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Electrochemical performance of
Na0.6[Li0.2Ni0.2Mn0.6]O2 cathodes with high-
Cite this: J. Mater. Chem. A, 2017, 5,
5858 working average voltage for Na-ion batteries
Ezequiel de la Llave,†a Prasant Kumar Nayak,†a Elena Levi,a Tirupathi Rao Penki,a
Shaul Bublil,a Pascal Hartmann,c Frederick-Francois Chesneau,c Miri Greenstein,a
Linda F. Nazarb and Doron Aurbach*a

Na0.6[Li0.2Ni0.2Mn0.6]O2 is synthesized by a self-combustion reaction (SCR) and studied for the first time as
a cathode material for Na-ion batteries. The Na0.6[Li0.2Ni0.2Mn0.6]O2 cathode presents remarkable high rate
capability and prolonged stability under galvanostatic cycling. A detailed analysis of X-ray diffraction (XRD)
patterns at various states of cycling reveals that the excellent structural stability is due to a primarily solid-
Received 9th December 2016
Accepted 26th February 2017
solution sodiation/desodiation mechanism of the material during cycling. Moreover, a meaningful
comparison with Na0.6MnO2 and Na0.6[Li0.2Mn0.8]O2 reveals that the Na0.6[Li0.2Ni0.2Mn0.6]O2 cathode
DOI: 10.1039/c6ta10577g
achieves a very high working-average voltage that outperforms most of the lithium-doped manganese-
rsc.li/materials-a oxide cathodes published to date.

metal cations into the interlayer space.26,27 Recently, it was found

1. Introduction that structural doping of lithium in manganese oxides enhances
Due to their high energy density and long life cycle, Li-ion the structural stabilization of the cathodes upon cycling.26–32
batteries have become a major power source, with applications Nonetheless, these oxide cathodes exhibit a low average
in portable electronic devices.1–4 Despite their high energy discharge voltage compared to their Li-based cathode analogs,
density, however, the anticipated rise in cost of Li (driven by the limiting their practical energy density.
increasing market intrusion of Li-ion batteries for vehicular In this paper, we report for the rst time the electrochemical
transport) has prompted the search for other intercalation performance of Na0.6[Li0.2Ni0.2Mn0.6]O2 synthesized by a self-
materials. Sodium-based batteries are considered as the most combustion reaction (SCR) as a cathode for NIB. The material
promising alternative. Although rechargeable sodium-ion analyzed here outperforms any other lithium-doped manganese
batteries (NIB) are inferior to Li-ion technology in terms of oxide published to date, in terms of average working voltage and
energy density, they appear to be one of the most promising rate capability, as well as high structural stability.
technologies for stationary large-scale energy storage devices.5,6
Because of the high abundance and low cost of sodium compared
to lithium, there is increasing interest in these systems.7–9
2. Experimental section
Layered oxide compounds are particularly promising as the Analytical-grade chemicals, namely, Mn(NO3)2 (Fluka),
positive electrodes of NIB due to their high specic capacities.10–15 Ni(NO3)2, NaNO3, LiNO3 (Aldrich), sucrose, poly(vinylidene
While these materials were introduced during the early 1980s,16–19 uoride) (PVDF) and 1-methyl-2-pyrrolidinone (NMP) (Aldrich)
it was recently discovered that Na-ion-based cathodes can exhibit were used as received. Double distilled (DD) water was used to
specic capacities close to 200 mA h g
1, making them an dissolve the metal nitrates and sucrose.
excellent choice for energy-storage devices.20–23 Although prom-
ising, layered transition metal oxides exhibit capacity fading due
to the structural instability resulting from the intercalation/de- 2.1. Synthesis and structural characterization
intercalation of large Na+ ions24–26 and migration of transition In a typical SCR synthesis,33 1.454 g of Ni(NO3)2, 3.765 g of
Mn(NO3)2, 0.362 g of LiNO3 and 1.340 g of NaNO3 were dis-
solved in 80 ml of DD water. About 5 wt% of Li and Na nitrates
a
Chemistry Department, Bar-Ilan University, Ramat-Gan 5290002, Israel. E-mail:
were used in excess to compensate for the Li and Na loss upon
Doron.Aurbach@biu.ac.il
b
Department of Chemistry, Waterloo Institute of Nanotechnology, University of
annealing at a high temperature. Sucrose was then added to the
Waterloo, Waterloo, ON N2L 3G1, Canada continuously stirred solution. Next, the water was evaporated by
c
BASF SE, Ludwigshafen 67056, Germany heating at about 100  C for a few hours to obtain a syrupy mass,
† Equal contributions. which on further heating at 350  C resulted in the self-ignition

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of the reactants and, subsequently, an amorphous compound.

