January 22, 2008 | 6 Comments
Lithium Ion batteries have a reputation of heating and cooling over the course of their charge and discharge cycles. With several new chemical innovations in trial for improving the activity in Lithium-ion batteries as the heating issue is addressed, one stands out not for the control of the heating and potential trouble that causes, but serves to increase the total charge capacity by some multiples. The press release people are talking 10x or an order of magnitude, but lets look into the facts and see.
Batteries are chemical reactors that release electrical energy from the reactions. Charging does a near reversal by accepting electrical energy compelling a chemical reaction. This cycle of energy flowing in and out and the chemical reactions over time just wear out batteries at the molecular level. The costs of the more sophisticated technologies such as Lithium-ion chemistry make the total life cycles a critical issue. A battery with 1000 cycles at near 100% charge is worth twice what a 500-cycle battery is worth, etc. Then there is the capacity of the charge that is useful. A battery that offers 80% of its total capacity at useful voltage is worth more than one offering 40% of the charge. These are key to the value of a particular design.
The anode and cathode, the contacts within the electrolyte chemicals in a battery are exposed to the chemical reactions and are the points of breakdown that “wears out” in batteries. They also affect the rate at which a battery can charge and discharge and depth of which the chemical reactions can provide voltage. The interface is where the action is, so to speak, so here is where it gets hot inside a battery. In a nutshell the matters of capacity, the range of useful charge and the heat factor are the tasks to solve in battery breakthroughs.
When the news release came out about a month ago claiming a 10-fold increase in capacity I was suspicious. As more of the hard numbers are published, we can get a look into just what the potential is.
Now the anodes are commonly made of graphite and some technologies in addressing the heat matter are anodes made of iron. Silicon is an attractive material as it offers a low discharge potential and has the highest known theoretical charge capacity, some 10x the capacity of common graphite anodes. The catch is silicon’s volume changes 400% in course of the lithium reactions as shown in research using films and particles. This is where the silicon nano wire innovation enters the hunt.
Rather than press a film on the conductor out of the battery or attach particle to the conductor the Stanford team has grown silicon fibers at the nano scale on the conductor. The volume change issue over the course of the lithium reactions just wrecks the films and particles and the connections to the conductor. The silicon nano wires on the other hand show a good resistance to the reaction destruction and remain attached to the conductor. The reason is explained by the size of the silicon, so small that the growth and reduction effect isn’t damaging the silicon structures whose dimensions are below the threshold of large dimensional pulverization. Thus, the design offers a 10x increase in the anode side of the battery.
There is also the cathode side. Introducing the silicon nano wire anode offers the advantage that in the same weight or volume a battery would need less assigned to the anode allowing a larger cathode which offers a large improvement. Dr. Chi offers that it wouldn’t get the media claims of 10x but would offer several times the capacity.
This leaves the issue observed by the team that the first cycle from a new battery build runs essentially 100% capacity at useful voltage, but looses 20% in succeeding cycles. 80% is great and it is impressive the team is working to improve this characteristic.
The process used to build the silicon wires on the conductor is a known process that can scale up. This means the technology can get into products quickly when the legal and other issues are cleared up.
That leaves the cathode for a big improvement. With an improved anode giving a multiple improvement of double or triple, this is a breakthrough. Should good design yield 4, 5, or more multiples the impact will be huge. Should a breakthrough come on the cathode side the future for electrical storage will be a very different matter in the total energy and fuel arena.