Construction of LiMn₂O₄ microcubes and spheres via the control of the (104) crystal planes of MnCO₃ for high rate Li-ions batteries

Abstract

We have studied a synthetic route to control the morphology of MnCO₃ precursors. Taking the (104) crystal planes in the structure of MnCO₃ as the research point, the hydrothermal method was used to synthesize MnCO₃ cubes with highly exposed (104) crystal planes and densely crystallized MnCO₃ spheres by changing the water–ethanol reaction system. The MnCO₃ cubes and spheres were used as self templates to prepare spinel LiMn₂O₄ by thermal decomposition and topological crystallization. The formation mechanism of MnCO₃ and LiMn₂O₄ was analyzed using characterization methods such as X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. Electrochemical tests evidenced that the electrochemical performance of the as-made cubic and spherical LiMn₂O₄ significantly improved compared with that of pristine LiMn₂O₄. The results manifested that the LiMn₂O₄ cubes and spheres have superior discharge capacity, delivering first discharge capacities of 130 and 115.1 mA h g⁻¹ at 0.5C, and 96.4 and 88.3 mA h g⁻¹ even at a high rate of 20C, respectively. After calculating the Li⁺ diffusion coefficients of the samples, the results elicited that the diffusion ability of the Li⁺ in the cubic and spherical LiMn₂O₄ was significantly improved.

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1. Introduction

As a variety of lithium ion battery cathode material, spinel LiMn₂O₄ is one of the most promising candidates because of its low cost, low toxicity, and environmental friendliness.^1–5 However, its low charge transfer rate and slow Li⁺ diffusion ability result in poor rate performance,^6–8 which seriously restricts the application of this material in hybrid electric vehicles. To the best of our knowledge, the rate performance is closely related to the charge transfer rate, and the charge transfer rate is limited by the diffusion of Li⁺ in the material.⁹ The characteristic diffusion time of Li⁺ is known from the equation τ = l²/D,^9,10 which is proportional to the diffusion distance (l) and inversely proportional to the Li⁺ diffusion coefficient (D). To reduce the diffusion time, either a shortened diffusion distance or increased diffusion coefficient is needed. The preparation of LiMn₂O₄ as a nano-material with a certain morphology is a method that can effectively shorten the diffusion distance between lithium ions.¹¹ For example, it has been prepared as nanoparticles,¹² nanowires,⁷ nanotubes,¹³ nanorods,¹⁴ nanospheres,¹⁵ and so on. However, some nanomaterial shapes are complicated to prepare and may encounter some undesirable side reactions due to the high specific surface area in the operating environment of the batteries.^15,16 Therefore, preparation of a porous micro-sized cathode material with a certain morphology is still one of the strategies for effectively improving the electrochemical performance.

Among the synthetic methods used to obtain a certain morphology, the hard template method and the self-template method have been most commonly used. The hard template method is costly to use and usually requires attention to the additional reaction against LiMn₂O₄.¹⁷ For the self-template method, precursors with a certain morphology can be retained before and after the reaction. Among these synthesis strategies of LiMn₂O₄, MnCO₃ is a relatively common reaction precursor. That is to say, LiMn₂O₄ produced using the self-template method commonly possesses a similar morphology to its corresponding precursor MnCO₃ during the thermal decomposition and lithiation processes, which is reasonable and beneficial for controlling the structures of the final products.

To fabricate MnCO₃ precursors, at present the hydrothermal/solvothermal method and facile room temperature precipitation method are generally employed.^18–24 Rapid precipitation and aggregation usually leads to spherical shaped MnCO₃. Besides this, Huang et al.²¹ explored the pH value of the precipitation system and prepared cubic and spherical MnCO₃ precursors. Wu et al.²² fabricated LiMn₂O₄ from cubic MnCO₃ and spherical MnCO₃ precursors by adding cetyl trimethylammonium bromide (CTAB) and adjusting the pH value of the precipitation system. However, in order to ensure uniform precipitation of the precursor, the precipitation temperature and the pH value must be strictly controlled, and the pH value boundary between the two morphologies is narrow.

