Ammonia Treatment of 0.35Li2MnO3Â·0.65LiNi0.35Mn0.45Co0.20O2

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Ammonia Treatment of 0.35LiMnO·0.65LiNi Mn Co O Material: Insights from Solid-State NMR Analysis

Nicole Leifer, Irina Matlahov, Evan M. Erickson, Hadar Sclar, Florian Schipper, Ji-Yong Shin, Christoph Erk, Frederick-Francois Chesneau, Jordan Lampert, Boris Markovsky, Doron Aurbach, and Gil Goobes J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12269 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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The Journal of Physical Chemistry

Ammonia Treatment of 0.35Li2MnO3·0.65LiNi0.35Mn0.45Co0.20O2 Material: Insights from Solid-State NMR Analysis Nicole Leifer1, Irina Matlahov1, †, Evan M. Erickson1, ‡, Hadar Sclar1, Florian Schipper1, Ji-Yong Shin2, Christoph Erk2, Frederick-François Chesneau2, Jordan Lampert2, Boris Markovsky1, Doron Aurbach1 and Gil Goobes1 ,* 1 2

Department of Chemistry, Faculty of Exact Sciences, Bar-Ilan University, Ramat-Gan, 5290002 Israel BASF SE, Ludwigshafen am Rhein, Rheinland-Pfalz, Germany

ABSTRACT: Li-rich cathode materials of the formula xLi2MnO3•yLiNiaCobMncO2 (x + y = 1, a + b + c = 1) boast very high discharge capacity, ca. 250 mAh/g. Yet, they suffer capacity decrease and average voltage fade during cycling in Li-ion batteries that prohibit their commercialization. Treatment of the materials with NH3(g) at high temperatures produces improved electrodes with higher stability of capacity and average voltage. The present study follows the changes occurring in the materials upon treatment with ammonia gas, through 6Li and 7Li solid-state NMR investigations of the untreated and ammonia treated 0.35Li2MnO3·0.65LiNi0.35Mn0.45Co0.20O2 as well as its constituent phases, Li2MnO3 and LiNi0.4Co0.2Mn0.4O2. The NMR analysis demonstrates the biphasic nature of these materials. Furthermore, it shows that the Li2MnO3 component phase in the integrated material is the phase mostly being affected by the gas treatment. A thickening of a protective surface film in the integrated material, with the right exposure time to the reactive gas, is observed, which further precludes Ni leach out from the bulk and leads to improved electrode performance. Formation of minor electrochemically inactive oxide phases in the integrated material and similarly in the Li2MnO3 alone upon longer exposure to the gas suggests that the performance deterioration observed can be linked to the rearrangement of ions in the Li2MnO3 constituent phase in the integrated material.

INTRODUCTION Li-rich materials of the family xLi2MnO3•yLiNiaCobMncO2 (x + y = 1, a + b + c = 1) are of great interest as cathodes in Li-ion batteries due to their large discharge capacities, reaching 250 mAh/g. Their characteristic ”integrated” structure is thought to be comprised of a layered Li2MnO3 (space group C/2m) phase and a layered Li(TM)O2 (TM=Ni, Mn, Co) (space group R-3m) closely interconnected on an atomic level.1 In these materials, an initial activation step of the Li2MnO3 phase is carried out during the first lithium extraction step by charging electrodes to 4.7 – 4.8 V. The activation was found to dramatically improve material performance, reaching charge capacities as high as 500 mAh/g, with reversible discharge capacities up to ∼300 mAh/g.2,3 This is understood to be a consequence of Li2O removal from the Li2MnO3 phase, inducing a structural re-organization which renders this phase electrochemically active. Integrated materials have not reached the performance levels required for consideration as alternative commercially viable cathode materials due to a voltage and capacity decrease over extended cycling. Nevertheless, they remain contemporary practical candidates. Among the factors causing performance deterioration are: (i) the decrease of lithium ion re-intercalation into the transition metal (TM) layer, 4 (ii) the decrease in the Ni2+/4+ redox reactions due to Li+/Ni2+ mixing in the Li layers,5 and (iii) the partial structural transformation from layered to spinel-like, occurring mainly at particle surfaces.6 Various modifications and surface treatments have been proposed for correcting the voltage fade occurring during cycling of Li-rich electrodes. One approach which has proven successful for lithium

