How do lithium-ion batteries work?

Changes from lithium metal to lithium ions

The study of lithium batteries began in GN Lewis in 1912, but it was not until the early 1970s that lithium primary batteries were commercialized for the first time. In the 1980s, scientists began to try to develop lithium secondary batteries, but the lithium metal used as the negative electrode material has great instability, and the limitation of raw materials leads to slow progress.

Lithium is undoubtedly the lightest of all metals, so it has the highest electrochemical potential and maximum specific energy per unit weight, and the secondary battery with lithium metal as the anode (negative electrode) [1] has a very high energy density. However, in the mid-1980s, it was found that metallic lithium anodes produced harmful dendrites during battery cycling, and it was easy to pierce the separator during dendritic growth to cause short-circuiting of the battery. Then, the battery temperature rises rapidly and approaches the melting point of lithium, and eventually the thermal runaway causes the battery to catch fire and even cause an explosion. For example, in 1991, due to burns caused by flammable gases released during the use of mobile phone lithium batteries, a large number of metal lithium secondary batteries sold to Japan were recalled.

Metal lithium has inherent instability and is particularly evident during the charging process, so researchers have shifted their focus to the study of lithium ions in non-metallic solutions. Although lithium-ion batteries have lower specific energy than lithium metal, as long as battery manufacturers and battery packs are implemented in accordance with safety regulations while maintaining a safe level of voltage and current, the safety of lithium-ion batteries is guaranteed. . Since the commercialization of the first lithium-ion batteries by Sony in 1991, lithium-ion batteries have become the most promising and fastest-growing market. At the same time, however, the researchers still did not give up the development of safe metal lithium batteries.

The discovery of lithium cobalt oxide as a positive electrode material was attributed to John Goodenough (1992). It is said that John Goodenough worked with a graduate employed by NTT Japan. Shortly after John Goodenough invented the lithium-ion battery, the student brought the invention back to Japan. In 1991, Sony announced that it had obtained an international patent for a lithium cobalt oxide cathode material. After many years, lawsuits followed, but Sony was still able to hold patents and John Goodenough had nothing.

Flash point of lithium-ion battery system

Lithium-ion batteries have twice the specific energy of nickel-cadmium batteries, and have a higher theoretical voltage (3.60V) than the nickel system's 1.20V. The former is more beneficial to the theoretical specific energy increase. At the same time, improvements in electrode active materials have greater potential for increasing energy density. The load performance of lithium-ion batteries is very good. The ideal single-cell has a flat discharge curve in the voltage range of 3.7 to 2.8V, showing good energy reserve performance. However, nickel-based single battery only has a narrow flat discharge range from 1.25 V to 1.0V.

In 1994, the 18650 model [2] had a cylindrical lithium-ion battery with a capacity of only 1100 MAH, which cost more than $10. By 2001, the cost was reduced to $2 and the capacity rose to 1900 MAH. Today, the high energy density 18650 cylindrical battery provides more than 3000 MAH of capacity and is less expensive. The reduction of cost, the increase of specific energy and the absence of toxic substances have made the application of lithium-ion batteries on portable devices generally recognized, and gradually moved from the initial consumer goods market to the heavy industry including electric vehicle power systems.

In 2009, approximately 38% of battery revenue was contributed by lithium-ion batteries. Lithium-ion batteries are easy to maintain and are unmatched by many other chemical batteries. Lithium-ion batteries have no memory effect, do not require full charge and discharge to maintain performance, and the self-discharge rate is less than half of that of nickel-based batteries, which makes lithium batteries well used in fuel gauges. In addition, the lithium-ion battery has a rated voltage of 3.60V, and can be directly used as a battery for mobile phones and digital cameras through the battery pack design, simplifying the process and reducing the cost. But the downside is that you need to protect the circuit against leakage and avoid high prices.

Classification of lithium ion batteries from the perspective of materials

Similar to lead-based and nickel-based batteries, lithium ions use a positive electrode (cathode), a negative electrode (anode), and an electrolyte as a conductor. The positive electrode is a metal oxide and the negative electrode is composed of porous graphite. During discharge, lithium ions move from the negative electrode to the positive electrode through the electrolyte and the separator; during charging, lithium ions flow from the positive electrode to the negative electrode in opposite directions, as shown in FIG.