MnCO₃ is similar in structure to CaCO₃, one of the most widely studied biominerals, as they belong to the same MCO₃ (M = Ca, Mn, Ba, etc.) crystalline type. Generally, polycrystalline biominerals exhibit various morphologies, with small crystal building blocks that can be used to compose complex structures.^25,26 Herein, the most common CaCO₃ crystal morphology is calcite-type rhombohedra,²⁷ with the characteristic (104) crystal planes of the morphology having the lowest surface energy.²⁸ Additives can be adsorbed onto the specific crystal planes of nanocrystals, thus changing the subsequent aggregation process.²⁹ In particular, some organic substances (for example, ethanol) have strong interactions between the (104) crystal planes.^27,30–34 Based on the relative theories above, we prepared a MnCO₃ cube precursor with highly exposed (104) crystal planes using a hydrothermal method without strict control of the pH and the addition of a structure directing agent, and changed the crystal stacking mode by simply adding ethanol solvent into the spherical material without changing the other conditions. The corresponding microporous cubic and spherical LiMn₂O₄ materials were then obtained by self-templated thermal decomposition and lithiation reactions and used as cathode materials for lithium ion batteries, wherein the cubic and spherical LiMn₂O₄ exhibited good cycling performance and rate performance, and the lithium ion diffusion ability was significantly improved. We have also discussed in detail the formation mechanism of the morphological diversity of MnCO₃ and LiMn₂O₄.

2. Experimental

2.1. Synthesis of MnCO₃ cubes and spheres

The cubic and spherical MnCO₃ precursors were prepared using a hydrothermal method. Firstly, 1 mmol of MnSO₄·H₂O and 0.1 mol of urea were dissolved in 120 mL of deionized water, and then 80 mL of the solution mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave. The sealed reactor was maintained at 150 °C for 10 h and then cooled down to ambient temperature. After filtration, the precipitate was washed four times with deionized water and ethanol, then dried at 120 °C over 12 h to obtain the MnCO₃ cubes. The preparation of the MnCO₃ spheres was similar to that of the MnCO₃ cubes, using the same amounts of MnSO₄ and urea, reaction temperature and time as those of the MnCO₃ cubes. The only difference was the use of mixed solvents (72 mL of distilled water and 48 mL of ethanol to dissolve MnSO₄ and urea). After the solvothermal reaction, the precipitated product was also washed and dried using the same conditions as those used to obtain MnCO₃ spheres.

2.2. Preparation of cubic and spherical LiMn₂O₄

The two different morphologies of MnCO₃ were pre-sintered at 550 °C for 3 h for conversion into Mn₂O₃. Thereafter, the oxide powders were mixed with Li₂CO₃ (Li₂CO₃ : Mn₂O₃ = 0.54 : 1, molar ratio) and then at 750 °C for 12 h in the air to obtain the final cubic and spherical LiMn₂O₄ products, respectively.

2.3. High temperature solid state preparation of pristine LiMn₂O₄

MnO₂ and Li₂CO₃ were weighed according to the stoichiometric ratio, carefully ground and uniformly mixed, and then placed in a high-temperature tube furnace and calcined at a temperature of 750 °C for 12 h to obtain pristine LiMn₂O₄.

