Introduction to Lithium Batteries
Lithium-ion batteries are a type of battery that uses lithium metal or lithium alloys as the negative electrode material and a non-aqueous electrolyte solution. The earliest lithium metal battery was proposed and studied by Gilbert N. Lewis in 1912. In the 1970s, MS Whitington proposed and began researching lithium-ion batteries. Due to the highly reactive chemical properties of lithium metal, its processing, storage, and use require very strict environmental controls. Therefore, lithium-ion batteries were not widely used for a long time. With the development of science and technology, lithium-ion batteries have now become mainstream.
Lithium batteries can be broadly classified into two categories: lithium metal batteries and lithium-ion batteries. Lithium-ion batteries do not contain metallic lithium and are rechargeable. The fifth generation of rechargeable batteries, lithium metal batteries, was developed in 1996. They offer superior safety, specific capacity, self-discharge rate, and price-performance ratio compared to lithium-ion batteries. Due to their high technological requirements, only companies in a few countries currently produce lithium metal batteries.
Lithium-ion battery structure
Lithium-ion batteries are a replacement for lithium metal batteries that appeared a few years ago. The main components of a battery are positive and negative electrodes, electrolyte, separator, and casing.
The positive electrode uses a carbon electrode that can absorb lithium ions. During discharge, lithium is converted into lithium ions, which leave the anode and reach the cathode of the lithium-ion battery.
For the negative electrode, materials are selected that can be intercalated into lithium compounds with a potential as close as possible to that of lithium, such as various carbon materials including natural graphite, synthetic graphite, carbon fibers, mesophase microspheres of carbon, and metal oxides.
Electrolyte---a mixed solvent system using alkyl carbonates such as ethylene carbonate, propylene carbonate and low-viscosity diethyl carbonate with LiPF6.
The diaphragm is made of polyolefin microporous membranes such as PE, PP or composite membranes of them. In particular, the PP/PE/PP three-layer diaphragm not only has a low melting point, but also has high puncture resistance, thus playing a role in thermal protection.
The outer casing is made of steel or aluminum, and the cover assembly has an explosion-proof and power-off function.
Working principle of lithium-ion batteries
The working principle of a lithium-ion battery refers to its charging and discharging principle.
When a battery is charged, lithium ions are generated at the positive electrode. These lithium ions then move through the electrolyte to the negative electrode. The carbon layer at the negative electrode has a layered structure with many micropores. The lithium ions that reach the negative electrode embed themselves into these micropores. The more lithium ions embedded, the higher the charging capacity. The chemical reaction occurring at the positive electrode at this time is as follows:
Similarly, when the battery is discharged (i.e., when we use the battery), lithium ions embedded in the carbon layer of the negative electrode are released and move back to the positive electrode. The more lithium ions return to the positive electrode, the higher the discharge capacity. The battery capacity we usually refer to is the discharge capacity. The chemical reaction occurring at the negative electrode at this time is:
It's easy to see that during the charging and discharging process of a lithium-ion battery, lithium ions are in a state of motion, moving from the positive electrode to the negative electrode and back to the positive electrode. If we figuratively compare a lithium-ion battery to a rocking chair, with the two ends of the chair representing the two electrodes, then the lithium ions are like excellent athletes running back and forth between the two ends of the chair. Therefore, experts have given lithium-ion batteries another endearing name: rocking chair batteries.
Introduction to soft-pack lithium batteries
A pouch lithium battery is simply a liquid lithium-ion battery encased in a polymer shell. Structurally, it uses aluminum-plastic film packaging, so in the event of a safety hazard, a pouch battery will at most swell and crack.
Parameters of soft-pack lithium batteries
Nominal voltage: 3.7V
Operating voltage: 2.4~4.2V
Nominal capacity: 1250mAh
Standard discharge duration current: 0.2C
Maximum discharge duration current: 0.5C
Operating temperature: Charging: 0~45℃
Discharge: -20~60℃
Product dimensions: MAX 9.5*35*52mm
Finished product internal resistance: ≤150mΩ
Over-discharge protection voltage: 2.5 ± 0.05 V
Lead wire type: GB standard wire UL1007/24#, wire length 55mm. Protection parameters: Overcharge protection voltage/per string 4.325±0.025V.
Overcurrent value: 2~4A
Advantages of pouch batteries
1. Good safety performance
Soft-pack batteries, unlike steel-cased or aluminum-cased batteries, are not prone to explosion.
2. Lightweight
The soft-pack battery is 40% lighter than a steel-cased lithium battery of the same capacity and 20% lighter than an aluminum-cased battery.
3. Large capacity
Soft-pack batteries have 10-15% higher capacity than steel-cased batteries of the same size and 5-10% higher capacity than aluminum-cased batteries.
4. Low internal resistance
The internal resistance of pouch batteries is lower than that of lithium batteries. Currently, the internal resistance of domestically produced pouch battery cells can be as low as 35mΩ, which greatly reduces the self-discharge of the battery.
5. Flexible design
The shape of pouch batteries can be customized according to customer needs, and new cell models can be developed.
Disadvantages of pouch batteries
The existing pouch battery cell models are limited and cannot meet market demand;
Developing new models is costly.
The difference between soft-pack and hard-pack lithium batteries
The main difference between soft-pack and hard-pack lithium batteries lies in the material of the battery casing. If there is packaging, it is not easy to tell the difference; you must see the battery itself. Steel-cased batteries are heavier than batteries of the same volume. However, it is not easy to distinguish between aluminum-cased and soft-pack batteries. Soft-pack batteries are slightly softer than aluminum-cased batteries and are easily deformed by hand.
