Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2019, 10, 3359−3368
Role of Manganese in Lithium- and Manganese-Rich Layered Oxides Cathodes Laura Simonelli,* Andrea Sorrentino, Carlo Marini, Nitya Ramanan, and Dominique Heinis ALBA Synchrotron Light Facility, Carrer de la Llum 2-26, 08290 Cerdanyola del Vallès, Spain
Wojciech Olszewski Downloaded via UNIV OF SOUTHERN INDIANA on July 18, 2019 at 17:34:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
ALBA Synchrotron Light Facility, Carrer de la Llum 2-26, 08290 Cerdanyola del Vallès, Spain Faculty of Physics, University of Bialystok, ul. K. Ciolkowskiego 1L, 15-245 Bialystok, Poland
Angelo Mullaliu Department of Industrial Chemistry, Toso Montanari University of Bologna, Viale del Risorgimento 4, 40136 Bologna, Italy
Agnese Birrozzi and Nina Laszczynski Helmholtz Institute Ulm (HIU), Electrochemistry I, Helmholtzstraβe 11, 89081 Ulm, Germany Karlsruhe Institute of Technology (KIT), PO Box 3640, 76021 Karlsruhe, Germany
Marco Giorgetti Department of Industrial Chemistry Toso Montanari University of Bologna, Viale del Risorgimento 4, 40136 Bologna, Italy
Stefano Passerini Helmholtz Institute Ulm (HIU), Electrochemistry I, Helmholtzstraβe 11, 89081 Ulm, Germany Karlsruhe Institute of Technology (KIT), PO Box 3640, 76021 Karlsruhe, Germany
Dino Tonti Institut de Ciència de Materials de Barcelona, Consejo Superior de Investigaciones Cientìficas, Campus UAB Bellaterra, Spain S Supporting Information *
ABSTRACT: Lithium-rich transition-metal-oxide cathodes are among the most promising materials for next generation lithium-ion-batteries because they operate at high voltages and deliver high capacities. However, their cycle-life remains limited, and individual roles of the transition-metals are still not fully understood. Using bulk-sensitive X-ray absorption and emission spectroscopy on Li[Li0.2Ni0.16Mn0.56Co0.08]O2, we inspect the behavior of Mn, generally considered inert upon the electrochemical process. During the first charge Mn appears to be redox-active showing a partial transformation from high-spin Mn4+ to Mn3+ in both high and low spin configurations, where the latter is expected to favor reversible cycling. The Mn redox-state with cycling continues changing in opposition to the expected charge compensation and is correlated with Ni oxidation/reduction, also spatially. The findings suggest that strain induced on the Mn−O sublattice by Ni oxidation triggers Mn reduction. These results unravel the Mn role in controlling the electrochemistry of Li-rich cathodes.
T
he wide use of rechargeable lithium-ion batteries and the continuously growing demands of increased energy and power densities stimulate the investigation of novel high-voltage cathode materials.1,2 © 2019 American Chemical Society
Received: April 24, 2019 Accepted: May 29, 2019 Published: May 29, 2019 3359
DOI: 10.1021/acs.jpclett.9b01174 J. Phys. Chem. Lett. 2019, 10, 3359−3368
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The Journal of Physical Chemistry Letters
Figure 1. Mn K- (a,b) and O K-edge (c,d) spectra collected on Li[Li0.2Ni0.16Mn0.56Co0.08]O2 samples at different charge states. The spectra are normalized to the atomic absorption estimated by the linear fit of the data in the extended X-ray absorption fine structure region after the pre-edge background subtraction. The insets on the left panels show the zoom over the Mn K-edge pre-edge peak region after the subtraction of the rising-edge. The spectra error bars are below the symbols dimension. (e,f) Representative TXM images obtained overlapping the average signal from oxygen and manganese postedge energy regions and the average signal at V L3 peak. The Mn, O, and V spatial distribution is reported in green, red, and blue, respectively. The image contrasts have been adjusted to emphasize the heterogeneities. Top and bottom correspond to uncoated and VOx-coated samples, respectively. (g) Voltage profile upon the first charge/discharge cycle at C/10 of uncoated and VOx-coated Li[Li0.2Ni0.16Mn0.56Co0.08]O2,15 where the main charge points investigated in the present study have been indicated (P01, P03, P04, P05, and P08). (h−k) O and Mn pre-edge peak once it is subtracted the rising edge modeled by a pseudo-Voigt tail. The Mn K-edge spectra (black) have been shifted in energy (of around 6012 eV) and the prepeaks have been rescaled in intensity for comparison. The full and open symbols correspond to the uncoated and VOx-coated samples, respectively.
