【TECHNICAL FIELD】
Cross Citation with Related Application(s)
This application claims the benefit of Korean Patent Application No. 10-2021-0023120
filed on February 22, 2021 with the Korean Intellectual Property Office, the content of which is
incorporated herein by reference in its entirety.
10 The present disclosure relates to a positive electrode for a lithium secondary battery with
a primer layer comprising lithium iron phosphate, and a lithium secondary battery comprising the
same
【BACKGROUND】
Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative
15 energy or clean energy is increasing, and as part thereof, the fields that are being studied most
actively are the fields of power generation and power storage using electrochemistry.
At present, a secondary battery is a representative example of an electrochemical device
that utilizes such electrochemical energy, and the range of use thereof tends to be gradually
expanding.
20 Recently, along with the increase of the technological development and demand for mobile
devices such as portable computers, portable phones, and cameras, the demand for secondary
batteries has also sharply increased as an energy source. Among such secondary batteries is a
lithium secondary battery exhibiting high charge/discharge characteristics and lifespan
characteristics and being environmentally friendly, in which much research has been carried out
25 and which is now commercialized and widely used.
Generally, the secondary battery is manufactured by impregnating a non-aqueous
electrolyte solution into an electrode assembly comprising a positive electrode, a negative
electrode, and a porous separator.
A carbon material is mainly used as a negative electrode active material of such a lithium
secondary battery, and the use of lithium metal, sulfur compounds, etc. is also being considered.
Also, as a positive electrode active material, lithium-containing cobalt oxide (LiCoO2) is mainly
used, and additionally, the use of lithium-containing manganese oxides such as LiMnO2 having a
5 layered crystal structure and LiMn2O4 having a spinel crystal structure, and lithium-containing
nickel oxide (LiNiO2) is being considered.
Moreover, in recent years, a lithium iron phosphate (LiFePO4)-based compound having
excellent thermal stability and being relatively inexpensive may be used as a positive electrode
active material.
10 However, such this lithium iron phosphate has low energy density compared to other
materials despite the advantages of excellent thermal stability and low price, and is therefore
unsuitable for use in products requiring high energy density.
Further, since the lithium iron phosphate has a drawback of low electrical conductivity and
ionic conductivity, it is applied by coating the surface of the lithium iron phosphate with carbon
15 to improve electrical conductivity, and reducing the particle size to improve the ionic conductivity.
However, in the case of reducing the particle size in this manner, the specific surface area increases
and also aggregation occurs severely, which causes that the dispersibility is lowered by the existing
mixing process and the slurry solid content is lowered.
Therefore, there is an urgent need to develop a technology for a positive electrode that can
20 have excellent characteristics as a whole without deteriorating special performance of the lithium
secondary battery.
【DETAILED DESCRIPTION OF THE INVENTION】
【Technical Problem】
The present disclosure has been designed to solve the above-mentioned problems and other
25 technical problems that have yet to be resolved.
Specifically, an object of the present disclosure is to provide a positive electrode for a lithium
secondary battery which can realize high energy density while improving thermal stability, and at
the same time, has excellent slurry characteristics for the production of positive electrodes, and a
lithium secondary battery comprising the same
【Technical Solution】
In order to achieve the above object, according to one embodiment of the present disclosure,
there is provided a positive electrode for a lithium secondary battery comprising:
a positive electrode current collector,
5 a primer coating layer formed on the positive electrode current collector, and
a positive electrode active material layer formed on the primer coating layer,
wherein the primer coating layer comprises lithium iron phosphate, a binder, a conductive
material and a dispersant,
wherein the lithium iron phosphate is represented by the following Chemical Formula 1,
10 wherein the dispersant is a hydrogenated nitrile butadiene rubber (HNBR), and
wherein the hydrogenated nitrile butadiene rubber is contained in an amount of 0.4 to 2.0
wt% based on the weight of lithium iron phosphate.
[Chemical Formula 1]
Li1+aFe1-sMs(PO4-b)Xb
15 wherein,
M is at least one element selected from Co, Ni, Al, Mg, Ti and V,
X is F, S, or N, and
0≤s≤0.5; -0.5≤a≤+0.5; 0≤b≤0.1.
