Lithium-Polymer Batteries

Drone Battery

ENOV High-Energy drone batteries power industrial and commercial drones. Delivering 220–320 Wh/kg energy density, they enable long flight times (30+ mins) and support fast charging (2C). Perfect for aerial photography, surveillance, and delivery drones.

1. Definition and Classification

Lithium-polymer batteries (abbreviated as LiPo batteries), also known as polymer lithium-ion batteries, are a crucial branch in the lithium-ion battery technology system. Their core feature is the use of polymer electrolytes instead of traditional liquid electrolytes to optimize battery performance and safety. Based on the differences in electrolyte forms, they can be divided into two categories: narrow-sense and broad-sense lithium-polymer batteries.

• Narrow-sense lithium-polymer batteries (all-solid-state polymer lithium batteries): The electrolyte is entirely composed of polymers without any liquid components. These batteries are the current focus of research and development, with great potential in terms of safety and energy density. However, limited by technical bottlenecks, their mass-production technology is not yet mature, and they have not been widely commercialized.

• Broad-sense lithium-polymer batteries (gel polymer lithium batteries): The electrolyte is a mixed system of polymers and liquid electrolytes, where the polymer forms a gel network to encapsulate the electrolyte. Most of the “soft-packed lithium batteries” commonly available in the market today fall into this category. They are also the mainstream form of “lithium-polymer batteries” in the public’s perception and are widely used in various consumer electronics and emerging devices.

2. Structure and Working Principle

2.1 Structural Composition

Overall Structure: It consists of five layers of thin films, with a total thickness that can be as thin as 0.1mm. The first layer is a metal foil current collector, which is used to collect current. The second layer is the negative electrode, usually made of materials such as graphite. The third layer is a solid or gel-like polymer electrolyte, which is responsible for the conduction of lithium ions. The fourth layer is the positive electrode, commonly using lithium cobalt oxide, ternary materials, etc. The fifth layer is an insulating layer, which prevents direct contact between the positive and negative electrodes to avoid short circuits.

Key Components

• Electrolyte: It is divided into four types: solid polymer electrolyte (SPE), gel polymer electrolyte (GPE), composite polymer electrolyte (CPE), and single-ion conducting (SIC) polymer electrolyte. All of them take polymers as the core matrix and have different physical states and conductive properties.

• Electrodes: The positive electrode mostly uses lithium transition metal oxides, such as lithium cobalt oxide (LiCoO₂), lithium manganese oxide, lithium-ion ternary materials, lithium iron phosphate (LiFePO₄), etc. The negative electrode usually uses carbon materials, and some high-end products will explore other new types of negative electrode materials.

• Separator/Barrier: The insulator between the electrodes in a lithium-polymer battery is made of microporous polymers. Some barrier designs include a polyethylene layer and a polypropylene layer. The polyethylene layer can block the current flow when the temperature exceeds a threshold value, while the polypropylene layer provides mechanical support and at the same time isolates the electrodes and allows the transmission of lithium ions.

• Packaging: It usually adopts an aluminum-plastic soft package (soft package/bag type), which is different from the stainless steel or aluminum hard shell of traditional lithium-ion batteries. The soft-package design makes the battery more malleable, enabling ultra-thin and special-shaped manufacturing, and it is also lighter in weight.

2.2 Working Principle

The working principle of lithium-polymer batteries is similar to that of lithium-ion batteries. It is based on the “rocking chair mechanism” and relies on the movement of lithium ions between the positive and negative electrodes to realize the charging and discharging process.

• Charging Stage: The external circuit provides electrical energy. Lithium ions are deintercalated from the positive electrode material, pass through the conduction channel formed by the polymer electrolyte, and are intercalated into the lattice structure of the negative electrode material. At this time, the negative electrode is in a lithium-rich state, and the positive electrode is in a lithium-poor state.

• Discharging Stage: The battery supplies power to the outside. The lithium ions intercalated in the negative electrode are deintercalated, migrate back to the positive electrode through the polymer electrolyte again, and are re-intercalated. During this process, electrons flow from the negative electrode to the positive electrode through the external circuit to form a current, which supplies power to external devices.

