Irreversible Side Reactions Research Articles

Enhanced Long-Term Stability of Organic Electrode Materials by a Trap Filler Strategy.

The sensitivity of organic electrode materials to water and oxygen has long been the bottleneck of their further development. The residual and penetrative water and oxygen in electrochemical cells form electron traps that trigger irreversible side reactions, which is detrimental to their long-term stability. A trap filler strategy by introducing molecules with low ionization energy in a cell, bis(pentamethylcyclopentadienyl)cobalt(II) (DMC) as an example, is demonstrated to deactivate traps spontaneously by donating electrons to traps without causing undesirable reactions with electrode materials. The electrode materials BthCz and AQCz, with lowest unoccupied molecular orbital levels above or near the electron traps (-3.6 to -3.8 eV), exhibit conspicuous stability increment of 68.6 and 26.3%, respectively, with the optimized DMC concentration of 5 × 10-4 M in acetonitrile electrolyte.

Effect of Fluorine Substitution on the Electrochemical Property and Structural Stability of a Lithium-Excess Cation Disordered Rock-Salt CathodeSupported by National Natural Science Foundation of China (Grant Nos. 51602191 and 52072233), and the Beijing National Laboratory for Condensed Matter Physics.

Lithium-excess cation disordered rock-salt materials have received much attention because of their high-capacity as a candidate for cathodes for lithium-ion batteries. The ultra-high specific capacity comes from the coordinated charge compensation of both transition metal and lattice oxygen. However, the oxygen redox at high voltage usually leads to irreversible oxygen release, thereby degrading the structure stability and electrochemical performance. Lithium-excess Li1.14Ni0.57+0.5 x Ti0.19 – 0.5 x Mo0.10O2 – x F x (x = 0, 0.05, 0.10, 0.15, and 0.20) with different amounts of fluorine substitution were synthesized. Among them, Li1.14Ni0.620Ti0.140Mo0.10O1.85F0.15 exhibits a lower capacity decline, better rate performance, and lower structure damage. The effects of fluorine substitution on the electrochemical property and structural stability were systematic studied by x-ray photoelectron spectroscopy and in situ XRD etc. Results show that fluorine substitution reduces the average valence of the anion, allowing a larger proportion of low-valent redox active transition metals, increasing the transition metal redox capacity, inhibiting irreversible oxygen release and side reaction. Fluorine substitution further improves the structural stability and suppresses lattice deformation of the material.

Unraveling gas evolution in sodium batteries by online electrochemical mass spectrometry

Identification of gaseous decomposition products from irreversible side-reactions enables understanding of inner working of rechargeable batteries. Unlike for Li-ion batteries, the knowledge of the gas-evolution processes in Na-ion batteries is limited. Therefore, in this study, we have performed online electrochemical mass spectrometry to understand gassing behavior of model electrodes and electrolytes in Na-ion cells. Our results show that a less stable solid–electrolyte interphase (SEI) layer is developed in Na-ion cells as compared with that in Li-ion cells, which is mainly caused by higher solubility of SEI constituents in Na-electrolytes. Electrolyte reduction on the anode has much larger contribution to the gassing in the Na-ion cells, as gas evolution comes not only from direct electrolyte reduction but also from the soluble species, which migrate to the cathode and are decomposed there. During cell cycling, linear carbonates do not form an SEI layer on the anode, resulting in continuous electrolyte reduction, similar to Li-ion system but with much higher severity, while cyclic carbonates form a more stable SEI, preventing further decomposition of the electrolyte. Besides the standard electrolyte solvents, we have also assessed effects of several common electrolyte additives in their ability to stabilize the interphases. The results of this study provide understanding and guidelines for developing more durable electrode–electrolyte interphase, enabling higher specific energy and improved cycling stability for Na-ion batteries.

