Twenty-four new functionalized ionic liquids (ILs) based on trialkylimidazolium cations with the alkoxymethyl group at the N-1 position were synthesized and characterized. Physicochemical properties of these ILs, such as melting point, thermal stability, density, viscosity, conductivity, and electrochemical stability, were studied systematically. Twenty-one ILs appeared in the liquid state at room temperature, and 14 ILs showed a melting point lower than −60 °C. Introduction of the alkoxymethyl group at the N-1 position of trialkylimidazolium cations could not be more beneficial than that of the alkoxyethyl group for reducing viscosity. Li/LiFePO4 cells employing three trialkylimidazolium ILs with methoxymethyl-group-based electrolytes showed good discharge capacity and cycle stability at a current rate of 0.1 C. This is the first report of ILs with the alkoxymethyl group used as electrolytes without an additive for a lithium ion battery.

In this work, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (F-EPE), a kind of non-flammable hydrofluoroether, was initially mixed with lithium bis(oxalato)borate (LiBOB) and gamma-butyrolactone (GBL) to formulate a safe electrolyte for lithium-ion batteries. It was found that the addition of F-EPE endowed this novel electrolyte with a high safety level, low surface tension and good wettability to the separator and electrodes. More importantly, this safe electrolyte supported the graphite/LiCo1/3Mn1/3Ni1/3O2 full cell in achieving excellent electrochemical performances. During the prolonged cycle test at room temperature, its capacity retention after 500 cycles could reach 80.6%, which was comparable to that of a commercial electrolyte. Its rate performance at room temperature and cycle performance at elevated temperature (60 °C) surpassed those of the commercial electrolyte. In particular, its low-temperature performance was remarkable, and the cell could deliver a capacity as high as 74.2 mA h g−1 at −40 °C. In view of these results, such a safe electrolyte showed tremendous potential for practical application.

Assembled microspherical cathodes have attracted great attention thanks to their high tap density, good rate capability and cycling stability. However, for layered Li-rich transition-metal oxides (LROs), the preparation of uniformly assembled microspheres still faces many challenges due to harsh synthetic conditions and the nature of multiple metal elements. In this work, Li1.17Mn0.50Ni0.16Co0.17O2 assembled microspheres have been prepared by a new route tactfully combining a solvothermal process and a molten-salt method. The use of a solvothermal process is helpful for the preparation of precursors with assembled microspherical morphology, and the addition of complex salts (NaCl and KCl), can increase the uniformity of cation distribution. The product obtained at 800 °C delivers the best electrochemical performances among all samples. At a current density of 300 mA g−1, its initial discharge capacity is larger than 228 mA h g−1, corresponding to a capacity retention ratio of 86.8% after 200 cycles. Even if the current density increases to 2000 mA g−1, its discharge capacity is still as large as 156 mA h g−1. What's more, we discover the moving rate of Li-ions during the sintering process will affect the uniformity of Li2MnO3-like and LiMO2 components in LRO assembled microspheres. This discovery is helpful for the preparation of LRO assembled microspheres with excellent electrochemical performances.

•Two nanoplates with exposed (101) or (001) facets are prepared.•Effect of exposed facets on voltage decay and capacity fading of LLOs is unraveled.•Large area of active surface will accelerate electrochemical corrosion of LLOs.•Phase transformation of LLOs can be mitigated by increasing the area of inactive surface.•We discover Co and Ni are easier to react with electrolyte than Mn.In this work, the effect of exposed facets on voltage decay and capacity fading of Li-rich layered oxides (LLOs) is unraveled via two nanoplates which are prepared by very similar co-precipitated-precursor method. The top-bottom surface of one nanoplate is (101) facet (S101 sample, active plane) and the other is exposed (001) facet (S001 sample, inactive plane). Although S101 sample delivers an excellent rate capability (149 mA h g−1 at 2000 mA g−1), both its voltage and capacity decrease faster than S001 sample. TEM, HRTEM, XPS and XRD of cycled samples demonstrate: (1) Large area of active surface will accelerate electrochemical corrosion and phase transformation of LLOs and (2) electrochemical corrosion and phase transformation of LLOs with large area of inactive surface can be mitigated, leading to slow voltage decay and capacity fading. In addition, we discover Co and Ni are easier to react with electrolyte than Mn. Therefore, suitable facets and compositions may be more useful for improving the electrochemical performance of LLOs.Voltage decay and capacity fading of Li-rich layered oxides can be mitigated by large area of exposed inactive surface.Download high-res image (198KB)Download full-size image

