Li-rich cathode materials synthesized through the solution combustion method essentially ... Lee et al.,26 reported high capacity for the Li-rich Li[Li0.2Ni0.2-.
RSC Advances PAPER
Cite this: RSC Adv., 2014, 4, 40359
Microstructure – twinning and hexad multiplet(s) in lithium-rich layered cathode materials for lithiumion batteries P. Manikandan,a P. Periasamya and R. Jagannathan*b Li-rich cathode materials synthesized through the solution combustion method essentially exhibits a superlattice layered structure and appears to have an edge over conventional cathode materials in terms of a good capacity with fade control. The cobalt content in this Li-rich cathode material is vital in determining the morphology, microstructure, high reversible redox peak and stable charge–discharge cycling. A typical Li-rich cathode with a Li1.2Ni0.13Mn0.54Co0.13O2 composition exhibits strange microstructure features in the SAED pattern viz., the simultaneous occurrence of twinning, and the hexad multiplets patterns yielded a good discharge capacity of 231 mA h g1. Whilst it is the
Received 20th June 2014 Accepted 12th August 2014
microstructure features, which have a profound dependence on the cobalt content appears to have control on the capacity. On the basis of these microstructural changes revealed in the SAED pattern and
DOI: 10.1039/c4ra06031h
cation ordering in the superlattice structure, an attempt has been made to explain the dependence of
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cobalt content vs. capacity performance in Li-ion batteries.
1. Introduction In the research and development of Li-ion batteries,1–3 there is an ever increasing zeal to achieve higher capacity, which is closer to the theoretical capacity in order to answer the question demanding high power suitability for applications in hybrid electric vehicles and electric vehicles.1,4,5 The Li-ion battery community has been endeavoring to achieve high capacity through several synthetic strategies,6–8 combinatorial chemistry,9,10 tuning structural and morphological features.11,12 In the design of cathodes yielding high capacity, the toxicity issue constitutes an important task, which can be comfortably addressed through the Li-rich cathode compositions having distinct edge over conventional cathode compositions such as LiCoO2, LiMO2 (M ¼ Ni, Mn, Co), LiFePO4 and LiMn2O4; offering moderate capacity of 180 mA h g1.12–15 Very recently, it has been established that a Li2MnO3 stabilized LiMO2 (M ¼ Ni, Mn, Co) system known by the generic formula xLi2MnO3$(1 x)LiMO2, labeled as Li-rich cathode materials16–18 perform considerably superior (>200 mA h g1) compared to individual constituent phases in terms of its electrochemical capacity with good capacity retention.19–22 Lirich cathode materials have rocksalt-type structures,20 in which excess lithium is accommodated within the transition-metal
Lithium Batteries – Electrochemical Power Sources Division, CSIR – Central Electrochemical Research Institute, Karaikudi – 630 006, Tamilnadu, India
a
b Lithium Batteries – Functional Materials Division, CSIR – Central Electrochemical Research Institute, Karaikudi – 630 006, Tamilnadu, India. E-mail: jags57_99@ yahoo.com; Fax: +914565227713; Tel: +919487167780
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layers at the expense of few M ions (M sites).20 In addition, the presence of cobalt in the transition metal layers reduces the electrode polarization and improves the activation of the Li2MnO3 phase.23 During the course of initial charging, the extraction of the lithium ion from the LiMO2 phase occurs in the voltage range of 2–4.4 V because of the oxidation of the Mn+ (Ni2+, Mn4+, Co3+) ions followed by the transformation of the electrochemically inert Li2MnO3 phase into electrochemically active MnO2 in the voltage region above 4.5 V with simultaneous extraction of the lithium and oxygen ions from the Li2MnO3 phase.19–25 In LiMO2 compounds, the average oxidation state of the M species is three (Ni2+, Mn4+ and Co3+) along with tetravalent manganese and monovalent lithium comprising the M layer in Li2MnO3 (Li[Li1/3Mn2/3]O2) structure, which acts as a stabilizing unit in the electrode structure.20–24 Li-rich cathode material are unique in terms of generating specic patterns in the electron diffraction pattern, which can be explained under the ambit of the hexad type patterns.20 The ne tuning of Li-rich cathode materials with good capacity through Co substitution in the Ni and Mn sites of Li-rich Li [Li0.2Ni0.2Mn0.6]O2 leads to cathode materials, which exhibit denite enhancement in their electrochemical features.20,23,25 Lee et al.,26 reported high capacity for the Li-rich Li[Li0.2Ni0.2Mn0.6]O2 cathode without any cobalt in its composition and prepared through a completely different carbonate co-precipitation method underlying subtle chemical processes involved during the synthesis. In this scenario, microstructure changes was evident from twinning the hexad multiplets of the electron diffraction spots in the SAED pattern reported in this
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investigation and it appears to play a crucial role in determining the capacity and performance of Li-ion battery. This may facilitate an understanding of the mechanism, which leads to a good capacity in this emerging Li-rich cathode material hitherto unknown.
