Overcoming "expansion"! Silicon and carbon "run in both directions"! Does a "small body" battery contain great energy?

□ Popular Science Times reporter Zhang Yingxian

Recently, the topic "Zhang Chaoyang talks about the future development of mobile phone batteries" has become a hot search. Mobile phone batteries directly affect the user experience. Zhang Chaoyang mentioned that compared with traditional graphite batteries, the energy density of silicon-carbon negative electrode batteries can be increased by an order of magnitude. What is silicon-carbon negative electrode battery technology? What are the advantages of silicon-carbon negative electrode batteries in mobile phones and cars? The reporter conducted an interview on this.

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Silicon and carbon "run in both directions"

When traveling, the "anxiety" of mobile phone battery affects almost everyone. At present, most of the mobile phone batteries we use are lithium-ion batteries. In places that we cannot see with the naked eye, the chemical reaction in the battery is proceeding rapidly.

"Lithium-ion batteries have a nickname called 'rocking chair batteries.' When they are charging, lithium ions come out of the positive electrode and run to the negative electrode; when they are discharging, lithium ions come out of the negative electrode and run to the positive electrode." Gao Peng, a doctoral student and associate professor in materials science at Harbin Institute of Technology, told Science Times that the lithium storage capacity of the positive and negative electrodes is very important, which determines how much energy a lithium-ion battery can store.

As an important component of lithium-ion batteries, high-capacity negative electrode materials can effectively improve the energy density of batteries. At present, the negative electrodes of mainstream lithium batteries on the market are all made of graphite materials, but after decades of development, the specific capacity of graphite materials has been very close to the theoretical limit. Therefore, continuing to improve the performance of graphite negative electrodes has very limited impact on the performance of lithium-ion batteries. People have found that silicon materials have a much higher lithium storage capacity than graphite.

When discussing the topic of "Looking at China's Leading Mobile Phone Technology from the Perspective of Physics", Zhang Chaoyang explained that graphite has six carbon atoms to accommodate one lithium ion, but one silicon atom can accommodate nearly four lithium ions. The lithium storage capacity of silicon materials is more than 10 times that of carbon materials. Therefore, if silicon materials are added to the battery, the energy density can be increased by an order of magnitude.

So, is it better to put more silicon in it? "During the charging and discharging process, the volume expansion effect of silicon is very serious. When lithium ions are embedded, the volume of silicon expands, and when they are removed, the volume shrinks again, and the expansion can be up to 300%. After multiple cycles, the silicon material will break and powder, resulting in a decrease in performance." Gao Peng admitted that low conductivity and high resistance are not conducive to conducting current, which is also one of the problems of silicon materials.

Scientists have discovered that carbon materials can make up for the shortcomings of silicon materials, so they combined silicon and carbon to produce silicon-carbon negative electrode materials with high conductivity and low volume expansion rate.

Battery capacity increased by 20%

As a new type of battery material technology, silicon-carbon negative electrode material technology was applied to automotive power batteries and then used by mobile phone manufacturers in mobile phone batteries.

"Current carbon materials are all small microspheres with diameters of several hundred nanometers to several microns, but for silicon, the smaller the particles, the better, because the larger the silicon particles, the easier they are to break, and the smaller the particles, the less likely they are to break. Therefore, in order to prevent the silicon particles from breaking, people try to make them as small as tens of nanometers, which is called nano-silicon," Gao Peng introduced.

There are many ways to composite silicon and carbon. Currently, there are two main popular technologies. One is to coat carbon on the surface of silicon microspheres, and the other is to embed silicon particles into porous carbon or deposit silicon in porous carbon.

Gao Peng introduced that in order to prevent volume expansion, the proportion of silicon in the current silicon-carbon negative electrode material will not be too high, and the lithium storage capacity is roughly twice that of the current carbon negative electrode. When matched with the high-energy positive electrode, it can increase the overall energy density of the battery by about 20%. In other words, for mobile phones or electric vehicles, the same size battery can have 20% more power, greatly improving the battery's endurance.

The latest research results show that compared with graphite negative electrode materials, silicon-carbon negative electrode materials have better cycle stability at low temperatures. Therefore, the use of silicon-carbon negative electrode materials can help improve the low-temperature performance of batteries and also improve the common power-off problem of mobile phones and electric vehicles in winter. "Silicon-carbon negative electrode technology is still a developing technology. In the future, with the advancement of technology, the proportion of silicon in silicon-carbon negative electrode materials will increase, which can also bring higher performance improvements to batteries." Gao Peng introduced.