Article pubs.acs.org/cm
LiNi0.8Co0.15Al0.05O2 Cathode Material: New Insights via 7Li and Magic-Angle Spinning NMR Spectroscopy
27
Al
Nicole Leifer, Onit Srur-Lavi, Irina Matlahov, Boris Markovsky, Doron Aurbach, and Gil Goobes* Department of Chemistry, Bar Ilan University, Ramat Gan 5290002, Israel S Supporting Information *
ABSTRACT: Aluminum doped mixed metal oxides are popular positive electrode materials for Li-ion batteries. They are used extensively in many applications, yet their operation and limitations are not entirely understood. This work shows the advantage of using solid-state 7Li and 27Al NMR for monitoring the electrochemical properties of the doped nickel−cobalt oxide cathode material, LiNi0.8Co0.15Al0.05O2 (NCA), particularly during the first few charge/ discharge cycles. The changes in the state of the material as lithium ions are intercalated and deintercalated during discharge and charge, respectively, are highlighted via the Li nuclei as a dynamic reporter and the Al nuclei as a static, material-embedded reporter. In particular, the NMR view of the cyclic change of Ni ions between paramagnetic and diamagnetic oxidation states is enhanced by monitoring both nuclei. Two protocols of cycling the NCA electrode are compared: one employing a smaller voltage window, cycled against graphite as anode, and one using a wider voltage window, cycled against a lithium metal anode. The NMR analysis unveils notable differences in the reversibility of the changes in the Ni oxidation states as charge carriers are shuttled in and out of the cathode material. The 27Al NMR data of the pristine material shows the existence of at least two distinct configurations of Ni ions around the Al dopant ions, suggesting coexistence of two disparate phases, which remain intact upon cycling. The protocol employing slower cycling versus Li anode delivers better cathode performance in the sense that more extensive relithiation occurs, and here, it is shown that the return of the local environments to their pristine electronic configurations is more complete. The 27Al and 7Li NMR results are integrated into a simple scheme exemplifying how better understanding of the local electronic changes in paramagnetic electrode materials can be captured in simple progressive plots.
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INTRODUCTION Lithium battery research today is highly driven by consumer demand for longer-lasting and better performing energy storage systems for use in many devices, from portable electronics to electric cars. Transition metal (TM) layered oxide materials, particularly of the form LiMO2 (M = Mn, Ni, Co), exhibit the most appropriate cathode properties in terms of specific capacity, energy density, relative safety, and cost. This body of materials, initially proposed in 1980 starting with LiCoO2, has evolved to include the huge variety of transition metal oxide combinations that exists today. The main drawbacks of the initially developed layered LiCoO2 cathodes are their high price and cobalt toxicity, but the primary limitation is their limited practical capacity due to oxygen loss in the partially delithiated cathodes. Consequently, the capacity achieved, 140 mAh/g, is only half of the theoretical value. The addition of nickel to the cobalt oxide system (e.g., LiNixCo1−xO2) serves to reduce the cost of the material and increase the capacity to 180−200 mAh/g, in part because these cathodes do not appear to lose oxygen until the lithium stoichiometry falls below 0.3. However, the substitution of cobalt for nickel introduces a structural instability under mild heat due to migration of the nickel ions from the nickel plane © 2016 American Chemical Society
(3a) to the lithium plane (3b), which can lead to a subsequent transformation to spinel structure.1 One successful way that this issue is addressed is by doping the cathode with aluminum cations, a common choice of dopant for many different transition metal oxide systems. Aluminum has a thermal stabilization effect due to partial substitution for nickel.2 The aluminum ions, having similar radii to Ni and Co (rCo+3 = 0.545 Å, rNi+3 = 0.56 Å, rAl+3 = 0.53 Å),3 occupy the TM sites in the layered R3̅m crystal structure.4 First-principles calculations suggest that at room temperature Li(Ni,Al)O2 is in the form of a solid solution,5 meaning that Al and Ni are spread evenly in the TM layer. Additional studies have shown that aluminum substitution up to 15% and in some cases up to 30% in the nickel layers indeed results in the formation of a solid solution.6−8 In contrast, in LiNiO2 with aluminum content of 10−50%, Xray diffraction (XRD) measurements have indicated that dopant aluminum ions are distributed inhomogeneously.9 Further support for the segregation tendency of nickel and Received: April 8, 2016 Revised: September 13, 2016 Published: September 13, 2016 7594
DOI: 10.1021/acs.chemmater.6b01412 Chem. Mater. 2016, 28, 7594−7604
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Chemistry of Materials
of the nucleus, NMR is applicable even in the case of highly disordered systems. NMR is very sensitive to the electronic surrounding of the nuclei probed including the influence of neighboring ions. It enables the monitoring of the effects of electrochemical cycling on the ions directly and indirectly involved in the electrochemical processes (in this case, 6,7Li and 27 Al).31 The NMR properties of lithium nuclei in transition metal oxide-based cathode materials are markedly affected by a strong interaction with TM ions containing unpaired electrons in their orbital structure (i.e., paramagnetic). The electronic environment around the Li nuclei becomes highly deshielded due to hyperfine coupling, or Fermi contact interaction, in which the paramagnetic spin density on the transition metal (in this case, Ni) ions is transferred to the Li 1s orbital via the overlapping oxygen 2p orbitals. The resultant Li NMR spectra are then characterized by heavily shifted resonance lines.32,33 Paramagnetic effects in 27Al NMR have been reported as well, both from direct contact with a neighboring paramagnetic ion and through-bond TM--O--Al spin−polarization mechanisms.34−38 However, these reports are much less common than Li NMR, presumably due to the larger quadrupolar interactions in 27Al which make the NMR measurements and data interpretation much more challenging. Several NMR studies have been conducted specifically on the family of Ni/ Co/Al lithium-intercalation compounds.4,39−45 Insights have been gleaned from NMR studies of the Al-free and/or Co-free analogs of the NCA material as well, but only a handful focused on the standard NCA material itself. These are discussed in context, below. In this study, two different electrochemical cycling protocols were performed in order to investigate different aspects of the NCA cathode material. The first protocol was performed on full cells (NCA vs. graphite), in accordance with the PNGV Battery Test Manual.46 The second protocol focused exclusively on the cathode by using a half-cell configuration (vs Li metal) and charging to a lower voltage to ensure a more complete lithium intercalation upon discharge. 27 Al and 7Li NMR were conducted on the cathode materials extracted from disassembled cells that were terminated at different states of charge (SOC) throughout the two abovementioned cycling protocols. As outlined above, the surrounding paramagnetic Ni ions induced large shifts in the probed nuclei, whose resonance frequencies are then a function of the SOC of the NCA material. In contrast to the usual large positive shifts in the 7Li spectra of lithiated NCA due to surrounding paramagnetic Ni ions, unusually large negative shifts are observed in 27Al spectra. This is demonstrated below and discussed in terms of the hyperfine interaction with the paramagnetic Ni ions. The 27Al nuclei are shown to serve as a valuable static probe of the changes occurring in the layered material at various SOC and through different cycles. The results obtained complement the information obtained from the 7Li nuclei of charge-carrying Li ions, which deintercalate and reintercalate into the cathode during each battery cycle.
