JP2013165030A - Steel material for nonaqueous electrolyte secondary battery case - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive steel material free from corrosion of a case and degradation of battery performance associated with metal elution even when a potential rise of a case occurs in a metal outer package case of negative electrode connection or neutral.SOLUTION: The steel material is provided with a Cr plating layer on the surface having an oxide film whose thickness is 10 to 500 nm on the surface. Preferably, an oxide film contains an element selected from Al and P. Preferably also, a Ni plating layer is provided under the Cr plating layer. Preferably, the oxide film is formed by anode electrolysis processing.

Description

  The present invention relates to an electrode group in which a negative electrode capable of inserting and extracting lithium ions and a positive electrode capable of inserting and extracting lithium ions are opposed to each other via a separator, and a lithium salt as a solute in an organic solvent. The present invention relates to a case of a non-aqueous electrolyte secondary battery including a dissolved non-aqueous electrolyte, and particularly relates to an inexpensive steel material for a case that has excellent corrosion resistance in the non-aqueous electrolyte.

  In recent years, with the miniaturization and high functionality of consumer mobile devices, there has been a demand for a secondary battery that can be charged and discharged for a long time with a small size, light weight and high energy density as its power source. As a result, in place of conventional nickel-cadmium batteries and nickel-hydrogen batteries, non-aqueous electrolyte secondary batteries such as lithium ion batteries having higher energy density and output density have come into widespread use. Recently, lithium ion batteries have already entered the practical stage as secondary batteries for vehicles, and have begun to spread as power sources for motors of hybrid vehicles and electric vehicles.

  In order to manufacture a non-aqueous electrolyte secondary battery at a low cost, a low-cost and highly reliable exterior case material is required. Although steel materials are promising as a base material that satisfies the press formability, weldability, corrosion resistance, strength, and the like necessary for the application and is low in cost, the application has the following problems.

When a steel material is used for the outer case of the nonaqueous electrolyte secondary battery, the surface is plated with Ni or the like for the purpose of corrosion protection.
When the metal outer case using this Ni-plated steel material is used with the negative electrode connected, there is no problem with the corrosion resistance in a normal situation.
However, when the potential of the battery case rises due to overdischarge of the battery, the plated Ni may be eluted. Also, when a metal outer case using Ni-plated steel is used as a neutral case insulated from the battery element, there is no problem with the corrosion resistance in a normal situation.
However, when the potential of the battery case increases due to the action of an oxidizing agent in the electrolyte, Ni may be eluted.

  When the plated metal such as Ni elutes in the electrolyte, when the battery is charged and discharged, the eluted metal is deposited and grows on the surface of the negative electrode. It causes a minute short circuit. When a micro short circuit occurs, the battery voltage is lowered, so that necessary battery performance cannot be obtained, leading to a decrease in battery yield. Further, the corrosion of the metal outer case itself progresses, causing electrolyte leakage.

  On the other hand, Patent Document 1 shows that the fluorine resin-dispersed Ni plating improves the corrosion resistance, but the fluorine resin-dispersed Ni plating is expensive and has little effect on improving the corrosion resistance. Further, Patent Document 2 shows that the corrosion resistance is improved by forming a fluoride film on Ni. However, a large-scale facility for using fluorine gas is required, and there is a problem that the cost is also high. . Patent Document 3 discloses a case material having a Ni—Cu diffusion layer, but there is a problem that Cu is likely to be eluted when the potential is increased. Patent Document 4 discloses a lithium ion battery having corrosion resistance in a case made of a Ni-plated steel material, but is not universal because it is based on the premise that a special compound is contained in the electrolytic solution.

  From the above point of view, the current situation is that aluminum or stainless steel must be used for the metal outer case where the corrosion resistance becomes a problem while having a cost problem.

JP 2002-231195 A JP 2003-229099 A JP 2011-9154 A JP 2011-70861 A

  In the case of negative electrode connection or a neutral metal outer case, the battery performance is deteriorated due to metal elution even when the battery case has a potential increase due to overdischarge of the battery or the action of an oxidizing agent in the electrolyte. The purpose is to provide an inexpensive steel material that is free from corrosion.

The gist of the present invention is that
(1) Non-water characterized by having a Cr plating layer on the surface of a steel material, an oxide film having a thickness of 10 to 500 nm on the surface, and the oxide film containing either Al or P or both. Steel material for electrolyte secondary battery case

(2) The steel material for a nonaqueous electrolyte secondary battery case according to (1), which has a Ni plating layer under the Cr plating layer.

