Infrared Spectroscopy Signatures of Aluminium Segregation and

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Infrared spectroscopy signatures of Aluminium Segregation and Partial Oxygen Substitution by Sulphur in LiNi Co Al O 0.8

0.15

0.05

2

Sasikala Natarajan, Sahana B Moodakare, Vasu Shanmugam, Prathap Haridoss, and Raghavan Gopalan ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00262 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Applied Energy Materials

Infrared Spectroscopy Signatures of Aluminium Segregation and Partial Oxygen Substitution by Sulphur in LiNi0.8Co0.15Al0.05O2 Sasikala Natarajan

a,b

, Sahana B Moodakare a*, Vasu Shanmugam

a,b

, Prathap Haridoss b, and

Raghavan Gopalan a a

Centre for Automotive Energy Materials, International Advanced Research Centre for Powder

Metallurgy and New Materials, IITM Research Park, Kanagam, Taramani, Chennai-600113, India. b

Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras,

Chennai 600036, India. * Corresponding Author Email: [email protected]

Abstract: Substitution of aluminium in nickel-rich layered oxides play a vital role in structural and thermal stability. Hence comprehension of aluminium distribution in nickel-rich layered oxides such as LiNi0.8Co0.15Al0.05O2 (LNCA) is crucial. However, investigation of aluminium distribution in LNCA is extremely challenging and sophisticated techniques such as

27

Al 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 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 aluminium and anion substitution at oxygen site in LNCA synthesized by the co-precipitation assisted solid state reaction. The influence of metal salts used for the co-precipitation of α/β interstratified Ni1-xyCoxAly(OH)2

(x=0.15, y=0/0.05) on anion substitution at oxygen site in LNCA was investigated.

While no anion substitution is observed in LNCA synthesized using nitrate metal salts, sulphur substitutes at oxygen site when sulphate 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 sulphur substitution. While uniform Al distribution reduces the voltage fading due to lower Ni2+ concentration in Li+ site, sulphur substitution increases the cyclic stability.

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ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Keywords: Lithium ion battery, co-precipitation, α/β interstratified Ni0.8Co0.15Al0.05(OH)2, LiNi0.8Co0.15Al0.05O2, precursor dependent Al segregation, oxygen substitution by sulphur, Vacuum Infrared spectroscopy Introduction: LiNi1-x-yCoxAlyO2 referred 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 R m, 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 stability1-2. However, the structural stability imparted by Al is sensitive to its uniform distribution in the structure. Especially when the cut off voltage is beyond 2.8 to 4.3V 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 nano-domains when Al substitution is more than 0.15. The lithium ion intercalation/de-intercalation 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

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Al and 7Li magic-angle spinning nuclear magnetic

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 co-precipitated 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 precursors used for co-precipitation and solid state synthesis. In the synthesis of LNCA, either Al co-precipitated along with Ni and Co 1011

or Al(OH)3 is used as Al precursor

2

for the solid state reaction. The co-precipitated metal

hydroxides [M(OH)2] can stabilise in any one of the following phases (i) α-M(OH)2, (ii) βM(OH)2, (iii) layered double hydroxide (LDH) or (iv) α/β interstratified metal hydroxide phases. 2


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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 Å 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 comprised of α, β and LDH phases, it is referred 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 aluminium distribution and anion incorporation, both in the co-precipitated metal hydroxide and LNCA. The extent of α/β interstratification in Ni, Co hydroxides and Ni, Co, Al hydroxides synthesized using sulphate and nitrate salts are semi quantitatively 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 sulphate or nitrate salts used for co-precipitation. The distribution of aluminium is uniform, when Al(OH)3 is used as aluminium precursor for solid-state reaction, instead of Ni, Co, Al mixed metal hydroxide. Partial substitution of oxygen by sulphate in LNCA is observed in the samples synthesized using sulphate salts. In this paper, we report the influence of aluminium distribution and sulphur substitution on the average intercalation/de-intercalation voltage with cycling and cyclic stability when the charge/discharge cut-off voltage is 2.5 to 4.4 V vs Li/Li+. The Al segregated LNCA shows increased average intercalation/de-intercalation voltage difference with charging/discharging cycles. Uniform Al distribution and sulphur doped LNCA shows superior specific energy retention with cycling compared LNCA with non-uniform Al distribution.

