MATERIAL FOR LITHIUM SECONDARY BATTERY OF HIGH
PERFORMANCE
FIELD OF THE INVENTION
The present invention relates to a Ni-based lithium mixed transition metal oxide and a cathode active material for a secondary battery comprising the same. More specifically, the Ni-based lithium mixed transition metal oxide according to the present invention has a given composition and exhibits intercalation/deintercalation of lithium ions into/from mixed transition metal oxide layers ("MO layers") and interconnection of MO layers via the insertion of some MO layer-derived Ni ions into intercalation/deintercalation layers (reversible lithium layers) of lithium ions, thereby improving the stability of the crystal structure upon charge/discharge to provide an excellent sintering stability. In addition, a battery comprising such a cathode active material can exert a high capacity and a high cycle stability. Further, due to substantially no water-soluble bases, such a lithium mixed transition metal oxide exhibits excellent storage stability and chemical resistance, and decreased gas evolution, thereby resulting in an excellent high-temperature stability and the feasibility of mass production at low costs.
BACKGROUND OF THE INVENTION
Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source.
Among other things, lithium secondary batteries having a high-energy density and voltage, a long cycle lifespan and a low self-discharge rate are commercially available and widely used.
As cathode active materials for the lithium secondary batteries, lithium- containing cobalt oxide (LiCoO2) is largely used. In addition, consideration has been made of using lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure, and lithium-containing nickel oxides (LiNiO2).
Of the aforementioned cathode active materials, LiCoO2 is currently widely used due to superior general properties including excellent cycle characteristics, but suffers from low safety, expensiveness due to finite resources of cobalt as a raw material, and limitations in practical and mass application thereof as a power source for electric vehicles (EVs) and the like.
Lithium manganese oxides, such as LiMnO2 and LiMn2O4, are abundant resources as raw materials and advantageously employ environmentally-friendly manganese, and therefore have attracted a great deal of attention as a cathode active material capable of substituting LiCoO2. However, these lithium manganese oxides suffer from shortcomings such as a low capacity and poor cycle characteristics.
Whereas, lithiutn/nickel-based oxides including LiNiO2 are inexpensive as compared to the aforementioned cobalt-based oxides and exhibit a high discharge
capacity upon charging Io 4.3 V. The reversible capacity of doped LiNiO2 approximates about 200 mAh/g which exceeds the capacity of LiCoO2 (about 165 mAh/g). Therefore, despite a slightly lower average discharge voltage and a slightly lower volumetric density, commercial batteries comprising LiNiO2 as the cathode active material exhibit an improved energy density. To this end, a great deal of intensive research is being actively undertaken on the feasibility of applications of such nickel-based cathode active materials for the development of high-capacity batteries. However, the LiNiO2- based cathode active materials suffer from some limitations in practical application thereof, due to the following problems.
First, LiNiθ2-based oxides undergo sharp phase transition of the crystal structure with volumetric changes accompanied by repeated charge/discharge cycling, and thereby may suffer from cracking of particles or formation of voids in grain boundaries. Consequently, intercalation/deintercalation of lithium ions may be hindered to increase the polarization resistance, thereby resulting in deterioration of the charge/discharge performance. In order to prevent such problems, conventional prior arts attempted to prepare a LiNiO2-based oxide by adding an excess of a Li source and reacting reaction components under an oxygen atmosphere. However, the thus-prepared cathode active material, under the charged state, undergoes structural swelling and destabilization due to the repulsive force between oxygen atoms, and suffers from problems of severe deterioration in cycle characteristics due to repeated charge/discharge cycles.
Second, LiNiO2 has a problem associated with evolution of an excess of gas during storage or cycling. That is, in order to smoothly form the crystal structure, an excess of a Li source is added during manufacturing of the LiNiO2-based oxide, followed by heat treatment. As a result, water-soluble bases including Li2CO3 and LiOH
as reaction residues remain between primary particles and thereby they decompose or react with electrolytes to thereby produce CO2 gas, upon charging. Further, LiNiO2 particles have an agglomerate secondary particle structure in which primary particles are agglomerated to form secondary particles and consequently a contact area with the electrolyte further increases to result in severe evolution of CO2 gas, which in turn unfortunately leads to the occurrence of battery swelling and deterioration of the high- temperature safety.
Third, LiNiO2 suffers from a sharp decrease in the chemical resistance of a surface thereof upon exposure to air and moisture, and gelation of slurries by polymerization of a NMP-PVDF slurry due to a high pH value. These properties of LiNiO2 cause severe processing problems during battery production.
Fourth, high-quality LiNiO2 cannot be produced by a simple solid-state reaction as is used in the production of LiCoO2, and LiNiMeO2 (Me = Co, Mn or Al) cathode active materials containing an essential dopant cobalt and further dopants manganese and aluminum are produced by reacting a lithium source such as LiOH-H2O with a mixed transition metal hydroxide under an oxygen or syngas atmosphere (i.e. CO2-deficient atmosphere), which consequently increases production costs. Further, when an additional step, such as intermediary washing or coating, is included to remove impurities in the production of LiNiO2, this leads to a further increase in production costs.
In order to solve such problems, the present invention, as will be illustrated hereinafter, provides a lithium mixed transition metal oxide having a novel structure with a specific composition formula and interconnection of reversible lithium
intercalation/deintercalation layers via the insertion of some of mixed transition metal oxide layer-derived Ni ions into the reversible lithium layers.
In this connection, Japanese Unexamined Patent Publication Nos. 2004- 281253, 2005-150057 and 2005-310744 disclose oxides having a composition formula of Li8MnxMyM2O2 (M = Co or Al, l<a<1.2, 0≤x<0.65, 0.35<y≤l, 0<z≤0.65, and x + y + z = l). Even though the aforementioned oxide partially overlaps with the present invention in the composition range, it was confirmed through various experiments conducted by the inventors of the present invention that the aforesaid prior art oxide has a structure different than that of the lithium mixed transition metal oxide of the present invention, that is, suffers from significant problems associated with severe gas evolution at high temperatures and structural stability, resulting from the inclusion of large amounts of impurities such as lithium carbonates, instead of having the reversible lithium layer with partial insertion of Ni ions.
Therefore, there is an urgent need in the art for the development of a technology which is capable of achieving a stable crystal structure while utilizing a lithium/nickel-based cathode active material suitable for realization of a high capacity and is also capable of securing a high-temperature safety due to a low content of impurities.
SUMMARY OF THE INVENTION
Therefore, the present invention has been made to solve the above problems and other technical problems that have yet to be resolved.
