A diisocyanate/sulfone binary electrolyte based on lithium difluoro(oxalate)borate for lithium batteries Feng Wu, ab Qizhen Zhu, a Li Li, * ab Renjie Chen * ab and Shi Chen ab A new binary electrolyte containing tetramethylene sulfone (TMS) and hexamethylene diisocyanate (HDI) with lithium difluoro(oxalate)borate (LiODFB) as the lithium salt has been prepared and investigated for physicochemical properties. A linear relationship between the frontier molecular orbital energies and the oxidation/reduction potentials is preliminarily confirmed. Compared to the pure TMS electrolyte, a mixture of TMS and HDI exhibits a wider electrochemical stability window, better wettability and an improved low temperature performance. Combined with the mixed electrolyte, LiCoO 2 and LiNi 1/3 Mn 1/3 Co 1/3 O 2 cathode materials show specific capacities of nearly 134.5 mA h g 1 and 168.3 mA hg 1 after 50 cycles, respectively, which is superior to those containing the traditional electrolyte. Furthermore, the composite electrolyte exhibits a good compatibility with the high voltage LiNi 0.5 Mn 1.5 O 4 cathode material which has a specific capacity close to 120 mA h g 1 after 50 cycles. The enhanced battery performance is mainly due to HDI, which has a high oxidation potential (5.2 V), good wettability, a low melting point and an outstanding ability to form effective solid electrolyte interface layers. In addition, LiODFB makes a contribution to the compatibility of the electrolyte due to its passivation toward aluminum, its high solubility and its ability to support reversible metallic lithium cycling. All of the properties above indicate that the LiODFB/HDI/TMS mixed electrolyte is a promising material and can have applications in the field of lithium batteries. 1 Introduction In recent years, lithium batteries have been widely expanded into novel elds, such as intermittent energy storage and (hybrid) electric vehicles. However, most of the current lithium batteries suffer an inherent hazard arising from the use of ammable organic carbonate compounds as electrolyte solvents. 1 As a result, the exploration of novel electrolyte materials with incombustibility, 2–4 a wide electrochemical stability window 5,6 and outstanding electrode compatibility 7 is of prime importance to many research teams dealing with lithium batteries. Improvements in safety and a high operating voltage can be achieved by using nitrile-based 8,9 or sulfone-based 10 organic solvents as main components in the electrolytes. However, the application of such electrolytes in lithium battery systems is currently limited not only due to their high melting points, but also due to their inability to form a stable solid electrolyte interface (SEI) layer on the anode surface resulting in the progressive performance deterioration of the battery. Moreover, the toxicity of nitrile-based electrolytes and the conned wettability of sulfone-based electrolytes are risk factors. In order to solve these problems, mixed electrolytes of sulfone-based organic solvents and additives have been suggested. 11,12 It has been reported that electrolytes composed of tetra- methylene sulfone (TMS) and isocyanate compounds are safe, have good electrochemical stability and have the ability to form an SEI layer. 13–15 In our previous study, a mixture of TMS and p- toluenesulfonyl isocyanate (PTSI) showed excellent wettability and compatibility with electrode materials. 16 However, the mixed electrolyte has several disadvantages which restrict its application, such as a narrow liquidus range and an unsatis- factory oxidation potential (4.9 V versus Li/Li + ), leading to incompatibility with high voltage cathode materials. The compatibility between the electrolyte and electrode materials can be improved by using lithium diuoro(oxalate) borate (LiODFB) as a lithium salt in lithium batteries. LiODFB consisting of half molecular moieties of lithium bis(oxalato) borate (LiBOB) and lithium tetrauoroborate (LiBF 4 ) strongly facilitates the formation of an effective SEI layer on the surface of anode materials. 17–19 Consequently, a composite electrolyte containing LiODFB, sulfone-based organic solvent and addi- tives with a low melting point, a high oxidation potential and a good compatibility with electrode materials presents an inter- esting target for investigation. Here, we report a novel electrolyte system containing TMS, hexamethylene diisocyanate (HDI) and LiODFB as the lithium a School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing Key Laboratory of Environmental Science and Engineering, Beijing, 100081, China. E-mail: [email protected]; [email protected]b National Development Center of High Technology Green Materials, Beijing 100081, China Cite this: J. Mater. Chem. A, 2013, 1, 3659 Received 21st November 2012 Accepted 10th January 2013 DOI: 10.1039/c3ta01182h www.rsc.org/MaterialsA This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. A, 2013, 1, 3659–3666 | 3659 Journal of Materials Chemistry A PAPER Downloaded by Washington University in St. Louis on 19/05/2013 14:02:50. Published on 10 January 2013 on http://pubs.rsc.org | doi:10.1039/C3TA01182H View Article Online View Journal | View Issue
Transcript
Journal ofMaterials Chemistry A
PAPER
Dow
nloa
ded
by W
ashi
ngto
n U
nive
rsity
in S
t. L
ouis
on
19/0
5/20
13 1
4:02
:50.
Pu
blis
hed
on 1
0 Ja
nuar
y 20
13 o
n ht
tp://
pubs
.rsc
.org
| do
i:10.
