"Lithium ions are employed in the host crystal of positive electrode materials," explains team leader Kisuk Kang of Seoul National University and the Institute of Basic Research in Seoul. “Upon battery charging (or discharging), these ions are extracted from (or reinserted into) the host crystal. Intrinsic lithium and a lithium conduction path are therefore regarded as essential characteristics of a positive electrode material. This constraint has significantly limited the choice of materials possible and hindered the development of new types of positive electrodes for lithium-ion batteries.

“In our work, we show that transition metal oxides (MO, where M = Mn, Fe or Co) that contain neither intrinsic lithium nor lithium conduction paths in their structure can be converted into high-capacity positive electrode materials when they are blended with nano-sized lithium fluoride (LF) in the electrode,” says Kang. “Our results imply that a variety of other transition metal compounds could be used as positive electrode materials, regardless of their crystal structures. This opens up new opportunities for discovering novel high-performance electrode materials.

MOs function as positive electrodes

Transition metal oxides were only known to be electrochemically active in negative electrodes until now. Kang and colleagues have proved that, when blended with nanosized LiF, they not only donate lithium ions to the negative electrodes but also function as positive electrodes.

“An unconventional charge/discharge mechanism occurs in electrodes made from MO thanks to the surface conversion reaction in which the metal monoxide is electrochemically activated as a result of simultaneous decomposition of the nanosized lithium ionic compound (LiF),” explains Kang. “The decomposition of this compound produces a reactive F that then migrates to the MO electrode and forms a surface fluorinated F– M3+O compound in situ. In this reaction, flourine ions interacting with the metal compound on the surface are responsible for the reversible redox reaction at the electrode and subsequent charge/discharge of the battery (LiF + M2+O ↔ Li + F– M3+O). The researchers studied Mn in this work, but Fe and Co should behave in the same way.

Nanoscale mixing between LiF and MO is important

“We believe that the intimate mixing between LiF and MnO on the nanoscale plays the key role in this unusual electrochemical behaviour,” Kang tells nanotechweb.org. “The complementary relation between the two is also important – neither LiF nor MnO is electrochemically reactive as a positive electrode on its own but they become so when combined.

“LiF itself is difficult to decompose electrochemically because of its large formation energy, but here it functions not only as a Li source but also as an anion source for charge compensation when the oxidation and reduction of Mn occur in the nanocomposite electrode. As a result, LiF can be regarded as a ‘stabilizer” for the oxidized Mn+ ions by providing F- ions. It can also be regarded a ‘promoter’ for the decomposition of LiF.”

Extending to other compounds

Kang and colleagues say that their approach might be extended to other compounds, such as sulphides, phosphates and nitrides or perhaps even to any crystalline material.

The Seoul team, reporting its work in Nature Energy doi:10.1038/nenergy.2016.208, will now be looking into such compounds. “Potential candidates include lithium and the 4d and 5d transition metals,” says Kang. “We are also carrying out more fundamental research into the newly discovered chemistry found in our system – for example, the factors affecting the first charge activation process and the different reaction mechanisms that occur when the LiF and MO are combined.”