Compatibility of quaternary ammonium-based ionic liquid electrolytes with electrodes in lithium ion batteries

Electrochemical intercalation/deintercalation behavior of lithium into/from electrodes of lithium ion batteries was comparatively investigated in 1 mol/L LiClO₄ ethylene carbonate-diethyl carbonate (EC-DEC) electrolyte and a quaternary ammonium-based ionic liquid electrolyte. The natural graphite anode exhibited satisfactory electrochemical performance in the ionic liquid electrolyte containing 20 vol.% chloroethylenene carbonate (Cl-EC). This is attributed to the mild reduction of solvated Cl-EC molecules at the graphite/ionic electrolyte interface resulting in the formation of a thin and homogenous SEI on the graphite surface. However, rate capability of the graphite anode is poor due to the higher interfacial resistance than that obtained in 1 mol/L LiClO₄/EC-DEC organic electrolyte. Spinel LiMn₂O₄ cathode was also electrochemically cycled in the ionic electrolyte showing satisfactory capacity and reversibility. The ionic electrolyte system is thus promising for 4 V lithium ion batteries based on the concept of “greenness and safety”.     

Study of the formation of a solid electrolyte interphase (SEI) in ionically crosslinked polyampholytic gel electrolytes

An ionic complex of anionic and cationic monomers was obtained by protonation of (N,N-diethylamino)ethylmethacrylate with acrylic acid. A novel ionically crosslinked polyampholytic gel electrolyte was prepared through the free radical copolymerization of the ionic complex and acrylamide in a solvent mixture of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate (1:1:1, v/v) containing 1 mol/L of LiPF₆. The impedance analysis indicated that the ionic conductivity of the polyampholytic gel electrolyte was rather close to that of solution electrolytes in the absence of a polymer at the same temperature. The temperature dependence of the conductivity was found to be well in accord with the Arrhenius behavior. The formation processes of the solid electrolyte interphase (SEI) formed in both gel and solution electrolytes during the cycles of charge-discharge were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The cyclic voltammetry curves show a strong peak at a potential of 0.68 V and an increase of the interfacial resistance from 17.2 Ω to 35.8 Ω after the first cycle of charge-discharge. The results indicate that the formation process of SEI formed in both gel and solution electrolytes was similar which could effectively prevent the organic electrolyte from further decomposition and inserting into the graphite electrode. The morphologies of SEI formed in both gel and solution electrolytes were analyzed by field emission scanning electron microscopy. The results indicate that the SEI formed in the gel electrolyte showed a rough surface consisting of smaller solid depositions. Moreover, the SEI formed in the gel electrolyte became more compact and thicker as the cycling increased.     

In situ Li nuclear magnetic resonance observation of the electrochemical intercalation of lithium in graphite; 1st cycle

We show a continuous, in situ nuclear magnetic resonance (NMR) experiment on a lithium/graphite electrochemical cell. The objective is to study a commercial graphite currently used as negative electrodes in secondary lithium batteries. A plastic cell is made, with metallic lithium as the counter electrode and 1 mol dm⁻³ LiPF₆/ethylene carbonate (EC) + diethylcarbonate (DEC) electrolyte. The reversible capacity is 346 mAh/g and the irreversible capacity 55 mAh/g, measured in the galvanostatic mode, at a rate of C/20 (20 h for the theoretical capacity of LiC₆) for the first cycle. We show the first discharge and the first charge of the cell inside the magnet and record simultaneously and regularly (in real time) static ⁷Li NMR spectra. As expected, we observe the quadrupolar lines characteristic of the lithium graphite intercalation compounds (GICs). During the discharge, the two types of in-plane densities of Li are successively found that correspond to the dilute LiC₉, then to the dense LiC₆ configuration; during the charge, we observe the successive decrease of these states. The galvanostatic curve helps to identify the stages NMR signature and the stages coexistence.     