This product was ground to a ne powder, annealed at 500  C
for 3 h and cooled down to room temperature. The obtained
powder was then ground and nally annealed at 900  C for 15 h,
to obtain the desired oxide material.
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Elemental analysis of the synthesized material was carried

out using the inductive coupled plasma technique (ICP-AES,
spectrometer Ultima-2 from JobinYvon Horiba). X-ray diffrac-
tion (XRD) studies were performed with a Bruker Inc. (Germany)
AXS D8 ADVANCE diffractometer (reection q–q geometry, Cu
Ka radiation, receiving slit 0.2 mm, high-resolution energy-
dispersive detector). The morphology of the products was
investigated by a Magellan XHR 400L FE-SEM-FEI scanning
electron microscope.
Electrode preparation. The cathode electrodes were fabri-
cated by mixing 80 wt% of the cathode material, 10 wt% of Fig. 1 XRD pattern of Na0.6Li0.2Ni0.2Mn0.6O2 powder. Inset: SEM
image of the sample.
carbon black and 10 wt% of PVDF binder. A planetary centrifugal
vacuum mixer was used to shake the vials at 2000 rpm for 10 min
under vacuum. The obtained slurry was cast onto aluminum foil,
followed by coating with a doctor blade. The electrodes were then the powder-chemical composition is found to be Na0.60[Li0.22-
dried overnight at 80  C under vacuum. The resulting foil was cut Ni0.20Mn0.58]O2, which is very close to the target composition.
into discs 14 mm in diameter, which were weighed and dried at The electrochemical behavior of Na0.6[Li0.2Ni0.2Mn0.6]O2 as
110  C overnight in vacuum, in order to remove the moisture cathode material for sodium-ion batteries is shown in Fig. 2,
adsorbed during the preparation process. A typical loading of the which is illustrative of typical galvanostatic charge–discharge
active mass was 3–4 mg cm
2. The electrochemical performance curves (panel a) and differential dQ/dV vs. V proles (panel b) for
was examined using 2325 coin-type cells assembled in an argon- our cathode material at different cycling stages. The cells were
lled dry glovebox (MBraun). Na metal foil was used for the charged to 4.6 V (vs. Na) and then discharged to 2.0 V (vs. Na) at
counter and reference electrodes. a constant rate (12 mA g
1  C/15 rate) in standard coin-type
Battery testing was carried out using multichannel battery cells. The rst-cycle voltage prole gradually slopes upward, fol-
testers (Arbin Inc.). The galvanostatic charge–discharge cycling lowed by a plateau at about 4.4 V vs. Na. This same feature can be
experiments were carried out at 12 mA g
1 (C/15 rate) over the clearly seen as a sharp peak at about 4.4 V in the differential
potential range of 2.0–4.6 V vs. Na. Electrochemical impedance capacity plot in Fig. 2b, along with a smaller peak at about 4.0 V
spectroscopy (EIS) experiments were measured using a VMP which is discussed below. Together with the voltage prole, the
Bio-Logic Potentiostat with a 1287/1260 FRA system from differential capacity plot reveals that the 4.4 V process only takes
Solartron. For all the experiments, the selected electrolyte place during the rst cycle. This process can be ascribed to the
solution was 0.5 M NaPF6 in PC. partial activation of the Li2MnO3 secondary phase observed in
the XRD pattern of Fig. 1, which is well known to occur in lithium
and manganese-rich cathode materials for LIB.34–44
3. Results and discussion From the voltage prole it can be clearly seen that the initial
The XRD pattern of the Na0.6[Li0.2Ni0.2Mn0.6]O2 powder is specic capacity increases from about 130 mA h g
1 to roughly
shown in Fig. 1. The crystal structure can be assigned to 150 mA h g
1 during the rst cycles. Aer the 50th cycle, the
a hexagonal P2 structure within the P63/mmc space group, capacity slightly decreases, reaching a constant value at about
which is isostructural with P2-Na2/3CoO2, as previously reported 120 mA h g
1 for more than 100 cycles. Several broad peaks
by Paulsen and Dahn.10 All the observed Bragg diffraction peaks correspond to the intercalation/deintercalation of Na+ ions to/
exhibit a long coherence length (beyond that detectable by XRD from the crystal lattice in the differential capacity plot. Recent
broadening), which shows the high degree of crystallinity of the XAS experiments on similar materials proves that Ni is the only
synthesized sample. The XRD pattern resembles other previ- electrochemically active species involved on the charge
ously published lithiated manganese oxides with a hexagonal compensation during charge/discharge process, whereas Mn
P2 structure.26–32 The weak peak near 19 (marked by an asterisk remains inactive.28,29 The incremental increase in capacity
in Fig. 1) can be assigned to the presence of a secondary during the rst cycle observed in our material is probably due to
Li2MnO3 phase, as previously reported for a similar material.31 a partial activation of the manganese on the Li2MnO3 secondary
The morphology of the powder is presented in the scanning- phase, unleashed during charge at high voltage.
electron-microscopy (SEM) image in the inset. The image Aer the rst step, no signicant changes are observed in the
shows well-crystallized particles of dimensions in the range of shape of the voltage proles or the differential capacity plot,
300–500 nm, in good agreement with the morphology of the which is evidence of the structural stability of the cathode
Na0.6MnO2 and Na0.6[Li0.20Mn0.80]O2 cathodes recently synthe- material. More interestingly, the smooth voltage prole and the
sized by our group using SCR.26 From ICP elemental analysis, broader peaks on the differential capacity plots suggest that the

