Cycling Performance Of Lithium Ion Batteries Research Articles

Organosilane Coatings for Ni-Rich High-Voltage Lithium Ion Batteries

One of the more promising cathode materials for high-energy high-voltage lithium ion batteries (LIBs) are Ni-rich layered cathode materials such as LiNixMnyCozO2 (where x + y + z = 1). Although these cathode materials are attractive from a high capacity standpoint, they commonly suffer from poor structural stability at highly delithiated (charged) states leading to poor electrochemical performance.1-3 While incorporation of additives into the electrolyte provides one way to improve cycling performance in LIBs, surface coatings are becoming a widely adopted method to improve the cycling performance of high-voltage cathode materials. More widely investigated coating materials include oxide coatings (Al2O3, TiO2, etc.)4 while comparatively fewer studies have investigated organic monomer or polymer coatings (PEDOT, polyimide, etc.).5 Additionally, the mechanism of the coating interaction with the material is intriguing due to its importance in creating a performance-enhancing coating. In this work, the surface of LiNi0.5Mn0.3Co0.2O2 cathode materials were coated with organosilane reagents to investigate their effect on cell electrochemical performance. The impact of the cathode pretreatment, the weight percentage of organosilane used and the functionality of the organosilane used will be presented. Characterization of the organosilane coating using SEM shows a slight morphological change in the cathode surface while EDX and FTIR confirm the presence of a Si-containing coating. Half cells (Li/ LiNi0.5Mn0.3Co0.2O2) using the organosilane coated cathodes operating in the voltage window of 3.0 - 4.4 V showed similar cycling behavior to that of non-coated material. In conclusion, a new type of organic coating was utilized on Ni-rich cathode materials and this research explores a new type of coating for high voltage cathodes. References 1) Son, I. H.; Park, J. H.; Kwon, S.; Mun, J. and Choi, J. W. Chem. Mat. 2015, 27, 7370-7379. 2) Bak, S.-M.; Hu, E.; Zhou, Y.; Yu, X.; Senanayake, S. D.; Cho, S.-J.; Kim, K.-B.; Chung, K. Y.; Yang, X.-Q. and Nam, K.-W. ACS Appl. Mater. Interfaces 2014, 6, 22594-22601. 3) Lin, F.; Markus, I. M.; Doeff, M. M.; Xin, H. L. Sci. Rep., 2014, 4, 5694. 4) Xu, S.; Jacobs, R. M.; Nguyen, H. M.; Hao, S.; Mahanthappa, M.; Wolverton, C.; Morgan, D. J. Mater. Chem. A, 2015, 3, 17248. 5) Zhang, J.; Lu, Q.; Fang, J.; Wang, J.; Yang, J. and NuLi, Y. ACS Appl. Mater. Interfaces 2014, 6, 17965-17973.

V2O5 self-assembled nanosheets as high stable cathodes for Lithium-ion batteries

The V2O5 self-assembled nanosheets (SANsV2O5) were synthesized by an additive-free ultrasonic method with subsequent thermal decomposition. The loose nanostructured V2O5 is multi-layer structure stacked by 4∼6 layers with a thickness of 200∼300nm and each layer is composed by abundant nanoparticles in order. Due to this unique layered structure, the SANs V2O5 exhibits remarkable rate performance and excellent cycling stability. Specifically, it delivers reversible discharge capacities of 157.2 and 111.6mAhg−1 at current densities of 2C and 5C. After 300 cycles, it still maintained high capacities of 123.5 and 85.1mAhg−1, which corresponds to a capacity loss of 0.07% and 0.08% per cycle, respectively. In addition, even at 10C, it displays a capacity of 68.8mAhg−1 and 87.9% of the capacity (60.5mAhg−1) is remained after 300 cycles. All the results indicate the V2O5 self-assembled nanosheets could be a promising candidate as cathode active material for long-term cycling performance in Lithium-ion batteries.

