Li ion batteries

The impedance spectrum of polymer and gel electrolyte appears as a depressed semicircle in the frequency region between 100 kHz and 0.1 Hz, which can be analyzed using the Cole and Cole [1941] approach, as described in Section 2.I.2.3. Typically, polymeric, plasticized, and gel Li-ion conductors show abnormally low conductivity as compared to that expected from self-diffusion coefficients calculated using other methods such as PMFG-NMR (Clericuzio et al. [1995]). In addition to the usual attribution of this effect to ion association, the incomplete removal of the electrode impedance effect during analysis can contribute to an apparent increase in the electrolyte resistance. 

Impedance is typically measured in a two-electrode configuration where the electrolyte is compressed between two blocking (steel, platinum) or nonblocking Li-electrodes (Qian et al. [2(X)2]). Analysis of electrolyte impedance in the presence of electrode impedance is complicated and usually assumes that the electrolyte is responsible for the highest frequency region of the spectrum, about IkHz. To improve confidence in the conductivity estimation, measurements with several layer thicknesses should be performed. To remove the effect of the electrode impedance in a test setup, four-electrode measurements have also been proposed (Bruce et al. [1988]). Typically, two pseudoreference electrodes made of Li-foil strips are pressed through a cavity in the middle of circular main electrodes to the surface of the polymer electrolyte under test. 

The separator in a Li-ion battery is typically a thin (15/xm) microporous polypropylene film. It prevents the electrodes from shorting directly or through Li microdendrite growth on overcharge, and it also serves as a thermal shut-down safety device. When heated above ISO C (for example due to an internal short in a cell) the separator melts and its pores close, thus preventing current flow and thermal runaway. It is common to investigate the shut-down behavior of separators by measurement of cell impedance at selected frequencies, such as 1 kHz, dependent on temperature (Uchida [2003]). 

Separate impedance investigation of electrodes is possible using a three-electrode configuration, where the electrode of interest (anode or cathode) is 

In this case, electrodes are compressed between two glass plates with a spring. Placement of the reference electrode through a hole in the counter electrode provides the most symmetric position, important for measurement reproducibility and for preventing inductive artifacts often associated with a conductive path to the reference electrode coinciding with that of the current flow between the main electrodes. Detailed discussion of three-electrode measurement is given in Dolle et al. [2001], 

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It is clear that there is enormous activity in the the search for better and cheaper anode materials for Li-ion batteries. In fact, it is not certain at this time whether carbon will remain the material of choice for this application. Nevertheless, large strides toward the optimization and understanding of carbons for Li-ion batteries have been made in the last 5 to 10 years. If continued progress is made, we can expect to see carbon materials in Li-ion batteries for a long timx to come. 

Chapter 11 reports the use of carbon materials in the fast growing consumer eleetronies applieation of lithium-ion batteries. The principles of operation of a lithium-ion battery and the mechanism of Li insertion are reviewed. The influence of the structure of carbon materials on anode performance is described. An extensive study of the behavior of various carbons as anodes in Li-ion batteries is reported. Carbons used in commereial Li-ion batteries are briefly reviewed. 

A great variety of polyolefin separator types are now used in Li ion batteries. They must be stable in the organic electrolytes. Typically they may not be properly wetted by the electrolytes of the optimized composition, e. g., mixtures with PC, PE, and others. Therefore some proprietary treatments are needed to provide hydrophilic behavior. Generally, a micro-porous nonwoven morphology with a large surface gives a good wettability. 

A-T Battery Co. is a joint venture between Asahi and Toshiba, to produce Li ion batteries. Fuji Electric and Fuji Film, Hitachi-Maxell (Li-thionyl cells, and now also Li ion cells), Japan Storage Battery Co. (prismatic cells), and Matsushita Battery Co. cover most systems. Mitsubishi Electric, Mitsui, and Sanyo are major producers of the Li - Mn02 system. Sony Energy... 

Tec. produces Li ion batteries. Yuasa works with Hydro-Quebec on polymer systems. 

Li ion batteries are heavily advertised as the future power sources for electric vehicles. This seems premature because the technology of heat management and many questions of safety are not solved. Fuel cells and several types of secondary batteries have a long history in the field of electric vehicle propulsion, with successes and failures. For information on electric vehicle batteries, see [16-22],... 

The preparation and properties of a novel, commercially viable Li-ion battery based on a gel electrolyte has recently been disclosed by Bellcore (USA) [124]. The technology has, to date, been licensed to six companies and full commercial production is imminent. The polymer membrane is a copolymer based on PVdF copolymerized with hexafluoropropylene (HFP). HFP helps to decrease the crystallinity of the PVdF component, enhancing its ability to absorb liquid. Optimizing the liquid absorption ability, mechanical strength, and processability requires optimized amorphous/crystalline-phase distribution. The PVdF-HFP membrane can absorb plasticizer up to 200 percent of its original volume, especially when a pore former (fumed silica) is added. The liquid electrolyte is typically a solution of LiPF6 in 2 1 ethylene carbonate dimethyl car-... 

