KR102351971B1 - Mellitic triimide as electrode active material for lithium secondary battery and lithium secondary battery using the same - Google Patents

Abstract

The disclosed content relates to an electrode active material for a lithium secondary battery and a lithium secondary battery using the same, and more specifically, the electrode active material for a lithium secondary battery is manufactured using an organic compound based on a single molecule of mellitic triimide derivative.
The electrode active material for a lithium secondary battery prepared from the organic compound based on the melittic triimide derivative monomolecular is stable redox, has excellent specific capacity, and at the same time is easy to control the reduction potential, and has excellent dissolution resistance. provides the

Description

Electrode active material for lithium secondary battery containing mellitic triimide and lithium secondary battery using same

The disclosed content relates to an electrode active material for a lithium secondary battery and a lithium secondary battery using the same, and more particularly, a novel electrode active material using an organic compound capable of stable oxidation-reduction and excellent specific capacity, a novel electrode active material having excellent dissolution resistance, and the same It relates to a lithium secondary battery used.

Recently, lithium secondary batteries are being used in various fields including portable electronic devices such as mobile phones and laptop computers. In particular, as interest in environmental issues grows, fossil fuels such as gasoline and diesel vehicles, which are one of the main causes of air pollution, are used. Research on lithium secondary batteries having high energy density and discharge voltage as a driving source of an electric vehicle that can replace the vehicle being used is being actively conducted, and some of them are in the commercialization stage.

In the past, metal oxide-based lithium secondary batteries have been commercially successful with many developments, but at a time when higher energy density is required due to the growth of the Energy Storage System (ESS) and Electric Vehicle industries, the The theoretical capacity limit, the need to use a limited amount of rare metals, and environmental problems due to the use of heavy metals are emerging.

Meanwhile, in order to use a lithium secondary battery as a driving source for such an electric vehicle, it must be able to maintain high output and stably output in a wide range of state of charge (SOC).

As a cathode material for a high-capacity lithium secondary battery, LiCoO ₂ , which is a typical conventional cathode material, has reached the practical limit of an increase in energy density and output characteristics. In the state, the oxygen in the structure is released along with the structural change, causing an exothermic reaction with the electrolyte in the battery, which is the main cause of battery explosion. In order to improve the safety problem of _{LiCoO 2 , the use} of lithium-containing manganese oxide such _{as LiMnO 2 having a layered crystal structure, LiMn 2} O _{4 having a} spinel crystal structure, _{and lithium-containing nickel oxide such as LiNiO 2} has been considered, and recently, high-capacity materials have been considered. A lot of research has been done on lithium manganese oxide (hereinafter also abbreviated as “Mn-rich”), which adds manganese (Mn) as an essential transition metal in a layered structure lithium manganese oxide in a larger amount than other transition metals (except lithium). is in progress

In addition, in addition to the above lithium manganese oxide, research to apply an organic compound-based active material that can be produced at a low price with better performance to a lithium secondary battery is increasing, and research is being actively conducted now. Researches have been conducted on organic compound active materials that are relatively inexpensive and eco-friendly, and some material groups have come close to commercialization.

The most representative organic compound active material is a group of carbonyl materials, and although benzoquinone has a high theoretical capacity of 500 mAh/g, it has a very small molecular weight, so it is difficult to be commercialized because it dissolves well in an electrolyte. In terms of energy density, research on a new active material group is needed because it has not yet reached a level that can replace metal oxide active materials. is becoming

In the case of a diimide-based compound, it is known that stable oxidation-reduction can be performed by exchanging two electrons. However, since the redox voltage range of the imide ^{is only 2~3V compared to Li/Li +} , it is insufficient in terms of capacity compared to the existing metal oxide-based active materials in terms of energy density.

Disclosed is to provide an electrode active material capable of stable oxidation-reduction, excellent specific capacity, easy control of reduction potential, and excellent dissolution resistance, and a lithium secondary battery using the same.

