KR100364659B1 - CATHODE MATERIAL, Li(Mn1-δCoδ)2O4 FOR LITHIUM-ION SECONDARY BATTERY AND METHOD FOR PREPARING THE SAME - Google Patents

Abstract

The present invention has a stable crystal structure, and a composition in which a small amount of cobalt is substituted for manganese oxide instead of manganese in lithium manganese oxide, which is a positive electrode active material for 4V lithium secondary batteries, which can satisfy high capacity, stability, low cost, and pollution-free. The present invention relates to a simple manufacturing method, which eliminates the disadvantages of lithium cobalt oxide used in the related art and greatly improves the low discharge capacity and cycle performance of pure lithium manganese oxide by substituting a small amount of cobalt instead of manganese.

Description

Li (Mn1-δCｏδ) 2O4 composition, a cathode active material for a lithium secondary battery, and a method of manufacturing the same

The present invention relates to the composition of a cathode active material, which is a core material of a lithium secondary battery, and a manufacturing method thereof, and a lithium manganese having an AB ₂ O ₄ (A, B = metal) spinel structure, which is a cathode active material for a lithium secondary battery. The present invention relates to a method of simply preparing a composition in which a small amount of cobalt is substituted for manganese in an oxide (LiMn ₂ O ₄ ) by a solid phase reaction method.

LiMn ₂ O ₄ is a material to replace LiCoO ₂ , which is commercialized as a cathode material of a lithium secondary battery, and a lot of research has been conducted with LiNiO ₂ . LiCoO ₂ and LiNiO ₂ have a risk of explosion due to the generation of oxygen during overcharging and need to install vent cap in the battery, but LiMn ₂ O ₄ has no oxygen generation during overcharging and the price of Mn, which is a relatively rich element compared to Co This has the advantage of being cheap. However, LiMn ₂ O ₄ has a theoretical charge capacity of 148 mAh / g, which is lower than LiCoO ₂ (274 mAh / g), and has a disadvantage in that the discharge capacity decreases rapidly as the charge and discharge cycles are repeated. According to W. Liu et al. (J. Electrchem. Soc., Vol. 143, No. 11, pp. 3590-3596, 1996), the reason why this cycle performance is poor is that lithium ion at the positive electrode as the charge and discharge continues. Li ₂ Mn ₂ O ₄ is formed due to the insertion, it is known that the phase transition to tetragonal. Due to lithium insertion, the average valence of Mn ions falls below 3.5, and strong Jahn-Teller strains (G. Pistoia et al., Solid State Ionics, Vol. 78, pp. 115-122, 1995) act to crystallize the crystal structure. cubic) is transformed into tetragonal, and as a result, further removal and insertion becomes difficult. In other words, the array of trivalent Mn ions (in spinel) (t ³ 2g · e ¹ g, high spin) is a strong elongation of the octahedron, which increases the c / a ratio by 16% per unit cell and causes the charge and discharge to repeat While maintaining the structural equilibrium of the anode. In addition, in the valence structure of Mn ³⁺ (3d ⁴ ) which occurs when the valence returns to 3.5 when lithium is released, it is known that the charge and discharge capacity is drastically reduced due to the difficulty of discharge.

In order to improve this problem, a part of Mn is substituted with another element to suppress the crystal structure transition caused by the Jahn-Teller effect, a small amount of Mn is substituted with Li (Li _{1 + δ} Mn _2-δ O ₄ ), or a cation There is a method of forming a defective spinel (Li _1-δ Mn _2-2δ O ₄ ) to maintain an average valence of Mn ions of 3.5 or more.

In order to solve the problem of LiMn ₂ O ₄ , which has many advantages but disadvantages of poor cycle performance, the Jahn-Teller strain is inhibited to prevent phase transition and the average valence of Mn ions is maintained at 3.5 or higher. It is a main object to provide a method for producing a new composition in which a small amount of Co is substituted for Mn in order to increase the discharge capacity by a solid phase reaction method, which is a simple and convenient method as the most typical manufacturing process.

1 is a manufacturing process chart of the positive electrode active material prepared by the solid-phase reaction method according to an embodiment of the present invention.

2 is an X-ray diffraction diagram of the positive electrode active material powder prepared according to the embodiment of the present invention.

