JP2020017438A - Method for charging/discharging lithium secondary battery - Google Patents

Abstract

To provide a method for charging/discharging a lithium secondary battery, capable of suppressing or preventing a dendrite growth in lithium during charge/discharge, and further achieving an improvement in charge/discharge cycle characteristics and high capacity by increasing output.SOLUTION: A lithium secondary battery includes a positive electrode, and a negative electrode containing metal lithium. The positive electrode includes a current collector and a positive electrode layer formed on one side or both sides of the current collector. The positive electrode layer contains a positive electrode active material and graphene having an oxygen content of 30 atom% or more and 50 atom% or less. In a method for charging/discharging the lithium secondary battery, charge/discharge after the lithium secondary battery has been assembled is started from discharge, and a discharge cut-off voltage in initial discharge is set to 1.0 V or more and 2.0 V or less based on the oxidation-reduction potential of lithium.SELECTED DRAWING: Figure 1

Description

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

  A lithium secondary battery is a high-performance secondary battery having the highest energy density among secondary batteries currently in practical use, and is used as 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, long life, etc. as these power supplies.

Such a lithium secondary battery performs charge and discharge by moving lithium ions between a positive electrode and a negative electrode. Each of the positive electrode and the negative electrode includes a current collector that supports a positive electrode active material and a negative electrode active material. As a positive electrode active material of a lithium secondary battery, at present, lithium cobalt oxide (LiCoO ₂ ), lithium manganate (LiMn ₂ O ₄ ), lithium nickelate (LiNiO ₂ ), lithium iron phosphate (LiFePO ₄ ) and the like are used. Metal oxides or metal phosphorus oxides containing are commercially available or are being developed for 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 a positive electrode and a negative electrode. Generally, a microporous thin film made of polyolefin is used for the separator. An electrode group including a positive electrode, a negative electrode, and a separator is housed in a battery container together with a nonaqueous electrolyte. As the non-aqueous electrolyte, a non-aqueous electrolyte 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 electrolytes or solid electrolytes have also attracted attention.

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

  However, in a lithium secondary battery using metal lithium as the negative electrode active material, lithium grows in a dendrite shape from the metal lithium surface of the negative electrode during repeated charging and discharging, and the dendritic lithium forms a separator interposed between the positive electrode and the negative electrode. There was a problem that the electrode penetrated to the positive electrode to cause an internal short circuit.

  For this reason, for example, Patent Literature 1 discloses that a lithium-containing compound (eg, manganese dioxide), which is a secondary active material and can be discharged from the beginning, in addition to a lithium-containing compound that is a main active material, is used as a positive electrode active material. A secondary battery is described. Such a lithium secondary battery of Patent Document 1 can be discharged from the first time after assembly. That is, metal lithium can be released as lithium ions from the negative electrode during the first discharge. Therefore, an inert coating such as lithium carbonate or lithium hydroxide formed on the surface of the lithium metal of the negative electrode immediately after the battery is assembled is removed. As a result, at the time of charging after the initial discharge, lithium ions are reduced and deposited on the metal lithium surface of the negative electrode having a favorable surface state, so that it is possible to suppress the lithium from growing in a dendritic form from the metal lithium surface of the negative electrode. Become.

Further, Patent Document 2 discloses a material which can be discharged from the beginning, Li ₄ Mn _5-x M1 _x O ₁₂ (where M1 is Co, Ni, Fe, Cu, Mg) , Zn, Al, Cr and Ga, x is 0 ≦ x <5), for example, a lithium secondary battery using Li ₄ Mn ₅ O ₁₂ is described. ing. Similarly to the lithium secondary battery of Patent Literature 1, the lithium secondary battery of Patent Literature 2 can discharge electricity from the first time after assembly. Further, Non-Patent Document 1 describes that the amount of electricity per unit weight of the Li ₄ Mn ₅ O ₁₂ is about 250 mAh / g.

