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RARE METALSVol. 30, No. 2, Apr 2011, p. 120 DOI: 10.1007/s12598-011-0209-5 Corresponding author: LIU Yunjian E-mail: lyjian122331@Improving the electrochemical performance of LiMn 2O 4/graphite batteries using LiF additive during fabricationLIU Yunjian a, b , GUO Huajun b , LI Xinhai b , and WANG Zhixing ba School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, ChinabSchool of Metallurgical Science and Engineering, Central South University, Changsha 410083, ChinaReceived 16 March 2010; received in revised form 19 May 2010; accepted 25 May 2010 © The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg 2011AbstractLiMn 2O 4/graphite batteries using LiF additive were fabricated and their electrochemical performance including discharge, cycling and stor-age performances were tested and compared with LiF-free LiMn 2O 4/graphite batteries. The LiMn 2O 4/graphite battery with LiF added shows better capacity (107.5 mAh/g), cycling performance (capacity retention ratio of 93% after 100 cycles), and capacity recovery ratio (98.1%) than the LiF-free battery. The improvement in electrochemical performance of the LiF-added LiMn 2O 4/graphite battery was due to the fact that LiF can restrain the dissolution of Mn from the spinel LiMn 2O 4 cathode into the electrolyte, leading to a smaller resistance and polariza-tion.Keywords: lithium batteries; lithium fluoride; additives; electrochemical properties1. IntroductionLithium ion secondary batteries have always been an at-tractive power source for a wide variety of applications.Spinel-LiMn 2O 4 and its derivatives are considered to bepromising materials for the positive electrodes of lithium rechargeable batteries. This is due to their good thermal sta-bility, low cost, and environmental friendliness. However, cells using spinel LiMn 2O 4 as a cathode material have been known to cause severe capacity fading on charge-discharge cycling and storage [1-2]. The mechanism of capacity fading during charge- discharge cycling and storage has not been completely clari-fied, although some associated factors have been discussed: (1) electrochemical reactions of the electrolyte solution at high voltage (above 4.0 V); (2) irreversible phase and struc-ture transition [3-4] (i.e. Jahn-Teller distortion at the dis-charged state, transformation of an unstable two-phase structure in the high-voltage region to a more stable sin-gle-phase structure through loss of MnO); (3) Mn dissolu-tion of a LiMn 2O 4 cathode into the electrolyte according to the disproportionate reaction of 2Mn 3+ → Mn 4+ + Mn 2+ [5-6].Among these factors, the one most influenced by the celltemperature is Mn dissolution, which is mainly caused byHF contained in the electrolyte [7-8]. Many research groups attempted to stabilize the structure of LiMn 2O 4 powders during cycling by substituting a small fraction of the manganese-ions with several divalent or tri-valent metal ions in the 16d sites or by coating with oxides [9-12]. Recently, metal ions and F − co-substitution or F − substitution were studied to improve the electrochemical performance of LiMn 2O 4. The cycling performance was im-proved, but the initial capacity was a little lower [13-18], but they did not study the storage performance of the LiMn 2O 4 battery. Thus it is necessary to enhance the initial capacity, capacity retaining ratio after cycling, and the capacity re-covery ratio after storage through other methods. In this work, a new approach through adding LiF into thecathode slurry during fabrication was introduced to improve the electrochemical performance of the LiMn 2O 4 battery. The discharge capacity, charge-discharge cycling, and stor-age performance of the LiF-containing batteries were stud-ied and compared with those of the common commercial LiMn 2O 4 battery. The possible mechanism for the im-provement in the electrochemical properties was presented.2. Experimental2.1. Battery preparation Commercially available 204468 type Li-ion batteriesLiu Y.J. et al., Improving the electrochemical performance of LiMn2O4/graphite batteries using LiF additive during (121)consisting of LiMn2O4 as the cathode and graphite as the anode were used as 2 wt.