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This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the author guidelines. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the ethical guidelines, outlined in our author and reviewer resource centre, still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. Accepted Manuscript rsc.li/chemcomm ChemComm Chemical Communications www.rsc.org/chemcomm ISSN 1359-7345 COMMUNICATION Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyen et al. Reactivity differences of Sc3N@C2n (2n= 68 and 80). Synthesis of the first methanofullerene derivatives of Sc3N@D5h-C80 Volume 52 Number 1 4 January 2016 Pages 1–216 ChemComm View Article Online View Journal This article can be cited before page numbers have been issued, to do this please use: D. C. Hannah, G. Sai Gautam, P. Canepa, Z. Rong and G. Ceder, Chem. Commun., 2017, DOI: 10.1039/C7CC01092C.

Transcript of View Article Online ChemComm - CEDER Group at Berkeley and...

This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication.

Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available.

You can find more information about Accepted Manuscripts in the author guidelines.

Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the ethical guidelines, outlined in our author and reviewer resource centre, still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

Accepted Manuscript

rsc.li/chemcomm

ChemCommChemical Communicationswww.rsc.org/chemcomm

ISSN 1359-7345

COMMUNICATIONMarilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyen et al. Reactivity diff erences of Sc

3N@C

2n (2n = 68 and 80). Synthesis of the

fi rst methanofullerene derivatives of Sc3N@D

5h-C

80

Volume 52 Number 1 4 January 2016 Pages 1–216

ChemCommChemical Communications

View Article OnlineView Journal

This article can be cited before page numbers have been issued, to do this please use: D. C. Hannah, G.

Sai Gautam, P. Canepa, Z. Rong and G. Ceder, Chem. Commun., 2017, DOI: 10.1039/C7CC01092C.

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Fig. 1 (a) The lowest-energy ordering of V and Ti atoms in a

Na4V5Ti3O16 unit cell based on a ranking of the DFT energy of possible

orderings whose structures were generated using a previously published

algorithm. 18 (b) DFT-derived energies above the convex hull (Ehull) for

AxV2−yTiyO4 (A = vacancy, Na, Mg) as a function of Na (green) or Mg

(blue) concentration. Three different Ti concentrations are shown: y =

0 (dotted line), y = 0.75 (solid line), y = 2 (dashed line). The partially

sodiated/magnesiated structures are the lowest-energy vacany/(Na/Mg)

orderings in the respective AxV2−yTiyO4 unit cells.

contraction depending upon the V/Ti ratio. Finally, we find Namobility to be acceptable (< 650 meV migration barrier in mostcases), with excellent Mg mobility (< 300 meV migration barrier)expected. However, low barriers to Mg migration can be a resultof the inherent instability of Mg in the V/Ti post-spinel lattice, andwe predict a strong thermodynamic driving force for conversionreactions to occur, which would result in the decomposition of thepost-spinel.

Figure 1a displays the lowest-energy V/Ti ordering in theCaFe2O4-type crystal structure among all possible V/Ti orderingson a 3×1×1 supercell of Na4V5Ti3O16. Figure 1b shows thecalculated stability of each compound considered in this study,based on the Materials Project database25. Specifically, the en-ergy above the ground state hull (Ehull) is shown for AxV2−yTiyO4

(0 ≤ x ≤ 1, 0 ≤ y ≤ 2, A = Na, Mg, or vacancy). Ehull de-scribes the amount of energy released by the decomposition of

a compound into the most stable compound(s) at that chemi-cal composition. Stable compounds have Ehull

= 0 eV.25 A pos-itive Ehull does not necessarily mean a material cannot be syn-thesized; metastable phases are often attainable through specialsynthetic routes. Indeed, metastable phases are frequently ob-served in cathode materials, including spinel-Mn2O4

30 obtainedby delithiation of LiMn2O4 and ε-Mg0.5V2O5 observed on magne-siating α-V2O5.31 As a guideline, ∼80% of the compounds in theICSD exhibit Ehull up to ∼ 40 meV/atom.26

From inspection of Fig. 1b, it is clear that sodiation stabilizesthe post-spinel framework, whereas magnesiation destabilizes it.The fully Na-intercalated structures are predicted to be stable(Ehull < 2 meV/atom for NaTi2O4 and NaV1.25Ti0.75O4), whileNaV2O4 is metastable (Ehull = 22 meV/atom). The stabilizationof NaV1.25Ti0.75O4 with respect to NaV2O4 is consistent with theexperimental finding that only the former compound is stable atambient pressure.21 Two hypotheses exist to explain the stabiliza-tion mechanism of the NaV1.25Ti0.75O4 phase: (i) The substitutionof Ti4+ for V4+ dilutes the frustrated magnetic network predictedto exist in NaV2O4,21,27 and (ii) The substitution of a larger B-siteion should make the CaFe2O4-type structure more favorable.14,28

The empty and magnesiated forms of the V/Ti post-spinels areconsiderably less stable (Ehull

= 113, 117, 120 meV/atom forMgV2O4, MgV1.25Ti0.75O4, and MgTi2O4, respectively, and Ehull

