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Electrochimica Acta 117 (2014) 99– 104

Contents lists available at ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

rtificial Interface Deriving from Sacrificialris(trimethylsilyl)phosphate Additive forithium Rich Cathode Materials

ie Zhang, Jiulin Wang ∗, Jun Yang, Yanna NuLichool of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China

r t i c l e i n f o

rticle history:eceived 15 August 2013eceived in revised form 4 November 2013ccepted 5 November 2013vailable online 28 November 2013

a b s t r a c t

Tris(trimethylsilyl)phosphate (TMSP) has been investigated as an additive to form a modified solidelectrolyte interface (SEI) on lithium rich cathode material Li[Li0.2Ni0.13Mn0.54Co0.13]O2 and improve itselectrochemical performances. Linear sweep voltammetry (LSV) results show that TMSP additive decom-poses at the potential ca. 4.1 V, lower than that of electrolyte solvent decomposition. The morphologyimages via TEM clearly demonstrate a continuous interfacial layer formed on the cathode surface after

eywords:ithium rich materialslectrolyte additiveolid electrolyte interfaceithium-ion battery

initial cycles. XPS results prove that the components of SEI are mainly derived from the decomposition ofTMSP. The Li[Li0.2Ni0.13Mn0.54Co0.13]O2 cathode materials cycled in 1.0 wt% TMSP-containing electrolytedemonstrate obvious enhancement in its cycling stability and capacity retention reaches 91.1% after 50cycles. The improved performances are ascribed to modified SEI which tightly covers on cathode particle,and effectively avoids a direct contact between cathode active material and electrolyte, leading to thestabilized interfacial structures.

© 2014 Elsevier Ltd. All rights reserved.

. Introduction

In recent years, lithium rich materials represented by the chem-cal formula zLi2MnO3-(1-z)LiMO2 (M = Co, Ni, Mn, etc.), seem to bene of the most promising positive electrode materials for lithium-on batteries (LIBs) applied to plug-in hybrid electric vehiclesPHEVs) and electric vehicles (EVs) because of their high reversibleapacity (ca. 250 mAh g−1), environmental friendliness and lowost [1–3]. However, one of the drawbacks with such materialss their poor cycling performance which is caused by deteriora-ion of electrode/electrolyte interface due to side reactions withlectrolyte at high voltage and structure rearrangement leadingo voltage decay during initial activation of Li2MnO3 componentnd subsequent cycles [1,4,5]. When working on high voltage (gen-rally, ≥4.5 V) beyond the electrochemical stability window ofonventional carbonated-based electrolytes, solvents decomposend form a solid electrolyte interface (SEI) layer on the cathode

urface with high impedance [4,6,7]. Besides, the active transitionetal species, such as Ni4+, Co4+ and Mn4+, can oxidize the con-

entional carbonated-based electrolytes and rapidly degrade cellerformance at high cell voltage. To stabilize the interface between

∗ Corresponding author.E-mail address: wangjiulin@sjtu.edu.cn (J. Wang).

013-4686/$ – see front matter © 2014 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.11.024

lithium rich cathode and electrolyte, lots of reported strategiesfocused on the pre-treatment of cathode materials. Typicallysurface coatings such as Al2O3 [8], MgO [9], MnOx [10], AlPO4[11,12] and AlF3 [13,14] have been proposed to prevent direct con-tact of the active material with electrolytes.

Besides surface treatments, optimizations of electrolyte hasalso been reported as more facile and effective methods toenhance the electrochemical performances of lithium rich cath-ode materials. Recently, Abraham et al. [15,16] investigatedLi1.2Ni0.15Mn0.55Co0.1O2/graphite cells with 2 wt% lithium diflu-oro(oxalato)borate (LiDFOB) in an ethylene carbonate/dimethylcarbonate (EC:EMC) based electrolyte and suggested that the sac-rificial oxidation of LiDFOB to form a thick passivation layer onthe positive electrode that suppressed electrolyte oxidation andtransition metal dissolution from the oxide. Tri(hexafluoro-iso-propyl)phosphate (HFiP) was proposed as an electrolyte additivewhich effectively improved the cycling performance of lithium-rich cathode material Li[Li0.2Mn0.56Ni0.16Co0.08]O2 [17]. Amineet al. [18] reported that addition of 3-hexylthiophene to a con-ventional carbonate-based electrolyte can improve the cyclingperformance of high capacitance and high voltage cathodes, whichwas ascribed to a conductive poly(3-hexylthiophene) layer formed