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Fig. 2 Electrochemical performance of Na0.6[Li0.2Ni0.2Mn0.6]O2 cathodes. Panel (a): galvanostatic charge/discharge curves. Panel (b): dQ/dV vs.
V (voltage profile derivatives). The experiments were performed at 30  C at a rate of C/15 (12 mA g
1) in 0.5 M NaPF6 PC electrolyte solution.

principal mechanism involved in the sodiation/desodiation transferred to the X-ray diffractometer in hermetically sealed
process is based on a solid-solution process instead of two- holders in order to avoid any reaction with the atmosphere prior
phase transitions. In recent work,25,26 we showed that, during to the XRD pattern acquisition. Fig. 3 also shows the XRD
cycling, the sodium manganese oxides undergo a detrimental patterns of both the pristine cathode and the cathode aer
multi-phase transition which induces structural instability. prolonged cycling.
Also, we proved that this structural instability can be overcome The XRD pattern on charge state conserve the main features
by lithium-structural doping, which has a direct effect on the of the pristine P2-phase, revealing that no phase transition take
insertion/deinsertion mechanism. In addition, we demon- place during charge. The determination of the unit cell
strated that the presence of lithium dopant effectively reduces parameters by prole matching, showed an increase in the
the extent of the detrimental Jahn–Teller distortions in the interlayered distance of about 1.9% in charged state, with
material, due to the increase in the average manganese oxida- respect to the pristine sample (11.042 Å vs. 11.253 Å). This is
tion state.26 Similar considerations are valid for the Na0.6[Li0.2- probably due to the enhancement of the repulsion between the
Ni0.2Mn0.6]O2 cathode analyzed in this paper. MO2 sheets, which increases as de-sodiation takes place.
In order to further explore the sodiation/desodiation mech- At a fully-discharged state, the XRD pattern is virtually
anism, ex situ XRD measurements at charge (4.6 V) and identical to that of the pristine sample, conrming the full
discharge (2.0 V) states were examined (Fig. 3). Aer cycling, the reversibility of the charge/discharge process described above.
cells were opened inside a dry glovebox under a pure argon However, the XRD pattern evolution reveals that the small peak
atmosphere, thoroughly washed with dry PC solvent several located around 19 , which was identied as a secondary
times and le to dry inside the glovebox. The electrodes were Li2MnO3 phase, signicantly decreased in intensity during
cycling. This nding gives further conrmation to the activation
process proposed above. Fig. 3 also presents the XRD pattern
aer 100 galvanostatic cycles at a fully discharged state. In
comparison to the pristine cathode, the main features of the
XRD pattern aer the prolonged cycle are conserved, conrm-
ing the excellent structural stability of the cathode material.
Also, no evidence of the presence of the Li2MnO3 phase was
found. A slight shis and peak broadening are observed,
probably due to the accumulation of stacking faults aer pro-
longed sodiation/desodiation cycling, as previously reported for
similar materials.24
To provide a comprehensive picture of the structural stability
of our cathode material, the electrochemical behavior of the
cathodes, under prolonged galvanostatic cycles, is presented.
Fig. 4 shows both the discharge capacity and the coulombic
efficiency, of the Na0.6[Li0.2Ni0.2Mn0.6]O2 cathode in conven-
tional coin-type half cells with 0.5 M NaPF6 PC solutions at
Fig. 3 Ex situ XRD patterns. Charge (4.6 V) and discharge (2.0 V) state 30  C. The cell was charged up to 4.6 V and then discharged to
during the second galvanostatic cycle were investigated. The pattern 2.0 V (vs. Na), and cycled at a rate of C/15.
of the pristine cathode (black curve) and the cathode after 100 gal- The rst cycle delivery possesses an initial discharge capacity
vanostatic cycles in discharge state (purple curve) are plotted for of 126 mA h g
1, with a coulombic efficiency of 81%. This low
comparison. Black asterisks denote the signals related to the
coulombic efficiency is linked to the process occurring during the
aluminium current collector.