As a result of a variety of extensive and intensive studies and experiments to solve the problems as described above, the inventors of the present invention have
discovered that when a lithium mixed transition metal oxide, as will be illustrated hereinafter, has a given composition and a specific atomic-level structure, it is possible to realize superior thermal stability, and high cycle stability in conjunction with a high capacity, due to improvements in the stability of the crystal structure upon charge/discharge. Further, such a lithium mixed transition metal oxide can be prepared in a substantially water-soluble base-free form and therefore exhibits excellent storage stability, reduced gas evolution and thereby excellent high-temperature safety in conjunction with the feasibility of industrial-scale production at low production costs. The present invention has been completed based on these findings.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a schematic view showing a crystal structure of a conventional Ni- based lithium transition metal oxide;
FIG. 2 is a schematic view showing a crystal structure of a Ni-based lithium mixed transition metal oxide according to an embodiment of the present invention;
FIGS. 3 and 4 are graphs showing a preferred composition range of a Ni-based lithium mixed transition metal oxide according to the present invention;
FIG. 5 is an FESEM (Field Emission Scanning Electron Microscope) image (x
2000) showing LiNiMO2 according to Example 1 of the present invention. 5A: 850 °C; 5B: 900 °C; 5C: 950 °C ; and 5D: 1000°C;
FIG. 6 is an FESEM image showing commercial LiMO2 (M = Nio.8Co02) according to Comparative Example 1. 6A: FESEM image of a sample as received, and 6B: FESEM image of a sample after heating to 850 °C in air;
FIG. 7 is an FESEM image showing the standard pH titration curve of commercial high-Ni LiNiO2 according to Comparative Example 2. A: Sample as received, B: After heating of a sample to 800 °C under an oxygen atmosphere, and C: Copy of A;
FIG. 8 is a graph showing a pH titration curve of a sample according to Comparative Example 3 during storage of the sample in a wet chamber. A: Sample as received, B: After storage of a sample for 17 hrs, and C: After storage of a sample for 3 days;
FIG. 9 is a graph showing a pH titration curve of a sample according to Example 2 during storage of the sample in a wet chamber. A: Sample as received, B: After storage of a sample for 17 hrs, and C: After storage of a sample for 3 days;
FIG. 10 is an SEiM image of a sample according to Example 3;
FIG. 11 shows the Rietveld refinement on X-ray diffraction patterns of a sample according to Example 3;
FIG. 12 is a graph showing electrochemical properties Of LiNiMO2 according to Example 3 in Experimental Example 1. 12A: Graph showing voltage profiles and rate characteristics at room temperature (1 to 7 cycles); 7B: Graph showing cycle stability at 25 °C and 60 °C and a rate of C/5 (3.0 to 4.3V); and 7C: Graph showing discharge profiles (at C/10 rate) for Cycle 2 and Cycle 31, obtained during cycling at 250C and 60 °C;
FIG. 13 is a graph showing DCS values for samples of Comparative Examples 3 and 4 in Experimental Example 2. A: Commercial Al/Ba-modified LiMO2 (M = Ni) of Comparative Example 3, and B: Commercial AlPO4-coated LiMO2 (M = Ni) of Comparative Example 4;
FIG. 14 is a graph showing DCS values for LiNiMO2 according to Example 3 in Experimental Example 2;
FIG. 15 is a graph showing electrophysical properties of a polymer cell according to one embodiment of the present invention in Experimental Example 3;
FIG. 16 is a graph showing swelling of a polymer cell during high-temperature storage in Experimental Example 3; and
FIG. 17 is a graph showing lengths of a-axis and c-axis of crystallographic unit cells of samples having different ratios of Li:M in Experimental Example 4.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a lithium mixed transition metal oxide having a composition represented by Formula I below, wherein lithium ions undergo intercalation/deintercalation into/from mixed transition metal oxide layers ("MO layers") and some of MO layer-derived Ni ions are inserted into intercalation/deintercalation layers of lithium ions ("reversible lithium layers") thereby resulting in the interconnection between the MO layers.
LixMyO2 (I)
wherein:
M
0.65 < a+b < 0.85 and 0.1 < b < 0.4;
A is a dopant;
0 ≤ k < 0.05; and
x+y ~ 2 and 0.95 < x < 1.05.
A conventional lithium transition metal oxide has a layered crystal structure as shown in FIG. 1, and performs charge/discharge processes through lithium insertion and desertion into/from the MO layers. However, when the oxide having such a structure is used as a cathode active material, desertion of lithium ions from the reversible lithium layers in the charged state brings about swelling and destabilization of the crystal structure due to the repulsive force between oxygen atoms in the MO layers, and thus suffers from problems associated with sharp decreases in the capacity and cycle characteristics, resulting from changes in the crystal structure due to repeated charge/discharge cycles.
As a result of a variety of extensive and intensive studies and experiments to solve the problems as described above, the inventors of the present invention have confirmed that the crystal structure stabilizes contrary to conventionally known or accepted ideas in the related art that intercalation/deintercalation of lithium ions will be hindered when some of nickel ions go down to and immobilize in the reversible lithium layers as shown in FIG. 2, so it is possible to prevent problems associated with collapse of the crystal structure caused by the intercalation/deintercalation of lithium ions. As a result, since the lifespan characteristics and safety are simultaneously improved due to no occurrence of additional structural collapse by oxygen desorption and prevention of
further formation of Ni2+, the battery capacity and cycle characteristics can be significantly improved and a desired level of rate characteristics can be exerted. Therefore, it can be said that such a concept of the present invention is a remarkable one which completely opposite to and overthrow the conventional idea.
That is, due to the insertion of some Ni ions into the reversible lithium layers, the lithium mixed transition metal oxide in accordance with the present invention does not undergo disintegration of the crystal structure with maintenance of the oxidation number of Ni ions inserted into the reversible lithium layers, thereby being capable of maintaining a well-layered structure, even when lithium ions are released during a charge process. Hence, a battery comprising the lithium mixed transition metal oxide having such a structure as a cathode active material can exert a high capacity and a high-cycle stability.
Further, the lithium mixed transition metal oxide in accordance with the present invention is very excellent in the thermal stability as the crystal structure is stably maintained even upon sintering at a relatively high temperature during a production process.
Generally, when conventional high-nickel LiMO2 is subjected to high- temperature sintering in air containing a trace amount of CO2, LiMO2 decomposes with a decrease of Ni3+ as shown in the following reaction below, during which amounts of impurities Li2CO3 increase.
LiM3+ O2 + CO2 ■» a Lii-xMi+x 3+'2+ O2 + b Li2CO3 + c O2
Further, since continuous degradation of the crystal structure occurs with increased cation mixing, an increased lattice constant and a decreased c:a ratio, and the
molten Li2CO3 segregates particles, primary particles lose a contact state therebetween, thereby resulting in the disintegration of secondary particles.
However, unlike the conventional high-nickel LiMO2, the lithium mixed transition metal oxide in accordance with the present invention, due to the stability of the atomic-level structure, does not involve formation of Li2CO3 impurities resulting from oxygen deficiency due to decrease and decomposition of Ni3+ as shown in the above reaction, in conjunction with no degradation of the crystal structure, even when it is subjected to high-temperature sintering under an air atmosphere. Further, there are substantially no water-soluble bases, as the lithium mixed transition metal oxide of the present invention can be prepared without addition of an excess of a lithium source. Accordingly, the lithium mixed transition metal oxide of the present invention exhibits excellent storage stability, decreased gas evolution and thereby excellent high- temperature stability simultaneously with the feasibility of industrial-scale production at low production costs.
Hereinafter, where appropriate throughout the specification, the term "lithium mixed transition metal oxide in accordance with the present invention" is used interchangeably with the term "LiNiMO2", and the term "Ni inserted and bound into the reversible lithium layers" is used interchangeably with the term "inserted Ni". Therefore, NiM in LiNiMO2 is a suggestive expression representing a complex composition of Ni, Mn and Co in Formula I.
