Comparative study of lithium bis(oxalato)borate and lithium bis(fluorosulfonyl)imide on lithium manganese oxide spinel lithium-ion batteries

Accepted Manuscript Comparative study of lithium bis(oxalato)borate and lithium bis(fluorosulfonyl)imide on lithium manganese oxide spinel lithium-ion batteries Renheng Wang, Xinhai Li, Zhixing Wang, Huajun Guo, Mingru Su, Tao Hou PII: DOI: Reference:

S0925-8388(14)02746-7 http://dx.doi.org/10.1016/j.jallcom.2014.11.098 JALCOM 32652

To appear in:

Journal of Alloys and Compounds

Received Date: Revised Date: Accepted Date:

15 September 2014 11 November 2014 13 November 2014

Please cite this article as: R. Wang, X. Li, Z. Wang, H. Guo, M. Su, T. Hou, Comparative study of lithium bis(oxalato)borate and lithium bis(fluorosulfonyl)imide on lithium manganese oxide spinel lithium-ion batteries, Journal of Alloys and Compounds (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.11.098

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Comparative study of lithium bis(oxalato)borate and lithium bis(fluorosulfonyl)imide on lithium manganese oxide spinel lithium-ion batteries Renheng Wanga, Xinhai Lia*, Zhixing Wanga, Huajun Guoa, Mingru Sua, Tao Houb

a

School of Metallurgy and Environment, Central South University, Changsha, 410083, PR China b

Jiangxi Youli New Materials Co., Ltd, Pingxiang, Jiangxi, 337000, PR China * Corresponding author, Tel (Fax): +86 731 88836633 E-mail address: [email protected] [email protected]

1

Abstract The comparative study of lithium bis(oxalato)borate (LiBOB) and lithium bis(fluorosulfonyl) imide (LiFSI) used as an additive in the performance of lithium manganese oxide spinel (LiMn2O4) cathode was systematically investigated at elevated temperature. The results indicated that a solid-electrolyte interphase (SEI) film on cathode produced by the oxidation of the LiFSI additive is more robust and stable against Mn dissolution problem during cycling at 55 °C compared with the SEI formed by the oxidation of the Blank and the LiBOB-added electrolyte. LiFSI aids in stabilizing the electrolyte by trapping the PF5, i.e., sequestering the radical which tends to oxidize EC and DEC electrolyte solvents. Thus, oxidation is suppressed on the cathode, as evidenced by scanning electron microscopy (SEM), inductively coupled plasma-atomic emission spectrometry (ICP-AES) and X-ray photoelectron spectroscopy (XPS). As a result, HF generation is suppressed, which results in less Mn dissolution from the spinel cathode.

Keywords

Lithium

bis(oxalato)borate;

Lithium

bis(fluorosulfonyl)imide;

Non-aqueous electrolyte; Elevated temperature; Lithium manganese oxide spinel

2

1. Introduction Among all candidate cathode materials, spinel lithium manganese oxide (LiMn2O4) becomes a promising cathode material for the lithium-ion batteries due to its high safety, good rate capability and non-toxicity [1−4]. However, a significant weakness of LiMn2O4 is that the dissolution of Mn2+ at low cathode potentials and the deposition of metallic Mn by the hydrofluoric acid (HF) on the surface of the anodes are responsible for the fast capacity fading of the cells [5−6]. Presently, the most commonly used electrolytes are carbonate solvent blends with a lithium hexafluorophosphate (LiPF6, see Fig. 1a) salt. However, LiPF6 is thermally unstable and easy to decompose into undesired products as HF. The acidity of the electrolyte increases at elevated temperature, which has been shown to be particularly detrimental for the operation of elevated-temperature spinel cathodes [7−8]. To improve the electrolyte stability, efforts have been devoted to develop lithium salts and additives for commercial lithium-ion batteries [9−11]. Among them, lithium bis(oxalate)borate (LiBOB, see Fig. 1b) has attracted interest as one of the promising salts. Li et al. reported that LiBOB-EC/EMC/DEC electrolyte had good compatibility with LiFePO4, LiMn2O4 and LiNi0.5Mn1.5O4 cathodes [12]. Xu al. demonstrated that the oxidation of LiBOB on the cathode resulted in the generation of CO2 and a cathode passivation film containing borate oxalates, which inhibited oxidation of the bulk electrolyte at high potential (>4.8 V vs Li/Li+) [13]. Zhang et al. showed that thermal stability of fully charged Li1-xNi1/3Co1/3Mn1/3O2 cathode was improved in the presence of 0.5 wt.% LiBOB [14]. 3

