Oxalate precursor preparation of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ for lithium ion battery positive electrode

Abstract

Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ powders have been prepared through co-precipitation of metal oxalate precursor and subsequent solid state reaction with lithium carbonate. X-ray diffraction pattern shows that the massive rock-like structure has a good layered structure and solid solution characteristic. Scanning electron microscope and transition electron microscope images reveal that the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ composed of nanoparticles have the size of 1–2 μm. As a lithium ion battery positive electrode, the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ has an initial discharge capacity of 285.2 mAh g⁻¹ at 0.1 C within 2.0–4.8 V. When the cutoff voltage is decreased to 4.6 V, the cycling stability of product can be greatly improved, and a discharge capacity of 178.5 mAh g⁻¹ could be retained at 0.5 C after 100 cycles. At a high charge–discharge rate of 5 C (1,000 mAh g⁻¹), a stable discharge capacity of 121.4 mAh g⁻¹ also can be reached. As the experimental results, the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ prepared from oxalate precursor route is suitable as lithium ion battery positive electrode.

Introduction

Development of positive electrode materials with high capacity, long cycle life, and low cost has been one of the most important subjects for high-performance lithium ion batteries [1, 2]. Recently, the lithium-rich manganese-based oxides xLi₂MnO₃ · (1-x)LiMO₂ (M = Ni, Co, Mn, Fe, Cr, Ni_1/2Mn_1/2, Ni_1/3Co_1/3Mn_1/3…) have received many attentions due to their high reversible capacity (230–300 mAh g⁻¹) [3–8]. As for a solid solution electrode, the Li₂MnO₃ component is considered to stabilize the crystal structure and enhance the discharge capacity through extracting the lithium ions concomitant with release of oxygen at charge voltage within 4.5–4.6 V, to form an active MnO₂ component. Therefore, the solid solutions could reach special high capacity and retain good cycling stability upon cycling [3–11].

Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ (or described as 0.5Li₂MnO₃∙0.5LiNi_1/3Co_1/3Mn_1/3O₂) stands out from other solid solution materials due to its high discharge capacity, moderate cycling performance, and good rate capability. Therein, the C2/m Li₂MnO₃ can well integrate with equivalent R-3m LiNi_1/3Co_1/3Mn_1/3O₂ [4–6], and the improved electrochemical performances of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ should be ascribed to the good synergistic effect of Li₂MnO₃ and LiNi_1/3Co_1/3Mn_1/3O₂ components and the appropriate existence of cobalt [12–15]. As the experimental results, the initial discharge capacity of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ can reach as high as 260–310 mAh g⁻¹ at low current density, and its capacity retention also reaches 70~90 % after a dozen or even a hundred cycles [16–21]. Conventional inorganic preparation methods such as sol–gel [16, 20], solid state [22], sucrose combustion [16], molten salt [17], and polymer gel [23] have been successfully introduced to prepare the good solid solution material, showing the structural and electrochemical properties of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ which are determined by the synthesis methods and preparation conditions [19].

Recently, the preparation technologies through initial co-precipitation and subsequent solid state reaction have become one of the most promising routes to synthesize multiple oxides such as Li_xNi_yCo_zMn_2-x-y-zO₂ [15, 18, 24, 25]. The Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ with controlled morphologies and sizes also can be readily obtained by using suitable multiple metal salts templates such as Ni_0.13Co_0.13Mn_0.54(OH)_1.6 [18] and Ni_0.13Co_0.13Mn_0.54(CO₃)_0.8 [25]. In the present study, the mixed synthesis route including initial co-precipitation of metal oxalate precursor and subsequent solid state reaction with lithium carbonate is used to prepare Li_1.2Ni_0.13Co_0.13Mn_0.54O₂. The purpose of the mixed oxalate route is to produce micro-size Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ composed of nanoparticles [25]. Correspondingly, the structures, morphologies, and electrochemical performances of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ particles are also discussed in the text.