aluminum in the layered lithium nickel oxides was inferred from a more recent study utilizing synchrotron XRD, electron diffraction, energy-dispersive X-ray diffraction (EDX), and electron energy loss spectroscopy analyses.10 Aluminum serves to reduce oxygen release from the system by suppressing the exothermic reactions of the nickel oxides with the electrolyte.16 This reduction of material breakdown has also been suggested to be a result of the stabilizing effect of aluminum on the cathode’s charge-transfer impedance.17 A related consequence of the presence of aluminum in these systems is the reduced variation in the unit cell c-parameter in the crystal lattice. In a study of LiNi0.8Co0.16Al0.04O2 cycled to 4.5 V, the lateral cell parameters (a,b) were unaffected, but the c-parameter indicated a 0.7% change, versus a 3.5% change in the Al-free analog.14,15 In all cases, the Al3+ ions remain electrochemically inactive throughout cycling8,11 while maintaining high practical capacity.8,12,13 Overall, the LiNi0.8Co0.15Al0.05O2 (NCA) system is currently an important material for commercial use. The most commonly used form of NCA is LiNi0.8Co0.15Al0.05O2, with an aluminum content limited to 5%. Its main advantages are relatively high specific energy, reasonably good specific power, and a long life span. The exact reasons for the favorable effects of the aluminum are only partly understood, and further investigations may lead to improvement in cathode performance. As typical of most of the nickel-based systems, the first charge/discharge cycle of NCA is characterized by a large irreversible specific charge loss, with subsequent cycles showing very stable capacity.11,15,18,19 This phenomenon has been investigated in several studies, in both the Al-free20 and Aldoped systems.19,21,22 Because of the importance of the first cycle in these systems, the cycling parameters, particularly in the voltage window used (3.0−4.1 V), were carefully considered. Several studies have concluded that, above 4.1/4.2 V, the Ni-based materials undergo some irreversible phase transitions, such as a drastic increase in defect sites,23 nickel ion leaching, and/or a twophase reaction, which severely degrade the rechargeability of the material.8 The use of aluminum as a dopant at moderate percentages helps reduce some of these undesired effects.8,12,13 Additional reported effects of high voltage cycling (above 4.2 V) in the LiNiO2 system include the breaking of cathode particles and associated loss of electrical connection,15 which in the NCA material leads to a faster material degradation.24 Therefore, the two cycling protocols used in this study were set with voltage windows of 3.0−4.1 and 2.7−4.2 V. Other studies have been conducted in this more limited voltage range9,25 using various current densities (C rates) during cycling.17,26,27 The uncycled material was initially thought to contain the nickel as Ni3+. A later study reported an additional minor population of Ni2+ in the layered nickel-oxide materials.28 This was further corroborated using NEXAFS measurements showing that Ni2+ ions are found on the surface of the NCA electrode.29,30 However, the relative fraction of Ni2+ ions in NCA cathode material was not yet determined; therefore, it is not known whether they have an important role electrochemically. Solid state nuclear magnetic resonance (NMR) has been employed very usefully in the last two decades to probe many different types of electrochemically active materials, mainly, but not limited to transition metal oxide cathodes and carbon-based anodes. Due to its ability to probe the immediate environment
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EXPERIMENTAL SECTION
Preparation of Sample Cells. The LiNi0.8Co0.15Al0.05O2 (NCA) studied in this work is a cathode material used by Tadiran Batteries in N-type cells. This material was used as the positive electrode in the preparation of the electrochemical cells. The electrodes comprised 88% NCA and the remainder polyvinyl difluoride (PVDF) and carbon
7595
DOI: 10.1021/acs.chemmater.6b01412 Chem. Mater. 2016, 28, 7594−7604
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Chemistry of Materials black. The electrodes were double-side coated on aluminum foil with an average loading of 9.8 mg/cm2 and a total thickness of approximately 80 μm. Prior to cell assembly, the electrodes were dried in a vacuum oven overnight at 120 °C. Pouch-cells were used in which the cathode and the anode were separated by polypropylene separators (Celgard). The cells were filled with an electrolyte solution of 1 M LiPF6 in EC/EMC = 3:7 (by weight) and sealed in an argon-filled glovebox. We assembled two types of cells comprising NCA cathodes, one with a graphite anode (full cells) and one with a lithium metal anode (halfcells). The full cells were cycled between 3.0 and 4.1 V at a C/5 rate for the first cycle and a C/3 rate for subsequent cycles (protocol A). The half-cells were charged to 4.2 V at a C/15 rate for the first cycle and cycled between 2.7 and 4.2 V at a C/10 rate in subsequent cycles (protocol B). Seven NCA electrode samples were prepared under protocol A, and six samples under protocol B, for analysis by 7Li and 27Al magic-angle spinning (MAS) NMR. The samples, collected at subsequent stages of cycling, are annotated by cycle number and protocol type (see Figure 1). The uncycled electrode was also analyzed for comparison (samples
Table 1. Details of Lithium Percentage in the NCA Cathode Samples at Each Voltage, As Calculated from the Capacity Data (Not Shown), and the Corresponding Average Oxidation State of the Nickel Ions Calculated from the Electrochemical Results, and Calculated from the 7Li NMR Shifts for Protocols A and B, as Shown in Figure 1 average Ni oxidation state sample
final voltage
1A 2A 3A 4A 5A 6A 7A
4.10 3.60 3.00 4.10 3.00 3.00
100 44.0 61.0 79.0 43.0 79.0 77.5
± ± ± ± ± ±
2.8 3.1 4.0 2.9 4.0 3.9
3.00 3.70 3.49 3.26 3.71 3.26 3.28
± ± ± ± ± ±
0.04 0.04 0.05 0.04 0.05 0.05
3.63 3.31 3.01 3.66 3.06 3.06
1B 2B 3B 4B 5B 6B
4.20 3.75 2.70 4.20 2.70
100 21.4 62.2 99.8 24.8 86.0
± ± ± ± ±
3.9 3.1 5.0 2.8 4.3
3.00 3.98 3.47 3.00 3.94 3.18
± ± ± ± ±
