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Cite This: ACS Appl. Energy Mater. 2018, 1, 2536−2545
Infrared Spectroscopy Signatures of Aluminum Segregation and Partial Oxygen Substitution by Sulfur in LiNi0.8Co0.15Al0.05O2 Sasikala Natarajan,†,‡ Sahana B. Moodakare,*,† Vasu Shanmugam,†,‡ Prathap Haridoss,‡ and Raghavan Gopalan† †
ACS Appl. Energy Mater. 2018.1:2536-2545. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 11/06/18. For personal use only.
Centre for Automotive Energy Materials, International Advanced Research Centre for Powder Metallurgy and New Materials, IITM Research Park, Kanagam, Taramani, Chennai 600113, India ‡ Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India S Supporting Information *
ABSTRACT: Substitution of aluminum in nickel-rich layered oxides plays a vital role in structural and thermal stability. Hence comprehension of aluminum distribution in nickel-rich layered oxides such as LiNi0.8Co0.15Al0.05O2 (LNCA) is crucial. However, investigation of aluminum distribution in LNCA is extremely challenging, and sophisticated techniques such as 27Al and 7Li MAS NMR, individual atom probe tomography, X-ray and neutron diffraction, and SQUID magnetic susceptibility measurements are recently employed. We demonstrate the use of a combination of versatile techniques such as X-ray diffraction, energy dispersive X-ray analysis mapping, and vacuum Fourier transform infrared spectroscopy to identify the distribution of aluminum and anion substitution at the oxygen site in LNCA synthesized by the coprecipitation-assisted solid-state reaction. The influence of metal salts used for the coprecipitation of α/β interstratified Ni1−x−yCoxAly(OH)2 (x = 0.15, y = 0/0.05) on anion substitution at the oxygen site in LNCA was investigated. While no anion substitution is observed in LNCA synthesized using nitrate metal salts, sulfur is substituted at the oxygen site when sulfate metal precursors are used. The distribution of Al in LNCA is uniform when Al(OH)3, NiCo(OH)2 are used as Al and Ni, Co precursors, respectively, during the solid-state reaction with LiOH. Segregation of Al is observed in LNCA when α/β interstratified Ni0.8Co0.15Al0.05(OH)2 is used as Ni, Co, and Al precursor. Electrochemical properties of LNCA are strongly influenced by Al distribution and sulfur substitution. While uniform Al distribution reduces the voltage fading due to lower Ni2+ concentration in Li+ site, sulfur substitution increases the cyclic stability. KEYWORDS: lithium ion battery, coprecipitation, α/β interstratified Ni0.8Co0.15Al0.05(OH)2, LiNi0.8Co0.15Al0.05O2, precursor-dependent Al segregation, oxygen substitution by sulfur, vacuum infrared spectroscopy
1. INTRODUCTION LiNi1−x−yCoxAl yO2 , referred to as LNCA, is a widely investigated layered Ni-rich mixed transition metal oxide as a cathode material for lithium-ion batteries for electric vehicle applications in recent years. The parent compound LiNiO2 is isostructural to α-NaFeO2 with space group R3̅m, where Li occupies 3a, Ni in 3b, and O in 6c sites. In LNCA, Ni3+ is partially substituted by Co3+ and Al3+ which helps to improve the structure and thermal stability.1,2 However, the structural stability imparted by Al is sensitive to its uniform distribution in the structure. Especially when the cutoff voltage is beyond 2.8 to 4.3 V vs Li/Li+ range, the concentration and the distribution of Al3+ influences the cyclic stability, power capability, and specific capacity.3−6 In LiNi1−yAlyO2, there is a tendency for Ni and Al segregation into nanodomains when Al substitution is more than 0.15. The lithium ion intercalation/deintercalation voltages, structure, and thermal stability of layered oxide strongly depend on the distribution of transition metal ions.7 Elaborative combination of various characterization techniques such as 27Al and 7Li magic-angle spinning nuclear magnetic © 2018 American Chemical Society
resonance (MAS NMR), local electrode atom probe tomography, X-ray and neutron diffraction, and squid magnetic susceptibility measurements are proposed to investigate Al distribution in LNCA.8 However, these techniques cannot be used routinely to characterize the local distribution of Al in mass-produced LiNi0.8Co0.15Al0.05O2. Fourier transform infrared spectroscopy (FTIR) has the capability of probing the localized region and versatile technique for investigating the quality control of the mass-produced material. The short-range environment of oxygen coordination around the cations in layered oxides can be studied from the peak position of IR absorption.9 LNCA is mainly synthesized by the solid-state reaction of Li salt with coprecipitated layered metal hydroxides (M(OH)2). The structure and morphology of LNCA depend on the mixed metal hydroxide properties which, in turn, is influenced by the Received: February 22, 2018 Accepted: May 31, 2018 Published: May 31, 2018 2536
DOI: 10.1021/acsaem.8b00262 ACS Appl. Energy Mater. 2018, 1, 2536−2545
Article