•A molten-salt method using KCl and NaCl as complex flux is designed.•Li1.18Mn0.56Ni0.13Co0.13O2 cathodes possess excellent rate and cycling properties.•The outstanding properties are due to the uniform distribution of TM-elements.•The uniformity of TM elements can be improved by molten-salt method.Li-rich layered oxides, one of the most promising cathodes for high energy Li-ion batteries, commonly undergo some issues such as poor rate performance, low Coulombic efficiency, voltage degradation and so on. In this work, Li1.18Mn0.56Ni0.13Co0.13O2 cathodes with excellent rate capability and cycling stability are prepared by a new molten-salt method using KCl and NaCl as complex flux. The electrochemical tests show that these cathodes can deliver an initial discharge specific capacity of 224.3 mA h g−1 at 300 mA g−1. Meanwhile, the capacity retention ratio remains 92.1% after 100 cycles. The outstanding electrochemical performance is mainly attributed to the uniform distribution of TM-elements in Li1.18Mn0.56Ni0.13Co0.13O2 cathodes, which can promote the electrochemical activation of Li2MnO3-like component. Especially, it is found that the uniformity of TM elements in Li-rich cathodes can be improved by choosing molten-salt method as the synthetic strategy.

•Mg-doped Li2FeSiO4/C is synthesized by sol-gel method with Fe2O3 nanoparticle.•Rietveld refinement confirms that Mg is doped in Fe site in Li2FeSiO4 lattice.•Mg-doped product shows high capacity retention of 96% after 100 cycles at 0.1 C.•Doping Mg can effectively improve the electrochemical performance of Li2FeSiO4/C.Mg-doped Li2FeSiO4/C is synthesized by using Fe2O3 nanoparticle as iron source. Through Rietveld refinement of X-ray diffraction data, it is confirmed that Mg-doped Li2FeSiO4 owns monoclinic P21/n structure and Mg occupies in Fe site in the lattice. Through energy dispersive X-ray measurement, it is detected that Mg element is distributed homogenously in the resulting product. The results of transmission electron microscopy measurement reveal that the effect of Mg-doping on Li2FeSiO4 crystallite size is not obvious. As a cathode material for lithium-ion battery, this Mg-doped Li2FeSiO4/C delivers high discharge capacity of 190 mAh g−1 (the capacity was with respect to the mass of Li2FeSiO4) at 0.1C and its capacity retention of 100 charge-discharge cycles reaches 96% at 0.1C. By the analysis of electrochemical impedance spectroscopy, it is concluded that Mg-doping can help to decrease the charge-transfer resistance and increase the Li+ diffusion capability.

In this work, Li1.15Mn0.49Ni0.18Co0.18O2 nanoplates with exposed (012) facet are prepared for the first time by co-precipitation method under the assistance of cetyltrimethyl ammonium bromide. As cathode materials of lithium-ion batteries, the Li1.15Mn0.49Ni0.18Co0.18O2 nanoplates can deliver the initial discharge capacities of 219.8 and 192 mA h g−1 at 300 and 700 mA g−1, respectively. It suggests the Li1.15Mn0.49Ni0.18Co0.18O2 nanoplates possess an excellent rate capability. After 200 cycles, the capacity retention ratio at 700 mA g−1 is still as large as 82.6%. The superior rate capability can be attributed to the shorter transport distance of lithium ions in these nanoplates with exposed (012) facet. The above results also indicate that the electrochemical performances of Li-rich layered oxides can be improved by allocating proper facets.

•New FSI-based imidazolium ILs are proposed as electrolytes for lithium-ion batteries.•Neat IL electrolytes are evaluated by full cells with commercial electrodes.•Ether functionalization brings the prominent improvement of performance.•MCMB/LiFePO4 full cells can show excellent cycling performance.Neat ionic liquid electrolytes based on functionalized 1,3-dialkylimidazolium cation and bis(fluorosulfonyl)imide anion were investigated in MCMB/LiFePO4 full cells with commercial electrodes for the first time. Ether functionalization could bring the prominent improvement of initial efficiency and the comparable cycle performance to a conventional carbonate-based electrolyte. In view of full cells, it was inferred that the further oxidation on cathode of the reduction products on anode during the charge process might result in the serious capacity loss of initial cycle.

