Yoshinori Kida

Electrochemical characteristics of graphite, coke and graphite/coke hybrid carbon as negative electrode materials for lithium secondary batteries

Electrochemical characteristics of various carbon materials have been investigated for application as a negative electrode material in lithium secondary batteries with long cycle life. Natural graphite electrodes show large discharge capacity in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC). However, their charge/discharge performance is largely influenced by electrolytes. There is a possibility that a rapid rise in the discharge potential of the natural graphite electrode at the end of the discharge would cause a side reaction such as decomposition of the electrolyte because of an unequal reaction over an electrode. In order to improve the cycle performance of natural graphite electrodes, mixtures of graphite and coke electrodes are prepared by adding coke to natural graphite. It is found that the mixture of graphite and coke electrode shows a better cycle performance than that of a natural graphite or coke electrode. The deterioration ratio of the mixture of graphite and coke negative electrode measured by using AA-type test cells is 0.057%/cycle up to the 500th cycle. The mixture of graphite and coke is a promising material for a negative electrode in long-life lithium secondary batteries for energy storage systems because of its excellent cycle performance and large discharge capacity.

Layered Cathode for Improving Safety of Li-Ion Batteries

To improve the safety performance of Li-ion batteries using LiCoO 2 cathodes without sacrificing battery capacity, a new electrode concept using a multilayered cathode as an alternative to a mixture of different kinds of powder is introduced. Results from a double-layered cathode consisting of a LiCoO 2 main layer with a LiFePO 4 sublayer on top of an aluminum current collector are presented. The mechanism of how the LiFePO 4 layer improves the tolerance of the batteries against overcharging is discussed.

Influence of solvent species on the charge-discharge characteristics of a natural graphite electrode

Abstract The charge–discharge characteristics of a natural graphite electrode are examined in a mixed solvent composed of ethylene carbonate (EC) and propylene carbonate (PC). The characteristics are influenced largely by the solvent species. Natural graphite electrode displays good charge–discharge characteristics in an electrolyte containing EC with a high volume fraction. In an electrolyte containing PC, however, the electrode cannot be charged and the solvent is decomposed. X-ray photoelectron spectroscopy is used to obtain information about the surface of natural graphite. A thin LiF layer, the decomposition product of lithium hexafluorophosphate (LiPF6), is formed on the surface of the natural graphite charged to 0.5 V (vs. Li/Li+) in an electrolyte containing a high volume fraction of EC. On the other hand, LiF and a carbonate compound are formed in the bulk and on the surface of natural graphite when the volume fraction of PC is high. These results suggest that the thin LiF layer, which is produced at a potential higher than 0.5 V (vs. Li/Li+) on the surface of natural graphite, enables the lithium ions to intercalate into the natural graphite without further decomposition of the electrolyte.

Electrochemical behaviour of carbon electrodes in some electrolyte solutions

Abstract The electrochemical properties of coke and natural graphite in some electrolyte solutions containing diethylcarbonate (DEC) are studied. It is found that natural graphite exhibits am excellent performance, such as high discharge capacity (370 mAh −1 g), when a mixed solvent composed of ethylene carbonate (EC) and DEC is used. The charge/discharge characteristics of the coke electrode are mot influenced by the species of the electrolyte solution, but those of the natural graphite electrode are very much influenced by the species of the electrolyte solution. It is confirmed that there are three patterns in the behaviour of the graphite electrode in the electrolyte solutions tested in this investigation. In the first pattern, natural graphite can be charged to C 6 Li and them discharged. In the second pattern, the charging and discharging of the natural graphite electrode is impossible and destruction of the natural graphite crystal structure is observed. In the third pattern, lithium is intercalated into the graphite layer but the de-intercalation of lithium does not take place.

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 LiNi0.7Co0.3O2 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 LiNi0.7Co0.3O2 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 LixNi0.7Co0.3O2 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.

Carbon Hybrids Graphite-Hard Carbon and Graphite-Coke as Negative Electrode Materials for Lithium Secondary Batteries Charge/Discharge Characteristics

Electrochemical characteristics of the hybrid carbon (HC) graphite-hard carbon and graphite-coke have been investigated for the application of these materials as negative electrodes in lithium secondary batteries with a long cycle life. The graphite-hard carbon HC showed a higher reversible lithium capacity and better cycle performance than did the graphite-coke HC. X-ray photoelectron spectroscopy and nuclear magnetic resonance spectroscopy were used to analyze the deterioration mechanisms of the graphite-HC and graphite-coke HC. The decomposition products after the charge/discharge cycles were considered to be LiF and a carbonate compound, and the increase in inactive lithium in the decomposition products for the graphite-hard carbon HC was smaller than that for the graphite-coke HC. Therefore, it was thought that the graphite-hard carbon HC negative electrode suppressed the decomposition of the electrolyte and showed better cycle performance than did the graphite-coke HC negative electrode. Consequently, graphite-hard carbon HC is a promising negative electrode material for long-life lithium secondary batteries for dispersed-type energy storage systems.

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

Abstract 10 Wh-class (30650 type) lithium secondary batteries were fabricated using LiNi0.7Co0.3O2 as the positive electrode material and graphite/coke hybrid carbon as the negative electrode material. In our previous work, we found that LiNi0.7Co0.3O2 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 LiNi0.7Co0.3O2 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.