manganese oxide materials was to pre-treat them with gaseous ammonia, (NH3 (g)).7–9 In a recent, related study, we showed that NH3(g) treatment improves the discharge capacity of Li-rich electrodes and decreases voltage fading during cycling.10 Using XRD, XPS, XANES, micro-Raman, EXAFS, TGA and electron diffraction analysis, we indicated that while ammonia treatment causes a reduction of the Co and Mn metal ions, it does not alter the structure of the bulk material. Instead, the reducing gas induces the formation of a spinel-like surface phase, and the precipitation of lithium oxide and carbonate salts on the surface of the material. The spinel-like phase may help reduce fading by circumventing Ni and Co ion migration from the TM layer into the Li layer, oxygen evolution, and by blocking the Li+ ions from populating the TM layer. The lithium salts, e.g. Li2CO3, LiOH11 and Li2O, act as a passivating yet Li+-conducting surface film on the material, preventing further detrimental reactions on the surface of the electrode during cycling. This manuscript expands our insights from the previous work by analyzing the NH3 (g) treatment effect on the integrated electrode material and its constituents. In particular, since this material is generated as a binary complex, we sought to investigate whether one of the constituents is more susceptible to the gas treatment, particularly as the treatment is applied before the first electrochemical activation step, which mainly alters only the Li2MnO3 phase. Using 6Li and 7Li MAS NMR measurements of ammonia treated Li-rich material 0.35Li2MnO3·0.65LiNi0.35Mn0.45Co0.20O2, Li2MnO3 and LiNi0.4Co0.2Mn0.4O2, (NCM424), which is similar to the second constituent phase, we could determine the phase mostly affected by the treatment, and the changes it undergoes. The

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results are discussed in the context of our conclusions of the previous study.10

EXPERIMENTAL SECTION The experimental procedures of the sample preparation were described in detail previously.10 Briefly, the Li-rich material, 0.35Li2MnO3·0.65LiNi0.35Mn0.45Co0.20O2 (Li-rich, BASF) , Li2MnO3 (Leap Labchem Company, Ltd. China), D50 14.433 µm and NCM424 (BASF) were treated with NH3 at 400 oC for 1, 2 and 4 h in a temperature controlled oven “Rotovap” from BASF. For the electrochemistry measurements shown, the cells were cycled from 2.8 to 4.6 V at a C/3 rate (1C = 250 mAh/g), with a 30-minute constant voltage step at 4.6 V, in coin cells with a Cel-

Complementary information is obtained by acquiring data from both Li isotopes owing to the differences in resonance frequencies, natural abundance and spin value of the two isotopes as well as the transverse relaxation properties of different Li species in the samples. Line deconvolutions of the 6Li spectra of integrated and Li2MnO3 materials were performed using the Mestre-C NMR processing program from Mestrelab Research. The spinning sideband patterns in the 7Li spectra were fitted using the DMfit program12 and were used to calculate the anisotropy and asymmetry parameters, and integrated signal intensities via summation of the isotropic peak and sidebands. It is important to note that the ratio of the NCM component material used in this study is slightly different from that of the compo-

Figure 1: 6Li Hahn echo MAS NMR spectra of untreated Li-rich integrated material (blue), NCM424 (red) and Li2MnO3 (green) recorded at spinning rate 22 kHz, using a pulse width of 4 µs, and a recycle delay of 1 s. In Li2MnO3 and NCM424 spectra, * indicate spinning sidebands of main peaks. In the integrated material spectrum, *, + indicate spinning sidebands of the two isotropic peaks at 693 and 548 ppm, respectively. Dashed lines indicate positions of the main constituent peaks. Inset shows spectra overlapped for easier comparison of the resonances. See Fig. S2 for line deconvolution of Li2MnO3 and the Li-rich integrated material.

gard 2500 separator, Li counter electrodes and 1 M LiPF6, 3:7 ethylene carbonate to ethyl methyl carbonate electrolyte solution. NMR measurements were performed on the as-prepared materials, with natural lithium isotopic abundance using a Bruker AVANCEIII spectrometer operating at a magnetic field of 4.7T. 7 Li NMR spectra were recorded at spinning rates of 12-15 kHz, using a single-pulse scheme and a recycle delay of 16 s. 90° pulse width of 1.9 µs was used. 6Li NMR spectra were recorded at spinning rates of 20-24 kHz, using a rotor-synchronized Hahn echo pulse sequence with respective 90° and 180° pulse widths of 4 and 8 µs and a recycle delay of 0.5 s.