Studies of the Effect of High Voltage on the Impedance and Cycling Performance of Li[Ni0.4Mn0.4Co0.2]O2/Graphite Lithium-Ion Pouch Cells

Introduction Electrolyte additives are the most effective way to improve the calendar life and cycling performance of lithium-ion batteries. Vinylene carbonate (VC) is perhaps the most famous and widely used additive and has been shown to improve cycle and calendar life of Li-ion cells [1]. VC is less effective, however, when used in cells cycling to potentials above 4.2 V [2] or at elevated temperatures [3]. Sulfur-containing additives have recently been investigated by several research groups in the hopes of overcoming the temperature sensitivity of VC and extending the usable voltage range of Li-ion cells [4-6]. LiCoO2/graphite cells with upper cutoff potentials of 4.4 V are now common in the marketplace, but Li[Ni0.4Mn0.4Co0.2]O2 (NMC442)/graphite cells that can be charged to 4.4 V are not, since NMC/graphite cells normally do not function well when charged to high potential. One possible factor leading to severe capacity fade in NMC cells, particularly when charged to high potential, is large impedance growth. It is therefore important and valuable to measure cell impedance as a function of cycle number, time, and voltage. Experimental NMC442/graphite pouch cells containing various electrolyte additives, either singly or in combination, were studied using cycling experiments up to 4.4 and 4.5 V coupled with simultaneous electrochemical impedance spectroscopy (EIS) measurements using a frequency response analyzer (FRA). The impacts of adding prop-1-ene-1,3-sultone (PES), VC, triallyl phosphate (TAP), methylene methane disulfonate (MMDS), ethylene sulfate (DTD) and/or tris(-trimethyl-silyl)-phosphite (TTSPi) to 1M LiPF6 ethylene carbonate:ethyl methyl carbonate (EC:EMC) electrolyte were studied. The additive combination 2% PES + 1% MMDS + 1% TTSPi, named PES211, is highlighted. Results Cells underwent either continuous charge-discharge cycling or charge-hold-discharge cycling in which the cells were held at either 4.4 or 4.5 V for 20 hours. The FRA measured cell impedance every 0.1 V of one cycle during regular intervals. Figure 1 shows a comparison of the discharge capacity (top panels) and Rct, a combination of the charge transfer resistances from both the positive and negative electrodes as well as resistance due to the motion of ions through the SEI layers at both the positive and negative electrodes, as a function of voltage (bottom panels) for NMC442/graphite cells containing PES211. Figure 1a shows the discharge capacity as a function of cycle number for cells containing PES211 undergoing continuous cycling and cycling with a 24 hr hold at 4.4 V. Despite the excellent capacity retention of the PES211–containing cell during continuous cycling, the hold at high voltage leads to severe discharge capacity fade. This effect is also seen in Figure 1b, which shows the discharge capacity as a function of time. Figure 1c shows Rct as a function of voltage for the PES211–containing cell undergoing charge-hold-discharge cycling. This cell exhibited severe impedance growth over 50 cycles (~1700 hours). Interestingly, the impedance was reversible over one cycle, while the impedance was irreversible over several cycles. Figure 1d shows Rct as a function of voltage for the PES211–containing cell undergoing continuous cycling. This cell had very low impedance at all voltages and all 285 cycles (~3000 hours). Figure 1 dramatically demonstrates that holding NMC442/graphite cells at high potentials for extended periods (as might be expected in real-life charging situations in portable electronics, for example) leads to severe impedance growth issues, at least with PES211 electrolyte, which might be overcome through the use of better additive combinations. The importance of performing experiments representative of “real-life” Li-ion cell use is demonstrated. Practical applications that use Li-on batteries often leave the cells at high voltages for extended periods of time after charging. Therefore, studying the effect of extended periods of time at high voltage on the performance and impedance growth of Li-ion cells is imperative.