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the Li-rich cathodes. Performance of the Li cells was evaluated through charge–discharge studies in the range of 2.0–4.8 V at a rate of 0.1 C at room temperature.
3. 2.
Experimental aspects
Li-rich cathode materials were synthesized through a glycine fuel based solution combustion method.27 The stoichiometric amounts of lithium nitrate, nickel nitrate, manganese nitrate and cobalt nitrate were dissolved in a minimum amount of water along with glycine dissolved separately in a minimum amount of water. The nitrates and the fuel were mixed together and transferred into a 300 mL alumina crucible, and the mixture was placed into a muffle furnace maintained at 400 C for 30 minutes and then allowed to cool naturally. The resulting amorphous product was ground well and calcined at 900 C for 12 h. Thus, in this work, the synthesized Li-rich cathode samples have the following compositions: Li1.2Ni0.2Mn0.6O2, Li1.2Ni0.16Mn0.56Co0.08O2, Li1.2Ni0.13Mn0.54Co0.13O2 and Li1.2Ni0.12Mn0.52Co0.16O2. The structural purity of the as-prepared Li-rich cathode samples were conrmed through powder X-ray diffraction (XRD) studies using a Bruker D8 Advance X-ray diffractometer ˚ with corundum employing a Cu Ka X-ray source (l ¼ 1.5418 A, Al2O3 as the internal standard) and the measurements were recorded at 2q range 10 –80 , and the XRD data were rened using a least squares data renement program. The chemical compositional stoichiometry of these cathode materials were analyzed using inductively coupled plasma mass spectrometry (Thermo Scientic – XSERIES 2). The morphology of the Li-rich cathode materials was examined using the eld-emission scanning electron microscopy (FE-SEM) MODEL ZEISS SUPRA™ 55VP, transmission electron microscopy (TEM) MODEL Hitachi S-3000H and the corresponding selected area electron diffraction patterns (SAED) were also recorded. The design of Li cells, coin cells were assembled using lithium foil (thickness: 0.75 mm) as the anode and the asprepared Li-rich cathode materials coated on aluminum foil as the cathode. In this cell design, a 1 M solution of LiPF6 in a 1 : 1 (v/v) EC–DMC mixture was used as the electrolyte. In this attempt, the cathode was fabricated using a doctor bladecoating slurry of 80% active materials (Li-rich cathode materials), 15% SP-carbon (Timcal) and 5% PVDF in NMP coated over aluminum foil (15 mm) and dried at 85 C for 12 h in a vacuum oven. An argon-lled glove box (mBRAUN MB200G) with oxygen and moisture levels less than 0.1 ppm was used during the fabrication of these Li cells. Moving to the electrochemical characterization of Li-rich cathode materials, cyclic voltammograms (CV) were recorded at a scan rate of 0.1 mV s1 in the voltage range 2.0 to 4.8 V under CR2032 cell conguration using a VMP3Z (Biological) multi-channel potentiostat/galvanostat electrochemical workstation, by maintaining the same active material weight. Galvanostatic charge–discharge studies were carried out using an Arbin multi-channel cycler instrument (BT2000) with similar active material weights for all
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Results and discussion
In this work, clear enhancement in the electrochemical performance was observed for the new Li-rich cathode materials. The electrochemical performance appeared to have a signicant dependence on the structural, microstructural information coupled with physico-, electrochemical properties. For this reason, in the following sections we present the results centering on the structural (XRD), microstructural (TEM) and physic-, electrochemical (CV, charge–discharge) studies, which will facilitate precise insight into the mechanism for explaining the good capacity observed.
