Role of Disproportionation in the Dissolution of Mn from Lithium

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Role of Disproportionation in the Dissolution of Mn From Lithium Manganate Spinel Roy Benedek J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05940 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017

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The Journal of Physical Chemistry

Role of Disproportionation in the Dissolution of Mn from Lithium Manganate Spinel Roy Benedek* Argonne National Laboratory, Argonne, IL 60439 ABSTRACT: Dissolution of Mn from lithium-manganate spinel has hindered its commercialization as a cathode material in Li-ion batteries. This dissolution has been widely attributed to disproportionation of near-surface Mn(III), in the presence of acid, with the resultant divalent Mn being vulnerable to dissolution. To what extent moisture in the cell (as opposed to the organic electrolyte) acts as the solvent for Mn ions has not been established. Simulations by Leung show that a small displacement of trivalent Mn from its equilibrium site at an LiMn2O4 (001)/ ethylene carbonate interface leads to its reduction to Mn(II). In the present work, Thermodynamic Integration is performed, based on first-principles molecular dynamics simulations within the Blue-Moon ensemble, to investigate the detachment of Mn ions at the LiMn2O4 (001)/water interface. The results show that reduction of Mn(III) to Mn(II) occurs also in the case of an aqueous interface. The simulations were performed for both neutral and acidic water (in the presence of HF), with the coordination number of the dissolving Mn ion (to substrate oxygen ions) taken as the reaction coordinate. The simulations indicate that an Ϝ ion strongly binds to a surface Mn(III) ion, and weakens its adhesion to the substrate. Owing to this weakening, a surface Mn-F complex traverses regions of phase space at room temperature where disproportionation becomes energetically favorable. Although this disproportionation occurs close to the substrate, where the Mn coordination number is only slightly lowered from its equilibrium value, we argue that the likelihood of reattachment after disproportionation is small (Leung arrived at a similar interpretation in the case of the LiMn2O4 (001)/ EC interface). We suggest that the critical role of Ϝ in promoting dissolution is to weaken the Mn binding to the substrate so as to enable disproportionation. The partially detached MnF complex may then undergo additional interaction with the solvent to form, e.g., MnF2, which would enable transport away from the substrate. The EPR measurements of Shilina et al. that appear to show Mn(III) as the predominant solvated species are discussed.

I. Introduction Lithium-manganate spinel, LiMn2O4 (LMO), is an attractive positive electrode material for lithium-ion batteries1. The tendency of LMO to dissolve in the presence of the most commonly employed lithium-ion-battery electrolyte/electrolyte salt combinations2, however, has been an obstacle to its commercial utilization. Work by Hunter3 showed the effectiveness of acid (aqueous HCl) in dissolving at least a fraction of the Mn in LMO. In LMO-based battery cells, the high-rate of Mn dissolution has therefore often been attributed to acid attack in the presence of inadvertent moisture in a battery cell, particularly in cells that employ LiPF6 as electrolyte salt. In Hunter’s picture3, dissolution is promoted by the disproportionation of Mn(III) into tetravalent and divalent states, with solvation of the latter species. Even if stray water could be entirely suppressed, however, dissolution of Mn into an organic electrolyte may still occur, as shown in simulations by Leung4,5. Efforts to understand the mechanism of Mn dissolution are motivated in part by the quest for an effective remedy, e.g., by a judicious choice of dopants6. The presence of dissolved Mn(II), as a consequence of disproportionation, or otherwise, was brought into question by recent EPR and x-ray XANES measurements7,8, which appear to show the trivalent state to be the predominant Mn-ion species dissolved from LiMn2O4. The latter result, however, is difficult to reconcile with the conventional expectation of only trace solubility of Mn(III). Aqueous Mn(III) does occur in some circumstances, e.g., in the presence of suitable chelation ligands9, but had not previously been demonstrated to occur in lithium-ion-battery electrolytes. Further discussion is given in section IV. To model the dissolution of LiMn2O4 is complicated by the large number of variables that influence the process, such as the spinel stoichiometry and structure, the structure of the solvent, the solvent-substrate interface, etc. Moreover, dissolution (like crystal growth) likely occurs primarily at low-symmetry sites, such as jogs, kink sites10 and steps11, which are especially challenging to model from first principles. Surface phases, such as Mn3O4, also come into consideration12.13. A comprehensive model of spinel dissolution thus appears to be a remote prospect, but the identification of “plausible scenarios”5 may still be a worthwhile objective. First-principles simulations are applied in this work to model dissolution of Mn ions from an LiMn2O4 (001) slab embedded in water that contains a single HF molecule; a reference calculation is also presented for a (neutral-water) cell without ACS Paragon Plus Environment


