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Trimethylsilylcyclopentadiene as a novel electrolyte additive for high temperature application of lithium nickel manganese oxide cathode
Journal of Power Sources 364 (2017) 23e32
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Trimethylsilylcyclopentadiene as a novel electrolyte additive for high temperature application of lithium nickel manganese oxide cathode Wenqiang Tu a, Changchun Ye a, Xuerui Yang a, Lidan Xing a, b, Youhao Liao a, b, Xiang Liu a, b, *, Weishan Li a, b, ** a
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China Engineering Research Center of MTEES (Ministry of Education), Research Center of BMET (Guangdong Province), Engineering Lab. of OFMHEB (Guangdong Province), Key Lab. of ETESPG(GHEI), and Innovative Platform for ITBMD (Guangzhou Municipality), South China Normal University, Guangzhou 510006, China
b
h i g h l i g h t s SE is an effective electrolyte additive for high temperature application of LiNi0.5Mn1.5O4. A protective film can be formed on LiNi0.5Mn1.5O4 due to preferential oxidation of SE. The film can suppress the dissolution of transition metal ions from LiNi0.5Mn1.5O4.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 May 2017 Received in revised form 20 July 2017 Accepted 6 August 2017
Electrolyte additives are necessary for the application of high potential cathode in high energy density lithium ion batteries, especially at elevated temperature. However, the electrolyte additives that can effectively suppress the dissolution of transition metal ions from cathode have seldom been developed up to date. In this work, we propose a novel electrolyte additive, trimethylsilylcyclopentadiene (SE), for high temperature application of a representative high potential cathode, lithium nickel manganese oxide (LiNi0.5Mn1.5O4). It is found that the dissolution of Mn and Ni from LiNi0.5Mn1.5O4 can be effectively suppressed by applying SE. With applying 0.25% SE, the dissolved amount of Mn and Ni is decreased by 97.4% and 98%, respectively, after 100 cycles at 55 C. Correspondingly, the cyclic performance of LiNi0.5Mn1.5O4 is significantly improved. Physical characterizations and electrochemical measurements show that SE can be preferentially oxidized and generate a protective film on LiNi0.5Mn1.5O4. The resulting film inhibits the electrolyte decomposition and the transition metal ion dissolution. © 2017 Elsevier B.V. All rights reserved.
Keywords: Trimethylsilylcyclopentadiene Electrolyte additive Protective film High potential cathode Lithium ion battery
1. Introduction Lithium ion batteries have been extensively applied in digital products including mobile phone, laptop and model airplane due to their higher energy density than other commercial rechargeable batteries [1e4]. However, the energy densities of lithium ion batteries are not high enough and they are too expensive for their applications in electric vehicles. Raising work voltage or applying
* Corresponding author. School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China. ** Corresponding author. School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China. E-mail addresses: [email protected] (X. Liu), [email protected] (W. Li). http://dx.doi.org/10.1016/j.jpowsour.2017.08.021 0378-7753/© 2017 Elsevier B.V. All rights reserved.
high potential cathodes is a direct and effective approach to enhance the energy densities of lithium ion batteries. Compared to the low working potential and high price of commercially available cathodes, including LiCoO2 (~3.6 V vs. Liþ/Li) and LiFePO4 (~3.2 V), LiNi0.5Mn1.5O4 has a high working potential (~4.7 V) and low price and has received much attention recently. However, there remain issues such as serious electrolyte decomposition and transition metal ion dissolution that should be solved before LiNi0.5Mn1.5O4 is put into practical application [5e8]. These issues become more challenging when LiNi0.5Mn1.5O4 is run at elevated temperature [9e12]. The conventional carbonate-based electrolytes with LiPF6 as salt are unstable at the potential higher than 4.2 V, tending to be decomposed and generating corrosive species HF that causes transition metal ion dissolution [13e16].
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Coating cathode materials with inert inorganic compounds is one of effective approaches to inhibit the electrolyte decomposition on high potential cathodes [17e21], but this approach has some shortages. Firstly, coating process is complex and expensive. Secondly, the introduced inert compounds reduce the specific capacity and increase the interfacial impedance between cathode and electrolyte. Substituting more stable solvents for carbonates has been adopted to solve the issues resulting from unstable carbonatebased electrolytes [22,23], but these new solvents have detrimental effects: lower ionic conductivity, higher viscosity and poorer compatibility with graphite compared to carbonates. Introducing some electrolyte additives into carbonate-based electrolytes can significantly improve the electrochemical performances of lithium ion batteries [15,24e28]. Several electrolyte additives have been proposed for the application of high potential cathode cycled in conventional carbonate-based electrolytes. For example, Choi et al. reported that TMSPi can improve the capacity retention of LiNi0.5Mn1.5O4 from 73% to 90% at 1 C rate after 100 cycles at 60 C [29], and Sun et al. revealed that LiBFMB could improve the capacity retention of LiNi0.5Mn1.5O4 from 68% to 86% after 100 cycles at room temperature [30]. Generally, the improved cyclic stabilities of LiNi0.5Mn1.5O4 by applying electrolyte additives are attributed to the protective film generated from the preferential oxidation of additives on cathodes. Unfortunately, the electrolyte additives that have been reported in literature do not show their effective inhibition of transition metal ion dissolution from LiNi0.5Mn1.5O4, although significantly improved cyclic stability of LiNi0.5Mn1.5O4 has been achieved by applying these additives. The transition metal ion dissolution is detrimental to the performance of full cell with graphite anode, because the dissolved metal ions will deposit on graphite and destroy the solid electrolyte interphase film. The chemical composition and molecular structure of additives are critical for the oxidation preference of the additives and protection effectiveness of the resulting film [31e34]. For example, the additives containing B or B-O structure are beneficial for lithium transportation in the film, while the additives containing Si or Si-O bond provides the film with improved chemical and thermal stability [35e40]. Comparatively, the chemical and thermal stability of a film is more important than its ionic transportation property for the cyclic performance improvement of a high potential cathode, because the film generated by electrolyte additives is so thin that the film impedance hardly affect the cathode performance. With this knowledge, we proposed a novel Si-containing additive, trimethylsilylcyclopentadiene (SE), for high temperature application of LiNi0.5M1.5O4. Physical and electrochemical results demonstrate that a thin film can be generated on LiNi0.5Mn1.5O4 by applying SE, which significantly inhibits the dissolution of transition metal ions. With the application of SE, the cyclic stability of not only LiNi0.5Mn1.5O4 cathode but also LiNi0.5Mn1.5O4/graphite full cell is effectively improved. 2. Experiments 2.1. Preparations and assemblies Mixture of 80 wt% LiNi0.5Mn1.5O4 powders (Sichuan Xingneng Co. Ltd. China), 10 wt% acetylene black (AB) and 10 wt% polyvinylidene fluoride (PVDF) was coated on Al foil to obtain LiNi0.5Mn1.5O4 electrode with a loading of ~3.1 mg cm2 LiNi0.5Mn1.5O4. Mixture of artificial graphite (AG) (Dongguan Kaijin New Energy Technology Co. Ltd, China), Super-P (SP) and PVDF in a weight ration of AG/SP/PVDF ¼ 9/0.5/0.5 was coated on Cu foil to obtain graphite electrode with a loading amount of 1.32 mg cm2. The standard (STD) electrolyte used in this work was 1 M LiPF6 in