JP 2017-16905 A JP-A-2017-68966

Masaki Okada, Yuya Sakaguchi “Study on High-Capacity LiMn Oxide Cathode Materials for Lithium-Ion Secondary Batteries” Tosoh Research and Technical Report 2016 Vol. 60 p. 21-28

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

Further, Li ₄ Mn ₅ O _{12, which} is a material that can be discharged from the first time as a sub-active material and is disclosed in Patent Document 2, has a quantity of electricity per unit weight of about 250 mAh / g as described above. Is a relatively low value. A lithium secondary battery provided with a positive electrode containing Li ₄ Mn ₅ O ₁₂ having such an amount of electricity per unit weight has a function of removing an inert film formed on the surface of lithium metal of the negative electrode during the first discharge. In order to secure a necessary discharge capacity, the proportion of the Li ₄ Mn ₅ O ₁₂ in the positive electrode active material must be increased. Therefore, also in the positive electrode active material containing Li ₄ Mn ₅ O ₁₂ as a secondary active material, the ratio of the main active material is relatively reduced, and there is a problem that the discharge capacity of the lithium secondary battery is reduced.

  Therefore, the present invention solves the above-mentioned problems, and can suppress or prevent the dendrite-like growth of lithium during charge / discharge, and further improve the charge / discharge cycle characteristics and increase the capacity of the lithium secondary battery by increasing the output. A charge / discharge method is provided.

  According to the present invention, there is provided a method for charging and discharging a lithium secondary battery including a positive electrode and a negative electrode including metallic lithium, wherein the positive electrode includes a current collector and the current collector. A positive electrode layer formed on one or both surfaces of the lithium secondary battery, wherein the positive electrode layer includes graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less, and a positive electrode active material; A rechargeable lithium battery comprising: starting charging / discharging after assembling from a discharge, and setting a discharge cutoff voltage at the time of an initial discharge to 1.0 V or more and 2.0 V or less based on a redox potential of lithium. Is provided.

  According to the present invention, there is provided a charge / discharge method for a lithium secondary battery which can suppress or prevent the dendrite-like growth of lithium during charge / discharge, and further achieve improvement in charge / discharge cycle characteristics and high capacity by increasing output. it can.

FIG. 1 is a cross-sectional view illustrating an example of a coin-type lithium secondary battery according to the 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 was discharged to 1.5V.

  Hereinafter, a method for charging and discharging 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 the embodiment includes a positive electrode, and a negative electrode including lithium metal.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 includes graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less, and a positive electrode active material, and starts charging and discharging after assembling a lithium secondary battery from discharge, and at the time of initial discharge. The discharge cutoff voltage is set to 1.0 V or more and 2.0 V or less based on the oxidation-reduction potential of lithium. According to the method of charging / discharging a 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 charge / discharge 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 including the graphene-containing positive electrode can start charging and discharging after the battery assembling from discharge, and at the time of initial discharge, a sufficient amount of lithium as ions from the surface of the metal lithium of the negative electrode. Has the necessary discharge capacity to release. In addition, since the discharge cutoff voltage at the time of the first discharge is set to be 1.0 V or more and 2.0 V or less based on the oxidation-reduction potential of lithium, the amount of lithium ions released from the metal lithium surface of the negative electrode is increased. it can. Since the amount of lithium ions released from the surface of the lithium metal of the negative electrode can be increased in this manner, an inactive coating such as lithium carbonate or lithium hydroxide on the surface of the lithium metal of the negative electrode can be broken and removed. As a result, in the charging after the initial discharge, when the lithium ions released from the positive electrode active material reduce and deposit lithium on the metal lithium surface of the negative electrode, the inactive film is removed from the metal lithium surface, so that lithium is It is not uniformly deposited on the lithium surface but is uniformly deposited on the metallic lithium surface. Therefore, with the repetition of the charge / discharge cycle, lithium is suppressed or prevented from growing on the negative electrode from the lithium metal surface in the form of dendrite, and an internal short circuit between the negative electrode and the positive electrode due to the dendritic growth of lithium is prevented. it can.
In the initial discharge, lithium (Li) is released as ions from the surface of the metallic lithium of the negative electrode to the nonaqueous 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 reduced, the graphene included in the positive electrode layer has higher electrical conductivity than before reduction, and thus functions as a conductive agent after the initial discharge. Graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less can be combined with oxygen (e.g., bonded to graphene) in the graphene in a polar solvent such as N-methyl-2-pyrrolidone (NMP). Is charged negatively, the graphenes are repelled and dispersed. That is, the graphene hardly aggregates in a polar solvent and exhibits high dispersibility. A positive electrode slurry is prepared by adding a positive electrode active material or the like to such a solvent containing graphene, and the positive electrode slurry is applied to one or both surfaces of a positive electrode current collector and dried to obtain graphene. Can be formed on the current collector. As a result, graphene functioning as a conductive agent reduced at the time of the first discharge is not localized in a part of the positive electrode layer, but between the current collector of the entire positive electrode layer and the positive electrode active material, and between the positive electrode active material. Can be interposed. In addition, since the graphene has a layered structure in which carbon atoms bonded by an SP2 bond are arranged in a plane, the graphene can be easily bonded to another adjacent graphene and a conductive additive further mixed in the positive electrode layer. Therefore, a conductive network that electrically connects the current collector and the positive electrode active material and between the positive electrode active materials can be formed using the graphene. Therefore, in the positive electrode, the diffusion of ions and the transfer of electric charges by electronic conduction can be improved by the conductive network, so that the output of the lithium secondary battery can be increased.

