KR19990052170A - Lithium-cobalt-manganese oxide (LICOXMN2-XO4) of cathode material of 5V class lithium secondary battery and its manufacturing method - Google Patents

Abstract

The present invention relates to a positive electrode material of a lithium secondary battery, and proposes a lithium-cobalt-manganese oxide as a positive electrode material, and a production method thereof and a lithium secondary battery using the same. Generally, a lithium secondary battery is composed of a cathode, an electrolyte, and a cathode. Lithium-manganese oxide, known as a conventional 4V anode material, is inexpensive and has a relatively high capacity of 110 mAh / g. However, it is difficult to produce a stoichiometric compound, and a reduction in capacity There is a disadvantage that electrode characteristics are deteriorated. The present invention proposes a lithium-cobalt-manganese compound in which cobalt is substituted for manganese in order to improve the above disadvantages. The lithium-cobalt-manganese compound is structurally stable as compared with the lithium-manganese compound, so that the reduction in capacity due to cycle repetition is small and can be applied to a high voltage.

Description

Lithium-cobalt-manganese oxide (LiCoxMn2-xO4) as a cathode material of 5V-class lithium secondary battery and method for manufacturing the same

The present invention relates to a positive electrode material of a lithium secondary battery, and more particularly, to a lithium-cobalt-manganese oxide for use as a positive electrode material of a lithium secondary battery and a method for producing the same, The present invention relates to a battery.

The lithium-manganese oxide used as a positive electrode of a conventional lithium secondary battery has a spinel structure. That is, half of the octahedral and tetrahedral sites are empty in the cubic close-packed oxygen arrangement, so that other ions can be assigned to vacancies in octahedral or tetrahedral spaces. This structure is called a "three-dimensional [1 × 1] tunnel structure" because the vacant space of this octahedral space serves as a three-dimensional empty tunnel structure in the spinel structure.

LiMn ₂ O ₄ as a positive electrode is charged by discharging Li ions from the LiMn ₂ O ₄ active material. When overcharging occurs, the LiMn ₂ O ₄ structure is extremely changed to λ-MnO _{2. If} the LiMn ₂ O _{4 is} discharged again, LiMn ₂ O ₄ A return to the structure is made. Theoretically, the theoretical capacity of LiMn ₂ O ₄ is 148 mAh / g, which is lower than that of LiCoO ₂ or LiNiO ₂ , but it is highly competitive in terms of price, so expectation of lithium secondary battery as a cathode active material is very high. Under equilibrium conditions, Li / LiMn ₂ O ₄ cells are inserted into the spinel framework of Mn ₂ O ₄ at about 4 V during discharge to form an isotropic structure of LiMn ₂ O ₄ . In the charge / discharge curve, a flat curve appears near 4V. At this time, the surface of the Li _x Mn ₂ O ₄ electrode is formed with Li _{1 + δ} Mn ₂ O ₄ and the average oxidation number of manganese is less than 3.5, The average molecular formula is Li _{1 - δ} Mn ₂ O ₄ , and the oxidation number of manganese is larger than 3.5. As a result, there is a difference in Jahn-Taller effect between the surface and the interior of the electrode. .

What is pointed out as a problem of spinel LiMn ₂ O ₄ which has been used as a positive electrode material of a conventional lithium secondary battery is a drastic decrease in discharge capacity due to repetition of charging and discharging cycles. The reason is that the diffusion rate of Li ions moving through the three-dimensional path structurally is smaller than that of a layered material such as LiNiO ₂ or LiCoO ₂ having a two-dimensional Li ion path in a layered structure, , It is known that the effective space for the intercalation / deintercalation of Li becomes small due to the change of the Li content, or the dissolution of Mn into the electrolyte during the charging and discharging process. In addition, the polarization is relatively strongly caused by the diffusion rate of Li ions, thereby increasing the internal resistance of the whole battery and decreasing the characteristics thereof.

Therefore, the present invention proposes a lithium-cobalt-manganese oxide which overcomes the above disadvantages. The above lithium-cobalt-manganese oxide is structurally stable as compared with the lithium-manganese oxide known as the conventional 4V-type cathode material, so that the capacity decrease due to cycle repetition is small and the capacity in the high voltage region (5V) It can be used as anode material of secondary battery. Accordingly, an object of the present invention is to provide a lithium secondary battery improved in capacity and performance by using the lithium-manganese oxide.

In the present invention, lithium-cobalt-manganese oxide in which a part of Mn is substituted with Co is used as a positive electrode material of a lithium secondary battery to constitute a lithium secondary battery.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart showing a method for producing lithium-cobalt-manganese oxide according to the present invention. FIG.

FIG. 2 is a characteristic diagram showing an X-ray diffraction analysis pattern of the lithium-cobalt-manganese oxide according to the present invention,

FIG. 2A is a characteristic diagram of lithium-cobalt-manganese oxide prepared at 850 ° C. using EMD. FIG.

FIG. 2B is a characteristic diagram of lithium-cobalt-manganese oxide produced at 800 ° C. using EMD.

FIG. 3 is a characteristic diagram showing a capacity change according to the replacement amount of cobalt in the lithium-cobalt-manganese oxide of the present invention,

3A and 3B are characteristic diagrams of lithium-cobalt-manganese oxide prepared at 850 ° C. using EMD.

