Structural Stability of LiNiO2 Cycled above 4.2 V Chong S. Yoon,† Do-Wook Jun,‡ Seung-Taek Myung,∥ and Yang-Kook Sun*,‡ †
Department of Material Science and Engineering, Hanyang University, Seoul 133-791, South Korea Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea ∥ Department of Nano Engineering, Sejong University, Seoul 143-747, South Korea ‡
S Supporting Information *
ABSTRACT: A spherical stoichiometric LiNiO2 particle, which was composed of compactly packed nanosized primary particles, was prepared and cycled at different cutoff voltages to explicitly demonstrate the effect of phase transitions during Li deintercalation/intercalation on the Li-ion intercalation stability of LiNiO2. The capacity retention was greatly improved by suppressing the H2 → H3 phase transition at 4.1 V, such that 95% of the initial capacity (164 mAh g−1) was retained after 100 cycles when cycled at 4.1 V. At 4.2 and 4.3 V, continuous capacity loss (81% of 191 mAh g−1 at 4.2 V and 75% of 232 mAh g−1 at 4.3 V after 100 cycles) was observed during cycling, and these electrodes incurred extensive structural damages (micro-, hairline and nanoscale cracks observed by transmission electron microscopy) from the repeated lattice contraction and expansion accompanying the H2 → H3 transition, in agreement with the cycling data.
B
underwent four phase transitions consisting of three singlephase regions for 0.0 ≤ x ≤ 0.75: a rhombohedral phase (H1) for 0.0 ≤ x ≤ 0.25, a monoclinic phase (M) for 0.25 ≤ x ≤ 0.55, and a rhombohedral phase (H2) for 0.55 ≤ x ≤ 0.75; one twophase coexistence region; and two rhombohedral phases (H2 + H3) for 0.75 ≤ x ≤ 1.0. Li+ ion intercalation stability in the single phase was outstanding up to x = ∼0.75 due to a small variation of the interlayer distance between NiO2 sheets from ∼4.73 to ∼4.80 Å. Further charging, however, induced ∼0.3 Å shrinkage in the interlayer distance due to emergence of a new H3 phase brought by Ni4+ with smaller ionic radius, which deteriorated the cycling performance. The requirement for significant improvement in energy density of the LIBs for EVs has recently rekindled the interest in high-capacity LNO. In this study, stoichiometric LNO is prepared to demonstrate explicitly the effect of the phase transitions, especially the formation of the H3 phase, on the cycling stability by charging the LNO cathode to a series of increasing upper cutoff voltages. The cycling performance of the LNO at different charging voltages is correlated to the structural damages incurred during cycling by examining the cycled electrodes using transmission electron microscopy (TEM) and X-ray diffraction (XRD). Figure 1a shows the SEM images of the as-prepared LNO particles synthesized by lithiation of the Ni(OH)2 with LiOH·
ecause of their high-energy density, lithium-ion batteries (LIBs) are able to power a large array of applications for portable electronic devices and to provide an energy storage medium to resolve the intermittent power supply problem of renewable energy sources. LIBs have also emerged as the most promising battery for electric vehicles (EVs). Although LIB-driven EVs are currently available, widespread use of electromobility still remains elusive mostly due to the limitation of the LIB’s technology.1 The most important technical challenges facing the current state-of-the-art LIBs for EVs are the cost, durability, and driving range, which are strongly dependent on the cathode performance. Among the existing cathodes for LIBs, the LiNiO2 (LNO) layered oxide, isostructural with LiCoO2 (LCO), has been considered as an alternative cathode because of its high reversible capacity, good rate capability, and relatively low material cost compared to LCO. The LNO cathode was extensively studied in the 1990s;2−10 however, it was notoriously difficult to synthesize stoichiometric LNO because the material decomposed to a lithium-deficient Li1−xNi1+xO2 during high-temperature calcination due to the instability of Ni3+ ions.6,9,10 The tendency for formation of antisite defects from occupation of Ni2+ ions in the lithium sites, which hinders Li+ diffusion, decreased the overall cell performances.9 In addition, unlike LCO, LNO undergoes several reversible phase transitions during Li+ intercalation and deintercalation,4,11,12 which can lead to severe capacity fading and adversely impact the Coulombic efficiency. The structural variation of the LNO has been intensively investigated using ex situ XRD by Ohzuku et al.4 who reported that Li1−xNiO2 © 2017 American Chemical Society