As one embodiment, the disclosure describes an electrode active material for a lithium secondary battery, which includes a single molecule of a melittic triimide derivative represented by Formula 1 below.

[Formula 1]

In Formula 1, R is the same or different substituents, and in Formula 1, R is each independently hydrogen, halogen, an alkyl group, an alkenyl group, an alkynyl group, a carbonyl group, a cycloalkyl group, an aryl group, an aralkyl group, a heterocyclic group, a heterocyclic group It may be selected from the group consisting of an aryl group, an alkoxy group, an acyl group, a hydroxyl group, an alkylcarbonyl group, and an alkoxycarbonyl group.

Preferably, the electrode active material may further include a porous carbon material.

More preferably, the monomolecular melic triimide derivative and the porous carbon material may have a weight ratio of 1:0.5 to 2.

Even more preferably, the weight ratio of the monomolecular melic triimide derivative and the porous carbon material may be 1:2.

Even more preferably, the porous carbon material may be made of CMK-3.

As another embodiment, the disclosure describes an electrode for a lithium secondary battery comprising an aluminum foil coated with a single molecule of the melic triimide derivative.

Preferably, the electrode for the lithium secondary battery is prepared by mixing the melic triimide derivative single molecule, Super-P and polyvinylidene fluoride in dimethylformamide to prepare a mixture, and then coating the mixture on aluminum foil, , can be prepared by drying.

As another embodiment, the content of this disclosure describes a lithium secondary battery comprising the electrode for the lithium secondary battery, a counter electrode made of lithium metal, an electrode separator made of a porous polypropylene membrane, and an electrolyte.

Preferably, the electrolyte solution may consist of a solution in which 2M lithium bis(trifluoromethane)sulfonimide (LiTFSi) is dissolved in tetraethylene glycol dimethyl ether (TEGDME).

The disclosed electrode active material for a lithium secondary battery and a lithium secondary battery using the same are capable of stable oxidation-reduction, excellent specific capacity, easy to control reduction potential, and excellent dissolution resistance. show the effect.

1 is a graph showing the measurement of current according to a voltage varying at a constant rate in a solution of a single molecule of a pyromellitic diimide derivative represented by Formula 4 and a single molecule of a mellitic triimide derivative represented by Formula 2 disclosed herein.
2 is a graph showing the evaluation of the characteristics of the lithium secondary battery prepared through the disclosed Examples 1 to 3;
3 is a graph showing the evaluation of the characteristics of the lithium secondary battery prepared through the disclosed Example 2 and Comparative Example 1.
4 is a graph showing the evaluation of the characteristics of the lithium secondary battery prepared through the disclosed Example 4.
5 is a graph showing the mechanism analysis of the melittic triimide derivative monomolecule represented by Chemical Formula 1 disclosed herein.
6 is a comparison of the theoretical specific capacity of the disclosed monomolecular melittic triimide derivative molecule with that of the monomolecular diimide derivative, and analyzing the redox reaction mechanism between the monomolecular diimide derivative and the mellitic triimide derivative. it has been shown

Hereinafter, a preferred embodiment of the present invention and the physical properties of each component will be described in detail, which is intended to describe in detail enough that a person of ordinary skill in the art to which the present invention pertains can easily carry out the invention, This does not mean that the technical spirit and scope of the present invention is limited.

The disclosed electrode active material for a lithium secondary battery is prepared from an organic compound based on a single mellitic triimide derivative molecule, and the mellitic triimide derivative single molecule is represented by Formula 1 below.

[Formula 1]

In Formula 1, R is the same or different substituents, and in Formula 1, R is each independently hydrogen, halogen, an alkyl group, an alkenyl group, an alkynyl group, a carboxy group ), a carbonyl group, a cycloalkyl group, an aryl group, an aralkyl group, a heterocyclic group, a heteroaryl group, an alkoxy group ), an acyl group, a hydroxy group, an alkylcarbonyl group, and an alkoxycarbonyl group. The substituent may have 1 to 20 carbon atoms, but is not limited thereto.