FIGS. 3 (a) and 3 (b) are graphs showing the characteristics of lithium-cobalt-manganese oxide prepared at 800 ° C. using EMD.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

A spinel compound of lithium-chromium-manganese oxide (LiCo _x Mn _2-x O ₄ , 0? _X ? 0.5) was prepared using LiOH, Co ₃ O ₄ , and MnO ₂ as starting materials. For the preparation of stoichiometric compounds, CO ₃ O ₄ and MnO ₂ were weighed at precise molar ratios, and LiOH was mixed with 10% excess because the atomic mass of Li was very small and the vapor pressure was high. The powders were mixed with each other in a mortar to prepare specimens. The powders were reacted at 800 ° C to 850 ° C for 18 hours twice and cooled to room temperature at a rate of 50 ° C / hour to obtain lithium-cobalt-manganese oxide (LiCo _x Mn _{2- x} O ₄ (0.0? _x? 0.5)). That is, each of the above-mentioned samples is ground into powders, followed by calcination, sintering and cooling to produce desired oxides. In the intermediate process, the samples are re-ground, mixed, and reacted to produce desired oxides. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart showing a process for producing lithium-cobalt-manganese oxide.

The lattice parameter was calculated by XRD and the crystal system was confirmed. The particle size of the sample was measured by SEM.

As a result of X-ray diffraction analysis of the lithium-cobalt-manganese oxide thus prepared, it was confirmed that the sample prepared was a spinel having a space group Fd3m (FIG. 2). As a result of X-ray diffraction analysis, the lattice constant according to the substitution amount of Co tended to decrease gradually. The lattice constant of the sample synthesized at 850 ℃ was small and the lattice constant was abruptly decreased when the content of Co was over 0.2. It is believed that this is due to the evaporation of Li in the synthesis conditions at 850 ° C and shows a non-stoichiometric composition. As the amount of Co increases, a sharp decrease in the lattice constant occurs due to a smaller ionic radius of Co ³⁺ than that of Mn ³⁺ .

To investigate the electrode characteristics of lithium-cobalt-manganese oxide, a half cell of Li // LiCo _x Mn _2-x O ₄ was constructed as follows. Acetylene black (wt. 10%) and PTFE (polytetrafluoro-ethylene, wt. 1%) were used as a conductive agent in LiCo _x Mn _2-x O ₄ (wt. , Lithium metal as a cathode and 1M LiPF ₆ as an electrolyte were dissolved in a mixed solvent of ethylene carbonate (EC) + dimethyl carbonate (DMC) in a volume ratio of 2: 1 to form a half cell . FIG. 3A shows discharge voltage curves of LiCo _x Mn _2-x O ₄ synthesized at 850 ° C and discharge curves according to cycles. As the Co substitution amount increased, the discharge capacity decreased as well as at 800 ° C. When the Co was replaced by more than 0.2, the abrupt capacity decreased, but the discharge capacity according to the cycle remained unchanged .

FIG. 3B shows the discharge voltage curve of LiCo _x Mn _2-x O ₄ synthesized at 800 ° C and the change in capacity according to the cycle. In LiCo _x Mn _2-x O ₄ , the discharge capacity was maintained at a substitution amount of Co of 0.1 and the capacity was decreased at a value of 0.1 or more. However, the discharging capacity according to the cycle was maintained at a substantially constant level and exhibited excellent reversible characteristics . From the above results, a small amount of Li vacancy can be predicted at a site of Li than at a sample prepared at 850 ° C at a temperature of 800 ° C. That is, the above-mentioned lithium-cobalt-manganese oxide substituted with cobalt stabilizes the phase of 5V, thereby increasing the capacity at the high voltage and decreasing the capacity at the 4V region, but the overall capacity is preserved.

Unlike the conventional lithium secondary battery, when the lithium-cobalt-manganese oxide is used as the positive electrode, the capacity decrease due to cyclic repetition is small and the cycle characteristics are improved, and the battery is stable in a high voltage range.

Claims (4)

In the oxide for a positive electrode material of a lithium secondary battery, (MnO ₂ ) and cobalt oxide (Co ₃ O ₄ ) selected from LiOH, LiCO ₃ and LiNO ₃ , mixing the powders, heating the mixture at a temperature of 800 to 850 ° C. for 18 to 20 hours Cobalt-manganese oxide (LiCo _x Mn _2-x O ₄ , (0 < _x &lt; = 0.5)) for a cathode material of a lithium secondary battery. An oxide of 0 < _x? 0.5 in lithium-cobalt-manganese oxide (LiCo _x Mn _2-x O ₄ ) is used as the anode and a compound containing lithium metal or carbon compound or lithium is used as the cathode, A lithium secondary battery operated at 5 V or higher, which is fabricated using a polymer electrolyte. LiOH, MnO _2, further comprising: a basis weight of Co ₃ O ₄ in the preparation of a stoichiometric compound as MnO ₂ and Co ₃ O ₄ and the exact molar ratio, Mixing the powders in a mortar so as to mix them well, Reacting the mixed powder at 800 DEG C to 850 DEG C for 18 to 20 hours twice, Cooling the reacted sample to room temperature at a rate of 50 ° C / hour Cobalt-manganese oxide (LiCo _x Mn _2-x O ₄ (0.0 _{? X?} 0.5)) for use as a positive electrode material of a lithium secondary battery is produced by performing the above- Lithium-manganese-iron oxide manufacturing method. 4. The method of claim 3, wherein the weighing step comprises: Wherein the LiOH is mixed with an excess of 10%.