JP2015228289A - Initial charging method of lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an initial charging method of lithium ion secondary battery capable of suppressing reduction in the capacity retention rate.SOLUTION: An initial charging method of lithium ion secondary battery starts initial charging while retaining a lithium ion secondary battery 10 from the outside along the lamination direction of an electrode assembly, and performs low rate charging in a charging range from the start of charging before a change in the stage structure appearing at first. After low rate charging, the lithium ion secondary battery 10 thus retained is released, thereafter it is retained again from the outside, and recharged at a higher rate than that in the low rate charging.

Description

  The present invention relates to an initial charging method for a lithium ion secondary battery having an electrode assembly and an electrolytic solution.

  Lithium ion secondary batteries can be recharged and can be used repeatedly, so that they are widely used as power sources. Generally, a lithium ion secondary battery includes a case, and an electrode assembly and a non-aqueous electrolyte are accommodated in the case. The electrode assembly has a positive electrode, a negative electrode, and a separator that insulates the positive electrode from the negative electrode, and has a laminated structure in which a separator is interposed between the positive electrode and the negative electrode. In a lithium ion secondary battery, lithium ions in the positive electrode active material move to the negative electrode active material side through the non-aqueous electrolyte during charging and are inserted into the negative electrode active material, and conversely desorb from the negative electrode active material during discharge. The lithium ions thus moved to the positive electrode active material side and are occluded by the positive electrode active material.

  In the manufacture of a lithium ion secondary battery, after the electrode assembly is housed in the case, a non-aqueous electrolyte is injected into the case from a liquid injection hole provided in the case, and the liquid injection hole is sealed. And after performing the process of making a non-aqueous electrolyte osmose | permeate a positive electrode active material and a negative electrode active material, the conditioning process which repeats charging / discharging is performed.

  One purpose of performing this conditioning process is to form an SEI film on the surface of the negative electrode active material, and the purpose of forming the SEI film is to suppress further decomposition of the electrolyte components on the surface of the negative electrode active material. is there. This SEI film is formed by the decomposition of the additive contained in the electrolyte during initial charging, and the battery capacity is stabilized when the SEI film is uniformly formed on the surface of the negative electrode active material. .

  However, in the conditioning process, gas is also generated with the decomposition of the additive. The generated gas becomes bubbles and enters between the layers of the electrode assembly, and the bubbles inhibit the formation of the SEI film described above, and unevenness of the SEI film occurs on the surface of the negative electrode active material. Therefore, it is conceivable to release the gas generated during the initial charging out of the case (see, for example, Patent Document 1).

JP 2010-262867 A

  By the way, if the negative electrode potential reaches the decomposition potential of the additive after the start of the initial charging, gas is generated and enters the layers of the electrode assembly. In Patent Document 1, after the initial charging is completed, the generated gas is released out of the case, but the gas generated during the initial charging becomes a bubble during the initial charging and becomes a bubble of the electrode assembly. It goes into the interlayer. For this reason, in Patent Document 1, during the initial charge, the generated gas inhibits the formation of the SEI film, resulting in unevenness in the formation of the SEI film. Then, in the charging after conditioning, since the SEI film is newly formed at a place where the SEI film is small, lithium is consumed. As a result, the amount of movable lithium decreases, and the capacity retention rate of the battery decreases.

  The present invention has been made to solve the above-described problems, and an object thereof is to provide an initial charging method for a lithium ion secondary battery capable of suppressing a decrease in capacity retention rate.

  An initial charging method of a lithium ion secondary battery for solving the above-mentioned problems includes a positive electrode having a positive electrode active material capable of occluding and releasing lithium ions, and a silicon-based or carbon-based material capable of inserting / extracting lithium ions. An electrode assembly in which a negative electrode having a negative electrode active material is laminated in a state in which they are insulated from each other, and an initial charging method in a lithium ion secondary battery having an electrolyte solution, the lamination direction of the electrode assembly The initial charge is started in a state in which the lithium ion secondary battery is constrained from the outside along the line, and the low-speed charge is performed in a charge range from the start of charge to at least the timing at which the potential plateau occurs at the negative electrode potential. After releasing the restraint of the lithium ion secondary battery, after releasing the restraint, restraining the lithium ion secondary battery again, the low speed And gist to carry out recharging at a high rate than at conductive.

