Preparation of LiBOB via rheological phase method and its application to mitigate voltage fade of Li1.16[Mn0.75Ni0.25]0.84O2 cathode

Article abstract of DOI:10.1039/c5ra18520c

Lithium bis(oxalato)borate (LiBOB) was synthesized via a novel rheological phase reaction method without any recrystallization procedure. The purity of the as-obtained LiBOB has been identified in comparison with the commercial sample and our sample prepared from solid-state reaction method. The results of XRD, ICP, and ¹¹B NMR demonstrate that high pure LiBOB has been synthesized via rheological phase reaction method with significantly simplified synthetic process. Moreover, LiBOB sample has been investigated as electrolyte additive to improve the electrochemical performances of high-energy lithium-rich layered oxide. The cycling performances imply that 0.03 M and 0.05 M LiBOB additive can mitigate discharge voltage fade and enhance the cycle stability of Li_1.16[Mn_0.75Ni_0.25]_0.84O₂ material. The CV, EIS and XPS data indicate that LiBOB oxidizes at ～4.3 V (vs. Li/Li⁺) on the cathode surface during the first charge to form a specific SEI layer with larger amount of organic species and fairly less content of LiF, which decreases the interfacial polarization and protects the active material from surface degradation, thereby mitigates the voltage-fade of Li-rich cathode.

Full text of DOI:10.1039/c5ra18520c

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: F. lian, Y. Li, Y. He,
H. guan, K. Yan, W. Qiu, K. Chou, P. Axmann and M. Wohlfahrt-Mehrens, RSC Adv., 2015, DOI:
10.1039/C5RA18520C.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Please do not adjust margins
RSC Advances
Page 1 of 9
View Article Online
DOI: 10.1039/C5RA18520C
Journal Name
ARTICLE
Preparation of LiBOB via rheological phase method and its
application to mitigate voltage fade of Li1.16[Mn0.75Ni0.25]0.84O2
cathode
Received 00th January 20xx,
Accepted 00th January 20xx
Fang Lian,a*, Yang Li,a Yi He,a Hongyan Guan,a Kun Yan,a Weihua Qiu,a Kuo-Chih Chou,a Peter
Axmannb and Magret Wohlfahrt-Mehrensb
DOI: 10.1039/x0xx00000x
www.rsc.org/
Lithium bis(oxalato)borate (LiBOB) were synthesized via a novel rheological phase reaction method without any
recrystallization procedure. The purity of the as-obtained LiBOB has been identified in comparison with the commercial
sample and our sample prepared from solid-state reaction method. The results of XRD, ICP, and 11B NMR demonstrate that
high pure LiBOB has been synthesized via rheological phase reaction method with significantly simplified synthetic process.
Moreover, LiBOB sample has been investigated as electrolyte additive to improve the electrochemical performances of
high-energy lithium-rich layered oxide. The cycling performances imply that 0.03M and 0.05M LiBOB additive can mitigate
discharge voltage fade and enhance the cycle stability of Li1.16[Mn0.75Ni0.25]0.84O2 material. The CV, EIS and XPS data indicate
that LiBOB oxidizes at ~4.3 V (vs. Li/Li+) on the cathode surface during the first charge to form a specific SEI layer with
larger amount of organic species and fairly less content of LiF, which decreases the interfacial polarization and protects the
active material from surface degradation, thereby mitigates the voltage-fade of Li-rich cathode.
tetramethanolatoborate LiB(OCH3)4 and di(trimethylsilyl)
oxalate (CH3)3SiOOCCOOSi(CH3)3 in anhydrous acetonitrile (AN).
And then LiBOB was recrystallized from boiling AN/toluene
(1:1 mixture), cooled to -20°C to obtain pure product. As
1. Introduction
The electrolyte compositions (lithium hexafluorophosphate
LiPF6 and carbonate mixtures) have remained largely
unchanged since the first lithium ion batteries were
commercialized.1 However, chemical and thermal instabilities
of LiPF6 restrict the application of lithium ion batteries
especially in hybrid and electric vehicles, which generally
require a service life of 10 years or longer under sustained
electrochemical cycling2, 3. Even at room temperature LiPF6
decomposes, resulting in the formation of solid deposition LiF
and the strong Lewis acid PF5, and the latter is reactive with
organic solvents in the electrolytes. Both LiPF6 and PF5 will
hydrolyze to form HF when trace amount of moisture exists.4, 5
To solve the above issues, lithium bis(oxalato)borate (LiBOB)
was first proposed as a promising candidate for LiPF6 because
of its advantages such as wide electrochemical window, high
thermal stability, fluorine free in the structure, ability to
passivate aluminum current collector, and ability to form a
stable solid-electrolyte interface (SEI) at the surface of
graphitic anode even in pure propylene carbonate (PC)
solvent.6, 7
reported, the common method for preparing LiBOB is based
on organic solution reactions including the procedures such as
dissolution, reflux, evaporation, etc., which are difficult to
perform and commercialize. Our group employed the solid-
state reaction to successfully synthesize LiBOB by heating the
mixtures of oxalic acid dihydrate, lithium hydroxide and boric
acid at 240°C for 6 hours in an open system, which is a simple,
feasible and more environmentally friendly method.11
However, the product obtained from the solid state reaction
method contains impurities due to the open system and high
heating temperature, which requires subsequent rigorous
recrystallization to improve its purity. In the research of
advance solutions to it, we propose a novel rheological phase
reaction method to synthesize pure LiBOB without further
recrystallization procedure for purification. The reaction occurs
in a closed system at a lower heating temperature, while
crystalline water and the water generated during the reaction
process act as the solvent, preventing the introduction of
impurities from additional reagent.