(3) The steel material for a nonaqueous electrolyte secondary battery case according to (1) or (2), wherein the Cr coating amount of the Cr plating layer is 0.01 to 0.5 g / m ^2.

(4) The steel material for a nonaqueous electrolyte secondary battery case as described in (2) or (3), wherein the Ni adhesion amount of the Ni plating layer is 0.1 to 15 g / m ^2.

(5) The steel material for a non-aqueous electrolyte secondary battery case according to any one of (1) to (4), wherein the oxide film is formed by an anodic electrolytic treatment.

  According to the present invention, even when there is an increase in the potential of the battery case due to the overdischarge of the battery or the action of an oxidant in the electrolyte, the steel material is free from deterioration of battery performance due to metal elution from the case and corrosion of the case. can get.

  The steel material of the present invention is one of the gist of the invention, and is not suitable for the provision of an inexpensive steel material. Therefore, the expensive stainless steel is excluded. It is sufficient to use a steel material that meets the required workability.

  The steel material of the present invention is characterized by having a Cr plating layer and an oxide film having a thickness of 10 to 500 nm on the surface thereof, and the oxide film contains either Al or P or both.

  The Cr plating layer is relatively stable in the non-aqueous electrolyte, but may be slightly dissolved at a high potential. Further, when the thickness of the Cr plating layer is increased, cracks are likely to occur. When the thickness is thin, plating pinholes are unavoidable, so that Fe is likely to be eluted from the base steel through the cracks or pinholes. By having the predetermined oxide film of the present invention, it is possible to prevent any of the aforementioned Cr or Fe elution, and even if the potential is increased, the elution of Cr and Fe is effective up to about 5 V on the basis of lithium. Can be suppressed.

  A natural oxide film naturally formed in the atmosphere on the Cr plating surface has several nanometers. However, the natural oxide film is not stable at a high potential, so that the effect of the present invention is not achieved.

If the thickness of the oxide film is less than 10 nm, the metal elution suppressing effect is insufficient, so it is necessary to set the thickness to 10 nm or more.
From the viewpoint of suppressing metal elution, there is no upper limit on the thickness of the oxide film. However, if the thickness of the oxide film exceeds 500 nm, cracking during processing becomes a problem, so the practical thickness is 500 nm or less, which is a practical upper limit.

  The thickness of the oxide film is measured by measuring the oxygen profile in the depth direction from the surface layer using analytical methods such as AES (Auger Electron Spectroscopy) and GDS (Glow Discharge Spectroscopy), and measuring the depth at which the oxygen intensity decreases to the background level. Is possible.

The oxide film of the present invention contains an element selected from Al and P. The oxide film containing these is stable to a high potential in the non-aqueous electrolyte and can effectively suppress metal elution.
The oxide film of the present invention may contain Fe from steel, Cr of Cr plating, or Ni from the Ni plating layer when Ni plating described later is applied.

  The concentration of Al and P in the oxide film of the present invention is 0.1 to 10% by mass, more preferably 1 to 5% by mass. In addition, when both Al and P are contained, it is good to set it as the said density | concentration with the sum total of Al + P. If the concentrations of Al and P are too low, metal dissolution occurs at a relatively low potential. On the other hand, if it is too high, damage to the oxide film during processing tends to increase, and as a result, metal dissolution tends to occur.

As the Cr plating adhesion amount of the present invention, 0.01 to 0.5 g / m ² is desirable. If it is less than 0.01 g / m ² , the metal elution suppressing effect is poor, and if it exceeds 0.5 g / m ² , not only the cost increases, but also processing damage tends to increase.

  When subjected to more severe processing or when stability for a longer time is required, it is desirable to provide a Ni plating layer under the Cr plating layer.

When the Ni plating is provided in the lower layer of the Cr plating, the Ni adhesion amount is preferably 0.1 to 15 g / m ² . If it is less than 0.1 g / m ² , the metal elution suppressing effect is poor, and if it exceeds 15 g / m ² , not only the cost increases, but also the processing damage tends to increase.

  The Ni plating layer is heat-diffused after Ni plating, and the Ni-Fe diffusion layer is made part or all of the Ni plating layer by diffusing Fe into the Ni plating layer, so that the corrosion resistance after processing is excellent. More preferred.

