KR101264435B1 - Electrolyte for improving storage performance at high temperature and secondary battery comprising the same - Google Patents

Abstract

(A) an electrolyte salt; (b) an electrolyte solvent; And (c) an ammonium-containing compound having a lower reduction potential than the electrolyte solvent, wherein the ammonium-containing compound is capable of forming a solid electrolyte interface (SEI) film on the cathode by electrical reduction. The secondary battery provided with the phosphorus battery electrolyte solution and the said electrolyte solution is provided.

In the present invention, by using an ammonium-containing compound having a lower reduction potential than the conventional electrolyte non-aqueous solvent as one component of the electrolyte, high temperature storage characteristics and long life characteristics of the battery can be provided.

Electrolyte, ammonium, solid electrolyte interface, performance, secondary battery

Description

ELECTROLYTE FOR IMPROVING STORAGE PERFORMANCE AT HIGH TEMPERATURE AND SECONDARY BATTERY COMPRISING THE SAME

1 is a diagram comparing reduction voltages of the electrolyte solutions of Examples 1 and 2 and Comparative Examples 1 and 2, respectively.

FIG. 2 shows charge and discharge of the lithium secondary battery of Example 2 including ammonium benzoate as an electrolyte component and the lithium secondary battery of Comparative Example 1 including a conventional electrolyte solution, and then the negative electrode was collected and analyzed. calorimetry) results.

FIG. 3 shows the battery of Example 1 containing ammonium hexafluorophosphate as the electrolyte component and the battery of Comparative Example 1 containing the conventional electrolyte solution, respectively, after long-term storage (12 weeks) at high temperature (60 ° C.). It is a graph showing a change in power conservation rate.

The present invention provides a secondary battery having excellent thermal stability, including a battery electrolyte containing an electrolyte additive capable of forming a solid and uniform SEI film, and the electrolyte including the electrolyte, and long-life characteristics and high temperature storage characteristics of the battery. It is about.

Recently, miniaturization and lighter weight of electronic equipment have been realized and use of portable electronic devices has become common, so researches on lithium secondary batteries having high energy density have been actively conducted. Lithium ion secondary batteries are manufactured by using a material capable of inserting and detaching lithium ions as a negative electrode and a positive electrode, and filling an organic electrolyte or polymer electrolyte between the positive electrode and the negative electrode, and lithium ions are inserted and removed from the positive electrode and the negative electrode. Electrical energy is generated by oxidation and reduction reactions.

Lithium secondary batteries are also called rocking chair batteries because lithium ions reciprocate the positive and negative poles like a rocking chair to transfer energy. The surface and the electrolyte react to form a solid electrolyte interface (SEI film). The formed SEI film serves to stabilize the battery by inhibiting decomposition of the carbonate-based electrolyte on the surface of the carbon particles, which are negative electrodes, but increase or decrease the electrochemical properties when the battery is continuously charged or discharged, or especially at high temperatures in a full charge state. As the SEI film gradually collapses due to energy and thermal energy, side reactions in which the exposed surface of the negative electrode active material and the electrolyte solvent react to be decomposed continuously occur, thereby increasing the resistance of the electrode and deteriorating overall performance of the battery. In addition, such side reactions lead to gas generation inside the battery, and this continuous gas generation not only causes the internal pressure of the lithium secondary battery to expand at high temperatures, thereby expanding the battery thickness, but also reducing the lifespan and weight of the battery. This creates a huge problem.

When the present inventors use an ammonium-based compound soluble in a non-aqueous solvent as one component of an electrolyte, the inventors have a lower reduction potential than that of a conventional carbonate-based electrolyte solvent, so that a solid electrolyte interface (SEI) film is easily formed. It has been found that the thermal and structural stability of the formed SEI film can be excellent to improve overall performance of the battery.

Accordingly, an object of the present invention is to provide a battery electrolyte containing the ammonium-containing compound and a secondary battery having the electrolyte.

