JP7031097B2 - Charging / discharging method of lithium secondary battery - Google Patents

Description

The present invention relates to a charging / discharging method for a lithium secondary battery.

The lithium secondary battery is a high-performance secondary battery having the highest energy density among the secondary batteries currently in practical use, and is a power source for portable electronic devices such as mobile phones and notebook personal computers, electric vehicles, and the like. It is installed as. Lithium secondary batteries are required to have high capacity, high performance, high safety, and long life as their power sources.

In such a lithium secondary battery, lithium ions are transferred between the positive electrode and the negative electrode to charge and discharge. The positive electrode and the negative electrode include a current collector that supports the positive electrode active material and the negative electrode active material, respectively. Lithium as a positive electrode active material for a lithium secondary battery is currently lithium such as lithium cobalt oxide (LiCoO ₂ ), lithium manganate (LiMn ₂ O ₄ ), lithium nickel oxide (LiNiO ₂ ), and lithium iron phosphate (LiFePO ₄ ). Metal oxides or metal phosphorus oxides containing lithium oxide have been put into practical use or are being developed with the aim of commercialization. As the negative electrode active material, a carbon material such as graphite or lithium titanium oxide (Li ₄ Ti ₅ O ₁₂ ) is used. Aluminum foil is generally used for the positive electrode current collector, and copper foil is generally used for the negative electrode current collector.

In the lithium secondary battery, a separator is further interposed between the positive electrode and the negative electrode. As the separator, a microporous thin film made of polyolefin is generally used. The electrode plate group including the positive electrode, the negative electrode and the separator is housed in the battery container together with the non-aqueous electrolyte. As the non-aqueous electrolyte, a non-aqueous electrolyte solution in which an electrolyte such as a lithium salt is dissolved in a non-aqueous solvent is generally used. As other non-aqueous electrolytes, gel-like electrolytes or solid electrolytes are also attracting attention.

By the way, metallic lithium, which is a negative electrode active material, has a large characteristic that the amount of electricity per unit weight is 3.86 Ah / g. Therefore, in order to realize a high-capacity lithium secondary battery having a high energy density, research on using metallic lithium as a negative electrode active material is being promoted again.

However, in a lithium secondary battery that uses metallic lithium as the negative electrode active material, lithium grows in a dendrite shape from the metallic lithium surface of the negative electrode during repeated charging and discharging, and dendrite-like lithium intervenes between the positive electrode and the negative electrode. There was a problem that it penetrated and reached the positive electrode, causing an internal short circuit.

For this reason, for example, in Patent Document 1, in addition to the lithium-containing compound which is the main active material as the positive electrode active material, lithium which is a side active material and can be discharged from the first time (for example, manganese dioxide) is used. Secondary batteries are listed. Such a lithium secondary battery of Patent Document 1 can be discharged from the first time after assembly. That is, metallic lithium can be released as lithium ions from the negative electrode at the time of initial discharge. Therefore, the inert film such as lithium carbonate or lithium hydroxide formed on the metallic lithium surface of the negative electrode immediately after battery assembly is removed. As a result, lithium ions are reduced and precipitated on the metallic lithium surface of the negative electrode, which has a good surface condition during charging after the initial discharge, so that it is possible to suppress the growth of lithium from the metallic lithium surface of the negative electrode in a dendrite manner. Become.

Further, in Patent Document 2, a material that can be discharged from the first time, which is a by-active material as a positive electrode active material, Li ₄ Mn _5-x M1 _x O ₁₂ (where M1 is Co, Ni, Fe, Cu, Mg). , Zn, Al, Cr and at least one element selected from the group consisting of Ga, x is 0 ≦ x <5), for example lithium secondary batteries using Li ₄ Mn ₅ O ₁₂ are described. ing. Like the lithium secondary battery of Patent Document 1, the lithium secondary battery of Patent Document 2 can be discharged from the first time after assembly. Further, Non-Patent Document 1 describes that the amount of electricity per unit weight of the above Li ₄ Mn ₅ O ₁₂ is about 250 mAh / g.