  According to this, at the time of initial charging, when the negative electrode potential reaches the decomposition potential of the additive contained in the electrolytic solution, decomposition of the additive is started, and the SEI film is formed on the surface of the negative electrode active material. Gas is generated, and the gas enters between adjacent layers in the stacking direction of the electrode assembly. Here, after completion of the low-speed charging during the initial charging, the restraint of the lithium ion secondary battery is released. Then, the interlayer of the electrode assembly spreads, the gas that has entered can flow between the layers, the gas is pushed up from the interlayer, and there is no gas remaining between the layers of the electrode assembly.

  Therefore, during the initial charging, the inhibition of the formation of the SEI film by the gas is suppressed, and the SEI film can be uniformly formed on the surface of the negative electrode active material. As a result, in charging after conditioning, lithium is suppressed from being consumed for forming the SEI film, and a decrease in the capacity maintenance rate of the lithium ion secondary battery can be suppressed.

Note that the slow charge is performed at least before the timing at which the potential plateau occurs at the negative electrode potential.
Further, regarding the initial charging method of the lithium ion secondary battery, in the lithium ion secondary battery using the negative electrode having the carbon-based negative electrode active material, a stage structure in which lithium ions are inserted into the negative electrode active material from the start of charging. Low speed charging is performed up to the potential corresponding to the change.

  According to this, in a lithium ion secondary battery using a negative electrode having a carbon-based negative electrode active material, in the initial (first) charge, the formation of the SEI film is the potential plateau that appears first from the start of charge, that is, the negative electrode active. It is considered to occur up to a potential corresponding to the stage structure change in which lithium ions are inserted into the substance, and slow charging is performed in this range. Then, by performing low-speed charging in the charging range where the SEI film is formed, the additive is gradually decomposed, and the SEI film can be formed in a stable state.

Regarding the initial charging method of the lithium ion secondary battery, the additive contains vinylene carbonate.
According to this, the SEI film can be uniformly formed by the additive.

  As for the initial charging method of the lithium ion secondary battery, in the lithium ion secondary battery using the negative electrode having the silicon-based negative electrode active material, from the start of charging until the potential plateau due to the generation of lithium silicate is observed. Slow charging is performed.

  According to this, the additive is decomposed by performing low-speed charging in a charging range in which the amount of change in the negative electrode potential is large, and the SEI film can be formed in a stable state.

  According to the present invention, it is possible to suppress a decrease in capacity maintenance rate.

The disassembled perspective view which shows the secondary battery of embodiment. The disassembled perspective view which shows the component of an electrode assembly. The perspective view which shows the state which restrained the secondary battery with the restraining jig | tool. The graph which shows the relationship between a negative electrode potential and SOC. (A) is a partial cross-sectional view showing an electrode assembly in a restrained state, (b) is a partial cross-sectional view showing an electrode assembly in a state where the restraint is released, and (c) shows the electrode assembly in a restrained state again. FIG.

Hereinafter, an embodiment embodying an initial charging method of a lithium ion secondary battery will be described with reference to FIGS.
As shown in FIG. 1, the lithium ion secondary battery 10 includes a case 11, and the case 11 contains an electrode assembly 14 and a non-aqueous electrolyte. The case 11 includes a bottomed square cylindrical case main body 12 and a flat lid 13 that closes an opening 12 a for inserting the electrode assembly 14 into the case main body 12.

  The case main body 12 is erected from a rectangular plate-like bottom wall 12b, a long side wall 12d erected from a pair of opposed long side edges of the bottom wall 12b, and a pair of erected short side edges of the bottom wall 12b. And a short side wall 12c. The front side view shape of the short side wall 12c is a rectangular shape whose side connected to the bottom wall 12b is a short side, and the front side view shape of the long side wall 12d is a rectangular shape whose side connected to the bottom wall 12b is a long side. The inner surface of the case body 12 is covered with an insulating layer 15a. Both the case main body 12 and the lid body 13 are made of metal (for example, made of stainless steel or aluminum). The lid 13 has a liquid injection hole 13a for injecting an electrolytic solution into the case 11 (case main body 13). The liquid injection hole 13 a is closed by a sealing plug 19.

