Novel nanostructured LiMn₂O₄ microspheres for high power Li-ion batteries

Abstract

A novel type of nanostructured LiMn₂O₄ microsphere was successfully synthesized by wet ball milling combined with high-temperature calcinations. The prepared materials were characterized by XRD, SEM, BET surface area measurement, electrochemical galvanostatic cycling tests, EIS, and XRF. The results show that the well-shaped microspheres with a diameter of approximately 0.5–2 μm are composed of reorganized nanoparticles. The LiMn₂O₄ microspheres have a high tap density of 2.8 g cm⁻³, and as electrodes, these micro/nanostructured spheres show excellent rate capability and cycle stability, resulting in a high volumetric energy density, even at elevated temperature. These materials are promising for power battery applications.

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Introduction

Lithium ion batteries, which are used in the portable electronics market, are promising candidates for power applications in electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).¹ The ordered spinel lithium manganese oxide, LiMn₂O₄, has been extensively and intensively studied as a cathodic material for power lithium-ion batteries. Its advantages include low cost, environmental friendliness, easy preparation, high-potential plateaus, and intrinsic thermal stability under severe environmental conditions.² However, this material has the problem of severe capacity fading during charge–discharge cycles,³ and numerous studies have determined that the capacity fading is caused by various factors such as Jahn–Teller distortion, a two-phase unstable reaction,⁴ slow dissolution of manganese into the electrolyte,⁵ lattice instability,⁶ and particle-size distribution.⁷ The rate performance and cyclability of LiMn₂O₄ have been improved by coating with carbon or a metallic conducting layer, by doping with isovalent ions, or reducing the particle size to nanoscale. Additionally, coating and downsizing strategies will adversely affect the tap density and volumetric energy density.^8–10

Therefore, microsphere powder with a micro/nanostructure is highly desired for designing high-performance lithium-ion batteries with high volumetric energy density and good rate capability.¹¹ Recently, Sun et al. reported the synthesis and electrochemical performance of LiMn₂O₄ nanosheets with high porosity and reactivity, but unfortunately, there was no obvious improvement in tap density after nanosizing.¹² Guo et al. have pointed out that LiMn₂O₄ microspheres prepared from porous Mn₃O₄ with sizes ranging from 7–9 μm exhibit a high tap density with excellent cycling performance and rate capability for lithium-ion batteries.¹³ Generally, manganese dioxides are good raw materials both as Mn sources and templates to synthesize novel nanostructured LiMn₂O₄ cathode materials for lithium-ion batteries.^14,15 It has been reported that nanostructured crystallites of MnO₂ can be exfoliated from layered protonic manganese oxide in a solution of tetrabutylammonium (TBA) hydroxide, and nanoporous LiMn₂O₄ materials prepared from these exfoliated nano-MnO₂ exhibit superior electrochemical properties as compared to the bulk material.^12,16,17 Additionally, Jang et al. have previously carried out an extensive study on the exfoliation of various types of layered materials and announced that ball milling can be used to exfoliate layered materials to produce nanostructure.¹⁸

To our knowledge, there are few reports on the use of MnO₂ as a precursor to yield LiMn₂O₄ spheres composed of nanostructured particles with a total particle size on the micrometre scale. Herein, we report a facile ball milling approach combined with high-temperature calcinations to synthesize LiMn₂O₄ microspheres, which consist of primary nanoscale particles with an open 3D microstructure and which have the potential for broad applications. Our results suggest an interesting approach for controlling microsphere morphology by exfoliating the MnO₂ precursors into “cubic shaped nanoparticles” and growth of crystals.

Experimental section

Synthesis of the cathode material

The nanostructured LiMn₂O₄ microspheres were synthesized by a simple high-energy ball milling approach combined with high-temperature calcinations. Using a typical synthesis procedure, stoichiometric amounts of MnO₂ (Sinopharm Chemical Reagent, AR) and Li₂CO₃ (SIMBOL Materials, 99.99%) were mixed by high-energy ball milling for 8 h with deionized water as the dispersant. The mixture was dried and calcinated at 800 °C for 9 h under an air atmosphere, and then annealed at room temperature to obtain well-crystallized LiMn₂O₄ microspheres. To illustrate the effects of this novel structure, bulk LiMn₂O₄ materials (micrometre scale particles) were synthesized via a sol–gel method combined with high-temperature calcinations according to our previous work.¹⁹