(A) an electrolyte salt; (b) an electrolyte solvent; And (c) an ammonium-containing compound having a lower reduction potential than the electrolyte solvent, wherein the ammonium-containing compound is capable of forming a solid electrolyte interface (SEI) film on the cathode by electrical reduction. The secondary battery provided with the phosphorus battery electrolyte solution and the said electrolyte solution is provided.

In addition, the present invention provides an electrode having a solid electrolyte interface (SEI film) film containing an ammonium-containing compound or a chemical reaction product thereof formed on part or all of a surface thereof, and a secondary battery having the electrode.

Furthermore, the present invention provides an electrolyte additive for forming an SEI film containing ammonium ions as a compound capable of being reduced on the negative electrode of a secondary battery to form a solid electrolyte interface (SEI) film.

Hereinafter, the present invention will be described in detail.

The present invention is not only easy to dissolve or dissociate in a non-aqueous solvent, but also an aqueous solvent, and uses an ammonium-containing compound having a lower reduction potential than an electrolyte carbonate solvent as one component of a battery electrolyte. do.

In the case of using a battery electrolyte containing such an ammonium-containing compound of the physical properties, it exhibits excellent high-temperature storage characteristics and lifetime characteristics, its working principle is estimated as follows.

1) The performance of the battery is highly dependent on the basic electrolyte composition and the solid electrode interface (SEI) formed by the reaction between the electrolyte and the electrode.

In the conventional lithium secondary battery, the surface of carbon particles, which are negative electrode active materials, and an electrolyte react with each other to form a solid electrolyte interface (SEI) film during a first charging process. The formed SEI film is a side reaction between a carbon material and an electrolyte solvent. ; And not only prevent the collapse of the negative electrode material due to co-intercalation of the electrolyte solvent into the negative electrode material, but also faithfully perform a conventional lithium ion tunnel, thereby minimizing performance degradation of the battery. However, SEI membranes formed by conventional carbonate organic solvents, fluorine salts or other inorganic salts are weak, porous and not dense so that lithium ions cannot be moved smoothly. This increases the capacity and lifetime characteristics of the battery.

In contrast, the ammonium-containing compound contained in the electrolyte of the present invention is reduced on the surface of the negative electrode material before other components during initial charging of the battery, thereby forming a SEI film that is not only firm and dense but also excellent in thermal stability (FIG. 1 and 2). Therefore, the conventional carbonate solvent prevents side reactions such as co-intercalation in the layered active material layer or decomposition of the solvent, thereby increasing the initial efficiency of the battery, and the SEI film formed selectively absorbs only lithium ions (Li ⁺ ). And a passivation layer having low chemical reactivity to emit, exhibiting high stability even in a long cycle and achieving long life characteristics.

2) In addition, in the conventional lithium secondary battery, a sudden deterioration of the battery performance occurs especially under a high temperature environment, which not only causes a sudden collapse of the SEI film formed on the surface of the negative electrode, but also increases side reaction between the electrode and the electrolyte and increases the resistance of the electrode. It is also estimated to occur rapidly.

On the contrary, the SEI film formed in the present invention hardly decomposes at a high temperature as it has excellent thermal stability, and is rapidly regenerated and maintained at a low potential even if decayed by a high temperature, thereby maintaining reactivity due to high temperature or the like. The occurrence of side reactions between the increased electrode and the electrolyte is reduced, which may result in improved overall performance and high temperature characteristics of the battery.

One of the elements constituting the battery electrolyte according to the present invention is a compound containing ammonium, and there is no particular limitation as long as it is a material capable of forming a solid electrolyte interface (SEI) film with a lower reduction potential than a mixed electrolyte solvent.

Conventional ammonium cations (ie, substituents that combine with nitrogen are all substituted with hydrocarbon groups) and room temperature molten salts (RTIL) containing inorganic anions are used as battery electrolytes in the ammonium-containing compounds. In other words, it was only intended to improve the safety of the battery by using non-volatile, non-toxic, non-flammable, etc. In fact, the battery caused by the high reduction potential, high viscosity and high solvation ability of the molten salt at room temperature We were not able to solve the problem of performance degradation fundamentally.