In one preferred embodiment, the lithium mixed transition metal oxide may have a structure wherein Ni2+ and Ni3+ coexist in the MO layers and some of Ni2+ are inserted into the reversible lithium layers. That is, in such a structure of the metal oxide,
all of Ni ions inserted into the reversible lithium layers are Ni2+ and the oxidation number of Ni ions is not changed in the charge process.
Specifically, when Ni2+ and Ni3+ coexist in a Ni-excess lithium transition metal oxide, an oxygen atom-deficient state is present under given conditions (reaction atmosphere, Li content, etc.) and therefore insertion of some Ni2+ ions into the reversible lithium layers may occur with changes in the oxidation number of Ni.
The composition of the lithium mixed transition metal oxide should satisfy the following specific requirements as defined in Formula I:
(i) Nii.(a+b)(Ni1/2Mn1/2)aCθb and 0.65 < a+b < 0.85
(ii) 0.1 ≤ b < 0.4, and
(iii) x+y ~ 2 and 0.95 < x < 1.05
Regarding the aforementioned requirement (i), Nii.(a+b) means a content of Ni3+. Therefore, if a mole fraction of Ni3+ exceeds 0.35 (a+b < 0.65), it is impossible to implement an industrial-scale production in air, using Li2Cθ3 as a raw material, so the lithium transition metal oxide should be produced using LiOH as a raw material under an oxygen atmosphere, thereby presenting a problems associated with decreased production efficiency and consequently increased production costs. On the other hand, if a mole fraction of Ni3+ is lower than 0.15 (a+b > 0.85), the capacity per volume of LiNiMO2 is not competitive as compared to LiCoO2.
Meanwhile, taking into consideration both of the above requirements (i) and
(ii), the total mole fraction of Ni including Ni2+ and Ni3+ in LiNiMO2 of the present invention is preferably a relatively nickel-excess as compared to manganese and cobalt and may be in a range of 0.4 to 0.7. If a content of nickel is excessively low, it is
difficult to achieve a high capacity. Conversely, if a content of nickel is excessively high, the safety may be significantly lowered. In conclusion, the lithium transition metal oxide (LiNiMO2) exhibits a large volume capacity and low raw material costs, as compared to lithium cobalt-based oxides.
Further, if the mole fraction of Ni2+ is too high relative to the Ni content, the cation mixing increases to thereby result in formation of a rock salt structure that is locally and electrochemically non-reactive and such a rock salt structure not only hinders charge/discharge and but also may bring about a decrease in a discharge capacity. On the other hand, if the mole fraction of Ni2+ is too low, this may lead to an increase in the structural instability to thereby lower the cycle stability. Therefore, the mole fraction of Ni2+ should be appropriately adjusted taking into consideration such problems that may occur. Preferably, within the range as shown in FIG. 3, the mole fraction of Ni2+ may be in a range of 0.05 to 04, based on the total content of Ni.
Therefore, since Ni2+ is inserted into and serves to support the MO layers, Ni2+ is preferably contained in an amount enough to stably support between MO layers such that the charge stability and cycle stability can be improved to a desired level, and at the same time it is inserted in an amount not so as to hinder intercalation/deintercalation of lithium ions into/from the reversible lithium layers such that rate characteristics are not deteriorated. Taken altogether, the mole fraction of Ni2+ inserted and bound into the reversible layers may be preferably in a range of 0.03 to 0.07, based on the total content ofNi.
The content of Ni2+ or the content of inserted Ni2+ may be determined by controlling, for example, a sintering atmosphere, the content of lithium, and the like.
Therefore, when a concentration of oxygen in the sintering atmosphere is high, the content OfNi2+ will be relatively low.
With regard to Ihe aforementioned condition (ii), a content of cobalt (b) is in a range of 0.1 to 0.4. If the content of cobalt is excessively high (b > 0.4), the overall cost of a raw material increases due to a high content of cobalt, and the reversible capacity somewhat decreases. On the other hand, if the content of cobalt is excessively low (b < 0.1), it is difficult to achieve sufficient rate characteristics and a high powder density of the battery at the same time.
With regard to the aforementioned condition (iii), if a content of lithium is excessively high, i.e. x > 1.05, this may result in a problem of decreased stability during charge/discharge cycling, particularly at T = 60 °C and a high voltage (U = 4.35 V). On the other hand, if a content of lithium is excessively low, i.e. x < 0.95, this may result in poor rate characteristics and a decreased reversible capacity.
Optionally, LiNiMO2 may further comprise trace amounts of dopants. Examples of the dopants may include aluminum, titanium, magnesium and the like, which are incorporated into the crystal structure. Further, other dopants, such as B, Ca, Zr, F, P, Bi, Al, Mg, Zn, Sr, Ga, In, Ge, and Sn, may be included via the grain boundary accumulation or surface coating of the dopants without being incorporated into the crystal structure. These dopants are included in amounts enough to increase the safety, capacity and overcharge stability of the battery while not causing a significant decrease in the reversible capacity. Therefore, a content of the dopant is in a range of less than 5% (k < 0.05), as defined in Formula I. In addition, the dopants may be preferably added in an amount of <1%, within a range that can improve the stability without causing deterioration of the reversible capacity.
As a ratio (Li/M) of Li to the transition metal (M) decreases, the amount of Ni inserted into the MO layers gradually increases. Therefore, if excessive amounts of Ni ions go down into the reversible lithium layers, a movement of Li+ during charge/discharge processes is hampered to thereby lead to problems associated with a decrease in the reversible capacity or deterioration of the rate characteristics. On the other hand, the Li/M ratio is excessively high, the amount of Ni inserted into the MO layer is excessively low, which may undesirably lead to structural instability, thereby presenting decreased safety of the battery and poor lifespan characteristics. Further, at an excessively high Li/M value, amounts of unreacted Li2CO3 increase to thereby result in a high pH-titration value, i.e. production of large amounts of impurities, and consequently the chemical resistance and high-temperature stability may be deteriorated. Therefore, in one preferred embodiment, the ratio of Li:M in LiNiMO2 may be in a range of 0.95 to 1.04: 1.
In one preferred embodiment, the lithium mixed transition metal oxide in accordance with the present invention is substantially free of water-soluble base impurities, particularly Li2CO3.
Usually, Ni-based lithium transition metal oxides contain large amounts of water-soluble bases such as lithium oxides, lithium sulfates, lithium carbonates, and the like. These water-soluble bases may be bases, such as Li2CO3 and LiOH, present in LiNiMO2, or otherwise may be bases produced by ion exchange (H+ (water) <r -> Li+ (surface, an outer surface of the bulk)), performed at the surface Of LiNiMO2. The bases of the latter case are usually present as a negligible level.
The former water-soluble bases are formed primarily due to unreacted lithium raw materials upon sintering. This is because as a conventional Ni-based lithium
transition metal oxide employs relatively large amounts of lithium and low-temperature sintering so as to prevent the collapse of a layered crystal structure, the resulting metal oxide has relatively large amounts of grain boundaries as compared to the cobalt-based oxides and lithium ions do not sufficiently react upon sintering.