It is reported that LiBOB aided in stabilizing the electrolyte by trapping the PF5, i.e., sequestering the radical which tends to oxidize EC and DEC electrolyte solvents [15−16]. As a result, HF generation is effectively suppressed. Lithium bis(fluorosulfonyl)imide (LiFSI, see Fig. 1c) is another promising salt for replacing the LiPF6 salt [17]. Recent papers indicate that battery electrolytes with LiFSI have very promising properties—i.e., a relatively high thermal and hydrolytic stability (relative to LiPF6), a high conductivity (comparable to electrolytes with LiPF6) and a wide liquidus range [18−21]. LiFSI outperforms LiPF6 [22] and exhibits good electrochemical performances when used as salt in nonaqueous carbonates solvents in half-cells: LiFePO4/Li [23], LiCoO2/Li [17], Li/graphite [23−24] or in the Li/Si cell [25]. Data collected with LiMn2O4/Li cells are, however, inadequate to identify the main degradation source of the elevated-temperature spinel. Therefore, the effect of the LiFSI and LiBOB additive on elevated-temperature spinel cathodes can be understood with LiMn2O4/Li cells. Toward this goal, we report here a systematic investigation of the impact of LiFSI and LiBOB additive on the performance of elevated-temperature LiMn2O4/Li cells. 2. Experimental 2.1 Preparation of the cells The commercial electrolytes (labeled Blank) of 1 M LiPF6 in a 1:1:1 (by weight ratio) EC/EMC/DEC was provided by Jiangxi Youli New Materials Co., Ltd. LiBOB (1 wt.%) and LiFSI (1 wt.%) were separately added in the commercial electrolyte, 4

which were prepared in an Ar-filled glove box. In the Ar-filled glove box the oxygen and water content were less than 1 ppm. The quantities of LiBOB and LiFSI were added to electrolytes by weight ratio on the paper. The spinel LiMn2O4 was obtained from Hunan Shanshan Toda Advanced materials Co., Ltd., China, and used without further treatment. The LiMn2O4 electrodes were prepared by mixing the LiMn2O4 powder (80 wt.%), poly(vinylidene fluoride) (PVDF, 10 wt.%) and acetylene black (AB, 10 wt.%) thoroughly followed by being dispersed in N-methyl pyrrolidinone (NMP). Subsequently, the mixed slurry was spread uniformly on a thin aluminum foil and dried in vacuum for 12 h at 120 ◦C. The counter and reference electrodes were lithium foils. The disc-like electrodes with a diameter of 14 mm were then punched out for assembling 2025 coin-type cells using the Celgard 2400 separator. 2.2 Measurements The water and free acid (HF) contents in the electrolyte were controlled to be less than 20 and 50 ppm, which were determined by Karl Fischer 831 Coulometer (Metrohm) and Karl Fischer 798 GPT Titrino (Metrohm), respectively. The charge-discharge behavior of the cells at room temperature (25 ◦C) and high temperature (55 ◦C) was tested on Newear tester (Shenzhen, China) at the constant current mode in the voltage range of 3.0−4.35 V. The cells were cycled firstly at 0.1 C three times, and then turned to 0.5 C for cycling performance test. The cyclic voltammetry (CV) and linear sweep voltammetry (LSV) was carried out on a Chenhua Instrumental Electrochemical Workstation (CHI604E) with a 5

three-electrode system incorporating LiMn2O4 as the working electrode and Li foils as counter and reference electrodes at the scanning rate of 0.1 mV s−1. The Pt electrode was made directly by sealing a Pt wire (10 mm diameter) in a glass tube. The plots are obtained at the scanning rate of 0.1 mV s−1 between 3.0 and 4.5 V vs. Li/Li+. All the electrodes subject to cyclic voltammetry measured with a similar electrode area in the same solutions and thus contain the same amount of active materials. Electrochemical impedance spectroscopy (EIS) analyses of the LiMn2O4/Li cells were performed under the full discharge state with an open circuit voltage of about 3.0 V. A sinusoidal amplitude modulation was used over the frequency range from 10−2 Hz to105 Hz, and the perturbation amplitude was 5 mV. For the Mn dissolution experiments, the LiMn2O4/Li half cells were cycled two times in the voltage range of 3.0−4.35 V and then fully charged in the third cycle. Afterwards, the cells were disassembled in an argon filled glove box, followed by opening the cells and recovering the LiMn2O4 electrodes. The obtained LiMn2O4 electrodes with 100% state of charge (SOC) were gently washed with fresh electrolyte and transferred to high density polypropylene bottles containing fresh electrolytes with the different electrolytes. The amount of 2 mL electrolyte was added to the dried electrodes and the closed bottles were stored in thermal chambers at 60 °C for 15 days. After the storage, each vessel was opened in a glove box to sample the electrolytes, and the resulting electrolytes were analyzed with inductively coupled plasma-atomic emission spectrometry (ICP-AES, IRIS intrepid XSP, Thermo Electron Corporation). To analyze the microstructure of the LiMn2O4 electrodes after charge-discharge 6

cycling measurements, the cells were disassembled in an argon-filled glove box. The LiMn2O4 electrodes were rinsed with pure DMC several times to remove the residual salts and dried in a vacuum oven at 60 ◦C for 4 h. The morphologies of the electrodes were visualized by scanning electron microscopy (SEM, JEOL, JSM-5600LV) with an accelerating voltage of 20 kV. A high-low temperature test-chamber (GDH-2005C) was used to provide a constant temperature environment for testing.