2.4. Characterization

The structures of the prepared materials were characterized by X-ray diffractometer (TD-3000, Dandong Tongda Co, LTD, China) equipped with a Cu Kα radiation source with continuous scanning in the 2θ range of 10–90°, at a scan rate of 0.02° per minute. Morphological characterization of the hydrothermal precursors and final calcined products was achieved by scanning electron microscopy (SEM, JMS-7610F, JEOL), and high-resolution transmission electron microscopy (HRTEM: JEM-2100, 180 kV). The final products were assembled into CR2032 coin-cell type batteries and their electrochemical performances were tested. The cathode material slurry was composed of 70 wt% active materials, 20 wt% acetylene black and 10 wt% polyvinylidene fluoride in N-methyl-2-pyrrolidone, carefully mixed and uniformly coated onto aluminum foil and then vacuum dried at 120 °C for 12 h. Lithium metal foil was used as the battery anode material and LiPF₆ as the electrolyte, and the assembly was carried out in an argon filled glove box, keeping the oxygen and moisture content under 0.01 ppm. The electrochemical performances of the test cells were measured using a battery testing system (BTS-5 V/10 mA, Neware Technology Limited Corporation, China) over a cycling voltage range of 3.0–4.5 V and rate voltage range of 3.0–4.3 V. (1C = 148 mA h g⁻¹). Electrochemical impedance measurements were conducted using an electrochemical workstation (CHI-760D, Shanghai Chenhua Instrument Limited Corporation, China) over a frequency range of 0.01 Hz to 100 kHz. The impedance fitting was accomplished utilizing Zview software, limiting the systematic error of the calculations to under 10%.

3. Results and discussion

Cubic and spherical LiMn₂O₄ were prepared using a hydrothermal and temperature solid phase method. A schematic diagram of the preparation method is shown in Fig. 1.

Fig. 1 Schematic of the preparation process of the cubic and spherical LiMn₂O₄ products.

The following chemical reactions mainly occur in the hydrothermal process:³⁵

NH₂CONH₂ + H₂O → 2NH₃ + CO₂ (1)

NH₃ + H₂O → NH₄⁺ + OH⁻ (2)

CO₂ + 2OH⁻ → CO₃²⁻ + H₂O (3)

CO₃²⁻ + Mn³⁺ → MnCO₃ (4)

In the first hydrothermal step shown in Fig. 1, MnSO₄ provides Mn²⁺ ions for the reaction, and urea acts as a precipitant. When the solvent is pure water, the macroscopic morphology of the MnCO₃ precipitate was cubic after the reaction. In contrast, after adding ethanol into the pure water reaction system, a series of reactions occurred that led to a final product of MnCO₃ spheres. The XRD characterization of the hydrothermal products derived from different solvent systems is shown in Fig. 2a.

Fig. 2 Characterization of the MnCO₃ cubes and spheres. (a) X-ray diffraction (XRD) patterns of the MnCO₃ cubes and spheres. (b) XRD characteristic peak intensity comparison of the cubic and spherical MnCO₃. SEM images of the MnCO₃ (c) cubes and (d) spheres. HRTEM and Fast Fourier Transform (FFT, inset) images of the MnCO₃ (e) cubes and (f) spheres.

The typical characteristic peaks of MnCO₃ are shown in Fig. 2a. The prepared MnCO₃ cubes and spheres are in the R3̄m space group, and can be well indexed to PDF#44-1472. The average crystallite sizes of the MnCO₃ cubes and spheres were calculated by the Debye–Scherrer equation from the full width half maximum (FWHM) of the XRD diffraction peaks and were found to be 42.8 and 34.6 nm for the cubes and spheres, respectively. Due to the heteromorphism of carbonate minerals, vaterite usually exhibits a spherical morphology,²⁶ but in this experiment, the addition of ethanol did not affect the overall crystal structure. In the meantime, as shown in the histogram of Fig. 2b, combining the results of I_Spheres/I_Cubes in the inset, the intensity of the XRD characteristic peaks of the MnCO₃ spheres after ethanol participation was remarkably lowered, especially for the (104) crystal planes. For the MnCO₃ crystal, its structure and crystal growth pattern are similar to those of CaCO₃. Among the numerous crystal planes of MnCO₃, the (104) crystal planes are the characteristic crystal planes,³⁶ composed of Mn²⁺ and CO₃²⁻, densely distributed, and have the lowest surface energy. According to the Bravais law,³⁷ the crystal growth rate is inversely proportional to the density of the surface network. During the growth of the MnCO₃ cubes, the growth direction is mainly in the direction of the vertical (104) crystal planes. After the crystal growth is completed, the crystal planes are completely retained, and the morphology of the final product is a thermodynamically stable calcite rhombohedral crystal, as shown in Fig. 2c. In this formation process, the least dense phase is formed first and transforms to the next stage phase, and finally, the most dense phase, which is the most stable phase. This kinetic transformation process is strictly in order of increasing thermodynamic stability.²⁹ The energy changes during the formation follow Wolfgang Ostwald's “steps rule”.³⁸ The dense surface of the block is smooth and the particle size is 2–5 μm.

