Electrochemical reaction mechanism
NiMH battery is related to the old NiCd battery, 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. In 1947, NiCd batteries were fully densified and have been used ever since. Long-term applications have shown that NiCd batteries are a high-performance and highly reliable 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 electrode, to the heavy metal cadmium Cd as the negative electrode, together with the electrolyte (KOH solution), after sealing the composition of the battery. Inside the battery container, the electrochemical reaction is as follows:
The electrochemical reaction is characterized by the fact that KOH, which is a component of the electrolyte, is not directly involved in the electrochemical reaction. Because the capacity of the negative electrode is greater than that of the positive electrode when the battery is manufactured, only the oxygen (O2) produced by the positive electrode can be seen when the battery is overcharged; because the negative electrode retains the uncharged portion of the battery, no hydrogen (H2) is produced; and because the oxygen (O2) produced is absorbed by the negative electrode, the battery can be sealed.
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 characteristic is that the discharge capacity is not low even when discharged with high current (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
Both the United States and the Netherlands have conducted research on the alloy MH (Hydrogen Storing alloy metal), which is capable of absorbing hydrogen, and have attempted 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 by practice, through the appropriate combination of La, Ce, Pr and Nd and other rare earth elements can form hydrogen-absorbing alloy metal, which can release/absorb the amount of hydrogen H2 is quite large, for example, 1 cc of liquid hydrogen can be turned into 784 cc of hydrogen, while 1 cc volume of hydrogen-absorbing alloy metal can be released 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:
As the design can be like NiCd battery, also the negative electrode of the capacity of the MH is made large enough, when the overcharge of the positive electrode of the release of oxygen by the MH in the reduction of hydrogen, so that the negative electrode is also made into a large enough, when overcharged by the positive electrode of the release of oxygen by MH in the reduction of hydrogen, the MH in the Like NiCd batteries, NiMH batteries can maintain a stable terminal voltage of 1.2V during high-current 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 negative material structure and electrochemical reaction mechanism is different from the NiCd battery, its energy density and service life are superior to 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 Research Institute of Nonferrous Metals researchers have carried out in-depth research on MH alloys, and obtained new progress.
3. LIB battery
The lithium metal Li as the negative electrode of the disposable battery, the reputation is very good. Therefore, the industrialized countries are trying to make use of Li manufacturing battery, in 1979, Canada MoLi-Energy lithium metal battery in the cell phone fire accident, had forced lithium metal battery once out of the market. However, because lithium Li metal as the negative electrode battery has ideal performance, countries are still research and development.
Now, the market popular lithium-ion battery (LIB) is to sacrifice the battery performance to obtain safety and service life of the compromise, its electrochemical reaction is as follows:
LIB is coated with LiCoO2 active material as the positive electrode of the aluminum collector, carbon (graphite or activated carbon) and dissolved in the organic solution of LiPF6 composition. When charging, Li ions in the layered structure of LiCoO2 are adsorbed to the negative electrode 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 longer service life.
The main features of LIB are high energy density by weight, a smooth discharge voltage of 3.6 V, the ability to operate in temperatures ranging from -20°C to 60°C, no storage effect, and a low self-discharge rate (and thus no high-current discharge). In order to use LIBs safely, protection against overcharging and overdischarging is required.
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), energy density (subdivided into weight energy density and volume energy density) and power density, etc., are expressed in quantitative values. For example, the Vdc of NiCd and NiMH = 1.2 V, while the Vdc of LIB is as high as 3.6 V. When a supply voltage of 3.6 V is required, people prefer to use one LIB instead of three NiCd (or NiMH) batteries in series. This example shows that the use of quantitative parameters can be a horizontal comparison of various types of 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.
According to 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, that each kg of battery can provide Wh (watt hours) power; Wh/Liter is the volume of the battery energy density, that each liter (Liter) battery can provide Wh power; W/kg indicates the power density of the battery, that each kg of battery can provide the number of watts (W), i.e., the power; Price Ratio is the battery between the power conversion devices and power storage devices of various parameters listed in Table 1, so that users can choose. Price Ratio is the price ratio between batteries, indicating the relative price of various batteries.
From Table 1 can be clearly seen, NiCd, NiMH, LIB and double charge layer capacitors have their own strengths and weaknesses, the parameters are perfect device, the current market has not yet appeared. Therefore, the selection of battery devices, must be combined with the specific application of the actual choice, reasonable with the use.
Device type Vdc Wh/kg Wh/Liter W/kg Recharges PriceRatio
Remarks
NiCD battery 1.2V 50 134 160 300 ~ 500 times 80% of the storage effect
NiMH battery 1.2V 60 300 260 500 times 100% NiCd and NiMH battery 1.2V 60 300 260 500 times ~ 1000 times 100% NiCd and NiMH batteries. ~1000 times 100% NiCd and NiMH can be **** with simple charging facilities
LIB battery 3.6V 100 287 150 300 to 400 times 200% Strictly prevent overcharge/overdischarge
Double Charge Layer Capacitor EDLC 1.2V 15 - 518 Semi-permanent 10% High tapping rate density, low cost
Application of Storage Battery
The most serious problem of NiCd storage battery is that its waste causes serious pollution to the environment and endangers human health. Since the recycling mechanism has been established in Europe, America and Japan, 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 recharged can be avoided; otherwise, if the NiCd battery in the discharge is very shallow in the case of charging, it will memorize the depth of discharge, not long to use and then need to charge.
In addition to the above shortcomings, NiCd batteries still have certain advantages, such as the price is quite cheap, voltage control and temperature control 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 an affordable battery. It has a wide range of applications and can be used in transceivers, cordless telephones, portable AV equipment, and electric motors, 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 at high current, allowing the number of rechargeable up to 500 ~ 1000 times, the price is becoming more reasonable (expected in the next 3 ~ 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 and NiCd batteries, with a cylindrical shape (AAA, AA AAA, AA, A, C, D, F and M), square and button cells. These NiMH batteries can be assembled into a variety of battery packs, which can meet the growing demand for portability of electronic devices. For example, NiMH batteries are ideally suited for high-current discharge requirements, such as portable printers, medical equipment, telecommunication equipment, notebook computers and digital AV machines (digital cameras, digital video cameras, digital audio players), etc., all of which can utilize 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, after the LIB is utilized, the situation is reversed, which will be introduced later.
NiMH battery due to the hydrogen-absorbing alloy MH specific gravity is very large, resulting in Wh/kg is only 60 or so; although NiMH Wh/Liter can reach 300 or even 400, W/kg as high as 160 or more, but its application prospects are limited to the application of heavy loads are not strictly weight, such as hybrid electric vehicles (hybrid electric vehicles), electric vehicles, military and military. electric vehicles), electric vehicles, military camping, anti-disaster (floods, earthquakes, etc.) on-site power and so on will play an irreplaceable important role. Because of the characteristics of 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, limited to driving as a power, and start and 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, acceleration of capacitors to provide a pulse high-current drive; solar panels and NiMH battery packs and double charge layer capacitors EDLC, portable wind generators and other composite systems. Solar panels and NiMH battery combination power supply system, relying on solar cells during the day to generate electricity for NiMH battery charging, discharged by the battery at night; wind generator and NiMH battery combination power supply system, wind generator for NiMH battery charging, no wind when the NiMH battery discharged by the NiMH battery.
LIB battery Vdc = 3.6V, rechargeable up to 300 ~ 400 times, energy density of 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 expensive, for the pursuit of lightweight and efficiency of the use of mobile communication cell phone users, is still the LIB battery love. In the field of mobile communication, LIB battery will replace NiCd and NiMH battery completely.
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.