Novel SEI formation of maleimide-based additives and its improvement of capability and cyclicability in lithium ion batteries

This investigation elucidates three maleimide (MI)-based aromatic molecules as additives in electrolyte that is used in lithium ion batteries. The 1.1 M LiPF₆ in ethylene carbonate (EC):propylene carbonate (PC):diethylene carbonate (DEC) (3:2:5 in volume) containing MI-based additives can prompt the formation of a solid electrolyte interface (SEI); and inhibit the entering into the irreversible state during lithium intercalation and co-intercalation. The reduction potential is 0.71-0.98 V versus Li/Li⁺ as determined by cyclic voltammetry (CV). The morphology and element analysis of the positive and negative electrode after the 100th charge-discharge cycle are examined by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS). Moreover, the MI was used in lithium ion batteries and provided 4.9% capacity increase and 16.7% capacity retention increase when cycled at 1C/1C. The MI-based additive also ensures respectable cycle-ability of lithium ion batteries. MI is decomposed electrochemically to form a long winding narrow SEI strip on the graphite surface. This novel SEI strip not only prevents exfoliation on the graphite electrode but also stabilizes the electrolyte. The MI-based additive also ensures respectable cycle-ability of lithium ion batteries.     

1-Alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids as highly safe electrolyte for Li/LiFePO4 battery

Several 1-alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids (alkyl-DMimTFSI) were prepared by changing carbon chain lengths and configuration of the alkyl group, and their electrochemical properties and compatibility with Li/LiFePO₄ battery electrodes were investigated in detail. Experiments indicated the type of ionic liquid has a wide electrochemical window (−0.16 to 5.2 V vs. Li⁺/Li) and are theoretically feasible as an electrolyte for batteries with metallic lithium as anode. Addition of vinylene carbonate (VC) improves the compatibility of alkyl-DMimTFSI-based electrolytes towards lithium anode and LiFePO₄ cathode, and enhanced the formation of solid electrolyte interface to protect lithium anodes from corrosion. The electrochemical properties of the ionic liquids obviously depend on carbon chain length and configuration of the alkyl, including ionic conductivity, viscosity, and charge/discharge capacity etc. Among five alkyl-DMimTFSI-LiTFSI-VC electrolytes, Li/LiFePO₄ battery with the electrolyte-based on amyl-DMimTFSI shows best charge/discharge capacity and reversibility due to relatively high conductivity and low viscosity, its initial discharge capacity is about 152.6 mAh g⁻¹, which the value is near to theoretical specific capacity (170 mAh g⁻¹). Although the battery with electrolyte-based isooctyl-DMimTFSI has lowest initial discharge capacity (8.1 mAh g⁻¹) due to relatively poor conductivity and high viscosity, the value will be dramatically added to 129.6 mAh g⁻¹ when 10% propylene carbonate was introduced into the ternary electrolyte as diluent. These results clearly indicates this type of ionic liquids have fine application prospect for lithium batteries as highly safety electrolytes in the future.     

Vinyl ethylene carbonate as an additive to ionic liquid electrolyte for lithium ion batteries

In this research, we investigated the potential application of vinyl ethylene carbonate (VEC) and ethylene carbonate (EC) as solid electrolyte interface (SEI) film-forming additive in 1-ethyl-3-methylimidazolium (EMI)-bis(trifluoromethyl-sulfonyl) imide (TFSI)-LiTFSI ionic liquid electrolyte (IL). The electrochemical performance of natural graphite (NG7) was studied in LiTFSI/EMI-TFSI containing different weight percent of EC/VEC via cyclic voltammetry (CV), electrochemical impedance spectrum (EIS), and galvanostatic charge/discharge cycles. Temperature effect on the discharge/charge performance of NG7 electrode in the researched IL electrolyte was also discussed.     