On the contrary, in the present invention, instead of forming a molten salt at room temperature using the ammonium-containing compound, an ammonium-containing compound having a lower reduction potential than that of the electrolyte solvent is used as one component of the electrolyte solution. Not only does this not occur fundamentally, but it is differentiated in that the overall performance is improved by the SEI film having excellent properties including ammonium.

The ammonium-containing compound is preferably easily dissolved or dissociated in a non-aqueous solvent used as a battery electrolyte solvent in order to form an SEI film having excellent physical properties. For example, the ammonium-containing compound may dissolve or dissociate at least 0.1% relative to 100% of a non-aqueous solvent. It may be a salt. Non-limiting examples of ammonium containing compounds that can be used include ammonium hexafluorophosphate (AP), ammonium benzoate (AB), ammonium fluoride (NH ₄ F), ammonium chloride (NH ₄ Cl), ammonium bromide (NH ₄ Br ) Or mixtures thereof. In addition, ammonium containing compounds having the above-described physical properties are also within the scope of the present invention.

The content of the ammonium-containing compound can be adjusted according to the goal of improving the overall performance of the battery, but preferably 0.01 to 10 parts by weight per 100 parts by weight of the electrolyte. If the amount is less than 0.01 parts by weight, the desired life characteristics and the high temperature storage characteristics are insignificant, and if it is more than 10 parts by weight, the capacity of the battery is reduced due to the side reaction of the surplus compound, the viscosity of the electrolyte is increased, and the ion conductivity is reduced. Performance degradation will occur.

The battery electrolyte to which the ammonium containing compound is to be added together includes conventional electrolyte components known in the art, such as electrolyte salts and organic solvents.

Using the electrolyte salts is A ⁺ B ^- A salt of the structure, such as, A ⁺ is Li ^+, Na ^+, K ⁺ comprises an alkaline metal cation or an ion composed of a combination thereof, such as, and B ^- is PF ₆ ^-, BF _{^{4 -, Cl -, Br -}} , I -, ClO 4 -, AsF 6 -, CH 3 CO 2 -, CF 3 SO 3 -, N (CF 3 SO 2) 2 -, C (CF 3 SO 2) 3 ^- is a salt containing an anion ion or a combination thereof, such as. In particular, a lithium salt is preferable. Non-limiting examples of lithium salts include LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, chloroborane lithium, lower aliphatic lithium carbonate, lithium phenyl borate and the like.

Organic solvents may be used conventional solvents known in the art, such as cyclic carbonates and / or linear carbonates, non-limiting examples of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl Carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethyl carbonate (EMC) Gamma butyrolactone (GBL), fluoroethylene carbonate (FEC), methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, pentyl acetate, methyl propionate, ethyl propionate, ethyl propionate, butyl propionate or these And mixtures thereof. Moreover, the halogen derivative of the said organic solvent can also be used.

In addition, for the purpose of improving charge and discharge characteristics, flame retardancy, etc., the electrolyte includes, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, Nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol, vinylene carbonate, aluminum trichloride, etc. This may be added. In some cases, in order to impart nonflammability, a halogen-containing solvent such as carbon tetrachloride and ethylene trifluoride may be further included, or carbon dioxide gas may be further included to further improve the high temperature storage characteristics.

The present invention also provides an electrode in which a solid electrolyte interface (SEI film) film containing an ammonium containing compound or a chemical reaction product thereof is formed on part or all of the surface thereof. In this case, the SEI membrane may include ammonium ions and lithium ions at the same time.

When the electrode is charged and discharged using the above-described electrolyte, the ammonium-containing compound in the electrolyte may be automatically formed on the surface of the electrode active material together with the reversible lithium ions, or the compound is coated on the surface of the electrode active material, or the electrode material It may be made in combination with, or may be made by coating on the other electrode surface prepared.

As described above, a secondary battery having an electrode including the ammonium-containing compound or a reduction product of the compound formed on part or all of the surface of the electrode active material stabilizes the carbon material, the transition metal, and the transition metal oxide in the electrode, thereby performing the electrode active material during charge and discharge. In addition to preventing the dissolution of a portion of the transition metal from the battery, it effectively controls the exothermic reaction caused by the electrolyte reacting directly with the electrode surface when a physical shock is applied from the outside, and delays the structure collapse of the electrode active material. It can prevent ignition and rupture due to the internal temperature rise.

