The Application Research of Silicon Negative Electrode in Lithium-ion Batteries

Ⅰ. Background

Due to the limitation of interlayer embedding mechanism, under fast charging conditions (charging over 80% in 15 minutes), lithium ions are easily reduced to lithium metal on the surface of graphite negative electrode, forming lithium dendrites, which affects the battery cycle performance and safety. Silicon negative electrode has high specific capacity and can replace graphite negative electrode to improve the energy density of lithium-ion batteries. Previous studies have focused on preparing new silicon-carbon negative electrodes to improve performance, and paid less attention to the fast charging performance of silicon negative electrode itself, especially the reversibility of electrode after lithium evolution, which is of great significance for analyzing the failure mechanism of full battery under fast charging and limited N/P ratio.

Ⅱ. Research findings and results

Professor Wang Feng from Beijing University of Chemical Technology and a team from Oxford University found that under the condition of limiting the silicon volume effect, the silicon negative electrode can not only improve the reversible capacity of the negative electrode of the lithium-ion battery, but also improve the fast charging performance of the battery. By comparing the electrochemical behavior of the graphite negative electrode and the silicon negative electrode (same electrode parameters) after lithium deposition, it was found that the silicon negative electrode can not only inhibit the formation of dendrites, but also show a unique "self-dissolution" phenomenon after lithium deposition. Combining theoretical calculations with in-situ characterization, it is known that compared with the graphite negative electrode, the silicon negative electrode has lower desolvation energy, faster SEI ion transmission, and higher migration rate on the surface of the lithiated negative electrode, which makes up for the lack of research on the fast charging performance of the whole battery and provides a more systematic method for the fast charging research of electrode materials.

Based on the differences in the fast-charging mechanisms of the two negative electrodes, the team designed a gradient negative electrode (Si/Gr-Grad). The assembled full battery can be cycled stably for more than 500 times. The capacity retention rate of the Ah-level pouch-type full battery assembled under 4C fast-charging conditions is as high as 97.9% after 300 cycles. The relevant results were published in "Energy Storage Materials".

Ⅲ. Performance comparison and cause analysis

（1） Performance comparison : Under the same electrode parameters, the capacity retention rate of the traditional graphite negative electrode is higher at 1C because there is no lithium plating problem; but when the charging rate rises to 4C, the coulombic efficiency of the graphite negative electrode is greatly reduced, and the capacity retention rate after 100 cycles is lower than that of the silicon negative electrode full battery, because the dendrites of the graphite negative electrode break during the stripping process after lithium evolution.

（2） Cause analysis : Through theoretical calculations, activation energy tests, etc., a comparative analysis of silicon negative electrode and graphite negative electrode (same electrolyte) was conducted, and it was found that the lithium ion coordination number of the silicon negative electrode was significantly lower than that of the graphite negative electrode, and the desolvation process was easier to carry out. DFT calculations showed that the silicon negative electrode preferentially adsorbed PF6- ions, which was conducive to the formation of an inorganic phase SEI containing LiF, enhancing the electronic insulation and mechanical strength of the SEI, alleviating the expansion of the negative electrode surface caused by lithium metal deposition, and improving the fast charging performance.

Ⅳ. SEI composition analysis and comparison of lithium adsorption and migration

（1） SEI composition analysis : The surface of the silicon negative electrode has a unique double-layer SEI structure, which is rich in organic phase on the outside and rich in LiF on the inside. It can not only ensure rapid ion transmission and good electronic insulation, but its unique double-layer structure can also stabilize SEI and inhibit dendrite growth.

（2） Comparison of lithium adsorption and migration : Theoretical calculations compare the lithium ion adsorption energy and surface migration energy of lithium-based graphite and silicon lithium compounds. Silicon lithium compounds have higher adsorption energy and lower surface migration energy, which is conducive to the rapid transmission of ions on the surface of lithium-based negative electrodes. In addition, the silicon negative electrode surface has a stronger ability to resist the growth of lithium dendrites.

Ⅴ. Silicon "self-dissolution" phenomenon and fast charging lithium storage behavior difference

（1） Silicon "self-dissolution" phenomenon : In situ optical characterization found that the lithium metal deposited on the surface of the silicon negative electrode will dissolve into the interior of the electrode as the static time increases, while the lithium deposited on the surface of the graphite negative electrode always exists.

（2） Differences in fast-charging lithium storage behaviors : Under fast-charging conditions, based on the excellent SEI structure, it is easier to desolventize the silicon negative electrode and the SEI ion transmission is faster. After lithium deposition, the silicon lithium negative electrode surface diffuses faster, achieving uniform deposition of metallic lithium. In the subsequent stripping process, the stripping efficiency of metallic lithium from the silicon negative electrode is higher, and almost no dead lithium is produced.

Ⅵ. Design and advantages of gradient silicon/graphite composite anode (Si/Gr-Grad)

A new silicon/graphite composite negative electrode with a gradient distribution of silicon content was designed (the silicon content is high on the side close to the current collector and low on the side close to the diaphragm). The introduction of a gradient distribution of silicon content can induce lithium deposition under fast charging conditions and avoid the appearance of lithium dendrites on the surface. The first deposition stripping test, non-in-situ SEM test, in-situ optical test, and COMSOL simulation confirmed that it can avoid the growth of surface dendrites. Electrochemical tests showed that the assembled LFP full battery and ternary positive electrode full battery can be stably cycled for multiple times under 4C fast charging conditions with good capacity retention. TOF-SIMS test characterization showed that there was no lithium evolution phenomenon. The 1Ah-level pouch battery was stably cycled for more than 300 times under 4C fast charging conditions with a capacity retention rate of up to 97.9%. Infrared thermal imaging monitoring showed that the maximum temperature of the battery during the fast charging process did not exceed 34°C, which is highly safe.