Industrialization of tailoring spherical cathode material towards high-capacity, cycling-stable and superior low temperature performance for lithium-ion batteries

Abstract

Three different types of spherical cathodes (Li[Ni_0.6Co_0.2Mn_0.2]O₂) were synthesized via hydroxide co-precipitation method coupled with high temperature lithiation process. The particle size, nanostructure, specific surface area and pore distributions can be controlled as expected. X-ray diffraction patterns revealed that the as-obtained cathode materials had a typical hexagonal α-NaFeO₂ layered structure with a space group R3̄m. The electrochemical measurements demonstrate that Li[Ni_0.6Co_0.2Mn_0.2]O₂ with 3 μm-size in diameter exhibited higher initial coulombic efficiency (94.9%), rate capacity (156 mA h g⁻¹ at 900 mA g⁻¹), and low-temperature property (157 mA h g⁻¹ at 180 mA g⁻¹, 0 °C) in comparison with the larger one (12 μm). Most impressively, an ultra-stable capacity of 156 mA h g⁻¹ can be retained at 180 mA g⁻¹ even after 300 cycles at 0 °C. As is known, Li[Ni_0.6Co_0.2Mn_0.2]O₂ with 3 μm-size has the best result among the reported Li[Ni_0.6Co_0.2Mn_0.2]O₂-based cathode materials. The excellent electrochemical performance of the smaller size cathode results from the advantageous hierarchical nanorods architecture, porous characteristics, and reduced ions/electrons transport path.

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1. Introduction

Lithium-ion battery, by virtue of its higher operating voltage, higher energy efficiency, and longer cycle life, represents the state-of-the-art technology among the rechargeable batteries, which has attracted considerable attention of researchers.^1–10 The Ni-rich layered oxide cathode materials Li[Ni_xCo_yMn_1−x−y]O₂ (0 ≤ x ≤ 1, 0 ≤ y ≤ 1, LNCM) are promising candidates for high energy density battery applications in electric vehicles (EVs) or hybrid electric vehicles (HEVs) because of their high capacity of about 200 mA h g⁻¹.^11–13 However, the current commercial LNCMs (e.g., Li[Ni_0.6Co_0.2Mn_0.2]O₂) are known to have spherical morphologies with 10–15 μm diameter consisting of primary nanoparticles. Although these cathode materials can provide a high energy density, properties of quick charge and low temperature of the current lithium battery are severely hindered by the sluggish ions and electron diffusion kinetics problems.^14–17 To address these issues, great efforts have been made, including surface coating lithium ion conductive oxide,^18–20 anion/cation substitution,^21,22 reducing particle size (micro/nano level) and constructing special particle morphologies.^23–27 Among these approaches, nanostructured cathodes have been intensively investigated, and they indeed exhibit excellent electrochemical performances in half cell tests. It appears that nanosized electrodes might be perfect candidates among various electrode materials. However, considering practical battery application, nanosized electrodes have too low tap and packing density to yield enough volumetric and mass energy density, but they possess very high specific surface area and high surface energy, which may give rise to security issues.²⁸

According to the previous literature reports, cathodes with micro/nano hierarchical structures may be the most appropriate candidates because they can possess both the advantages of nanometer-sized building blocks and microsized assemblies, in which the former provide shorter ions/electrons transport path and the latter guarantee good structural stability.^29,30 Despite the laudable works in constructing micro/nano hierarchical structured electrode materials, the large-scale industrial production of LNCM cathode materials with smaller size (3 μm) remains a great challenge.

Herein, we report a general size-controlled strategy to fabricate cathode material Li[Ni_0.6Co_0.2Mn_0.2]O₂ with 3 μm in diameter by a hydroxide co-precipitation and high temperature lithiation process. For comparison, we also prepared cathode materials Li[Ni_0.6Co_0.2Mn_0.2]O₂ with 6 μm and 12 μm diameter. The obtained results indicate that Li[Ni_0.6Co_0.2Mn_0.2]O₂ having smaller size (3 μm) showed more outstanding performances compared with that of the larger Li[Ni_0.6Co_0.2Mn_0.2]O₂ (12 μm) in terms of rate capacity, low temperature characteristics and cycling life. The proposed smaller size cathodes strategy can also be extended to design and prepare other cathodes towards superior performances for LIBs.