ACS Applied Energy Materials precursors used for coprecipitation and solid-state synthesis. In the synthesis of LNCA, either Al coprecipitated along with Ni and Co10,11 or Al(OH)3 is used as Al precursor2 for the solidstate reaction. The coprecipitated metal hydroxides [M(OH)2] can stabilize in any one of the following phases (i) α-M(OH)2, (ii) β-M(OH)2, (iii) layered double hydroxide (LDH), or (iv) α/β interstratified metal hydroxide phases. The interlayer distance of hexagonally arranged M(II) and (OH)− ions stacked along the c-axis in β- and α-M(OH)2 are 4.6 and 7.6 Å respectively. The larger interlayer distance in α-M(OH)2 is due to the incorporation of water molecules between the layers of metal hydroxides.12 The LDH is usually represented as [MII1−xMIIIx(OH)2]x+[Ax/n]n−·yH2O, where An− is an n− valence anion, MII is a divalent ion, and MIII is trivalent metal ions.13 The LDH has very similar arrangement of α-metal hydroxide layers, except that anions are also intercalated along with water molecules for the charge balance. When metal hydroxide is composed of α, β, and LDH phases, it is referred to as α/β interstratified, where α stands for the contributions from both α and LDH phases.14 The percentage of α, β, LDH phases, and the local arrangement of Ni, Co, Al in α/β interstratified hydroxide will influence the distribution of metal ions in LNCA. Therefore, the detailed understanding on the distribution of cations and anions in metal hydroxide precursors is essential for the development of LNCA with uniformly distributed metal ions. In this work, we demonstrate the application of FTIR in investigating aluminum distribution and anion incorporation, both in the coprecipitated metal hydroxide and LNCA. The extent of α/β interstratification in Ni, Co hydroxides and Ni, Co, Al hydroxides synthesized using sulfate and nitrate salts are semiquantitatively estimated using FTIR and X-ray diffraction (XRD). Segregation of Al is observed in LNCA synthesized with Ni, Co, Al mixed metal hydroxide irrespective of sulfate or nitrate salts used for coprecipitation. The distribution of aluminum is uniform, when Al(OH)3 is used as aluminum precursor for solid-state reaction, instead of Ni, Co, Al mixed metal hydroxide. Partial substitution of oxygen by sulfate in LNCA is observed in the samples synthesized using sulfate salts. In this paper, we report the influence of aluminum distribution and sulfur substitution on the average intercalation/deintercalation voltage with cycling and cyclic stability when the charge/discharge cutoff voltage is 2.5 to 4.4 V vs Li/ Li+. The Al segregated LNCA shows increased average intercalation/deintercalation voltage difference with charging/ discharging cycles. Uniform Al distribution and sulfur doped LNCA shows superior specific energy retention with cycling compared LNCA with nonuniform Al distribution.
Table 1. Details of Metal Salts Used for Mixed Metal Hydroxide Co-Precipitation and Sample Designation of Hydroxides along with Corresponding Layered Oxide precursor used for coprecipitation
hydroxide sample ID
precursors used for solidstate synthesis of LNCA
LNCAoxide sample ID
Ni, Co sulfates Ni, Co, Al sulfates Ni, Co nitrates Ni, Co, Al nitrates
S-NCH S-NCAH N-NCH N-NCAH
S-NCH, Al(OH)3, LiOH S- NCAH, LiOH N-NCH, Al(OH)3, LiOH N- NCAH, LiOH
S750 SA750 N750 NA750
deionized water (1M) were added dropwise to 30% NaOH-NH3OH solution mixture under continuous stirring. The pH of the overall solution was maintained between 11 and 12 by adding required amount of base solution intermittently. After completion of the precipitation, aging of the hydroxides was carried out in the mother liquor for 24 h with constant stirring. The precipitate was filtered, washed, and finally dried in a hot air oven at 120 °C overnight. The dried precipitate was mixed with Al(OH)3 and/or LiOH·H2O and heat treated in air at 450 °C for 12 h followed by 750 °C for 20 h. A smart lab X-ray diffractometer from Rigaku with Cu Kα radiation (λ = 1.5406 Å) was used to collect the powder diffraction pattern of hydroxides and oxides. The microstructure of the oxide powders was examined using Scanning Electron Microscopy (SEM; Quanta 400FEG operating with a field emission gun at 30 kV). Thermogravimetric analysis (TGA) of mixed metal hydroxides mixed with Al(OH)3 and/ or LiOH·H2O was carried out using STA 449, NETZSCH. FTIR of the hydroxide and oxide samples were performed using Bruker vacuum VERTEX 70v under ATR/FTIR mode with 32 scans and resolution of 2 cm−1. The electrochemical charging/discharging test was conducted in a half cell Swagelok configuration with LNCA powder mixed with carbon black in 90:10 ratio as the working electrode and lithium metal as reference and counter electrode. One M LiPF6 in 1:1:1 mixture (% vol.) of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) was used as the electrolyte. Arbin BT 2000 USA was used to carry out the galvanostatic charge−discharge cyclic stability of the electrochemical cells.