Fig. 2d shows MnCO₃ spheres prepared using a mixed solution system of ethanol and water under the same reaction conditions. It can be observed that the spheres mainly feature the stacking of cubic blocks. Herein, we have analyzed the formation process of the MnCO₃ spheres. When ethanol is added at the beginning of the hydrothermal reaction, the overall viscosity of the system changes first, which can be described by the exponential expression:³⁹

η = η_IL exp[−x_c/a] (5)

and the diffusion of ions follows the Stokes–Einstein equation:³⁵

D = RT/6πηr (6)

In eqn (5), x_c is the mole fraction of water, α is a characteristic constant of the mixture, and η_IL is the viscosity of the pure ionic liquid.³⁹ In eqn (6), D is the diffusion constant, η is the solution viscosity, and r is the particle radius.³⁵ It can be seen from eqn (6) that the diffusion coefficient is proportional to the reciprocal of the liquid viscosity, and ethanol plays a role in the crystallization process. In the hydrothermal reaction, when MnCO₃ is precipitated, the ethanol molecules in the solution instantaneously adsorb to the precipitated MnCO₃, especially to the (104) crystal planes of MnCO₃. The mechanism is shown in Fig. 3. Fig. 3a shows the overall effect of ethanol molecules on the MnCO₃ crystals, where the red planes in Fig. 3b are the {104} planes of the MnCO₃ crystal. The hydrophilic hydroxyl group of the ethanol molecule strongly interacts with the oxygen on the (104) crystal planes, and at the same time, the hydrophobic group –CH₃ is exposed to the outside to form an extremely stable and orderly hydrophobic adsorption layer.^27,31,33 The existence of the adsorption layer seriously reduces the growth ability of the (104) crystal planes in the vertical direction,³⁰ changes the surface energy and the MnCO₃ particles gradually undergo directional crystallization, and the blocks are densely packed into spheres. Also, the hydroxyl group is present in the pure water reaction system, where the lowest surface energy is stabilized, especially if its adsorption capacity is insufficient to change the equilibrium shape.⁴⁰ Therefore, the addition of ethanol plays an important role in the morphology control.

Fig. 3 Schematic of the effects of the ethanol molecules and MnCO₃ crystals. (a) The overall effect of the ethanol molecules on the MnCO₃ crystals. (b) Effects of the ethanol molecules on the MnCO₃ {104} crystal planes. The oxygen atoms are shown in red; manganese, green; hydrogen, titanium white; and carbon, lake blue or indigo.

The MnCO₃ precursor of the obtained sample was further examined by high resolution transmission electron microscopy (HRTEM) and its Fast Fourier transform (FFT) pattern was recorded. From the HRTEM images of the lattice fringes (Fig. 2e and f), a lattice distance of 0.285 nm can be observed, which is in squares showing the d-spacing of the (104) crystal planes of the MnCO₃ and the highly crystalline crystals have a relatively regular boundary. From the FFT pattern studies, it is apparent that the crystallinity of the product obtained by adding ethanol is reduced in quality. This result is in agreement with the XRD analysis.

The chemical reaction equation for the calcining of the MnCO₃ precursor in an air atmosphere at 550 °C is as follows:¹⁵

2MnCO₃ + O₂ → 2MnO₂ + 2CO₂ (7)

2MnO₂ → Mn₂O₃ + 0.5O₂ (8)

