KR102568649B1 - Anode for lithium secondary battery and lithium secondary battery comprising the same - Google Patents

Abstract

An object of the present invention is to provide a positive electrode for a lithium secondary battery including a positive electrode additive compensating for an irreversible capacity of the negative electrode, a lithium secondary battery including the same, a battery pack including the same, and a device including the same. In order to achieve the above object, the present invention is a positive electrode for a lithium secondary battery, characterized in that it comprises a positive electrode active material and a positive electrode additive represented by the following formula (1) for compensating for the irreversible capacity of the negative electrode:
<Formula 1>
Li _8-x Si _1-y M _y O _6-z
(In the above formula, 0≤x≤1, 0≤y<1, 0≤z<1, M is Al, P, Ti, V, Mn, Fe, Co, Ni, Mo, Ga, Ge, As, Sn, and one metal selected from the group consisting of Sb), and a lithium secondary battery including the same, a battery pack including the same, and a device including the same. According to the present invention, the energy density reduction due to the irreversible capacity of the initial negative electrode can be effectively offset, ultimately greatly improving the characteristics of a lithium secondary battery, and the positive electrode additive used at this time has a large capacity, is stable, and furthermore Since it is inexpensive compared to materials, it has excellent advantages in terms of manufacturing process as well as characteristics of a lithium secondary battery including the same.

Description

Anode for lithium secondary battery and lithium secondary battery including the same

The present invention relates to a positive electrode for a lithium secondary battery, a lithium secondary battery including the same, a battery pack including the same, and a device.

Recently, as the technology development and demand for portable devices such as portable computers, mobile phones, smart pads, and wearable devices increase, the demand for secondary batteries as an energy source is rapidly increasing, and among such secondary batteries, high energy density and operating potential A lot of research has been conducted on a lithium secondary battery having a long cycle life and a low self-discharge rate, and it is also commercialized and widely used.

Such a lithium secondary battery has a structure in which an electrolyte containing a lithium salt is impregnated in an electrode assembly in which a porous separator is interposed between a positive electrode and a negative electrode on which an electrode active material is applied on a current collector, respectively.

Lithium-containing cobalt oxide (LiCoO ₂ ) is mainly used as a cathode active material for lithium secondary batteries, and in addition, lithium-containing manganese oxides such as LiMn ₂ O ₄ having a spinel crystal structure and lithium-containing nickel oxide (LiNiO ₂ ) are also used. .

Carbon materials are mainly used as the anode active material, and the use of lithium metal and sulfur compounds is also being considered. In particular, the theoretical specific capacity of pure silicon (Si) is 4200 mAh/g, compared to 372 mAh of graphite carbon. / g, lithium secondary batteries using the Si-based active material have attracted much attention, and some are used as electrodes mixed with carbon materials.

However, the Si-based active material as described above has a high irreversible capacity, so lithium ions are consumed, which can negatively affect the capacity of the battery, and as the number of cycles progresses, lithium ions are consumed, resulting in a decrease in cycle life. .

Therefore, when manufacturing an electrode using an active material with high irreversible capacity, a pre-lithiation process must be included so that lithium can be inserted into the irreversible capacity part. There are problems in that there is a risk, the process is complicated, and excessive costs are incurred.

Therefore, there is a high need for a technology capable of solving these problems.

In this regard, Korean Patent Laid-open Publication No. 10-2005-0030588 discloses a method for controlling a cathode termination voltage during overdischarge and a cathode active material for a lithium secondary battery, specifically Li _2+x Ni _1-y M _y O ₂ Preparation of a positive electrode including an additive represented by a chemical formula of _+a is described. According to the description of the above document, by administering a positive electrode active material additive for improving overdischarge performance, the positive electrode active material additive provides enough lithium ions or more lithium ions to compensate for the irreversibility of the negative electrode, and in particular, irreversibility toward the positive electrode during overdischarge There is an excellent effect of showing a capacity recovery of 90% or more even after overdischarge by increasing the anode voltage to prevent the cathode voltage from rising by first lowering the anode voltage. In addition, as a cathode active material additive, lithium nickel oxide of the above formula containing lithium nickel oxide in which some nickel is substituted with another element (here, y = 0 is excluded) or surface coated with an oxide other than lithium nickel oxide Lithium nickel oxide of the above formula 1 By providing a positive electrode active material for a lithium secondary battery containing, the capacity does not significantly decrease even after overdischarge without impairing the overall performance of the battery, the capacity recoverability after overdischarge is excellent, and the phenomenon of high-temperature parts can be prevented. However, the additive represented by the chemical formula Li _2+x Ni _1-y M _y O _2+a has a problem in that the capacity is not sufficient to improve the characteristics of the lithium secondary battery.

