Lepidocrocite-type Layered Titanate Structures: New
Lithium and Sodium Ion Intercalation Anode Materials
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Abstract
The
electrochemical characteristics of lepidocrocite-type titanates
derived from K<sub>0.8</sub>Ti<sub>1.73</sub>Li<sub>0.27</sub>O<sub>4</sub> are presented for the first time. By exchanging sodium ions
for potassium, the practical specific capacity of the titanate in
both sodium and lithium half cells is considerably enhanced. Although
the gross structural features of the titanate framework are maintained
during the ion exchange process, the symmetry changes because sodium
occupies different sites from potassium. The smaller size of the sodium
ion as compared to potassium and the change in site symmetry allow
more alkali metal cations to be inserted reversibly into the structure
during discharge in sodium and lithium cells than in the parent compound.
Insertion of lithium cations takes place at an average of about 0.8
V vs Li<sup>+</sup>/Li while sodium intercalation occurs at 0.5 V
vs Na<sup>+</sup>/Na, with sloping voltage profiles exhibited for
both cell configurations, implying single-phase processes. Ex situ
synchrotron X-ray diffraction measurements show that a lithiated lepidocrocite
is formed during discharge in lithium cells, which undergoes further
lithium insertion with almost no volume change. In sodium cells, insertion
of sodium initially causes an overall expansion of about 12% in the <i>b</i> lattice parameter, but reversible uptake of solvent minimizes
changes upon further cycling. In the case of the sodium cells, both
the practical capacity and the cyclability are improved when a more
compliant binder (polyacrylic acid) that can accommodate volume changes
associated with insertion processes is used in place of the more common
polyvinylidene fluoride. The ability to tune the electrochemical properties
of lepidocrocite titanate structures by varying compositions and utilizing
ion exchange processes make them especially versatile anode materials
for both lithium and sodium ion battery configurations