Experimental

Synthesis of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂

First, 1.39 g of Mn(AC)₂·4H₂O, 0.34 g of Co(AC)₂·4H₂O and 0.34 g of Ni(AC)₂·4H₂O were dissolved into 40-mL equal volume mixed solution of water and ethanol, and then 40 mL of the same water and ethanol mixed solution containing 1.52 g H₂C₂O₄·2H₂O was added drop by drop under stirring. The obtained suspension was transferred into a 100-mL teflon-lined autoclave, and kept at 80 °C for 5 h. The precipitated metal oxalate was filtered, washed thoroughly with ultra-pure water and ethanol, and then dried at 80 °C for 5 h. Then, 0.6 g freshly prepared metal oxalate was mixed well with 0.1844 g Li₂CO₃ (3 % excess) in an agate mortar using ethanol as dispersant. The obtained mixture was pre-calcined at 450 °C for 4 h, calcined at 850 °C for 12 h, and cooled naturally to room temperature.

Crystal characterization

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were carried out on a Mettler (SDTQ-600) to monitor the weight loss and heat flow of metal oxalate precursor and metal oxalate-Li₂CO₃ mixture at heating rate of 10 °C min⁻¹ from room temperature to 800 or 900 °C under an air atmosphere. X-ray diffractometer (XRD) patterns of 80 °C-treated hydrothermal precursor, corresponding to Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ and cycled product were collected on a Rigaku D/max-2400 powder XRD, (0.08 ° pot/s). SEM (JEOL JSM-7600F, 5 kV) and TEM (JEM-100CX11, 100 kV) were conducted to characterize the morphologies and sizes of precursor metal oxalate and product Li_1.2Ni_0.13Co_0.13Mn_0.54O₂.

Electrochemical characterization

CR 2032 coin cells of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂/Li were used for electrochemical experiments performed at room temperature. The working electrodes were prepared as the following: after mixing of electrode materials, acetylene black and poly(vinyl difluoride) at a weight ratio of 80:10:10, the resulting mixtures were slurried with N-methyl-2-pyrrolidone, pasted onto aluminum foils, dried at 80 °C for 5 h, and cut into disks with a diameter of 12 mm with a positive electrode material loading density of 2.62 ± 0.35 mg cm⁻². Glass fibers (Whatman) were used as separators, and the electrolyte (LBC 305–01, Shenzhen Xinzhoubang) is the LiPF₆ in EC, EMC, and DMC-mixed solvent. The cells were assembled in an argon-filled glove box. Galvanostatic cycling tests were conducted on a Land CT2001A battery system at various current rates (1 C = 200 mAh g⁻¹) within 2.0–4.6 and 2.0–4.8 V. After the 100-cycle tests, parts of the lithium ion batteries were unpacked in an air–atmosphere ventilating cabinet, therein the active materials of the working electrodes were washed with acetone and then ethanol three times, dried at 80 °C, and then used for tests.

Results and discussion

Figure 1 shows the TGA-DSC curves of metal oxalate precursor and metal oxalate-Li₂CO₃ mixture from room temperature to 800 or 900 °C at a heating rate of 10 °C min⁻¹. In Fig. 1a, the first weight loss of ca. 20 wt.% from room temperature to 250 °C with broad endothermic peak in DSC curve would be related with dehydration of metal oxalate, and the weight loss is close to the value of 19.9 wt.%, which is calculated from the dehydration of Ni_0.13Co_0.13Mn_0.54(C₂O₄)_0.8·1.6H₂O. When the temperature is increased above 300 °C, the anhydrous metal oxalate is decomposed into metal oxide and carbon dioxide accompanied with the release of heat. In Fig. 1b, the weight loss before 300 °C should be the dehydration and decomposition of metal oxalate [26, 27]. After that, a weak exothermic peak nearby 400 °C may be attributed to the beginning of crystallization of target product Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ [26]. Furthermore, From the TGA profile, the mass loss is almost stopped until 800 °C, indicating that the crystallization temperature of Li_1.2Co_0.13Ni_0.13Mn_0.54O₂ should be above 800 °C, and the total weight loss of metal oxalate-Li₂CO₃ mixture is 55.6 % from room temperature to 900 °C, which agree well with the theoretical data of 54.9 % according to the total chemical reaction:

Fig. 1

TGA-DSC analysis of metal oxalate precursor (a) and metal oxalate-Li₂CO₃ mixture (b) from room temperature to 800 or 900 °C at a heating rate of 10 °C min⁻¹

$$ \begin{array}{l}{\mathrm{Ni}}_{0.13}{\mathrm{C}\mathrm{o}}_{0.13}{\mathrm{Mn}}_{0.54}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_{0.8}\cdot 1.6{\mathrm{H}}_2\mathrm{O}+0.6{\mathrm{Li}}_2{\mathrm{C}\mathrm{O}}_3+0.7{\mathrm{O}}_2\to \\ {}\kern10.25em {\mathrm{Li}}_{1.2}{\mathrm{Ni}}_{0.13}{\mathrm{C}\mathrm{o}}_{0.13}{\mathrm{Mn}}_{0.54}{\mathrm{O}}_2+1.6{\mathrm{H}}_2\mathrm{O}\uparrow +2.2{\mathrm{C}\mathrm{O}}_2\uparrow \end{array} $$

According to the TGA-DSC results in Fig. 1b and early papers on calcination temperature [19, 22, 23], the mixture is decomposed at 450 °C for 4 h and crystallization treatment at 850 °C for 12 h to obtain the target product.

The XRD patterns of precursor Ni_0.13Co_0.13Mn_0.54(C₂O₄)_0.8·1.6H₂O and product Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ are revealed in Fig. 2. The diffraction peaks of precursor in Fig. 2a can be indexed to monoclinic structure MnC₂O₄·2H₂O (JCPDS: 25–0544) [27, 28]. The further calcination of metal oxalate with Li₂CO₃ at 850 °C results in the formation of Li_1.2Co_0.13Ni_0.13Mn_0.54O₂, and the corresponding XRD pattern with the Miller indices is showed in Fig. 2b. All of the diffraction peaks can be attributed to hexagonal layered structure with space group of R $\overline{3}$ m except the weak diffraction peaks marked by asterisk. In fact, the Li₂MnO₃ component in a solid solution can also be referred to as the LiMO₂ (i.e., M³⁺ = Li⁺ _1/3Mn⁴⁺ _2/3) formula, meaning that tetravalent manganese and monovalent lithium construct the trivalent M layer in LiMO₂ lattice structures. Especially, the alternating arrangement of Li⁺ and Mn⁴⁺ ions in the Li⁺ _1/3Mn⁴⁺ _2/3 layer may induce the appearance of XRD superlattice signals. The asterisk-marked weak diffraction peaks correspond to the XRD characteristics of LiMO₂ superlattice structures within the 2θ range of 20 and 25 ^o, indicating the coexistence of crystalline Li₂MnO₃ phase in solid solution [12–15]. Also, the strong and sharp diffraction peaks of Fig. 2b indicate a good crystallinity of products.

Fig. 2

XRD patterns of precursor Ni_0.13Co_0.13Mn_0.54(C₂O₄)_0.8·1.6H₂O (a) and resulting Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ (b)

Table 1 reveals lattice parameters, the ratios of I₍₀₀₃₎/I₍₁₀₄₎ and the estimated crystallite size for Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ samples calculated from the XRD data using the R $\overline{3}$ m space group. The calculated lattice parameters are closed to the layered LiNi_1/3Co_1/3Mn_1/3O₂ [29]. As we know, the integrated intensity ratio of the (003) to (104) peak (I₍₀₀₃₎/I₍₁₀₄₎) in XRD pattern can be used as a measurement of layered structure and cation mixing, and a value greater than 1.2 is an indication of good layered structure and low cation mixing. Herein, the I₍₀₀₃₎/I₍₁₀₄₎ of as-prepared Li_1.2Co_0.13Ni_0.13Mn_0.54O₂ can reach 1.49, indicating the material has good layered structure, which also can be further confirmed by well peaks splits of (006)/(102) and (108)/(110) diffraction peaks [30, 31].