Lithiated transition-metal-oxides are under intensive investigation as cathode materials for Li-ion batteries. The best performing cathodes show an ordered layered structure, which locates the Li ions in between the metal−oxygen layers.3,4 Generally these materials offer the best cycle life when the layered structure is maintained during the delithiation/lithiation process. Among Mn, Co, and Ni, only Co3+ and Ni3+ enable 2D layered Li-based oxides. LiNiO2, however, has various drawbacks related to its crystal structure: difficulty to be synthesized, poor cycling performance, and poor thermal stability.5 As a matter of fact, LiCoO2 is the 2D layered oxide showing the best electrochemical performance and the most commonly used material in Li-ion batteries.6 The delithiation process in LiCoO2/C appears to be limited to slightly less than 0.5 Li, meaning that only about 50% of the total material capacity can be used, which corresponds to around 140 mAh/g. Attempts to go beyond such a limit are not reversible because the strong
repulsion between the CoO2 layers is no longer sufficiently screened by the Li+ cations, finally resulting in the material degradation. To overcome this limitation, chemical substitution at the transition metal (M) sites has been explored, resulting in several M combinations offering decreased production costs, increased safety, and enhanced energy densities. In particular, when the Co3+ is partially replaced by Mn4+, Ni2+, and Li+, a new material is formed, herein called “Li-rich-NMC”, with promising electrochemical performances.7−10 This new material combines the different beneficial effects of Ni, Co, and Mn with the possibility of storing extra Li in the transition metal layers in addition to the Li present in the van der Waals gap. Its structure can be described as the superposition of the layered rhombohedral structure with a monoclinic Li2MnO3 superlattice. It shows capacities exceeding 280 mAh/g,3 about twice that of conventional LiCoO2.6 Despite the great potential, the implementation of Li-rich NMC in practical Li-ion batteries is 3360
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and VOx-coated Li[Li0.2Ni0.16Mn0.56Co0.08]O2 is reproduced in Figure 1g, where the charge points investigated in the present study are indicated, following the labeling of ref 13 (P01, P03, P04, P05, and P08). The charge capacity is slightly higher for VOx-coated sample. It has been proposed that, upon cell operation, the coating delays the transformation of the active layer material into the spinel phase, thus delaying its typical capacity fade upon cycling, and contributes to the overall material capacity.14 Figure S1 shows the voltage profile of the uncoated system, where also the charge points P12 and P16 have been indicated. Upon charge, from Figure 1a,b it is possible to observe a partially irreversible increase of intensity of the whole Mn Kedge pre-edge region that most likely corresponds to increased deformations of the MnO6 octahedra and the consequent increase in the Mn 3d-4p orbital hybridizations.31 On the Mn Kedge absorption spectra, the energy position of the main- (Mn 1s → 4p) and pre-edge (Mn 1s → 3d/4p) features, sensitive to changes in shielding of the nuclear charge provided by the valence electrons, may be used for the determination of the Mn valence. Unfortunately, it is not always straightforward because these spectral features also depend on the local magnetic properties, site symmetry, and nature of bonding with surrounding ligands (Figure S6). Moreover, considering that the Li2MnO3-like component is expected to be fully converted only at the end of the first charge,32 the Mn site of the pristine electrode most likely corresponds to two components, the monoclinic Li 2 MnO 3 -like superlattice and the layered LiMn0.4Ni0.4Co0.2O2. In this complex scenario, complementary information can be extracted from the O K-edge XANES (Figure 1c,d). The O K-edge absorption spectra have been extracted from full field transmission soft X-ray microscopy (TXM) images collected varying the energy across the O Kedge. The O K-pre-edge peak region (around 530 eV) corresponds to the transition from O 1s to hybridized states of M (M = Mn, Ni, and Co) 3d and O 2p orbitals. Similarly to the Mn, the O absorption prepeak shows a not fully reversible trend, confirming the occurrence of some irreversible reactions along the first charge/discharge. Instead, the broad band above 534 eV corresponds to the hybridized M 4p and O 2p orbitals, which is expected to be sensitive to the oxygen oxidation state and the metal−ligand bond distance.33,34 Upon cycling, a reversible evolution of the broad band above 534 eV in the O K-edge spectra is detected. Within the ionic picture, this trend is compatible with both, the O2−/O− redox reactions, which have been recently proposed to take place during charge,28−30 and the shortening of Ni−O and Co−O bonds upon Ni and Co oxidation.35 Representative element maps obtained composing images at different energies are reported in Figure 1e,f. Mn spatial distribution is reported in green, oxygen in red, and vanadium (for the VOx-coated samples) in blue. The yellow color results from the superimposition of green and red. The appearance of systematic reddish borders on the imaged sample particles suggests a higher O to Mn ratio in these regions. Additional oxygen on the particle edges is evident on both uncoated and VOx-coated samples because of a native precipitate, or the formations of a shell of solid electrolyte interphase (SEI) after exposition to the electrolyte, in agreement with previous work.36 This is confirmed by the spectral differences at the O K-edge pre-edge regions, bulk with respect to the border of isolated particles (Figure S15), which resulted smaller for the VOxcoating, supporting some hindered SEI formation from reaction