3. Core Characteristics

3.1 Basic Performance Parameters

Characteristic	Specific Description
Electrolyte Form	Uses solid or gel-like polymer electrolytes without free-flowing liquids, reducing the risk of liquid leakage
Packaging Form	Usually adopts an aluminum-plastic soft package, which has strong malleability and supports ultra-thin and special-shaped designs
Monomer Voltage	The nominal voltage is 3.7V, and the full-charge voltage is 4.2V; for batteries based on lithium iron phosphate, the nominal voltage is 3.6-3.8V during charging and 1.8-2.0V during discharging
Energy Density	The mass energy density is generally 100-265Wh/kg (some high-end models such as enovbattery can reach 320Wh/kg), and the volume energy density is 250-670Wh/L, which is slightly lower than that of high-end liquid lithium-ion batteries (such as the NCM system)
Discharge Rate	Supports high-rate discharge, up to 20C-50C, suitable for high-power demand scenarios and can provide instantaneous large-current output
Cycle Life	Usually 300-500 cycles, and high-quality products can reach 800 cycles, which is lower than that of liquid lithium-ion batteries (500-1000 cycles)
Self-Discharge Rate	Relatively low, with a monthly self-discharge rate of about 2-3% (some data mention about 5%), and it can still maintain a high charge level when idle for a long time

3.2 Outstanding Features

• Thinness, Lightness, and Flexibility: It can be made into an ultra-thin form (with a thickness of less than 100 microns, and the thinnest can reach 0.1mm) or a special-shaped structure (such as a curved shape), which can adapt to the space requirements of different devices, especially suitable for products such as wearable devices and flexible screens.

• Lightweight: The soft-package structure does not require a thick metal shell, so it is about 20% lighter than liquid lithium-ion batteries. At the same time, the polymer electrolyte can be made into a thin film and also has the function of a separator, which further reduces the overall weight of the battery and is suitable for application scenarios sensitive to weight.

• Design Flexibility: It can be customized in terms of capacity, size, weight, etc., according to customer needs, and can be produced into almost any desired shape to meet the personalized design needs of different devices, such as consumer electronics and drones.

• Operating Temperature Range: It has a relatively wide operating temperature range, generally between-20℃ and 60℃, but its discharge performance at low temperatures (below 0℃) is worse than that of traditional lithium-ion batteries.

4. Advantages and Disadvantages Analysis

4.1 Advantages

• High Safety: There is no liquid electrolyte, which greatly reduces the risk of liquid leakage. Even when damaged, the risk of thermal runaway is lower than that of liquid lithium-ion batteries, and it is not easy to catch fire or explode when punctured (but there is still a certain risk). Some barrier designs have an overheating protection function, which can cut off the current when the temperature is too high.

• No Memory Effect: It can be charged without being fully discharged, and can be charged and used at any time, which is convenient to use and does not affect the battery capacity and service life.

• High Discharge Capacity: It supports high-rate discharge and can provide an instantaneous large current, which meets the power needs of high-power devices such as drones, electric models, and racing drones, and helps the devices achieve performance such as rapid take-off and high-speed operation.

• Low Self-Discharge Rate: The power loss is slow when stored for a long time, which is suitable for scenarios where devices are not used frequently, such as emergency power supplies, remote controls, and seasonal use devices, and they can still be used normally after storage.

• Good Electrochemical Stability: The polymer electrolyte has higher electrochemical oxidation resistance and is more suitable for high-voltage application scenarios compared with many electrolytes based on solvents.

• Strong Electrode-Electrolyte Adhesion: The polymer electrolyte can make better contact with the electrodes, reduce the interface impedance, and improve the overall performance and stability of the battery.

4.2 Disadvantages

• High Cost: The manufacturing process is complex, the material cost and production difficulty are higher than those of traditional liquid lithium-ion batteries, and the unit energy cost is 10-30% more expensive than that of liquid lithium-ion batteries.