  Metal lithium has a large amount of electricity per unit weight of 3.86 Ah / g. Therefore, a lithium secondary battery including a negative electrode containing lithium metal as a negative electrode active material can have a high capacity.

  According to the method of charging / discharging a lithium secondary battery according to the embodiment described above, it is possible to suppress or prevent lithium dendrite-like growth during charging / discharging, and to further improve charge / discharge cycle characteristics and increase capacity by increasing 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 a known or commercially available positive electrode current collector can be used. The positive electrode current collector is, for example, a metal foil of aluminum or the like, a lath-processed or etched metal foil, or the like.

  The positive electrode layer includes 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 using X-ray photoelectron spectroscopy (X-ray photoelectron spectroscopy: XPS). Graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less is, for example, 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 has high dispersibility in a polar solvent, and thus is well dispersed in the positive electrode layer, is reduced at the time of the first discharge, and has a conductive agent. And a conductive network can be formed in the positive electrode layer.
Graphene having an oxygen content of less than 30 atomic% is not sufficiently charged in a polar solvent such that the oxygen in the graphene is negatively charged but the graphenes repel each other and disperse. Therefore, graphene aggregates in the positive electrode layer, and it becomes difficult to form the above-described conductive network even if graphene functions as a conductive agent after the first discharge. On the other hand, in graphene having an oxygen content of more than 50 atomic%, it is difficult to function as a conductive agent because all of the oxygen contained therein remains without being reduced during the 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 ratio of 1.0 mass% to 10 mass% based on the total amount of the positive electrode layer. A more preferable content ratio of the graphene is 1.0% by mass or more and 5.0% by mass or less based on the total amount of the positive electrode layer, and a more preferable content ratio of the graphene is 2.0% by mass based on the total amount of the positive electrode layer. Not less than 5.0% by mass.
In a lithium secondary battery including a positive electrode layer containing less than 1.0% by mass of graphene based on the total amount of the positive electrode layer, it is difficult to start charging and discharging after the battery assembly from discharging. On the other hand, in a lithium secondary battery including a positive electrode layer containing graphene exceeding 10% by mass with respect to the total amount of the positive electrode layer, the ratio of graphene in the positive electrode layer increases and the content ratio of the positive electrode active material decreases relatively. Therefore, the capacity may be reduced.

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, concentrated sulfuric acid solution, hydrogen peroxide solution and the like are added to the graphite powder to oxidize the graphite powder to prepare a dispersion solution containing graphite oxide. Graphite oxide is 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 carbon atom layers to prepare a dispersion solution containing graphene oxide. Since graphite oxide has a larger interlayer between carbon atoms than graphite, it can be easily cleaved between layers of carbon 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 inserting and extracting 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 of 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 ferrate (LiCo _0.5 Fe _0.5 O). _2), lithium nickel cobalt manganese oxide _{(Li (Ni x Co y Mn} 1-x-y) O 2 (0 <x <1,0 <y <1)), lithium manganese iron phosphate _(LiMn x _{Fe 1- x PO 4) (0 <x} <1), lithium iron phosphate (LiFePO _4), lithium cobalt phosphate (LiCoPO _4), and a lithium manganese phosphate (LiMnPO ₄₎ or the like.