As shown in FIG. 2, the electrode assembly 14 includes a negative electrode 21, a positive electrode 24, and a separator 27 that insulates the positive electrode 24 from the negative electrode 21.
The negative electrode 21 includes a negative electrode metal foil 22 (copper foil) and a negative electrode active material layer 23 configured by applying a negative electrode active material to both surfaces of the negative electrode metal foil 22. The negative electrode 21 has a negative electrode uncoated portion 22 d made of a negative metal foil 22 along one side 21 a. A negative electrode tab 29 protrudes from a part of one side 21 a of the negative electrode 21.

  The negative electrode active material layer 23 is formed from an active material mixture including a negative electrode active material, a conductive agent, a binder, and a solvent. As the negative electrode active material, a material capable of inserting / extracting lithium ions is used, and a carbon-based material is used. Specific examples include amorphous carbon and graphite. A well-known thing is used as a electrically conductive agent, a binder, and a solvent.

  The positive electrode 24 includes a rectangular positive metal foil 25 (aluminum foil) and a positive electrode active material layer 26 configured by applying a positive electrode active material to both surfaces of the positive electrode metal foil 25. The positive electrode 24 has a positive electrode uncoated portion 25d made of a positive metal foil 25 along one side 24a. A positive electrode tab 28 protrudes from a part of one side 24 a of the positive electrode 24. The positive electrode active material layer 26 of the positive electrode 24 is formed from an active material mixture containing a positive electrode active material, a conductive agent, a binder, and a solvent.

  As the positive electrode active material, a material capable of inserting and extracting lithium ions is used, and a lithium-containing oxide or the like is preferably used. Specific examples include compounds used for the positive electrode active material of lithium ion secondary batteries, such as lithium manganese oxide, lithium nickel oxide, lithium cobalt oxide, and lithium iron oxide.

  As the solvent for the non-aqueous electrolyte, one or more selected from various organic solvents used in lithium ion secondary batteries are used. Examples of the organic solvent include ethylene carbonate (EC), propylene carbonate (PC), Examples include vinylene carbonate (VC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). As the electrolyte of the non-aqueous electrolyte, various lithium salts used in conventional lithium ion secondary batteries are used. As the nonaqueous electrolytic solution, a solution in which an electrolyte is dissolved in a selected organic solvent is used. In this embodiment, at least vinylene carbonate is included as an additive.

  The electrode assembly 14 has a rectangular parallelepiped shape in which a plurality of positive electrodes 24 and a plurality of negative electrodes 21 are alternately stacked, and a separator 27 is interposed between the electrodes 21 and 24. The direction in which the negative electrode 21, the positive electrode 24, and the separator 27 are stacked is defined as the stacking direction of the electrode assembly 14.

  As shown in FIG. 1, the electrode assembly 14 has flat planes 14a having a rectangular shape in plan view at both ends in the stacking direction, and both the flat planes 14a are opposed to the long side wall 12d through the insulating layer 15a. In the electrode assembly 14, the positive electrode tabs 28 are arranged in a row along the stacking direction, and the negative electrode tabs 29 are arranged in a row along the stacking direction at positions that do not overlap the positive electrode tab 28. The electrode 24 and the negative electrode 21 are laminated. The positive electrode tab 28 and the negative electrode tab 29 are bent in a state where they are collected within a range from one end to the other end in the stacking direction of the electrode assembly 14.

  The positive electrode terminal 16 is electrically connected to the positive electrode tab 28, and the negative electrode terminal 15 is electrically connected to the negative electrode tab 29. A part of each of the positive terminal 16 and the negative terminal 15 is exposed outside the case 11 through the hole 13 c of the lid 13. The positive electrode terminal 16 and the negative electrode terminal 15 are each attached with a ring-shaped insulating ring 17 a for insulation from the case 11.