In addition, international patent application WO2016/191563 describes the use of lithium nitride as a cathode additive to reduce the decrease in reversible capacity and energy density, and specifically, a compound represented by Li _a Q _b X is used as a cathode Disclosed is a lithium secondary battery including a positive electrode used as an additive. However, materials such as Li ₃ N disclosed in the above document are highly reactive and highly ignitable, resulting in a problem in stability.

Accordingly, the inventors of the present invention have researched a positive electrode additive having a large capacity and stability while being able to compensate for the initial irreversible capacity of the negative electrode in a lithium secondary battery, leading to the present invention.

Republic of Korea Patent Publication No. 10-2005-0030588 International patent application WO2016/191563

An object of the present invention is to provide a positive electrode for a lithium secondary battery including a positive electrode additive compensating for an irreversible capacity of the negative electrode, a lithium secondary battery including the same, a battery pack including the same, and a device including the same.

In order to achieve the above object, the present invention is a positive electrode for a lithium secondary battery, characterized in that it comprises a positive electrode active material and a positive electrode additive represented by the following formula (1) for compensating for the irreversible capacity of the negative electrode:

<Formula 1>

Li _8-x Si _1-y M _y O _6-z

(In the above formula, 0≤x≤1, 0≤y<1, 0≤z<1, M is Al, P, Ti, V, Mn, Fe, Co, Ni, Mo, Ga, Ge, As, Sn, and one metal selected from the group consisting of Sb), and a lithium secondary battery including the same, a battery pack including the same, and a device including the same.

According to the present invention, the energy density reduction due to the irreversible capacity of the initial negative electrode can be effectively offset, ultimately greatly improving the characteristics of a lithium secondary battery, and the positive electrode additive used at this time has a large capacity, is stable, and furthermore Since it is inexpensive compared to materials, it has excellent advantages in terms of manufacturing process as well as characteristics of a lithium secondary battery including the same.

1 is an XRD graph of Li ₈ SiO ₆ according to the present invention;
2 is an image showing the structure of Li ₈ SiO ₆ in the (110) direction according to the present invention;
3 is a scanning electron microscope showing the particle size of Li ₈ SiO ₆ before and after ball mill treatment; is a picture,
4 and 5 are graphs showing charge and discharge characteristics of the lithium secondary battery of the present invention, and
6 is a graph showing charge/discharge characteristics of a secondary battery including a cathode using only carbon.

The present invention provides a positive electrode for a lithium secondary battery comprising a positive electrode additive compensating for the irreversible capacity of the negative electrode, and more specifically, a lithium secondary battery comprising a positive electrode active material and a positive electrode additive represented by the following Chemical Formula 1 for compensating for the irreversible capacity of the negative electrode. Anode for battery:

<Formula 1>

Li _8-x Si _1-y M _y O _6-z

(In the above formula, 0≤x≤1, 0≤y<1, 0≤z<1, M is Al, P, Ti, V, Mn, Fe, Co, Ni, Mo, Ga, Ge, As, Sn, and one metal selected from the group consisting of Sb).

Hereinafter, the positive electrode for a lithium secondary battery of the present invention will be described in more detail for each configuration.

The first irreversible capacity occurring in the conventional lithium secondary battery anode is one of the factors that reduce the energy density of the entire cathode cell in the initial cycle. The material represented by Formula 1 is a material that loses reactivity after the first oxidation, that is, a large amount of lithium is released during charging, and is a material that can effectively offset the decrease in energy density due to the irreversible capacity of the initial negative electrode. That is, even if a small amount is added to the anode side, a large amount of lithium is provided to the anode side during the first charge, which effectively offsets the decrease in energy density of the entire cell due to the initial irreversible capacity of the anode, ultimately increasing the energy density of the cell. has the effect of increasing Hereinafter, the present invention will be described in more detail.

The positive electrode for a secondary battery according to the present invention includes the compound of Formula 1 as a positive electrode additive, thereby compensating for the irreversible capacity of the negative electrode without a separate pre-lithiation process, thereby shortening the process time, improving production efficiency, and reducing manufacturing cost. has a saving effect.