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Experimental: 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 sulphate salts as precursors by co-precipitation. 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 the four mixed metal hydroxides were precipitated under identical conditions of pH, temperature and aging duration. Metal precursors dissolved in deionised water (1M) were added drop wise to 30% NaOH-NH3OH solution mixture under continuous stirring. The pH of the overall solution was maintained between 11-12 by adding required amount of base solution intermittently. After completion of the precipitation, aging of the hydroxides was carried out in mother liquor for 24 hr with constant stirring. The precipitate was filtered, washed and finally dried in 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 hr followed by 750 °C for 20 hr.

Table 1. Details of metal salts used for mixed metal hydroxide co-precipitation and sample designation of hydroxides along with corresponding layered oxide. Precursors used for Co-precipitation Ni, Co Sulphates Ni, Co, Al Sulphates Ni, Co Nitrates Ni, Co, Al Nitrates

Hydroxides sample ID S-NCH S-NCAH N-NCH N-NCAH

Precursors used for solid state synthesis of LNCA S-NCH, Al(OH)3, LiOH S- NCAH, LiOH N-NCH, Al(OH)3, LiOH N- NCAH, LiOH

LNCA-Oxide sample ID S750 SA750 N750 NA750

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 400-FEG operating with a field emission gun at 30 kV). Thermo gravimetric 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 4


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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. 1 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 co-precipitated mixed metal hydroxide

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 reference 12 ( Pseudo-Voigt 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.

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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 in between 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 in 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 non-uniform broadening of the reflections in XRD patterns of Ni(OH)2 derivatives. The XRD patterns of S-NCH, S-NCAH, N-NCH and NNCAH along with peak position of the calculated α/β interstratified Ni(OH)2 as given in Ni(OH)2 from the reference

12

15

, α-

and 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 S-NCAH exhibit α/β interstratified Ni1x-yCoxAly(OH)2

structure

12

, N-NCAH possesses close to α- Ni1-x-yCoxAly(OH)2 structure. The

XRD peaks of α/β interstratified Ni1-x-yCoxAly(OH)2 are fitted with pseudo-Voigt function and are given in Figure 1 along with respective XRD patterns. The estimated full width at half maximum (FWHM) of (001), (101), (102) (100) and (110) reflection from S-NCH, N-NCH and S-NCAH of 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 Å to 4.3 Å with 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 full-width of half- maximum (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

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hydroxide 12. Therefore the XRD analysis suggests that the percentage of the interstratification is in the order of S-NCH < N-NCH < S-NCAH < 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 900- 1700 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.

(i) α/β interstratification: Interstratification can be semi-quantitatively 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 cm-1 and 496 cm-1 respectively 12. However, the various peak positions in the range from 3630 cm-1 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 de-convoluted 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 centred at 530 cm-1, 570 cm-1, and 620 cm-1. The intensity ratio of the peaks corresponding to in-plane 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 N-NCAH.

.

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Figure 2. FTIR spectra of co-precipitated hydroxides in the range (a) 200-800 cm-1 ( β, α1, α2, α3 corresponds to peaks centred at ~ 490cm-1, 530 cm-1, 570 cm-1, and 620 cm-1) (b) 2500–3800cm-1 (Sharp peak βˊ corresponds to β-M(OH)2, the remaining broad peaks α1ˊ, α2ˊ, α3ˊ 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 (Iαˊ = I α1ˊ+I α2ˊ +I α3ˊ) and (d) FTIR spectra of hydroxides taken in the range of 900 – 1700 cm-1 .

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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 centred close to 3630 cm-1 corresponds to β-M(OH)2, the remaining broad peaks are from the OH stretching modes of αM(OH)2 . The summed up intensity of all the peaks corresponding to α-M(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 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 OH-stretching and in plane-mode corresponding to β phase with respect to α phase is in S-NCH, N-NCH, S-NCAH and N-NCAH 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

Infrared Spectroscopy Signatures of Aluminium Segregation and

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