• Short Service Life: The cycle life is generally lower than that of liquid lithium-ion batteries, and the capacity decays significantly after long-term use. Moreover, it is sensitive to use conditions, and improper maintenance will further shorten the service life.

• Easy to Bulge: Under high temperature, over-charging, over-discharging or long-term use, the electrolyte may vaporize, causing the battery to expand. When the expansion pressure exceeds 50kPa, it may rupture, which affects the use of the device and poses a safety hazard. Bulging is also an important precursor to battery failure.

• Difficult to Recycle: The polymer structure is complex, and it is difficult to separate the metals, plastics and other components in the battery, resulting in a low recycling rate, which is not conducive to resource recycling.

• Sensitive to Use Conditions: It is more sensitive to over-charging, deep discharging and high-temperature environments. Deep and rapid discharging may cause the battery to expand, burn or even explode. When stored for a long time, the charge level and environmental conditions must be strictly controlled, otherwise the battery is easily damaged.

5. Detailed Explanation of Electrolyte Types

5.1 Solid Polymer Electrolyte (SPE)

• Definition: Also known as solvent-free polymer electrolyte, it is composed of a polymer matrix and inorganic salts (lithium salts). The lithium salts are dissolved in the polymer matrix to provide ionic conductivity, and there are no liquid components.

• Common Materials: The polymer matrix includes polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), etc. It combines with alkali metal salt complexes to form Li⁺ conductive materials.

• Advantages and Disadvantages

Advantages: It is safer than liquid/gel electrolytes, has no risk of liquid leakage, and low flammability. It also has high mechanical and electrochemical stability and is easy to manufacture.

Disadvantages: The ionic conductivity at room temperature is limited, and the movement speed of lithium ions is slow (caused by the coordination of Li⁺ with the Lewis base sites of the polymer chain). In addition, it has poor interface compatibility with the electrodes, which affects the overall performance of the battery.

5.2 Gel Polymer Electrolyte (GPE)

• Definition: It is composed of a polymer matrix, a liquid electrolyte (plasticizer), lithium salts and additives such as inorganic fillers. The liquid electrolyte is encapsulated by a small amount of polymer network, and its performance is between that of liquid and solid electrolytes.

• Common Materials: The polymer matrix is similar to that of SPE, including PEO, PAN, PVDF, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), PMMA, etc. The plasticizers include carbonates (such as propylene carbonate PC, ethylene carbonate EC), ethers (such as tetraethylene glycol dimethyl ether TEGDME) and ionic liquids.

• Advantages and Disadvantages

Advantages: It has high ionic conductivity, enabling fast charging and discharging. It also has flexibility and good interface compatibility with the electrodes. In addition, it has high thermal stability, low volatility and higher safety than liquid electrolytes.

Disadvantages: It has hygroscopicity and is sensitive to environmental humidity. Temperature changes will affect its performance. There may be electrolyte leakage (though lower than that of liquid electrolytes). Moreover, there are problems such as solvent residue during the manufacturing process.

5.3 Composite Polymer Electrolyte (CPE)

• Definition: It is composed of a polymer matrix, inorganic fillers, and lithium salts. The inorganic fillers are divided into non-ionically conductive (passive) fillers and ionically conductive (active) fillers, and the conductive properties are affected by multiple components together.

• Common Materials: The polymer matrix is similar to that of the previous two types of electrolytes. The non-ionically conductive inorganic fillers include aluminum oxide (Al₂O₃), silicon oxide (SiO₂) nanoparticles (ceramics), and some ferroelectric ceramic fillers can increase the dipole moment of the polymer chain. The ionically conductive fillers include garnet-type (such as Li₇La₃Zr₂O₁₂, LLZO), NASICON-type (such as Li₁.₅Al₀.₅Ge₁.₅(PO₄)₃), and sulfide-type (such as Li₁₀GeP₂S₁₂) materials.

