Silyl-group functionalized organic additive for high voltage Ni-rich cathode material

To allow stable cycling of layered nickel-rich cathode material at high voltage, silyl-functionalized dimethoxydimethylsilane is proposed as a multi-functional additive. In contrast to typical functional additive, dimethoxydimethylsilane does not make artificial cathode-electrolyte interfaces by electrochemical oxidation because it is quite stable under anodic polarization. We find that dimethoxydimethylsilane mainly focuses on scavenging nucleophilic fluoride species that can be produced by electrolyte decomposition during cycling, leading to improving interfacial stability of both nickel-rich cathode and graphite anode. As a result, the cell cycled with dimethoxydimethylsilane-controlled electrolyte exhibits 65.7% of retention after 100 cycle, which is identified by systematic spectroscopic analyses for the cycled cell.     

Ethylene sulfate as film formation additive to improve the compatibility of graphite electrode for lithium-ion battery

Ethylene sulfate (DTD) is investigated as a novel film formation electrolyte additive for graphite anode material in lithium-ion battery. The CV results reveal that DTD is reduced prior to ethylene carbonate (EC) at the interface between graphite and electrolyte, while it cannot prevent the sustained reduction of propylene carbonate (PC) when the amount of DTD is lesser than 3 wt% in the PC-based electrolyte. XPS analyses demonstrate that the reduction products of DTD, Li₂SO₃, and ROSO₂Li are formed at the surface of graphite in the EC-based electrolyte, which is beneficial to lower the interfacial resistance as suggested by the EIS results. In addition, SEM images show a smoother and homogeneous surface film at the surface of graphite when DTD is incorporated into the electrolyte. Consequently, the Li/graphite half cells cycled in EC-based electrolyte containing DTD exhibit higher specific capacity and improved cycling capability than that without DTD.     

Design of ionic liquid-based hybrid electrolytes with additive for lithium insertion in graphite effectively and their effects on interfacial properties

To address the challenge of the IL-based electrolyte cannot be effectively intercalated in graphite anode, and especially the urgent needs for the compatibility between high performance and high security, the IL-based hybrid electrolyte systems with ethylene carbonate/propylene carbonate (EC/PC) as a co-solvent and vinylene carbonate (VC) as an additive were designed. The high dielectric constant of EC/PC significantly increased the ionic conductivity and lithium ion migration of the electrolyte system. Meanwhile, the presence of VC can form SEI preventing EC and PYR₁₄⁺ reductive decomposition on the electrode interface, and at the same moment, the SEI promotes effective Li cation insertion into the graphene interlayer. The Li/C half-cells showed high reversible capacity, cycling efficiency, and good cycle stability with the IL-based hybrid electrolyte. It is worth to highlight the better performance, in terms of the excellent thermal stability and high safety. Thus, the IL-based hybrid electrolyte combined with good electrochemical performance holds substantial promise for lithium-ion battery, and should have broad application prospects in the high energy density, especially high-security requirements, of the new lithium-ion battery.     

Effect of fluoroethylene carbonate as an electrolyte additive on the cycle performance of silicon-carbon composite anode in lithium-ion battery

The cycling performance of silicon-carbon anodes in the electrolyte with different content (0, 2, 5, 10 wt%) fluorinated ethylene carbonate (FEC) was studied. Among all the electrolytes the injection of 2 wt% FEC into carbonate electrolyte, the retention capacity of silicon carbon anode enhanced from 54.81 to 83.82% after 50 cycles. The performance of SEI layer on the anode was characterized by SEM, EIS, FTIR, and XPS analysis. These studies reveal that the SEI layer formed in the FEC-containing electrolyte effectively reduced the capacity loss of the material and reduced the interfacial impedance.     

Synergistic film-forming effect of oligo(ethylene oxide)-functionalized trimethoxysilane and propylene carbonate electrolytes on graphite anode

