface and electrolyte, thus enhancing the stability of the structure and reducing the extent of manganese dissolution. Further studies have shown that the contribution of the Li3PO4 layer promotes the diffusion coefficient of lithium ions, which increases from 7.89 × 10–12 to 4.43 × 10–11 cm2 s–1. Meanwhile, the Li3PO4 coating regulates the concentration of oxygen vacancies on the surface of the spinel, effectively reducing the transfer resistance and increasing the electronic conductivity to 3.50 × 10–5 cm–1. Specifically, the discharge specific capacity of the 1.0 wt % Li3PO4-modified electrode is still 115.4 mAh g–1 after 300 cycles, with the capacity retention increased to 90.8%. Even at a high rate of 10C, it still demonstrates excellent rate capability, with the discharge specific capacity more elevated than that of the unmodified material at about 40 mAh g–1. This strategy can simultaneously solve the interfacial instability and bulk structure degradation, thus improving the electrochemical performance of lithium manganate materials.

ZnS-rGO/CNF Free-Standing Anodes for SIBs: Improved Electrochemical Performance at High C-Rate.

ZnS-graphene composites (ZnSGO) were synthesized by a hydrothermal process and loaded onto carbon nanofibers (CNFs) by electrospinning (ZnS-GO/CNF), to obtain self-standing anodes for SIBs. The characterization techniques (XRPD, SEM, TEM, EDS, TGA, and Raman spectroscopy) confirm that the ZnS nanocrystals (10 nm) with sphalerite structure covered by the graphene sheets were successfully synthesized. In the ZnS-GO/CNF anodes, the active material is homogeneously dispersed in the CNFs' matrix and the ordered carbon source mainly resides in the graphene component. Two self-standing ZnS-GO/CNF anodes (active material amount: 11.3 and 24.9 wt%) were electrochemically tested and compared to a tape-casted ZnS-GO example prepared by conventional methods (active material amount: 70 wt%). The results demonstrate improved specific capacity at high C-rate for the free-standing anodes compared to the tape-casted example (69.93 and 92.59 mAh g-1 at 5 C for 11.3 and 24.9 wt% free-standing anodes, respectively, vs. 50 mAh g-1 for tape-casted). The 24.9 wt% ZnS-GO/CNF anode gives the best cycling performances: we obtained capacities of 255-400 mAh g-1 for 200 cycles and coulombic efficiencies ≥ 99% at 0.5 C, and of 80-90 mAh g-1 for additional 50 cycles at 5 C. The results suggest that self-standing electrodes with improved electrochemical performances at high C-rates can be prepared by a feasible and simple strategy: ex situ synthesis of the active material and addition to the carbon precursor for electrospinning.

Overcoming the ultra-high C-rate limitation of LiFeBO3 with careful optimization of carbothermal synthesis conditions

Carbon coating with sucrose as the precursor under a moderate synthesis temperature of 500 °C results in a high-purity monoclinic LiFeBO3 phase. This material has a high specific discharge capacity of 200 mAh/g at 0.02 C, which is close to its theoretical capacity. However, its poor performance at high C-rates limits its utility in electric vehicles. Achieving ultra-high C-rate capability of more than 5 C for LiFeBO3 cathode for lithium-ion batteries is uncommon. To overcome this challenge, this study carefully optimizes the synthesis conditions, such as temperature and reducing atmosphere. In this work we utilize carbothermal synthesis method for modulating the reducing conditions. Usually, the carbothermal reaction is carried out using carbon at high temperatures which leads to the formation of CO and CO2 gases. In our work, we have utilized a mixture of sucrose and oxalic acid as the carbon source, that is intimately mixed with the other precursors to create moderate reducing conditions. Oxalic acid decomposes to generate CO which helps to attain the limited reducing atmosphere condition necessary to retain the low valence state of iron. This is important because the low valence state of iron is essential for a good rate-performance. However, if the amount of carbon source is too low, it can lead to poor rate-performance. On the other hand, if the amount of carbon source is too high, it can generate impurity phases, which can be detrimental to the material’s properties. The optimized sample can deliver electrochemical activity at ultra-high C-rates, even up to 30 C, which is achieved for the first time in this material. Comprehensive spectroscopic analyses reveal that multitude of factors, including improved particle crystallinity and better graphitization, favor the retention of the monoclinic phase during electrochemical cycling. A stable specific discharge capacity of 104 mAh/g at 10 C sets it as a benchmark for this material.

