The storage of electrical energy at high charge and discharge rate is an important technology in today's society. It can enable hybrid and plug-in hybrid electric vehicles and provide back-up for wind and solar energy.
It is typically believed that in electrochemical systems, very high power rates can only be achieved with supercapacitors, which trade high power for low energy density as they only store energy by surface adsorption reactions of charged species on an electrode material
A group of scientists from the Massachusetts Institute of Technology shows that batteries which obtain high energy density by storing charge in the bulk of a material can also achieve ultra high discharge rates, comparable to those of supercapacitors [Kang, B., and Ceder, G., Nature (2009) 458, 190].
In a departure from previous approaches, the scientists have created a lithium phosphate coating on the surface of nanoscale LiFePO4 and showed that this results in extremely high rates of performance. In particular, glassy lithium phosphates are well known to be good, stable Li conductors and can be doped with transition metals to achieve electronic conduction.
The ability to charge and discharge batteries in a matter of seconds rather than hours may make possible new technological applications and induce lifestyle changes. Such changes may first take place in the use of small devices, where the total amount of energy stored is small.
Only 360 W is required to charge a 1 W.h−1 cell phone battery in 10 s (at a 360 C charging rate). On the other hand, the rate at which very large batteries such as those planned for plug-in hybrid electric vehicles can be charged is likely to be limited by the available power: 180 kW is needed to charge a 15 kW.h−1 battery (a typical size estimated for a plug-in hybrid electric vehicle) in 5 min.
The fact that the material scientists can obtain power densities similar to those of supercapacitors is consistent with there being a fast bulk process. For LiFePO4, bulk lithium transport is so fast that the charging is ultimately limited by the surface adsorption and surface transfer, which is also the rate-limiting step in supercapacitors.
According to Kang, the findings “dispel the myth that batteries need to be slow in charging or discharging. There are clearly no material limitations to making batteries that can be charged very rapidly”.