limited by the poor kinetics and reversibility and the large voltage decay upon cycling. To circumvent these issues, many synthesis efforts have been performed;4,7,8,11−21 however, understanding the problem from the structural/chemical point of view is still limited. The exploration of Li-rich materials formed by 4d rather than 3d metals suggests that, to avoid the voltage decay upon cycling, it is mainly necessary to control the size of the metal ionic radii to prevent the possible trapping of metal ions in interstitial tetrahedral sites.22 The relative M size depends on the atomic number, oxidation state, and spin state, and it has been proposed, ones compared with the alkaline metal size, to be the main parameter governing the structural aspects in layered oxides.3 In all Li-rich-NMC materials, the first charge involves severe structural and chemical rearrangements that are still not fully understood. Some irreversible reactions occur as demonstrated by the disappearance of the voltage plateau at 4.4 V in the following discharge and next cycles.13,14 It has been proposed that, in the first charge, two reactions occur in series: (i) one involving the LiMO2 component, i.e., the Ni2+/Ni4+ and Co3+/Co4+ redox reactions, while Mn is expected to remain in the Mn4+ oxidation state, and (ii) a second one, involving the activation reaction of the Li2MnO3 phase, where manganese is not expected to change the oxidation state (Mn4+), but the oxide anion is oxidized with possible oxygen release. Furthermore, it has been shown that, after the first delithiation, the following recharges are highly reversible,23,24 where the involvement of the Ni2+/Ni4+, Co3+/Co4+, and Mn3+/Mn4+ redox reactions are expected on the basis of the experimentally released capacity.13 Despite all the experimental efforts, it looks that this model is not able to describe completely the charge compensation mechanism in Li-rich-NMC material, where the role of manganese, which at least during delithiation is generally foreseen to merely act as a spectator, is still controversial. In fact, some authors have reported an unexpected Mn reduction, mainly during the first charge, at the surface of the active material,25−27 associated with the formation of oxygen vacancies. Others proposed the formation of a little amount of Mn3+ in the bulk at the end of the first charge.28,29 In this work, we directly access the Mn electronic and magnetic properties evolution as a function of charge state in Li[Li0.2Ni0.16Mn0.56Co0.08]O2, a cobalt-poor and lithium-richNMC material that shows promising long-term cycling and rate performances.23 We also investigate the influence of a VOxcoating on the electronic properties of the host compound during cycling. By means of multiple synchrotron bulk-sensitive X-ray techniques, we finally reveal the Mn oxidation state evolution, surprisingly constantly opposite to the main charge compensation mechanism. The detected partial reduction of Mn during charge indicates a role of Mn in the charge compensation mechanism and confirms as well the recent proposition that oxygen is oxidizing during charge,28−30 in an even higher extent to compensate as well the Mn reduction. Moreover, the data suggest that the strain induced on the Mn−O network by the strong Ni size variation during its oxidation is triggering the Mn reduction at the nanoscale. Finally we report important and novel insights on the Mn spin state, where the combination with the Mn oxidation state is key in defining the Mn size and then in controlling the structural properties, i.e., the cathode performances. Mn and O K-edge X-ray absorption near edge structure (XANES) collected on the uncoated and VOx-coated samples at different states of charge are depicted in Figure 1. The voltage profile upon the first charge/discharge cycle at C/10 of uncoated 3361
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Figure 2. (a) Integrated intensity of features (I) for the O K-edge data set. (b) Relative amount of feature (II), qualitatively corresponding to the relative Mn4+ fraction over the total Mn. (c) Estimated Mn4+ fraction as a function of the integrated intensity of features (I) (O K-edge data set). Where not indicated, the error bars are of the symbols dimensions. The full and open symbols correspond to the uncoated and VOx-coated samples, respectively. Charge states 1 to 8 on the horizontal axis correspond to the P01 to P08 sample notation.