Electrospun Cellulose Nanofiber Membranes as Multifunctional Separators for High Energy and Stable Lithium-Sulfur Batteries

Although Li–S batteries (LSB) are one of the most promising electrochemical energy storage technologies, their practical applications are limited by their rapid capacity decay and uncontrolled lithium dendrite formation. In addition to the well-known role of a separator in lithium-ion batteries, LSB separators must perform additional functions. Using a facile electrospinning method, we developed an eco-friendly separator prepared from natural cellulose with an interconnected fibrous and porous structure rich in polar oxygen-containing functional groups. These polar functional groups enhance electrolyte wettability, polysulfide adsorption, and lithiophilicity, thus boosting LSB performance. A cellulose separator with a thickness (22 μm) comparable to that of a commercial polypropylene (Celgard) separator delivers an initial discharge capacity of 1458 mAh·g−1 at 0.1C with a high sulfur utilization of 87%, including a high reversible discharge capacity of 1091 mAh·g−1 for 100 cycles, exhibiting a 1.8-times greater capacity retention than those of Celgard-containing LSBs. In addition, an excellent rate capability of 908 mAh·g−1 can be achieved at a high rate of 1C. These intriguing characteristics indicate that these separators could replace conventional synthetic polymer-based separators for commercial LSBs.

BattX: An equivalent circuit model for lithium-ion batteries over broad current ranges

Advanced battery management is as important for lithium-ion battery systems as the brain is for the human body. Its performance is based on the use of fast and accurate battery models. However, the mainstream equivalent circuit models and electrochemical models have yet to meet this need well, due to their struggle with either predictive accuracy or computational complexity. This problem has acquired urgency as some emerging battery applications running across broad current ranges, e.g., electric vertical take-off and landing aircraft, can hardly find usable models from the literature. Motivated to address this problem, we develop an innovative model in this study. Called BattX, the model is an equivalent circuit model that draws comparisons to a single particle model with electrolyte and thermal dynamics, thus combining their respective merits to be computationally efficient, accurate, and physically interpretable. The model design pivots on leveraging multiple circuits to approximate major electrochemical and physical processes in charging/discharging. Given the model, we develop a multipronged approach to design experiments and identify its parameters in groups from experimental data. Experimental validation proves that the BattX model is capable of accurate voltage prediction for charging/discharging across low to high C-rates.

Elastomeric Electrolyte for High Capacity and Long-Cycle-Life Solid-State Lithium Metal Battery.

High room-temperature ionic conductivity and good compatibility with lithium metal and cathode materials are prerequisites for solid-state electrolytes used in lithium metal batteries. Here, the solid-state polymer electrolytes (SSPE) are prepared by combining the traditional two-roll milling technology with interface wetting. The as-prepared electrolytes consisting of elastomer matrix and high-mole-loading of LiTFSI salt show a high room temperature ionic conductivity of 4.6×10-4 S cm-1 , a good electrochemical oxidationstability up to 5.08V, and improved interface stability. These phenomena are rationalized with the formation of continuous ion conductive paths based on sophisticated structure characterization including synchrotron radiation Fourier-transform infrared microscopy, wide- and small-angle X-ray scattering. Moreover, at room temperature, the Li||SSPE||LFP coin cell shows a high capacity (161.5 mAh g-1 at 0.1 C), long-cycle-life (retaining 50% capacity and 99.8% Coulombic efficiency after 2000 cycles), and good C-rate compatibility up to 5 C. This study, therefore, provides a promising solid-state electrolyte that meets both the electrochemical and mechanical requirements of practical lithium metal batteries.

Modelling and experimental investigation of Nb2O5 as a high-rate battery anode material

