Scientists at the Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW) (Center for Solar Energy and Hydrogen Research Baden-Württemberg) have developed Li-ion batteries that have already demonstrated a very high cycle life of more than 10,000 full cycles with retention of more than 85% of original capacity. The cells have a power density of 1,100 W/kg.
The active materials for the batteries exclusively originate from German companies. ZSW designed the cells, developed the manufacturing process and produced a small sample series in the 18650 format. The technology has also created the basis for manufacturing large-size pouch and prismatic cells. The Lithium-ion batteries are intended for use in electric vehicles and as solar power storage systems.
After 10,000 complete charging and discharging cycles with a complete charge and discharge cycle per hour (2 C), our lithium batteries still have more than 85 % of the initial capacity. That also provides excellent prospects for a long calendar life.—Dr. Margret Wohlfahrt-Mehrens, Head of the Accumulator Material Research Department in Ulm
In a 2012 paper (Tran et al.) published in the Journal of Power Sources, ZSW researchers noted that electrode manufacturing for lithium-ion batteries is based on a complex process chain with several influencing factors and that a proper tailoring of the electrodes can greatly improve both the electrochemical performances and the energy density of the battery.
In that paper, which specially investigated an NCA cathode material (LiNi0.8Co0.15Al0.05O2) they showed that electrode thickness, the degree of compacting, and the conductive agent type and mixing ratio have a strong impact on the electrochemical performances of the composite electrodes, especially on their behavior at high C-rates.
The ZSW researchers outlined the basics for optimizing cycling stability through the balancing of anode and cathode in a presentation later that year at the Honolulu PRiME 2012 meeting of The Electrochemical Society.
The balancing of electrodes means that the electrodes must be adjusted because the complete lithium is introduced by the positive electrode. Otherwise, when the electrodes are not adjusted this could lead to a lithium deposition on the surface of the negative electrode. Therefore the negative electrode (anode) was oversized to avoid the lithium deposition on the anode surface. The balancing of the electrode must be performed as accurately as possible. A high oversizing of the anode leads to a loss of energy density of the complete cell. One the other hand an oversizing of the cathode results in lithium deposition on the anode surface.
The problems with the accurately balancing of the electrodes are the different kinetics of the electrodes, the irreversible losses which can not be exactly recorded with half cell measurements, etc.—Wilka et al.
The small cells are produced semi-automatically in a plant in the ZSW Laboratory for Battery Technology (eLaB) in Ulm. The cell development has been funded by the German Federal Ministry of Education and Research (BMBF) and the German Federal Ministry of Economics and Technology (BMWi). Such research contracts enable interested companies to have new materials and processes evaluated without having to stop their own production.
In a next stage, the researchers at ZSW want to develop electrodes for large prismatic lithium cells together with partners from industry.
It’s essential to master the currently demonstrated cell technology before going on to produce large cells.—Margret Wohlfahrt-Mehrens
Wohlfahrt-Mehrens says that although further research and development work is necessary for the technology’s implementation in large cells, upscaling is in principle possible.
Marcel D. Wilka, Alice Hoffmann, Rainer Stern and Margret Wohlfahrt-Mehrens (2012) Influence of Anode/Cathode Balancing on Cycling Stability of Lithium Ion Cells (ECS Honolulu PRIME 2012)
Hai Yen Tran, Giorgia Greco, Corina Täubert, Margret Wohlfahrt-Mehrens, Wolfgang Haselrieder, Arno Kwade (2012) Influence of electrode preparation on the electrochemical performance of LiNi0.8Co0.15Al0.05O2 composite electrodes for lithium-ion batteries, Journal of Power Sources, Volume 210, Pages 276-285 doi: 10.1016/j.jpowsour.2012.03.017