ethylene carbonate (EC)/ethyl methyl carbonate (EMC)/diethyl carbonate (DEC) (3/5/2 in volume, provided by Dongguan Kaixin Materials Technology Co. Ltd. China), and the additive-containing electrolytes were prepared by adding 0.25% or 0.5% (in weight) SE (purchased from Tokyo Chemical Industry (shanghai) Co. Ltd, China) into the STD electrolyte. The Li/LiNi0.5Mn1.5O4 cells were assembled with LiNi0.5Mn1.5O4 electrode, Celgard 2300 separator and Li metal plate electrode in 2025 coin cell. The Li/graphite cells were assembled with AG electrode, Celgard 2300 separator and Li metal plate electrode in 2025 coin cells. The graphite/ LiNi0.5Mn1.5O4 full cells were assembled with LiNi0.5Mn1.5O4 electrode, Celgard 2300 separator and graphite electrode in 2025 coin cells. Electrolyte cells were also assembled with platinum or LiNi0.5Mn1.5O4 electrode and Li metal plate electrode without using separator, as described in our previous report [31e33]. Electrolyte preparation and cell assembly were conducted in a dry argon glove box where the water and oxygen concentrations were controlled to be less than 0.1 ppm. 2.2. Characterizations and measurements The cyclic stability of the cells was measured on a LAND cell test system (Land CT 2001A, China). Li/LiNi0.5Mn1.5O4 half cell was charged to 4.9 V at 1 C (1 C ¼ 120 mA g1), followed by a constant voltage at 4.9 V for 10 min, and then discharged at 1 C to 3.0 V under room (25 C, RT) and elevated (55 C, ET) temperature. Before cycling test, the cells were charged/discharged for three cycles at 0.5 C under room temperature. Li/graphite half cell was charged and discharged at 0.1 C (1C ¼ 350 mA g1) for the first cycle and then at 0.2 C rate for the subsequent cycles. Graphite/LiNi0.5Mn1.5O4 full cells were charged/discharged at 0.1 C (1C ¼ 120 mA g1) for the initial three cycles and then at 0.5 C for the subsequent cycles. Electrochemical impedance spectroscopy (EIS) was performed on the electrochemical station (Autolab PGSTAT-302N, Metrohm Co. Switzerland) in Li/LiNi0.5Mn1.5O4 half cell with LiNi0.5Mn1.5O4 as working electrode and lithium electrode as counter and reference electrodes. To ignore the contribution of lithium electrode, the impedance measurements were performed immediately after cycling at the discharge stage (3.0 V) in a frequency range from 100 KHz to 10 mHz with a potential amplitude of 5 mV. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) tests were performed on Solartron 1480 instrument (England). Coin cells were used for CV at a scan rate of 0.1 mV s1, while electrolyte cells were used for LSV by using platinum disk (2 mm in diameter) at a scan rate of 1 mV s1 or LiNi0.5Mn1.5O4 electrode (1 2 cm) at a scan rate of 0.1 mV s1 as working electrode. Chronoamperometry was carried out in Li/LiNi0.5Mn1.5O4 coin cells, which were charged at 0.5 C to 4.9 V and then kept at 4.9 V for 18 h. To understand the effect of cycling on the structure and composition of LiNi0.5Mn1.5O4 in SE-containing and STD electrolytes, the electrodes were dismantled from the cycled cells in glove box, washed by DMC for three times, and dried in vacuum box for overnight time at room temperature. The crystal structure was evaluated by X-ray diffraction (XRD, Bruker D8 Advance, Germany) operated at 60 KV and 80 mA using Cu Ka radiation (l ¼ 1.5405 nm) in the 2q range of 10e80 . The scanning electron microscopy (SEM) was performed on JEM-6510 (Japan) and transmission electron microscopy (TEM) was performed on JEM-2100HR (Japan). The surface element compositions on cathode were analyzed by XPS on Thermo Fisher Scientific (UK) with a monochromatic Al Ka X-ray source (excitation energy ¼ 1468.6 eV). The data were collected from 0 to 1350 eV using an X-ray spot size of 400 mm with a pass energy of 100 eV for wide scan and 30 eV for individual elements. Binding energies were corrected based on the carbon 1s signal at 284.8 eV. The infrared spectroscopy was performed at a Nicolet
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67000 (America) spectrometer. To analyze the content of transition metal deposited on the counter electrode, the lithium electrode was dismantled from cycled Li/LiNi0.5Mn1.5O4 cells, washed by DMC for three times, dissolved in 5 ml 3% HNO3 solution. The solution was diluted to 25 ml for the ICP-AES analysis (ICAP 6500 Duo, USA). 2.3. Theoretical calculations The theoretical calculations were performed in Gaussian 09 package [13,42,43]. All the molecular structures were optimized by Density Functional Theory (DFT) method at B3LYP (6e333þþG (d)) level. Polarized continuum models (PCM) with a dielectric constant of 20.5 (acetone) were applied to investigate the bulk solvent effect. And frequency calculations were performed at the same level. 3. Results and discussions 3.1. Electrochemical performances The effectiveness of SE on the performance of LiNi0.5Mn1.5O4 was evaluated by cycling Li/LiNi0.5Mn1.5O4 half cells. The obtained results are presented in Fig. 1. As shown in Fig. 1a, LiNi0.5Mn1.5O4 suffers a fast capacity fading under elevated temperature, from an initial capacity of 120 mAh g1 to 18 mAh g1 with a capacity retention of only 15% after 75 cycles. This is the main issue that limits the application of LiNi0.5Mn1.5O4 in practice, which is caused by the serious decomposition of the STD electrolyte and the dissolution of transition metal ions. When SE is applied, the capacity retention of LiNi0.5Mn1.5O4 is significantly improved, to 88% and 84%, respectively, in 0.25% and 0.5% SE-containing electrolytes after 100 cycles, indicating that SE is effective for the cyclic stability improvement of LiNi0.5Mn1.5O4 under high temperature application. This effect should be associated with the protective film on
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LiNi0.5Mn1.5O4, which effectively suppresses the electrolyte decomposition. The suppression of the electrolyte decomposition can be confirmed by the more stable and far higher coulombic efficiency of LiNi0.5Mn1.5O4 in SE-containing electrolytes than that in STD electrolyte, as shown in Fig. 1b. The cycle performances of LiNi0.5Mn1.5O4 in the electrolytes with or without SE were also evaluated under room temperature. The obtained results are presented in Fig. 1c and d. It can be found from Fig. 1c and d that whether capacity retention or coulombic efficiency of LiNi0.5Mn1.5O4 maintains a high level in STD and SEcontaining electrolytes. These results confirm that STD electrolyte does not favor the high temperature application of LiNi0.5Mn1.5O4 and suggest that the application of SE as an electrolyte additive hardly affects the capacity delivery of LiNi0.5Mn1.5O4 under room temperature. Nevertheless, increasing the concentration of SE will slightly reduce the capacity delivery of LiNi0.5Mn1.5O4, as shown in Fig. 1a. This phenomenon can be explained by the increased interfacial impedance due to the protective film formed by SE. The film will become thicker with increasing SE concentration and increase the interfacial impedance. Therefore, 0.25% SE was considered for further investigations. To understand the mechanism on the effectiveness of SE for the cyclic stability improvement of LiNi0.5Mn1.5O4, the electrochemical performance of LiNi0.5Mn1.5O4 in STD and SE-containing electrolytes were further analyzed in detail. The initial charge/discharge profiles at 0.5 C are presented in Fig. 2a and b. Initially in the STD electrolyte, as shown in Fig. 2a, the potential of LiNi0.5Mn1.5O4 quickly rises to 4.7 V and keeps stable at about 4.8 V. This long charge plateau at 4.8 V corresponds to the oxidation of Ni2þ, which contributes to the main capacity of LiNi0.5Mn1.5O4 [6,44]. Correspondingly, there is a long discharge potential plateau at about 4.7 V. Additionally, a small discharge potential plateau appears at about 4.0 V, corresponding to the reduction of and Mn4þ to Mn3þ.
Fig. 1. Cyclic performances of LiNi0.5Mn1.5O4 at 1 C in STD and SE-containing electrolytes after three cycles at 0.5 C. Cyclic stability (a, c) and coulombic efficiency (b, d) at elevated (50 C, ET) and room temperature (25 C, RT).
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Fig. 2. Electrochemical performances of LiNi0.5Mn1.5O4 with and without SE additive. The first (a) and third (b) charge/discharge profiles of LiNi0.5Mn1.5O4 at 0.5 C; linear sweep voltammograms of platinum (c) and LiNi0.5Mn1.5O4 (d) electrodes; and cyclic voltammograms of LiNi0.5Mn1.5O4 electrodes in STD (e) and SE-containing (f) electrolytes.