3.1. XRD patterns, cell parameters and superlattice reections The Li-rich cathode materials were synthesized with varying cobalt contents at a pre-optimized temperature of 900 C for 12 h and were well characterized under respective space group m, C2/m symmetries of the constituent LiMO2, Li2MnO3 R3 phases, adopting a hexagonal a-NaFeO2 type structure (Fig. 1).19,20,28,29 Furthermore, least squares rened crystallographic cell parameter values obtained for the Li-rich cathode material is in good agreement (Table 1) with the respective standard values of the LiMO2 phase (JCPDS# 01-0871564).19,20,22 In addition, we observed that line width (b003) of the most intense line corresponding to (003) peak does not change at all for all compositions indicating almost the same crystallite size for all these samples (Table 1). In the XRD patterns, all the strong peaks are indexed under the rhombohedral phase (labeled as R) corresponding to the LiMO2 phase in line with earlier reports.19 On the other hand the weak peaks can be tallied with the Li2MnO3 monoclinic phase19,20 labeled as “M”. These XRD peaks were labeled using “R” or “M” in considering that either they correspond to the former or latter phases, respectively. Moreover, we observed a weak change (2q around 20.9 ) in the peak width with cobalt concentration as depicted in Fig. 1 (marked #) and its corresponding expanded version is shown in the right panel in Fig. 1. The perfect matching of the XRD patterns of these constituent phases both having layered structures co-existing together suggests the clear possibility of a super-lattice structure30–32 for this Li-rich cathode material. Closer investigation of the XRD patterns (Fig. 1) reveal some sharp splitting pairs corresponding to (006), (102) and (108), (110) peaks, which may be a clear indication of the perfect hexagonal ordering with good crystallinity. Furthermore, the relative intensity ratio of I(003)/I(104) for these Li-rich cathode materials is very important, especially when it is higher than 1.2 and favorable for least cation mixing owing to perfect hexagonal ordering coupled with pronounced lithium (de)intercalation processes as reported in literature.33–35
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sections. Moreover, the size of these cathode particles could be conrmed using the TEM images (Fig. 3). The TEM images of these Li-rich cathode materials depict particles having faceted edges in their morphology. In addition, it is signicant to note that the sample with the composition Li1.2Ni0.13Mn0.54Co0.13O2 yields some particles, which are hexagonal in shape25 as shown in Fig. 3c (inset #). 3.3. Microstructure-twinning through SAED analysis
Powder X-ray diffraction patterns of the Li-rich cathode phases synthesized through solution combustion method (at 900 C for 12 h) compared with standard patterns: (a) JCPDS# 01-087-1564, (b) JCPDS# 01-073-0152, (c) Li1.2Ni0.2Mn0.6O2, (d) Li1.2Ni0.16Mn0.56Co0.08O2, (e) Li1.2Ni0.13Mn0.54Co0.13O2 and (f) Li1.2Ni0.12Mn0.52Co0.16O2. Fig. 1
3.2. Morphological features of Li-rich cathode systems The particle size and morphology of the cathodic phases hold the key in achieving good electrochemical performances for eventual Li-ion batteries.36 Because the precise analysis and evaluation of the particle morphology are crucial, a more powerful and reliable microscopic analytical method based on FE-SEM has been employed in this study. It appears that the particle features of the synthesized Li-rich cathode materials (Fig. 2) resemble each other in terms of both particle morphology and size (200 nm) with the exception of the Li1.2Ni0.13Mn0.54Co0.13O2 sample having a smaller (100 nm) and highly agglomerated particles (Fig. 2c). Although most of these cathode particles yield faceted particles, the latter two samples (Li1.2Ni0.13Mn0.54Co0.13O2 and Li1.2Ni0.12Mn0.52Co0.16O2) have particles decorated with ne particles (Fig. 2c and d), while the former Li1.2Ni0.2Mn0.6O2 and Li1.2Ni0.16Mn0.56Co0.08O2 compositions are made of particles with relatively smooth surfaces (Fig. 2a and b). This distinguishing feature of these particles having ne particles decorated on the surface of the bigger cathode particles is that they have a greater stake in the electrochemical performance as elaborated in the subsequent