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related to the diffusion of Na+ ions through a surface lm, as

well as bulk charge transfer resistance and solid-state diffusion
kinetics of Na+ into the active mass.45 The impedance was
measured at several equilibrium potentials during charging
aer the completion of 30 charge–discharge cycles at a rate of C/
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15 (Fig. 5a). The Nyquist plots present a semicircle at high to

medium frequency ranges, representing the parallel combina-
tion of resistance (due to surface lm and charge-transfer at
electrode/electrolyte interface) and double-layer capacitance
(Cdl). A low frequency linear spike appears, due to the diffusion
of Na+ ions into the active mass. It can be seen that the diameter
of the semicircle gradually decreases upon charging to higher
potentials, indicating that the resistance due to surface and
bulk decreases upon charging. Moreover, a sharp change on the
slope of the spectra in the Warburg region (low frequencies) is
Fig. 4 Electrochemical performance of Na0.6Li0.2Ni0.2Mn0.6O2 cath- observed when the cell is charged above 4.0 V (Fig. 5a). This
odes. Discharge capacity (red curve) and coulombic efficiency (blue drastic change of the Na+-ion diffusion in the bulk of the
curve) over the galvanostatic cycle between 4.6 and 2.0 V (vs. Na) at 12 material is a direct consequence of the expansion of the P2-
mA g
1 (C/15 rate) in standard coin-type cells. The experiments were
performed at 30  C in 0.5 M NaPF6 PC electrolyte solution.
phase observed at low sodium content described above (Fig. 3).
It is essential to compare the impedance upon prolonged
cycling with that obtained aer only a few cycles, which can give
activation process during rst cycle described above. Aer the useful information about the stability of the material. The
rst cycle, the total capacity of the cathode steadily increased up impedance was measured at a discharged state (about 2.2 V) as
to 147 mA h g
1 at the 30th cycle. This capacity remained stable well as a charged state (4.6 V) aer completing 10, 50 and 100
until the 50th cycle, slightly decreasing aer that, reaching cycles (Fig. 5b and c). It can be clearly seen that the diameter of
a constant value of about 120 mA h g
1 for more than 100 cycles, the semicircle initially decreases, indicating the decrease in
with a remarkable coulombic efficiency close to 100% during resistance due to lowered surface resistance as well as bulk
cycling. Recently, we showed that the specic capacity of resistance upon cycling from 10 to 50 cycles. The decrease in the
Na0.6MnO2 decreases rapidly upon cycling. However, the doping resistance is in direct relationship to the increase in capacity
of Li in the transition metal layer results in the stabilization of the observed during the rst cycles. Upon further cycling, the
structure which, in turn, results in improved electrochemical Nyquist plots remain almost unchanged, revealing the excellent
performance, as reported for Na0.6[Li0.2Mn0.8]O2.26 stability of our cathode material, allowing prolonged cycling in
Further conrmation of the cathode stability by lithium a wide potential range. Thus, the impedance correlates well
doping was obtained by EIS. EIS is an important electro- with the cycling stability of Na0.6[Li0.2Ni0.2Mn0.6]O2, as dis-
analytical technique for evaluating the surface phenomenon cussed above.

