About Lithium Polymer Battery
1. Introduction to lithium polymer battery
Before introducing lithium polymer batteries, let us first know about the relevant knowledge of lithium batteries. So that we can find out the reasons for the emergence of lithium polymer batteries.
Lithium-ion batteries have many advantages. High energy density and low self-discharge rate give it certain advantages over other batteries, but it still faces some problems that affect its service life and safety.
For the problems that may affect the performance of lithium-ion batteries, lithium-polymer batteries (Li-Polymer) have recently come out for this, as shown in the figure below.
The essence of lithium polymer batteries is also lithium ion batteries, and the so-called lithium polymer batteries use polymer materials in at least one of the main structures such as electrolytes and electrode plates.
1.1 Characteristics of lithium polymer battery
Compared with lithium-ion batteries, the characteristics of lithium polymer batteries are as follows:
(1) The problem of battery leakage is relatively improved but not thorough.
(2) It can be made into a thin battery. For a capacity of 3.6V and 250mAh, the thickness of the battery can be as thin as 0.5mm.
(3) The battery can be designed in various shapes.
(4) It can be made into a single high-voltage battery. Liquid electrolyte batteries can only obtain high voltage by connecting several batteries in series. While lithium polymer batteries can be made into multi-layer combinations in a single cell to achieve high voltage because they have no liquid.
(5) The theoretical discharge capacity is about 10% higher than that of a lithium-ion battery of the same size.
In lithium polymer batteries, the electrolyte performs the dual function of separator and electrolyte:
On the one hand, it can isolate the positive and negative electrode materials like a diaphragm, so that self-discharge and short circuit do not occur inside the battery.
On the other hand, it conducts lithium ions between the positive and negative electrodes like an electrolyte.
Advantages of polymer electrolytes:
Polymer electrolytes not only have good electrical conductivity, but also have the characteristics of light weight, good elasticity, and easy film formation that are unique to polymer materials.
It also conforms to the development trend of chemical power sources with light weight, small size, safety, high efficiency and environmental protection.
1.2 Safety issues of lithium polymer batteries
For all lithium-ion batteries, previous or current, problems like internal short circuit, external short circuit, and overcharge are worrying.
Because the chemical properties of lithium are very active, it is easy to burn.
When the battery is discharged or charged, the internal temperature of the battery will continue to rise, and the gas generated during the activation process will expand.
And that will increase the internal pressure of the battery. And when the pressure reaches a certain level, if there is a scar on the shell, it will rupture, causing liquid leakage, fire, or even explosion.
In order to alleviate or eliminate the danger of lithium-ion batteries, technicians have added components that can inhibit the activity of lithium elements (such as cobalt, manganese, iron, etc.).
But these cannot essentially eliminate the danger of lithium-ion batteries.
1.3 Construction of lithium polymer battery
Lithium polymer batteries have a special structure consisting of five layers of thin films:
- The 1st layer uses metal foil as the collector.
- The 2nd layer is the negative electrode.
- The 3rd layer is the solid electrolyte.
- The 4th layer uses aluminum foil as the positive electrode.
- The 5th layer is the insulating layer.
The total thickness of the five layers stacked is 0.1mm.
In order to prevent the battery from overheating when it outputs a large current instantaneously, there is a strict thermal management system to control the normal operating temperature of the lithium polymer battery.
The main advantage of lithium polymer batteries is the elimination of liquid electrolyte, which can avoid the pollution caused by electrolyte overflow when the battery fails.
2. Working principle of lithium polymer battery
Among the three elements of the battery – the positive electrode, the negative electrode and the electrolyte, at least one or more elements of the lithium polymer battery are made of polymer materials.
In lithium polymer batteries, polymer materials are mostly used in positive electrodes and electrolytes.
- The positive electrode is made of conductive high molecular polymer or inorganic compound commonly used in lithium-ion batteries.
- The negative electrode adopts lithium metal or lithium carbon interlayer compound.
- The electrolyte adopts solid or colloidal polymer electrolyte, or organic electrolyte, so the specific energy is higher.
For example, the specific energy of lithium polyaniline battery can reach 350Wh/kg, but the specific power is only 50~60W/kg. Since there is no excess electrolyte in lithium polymer, it is more reliable and stable.