0.05 0.04 0.06 0.04 0.05
3.92 3.17
% Li in cathode
calculated from capacity
calculated from 7Li NMR shift
3.81
a
Note that the partially discharged samples (3A and 3B), despite their different states of charge (maximum final voltages of 3.6 and 3.75 V, respectively), resulted in nearly equal percentages of Li (partial) reintercalation: 61.0 ± 3.1% and 62.2 ± 3.1%, respectively. The larger differences in lithium reinsertion appear after one cycle in samples 4A and 4B and are a consequence of the anodes used (graphite vs lithium metal, respectively). recorded on a Bruker AvanceIII 500 MHz spectrometer at a Larmor frequency of 130.32 MHz. The spectra were collected with no external temperature control. All 27Al measurements were externally referenced to Al(NO3)2(aq). All 7Li measurements were externally referenced to LiCl(aq). 7 Li spectra were recorded using a spinning rate of 12.5 kHz, and 27 Al spectra were recorded at spinning rates of 9, 10, 10, 15, 15, 15, and 12.5 kHz for samples 1A, 2A, 3A, 4A, 5A, 6A, and 7A, respectively, and at 12.5 kHz for samples 1B−6B. The 7Li spectra were collected using single-pulse and rotor-synchronized Hahn-echo (90x-τ-180y-τacquire) sequences, employing a recycle delay of 1 s and a 90° = 1.9 μs. The 27Al spectra were collected using a rotor-synchronized solidecho (90x-τ-90y-τ-acquire) sequence, a recycle delay of 0.3s, and a 90° = 2.0 μs and processed using exponential multiplication with 1000 Hz line broadening. The uncycled NCA material 27Al solid-echo spectra, acquired at 130.32 MHz, were collected using frequency stepping of 1000 ppm (from −2000 to 2000 ppm), a 90° pulse of 5.5 μs, spinning rate of 12 kHz, 9k scans, echo delay period of 83.3 μs equal to the rotor period, and a recycle delay of 0.3 s. The signal was acquired with a dwell time of 1.6 μs and processing included a left shift of 202 points followed by exponential multiplication using a 4000 Hz line broadening. The uncycled NCA material 27Al solid-echo spectrum, acquired at 52.11 MHz, employed a 90° pulse of 2.0 μs, a spinning rate of 12.5 kHz, 196k scans, a transmitter frequency offset of −1000 ppm, an echo delay period of 80 μs equal to the rotor period, and a recycle delay of 0.3 s. The signal was acquired with a dwell time of 1.6 μs and processing included a left shift of 180 points followed by exponential multiplication using a 4000 Hz line broadening. Solid-echo measurements of delithiated material sample 5B (Figure 4 bottom) were done at 130.32 and 52.11 MHz using 90° pulses of 5.5 and 2.0 μs, respectively, and an interpulse delay of one rotor period. In both measurements, spinning rate was adjusted to 12.5 kHz, recycle delay was 0.3 s, and exponential multiplication with 400 Hz line broadening was used in processing.
Figure 1. (a, b) Potential profiles measured from NCA electrodes using cycling protocols A and B, respectively. Protocol A entailed a first charge at a C/5 rate and subsequent cycles at a C/3 rate. Protocol B entailed a first charge at a C/15 rate and subsequent cycles at a C/10 rate. The dots indicated on the curves are the points during cycling at which the cells were terminated and samples taken for NMR measurements. 1A and 1B). The experiments were conducted ex situ. For each sample, a cell was prepared and cycled separately in the constant current− constant voltage mode to the cutoff voltage labeled on the profile. The cells were then each held potentiostatically for 5 h at the specified potentials (see Figure 1a,b; constant voltage steps are not shown) to reach quasi-equilibrium before disassembly inside an oxygen-free drybox. The samples were then extracted from the cells, rinsed with dimethyl carbonate, and dried under vacuum. The samples were then packed inside the box into airtight NMR rotors for analysis. The values for the Ni oxidation states at each electrode state-ofcharge are reported in Table 1a,b and in Figure 5. They were calculated by subtracting the equivalents of remaining intercalated Li+ ions from the unchanged oxidation states of the O (−2 × 2 = −4), the Co (+3 × 0.15), and the Al (+3 × 0.05) and dividing by the number of equivalents of Ni (0.8). These calculations assumed no change in the Co or O oxidation states within the voltage window used. It is also reasonable, given the moderate cycling conditions employed, to neglect side reactions, which otherwise may have affected calculations of the Ni oxidation states. MAS NMR Measurements. 7Li and 27Al MAS NMR spectra were recorded on a Bruker AvanceIII 200 MHz spectrometer with respective Larmor frequencies of 77.73 and 52.11 MHz equipped with a 4 mm VTN CPMAS probe. Additional 27Al MAS NMR spectra were 7596
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Figure 2. 7Li single-pulse MAS NMR spectra for (left) samples 1A−7A run at a spinning rate of 15 kHz and (right) samples 1B−6B (data not normalized) run at a spinning rate of 10 kHz. Approximate peaks of paramagnetically shifted Li species are marked by empty diamonds and circles, diamagnetic Li signals (at ∼0 ppm) by the dashed line, and spinning sidebands of 0 ppm peaks by an asterisk. The 7Li NMR data were processed and fitted using the MestReNova program version 7.1.2−10008, from MestreLab Research. The 27Al NMR data were processed using the Bruker software, TopSpin, version 3.2. The 7Li single-pulse experiments were processed using no left shift, in order to capture the maximum amount of signal. The sample signal was recorded after a standard 6.5 μs dead time delay to avoid overlap with the ring-down; therefore, some baseline distortions in the spectra were encountered. These distortions were removed using a standard polynomial baseline correction algorithm in most cases and using a spline function in few spectra. 7 Li Hahn-echo experiments conducted on these samples (Figure S3a,b) exhibited differences in relative signal intensities from Li in diamagnetic environments and Li in paramagnetic environments, as compared to the single-pulse data.45 The signal intensities from surface versus bulk Li species in the samples are affected by disparate anisotropic bulk susceptibilities and T2 spin−spin relaxation times which make any reliance on intensity challenging.
