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Synthetic Pathway Determines the Nonequilibrium Crystallography
of Li- and Mn-Rich Layered Oxide Cathode Materials
Ashok S. Menon, Seda Ulusoy, Dickson O. Ojwang, Lars Riekehr, Christophe Didier,
Vanessa K. Peterson, Germań Salazar-Alvarez, Peter Svedlindh, Kristina Edström, Cesar Pay Gomez,
and William R. Brant*
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ABSTRACT: Li- and Mn-rich layered oxides show significant
promise as electrode materials for future Li-ion batteries. However,
an accurate description of its crystallography remains elusive, with
both single-phase solid solution and multiphase structures being
proposed fo r h igh per fo rming mate r i a l s such as
Li1.2Mn0.54Ni0.13Co0.13O2. Herein, we report the synthesis of single-
and multiphase variants of this material through sol−gel and solid-
state methods, respectively, and demonstrate that its crystallog-
raphy is a direct consequence of the synthetic route and not
necessarily an inherent property of the composition, as previously
argued. This was accomplished via complementary techniques that
probe the bulk and local structure followed by in situ methods to
map the synthetic progression. As the electrochemical performance
and anionic redox behavior are often rationalized on the basis of the presumed crystal structure, clarifying the structural ambiguities
is an important step toward harnessing its potential as an electrode material.
KEYWORDS: Li- and Mn-rich layered oxides, Li-ion battery cathodes, synthesis−structure relationships, anionic redox materials,
stacking faulted materials
■ INTRODUCTION and as a multiphase (MP) material, existing as an intergrowth9
The search for novel high energy density positive electrode of cation-ordered monoclinic Li2MnO3 and transition metal
10
materials for Li-ion batteries has led to the discovery of several (TM)-disordered hexagonal LiNi0.33Mn0.33Co0.33O2 phases
promising but increasingly complex materials, such as the Li- (x[Li2MnO3]·(1 − x)[LiTMO ], x = 0.5).11−132 These phases
and Mn-rich layered transition metal oxide system.1 In are said to exist as domains integrated through a shared cubic
particular, Li1.2Mn0.54Ni0.13Co0.13O2 (LMNCO), is considered close-packed (ccp) O2− substructure. Although both models
a likely candidate for commercialization due to its high specific possess long-range Li-TM superstructure ordering, the
capacity (∼300 mAh/g), facilitated by the joint participation of manifestation of this ordering in the TM layers differs. In
cations and anions in its functional redox process.2 However,
harnessing the high capacity comes at the cost of irreversible the single-phase model, superstructure arises from preferential
capacity loss and voltage hysteresis over continued electro- occupation of 2b and 4g (C2/m) crystallographic sites by Li
chemical cycling originating from structural transformations in and Mn, respectively, with Co and Ni distributed across the
the material.2,3 Among other approaches, crystallographic two sites.14 In the multiphase model, the superstructure is
modifications have been successful in improving the electro- formed by Li and Mn ordering within the Li2MnO3 phase/
chemical performance of LMNCO, although much work
− domain, where Li in the TM layer is surrounded exclusively byremains to be done.4 6 Mn.11,13 However, these models are idealized disorder-free
Efforts aimed at further developing the LMNCO system
must be complemented by fundamental investigations of
physical characteristics and properties. This is especially Received: December 4, 2020
relevant because of the chemical and structural complexity of Accepted: February 1, 2021
LMNCO, where gaps in our knowledge of the crystallographic Published: February 10, 2021
structure exist. LMNCO is argued to exist in multiple
crystallographic forms (Figure 1a): as a single-phase (SP)
solid solution, expressed as Li[Li0.2Mn
7,8
0.54Ni0.13Co0.13O2]O2,
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Figure 1. (a) Single- and multiphase LMNCO structure models. The Li−transition metal (TM) and Li−Mn ordering in the corresponding models
are also shown. Dashed hexagons represent superstructures. (b) Structural defects that can occur in Li- and Mn-rich layered transition metal oxides.
(c) Stacked normalized X-ray diffraction patterns of Li2MnO3, Li1.2Mn0.54Ni0.13Co0.13O2, and LiNi0.33Mn0.33Co0.33O2. The superstructure reflections
in Li2MnO3 and Li1.2Mn0.54Ni0.13Co0.13O2 are indexed with C2/m space group symmetry.
representations, and in reality, structural disorder occurs materials originates from the disruption of periodicity in the
leading to variation from the ideal case.7,14 c direction due to TM layer stacking faults.8,16 This disruption
Structural characterization of LMNCO is complicated by can manifest in multiple ways in LMNCO. For example, in the
three kinds of disorders that manifest across different MP model, stacking faults may be caused when Li2MnO3-like
crystallographic length scales; (1) Li−TM site mixing (in the (Li−Mn) stacking is interrupted by a LiNi0.33Mn0.33Co0.33O2-
TM layer), (2) stacking faults, and (3) interlayer Li+−Ni2+ like TM-only layer, in addition to irregular stacking of similar
mixing. These are schematically illustrated in Figure 1b and layer types.