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HF. Mn ions at the selected surface were predicted to be trivalent in previous work14. We find that, with an Ϝ ion adsorbed to a surface Mn ion, the energy barrier to partially break one (Mn-O) bond with the substrate is considerably lowered, relative to breaking the bond in a neutral water solvent; when the bond of Mn to the substrate is partially broken, disproportionation of the Mn is predicted, and results in an MnF complex with net charge 1+. We argue that, after disproportionation, the MnF complex is much less likely to reattach to the substrate, and, thus the Mn ion is, in effect, dissolved, despite the residual attraction to the substrate. This would suggest that the most critical step of Mn dissolution is the displacement of an MnF complex from the substrate to a point at which disproportionation occurs, leaving an (MnF)+1 complex in the process of detachment from the substrate and a trivalent substrate Mn ion oxidized to 4+. The LiMn2O4 (001)/EC interface treated recently by Leung4,5 is subject to a different set of reactions (e.g., EC decomposition4) than the aqueous interface, however, some common features hold for both water and organic solvents, particularly the electron transfer (disproportionation) to create Mn(II) as the Mn ion (or MnF complex) is partially detached from its equilibrium site. The present work employs thermodynamic integration15 to investigate the energy barriers to the displacement of an ion or complex from a substrate into a solvent. (The work by Leung5 employed Umbrella Sampling, an alternative method for the simulation of rare events). In previous work16, the free energy required to detach Mn ions from LiMn2O4 (001) in neutral water was simulated. This prior work was intended to gain experience with the application of thermodynamic integration to this material, without the extra complication of acidity. The dissolution of LiMn2O4 in neutral H2O, however, is negligible, owing to high-energy barriers, and the absence of a thermodynamic driving force. Acid, however, provides a thermodynamic driving force for dissolution17, and, given that dissolution is observed experimentally, the energy barriers for Mn detachment from LiMn2O4 must be low enough for the reaction to proceed. The present work suggests a plausible path to Mn dissolution at the LiMn2O4 (001)/water interface. II. Calculations The approach employed in this work follows closely methods used in previous work16. Simulations were performed for a unit cell that consisted of a LiMn2O4 (001) spinel slab separated from its adjacent periodic images by 10 Å, with the gap between slabs filled by water. Tritium (T) was employed instead of H, to enable a longer molecular-dynamics time-step (1 fs). (Although the larger mass of T, relative to H, is expected to have some influence on the dynamics, this effect is unlikely to alter the conclusions drawn in this paper.) The spinel slab contained 8 LiMn2O4 formula units, and the gap (between adjacent slabs) contained 48 T2O molecules. The slab