Comparatively, there appears an additional oxidation potential plateau at about 4.5 V for the LiNi0.5Mn1.5O4 in SE-containing electrolyte (Fig. 2a), showing that SE is preferentially oxidized. Correspondingly, there is a lower efficiency (75.8%) in SEcontaining electrolyte than in STD electrolyte (83.6%). This preferential oxidation of SE is important for the formation of a protective film that can effectively inhibit the electrolyte decomposition. At the third cycling, the oxidation potential plateau of SE disappears, as shown in Fig. 2c, suggesting that the oxidation of SE is also inhibited by the protective film formed at the first cycle. The preferential oxidation of SE can be directly demonstrated by LSV on platinum and LiNi0.5Mn1.5O4 in electrolytic cells. As shown in Fig. 2c, the voltammogram of platinum electrode in the SEcontaining electrolyte is different completely from that in the STD electrolyte. Before 4.5 V, the oxidation current in the STD electrolyte is small, suggesting that STD electrolyte is relatively stable. This is why carbonate-based electrolytes can be widely used in commercial lithium ion batteries [27]. However, fast increasing oxidation current can be observed when the potential is enhanced further, demonstrating the instability of carbonate-based electrolyte under high potential. Interestingly, the oxidation current for
the electrolyte decomposition on platinum can be significantly reduced and a small current peak appears at about 3.75 V when SE is applied (Fig. 2c). This difference suggests that SE is oxidized preferentially and the electrolyte decomposition can be inhibited due to the SE oxidation. Similar behaviors can be observed on LiNi0.5Mn1.5O4 electrode in the electrolytic cell, as shown in Fig. 2d. In STD electrolyte, there are two oxidation peaks (~4.0 V and ~5 V), corresponding to the redox reactions of Mn3.5þ/Mn4þ and Ni2þ/Ni4þ, respectively. At higher potential, the increasing current for the electrolyte decomposition can be observed for LiNi0.5Mn1.5O4 electrode in the STD electrolyte. In the SE-containing electrolyte, an obvious oxidation current is observed at the potential just higher than that for the oxidation of Mn3.5þ and lower than that for Ni2þ, indicative of the oxidation of SE. Additionally, the oxidation current is reduced for the LiNi0.5Mn1.5O4 electrode in the SE-containing electrolyte with increasing the voltage, indicative of the suppression of electrolyte decomposition by SE. It can be found by comparing Fig. 2c with d that the oxidation potential of SE is more negative on platinum than that on LiNi0.5Mn1.5O4. This difference can be explained by the catalysis of platinum toward the oxidation of SE.
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The preferential oxidation of SE on LiNi0.5Mn1.5O4 electrode can be more clearly identified in coin cell where the electrode was pressed compared to the electrode in the electrolytic cells where the electrode has larger polarization. Fig. 2e shows cyclic voltammograms of LiNi0.5Mn1.5O4 electrode in the STD electrolyte. There are two redox peaks at 4.0 and 4.7 V, corresponding to the couples of Mn3.5þ/Mn4þ and Ni2þ/Ni4þ, respectively [31,45]. These redox peaks remain but additional oxidation peak at 4.2 V appears in the SE-containing electrolyte, indicative of the oxidation of SE. This oxidation peak cannot be recorded in the subsequent cycles, suggesting that the oxidation of SE is stopped when a protective film is formed on LiNi0.5Mn1.5O4. The electrolyte decomposition products involve polymers, gases and acid (HF). The polymers deposit on electrode, which will increase the interfacial impedance of electrode/electrolyte and the polarization for charge/discharge processes. The potential difference between discharge plateau potentials of the initial and the 70th cycles for the redox of Ni2þ/Ni4þ in the STD electrolyte is 0.19 V (Fig. 3a), which is far larger than that in the SE-containing
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electrolyte (0.04 V, Fig. 3b), indicating the polarization resulting from the electrolyte decomposition. The impedance evolution of LiNi0.5Mn1.5O4 electrode was understood in Li/LiNi0.5Mn1.5O4 cell by EIS. As shown in Fig. 3(c and d), the EIS profiles consist of a pressed semicircle in high and medium frequencies, corresponding to the interfacial impedance including film resistance (Rf) and chargetransfer resistance (Rct), and a slope line in low frequencies representing Warburg impedance [40,46]. The experimental results can be fitted by the equivalent circuit (shown in the inset of Fig. 3d). The obtained resistance of LiNi0.5Mn1.5O4 electrode in STD electrolyte (Rf ¼ 11 U and Rct ¼ 93 U) is lower than that in SE-containing electrolyte (Rf ¼ 38 U and Rct ¼ 78 U) after 5 cycles (Fig. 3c), showing that the initially SE-developed film has larger resistance that leads to the lower capacity for the LiNi0.5Mn1.5O4 in SEcontaining electrolyte at the initial cycles (Fig. 2a). After 100 cycles, however, the resistance of LiNi0.5Mn1.5O4 has less change (Rf ¼ 38 U and Rct ¼ 102 U) in SE-containing electrolyte but has a significant increase (Rf ¼ 45 U and Rct ¼ 152 U) in STD electrolyte. This difference confirms that the electrolyte decomposition
Fig. 3. Selected discharge profiles of LiNi0.5Mn1.5O4 in STD (a) and SE-containing (b) electrolytes from Fig. 1a; electrochemical impedance spectra of LiNi0.5Mn1.5O4 after 5 cycles (c) and 100 cycles (d); chronoamperometric profiles of LiNi0.5Mn1.5O4 at 4.9 V (e).
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products increase the interfacial impedance and the film developed by SE inhibits the electrolyte decomposition. The inhibition of electrolyte decomposition by SE can be indicated by chronoamperometry. Fig. 3e presents the chronoamperometric responses of LiNi0.5Mn1.5O4 electrode at 4.9 V. It can be found from Fig. 3e that the residue current of LiNi0.5Mn1.5O4 in SE-containing electrolyte is lower than that in STD electrolyte. 3.2. Protective film generated from SE The oxidation mechanism of SE can be inferred by theoretical calculations combining surface analyses [33,42]. Fig. 4a shows the optimized geometric structure of SE and its product after one electron oxidation, labeled with some special bond lengths (Å). The calculated oxidation potential of SE is 4.37 V, which is lower than that of EC, EMC and DEC (7.02, 6.85 and 6.78 V, respectively), confirming that SE can be oxidized far more easily than the solvent in the electrolyte. It can be found from Fig. 4a that the length of C¼C bonds in SE increases the most significantly after one electron, suggesting that the oxidation of SE takes place through breaking of the C¼C bond in cyclopentadiene group (marked by red dotted cycle). In fact, cyclopentadiene has a similar oxidation potential (4.62 V). Therefore, the cyclopentadiene group provides SE with preferentially oxidative activity, which is important for the formation of a protective film. Fig. 4b shows the infrared (IR) spectrum of the cycled LiNi0.5Mn1.5O4 electrode in SE-containing electrolyte, compared with the spectra of pure SE and the fresh electrode. It can be found from Fig. 4b that the cycled electrode shows its IR spectrum very similar to that of pure SE, except for the peaks of PVDF (870, 960, 1162 and 1386 cm1 [27,47]), suggesting that the oxidation product of SE has been incorporated into the film on LiNi0.5Mn1.5O4. In the IR spectrum of pure SE additive, the peaks at 747, 830 and 1244 cm1 correspond to the -Si(CH3)3 group (marked by blue stars) [48,49], the peaks at 1444 and 2948 cm1 are related to the methyl group (-CH3) vibration, the peaks at 1000e1080 and 3020-3050 cm1 are attributed to the unsaturated hydrocarbon (-CH-), and the peak at
1590 cm1 is related to the C¼C bond. In the IR spectrum of cycled LiNi0.5Mn1.5O4 electrode, the peak positions for -Si(CH3)3 group do not shift, suggesting that -Si(CH3)3 group is not changed after the oxidation of SE. The peak of C¼C shifts from 1590 to 1735 cm1. The red shift phenomenon dissappeares due to the transformation of a conjugated double bond (-C¼C-C¼C-) to a single C¼C bond, which is in agreement with the theoretical calculation. With these analyses, the oxidation mechanism of SE can be illustrated in Fig. 4c. SE is oxidized generating a radical cation that combines itself yielding polymer. This polymer provides the film with effective protection due to the introduction of the stable Si-containing group into the film. The protection that SE provides can be confirmed by analyzing the structure of the cycled LiNi0.5Mn1.5O4 electrodes in Fig. 1a and the contents of dissolved transition metal ions. Fig. 5 presents the XRD patterns of the cycled electrodes with a comparison of fresh one. It can be seen from Fig. 5a that the fresh LiNi0.5Mn1.5O4 shows a typical Fd3m spinel structure [21,50,51]. After cycling in STD electrolyte, however, these typical diffractions almost disappear and new diffraction peaks (marked by green stars) appear. Obviously, the lattice structure of LiNi0.5Mn1.5O4 has been destroyed when it is performed under elevated temperature. The structural destruction is associated with the transition metal ion dissolution. At elevated temperature, the decomposition of electrolyte generates acid species (HF) [8,13,43], which can etch LiNi0.5Mn1.5O4 and cause the dissolution of transition metal ions. After cycling in SE-containing electrolyte, completely different results were obtained. All the typical diffraction peaks of fresh LiNi0.5Mn1.5O4 can still be identified clearly for the electrode cycled in SE-containing electrolyte, indicating that the film generated from SE yields an effective protection for the structural integrity of LiNi0.5Mn1.5O4. The dissolved transition metal ions from LiNi0.5Mn1.5O4 cathode will be distributed in electrolyte and anode. Since the cell is performed with deep cycling. Most dissolved transition metal ions in the electrolyte will transport to and deposit on anode during charge processes [41]. Therefore, the contents of transition metals on anode are indicative of their dissolution from cathode. Fig. 5b presents the analysis
Fig. 4. (a) Optimized structure of SE (above) and cyclopentadiene group (bottom) before and after one electron oxidation; (b) infrared spectra of pure SE and fresh and cycled LiNi0.5Mn1.5O4 electrodes in SE-containing electrolyte; (c) possible oxidation reaction of SE on LiNi0.5Mn1.5O4.