For example, in Formula 1, the compound in which R is all hydrogen has a molecular weight of 300 and a theoretical specific capacity of 282 mAh/g.

Preferably, the heterocyclic group may be at least one compound selected from the group consisting of a pyridyl group, a pyrimidyl group, a thiophenyl group, a furanyl group, a pyriryl group and an imidazole group.

<Scheme 1>

As shown in Scheme 1 above, the melic triimide derivative monomolecule used in the disclosure is a triimide-based material comprising three imide functional groups capable of stably exchanging electrons and capable of redox, and shares one phenyl ring. and has a C3 symmetric structure. By exchanging one electron from each imide group, up to three electrons can participate in the redox reaction, and since the three imide groups share only one phenyl ring, the elements other than the redox functional group are minimized, so specific capacity (specific capacity) ) is advantageous in terms of

In addition, the monomolecular melittic triimide derivative possesses a large number of imide groups having excellent electron-attracting ability, thereby stabilizing the lowest unoccupied molecular orbital function (LUMO), and thus may have a high reduction potential. In particular, in a lithium-based electrolyte, the lowest unoccupied molecular orbital function (LUMO) is stabilized and the reduction potential can be further increased by combining two adjacent oxygens of two different imide groups with lithium ions to form a 7-membered ring.

In addition, by introducing a substituent at the N position of each imide, it is possible to not only provide a secondary battery with improved performance by controlling the reduction potential, but also improve the dissolution resistance of the material by controlling the molecular weight to improve the stability of charging and discharging. have.

The melittic triimide derivative monomolecules exhibiting the above effects may be prepared, for example, in the form of Chemical Formulas 2 to 3 below.

[Formula 2]

As shown in Formula 2 (ETTI), the mellitic triimide derivative monomolecule used in the disclosed electrode active material for a lithium secondary battery may be prepared in a form in which an ethyl group is introduced at the N-position. ETTI of Formula 2 has a molecular weight of 369 and a specific specific capacity of 217 mAh/g.

In this case, the method for synthesizing the monomolecular melittic triimide derivative represented by Chemical Formula 2 as described above is performed through the process of Scheme 2 below.

<Scheme 2>

[Formula 3]

In this case, the method for synthesizing the monomolecular melittic triimide derivative represented by Chemical Formula 3 as described above is performed through the process of Scheme 3 below.

<Scheme 3>

In addition, as shown in Formula 3 (BZTI), the mellitic triimide derivative monomolecule used in the disclosed electrode active material for a lithium secondary battery may be prepared in a form in which a benzyl group is introduced at the N-position. BZTI of Formula 3 has a molecular weight of 555 and a theoretical specific capacity of 145 mAh/g.

As a comparative group of the monomolecular melittic triimide derivative, a pyromellitic diimide derivative monomolecule represented by the following Chemical Formula 4 was synthesized and used.

[Formula 4]

In this case, the method for synthesizing the pyromellitic diimide derivative monomolecule represented by Chemical Formula 4 as described above is performed through the process of Scheme 4 below.

<Scheme 4>

In addition, as shown in Chemical Formula 4 (ETDI), the pyromellitic diimide derivative monomolecule used in the disclosed electrode active material for a lithium secondary battery may be prepared in a form in which an ethyl group is introduced at the N position. ETDI of Chemical Formula 4 has a molecular weight of 272 and a theoretical specific capacity of 197 mAh/g.

The reduction potentials of the monomolecular pyromellitic diimide derivative represented by Chemical Formula 4 and the mellitic triimide derivative monomolecule represented by Chemical Formula 2 were measured according to current and are shown in FIG. 1 below.

In this case, the electrode active material for the lithium secondary battery contains N-methyl in a weight ratio of 2 to 6:2 to 6:1, respectively, of the monomolecular melic triimide derivative of Formulas 1 to 3, graphene, and polyvinylidene fluoride. After preparing a slurry of an organic compound based on a single molecule of melittic triimide derivative by mixing with pyrrolidone, the organic compound is coated on aluminum foil and dried in a vacuum oven at 105° C. to 135° C. for 7 to 9 hours. can be

At this time, the organic compound based on the monomolecular melittic triimide derivative and graphene, which is a porous carbon material, are mixed in a weight ratio of 1:0.5 to 2, and most preferably in a weight ratio of 1:2.