Received: April 7, 2017 Accepted: April 24, 2017 Published: April 24, 2017 1150
DOI: 10.1021/acsenergylett.7b00304 ACS Energy Lett. 2017, 2, 1150−1155
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type half-cell at a rate of 0.1 C with an upper cutoff voltage ranging from 4.1 to 4.3 V. The initial charge−discharge curves are shown in Figure 2a. At 4.1 V, the discharge capacity was 179 mAh g−1, which progressively increased as the cutoff voltage was raised such that 227.0 mAh g−1 for 4.2 V and 246.6 mAh g−1 for 4.3 V were extracted. The cycling data shown in Figure 2b show that the discharge capacity of the cell charged at 4.2 and 4.3 V dropped continuously as the cycling proceeded, resulting in capacity retention of 81.1% for 4.2 V and 75.2% for 4.3 V after 100 cycles. Meanwhile, the cells charged to 4.1 V nearly maintained their respective initial discharge capacities of 95% after 100 cycles; hence, there was a clear demarcation cutoff voltage below which the LNO cathode will cycle reversibly with minimal capacity loss. It has previously been reported that the maximum rechargeable capacity for LNO would be 200 mAh g−1, which corresponds to x = 0.72 in Li1−xNiO2, because further extraction of Li+ ions led to a large contraction in the interlayer distance at x = ∼0.8. Abrupt extension of the lattice during the following discharging process will likely induce structural damage and cause subsequent large irreversible capacity loss.4 For the cells charged above 4.1 V in this work, the initial charge capacities were beyond this limit, x = 0.86 for 4.2 V and x = 0.93 for 4.3 V; hence, the cathodes went through a detrimental phase transition during discharging, resulting in the continuous capacity loss observed in Figure 2b. To explicitly show the series of structural changes of LNO during charging/discharging, differential capacity curves (dQ dV−1) profiles obtained by differentiating the initial charge/ discharge curves are shown in Figure 2c−e. For the dQ dV−1 curve for the LNO cathode charge to 4.1 V, as can be seen from Figure 2c, the H3 phase transition was completely suppressed and this suppression of the detrimental
Figure 1. (a) SEM images of the as-prepared LNO cathode particles with the inset showing the nanosized primary particles and (b) the resulting Rietveld refinement data of the XRD spectrum for the asprepared LNO powders: a = 2.8794(1) Å, c = 14.2120(2) Å, unit volume = 102.046 (4) Å3, Ni2+ in Li layer = 2.2%, Rp = 1.46%, and Rwp = 2.49%.
H2O. The LNO particles are spherical with a particle diameter of ∼10 μm, and each particle, in turn, is composed of ∼200 nm sized secondary particles that are packed compactly into a spherical geometry, as shown in the inset in Figure 1a. Such secondary structure would minimize the contact surface between the electrolyte and cathode and thus minimize the damaging parasitic reactions of the electrolyte on the cathode surface.13,14 The XRD spectrum in Figure 1b indicates that the LNO particles had a typical R3̅m layered structure with no impurity phases and lattice parameters of a = 2.8794(1) Å and c = 14.2120(2) Å, which agree with previous literature values for LNO.15,16 The stoichiometry of the sample was also verified by inductively coupled plasma (ICP, OPIMA 8300, PerkinElmer) to be Li1.01±0.02NiO2. To study the effect of the cutoff voltage on the electrochemical activity, the LNO cathode was cycled in a 2032 coin-
Figure 2. Electrochemical performances of LNO cathodes at different upper cutoff voltages from 2032 coin-type half-cells adopting Li metal as the anode: (a) initial charge and discharge curves at 0.1 C (18 mA g−1), (b) cycling performance at 0.5 C. (90 mA g−1), and differential capacity vs voltage curves from the initial charge and discharge curves at upper cutoff voltages of (c) 4.1, (d) 4.2, and (e) 4.3 V. 1151