Angell and co-workers8-10 synthesized LiBOB using lithium
Nevertheless, the shortcomings of LiBOB have been exposed
with researches going on: low solubility and conductivity when
used with typical solvent systems, which renders the
electrolyte system with poor rate capability and low-
temperature performance.12 Therefore, research interests
have been transferred to focus on the application of LiBOB as
functional electrolyte additive.13-15 The high-energy lithium-
a.School of Materials Science and Engineering, University of Science and Technology
Beijing, Xueyuan Road, Haidian District, 100083 Beijing, P.R. China. E-mail:
lianfang@mater.ustb.edu.cn
b.Centre of Solar Energy and Hydrogen Research (ZSW) Baden-Württemberg,
Helmholtzstr. 8, Ulm, 89081 Germany.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 2 of 9
View Article Online
DOI: 10.1039/C5RA18520C
ARTICLE
Journal Name
rich cathode Li1+x[NiyMn1-y]1-xO2 is believed to be a promising water. The reaction for the synthesis of r-LiBOB is shown in
next generation cathode material due to its high specific Equation 1.
capacity (>250 mAh/g) with larger charging voltages (>4.5 V vs.
Li/Li+).16 As known, its problem on voltage- and capacity-fade
2H2C2O4·2H2O + LiOH·H2O + H3BO3
→LiBOB + 9H2O (1)
The LiBOB sample (denoted as coarse-s-LiBOB) was also
during cycling still remains unsolved and limits the large-scale synthesized via solid-state reaction method on the basis of our
application. The aggressive oxidization and irreversible previous work11 according to Equation 1, which was purified by
decompose of the commercial liquid electrolyte 1M LiPF6 in further recrystallization to obtain the pure sample (denoted as
EC/DMC at high cut-off voltage, moreover oxygen evolved s-LiBOB).
from the activation of Li2MnO3 during the charge could induce
or accelerate the deterioration of the high-energy
cathode/electrolyte interface. The formation of a SEI layer
with low conductivity and the attack of acidic species, such as
HF, are believed to be the main reason for the deteriorated
electrochemical performance of the Li-rich material during
cycling. The application of SEI film-formation additive can
effectively stabilize cathode/electrolyte interface and improve
the performance of the cathodes.15 For example, our previous
Scheme 1. The schematic diagram for the synthetic process of
work confirmed that fluoroethylene carbonate (FEC) could
improve the electrochemical performances of the lithium-rich
rheological phase reaction method
cathode Li1.16[Mn0.75Ni0.25 0.84
O2.17 Choi et al reported that
]
2.2 Cell preparation
cycling stability of lithium-rich cathode Li1.17Ni0.17Mn0.5Co0.17O2
could be enhanced by LiBOB additive which attributed to the
LiBOB-derived protective layer on the cathode surface.18
However, the impact of LiBOB additive on the discharge
The active material Li1.16[Mn0.75Ni0.25]0.84
O2 was prepared
through a co-precipitation process using LiOH·H2O,
Ni(NO3)2·6H2O (all are 99.9% in purity) and Mn(NO3)2 (50%
aqueous solution) as raw materials.16 The mixed aqueous
solution of 2.0 M transition metal nitrate and 2.0 M Na2CO3
solution were simultaneous added dropwise by peristaltic
pump into a reactor, in which distilled water was under
vigorous stirring. The resulting precipitates were filtered and
washed three times to remove residual Na+, and then dried
under nitrogen at 120 °C for 24 hours. The as-obtained
precursor was subsequently mixed with LiOH·H2O using mortar
and pestle. The mixture was sintered at 850 °C for 12 hours in
muffle furnace under air.
voltage fade of Li-rich cathode Li1+x[NiyMn1-y]1-xO2 has still been
significantly less investigated to our knowledge.
In our work, LiBOB sample obtained via rheological phase
reaction method (r-LiBOB) was evaluated by comparing with
the commercial sample (c-LiBOB, Chemtall) and the one
prepared from solid-state reaction method (s-LiBOB). As an
additive of the state-of-the-art LiPF6-based electrolyte, r-LiBOB
was applied in lithium-rich cathode Li1.16[Mn0.75Ni0.25]0.84O2
system to investigate the effect of LiBOB on its electrochemical
performances, especially the discharge voltage fade issue.
