JP2013165018A - 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 for a nonaqueous electrolyte secondary battery case is provided with an oxide film whose surface thickness is 10 to 500nm, where the oxide film contains an element selected from Mo and W. Preferably, an oxide film is formed via a plating layer containing an element selected from Ni and Cr. Preferably, the oxide film is formed by anode electrolysis processing.

Description

  The present invention includes a negative electrode capable of inserting and extracting lithium ions and a positive electrode capable of inserting and extracting lithium ions facing each other through a separator, and a lithium salt dissolved as a solute in an organic solvent. The present invention relates to a case of a non-aqueous electrolyte secondary battery provided with a non-aqueous electrolyte, and particularly relates to a steel material for a case which is excellent in corrosion resistance in a non-aqueous electrolyte and is inexpensive.

  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 been put into practical use as in-vehicle secondary batteries 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 an outer case of a 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) A steel material for a nonaqueous electrolyte secondary battery case having an oxide film having a thickness of 10 to 500 nm on the surface of the steel material, wherein the oxide film contains one or both of Mo and W.

(2) The steel material for a nonaqueous electrolyte secondary battery case according to (1), wherein a plating layer containing one or both of Ni and Cr is provided between the oxide film and the steel material surface.

(3) When the plating layer contains Ni, the Ni adhesion amount is 0.1 to 10 g / m ² , and when it contains Cr, the Cr adhesion amount is 0.01 to 0.5 g / m ^2. The steel for non-aqueous electrolyte secondary battery case described in (2)

(4) The steel material for a nonaqueous electrolyte secondary battery case according to any one of (1) to (3), wherein the oxide film is formed by anodic electrolysis.

  According to the present invention, even when there is an increase in the potential of the battery case due to overdischarge of the battery or the action of an oxidant in the electrolyte, a steel material that does not deteriorate in battery performance due to metal elution from the case or corrosion of the case is obtained. It is done.

  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 has a thickness of 10 to 500 nm on the surface and has an oxide film containing either one or both of Mo and W.

  The oxide film is stable in the non-aqueous electrolyte, and even when the battery case potential rises, it is possible to effectively suppress metal elution from the steel material up to about 4 V on the basis of lithium. In addition, although the natural oxide film naturally formed in air | atmosphere several nanometers exists on the steel material surface, this natural oxide film does not have the effect of this invention.

  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 requires either Mo or W, but when it is subjected to Fe from steel, or Ni or Cr plating described later, it further contains Ni and Cr from the plating layer. It doesn't matter. Further, the inclusion of other elements such as Co, Mn, P, Si, and Al is not excluded as long as either Mo or W is essential.

  The concentration of Mo and W in the oxide film of the present invention is 0.1 to 10% by mass, more preferably 1 to 5% by mass. When both Mo and W are contained, the concentration is preferably the sum of Mo + W. If the concentrations of Mo and W 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.

  When stability to a higher potential or longer time is required, a plating layer containing either (or both) of Ni and Cr is not formed directly on the steel surface. It is desirable to form an oxide film through the film. In this case, the elution of Fe from the steel material, Ni and Cr in the plating layer can be effectively suppressed when the potential is about 5 V on the basis of lithium. It is presumed that an oxide film containing either Mo or W as an essential component has an effect that the film is hardly destroyed even when the potential is increased.

  Even if the potential is kept at about 4 V for a long time, the metal does not elute. The plating layer containing one or both of Ni and Cr may be Ni plating, Cr plating, or alloy plating mainly composed of these. Moreover, about these plating, you may carry out the thermal-diffusion process after plating, and the element contained in a plating base material may be made to mutually diffuse.

  The optimum oxide film thickness is slightly different depending on the presence or absence of a plating layer containing one or both of Ni and Cr. When there is no plating layer, the film thickness is preferably 20 to 200 nm, and when there is a plating layer, the film thickness is preferably 10 to 100 nm.

When the plating layer containing any one or both of Ni and Cr is Ni-containing plating, the Ni adhesion amount is preferably 0.1 to 10 g / m ² . If it is less than 0.1 g / m ² , the metal elution suppression effect is poor, and if it exceeds 10 g / m ² , not only the cost increases but also the processing damage tends to increase.

When the plating layer contains plating containing Cr, the Cr adhesion amount is preferably 0.01 to 0.5 g / m2. 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.

  In the present invention, the plating layer formed between the oxide film and the steel material surface is characterized by a relatively low adhesion amount, which 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 areas where the underlying steel material is exposed, but the oxide film formed on the surface layer of the plating is Not only the surface but also the surface of the steel exposed portion of the plating defect portion is covered, and metal elution can be effectively suppressed.