Although any differences in the particle morphology for these samples are barely decipherable, the corresponding SAED patterns (Fig. 4a–d) exhibit obvious differences suggesting the possibility of drastic structural changes in these cathode compositions at nanoscale. The spotty SAED patterns (Fig. 4a– d) with directionality,37,38 which was observed for all these samples, indicate the single crystalline nature of the samples in contrast to the spotty rings expected for conventional polycrystalline samples. Closer study of the SAED patterns make following generalizations possible: (i) these spotty patterns are basically concentric hexagons (Fig. 4a–d); (ii) the relative intensities of the vertices of these hexagons changes with the cobalt content (Fig. 4b and c); (iii) with an optimum cobalt content the SAED pattern exhibits twinning of the diffraction spots readily suggesting the presence of a microstructure (Fig. 4c and d); (iv) the relative intensities of the vertices of these hexagons is nearly absent (Fig. 4a and d). These observations in conjunction with the super-lattice structure have a signicant dependence on the cobalt content under investigation, and their corresponding electrochemical performances as will be discussed in the subsequent section. It is important to make a note of the several work highlighting the cation distribution20,30,38 and short range ordering39 in the Li-rich cathode materials. These underlie the importance of the cation environment-ordering through the microstructural changes of these Li-rich cathode compositions. 3.4. CV investigation on Li-rich cathode systems In the design of a lithium battery for its application, the performance of a cathode is very crucial and therefore extensive evaluation, using precise electrochemical techniques such as CV and charge–discharge studies on the cathode material, is necessary and is explained as follows: a typical CV run on the Lirich cathode materials in the present study provides information on the Ni2+44+, Co3+44+ and oxygen loss processes
Table 1 Comparison of crystallographic refined cell parameters and the XRD line intensity ratio of the Li-rich cathode materials viz., Li1.2Ni0.2Mn0.6O2, Li1.2Ni0.16Mn0.56Co0.08O2, Li1.2Ni0.13Mn0.54Co0.13O2 and Li1.2Ni0.12Mn0.52Co0.16O2
Cathode composition
˚) a (A
˚) c (A
c/a
˚ )3 V (A
I003 I104
b003 103 rad
b020a 103 rad
Li1.2Ni0.2Mn0.6O2 Li1.2Ni0.16Mn0.56Co0.08O2 Li1.2Ni0.13Mn0.54Co0.13O2 Li1.2Ni0.12Mn0.52Co0.16O2
2.8545 2.8519 2.8459 2.8429
14.2065 14.2363 14.2053 14.1741
4.9768 4.9918 4.9914 4.9857
100.25 100.27 99.63 99.21
1.8680 1.9533 1.7530 1.8220
2.0 2.0 2.0 2.0
2.4 2.7 5.4 1.7
a
Indicates pronounced change in XRD line-width values.
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Fig. 2 Particle morphology micrographs using FE-SEM for Li-rich cathode materials with composition (a) Li1.2Ni0.2Mn0.6O2, (b) Li1.2Ni0.16Mn0.56Co0.08O2, (c) Li1.2Ni0.13Mn0.54Co0.13O2 and (d) Li1.2Ni0.12Mn0.52Co0.16O2.
Fig. 3 Particle morphology micrographs using TEM for Li-rich cathode materials with composition (a) Li1.2Ni0.2Mn0.6O2, (b) Li1.2Ni0.16Mn0.56Co0.08O2, (c) Li1.2Ni0.13Mn0.54Co0.13O2 (inset # – particles having hexagonal morphology) and (d) Li1.2Ni0.12Mn0.52Co0.16O2.
Fig. 4 Corresponding SAED patterns (TEM) of Li-rich cathode materials with composition (a) Li1.2Ni0.2Mn0.6O2, (b) Li1.2Ni0.16Mn0.56Co0.08O2, (c) Li1.2Ni0.13Mn0.54Co0.13O2 and (d) Li1.2Ni0.12Mn0.52Co0.16O2.
(Fig. 5a–d). On comparing the CV proles of the cobalt free vs. the cobalt substituted compositions, the CV proles corresponding to the abovementioned processes acquires considerable importance. The composition of Li1.2Ni0.13Mn0.54Co0.13O2 with a critical cobalt concentration shows good electrochemical performance and the corresponding CV traces indicate a highly
reversible process with the marked characteristic redox peak. For the cobalt free Li1.2Ni0.2Mn0.6O2 material (Fig. 5a), two anodic-oxidation peaks are obtained at 4.0 V and 4.6 V in the rst scan, which may correspond to the oxidation of Ni2+ and the loss of oxygen, but in the reverse cathodic scan only one peak is observed at 3.7 V representing the reduction of
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Fig. 5 Cyclic voltammograms of lithium cells made with cathode (a) Li vs. Li1.2Ni0.2Mn0.6O2, (b) Li vs. Li1.2Ni0.16Mn0.56Co0.08O2, (c) Li vs. Li1.2Ni0.13Mn0.54Co0.13O2 and (d) Li vs. Li1.2Ni0.12Mn0.52Co0.16O2 in the voltage range from 2.0 to 4.8 V (1st to 5th cycles) at 0.1 mV s1 (1 M LiPF6 in 1 : 1 EC–DMC solvents).
Fig. 6 Voltage vs. capacity profile of lithium cells made with cathode (a) Li vs. Li1.2Ni0.2Mn0.6O2, (b) Li vs. Li1.2Ni0.16Mn0.56Co0.08O2, (c) Li vs. Li1.2Ni0.13Mn0.54Co0.13O2 and (d) Li vs. Li1.2Ni0.12Mn0.52Co0.16O2 in the voltage range from 2.0 to 4.8 V (1st to 50th cycles) at 0.1 rate (1 M LiPF6 in 1 : 1 EC–DMC solvents).