3. RESULTS AND DISCUSSION 3.1. Structural Studies of Coprecipitated Mixed Metal Hydroxide. Derivatives of Ni(OH)2 such as Ni1−x−yCoxAly(OH)2 are the complex systems, and a small modification to the synthesis leads to significant change in the phase formation, crystallinity, and defects in the system. These variations in Ni1−x−yCoxAly(OH)2 will play a crucial role in determining the structure and morphology of the final LNCA prepared by the solid-state reaction between the metal hydroxides. The relative quantity of the anions that are intercalated between a layered metal hydroxide was estimated from corroborative analysis of XRD and FTIR Ni(OH)2 derivatives can accommodate numerous types of disorders such as stacking faults, the inclusion of foreign cations and anions, and interstratification and mechanical stress. X-ray diffraction analysis enables analyzing and categorizing the type of disorder present in Ni(OH)2 and its derivatives.12,14 DIFFaX simulations by Ramesh et al.15 suggest that various factors like (i) interstratification, (ii) cation vacancies, (iii) turbostraticity, (iii) stacking fault, (iv) crystallite size, and (v) disorder due to the intercalated species leads to nonuniform broadening of the reflections in XRD patterns of Ni(OH)2 derivatives. The XRD patterns of S-NCH, S-NCAH, N-NCH, and N-NCAH along with the peak position of the calculated α/β interstratified Ni(OH)2 as given in ref 15, α-Ni(OH)2 from ref 12, and the standard pattern of β Ni(OH)2 (ICDD number 98-011-4725) are given in Figure 1. While the samples S-NCH, N-NCH, and
2. EXPERIMENTAL SECTION LNCA was synthesized by the solid-state reaction of Ni0.8Co0.15Al0.05(OH)2 with Li(OH)·H2O and Ni0.85Co0.15(OH)2 with Al(OH)3 and Li(OH)·H2O. Mixed metal hydroxides were synthesized using nitrate and sulfate salts as precursors by coprecipitation. The details of the salts used for the preparation of mixed metal hydroxides and sample designation of four mixed metal hydroxides and the corresponding LNCA oxides are summarized in Table 1. Reagents used in this investigation were NiSO4·6H2O (Merck, 97%), Al2(SO4)3·16H2O (Merck, 98%), CoSO4·7H2O (Merck, 98%), Ni(NO3)2·6H2O (Merck, 98%), Co(NO3)2·6H2O (Merck, 98%), Al(NO3)3·9H2O (Merck, 95%), NaOH Pellets (Merck, 97%), and ammonia (Merck, 28.0- 30.0%). All four mixed metal hydroxides were precipitated under identical conditions of pH, temperature, and aging duration. Metal precursors dissolved in 2537
DOI: 10.1021/acsaem.8b00262 ACS Appl. Energy Mater. 2018, 1, 2536−2545
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ACS Applied Energy Materials
Figure 1. XRD patterns of (a) S-NCH, (b) S-NCAH, (c) N-NCH, (d) N-NCAH, (e) standard pattern of β Ni(OH)2 (ICDD number 98-011-4725), (f) peak position of α-Ni(OH)2 from the reference,15 (g) peak position of calculated α/β interstratified Ni(OH)2 as given in reference12 (PseudoVoigt fit of the XRD peaks of S-NCH, S-NCAH, N-NCH are given along with their corresponding XRD patterns), and (h) the estimated full width at half-maximum (fwhm) of (001), (101), (102) (100), and (110) reflections from S-NCH, N-NCH, and S-NCAH.