Next, a method for manufacturing the lithium ion secondary battery 10 will be described.
First, the electrode assembly 14 is accommodated in the case body 12. Next, the lid 13 is provided so as to close the opening 12 a of the case body 12. At this time, the negative electrode terminal 15 and the positive electrode terminal 16 are inserted through the insulating ring 17a. The case 11 is formed by welding the lid 13 to the case main body 12.

  Next, a non-aqueous electrolyte is injected into the case 11 from the liquid injection hole 13a. Then, after injecting the non-aqueous electrolyte, the liquid injection hole 13a is temporarily sealed with a sealing plug. Next, a conditioning process for repeatedly charging and discharging the electrode assembly 14 is performed, and then the lithium ion secondary battery 10 is left in a predetermined temperature environment to perform an aging process. After the aging process is finished, the sealing plug used for the temporary sealing is removed from the liquid injection hole 13a, and the gas in the case 11 is released to the outside of the case 11. After the gas discharge is completed, the injection hole 13a is fully sealed by the sealing plug 19 to complete the lithium ion secondary battery 10.

Next, the conditioning process will be described in detail.
As shown in FIG. 3, in the conditioning process, first, the case 11 of the lithium ion secondary battery 10 is restrained by the restraining jig 30. The restraining jig 30 includes a pair of restraining plates 31, bolts 32 that connect the restraining plates 31, and nuts 33 that are screwed into the bolts 32. The restraining jig 30 restrains the lithium ion secondary battery 10 by pressing the two long side walls 12d of the case 11 from the outer surface side along the width direction of the case 11, that is, the stacking direction of the electrode assembly 14. It is.

  Then, when the lithium ion secondary battery 10 is restrained by the restraining jig 30 from the width direction of the case 11, as shown in FIG. 5A, the interlayer of the electrode assembly 14 (the negative electrode 21 and the separator 27, And the gap S2) between the positive electrode 24 and the separator 27 become narrower than before the restraint.

  Next, initial (initial) charging is performed in a state where the lithium ion secondary battery 10 is restrained by the restraining jig 30. At the time of charging in the first cycle, a period from the start of charging to before the stage structure change that appears first is defined as a low-speed charging period. In the low speed charging period, the charging current is set to 0.025 to 0.1 C, and charging is performed at a relatively low speed (low speed charging is performed).

  As shown by the curve in FIG. 4, the negative electrode potential from the start of the slow charge to before the “first stage structure change” has a large change rate of the negative electrode potential. On the other hand, the potential at which “the stage structure change in which lithium ions are inserted into the negative electrode active material” occurs has a small negative electrode potential change rate in the curve of FIG. ) Is the observed potential. In this stage, since lithium ions are inserted at a constant negative electrode potential, the negative electrode potential does not change and becomes flat (the negative electrode potential change rate is zero). In the present embodiment, the potential at the “stage where lithium ions are inserted into the negative electrode active material” is about 0.2 V, and the slow charge period is from the start of charging until the negative electrode potential reaches around 0.2 V. ing.

  The low-speed charging period is also a period in which the SOC that represents the amount of electricity that is currently charged with respect to the electric capacity (full charge) of the electrode assembly 14 is 0% to 15%. That is, the slow charging period is from the start of SOC 0% charging to SOC 15%. The timing at which the low-speed charging is terminated, that is, the timing at which the SOC reaches 15%, and the timing at which the potential plateau is reached is that the decomposition rate of vinylene carbonate (VC), which is an additive (organic solvent) of the non-aqueous electrolyte, It is also the time to relax.

  In the initial charge, when the slow charge is started, the negative electrode potential reaches the decomposition potential of vinylene carbonate (0.8 V in this embodiment). Then, the decomposition of vinylene carbonate is started, the formation of the SEI film on the surface of the negative electrode active material is started, and the generation of gas is started. Then, vinylene carbonate is rapidly decomposed until it reaches the decomposition potential and immediately before the completion of the low-speed charging, and at the timing when the low-speed charging is completed (near the negative electrode potential of 0.2 V), the decomposition rate becomes slow.