The crystal structure of the compound of Formula 1 may be a hexagonal system (space group P 6 ₃ cm ), and cell parameters may be as follows:

5.3<a=b<5.6 Å, 10.5<c<10.8 Å α=β= 90°, γ = 120°.

Specifically, the positive electrode additive of Chemical Formula 1 may compensate for the irreversible capacity of the negative electrode active material by irreversible release of lithium during the initial charging and discharging process for activation of the secondary battery.

In lithium secondary batteries, lithium ions from the cathode active material are released and inserted into the carbon layer of the anode during charging, and during discharging, lithium ions from the anode carbon layer are released and inserted into the cathode active material. It serves as a medium for moving lithium ions. Such a lithium secondary battery should basically be stable in the operating voltage range of the battery and have the ability to transfer ions at a sufficiently high speed.

However, gas may be generated as the electrolyte is decomposed on the surface of the negative electrode active material during continuous charging and discharging processes. In general, lithium secondary batteries form an SEI film on the surface of the negative active material during the initial charging and discharging process to suppress additional gas generation.

The positive electrode according to the present invention includes a positive electrode additive capable of irreversibly releasing lithium during the initial charge/discharge process, thereby compensating for the irreversible capacity of the negative electrode active material without a separate prelithiation process.

As described above, the total energy density of the battery is increased by compensating for the initial irreversible capacity of the negative electrode.

The inventors of the present invention found that the positive electrode additive, which is the lithium silicon-based oxide of Formula 1, is a lithium-rich oxide, and excessive ions can be irreversibly released during the first charge based on the oxidation mechanism of oxygen ions to exhibit high capacity, After that, a small amount of capacity was observed during the discharge of the second cycle, and it was confirmed that there was almost no reaction during charge/discharge from the next cycle. , it was confirmed that the characteristics of the lithium secondary battery can ultimately be greatly improved by compensating for the initial irreversible capacity of the negative electrode.

In Chemical Formula 1 of the positive electrode additive included in the positive electrode for a lithium secondary battery according to the present invention, when x exceeds 1, there is a problem in that the capacity decreases, and when z is greater than 1, the amount of oxygen that is bonded so that lithium can exist in the structure Due to the reduction, the instability of the structure increases and the amount of oxygen that can undergo an oxidation reaction decreases, so there is a problem in that the capacity decreases as above, and the above components listed for M combine with four oxygens to form a tetrahedron. It is preferable in that it is suitable for a small amount of substitution in place.

Meanwhile, the positive electrode additive included in the positive electrode for a lithium secondary battery according to the present invention is preferably Li ₈ SiO ₆ . The material is preferably used as a cathode additive in the present invention in that it is based on Si, which is a relatively light element, and has a large amount of lithium per molecular weight, so that it can realize a higher capacity than other cathode additives during initial oxidation.

The positive electrode for a lithium secondary battery according to the present invention preferably includes 0.1 to 10 parts by weight of a positive electrode additive based on 100 parts by weight of the positive electrode active material. If the amount of the positive electrode additive is less than 0.1 parts by weight, there is a problem in that energy density reduction due to the irreversible capacity of the initial negative electrode cannot be effectively offset, and if it exceeds 10 parts by weight, energy density decreases.

Meanwhile, the positive electrode for a lithium secondary battery of the present invention preferably further includes a binder and a conductive material.

The conductive material may be added in an amount of 1 to 30% by weight based on the total weight of the mixture including the cathode active material. Such a conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery, and examples thereof include graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskeys such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives may be used.

The binder is a component that assists in the binding of the active material and the conductive material and the binding to the current collector, and may be typically added in an amount of 1 to 20% by weight based on the total weight of the mixture including the positive electrode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butyrene rubber, fluororubber, various copolymers, and the like.

In some cases, a filler may optionally be further included as a component that suppresses expansion of the positive electrode, and the filler is not particularly limited as long as it is a fibrous material without causing chemical change in the battery. For example, polyethylene, olefin polymers such as polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

In addition, the present invention provides a lithium secondary battery including the positive electrode, the negative electrode, a separator positioned between the positive electrode and the negative electrode, and an electrolyte.