S-NCAH exhibit α/β interstratified Ni1−x−yCoxAly(OH)2 structure,12 N-NCAH possesses close to αNi1−x−yCoxAly(OH)2 structure. The XRD peaks of α/β interstratified Ni1−x−yCoxAly(OH)2 are fitted with pseudoVoigt function and are given in Figure 1 along with the respective XRD patterns. The estimated full width at halfmaximum (fwhm) of (001), (101), (102), (100), and (110) reflection from S-NCH, N-NCH, and S-NCAH are given in Figure 1h. It is observed that the broadening of reflections (001), (101), (102) gradually increases in the following sequence S-NCH, N-NCH, S-NCAH, whereas (100) and (110) reflections are almost narrow in all the samples. Interlayer spacing corresponding to (001) decreases from 4.6 to 4.3 Å with an increase in the interstratification. Disorders in the crystal lattice can be characterized by the preferential shift in some of the (hkl) reflection and variation in the fwhm values of the reflections.16 The increase in the fwhm of (001), (101), (102) and the decrease in d-value corresponding to (001) is attributed to the increase in the interstratification. Further, the XRD peak positions of N-NCAH corresponds to αNi1−x−yCoxAly(OH)2, where the intercalation of water and anions is in almost all the layers of metal hydroxide.12 Therefore, the XRD analysis suggests that the percentage of the interstratification is in the order of S-NCH < N-NCH < SNCAH < N-NCAH. FTIR of the hydroxides were taken to investigate (i) the extent of α/β interstratification, (ii) the nature of M−O-H bonds and (iii) Intercalated anions. The FTIR spectra in the frequency range 200−800 cm−1, 2500−3800 cm−1 and 9001700 cm−1 are given in Figure 2 along with the deconvoluted peaks of stretching and in-plane OH modes. Table 2 presents the observed FTIR peaks and its assignments. 3.1.1. α/β Interstratification. Interstratification can be semiquantitatively determined using the shape and position of stretching and in-plane OH modes whose calculated band positions in defect-free β-Ni(OH)2 are at 3643 and 496 cm−1
respectively.12 However, the various peak positions in the range from 3630 to 3650 cm−1 for OH stretching and 510 cm−1 to 553 cm−1 for in-plane OH mode are reported experimentally.12 The main difference between α -M(OH)2 and β-M(OH)2 is that in β phase the OH group is free, while in α phase the hydrogen atom is bonded to the oxygen atoms of the intercalated anions or the water molecule. A shift to the higher frequency of in-plane OH deformations peak up to 620 cm−1 is reported by Kamat et al. and to 685 cm−1 by Acharya et al. in extensive hydrogen bonded α -Ni(OH)2 due to the hindrance for − OH deformation.17,18 In α/β-interstratified samples, the extent of the hindrance to the OH in-plane deformation varies within the structure, leading to different extent of blue shift of FTIR band. Therefore, the FTIR spectra of α/β interstratified M(OH)2 is comprised of superimposed sharp and broad peaks (∼490 cm−1 to 700 cm−1) corresponding to in-plane OH mode. The in-plane OH mode of all the samples are deconvoluted into, one defect free β-M (OH)2 peak centered at ∼490 cm−1 and three α-M(OH)2 peaks with different extent of hydrogen bonding centered at 530 cm−1, 570 cm−1, and 620 cm−1. The intensity ratio of the peaks corresponding to inplane OH mode of β-M (OH)2 to α -M (OH)2 (Iβ‑M (OH)2/ Iα‑M (OH)2), which predicts the amount of β-M (OH)2 in the α/ β interstratified M(OH)2 are given in Figure 2c for all the samples. The gradual decrease in the I β‑M (OH)2/Iα‑M (OH)2 is observed in the sequence of S-NCH,N-NCH,S-NCAH and NNCAH. To corroborate the above observation, the spectral band profile of the OH stretching modes are deconvoluted and are given in Figure 2b. While, the sharp peak centered close to 3630 cm−1 corresponds to β-M(OH)2, the remaining broad peaks are from the OH stretching modes of α-M(OH)2. The summed up intensity of all the peaks corresponding to αM(OH)2 is the measure of percentage of α phase in the α/β interstratified hydroxides. The intensity ratio of the OH stretching mode corresponding to β to α is given in Figure 2538
DOI: 10.1021/acsaem.8b00262 ACS Appl. Energy Mater. 2018, 1, 2536−2545
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Figure 2. FTIR spectra of coprecipitated hydroxides in the range (a) 200−800 cm−1 (β, α1, α2, α3 corresponds to peaks centered at ∼490 cm−1, 530 cm−1, 570 cm−1, and 620 cm−1). (b) 2500−3800 cm−1 (Sharp peak β′ corresponds to β-M(OH)2, the remaining broad peaks α1′, α2′, α3′ corresponds to the OH stretching modes of α-M(OH)2. (c) The intensity ratio of the OH stretching mode corresponding to β to α along with the OH in-plane mode (Iα′ = I α1′+I α2′+I α3′). (d) FTIR spectra of hydroxides taken in the range of 900−1700 cm−1.
