Article
Cite This: ACS Appl. Energy Mater. 2019, 2, 4521−4534
www.acsaem.org
Improving Performance of LiNi0.8Co0.1Mn0.1O2 Cathode Materials for
Lithium-Ion Batteries by Doping with Molybdenum-Ions: Theoretical
and Experimental Studies
Francis Amalraj Susai,† Daniela Kovacheva,‡ Arup Chakraborty,† Tatyana Kravchuk,§ R. Ravikumar,†
Michael Talianker,∥ Judith Grinblat,† Larisa Burstein,⊥ Yaron Kauffmann,# Dan Thomas Major,*,†
Boris Markovsky,† and Doron Aurbach*,†
Downloaded via BUFFALO STATE on July 22, 2019 at 12:14:19 (UTC).
See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
Department of Chemistry and Institute for Nanotechnology and Advanced Materials (BINA), Bar-Ilan University, Ramat-Gan
52900, Israel
‡
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
§
Solid State Institute, TechnionIsrael Institute of Technology, Haifa 32000, Israel
∥
Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
⊥
The Wolfson Applied Materials Research Centre, Tel-Aviv University, Tel-Aviv 69978, Israel
#
Department of Materials Science and Engineering, TechnionIsrael Institute of Technology, Haifa 32000, Israel
S Supporting Information
*
ABSTRACT: The work reported herein is an important continuation of our recent experimental and computational studies on
Li[NixCoyMnz]O2 (x + y + z = 1) cathode materials for Li-ion
batteries, containing minor amounts of multivalent cationic dopants
like Al3+, Zr4+, W6+, Mo6+. On the basis of DFT calculations for
LiNi0.8Co0.1Mn0.1O2, it was concluded that Mo6+ cations preferably
substitute Ni cations in the layered structure due to the lowest
substitution energy compared to Li, Co, and Mn. It was established
that the electrochemical behavior of LiNi0.8Co0.1Mn0.1O2 as a
positive electrode material for Li-ion batteries can be substantially
improved by doping with 1−3 mol % of Mo6+, in terms of lowering
the irreversible capacity loss during the first cycle, increasing
discharge capacity and rate capability, decreasing capacity fade upon
prolonged cycling, and lowering the voltage hysteresis and chargetransfer resistance. The latter is attributed to the presence of additional conduction bands near the Fermi level of the doped
materials, which facilitate Li-ions and electron transfer within the doped material. This is expressed by a lower charge-transfer
resistance of Mo-doped electrodes as shown by impedance spectroscopy studies. We also discovered unique segregation
phenomena, in which the surface concentration of the transition metals and dopant differs from that of the bulk. This near
surface segregation of the Mo-dopant seems to have a stabilization effect on these cathode materials.
KEYWORDS: lithium batteries, Ni-rich cathode materials, Mo6+ doping, electrochemical behavior, computational modeling,
dopant segregation
1. INTRODUCTION
the electrode stability is during cycling. Consequently,
significant efforts have been devoted to improving intrinsic
characteristics of Li[NixCoyMnz]O2 materials by lattice doping
with cations or anions.9−13 Amine et al. have shown that small
amounts of Al-doping of LiNi0.8Co0.2−xAlxO2 (0 < x < 0.1)
significantly stabilized the electrode behavior, lowered their
impedance, and improved power performance as well.14 It was
also shown that double doping of LiNi0.6 Co0.2 Mn 0.2O 2
The most promising cathode materials for lithium-ion batteries
(LIBs) are the lithium intercalated layered transition metal
oxides of layered structure having the general formula
Li[NixCoyMnz]O2 (x + y + z = 1) (NCM). NCM materials
can exhibit a specific capacity >200 mAh g−1 at Ni-content of
80% and higher. They can also demonstrate high rate
capability and may reach relatively low costs.1−8 However,
these NCM materials have disadvantages like low electronic
and ionic conductivities and structural instability; their
capacity also decays quickly upon cycling.1 In fact, the higher
the Ni-content is, and thus the specific capacity is, the lower
© 2019 American Chemical Society
Received: April 16, 2019
Accepted: May 22, 2019
Published: May 22, 2019
4521
DOI: 10.1021/acsaem.9b00767
ACS Appl. Energy Mater. 2019, 2, 4521−4534
Article
ACS Applied Energy Materials
fraction, XPS, and DFT calculations. The novelty of this
research is that, by employing the computational tools, we find
out how incorporation of the highly charged Mo6+ dopant into
the lattice affects the electronic structure of the material and
the relative distribution of the Ni2+-, Ni3+-, and Ni4+-ions
therein, aiming at understanding the effects of this doping on
the electrochemical behavior of NCM811 electrodes. While
there are several recent descriptions in the literature (as cited
herein) of Mo-doped NCM cathodes demonstrating their
advantage over reference systems, we believe that the present
work is important because it supports very strongly the benefit
of doping Ni-rich materials by molybdenum cations at low
concentration.
We consider this study as an important step further in the
continuous and extensive efforts to optimize cathodes for
advanced Li-ion batteries, in light of the great challenge of the
electromobility revolution. Its success depends so strongly on
the availability of suitable high energy density, stable, and
durable Li-ion battery technology.
(NCM622) by minor contents of aluminum and iron (0.025
at. %) reduced Li+/Ni2+ cation mixing, stabilized the structure,
and enhanced electrode behavior.15 Recently, we have
demonstrated that substantial improvements in the NCM523
electrode’s performance upon cycling and aging at 60 °C can
be achieved by doping of LiNi0.5Co0.2Mn0.3O2 with Al (0.01 at.
% at the expense of Ni, Co, and Mn).16 Using density
functional theory (DFT), Ni-sites were shown to be the most
preferred for Al-doping in this cathode material. Our
theoretical studies revealed that one of the main advantages
of this dopant is stabilization of the layered structure.17
Importantly, doping the NCM622 cathode material by highcharge state Zr4+ cations resulted in improved electrochemical
characteristics as well as in partial inhibition of the structural
transformation of layered phase R3̅m to spinel Fd3̅m during
cycling.18 Recently, a few works have been devoted to studying
the role of Zr4+ cations as dopant in NCM52319 and LiNiO220
electrodes. Although high-charge state W6+ and Mo6+ cationic
dopants are promising for NCM materials, their behavior has
been less studied.21−24 In a recent study, Kim et al.
demonstrated stable Li-ion battery prototypes comprising Nirich (Ni ≥ 80 at. %) cathodes doped with W6+ cations (1 mol
%).25 In this work, W-doping was found to enhance rock-salt
formation at the particles’ surface. This surface segregation was
described as one of the major reasons for the clearly observed
stabilization of W6+-doped LiNiO2 cathodes. Konishi et al.22
demonstrated that partial substitution of manganese by Mo6+
in Ni-rich materials (Ni-content 80%) resulted in suppression
of the structural transformation from spinel to rock-salt phase
and also in improvement of the thermal stability of the doped
samples in reactions with electrolyte solutions. In a recently
published paper,26 we have shown that doping NCM523 with
minor amounts of Mo6+ (1 mol %) pronouncedly affects its
structure and surface properties, and electrochemical behavior
in terms of increased rate capability and decreased capacity
fade during cycling. It was suggested that lower charge-transfer
interfacial resistance (Rct) of Mo-doped electrodes can be
explained by the formation of additional conduction bands
near the Fermi level elucidated from the calculated density of
states.
In spite of intensive efforts dedicated to Ni-rich layered
structure materials doped with foreign cations or anions, more
studies with high-charge dopants like Mo6+ are required in
order to understand better the effect of doping on capacity,
rate capability, structural and cycling stability, and possible
surface phenomena, like dopant and transition metal
segregation and structural changes induced by the presence
of Mo6+ in the lattice. In the work reported herein, we have
continued our efforts to improve the long-term stability of Nirich NCM cathode materials, which can be considered as their
main problem. It is clear that a promising route for that is
doping by multivalent cations like Al3+, Zr4+, and Mo6+ which
can reduce Li/Ni cation mixing as was shown in our previous
work.16,17,26,27 It is important to note that cation-doped NCM
cathode materials are multiparameter systems. Each parameter,
like composition (level of Ni content, the choice of dopant, its
concentration), the choice of synthetic route and the nature of
the electrolyte solution, may dominate the electrochemical
behavior. The present study is concentrated on LiNi0.8Co0.1Mn0.1O2 (NCM811, which can be considered as a very
important cathode material) and the effect of its doping by
Mo6+ cations. The tools included a variety of electrochemical
techniques, high-resolution microscopy, XRD, electron dif-
2. EXPERIMENTAL METHODS
2.1. Synthesis of Materials. We synthesized lithiated oxides of
transition metals Ni, Co, and Mn of a layered structure with the
following undoped composition: LiNi0.8Co0.1Mn0.1O2. Samples
include those that are molybdenum-doped: in the cost of Ni (1 and
3 mol %), LiNi 0.79 Mo 0.01 Co 0.1 Mn 0.1 O 2 and LiNi 0.77 Mo 0.03 Co0.1Mn0.1O2; and in the cost of both Ni (1 and 2 mol %) and Mn
(1 mol %), LiNi0.79Mo0.01Co0.1Mn0.09Mo0.01O2 and LiNi0.78Mo0.02Co 0.1Mn0.09 Mo 0.01O2 . These were synthesized by a solution
combustion method as described previously.18,28,29 The initial
compounds (precursors) were analytical grade LiOH·H2O, Ni(NO3)2·6H2O, Co(NO3)2·6H2O, Mn(NO3)2·4H2O, ammonium
molybdate tetrahydrate (NH4)6Mo7O24·4H2O (doping agent), and
sucrose C12H22O11. A small excess of lithium was provided (Li1.02)
according to our previous experience. The metal nitrates, LiOH·H2O,
and sucrose were dissolved in distilled water until a clear solution was
obtained. The latter was placed in a heater for slow evaporation of the
water. After the solution dries, an increase of the temperature is
observed and the self-ignition starts. The combustion process resulted
in a voluminous amorphous mass, which was heat treated in air at 400
°C for 1 h to ensure the burning of residual organic components. The
mechanism of self-combustion reactions to form lithiated transition
metal oxides was already discussed in detail.30,31 This method is very
convenient for fast combinatorial studies of many compositions in lab
scales. This allows us to reach very well the desirable stoichiometry of
compounds at very uniform compositions. The final stage is
calcination, namely, heat treatment under an oxygen atmosphere at
high temperatures (750−900 °C), which brings the materials to their
final stable structure (confirmed always by crystallographic studies).
