FIELD

Embodiments described herein relate generally to a lithium zinc secondary battery, a battery pack, a vehicle, and a stationary power supply.

BACKGROUND

Recently, as a battery having a high energy density, batteries formed using a nonaqueous solvent have been developed, such as a lithium ion secondary battery. The lithium ion secondary battery is excellent in energy density and cycle performance compared to a lead storage battery and a nickel hydrogen secondary battery, and is anticipated for use as a large electricity storage power source, such as a power source for vehicles such as hybrid vehicles and electric vehicles. From the viewpoint of providing a large potential window, a nonaqueous solvent such as ethylene carbonate, diethyl carbonate or propylene carbonate is used as an electrolyte of the lithium ion secondary battery. Since these solvents are flammable, there are safety problems. If the solvents can be replaced with an aqueous electrolyte, the problems can fundamentally be solved. In addition, the aqueous electrolyte is inexpensive, compared to a nonaqueous solvent-based electrolyte. Further, no longer would production process need be performed in an inert atmosphere. Therefore, by replacing the nonaqueous solution-based electrolyte with the aqueous electrolyte, large cost reduction is expected. However, there is a large problem in the use of the aqueous electrolyte for the lithium ion secondary battery. The problem is that since the theoretical decomposition voltage calculated from the chemical equilibrium of water is 1.23 V, when a battery is configured to have a greater design voltage, oxygen is generated in a positive electrode and hydrogen is generated in a negative electrode. In order to solve the problem, the oxygen overvoltage can be increased at the oxidation side (positive electrode side), the hydrogen overvoltage can be increased at the reduction side (negative electrode side), and the design of the battery can be improved in viewpoint of kinetics.

In the lithium ion secondary battery formed by using an aqueous solution, the charge-discharge process of the positive electrode is relatively easy. There are many reported examples of positive electrode active materials such as LiCoO2, LiMn2O4, and LiFePO4. Meanwhile, for the negative electrode active material, although there have been reported examples such as LiTi2(PO4)3, TiP2O7, and VO2(B), which have a lithium insertion/extraction potential of about −0.5 V (vs. SHE), there are only few reported examples using active materials which operate at lower potentials. This is due to charge-discharge being difficult because of hydrogen generation proceeding in the negative electrode. Even when a battery is configured, for example, using the above-described active materials as the negative electrode active material, the average operating potential is less than 2 V. In this case, it is difficult for the battery to achieve a high energy density, and therefore, there is currently no aqueous secondary battery which has an energy density greater than that of the lead storage battery or the nickel hydrogen secondary battery.

Therefore, among aqueous lithium ion secondary batteries, attention is attracted to lithium-zinc secondary batteries, where used for a positive electrode is an oxide material into and out from which lithium is inserted and extracted, and used for a negative electrode a zinc metal. The theoretical capacity of the zinc metal is as large as 820 mAh/g. Therefore, the lithium-zinc secondary batteries having a combination of the negative electrode using the zinc metal and the positive electrode into and out from which lithium is inserted and extracted are anticipated as one of candidates for secondary batteries having high energy density among secondary batteries using an aqueous solution. However, dendrites are precipitated on the surface of the electrode during a zinc dissolution-precipitation reaction. Therefore, there is a problem in that life performance deteriorates due to an increase in overvoltage at the time of charge and discharge.