generally due to two main reasons. One is the anode used, Li metal vs graphite. The other is the C rate used. The slower rate in protocol B allows for more efficient and therefore more complete relithiation. The voltage window used is large enough to have an insignificant effect on the Li content in the cathode. In terms of the cells’ deintercalation steps (charge), the anodes used (graphite or Li metal) play a minor role. The different upper charge voltages used and the different currents applied dictate the amount of lithium deintercalated from the cell. Therefore, the cells from the two protocols can be compared on that basis alone. Since it is only the NCA material that is under investigation in this study, it is only the amount of Li remaining in the material that is of relevance to our analysis. The cycling profile in protocol B is similar to previously reported NCA cathode cycling profiles.19,47 The cycling profile in protocol A exhibits some differences from these reported profiles due to the changes in the dynamics of lithiation when cycling is performed against graphite with relatively high C rates. The calculated number of Li equivalents and average nickel oxidation state (OS) at each state-of-charge (Table 1) were derived from the capacity value at each state. The OS was calculated as described in the Experimental Section. In protocol A, sample 2A was charged to 4.1 V and only 56.0 ± 2.8% of the lithium was extracted. Sample 2B was charged to 4.2 V resulting in 78.6 ± 3.9% of the Li being deintercalated. The latter sample reached near complete delithiation of the material, since 20% of the lithium must remain intercalated within the material at full charge due to the presence of 5% of Al and 15% cobalt, which are electrochemically inactive in this voltage range. Cathode samples 5A and 5B were taken at the end of the second and fifth charge to their cutoff voltages, respectively (4.1 V for 5A vs 4.2 V for 5B). The percentages of remaining lithium charge carriers in the cathodes at these states relative to the values after the first charge were: 43.0 ± 2.9% in 5A vs 44.0 ± 2.8% in 2A and 24.8 ± 3.8% in 5B vs 21.4 ± 3.9% in 2B. This indicates that, in each protocol separately, Li ions intercalated and deintercalated quite reversibly as the cycling proceeded. Samples 7A and 6B similarly reflect the state of the NCA material in the discharged state at the end of the fifth cycle. They both showed the expected decrease in reversible lithiation. A larger drop of 22.5 ± 3.9% was observed for the full cell in protocol A, compared to 14.0 ± 4.3% for the half-cell
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RESULTS AND DISCUSSION Electrochemical Cycling. Cycling profiles of NCA cathode material are shown in Figure 1 with points along the charge− discharge curves at which samples were taken for NMR analysis. These profiles represent comparison between two protocols, protocol A matching standard requirements in commercial cycling tests and protocol B chosen to follow typical laboratory cycling procedures. Protocol A employed cyclic charging to 4.1 V and discharging to 3.0 V, and protocol B employed charging to 4.2 V and discharging to 2.7 V, as shown in the figure. Additional differences in C rates and counter electrodes in the two protocols are given in the Experimental Section. Because of the differences in cycling conditions in the two protocols (the voltage window, cycling rates, and the counter electrode used), valid comparisons can be made as long as appropriate considerations are taken of the differences expected for each cycling procedure. Detailed descriptions of these measurements are given in the Experimental Section. It is evident from the potential profile in Figure 1b that the electrochemically active voltage window contributing to nearly all of the capacity is from 4.2 to 3.5 V, and in the range from 3.4 to 2.7 V, the number of Li ions deintercalated or intercalated is negligible. The latter phenomenon is represented in the curve by the constant vertical regions. The differences in content of charge carriers along similar steps in the two cycling profiles are 7597
DOI: 10.1021/acs.chemmater.6b01412 Chem. Mater. 2016, 28, 7594−7604
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Chemistry of Materials
discharged) samples contain less of the paramagnetic species Ni3+ and Ni2+, they exhibit less of this broadening phenomenon as indicated earlier for the NCA material27 and for the Al-free analog.40 The shift values of these resonances for the different samples are given in Tables S1 and S2. The upfield shift typically occurring upon successive Li deintercalation was observed, as a result of the oxidation of the Ni2+/Ni3+ to Ni4+, as was the subsequent downfield shift which occurs upon Li reintercalation during each discharge cycle and its associated conversion of the nickel back to low oxidation states. The largest shifts were seen for the pristine samples 1A and 1B, for which the average oxidation state of the Ni ions is 3.00, and for the discharged samples 4A, 4B, and 6B. Changes in the resonance shifts were observed in the electrode material with progressive cycling. The partially charged (delithiated) samples 3A and 3B show intermediate shifts, and the charged samples 2A, 5A and 2B, 5B exhibit the lowest shift values. The cycling conditions used in protocol A are very similar to those used in a previous 7Li NMR study of NCA.27 The resonance shifts measured (see Table S1) are also comparable to shifts reported for samples similar in both state-of-charge and capacity fade.27 When the cells are fully charged and all Ni ions nominally oxidize to diamagnetic Ni4+, the remaining lithium is not expected to exhibit a shift at all and, instead, is expected to resonate at or around 0 ppm. In the maximally charged samples from protocol B (2B and 5B), the lithium remaining in the system does not resonate at 0 ppm. In the maximally charged samples from protocol A (2A and 5A), complete delithiation was not reached and a significant Li shift is observed instead, reflecting the relative quantity of Ni2+/3+ that remains in reduced form. The shifts correlate with the difference in the average nickel oxidation states in the cells (further elaborated below) and highlight the marked differences upon charging to 4.1 V (protocol A) vs. charging to 4.2 V (protocol B). 7 Li NMR studies of similar LiNiO2 materials have shown that the large Li resonance shifts are the result of cumulative individual shifts imparted by nearest neighbor (n.n.) paramagnetic nickel ions in either the first or second coordination shell around the lithium ion. The differences between Ni2+ and Ni3+ paramagnetism (with d8 and d7 electron configurations, respectively) result in Fermi contact shifts that are twice as large in the former.51 Thus, each Ni3+ ion in the first coordination shell (1st n.n., a 90° Ni−O−Li interaction) shifts the resonance by −15 ppm and each Ni3+ in the second coordination shell (2nd n.n., a 180° Ni−O−Li interaction) by +110 ppm. Each n.n. Ni2+ ion in the first shell shifts by −30 ppm and each n.n. Ni2+ in the second shell by +170 ppm.40,51 However, a discrete distribution of the different lithium environments is mostly not seen, because, as the Ni3+ and Ni2+ ions are oxidized to Ni4+, electron hopping is enabled between the nickel states, and this occurs on a time-scale faster than the NMR sampling rate; therefore “average” shifts are observed.33,42 Assuming a defect-free layered structure, each lithium ion is surrounded by six first shell n.n. TM ions and six second shell n.n. TM ions. Since nickel ions constitute 80% of the TM ions, this translates into an average of 9.6 Ni ions, half in the first and the other half in the second shell surrounding the Li ion. Then, 7Li shifts can be calculated for the pristine materials. Assuming all Ni ions are trivalent, the average value of the first and second nearest neighbor shifts would result in a 456 ppm shift (4.8 × 110 + 4.8 × −15 ppm).