On the other hand, since LiNiMO2 in accordance with the present invention, as discussed hereinbefore, stably maintains the layered crystal structure, it is possible to carry out a sintering process at a relatively high-temperature under an air atmosphere and thereby LiNiMO2 has relatively small amounts of grain boundaries. In addition, as retention of unreacted lithium on surfaces of particles is prevented, the particle surfaces are substantially free of lithium salts such as lithium carbonates, lithium sulfates, and the like. Further, there is no need to add an excess of a lithium source upon production of LiNiMO2, so it is possible to fundamentally prevent a problem associated with the formation of impurities due to the unreacted lithium source remaining in the powder.
As such, it is possible to fundamentally solve various problems that may occur due to the presence of the water-soluble bases, particularly the problem that may damage the battery safety due to evolution of gas arising from acceleration of electrolyte decomposition at high temperatures.
As used herein, the phrase "is (are) substantially free of water-soluble bases" refers to an extent that upon titration of 200 mL of a solution containing the lithium mixed transition metal oxide with 0.1M HCl, a HCl solution used to reach a pH of less than 5 is preferably consumed in an amount of less than 20 mL, more preferably less than 10 mL. Herein, 200 mL of the aforementioned solution contains substantially all kinds of the water-soluble bases in the lithium mixed transition metal oxide, and is
prepared by repeatedly soaking and decanting 1O g of the lithium mixed transition metal oxide.
More specifically, first, 5 g of a cathode active material powder is added to 25 mL of water, followed by brief stirring. About 20 mL of a clear solution is separated and pooled from the powder by soaking and decanting. Again, about 20 mL of water is added to the powder and the resulting mixture is stirred, followed by decanting and pooling. The soaking and decanting are repeated at least 5 times. In this manner, total 100 mL of the clear solution containing water-soluble bases is pooled. A 0.1M HCl solution is added to the thus-pooled solution, followed by pH titration with stirring. The pH profile is recorded as a function of time. Experiments are terminated when the pH reaches a value of less than 3, and a flow rate may be selected within a range that titration takes about 20 to 30 min. The content of the water-soluble bases is given as an amount of acid that was used until the pH reaches a value of less than 5.
In this manner, it is possible to determine the content of the water-soluble bases contained in the aforesaid powder. At this time, there are no significant influences of parameters such as a total soaking time of the powder in water.
A method of preparing the lithium mixed transition metal oxide in accordance with the present invention may be preferably carried out under an oxygen-deficient atmosphere. The oxygen -deficient atmosphere may be an atmosphere with an oxygen concentration of preferably 10% to 50%, more preferably 10% to 30%. Particularly preferably, the lithium mixed transition metal oxide can be prepared by a solid-state reaction of Li2CO3 and mixed transition metal precursors in the air atmosphere. Therefore, it is also possible to solve various problems associated with increased production costs and the presence of large amounts of water-soluble bases, which are
suffered by conventional arts of making the lithium transition metal oxide involving adding, mixing and sintering of LiOH and each transition metal precursor under an oxygen atmosphere. That is, the present invention enables production of the lithium mixed transition metal oxide having a given composition and a high content of nickel via a simple solid-state reaction in air, using a raw material that is cheap and easy to handle. Therefore, it is possible to realize a significant reduction of production costs and high-efficiency mass production.
That is, due to thermodynamic limitations in the conventional art, it was impossible to produce high-nickel lithium mixed transition metal oxide in the air containing trace amounts of carbon dioxide. In addition, the conventional art suffered from problems in that utilization of Li2COs as a precursor material brings about evolution of CO2 due to decomposition of Li2CO3, which then thermodynamically hinders additional decomposition of Li2CO3 even at a low partial pressure, consequently resulting in no further progression of the reaction. For these reasons, it was impossible to use Li2CO3 as the precursor in the practical production process.
On the other hand, the lithium mixed transition metal oxide in accordance with the present invention employs Li2CO3 and the mixed transition metal precursor as the reaction materials, and can be prepared by reacting reactants in an oxygen-deficient atmosphere, preferably in air. As a result, it is possible to achieve a significant reduction of production costs, and a significant increase of the production efficiency due to the feasibility to produce a desired product by a relatively simple process. This is because the lithium mixed transition metal oxide in accordance with the present invention is very excellent in the sintering stability at high temperatures, i.e. the thermodynamic stability, due to a stable crystal structure.
Further, since the lithium source material and the mixed transition metal material may be preferably added in a ratio of 0.95 to 1.04:1 when producing the lithium mixed transition metal oxide in accordance with the present invention, there is no need to add an excess of a lithium source and it is possible to significantly reduce the possibility of retention of the water-soluble bases which may derive from the residual lithium source.
In order to carry out a smooth reaction via high air circulation upon production of the lithium mixed transition metal oxide by the large-scale mass production process, preferably at least 2 m3 (a volume at room temperature) of air, per kg of the final lithium mixed transition metal oxide, may be pumped into or out of a reaction vessel. For this purpose, a heat exchanger may be employed to further enhance the efficiency of energy utilization by pre-warming the in-flowing air before it enters the reactor, while cooling the out-flowing air.
In accordance with a further aspect of the present invention, there is provided a cathode active material for a secondary battery comprising the aforementioned lithium nickel-based oxide.
The cathode active material in accordance with the present invention may be comprised only of the lithium mixed transition metal oxide having the above-specified composition and the specific atomic-level structure, or where appropriate, it may be comprised of the lithium mixed transition metal oxide in conjunction with other lithium- containing transition metal oxides.
Examples of the lithium-containing transition metal oxides that can be used in the present invention may include, but are not limited to, layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds
substituted with one or more transition metals; lithium manganese oxides such as compounds of Formula Li1+yMn2-y04 (0<y<0.33), LiMnC<3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3Os, V2O5, and Cu2V2O7; Ni-site type lithium nickel oxides of Formula LiNii.yMyO2 (M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and 0.01≤y<0.3); lithium manganese composite oxides of Formula LiMn2.yMyO2 (M = Co, Ni, Fe, Cr, Zn, or Ta, and O.Ol≤y≤O.l), or Formula Li2Mn3MO8 (M = Fe, Co, Ni, Cu, or Zn); LiMn2O4 wherein a portion of Li is substituted with alkaline earth metal ions; disulfide compounds; and Fe2(MoO4)3, LiFe3O4, etc.
In accordance with a still further aspect of the present invention, there is provided a lithium secondary battery comprising the aforementioned cathode active material. The lithium secondary battery is generally comprised of a cathode, an anode, a separator and a lithium salt-containing non-aqueous electrolyte. Methods for preparing the lithium secondary battery are well-known in the art and therefore detailed description thereof will be omitted herein.
The cathode is, for example, fabricated by applying a mixture of the above- mentioned cathode active material, a conductive material and a binder to a cathode current collector, followed by drying. If necessary, a filler may be further added to the above mixture.
The cathode current collector is generally fabricated to have a thickness of 3 to 500 μm. There is no particular limit to materials for the cathode current collector, so long as they have high conductivity without causing chemical changes in the fabricated battery. As examples of the materials for the cathode current collector, mention may be made of stainless steel, aluminum, nickel, titanium, sintered carbon, and aluminum or stainless steel which was surface-treated with carbon, nickel,
titanium or silver. The current collector may be fabricated to have fine irregularities on the surface thereof so as to enhance adhesion to the cathode active material. In addition, the current collector may take various forms including films, sheets, foils, nets, porous structures, foams and non- woven fabrics.