The electrode active material for a lithium secondary battery prepared through the above process has excellent dissolution resistance because it is prepared by penetrating graphene, a porous carbon material, with an organic compound based on a monomolecular melittic triimide derivative.

Hereinafter, a method for manufacturing an electrode for a lithium secondary battery to which the disclosed electrode active material for a lithium secondary battery is applied and physical properties of a lithium secondary battery to which the electrode is applied will be described with reference to Examples.

<Preparation Example 1> Preparation of electrode for lithium secondary battery

As shown in Formula 2, a single molecule of a mellitic triimide derivative having an ethyl group introduced at the N position, graphene, and polyvinylidene fluoride were mixed with N-methyl-2-pyrrolidone in a weight ratio of 6:3:1, respectively. After preparing the mixture, the mixture was coated on aluminum foil using a doctor blade, and dried in a vacuum oven at 120° C. for 8 hours to prepare an electrode for a lithium secondary battery (ETTI 631).

<Preparation Example 2> Preparation of electrode for lithium secondary battery

Proceed in the same manner as in Preparation Example 1, except that the monomolecular melittic triimide derivative, graphene, and polyvinylidene fluoride were mixed in N-methyl-2-pyrrolidone in a weight ratio of 4:4:2, respectively, and lithium secondary An electrode for a battery (ETTI 442) was manufactured.

<Preparation Example 3> Preparation of electrode for lithium secondary battery

Proceed in the same manner as in Preparation Example 1, except that the monomolecular melic triimide derivative, graphene, and polyvinylidene fluoride were mixed in N-methyl-2-pyrrolidone in a weight ratio of 3:6:1, respectively, and lithium secondary An electrode for a battery (ETTI 361) was prepared.

<Preparation Example 4> Preparation of electrode for lithium secondary battery

An electrode for a lithium secondary battery (BZTI 442) was prepared in the same manner as in Preparation Example 2, except that an organic compound based on a single molecule of the melic triimide derivative represented by Chemical Formula 3 was used.

<Comparative Preparation Example 1> Preparation of electrode for lithium secondary battery

An electrode for a lithium secondary battery (ETDI 442) was prepared in the same manner as in Preparation Example 2, except that an organic compound based on a single molecule of a pyromellitic diimide derivative represented by Chemical Formula 4 was used.

<Example 1>

The electrode for a lithium secondary battery prepared in Preparation Example 1, a lithium metal as a counter electrode, a porous polypropylene membrane as an electrode separator, and 2M lithium bis (trifluoro) in tetraethylene glycol dimethyl ether (TEGDME) as an electrolyte A CR2032 coin-type lithium secondary battery was prepared using a solution in which methane)sulfonimide (LiTFSi) was dissolved.

<Example 2>

A CR2032 coin-type lithium secondary battery was prepared in the same manner as in Example 1, except that the electrode for a lithium secondary battery prepared in Preparation Example 2 was used.

<Example 3>

A CR2032 coin-type lithium secondary battery was prepared in the same manner as in Example 1, except that the electrode for a lithium secondary battery prepared in Preparation Example 3 was used.

<Example 4>

A CR2032 coin-type lithium secondary battery was prepared in the same manner as in Example 1, except that the electrode for a lithium secondary battery prepared in Preparation Example 4 was used.

<Comparative Example 1>

A CR2032 coin-type lithium secondary battery was prepared in the same manner as in Example 1, except that the electrode for a lithium secondary battery prepared in Comparative Preparation Example 1 was used.