nent material (i.e. 3.5:2.0:4.5 vs 4.0:2.0:4.0). However, the Ni/Mn rich NCM materials generally indicate very broad (>10 kHz) and featureless NMR spectra, even at very high MAS spinning rates, therefore the use of a slightly different ratio of the NCM (4.0/4.0/2.0) material was considered a valid comparison for these studies.13–15 Small differences between Li resonance shifts reported here and previously16 are from disparate sample temperatures as a result of using different spinning rates and the temperature sensitivity of the Fermi contact interaction.

RESULTS AND DISCUSSION


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The Journal of Physical Chemistry Electrochemical cycling results of electrodes prepared from untreated and 2 h NH3 treated Li-rich 0.35Li2MnO3·0.65LiNi0.35Mn0.45Co0.20O2 are reproduced from our previous work, and shown with new data on treated and untreated LiNi0.4Co0.2Mn0.4O2 in Fig. S1. For the 0.35Li2MnO3·0.65LiNi0.35Mn0.45Co0.20O2 material, called “the integrated material” hereafter, discharge capacities were averaged from 3 – 4 cells for statistical purposes with the standard deviation used as error bars. Black curves represent the untreated material, which shows markedly higher capacity fading (0.477 mAhg1 cycle-1) than the 2 h NH3 treated material electrodes (0.228 mAhg-1cycle-1). From our earlier study, this greater than two-fold enhancement in cycle stability was attributed to the bulk reduction of the Li-rich material by the NH3, resulting in a surface passivation, forming surface species of oxygen deficient spinel or “spinel-like” material and lithium salts such as Li2CO3, LiOH and Li2O.1 Fig. S1b depicts the discharge capacity of LiNi0.4Co0.2Mn0.4O2 (NCM424) electrodes vs. Li measured in similar cells to the integrated materials. Whereas Li-rich electrodes show enhanced cycle stability due to NH3 treatments, NCM424 electrodes demonstrate enhanced capacity but lower cycling stability than the untreated version. The electrochemical results provided with motivation for further analysis of the gas treatment on integrated electrode constituents using solid state NMR.

of the Li ions, in each of these three materials, are expected to reside in this layer. In the spectrum of the integrated material (green), the 693-ppm peak (marked with the left dashed line), appears at a similar shift value as the main peak of Li2MnO3 (689 ppm). It is thus attributed to the “Li2MnO3-like” domains suggested to exist in these materials.13 The peak width in the integrated material (4.2 kHz) is three times larger than the analogous peak in Li2MnO3 (1.3 kHz). It is independent of spinning rate,16 which confirms that the source of the broadening is an increase in the dispersion of chemical environments rather than a stronger paramagnetic effect. Larger resonance dispersion is indicative of a decrease in crystalline domain size to nanometer scale,1 implying smaller Li2MnO3 domains in the integrated material than individual crystallites in Li2MnO3. The size of the Li2MnO3 domains in the integrated materials has been reported to be restricted by the presence of the cobalt, even to the point of their invisibility in X-ray diffraction analysis.13 The difficulty in the detection of these domains is part of the basis for the ongoing debate as to whether these integrated materials are more accurately described as homogeneous solids i.e. "solid solutions",17 or as heterogeneous materials with distinctly different phases1. Solid state NMR, as a sensitive probe of local atomic environment around the nucleus, has been useful in resolving these issues.1,18,19 The 6Li NMR spectrum of the integrated mate-

