EP-3771016-B1 - LITHIUM SECONDARY BATTERY

Inventors

• OKAZAKI, TOMOHISA
• SADAKANE, TAKUYA

Dates

Publication Date

20260513

Application Date

20190213

Claims (9)

1. A lithium secondary battery having a positive electrode, a separator, a negative electrode facing the positive electrode with the separator interposed between the negative electrode and the positive electrode, and an electrolyte, wherein metallic lithium deposits on the negative electrode during charge, to form a lithium metal layer, the electrolyte comprising: an oxalate salt containing an oxalate complex as an anion, and a lithium ion as a cation; and a dicarboxylic acid diester compound, the dicarboxylic acid diester compound being contained in an amount of 0.1 mass% or more and less than 10 mass% in the electrolyte.
2. The lithium secondary battery of claim 1, wherein the oxalate salt is contained in an amount of 0.1 mass% or more and 20 mass% or less in the electrolyte.
3. The lithium secondary battery of claim 1 or 2, wherein the dicarboxylic acid diester compound is represented by the following formula (1): (where R 1 represents a single bond or a hydrocarbon group having 1 to 3 carbon atoms, R a and R b each represent a hydrocarbon group having 1 to 4 carbon atoms).
4. The lithium secondary battery of any one of claims 1 to 3, wherein the oxalate salt includes lithium difluorooxalate borate.
5. The lithium secondary battery of any one of claims 1 to 4, wherein the negative electrode includes a negative electrode current collector, and the negative electrode current collector includes a metal material that does not react with metallic lithium.
6. The lithium secondary battery of claim 5, wherein the metal material is copper or a copper alloy.
7. The lithium secondary battery of any one of claims 1 to 6, wherein in a fully discharged state, the negative electrode has no lithium metal that can be substantially discharged.
8. The lithium secondary battery of claims 5, wherein the negative electrode active material layer is formed by attaching a foil of lithium metal on the current collector, or electrodeposition or vapor deposition of lithium metal on the current collector.
9. The lithium secondary battery of any one of claims 1 to 8, wherein an electrode group formed by winding the positive electrode and the negative electrode with the separator interposed therebetween is housed together with the electrolyte in an outer case.

Description

[Technical Field] The present invention relates to a lithium secondary battery including lithium metal as a negative electrode active material. [Background Art] Non-aqueous electrolyte secondary batteries have been widely used for ICT devices, such as personal computers and smart phones, automobiles, power storage systems, and other applications. For the non-aqueous electrolyte secondary batteries used for such applications, further improvement in their capacity has been required. A lithium ion battery is known as a high-capacity non-aqueous electrolyte secondary battery. The capacity of the lithium ion battery can be further improved by using, for example, graphite and an alloy-type active material, such as a silicon compound, in combination as negative electrode active materials. However, the improvement in capacity of the lithium ion battery is approaching to the limit. A lithium secondary battery is seen as promising as a non-aqueous electrolyte secondary battery superior in capacity to the lithium ion battery. In the lithium secondary battery, metallic lithium deposits on the negative electrode during charge, and the metallic lithium dissolves in the electrolyte during discharge. Patent Literature 1, which relates to a secondary battery including an anode containing lithium metal, proposes using an electrolyte containing an oxalate complex as an anion. [Citation List] [Patent Literature] [PTL 1] Japanese Laid-Open Patent Publication No. 2018-501615 [Summary of Invention] According to Patent Literature 1, the capacity attenuation associated with cycling operations can be suppressed. The suppression, however, is not sufficient. One aspect of the present invention relates to a lithium secondary battery having a positive electrode, a separator, a negative electrode facing the positive electrode with the separator interposed between the negative electrode and the positive electrode, and an electrolyte, wherein metallic lithium deposits on the negative electrode during charge, the electrolyte including: an oxalate salt containing an oxalate complex as an anion, and a lithium ion as a cation; and a dicarboxylic acid diester compound, the dicarboxylic acid diester compound being contained in an amount of 0.1 mass% or more and less than 10 mass% in the electrolyte. The lithium secondary battery of the present invention has excellent cycle characteristics. [Brief Description of Drawing] [FIG. 1] A partially cut-away schematic oblique view of a lithium secondary battery according to one embodiment of the present invention. [Description of Embodiments] A lithium secondary battery according to the present embodiment has a positive electrode, a separator, a negative electrode facing the positive electrode with the separator interposed therebetween, and an electrolyte. Metallic lithium deposits on the negative electrode during charge. The deposited metallic lithium on the negative electrode dissolves in the form of lithium ions in the electrolyte during discharge. In a typical lithium secondary battery, metallic lithium tends to deposit in a dendrite form on the negative electrode. This increases side reactions, which reduces the charge-discharge efficiency, and deteriorates the cycle characteristics. When the electrolyte includes an oxalate salt containing an oxalate complex as an anion and a lithium ion as a cation, the oxalate complex serving as the anion interacts with lithium, allowing metallic lithium to uniformly deposit in a fine particulate form. Accordingly, the formation of dendrites tends to be suppressed. However, with the oxalate salt alone, the formation of dendrites cannot be suppressed sufficiently. To address this, in the present embodiment, in addition to the oxalate salt, a dicarboxylic acid diester compound is added to the electrolyte in an amount of 0.1 mass% or more and less than 10 mass%. This can further suppress the formation of dendrites on the negative electrode. Although it remains unclear by what mechanism the addition of a dicarboxylic acid diester compound can further suppress the formation of dendrites, one or more dicarboxylic acid diester compounds are considered to further coordinate around the lithium ion to which the oxalate complex is coordinated. This apparently makes the lithium ion bulkier. The lithium ions are thus unlikely to come in proximity with each other, and reach the negative electrode over time. This leads to a uniform deposition of metallic lithium on the negative electrode. In other words, the dicarboxylic acid diester compound enhances the action of the oxalate salt on the lithium ions. The dicarboxylic acid diester compound can coordinate to an independent lithium ion. However, with the dicarboxylic acid diester compound alone, the effect of suppressing the formation of dendrites is extremely small. During the deposition of metallic lithium, side reactions occur between the deposited metallic lithium and the electrolyte, which may cause byproducts to deposit at