Lithium-Ion Polymer Batteries Assembled with a Cross-Linked Composite Polymer Electrolyte Using a Vinyl-Functionalized SiO2 Nanoparticles

Recently, a safety of lithium-ion batteries has been a critical issue as the demand of high capacity batteries for electric vehicles and energy storage systems has increased. The electrolyte commonly used in current lithium-ion batteries is lithium salts (mostly LiPF6) in a mixture of carbonate-based organic solvents. However, the high flammability of the organic solvents in liquid electrolyte can lead to fires and explosions when short circuits or local overheating accidentally occurs [1]. Furthermore, HF can be generated by thermal decomposition and hydrolysis of LiPF6 by trace moisture in the electrolyte solution, which leads to degradation of the cycling performance of lithium-ion batteries at high temperature [2, 3]. Among the various electrolyte systems for lithium-ion batteries, the cross-linked polymer electrolyte is one of the promising candidates due to its high ionic conductivity, good interfacial properties and thermal safety with no electrolyte leakage. In this work, we synthesized the vinyl-functionalized SiO2 particles and coated them onto the fibrous polymer membrane or the microporous polyolefin separator. The SiO2 particles on polymer participated in free radical polymerization with tri(ethylene glycol) diacrylate in a gel electrolyte precursor to form a cross-linked composite polymer electrolyte. An in-situ cross-linking reaction using these reactive SiO2-coated separators promoted strong interfacial adhesion between electrodes and separator, and allowed effective encapsulation of liquid electrolyte, resulting in low interfacial resistances. In addition, the silica nanoparticles played a role as HF scavengers to reduce HF content in the electrolyte at high temperature. The lithium-ion polymer cells assembled with a SiO2-coated separator exhibited superior cycling performance to cell prepared with a conventional polyethylene separator due to the strong interfacial adhesion between the electrodes and separator, as well as suppression of deleterious reactions during cycling.

Effectively Suppressing Dissolution of Manganese from LiMn2O4 Spinel Via a Novel Nanoscale Surface-Doping Approach

The capacity fade of LiMn2O4-based cells is directly associated with the dissolution of Mn from the cathode/electrolyte interface due to the disproportion reaction of Mn(III), and the subsequent deposition of Mn2+ onto the anode leading to an increase in impedance of the cell. Suppressing the dissolution of Mn from the cathode is therefore critical to reducing capacity fade of LiMn2O4–based cells. In this work, we report a novel nanoscale surface-doping approach that minimizes Mn dissolution from LiMn2O4. This new approach exploits advantages of both bulk doping and surface coating methods by a) stabilizing the surface crystal structure of LiMn2O4 through cationic doping while maintaining the bulk spinel LiMn2O4 structure, and b) protecting the bulk LiMn2O4 from electrolyte corrosion while maintaining ion and charge transport channels on the surface through the electrochemically active doping layer. As a consequence, the surface-doped LiMn2O4 demonstrates significantly enhanced electrochemical performance in terms of cycleability and capacity at elevated temperature. This study provides encouraging evidence that surface doping could be a promising alternative to improve the cycling performance of lithium-ion batteries.

A polyaniline-coated mechanochemically synthesized tin oxide/graphene nanocomposite for high-power and high-energy lithium-ion batteries

Although intensive efforts have been made during the past decades, development of an anode material with high specific capacity and stable cycling performance for lithium-ion batteries (LIBs) using a cost-effective preparation method still remains challenging. Herein, we report a polyaniline (PANI)-coated mechanochemically synthesized SnO2/graphene (SG) nanocomposite via in situ polymerization. PANI-coated nanocomposites are successfully prepared with different raw material mass ratios (aniline:SG, 0.15:1, 0.2:1, 0.25:1). The nanocomposite with initial aniline:SG mass ratio of 0.2:1 (20%PANI-SG) contains an optimal structure housing genuine PANI nanofibers as conductive bridges and a relatively high surface area of 158.5 m2 g−1; furthermore, it exhibits a stable cycling performance over 100 cycles at high current density (1000 mA g−1) with a specific capacity of more than twice that of the starting SG electrode at the 100th cycle. Additionally, this material achieved an outstanding cycling rate with current densities changing stepwise from 100 to 3000 mA g−1 and back, and exhibited a specific capacity of 467 mA h g−1 even at 2000 mA g−1. In terms of the electrochemical stability, rate capability and cost-effective preparation process, the PANI-SG nanocomposite is a viable anode material for next-generation high-power and high-energy LIBs.