The chemical

components of the LiMn2O4 cathode surface were measured using X-ray photoelectron spectroscopy (XPS, K-Alpha 1063, Thermol Fisher). 3. Results and discussion 3.1 LSV of Pt electrode and LiMn2O4 electrode in different electrolytes A three-electrode system was used for linear sweep voltammetry measurement. Fig. 2 displays LSV curves using Pt electrode with the Blank (no additive), 1% LiBOB and 1% LiFSI, respectively. As we can see from Fig. 2, during the anodic potential sweeping, the oxidation reactions happen. The result indicates that the decomposition potential of 1 M LiPF6 electrolyte is a little lower than that of the electrolytes containing 1% LiBOB or 1% LiFSI. The decomposition current of the Blank starts to increase rapidly at 4.75 V vs. Li/Li+, while the decomposition current of the others grows more slowly until the potential reaches to higher than 5.60 V vs. Li/Li+. The results suggest that both the electrolytes containing 1% LiFSI and 1% LiBOB show better electrochemical stability than the Blank, especially for the electrolyte containing 1% LiFSI. Fig. 3 shows the first LSV curves of LiMn2O4 electrodes with different 7

electrolytes at 55 ◦C. As shown in Fig. 3, during the anodic potential sweeping, there are two obvious anodic peaks in each curve, which should be associated with the two-step process of lithium-ion extraction from LiMn2O4 cathode [26]. LiMn2O4 ⇌ Li1−χMn2O4 + χ Li+ + χ e−

(1)

Li1−χMn2O4 ⇌ Mn2O4 + (1−χ) Li+ + (1−χ) e−

(2)

With the potential increasing the current delivered by the cell containing 1% LiBOB additive is relatively low until the potential reaches up to 4.70 V vs. Li/Li+, which is a little higher than that of the Blank. While the potential of the two peaks in the cell containing 1% LiFSI additive is much higher than that of the cell without additive and it is a smaller current when the potential reaches about 4.80 V vs. Li/Li+. It may be attributed that the nature of FSI− has been shown to facilitate excellent lithium deposition/stripping on a lithium-metal anode [27−28]. From the results, it is confirmed

that

there

is

good

electrochemical compatibility between the

LiFSI-containing electrolyte and LiMn2O4 electrode in range of the working-voltage at elevated temperature. 3.2 CV measurements of LiMn2O4 electrode in the different electrolytes. Fig. 4 displays the cyclic voltammograms of LiMn2O4 electrode. As we can see from Fig. 4, there are two pairs of peak reflecting the typical oxidation processes of LiMn2O4 about 4.09 and 4.21 V vs. Li/Li+ and the reduction processes at 3.83 and 4.09 V vs. Li/Li+ in the Blank (1) and LiBOB-containing electrolyte (2). For the LiFSI-containing electrolyte (3), cathodic peaks are at 3.87 and 4.03 V vs. Li/Li+ which involves phase transitions. In addition, the peaks of reduction process in (3) are 8

stronger. What is more, the peak splitting of the cell with electrolyte (3) is not obvious between the two phase transitions. This confirms that Li+ can transfer more easily in the (3) electrolyte than in the others. 3.3 Cycling performance of LiMn2O4/Li cells Fig. 5 presents the impact of three electrolytes on cycling performance of LiMn2O4/Li cells under 25 ◦C at 0.5 C. It can be observed from Fig. 5a, the LiMn2O4/Li cell without additive initially delivered a discharge capacity of 118.5 mAh g−1 and the capacity retention is 88.9% after 100 cycles. The cell with LiBOB additive delivered a discharge capacity of 115.2 mAh g−1 and the capacity retention is 87.9.9% after 100 cycles. While the cell with LiFSI additive delivered a discharge capacity of 118.2 mAh g−1 and the capacity retention is 89.6% after 100 cycles. The discharge capacity increased after around the 20th cycle. This is believed to be indicative of electrolyte decomposition and growth of electrode/electrolyte interface films, which were effectively modified. Currently, such behavior could not be fully explained and is under investigation. The LiMn2O4 delivered the best battery performance with the LiFSI-containing electrolyte than others among in them. Fig. 5b compares the coulombic efficiency (CE) of the cells with different electrolytes. The cells with three electrolytes all show stable CE during cycling. While the initial CE of the cell with 1% LiBOB was 92.1%, which was lower than that without (95.6%) additive and with LiFSI (95.0%) additive. This can be partially attributed to a larger initial capacity loss during either reduction or oxidation of the LiBOB and LiFSI as indicated by the lower initial CE. 9

