TECHNICAL FIELD
The present disclosure relates to a positive electrode and a lithium secondary
5 battery including the same. Particularly, the present disclosure relates to a positive
electrode showing high initial efficiency and excellent quick charging performance, and a
lithium secondary battery including the same.
The present application claims priority to Korean Patent Application No. 10-2021-
0002853 filed on January 8, 2021 in the Republic of Korea, the disclosures of which are
10 incorporated herein by reference.
BACKGROUND ART
As technical development and needs for mobile instruments have been increased,
rechargeable secondary batteries that can be downsized and provided with high capacity
15 have been increasingly in demand. In addition, among such secondary batteries, lithium
secondary batteries having high energy density and operating voltage have been
commercialized and used widely.
A lithium secondary battery has a structure including an electrode assembly
having a positive electrode and a negative electrode, each of which includes an active
20 material coated on an electrode current collector, and a porous separator interposed
between both electrodes; and a lithium salt-containing electrolyte injected to the electrode
assembly. The electrode is obtained by applying a slurry including an active material, a
binder and a conductive material dispersed in a solvent to a current collector, followed by
drying and pressing.
25 A lithium secondary battery is a secondary battery in which lithium ions
participate in electric conduction between electrodes during charge/discharge, and shows
higher energy density and a lower memory effect, as compared to the other secondary
batteries, such as a nickel metal hydride battery or a nickel cadmium battery. Therefore,
the use of such a lithium secondary battery has been extended from a compact power
30 source for portable electronic devices, household electric instruments, or the like, to
medium- or large-scale power sources for devices, such as electric power storage devices,
UPS devices, electric power leveling devices, etc., or power sources for driving ships,
railroad cars, hybrid vehicles, electric vehicles, etc., and there is a need for improvement of
3
the performance of such a battery. For example, in the use for cars, such as hybrid
vehicles or electric vehicles, there is a need for providing a secondary battery with high
energy density in order to realize a long range of driving or with high output in order to
improve acceleration response.
5 To realize a secondary battery for electric vehicles with high output and high
energy density, a positive electrode having a high nickel (Ni) content has been used to
obtain an electrode.
However, when using such a Ni-enriched positive electrode, the electrode affects a
change in resistance depending on the state-of-charge (SOC) of a battery, as compared to a
10 positive electrode having a low Ni content, and most secondary batteries show an increase
in resistance. To solve the above-mentioned problem, there is still a need for studies of
controlling the resistance of an electrode or cell through the structure of a positive
electrode or the design of the composition of a positive electrode.
15 DISCLOSURE
Technical Problem
The present disclosure is designed to solve the problems of the related art, and
therefore the present disclosure is directed to providing a positive electrode which prevents
an increase in resistance at a low SOC during discharge, and a secondary battery including
20 the same.
Technical Solution
In one aspect of the present disclosure, there is provided a positive electrode
according to any one of the following embodiments.
25 According to the first embodiment, there is provided a positive electrode
including:
a positive electrode current collector; and
a positive electrode active material layer disposed on at least one surface of the
positive electrode current collector, and having a lower layer region facing the positive
30 electrode current collector and containing a first positive electrode active material and a
first binder polymer, and an upper layer region facing the lower layer region and
containing a second positive electrode active material and a second binder polymer,
wherein the first positive electrode active material in the lower layer region is
4
represented by the following Chemical Formula 1, the second positive electrode active
material in the upper layer region is represented by the following Chemical Formula 2, and
the Ni content of the second positive electrode active material is larger than the Ni content
of the first positive electrode active material:
5 [Chemical Formula 1]
Li1+a[NixMnyCoz]M1tO2
wherein 0 ≤ a ≤ 0.2, 0.4 ≤ x ≤ 0.9, 0 < y < 1, 0 < z < 1, 0 ≤ t < 0.1 and x + y + z + t
= 1, and M1 is at least one element selected from the group consisting of Co, Mn, Ni, Al,
Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd,
10 [Chemical Formula 2]
Li1+b[NiuMnvCow]M2sO2
wherein 0 ≤ b ≤ 0.2, 0.4 ≤ u ≤ 0.9, 0 < v < 1, 0 < w < 1, 0 ≤ s < 0.1 and u + v + w +
s = 1, and M2 is at least one element selected from the group consisting of Co, Mn, Ni, Al,
Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd.
15 According to the second embodiment, there is provided the positive electrode as
defined in the first embodiment, wherein the Ni content of the first positive electrode
active material is 40-75 mol% based on the total transition metals of the first positive
electrode active material, and the Ni content of the second positive electrode active
material is 80-90 mol% based on the total transition metals of the second positive electrode
20 active material.
