Peculiarities of Phase Formation in Mn-Based Na SuperIonic Conductor (NaSICon) Systems: The Case of Na_1+2xMn_xTi_2–x(PO₄)₃ (0.0 ≤ x ≤ 1.5)

Abstract

NAtrium SuperIonic CONductor (NASICON) structured phosphate framework compounds are attracting a great deal of interest as suitable electrode materials for “rocking chair” type batteries. Manganese-based electrode materials are among the most favored due to their superior stability, resource non-criticality, and high electrode potentials. Although a large share of research was devoted to Mn-based oxides for Li- and Na-ion batteries, the understanding of thermodynamics and phase formation in Mn-rich polyanions is still generally lacking. In this study, we investigate a bifunctional Na-ion battery electrode system based on NASICON-structured Na 1+2 x Mn x Ti 2– x (PO 4 ) 3 (0.0 ≤ x ≤ 1.5). In order to analyze the thermodynamic and phase formation properties, we construct a composition–temperature phase diagram using a computational sampling by density functional theory, cluster expansion, and semi-grand canonical Monte Carlo methods. The results indicate finite thermodynamic limits of possible Mn concentrations in this system, which are primarily determined by the phase separation into stoichiometric Na 3 MnTi(PO 4 ) 3 ( x = 1.0) and NaTi 2 (PO 4 ) 3 for x < 1.0 or NaMnPO 4 for x > 1.0. The theoretical predictions are corroborated by experiments obtained using X-ray diffraction and Raman spectroscopy on solid-state and sol–gel prepared samples. The results confirm that this system does not show a solid solution type behavior but phase-separates into thermodynamically more stable sodium ordered monoclinic α-Na 3 MnTi(PO 4 ) 3 (space group C 2) and other phases. In addition to sodium ordering, the anti-bonding character of the Mn–O bond as compared to Ti–O is suggested as another important factor governing the stability of Mn-based NASICONs. We believe that these results will not only clarify some important questions regarding the thermodynamic properties of NASICON frameworks but will also be helpful for a more general understanding of polyanionic systems.

Na3MnTi(PO4)3/C Nanofiber Free-Standing Electrode for Long-Cycling-Life Sodium-Ion Batteries

“…However, to better account for The MnTi spectrum displays some broad signals in the low-frequency region (see inset). According to [56], the most prominent features at about 565 and 680 cm −1 could be ascribed to P-O and P-O-Na vibrations together with possible contribution by Ti-O ones, while less intense modes are visible below 300 cm −1 and they could arise from the vibrations involving Mn ions.…”

Unlocking the multi-electron transfer reaction in NASICON-type cathode materials

“…Manganese-based electrode materials are attractive due to their excellent stability, resource non-criticality, and high electrode potential [24,81]. Currently, Mn-rich NASICON-type phosphates provide high electrode potentials and robust anionic redox-free frameworks, such as Na 3 MnTi(PO 4 ) 3 [25,[81][82][83][84][85][86][87][88][89], Na 3 MnZr(PO 4 ) 3 [47,90], and Na 4 MnCr(PO 4 ) 3 [91][92][93][94][95] (figure 3(a)). In 2013, Pan et al [24] first proposed Na 3 MnTi(PO 4 ) 3 and Na 3 MnZr(PO 4 ) 3 as cathode materials for NIBs.…”

Vanadium-free NASICON-type electrode materials for sodium-ion batteries

“…30,47 Therefore, these materials can provide superior energy density. Many manganese-based materials have been reported recently, such as Na 3 MnZr(PO 4 ) 3 , 48,49 Na 3 MnTi(PO 4 ) 3 , 50,51 Na 4 MnCr(PO 4 ) 3 , 52–54 Na 4 MnAl(PO 4 ) 3 55 and Na 4 Mn 3 (PO 4 ) 2 (P 2 O 7 ). 56,57…”