was terminated by MnO layers, half of whose sites were vacant, to enable the LiMn2O4 stoichiometry in the pristine cell to be maintained. LiMn2O4 (001) had been predicted to be the lowest-energy surface orientation14, however, a lower energy was subsequently calculated for the (111) surface18,19. Different surface orientations exhibit different Mn oxidation states. Simulations for the MnO terminated (001) surface showed trivalent Mn at the termination layer14,16. The present simulations therefore pertain to dissolution of Mn(III) at the termination layer of an LMO slab embedded in water. The calculations employed the PAW implementation20 of the VASP code21,22, at the GGA+U level of density functional theory, with Ueff(Mn) = 4.84 eV, and the PW91 correlation functional23. The MD simulations employed the low-precision option of VASP, with a single k-point. The magnetic moment calculations employed the high-precision option, applied to atomic configurations generated in the MD simulations. Energy barriers were calculated by thermodynamic integration, within the “Blue-Moon” ensemble24, which was incorporated into the VASP code by T. Bucko. The constrained MD simulations were performed at temperature T = 450 K. The selection of this elevated temperature was intended to (i) compensate for the known underestimation of water diffusion in the Born-Oppenheimer approximation25, and (ii) to accelerate phase-space sampling. Simulations were performed for two cells: Li8Mn16O32•T96O48 and Li8Mn16O32•T95O47F. The first corresponds, in effect, to neutral (tritiated) water, and the second to replacing a hydroxyl ion, in the vicinity of the Mn to be detached, with a fluorine ion. Both cells were initialized by running unconstrained MD for several ps, to equilibrate. III. Results As reaction coordinate, we employ the coordination number CN of the dissolving (tagged) Mn ion16. The Mn ions at the termination layer of LiMn2O4 (001) are coordinated to four substrate O ions, so that CN ≈ 4 before dissolution (Fig. 1, top panel). The mathematical expression selected16 to represent CN varies continuously as the Mn ion moves away (perpendicular to the slab or sideways) from its equilibrium lattice site on the surface of LiMn2O4 (001). At (unconstrained) equilibrium, CNeq ≈ 3.9, both in the presence of neutral water, and with an Ϝ ion adsorbed to a surface Mn ion. In the lower panel of Fig. 1, CN ≈ 3.5, which corresponds to one partially broken Mn-O bond. The constrained MD simulations are designed to provide an estimate of the thermodynamic force, f = dG/d(CN), along the reaction coordinate, where G is the free energy. Constrained MD simulations for 1000 MD time-steps are performed with fixed values of CN. After one (1000-step) MD run is completed, the value of CN is changed slightly by adding a small displacement to the final coordinates of the dissolving MnF complex; the resultant configuration, including the final coordinates of the other ions from the previous run, becomes the initial configuration for the next MD run, at slightly altered CN.

