Non-manufacturing reasons for failure of VRLA batteries in electric bicycles

The demand for high energy density and high performance batteries is increasing, which puts higher requirements on the performance of positive electrode active materials[1]. In order to improve the electrochemical performance of positive electrode materials, surface modification or mechanical addition is usually adopted to form a conductive network. There are two main methods for surface modification of spherical Ni(OH)2: ① coating a layer of metal nickel film on the surface by chemical plating; ② coating a layer of Co(OH)2[2] or Co(OH)3 (referred to as cobalt coating) on ​​its surface to generate a stable, highly conductive material that is not reduced during discharge[3-4]. The authors of this paper first formed a uniform cobalt salt layer on the surface of spherical Ni(OH)2 by drying and crystallizing, and then alkalized it to convert it into Co(OH)2 to achieve the purpose of cobalt coating.

1 Experiment

1.1 Preparation of materials

Put spherical Ni(OH)2 (produced in Yixing, nickel content ≥56.0%) into a reversible reactor, add a pre-prepared aqueous solution of cobalt salt (produced in Jilin, cobalt content ≥21.5%) by spraying, and evaporate while reversing, controlling the evaporation temperature to (80±5)℃ until it evaporates into dry powder or colloid; then add a pre-prepared NaOH solution (produced in Jiangsu, ≥32.0%), and react while reversing, controlling the reaction temperature to (70±5)℃. After the reaction is completed, filter, wash, dry and sieve to obtain a spherical Ni(OH)2 product with surface coated with Co(OH)2. Its tap density is measured to be 2.05g/ml. The material prepared by the integral feeding cobalt coating method is used as a comparative sample[5].

1.2 Electrochemical performance test of materials

The sample and nickel powder were mixed evenly at a mass ratio of 1:4. About 0.2g of the mixture was weighed and applied on nickel foam (produced in Jiangyin, with a surface density of 300g/m2), pressed into small thin sheets, and immersed in a 6mol/L KOH (produced in Jiangsu, ≥90.0%) solution. Cyclic voltammetry was performed on a LK98C electrochemical test system (produced in Tianjin) using a three-electrode system. The counter electrode was a Pt electrode, the reference electrode was a HgR/HgO electrode, the scanning speed was 0.001V/s, and the scanning range was -0.150~0.450V. The same mass of positive electrode material was made into sample batteries according to the SC type 1500mAh battery manufacturing process. The batteries made of the prepared cobalt-coated Ni(OH)2 are called sample batteries No. 1, No. 2, No. 3 and No. 4, respectively. The battery made of the material prepared by the integral feeding cobalt coating method is called sample battery No. 5, and the battery made by the mechanical method of externally doping conductive agent (CoO) is called sample battery No. 6 and No. 7. The test instrument is BS9360 battery performance test device (produced in Guangzhou).

2 Results and discussion

2.1 XRD test

The XRD patterns of spherical Ni(OH)2 without cobalt coating and 3.0% cobalt coating are shown in Figure 1.

As can be seen from Figure 1, the spherical Ni(OH)2 without cobalt coating is a β-type structure, with characteristic peaks appearing at 19.20, 33.40 and 38.90, and the characteristic peaks of spherical Ni(OH)2 with 3.0% cobalt coating appear at 19.10, 33.30 and 38.70, and no other impurity peaks are found, indicating that the structure of spherical Co(OH)2 with 3.0% cobalt coating is consistent with that of the spherical Ni(OH)2 without cobalt coating.

2.2 Cyclic voltammetry test

The cyclic voltammetry curves of spherical Ni(OH)2 without cobalt coating and 3.0% cobalt coating are shown in Figure 2.

As shown in Figure 2a, pure spherical Ni(OH)2 has an oxidation peak at 0.312V, with a peak current of 17.362mA, and a reduction peak at 0.064V, with a peak current of -11.882mA. As shown in Figure 2b, spherical Ni(OH)2 with 3.0% cobalt coating has an oxidation peak at 0.306V, with a peak current of 21.165mA, and a reduction peak at 0.078V, with a peak current of -14.945mA. The △E of Ni(OH)2 with 3.0% cobalt coating is 0.020V lower than that of spherical Ni(OH)2 without cobalt coating, while the current is 3-4mA larger, indicating that spherical Ni(OH)2 with 3.0% cobalt coating has good cycle performance and conductivity.

2.3 Battery performance test

Table 1 shows the effect of the cobalt coating method and the amount of cobalt coating on the specific capacity.