Japanese Unexamined Patent Publication No. 2017-16905 Japanese Unexamined Patent Publication No. 2017-68966

Masaki Okada, Yuya Sakaguchi "Consideration on High Capacity LiMn Oxide Positive Electrode Materials for Lithium Ion Secondary Batteries" Tosoh Research and Technology Report 2016 Vol. 60 p. 21-28

However, manganese dioxide, which is a material that can be discharged from the first time as a by-active substance disclosed in Patent Document 1, does not participate in the charge / discharge after the first discharge. Therefore, the positive electrode active material containing manganese dioxide as a side active material has a problem that the ratio of the main active material is relatively reduced and the discharge capacity of the lithium secondary battery is lowered.

Further, the material Li ₄ Mn ₅ O ₁₂ , which is disclosed in Patent Document 2 and can be discharged from the first time as a by-active substance, has an electric amount of about 250 mAh / g per unit weight as described above, and the value thereof. Is a relatively low value. A lithium secondary battery including a positive electrode containing Li ₄ Mn ₅ O ₁₂ having such an amount of electricity per unit weight is used to remove the inert film formed on the metallic lithium surface of the negative electrode at the time of initial discharge. In order to secure the required discharge capacity, the proportion of the Li ₄ Mn ₅ O ₁₂ in the positive electrode active material must be increased. Therefore, the positive electrode active material containing Li ₄ Mn ₅ O ₁₂ as the side active material also has a problem that the ratio of the main active material is relatively reduced and the discharge capacity of the lithium secondary battery is lowered.

Therefore, the present invention solves the above-mentioned problems, suppresses or prevents the dendrite-like growth of lithium during charging / discharging, and further improves the charge / discharge cycle characteristics and increases the capacity by increasing the output of the lithium secondary battery. It provides a charging / discharging method.

In order to solve the above problems, according to the present invention, there is a charging / discharging method of a lithium secondary battery including a positive electrode and a negative electrode containing metallic lithium, wherein the positive electrode is a current collector and the current collector. The positive electrode layer includes a positive electrode layer formed on one or both surfaces, and the positive electrode layer contains graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less, and a positive electrode active material, and the lithium secondary battery. The lithium secondary battery is characterized in that the charge / discharge after assembly is started from the discharge, and the discharge cutoff voltage at the time of the first discharge is set to 1.0 V or more and 2.0 V or less based on the oxidation-reduction potential of lithium. A charging / discharging method is provided.

INDUSTRIAL APPLICABILITY According to the present invention, there is provided a charging / discharging method for a lithium secondary battery, which can suppress or prevent the dendrite-like growth of lithium during charging / discharging, and further improve the charging / discharging cycle characteristics and increase the capacity by increasing the output. can.

FIG. 1 is a cross-sectional view showing an example of a coin-type lithium secondary battery according to an embodiment. FIG. 2 is a scanning electron micrograph of a part of the positive electrode layer of the positive electrode A. FIG. 3 is a scanning electron micrograph of a part of the positive electrode layer of the positive electrode E. FIG. 4 is a graph showing a constant current discharge curve in which each evaluation cell having positive electrodes A to D is discharged to 1.5 V.

Hereinafter, the charging / discharging method of the lithium secondary battery according to the embodiment will be described in detail.

The method for charging and discharging a lithium secondary battery according to an embodiment includes a positive electrode and a negative electrode containing metallic lithium, and the positive electrode includes a current collector and a positive electrode layer formed on one or both surfaces of the current collector. The positive electrode layer contains graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less, and a positive electrode active material. The discharge cutoff voltage is set to 1.0 V or more and 2.0 V or less with respect to the oxidation-reduction potential of lithium. According to the charging / discharging method of the lithium secondary battery according to such an embodiment, it is possible to suppress or prevent the dendrite-like growth of lithium during charging / discharging, and further improve the charging / discharging cycle characteristics and increase the capacity by increasing the output. Can be achieved.