Oligo(ethylene oxide)-functionalized trialkoxysilanes can be used as novel electrolytes for high-voltage cathode, such as LiCoO₂ (4.35 V) and Li_1.2Ni_0.2Mn_0.6O₂ (4.6 V); however, they are not well compatible with graphite anode. In this study, a synergistic solid electrolyte interphase (SEI) film-forming effect between [3-[2-(2-methoxyethoxy)ethoxy]propyl]-trimethoxysilane (TMSM2) and propylene carbonate (PC) on graphite electrode was investigated. Excellent SEI film-forming capability and cycling performance was observed in graphite/Li cells using the electrolyte of 1 M LiPF₆ in the binary solvent of TMSM2 and PC, with the PC content in the range of 10–30 vol.%. Meanwhile, the graphite/Li cells delivered higher specific capacity and better capacity retention in the electrolyte of 1 M LiPF₆ in TMSM2 and PC (TMSM2:PC = 9:1, by vol.), compared with those in the electrolyte of 1 M LiPF₆ in TMSM2 and EC (TMSM2:EC = 9:1, by vol.). The synergistic SEI film-forming properties of TMSM2 and PC on the surface of graphite anode was characterized by electrolyte solution structure analysis through Raman spectroscopy and surface analysis detected by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Fourier transform infrared spectroscopy (FT-IR) analysis.     

Tea polyphenols as a novel reaction-type electrolyte additive in lithium-ion batteries

In this study, we reported tea polyphenols (TP) as a novel, cheap, environment-friendly and easy dissolution in common electrolytes reaction-type electrolyte additive for the graphite anode of the lithium-ion batteries. The TP can capture less stable radical anions that are harmful to oxidation stability of ethylene carbonate (EC) to form stable polymer. To a certain extent, it improved the electrochemical performance of the graphite electrode such as reversible capacity and cyclic stability by charge-discharge test, cyclic voltammetry (CV), scanning electron microscope (SEM), and electrochemical impedance microscope (EIS). The first charge capacities of the graphite electrodes in electrolytes without and with TP were 327.1 and 349.1 mAh g^?1, respectively. The charge capacities were 306.8 and 344.2 mAh g^?1 after 100 cycles and the capacity retention were 93.79 and 98.60%, respectively. The improvement was benefited from the effective scavenging the less stable radical anions and improvement the oxidation stability of EC and formation of a stable, compact and thin solid electrolyte interface (SEI) film with lower resistance.     

Vinyl ethylene carbonate as an electrolyte additive for high-voltage LiNi<Subscript>0.4</Subscript>Mn<Subscript>0.4</Subscript>Co<Subscript>0.2</Subscript>O<Subscript>2</Subscript>/graphite Li-ion batteries

Vinyl ethylene carbonate (VEC) is investigated as an electrolyte additive to improve the electrochemical performance of LiNi_0.4Mn_0.4Co_0.2O₂/graphite lithium-ion battery at higher voltage operation (3.0–4.5 V) than the conventional voltage (3.0–4.25 V). In the voltage range of 3.0–4.5 V, it is shown that the performances of the cells with VEC-containing electrolyte are greatly improved than the cells without additive. With 2.0 wt.% VEC addition in the electrolyte, the capacity retention of the cell is increased from 62.5 to 74.5 % after 300 cycles. The effects of VEC on the cell performance are investigated by cyclic voltammetry(CV), electrochemical impedance spectroscopy(EIS), x-ray powder diffraction (XRD), energy dispersive x-ray spectrometry (EDS), scanning electron microscopy (SEM), and attenuated total reflectance-Fourier transform infrared (ATR-FTIR). The results show that the films electrochemically formed on both anode and cathode, derived from the in situ decomposition of VEC at the initial charge–discharge cycles, are the main reasons for the improved cell performance.     

Dinitrile compound containing ethylene oxide moiety with enhanced solubility of lithium salts as electrolyte solvent for high-voltage lithium-ion batteries

A dinitrile compound containing ethylene oxide moiety (4,7-dioxa-1,10-decanedinitrile, NEON) is synthesized as an electrolyte solvent for high-voltage lithium-ion batteries. The introduction of ethylene oxide moiety into the conventional aprotic aliphatic dinitrile compounds improves the solubility of lithium hexafluorophosphate (LiPF₆) used commercially in the lithium-ion battery industry. The electrochemical performances of the NEON-based electrolyte (0.8 M LiPF₆?+?0.2 M lithium oxalyldifluoroborate in NEON:EC:DEC, v:v:v?=?1:1:1) are evaluated in graphite/Li, LiCoO₂/Li, and LiCoO₂/graphite cells. Half-cell tests show that the electrolyte exhibits significantly improved compatibility with graphite by the addition of vinylene carbonate and lithium oxalyldifluoroborate and excellent cycling stability with a capacity retention of 97 % after 50 cycles at a cutoff voltage of 4.4 V in LiCoO₂/Li cell. A comparative experiment in LiCoO₂/graphite full cells shows that the electrolyte (NEON:EC:DEC, v:v:v?=?1:1:1) exhibits improved cycling stability at 4.4 V compared with the electrolyte without NEON (EC:DEC, v:v?=?1:1), demonstrating that NEON has a great potential as an electrolyte solvent for the high-voltage application in lithium-ion batteries.     

