New Iron-Based Mixed-Polyanion Cathodes for Lithium and Sodium Rechargeable Batteries: Combined First Principles Calculations and Experimental Study

Abstract

New iron-based mixed-polyanion compounds Li(x)Na(4-x)Fe(3)(PO(4))(2)(P(2)O(7)) (x = 0-3) were synthesized, and their crystal structures were determined. The new compounds contained three-dimensional (3D)sodium/lithium paths supported by P(2)O(7) pillars in the crystal. First principles calculations identified the complex 3D paths with their activation barriers and revealed them as fast ionic conductors. The reversible electrode operation was found in both Li and Na cells with capacities of one-electron reaction per Fe atom, 140 and 129 mAh g(-1), respectively. The redox potential of each phase was ∼3.4 V (vs Li) for the Li-ion cell and ∼3.2 V (vs Na) for the Na-ion cell. The properties of high power, small volume change, and high thermal stability were also recognized, presenting this new compound as a potential competitor to other iron-based electrodes such as Li(2)FeP(2)O(7), Li(2)FePO(4)F, and LiFePO(4).

Exploring the Ni redox activity in polyanionic compounds as conceivable high potential cathodes for Na rechargeable batteries

“…The crystal structures of Na 4 6 octahedra and (PO 4 ) 3 − tetrahedra in the bc plane that are bridged by the (P 2 O 7 ) 4 − dimers generating large channels along the b-axis with a low activation barrier energy for Na + ion diffusion. 23,29 The Na ions occupy four distinguishable sites, among which two (Na1 and Na4) are located in the large channels parallel to the b-axis. The Ni-based polyanionic materials have so far been considered as an electrochemically inactive cathode material in both Li and Na systems because of the low electronic conductivity resulting in the poor alkali ion diffusion activity.…”

“…Indeed, first principle calculations confirmed a relatively low migration barrier for diffusion of Na ions. 15 In this context, a new polyanionic compound of the general formula Na 4 M 3 (PO 4 ) 2 (P 2 O 7 ) with the Fe 2+/3+ redox couple has been proposed by Kang and colleagues 22,23 as a promising cathode in terms of its superior Na mobility and thermal stability. The material has a theoretical capacity of 129 mAh g − 1 and an operating potential of ∼ 3.2 V vs Na + /Na, higher than the potential exhibited by NaFePO 4 and Na 2 FeP 2 O 7 .…”

Phosphate Framework Electrode Materials for Sodium Ion Batteries

“…The materials show double chains built up from PO 4 tetrahedron and MO 6 octahedra sharing corners with interlayer linkages via P–O–P bridges of the pyrophosphate groups in such a way that large tunnels extending along the [010] and [001] directions occur between two neighboring sheets 226. Kang's group first studied the Li and Na storage performance of Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) through a combined first principles calculations and experiments and reported a reversible capacity of 129 mAh g –1 and average potential of 3 V for the Na‐ion cell ( Figure 13 a) 229. The Na storage mechanism of Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) was testified to be a one‐phase reaction accompanying an exceptionally small volumetric change of less than 4% 230.…”

Three-dimensional-network Li3V2(PO4)3/C composite as high rate lithium ion battery cathode material and its compatibility with ionic liquid electrolytes

“…Other lithium transition metal phosphates including LiMPO 4 (M = Mn, Co, Ni) [3,4], Li 3 V 2 (PO 4 ) 3 [5,6], LiVOPO 4 [7,8], Li 9 M 3 (PO 4 ) 2 (P 2 O 7 ) 3 (M = V, Cr, Al, Ga) [9,10], Li 2 MP 2 O 7 (M = Mn, Co, Fe) [11,12] and their derivatives [13][14][15], have also received a lot of attention due to their great thermal stability and competitive energy density. For one particular, Li 3 V 2 (PO 4 ) 3 shows higher theoretical capacity (~ 197 mAh g -1 vs. ~ 166 mAh g -1 for LiFePO 4 ) and higher average operation voltage plateau (~ 4.0 V vs. ~ 3.5 V for LiFePO 4 ), leading to its higher energy density (~ 800 Wh g -1 vs. ~ 560 Wh g -1 for LiFePO 4 ) [1,5].…”