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August 2015 • The PCB Design Magazine 51 • Additional power planes need to be exploited and these need to be split to accommodate all supplies plus GND. • The impedance plots project the correct values of all variables for the chosen material to achieve the target impedance. PCBDESIGN References 1. Barry Olney Beyond Design columns: Material Selection for SERDES Design, Material Selection for Digital Design, The Perfect Stack- up for High-Speed Design, and Embedded Sig- nal Routing. STACKUP PLANNING, PART 3 continues 2. Henry Ott: Electromagnetic Compatibil- ity Engineering. 3. To download the ICD Stackup and PDN Planner, visit beyond design Barry Olney is managing director of in-Circuit Design Pty ltd (iCD), Australia. The compa- ny developed the iCD stackup Planner and iCD PDn Planner software, is a PCB Design service Bureau and specializes in board level simulation. To read past columns, or to contact olney, click here. imagine a cell phone that charges in less than an hour and lasts for three to four days, or an electric car that runs for hundreds of miles before needing to be plugged in. Researchers at the u.s. Department of energy's Argonne national laboratory are working to make this dream a reality by developing lithium-ion bat - teries containing silicon-based materials. The most commonly used commercial lithium-ion batteries are graphite-based, but scientists are becoming in- creasingly interested in silicon because it can store roughly 10 times more lithium than graphite. There's just one problem: current batteries based on silicon materials don't last long. The problem lies in the battery's chemistry. The electrolyte inside the battery transports lithium ions back and forth between positive and negative electrodes as the battery charges and discharges. lithium ions react with the negative elec - trode to form a new compound, causing the electrode to ex- pand, while the elec- trolyte produces a protective coating called the solid electrolyte interphase. "The ideal solid electrolyte interphase should halt the reaction between the electrode and electrolyte, while allowing the lithium to come through," said ilya shkrob, a chemist in the Chem- ical sciences and engineering Division. But the coating also needs to expand and con- tract with the electrode, or else it will crack and the battery won't work. "When the protective layer cracks, the elec- trode surface reacts and consumes the electro- lyte," shkrob said. "if the electrolyte is completely consumed, then the battery won't work." in today's graphite-based lithium-ion batter- ies, the electrode expands about 10%—a small enough change that cracks in the coating aren't an issue. But the electrode in a silicon-based lithium-ion battery expands up to 300%. These batteries need a different electrolyte in order to produce an elas- tic shell. The researchers found that when fluo- rine is added to eth- ylene carbonate, the resulting electrolyte forms a coating that can stretch and ac- commodate the vol- ume changes in the electrode. Protective Shells May Boost Silicon Lithium-ion Batteries

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