Lithium ion secondary cell and method for producing active material
The art disclosed herein provides a lithium ion secondary cell in which the internal resistance of the secondary cell is further reduced. A lithium ion secondary cell includes electrodes including an active material. The active material includes, on the surface, two coating layers of a metal oxide layer including a metal oxide and an ion conductive layer including a lithium ion conductor. The metal oxide layer and the ion conductive layer are adjacent to each other.
1. A lithium ion secondary cell comprising:
an electrode including an active material, the active material comprising on the surface thereof, two coating layers of a metal oxide layer including a metal oxide and an ion conductive layer including a lithium ion conductor; wherein
the metal oxide layer and the ion conductive layer are adjacent to each other; and
where the diameter of the active material particle in the cross section is denoted by D, the length 1 of the contact surface between the active material particle and the ion conductive layer in the cross section of the electrode material is 1.6×D or less.
2. The lithium ion secondary cell according to claim 1 , wherein the metal oxide includes at least one selected from the group consisting of titanium oxide, aluminum oxide, silicon dioxide, and tungsten oxide.
3. The lithium ion secondary cell according to claim 1 , wherein the lithium ion conductor includes at least one selected from the group consisting of lithium phosphate, lithium sulfate, lithium tungstate, lithium silicate, lithium cobaltate, lithium aluminate, and lithium titanate.
4. The lithium ion secondary cell according to claim 1 , wherein where an electronegativity of a central element of the metal oxide is denoted by χM and an electronegativity of a central element of the lithium ion conductor is denoted by χM′, these satisfy χM<χM′.
5. The lithium ion secondary cell according to claim 1 , wherein where the active material is taken as 100 parts by mass, the ion conductive layer is 0.05 parts by mass or more and 5 parts by mass or less.
6. The lithium ion secondary cell according to claim 1 , wherein when a cross section of the active material is observed, the product l×m (nm 2 ) of a length l (nm) of a contact surface between the ion conductive layer and the active material and the maximum thickness m (nm) of the ion conductive layer satisfies 100≤l×m 20,000.
7. A method for producing an active material for a lithium ion secondary cell, comprising:
coating a part of a surface of active material particles capable of occluding and releasing lithium ions with a lithium ion conductor to form an ion conductive layer, and
then coating another part of the surface of the active material particles where the ion conductive layer has been formed with a metal oxide to form a metal oxide layer adjacent to the ion conductive layer;
where the diameter of the active material particle in the cross section is denoted by D, the length l of the contact surface between the active material particle and the ion conductive layer in the cross section of the electrode material is 1.6×D or less.
8. The lithium ion secondary cell according to claim 1 , wherein a part of the surface of the active material is exposed.
9. The lithium ion secondary cell according to claim 3 , wherein the ion conductive layer does not contain vanadium.