1039
/C3T
A01
182H
View Article OnlineView Journal | View Issue
aSchool of Chemical Engineering and Envir
Beijing Key Laboratory of Environmental S
China. E-mail: [email protected]; chenrj@bNational Development Center of High Tech
China
Cite this: J. Mater. Chem. A, 2013, 1,3659
Received 21st November 2012Accepted 10th January 2013
DOI: 10.1039/c3ta01182h
www.rsc.org/MaterialsA
This journal is ª The Royal Society of
A diisocyanate/sulfone binary electrolyte based onlithium difluoro(oxalate)borate for lithium batteries
Feng Wu,ab Qizhen Zhu,a Li Li,*ab Renjie Chen*ab and Shi Chenab
A new binary electrolyte containing tetramethylene sulfone (TMS) and hexamethylene diisocyanate (HDI)
with lithium difluoro(oxalate)borate (LiODFB) as the lithium salt has been prepared and investigated for
physicochemical properties. A linear relationship between the frontier molecular orbital energies and
the oxidation/reduction potentials is preliminarily confirmed. Compared to the pure TMS electrolyte, a
mixture of TMS and HDI exhibits a wider electrochemical stability window, better wettability and an
improved low temperature performance. Combined with the mixed electrolyte, LiCoO2 and
LiNi1/3Mn1/3Co1/3O2 cathode materials show specific capacities of nearly 134.5 mA h g�1 and 168.3 mA
h g�1 after 50 cycles, respectively, which is superior to those containing the traditional electrolyte.
Furthermore, the composite electrolyte exhibits a good compatibility with the high voltage
LiNi0.5Mn1.5O4 cathode material which has a specific capacity close to 120 mA h g�1 after 50 cycles. The
enhanced battery performance is mainly due to HDI, which has a high oxidation potential (5.2 V), good
wettability, a low melting point and an outstanding ability to form effective solid electrolyte interface
layers. In addition, LiODFB makes a contribution to the compatibility of the electrolyte due to its
passivation toward aluminum, its high solubility and its ability to support reversible metallic lithium
cycling. All of the properties above indicate that the LiODFB/HDI/TMS mixed electrolyte is a promising
material and can have applications in the field of lithium batteries.
1 Introduction
In recent years, lithium batteries have been widely expandedinto novel elds, such as intermittent energy storage and(hybrid) electric vehicles. However, most of the current lithiumbatteries suffer an inherent hazard arising from the use ofammable organic carbonate compounds as electrolytesolvents.1 As a result, the exploration of novel electrolytematerials with incombustibility,2–4 a wide electrochemicalstability window5,6 and outstanding electrode compatibility7 isof prime importance to many research teams dealing withlithium batteries.
Improvements in safety and a high operating voltage can beachieved by using nitrile-based8,9 or sulfone-based10 organicsolvents as main components in the electrolytes. However, theapplication of such electrolytes in lithium battery systems iscurrently limited not only due to their high melting points, butalso due to their inability to form a stable solid electrolyteinterface (SEI) layer on the anode surface resulting in theprogressive performance deterioration of the battery. Moreover,the toxicity of nitrile-based electrolytes and the conned
onment, Beijing Institute of Technology,
cience and Engineering, Beijing, 100081,
bit.edu.cn
nology Green Materials, Beijing 100081,
Chemistry 2013
wettability of sulfone-based electrolytes are risk factors. In orderto solve these problems, mixed electrolytes of sulfone-basedorganic solvents and additives have been suggested.11,12
It has been reported that electrolytes composed of tetra-methylene sulfone (TMS) and isocyanate compounds are safe,have good electrochemical stability and have the ability to forman SEI layer.13–15 In our previous study, a mixture of TMS and p-toluenesulfonyl isocyanate (PTSI) showed excellent wettabilityand compatibility with electrode materials.16 However, themixed electrolyte has several disadvantages which restrict itsapplication, such as a narrow liquidus range and an unsatis-factory oxidation potential (4.9 V versus Li/Li+), leading toincompatibility with high voltage cathode materials.
The compatibility between the electrolyte and electrodematerials can be improved by using lithium diuoro(oxalate)borate (LiODFB) as a lithium salt in lithium batteries. LiODFBconsisting of half molecular moieties of lithium bis(oxalato)borate (LiBOB) and lithium tetrauoroborate (LiBF4) stronglyfacilitates the formation of an effective SEI layer on the surfaceof anode materials.17–19 Consequently, a composite electrolytecontaining LiODFB, sulfone-based organic solvent and addi-tives with a low melting point, a high oxidation potential and agood compatibility with electrode materials presents an inter-esting target for investigation.
Here, we report a novel electrolyte system containing TMS,hexamethylene diisocyanate (HDI) and LiODFB as the lithium
salt. The molecular structures of TMS and HDI are shown inFig. 1. HDI is used as a cosolvent in the electrolyte and exhibits alow melting point, high boiling and ash points and a wideelectrochemical stability window. It is anticipated that suchelectrolytes would provide a wide operating temperature rangeand a good compatibility with electrodes. The charge–dischargecapacities of lithium batteries fabricated with different elec-trodes and the mixed electrolyte are investigated. Furthermore,the effect of HDI and LiODFB on the performance of the cells isstudied, and the safety of the mixed electrolyte is discussed.