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Fig. 5. XRD patterns of LiNi0.5Mn1.5O4 cathode (a) and contents of transition metals deposited on anode for the cells after cycling tests in Fig. 1a.
results by ICP-AES. It can be found from Fig. 5b that massive transition metal ions can be detected on the anode cycled in STD electrolyte but hardly detected on that cycled in SE-containing electrolyte. The dissolved amount of Mn and Ni in SE-containing electrolyte is decreased by 97.4% and 98%, respectively. Obviously, the film generated from SE yields an effective protection for LiNi0.5Mn1.5O4. SE as an electrolyte additive for the high temperature application of LiNi0.5Mn1.5O4 behaves best compared to the additives that have been reported in literature. For example, the Ni and Mn contents detected on LiNi0.5Mn1.5O4 in 0.075% quercetincontaining electrolyte after 100 cycles at 60 C are decreased by 50% and 60%, respectively [25]. This effectiveness of SE should be
attributed to its special molecular structure, which contains a stable Si-containing group and an easily oxidized double bond. The existence and the contribution of the protective film generated from SE can be further demonstrated by SEM, TEM and XPS analyses. Fig. 6 shows the SEM and TEM images of the cycled LiNi0.5Mn1.5O4 electrode with a comparison of fresh one. As shown in Fig. 6a and b, the fresh LiNi0.5Mn1.5O4 electrode is composed of LiNi0.5Mn1.5O4 particles that has a spinel crystal morphology and smooth surface and conductive agents. After 100 cycles at elevated temperature, however, there appear lots of electrolyte decomposition deposits on LiNi0.5Mn1.5O4 particles (Fig. 6c). Additionally, cracks can be observed in LiNi0.5Mn1.5O4 particle, as marked by the
Fig. 6. SEM and TEM images of LiNi0.5Mn1.5O4. Fresh LiNi0.5Mn1.5O4 (a, b), LiNi0.5Mn1.5O4 after 100 cycles in STD (c, d) and SE-containing (e, f) electrolytes.
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red arrows in Fig. 6d. This morphological change confirms that the electrolyte suffers serious decomposition and LiNi0.5Mn1.5O4 suffers structural collapse. Differently, less deposit exists (Fig. 6e) and a thin and uniform film (about 15 nm thick, Fig. 6f) can be identified on LiNi0.5Mn1.5O4 particles cycled in SE-containing electrolyte. Obviously, a protective film is generated by applying SE, which can suppress the electrolyte decomposition. Fig. 7 presents the XPS profiles of the cycled LiNi0.5Mn1.5O4, with a comparison of fresh one. It can be found that the peaks related to
the electrolyte decomposition products, including C¼O (289 eV, polycarbonates and carbonate salts) in C1s, O¼C (531.5 eV, carbonate salts) in O1s, and LiF (685.5 eV) in F1s, are stronger, while those related to the electrode materials including C-C (284.8 eV, conductive carbon), C-H (286.5 eV, PVDF) and C-F (291 eV, PVDF) in C1s [26,52e54], and M-O (529.7 eV, LiNi0.5Mn1.5O4) in O1s, are weaker for the electrode cycled in STD electrolyte than in SEcontaining electrolyte. These differences indicate that more deposits exist on the electrode cycled in STD electrolyte than in SE-
Fig. 7. XPS patterns of LiNi0.5Mn1.5O4 before and after cycling in STD and SE-containing electrolytes.
Fig. 8. Electrochemical performances of graphite: the initial charge/discharge profiles and cyclic stability for Li/graphite half cells (a, b) and graphite/LiNi0.5Mn1.5O4 full cells (c, d) with or without SE.
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containing electrolyte. Most importantly, the peak for Si-C in Si 2p can be identified on the electrode cycled in SE-containing electrolyte but cannot be observed in STD electrolyte, confirming that the oxidation products of SE have been incorporated into the protective film. 3.3. Effect of SE on graphite anode Graphite is an important anode material of lithium ion batteries. The additives applied in electrolyte may affect the performance of graphite anode. To understand the effect of SE on graphite anode, Li/graphite half cell and graphite/graphite full cell were assembled and their cyclic performances were evaluated. Fig. 8a presents the initial charge/discharge profiles of Li/graphite half cell in STD and SE-containing electrolyte. It can be found that the capacity of graphite at the first is reduced to some extent when SE is applied. This phenomenon can be explained by the poor wettability of SE on graphite. This negative effect of SE can be avoided after cycling. The capacity of graphite in SE-containing electrolyte is recovered at the second cycle and there is insignificant cyclic stability between the electrodes in STD and SE-containing electrolyte (Fig. 8b). Similar results can be observed in graphite/LiNi0.5Mn1.5O4 full cells, as shown in Fig. 8c. Initially, the full cell in SE-containing electrolyte shows the voltage plateau at 3.0 V for the SE oxidation and delivers a lower capacity than the cell in STD electrolyte. Similarly to the effect of SE on Li/LiNi0.5Mn1.5O4 half cell (Fig. 1), the full cell shows less difference in cyclic stability between STD and SE-containing electrolyte under room temperature, but the capacity decays more quickly for the cell in STD electrode than in SEcontaining electrolyte under elevated temperature. These comparisons indicate that SE hardly affects the performance of graphite anode. 4. Conclusion The cyclic performance of LiNi0.5Mn1.5O4 at elevated temperature (55 C) can be significantly improved by adding 0.25% SE additive. This improvement is attributed to the special molecular structure of SE, which consists of a stable Si-containing group and an easily oxidized double bond. With these characteristics, SE can be oxidized preferentially compared to the baseline electrolyte, forming a protective film on LiNi0.5Mn1.5O4. The resulting film effectively suppresses the decomposition of the baseline electrolyte on LiNi0.5Mn1.5O4 electrode and the dissolution of transition metal ions from LiNi0.5Mn1.5O4. Compared with the electrolyte additives for the application of LiNi0.5Mn1.5O4, SE is best in inhibition of transition metal ion dissolution. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No.21303061), the key project of Science and Technology in Guangdong Province (Grant Nos. 2013B090800013 and 2016B010114001), Guangzhou City Project for Cooperation among Industries, Universities and Institutes (Grant No. 201604016011), and the Innovation Project of Graduate School of South China Normal University (Grant No. 20161kxm07). References [1] X. Dong, L. Chen, X. Su, Y. Wang, Y. Xia, Angew. Chem. Int. Ed. 55 (2016) 7474e7477. [2] F. Cheng, J. Liang, Z. Tao, J. Chen, Adv. Mater. 23 (2011) 1695e1715. [3] W. Tang, Y.S. Zhu, Y.Y. Hou, L.L. Liu, Y.P. Wu, K.P. Loh, H.P. Zhang, K. Zhu, Energy & Environ. Sci. 6 (2013) 2093e2104.