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ACS Energy Letters phase transition was clearly reflected in the excellent Li+ ion intercalation stability for the cells charged to 4.1 V. The dQ dV−1 profiles of the cells charged to 4.2 and 4.3 V in Figure 2d,e, on the other hand, exhibited well-resolved redox peaks corresponding to M, H2, and H3 transitions, with the H3 transition occurring at 4.15 V. The measured dQ dV−1 curve matched well the previous results.11,12 The appearance of the H3 transition peak for the cells charged to 4.2 and 4.3 V coincided with the quick deterioration of the discharge capacity observed in Figure 2b, suggesting that the H3 phase transition is indeed responsible for the capacity fading above 4.1 V. To confirm the series of structural changes during cycling, the dQ dV−1 curves were measured during charge/discharge until 100 cycles at 4.1, 4.2, and 4.3 V (see Figure S1). XRD analysis of the LNO cathodes after charging was also used to confirm the phase transition (see Figure S2). After charging to 4.1 V, the c-axis lattice parameter expanded to 14.4012(6) Å from 14.2120(4) Å in the as-prepared state. When charged to 4.2 and 4.3 V, the interlayer distance, however, considerably contracted to 13.8897(8) and 13.5867(8) Å, respectively. The lattice contraction agrees well with the progressive appearance of the H3 phase when charged to increasingly higher cutoff voltages. Note that abnormal broadening of XRD peaks and inversion of (101) and (104) peaks in intensity were observed particularly for the electrodes charged to 4.2 and 4.3 V (Figure S2). For 4.3 V, such a tendency is more pronounced, as evidenced by the further increase of the intensity of the (101) peak relative to the (104) peak. Rietveld refinement of the XRD spectrum for Li0.06NiO2 charged to 4.3 V indicates that the (101) and (110) peaks did not fit well to the R3̅m space group, which is the crystal structure of the H3 phase. The refinement was also carried out assuming two phases, R3̅m (O3) for Li0.06NiO2 (a = 2.8165(2) Å and c = 13.5867(8) Å) and P3̅m1 (O1) (a = 2.8142(6) Å and c = 4.3125(8) Å) because the O1 structure was also observed in the deeply delithiated LixCoO2;17 however, the two-phase refinement was not successful for the present case. Croguennec et al.18 pointed out that the discrepancy of the (101) peak in delithiated Li0.06NiO2 was likely associated with the presence of a NiO2 phase that produced an O1 structured AB stacking fault in the O3 layer structure. High-resolution TEM (Figure 3) shows the presence of both a single-phased Li0.06NiO2 (R3m ̅ , region 1) region and an R3m ̅ region with numerous stacking faults (region 2). From the result, it is conjectured that the formation of the NiO2 phase with a smaller interlayer distance was likely related to the cause of stacking faults in the highly delithiated Li0.06NiO2, and this phase transition accompanying the drastic variation in the caxis, in turn, could be a reason for acceleration of the structural distortion at the microscopic level. The dramatic effect of the detrimental H3 phase transition on the mechanical stability of the electrode can be seen at the secondary particle level, as shown in the cross-sectional SEM images of the LNO particles at the first charge end at 4.1−4.3 V in Figure 4 and after 10 cycles (see Figure S3). While the LNO particles were undamaged when charged to 4.1 V, some of the particles from the LNO cathodes charged to 4.2 and 4.3 V contained developed large cracks transversing the entire particle even at the initial charge state19,20 and after 10 cycles. In addition to the mechanical damages observed in Figure 4, to correlate the cycling data with the microscopic-level structural damage incurred by the LNO cathode during cycling, the LNO cathodes after 100 cycles were recovered and the
Figure 3. (a) Electrochemically delithiated Li0.06NiO2 cathode by charging to 4.3 V (first cycle); the Fourier transformed images shown as insets attest to the structural damage incurred by the cathode even in the first charge end. Region I maintained the R3̅m structure, whereas region II consisted of numerous stacking faults resulting from formation of the P3̅m1 (O1) phase. (b,c) Crystal structure of regions I (O3) and II (O3 + O1).
Figure 4. Cross-sectional SEM images of the initial charge-ended LNO cathode at (a) 4.1, (b) 4.2, and (c) 4.3 V at different magnifications.