•Li1.14Mn0.48Ni0.19Co0.19O2 nanoplate exposed (001) is prepared.•A surface protective layer (SPL) is discovered at the outer edge of nanoplate.•The direction and order of TM ions migration during cycling have been demonstrated.•Capacity and voltage degradation of Li-rich layered cathodes is countered.Capacity and voltage degradation is a crucial factor that restricts the commercialization of Li-rich layered cathode materials. Recently, it has been demonstrated that the degradation results from the structural evolution from layered to spinel-like phase, caused by the migration of transition metal (TM) ions. However, the direction and order of TM ions migration is hard to identify. In this study, a surface protective layer (SPL) is discovered for the first time at the outer edge of Li1.14Mn0.48Ni0.19Co0.19O2 nanoplate by the research on the surface condition of nanoplates after charge–discharge cycles. More importantly, by the detailed analysis for the SPL, we further reveal that the formation of SPL is active facet dependent, TM ions migrate toward the active facets, and Ni and Co ions hop more preferentially than Mn ions during the cycling. These discoveries can help to understand the fading mechanism of Li-rich layered cathode materials. In addition, electrochemical test indicates that the SPL greatly delays the corrosion of active surfaces in electrolyte and improves the cycling stability of Li-rich layered cathode materials. This insight provides a new thought for preparing long-life cathodes of high energy Li-ion batteries.Figure optionsDownload full-size imageDownload as PowerPoint slide

Low-cost sorbitanlaurat is used as new carbon source to synthesize Li2FeSiO4/C by sol–gel method. Four Li2FeSiO4/C products with different carbon contents are pursued as the cathode material of lithium-ion battery. The structures and morphologies of these products are characterized by X-ray diffraction, scanning electron microscope and transmission electron microscope. The particle size of these Li2FeSiO4 products is about 10–20 nm. The product with 8.4 wt% carbon delivers an initial discharge capacity of 187 mA h g−1 at 0.1 C. This product also exhibits good rate and cycle performances. By Raman spectroscopy, cyclic voltammogram and impedance spectroscopy, the factor which can affect the rate performance of these Li2FeSiO4/C products is analyzed.

•Pure-phased Li2MnSiO4/C is prepared by sol–gel method with Mn3O4 nanoparticle.•It owns nano-sized active material particle with 20–30 nm.•It delivers an initial discharge capacity of 240 mAh g−1 at room temperature.•Irreversible distortion might mainly effect on the cycle performance of Li2MnSiO4/C.Li2MnSiO4/C composite is prepared by sol–gel method with Mn3O4 nanoparticle, and its carbon content, structure, and morphology are characterized. The results show that Li2MnSiO4/C exhibits pure phase with orthorhombic structure and the size of Li2MnSiO4 (20–30 nm) is smaller than Mn3O4 nanoparticle. As the cathode material of lithium-ion battery, Li2MnSiO4/C delivers an initial discharge capacity of about 240 mAh g−1 at the current density of 8 mA g−1, corresponding to 1.44 mol of Li+ per formula unit. The cycle performance of Li2MnSiO4/C at different current densities from 8 mA g−1–320 mA g−1 is studied, and it is found that the capacity retention is improved with the increasing of current density. Basing on the results of ex-situ X-ray diffraction measurement, it is inferred that low degree of irreversible distortion for Li2MnSiO4 may result in the improved capacity retention at high current density.

•New functionalized quaternary ammonium ILs with two ether groups are reported.•They have low viscosity and good electrochemical stability.•Li/LiFePO4 cells using these IL electrolytes have good electrochemical performance.New functionalized ILs based on quaternary ammonium cations with two ether groups and bis(trifluoromethanesulfonyl)imide (TFSA−) anion are synthesized and characterized. Physical and electrochemical properties, including melting point, thermal stability, viscosity, conductivity and electrochemical stability are investigated for these ILs. All these ILs are liquids at room temperature except N,N-diethyl-N,N-bis(2-ethoxyethyl)ammonium TFSA (N22(2o2)(2o2)-TFSA, Tm = 29.7 °C), and the viscosities of N-methyl-N-ethyl-N-(2-methoxyethyl)-N-(2-ethoxyethyl)ammonium TFSA (N12(2o1)(2o2)-TFSA) and N-methyl-N-ethyl-N,N-bis(2-ethoxyethyl)ammonium TFSA (N12(2o2)(2o2)-TFSA) are 68.0 cP and 63.0 cP at 25 °C, respectively. N-Methyl-N,N-diethyl-N-(2-methoxyethyl)ammonium TFSA (DEME–TFSA) and five ILs with lower viscosity are chosen to dissolve 0.6 mol kg−1 of LiTFSA as IL electrolytes without additive for lithium battery. Lithium plating and striping on Ni electrode can be observed in these IL electrolytes, and cycle performances of lithium symmetrical cells are also investigated for these IL electrolytes. Li/LiFePO4 cells using these IL electrolytes without additives have good cycle property at the current rate of 0.1 C, and the N-methyl-N-ethyl-N,N-bis(2-methoxyethyl)ammonium TFSA (N12(2o1)(2o1)-TFSA) and N12(2o2)(2o2)-TFSA electrolytes own better rate property than DEME–TFSA electrolyte.

