Ubiquitous in portable electronics, rechargeable lithium-ion batteries consist of two electrodes – anode and cathode – separated by an electrolyte. When the battery is being charged with electrical energy, lithium ions move from the cathode through the electrolyte to the anode, where they are absorbed into the bulk of the anode material.

Expansion and contraction

When the battery is discharged, lithium ions come out of the anode and return to the cathode. This makes the anode first expand and then contract, which can damage the anode over repeated charge/discharge cycles. Anodes made from graphite, though, are resistant to this damage, which is why this material has been used in commercial batteries for three decades.

As portable devices become more energy-hungry, however, researchers have sought to boost the amount of energy that can be stored in lithium-ion batteries by developing anodes made from silicon. As well as being cheap and easy to work with, silicon can absorb 10 times more lithium ions per unit mass than graphite. Unfortunately, the volume of silicon expands by a factor of four when it absorbs lithium, which makes the silicon anodes prone to fracture and failure.

Cracking and coating

One way round this problem is to make the anode from an agglomeration of tiny spheres of silicon – each about 100 nm diameter – that are more resistant to cracking. But this approach also has its own challenges. Silicon is a semiconductor and to be an effective anode it must be coated with an electrical conductor. This coating must also remain intact as the nanospheres expand and contract.

Now Mark Rümmeli and colleagues at the Institute for Basic Science in Korea, at Samsung and at the Korea Advanced Institute of Technology have devised a way to coat silicon nanoparticles with multiple layers of graphene. Graphene is a layer of carbon just one atom thick that is both a good electrical conductor and an extremely strong material. These two properties combine to make the coated nanoparticles very good conductors that are able to increase in size without damage to the coating or to the nanoparticles.

An important challenge for Rümmeli and colleagues was how to coat silicon with graphene without creating a thin layer of silicon carbide between the two materials. This is because silicon carbide is an electrical insulator and also inhibits the flow of lithium ions. The team achieved silicon-carbide-free growth by heating the nanoparticles in the presence of methane and carbon dioxide.

High conductivity

Thanks to the graphene coating, a powder sample of nanoparticles has a conductivity that is 100 million times greater than a powder sample of uncoated particles. The team then made anodes from the coated nanoparticles and tested them in otherwise standard lithium-ion batteries. During the first charge–discharge cycle they found that the batteries held 1.8 times more energy than a battery with a conventional graphite anode. After 200 cycles, the batteries were still able to store 1.5 times more energy than a conventional device.

When the team took a closer look at individual nanoparticles using an electron microscope, the researchers found that each layer of graphene did not completely encapsulate a nanoparticle. This allowed the graphene layers to slide across each other as the nanoparticle grew in size, thereby creating an expandable shell. Rümmeli told physicsworld.com that a similar sliding effect has been seen in multiwalled carbon nanotubes – rolled up sheets of graphene – which can extend telescopically.

The team also believes that the sliding is offset by an inward "clamping" force that maintains the integrity of the graphene coating and reduces cracking in the nanoparticles. The incomplete layers also provide paths for the lithium ions to travel through the graphene coating to reach the anode.

The research is described in Nature Communications.