Carbon-coated silicon/crumpled graphene composite as anode material for lithium-ion batteries

Silicon is a promising anode material for high-capacity Li-ion batteries (LIBs). However, its insulating property and large volume change during the lithiation/delithiation process result in poor cycling stability and in pulverization of Si. In this work, glucose-derived carbon-coated Si nanoparticles (C–Si NPs) are in conjunction with crumpled graphene (cGr) particles by a spray-drying method to prepare a novel composite (C–Si/cGr) material. The prepared C–Si NPs are uniformly embedded in the ridges of the cGr particles. The carbon layer of C–Si can make a good contact with the graphene sheet, resulting in enhanced electrical conductivity and fast charge transfer. In addition, the unique crumpled structure of the cGr can buffer the large volume change upon cycling process and facilitate the diffusion of electrolyte into the composite material. When employed as an anode electrode of LIBs, the C–Si/cGr composites deliver enhanced electrochemical performance, including stable cycling with a discharge capacity of 790 mAh·g⁻¹ after 100 cycles and a rate capability of 654 mAh·g⁻¹ at 2C. The synergistic effect of the carbon layer coating of Si NPs and the crumpled structure of the cGr particles results in a composite with improved the electrochemical performance, which is likely related to its high electrical conductivity and good mechanical stability of composite material.     

Allyl cyanide as a new functional additive in propylene carbonate-based electrolyte for lithium-ion batteries

Allyl cyanide (AC) was investigated as a film-forming additive in propylene carbonate (PC)-based electrolytes for graphite anode in lithium-ion batteries. The film-forming behavior of AC was characterized with cyclic voltammetry, electrochemical impedance spectroscopy, scanning electron microscopy, and Fourier transform infrared spectroscopy. By adding 2 wt% AC in the electrolyte of 1 M LiPF₆-PC/DMC (1:1, in vol), the exfoliation of graphite anode was effectively suppressed over cycling. Graphite/Li half-cell showed an initial coulombic efficiency of 75 % and a specific capacity of 300 mAh/g after 48 cycles. A possible reductive polymerization mechanism of AC on the surface of graphite was proposed.     

Effect of SO<Subscript>2</Subscript> and CO<Subscript>2</Subscript> additives on the cycle performances of commercial lithium-ion batteries

The effects of SO₂ and CO₂ additives in electrolytes on the cycle properties of liquid-state Al-plastic film lithium-ion batteries were first investigated. The experimental electrolytes were added with different amounts of SO₂ and CO₂. The baseline electrolyte was 1 mol L⁻¹ LiPF₆ in ethylene carbonate/dimethylcarbonate/ethyl-methyl carbonate (1:1:1, by volume), and graphite was used as anode. The main analysis tools were cycling test, rate capability, internal resistance test, low-temperature performance, and thermal stability. The results showed that both of the additives could promote to form an excellent solid electrolyte interface film on the surface of graphite anode, leading to excellent cycle performances, the capacity retentions of CO₂ and S5 were 94% and 97% after 400 cycles, respectively. Besides, the results also exhibited that the electrochemical performances of internal resistance, rate capability, low-temperature performance, and thermal stability were not changed significantly by the use of SO₂ and CO₂ as electrolyte additives.     

Montmorillonite-based ceramic membranes as novel lithium-ion battery separators

Novel montmorillonite-based ceramic membrane (CM) has been prepared with poly(vinylidene fluoride-co-hexafluoropropene) (PVdF-HFP) copolymer as binder. Physical properties such as surface morphology, porosity, liquid electrolyte uptake and thermal stability were analysed. The ceramic membrane was activated by soaking it in a non-aqueous liquid electrolyte (1.0 M LiPF₆ solution in 1/1 v/v ethylene carbonate/diethyl carbonate mixture) for 10 min. The compatibility of the membrane with lithium metal anode as a function of storage time was analysed by assembling a Li/CM/Li symmetric cell. Finally, a lab-scale cell composed of Li/CM/LiFePO₄ is assembled and its cycling performance analysed at different C-rates. Although the ceramic membrane is not flexible, it shows high thermal stability and stable interfacial properties when in contact with the lithium metal anode. A stable cycling behaviour is demonstrated even at 1C-rate with limited fade in capacity.     