2. Experimental

2.1 Materials

Nickel sulfate hexahydrate (NiSO₄·6H₂O), manganese sulphate monohydrate (MnSO₄·H₂O), cobalt sulphate heptahydrate (CoSO₄·7H₂O), sodium hydroxide (NaOH), and ammonium hydroxide (NH₄OH) are of analytical grade and used as received without any further purification.

2.2 Preparation of precursors and lithiated layered oxides

Spherical precursors [Ni_0.6Co_0.2Mn_0.2](OH)₂ with different sizes were synthesized via hydroxide co-precipitation method, as shown in Fig. 1.

Fig. 1 Schematic of the synthesis procedure for cathode material Li[Ni_0.6Co_0.2Mn_0.2]O₂.

Synthesis of spherical precursors [Ni_0.6Co_0.2Mn_0.2](OH)₂ (NCM-3, NCM-6, NCM-12). Taking NCM-3 as an example, firstly, 4 M NH₄OH solution was added into the continuously stirred tank reactor (CSTR) as the base solution. Then, 2.5 L of 2 M metal sulfates (Ni : Co : Mn = 6 : 2 : 2, molar ratio) was dissolved in deoxidized water and pumped into the CSTR under a nitrogen atmosphere. Moreover, 2.5 L of 4 M NaOH solution and 2 M NH₄OH solution were pumped into the reactor under a constant specific pH value (11.8). During the reaction, the temperature was kept at 60 °C, stirring speed was controlled at 1000 rpm min⁻¹, feeding time was 4 h and ageing time was 10 h. After the reaction, the hydroxide precursor (NCM-3) was obtained after washing with distilled water and finally dried at 110 °C for 24 h. The other two precursors (NCM-6 and NCM-12) were prepared via a similar co-precipitation route except that different amounts of base solution (3 M NH₄OH, 2 M NH₄OH, respectively), chelating agent (1.5 M NH₄OH, 1 M NH₄OH, respectively), pH values (11.4, 11.0, respectively), stirring speed (700 rpm min⁻¹, 500 rpm min⁻¹, respectively), and feeding time (10 h, 26 h, respectively) were employed.

Synthesis of spherical precursors Li[Ni_0.6Co_0.2Mn_0.2]O₂ (LNCM-3, LNCM-6, LNCM-12). The obtained [Ni_0.6Co_0.2Mn_0.2](OH)₂ precursors (NCM-3, NCM-6, NCM-12) were thoroughly mixed with LiOH·H₂O with a molar ratio of 1 : 1.05 and were calcined at 800 °C for 15 h in air to obtain spherical lithiated layered oxides (LNCM-3, LNCM-6, LNCM-12).

2.3 Characterization

The chemical compositions of as-obtained precursors were determined by inductively coupled plasmas spectrometer (ICP, 6000 Series, Thermo SCIENTIFIC). Powder X-ray diffraction (BRUKER D8 Focus Diffractometer) with Cu Kα radiation was used to identify the crystalline phase of the prepared materials in the range of 10–80° (2θ) with a step size of 0.03°. The morphology of the precursors and lithiated oxides was observed by means of scanning electron microscope (SEM, Philips XL30) at an accelerating voltage of 10.0 kV. BET surface area and pore size measurements were carried out by N₂ adsorption at 77 K on an Autosorb iQ Station 2. Tap densities were determined using a tap-density tester (ZS-201, Liaoning Instrument Research Institute Co. Ltd.).

