Structure and cycle stability of SrHPO4-coated LiMn2O4 cathode materials for lithium-ion batteries

Electrochimica Acta 145 (2014) 201–208

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Structure and cycle stability of SrHPO4 -coated LiMn2 O4 cathode materials for lithium-ion batteries Xiusheng Zhang, Yunlong Xu ∗ , Huang Zhang, Chongjun Zhao, Xiuzhen Qian Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China

a r t i c l e

i n f o

Article history: Received 19 May 2014 Received in revised form 7 July 2014 Accepted 6 August 2014 Available online 23 August 2014 Keywords: Lithium manganese oxide Coating Manganese dissolution Elevated temperature performance

a b s t r a c t The SrHPO4 -coated LiMn2 O4 composite materials are prepared through co-precipitation method. The phase structures and morphologies of pristine and SrHPO4 coated LiMn2 O4 are characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The cycling performances are thoroughly investigated and discussed both at room and elevated temperature. The results indicate that 2.0wt% SrHPO4 coated LiMn2 O4 can efficiently improve the cycling performance with capacity retention of 92.3% and 83.6% under room temperature (25 ◦ C) and elevated temperature (55 ◦ C) after 100 cycles at 1 C rate, respectively, which are much better than those of the pristine materials. The CV, EIS and XRF measurements reveal that the enhanced stabilization in the cycling performance can be attributed to the suppression of manganese dissolution into electrolyte with the contribution of SrHPO4 coating on the surface of LiMn2 O4 . © 2014 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, lithium-ion batteries with high energy density and capacity have attracted widespread interest due to their potential applications in both hybrid electric vehicles (HEV) and full electric vehicles (EV) [1–3]. The spinel LiMn2 O4 has been regarded as the most prospective material for the rechargeable lithiumion batteries due to its merits of easy preparation, environmental friendly, low cost and natural abundance compared to other cathode materials [4,5]. However, the performance of the material is not ideal enough to be used as a commercial cathode, and many research groups have found that it suffers from severe capacity fading during charge/discharge cycles, especially at elevated temperature (50∼60 ◦ C). The capacity loss has been ascribed to several factors such as (i) Jahn-Teller distortion [6], (ii) the dissolution of manganese ions into the electrolyte [7], (iii) lattice instability [8], and (iv) particle size distribution [9]. In order to overcome its capacity fading, many methods have been taken into account. Among them, the substitution with heterogeneous ions into the lattice and surface coating are more promising to put into reality. The capacity fading mechanism at room temperature has been proved to be correlated with the

∗ Corresponding author. Tel.: +86 021 64252019; fax: +86 021 64250838. E-mail address: [email protected] (Y. Xu). http://dx.doi.org/10.1016/j.electacta.2014.08.043 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

Jahn-Teller distortion caused by the presence of Mn3+ [6], which can be minimized with the substitution of a small fraction of transition metals in the 16d sites [10–12]. Despite the improvement of cycle performance at room temperature by doping with transition metals such as Cr [13], Ni [14], Mg [15], Al [16], etc, LiMn2 O4 still suffered from significant capacity attenuation at elevated temperature. Researches show that the main reason was associated with Mn dissolution induced by HF acid, which was generated from temperature-enhanced electrolyte decomposition [17–19]. Therefore, surface coating has been introduced to suppress Mn dissolution. In previous works, LiMn2 O4 has been modified by surface coating with various materials like Al2 O3 [20], LiTi5 O12 [21], ZrO2 [22], Li2 O-2B2 O3 [23], Li2 ZrO3 [24], Li3 PO4 [25] and AlPO4 [26]. These studies showed that the capacity retention of LiMn2 O4 at elevated temperature was significantly improved after coating with thin layers of the electrochemically inert metal oxides, which is attributed to the minimization of contact interfaces between LiMn2 O4 /electrolyte. Overcoming the major problem of capacity fading requires a better understanding of the mechanisms of manganese dissolution, migration and deposition. Moreover, Li ion penetration through the coated layer will be affected with the coating materials. Therefore, further research is very necessary to look for an excellent coating material to enhance the cycling performance of LiMn2 O4 , particularly at elevated temperature. SrHPO4 has been claimed as a catalyst, proton conductor, and surface conditioner as well as for application in batteries, fuel cells,

