How nice it would be if one could charge the battery of an electric car and drive without a care for 1,000 km!
A battery so dense with energy that can last really long is at the heart of electric mobility. To get there, it is necessary to have energy-packed electrochemical cells (a battery is made of cells.)
When you charge a cell, the electrons from the cathode split from their parent atoms; the cathode then is left with positive-charged ions. The negatively charged electrons run along an external circuit and reach the anode; meanwhile, the ions run through the electrolyte and also reach the anode.
The crux of making a battery that lasts for 500 km is in making an anode that can accommodate a heck of a lot of ions. The more room it has for the ions, the more energy it can pack.
Today, carbon (in the form of graphite) is the anode material of choice. For cathode, the common choices are different compounds of lithium. The lithium is ‘housed’ in other materials; when its ions cross over, they again get housed in the anode, which is almost all the time. Now, the search is for anode materials that have more room to seat lithium ions. There are many known alternatives for graphite, but each comes with its own flipside — which is why graphite is still the king. But some nice flavours are emanating from research labs in the recent times. Businesses have begun to take these new anodes seriously enough to work out business models and some investments are also beginning to happen. The lithium-ion batteries that use these new anodes are called ‘Advanced Li-batteries. In this article, we will take a look at two of them.
Our old friend, Silicon
This ubiquitous stuff from sand, a big friend of mankind, in use everywhere from glass to semiconductors, is a darn good anode material. Pure silicon can accommodate ten times as many ions as graphite does. Unlike with graphite, where lithium ions embed themselves in its ‘pores’ (called ‘intercalation’), the ions form an alloy with silicon (called ‘conversion’) — where the energy is stored in the bonds between lithium and silicon.
This use of silicon has been known to scientists for over six decades. The industry has been sniffing at silicon to make anodes for about twenty years. Many companies have considered it and given it up.
Why? Because silicon has a few disadvantages. As it takes in lithium-ions while charging, it expands. While discharging, it contracts. This causes pulverisation of large silicon particles, which affects the integrity of the anode. Second, silicon has low electrical conductivity (ions do not swim through it easily, unlike in carbon). Third, it affects the Solid Electrolyte Interphase (SEI) — a protective film that quickly forms on the surface of the anode that touches the electrolyte and then permits only ions and shuts out any wandering electrons.
Due to these issues, some companies just sighed and walked away (towards the other promising anode that we will discuss later in this article.)
In a 2021 whitepaper titled ‘National Blueprint for Lithium Batteries’, the Federal Consortium for Advanced Batteries, US, does not mention the word ‘silicon’ even once. A point to note is that even batteries of today have a little bit of silicon in their anodes, to increase energy density.
The solution
Some companies, however, have been doggedly pursuing silicon to make anodes for several years now. “Many companies have tried to address the limitations of silicon as an anode in different ways,” notes Dr Rahul Gopalakrishnan, CEO at ABEE group of Belgium, “but still these are at lab level or pre-manufacturing stage, somewhere at TRL 4-6.” The Abee group is into the other type of anodes — lithium metal.
He notes that the trick is in scaling up. Leaping from lab to industry is fraught with challenges, he says. “If this bridge is crossed we will see widespread adoption of silicon as a viable alternative to graphite as an anode,” Gopalakrishnan tells quantum.
But it appears that some companies have cracked the code of scaling up.
Sicona, an Australian start-up, is building a silicon carbon anode plant in the US. Last year, the Kolkata-headquartered Himadri Speciality took a 12.8 per cent stake in Sicona.
Sicona has not given out much about its technology, but it is known that the core of its technology is based on silicon nanoparticles and a “specialised coating process to create unique material qualities.” Nano particles are so small that while they may expand and contract, they won’t pulverize — you can’t pulverize powder. Also, clearly, Sicon’s anode is not pure silicon, as it says it will deliver a 20 per cent increase in energy density.
Another notable company is the California based Sila Nanotechnologies, founded by Gene Berdichevsky, an ex-Tesla employee. Bessemer Venture and Canada Pension backed Sila describes its technology thus: “What’s required to replace graphite entirely is a material that compensates for the swelling of silicon through the design of an engineered particle structure. If you can create a particle that allows the swelling and contraction of silicon to happen inside the particle, while keeping the electrolyte outside of the particle, you could cycle the material reversibly 1,000’s of times and perhaps 10,000 times.”
Sila is already in production; in December 2023, it signed a commercial agreement with Panasonic to supply silicon anode materials. (Quantity and price have not been disclosed.)
Yet another company into silicon anode is Amprius Technologies, USA, which also speaks of silicon nanowires and special coating. Amprius says that its battery of energy density of 370 Wh/kg of silicon anode material is already commercially available; the battery is capable of “extreme fast charge”— up to 80 per cent within six minutes.
So, it does appear that silicon has been tuned for batteries.
Lithium metal
The companies that have said “to hell with silicon” have turned to the other promising anode — lithium itself. By having a copper foil on the anode end and charging, the lithium ions — instead of intercalating (graphite) or forming bonds (silicon) — just plates itself on the foil.
The beauty of a lithium metal anode (or, anode-less batteries) is its high energy density — 4,600 Wh/kg. But, as with silicon, there are issues — mainly formation of dendrites (spikes) that can pierce the electrolyte and touch the cathode. “The large scale of lithium metal also faces engineering problems such as mechanical stress by coiling and slitting, electrolyte consumption and volume swelling stress,” says a recent scientific paper. These issues are not insurmountable but they call for a new supply chain. “The commercial use of lithium metal batteries has a long way to run,” the paper notes.
Yet, as in the case of silicon, the challenges have not totally deterred businesses from trying their hand at lithium metal anodes. ABEE is an example.
Beyond lithium
Is the world resigned to having (the scarce) lithium for cathode? Not quite. Today, lithium is the best — challengers such as sodium, aluminium and zinc can at best be niche players. But you can’t rule out other equal matches for lithium. Scientists are thinking in terms of metal fluorides (iron fluoride, copper fluoride) and sulphur-based cathodes.
Research is also on to make solid electrolytes to replace the incumbent liquid — an area of research that has both strong supporters and sceptics.
The broad message is clear. In the not-too-distant future, you would be able to travel 1,000 km on one charge.