  The oxide film of the present invention is desirably formed by an anodic oxidation treatment. The oxide film thus formed is more stable in the nonaqueous electrolyte. Also, when an oxide film is formed through a plating layer, if anodization is used, the steel material in which the plating defect portion is exposed more than on the Ni or Cr plating layer even if pinhole defects exist in the plating layer The oxidation reaction proceeds preferentially in the part, and an oxide film is formed thicker on the steel material in the plating defect part than the plating surface part, which 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. In forming an oxide film containing an element selected from Mo and W, an oxyacid anion of the element, for example, molybdate ion, tungstate ion, or the polyacid ion, and further against the polyacid It is desirable to perform anodic electrolytic treatment in an aqueous solution containing a heteropolyacid ion having a heteroatom such as Si or P inserted therein, for example, a phosphomolybdate ion, a silicate molybdate ion, a phosphotungstenate ion, or a phosphotungstenate ion.

  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 to 17 and Comparative Example 1)
Using cold-rolled steel sheets having the components shown in Table 1 as original sheets, anodic electrolytic oxidation treatment was performed in various aqueous solutions shown in Table 3. The conditions of the anode electrolytic oxidation treatment 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)
The cold-rolled steel sheet shown in Table 1 was used for evaluation as it was.

(Comparative Example 3)
The cold-rolled steel sheet shown in Table 1 was plated with 5 g / m ² of Ni in the plating bath shown in Table 2.

(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 contained in the oxide film was confirmed by the same method, and the concentrations thereof were quantified and shown in Table 3.

・ Test each steel material as it is or after pressing into a cylindrical drawn can corresponding to standard 18650 (diameter 18 mm x length 65 mm) of a cylindrical lithium ion secondary battery, cutting out the inner surface, and tape-sealing the edge and back surface A material was used. 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 reference) at 25 ° C., and the flowing current density was measured. The potential at which the current density becomes 10 μA / cm ² (lithium reference) was defined as the dissolution potential, and the dissolution potential was determined for each specimen.

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

(Examples 18 to 32)
Using the cold-rolled steel sheets having the components shown in Table 1 as original sheets, various plating amounts of Ni plating shown in Table 7 were performed under the plating conditions shown in Table 2. Thereafter, anodic electrolytic oxidation treatment was performed in various aqueous solutions shown in Table 7. 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 33)
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, an anodic electrolytic oxidation treatment was performed in an aqueous solution shown in Table 7. 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 34)
Using a cold-rolled steel plate having the components shown in Table 1 as an original plate, 1 g / m ² of Ni—P alloy plating was performed under the plating conditions shown in Table 4. Thereafter, an anodic electrolytic oxidation treatment was performed in an aqueous solution shown in Table 7. 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.

(Examples 35-39)
Using the cold-rolled steel sheet having the components shown in Table 1 as an original sheet, various plating amounts of Cr plating shown in Table 7 were performed under the plating conditions shown in Table 5. Thereafter, an anodic electrolytic oxidation treatment was performed in an aqueous solution shown in Table 7. 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 40)
A cold-rolled steel plate having the components shown in Table 1 was used as a base plate, and 0.3 g / m ² Fe—Cr alloy plating was performed under the plating conditions shown in Table 6. Thereafter, an anodic electrolytic oxidation treatment was performed in an aqueous solution shown in Table 7. 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)
The cold-rolled steel sheet of the components shown in Table 1 as original plate performs Cr plating of 0.05 g / m ² by plating conditions shown in Table 5, were Ni plating 1 g / m ² further plating conditions shown in Table 2 . Thereafter, a thermal diffusion treatment at 800 ° C. for 10 seconds was performed in a 5% hydrogen-nitrogen atmosphere to change the plating layer into a Ni—Cr—Fe diffusion layer. Thereafter, an anodic electrolytic oxidation treatment was performed in an aqueous solution shown in Table 7. 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 Examples 4 and 5)
Using the cold-rolled steel sheets having the components shown in Table 1 as original sheets, various plating amounts of Ni plating shown in Table 7 were performed under the plating conditions shown in Table 2.

(Comparative Example 6)
Using the cold-rolled steel sheet having the components shown in Table 1 as an original sheet, various plating amounts of Cr plating shown in Table 7 were performed under the plating conditions shown in Table 5.

(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 7 after identifying and quantifying elements other than Fe and plated metal (Ni or Cr).

Constant-potential electrolysis: After pressing into a cylindrical drawn can corresponding to the standard 18650 (diameter 18 mm x length 65 mm) of a cylindrical lithium ion secondary battery, the inner surface is cut out, and the edges and back are tape sealed and did. 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 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 “◯”.

  Table 7 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 (4)

  A steel material for a non-aqueous electrolyte secondary battery case, comprising an oxide film having a thickness of 10 to 500 nm on a steel material surface, wherein the oxide film contains either Mo or W or both.   2. The steel material for a nonaqueous electrolyte secondary battery case according to claim 1, further comprising a plating layer containing one or both of Ni and Cr between the oxide film and the steel material surface. When the plating layer contains Ni, the Ni adhesion amount is 0.1 to 10 g / m ² , and when the plating layer contains Cr, the Cr adhesion amount is 0.01 to 0.5 g / m ^2. The steel material for nonaqueous electrolyte secondary battery cases according to claim 2.   The steel material for a nonaqueous electrolyte secondary battery case according to any one of claims 1 to 3, wherein the oxide film is formed by anodic electrolytic treatment.