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The free energy G(CN) is obtained by numerical integration of the forces, f(CN) [cf. Fig. 2]. The green curve corresponds to the cell with a single (dissociated) HF molecule (with Ϝ adsorbed to a surface Mn ion), and the blue curve to the results for neutral water. The abscissa scale represents a renormalization of the respective coordination numbers (dividing them by a factor of about 4) such that a value of unity corresponds to equilibrium. Although the resultant energy curves G(CN) are approximate, owing to the statistical scatter in the simulated forces f(CN) (not shown), the qualitative feature are expected to be robust. We observe that the initial barrier to partially detach Mn from the substrate is lower for an MnF complex as compared to Mn. This is not surprising, because the adsorbed Ϝ ion is expected to weaken bonding of Mn to the substrate O ions. The initial energy barriers (0.4 eV for MnF and 0.8 eV for the Mn), enable estimates of the probability of thermal fluctuations at the operating temperature to achieve partial detachment. The rate at which a barrier of 0.4 eV for a dissolving MnF complex is surmounted so that coordination is decreased to about CN= 3.9 x 0.92 ≈ 3.6 is about 106/s. In the case of a barrier of 0.8 eV, the rate of fluctuations that lower coordination to CN = 3.6 , is about 10-1 /s . Thus, a thermal fluctuation that results in CN as low as 3.6 is vastly more likely in the presence of acid. When the coordination number of the dissolving Mn (bound in an Mn-F complex) dips below about 3.6 in the acid cell (or about 3.2 in the neutral-water cell), disproportionation occurs [Fig. 3]: the effective electron affinity of the dissolving Mn exceeds the ionization potential of one of the substrate Mn ions, so that the reaction 2 Mn3+ -> Mn2+ + Mn4+ becomes thermodynamically favorable. Oxidation states of Mn ions can be conveniently monitored in the calculations by their magnetic moments. In Fig. 3, magnetic moments are plotted of the dissolving Mn versus CN in both the acid (with Mn bound to F) and neutral-water cells. The figure shows calculated Mn magnetic moments for the atomic configuration at the end of each (1000-step) MD run. Magnetic moments of Mn ions, assumed to be in high spin states, are inversely proportional to their oxidation state. Calculated magnetic moments in the vicinity of 2.8 (Bohr magnetons) are assigned to oxidation state 3+, and those in the vicinity of 3.3 to 2+. Although the data exhibit considerable scatter, the trends appear clear. We note that identification of the disproportionation partner, i.e., the substrate Mn ion oxidized from 3+ to 4+, is not straightforward, because the system is far from equilibrium, and the electronic orbital calculation is approximate. In general, one expects the disproportionation partner to be located at the termination layer or in a shallow layer below the surface. IV. Discussion A. Trivalent Mn Our analysis indicates that the bonding of Ϝ to a trivalent Mn ion at a LiMn2O4 (001)/water interface weakens the adhesion of the Mn ion to the substrate sufficiently to enable an MnF complex to surmount near-surface energy barriers via thermal fluctuation; the MnF complex then samples a region of phase space in which disproportionation is likely. This process creates an MnF complex with charge -1, and Mn coordination number is lowered to CN ≈ 3.6. This process (partial detachment of Mn followed by disproportionation) can lead to dissolution, however, only if the remaining steps in the dissolution process can readily occur. Thus, if dissolution required breaking several additional bonds, each of magnitude hundreds of millivolts16, the Mn ion would likely return to its initial site, and dissolution would not occur. Although our simulations don’t address the remaining steps in the dissolution process directly, we propose a mechanism by which the latter can be readily accomplished. First, we observe that the likelihood of reattachment of the MnF complex to the substrate following disproportionation is even less than Fig. 2 would suggest. The formulation of thermodynamic integration in terms of reaction coordinate CN(Mn) only indirectly includes the repulsion between the Ϝ ion and the substrate. An electrostatic interaction exists between the Ϝ ion and the (negatively charged) substrate, which, may be regarded a function of the Ϝ ion coordinate zF normal to the surface. Therefore, if the strongly-bonded MnF complex is regarded as a rigid entity, the total force on the complex includes, in addition to fMn, a contribution fF ≈ G/zF • zF/(CN),