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Ni4+.19,24 With the addition of cobalt in the Li-rich cathode material these two peaks gain importance indicating the dominance of the electro-active processes. For the Li1.2Ni0.16Mn0.56Co0.08O2 material (Fig. 5b) there are two well resolved oxidation peaks at 4.0 V and 4.6 V, respectively representing the oxidation of Ni2+ (also partial oxidation17 of Co3+) and the loss of oxygen. While during the cathodic scan, a peak at 3.7 V and a small hump at nearly 3.3 V are observed, which may correspond to the reduction of the Ni4+ and Mn4+, respectively, owing to the loss of oxygen. This could be inferred from the intense peak at 4.6 V during the rst anodic scan. In the subsequent scans, this hump disappears and could be ascribed to the possibility of insufficient cobalt species (Fig. 5b). Further, increase in the cobalt content with compositions
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Li1.2Ni0.13Mn0.54Co0.13O2 and Li1.2Ni0.12Mn0.52Co0.16O2 exhibit two strong characteristic peaks viz., the oxidation of Ni2+ (also partial oxidation of Co3+) and also the loss of oxygen during the rst scan.19,24 During the rst reverse-cathodic scan on Li1.2Ni0.13Mn0.54Co0.13O2 and Li1.2Ni0.12Mn0.52Co0.16O2 materials, there are three reduction peaks decipherable at 4.3 V, 3.7 V and 3.3 V. Of these, the rst two peaks can be attributed to the reduction of Ni4+ and Co4+, while the third peak at 3.3 V can be attributed to the reduction of Mn4+, which is rationalized in terms of loss of oxygen during the rst anodic scan.19,24 Moreover, in the subsequent scans, the CV features are remarkably different. Such that the dominant peak at 4.6 V representing the loss of oxygen disappears leaving only a feeble step around 4.5 V. While the peak at 3.9 V representing the oxidation of Ni2+ goes down in intensity.19,24 Furthermore, a stable high reversible redox peak occurring at 3.3 V from the second cycle onwards can be attributed to the Mn4+ redox process. While the high cobalt material presents a different picture, in which the complete loss of the oxygen peak is accompanied with a signicant drop in intensity of the other characteristic peaks. Thus, explaining the low performance as discussed in the next section. In brief, the results of the CV studies, a typical Li-rich cathode of Li1.2Ni0.13Mn0.54Co0.13O2 composition yielded impressive electrochemical features in terms of a superior (de) intercalation process, high reversibility and complete overlap of the CV scan.
3.5. Charge–discharge studies of Li-rich cathode systems
Fig. 7 Charge–discharge studies of lithium cells made with Li-rich
cathode materials: (a) voltage vs. capacity profile for first cycle (a) Li vs. Li1.2Ni0.2Mn0.6O2, (b) Li vs. Li1.2Ni0.16Mn0.56Co0.08O2, (c) Li vs. Li1.2Ni0.13Mn0.54Co0.13O2 and (d) Li vs. Li1.2Ni0.12Mn0.52Co0.16O2; (b) capacity vs. cycle number performance in the voltage range from 2.0 to 4.8 V (1–50 cycles) at 0.1 rate (1 M LiPF6 in 1 : 1 EC–DMC solvents). Discharge capacity fade DQLi (negative sign indicating capacity decrease upon cycling) indicated in respective cases of cathode materials.