2.4 Electrochemical measurements

The positive electrode was prepared by blending the cathode Li[Ni_0.6Co_0.2Mn_0.2]O₂, acetylene black, and PVDF (80 : 10 : 10 in weight ratio) in NMP. The slurry was cast on Al foil with the areal mass loading of 5.5 ± 0.5 mg cm⁻², and dried at 120 °C for 12 h under vacuum, followed by roll-pressing. Pure lithium foil was used as the anode. The electrolyte consisted of 1 M LiPF₆ in ethylene carbonate (EC) and diethyl carbonate (DEC) (1 : 1 in volume). A glass microfiber filter (934-AH, Whatman, UK) was used as the separator. The half-cells were assembled in an Ar-filled glove-box with CR2032 coin half cells. Charge and discharge measurements were carried out at different current densities in the voltage range of 3.0–4.4 V (vs. Li⁺/Li) using a NEWARE battery test system (CT-3008) at 25 °C and 0 °C. Electrochemical impedance measurements were carried out using a Solartron 1255 B frequency response analyzer (SolartronInc., UK) in the frequency range of 100 kHz to 10 mHz, applying an AC signal of 10 mV amplitude voltage.

3. Results and discussion

3.1 Material characterization

Atomic ratios of Ni, Co, Mn in as-prepared precursors are analyzed by ICP, and the values are listed in Table S1, ESI.† From the ICP results, we can confirm that the chemical compositions of the measured samples are approximately [Ni_0.6Co_0.2Mn_0.2](OH)₂. Fig. 2 exhibits the XRD patterns of the as-prepared precursors and cathode materials with different sizes. It can be clearly seen that the XRD pattern of the precursors is in accordance with the typical M(OH)₂ (M = Ni, Co, Mn) structure. The absence of impurity phases indicates that Ni, Co and Mn have been homogeneously distributed in the precursor particles.³² Moreover, it is also found that all the peaks of the three cathode samples can be indexed with a typical layered structure based on the hexagonal α-NaFeO₂ structure with space group R3̄m without any impurity phases, as shown in Fig. 2b. In addition, it is observed that the clear splitting of the (006)/(102) and (108)/(110) pairs is an indication of the formation of a well-crystallized layered structure for the three samples.^31,32 The intensity ratio of I₀₀₃/I₁₀₄ is indicative of undesirable cation mixing and the ratio lower than 1.2 indicates the migration of some Ni²⁺ ions to Li⁺ sites, blocking the channel for Li⁺ ion transport.³³ As expected, the ratio of I₀₀₃/I₁₀₄ of these cathodes are higher than 1.2 (Table 1). From these results, we can conclude that the difference in the particle size does not significantly change crystal structure.

Fig. 2 Powder XRD patterns of (a) the precursors [Ni_0.6Co_0.2Mn_0.2](OH)₂, and (b) the cathodes Li[Ni_0.6Co_0.2Mn_0.2]O₂ with different sizes.

Table 1 Intensity ratio of I₀₀₃/I₁₀₄ of the Li[Ni_0.6Co_0.2Mn_0.2]O₂ before and after cycles

Sample	Before cycle	After cycle
	I003/I104	I003/I104
LNCM-3	1.4423	1.4368
LNCM-6	1.3246	—
LNCM-12	1.3389	0.8289

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Fig. 3 shows the comparison of the SEM images of the precursors and cathodes Li[Ni_0.6Co_0.2Mn_0.2]O₂ with different sizes. Every kind of precursor and cathode shows homogeneous spherical secondary particles assembled with densely aggregated nanorods (Fig. 3a and d), nanoneedles (Fig. 3b and e) and nanoparticles (Fig. 3c and f), and average sizes are 3 μm (Fig. 3g), 6 μm (Fig. 3h) and 12 μm (Fig. 3i), respectively. The formation mechanism of the different nanostructures might be attributed to different amounts of NH₄OH and stirring speed. Adding large amounts of NH₄OH and high stirring speed are in favor to formation of the primary grain nanorods and assembling into the secondary particles (NCM-3). In contrast, low concentration of NH₄OH and low stirring speed are prone to form secondary particles composed of primary nanoparticles (NCM-12). However, NCM-6 is in between NCM-3 and NCM-12, which is in accordance with the previous relevant literature.³⁴ The size statistic results are based on the analysis of SEM images in Fig. S1, ESI.† The tap densities of the three cathodes (LNCM-3, LNCM-6, LNCM-12) are 1.9, 2.3 and 2.6 g cm⁻³, respectively. The specific surface area and pore size of LNCM-3 and LNCM-12 are measured to be 139.8 m² g⁻¹, 4.0 nm and 85.9 m² g⁻¹, 3.8 nm, respectively (Fig. 4). These results demonstrate that the particle size and microstructure of the hydroxide precursors can be easily controlled via hydroxide co-precipitation method.

