Success in Materials Design of High Performance Lithium Ion Rechargeable Battery for the First Time in the Word

Aiming at the increase in the capacity of lithium ion rechargeable battery, we have attempted to investigate theoretically the optimum structure of carbon materials at negative electrode. As a result, we have succeeded in the design of carbon materials for lithium ion rechargeable batteries at negative electrode for the first time in the world.

Significance and Objectives of the Research

Recently, due to the rapid increase in the use of lithium ion rechargeable batteries for portable computers, cellular phones, electric vehicles, etc., there is an increasing demand for larger capacity, smaller size, lighter weight and lower price for this kind of batteries. Therefore, researches have been made to investigate materials for both positive and negative electrodes as well as electrolyte solutions aiming at improving the performance of lithium ion rechargeable atteries.

Our group investigated the theoretical design of carbon materials at negative electrode in order to improve the performance of lithium ion rechargeable batteries. Although different carbon materials are used for negative electrode, the mechanisms or structures of lithium ion inclusion or extrusion into carbon materials have not been made clear. The improvement of negative electrode is indispensable to realize higher performance, and for that purpose, it is imperative that we should understand the behavior of lithium ion in further detail.

On the other hand, because the forms of carbon materials at negative electrodes are irregular, investigation by actual experiments is quite difficult. Therefore, at present, materials are investigated on rather inefficient error and trial bases. With this in mind, we set our objectives to present a guide to the materials investigation, and have worked out the materials design for negative electrode of lithium ion rechargeable batteries by taking theoretical approaches.

Research Method

We used the program Q-Chem of ab initio molecular orbital method to calculate the electronic states of various sizes and shapes of carbon clusters in order to investigate the optimum structures of carbon clusters.

Results

(1) Positions of lithium ion inclusion
Except for some hypothesis that had been proposed, the positions where lithium ion are included into have not been made clear. Thus, we have attempted to calculate the positions using ab initio molecular orbital method, and as a result, for the first time in the world, we have discovered the new and unknown positions where lithium ion is included into.

(2) About optimum structures of carbon clusters
From the above results, in order to increase the capacity of lithium ion rechargeable batteries, our calculation has led the conclusion that not only the absorption positions at the planes between carbon clusters, but also the positions around and inside the same planes of carbon clusters are favorable. This enables the inclusion of greater amount of lithium ion.

On the bases of the above findings, the optimum structures to include and extrude lithium ion electron are investigated by calculating electronic states of carbon clusters having various sizes and shapes (as shown in the following) using ab initio molecular orbital method. As a result, we found some optimum carbon cluster structure and at the same time discovered that if carbon materials having these structures shall be synthesized, the improvement of the performance of the lithium ion rechargeable batteries to several-fold greater capacity will be possible.