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Development of Inter-Digital Lithium Micro Batteries

Dr. Robert Hahn
Fraunhofer IZM, Berlin

Abstract

During the next years many energy autarkic micro systems will be developed, which rely on energy harvesting. In most cases the time, duration and power level of the converted ambient energy is different from the systems power demand.  Therefore a means of temporary energy storage is required in nearly all energy harvesting configurations.  For that purpose, capacitors, super capacitors or secondary batteries can be used as an electrical buffer.

Lithium ion battery technology has matured vast during recent years. Current capability, life time and cycles of several materials are sufficient for energy autarkic systems. On the other hand pouch packaging of lithium polymer batteries and coin cell housings cannot be miniaturized and integrated as desired. A better miniaturization can be obtained with solid state thin film batteries.

The presentation is divided into three parts: At first the electrochemical basics, materials and construction of lithium ion batteries will be explained.

Second, a review of state of the art lithium secondary batteries is given with focus on micro batteries for energy harvesting. These batteries should show a high power-pulse capability, high energy density, long life time, low self-discharge and should be packaged for integration into the system.

The third paragraph focusses on current micro battery research. Several fabrication methods and structures which would be inapplicable for mass fabrication of large battery cells can be used for micro batteries.  For example in case of solid state thin film batteries, all battery layers are deposited in a vacuum process. Since liquid electrolyte is eliminated, the batteries show high temperature stability and very good long term behavior.

Micro patterning technologies are used to realize 3D battery concepts. Here the electrode surfaces are increased significantly which translates to high power performance.

At Fraunhofer IZM a concept has been developed which combines state of the art lithium battery electrode materials with 3D silicon wafer level processing. All available high performance electrode materials can be used as long as they can be processed in form of pastes for dispensing or screen printing. Inter digital electrode structures were fabricated with micro patterned separator structures. Manufacturing is straight forward since all electrodes are on the same planar layer at the wafer and can be deposited simultaneously.

Biographical Information

 

Robert Hahn is currently head of the micro energy systems group at Fraunhofer IZM in Berlin. He received the M.S. and the PhD degree in electrical engineering in 1986 and 1990 respectively from the Technische Universit├Ąt Dresden. He has taken over the coordination of several national and European research projects for the development of new batteries, micro-fuel cells and integrated power supplies for micro systems and energy autarkic electronics. Dr. Hahn has filed 30 patents in the area of micro energy systems and authored and co-authored more than 100 journal and conference publications as well as book chapters. His research focusses on the development of wafer integrated micro batteries, 3D silicon processing, micro fuel cells and hydrogen sources. He received the f-cell award for the Fraunhofer micro fuel cell development in 2005. He is coordinator of the FP7 project MATFLEXEND that develops integrated micro batteries and capacitive harvesters and the Fraunhofer MicroLiB project. As a teacher, he gives lectures in micro energy harvesting and storage at the Technische Universit├Ąt Berlin, Technische Akademie Esslingen and HDT.
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