Study on capacity fade factors of lithium secondary batteries using LiNi0.7Co0.3O2 and graphite-coke hybrid carbon

In our previous work, 10 Wh-class (30650 type) lithium secondary batteries, which were fabricated with LiNi_0.7Co_0.3O₂ positive electrodes and graphite-coke hybrid carbon negative electrodes, showed an excellent cycle performance of 2350 cycles at a 70% state of charge charge-discharge cycle test. However, this cycle performance is insufficient for dispersed energy storage systems, such as home use load leveling systems. In order to clarify the capacity fade factors of the cell, we focused our investigation on the ability discharge capacity of the positive and negative electrodes after 2350 cycles. Although the cell capacity deteriorated to 70% of its initial capacity after 2350 cycles, it was confirmed that the LiNi_0.7Co_0.3O₂ positive electrode and graphite-coke hybrid negative electrode after 2350 cycles still have sufficient ability discharge capacity of 86 and 92% of their initial capacity, respectively. Accompanied by the result for a composition analysis of the positive electrode material by inductively coupled plasma (ICP) spectroscopy and atomic absorption spectrometry (AAS), electrochemical active lithium decreased and the Li_xNi_0.7Co_0.3O₂ positive electrode could be charged-discharged in a narrow range of between x=0.41 and 0.66 in the battery, although it had enough ability discharge capacity that can use between x=0.36 and 0.87. It is predicted that solid electrolyte interface formation by electrolyte decomposition on the carbon negative electrode during the charge-discharge cycle test is a main factor of the decrease of electrochemical active lithium.     

废锂离子电池负极活性材料的分析测试

目前关于废锂离子电池资源化的研究主要集中在正极贵金属和负极铜材料的分离回收和精制方面,但对负极活性材料的资源化研究很少。本文采用XRD、SEM、GC-MS、ICP-AES等检测手段对废锂离子电池负极活性材料中石墨的结构、有机物的种类以及Li、Gu等金属的含量进行测试分析。结果显示,其主要组分石墨的本体结构基本无变化,仍保持完整的层状结构,但是其中含有一定量的有机物质,如有机电解质及增塑剂等。经过提纯,可以将其作为石墨原料进行资源化再利用;此外,稀有金属Li含量较高,为31.03 mg/g,分离回收的价值较高。     

制备锂电池电解质六氟磷酸锂的工艺探讨

分析了目前国内外锂电池的发展状况,并对其中一种电解质六氟磷酸锂的制备工艺作了一些探讨。简要介绍了国外已经研究出的六氟磷酸锂的替代产品———氟烷基磷酸锂用作锂电池电解质。     

Cathode materials modified by surface coating for lithium ion batteries

Recent research results confirm the importance of structural surface features of cathode materials for their electrochemical performance. Modification by coating is an important method to achieve improved electrochemical performance, and the latest progress was reviewed here. When the surface of cathode materials including LiCoO₂, LiNiO₂, LiMn₂O₄ and LiMnO₂ is coated with oxides such as MgO, Al₂O₃, SiO₂, TiO₂, ZnO, SnO₂, ZrO₂, Li₂O·2B₂O₃-glass and other materials, the coatings prevent the direct contact with the electrolyte solution, suppress phase transition, improve the structural stability, and decrease the disorder of cations in crystal sites. As a result, side reactions and heat generation during cycling are decreased. Accompanying actions such as suppression of Mn²⁺ dissolution, increase in conductivity and removal of HF in electrolyte solutions have been observed. Consequently, marked improvement of electrochemical performance of electrode materials including reversible capacity, coulomb efficiency in the first cycle, cycling behavior, rate capability and overcharge tolerance has been achieved. In conclusion, further directions are suggested for the surface modification of electrode materials. With further understanding of the effects of the surface structure of cathode materials on lithium intercalation and de-intercalation, better and/or cheaper cathode materials from surface modification will come up in the near future.     