The cathode was prepared from 85 wt.% active material
Li1.16[Mn0.75Ni0.25]0.84O2, 10 wt.% carbon black and 5 wt.%
polyvinylidene difluoride (PVDF) dissolved in N-methyl-2-
pyrrolidone (NMP). The slurry was coated on Al foil, and then
heated at 120 °C in vacuum for 24 hours. Coin cells (2032)
were fabricated in a dry argon filled glove box (MBraun) with
metallic lithium and porous polypropylene films as anode and
separator, respectively. Four electrolyte samples were
involved herein: pristine 1M LiPF6 in EC/ DMC/ DEC (1:1:1 in
volume) electrolyte and the ones with various LiBOB contents
(0.01M, 0.03M and 0.05M) in the pristine solution.
2. Experimental
2.1 Synthesis of high-pure LiBOB
LiBOB was prepared via the novel rheological phase reaction
method shown in Scheme 1 using the raw materials oxalic acid
dihydrate H2C2O4·2H2O (≥ 99.5%), lithium hydroxide
monohydrate LiOH·H2O (≥ 96%) and boric acid H3BO3 (≥ 99.5%)
with a mole ratio of 2:1:1. In detail, 12.6 g of H2C2O4·2H2O and
2.1 g of LiOH·H2O were mechanically mixed in a stirrer for 5
minutes; then, 3.1 g of H3BO3 was added and stirred for
another 5 minutes. The mixture was transferred into a sealed
reactor and placed in a thermautostat at 110 °C for 6 hours.
The gas generated during the heating process was released
through the valve when the pressure in the sealed reactor
exceeded 0.05 MPa. After 6 hours, the reaction system was
cooled quickly in air, and then the coarse LiBOB precipitate
was ground to a fine powder. The resulting samples were
sequentially treated by microwave drying for 4 minutes at
70 °C, then by vacuum drying at room temperature for 5 hours
to remove the residual adsorbed water, and finally by vacuum
tube drying at 130 °C for 48 hours to eliminate the crystalline
2.3 Measurements
The r-LiBOB, s-LiBOB, and c-LiBOB samples were analyzed by
XRD, ICP, and 11B NMR spectroscopy to identify the purity. XRD
measurements of powder samples were performed on a
XD2618N X-ray diffraction analyzer (Japan) at room
temperature. ICP tests were measured by a Vista-MPX CCD
inductive coupled plasma emission spectrometer (United
States). 11B NMR spectra were acquired using a Bruker AV600
nuclear magnetic resonance instrument (Germany) with a
mixture solution of propylene carbonate (PC): ethylene
carbonate (EC): dimethyl carbonate (DMC): γ-butyrolactone
(GBL) (1:1:1:1, in volume) as solvent.
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 3 of 9
View Article Online
DOI: 10.1039/C5RA18520C
Journal Name
Li/Li1.16[Mn0.75Ni0.25 0.84
ARTICLE
]
O2 cells with different electrolytes reported earlier in the literature,21 which is contributed to the
were cycled between 2.5V and 4.7V at a constant current introduction of different solvents for the test LiBOB samples.
density of 100mA/g (0.5C) using Land CT 2001 battery test Therefore, LiBOB sample obtained using the rheological phase
system (China) at 25°C. Cyclic voltammetry (CV) and method possesses high purity and well meets the requirement
electrochemical impedance spectroscopy (EIS) were carried of electrolyte for lithium ion batteries.
out on the Princeton VersaSTAT3 electrochemical work station
(United States). CVs were performed with sweep rate of
0.1mV/s between 2.0V and 4.8V at 25°C using three-electrode
cells (working electrode: Li1.16[Mn0.75Ni0.25]0.84O2 cathode
(Ф=1.6mm), counter electrode and reference electrode: Li foil).
EIS were investigated at charged state of 4.0V at the 15th, 30th,
50th, and 100th cycles at frequency ranging from 100kHz to
0.01Hz under amplitude of 5mV, and the obtained impedance
spectra were fitted by Zview program. The discharged
Li1.16[Mn0.75Ni0.25]0.84O2 electrodes were disassembled from the
cells and then washed by DMC solvent to remove the residual
electrolyte, finally dried under vacuum. The surface chemical
components of the cycled electrodes were analyzed by Kratos
AXIS Ultra DLD X-ray photoelectron spectroscopy (XPS) Fig. 1.XRD patterns of LiBOB samples: (a) r-LiBOB, (b) impure-s-
instrument (Japan) with Al Kα line as an X-ray source.
LiBOB, (c) s-LiBOB, (d) c-LiBOB.