Cycling Performance of Lithium-Ion Batteries with Non-Uniform Size Particles in the Carbon Anode

Lithium-ion batteries (LIBs) are being considered as portable power sources for modern electric and hybrid vehicles due to their high gravimetric and volumetric energy density. However, the discharge capacity of LIBs decreases with continuous cycling due to various associated side reactions. The intensity of degradation caused by side reactions might be reduced through control of design and operational parameters i.e. charge & discharge rate, operational voltage, electrode thickness and particle size. Mathematical modeling is an important tool to analyze the effect of these variables on the performance of LIBs, while measuring their influence through experiments is difficult and expensive. In a recent study1, a mathematical model was developed to quantify the solid-electrolyte interface (SEI) film thickness and gas volume in a carbon electrode for different discharge rates up to a 1000 cycles; however, it was assumed that the carbon electrode is composed of identical particles. In this work, the model developed in ref. [1] is being extended to analyze the effect of cycling lithium-ion cells with carbon negative electrodes having different particle configurations and to observe the impact of electrode morphology over cell performance. For this purpose, the carbon electrode will be split into three equal sub-parts such that within each layer particles of the same size exist. The radius of particles is either 0.7Rp, Rp or 1.3 Rp (so that the interlayer sizing is non-uniform), where Rp is radius of the reference case considered in ref. [1]. All possible combinations of the final electrode morphologies to be considered are shown in Fig. 1. One representation of fresh and cycled negative electrode is shown in Fig. 2, with smallest particle section sandwiched between medium and largest particle layers at the current collector and separator ends, respectively. As is given in Eq. (1), cycling of the cell results in the decomposition of the electrolyte at the negative electrode leading to SEI film formation and generation of ethylene gas as a by-product of the side reaction. The SEI film increases the resistance for lithium diffusion in the carbon active material and the gas generated occupies the space previously occupied by the pores of the electrode (as shown in Fig. 2) slowing the movement of Li-ions in the electrolyte, both leading to reduction in cell capacity. The governing equations of the pseudo-2D model2-3 along with SEI formation and gas evolution1 will be solved using COMSOL Multiphysics 4.3a for all morphologies of MCMB anode shown in Fig. 1 with LiMn2O4 cathode. The cell will be cycled till either of 80% loss (end of life) of rated capacity or 1000 cycles is reached. A comparative analysis of discharge capacity, cycling performance, thickness of SEI film and gas generation will be demonstrated. It is expected that this study will pave the way for design of carbon electrodes for achieving higher discharge capacity. From preliminary data results obtained so far, B-A-C or C-A-B based negative electrodes have shown much better performance than the remaining four combinations. In the latter cases, the battery loses 80% of the rated capacity before the completion of 1000 cycles, whereas a high discharge capacity, low film thickness and lesser gas is generated after the same number of cycles in the former cases.

Improving the long-term cycling performance of lithium-ion batteries at elevated temperature with electrolyte additives

The effects of vinylene carbonate-based and prop-1-ene-1,3-sultone-based electrolyte additives on the cycling behavior of Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch type cells at elevated temperature have been systematically studied. Capacity fade during cycling, charge-transfer resistance before and after cycling as well as gas evolution during formation and also during cycling were examined and compared. For vinylene carbonate-based additive blends, only 3% vinylene carbonate, 2% vinylene carbonate+1% 1,3,2-dioxathiolane-2,2-dioxide+1% tris(trimethylsilyl) phosphite or 2% vinylene carbonate + 1% methylene methyl disulfonate + 1% tris(trimethylsilyl) phosphite showed less capacity fade than 2% vinylene carbonate alone. Cells with all of these vinylene carbonate-based electrolyte additive blends lost more than 20% of their initial capacity after ∼1000 cycles at 55 °C and all the vinylene carbonate-based cells swelled more than 10% of their initial volume during this test. Cells containing all prop-1-ene-1,3-sultone-based additive blends generally produced much less gas than the vinylene carbonate-based blends. Many cells containing prop-1-ene-1,3-sultone-based additive blends lost less than 20% of their initial capacity after 1000 cycles. Moreover, the impedance of these prop-1-ene-1,3-sultone-based electrolytes decreased after long-term cycling. These results suggest that prop-1-ene-1,3-sultone-based electrolytes are more useful than vinylene carbonate-based electrolytes at high temperatures in Li[Ni1/3Mn1/3Co1/3]O2/graphite cells.