• Advantages and Disadvantages

Advantages: It has high mechanical strength, can withstand high mechanical stress and strain, and solves the problem of liquid electrolyte leakage. It also has good thermal stability, relatively high ionic conductivity, and some types also have a certain degree of flexibility. The non-ionically conductive fillers can enhance the electrochemical stability of the polymer through Lewis acid-base interactions.

Disadvantages: The manufacturing process is complex. The electrolyte is prone to drying problems. It has strong temperature sensitivity and poor interface compatibility with the electrodes. The inorganic fillers are prone to agglomeration, which affects the performance. In addition, the cost is relatively high.

5.4 Single-Ion Conducting (SIC) Polymer Electrolyte

• Definition: The anions are covalently bonded to the polymer. During the battery operation, the only mobile ions are lithium ions, which is different from other electrolytes where both lithium cations and counter anions can move.

• Performance Characteristics: It avoids the accumulation of anions on the electrodes, reduces battery polarization, and prolongs the cycle life. The movement of lithium ions is uniform during the charging and discharging process, which can inhibit the formation of lithium dendrites and improve battery safety. It solves the problem that the movement of lithium cations in traditional electrolytes only accounts for 20% of the total ionic current (the movement speed of anions is at least 4 times that of lithium cations).

• Application Potential: It has a good application prospect in scenarios with high requirements for battery cycle life and safety, such as electric vehicles and energy storage systems. However, the current technology is still under continuous research and optimization.

6. Application Scenarios

6.1 Consumer Electronics Field

• Smartphones: Early models such as the iPhone and many mainstream smartphones use lithium-polymer batteries. Their thin, light and special-shaped design characteristics can adapt to the slim body of smartphones and meet the daily battery life needs.

• Laptops: Especially ultra-thin laptops, by virtue of the lightweight and high energy density advantages of lithium-polymer batteries, they can reduce the weight of the device while extending the battery life and improving portability.

• Wearable Devices: Devices such as smart watches, fitness trackers and Bluetooth headsets are small in size and have strict requirements on the shape and weight of the battery. The flexible design and lightweight characteristics of lithium-polymer batteries can be well adapted to them, and the low self-discharge rate is suitable for intermittent use scenarios.

• Other Devices: Tablets, portable media players, wireless controllers for video game consoles, wireless PC peripherals, electronic cigarettes, etc., all use the advantages of small size and high energy density of lithium-polymer batteries to meet the miniaturization and battery life needs of the devices.

6.2 Emerging Equipment Field

• Flexible Devices: Flexible electronic devices, such as Samsung foldable screen mobile phones rely on the bending and special-shaped characteristics of lithium-polymer batteries to realize the folding function of the devices and ensure a stable power supply during use.

• Medical Equipment: For small portable medical instruments, such as portable oxygen concentrators, defibrillators, and medical implant devices, the portability, low self-discharge rate, and high safety of lithium-polymer batteries can meet the strict requirements of medical equipment for power supply reliability and portability.

6.3 UAV and Model Field

• UAVs: Lithium-polymer batteries are widely used in consumer-level small UAVs to professional-level large UAVs (such as some models of DJI, film photography UAVs, military and industrial unmanned aerial vehicles (UAVs)). Their high discharge rate, high energy density and lightweight characteristics can provide strong power for UAVs, ensuring rapid take-off, long flight time and high load capacity. FPV racing UAVs even rely on their high-power output performance.

• Remote-Control Models: For amateur model devices such as remote-control cars, remote-control boats and remote-control airplanes, the high discharge current and high energy density advantages of lithium-polymer batteries can significantly improve the running speed and battery life of the models. Compared with traditional nickel-metal hydride batteries, their performance is significantly improved (such as higher firing rate of soft air guns).

6.4 Transportation Field

• Electric Vehicles and Bicycles: Some car companies such as Chery, Geely, Toyota, Nissan, Kia (Kia Soul model) and Hyundai use lithium-polymer batteries in their battery-electric and hybrid vehicles. Some electric bicycles also use this type of battery. By using its high energy density and lightweight characteristics, the battery life and handling of the vehicles are improved.