That is, graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less has a discharge capacity of more than 1000 mAh / g. Therefore, a lithium secondary battery having a positive electrode containing the graphene can start charging / discharging after assembling the battery from a discharge, and at the time of the first discharge, a sufficient amount of lithium is used as ions from the metallic lithium surface of the negative electrode. Has the required discharge capacity to discharge. Further, since the discharge cutoff voltage at the time of initial discharge is set to 1.0 V or more and 2.0 V or less based on the redox potential of lithium, the amount of lithium ions released from the metallic lithium surface of the negative electrode is increased. can. Since the amount of lithium ions released from the metallic lithium surface of the negative electrode can be increased in this way, an inert film such as lithium carbonate or lithium hydroxide on the metallic lithium surface of the negative electrode can be destroyed and removed. As a result, in charging after the initial discharge, when lithium ions released from the positive electrode active material reduce and precipitate lithium on the metallic lithium surface of the negative electrode, the inert film is removed on the metallic lithium surface, so that lithium is a metal. It does not precipitate unevenly on the lithium surface, but uniformly precipitates on the surface of metallic lithium. Therefore, it suppresses or prevents the growth of lithium from the surface of metallic lithium on the negative electrode in a dendrite-like manner with the repetition of the charge / discharge cycle, and prevents the internal short circuit between the negative electrode and the positive electrode due to the dendrite-like growth of lithium. can.
In the initial discharge, lithium (Li) is released as ions from the metallic lithium surface of the negative electrode to the non-aqueous electrolyte and moves to the positive electrode side through the separator. The transferred lithium ions reduce oxygen in graphene contained in the positive electrode layer (for example, a functional group containing an oxygen atom bonded to graphene). When graphene contained in the positive electrode layer is reduced, it becomes highly electrically conductive as compared with that before reduction, and therefore functions as a conductive agent after the initial discharge. Further, graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less binds to oxygen (for example, graphene) in the graphene in a polar solvent such as N-methyl-2-pyrrolidone (NMP). The graphenes (functional groups containing oxygen atoms) are negatively charged, so that the graphenes repel each other and disperse. That is, the graphene is less likely to aggregate in a polar solvent and exhibits high dispersibility. Graphene is prepared by adding it to a solvent containing graphene together with a positive electrode active material, applying the positive electrode slurry to one or both surfaces of the positive electrode current collector, and drying the graphene. Can form a well-dispersed positive electrode layer in the current collector. As a result, graphene, which functions as a conductive agent reduced during the initial discharge, does not localize to a part of the positive electrode layer, but is between the current collector and the positive electrode active material of the entire positive electrode layer, and between the positive electrode active material. Can be intervened in. Further, since the graphene has a layered structure in which carbon atoms bonded by SP2 bond are arranged in a plane, it can be easily bonded to other adjacent graphene and a conductive auxiliary agent further blended in the positive electrode layer. Therefore, the graphene can form a conductive network that electrically connects the current collector and the positive electrode active material and the positive electrode active materials, respectively. Therefore, in the positive electrode, the diffusion of ions and the transfer of electric charges by electron conduction can be improved by the conductive network, so that the output of the lithium secondary battery can be increased.

Metallic lithium has a large amount of electricity per unit weight of 3.86 Ah / g. Therefore, the capacity of a lithium secondary battery including a negative electrode containing metallic lithium as a negative electrode active material can be increased.

According to the charging / discharging method of the lithium secondary battery according to the above-described embodiment, it is possible to suppress or prevent the dendrite-like growth of lithium during charging / discharging, and further improve the charging / discharging cycle characteristics and increase the capacity by increasing the output. Can be achieved.

Next, the configuration and charge / discharge conditions of the lithium secondary battery used in the charge / discharge method according to the embodiment will be described.

<Positive electrode>
The positive electrode includes a positive electrode current collector and a positive electrode layer formed on one or both surfaces of the positive electrode current collector. The positive electrode current collector is not particularly limited, and known or commercially available current collectors can be used. The positive electrode current collector is, for example, a metal foil such as aluminum, a lath-processed or etched metal foil, or the like.

The positive electrode layer contains graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less, and a positive electrode active material. The content ratio of oxygen contained in such graphene is measured by using X-ray photoelectron spectroscopy (XPS). A graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less is a graphene having a functional group containing an oxygen atom such as an epoxy group, a carbonyl group, a carboxyl group, or a hydroxyl group.