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Trimethylsilylcyclopentadiene as a novel electrolyte additive for high temperature application of lithium nickel manganese oxide cathode Wenqiang Tu a, Changchun Ye a, Xuerui Yang a, Lidan Xing a, b, Youhao Liao a, b, Xiang Liu a, b, *, Weishan Li a, b, ** a
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China Engineering Research Center of MTEES (Ministry of Education), Research Center of BMET (Guangdong Province), Engineering Lab. of OFMHEB (Guangdong Province), Key Lab. of ETESPG(GHEI), and Innovative Platform for ITBMD (Guangzhou Municipality), South China Normal University, Guangzhou 510006, China
b
h i g h l i g h t s SE is an effective electrolyte additive for high temperature application of LiNi0.5Mn1.5O4. A protective film can be formed on LiNi0.5Mn1.5O4 due to preferential oxidation of SE. The film can suppress the dissolution of transition metal ions from LiNi0.5Mn1.5O4.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 May 2017 Received in revised form 20 July 2017 Accepted 6 August 2017
Electrolyte additives are necessary for the application of high potential cathode in high energy density lithium ion batteries, especially at elevated temperature. However, the electrolyte additives that can effectively suppress the dissolution of transition metal ions from cathode have seldom been developed up to date. In this work, we propose a novel electrolyte additive, trimethylsilylcyclopentadiene (SE), for high temperature application of a representative high potential cathode, lithium nickel manganese oxide (LiNi0.5Mn1.5O4). It is found that the dissolution of Mn and Ni from LiNi0.5Mn1.5O4 can be effectively suppressed by applying SE. With applying 0.25% SE, the dissolved amount of Mn and Ni is decreased by 97.4% and 98%, respectively, after 100 cycles at 55 C. Correspondingly, the cyclic performance of LiNi0.5Mn1.5O4 is significantly improved. Physical characterizations and electrochemical measurements show that SE can be preferentially oxidized and generate a protective film on LiNi0.5Mn1.5O4. The resulting film inhibits the electrolyte decomposition and the transition metal ion dissolution. © 2017 Elsevier B.V. All rights reserved.
Keywords: Trimethylsilylcyclopentadiene Electrolyte additive Protective film High potential cathode Lithium ion battery
1. Introduction Lithium ion batteries have been extensively applied in digital products including mobile phone, laptop and model airplane due to their higher energy density than other commercial rechargeable batteries [1e4]. However, the energy densities of lithium ion batteries are not high enough and they are too expensive for their applications in electric vehicles. Raising work voltage or applying
* Corresponding author. School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China. ** Corresponding author. School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China. E-mail addresses: [email protected] (X. Liu), [email protected] (W. Li). http://dx.doi.org/10.1016/j.jpowsour.2017.08.021 0378-7753/© 2017 Elsevier B.V. All rights reserved.
high potential cathodes is a direct and effective approach to enhance the energy densities of lithium ion batteries. Compared to the low working potential and high price of commercially available cathodes, including LiCoO2 (~3.6 V vs. Liþ/Li) and LiFePO4 (~3.2 V), LiNi0.5Mn1.5O4 has a high working potential (~4.7 V) and low price and has received much attention recently. However, there remain issues such as serious electrolyte decomposition and transition metal ion dissolution that should be solved before LiNi0.5Mn1.5O4 is put into practical application [5e8]. These issues become more challenging when LiNi0.5Mn1.5O4 is run at elevated temperature [9e12]. The conventional carbonate-based electrolytes with LiPF6 as salt are unstable at the potential higher than 4.2 V, tending to be decomposed and generating corrosive species HF that causes transition metal ion dissolution [13e16].
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Coating cathode materials with inert inorganic compounds is one of effective approaches to inhibit the electrolyte decomposition on high potential cathodes [17e21], but this approach has some shortages. Firstly, coating process is complex and expensive. Secondly, the introduced inert compounds reduce the specific capacity and increase the interfacial impedance between cathode and electrolyte. Substituting more stable solvents for carbonates has been adopted to solve the issues resulting from unstable carbonatebased electrolytes [22,23], but these new solvents have detrimental effects: lower ionic conductivity, higher viscosity and poorer compatibility with graphite compared to carbonates. Introducing some electrolyte additives into carbonate-based electrolytes can significantly improve the electrochemical performances of lithium ion batteries [15,24e28]. Several electrolyte additives have been proposed for the application of high potential cathode cycled in conventional carbonate-based electrolytes. For example, Choi et al. reported that TMSPi can improve the capacity retention of LiNi0.5Mn1.5O4 from 73% to 90% at 1 C rate after 100 cycles at 60 C [29], and Sun et al. revealed that LiBFMB could improve the capacity retention of LiNi0.5Mn1.5O4 from 68% to 86% after 100 cycles at room temperature [30]. Generally, the improved cyclic stabilities of LiNi0.5Mn1.5O4 by applying electrolyte additives are attributed to the protective film generated from the preferential oxidation of additives on cathodes. Unfortunately, the electrolyte additives that have been reported in literature do not show their effective inhibition of transition metal ion dissolution from LiNi0.5Mn1.5O4, although significantly improved cyclic stability of LiNi0.5Mn1.5O4 has been achieved by applying these additives. The transition metal ion dissolution is detrimental to the performance of full cell with graphite anode, because the dissolved metal ions will deposit on graphite and destroy the solid electrolyte interphase film. The chemical composition and molecular structure of additives are critical for the oxidation preference of the additives and protection effectiveness of the resulting film [31e34]. For example, the additives containing B or B-O structure are beneficial for lithium transportation in the film, while the additives containing Si or Si-O bond provides the film with improved chemical and thermal stability [35e40]. Comparatively, the chemical and thermal stability of a film is more important than its ionic transportation property for the cyclic performance improvement of a high potential cathode, because the film generated by electrolyte additives is so thin that the film impedance hardly affect the cathode performance. With this knowledge, we proposed a novel Si-containing additive, trimethylsilylcyclopentadiene (SE), for high temperature application of LiNi0.5M1.5O4. Physical and electrochemical results demonstrate that a thin film can be generated on LiNi0.5Mn1.5O4 by applying SE, which significantly inhibits the dissolution of transition metal ions. With the application of SE, the cyclic stability of not only LiNi0.5Mn1.5O4 cathode but also LiNi0.5Mn1.5O4/graphite full cell is effectively improved. 2. Experiments 2.1. Preparations and assemblies Mixture of 80 wt% LiNi0.5Mn1.5O4 powders (Sichuan Xingneng Co. Ltd. China), 10 wt% acetylene black (AB) and 10 wt% polyvinylidene fluoride (PVDF) was coated on Al foil to obtain LiNi0.5Mn1.5O4 electrode with a loading of ~3.1 mg cm2 LiNi0.5Mn1.5O4. Mixture of artificial graphite (AG) (Dongguan Kaijin New Energy Technology Co. Ltd, China), Super-P (SP) and PVDF in a weight ration of AG/SP/PVDF ¼ 9/0.5/0.5 was coated on Cu foil to obtain graphite electrode with a loading amount of 1.32 mg cm2. The standard (STD) electrolyte used in this work was 1 M LiPF6 in