3. Results and discussion
3.1 Characterization of LiBOB samples
XRD patterns of r-LiBOB, s-LiBOB (also including the sample
before purification, coarse-s-LiBOB) and c-LiBOB are compared
in Fig. 1. The r-LiBOB sample shows similar diffraction pattern
with the c-LiBOB and s-LiBOB. All the peaks of r-LiBOB sample
can be indexed based on an orthorhombic structure (space
group: Pnma, No. 62).19 Besides, the main diffraction peaks
intensities of r-LiBOB sample are stronger, indicating a higher
degree of crystallization for the as-synthesized r-LiBOB salt. As
shown in Fig. 1b, several impurity peaks are observed in the
XRD pattern for the coarse-s-LiBOB sample:20 peak at 11.25°
associated with crystalline water; peaks at 20.08° and 28.01°
related to HBO2 (JCPDS card no. 22-1109), which is a product
of H3BO3 decomposition; peak at 22.27° and 29.69° indexed to
Li2C2O4 (JCPDS card no. 24-0646), which is a side product of the
reaction between H2C2O4·2H2O and LiOH·H2O. The s-LiBOB
with high purity (Fig. 1c) was acquired after the removal of the
impurities by recrystallization. Excitingly, these impurity peaks
cannot be observed in the patterns of the r-LiBOB sample
obtained via the rheological phase reaction method without
any further purification.
Fig. 2 11B NMR spectra from: (a) r-LiBOB, (b) pure s-LiBOB, (c)
c-LiBOB.
3.2 Mitigating voltage fade of Li-rich cathode by LiBOB as
electrolyte additive
Elemental analysis (ICP) results show that the r-LiBOB
sample contains Li at 3.79 mass % (3.58% in theory), B at 5.70
mass % (5.58% in theory), and C2O4 at 90.51 mass % (90.84% in
theory). It can be calculated that the atom ratio of Li to B to
C2O4 was 1.04:1: 1.91, which was very close to the theoretical
value of 1:1:2.
The LiBOB samples were further studied using 11B NMR to
compare their purity, as shown in Fig. 2. The 11B spectrum for
each sample (r-LiBOB, s-LiBOB and c-LiBOB) shows analogous
features, corresponding to a chemical shift of ~7.6 ppm, which
is the only contribution from BOB anion. The chemical shift of
7.677 ppm for the r-LiBOB sample is very close to the 11B shift
near 7.70 ppm of LiBOB using tetrahydrofuran (THF) solvent as
The electrochemical performances of Li1.16[Mn0.75Ni0.25]0.84O2
cathode were evaluated in the electrolyte system with and
without LiBOB prepared via the rheological phase reaction
method. The initial charge/discharge profiles in Fig. 3a appear
to be similar, revealing that LiBOB as additive has not altered
the lithium intercalation/de-intercalation behavior. The
charging voltage slope below 4.5 V is attributed to Li+
deintercalation along with the oxidation of Ni2+ to Ni3+ and
then to Ni4+, and the voltage plateau above 4.5 V is assigned to
the activation of Li2MnO3 component accompanied with the
irreversible removal of Li2O. After activation of the Li2MO3
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 4 of 9
View Article Online
DOI: 10.1039/C5RA18520C
ARTICLE
Journal Name
component, the cathode material can deliver a high capacity in the pristine electrolyte and with 0.01M, 0.03M and 0.05M
the subsequent cycles. During the discharge process, the LiBOB, respectively. The phenomenon is attributed to the
reduction of Ni4+ to Ni2+ occurs before 3.5V, and the reaction in rearrangement of transition metal ions in the delithiated host
the low voltage region (< 3.5V) can be attributed to the structure, which make up for the structural deficiencies of the
reduction of Mn4+ to Mn3+.22, 23 The capacity and coulombic cathode material possibly derived from preparation process
efficiency of the initial cycle at the current rate of 0.5C have such as one-step calcination.
been listed in Table 1. It can be seen that the initial irreversible
Fig. 4a-d show the evolution for charge/discharge profiles of
O2 cathode in electrolytes with and
capacity (Cirr) of the cell slightly increases with an increase of Li1.16[Mn0.75Ni0.25]0.84
LiBOB content, in detail, the cells with 0.03 M and 0.05 M without LiBOB additive. The cell with the pristine electrolyte
LiBOB show a little bit larger initial Cirr than the one cycled in shows severe discharge voltage fade upon cycling in Fig. 4a. As
the pristine electrolyte, which could be ascribed to the the LiBOB content increases from 0.01M to 0.05M, the value
formation of LiBOB-involved SEI layer.
of voltage fade during cycling decreases monotonously (Fig.
4b-d). Moreover, detailed comparison of median discharge
voltage for different electrolyte system is shown in Fig. 4e.
After 60 cycles the material in electrolytes with 0.03M and
0.05M LiBOB exhibits the apparently suppressed growth in
median discharge voltage fade as compared with the material
cycled in pristine electrolyte and the system with 0.01M LiBOB.