Fig. 5 EIS of Na0.6Li0.2Ni0.2Mn0.6O2 cathodes in sodium half-cells. Nyquist plots at different potentials after 30 galvanostatic cycles (panel (a)),
and at different cycling steps in discharge (panel (b)) and charge (panel (c)) states. The experiments were performed at 30  C in 0.5 M NaPF6 PC
electrolyte solution.

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Fig. 6 Discharge capacity and average voltage (inset) of Na0.6Li0.2- Fig. 7 Voltage profile for Na0.6MnO2 (blue), Na0.6Li0.2Mn0.8O2 (red)
Ni0.2Mn0.6O2 cathodes in sodium half-cells at different current rates and Na0.6Li0.2Ni0.2Mn0.6O2 (black). The arrows indicate the average
(C/10  16 mA g
1). The experiments were performed at 30  C in 0.5 M voltage in each case.
NaPF6 PC electrolyte solution.

Na0.6MnO2 and Na0.6Li0.2Mn0.8O2 cathodes, we refer the reader

The cathode material furthermore presents very good rate to our recent paper.26
capability (Fig. 6). The cell was rst run over 20 cycles at a low Even though the nickel manganese lithium-doped material
rate (C/20) until it reached a stable reversible capacity of 145 has the lowest capacity of the tested cathodes – Na0.6Li0.2-
mA h g
1. Increasing the rates to C/10 and C/5 shows a reduc- Mn0.8O2  190 mA h g
1, Na0.6MnO2  160 mA h g
1 and
tion of 8% (134 mA h g
1) and 21% (120 mA h g
1) in capacity, Na0.6Li0.2Ni0.2Mn0.6O2  150 mA h g
1 – the presence of nickel
respectively. Interestingly, exceptional capacity values were offers a signicant improvement in the cell average voltage. The
found at high rates, i.e. 112, 105, 98 and 92 mA h g
1 for C/2, C, non-doped material (Na0.6MnO2) shows an average voltage of
1.5C and 2C, respectively. Upon returning to a rate of C/20, the 2.7 V, which rises to 2.9 V on lithium substitution (Na0.6-
initial high specic capacity was recovered, indicating that the Li0.2Mn0.8O2). Moreover, the manganese and nickel cathode
high-rate cycling does not deteriorate the electrochemical (Na0.6Li0.2Ni0.2Mn0.6O2) presents an average voltage of 3.4 V,
stability of the Na0.6[Li0.2Ni0.2Mn0.6]O2 material. This remark- outperforming the non-doped Na0.6MnO2 by 0.7 V. This average
able rate-dependent behavior is accompanied by a highly stable voltage is among the highest reported in the literature for
average potential. At the lower rate tested (C/20), the average sodium oxides as cathodes in sodium-ion batteries.24 In a recent
charge and discharge voltage were 3.55 and 3.13 V, respectively, paper Hasa et al.27 compared the electrochemical behavior of
while at the higher rate (2C), the average charge and discharge different P-type layered NaxMO2 with M ¼ Ni, Fe, Mn. In the
voltage changed by only about 7% (3.85 V and 2.90 V, respec- same line with our ndings, they showed that the nickel content
tively) (Fig. 6, inset). remarkably inuences the average working potential of the
The excellent rate capabilities of the material appear to arise batteries and that the Ni-rich cathodes present the higher
from a combination of factors; one being the quasi-solid solu- structural stability during cycling.
tion behavior over much of the electrochemical prole, and the As discussed previously, the increase in the average voltage
other being the partial substitution of manganese for nickel in on the lithiated cathodes may be due not only to the presence of
the lattice. The latter conclusion is well supported by the fact nickel, but also to the contribution of oxide ions in the redox
that the increase in Ni content results in an increase in the rate reactions.26 This hypothesis is being explored in a parallel study
capability and discharge potential in both LIB40 and NIB,27 and is beyond the scope of this work.
although this is still not well understood. In order to provide
a clear picture of the effect of lithium and nickel substitution on
the average working voltage of the manganese oxide cathodes in 4. Conclusions
NIB, we compared the voltage prole of the Na0.6MnO2 and
We demonstrate the advantages of the use of lithiated sodium
Na0.6Li0.2Mn0.8O2 cathodes to our Na0.6Li0.2Ni0.2Mn0.6O2 mate-
nickel–manganese oxides as cathodes for sodium-ion batteries.
rial (Fig. 7). The cells were charged to 4.6 V (vs. Na) and then
Na0.6[Li0.2Mn0.6Ni0.2]O2 was synthesized by SCR, and the elec-
discharged to 2.0 V (vs. Na) at the same rate (12 mA g
1  C/15
trochemical behavior as a cathode on sodium-ion cells has been
rate), in standard coin-type cells. The voltage proles of the
reported for the rst time. The incorporation of lithium dopant
cathodes are plotted aer they reach a stable reversible capacity,
improves the structural stability, allowing the retention of
namely, aer 40 cycles for the Na0.6Li0.2Mn0.8O2 and Na0.6-
stable capacity for more than 100 cycles. Through an ex situ XRD
Li0.2Ni0.2Mn0.6O2 cathodes, and aer ve cycles for Na0.6MnO2.
analysis at different charge/discharge states, we demonstrated
For more details on the electrochemical behavior of the
that the principal intercalation mechanism that the material