Figure 3 is a scanning electron micrograph comparing the microstructure before and after the charge and discharge for the positive electrode active material and pure lithium manganese oxide prepared according to an embodiment of the present invention [a. Before charging and discharging experiment LiMn ₂ O ₄ , b. After 7 charge and discharge experiments, LiMn ₂ O ₄ , c. Before charging / discharging experiments Li (Mn _0.9 Co _0.1 ) ₂ O ₄ , b. After 7 charge / discharge experiments, Li (Mn _0.9 Co _0.1 ) ₂ O ₄ ].

Figure 4 is a graph comparing the change in voltage according to the charge and discharge experiments for the positive electrode active material Li (Mn _0.9 Co _0.1 ) ₂ O ₄ prepared according to an embodiment of the present invention.

5 is a graph showing a change in discharge capacity according to the number of cycles by performing 20 charge and discharge experiments on the cathode active material and pure lithium manganese oxide prepared according to the embodiment of the present invention.

A feature of the present invention is to prepare a lithium secondary battery cathode active material Li (Mn _1-δ Co _δ ) ₂ O ₄ by a solid phase reaction method. In general, the solid phase synthesis method is a hydrothermal synthesis method, a Pechini process, a precipitation method, a wet method such as an evaporation method, and the like. Compared with the production method, the synthesized powder has a larger particle size, and thus, the density of the anode coated with the powder is also small, and thus the charge and discharge capacity is low. However, since the synthesis and manufacturing process is easy, and the cost and manufacturing time are largely saved, it is advantageous in terms of economics and competitiveness in industrial applications. The present invention provides a method for producing a cathode active material for a lithium secondary battery having excellent performance even in a solid phase reaction method using the substitution amount of Co and all manufacturing processes as optimum conditions.

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

(Example)

(A) Preparation of positive electrode active material

As a starting material for the cathode active material Li (Mn _1-δ Co _δ ) ₂ O ₄ for a lithium secondary battery, high purity reagents Li ₂ CO ₃ , MnO ₂ , and CoO are used. At this time, δ is measured according to the stoichiometric composition ratio such that 0, 0.05, 0.1, and 0.2, respectively. In the mixing and grinding process, the ball mill was used for 24 hours using alumina balls, followed by drying. Then, the resultant was press-molded into pellets, calcined in an air atmosphere at 780 ° C. for 48 hours, and then pulverized to prepare a cathode active material. do. The general manufacturing process is shown in FIG. Fig. 2 shows the results of phase analysis analyzed by the X-ray diffractometer for the synthesized powder. At δ = 0, i.e. for pure LiMn ₂ O ₄ , diffraction peaks of (111), (311), (222), (400), (511), (440) and (511) planes were observed, respectively. The space group was found to have a spinel structure with Fd3m (cubic, no. 227).

The samples substituted with Co all have the same pattern of pure LiMn ₂ O ₄ in the materials of δ = 0.05, 0.1, and 0.2, and the peaks coincide with each other to form a completely substituted solid solution in the above composition ratio.

(B) Manufacture of half cell

NMP containing 2% by weight of polyvilylidene fluoride (PVDF) in a mixed powder of 93% by weight of Li (Mn _1-δ M _δ ) ₂ O ₄ and 5% by weight of acetylenized carbon black for the production of a charge / discharge positive electrode After dispersing in a (n-methyl-pyrrolidinone) solution to make a slurry, it is applied to ex-met (SUS 316, 325 mesh) of 1.5cm ² and dried for 1 hour at 120 ℃ in vacuum to prepare an electrode. In the charge / discharge experiment, a half cell including Li metal as a negative electrode and a reference electrode was measured in a 2 hour step. In this case, the distance between the cathode and the reference electrode is 1.5 cm, and the distance between the anode and the reference electrode is 1.0 cm.

(C) Photograph of the surface of the positive electrode active material before and after the charge and discharge test