have been reported in several works, irrespective of the The determination of LMNCO as single- or multi-phase is
structure model employed.7,8,11,12 Figure 1c shows the X-ray nontrivial as the synthesis method has a thermodynamic
diffraction patterns of LR-TMOs, with that of LiNi0.33Mn0.33- influence on the material structure. Shukla et al. have shown
Co0.33O2. The underlying similarity between these compounds that the bulk structure of LMNCO is composed of monoclinic
due to their layered structure is apparent. The primary phases with randomly stacked domains.7 However, the
difference between the patterns are the superstructure existence of separate Li2MnO3 and Li-TM-O2 domains/phases
reflections in the 20°−35° (2θ) Cu Kα range (1.4−2.4 Å−1 in LMNCO has also been reported by Yu et al. (using solid-
in Q-space) in the Li-rich systems. The di raction pattern of state method), among others.11,13ff In addition to slight
LiNi0.33Mn0.33Co0.33O2, on the other hand, does not possess stoichiometric variations, these studies employ different
superstructure reflections due to the random distribution of material synthesis protocols. It is also worth noting that
TM ions in the TM layer.10 The asymmetric broadening of the although the effects of parameters such as sintering temper-
superstructure reflections (Warren fall15) in the Li-rich ature and synthesis route on the properties of LMNCO have
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been investigated,17−19 a specific structural model was assumed measurements with a PerkinElmer ICP-OES Avio 200 system. The
for the analysis. However, because of the compositional and powders were dissolved in a HCl:HNO3 (3:1 v/v) solution (ICP
crystallographic complexity of LMNCO, it is reasonable to grade) and diluted to the required concentration by using a solution
assume that synthetic variations can lead to dissimilar of 5 vol % HNO3 in ultrapure Milli-Q water (blank) prior to the
nonequilibrium crystallographic structures, resulting in the analysis. The PerkinElmer Pure Plus Multielement calibration
20 standard was used as the reference for the ICP-OES measurements.aforementioned contradictory observations. Considering that The particle size and morphology were studied by using a Zeiss
a thermodynamically stable product is not reached (due to LEO 1550 scanning electron microscope (SEM). The powdered
limited heat treatment), a single structure model is often samples were spread on carbon tape and coated with a thin layer of
insufficient to describe this system. AuPd alloy to prevent charging. The images were obtained at an
The present work investigates the hypothesis that the accelerating voltage of 5 kV by using the InLens detector. Powder
LMNCO synthetic pathway defines the observed crystal samples for transmission electron microscopy were prepared by
structure. Toward this, LMNCO was intentionally synthesized crushing the powder in a mortar followed by sonication in anhydrous
via two approaches with extremely contrasting degrees of ethanol and drop casting the dispersion on a holey-carbon copper
precursor mixing, solid-state and sol−gel, with the intent that grid. Scanning transmission electron microscopy and X-ray energy-
dispersive spectroscopy (STEM-EDX) maps were recorded by using a
each would produce significant crystallographic and morpho- probe corrected FEI Titan Themis 200 microscope operating at 200
logical differences. The products were characterized over kV equipped with a four-detector Super-X EDS system. The EDS
different structural length scales by using X-ray and neutron images were acquired and evaluated with the software ESPRIT 1.9
powder diffraction, Raman spectroscopy, electron microscopy, from Bruker. Quantification was performed standard-less with the
and magnetic measurements, thus providing a complete Cliff−Lorimer method using theoretical k-factors provided by the
structural perspective beyond the “single- vs multi-phase” software.
debate surrounding this material. The observed differences Thermogravimetric and differential thermal analysis (TG-DTA)
were rationalized through investigation of the synthesis process were performed simultaneously by using a Netzsch STA 409 thermal
in situ through thermal and powder di raction analysis, and analyzer. The precursor mixture was placed in an alumina crucible andff
synthesis−structure relationships are established. heated at 5 °C/min from 25 to 900 °C in air (60 mL/min flow rate).Synchrotron X-ray diffraction (XRD) experiments were performed
on the Powder Diffraction beamline22 at the Australian Synchrotron.
■ EXPERIMENTAL SECTION The powder samples were packed in 0.5 mm (diameter) quartz
Synthesis. Li Mn Ni Co O (LMNCO) samples were capillaries and data collected in transmission mode by using the1.2 0.54 0.13 0.13 2
synthesized by using solid-state and sol−gel methods. For solid- Mythen II detector from 1° to 81° (2θ) using a wavelength of
state synthesis, the precursors:lithium carbonate (7Li CO , Sigma- 0.7736831(8) Å (∼16 keV). Two data sets were collected for 40 s2 3
Aldrich, 99% 7Li), manganese(IV) dioxide (MnO2, Alfa Aesar, 98%), each with the detector set 0.5° apart to cover gaps between the
nickel(II) oxide (NiO, Alfa Aesar, 99%), and cobalt (II, III) oxide detector modules; these were then merged by using the in-house
(Co3O4 Alfa Aesar, 99.7%), were thoroughly mixed by using a mortar software, PDViPeR. The wavelength and instrumental parameters
and pestle. 7Li2CO3 was used to reduce neutron absorption by
6Li in were determined by using data collected for the NIST 660b LaB6
natural Li. An ∼10 wt % excess of Li2CO3 was used to compensate for standard reference material. Constant wavelength neutron powder
the loss of Li during high-temperature annealing. diffraction (NPD) data were collected on the high-resolution neutron
23
The sol−gel precursor was prepared through a modified Pechini powder diffractometer, Echidna, at the Australian Nuclear Science
sol−gel based method.21 Stoichiometric amounts of lithium acetate and Technology Organisation (ANSTO). The solid-state and sol−gel
dihydrate (CH3COOLi·2H2O, Sigma-Aldrich, reagent grade), samples were measured by using neutron wavelengths of 1.62183(2)
manganese(II) acetate tetrahydrate ((CH3COO)2Mn·4H2O, Sigma- and 1.62189(2) Å, respectively. For the measurement, ∼2.1 g of the
Aldrich, ≥99%), nickel(II) acetate tetrahydrate ((CH3COO)2Ni· solid-state sample and ∼0.38 g of the sol−gel sample were packed into
4H2O), Sigma-Aldrich, ≥99%), and cobalt(II) acetate tetrahydrate 6 and 9 mm (diameter) vanadium cans, respectively. Data were
((CH3COO)2Co·4H2O, Sigma-Aldrich, ≥99%) were dissolved in 300 collected over a 2θ range of 5°−164° for a duration of 4 h for the
mL of deionized water. An excess of the Li source, ∼2.5 wt %, was solid-state sample and 10 h for the sol−gel sample. The wavelength
again used to account for Li loss during annealing. Similarly, a 300 mL and the instrumental parameters were determined by using the NIST
aqueous solution of citric acid (Sigma-Aldrich, ≥99.5%) and EDTA 660b La11B6 standard reference material.
