CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a National Stage Entry of International Application No. PCT/JP2017/009429, filed Mar. 9, 2017, which claims priority from Japanese Patent Application No. 2016-047396, filed Mar. 10, 2016. The entire contents of the above-referenced applications are expressly incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a secondary battery, a method for manufacturing the same and a vehicle using the same.

BACKGROUND ART

Lithium ion secondary batteries, which feature small size and large capacity, have been widely used as power supplies for electronic devices such as mobile phones and notebook computers and have contributed to enhancing convenience of mobile IT devices. In recent years, larger-scale applications, such as power supplies for driving motorcycles and automobiles and storage cells for smart grids, have attracted attention. As the demand for lithium ion secondary batteries has increased and they have been used in various fields, the batteries have been required to have characteristics, such as further higher energy density, lifetime characteristics that can withstand long-term use, and usability under a wide range of temperature conditions.

Although carbon-based materials are usually used in a negative electrode of the lithium ion secondary battery, in order to increase the energy density of the battery, using silicon materials, absorbing and desorbing a large amount of lithium ions per unit volume, in a negative electrode are studied. However, batteries using the silicon material have a problem with cycle characteristics, because the silicon material expands and contracts by repeating charge and discharge of lithium and thereby deteriorates.

Various proposals have been made to improve cycle characteristics of a lithium ion secondary battery using the silicon material in a negative electrode. Patent Document 1 discloses that the initial capacity, the initial efficiency and cycle characteristics of a battery can be improved by using plate like graphite having a thickness of 0.03 to 100 nm together with a silicon material as negative electrode active materials.

CITATION LIST

Patent Document

Patent Document 1: Japanese patent laid-open No. 2015-88462

SUMMARY OF INVENTION

Technical Problem

However, the battery of the prior art document still has the problem that its capacity decreases with charge and discharge cycles, and further improvement in cycle characteristics is necessary. The silicon materials, which have low conductivity, are generally used in combination with a conductive assisting agent, such as carbon black or the like. Degradation of the conductive assisting agent due to repeating charge and discharge cycles is also one of factors causing the deterioration of cycle characteristics of the lithium ion secondary battery using the silicon material. An object of the present invention is to provide a lithium ion secondary battery excellent in cycle characteristics despite using a negative electrode containing the silicon material.

Solution to Problem

The lithium ion secondary battery according to the present invention is characterized in comprising a negative electrode comprising a plate-like artificial graphite and a material comprising silicon as a constituent element, wherein at least some of particles of the plate-like artificial graphite are bent and have a crease on a plate face.

Advantageous Effect of Invention

According to the present invention, cycle characteristics of a lithium ion secondary battery using a material comprising silicon as a constituent element as a negative electrode active material can be improved.