The effect of electrolyte temperature on the passivity of solid electrolyte interphase formed on a graphite electrode

The effect of electrolyte temperature on the passivity of solid electrolyte interphase (SEI) was investigated in 1 M LiPF₆-ethylene carbonate/diethyl carbonate (50:50 vol.%) electrolyte, using galvanostatic charge-discharge experiment, and ac-impedance spectroscopy combined with Fourier transform infra-red spectroscopy, and high resolution transmission electron microscopy (HRTEM). The galvanostatic charge-discharge curves at 20 °C evidenced that the irreversible capacity loss during electrochemical cycling was markedly increased with rising SEI formation temperature from 0 to 40 °C. This implies that the higher the SEI formation temperature, the more were the graphite electrodes exposed to structural damages. From both increase of the relative amount of Li₂CO₃ to ROCO₂Li and decrease of resistance to the lithium transport through the SEI layer with increasing SEI formation temperature, it is reasonable to claim that, due to the enhanced gas evolution reactions during transformation of ROCO₂Li to Li₂CO₃, the rising SEI formation temperature increased the number of defect sites in the SEI layer. From the analysis of HRTEM images, no significant structural destruction in bulk graphite layer was observed after charge-discharge cycles. This means that solvated lithium ions were intercalated through the defect sites in the SEI, at most, into the surface region of the graphite layer.     

Effect of fluoroethylene carbonate on high temperature capacity retention of LiMn2O4/graphite Li-ion cells

In order to overcome severe capacity fading of LiMn₂O₄/graphite Li-ion cells at high temperature at 60 °C, fluoroethylene carbonate (FEC) was newly evaluated as an electrolyte additive. With 2 wt.% FEC addition into the electrolyte (EC/DEC/PC with 1 M LiPF₆), the capacity retention at 60 °C after 130 cycles was significantly improved by about 20%. To understand the underlying principle on the capacity retention enhancement, the electrochemical properties of the cells including cell performance, impedance behavior as well as the characteristics of the interfacial properties were examined. Based on these results, it is suggested that the improved capacity retention of LiMn₂O₄/graphite Li-ion cells with addition of FEC especially at high temperature is mainly originated from the thin and stable SEI layer formed on the graphite anode surface.     

Characterisation of the SEI formed on natural graphite in PC-based electrolytes

The origin of the different Li⁺ intercalation behaviour of raw and jet-milled natural graphite has been investigated. Jet-milled graphite is found to cycle reversibly in equal solvent mixture of propylene carbonate (PC) and ethylene carbonate (EC), whereas raw graphite does not. Using both Al Kα and synchrotron radiation (SR) Photoelectron Spectroscopy, new insight is obtained into the formation of the solid electrolyte interphase (SEI) on the two different graphite materials during electrochemical cycling in 1 M LiPF₆ in either PC:EC (1:1) or in PC with 5% vinylene carbonate (VC) as additive. Solvent reduction products are found at the surface of both raw and jet-milled graphite cycled in PC:EC (1:1), but differed in composition. The addition of VC reduces primarily the quantities of salt reaction products (LiF and Li_xPF_y compounds) and produces a mainly organic SEI layer. Electron diffraction from the edges for raw and jet-milled graphite particles shows a more disordered surface structure in the jet-milled particles than in the raw graphite. The more disordered surface structure can serve as a physical barrier hindering PC co-intercalation and facilitating the formation of a stable SEI layer.     

Stabilisation of lithiated graphite in an electrolyte based on ionic liquids: an electrochemical and scanning electron microscopy study

1-Ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide (EMI-TFSI) is shown to reversibly permit lithium intercalation into standard TIMREX^® SFG44 graphite when vinylene carbonate (VC) is used in small amounts as additive. The best performance was obtained when 5% of VC was added to a 1 M solution of LiPF₆ in EMI-TFSI. Intercalation of lithium in the SFG44 graphite host was demonstrated over 100 cycles without noticeable capacity fading. The reversible charge capacity was around 350 mA h g⁻¹ and an only small irreversible capacity loss per cycle could be observed. Li₄Ti₅O₁₂ was used as counter electrode material. Scanning electron microscopy indicates the reduction of the electrolyte without graphite exfoliation in the neat electrolyte and the formation of a passivation film in the case of a VC-containing electrolyte. Other additives that were tested comprise ethylene sulphite and acrylonitrile which show also a positive effect, but a smaller one than vinylene carbonate. LiCoO₂ positive electrodes were cycled in a 1 M solution of LiPF₆ in EMI-TFSI with good charge capacity retention over more than 300 cycles, when Li₄Ti₅O₁₂ was used as counter electrode. The formation of a passivation film is proven on the LiCoO₂-electrodes, when the electrolyte contained VC, but not in the neat ionic liquid. Finally, the stable cycling of a full cell configuration is proven in this electrolyte system. An ammonium-containing ionic liquid (methyltrioctylammonium-bis(trifluoromethylsulfonyl)-imide, MTO-TFSI) is shown to permit the cycling of both, graphite and lithium cobalt oxide when VC is used as additive in small amounts, but at slightly elevated temperatures.     