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for flame proofing, and thermal cathodes because of its various advantages such as environmental friendliness, low cost and thermal stability [27–30], which makes it a desirable coating material. J. Kim [30] et al. has reported the SrHPO4 -coated LiCoO2 cathode shows improved cycle life and higher Li diffusivity compared with the uncoated one, indicating that the formation of an electrically resistive film was suppressed. But, the systematic study about SrHPO4 -coated LiMn2 O4 has not been reported yet. In this paper, the various contents of SrHPO4 -coated LiMn2 O4 were successfully synthesized via co-precipitation process. The effects of surface modification with SrHPO4 on the electrochemical performance and structural stability of LiMn2 O4 cathode material were sufficient investigated both at room and elevated temperatures. 2. Experimental 2.1. Prepare of bare LiMn2 O4 Bare LiMn2 O4 powders were synthesized via a sol-gel method using C6 H5 Li3 O7 •4H2 O (GR) and C4 H6 MnO4 •4H2 O (AR) as the starting materials. Stoichiometric amounts of the precursors were stirred and dissolved in deionized water. After thorough dissolved, the solution was dried in a microwave oven (2.5 GHz, 500 W) until a transparent gel was obtained. Finally the gel precursor decomposed at 350 ◦ C for 3 h followed by calcinations at 800 ◦ C for 10 h in an ambient atmosphere, and then, the pure LiMn2 O4 powders were obtained. 2.2. synthesis of SrHPO4 -coated LiMn2 O4 To obtained the SrHPO4 -coated LiMn2 O4 , Sr(NO3 )2 (AR) and (NH4 )2 HPO4 (AR) were selected as raw materials for the coating layers. The synthesis process was as followed: Sr(NO3 )2 was first dissolved into deionized water, then the as-synthesized bare LiMn2 O4 powders were incorporated into the solution. The weight ratio of Sr(NO3 )2 to LiMn2 O4 powder was fixed to 0.5, 1.0, 2.0 and 3.0wt%, respectively. The mixture solution was ultrasonically agitated for 30 min followed by vigorous stirring for 1 h to obtain a suspension. While a stoichiometric amount of (NH4 )2 HPO4 solution was slowly added into the suspension with vigorous stirring and aged at room temperature for 3 h. The final powder was filtered and washed with deionized water, dried at 120 ◦ C for 2 h and followed by heat-treating at 500 ◦ C for 5 h in ambient atmosphere to obtain SrHPO4 -coated LiMn2 O4 material. 2.3. Characterization The phase identification of powders was conducted with Xray diffraction measurement (XRD, D/MAX 2550 V, Japan) using Cu K␣ radiation (␭=0.15418 nm). The morphology was evaluated by field emission scanning electron microscopy (FESEM, Hitachi S4800, Japan) and field emission transmission electron microscopy (FETEM, JEM-2100, Japan). The electrochemical behaviors measurements were carried out using CR2032 coin cell. The cathode slurries were prepared by dispersing 80 wt% active material, 10 wt% acetylene black (AB, Shanghai Haohua Chemical Co. Ltd.) and 10wt% polyvinylidene fluoride (PVDF, Shanghai Ofluorine Chemical Technology Co. Ltd.) in N-methyl-2-pyrrolidone (NMP, Shanghai Lingfeng Chemical Reagent Co. Ltd.) solvent and coated onto Al foil, then dried in a vacuum overnight. The load of active material in the working electrodes was 1.625 mg/cm2 . Metallic lithium foil was used as the anode electrode, 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1, V/V) (Guangzhou Tinci Materialt Technology Co. Ltd.) as the electrolyte. The cells were assembled in an Argon-filled gloved box (Super 1220/750, Mikrouna China Co.