Ion-exchange surface modification enhances cycling stability and kinetics of sodium manganese hexacyanoferrate cathode in sodium-ion batteries

Sodium manganese hexacyanoferrate (NaMnHCF) with low cost and high energy density is a promising cathode in Na-ion batteries (NIBs); however, the Jahn-teller effect of Mn3+ and large amounts of Fe(CN)64− defects significantly reduces the cycling life of the battery. Here, we introduce a chemically less soluble and structurally more stable layer to the NaMnHCF surface with a simple ion-exchange method to mitigate the irreversible structural change and side reactions associated with both the low-spin FeII/FeIII and the high-spin MnII/MnIII redox. The NaMnHCF cathode treated in Cu2+ solution shows much better cycling stability (80 vs. 30 mA h g−1 after 1000 cycles at 1 C) and rate performance than the unmodified NaMnHCF. The as-proposed chemical ion-exchange is a well-compatible and low-cost technology to realize surface modification of NaMnHCF and other hexacyanoferrates, which is of great significance for the development of high-performance NIB cathodes.

The roles of electrolyte chemistry in hard carbon anode for potassium-ion batteries

Potassium-ion battery (PIB) is a rising star in the rechargeable battery field due to its potential low cost and high energy for large-scale applications. Hard carbon (HC) is one of the most popular anodes for practical PIBs due to its high K-ion storage and relatively low material cost. However, the role of electrolytes in determining the HC performance and K-ion storage mechanism has been rarely investigated. Herein, we systematically studied the influence of four electrolyte systems, i.e., two K salts (i.e., KPF6 and KFSI) in carbonate ester and ether solvents, on the K-ion mobility, cycling stability, and charge transfer kinetics of the HC anode in PIBs. It is found that HC anode achieved the best cycling stability and kinetics performance in the KFSI EC/DEC electrolyte. Mechanismic study disclosed that the improved performance could be ascribed to the formation of robust KF-rich SEI resulting from FSI- decomposition, which effectively prevented irreversible side reactions and severe structural decay (e.g., exfoliation and pulverization). The degradation mechanisms of other electrolyte systems are also explained from the viewpoints of SEI formation and solvation/desolvation effect. It is expected that this work will provide guidance on the anode and electrolyte selection and design for PIBs in the near future.

Stabilizing zinc deposition with sodium lignosulfonate as an electrolyte additive to improve the life span of aqueous zinc-ion batteries

Thanks to high safety and low cost, rechargeable zinc-ion batteries (RZIBs) have become a promising candidate for grid-scale energy storage systems. However, zinc anodes suffer from severe dendrite growth and irreversible side reactions, leading to poor cyclability of RZIBs. In this work, low-cost sodium lignosulfonate (SL) is utilized as the electrolyte additive to solve this problem. The added amount of SL is optimized to be 0.02%, which enables the Zn//α-MnO2 battery to deliver a large capacity of 146 mAh g−1 after 1000 cycles at 1 A g−1, corresponding to a high capacity retention of 83.5%. The Zn//Zn symmetric cell with the modified electrolyte also shows excellent cyclability even under a commercial level of areal specific capacity (4 mAh cm−2). Overall, the results of this study confirm that the SL additive can improve the ionic conductivity of electrolyte, restrict the two-dimensional planar diffusion of Zn2+ ions at the electrode/electrolyte interface, lower the nucleation overpotential of Zn2+ ions, prevent side reactions, and inhibit the corrosion of Zn metal. Therefore, the dendrite growth and byproduct formation can be effectively suppressed. This study provides new insights into protecting metal electrodes of electrochemical energy storage devices.

Bonding the Terminal Isocyanate-Related Functional Group to the Surface Manganese Ions to Enhance Li-Rich Cathode's Cycling Stability.

Capacity fading of Li-rich cathodes in the cycling process is mainly caused by the irreversible side reactions at the interface of electrode and electrolyte by reason of the lack of a corrosion resistant surface. In this work, isocyanate-related functional groups (-N═C═O groups and polyamide-like groups) were tightly bonded on the surface of Li-rich oxides through a urea decomposition gas heat-treatment. The surface isocyanate functionalization inhibits the side reaction of PF5 hydrolysis to give LixPFyOz and HF species at the surface of Li-rich materials in the cycle process. As compared to the untreated Li-rich sample U0, the samples with the spinel-like layer and isocyanate functionalized surface exhibited an enhanced cycle stability. The capacity retention of the treated sample U3 reached as high as 92.6% after 100 cycles at the current density of 100 mA/g, larger than 66.8% for the untreated sample. Even at a higher current density of 1000 mA/g, sample U3 gives a capacity retention of 81.7% after 300 cycles. The findings of this work reveal the importance of surface isocyanate functionalization in restraining the surface side reactions and also suggest an effective method to design Li-rich cathode materials with better electrochemistry performance.