Fig. 3 SEM images of low- and high-magnification of [Ni_0.6Co_0.2Mn_0.2](OH)₂ (a–c) and Li[Ni_0.6Co_0.2Mn_0.2]O₂ (d–f); (a and d) 3 μm; (b and e) 6 μm; (c and f) 12 μm, and the corresponding size distribution of Li[Ni_0.6Co_0.2Mn_0.2]O₂ (g–i).

Fig. 4 (a) N₂ adsorption/desorption isotherms of LNCM-3 and LNCM-12; (b) pore size distribution of LNCM-3 and LNCM-12.

3.2 Electrochemical properties

To study the size and microstructure effects of the cathodes on electrochemical performances, we take two typical size cathodes (3 μm and 12 μm) as example to evaluate their electrochemical performances via 2032 coin half cells at room temperature (25 °C) and low temperature (0 °C). Fig. 5a and c show the initial charge–discharge curves of both Li[Ni_0.6Co_0.2Mn_0.2]O₂ cathodes from 18 mA g⁻¹ (0.1C) to 900 mA g⁻¹ (5C) between 3.0 and 4.4 V at 25 °C. Notably, both cathodes have almost the same initial charge capacity of about 208 mA h g⁻¹, the cathode with smaller size (3 μm) had a higher discharge capacity of 196 mA h g⁻¹ accompanied with higher initial coulombic efficiency (94.4%) as shown in Fig. 5a. However, LNCM-12 has a relatively lower discharge capacity of 189 mA h g⁻¹ with relatively lower coulombic efficiency of 90.4% (Fig. 5c). The same phenomenon can also be observed under the low temperature environment test, as shown in Fig. 5b and d. Furthermore, the rate capacity of both cathodes is also evaluated at varying current rates ranging from 0.1C to 5C (Fig. 5e and f) at 25 °C and 0 °C. As expected, LNCM-3 delivers a much higher capacity than LNCM-12 at each current rate. At 25 °C, the average specific capacities for LNCM-3 were 196, 185, 178, 170, 165, and 156 mA h g⁻¹ at current rates of 0.1C, 0.2C, 0.5C, 1C, 2C and 5C, respectively. Surprisingly, the initial discharge capacity of LNCM-3 is 157 mA h g⁻¹ at a rate of 1C at 0 °C, which is much higher than that of LNCM-12 at the same rate (123 mA h g⁻¹). This excellent low-temperature capacity can be comparable with that of commercially available LiCoO₂ and LiFePO₄ under the condition of the room temperature.

Fig. 5 Typical charge–discharge profiles of LNCM-3 (a) at 25 °C and (b) 0 °C; and LNCM-12 (c) at 25 °C and (d) 0 °C with different rates. Rate capacity of LNCM-3 and LNCM-12 (e) at 25 °C and (f) 0 °C; cycling performance of LNCM-3 and LNCM-12 (g) at 25 °C and (h) at 0 °C between 3.0 and 4.4 V.

Inspired by the smaller size cathode performance, long cycling performances of 3 μm- and 12 μm-size cathode materials at current density of 1C were studied and the results are shown in Fig. 5g and h. LNCM-3 shows excellent cycling performance over the LNCM-12 with capacity retention of 96% and 98% after 300 cycles at 25 °C and 0 °C, respectively. In contrast, LNCM-12 shows a rapid capacity loss especially at low temperature, resulting in capacity retention of 88% and 50% at 25 °C and 0 °C, respectively. The properties of LNCM-3 in terms of cycle, rate capacity, and low-temperature are much superior to those of the reported Li[Ni_0.6Co_0.2Mn_0.2]O₂-based cathodes (Table S2, ESI†), suggesting the effectiveness of our strategy in improving the electrochemical performance. Moreover, post-mortem SEM images show that the spherical morphologies of LNCM-3 can still be well maintained even after 300 cycles (Fig. 6a and b).