Cycling performance of the LiMn2O4/Li cells at 55 ◦C is summarized in Fig. 6. All cells underwent three 0.1 C formation cycles at 55 ◦C in the voltage range of 3.0−4.35 V. The cell with the Blank experienced rapid cell failure. The results revealed that the amount of Mn dissolution from the LiMn2O4 cathode increased with temperature. In contrast, cells with 1% LiBOB delivered ∼92.3% CE and improved capacity retention (∼82.3% after 100 cycles), and 1% LiFSI delivered ∼94.8% CE and improved capacity retention (∼85.2% after 100 cycles). While in the absence of additive, the cells show unstable CE during cycling. From above discussion, the cycling performances of the cells with LiFSI additive are better than the cells without and with LiBOB additive both at room temperature and 55 ◦C. 3.4 Rate capability analysis Fig. 7 shows discharge capacities vs cycle number of LiMn2O4/Li cells during different current rates from 0.1 C to 10 C (rate capability test) at 55 ◦C. The discharge capacities of the first cycle are similar for both salts (∼118 mAh g−1). Upon the first five cycles at 0.1 C, they decrease up to 113−115 mAh g−1. This decrease is slower with LiFSI than without additive and with LiBOB, in agreement with the capacity decrease already observed in Fig. 6a. This capacity decrease is due to the disproportionation of Mn3+ into Mn4+ and Mn2+ and the dissolution of Mn2+ [5−6]. During the following steps at higher cycling rates, we can observe a significant decrease of the capacities as a function of the applied current, and for the highest rates,

10

a rapid capacity fading as a function of the cycle number. However, we can notice that the rate capability is better with LiFSI than those of other samples. When a low current rate is applied again (0.1 C), the capacities of 107.1 mAh g−1 (no additive), 110.1 mAh g−1 (with LiBOB) and 111.7 mAh g−1 (with LiFSI) are recovered. Then in the following cycles the capacity stabilizes after 5 cycles at ∼107, 110.2 and 111.5 mAh g−1 without additive, with LiBOB and with LiFSI, respectively. Such capacity recovery suggests that the electrode material is not significantly damaged during fast cycling with both salts. Finally, this electrochemical test clearly shows the better capacity retention at high current rate when LiFSI salt is used. 3.5 Impedance analysis EIS is one of the most important, highly-resolved electroanalytical techniques that may provide unique information about the nature of electrode processes related to a wide range of time constants [29−30]. The impedance spectrum of Li2MnO4 electrodes in three electrolytes at discharged state before cycling and after 100 cycles at 55 ◦C are shown in Fig. 8. As shown in Fig. 8a (before cycling), EIS of the newly assembled cells are mainly composed of a semicircle at high frequency and medium frequency and a straight sloping line at the low frequency end. The EIS of cells with additive-containing electrolytes is very similar in the EIS of cell without additive electrolyte. It is suggested that the surface film is not formed yet, because of the absence of passive film resistance and its relative capacitance. AC of cells after cycling formed two overlapping semicircles at the high and medium frequency ranges, and a sloping line at the low frequency range. 11

Fig. 8b (after cycling at 55 ◦C) shows that the overall impedance of LiMn2O4/Li cells increase visibly. The interface-film resistances of cells without additive and with 1% LiBOB are much higher than that with 1% LiFSI. It is believed that LiFSI aids in stabilizing the electrolyte, sequestering the radical which tends to oxidize EC and DEC electrolyte solvents [22]. Consequently, lithium-ion could move more effectively in the cell with LiFSI at elevated temperature. It can be observed that the resistance

of the

solution increases,

which may be attributable

to the

disproportionation reaction [2Mn3+ → Mn2+ + Mn4+] and Mn2+ is soluble in the non-aqueous electrolyte and Mn4+ tends to stay in the cathode in a form of λ-MnO2 [5−6], while passive film resistance increases. Accordingly, the cycling performance of the cells is bad, just as it was previously commented. The EIS suggests that the Li+ ion diffusion in bulk LiMn2O4 proceeds quickly [31]. From the results, it is suggested that the electrolyte can not only influence the ohmic resistance but also influence the charge transfer resistance of the electrode. So we can conclude that 1% LiFSI salt electrolyte used in the LiMn2O4/Li cell has excellent filming property, especially at elevated temperature. Fig. 9 presents the impedance spectra of the LiMn2O4 cells containing three different electrolytes before and after three formation cycles at the charge potential status. As shown in Fig. 9a, the impedance spectra of the cells with additive is smaller than that of cell without additive. As we know, the formation of the SEI film will result in the decrease of the initial charge-discharge capacity, because HF can consume some lithium salts. When the LiMn2O4 cells were charge-discharged after 12