with the electrolyte, much more significant with the uncoated samples. The difference of bulk and border spectra integral around the pre-edge region as a function of the charge state (Figure S15g) strongly suggests formation of compounds with the oxygen expected to be released at the end of the voltage plateau in the uncoated material, which seems suppressed by the VOx-coating.14 In the following paragraphs, we continue to focus on the preedge absorption features, which are less influenced by the chemical environment, being more directly connected to the M valence and magnetic state. In Figures 1h−k and S2, we compare the O and Mn pre-edge peak features. The Mn K-edge spectra (black) have been shifted in energy for comparison. A common double peak structure is evident, which corresponds to the Mn4+ phase, as expected for the investigated systems.13,37,38 We labeled as (II) and (IV) the two features composing the Mn4+ spectral contribution. In the O pre-edge region, more features can be appreciated. Feature (I) corresponds to the O 1s → Ni4+ 3d hybridized with O 2p transition,39−41 which is expected in the charge states in agreement with Ni K-edge absorption investigation.13 The presence of Mn3+ can be hypothesized in the cycled samples,37 where features (III) and (V) become visible, despite the possible contributions from oxidized O species that may also overlap in this energy range, in agreement with the Mn L-edge spectra reported in Figure S7. To disentangle the contribution of the different components from the two sets of collected spectra, the same five-Gaussian model fit has been used to fit both the Mn and O K-edge preedge peaks (details in Figures S3, S4, S5 and Table S1). The
empirically determined width values reproduce well the different experimental energy resolutions. The energy differences among (II), (III), and (IV) components are similar for the two sets of spectra as a function of charge, confirming the main Mn character of the corresponding excitations in the O K-edge spectra. The behavior of the individual Mn K-edge pre-edge peak features intensity is similar and can be represented by the evolution of the total integrated intensity. It grows as a function of charge to decrease again upon discharge without, however, reaching the original value (insets in Figure 1a,b). The uncoated and VOx-coated materials present a similar behavior, with, except for the pristine state (P01), systematically lower values for the VOx-coated samples. Delithiation (charge, P01 → P05) seems to favor the Mn 3d-4p hybridization, in a partially reversible way (P08 ≠ P01).31 The strains induced by the VOxcoating could potentially affect the Mn 3d-4p hybridization. From Mn K-edge extended X-ray absorption fine structure (EXAFS) data, it results that the strains induced by the VOxcoating strongly affect the Mn local structure in the pristine compound, but relax during the first charge cycle, being negligible in the charged state (Figure S9). The detected Mn 3d-4p hybridization reduction by VOx-coating appears to not correlate to structural strains. The O pre-edge peak feature integrated intensities as a function of charge evolve differently, reflecting the main effect of the M 3d−O 2p hybridization. Being the Ni4+ contribution (I) isolated from the Mn one, the O K-edge data set allows to follow directly the Ni4+ formation. In Figure 2a, the evolution as a 3362
DOI: 10.1021/acs.jpclett.9b01174 J. Phys. Chem. Lett. 2019, 10, 3359−3368
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Figure 3. (a,b) Representative TXM images of P04 uncoated and P04 VOx-coated samples (field of view of few micrometers) obtained by dividing the average of three images collected around the spectral feature (I) (Ni4+) by the sum of the average images collected around feature (II) and (III) (Mn), therefore representing the spatial distribution of Ni4+ with respect to Mn. White and black correspond to higher and lower intensity, respectively. In a similar way, the averaged images collected around feature (I), (II), and (III) have been used to extract the representative distribution profiles over isolated particles reported in panels c and d for uncoated and VOx-coated samples, respectively. The distribution profiles report the TXM image intensity along a line in representative isolated particles toward the end of the first charge cycle (P04 and P05). The total Mn (IIII + III, black squares), relative amount of Ni4+ respect Mn (II/(III + IIII), red circles), and relative amount of Mn3+ with respect to Mn4+ ((IIII − III)/(III + IIII), blue triangles) distributions have been extracted by treating directly the images collected at incoming energy around the different absorption features (I, II, and III). The Y axis corresponds to the intensity of the plotted values rescaled to fit more than one particle on the same plot. Each profile point is the average of three pixel values, and the corresponding error bar is calculated as the corresponding average deviation.