Figure 2: 7Li single pulse MAS NMR spectra of integrated material sample, untreated (blue), after 2h treatment (red) and after 4h treatment with NH3 gas (green) recorded at a spinning rate of 15kHz, using a pulse width of 1.9 us, a recycle delay of 16 s, and 128 transients. The signal intensity is normalized by mass of the samples and has arbitrary units. Fig. 1 presents the 6Li Hahn-echo spectra of the integrated material, Li2MnO3 and NCM424 before ammonia treatment. The spectra of all samples indicate peaks at 500-700 ppm representing lithium ions in the lithium layer. The spectra of the integrated and Li2MnO3 materials show smaller peaks at 1300-1400 ppm representing lithium in the transition metal layer, which are more clearly seen using peak deconvolution (Fig. S2) due to partial overlap with sidebands of the lithium layer peak. The 6Li signal from the lithium layer is much more intense, as approximately 80 – 90 %

rial (Fig. 1) is similar to that published recently,16 in that a comparable set of peaks is seen in the line deconvolution of the lithiumlayer region (Fig. S2a). A Li-layer peak observed at approximately 560 ppm was similarly reported in that study. However, while in that study it was attributed to a disordered lithium in the vicinity of a combination of transition metals and lithium cations 16, here it is specifically attributed to lithium in an “NCM424-like” phase. This assignment is alluded to by comparing the integrated material spectrum to that of the NCM424 material (Fig. 1, in



blue), which exhibits a prominent peak at almost the same frequency (~550 ppm). This assignment is also supported by the published 6Li NMR data of an integrated material prepared via the combination of the two material constituents under a molten lithium-salt flux. 19 As was shown therein, the spectrum of the electrochemically poor synthetic variant did not exhibit this narrow “NCM – like” component at ~ 500 ppm; it was only seen in a well-performing integrated material variant. Therefore the current data are consistent with the description of these integrated materials as biphasic 1,18,20,21, and is also justified by the established assignment of the “Li2MnO3 – like” peak. The NCM424-like peak in the integrated material spectrum is about twice narrower (8.8 kHz), than in the NCM424 material alone (~ 20 kHz), as also noted before 19. This is consistent with the differences in the cationic ordering reported

Fig. 2 shows the single-pulse 7Li spectra of the integrated material samples, untreated, after 2 h and 4 h ammonia treatment, normalized to sample mass and number of scans collected. The 7Li nucleus, with its higher gyromagnetic ratio than 6Li nuclei, experiences a similarly stronger hyperfine coupling with the unpaired electrons (paramagnetic interaction). The stronger interaction is manifested by severe attenuation of the bulk lithium-layer resonance and by prominence of a Li resonance centered at 0 ppm and having extensive spinning sidebands. The latter resonance value was confirmed via experiments run at multiple spinning rates and shown in Fig. S3. This 0 ppm peak, flanked by the large sideband pattern (from ca. 1200 to -1200 ppm), represents lithium in a diamagnetic environment, experiencing dipolar interactions with paramagnetic ions located several bonds away, in the bulk material. Diamagnetic lithium salts, by-products of the synthesis, can typically be found as LiOH, Li2CO3, Li2O, and other unreacted Li

Figure 3: 6Li Hahn-echo MAS NMR spectra of the 0.35Li2MnO3·0.65LiNi0.35Mn0.45Co0.20O2 material: untreated (blue), after 2 h (red) and after 4 h (green) NH3 treatment recorded at a spinning rate of 22 kHz, using a pulse width of 4 µs, a recycle delay of 1 s, and 70k transients. *, + indicate the spinning sidebands of the main isotropic peaks at 693 and 548 ppm, respectively. The grey area indicates where the changes take place. The inset shows a zoomed in region of the spectra in which the extra peaks between 75 - 250 ppm can be seen more clearly. for each of these species, as evidenced by neutron diffraction and magnetic susceptibility data: the LiMO2 phase exhibits a random distribution of the cations in the TM layer, giving rise to a bulk magnetic susceptibility anisotropy22,23, whereas the integrated material indicates a clear magnetic ordering24,25. This is also in line with an earlier work in which a similarly broad peak was measured on LiNi0.33Co0.33Mn0.33O2 (NCM333) and shown to comprise of three resonances based on three possible configurations of the Ni, Co, and Mn around the Li ions.15 The main, central resonance, attributed to an arrangement of 2Ni2+, 2Co3+ and 2Mn4+ in the first coordination sphere of the Li ions, was at ~520 ppm, and had a linewidth of 10.3 kHz. Therefore, the reduced linewidth of this “NCM424-like” phase in the current data reflects a particular cationic ordering rather than a random distribution, which has been attributed to the Li2MnO3 phase exerting the ordering on the LiMO2 layers.26