In one specific embodiment, the lithium iron phosphate may be composed of primary
20 particles, or may be a mixture of the primary particles and secondary particles in a state where the
primary particles are aggregated.
The primary particles of the lithium iron phosphate may have an average diameter (D50)
of 0.2 to 3.0 ㎛, and the secondary particles may have an average diameter (D50) of 7 to 25 ㎛.
The lithium iron phosphate may be in a state where the primary particles and/or secondary
25 particles are carbon-coated.
In one specific embodiment, the content of acrylonitrile (AN) of the hydrogenated nitrile
butadiene rubber may be 20 to 50 wt%, and the residual double bond (RDB) present in the
hydrogenated nitrile butadiene rubber may be contained in an amount of 30% or less.
Further, the hydrogenated nitrile butadiene rubber may have a weight average molecular
weight (Mw) of 10,000 to 250,000.
In one specific embodiment, the binder included in the primer coating layer may be
polyvinylidene fluoride (PVdF) having a weight average molecular weight (Mw) of 500,000 to
5 1,200,000.
In one specific embodiment, the primer coating layer may be formed to a thickness of 1
to 5 ㎛, and the positive active material layer may be formed to a thickness of 50 to 300 ㎛.
In one specific embodiment, the positive electrode active material layer may include a
lithium transition metal oxide as a positive electrode active material, and the lithium transition
10 metal oxide may include at least one transition metal selected from the group consisting of Ni, Mn,
and Co.
According to one embodiment of the present disclosure, there is provided a lithium
secondary battery comprising the positive electrode for a lithium secondary battery.
【DETAILED DESCRIPTION OF THE EMBODIMENTS】
15 Hereinafter, the present disclosure will be described in more detail for a better understanding
of the present disclosure.
Terms or words used in the present specification and claims should not be construed as limited
to ordinary or dictionary terms, and the present disclosure should be construed with meanings and
concepts that are consistent with the technical idea of the present disclosure based on the principle
20 that the inventors may appropriately define concepts of the terms to appropriately describe their
own disclosure in the best way.
The technical terms provided herein is merely used for the purpose of describing particular
embodiments only, and is not intended to be limiting of the present disclosure. The singular forms
“a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly
25 indicates otherwise.
As used herein, the terms "comprise", "include" or "have", etc. are intended to designate the
existence of a specific feature, number, step, constitutional element and a combination thereof, but
does not exclude the presence or addition of a different specific feature, number, step,
constitutional element and a combination thereof step.
According to one embodiment of the present disclosure, there is provided a positive electrode
for a lithium secondary battery comprising:
a positive electrode current collector,
a primer coating layer formed on the positive electrode current collector, and
5 a positive electrode active material layer formed on the primer coating layer,
wherein the primer coating layer comprises lithium iron phosphate, a binder, a conductive
material and a dispersant,
wherein the lithium iron phosphate is represented by the following Chemical Formula 1,
wherein the dispersant is a hydrogenated nitrile butadiene rubber (HNBR), and
10 wherein the hydrogenated nitrile butadiene rubber is contained in an amount of 0.4 to 2.0
wt% based on the weight of lithium iron phosphate.
[Chemical Formula 1]
Li1+aFe1-sMs(PO4-b)Xb
wherein,
15 M is at least one element selected from Co, Ni, Al, Mg, Ti and V,
X is F, S, or N, and
0≤s≤0.5; -0.5≤a≤+0.5; 0≤b≤0.1.
Here, the primer coating layer according to the present disclosure not only improves the
adhesive force and electrical conductivity between the current collector and the positive electrode
20 active material layer, but also is a layer applied to improve thermal stability, which is coated with
a thin thickness different from that of a general positive electrode active material layer.
The primer coating layer for this purpose includes lithium iron phosphate, a binder, a
conductive material, and a dispersant.
At this time, the lithium iron phosphate may be specifically LiFePO4, and the lithium iron
25 phosphate may be composed of primary particles or a mixture of the primary particles and
secondary particles in a state where the primary particles are aggregated.