relative Mn4+ fraction reduces again in favor of the Mn3+ one during the charges, contrarily to the main charge compensation process and to what assumed in the past. The Mn evolution, constantly opposite to the main charge compensation mechanism during lithium extraction and insertion, can be considered as a collateral effect occurring in parallel to Ni, Co, and O oxidation and subsequent reduction. Interestingly, by comparing the evolution as a function of the charge state of the spectra collected over uncoated and VOxcoated samples, the Ni2+/Ni4+ redox reaction (Figure 2a) seems correlated to the observed Mn reduction (Figure 2b). Figure 2c reports the relative intensity of feature (II) as a function of the integrated intensity of feature (I). A linear tendency is clearly visible, evidencing the simultaneity of the two effects, whose overall evolution is different in uncoated and VOx-coated samples. TXM reveals that there is also a spatial correlation between Mn3+ and Ni4+ states. We exploited the images collected around the (I), (II), and (III) spectral features to build maps of the total Mn, relative Ni4+, and relative Mn3+ distributions. Figure 3a,b reports the images representing the relative Ni4+ distribution for the charge point P04 for uncoated and VOx-coated cathodes. Ni4+ results clearly present in higher amount in the bulk of the uncoated particles, which is also consistent with the results reported by Gent and co-workers.42 With the VOx-coating instead, the distribution seems more homogeneous. Figure 3c,d reports the distribution profiles of the total Mn, relative amount
function of the charge state of the integrated intensity of feature (I) is reported. It appears that Ni2+ is in part irreversibly converted into Ni4+ during the charge. The Ni oxidation results almost completed at the beginning of the voltage plateau for the uncoated sample, while it spans over the whole plateau for the coated one. Assuming features (II) and (III) to correspond principally to Mn4+ and Mn3+ phases,37 the relative intensity of the feature (II) is related to the Mn4+ fraction of Mn. In Figure 2b, the relative intensity of feature (II) as a function of the charge state for the two sets of data shows similar trends, with the results obtained from the O absorption data being slightly affected by the presence of contributions coming from Ni and Co and by total absorption effects. Differences in the orbital hybridizations and density of states of the two Mn oxidation species permit only qualitative considerations. The parallel evolution of the relative fractions of (II) and (III) components at the Mn and O K-edge demonstrates that, during the first charge, Mn4+ is on average partially reduced to Mn3+, even if an overall oxidation process is taking place, as supported by the Mn L-edge spectra reported in Figure S7. In P01, Mn appears to be in the pure Mn4+ phase, which is continuously, although partially, reduced upon charging, with a faster trend for the uncoated material. Upon the following discharge (reduction process), the produced Mn3+ is partially reoxidized, which agrees with some previous reports.25−27 In the following cycles, which seem to be reversible within the error bar (P12 and P16 compared with P05), the 3363
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Figure 4. (a) Mn Kβ emission line for the uncoated and VOx-coated Li[Li0.2Ni0.16Mn0.56Co0.08]O2 samples, compared with the MnO2 and MnO references. (b) Evolution of the extracted Mn local magnetic moment as a function of the charge state. (c,d) Indicative Ni4+, total Mn3+, and LS Mn3+ fractions as a function of charge state. The total Mn3+ fraction has been derived as 1 − III/(III − IIII), while the Mn3+ LS fraction as [1 − III/(III − IIII)] − (μMn − 2.2·III/(III − IIII))/3.45], where 2.2 and 3.45 μB are the μMn obtained empirically for the Mn4+ HS phase and the ones reported in the literature for the Mn3+ HS phase, respectively.47 The shaded areas evidence the main Mn3+ LS formation along the first charge. Charge states 1 to 8 on the horizontal axis correspond to the P01 to P08 sample notation.