The specific capacity and rate characteristics of the lithium secondary batteries prepared in Examples 1 to 3 were measured and shown in FIG. 2 below. FIG. 2(a) graph shows the values measured at a current rate of 0.1C for the initial constant current charge/discharge voltage compared to Li/Li+ in the 1.5-3.5V range of the electrodes prepared according to Preparation Examples 1 to 3, and FIG. 2( b) The graph shows the change in specific capacity during discharge according to the charge/discharge cycle at 0.1C current rate.

Referring to FIG. 2 below, in the case of a monomolecular melittic triimide derivative, it was calculated that each of the three imide functional groups exchanged one electron to exchange a maximum of three electrons, and charging and discharging as shown in the graph (a) In the experiment, each ratio exhibited three plateaus. In the case of the ETTI electrode, the theoretical specific capacity is 217 mAh/g, and the 4:4:2 ratio electrode shows a specific capacity of 220 mAh/g, so it can be confirmed that all capacities corresponding to the theoretical specific capacity are expressed.

The characteristics of the lithium secondary battery prepared in Example 2 and Comparative Example 1 were evaluated and shown in FIG. 3 below. 3(a) is a graph showing the initial constant current charge/discharge voltage of Li/Li+ in the 1.5-3.5V range of the ETTI and ETDI electrodes prepared according to Preparation Example 2 and Comparative Preparation Example 1 at a current rate of 0.1C. 3(b) graph shows the change in specific capacity during discharge according to the charge/discharge cycle at the 0.1C current rate.

Referring to FIG. 3 below, in the case of the ETTI electrode in FIG. 3(a), all specific capacities corresponding to the theoretical specific capacities were expressed while clearly showing three flat regions, but in the case of the ETDI electrode used as a control, the initial cycle In the case of the ETDI electrode, the specific capacity was only 92 mAh/g, indicating only about half of the theoretical specific capacity of 197 mAh/g. It can be seen that the actual activation ratio to the theoretical specific capacity of the mellitic triimide electrode is significantly higher than the pyromellitic diimide electrode used as the comparative preparation example.

In terms of discharge voltage, the ETDI electrode showed an average voltage of 2.18V, while the ETTI electrode showed an increased discharge voltage of 2.41V on average. This is the reason why the lowest unoccupied molecular orbital of the triimide system is lower than that of the diimide system. This is because two adjacent oxygen and lithium ions of the imide group form a 7-membered ring and exist in a more stable state.

The characteristics of the lithium secondary battery prepared in Example 4 were evaluated and shown in FIG. 4 below. FIG. 4(a) graph shows the values measured at a current rate of 0.1C for the initial constant current charge/discharge voltage compared to Li/Li+ in the 1.5-3.5V range of the BZTI electrode manufactured according to Preparation Example 4, FIG. 4(b) ) The graph shows the change in specific capacity during discharge according to the charge/discharge cycle at 0.1C current rate.

Referring to FIG. 4 below, the BZTI introduced with a benzyl group at the N position of the melic triimide increased the dissolution resistance due to the improved molecular weight, thereby increasing the cycle retention. In the graph of FIG. 4(a), a specific capacity of 171 mAh/g is shown in the initial cycle, but the specific capacity slightly increased compared to the theoretical capacity of 145 mAh/g of the BZTI electrode is the ratio due to the formation of the SEI (solid electrolyte interphase) layer. Because of the increase in capacity, the second cycle test confirmed that 100% of the theoretical capacity was expressed at 146mAh/g. In addition, the dissolution resistance was improved, and it showed an excellent specific capacity retention ability even when the cycle progressed.

Comparing the specific capacity retention capacity of Examples 2 and 4, the ETTI electrode exhibited a specific capacity of 83 mAh/g in the 20th cycle, and the BZTI electrode exhibited a specific capacity of 117 mAh/g in the 20th cycle, compared to the theoretical capacity of 80 % or more. BZTI electrode has a loss in specific capacity due to improved molecular weight, but has excellent cycle retention due to increase in dissolution resistance.

A mechanistic analysis of the monomolecular melittic triimide derivative was carried out. FT-IR for each electrode in the initial state (pristine, electrode not charged or discharged), discharged state (discharged, electrode after full discharge), and recharged state (recharged electrode after full discharge and fully charged again) The analysis is shown in FIG. 5 .