Composite anode material of silicon/graphite/carbon nanotubes for Li-ion batteries

A composite anode material of silicon/graphite/multi-walled carbon nanotubes (MWNTs) for Li-ion batteries was prepared by ball milling. This composite anode material showed a discharge capacity of 2274 mAh/g in the first cycle, and after 20 charge-discharge cycles, a reversible capacity of 584 mAh/g was retained, much higher than 218 mAh/g for silicon/graphite composite. It was observed that silicon particles were homogeneously embedded into the “lamellar structures” of flaked graphite particles, and the silicon/graphite composite particles were further wrapped by a MWNTs network. The improvement in the electrochemical properties of the composite anode material was mainly attributed to the excellent resiliency and good electric conductivity of the MWNTs network.     

A comparison of solid electrolyte interphase (SEI) on the artificial graphite anode of the aged and cycled commercial lithium ion cells

The commercial lithium ion cells with LiCoO₂ as cathode, artificial graphite as anode and 1 M LiPF₆/EC-DEC-EMC (ethylene carbonate-diethyl carbonate-dimethyl carbonate) (1:1:1, v/v/v) with additives (1 wt.% vinylene carbonate (VC) + 1 wt.% propylene sulfite (PS)) as electrolyte were aged at 60% and 100% state of charge (SOC) for 6 months at room temperature and the corresponding cycle performance was measured. Charge/discharge results showed that the capacity retentions after 100 cycles were in the order of fresh cell >60% SOC > 100% SOC. The composition of SEI on the anode was analyzed by X-ray photoelectron spectroscopy (XPS) and the sulfur atom in PS was used as a tagged atom in XPS analysis. The results suggested that the transformation of organic species to inorganic species and the species containing sulfur atom from the reduction of PS was dissolved for the cells aged at 60% and 100% SOC. The SEM and XPS surface and depth profile analysis showed that the increase of the thickness of SEI layer and the variation of compositions on storage or cycling, is one of the most important reasons that results in the deterioration of the cycle performance of commercial lithium ion cells aged at 60% and 100% SOC at room temperature for 6 months.     

Interfacial structural stabilization on amorphous silicon anode for improved cycling performance in lithium-ion batteries

Interfacial structures of electrode-current collector and electrode-electrolyte have been designed to be stabilized for improved cycling performance of amorphous silicon (Si) that is considered as an alternative anode material to graphite for lithium-ion batteries. Interfacial structural stabilization involves the interdigitation of Si electrode-Cu current collector substrate by anodic Cu etching with thiol-induced self-assembly, and the formation of self-assembled siloxane on the surface of Si electrode using silane. The novel interfacial architecture possesses promoted interfacial contact area between Si and Cu, and a surface protective layer of siloxane that suppresses interfacial reactions with the electrolyte of 1 M LiPF₆/ethylene carbonate (EC):diethylene carbondate (DEC). FTIR spectroscopic analyses revealed that a stable solid electrolyte interphase (SEI) layer composed of lithium carbonate, organic compounds with carboxylate metal salt and ester functionalities, and PF-containing species formed when having siloxane on Si electrode. Interfacially stabilized Si electrode exhibited a high capacity retention 80% of the maximum discharge capacity after 200 cycles between 0.1 and 1.5 V vs. Li/Li⁺. The data contribute to a basic understanding of interfacial structural causes responsible for the cycling performance of Si-based alloy anodes in lithium-ion batteries.     