The final product was obtained by calcining the manganese oxide with Li₂CO₃ at 750 °C in air, and their corresponding XRD patterns are shown in Fig. 4. The diffracted characteristic peaks of the LiMn₂O₄ cubes, spheres and pristine sample are completely coincident with those of spinel LiMn₂O₄ (PDF#35-0782), and there is no impurity phase detected, with ideal crystallinity. Fig. 5a and b show the SEM images of the LiMn₂O₄ cubes and LiMn₂O₄ spheres, respectively, and it can be seen that the particle size after calcination did not change significantly compared to the size before calcination. The surfaces of the LiMn₂O₄ cubes and LiMn₂O₄ spheres are rough and have a porous structure with a particle size of about 3 μm. Since the decomposition of the Li₂CO₃ during calcination releases CO₂, the retention of the morphology of the final product is affected, but the morphology of most of the product is retained. The calcination process can be explained by a template formation mechanism. During the calcination process, Li₂CO₃ thermally decomposes to form Li₂O,⁴¹ and Li₂O undergoes a topological crystallization reaction with thermodynamically stable Mn₂O₃,¹⁵ thereby largely retaining the morphology consistent with that of the precursor.

Fig. 4 XRD patterns of the LiMn₂O₄ cubes, spheres and pristine sample.

Fig. 5 SEM images of the LiMn₂O₄ cubes and spheres: (a) LiMn₂O₄ cubes, (b) LiMn₂O₄ spheres. HRTEM and fast Fourier Transform (FFT) images of the (c) LiMn₂O₄ cubes and (d) LiMn₂O₄ spheres.

The HRTEM characterization and FFT patterns of the LiMn₂O₄ cubes and spheres were carried out and the results are shown in Fig. 5c and d. Lattice fringes with a spacing of 0.48 nm can be observed, perfectly matching the distance between the (111) crystal planes of spinel LiMn₂O₄. Moreover, in the FFT patterns of the LiMn₂O₄ cubes and spheres, the LiMn₂O₄ cubes exhibit better crystallinity on the (111) crystal plane than the LiMn₂O₄ spheres. For spinel LiMn₂O₄, the crystal plane (111) facilitates Li⁺ insertion/deinsertion behaviour, which increases the rate and cycling capabilities of spinel LiMn₂O₄, and the crystal plane (111) has lesser Mn dissolution and a more stable reconstructed surface structure.^42,43 Hence, the LiMn₂O₄ cubes show better electrochemical activity and stability than the LiMn₂O₄ spheres.

Fig. 6a and b show the morphology of pristine LiMn₂O₄ synthesized via a one-step high temperature solid state method. It can be observed that a large number of irregular aggregates exist due to the non-uniform crystallization in the sintering reaction.

Fig. 6 SEM images of pristine LiMn₂O₄.

Fig. 7a shows the cycling capabilities of the LiMn₂O₄ cubes, spheres, and pristine LiMn₂O₄ electrodes at a current density of 0.5C in the voltage range of 3.0–4.5 V, where the LiMn₂O₄ cubes, spheres and pristine samples have first discharge capacities of 130, 115.1 and 106 mA h g⁻¹, respectively. As shown in Fig. 7b, after 100 cycles, the capacity retention rates are 87.9%, 80.49%, and 43.96%, respectively. The first capacity was increased by 22.6% and 8.5%, respectively, compared to that of the pristine LiMn₂O₄.

Fig. 7 (a) Cycling performance of the LiMn₂O₄ cubes, spheres, and pristine LiMn₂O₄ at a current rate of 0.5C. (b) Discharge capacity retention of the LiMn₂O₄ cubes, spheres, and pristine LiMn₂O₄.

It is worth mentioning that the discharge capacity of the LiMn₂O₄ cubes at a current density of 0.5C is very close to the theoretical capacity. Compared to the pristine LiMn₂O₄, the capacity increase is very obvious.

Fig. 8a and b show the first discharge curves for the LiMn₂O₄ cubes and spheres at different rates of 0.5, 1, 2, 5, 10 and 20C, respectively. As can be seen, both the LiMn₂O₄ cubes and LiMn₂O₄ spheres exhibit two voltage plateaus in their discharge curves, characteristic of typical spinel LiMn₂O₄.⁴⁴ As the current density increases, the curve gradually becomes smoother, such as at 20C. This is due to an increase in cell polarization under high current charge and discharge.⁴⁵

Fig. 8 Discharge curves at different current densities for the LiMn₂O₄ (a) cubes and (b) spheres. (c) Rate capabilities of the LiMn₂O₄ cubes, spheres, and pristine LiMn₂O₄ and (d) rate capacity retention of the LiMn₂O₄ cubes, spheres, and pristine LiMn₂O₄.