% LiF was added in the cathode slurry to prepare the LiF containing batteries. The electrolyte was 1 mol/L LiPF6 in a mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) with a volume ratio of 1:1:1.2.2. Electrochemical performance testThe capacity test of the battery was charged at 1/3C rate to 4.2 V and then discharged at 1/3C rate to 3.0 V at 25°C. For the cycling test, the current for charge-discharge was held at 0.5C.The electrochemical impedance spectroscopy (EIS) of the cell, which was full charged, was measured using CHI660A (Chenghua Instrument Co. Ltd, Shanghai). The amplitude of the AC signal was 5 mV over the frequency range between 100 kHz and 0.01 Hz.2.3. Storage performance testThe batteries were charged to half charged state, and then stored at room temperature (25°C) for 28 d. Afterwards, the batteries were discharged to 3.0 V at 0.2C, and then charged to 4.2 V and finally discharged to 3.0 V at 0.2C rate to measure their capacity.2.4. Structure and surface analysisFor evaluating the cell materials after charge-discharge cycling and storage, the cathode powders were collected by disassembling the tested cells in an argon-filled glove box. The cathode powders were washed with EC+DEC mixed solution to remove the electrolyte salt.The crystal structures of the cathode and anode were characterized by X-ray diffraction (XRD) with a Cu Kα ra-diation monochromator and a step width of 0.02°. The mor-phology was characterized by a scanning electron micro-scope (SEM, JEOL, JSM-5600LV) with an accelerating voltage of 20 kV.3. Results and discussionFig. 1 compares the discharge curves of the batteries with and without adding LiF. The operating cut-off voltages were 3.0 and 4.2 V at 0.2C. The LiMn2O4/graphite battery with LiF added shows a better capacity (107.5 mAh/g) compared to the pristine LiMn2O4/graphite battery (105 mAh/g), indi-cating that LiF can enhance the capacity of LiMn2O4. The two discharge curves show two distinct discharge plateaus, which should be attributed to orderly intercalating of lithium ions in the tetrahedral (8a) sites at 4.1 V and disorderly in-tercalating lithium ions at 3.9 V [19].Fig. 1. Discharge curves of the LiMn2O4 batteries.Fig. 2 shows the electrochemical cycling performance at 0.5C between 3.0 and 4.2 V for two batteries. LiMn2O4 with LiF added shows a better cycling performance, with the 93.0% capacity retention, while the pristine LiMn2O4 exhib-its the 91.1% capacity retention ratio after 100 cycles. It fol-lows that the additive LiF not only improves the initial ca-pacity, but also can improve the cycling performance of the spinel manganese battery. This result was not achieved in previous reports [13-18]. This is a salient result for the prac-tical use of the LiMn2O4 battery, although the capacity re-tention ratio is only enhanced by about 2% after 100 cycles. In addition, it provides a viable alternative to improve the electrochemical performance of the cathode for Li-ion bat-teries. Park et al. [20-21] found that adding amphoteric ox-ides in the slurry can also improve the cycling performance of the LiMn2O4 battery, and they just considered that the amphoteric oxide could scavenge HF effectively. In addition, the amphoteric oxide adhered to LiMn2O4 and the attached ZrO2 particles also appeared to reduce the effective surface area exposed in the chemical reaction. In our work, we just expected that LiF can restrain the appearance of HF from the electrolyte, and reduce the effective surface area between LiMn2O4 particles and the electrolyte, resulting in better ca-pacity and cycling performance.Fig. 2. Cycling performance of the LiMn2O4 batteries at room temperature.The storage performance of the LiMn2O4 battery at room temperature was evaluated. Typical discharge curves for122 RARE METALS, Vol. 30, No. 2, Apr 2011LiMn 2O 4 batteries after storage are shown in Fig. 3. The discharge capacities of the pristine and LiF-added LiMn 2O 4 batteries are 101.0 and 105.3 mAh/g, respectively. The ca-pacity retention of the pure LiMn 2O 4 battery is 96.2% and that of the LiMn 2O 4 battery with LiF added is 98.0%, indi-cating that the capacity recovery ratio of the LiMn 2O 4 bat-tery after storage is significantly improved. The additive LiF not only enhances the capacity of LiMn 2O 4 and the cycling performance of the LiMn 2O 4 battery, but also improves the capacity recovery ratio of the LiMn 2O 4 battery after storage. The approach in which LiF was added into the cathode slurry during fabrication is more convenient than doped LiMn 2O 4.Fig. 3. Discharge curves of the LiMn 2O 4 batteries after stor-age.Fig. 4(a) shows the surface morphology of the LiMn 2O 4 battery cathode before storage, and Figs. 4(b) and 4(c) show the surface morphologies after storage for the LiMn 2O 4 bat-teries without and with LiF added, respectively. The small particles of LiMn 2O 4 on the surface in Fig. 4(a) disappeared after storage because of electrolyte erosion. The surface of LiMn 2O 4 after storage is eroded seriously by electrolytes while the surface of LiF-containing LiMn 2O 4 is relatively smooth. It is expected that the LiF additive can restrain the electrolytes from eroding the LiMn 2O 4 electrode.Fig. 5(A) shows the XRD patterns of the LiMn 2O 4 elec-trodes. All peaks in the XRD patterns of the LiMn 2O 4 cath-ode can be indexed as the spinel phase (JCPDS: 35-0782) except the characteristic peak of graphite at 26.5°, indicating that LiMn 2O 4 retains its spinel structure after storage.Fig. 5(B) shows the magnified patterns from 35.5 to 40° for 2θ. Diffraction peaks of all LiMn 2O 4 electrodes shift to a higher angle after storage, indicating shrinkage of the crystal lattice [22]. The shift degree of LiMn 2O 4 in the pristine bat-tery is larger than that in the LiF-added LiMn 2O 4, showing that the spinel structure of LiMn 2O 4 in the battery with LiF added was held better than that in the pristine LiMn 2O 4 bat-tery.It is also well-known that shrinkage of the crystal lattice is commonly linked to Mn dissolution. Fig. 5 indicates that the amount of soluble Mn in the pristine LiMn 2O 4batteryFig. 