The conductive material is typically added in an amount of 1 to 50% by weight, based on the total weight of the mixture including the cathode active material. There is no particular limit to the conductive material, so long as it has suitable conductivity without causing chemical changes in the fabricated battery. As examples of conductive materials, mention may be made of conductive materials, including graphite such as natural or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal black; conductive fibers such as carbon fibers and metallic fibers; metallic powders such as carbon fluoride powder, aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.
The binder is a component assisting in binding between the electrode active material and the conductive material, and in binding of the electrode active material to the current collector. The binder is typically added in an amount of 1 to 50% by weight, based on the total weight of the mixture including the cathode active material. As examples of the binder, mention may be made of polyvinylidene fluoride, polyvinyl alcohols, carboxymethyleellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinyl pyrollidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluoro rubber and various copolymers.
The filler is an optional ingredient used to inhibit cathode expansion. There is no particular limit to the filler, so long as it does not cause chemical changes in the fabricated battery and is a fibrous material. As examples of the filler, there may be used olefin polymers such as polyethylene and polypropylene; and fibrous materials such as glass fiber and carbon fiber.
The anode is fabricated by applying an anode active material to an anode current collector, followed by drying. If necessary, other components as described above may be further included.
The anode current collector is generally fabricated to have a thickness of 3 to 500 μm. There is no particular limit to materials for the anode current collector, so long as they have suitable conductivity without causing chemical changes in the fabricated battery. As examples of materials for the anode current collector, mention may be made of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel having a surface treated with carbon, nickel, titanium or silver, and aluminum-cadmium alloys. Similar to the cathode current collector, the anode current collector may also be processed to form fine irregularities on the surfaces thereof so as to enhance adhesion to the anode active material. In addition, the anode current collector may be used in various forms including films, sheets, foils, nets, porous structures, foams and non-woven fabrics.
As examples of the anode active materials utilizable in the present invention, mention may be made of carbon such as non-graphitizing carbon and graphite-based carbon; metal composite oxides such as LIyFe2O3 (0<y<l), LIyWO2 (O≤y≤l) and SnxMe1-XMeVO2 (Me: Mn, Fe, Pb or Ge; Me1: Al, B, P, Si, Group I, Group II and Group III elements of the Periodic Table of the Elements, or halogens; 0<x<l; l≤y<3; and
l<z<8); lithium metals; lithium alloys; silicon-based alloys; tin-based alloys; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, and Bi2O5; conductive polymers such as polyacetylene; and Li-Co-Ni based materials.
The separator is interposed between the cathode and anode. As the separator, an insulating thin film having high ion permeability and mechanical strength is used. The separator typically has a pore diameter of 0.01 to 10 im and a thickness of 5 to 300 μm. As the separator, sheets or non-woven fabrics made of an olefin polymer such as polypropylene and/or glass fibers or polyethylene, which have chemical resistance and hydrophobicity, are used. When a solid electrolyte such as a polymer is employed as the electrolyte, the solid electrolyte may also serve as both the separator and electrolyte.
The lithium salt-containing non-aqueous electrolyte is composed of an electrolyte and a lithium salt. As the electrolyte, a non-aqueous organic solvent, an organic solid electrolyte, or an inorganic solid electrolyte may be utilized.
As the non-aqueous organic solvent that can be used in the present invention, for example, mention may be made of aprotic organic solvents such as N-methyl-2- pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyro lactone, 1,2-dimethoxy ethane, tetrahydroxy Franc, 2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxy methane, dioxolane derivatives, sulfolane, methyl sulfolane, l,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate.
As examples of the organic solid electrolyte utilized in the present invention, mention may be made of polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphoric acid ester polymers, poly agitation lysine, polyester sulfide, polyvinyl alcohols, polyvinylidene fluoride, and polymers containing ionic dissociation groups.
As examples of the inorganic solid electrolyte utilized in the present invention, mention may be made of nitrides, halides and sulfates of lithium such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH, and Li3PO4-Li2S-SiS2.
The lithium salt is a material that is readily soluble in the above-mentioned non-aqueous electrolyte and may include, for example, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB10Cl10, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CFsSO3Li, (CF3SO2)2NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate, and imide.
Additionally, in order to improve charge/discharge characteristics and flame retardancy, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol, aluminum trichloride, or the like may be added to the non-aqueous electrolyte. If necessary, in order to impart incombustibility, the non-aqueous electrolyte may further include halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride. Further, in order to improve high-temperature storage characteristics, the non-aqueous electrolyte may additionally include carbon dioxide gas.
In accordance with yet another aspect of the present invention, there is provided a lithium mixed transition metal oxide comprising a mixed transition metal of Ni, Mn and Co, wherein lithium ions undergo intercalation/deintercalation into/from mixed-transition metal oxide layers ("MO layers") and some of MO layer-derived Ni ions are inserted into intercalation/deintercalation layers of lithium ions ("reversible lithium layers") thereby resulting in interconnection of the MO layers.
Generally, the use of nickel-based lithium mixed transition metal oxide as the cathode active material may bring about the collapse of the crystal structure upon intercalation/deintercalation of lithium ions. However, the lithium mixed transition metal oxide in accordance with the present invention exhibits stabilization of the crystal structure due to nickel ions inserted into the reversible lithium layers and therefore does not undergo additional structural collapse that may result from oxygen desorption, thereby simultaneously improving the lifespan characteristics and safety.
Further, the present invention provides a lithium mixed transition metal oxide having a composition represented by Formula II below, wherein a cation mixing ratio with location of some Ni in lithium sites is in a range of 0.02 to 0.07 based on the total content of Ni, to thereby stably support the layered crystal structure.
Lix-M9VAVO2 (II)
wherein
0.95 < x' < 1.05, 0 < z' < 0.05, x'+y' ~ 2, and y'+z' = 1 ;
M" is a mixed transition metal of Ni, Mn and Co; and
A' is at least one element selected from the group consisting of B, Ca, Zr, S, F, P, Bi, Al, Mg, Zn, Sr, Cu, Fe, Ga, In, Cr, Ge and Sn.
According to the experimental confirmation by the present inventors, the cation mixing ratio is particularly preferably in a range of 0.02 to 0.07, upon considering both of the charge and cycle stability and the rate characteristics. When the cation mixing ratio is within the above-specified range, the crystal structure can be further stably maintained and intercalation/deintercalation of lithium ions can be maximized.
Further, the present invention provides a lithium mixed transition metal oxide having a composition of Formula II, wherein a ratio of lithium to a mixed transition metal (M) is in a range of 0.95 to 1.04, and some Ni are inserted into lithium sites to thereby stably support a layered crystal structure.
Therefore, there is no need to add an excess of a lithium source and it is also possible to significantly reduce the possibility of retention of the water-soluble base impurities which may derive from the lithium source.
When the ratio of lithium to the mixed transition metal (M) is in a range of 0.95 to 1.04, this may lead to appropriate cation mixing to thereby further enhance the structural stability and minimize a hindrance of Li+ migration during the charge and discharge process. Therefore, the lithium mixed transition metal oxide exhibits excellent stability, lifespan characteristics and rate characteristics. Further, this metal oxide has a high capacity, as well as high chemical resistance and high-temperature stability due to substantially no water-soluble bases.