Silver-modified hollow ZnSnO3 boxes as high capacity anode materials for Li-ion batteries

Silver particles were dispersed on the hollow ZnSnO3 cubes by a thermal decomposition method. With good cycling performance for lithium-ion batteries anode materials, the impacts of the Ag content on the electrochemical properties of the electrodes were compared. The results showed that the silver-modified hollow ZnSnO3 boxes composites exhibit higher specific capacity and better cycling performance than hollow ZnSnO3 boxes. When Ag content reaches at about 5wt%, the retain capacity is 464.5mAh/g after 45 cycles at a current density of 300mA/g, which is higher than that of hollow ZnSnO3 boxes.

Nitrogen-doped Carbon-coated SnxOy (x = 1 and y = 0 and 2) Nanoparticles for Rechargeable Li-Ion Batteries

Polyacrylonitrile was used as a precursor to coat N-doped carbon (NC) on SnxOy (x=1 and y=0 and 2) nanoparticles. The effect of NC coating on electrochemical performance of SnxOy anodes was investigated. Fine dispersion of metal nanoparticles coated with carbon was observed from microscopic analysis. Systematic studies were performed to understand the electrochemical behavior as well as Li-ion battery performance of the prepared materials. Noticeable capacity improvements for NC-SnO2 and NC-Sn were achieved over carbon-coated particles reported previously in the literature. Among the investigated materials, 50wt% NC-SnO2 nanoparticles exhibited the best reversible capacity of 621mAhg−1 while 60wt% NC-Sn exhibited the highest capacity of 450mAhg−1 at 100mA/g after 50 cycles. Electrochemical studies revealed that the carbon shell can effectively increase the stability of the active materials and improve the cycling performance for lithium-ion batteries. NC-protected Sn nanoparticles were also synthesized using poly(vinylpyrrolidone) as a precursor and this material exhibited a significantly high discharge capacity of 650mAhg−1 at 100mA/g after 50 cycles. The studies performed on the Sn-based anodes show the potential of carbon content/coating level and metal particle size for improved Li-ion battery performance.

Building sponge-like robust architectures of CNT–graphene–Si composites with enhanced rate and cycling performance for lithium-ion batteries

High-performance robust CNT–graphene–Si composites are designed as anode materials with enhanced rate capability and excellent cycling stability for lithium-ion batteries. Such an improvement is mainly attributed to the robust sponge-like architecture, which holds great promise in future practical applications.

Effectively suppressing dissolution of manganese from spinel lithium manganate via a nanoscale surface-doping approach.

The capacity fade of lithium manganate-based cells is associated with the dissolution of Mn from cathode/electrolyte interface due to the disproportionation reaction of Mn(III), and the subsequent deposition of Mn(II) on the anode. Suppressing the dissolution of Mn from the cathode is critical to reducing capacity fade of LiMn2O4-based cells. Here we report a nanoscale surface-doping approach that minimizes Mn dissolution from lithium manganate. This approach exploits advantages of both bulk doping and surface-coating methods by stabilizing surface crystal structure of lithium manganate through cationic doping while maintaining bulk lithium manganate structure, and protecting bulk lithium manganate from electrolyte corrosion while maintaining ion and charge transport channels on the surface through the electrochemically active doping layer. Consequently, the surface-doped lithium manganate demonstrates enhanced electrochemical performance. This study provides encouraging evidence that surface doping could be a promising alternative to improve the cycling performance of lithium-ion batteries.