salts, as was indicated by the XRD, electron diffraction and XPS results from our previous study on these materials.10 Here, they can also represent Li-containing species, formed by the ammonia gas treatment and its superficial corrosive effect on the oxide material. The integrated intensity of the 0 ppm peak, obtained via a summation of the spinning sideband manifold (see Fig. S4, and Table S1) increases from 12 % to 34 % of the total integrated intensity after 2 h treatment with NH3 and then to 38 % as treatment is extended to 4 h. The monotonic increase evidently reflects an increase in these types of lithium species with surface exposure to the gas.27 Additional sideband manifold peaks from bulk Li are seen between 1200-2800 ppm in the spectra as well. These bulk-Li peaks exhibit a decrease in the 2 h sample from 61 % to 31 % of the total integrated signal, and a further decrease to 18% in the 4 h


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The Journal of Physical Chemistry sample (see Fig. S4 and Table S1). This monotonous decrease cannot be explained by migration of Li ions to the surface and formation of a surface film alone, since the 4 h sample exhibits only a very minor change in surface Li content. Instead, the bulk TM Li decrease in the 4 h sample can be explained by minor ion rearrangement and partial transformation to a spinel-like phase, as evidenced by the data shown below in Fig. 4. 7 Li NMR spectra of the NCM424 component material prior to treatment, and after 2 h NH3 treatment are shown in Fig. S5. These spectra are also dominated by the 0 ppm peak of diamagnetic

sotropy (ENDCA) and asymmetry (η) parameters for each sample (Fig. S4 and Table S1). Note that these parameters represent the cumulative effect of surrounding dipolar couplings to vicinal paramagnetic ions. At least two major peaks with sideband manifolds were determined to comprise the spectra, including a surface lithium peak at 0 ppm (see discussion above, and Fig. S3) and a TM lithium peak at ~2160 ppm (see Fig. S6). An additional peak with intensity between -400 to +700 ppm was required for adequate fit. The latter peak could not be determined unequivocally due to the complexity of the spectrum. One possibility is that the isotropic

Figure 4: 6Li Hahn echo MAS NMR spectra of Li2MnO3: untreated (blue) and after 2h (red) and 4h (green) NH3 treatment recorded at a spinning rate 22kHz, using a pulse width of 4 µs, a recycle delay of 1 s, and 4k transients. The * indicate the spinning sidebands of the main central resonances at ~700 ppm, and the dashed line indicates the isotropic peak of the lithium in the transition metal layer. The green area indicates where the most prominent changes take place. The inset shows a zoomed in region of the spectra in which the extra peaks between 75 - 150 ppm can be seen more clearly.

Li ions with extensive spinning sideband manifolds due to dipolar couplings to surrounding paramagnetic ions, as observed in the spectra of the integrated material. The diamagnetic surface Li peak, of salts resulting from material synthesis, decreases in intensity by approximately 25% upon treatment (accounting for differences in sample mass). The decrease may be attributed to reaction of the salts with the NH3(g) and/or to exposure to the high temperatures used during treatment. Similar residual surface species were also evident in the Li2MnO3 material, but there was no noticeable change in their quantity after NH3 treatment (spectra not shown). Comparing the treatment effect on the surface film in integrated material and in the component materials indicates that the NCM is much less affected by the corrosive gas than Li2MnO3 either due to better protection by surface film or due to lower reactivity of the bulk oxide. Surface film growth upon treatment of the integrated material is therefore attributed to changes in the Li2MnO3 constituent phase. Fittings of the sideband patterns28 of the different peaks were used to determine the effective electron-nucleus dipolar coupling ani-

peak is around 200 ppm, which would correspond to the shift of one of the peaks in the NCM material assigned15 as a Li ion in the lithium layer surrounded by 4 Co, 1 Mn, and 1 Ni ion.15 The selected fit which resulted in a similarly adequate reproduction of experimental spectrum was using a single broad Gaussian peak. The bulk signal peak in the untreated material had an ENDCA of -1481 ppm and in the 2 h treated sample changed to -1681 ppm and to 1650 ppm in the 4 h treated sample. This suggests that the initial 2 h NH3 treatment caused some noticeable changes to the TM Li environment, evidently by small TM ion re-arrangements. The sign flip of the anisotropy when the asymmetry parameter is close to unity, is trivial, and concurrently with the minor change in absolute value indicate that the longer treatment did not induce further changes that are significant, to the material. For the surface lithium, fittings of the diamagnetic species' sideband patterns in the untreated sample gave an ENDCA of -755 ppm, the 2 h sample showed lower and sign-flipped value of 694 ppm which did not change in the 4 h sample. The marked change in the anisotropy for the surface charge carriers indicates a major rear-