Characterization

The phase identification of powders was conducted using X-ray diffraction measurement (XRD, D/MAX 2550V, Japan) with Cu Kα radiation (λ = 0.15423 nm). The morphologies of the precursors and prepared sample were evaluated by field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan). The surface areas of as-prepared samples were determined via nitrogen adsorption using a Micromeritics TriStar surface area and porosity analyser (BET, BJH, TriStar 3000, USA). The samples were degassed under vacuum (100 mTorr) at 150 °C for 16 h prior to analysis. Then the surface area was calculated using the Brunauer–Emmet–Teller (BET) method. The tap density of the LiMn₂O₄ powders was measured as follows: a certain quantity of powder was added to a dry measuring cylinder, and the measuring cylinder was then tapped until the volume of the powders did not change. Then, the tap density was calculated by determining the ratio of mass-to-volume of the powders. Electrochemical studies were conducted in standard two electrode CR2032 coin-cell configurations. The composite electrodes were prepared by mixing active materials (LiMn₂O₄) with carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 80 : 10 : 10 in N-methyl-2-pyrrolidone (NMP). The cells were assembled in an argon-filled glove box (Super1220/750, Mikrouna) using lithium metal as the anode electrode, a polypropylene microporous film (Cellgard2400) as the separator, and 1 M LiPF₆ in ethylene carbonate (EC) and diethyl carbonate (DEC) (1 : 1, v/v) (Guangzhou Tinci, China) as the electrolyte. The LAND cell tester (LAND CT2001A, Wuhan Jinnuo, China) was used to perform the galvanostatic charge–discharge tests between 3.5 and 4.3 V (versus Li/Li⁺) at room temperature (25 °C) and 55 °C. Electrochemical impedance spectroscopy (EIS) measurements were performed using an electrochemical workstation (CHI660D, Shanghai Chenhua, China). The EISs were potentiostatically measured at the cell's open circuit voltage (OCV) with an AC oscillation of 5 mV amplitude over the frequencies from 10⁵ Hz to 10⁻² Hz. A stable OCV was obtained by cycling the cell at a constant charge–discharge rate to the desired value and then leaving the circuit open for 10 min. The impedance parameters are derived from fitting the collected EISs using ZSimpWin software.

To investigate the dissolution of Mn from the surface of LiMn₂O₄ into the electrolyte, the lithium anodes used as counter electrodes for the LiMn₂O₄ microspheres were examined by X-ray fluorescence (XRF) after 100 cycles at 25 °C and 55 °C. The cell we adopted in this experiment is a removable battery model, and the resulting lithium anodes were dried under vacuum after cycling and analysed using X-ray fluorescence to determine the amounts of Mn-containing complexes deposited in the SEI layer on the surface of the lithium anode.

Results and discussion

Fig. 1 shows the XRD and SEM images of the obtained product. The XRD pattern of the LiMn₂O₄ microspheres in Fig. 1(a) displays features of the spinel structure with the Fd3m space group (JCPS card no. 35-0782), and no peaks of an impurity phase were detected. The parameters were calculated by least-squares fitting with the peak-top values in the pattern. Table 1 shows the unit cell parameters from the powder XRD pattern and BET surface areas of the standard, bulk, and microspheric LiMn₂O₄ materials. It can be seen that the decreasing crystallite size leads to a small shrinkage of cell volume (a and V). The decrease in unit cell parameters for LiMn₂O₄ is frequently ascribed to substitution of the Mn-ion by the Li-ion (Li^8a[Li_xMn_2−x]^16dO₄).²⁰

Fig. 1 (a) Powder X-ray diffraction pattern, (b and c) SEM images of the microsphere's overall morphology, and (d) close-up SEM image of individual microspheres.