Co-Estimation of State-of-Charge and State-of- Health for Lithium-Ion Batteries Using an Enhanced Electrochemical Model

Real-time electrochemical state information of lithium-ion batteries attributes to a high-fidelity estimation of state-of-charge (SOC) and state-of-health (SOH) in advanced battery management systems. However, the consumption of recyclable lithium ions, loss of the active materials, and the interior resistance increase resulted from the irreversible side reactions cause severe battery performance decay. To maintain accurate battery state estimation over time, a scheme using the reduced-order electrochemical model and the dual nonlinear filters is presented in this article for the reliable co-estimations of cell SOC and SOH. Specifically, the full-order pseudo-two-dimensional model is first simplified with Padé approximation while ensuring precision and observability. Next, the feasibility and performance of SOC estimator are revealed by accessing unmeasurable physical variables, such as the surface and bulk solid-phase concentration. To well reflect battery degradation, three key aging factors including the loss of lithium ions, loss of active materials, and resistance increment, are simultaneously identified, leading to an appreciable precision improvement of SOC estimation online particular for aged cells. Finally, extensive verification experiments are carried out over the cell's lifespan. The results demonstrate the performance of the proposed SOC/SOH co-estimation scheme.

Parameter identification of fractional‐order model with transfer learning for aging lithium‐ion batteries

The aging of lithium-ion batteries (LiBs) is inevitable during their operation owing to their irreversible side reactions. It is practical to capture only the dominant physicochemical processes with a physics-based model for engineering applications, as the degradation mechanism of LiBs is complex and interconnected. Numerous factors dramatically affect the performance of LiBs; thus, it is necessary to use the real-time operational data to estimate the model parameters online. The combination of the degradation mechanism and real-time data can effectively reduce the complexity of the parameter identification, and offers practicality from the perspective of engineering applications. In this study, a transfer learning method based on a back propagation neural network (BPNN) is introduced as the data-driven parameter identification method for the physics-based fractional-order model (FOM) of the LiBs. Accelerated aging tests are designed for commercial LiBs to reduce the experimental time, and the reference performance tests under different aging states are implemented to capture the degradation modes with increasing cycle numbers. The initial key parameters of the fresh FOM are estimated based on the BPNN with two hidden layers. The model-based transfer learning is proposed to estimate the key parameters of the aging FOM. The parameter identification results at different aging states are validated under various working loads. The good fit with the experimental data indicated that the parameters can be identified accurately using the transfer learning with BPNN. The outstanding results show the effectiveness of the parameter identification method based on transfer learning.

Examining CO2 as an Additive for Solid Electrolyte Interphase Formation on Silicon Anodes

We demonstrate that the addition of CO2 to a standard 1.0 M LiPF6 3:7 wt% ethylene carbonate:ethyl methyl carbonate electrolyte results in the formation of a thinner insoluble solid electrolyte interphase (SEI) that is dominated by the presence of LiF. In contrast, cells without CO2 result in a thicker insoluble SEI layer containing more organic constituents. The CO2 is incorporated in the dimethyl carbonate soluble part of the SEI composed primarily of polymeric poly(ethylene oxide) (PEO) on the surface of a thin inorganic layer. This combination of properties from CO2 addition provides an improved cycling performance through the reduction of irreversible side reactions, leading to higher coulombic efficiency. The results indicate that CO2 incorporates into the SEI and plays a role similar to additives like fluorinated ethylene carbonate and vinylene carbonate with respect to polymeric components.

