Electrochemical Performances of the Fluorine-Substituted on the 0.3Li₂MnO₃·0.7LiMn_0.60Ni_0.25Co_0.15O₂ Cathode Material

Abstract

The fluorine-substituted $0.3Li_2MnO_3{\cdot}0.7Li[Mn_{0.60}Ni_{0.25}Co_{0.15}]O_{2-x}F_x$ cathode materials were synthesized by using the transition metal precursor, $LiOH{\cdot}H_2O$ and LiF. This was to facilitate the movement of lithium ions by forming more compact SEI layer and to reduce the dissolution of transition metals. The $0.3Li_2MnO_3{\cdot}0.7Li[Mn_{0.60}Ni_{0.25}Co_{0.15}]O_{2-x}F_x$ cathode material was sphere-shaped and each secondary particle had $10{\sim}15{\mu}m$ in size. The fluorine-substituted cathodes initially delivered low discharge capacity, but it gradually increased until 50th charge-discharge cycles. These results indicated that fluorine substitution gave positive effects on the structural stabilization and resistance reduction in materials.

Introduction

Secondary lithium batteries have been extensively used in portable electronic devices due to its high voltage and stable charge-discharge properties. Recently, their needs and market have been spread out to other application fields such as electric cars and energy storage devices. Secondary lithium batteries are composed of many different materials and parts; of these, anode and cathode materials play a major role by governing the battery’s characteristics and energy density. Commercialized cathode material so far include layered structured LiCoO2 [1,2] LiNixCoyAlzO2 [3], spinel structured LiMn2O 4 [4], olivine structured LiFePO4 [5], and so on.

Of these cathode materials, the layered structured LiCoO2 has good cell performances and can be prepared very easily. However, due to limited cobalt deposit, high price, and toxicity, alternative materials have been developed for a long while. As a result materials with stabilized structure such as Li[MnxNiyCoz]O2 [6] have been developed. This material substitutes some of the Co with Mn and Ni, thus improving some problems such as low capacity, high price, and toxicity.

Recently, Thackeray and co-workers [7] have suggested very high capacity materials, a complex layered compound in which Li2MnO3 coexists with Li[MnxNiyCoz]O2. There are several different ways to synthesize Li[MnxNiyCoz]O2, including a sol-gel, a co-precipitation, a hydrothermal, and a solid state method. The coprecipitation method precipitates different ions at the same time within dissolved or non-dissolved solution, and is the most suitable method for synthesizing complex cathode material. This method has the advantage of being able to control the shape and size of the material, as well as achieving even particle distribution. Li[MnxNiyCoz]O2 is a solid solution where Mn and Ni are bound to Co lattice positions on the R-3m rhombohedral structure of LiCoO2. This structure forms a layered structure in which lithium, transition metal and oxygen form a repeated O-Li-O-M-O-Li-O-M-O arrangement. Adding Li2MnO3 onto this structure yields Li2MnO3·LiMnxNiyCozO2, in which the Li ion disassembled from Li2MnO3 moves onto LiMnxNiyCozO2 and stabilizes the structure [7,8,9]. This material also shows high thermal stability and large theoretical capacity. However, they showed a large irreversible capacity, electrolyte oxidation and poor rate capability, which hindered their practical application.

In this study, we attempts to facilitate the movement of lithium ions by forming a more compact SEI layer and to reduce transition metal dissolution through the fluorine substitution [10,11,12]. The 0.3Li2MnO3·0.7Li[Mn0.60Ni0.25Co0.15]O2-xFx cathode materials were synthesized to using the transition metal precursor from ECOPRO, LiOH·H2O and LiF through sintering. XRD and FE-SEM were used to analyse of structure and morphology of the cathode materials. 2032 coin-cells were assembled to determine the electrochemical properties.

 

Experimental

The transition metal precursor was supplied from ECOPRO Co. and then it was ground with lithium hydroxide (LiOH·H2O) and LiF. The prepared powder was sintered at 500℃ for 8 hrs and at 900℃ for 6 hrs to yield composite layered compound of the 0.3Li2MnO3·0.7Li[Mn0.60Ni0.25Co0.15]O2-xFx cathode material. Amount of substituted fluorine was x = 0.000, 0.025, 0.050, and 0.100.

Powder surface morphologies were studied with a scanning electron microscopy (FE-SEM, Hitachi, S-4800), an X-ray diffraction (XRD, Philips, X-pert PRO MPD) were analyzed with Cu-Kα radiation at 40 kV. Diffraction data were collected at 0.02°/sec with width ranging from 10 to 90° to determine crystallization and crystal structure.