The active material included in the negative electrode may include carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene, and activated carbon; silicon-based materials such as silicon, silicon oxide, and silicon-carbon-based materials; Li _x Fe ₂ O ₃ (0≤x≤1), Li _x WO ₂ (0≤x≤1), Sn _x Me _1-x Me' _y O _z (Me: Mn, Fe, Pb, Ge; Me' : Metal complex oxides such as Al, B, P, Si, elements of groups 1, 2, and 3 of the periodic table, halogens, 0<x0≤1;0≤y≤3;0≤z≤8); lithium metal; lithium alloy; silicon-based alloys; tin-based alloys; SnO, SnO ₂ , PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , Sb ₂ O ₃ , Sb ₂ O ₄ , Sb ₂ O ₅ , GeO, GeO ₂ , Bi ₂ O ₃ , Bi ₂ O ₄ , and metal oxides such as Bi ₂ O ₅ ; conductive polymers such as polyacetylene; Li-Co-Ni based materials; titanium oxide; It may include lithium titanium oxide and the like, but is not limited thereto.

In one specific example, the anode may include a silicon (Si)-based material as an anode active material, and the silicon-based material may be a composite of silicon and silicon oxide and/or a silicon alloy.

The silicon-based material has a high irreversible capacity, so it is common to undergo a prelithiation process, but the secondary battery according to the present invention has the advantage of not requiring a separate prelithiation process because a specific cathode additive is included in the cathode. .

When another negative active material is further included in addition to the silicon-based material, the other negative active material may be included in an amount of 20% to 80% by weight based on the total weight of the negative electrode active material. Such additional negative electrode active materials may be selected from the materials exemplified above, and specifically, may be carbon-based materials.

The separator is located between an anode and a cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally 001 μm to 10 μm, and the thickness is generally 5 μm to 300 μm. Examples of such a separator include olefin-based polymers such as chemical-resistant and hydrophobic polypropylene; A sheet or non-woven fabric made of glass fiber or polyethylene is used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may serve as a separator.

Various electrolytes may be used as the electrolyte, for example, a lithium salt-containing non-aqueous electrolyte, an organic solid electrolyte, an inorganic solid electrolyte, and the like may be used. Accordingly, the lithium secondary battery according to the present invention may be a lithium ion secondary battery, a lithium ion polymer secondary battery, or a lithium polymer secondary battery.

The present invention also provides a battery pack including the secondary battery as a unit cell, and a device including the battery pack as a power source.

The device may be selected from a mobile phone, a laptop computer, a tablet PC, a wearable electronic device, an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or a power storage system, but is not limited thereto.

Since the structures and manufacturing methods of these devices are well known in the art, a detailed description thereof is omitted in the present specification.

Hereinafter, the present invention will be described in more detail through Examples and Experimental Examples. However, the following examples and experimental examples are only intended to specifically explain the present invention, and the scope of the present invention is not limited by the description below.

<Production Example>

Manufacture of positive electrode additives

Li2O and SiO2 were mixed in a molar ratio of 4:1, and then pulverized using a ball-mill. Heat treatment was performed at a temperature of 700 °C for 12 hours at a rate of 2 °C per minute (2 °C/min) in an oxygen atmosphere using a tube-type furnace. Carbon (Super C carbon, TIMCAL) was mixed with the heat-treated powder at a weight ratio of Li ₈ SiO ₆ :C = 1:3 and ball milled to adjust the particle size. XRD was confirmed for the prepared Li8SiO6 by the following process, and the results are shown in FIG. 1. According to the XRD graph of FIG. 1, it can be seen that it was well synthesized in agreement with the XRD pattern of the previously reported Li8SiO6 crystal structure information [ICSD-65176].

2 is an image showing the structure of Li ₈ SiO ₆ prepared in the present invention in the (110) direction. According to FIG. 2, it can be confirmed that in addition to the oxygen atoms bonded to Si in a tetrahedral form, oxygens not bonded to Si exist in the structure, and since the oxygens are relatively easy to oxidize, it can be expected that lithium will be extracted during charging. It can be seen that the crystal structure of

3 is a scanning electron micrograph comparing Li ₈ SiO ₆ prepared in the present invention before and after carbon addition and ball mill treatment. According to FIG. 3 , it can be seen that compared to the Li ₈ SiO ₆ particle (a), the size of the carbon ball-milled particle (b) is uniformly reduced as a whole.

<Example>

Manufacture of lithium secondary battery

10 mg of the prepared Li ₈ SiO ₆ :carbon mixture was pressed under a pressure of 8000 psi to prepare a pellet type electrode. This electrode was used as an anode and lithium metal as a cathode, and a glass fiber separator was used. As an electrolyte, a cell was prepared using 2 ml of 1 M LiPF6 EC:DMC (1:1 v/v).