At present, when the common liquid lithium-ion battery is overcharged, it is easy to cause the safety valve to rupture and thus catch fire and explode, which is very dangerous.
Therefore, a protection circuit must be installed to ensure that the battery will not be overcharged.
Compared with liquid lithium-ion batteries, polymer lithium polymer batteries have better charge and discharge characteristics. And the requirements for additional protection of IC circuits can be appropriately relaxed.
In addition, in terms of charging, lithium polymer batteries can be charged with IC constant current. Compared with the “constant current-constant voltage” charging method adopted by lithium-ion batteries, it can shorten the waiting time for charging.
The new generation of Li-polymer batteries has done an excellent job of polymerizing. Therefore, it can be made very thin in shape (the thinnest is 0.5mm), and it can also realize arbitrary area and arbitrary shape.
This greatly improves the flexibility of battery modeling design, so that batteries of any shape and capacity can be made to meet product requirements.
At the same time, the unit energy of the lithium polymer battery is 50% higher than that of the current general lithium ion battery.
Its capacity, charge and discharge characteristics, safety, operating temperature range, cycle life and environmental protection performance have been greatly improved compared with lithium-ion batteries, and are favored by people.
3. The use of lithium polymer batteries
(1) The lithium polymer battery needs to be equipped with a corresponding protection circuit board.
It has functions such as overcharge protection, overdischarge protection, overcurrent (or overheating) protection, and positive and negative short circuit protection.
At the same time, there are current and voltage equalization functions in the battery pack to ensure the safety of the battery.
(2) The lithium polymer battery needs to be equipped with a corresponding charger to ensure that the charging voltage is within the range of 4.2v±0.05V.
Never use another lithium battery charger to charge it.
(3) Do not discharge deeply (discharge to 2.75V). Shallow discharge can improve battery life (it has no memory effect). And it is more appropriate to use shallow discharge (discharge to 3V).
(4) Do not mix with other types of batteries or different types of lithium polymer batteries.
(5) Do not squeeze or bend the battery, otherwise it will cause damage.
(6) Do not place near heaters and fire sources, otherwise the battery will be damaged.
(7) When not in use for a long time, it should be charged regularly to keep the voltage above 3.0V.
(8) Note that different discharge rates are related to the size of the discharge capacity, and the relationship between them is listed in the table below.
The relationship between discharge rate and discharge capacity of lithium polymer battery
Discharge rate | 1C | 2C | 5C | 10C | 12C |
Discharge capacity ratio /% | 99 | 98 | 95 | 90 | 70 |
4. Charge and discharge characteristics of lithium polymer batteries
It is generally believed that the charge capacity of lithium polymer batteries in the storage state is most suitable between 40% and 60%.
Of course it is difficult to do this all the time. Idle lithium polymer batteries are also plagued by self-discharge, and prolonged self-discharge can cause the battery to over-discharge.
Therefore, preparations should be made for the self-discharge phenomenon:
- One is to charge regularly to keep its voltage between 3.6V and 3.9V. Lithium polymer batteries can be charged at any time because they have no memory effect.
- The second is to ensure that the end-of-discharge voltage is not broken. If there is an alarm of insufficient power during use, the corresponding equipment should be stopped decisively.
4.1 Discharge
(1) Ambient temperature
Discharging is the working state of the lithium polymer battery, and the temperature requirement at this time is -20°C~60°C.
(2) Discharge termination voltage
The current common standard is 2.75V, and some can be set to 3V.
(3) Discharge current
Lithium polymer batteries also have types such as high current and large capacity. For lithium polymer batteries capable of high-power discharge, the current should be controlled within the range of the product specification.
4.2 Charging
The working characteristics of the lithium polymer battery charger should conform to the characteristics of the three stages of lithium battery charging. That is:
It can realize the charging requirements of the three stages of pre-charging, constant current charging and constant voltage charging. Therefore, the original charger is the best choice.
(1) Ambient temperature
The ambient temperature when charging the lithium polymer battery should be controlled within the range of 0~40°C.
(2) Charging cut-off voltage
The charging cut-off voltage of the lithium polymer battery is 4.2V. Even if multiple battery cells are combined in series, the balanced charging method should be adopted to ensure that the voltage of a single battery cell will not exceed 4.2V.
(3) Charging current
Lithium polymer batteries can be charged at 0.2C in non-emergency situations, and generally cannot exceed 1C.