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

The conductive agent is not particularly limited, and a known or commercially available conductive agent can be used. Examples of the conductive assistant include carbon black such as acetylene black and Ketjen black, carbon nanotubes, carbon fibers, activated carbon, and graphite.
Note that if the above-described graphene is added to the positive electrode layer in an appropriate ratio, the addition 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 a known or commercially available binder can be used. Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), and ethylene-propylene copolymer. Coal, styrene butadiene rubber (SBR), acrylic resin, and the like.

  The positive electrode can be manufactured, for example, by the following method. First, a positive electrode slurry is prepared by dispersing the above-described graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less, a positive electrode active material, a conductive additive, and a binder in a solvent. Subsequently, after applying the positive electrode slurry to one or both surfaces of the positive electrode current collector, the positive electrode is manufactured by drying at 80 ° C., for example, under vacuum 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. In addition, when using polyvinylidene fluoride (PVDF) as a binder, it is preferable to use N-methyl-2-pyrrolidone (NMP) as a solvent.

<Negative electrode>
The negative electrode includes, for example, a negative electrode current collector and lithium metal as 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 a known or commercially available negative electrode current collector can be used. The negative electrode current collector is, for example, a rolled foil or an electrolytic foil made of copper or a copper alloy.

<Non-aqueous electrolyte>
When the non-aqueous electrolyte is in a liquid state, it includes a non-aqueous solvent and an electrolyte.

  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 electrolyte includes, for example, LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (C _m F _{2m + 1} SO ₂ ) (C _n F _{2n + 1} SO ₂ ) (m and n are integers of 1 or more), and LiC (C _{p F 2p + 1 SO 2)} (C q F 2q + 1 SO 2) (C r F 2r + 1 SO 2) (p, q, r is an integer of 1 or more), a lithium difluoro (oxalato) borate. These electrolytes may be used alone or in combination of two or more. It is desirable that this electrolyte be dissolved in the 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 film or a nonwoven fabric of a polyolefin resin such as a polyethylene resin or a polypropylene resin can be used. The microporous membrane or nonwoven fabric may have a single layer or a multilayer structure. In particular, the separator is preferably a microporous polypropylene membrane.

<Charging and discharging conditions>
The charge / discharge after the assembly of the lithium secondary battery 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.
When the discharge cutoff voltage at the time of the first discharge exceeds 2.0 V based on the oxidation-reduction potential of lithium, at the time of the first discharge, a discharge capacity necessary for removing the inactive film on the metal lithium surface of the negative electrode is obtained. Can not do.
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 released from the negative electrode at the time of the first discharge. In the charge and discharge after the initial discharge, the amount of the available positive electrode active material contained in the positive electrode may be reduced by the reduced lithium ions.

  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 stacked type, a cylindrical type, a rectangular shape, a flat type, and the like.

  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 illustrating 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 disposed 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 a first external terminal 5 located on the lower side and a second external terminal 6 located on the upper side. The contact portions of the first external terminal 5 and the second external terminal 6 are insulated by a gasket 7.
The positive electrode 2 includes a positive electrode current collector 21 located on the inner surface of the first external terminal 5 and connected thereto, 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 includes a negative electrode current collector 31 located on the inner surface of the second external terminal 6 and connected thereto, and a negative electrode layer made of metallic lithium provided on a surface of the negative electrode current collector 31 facing the separator 4. 32. 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 end wrapped at its lower end and both side surfaces by the gasket 7, and the open end of the lower first external terminal 5 is placed on the gasket 7 side. The second external terminal 6 is caulked and fixed to the first external terminal 5 by bending, and a contact portion between the first external terminal 5 and the second external terminal 6 is 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)
[Production of Positive Electrode A]
A mixture of 1.0 mass% of graphene having an oxygen content of 50 atomic% and 80 mass% of lithium iron manganese phosphate (LiMn _0.7 Fe _0.3 PO ₄ ) as a positive electrode active material is mixed. Was prepared. Subsequently, 9.0% by mass of acetylene black as a conductive additive and 10% by mass of polyvinylidene fluoride (PVDF) as a binder were added to the positive electrode layer material and mixed, and then N 2 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 on an aluminum foil as a current collector, and dried at 80 ° C. Thereafter, a positive electrode A was produced by press working until the electrode density became 1.8 g / cc.