As described above, graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less exhibits high dispersibility in a polar solvent, so that it is well dispersed in the positive electrode layer and is reduced at the time of initial discharge to be a conductive agent. And can form a conductive network in the positive electrode layer.
Graphene having an oxygen content of less than 30 atomic% is negatively charged in the polar solvent, but is not sufficiently charged so that the graphenes repel each other and disperse. Therefore, graphene aggregates on the positive electrode layer, and even if graphene functions as a conductive agent after the initial discharge, it becomes difficult to form the above-mentioned conductive network. On the other hand, graphene having an oxygen content of more than 50 atomic% is difficult to function as a conductive agent because all of the contained oxygen remains without being reduced at the time of initial discharge.

In some embodiments, graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less is preferably contained in a proportion of 1.0% by mass or more and 10% by mass or less with respect to the total amount of the positive electrode layer. The more preferable content ratio of the graphene is 1.0% by mass or more and 5.0% by mass or less with respect to the total amount of the positive electrode layer, and the more preferable content ratio of the graphene is 2.0% by mass with respect to the total amount of the positive electrode layer. It is 5.0% by mass or less.
A lithium secondary battery including a positive electrode layer containing graphene in an amount of less than 1.0% by mass based on the total amount of the positive electrode layer makes it difficult to start charging / discharging after assembling the battery from discharging. On the other hand, a lithium secondary battery provided with a positive electrode layer containing graphene in an amount of more than 10% by mass with respect to the total amount of the positive electrode layer has a large proportion of graphene in the positive electrode layer and a relatively small proportion of the positive electrode active material. Therefore, the capacity may decrease.

Such graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less can be produced by an oxidation method such as the Hummers method using graphite powder as a starting material described below.
First, potassium permanganate, a concentrated sulfuric acid solution, a hydrogen peroxide solution and the like are added to the graphite powder, and the graphite powder is oxidized to prepare a dispersion solution containing graphite oxide. Graphite oxide is a graphite having a functional group such as an epoxy group, a carbonyl group, a carboxyl group, or a hydroxyl group.
Next, ultrasonic waves are applied to the dispersion solution containing graphite oxide, and the graphite oxide in the dispersion solution is cleaved between the carbon atom layers to prepare a dispersion solution containing graphene oxide. Since graphite oxide has a larger carbon atom layer than graphite, the carbon atom layer can be easily cleaved by applying ultrasonic waves. Subsequently, the solvent is removed from the dispersion solution containing graphene oxide and dried to produce graphene oxide.

As the positive electrode active material, for example, a lithium-containing compound capable of occluding and releasing lithium can be used. The lithium-containing compound is not particularly limited as long as it is a compound generally used as a positive electrode active material for a lithium secondary battery, such as a lithium-containing metal oxide or a lithium-containing metal phosphorus compound.
Examples of the lithium-containing compound include lithium cobaltate (LiCoO ₂ ), lithium manganate (LiMnO ₂ , LiMn ₂ O ₄ ), lithium nickelate (LiNiO ₂ ), and lithium cobalt ironate (LiCo _0.5 Fe _0.5 O). ₂ ), Lithium nickel cobalt manganate (Li (Ni _x Coy Mn _{1-xy )} O ₂ (0 <x <1, 0 <y <1)), Lithium manganese iron phosphate (LiMn _x Fe _1- ) _{xPO 4} ) (0 <x <1), lithium iron phosphate (LiFePO ₄ ), lithium cobalt phosphate (LiCoPO ₄ ), lithium manganese phosphate (LiMnPO ₄ ), and the like.

The positive electrode layer may further contain a conductive auxiliary agent and a binder.

The conductive auxiliary agent is not particularly limited, and known or commercially available ones can be used. Examples of the conductive auxiliary agent include carbon black such as acetylene black and Ketjen black, carbon nanotubes, carbon fibers, activated carbon, and graphite.
If the above-mentioned graphene is blended in the positive electrode layer at an appropriate ratio, the blending of the conductive agent can be omitted, and the amount of the positive electrode active material in the positive electrode layer can be increased.

