Principal Research Results in Fiscal 1995
(source: Annual Research Report/1996)
6. Regional Activities and Customer Service
- An Internal Diagnosis Method for Assembled Lithium Ion Cells -
An Internal Diagnosis Method for Assembled Lithium Ion Cells
Background
Among lithium secondary batteries, small capacity lithium ion cells which use carbon as the active material for the anode are at the stage of practical development. To develop large capacity long life lithium ion batteries, internal battery diagnosis methods such as the clarification of internal changes during charge and discharge will be a key technology. However, assembled cells are packed so tightly that until now it has been difficult for users to analyze the characteristics of their internal materials during charging or discharging.
Purpose
The purpose of this study was to develop new techniques to evaluate the utilization efficiencies of active materials, behavior of cathode/anode potentials, and stability of components in assembled lithium ion cells. The purpose of these techniques was to provide a way for users to easily analyze battery design to improve battery life and enlarge capacity. The techniques we sought to develop were calorimetric measurement for analysis of the heat of chemical reactions in a cell, and observation of individual potentials of the cathode and anode by inserting a reference electrode into an assembled cell.
Principal Results
1. Development of Precise Calorimetric Measurement of Internal Reactions in a Cell
We designed and manufactured a special vessel which uses a conduction type calorimeter to perform precise nondestructive measurement of thermal properties accompanying charging and discharging (Fig. 1). Using this vessel, we developed a method for observing the internal thermal reactions in lithium ion cells. We applied our method to assembled lithium ion cells. With lithium cobalt oxide (LiCoO2) as the active material of the cathode, we observed endothermic behavior during the first half of charge, and exothermic behavior at the end of discharge (Fig. 2 A, B). With the cell voltage near 4.1 V, we observed extremely symmetrical endothermic/exothermic peaks, indicating a reversible phase transition of the cathode (Fig. 2 C).
2. Development of a Method for Measuring Individual Potentials in an Assembled Lithium Ion Cell
We developed a new technique which allows simultaneous measurements of individual cathode and anode potentials, and the temperature of the center of the cell (Fig. 3). The technique was accomplished by taking advantage of the spiraly wounded electrodes' structure, to insert lithium metal and a thermocouple in the cell center. The modified cells could perform stable charge/discharge cycles for more than 2 months during the study. We applied the technique to unused and cycle-degraded assembled lithium ion cells (Figs. 4, 5). As a result, from the behavior of the single electrode potentials, we determined that the causes for the degradation of the lithium ion cells were the amount of lithium ions entering and leaving the cathode, and the decrease of electrical conductivity.
3. Effectiveness of Developed Internal Diagnosis
We used our calorimetric measurement method to verify that the reported phase transition of cathode material occurred at an operation voltage range within the range of practical use. From measurement of single electrode potentials, under the degradation conditions of this experiment, degradation of the cathode was the main reason for the degradation of cell performance. This result suggests that higher performance lithium ion batteries could be developed by improving the electric conductivity of the cathode or increasing the surface area of the active cathode material.
Future Initiatives
In future we shall apply the techniques developed in this study to lithium ion cells degraded under various conditions, to clarify the causes of cell performance degradation and improve the capacity and life of lithium ion cells.
Coordinator:
Hajime Miyashiro
Komae Research Laboratory, Physical Chemistry Department, Electrochemistry Group
Reference
Report: "Development of a Method for Evaluating Single Electrode Potentials of Lithium Ion Cells" CRIEPI Report No. T95086 (1996)
Fig. 1 The Vessel Designed for Calorimetry Measurement of Lithium Ion Cell
Fig. 2 Changes in Thermal Behavior of Lithium Ion Battery During Charge/Discharge
(Test conditions: Charging: 4.2 V, 10 mA, V control. Discharging: 2.5 V, 10 mA, V control. A: Endothermic reaction during charge cycle. B: Exothermic reaction during discharge cycle. C: Symmetrical thermal properties correspond to cathode phase transition. During the charge cycle, the endothermic behavior exceeded the resistance exothermic behavior due to a chemical reaction. During the discharge cycle, the resistance exothermic behavior was added to the chemical reaction heat.)
Fig. 3 Schematic Diagram of Lithium Ion Cell With Reference Electrode/Thermocouple
Fig. 4 Behavior of Cathode/Anode Potential and Temperature of Unused Cell During Charge/Discharge Cycle
(Test conditions: Charging: 4.2 V, 10 h, 156 mA, V + T control. Discharging: 2.5 V, 10 h, 156 mA, V control. The cathode potential rose smoothly up to about 4.4 V accompanying the charge cycle, indicating a charge/discharge capacity of about 1.27 Ah.)
Fig. 5 Behavior of Cathode/Anode Potential and Temperature of Degraded Cell During Charge/Discharge Cycle
(Test conditions: Charging: 4.2 V, 10 h, 156 mA, V + T control. Discharging: 2.5 V, 10 h, 156 mA, V control. Degradation conditions: Charging: 4.2 V, 1 h, 10 A, V + T control. Discharging: 2.5 V, 1 h, 10 A, V + T control. The cathode potential rose suddenly up with the beginning of the charge cycle, reaching a maximum of 4.7 V in just under 2 hours. The cell voltage was determined almost entirely by the change in cathode potential, and cell capacity decreased to roughly 0.34 Ah.)