In one preferred, embodiment, the lithium mixed transition metal oxide having a composition of Formula II and the ratio of lithium to the mixed transition metal (M) of 0.95 to 1.04 may be a lithium mixed transition metal oxide wherein a cation mixing ratio with location of some Ni in lithium sites is in a range of 0.02 to 0.07 based on the
total content of Ni to thereby stably support the layered crystal structure. A range of the cation mixing ratio is as defined above.
Further, the present invention provides a lithium mixed transition metal oxide comprising mixed transition metal oxide layers ("MO layers") of Ni, Mn and Co, and lithium-ion intercalation/deintercalation layers (reversible lithium layers), wherein the MO layers and the reversible lithium layers are alternately and repeatedly disposed to form a layered crystal structure, the MO layers contain Ni3+ and Ni2+, and some Ni2+ ions derived from the MO layers are inserted into the reversible lithium layers.
Such a structure is not known yet in the art and makes a great contribution to the stability and lifespan characteristics.
Further, the present invention provides a lithium mixed transition metal oxide having a composition of Formula I and showing intercalation/deintercalation of lithium ions into/from mixed transition metal oxide layers ("MO layers"), wherein the MO layers contain Ni3+ and Ni2+, and some Ni2+ ions derived from the MO layers are inserted into the reversible lithium layers, due to preparation of the lithium mixed transition metal oxide by a reaction of raw materials under an 02-deficient atmosphere.
A conventional lithium transition metal oxide has a crystal structure with no insertion of Ni2+ ions into the MO layer by the reaction of raw materials under an oxygen atmosphere, whereas the lithium mixed transition metal oxide in accordance with the present invention has a structure wherein relatively large amounts of Ni2+ ions are produced by carrying out the reaction under an oxygen-deficient atmosphere and some of the thus-produced Ni2+ ions are inserted into the reversible lithium layers. Therefore, as illustrated hereinbefore, the crystal structure is stably maintained to
thereby exert superior sintering stability, and a secondary battery comprising the same exhibits excellent cycle stability.
EXAMPLES
Now, the present invention will be described in more detail with reference to the following Examples. These examples are provided only for illustrating the present invention and should not be construed as limiting the scope and spirit of the present invention.
[Example 1]
A mixed hydroxide of Formula MOOH (M = Ni4Z1S(Mn1Z2Ni1Z2)Sz1SCOo2) as a mixed transition metal precursor and Li2CO3 were mixed in a stoichiometric ratio (Li:M = 1.02:1), and the mixture was sintered in air at various temperatures of 850 to 1000°C for 10 hours, thereby preparing a lithium mixed transition metal oxide. Herein, secondary particles were maintained intact without being collapsed, and the crystal size increased with an increase in the sintering temperature.
X-ray analysis confirmed that all samples have a well-layered crystal structure.
Further, a unit cell volume did not exhibit a significant change with an increase in the sintering temperature, thus representing that there was no significant oxygen-deficiency and no significant increase of cation mixing, in conjunction with essentially no occurrence of lithium evaporation.
The crystallographic data for the thus-prepared lithium mixed transition metal oxide are given in Table 1 below, and FESEM images thereof are shown in FIG. 5. From these results, it was confirmed that the lithium mixed transition metal oxide is
LiNiMO2 having a well-layered crystal structure with the insertion of nickel at a level of 3.9 to 4.5% into a reversible lithium layer. Further, it was also confirmed that even though Li2CO3 was used as a raw material and sintering was carried out in air, proper amounts of Ni2+ ions were inserted into the lithium layer, thereby achieving the structural stability.
Particularly, Sample B, sintered at 900 °C, exhibited a high c:a ratio and therefore excellent crystallinity, a low unit cell volume and a reasonable cation mixing ratio. As a result, Sample B showed the most excellent electrochemical properties, and a BET surface area of about 0.4 to 0.8 m2/g.
<Table 1>
[Comparative Example 1]
50 g of a commercial sample having a composition of LiNio.sCoo^Mno.^ represented by Formula LiNi1-xMxO2 (x = 0.3, and M = Mn1Z3Ni1Z3Co1Z3) was heated in air to 750 °C, 850 °C, 900 "C and 950 °C (10 hrs), respectively.
X-ray analysis was carried out to obtain detailed lattice parameters with high resolution. Cation mixing was observed by Rietveld refinement, and morphology was analyzed by FESEM. The results thus obtained are given in Table 2 below. Referring to Table 2, it can be seen that all of the samples heated to a temperature of T>750°C exhibited continuous degradation of a crystal structure (increased cation mixing,
increased lattice constant and decreased c:a ratio). FIG. 6 shows a FESEM image of a commercial sample as received and a FESEM image of the same sample heated to 850 "C in air; and it can be seen that the sample heated to a temperature of T≥850°C exhibited structural collapse. This is believed to be due to that Li2CO3, formed during heating in air, melts to thereby segregate particles.
<Table 2>
Therefore, it can be seen that it is impossible to produce a conventional lithium mixed transition metal oxide having the above-specified composition in the air containing trace amounts of carbon dioxide, due to thermodynamic limitations. In addition, upon producing the lithium transition metal oxide having the above composition according to a conventional method, the use of Li2CO3 as a raw material is accompanied by evolution of CO2 due to decomposition of Li2CO3, thereby leading to thermodynamic hindrance of additional decomposition of Li2CO3, consequently resulting in no further progression of the reaction. For these reasons, it was confirmed that such a conventional method cannot be applied to the practical production process.
[Comparative Example 2]
The pH titration was carried out at a flow rate of > 2 L/min for 400 g of a commercial sample having a composition of LiNio.sCoo.2O2. The results thus obtained are given in FIG. 7. In FIG. 7, Curve A represents pH titration for the sample as
received, and Curve B represents pH titration for the sample heated to 800 °C in a flow of pure oxygen for 24 hours. From the analysis results of pH profiles, it can be seen that the contents of Li2CO3 before and after heat treatment were the same therebetween, and there was no reaction of Li2CO3 impurities. That is, it can be seen that the heat treatment under an oxygen atmosphere resulted in no additional production of Li2CO3 impurities, but Li2COs impurities present in the particles were not decomposed. Through slightly increased cation mixing, a slightly decreased c:a ratio and a slightly decreased unit cell volume from the X-ray analysis results, it was confirmed that the content of Li slightly decreased in the crystal structure of LiNiO2 in conjunction with the formation of a small amount of Li2O. Therefore, it can be seen that it is impossible to prepare a stoichiometric lithium mixed transition metal oxide with no impurities and no lithium-deficiency in a flow of oxygen gas or synthetic air.
[Comparative Example 3]
LiAlo.o2Nio.78Coo.202 containing less than 3% aluminum compound, as commercially available Al/Ba-modifϊed, high-nickel LiNiO2, was stored in a wet chamber (90% RH) at 60 °C in air. The pH titration was carried out for a sample prior to exposure to moisture, and samples wet-stored for 17 hrs and 3 days, respectively. The results thus obtained are given in FIG. 8. Referring to FIG. 8, an amount of water- soluble bases was low before storage, but substantial amounts of water-soluble bases, primarily comprising Li2CO3, were continuously formed upon exposure to air. Therefore, even when an initial amount of Li2CO3 impurities was low, it was revealed that the commercially available high-nickel LiNiO2 is not stable in air and therefore rapidly decomposes at a substantial rate, and substantial amounts of Li2CO3 impurities are formed during storage.