on the cathode material surface. Yan et al. [19] investigatedtris(trimethylsilyl)phosphate (TMSP) as a film-forming additive forLiNi0.5Co0.2Mn0.3O2 cycling in LiPF6-based electrolyte between 3.0and 4.5 V.

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clearly display a smooth voltage ramp in the range 3.9–4.4 V,a long flat plateau at ∼4.5 V, and a small slope when chargedabove 4.6 V. The voltage ramp observed below 4.4 V correspondsto the oxidation of Ni2+ to Ni4+ and Co3+ to Co4+, accompanying

Fig. 1. Linear sweep voltammograms of platinum electrode at a scanning rate of 0.1mv s−1 in the reference electrolyte (a) and electrolyte with 2.0 wt% TMSP additive(b).

00 J. Zhang et al. / Electroch

Based on the knowledge that functional electrolyte with noveldditive is a facile and self-healing method to stabilize the interfaceetween lithium rich cathode and electrolyte, the TMSP additive is

nvestigated in this work to stabilize the electrochemical perform-nces of Li[Li0.2Ni0.13Mn0.54Co0.13]O2 cathode materials.

. Experimental

.1. Preparation

Li1.2Ni0.13Mn0.54Co0.13O2 was prepared through a sprayrying-pyrolysis process [20]. Stoichiometric amounts ofi(CH3COO)2·4H2O, Co(CH3COO)2·4H2O, Mn(CH3COO)2·4H2Ond LiCH3COO·2H2O (5% excess) were selected as starting mate-ials which were dissolved in distilled water, while citric acidas added simultaneously. After adjusting pH to 7 by ammoniumydroxide, spray drying process was carried out and precursor wasbtained. Then, the precursor was initially pre-heated at 400 ◦C for

h and sintered at 900 ◦C in air for 12 h.The traditional electrolyte of 1 M LiPF6 in ethylene car-

onate/dimethyl carbonate (EC/DMC = 1:1, v/v) was defined aseference electrolyte in this work. The TMSP additive purchasedrom Fujian Chuangxin Science and Technology Develops Co. Ltd.China), was added to the reference electrolyte at the concentra-ions varying from 1.0 wt% to 2.0 wt%.

The cathode electrode composed of 80 wt% active material with0 wt% Super P and 10 wt% polyvinylidene difluoride (PVDF) on theluminum current collector with the diameter of 1.2 cm. The coinells were assembled with as-prepared cathode electrode, lithiumetal as anode, and a polypropylene separator in a glove box filledith pure argon.

.2. Characterization

Galvanostatic charge/discharge tests were performed usingR2016 coin-type cells on a Land CT2001 battery test system at5 ◦C. Cells were cycled in a voltage range of 2.0–4.8 V using aonstant current density of 20 mA g−1. The rate capability of cellsas performed with different discharge current densities of 20, 40,

00, 200 and 400 mA g−1 after charge at 20 mA g−1. Linear sweepoltammetry (LSV) was carried out using a three-electrode cell on alatinum working electrode (d = 10 mm) with lithium counter andeference electrodes at a scan rate of 0.1 mV s−1. Electrochemi-al impedance spectroscopic measurement (EIS) was conducted onHI650 C in the frequency range of 100 kHz to 0.01 Hz. The potentialalues mentioned in this work are referred to Li/Li+ redox couple.

