The Coating Effects of Al₂O₃ on a Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ Surface Modified with (NH₄)₂SO₄

Abstract

A series of 20 wt % $(NH_4)_2SO_4$ and 3 wt % $Al_2O_3$ surface treatments were applied to $Li[Li_{0.2}Mn_{0.54}Co_{0.13}Ni_{0.13}]O_2$ substrates. The $Li[Li_{0.2}Mn_{0.54}Co_{0.13}Ni_{0.13}]O_2$ substrates were synthesized using a co-precipitation method. Sample (a) was left pristine and variations of the 20 wt % $(NH_4)_2SO_4$ and 3 wt % $Al_2O_3$ were applied to samples (b), (c) and (d). XRD was used to verify the space group of the samples as R$\bar{3}$m. Additional morphology and particle size data were obtained using SEM imagery. The $Al_2O_3$ coating layers of sample (b) and (d) were confirmed by TEM images and EDS mapping of the SEM images. 2032-type coin cells were fabricated in a glove box in order to investigate their electrochemical properties. The cells were charged and discharged at room temperature ($25^{\circ}C$) between 2.0V and 4.8V during the first cycle. The cells were then charged and discharged between 2.0V and 4.6V in subsequent cycles. Sample (d) exhibited lower irreversible capacity loss (ICL) in the first charge-discharge cycle as compared to sample (c). Sample (d) also had a higher discharge capacity of ~250 mAh/g during the first and second charge-discharge cycles when compared with sample (c). The rate capability of the $Al_2O_3$-coated sample (b) and (d) was lower when compared with sample (a) and (c). Sample (d), coated with $Al_2O_3$ after the surface treatment with $(NH_4)_2SO_4$, showed an improvement in cycle performance as well as an enhancement of discharge capacity. The thermal stability of sample (d) was higher than that of the sample (c) as the result of DSC.

Introduction

The layered LiCoO2 has received extensive interest, however, it suffers from the low energy density, high cost, and toxicity. In this regard, solid solutions between layered Li(Li1/3Mn2/3)O2 (commonly designated as Li2MnO3) and LiMO2 (M = Mn, Co, Ni) have become an area of increased interest in recent years, as they offer high capacities about 250 mAh/g with a significant reduction in cost and improve-ment in safety when compared to the LiCoO2 cathode.1-5 These materials have the potential to increase the overall capacity and energy density of Li ion batteries; however, the high discharge capacity of the materials is only available at a low discharge rate (typically C/20). When the discharge current is increased, the available discharge capacity of the materials is drastically reduced, with a typical discharge capacity below 200 mAh/g at a 1C rate.6-8 This is due to the creation of sites of two-dimensional (2D) diffusion-layered structures, and because the rate capability of a material is generally affected by bulk Li diffusion and transport through the surface of the particles. Recently, it has been determined that rate ability can be enhanced with (NH4)2SO2 treatment. Li-excess Mn-based layered materials treated with (NH4)2SO2 have a surface structure similar to the spinel. Spinel structure, as in LiMn2O4, LiNi0.5Mn1.5O4, and Li4Mn5O12, has a three-dimensional (3D) diffusion path and good reversibility of Li-ion intercalation or deintercalation. Spinel structure is also structurally compatible with layered structure, with the same oxygen arrangement, and differs only in the arrangement of Li and transition-metal atoms. The two structures can thus be integrated without a distinct boundary.9-11 According to previous study, the active material treated with 20 wt % (NH4)2SO2 exhibited a better enhanced discharge capacity and rate ability than among various percentage of (NH4)2SO2 as amounts. The improvement in cycle performance, how-ever, was not significant, indicating that surface treatment does not significantly affect cycle performance, since the surface treatment method does not insulate the active material from direct contact with the electrolyte. It is clear that the structure of the surface is of great importance to the electrochemical performance of cathode materials. There-fore, additional surface coating is required to improve cycle performance. As a coating material, Al2O3 can prevent the direct contact of electrolyte with cathode materials and it shows the best cyclability on the spinel structure among MgO, TiO2, ZrO2, and other compounds or composites, according to the preceded researches.12 In addition, surface modification of active materials by Al2O3 is needed to achieve higher thermal stability.13 But Al2O3 has no conduc-tivity and no ability for capacity. Minimum amount of Al2O3 for best performance was revealed below 3% as molar ratios from previous study.