2 Experimental section2.1 Materials
To prepare the LiODFB/HDI/TMS composite electrolyte, HDI(>99%, Acros) and TMS (>99%, Acros) were simply mixed invarious volume ratios, and a concentration of 1 M lithium salt,LiODFB(99%, Fosai) was added in an argon-lled glovebox (H2O< 1 ppm, Super (1220/750), Mikrouna). The reference electrolyteswere prepared by mixing HDI and TMS in various volume ratiosusing 1 M LiPF6 as the lithium salt. Benzyl isocyanate (BI, >99%,Acros), p-tolyl isocyanate (PTI, 99%, Acros), ethyl isocyanate (EI,98%, J&K Chemical), 4-uoro phenyl isocyanate (FPI, 99%,Acros), LiCoO2 (Easpring Material) and LiNi1/3Mn1/3Co1/3O2
(Shanshan Tech) electrode materials were used as received. TheLiNi0.5Mn1.5O4 electrodematerial was prepared according to themethod reported and described in detail in a previous work.20
2.2 Preparation of the electrodes and the construction ofcells
Cathodes used in electrochemical half-cells of Li/LiCoO2 con-sisted of 80 wt% LiCoO2, 10 wt% acetylene black and 10 wt%polyvinylidene uoride (PVDF). The mixture was spread onto analuminum foil current collector and dried. The other cathodeswere prepared in a similar way.
An electrochemical half-cell was constructed with a diskcathode, a disk lithium foil anode, a conventional separator(Celgard�2300) and the prepared electrolyte in the gloveboxunder an argon atmosphere with the water content below1 ppm.
2.3 Instruments
The oxidation/reduction potentials for the various electrolyteswere determined using a linear sweep voltammogram on anelectrochemical workstation (CHI604D, Shanghai ChenhuaCompany) at a scan rate of 1 mV s�1 at 25 �C. The electrolyte wassealed in a glass cell with a platinumwire (99.9%,ؼ 0.1mm) as
Fig. 1 The molecular structures of TMS and HDI.
3660 | J. Mater. Chem. A, 2013, 1, 3659–3666
the working electrode and Li foil (99.9%) as the reference andcounter electrodes. For cyclic voltammetry (CV), using the sameworking electrode, counter and reference electrodes, was con-ducted on the same electrochemical workstation in the voltagerange of�0.5 V to 6.0 V at a scan rate of 1 mV s�1. Two-electrodecells were cycled with a constant current on a Land cell tester(CT2001A, Wuhan Jinnuo Company). For Li/LiCoO2,Li/LiNi1/3Mn1/3Co1/3O2 and Li/LiNi0.5Mn1.5O4 half-cells, thecharge cutoff voltages were set at 4.2 V, 4.5 V and 4.95 V, and thedischarge cutoff voltages were set at 2.7 V, 2.8 V and 3.5 V,respectively. The constant current charge–discharge experi-ments were performed at room temperature and at low temper-atures on a temperature testing chamber (GDJS-100, WuxiSuoyate Company).
Differential scanning calorimeter (DSC) measurements wereobtained to investigate the thermal behaviour using a MDSC2910 DSC (TA Instruments, USA). The samples (about 10 mg)used for measurements were lled and sealed in specialaluminum pans. The pans containing the electrolytes weretransferred into DSC sample holders, cooled to �80 �C, andthen heated to 100 �C at a rate of 10 �C min�1 under a N2
atmosphere. Thermal stability measurements were measuredusing a DSC-TGA analyzer (SDT-Q600, TA Instruments, USA).The temperature measurement ranges from room temperatureup to 400 �C at a scan rate of 10 �C min�1 under a N2
atmosphere.The calculations were set with the DMol3 module of the
Materials Studio 4.2 program. Molecular structures of theorganic molecules were optimized and calculated employingthe nonlocal density functional theory with the BLYP functionalbased on DNP group. This program was also applied to calculatethe frontier molecular orbital energies of each organicmolecule.
3 Results and discussion3.1 Molecular orbital calculations
On the basis of molecular orbital theory, the energies of thehighest occupied molecular orbital (HOMO) and lowest unoc-cupied molecular orbital (LUMO) determine the ability ofmolecules to lose and gain electrons.21–23 The frontier molecularorbitals of various electrolytes and their energies were calcu-lated and are shown in Fig. 2(a). The energy of the HOMO ofHDI and EI is lower than that of other isocyanate compounds,while the energy of their LUMO is higher. This creates widerelectrochemical stability windows in the HDI and EI moleculesand higher oxidation potentials and lower reduction potentialsare probably indicated. It is observed that the HOMO of thebenzene compound molecules is mainly distributed over thebenzene ring, which makes it clear that the benzene ringreduces the oxidation potential of the complex. Furthermore,the EHOMO of FPI is �0.2138 Ha, which is lower than that of PTI(�0.2036 Ha). The existence of uorine is indicative of a lowenergy HOMO of the compound molecule, and this raises itsoxidation potential.