According to the third embodiment, there is provided the positive electrode as
defined in the first or the second embodiment, wherein the Ni content of the first positive
electrode active material is 50-75 mol% based on the total transition metals of the first
positive electrode active material, and the Ni content of the second positive electrode
25 active material is 81-90 mol% based on the total transition metals of the second positive
electrode active material.
According to the fourth embodiment, there is provided the positive electrode as
defined in any one of the first to the third embodiments, wherein the Ni content of the first
positive electrode active material is 65-70 mol% based on the total transition metals of the
30 first positive electrode active material, and the Ni content of the second positive electrode
active material is 86-90 mol% based on the total transition metals of the second positive
electrode active material.
According to the fifth embodiment, there is provided the positive electrode as
5
defined in any one of the first to the fourth embodiments, wherein the weight ratio of the
lower layer region of the positive electrode active material layer to the upper layer region
thereof is 20:80-80:20.
According to the sixth embodiment, there is provided a method for manufacturing
5 a positive electrode, including the steps of:
preparing a slurry for a lower layer containing a first positive electrode active
material represented by the following Chemical Formula 1, a first binder polymer and a
first dispersion medium and a slurry for an upper layer containing a second positive
electrode active material represented by the following Chemical Formula 2, a second
10 binder polymer and a second dispersion medium, wherein the Ni content of the second
positive electrode active material is larger than the Ni content of the first positive electrode
active material;
coating the slurry for a lower layer on one surface of a positive electrode current
collector, and coating the slurry for an upper layer on the slurry for a lower layer; and
15 drying the coated slurry for a lower layer and slurry for an upper layer at the same
time to form a positive electrode active material layer:
[Chemical Formula 1]
Li1+a[NixMnyCoz]M1tO2
wherein 0 ≤ a ≤ 0.2, 0.4 ≤ x ≤ 0.9, 0 < y < 1, 0 < z < 1, 0 ≤ t < 0.1 and x + y + z + t
20 = 1, and M1 is at least one element selected from the group consisting of Co, Mn, Ni, Al,
Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd,
[Chemical Formula 2]
Li1+b[NiuMnvCow]M2sO2
wherein 0 ≤ b ≤ 0.2, 0.4 ≤ u ≤ 0.9, 0 < v < 1, 0 < w < 1, 0 ≤ s < 0.1 and u + v + w +
25 s = 1, and M2 is at least one element selected from the group consisting of Co, Mn, Ni, Al,
Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd.
According to the seventh embodiment, there is provided the method for
manufacturing a positive electrode as defined in the sixth embodiment, wherein the Ni
content of the first positive electrode active material is 40-75 mol% based on the total
30 transition metals of the first positive electrode active material, and the Ni content of the
second positive electrode active material is 80-90 mol% based on the total transition metals
of the second positive electrode active material.
According to the eighth embodiment, there is provided the method for
6
manufacturing a positive electrode as defined in the sixth or the seventh embodiment,
wherein the Ni content of the first positive electrode active material is 50-75 mol% based
on the total transition metals of the first positive electrode active material, and the Ni
content of the second positive electrode active material is 81-90 mol% based on the total
5 transition metals of the second positive electrode active material.
According to the ninth embodiment, there is provided the method for
manufacturing a positive electrode as defined in any one of the sixth to the eighth
embodiments, wherein the Ni content of the first positive electrode active material is 65-70
mol% based on the total transition metals of the first positive electrode active material, and
10 the Ni content of the second positive electrode active material is 86-90 mol% based on the
total transition metals of the second positive electrode active material.
According to the tenth embodiment, there is provided a lithium secondary battery
including the positive electrode as defined in any one of the first to the fifth embodiments.
15 Advantageous Effects
The positive electrode according to an embodiment of the present disclosure
includes a positive electrode active material layer, which has a lower layer region facing
the positive electrode current collector and containing a first positive electrode active
material and a first binder polymer, and an upper layer region disposed on the lower layer
20 region, wherein the Ni content of the second positive electrode active material in the upper
layer region is controlled to be larger than the Ni content of the first positive electrode
active material in the lower layer region. In this manner, it is possible to prevent an
increase in resistance which may cause rapid degradation of output at a low SOC, and to
provide a positive electrode and a secondary battery including the same with improved
25 high-temperature cycle characteristics.
In addition, according to an embodiment of the present disclosure, the aboveeffects may be realized more predominantly in a high-loading positive electrode having a
high coating amount of positive electrode active material. As a result, it is possible to
realize a battery for electric vehicles (EV) having high capacity and high energy density.