in protocol B. The former value reflects the partial discharge already noted above. The decrease in protocol B was slightly higher than expected for the material after only five cycles. The corresponding differences in the 7Li and 27Al NMR results of these samples will be described below. The differences in Li content and average Ni oxidation state are used in conjunction with NMR measurements in a semiquantitative manner to show different partitioning of Ni ions between diamagnetic (Ni4+) and paramagnetic (Ni3+, Ni2+) states at different cycling stages in the two protocols. NMR Studies of Pristine and Cycled Material: 7Li NMR. The 7Li spectra of the samples from protocols A and B are shown in Figure 2 with spectra representing analogous points along the cycling profiles shown in parallel and colored similarly. The peak at 0 ppm (marked with a dashed vertical line) of lithium in diamagnetic environments is attributed to unreacted precursor salts on the surface of the cathode or Li surrounded solely by Co and Al neighbors in the first and second coordination spheres, suggesting inhomogeneous TM ion distribution. The differences in the 0 ppm peak intensities in the spectra of the uncycled materials, 1A and 1B, are due to the presence of unreacted lithium-containing precursor materials (e.g., LiOH) indicating slight differences in sample preparation. The differences in the paramagnetically shifted peak are due to the different spinning rates used in the NMR experiments causing differences in the sample temperatures, which have an inverse effect on the paramagnetic shift.48,49 In the cycled samples, with Ni4+ formation, the 0 ppm peak can result from either lithium surrounded by diamagnetic TM ions (Ni4+, Al3+, Co3+) or Li-containing surface breakdown products such as LiOH, LiF, and others forming the solid electrolyte interphase (SEI). The diamagnetic peaks in the various spectra exhibit broad spinning sideband manifolds (marked by an ∗) which span up to 2000 ppm in some samples. These very broad patterns are the result of the through-space interactions of the Li nuclei with unpaired electrons of Ni3+/ Ni2+ ions that are distant (beyond the second coordination sphere) and therefore are not causing a Fermi contact shift. The breadth of the manifold reflects the strength of the effective dipolar-like interaction, which depends on the number of Ni3+/ Ni2+ ions and their distances from the Li nucleus.42 In both protocols, these sideband manifolds narrowed substantially with each delithiation step (2A, 3A, 5A and 2B, 3B, 5B) as the amount of paramagnetic Ni decreased and the distances to the Li nuclei increased with charge carrier extraction. The manifolds broadened again with each relithiation step (4A, 6A, 7A, 4B, 6B) as charge carriers were inserted back into the NCA material and Ni was reduced. Apart from the sharp line at 0 ppm common to all spectra, at least one other prominent line was observed that varied in position and line width between different samples. To determine the resonance shifts, careful line deconvolutions were conducted on all samples. Samples 2A, 3A, 5A, 2B, 3B, and 5B were fitted reliably. The very broad spectra of the pristine and relithiated samples were fitted crudely, in order to determine an approximate value for the main line intensity. These fittings are indicated in Figure S1. The changes observed in the line width of the main (nonzero) peak include a narrowing from ∼15 to ∼7 kHz and are also the result of neighboring paramagnetic Ni ions. The dipolar broadening mechanism is known to affect the 7Li signal,50 and as the cycled (fully charged and partially 7598
DOI: 10.1021/acs.chemmater.6b01412 Chem. Mater. 2016, 28, 7594−7604
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below the Ni OS derived from the electrochemical data. This low value of NMR calculated Ni OS, as aforementioned, may result from existence of an electrochemically inactive phase. The spectra of the relithiated samples having similar SOC, i.e., 4A vs 4B, as well as 6A, 7A vs 6B are interesting to compare vis-a-vis. In the “A” series, the 21.0% decrease in reversible lithiation is evident in the NMR spectra, where the main bulk shift is reduced in intensity reflecting less lithium ions reintercalated in the bulk material. However, the spectra of 6A and 7A are almost identical to that of 4A. In terms of the signal intensity, this is expected, because the amount of lithium does not change considerably. Also, no additional resonance shift is seen, indicating that the system has reached an invariant state and so the lithium environment remains unchanged. In the “B” series, a shift is not seen in spectrum of 4B as compared to that of the pristine material. This is expected because the relithiation is 99.8 ± 5.0%. However, in the spectrum of 6B, fully discharged after five cycles, the lithiation is only 86.0 ± 4.3%, yet the 7Li NMR spectrum, while lower in intensity, has practically the same resonance shift as in the spectrum of the pristine material. An overlay of these spectra, normalized to equal intensity, renders them indistinguishable (see Figure S2b). This suggests, then, that even with 86% reintercalation in the fifth cycle, in protocol B, the Li ions reattain the same environment as in the pristine state. This is in contrast to the data from protocol A (see Figure S2a) in which the spectra of the relithiated samples are very similar to each other but very different from the pristine state. Additional relevant information about the changes to the bulk spectra were revealed in the Hahn echo data collected (shown in Figure S3a,b). In protocol A, while a large signal centered around 450 ppm is seen in the pristine sample (1A), almost no bulk signal is seen in the relithiated samples (4A, 6A, and 7A). In contrast, the relithiated samples in protocol B (4B and 6B) exhibit significant intensity, equivalent to approximately 80% and 65%, respectively, of the original intensities. The explanation for these differences is due to the mobility of the 7Li ions in the bulk material.42 Because of the changes induced in the spin system during the time between the echo pulses (τ), highly mobile 7Li ions may not recover by the echo. Thus, the relithiated samples from protocol A exhibit charge carriers that are more mobile, as compared to those from protocol B. Increased motions of Li ions in layered oxide materials is generally expected upon delithiation, as the number of holes available for ion-hopping is increased. Thus, some differences in the samples with 99.8% lithium versus 79.0% are to be expected. However, it is quite surprising to compare the signals of 6A and 6B, whose lithium contents are 79.0 ± 4.0% and 86.0 ± 4.3%, respectively, differing by only 7.0 ± 5.8%, but whose respective 7Li NMR responses may indicate very different Li motions. Relaxation measurements are required to further assess that explanation. NMR Studies of Pristine and Cycled Material: 27Al NMR. While the 7Li signal followed a predicted pattern, the 27 Al NMR results of the material over the course of the first few charge/discharge cycles were in some cases unexpected and potentially illuminating in terms of the local changes occurring over the course of the cell cycling. All of the prepared samples showed a peak in their 27Al spectra at approximately 1640 ppm (not shown) which was the signal from metallic aluminum shifted downfield by Knight shift,52 a contamination originating from scraping off the
However, the shifted peaks in the pristine materials are centered at a somewhat higher value, i.e., ∼500 ppm. The additional shift can possibly be explained either by an overall 0.8:0.2 partitioning of Ni3+/Ni2+ (0.8 × 456 + 0.2 × 672) or by existence of biphasic distribution of Ni3+ in material, with 39% in a Co/Al free phase and 61% in a stoichiometric NCA phase (0.61 × 456 + 0.39 × 570). Different scenarios with a mix of several phases and other fractions of divalent Ni, of course, can explain the resonance shift obtained. The difficulty in accurately determining experimentally this resonance shift from the 7Li spectra leaves this figure as an estimated value only. Moreover, since they involve either inhomogeneous distribution of ions or existence of divalent Ni ions, the average OS of these samples does not appear in Table 1. Sample 2A, charged to 4.1 V, indicates a peak at 171 ppm. Assuming now that all Ni2+ ions have oxidized at this voltage means that the shift depends only on the Ni3+ amount. Then, the 171 ppm corresponds to 3.6 n.n. Ni3+ ions and therefore 6 n.n. Ni4+ ions, giving an average Ni OS of 3.63, which is almost within error of the OS derived from the capacitance (3.70 ± 0.04). Sample 2B, charged to 4.2 V, indicates a peak at 36 ppm, corresponding to a nearly fully discharged state. Using a similar argument, the calculated average Ni OS from the NMR shift gives 3.92, also slightly outside of the error of the electrochemically calculated value. After multiple cycles, the resonance shift of the lithium retained in the cathode changes. After two cycles under protocol A, the shift decreases from 171 (2A) to 155 ppm (5A), corresponding to the decrease in calculated lithium (44.0% to 43.0%), while in protocol B, after five cycles, the peak increases from 36 (2B) to 84 ppm (5B), in correspondence with the increase in lithium from 21.4% to 24.8%. Again no Ni2+ is expected to remain at this voltage, and thus, the Ni OS is calculated from the NMR shifts by Ni3+ ions as 3.66 (5A) and 3.81 (5B). The value for 5A is within error of the electrochemically calculated value, while the 5B value is somewhat lower. It was suggested earlier that, in cycled NCA, regions that are electrochemically less active are formed.30 These regions with Ni ions in the reduced state would be invisible to the electrochemical measurement but would still contribute to the shifted Li resonance and lead to the lower Ni OS obtained from the NMR. Spectra from samples 2A and 5A both indicate small shoulder peaks. Sample 2A indicates a shoulder at 111 ppm constituting ∼8.3% of the integrated intensity of the main peak, which translates into a Ni OS of 3.76. Sample 5A indicates a shoulder at 102 ppm that is 5% of the total intensity corresponding to a Ni OS of 3.78. These data are indications of segregated regions within the bulk material, which have oxidized differently. The decrease in this ratio from 2A to 5A may suggest a reduction of these segregated regions, i.e., a homogenization of the material over the course of the second discharge/charge cycle. Samples 3A and 3B, which have nearly equal amounts of lithium, both indicate shifts reflective of their partially discharged state. The difference in their shifts, 313 vs 376 ppm, is mainly attributed to the disparate spinning rates used in the measurements. These samples are similar to the 40% SOC sample in the Kerlau study,27 which underwent two cycles during preparation before the final (partial) discharge and indicated a 7Li shift at 330 ppm. Again, neglecting Ni2+ in the relithiated state, the NMR calculated average Ni OS of these samples is 3.31 and 3.17, respectively. Both of these values fall 7599
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Figure 3. 27Al solid-echo MAS NMR spectra recorded at spinning rates given in the Experimental Section for samples 1A−7A (left) and for samples 1B−6B (right). Open circles, diamonds, and squares indicate approximate locations of isotropic resonances; asterisks (∗) indicate spinning sidebands. Data 1B−6B were not normalized.