Improving ionic conductivity of polymer-based solid electrolytes for lithium metal batteries

Because of its superior safety and excellent processability, solid polymer electrolytes (SPEs) have attracted widespread attention. In lithium based batteries, SPEs have great prospects in replacing leaky and flammable liquid electrolytes. However, the low ionic conductivity of SPEs cannot meet the requirements of high energy density systems, which is also an important obstacle to its practical application. In this respect, escalating charge carriers (i.e. Li⁺) and Li⁺ transport paths are two major aspects of improving the ionic conductivity of SPEs. This article reviews recent advances from the two perspectives, and the underlying mechanism of these proposed strategies is discussed, including increasing the Li⁺ number and optimizing the Li⁺ transport paths through increasing the types and shortening the distance of Li⁺ transport path. It is hoped that this article can enlighten profound thinking and open up new ways to improve the ionic conductivity of SPEs.     

Thin films of lithium manganese oxide spinel as cathode materials for secondary lithium batteries

The miniaturization of rechargeable lithium-ion batteries requires high quality thin-film electrodes. Electrostatic spray deposition (ESD) technique was used to fabricate LiMn₂O₄ thin-film electrodes with three different morphologies: sponge-like porous, fractal-like porous, and dense structures. X-ray diffraction (XRD) and scanning electron microscopy were used to analyze the structures of the electrodes. These electrodes were made into coin cells against metallic lithium for electrochemical characterization. Galvanostatic cycling of the cells revealed different rate capability for the cells with LiMn₂O₄ electrodes of different morphologies. It is found that the cells with LiMn₂O₄ electrodes of porous, especially the sponge-like porous, morphology better rate capability than those with dense LiMn₂O₄ electrodes. Electrochemical impedance spectroscopy (EIS) study indicates that the large surface area of the porous electrodes should be attributed to the smaller interfacial resistance and better rate capability.     

A novel method for preparation of macroposous lithium nickel manganese oxygen as cathode material for lithium ion batteries

A simple one-step route using gas template method is applied to synthesize macroporous LiNi_0.5Mn_0.5O₂ which is characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Telle (BET) surface area, charge–discharge tests and electrochemical impedance spectroscopy (EIS) measurements. The as-synthesized material shows pure crystalline phase of LiNi_0.5Mn_0.5O₂, while the microstructure is comprised of macrospores ranging from 0.2 to 0.5 μm. The first discharge capacity is of 174 mAh g⁻¹ at 0.1 C rate, which is much higher than that of the material synthesized by the conventional solid state reaction method. Furthermore, the macroporous LiNi_0.5Mn_0.5O₂ material shows remarkable rate capacity and cycle stability, which may be attributed to the shorter lithium ion diffusion distance and better electrolyte penetration.     

锂离子电池正极材料LiFePO4的改性研究进展

橄榄石型结构的磷酸亚铁锂( LiFePO4)作为备受关注的锂离子电池正极材料,可望成为新一代首选的可代替钴酸锂的锂离子二次电池正极材料.详细地叙述了近年来国内外对LiFePO4改性所做的研究,着重介绍了导电剂掺杂包覆、金属离子掺杂和合成方法对LiFePO4电化学性能的影响,以及这些改性方法存在的问题.     

CuO/C microspheres as anode materials for lithium ion batteries

CuO/C microspheres are prepared by calcining CuCl₂/resorcinol-formaldehyde (RF) gel in argon atmosphere followed by a subsequent oxidation process using H₂O₂ solution. The microstructure and morphology of materials are characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and transition electron microscopy (TEM). Carbon microspheres have an average diameter of about 2 μm, and CuO particles with the sizes of 50–200 nm disperse in these microspheres. The electrochemical properties of CuO/C microspheres as anode materials for lithium ion batteries are investigated by galvanostatic discharge–charge and cyclic voltammetry (CV) tests. The results show that CuO/C microspheres deliver discharge and charge capacities of 470 and 440 mAh g⁻¹ after 50 cycles, and they also exhibit better rate capability than that of pure CuO. It is believed that the carbon microspheres play an important role in their electrochemical properties.     