To analyze the composition of surface layer on the electrodes,he cycled cells were disassembled in glove box under Ar atmo-phere. The Li1.2Ni0.13Mn0.54Co0.13O2 electrodes were rinsed withnhydrous DMC for 3 times to remove residual electrolyte, fol-owed by dried under vacuum condition at room temperature. Thelement composition on the surface of cathode was analyzed by-ray photoelectron spectroscopy (XPS) acquired with a Kratosxis UltraDLD spectrometer (Kratos Analytical-A Shimadzu groupompany) using a monochromatic Al K� source (1486.6 eV). Thenalyzer uses hybrid magnification mode (both electrostatic andagnetic) and take-off angle is 90◦. Under slot mode, the analysis

rea is 700 × 300 �m and analysis chamber pressure is less than × 10−9 Torr. The pass energy of 40 eV and the energy step sizesf 0.1 eV were chosen for narrow scan spectra. The binding energy

as calibrated according to the C 1s peak (284.8 eV) of adventi-

ious carbon on the analyzed sample surface. Transmission electronicroscopy (TEM, JOEL JEM-2100) operated with an accelerating

oltage of 200 kV was used to characterize the surface morphologyf the cycled cathode powder.

Acta 117 (2014) 99– 104

3. Results and discussion

In order to evaluate the TMSP as electrolyte additive, electro-chemical performance of electrolytes with and without additivewas measured by LSV at platinum foil with three-electrode cell. Asshown in Fig. 1, an oxidative peak is near 4.1 V for TMSP-added elec-trolyte, while below this potential the corresponding peak is notavailable for the reference electrolyte. Although the reference elec-trolyte is stable below 4.1 V, the current increases almost linearlywith the potential starting from 4.2 V, which can be ascribed to thegradually decomposition of solvents. At the potential of 5.5 V, theoxidative current of the reference electrolyte is much higher that ofthe TMSP-added electrolyte. Fig. 1 also shows a stable current forthe TMSP-added electrolyte until the potential reaches 5.5 V. Theseresults clearly indicate that the additive of TMSP can be oxidizedat a relatively low potential to form a SEI layer, which effectivelysuppress electrolyte decomposition under high potential.

Fig. 2 shows the initial charge/discharge profiles ofLi[Li0.2Ni0.13Mn0.54Co0.13]O2/Li cell in electrolytes with and with-out TMSP additive. It is obviously that all the first charge curves

Fig. 2. The initial charge/discharge curves of Li[Li0.2Ni0.13Mn0.54Co0.13]O2/Li cellin electrolytes containing 0%, 1.0 wt% and 2.0 wt% TMSP at a current density of20 mA g−1 in the voltage range of 2.0-4.8 V.

J. Zhang et al. / Electrochimica Acta 117 (2014) 99– 104 101

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ig. 3. Comparison of the initial 5 charge/discharge curves of Li[Li0.2Ni0.13Mn0.54Coycling performance (d) at a current density of 20 mA g−1 in the voltage range of 2.0

he de-insertion of lithium ion from the LiNi1/3Mn1/3Co1/3O2omponent. The long flat plateau locates at ∼4.5 V can be ascribedo the extraction of Li+ and oxygen from the inactive Li2MnO3omponent, which leads to a large initial irreversible capacity loss.he small slope plateau may be derive from electrolyte oxidation21,22].

There are no significant differences in the initialharge/discharge curves of Li1.2Ni0.13Mn0.54Co0.13O2/Li cell inhe electrolytes with and without TMSP additive. The initialischarge capacities of cells containing TMSP additive are slightly

ower than that without additive. The cell cycled in the referencelectrolyte shows an initial discharge capacity of 254.4 mAh g−1,hile initial discharge capacities of the cells containing 1.0 wt%

nd 2.0 wt% TMSP additive are 247.7 mAh g−1 and 243.9 mAh−1, respectively. When comparing the coulombic efficiency, theell cycled in the reference electrolyte has a lower value (75.6%)han cells cycled in electrolytes containing 1.0 wt% (76.2%) and.0 wt% (76.6%) TMSP additive. The small differences in the initialischarge capacity and coulombic efficiency can be ascribed to theormation of SEI layer after the addition of TMSP, which stabilizeshe electrolyte over high voltage and has influences on the cellesistance analyzed later in EIS section.