The first discharge capacities of the LiMn₂O₄ cubes are 121.3, 116.7, 110.9, 107.4, 101.9 and 96.4 mA h g⁻¹ at 0.5, 1, 2, 5, 10 and 20C, respectively, while the corresponding values for the LiMn₂O₄ spheres are 116.4, 107.8, 105, 101.3, 94.6 and 88.3 mA h g⁻¹. Fig. 8c shows the rate capabilities of the LiMn₂O₄ cubes, spheres, and pristine LiMn₂O₄. As the current density increases, the capacities of all of the samples decrease. The pristine LiMn₂O₄ exhibits a first discharge capacity of 95.9 mA h g⁻¹ at 0.5C and a dramatically decreased value of 35.3 mA h g⁻¹ at a high current of 20C. However, the LiMn₂O₄ cubes and LiMn₂O₄ spheres maintain a highly discharge capacity and capacity retention rate during the high current charge and discharge tests. Fig. 8d shows a tendency for the discharge capacity to decrease as the current density increases. The downward trend in the volume of the pristine LiMn₂O₄ is quite obvious, and the LiMn₂O₄ cubes and spheres show a trend towards lower capacity loss. It is obvious that the LiMn₂O₄ cubes and LiMn₂O₄ spheres show better structural stability in the high rate current tests. The ordered morphology and loose porous structure of the samples effectively shortens the diffusion distance of Li⁺ in the electrochemical reaction, effectively alleviating the volume change caused by charge and discharge. The data in Fig. 9 is compared to the discharge capacities of LiMn₂O₄ electrodes reported in ref. 46–49, where it can be seen that the LiMn₂O₄ cube and sphere electrodes exhibited a superior high rate performance at rates of 5–20C.

Fig. 9 Discharge capacity compared with that of LiMn₂O₄ electrodes reported in ref. 46–49.

Fig. 10a depicts the electrical impedance spectra (EIS) using the equivalent circuit by the Zview software. The horizontal axis part is the real part impedance Z′ and the vertical axis part is the imaginary part impedance Z′′. The EIS spectrum is mainly composed of a semicircular curve in the high frequency and medium frequency regions and a straight line in the low frequency region. The intercept of the arc curve and the horizontal axis is the ohmic resistance R_s, the semicircular curve of the high frequency and medium frequency regions represents the charge transfer resistance R_ct, and the straight line in the low frequency region and the Li⁺ in the LiMn₂O₄ electrode material relate to the spread, represented by the Warburg impedance (Z_w).^15,50,51 The Zview software is used to fit the impedance parameters using the equivalent circuit to obtain the parameters. The R_ct values of the LiMn₂O₄ cubes, spheres, and pristine LiMn₂O₄ were calculated to be 176.6, 193.2, and 219.7 Ω, respectively, and the Warburg coefficient and diffusion coefficient of lithium ions in the low frequency region were calculated using the following equation:^15,50

|Z| = R_s + R_ct + σ_wω^−1/2 (9)

D_Li⁺ = R²T²/2A²n⁴F⁴C²σ² (10)