(ethylenediaminetetraacetic acid, ACS reagent) was also prepared. In situ synchrotron XRD measurements were performed at the I11
The cation:citric acid:EDTA molar ratio was approximately 1:1.5:1. High Resolution Powder Diffraction beamline24 at the Diamond Light
The two solutions were thoroughly mixed by magnetic stirring for 1 h, Source with a wavelength of 0.8265203(3) Å. The precursor mixture
after which the pH was adjusted to ∼7.5 by using ammonium was loaded into a 0.5 mm (diameter) quartz capillary and heated by a
hydroxide solution (NH4OH, Sigma-Aldrich, 28−30%). The solution Cyberstar hot-air blower. The capillary, under rotation, was initially
was heated at 120 °C overnight while stirring, which led to the heated to 400 °C at ∼12 °C/min and then at ∼6 °C/min until the
formation of a dry gel that was then crushed into a powder. This end. Diffraction data were collected with an acquisition time of 20 s
powder was then transferred to an alumina crucible and heated in a throughout the heating by using the Mythen position sensitive
muffle furnace in air at 500 °C (5 °C/min ramp) for 5 h and allowed detector. Data collection was stopped at ∼800 °C due to reaction
to cool to room temperature in the furnace. between the sample and capillary. The wavelength and instrumental
The two precursors (mixture of powder precursors for the solid- parameters were determined by using data for the NIST 640c Si
state method and the preheated precursor for the sol−gel method) standard reference material. In situ NPD experiments were performed
were separately transferred to an alumina crucible and annealed in air at the high-intensity neutron powder di ractometer, Wombat,25ff at
at 900 °C (5 °C/min ramp) for 12 h by using a muffle furnace. After ANSTO over a 2θ range (16°−136°). The solid-state and sol−gel
annealing, they were quenched to room temperature by bringing the samples were measured by using neutron wavelengths of 2.41656(7)
crucibles in contact with an aluminium plate. and 2.41580(7) Å, respectively. The precursors were packed in
Two additional samples, Li2MnO3 and LiNi0.33Mn0.33Co0.33O2, cylindrical Pt cans, which were then heated in a high-temperature
were also studied for comparative purposes. Li2MnO3 was synthesized furnace (ILL type, niobium element vacuum furnace) equipped with a
in a similar way to sol−gel LMNCO, whereas LiNi0.33Mn0.33Co0.33O2 Pt tube insert. The solid-state sample was heated from room
was obtained commercially from Custom Cells Itzehoe GmbH. temperature to 300 °C at 10 °C/min, while the sol−gel sample was
Characterization. Elemental analysis was performed by induc- heated to the same temperature at 5 °C/min. They were then heated
tively coupled plasma−optical emission spectroscopy (ICP-OES) to 900 °C at 5 °C/min and annealed for 6 h, after which the furnace
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Figure 2. (a) SEM images (scale bars represent 1 μm and 200 nm for SS-LMNCO and SG-LMNCO, respectively). Stack plots of (b) X-ray and (c)
neutron diffraction patterns of the LMNCO samples (intensities are normalized to highest values) along with their (d) Raman spectra. The insets
in (b) and (c) show a Q-space region with superstructure reflections.
was allowed to cool. Diffraction data were recorded every minute Additional details of the refinement procedures and the refined
during the thermal treatment. The wavelength and the instrumental values are provided in the Supporting Information, sections S6 and
parameters were determined by using the data for the NIST 660b S7. Crystallographic structures were visualized by using the VESTA
La11B standard reference material. software.326 It should be noted that diffraction data have been plotted
Instrumental parameters of the diffractometers were determined by in terms of the reciprocal space scattering vector, Q (Å−1), to facilitate
Pawley refinement26 of the corresponding unit cells against data direct comparison between the different data sets. Q is related to the
collected from the standard reference materials. Refinement of the scattering angle (2θ) by Q = (4π sin θ)/λ, where λ is the wavelength
hexagonal (R3m) and monoclinic (C2/m) unit cell parameters of the of incident radiation.
samples against X-ray diffraction data was performed by using TOPAS The magnetic properties were measured with a Quantum Design
Academic (V6) software.27 The monoclinic cell is a supercell of the magnetic property measurement system (MPMS-XL). The temper-
hexagonal cell, and they are related via the following equation, where ature dependence of constant field DC magnetization was measured
a,̅ b̅, and c ̅ are the unit cell parameters. from 300 to 2 K. Each sample was first cooled to 2 K in zero field,
i− y then a field of 100 Oe was applied, and data were collected between 2jjj 2 0 2/3j zzzz and 300 K (zero-field-cooling mode, ZFC). The sample was then(a ̅ b ̅ c ̅)C2/m = (a ̅ b ̅ c ) j z̅ R3̅m ·jjjj−1 −3 1/3zzzz cooled under the same applied field from 300 to 2 K, whilejj0 0 1/3zz magnetization was measured (field-cooling mode, FC). Isothermalk { magnetization curves were measured at 5 K in magnetic fields up to
Refinement (Rietveld28,29) of the LMNCO structures against X-ray ±50000 Oe. The temperature dependent sinusoidally varied (AC)
and neutron diffraction data was performed with FAULTS30 and susceptibility χ = χ′ + iχ″, where χ′ is the in-phase component and χ″
TOPAS, respectively. FAULTS facilitates the refinement of stacking is the out-of-phase component of the AC susceptibility, was measured
faulted structures, thereby enabling an investigation of the degree of in an AC magnetic field of 4 Oe at various frequencies (1.7, 17, and
faulting within the structure in addition to other structural parameters. 170 Hz) within the temperature range 250 to 2 K. The inverse
The single-phase stacking-faulted LMNCO structure model was magnetic susceptibility curves were fitted to the Curie−Weiss law (χ =
obtained by modifying a previously reported Li MnO 312 3 structure to C/(T − θ), where C is the Curie constant, T is the temperature, and θ
the LMNCO structure and approximating the TM species to Mn (i.e., is the Curie−Weiss temperature) by the SciPy33 “curve_fit”
Li1.2Mn0.54Ni0.13Co0.13O2 = Li1.2Mn0.8O2), to avoid overparametriza- optimization function.