Anodic titania films as anode materials for lithium ion batteries

Titania thin films were prepared through the anodisation of titanium metal in a 1.0 M sulphuric acid solution at 80 °C utilising a series of pulsed dc constant currents of increasing magnitude. Films were then tested as a potential anode material for lithium batteries using a variety of techniques. Electrochemical testing revealed that the films (3.8 cm²) offered good rate capabilities affording a constant capacity of 48 μAh for a constant current of 10 μA which decreased to 25 μAh on increasing the current to 1250 μA. Cyclic voltammetry was conducted over a range of scan rates from which capacitive currents were examined and rate constants, transfer coefficients and diffusion coefficients calculated. Electrochemical impedance spectroscopy was conducted over six potentials in the range 0.1-2.7 V with the experimental data successfully modelled using an equivalent circuit with the notation R(Q(RW))C. TEM observation of focussed ion beam milled cross-sections showed significant structural differences between the as-anodised film and those cycled in a lithium battery. Raman spectroscopy showed that the films had an anatase character that transformed into an unidentified lithium-containing, titanate phase on cycling. Based on a film thickness of 100 nm, and assuming density of 4 g cm⁻³ such films offered a stable capacity of 316 mAh g⁻¹.     

A study on the cycle performance of lithium secondary batteries using lithium nickel-cobalt composite oxide and graphite/coke hybrid carbon

10 Wh-class (30650 type) lithium secondary batteries were fabricated using LiNi_0.7Co_0.3O₂ as the positive electrode material and graphite/coke hybrid carbon as the negative electrode material. In our previous work, we found that LiNi_0.7Co_0.3O₂ and graphite/coke hybrid carbon each provide a longer cycle life among several candidates (Kida et al., J. Power Sources 94 (2001) 74; Kida et al., in preparation; Kinoshita et al., J. Power Sources 102 (2001) 284). In this study, the cycle performance of cells using both LiNi_0.7Co_0.3O₂ and graphite/coke hybrid carbon was examined and the deterioration factor of the discharge capacity was investigated during charge/discharge tests. We then focused our interest on the negative electrode and analyzed it using ⁷Li nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS). After the discharge capacity of the battery deteriorated to 70% of the rated capacity after 2000 cycles, the graphite/coke hybrid carbon showed 91% of initial discharge capacity. When the solid electrolyte interface (SEI) (LiF, Li₂CO₃ and polymers) (E. Peled, J. Electrochem. Soc. 126 (1979) 2047) on the carbon negative electrode became thicker in the charge/discharge cycle test, the impedance was considered to have increased. This suggests that the deterioration of the graphite/coke hybrid carbon material is not so large, but that the production of the SEI on the negative electrode and impedance change of the negative electrode are factors of the capacity fade.     

Porous polythiophene as a cathode material for lithium batteries with high capacity and good cycling stability

Polythiophene (PTh) has been synthesized by chemical oxidative polymerization and used as an active cathode material in lithium batteries. The lithium batteries are characterized by cyclic voltammetry (CV), galvanostatic charge/discharge cycling and electrochemical impedance spectroscopic studies (EIS). The lithium battery with the PTh cathode exhibits a discharge voltage of 3.7 V compared to Li⁺/Li and excellent electrochemical performance. PTh can provide large discharge capacities above 50 mA h g⁻¹ and good cycle stability at a high current density 900 mA g⁻¹. After 500 cycles, the discharge capacity is maintained at 50.6 mA h g⁻¹. PTh is a promising candidate for high-voltage power sources with excellent electrochemical performance.     

Separator grafted with siloxane by electron beam irradiation for lithium secondary batteries

Polyethylene (PE) separator grafted with 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (siloxane) was newly prepared by electron beam irradiation. The degree of grafting and morphology of the grafted separators were characterized by FT-IR and scanning electron microscopy (SEM). The polymer electrolytes based on the grafted separators were prepared by immersing the separators in the electrolyte containing 1 M LiPF₆ in EC/DMC (1:1 by volume). The ionic conductivity of the grafted separators was changed with the degree of grafting and showed the highest value of 7 × 10⁻⁴ S cm⁻¹ at the degree of grafting of 6%. The electrochemical stability limit of the grafted separator with the degree of grafting of 6% was increased to 5.2 V. The Li ion cell using the grafted separator also showed an improved performance, suggesting that the grafted separator is a good candidate for the separator of lithium batteries at high voltage operation.     