Table 1 Unit cell parameters from the powder XRD pattern and BET surface areas of the standard, bulk, and microspheric LiMn₂O₄ materials

Samples	Space	a (Å)	Volume (Å^3)	SBET (m^2 g^−1)
Standard LMO	Fd3m	8.245	560.9	None
Bulk LMO	Fd3m	8.305	572.8	3.8
Microsphere LMO	Fd3m	8.238	559.1	24.7

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Fig. 1(b)–(d) shows the SEM images of the LiMn₂O₄ sample. The overall morphology in Fig. 1(b) and (c) reveals that the LiMn₂O₄ sample consists of well-distributed microspheres with diameters of approximately 0.5–2 μm. From the close-up SEM image of individual microspheres in Fig. 1(d), it can be clearly seen that these assembled microspheres are composed of many cubic-shaped nanoparticles that were assembled by the primary nanocrystallites. It is also very interesting to see that some of the cubic nanoparticles have a “twin” structure composed of cubic particles that are forced together to form a spherical structure. Such a well-organized structure is expected to facilitate electrolyte penetration into the electrode, thus providing more interface area between the electrode material and the electrolyte. Combined with the results of the BET surface area measurement in Table 1, we can confirm that the LiMn₂O₄ microspheres exhibit a higher surface area than the bulk material, which is contributed by the irregular surface and protrusions of the microspheres.

To identify the basic structural/compositional information and growth mechanism of the nano/microstructural LiMn₂O₄ spheres, the SEM images of the bulk MnO₂ and ball-milled precursors are presented in Fig. 2. Fig. 2(a) and (b) shows that before the ball milling process, the bulk MnO₂ particles exhibit an irregular cubic structure in microscale. However, after ball milling for a long period of time, the mixed precursors form a spherical structure consisting of the nanostructured MnO₂ with a particle size ranging from 20–100 nm and Li sources (Fig. 2(c) and (d)). On the basis of the above experimental evidence, the formation of nanostructured LiMn₂O₄ microspheres can be ascribed to the protonation of MnO₂ and ball milling. With the high-energy mechanical forces contributed by ball milling, the insertion of lithium ions and water molecules can form a type of protonic manganese oxide, Li_xMnO₂·yH₂O,¹⁷ and the MnO₂ nanoparticles peel off due to the swelling and exfoliation behavior of these cubic nanomaterials. Then, the delaminated MnO₂ with free Li ions (mostly Li₂CO₃ molecules) was restacked into inorganic microsphere precursors. Therefore, the overall shape of the resultant products as well as the prepared precursors is typically spherical.

Fig. 2 SEM images of the bulk MnO₂ (a and b) and ball-milled precursors (c and d).

At present, the batteries for electric vehicles are still large, and the tap density of electrode materials is directly related to the volumetric energy density of the battery.²¹ The tap density of LiMn₂O₄ with nanoparticles is generally less than 2.0 g cm⁻³; however, the tap densities of these nanosheets consisting of closely packed microspheres can reach a certain stage of 2.8 g cm⁻³. This characteristic enables LiMn₂O₄ microspheric electrodes to have a considerably high volumetric energy density.

The electrochemical properties of LiMn₂O₄ microspheres were evaluated in a half-cell configuration between 3.5 and 4.3 V. The discharge curves of the spinel microspheres are shown in Fig. 3(a). The discharge curves of bulk and microspheric LiMn₂O₄ materials exhibit two pseudo plateaus at approximately 3.9 and 4.1 V, which represent the typical electrochemical behavior of spinel LiMn₂O₄.¹⁴ As shown in Fig. 3(a), these nano/microstructure-based compounds achieve a higher capacity than the bulk materials at micrometer scale, which can be explained by the short lithium-ion diffusion length. The reversible specific capacities of LiMn₂O₄ microspheres as displayed in Fig. 3(b) are 128.3 mA h g⁻¹, 122.6 mA h g⁻¹, 108.3 mA h g⁻¹, and 68.7 mA h g⁻¹ at 0.1 C, 1 C, 10 C, and 50 C charge–discharge rate, respectively, and remain at approximately 100 mA h g⁻¹ at 10 C for approximately 70 cycles and 54 mA h g⁻¹ even at 50 C for a total of 170 cycles, which indicates that at a higher rate, the superior behavior of the LiMn₂O₄ microsphere is revealed. Although the LiMn₂O₄ microspheric materials initially exhibit a slightly reduced discharge specific capacity, they exhibit good cycle stability, as shown in Fig. 3(c). We also tested the as-prepared material as a cathode in a half-cell to determine its capacity and cyclability at elevated temperatures (55 °C). Compared with room temperature performance, it is apparent that although the LiMn₂O₄ microspheric materials initially exhibit a slightly reduced discharge specific capacity, they show good cycle stability from the 40^th cycle. The materials render an 85.4 mA h g⁻¹ discharge capacity, and the retention is 80.3% after 70 cycles, in contrast to the results obtained under ambient temperature of 100.2 mA h g⁻¹ and 92.5%, respectively. It was observed that the discharge capacity still remains at approximately 81.5 mA h g⁻¹ after 100 cycles (76.6% of the initial capacity at 10 C). This excellent cyclability of the materials without any surface modification is mainly attributed to the tight stacking of LiMn₂O₄ nanocrystallites and structural robustness of the assembled microspheres.