tion. The difference between the TM electronic charges before and Raman spectra were measured on a Renishaw InVia Raman
after this approximation is ∼5.8% and therefore is reasonable. For SS- microscope with an excitation wavelength of 532 nm over the range
LMNCO, a two-phase model comprising of stacking-faulted 1000 to 100 cm−1. Prior to the measurements, instrument calibration
Li MnO 31 and LiNi Mn Co O 102 3 0.33 0.33 0.33 2 phases was used, with the was performed by using the internal Si reference standard (520.6 ±
latter being incorporated as a background phase. Refinements against 0.1 cm−1). To improve the data quality, ten spectra with an individual
neutron diffraction data were performed without using stacking- 15 s exposure time were averaged for each sample.
faulted structure models. The single-phase structure model was Galvanostatic cycling was conducted by using Swagelok cells
obtained by modifying the LMNCO structure model reported by prepared in an argon-filled glovebox in half-cell configuration. The
Whitfield et al.14 to fit the stoichiometry of the LMNCO samples in working electrode was prepared by mixing ∼75 wt % of the active
this study. Multiphase LMNCO structure refinements were material (LMNCO) and ∼25 wt % of carbon black (Super P
performed by using Li 92MnO3 and LiNi
10
0.33Mn0.33Co0.33O2 struc- Conductive, Alfa Aesar, 99%) with a mortar and pestle. This mixture
tures, similar to conventional multiphase Rietveld refinements. was dried overnight in a vacuum oven inside the glovebox at 120 °C.
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Figure 3. Bright field (BF) STEM data with EDX maps of (a) SG-LMNCO and (b) SS-LMNCO. Quantified elemental maps are shown at the
bottom.
Half-cells were prepared by using Li metal as a counter electrode and and the different peak amplitude of the 108 and 110 reflections
two glass fiber separators (dried at 150 °C for 6 h in a vacuum inside (R3m symmetry, observable at ∼4.5 Å−1) unambiguously
the glovebox), with a standard electrolyte solution of 1 M LiPF6 in evidence a monoclinic symmetry. Pawley refinement of a
ethylene carbonate (EC):diethyl carbonate (DEC) (1:1 vol %)
(Sigma-Aldrich, 99%). The cells were cycled on the Land BT2000 monoclinic (C2/m) unit cell is tabulated in Table S3.
battery testing system between 2.0 and 4.8 V at 5 mA/g under Despite the close composition and bulk crystallographic
ambient conditions (∼22 °C), with an initial resting step at the open structure of the two samples, the differences in Raman spectra
circuit voltage (OCV) for 5 h. of the samples (Figure 2d) are quite distinct. However, as the
deconvolution of the spectra is complicated by the elemental
■ RESULTS AND DISCUSSION composition, structural disorder, and microstructural differ-ences, conclusions that can be drawn from it are limited. A
Morphology, Stoichiometry, and Long-Range Crys- qualitative analysis of the spectra, presented in the Supporting
tallographic Structure. The as-synthesized samples notice- Information, section 3, points toward incomparable local TM-
ably differed in their morphology (Figure 2a). The solid-state O coordination environments in the samples, with the SS-
LMNCO sample (SS-LMNCO) had heterogeneous secondary LMNCO sample suggesting the possible existence of multiple
particles several micrometers in size formed from tightly phases. Taken together, these results establish that the two
packed primary particles of varying sizes, with particles at the
∼ − ∼ − LMNCO samples have comparable stoichiometry and long-surface ( 1 3 μm) larger than interior ones ( 0.5 1 μm).
The sol−gel sample (SG-LMNCO) was predominantly range average structure but dissimilar local structural features.
composed of loosely bound homogeneously shaped particles, Differences in Local TM Distribution. As the two
100−200 nm in size. The Li:Mn:Ni:Co stoichiometry was LMNCO models have identical average structures, character-
determined by inductively coupled plasma optical emission ization techniques sensitive to the local (Li-)TM ordering must
s p e c t r o s c o p y ( I C P - O E S ) a n a l y s i s t o b e be employed to investigate the structural differences. Here, the
1.216(13):0.533(19):0.125(9):0.12(1) and 1.2101(33):0.536- TM distributions of the samples were probed at different
(16):0.128(34):0.124(31) for SS-LMNCO and SG-LMNCO, length scales by using scanning transmission electron
respectively (Table S1). The two compositions are therefore microscopy−X-ray energy dispersive spectroscopy (STEM-
comparable, with a Li content slightly higher than expected EDX) and magnetic measurements. EDX mapping was
due to the excess used in synthesis. performed at microscopic length scales to probe the chemical
The diffraction data in Figure 2 reveal an overall structural homogeneity of the samples. The SG-LMNCO map revealed a
similarity between the samples, with the exception of the homogeneous distribution of TMs without microscopic
superstructure reflections (insets in Figures 2b,c). The parent segregation of any species, including oxygen which was
hexagonal (R3m) unit cells of the two materials were uniformly distributed and close to the expected ∼71 mol %.