The binder is not particularly limited, and known or commercially available binders can be used. The binder is, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), ethylene-propylene co-weight. Combined, styrene-butadiene rubber (SBR), acrylic resin, etc.

The positive electrode can be produced, for example, by the method shown below. First, the above-mentioned graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less, a positive electrode active material, a conductive auxiliary agent and a binder are dispersed in a solvent to prepare a positive electrode slurry. Subsequently, after applying the positive electrode slurry to one or both surfaces of the positive electrode current collector, the positive electrode is produced by, for example, drying under vacuum at 80 ° C. to form a positive electrode layer.

The solvent is not particularly limited, and a solvent generally used in a lithium secondary battery can be used. The solvent is, for example, N-methyl-2-pyrrolidone (NMP), N, N-dimethylformamide (DMF), N, N-dimethylacetamide (DMA) and the like. When polyvinylidene fluoride (PVDF) is used as the binder, it is preferable to use N-methyl-2-pyrrolidone (NMP) as the solvent.

<Negative electrode>
The negative electrode includes, for example, a negative electrode current collector and metallic lithium which is a negative electrode active material formed on one or both surfaces of the negative electrode current collector.

The negative electrode current collector is not particularly limited, and known or commercially available current collectors can be used. The negative electrode current collector is, for example, a rolled foil made of copper or a copper alloy, an electrolytic foil, or the like.

<Non-water electrolyte>
Non-aqueous electrolytes, when in liquid form, include non-aqueous solvents and electrolytes.

The non-aqueous solvent contains a cyclic carbonate and a chain carbonate as main components. The cyclic carbonate is preferably at least one selected from ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). The chain carbonate is preferably at least one selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and the like.

The electrolyte is not particularly limited, and a lithium salt electrolyte generally used in a lithium secondary battery can be used. The electrolytes are, for example, LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (Cm F _{2m + 1} SO ₂ ) (C _n F _{2n + 1} SO ₂ ) (m, _n are integers of 1 or more), LiC (C). _p F _{2p + 1} SO ₂ ) (C _q F _{2q + 1} SO ₂ ) (Cr F _{2r + 1} SO ₂ ) (p, q, _r are integers of 1 or more), difluoro (oxalate) lithium borate, and the like. These electrolytes may be used alone or in combination of two or more. Further, it is desirable that this electrolyte is dissolved in a non-aqueous solvent at a concentration of 0.1 to 1.5 mol / L, preferably 0.5 to 1.5 mol / L.

<Separator>
As the separator, a microporous membrane of a polyolefin resin such as polyethylene resin or polypropylene resin or a non-woven fabric can be used. The microporous membrane or non-woven fabric may have a single layer or a multi-layer structure. In particular, the separator is preferably a microporous polypropylene film.

<Charging / discharging conditions>
The charge / discharge after assembly of the lithium secondary battery is started from the discharge, and the discharge cutoff voltage at the time of the initial discharge is set to 1.0 V or more and 2.0 V or less based on the redox potential of lithium.
When the discharge cutoff voltage at the time of the first discharge exceeds 2.0 V with respect to the redox potential of lithium, the discharge capacity required for removing the inert film on the metallic lithium surface of the negative electrode is obtained at the time of the first discharge. Can't.
On the other hand, when the discharge cutoff voltage at the time of the first discharge is set to less than 1.0 V based on the oxidation-reduction potential of lithium, for example, Fe ²⁺ or Mn ²⁺ in the positive electrode active material is discharged from the negative electrode at the time of the first discharge. It is reduced by the lithium ions, and there is a risk that the amount of available positive electrode active material contained in the positive electrode will decrease in the charge and discharge after the initial discharge.

The shape of the lithium secondary battery according to the embodiment is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a square type, and a flat type.

Hereinafter, the structure of the lithium secondary battery according to the embodiment will be described with reference to the drawings, taking a coin-type lithium secondary battery as an example. FIG. 1 is a cross-sectional view showing an example of a coin-type lithium secondary battery.

