Materials Engineering for better lithium ion batteries 
 
Hyderabad, : 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.  In particular, they hold the potential to free the automobile industry from the strong hold of depleting petroleum reserves. Little wonder then, that the Indian government has set a goal of 30% electric vehicles (EVs) on the Indian road by 2030. The realisation of this goal and the development of next generation lithium ion batteries hinges on improvements in the cell components and there is continuing research all over the world in developing better batteries with higher energy densities than possible in current systems. 
New electrodes are being developed by Dr. Surendra K. Martha and his team at the Indian Institute of Technology, Hyderabad, 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.   Dr. Martha’s studies have been published in the Journal of The Electrochemical Society, Journal of Power Sources, Ionics, Journal of Energy Storage and the American Chemical Society’s open access journal, ACS-Omega. 
The lithium ion cell, in its simplified form, consists of three parts: positive electrode (cathode), 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 synthesized 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.  
While carbon is presently used as anode material, there has been increasing interest in using silicon instead of carbon because of its outstanding theoretical capacity of 4200 mAhg-1.  This capacity improvement over carbon arises because one silicon atom can bond with more than four lithium ions, whereas six carbon atoms are needed to bind to a single ion of lithium. The main drawback of silicon is its poor physical integrity; silicon expands and contracts during lithium ion insertion and extraction and breaks down with repeated cycling.  
Dr. Martha’s team, in collaboration with Oak Ridge National Laboratory USA, has developed a unique organic-binder-less, additive-free 3D electrode architecture made of silicon and carbon in nanodimensions 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. 
 
Dr. Martha’s 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.