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As discussed in the previous sections, a Li-rich cathode material is a combination of two phases viz., Li2MnO3 and LiMO2 explained under the generic formula xLi2MnO3$(1 x)LiMO2.16 From the literature the Li-rich cathode material, Li2MnO3 plays an important role in ensuring the structural stability and electrochemical performance during charge and discharge cycling.19–22,40 On the basis of the CV results, galvanostatic charge–discharge studies were carried out for the Li vs. Li-rich cathode material (CR 2032 conguration) at a rate of 0.1 C in the voltage range from 2.0 to 4.8 V for 50 cycles (Fig. 6 & 7). The voltage vs. capacity prole clearly shows a reduced electrode polarization and increased charge–discharge capacity with varying Li-rich cathode materials (Fig. 6a–d). The initial charge– discharge capacities for compositions Li1.2Ni0.2Mn0.6O2, Li1.2Ni0.16Mn0.56Co0.08O2, Li1.2Ni0.13Mn0.54Co0.13O2 and Li1.2Ni0.12Mn0.52Co0.16O2 of the Li-rich cathode materials were found to be 133/113 mA h g1, 218/162 mA h g1, 287/231 mA h g1 and 281/ 211 mA h g1, respectively. Of these four Li-rich cathode materials, a typical Li-rich cathode Li1.2Ni0.13Mn0.54Co0.13O2 composition resulted in the good discharge capacity of 231 mA h g1 the voltage range 2.0–4.8 V in the 1st cycle (Fig. 6c). It can also be seen that the voltage drop and polarization of Li1.2Ni0.13Mn0.54Co0.13O2 material at the 50th cycle was less compared with other Li-rich materials for the same 50th cycle as depicted in Fig. 6a–d. On close examination of the voltage vs. capacity proles (initial charge) of Li-rich cathode materials with varying cobalt content given in Fig. 7a, there are two clear regions, which can
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be specically attributed (i) the region below 4.5 V corresponding to electrochemically active components of the Mn+ (Ni2+, Mn4+, Co3+) species; (ii) the region above 4.5 V corresponding to the activation of Li2MnO3 following the removal of lithium and oxygen, eventually leading to the formation of Li2O and MnO2.19,20,24,25 Interestingly, the rst region does not show considerable change in the prole while the second region seen above 4.5 V exhibits a prolonged voltage plateau with an increase in the cobalt content.20,25 However, a further increase in the cobalt content (Li1.2Ni0.13Mn0.54Co0.13O2 material) curtails the plateau region indicating a drop in the capacity. Thackeray et al. explained the importance of the critical concentration of cobalt for obtaining a reasonable capacity for Li-rich cathode materials based on order–disorder considerations through structural schematics,20 which is in good agreement with our present results. In the galvanostatic cycling performance of these Li-rich cathode materials (Fig. 7b), it is observed that the Li cells (Li vs. Li-rich cathode material) exhibit a gradual capacity fade upon cycling and the discharge capacity drop DQLi was about 27%, 33%, 10% and 15% for the cathode compositions Li1.2Ni0.2Mn0.6O2, Li1.2Ni0.16Mn0.56Co0.08O2, Li1.2Ni0.13Mn0.54Co0.13O2 and Li1.2Ni0.12Mn0.52Co0.16O2, respectively, at the 50th cycle. The cell corresponding to Li vs. Li1.2Ni0.13Mn0.54Co0.13O2 material showed the lowest DQLi of 10% at 50th cycle. The corresponding charge–discharge capacities for the 1st and 50th cycles are 287/231 mA h g1 and 207/206 mA h g1, respectively. Low inherent capacity fading was present in Li1.2Ni0.13Mn0.54Co0.13O2 Li-rich cathode material and can be correlated with the progressive increase in surface passivation on the Li1.2Ni0.13Mn0.54Co0.13O2 electrode causing an ohmic drop upon cycling. To explain the mechanism of capacity fading; it is possible that
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the important cause could be the dissolution of the Li1.2Ni0.13Mn0.54Co0.13O2 active species (Mn+) in the electrolyte during cycling, which can be attributed to the existence of HF, which is easily formed when LiPF6 is used as the electrolyte salt.41,42 The charge–discharge results of the present studies are found to be comparable to the capacity fade reported earlier for Li-rich cathode materials.22,24 3.6. Twinning and hexad multiplet(s) exhibiting microstructure features for good capacity It is well established that cobalt substitution in the LiMO2 phase23 of the Li-rich cathode materials generates a reasonable electrochemical capacity, eventually paving the way for superior cathode materials in Li-ion batteries.16–22 Along these lines the previous results presented in this report also conrm good discharge capacity. This signicant enhancement in electrochemical performance has been attributed to increased cation disorder explained using several structural schemes with and without cobalt.20,23,25 Nevertheless, the present study focused on local structure modication, following cobalt substitution and the SAED patterns indeed conrm the profound changes in the physicochemical structure of the constituent phase manifesting as the micro structural changes depicted in Fig. 8. Clear insights on such microstructure may facilitate a possible mechanism to explain the obtained good capacity. On listing the observable features in these SAED patterns: (i) we have hexad multiplets viz., the rst kind having closely spaced diffraction spots as vertices (labeled as hexadN),37,38 while the second one is bigger in size having relatively brighter spots (labeled as hexadB) and so on; (ii) these diffraction spots either appear as single or twinned. Occurrence of hexad feature suggests the ordered arrangement of the constituent atoms in a hexagonal symmetry
Fig. 8 SAED pattern and corresponding schematic projection illustrating the presence microstructure revealing the two types of hexad pattern and twinning of spots for the optimized Li-rich cathode composition (Li1.2Ni0.13Mn0.54Co0.13O2).