A family of new ether-functionalized ionic liquids based on 1,3-dialkylimidazolium cation with alkoxymethyl or alkoxyethyl group and TFSA anion were synthesized and characterized. The properties of these ionic liquids, including melting point, thermal stability, density, viscosity, conductivity, and electrochemical window, were determined and compared with those of 3 typical 1,3-dialkylimidazolium ionic liquids without ether group. The effect of ether group on the properties was systematically investigated. All these ionic liquids were liquid at room temperature, and their melting points were lower than −60 °C. It was demonstrated that alkoxyethyl group was favorable to decreasing viscosity of 1,3-dialkylimidazolium ionic liquids, and alkoxymethyl group was not helpful for decreasing viscosity. At room temperature, 6 new ionic liquids had the viscosities lower than 45 cP, and the viscosity of IM2o2-2-TFSA was only 31.3 cP.

Six low-viscosity ionic liquids based on trialkylimidazolium cation with one or two ether groups and TFSA− anion are used as new electrolytes for lithium battery, and compared with three typical trialkylimidazolium ILs without ether group. It is found that ether group in trialkylimidazolium cation can have an obvious effect on properties of electrolyte and performances of lithium battery. Introducing of ether group into trialkylimidazolium cation can be benefit for lithium redox behavior on Ni electrode, and affect passivation layer between IL electrolyte and lithium metal. Li/LiFePO4 cells using these ether-functionalized IL electrolytes without additive have good battery performance, and IM(2o1)1(2o2)-TFSA electrolyte owns better rate property.Highlights► New trialkylimidazolium ILs are used as electrolytes for lithium battery. ► Ether group in IL cation can have effect on properties of IL electrolyte. ► Li/LiFePO4 cells using these IL electrolytes have good electrochemical performance.

A new family of C-2 functionalized trialkylimidazolium ionic liquids with alkoxymethyl groups were synthesized and characterized. Their physicochemical properties including melting point, thermal stability, viscosity, conductivity, and electrochemical windows were systematically investigated. All of these functionalized trialkylimidazolium ionic liquids were liquids at room temperature, and most of them had melting points lower than −60 °C. At room temperature, 24 ionic liquids had viscosities lower than 90 cP, and the viscosities of IM2(1o2)2TFSA and IM1(1o2)2TFSA were 50.3 and 53.3 cP.

•Pure-phased Li2FeSiO4/C is prepared by the sol–gel method with Fe2O3 nanoparticle.•The particle size of Li2FeSiO4 is close to Fe2O3 nanoparticle.•It owns stable cycle performance at 0.1C to 5C rate.•It has good rate performance.Li2FeSiO4/C has been prepared by the ultrasonic-assisted sol–gel method with about 50 nm Fe2O3 particle as Fe source. The structures, morphologies, and carbon content of the product are characterized. Li2FeSiO4/C owns pure phase and the particle size of Li2FeSiO4 is close to Fe2O3 nanoparticle. As the cathode material of Li-ion battery, the product shows stable cycle performance at the wide range of rate and good rate performance. The discharge capacities of the 100th cycle are 140 mAh g−1, 125 mAh g−1, 120 mAh g−1, 110 mAh g−1, and 90 mAh g−1 at 0.1C, 0.5C, 1C, 2C, and 5C rate, respectively.

Fe2O3 microsphere is used as iron source and template to prepare Li2FeSiO4/C nanocomposite by a sol–gel method. The carbon content, structure, and morphology of the sample are characterized by carbon–sulfur inferred analysis (CSI), X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscopy (TEM) techniques. The results show that the Li2FeSiO4/C owns pure phase and sphere-like morphology as a result of an agglomeration of nanoparticles with an average particle size of about 50 nm. The Li2FeSiO4/C exhibits stable cycle performance at different rates from 0.1 C to 2 C, and its capacity at 0.1 C rate can reach 160 mAh g−1.Highlights► Li2FeSiO4/C nanocomposite is prepared by sol–gel method with Fe2O3 microsphere. ► It exhibits sphere-like morphology. ► It owns stable cycle performance at 0.1 C–2 C rate.