In this study, in order to improve the cycle-life and thermal stability of Li[Li0.2Mn0.54Co0.13Ni0.13]O2 substrates treated with (NH4)2SO4, the substrates were surface-coated with Al2O3. To measure the coating effect of Al2O3 on the Li[Li0.2Mn0.54Co0.13Ni0.13]O2 surface modified by (NH4)2SO4, we compared the electrochemical performance of four samples: Li[Li0.2Mn0.54Ni0.13Co0.13]O2 pristine (sample (a)), Al2O3-modified Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (sample (b)), (NH4)2SO4-treated Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (sample (c)), and Al2O3-modified Li[Li0.2Mn0.54Ni0.13Co0.13]O2 treated with (NH4)2SO4 (sample (d)). We expect enhanced cyclability and thermal stability in sample (d), due to bulwarks against electrolytes from the Al2O3 surface coating.

 

Experimental

Lithium hydroxide monohydrate (assay ≥ 98.0%), manga-nese (II) acetate tetrahydrate (assay ≥ 99.0%), nickel (II) nitrate hexahydrate (assay ≥ 98.5%), and cobalt (II) acetate tetrahydrate (assay ≥ 98.0%) were purchased from Sigma Chemical Co. All chemicals and solvents used were analyt-ical grade and used as received. Double distilled water was used throughout this study. Sample (a) (pristine) consisted of Li[Li0.2Mn0.54Co0.13Ni0.13]O2 synthesized via a co-precipita-tion method. Required amounts of the transition metal acetates were dissolved in deionized water and added by dropper to a 0.1 M NaOH solution to form the co-pre-cipitated hydroxides of Mn, Co, and Ni. After drying overnight at 100 °C in an air oven, the coprecipitated hydroxides were mixed with a required amount of lithium hydroxide, and fired in air at 900 °C for 24 h. Sample (b) consisted of Al2O3-coated Li[Li0.2Mn0.54Co0.13Ni0.13]O2 and was prepared via the traditional Al2O3 coating process. The Li[Li0.2Mn0.54Co0.13Ni0.13]O2 powder (pristine) was poured into a 3% Al(NO3)3 solution with continuous stirring for 1 h. The Al content was set at molar ratios of Al/(Ni + Co +Mn) = 3% by controlling the amount of Al(NO3)3. NH4OH was then slowly added, and pH was controlled at 8. This mixture was kept at 80 °C for 5 h until most of the solvent was evaporated. The powder was dried at 120 °C for 12 h and sintered at 500 °C for 5 h. Sample (c) consisted of the Li[Li0.2Mn0.54Co0.13Ni0.13]O2 powder (pristine) treated with (NH4)2SO4 in water. The amount of (NH4)2SO4 used relative to the weight of the active material was (NH4)2SO4/Li[Li0.2Mn0.54Co0.13Ni0.13]O2 = 20 wt %. (NH4)2SO4 was first dissolved in distilled water. The active material (pristine) was then mixed into the solution and dried at 80 °C. The resulting powder was annealed at 300 °C for 10 h to decom-pose the (NH4)2SO4. The final material was washed with distilled water, filtered, and dried to remove impurities.2 The (NH4)2SO4 surface modification of the thus-treated active material was carried out by dispersing the powders from sample (c) into the 3 wt % Al(NO3)3 solution, followed by the addition of NH4OH to precipitate aluminum hydroxide. This process formed sample (d). The products were heated at 500 °C in air for 5 h.

Sample structures were characterized using X-ray powder diffraction (XRD). XRD was performed using the Rigaku ultra-x (Rigaku Co., CuKα radiation (λ = 1.5418 Å), 40 kV, 120 mA) in the 2θ value range of 15-80° with a step of 0.02° sec˗1. Morphology and particle size data were obtained from field emission scanning electron microscopy (FE-SEM, Supra 40, Carl Zeiss Co., Ltd.). The Al2O3 coating layers of samples (b) and (d) were confirmed with transmission electron microscopy images (FE-TEM, JEM 2100F, JEOL Ltd., 200 kV) and energy dispersive spectrometer mapping (EDS, Supra 40, Carl Zeiss Co., Ltd.) of the SEM imagery. To measure a reduced amount of lithium by the reaction with (NH4)2SO4 in the active materials, the molar ratios of the metal elements present in samples (a) and (c) were evaluated by inductively-coupled plasma emission spectroscopy (ICP, Varian).

