What exactly affects the life of lithium batteries?

ithium batteries have become an indispensable part of our lives. From mobile phones to cars, everyone is more or less exposed to lithium batteries. Many people will be concerned about the life span of lithium batteries? What determines its lifespan

Internal material

The decrease in the capacity of lithium batteries is mainly due to the loss of electrode materials and the decomposition of the electrolyte. Of course, improper operation of lithium batteries can also lead to a sharp decrease in life.

Starting from the inside of the battery, the current lithium batteries basically use graphite as the negative electrode. The graphite material of the negative electrode cannot be fused with the electrolyte stably, and a solid electrolyte interface (SEI) film (solid electrolyte interface) will be formed when the two are in contact.

The voids on the SEI film allow lithium ions to pass in and out, but prohibit electrons from passing through, and at the same time isolate the electrolyte from the graphite. This structure is perfect for lithium batteries, and SEI is one of the important structures to maintain the stability of lithium batteries.

SEI is not static. Under normal circumstances, SEI will gradually thicken and have a certain degree of damage, and the damaged part will form a new SEI film due to the re-contact of graphite and electrolyte. This process will consume lithium ions. When the battery is deformed due to external factors (humidity, temperature, etc.), the internal interaction force, that is, stress, will deform the SEI hole and make the ion channel no longer open.

These microscopic changes have caused the battery to exhibit increased internal resistance, decreased capacity, and deterioration of charging capabilities, and other life-deterioration phenomena.

The main source of lithium ions in lithium batteries is the cathode material. Lithium ions are stored in the lattice structure of the material, and are removed or inserted during charging and discharging. There are two main aging forms of cathode materials. One is the decrease in the total amount of active material caused by the collapse of the lattice structure and the partial material falling off from the whole; the other is the consumption of side reactions between the electrolyte and the positive electrode material. As a result, the number of lithium ions that can be extracted and the number of vacancies for storing lithium ions are correspondingly reduced. In the event of improper operation and abuse, large-scale rupture of the crystals of the positive electrode material due to various stress effects will result in a large amount of active material loss in a short period of time. The direct performance of the battery's external characteristics is the decrease in capacity. As mentioned before, the protection of the SEI film is needed to reduce the reaction between the electrolyte and graphite: the electrolyte and the cathode also have minor side reactions, and the side reactions will increase with the increase of temperature. These side reactions consume electrolyte, reduce conductive ions, and generate gas.

The above is the discussion of the factors affecting the life of the battery at the microscopic level. The following explains the influence of the outside world on the battery life.

Temperature can be said to be the most important factor affecting lithium batteries. There are complex electrochemical processes inside lithium batteries, and temperature determines the activity of most chemical reactions.

At high temperatures, the electrochemical reaction is more active, and the battery has better performance, such as enhanced discharge capacity and reduced internal resistance. However, high temperature will aggravate the side reactions between the electrolyte and the anode and cathode materials, and accelerate the consumption of the electrolyte, the anode and cathode materials and lithium ions. This loss is permanent and will cause the battery capacity to attenuate and the internal resistance to increase.

The low temperature side reaction level is low, and the internal chemical activity of the battery is also reduced. If it is discharged with a high current in this case, the electrode material cannot meet the load requirement, which will cause structural damage; charging at a low temperature and high current may cause lithium evolution and dendrite growth problems.

During overcharge, the battery voltage rises rapidly with the increase in polarization, which will cause irreversible changes in the structure of the positive electrode active material and decomposition of the electrolyte, generating a large amount of gas, releasing a large amount of heat, and causing a sharp increase in battery temperature and internal pressure. Melting or shrinking will cause the positive and negative electrode materials to contact and short-circuit, and there will be hidden dangers such as explosion and combustion.

Over-discharge means that the battery has discharged the internally stored power, and the voltage reaches the rated voltage and continues to discharge, which may bring catastrophic consequences to the battery. Generally speaking, over-discharge will increase the internal voltage of the battery and activate the positive and negative electrodes. The reversibility of the material is destroyed, the electrolyte is decomposed, the negative electrode is deposited with lithium, and the resistance increases. Even if it is charged, it can only be partially restored, and the capacity will be significantly attenuated.

There are many factors that affect the life of lithium batteries, and BMS is to reduce these effects, but good usage habits are also very important.