The cathode electrode was consisted of a cathode active material, a conductor (carbon black), and a binder (poly-vinylidene fluorine) at 84: 8: 8 weight ratio. The slurry was dispersed and coated on an aluminum foil. This was dried at 100℃ for 24 hrs, and pressed at 110℃ using a hot roll press. The 0.3Li2MnO3·0.7Li[Mn0.60Ni0.25Co0.15]O2-xFx material was used as a cathode, and lithium foil was used as an anode. Cell-guard 2300 membrane was used as a separator, and 1 M LiPF6 dissolved in EC (ethylene carbonate) / DEC (dimethyl ethyl carbonate) (1/1 vol.%) was used as an electrolyte. All processes related to cell preparation were carried out in a dry room to prevent hydration and oxidation of lithium metals.

Electrochemical characteristics within a voltage range of 2.0~4.6 V were analyzed using TOCAT-3100 (Toyo System) as a charger-discharger. Rate capability was measured with a current rate of 0.1 C, 0.2 C, 1 C, 2 C, 5 C, and 10 C. Cyclic characteristics were measured with a current rate of 0.5 C up to 50th charge/discharge cycles.

 

Result and discussion

Figure 1 depicted scanning electron microscopic images of the Li2MnO3·Li[Mn0.60Ni0.25Co0.15]O2-xFx (x = 0, 0.025, 0.05, and 0.1). All samples were sphere-shaped and homogeneously distributed. The fluorine substitution did not affect the morphology of the samples. Fig. 1 (a) shows that the pristine material has an average particle size of 15 μm. However, the fluorine-substituted materials has smaller size of 10 μm than the non-substituted materials. Most particles were agglomerated with the primary particles except for Fig. 1(d). Fig. 1(b) shows the surface of the non-fluorine-substituted materials. However, in Fig. 1(d), particles are not agglomerated, and the surface looks rough. Fig. 1(f) and (h) showed that the fluorinesubstituted material contained agglomerated particles.

Fig. 1.FE-SEM images of Li2MnO3·Li[Mn0.60Ni0.25Co0.15]O2-xFx: (a) and (b) are x = 0, (c) and (d) are x = 0.025, (e) and (f) are x = 0.050, (g) and (h) are x = 0.100.

Figure 2 showed the results of the X-ray diffraction patterns of the Li2MnO3·Li[Mn0.60Ni0.25Co0.15]O2-xFx (x = 0, 0.025, 0.05, and 0.1) material. The symmetry of the crystal lattice was approximated to be trigonal (R-3m) [13,14,15,16]. The (020) reflection peak at ~21°2θ was observed, which is characteristic of the integrated monoclinic Li2MnO3-like component (C2/m) [17,18,19]. The peak at 36° indicated the spinelphase LixMn2O4 (010), which didn’t show in the pristine substituted materials, and increased with increasing in the amount of fluorine. This indicated that X-ray diffraction analysis showed the degree of fluorine-substitution influenced on the crystal structure of the material. This meant that the spinel-phase combines as an impurity to the original complex layered compound, forming a more complex structure. The spinel-phase LixMn2O4 causes the discharge capacity of the materials decrease.

Fig. 2.XRD patterns of Li2MnO3·Li[Mn0.60Ni0.25Co0.15]O2-xFx: (a) x = 0, (b) x = 0.025, (c) x = 0.050, and (d) x = 0.100.

There were peaks of fluorine substitution in the diffraction pattern, which is consistent with previous reports [12,20,21,22,23]. In addition, it was reported that the small ionic radius of fluorine leads to a change in the lattice constant. We observed EDX mapping in order to confirm the each elements as shown in Fig. 3. In EDX mapping, we conclude that fluorine was substituted in the lattice of transition metal.

Fig. 3.The elemental mapping of Li2MnO3·Li[MnNiCo]O1.975F0.025 at (a) SEM micrographs, (b) O, (C) Mn, (d) Ni, (e) Co and (f) F.

Figure 4 showed the charge-discharge profiles in a voltage range of 2.0~4.6 V for up to 2nd charge-discharge cycles. All samples showed large irreversible capacities during the first cycle. The large irreversible capacity loss was mainly attributed to the electrochemical removal of two lithium ions and to the reinsertion of only one lithium ion during discharge. A plateau was observed around 4.0 V during charging, which corresponds to removal of Li from layered Li[MnxNiyCoz]O2. Another plateau was observed around 4.5 V as well, which corresponded to the removal of Li from Li2MnO3. The discharge capacity of the fluorine-substituted electrode (x = 0.025) was 193 mAhg–1 and the coulombic efficiency was 82 %. Its coulomb efficiency was inversely related to the fluorine-substitution amount. The discharge capacity of the non-substituted electrode was 215 mAhg–1 and the coulomb efficiency was 76%. The discharge capacity of the non-substituted electrode was larger than that of the fluorine-substituted electrode. This can be explained with a difference of the bonding energy between two elements: the bonding energy between Li and F (557 kJ/mol) is greater than that between Li and O (341 kJ/mol), which hinders the insertion and removal of Li [24].

Fig. 4.Charge and discharge curves of Li/ Li2MnO3·Li[Mn0.60Ni0.25Co0.15]O2-xFx cells: (a) x = 0, (b) x = 0.025, (c) x = 0.050, and (d) x = 0.100.