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Paper Journal of Materials Chemistry A

undergoes during galvanostatic cycling is predominately solid- 17 J.-J. Braconnier, C. Delmas and P. Hagenmuller, Mater. Res.
solution. The presence of the lithium dopant appears to be an Bull., 1982, 17, 993–1000.
effective method to partially suppress both the extent of the 18 S. Miyazaki, S. Kikkawa and M. Koizumi, Synth. Met., 1983, 6,
destructive cooperative Jahn–Teller distortions and detrimental 211–271.
phase transitions. Moreover, the Na0.6[Li0.2Ni0.2Mn0.6]O2 19 A. Maazaz, C. Delmas and P. Hagenmuller, J. Inclusion
Published on 27 February 2017. Downloaded by Thueringer Universitats Landesbibliothek Jena on 22/08/2017 15:26:21.

cathode presents an exceptionally high working average voltage, Phenom., 1983, 1, 45–51.
outperforming most of the lithium-doped manganese-oxide 20 R. Kataoka, T. Mukai, A. Yoshizawa and T. Sakai, J.
cathodes published to date. This work serves as a proof of Electrochem. Soc., 2013, 160, A933–A939.
concept that the careful selection of the proper dopants 21 N. Yabuuchi, R. Hara, M. Kajiyama, K. Kubota, T. Ishigaki,
improves the properties of the manganese oxide intercalation A. Hoshikawa and S. Komaba, Adv. Energy Mater., 2014, 4,
materials, opening the door to the development of cost-effective 1301453.
practical sodium-ion batteries. 22 N. V. Nghia, P.-W. Ou and I.-M. Hung, Electrochim. Acta,
2015, 161, 63–71.
23 J. Xu, D. H. Lee, R. J. Clément, X. Yu, M. Leskes, A. J. Pell,
Acknowledgements G. Pintacuda, X.-Q. Yang, C. P. Grey and Y.-S. Meng, Chem.
Mater., 2014, 26, 1260–1269.
L. F. N. and D. A. gratefully acknowledge funding from BASF SE
24 R. J. Clement, P. G. Bruce and C. P. Grey, J. Electrochem. Soc.,
for ongoing support through the BASF Research Network in
2015, 162, A2589–A2604.
Electrochemistry and Batteries. Partial support for this work
25 E. de la Llave, V. Borgel, K. J. Park, J. Y. Hwang, Y.-K. Sun,
was obtained from the Israel Science Foundation (ISF) as part of
P. Hartmann, F. F. Chesneau and D. Aurbach, ACS Appl.
the INREP project, and from the Israel Ministry of Science and
Mater. Interfaces, 2016, 8, 1867–1875.
Technology as part of the Israel–India cooperation program.
26 E. de la Llave, E. Talaie, E. Levi, P. K. Nayak, M. Dixit,
P. T. Rao, P. Hartmann, F. F. Chesneau, D. T. Major,
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