The method for manufacturing the electrode according to the present invention is not particularly limited, but is prepared by applying and drying a conventional method known in the art as an example, that is, an electrode slurry including a positive electrode active material or a negative electrode active material on a current collector. . At this time, a small amount of conductive agent and / or binder may be optionally added.

The positive electrode active material may be a conventional positive electrode active material that can be used in the positive electrode of a conventional secondary battery, non-limiting examples of lithium such as LiM _x O _y (M = Co, Ni, Mn, Co _a Ni _b Mn _c ) Transition metal composite oxides (for example, lithium manganese composite oxides such as LiMn ₂ O ₄ , lithium nickel oxides such as LiNiO ₂ , lithium cobalt oxides such as LiCoO ₂ , and some of the manganese, nickel and cobalt oxides of these oxides And the like, or a vanadium oxide containing lithium) or a chalcogen compound (for example, manganese dioxide, titanium disulfide, molybdenum disulfide, and the like). Preferably _{LiCoO 2, LiNiO 2, LiMnO 2} , LiMn 2 O 4, Li (Ni a Co b Mn c) O 2 (0 <a <1, 0 <b <1, 0 <c <1, a + b _{+ c = 1), LiNi 1} -Y Co Y O 2, LiCo 1-Y Mn Y O 2, LiNi 1-Y Mn Y O 2 ( here, 0≤Y <1), Li ( Ni a Co b Mn _c ) O ₄ (0 <a <2, 0 <b <2, 0 <c <2, a + b + c = 2), LiMn _{2 -z} Ni _z O ₄ , LiMn _{2 -z} Co _z O ₄ here, the 0 <Z <2), LiCoPO 4, LiFePO 4 or a mixture thereof.

In particular, the lithium secondary battery according to the present invention can exhibit excellent rate characteristics required for power sources such as vehicles. Therefore, the cathode active material is inexpensive and can be used in a large amount of active materials, and lithium manganese oxide having excellent high temperature safety is more preferable. Non-limiting examples of such lithium manganese oxides include Li _{1 + a} Mn _2-a O ₄ (where a is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ , LiMn _2-a M _a O ₂ , where M = Co, Ni, Fe, Cr, Zn or Ta, and a = 0.01 to 0.1, Li ₂ Mn ₃ MO ₈ , where M = Fe, Co, Ni, Cu, or Zn And LiMn ₂ O _{4 in} which a part of Li is substituted with alkaline earth metal ions, but is not limited thereto. These may be used in one or two or more mixtures.

The negative electrode active material can be a conventional negative electrode active material that can be used for a negative electrode of a conventional secondary battery. Examples of the negative electrode active material include lithium metal or lithium alloy, carbon, petroleum coke, activated carbon, (graphite) or other carbon-based materials. Non-limiting examples of the positive electrode current collector is a foil made by aluminum, nickel or a combination thereof, and non-limiting examples of the negative electrode current collector by copper, gold, nickel or copper alloy or a combination thereof Foils produced.

Conventional binders may be used, and non-limiting examples thereof include polyvinylidene fluoride (PVDF) or styrene butadiene rubber (SBR).

The present invention provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte is an electrolyte containing an ammonium-containing compound; The positive electrode, the negative electrode or the positive electrode is a secondary electrode characterized in that the solid electrolyte interface (SEI film) film containing the ammonium-containing compound or the chemical reaction product of the compound is formed on part or all of the surface Provide a battery.

The secondary battery is preferably a lithium secondary battery, and non-limiting examples of the lithium secondary battery include a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

The secondary battery according to the present invention can be preferably used as a power source for a vehicle, in particular, a hybrid electric vehicle, in consideration of excellent low-temperature output characteristics, high temperature storage characteristics, rating characteristics and the like. However, the present invention is not limited thereto.

The secondary battery of the present invention can be prepared by putting a porous separator between the positive electrode and the negative electrode in a conventional manner known in the art and the electrolyte.

