Why do Li-ion batteries age?

Even the best Li-ion batteries degrade with time. A reason for this was now identified by researchers at HZB. They could directly observe at BESSY II and DORIS atomic rearrangements occurring in the cathode material of Li-ion batteries during charge and discharge processes. Such repetitive changes in atomic arrangements can lead to the breakdown of the crystal structure of a material and are the major causes of “ageing” of Li-ion batteries during cycling.

“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, and “M” is a transition metal like Manganese, Chromium or Iron. These materials are the best candidates for the next generation of Lithium-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 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. “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.

More information: Jatinkumar Rana et al.  “Structural Changes in Li2MnO3 Cathode Material for Li-Ion Batteries”, Advanced Energy Materials,  DOI: 10.1002/aenm.201300998