Nowadays, the common storage batteries are NiMH, NiCd and LIB batteries. Because of their respective electrochemical reaction mechanism is not the same, so also each has its own characteristics and different application areas. This article introduces their characteristics and corresponding application areas according to their electrochemical reaction mechanism.
Electrochemical reaction mechanism
NiMH battery and the old NiCd battery are related to each other, so we will introduce NiCd battery first, followed by NiMH battery, and finally LIB.
1. NiCd battery
NiCd battery was invented as early as 1899, and the fully densified NiCd battery was realized in 1947. Completely densified NiCd batteries have been used until now. The long time application shows that NiCd battery is a kind of high performance and high reliability battery.
Today's NiCd battery, in the foam nickel or nickel fiber-like substrate attached to a large number of NiOOH active material as the positive pole, to the heavy metal cadmium Cd as the negative pole, together with the electrolyte into the electrolyte (KOH solution), after sealing the composition of the battery. Inside the battery container, the electrochemical reaction takes place as follows:
The electrochemical reaction is characterized by the fact that KOH, which is clearly seen as a component of the electrolyte, is not directly involved in the electrochemical reaction. Since the capacity of the negative electrode is made larger than that of the positive electrode when the battery is manufactured, only oxygen (O2) produced by the positive electrode can be seen when the battery is overcharged; hydrogen (H2) is not produced because the negative electrode retains an uncharged portion; and sealing is possible because the oxygen (O2) produced is absorbed by the negative electrode.
From the electrochemical reaction mechanism of NiCd battery, we know that it relies on the rapid movement of OH- ions, and the reaction is smoother than that of aluminum-acid battery. Therefore, its important feature is that the discharge capacity does not appear low even when the large electric discharge is being expelled (1.2 V terminal voltage can be maintained). The crystalline structure basically does not change due to charging and discharging, and has a longer service life.
2. NiMH battery
The United States and the Netherlands both conducted research on the alloy MH (Hydrogen Storing alloy metal), which can absorb hydrogen, and tried to use it for the development of storage batteries. NiMH battery products appeared in the world in the early 1990s, but the development is very rapid. It has been proved in practice that the alloy metal of hydrogen storing can be formed by the appropriate combination of rare earth elements such as La, Ce, Pr and Nd, and the amount of hydrogen H2 it can release/storing is quite large, for example, 1 cc of liquid hydrogen can be turned into 784 cc of hydrogen, while 1 cc volume of the alloy metal of hydrogen storing can release 1,000 cc of hydrogen.
In NiCd battery, as long as the use of hydrogen-absorbing alloy MH to replace the toxic heavy metal Cd (cadmium), the formation of environmentally non-polluting green battery NiMH, the electrochemical reaction is as follows:
Since the design can be like the NiCd battery, also the capacity of the negative electrode MH is made large enough, when the overcharge of the positive electrode by the oxygen released by the MH in the reduction of hydrogen, making the battery NiMH batteries, like NiCd batteries, can maintain a stable terminal voltage of 1.2V during large discharge. It is commendable that the waste of NiMH battery does not pollute the environment, while the waste of NiCd battery (if not recycled) will cause environmental pollution.
NiMH battery's anode material structure and electrochemical reaction mechanism is different from NiCd battery, its energy density and service life are superior than NiCd battery, which can also open up a broader application market. It is because of this reason, the world's industrialized countries attach great importance to the research and development of NiMH battery. According to reports, China's non-ferrous metal research institute of scientific research staff on the MH alloy has been carried out very advanced research, and to obtain the new progress.
3. LIB storage battery
The disposable battery with lithium metal Li as negative electrode has a good reputation. Therefore, the developed countries in various industries have tried to use Li to manufacture storage batteries, and in 1979, the accident in which the lithium metal storage battery of Canada's MoLi-Energy caught fire in a cell phone had forced lithium metal storage batteries to withdraw from the market for a time. However, due to the ideal performance of lithium Li metal as the negative electrode of the battery, countries are still researching and developing.
Now, the market popular lithium-ion battery (LIB) is to sacrifice the battery performance to obtain the safety and longevity of the compromise, its electrochemical reaction is as follows:
LIB is coated with LiCoO2 active material of aluminum collector as the positive electrode, carbon (graphite or activated carbon), and dissolved in the organic solution of LiPF6 composition. When charging, Li ions in the layered structure of LiCoO2 swim to the negative electrode and are adsorbed by the layered structure of carbon; when discharging, the lithium ions adsorbed in the layered structure of carbon return to the positive electrode, so the positive electrode is restored to the layered structure of LiCoO2, and the negative electrode is also restored to the layered structure of carbon. In other words, in the charging and discharging process of this battery, only lithium ions rather than active lithium metal appear. Therefore, LIB has better safety and serviceable life.
The main features of LIB are high weight energy density, a smooth discharge voltage of 3.6 V, the ability to work in the temperature range of -20 ° C to 60 ° C, no storage effect, and a low rate of self-discharge (and thus can not be a large power discharge by power). In order to use the LIB safely, it is required to have protection facilities against overcharging and overdischarging.