[Production of Positive Electrode B]
2.0 mass% of graphene having an oxygen content of 50 atomic%, 80 mass% of LiMn _0.7 Fe _0.3 PO _{4 as} a positive electrode active material, and 8.0 mass% of acetylene black as a conductive additive. A positive electrode B was prepared in the same manner as the positive electrode A except that 10% by mass of PVDF as a binder was used.

[Preparation of positive electrode C]
5.0 mass% of graphene having an oxygen content of 50 atomic%, 80 mass% of LiMn _0.7 Fe _0.3 PO _{4 as} a positive electrode active material, and 5.0 mass% of acetylene black as a conductive additive. A positive electrode C was prepared in the same manner as in the positive electrode A except that 10% by mass of PVDF as a binder was used.

[Production of positive electrode D]
After adding and mixing 80% by mass of LiMn _0.7 Fe _0.3 PO _{4 as} a positive electrode active material, 10% by mass of acetylene black as a conductive additive, and 10% by mass of PVDF as a binder, N-methyl-2-pyrrolidone (NMP) was added as a solvent to the mixture to prepare a positive electrode slurry. The positive electrode slurry does not contain graphene. Next, the positive electrode slurry was applied on an aluminum foil as a current collector, and dried at 80 ° C. Then, the positive electrode D was produced by press working until the electrode density became 1.8 g / cc.

[Preparation of positive electrode E]
5.0 mass% of graphene having an oxygen content of 28 atomic% (less than 30 atomic%), 80 mass% of LiMn _0.7 Fe _0.3 PO _{4 as} a positive electrode active material, and acetylene black as a conductive auxiliary agent Positive electrode E was produced by the same method as that for producing positive electrode A, except that 5.0% by mass and 10% by mass of PVDF as a binder were used.

[Preparation of negative electrode]
As one of the negative electrode current collectors, metal lithium as a negative electrode active material was formed on one surface of a rolled copper alloy foil to produce a negative electrode.

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

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

[Constant current discharge test]
An evaluation cell including positive electrodes A to C having a positive electrode layer containing 1.0 mass%, 2.0 mass%, and 5.0 mass% of graphene having an oxygen content of 30 atomic% to 50 atomic%; Each of the evaluation cells provided with the positive electrode D having the positive electrode layer containing no was discharged at a constant current to 1.5 V. As a result, as shown in FIG. 4, it was confirmed that the discharge capacity at the time of the first discharge was increased by increasing the proportion of graphene containing 30 atomic% or more and 50 atomic% or less of oxygen.

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

The equation for calculating the discharge capacity retention ratio is shown in equation (1).
Discharge capacity maintenance rate (%) =
(Discharge capacity at 180th cycle / discharge capacity at fourth cycle) × 100 (1)
(Charging and discharging conditions 1)
First time: 0.1 C discharge to 1.5 V (1 time)
Activation: 0.1C charge to 4.5V, 0.1C discharge to 2.5V (3 times)
Cycle: 1.0C charge to 4.5V, 1.0C discharge to 2.5V (180 times)
(Charging and discharging conditions 2)
First time: 0.1C discharge to 1.9V (1 time)
Activation: 0.1C charge to 4.5V, 0.1C discharge to 2.5V (3 times)
Cycle: 1.0C charge to 4.5V, 1.0C discharge to 2.5V (180 times)
(Charging and discharging conditions 3)
First time: 0.1C discharge to 0.5V (1 time)
Activation: 0.1C charge to 4.5V, 0.1C discharge to 2.5V (3 times)
Cycle: 1.0C charge to 4.5V, 1.0C discharge to 2.5V (180 times)
(Charging and discharging conditions 4)
First time: 0.1C charge to 4.5V, 0.1C discharge to 2.5V (1 time)
Activation: 0.1 C charge to 4.3 V, 0.1 C discharge to 2.5 V (twice)
Cycle: 1.0C charge to 4.3V, 1.0C discharge to 2.5V (180 times)