That is, the lithium iron phosphate tends to aggregate, and is mainly used in the form of
secondary particles, but according to the present disclosure, the ionic conductivity can be increased
by using primary particles having a relatively small size.
At this time, the primary particles have an average diameter (D50) of 0.2 to 3.0 ㎛, specifically
0.2 to 1.0 ㎛, more specifically, 0.3 to 0.8 ㎛, and the secondary particles may have an average
diameter (D50) of 2 to 10 ㎛, specifically 2 to 5 ㎛.
When the primary particle is too small outside the above range, the dispersibility is
5 significantly reduced and aggregated, which makes it difficult to prepare the primary particles, and
when the primary particle is too large, the difference from the secondary particles is small and the
ionic conductivity may be lowered, which is not preferable. Even when the secondary particles
are large, it is difficult to make the coating layer of the present disclosure uniform, and the ionic
conductivity may be lowered, which is not preferable.
10 The average diameter (D50) means a particle diameter corresponding to a point of n% in the
cumulative distribution of the number of particles relative to the particle diameter. That is, D50 is
the particle diameter corresponding to a point of 50% in the cumulative distribution of the number
of particles relative to the particle diameter.
The average diameter (D50) can be measured by using a laser diffraction method.
15 Specifically, the powder to be measured is dispersed in a dispersion medium, and then introduced
into a commercially available laser diffraction particle size analyzer (e.g., Mastersizer 3000
available from Malvern). When the particles pass through the laser beam, the diffraction pattern
difference according to the particle size is measured to calculate the particle size distribution. The
D50 can be measured by calculating the particle diameter corresponding a point of 50% in the
20 cumulative distribution of the number of particles relative to the particle diameter in the analyzer.
Meanwhile, since the lithium iron phosphate oxide is used for the primer coating layer, it
is preferably excellent in electrical conductivity. Thus, specifically, the primary particles and/or
secondary particles of the lithium iron phosphate may be in a carbon-coated form.
In this manner, the lithium iron phosphate used for the primer coating layer has a small
25 size, and has a problem that the dispersibility is greatly lowered only by the existing slurry mixing
step and the solid content of the slurry is lowered, and thus improvement thereof is needed.
Therefore, the primer coating layer according to the present disclosure contains a
dispersant together with the lithium iron phosphate, wherein the dispersant may be a hydrogenated
nitrile butadiene rubber (HNBR).
The hydrogenated nitrile butadiene rubber (HNBR) means that the double bond originally
contained in the nitrile butadiene rubber (NBR) is changed to a single bond by hydrogenating the
nitrile butadiene rubber (NBR).
At this time, the content of acrylonitrile (AN) of the hydrogenated nitrile butadiene rubber
5 may be 20 to 50 wt%, specifically 30 to 40 wt%, based on the total weight of the hydrogenated
nitrile butadiene rubber (HNBR).
The acrylonitrile (AN) is polar, and the hydrogenated nitrile butadiene is non-polar. Thus,
when the content of the acrylonitrile (AN) is too small or too large, dispersion in a solvent is not
preferable when preparing the primer coating layer slurry.
10 Further, the residual double bond (RDB) present in the hydrogenated nitrile butadiene
rubber may be 30% or less, specifically 20% or less, more specifically 10% or less, and most
specifically 5% or less.
When the ratio of the residual double bond (RDB) is too large outside the above range, it
may decrease the phase stability such as an increase in the possibility of side reactions with the
15 electrolyte and gelation of the primer coating layer slurry, which is not preferable.
Further, the weight average molecular weight (Mw) of the hydrogenated nitrile butadiene
rubber may be 10,000 to 250,000, specifically, 150,000 to 250,000.
Meanwhile, in order to minimize the problem of acting as a resistor while improving the
dispersibility of the lithium iron phosphorus oxide, the hydrogenated nitrile butadiene rubber may
20 be contained in an amount of 0.4 to 2.0 wt%, specifically, 0.5 to 1.0 wt% based on the weight of
lithium iron phosphate.