Fig. 1. XRD patterns of (a) pristine, (b) 0.5wt%, (c) 2.0wt%, (d) 3.0wt%, (e) 3.0wt% SrHPO4 -coated LiMn2 O4 and isolated SrHPO4 . Table 1 Lattice parameters of the samples obtained from the XRD patterns. sample

Pristine

0.5wt%

1.0wt%

2.0wt%

3.0wt%

Lattice parameter/Å

8.2305

8.2311

8.2301

8.2308

8.2313

Ltd.). Galvanostatic charge/discharge measurement were carried out in the voltage range of 3.0∼4.3 V using a Land CT2001 battery program-control cell tester (Wuhan Land Electronic Co. Ltd., China) at room temperature (25 ◦ C) and elevated temperature (55 ◦ C) at a current density of 1 C rate. Cyclic voltammetry (CV) was measured by an electrochemical working station (CHI660D, Shanghai Chenhua Co. Ltd., China) at scan rate of 0.1mVs−1 between 3.0 and 4.4 V. Electrochemical impedance spectroscopy (EIS) was potentiostatically conducted on the electrochemical working station with an AC oscillation of 5 mV amplitude over the frequencies between 105 and 10−2 Hz. To investigate the effect of the SrHPO4 coating layer on decreasing dissolution of Mn from the surface of LiMn2 O4 into electrolyte, the lithium anodes were used as counter electrodes of the pristine and the 2.0wt% SrHPO4 -coated LiMn2 O4 were examined by X-ray Fluorescence (XRF) after 100th cycles at 25 ◦ C and 55 ◦ C, respectively. The cell we adopt in this experiment is a removable battery model, the resulted lithium anodes was dried under vacuum after cycles and analyzed using an X-ray Fluorescence to determine the amounts of Mn-containing complexes deposited in the SEI layer on the surface of lithium anode. 3. Results and Discussion 3.1. structure and morphology The X-ray diffraction patterns of the pristine LiMn2 O4 , 0.5wt%, 1.0wt%, 2.0wt% and 3.0wt% SrHPO4 -coated LiMn2 O4 are shown in Fig. 1(a∼e). All of the diffraction peaks correspond well to a cubic spinel structure with space group Fd3 m. It implies that the bulk crystal structure of LiMn2 O4 has not been destroyed after surface modification. No diffraction peaks from SrHPO4 are observed in the XRD patterns, suggesting that the content of SrHPO4 may be too low to be detected. Previous researches have been displayed that substitution of Mn3+ in LiMn2 O4 with transition mental ions will significantly change the lattice parameters and cause the peak shift [31–33]. Table 1 shows the lattice parameters calculated from the characteristic XRD peaks, and no significant changes in lattice parameter and peaks shift were observed after SrHPO4 coating. The

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Fig. 2. FE-SEM images of (a) pristine, (b) 0.5wt%, (c) 1.0wt%, (d) 2.0wt% and (e) 3.0wt% SrHPO4 -coated LiMn2 O4 .

XRD diffraction pattern of the isolated SrHPO4 after heat-treatment at 500 ◦ C was showed in Fig. 1. The XRD diffraction pattern of the asprepared material indicates that SrHPO4 represents in a pure phase, and there was no impurity phase observed in the XRD pattern of the SrHPO4 -coated LiMn2 O4 , which shows that SrHPO4 would not change to other forms like its oxides [30]. The morphologies of pristine and SrHPO4 -coated LiMn2 O4 were presented in Fig. 2. It can be clearly observed that there is no significant particle size difference between the bare and SrHPO4 -coated LiMn2 O4 . However, the surface morphology of the particles has changed after coating with fuzzy surface and edges occurring and more fragments growing on the surface, which of pristine LiMn2 O4 is smooth. And also it is obvious that the aggregation occurs when the coating content increases. Comparison of SEM images of all the as-prepared samples suggests that the Sr-contained layer exists on the surface of LiMn2 O4 particles which causes the surface changing after coating. In order to confirm it, EDAS analyses in the selected