Fig. 4f shows the energy density calculated through C
×V of
the Li1.16[Mn0.75Ni0.25]0.84O2 electrodes, where C is the
discharge specific capacity and V is the median discharge
voltage. The value of energy density for the cell with pristine
electrolyte exhibits
a fast decrease after 100 cycles to
Fig.
3
Charge/discharge
characteristics
of
630.2Wh/kg with energy retention of 79.81% (versus the
maximum energy density), while the cells after 100 cycles with
0.01M, 0.03M and 0.05M LiBOB retain the energy density of
691.6Wh/kg, 724.5Wh/kg and 732.8Wh/kg, corresponding to
87.98%, 90.84% and 92.32%, respectively. The results indicate
that the energy degradation of Li-rich electrodes is mitigated
significantly with LiBOB as electrolyte additive. In order to
Li/Li1.16[Mn0.75Ni0.25]0.84
O2 cells cycled in the electrolytes with
and without LiBOB at 25°C, 2.5-4.7V, 0.5C: (a)
charge/discharge profiles of the 1st, (b) cycling performance.
Table 1 The initial cycle data of Li/ Li1.16[Mn0.75Ni0.25]0.84O2 cells
in the pristine and electrolytes with various contents of LiBOB:
at 25°C, 2.5-4.7V, 0.5C.
further analysis, discharge capacity of Li1.16[Mn0.75Ni0.25]0.84O2
cathode above and below 3.5 V during cycling is displayed in
Fig. 5. A decrease on the capacity delivered above 3.5 V (Fig.
5a) and a simultaneous increase on the capacity below 3.5V
after 10 cycles (Fig. 5b) for the electrodes reflect the gradual
phase transformation of the material during cycling. Generally,
there are two main reasons contributing to voltage fade of
lithium-rich cathode materials during cycling: one is the
deterioration of the electrode/electrolyte interface associated
with the parasitic side reactions between electrode and
electrolyte at high voltages; the other can be attributed to the
structural evolution from layered to spinel-like phase occurs at
the surface of the particles and evolves gradually in the
internal structure during cycling.24, 25 As shown in Fig. 5a, the
cathode cycled in electrolytes with 0.03M and 0.05M LiBOB
exhibits significantly reduced capacity decay above 3.5 V,
indicating a suppression of phase transformation.25 The results
indicate that the phase transformation of Li-rich material can
be suppressed obviously, but could not be totally avoided in
the presence of LiBOB as electrolyte additive. Interestingly, the
capacity drop at >3.5 V region is compensated by the capacity
increase at <3.5 V region, so the cells with LiBOB additive
exhibit an excellent capacity stability. However, the
undetectable change in voltage fade of the material with
0.01M LiBOB appears by compared with the one in pristine
electrolyte. The fact indicates that the content of 0.01M is too
small to create a desirable SEI layer, while 0.03M and 0.05M
First
charge
capacity
(mAh/g)
293.6
284.9
297.7
285.2
First
discharge
capacity
(mAh/g)
196.8
192.2
200.2
184.8
Amount of
LiBOB
Coulombic
efficiency
(%)
Cirr
(mAh/g)
（mol/L）
0
96.8
92.7
97.5
67.0
67.5
67.3
64.8
0.01
0.03
0.05
100.4
The cycling performance in Fig. 3b demonstrates that
Li1.16[Mn0.75Ni0.25 0.84O2 material cycled in the pristine
]
electrolyte displays fast capacity degradation during cycling,
the discharge capacity decreases to 196.4mAh/g after 100
cycles at 0.5C with capacity retention of 88%; while the ones
cycled in the electrolytes with LiBOB show the improved
cycling stability, discharge capacities of the cells after 100
cycles with 0.01M, 0.03M and 0.05M LiBOB are 216.4mAh/g,
224.2mAh/g and 225.4mAh/g, corresponding to capacity
retention of 96.52%, 97.95% and 98.64% (versus the maximum
discharge
capacity),
respectively.
In
addition,
Li1.16[Mn0.75Ni0.25]0.84
O2 shows the increasing discharge capacity
concentrated in the 3.5–3.0 V range during the early cycles, i.e.,
the discharge capacities delivered between 3.5V and 3.0V
increase from 52, 45, 45 and 41.6 mAh/g initially to 80.1, 76.7,
78.3 and 69.9 mAh/g of the 10th cycle for the cathode cycled in
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 5 of 9
View Article Online
DOI: 10.1039/C5RA18520C
Journal Name
ARTICLE
LiBOB as electrolyte additive are more appropriate to improve electrolyte additives preferentially decompose during the
the performances of Li1.16[Mn0.75Ni0.25 0.84O2. It is deduced that initial cycle in order to form a stable electrode/electrolyte
]
LiBOB additive may decompose to form a stable SEI layer interface, which occurs prior to the decomposition of the
which inhibits the deterioration of the electrode/electrolyte electrolyte solvents, and thereby guarantees the stability of
27
Fig. 6d exhibits the
interface and also mitigates structural degradation of the electrolyte and electrodes.26,
active material.
magnification of CV curves for the cells cycled in electrolytes
with and without LiBOB additive. An oxidation peak appears at
~4.3 V (vs. Li/Li+) for the cell with LiBOB additive in the initial
charge, which becomes more obvious with an increase of
additive content. And the peak almost disappears in the
subsequent cycle, indicating the additive oxidation mainly
occurs in the first cycle. The results suggest that LiBOB additive
does sacrificially decompose on the cathode surface involving
in the formation of SEI layer, which inhibits the side reactions
on the interface even at high voltage.