Effects of particle size on the thermal stability of lithiated graphite anode

The thermal behavior of fully lithiated natural graphite flakes with different particle sizes has been investigated using differential scanning calorimetry (DSC). For DSC measurements, a fully lithiated graphite anode was heated in a hermetically sealed high pressure pan with a poly vinylidene diflouride (PVdF) binder and 1 M LiPF₆ solution in ethylene carbonate (EC)-diethyl carbonate (DEC) mixture. It has been founded that the particle size has a strong influence on the thermal stability of the lithiated graphite anode. The heat generation due to the solid electrolyte interface (SEI) decomposition increases with decreasing the particle size. The onset temperatures for exothermic reactions after initial SEI decomposition appear to be lower for graphite electrodes with smaller particle sizes. This is attributed to a thermal induced delithiation facilitated by reduced diffusion path and higher surface area in smaller graphites. The structural changes in graphites during DSC scan have been investigated by ex situ X-ray diffraction (XRD) and Raman spectrometer.     

Formation of artificial solid electrolyte interphase by grafting for improving Li-ion intercalation and preventing exfoliation of graphite

Various grafting methods for surface modification of graphite powder are reported. p-Carboxylic benzene diazonium salt has been grafted on graphite powder using different grafting methods namely electrochemical grafting, in situ grafting aqueous, in situ grafting non-aqueous, and spontaneous grafting. Each grafting method results in a different loading of the surface organic layer, which is quantified by X-ray photoelectron spectroscopy. Grafted graphite powders exhibit controlled solid electrolyte interphase (SEI) morphology, Li-ion intercalation, and averted exfoliation. Grafted graphite powders were further modified by transformation of the terminal functional group of the grafted surface layer by chemical or electrochemical treatments. This step tunes the reactivity of grafted material towards non-aqueous electrolyte, which is reflected in modified electrochemical properties like formation of SEI, overall Li-ion intercalation/deintercalation kinetics, cyclability, practical specific charge, and eventual exfoliation of graphite. The morphology of the grafted samples is retained after electrochemical cycling as well as after functional group transformation steps. The morphology of the SEI is also seen to be similar to that of the grafted layer in all the cases. These methods offer a pathway for tuning SEI properties and preventing exfoliation of graphite in propylene carbonate based electrolyte.     

Properties of Li-graphite and LiFePO4 electrodes in LiPF6–sulfolane electrolyte

Sulfolane (also referred to as tetramethylene sulfone, TMS) containing LiPF₆ and vinylene carbonate (VC) was tested as a non-flammable electrolyte for a graphite |LiFePO₄ lithium-ion battery. Charging/discharging capacity of the LiFePO₄ electrode was ca. 150 mAh g⁻¹ (VC content 5 wt%). The capacity of the graphite electrode after 10 cycles establishes at the level of ca. 350 mAh g⁻¹ (C/10 rate). In the case of the full graphite |1 M LiPF₆ + TMS + VC 10 wt% |LiFePO₄ cell, both charging and discharging capacity (referred to cathode mass) stabilized at a value of ca. 120 mAh g⁻¹. Exchange current density for Li⁺ reduction on metallic lithium, estimated from electrochemical impedance spectroscopy (EIS) experiments, was j_o(Li/Li⁺) = 8.15 × 10⁻⁴ A cm⁻². Moreover, EIS suggests formation of the solid electrolyte interface (SEI) on lithium, lithiated graphite and LiFePO₄ electrodes, protecting them from further corrosion in contact with the liquid electrolyte. Scanning electron microscopy (SEM) images of pristine electrodes and those taken after electrochemical cycling showed changes which may be interpreted as a result of SEI formation. No graphite exfoliation was observed. The main decomposition peak of the LiPF₆ + TMS + VC electrolyte (TG/DTA experiment) was present at ca. 275 °C. The LiFePO₄(solid) + 1 M LiPF₆ + TMS + 10 wt% VC system shows a flash point of ca. 150 °C. This was much higher in comparison to that characteristic of a classical LiFePO₄ (solid) + 1 M LiPF₆ + 50 wt% EC + 50 wt% DMC system (T_f ≈ 37 °C).     