Fig. 3 (a) Characteristic low rate (1 C) discharge curves, (b) rate performance studies, (c) long-term cyclability at a high rate (10 C and 50 C) at room temperature and 55 °C, and (d) electrochemical impedance spectroscopy (EIS) results of LiMn₂O₄ microspheres in the half-cell.

Fig. 3(d) shows a series of electrochemical impedance spectroscopy (EIS) results. Table 2 shows the impedance parameters derived using an equivalent circuit model for bulk LiMn₂O₄ and LiMn₂O₄ microspheres. The EIS spectra consist of semi-circles and a slope. The semicircle in the high middle frequency region is attributed to the lithium-ion migration through the SEI film and charge transfer reaction, and the slope in the low frequency region is attributed to lithium-ion diffusion in the bulk electrode. Combined with the impedance parameters in Table 2 derived from the Nyquist plots, the LiMn₂O₄ microspheres exhibited a lower charge transfer resistance (R_ct) than the bulk materials, which is attributed to the lithium-ion diffusion in the electrode. This result convincingly demonstrates the excellent reversibility and cyclability of nanostructured LiMn₂O₄ microspheres at a high rate.

Table 2 Impedance parameters derived using an equivalent circuit model for bulk LiMn₂O₄ and LiMn₂O₄ microspheres (fully discharged)

Sample	Rf (Ω)		Rct (Ω)
	1 C	10 C	1 C	10 C
Bulk LiMn2O4	10.7	12.5	124.5	257.2
LiMn2O4 microspheres	9.8	11.2	67.8	103.4

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Our previous work disclosed that the Mn dissolution of bulk LiMn₂O₄ increases more remarkably than that of ion-doped LiMn₂O₄, and it is the most important cause of cycle performance deterioration.^19,22 As shown in Fig. 4, the lower Mn content on the surface of the lithium anode was achieved using the nanosheet LiMn₂O₄ microspheres as the cathode rather than bulk LiMn₂O₄. The higher the discharge rate, the easier it is to restrain Mn dissolution. The XRF analysis shows that the micro/nanostructure can inhibit Mn dissolubility from the active electrode into the electrolyte, which will deposit on the anode and lower the cyclability.

Fig. 4 Quantitative XRF analysis of Mn and/or the Mn-containing complexes on the lithium anode surface at different rates and at room temperature after 100 cycles.

The remarkable advantage of the LiMn₂O₄ materials is their excellent rate capability and cyclability with an improved volumetric energy density. These excellent properties may be the result of the following: (a) a fast reaction and ionic diffusion kinetics of nanostructured LiMn₂O₄ with a high surface area; (b) the well-crystallized and closely assembled nanocrystallites; and (c) the structural robustness of the LiMn₂O₄ microspheres. The electrochemical performance of the LiMn₂O₄ microspheres might be further optimized by surface coating and ion doping.

Conclusions

In summary, we have developed a facile two-step process for preparing nanostructure-based LiMn₂O₄ microspheres with a well-defined spinel crystal structure. This novel morphology with an irregular surface and protrusions on the surface provides interlaminar space that allows electrolyte penetration, thus reducing the diffusion path and improving the ionic conductivity of lithium ions. This feature, combined with the high tap density, results in electrodes that have a high volumetric energy density and excellent rate capability. This novel method can be extended to obtain various nanostructure-based microspheric materials (e.g., LiMO₂, LiMPO₄) for use in energy storage and conversion.

Acknowledgements

The work is supported by the Shanghai Leading Academic Discipline Project (B502) and Shanghai Key Laboratory Project (08DZ2230500).

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