Fig.
4
Charge/discharge
profile
evolutions
of
Fig. 6 Cyclic voltammetry (CV) curves of Li1.16[Mn0.75Ni0.25]0.84O2
Li/Li1.16[Mn0.75Ni0.25]0.84
O2 cells cycled in (a) pristine electrolyte
cycled in (a) pristine electrolyte and electrolytes with (b)
0.03M and (c) 0.05M LiBOB at 25°C, 2.0-4.8V, 0.1mV/s rate, (d)
magnification CV curves of the 1st and 2nd cycles for the three
types of electrolytes at 4.2-4.5 V.
and electrolytes with (b) 0.01M (c) 0.03M (d) 0.05M LiBOB; (e)
normalized median discharge voltage and (f) energy density of
the cells with and without LiBOB. The inset shows the
corresponding median discharge voltage.
EIS measurements of Li/Li1.16[Mn0.75Ni0.25]0.84O2 cells were
carried out to further evaluate the electrode/electrolyte
interfacial behavior. The cells for EIS measurement firstly
underwent specific cycles between 2.5-4.7 V at 1C rate and
then charged to 4.0 V at 0.2C rate. As shown in Fig. 7, all the
Nyquist plots of the cells with and without LiBOB include two
semicircles in the frequency region. The semicircle in the high
frequency (Rf) is associated with Li+ migration through the SEI
layer on the cathode surface, and the medium-frequency
semicircle is attributed to the charge-transfer reaction through
the electrode/electrolyte interface (Rct). The values of Rf and
Rct were determined by fitting the EIS spectra using the
equivalent circuit (Scheme 216), where Rs represents the
solution resistance, Rf and CPEf stand for the resistance and
capacitance of SEI layer, Rct and CPEdl correspond to the
charge-transfer resistance and double layer capacitance,
respectively. The obtained data were summarized in Fig. 8. It
can be seen that Rf of the cells without LiBOB decreases from
28.6Ω of the 15th cycle to 21.8Ω of the 30th cycle, and then
increases to 30.1Ω of the 50th cycle, especially at 100th cycle
surges to 104.1Ω. The dramatic growth of Rf implies that a
Fig. 5 Discharge capacity (a) above and (b) below 3.5 V during
cycling of Li/Li1.16[Mn0.75Ni0.25]0.84O2 cells cycled in the
electrolytes with and without LiBOB at 25°C, 2.5-4.7V, 0.5C.
3.3 Effect of LiBOB additive on electrolyte/Li-rich cathode
interface
In order to get insight of the functional mechanism of LiBOB
additive on mitigating voltage fade of Li1.16[Mn0.75Ni0.25]0.84O2
cathode, CV tests were performed to compare the oxidation
potential of electrolytes as shown in Fig. 6. As known, some
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 6 of 9
View Article Online
DOI: 10.1039/C5RA18520C
ARTICLE
Journal Name
thick and less conductive SEI layer forms on the cathode contributed to the absorbed carbon from the atmosphere
surface in 100 cycles for the cells with the pristine electrolyte. (284.8 eV) and residual PVDF (C-F at 291.3 eV) on the surface
By contrast, the cells with 0.01M and 0.03M LiBOB exhibit of Li1.16[Mn0.75Ni0.25 0.84O2 cathode. The peaks at 289.3 eV (C=O)
]
small variations in Rf over 100 cycles, which indicates that the and 286.2 eV (C-O)28 may be attributed to the species with C–
LiBOB-involved SEI layer is more stable and effectively O and O–C=O groups such as polyethylene carbonate (PEO),
suppresses the continuous decomposition of electrolyte on the lithium alkyl carbonates (LiOR, ROCO2Li) and oxalates.
cathode surface. Although all cells show an increase of Rct Moreover, the system with 0.03M LiBOB shows the higher
upon cycling, the cell with LiBOB exhibits much lower increase intensity of C=O and C-O peaks than the one cycled in the
in Rct than the ones without LiBOB, which demonstrates that pristine electrolyte, which may be derived from the
SEI layer derived from LiBOB protects the active material from decomposition products of LiBOB. The F1s spectrum
severe structural degradation. The results confirm that LiBOB corresponding only one peak at 688eV for the fresh cathode is
has intervened in the electrolyte/cathode interfacial behavior, ascribed to PVDF.29 While, the peak at 685.6eV appears in the
and thereby suppresses the cell polarization and mitigates the cathode cycled with and without LiBOB, which demonstrates
voltage fade of Li-rich cathode.
the presence of LiF associated with the decomposition of
LiPF6.29 It is worthy to note that the intensity of LiF peak
reduces obviously in the system with 0.03M LiBOB, which
results in a conductive SEI layer as EIS measurements shown.