rangement of the paramagnetic ions near the surface layer, putatively as a result of oxygen loss from the NH3 treatment. The ENDCA can be simulated to extract the TM ion arrangement in both locations, however, it is complicated by the presence of additional constituents with unknown ratios and is therefore outside the scope of this work. 7 Li NMR spectra of the NCM424 component material prior to treatment, and with 2 h NH3 treatment are shown in Fig. S5. The spectra are dominated by the 0 ppm peak of diamagnetic Li affected by ENDCA from surrounding paramagnetic ions, as observed in the spectra of the integrated material. This peak manifold decreases in intensity by approximately 30% upon treatment with ammonia. The signal from Li ions in the bulk of the material is barely detectable here. Fig. 3 shows the resultant spectra from 6Li Hahn-echo measurements of the integrated material samples before treatment, after 2 h treatment, and after 4 h NH3 treatment. These measurements, collected at higher rates (22 kHz) and influenced by weaker FCS interactions, efficiently recover the two bulk Li signals, as seen in Fig. 1. These peaks are more clearly seen in the spectral deconvolution indicated in Fig. S2a, in which two additional minor peaks can be identified: at 548 ppm (overlapping with the “NCM424” peak, and at 783 ppm. It is evident that both peaks decrease upon successive NH3 treatments, indicating a monotonic decrease of Li ions in the lithium layer, presumably due to the ions leaching out to the surface, and undergoing some degradation/ionic rearrangement. No chemical shift changes to the bulk peaks are seen, which is in correspondence with the XRD results on these same samples as we have shown before 10. The 4 h sample indicates additional peaks at the 100-150 ppm region, a region associated with a monoclinic layered phase of lithium manganese oxide and a tetragonal spinel phase (m-LiMnO2 and t-Li2Mn2O4, respectively).29 These additional peaks indicate the detrimental effect of extended exposure to NH3. This was also shown in our previous study whereby the electrochemically inert resistive phase Ni6MnO8 was detected via XRD measurements after 4 h treatment.10 Current results hence complement our previous conclusions.10 Fig. 4 shows the 6Li Hahn-echo results of the Li2MnO3 material without treatment, after 2 h and after 4 h of NH3 treatment. Three main peaks are seen in the untreated material. The main peak is at 694 ppm, and is actually comprised of two peaks at 689 and 704 ppm, in a ratio of 3:2 (see spectral deconvolution in Fig. S2b), and attributed to the 4h and 2c Wyckoff sites in the C2/m structure, respectively.30–32 A further shifted, smaller resonance at approximately 1385 ppm is assigned to the 2b sites (marked with a dashed line),30 and an additional minor peak at 35 ppm (~1 % of total integrated area) is attributed to the presence of an orthorhombic phase (o-LiMnO2).29 The shoulder visible on the up-field side of the main peak is a spinning sideband of the 1385 ppm peak, as indicated in the figure. The 6Li spectra of Li2MnO3 before and after 2 h NH3 treatment are nearly identical with only small intensity changes as a result of the NH3 treatment. Upon successive NH3 treatment, the main 693 ppm peak decreases by 10% in the 2 h sample, and another 5% in the 4 h sample, indicative of Li ion loss from the Li layer in the monoclinic structure. The intensities from the different Li environments maintain approximately the same ratio, suggesting that the Li comes out of the two main sites (lithium layer and transition metal layer) in similar proportion. The two minor peaks appear unchanged upon treatment. The 4 h treated sample indicates extra peaks between 75-150 ppm (as indicated more clearly in the figure inset), a region associated with a monoclinic layered phase and/or tetragonal spinel phase (m-LiMnO2, t-Li2Mn2O4).29 Similar peaks were observed in the integrated material after 4 h of treatment. This suggests that the structural rearrangement due to the

NH3 treatment that occurs in the integrated material takes place mainly in the Li2MnO3 phase of that material.