The electrodes were prepared by casting and pressing a mixture of 80 wt % of the obtained active material, 10 wt % of PVdF (polyvinylidene difluoride) binder, and 10 wt % of carbon black (Super P) in N-methyl pyrrolidinone (NMP) solvent on aluminum foil, followed by drying for 24 h at 80 °C. The coin-type cells (CR 2032) were fabricated in an argon-filled glove box with a 1.15 M LiPF6 electrolyte in an ethylene carbonate-diethyl carbonate-dimethyl carbonate (EC-DEC-DMC, 3:2:5 volume ratio) solution. A celgard 2400 film separated the positive electrode from the lithium metal negative electrode.

To investigate the electrochemical properties, the cells were charged and discharged in the first cycle at room temperature (25 °C) between 2.0 V and 4.8 V. Cells were then charged and discharged between 2.0V and 4.6 V. At this time, the current density of 20 mA/g (0.08 C) was applied. Discharge current densities measured during the rate capability test were 100 mA/g (0.4 C), 200 mA/g (0.8 C), and 300 mA/ g (1.2 C). The discharge current of the cycle life test was 300 mA/g (1.2 C), but the charge current density was fixed at 20 mA/g (0.08 C) for both rate capability and cycle life. Cyclic voltammogram (CV, Wonatech) data of the cells were obtain-ed in the range of 2.0-4.8 V at a scan rate of 0.03 mV/s.

The thermal stability of the sample (c) and (d) was mea-sured by using differential scanning calorimetry (DSC, Q20, TA Instruments Co.). For DSC measurements, the Li ion cells were fully charged to 4.8 V, the positive electrode was taken out of the cell, and the electrolyte was wiped with filter paper. A piece of crushed pellet was mechanically sealed in an aluminum cell used to collect samples of ap-proximately 7 mg.

 

Results and Discussion

Lattice constants of the samples were determined by fitting the peak positions with an R3m space group; the results are shown in Table 1. Figure 1(a) shows the fitted XRD peaks of sample (a) (pristine) by rietveld refinement, representatively. Figure 1(b) shows the XRD patterns of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (sample (a)), Al2O3-modified Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (sample (b)), (NH4)2SO4-treat-ed Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (sample (c)), and Al2O3-modified Li[Li0.2Mn0.54Ni0.13Co0.13]O2 treated with (NH4)2SO4 (sample (d)). XRD profiles of the samples show a charac-teristic pattern corresponding to Li-excess Mn-based materials, with a superlattice peak near 21° caused by the ordering of Li and Mn in the transition-metal layer. After modification and treatment, XRD peak position remained almost the same, indicating that the modification and treatment processes did not alter the bulk structure.9

Table 1.Comparison of structure parameters of the samples

The surface morphology of samples (a)-(d) is shown in the SEM images in Figure 2. Primary and secondary particle sizes were measured at approximately 200-300 nm and 10-20 μm in diameter, respectively. The (NH4)2SO4-treated Li[Li0.2Mn0.54Ni0.13Co0.13]O2 substrate (sample (c)) had a rougher surface than that of the pristine substrate (sample (a)). The secondary particle size of the active material remained approximately the same before and after treatment, indicating that the (NH4)2SO4 surface treatment process did not break up the secondary particles. The Al2O3-modified Li[Li0.2Mn0.54Ni0.13Co0.13]O2 surface (sample (b)) is smoother than the surface of the pristine substrate (sample (a)). Also, sample (d), Li[Li0.2Mn0.54Ni0.13Co0.13]O2 treated with (NH4)2SO4 and modified by coating with Al2O3, is smoother when compared with sample (c). EDS mapping results concerning transition metals in active materials support Al2O3 surface coating, as shown in Figure 2.

Figure 1.(a) Fitted XRD peaks of sample (a) (pristine) by rietveld refinement. (b) XRD patterns of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (sample (a)), Al2O3-modified Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (sample (b)), (NH4)2SO4-treated Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (sample (c)) and Al2O3-modified Li[Li0.2Mn0.54Ni0.13Co0.13]O2 treated with (NH4)2SO4 (sample (d)).

Figure 2.SEM and EDS images of (a) sample (a), (b) sample (b), (c) sample (c), and (d) sample (d).