In the synthesis for the present study, the final calcination stage
included heating at 900 °C for 3 h in air, followed by a heat treatment
at 800 °C in pure oxygen atmosphere during 15 h. We represent the
results of the chemical analysis of NCM811 materials in Table S1.
2.2. Electrochemical Measurements. Preparations of working
electrodes (cathodes) containing LiNi0.8Co0.1Mn0.1O2 undoped or
Mo-doped materials for electrochemical cells (2325 coin-type
configuration, Li anodes), as well as their assemblies, were described
in detail previously.16,18,26 We used the electrolyte solutions (Li
battery grade) comprising ethyl-methyl carbonate (EMC) and
ethylene carbonate (EC), in a weight ratio 7:3 with 1 M LiPF6. For
statistical purposes, we studied simultaneously the electrochemical
perfromance of at least 2−3 cells of undoped and Mo-doped
electrodes, and the results were averaged. For the electrochemical
measurements a multichannel Maccor-2000 battery cycler and a
battery test unit (BTU-1470), coupled with a frequency response
analyzer (FRA-1255) from Solartron, Inc., were used.16 All the
potentials in this paper are given vs Li+/Li.
4522
DOI: 10.1021/acsaem.9b00767
ACS Appl. Energy Mater. 2019, 2, 4521−4534
Article
ACS Applied Energy Materials
Figure 1. Schematic structure of NCM811 undoped (a) and Mo-doped (b) materials. Mo-dopant that preferably substitutes Ni is shown as yellow
balls.
(COHP) as implemented in the LOBSTER code.38−41 This entails
postprocessing of the electronic structure to find COHP and
integrated COHP (ICOHP). To estimate the bond strength between
two TM atoms, we averaged over all possible interactions between
pairs of TMs within finite distances (2.0−3.1 Å for Ni−Ni, Ni−Co,
Ni−Mn; 4.0−6.0 Å for Co−Co, Co−Mn, Mn−Mn (considering
nearest one)).
Other experimental methods used in this work [chemical analysis
by inductively coupled plasma (ICP), X-ray diffraction (XRD),
scanning electron microscopy (SEM), and transmission electron
microscopy (TEM); differential scanning calorimetry (DSC); time-offlight secondary-ion-mass-spectroscopy (TOF-SIMS); X-ray photoelectron spectroscopy (XPS)] are presented in the Supporting
Information.
2.3. Computational Methods. The electronic structure calculations were performed using plane-wave-based DFT as implemented
in the Vienna ab initio simulation package32,33 (VASP) in conjunction
with projector augmented wave34 (PAW) potentials. The exchangecorrelation terms are treated within the generalized gradient
approximation (GGA) using the Perdew, Burke, and Ernzerhof35
(PBE) functional. A 2 × 2 × 1 k-mesh is applied for the integration in
k-space. Further, the effective Hubbard U parameter, i.e., the on-site
Coulombic interactions, were chosen to be 5.00, 5.96, 5.00, and 5.10
eV for Mo, Ni, Co, and Mn, respectively.26 Nonbonded van der Waals
interactions were incorporated using the D3 dispersion correction
approach of Grimme together with PBE+U.26,36 To optimize the
geometry of the systems, the coordinates, as well as the volume of the
unit cell, were relaxed until the force per atom was less than 0.01 eV/
Å.
For our computational model, we adopted NCM811 in R3̅m
symmetry, where transition metals (TMs) occupy 3a sites and Li-ions
occupy 3b sites in alternating layers along the c-axis. Specifically, we
adopted the structure reported by Dixit et al.37 as our initial structure.
The unit cell of NCM811 is displayed in Figure 1. In the unit cell, we
have a total of 60 TMs, and we substituted one Mo and two Mo
atoms in both 3a and 3b sites to reach 1.66% and 3.32% doping
concentrations, respectively.
To estimate the intercalation voltage from the calculated total
energy of the system with different delithiation limits, we applied the
following formula.
v=−
[E(Lix + dxNCMO2 ) − E(LixNCMO2 )]
+ E(Libcc)
dx
3. RESULTS AND DISCUSSION
3.1. Structural Analysis and Morphology of Ni-Rich
Materials. Structural models of undoped and Mo-doped
NCM811 are shown in Figure 1a,b, respectively. Powder X-ray
diffraction patterns of the synthesized Ni-rich samples are
presented in Figure S1. Undoped LiNi0.8Co0.1Mn0.1O2 and 0.01
Mo-doped samples are single phase, possessing a rhombohedral structure, in the R3̅m space group, while that with 0.03 Mo
shows traces of cubic spinel Li(TM)2O4 (small peaks at 18.4°,
30.1°, and 35.4° 2θ) and Li2MoO3 (small peaks at 18.4°, 36°,
and 43.6° 2θ). The latter has a disordered NaFeO2 structure
(space group R3̅m) with the Mo-ions present as Mo3O13
clusters in the [Li1/3Mo2/3] layer.42 To assess the perfectness
of the layered structure of the undoped pristine material, the
ratio of (003) to (104) peak intensities was calculated on the
basis of the results of the Rietveld analysis of the XRD profile.
The calculations have shown that the estimated ratio of
intensities of (003)/(104) peaks was about 1.5, which is
greater than the value 1.2, thus providing an indication of the
pronounced layered character of the structure,43 which means
there is a good separation of transition-metal-ions and Li-ions
(1)
Here, E(Lix+dxNCMO2) and E(LixNCMO2) are the energies per
formula unit for the system with x + dx and x Li-ions limits. E(Libcc) is
the energy per formula unit of bulk Li metal. It is known that PBE can
provide accurate trends for intercalation potentials yet it underestimates the absolute voltage values. Therefore, we performed a rigid
shift of 0.51 and 0.9 V for the plot of discharging and charging voltage
profile to match the experimental values.
We further analyzed the strength of chemical bonding between two
TM atoms from the calculation of crystal orbital Hamilton population
4523
DOI: 10.1021/acsaem.9b00767
ACS Appl. Energy Mater. 2019, 2, 4521−4534
Article
ACS Applied Energy Materials
ions at different TM-sites. We considered several different Moconfigurations either in the same or in different TM layers. The
relative FEs for these configurations are shown in Figure S2b,
where a configuration with two Mo-ions replacing two Ni-ions
in the same layer is taken as the reference energy level. The
calculations of formation energy reveal that Mo-doping at Nisites is energetically more favorable. We note that the relative
Mo-doping FE values are independent of the dopant precursor
(i.e., (NH4)6Mo7O24·4H2O) used in the experiment.
The results from Rietveld refinement of the crystal structure
of LiNi0.79Mo0.01Co0.1Mn0.1O2 and LiNi0.77Mo0.03Co0.1Mn0.1O2
are presented in Tables S2 and S3.
Further, we calculated the optimized lattice parameters for
undoped and 1.66% and 3.32% Mo-doped NCM811 (Table
S4). We observe a substantial increase in the c lattice
parameter due to Mo-doping, regardless of the detailed
computational approach (i.e., PBE and PBE+U+D3), in
agreement with the experimental values (Table S4).