When the hydrogenated nitrile butadiene rubber (HNBR) is contained in an amount of less
than 0.4%, the surface area of lithium iron phosphate increases as the dispersed particle size
decreases, and the dispersant does not sufficiently cover the surface of the increased lithium iron
25 phosphate and thus, the slurry viscosity may increase significantly. When the content is more than
2.0%, an excess of the dispersant that is not adsorbed to the surface of the lithium iron phosphate
may be present in the solvent, which may cause an increase in slurry viscosity.
Further, the primer coating layer includes a conductive material and a binder in addition to
the lithium iron phosphate and the dispersant.
The conductive material is not particularly limited as long as it has conductivity without
causing a chemical change in the corresponding battery, and for example, such as natural graphite
or artificial graphite; carbon blacks such as carbon black, acetylene black, ketjen black, channel
black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and
5 metal fiber; metal powders such as carbon fluoride powder, aluminum powder, and nickel powder;
conductive whiskey such as zinc oxide and potassium titanate; conductive metal oxides such as
titanium oxide; conductive materials such as polyphenylene derivatives can be used.
Specific examples of commercially available conductive materials include acetylene black
series products available from Chevron Chemical Company, Denka Singapore Private Limited,
10 Gulf Oil Company, Ketjen black, EC series products available from Armak Company, Vulcan XC72 available from Cabot Company and Super P available from Timcal, and the like.
Wherein, the content of the conductive material may be contained in an amount of 1 to 30
wt%, specifically 1 to 10 wt%, more specifically 1 to 5 wt% based on the total weight of the primer
coating layer.
15 The binder is a type of binder known in the art, and is not limited as long as it is a type
capable of improving the adhesive force of the electrode components. Examples thereof may be
at least one selected from the group consisting of polyvinylidene fluoride, polyvinylidene fluoridehexafluoropropylene, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropyl
cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene,
20 polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene
rubber, and fluorine rubber.
Specifically, the binder may be polyvinylidene fluoride (PVDF), and more specifically,
polyvinylidene fluoride (PVDF) having a weight average molecular weight (Mw) of 500,000 to
1,200,000, specifically 700,000 to 1,000,000.
25 When the PVDF is contained, it can have the best phase stability with lithium iron
phosphate, which is preferable.
When PVDF having a weight average molecular weight satisfying the above range is used,
problems such as a decrease in solid content due to increase in the viscosity of the primer coating
layer slurry do not occur while being able to have high adhesive force, which is preferable.
Wherein, the binder may be contained in an amount of 1 to 10 wt%, specifically 1 to 5
wt%, based on the total weight of the primer coating layer. When the content of the binder is too
small outside the above range, the adhesive force is too low, making it difficult to maintain the
coating layer, and when the content is too large, the resistance can be high.
5 The primer coating layer may be formed to a thickness of 1 to 20 ㎛, specifically 1 to 10
㎛, and more specifically 1 to 5 ㎛.
When the primer coating layer is formed too thin outside the above range, it is not possible
to sufficiently secure the thermal stability to be obtained by applying it according to the present
disclosure, and when the primer coating layer is formed too thick, it acts like an active material
10 layer rather than serving as a primer layer, whereby the volume of the positive electrode active
material layer of the present disclosure is relatively reduced in the same volume, and therefore,
the content of the positive electrode active material capable of increasing the energy density is
reduced, which is not preferable in terms of secondary battery performance.
Meanwhile, the positive electrode active material layer formed on the primer coating layer
15 may further include a positive electrode active material, and may further include the conductive
material and the binder, and optionally, may further include an additive such as a filler.
In this case, examples of the conductive material and the binder are the same as described
above, and may include the same or different materials from the conductive material and the
binder included in the primer coating layer.