Novel nanosized adsorbing sulfur composite cathode materials for the advanced secondary lithium batteries

A novel conductive sulfur-containing nanocomposite cathode material was synthesized by heating the mixture of sublimed sulfur and multi-walled carbon nanotubes (MWNTs) in certain conditions. The cathode with MWNTs-sulfur nanocomposite (MSN) material shows the improvement of not only the charge-discharge capacity but also cycle durability. From the results, it is confirmed that the MWNTs shows a vital role on adsorbing sublimed sulfur and the polysulfides within the cathode and is an excellent electric conductor for the lithium-sulfur rechargeable system. It can effectively prevent the shuttle behavior of the lithium-sulfur battery.     

A soft chemical route to multicomponent lithium transition metal oxide nanowires as promising cathode materials for lithium secondary batteries

We have synthesized 1D nanowires of lithium nickel manganese oxides with two different crystal structures through the chemical oxidation reaction of solid-state precursor LiMn_0.5Ni_0.5O₂ under hydrothermal condition. According to X-ray diffraction and elemental analyses, the nanowires obtained by persulfate treatments at 65 and 120 °C crystallize with a hexagonal layered and an α-MnO₂-type structure, respectively, in which nickel and manganese ions exist in octahedral sites. Electron microscopic analyses reveal that the platelike crystallites of the precursor are changed into nanowires with the diameter of ∼20 nm after the persulfate treatment. Thermal and infrared spectroscopic analyses clearly demonstrate that, in comparison with α-MnO₂-structured nanowires, the hexagonal layered nanowires contain less water molecules in the lattice, which makes them suitable for the application as electrode materials for lithium secondary batteries. According to electrochemical measurements, the hexagonal layered nanowires show a larger discharge capacity and an excellent cyclability with respect to repeated Li intercalation-disintercalation process. X-ray diffraction and electron microscopic analyses on the samples subjected to electrochemical analysis reveal that the layered structure and 1D morphology of the nanowires are still maintained after the electrochemical cyclings, which is responsible for their excellent electrochemical performances.     

Electrochemical characteristics of copper silicide-coated graphite as an anode material of lithium secondary batteries

Copper silicide-coated graphite as an anode material was prepared by the sequential employments of plasma enhanced chemical vapor deposition (PECVD) and radio frequency magnetron sputtering (RFMS) method at 300 °C. The silicon-coated graphite exhibited an initial discharge capacity of 540 mAh/g with 76% coulomb efficiency, and the discharge capacity was sharply decreased down to 50% of initial capacity after 30 cycles, probably due to large volume changes during the charge-discharge cycling. Copper silicide-coated graphite, however, exhibited an initial discharge capacity of 480 mAh/g with higher retention capacity of 87% even after 30 cycles, probably due to the enhanced interfacial conductivity. The copper silicide film on the graphite surface played as the active anode material of lithium secondary batteries via the reduction of interfacial resistance and mitigation of volume changes during repeated cycles.     

High lithium ion conducting Li2S–GeS2–P2S5 glass–ceramic solid electrolyte with sulfur additive for all solid-state lithium secondary batteries

Glass–ceramic Li₂S–GeS₂–P₂S₅ electrolytes were prepared by a single step ball milling (SSBM) process. Various compositions of Li_4−xGe_1−xP_xS₄ from x = 0.70 to x = 1.00 were systematically investigated. Structural analysis by X-ray diffraction (XRD) showed gradual increase of the lattice constant followed by significant phase change with increasing GeS₂. All-solid-state LiCoO₂/Li cells were tested by constant-current constant-voltage (CCCV) charge–discharge cycling at a current density of 50 μA cm⁻² between 2.5 and 4.3 V (vs. Li/Li⁺). In spite of the high conductivity of the solid-state electrolyte (SSE), LiCoO₂/Li cells showed a large irreversible reaction especially during the first charging cycle. Limitation of instability of Li₂S–GeS₂–P₂S₅ in contact with Li was solved by using double layer electrolyte configuration: Li/(Li₂S-P₂S₅/Li₂S–GeS₂–P₂S₅)/LiCoO₂. LiCoO₂ with SSEs heat-treated with elemental sulfur at elevated temperature exhibited a discharge capacity of 129 mA h g⁻¹ at the second cycle and considerably improved cycling stability.     

