Sodium-ion diffusion and ordering in single-crystalP2-NaxCoO2

Abstract

Sodium-ion self-diffusion in P2͑␥͒-Na x CoO 2 has been studied using a single crystal as an electrode. Mass transport and thermodynamic properties of Na ions were measured by the potentiostatic intermittent titration technique on a single-crystal electrode with greater accuracy under well-defined diffusion geometry. Excluding the significantly slower diffusion behavior near particular Na ordered phases, average diffusion coefficients D Na are extracted to be 1.2Ϯ 0.5ϫ 10 −7 cm 2 / s for x Ͼ 0.5 and 4.1Ϯ 0.5ϫ 10 −8 cm 2 / s for x Ͻ 0.5, respectively. The existence of stable phases with special Na ordering of x ϳ 0.25, 0.33, 0.43, 0.5, 0.55, and 0.71 are verified by either the maximum or diverging signature of the x-dependent self-diffusion coefficients.

Staging model of the ordered stacking of vacancy layers and phase separation in layeredNaxCoO2(xShu1, Huang2, Chu3 et al. 2009Phys. Rev. BSelf Cite1413402View full textAdd to dashboardCite

“…Such coexisting pattern of two phases can only be observed in the narrow regions of 0.76-0.82 and 0.83-0.86, although O3-type secondary phase is commonly found for x տ 0.85 from melt growth. 9,12 Pure x ϳ 0.86 phase cannot be prepared using electrochemical technique, partly due to the fact that it becomes harder for Na to intercalate into the host structure of compressed c axis. 5,7 Enforcing more Na into the matrix electrochemically destroys its ordering as indicated by the broadened diffraction peaks that correspond to x տ 0.86 as shown in Fig.…”

“…5,7,12 The exact Na content has been cross checked with c axis vs x linear relationship constructed from combined high angle x-ray diffraction ͑008͒ peak position, inductively coupled plasma and electron probe microanalysis ͑EPMA͒ techniques. 1,5,7 In particular, current study uses c͑x͒ linear function that is further calibrated by the phase-separated boundaries and EPMA is averaged out from freshly cleaved crystal surface for more than 100 points.…”

Voltage, stability and diffusion barrier differences between sodium-ion and lithium-ion intercalation materials

“…The LiMO 2 layered oxides are O3-type structures, where the oxygen planes have an ABCABC stacking sequence, while the Na equivalents typically exist in several polytypes (e.g., Na x CoO 2 exists in three polytypes, O3, P2 and P3, depending on the amount of Na intercalated). 16,19,21,[33][34][35][36][37] These polytypes differ in the stacking of the oxygen layers (ABCABC for O3, ABBA for P2 and ABBCCA for P3), resulting in different intercalation sites for the alkali cation. After complete delithiation, the MO 2 layers are weakly bound by van der Waals forces.…”

“…[7][8][9][10][11][12] Since then, however, research into sodium-ion (Na-ion) batteries have been sporadic, [13][14][15] due in no small part to the successes of Li-ion battery chemistry. Nevertheless, there has been a resurgence of research interest in Na-ion battery chemistries in recent years [16][17][18][19][20][21][22][23][24] because of its potential cost advantages. Sodium is far more abundant than lithium, though it has not been conclusively demonstrated that lithium reserves would be an issue in the foreseeable future.…”

“…16,[25][26][27] First principles calculations have been used to great success in the Li-ion battery field, [28][29][30][31][32] and the methodologies and techniques developed in that field can similarly be applied to the Na-ion battery field.…”