three formation cycles, the change of the impedance spectra of the cells with additive is smaller than that of control cell (Fig. 9b), which can also indicate that the SEI of the cell with additive is thinner than that of the cell without additive. The ring-opening reaction of EC is catalyzed by the PF5, which leads to its polymerization into poly(ethylene carbonate) (PEC) or poly(ethylene oxide) (PEO)-like products. When an additive was added into the electrolyte, EC was effectively suppressed to product to PEC or (PEC)-like products. It is concluded that LiBOB and LiFSI as an additive participate in the formation process of the cathode electrolyte interface film on LiMn2O4 cathode surface, which is effectively suppressed to oxidize the Blank electrolyte solvents including EC and DEC and improves the capacity retention of cells. 3.6 Impact of LiBOB and LiFSI on the electrolyte stability Fig. 10 compares the amounts of Mn dissolution from the LiMn2O4 electrodes stored in the Blank electrolyte (no additive), electrolyte with 1% LiBOB and electrolyte with 1% LiFSI. For the Blank electrolyte and the 1% LiBOB added electrolyte, the Mn concentrations were respectively 205 and 159 µg L−1. In contrast, the Mn concentration decreased to 142 µg L−1 for the 1% LiFSI added electrolyte. The reduction of dissolved Mn during storage is believed to be related to the role of additive sequestering the PF5, which is attributed to the following reaction at elevated temperatures: LiPF6 ↔ LiF + PF5

(3)

H2O + PF5 → POF3 + HF

(4) 13

H2O + POF3 → PO2F + 2 HF

(5)

The PF5 is an initiator for electrolyte oxidation, resulting in the production of HF with H2O reaction pathways. In addition, the PF5 initiates the decomposition reactions of EC and DEC as described in Scheme 1 [15]. The ring-opening reaction of EC is catalyzed by the PF5, which leads to its polymerization into poly(ethylene carbonate) (PEC) or poly(ethylene oxide) (PEO)-like products [32−33]. Kawamura et al. [34] demonstrated that DEC electrolyte decomposition reactions resulting from PF5 attacked produce HF by several side reaction steps. The resulting HF can dissolve the Mn from LiMn2O4 spinel oxide, as reported in the literature [7]. Han et al. [18] presented that the stability of LiFSI toward hydrolysis was better than LiPF6. Eshetu et al. [22] reported that the positive attributes of LiFSI salt in avoiding Lewis acid and lowering the HF release upon heating and combustion respectively. By reducing the reactivity of PF5, the LiFSI additive could effectively suppress HF formation, decreasing the amount of Mn dissolution. Thus, the decrease in Mn dissolution due to the presence of LiFSI additive improves the good cyclability of LiMn2O4/Li cells as shown in Figs. 5 and 6. 3.7 Self-discharge rate analysis In

this

section,

another

possible

improvement

mechanism

of

the

electrode/electrolyte interface films will be discussed. The degree of electrolyte oxidation can be estimated by measuring self-discharge of the LiMn2O4 spinel electrodes. The self-discharge of the LiMn2O4 electrode occurs as a result of electrolyte oxidation as follows: 14

Mn2O4 + χ Li+ + χ electrolyte → LiχMn2O4 + χ electrolyte+

(6)

Therefore, the amount of self-discharge, resulting from the amount of electrolyte oxidation, will occurs at a given storage time. The LiMn2O4/Li cells were firstly cycled two cycles and then charged to 4.35 V, followed by storing for 15 days at 60 °C. The variation of open-circuit voltage (OCV) of the cells with storage time is shown in Fig. 11b. The cell without additive showed rapid OCV drop, which indicated its fast relithiation via self-discharge (Eq. 6) compared with the additive-containing electrolytes. In Fig. 11c, the cell without additive exhibited lower discharge capacity than that of the cells with additive due to the high self-discharge capacity. The amount of the self-discharge was determined by subtracting the capacity value from normal cycling. As the previous discussion in Fig. 9, FSI− and BOB− anion would help to form the stable SEI and reduce the impedance spectra of cells, which are attributed to effectively suppress the Blank electrolyte solvents including EC and DEC oxidized. The results demonstrated that the stable SEI can inhibit Mn dissolution from HF induced reaction and improves the capacity retention of cells. Dalavi et al. [35] proposed that oxidation of the LiBOB alters the surface chemistry of the cathode by forming thinner surface films containing oxalate species. Li et al. [24] found that the binding energies for LiFSI and LiPF6 increase in the order of LiFSI
The SEM images of LiMn2O4 electrodes 100 cycled at 55 ◦C with three electrolytes are shown in Fig. 12. In Fig. 12 (d) 1kx (d1)10kx, the covering of film on the cathode surface containing LiFSI electrolyte is still compacted, while the covering of film on the cathode surface is more uncompacted after 55 ◦C cycling in Blank electrolyte (Fig. 12(b) 1kx (b1) 10kx) or electrolyte containing 1% LiBOB (Fig. 12(c) 1kx (c1)10kx). On the surface of the electrode in Blank cell, there is very rough and the electrode was broken. The covering becomes more irregular. The cathode surface in 1% LiBOB electrolyte is also destroyed on the surface. While the cathode surface in 1% LiFSI electrolyte is maintained relatively smoother. This indicates that LiFSI-containing electrolyte slightly erodes the electrode, and the higher capacity is agreed with Fig. 6. The cycling stability of the cells may be caused by the corrosion of aluminum (Al) current collector [36], which may be more likely to happen at high temperatures and will lead to apparent effects on the cell performance. The SEM images of Al current collectors are shown in Fig. 13. It is clear that the Al current collector for the electrolyte of LiPF6 before cycled (Fig. 13a) is smooth and the one after cycled at 55 ◦C is damaged by the large pitting-corrosion holes (Fig. 13b) occurring of the surface of Al. The corrosion on the surface of Al for the electrolytes of LiBOB is roughness and some spheroid protrusions (Fig. 13c), whereas the Al current collector for the electrolyte (Fig. 13d) is slightly corroded with some ravings appearing. Han et al. [17] stated that LiFSI was not an “aggressive” salt towards Al, and the passivation of Al could be achieved in the LiFSI-based electrolyte solutions. Only Cl− 16