Once in an operational battery, the separator should be physically and chemically stable to the electrochemical environment inside the cell. The separator should prevent migration of particles between electrodes, so the effective pore size should be less than 1pm. Typically, a Li-ion battery might be used at a C rate, which corresponds to 1-3 mAcm2, depending on electrode area the electrical resistivity of the separator should not limit battery performance under any conditions. 

Zhu et al. [94] reported the synthesis of Sn02 semiconductor nanoparticles by ultrasonic irradiation of an aqueous solution of SnCLj and azodicarbonamide under ambient air. They found that the sonochemically synthesized Sn02 nanoparticles improved remarkably the performance of Li ion batteries such that there was about threefold increase (from 300 to 800 mAh/g) in the reversible capacity in the first lithiation to delithiation cycles. Similarly the irreversible capacity also increased by about 70% (from 800 to 1400 mA h/g). Wang et al. [95] reported the synthesis of positively charged tin porphyrin adsorbed onto the surface of silica and used as photochemically active templates to synthesise platinum and palladium shell and... 

The Li-Ion system was developed to eliminate problems of lithium metal deposition. On charge, lithium metal electrodes deposit moss-like or dendrite-like metallic lithium on the surface of the metal anode. Once such metallic lithium is deposited, the battery is vulnerable to internal shorting, which may cause dangerous thermal run away. The use of carbonaceous material as the anode active material can completely prevent such dangerous phenomenon. Carbon materials can intercalate lithium into their structure (up to LiCe). The intercalation reaction is very reversible and the intercalated carbons have a potential about 50mV from the lithium metal potential. As a result, no lithium metal is found in the Li-Ion cell. The electrochemical reactions at the surface insert the lithium atoms formed at the electrode surface directly into the carbon anode matrix (Li insertion). There is no lithium metal, only lithium ions in the cell (this is the reason why Li-Ion batteries are named). Therefore, carbonaceous material is the key material for Li-Ion batteries. Carbonaceous anode materials are the key to their ever-increasing capacity. No other proposed anode material has proven to perform as well. The carbon materials have demonstrated lower initial irreversible capacities, higher cycle-ability and faster mobility of Li in the solid phase. 

Two different natural graphites manufactured by Superior Graphite Co. (SL-20 and LBG-73) were tested as received on the possibility of using as anode of a cylindrical Li-ion battery. For comparison, typical synthetic graphite KS-15 from Lonza was examined. 

The model cylindrical Li-ion battery (AA-size) was manufactured using SL-20 graphite as anode active material. The general appearance of the cells is shown by Figure 2 for more detailed description of the cells see the experimental part of the paper. 

There is no question that the development and commercialization of lithium ion batteries in recent years is one of the most important successes of modem electrochemistiy. Recent commercial systems for power sources show high energy density, improved rate capabilities and extended cycle life. The major components in most of the commercial Li-ion batteries are graphite electrodes, LiCo02 cathodes and electrolyte solutions based on mixtures of alkyl carbonate solvents, and LiPF6 as the salt.1 The electrodes for these batteries always have a composite structure that includes a metallic current collector (usually copper or aluminum foil/grid for the anode and cathode, respectively), the active mass comprises micrometric size particles and a polymeric binder. 

MECHANISMS OF REVERSIBLE AND IRREVERSIBLE INSERTION IN NANOSTRUCTURED CARBONS USED FOR Li-ION BATTERIES... 

Composite electrodes made of two carbon components were evaluated experimentally as anodes for Li-ion batteries. The electrochemical activity of these electrodes in the reaction of reversible lithium intercalation ffom/to a solution of LiPF6 in ethyl carbonate and diethyl carbonate was studied. Compositions of the electrode material promising for the usage in Li-ion batteries were found. 

Therefore, there is a wide spectrum of carbon materials suitable for the usage in Li-ion batteries the choice of a specific one determined by many factors. According to Ref. 7, the percentage of various carbon materials used in commercial Li-ion batteries was as follows graphites - 43 %, hard carbons - 52 %, soft carbons - 5 %. 

Thus, the electrochemical properties of the individual carbon materials are not so high as to enable their commercial usage in Li-ion batteries. In order to improve the performance, we started making composite materials from two individual carbon ingredients. Figure 1 shows a typical result of electrochemical tests of an electrode made of a blend of graphite and soft carbon treated at 1100°C (Cl 100) in comparison with the discharge curves of the individual constituents. 

SURFACE TREATED NATURAL GRAPHITE AS ANODE MATERIAL FOR HIGH-POWER LI-ION BATTERY APPLICATIONS... 

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