The increase in unit cell parameters with increasing
substituting level is a clear indication that Mo is present in
the crystal structure of the layered phase. The increase is likely
due to the increasing Ni2+ content in the structure upon partial
substitution, as Ni2+ has greater ionic radius than Mo6+ (69 vs
59 pm).44 From the calculations of moment per atomic sites,
we can estimate the oxidation state of the atoms. In the case of
undoped NCM811, we observed three different types of Niions, corresponding to oxidation states 2+, 3+, and 4+. The
distribution of different oxidation states of Ni-ions for undoped
and 1.66% and 3.32% Mo-doped at Ni-sites is presented in
Figure 2. The Ni3+-ion population is greater than those of Ni2+
onto their respective planes in the layered Li[NixCoyMnz]O2
structure.26 The result of the calculations of lithium occupancy
on the 3b site is also in keeping with the conclusion that the
analyzed NCM811 pristine material has a distinct layered
structure. According to the Rietveld analysis, the contamination of the lithium layer turned out to be minimal: the
normalized occupancies of the 3b Li-site by Li- and Ni-ions
are, respectively, 0.9640 for Li and 0.0360 for Ni.
Rietveld refinement of the crystal structure of LiNi0.79Mo0.01Co0.1Mn0.1O2 and LiNi0.77Mo0.03Co0.1Mn0.1O2 was performed
to identify the location of molybdenum-ions in the layered
LiNiO2-type structure. The determination of the distribution
of Mo-ions between lithium (3b) and 3d-metal (3a) positions
on the basis of XRD pattern is a difficult task, since the
quantity of Mo in the structure is extremely small. Thus, we
performed some refinement with different fixed distributions of
Mo over the two positions, allowing Li and Ni occupancies to
vary in a constrained manner to fill both cation positions. All
variable parameters were refined in a uniform manner for all
models. The quality of the solutions was evaluated on the basis
of general statistical criteria (the values of the numerical
criteria of fit), and the physical reliability of the obtained
structural parameters is listed in Table 1.
Table 1. Conventional Rietveld R-Factors for Different
Models of the Mo-Dopant Distributions in Li or TM-Layers
fixed
Mo0.01
Mo0.01
Mo0.03
Mo0.03
in
in
in
in
Li-layer
TM-layer
Li-layer
TM-layer
Rp
Rwp
Rexp
χ2
Rf
RBragg
11.6
11.6
13.6
13.6
7.21
7.22
8.25
8.27
3.41
3.42
3.67
3.68
4.48
4.45
5.05
5.04
2.00
1.99
2.24
2.31
2.38
2.37
2.88
2.91
As can be seen from Table 1, the differences between the
model placing Mo-ions in the 3b position and that placing Moions in the 3a position are minute. For the Mo0.01 sample, it is
not possible to prefer one to the other model. For Mo0.03doped material, the model with Mo in the 3b positions shows
systematically lower values for the conventional R-factors.
Additionally, the model with Mo0.03 placed in the TM metal
layer (position 3a) resulted in negative isotropic temperature
factor values for the ions in 3b positions. This result supports
the view that Mo-ions likely occupy the 3b position of the
crystal structure. Further, we performed DFT calculations to
probe the most likely doping position of Mo. To find the
energetically favorable position for the Mo in NCM811, we
calculated the formation energy (FE) using the following
equation:
Figure 2. Distribution of Ni-ions in different oxidation states Ni2+,
Ni3+, and Ni4+ for undoped and Mo-doped NCM811 materials.
FE = [E(Mo@NCM811) + E(M) + 2E(Li) + 2E(O2 )]
− [E(NCM811) + E(Li 2MoO4 )]
and Ni4+, in both undoped and doped materials. Importantly,
the amount of Ni3+ is reduced upon doping due to charge
compensation, similar to our earlier observations.26,37 The
influence of Mo-doping on reducing the amount of Ni3+ was
confirmed by the XPS results presented in Figure 3a,b. The
curve fitting of the Ni 2p spectra for undoped (Figure 3a) and
Mo-doped (Figure 3b) samples clearly demonstrates an
increase in NiO content after Mo-doping, from 59.8% in an
undoped sample to 67.0% in a Mo-doped material. The
decrease in Ni3+ cannot be revealed from the XPS measurements as the Ni2O3 peak (Ni3+) appears to be in a very close
vicinity to the Ni(OH)2 peak (Ni2+). It seems that an increase
in the Ni(OH)2 component is dominant and, hence, screens
the possible changes/reduction in Ni2O3 quantity with Mo-
(2)
Here E values are the energies of the systems written in
parentheses, and x indicates the concentration of Mo-dopant.
This equation represents Mo-doping at all metal sites, M. We
calculated the FE for four different configurations by replacing
Mo at Ni, Co, Mn, and Li-sites. Inspection of the data reveals
that Mo-doping is favored at Ni-sites, as was found previously
for high-valence doping in other NCM materials.26 We probed
six different Mo-configurations at Ni-sites. The calculated FEs
of Mo-doping at Co-, Mn-, and Li-sites, relative to Ni-sites, are
shown in Figure S2a for 1.66% Mo-doping in NCM811. In the
case of 3.32% Mo-doping in NCM811, we substituted two Mo4524
DOI: 10.1021/acsaem.9b00767
ACS Appl. Energy Mater. 2019, 2, 4521−4534
Article
ACS Applied Energy Materials
materials clearly show the increased bond lengths for Modoped samples indicating thus incorporation of the dopant into
the lattice (Table S5). SEM and TEM imaging further reveals
that Mo-doping slightly influences the morphology and particle
size of LiNi 0.79 Mo 0.01 Co 0.1 Mn 0.1 O 2 and LiNi 0.77 Mo0.03 Co0.1Mn0.1O2 (Figure S4). The particle size of the Mo-doped
samples is somewhat smaller, probably due to inhibition of the
lithiated oxide crystals growth during synthesis. We suggest
that even a minute concentration of additives, like Mo-dopant,
accumulates at the crystallization front during the synthesis,
blocking the interface of the nuclei and slowing down the
attachment of further building units.45 This process of dopantinduced inhibition of particle growth in the early stages is a
general phenomenon for nanocrystalline materials, and this
influences particle size, structure, and morphology.46
3.2. Electronic Structure of Undoped and Mo-Doped
NCM811. We further calculated the total and partial density of
states (DOS) for undoped and 1.66% and 3.32% Mo-doped
NCM811 using PBE (Figure 4a−c, respectively). We observed
strong hybridization between Ni-3d states and O-2p states for
both undoped and doped materials, similar to Mo-doped
NCM523.26 The DOS of Ni2+ near the Fermi level is greatly
increased due to Mo-doping, and this is expected to be
important for electrochemical performance. From the DOS for
undoped NCM811 (Figure 4a), we conclude that t2g of Ni2+ is
fully occupied (↑↓|↑↓|↑↓), while eg is partially occupied (↑|↑),
yielding ca. 2 μB per Ni (i.e., Ni2+ is in a high-spin (HS) state).
Ni3+ is in a low-spin (LS) state, with t2g fully occupied (↑↓|↑↓|↑
↓) and eg singly occupied (↑|), with a net moment per Ni of ca.
1 μB Ni4+-ions appearing in an LS state, providing zero
moment per Ni-site, as t2g is fully occupied (↑↓|↑↓|↑↓), while eg
is unoccupied (|). Additionally, t2g of Co-ions is fully occupied
(↑↓|↑↓|↑↓) and eg of Co-ions is unoccupied (|), in line with net
moment of zero μB. The t2g of Mn-ions is partially filled (↑|↑
|↑), and eg is empty with net moment of ca. 3 μB per Mn (see
Figure 3); i.e., Mn is in an HS state. Inspection of the DOS
(Figure 4b,c) does not indicate any significant change in the
relative position of the t2g and eg bands of Ni, Co, and Mn, with
the exception of the appearance of mostly unoccupied Mo-3d
states near the conduction band minima.
3.3. Transition Metals and Mo-Dopant Segregation at
the Surface in NCM811. We have established that the Modopant and TMs segregate at the surface of NCM811 as is
Figure 3. Results of the XPS studies for Ni 2p3/2 of NCM811
undoped (a) and 3 mol % Mo-doped (b) samples.
doping. The peak at the low-energy knee in both doped and
undoped samples, at the binding energy that is slightly lower
than the NiO component, is supposed to be the NCM-matrixrelated component, with x < 1. This redistribution of Ni-ions
in different oxidation states for Mo-doped materials enhances
electrochemical behavior of the doped electrodes, as discussed
in detail below. It may be seen that the increase of the unit cell
parameters upon Mo substitution is mainly due to the increase
of the metal−oxygen bond lengths within the layers. To find
the average bond lengths between oxygen atoms and the 3a
and 3b sites, we calculated the radial distribution function for
the all interatomic distances from the optimized structures
(Figure S3). The results using PBE and PBE+U+D3 were
similar, and here, we only discuss results using the latter
method. From our analysis, we observe a slight increase in both
3a−O and 3b−O bond lengths upon Mo-doping in NCM811
(Table S5). The calculated radial distribution functions for
TM(3a)−oxygen and Li(3b)−oxygen distances in NCM811
Figure 4. Calculated density of states for NCM811 undoped (a) and for 1.66% and 3.32% Mo-doped (b, c) materials, using PBE.