In order to clarify the inuence of the isocyanate molecularstructure on its electrochemical stability, a linear scan test was
This journal is ª The Royal Society of Chemistry 2013
Fig. 2 (a) Frontier molecular orbitals and energies of TMS and various isocyanate compounds, and the relation between the frontier molecular orbitals energies ((b)HOMO and (c) LUMO) and their oxidation/reduction potentials.
Fig. 3 CV of the 1 M LiODFB/HDI/TMS mixed electrolyte. The volume percent ofHDI is 5%. Working electrode: Pt; counter and reference electrodes: Li; scan rate:1 mV s�1.
Paper Journal of Materials Chemistry A
Dow
nloa
ded
by W
ashi
ngto
n U
nive
rsity
in S
t. L
ouis
on
19/0
5/20
13 1
4:02
:50.
Pu
blis
hed
on 1
0 Ja
nuar
y 20
13 o
n ht
tp://
pubs
.rsc
.org
| do
i:10.
1039
/C3T
A01
182H
View Article Online
assembled using a glass cell with a platinum wire as theworking electrode and Li foil as the reference and counterelectrodes to determine their oxidation/reduction potentials.The oxidation/reduction potentials for various isocyanatecompounds are shown in Fig. 2(b) and (c). HDI and EI possessthe widest electrochemical window of 4 V or so, and haveoxidation potentials of 5.2 V and 5.1 V, respectively. Bothreduction potentials are about 1.1 V. The other three isocyanatecompounds show a narrower electrochemical stability window,and the narrowest one of PTI is less than 2.5 V. It is worthwhileto note that the test results above are basically consistent with
This journal is ª The Royal Society of Chemistry 2013
our theoretical analyses, which demonstrates that electro-chemical stability of the molecule can probably be predictedusing molecular orbital calculations.
A relationship exists between the molecular orbitals energylevels and the complexes electrochemical stability. It is reportedthat there is a linear relationship between the reductionpotential and the LUMO energy level of some lm-formingadditive molecules.24 Here, a preliminary descriptive approachto the relationship between the molecular orbitals energy levelsand the oxidation/reduction potentials for various isocyanatemolecules is conducted. The results indicate that the HOMOand LUMO energy levels of the ve isocyanate molecules arelinearly related to their oxidation and reduction potentials,respectively. The relationship might offer a useful starting pointto predict the electrochemical stability of molecules usingtheoretical calculations.
3.2 Electrochemical stability of the LiODFB/HDI/TMSelectrolyte
A glass cell was assembled to further study the electrochemicalstability of the 1 M LiODFB/HDI/TMS electrolyte containing 5vol% HDI by CV (Fig. 3). As expected, the mixed electrolyte has ahigh oxidation potential of 5.6 V which is ascertained accordingto the crossover point where the horizontal line with a currentdensity of 0 cuts the tangent of the reduction peak, and it ismainly attributed to the good antioxidation protection of TMSand HDI. The reduction peaks around 1–1.6 V correspond to thereduction of LiODFB.25 LiODFB advances a thin lm like SEI
Fig. 4 The electrochemical performance of Li/LiCoO2 half-cells containing the mixed electrolyte. (a) Discharge capacities of the cells at a 0.2 C current densitycontaining the 1 M LiODFB/HDI/TMS mixed electrolyte with various volume percents of HDI. (b) Coulombic efficiencies of the cells containing the 1 M LiODFB/5 vol%HDI/TMS mixed electrolyte. (c) Rate performance of the cells containing the 1 M LiODFB/5 vol% HDI/TMS mixed electrolyte. (d) Discharge capacities and coulombicefficiencies of cells at a 0.2 C current density containing the 1 M LiPF6/5 vol% HDI/TMS mixed electrolyte.
Journal of Materials Chemistry A Paper
Dow
nloa
ded
by W
ashi
ngto
n U
nive
rsity
in S
t. L
ouis
on
19/0
5/20
13 1
4:02
:50.
Pu
blis
hed
on 1
0 Ja
nuar
y 20
13 o
n ht
tp://
pubs
.rsc
.org
| do
i:10.
1039
/C3T
A01
182H
View Article Online
formed on Pt electrode surface, and consequently broadens theelectrochemical window of the electrolyte.26
3.3 Room-temperature cell performance of the LiODFB/HDI/TMS electrolyte
The electrochemical compatibility between the LiODFB/HDI/TMS mixed electrolyte and various electrode materials isstudied. Fig. 4(a) shows the electrochemical performance ofcommercial LiCoO2 in the 1 M LiODFB/DHI/TMS mixed elec-trolyte containing various volume percents of HDI at a 0.2 Ccurrent density. The best cell performance is achieved when thevolume percent of HDI is 5%. The discharge capacity andcoulombic efficiency of this cell are 134.5 mA h g�1 and about99.5% aer 50 cycles (Fig. 4(b)). The improvement in the
Fig. 5 The electrochemical performance of Li/LiNi1/3Mn1/3Co1/3O2 half-cells contadensity containing the 1 M LiODFB/HDI/TMS mixed electrolyte with various volumevol% HDI/TMS mixed electrolyte, the insert is the rate performance of the cells con
3662 | J. Mater. Chem. A, 2013, 1, 3659–3666
electrochemical performance on the addition of HDI is mainlydue to the fact that HDI in the electrolyte enhances wettability,facilitates the formation of an SEI layer and gives the electrolyteexcellent electrochemical stability. The discharge capacities ofLi/LiCoO2 cells at a 0.2 C, 0.5 C, 1 C and 2 C current densitycontaining the 1 M LiODFB/5 vol% HDI/TMS mixed electrolyteare compared in Fig. 4(c). The discharge capacities of the cells ata 1 C and 2 C current density attain 130.6 mA h g�1 and 82.8 mAhg�1, respectively, indicating a satisfactory rate performance.