As can be seen from Table 1, with the increase of cobalt content, the discharge specific capacity of the battery increases; with the same cobalt content, the discharge specific capacity of the present cobalt coating method is about 7% higher than that of mechanical external cobalt addition, and slightly higher than that of the cobalt coating method using integral feed. Similarly, there are obvious differences in the 10C discharge performance and the discharge performance after 3C fast charging, which shows that the product obtained by the present cobalt coating method has better high-rate charge and discharge performance.

As can be seen from Figure 3a, the discharge platform and capacity of the sample battery prepared by the present cobalt coating method are higher than those of other sample batteries prepared by external cobalt addition, indicating that the material of the present cobalt coating method has higher conductivity than other external cobalt additions. As can be seen from Figure 3b, the sample battery prepared by the present cobalt coating method has excellent high-rate charge and discharge cycle performance.

3 Conclusions

The dry crystallization alkalization method is used to first coat the surface of spherical Ni(OH)2 with a cobalt salt layer, and then alkalize to form Co(OH)2. The cobalt coating layer has a strong bonding force with the spherical Ni(OH)2, and has good conductivity after oxidation, which effectively improves the utilization rate of the material and improves the high-rate charge and discharge cycle performance of the material. Compared with other cobalt coating methods, this method is simple to operate, has a stable and reliable cobalt coating effect, and has good practical value.

References:

[1] ZHOU Han-zhang, LIU Ming-jun, XU Qing. Surface coating of cobalt hydroxide on positive active material of alkaline battery and preparation method [P]. CN:200310109865.3, 2005-07-06.

[2] Ding Y C, Yuan J L, Wang Z Y, et al. Effects of surface modification of Ni(OH)2 powders on the performance of nickel cathodes [J]. J

Power Sources, 1997, 66(1-2): 55-59．

[3] TANG Zhi-yuan, XU Zheng-rong, RONG Qiang, et al. Coating of CoOOH and its performance study [J]. Battery Bimonthly, 2004, 34(2): 96-98．

[4] LI Fang, YANG Yi-fu, WEI Ya-hui, et al. Effect of Ni(OH)2 surface coating on the performance of MH/Ni battery [J]. Battery Bimonthly, 2004, 34(3): 178-179．

[5] DU Xiao-hua, JIANG Chang-yin, ZHANG Quan-rong, et al. Surface cobalt coating process of high-density spherical nickel hydroxide [P]. CN:99107434.3, 1999-11-10.
sp;

As shown in Figure 1, the spherical Ni(OH)2 without cobalt coating has a β-type structure, with characteristic peaks at 19.20, 33.40 and 38.90. The characteristic peaks of the spherical Ni(OH)2 with 3.0% cobalt coating appear at 19.10, 33.30 and 38.70, and no other impurity peaks are found, indicating that the structure of the spherical Co(OH)2 with 3.0% cobalt coating is consistent with that of the spherical Co(OH)2 without cobalt coating.

2.2 Cyclic voltammetry test

The cyclic voltammetry curves of spherical Ni(OH)2 without cobalt coating and 3.0% cobalt coating are shown in Figure 2.

As shown in Figure 2a, pure spherical Ni(OH)2 has an oxidation peak at 0.312V, with a peak current of 17.362mA, and a reduction peak at 0.064V, with a peak current of -11.882mA. As shown in Figure 2b, spherical Ni(OH)2 with 3.0% cobalt coating has an oxidation peak at 0.306V, with a peak current of 21.165mA, and a reduction peak at 0.078V, with a peak current of -14.945mA. The △E of Ni(OH)2 with 3.0% cobalt coating is 0.020V lower than that of spherical Ni(OH)2 without cobalt coating, while the current is 3-4mA larger, indicating that spherical Ni(OH)2 with 3.0% cobalt coating has good cycle performance and conductivity.

2.3 Battery performance test

Table 1 shows the effect of the cobalt coating method and the amount of cobalt coating on the specific capacity.

As can be seen from Table 1, as the cobalt content increases, the discharge specific capacity of the battery increases; for the same cobalt content, the discharge specific capacity of the cobalt coating method is about 7% higher than that of mechanical external cobalt addition, and slightly higher than that of the cobalt coating method using integral feed. Similarly, there are obvious differences in the 10C discharge performance and the discharge performance after 3C fast charging, which shows that the product obtained by this cobalt coating method has better high-rate charge and discharge performance.