Enhancing the structure stability of Ni-rich LiNi0.6Co0.2Mn0.2O2 cathode via encapsulating in negative thermal expansion nanocrystalline shell

Ni-rich cathode materials have attracted considerable attention due to the high electrochemical capacities and prominent price advantage, while their extensive applications are still impeded by the poor structure stability induced by the intrinsic Li+/Ni2+ cation mixing and mechanical stress accumulation upon cycling. To address these issues, a typical negative thermal expansion (NTE) material ZrV2O7 (ZVO) is firstly employed as coating layer on LiNi0.6Co0.2Mn0.2O2 (NCM622) particles to promote the electrochemical performances through enhancing the structure stability upon repeated cycling and operating at high temperature. The [email protected] wt% ZVO sample exhibits excellent cyclability (71.0% capacity retension after 500 cycles under 25 °C and 67.0% capacity retension after 200 cycles under 55 °C) and rate capability (60.7% at 5 C). Further in-situ XRD analysis confirms that the structure stability of NCM622 could be prominently stabilized both under electrochemical and thermal treatments after ZVO decoration. Intensive investigation implies the unique functionalities of ZVO in restraining the stress accumulation and volume expansion, thus contributing to the bulk structure stability and interface compatibility of modified NCM622, besides the traditional effect of surface deposition layers (physical separation between solid electrode/electrolyte and inhibited irreversible surface side reactions).

Evaluation of Emissions of 2-Amino-2-Methyl-1-Propanol Degradation Products by adding Degradation Reactions to the Carbon Dioxide Capture Unit

Degradation is a major problem which poses lots of emission risk during chemical absorption process with amine solvents. Degradation occurs through irreversible side reactions with CO2 and other flue gas components, forming into products that cannot easily regenerate. The degradation products then react with amines to form thermally stable salts, which accumulate in the system over time. The problems associated with degradation include decreased plant equipment life, foaming, corrosion, high solution viscosity, and increased operating cost. Amines capture about 70 90% CO2 from commercial power stations. These high removal rates have many environmental impacts due to their degradation products. Researchers have therefore shown interest in characterising and quantifying atmospheric emissions of amines and their degradation products. In this study, 2-Amino-2-Methyl-1-Propanol (AMP) degradation reactions were included into a largescale capture plant model to evaluate the influence of process variables, the emissions of AMP and its degradation products. Steadystate simulations were performed using Aspen Plus® V8.4 software to provide a full assessment of the degradation products and their impact on the capture process. This assessment is important because it identifies and quantifies all pollutants emitted from the process plant. The results of the simulation indicate that AMP emissions are 3.04E+03mg/Nm3 of CO2 lean flue gas, while the quantity of AMP lost due to degradation was 37.88kg/s for the largescale capture plant. The results further showed that among the gases emitted, ammonia was highest, while acetone was the highest gas formed. In this study, 2-amino-2-methyl-1-propanol (AMP) degradation reactions were included into a largescale carbon dioxide (CO2) capture plant model to evaluate the influence of process variables, AMP emissions and its degradation products. Steadystate simulations were performed using Aspen Plus® V8.4 software to provide a full assessment of the degradation products and their impact on the largescale AMP capture process. The results of the equilibrium model developed in this study revealed that AMP emissions are 3.04E+03mg/Nm3 of CO2 lean flue gas, while the quantity of AMP lost due to degradation was 37.88kg/s for the largescale capture plant. More importantly, the emissions obtained from the PWOD and PWD are 7.80E+03 mg/Nm3 and 9.82E+03 mg/Nm3 of CO2 respectively.
 Keywords: oxidative degradation, 2amino2methyl1 propanol, emissions, modelling