compared by using Pawley analysis of the X-ray diffraction The quantified values for the constituent elements are
(XRD) data. The unit cell parameters of the samples show comparable to the composition of LMNCO (Tables S4 and
slight differences0.16% and 0.04% for a(b) and c lattice S5). On the other hand, the SS-LMNCO sample is
parameters, respectivelyand are tabulated in Table S2. The inhomogeneous and composed of at least three chemically
c/3a value, a measure of the deviation of the hexagonal lattice distinct particle types or regions, which are shown in Figure 3b
from the ideal cubic close-packed (ccp) arrangement (c/3a = (and highlighted in Figure S4), with the corresponding
1.633), is comparable (difference of ∼0.2%) between the compositions tabulated in Table S5. Region 1 is predominantly
samples and to other layered LiNixMnyCo1−x−yO2 systems,
34 composed of Ni and Co. The O content was quantified to be
signifying that the samples have a well-crystallized layered ∼62%, which is lower than that of LiNi0.33Mn0.33Co0.33O2
structure. However, the presence of superstructure reflections (66%). Region 2, almost devoid of Ni and Co, has O and
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Figure 4. (a) Temperature-dependent constant field (DC) magnetic susceptibility (χ) of the samples. Field-cooled (FC) and zero-field-cooled
(ZFC) susceptibilities are shown as filled and empty symbols, respectively. (b) Reciprocal FC susceptibilities with their fits (dashed lines) to the
Curie−Weiss law. Temperature dependences of the imaginary/out-of-phase magnetic susceptibility (χ″) of SG-LMNCO (c) and SS-LMNCO (d).
Points are connected by lines for clarity.
Mn contents of ∼72.5% and ∼25%, which are comparably LiNi0.33Mn0.33Co0.33O2, where the spin glass behavior is
close to that of Li2MnO3. Region 3 has a considerable amount realized through configurational disorder facilitated by a
of all species, with the Mn and O values conforming to the random distribution of TMs in the TM layer.36
values expected from LMNCO. However, the Ni and Co This reasoning may be extended to explain the magnetic
values are lower than expected. This suggests either that the response of SG-LMNCO, where a structural configuration with
Li2MnO3-like phase is present in excess and the composition of random distribution of TM ions (with respect to Li) precludes
the Li−Ni−Mn−Co−O phase is manganese-deficient or that the formation of magnetic ordering within the sample above 2
the Li2MnO3-like phase is overrepresented within the area K. The layered (rock salt) structure with its stacking of two-
probed. These results suggest that the sample could be dimensional triangular edge-sharing planes imparts the geo-
composed of Li2MnO3 and Li[NiyCozMn1−y−z]O2 (y, z ≥ metric frustration necessary to realize a spin glass state. An
0.33) phases integrated heterogeneously, ranging from atomic- empirical criterion for the realization of a spin glass with
scale intergrowths to segregated Li2MnO3 and Li- magnetic frustration is that the |θ|/Tf value should be greater[NiyCozMn1−y−z]O2 particles. Considering that the ICP-OES than 10, where θ is the Curie−Weiss temperature and T is the
results establish the conformity of the overall stoichiometry to ffreezing temperature.35 As shown in Figure 4b, the Curie−
the expected value, SS-LMNCO may be represented as
· − ≤ ≤ Weiss temperature for SG-LMNCO is −57.72 K, which results(x)Li2MnO3 (1 x)Li[NiyCozMn1−y−z]O2 where 0.5 x
1 and y, z ≥ 0.33. This agrees well with the Raman spectra, in |θ|/Tf of 7.21, suggesting that SG-LMNCO, although not a
which show the peaks for pure Li MnO and LiNi Mn - perfect spin glass system, is close to a state of configurational2 3 0.33 0.33
Co O phases (Figure S3). With the EDX data clearly disorder with respect to the TMs. The out-of-phase (χ″)0.33 2
evidencing di erent TM distributions in the two samples at a component of the AC magnetic susceptibility of SG-LMNCOff
microscopic scale, magnetic measurements were performed to shows a frequency-dependent sharp onset of dissipation at ∼8
probe the distribution within the bulk. K (Figure 4c). This onset is found to shift toward lower
The temperature-dependent DC magnetic susceptibilities temperature with lower frequency, as typical of spin glass
(χ) of the LMNCO samples show pronounced differences, as systems, further evidencing the absence of magnetic/cation
seen in Figure 4a. In SG-LMNCO, the ZFC and FC curves clustering in this sample. Therefore, the magnetic response of
trace the same path until ∼8 K, where the plots diverge and a SG-LMNCO, in corroboration with the EDX results, does not
cusp is visible in the ZFC susceptibility (Figure S5). This is provide evidence for any TM segregation in the structure. This
typical of spin glass systems that are in a state of quenched conclusively rules out the existence of Li2MnO3 domains in the
magnetic disorder due to the presence of randomly oriented structure and suggests that SG-LMNCO is similar to the
magnetic moments.35,36 Comparable behavior is observed in single-phase LMNCO model.
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The magnetic response of SS-LMNCO is more complex.
The FC and ZFC curves diverge at ∼200 K, and on further
cooling, the FC curve increases strongly while the ZFC curve
increases only slowly, displaying an antiferromagnetic-like
transition at ∼50 K. This divergent behavior of the ZFC-FC
curves is characteristic of cluster glass systems composed of
phase-separated magnetic domains,37,38 suggesting that SS-
LMNCO is a multiphase system. The presence of Li2MnO3
phase is revealed by the antiferromagnetic transition at ∼50 K
in the ZFC curve, which is characteristic of this phase (Figure
S6). The significant increase of magnetic susceptibility on
continued cooling is due to different types of short-range
magnetic ordering, including ferromagnetic ordering with a
180° Ni2+ 4+Li layer−O−Mn TM layer interaction, which can be
introduced by Ni2+ in the Li layer in Li[NiyCozMn1−y−z]O2/
LiNi 360.33Mn0.33Co0.33O2 domains. On the basis of Good-
enough’s rules, the antiferromagnetic Mn−O−Li−O−Mn
superexchange interaction in the Li2MnO3 domains is
considered the dominant mechanism.39 Similar observations
fo r t he compos i t i ona l l y s im i l a r ( commerc i a l )
Li1.2Mn0.55Ni0.15Co0.10O2 were reported by Mohanty et al.,
13
including a magnetic transition at ∼50 K in the ZFC curve.