(1)

where CN is the coordination number of Mn. Although we have not evaluated fF explicitly, it is expected to be positive, so that this force component tends to draw the MnF complex away from the substrate. The total force ftot = fMn + fF exerted on the MnF complex by the substrate is therefore likely to be at most weakly attractive, and certainly vastly less attractive than that on Mn in the absence of acid. Leung5 considers other mechanisms, such as Li migration to the vacated Mn site, which would tend to make reattachment of the dissolving Mn ion less likely. We suggest furthermore that, with Ϝ ions in the vicinity of the substrate, an MnF complex may react to form neutral MnF2 molecules, whose interaction with the substrate is further reduced. The formation of MnF2, in this scenario, would facilitate Mn transport away from the substrate. B. Alternative Mn oxidation states The results presented here for the spinel/water interface complement those of Leung4,5 for the spinel/EC interface. At interfaces of LiMn2O4 (001) with either water or EC4,5, trivalent Mn is predicted to convert to Mn(II) when it is displaced, by thermal fluctuation, only a small distance from its equilibrium site, thereby promoting dissolution. If the Mn ion at the cath-


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ode surface is already divalent, e.g., for interface orientations of LiMn2O4 other than (001)14, or for surface interphases of Mn3O4 [12, 13], dissolution would be expected to occur even more readily. Shilina et al. recently presented EPR7 as well as x-ray XANES8 measurements on a Li-ion battery cell with a spinel cathode. Their analysis provides substantial, but, in my opinion, not conclusive, evidence for the predominance of Mn(III) as the dissolved manganese species, contrary to the interpretation presented in this paper and in5. The interpretation of the EPR measurements7 relies on the EPR-silence of Mn(III). Divalent Mn in complexes MnF and MnF2, however, whose presence is suggested by ref.5 and the present work, may also be relatively EPR-silent. V. Conclusions We have investigated the HF-promoted dissolution of Mn(III) from an aqueous interface of lithium manganate spinel. We find that an Ϝ ion binds to a Mn(III) ion at LiMn2O4 (001). Adhesion of an MnF complex to the substrate is sufficiently weakened so that thermal fluctuations enable the Mn-ion to traverse regions of phase space at room temperature at which disproportionation is likely. Arguments were presented that the likelihood of Mn reattachment to the substrate after disproportionation are small, so that the Mn ion is destined to dissolve once disproportionation has occurred. This work suggests the plausibility of Mn dissolution promoted by the susceptibility of the MnF to participate in a disproportionation reaction. It remains to be shown, however, whether this interpretation can be reconciled with EPR experiments7. AUTHOR INFORMATION *E-mail: [email protected].

ACKNOWLEDGMENTS I am indebted to H. Iddir and J. Croy for comments on the manuscript. Support from the Battery Materials Research (BMR) Program, U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, is gratefully acknowledged. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (”Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. A computer time allocation at the Blues Computer Facility, Argonne National Laboratory, is gratefully acknowledged. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. REFERENCES (1) Thackeray, M. M. Manganese Oxides for Lithium Batteries, Progress in Solid State Chemistry 1997, 25, 1-71. (2) Choi, W.; Manthiram, A. Comparison of Metal Ion Dissolutions from Lithium-ion-Battery Cathodes, Journal of the Electrochemical Society 2006, 153, A1760-A1764. (3) Hunter, J. C. Preparation of a new Crystal form of Manganese-Dioxide-λ-MnO2, J. Solid State Chem. 1981, 39, 142-147. (4) Leung, K. First Principles Modeling of the Initial Stages of Organic Solvent Decomposition on LixMn2O4(100) Surfaces, J. Phys. Chem. C 2012, 116, 9852-9861. (5) Leung, K. First-Principles Modeling of Mn(II) Migration above and Dissolution from LixMn2O4 (100) Surfaces, Chem. Mater. 2017, 29, 2550-2562. (6) Lee, Y. K.; Park, J.; Lu, W. Electronic and Bonding Properties of LiMn2O4 Spinel with Different Surface Orientations and Doping Elements and Their Effects on Manganese Dissolution, Journal of the Electrochemical Society 2016, 163, A1359-A1368. (7) Shilina, Y.; Ziv, B; Meir, A.; Banerjee, A.; Ruthstein, S.; Luski, S.; Aurbach, D.; Halalay, I. C. Combined Electron Paramagnetic Resonance and Atomic Absorption Spectroscopy/Inductively Coupled Plasma Analysis as Diagnostics for Soluble Manganese Species from Mn-Based Positive Electrode Materials in Li-ion Cells, Anal. Chem. 2016, 88, 4440-4447. (8) Banerjee, A.; Shilina, Y.; Ziv, B; Ziegelbauer, J. M.; Luski, S.; Aurbach, D.; Halalay, I. C. On the Oxidation State of Manganese Ions in Li-Ion Battery Electrolyte Solutions, J. Am. Chem. Soc. 2017, 139, 1738-1741. (9) Trouwborst, R. E.; Clement, B. G.; Tebo, B. M.; Glazer, B. T.; Luther, G. W. Soluble Mn(III) in Suboxic Zones, Science 2008, 313, 1955-1957. (10) Stack, A. G. Molecular Dynamics Simulations of Solvation and Kink Site Formation on the {001} Barite-Water Interface, J. Phys. Chem. C 2009, 113, 2104-2110.