20 The positive active material may include a lithium transition metal oxide.
The lithium transition metal oxide may include, for example, a layered compound such as
lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2) or a compound substituted with
one or more transition metals; lithium manganese oxides such as chemical formulae Li1+xMn2-xO4
(where, x is 0 ~ 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium
25 oxides such as LiV3O8; lithium iron oxides such as LiFe3O4, a Ni-site type lithium nickel oxide
represented by chemical formula LiNi1-xMxO2 (where, M = at least one selected from Co, Mn, Al,
Cu, Fe, Mg, B and Ga, and x= 0.01 ~ 0.3); lithium manganese composite oxide represented by
chemical formulae LiMn2-xMxO2 (where, M = at least one selected from Co, Ni, Fe, Cr, Zn or Ta,
and x = 0.01 ~ 0.1) or Li2Mn3MO8 (where, M = at least one selected from Fe, Co, Ni, Cu or Zn );
lithium manganese composite oxide having a spinel structure represented by LiNixMn2-xO4,
LiMn2O4 in which a portion of Li in the chemical formula is substituted with an alkaline earth
metal ion, and the like.
Specifically, it may be a lithium transition metal oxide including at least one transition
5 metal selected from the group consisting of Ni, Mn, and Co.
That is, according to the present disclosure, a positive electrode active material layer is
formed on a primer coating layer containing lithium iron phosphate and a dispersing agent, and
the positive electrode active material layer has a structure that does not contain lithium iron
phosphate and includes lithium transition metal oxide as a positive electrode active material. If
10 the positive electrode active material layer also uses lithium iron phosphate as the positive
electrode active material, it can be used to a voltage drive range of 3.6V or less, wherein the energy
density is lowered. Thus, in the present disclosure, the lithium iron phosphate is preferably
included only in the primer coating layer.
The thickness of the positive active material layer is not limited, but, for example, may be
15 formed to a thickness of 50 to 300 ㎛.
Meanwhile, the positive electrode current collector is not particularly limited as long as it has
conductivity while not causing chemical changes to the battery, and for example, stainless steel,
aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel having a surface
treated with carbon, nickel, titanium, silver, and the like can be used. In addition, the first positive
20 electrode current collector may have a thickness of 3 to 500 ㎛, and may have fine irregularities
formed on the surface of the current collector to increase the adhesive force of the first positive
electrode active material. For example, it may be used in various forms such as films, sheets, foils,
nets, porous bodies, foams, and nonwoven fabrics.
Meanwhile, according to another embodiment of the present disclosure, there is provided
25 a lithium secondary battery including the positive electrode for the lithium secondary battery.
The lithium secondary battery has a structure in which an electrode assembly including a
negative electrode together with the positive electrode and a separator interposed between the
positive electrode and the negative electrode is incorporated in a battery case together with a
lithium non-aqueous electrolyte.
Other components and structures, etc. of such a lithium secondary battery are incorporated
in the present disclosure with the contents known in the art, and, therefore, detailed description
thereof will be omitted.
Hereinafter, preferred examples of the present disclosure, comparative examples for
5 comparing them, and experimental examples for evaluating them are described. However, it will
be obvious to those skilled in the art that these examples are for illustrative purposes only and
various changes and modifications can be made without deviating from the scope and spirit of the
present description, and it goes without saying that such modifications and modifications fall
within the scope of the appended claims.
10

Primary particle LiFePO4 (average diameter (D50): 0.5±0.15 ㎛), HNBR dispersant (Mw:
220,000±30,000, AN content: 35±3wt%, RDB 0~5%) were mixed in a weight ratio of 100:1 in a
NMP solvent so that the solid content was 61%, thereby preparing a slurry.
15

Primary particles LiFePO4 (average diameter (D50): 0.5±0.15 ㎛) were mixed in an NMP
solvent so that the solid content was 61%%, thereby preparing a slurry.
20
The viscosity of the slurry prepared in Preparation Examples 1 and 2, and the average
diameter (D50) of the LFP particles were measured, and shown in Table 1 below.
The average diameter of the LFP particles was measured by using a laser diffraction
particle size analyzer (e.g., Mastersizer 3000 available from Malvern). When passing the particles
25 through the laser beam, the diffraction pattern difference according to the particle size was
measured to calculate the particle size distribution. The D50 was measured by calculating the
particle diameter corresponding a point of 50% in the cumulative distribution of the number of
particles relative to the particle diameter in the analyzer.