The slight hysteresis observed in the M−H curve for SS-
LMNCO may be attributed to the increased magnetization, as
opposed to SG-LMNCO where no hysteresis is observed
(Figure S7). From the inverse susceptibility (FC) plot in
Figure 4b, it is evident that SS-LMNCO follows the Curie−
Weiss law until ∼200 K, below which it begins to deviate due
to the onset of magnetic (ferromagnetic and antiferromag-
netic) ordering in different domains. In the AC susceptibility
curves (Figure 4d), the broad maximum of the χ″ component
around 50 K represents dissipation in the vicinity of the
expected phase transition and further confirms the existence of Figure 5. Refinement plots of stacking faulted structure models
antiferromagnetic Li MnO domains in the structure. Addi- against XRD data. The observed and calculated intensities are shown2 3
tionally, a feature is also observed around 200 K in the χ″ as colored circles and black lines, respectively. The difference curve is
component, signifying the dissipation of ferromagnetic or shown in blue and the positions of the Bragg reflections as vertical tickmarkers. In (a) the black and red markers denote Li MnO and
ferrimagnetic clusters. The Curie−Weiss fit of the samples and 2 3LiNi0.33Mn0.33Co0.33O2 phases, respectively. In (b), the markers
calculation of the effective magnetic moments are provided in denote the LMNCO phase. The definitions of the R-Factor and χ2
the Supporting Information (section S5.1). can be found in the FAULTS manual.
Structural Analysis Using Powder Diffraction Data.
The structural differences highlighted by the EDX and “single phase”, even if in practice a “single” LMNCO phase
magnetic measurements should be visible in diffraction data, model is used for refinements. For SS-LMNCO, the
the analysis of which can further corroborate the results percentage area of the phases (indicative of the phase
obtained thus far. Considering the different X-ray and neutron composition) after refinement was ∼65% and ∼35% for
scattering of constituent elements (Table S7) and risk of Li2MnO3 and LiNi0.33Mn0.33Co0.33O2, respectively, indicating
model overparametrization, structural refinements against an excess of Li2MnO3, further corroborating the EDX data
powder diffraction data must be constrained to produce where the phase was found to be overrepresented. That it is
statistically reliable results. Complementary techniques like also in excess from modeling of the diffraction data implies that
magnetic measurements are useful in guiding this constraint. the result obtained from EDX is likely applicable to the bulk.
Therefore, refinements of stacking-fault incorporated single- SG-LMNCO and SS-LMNCO XRD data were also intention-
and multiphase LMNCO structure models were performed ally fit to the multi- and single-phase models, respectively, to
against SG-LMNCO and SS-LMNCO XRD data, respectively, confirm the refinement results. This resulted in chemically
by using FAULTS.30 For SS-LMNCO, refinements were invalid models in either case, thereby justifying the initial
performed using faulted-Li2MnO3 and LiNi0.33Mn0.33Co0.33O2 choice of structure models. Refinement of the structure models
phases, with the latter incorporated as background. As seen in against the neutron diffraction data offered further validation of
Figure 5, satisfactory fits are obtained, and the degree of the results, in addition to confirming small amounts of Li+−
faulting (explained in the Supporting Information, section Ni2+ interlayer mixing in the samples. The structure refinement
S6.2) in SS-LMNCO and SG-LMNCO is calculated to be methodology and the refined values are provided in the
25.77(10)% and 48.15(20)%, respectively. While satisfactory, Supporting Information, section S6 (X-ray) and section S7
the fit is less good for SS-LMNCO due to the variation of (neutron). The results obtained thus far confirm the initial
faulting within the structure as previously reported for hypothesis that the crystallography of LMNCO is a
LMNCO and other Li-rich layered oxides.7,16 This variation consequence of synthesis pathway, given the identical
of faulting implies that this material cannot be considered as a composition and heat treatment. To investigate the underlying
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Figure 6. TG-DTA plots of (a) SG-LMNCO and (b) SS-LMNCO precursors. (c) Ex situ XRD data of the SG-LMNCO precursor after preheating
and final annealing. The 020 (C2/m) superstructure reflection is highlighted in the inset in (c). The asterisk denotes reflections from precursors
due to incomplete synthesis reactions. (d) In situ XRD data of the SS-LMNCO precursor at selected temperatures (offset in y), with the 020
reflection highlighted in the inset (plots are overlaid in the inset). In situ NPD data of the (e) SG-LMNCO and (f) SS-LMNCO precursors (offset
in y) at selected temperatures. In (d), (e), and (f), the LMNCO phase is shown with colored triangular markers. Fully indexed diffraction patterns
of the precursor mix (before the in situ heating) are shown in Figures S12 and S13.
reasons for these differences, the structures were studied the decomposition of organic matter is all that occurs during
during their synthesis through thermal analysis and in situ the final heating.