particles that remained largely intact were selected and examined using TEM. Bright-field STEM images of the cycled LNO cathodes that were charged to different cutoff voltages shown in Figure S4a−c clearly evince a major crack, initiated from the surface, traversing across the primary particle to the particle core for the cathode charged to 4.3 V. Meanwhile, no easily discernible cracks were observed from the cathode charged to 4.1 and 4.2 V after 100 cycles. However, upon close examination by dark-field STEM imaging, which accentuates the changes in sample thickness, the cathode charge to 4.2 V also sustained considerable mechanical damage arising from the H3 transformation, as evidenced by numerous hairline cracks observed in Figure S4d. To highlight the hairline cracks observed from the cathode charged to 4.2 V using conventional TEM, a bright-field image was slightly defocused and the defocused image in Figure 5a revealed numerous cracks as marked by the arrows. In addition to the visible cracks, a TEM image of a primary particle from the cathode charged to 4.2 V in Figure 5b seemingly contained no visible defects; however, the indexed electron pattern of the primary particle in Figure 5c exhibited split spots (e.g., (105) and (101̅) spots). The consequence of the split spots can be seen in the high-resolution TEM image of the primary particle shown in Figure 5d. Fourier transformed images of regions I 1152
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surface, which is the main capacity fading mechanism for NCM cathodes, increases the charge transfer resistance and adversely affects the capacity retention.25−27 In the case of the LNO cathode, the surface damage would be accelerated by the abundant presence of Ni4+ ions such that the LNO cathode will eventually experience capacity fading even if cycled at 4.1 V. Hence, it appears that capacity retention of the LNO cathode cycled above 4.1 V was severely limited mostly from the microcracking resulting from the repeated H2 → H3 transition, and below 4.1 V, although the detrimental transition was suppressed, the long-term cycling will be eventually limited by the inherent chemical instability of LNO. Surface damage incurred during cycling was reflected in the electrochemical impedance spectroscopy (EIS) of the cells, as shown in Figure 6. The charge transfer resistance, Rct, estimated
Figure 5. (a) Defocused TEM image of the LNO cathode cycled at 4.2 V. (b) Bright-field TEM image of a primary particle from the LNO cathode cycled at 4.2 V and (c) the corresponding electron diffraction pattern, (d) high-resolution TEM image of the primary particle in (b) and Fourier transformed images from the selected regions, and (e) high-resolution TEM image of a primary particle from the LNO cathode cycled at 4.1 V and corresponding Fourier transformed images from the selected regions.
and II (marked in Figure 5d) within the single crystal indicate that two regions were slightly out of angle approximately by 2.9°. During cycling, the internal stress arising from the H3 lattice contraction and expansion tore apart the primary particle and pushed each half together so that the region in the middle (marked as region III) exhibited moire fringes, which are a manifestation of two overlapping crystalline regions. Existence of the moire fringes can be also verified by the extra spots in the accompanying Fourier transform of the middle region (region III). Therefore, charging the LNO cathode above 4.1 V induced a range of structural degradations due to the detrimental H3 transition. At 4.3 V, repeated strain on the cathode developed large cracks propagating through the particle core, whereas at 4.2 V, at which the H3 transition was partially suppressed, the effect was subtle as the fine hairline cracks and local structural distortion visible only with high magnification were generated. These structural degradations observed in the cathodes cycled at 4.2 and 4.3 V were reflected in the precipitous capacity fading during cycling. On the other hand, the cathode cycled at 4.1 V did not show comparable structural damages during cycling, in agreement with the excellent capacity retention at this cutoff voltage. A typical HR-TEM image of the LNO cathode, shown in Figure 5e, displayed no visible structural distortions. Instead, a surface-damaged layer was clearly observed as the Fourier transform of regions I, II, and II marked in Figure 5e revealed progressive transformation from the original R3m ̅ structure in the particle interior to the NiO-like Fm3̅m structure at the surface, as observed in a Li[NixCoyMn1−x−y]O2 (NCM) cathode.21 The thickness of the damaged layer was confined to ∼20 nm. The NiO-like surface layer in Ni-rich NCM or LiNi0.8Co0.15Al0.05O2 cathodes is believed to have formed from the presence of reactive Ni4+ generated in the highly delithiated state during charging.17,22−24 The structural deterioration of the
Figure 6. Nyquist plots of the LNO cathode at different upper cutoff voltages of (a) 4.1, (b) 4.2, and (c) 4.3 V as a function of the number of cycles.