4525
DOI: 10.1021/acsaem.9b00767
ACS Appl. Energy Mater. 2019, 2, 4521−4534
Article
ACS Applied Energy Materials
Figure 5. Depth profiles of Ni+, Co+, and Mn+ measured by ToF-SIMS from NCM811 undoped LiNi0.8Co0.1Mn0.1O2 (a) and 1 mol % Mo-doped
LiNi0.79Mo0.01Co0.1Mn0.1O2 (b) and 3 mol % Mo-doped LiNi0.77Mo0.03Co0.1Mn0.1O2 (c) materials. Due to high roughness of the powder samples,
“Surface” of 15−20 Å (light blue bars) and “Bulk” regions are shown only schematically, as an eye guide. The sputter depth was calculated assuming
that the samples are made of Li and using the current of the sputter beam. (d) Typical high-resolution TEM image of the 1 mol % Mo-doped
NCM811 material measured by HAADF-STEM at the surface of the particles and in the bulk indicated with the red rectangular areas 1, 3, and 2,
respectively. The content of the Mo-dopant in these areas is indicated. The samples for microscopic studies were prepared by the focused ion beam
technique.26
ToF-SIMS studies revealed that these TMs segregate at the
surface in NCM811, while in the 1 mol % Mo-doped material
Ni and Co segregate to a lesser extent and the Mn-segregation
is substantially diminished (Figure 5b). Similar results were
obtained in the case of 3 mol % Mo-doped material (Figure
5c). The segregation of the Mo-dopant at the surface was
confirmed by HAADF-STEM-EDS studies of the doped
NCM811 sample, as is evident from the STEM image in
Figure 5d. This demonstrates that the atomic concentration of
Mo at the surface (areas 1 and 3 marked with the red
rectangular) is several times higher compared to that in the
bulk of the particle (area 2). These results are in close
agreement with our previous work on the Mo-doped NCM523
materials in which the dopant segregation was established
using the above techniques.26
We analyzed the strengths of different TM−TM chemical
bonds by calculating their ICOHP. A schematic diagram for
evident from the depth profiles recorded using ToF-SIMS in
Figure 5a−c. The depth profiles indicate enrichment of TMs
due to segregation at the outermost surface layer of 15−20 Å.
Inspection of Figure 5b reveals that Mo segregates at the
surface, and then its content decreases and levels off, in line
with what we observed in our earlier studies on
LiNi0.49 Mo0.01Co0.2 Mn0.3O 2 Mo-doped cathode material
NCM523.26 However, in contrast to this mildly Ni-rich
material, the depth profiles measured from NCM811 samples
demonstrate also segregation of Ni, Co, and Mn, in agreement
with literature reports of Liang et al.47,48 These authors
suggested that the reason for Ni surface segregation is weak
Ni−Ni bonding (effective interactions) in Ni-rich NCM
materials that breaks, resulting in Ni-segregation. In a proposed
new bond model, they attribute the origin of instability and
deterioration of Ni-rich NCM (Ni ≥ 80 at. %) to the Co- and
Mn-segregation at the surface and formation of clusters. Our
4526
DOI: 10.1021/acsaem.9b00767
ACS Appl. Energy Mater. 2019, 2, 4521−4534
Article
ACS Applied Energy Materials
Figure 6. (a) Results of the rate capability tests obtained at 30 °C from NCM811 undoped LiNi0.8Co0.1Mn0.1O2 and 1 mol % Mo and 3 mol % Modoped LiNi0.79Mo0.01Co0.1Mn0.1O2 and Li1Ni0.77Mo0.03Co0.1Mn0.1O2 electrodes, as indicated. (b) Cycling performance of these electrodes at a C/3
rate using the CC−CV mode in the potential range 2.8−4.3 V with potentiostatic steps at 4.3 V for 0.5 h. Capacity retention (Q) values as ratios of
the corresponding discharge capacities measured for 100th and 30th cycles are shown. (c) Results of the rate capability tests obtained at 30 °C from
NCM811 electrodes: undoped LiNi0.8Co0.1Mn0.1O2 and 2 mol % Mo-doped and 3 mol % Mo-doped (by Ni and Mn partial substitution)
LiNi0.79Mo0.01Co0.1Mn0.09Mo0.01O2 and LiNi0.78Mo0.02Co0.1Mn0.09Mo0.01O2, as indicated. (d) Cycling performance of these electrodes at a C/3 rate
using the CC−CV mode (2.8−4.3 V range) with potentiostatic steps at 4.3 V for 0.5 h. Capacity retention (Q) values as ratios of the corresponding
discharge capacities measured for 100th and 30th cycles are shown.
for Ni atoms in proximity to Mo-dopant atoms. This change in
Ni−Ni bond strength is accompanied by a reduction in bond
strength of Ni−O bonds near Mo-dopant sites. It should be
noted, however, that segregation of Ni, Co, and Mn established
in this work reflects an average trend, while the homogeneity of
the distribution on the surface of Ni-rich NCM may differ
greatly on polar (012), (001), (111), and nonpolar (100),
(110), (104) surfaces. 48,49 Taking into account that
segregation of TMs originates from the nature of the TM−
TM bonds in LiNi0.8Co0.1Mn0.1O2,47 segregation of Ni, Co,
and Mn in the undoped material (Ni = 80 at. %) can explain,
to some extent, deteriorating performance of these Ni-rich
electrodes during cycling, which is a common phenomenon of
Ni-rich cathode materials (Ni ≥ 80 at. %).1,50 In contrast,
more stable electrochemical behavior of Mo-substituted
(doped) electrode materials with Ni = 77−79 at. % shown
in the following section can be attributed (according to the
the average ICOHP values between different TM−TM bonds
is displayed in Figure S5. We note that more negative values of
ICOHP indicate stronger bonds. A main observation is that
the greatest changes to bonding are observed for bonds
involving Ni-ions, and this may be ascribed to changes in
oxidation states on doping. Bonds involving Mn-ions hardly
change on doping, as the Mn-oxidation state is unaffected by
doping. The average ICOHP values for Ni−Mn and Ni−Co
bonds are more negative compared to Ni−Ni bonds in
undoped NCM811, consistent with earlier reports on
NCM811.47,48 Additionally, Ni−Mn is found to be the
strongest bond, and the Mn−Mn bond is the weakest bond
among all TM−TM bonds. This indicates that there can be
segregation of Ni at the surface. Further, the strength of the
Ni−Ni bond enhanced due to Mo-doping and possibly
suppresses Ni-segregation at the surfaces. The Mo-doping
induced increase in Ni−Ni bond strength is more prominent
4527
DOI: 10.1021/acsaem.9b00767
ACS Appl. Energy Mater. 2019, 2, 4521−4534
Article
ACS Applied Energy Materials
bond theory) to their stabilization by Ni2+−Mn4+, Ni3+−Mn4+,
and Co3+−Mn4+ bonds. It can be proposed that the presence of
Mo stabilizes TM−TM bonding and, therefore, induces
formation of a more stable modified electrochemical interface
and improved behavior of the Mo-doped NCM811 electrodes.
In the following section, we discuss these issues related to
comparative electrode behaviors of undoped and Mo-doped
samples.
3.4. Comparative Analysis of the Electrochemical
Behavior of LiNi0.8Co0.1Mn0.1O2 Undoped and MoDoped Electrodes. In Figure 6, we demonstrate results of
the rate capability studies (a, c) and of cycling behavior (b, d),
respectively, of NCM811 undoped and Mo-doped electrodes
at 30 °C, as indicated. Note that this figure compares the
electrochemical performance of Ni-substituted samples (by 1
and 3 mol % of Mo LiNi0.79 Mo0.01Co0.1 Mn 0.1O 2 and
LiNi0.77Mo0.03Co0.1Mn0.1O2, respectively) and of those in
which Ni (1 and 2 mol %) and Mn (1 mol %) were both
substituted by Mo, namely, LiNi0.78Mo0.02Co0.1Mn0.09Mo0.01O2
and LiNi0.79Mo0.01Co0.1Mn0.09Mo0.01O2. We also computed the
voltage profiles using DFT, and inspection of Figure 7a,b
respectively). This can be explained by the increased c lattice
parameter for the doped materials, as shown by the Rietveld
refinement of their XRD patterns (Table S4). The Mo-doped
electrodes deliver increased discharge capacities on prolonged
cycling at a C/3 rate and demonstrate higher capacity
retention (Q), expressed as ratio of the capacities obtained
in the 100th and 30th cycles (Figure 6b,d). We conclude
therefore that these results are characteristic of doped
electrodes and can be ascribed to the fact that incorporation
of highly charged dopant ions like Mo6+ leads to redistribution
of charge among TM cations in the NCM lattice, as shown in
Figure 2. This, in turn, results in decreasing the relative
number of Ni3+-ions and increasing the number of Ni2+ and
electrochemically active Ni2+/Ni3+ redox couples responsible
mainly for the electrode capacity.18,26,27,50 We also conclude
from the results of the cycling behavior that even minor levels
of Mo-doping (1 mol %) can be considered as optimal for
NCM811 electrodes (i.e., LiNi0.79Mo0.01Co0.1Mn0.1O2), as
reflected in the higher discharge capacity and reduced fading
upon cycling in Li-cells. This is in agreement with the recent
studies of 1% Mo-doped NCM811 cathodes by Su et al.51 In
addition, we have established that the voltage hysteresis
calculated as the difference of mean voltage in charge and
mean voltage in discharge is lower for the Mo-doped samples
during cycling as shown in Figure S6. The above parameters
were also substantially improved especially for the 3 mol %
Mo-doped LiNi0.77Mo0.03Co0.1Mn0.1O2 electrodes during cycling at a C/3 rate at an elevated temperature of 45 °C (Figure
8a,b). As expected, the Ni-rich undoped electrodes typically
exhibited an increased capacity degradation during cycling at
45 °C compared to that at 30 °C, likely due to intense
structural instability and surface interactions with solution
species, in agreement with the literature reports.1,49,52 The
results of Figure 8a allow the conclusion that though undoped
and 1 mol % and 3 mol % Mo-doped electrodes exhibited
similar discharge capacities around 185−195 mAh/g at the
beginning of cycling, undoped samples showed an immediate
sharp capacity decrease, as expected, while the Mo-doped ones
retain their capacity. In addition, the effect of Mo-substitution
(doping) is more pronounced at 45 °C for electrodes
comprising 3 mol % Mo-doped LiNi0.77Mo0.03Co0.1Mn0.1O2.