Performance of developed natural vein graphite as the anode material of rechargeable lithium ion batteries

Electrochemical performance of natural vein graphite as an anode material for the rechargeable Li-ion battery (LIB) was investigated in this study. Natural graphite exhibits many favorable characteristics such as, high reversible capacity, appropriate potential profile, and comparatively low cost, to be an anode material for the LIB. Among the natural graphite varieties, the vein graphite typically possesses very high crystallinity together with extensively high natural purity, which in turn reduces the cost for purification. The developed natural vein graphite variety used for this study, possessed extra high purity with modified surface characteristics. Half-cell testing was carried out using CR 2032 coin cells with natural vein graphite as the active material and 1 M LiPF₆ (EC: DMC; vol. 1:1) as the electrolyte. Galvanostatic charge–discharge, cyclic voltammetry, and impedance analysis revealed a high and stable reversible capacity of 378 mA h g^?1, which is higher than the theoretical capacity (372 mA h g^?1 for LiC₆). Further, the observed low irreversible capacity acquiesces to the high columbic efficiency of over 99.9%. Therefore, this highly crystalline developed natural vein graphite can be presented as a readily usable low-cost anode material for Li-ion rechargeable batteries.     

Effect of vinyl ethylene carbonate on the compatibility between graphite and the flame-retarded electrolytes containing dimethyl methyl phosphonate

Effect of the content of vinyl ethylene carbonate (VEC) on the compatibility between the graphite anode and the electrolytes containing 10–30% dimethyl methyl phosphonate (DMMP) was investigated by cyclic voltammetry measurement. Impact of the contents of VEC and DMMP on the formation of the solid electrolyte interface layer was discussed, and a competitive mechanism between the destructive effect of DMMP decomposition and the positive effect of VEC was proposed. In the LiCoO₂/graphite cells, the electrolytes modified by DMMP and VEC exhibited satisfying cell performances, especially for the electrolyte with 10% DMMP and 2% VEC.     

Characterization of a hybrid Li-ion anode system from pulsed laser deposited silicon on CVD-grown multilayer graphene

A hybrid anode system for lithium (Li) ion battery applications based on pulsed laser deposited silicon films on chemical vapor deposited multilayer graphene (MLG) layers on a nickel foam substrate was electrochemically characterized. The as-grown material was directly fabricated into an anode without a binder, and tested in a half-cell configuration. There is evidence of the participation of both the multilayer graphene and the Si in the transport of Li ions. Even when cycled under stressful voltage limits that accelerate degradation, the MLG–Si films displayed higher stability than Si-only anodes, especially at higher cycling rates. Unlike the Si cells that display capacity fade even within the first few cycles, the MLG–Si cells show a very narrow spread in capacity, indicative of the role of the graphene layers in improving adhesion of the Si and acting as a compliant buffer for its volume expansion. Stable average specific capacities of ~1,200 mAh/g per total weight of MLG + Si, over 80 cycles at C/5 rate, were obtained for the MLG–Si anode. Pre- and post-cycling characterization of the anode materials revealed the differences between the two systems.     

Carbon-coated Si/graphite composites with combined electrochemical properties for high-energy-density lithium-ion batteries

Carbon-coated Si/graphite composites with different Si/graphite weight ratio have been fabricated using solid-state reaction with aim to improve the cyclic stability, coulombic efficiency, and rate capability simultaneously. Microstructural investigation reveals that the Si particles are covered by amorphous carbon and attached to the carbon-coated graphite surface. Electrochemical evaluation has been performed using cyclic voltammetry and charge/discharge cycling at different current densities, which indicate that addition of graphite can not only enhance the first-cycle coulombic efficiency to 90 % but also improve the cyclic stability drastically. The carbon-coated Si/graphite composite with appropriate contents of Si, graphite, and carbon is expected to be promising candidate as anode materials for high-energy-density lithium-ion batteries.     

