Lithium secondary battery

Abstract

A lithium secondary battery of the present invention has a positive electrode is provided with a positive electrode mix layer that includes a positive electrode active material and a conductive material. The positive electrode mix layer has two peaks, large and small, of differential pore volume over a pore size ranging from 0.01 μm to 10 μm in a pore distribution curve measured by a mercury porosimeter. A pore size of the smaller peak B of the differential pore volume is smaller than a pore size of the larger peak A of the differential pore volume.

Description

TECHNICAL FIELD[0001]The present invention relates to a lithium secondary battery and a production method of the battery, and more particularly, to a positive electrode of the battery.BACKGROUND ART[0002]Recent years have witnessed the growing importance of secondary batteries, such as lithium secondary batteries and nickel-hydride batteries, as vehicle-mounted power sources having electricity as a drive source, as well as in power sources provided in, for instance, portable terminals and other electronic devices. In particular, lithium secondary batteries (typically lithium ion batteries) that are lightweight and afford high energy density are expected to be used as preferred high-output power sources installed in vehicles (for instance, automobiles, in particular hybrid automobiles and electric automobiles).[0003]Batteries that afford good electric performance over long periods of time, i.e. batteries having superior durability (cycle characteristic), are demanded in lithium secon...

AI Technical Summary

Benefits of technology

[0016]The conductive material comprising a conductive powder material such as carbon powder or the like, from among the materials that make up the above positive electrode mix layer, is of high bulkiness and has a very small particle size (typically, no greater than 1 μm, for instance ranging from 0.001 μm to 1 μm). By contrast, the material used in the positive electrode active material that uses a lithium-transition metal complex oxide has a particle size greater than the particle size of the conductive material (typically, from 1 μm to 50 μm, preferably from 2 μm to 20 μm, for instance from 3 μm to 8 μm). Therefore, it is deemed that, in the above-described pore distribution curve, the large pore size peak A denotes roughly pores that are derived from gaps between positive electrode active material particles, and the small pore size peak B does roughly pores derived from gaps between conductive material particles. In the present invention, forming thus pores that comprise a large pore size peak A and a small pore size peak B, such as the above-described ones, in the positive electrode mix layer, has the effect of enhancing the holding force of the nonaqueous electrolyte solution in the pores. As a result, lithium ions migrate efficiently via the electrolyte solution that impregnates (is held in) the pores. In consequence, a lithium secondary battery can be provided that has superior battery performance (cycle characteristic or high-rate characteristic) even when used in a mode of repeated high-rate charge and discharge.

[0033]The small pore size peak B from among the two peaks, large and small, in the pore distribution curve, denotes roughly pores that are formed by gaps between conductive material particles. Therefore, a suitable amount of nonaqueous electrolyte solution can be sufficiently held in pores formed by gaps between conductive material particles in a lithium secondary battery wherein the total pore volume per unit mass of conductive material satisfies 0.18 cm3 / g to 0.8 cm3 / g for pores that have a pore size smaller than the pore size P [μm] and that encompass the small pore size peak B (typically, a pore size ranging from 0.01 μm to P [μm]). Therefore, ion diffusivity and salt concentration homogeneity in a pole group are enhanced thereby, and a lithium secondary battery having superior battery performance (cycle characteristic or high-rate characteristic) can be produced as a result.

Problems solved by technology

However, Patent Literature 1 is not found to sufficiently address the technical issue relating to the structure of a positive electrode mix layer having good conductivity, which is arguably another issue that demands consideration in lithium secondary batteries that are excellent in high-rate characteristic or cycle characteristic.

As a result, good conductive paths (conduction paths) may fail to be formed in the positive electrode mix layer, and thus internal resistance may rise upon use of such a lithium secondary battery that is configured for repeated high-rate charge and discharge.

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