Modelling and understanding the battery electrochemical performance at high rates is a great challenge. Known for its fast rate and good cyclability, niobium pentoxide (Nb2O5) is a promising anode material for lithium-ion batteries and is specifically modelled and investigated in this work. Commercially sourced Nb2O5 was characterised using scanning electron microscopy, X-ray diffraction, and micro-computed tomography. The Nb2O5 material was found to contain large rod- and ball-like polycrystalline particles of tens of microns in size and have mixed T-Nb2O5 and H-Nb2O5 phases. The electrochemical performance of the material after ball milling was tested via cyclic voltammetry and constant-current cycling at different C-rates up to 50C (10,000 mA g−1). The material achieved a similar charge capacity (143 mAh g−1) to T-Nb2O5 at 0.5C and this capacity could be retained by more than 55% when C-rate was increased to 10C. The experimental results were used to support the development of the Doyle-Fuller-Newman electrochemical model for Nb2O5. By model parameterization, the reference exchange-current density and solid-state diffusivity of the present Nb2O5 were estimated to be 9.6 × 10−4 A m−2 and 6.2 × 10−14 m2 s−1, respectively. The model achieved accurate prediction of the battery performance up to currents of 5C with the obtained constant properties. However, the properties of Nb2O5 were found to be rate-dependent at higher C-rates when good agreements between the model and experiment were maintained. The decrease of the two properties at 10−50C revealed that there was a change of dominant charge storage mechanism from diffusion-controlled lithium insertion to capacitive effects, which was experimentally observed in the cyclic voltammetry.

Review and Stress Analysis on the Lithiation Onset of Amorphous Silicon Films

This work aims to review and understand the behavior of the electrochemical lithiation onset of amorphous silicon (a-Si) films as electrochemically active material for new generation lithium-ion batteries. The article includes (i) a review on the lithiation onset of silicon films and (ii) a mechanochemical model with numerical results on the depth-resolved mechanical stress during the lithiation onset of silicon films. Recent experimental studies have revealed that the electrochemical lithiation onset of a-Si films involves the formation of a Li-poor phase (Li0.3Si alloy) and the propagation of a reaction front in the films. The literature review performed reveals peculiarities in the lithiation onset of a-Si films, such as (i) the build-up of the highest mechanical stress (up to 1.2 GPa) during lithiation, (ii) a linear increase in the mechanical stress with lithiation which mimics the characteristics of linear elastic deformation, (iii) only a minute volume increase during Li incorporation, which is lower than expected from the number of Li ions entering the silicon electrode, (iv) the largest heat generation appearing during cycling with only a minor degree of parasitic heat contribution, and (v) an unexpected enhanced brittleness. The literature review points to the important role of mechanical stresses in the formation of the Li-poor phase and the propagation of the reaction front. Consequently, a mechanochemical model consisting of two stages for the lithiation onset of a-Si film is developed. The numerical results calculated from the mechanochemical model are in good accord with the corresponding experimental data for the variations in the volumetric change with state of charge and for the moving speed of the reaction front for the lithiation of an a-Si film of 230 nm thickness under a total C-rate of C/18. An increase in the total C-rate increases the moving speed of the reaction front, and a Li-rich phase is likely formed prior to the end of the growth of the Li-poor phase at a high total C-rate. The stress-induced phase formation of the Li-poor phase likely occurs during the lithiation onset of silicon electrodes in lithium-ion battery.

Operando Calorimetry Investigation of Particle Size Effects on Heat Generation in Wadsley–Roth (W0.2V0.8)3O7-Based Electrodes

The Wadsley–Roth shear phase compound (W0.2V0.8)3O7 is a promising fast-charging electrode material with the ability to engage in multielectron redox reactions and to sustain high specific capacity under large currents. Here, solid-state methods and sol–gel methods combined with freeze-drying were used to synthesize (W0.2V0.8)3O7 microparticles and nanoparticles, respectively. Cyclic voltammetry and galvanostatic cycling demonstrated that electrodes made of (W0.2V0.8)3O7 nanoparticles have superior electrochemical performance compared to those made of microparticles, but the origin of this difference was not well understood. Here, entropic potential measurements performed at slow C-rate indicated that both nanoparticles and microparticles undergo a semiconductor to metal transition. However, the nanoparticles display a two-phase coexistence over a narrower range of compositions than the microparticles. Operando calorimetry measurements at high C-rate established that, regardless of particle size, the heat generation rate at the (W0.2V0.8)3O7 electrode increased with lithiation due to an increase in charge transfer resistance. The time-averaged irreversible heat generation rate was slightly but systematically smaller at the electrode made of nanoparticles. However, the specific dissipated energy and the contribution from enthalpy of mixing caused by the lithium concentration gradient were notably smaller for the (W0.2V0.8)3O7 nanoparticles. These observations were attributed to the fact that nanoparticles were less ionically resistive and able to accommodate more lithium while lithium-ion intercalation therein was more kinetically favorable.

Conformal Coating of a High-Voltage Spinel to Stabilize LiCoO2 at 4.6 V.