  The plating layer in the present invention has an oxide film containing either Al or P or both on the surface, and even if the amount of Cr or Ni plating is small compared to the structure of plating single plating without an oxide film. It is characterized by excellent performance and is advantageous in terms of cost. On the other hand, due to the small amount of adhesion of the plating layer, defects such as plating pinholes are likely to occur, and there may be a region where the underlying steel material is exposed, but either Al or P formed on the Cr plating surface can be used. The contained oxide film covers not only the surface of the plating layer but also the surface of the steel material part of the plating defect part, and metal elution can be effectively suppressed.

  The oxide film containing either or both of Al and P according to the present invention is preferably formed by an anodic oxidation treatment. The oxide film thus formed is more stable in the nonaqueous electrolyte. In addition, when an oxide film is formed through a plating layer, if anodizing treatment is used, even if pinhole defects exist in the plating layer, the steel material portion where the plating defect portion is exposed than on the Cr plating layer is used. Since the oxidation reaction proceeds preferentially and an oxide film is formed thicker on the steel material in the plating defect portion than the plating surface portion, it is more advantageous in terms of corrosion resistance.

  When performing the anodic oxidation treatment, it is desirable to carry out the treatment in a neutral to alkaline aqueous solution. An oxide film containing either or both of Al and P can be formed by performing anodic electrolytic treatment in an aqueous solution containing oxygen acid anions of Al and P, such as aluminate ions and phosphate ions. It is also preferable to add a pH adjuster or a conductive aid to the treatment liquid.

The conditions for the anodic electrolysis are not particularly limited, but it is desirable to perform constant current electrolysis at a temperature of about room temperature to about 80 ° C. and a current density of about 10 to 100 A / dm ² .

(Examples 1-20 and Comparative Example 1)
A cold-rolled steel sheet having the components shown in Table 1 was used as a base plate, and various amounts of Cr plating were performed under the conditions shown in Table 2, followed by anodic electrolytic oxidation treatment in various aqueous solutions shown in Table 3. The anode electrolytic treatment conditions were a treatment temperature of 70 ° C. and a current density of 50 A / dm ^2, and the treatment time was adjusted so as to obtain a predetermined oxide film thickness.

(Comparative Example 2)
Using cold-rolled steel sheets having the components shown in Table 1 as original sheets, Cr plating of 0.1 g / m ² was performed under the conditions shown in Table 2 for evaluation.

(Comparative Example 3)
The cold rolled steel sheet shown in Table 1 was directly subjected to anodic electrolytic oxidation treatment without using Cr plating. The anode electrolytic treatment conditions were a treatment temperature of 70 ° C. and a current density of 50 A / dm ^2, and the treatment time was adjusted so as to obtain a predetermined oxide film thickness.

(Evaluation method)
-Characterization of the oxide film of each steel material used both AES (Auger electron spectroscopy) and GDS (glow discharge spectroscopy). AES was used when the oxide film was thin, and GDS was used when it was thick. Further, the presence of metal components other than Fe and Cr contained in the oxide film was confirmed by the same method and the concentrations thereof were quantified and shown in Table 3.

-The edge and the back of each steel material were tape-sealed to make test materials. In a glove box with an argon atmosphere (dew point −60 ° C.), a three-electrode cell was assembled with the test material as a working electrode, metallic lithium as a counter electrode and a reference electrode. The electrolyte used was 1M LiPF6 dissolved in a 1: 1 mixed solvent of ethylene carbonate and diethyl carbonate by volume. The cell was subjected to anodic polarization at a rate of 5 mV / sec from a natural potential to 5 V (lithium standard) at 25 ° C., and was cathodic polarized to a natural potential by folding back at 5 V (lithium standard). . The cycle was repeated five times, and the potential (lithium reference) at which the current density when scanning in the anode direction was 10 μA / cm ² was defined as the dissolution potential, and the first and fifth dissolution potentials were evaluated.

  Table 3 shows the evaluation results. In the examples of the present invention, the dissolution potential could be greatly improved.

(Examples 21 to 40)
Using cold-rolled steel sheets having the components shown in Table 1 as original sheets, various plating amounts of Ni plating were performed under the plating conditions shown in Table 4, and then various amounts of Cr plating were performed under the conditions shown in Table 2. Thereafter, anode electrolytic oxidation treatment was performed in various aqueous solutions shown in Table 5. In the anodic electrolytic treatment, the treatment temperature was 70 ° C., the current density was 50 A / dm ^2, and the treatment time was adjusted so as to obtain a predetermined oxide film thickness.