Fig. 3a 3b and 3c present the initial 5 charge/discharge curvesf Li[Li0.2Mn0.54Ni0.13Co0.13]O2/Li cell cycled in the electrolytesontaining 0%, 1.0 wt% and 2.0 wt% TMSP. For the cell withoutdditive, the capacity of cathode material shows steady degrada-

ion during cycling, due to the deterioration of electrode/electrolytenterface. In comparison, the Li[Li0.2Mn0.54Ni0.13Co0.13]O2/Li cellsycled in electrolytes containing 1.0 wt% and 2.0 wt% TMSP addi-ive exhibit a little fast capacity loss at the initial 3 cycles, probably

2/Li cell in the electrolytes containing 0% (a), 1.0 wt% (b) and 2.0 wt% (c) TMSP and.

ascribed to the decomposition of TMSP additive and formationof SEI layer. However, cells with TMSP added electrolytes dis-play much more stable cycling performance than the additivefree cell after 3 cycles. Fig. 3d shows the cycling performance ofLi[Li0.2Mn0.54Ni0.13Co0.13]O2/Li cells containing 0%, 1.0 wt% and2.0 wt% TMSP additive. After 50 cycles, the discharge capacity ofLi[Li0.2Mn0.54Ni0.13Co0.13]O2/Li cells containing 0%, 1.0 wt% and 2.0wt% TMSP additive are 212.7 mAh g−1, 225.0 mAh g−1 and 203.4mAh g−1, respectively. The results reveal that cycling performanceof Li[Li0.2Mn0.54Ni0.13Co0.13]O2/Li cells is improved with the addi-tion of TMSP.

Fig. 4 illustrates the rate capability of Li[Li0.2Mn0.54Ni0.13Co0.13]O2/Li cells cycled in the electrolytes with and withoutTMSP additive ranging from 20 to 400 mA g−1. Although theLi[Li0.2Mn0.54Ni0.13Co0.13]O2/Li cell cycled in 1.0 wt% TMSP-addedelectrolyte shows a slightly lower discharge capacity at 20 mA g−1

than that without additive, it delivers a higher capacity of 158.2mAh g−1 at 400 mA g−1. Besides, the Li[Li0.2Mn0.54Ni0.13Co0.13]O2/Licell cycled in 2.0 wt% TMSP-added electrolyte performs the worstperformance thoroughly, which may be ascribed to too thick SEIlayer formed gradually during cycling.

To investigate the electrode kinetics, electrochemicalimpedance spectroscopy (EIS) of the Li[Li0.2Mn0.54Ni0.13Co0.13]O2/Li cells charged to 4.8 V was measured to evaluate the effect ofTMSP after the initial cycle. As shown in Fig. 5, all Nyquist plotsinclude two semicircles in the frequency region. The one in the high

frequency region is associated with SEI layer and the appearanceof semicircle in the low frequency is related to the charge transferreaction [23]. The impedance spectra are analyzed by Zsimpwinsoftware using the equivalent circuit, where Re stands for solution

102 J. Zhang et al. / Electrochimica

Fig. 4. Rate capabilities of Li[Li0.2Ni0.13Mn0.54Co0.13]O2 cathode in electrolytes con-taining 0%, 1.0 wt% and 2.0 wt% TMSP additive at 0.1, 0.2, 0.5, 1 and 2 C rates(1C = 200 mA g−1) in the voltage range of 2.0-4.8 V.

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ig. 5. Nyquist plots of Li[Li0.2Ni0.13Mn0.54Co0.13]O2/Li cell charged to 4.8 V after thenitial cycle in electrolytes containing 0%, 1.0 wt% and 2.0 wt% TMSP.

esistance, Rf and CPEf represent the resistance and capacitancef SEI layer, Rct and CPEct correspond to the charge-transfer resis-ance and double layer capacitance. According to the fitted results,he surface film resistance increases significantly from 110.6 � ineference electrolyte to 146 � and 181.2 � in 1.0 wt% and 2.0 wt%MSP added-electrolytes, respectively. This suggests that the TMSPdditive decomposes and a protective film forms on the cathode

urface. However, the charge transfer resistance (Rct) decreaseslightly from 2417 � in reference electrolyte to 2132 � and 2052 �n 1.0 wt.% and 2.0 wt.% TMSP added-electrolytes, respectively. Inhe reference electrolyte, active material directly contacts with

