Sodium-ion batteries can be made for afraction of the cost of today's lithium ion cells, and have particular potential in Saudi Arabia, where sodium is readily available and easily extracted as a byproduct of water desalination.
The challenge is that the traditional graphite anode struggles to store or intercalate sodium ions because sodium ions are larger than lithium ions. An alternative is hard carbon, a disordered graphite that can store more sodium ions and so increase battery capacity. The problem is that making hard carbon requires temperatures of almost 1000°C.
The KAUST team led by Husam Alshareef has developed a process using a simple bench-top laser to make three-dimensional hard carbon directly on copper collectors without excessive temperatures or additional coating steps.
The team formed a polymer (urea-containing polyimide) sheet on copper and then exposed this sheet to strong laser light. By introducing nitrogen gas during the process, the team could replace some of the carbon atoms with nitrogen atoms, reaching an extremely high nitrogen level (13 atomic %), which is unattainable by other techniques. Thus, the three-dimensional graphene was more conductive, had expanded atomic spacing, and was directly bonded to the copper current collectors, eliminating the need for additional processing steps.
"We wanted to find a way to make three dimensional hard carbons without having to excessively heat our samples. This way we could form the hard carbon directly on copper collectors," said Fan Zhang, a student in Alshareef's group.
The KAUST researchers fabricated sodium-ion batteries using their laser-formed anode material. Their device exhibited a coulombic efficiency that exceeds most reported carbonaceous anodes, such as hard and soft carbon, and a sodium-ion capacity better than most previous carbon anodes in sodium-ion batteries.