The separator that can be used in the present invention is not particularly limited, but a porous separator may be used, for example, a polypropylene-based, polyethylene-based, or polyolefin-based porous separator. Alternatively, a porous separator into which inorganic particles are introduced may also be used.

The shape of the secondary battery manufactured by the above method is not limited, but may be cylindrical, square, pouch type or coin type using a can.

In addition, the present invention provides an electrolyte additive for forming an SEI film containing ammonium ions as a compound capable of being reduced on the negative electrode of a secondary battery to form a solid electrolyte interface (SEI) film.

Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples. However, the following examples serve to illustrate the present invention, and the scope of the present invention is not limited thereto.

[Examples 1 and 2]

Example  One

(Anode manufacture)

LiMn ₂ O ₄ was used as a cathode active material, and a cathode slurry was prepared by adding a conductive agent and a binder to N-methyl-2-pyrrolidone (NMP), and then coated on an aluminum current collector to prepare a cathode. It was.

(Cathode production)

Hard carbon was used as a negative electrode active material, a negative electrode slurry was prepared by adding a binder to NMP, and then coated on a copper (Cu) current collector to prepare a negative electrode.

(Electrolytic solution)

For electrolyte, EC / DMC / DEC solution in 1M LiPF ₆ 2 parts by weight of vinylene carbonate and 0.2 parts by weight of ammonium hexaflurophosphate were added to the electrolyte.

(Battery manufacturing)

After interposing a polyolefin-based separator between the prepared positive electrode and negative electrode, the electrolyte was injected to prepare a battery.

Example 2

A lithium secondary battery was manufactured in the same manner as in Example 1, except that 0.1 part by weight of ammonium benzoate was used instead of ammonium hexafluorophosphate.

[Comparative Examples 1 and 2]

Comparative Example 1

A lithium secondary battery was manufactured in the same manner as in Example 1, except that no electrolyte additive was used in the electrolyte.

Comparative Example 2

A lithium secondary battery was manufactured by the same method as Example 1, except that 0.1 part by weight of lithium benzoate was used instead of ammonium hexafluorophosphate.

Experimental Example 1. Comparison of Reduction Voltage of Electrolyte

In order to compare the reduction voltage of the electrolyte solution containing an ammonium-containing compound according to the present invention, the following experiment was performed.

The lithium secondary batteries of Example 1 and Example 2 having an electrolyte solution to which an ammonium-containing compound was added were used, and the battery of Comparative Example 1, in which no electrolyte solution additive was used as a control thereof, and an ammonium-free additive The battery of Comparative Example 2 was used. The dQ / dV plot was obtained while charging the prepared battery to 4.2V at 0.1C, and the reduced peak voltage measured therefrom is shown in FIG. 1.

As a result, the batteries of Comparative Examples 1 and 2, in which no electrolyte additive was used or electrolyte additives containing no ammonium ions were introduced, had a significantly high reduction voltage, whereas the present invention containing an ammonium-containing compound was carried out. It was confirmed that the electrolyte solution of the example shows a significantly low reduction voltage (see Fig. 1).

Experimental Example 2. Confirmation of Cathode SEI Film Formation by Ammonium Compound

In order to confirm the formation of the SEI film on the negative electrode by the ammonium-containing compound according to the present invention was carried out the following experiment.

As the ammonium-containing compound, a lithium full cell of Example 2 having an electrolyte solution to which ammonium benzoate was added was used, and a battery of Comparative Example 1, in which an electrolyte additive was not used as a control thereof, was used. At this time, the concentration of ammonium benzoate was changed to 0.02% by weight and 0.1% by weight, respectively.

Each battery was charged and discharged at 0.2 C at 23 ° C. three times, and then the battery was disassembled in a discharged state to collect a negative electrode. Thereafter, DSC (differential scanning calorimetry) analysis was performed on the collected negative electrode, and the results are shown in FIG. 2. For reference, the exothermic reaction observed between 100 and 140 ° C. is believed to be due to the thermal collapse of the SEI film on the surface of the cathode.