ions are up to 50 ppm in the LiFSI-based electrolytes will Al be seriously corrode. This indicates that LiFSI-containing electrolyte slightly erodes the Al current collector because of Cl− < 50 ppm in the electrolyte with 1% LiFSI. 3.9 XPS analysis The X-ray photoelectron spectroscopy (XPS) spectra of the LiMn2O4 electrode in 1 M LiPF6-EC/DEC/EMC (1:1:1, weight ratio) without and with additive after 100 cycles at 55 ◦C are shown in Fig. 14. The C 1s, O 1s, F 1s, Mn 2p, B 1s, N 1s and S 1s XPS spectra of the cycle cathode with and without additive are analyzed. The C 1s spectra (see Fig. 14a) shows slight differences between the cathode cycled without additive and LiBOB additive, whereas the intensities of C−C, C−H and C=O bonds observed on the cathode with LiFSI additive are much weaker than that of others. This indicates that less R-CH2OCO2-Li or polycarbonates were formed on the surface of the LiMn2O4 electrode containing LiFSI additive. In addition, the intensity of Li2CO3 observed on the surface of the LiMn2O4 electrode containing LiFSI additive is lower than that of others, which suggests that the electrode cycled with the LiFSI additive was covered with less inorganic degradation products. Difference are observed in O 1s spectra with or without additive shown in Fig. 14b. With addition of LiFSI in the electrolyte, the relative intensity of the C-O decreases and C=O increases. This means the LiFSI can hinder the polymerization of EC and DEC solvent efficiently, which is agreed with “3.6” section. The F 1s peak corresponds to the C−F (687.7 eV) in the PVDF binder and the peak (684.5 eV) is assigned to be LiF [37]. The peak of LiF of the LiMn2O4 electrode 17

cycled with LiFSI shows slightly lower intensity than that of others, which indicates less compact and resistive substance is formed on the electrode surface [38−39]. The improved battery performance with LiFSI-containing electrolyte can be attributed to the higher ionic film formed on the LiMn2O4 surface with lower content of LiF, which improves charge transport at the electrode active particles/liquid electrolyte interface [40]. As a result, the capacity cycle of a cell during following cycles is enhanced. The Mn 2p spectra of the electrodes after 100 cycles at 55 ◦C are shown in Fig. 14d. There are Mn3+ (641.7 eV), Mn4+ (642.9 eV) and Mn 2p1/2 (653.6 eV) peaks in Mn 2p spectra [41]. It is shown in Fig. 14d that the intensities of both Mn3+ and Mn4+ peaks observed on the surface of the LiMn2O4 electrode cycled with LiFSI additive are stronger than that of others. That is to say, the HF induced metal dissolution can be suppressed. To further confirm the fact that LiBOB and LiFSI participated in the formation of SEI on the surface of the cathode, the B 1s, S 1s and N 1s spectra of the electrode with LiBOB and LiFSI were measured (see Fig. 14e, f and g). It is clearly observed that there are two B−O (191.6 eV, 194 eV) bonds in the B 1s spectra [42], N−C bond (399.5 eV) and N=C (401.3 eV) bond in the N 1s spectra [43], respectively. Moreover, the S 2p spectra recorded after discharge (see Fig. 11) have revealed the presence of a degradation compound with an S 2p3/2 peak at 169 eV and 170.2 eV [25]. These results indicate that the LiBOB and LiFSI additive participated in the formation of SEI. The improvement of SEI will reduce the dissolution of Mn2+ into electrolyte, resulting in good cycle performance of the battery. The similar results on the 18