To clarify the role of LiODFB in ameliorating the perfor-mance of the mixed electrolyte, the electrochemical perfor-mance of the cells containing LiPF6 instead of LiODFB aslithium salt has been studied. As shown in Fig. 4(d), thedischarge capacity of the Li/LiCoO2 half-cell containing the 1 MLiPF6/5 vol% HDI/TMS mixed electrolyte is about 112.4 mA h
ining the mixed electrolyte. (a) Discharge capacities of the cells at a 0.2 C currentpercents of HDI. (b) Coulombic efficiencies of the cell containing the 1 M LiODFB/2taining the 1 M LiODFB/2 vol% HDI/TMS mixed electrolyte.
This journal is ª The Royal Society of Chemistry 2013
Fig. 6 The electrochemical performance of the high voltage Li/LiNi0.5Mn1.5O4 half-cells containing the mixed electrolyte. (a) Initial charge–discharge curves and (b)discharge capacities at a 0.2 C current density of the cells containing the 1 M LiODFB/HDI/TMS mixed electrolyte with various volume percents of HDI.
Fig. 7 The DSC curves of TMS, 10 vol% HDI/TMS and 5 vol% HDI/TMS.
Paper Journal of Materials Chemistry A
Dow
nloa
ded
by W
ashi
ngto
n U
nive
rsity
in S
t. L
ouis
on
19/0
5/20
13 1
4:02
:50.
Pu
blis
hed
on 1
0 Ja
nuar
y 20
13 o
n ht
tp://
pubs
.rsc
.org
| do
i:10.
1039
/C3T
A01
182H
View Article Online
g�1 aer 50 cycles, which only retains 82% of its initial capacity,and the coulombic efficiency is over 99.5%. Therefore, LiODFBplays an important part in improving the capacity retentionrate. According to previous papers, there are several probablereasons. (1) LiODFB is more soluble than other salts in linearcarbonate solvents that are essential to lower the viscosity andincrease the wettability of the lithium electrolyte.25 (2) LiODFBhas the ability to passivate aluminum cathode current collectorsat high potentials.17 (3) LiODFB supports reversible metalliclithium cycling,17 and then stabilizes the lithium electrode.
Fig. 8 Initial discharge curves of (a) Li/LiCoO2 and (b) Li/LiNi1/3Mn1/3Co1/3O2 halftrolyte with various volume percents of HDI at 0 �C.
This journal is ª The Royal Society of Chemistry 2013
Cycling performance and rate performance of theLi/LiNi1/3Mn1/3Co1/3O2 cells containing the 1 M LiODFB/HDI/TMS mixed electrolyte are exhibited in Fig. 5(a) and (b),respectively. A high compatibility is observed between theLiNi1/3Mn1/3Co1/3O2 electrode and the mixed electrolyte whichis similar to the Li/LiCoO2 cells. The optimum concentration ofHDI is 2 vol% in the mixed electrolyte, when the highestdischarge capacity for the 50th cycle is 168.3 mA h g�1 retainingabout 96% of the initial capacity of 175.6 mA h g�1, and thecoulombic efficiency is over 99%. However, the electrochemicalperformance of the cells at a 0.5 C and 1 C current density isunsatisfactory and the discharge capacities are 150.9 and95.2 mA h g�1 respectively. These values may be due to the lowconductivity of the electrolyte.
LiNi0.5Mn1.5O4 is a cathode material with a spinel structure,a high potential plateau of 4.75 V and a stable charge–dischargecapacity of above 120 mA h g�1. Due to its high energy densityand power density, LiNi0.5Mn1.5O4 is considered a cathodematerial with extensive prospects for applications in lithiumbatteries used in electric vehicles.27–29
Fig. 6(a) shows the initial charge–discharge curves ofLi/LiNi0.5Mn1.5O4 half-cells containing the 1 M LiODFB/2 vol%HDI/TMSmixed electrolyte and the 1M LiODFB/TMS electrolytewith a constant current of 0.2 C and cutoff voltages of 4.95 V and3.5 V. There are two distinct potential plateaus on the charge–discharge curve for the mixed electrolyte containing HDI, whichare at 4.7 V and 4.0 V and correspond to the redox processes of
-cells at a 0.1 C current density containing the 1 M LiODFB/HDI/TMS mixed elec-
Fig. 9 Initial discharge curves of (a) Li/LiCoO2 and (b) Li/LiNi1/3Mn1/3Co1/3O2 half-cells at a 0.1 C current density containing the 1 M LiODFB/HDI/TMS mixed elec-trolyte with 10 vol% HDI at various temperatures.