Studies of the Effect of Varying Prop-1-Ene-1,3-Sultone (PES) Concentration on Li-Ion Batteries

Introduction Electrolyte additives are the most effective way to improve the calendar life and cycling performance of lithium-ion batteries. Vinylene carbonate (VC) is perhaps the most well-known additive for Li-ion batteries and has been shown to be useful during the solid electrolyte interphase (SEI) formation [1], which can result in longer calendar life and reduced irreversible capacity (IRC). However, it has been shown that the performance of cells containing VC decreases at high temperatures and high voltages. Sulfur-containing additives have been studied in hopes to overcome the temperature sensitivity of VC. Prop-1-ene-1,3-sultone (PES) was found to have smaller IRC than VC-containing cells and was suggested to be an SEI forming additive [2]. In this study, we show that PES is a useful additive for Li-ion pouch cells and suggest that PES, when used in combination with other additives, may be more effective than VC at achieving long lifetimes and high cycling performance. Experimental Li[Ni1/3Mn1/3Co1/3]O2 (NMC)/graphite pouch cells made with 1M LiPF6in EC:EMC 3:7 (by weight) and varying amounts of PES were studied using ultra high precision coulometry and storage experiments. In addition, gas evolution measurements employing Archimedes principle and electrochemical impedance spectroscopy (EIS) measurements were performed. The PES-containing cells were compared to the same control electrolyte containing 2% VC. Results Figure 1 shows a preliminary set of results from both storage and cycling experiments of PES-containing cells compared with control electrolyte and 2% VC-containing cells. The dotted line in Figures 1a, d, and e indicates the value for 2% VC. Figure 1a shows the gas evolution during formation at 40°C and Figures 1b and c show gas evolution during storage at 60°C and 40°C, respectively. Cells containing 2, 4 and 6% PES show smaller gas during formation than that of 2% VC and all concentrations of PES studied show significantly smaller gas during 40°C and 60°C storage than VC-containing cells. The most prominent advantage of PES compared to VC is the extremely low gas evolution during high temperature storage. Figure 1d shows the voltage drop during storage from 4.2 V at 40°C for 500 hours. Figure 1e shows the average coloumbic inefficiency (CIE) during the last three cycles of cycling between 2.8 V and 4.2 V at C/20 and 40°C. Through the voltage drop and CIE, it is clear that 2% PES appears to be as good as 2% VC. This work suggests that PES is a viable alternative to VC and is superior during high temperature storage. PES may also prove to be beneficial in combination with other additives.

Effects of polyaniline coating of cryptomelane-type KMn8O16 on electrochemical performance for lithium-ion batteries

KMn8O16 nanorods were synthesized via a reflux method, and polyaniline-coated KMn8O16 nanorods were prepared via oxidative polymerization of aniline in acidic medium. The microstructures and morphologies of the KMn8O16 nanorods and PANI-coated KMn8O16 nanorods were characterized using powder X-ray diffraction, thermogravimetric analysis, Raman measurements, scanning electron microscopy, and transmission electron microscopy. The as-prepared KMn8O16 and PANI-coated KMn8O16 are all mainly composed of nanorods with a diameter around 5–20 nm and a length of around 100–300 nm. The electrochemical measurements demonstrated that the KMn8O16 nanorods as cathode material have both higher reversible capacity of 168 mAh g−1 at 50 mA g−1 current density and excellent cycling performance in lithium-ion batteries at room temperature over 100 cycles, in the range 1.5–4.2 V. PANI-coated KMn8O16, in addition to its excellent cyclability, presented a much higher reversible discharge capacity. The PANI-coated KMn8O16 composite could retain a discharge capacity of 183 mAh g−1 after 100 cycles.

Nitrogen-doped carbon coated TiO2 nanocomposites as anode material to improve cycle life for lithium-ion batteries

Anatase TiO2 has attracted much attention as a safe anode material for lithium-ion battery applications, due to its low electronic conductivity and severe aggregation during Li+ insertion/extraction processes, the practical application of the anatase TiO2 is still hindered by poor long-term cycling stability. In order to improve the cycling performance of lithium-ion batteries, a uniform thin nitrogen-doped carbon layer with a thickness of 3–6nm is successfully coated on the surface of the anatase TiO2 nanoparticles by using polydopamine as carbon precursor. Compared with the pristine TiO2 electrode, the carbon coated TiO2 nanocomposites electrodes show very good capacity retention and cycling performance.