These electrodes demonstrate similar discharge capacity during
∼70 cycles, yet the capacity retention of Q100/Q30 = 72% is
higher compared to that of 1 mol % doped electrodes (Q100/
Q30 = 64%). This can be attributed to a synergetic effect of the
following factors: slightly lower Ni-content (77 at. %) and the
stabilizing impact of Mo-doping in the lattice. The presence of
the Li-rich Li2MoO3 phase (though in minor content) in the
above material, as discussed in the section on Structural
Analysis, should also be taken into account. Indeed, recent
experimental results demonstrated that incorporation of
Li2MoO3 in NCM cathodes effectively stabilized their cycling
behavior.52 As shown in Figure 8b, voltage hysteresis measured
as mean voltage in charge−mean voltage in discharge of the
above NCM811 Mo-doped electrodes is much lower
compared to that of the undoped samples during prolonged
cycling. Cycling and structural stability of the 3 mol % Modoped LiNi0.77Mo0.03Co0.1Mn0.1O2 electrodes is evident from
comparison of the differential capacity dQ/dV vs V plots of
undoped and Mo-doped samples measured at 45 °C for cycles
5, 50, and 75 (Figure 9). In these plots, the Mo-doped
electrodes demonstrate faster electrochemical kinetics, and the
redox couple at ∼4.15 V exhibits higher reversibility of these
Figure 7. Voltage profiles of charging (a) and discharging (b) for
undoped (black) and 1% (red) and 3% Mo-doped (blue) NCM811
electrodes. Profiles shown with steps are from theoretical calculations,
and curves with empty circles are from experiment.
reveals good agreement between theory and experimental
results. We established that doped electrodes exhibited 12−
20% higher discharge capacities than the undoped ones,
especially at high rates of 4C, in line with our recent results on
Zr- and Mo-doped NCM electrodes.18,26,27 We emphasize that
all Mo-doped Ni-rich electrodes studied in the present paper,
Ni-substituted as well as Ni- and Mn-substituted, outperform
the undoped ones in terms of the rate capability (Figure 6a,c,
4528
DOI: 10.1021/acsaem.9b00767
ACS Appl. Energy Mater. 2019, 2, 4521−4534
Article
ACS Applied Energy Materials
electrodes, in which both Ni and Mn were partially substituted
by the Mo-dopant. The total doping levels were 2 and 3 mol %,
respectively, in these samples. These voltage profiles are typical
for NCM electrodes exhibiting a sharp increase of the potential
at the beginning of charging from OCV to 3.6−3.7 V and a
continuous smooth increase up to cutoff 4.3 V. At potentials
∼4.1−4.2 V they exhibit a plateau-like region that corresponds
to structural phase transitions of a hexagonal H2 to H3, typical
for Ni-rich electrodes.50,54
It is important to stress that Mo-doped electrodes exhibit
significantly less irreversible capacity loss in the first cycle.
Additionally, the remaining currents developed during the
potentiostatic steps for 3 h after initial charging to 4.3 V are
also notably smaller (Table S6), implying lesser side reactions
of doped materials with the solution species and formation of a
more stable interface. This is evident also from comparison of
the total heat evolved during chemical reactions of NCM811
undoped and Mo-doped materials with the battery solutions
EC-EMC/LiPF6. Our DSC studies demonstrated that less heat
was evolved for the 1 and 3 mol % doped samples (352 and
299 J/g, respectively) compared to the undoped one (433 J/g)
(Figure S8). We also established that undoped electrodes
exhibited ∼180 mV higher potential (“overvoltage”) upon the
Li+ extraction both in the initial cycle at slow rate and by 75
mV in the 12th cycle at 1C rate (Figure S7), while the charging
curves of the Mo-doped samples lie at lower potentials
compared to the undoped ones, suggesting reduced resistance.
This is also reflected in impedance spectra measured at OCV =
4.0 V (during charging) from undoped and doped electrodes
after 10, 50, and 100 galvanostatic cycles, as shown in Figure
10a−c, respectively. This finding is one of the advantages of
Mo-doped electrodes, which remain less resistive upon cycling
compared to the undoped samples. Overall, we conclude that
Mo-doping results in lower interfacial charge-transfer resistance, calculated from fitting of the impedance spectra (Nyquist
plots) and represented in Figure 10d. The results obtained can
be attributed to a modified electrode/solution interface due to
dopant segregation at the surface (Figure 5). The lower
interfacial resistance results from the additional conduction
bands formed near the Fermi level in doped materials,26 as
demonstrated in Figure 4. Therefore, the transfer of Li-ions
and electrons is facilitated, increasing thus the exchange
current density (i0) due to the decreased Rct. Similar results
were obtained in Mo6+-doped NCM523 materials26 and in the
W-doped TiO2 and spinel materials.55,56
3.5. Structural Analysis by XRD and TEM of Cycled
Electrodes Comprising LiNi0.8Co0.1Mn0.1O2 (Undoped)
and Mo-Doped LiNi0.79Mo0.01Co0.1Mn0.1O2 Materials. A
comparison of XRD profiles of undoped and Mo-doped NCM811 electrodes in conditions before and after 100 charge/
discharge cycles (C/3, 30 °C) is presented in Figure S9. Except
the basic rhombohedral phase peaks, the XRD profiles show
the presence of additional peaks at 2θ = 26.6° and 2θ = 28.7°.
We attribute these two peaks, respectively, to the (002)
reflection caused by graphite component of the electrode and
to the strongest (110) peak of the tetragonal β-MnO2 phase
(pyrolusite), which can be considered as an impurity phase. A
strong peak labeled Al220 arises from the Al-current collector of
the electrode. The results of Figure S9 show no essential
difference between the XRD patterns of the electrodes
comprising undoped LiNi0.8Co0.1Mn0.1O2 and Mo-doped
LiNi0.79Mo0.01Co0.1Mn0.1O2 materials, as well as between the
profiles corresponding to cycled and uncycled conditions. The
Figure 8. (a) Cycling performance at 45 °C of NCM811 electrodes:
undoped LiNi0.8Co0.1Mn0.1O2 and 1 mol % Mo and 3 mol % Modoped LiNi0.79Mo0.01Co0.1Mn0.1O2 and LiNi0.77Mo0.03Co0.1Mn0.1O2, as
indicated. Potential range used was from 2.8 to 4.3 V at a C/3 rate.
Every 20th cycle the charge/discharge current was increased
corresponding to a C/10 rate. Inset: Discharge capacity measured
from the above electrodes every 20th cycle at a C/10 rate. Constant
current−constant voltage (CC−CV) mode with potentiostatic steps
at 4.3 V for 0.5 h. Capacity retention (Q) values as ratios of the
corresponding discharge capacities measured for 100th and 30th
cycles are shown. (b) Voltage hysteresis measured as mean voltage in
charge − mean voltage in discharge of the above NCM811 electrodes.
electrodes compared to the undoped ones. This indicates
diminishing, to some extent, of a possible phase transformation
from hexagonal phases H2 + H3 to H3 (at 4.2−4.3 V) that is
mainly responsible for structural degradation of Ni-rich
materials.1 We note that the question of possible phase
transitions in Ni-rich (80 at. %) cathodes upon cycling requires
more experimental and theoretical studies. However, as is
evident from in situ XRD measurements,53 there is a
correlation among evolution of the cell parameters a and c
during charge/discharge, dQ/dV plots, and possible transformations of the original phase H1 to another hexagonal
phase with a lower Li-ion content, and subsequently to
domains comprising both of these two phases and their
transformation to H3.
The voltage profiles measured at 30 °C in the 1st and 12th
cycles of NCM811 undoped and 1% and 3% Mo-doped
electrodes LiNi0.79Mo0.01Co0.1Mn0.1O2 and LiNi0.77Mo0.03Co0.1Mn0.1O2, respectively, are demonstrated in Figure S7a,b.