[Example 2]
The pH titration was carried out for a sample of the lithium mixed transition metal oxide in accordance with Example 2 prior to exposure to moisture, and samples stored in a wet chamber (90% RH) at 60 °C in air for 17 hours and 3 days, respectively. The results thus obtained are given in FIG. 9. Upon comparing the lithium mixed transition metal oxide of Example 2 (see FIG. 9) with the sample of Comparative Example 3 (see FIG. 8), the sample of Comparative Example 3 (stored for 17 hours) exhibited consumption of about 20 mL of HCl, whereas the sample of Example 2 (stored for 17 hours) exhibited consumption of 10 mL of HCl, thus showing an about two-fold decrease in production of the water-soluble bases. Further, in 3-day-storage samples, the sample of Comparative Example 3 exhibited consumption of about 110 mL of HCl, whereas the sample of Example 2 exhibited consumption of 26 mL of HCl, which corresponds to an about five-fold decrease in production of the water-soluble bases. Therefore, it can be seen that the sample of Example 2 decomposed at a rate about five-fold slower than that of the sample of Comparative Example 3. Then, it can be confirmed that the lithium mixed transition metal oxide of Example 2 exhibits superior chemical resistance even when it is exposed to air and moisture.
[Comparative Example 4]
A high-nickel LiNiO2 sample having a composition of LiNio.8Mno.05Coo.t5O2, as a commercial sample which was surface-coated with AlPO4 followed by gentle heat treatment, was subjected to pH titration before and after storage in a wet chamber. As a result of pH titration, 12 mL of 0.1 M HCl was consumed per 1O g cathode, an initial content of Li2COs was low, and the content of Li2CO3 after storage was slightly lower (80 to 90%) as compared to the sample of Comparative Example 3,
but a higher content of Li2CO3 was formed than in the sample of Example 2. Consequently, it was confirmed that the aforementioned high-Ni LiNiO2 shows no improvements in the stability against exposure to the air even when it was surface- coated, and also exhibits insignificant improvements in the electrochemical properties such as the cycle stability and rate characteristics.
[Example 3]
A mixture of Li2Cθ3 with mixed hydroxide of Formula MOOH (M =
was introduced into a furnace with an about 20 L chamber and sintered at 9200C for 10 hours, during which more than 10 m3 of air was pumped into the furnace, thereby preparing about 5 kg of LiNiMO2 in one batch.
After sintering was complete, a unit cell constant was determined by X-ray analysis, and a unit cell volume was compared with a target value (Sample B of Example 1: 33.921 A3). ICP analysis confirmed that a ratio of Li and M is very close to 1.00, and the unit cell volume was within the target range. FIG. 10 shows an SEM image of the thus-prepared cathode active material and FIG. 11 shows results of Rietveld refinement. Referring to these drawings, it was confirmed that the sample exhibits high crystallinity and well-layered structure, a mole fraction of Ni2+ inserted into a reversible lithium layer is 3.97%, and the calculated value and the measured value of the mole fraction of Ni2+ are approximately the same.
Meanwhile, upon performing pH titration, less than 10 mL of 0.1 M HCl was consumed to titrate 10 g of a cathode to achieve a pH of less than 5, which corresponds to a Li2CO3 impurity content of less than about 0.035 wt%. Hence, these results show
that it is possible to achieve mass production of Li2CO3-free LiNiMO2 having a stable crystal structure from the mixed hydroxide and Li2CO3 by a solid-state reaction.
[Experimental Example 1 ] Test of electrochemical properties
Coin cells were fabricated using the lithium mixed transition metal oxide of Example 3, LiNiMO2 of Comparative Examples 2 to 4, and commercial LiMO2 with M
= (Ni1Z2Mn1Z2)I-XCox and x = 0.17 (Comparative Example 5) and x = 0.33 (Comparative
Example 6), respectively, as a cathode, and a lithium metal as an anode.
Electrochemical properties of the thus-fabricated coin cells were tested. Cycling was carried out at 25 °C and 60 °C , primarily a charge rate of C/5 and a discharge rate of C/5 (I C = 150 niA/g) within a range of 3 to 4.3 V.
Experimental results of the electrochemical properties for the coin cells of Comparative Examples 2 to 4 are given in Table 3 below. Referring to Table 3, the cycle stability was poor with the exception of Comparative Example 3 (Sample B). It is believed that Comparative Example 4 (Sample C) exhibits the poor cycle stability due to the lithium-deficiency of the surface. Whereas, even though Comparative Example 2 (Sample A) and Comparative Example 3 (Sample B) were not lithium-deficient, only Comparative Example 4 (Sample C) exhibited a low content Of Li2CO3. The presence of such Li2CO3 may lead to gas evolution and gradual degradation of the performance (at 4.3 V, Li2CO3 slowly decomposes with the collapse of crystals). That is, there are no nickel-based active materials meeting both the excellent cycle stability and the low- impurity content, and therefore it can be confirmed that the conventional nickel-based active materials suffer from poor cycle stability and low stability against exposure to air, in conjunction with a high level OfLi2CO3 impurities and high production costs.
<Table 3>
On the other hand, the cells of Comparative Examples 5 and 6 exhibited a crystallographic density of 4.7 and 4.76 g/cm3, respectively, which were almost the same, and showed a discharge capacity of 157 to 159 niAh/g at a C/10 rate (3 to 4.3 V). Upon comparing with LiCoO2 having a crystallographic density of 5.04 g/cm3 and a discharge capacity of 157 mAh/g, a volume capacity of the cell of Comparative Example 5 is equal to a 93% level of LiCoO2, and the cell of Comparative Example 6 exhibits a crystallographic density corresponding to a 94% level of LiCoO2. Therefore, it can be seen that a low content of Ni results in a poor volume capacity.
Whereas, a crystallographic density OfLiNiMO2 in accordance with Example 3 was 4.74 g/cm3 (cf. LiCoO2: 5.05 g/cm3). A discharge capacity was more than 170 mAh/g (cf. LiCoO2: 157 mAh/g) at C/20, thus representing that the volume capacity of LiNiMO2 was much improved as compared to LiCoO2.
Table 4 below summarizes electrochemical results of coin cells using LiNiMO2 in accordance with Example 3 as a cathode, and FIG. 12 depicts voltage profiles, discharge curves and cycle stability.
<Table 4>
[Experimental Example 2] Determination of thermal stability
In order to examine the thermal stability for the lithium mixed transition metal oxide of Example 3 and LiNiMO2 in accordance with Comparative Examples 3 and 4, DSC analysis was carried out. The thus-obtained results are given in FIGS. 13 and 14. For this purpose, coin cells (anode: lithium metal) were charged to 4.3 V, disassembled, and inserted into hermetically sealed DSC cans, followed by injection of an electrolyte. A total weight of the cathode was in a range of about 50 to 60 mg, A total weight of the electrolyte was approximately the same. Therefore, an exothermic reaction is strongly cathode-limited. The DSC measurement was carried out at a heating rate of 0.5 K/min.