of nitrate ions are required while compared to sulfate ion in M(OH)2. The introduction of Al3+ in coprecipitation requires additional anions for the charge balance, which increases the αphase further. 3.1.2. Nature of M−O−H Bonds. The exact band positions of intermolecular vibrational modes (both rotational and translational) depend on the transition metal involved in M− O-H bonding. M−O-H bands corresponding to β-NiCo(OH)2 at 338 cm−1, 427 cm−1 are observed in all four samples.19 In SNCAH and N-NCAH peaks corresponding to Al−O−OH (∼380 cm−1)20 are also observed, confirming the incorporation Al in M(OH)2. 3.1.3. Nature of the Incorporated Anions. A broad peak at approximately ∼1640 cm−1 arises from O−H bending mode of H2O that is either absorbed on the material surface or trapped
2c along with the OH in-plane mode. It can be inferred from Iβ‑M(OH)2/Iα‑M(OH)2, that the relative intensity of the OHstretching and in-plane mode corresponding to β phase with respect to α phase is in S-NCH, N-NCH, S-NCAH and NNCAH order. Therefore, from the nature of the stretching and in-plane vibration of OH group, it can be discerned that the presence of α content is in S-NCH < N-NCH < S-NCAH < NNCAH sequence. The α-phase is present in all the four samples due to the intercalation of the anions (either sulfate or nitrate) for charge balancing of Co3+. The use of nitrate salts facilitates a higher concentration of the α-phase in the hydroxide compared to that of the samples prepared using sulfate salts. While in nitrate precursors, for every metal ion, there are two anions, the sulfate precursor has only one anion. Hence for a given concentration of Co3+, for the charge balance, twice the number 2539
DOI: 10.1021/acsaem.8b00262 ACS Appl. Energy Mater. 2018, 1, 2536−2545
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α/β Ni1−x−yCox(OH)2, Al(OH)3, and Li(OH)·H2O, whereas in S-NCAH and N-NCAH, the loss is from α/β Ni1−x−yCoxAly(OH)2 and LiOH.H2O. Because weight losses due to dehydroxylation of above-mentioned hydroxides are partially overlapping, understanding the exact thermal activation pathway is not possible from the TGA of the mixture. However, the melting and dehydroxylation temperatures of Al(OH)3, Li(OH), and polymorphs of Ni(OH)2 derivatives are well-documented.12,22 However, Li(OH) melts and decomposes at 425 and 473 °C, respectively,22 Al(OH)3 dehydroxylates at ∼275 °C. Decomposition of β-Ni(OH)2 to NiO12 takes place at 170 < T < 525 C, while the dehydroxylation of LDH containing Al to spinel-like mixed oxides (Ni1−xCoxAl2O4) takes place at temperatures below 300 °C.23 Compared with S-NCH and N-NCH, slightly higher weight loss is observed in S-NCAH and N-NCAH from 120 to 300 °C because of the presence of LDH phase containing Al. In addition to this, weight loss due to the decomposition of intercalated nitrate has been observed from 575 to 625 °C in N-NCH and N-NCAH.21 However, no apparent weight loss due to intercalated sulfate ions is observed in S-NCH and SNCAH.21 3.2. Structural Studies of LiNi0.8Co0.15Al0.05O2. X-ray diffraction pattern along with Rietveld refinement of four LNCA samples are shown in Figure 4. The XRD pattern of all
Table 2. Wavenumber of the FTIR Vibrational Modes That Are Observed in Co-Precipitated Hydroxide and Their Assignments S-NCH (cm−1) 3636
SNCAH (cm−1)
1633 1360
3639 3000 1639 1358
1106 622 498
1106 621 497
538 426
529 431 380 345
334
N-NCH (cm−1) 3636
NNCAH (cm−1)
1640
3627 3468 1635
1335
1337
621 494
621 487
530 427
530 430 381 334
337
peak assignment (TO) O−H stretch, β-NiOH O−H stretch, α-NiOH O−H bending carbonate ν2 mode of nitrate ν3 mode of free sulfate lattice modes in-plane OH deformations peak in defect-free β-M(OH)2 in-plane OH mode β-NiCo(OH)2 Al−O−OH β-NiCo(OH)2
within the structure.12 A strong vibration at 1000 cm−1 to 1500 cm−1 is observed due to intercalated anions corresponding to carbonates at 1358 cm−1 in all the samples, nitrates at 1337 cm−1 (N-NCAH and N-NCH) and sulfate at 1106 cm−1 (SNCAH and S-NCH). The use of different precursors in the synthesis of Ni1−x−yCoxAly(OH)2 has led to change in the percentage of α/β composition due to variation in the oxidation state of the transition metal and anions such as NO3−, SO42−, and CO32−. Because the thermal activation of α/β interstratified Ni1−x−yCoxAly(OH)2 and Al(OH)3 is different, TG of LiOH and Al(OH)3 or Li(OH) mixed with coprecipitated hydroxide is taken under air ambient. Figure 3 shows the TG of the four samples taken from room temperature to 800 °C. The TG curves of all the samples show weight loss due to absorbed and intercalated water at temperatures below 120 °C.21 The weight loss due to dehydroxylation observed in the temperature range of 120 to 500 °C, in the case of S-NCH and N-NCH, is from
Figure 4. XRD of S750, SA750, N750, NA750 along with Rietveld simulated spectra and the difference between the experimental and simulated intensity.
