Hyderabad: Dr Surendra K Martha and his team at the Indian Institute of Technology(IIT), Hyderabad have developed new electrodes for producing rechargeable lithium-ion batteries with high energy densities. A combination of nanoengineering and new materials processing techniques promise batteries with energy densities twice as high as commercially available products.
IIT-H develops new electrodes for producing rechargeable batteries
According to an IIT-H communique, it assumes significance following Li-ion batteries have become the power source of choice for a range of applications that involve portable electronics, power tools, and hybrid/full electric vehicles. Further, they hold the potential to free the automobile industry from the stronghold of depleting petroleum reserves.
The lithium-ion cell, in its simplified form, consists of three parts: a positive electrode (cathode), a negative electrode (anode), and electrolyte that conducts the lithium ions between the two electrodes. Lithium-ion shuttles from the cathode to the anode during charging and in reverse during discharge, i.e., when current is being drawn from the battery to, say, operate your cell phone, through the electrolyte. The energy density of the lithium-ion battery is decided by the “specific capacity” of the electrode material, defined as “the amount of charge that can be stored in the material per unit mass”, and voltage.
The material currently used for making cathodes, usually mixed oxides and phosphates of certain metals, have specific capacity in the range of 140-210 mAhg-1 and the anode material, usually graphite, around 350 mAhg-1. The energy density of cells made using such material has hovered around 100–265 Wh kg-1. Dr Martha and his team at IIT Hyderabad have developed new electrode (both cathode and anode) material with higher specific capacities than conventionally used electrodes.
Dr Martha’s lab has developed two kinds of cathode material with better capacity than existing systems. In one, they have synthesised mixtures of transition metal oxide and carbon-coated lithium manganese phosphate to form “blends” that show excellent stability under repeated cycling and very little energy loss over cycle life.
In addition, they have used nanoengineering, in which the material used to fabricate the cathode is a few nanometre in dimensions (for reference: a human hair is approximately 80,000-100,000 nanometre in diameter). The capacity of these electrodes is around 225 mAhg-1, clearly higher than those of current cathode material. In a further development, the team has doped cathode material with fluorine and magnesium, to result in capacities ~280 mAh g-1 without energy loss during cycling, the communique said.
Dr Martha’s team, in collaboration with Oak Ridge National Laboratory the USA, has developed a unique organic-binder-less, additive-free 3D electrode architecture made of silicon and carbon in nano dimensions and have coated it on a current collector made of carbon fibre, instead of copper foil that is used in conventional cells. The advantage of this material is that there is enough space between silicon and the surrounding carbon coating, which allows for volume expansion and contraction without pulverisation of the silicon. Reversible capacities over 2000 mAh g-1 at C/10 rate have been obtained for these electrodes. A provisional Indian patent has been filed for this work.
Also, the new electrodes can result in better-performing lithium batteries with higher energy density than currently possible. Indeed, the team has already shown that combining the cathode and anode materials developed by them in coin-type lithium ion cell results in energy densities greater than 500 Wh kg-1. This is more than twice the energy density seen in commercial lithium-ion cells. “I am in the process of scaling up and designing pouch and prismatic batteries from this small product”, says Dr Martha. This, in R&D parlance, is the first step in the path of commercialisation of high energy batteries that could power future electric vehicles on the Indian road.