[Table 1]
Viscosity (cps) LFP PSD(D50)
Preparation Example 1 1980 0.88
Preparation Example 2 9370 1.50
Referring to Table 1 above, it can be confirmed that due to the inclusion of the dispersant,
LFP is well dispersed and there is less aggregation between particles, so that D50 is still similar to
the size of the primary particles and the viscosity is not high, whereas Preparation Example 2 does
not include the dispersant, the particle diameter and viscosity increase as the LFP aggregates with
5 each other.

Primary particles LiFePO4 (average diameter (D50): 0.5±0.15 ㎛), HNBR dispersant (Mw:
220,000±30,000, AN content: 35±3 wt%, RDB 0~5%), a conductive material (carbon black) and
10 a binder (PVDF, Mw: 700,000±50,000) were mixed at a weight ratio of 94.5:1:1.5:3 in an NMP
solvent to prepare a primer slurry. At this time, the solid content was set to 45%.
The slurry was coated onto Al foil (thickness: 12 ㎛) to a thickness of 5 ㎛ and dried to
form a primer coating layer.
Then, LiCoO2 as a positive electrode active material, a conductive material (carbon black),
15 and a binder (PVDF, Mw: 700,000±50,000) were mixed at a weight ratio of 95.5:1.5:3 in an NMP
solvent to prepare an active material slurry.
The active material slurry was coated onto the primer coating layer to a thickness of 100
㎛ and dried to form an active material layer, thereby producing a positive electrode.
20
A positive electrode was produced in the same manner as in Example 1, except that in
Example 1, primary particles LiFePO4 (average diameter (D50): 0.5±0.15 ㎛), HNBR dispersant
(Mw: 220,000±30,000, AN content: 35±3wt%, RDB 0~5%), a conductive material (carbon black)
and a binder (PVDF, Mw: 700,000±50,000) were mixed at a weight ratio of 88.1:0.9:3:8 in an
25 NMP solvent to prepare a primer slurry, wherein the solid content was set to 45%.

A positive electrode was produced in the same manner as in Example 1, except that in
Example 1, primary particles LiFePO4 (average diameter (D50): 0.5±0.15 ㎛), a conductive
5 material (carbon black) and a binder (PVDF, Mw: 700,000±50,000) were mixed at a weight ratio
of 95.5:1.5:3 in an NMP solvent to prepare a primer slurry, wherein the solid content was set to
45%.

10 A positive electrode was produced in the same manner as in Example 1, except that the
primer slurry was coated to a thickness of 25 ㎛, and the active material slurry was coated to a
thickness of 80 ㎛.

15 A positive electrode was produced in the same manner as in Example 1, except that the
primer slurry was coated to a thickness of 10 ㎛, and the active material slurry was coated to a
thickness of 90 ㎛.

20 A positive electrode was produced in the same manner as in Example 1, except that the
primer slurry was not coated, and the active material slurry was coated to a thickness of 105 ㎛.

A positive electrode was produced in the same manner as in Example 1, except that the
25 primer slurry prepared in Example 1 was coated and dried to a thickness of 105 ㎛, and the active
material slurry was not applied.

MCMB (mesocarbon microbead), which is artificial graphite as a negative electrode active
material, carbon black conductive material, SBR binder and CMC were mixed in a weight ratio of
95.8:0.5:2.5:1.2 in H2O solvent to prepare a negative electrode active material slurry, which was
then applied to a thickness of 65 ㎛ onto a copper foil (thickness: 8 ㎛) to produce a negative
5 electrode.
A separator of porous polyethylene was interposed between the positive electrode and the
negative electrode produced in Examples 1 to 3 and Comparative Examples 1 and 2 as described
above to prepare an electrode assembly. The electrode assembly was placed inside a case, and an
electrolyte solution was injected into the case to produce a lithium secondary battery. At this time,
10 the electrolyte solution was prepared by dissolving 1.0M concentration of lithium
hexafluorophosphate (LiPF6) in an organic solvent consisting of ethylene carbonate / dimethyl
carbonate / ethyl methyl carbonate (mixed volume ratio of EC / DMC / EMC = 3 / 4 / 3).