Fig. 6 SEM images of (a) LNCM-3 and (b) LNCM-12 after 300 cycles. (c) XRD patterns of LNCM-3 and LNCM-12 after 300 cycles.

Such excellent electrochemical performance is mainly attributed to its advantageous structural stability of hierarchical nanorods assembled with 3 μm spherical secondary particles and the faster kinetics of ions and electrons transport compared to that of the 12 μm-size particles. In addition, the larger specific surface area and pore size of LNCM-3 might also contribute to the high capacity of reversibility via providing sufficient contact between electrolyte and inner particles.^35,36 XRD tests are also carried out with the sample after 300 cycles to further study the structural stability of LNCM-3 and LNCM-12. As shown in Fig. 6c, the lattice parameters of LNCM-3 cathode are almost identical to the pristine electrode, the clear peak splitting of the (006)/(102) and (108)/(110) pairs indicates that the well layered structure is maintained after 300 cycles. However, the LNCM-12 cathode has a severely weak peak intensity of I₍₀₀₃₎/I₍₁₀₄₎ compared to that of the LNCM-3 cathode, as shown in Table 1, owing to the changes of crystal structure from Li⁺/Ni²⁺ mixing. The occupation of Ni²⁺ in the Li layer leads to the gradual declination of the capacity upon cycling.

To further understand the electrode reaction kinetics of both the samples, the EIS spectra is given in Fig. 7. The solid dots were the experimental results and the solid lines were fitted by ZSimpWin software with the proposed equivalent circuit model (inset of Fig. 7). R_s is the solution resistance. R_sf and C_sf are the resistance and the associated constant phase element of the surface film, respectively. R_ct and C_dl are the charge transfer resistance and constant phase element at the electrode/electrolyte interface, respectively. Z_w is Warburg impedance. Apparently, the EIS spectra of both electrodes are almost overlapped before the cycle. However, after 300 cycles, the R_sf and R_ct of the LNCM-3 cathode are much smaller than that of the LNCM-12 cathode, indicating a lower film resistance and charge transfer resistance for the LNCM-3, which is beneficial to its electrochemical performances.

Fig. 7 EIS spectra of (a) LNCM-3 and (b) LNCM-12 cathodes before and after 300 cycles.

4. Conclusions

In summary, three cathodes (Li[Ni_0.6Co_0.2Mn_0.2]O₂) were successfully synthesized via a co-precipitation method followed by a high temperature lithiation process. The size and microstructure of the precursors can be easily tailored by controlling feeding time, ammonium hydroxide concentration, pH value, and stirring rate during the reaction process. The smaller size particle, unique microstructure, and porous characteristics contribute to the excellent electrochemical performances. Impressively, when evaluated as cathode materials for LIBs, Li[Ni_0.6Co_0.2Mn_0.2]O₂ with 3 μm-size exhibits a high reversible capacity (194 mA h g⁻¹ at 18 mA g⁻¹), excellent rate capacity (156 mA h g⁻¹ at 900 mA g⁻¹), remarkable low-temperature property (157 mA h g⁻¹ at 180 mA g⁻¹), and ultralong low temperature cycling life (300 cycles, capacity retention of 99% at a current density of 180 mA g⁻¹). The proposed smaller size cathodes strategy can be extended to design and prepare other cathodes for superior power LIBs.

Acknowledgements

The authors are most grateful to the NSFC, China (21225524, 21505127 and 21501169), the Department of Science and Techniques of Jilin Province (20150203002YY, 20150201001GX and 20150204065GX) and special funds for the Construction of Taishan Scholars (No. ts201511058).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22040a

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