A comprehensive picture of the current rate dependence of the structural evolution of P2-Na_2/3Fe_2/3Mn_1/3O₂

Abstract

Cathodes that feature a layered structure are attractive reversible sodium hosts for ambient temperature sodium-ion batteries which may meet the demands for large-scale energy storage devices. However, crystallographic data on these electrodes are limited to equilibrium or quasi-equilibrium information.Here we report the current-dependent structural evolution of the P2-Na 2/3 Fe 2/3 Mn 1/3 O 2 electrode during charge/discharge at different current rates. The structural evolution is highly dependent on the current rate used, e.g., there is significant disorder in the layered structure near the charged state at slower rates and following the cessation of high-current rate cycling. At moderate and high rates this disordered structure does not appear. In addition, at the slower rates the disordered structure persists during subsequent discharge. In all rates examined, we show the presence of an additional two-phase region that has not been observed before, where both phases maintain P6 3 /mmc symmetry but with varying sodium contents. Notably, most of the charge at each current rate is transferred via P2 (P6 3 / mmc) phases with varying sodium contents. This illustrates that the high-rate performance of these electrodes is in part due to the preservation of the P2 structure and the disordered phases appear predominantly at lower rates. Such current-dependent structural information is critical to understand how electrodes function in batteries which can be used to develop optimised charge/discharge routines and better materials.

In operando observation of sodium ion diffusion in a layered sodium transition metal oxide cathode material, P2 NaxCo0.7Mn0.3O2

“…40 Unusual evolution of sodium loadings for P2 type cathode materials have been reported before and shown to be rate dependent. 37 Here, highly non equilibrium structural evolution is shown to occur for P2-NCMO even at a relatively low current rate of ~C/19. The sodium loading of the P2-NCMO cathode material, as atomic fraction per formula unit (x), is presented in Fig.…”

“…Different rates of extraction dependent on the relative stability of the sites can be expected but an increased loading of a more unstable site is highly unexpected. 37,39,41,42 To our knowledge this has only been shown once before, for P2 type Na2/3Fe2/3Mn1/3O2 at relatively high rate of 1C. 37 It has been postulated that sodium ion migration pathways in P2 sodium transition metal oxide materials is a direct pathway between adjacent prismatic sites, Nae and Naf.…”

“…[20][21][22][23][24][25][26] Since the first in operando PXRD study of the P2 type material in 2001, 27 many studies of P2 materials with various compositions have been reported in the literature. 12, 15, 24, [28][29][30][31][32][33][34][35][36][37][38][39] All these studies report reversible expansion/contraction of the c axis and contraction/expansion of the a/b axis upon charge/discharge. Some studies also report reversible phase transformations at high and/or low potential.…”

Layered Oxide Cathodes for Sodium‐Ion Batteries: Phase Transition, Air Stability, and Performance

“…During deintercalation above 4.0 V, P2-Na 2/3 Fe 2/3 Mn 1/3 O 2 undergoes a phase change to the so-called "Z" or OP4 phase, which is highly disordered. [169] This phase transformation can be prevented in P2-Na 2/3 Fe 0.4 Mn 0.6 O 2 at high potentials with higher Mn content. The P2-type phase is maintained to the discharged state, but coexists with the distorted orthorhombic P′2 phase and at least two other phases.…”

Recent Progress of Layered Transition Metal Oxide Cathodes for Sodium‐Ion Batteries

“…As a result, reversible extraction and insertion of Na + was realized over a wide range (0.15 < x < 0.9 in Na x Fe 0.5 Mn 0.5 O 2 ) corresponding to capacity of approximately 200 mAh g −1 with a 65% capacity retention after 50 cycles . Similarly, P2‐Na 2/3 Fe 2/3 Mn 1/3 O 2 also undergoes a phase transformation from P2 to highly disordered Z or OP4 phase when charged above 4.0 V . This phase change can be prevented in P2‐Na 2/3 Fe 0.4 Mn 0.6 O 2 with higher Mn content, for which the P2 phase can be retained to the discharged state, but another distorted P′2 phase and more than two other phases are formed …”