powder diffraction. This suggests that the LMNCO phase must have already
In Situ Investigation of the Material Synthesis. The formed during the intermediate annealing step. The thermal
thermal gravimetric−differential thermal analysis (TG-DTA) response of the as-synthesized sol−gel precursor upon heating
and in situ diffraction data for LMNCO precursors are shown to 550 °C (representative of the intermediate heating step) is
in Figure 6. As seen in Figure 6a, the gradual weight loss due to provided in Figure S11. The response can be divided into two
1931 https://dx.doi.org/10.1021/acsaem.0c03027
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stages. The first stage centered around 175 °C arises from the Li2MnO3 phase. This leads to the formation of Li2MnO3 and
loss of aqueous and acidic species, and as the temperature Li[NiyCozMn1−y−z]O2 (y, z ≥ 0.33) phases that are integrated
reaches 450 °C, ∼60% mass loss has occurred. Between 450 to varying degrees, ranging from crystallographic intergrowths
and 500 °C, there is a mass loss of about 35% because of the (within a particle) to instances where they exist as different
decomposition of organic matter and its removal as gaseous primary particles. Hence, SS-LMNCO has a multiphase
products. This decomposition proceeds through breaking of LMNCO structure that may be represented as (x)Li2MnO3·
chemical bonds and is highly exothermic. Although in situ (1 − x)Li[NiyCozMn1−y−z]O2 where 0.5 ≤ x ≤ 1 and y, z ≥
diffraction studies are required to understand the crystallization 0.33. Considering the above mechanism, it is clear that the
pathway, comparing the ex situ XRD patterns (Figure 6c) of specific crystallization pathway of SS-LMNCO will be
the intermediate and final SG-LMNCO samples, it is clear that dependent on the choice of precursors and temperatures at
the LMNCO phase has already formed after the intermediate which they begin to react. This offers additional possibilities
heating, with the crystallization happening concomitantly with through which the crystallography can be controlled.
the organic matter decomposition. Additional reflections in the Coprecipitation is another method to synthesize multi-cation
intermediate sample XRD data (highlighted with asterisks in systems like LMNCO, even more so than the solid-state
Figure 6c, inset) indicate that the synthesis is not complete. method. In terms of precursor mixing, it represents an
Superstructure reflections are also already visible in the intermediate case (with separate Li and TM sources) between
intermediate sample, signifying some degree of Li-TM sol−gel and solid-state methods, which provide a high and low
ordering. On the basis of these results, it can be understood degree of mixing, respectively. As described in this work, in
that during final annealing step the crystallinity of the already- addition to the local cation ordering, the different synthesis
formed LMNCO phase increases through atomic ordering, routes can also affect the degree of structural integration
together with the growth of crystallites. As the crystallization between the two phases (as in the multiphase model). This has
occurs from a metal−citrate matrix with a homogeneous recently been shown to affect the structural and electro-
distribution of cations, the probability for Mn to preferentially chemica l p roper t i e s o f so l id - s t a t e syn thes i zed
cluster around Li is reduced, hindering the formation of Li 431.2Mn0.6Ni0.2O2 and therefore may be thought to influence
Li2MnO3 domains and subsequent phase segregation in the LMNCO as well.
structure. In contrast to SS-LMNCO (Figure 6d, inset), the The differences in synthesis routes are also reflected in the
intensity of the 020 (C2/m) superstructure reflection in the degree of faulting observed in the samples. The reduced
forming SG-LMNCO material does not increase substantially stacking disorder in SS-LMNCO indicates increased perio-
over the course of the final annealing, suggesting an increased dicity in the Li−Mn layer along the c direction in the Li2MnO3
kinetic barrier toward Li-TM ordering. phase. This is achieved as the cation ordering involves only two
Figure 6b shows the TG-DTA and in situ diffraction data species (Li and Mn), and therefore order between consecutive
during heating of the SS-LMNCO precursor mix containing TM layers is thermodynamically favorable relative to SG-
Li2CO3, MnO2, NiO, and Co3O4. Indexed diffraction data of LMNCO, where four cations (Li with Mn, Ni, and Co) are
the precursor mix prior to heating are provided in Figures S12 involved. This imparts more degrees of freedom and entropy
and S13. The TG-DTA plots reveal that the synthesis proceeds drives SG-LMNCO toward a more disordered state. Addition-
through three stages corresponding to the decomposition of ally, the presence of organic matter may hinder the formation
MnO ,402 Li2CO3,
41 and Co3O4.
42 The mass loss at ∼450 °C of a well-layered structure, as shown for sol−gel synthesized
corresponds to the onset of the decomposition of MnO2 into Li2MnO3 in our previous work.
16
Mn2O3 accompanied by O2 gas evolution. The Li-rich phase This work establishes that the phase composition of
emerges between 500 and 600 °C, as seen in both the X-ray LMNCO varies significantly depending on the synthetic
(001 at ∼1.3 Å−1C2/m ) and neutron (131C2/m and 200C2/m at route. The samples in this work were synthesized by using
∼2.7 Å−1) data, and continues to grow with heating. At these identical final annealing protocols, and therefore the primary
temperatures, the Co3O4 and NiO reflections are unaffected difference between the synthesis methods is the degree of
whereas the intensities of the Li2CO3 and MnO2 reflections precursor mixing. However, since limited heat treatment
decrease substantially, as shown in the Figure S14, suggesting protocols were used, both structural forms could be
that the Li-rich phase is Li2MnO3. Starting at ∼620 °C, the metastable. This leads to questions concerning the most
020C2/m superstructure reflection is clearly seen in the XRD thermodynamically stable LMNCO configuration and its
data (Figure 6d, inset), evidencing Li−Mn ordering in the formation mechanism. Are there thermodynamic drivers for
Li2MnO3 phase. The asymmetric broadening of these phase segregation, or does entropy stabilization driven by
reflections is also clearly visible as they grow, indicating the configurational disorder lead to solid solution-like single-phase
presence of stacking faults in the Li2MnO3 phase. As the structures?