Fig. 7 EIS spectra of 50% DOC Li1.16[Mn0.75Ni0.25]0.84O2 cells
without LiBOB and with 0.01M, 0.03M and 0.05M LiBOB at
25°C after (a) 15 cycles, (b) 30 cycles, (c) 50 cycles and (d)
100cycles.
Fig. 9 C1s (a, b, c) and F1s (d, e, f) XPS spectra of fresh
Scheme 2 The equivalent circuit model for EIS data fitting
Li1.16[Mn0.75Ni0.25]0.84O2 electrode and electrodes cycled in
pristine electrolyte and electrolyte with 0.03M LiBOB at 0.5C
after 100 cycles.
It is widely accepted that SEI formed on the cathode surface
in the LiPF6-EC-based electrolyte consists of inorganic species
(LiF, Li2CO3, LixPOyFz, LixPFy) as the matrix of SEI layer and
organic species (PEO, LiOR and ROCO2Li), which determines
the elasticity and flexibility characters of SEI.30, 31 It can be seen
from the results that the SEI film in the cells with LiBOB
contains a much larger amount of organic substance and less
LiF content. In conclusion of the experimental results, the
functional mechanism of LiBOB is proposed as shown in Fig. 10.
Fig. 8 The fitting results of (a) Rf and (b) Rct from EIS spectra.
XPS spectra of C1s and F1s were carried out to study the BOB anion may follow a 1-electron oxidation process to
surface composition of Li1.16[Mn0.75Ni0.25 0.84O2 cathode cycled eliminate CO2 and generate borate radical, which performs the
]
in different electrolytes at a current density of 100mA/g after radical coupling reaction to form cross-linking compounds on
100 cycles. As displayed in Fig. 9, all C1s spectra are the cathode surface.32 The LiBOB-derived SEI layer provides
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 7 of 9
View Article Online
DOI: 10.1039/C5RA18520C
Journal Name
ARTICLE
the stable interface between the electrolyte and the electrode
material, where Li+ diffusion is also facilitated. Although the
oxygen release during activation process of the Li2MnO3
component at high voltage, the LiBOB-derived SEI layer could
minimize the reaction between electrolyte components and
References
1
J. L. Allen, S.-D. Han, P. D. Boyle and W. A. Henderson, J.
Power Sources, 2011, 196, 9737-9742.
2
Z. Ma, T. Li, Y. L. Huang, J. Liu, Y. Zhou and D. Xue, RSC Adv.,
2013, 3, 7398-7402.
-
the evolved oxygen species, such as superoxide anion O2 , and
3
4
M. Armand and J. M. Tarascon, Nature, 2008, 451, 652-657.
S. Li, Y. Zhao, X. Shi, B. Li, X. Xu, W. Zhao and X. Cui,
Electrochim. Acta, 2012, 65, 221-227.
thus decrease the oxidation of electrolyte. Additionally, the
acidic species from the electrolyte decomposition may lead to
the corrosion of the cathode form the surface to bulk. It is
found that the content of acidic species on the surface is also
reduced by the LiBOB-derived SEI layer, which suppresses
degradation of the cathode structure.33, 34
5
6
K. Xu, Chem. Rev., 2004, 104, 4303-4417.
V. Aravindan, J. Gnanaraj, S. Madhavi and H.-K. Liu, Chem.
Eur. J., 2011, 17, 14326-14346.
7
8
9
J.-C. Panitz, U. Wietelmann, M. Wachtler, S. Ströbele and M.
Wohlfahrt-Mehrens, J. Power Sources, 2006, 153, 396-401.
W. Xu and C. A. Angell, Electrochem. Solid-State Lett., 2001,
4
, E1-E4.
K. Xu, S. S. Zhang, U. Lee, J. L. Allen and T. R. Jow, J. Power
Sources, 2005, 146, 79-85.
10 S.-T. Myung, H. Natsui, Y.-K. Sun and H. Yashiro, J. Power
Sources, 2010, 195, 8297-8301.
11 B.-T. Yu, W.-H. Qiu, F.-S. Li and G.-X. Xu, Electrochem. Solid-
State Lett., 2006, 9, A1-A4.
12 K. Xu, J. Electrochem. Soc., 2008, 155, A733-A738.
13 X. Wu, S. Wang, X. Lin, G. Zhong, Z. Gong and Y. Yang, J.
Mater. Chem. A, 2014,
14 G. Xu, Z. Liu, C. Zhang, G. Cui and L. Chen, J. Mater. Chem. A,
2015, , 4092-4123.