the samples is consistent with the standard reflections for LNCA (ICDD 98-007-9167). The stoichiometric layered LNCA possesses the rhombohedral structure with trigonal symmetry (space group: R3̅m) and can be visualized as interpenetrating close-packed FCC sublattices of oxygen and metal ions along (111) planes. In stoichiometric LNCA, the FCC sublattice of metal ions are alternatively occupied by Li and Ni3+/Co3+/Al3+. However, 2+ being a stable oxidation state for Ni, a small concentration of Ni2+ is always observed in
Figure 3. TGA curves of the mixture of hydroxide precursors used for solid-state synthesis of LNCA. 2540
DOI: 10.1021/acsaem.8b00262 ACS Appl. Energy Mater. 2018, 1, 2536−2545
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ACS Applied Energy Materials Table 3. Rietveld Refined Structural Data of LNCA Samples sample
a (Å)
c (Å)
a/c (Å)
I003/I104
Li+
Ni2+
Ni3+
Al3+
Co3+
O2−/SO42−
Rwp
S750 SA750 N750 NA750
2.866 2.865 2.865 2.864
14.177 14.180 14.181 14.183
4.947 4.949 4.949 4.951
1.31 1.28 1.35 1.23
0.9652 0.9475 0.963 0.954
0.0347 0.0525 0.037 0.046
0.7932 0.7979 0.7166 0.8158
0.0522 0.0497 0.0518 0.0389
0.1634 0.1502 0.1533 0.1561
0.9879/0.0344 0.9600/0.0699 1 1
9.7 8.4 7.6 9.3
Figure 5. (a) SEM images of S750, SA750, N750, NA750. (b) Corresponding histogram to quantify the particle size distribution.
Figure 6. SEM-EDAX elemental mapping of S750, SA750, N750, NA750.
XRD pattern,24 higher the intensity ratio lower the nonstoichiometry. The value of I(003)/I(104)) (Table 3) is found to be above 1.3 for S750 and N750, and above 1.2 for SA750 and NA750 supporting the Rietveld refinement values of Ni2+. Additionally, in samples prepared using sulfate salts (S750 and SA750), small concentration of sulfur doping is observed in oxygen site. From the SEM micrographs (Figure 5a), it can be observed that particles are in uniform size in S750 and N750, while in SA750 and NA750, particles have broad distribution in size.
LNCA and occupies Li layers due to similarity in ionic radius of the two, resulting in rock salt cubic structure. This nonstoichiometric LNCA can be represented as (Li1−zNiz)3a(Ni1−x−yCoxAly)3b(O2)6c.24 The Rietveld refinement of the XRD patterns show lower concentration of Ni2+ in Li+ site in S750 and N750, compared with that of SA750 and NA750, respectively (Table 3). Deviation from the stoichiometry of LiNi1−x−yCoxAlyO2 and the concentration of Ni2+ in 3a site can also be determined from the intensity ratio of (003) and (104) (I(003)/I(104)) peaks in 2541
DOI: 10.1021/acsaem.8b00262 ACS Appl. Energy Mater. 2018, 1, 2536−2545
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ACS Applied Energy Materials
Figure 7. FTIR spectra of S750, SA750, N750, NA750 in the wavelength range (a) 100−1600 cm−1 ; the regions marked corresponding to vibrational modes of (I) LiO2 layers, (II) NiCoAlO2 layers, (III) anion dopants (b) 600−650 cm−1 and (c) 900−1600 cm−1.