Fig. 6. TEM images of cycled Li[Li0.2Ni0.13Mn0.54Co0.13]O2 electrodes i

Acta 117 (2014) 99– 104

electrolyte during cycling and severe damage might be occurred athigh voltage, resulting in high charge transfer resistance. However,artificial SEI layer derived from TMSP protects the surface of activematerial from electrolyte, leading to smaller charge transfer resis-tance than that in reference electrolyte. It is clear that the additionof TMSP slightly decreased the initial capacity of the cathode asshown in Fig. 2 and Fig. 3d. It is reasonable that higher impedance,lower electrochemical activity. However, this artificial SEI layer onthe electrode surface deriving from TMSP decomposition stabilizedthe electrolyte/electrode interface, leading to better cycling andrate performances.

Fig. 6 displays the TEM images of the Li[Li0.2Mn0.54Ni0.13Co0.13]O2 electrode on charged state (4.8 V) with andwithout TMSP additive after 3 cycles. SEI layer is not detectedon the edge of the Li1.2Ni0.13Mn0.54Co0.13O2 particles cycled inthe reference electrolyte, which agrees with previous reports.Hong et al. [24] reported that SEI layer including Li2CO3 andorganic compounds formed during the discharge process due tothe decomposition of carbonated-based electrolyte and decom-posed during the charging process causing severe damageon the surface of the electrode at high voltage. Interestingly,Li[Li0.2Mn0.54Ni0.13Co0.13]O2 electrode cycled in the TMSP-addedelectrolyte remains a distinguishable and amorphous SEI layer onthe surface of particles. In the reference electrolyte, the amountof SEI layer increases with cycle number due to the decom-position of electrolyte and no or only partial coverage of theLi[Li0.2Mn0.54Ni0.13Co0.13]O2 surface is achieved at the beginningcycles [4]. Hence, the surface film does not protect the cathodesurface completely. However, it is evident that the SEI layer formedon Li[Li0.2Mn0.54Ni0.13Co0.13]O2 surface in TMSP-added electrolyteduring initial 3 cycles. Therefore, the use of TMSP additive resultsin a surface layer on the cathode and the better electrochemicalperformance.

To clarify the composition of SEI layer, the Li[Li0.2Mn0.54Ni0.13Co0.13]O2 electrodes after 50 cycles with different electrolyteswere analyzed by XPS. The C1s, O1s, F1s, P2p and Si2p spectra ofLi[Li0.2Mn0.54Ni0.13Co0.13]O2 electrodes cycled in electrolytes withand without TMSP additive at a current density of 20 mA g−1 areshown in Fig. 7. The C1s spectrum of the pristine electrode con-tains intensities from graphite carbon (Super P) at 284.8 eV andthe PVdF binder (C-C and C-H at 286.2 eV, C-F at 291.2 eV). TheF1s spectrum of the pristine electrode mainly consists of strongpeaks from PVdF (688.3 eV). The O1s spectrum consists of two mainpeaks assigned to lattice oxygen from Li[Li0.2Mn0.54Ni0.13Co0.13]O2(529.7 eV), Li2CO3 or hydroxyls (-OH) (531.8 eV) [4,25]. After cycledin electrolytes with and without TMSP additive, intensity reduc-tions are observed for the graphite and PVdF peaks in the C1s

spectra, PVdF peak in the F1s spectra and lattice oxygen in theO1s peak. Compared with electrode cycled in reference electrolyte,these intensity reductions are greater for the 2.0 wt.% TMSP addedelectrolyte.

n the reference electrolyte (a) and TMSP-added electrolyte (b).