As can be seen in Figure 2 it can be seen that the exothermic reaction mode of the negative electrode is different from each other according to the type of the electrolyte additive. In this case, Figure 2 shows the exothermic onset temperature by the thermal collapse reaction of the SEI film, respectively, the high exothermic onset temperature means that the thermal stability of the SEI film formed on the cathode is excellent.

As a result, Example 1 and Example 2 having an electrolyte solution to which ammonium benzoate was added showed a significantly high exothermic onset temperature, while Comparative Example 1, in which no electrolyte additive was used, showed an extremely low exothermic onset temperature. This means that the negative electrode SEI film formed by the conventional carbonate solvent, vinylene carbonate (VC), etc. is relatively poor in terms of thermal stability. Furthermore, the data described above can be considered experimental evidence that the ammonium containing compound of the present invention is involved in the formation of the SEI film of the negative electrode.

In addition, as the amount of ammonium benzoate in the electrolyte increased, the exothermic onset temperature was higher, which means that the thermal stability of the formed SEI film is increased as the amount of the ammonium-containing compound contributing to SEI film formation is increased.

Experimental Example 3. Performance Evaluation of Lithium Secondary Battery

In order to evaluate the performance of the lithium secondary battery including the ammonium-containing compound as one component of the electrolyte according to the present invention, the following experiment was conducted.

The lithium secondary battery of Example 1 prepared by adding ammonium phosphate to the electrolyte solution was used, and the battery of Comparative Example 1 using a conventional electrolyte solution without an electrolyte additive was used as a control.

Each battery was charged in a section of 4.2 to 3V with a current of 0.5C, and the initial capacity was measured, and then stored for 12 weeks at a temperature of 60 ° C. The power conservation rate of was measured.

As a result of the experiment, it was confirmed that the battery of Example 1 using ammonium phosphate as the electrolyte solution additive had a significantly higher power conservation after the high temperature storage than the battery of Comparative Example 1 having a conventional electrolyte solution (see FIG. 3). In fact, after 12 weeks of storage at 60 ° C., the power assist ratio of the cell of Comparative Example 1 was 80.48%, whereas the battery of Example 1 containing ammonium containing compound as the electrolyte component showed an increase of about 10% or more, 90.89%. It was. This indicates that the ammonium containing compound not only can form a thermally stable and high performance uniform SEI film, but also has the property of reducing film collapse at high temperatures.

The lithium secondary battery according to the present invention can provide an improvement in long-life characteristics and high temperature storage characteristics of the battery by using an ammonium-containing compound capable of forming an SEI film with a low reduction potential as an electrolyte solution component.

Claims (10)

(a) an electrolyte salt; (b) an electrolyte solvent; And (c) an ammonium-containing compound having a lower reduction potential than the electrolyte solvent, including ammonium hexafluorophosphate, wherein the ammonium-containing compound forms a solid electrolyte interface (SEI) coating on the cathode by electrical reduction. A battery electrolyte characterized by being able to form. The electrolyte of claim 1, wherein the ammonium-containing compound is a salt capable of dissolving or dissociating at least 0.1% relative to 100% of a non-aqueous solvent. delete The electrolyte of claim 1, wherein the content of the ammonium-containing compound is in the range of 0.01 to 10 parts by weight relative to 100 parts by weight of the electrolyte. An electrode in which a solid electrolyte interface (SEI film) film containing ammonium hexafluorophosphate or a chemical reaction product thereof is formed on part or all of a surface thereof. 6. The electrode according to claim 5, wherein the solid electrolyte interfacial film contains ammonium ions and lithium ions at the same time. A secondary battery comprising a positive electrode, a negative electrode, an electrolyte and a separator, wherein the secondary battery comprises the electrolyte solution of claim 1. The secondary battery of claim 7, wherein the positive electrode comprises (a) a lithium manganese oxide capable of occluding and releasing lithium ions, and the negative electrode includes amorphous carbon. The secondary battery of claim 7, wherein the secondary battery is a lithium secondary battery. An electrolyte solution additive for forming an SEI film containing ammonium hexafluorophosphate as a compound capable of being reduced on a negative electrode of a secondary battery to form a solid electrolyte interface (SEI) film.

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