EP-4459738-B1 - METHOD FOR CHARGING SECONDARY BATTERY

Inventors

• DOI, SHOTARO
• ONO, YOSHITAKA
• TANAKA, HIROYUKI
• OSHIHARA, KENZO

Dates

Publication Date

20260506

Application Date

20221216

Claims (3)

1. A method for charging a secondary battery including a positive electrode current collector, a positive electrode active material layer, a solid electrolyte layer, and a negative electrode current collector in this order and utilizing a deposition-dissolution reaction of metallic lithium as a reaction of a negative electrode, the method having a multi-stage charging step, and comprising at least: a first charging step in which the secondary battery is charged at a first current density I1 to deposit metallic lithium on a surface on the solid electrolyte layer side of the negative electrode current collector to form a deposited Li layer that is a part of a negative electrode active material layer and that comprises the metallic lithium; and a second charging step in which the secondary battery is charged at a second current density I2 greater than the first current density I1 after the first charging step to increase a thickness of the deposited Li layer, wherein the first charging step includes performing pausing at least once or discharging at least once, in which the secondary battery is charged at the first current density I1 so that an SOC does not exceed 4.5%, and the first current density I1 is less than 0.22 (mA/cm 2 ).
2. The method for charging a secondary battery according to claim 1, wherein the first charging step includes pausing at least once, and a pausing time in the pausing is 1.5 hours or less.
3. The method for charging a secondary battery according to claim 1 or 2, wherein the first charging step includes discharging at least once, and an end-of-discharge voltage in the discharging is 2.5 V or less.

Description

TECHNICAL FIELD The present invention relates to a method for charging secondary battery. BACKGROUND ART In recent years, in order to fight global warming, there is a strong need for reduction of the amount of carbon dioxide. In the automobile industry, there are increasing expectations for a reduction of carbon dioxide emissions by introduction of electric vehicles (EV) and hybrid electric vehicles (HEV), and development of secondary batteries such as secondary batteries for motor driving, which are key to practical application of such vehicles, has been actively conducted. A secondary battery for motor driving is required to have extremely high output characteristics and high energy as compared with a lithium secondary battery for consumer use used in a mobile phone, a notebook computer, and the like. Therefore, a lithium secondary battery having the highest theoretical energy among all practical batteries has attracted attention, and is currently being rapidly developed. Here, lithium secondary batteries that are currently widespread use a combustible organic electrolyte solution as an electrolyte. In such liquid-based lithium secondary batteries, safety measures against liquid leakage, short circuit, overcharge, and the like are more strictly required than other batteries. Therefore, in recent years, research and development on an all-solid-state lithium secondary battery using an oxide-based or sulfide-based solid electrolyte as an electrolyte have been actively conducted. The solid electrolyte is a material mainly made of an ion conductor capable of ion conduction in a solid. Therefore, in the all-solid-state lithium secondary battery, in principle, various problems caused by the combustible organic electrolyte solution do not occur unlike the conventional liquid lithium secondary battery. Further, in general, when a positive electrode material having a high potential and a large capacity and a negative electrode material having a large capacity are used, the power density and the energy density of the battery can be significantly improved. An all-solid-state lithium secondary battery using a sulfide-based material as a positive electrode active material and using metallic lithium or a lithium-containing alloy as a negative electrode active material is a promising candidate. Meanwhile, in the lithium secondary battery, the negative electrode potential decreases with the progress of charging. When the negative electrode potential decreases to be lower than 0 V (vs. Li/Li+), metal lithium is precipitated at the negative electrode, and dendrite (tree-like) crystals are precipitated (this phenomenon is also referred to as "electrodeposition of metal lithium"). When electrodeposition of metallic lithium occurs, there is a problem that a deposited dendrite penetrates the solid electrolyte layer to cause an internal short circuit in the battery. For example, JP 2020-009724 A discloses a charging method in a secondary battery utilizing a deposition-dissolution reaction of metallic lithium as a reaction of a negative electrode for the purpose of achieving both suppression of a short circuit in the battery and shortening of a charging time. The charging method includes a first charging step in which the secondary battery is charged at a first current density I1, and a second charging step in which the secondary battery is charged at a second current density I2 greater than the first current density I1 after the first charging step. Then, the first charging step is characterized in that when the roughness height on the surface on the negative electrode current collector foil side of the solid electrolyte layer is represented by Y (um) and the thickness of the roughness coating layer is represented by X (µm), the secondary battery is charged at the first current density I1 until X/Y becomes 0.5 or more. SUMMARY OF INVENTION However, according to the study by the present inventors, when the charging method described in JP 2020-009724 A is applied to a secondary battery, it has been found that a short circuit in the battery sometimes cannot be sufficiently suppressed. Therefore, an object of the present invention is to provide a means for more effectively suppressing a short circuit in a secondary battery. The present inventors have conducted intensive studies in order to solve the above problems. As a result, the present inventors have found that the problem can be solved by performing pausing and/or discharging at least once and performing charging so that the SOC (State of Charge) does not exceed 4.5% in a charging step corresponding to the "first charging step" described in JP 2020-009724 A, and thus completed the present invention. In the present description, an uncharged state is defined as an SOC of 0%, and a state in which a current value becomes 20% or less of a constant current value by charging at a constant current and charging at a constant voltage after 4.2 V is reached is defined as an SOC of 100%. A