Table 4. Variation in the Specific Capacity of LNCA with Cutoff Voltage along with the Reference synthesis method
composition
discharge capacity (mAh/g)
cutoff voltage
C rate
reference
coprecipitation
LiNi0.8Co0.15Al0.05O2
182.98 (1st) 91.72 (100th) 189.96 (1st) 171.36 (100th) 180 (1st) 189.9 (1st) 112.93 (150th) 177.6 (1st) 144.40 (150th) 182 (1st) 141.6 (50th) 190 (1st) 128 (50th) 177 (1st) 153.6 (50th) 182 (1st) 144 (50th) 200 (1st) 207 (1st) 199 (1st)
2.5−4.5 V
1C
3
2.8−4.4 V 2.8−4.5 V
1C
4
1C
5
0.1C
6
TiO2-coated LiNi0.8Co0.15Al0.05O2 sol−gel
LiNi0.8Co0.15Al0.05O2
Al2O3-coated LiNi0.8Co0.15Al0.05O2 coprecipitation and solid-state synthesis
LiNi0.8Co0.15Al0.05O2
fluorine-doped LiNi0.8Co0.15Al0.05O2
coprecipitation and solid-state synthesis
LiNi0.89Co0.11O2 LiNi0.85Co0.10Al0.05O2 LiNi0.81Ni0.10Al0.09O2
2.8−4.5 V 2.8−4.4 V 2.8−4.5 V 2.8−4.4 V 2.8−4.5 V 3−4.5 V
given in Figure 7b. The position and the intensity of the bending modes of O−M−O bonds are sensitive to the nature of the transition metal (mass and ionic radius). A large number of bands are observed from 600 to 630 cm−1 suggesting that there is local inhomogeneity in Ni, Co, and Al concentration.24,25 In both SA750 and NA750, an additional peak at 610 cm−1, corresponding to Al−O2 TO bending mode is observed suggesting that there is segregation of aluminum.26 Therefore, corroborative EDAX, FTIR, TGA, and XRD refinements suggest that in large particles, aluminum is segregated as Ni1−xCoxAl2O4 spinel-like phase. Aluminum substitution of around 5% in LiNi1−xCoxO2 prevents the occupation of Ni2+ in Li+ site.27 The localized segregation reduces the concentration of aluminum in LNCA leading to a higher concentration of Ni2+ in Li site in SA750 and NA750. However, in S750 and N750, the distribution of aluminum is uniform. The segregation of aluminum in SA750 and NA750 and uniform distribution of aluminum in S750 and N750 can be attributed to the difference in the decomposition pathway of layered double hydroxide and Al(OH)3. During the thermal activation of α/β interstratified, Ni1−x−yCoxAly(OH)2 melts and decomposes at a temperature
From the particle size histogram of the samples (Figure 5b), it can be seen that the nonuniformity in the particle size is in the order of S750≅N750 < SA750≅NA750. The SEM and XRD studies illustrate that the nonuniformity in the particle size and the concertation of Ni2+ in Li+ site is higher in the case of samples prepared using Ni1−x−yCoxAly(OH)2, compared with that of using Ni1−xCox(OH)2 and Al(OH)3. To understand the correlation between the particle sizes, the nonstoichiometry of LNCA, and aluminum precursor used, EDAX mapping of the samples is carried out and is shown in Figure 6. From the EDAX mapping, it can be inferred that the large particles present in SA750 and NA750 contain a higher concentration of aluminum and oxygen while compared to the smaller particles. The FTIR spectra of LNCA taken from 100−1600 cm−1 show three main features corresponding vibrational modes (i) LiO2 layers (100−400 cm−1), (ii) NiCoAlO2 layers (400−700 cm−1), and (iii) anion dopants (900−1600 cm−1) as marked in the Figure 7a. The sharp band positioned around 620 cm−1 is attributed to the bending modes of O−M−O bonds.24 To investigate the local arrangement of Ni, Co, and Al in LNCA, the FTIR from 600 to 650 cm−1 is carried out and is 2542
DOI: 10.1021/acsaem.8b00262 ACS Appl. Energy Mater. 2018, 1, 2536−2545
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ACS Applied Energy Materials
Figure 8. Electrochemical characterization of S750, SA750, N750, NA750 at different cycles. (a) Charge/discharge curve at different cycles. (b) Charge/discharge specific capacity vs cycle number. (c) Average voltage of charge−discharge and average voltage difference between charge− discharge. (d) Energy density vs cycle number (■ S750C, red ● S750D, blue ▲ SA750C, magenta ▼ SA750D, green ⧫ N750C, navy ◀ N750D, purple ▶ NA750C, purple ● NA750D C-Charge D-Discharge) (Vac−Vad for brown ■ S750, gray ⧫ N750, dark purple ▲ SA750, purple ● NA750).