Facile synthesis of cookies-shaped LiV3O8 cathode materials with good cycling performance for lithium-ion batteries

Cookies-shaped LiV3O8 materials were successfully synthesized by a facile ethylene glycol-assisted sol–gel method. The LiV3O8 compound fabricated at 550 °C delivers an initial specific discharge capacity of 255.2 mAh g−1 between 2.0 and 4.0 V at a current density of 50 mA g−1, and possesses a capacity retention of 90.2% after 50 cycles and up to 85% at a current density of 120 mA g−1 after 100 cycles. Furthermore, the compound with a proper particle size and high crystallinity also shows high electrochemical reversibility and structural stability, leading good rate capability. By analysis of inductively coupled plasma emission spectrometer (ICP), the cookies-like LiV3O8 has very little dissolution of vanadium in the electrolyte after 100 cycles, indicating that the well-formed crystal can protect the structure damage to some degree during cycling.

Cycle Performance of Lithium-Ion Batteries Containing Ionic Liquids with Improved Reduction Stability

The high-temperature cycling performance of lithium-ion batteries containing a new series of ionic liquids based on an aliphatic quaternary ammonium cation and a bis(fluorosulfonyl)amide anion were investigated, by conducting charge-discharge cycling tests of graphite / LiFePO4 cells. The new series of ionic liquids were 1,3-dimethyl-1-propylpiperidinium bis(fluorosulfonyl)amide (3mPP13-FSA) and 7-methyl-5-azoniaspiro[4,5]decane bis(fluorosulfonyl)amide (7mAS45-FSA), which were obtained by introducing methyl groups to known compounds. The relationship between a change in electronic structure and reduction stability was researched. Introducing methyl group improved reduction stability as electron-donating ability. Further, melting points were lowered owing to the asymmetric structures of the cations. The electrochemical properties of these ionic liquids were evaluated by linear sweep voltammetry. The cycle performance of graphite/LiFePO4 cells containing these ionic liquids for the electrolytes was improved compared with that of cells containing organic electrolytes based on LiPF6/ECDEC, and the capacity retention ratios were more than 50% at the 500th cycle at 60°C.

Improvement of Cycle Performance of Lithium-Ion Batteries at Elevated Temperature of 60oC Using Graphite Coated with Metal Oxide

Improvement of cycle performance at 60oC by using SiOx modified graphite has been investigated. A main degradation mode of LiFePO4/Graphite cell was investigated. In LiFePO4 and graphite, the degradation of LIB is mainly due to the loss of Li+, and probably not due to the damage to active material analyzed by XRD pattern and charge-discharge curve. To suppress the loss of Li+, surface modification was examined. SEI formation on Si film is suppressed by modification of SiOx and AlOx. SEI thickness on the Si thin film coated with metal oxide (MOx) has critical influence on resistance of MOx. In the cyclic voltammetry, the intensity of the charge peak at 0.9V during the first charge indicates decomposition of an electrolyte. In SiOx modified graphite, the capacity estimated from 1 V to 0.7 V is decreased compared to the unmodified graphite. Cycle performance was improved by SiOx modified graphite.

A facile strategy to prepare nano-crystalline Li4Ti5O12/C anode material via polyvinyl alcohol as carbon source for high-rate rechargeable Li-ion batteries

Li4Ti5O12/C nano-particles is synthesized at 800°C, and investigated by XRD, SEM, TEM and HRTEM, and the carbon in the composite is detected by Raman spectroscopy. Experimental results indicate that compared with those synthesized at 600, 700 and 900°C and pure Li4Ti5O12 the Li4Ti5O12/C nanoparticles synthesized at 800°C remarkably enhances the rate performance and cycling performance of lithium-ion batteries. Lithium-ion batteries with nanocrystalline Li4Ti5O12 used as anode exhibit an excellent reversible capacity of 169, 141, 132, 106 and 82mAhg−1, at the current density of 1C, 10C, 20C, 40C and 80C, respectively.

Improvement of Cycle Performance of Lithium-Ion Batteries at Elevated Temperature of 60° Using Graphite Coated with Metal Oxide

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