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with the cation order–disorder arrangements changing in accordance with substituent(s) species. While, the twinning of the diffraction spots indicates a super lattice structure constituted by two closely matching phases with a layered structure.30–32 Close evaluation of the SAED patterns revealed that the Li1.2Ni0.13Mn0.54Co0.13O2 composition yields a good capacity and has the hexad multiplets feature37,38 along with complete twinning of the diffraction spots. In addition, for other compositions either the hexad multiplets feature or the twinning of the diffraction spots could be observed either alone or together. The simultaneous existence of the hexad multiplets feature along with twinned spots is unique to the Li1.2Ni0.13Mn0.54Co0.13O2 sample having the optimum cobalt content and appears to have a special microstructure comprising of two closely spaced layers with the cations having high disorder. This can be rationalized in terms of the pronounced two fold increase in the XRD line width of the (020) line as given in Table 1. Our hypothesis is consistent with other reports based on the disorder in the cation distribution,20,30,38 NMR and pair distribution function analysis.39 Furthermore, it should be noted that a good capacity from Li-rich cathode materials can be correlated with the occurrence of the microstructure as revealed through the twinning of spots in the SAED pattern similar to superlattice features.19,30,31
4. Conclusion Li-rich cathode materials having an edge over conventional cathode materials have been synthesized through a solution combustion method. For harnessing a good capacity from these Li-rich cathode materials cobalt is used. Cobalt as a constituent plays a very critical role in terms of signicantly modifying the morphology, microstructure, impressive electrochemical features viz., superior (de)intercalation process, high reversibility and complete overlap of the CV scan. The optimum Lirich cathode Li1.2Ni0.13Mn0.54Co0.13O2 composition delivered a good discharge capacity of 231 mA h g1 and good control of the capacity fade becomes possible. Thus a good discharge capacity of the Li-rich cathode materials has been explained through cation ordering, microstructure patterning – twinning and hexad multiplets in the SAED pattern of this Li-rich layerstructured superlattice for Li-ion batteries.
Acknowledgements The rst author, P. Manikandan, acknowledges the Council of Scientic and Industrial Research (CSIR), New Delhi for granting the nancial support for the above work in the form of a senior research fellowship and also our thanks to the Director, CSIR-CECRI, Karaikudi.
References 1 M. Armand and J. M. Tarascon, Nature, 2008, 451, 652–657. 2 M. Whittingham, Chem. Rev., 2004, 104, 4271–4301. 3 J. Tarascon and M. Armand, Nature, 2001, 414, 359–367.
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4 A. S. Arico, P. Bruce, B. Scrosati, J. M. Tarascon and W. Van Schalkwijk, Nat. Mater., 2005, 4, 366–377. 5 J. B. Goodenough and Y. Kim, Chem. Mater., 2010, 22, 587– 603. 6 J. H. Lim, H. Bang, K. S. Lee, K. Amine and Y. K. Sun, J. Power Sources, 2009, 189, 571–575. 7 J. H. Kim and Y. K. Sun, J. Power Sources, 2003, 119, 166–170. 8 L. Zhang, T. Muta, H. Noguchi, X. Wang, M. Zhou and M. Yoshio, J. Power Sources, 2003, 117, 137–142. 9 C. H. Chen, J. Liu, M. E. Stoll, G. Henriksen, D. R. Vissers and K. Amine, J. Power Sources, 2004, 128, 278–285. 10 S. H. Kang and K. Amine, J. Power Sources, 2005, 146, 654– 657. 