Power batteries usher in the biggest disruptive innovation in history? Decoding silicon nanowire batteries

The highest energy density battery currently! Beating Tesla 4680, breaking the ceiling of power battery energy density, fully charged to 80% in 6 minutes. Amprius has completed its first batch of lithium batteries with a cell energy density of 450Wh/kg, promoting the commercialization of electric aircraft. With the addition of silicon nanowire technology from Stanford, Amprius may stir up the power battery industry.

The energy density is 50% higher than that of Tesla 4680!

According to foreign media reports, the US startup Amprius has produced its first batch of high-energy-density lithium batteries, with a cell energy density of 450 Wh/kg and 1150 Wh/L.

For reference, the highly anticipated Tesla 4680 battery, as a representative of current high-energy-density, low-cost batteries, has a cell energy density of approximately 300Wh/kg.

Data shows that the general energy density of ternary lithium batteries in 2021 hovered around 150Wh/kg.

As the name implies, the higher the energy density, the smaller the weight or volume of the battery will be to store the same amount of energy, depending on the unit used.

Under the premise that the space or weight left for the battery in a car is fixed, the higher the energy density, the more power can be stored and the longer the range.

As far as the current situation is concerned, energy density is almost the most obvious breakthrough for the battery industry, and even the electric vehicle industry, to take a big step forward.

But achieving this breakthrough is not easy.

And how could Amprius, an almost unknown startup, achieve a qualitative leap?

The secret lies in the silicon nanowire negative electrode material used by Amprius.

Why can the use of silicon nanowire negative electrode materials achieve a significant increase in energy density?

Silicon Nanowire Technology Principle

To answer this question, let's first look at the structure of lithium-ion power batteries.

Lithium-ion power batteries are generally composed of a negative electrode current collector, a negative electrode, a separator, a positive electrode, a positive electrode current collector, a tab, an electrolyte, an insulating sheet, a casing, etc.

The operating principle of lithium batteries is:

During charging, the lithium atoms on the positive electrode lose electrons and are oxidized into lithium ions. They then move in the electrolyte, run to the negative electrode, combine with electrons flowing through the external power source, are reduced to lithium atoms, and embed into the micropores of the negative electrode material.

Discharging is the opposite process. The lithium atoms embedded in the negative electrode material lose electrons, are oxidized into lithium ions, return to the electrolyte, move to the positive electrode, combine with the electrons flowing through, and are reduced to lithium atoms again.

Therefore, increasing the amount of lithium atoms embedded in the positive and negative electrode materials can also increase the power storage capacity of the entire battery cell.

Common positive electrode materials are lithium nickel cobalt manganese oxide and lithium iron phosphate, which are often referred to as ternary lithium batteries and lithium iron phosphate batteries.

The current negative electrode materials are mainly graphite, and some also use lithium titanate and silicon-based materials.

At present, the gram capacity of carbon negative electrode materials mainly composed of graphite has reached 360mAh/g, which is very close to the theoretical gram capacity (372mAh/g), and there is not much room for improvement.

The gram capacity of silicon-based negative electrode materials reaches 3500mAh/g, and its lithium storage capacity is almost 10 times that of carbon materials.

Theoretically, the energy density of lithium batteries using silicon-based negative electrode materials can reach ten times that of carbon-based lithium batteries.

The problem is that the silicon put into the battery expands as it absorbs positively charged lithium atoms during charging, and then contracts during use as the lithium is pulled out of the silicon. This expansion/contraction cycle often causes the silicon (usually in the form of particles or films) to pulverize, degrading battery performance.

To solve this problem, Amprius uses silicon nanowire technology.

What is Silicon Nanowire Technology?

Amprius' silicon nanowire technology is to grow silicon nanowires directly on the battery electrode. After absorbing lithium atoms, they expand to four times their normal volume. However, unlike general silicon structures, silicon materials with this structure can release stress well through axial expansion without causing cracking or damage to the nanowires, thereby preventing the powdering of the electrode.

In addition, the thickness of the negative electrode using silicon nanowires is only half the thickness of the negative electrode made of carbon materials.

Since silicon is the material with the best energy density, using 100% silicon material means that the highest energy density lithium-ion battery can be achieved.