Table 1 Lattice parameters, the ratios of I₍₀₀₃₎/I₍₁₀₄₎ and the estimated crystallite size for Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ samples calculated from the XRD data using the R $\overline{3}$ m space group

The morphologies and surface structures of precursor Ni_0.13Co_0.13Mn_0.54(C₂O₄)_0.8·1.6H₂O and product Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ are shown in Fig. 3. The micro-size precursor has the size of 1–2 μm, and its particle surface is very smooth in Fig. 3a, b. Interestingly, some terrace-like structures can be observed for a single oxalate particle, which may be attributed to special nucleation habit of metal oxalate comparing with other precipitations such as metal carbonates and metal hydroxides [16, 18, 25]. As we expected, the morphology of precursor can be partially maintained after calcination with Li₂CO₃ at high temperature in Fig. 3c. The particles size of resulting Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ is slightly smaller than that of precursor and the average diagonal length decrease from 1.53 to 1.38 μm after calcination. From the enlarged particle picture in Fig. 3d, it can be found that the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ is aggregated by nanoparticles. Figure 4 reveals the TEM image of the edge of a single Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ particle. Some nanoparticles with a sharp corner and edge can be found in the picture. These nanoparticles have the size of 60–150 nm, which is a little larger than the estimated crystallite size (about 57 nm) based on full width at half maximum of (003) crystal plane and Sheerer equation (D = kλ/Βcosθ) in Table 1.

Fig. 3

SEM images of precursor Ni_0.13Co_0.13Mn_0.54(C₂O₄)_0.8·1.6H₂O (a, b) and resulting Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ (c, d)

Fig. 4

TEM image of the edge of single Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ particle

Figure 5 shows the energy-dispersive x-ray spectroscopy (EDS) spectrum of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ prepared from oxalate precursor route. The estimated element ratio of Ni/Co/Mn is 0.12(4):0.13(0):0.58 (1). Considering the allowable error (±10 %) of the EDS result, the measured nanoparticle is approximately homogeneous in chemical composition as that of solid solution Li_1.2Ni_0.13Co_0.13Mn_0.54O₂.

Fig. 5

EDS analysis of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂

Based on the above results, the formation process of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ can be described as follows: in the first step, after adding precipitant oxalic acid into mixed metal salts solution, the nucleation and crystal growth of metal oxalate will start, and the precursor can be obtained. In the next step, the oxalate precursor plays a role as sacrificed template on the formation of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂, and the structure composed of nanoparticles can be produced due to the generation of carbon dioxide and vapor. As an experimental result, the well-defined nanoparticles can provide a solid framework during the intercalation/decalation of lithium ions. Also, these nanoparticles can shorten the diffusion route of lithium ions and electrons, especially for lithium-rich manganese-based oxides, which have lower electrochemical conductivity [16, 19, 32].

The initial charge discharge profiles of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ at 0.1C (20 mA g⁻¹) within 2.0–4.6 and 2.0–4.8 V are shown in Fig. 6a. During the charging at the potential below 4.4 V, the capacity can be ascribed to deintercalation of Li ions accompanied by the oxidation of Ni²⁺ ions within LiNi_xCo_yMn_1-x-yO₂ component [14, 31]. The plateau nearby 4.5–4.6 V corresponds to both the loss of Li₂O from Li₂MnO₃ phases and the oxidation of Co³⁺ ions within active LiNi_xCo_yMn_1-x-yO₂ components. In the discharge profile, the corresponding 4.5-V plateau cannot be found, indicating that the Li₂MnO₃ would not be recovered and irreversible Li₂MnO₃ → MnO₂ → LiMnO₂ transformation accounts for the high-capacity feature of solid solution material [3–15]. The discharge capacity and coulombic efficiency of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ can be partially determined by cutoff voltage. At the voltage window of 2.0–4.6 V, a discharge capacity of 242.7 mAh g⁻¹ with coulombic efficiency of 77.6 % can be obtained. When the cutoff voltage is elevated to 4.8 V, the discharge capacity increases to 285.2 mAh g⁻¹, and the coulombic efficiency decreases to 75.8 %.