improvement of the cyclability and capacity retention of lithium-ion batteries were also reported [15, 17 and 25]. The results, obtained from XPS and SEM, indicate that the addition of LiFSI is help to form a stable and favorable SEI film, which accounts for the excellent cycleability of the LiMn2O4/Li. The results are consistent with those of the cyclic voltammetry and constant current charge-discharge study. 4. Conclusions In this study we have shown the beneficial role of LiFSI salt and LiBOB salt to improve the electrochemical performances of LiMn2O4/Li cells. This beneficial role is mainly attributed to stabilize the electrolyte and the LiMn2O4 electrode against the degradative effects of Mn dissolution. 1. LiBOB and LiFSI additive in the electrolyte was preferentially oxidized relative to solvents (EC and DEC) during cycling. By reducing the reactivity of PF5 of electrolyte oxidation, the LiFSI additive could more effectively suppress HF formation than the LiBOB additive. 2. Mn concentration reduced from 205 µg L−1 to 142 µg L−1 for the 1% LiFSI added electrolyte after storing the LiMn2O4 electrode at 60°C for 15 days. While Mn concentration reduced from 205 µg L−1 to 159 µg·L−1 for the 1% LiBOB added electrolyte. The LiFSI may have better suppressed HF generation than the LiBOB. 3. SEM images suggested that the SEI film on the LiMn2O4 electrode surface with LiBOB and LiFSI additive was detected. Compared with the Al current 19

collector for without additive and with LiBOB additive, the Al current collector for the LiFSI-containing electrolyte is slightly corroded with some ravings appearing. XPS indicate that the addition of LiFSI is help to form a stable and favorable SEI film. Acknowledgments This work is financially supported by the National Basic Research Program of China (973 Program, 2014CB643406). References [1] M.M. Thackeray, W.I.F. David, P.G. Bruce, J.B. Goodenough, Mater. Res. Bull. 18 (1983) 461−472. [2] P. Bruce, B. Scrosati, J.M. Tarascon, Angew. Chem., Int. Ed. 47 (2008) 2930. [3] Y.L. Ding, J. Xie, G.S. Cao, T.J. Zhu, H.M. Yu, X.B. Zhao, J. Phys. Chem. C 115 (2011) 9821−9825. [4] Y. Wu, Z. Wen, H. Feng, J. Li, Small 8 (2012) 858−862. [5] D.C. Tang, Y. Sun, Z.Z. Yang, L.B. Ben, L. Gu, X.J. Huang, Chem. Mater. 26 (11) (2014) 3535–3543. [6] D.R. Gallus, R. Schmitz, R. Wagner, B. Hoffmann, S. Nowak, I. Cekic-Laskovic, R.W. Schmitz, M. Winter, Electrochim Acta 134 (2014) 393-398. [7] K. Xu, Chem. Rev. 104 (2004) 4303−4417. [8] Y.J. Liu, X.H. Li, H.J. Guo, Z.X. Wang, W.J. Peng, Q.Y. Hu, Y. Yang, J. Power Sources 189(1) (2009) 721−725. [9] R. J. Chen, L. Zhu, F. Wu, L. Li, R. Zhang, S. Chen, J. Power Sources 245 (2014) 20

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23

Figure captions Scheme 1 Decomposition reactions of EC and DEC catalyzed by PF5. Fig. 1 Chemical structures of (a) LiPF6, (b) LiBOB and (c) LiFSI. Fig. 2 LSV of Pt electrode using the Blank electrolyte, 1% LiBOB and 1% LiFSI additive electrolyte, at a scan rate 0.1 mV s−1. Fig. 3 LSVs of LiMn2O4 electrode using the Blank electrolyte, 1% LiBOB and 1% LiFSI additive electrolyte, at a scan rate 0.1 mV s−1. Fig. 4 CV curves of LiMn2O4 electrode using the Blank electrolyte, 1% LiBOB and 1% LiFSI additive electrolyte. The electrode is fresh. Fig. 5 (a) Cycle lives and (b) coulombic efficiencies of the LiMn2O4/Li cells at 0.5 C in the voltage range of 3.0−4.35 V at 25 ◦C. The interval between charge and discharge (rest time) was 5 min. Fig.6 (a) Cycle lives and (b) coulombic efficiencies of the LiMn2O4/Li cells at 0.5 C in the voltage range of 3.0−4.35 V at 55 ◦C. The interval between charge and discharge (rest time) was 5 min. Fig. 7 Rate performance of LiMn2O4/Li cells with different electrolytes between 3.0 and 4.35 V during different current rates from 0.1 C to 10 C at 55 ◦C. Fig. 8 Impedance spectrum of the LiMn2O4 cells containing the different electrolytes (a) before and (b) after 100 cycles at 55 ◦C. Fig. 9 Impedance spectrum of the LiMn2O4 cells containing the different electrolytes (a) before and (b) after three cycles at the charge potential of 4.35 V. Fig. 10 Mn dissolution amount in the electrolyte with and without additive after 24