Porous Si-C Composite As High-Performance Anode Material for Li-Ion Batteries

Li-ion batteries (LIBs) have received widespread recognition as power sources for various potable electronics, electric vehicles, and large-scale energy storage systems. To meet ever-increasing demand for energy storage, it is necessary to develop next generation anode materials with a high capacity. Carbon-based materials are widely used as anodes for LIB and however, the theoretical capacity for graphite (372 mAh g–1) is limited due to its intercalation chemistry. Si has been considered as one of the most promising next generation materials as LIB anodes to replace commercial graphite because of its outstanding theoretical gravimetric capacity of 3580 mAh g–1 (for Li15Si4 phase), which can be obtained through the electrochemical alloying reaction with Li ion. Currently, the main challenges for using Si-based active materials are structural transition, low electrical conductivity, and unstable solid electrolyte interphase (SEI) formation caused by the large volume change (~400%) during alloying and dealloying cycle and irreversible side reactions with electrolyte. The large volume changes lead to the pulverization of electrode materials, resulting in degradation of capacity upon cycling. To overcome these drawbacks, many researches have been performed and various strategies have been proposed in terms of structural aspects such as hollow sphere, core-shell structure, and porous structure. In this study, porous Si-C composites were prepared through a simple annealing method of commercial amorphous silica (a-SiO2) and magnesium silicide (Mg2Si). Then, a chemical wet etching process was performed to remove formed magnesium oxide (MgO) and a carbon layer coating was finally carried out. The prepared material exhibited high reversible capacity and good cycle performance with very high initial coulombic efficiency. The enhanced electrochemical properties can be attributed to the porous structure with thin carbon coating.

Characteristics of Gel Electrolyte Composed of PEO-Based Polymer / Solvated Ionic Liquid for High Performance Li-S Batteries

IntroductionLithium-sulfur (Li-S) batteries as new secondly battery system type are attracted attention. The sulfur positive electrode of this system have theoretical capacity of 1672 mAhg-1, which shows approximately 10 times higher than conventional Li-ion battery positive electrode material (LiCoO2). The problem of this Li-S batteries is dissolution of sulfur component of positive electrode which reached a state of reaction intermediate (Li2S m : m=2~8) into electrolyte during charge / discharge processes. The dissolution and diffusion of Li2S m into electrolyte solution lead significant degradation of cycle characteristics and irreversible side reaction of Li-S batteries. Currently, as functional electrolyte solution that suppresses dissolution of Li2S m , “Solvate Ionic Liquid (SIL)” has been proposed [1]. SIL is equimolar mixed electrolyte of glyme solvent and Li salt, and has strongly interaction between glyme / Li+ to form complex cation. SIL was suppressed dissolution of Li2S m because of weak Lewis acidity / basesity. By application of SIL electrolyte, Li-S battery exhibited stable charge / discharge reaction more than 800 cycles [2]. However, equimolar composition of SIL was destroyed at electrolyte / electrode interface during charge and discharge, and free glyme which have not coordinated Li+ is temporary formed. Thus, it is dynamically possible that Li2S m dissolution into generated free glyme [3]. To solve this problem, we attempted increasing carrier cation (suppression of free glyme generation), solidification of SIL electrolyte by introduction of polymer matrix (diffusion suppression of dissolved chemical species).ExperimentalIn this study, all procedures of sample preparation were conducted in argon-filled glovebox. At first, as highly concentrated SILs, [Li a (G4)1]TFSA a (a = 1 and 1.25) (G4 : CH3-O-(C2H4O)4-CH3, LiTFSA: LiN(SO2CF3)2 = 1 : a) were prepared [4]. Gel electrolyte were prepared by mixing of SILs (a = 1, 1.25), polyether-based P(EO/PO) macromonomer and photo initiator. Composition ratio of gel electrolyte were SILs : polymer = 7 : 3, 8 : 2, 9 : 1 (wt%), respectively. After stirring, samples casted to glass substrate and polymerized by UV irradiation to obtain thin-films, which was transparent and self-standing gel electrolyte films. Prepared electrolyte films ware evaluated by differential scanning calorimetry (DSC) and AC impedance measurements.Result and discussionFig.1(a) show DSC thermograms of prepared liquid and solid electrolytes. All electrolytes exhibited glass transition temperatures (T gs). Gel electrolyte showed lower T g than that neat polymer electrolyte (without SIL). From the results, it was considered that mobility of PEO chain was increased by SIL electrolyte solutions. With increasing SIL amount into the gel electrolyte, T g appeared low temperature side, and therefore, the gel electrolyte with high SIL composition should indicate high ionic conductivity.Fig.1(b) shows temperature dependence of the ionic conductivity (σ) of gel electrolytes, respectively. Owing to increase of viscosity with high Li salt concentration, σ value of high Li salt composition SIL electrolyte (a=1.25) was lower than equimolar SIL (a=1). At 303 K, σ of [Li1(G4)1]TFSA1 : polymer = 8 : 2 electrolyte was 1.5×10−3 Scm−1, which was almost same σ value with SIL electrolyte solution.Fig.1(c) shows charge/discharge curves of Li-S cell using gel electrolyte of [Li1(G4)1]TFSA1 : polymer = 8 : 2. The Li-S battery using the equimolar SIL of gel electrolyte was confirmed first discharge capacity of 1100 mAhg-1. However, this system showed unstable voltage profiles during charge process, and this charge capacity exhibited larger than that discharging capacity. From the unstable and large voltage in this charging process, Li2S m was dissolved into gel electrolyte and side reactions occurred, was considered. This result correlates with the report that the co-migration of glyme molecule and Li cation is destroyed in when PEO matrix introduction by NMR spectroscopy method measurement [5].