Fig. 10 TGA and DSC curves of the mixed electrolytes containing 1 M LiODFB/5vol% HDI/TMS or 1 M LiPF6/EC/DMC (1 : 1).
Journal of Materials Chemistry A Paper
Dow
nloa
ded
by W
ashi
ngto
n U
nive
rsity
in S
t. L
ouis
on
19/0
5/20
13 1
4:02
:50.
Pu
blis
hed
on 1
0 Ja
nuar
y 20
13 o
n ht
tp://
pubs
.rsc
.org
| do
i:10.
1039
/C3T
A01
182H
View Article Online
Ni2+/Ni4+ and Mn3+/Mn4+, respectively.30 In comparison, for theelectrolyte without HDI, the charging plateau is signicantlyhigher and the discharging plateau is slightly lower. Thecoulombic efficiency of the Li/LiNi0.5Mn1.5O4 half-cell with theLiODFB/TMS electrolyte is only 74%, which is considerablyreduced from the cell with the LiODFB/2 vol% HDI/TMS elec-trolyte due to the fact that the wettability of the electrolytewithout HDI is much weaker. The comparison of the cyclingperformance of Li/LiNi0.5Mn1.5O4 half-cells containing 1 MLiODFB/2 vol% HDI/TMS, 1 M LiODFB/TMS and the traditionalelectrolyte is displayed in Fig. 6(b). The discharge capacity of theLi/LiNi0.5Mn1.5O4 half-cell with the isocyanate/sulfone electro-lyte is close to 130 mA h g�1 over the rst 15 cycles, which ishigher than that of the traditional electrolyte, which is close to120 mA h g�1 aer 50 cycles. The coulombic efficiency of theLi/LiNi0.5Mn1.5O4 half-cell with the isocyanate/sulfone electro-lyte is close to 98%, which is equal to that of the traditionalelectrolyte. However, the discharge capacity of the 1 M LiODFB/TMS electrolyte fades rapidly and is about 100 mA h g�1 aer 50cycles with a coulombic efficiency of only 95%. The aboveresults show that HDI can improve the compatibility of theelectrolyte with the LiNi0.5Mn1.5O4 electrode, which could makeit a promising electrolyte additive for use in lithium batterieswith a high potential LiNi0.5Mn1.5O4 electrode.
3664 | J. Mater. Chem. A, 2013, 1, 3659–3666
3.4 Low temperature cell performance of the LiODFB/HDI/TMS electrolyte
The low temperature cell performance is improved by addingHDI to the mixed electrolyte because HDI possesses a lowmelting point. Fig. 7 shows the DSC curves of pure TMS and themixture of TMS and HDI in various volume ratios. Pure TMS hasa higher melting point for its phase transformation of about10 �C. In contrast, absorption of heat for 5 vol% HDI/TMS startsat �10 �C or so, and the phase transformation of 10 vol% HDI/TMS begins at about �15 �C. Therefore, even a small amount ofHDI mixed with the electrolyte can signicantly reduce itsmelting point and advance its application at low temperature.
Fig. 8 shows the electrochemical performance of Li/LiCoO2
and Li/LiNi1/3Mn1/3Co1/3O2 half-cells with the mixed electrolytecontaining various volume percents of HDI at 0 �C. The resultsreveal that cells containing the electrolyte without HDI cannotoperate at 0 �C while cells containing the mixed electrolyte withHDI have normal discharge curves and potential plateaus at0 �C. The highest initial discharge capacities of Li/LiCoO2 andLi/LiNi1/3Mn1/3Co1/3O2 half-cells are 136.3 mA h g�1 and 163.4mA h g�1 respectively, when the electrolyte contains 5 vol%HDI. The electrochemical performance of the cells with themixed electrolyte containing 10 vol% HDI at various tempera-tures is shown in Fig. 9. The cells operate properly even at lowtemperatures of �5 �C or �10 �C. The initial discharge capac-ities of Li/LiCoO2 half-cells achieve 119 and 101 mA h g�1 andthose of Li/LiNi1/3Mn1/3Co1/3O2 half-cells reach up to 142.2 mAh g�1 and 122 mA h g�1 at �5 �C and �10 �C, respectively. Thedischarging plateaus are lower when the temperature is reduceddue to a continuous reduction in the electrolyte conductivity.
3.5 Thermal stability of the LiODFB/HDI/TMS electrolyte
Fig. 10 shows TGA and DSC curves of the 1 M LiODFB/5 vol%HDI/TMS and 1 M LiPF6/EC/DMC (1 : 1) electrolytes. Thetraditional electrolyte shows an obvious loss of mass even atroom temperature, whereas the 1 M LiODFB/5 vol% HDI/TMShybrid electrolyte is thermally stable until 165 �C. In addition,the DSC curve shows that the exothermic reaction of the tradi-tional electrolyte begins at 213.6 �C, nevertheless the tempera-ture for the HDI/TMS hybrid electrolyte is 221.6 �C. These
This journal is ª The Royal Society of Chemistry 2013
results suggest that the HDI/TMS hybrid electrolyte system hasa better thermal stability than the traditional electrolyte, hencethe isocyanate/sulfone electrolyte has enormous potential as asafe electrolyte in lithium batteries.