region were performed as show in Fig. 3(a) and (b). Sr can be clearly detected on the surface of the coated LiMn2 O4 , suggesting that SrHPO4 may exist in the surface-treated LiMn2 O4 . The accurate content of SrHPO4 in the four coated LiMn2 O4 samples obtained by ICP-AES measurement are 0.48%, 0.95%, 1.97% and 2.93%, respectively. Nevertheless, the coated samples are still called 0.5, 1.0, 2.0 and 3.0wt% SrHPO4 -coated LiMn2 O4 in the following discussion. The FE-TEM images of the pristine and 2.0wt% SrHPO4 -coated LiMn2 O4 were exhibited in Fig. 3(c) and (d) to further clarify the microstructure. These images indicate that SrHPO4 coating forms over the pristine LiMn2 O4 particles with an average thickness of around 15 nm and the SrHPO4 -coated layer is structurally loose. Since the structure of coated layer forms a novel micropore structure, Li ions can easily penetrate the coated layer into the spinel during the process of charge or discharge. Furthermore, the surface modification of LiMn2 O4 with SrHPO4 could decrease the direct contact area between active materials and electrolyte, which will

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Fig. 3. EDAS results of (a) pristine and (b) 2.0wt% SrHPO4 -coated LiMn2 O4 ; FE-TEM images of (c) pristine and (d) 2.0wt% SrHPO4 -coated LiMn2 O4 .

Fig. 4. The initial charge/discharge curves of pristine and 0.5, 1.0, 2.0, 3.0wt% SrHPO4 -coated LiMn2 O4 .

accordingly prevent Mn dissolving from active materials into electrolyte and achieve better cyclic performance. 3.2. Electrochemical behavior Fig. 4 shows the initial charge/discharge profiles of pristine and surface treated LiMn2 O4 sample at current density of 148mAhg−1 (1 C) between the potential rang 3.0-4.3 V (vs Li/Li+ ) at room temperature. Typically, it can be obviously seen that all the samples display two voltage plateaus in the potential region of 3.9-4.2 V that can be ascribed to the remarkable characteristic of welldefined spinel LiMn2 O4 . The voltage plateaus indicate that the

insertion and extraction of lithium ions occur in two states [34]. The pristine spinel exhibits a capacity of 116.3mAhg−1 , and the capacities of 0.5wt%, 1.0wt%, 2.0wt%, 3.0wt% coated LiMn2 O4 are 116, 115.8, 115.2, 110.9 mAhg−1 , respectively. Moreover, it also can be observed that the initial discharge capacity declines slightly with the amount of coating increased to 3.0wt%. It is mainly caused by the addition of electrochemically inert SrHPO4 coating to the active spinel LiMn2 O4 . The effect of surface modification of pristine LiMn2 O4 on the cycle performance under 1 C charge/discharge rate at both room temperature (25 ◦ C) and elevated temperature (55 ◦ C) are exhibited in Fig. 5. The first and 100th cycle discharge capacities and capacity retention ratios of SrHPO4 -coated and pristine LiMn2 O4 are summarized in Table 2. As shown in Fig. 5a, the pristine LiMn2 O4 possess an initial capacity of 116.3mAhg−1 which decays to 90.5mAhg−1 after 100 cycles with the retention of 77.8%. Oppositely, the SrHPO4 -coated LiMn2 O4 samples significantly display improved cycling behavior. The 0.5wt%, 1.0wt%, 2.0wt%, 3.0wt% coated LiMn2 O4 show the initial discharge capacities of 116, 115.8, 115.2 and 110.9mAhg−1 with capacity retention of 83.2%, 88.7%, 92.3% and 90.1%, which proves that the coating of SrHPO4 could improve the cycle stability of LiMn2 O4 . Additionally, among all the samples, the 2.0wt% SrHPO4 coated materials possess the superior cycling performance. When the coating content increases from 2.0wt% to 3.0wt%, the cycling performance is deteriorating. This manifests that the SrHPO4 coating can facilitate the diffusion of lithium ions for an appropriate content. As the SrHPO4 content increases over 2.0wt%, the excessive coating will hinder the transportation of lithium ions, leading to the evident decay of the specific capacity. From the as-presented cycle performance shown in Fig. 5b, it can be clearly observed that the capacity fading of the pristine spinel is accelerated at elevated temperature (55 ◦ C),