electrode from its Al foil current collector. The 27Al NMR spectra of the pristine NCA material exhibited extremely large negative shifts (−770 to −1150 ppm), suggesting that a strong electron-shielding interaction is affecting the 27Al nuclei (see Figure 3). A recent study of NCA materials reported 27Al spectra with similarly large negative shifts.45 Several Li NMR studies on lithium transition metal oxide materials have indicated negative shifts from through-bond hyperfine interactions with paramagnetic ions.40,42,55−57 In one study, the 6Li MAS NMR of La4LiMnO8 and La3SrLiMnO8 indicated resonances with large negative hyperfine shifts of −491 and −500 ppm, respectively.57 In a 119Sn NMR study of a series of lanthanide stannates (Ln2Sn2O7), negative Fermi contact shifts as large as −4150 ppm were reported.55 The resonance shift is associated with a hyperfine (Fermi contact) coupling similar to the 7Li nuclei. However, since the Al ions occupy TM positions in the layered structure, its orbital overlap with vicinal Ni assumes a 90° Ni−O−Al configuration. In this form of interaction, the Ni electrons induce a net negative spin density on the electronic orbitals of the Al ion. The latter is reflected by a substantial shielding effect on the nuclei and a large negative shift.53−55 These shifts are larger than those observed for Li nuclei in a similar electronic configuration (90° Ni−O−Li). A more in-depth explanation of these interactions and their calculations from first-principles based on the Goodenough-Kanamori super exchange rules can be found elsewhere.42,53,58 To corroborate that the hyperfine interaction is the predominant factor causing these large shifts, the sample was run at elevated temperatures and at two different magnetic fields. The elevated temperature (data not shown) caused a clear downfield shift, in accordance with the inverse temperature dependence of the Fermi contact shift. The measurements at 11.7 and 4.7 T were used to explore quadrupolar vs paramagnetic mechanisms of line broadening. 27 Al nuclei (I = 5/2) have a significant quadrupole moment Cq = 14.66 Q/fm2, resulting in a large coupling interaction that markedly affects the spectra of aluminum found in nonsymmetric electronic environments. The quadrupole coupling in 27Al is typically 100 kHz to 2 MHz. Therefore, the NMR
central transition (−1/2 to 1/2) resonance is shifted and broadened by a non-negligible second-order quadrupolar term. While this interaction can potentially reveal further information on bond structure around the nucleus, the extra shift and reduced resolution add further complication to interpretation of the 27Al signals. In the presence of another large interaction, e.g., the Fermi contact coupling, it is challenging to isolate and determine individual contributions. The rotor-synchronized solid-echo pulse sequence (θ1 = 90°x, θ2 = 90°y) was employed to measure the 27Al spectra of the NCA samples shown in Figure 3. The sequence, which serves to refocus some of the quadrupolar broadening and suppress the broadening from quadrupole satellite transitions, generated sensitive spectra and showed the off-resonant signals of the bulk aluminum barely discernible in the single pulse experiments. The line widths in the spectra were about 10−40 kHz. Due to the limited resolution of samples 1A and 1B, the resonance shifts listed in Table S2a,b and plotted in Figure 5 have a substantial degree of uncertainty. To examine the contributions of hyperfine versus the quadrupolar couplings to line width, solid-echo measurements were performed on the pristine NCA 1B sample (Figure 4 top) and on the cycled, charged 5B sample (Figure 4 bottom) at 4.7 and 11.7 T. The spectrum of 1B at 11.7 T is a panoramic reconstruction of four spectra measured at four different resonance frequencies as explained in the Experimental Section.59 The 27Al spectra of the pristine sample indicate a similar resonance frequency for the negatively shifted line at around −1000 ppm. The spectrum recorded at 4.7 T (blue) appears broader on the ppm scale but narrower on an absolute frequency scale (see Figure S4). This increase in line width with field strength points to the hyperfine coupling (which grows with the field) as the main source of line broadening in the pristine material spectrum. Line width dominated by the second order quadrupolar interaction (which scales inversely with the field) would be expected to decrease going to higher magnetic field. For sample 5B, the 27Al spectra indicate a shift in the main peak from 1.3 ppm (at 4.7 T) with line width of 4.5 kHz to 47.4 7600
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shifts. The Al ion, occupying an octahedral site in the TM layer, is in a 90° Ni−O−Al orbital configuration, thus experiencing a negative spin−polarization interaction. This interaction was assigned a shift of the Al resonance by ∼−100 ppm.45 The cumulative shift is obtained by adding up all interactions with the n.n. Ni3+ ions. Neglecting possible effects of the presence of Ni2+ (which would likely cause a larger shift, as seen for the 7Li) and assuming 12 interactions with the 6 surrounding ions, this could explain the two shifts, i.e., −980 and −1070 ppm, as two phases in the NCA. Nondistorted phase of homogeneously distributed Ni/Co/Al would result in a shift of −960 ppm (80% of the 12 nearest neighbors being Ni3+), explaining the −980 ppm resonance observed in our data. While a phase completely devoid of Co ions, e.g., with only Ni3+ ions surrounding the Al dopant would predict a peak at around −1200 ppm, the −1070 ppm peak seen herein, therefore, would correspond to partial (∼50%) cobalt segregation, approximately 10.5 n.n. Ni3+, meaning ∼87%. This existence of the Ni-rich second phase, in turn, implies partial cobalt segregation, a phenomenon reported in the Alfree alloy of such materials and could explain the diamagnetic Al line in the pristine material spectra in Figure 3. Unlike in the lithiated case, the 27Al shifts in the delithiated samples could not be employed, on the basis of the proposed value of −100 ppm, to extract calculated Ni OS, and therefore, their analysis is left out. However, it is noteworthy that, in the relithiated (recharged) samples, these two distinct peaks (NCA and cobalt-depleted NCA) reappear, implying that these phases remain intact even after several charge/discharge cycles. These two regions may correspond to the phases identified in the pristine material from XRD61 and XANES measurements,19,30 though the issue of having divalent Ni ions and their ability to shift the Al resonance needs further examination. Upon electrochemical charging of the NCA material, the 27Al signals shift downfield to less negative resonance values (see Figure 3). As with the gradual shift seen in the 7Li NMR measurements, the change in the shift of the 27Al signal also reflects the gradual oxidation of Ni2+/3+ to diamagnetic Ni4+. In contrast to the 7Li data, almost no intensity was seen at 0 ppm in the 27Al data of the pristine material, indicating that all of the aluminum is in the bulk; no unreacted aluminum salts remain, nor do they form upon cycling. This is unlike results reported in a recent study on the NCA cathode where prominent diamagnetic species were observed in the 27Al spectra.45 A few comparisons of the samples from the two different protocols are worth noting. It is important to point out that most of the variations seen in the spectra, particularly with respect to the extra peaks seen in protocol B samples compared to the spectra from protocol A, are due to the decrease in the spinning rate used, so that more spinning sidebands are visible, and anisotropic interactions are less efficiently averaged out. Samples 2A and 2B both indicate a positive resonance shift value of 15 and 222 ppm, respectively, and a substantial reduction in peak width. The sizable decrease in peak width suggests that it is mostly diamagnetic Ni4+ ions that are located in the two nearest coordination shells of the 27Al nuclei in these samples. The major downfield shift to a slightly deshielded environment may suggest that residual paramagnetic ions found in the Al coordination shells in these samples are in a different TM−O−M+ configuration so as to induce a positive spin polarization on the nuclei. The incomplete charge associated with protocol A and the subsequent limited oxidation of
Figure 4. 27Al solid-echo spectra recorded at 130.3 MHz (11.7 T) (black) and 52.1 MHz (4.7 T) (blue) of (top) uncycled NCA material and (bottom) delithiated material (sample 5B). In the top spectra of the pristine material, the experiment at 11.7 T was collected using frequency stepping and at 4.7 T was recorded normally, using a transmitter frequency offset of −1000 ppm.