4 Conclusion
A new HDI/TMS binary electrolyte containing LiODFB as alithium salt is prepared. Its physicochemical properties andelectrochemical application in lithium batteries are studied indetail. A linear correlation between the energies of the HOMO/LUMO level and the oxidation/reduction potential is prelimi-narily conrmed. The mixed electrolyte containing HDI exhibitsexcellent electrochemical stability, excellent wettability, a goodcompatibility with various electrode materials and a lowtemperature electrochemical performance because HDI has ahigh oxidation potential (5.2 V), good wettability, anoutstanding ability to form an SEI layer and a lowmelting point.It is found that the mixed electrolyte exhibits a better cyclingperformance than commercial electrolytes in both Li/LiCoO2
and Li/LiNi1/3Mn1/3Co1/3O2 half-cells. The reversible capacitiesof Li/LiCoO2 and Li/LiNi1/3Mn1/3Co1/3O2 half-cells are nearly134.5 mA h g�1 and 168.3 mA h g�1, respectively, with excellentcapacity retention of above 95%, and a high coulombic effi-ciency of over 99% aer 50 cycles. The discharging plateau ofthe Li/LiNi0.5Mn1.5O4 half-cells containing the mixed electrolyteis at 4.7 V. The battery with the mixed electrolyte operatesproperly at low temperature of �5 �C and �10 �C by addition ofHDI. LiODFB could improve the compatibility of mixed elec-trolytes with cathode materials. This is probably due to its goodpassivation toward aluminum, its high solubility and its abilityto support reversible metallic lithium cycling. Furthermore, theisocyanate/sulfone electrolyte have increased safety due to theirgood thermal stability. In summary, the LiODFB/HDI/TMSmixed electrolyte with a wide electrochemical stability window,good compatibility with electrodes and high safety has a goodapplication prospect in the eld of lithium batteries.
Acknowledgements
This study was supported by the National Key Program for BasicResearch of China (No. 2009CB220100), the International S&TCooperation Program of China (2010DFB63370), the National863 Program (2011AA11A256), the New Century EducationalTalents Plan of Chinese EducationMinistry (NCET-10-0038) andthe Beijing Novel Program (2010B018).
References
1 Z. X. Zhang, H. Y. Zhou, L. Yang, K. Tachibana, K. Kamijimaand J. Xu, Asymmetrical dicationic ionic liquids based onboth imidazolium and aliphatic ammonium as potentialelectrolyte additives applied to lithium secondary batteries,Electrochim. Acta, 2008, 53, 4833–4838.
2 S. H. Fang, Y. F. Tang, X. Y. Tai, L. Yang, K. Tachibana andK. Kamijim, One ether-functionalized guanidinium ionic
This journal is ª The Royal Society of Chemistry 2013
liquid as new electrolyte for lithium battery, J. PowerSources, 2011, 196, 1433–1441.
3 G. B. Appetecchi, M. Montanino, M. Carewska, M. Moreno,F. Alessandrini and S. Passerini, Chemical–physicalproperties of bis(peruoroalkylsulfonyl)imide-based ionicliquids, Electrochim. Acta, 2011, 56, 1300–1307.
4 T. Ruther, T. D. Huynh, J. H. Huang, A. F. Hollenkamp,E. A. Salter, A. Wierzbicki, K. Mattson, A. Lewis andJ. H. Davis, Jr, Stable cycling of lithium batteries usingnovel boronium-cation-based ionic liquid electrolytes,Chem. Mater., 2010, 22, 1038–1045.
5 K. Kubota, T. Nohira and R. Hagiwara, New inorganic ionicliquids possessing low melting temperatures and wideelectrochemical windows: ternary mixtures of alkalibis(uorosulfonyl)amides, Electrochim. Acta, 2012, 66, 320–324.
6 N. Shao, X. G. Sun, S. Dai and D. Jiang, Oxidation potentialsof functionalized sulfone solvents for high-voltage Li-ionbatteries: a computational study, J. Phys. Chem. B, 2012,116, 3235–3238.
7 Y. X. An, P. J. Zuo, X. Q. Cheng, L. X. Liao and G. P. Yin, Theeffects of LiBOB additive for stable SEI formation ofPP13TFSI-organic mixed electrolyte in lithium ionbatteries, Electrochim. Acta, 2011, 56, 4841–4848.
8 Y. Abu-Lebdeh and I. Davidson, High-voltage electrolytesbased on adiponitrile for Li-ion batteries, J. Electrochem.Soc., 2009, 156, A60–A65.
9 Y. Abu-Lebdeh and I. Davidson, New electrolytes based onglutaronitrile for high energy/power Li-ion batteries, J.Power Sources, 2009, 189, 576–579.
10 A. Abouimrane, I. Belharouak and K. Amine, Sulfone-basedelectrolytes for high-voltage Li-ion batteries, Electrochem.Commun., 2009, 11, 1073–1076.