J. Zhang et al. / Electrochimica Acta 117 (2014) 99– 104 103

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TMSP added electrolyte, respectively. Similar change is observedfor the phosphorus content increasing from 0% to 3.15% and 9.75%,respectively. These indicate that a thicker SEI layer formed on the

Table 1Element concentrations (atom %) of pristine and cycled electrode calculated by XPSdata.

ig. 7. XPS spectra of C1s, F1s, O1s, P2p and Si2p measured from Li[Li0.2Ni0.13Mn0.5

As observed in C1s, O1s, F1s and P2p spectra, new species cor-esponding to the electrolyte decomposition products are formedn the surface of cycled electrodes. SEI layer consists of ethers (C-Ot 533.4 eV in O1s and at 287 eV in C1s,), lithium alkyl carbonatesr polycarbonates (C-O at 533.4 eV and C = O at 531.8 eV in O1s, C-

at 286.2 eV and C = O at 289.8 eV in C1s), LixPFy (P-F at 687.6 eVn F1s and 135.5 eV in P2p), LixPFyOz (P-O-F at 687.6 eV in F1s and34.4 eV in P2p) and LiF (Li-F at 685.2 eV in F1s) is formed on theurface of cycled electrode in reference electrolyte [4,24,26]. Inter-stingly, the electrode cycled in TMSP added electrolyte shows

ignificant increases in intensity of O1s and P2p spectra ascribedo the decomposition of TMSP. Table 1 shows element concentra-ions of pristine electrode and cycled electrodes in reference and.0 wt% TMSP added electrolytes. The oxygen content increases

]O2 electrodes after 50 cycles in electrolyte with and without TMSP additive.

from 12.89% (atom %) on the surface of pristine electrode to 23.35%and 31.58% for the cycled electrode in reference electrolyte and

C1s F1s O1s P2p Si2p

Pristine 75.43 11.68 12.89 0 0reference 61.06 12.44 23.35 3.15 0TMSP 46.04 11.09 31.58 9.75 1.54

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lectrode surface upon cycling with TMSP additive than that with-ut additive.

To further confirm the fact that TMSP participated in the for-ation of SEI layer on the surface of Li[Li0.2Ni0.13Mn0.54Co0.13]O2

lectrode, Si2p spectrum of cycled electrode with TMSP additives measured. It is clearly observed that there is one main peak inhe Si2p spectrum dominated by Si-O bond at 102.3 eV and siliconontent is about 1.54%, which indicates TMSP additive participatedn the formation of SEI layer. The improvement of SEI layer will sta-ilize the electrode/electrolyte interface, which resulted in goodlectrochemical performances of the battery.

Based on XPS results on C1s, F1s, O1s, P2p and Si2p spectra,t can be found that modified SEI layer is covered on the surfacef Li[Li0.2Ni0.13Mn0.54Co0.13]O2 electrode cycled in TMSP-addedlectrolyte, compared with the electrode cycled in reference elec-rolyte. This artificial SEI layer separates the direct contact betweeni[Li0.2Ni0.13Mn0.54Co0.13]O2 with carbonate electrolyte, and stabi-izes it over high voltage, leading to the improved electrochemicalerformances of Li[Li0.2Ni0.13Mn0.54Co0.13]O2 lithium rich cathodeaterials.

. Conclusions

Tris(trimethylsilyl)phosphate (TMSP) was investigated as andditive to improve the electrochemical performance of lithium-ich cathode material Li[Li0.2Ni0.13Mn0.54Co0.13]O2. EIS and TEMonfirm the existence of artificial SEI layer when TMSP added. XPSesults show the compositions of modified SEI layers on cycledlectrodes with and without TMSP additive. Ascribed to the modi-ed SEI layer, capacity retention of the Li[Li0.2Ni0.13Mn0.54Co0.13]O2ycled in 1.0 wt% TMSP-added electrolyte after 50 cycles reaches1.1%, which is higher than that in the reference electrolyte (84.7%).

t is clear that TMSP with 1.0 wt% addition decomposes and forms protective layer on the surface of cathode material, which sta-ilizes the electrode/electrolyte interface and prevents the sideeaction between electrode and electrolyte, resulting in obviouslynhanced cycling stability and slightly improved high power rateerformances Li[Li0.2Ni0.13Mn0.54Co0.13]O2 cathode materials.

cknowledgement

This work was supported by the National Natural Science Foun-ation of China (21333007, 51272156 and 21273146).

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