Nickel-cadmium batteries are currently the most widely used small secondary batteries. Compared with other secondary batteries, nickel-cadmium batteries are widely used in many fields of the national economy due to their excellent performance and price ratio.
Cylindrical sealed nickel-cadmium batteries are widely used in household appliances as portable power sources. Experts predict that nickel-cadmium batteries will occupy a major market share of the power supply for some equipment such as safety emergency equipment, power tools and video recorders in the future.
1. Principle, structure and material composition of nickel-cadmium battery
The negative active material of the nickel-cadmium battery is a sponge-like metal pot, the positive active material is nickel oxyhydroxide (NiOOH), and the electrolyte is KOH or NaOH aqueous solution. The electrode material is supported by a nickel mesh. In order to increase the charge and discharge performance of the battery, the battery also contains various additives.
The battery symbol for nickel-cadmium batteries is
(-) Cd | Cd(OH)2 | KOH| Ni(OH)2 | NiOOH(+)
The electrode reaction and the battery reaction are as follows.
Negative electrode: Cd+2OH-→Cd(OH)2+2e-
Positive electrode: NiOOH+H2O+e-→Ni(OH)2+OH-
Battery response: Cd+2NiOOH+2H2O=Cd(OH)2 +2Ni(OH)2
When the nickel-cadmium battery is fully charged, the positive and negative active materials are NiOOH and Cd, respectively; when the battery is fully discharged, the positive and negative active materials are converted to Ni(OH)2 and Cd(OH)2. It can be seen from the reaction principle and material composition of the battery that Cd(OH)2 and Ni(OH)2 will occupy a large part of the nickel-cadmium battery after long-term use.
The structure of a commonly used cylindrical small nickel pot battery is shown in Figure 1. As can be seen from Figure 1, the positive and negative electrodes of the nickel-cadmium battery are tightly rolled together with the separator paper as the insulating medium; moreover, during the continuous use of the battery, the positive and negative electrodes will be severely corroded, and the electrode materials will be in a state of interweaving and mixing.
Figure 1 - Structure diagram of NiCd battery
2. Production and consumption of nickel-cadmium batteries
In 2000, the sales of nickel-cadmium batteries in the world reached 1.4 billion US dollars, and the sales volume exceeded 1.4 billion, ranking first in the sales volume of small secondary batteries. In recent years and the next few years, nickel-cadmium batteries will not only not withdraw from the market, but will occupy an important share of the secondary battery market.
China is the world's largest producer and consumer of batteries, and an important producer and consumer of nickel-cadmium batteries in the world. China's nickel-cadmium battery production has a history of more than 40 years, forming a complete power system industry from the design and manufacture of various types of plates, components, battery cells and battery packs to battery production and testing equipment. The R&D and production units are located in 19 provinces and cities, and the products are exported to more than 60 countries and regions. In 2000, China's annual output of nickel-cadmium batteries was about 350 million to 400 million, with sales of nearly 200 million US dollars, accounting for about 6% of the world's nickel-cadmium battery sales.
The total output of small secondary batteries in China will grow at an average annual rate of over 40% from 2000 to 2005. According to the main development goals of China's battery industry during the "Tenth Five-Year Plan" period, the output of China's nickel-cadmium batteries will gradually decrease in general. However, due to the good performance, price and irreplaceability of nickel-cadmium batteries in some applications, the production volume of nickel-cadmium batteries will still be higher than that of nickel-metal hydride batteries and lithium batteries and will remain above 300 million in the next few years.
3. Production of waste nickel-cadmium batteries
The production and consumption of nickel-cadmium batteries in China is huge, and it is very difficult to accurately count the use of batteries and the generation of waste batteries. Generally, the model is used to estimate by comprehensively considering factors such as battery production, import and export, and battery life. According to China's battery production volume in recent years and the main development goals of the battery industry during the "Tenth Five-Year Plan" period, the estimated results of the waste nickel-cadmium battery production show that from 1999 to 2005, the average annual production and sales volume will exceed 400 million Only; if the import and export volume is basically flat, the annual production of waste nickel-cadmium batteries in China will be more than 400 million.
4. Environmental problems of waste nickel-cadmium batteries
Nickel-cadmium batteries contain heavy metals such as cadmium, nickel, and strong alkaline electrolytes. If they are not disposed of properly, they may cause serious harm to the ecological environment and human health.
