Katsunori Yanagida

Electrochemical Activities in Li2MnO3

Li 2 MnO 3 is shown to be electrochemically active, with a maximum charge capacity of ~ 350 mAh/g and a discharge capacity of ~260 mAh/g at 25°C. A total of I mole of Li can be extracted from Li[Li 1/3 Mn 2/3 ]O 2 , and the first cycle efficiency is ∼66% regardless of state of charge. Larger charge-discharge capacity is obtained from materials with smaller particle size and larger amount of stacking faults. Composition and structural analyses indicate that Li are removed from both the Li and transitional metal layers of the material during charging. Results from X-ray-absorption fine-structure measurements suggest that the valence of Mn remains at 4+ during charging but is reduced during discharging. Charging is accompanied by gas generation: at 25°C, oxygen is the main gas detected, and the total amount accounts for ∼ 1/8 mole of O 2 generation from Li[Li 1/3 Mn 2/3 ]O 2 . At an elevated temperature, amount of CO 2 increases due to electrolyte decomposition. Li 2 MnO 3 shows poor cycle performance, which is attributed to phase transformation and low charge-discharge efficiency during cycling. Low first-cycle efficiency, gas generation, and poor cycle performance limit the usage of Li 2 MnO 3 in practical batteries.

Structural Analysis of Li2MnO3 and Related Li-Mn-O Materials

Structure of Li 2 MnO 3 during charge and discharge is studied by x-ray diffraction, Raman spectroscopy and x-ray absorption spectroscopy (XAS). During electrochemical delithiation, development of a local spinel-like structure and an increase in disordering of Mn in Li 2 MnO 3 is observed from Raman spectroscopy and XAS, respectively. However, the charge-discharge curves show a sloping profile, which is associated with Mn atoms in the transition metal layer. Upon cycling, plateaus develop in the 3 and 4 V regions, suggesting transformation of Li 2 MnO 3 to a spinel structure. The formation of spinel phase is linked to Mn re-arrangement in the lattice and lowering of oxygen content, as indicated by results of Li-Mn-O compounds made by varying Li/Mn ratio during synthesis and by chemically (acid treatment) extracting Li from Li 2 MnO 3 . Our results suggest the better cycle performance of Li-excess materials with a solid solution of Li2MnO 3 -LiMnO 2 is due to stabilization of the Li 2 Mn0 3 structure by addition of a layered component into the structure.

Surface Modification of Li-Excess Mn-based Cathode Materials

Rate capability of Li-excess Mn-based layered cathode materials is improved by treating with (NH 4 ) 2 SO 4 . After treatment, discharge capacity of as high as 230 mAh/g can be obtained at a rate of 300 mA/g (~ 1.2C). The improvement is attributed to the modification of the surface of the layered material into a spinel-like structure with the treatment, as suggested by Raman spectroscopy and electrochemical charge-discharge. X-ray diffraction results show no change in bulk lattice parameters, indicating that the structural modification is only on the surface of the active material. Chemical analyses show that both lithium and oxygen are extracted from the active material, supporting the formation of a surface spinel layer with the treatment.

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.

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.