3 is a scanning electron microscope (SEM) photograph of the surface of each positive electrode specimen before and after 7 charge / discharge tests of LiMn ₂ O ₄ and Li (Mn _0.9 Co _0.1 ) ₂ O ₄ . As shown in the photograph, in the first specimen, the crystallized powder particles can be seen in FIGS. 3a and c. However, it was confirmed that Li ions with weak crystallinity did not completely escape from the crystals of the anode due to lithium dissociation due to the charging experiments, and began to form metal layers on the surface thereof. Reaction with the Li metal layer accumulated in the layer is occurring. That is, as the lithium ions released from the cathode are inserted into the anode, some of them are caught in the Li dendrite layer formed on the surface layer and reprecipitated, thereby increasing the thickness of the Li layer on the anode surface. It is believed that the Li layer becomes a so-called dead metal that prevents the diffusion of Li ions into the anode and cathode and does not contribute to further charging and discharging while repeating reciprocation only on the surface of the anode (M. Water et al., J. Electrochem). Soc., Vol. 145, No. 2, pp. 428-435, 1998). 3b and d, which are photographs of the specimens after seven charge and discharge experiments, show that the metal Li layer is hot enough to be difficult to find the crystal surface. Can be seen that the Li layer of pure LiMn ₂ O ₄ surface _{Li (Mn 0.9 Co 0.1) 2} O 4 stacked much more than Li layer on the surface, this Li than that in pure LiMn ₂ O ₄ de-insertion of Li ( Mn _0.9 Co _0.1 ) ₂ O _{4 It} seems to be because it is smoother. As expected, this phenomenon is due to the insertion of lithium, the average valence of Mn ions falls below 3.5, and the strong Jahn-Teller strain acts to transform the crystal structure from cubic to tetragonal, making it more difficult to remove. In addition, it is thought that the effective Li elimination was prevented by the difficulty of discharge due to the Mn ³⁺ (3d ⁴ ) valence structure when the valence returns to 3.5 upon lithium elimination. And, it can be predicted that the formation of such a Li dendritic layer is a decisive cause of reducing the capacity and cycle performance, especially at the cathode, but also the effect is not small at the anode.

(D) Charging and discharging characteristics

In Li (Mn _1-δ Co _δ ) ₂ O ₄ compositions, the charge and discharge capacity was highest at δ = 0.1. Charging and discharging characteristics of Li (Mn _0.9 Co _0.1 ) ₂ O ₄ at cut-off voltage of 3.5 V to 4.5 V are shown in FIG. 4, and initial charge and discharge capacities were 118.76 mAh / g and 112.69 mAh / g, respectively. 80% and 76% of theoretical capacity were shown. In addition, the potential plateau during charging was found to be between 4.05 V and 4.20 V, and during discharge, between 4.05 V and 3.85 V. The potential flatness area of 4.05 V during charging and discharging coincides with the flat area of a typical lithium manganese battery, and the flat potential between charging and discharging is almost symmetrical and stable.

5 shows the change in discharge capacity according to the number of cycles when the charge-discharge experiment was conducted with the cut-off voltage from 3.0 V to 4.2 V. FIG. It can be seen that the cycle performance of Li (Mn _0.9 Co _0.1 ) ₂ O ₄ is much higher than that of unstable discharge capacity or low discharge capacity of pure LiMn ₂ O ₄ . The cycle performance of pure LiMn ₂ O ₄ is poor due to the effect of deformation due to the collapse of crystallinity during charging and discharging and the formation of thick Li layer on the anode surface. Prepared as shown in the embodiment of the present invention, a substituted anode a part of Mn by Co active material, Li (Mn _0.9 Co _{0.1) 2} capacity and cycle performance of the O ₄ is LiMn ₂ O produced in a number of conventional types of wet production method _It is thought to be superior to or equivalent to the capacity and cycle performance of the positive electrode material for a _four -phase lithium secondary battery (W. Liu et al., J. Electrchem. Soc., Vol. 143, No. 11, pp. 3590-3596, 1996, W Yang et al., Solid State Ionics, Vol. 121, pp. 79-84, 1999).

Li (Mn _1-δ Co _δ) in the lithium manganese oxide based LiCoO ₂ alternative material, which is a commercially available cathode active material for lithium secondary batteries, is replaced with Li (Mn _1-δ Co _δ) , in which a pure lithium manganese oxide or a part of Mn is substituted with other elements such as Ni, Ti, and Nb. ) ₂ O ₄ (0 <δ <0.2) The composition has excellent charge / discharge capacity and cycle performance. The cathode active material synthesis process is simpler and easier than the complicated wet manufacturing method, and thus the cathode active material having excellent charge and discharge characteristics can be produced by the solid phase reaction method proposed in the present invention.

Claims (1)

As a cathode active material for 4V class lithium secondary batteries, Li (Mn _1-δ Co _δ ) ₂ O ₄ (0 <δ <0.2) was prepared using Li ₂ CO ₃ , MnO _2, and CoO powder, and the powder thus prepared. After mixing, finely pulverized and press-molded into pellets, calcined (calcination) in an air atmosphere for 48 hours at a temperature of 780 ℃ and then pulverized to produce a positive electrode active material powder lithium secondary battery positive electrode Active material composition and its manufacturing method.