CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority based on Japanese Patent Application No. 2019-074407, filed on Apr. 9, 2019, the entire contents whereof are incorporated in the present specification by reference.

BACKGROUND OF THE DISCLOSURE

1. Field

The art disclosed herein relates to a lithium ion battery.

2. Background

Lithium ion batteries are lightweight and afford high energy densities, and therefore have come to be preferably used for instance as portable power sources, and as high-output power sources installed in vehicles. Such lithium ion batteries are provided with a spiral electrode body in which a power storage element, having a configuration where a positive electrode and a negative electrode are insulated by a separator or the like, is stacked and wound, to yield a cylindrical shape or an elliptical cylindrical shape, within one battery case. The batteries are configured as a result so that a large amount of power per one charge/discharge can be extracted from the positive electrode and the negative electrode having a large surface area.

However, for instance secondary batteries for high-output applications where the battery is charged and discharged at a high rate are characteristically prone to generate heat also during ordinary use (during charge and discharge). In particular, an uncoated section of a positive electrode collector in which a positive electrode active material layer is not provided constitutes a collecting section in which current density is high and that becomes readily oxidized, as a result of which the uncoated section is prone to generating heat. Generation of heat by lithium ion batteries is an issue to be improved upon in order to increase safety at the time of use of the battery.

It is an object of the art disclosed herein, arrived at in the light of the above considerations, to provide a lithium ion battery in which generation of heat can be suitably suppressed.

SUMMARY

As a solution to the above problem, the art disclosed herein provides a lithium ion battery having a positive electrode, a negative electrode, and a nonaqueous electrolyte. In this lithium ion battery, the positive electrode has a positive electrode collector, a positive electrode active material layer provided on part of the surface of the positive electrode collector, and an insulating layer provided on another part of the surface of the positive electrode collector, so as to be adjacent to the positive electrode active material layer. Said insulating layer contains an inorganic filler and trilithium phosphate (Li3PO4; hereafter also referred to simply as “LPO”), and a binder, such that at least part of the surface of the inorganic filler is covered with LPO.

It is known that acids are generated as a result of oxidative decomposition of electrolyte solutions when the potential of the positive electrode is high. In a case where the positive electrode active material layer contains LPO, as illustrated in Japanese Patent Application Publication No. 2015-103332, LPO undergoes an acid-base reaction with such an acid, and the resulting phosphate ions (PO43−) leach out. The phosphate ions have the function of enhancing for instance the overcharge resistance of the battery by reaching the negative electrode and forming a coating film that suitably suppresses exothermic reactions in the negative electrode, to suppress generation of heat by the battery. Studies by the inventors have revealed that exothermic reactions in the negative electrode can be effectively suppressed by an LPO-derived coating film that becomes formed, on the negative electrode, through arrangement of LPO by providing an insulating layer on an uncoated section of the positive electrode that is readily brought to a high-potential state as a result of current concentration. The inventors found that this heat generation suppression effect can be brought out yet more effectively by controlling the manner in which LPO is formulated. The art disclosed herein was perfected on the basis of such findings.

In a preferred implementation of the lithium ion battery according to the present art, a ratio of the thickness of LPO that covers the inorganic filler with respect to the average particle size of the inorganic filler lies in the range of 0.1 to 1.2. Such a configuration is preferable since it allows balancing the heat generation suppression effect derived from LPO with suppression of increases in resistance through addition LPO to the insulating layer.

In a preferred implementation of the lithium ion battery according to the present art, the average thickness of the insulating layer is 10 μm or less. The lithium ion battery may be embodied so that the bulk and amount of the insulating layer provided on the positive electrode collector are small, from the viewpoint of reducing weight and costs. By incorporating LPO in an insulating layer in a form where the inorganic filler is coated with LPO, it becomes possible to elicit a predetermined heat generation suppression effect using a yet smaller amount of LPO, and to reduce the thickness (i.e. bulk and amount) of the insulating layer. As a result a lithium ion battery is provided which is safe and in which reductions in both weight and cost are achieved.

In the present art, the term “average thickness” is an arithmetic mean value obtained by measuring, at 3 or more points, the thickness (dimension in a direction substantially perpendicular to the surface of a base material) of a target site, in a cross-sectional observation of the target site, using for instance an electron microscope. For instance, the average thickness of the “insulating layer” is a dimension of the insulating layer in a direction perpendicular to the surface of the positive electrode collector.

In a preferred implementation of the lithium ion battery according to the present art, the inorganic filler is at least one selected from the group consisting of a boehmite powder, a zeolite powder, and an alumina powder. More preferably, the inorganic filler is a boehmite powder. An insulating layer of superior heat resistance can be produced conveniently through the use of such a powder as the inorganic filler. The use of a boehmite powder having a lower Mohs hardness than that of alumina allows suppressing generation of metallic foreign matter arising for instance from scraping of equipment contact sections by portions of inorganic filler not covered with LPO, during the production process.

The lithium ion battery is thus provided as a battery that is highly safe and in which short circuits between a positive electrode collector (uncoated section) and a negative electrode active material layer are suppressed, even in a case where for instance metal foreign matter is mixed in the battery. Such safety in cases where the battery temperature rises due to overcharge or the like can be suitably applied to for instance secondary batteries having a multilayer structure (including multilayer electrode bodies and wound electrode bodies) in which multiple power storage elements are stacked, in applications where the temperature of the lithium ion battery itself is prone to rise on account of repeated charging and discharge with large currents at a high rate. In addition, the above high safety can be suitably exploited in secondary batteries that are used in close proximity to a person, in applications where high safety is demanded. Therefore, the lithium ion battery disclosed herein can be suitably used as a drive power source in vehicles (main power source); among the foregoing, the lithium ion battery disclosed herein can be suitably used as a drive power source in plug-in hybrid vehicles or the like.

The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the embodiments with reference to the attached drawings.