4. Surface morphologies of LiMn 2O 4(SEM): (a) before storage; (b) after storage; (c) after storage and with LiF added.Fig. 5. XRD patterns of LiMn 2O 4 (A) and the magnified between 35.5 to 40° for 2θ (B): (a) before storage; (b) after storage; (c) af-ter storage and with LiF added.Liu Y.J. et al., Improving the electrochemical performance of LiMn2O4/graphite batteries using LiF additive during (123)should be larger than that of the battery with LiF added in the slurry.Although the mechanism of capacity fading during cy-cling and storage has not been completely clarified, the dis-solution of Mn should be one of the reasons. The dissolution of Mn is generally attributed to the existence of HF, which is easily formed when using LiPF6 as the electrolyte salt. The correlation of HF formation and Mn dissolution has been experimentally reported [7]. LiPF6 itself contains a small amount of HF during the manufacturing process [23], and the salt can easily react with water, which exists unavoid-ably at very low concentrations in the electrolyte, as shown in the following reaction equations:LiPF6+ H2O → POF3+ LiF + 2HF (1) POF3+ H2O → PO2F2−+ HF + H+ (2) PO2F2−+ H2O → PO3F2−+ HF + H+ (3) PO3F2−+ H2O → PO43−+ HF + H+(4) LiPF6 salt decomposes at high temperature, especially over 55°C,according to the following reactions:LiPF6→ LiF + PF5(5) PF5+ H2O → POF3+ 2HF (6) The combination of HF-H2O in the presence of air is known to react on the LiMn2O4 in an aqueous medium ac-cording to Eq. (7) [24]:2LiMn2O4+ 4H+→3λ-MnO2(s) + Mn2+(l)+2Li+(l) + 2H2O (7) As shown in Eq. (7), Mn2+ enters into the electrolyte, while MnO2 is the byproduct of Mn dissolution. The solid state MnO2 should be deposited on the surface of the LiMn2O4 electrode and separates the LiMn2O4 electrode from the electrolyte, which is good for the electrochemical performance. The deposited MnO2 was detected by XPS in our previous paper [25].The solid electrolyte interface on the surface of the LiMn2O4 electrode has been validated in recent research [26]. The film is made up of a Li x PF y-type compound, LiF, Li2CO3, R-CO3Li, and MnO2 [15]. Therefore, the solid elec-trolyte interface film can be improved by the added LiF ad-hering to the LiMn2O4 particle. It is expected that the addi-tive LiF can restrain the process in Eqs. (1) and (5) on the surface of the LiMn2O4 particle according to the chemical balance principle. Thus, the amount of HF in the electrolyte is also restrained, which results in less soluble Mn.The manganese concentration in the electrolyte solution was measured to confirm our expectation. LiMn2O4/graphite cells were disassembled at a half-charged state, and the cathode electrodes were immersed in a specified amount of electrolyte solution at 35°C for 48 h. The amount of dis-solved manganese into the electrolyte from the cathode was determined with atomic absorption spectroscopy. The amount of soluble Mn in the electrolyte for a pristine LiMn2O4 electrode was 1.189 mg/L. Significant improve-ment in the manganese dissolution was shown for the LiMn2O4 battery with LiF added in the cathode, and the amount was 0.918mg/L. This result is in agreement with the XRD results. It is evident that the LiF additive restrains the dissolution of Mn.Fig. 6 shows the cycling performance of LiMn2O4 batter-ies after storage. A pristine LiMn2O4 battery shows a better cycling performance during 200 cycles with the 89.2% ca-pacity retention. Also, the LiF-added LiMn2O4 battery shows the 86.2% capacity retention. Compared with Fig. 2, the cycling performance of LiMn2O4 batteries are improved after storage. However, the cycling performance of the pris-tine LiMn2O4 battery is better than that of the LiF-added LiMn2O4 battery after storage. This can be ascribed to lower capacity, crystal shrinkage, and a thicker covered MnO2 film, which is the result of a greater Mn dissolution.Fig. 6. Cycling performance of LiMn2O4 after storage.The amount of Li+ inserting into Li x Mn2O4 is lower, which can result in a higher value of Mn in Li x Mn2O4 and reducing the Li+ concentration in the vicinity of the material surface. Therefore, the Jahn-Teller distortion is restrained [22], resulting in a better cycling performance. Fig. 5 indi-cates that the crystal lattice of LiMn2O4 with LiF added is bigger than that of