Figure 5 showed the dQ/dV plots of Li/0.3Li2MnO3·0.7Li[Mn0.60Ni0.25Co0.15]O2-xFx electrodes, measured between 2.0~4.6 V for 2nd charge/discharge cycle. All four samples have shown similar curves. Oxidation peaks were observed at 4.1 V for the first cycle, which corresponded to the removal of Li from Li[MnxNiyCoz]O2. The peak at 4.1 V was shifted at 2nd cycle. Also, insertion of Li into Li[MnxNiyCoz]O2 was shown as a plateau at 3.4 V. Oxidation peak was observed at 4.5 V area as well, which was the removal of Li from Li2MnO3. In the first cycle, the peaks were strong, which corresponded to the irreversible capacity of the first cycle. However, peaks were reduced greatly in the second cycle. However, the spinel phase (LixMn2O4) was not observed in the plots, which was detected in XRD analysis in Fig. 2 as a function of fluorine amount. This was probably because it did not have noticeable effect in chargedischarge characteristics.

Fig. 5.dQ/dV plots of Li/ Li2MnO3·Li[Mn0.60Ni0.25Co0.15]O2-xFx cells: (a) x = 0, (b) x = 0.025, (c) x = 0.050, and (d) x = 0.100.

Figure 6 showed the rate capability of the Li/ Li2MnO3·Li[Mn0.60Ni0.25Co0.15]O2-xFx cells at a currents rate of 0.1, 0.2, 1.0, 2.0, 5.0, and 10 C. The cell was charged at a currents rate of 0.1C before each discharge test. The non-fluorine-substituted material exhibited the capacity retention of 30% at 10 C rate against at 0.1 C. The capacity retention ratio of the fluorinesubstituted electrode (x = 0.025) was 29% and it was decreased with the fluorine-substitution amount.

Fig. 6.Rate capability of Li/ Li2MnO3·Li[Mn0.60Ni0.25Co0.15]O2-xFx cells: (a) x = 0, (b) x = 0.025, (c) x = 0.050, and (d) x = 0.100.

To understand the cycle performances of the Li/Li2MnO3·Li[Mn0.60Ni0.25Co0.15]O2-xFx, the cells were tested at 0.5 C rate at a voltage range of 2.0~4.6 V and presented in Fig. 7. The fluorine-substitution led to changes in cycle properties and capacities of the samples. The initial charge-discharge capacities were low as the amount of fluorine-substitution increased (Fig. 1). The discharge capacities decreased with increasing cycle number in the pristine material. However, the electrode with fluorine substitution, the discharge capacities gradually increased with cycle numbers. In particular, when fluorine-substitution was x = 0.025, only minor differences were observed in the discharge capacities between the fluorine-substituted and non-substituted cells.

Fig. 7.Cycling performance of Li/Li2MnO3·Li[Mn0.60Ni0.25Co0.15]O2-xFx cells: (a) x = 0, (b) x = 0.025, (c) x = 0.050, and (d) x = 0.100.

Figure 8 was showed the charge-discharge profiles in a voltage range of 2.0~4.6 V to obtained at 2nd cycle and after 50th cycles. A plateau was observed around 4.0 V during charging, which corresponds to removal of Li from layered Li[MnxNiyCoz]O2. This plateau of the Li2MnO3·Li[Mn0.60Ni0.25Co0.15]O2 electrodes was showed almost same profile after 2nd cycle and 30th cycles. However, Li2MnO3·Li[MnNiCo]O1.975F0.025 electrodes showed to better plateau and increased charge capacity, which made influence on the cycle performance. As a result, the discharge capacity has increased as a function of cycle numbers.

Fig. 8.Charge and discharge curves of Li/ Li2MnO3·Li[Mn0.60Ni0.25Co0.15]O2-xFx cells: (a) x = 0 and (b) x = 0.025.

It suggested that the fluorine-substituted cells showed larger discharge capacities with increasing cycle number. The reason that capacity stability improved was attributed to the fluorine substituted on the Li[MnxNiyCoz]O2. And, the enhanced electrical conductivity of surface layer could decrease the impedance, resulting in improving cycle performance [20,22,23].

 

Conclusion

To achieve structural stability of the Li2MnO3·LiMO2 (M = Mn, Ni, Co) cathode material, the Li2MnO3·Li[Mn0.60Ni0.25Co0.15]O2-xFx (x = 0, 0.025, 0.050 and 0.100) was synthesized by a co-precipitation method. The spinel phase peak observed in XRD patterns as a function of fluorine amount (x = 0.050, 0.100). In electrochemical performances, capacity fade was observed with increasing fluorine substitution and the highest charge-discharge capacity (215 mAhg–1) was observed with no fluorine substitution.

The initial discharge capacity of the fluorine-substituted electrode is low, but cycle properties were improved and higher capacities are observed with increased cycle numbers.