ethylene carbonate (EC)/ethyl methyl carbonate (EMC)/diethyl carbonate (DEC) (3/5/2 in volume, provided by Dongguan Kaixin Materials Technology Co. Ltd. China), and the additive-containing electrolytes were prepared by adding 0.25% or 0.5% (in weight) SE (purchased from Tokyo Chemical Industry (shanghai) Co. Ltd, China) into the STD electrolyte. The Li/LiNi0.5Mn1.5O4 cells were assembled with LiNi0.5Mn1.5O4 electrode, Celgard 2300 separator and Li metal plate electrode in 2025 coin cell. The Li/graphite cells were assembled with AG electrode, Celgard 2300 separator and Li metal plate electrode in 2025 coin cells. The graphite/ LiNi0.5Mn1.5O4 full cells were assembled with LiNi0.5Mn1.5O4 electrode, Celgard 2300 separator and graphite electrode in 2025 coin cells. Electrolyte cells were also assembled with platinum or LiNi0.5Mn1.5O4 electrode and Li metal plate electrode without using separator, as described in our previous report [31e33]. Electrolyte preparation and cell assembly were conducted in a dry argon glove box where the water and oxygen concentrations were controlled to be less than 0.1 ppm. 2.2. Characterizations and measurements The cyclic stability of the cells was measured on a LAND cell test system (Land CT 2001A, China). Li/LiNi0.5Mn1.5O4 half cell was charged to 4.9 V at 1 C (1 C ¼ 120 mA g1), followed by a constant voltage at 4.9 V for 10 min, and then discharged at 1 C to 3.0 V under room (25 C, RT) and elevated (55 C, ET) temperature. Before cycling test, the cells were charged/discharged for three cycles at 0.5 C under room temperature. Li/graphite half cell was charged and discharged at 0.1 C (1C ¼ 350 mA g1) for the first cycle and then at 0.2 C rate for the subsequent cycles. Graphite/LiNi0.5Mn1.5O4 full cells were charged/discharged at 0.1 C (1C ¼ 120 mA g1) for the initial three cycles and then at 0.5 C for the subsequent cycles. Electrochemical impedance spectroscopy (EIS) was performed on the electrochemical station (Autolab PGSTAT-302N, Metrohm Co. Switzerland) in Li/LiNi0.5Mn1.5O4 half cell with LiNi0.5Mn1.5O4 as working electrode and lithium electrode as counter and reference electrodes. To ignore the contribution of lithium electrode, the impedance measurements were performed immediately after cycling at the discharge stage (3.0 V) in a frequency range from 100 KHz to 10 mHz with a potential amplitude of 5 mV. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) tests were performed on Solartron 1480 instrument (England). Coin cells were used for CV at a scan rate of 0.1 mV s1, while electrolyte cells were used for LSV by using platinum disk (2 mm in diameter) at a scan rate of 1 mV s1 or LiNi0.5Mn1.5O4 electrode (1 2 cm) at a scan rate of 0.1 mV s1 as working electrode. Chronoamperometry was carried out in Li/LiNi0.5Mn1.5O4 coin cells, which were charged at 0.5 C to 4.9 V and then kept at 4.9 V for 18 h. To understand the effect of cycling on the structure and composition of LiNi0.5Mn1.5O4 in SE-containing and STD electrolytes, the electrodes were dismantled from the cycled cells in glove box, washed by DMC for three times, and dried in vacuum box for overnight time at room temperature. The crystal structure was evaluated by X-ray diffraction (XRD, Bruker D8 Advance, Germany) operated at 60 KV and 80 mA using Cu Ka radiation (l ¼ 1.5405 nm) in the 2q range of 10e80 . The scanning electron microscopy (SEM) was performed on JEM-6510 (Japan) and transmission electron microscopy (TEM) was performed on JEM-2100HR (Japan). The surface element compositions on cathode were analyzed by XPS on Thermo Fisher Scientific (UK) with a monochromatic Al Ka X-ray source (excitation energy ¼ 1468.6 eV). The data were collected from 0 to 1350 eV using an X-ray spot size of 400 mm with a pass energy of 100 eV for wide scan and 30 eV for individual elements. Binding energies were corrected based on the carbon 1s signal at 284.8 eV. The infrared spectroscopy was performed at a Nicolet
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67000 (America) spectrometer. To analyze the content of transition metal deposited on the counter electrode, the lithium electrode was dismantled from cycled Li/LiNi0.5Mn1.5O4 cells, washed by DMC for three times, dissolved in 5 ml 3% HNO3 solution. The solution was diluted to 25 ml for the ICP-AES analysis (ICAP 6500 Duo, USA). 2.3. Theoretical calculations The theoretical calculations were performed in Gaussian 09 package [13,42,43]. All the molecular structures were optimized by Density Functional Theory (DFT) method at B3LYP (6e333þþG (d)) level. Polarized continuum models (PCM) with a dielectric constant of 20.5 (acetone) were applied to investigate the bulk solvent effect. And frequency calculations were performed at the same level. 3. Results and discussions 3.1. Electrochemical performances The effectiveness of SE on the performance of LiNi0.5Mn1.5O4 was evaluated by cycling Li/LiNi0.5Mn1.5O4 half cells. The obtained results are presented in Fig. 1. As shown in Fig. 1a, LiNi0.5Mn1.5O4 suffers a fast capacity fading under elevated temperature, from an initial capacity of 120 mAh g1 to 18 mAh g1 with a capacity retention of only 15% after 75 cycles. This is the main issue that limits the application of LiNi0.5Mn1.5O4 in practice, which is caused by the serious decomposition of the STD electrolyte and the dissolution of transition metal ions. When SE is applied, the capacity retention of LiNi0.5Mn1.5O4 is significantly improved, to 88% and 84%, respectively, in 0.25% and 0.5% SE-containing electrolytes after 100 cycles, indicating that SE is effective for the cyclic stability improvement of LiNi0.5Mn1.5O4 under high temperature application. This effect should be associated with the protective film on
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LiNi0.5Mn1.5O4, which effectively suppresses the electrolyte decomposition. The suppression of the electrolyte decomposition can be confirmed by the more stable and far higher coulombic efficiency of LiNi0.5Mn1.5O4 in SE-containing electrolytes than that in STD electrolyte, as shown in Fig. 1b. The cycle performances of LiNi0.5Mn1.5O4 in the electrolytes with or without SE were also evaluated under room temperature. The obtained results are presented in Fig. 1c and d. It can be found from Fig. 1c and d that whether capacity retention or coulombic efficiency of LiNi0.5Mn1.5O4 maintains a high level in STD and SEcontaining electrolytes. These results confirm that STD electrolyte does not favor the high temperature application of LiNi0.5Mn1.5O4 and suggest that the application of SE as an electrolyte additive hardly affects the capacity delivery of LiNi0.5Mn1.5O4 under room temperature. Nevertheless, increasing the concentration of SE will slightly reduce the capacity delivery of LiNi0.5Mn1.5O4, as shown in Fig. 1a. This phenomenon can be explained by the increased interfacial impedance due to the protective film formed by SE. The film will become thicker with increasing SE concentration and increase the interfacial impedance. Therefore, 0.25% SE was considered for further investigations. To understand the mechanism on the effectiveness of SE for the cyclic stability improvement of LiNi0.5Mn1.5O4, the electrochemical performance of LiNi0.5Mn1.5O4 in STD and SE-containing electrolytes were further analyzed in detail. The initial charge/discharge profiles at 0.5 C are presented in Fig. 2a and b. Initially in the STD electrolyte, as shown in Fig. 2a, the potential of LiNi0.5Mn1.5O4 quickly rises to 4.7 V and keeps stable at about 4.8 V. This long charge plateau at 4.8 V corresponds to the oxidation of Ni2þ, which contributes to the main capacity of LiNi0.5Mn1.5O4 [6,44]. Correspondingly, there is a long discharge potential plateau at about 4.7 V. Additionally, a small discharge potential plateau appears at about 4.0 V, corresponding to the reduction of and Mn4þ to Mn3þ.
Fig. 1. Cyclic performances of LiNi0.5Mn1.5O4 at 1 C in STD and SE-containing electrolytes after three cycles at 0.5 C. Cyclic stability (a, c) and coulombic efficiency (b, d) at elevated (50 C, ET) and room temperature (25 C, RT).
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Fig. 2. Electrochemical performances of LiNi0.5Mn1.5O4 with and without SE additive. The first (a) and third (b) charge/discharge profiles of LiNi0.5Mn1.5O4 at 0.5 C; linear sweep voltammograms of platinum (c) and LiNi0.5Mn1.5O4 (d) electrodes; and cyclic voltammograms of LiNi0.5Mn1.5O4 electrodes in STD (e) and SE-containing (f) electrolytes.