CONCLUSIONS The NMR investigations carried out here offer several important insights, both into the Li-rich 0.35Li2MnO3·0.65LiNi0.35Mn0.45Co0.20O2 material structure and into the effects of electrochemically enhancing ammonia treatment. The material is shown to comprise two segregated oxide phases rather than a solid solution. The spectrum of the integrated Li-rich material indicates two main resonances associated with lithium in the lithium layer. The higher shifted resonance, consistent with previous studies, is associated with a “Li2MnO3–like” phase, while the second resonance is assigned to a “NCM424like” phase. The “Li2MnO3–like” phase indicates a significant increase in linewidth in the integrated material, consistent with a significant restriction on the domain size of this phase. The resonance assigned to the “NCM424-like” phase, on the other hand, indicates a significant reduction in linewidth, possibly implying a more defined cationic distribution around the Li ions in this phase. The thickness of the diamagnetic surface Li film (comprising salts like Li2O, LiOH, Li2CO3, etc.) increases with initial 2 h NH3 treatment and is related to the improved electrode performance. Further treatment for 4 h, however, appears to degrade this film and reduce layer thickness. New minor phases are formed, as was similarly noted in our previous study, in which delithiated resistive Ni6MnO8 species was detected via XRD.10 An associated decrease in the bulk lithium signal is seen after 2 h treatment, and more so in after 4 h of treatment. The two observations indicate that under continued treatment, the surface layer does not continue to increase and the continued leach out of Li ions from the bulk layers results in structural rearrangement into different phases of oxide material with minor prevalence in the material. These insights supports a similar conclusion from our previous study, that 2 h treatment is the optimal treatment duration from electrochemical standpoint, resulting in stabilization of the cycling behavior of 0.35Li2MnO3·0.65LiNi0.35Mn0.45Co0.20O2 electrodes with discharge capacities of ∼220 mAhg-1 at a C/3 rate and smaller differences in the mean voltage over repeated cycles.10 The change of surface film thickness upon successive treatment in the NCM424 component is opposite that of the integrated material, suggesting that it is not the phase most affected by the gas treatment. The Li2MnO3 component shows no change to Li in the surface film, however, since in the integrated material it is found and smaller domains, these domains may be more susceptible to changes not replicated in the material alone. The decrease in lithium-layer content in the integrated material upon successive NH3 treatments and concomitant observation, in the 4 h treated sample, of a monoclinic layered phase m-LiMnO2 and/or tetragonal spinel phase t-Li2Mn2O4 formation.29 correlates well with the identification of a surface spinel-like phase via micro-Raman and XPS results in our previous study. As this new phase was also seen in the separately treated Li2MnO3, with a leach out of ions from the lithium layer, it suggests that the rearrangements that occurs in the integrated material involves predominantly the Li2MnO3 constituent phase in the material. As these additional minor phases (i.e. the m-LiMnO2 and t-Li2Mn2O4) are not seen in the pristine, nor in the 2 h sample, this picture is consistent with the detrimental effect of extended exposure to the reactive gas. This prolonged treatment results in the formation of some minor electrochemically inert phases and in accordance with a reduced electrochemical performance as we have shown.10 The data from the NCM424 material does not indicate differences upon treatment, but it must be noted that the spec-


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The Journal of Physical Chemistry tra of this material are extremely broad, and therefore some minor rearrangements could be taking place that are not discernable in these data. (11)

ASSOCIATED CONTENT Supporting Information. Electrode materials cycling curves, line fitting of Li spectra and table of the electron-nucleus dipolar coupling anisotropies for the materials. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author *email: [email protected]

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Present Addresses Addresses † Department of Structural Biology, University of Pittsburgh School of Medicine, 3501 Fifth Avenue, Pittsburgh, Pennsylvania 15213, USA. ‡ Department of Chemical Engineering, University of Texas, Austin, Texas 78712-1589, USA.

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ACKNOWLEDGEMENTS

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Partial support for this work was obtained from BASF, Germany, and from the Israel Committee of Higher Education and Ministry of the Prime Minister, Israel, in the framework of the Israel National Research Center for Electrochemical Propulsion (INREP) project.

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Ammonia Treatment of 0.35Li2MnO3Â·0.65LiNi0.35Mn0.45Co0.20O2

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