=

87, 77, 74 meV/atom for V2O4, V1.25Ti0.75O4, and Ti2O4, respec-tively, as shown in Fig. 1b). Some kinetic stabilization againstphase transitions during cycling might be expected on the basisof poor B-site ion mobility15 and the requirement for substan-tial re-arrangement of the underlying oxygen lattice to occur (aswould be the case, for example, in a post-spinel → spinel transi-tion).29 In particular, if the empty structure can be obtained viadesodiation, the V1.25Ti0.75O4 host material is as stable as the 50%magnesiated structure, indicating that some degree of reversiblemagnesiation may be feasible. Nevertheless, the Ehull computedvalues suggest there is a substantial thermodynamic driving forcefor decomposition, especially for the fully magnesiated cathodes(see decomposition reactions in the SI). Decomposition reactionscan be detrimental not only due to the destruction of cathodematerial, but also because many products (such as MgO) are pas-sivating with regard to ionic conductivity and can prevent (de-)intercalation of remaining material.32

Figure 2 reports the calculated properties of Na/Mg (de-)intercalation into the V2−xTixO4 system. Figure 2a shows thevariation of unit-cell volume upon Na or Mg insertion (y-axis)into the charged ("empty") structure as a function of Ti concen-tration (x-axis). A non negligible increase (∼ 4–6%) is noted forall three compounds upon Na insertion, with the largest increaseobserved for the mixed V/Ti system. Insertion of Mg yields asmaller increase (∼ 2%) for both V-containing compounds, but adecrease in volume (∼ 2%) for Ti2O4. For both Na and Mg in-sertion the pure Ti2O4 compound has the lowest volume change.This is consistent with the low volume change associated withLi intercalation in Li[Li1/3Ti5/3]O4 (LTO), attributed to the non-bonding nature of the t2g electron which is added when Ti4+ isreduced to Ti3+. Indeed, LTO is one of only two known zero-strain intercalation electrodes.33,34 Similarly, Figures 2b and 2c

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compounds. This is probably related to a difference in stable sites:Mg occupies a different site in the channel as compared to Na, re-sulting in a different migration path (see SI). These differencesbecome more pronounced as the Ti concentration increases ow-ing to a concomitant increase in channel size. The overall barriersfor Mg migration in all three structures are much lower than forNa. Magnesium migration barriers are likely lower because themagnesiated post-spinels are much less stable than the sodiatedpost-spinels (Fig. 1a), indicating a higher-energy starting site forthe Mg migration path and thus reduced activation energy.6

The migration barriers (Em) in Figure 3 can be used to pro-vide an estimate of the ionic diffusivity. Using a simple randomwalk analysis and a vibrational frequency of 1012 s−1, an Em of∼ 525 meV corresponds to a room-temperature ionic diffusivityof ∼ 10−12 cm2s−1.6 Considering the relationship between diffu-sivity and particle size, migration barriers up to ∼ 650 meV yieldreasonable diffusivity in 100 nm particulates, with larger particlesnecessitating lower migration barriers. As a result, the migrationbarriers presented in Figure 3 are well within the range of accept-able values and indicate that the extraction of Na and subsequentinsertion of Mg should be feasible from a kinetic standpoint.

While the diffusivity of Na and Mg in the V/Ti-compounds arepredicted to be favorable, it is important to note that the ther-modynamic driving force for decomposition (Figure 1b) is largeenough that the expected kinetic stabilization may not be suffi-cient to prevent conversion reactions (Equations S1–3) from oc-curring. As the sodiated forms of the V/Ti and Ti post-spinels arestable at ambient pressure and Na mobility is sufficient to permitextraction, experimental testing of magnesiation of post-spinelsshould be possible.

To conclude, we have utilized first-principles calculations toassess, for the first time, the performance of post-spinel phaseV and Ti oxides as cathodes for magnesium batteries. Becausethe preparation of these compounds would likely require a sodi-ated starting material, we have also studied the thermodynam-ics and kinetics of these compounds with sodium. Consistentwith previous reports on manganese oxide post-spinels, Na+ andMg2+ migration generally exhibit a sufficiently low energetic bar-rier for extraction/intercalation to be feasible. Importantly, thesematerials differ from the manganese oxide post-spinels in thatNaV1.25Ti0.75O4 and NaTi2O4 are stable at ambient pressure, pro-viding a much easier route to synthesis. While the post-spinellattice is expected to be kinetically stabilized, the thermodynamicinstability (Ehull > 100 meV/atom) suggests that experimental ef-forts to intercalate Mg into V2−xTixO4 should be monitored care-fully for the occurrence of parallel conversion reactions leadingto the decomposition of the post-spinel structures. As the require-ment for high pressures in the GPa range has so-far been a majorobstacle to the experimental investigation of magnesium mobilityin the post-spinel framework, the results presented here suggestthat the V and Ti oxide post-spinels may represent a means ofachieving the milestone of high Mg-mobility.

The current work is fully supported by the Joint Center forEnergy Storage Research (JCESR), an Energy Innovation Hub

funded by the U.S. Department of Energy, Office of Science andBasic Energy Sciences. This study is supported by Subcontract3F-31144. The authors thank the National Energy Research Sci-entific Computing Center (NERSC) for computing resources. Theauthors would also like to thank Jordi Cabana and Linda Nazarfor helpful discussions regarding the synthesis of post-spinel.

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Em,Na = 400 meVEm,Mg = 350 meV

Caption: First-principles modeling of a new mixed V/Ti post-spinel phase reveals a possible route to ambient-pressure oxide cathodes exhibiting fast Mg diffusion.

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