Comparatively, there appears an additional oxidation potential plateau at about 4.5 V for the LiNi0.5Mn1.5O4 in SE-containing electrolyte (Fig. 2a), showing that SE is preferentially oxidized. Correspondingly, there is a lower efficiency (75.8%) in SEcontaining electrolyte than in STD electrolyte (83.6%). This preferential oxidation of SE is important for the formation of a protective film that can effectively inhibit the electrolyte decomposition. At the third cycling, the oxidation potential plateau of SE disappears, as shown in Fig. 2c, suggesting that the oxidation of SE is also inhibited by the protective film formed at the first cycle. The preferential oxidation of SE can be directly demonstrated by LSV on platinum and LiNi0.5Mn1.5O4 in electrolytic cells. As shown in Fig. 2c, the voltammogram of platinum electrode in the SEcontaining electrolyte is different completely from that in the STD electrolyte. Before 4.5 V, the oxidation current in the STD electrolyte is small, suggesting that STD electrolyte is relatively stable. This is why carbonate-based electrolytes can be widely used in commercial lithium ion batteries [27]. However, fast increasing oxidation current can be observed when the potential is enhanced further, demonstrating the instability of carbonate-based electrolyte under high potential. Interestingly, the oxidation current for
the electrolyte decomposition on platinum can be significantly reduced and a small current peak appears at about 3.75 V when SE is applied (Fig. 2c). This difference suggests that SE is oxidized preferentially and the electrolyte decomposition can be inhibited due to the SE oxidation. Similar behaviors can be observed on LiNi0.5Mn1.5O4 electrode in the electrolytic cell, as shown in Fig. 2d. In STD electrolyte, there are two oxidation peaks (~4.0 V and ~5 V), corresponding to the redox reactions of Mn3.5þ/Mn4þ and Ni2þ/Ni4þ, respectively. At higher potential, the increasing current for the electrolyte decomposition can be observed for LiNi0.5Mn1.5O4 electrode in the STD electrolyte. In the SE-containing electrolyte, an obvious oxidation current is observed at the potential just higher than that for the oxidation of Mn3.5þ and lower than that for Ni2þ, indicative of the oxidation of SE. Additionally, the oxidation current is reduced for the LiNi0.5Mn1.5O4 electrode in the SE-containing electrolyte with increasing the voltage, indicative of the suppression of electrolyte decomposition by SE. It can be found by comparing Fig. 2c with d that the oxidation potential of SE is more negative on platinum than that on LiNi0.5Mn1.5O4. This difference can be explained by the catalysis of platinum toward the oxidation of SE.
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The preferential oxidation of SE on LiNi0.5Mn1.5O4 electrode can be more clearly identified in coin cell where the electrode was pressed compared to the electrode in the electrolytic cells where the electrode has larger polarization. Fig. 2e shows cyclic voltammograms of LiNi0.5Mn1.5O4 electrode in the STD electrolyte. There are two redox peaks at 4.0 and 4.7 V, corresponding to the couples of Mn3.5þ/Mn4þ and Ni2þ/Ni4þ, respectively [31,45]. These redox peaks remain but additional oxidation peak at 4.2 V appears in the SE-containing electrolyte, indicative of the oxidation of SE. This oxidation peak cannot be recorded in the subsequent cycles, suggesting that the oxidation of SE is stopped when a protective film is formed on LiNi0.5Mn1.5O4. The electrolyte decomposition products involve polymers, gases and acid (HF). The polymers deposit on electrode, which will increase the interfacial impedance of electrode/electrolyte and the polarization for charge/discharge processes. The potential difference between discharge plateau potentials of the initial and the 70th cycles for the redox of Ni2þ/Ni4þ in the STD electrolyte is 0.19 V (Fig. 3a), which is far larger than that in the SE-containing
27
electrolyte (0.04 V, Fig. 3b), indicating the polarization resulting from the electrolyte decomposition. The impedance evolution of LiNi0.5Mn1.5O4 electrode was understood in Li/LiNi0.5Mn1.5O4 cell by EIS. As shown in Fig. 3(c and d), the EIS profiles consist of a pressed semicircle in high and medium frequencies, corresponding to the interfacial impedance including film resistance (Rf) and chargetransfer resistance (Rct), and a slope line in low frequencies representing Warburg impedance [40,46]. The experimental results can be fitted by the equivalent circuit (shown in the inset of Fig. 3d). The obtained resistance of LiNi0.5Mn1.5O4 electrode in STD electrolyte (Rf ¼ 11 U and Rct ¼ 93 U) is lower than that in SE-containing electrolyte (Rf ¼ 38 U and Rct ¼ 78 U) after 5 cycles (Fig. 3c), showing that the initially SE-developed film has larger resistance that leads to the lower capacity for the LiNi0.5Mn1.5O4 in SEcontaining electrolyte at the initial cycles (Fig. 2a). After 100 cycles, however, the resistance of LiNi0.5Mn1.5O4 has less change (Rf ¼ 38 U and Rct ¼ 102 U) in SE-containing electrolyte but has a significant increase (Rf ¼ 45 U and Rct ¼ 152 U) in STD electrolyte. This difference confirms that the electrolyte decomposition
Fig. 3. Selected discharge profiles of LiNi0.5Mn1.5O4 in STD (a) and SE-containing (b) electrolytes from Fig. 1a; electrochemical impedance spectra of LiNi0.5Mn1.5O4 after 5 cycles (c) and 100 cycles (d); chronoamperometric profiles of LiNi0.5Mn1.5O4 at 4.9 V (e).
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products increase the interfacial impedance and the film developed by SE inhibits the electrolyte decomposition. The inhibition of electrolyte decomposition by SE can be indicated by chronoamperometry. Fig. 3e presents the chronoamperometric responses of LiNi0.5Mn1.5O4 electrode at 4.9 V. It can be found from Fig. 3e that the residue current of LiNi0.5Mn1.5O4 in SE-containing electrolyte is lower than that in STD electrolyte. 3.2. Protective film generated from SE The oxidation mechanism of SE can be inferred by theoretical calculations combining surface analyses [33,42]. Fig. 4a shows the optimized geometric structure of SE and its product after one electron oxidation, labeled with some special bond lengths (Å). The calculated oxidation potential of SE is 4.37 V, which is lower than that of EC, EMC and DEC (7.02, 6.85 and 6.78 V, respectively), confirming that SE can be oxidized far more easily than the solvent in the electrolyte. It can be found from Fig. 4a that the length of C¼C bonds in SE increases the most significantly after one electron, suggesting that the oxidation of SE takes place through breaking of the C¼C bond in cyclopentadiene group (marked by red dotted cycle). In fact, cyclopentadiene has a similar oxidation potential (4.62 V). Therefore, the cyclopentadiene group provides SE with preferentially oxidative activity, which is important for the formation of a protective film. Fig. 4b shows the infrared (IR) spectrum of the cycled LiNi0.5Mn1.5O4 electrode in SE-containing electrolyte, compared with the spectra of pure SE and the fresh electrode. It can be found from Fig. 4b that the cycled electrode shows its IR spectrum very similar to that of pure SE, except for the peaks of PVDF (870, 960, 1162 and 1386 cm1 [27,47]), suggesting that the oxidation product of SE has been incorporated into the film on LiNi0.5Mn1.5O4. In the IR spectrum of pure SE additive, the peaks at 747, 830 and 1244 cm1 correspond to the -Si(CH3)3 group (marked by blue stars) [48,49], the peaks at 1444 and 2948 cm1 are related to the methyl group (-CH3) vibration, the peaks at 1000e1080 and 3020-3050 cm1 are attributed to the unsaturated hydrocarbon (-CH-), and the peak at
1590 cm1 is related to the C¼C bond. In the IR spectrum of cycled LiNi0.5Mn1.5O4 electrode, the peak positions for -Si(CH3)3 group do not shift, suggesting that -Si(CH3)3 group is not changed after the oxidation of SE. The peak of C¼C shifts from 1590 to 1735 cm1. The red shift phenomenon dissappeares due to the transformation of a conjugated double bond (-C¼C-C¼C-) to a single C¼C bond, which is in agreement with the theoretical calculation. With these analyses, the oxidation mechanism of SE can be illustrated in Fig. 4c. SE is oxidized generating a radical cation that combines itself yielding polymer. This polymer provides the film with effective protection due to the introduction of the stable Si-containing group into the film. The protection that SE provides can be confirmed by analyzing the structure of the cycled LiNi0.5Mn1.5O4 electrodes in Fig. 1a and the contents of dissolved transition metal ions. Fig. 5 presents the XRD patterns of the cycled electrodes with a comparison of fresh one. It can be seen from Fig. 5a that the fresh LiNi0.5Mn1.5O4 shows a typical Fd3m spinel structure [21,50,51]. After cycling in STD electrolyte, however, these typical diffractions almost disappear and new diffraction peaks (marked by green stars) appear. Obviously, the lattice structure of LiNi0.5Mn1.5O4 has been destroyed when it is performed under elevated temperature. The structural destruction is associated with the transition metal ion dissolution. At elevated temperature, the decomposition of electrolyte generates acid species (HF) [8,13,43], which can etch LiNi0.5Mn1.5O4 and cause the dissolution of transition metal ions. After cycling in SE-containing electrolyte, completely different results were obtained. All the typical diffraction peaks of fresh LiNi0.5Mn1.5O4 can still be identified clearly for the electrode cycled in SE-containing electrolyte, indicating that the film generated from SE yields an effective protection for the structural integrity of LiNi0.5Mn1.5O4. The dissolved transition metal ions from LiNi0.5Mn1.5O4 cathode will be distributed in electrolyte and anode. Since the cell is performed with deep cycling. Most dissolved transition metal ions in the electrolyte will transport to and deposit on anode during charge processes [41]. Therefore, the contents of transition metals on anode are indicative of their dissolution from cathode. Fig. 5b presents the analysis
Fig. 4. (a) Optimized structure of SE (above) and cyclopentadiene group (bottom) before and after one electron oxidation; (b) infrared spectra of pure SE and fresh and cycled LiNi0.5Mn1.5O4 electrodes in SE-containing electrolyte; (c) possible oxidation reaction of SE on LiNi0.5Mn1.5O4.