Referring to FIG. 5 below, the left part shows the accumulated capacity according to the change of the constant current charging/discharging voltage, and the pristine state, the discharged state, and the recharged state correspond to which parts each corresponds to black, red, and blue circles, respectively. indicated. The FT-IR results in each state are shown in the right part of FIG. 5 .

In the case of the pristine state, the peak due to the carbonyl group of the monomolecular melic triimide derivative clearly ^{appears at 1,720 cm -1} and 1,773 cm ^-1 , and in the discharged state, the peak due to the existing carbonyl group decreases and at the same time, in the low wavenumber region (1,600 cm ^-1) was formed, a new large peaks, when the recharged state that the peak seen in the vicinity of 1,600cm ^-1 saenggyeotdeon in the discharged state disappears and it can be seen that represents only peak by carbonyl group looked at existing pristine state. ^{The peak near 1,600 cm -1} , which was not present in the pristine state, appeared largely due to the combination of triimide and lithium ions in the discharged state, and then disappeared again when the recharged state was reached. In addition, in the DFT calculation, it was confirmed that the vibration due to the carbonyl group shifted to a low wave number when the melittic triimide derivative monomolecule was in a discharged state to form an ETTI-3Li complex. As such, it was confirmed through ex-situ FT-IR analysis that the lithium secondary battery operates as the melittic triimide electrode is reversibly reduced during discharging and oxidized again during recharging.

Accordingly, the disclosed electrode active material for a lithium secondary battery and a lithium secondary battery using the same are capable of stable oxidation-reduction and easy control of the reduction potential, and provide an electrode active material having excellent dissolution resistance and a lithium secondary battery using the same.

Claims (10)

An electrode active material for a lithium secondary battery, characterized in that it contains a single molecule of a mellitic triimide derivative represented by the following formula (1).
[Formula 1]

(In Formula 1, each R is the same or different substituents)
The method according to claim 1,
In Formula 1, R is each independently hydrogen, halogen, alkyl group, alkenyl group, alkynyl group, carboxy group, carbonyl group, cycloalkyl group, aryl group, aralkyl group, heterocyclic group, heteroaryl group, alkoxy group, acyl group, hydroxyl group, An electrode active material for a lithium secondary battery, characterized in that it is selected from the group consisting of an alkylcarbonyl group and an alkoxycarbonyl group.
The method according to claim 1,
Electrode active material for a lithium secondary battery, characterized in that it further comprises a porous carbon material.
4. The method according to claim 3,
The electrode active material for a lithium secondary battery, characterized in that the melic triimide derivative monomolecule and the porous carbon material have a weight ratio of 1:0.5 to 2.
5. The method according to claim 4,
The electrode active material for a lithium secondary battery, characterized in that the melittic triimide derivative monomolecule and the porous carbon material have a weight ratio of 1:1 or 1:2.
6. The method according to any one of claims 3-5,
The porous carbon material is an electrode active material for a lithium secondary battery, characterized in that graphene.
An electrode for a lithium secondary battery comprising an aluminum foil coated with the monomolecular melic triimide derivative of claim 1 .
8. The method of claim 7,
The electrode for the lithium secondary battery is prepared by mixing the melic triimide derivative single molecule, graphene, and polyvinylidene fluoride in N-methyl-2-pyrrolidone to prepare a mixture, and then coating the mixture on aluminum foil, , An electrode for a lithium secondary battery, characterized in that manufactured by drying.
A lithium secondary battery comprising the electrode for a lithium secondary battery according to any one of claims 7 to 8, a counter electrode made of lithium metal, an electrode separator made of a porous polypropylene membrane, and an electrolyte.
10. The method of claim 9,
The electrolyte is a lithium secondary battery, characterized in that consisting of a solution in which lithium bis (trifluoromethane) sulfonimide (LiTFSi) of 2M in tetraethylene glycol dimethyl ether (TEGDME) is dissolved.

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