The coin-type lithium secondary battery 1 includes a positive electrode 2, a negative electrode 3, and a separator 4 arranged between the positive electrode 2 and the negative electrode 3. The positive electrode 2, the negative electrode 3, and the separator 4 are housed between the first external terminal 5 located on the lower side and the second external terminal 6 located on the upper side. The contact portion between the first external terminal 5 and the second external terminal 6 is insulated by a gasket 7.
The positive electrode 2 is composed of a positive electrode current collector 21 located on the inner surface of the first external terminal 5 and connected to the positive electrode current collector 21, and a positive electrode layer 22 provided on a surface of the positive electrode current collector 21 facing the separator 4. ing. The negative electrode 3 is a negative electrode layer made of metallic lithium provided on a surface facing the separator 4 of the negative electrode current collector 31 and a negative electrode current collector 31 located on the inner surface of the second external terminal 6 and connected to the negative electrode current collector 31. It is composed of 32 and. The separator 4 is impregnated with, for example, a non-aqueous electrolyte. The second external terminal 6 is inserted into the first external terminal 5 with its lower end and both side surfaces wrapped in the gasket 7, and the open end of the lower first external terminal 5 is on the gasket 7 side. The second external terminal 6 is curved and crimped and fixed to the first external terminal 5, and the contact portions of the first external terminal 5 and the second external terminal 6 are insulated by the gasket 7.

Hereinafter, the present invention will be described in detail with reference to examples.

(Examples 1 to 5 and Comparative Examples 1 to 3)
[Preparation of positive electrode A]
A positive electrode layer material in which 1.0% by mass of graphene having an oxygen content of 50 atomic% and 80% by mass of lithium manganese iron phosphate (LiMn _0.7 Fe _0.3 PO ₄ ), which is a positive electrode active material, are mixed. Was prepared. Subsequently, 9.0% by mass of acetylene black, which is a conductive auxiliary agent, and 10% by mass of polyvinylidene fluoride (PVDF), which is a binder, were added to the positive electrode layer material and mixed, and then N was added to the mixture as a solvent. -Methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode slurry. Next, the positive electrode slurry was applied onto the aluminum foil which is a current collector, and dried at 80 ° C. Then, the positive electrode A was produced by press working until the electrode density became 1.8 g / cc.

[Preparation of positive electrode B]
Graphene with an oxygen content of 50 atomic% is 2.0% by mass, LiMn _0.7 Fe _0.3 PO ₄₈₀ % by mass, which is a positive electrode active material, and acetylene black 8.0% by mass, which is a conductive auxiliary agent. The positive electrode B was produced by the same method as the method for producing the positive electrode A, except that 10% by mass of PVDF, which is a binder, was used.

[Preparation of positive electrode C]
Graphene with an oxygen content of 50 atomic% 5.0% by mass, LiMn _0.7 Fe _0.3 PO ₄₈₀ % by mass, which is a positive electrode active material, and acetylene black 5.0% by mass, which is a conductive auxiliary agent. The positive electrode C was produced by the same method as the method for producing the positive electrode A, except that 10% by mass of PVDF, which is a binder, was used.

[Preparation of positive electrode D]
LiMn _0.7 Fe _0.3 PO ₄₈₀ % by mass, which is a positive electrode active material, 10% by mass of acetylene black, which is a conductive auxiliary agent, and 10% by mass of PVDF, which is a binder, are added and mixed, and then the mixture is used. N-Methyl-2-pyrrolidone (NMP) was added to the mixture as a solvent to prepare a positive electrode slurry. Graphene is not contained in the positive electrode slurry. Next, the positive electrode slurry was applied onto the aluminum foil as a current collector and dried at 80 ° C. Then, a positive electrode D was produced by press working until the electrode density reached 1.8 g / cc.

[Preparation of positive electrode E]
Graphene with an oxygen content of 28 atomic% (less than 30 atomic%) is 5.0% by mass, LiMn _0.7 Fe _0.3 PO ₄₈₀ % by mass, which is a positive electrode active material, and acetylene black, which is a conductive auxiliary agent. A positive electrode E was produced by the same method as that for producing the positive electrode A, except that 5.0% by mass and 10% by mass of PVDF as a binder were used.

[Manufacturing of negative electrode]
As a negative electrode current collector, metallic lithium, which is a negative electrode active material, was formed on one surface of a rolled copper alloy foil to prepare a negative electrode.

