Lithium ion battery breakthrough and what it means for plug-in hybrids

Scientists at MIT recently had a major breakthrough in battery development. By adding a glassy coating to LiFePO4 particles they were able to increase the power density of the battery electrode material by two orders of magnitude. This means that they were able to develop a battery electrode with 100 times the power capabilities. Instead of charging or discharging a battery in 1 hour it could be done in less than a minute.

Lithium ion batteries work by shuttling lithium ions between anode and cathode materials that are composed of a host matrix material into which lithium ions can insert. The overall power capability of the battery is limited by how fast lithium ions can move through the system, so by making it easier for the lithium ions to enter the LiFePO4 particles, the overall rate capabilities can be significantly enhanced. (See howstuffworks.com for some more detailed diagrams of how lithium ion batteries work: http://electronics.howstuffworks.com/lithium-ion-battery1.htm).

LiFePO4 is a material with both poor ionic (lithium ion) and electronic conductivity. Traditionally this problem has been addressed through the use of nanotechnology, by making the particles smaller and smaller, and by coating the particles in carbon, a good electronic conductor. Nanoparticles provide the double benefit of reducing the diffusion length (distance the ions need to travel) and increasing the surface area for the battery reactions (the electron transfer reactions) to occur. Despite these material limitations, LiFePO4 is a popular material because it is more stable (safer) than other battery chemistries and has good energy density (can store a lot of lithium ions).

This MIT research takes the alternative approach of changing the surface characteristics of the particles to improve performance. Through computational simulations they determined that lithium ions should actually be able to diffuse quite rapidly through the LiFePO4 material. They then hypothesized that the observed slow diffusion rates were likely a surface effect due to the difficulty of lithium ions entering the material. LiFePO4, unlike many other electrode materials, only allows 1D diffusion of lithium. This means that the lithium ions must find these “tunnels” in order to enter the material. By allowing lithium ions to rapidly move over the surface of the material through the addition of this glassy coating, they can more quickly find the “tunnels” to enter the material. This is what allows the MIT electrode material to achieve such high power capabilities.

From a plug in hybrid electric vehicle standpoint, improved power density offers a number of benefits: 1) Vehicle acceleration performance can be improved due to more power being available to the electric motors, 2) Fuel economy can be increased by allowing a larger percentage of the energy during braking to be recovered. (Right now only a fraction of this energy is recovered due to the limitations in how much power lithium ion batteries can accept.) 3) Vehicles can be much more rapidly charged, opening up the possibility of electric charging stations where the battery can be recharged in 10-15 minutes (or faster) instead of overnight.

This new breakthrough does not however mean that we’ll all be driving plug in hybrid or electric vehicles tomorrow. There are still many other policy, economic, and technical factors influencing the market penetration of these vehicles. One of the biggest hurdles is the cost of the batteries, electric motors, and other associated electronic control systems, which make plug-in hybrids significantly more expensive. Another issue is the long term durability of batteries and making sure that the batteries last the life of the vehicle or at least long enough to be acceptable to the consumer. The cost of gasoline, now much lower, also significantly impacts the economics. The recent increased fuel economy regulations will almost certainly increase the market share of hybrid electric vehicles. The difference in cost between a hybrid and plug in hybrid will be less than between a plug-in hybrid and standard gasoline vehicle as hybrids already contain many of the same components (although on a smaller scale), which should help promote the growth of plug-in hybrid vehicles. Other policies or business opportunities, which enhance the value of plug-in hybrids, such as smart grids that allow plug-in hybrids to return energy to the grid during peak loads, government tax credits on plug-ins, or gasoline taxes could increase the market share of these vehicles as well.

Sources:

Kang, Byoungwoo, and Gerbrand Ceder. “Battery materials for ultrafast charging and discharging.” Nature 458.7235 (2009): 190-193.

Brumfiel, Geoff. “Lithium Ion Batteries Charge Ahead” Nature News, 11 March 2009 http://www.nature.com/news/2009/090311/full/news.2009.156.html