Four new functionalized ILs based on pyrazolium cations and bis(trifluoromethylsulfonyl)imide anions (TFSI−) are synthesized and characterized. These ILs show low-melting point and low-viscosity characteristics, and the viscosity of OEPZ–TFSI is 41.2 mPa s at 25 °C. These IL electrolytes with 0.4 mol kg−1 LiTFSI have good chemical stability against lithium metal. Li/LiFeO4 cells using these IL electrolytes without additives own good electrochemical performance, and the cells using OMPZ–TFSI and OEPZ–TFSI electrolytes own better rate property.Highlights► New ether-functionalized pyrazolium ILs are reported. ► They have low-viscosity and low-melting point characteristics. ► Li/LiFePO4 cells using these IL electrolytes have good electrochemical performance.

Two new functionalized ILs based on pyrazolium cations and bis(trifluoromethylsulfonyl)imide anions (TFSI−) were synthesized and characterized. Their physicochemical and electrochemical properties, including melting point, thermal stability, density, viscosity, conductivity, and electrochemical window were investigated. The two ILs had low melting points and wide electrochemical windows. Behaviors of lithium redox, chemical stability against lithium metal, and charge–discharge cycle performance of lithium battery were also examined for the two IL electrolytes with 0.4 mol kg−1 LiTFSI. The IL electrolytes showed good chemical stability against lithium metal, and Li/LiFeO4 cells using the two IL electrolytes without additives owned good capacity and cycle property at the current rate of 0.1 C.Highlights► New ether-functionalized pyrazolium ILs are reported. ► They have low melting point and good electrochemical stability. ► Li/LiFePO4 cells using the IL electrolytes have good capacity and cycle property.

A family of new ether-functionalized ILs based on trialkylimidazolium cations with one or two ether groups and TFSA– anion was synthesized and characterized. Their properties including melting point, thermal stability, viscosity, conductivity, and electrochemical windows were determined and compared to those of the trialkylimidazolium ILs without ether group. The relationship between the cations structure and IL physicochemical properties was systematically studied. Most of these ether-functionalized ILs were liquids at room temperature, and the melting points of 21 ILs were lower than −60 °C. At room temperature, 26 ILs owned the viscosities lower than 100 mPa s, and the viscosities of IM(2o1)12TFSA and IM(2o2)12TFSA were 57.4 and 54.4 mPa s.

Two new functionalized ionic liquids (ILs) based on guanidinium cation with two ether groups and TFSA− anion are synthesized and characterized. Their physicochemical and electrochemical properties, including melting point, thermal stability, density, viscosity, conductivity, and electrochemical window are determined. Both the ILs are liquids at room temperature, and the viscosities are about 60 mPa s at 25 °C. Behavior of lithium redox, chemical stability against lithium metal and charge–discharge characteristics of lithium battery, are also examined for the two ILs as electrolyte with 0.6 mol kg−1 LiTFSA. Though the cathodic limiting potentials of the two ILs are higher than 0 V versus Li/Li+, the lithium plating and stripping on Ni electrode can be observed in the two IL electrolytes, indicating their good chemical stability against lithium metal. Li/LiFePO4 cells using the two IL electrolytes without any additive showed good cycle property at the current rate of 0.2 C at 25 °C and 55 °C.Highlights► New functionalized guanidinium ILs with two ether groups are reported. ► They have low viscosity and good electrochemical stability. ► Li/LiFePO4 cells using these IL electrolytes have good capacity and cycle property.

Assembled microspherical cathodes have attracted great attention thanks to their high tap density, good rate capability and cycling stability. However, for layered Li-rich transition-metal oxides (LROs), the preparation of uniformly assembled microspheres still faces many challenges due to harsh synthetic conditions and the nature of multiple metal elements. In this work, Li1.17Mn0.50Ni0.16Co0.17O2 assembled microspheres have been prepared by a new route tactfully combining a solvothermal process and a molten-salt method. The use of a solvothermal process is helpful for the preparation of precursors with assembled microspherical morphology, and the addition of complex salts (NaCl and KCl), can increase the uniformity of cation distribution. The product obtained at 800 °C delivers the best electrochemical performances among all samples. At a current density of 300 mA g−1, its initial discharge capacity is larger than 228 mA h g−1, corresponding to a capacity retention ratio of 86.8% after 200 cycles. Even if the current density increases to 2000 mA g−1, its discharge capacity is still as large as 156 mA h g−1. What's more, we discover the moving rate of Li-ions during the sintering process will affect the uniformity of Li2MnO3-like and LiMO2 components in LRO assembled microspheres. This discovery is helpful for the preparation of LRO assembled microspheres with excellent electrochemical performances.