Negative electrode active material and negative electrode using the same and lithium ion secondary battery
In a negative electrode active material for a lithium ion secondary battery including a silicon oxide capable of absorbing and desorbing lithium ions, a silicon oxide having structural units each in the form of a tetrahedron in which a silicon atom is located at its center and silicon or oxygen atoms are located at its four vertices is used. The structural units are arranged randomly to form an amorphous structure. In the case that the number of oxygen atoms located at the four vertices in the structural units is represented by n (n=0, 1, 2, 3 or 4) and the structural units are represented by Si(n), the number of the structural units NSi(n) in the silicon oxide satisfies the following relations (1) to (3). [ Formula 1 ] NSi ( 0 ) ∑ NSi ( n ) ≥ 0.1 ( 1 ) Nsi ( 4 ) ∑ NSi ( n ) ≤ 0.1 ( 2 ) Nsi ( 1 ) + NSi ( 2 ) + Nsi ( 3 ) ≠ 0 ( 3 )
1. A method for producing a negative electrode active material for a lithium ion secondary battery, the method comprising steps of:
melting silicon to give molten silicon; and
evaporating said molten silicon as a sole silicon source into an oxygen atmosphere and depositing said evaporated silicon on a substrate having a temperature lower than an ambient temperature to give a negative electrode active material including a silicon oxide that is represented by SiO x where 0.1≦x≦1.2 and has structural units each in the form of a tetrahedron in which a silicon atom is located at its center and silicon or oxygen atoms are located at its four vertices, said structural units being arranged randomly to form an amorphous structure.
2. The method in accordance with claim 1 , wherein said structural units are totally and uniformly dispersed in the negative electrode active material.
3. A negative electrode active material for a lithium ion secondary battery, the active material consisting essentially of:
silicon oxide that has structural units each in the form of a tetrahedron in which a silicon atom is located at the center of each unit and silicon or oxygen atoms are located at four vertices of the each unit, said structural units being arranged randomly to form an amorphous structure, wherein:
said structural units are totally and uniformly dispersed in the negative electrode active material,
in the case that the number of oxygen atoms located at said four vertices in said structural units is represented by n, where n=0, 1, 2, 3 or 4, and said structural units are represented by Si(n), and
the number of said structural units NSi(n) in said silicon oxide satisfies the following relations (1) to (3) and (6):
[
Formula
1
]
NSi
(
0
)
∑
NSi
(
n
)
≥
0.1
,
(
1
)
NSi
(
4
)
∑
NSi
(
n
)
≤
0.1
,
(
2
)
NSi
(
1
)
+
NSi
(
2
)
+
NSi
(
3
)
≠
0
,
and
(
3
)
0.231
≤
NSi
(
1
)
+
NSi
(
2
)
+
NSi
(
3
)
∑
NSi
(
n
)
<
1.
(
6
)
4. The negative electrode active material for a lithium ion secondary battery in accordance with claim 3 , wherein said silicon oxide is represented by SiO x where 0.1≦x≦0.6.
5. The negative electrode active material for a lithium ion secondary battery in accordance with claim 3 , wherein in an X-ray photoelectron spectroscopy using Al-Kα ray of said silicon oxide in said negative electrode active material having been subjected to charge and discharge at least once, a peak of O 1s energy is observed at 530±1 eV.
6. A negative electrode for a lithium ion secondary battery comprising the negative electrode active material for a lithium ion secondary battery in accordance with claim 3 .
7. A lithium ion secondary battery comprising:
a positive electrode capable of absorbing and desorbing lithium ions,
the negative electrode in accordance with claim 6 ,
a separator interposed between said positive electrode and said negative electrode, and
an electrolyte with lithium ion conductivity.
8. The negative electrode active material for a lithium ion secondary battery in accordance with claim 3 , wherein the negative electrode active material is prepared by a method comprising steps of:
melting silicon to give molten silicon; and
evaporating said molten silicon as a sole silicon source in oxygen atmosphere and depositing said evaporated silicon on a substrate having a temperature lower than an ambient temperature.