(Example 41)
Using a cold-rolled steel plate having the components shown in Table 1 as an original plate, Ni plating of 5 g / m ² was performed under the plating conditions shown in Table 2. Thereafter, a thermal diffusion treatment at 700 ° C. for 10 seconds was performed in an atmosphere of 5% hydrogen-nitrogen to change the Ni plating layer into a Ni—Fe diffusion layer. Thereafter, Cr plating was performed under the conditions shown in Table 2, and then an anode electrolytic oxidation treatment was performed in an aqueous solution shown in Table 5. In the anodic electrolytic treatment, the treatment temperature was 70 ° C., the current density was 50 A / dm ^2, and the treatment time was adjusted so as to obtain a predetermined oxide film thickness.

(Comparative Example 4)
Using cold-rolled steel sheets having the components shown in Table 1 as original sheets, Ni plating was performed under the plating conditions shown in Table 4, and then Cr plating was performed under the conditions shown in Table 2.

(Comparative Example 5)
Using the cold-rolled steel sheet having the components shown in Table 1 as an original plate, Ni plating was performed under the plating conditions shown in Table 4, and anodic electrolytic oxidation treatment was performed in various aqueous solutions shown in Table 5. In the anodic electrolytic treatment, the treatment temperature was 70 ° C., the current density was 50 A / dm ^2, and the treatment time was adjusted so as to obtain a predetermined oxide film thickness.

(Evaluation method)
The characterization of the oxide film and the measurement of the dissolution potential were performed in the same manner as in the previous example. The elements contained in the oxide film are shown in Table 5 after identifying and quantifying elements other than Fe and plating metal (Ni or Cr).

Melting potential measurement: After pressing into a cylindrical drawn can corresponding to standard 18650 of a cylindrical lithium ion secondary battery, the inner surface was cut out, and the edge and back surface were tape-sealed to obtain a test material. Similar to the previous example, the first and fifth dissolution potentials were evaluated.

Constant-potential electrolysis: After pressing into a cylindrical drawn can corresponding to standard 18650 of a cylindrical lithium ion secondary battery, the inner surface was cut out, and the edge and back surface were tape-sealed to obtain a test material. In a glove box with an argon atmosphere (dew point −60 ° C.), a three-electrode cell was assembled with the test material as a working electrode, metallic lithium as a counter electrode and a reference electrode. The electrolyte used was 1M LiPF6 dissolved in a 1: 1 mixed solvent of ethylene carbonate and diethyl carbonate by volume. The cell was held at 25 ° C. for 24 hours with a working electrode potential of 4.2 V (based on lithium). The total energization amount was measured and the metal concentration eluted in the electrolyte was quantified. A substance in which no dissolved metal was detected in the electrolyte was evaluated as “◯”.

・作製条件および評価結果（その２）
表５に評価結果を示すが、本発明の実施例では、溶解電位を大きく向上することができるとともに、高電位で長時間保持した場合にも金属溶出は検出されなかった。

・ Production conditions and evaluation results (2)
Table 5 shows the evaluation results. In the examples of the present invention, the dissolution potential could be greatly improved, and metal elution was not detected even when held at a high potential for a long time.

  In order to manufacture non-aqueous electrolyte secondary batteries at low cost, low-cost and highly reliable exterior case materials are required, and the use of steel is desirable from the viewpoint of press formability, weldability, corrosion resistance, strength, etc. It is. The steel material of the present invention is useful as a case material for a non-aqueous electrolyte secondary battery because it does not elute even when held at a high potential in the non-aqueous electrolyte and is excellent in corrosion resistance.

Claims (5)

  A non-aqueous electrolyte secondary characterized in that it has a Cr plating layer on the surface of a steel material, an oxide film having a thickness of 10 to 500 nm on the surface, and the oxide film contains either Al or P or both. Steel for battery cases.   The steel material for a nonaqueous electrolyte secondary battery case according to claim 1, further comprising a Ni plating layer under the Cr plating layer. Non-aqueous electrolyte secondary battery case for steel according to claim 1 or 2 Cr deposition amount of the Cr plating layer, characterized in that a 0.01 to 0.5 g / m ^2. The non-aqueous electrolyte secondary battery case for steel according to claim 2 or 3 Ni deposition amount of the Ni plating layer is characterized in that it is a 0.1 to 15 g / m ^2.   The steel material for a nonaqueous electrolyte secondary battery case according to any one of claims 1 to 4, wherein the oxide film is formed by anodic electrolytic treatment.