• eVTOL Aircraft: The emerging urban air transportation solution-eVTOL aircraft has extremely high requirements for the power-to-weight ratio of the battery. Lithium-polymer batteries can meet its high-power output and lightweight needs and provide power support for flight.

• Emergency Start-up Equipment: Vehicle emergency starters or battery boosters use three or six series-connected lithium-polymer batteries (3S1P/6S1P) to provide 12V or 24V starting voltage when the vehicle is out of power. Compared with traditional lead-acid emergency starters, they are much smaller in size, lighter in weight and have greatly improved portability.

6.5 Industrial and Military Fields

• Satellites and Aerospace Equipment: Backup power supplies for satellites and some space exploration equipment use the high energy density, lightweight and high reliability characteristics of lithium-polymer batteries to work stably in the extreme space environment and ensure the operation of the equipment.

• Power Tools: For cordless power tools such as cordless drills and saws, the high discharge rate of lithium-polymer batteries can provide strong power to meet the high-intensity work needs of the tools, and the lightweight design is convenient for hand-held operation.

6.6 Energy Storage Field

• Portable Energy Storage Devices: For power banks, emergency power supplies, etc., the thinness, lightness, high energy density and low self-discharge rate of lithium-polymer batteries make them ideal power sources for portable energy storage devices, facilitating users’ daily charging and emergency power supply.

• Solar Energy Storage Systems: They are used to store the electrical energy generated by solar panels for use during non-sunlight hours. The high energy density and long cycle life (compared with some energy storage batteries) of lithium-polymer batteries can improve the efficiency and service life of the energy storage system.

• Uninterruptible Power Supply (UPS): In scenarios such as data centers and backup power supplies for key equipment, compared with traditional VRLA batteries, lithium-polymer batteries have higher power-to-size-to-weight ratio, longer cycle life, larger available energy (depth of discharge) and lower risk of thermal runaway. They can effectively reduce the space occupation and energy consumption of data centers and ensure the continuous operation of key equipment when the power is cut off.

7. Usage and Maintenance Suggestions

7.1 Charging Specifications

• Voltage Control: The charging voltage of a single battery should not exceed 4.2V to avoid over-charging. For batteries based on lithium iron phosphate, the corresponding voltage standard should be followed (3.6-3.8V for charging). It is recommended to use a balanced charger to ensure that the charge of each single battery in the battery pack is balanced and prevent over-charging or under-charging of some single batteries.

• Charging Rate: It is recommended to use the 0.5C constant current and constant voltage charging method. Avoid high-rate fast charging (unless the battery clearly supports it) to reduce the heat generation of the battery during charging and extend the battery life.

7.2 Discharge Precautions

• Voltage Protection: Avoid over-discharging the battery. If the voltage of a single battery is lower than 3.0V (for lithium iron phosphate batteries, it is lower than 1.8-2.0V), it may cause permanent damage. The device should be equipped with an over-discharge protection circuit to prevent over-discharging of the battery.

• Discharge Rate: The discharge rate should be limited according to the battery specifications, not exceeding the maximum allowable discharge rate of the battery (usually 20C-50C). When discharging at a high rate, attention should be paid to the heat dissipation of the device to avoid overheating of the battery.

• Low-Temperature Use: The discharge performance decreases in a low-temperature environment (below 0℃). Therefore, the use in a low-temperature environment should be avoided as much as possible. If it is necessary to use it, attention should be paid to the discharge current and time to prevent damage to the battery.

7.3 Storage Requirements

• Charge Control: When storing the battery for a long time, the battery should maintain a charge level of about 40% (corresponding to a voltage of 3.7-3.8V, and 3.6-3.9V is recommended for remote-control model batteries). Avoid storing the battery with a full charge or empty charge. Storing with a full charge is easy to cause the battery to bulge, and storing with an empty charge is easy to cause irreversible capacity loss.