of Ni4+, and relative amount of Mn3+ on representative isolated particles (300−150 nm) for uncoated and VOx-coated systems, respectively. The distribution profiles reflect the concentration of the Ni4+ in the particle bulk for the uncoated and a more homogeneous distribution for the VOx-coated samples, while the Mn3+ follows fairly closely the Ni4+ corresponding profiles, proving that Mn reduction is a bulk phenomenon. The correlation between Ni4+ and Mn3+ along the different samples and in the space suggests a possible cause-effect, which could be explained by the idea that the strain induced on the Mn−O network by the strong Ni size variation during its oxidation is triggering the Mn reduction. The strain induced by the VOx coating could instead hamper such structural rearrangements and cause the more even Ni4+ and Mn3+ distribution within the particles. Such stiffness is likely the same that delays Ni4+ formation as seen in Figure 2a. Complementary information on the Mn electronic structure and its evolution as a function of the charge state and coating have been obtained by looking at the Mn Kβ emission line (Figure 4a). We quantify the local Mn magnetic moment (μMn) from the integrated area of absolute X-ray emission Kβ line difference (IAD) with respect to a reference.43 For this purpose we compared the reported spectra to the MnO and MnO2 references. The energy position of the Kβ1,3, directly related to the spin state reflecting the effective number of unpaired 3d electrons,44 shows a similar trend as a function of the charge state, suggesting that the detected variations correspond to actual specific magnetic changes (Figure S8). μMn is equal in the P01 state of the uncoated and VOx-coated materials. By delithiation, the μMn of the VOx-coated samples stays almost
unchanged, while the one of the uncoated samples significantly drops in an irreversible way. This points to different proportions of high and low spin configurations within the produced Mn3+ states in uncoated and VOx-coated samples. Generally, Mn4+ and Mn3+ in octahedral coordination are both in high spin (HS) configuration, with the latter showing a higher spin state.45−47 The HS Mn3+ Jahn−Teller active phase has been correlated to the capacity loss in spinel LiMn2O4, with the Jahn−Teller distortions destabilizing the structure during electrochemical cycling.48−50 The strong μMn drop upon charge in the uncoated material suggests the formation of a Mn environment corresponding to that of layered rhombohedral r-LiMnO2, where theoretical calculations predict the Mn3+ in the not Jahn− Teller active low spin (LS) configuration.47 Interestingly, the theoretical simulation of this phase predicts several potential advantages for a good electrochemical activity and agrees with the results reported here, in fact it explains the shift of the main Mn K-edge absorption feature toward higher energy during the charge, generally associated with oxidation, while Mn is actually partially reducing. More details are reported in the Supporting Information. In the uncoated material, while the Mn 4+ → Mn 3+ transformation looks continuous during the first charge (Figure 2b), the strong μMn drop indicates that the main partially irreversible formation of the LS Mn3+ phase occurs between P01 and P03, where also the Co3+/Co4+ and Ni2+/Ni4+ redox reactions occur (dashed region in Figure 4c). During the voltage plateau, Mn4+ is instead partially converted to a mixture of LS and HS Mn3+ since the average μMn is not changing substantially. In the following cycles it appears that the LS Mn3+ phase is 3364
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The Journal of Physical Chemistry Letters partially reversibly cycling, LS Mn3+ → HS Mn4+. Instead, in the VOx-coated material, in the first step (P01 → P03), Ni and Mn seem almost inactive, and the clear formation of LS Mn3+ is suppressed, most likely because of the strain induced by the coating. From our results, it is tempting to hypothesize that, while increasing the extent of the Mn3+ HS formation, the coating is, however, suppressing the detrimental Jahn−Teller distortions that would be expected with the HS configuration and are naturally absent in the Mn3+ LS. In fact, the EXAFS Fourier transforms (FTs) of P05 are similar for the two systems, highlighting a similar averaged Mn local structure (Figure S9). In the VOx-coated case the Mn4+ → Mn3+ transformation leads to the Mn3+ phase formation in a mixed spin configuration mainly along the voltage plateau, where also the Ni2+/Ni4+ redox reaction occurs (dashed region in Figure 4d). At the end of the voltage plateau (P04 → P05) μMn appears to irreversibly increase, suggesting the formation of an inactive HS Mn3+ phase. Only part of the formed Mn3+ seems available for the subsequent reversible cycling, similarly to the uncoated case. Finally, the position of absorption feature (II), at slightly higher energy (Figure S5a), and the increase of the Mn 3d-4p hybridization in the uncoated cathodes (Figure S5c) most likely correspond both to the presence of the Mn3+ LS phase. The reported results are globally in agreement with the qualitative evolution of the EXAFS data (Figure S9). The intensity of the FT Mn−O peak decreases as a function of charge, highlighting an increasing disorder in the Mn−O shell, while the occurrence of coexisting different Mn sites by charge is expected to globally increase the local structural disorder as well as the increase of local distortions. Also, the partially irreversible formation of Mn3+ in the first charge is in agreement with the EXAFS data, which show no completely reversible evolution of the local structure by charging. In conclusion, the local manganese electronic and magnetic properties of uncoated and VOx-coated Li[Li0.2Ni0.16Mn0.56Co0.08]O2 materials have been studied in detail as a function of the charge state in a lithium battery. The active role of Mn in the lithiation/delithiation process is identified, with Mn oxidation state evolving constantly in opposition to the main charge compensation mechanism. This supposes a supplementary oxidation localized on the other elements, including O, by charging. In parallel, the evolution of the Mn spin state as a function of the charge state in the uncoated and VOx-coated systems is unveiled. The Mn oxidation and spin states are key parameters in controlling the reaction reversibility in lithium-rich NMC cathodes. Both these parameters are linked to the Mn size and magnetic interactions, which end determining the stability of the alternative possible phases in which this cation may be involved.3,50 In fact, it has been shown as well that the Mn spin polarization has a significant effect on the energies and relative stability of different structures.50 Importantly, we have observed that both Mn oxidation and spin state are strongly affected by the strains produced, for instance, by coating or by the oxidation/reduction of Ni. In particular, the results suggest the strain induced on the Mn−O network by the strong Ni size variation during its oxidation to trigger the partially irreversible Mn reduction and to favor a Mn3+ LS phase formation in the bulk. A HS Mn3+ phase, which could be related to spinel, most likely forms during and at the end of the voltage plateau. The VOx coating, most likely participating in the redox reaction,14 appears to slower the Ni2+/Ni4+ and the consequent Mn4+/Mn3+ redox reactions. Moreover, it looks as well to favor the Mn3+ HS phase, most likely because of the induced strains.
Even if further systematic studies are necessary to address quantitatively how the strain controls the Mn oxidation and spin state, this mechanism provides a fundamental way to elaborate design strategies that can be used to control the structure, for instance, hindering the spinel formation at the benefit of the electrode cycle life.
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EXPERIMENTAL METHODS Materials. The previously characterized uncoated13 and VOx-coated14 Li[Li0.2Ni0.16Mn0.56Co0.08]O2 samples were herein studied via ex situ techniques using samples at different state of charge taken from electrochemical cells assembled as detailed earlier. The samples were extracted from cycled electrodes after dismantling the respective cells inside an Ar filled glovebox, and washed with dimethylcarbonate (Sigma-Aldrich) in order to minimize interferences from salt and other possible soluble species in the electrolyte. The so extracted electrodes were loaded on the sample holder for the measurements. The samples investigated in the present study cover several charge/discharge cycles, focusing, however, over the first charge step. In particular, few key states of charge have been identified (see also Figure S1), which are the pristine state (P01), the beginning (P03), and the end (P04) of the high voltage plateau and the fully charged state (P05) during the first delithiation step, and the following fully discharged (P08), as well as fully charged after 5 (P12) and, again, fully charged after 50 (P16) cycles. Hard X-ray Absorption and Emission Measurements. Mn K-edge X-ray absorption and Kβ X-ray emission measurements were performed at the CLÆSS beamline of the ALBA CELLS synchrotron (Spain).51 The synchrotron radiation emitted by a wiggler source was monochromatized using a double crystal Si(311) (Si(111)) monochromator for absorption (emission) measurements. Higher harmonics were rejected choosing proper angles and coatings of the collimating and focusing mirrors. The absorption data were collected in transmission mode using three ionization chambers, for simultaneous measurements of the sample and the Mn foil used as a reference, within an incoming energy resolution below 0.3 eV. The spectra have been normalized