Fig. 6

Initial charge discharge profiles at 0.1 °C (a) and cycling performances (b) of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ within 2.0–4.6 and 2.0–4.8 V

After the initially preset one charge–discharge cycle at 0.1 C the electrochemical cycling stability of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ is measured at 0.5 C for the subsequent 100 cycles. As shown in Fig. 6b, when the cutoff voltage is set as 4.6 V, the second cycle discharge capacity is194.8 mAh g⁻¹, over the 100 cycles; the residual value is 178.5 mAh g⁻¹. Briefly, high capacity (178.5 mAh g⁻¹) and capacity retention (91.6 %) after 100 cycles can be obtained for Li_1.2Ni_0.13Co_0.13Mn_0.54O₂. However, when the cutoff voltage increases to 4.8 V, the discharge capacity of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ quickly drops from 212.3 (second cycle) to 154.7 mAh g⁻¹(100th cycle). According to the above results, a low cutoff voltage (i.e., 4.6 V) is more suitable for the electrochemical cycling purpose. Simultaneously, a summary for the electrochemical performances of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ obtained from different preparation routes and conditions is revealed in Table 2; by comparison, the electrochemical performances especially cycling stability of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ are not poor.

Table 2 Summary of the electrochemical performances of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ obtained from different preparation routes and conditions

In order to further study the electrochemical and structural changes at different cutoff voltages, typical 2nd, 10th, 50th, and 100th charge discharge curves and corresponding dQ/dV profiles of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ are revealed in Fig. 7. The solid solution material experiences voltage decay from the 2nd to the 101st cycle marked by arrow in Fig. 7a, c. Three pairs of redox peaks including layered Co⁴⁺/Co³⁺, Ni⁴⁺/Ni²⁺, and spinel Mn⁴⁺/Mn³⁺ can be observed in dQ/dV profiles as shown in Fig. 7b. Interestingly, as the cycles increase, the cathodic peak of layered Ni⁴⁺/Ni²⁺ nearby 3.8 V weaken gradually, and the cathodic peak of spinel Mn⁴⁺/Mn³⁺ nearby 3.0 V becomes stronger, meaning that the layered structure of solid solution is partially transformed into spinel structure during cycling [19]. In Fig. 7d, the structural phase transformation is more serious, and the serious transformation can account for the poor cycling stability when operating at the voltage window of 2.0–4.8 V. Figure 8 reveals the XRD patterns of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cycled after 100 cycles within 2.0–4.6 and 2.0–4.8 V. The peak intensity of 4.6 V cycled product is stronger than that of 4.8 V cycled product. Also, the value of I₍₀₀₃₎/I₍₁₀₄₎ decreases from 1.18 to 1.11 when the cutoff voltage increases from 4.6 to 4.8 V. These results suggest that the layered structure can be better retained at low cutoff, which delect is consistent with the result in Fig. 7.

Fig. 7

Typical 2nd, 10th, 50th, and 100th charge discharge curves and corresponding dQ/dV profiles of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ at 0.5 °C within 2.0–4.6 V (a, b) and 2.0–4.8 V (c, d)

Fig. 8

XRD patterns of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cycled after 100 cycles within 2.0–4.6 (a) and 2.0–4.8 V (b)

The rate capability of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ was also evaluated at different current densities and cutoff voltages as shown in Fig. 9. It is found that the rate capability performance is less affected by the different cutoff voltages in comparison with cycling stability. When 4.6 V is selected as the cutoff voltage, the 22nd specific discharge capacity of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ is 155.8 mAh g⁻¹ at 2 C and the 32nd reversible capacity at 5 C still keeps at a value of 128.3 mAh g⁻¹. More importantly, when the current density goes back to 0.1 C, the 52nd cycle discharge capacity returns to a high value of 246.1 mAh g⁻¹, which is 94.6 % of the initial discharge capacity at 0.1 C. These indicate that the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂, as a positive electrode material, could be a promising candidate for lithium ion batteries with good rate capacity.

Fig. 9

Rate capability of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ evaluated at different current densities within 2.0–4.6 and 2.0–4.8 V

Conclusion

In summary, the two-step preparation route including initial oxalate co-precipitation and subsequent solid state reaction with lithium carbonate is used to prepare high-capacity positive electrode material Li_1.2Ni_0.13Co_0.13Mn_0.54O₂. The structural measurements reveal that, the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ composed of nanoparticles has a good layered structure. As a lithium ion battery positive material, the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ shows good cycling stability and rate capability at the electrochemical window of 2.0–4.6 V. According to the above results, the good lithium storage ability of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ may be attributed to its ordered layered structure and special morphology.