storing LiMn2O4 electrodes at 60 ◦C for 15 days. Fig. 11 Self-discharge behaviors of LiMn2O4 with the Blank, 1% LiBOB and 1% LiFSI additive. The experiments were performed by measuring voltage profiles of LiMn2O4/Li cells: (a) charge, (b) resting for 10 days at fully charged state, and (c) following discharge at room temperature (25 ◦C). Fig. 12 SEM images of (a) pristine LiMn2O4 electrode (before cycling), ((b) 1kx, (b1) 10kx) 100 times cycled LiMn2O4 electrode with the Blank electrolyte (no additive), ((c) 1kx, (c1) 10kx) 100 times cycled LiMn2O4 electrode with 1% LiBOB additive and ((d) 1kx, (d1) 10kx) 100 times cycled LiMn2 O4 electrode at 55 ◦C with 1% LiFSI additive. Fig. 13 SEM images of the aluminum (Al) current collector after 100 cycles at 55 ◦C. Before the measurement, the sample was well rinsed by DMC in the glove box, and then dried at 60 ◦C under vacuum for 4 h. (a) fresh, (b) LiPF6, (c) LiBOB and (d) LiFSI. Fig.14 XPS spectra of LiMn2O4 electrode after 100 cycles at 55 ◦C in 1 M LiPF6-EC/DEC/EMC (1:1:1, weight ratio) without and with additive: (a) C 1s, (b) O 1s, (c) F 1s, (d) Mn 2p, (e) B 1s, (f) N 1s and (g) S 2p spectra.

25

Figure 1

(a)

(b)

(c)

26

Figure 2 -6

1.0x10

-7

Current (A)

8.0x10

Blank 1% LiBOB 1% LiFSI

-7

6.0x10

-7

4.0x10

-7

2.0x10

0.0 3.0

3.5

4.0

4.5

5.0

5.5 +

Potential (V vs. Li/Li )

27

6.0

Figure 3

Current (A)

0.0006

Blank 1% LiBOB 1% LiFSI

0.0004

0.0002

0.0000 2.5

3.0

3.5

4.0

4.5

5.0

5.5 +

Potential (V vs. Li/Li )

28

6.0

Figure 4 (1)Blank

0.0004

(2)1% LiBOB (3)1% LiFSI

Current (A)

0.0002 0.0000

(2)

-0.0002 (1) (3)

-0.0004 2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2 +

Potential (V vs. Li/Li )

29

4.4

4.6

Figure 5

30

Figure 6

31

Figure 7

32

Figure 8

33

Figure 9

34

Figure 10

-1

Concentration (ug L )

250 200 150 100 50 0

Blank

1% LiBOB

35

1% LiFSI

Figure 11 a (1) Blank (2) 1% LiBOB (3) 1% LiFSI

4.4 4.2

Voltage (V)

4.0 1, 2, 3

4.3

Voltage (V)

3.8 3.6

4.2

4.1

3.4

60

80

100

120 -1

Discharge capacity (mAh g

3.2 0

20

40

60

80

100

)

120

140

-1

Discharge capacity (mAh g )

b

4.4

Blank 1% LiBOB 1% LiFSI

4.2

4.1 0

2

4

6

8

10

Time (day)

c Blank 1% LiBOB 1% LiFSI

4.2 4.0

Voltage (V)

Voltage (V)

4.3

3.8 3.6 3.4 3.2 3.0 0

20

40

60

80

100 -1

Discharge capacity (mAh g )

120

Figure 12 a

10μm

b

b1

10μm

c

1μm

c1 ’

10μm

d

1μm

d1

10μm

1μm

Figure 13 fresh

a

b

5µm

c

55 ◦C

5µm

55 ◦C

d

5µm

5µm

38

55 ◦C

Figure 14 a

Blank LiBOB LiFSI

C-C

C 1s

b C=O

O 1s

C-H Li2CO3 C=O

296

292

288

C-O

Intensity (a.u.)

Intensity (a.u.)

Li2CO3

284

280

Mn-O

Blank LiBOB LiFSI

545

540

Binding energy (eV)

c

Intensity (a.u.)

PVDF

690

688

686

684

525

Blank LiBOB LiFSI

4+

Mn

Mn 2p1/2

LiF

692

3+

Mn

Mn 2p

LiPxOyFz

694

530

Intensity (a.u.)

LiPxFy

d

Blank LiBOB LiFSI

F 1s

535

Binding energy (eV)

682

680

660

655

Binding energy (eV)

650

645

640

635

Binding energy (eV)

f

e

N 1s B 1s

Intensity (a.u.)

Intensity (a.u.)

B-O

196

192

188

LiFSI

Intensity (a.u.)

LiFSI

168

164

404

400

396

Binding energy (eV)

g S 2p

N=C

408

184

Bingding energy (eV)

172

N-C

LiBOB

160

Bingding energy (eV)

39

392

Scheme 1.

40

Highlights

(1) Compared to conventional LiPF6, the electrochemical cycling properties of LiMn2O4 with the additive-containing electrolytes show much improved at 55 °C.

(2) The addition of LiBOB or LiFSI inhibits detrimental reaction of the electrolyte on the surface of LiMn2O4 cathodes at high temperature, and LiFSI performs better than LiBOB in this aspect.

(3) The 1% LiFSI-containing electrolyte can participate in the formation of SEI films by XPS and increase electrochemical stability and compatibility with LiMn2O4 cathode.

41