Comparison of various batteries
The above NiCd, NiMH and LIB batteries have different electrochemical reaction mechanisms, and the characteristics of each battery are different. In order to facilitate the comparison, it is necessary to use the evaluation of battery performance standards or parameters. Usually used evaluation parameters, such as the battery terminal voltage Vdc at equilibrium discharge, the number of recharges (Recharges) or the number of charge/discharge cycles, the price ratio (Price Ratio), the energy density (subdivided into the weight energy density and volume energy density) and the power density, etc., are expressed by quantitative values. For example, NiCd and NiMH have a Vdc = 1.2 V, while LIBs have a Vdc as high as 3.6 V. When a supply voltage of 3.6 V is required, one would prefer to use one LIB rather than three NiCd (or NiMH) batteries in series. This example shows that the use of quantitative parameters can be a horizontal comparison of various batteries, easy to choose the application.
In addition, the safety of the battery and whether the memory effect, etc., is also an important factor affecting the widespread use of batteries, it is worth noting.
Based on the above, can now commonly used power conversion devices and power storage devices of various parameters listed in Table 1, so that users can choose. Among them, Wh/kg is the weight of the battery energy density, said each kg of battery can provide Wh (watt hours) power; Wh/Liter is the volume of the battery energy density, said each liter (Liter) battery can provide Wh power; W/kg said the power density of the battery, said each kg of battery can provide wattage (W), that is, electric power; Price Ratio is the price ratio between batteries, indicating the relative price of various batteries.
From Table 1, it can be clearly seen, NiCd, NiMH, LIB and double charge layer capacitors have their own shortcomings, the parameters are perfect device, the market has not yet appeared. Therefore, the selection of battery devices, must be combined with the specific application of the actual choice, fair with the use.
Application of storage batteries
NiCd storage batteries is the most serious topic of its waste on the environment causing serious pollution, endangering human health. As in Europe and the United States and Japan have established a recycling mechanism, the environmental pollution problem has basically been solved. As for the NiCd battery storage (memory) effect, as long as the use of bear in mind that it must be fully discharged and then recharge can be avoided; otherwise, if the NiCd battery in the discharge is very shallow in the case of charging, it will be memorized in the depth of the discharge, with a few hours on the need to charge again.
In addition to the above shortcomings, NiCd batteries still have certain advantages, such as the price is quite cheap, voltage-controlled and temperature-controlled charging facilities are relatively simple, the discharge capacity of heavy loads, and a variety of models (high-capacity type, rapid charging type, etc.), etc., can be said to be affordable batteries. Its field of application is quite wide, and it can be used for transceivers, cordless telephones, portable AV machines and electric machines, etc., as long as its size and weight are not taken into account.
NiMH battery is a new development of NiCd battery, high volume energy density, and no pollution to the environment and no memory effect, welcomed by the majority of users. It has a high capacity, can be discharged by the power, promised to recharge as many as 500 to 1000 times, the price is becoming fair (expected in the next 3 to 5 years, the annual cost can be reduced by 3%), and can be utilized in the current NiCd battery charging facilities, and thus NiMH batteries have been widely used. NiMH batteries, like NiCd batteries, have a cylindrical (AAA, AA, A, C, D, F, A, C, D, F, D, D, D, F). A, A, C, D, F and M), square and button cells. These NiMH batteries can be assembled into a variety of battery packs that can meet the growing demand for portability in electronic devices. For example, NiMH batteries are very suitable for large discharge power needs, such as portable printers, medical equipment, telecommunication equipment, notebook computers and digital AV machines (digital cameras, digital video cameras, digital audio players), etc., can be applied to NiMH batteries. Originally, NiMH battery practical than lithium-ion battery LIB one step ahead, so in the field of mobile communications is also the world of NiMH battery. However, the situation was reversed after the LIB was commercialized, which will be described later.
NiMH batteries absorb hydrogen due to the alloy MH has a large specific gravity, resulting in Wh/kg is only 60 or so; although NiMH Wh/Liter can reach 300 or even 400, W/kg up to 160 or more, but its application prospects are limited to the application of heavy loads that do not strictly take into account the weight of the application, such as hybrid electric vehicles (hybrid electric vehicles), electric vehicles, military, and military. However, its application prospect is limited to heavy load applications without strict weight calculation, such as hybrid electric vehicles (hybrid electric vehicles), electric vehicles, military camping, anti-disaster (floods, earthquakes, etc.) on-site power and so on, which will play an irreplaceable and important role. Due to the characteristics of the NiMH battery decided that it can and solar panels, double charge layer capacitor EDLC, portable wind turbines and other composite systems. For example, hybrid electric vehicle gasoline engine power is small, is limited to driving as a power, and start and hill climbing with the help of NiMH batteries and double charge layer capacitors to provide electricity to drive the motor to achieve acceleration; future electric vehicles mainly rely on large NiMH battery packs and large double charge layer capacitor group compound charging mode, accelerated by the capacitor to provide a pulse high-current drive; solar panels and NiMH batteries. Solar panels and NiMH battery combination power supply system, relying on solar cell power generation for NiMH battery charging during the day, the battery discharged at night; wind generator and NiMH battery combination power supply system, generator for NiMH battery charging when there is wind, no wind when the NiMH battery discharged.
LIB battery Vdc = 3.6V, rechargeable up to 300 ~ 400 times, energy density up to 287Wh / Liter, is currently the world's most lightweight battery. Although it is charging and discharging, require a set of sophisticated control facilities to ensure safety, and the price is not expensive, for the pursuit of lightweight and efficient use of mobile communication cell phone users, is still on the LIB battery love. In the field of mobile communications, LIB batteries will eventually replace NiCd and NiMH batteries.
In short, NiCd, NiMH and LIB batteries have their own application fields due to their different mechanisms and characteristics, and will be developed in different fields in the future.