pristine LiMn2O4 after storage. The in-tensity of the M−O bond of LiMn2O4 in the LiMn2O4 battery with LiF added is weaker than that in pristine LiMn2O4 after storage. As a result, the M−O bond is easier to rupture dur-ing the cycling, leading to a worse cycling performance.The amount of soluble Mn in the pristine LiMn2O4 bat-tery is larger than that in the LiF-added LiMn2O4 battery, which results in the thicker MnO2 covering on the pristine LiMn2O4 electrode according to Eq. (7). A thicker MnO2 covering on the surface of pristine LiMn2O4 can reduce the contact area of the LiMn2O4 electrode and electrolyte more effectively and improve the cycling performance of the pris-tine LiMn2O4 battery. Thus, the cycling performances of124 RARE METALS, Vol. 30, No. 2, Apr 2011LiMn 2O 4 batteries after storage also confirm that the LiF additive restrains the dissolution of Mn.Refs. [27-29] reported the storage performance of LiMn 2O 4, and indicated that capacity fading was due to the poor conduction between the active material and the collec-tor except for the Mn dissolution. AC impedance measure-ments were carried out using three-electrode configuration to study the surface state of the LiMn 2O 4 batteries after storage in greater detail. In Fig. 7, the impedance spectra of LiMn 2O 4 with or without LiF after storage are combinations of two depressed semicircles in the high frequency region and a straight line in the low frequency region. An intercept at the Z ′ axis in the high frequency region corresponds to the ohmic resistance (R s ). The depressed semicircle in the high frequency range is related to the Li-ion migration resistance (R f ) through the solid electrolyte interface (SEI) film formed on the cathode surface. The second semicircle in the middle frequency range indicates the charge transfer resistance (R ct ). The inclined line in the lower frequency represents the Warburg impedance (W ), which is associated with lith-ium-ion diffusion in the LiMn 2O 4 particles. It is obvious that the ohmic and migration resistances produced by the stored LiMn 2O 4 electrode without LiF exceeds those of the LiMn 2O 4 electrode with added LiF. According to Eq. (7), it can be concluded that the increase in resistance could be at-tributed to excess MnO 2 from the Mn dissolution. The re-sults also confirm that LiF can restrain the dissolution of Mn. The results can support those in Fig. 6 well, and they are in good agreement with the results reported in Refs. [27-29]. Furthermore, the increased resistance can also cause higher cell polarization and poor conduction, which results in a greater resistance during the Li + insertion and incomplete charging. The above results show greater capacity lossesmacroscopically.Fig. 7. AC impedance of the LiMn 2O 4 electrode.4. ConclusionsLiF was added into the cathode slurry during battery fab-rication to improve the electrochemical performance of LiMn 2O 4/graphite batteries. Compared with pristine LiMn 2O 4 batteries, LiMn 2O 4/graphite batteries with LiF added shows better capacity and cyclic performance. The discharge capacity of LiMn 2O 4 with LiF added is 107.5 mA ⋅h −1 and the capacity retention ratio is 93.0% after 100 cycles, while those of pristine LiMn 2O 4 batteries are 105 mA ⋅h −1 and 91.1%, respectively. The capacity recovery ratio of LiMn 2O 4/graphite batteries with LiF added at room tem-perature is 98.0%, while that of pristine LiMn 2O 4/graphite batteries is 96.2%. XRD results show that the diffraction peaks of LiMn 2O 4 shift to higher angles after storage, and the shift degree of peaks for LiMn 2O 4 in pristine LiMn 2O 4 batteries is larger than that in LiF-added LiMn 2O 4 batteries. Atomic absorption spectroscopy results show that LiF can restrain Mn from dissolution. The additive LiF restrains HF production because of the chemical balance principle. The resistance and polarization of a pristine LiMn 2O 4/electrolyte is larger than that of LiMn 2O 4 with LiF added after storage due to the covered MnO 2. The additive LiF not only en-hances the capacity of LiMn 2O 4 and the cycling perform-ance of LiMn 2O 4 batteries, but also improves the capacity recovery ratio of LiMn 2O 4 batteries after storage. The ap-proach by which LiF was added into the cathode slurry dur-ing fabrication is more convenient than doped LiMn 2O 4, and the common commercial non-doped LiMn 2O 4 is cheaper.AcknowledgementsThe work was financially supported by the Advanced Person Fund of Jiangsu University (No. 10JDG041) and the Major State Basic Research Development Program of China (No. 2007CB613607).References[1] Iwata E., Takahashi K.I., Maeda K., and Mouri T., Capacityfailure on cycling or storage of lithium-ion batteries with Li-Mn-O ternary phases having spinel-framework structure and its possible solution, J. Power Sources , 1999, 81(2): 430. 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