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Table 2 Electrochemical capacities and retention ratios of pristine and SrHPO4 -coated LiMn2 O4 cathode electrodes. sample

pristine 0.5wt% coated 1.0wt% coated 2.0wt% coated 3.0wt% coated

25 ◦ C

55 ◦ C

Initial discharge capacity/mAhg−1

100th discharge capacity/mAhg−1

Retention rate/%

Initial discharge capacity/mAhg−1

100th discharge capacity/mAhg−1

Retention rate/%

116.3 116 115.8 115.2 110.9

90.5 96.5 102.7 106.3 99.9

77.8 83.2 88.7 92.3 90.1

116.9 116.6 116.1 115.5 110.4

74.7 82.0 92.8 96.6 89.8

63.9 70.3 79.9 83.6 81.3

Table 3 Electrochemical performance of 2.0wt% SrHPO4 -coated LiMn2 O4 spinel electrode. Rate of discharge

Initial discharge capacity/mAhg−1

100th discharge capacity/mAhg−1

Retention rate/%

1C

115.5

96.6

83.6

2C

109.8

92.4

84.2

5C

96.5

83.7

86.7

10 C

85.3

74.3

87.1

20 C

78.9

69.8

88.5

Fig. 6. Cycling performance with different rate for 2.0wt% SrHPO4 -coated LiMn2 O4 in the potential range of 3.0-4.3 V at 55 ◦ C.

Fig. 5. The cycling performance of pristine and 0.5, 1.0, 2.0, 3.0wt% SrHPO4 -coated LiMn2 O4 at 1 C at 25 ◦ C (a) and 55 ◦ C (b).

with capacity fading about 36.1% after 100 cycles. The electrochemical cycle performance displays a correspondent variation with the amount of coating to the cycle performance at elevated temperature. The 2.0wt% SrHPO4 -coated LiMn2 O4 shows the best cycling

stability after 100 cycles at 55 ◦ C, with a capacity retention of 83.6% (96.6mAhg−1 ), which is much higher than that of the pristine LiMn2 O4 (74.7mAhg−1 ). Fig. 6 shows the cycling performance of the 2.0wt% SrHPO4 -coated LiMn2 O4 at different discharge rates at elevated temperature (55 ◦ C) in the potential range between 3.0-4.3 V. The initial and 100th discharge capacities and the capacity retention with different discharge rates are summarized in Table 3. The initial discharge capacities for 2.0wt% SrHPO4 -coated LiMn2 O4 are 115.5mAh/g at 1 C, 109.8mAh/g at 2 C, 96.5mAh/g at 5 C, 84.3mAh/g at 10 C and 78.9mAh/g at 20 C rates, with capacity retention of 83.6, 84.2, 86.7, 87.1, 88.5% after 100 cycles, respectively, which proves the 2.0wt% SrHPO4 -coated LiMn2 O4 sample is an attractive material for practical applications, and excellent capacity retention may be obtained at moderate rates. Moreover, the coating material shows a better cycling performance with the increase of rate, which may be attributed to the reversible lithiation/delithiation under high rate as well as the effectively inhibited Mn dissolution from the inner core of LiMn2 O4 electrode to the electrolyte, resulting in a lower Mn dissolution. The result is very outstanding and valuable that further investigation should be taken in our future work.

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Fig. 8. The electrochemical impedance spectroscopy for pristine and 2.0wt% SrHPO4 -coated LiMn2 O4 at 1st and 100th cycle. Table 4 Rf , Rct and W calculation of the samples at 1st and 100th cycle on an equivalent circuit of the cell. sample

Pristine 2.0wt% coated

Fig. 7. The cyclic voltammograms of pristine (a) and 2.0wt% SrHPO4 -coated LiMn2 O4 (b) in the voltage of 3.0-4.3 V at 1st and 100th cycle.