with respect to the atomic absorption jump established by a linear fit far away from the absorption edge. The emission data were collected in back scattering horizontal geometry (parallel to the linear polarization vector of the incoming X-ray beam) by means of the CLEAR emission spectrometer. The spectrometer is based on a diced Si(333) analyzer crystal (bending radius R = 1 m) and a 1D position-sensitive Mythen detector. The emission spectra were acquired exciting the sample well above the Mn K edge and detecting the emitted Mn Kβ emission lines with a total energy resolution around 1 eV. The measurements were performed at ambient temperature under vacuum condition. Several X-ray absorption and emission scans were measured to ensure the reproducibility of the spectra and to obtain high signal-to-noise-ratio. Soft X-ray Absorption Measurements. Energy-resolved soft X-ray transmission microscopy (ER-TXM) was performed at the MISTRAL beamline of the ALBA synchrotron.52 Samples were scratched off from the electrode and deposited on carboncoated Au TEM grids inside an Ar filled glovebox. Dimethylcarbonate (Sigma-Aldrich) is used to wet the sample powder improving its adhesion on the carbon-coated TEM grid used as sample holder. The grids were then stored under Ar in cryogenic vials and transferred in the MISTRAL microscope in cryogenic condition (T < 110 K) under N2 vapor to avoid atmospheric 3365
DOI: 10.1021/acs.jpclett.9b01174 J. Phys. Chem. Lett. 2019, 10, 3359−3368
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The Journal of Physical Chemistry Letters ORCID
contamination. The samples were kept at cryogenic temperature and under high vacuum conditions during all the measurements. Representative regions of the samples were selected from areas of 100 μm × 100 μm recorded at different zones of the sample by the means of composite or mosaic images just at two energies, above and below the O K-edge for a first localization of oxygen compounds. Transmission images (2 s exposure time, effective pixel size 10 nm, field of view (FOV) 10 μm × 10 μm) were collected, varying the energy across the O K-edge with a variable spectral sampling (0.5−0.1 eV). The energy resolution was determined at the nitrogen K-edge measuring the visibility of nitrogen gas vibrational peaks at the exit slit of the monochromator following the strategy proposed in ref 53. The energy resolution at the O−K edge was extrapolated to be around 0.5 eV (full width half-maximum). A preliminary calibration of the absolute value of the energy was carefully performed before the experiment using standard references samples at different energies along the available energy range of the beamline (CaCO3, N2, TiO2, Mn2O3, and Fe2O3). No energy shift was applied to the measured sample spectra. The objective zone plate lens (outermost zone width of 25 nm, 1500 zones) and the back illuminated CCD detector (Pixis XO by Princeton Instruments with 1024 × 1024 pixels and 13 μm pixel size) positions were automatically adjusted to maintain the sample in focus and constant magnification (= 1300). The spatial resolution of the system was estimated at 520 eV using a Siemens star pattern with 30 nm smallest features to be 23 nm half pitch.54 The necessary total acquisition time for a single energy stack of transmission images was about 1.5 h, including the flat field acquisition at each energy step. After normalization, alignment and conversion to absorbance of the transmission energy stack, spectra were extracted only from pixels in the field of view (FOV) with a signal to noise ratio ≥ 2, where the noise is defined as the average single pixel spectra standard deviation in the pre-edge energy region (544 eV) and the average value in the pre-edge energy region. The noise is mainly due to small instabilities in the incident beam and is around 0.05 for all the measured spectra. The reported spectrum for each sample is then calculated as the average of the extracted single pixel spectra from the full FOV (Figure 1c,d) or from particular region of interest (Figure S15a,f). Spectra have been normalized with respect to the atomic absorption jump established by a linear fit far away from the absorption edge. The experimental error on the extracted spectra from the full FOV is discussed in the Supporting Information.
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Laura Simonelli: 0000-0001-5331-0633 Angelo Mullaliu: 0000-0003-2800-2836 Marco Giorgetti: 0000-0001-7967-8364 Stefano Passerini: 0000-0002-6606-5304 Dino Tonti: 0000-0003-0240-1011 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been partially financed by the Spanish Ministry of Economy and Competitiveness, through the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-20150496). The HIU authors kindly acknowledge the basic funding of the Helmholtz Association.
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