from spectra for the cell charged to 4.1 V was initially 4.2 Ω, which increased to 5.8 Ω after 100 cycles. Rct for the cell charged to 4.2 V was similar in magnitude at 7.0 Ω, which increased to only 8.2 Ω after 100 cycles. However, in the case of the cell charged to 4.3 V, the initial Rct of 6.3 Ω markedly rose to 61.8 Ω after 100 cycles. The EIS data suggest that when cycled at 4.3 V, LNO will sustain structural damages not only from the abrupt lattice contraction and expansion but also from the surface degradation as the breakdown of the electrolyte on the cathode surface is accelerated by the high cutoff voltage. The destructive nature of the H2 → H3 transition was well evinced by the SEM images of the electrodes after 100 cycles in Figure S5. At 4.3 V after 100 cycles, nearly most of the LNO particles were pulverized as the fractured particles and resulting debris can be clearly observed. The pulverization was less severe when cycled at 4.2 V as there were few particles that remained intact. At 4.1 V, however, the original spherical shape of the LNO particles was well preserved even after 100 cycles. The spherical stoichiometric LNO particle, which was composed of compactly packed nanosized primary particles, 1153
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was prepared by lithiation of the Ni(OH)2 precursor and cycled at different cutoff voltages. It was demonstrated that the Li+ ion intercalation stability can be greatly improved through suppression of the H2 → H3 phase transition at 4.15 V during charging by limiting the upper cutoff voltage to 4.1 V. Structural damage incurred by repeated H2 → H3 phase transition was shown by TEM analysis and correlated well with the electrochemical data. Above 4.1 V, the LNO cathode was susceptible to structural damage due to microcracking due to the H2 → H3 phase transition, which led to rapid capacity fading, whereas by avoiding the H2 → H3 phase transition below 4.1 V, the cycling stability was markedly improved. As the composition of the newly developed NCM cathodes is edging toward increasingly Ni-rich compositions, the detrimental phase transition at high voltages, as demonstrated here, needs be closely addressed to ensure long-term life of the NCM cathode-based battery.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chong S. Yoon: 0000-0001-6164-3331 Seung-Taek Myung: 0000-0001-6888-5376 Yang-Kook Sun: 0000-0002-0117-0170 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was mainly supported by the Global Frontier R&D Program (2013M3A6B1078875) on the Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, Information & Communication Technology (ICT) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2014R1A2A1A13050479).
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EXPERIMENTAL METHODS Material Synthesis. Spherical Ni(OH)2 precursor was synthesized by precipitation of a NiSO4·6H2O aqueous solution.28 The NiSO4·6H2O solution was continuously pumped into a batch reactor (47 L) filled with deionized water, NaOH(aq), and NH4OH(aq) under a nitrogen atmosphere with vigorous stirring. Concurrently, a 4.0 mol L−1 NaOH solution (aq) and an NH4OH chelating agent solution (aq) were pumped separately into the reactor. A Ni(OH)2 precursor powder was obtained through filtration, washing, and vacuum drying at 110 °C for 12 h. The precursor Ni(OH)2 was mixed with LiOH· H2O (Li/Ni = 1.01:1 in molar ratio) and calcined at 650 °C for 10 h under an oxygen atmosphere to obtain stoichiometric LNO. Analytical Techniques. The chemical composition of the LNO powder was verified using inductively coupled plasma spectrometry (ICP, OPIMA 8300, PerkinElmer). Powder Xray diffraction (XRD) (Rigaku, Rint-2000) using Cu Kα radiation was employed to identify the crystalline phases of the prepared powders. XRD data were obtained between 10 and 110° 2θ with a step size of 0.03°, and the collected XRD data were analyzed by the Rietveld refinement program Fullprof.29 Scanning electron microscopy (SEM, JSM-6340F) and TEM (JEOL, JEOL 2100F) were used to examine the morphology and structure of the cathode. TEM samples were prepared by a focused ion beam. Electrochemical Testing. For electrochemical testing, the LNO powder was mixed with carbon black and poly(vinylidene fluoride) in a weight ratio of 90:5.5:4.5 in N-methylpyrrolidinone. The slurry was spread onto Al foil, dried, and rollpressed. Cell tests were performed at a 0.5 C-rate at 30 °C in a 2032 coin-type half-cell using a lithium metal anode and 1.2 M LiPF6 in ethylene carbonate−ethyl methyl carbonate (EC/ EMC = 3:7 vol/vol) with 2 wt % vinylene carbonate (VC) as the electrolyte.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00304. dQ dV−1 curves of different upper cutoff voltage, XRD patterns, cross-sectional SEM images, bright-field STEM images, and SEM images of cycled cathodes (PDF) 1154
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