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). Source: HZB. Click to enlarge. Click to enlarge.

The researchers were interested in elucidating electrochemical processes in high-capacity Li-rich cathode materials: (x)Li₂MnO₃*(1-x)LiMO₂ (“M” is a transition metal such as manganese, chromium or iron). Despite their attractive properties, Li-rich cathode materials suffer from certain drawbacks such as a reduction in battery voltage upon cycling (“voltage-fade”), 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 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.

—Dr. Jatinkumar Rana, HZB, corresponding author

The scientists investigated charged-discharged samples of Li₂MnO₃ during the first and 33^rd cycles by X-ray absorption spectroscopy (XAS) using synchrotron facilities of BESSY II at HZB and DORIS at DESY.

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.

It is found that both the participation of oxygen anions in redox processes and Li⁺-H⁺ exchange play an important role in the electrochemistry of Li₂MnO₃. During activation, oxygen removal from the material along with Li gives rise to the formation of a layered MnO₂-type structure, while the presence of protons in the interslab region, as a result of electrolyte oxidation and Li⁺-H⁺ exchange, alters the stacking sequence of oxygen layers. Li re-insertion by exchanging already present protons reverts the stacking sequence of oxygen layers.

The re-lithiated structure closely resembles the parent Li₂MnO₃, except that it contains less Li and O. Mn⁴⁺ ions remain electrochemically inactive at all times. Irreversible oxygen release occurs only during activation of the material in the first cycle. During subsequent cycles, electrochemical processes seem to involve unusual redox processes of oxygen anions of active material along with the repetitive, irreversible oxidation of electrolyte species.

The deteriorating electrochemical performance of Li₂MnO₃ upon cycling is attributed to the structural degradation caused by repetitive shearing of oxygen layers.

—Rana et al.

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, Rana explained.

These observed structural changes in Li₂MnO₃ provide clues about the mechanism of electrochemical activation in Li-rich cathode materials.

A series of Li-rich cathode materials so far investigated by the team shows similar structural changes as observed in the original case of Li₂MnO₃, Rana said.

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.

—Dr. Rana

Resources

• Rana, J., Stan, M., Kloepsch, R., Li, J., Schumacher, G., Welter, E., Zizak, I., Banhart, J. and Winter, M. (2013) “Structural Changes in Li₂MnO₃ Cathode Material for Li-Ion Batteries”, Advanced Energy Materials, doi: 10.1002/aenm.201300998