(1) Main pollutants and release pathways in waste nickel-cadmium batteries
The heavy metals cadmium and nickel in waste nickel-cadmium batteries are harmful to the human body, especially cadmium is a toxic substance, which will cause great harm to human health when released into the environment.
Waste nickel-cadmium batteries are one of the important sources of heavy metal cadmium in municipal solid waste, and cadmium from waste nickel-cadmium batteries accounts for about 92% of the total cadmium in municipal waste.
If waste batteries are mixed in the composting process, it will seriously affect the quality of compost products; when mixed in the incineration process, a large amount of volatilization of cadmium will pollute the atmosphere and soil environment, and will also produce a large amount of ash that is difficult to handle due to the accumulation of heavy metals. In the case of low level of landfill construction, heavy metals cadmium and nickel in waste nickel-cadmium batteries during landfill treatment may pollute water or soil through leaching.
(2) Environmental hazards of cadmium
Once the heavy metal cadmium enters the environment, especially after entering the soil, it is difficult to eliminate it, and it will be organically formed in the living body, making the heavy metal pollution more harmful. Excessive cadmium will accumulate in the roots, stems, leaves, especially fruits of plants, which not only seriously affects the growth and development of plants, but also endangers animals and humans through the food chain.
The forms of cadmium in soil can be divided into exchangeable state, carbonate bound state, iron and manganese oxidation bound state, organic state and residual state. Generally, with the increase of the total cadmium content in the soil, the residual cadmium content decreases, and the exchangeable cadmium content increases, which will increase the relative toxicity of the pot. As the main decomposers in the natural environment, microorganisms play an important role in the cycle of carbon, nitrogen, phosphorus, and sulfur in soil sediments, as well as in the regeneration of inorganic nutrients and the transformation of nutrients. Cadmium can cause significant changes in the microbial population: the number and variety of microorganisms are reduced, the activity of microorganisms is reduced, and the microbial community is fluctuated.
The transfer process of cadmium in soil to plants is related to the form and content of cadmium, as well as the characteristics and species of soil and plants. Generally, plants poisoned by cadmium will not grow normally. The study found that with the increase of cadmium concentration in the culture medium of maize seedlings, the degree of inhibition of seedlings increased. After the seedlings were treated with the divalent cadmium ion solution, the growth of the seedlings was only 3% of the control, and the growth almost stopped. After being poisoned by cadmium, maize showed the symptoms of stunted growth, small leaves, and yellowish-brown color, swollen and blackened root tips, and rot; and the higher the concentration, the earlier the symptoms appeared. The results of studies using other plants were basically similar.
Pots in the environment are mainly introduced into humans and animals through plants. After nickel enters the body of humans and animals, it mainly exists in the form of organic cadmium in organs with large blood flow, such as liver, testis, gallbladder, kidney, hair and other parts. Cadmium can combine with macromolecular organic compounds containing hydroxyl, amino and sulfhydryl groups, which inhibits many enzyme systems, thereby affecting the normal function of enzyme systems in organs such as liver and kidney. In addition, cadmium also damages renal tubules, leading to glycosuria, proteinuria, aminoaciduria, and increased urinary calcium and uric acid excretion, resulting in renal insufficiency. Cadmium ions can easily cause brain poisoning and even death if they cross the blood-brain barrier.
Long-term consumption of cadmium-containing plants can induce diseases such as osteoporosis, osteomalacia, and kidney stones, and eventually lead to "bone pain disease". Cadmium also has strong teratogenic, carcinogenic and mutagenic effects on mammals. After being absorbed by organisms, natural excretion is very slow, and its biological half-life is 930 years.
(3) Environmental hazards of nickel
The heavy metal nickel also has a great impact on human health. People with nickel allergies can develop a contact rash when their skin comes into direct contact with nickel-containing jewelry or other substances. Inhaling a lot of nickel-containing air will cause chronic bronchitis and lung function decline, and even cause lung cancer and sinus cancer. Eating or drinking large amounts of nickel-containing food and water can cause lung disease, as well as affect the animal's stomach, liver, kidney, immune system, and metabolic system. Nickel and some nickel-containing compounds are classified as carcinogens, according to the U.S. Department of Health and Human Services (DHHS).