Above all, we can conclude that the capacity of electrode is stabilized as the presence of SrHPO4 on the surface of the LiMn2 O4 restrains the Mn dissolution by effectively reducing the direct contact between cathode material and electrolyte, which is regarded as one of the most important causes of the capacity loss of LiMn2 O4 at elevated temperature [17–19]. According to the above discussion, the change of surface layer of LiMn2 O4 should be responsible for such improvements. Combined with the results of SEM and TEM analysis, the surface of 0.5wt% and 1.0wt% coated LiMn2 O4 may not be covered efficiently, resulting in the slightly improved cycle performance. While the amount of SrHPO4 rises to 2.0wt%, the cycle stability is well improved, that can be attributed to the significant suppression of Mn dissolution into electrolyte. However, the 3.0wt% SrHPO4 coating may be excessive and that could obstruct the transportation of Li+ , leading to poorer electrochemical performance compared to the optimal coating sample [35]. In order to explore the effect of SrHPO4 coating on the degradation mechanism of spinel LiMn2 O4 , the typical cyclic voltammograms of pristine LiMn2 O4 and 2.0wt% SrHPO4 -coated LiMn2 O4 electrodes were carried out using lithium as a counter and reference electrode in the voltage range of 3.0-4.4 V at a scan rate of 0.1mVs−1 . Fig. 7 represents the cyclic voltammogramic (CV) profiles of bare and 2.0wt% SrHPO4 -coated LiMn2 O4 at 1st cycle and 100th cycles. It can be clearly observed that two couples of redox peaks with similar shapes are existing which can further prove the SrHPO4 coating does not change the electrochemical mechanism of LiMn2 O4 during cycles. The split of well-defined redox peaks into two couples indicates that the electrochemical intercalation and de-intercalation reactions of Li+ proceed in two steps, in agreement with the result of Fig. 4. Furthermore, both anodic and cathodic

1st cycle

100th cycle

Rf /

Rct /

W/−1 S−1

Rf /

Rct /

W/−1 S−1

10.9 12.5

107.5 91.2

78.9 65.4

24.3 14.9

552.3 188.5

457.3 138.6

peaks becomes much broader and closer to each other with similar intensity of peaks current in the 100 cycles compared to the first cycle, which can be ascribed to the possible Jahn-Teller distortion and Mn ions dissolution [36,37]. Additionally, the oxidation and reduction peaks related to 2.0wt% SrHPO4 -coated LiMn2 O4 electrode are much steadier compared to those of the bare electrode with 100 cycles, indicating that an accelerated electrode reaction after SrHPO4 coating. And the intervals between the oxidation and the corresponding reduction potentials are closer than those of 2.0wt% SrHPO4 -coated LiMn2 O4 , implying that the polarization is decreased after SrHPO4 coating. Those two points show that 2.0wt% SrHPO4 surface coating can effectively improve the kinetic properties of LiMn2 O4 . Based on the above analysis, it reveals that 2.0wt% SrHPO4 -coated LiMn2 O4 electrode is of reversibility better than bare spinel electrode, and SrHPO4 coating is effective to stabilize the structure of LiMn2 O4 during charge/discharge process, preventing the dissolution of Mn ions, which partly explains the improved capacity retention as shown in Fig. 5∼6. Furthermore, SrHPO4 coating is expected to reduce the formation of SEI film that can decrease the cathode/electrolyte interfacial impedance [30,38,39]. Electrochemical impedance spectroscopy (EIS) investigation was performed on the bare LiMn2 O4 and SrHPO4 -coated LiMn2 O4 at both the 1st and the 100th cycle to intensively explore the intrinsic difference in their cycle performances at the charged state of 4.3 V. The Nyquist plots and the result of simulation fitting based on the equivalent circuit for the bare LiMn2 O4 and SrHPO4 -coated LiMn2 O4 are shown in Fig. 8 and Table 4. The EIS spectra consist of semi-circles and a slope, the semicircle in the high and middle frequency region is attributed to the lithium-ion migration through the SEI film and charge transfer reaction (Rf ), and the slope in the low frequency region is attributed to the lithiumion diffusion in the bulk electrode (Rct ) [40–42], the inclined line in the low frequency region represents the Warburg impedance (W), which is associated with the diffusion of Li ion in electrode [43]. From the result of the 1st cycle, the SrHPO4 -coated LiMn2 O4 displayed a slightly large Rf than the bare LiMn2 O4 , which can be