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(11) Kim, J.-S.; Kim, K; Cho, W.; Shin, W. H.; Kanno, R. A Truncated Manganese Spinel Cathode for Excellent Power and Lifetime in Lithium-Ion Batteries, Nano Lett. 2012, 12, 6358-6365. (12) Amos, D. D.; Roldan, M.; Varela, M.; Goodenough, J. B.; Ferreira, P. J., Revealing the Reconstructed Surface of LiMn2O4, Nano Lett. 2016, 16, 2899-2906. (13) Fetisov, V. B.; Kozhina, G. A.; Ermakov, A. N. ; Fetisov, A. V.; Miroshnikova, E. G. Electrochemical Dissolution of Mn3O4 in Acid Solutions, J. Solid State Electrochem. 2007, 11, 1205-1210. (14) Benedek, R.; Thackeray, M. M. Simulation of the Surface Structure of Lithium Manganese Oxide Spinel, Phys. Rev. B 2011, 83, 195439. (15) Adeagbo, W. A.; Doltsinis, N. L.; Klevakina, K.; Renner, J. Transport Processes at a-Quartz-Water Interfaces: Insights from First-Principles Molecular Dynamics Simulations, ChemPhysChem 2008, 9, 994-1002. (16) Benedek, R.; Thackeray, M. M.; Low, J. ; Bucko, T. Simulation of Aqueous Dissolution of Lithium Manganate Spinel from First Principles, J. Phys. Chem. C 2012, 116, 4050-4059. (17) Benedek, R.; Thackeray, M. M. Reaction Energy for LiMn2O4 Spinel Dissolution in Acid, Electrochem. and Solid State Lett. 2006, 9, A265-A267. (18) Karim, A., Fosse, S., Persson, K. A. Surface Structure and Equilibrium Particle Shape of the LiMn2O4 Spinel from First Principles Calculation, Phys. Rev. B 2013, 87, 075322. (19) Warburton, R. E.; Iddir, H.; Curtiss, L. A.; Greeley, J. Thermodynamic Stability of Low- and High-Index Spinel LiMn2O4 Surface Terminations, ACS Appl. Material Interfaces 2016, 8, 11108-11121. (20) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method, Phys. Rev. B 1999, 59, 1758-1775. (21) Kresse, G.; Furthm ller, J. Efficiency of ab-initio Total-Energy Calculations for Metals and Semiconductors using a Plane-Wave Basis Set, Comput. Mater. Sci. 1996, 6, 15-50. (22) Kresse, G.; Furthm ller, J. Efficient Iterative Schemes for ab-initio Total-Energy Calculations using a Plane-Wave Basis Set, Phys. Rev. B 1996, 54, 11169-11186. (23) Perdew, J. P; Wang, P. Accurate Simple Representation of the Electron Gas Correlation Energy, Phys. Rev. B 1992, 45, 13244-13249. (24) Bucko, T. Ab initio Calculations of Free-Energy Reaction Barriers, J. Phys. Condensed Matt. 2008, 20, 064211. (25) Fernandez-Serra, M. V., Artacho, E. Network Equilibration and First-Principles Liquid Water, J. Chem. Phys. 2004, 121, 11136-11144.

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Fig. 1. Snapshots from MD simulations of an MnF complex at a spinel-water interface. Mn: purple; F: gray; O (in T2O): red; O (in LiMn2O4): brown; tritium: white; Li: green. Top panel: CN = 3.9 (the unconstrained system is near equilibrium, i.e., f ≈ 0). The tagged Mn (i.e., the one bound to F) is bonded to four spinel O ions. Bottom panel: depicts a configuration at a lower coordination number, CN = 3.5 (tagged Mn is bonded to three spinel O ions in the figure; the explicit formula16 for CN allows the coordination number to vary continuously, and assume non-integer values). At the lower coordination of the bottom panel, electron transfer has occurred from the substrate to the dissolving Mn, and has reduced it from trivalent to divalent, as discussed in the text.


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Fig.2. Free energy vs. CN, obtained by numerical integration of forces, f. Blue curve corresponds to dissolution of Mn ion in neutral water, green curve to an MnF complex in acid.


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Fig. 3. Magnetic moment of dissolving Mn ion (or MnF complex), in Bohr magnetons. Each point corresponds to the atomic configuration at the end of an MD run.

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Role of Disproportionation in the Dissolution of Mn from Lithium

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