Figure 3(a) and (b) display TEM images confirming the Al2O3-coating of sample (b) and (d). The images indicated that the amorphous layer of sample (b) and (d) were coated by Al2O3 on the surface of cathode particles. The thickness of the coating layers in sample (b) and (d) was approxi-mately 10 nm and 16 nm, respectively.

Figure 4(a) and (b) show the CV profiles of the samples during the first and second cycles with a scan rate of 0.03 mV/s. The samples show an initial charge peak at approxi-mately 4.1 V and 4.6 V vs Li/Li+, as shown in Figure 4(a). The 4.1 V peak is associated with charging of the Ni-Co-Mn component in the material, whereas the peak at 4.6 V is associated with activation of the Li2MnO3 component. The peaks of sample (c) and (d), active materials treated with (NH4)2SO4, are higher at 4.6 V compared to sample (a) and (b) during the first cycle, suggesting a higher Li extraction rate from the Li2MnO3 component in the active material after (NH4)2SO4 treatment. However, the peak for sample (d) after additional Al2O3-surface coating is lower than the peak of sample (c) around 4.6 V.

Figure 3.(a) TEM images of Al2O3-modified Li[Li0.2Mn0.54Ni0.13-Co0.13]O2 (sample (b)), (b) Al2O3-modified Li[Li0.2Mn0.54Ni0.13-Co0.13]O2 treated with (NH4)2SO4 (sample (d)).

This is due to the existence of a boundary between the active material and the surface coating, which may limit the effectiveness of Li extraction. (NH4)2SO4 surface treatments induce a change in structure at the surface of the active material to spinel-like regions by the exclusion of Li from the active material, which is confirmed by the second cycles of CV profiles and ICP results. Figure 4(b) shows the second CV cycles for sample (c) and (d), which are the active material treated with (NH4)2SO4. These materials have a redox pair at approximately 3.25 V and 3.0 V, marked by arrows in the figure, indicating a change in electrochemical potential. The redox pair is attributed to a spinel contribu-tion. The improvement in rate capability is attributed to a faster Li diffusion through the spinel surface layer.9

Table 2.Collected electrochemical cell data of samples

Figure 4.Cyclic voltammograms of the samples at 0.03 mV/s (a) first cycle and (b) second cycle.

Table 2 shows that Li was eliminated from the pristine material by the reaction between Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (pristine) and (NH4)2SO4, as shown in the following reaction mechanism.

0.6Li4/3Mn2/3O2•0.4LiNi1/3Co1/3Mn1/3O2 + y(NH4)2SO4

→ 2yNH3 + yLi2SO4 + yH2O + 0.6Li4/3˗2yMn2/3O2˗y • 0.4LiNi1/3Co1/3Mn1/3O2

Surface modification by (NH4)2SO4 decreased the ratio of Li to transition metal (Li/M), due to the formation of Li2SO4.

Figure 5(a) shows the first charge/discharge profiles of the cells with Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (sample (a)), Al2O3-modified Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (sample (b)), (NH4)2SO4-treated Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (sample (c)) and Al2O3-modified Li[Li0.2Mn0.54Ni0.13Co0.13]O2 treated with (NH4)2SO4 (sample (d)). The voltage range is between 2.0 and 4.8 V, and the current density is 20 mA/g. Both were charged and discharged at room temperature (25 °C). Treated sample (c) and sample (d) by (NH4)2SO4 increased the dischargecapacity compared with other samples then Al2O3-modified samples (d) exhibited lower irreversible capacity loss (ICL) in the first charge-discharge cycle when compared to sample (c). First cycle efficiency of sample (d) increased from about 69% for the sample (c) to over 72% with Al2O3 coating, due to a suppression of the reaction between the cathode surface and the electrolyte, as well as an optimization of the solid-electrolyte interface (SEI) layer.14 The slope of the discharge curve for sample (c) shows a higher discharge potential.

Figure 5.Charge-discharge curves of samples at 20 mA/g in the (a) first cycle and (b) second cycle.

Following the initial incline, the voltage decreased in a stair-step manner, a characteristic feature of a spinel-like component.9 Cells were charged and discharged between 2.0 V and 4.6 V during the second cycle, as shown in Figure 5(b). Sample (d) had higher discharge capacity at ~250 mAh/g in both the first and second charge-discharge cycles. The electrochemical results are listed in Table 2.