As a result, Comparative Example 3 (Al/Ba-modified LiNiO2) and Comparative Example 4 (AlPO4-coated LiNiO2) showed the initiation of a strong exothermic reaction at a relatively low temperature. Particularly, Comparative Example 3 exhibited a heat evolution that exceeds the limit of the device. The total accumulation of heat generation was large, i.e. well above 2000 kJ/g, thus indicating a low thermal stability (see FIG. 13).
Meanwhile, LiNiMO2 of Example 3 in accordance with the present invention exhibited a low total heat evolution, and the initiation of an exothermic reaction at a relatively high temperature (about 260 "C) as compared to Comparative Examples 3 and
4 (about 200 °C) (see FIG. 14). Therefore, it can be seen that the thermal stability of LiNiMO2 in accordance with the present invention is very excellent.
[Experimental Example 3] Test of electrochemical properties of polymer cells with application of lithium mixed transition metal oxide
Using the lithium mixed transition metal oxide of Example 3 as a cathode active material, a pilot plant polymer cell of 383562 type was fabricated. For this purpose, the cathode was mixed with 17% LiCoO2, and the cathode slurry was NMP/PVDF-based slurry. No additives for the purpose of preventing gelation were added. The anode was MCMB. The electrolyte was a standard commercial electrolyte free of additives known to reduce excessive swelling. Experiments were carried out at 60 °C and charge and discharge rates of C/5. A charge voltage was in a range of 3.0 to 4.3 V.
FIG. 15 shows the cycle stability of the battery of the present invention (0.8 C charge, 1C discharge, 3 to 4 V, 2 V) at 250C . An exceptional cycle stability (91% at C/l rate after 300 cycles) was achieved at room temperature. The impedance build up was low. Also, the gas evolution during storage was measured. The result thus obtained are given in FIG. 16. During a 4 h-90°C fully charged (4.2 V) storage, a very small amount of gas was evolved and only a small increase of thickness was observed. The increase of thickness was within or less than the value expected for good LiCoO2 cathodes tested in similar cells under similar conditions. Therefore, it can be seen that LiNiMO2 in accordance with the present invention exhibits very high stability and chemical resistance.
[Experimental Example 4]
Samples with different Li: M ratios were prepared from Formula MOOH
(M=Ni4Z1S(Mn1Z2Ni1Z2)SZIsCOc2)- Li2CO3 was used as a lithium source. 7 samples each of about 50 g with Li:M ratios ranging from 0.925 to 1.125 were prepared by a sintering process in air at a temperature of 910 to 920 °C . Then, electrochemical properties were tested.
Table 5 below provides the obtained crystallographic data. The unit cell volume changes smoothly according to the Li:M ratio. FIG. 17 shows its crystallographic map. AU samples are located on a straight line. According to the results of pH titration, the content of soluble base increases slightly with an increase of the Li:M ratio. The soluble base probably originates from the surface basicity (ion exchange) but not from the dissolution of Li2Cθ3 impurities as observed in Comparative Example 1.
Therefore, this experiment clearly shows that the lithium mixed transition metal oxide prepared by the method in accordance with the present invention is in the Li stoichiometric range and additional Li is inserted into the crystal structure. In addition, it can be seen that stoichiometric samples without Li2CO3 impurity can be obtained even when Li2CO3 is used as a precursor and the sintering is carried out in air.
That is, as the Li/M ratio decreases, the amount of Ni2+ inserted into the reversible lithium layer gradually increases. Insertion of excessively large amounts of Ni + into the reversible lithium layer hinders the movement of Li+ during the charge/discharge process, thereby resulting in decreased capacity or poor rate characteristics. On the other hand, if the Li/M ratio is excessively high, the amount of Ni2+ inserted into the reversible lithium layer is too low, which may result in structural instability leading to deterioration of the battery safety and lifespan characteristics.
Further, at the high Li/M value, amounts of unreacted IΛ2CO3 increase to thereby result in a high pH-titration value. Therefore, upon considering the performance and safety of the battery, the ratio of Li:M is particularly preferably in a range of 0.95 to 1.04 (Samples B, C and D) to ensure that the value of Ni2+ inserted into the lithium layer is in a range of 3 to 7%.
<Table 5>
[Example 4]
A mixed hydroxide of Formula MOOH (M =
as a mixed transition metal precursor and Li2CO3 were mixed in a ratio of Li:M = 1.01:1, and the mixture was sintered in air at 900 °C for 10 hours, thereby preparing 50 g of a lithium mixed transition metal oxide having a composition of LiNi0.53Coo.2Mno.2702.
X-ray analysis was carried out to obtain detailed lattice parameters with high resolution. Cation mixing was observed by Rietveld refinement. The results thus obtained are given in Table 6 below.
[Comparative Example 7]
A lithium transition metal oxide was prepared in the same manner as in Example 4, except that a ratio of Li:M was set to 1 : 1 and sintering was carried out under
an O2 atmosphere. Then, X-ray analysis was carried out and the cation mixing was observed. The results thus obtained are given in Table 6 below.
<Table 6>
As can be seen from Table 6, the lithium transition metal oxide of Comparative Example 7 exhibited a significantly low cation mixing ratio due to the heat treatment under the oxygen atmosphere. This case suffers from deterioration of the structural stability. That is, it can be seen that the heat treatment under the oxygen atmosphere resulted in the development of a layered structure due to excessively low cation mixing, but migration of Ni2+ ions was hindered to an extent that the cycle stability of the battery is arrested.
[Example 5]
A lithium mixed transition metal oxide having a composition of LiNio.4Cθo.3Mno.3θ2 was prepared in the same manner as in Example 4, except that a mixed hydroxide of Formula MOOH (M = Ni1ZIO(Mn1Z2Ni1Z2)OZ1OCoCs) was used as a mixed transition metal precursor, and the mixed hydroxide and Li2Cθ3 were mixed in a ratio of Li:M = 1:1. The cation mixing was observed by X-ray analysis and Rietveld refinement. The results thus obtained are given in Table 7 below.
<Table 7>
[Example 6]
A lithium mixed transition metal oxide having a composition of LiNio.65Cθo.2Mno.i5θ2 was prepared in the same manner as in Example 4, except that a mixed hydroxide of Formula MOOH (M = Ni5Z1O(Mn1Z2Ni1Z2)SZIoCOo2) was used as a mixed transition metal precursor, and the mixed hydroxide and Li2CO3 were mixed in a ratio of Li:M = 1:1. The cation mixing was observed by X-ray analysis and Rietveld refinement. The results thus obtained are given in Table 8 below.
<Table 8>
From the results shown in Tables 7 and 8, it can be seen that the lithium mixed transition metal oxide in accordance with the present invention provides desired effects, as discussed hereinbefore, in a given range.
INDUSTRIAL APPLICABILITY
As apparent from the above description, a lithium mixed transition metal oxide in accordance with the present invention has a specified composition and interconnection of MO layers via the insertion of some MO layer-derived Ni ions into
reversible lithium layers, whereby the crystal structure can be stably supported to thereby prevent the deterioration of cycle characteristics. In addition, such a lithium mixed transition metal oxide exhibits a high battery capacity and is substantially free of impurities such as water-soluble bases, thereby providing excellent storage stability, decreased gas evolution and consequently superior high-temperature stability.
Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.