  As is clear from Table 1, the evaluation cells including the positive electrodes A to C having the positive electrode layers containing graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less were used, and each cell was charged and discharged under the first charging / discharging condition 1 (first time). In Examples 1 to 3 in which charging and discharging were performed at a discharge cutoff voltage of 1.5 V with reference to the oxidation-reduction potential of lithium, the dendritic growth of lithium during charging and discharging was suppressed or prevented. Further, since the output characteristics were improved, it was found that both the high discharge capacity and the high discharge capacity maintenance ratio were exhibited at the 180th cycle, that is, the charge / discharge cycle characteristics were improved.

  Further, an evaluation cell 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 was used, and each cell was charged and discharged under condition 2 (the cut-off voltage of the first discharge was changed to In Examples 4 and 5, in which charging and discharging were performed at 1.9 V with reference to the oxidation-reduction potential of lithium), dendrite-like growth of lithium during charging and discharging was suppressed or prevented, and output characteristics were improved. Therefore, it can be seen that both the high discharge capacity and the high discharge capacity maintenance ratio were exhibited at the 180th cycle, that is, the charge / discharge cycle characteristics were improved.

  On the other hand, Comparative Example 1 in which the evaluation cell including the positive electrode D having the positive electrode layer containing no graphene is used and the cell is charged and discharged under the charging and discharging condition 4 (no initial discharge), the low discharge capacity at the 180th cycle It can be seen that both the low and high discharge capacity retention rates were exhibited. This is presumably because there was no first discharge immediately after the battery assembly, so that an inactive coating such as lithium carbonate or lithium hydroxide formed on the surface of the lithium metal of the negative electrode was destroyed and could not be removed.

Further, an evaluation cell 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 was used, and each cell was charged and discharged under condition 2 (the cut-off voltage of the first discharge was changed to It can be seen that Comparative Examples 2 and 3 in which charging and discharging were performed at a voltage of 0.5 V with reference to the oxidation-reduction potential of lithium also exhibited both a low discharge capacity and a low discharge capacity retention ratio at the 180th cycle. This is because, since the cut-off voltage at the time of the first discharge is less than 1.0 V, Fe ²⁺ or Mn ²⁺ in the positive electrode active material is reduced by lithium ions released from the negative electrode, and in charge and discharge after the first discharge, It is estimated that the amount of the available positive electrode active material contained in the positive electrode decreased.

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

  DESCRIPTION OF SYMBOLS 1 ... Lithium secondary battery, 2 ... Positive electrode, 21 ... Positive electrode collector, 22 ... Positive electrode layer, 3 ... Negative electrode, 31 ... Negative electrode current collector, 32 ... Negative electrode layer, 4 ... Separator, 5 ... 1st external terminal, 6 second external terminal, 7 gasket

Claims (3)

A positive electrode, and a method for charging and discharging a lithium secondary battery including a negative electrode including lithium metal,
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 includes graphene having an oxygen content of 30 atomic% or more and 50 atomic% or less, and a positive electrode active material,
Charging and discharging after assembling the lithium secondary battery is started from discharging, and a discharge cutoff voltage at the time of initial discharging is set to 1.0 V or more and 2.0 V or less based on the oxidation-reduction potential of lithium. Method for charging and discharging lithium secondary batteries.   The lithium according to claim 1, wherein the positive electrode active material is at least one lithium-containing compound selected from the group consisting of lithium cobaltate, lithium manganate, lithium nickel cobalt manganate, and lithium iron phosphate. A method for charging and discharging a secondary battery.   3. The method according to claim 1, wherein the graphene is contained in a ratio of 1.0% by mass to 10% by mass with respect to a total amount of the positive electrode layer. 4.

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