[Observation of the positive electrode layer with a scanning electron microscope]
Each of the positive electrode layers of the positive electrodes A and E was observed using a scanning electron microscope manufactured by JEOL Ltd .: model number JSM-7500. As a result, as shown in FIG. 2, it was observed that graphene having an oxygen content of 50 atomic% was dispersed in the positive electrode layer of the positive electrode A without agglomeration. On the other hand, as shown in FIG. 3, it was observed that graphene having an oxygen content of 28 atomic% was aggregated in the positive electrode layer of the positive electrode E.

[Assembly of evaluation cell]
The positive electrodes A to E and the negative electrode were punched out in a circular shape. The separator is made of a microporous polypropylene membrane. The non-aqueous electrolytic solution was prepared by dissolving LiPF ₆ in a mixed non-aqueous solvent (volume ratio, EC: DEC = 1: 2) of ethylene carbonate (EC) and diethyl carbonate (DEC) at 1.0 mol / L.
Using these positive electrodes A to E, the negative electrode, the separator, and the mixed non-aqueous solvent, evaluation cells (coin-type lithium) having positive electrodes A to E with the structure shown in FIG. 1 were assembled.

[Constant current discharge test]
An evaluation cell having positive electrodes A to C having a positive electrode layer containing 1.0% by mass, 2.0% by mass, and 5.0% by mass of graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less, and graphene. The evaluation cells having the positive electrode D having the positive electrode layer not containing the above-mentioned cells were discharged at a constant current up to 1.5 V, respectively. As a result, as shown in FIG. 4, it was confirmed that the discharge capacity at the time of the initial discharge increases as the ratio of oxygen containing graphene of 30 atomic% or more and 50 atomic% or less increases.

[Charge / discharge cycle test]
Using the evaluation cells provided with the positive electrodes A to E, the charge / discharge cycle test was performed under the following charge / discharge conditions 1 to 4. The test temperature was 30 ° C. Table 1 below shows the combinations of the evaluation cells provided with the positive electrodes A to E and the charge / discharge conditions 1 to 4, and the measurement results of the 180th cycle discharge capacity and the discharge capacity retention rate of each evaluation cell.

The calculation formula of the discharge capacity retention rate is shown in the formula (1).
Discharge capacity retention rate (%) =
(Discharge capacity at the 180th cycle / Discharge capacity at the 4th cycle) × 100 ... (1)
(Charging / discharging condition 1)
First time: 0.1C discharge up to 1.5V (once)
Activation: 0.1C charge up to 4.5V, 0.1C discharge up to 2.5V (3 times)
Cycle: 1.0C charge up to 4.5V, 1.0C discharge up to 2.5V (180 times)
(Charging / discharging condition 2)
First time: 0.1C discharge up to 1.9V (once)
Activation: 0.1C charge up to 4.5V, 0.1C discharge up to 2.5V (3 times)
Cycle: 1.0C charge up to 4.5V, 1.0C discharge up to 2.5V (180 times)
(Charging / discharging condition 3)
First time: 0.1C discharge up to 0.5V (once)
Activation: 0.1C charge up to 4.5V, 0.1C discharge up to 2.5V (3 times)
Cycle: 1.0C charge up to 4.5V, 1.0C discharge up to 2.5V (180 times)
(Charging / discharging condition 4)
First time: 0.1C charge up to 4.5V, 0.1C discharge up to 2.5V (once)
Activation: 0.1C charge up to 4.3V, 0.1C discharge up to 2.5V (twice)
Cycle: 1.0C charge up to 4.3V, 1.0C discharge up to 2.5V (180 times)

As is clear from Table 1, evaluation cells having positive electrodes A to C having a positive electrode layer containing graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less are used, and each cell is charged / discharged under condition 1 (first time). In Examples 1 to 3 in which the discharge cutoff voltage is set to 1.5 V based on the oxidation-reduction potential of lithium), the dendrite-like growth of lithium during charge / discharge is suppressed or prevented. Moreover, since the output characteristics were improved, it can be seen that both the high discharge capacity and the high discharge capacity retention rate were shown at the 180th cycle, that is, the charge / discharge cycle characteristics were improved.