Figure S7c,d compares similar profiles of LiNi0.79Mo0.01Co0.1Mn0.09Mo0.01O2 and LiNi0.78Mo0.02Co0.1Mn0.09Mo0.01O2
4529
DOI: 10.1021/acsaem.9b00767
ACS Appl. Energy Mater. 2019, 2, 4521−4534
Article
ACS Applied Energy Materials
Figure 9. Differential capacity plots dQ/dV vs V measured at 45 °C from NCM811 electrodes of undoped LiNi0.8Co0.1Mn0.1O2 and 1 mol % and 3
mol % Mo-doped LiNi0.79Mo0.01Co0.1Mn0.1O2 and LiNi0.77Mo0.03Co0.1Mn0.1O2, as indicated, after the 5th, 50th, and 75th cycles. Electrode
potentials of the redox couple related to the coexisting H2 and H3 hexagonal phases are indicated.
LiNi0.79Mo0.01Co0.1Mn0.1O2, it was concluded that two models
provide similar results for the Mo-ions residing either in the Lilayer or occupying the transition metal sites. In contrast, for
the dopant content of 3 mol %, we concluded that Mo-ions are
more likely to occupy the 3b position (lithium layer) in the
crystal structure. On the basis of the DFT calculations, it was
shown that Mo6+ cations preferably substitute Ni in the oxide
structure due to the lowest substitution energy compared to Li,
Co, and Mn. This conclusion agrees with the relatively low
crystallographic mismatch between ionic radii of Ni-host (rNi2+
= 69 pm) and Mo-guest (rMo6+ = 59 pm). Analysis of the unit
cell parameters a and c of the undoped and Mo-doped
materials that increase with increasing the substituting level
allowed us to conclude there was a successful incorporation of
the dopant into the lattice. This conclusion was supported by
the calculated values of the radial distribution functions for
“TM(3a)−oxygen” and “Li(3b)−oxygen” distances in
NCM811 materials that clearly show the increased bond
lengths for the Mo-doped samples. Studies of the electrochemical behavior of undoped and Mo-doped materials
demonstrated that Mo-doped electrodes exhibited much
lower irreversible capacity loss in the 1st cycle and lower the
remaining currents developed during the potentiostatic steps
following the initial charging to 4.3 V. We concluded therefore
the formation of a more stable electrode/solution interface and
lattice parameters of undoped and Mo-doped NCM811
electrodes are shown in Table S7. Although the XRD patterns
of the cycled doped and undoped NCM-811 samples did not
contain direct evidence of the transformation to cubic spinel
phase, our TEM examinations of the individual particles of the
cycled materials did show the presence of cubic LiMn2O4
(Fd3̅m) spinel. In some cycled Mo-doped samples, in addition
to the cubic LiMn2O4 spinel, we also observed completely
delithiated spinel Mn2O4. This is demonstrated on examples of
nanobeam electron diffractions in Figure 11a−c, which
presents spinel patterns in (a) undoped and (b) 1% Modoped NCM-811 material. The pattern in Figure 11c
corresponds to delithiated spinel observed in 1% Mo-doped
material. Also shown in Figure 11 are examples of the
diffraction patterns of the basic rhombohedral phase (Figure
11d) and the tetragonal β-MnO2 impurity phase (Figure 11e),
which were observed both in doped and undoped samples.
4. CONCLUSIONS
In this work, we have synthesized a family of five Ni-rich
lithiated oxides LiNi0.80−xMoxCo0.1Mn0.1−yO2 (x = 0.01−0.03;
y = 0.01), in which Ni or both Ni and Mn were partially
substituted upon the synthesis with a minor amount of the
molybdenum Mo6+-dopant. From the Rietveld refinement
analysis of XRD patterns of 1 mol % doped sample
4530
DOI: 10.1021/acsaem.9b00767
ACS Appl. Energy Mater. 2019, 2, 4521−4534
Article
ACS Applied Energy Materials
Figure 10. Impedance spectra measured at OCV = 4.0 V (charged state) after 10 (a), 50 (b), and 100 (c) cycles at a C/3 rate from NCM811
electrodes undoped LiNi0.8Co0.1Mn0.1O2 and 1 mol % Mo-doped LiNi0.79Mo0.01Co0.1Mn0.1O2, as indicated. Cycling was carried out at 30 °C using
CC−CV mode from 2.8 to 4.3 V with potentiostatic steps for 0.5 h at 4.3 V. Insets: high-frequency semicircles (100−1 kHz) of impedance spectra
attributed to the resistance of the Li-ion migration through the surface films formed on the electrodes.26 (d) Charge-transfer (interfacial) resistance
measured from impedance spectra (second semicircle at 1 kHz−10 Hz) of the above electrodes as a function of cycle number.
Figure 11. Results of postcycling analysis of NCM811 undoped LiNi0.8Co0.1Mn0.1O2 and 1 mol % Mo-doped LiNi0.79Mo0.01Co0.1Mn0.1O2 materials
by electron diffraction in TEM. Patterns of spinel phase (sp) observed in undoped and 1% Mo-doped NCM-811 materials are shown in parts a and
b, respectively. (c) Pattern from delithiated spinel observed in 1% Mo-doped material. (d) Example of the diffraction patterns of the basic
rhombohedral phase. (e) Example of the diffraction pattern of tetragonal β-MnO2 impurity phase, which was observed both in doped and undoped
samples.
4531
DOI: 10.1021/acsaem.9b00767
ACS Appl. Energy Mater. 2019, 2, 4521−4534
ACS Applied Energy Materials
■
fewer side reactions of doped materials with the solution
species. This is evident also from the lower charge-transfer
resistance of the above electrodes as well as from decreased the
total heat evolved during chemical reactions of undoped (433 J
g−1) and 1% Mo-doped (299 J g−1) materials with the battery
solutions in the range 30−350 °C, as measured by DSC. The
above findings correlate well with the segregation of transition
metals and the Mo-dopant at the surface established
experimentally, for the first time, in the above Ni-rich
materials. We have established that doped electrodes exhibited
higher discharge capacities compared to undoped ones, by 12−
20%, especially at high rates of 4C. This is in agreement with
our previous results for Zr- and Mo-doped NCM622 and
NCM523 electrodes, respectively. It was shown also that Modoped NCM811 electrodes demonstrated higher capacity
retention and lower voltage hysteresis during cycling (100
times at C/3 rate) at 30 and 45 °C. The results of the
electrochemical studies were attributed to the fact that
incorporation of highly charged Mo6+ dopant leads to the
redistribution of charge among transition metal cations
resulting in decreasing the relative number of Ni3+-ions and
increasing the number of Ni2+-ions and electrochemically
active Ni2+/Ni3+ redox couples responsible mainly for the
electrode capacity. Analysis of the nanobeam electron
diffraction patterns of cycled electrodes led us to a conclusion
on their structural stability since the initial rhombohedral R3̅m
structure was mainly preserved both for the undoped and 1
mol % Mo-doped samples. However, additionally it was
suggested the formation of a spinel phase (cubic, Fd3̅m)
detected on a few selected particles of cycled electrodes, due to
the layered-to-spinel transition in agreement with the literature
data.
■
REFERENCES
(1) Ryu, H. H.; Park, K. J.; Yoon, C. S.; Sun, Y. K. Capacity Fading
of Ni-Rich Li[NixCoyMn1-x-y]O2(0.6 ≤ x ≤ 0.95) Cathodes for
High-Energy-Density Lithium-Ion Batteries: Bulk or Surface Degradation? Chem. Mater. 2018, 30, 1155−1163.
(2) Liu, W.; Oh, P.; Liu, X.; Lee, M. J.; Cho, W.; Chae, S.; Kim, Y.;
Cho, J. Nickel-Rich Layered Lithium Transition-Metal Oxide for
High-Energy Lithium-Ion Batteries. Angew. Chem., Int. Ed. 2015, 54,
4440−4457.
(3) Schipper, F.; Erickson, E. M.; Erk, C.; Shin, J.-Y.; Chesneau, F.
F.; Aurbach, D. ReviewRecent Advances and Remaining Challenges
for Lithium Ion Battery Cathodes. J. Electrochem. Soc. 2017, 164,
A6220−A6228.
(4) Xu, J.; Lin, F.; Doeff, M.; Tong, W. A Review of Ni-Based
Layered Oxides for Rechargeable Li-Ion Batteries. J. Mater. Chem. A
2017, 5, 874−901.
(5) Susai, F. A.; Sclar, H.; Shilina, Y.; Penki, T. R.; Raman, R.;
Maddukuri, S.; Maiti, S.; Halalay, I. C.; Luski, S.; Markovsky, B.;
Aurbach, D. Horizons for Li-Ion Batteries Relevant to ElectroMobility: High-Specific-Energy Cathodes and Chemically Active
Separators. Adv. Mater. 2018, 30, 1801348.
(6) Kim, J.; Lee, H.; Cha, H.; Yoon, M.; Park, M.; Cho, J. Prospect
and Reality of Ni-Rich Cathode for Commercialization. Adv. Energy
Mater. 2018, 8, 1702028.