Layered lithium transition metal nitrides as novel anodes for lithium secondary batteries

We report the approach to overcome the deterrents of the hexagonal Li_2.6Co_0.4N as potential insertion anode for lithium ion batteries: the rapid capacity fading upon long cycles and the fully Li-rich state before cycling. Research reveals that the appropriate amount of Co substituted by Cu can greatly improve the cycling performance of Li_2.6Co_0.4N. It is attributed to the enhanced electrochemical stability and interfacial comparability. However, doped Cu leads to a slightly decreased capacity. High energy mechanical milling (HEMM) was found to effectively improve the reversible capacity associated with the electrochemical kinetics by modifying the active hosts’ morphology characteristics. Moreover, the composite based on mesocarbon microbead (MCMB) and Li_2.6Co_0.4N was developed under HEMM. The composite demonstrates a high first cycle efficiency at 100% and a large reversible capacity of ca. 450 mAh g⁻¹, as well as a stable cycling performance. This work may contribute to a development of the lithium transition metal nitrides as novel anodes for lithium ion batteries.     

锂离子电池锡基负极材料的研究进展

锡基负极材料与碳负极材料相比,具有容量密度高,安全性好等优点,成为动力锂离子电池用新型负极材料研究的热点之一。本文综述了近年来国内外针对锡基材料首次不可逆容量高、循环性能差等问题所进行的改性研究,分别从材料的制备方法、组成结构及电化学性能等方面进行比较分析,并对锡基负极材料的进一步研究、发展应用予以展望。     

Asymmetrical dicationic ionic liquids based on both imidazolium and aliphatic ammonium as potential electrolyte additives applied to lithium secondary batteries

Asymmetrical dicationic ionic liquids based on the combination of imidazolium and aliphatic ammonium cations with TFSI anion, MIC_nN₁₁₁-TFSI₂, have been synthesized for the first time, wherein MI represents imidazolium cation, N₁₁₁ represents trimethylammonium cation, and C_n represents spacer length. The physical and electrochemical properties of this family of ionic liquids were studied. 1-(3-Methylimidazolium-1-yl)ethane-(trimethylammonium) bi[bis(trifluoromethane-sulfonyl) imide] (MIC₂N₁₁₁-TFSI₂) shows solid-solid transition characteristics. 1-(3-Methylimidazolium-1-yl)pentane-(trimethylammonium) bi[bis(trifluoromethan-esulfonyl)imide] (MIC₅N₁₁₁-TFSI₂) has one of the lowest solid-liquid transformation temperatures among analogues, and belongs to the greatest thermal stable ionic liquids. Additionally, it has an order of conductivity of 10⁻¹ ms cm⁻¹, and electrochemical window of about 3.7 V at room temperature. To evaluate the potential of MIC₅N₁₁₁-TFSI₂ as an additive of electrolyte for lithium secondary batteries, cells composed of LiMn₂O₄ cathode/1 M LiPF₆ in EC:DMC (1:1, v/v) electrolytic solution containing 5 wt% of MIC₅N₁₁₁-TFSI₂/lithium metal anode have been prepared. The charge-discharge cycling test reveals that unlike the cases of Li/LiMn₂O₄ cells employing a conventional electrolyte with a monocationic ionic liquid, such as 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EtMeImTFSI) as an additive, the performances of Li/LiMn₂O₄ cells do not drop with the addition of MIC₅N₁₁₁-TFSI₂ at 1C rate, moreover, the cell exhibits better discharge capacity and cycle durability compared with the cell using the conventional electrolyte.