11 X. Luo, X. Wang, L. Liao, S. Gamboa and P. J. Sebastian, J. Power Sources, 2006, 158, 654–658. 12 P. Manikandan, P. Periasamy and R. Jagannathan, J. Power Sources, 2014, 245, 501–509. 13 F. Fu, G. L. Xu, Q. Wang, Y. P. Deng, X. Li, J. T. Li, L. Huang and S. G. Sun, J. Mater. Chem. A, 2013, 1, 3860–3864. 14 P. Manikandan, P. Periasamy and R. Jagannathan, J. Mater. Chem. A, 2013, 1, 15397–15405. 15 R. J. Gummow, A. D. Kock and M. M. Thackeray, Solid State Ionics, 1994, 69, 59–67. 16 C. S. Johnson, N. Li, C. Leef and M. M. Thackeray, Electrochem. Commun., 2007, 9, 787–795. 17 C. S. Johnson, N. Li, C. Leef, J. T. Vaughey and M. M. Thackeray, Chem. Mater., 2008, 20, 6095–6106. 18 N. Yabuuchi, K. Yoshii, S. T. Myung, I. Nakai and S. Komaba, J. Am. Chem. Soc., 2011, 133, 4404–4419. 19 B. Song, H. Liu, Z. Liu, P. Xiao, M. O. Lai and L. Lu, Sci. Rep., DOI: 10.1038/srep03094. 20 M. M. Thackeray, S. Kang, C. S. Johnson, J. T. Vaughey, R. Benedek and S. A. Hackney, J. Mater. Chem., 2007, 17, 3112–3125. 21 J. Yang, X. Zhang, X. Han, F. Cheng, Z. Tao and J. Chen, J. Mater. Chem. A, 2013, 1, 13742–13745. 22 J. Yang, F. Cheng, X. Zhang, H. Gao, Z. Tao and J. Chen, J. Mater. Chem. A, 2014, 2, 1636–1640. 23 J. Li, R. Klopsch, M. C. Stan, S. Nowak, M. Kunze, M. Winter and S. Passerini, J. Power Sources, 2011, 196, 4821–4825. 24 W. He, J. Qian, Y. Cao, X. Ai and H. Yang, RSC Adv., 2012, 2, 3423–3429. 25 O. TopraKci, A. K. TopraKci, Y. Li, L. Ji, L. Xue, H. Lee, S. Zhang and X. Zhang, J. Power Sources, 2013, 241, 522–528. 26 D. K. Lee, S. H. Park, K. Amine, H. J. Bang, J. Parakash and Y. K. Sun, J. Power Sources, 2006, 162, 1346–1350. 27 A. S. Prakash, P. Manikandan, K. Ramesha, M. Sathiya, J. M. Tarascon and A. K. Shukla, Chem. Mater., 2010, 22, 2857–2863. 28 C. H. Lei, J. Bareno, J. G. Wen, I. Petrov, S. H. Kang and D. P. Abraham, J. Power Sources, 2008, 178, 422–433. 29 A. Ito, K. Shoda, Y. Sato, M. Hatano, H. Horie and Y. Ohsawa, J. Power Sources, 2011, 196, 4785–4790. 30 Z. Lu, Z. Chen and J. R. Dahn, Chem. Mater., 2003, 15, 3214– 3220. 31 Z. Lu, L. Y. Beaulieu, A. Donaberger, C. L. Thomas and J. R. Dahn, J. Electrochem. Soc., 2002, 149, A778–A791.
This journal is © The Royal Society of Chemistry 2014
Paper
32 C. S. Johnson, J. S. Kim, C. Leef, N. Li, J. T. Vaughey and M. M. Thackeray, Electrochem. Commun., 2004, 6, 1085–1091. 33 Z. Wang, Y. Sun, L. Chen and X. Huang, J. Electrochem. Soc., 2004, 151, A914–A921. 34 X. Luo, X. Wang, L. Lao, X. Wang, S. Gamboa and P. J. Sebastian, J. Power Sources, 2006, 161, 601–605. 35 J. N. Reimers, E. Rossen, C. D. Jones and J. R. Dahn, Solid State Ionics, 1993, 61, 335–344. 36 X. Luo, X. Wang, L. Liao, S. Gamboa and P. J. Sebastian, J. Power Sources, 2006, 158, 654–658. 37 C. He, R. Wang, H. Fu and P. K. Shen, J. Mater. Chem. A, 2013, 1, 14586–14591.
This journal is © The Royal Society of Chemistry 2014
RSC Advances
38 M. M. Thackeray, S. H. Kang, C. S. Johnson, J. T. Vaughey and S. A. Hackney, Electrochem. Commun., 2006, 8, 1531– 1538. 39 J. Breger, N. Dupre, P. J. Chupas, P. L. Lee, T. Proffen, J. B. Parise and C. P. Grey, J. Am. Chem. Soc., 2005, 127, 7529–7537. 40 S. J. Shi, J. P. Tu, Y. Y. Tang, Y. X. Yu, Y. Q. Zhang and X. L. Wang, J. Power Sources, 2013, 221, 300–307. 41 H. F. Xiang, H. Wang, C. H. Chen, X. W. Ge, S. Guo, J. H. Sun and W. Q. Hu, J. Power Sources, 2009, 191, 575–581. 42 S. F. Lux, I. T. Lucas, E. Pollak, S. Passerini, M. Winter and R. Kostecki, Electrochem. Commun., 2012, 14, 47–50.
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