A few days ago, Tesla officially released a latest poster for Battery Day. On the same day, CEO Musk said on Twitter that Tesla would mass-produce batteries with longer lifespan within 3-4 years. And the energy density is increased by 50, which means the battery reaches 400wh/kg. After research by overseas media, they came to a conclusion: This... seems to be a structural diagram of silicon nanowires? !
We have previously speculated that Tesla may make a fuss about the cathode material and use single crystal technology; it may also use Maxwell’s dry electrode technology to pre-lithium the anode material, or it may use electrolysis. Add new additives to the liquid. As a result, Tesla’s nanomaterials preview is telling us that we still have a trump card in battery technology! .
In fact, Amprius, a company that cooperates with Tesla, has put this new type of battery into use as early as last year. The Zephyr drone was equipped with silicon nanowire batteries and flew in the stratosphere for more than 25 days.
This company is also very interesting. Now it has secretly been "secretly working" with Tesla. Pictures show that Amprius has moved to be a neighbor of Tesla's secretive "Tera" battery manufacturing plant, where Tesla Battery Day will also be held. Very good, those who are closer to the water are really first to the moon. No wonder Maxwell has to be a little behind.
Having said that, let’s talk about what this so-called “silicon nanowire” battery is. Pay attention to the sentence break, silicon/nanowire. In fact, this kind of battery essentially replaces traditional lithium ions with silicon nanowires. Graphite is the negative electrode material in batteries.
Then why not use ordinary silicon? Silicon is environmentally friendly and rich in resources. Its working voltage is also close to that of graphite. Its theoretical specific capacity has reached 4200mAh/g, which is also the highest among current alloy anode materials. Silicon is cheap and common, doesn’t it smell good?
Of course - that's not possible. During cycling, silicon undergoes severe volume changes. The capacity of a lithium-ion battery is related to the number of lithium ions that can be embedded in the negative electrode material. The greater the number of embedded lithium ions, the greater the battery capacity. The charging process is the lithium ion embedding process, and the silicon material expands; when the battery is working and discharging, the lithium ions are released from the silicon material.
The main problem with silicon is that when lithium is embedded, the volume expands by nearly 400% compared to its original size. Similar to previous batteries that would bulge, batteries with silicon as the negative electrode may bulge into their original size after expansion. of four large. Therefore, it is difficult for silicon to return to its original morphology after delithiation.
Volume expansion in turn places additional stress on the silicon material, causing material breakage and cracks. This process is the so-called electrode powdering. In layman's terms, it means that the electrode subjectively resists the process of causing it to expand, and as a result, the inside of the battery is broken due to use. During the powdering process, the broken silicon material will fall off from the electrodes, which will also lead to poor cycle stability of the final battery and a sharp drop in battery capacity. Silicon can be regarded as a reaction vessel. If the reaction vessel is gone, the battery reaction cannot proceed normally.
This silicon nanowire refers to a material with a width of about 10 nanometers and an unlimited length. It can be imagined as a silicon nanoribbon, which is a new type of one-dimensional semiconductor nanomaterial with an inner crystal core. It is single crystal silicon, coated with a layer of SiO2 on the outside.
By searching for patents, you can get a rough idea of ??Amprius' silicon nanowire production process: using a silicon-coated nanostructure template, using the PECVD process or thermal CVD method, silicon nanowires are grown directly on the conductive substrate. . The nanowire has a core-shell structure, and this preparation method is also suitable for compounding or doping other substances on the silicon nanowire to enhance its conductivity and strength.
Silicon nanowires can well release stress through lateral expansion without causing cracks or damage to the nanowires, thus preventing the electrode from powdering. There is a certain gap between the silicon nanowires. When the volume expands, there is also some space for it to expand without causing them to squeeze against each other.
In addition, growing nanowires directly on the current collector can also enhance the physical and electrical contact between the nanowires and the substrate. Generally speaking, the electrode material is adhered to the electrode through a binder , this method is equivalent to making the silicon "grow" out from the electrode, allowing the electrons on each nanowire to be well transmitted to the external circuit. The conductivity of nanowires can also be improved by cooperating with other materials in different ways, such as doping, alloying, core-shell materials, etc. There is no need to worry about the conductivity of silicon nanowires.
As of now, although silicon nanowires have so many advantages, I still insist that Tesla may use this silicon nanowire battery, but it cannot attribute all the capacity increase to this. a little. Why?
From a cost perspective, although silicon reserves are large and the cost is very low, the cost of the above-mentioned preparation process is not low. Whether it is nanotechnology or the chemical vapor deposition method (that is, CVD) mentioned above, they have virtually increased the cost of silicon nanowire anodes.
This process is a high-energy-consuming process, and its processing difficulty and manufacturing cost are higher than that of artificial graphite. If mass production is carried out, the cost will be unacceptable. The batteries in new energy vehicles are different from those in the aerospace field. They require large-scale mass production, but it is normal for aerospace materials to be burned for tens of billions. If costs cannot be reduced, they will eventually be passed on to consumers.
In terms of performance, although Amprius’ patent shows that silicon nanowires can still reach an energy density of 1000mAh/g after 180 cycles, the integration into the battery is not the sole responsibility of the negative electrode material. It must be paired with the positive electrode. And the electrolyte, the maximum capacity and cycle performance of silicon nanowire batteries are still a big question mark. Whether the cost is proportional to this improvement will not be answered until mass production.
Another key issue is that it is not the negative electrode material that limits the battery capacity of pure electric vehicles, but the positive electrode material. From NCM523, to NCM622 to NCM811, along the way, the development trend of batteries is to increase the proportion of nickel in the battery. Even the lithium iron phosphate blade batteries that have become popular this year are only improved from the module level.
Although there is a lot of fuss about the negative electrode material, GAC New Energy also promoted the various advantages of graphene as the negative electrode some time ago, but until now, no more information has been disclosed, but major manufacturers are really that stupid , can’t you think of improving the negative electrode to increase battery capacity?
The contribution of battery negative electrode capacity to the total specific capacity of the battery is far less than everyone imagines. The theoretical capacity of graphite is 372mAh/g. Tesla Model? 3 has a graphite negative electrode capacity mixed with 10 silicon-based materials. Approximately 550mAh/g. So has the cruising range of Model 3 been significantly improved?
The attitude of Tesla, a "scumbag", is that he doesn't admit it, doesn't deny it, and doesn't know. I can hint that he cooperated with Amprius and used silicon nanowire technology, but as long as I don't admit it, you We’ll have to wait until Battery Day on September 22 to find out what’s going on. What we can do, we’ll have to wait until Musk announces it.
This article comes from the author of Autohome Chejiahao and does not represent the views and positions of Autohome.