ppm (at 11.74 T) with line width of ∼4.2 kHz (see also Figure S5). The field independent line width implies mutual cancelation of the opposite effects from the hyperfine and the quadrupolar couplings on line width. In turn, it suggests that the resonance shift depends on the two interactions and precludes the use of the two shift values in the two fields for extraction of the quadrupole coupling constant using eq 1.60 ω (2)iso 1 1 = − − , 2 2
3ΩQ 2
3 ⎛ 1 ⎞ I(I + 1) − } 1 + η { 10ω 4 ⎝ 3 ⎠ L
⎜
2⎟
(1)
Interestingly, the contribution from quadrupolar coupling to the spectra could not be separated out from the paramagnetic one even for the extensively charged (mostly diamagnetic) sample. Coming back to the spectra in Figure 3, another prominent feature of the 27Al data is that the pristine material exhibits at least two distinguishable resonances, at approximately −980 and −1070 ppm, indicative of two different configurations of first and second n.n. Ni ions around the Al3+ ion. These isotropic shifts in the pristine material were determined via measurements at different spinning rates. In their recent 27Al measurements of the NCA material, Dogan et al.45 suggested a quantitative analysis of similar large negative 7601
DOI: 10.1021/acs.chemmater.6b01412 Chem. Mater. 2016, 28, 7594−7604
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Figure 5. Progressive 27Al (left) and 7Li (right) resonance shift plots as a function of cycling, showing the reversible/nonreversible changes in the spectra of the NCA material subjected to cycling protocol A (blue) and B (red) and demonstrating the larger shift values spanned by the latter protocol.
paramagnetic Ni ions result in a larger fraction of residual Ni2+/ Ni3+ in this sample versus 2B. The spectra of samples 3A and 3B are similar, as was the case with the 7Li data. One noticeable difference is that sidebands are unresolvable in 3A due to an additional broadening process. Both spectra indicate a negative shift reflecting relithiation and subsequent reduction of the Ni4+ to its paramagnetic state. However, both spectra also indicate some remaining intensity at 0 ppm, suggesting a nonhomogeneous relithiation in the vicinity of the Al3+ ions. Samples 4A and 4B indicate results that are analogous to their 7Li NMR counterpart spectra. As expected and in correlation with the extent of reversible lithium intercalation under the two charging conditions, the shift of the signal indicated in the sample from protocol A similarly does not return to its pristine state upon a full cycle of the material, while the signal from protocol B does. Samples 6A and 7A separated by three cycles exhibited similar 27Al spectra, demonstrating that the aluminum environment is nearly unchanged in the discharged state from the end of the second cycle forward, although this state is of partial discharge compared to the pristine material as judged by their smaller shift in absolute terms. Samples 7A and 6B exhibited interesting variations from the lithium. The 27Al signal in sample 7A (and 6A, in fact) did not show the same shift as in sample 4A, although the amount of Li is the same. This further suggests the inhomogeneous relithiation of the cells under protocol A. In contrast, the 27Al data of the relithiated cells in protocol B exhibited similar behavior to the 7Li results: the five-timescycled sample is almost identical to the data after only one cycle, demonstrating that the NCA material cycles back to a similar state as observed from both the dynamic charge carrier spectra (7Li) and dopant spectra (27Al). The NMR results and the respective effect on the resonance shift of the different paramagnetic Ni environments experienced by the charge carrier (7Li) and dopant nuclei (27Al) as a function of the two cycling protocols performed in this paper are summarized in Figure 5. This form of progressive chart is used to represent some of the effects seen in the NMR spectra and is suggested as a useful tool to illustrate the performance of the battery material. It also shows the reversibility or irreversibility of the changes imposed by the electrochemical cycling on the NCA material in terms of the changes to the partitioning of Ni ions between the different oxidation states.
It underscores the larger span of resonance shifts obtained for protocol B in accordance with a larger variation in Ni OS imposed by a larger potential difference and other cycling conditions and the deviation of sample 3B from the overall trend of the samples in protocol B. It also shows that, within the limited cycling window used in protocol A, aluminum serves as a better proxy of the progressive changes in the material than Li. Similar plots can be constructed for other cathode materials in order to infer concise informative data from the NMR on the cycling behavior of the oxide material that relates to its performance.
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CONCLUSIONS This work offers some novel insights into the commercially important NCA cathode material, which may be useful in improving its long-term performance in lithium-ion batteries. One of the findings is that, despite its inactivity, the 27Al serves as a distinct, static marker of the changes in the local electronic environments in the bulk material, analogous to the way 6,7Li has been used as a dynamic probe. The 27Al NMR data of the pristine material also indicates the existence of at least two distinct configurations of Ni ions around the Al dopant ions, which are not averaged out by electron hopping. These two configurations, approximately equal in magnitude, can be explained by significant cobalt segregation and/or the presence of a significant amount of Ni2+. It is furthermore indicated that these distinct domains remain intact upon cycling, even after several charge cycles. It is possible that the existence of these regions corresponds to the biphasic scenario suggested to exist in the pristine material via EXAFS.30 Neither the pristine configurations around the Li+ nor around the Al3+ environments are recovered in the charge− discharge cycle in protocol A, reflecting the irreversibility, on the electronic level, in samples cycled under these specific conditions. In contrast, the results from the slower cycling used in protocol B indicate that both the Li+ and Al3+ environments return to their pristine configurations upon subsequent discharge to low voltage. It was significant that, in correspondence with the changes in nickel oxidation states occurring during cycling, the 27Al NMR signal followed a similar (but opposite in sign) pattern of resonance shift variation typically seen in 7Li NMR of such materials. All the more so, comparative analysis underscored the differences that Al and Li ions are experiencing in terms of their electronic environments throughout the course of cycling and between different protocols. Progressive resonance shift 7602
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plots illustrate these effects vividly and provide a useful form of NMR data presentation, immediately expressing the variation with cycling in various electrodes in a compact form.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01412. Tables with 7Li and 27Al resonance shift values for all samples; line deconvolutions of samples 2A, 3A, 5A, 2B, 3B, and 5B; 7Li single pulse and Hahn echo spectra of pristine and relithiated samples; 27Al solid-echo spectra of samples 1B and 5B measured at 11.74 and 4.7 T (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Ronit Lavi for fruitful data discussions and Dr. Keren Keinan-Adamsky for assistance in operating the NMR spectrometers.
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REFERENCES
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Article
Chemistry of Materials
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DOI: 10.1021/acs.chemmater.6b01412 Chem. Mater. 2016, 28, 7594−7604