less than 300 °C, and from the LDH part of metal hydroxides, spinel-like NiCoAl2O4 phase crystallizes.28 However, melting and decomposition of LiOH occurs at a higher temperature of 410 to 600 °C. Because of the formation of spinel-like Ni1−xCoxAl2O4 phase before the melting of Li(OH), aluminum segregated large particles are observed in SA750 and NA750. In the FTIR spectra of all four samples, a strong peak at ∼1415 cm−1 and shoulder peak at ∼1473 cm−1 is observed, and they are assigned to C−O asymmetric and symmetric stretching modes of Li2CO3 respectively.29 Additionally, a broad peak at 1094 cm−130 is observed only in S750 and SA750, which corresponds to SO43− confirming the doping of Sulfur at oxygen site. 3.3. Electrochemical Characterization of LNCA. Depending upon the cycling cutoff potential, the practical first cycle specific capacity and cyclic stability of LNCA changes as depicted in Table 4 and Supplementary Table 1. However, when overcharged, the specific capacity decreases with cycling due to surface structural modification.31 To study the effect of aluminum distribution and sulfur doping on cyclic stability and
impedance development with moderately overcharged cycling, galvanostatic charge/discharge of LNCA are carried out from 2.5 to 4.4 V with respect to Li metal. Charging/discharging voltage profile at a current density of 27 mA/g for 1st, 2nd, 5th, 15th, and 30th are given in Figure 8a. The first cycle charge/ discharge capacity of all the samples are ranging from 200 to 210 mAhg−1 and 170 to 180 mAhg−1, respectively. The charge/ discharge specific capacity vs cycle number of the samples is given in Figure 8b. The first cycle charging capacity is in the order of reported best values.10,11 However, because of the higher cutoff voltage, the irreversibility in the first cycle is around ∼20 to 30 mAh/g. The discharge capacity retention after 30 cycles for S750, SA750, N750, and NA750 are 89%, 78%, 87%, and 74%, respectively. From Figure 8a, it can be observed that with the increase in cycle number, midpoint charging voltage (Vac) increases and the midpoint discharge voltage (Vad) decreases. To quantify the changes in charge/ discharge voltage with cycle number, the average voltage (specific energy divided by capacity) of each cycle is calculated. Charging average voltage (Vac), discharging average voltage 2543
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ACS Applied Energy Materials (Vad), and Vac−Vad for all the samples are given in Figure 8c. For all the samples, the first cycle Vac and Vad are ∼3.9 and 3.7 V, respectively. However, with the cycle number the Vac increases and Vad decreases leading to an increase in Vac−Vad. While this increase is minimum in S750 (0.15 V) and N750 (0.18 V), it is high in SA750 (0.3 V) and NA750 (0.45 V), suggesting higher polarization and impedance development in SA750 and NA750 with cycling. Recently, it has been reported that the reason for the increase in the polarization and impedance of LNCA is due to morphological changes like particle cracking with cycling.31 The presence of a higher concentration of NiO-like phase on the surfaces of individual primary particles in SA750 and NA750 accelerates the particle cracking.32 The cracking at the surface reduces the electrical contact between the active material and the conducting agent, and thus, it increases the polarization and impedance. Because of the higher concentration of Ni2+ in the Li site in SA750 and NA750, a rapid increase in the polarization and impedance with cycling is observed. Overall, the nonuniform distribution of aluminum reduces both specific capacity and average voltage with cycling, thereby reducing the total energy of LNCA as can be discerned by discharge energy density vs cycle given in Figure 8d. Even though aluminum distribution is uniform both in S750 and N750, S750 has better cyclic stability due to the small percentage of sulfur doping at oxygen site. It has been observed by Park et al. that a small concentration of sulfur doping in LiNiO2 reduces structural strain during Li+ transport due to the lower electronegativity of sulfur in comparison with oxygen.33,34
Notes
The authors declare no competing financial interest.
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REFERENCES
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CONCLUSIONS The α/β interstratified Ni, Co mixed metal hydroxide with and without Al was synthesized by coprecipitation using nitrate and sulfate precursors. The higher concentration of the α phase in the hydroxide is observed when aluminum is coprecipitated along with Ni and Co. Corroborative XRD, FTIR, EDAX mapping reveals that there is segregation of Al in LNCA synthesized with Ni, Co, Al mixed metal hydroxide irrespective of sulfate or nitrate salts used for coprecipitation. When Al(OH)3 is used as Al precursor for solid-state reaction instead of Ni, Co, Al mixed metal hydroxide, Al is distributed uniformly throughout the sample. Partial substitution of oxygen by sulfate in LNCA is observed in the samples synthesized using sulfate salts. The LNCA (having Al segregation) composed of a higher percentage of NiO type phase shows increased average intercalation/deintercalation voltage difference with charging/ discharging cycles. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00262. SI Table 1: The variation in the reported practical specific capacity, reversibility and cyclic stability depending on the C-rate, electrode composition, and voltage window (PDF)
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ACKNOWLEDGMENTS
We would like to thank the financial support received from the Department of Science and Technology, (DST) Government of India (No. AI/1/65/ARCI/2014). The authors are thankful to the Director, ARCI for his constant encouragement and discussion during the course of this work.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Sahana B. Moodakare: 0000-0002-2453-1223 2544
DOI: 10.1021/acsaem.8b00262 ACS Appl. Energy Mater. 2018, 1, 2536−2545
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