Further, using evaluation cells including positive electrodes A and B having a positive electrode layer containing graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less, each cell is charged / discharged under the charge / discharge condition 2 (the cutoff voltage of the initial discharge is set. Examples 4 and 5 in which charging and discharging were performed at 1.9V based on the oxidation-reduction potential of lithium also suppressed or prevented the dendrite-like growth of lithium during charging and discharging, and improved the output characteristics. Therefore, it can be seen that both the high discharge capacity and the high discharge capacity retention rate were shown at the 180th cycle, that is, the charge / discharge cycle characteristics were improved.

On the other hand, in Comparative Example 1 in which an evaluation cell including a positive electrode D having a positive electrode layer containing no graphene is used and the cell is charged / discharged under charge / discharge condition 4 (no initial discharge), the discharge capacity is low at the 180th cycle. It can be seen that both the low discharge capacity retention rate and the low discharge capacity retention rate were shown. It is presumed that this is because the inert film such as lithium carbonate or lithium hydroxide formed on the metallic lithium surface of the negative electrode could not be destroyed and removed because there was no initial discharge immediately after battery assembly.

Further, using evaluation cells including positive electrodes A and B having a positive electrode layer containing a graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less, each cell is charged / discharged under the charge / discharge condition 2 (the cutoff voltage of the initial discharge is set. It can be seen that Comparative Examples 2 and 3 in which charging / discharging is performed at 0.5 V based on the oxidation-reduction potential of lithium also show both a low discharge capacity and a low discharge capacity retention rate at the 180th cycle. This is because the cutoff voltage at the time of the initial discharge is less than 1.0 V, so that Fe ²⁺ or Mn ²⁺ in the positive electrode active material is reduced by the lithium ions released from the negative electrode, and in the charge / discharge after the initial discharge, It is presumed that the amount of available positive electrode active material contained in the positive electrode has decreased.

Further, using an evaluation cell having a positive electrode E having a positive electrode layer containing graphene having an oxygen content of 28 atomic% (less than 30 atomic%), the cell is charged / discharged under condition 1 (cutoff voltage of initial discharge is lithium). In Comparative Example 4 in which charging / discharging is performed at 1.5 V based on the oxidation-reduction potential of the above, an evaluation cell including a positive electrode C having a positive electrode layer containing graphene having an oxygen content of 50 atomic% is used. It can be seen that both the low discharge capacity and the low discharge capacity retention rate were shown in the 180th cycle as compared with Example 3 in which the cell was charged and discharged under the charge / discharge condition 1. It is presumed that this is because graphene aggregates on the positive electrode layer, and even if graphene functions as a conductive agent after the initial discharge, it becomes difficult to form the above-mentioned conductive network.

1 ... Lithium secondary battery, 2 ... Positive electrode, 21 ... Positive electrode current collector, 22 ... Positive electrode layer, 3 ... Negative electrode, 31 ... Negative electrode current collector, 32 ... Negative electrode layer, 4 ... Separator, 5 ... First external terminal, 6 ... 2nd external terminal, 7 ... Gasket

Claims (3)

A method for charging and discharging a lithium secondary battery including a positive electrode and a negative electrode containing metallic lithium.
The positive electrode includes a current collector and a positive electrode layer formed on one or both surfaces of the current collector.
The positive electrode layer contains graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less, and a positive electrode active material.
The feature is that the charge / discharge after assembly of the lithium secondary battery is started from discharge, and the discharge cutoff voltage at the time of initial discharge is set to 1.0 V or more and 2.0 V or less based on the oxidation-reduction potential of lithium. How to charge and discharge the lithium secondary battery. The lithium according to claim 1, wherein the positive electrode active material is at least one lithium-containing compound selected from the group of lithium cobalt oxide, lithium manganate, lithium nickel cobalt manganate, and lithium iron phosphate. How to charge and discharge the secondary battery. The charging / discharging method for a lithium secondary battery according to claim 1 or 2, wherein the graphene is contained in a proportion of 1.0% by mass or more and 10% by mass or less with respect to the total amount of the positive electrode layer.

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