Resistance: the lithium secondary battery prepared above was charged up to 4.35V/38mA
at 1C under constant current/constant voltage (CC/CV) conditions at a temperature of 25°C, and
15 then discharged up to 2.5V at 2C under constant current (CC) conditions to confirm the discharge
capacity. Next, after charging in the same manner as above, the discharge was performed only by
50% of the previously confirmed discharge capacity. Then, after discharging at 2.5C for 10
seconds, the resistance was calculated using the voltage difference before and after discharge.
High-temperature capacity retention rate (%): the lithium secondary battery prepared
20 above was charged up to 4.35V/38mA at 1C under constant current/constant voltage (CC/CV)
conditions at a temperature of 45°C, and then discharged up to 2.5 V at 2 C under constant current
(CC) conditions to measure the discharge capacity. This process was repeated by 1 to 200 cycles.
A value calculated by (capacity after 200 cycles/capacity after 1 cycle) x 100 was expressed as a
high-temperature capacity retention rate (%).
25 Impact: the lithium secondary battery prepared above was fully charged up to 4.35V/38mA
at 1C under constant current/constant voltage (CC/CV) conditions at room temperature, and then
9.8kg bar was freely dropped at a height of 61 cm to apply an impact to the cell. When maintained
for 1 hour after the impact was applied, it was judged as Pass if the temperature dropped to less
than 50 degrees.
Nail penetration: the lithium secondary battery prepared above was fully charged up to
4.35V/38mA at 1C under constant current/constant voltage (CC/CV) conditions at room
temperature, and then nail penetration experiment was performed under GB/T conditions (nail
diameter of 3mm, penetration speed of 150mm/sec). When maintained for 1 hour after nail
5 penetration, and it was judged as Pass if the temperature dropped to less than 50 degrees.
The measurement results are shown in Table 2 below.
[Table 2]
Resistance
(ohm)
Hightemperature
capacity
retention rate
(%)
Impact
(pass/experiment)
Nail penetration
(pass/experiment)
Example 1 1.20 94.2 3/3 3/3
Example 2 1.41 92.3 3/3 2/2
Comparative
Example 1
1.34 93.1 2/3 1/2
Example 3 1.47 85.2 3/3 1/2
Comparative
Example 4
1.36 88.6 3/3 2/3
Comparative
Example 2
1.18 93.9 0/3 0/3
Comparative
Example 3
1.86 81.7 3/3 3/3
Referring to Table 2, it can be confirmed that examining Example 1 and Comparative
Example 1 according to the present disclosure, by introducing a dispersant, the dispersibility is
10 improved, the resistance is also reduced and the thermal stability is improved, thereby effectively
solving the problems caused by impact and nail penetration.
Additionally, when the primer coating layer is not formed at all as in Comparative Example
2, it can be confirmed that it cannot pass both the impact and nail penetration tests, so the thermal
stability is significantly lowered.
Moreover, when only the positive active material layer is formed using a dispersant as in
Comparative Example 3, there is a problem that although the safety is ensured as compared with
the case of forming a primer coating layer thinly, the resistance is increased and the amount of
active material is relatively reduced, thereby reducing capacity retention rate.
5 On the other hand, comparing Example 1 and Example 2, when the content of the binder
of the primer coating layer increases, the resistance increases and may decrease in terms of
capacity, which is more preferably included in an amount of 5 wt% or less.
Additionally, examining Example 1 and Examples 3 and 4, the effect on impact and nail
penetration can be confirmed, but when forming a primer coating layer thickly as in Examples 3
10 and 4, the layer of the positive active material exhibiting high capacity is relatively reduced, while
the safety is rather decreased, and thus there is a drawback in that the resistance increases and the
capacity retention rate is reduced, and further it is also disadvantageous in terms of energy density.
【Industrial Applicability】
15 As described above, the positive electrode for a lithium secondary battery according to an
embodiment of the present disclosure improves thermal stability by applying lithium iron
phosphate as a primer coating layer, and at the same time, has an effect of improving energy
density by separately applying a positive electrode active material layer containing other lithium
transition metal oxides to a substantial capacity.
