Alkaline secondary batteries typified by nickel-hydrogen batteries and nonaqueous electrolyte secondary batteries typified by lithium ion batteries have been widely used as a power supply for driving portable electronic equipment, such as cell phones including smartphones, portable personal computers, PDAs, and portable music players. In addition, alkaline secondary batteries and nonaqueous electrolyte secondary batteries have been widely used as a power supply for driving electric vehicles (EVs) and hybrid electric vehicles (HEVs and PHEVs), and in stationary storage battery systems for suppressing output fluctuation of solar power generation and wind power generation, for example, and for a peak shift of system power that utilizes the power during the daytime while saving the power during the nighttime.
The use of EVs, HEVs, and PHEVs or the stationary storage battery system especially requires high capacity and high output characteristics. The size of each battery is therefore increased, and a plurality of batteries are connected in series or in parallel for use. Therefore, nonaqueous electrolyte secondary batteries have been generally used for these purposes in view of space efficiency. When physical strength is needed, a metal prismatic outer body with one side open, and a metal sealing plate for sealing this opening are generally adopted as an outer body of a battery.
Increasing longevity is essential in nonaqueous electrolyte secondary batteries used for the above-mentioned purposes. Therefore, various additives are added to a nonaqueous electrolyte in order to prevent degradation. For example, JP-A-2009-129541 discloses that, in a nonaqueous electrolyte secondary battery, a cyclic phosphazene compound and a salt having various oxalate complexes as an anion are added to a nonaqueous electrolyte. JP-T-2010-531856 and JP-A-2010-108624 describe adding lithium bis(oxalato)borate (Li[B(C2O4)2], hereinafter referred to as “LiBOB”), which is a lithium salt having an oxalate complex as an anion, as represented by the following structural formula (I).

Japanese Patent No. 3439085 discloses the invention of a nonaqueous electrolyte secondary battery in which lithium difluorophosphate (LiPF2O2) is added to a nonaqueous electrolyte in order to prevent self-discharge at charge storage and improve storage characteristics after charging.
When a cyclic phosphazene compound and a salt having various oxalate complexes as an anion disclosed in JP-A-2009-129541 are added to a nonaqueous electrolyte, fire resistance of the nonaqueous electrolyte is improved, which can provide a nonaqueous electrolyte secondary battery having excellent battery characteristics and high safety. When LiBOB disclosed in JP-T-2010-531856 and JP-A-2010-108624 is added to a nonaqueous electrolyte, a protective layer including a lithium ion conductive layer that is thin and extremely stable is formed on the surface of a carbon negative electrode active material of the nonaqueous electrolyte secondary battery. This protective layer is stable even in a high temperature, consequently preventing the carbon negative electrode active material from decomposing the nonaqueous electrolyte. This leads to an advantage of providing excellent cycling characteristics and improving the safety of a battery.
In the nonaqueous electrolyte secondary battery disclosed in Japanese Patent No. 3439085, LiPF2O2 and lithium react to form a high-quality protective covering on the interfaces of a positive electrode active material and a negative electrode active material. This protective covering prevents direct contact between an active material in a state of charge and an organic solvent, thereby preventing decomposition of a nonaqueous electrolyte due to the contact between the active material and the nonaqueous electrolyte and improving charge storage characteristics.
A nonaqueous electrolyte secondary battery having high capacity and high output characteristics uses a flat winding electrode assembly that is produced by winding a positive electrode with a large width, a negative electrode, and a separator. In a nonaqueous electrolyte secondary battery using such a flat winding electrode assembly that includes an electrode with a large width, a nonaqueous electrolyte is less likely to penetrate the winding electrode assembly in the width direction of the electrode and is even less likely to penetrate the inner periphery than the outer periphery. In particular, the viscosity of the nonaqueous electrolyte is high in a nonaqueous electrolyte secondary battery using a nonaqueous solvent that has a high content ratio of a cyclic carbonate or using a nonaqueous electrolyte that includes a lithium salt having an oxalate complex as an anion or LiPF2O2 added in the nonaqueous solvent. Consequently, the nonaqueous electrolyte is less likely to penetrate the inside of the flat winding electrode assembly. When a positive electrode, a negative electrode, and a separator are wound and then pressed to produce a flat winding electrode assembly, the positive electrode, the negative electrode, and the separator closely contact each other, and the effect of the viscosity of the nonaqueous electrolyte can be clearly seen.