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SrHPO4 coating in inhibiting the Mn dissolution. When the temperature rises to 55 ◦ C, the Mn content is also increasing that further certified that Mn dissolution is the most significant factor of the poor electrochemical performance of LiMn2 O4 at elevated temperature. However, it was demonstrated that the SrHPO4 coating was an effective way to decrease the dissolution of manganese to improve the high-temperature cyclic performance. 4. Conclusion

Fig. 9. Quantitative XRF analysis of Mn and/or the Mn-containing complexes deposited in the SEI layer on the surface of lithium anode and dissolved from (a) pristine LiMn2 O4 at 25 ◦ C, (b) 2.0 wt.% SrHPO4 -coated LiMn2 O4 at 25 ◦ C, (c) pristine LiMn2 O4 at 55 ◦ C and (d) 2.0 wt.% SrHPO4 -coated LiMn2 O4 at 55 ◦ C during 100 cycles.

attributed to the combination of the SEI film and SrHPO4 coating formed on the surface of the SrHPO4 -coated electrode. However, the 2.0wt% SrHPO4 -coated LiMn2 O4 shows a smaller charge transfer resistance (Rct ) than pristine LiMn2 O4 at fist cycle, and the charge transfer resistance (Rct ) values of the bare LiMn2 O4 (from 107.5  to 552.3 ) increases more sharply than that of the 2.0wt% SrHPO4 -coated LiMn2 O4 (from 91.2  to 188.5 ) after 100 cycles. This may be attributed to the reducing contact between the electrode and electrolyte and alleviating growth of SEI film due to the existence of SrHPO4 coating. Besides, the SrHPO4 -coating is in favor of alleviation of dissolution of Mn which is beneficial to the cycle performance. Additionally, it can be found that the Warburg impedances (W) of the 2.0wt% SrHPO4 -coated LiMn2 O4 after 100 cycles are much smaller than that of uncoated LiMn2 O4 , resulting in an easier diffusion of Li ion in electrode bulk. The obtained results show that the SrHPO4 surface coating can significantly facilitate the kinetics for lithium diffusion during the process of charge/discharge, which further prove that SrHPO4 surface coating can promote the electrochemical performance of LiMn2 O4 electrode. 3.3. Mn dissolution Amine et al. [44] suggested the possible degradation mechanisms of LiMn2 O4 at elevated temperatures is due to the disproportioned reaction (2Mn3+ →Mn2+ + Mn4+ ) and the Mn dissolution from the surface of LiMn2 O4 particles into the electrolyte are accelerated at an elevated temperature. Subsequently, the dissolved Mn migrated to the lithium anode, and then the reduction of the Mn occurred on the surface of anode. In this regard, it is expected that the dissolved Mn and/or the Mn-containing complexes were deposited on the surface of the lithium anode. Such complexes can be chemically formed by an electrolytic decomposition combined with Mn. Hence, the lithium anodes used as counter electrodes of the bare LiMn2 O4 and the 2.0wt% SrHPO4 coated LiMn2 O4 were examined by X-ray Fluorescence (XRF) after 100th cycles to measure the amounts of Mn-containing complexes deposited in the SEI layer on the surface of lithium anode. As shown in Fig. 9, the 2.0wt% SrHPO4 -coated LiMn2 O4 exhibited the lower amount of Mn-containing complexes both at room and elevated temperature compared with bare LiMn2 O4 . The Mncontaining complexes are significantly reduced by coating LiMn2 O4 with SrHPO4 , which agrees well with the expected role of the

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