Analysis of stress distribution and characteristics of NMC materials during discharge

NMC materials are widely used in power tools and other fields due to their high capacity and low cost. In recent years, with the rapid development of electric vehicles, NMC material lithium-ion batteries have been widely used in the field of power batteries. Therefore, NMC materials have attracted the attention of researchers. In the previous article, we also introduced the surface morphology of ternary materials NMC has a crucial impact on its electrochemical performance and cycle stability.

During the charge-discharge cycle, due to the phase change and Li content change inside the NMC material, the crystal will expand, and stress will be generated inside the particle. More serious, due to uneven current distribution on the electrode and inside the large particle, There is a large difference in the SOC state of different parts, which results in different stress states between different particles, resulting in link breakage between particles and generation of particle surface cracks.

The presence of these cracks will cause the transition metal elements inside the NMC to dissolve, the electrolyte to be oxidized, and the positive interface film to be produced and grown. The precipitation of the transition metal elements on the surface of the negative electrode will destroy the SEI film on the surface of the negative electrode, resulting in the NMC material during the cycle. Capacity degradation and voltage decay.

From the above analysis of the particles, it is seen that the stress state and variation characteristics of the NMC material during charge and discharge have an important influence on the long-term cycling stability of the NMC. Recently, Linmin Wu and others at Indiana University and Purdue University in Indianapolis jointly analyzed the stress generation process and variation characteristics of NMC during charge and discharge.

It has been found that during the charging and discharging process, the stresses on the concave and convex portions of the particles **, due to the stress, the joints of the particles may be broken, thereby generating particles insulated from the conductive network, resulting in capacity loss. Particles that are broken but not insulated from the conductive network are more susceptible to cracking on the surface of the particles. In the long-term circulation, due to the accumulation of phase transformation inside the particles, the stress of the material particles is gradually increased, which affects the long-term cycle stability of the material.

First, Linmin Wu reconstructed the structure of the NMC half-cell using synchronous acceleration X-ray, and then introduced a large number of mathematical formulas to describe the electrochemical reaction and stress generation. Then the model was used to study the chemical reaction of NMC materials at different magnifications. And stress is generated. Due to space limitations, the modeling process is not detailed, only a brief description of the important parts.

The battery modeling process is divided into two parts, electrochemical modeling and mechanical modeling. The electrochemical model of the battery is mainly used to describe the dynamic characteristics, material and charge transport of the battery. The electrochemical modeling process is mainly based on the work of Doyle and Fuller, and the mathematical simulation of the positive electrode particles, electrolyte and interface. . Mechanical modeling mainly simulates NMC particles, mainly analyzing the stress and deformation of the particles. Finally, the boundary conditions of the model and the characteristics of the material are set.

The simulation results show that the stress of the NMC particles gradually increases with the insertion of Li+ into the NMC material during the discharge process, and the stress reaches ** in the complete discharge state, and then the stress begins to decrease, which is mainly due to the complete discharge state. At this time, the material changes from a lamellar result to a spinel structure, resulting in a change in the stress of the particles. And this ** stress will increase with the increase of the discharge current density. For example, at a magnification of 2C, the ** stress is 271.52 MPa, which is about 2 times of the ** stress at 1 C rate, at 0.5 C rate. 4 times.

Once the stress reaches the yield strength of the material, the material structure will be destroyed and cause failure. Due to the lack of NMC yield strength data, the yield strength of LiCoO2 is about 200 MPa based on the yield strength of LiCoO2, so that is to say Complete discharge at a 2C rate will cause failure of the NMC material particles.

In order to further study the failure mode of NMC, the stress distribution of NMC materials was studied. It was found that: 1) the particles insulated from the conductive network will not generate stress; 2) the depressions and bulges of the particles are the places where the stress is most concentrated. It is much higher than other places; 3) Where the stress concentration is not considered, the stress on the surface of the particle is significantly higher than the stress inside the particle. Since the stresses at the depressions and projections of the particles are most concentrated, the joints of the particles are broken, which causes the particles to be insulated from the conductive network, resulting in capacity loss.

The study mainly came to the following conclusions:

1) As the discharge rate increases, the discharge voltage will decrease, which is mainly the result of an increase in battery impedance;

2) During the discharge process, the breakage of the particle joint causes the particle to be insulated from the conductive network to cause capacity loss, and those particles which are still in contact with the conductive network although the particle joint is broken, are more likely to form cracks on the surface of the particle;

3) As the discharge progresses, the stress of the particles gradually increases, reaching ** at the time of complete discharge, but then the stress is lowered due to the occurrence of the phase change. As the discharge rate increases, the ** stress also increases;

4) The stress at the depressions and projections of the NMC particles is the most concentrated, and the stress**. The stress on the surface of the particle is higher than the stress inside the particle;