• Environmental Conditions: The storage environment should have an appropriate temperature, avoiding high temperature, low temperature or severe temperature fluctuations. The environmental humidity should be lower than 50% to prevent the battery from being damaged by moisture. The storage location should be far away from fire sources, heat sources and corrosive substances.

7.4 Physical Protection and Inspection

• Prevention of Physical Damage: Avoid physical damage to the battery, such as impact, extrusion, and puncture. The soft-package shell of the lithium-polymer battery is easy to be damaged, which may lead to short circuits, liquid leakage, and even fire and explosion after damage.

• Regular Inspection: Regularly check the appearance of the battery. If the battery is found to be bulging, leaking, having a damaged shell, or abnormal heat generation, it should be stopped using immediately, and should not be used or charged continuously to prevent safety accidents. The state of the battery should also be checked before use to ensure that there is no abnormality.

• Pressure Maintenance: Applying an appropriate pressure to the lithium-polymer battery layer stack can maximize the contact between components, prevent delamination and deformation, improve the capacity retention rate, and avoid the increase of battery impedance and performance degradation.

8. Frequently Asked Questions

8.1 Can Lithium-Polymer Batteries Replace 18650 Batteries?

In scenarios that require high energy density or special-shaped designs (such as ultra-thin devices, flexible devices, and UAVs), lithium-polymer batteries can replace 18650 batteries. Their flexible design and lightweight advantages can be better adapted to such scenario needs. However, in the power battery field (such as Tesla electric vehicles), liquid lithium-ion batteries such as 18650 are still the mainstream choice due to their long cycle life, low cost and mature technology, and lithium-polymer batteries have not been widely used as a replacement.

8.2 Is the Bulging of Lithium-Polymer Batteries Dangerous?

The bulging of lithium-polymer batteries is dangerous and is a precursor to battery failure. Bulging is usually caused by the vaporization of the electrolyte and the generation of gas from internal reactions. When the expansion pressure exceeds 50kPa, the battery may rupture, which may further lead to liquid leakage, heat generation and even fire and explosion. Once the battery is found to be bulging, it should be stopped using immediately, and should not be used or charged continuously. It should be recycled and disposed of in the correct way.

8.3 What Should Be Noted When Storing Lithium-Polymer Batteries for a Long Time?

When storing lithium-polymer batteries for a long time, the charge should be maintained at about 40% (corresponding to a voltage of 3.7-3.8V, and 3.6-3.9V for remote-control model batteries). Avoid storing with a full charge or empty charge. The storage environment should have an appropriate temperature, avoiding high temperature, low temperature or severe fluctuations. The environmental humidity should be lower than 50% to prevent moisture. When storing, avoid squeezing or impacting the battery, keep it away from fire sources and corrosive substances, and regularly check the appearance of the battery to ensure that there is no abnormality such as bulging or liquid leakage.

8.4 Why Is a Balanced Charger Needed When Charging Lithium-Polymer Batteries?

A lithium-polymer battery pack is composed of multiple series-connected or parallel-connected single batteries. If the capacity and voltage of each single battery are inconsistent, some single batteries may be over-charged and some may be under-charged during charging, which affects the life and safety of the battery pack. A balanced charger can monitor the voltage of each single battery in real time and adjust the charging current to ensure that each single battery reaches the optimal charging state, avoid over-charging or under-charging, ensure the stable operation of the battery pack and extend its service life.

8.5 What to Do If a Lithium-Polymer Battery Is Punctured?

If a lithium-polymer battery is punctured, it may cause internal short circuits, liquid leakage, heat generation, and even fire. You should stay away from the battery immediately, avoid contact with the leaked electrolyte (which is corrosive), and transfer it to an open, ventilated area without flammable substances. If the battery does not catch fire immediately, do not continue to use or charge it, and do not try to squeeze or disassemble the battery. If the battery catches fire, use a dry powder fire extinguisher or a carbon dioxide fire extinguisher to put out the fire. Do not use water to put out the fire. After the fire is put out, dispose of it in accordance with the specifications for hazardous waste.