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Condens. Matter 2019, 4, 53 6 of 8 ReS2 is described by a Jahn–Teller distortion in analogy to the degenerate orbitals in C4H4. Here, the experimental stabilization of the 1T’ phase is confirmed by the presence of the J1 and J2 peaks in the Raman spectra (Figure 2), which originate from the phononic modes of the one-dimensional diamond Condens. Matter 2019, 4, x FOR PEER REVIEW 7 of 10 chains [14–16]. These modes are continually observed up to high capacity, confirming the robustness of the 1T’ structure. Figure 5. Schematic of the Na layer-doubling process used to generate specific capacities Figure 5. Schematic of the Na layer-doubling process used to generate specific capacities greater than of the stage 1 structure. (a) Voltage verses specific capacity for 146 mAh·g of the stage 1 structure. (a) Voltage verses specific capacity for both the 2H and 1T/1T’ −1 −1 greater than 146 mAh∙g sbtrouthctuthraelp2hHasaensdun1dTe/r1NTa’siotrnuicntuercallaptihoans.eTshuenvdoletragNeafoiroLniisnatelsrocagliavteinonfo.rTchomepvaorlitsaogne.(fbo)rSLtaigies 1alsstroucgtiuvrenwfhoerrecothmepMaorSisosnh.ee(bts)(rSetcatgaeng1lestwruitchtuvrieolewthsheardeinthge) aMreosSe2pasrhaetedtsb(yreocnteanlagylersowf Nitha 2 (vgioldlectisrchlaesd)ignigv)inagreasetopiachraiotmedetbriycroantieoloafy1e:r1obfetNwaee(ngoMldoacnirdclNesa). (gci)vAin‘gbeayosntodi’cshtiaogme1etsrtircucrtautrieo −1 doifsp1l:a1yibnegt2wlaeyeenrsMofoNanbdetwNeae.n(tch)e AMo‘Sbesyhoenetds’yisetladgineg1a csatpruacittuyroef 2d6i0spmlAayhi·ngg .2Tlhaeyuenrist coefllNfoar 2 each structure is highlighted with red line. −1 between the MoS2 sheets yielding a capacity of 260 mAh∙g . The unit cell for each structure is highlighted with red line. Figure 5a shows the calculated voltage measured with respect to bulk Na (Li) as a function of specific capacity for each structural phase. Both phases generally exhibit trends in voltage-capacity In order to compare the structural stability of the 2H and 1T phases, we calculated the curves similar to those seen in experiments, although the 1T’ phase exhibits a larger initial voltage in stabilization energy defined as the difference in total energies per molybdenum atom, Es = E2H − E1T agreement with experiment. In the high capacity regime, the voltage is found to level out and become as a function of ion intercalation. Figure 4a compares the stabilization energies so obtained for both marginal, indicating negligible formation energy gained per additional Na atom as in the experiment, Na (red) and Li (blue) intercalation. The stability of the 2H phase is seen to decrease with ion validating the Na multi-layer model. We also found a sensitive dependence of the voltage on the concentration up to stage 1. In contrast, the stabilization energy for the 1T phase is found to increase in-plane Mo-Mo buckling. with Na concentration, becoming the ground state for specific capacities above 51.98 mAh∙g-1. This Figure 6 shows the site-projected density of states (DOS) for various Na concentrations in the 2H result suggests that 1T-MoS2 is an attractive anode material since it avoids complications derived and 1T phases. As expected, the 2H phase is initially semiconducting with a band gap of 1.0 eV. Doped from structural phase transitions under cycling. electrons introduced by Na filling in the conduction band of the pristine material follow a simple rigid Physically, the stabilization of the 1T phase is mainly driven by a buckling of the Mo layer. The band model. The metallic character of the states is maintained up through high Na concentrations. ground state coordination of the transition metals in TMDs varies systematically with d electron The DOS in the 1T’ phase retains a predominant metallic character except at a few particular fillings count. For instance, Group 4 metals are all octahedrally coordinated and Group 5 metals mostly such as x = 1 due to a reorganization of the t2g states induced by the Jahn–Teller distortion. However, have octahedral structures, although some are trigonal-prismatically coordinated. For Group 6 this band gap is not robust against disorder effects present in the real material, leaving the macroscopic metals, however, the trigonal coordination is dominant with few octahedral exceptions. Group 7 system mostly metallic over the full range of Na concentrations. The observed good electron transport metals form distorted octahedra, where the distortion is characterized by the transition metal in the anode is thus explained by the predominantly metallic character of the material. slipping off the center of the regular octahedra in such a manner as to form infinite one-dimensional diamond chains. This phase is designated as 1T’ [20]. Therefore, as Na ions are introduced into the 1T-MoS2 matrix, electronic carriers accumulate in the Mo d-t2g states, effectively changing the d-electron count. This intercalation thus transforms Mo from a Group 6 to an effective Group 7 transition metal, thereby inducing Mo-Mo buckling characteristic of the 1T’ phase. As elucidated by Kertesz and Hoffmann [21], the 1T’ phase of ReS2 is described by a Jahn–Teller distortion in analogy to the degenerate orbitals in C4H4. Here, the experimental stabilization of the 1T’ phase is confirmed by the presence of the J1 and J2 peaks in the Raman spectra (Figure 2), which originate from the phononic modes of the one-dimensional diamond chains [14–16]. These modes are continually observed up to high capacity, confirming the robustness of the 1T’ structure. Figure 5a shows the calculated voltage measured with respect to bulk Na (Li) as a function of specific capacity for each structural phase. Both phases generally exhibit trends in voltage-capacity curves similar to those seen in experiments, although the 1T’ phase exhibits a larger initial voltage in agreement with experiment. In the high capacity regime, the voltage is found to level out andPDF Image | Understanding Phase Stability of Metallic 1T-MoS2 Anodes for Sodium-Ion Batteries
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