posted on 2019-01-22, 00:00authored byNam Tran, Brian D. Spindler, Andrey A. Yakovenko, Kamila M. Wiaderek, Karena W. Chapman, Shuping Huang, William H. Smyrl, Donald G. Truhlar, Andreas Stein
Li8ZrO6 is
a pseudolamellar compound with high lithium content. Even though it
is intrinsically a poor conductor and does not contain a transition
metal with easily variable oxidation states, a new synthetic approach
to preparing it in nanocomposite form with intimate contact to a conductive
carbon by mechanical delamination enabled galvanostatic cycling of
coin half-cells containing Li8ZrO6/C as the
cathode and Li metal as the anode at 221 mAh/g (which corresponds
to extracting 2 Li per formula unit) over at least 140 cycles. With
a higher capacity limit, a discharge capacity of 331 mAh/g (which
corresponds to extracting 3 Li per formula unit) was maintained over
15–20 cycles. Ex situ and operando X-ray diffraction (XRD)
studies of galvanostatically cycled cells showed that at these levels
of charge, delithiation follows a reversible, topotactic path with
only small distortions around Zr atoms. During this process, crystalline
grain sizes decrease continuously, shortening diffusion lengths within
grains but increasing the number of grain boundaries and electrode/electrolyte
interfaces. Charge storage in Li8ZrO6 appears
to involve partial oxidation of oxygen atoms and production of small-polaron
holes, as supported by XRD, X-ray photoelectron spectroscopy, and
pair-distribution function studies and predicted by quantum mechanical
calculations. At higher depths of charge, delithiation results in
amorphization of the active electrode material. The charge storage
mechanism in Li8ZrO6 is unusual among lithium-ion
battery electrode materials and involves a combination of mechanisms
that resemble intercalation and conversion reactions. With further
refinement, Li8ZrO6/C based materials open up
opportunities to develop new cathode materials for lithium-ion batteries
that may improve on currently existing capacity barriers.