Toshiyuki Nohma

Manganese oxides for a lithium secondary battery — composite dimensional manganese oxide (CDMO)

Abstract Manganese dioxide was adopted as a positive material for a lithium secondary battery that is both inexpensive and has a high discharge voltage. In this report, the rechargeability of several manganese oxides that contain lithium in their structures was investigated by cycle tests on flat cells and by X-ray diffractometry (XRD). The results were compared with the rechargeability of γ/β-MnO 2 . In a cycle test at a depth of 0.14 e/Mn, spinel LiMn 2 O 4 and CDMO (heat treated LiOH·MnO 2 ) showed much better rechargeability than γ/β-MnO 2 . At a depth of 0.26 e/Mn, the rechargeability of CDMO was better than that of spinel LiMn 2 O 4 . The crystal structure of CDMO consists of Li 2 MnO 3 and γ/β-MnO 2 (composite dimension). We also investigated the optimum lithium molar ratio from which CDMO could be prepared. We consider that the optimum molar ratio of lithium is between 30 and 50 mole percent.

Electrochemical characteristics of LiNiO2 and LiCoO2 as a positive material for lithium secondary batteries

Abstract The discharge characteristics of lithium nickel oxides have been improved by investigating their synthesizing conditions, such as their raw materials and heat-treating conditions. Lithium hydroxide (LiOH) and nickel hydroxide (Ni(OH)2) were found to be appropriate raw materials, and 750 °C in oxygen atmosphere was the most suitable heat-treating condition. Lithium nickel oxide synthesized under these suitable conditions showed a greater discharge capacity of more than 190 mAh/g than that of lithium cobalt oxide. The existence of lithium carbonate (Li2CO3) was detected, by the Fourier-transform infrared spectroscopy method, in lithium nickel oxide heat-treated in air. This suggests that the existence of Li2CO3 adversely affected the discharge characteristics of lithium nickel oxides.

High-Temperature Storage Performance of Li-Ion Batteries Using a Mixture of Li-Mn Spinel and Li-Ni-Co-Mn Oxide as a Positive Electrode Material

To improve the storage performance of Li-Mn spinel at high temperatures, we investigated a mixture of Li-Mn spinel (Li 1 . 1 Mn 1 . 9 O 4 ) and Li-Ni-Co-Mn oxide (LiNi 0 . 4 Co 0 . 3 Mn 0 . 3 O 2 ) as a positive electrode material. We found that a battery using such a mixture as a positive electrode material showed better storage characteristics at 45°C for 30 days than batteries using both a single material Li-Mn spinel and a single material Li-Ni-Co-Mn oxide. This improved storage performance resulted from the suppression of capacity loss of LiMn-spinel in the mixture. We compared the differences in Li-Mn spinel to clarify the reasons for its improved storage performance in the mixture and found two changes during charging in the potential and lattice constant of Li-Mn spinel in the mixture that were different from those of single material Li-Mn spinel. In single material, changes in potential and lattice constants of Li-Mn spinel showed two-step change when charged, while Li-Mn spinel in a mixture showed continuous change.

Lithium-containing manganese dioxide (composite dimensional manganese oxide: CDMO) as positive material for a lithium secondary battery

Abstract Lithium-containing manganese dioxide (CDMO) has been developed as the positive material for lithium secondary batteries. CDMO is prepared from lithium salt and manganese dioxide by heat treatment. It is a composite oxide of γ/β-MnO 2 and Li 2 MnO 3 . The influence on rechargeability of lithium salts, heat-treatment temperature, and manganese dioxide type has been investigated by conducting cycle tests with flat cells. Lithium hydroxide is more reactive with MnO 2 in the production of Li 2 MnO 3 than either Li 2 O or Li 2 CO 3 . The optimum condition for preparing CDMO is to heat treat LiOH and MnO 2 at about 375 °C. CDMO prepared from EMD (electrolytic manganese dioxide) yields a larger and more stable capacity than CDMO prepared from CMD (chemical manganese dioxide). Sodium-free EMD exhibits the largest discharge capacity.

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.

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.

Improvement of lithium-containing manganese dioxide (composite dimensional manganese oxide: CDMO) as positive material for lithium secondary batteries

Abstract Lithium-containing manganese dioxide (CDMO) has been developed as the positive material for lithium secondary batteries. CDMO is prepared from lithium salt and manganese dioxide by heat treatment. The material is a composite oxide of γ/β-MnO2 and Li2MnO3. The charge condition has been investigated in order to develop an improved CDMO that will exhibit a higher discharge voltage and a larger capacity. CDMO charged to a high potential (i.e., 3.6 V versus LiAl electrode) displays higher discharge voltage and larger capacity than CDMO subjected to normal charge (i.e., 3.3 V versus LiAl). It is concluded that when CDMO is charged to a high potential, lithium inserted not only by electrochemical reaction but also by heat treatment are removed from the γ/β-MnO2 phase. The optimum conditions for preparing improved CDMO is to heat treat LiOH and electrolytic manganese dioxide (EMD) at a Li/Li+Mn atomic ratio of 0.3 at ∼250 °C. The improved CDMO delivers a discharge capacity of over 200 mA h g−1. Also, excellent rechargeability is experienced, even when CDMO is charged to a high potential.

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.