44 This is an important consequence of this study to
temperature approaches 700 °C, the Co3O4 and NiO consider when tailoring the design of electrode materials as
reflections begin to lose intensity, indicating their entry into different metastable configurations will result in different
the reaction matrix, and on further annealing, the Ni and Co electrochemical responses. Furthermore, the anionic redox
species are incorporated into the Li2MnO3 phase, leading to behavior of LMNCO has been explained based on the
the formation of the LMNCO phase. Note that Li2CO3 is Li2MnO3 domains in the structure in several studies.
12,45,46
present in the XRD data (at ∼1.5 Å−1) at temperatures close to However, the single-phase SG-LMNCO shows electrochemical
750 °C, which is higher than its melting point. This is because and anionic redox behavior that is typical of LMNCO systems
of the localized (non-uniform) heating of the hot-air blower (Figure S15). This suggests that the anionic redox behavior is
used for the in situ XRD experiment. However, as seen in the not dependent on the presence of Li2MnO3 domains. The
EDX maps, the inhomogeneous contact between the figure also includes the first-cycle potential−capacity plots of
precursors (that leads to varying diffusion lengths) results in SS-LMNCO. Although the profiles bear a qualitative
the heterogeneous incorporation of Ni and Co into the resemblance to that of SG-LMNCO, the capacities are
1932 https://dx.doi.org/10.1021/acsaem.0c03027
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expectedly lower than that of SG-LMNCO, primarily because Christophe Didier − Australian Centre for Neutron
of the larger micrometer-sized particles and their heteroge- Scattering, Australian Nuclear Science and Technology
neous morphology. Organization, New South Wales 2232, Australia; School of
Chemistry, University of New South Wales, Sydney 2052,
■ CONCLUSION Australia
This work demonstrates that Li1.2Mn
Vanessa K. Peterson − Australian Centre for Neutron
0.54Ni0.13Co0.13O2
(LMNCO) can exist in multiple nonequilibrium crystallo- Scattering, Australian Nuclear Science and Technology
graphic forms, with the synthesis route being a major Organization, New South Wales 2232, Australia; Institute
determinant. The solid-state synthesized LMNCO (SS- for Superconducting & Electronic Materials, Faculty of
LMNCO) crystallizes as a multiphase structure, with Engineering, University of Wollongong, Wollongong 2522,
Li MnO and Li[Ni Co Mn ]O (y, z ≥ 0.33) phases Australia; orcid.org/0000-0002-5442-05912 3 y z 1−y−z 2
integrated to varying degrees ranging from crystallographic Germán Salazar-Alvarez − Department of Materials Science
intergrowths to distinct particles. This is a consequence of the and Engineering, Uppsala University, SE-75103 Uppsala,
synthetic pathway, where the initial reaction between the Sweden; orcid.org/0000-0002-0671-435X
Li CO and MnO precursors forms Li MnO , after which Co Peter Svedlindh − Department of Materials Science and2 3 2 2 3
and Ni are integrated into the structure heterogeneously Engineering, Uppsala University, SE-75103 Uppsala, Sweden
resulting in Li[Ni Co Mn ]O (y, z ≥ 0.33) phases. The Kristina Edström − Department of Chemistry-Ångströmy z 1−y−z 2
sol−gel synthesized sample (SG-LMNCO), on the other hand, Laboratory, Uppsala University, SE-75121 Uppsala,
has a single-phase structure with a homogeneous distribution Sweden; orcid.org/0000-0003-4440-2952
of transition metal (TM) with respect to Li in the TM layer. Cesar Pay Gomez − Department of Chemistry-Ångström
Here, Li MnO domains do not form as the LMNCO phase Laboratory, Uppsala University, SE-75121 Uppsala, Sweden2 3
crystallizes from a metal−citrate matrix where the cations are Complete contact information is available at:
uniformly distributed. It is envisaged that these results clarify https://pubs.acs.org/10.1021/acsaem.0c03027
the structural ambiguities of this promising electrode material
and, in doing so, pave the way for further advancement of Li- Funding
and Mn-rich layered oxides. This work also accentuates the This research was funded by the Swedish Foundation for
need for extra caution and complementary techniques during Strategic Research (SSF) within the Swedish national graduate
the structural characterization of novel complex materials, school in neutron scattering (SwedNess). The authors also
where the local structural and configurational (dis)order can gratefully acknowledge funding from the Strategic Research
lead to multiple metastable states entirely dependent on the Area StandUp for Energy and the Swedish Energy Agency. The
synthetic route. Swedish research council, VR, is also acknowledged (Grants
349-2014-3946 and 2016-06959).
■ ASSOCIATED CONTENT Notes
*sı Supporting Information The authors declare no competing financial interest.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsaem.0c03027. ■ ACKNOWLEDGMENTS
ICP-OES results, Pawley refinements−XRD, Raman The authors are grateful to Dr. Anita D’Angelo (Beamline
spectroscopy, quantification of X-ray energy-dispersive scientist) at the Powder Diffraction beamline, Australian
spectroscopy (XEDS) maps, magnetic measurements, Synchrotron, as well as Dr. Chiu Tang (principal beamline
structure analysis using FAULTS, Rietveld refinement scientist) and Dr. Stephen Thompson (senior beamline
using neutron diffraction data, in situ studies, electro- scientist) at the I11 High Resolution Powder Diffraction
chemistry (PDF) beamline at the Diamond Light Source (UK) for their help and
guidance. The powder diffraction beamlines and sample
■ AUTHOR INFORMATION environment team at ANSTO, Sydney are also acknowledged.A.S.M. is particularly grateful to colleagues Victor Pacheco, Dr.
Corresponding Author Adriano Francesco Pavan, and Dr. Ronnie Mogensen for their
William R. Brant − Department of Chemistry-Ångström support and help.
Laboratory, Uppsala University, SE-75121 Uppsala,
Sweden; orcid.org/0000-0002-8658-8938;
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