  As shown in FIG. 5A, during the low-speed charging period, the gas generated by the decomposition of vinylene carbonate becomes bubbles 40, the gap S1 (interlayer) between the negative electrode 21 and the separator 27, and the positive electrode 24. And the separator 27 enter the gap S2 (interlayer). Thereafter, the charging is temporarily stopped, and the restraint of the lithium ion secondary battery 10 by the restraining jig 30 is released.

  Then, as shown in FIG. 5 (b), the gap S1 between the negative electrode 21 and the separator 27 and the gap S2 between the positive electrode 24 and the separator 27 spread before the restraint by the restraining jig 30, and the bubble 40 is The gaps S1 and S2 can flow, and the bubbles 40 are pushed upward along the gaps S1 and S2. After that, by restraining the lithium ion secondary battery 10 again by the restraining jig 30, the bubbles 40 remaining between the layers of the electrode assembly 14 are eliminated as shown in FIG.

  Next, in a state in which the lithium ion secondary battery 10 is restrained by the restraining jig 30, charging is resumed at a charging current at a higher rate than at the time of low-speed charging, and late charging (recharging) is performed. In the present embodiment, late charging is performed with a charging current of 0.8C. Then, as shown in FIG. 4, since the negative electrode potential has already reached the decomposition potential of vinylene carbonate, vinylene carbonate is decomposed and an SEI film is formed on the surface of the negative electrode active material. At this time, vinylene carbonate is gradually decomposed and stably formed on the surface of the negative electrode active material, and since the bubbles 40 do not exist, the SEI film is uniformly formed on the surface of the negative electrode active material. Although gas is generated by the decomposition of vinylene carbonate, the amount of generation is smaller than that during low-speed charging, and the SEI film is already formed evenly, so that the influence of the gas on the formation of the SEI film is small.

  Then, when the voltage of the lithium ion secondary battery 10 reaches a predetermined voltage, charging in the first cycle is completed. Thereafter, discharging is performed, and thereafter, the charging and discharging are repeated and the conditioning process is completed.

According to the above embodiment, the following effects can be obtained.
(1) In the conditioning process, after the low-speed charging, the restraint of the lithium ion secondary battery 10 by the restraining jig 30 is once released so that the bubbles 40 existing between the layers of the electrode assembly 14 can be moved to restrain the restraint. The lithium ion secondary battery 10 was restrained again with the tool 30. For this reason, even if the bubble 40 is generated during the initial charging, the bubble 40 can be eliminated from the interlayer of the electrode assembly 14 by releasing the restraint by the restraining jig 30 and re-restraining in the middle. Therefore, the SEI film can be uniformly formed on the surface of the negative electrode active material by late charging after the bubbles 40 are eliminated. As a result, in charging after conditioning, it is possible to suppress the consumption of lithium for the formation of the SEI film, and it is possible to suppress a decrease in the capacity maintenance rate of the lithium ion secondary battery 10.

  (2) The timing at which the restraint of the lithium ion secondary battery 10 by the restraining jig 30 is released is the timing at which the “change in the stage structure that appears first from the start of charging” is reached. At the potential at which the stage structure change occurs, the amount of change in the negative electrode potential is small, vinylene carbonate is slowly decomposed, and the SEI film can be formed in a stable state, suppressing unevenness in the SEI film. it can. Therefore, the SEI film can be uniformly formed on the surface of the negative electrode active material by extruding the bubble 40 from the interlayer of the electrode assembly 14 by releasing the restraint and re-restraining, and setting the restraint releasing timing to a predetermined timing. .

  (3) Further, in the “stage structure change first appearing from the start of charging”, the amount of change in the negative electrode potential is small, and the decomposition of vinylene carbonate is almost suppressed. For this reason, the gas generation accompanying decomposition | disassembly of vinylene carbonate decreases. Then, at the timing when the change in the stage structure is reached, the restraint release and re-restraint of the lithium ion secondary battery are performed. For this reason, the gas is extracted from the electrode assembly 14 in a state where the gas is substantially generated during the low-speed charging period, and the SEI film can be formed while suppressing the amount of gas generated during the later charging. it can.

