TECHNICAL FIELD

The present invention relates to an energy storage device such as a lithium ion secondary battery, and a method for manufacturing the energy storage device.

BACKGROUND ART

A lithium ion secondary battery is conventionally known that includes an electrode having a current collector and an active material layer containing particles of an active material (for example, Patent Document 1).

In the battery disclosed in Patent Document 1, the active material layer contains particles of a monoclinic ß-type titanium composite oxide and particles of lithium titanate having a spinel structure. In the battery disclosed in Patent Document 1, a first peak P1 appears in a range of 0.3 μm or more and 3 μm or less and a second peak P2 appears in a range of 5 μm or more and 20 μm or less when a particle size frequency distribution of the particles contained in the active material layer is measured by a laser diffraction and scattering method, and a ratio FP1/FP2 of a frequency FP1 of the first peak P1 to a frequency FP2 of the second peak P2 is 0.4 or more and 2.3 or less.

The battery disclosed in Patent Document 1 sometimes has insufficient power performance at low temperature, and an energy storage device is required that has improved power performance at low temperature.

PRIOR ART DOCUMENT

Patent Document

Patent Document 1: JP-A-2013-105703

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

An object of one embodiment is to provide an energy storage device that has improved power performance at low temperature. Another object is to provide a method for manufacturing the energy storage device.

Means for Solving the Problems

An energy storage device according to an embodiment includes an electrode having an active material layer, the active material layer contains at least active material particles, the particles contained in the active material layer gives a volume-based particle size frequency distribution that has a first peak and a second peak appearing in a particle size larger than a particle size of the first peak, and particles having particle sizes equal to or smaller than a particle size Dx have a volume proportion of 49% or more and 62% or less in a volume of whole particles contained in the active material layer, with the particle size Dx defined as a particle size at a local minimum frequency between the first peak and the second peak in the particle size frequency distribution. This configuration enables provision of an energy storage device that has improved power performance at low temperature.

In the energy storage device, the active material layer contains secondary particles formed by aggregation of primary particles of an active material, and the secondary particles of the active material may have a pore formed therein, with the pore being equal to or larger than particle sizes of the primary particles constituting the secondary particles. This configuration forms inner surfaces of the secondary particles to increase the specific surface area of the secondary particles and thus efficiently progress a reaction on a surface of the active material during generation of power.

In the energy storage device, the particles having particle sizes equal to or smaller than the particle size Dx may have a volume proportion of 52% or more and 60% or less in the volume of the whole particles contained in the active material layer. This configuration improves the power performance at low temperature.

A method for manufacturing an energy storage device according to an embodiment includes preparing an electrode that has an active material layer containing at least active material particles, the preparing the electrode includes pressing the active material layer containing secondary particles formed by aggregation of primary particles of an active material, the pressing the active material layer includes crushing a part of the secondary particles to allow the particles contained in the active material layer pressed to give a volume-based particle size frequency distribution that has a first peak and a second peak appearing in a particle size larger than a particle size of the first peak, and crushing the part of the secondary particles to allow particles having particle sizes equal to or smaller than a particle size Dx to have a volume proportion of 49% or more and 62% or less in a volume of whole particles contained in the active material layer, with the particle size Dx defined as a particle size at a local minimum frequency between the first peak and the second peak in the particle size frequency distribution. This manufacturing method including such procedures is capable of providing an energy storage device that has improved power performance at low temperature.

Advantages of the Invention

According to the present embodiment, it is possible to provide an energy storage device having improved power performance at low temperature, and a method for manufacturing the energy storage device.