11 Y. Watanabe, S. Kinoshita, S. Wada, K. Hoshino,H. Morimoto and S. Tobishima, Electrochemical propertiesand lithium ion solvation behavior of sulfone-ester mixedelectrolytes for high-voltage rechargeable lithium cells, J.Power Sources, 2008, 179, 770–779.
12 X. G. Sun and C. A. Angell, Doped sulfone electrolytes forhigh voltage Li-ion cell applications, Electrochem.Commun., 2009, 11, 1418–1421.
13 C. Korepp, W. Kern, E. A. Lanzer, P. R. Raimann,J. O. Besenhard, M. H. Yang, K. C. Moller, D. T. Shieh andM. Winter, Isocyanate compounds as electrolyte additivesfor lithium-ion batteries, J. Power Sources, 2007, 174, 387–393.
14 S. S. Zhang, Aromatic isocyanate as a new type of electrolyteadditive for the improved performance of Li-ion batteries, J.Power Sources, 2006, 163, 567–572.
15 C. Korepp, W. Kern, E. A. Lanzer, P. R. Raimann,J. O. Besenhard, M. Yang, K. C. Moller, D. T. Shieh andM. Winter, Ethyl isocyanate – An electrolyte additive fromthe large family of isocyanates for PC-based electrolytes inlithium-ion batteries, J. Power Sources, 2007, 174, 628–631.
16 F. Wu, J. Xiang, L. Li, J. Z. Chen, G. Q. Tan and R. J. Chen,Study of the electrochemical characteristics of sulfonyl
isocyanate/sulfone binary electrolytes for use in lithium-ionbatteries, J. Power Sources, 2012, 202, 322–331.
17 S. S. Zhang, An unique lithium salt for the improvedelectrolyte of Li-ion battery, Electrochem. Commun., 2006, 8,1423–1428.
18 H. Q. Gao, Z. A. Zhang, Y. Q. Lai, J. Li and Y. X. Liu, Structurecharacterization and electrochemical properties of newlithium salt LiODFB for electrolyte of lithium ion batteries,J. Cent. South Univ. Technol., 2008, 15, 830–834.
19 Z. Zhang, X. J. Chen, F. Q. Li, Y. Q. Lai, J. Li, P. Liu andX. Y. Wang, LiPF6 and lithium oxalyldiuoroborate blendsalts electrolyte for LiFePO4/articial graphite lithium-ioncells, J. Power Sources, 2010, 195, 7397–7402.
20 H. Q. Lu, F. Wu, Y. F. Su, N. Li, S. Chen and L. Y. Bao,Preparation of LiNi0.5Mn1.5O4 as 5 V cathode materials forlithium ion battery and diffusion performance of Li+ in thematerials, Chem. J. Chin. Univ., 2011, 32, 946–951.
21 R. J. Chen, F. Wu, L. Li, Y. B. Guan, X. P. Qiu, S. Chen, Y. J. Liand S. X. Wu, Butylene sulte as a lm-forming additive topropylene carbonate-based electrolytes for lithium ionbatteries, J. Power Sources, 2007, 172, 395–403.
22 H. Yoshitake, K. Abe, T. Kitakura, J. B. Gong, Y. S. Lee,H. Nakanmura and M. Yoshio, The effect of nano-sized SEIlm formed by vinyl acetate additive for Li-ion batteries,Chem. Lett., 2003, 32, 134–135.
23 Z. M. Xue and C. H. Chen, Density functional theory study onlithium bis[1,2-benzenediolato(2�)-O,O0] borate and its
24 G. Park, H. Nakamura, Y. Lee and M. Yoshio, The importantrole of additives for improved lithium ion battery safety, J.Power Sources, 2009, 189, 602–606.
25 S. S. Zhang, Electrochemical study of the formation of a solidelectrolyte interface on graphite in a LiBC2O4F2-basedelectrolyte, J. Power Sources, 2007, 163, 713–718.
26 V. Borgel, E. Markevich, D. Aurbach, G. Semrau andM. Schmidt, On the application of ionic liquids forrechargeable Li batteries: high voltage systems, J. PowerSources, 2009, 189, 331–336.
27 Y. S. Lee, Y. K. Sun, S. Ota, T. Miyashita and M. Yoshio,Preparation and characterization of nano-crystallineLiNi0.5Mn1.5O4 for 5 V cathode material by compositecarbonate process, Electrochem. Commun., 2002, 4, 989–994.
28 H. S. Fang, Z. X. Wang, X. H. Li, H. J. Guo and W. J. Peng,Exploration of high capacity LiNi0.5Mn1.5O4 synthesized bysolid-state reaction, J. Power Sources, 2006, 153, 174–176.
29 C. Y. Lin, J. G. Duh, C. H. Hsu and J. M. Chen, LiNi0.5Mn1.5O4
cathode material by low-temperature solid-state methodwith excellent cyclability in lithium ion battery, Mater.Lett., 2010, 64, 2328–2330.
30 H. Y. Xu, S. Xie, N. Ding, B. L. Liu, Y. Shang and C. H. Chen,Improvement of electrochemical properties ofLiNi0.5Mn1.5O4 spinel prepared by radiated polymer gelmethod, Electrochim. Acta, 2006, 51, 4352–4357.
This journal is ª The Royal Society of Chemistry 2013