The ever-growing demand for portable electronic devices has put forward higher requirements on the energy density of layered LiCoO2 (LCO). The unstable surface structure and side reactions with electrolytes at high voltages (>4.5 V) however hinder its practical applications. Here, considering the high-voltage stability and three-dimensional lithium-ion transport channel of the high-voltage Li-containing spinel (M = Ni and Co) LiMxMn2-xO4, we design a conformal and integral LiNixCoyMn2-x-yO4 spinel coating on the surface of LCO via a sol-gel method. The accurate structure of the coating layer is identified to be a spinel solid solution with gradient element distribution, which compactly covers the LCO particle. The coated LCO exhibits significantly improved cycle performance (86% capacity remained after 100 cycles at 0.5C in 3-4.6 V) and rate performance (150 mAh/g at a high rate of 5C). The characterizations of the electrodes from the bulk to surface suggest that the conformal spinel coating acts as a physical barrier to inhibit the side reactions and stabilize the cathode-electrolyte interface (CEI). In addition, the artificially designed spinel coating layer is well preserved on the surface of LCO after prolonged cycling, preventing the formation of an electrochemically inert Co3O4 phase and ensuring fast lithium transport kinetics. This work provides a facile and effective method for solving the surface problems of LCO operated at high voltages.

Porous nanofibers comprising VN nanodots and densified N-doped CNTs as redox-active interlayers for Li–S batteries

A synthesis strategy to obtain hierarchically porous N-doped carbon (P–N–C) nanofibers (NFs) comprising VN nanodots and highly densified-entangled N-doped CNTs (N-CNTs) was visualized. The unique redox-active nanostructure was applied as a multifunctional interlayer for highly stable Li–S batteries (LSBs). The P–N–C skeleton obtained via selective removal of amorphous carbon (AC) and polystyrene (PS) not only guarantees efficient electrolyte percolation but also facilitates rapid diffusion of charged species during charge-discharge processes. Moreover, the N-CNTs constituting the fiber matrix increase the overall conductivity of the framework. Besides, highly conductive vanadium nitride (VN) nanodots act as chemisorption sites for efficient lithium polysulfide capturing and prohibit their diffusion towards the anode through efficient catalytic conversion, thus improving the elemental S utilization. The Li–S cells utilizing P–N–C@VN/N-CNT NF as a multifunctional barrier resulted in decent rate capabilities (559 mA h g−1 at 2.0 C) and long-term cycling stabilities at low and high C-rates (432 mA h g−1 after the 400th cycle at 0.1 C and 290 mA h g−1 at the end of 600th cycle at 1.0 C). This is attributed to the combined synergistic effects of the P–N–C framework, N-CNT matrix, and polar VN nanodots.

Effect of a Weak Coordination Solvent on a Kinetically Favorable Electrode Reaction in Concentrated Lithium-Ion Battery Electrolytes

We present highly concentrated electrolytes prepared with a fluorinated acetate solvent that has much weaker coordination power, allowing for a kinetically favorable electrode reaction in lithium (Li)-ion batteries (LIBs). Among the concentrated electrolytes using conventional organic solvents, the concentrated electrolyte containing a 2,2,2-trifluoroethyl acetate (TFEAc) solvent and 3.2 mol dm–3 LiFSA salt (FSA: bis(fluorosulfonyl)amide) had the lowest ionic conductivity but a wide electrochemical window (∼5 V), allowing for a LIB electrode reaction. Compared to an analogous electrolyte containing LiTFSA salt (TFSA: bis(trifluoromethanesulfonyl)amide), the concentrated LiFSA/TFEAc electrolyte demonstrated significantly improved charge–discharge behavior with a high C-rate performance on the graphite negative electrode. The activation energy (Ea) of the graphite electrode reaction (i.e., Li+-insertion reaction) was determined experimentally to be Ea = 19.9 kJ mol–1 in the concentrated LiFSA/TFEAc electrolyte, which was significantly lower than that reported previously for dilute carbonate-based electrolytes. Furthermore, a combination of high-energy X-ray total scattering experiments and all-atom molecular dynamics simulations revealed that Li ions are co-coordinated with TFEAc molecules and FSA anions to form intricate ion-ordered complexes in the bulk solution with no free solvent and counteranion. From a structural viewpoint, we discussed the characteristic Ea in the current concentrated electrolyte system and concluded that the weak interaction between Li-ion and TFEAc remaining in the activation state plays a key role in triggering energetically easier decoordination during the charge transfer process, achieving a kinetically unique graphite electrode reaction.