When the battery is charged and discharged, Li+ shuttles between the positive and negative electrodes. During discharge, the anode oxidizes, loses electrons, and the cathode is reduced to obtain electrons; when charging, the charge moves in the opposite direction.

There are many types of lithium ion batteries, depending on the electrode material. But when you choose different materials, the battery performance will vary greatly.

The positive electrode materials all contain Li+. Common lithium cobalt oxide (lithium cobalt oxide), lithium manganese oxide (also known as spinel or lithium manganate), lithium iron phosphate, nickel cobalt manganese ternary material (NMC) [3] and lithium nickel cobalt Aluminum oxide (NCA). All of these materials have a theoretical upper energy limit (lithium ions have a theoretical capacity of about 2000 kWh, which is more than 10 times the specific energy of commercial lithium-ion batteries).

Sony's original lithium-ion battery uses coke (a coal product) as a negative electrode material. Since 1997, most lithium-ion battery manufacturers, including Sony, have converted anode materials to graphite, resulting in a flat discharge curve. Graphite is a form of carbon that is used in pencils. It can store lithium ions well during charging, and has a long cycle and good stability. Of the carbon materials, graphite is the most common, followed by hard carbon and soft carbon. Other carbons, such as carbon nanotubes, have not yet found their commercial use. Figure 2 compares the voltage discharge curves of a modern lithium-ion battery with graphite as the negative electrode and a lithium-ion battery with an early coke negative electrode.

In the normal operating discharge range, the battery should have a flat voltage curve, which is better than earlier coke.

Anode materials are also evolving, and researchers are constantly experimenting with new materials, including silicon-based alloys. In this alloy, six carbon atoms are bonded to one lithium ion, and one silicon atom can bond four lithium ions. This means that the silicon negative electrode can theoretically store 10 times the energy of the graphite material. At present, silicon materials have increased by 20%-30% in specific capacity at the cost of reducing load potential and cycle life. However, the problem is that during the charging process, lithium ions are easily expanded in volume after being embedded in the silicon-based material (expanding to more than four times the initial volume).

The nanostructured lithium titanate salt has good cycle life and load capacity, excellent low temperature performance and good safety performance as a negative electrode material, but its specific capacity is low and the cost is high.

Tradeoffs between different manufacturers in the performance of batteries

Various studies on positive and negative materials allow manufacturers to consider the inherent performance of the battery, but the strengthening of one indicator is often at the expense of another performance sacrifice. In so-called " energy storage batteries ", battery manufacturers are more inclined to increase specific capacity for long-term use, but doing so may result in lower specific power and cycle life. In the "power battery ", a certain capacity may be sacrificed in order to achieve high power. The above properties of the "hybrid battery" are relatively balanced. "Longevity battery" was developed for long-term use. These special batteries are generally bulky and costly.

Manufacturers can easily obtain high specific capacity and low cost lithium-ion batteries if they replace nickel with nickel, but this will reduce battery stability. Although some newly established companies may pay more attention to the specific capacity of the battery in order to gain market recognition faster, safety and stability cannot be ignored. Reputable companies will put safety and long-term efficiency extremely important location.

Improving existing materials has a long way to go

The lithium-ion battery industry is mainly used in portable electronic products, and the long-term stability of its electric power system is still unknown. Cycle life, long-term performance and operating costs are only known after the electric car has been updated several times and accepted by the customer. Figure 3 below summarizes the advantages and limitations of lithium-ion batteries.

Taken together, these two challenges are particularly acute today, improving battery performance and finding better compounds. Overcoming any bottleneck will make batteries more decisive than near-free fossil fuels. Although the media has made extensive reports on major breakthroughs in batteries, it is still not time to write an article to praise the victory. Even if a certain development is confirmed and approved, it will take several years to enter the market and truly “fly into” the homes of ordinary people.

Annotation:

[1] When energy is consumed, such as in a diode, a vacuum tube or a rechargeable battery, the anode material is a positive electrode material; conversely, when discharging, such as a discharge process of a battery, the anode material is a negative electrode material.

[2] The cylindrical lithium-ion battery was developed in the mid-1990s. It has been measured to have a diameter of 18 mm and a length of 65 mm, which is mostly used in notebook computers.

[3] Some nickel manganese manganese cobalt battery systems were written as NCM, CMN, CNM, MNC and MCN. These systems are basically the same.

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