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Lithium-polymer cells, where the electrodes are separated by a thin polymer film, have long promised to be the stuff of batteries with high energy and power density. One major difficulty with such cells is the tendency for the lithium to form dendrites as the cell is charged. These dendrites are protrusions of lithium from the negative electrode which ultimately protrude through the polymer film, reach the positive electrode and internally short circuit the cell, ending the battery life. Viewed simply, the growth of dendrites is favored by the fact that the tip of the dendrite is a more favorable site for Li deposition in the potential or concentration gradients usually encountered in the cell. This favorability is offset by the fact that more energy is required for the deposition of a dendrite than for the (preferred) smooth surface. This energy requirement arises from the greater surface area, and therefore surface energy, of the dendritic deposit and from the fact that the dendrite must greatly deform the polymer as it grows. These ideas lead to the concept that a stiffer polymer (one with a higher modulus) might inhibit dendrite growth. Dr. Kerr and Mr. Han have put this concept to test by comparing the lifetimes (measured in charge/discharge cycles) of cells with the usual polyethylene oxide (PEO) film and ones with a highly cross-linked PEO which is much more resistant to elastic or plastic deformation. These cells were ones where both electrodes are Li and the figure below shows the potential across the cell when the following cycle is repeatedly applied:
1. Current is passed in one direction for two hours.
2. The current is turned off and the cell allowed to relax for a period (during which time, under normal circumstances, the cell voltage returns smoothly to zero).
3. Current is passed in the other direction for two hours.
4. Current is turned off for a period.
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