Sodium-ion battery charging and discharging causes rapid performance decline, scientists finally found the real reason

Sodium-ion batteries have attracted a lot of attention recently and are showing a trend of surpassing lithium-ion batteries to become the new generation of "traffic".

Ever since Ning Wang, the leader in the battery manufacturing industry, officially released the first generation of sodium-ion batteries, sodium-ion batteries have begun to appear frequently on domestic and foreign media websites. Many battery manufacturers have followed closely and are gradually deploying sodium-ion batteries. Some even predict that sodium-ion batteries will soon replace lithium-ion batteries and become the next generation of power source for electric vehicles.

However, scientific research shows that there is still some way to go before pure sodium-ion batteries can be commercialized.

The sodium-ion battery released by Ningwang is not a pure sodium-ion battery pack. It is a hybrid AB battery pack that combines sodium-ion batteries and ternary lithium batteries to complement each other's strengths.

As lithium metal prices soared, under cost pressure, the advantages of sodium-ion batteries became more prominent. Sodium is more abundant in the earth's crust, has lower costs, and is easy to obtain.

Battery manufacturers and academia are racing to develop sodium-ion batteries as a way to power the next generation of electric vehicles.

However, since sodium ions have a larger radius than lithium ions, they can easily cause fission of the positive electrode material during the charging and discharging process. This is also one of the reasons why its performance drops rapidly during rapid charging and discharging.

The embedding and de-embedding of ions in the positive and negative electrode materials of metal ion batteries will cause mechanical strain, which is a very important factor affecting the life of metal ion batteries. Research on this in lithium-ion batteries is relatively mature.

However, little is known about the strain caused by the movement of sodium ions inside sodium-ion batteries. It has always been generally believed that phase boundary reactions similar to those in lithium-ion batteries, namely irreversible phase changes and cathode-electrolyte reactions, are important factors that cause the battery life to decrease.

However, a recent study conducted by the Argonne National Laboratory in the United States and Stanford University found that during the charging and discharging process of sodium-ion batteries, the native lattice strain of the positive electrode plays an overwhelming role in the performance degradation and life of sodium-ion batteries, which runs counter to traditional understanding.

They first tested the capacity performance of the sodium-ion battery during the charge and discharge cycle. After 100 charge and discharge cycles, the capacity of the sodium-ion battery decayed to 30%, which is very low! They speculated that the mechanical strain of the positive electrode material inside the battery is very high.

Next, they used an in situ synchrotron X-ray probe to observe the changes in the positive electrode material during the charge and discharge of the sodium-ion battery and found that a phase change occurred in the positive electrode, but this phase change was reversible, which was different from the results previously observed in lithium-ion batteries, where the phase change is irreversible.

This puzzled the researchers.

To further explore the reasons, they used advanced transmission electron microscopy to detect the crystal structure of the oxide positive electrode during the charge/discharge cycle of the sodium-ion battery, and found that the lattice distortion in this process may accelerate the mixing or migration of sodium ions during the cycle, forming unexpected structural evolution, thereby leading to capacity decay!

Transmission electron microscopy after the sodium-ion battery was charged and discharged 100 times showed that the original lattice strain contour of the positive electrode material disappeared and the strain was relaxed. As a result, there were many stacking layer dislocations, which is why it is difficult for sodium ions to be re-embedded in the positive electrode.

They also observed that the areas of strain relaxation were not empty, but were filled with crystalline fragments, which is very similar to the process of an earthquake!

All of these are important reasons for the rapid capacity decay of sodium-ion batteries during the charging and discharging process. Especially in the case of rapid charging and discharging, this capacity decline is more serious!

Currently, their research paper has been published in the academic journal Nature Communications. Through this research, they hope to improve the positive and negative electrode materials by clearly understanding the changes in electrode materials during the charging and discharging process of sodium-ion batteries, so as to improve the performance of sodium-ion batteries by 20~40%, thereby extending battery life.