An energy storage device, especially a lithium secondary battery, has been widely used recently for a power source of a small-sized electronic device, such as a mobile telephone, a notebook personal computer, etc., and a power source for an electric vehicle or electric power storage. There is a possibility that such an electronic device or vehicle is used in a broad temperature range, such as a high temperature in the midsummer, a low temperature in the coldest season, etc., and therefore, it is required to improve electrochemical characteristics with good balance in a broad temperature range.
In particular, in order to achieve prevention of global warming, it is an urgent need to reduce the CO2 emissions. Among eco-friendly vehicles mounted with an energy storage system composed of an energy storage device, such as a lithium secondary battery, a capacitor, etc., early dissemination of hybrid electric vehicle (HEV), plug-in hybrid electric vehicle (PHEV), and battery electric vehicle (BEV) is demanded. In vehicles, a moving distance is long, and therefore, there is a possibility that the vehicles are used in regions of a broad temperature range of from a very warm region of the tropics to a coldest region. In consequence, in particular, such an onboard energy storage device is required such that even when used in a broad temperature range of from high temperatures to low temperatures, the electrochemical characteristics are not worsened.
In the present specification, the term “lithium secondary battery” is used as a concept also including a so-called lithium ion secondary battery.
A lithium secondary battery is mainly constituted of a positive electrode and a negative electrode, each containing a material capable of absorbing and releasing lithium, and a nonaqueous electrolytic solution including a lithium salt and a nonaqueous solvent; and a carbonate, such as ethylene carbonate (EC), propylene carbonate (PC), etc., is used as the nonaqueous solvent.
In addition, metal lithium, a metal compound capable of absorbing and releasing lithium (e.g., a metal elemental substance, a metal oxide, an alloy with lithium, etc.), and a carbon material are known as the negative electrode. In particular, a lithium secondary battery using a carbon material capable of absorbing and releasing lithium, for example, coke, artificial graphite, natural graphite, etc., is widely put into practical use.
For example, as for a lithium secondary battery using, as a negative electrode material, a highly crystallized carbon material, such as natural graphite, artificial graphite, etc., it is known that decomposed products or a gas generated when a solvent in a nonaqueous electrolytic solution is reductively decomposed on a surface of the negative electrode at the time of charging hinders a desired electrochemical reaction of the battery, worsening of cycle properties is caused. In addition, when the decomposed products of the nonaqueous solvent are accumulated, it becomes difficult to smoothly achieve absorption and release of lithium on and from the negative electrode, and when used in a broad temperature range, electrochemical characteristics are apt to be worsened.
Furthermore, it is known that a lithium secondary battery using a metal lithium or an alloy thereof, a metal elemental substance, such as tin, silicon, etc., or an oxide thereof as the negative electrode material may have a high initial battery capacity, but the battery capacity and the battery performance thereof, such as cycle properties, may be largely worsened because the micronized powdering of the material may be promoted during cycles, which brings about accelerated reductive decomposition of the nonaqueous solvent, as compared with the negative electrode formed of a carbon material. In addition, the micronized powering of such a negative electrode material is promoted, or when the decomposed products of the nonaqueous solvent are accumulated, it becomes difficult to smoothly achieve absorption and release of lithium on and from the negative electrode, and when used in a broad temperature range, electrochemical characteristics are apt to be worsened.
Meanwhile, as for a lithium secondary battery using, as a positive electrode, for example, LiCoO2, LiMn2O4, LiNiO2, LiFePO4, etc., it is known that decomposed products or a gas generated when a part of a nonaqueous solvent in a nonaqueous electrolytic solution is locally oxidatively decomposed on an interface between the positive electrode material and the nonaqueous electrolytic solution in a charged state hinders a desired electrochemical reaction of the battery, and thus, when used in a broad temperature range, the electrochemical characteristics are apt to be worsened, too.
In the light of the above, the movement of lithium ions is hindered, or the battery expands due to decomposed products or a gas generated when the nonaqueous electrolytic solution is decomposed on the positive electrode or negative electrode, so that the battery performance was worsened. Irrespective of the foregoing situation, the multifunctionality of electronic devices on which lithium secondary batteries are mounted is more and more advanced, and the electric power consumption tends to increase. The capacity of the lithium secondary battery is thus being much increased, and because of an increase of a density of the electrode, a reduction of a useless space capacity within the battery, and so on, a volume occupied by the nonaqueous electrolytic solution in the battery is becoming small. In consequence, it is the present situation that in the case of using the battery in a broad temperature range, the electrochemical characteristics are apt to be worsened due to decomposition of a bit of the nonaqueous electrolytic solution.
PTL 1 describes that a nonaqueous electrolytic solution containing, as an additive, a phosphate ester compound, such as triethyl phosphonoacetate, triethyl phosphonoformate, etc., is able to improve continuous charging properties and high-temperature storage properties and inhibit the gas generation.
PTL 2 describes that a nonaqueous electrolytic solution capable of realizing a lithium ion secondary battery having a higher positive electrode potential than that in a conventional one, being excellent in cycle properties, and generating little gas and a lithium ion secondary battery using the foregoing nonaqueous electrolytic solution can be provided.