(7) Manthiram, A.; Song, B.; Li, W. A Perspective on Nickel-Rich
Layered Oxide Cathodes for Lithium-Ion Batteries. Energy Storage
Mater. 2017, 6, 125−139.
(8) Myung, S. T.; Maglia, F.; Park, K. J.; Yoon, C. S.; Lamp, P.; Kim,
S. J.; Sun, Y. K. Nickel-Rich Layered Cathode Materials for
Automotive Lithium-Ion Batteries: Achievements and Perspectives.
ACS Energy Lett. 2017, 2, 196−223.
(9) Fergus, J. W. Recent Developments in Cathode Materials for
Lithium Ion Batteries. J. Power Sources 2010, 195, 939−954.
(10) Pouillerie, C.; Perton, F.; Biensan, P.; Pérès, J. P.; Broussely,
M.; Delmas, C. Effect of Magnesium Substitution on the Cycling
Behavior of Lithium Nickel Cobalt Oxide. J. Power Sources 2001, 96,
293−302.
(11) Huang, B.; Li, X.; Wang, Z.; Guo, H.; Xiong, X. Synthesis of
Mg-Doped LiNi0.8Co0.15Al0.05O2oxide and Its Electrochemical
Behavior in High-Voltage Lithium-Ion Batteries. Ceram. Int. 2014, 40,
13223−13230.
(12) Du, R.; Bi, Y.; Yang, W.; Peng, Z.; Liu, M.; Liu, Y.; Wu, B.;
Yang, B.; Ding, F.; Wang, D. Improved Cyclic Stability of
LiNi0.8Co0.1Mn0.1O2 via Ti Substitution with a Cut-off Potential
of 4.5V. Ceram. Int. 2015, 41, 7133−7139.
(13) Hu, G.; Zhang, M.; Liang, L.; Peng, Z.; Du, K.; Cao, Y. Mg−
Al−B Co-Substitution LiNi 0.5 Co 0.2 Mn 0.3 O 2 Cathode Materials
with Improved Cycling Performance for Lithium-Ion Battery under
High Cutoff Voltage. Electrochim. Acta 2016, 190, 264−275.
(14) Chen, C. H.; Liu, J.; Stoll, M. E.; Henriksen, G.; Vissers, D. R.;
Amine, K. Aluminum-Doped Lithium Nickel Cobalt Oxide Electrodes
for High-Power Lithium-Ion Batteries. J. Power Sources 2004, 128,
278−285.
(15) El Mofid, W.; Ivanov, S.; Konkin, A.; Bund, A. A High
Performance Layered Transition Metal Oxide Cathode Material
Obtained by Simultaneous Aluminum and Iron Cationic Substitution.
J. Power Sources 2014, 268, 414−422.
(16) Aurbach, D.; Srur-Lavi, O.; Ghanty, C.; Dixit, M.; Haik, O.;
Talianker, M.; Grinblat, Y.; Leifer, N.; Lavi, R.; Major, D. T.; Goobes,
G.; Zinigrad, E.; Erickson, E. M.; Kosa, M.; Markovsky, B.; Lampert,
J.; Volkov, A.; Shin, J.-Y.; Garsuch, A. Studies of Aluminum-Doped
LiNi0.5Co0.2Mn0.3O2: Electrochemical Behavior, Aging, Structural
Transformations, and Thermal Characteristics. J. Electrochem. Soc.
2015, 162, A1014−A1027.
(17) Dixit, M.; Markovsky, B.; Aurbach, D.; Major, D. T. Unraveling
the Effects of Al Doping on the Electrochemical Properties of LiNi 0.5
Co 0.2 Mn 0.3 O 2 Using First Principles. J. Electrochem. Soc. 2017,
164, A6359−A6365.
ASSOCIATED CONTENT
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsaem.9b00767.
Additional data and figures and a detailed description of
the experimental methods (PDF)
■
Article
AUTHOR INFORMATION
Corresponding Authors
*E-mail: Doron.Aurbach@biu.ac.il.
*E-mail: Dan-Thomas.Major@biu.ac.il.
ORCID
Francis Amalraj Susai: 0000-0003-3778-4200
Dan Thomas Major: 0000-0002-9231-0676
Boris Markovsky: 0000-0001-7756-0071
Doron Aurbach: 0000-0001-8047-9020
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
Partial support for the work discussed herein was provided by
the BASF SE through its Research Network on Electromobility, and the Israeli Committee for Higher Education
within the framework of the INREP project. The authors thank
Ortal Bruer for her assistance in the materials synthesis and Dr.
Vasiliy Rosen for his support in ICP analysis.
4532
DOI: 10.1021/acsaem.9b00767
ACS Appl. Energy Mater. 2019, 2, 4521−4534
Article
ACS Applied Energy Materials
(18) Schipper, F.; Dixit, M.; Kovacheva, D.; Talianker, M.; Haik, O.;
Grinblat, J.; Erickson, E. M.; Ghanty, C.; Major, D. T.; Markovsky, B.;
Aurbach, D. Stabilizing Nickel-Rich Layered Cathode Materials by a
High-Charge Cation Doping Strategy: Zirconium-Doped LiNi 0.6 Co
0.2 Mn 0.2 O 2. J. Mater. Chem. A 2016, 4, 16073−16084.
(19) Wang, D.; Li, X.; Wang, Z.; Guo, H.; Xu, Y.; Fan, Y.; Ru, J. Role
of Zirconium Dopant on the Structure and High Voltage Electrochemical Performances of LiNi 0.5 Co 0.2 Mn 0.3 O 2 Cathode
Materials for Lithium Ion Batteries. Electrochim. Acta 2016, 188, 48−
56.
(20) Yoon, C. S.; Choi, M. J.; Jun, D. W.; Zhang, Q.; Kaghazchi, P.;
Kim, K. H.; Sun, Y. K. Cation Ordering of Zr-Doped LiNiO2Cathode
for Lithium-Ion Batteries. Chem. Mater. 2018, 30, 1808−1814.
(21) Park, S. H.; Oh, S. W.; Sun, Y. K. Synthesis and structural
characterization of layered Li[Ni1/3+xCo1/3Mn1/3−2xMox]O2cathode materials by ultrasonic spray pyrolysis. J. Power Sources
2005, 146, 622−625.
(22) Konishi, H.; Yoshikawa, M.; Hirano, T. The Effect of Thermal
Stability for High-Ni-Content Layer-Structured Cathode Materials,
LiNi0.8Mn0.1-XCo0.1MoxO2(x = 0, 0.02, 0.04). J. Power Sources
2013, 244, 23−28.
(23) Zhang, Y.; Wang, Z. B.; Yu, F. Da; Que, L. F.; Wang, M. J.; Xia,
Y. F.; Xue, Y.; Wu, J. Studies on Stability and Capacity for Long-Life
Cycle Performance of Li(Ni0.5Co0.2Mn0.3)O2by Mo Modification
for Lithium-Ion Battery. J. Power Sources 2017, 358, 1−12.
(24) Büyükburç, A.; Aydinol, M. K. High Rate Capable Manganese
and Tungsten Doped Freeze Dried Lithium Cobalt Oxide Cathode
Material for Secondary Lithium-Ion Batteries. Technol. Rep. 2016, 24.
(25) Kim, U. H.; Jun, D. W.; Park, K. J.; Zhang, Q.; Kaghazchi, P.;
Aurbach, D.; Major, D. T.; Goobes, G.; Dixit, M.; Leifer, N.; et al.
Pushing the Limit of Layered Transition Metal Oxide Cathodes for
High-Energy Density Rechargeable Li Ion Batteries. Energy Environ.
Sci. 2018, 11, 1271−1279.
(26) Breuer, O.; Chakraborty, A.; Liu, J.; Kravchuk, T.; Burstein, L.;
Grinblat, J.; Kauffman, Y.; Gladkih, A.; Nayak, P.; Tsubery, M.;
Frenkel, A. I.; Talianker, M.; Major, D. T.; Markovsky, B.; Aurbach,
D. Understanding the Role of Minor Molybdenum Doping in LiNi
0.5 Co 0.2 Mn 0.3 O 2 Electrodes: From Structural and Surface
Analyses and Theoretical Modeling to Practical Electrochemical Cells.
ACS Appl. Mater. Interfaces 2018, 10, 29608−29621.
(27) Schipper, F.; Bouzaglo, H.; Dixit, M.; Erickson, E. M.; Weigel,
T.; Talianker, M.; Grinblat, J.; Burstein, L.; Schmidt, M.; Lampert, J.;
Erk, C.; Markovsky, B.; Major, D. T.; Aurbach, D. From Surface ZrO2
Coating to Bulk Zr Doping by High Temperature Annealing of
Nickel-Rich Lithiated Oxides and Their Enhanced Electrochemical
Performance in Lithium Ion Batteries. Adv. Energy Mater. 2018, 8,
1701682.
(28) Kovacheva, D.; Gadjov, H.; Petrov, K.; Mandal, S.; Lazarraga,
M. G.; Pascual, L.; Amarilla, J. M.; Rojas, R. M.; Herrero, P.; Rojo, J.
M. Synthesizing Nanocrystalline LiMn2O4by a Combustion Route. J.
Mater. Chem. 2002, 12, 1184−1188.