Recent advances toward high voltage,EC-free electrolytes for graphite-based Li-ion battery

Lithium-ion batteries are a key technology in today’s world and improving their performances requires, in many cases, the use of cathodes operating above the anodic stability of state-of-the-art electrolytes based on ethylene carbonate (EC) mixtures. EC, however, is a crucial component of electrolytes, due to its excellent ability to allow graphite anode operation–also required for high energy density batteries–by stabilizing the electrode/electrolyte interface. In the last years, many alternative electrolytes, aiming at allowing high voltage battery operation, have been proposed. However, often, graphite electrode operation is not well demonstrated in these electrolytes. Thus, we review here the high voltage, EC-free alternative electrolytes, focusing on those allowing the steady operation of graphite anodes. This review covers electrolyte compositions, with the widespread use of additives, the change in main lithium salt, the effect of anion (or Li salt) concentration, but also reports on graphite protection strategies, by coatings or artificial solid electrolyte interphase (SEI) or by use of water-soluble binder for electrode processing as these can also enable the use of graphite in electrolytes with suboptimal intrinsic SEI formation ability.     

Electrochemical and interfacial behavior of a FeSi2.7 thin film electrode in an ionic liquid electrolyte

A FeSi_2.7 thin film is deposited on a copper substrate by RF magnetron sputtering of a Fe–Si alloy target. The electrochemical behavior of the FeSi_2.7 electrode in ionic liquid electrolyte based on 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide is investigated and compared with that of a FeSi_2.7 electrode in conventional liquid electrolyte. The FeSi_2.7 electrode in the ionic liquid electrolyte delivers an initial discharge capacity of 756 mAh g⁻¹ at room temperature, and its discharge capacity is maintained to be 92% of the initial discharge capacity after the 100th cycle. AC impedance and FTIR analysis reveal that the formation of a stable solid electrolyte interphase (SEI) layer on the FeSi_2.7 electrode in the ionic liquid electrolyte leads to a good capacity retention. This study demonstrates that the FeSi_2.7 electrode exhibits stable cycling behavior and good interfacial characteristics in the ionic liquid electrolyte without any solvents and additives.     

Improving room-temperature electrochemical performance of solid-state lithium battery by using electrospun La2Zr2O7 fibers-filled composite solid electrolyte

Low ionic conductivity at room temperature and poor interfacial compatibility are the main obstacles to restrain the practical application of polymer solid electrolytes. In this work, lanthanum zirconate (LZO) fibers were prepared by electrospinning method and used for the first time as fillers in sandwich polypropylene carbonate (PPC)-based solid electrolyte. Meanwhile, a graphite coating was applied on one surface of the composite solid electrolyte (CSE) membrane. The results show that the LZO fibers significantly increases the room-temperature electrochemical performance of the CSE, and the graphite coating enhances the interfacial compatibility between electrolyte and lithium anode. Furthermore, an ultra-thin PPC-LZO CSE with a total thickness of 22 μm was prepared and used in NCM622/CSE/Li solid-state cell, which shows an initial discharge capacity of 165.6 mAh/g at the current density of 0.5C and a remaining capacity of 113.0 mAh/g after 250 cycles at room temperature. Rise to 1C, the cell shows an initial discharge capacity of 154.2 mAh/g with a remaining capacity of 95.6 mAh/g after 250 cycles. This ultra-thin CSE is expected to be widely applied in high energy-density solid-state battery with excellent room-temperature electrochemical performances.     

Nano-porous Si/C composites for anode material of lithium-ion batteries

Nano-porous silicon composite incorporated with graphite and pyrolyzed carbon was synthesized and investigated as a promising anode material for lithium-ion batteries. The nano-porous Si/graphite composite was prepared via two-step ball-milling followed by etching process. Then carbon was incorporated by using different approaches. The nano-porous Si/graphite/C composite exhibits a reversible capacity of about 700 mAh/g with no capacity loss up to the 120th cycle at a constant current density of 0.2 mA/cm². The superior electrochemical characteristics are attributed to the nanosized pores in Si particles, which suppress the volume effect, and buffering action as well as excellent electronic and ionic conductivity of carbon materials.