JPH04301366A - Lithium secondary battery - Google Patents

Abstract

PURPOSE:To improve rate characteristic of discharge by using a material, substituting H for partly Li of LiCoO2, as a positive pole active material. CONSTITUTION:Lithium carbonate is mixed in cobalt carbonate while crushed to bake a mixture in the air. The mixture, after it is baked, is slowly cooled to a room temperature to obtain LiCoO2. Next, this LiCoO2, after immersed in HNO3, is washed and dried to obtain a positive pole active material. Thus by displacing partly a structure of LiCoO2 with a structure of CoOOH of high conductivity, electronic conductivity is improved to inside the active material to increase a utilization factor at high rate discharge time.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly to a positive electrode thereof.

0002

BACKGROUND OF THE INVENTION Lithium batteries using lithium as a negative electrode active material have become widely used because of their characteristics of high voltage, high energy density, and high reliability, but these are primary batteries. Recently, research on secondary batteries has been actively conducted, and some have even been put into practical use. However, the characteristics of these batteries are still not sufficient. Typical positive electrode active materials that have been studied in the past include MoS2,
V2 O5 , MnO2 , NbSe3 , TiS2
and so on. Among these, MnO2 has attracted particular attention because it is inexpensive, and various modified manganese oxides have been proposed. For example, JP-A-1-67869 and JP-A-2
Although disclosed in Japanese Patent No. 183963, all of these manganese oxides have an operating voltage of about 3 V and a not so large capacity.

0003

[Problem to be solved by the invention] Recently, LiCoO2, which has a capacity similar to that of manganese oxide but has a high operating voltage of about 4V, has been attracting attention and research as an active material that can obtain high voltage and high energy density. ing. The characteristics required of a lithium secondary battery include high voltage, high energy density, and long life, as well as excellent discharge rate characteristics. However, when a discharge rate characteristic test was conducted using LiCoO2, it was found that there was a problem in that the utilization rate of the active material was low during high rate discharge. In order to solve the above-mentioned problems, the present invention aims to obtain LiCoO2 whose utilization rate is less reduced even during high rate discharge.

0004

[Means for Solving the Problems] The present invention provides LiCoO2
The above-mentioned problems are solved by a lithium secondary battery equipped with a positive electrode in which a portion of Li is replaced with hydrogen (H) as a positive electrode active material.

[0005]

[Operation] Part of Li in LiCoO2 is converted into hydrogen (H)
By replacing LiCoO2, which is a positive electrode active material,
A part of the structure is replaced with a highly conductive CoOOH structure. As a result, unlike the conventional case where a conductive agent is added and a conductive network structure is placed only on the surface, the conductive network structure is placed inside the active material, and electron conduction is conducted even inside the active material. It is thought that the decrease in the utilization rate can be suppressed to a small extent even during high rate discharge due to the improved performance.

[0006]

EXAMPLES The details of the present invention will be explained below based on examples.

(Example 1) First, in preparing a positive electrode active material, 37 g of lithium carbonate, a commercially available special grade reagent, and 119 g of cobalt carbonate were thoroughly mixed while being ground in a ball mill, and the mixture was placed in an alumina crucible and heated in air for 650 g. °C, 5
After being pre-baked for an hour, it was fired at 950°C for 20 hours. After firing, it was slowly cooled to room temperature, and the resulting powder was used as a positive electrode active material. The X-ray diffraction pattern of the obtained product is shown in FIG. From FIG. 1, it can be seen that the obtained product is LiCoO2. Furthermore, the obtained LiCo
50 g of O2 was soaked in 300 ml of 0.1N HNO3. Seven types of samples were prepared with immersion times of 0, 0.5, 1, 3, 5, 10, and 15 hours. After immersion, the powder was thoroughly washed with water and dried, and the resulting powder was used as a positive electrode active material. The electronic conductivity of each of the seven types of positive electrode active materials thus obtained was measured. The measurement results are shown in Table 1. From Table 1, it can be seen that as the immersion time becomes longer, active materials with better electronic conductivity are obtained.

[0008]

(Example 2) Using the seven types of positive electrode active materials obtained in Example 1, seven batteries were experimentally manufactured in the following manner. The positive electrode active material, acetylene black, and polytetrafluoroethylene powder were mixed at a weight ratio of 85:10:5, toluene was added, and the mixture was sufficiently kneaded. This was molded into a sheet with a thickness of 0.8 mm using a roller press. Next, this was punched into a 16 mm circular shape and heat-treated at 200° C. for 15 hours under reduced pressure to obtain a positive electrode.

[0010] The negative electrode was used by punching out a lithium foil with a thickness of 0.3 mm into a circle with a diameter of 15 mm, and pressing it onto a negative electrode can through a current collector. The non-aqueous electrolyte contains γ-butyrolactone and 1
A solution containing mol/l of LiBF4 was used, and a microporous thin film made of polypropylene was used as a separator. Using the above positive electrode, negative electrode, electrolyte and separator, the diameter is 20 mm.
A button-shaped lithium battery with a thickness of 1.6 mm was produced. The batteries thus produced are labeled A, B, C, D, E, F, and G in the order of decreasing active material immersion time.

[0011] Batteries A, B, C, and
A discharge rate characteristic test was conducted using D, E, F, and G. Test conditions are charging current 3mA, charging end voltage 4.5V.
, a discharge current of 15 mA, and a discharge end voltage of 3.0V. The results are shown in FIG. 2.

From FIG. 2, it can be seen that batteries F and G have a smaller discharge capacity than battery A, in which a portion of Li is not replaced with hydrogen (H). Also, batteries B, C, D and E
It can be seen that the discharge capacity of battery A is larger than that of battery A. From this, it is thought that in Batteries F and G, the replacement of Li and H progressed too much, the amount of Li involved in the battery reaction decreased, and the theoretical capacity of the active material itself decreased, resulting in a decrease in discharge capacity.

[0013] Furthermore, in batteries B, C, D, and E, the substitution of Li and hydrogen (H) progressed moderately, and a conductive network structure was formed inside the active material, which had not been used until now. It is thought that the discharge capacity increased due to the use of Li.

[0014] From these facts, it is presumed that there is an optimum value for the amount of substitution of Li and H. At least in this example, the reaction time is preferably 10 hours or less.

[0015]

[Effects of the Invention] As mentioned above, a positive electrode using an active material obtained by substituting a portion of Li in LiCoO2 with H can be used at a high rate of discharge compared to a positive electrode using unsubstituted LiCoO2 as an active material. usage rate is high. As a result, the lithium secondary battery including the positive electrode, negative electrode, and electrolyte according to the present invention can significantly improve the discharge rate characteristics of the conventional lithium secondary battery. Note that the present invention is not limited to the manufacturing method of the active material, the positive electrode, the negative electrode, the electrolyte, the separator, the battery shape, etc. described in the examples.
It is also applicable to those using other acids instead of NO3, those using organic fired bodies for the negative electrode, and those using solid electrolytes instead of electrolytes and separators.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an X-ray diffraction pattern of LiCoO2.

FIG. 2 is a relationship diagram between discharge capacity and voltage.

Claims (1)

[Claims] 1. A lithium secondary battery characterized in that a positive electrode active material is LiCoO2 in which part of the Li is replaced with hydrogen (H).