<Comparative Example>

Li ₈ SiO ₆ prepared in the above preparation example: Same method as in the above example, except that a pellet type electrode was prepared using 10 mg of the carbon (Super C carbon, TIMCAL) used in the preparation example instead of the carbon mixture A cell was made with

<Experimental Example 1>

For the lithium secondary battery manufactured according to the above example, a charge/discharge experiment was performed starting from charging with a constant current in a voltage range of 2.75 V - 4.5 V (vs Li/Li ⁺ ) and a current density of 10 mA/(g of Li8SiO6 ) 4 (a: charge/discharge graph, b: initial 10 cycle capacity). In addition, with respect to the lithium secondary battery manufactured according to the above example, the voltage range of 2.75 V - 4.2 V (vs Li/Li ⁺ ) and the current density of 10 mA/(g of Li ₈ SiO ₆ ) A /discharge experiment was performed, and the results are shown in FIG. 5 (a: charge/discharge graph, b: initial 10 cycle capacity).

According to FIGS. 4 and 5 , it can be seen that after the excess lithium ions are irreversibly discharged during the first charge, a small amount of capacity is observed during charge/discharge from subsequent cycles. As a result of charging up to an oxidation voltage of 4.5 V versus lithium in FIG. 4 and up to 4.2 V in FIG. 5, the capacities of about 450 mAh/g and 300 mAh/g in the first charging cycle, respectively, are shown. In particular, the voltage range of FIG. 5 is a condition consistent with the current commercially available charge/discharge voltage range, and therefore, the material of Chemical Formula 1 compensates for the initial irreversible capacity of the negative electrode to ultimately improve the characteristics of the lithium secondary battery. As a positive electrode additive that can be confirmed that it meets the purpose.

<Experimental Example 2>

In order to confirm the capacity of only carbon used as a conductive material in the present invention, an experiment was performed in the same manner as in Experimental Example 1 using a cell manufactured by Comparative Example (voltage range 2.75 V - 4.5 V (vs Li/ Li ⁺ )), and the results are shown in FIG. 6 (a: charge/discharge graph, b: initial 10 cycle capacity). Referring to FIG. 6 , it can be seen that the capacity of the conductive carbon is about 30 mAh/g at the time of first charge in the 4.5 V voltage range, and about 15 mAh/g is produced when limited to the 4.2 V voltage range. Based on this, if the capacity of only Li ₈ SiO ₆ excluding the effect of the conductive material is inferred, the first charge capacity in the 4.5 V range is at least about 420 mAh / g, and the first charge capacity in the 4.2 V range is at least about 285 mAh / g can confirm that In addition, the capacity of Li ₈ SiO ₆ only after the first cycle can be inferred to a lower value if the capacity due to the conductive material is taken into account, so the effect of Li ₈ SiO ₆ on the cell during subsequent charge/discharge is negligible. can be expected to

Claims (10)

In a lithium secondary battery including a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode, and an electrolyte,
The positive electrode includes a positive electrode active material and a positive electrode additive represented by Formula 1 to compensate for irreversible capacity of the negative electrode,
The crystal structure of the positive electrode additive is hexagonal (space group P6 ₃ cm),
Lithium secondary battery, characterized in that the negative electrode is not pre-lithiated:
<Formula 1>
Li _8-x Si _1-y M _y O _6-z
(In the above formula, 0≤x≤1, 0≤y<1, 0≤z<1, M is Al, P, Ti, V, Mn, Fe, Co, Ni, Mo, Ga, Ge, As, Sn, and one metal selected from the group consisting of Sb).
The lithium secondary battery according to claim 1, wherein the positive electrode additive is Li ₈ SiO ₆ .
The lithium secondary battery according to claim 1, wherein the cathode additive is included in an amount of 0.1 to 10 parts by weight based on 100 parts by weight of the cathode active material.
The lithium secondary battery according to claim 1, wherein the positive electrode for the lithium secondary battery further comprises a binder and a conductive material.
delete delete delete The lithium secondary battery according to claim 1, wherein the anode comprises a silicon (Si)-based material as an anode active material.
A battery pack comprising the lithium secondary battery according to claim 1 as a unit cell.
A device comprising the battery pack according to claim 9 as a power source.