(29) Amalraj, F.; Kovacheva, D.; Talianker, M.; Zeiri, L.; Grinblat, J.;
Leifer, N.; Goobes, G.; Markovsky, B.; Aurbach, D. Synthesis of
Integrated Cathode Materials xLi2MnO3·(1−x)LiMn1/3Ni1/3Co1/3O2
(x = 0.3, 0.5, 0.7) and Studies of Their Electrochemical Behavior. J.
Electrochem. Soc. 2010, 157, A1121.
(30) Haik, O.; Martha, S. K.; Sclar, H.; Samuk-Fromovich, Z.;
Zinigrad, E.; Markovsky, B.; Kovacheva, D.; Saliyski, N.; Aurbach, D.
Characterizations of Self-Combustion Reactions (SCR) for the
Production of Nanomaterials Used as Advanced Cathodes in Li-Ion
Batteries. Thermochim. Acta 2009, 493, 96−104.
(31) Martha, S. K.; Sclar, H.; Szmuk Framowitz, Z.; Kovacheva, D.;
Saliyski, N.; Gofer, Y.; Sharon, P.; Golik, E.; Markovsky, B.; Aurbach,
D. A Comparative Study of Electrodes Comprising Nanometric and
Submicron Particles of LiNi0.50Mn0.50O2, LiNi0.33Mn0.33Co0.33O2, and LiNi0.40Mn0.40Co0.20O2 Layered Compounds. J.
Power Sources 2009, 189, 248−255.
(32) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy
Calculations for Metals and Semiconductors Using a Plane-Wave
Basis Set. Comput. Mater. Sci. 1996, 6, 15−50.
(33) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamcis for Liquid
Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558.
(34) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B:
Condens. Matter Mater. Phys. 1994, 50, 17953−17979.
(35) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient
Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868.
(36) Grimme, S. Accurate Description of van Der Waals Complexes
by Density Functional Theory Including Empirical Corrections. J.
Comput. Chem. 2004, 25, 1463−1473.
(37) Dixit, M.; Markovsky, B.; Schipper, F.; Aurbach, D.; Major, D.
T. Origin of Structural Degradation During Cycling and Low Thermal
Stability of Ni-Rich Layered Transition Metal-Based Electrode
Materials. J. Phys. Chem. C 2017, 121, 22628−22636.
(38) Dronskowski, R.; Blöchl, P. E. Crystal Orbital Hamilton
Populations (COHP). Energy-Resolved Visualization of Chemical
Bonding in Solids Based on Density-Functional Calculations. J. Phys.
Chem. 1993, 97, 8617−8624.
(39) Deringer, V. L.; Tchougréeff, A. L.; Dronskowski, R. Crystal
Orbital Hamilton Population (COHP) Analysis as Projected from
Plane-Wave Basis Sets. J. Phys. Chem. A 2011, 115, 5461−5466.
(40) Maintz, S.; Deringer, V. L.; Tchougréeff, A. L.; Dronskowski, R.
Analytic Projection from Plane-Wave and PAW Wavefunctions and
Application to Chemical-Bonding Analysis in Solids. J. Comput. Chem.
2013, 34, 2557−2567.
(41) Maintz, S.; Deringer, V. L.; Tchougréeff, A. L.; Dronskowski, R.
LOBSTER: A Tool to Extract Chemical Bonding from Plane-Wave
Based DFT. J. Comput. Chem. 2016, 37, 1030−1035.
(42) Ma, J.; Gao, Y.; Wang, Z.; Chen, L. Structural and
Electrochemical Stability of Li-Rich Layer Structured Li2MoO3 in
Air. J. Power Sources 2014, 258, 314−320.
(43) Ohzuku, T.; Ueda, A.; Nagayama, M.; Iwakoshi, Y.; Komori, H.
Comparative Study of LiCoO2, LiNi1/2Col/2O2 and LiNiO2 For 4 V
Secondary Lithium Cells Secondary Lithium Cells. Electrochim. Acta
1993, 38, 1159−1167.
(44) Shannon, R. D. Revised Effective Ionic Radii and Systematic
Studies of Interatomic Distances in Halides and Chalcogenides. Acta
Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976,
32, 751−767.
(45) Thieme, K.; Avramov, I.; Rüssel, C. The Mechanism of
Deceleration of Nucleation and Crystal Growth by the Small Addition
of Transition Metals to Lithium Disilicate Glasses. Sci. Rep. 2016, 6,
25451.
(46) Farvid, S. S.; Dave, N.; Wang, T.; Radovanovic, P. V. DopantInduced Manipulation of the Growth and Structural Metastability of
Colloidal Indium Oxide Nanocrystals. J. Phys. Chem. C 2009, 113,
15928−15933.
(47) Liang, C.; Kong, F.; Longo, R. C.; KC, S.; Kim, J.-S.; Jeon, S.;
Choi, S.; Cho, K. Unraveling the Origin of Instability in Ni-Rich
LiNi1−2xCoxMnxO2(NCM) Cathode Materials. J. Phys. Chem. C
2016, 120, 6383−6393.
(48) Liang, C.; Longo, R. C.; Kong, F.; Zhang, C.; Nie, Y.; Zheng,
Y.; Cho, K. Ab Initio Study on Surface Segregation and Anisotropy of
Ni-Rich Ni-Rich LiNi1−2yCoyMnyO2(y ≤ 0.1) Cathodes. ACS Appl.
Mater. Interfaces 2018, 10, 6673−6680.
(49) Li, G.; Li, Q.; Li, L.; Fan, J.; Ge, Q.; Xie, D.; Zheng, J.; Li, G.
Surface Element Segregation and Electrical Conductivity of Lithium
Layered Transition-Metal Oxide Cathode Materials. Appl. Surf. Sci.
2018, 427, 226−232.
(50) Yoon, C. S.; Kim, U. H.; Park, G. T.; Kim, S. J.; Kim, K. H.;
Kim, J.; Sun, Y. K. Self-Passivation of a LiNiO2Cathode for a LithiumIon Battery through Zr Doping. ACS Energy Lett. 2018, 3, 1634−
1639.
(51) Su, Y.; Yang, Y.; Chen, L.; Lu, Y.; Bao, L.; Chen, G.; Yang, Z.;
Zhang, Q.; Wang, J.; Chen, R.; et al. Improving the Cycling Stability
of Ni-Rich Cathode Materials by Fabricating Surface Rock Salt Phase.
Electrochim. Acta 2018, 292, 217−226.
4533
DOI: 10.1021/acsaem.9b00767
ACS Appl. Energy Mater. 2019, 2, 4521−4534
Article
ACS Applied Energy Materials
(52) Self, E. C.; Zou, L.; Zhang, M.-J.; Opfer, R.; Ruther, R. E.;
Veith, G. M.; Song, B.; Wang, C.; Wang, F.; Huq, A.; Nanda, J.
Synthesis and Electrochemical and Structural Investigations of
Oxidatively Stable Li2MoO3and XLi2MoO3·(1 - X)LiMO2Composite Cathodes. Chem. Mater. 2018, 30, 5061−5068.
(53) Ghanty, C.; Markovsky, B.; Erickson, E. M.; Talianker, M.;
Haik, O.; Tal-Yossef, Y.; Mor, A.; Aurbach, D.; Lampert, J.; Volkov,
A.; et al. Li + -Ion Extraction/Insertion of Ni-Rich Li 1+ x (Ni y Co z
Mn z) w O 2 (0.005< x < 0.03; Y: Z = 8:1, w ≈1) Electrodes: In Situ
XRD and Raman Spectroscopy Study. ChemElectroChem 2015, 2,
1479−1486.
(54) Kondrakov, A. O.; Geßwein, H.; Galdina, K.; de Biasi, L.;
Meded, V.; Filatova, E. O.; Schumacher, G.; Wenzel, W.; Hartmann,
P.; Brezesinski, T.; Janek, J. Charge-Transfer-Induced Lattice Collapse
in Ni-Rich NCM Cathode Materials during Delithiation. J. Phys.
Chem. C 2017, 121, 24381−24388.
(55) Li, Y.; Hu, Y.; Shen, J.; Jiang, H.; Min, G.; Qiu, S.; Song, Z.;
Sun, Z.; Li, C. Rapid Flame Synthesis of Internal Mo6+doped
TiO2nanocrystals in Situ Decorated with Highly Dispersed
MoO3clusters for Lithium Ion Storage. Nanoscale 2015, 7, 18603−
18611.
(56) Huang, J.; Liu, H.; Zhou, N.; An, K.; Meng, Y. S.; Luo, J.
Enhancing the Ion Transport in LiMn 1.5 Ni 0.5 O 4 by Altering the
Particle Wulff Shape via Anisotropic Surface Segregation. ACS Appl.
Mater. Interfaces 2017, 9, 36745−36754.
4534
DOI: 10.1021/acsaem.9b00767
ACS Appl. Energy Mater. 2019, 2, 4521−4534