20 Additionally, in comparison with the case of applying a primer coating layer containing
only a general conductive material, the present disclosure not only has an effect that can contribute
to capacity increase by including lithium iron phosphate, but also solves the problem of the
lowering of the dispersibility of lithium iron phosphate by adding a small amount of the dispersant,
thereby solving the problem of slurry processability caused by the lowering of dispersibility, and
25 effectively exhibiting the performance improvement of the lithium secondary battery, such as
electrical conductivity according to the application of the primer coating layer.
Based on the above disclosure, various applications and modifications can be carried out
by those of ordinary skill in the art without deviating from the spirit and scope of the present
disclosure.
5
10
15
20
25
【CLAIMS】
【Claim 1】
A positive electrode for a lithium secondary battery comprising:
a positive electrode current collector,
5 a primer coating layer formed on the positive electrode current collector, and
a positive electrode active material layer formed on the primer coating layer,
wherein the primer coating layer comprises lithium iron phosphate, a binder, a conductive
material and a dispersant,
wherein the lithium iron phosphate is represented by the following Chemical Formula 1,
10 wherein the dispersant is a hydrogenated nitrile butadiene rubber (HNBR), and
wherein the hydrogenated nitrile butadiene rubber is contained in an amount of 0.4 to 2.0
wt% based on the weight of lithium iron phosphate.
[Chemical Formula 1]
Li1+aFe1-sMs(PO4-b)Xb
15 wherein,
M is at least one element selected from Co, Ni, Al, Mg, Ti and V,
X is F, S, or N, and
0≤s≤0.5; -0.5≤a≤+0.5; 0≤b≤0.1.
【Claim 2】
20 The positive electrode for a lithium secondary battery according to claim 1, wherein:
the lithium iron phosphate is composed of primary particles, or is a mixture of the primary
particles and secondary particles in a state where the primary particles are aggregated.
【Claim 3】
The positive electrode for a lithium secondary battery according to claim 2, wherein:
25 the primary particles of the lithium iron phosphate have an average diameter (D50) of 0.2
to 3.0 ㎛, and the secondary particles have an average diameter (D50) of 7 to 25 ㎛.
【Claim 4】
The positive electrode for a lithium secondary battery according to claim 2, wherein:
the lithium iron phosphate is in a state where the primary particles and/or secondary
particles are carbon-coated.
【Claim 5】
The positive electrode for a lithium secondary battery according to claim 1, wherein:
5 the content of acrylonitrile (AN) of the hydrogenated nitrile butadiene rubber is 20 to 50
wt%, and the residual double bond (RDB) present in the hydrogenated nitrile butadiene rubber is
contained in an amount of 30% or less.
【Claim 6】
The positive electrode for a lithium secondary battery according to claim 1, wherein:
10 the hydrogenated nitrile butadiene rubber has a weight average molecular weight (Mw) of
10,000 to 250,000.
【Claim 7】
The positive electrode for a lithium secondary battery according to claim 1, wherein:
the binder included in the primer coating layer is polyvinylidene fluoride (PVdF) having
15 a weight average molecular weight (Mw) of 500,000 to 1,200,000.
【Claim 8】
The positive electrode for a lithium secondary battery according to claim 1, wherein:
the primer coating layer is formed to a thickness of 1 to 5 ㎛.
【Claim 9】
20 The positive electrode for a lithium secondary battery according to claim 1, wherein:
the positive active material layer is formed to a thickness of 50 to 300 ㎛.
【Claim 10】
The positive electrode for a lithium secondary battery according to claim 1, wherein:
the positive electrode active material layer comprises a lithium transition metal oxide as a
25 positive electrode active material.
【Claim 11】
The positive electrode for a lithium secondary battery according to claim 10, wherein:
the lithium transition metal oxide comprises at least one transition metal selected from the
group consisting of Ni, Mn, and Co.
【Claim 12】
A lithium secondary battery comprising the positive electrode for a lithium secondary
battery according to any one of claims 1 to 11.