Even the best Li-ion batteries degrade with time. A reason for this Li-ion battery degradation was now identified by researchers at the Helmholtz-Zentrum Berlin for Materials and Energy (HZB).
“Rechargeable Li-ion batteries are the power sources of choice for various portable electronics such as cellphones, laptops, cameras etc. and are gradually finding applications in automobile industries”, Dr. Jatinkumar Rana from HZB says. The young scientist and his colleagues in collaboration with the group of Prof. Martin Winter at University of Münster were interested in elucidating electrochemical processes in high-capacity Li-rich cathode materials.
The cathode material is (x)Li2MnO3*(1-x)LiMO2, where “M” is a transition metal like manganese, chromium or iron. These materials are the best candidates for the next generation of Li-ion batteries, because they deliver twice the capacity of the commercially available cathode materials and exhibit exceptionally high rate capabilities, so that they can be charged or discharged in short times or at very high currents. “Besides, they contain lesser amounts of rare, toxic elements such as nickel and cobalt, which make them cost-effective and eco-friendly”, adds Rana.
However, despite their attractive properties, Li-rich cathode materials suffer from certain drawbacks such as a reduction in battery voltage upon cycling, known as the “voltage-fade” mechanism, which reduces energy density of a battery. In addition, there remains a fair amount of ambiguity especially about the role of the Li2MnO3 component in electrochemical processes of Li-rich materials, since it is believed to be electrochemically inactive. “The answers to these questions lie in thoroughly understanding electrochemical processes and associated structural changes in Li2MnO3”, explains Rana.
The original structure of the material has an ABCABC arrangement of oxygen layers (left) – Due to the Li+H+ exchange during the charging process it degrades to ABBCCA (right). (Credit: See citation at the end of this article)
The scientists investigated charged-discharged samples of Li2MnO3 during the first and 33rd cycles by X-ray absorption spectroscopy (XAS) using synchrotron facilities of BESSY II at HZB and DORIS at DESY. “The element selectivity of XAS provides a unique opportunity to probe electronic, chemical and structural changes occurring at and around individual atom types in a material. Such a combination of element-specific information is rather difficult to obtain from X-ray diffraction which provides average changes in long-range structure of a material”, says Rana. They observed oxygen removal from the material during the first charge and shearing of oxygen layers as a result of the Li+-H+ exchange. These phenomena were previously proposed by various research groups. “The cumulative effect of such repetitive shearing of atomic layers during cycling is that the material gradually loses periodicity in atomic arrangements and, as a result, the electrochemical performance of a battery degrades upon cycling”, claims Dr. Rana.
The observed structural changes in Li2MnO3 provide vital clues about the mechanism of electrochemical activation in Li-rich cathode materials and the causes for Li-ion battery degradation. “A series of Li-rich cathode materials so far investigated by us show similar structural changes as observed in the case of Li2MnO3. Now that the electrochemical processes in Li-rich cathode materials are becoming clearer to us, we can use this knowledge to improve the cycling performance of these cathode materials”, concludes Dr. Rana.
Rana, J., Stan, M., Kloepsch, R., Li, J., Schumacher, G., Welter, E., Zizak, I., Banhart, J., & Winter, M. (2013). Structural Changes in Li MnO Cathode Material for Li-Ion BatteriesAdvanced Energy Materials DOI: 10.1002/aenm.201300998