With the approaching of 2020, the majority of power battery manufacturers have to challenge the index of 300Wh / kg, from the current point of view we are basically the same technical line - high Ni ternary material + high-capacity silicon carbon material. With the improvement of the Ni content of the ternary material, the capacity of the material will also be increased accordingly. For example, the current specific capacity of the NMC811 material has reached about 200 mAh / g, but the space for increasing the capacity of the ternary material by increasing the Ni content is no longer Large, first with the Ni content increased, the material itself will be greatly increased the difficulty of production; In addition, too high Ni content will lead to material homogenization and production difficulties; Finally, too high Ni content is difficult to guarantee the material In the charge and discharge process of structural stability.Therefore, the cathode material is currently difficult to have a major breakthrough, people will focus on the development of high specific energy battery to high-capacity silicon carbon material research and development up.
Si material theoretical capacity of 4200mAh / g (Li4.4Si), graphite material is more than ten times, can be said to be an ideal lithium-ion battery anode material, but the Si material is facing a large volume expansion challenges (complete lithium Up to 300%), which not only leads to the breakage of the Si material particles during charging and discharging, but also leads to the destruction and regeneration of the negative electrode SEI film, which causes the cycle performance of the Si negative electrode to drop sharply. In order to solve the volume expansion of the Si material, Large issues, the most common means of nano-technology.With nano-means, can effectively reduce the absolute volume expansion of Si material, thereby enhancing the battery cycle performance, but the huge surface area of nanoparticles, will cause increased side effects, serious Affect the cycle life of the battery, so how to weigh the relationship between the two is particularly important.
Recently, Jianping Yang, Donghua University, who use amorphous TiO 2Nano-Si particles were coated (thickness of the coating is about 3nm), amorphous TiO2 good elastic properties of Si particles in the process of volume expansion during charging and discharging to provide a very good buffer, thus ensuring the Si particles The integrity of the nano-Si material significantly improves the cycle performance.
As shown in the above figure, Jianping Yang synthesizes amorphous TiO by sol-gel method 2Coated Si Nanoparticles - Si @ a-TiO 2, Amorphous TiO 2The good elasticity of the shell absorbs the volume expansion of the Si particles during charging and discharging. In the presence of amorphous TiO 2With the help of the housing, the material not only achieved a first efficiency of 86.1% but also exhibited excellent cycle performance - with a current density of 420mA / g, cycling 200 times with a capacity of 1720mAh / g, g high current density, the capacity of up to 812mAh / g, much higher than graphite materials.
The graph above shows the electrochemical performance of Si @ a-TiO2. From Fig. A, it can be seen that in addition to a current peak near 1.25 V (corresponding to SEI film formation) During the scanning, the peak of intercalation lithium current appeared in the vicinity of 0.185V, and the current peak in delithiation appeared at 0.54V. With the increase of scanning times, the intensity of current peak also increased gradually. As the lithium insertion process progressed, , Si @ a-TiO 2The lithium insertion kinetic conditions of the material become better and better.
As can be seen from the cycle performance test results of c above, no matter the amorphous TiO is used 2The Si nanoparticles were coated (Si @ a-TiO 2), Or using anatase TiO 2The nano-Si particles were coated (Si @ c-TiO 2), Can significantly improve the cycling performance of nano-Si material.Compared with the untreated nano-Si material, Si @ a-TiO 2The cycle performance of the material has been greatly improved, circulating at a current density of 420mA / g 200 times, Si @ a-TiO 2The material capacity is still able to reach 1720mAh / g (but the capacity retention rate is only about 56%, the cycle performance needs to continue to improve).
In addition to its excellent cycling properties, Si @ a-TiO 2The material also exhibits excellent rate performance (as shown in d above), increasing the current density from 0.14 A / g to 8.4 A / g and decreasing the material capacity from 3420 mAh / g to 812 mAh / g, Graphite materials, but also significantly higher than anatase TiO 2The coated nano-Si material.
Amorphous TiO 2Materials to enhance the cycle performance of nano-Si material principle As shown above, the surface of nano-Si particles coated amorphous TiO 2Can withstand the huge volume expansion of Si particles in the charge and discharge process to ensure the stability of the core-shell structure, thereby reducing the decomposition of the electrolyte and the loss of active substances. TEM TEM study also validated the above speculation, In the state of lithium intercalation, the nano-Si particles have undergone a great deformation, but still maintain a complete core-shell structure, and in the TiO 2The outer layer formed a layer of SEI film, and the cycle 200 times, the surface did not occur significant broken phenomenon. 2The high stability shell formed on the surface of nano-Si particles is the key factor to ensure the cycling stability of nano-Si material.
Jianping Yang, who developed this amorphous TiO 2Coating nano-Si material, a good solution to the large volume expansion of Si material, leading to instability of the interface: a good flexible amorphous TiO 2The coating ensures the stability of the surface of the nano-Si particles during charging and discharging, reduces the decline of the capacity and reduces the occurrence of side reactions, but the material still faces some problems. Although the material can reach a capacity of 3000 mAh / g or more, but its cycle performance still needs to be improved (200 cycles, capacity retention rate of only 56%), Xiaobian that the coating process can be optimized and Si particle size optimization to further enhance the stability of the interface , Improve cycle life.