  (4) After the lithium ion secondary battery 10 was restrained again by the restraining jig 30, it was charged at a higher rate than at the time of low speed charging. At the time of late charging, the SEI film is formed evenly and the capacity maintenance rate is difficult to decrease, so it can be charged at a high rate, and compared with the case of charging late charge at the same rate as low-speed charging. The time required for charging can be shortened.

In addition, you may change the said embodiment as follows.
○ A silicon-based material may be used as the negative electrode active material. As the silicon-based negative electrode active material, silicon oxide is used, and it is preferable to use lower silicon oxide (SiO _x ) satisfying x of 0.4 or more and 1.2 or less. The negative electrode active material includes a lower silicon oxide powder, an organic solvent, a conductive agent, and a binder.

  In a lithium ion secondary battery using a negative electrode using a silicon-based negative electrode active material, a potential plateau is observed when lithium silicate is uniformly generated during initial (initial) charging. Therefore, in the initial charging of a lithium ion secondary battery using a negative electrode using a silicon-based negative electrode active material, slow charging is performed from the start of charging until a potential plateau due to the generation of lithium silicate is observed.

Therefore, an active material in which a potential plateau due to the formation of lithium silicate is observed is used as the silicon-based negative electrode active material, such as amorphous silicon.
In late charging, charging may be performed at a rate other than 0.8C as long as it is higher than low-speed charging.

O The restraint may be released at the timing when the potential plateau at the negative electrode potential occurs from the start of charging.
○ The SOC during low-speed charging may be changed as appropriate.

The additive for forming the SEI film may be other than vinylene carbonate.
The electrode assembly 14 is a laminated type, but may be a wound type in which a strip-shaped separator is sandwiched between a strip-shaped negative electrode and a positive electrode, and these are wound in layers.

The number of the negative electrode 21 and the positive electrode 24 which comprise the electrode assembly 14 may be changed suitably.
In the embodiment, the negative electrode 21 has the negative electrode active material layer 23 on both sides of the negative electrode metal foil 22, but may have the negative electrode active material layer 23 only on one side of the negative electrode metal foil 22. Similarly, the positive electrode 24 has the positive electrode active material layer 26 on both sides of the positive electrode metal foil 25, but may have the positive electrode active material layer 26 only on one side of the positive electrode metal foil 25.

Next, the technical idea that can be grasped from the above embodiment and other examples will be described below.
(A) The electrolytic solution is a non-aqueous electrolytic solution.

  DESCRIPTION OF SYMBOLS 10 ... Lithium ion secondary battery, 14 ... Electrode assembly, 21 ... Negative electrode, 24 ... Positive electrode

Claims (4)

A positive electrode having a positive electrode active material capable of inserting / extracting lithium ions and a negative electrode having a silicon-based or carbon-based negative electrode active material capable of inserting / extracting lithium ions are laminated in an insulated state. An initial charging method in a lithium ion secondary battery having an electrode assembly and an electrolyte solution,
The initial charging is started in a state where the lithium ion secondary battery is restrained from the outside along the stacking direction of the electrode assembly, and the charging speed is low in the charging range from the start of charging to at least the timing at which the potential plateau occurs at the negative electrode potential. Charge
After the low-speed charging, release the restraint of the lithium ion secondary battery,
After the restraint is released, the lithium ion secondary battery is restrained again and recharged at a higher rate than in the low speed charge.   In the lithium ion secondary battery using the negative electrode having the carbon-based negative electrode active material, low-speed charging is performed from the start of charging to a potential corresponding to a stage structure change in which lithium ions are inserted into the negative electrode active material. Item 2. An initial charging method of a lithium ion secondary battery according to Item 1.   The initial charging method for a lithium ion secondary battery according to claim 2, wherein the electrolyte contains vinylene carbonate.   2. The lithium ion according to claim 1, wherein in the lithium ion secondary battery using the negative electrode having the silicon-based negative electrode active material, low-speed charging is performed from the start of charging until a potential plateau due to generation of lithium silicate is observed. An initial charging method for a secondary battery.