An electrochemical and structural investigation of porous composite anode materials for LIB

A porous composite anode for lithium ion battery (LIB) was investigated. The composite anode was prepared by electrodepositing Sn?CSb alloy on a template-like electrode and then annealing it in the atmosphere of N₂, whereas the porous template-like electrode was obtained by forming a sponge-like porous membrane on a copper foil via a mixed phase inversion process, followed by pre-plating Cu through membrane pores in it. SEM and XRD results showed that composite structure of the anode consisted of electrodeposited Sn?CSb alloy dispersed in a PAN-pyrolyzed conjugated conducting polymer gridding, which was tightly connected with the Cu foil through transition alloy layer formed by heat treatment. Due to its relatively reasonable microcosmic structure, the composite anode presented better cycling performance and specific capacity retention during charging and discharging at diverse rates. When cycled between 0 and 2.0?V (vs Li/Li⁺) at 0.5?C rate, the reversible charge/discharge capacity of the composite material remained 415 and 414.8?mAh?g^?1, respectively, after 30 cycles, corresponding to 82.9% of the capacity retention. When charging and discharging at 2?C rate, the composite material electrode showed 71.7% capacity retention at the 30th cycle.     

Investigation of the synergetic effects of LiBF<Subscript>4</Subscript> and LiODFB as wide-temperature electrolyte salts in lithium-ion batteries

Herein, we present the use of lithium tetrafluoroborate (LiBF₄) as an electrolyte salt for wide-temperature electrolytes in lithium-ion batteries. The research focused on the application of blend salts to exhibit their synergistic effect especially in a wide temperature range. In the study, LiCoO₂ was employed as the cathode material; LiBF₄ and lithium difluoro(oxalate)borate (LiODFB) were added to an electrolyte consisting of ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC). The electrochemical performance of the resulting electrolyte was evaluated through various analytical techniques. Analysis of the electrical conductivity showed the relationship among solution conductivity, the electrolyte composition, and temperature. Cyclic voltammetry (CV), charge-discharge cycling, and AC impedance measurements were used to investigate the capacity and cycling stability of the LiCoO₂ cathode in different electrolyte systems and at different temperatures. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were applied to analyze the surface properties of the LiCoO₂ cathode after cycling. The results indicated that the addition of a small amount of LiODFB into the LiBF₄-based electrolyte system (LiBF₄/LiODFB of 8:2) may enhance the electrochemical performance of the LiCoO₂ cell over a relatively wide temperature range and improve the cyclability of the LiCoO₂ cell at 60 °C.     

LiPF6-EC-MPC electrolyte for LiMn2O4 cathode in lithium-ion battery

Methyl propyl carbonate (MPC) is a promising single solvent for lithium-ion battery without addition of ethylene carbonate (EC), but it is unstable upon cycling because of exposure to the spinel LiMn₂O₄ cathode. Thus, we attempted to add EC to MPC in order to form LiPF₆-EC-MPC electrolyte; the effects of solvent ratio and salt concentration on the cycling performance of LiMn₂O₄ cathode were also investigated. The experiments were characterized by conductivity measurements, charge-discharge at a constant current density and voltage–capacity curves at low temperature. To further enhance our understanding of the performance improvement of LiMn₂O₄/Li cells, the electrochemical characterization techniques (such as, LSV, EIS) were performed on these cells. The results show that the ionic conductivity of the electrolyte and the cycling performance of the spinel LiMn₂O₄ cathode have been dramatically enhanced. From the point of view of operation at low temperature (− 20 °C), 1 M LiPF₆ EC/MPC (1/3) electrolyte is highly recommended for spinel LiMn₂O₄ cathode in lithium-ion battery.     

Identification of solid electrolyte interphase formed on graphite electrode cycled in trifluoroethyl aliphatic carboxylate-based electrolytes for low-temperature lithium-ion batteries

Trifluoroethyl aliphatic carboxylates with different length of carbon-chain in acyl groups have been introduced into carbonate-based electrolyte as co-solvents to improve the low-temperature performance of lithium-ion batteries, both in capacity retention and lowering polarization of graphite electrode. To identify the further influence of trifluoroethyl aliphatic carboxylates on graphite electrode, the components and properties of the surface film on graphite electrode cycled in different electrolytes are investigated using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. The IR and XPS results show that the chemical species of the solid electrolyte interphase (SEI) on graphite electrode strongly depend on the selection of co-solvent. For instance, among those species, the content of RCOOLi increases with an increasing number of carbon atoms in RCOOCH₂CF₃ molecule, wherein R was an alkyl with 1, 3, or 5 carbon atoms. We suggest that the thickness and components of the SEI film play a crucial role on the enhanced low-temperature performance of the lithium-ion batteries.