In eqn (9), σ is the Warburg impedance coefficient and ω is the angular frequency. In eqn (10), R is the molar gas constant 8.314 J mol⁻¹ K⁻¹, T is the temperature in Kelvin, A is the surface area of the electrode material, n is the number of electrons transferred during charging and discharging, F is the Faraday constant (96 485C mol⁻¹), and C is the molar concentration of lithium ions (0.0238 mol cm⁻¹⁻³ derived from the relationship between the LiMn₂O₄ density and relative molecular mass).^52,53 Fig. 10b shows the relationship between the reciprocal square root of the angular frequency of the low frequency region and the Z′ after the data is fitted. The slope of the fitted straight line is the Warburg coefficient. The Warburg coefficients of the LiMn₂O₄ cubes, spheres, and pristine LiMn₂O₄ were calculated to be 357, 546.4 and 913.3 Ω S^1/2, respectively. The diffusion coefficients of the lithium ions were then calculated according to eqn (10). The diffusion coefficients of the LiMn₂O₄ cubes, spheres, and pristine LiMn₂O₄ were calculated to be 2.074 × 10⁻¹⁶, 8.855 × 10⁻¹⁷, and 3.169 × 10⁻¹⁷ cm² s⁻¹, respectively. Therefore, the results of the tests and calculations show that the LiMn₂O₄ cubes and spheres exhibit lower charge transfer resistance and better lithium ion diffusion ability than the comparative pristine sample. This revealed that the morphology control improved the charge transfer resistance and the diffusion of lithium ions.

Fig. 10 (a) The fitted EIS curves of the LiMn₂O₄ cubes, spheres, and pristine LiMn₂O₄ and (b) fitting line function diagram of Z′ and ω^−1/2.

Compared with the data obtained for the carbonate precursor or the same morphology synthesized using different methods (as shown in Table 1), it could be seen that the hydrothermal and ethanol assisted hydrothermal routes are the ideal routes to use to synthesize a precursor with a different morphology and final products that have a better electrochemical performance. Using this method, the carbonate precursor can be obtained under mild reaction conditions, and a better discharge capacity of the LiMn₂O₄ electrode charge/discharge can be achieved at a higher current density.

Table 1 Data obtained for the carbonate precursor or the same morphology LiMn₂O₄ synthesized using different methods

Host	Preparation method (precursor)	Capacity (mA h g^−1)/rate	Ref.
LiMn2O4 hollow microspheres	Precipitation	106.7/5C	8
		88.7/10C
LiMn2O4 hollow microspheres	Precipitation	86.3/10C	54
Yolk-structured LiMn2O4 microspheres	Precipitation	94.7/10C	55
LiMn2O4 spheres	Precipitation	109/5C	15
		98/10C
		83/20C
LiAlxMn2−xO4 microspheres	Co-precipitation	82.4/10C	56
Si-modified LiNi0.5Mn1.5O4 microspheres	Co-precipitation	100/10C	57
LiNi0.5Mn1.5O4 spheres	Hydrothermal	58/5C	58
Porous core–shell LiMn2O4 microellipsoids	Hydrothermal	70.2/10C	1
LiMn2O4 microcubes	Hydrothermal	68/5C	6
LiNi0.5Mn1.5O4 microcubes	Co-precipitation	99/5C	59
LiMn2O4 nanocubes	Solution combustion	104/200 mA g^−1(1.35C)	60
LiMn2O4 microcubes	Hydrothermal	101.9/10C, 96.4/20C	This work
LiMn2O4 microspheres	Ethanol assisted hydrothermal	94.6/10C, 88.3/20C

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4. Conclusions

We have adopted a synthetic strategy for preparing MnCO₃ precursors with different morphologies. MnCO₃ cubes and spheres were prepared using the crystal planes (104) in the structure of MnCO₃ as the synthetic entry point. After thermal decomposition and lithiation, a LiMn₂O₄ cathode material was obtained. Even if the lithium source was Li₂CO₃, the morphology was still largely retained after high-temperature calcination. The macro-ordered surfaces of the prepared LiMn₂O₄ cubes and spheres are porous, which provides a space for the diffusion of lithium ions, significantly improving the lithium-ion solid-phase diffusion ability. And, the electrochemical performance was remarkably improved, especially the rate performance, without the need for any coating and doping with other elements. This synthetic strategy can be applied to the synthesis of other oxides and in other fields.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research has been supported by the Natural Science Foundation of Jilin Province (No. 20170101128JC), the Youth Foundation of Changchun University of Science and Technology (no. XQNJJ-2014-13, XJJLG-2014-10), the Industrial Technology Research and Development Project of Jilin Province Development and Reform Commission (2017C052-4), and the Science and Technology Planning Project of Changchun City (no. 2013064).

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