Radical polymer-grafted carbon nanotubes as high-performance cathode materials for lithium organic batteries with promoted n-/p-type redox reactions

Here we present the covalently grafting organic radical polymer, poly(2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl methacrylate) (PTMA), onto the surface of multi-walled carbon nanotubes (MWNT) via free radical polymerization, affording MWNT-g-PTMA. The free-standing MWNT-g-PTMA-based electrode exhibits a high specific capacity of 262.9 mAh g−1 and a >60% capacity retention (144 mAh g−1) after 250 cycles, superior to that of physically mixed MWNT/PTMA electrode. Investigating the charge/discharge process by ex situ electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS), the excellent electrochemical performance of MWNT-g-PTMA-based electrode is attributed to the mutual participation of both p- and n-type redox reactions of PTMA with both cationic and anionic ions insertion/extraction into/from the electrode. Moreover, the capacity fading mainly results from irreversible side reactions during the n-type doping process of PTMA as investigating the charge/discharge process in two separate voltage ranges. This study may contribute to the understanding of energy storage mechanisms of radical polymers in the right way.

Electrolyte for Improved Durability of Dynamic Windows Based on Reversible Metal Electrodeposition

Summary Electrochromic, “smart” windows allow users to control light and heat flow into and out of buildings or vehicles without sacrificing their view. Despite the appeal of dynamic windows, they have yet to achieve extensive commercialization due to their inability to simultaneously achieve fast, color-neutral tinting over a wide optical dynamic range at a low cost. Reversible metal electrodeposition (RME) is a promising alternative that addresses the drawbacks of existing technologies. Here, we study several possible RME electrolytes at various pHs with different supporting anions. We identify that acidic perchlorate electrolytes permit fully reversible metal electrodeposition without harming the substrate or introducing irreversible side reactions, which was not seen among all other electrolytic compositions. This perchlorate electrolyte shows promising long-term durability, demonstrating 10,000 stable cycles with no evidence of electrode etching. In addition, we construct 15 × 15 cm2 dynamic windows that tint uniformly with fast, color-neutral switching (>60% contrast in

3D assembly of MXene-stabilized spinel ZnMn2O4 for highly durable aqueous zinc-ion batteries

Although spinel ZnMn2O4 (ZMO) has been regarded as a potential cathode material for aqueous zinc-ion batteries (ZIBs), the unsatisfactory long-term cycling stability seriously restricts its commercial applications. To overcome this obstacle, it is urgent to clarify the energy storage mechanism and cycling degradation reason of spinel ZMO upon Zn2+ insertion/extraction. Herein, the phase and structure evolutions of spinel ZMO are deeply probed by means of in-situ and ex-situ investigations, which is closely related to the reversible phase transformation between spinel ZMO and MnO2 during charging-discharging, while irreversible formation of inactive ZnO byproduct could cause the capacity fading after repeated cycles. Guided by the clear electrochemical mechanism, a 3D assembly of Ti-MXene (Ti3C2Tx)-stabilized ZMO nanoparticles has been designed and synthesized, in which high-conductive Ti3C2Tx scaffold can effectively inhibit the irreversible structural degradation and side reaction of spinel ZMO. As a result, the ZMO@Ti3C2Tx composite cathode exhibits a large reversible specific capacity, excellent rate capability and long-term cyclic stability (capacity retention of ~92.4% after 5000 cycles), superior than previously reported ZMO-based cathodes in aqueous ZIBs. For real applications, a kind of flexible aqueous ZIBs are fabricated and represent stable electrochemical performance at various deformation states, indicating their potential applications in portable/wearable electronics.