Moreover, Amprius' silicon nanowire batteries have excellent cycle life, which has been demonstrated in real-world use by multiple organizations including U.S. national laboratories and major aerospace companies.

The first batch of silicon nanowire lithium batteries produced by Amprius will be supplied to a high-altitude pseudo-satellite company.

Origin and Application

Amprius' silicon nanowire technology originated from Stanford.

In 2007, Cui Yi, then an assistant professor in the Department of Materials Science and Engineering at Stanford, published a paper titled "High-performance lithium battery anodes using silicon nanowires" in Nature Nanotechnology, detailing the application of silicon nanowire technology in lithium batteries.

In 2008, Cui Yi and veteran venture capital partner Mark Platshon co-founded Amprius, whose board of directors includes Nobel Prize winner and former U.S. Secretary of Energy Steven Chu.

Today, Cui Yi, who is only in his forties, is not only a tenured professor at Stanford, but also the founder of four startups.

Amprius is headquartered in Silicon Valley and has established a large battery factory in Wuxi, China, in a joint venture with the government-owned investment company Wuxi Industrial Development Group.

Amprius' investors include Silicon Valley venture capital firms Trident Capital and Kleiner Perkins, Chinese private equity firm Softbank SAIF, Stanford University, Airbus, etc. In addition, former Google CEO Eric Schmidt is also one of the company's investors.

The company's current CEO, Dr. Kang Sun, was previously vice president of Honeywell and helped to found JA Solar, currently the world's largest solar panel manufacturer.

In December 2021, Amprius announced a breakthrough in battery charging efficiency, achieving a 0% to 80% charge rate in just 6 minutes. The company is working to achieve a battery mass production rate of hundreds of megawatt-hours per year and expects to begin mass production in 2024.

Its COO Jon Bornstein said that lithium batteries with a high energy density of 450Wh/kg will support aerospace products with higher power requirements, including new eVTOL aircraft (Electric Vertical Takeoff and Landing, also known as flying cars), which can extend flight time. In addition, he also said that Amprius expects to produce the first batch of 500 Wh/kg batteries later this year.

Bornstein also said Amprius is developing batteries for drone applications and has signed a contract with an undisclosed airline. Amprius has previously supplied batteries for Airbus' Zephyr solar drone.

In addition, according to Fortune, the US military is also using Amprius' batteries to test their application in wearable devices.

The Airbus contract is both a lifeline and a warning sign for Amprius. "We offered Airbus a sky-high price, ... this price is unsustainable," said CEO Sun Kang.

If it is to be used on a large scale in passenger cars, the high cost is obviously unacceptable.

Before Tesla's Battery Day in August 2020, Musk tweeted that Tesla would mass-produce batteries with longer life and a 50% increase in energy density, or 400wh/kg, within 3-4 years. At that time, the Panasonic 2170 battery cell used in the Model 3 had an energy density of 260wh/kg.

Subsequently, Tesla used a silicon nanowire structure diagram as the background image on the registration page for the Battery Day event on September 19.

In addition, Amprius' company address is just opposite Tesla, and speculation about Tesla's acquisition of Amprius was rife at the time.

But this technology was not unveiled on Battery Day.

Musk even stated on Twitter that nothing happened between Tesla and Amprius.

For Musk, cost is always one of the primary considerations.

According to data from Huabao Securities, the current market price of silicon-based negative electrode materials (a mixture of silicon-based materials and graphite) with a capacity of 420-450mAh/g is between 110,000 and 150,000 yuan/ton, with the median value being approximately 120,000 yuan/ton, while the price of high-end graphite is only 70,000 to 80,000 yuan/ton.

Perhaps when the cost of using silicon-based materials as negative electrode materials can be controlled to be comparable to that of graphite, something will finally happen between Amprius and Tesla.

The future of power batteries: solid-state batteries or silicon nanowire anodes?

The industry generally believes that the development of lithium-ion batteries has reached its limit. If we want to further increase the driving range, we must make major technological innovations.

Like the silicon nanowire negative electrode battery released by Amprius, one of the main purposes of the solid-state batteries that have been popular in the industry is to improve energy density.

It is generally believed that the energy density of liquid lithium batteries reaching 300Wh/kg is already an excellent performance, while solid-state batteries can generally reach 300-400Wh/kg.