New electrode material for high-capacity lithium batteries
10 May 2007
US researchers this week presented details of a new electrode material for rechargeable batteries which, they claim, can store almost twice as much charge as conventional electrodes. The material could lead to more efficient batteries for applications such as power tools and electric vehicles.
Chris Johnson and colleagues from the Argonne National Laboratory told delegates at a meeting of the Electrochemical Society in Chicago that their new 'layered-layered' nanocrystalline composite anode, rich in manganese, had a storage capacity of more than 250 milliamp hours per gram (mAh/g), compared with around 140 mAh/g of conventional cobalt-based electrodes. Furthermore the material remained stable for multiple charging and discharging cycles.
The new electrode consists of layers of two materials, Li2MnO3 and LiMO2, where M in the second material represents nickel, manganese and cobalt in a range of ratios. The materials are co-precipitated to create a layered structure, with each layer of the order of nanometres deep. The LiMO2 is the 'active' part of the electrode, which is charged with lithium ions that then vacate the crystalline lattice upon discharge of the battery. While such manganese-based materials are known to accommodate large numbers of lithium ions - the key to greater capacity - they are inherently unstable because the movement of the ions in and out of the lattice creates significant distortions, resulting in degradation.
According to Argonne researcher Jim Miller, the LiMnO3 acts as a rigid scaffold for the structure, increasing its stability significantly. 'One of the main difficulties with rechargeable battery systems is to provide a high charge capacity while maintaining stability of the electrode,' Miller told Chemistry World. 'When you dump lithium ions in and out of the material there is a large volume change that results in instability. By using this two-component approach we think we can achieve the stability we are looking for.'
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