2, 1006-1013.
3
15 S. S. Zhang, J. Power Sources, 2006, 162, 1379-1394.
16 M. Gao, F. Lian, H. Liu, C. Tian, L. Ma and W. Yang,
Electrochim. Acta, 2013, 95, 87-94.
Fig. 10 The possible decomposition process of LiBOB and
17 Y. Li, F. Lian, L. Ma, C. Liu, L. Yang, X. Sun and K. Chou,
Electrochim. Acta, 2015, 168, 261-270.
functions of LiBOB-derived SEI layer.
18 S. J. Lee, J. G. Han, I. Park, J. Song, J. Cho, J. S. Kim and N. S.
Choi, J. Electrochem. Soc., 2014, 161, A2012-A2019.
19 P. Y. Zavalij, S. Yang and M. S. Whittingham, Acta Cryst., 2003,
B59, 753-759.
4. Conclusions
The novel rheological phase reaction method is employed to
synthesize lithium salt LiBOB without purification procedure.
The results of XRD, ICP, and 11B NMR indicate that the as-
obtained LiBOB has high purity comparable to the commercial
samples and the one prepared from solid state reaction
method with purification. The rheological phase reaction
method realizes simplification of the synthetic process for pure
LiBOB. Furthermore, LiBOB as electrolyte additive in
20 S. Wang, W. Qiu, T. Li, B. Yu and H. Zhao, Int. J. Electrochem.
Sci., 2006, 1, 250-257.
21 U. Wietelmann, U. Lischka and M. Wegner, U.S. Pat., 6 506
516, 2003.
22 S. Rajarathinam, S. Mitra and R. K. Petla, Electrochim. Acta,
2013, 108, 135-144.
23 S. Park, S. Kang, C. Johnson, K. Amine and M. Thackeray,
Electrochem. Commun., 2007, 9, 262-268.
Li1.16[Mn0.75Ni0.25]0.84O2 cells has been investigated herein. The
24 J. Zheng, M. Gu, A. Genc, J. Xiao, P. Xu, X. Chen, Z. Zhu, W.
Zhao, L. Pullan, C. Wang and J. G. Zhang, Nano Lett., 2014,
14, 2628-2635.
results confirm that LiBOB additive participates in the reaction
for SEI film formation, which improves the interfacial stability
and mitigates the structural deterioration of lithium-rich
cathode. The issues in discharge voltage fade and energy
degradation can also be eased in presence of LiBOB as
electrolyte additive.
25 J. Zheng, M. Gu, J. Xiao, B. J. Polzin, P. Yan, X. Chen, C. Wang
and J. Zhang, Chem. Mater., 2014, 26, 6320-6327.
26 J. Zhang, J. Wang, J. Yang and Y. NuLi, Electrochim. Acta,
2014, 117, 99-104.
27 S. Tan, Z. Zhang, Y. Li, J. Zheng, Z. Zhou and Y. Yang, J.
Electrochem. Soc., 2013, 160, A285-A292.
Acknowledgements
28 S. K. Martha, J. Nanda, G. M. Veith and N. J. Dudney, J. Power
Sources, 2012, 216, 179-186.
This work was financially supported by the National 863
Program of China (No.2013AA050901).
29 L. Liao, X. Cheng, Y. Ma, P. Zuo, W. Fang, G. Yin and Y. Gao,
Electrochim. Acta, 2013, 87, 466-472.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 8 of 9
View Article Online
DOI: 10.1039/C5RA18520C
ARTICLE
Journal Name
30 L. Baggetto, D. Mohanty, R. A. Meisner, C. A. Bridges, C.
Daniel, D. L. Wood Iii, N. J. Dudney and G. M. Veith, RSC Adv.,
2014, 4, 23364–23371.
31 Y.-M. Lee, K.-M. Nam, E.-H. Hwang, Y.-G. Kwon, D.-H. Kang,
S.-S. Kim and S.-W. Song, J. Phys. Chem. C, 2014, 118, 10631-
10639.
32 M. Xu, N. Tsiouvaras, A. Garsuch, H. A. Gasteiger and B. L.
Lucht, J. Phys. Chem. C, 2014, 118, 7363-7368.
33 N.-S. Choi, J.-G. Han, S.-Y. Ha, I. Park and C.-K. Back, RSC Adv.,
2015, 5, 2732-2748.
34 J. Zheng, M. Gu, J. Xiao, P. Zuo, C. Wang and J.-G. Zhang,
Nano Lett., 2013, 13, 3824-3830.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 9
RSC Advances
View Article Online
DOI: 10.1039/C5RA18520C
High-purity LiBOB, synthesized via novel rheological phase reaction method without further
recrystallization, mitigated voltage-fade of Li-rich cathode as electrolyte additive.