Figure 6 shows the rate capability of the samples. Tests were performed at room temperature (25 °C) between 2.0 V and 4.6 V after the first and second cycle. The charge current was fixed at 30 mA/g, and the discharge currents were set at 100 mA/g (~0.4C), 200 mA/g (~0.8C), and 300 mA/g (~1.2C). Sample (c) and (d) showed improved rate ability after treatment with (NH4)2SO4 when compared with the pristine substrate due to enhanced diffusion velocity, sug-gesting that the higher rate capability is associated with this spinel-like behavior.9

The rate capability of a sample (d) decreased in com-parison with sample (c), because the existence of a boundary between the active material and the surface coating may limit the effectiveness of the coating. In addition, surface coating adds an inactive layer on the surface of the active material, often resulting in a decrease in the overall capacity at low rates.

Figure 6.Rate ability of the samples at 100 mA/g, 200 mA/g, and 300 mA/g as discharge current density.

A comparison of cycling performance for the prepared samples is displayed in Figure 7. The cells were cycled at a constant current of 300 mA /g (~1.2C), with discharge in the voltage range of 2.0–4.6 V and charge current fixed at 30 mA/g. Sample (a), the pristine material, showed a dramatic decrease in discharge capacity, which was attributed to the dissolution of transition metal from the active material during cycling. The Al2O3-coated sample (b) and (d) showed con-sistent discharge capacity, as Al2O3 can prevent the direct contact of electrolyte solutions with cathode materials. The Al2O3-coated sample (d) showed an improvement in both cycle performance and enhancement of discharge capacity. For example, sample (a), (b), (c), and (d) show capacity retentions of 67.7%, 96.3%, 82.3%, and 94.7%, respectively. The results of the cycle test are listed in Table 3.

Figure 7.Cycle performance of samples at 300 mA/g as current density of discharge.

Differential scanning calorimetry was performed for com-parison of thermal stability in sample (a), (c), and (d) powders, particularly in the charged 4.8 V state (Fig. 8). The exo-thermic peak area indicates the level of heat generation (related to oxygen generation) of the decomposed cathode following reaction with the electrolyte. Pristine, sample (a), shows a peak of decomposition at 229.02 ℃ (99.66 J/g). The Al2O3-coated sample (d) displayed an increased temperature of decomposition at 253.81 °C (141.8 J/g). The improved thermal stability of sample (d) compared to the sample (a) and (c) is due to Al2O3, which provides the protection of active material from reaction with electrolytes.9,12 Sample (c) shows the onset of decomposition at 238.46 °C. This sample has the higher calorific value of approximately 166.9 J/g than the pristine and sample (d), which may be attributed to the added side reaction with electrolytes by the increased surface area from the reaction with (NH4)2SO4. Sample (d) provides better thermal stability in the charged state.

Table 3.Cycle performances of samples at 300 mA/g after 50 cycles

Figure 8.DSC curves of sample (a), (c), and (d).

 

Conclusion

The cathode material Li[Li0.2Mn0.54Co0.13Ni0.13]O2 was synthesized using a co-precipitation method. In order to improve the cycle-life and thermal stability of Li[Li0.2-Mn0.54Co0.13Ni0.13]O2 treated with (NH4)2SO4, substrates were surface-coated with Al2O3. The Al2O3-coated sample (d) shows lower ICL in the first charge-discharge cycle than other samples due to a suppression of the reaction between the cathode surface and the electrolyte, as well as an optimi-zation of the SEI layer. In addition, sample (d) showed an improvement in cycle performance and an enhancement of discharge capacity (192.9 mAh/g after 50 cycles at a rate of 300 mA/g). The decrease in rate capability of sample (d), as compared to sample (c), is caused by the boundary between the active material and the surface coating, which may limit the effectiveness of the coating. In addition, surface coating adds an inactive layer on the surface of the active material. The improved thermal stability of sample (d) can be attribut-ed to Al2O3, which provided protection for the active material from reaction with electrolytes. Though the Al2O3-coated sample (d) showed lower rate capability than sample (c), it also showed improved cycle-life and thermal stability. These results suggest that electrochemical performance depends on both surface structure and surface modification. To maintain a cycle-life and a rate capability, further work is needed to use the material with a spinel-like structure as a coating layer.