The Charging and Discharging Principles of Lithium - Ion Polymer Small Batteries

In our daily lives, lithium - ion polymer small batteries are everywhere. They power our smartwatches that track our fitness, our wireless 

earbuds that let us enjoy music on the go, and our portable Bluetooth speakers that bring sound to our outdoor activities. Have you ever 

wondered how these small but powerful energy sources work when they are being charged and discharged? Let's take a deep dive into 

the fascinating world of lithium - ion polymer small batteries and explore their charging and discharging principles.

Let's start with the basic structure of a lithium - ion polymer small battery, as it's essential for understanding the subsequent processes. A 

typical lithium - ion polymer small battery consists of several key components: a positive electrode, a negative electrode, an electrolyte, 

and a separator.

The positive electrode, often made of materials like lithium cobalt oxide (LiCoO₂), lithium manganese oxide (LiMn₂O₄), or lithium iron 

phosphate (LiFePO₄), is where important chemical reactions take place during charging and discharging. These materials have a unique 

crystal structure that allows them to store and release lithium ions.

The negative electrode is commonly composed of graphite. Graphite has a layered structure, which is like a stack of sheets, and this 

structure is perfect for accommodating lithium ions. It can effectively absorb and hold lithium ions during charging and release them when 

needed during discharging.

The electrolyte in lithium - ion polymer batteries is usually a polymer gel or a liquid electrolyte trapped in a polymer matrix. Its main role is to 

conduct lithium ions between the positive and negative electrodes. Unlike some other batteries, the electrolyte here is designed to be stable 

and not conduct electrons, which is crucial for the proper functioning of the battery.

The separator is a thin, porous membrane that physically separates the positive and negative electrodes to prevent a short circuit. At the 

same time, its porous nature allows lithium ions to pass through freely, ensuring the continuity of the ion flow.

Now, let's explore the charging process. When we connect a lithium - ion polymer small battery to a charger, an external voltage is applied. 

This voltage drives a series of reactions. At the positive electrode, under the influence of the external voltage, the lithium - containing 

compound starts to undergo a oxidation reaction. The lithium ions (Li⁺) in the positive electrode material are separated from their original 

positions.

These liberated lithium ions then move through the electrolyte. The separator, with its porous structure, doesn't block their path. Guided by 

the electric field created by the external voltage, the lithium ions travel towards the negative electrode.

When the lithium ions reach the negative electrode, which is made of graphite, they are inserted into the layered structure of the graphite. 

This process is called intercalation. At the same time, electrons are generated at the positive electrode during the oxidation reaction. Since 

the electrolyte doesn't conduct electrons, these electrons have to flow through the external circuit (including the charger and the wires) to 

the negative electrode. This electron flow is what we measure as electric current during charging. As more and more lithium ions are 

intercalated into the graphite of the negative electrode, the battery is being charged, and it stores more and more energy.

It's important to note that during charging, there are some safety - related considerations. For example, over - charging can be very 

dangerous. If the battery is charged beyond its recommended capacity, the positive electrode may release too many lithium ions, and the 

negative electrode may not be able to accommodate them all. This can lead to the formation of lithium metal dendrites on the surface of 

the negative electrode. These dendrites are like tiny needles, and they can pierce the separator, causing a short circuit inside the battery. 

A short circuit can generate a lot of heat, which may even cause the battery to catch fire or explode, especially considering that some 

electrolytes are flammable. That's why modern lithium - ion polymer batteries are equipped with protection circuits. These circuits monitor 

the voltage and current during charging and cut off the charging process when the battery reaches its full capacity, preventing over - 

charging.

Now, let's turn to the discharging process, which is when the battery provides power to our devices. When we use a device powered by a 

lithium - ion polymer small battery, the battery acts as a power source, and a circuit is formed between the battery and the device. The 

stored energy in the battery is released through a series of chemical reactions.

At the negative electrode, the lithium ions that were intercalated into the graphite during charging start to move out of the layered structure. 

This is because the chemical potential difference between the two electrodes drives them. The lithium ions then travel back through the 

electrolyte and the separator towards the positive electrode.

At the same time, electrons are released from the negative electrode. Since the external circuit (the device) provides a path for them, the 

electrons flow from the negative electrode through the device to the positive electrode. This electron flow is the electric current that powers 

the device, making our smartwatches display time, our earbuds play music, and so on.

When the lithium ions reach the positive electrode, they are reinserted into the crystal structure of the positive electrode material. This is a 

reduction reaction. As the lithium ions move back to the positive electrode and the electrons flow through the external circuit, the battery 

discharges, and the energy it stored during charging is converted into electrical energy to power the device.

Similar to charging, there are also considerations during discharging. Over - discharging is not good for the battery. If the battery is 

discharged too much, the positive electrode may not have enough lithium ions to maintain its structure, and the negative electrode may 

be damaged. This can reduce the battery's capacity and lifespan over time. Again, the protection circuits in the battery help here. They 

can detect when the battery voltage drops to a certain low level and cut off the discharge to prevent over - discharging.

Another aspect to consider is the rate of charging and discharging. Charging or discharging at a very high rate can affect the battery's 

performance and lifespan. A high charging rate may cause the lithium ions to move too quickly, and some of them may not be able to be 

properly intercalated into the negative electrode, leading to the formation of lithium dendrites. A high discharging rate can cause a significant 

voltage drop in the battery, making the device work improperly, and it can also generate more heat, which is not good for the battery's stability.

In conclusion, the charging and discharging processes of lithium - ion polymer small batteries are based on the movement of lithium ions 

between the positive and negative electrodes and the corresponding flow of electrons through an external circuit. During charging, lithium 

ions move from the positive electrode to the negative electrode, and electrons flow through the external circuit to the negative electrode, 

with the battery storing energy. During discharging, lithium ions move back to the positive electrode, and electrons flow through the external 

circuit to power the device, with the battery releasing energy. Understanding these principles not only helps us use our devices' batteries 

more wisely but also makes us appreciate the advanced technology behind these small energy powerhouses. By taking good care of our 

lithium - ion polymer small batteries, such as avoiding over - charging and over - discharging and using appropriate chargers, we can extend 

their lifespan and ensure their safe and efficient operation.