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Fig. 5. XRD patterns of LiNi0.5Mn1.5O4 cathode (a) and contents of transition metals deposited on anode for the cells after cycling tests in Fig. 1a.
results by ICP-AES. It can be found from Fig. 5b that massive transition metal ions can be detected on the anode cycled in STD electrolyte but hardly detected on that cycled in SE-containing electrolyte. The dissolved amount of Mn and Ni in SE-containing electrolyte is decreased by 97.4% and 98%, respectively. Obviously, the film generated from SE yields an effective protection for LiNi0.5Mn1.5O4. SE as an electrolyte additive for the high temperature application of LiNi0.5Mn1.5O4 behaves best compared to the additives that have been reported in literature. For example, the Ni and Mn contents detected on LiNi0.5Mn1.5O4 in 0.075% quercetincontaining electrolyte after 100 cycles at 60 C are decreased by 50% and 60%, respectively [25]. This effectiveness of SE should be
attributed to its special molecular structure, which contains a stable Si-containing group and an easily oxidized double bond. The existence and the contribution of the protective film generated from SE can be further demonstrated by SEM, TEM and XPS analyses. Fig. 6 shows the SEM and TEM images of the cycled LiNi0.5Mn1.5O4 electrode with a comparison of fresh one. As shown in Fig. 6a and b, the fresh LiNi0.5Mn1.5O4 electrode is composed of LiNi0.5Mn1.5O4 particles that has a spinel crystal morphology and smooth surface and conductive agents. After 100 cycles at elevated temperature, however, there appear lots of electrolyte decomposition deposits on LiNi0.5Mn1.5O4 particles (Fig. 6c). Additionally, cracks can be observed in LiNi0.5Mn1.5O4 particle, as marked by the
Fig. 6. SEM and TEM images of LiNi0.5Mn1.5O4. Fresh LiNi0.5Mn1.5O4 (a, b), LiNi0.5Mn1.5O4 after 100 cycles in STD (c, d) and SE-containing (e, f) electrolytes.
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red arrows in Fig. 6d. This morphological change confirms that the electrolyte suffers serious decomposition and LiNi0.5Mn1.5O4 suffers structural collapse. Differently, less deposit exists (Fig. 6e) and a thin and uniform film (about 15 nm thick, Fig. 6f) can be identified on LiNi0.5Mn1.5O4 particles cycled in SE-containing electrolyte. Obviously, a protective film is generated by applying SE, which can suppress the electrolyte decomposition. Fig. 7 presents the XPS profiles of the cycled LiNi0.5Mn1.5O4, with a comparison of fresh one. It can be found that the peaks related to
the electrolyte decomposition products, including C¼O (289 eV, polycarbonates and carbonate salts) in C1s, O¼C (531.5 eV, carbonate salts) in O1s, and LiF (685.5 eV) in F1s, are stronger, while those related to the electrode materials including C-C (284.8 eV, conductive carbon), C-H (286.5 eV, PVDF) and C-F (291 eV, PVDF) in C1s [26,52e54], and M-O (529.7 eV, LiNi0.5Mn1.5O4) in O1s, are weaker for the electrode cycled in STD electrolyte than in SEcontaining electrolyte. These differences indicate that more deposits exist on the electrode cycled in STD electrolyte than in SE-
Fig. 7. XPS patterns of LiNi0.5Mn1.5O4 before and after cycling in STD and SE-containing electrolytes.
Fig. 8. Electrochemical performances of graphite: the initial charge/discharge profiles and cyclic stability for Li/graphite half cells (a, b) and graphite/LiNi0.5Mn1.5O4 full cells (c, d) with or without SE.
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containing electrolyte. Most importantly, the peak for Si-C in Si 2p can be identified on the electrode cycled in SE-containing electrolyte but cannot be observed in STD electrolyte, confirming that the oxidation products of SE have been incorporated into the protective film. 3.3. Effect of SE on graphite anode Graphite is an important anode material of lithium ion batteries. The additives applied in electrolyte may affect the performance of graphite anode. To understand the effect of SE on graphite anode, Li/graphite half cell and graphite/graphite full cell were assembled and their cyclic performances were evaluated. Fig. 8a presents the initial charge/discharge profiles of Li/graphite half cell in STD and SE-containing electrolyte. It can be found that the capacity of graphite at the first is reduced to some extent when SE is applied. This phenomenon can be explained by the poor wettability of SE on graphite. This negative effect of SE can be avoided after cycling. The capacity of graphite in SE-containing electrolyte is recovered at the second cycle and there is insignificant cyclic stability between the electrodes in STD and SE-containing electrolyte (Fig. 8b). Similar results can be observed in graphite/LiNi0.5Mn1.5O4 full cells, as shown in Fig. 8c. Initially, the full cell in SE-containing electrolyte shows the voltage plateau at 3.0 V for the SE oxidation and delivers a lower capacity than the cell in STD electrolyte. Similarly to the effect of SE on Li/LiNi0.5Mn1.5O4 half cell (Fig. 1), the full cell shows less difference in cyclic stability between STD and SE-containing electrolyte under room temperature, but the capacity decays more quickly for the cell in STD electrode than in SEcontaining electrolyte under elevated temperature. These comparisons indicate that SE hardly affects the performance of graphite anode. 4. Conclusion The cyclic performance of LiNi0.5Mn1.5O4 at elevated temperature (55 C) can be significantly improved by adding 0.25% SE additive. This improvement is attributed to the special molecular structure of SE, which consists of a stable Si-containing group and an easily oxidized double bond. With these characteristics, SE can be oxidized preferentially compared to the baseline electrolyte, forming a protective film on LiNi0.5Mn1.5O4. The resulting film effectively suppresses the decomposition of the baseline electrolyte on LiNi0.5Mn1.5O4 electrode and the dissolution of transition metal ions from LiNi0.5Mn1.5O4. Compared with the electrolyte additives for the application of LiNi0.5Mn1.5O4, SE is best in inhibition of transition metal ion dissolution. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No.21303061), the key project of Science and Technology in Guangdong Province (Grant Nos. 2013B090800013 and 2016B010114001), Guangzhou City Project for Cooperation among Industries, Universities and Institutes (Grant No. 201604016011), and the Innovation Project of Graduate School of South China Normal University (Grant No. 20161kxm07). References [1] X. Dong, L. Chen, X. Su, Y. Wang, Y. Xia, Angew. Chem. Int. Ed. 55 (2016) 7474e7477. [2] F. Cheng, J. Liang, Z. Tao, J. Chen, Adv. Mater. 23 (2011) 1695e1715. [3] W. Tang, Y.S. Zhu, Y.Y. Hou, L.L. Liu, Y.P. Wu, K.P. Loh, H.P. Zhang, K. Zhu, Energy & Environ. Sci. 6 (2013) 2093e2104.
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