Porous Si-C Composite As High-Performance Anode Material for Li-Ion Batteries

Li-ion batteries (LIBs) have received widespread recognition as power sources for various potable electronics, electric vehicles, and large-scale energy storage systems. To meet ever-increasing demand for energy storage, it is necessary to develop next generation anode materials with a high capacity. Carbon-based materials are widely used as anodes for LIB and however, the theoretical capacity for graphite (372 mAh g–1) is limited due to its intercalation chemistry. Si has been considered as one of the most promising next generation materials as LIB anodes to replace commercial graphite because of its outstanding theoretical gravimetric capacity of 3580 mAh g–1 (for Li15Si4 phase), which can be obtained through the electrochemical alloying reaction with Li ion. Currently, the main challenges for using Si-based active materials are structural transition, low electrical conductivity, and unstable solid electrolyte interphase (SEI) formation caused by the large volume change (~400%) during alloying and dealloying cycle and irreversible side reactions with electrolyte. The large volume changes lead to the pulverization of electrode materials, resulting in degradation of capacity upon cycling. To overcome these drawbacks, many researches have been performed and various strategies have been proposed in terms of structural aspects such as hollow sphere, core-shell structure, and porous structure. In this study, porous Si-C composites were prepared through a simple annealing method of commercial amorphous silica (a-SiO2) and magnesium silicide (Mg2Si). Then, a chemical wet etching process was performed to remove formed magnesium oxide (MgO) and a carbon layer coating was finally carried out. The prepared material exhibited high reversible capacity and good cycle performance with very high initial coulombic efficiency. The enhanced electrochemical properties can be attributed to the porous structure with thin carbon coating.

Hydrogen Sorption and Reversibility of the LiBH4-KBH4 Eutectic System Confined in a CMK-3 Type Carbon via Melt Infiltration

Metal borohydrides have very high hydrogen densities but their poor thermodynamic and kinetic properties hinder their use as solid hydrogen stores. An interesting approach to improve their functionality is nano-sizing by confinement in mesoporous materials. In this respect, we used the 0.725 LiBH4–0.275 KBH4 eutectic mixture, and by exploiting its very low melting temperature (378 K) it was possible to successfully melt infiltrate the borohydrides in a mesoporous CMK-3 type carbon (pore diameter ~5 nm). The obtained carbon–borohydride composite appears to partially alleviate the irreversibility of the dehydrogenation reaction when compared with the bulk LiBH4-KBH4, and shows a constant hydrogen uptake of 2.5 wt%–3 wt% for at least five absorption–desorption cycles. Moreover, pore infiltration resulted in a drastic decrease of the decomposition temperature (more than 100 K) compared to the bulk eutectic mixture. The increased reversibility and the improved kinetics may be a combined result of several phenomena such as the catalytic action of the carbon surface, the nano-sizing of the borohydride particles or the reduction of irreversible side-reactions.

A novel quantitative electrochemical aging model considering side reactions for lithium-ion batteries

A novel quantitative electrochemical aging model for lithium-ion batteries considering side reactions is proposed in this paper. The resistance of solid electrolyte interphase and the thickness of deposited layer caused by side reactions are utilized as degradation representatives to explicitly quantify the aging effects. The aging model is established through deriving the transfer function relationship between the aging representatives and input current history. Therefore, the gap between macroscopic (battery operating mode) and microscopic (aging mechanism) can be well bridged. The aging mechanisms for the lithium-ion batteries are well identified by comprehensive post-mortem analysis. The experimental results demonstrate that the irreversible side reactions occurring at the surface of anode particles are the primary cause for performance degradation in this study. To verify the proposed aging model, the comparisons are made between experimental and simulated results at both macroscopic cell-level (cell voltage response, capacity fade, and solid-electrolyte interphase resistance increase) and microscopic-level (deposited-layer growth). The capacity decay error is bounded to 3% up to 400 cycles. The results demonstrate that the presented transfer-function type aging model is capable of predicting battery degradation severity precisely.