One-time sintering process to synthesize ZrO₂-coated LiMn₂O₄ materials for lithium-ion batteries

Abstract

Herein, different amounts of ZrO₂-coated LiMn₂O₄ materials are successfully prepared by one-time sintering ZrO₂-coated Mn₃O₄ and Li₂CO₃. Scanning and transmission electron microscopy results confirm that the ZrO₂ coating layer on the surface of Mn₃O₄ can still be maintained on the surface of the final LiMn₂O₄ particles even after long-term high-temperature heat-treatment. Three key factors to realize ZrO₂-coated LiMn₂O₄ materials via the one-time sintering process are as follows: (i) the Mn₃O₄ precursor is coated by ZrO₂ in advance; (ii) the ionic radius of Zr⁴⁺ is much larger than those of Mn³⁺ and Mn⁴⁺; (iii) the pre-calcination temperature is set in the reaction temperature range between Li₂CO₃ and Mn₃O₄ and lower than that between Li₂CO₃ and ZrO₂. The 3 wt% ZrO₂-coated LiMn₂O₄ material exhibits excellent electrochemical properties with an initial specific discharge capacity of 118.8 mA h g⁻¹ and the capacity retention of 90.1% after 400 cycles at 25 °C and 88.9% after 150 cycles at 55 °C. Compared with the conventional coating method, the one-time sintering process to synthesize ZrO₂-coated LiMn₂O₄ materials is very simple, low-cost, environmentally friendly, and easy to scale up for large-scale industrial production, which also provides a valuable reference for preparing other coating-type cathode materials for lithium-ion batteries.

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Introduction

Rechargeable lithium-ion batteries (LIBs) have achieved great success as the power sources for portable electronic devices due to their high energy density and long cycle life. Recently, the demand for LIBs has been increasingly shifting from small portable power systems to electric vehicles (EVs) and large-scale energy storage systems (ESSs).^1,2 The safety and cost issues are mostly concerned for EVs and ESSs.^3,4 LiMn₂O₄ is one of the most suitable cathode materials owing to its high abundance, low material cost, and high safety.⁵ However, the practical application of LiMn₂O₄ is greatly limited due to their poor cycling performance, especially at elevated temperatures (≥55 °C).⁶ The reasons can be summarized as follows: (1) the dissolution of Mn²⁺ from cathode to electrolyte via a disproportionation reaction (2Mn³⁺_solid → Mn⁴⁺_solid + Mn²⁺_electrolyte);^7–9 (2) structural transformation from cubic to tetragonal phase, induced by the Jahn–Teller distortion of Mn³⁺;^10,11 (3) the electrolyte decomposition on the electrode surface at high voltage.¹²

To solve the above problems, two strategies, namely the doping and coating techniques, are usually employed to improve the room-temperature (RT) and high-temperature (HT) cycling performance of LiMn₂O₄ materials. Partial substitution of Mn³⁺ with other metal ions, such as Li⁺, Al³⁺, Co³⁺, and Cr³⁺, can stabilize the structure of LiMn₂O₄ materials, which is attributed to the strong metal–oxygen bonds formed by the doping metals.^5,13–15 With regard to the synthetic operations, the doping elements can be easily added in the precursor-synthesizing or the raw-material-mixing processes.^16–20 Therefore, no extra processes are needed and the production cost would not be greatly increased.

The coating technology is another effective strategy to improve the cycling performance of LiMn₂O₄ materials. The coating layer can protect the inner active materials from attack by the acidic species such as HF and oxidation of carbonate solvents at high voltages.²¹ Many electrochemical inert materials^12,22–24 such as ZrO₂, Al₂O₃, TiO₂, AlPO₄, AlF₃ and many coating methods^6,25–29 such as sol–gel, wet-coating, spray drying, atomic layer deposition (ALD) have been widely studied. Until now, the coating-type LiMn₂O₄ materials are usually synthesized by sintering two times: the pristine LiMn₂O₄ is firstly prepared by calcination at ∼800 °C, and then the LiMn₂O₄ is coated with some inert materials and followed by re-calcination between 400 °C and 600 °C. Although the coating techniques can effectively improve the cycling performance, they still have some drawbacks. For example, the production cost of ALD is very high, the sol–gel and spray drying methods are of high-energy consumption, and wet-coating method would generate a large amount of wastewater. Therefore, the industrial applications of the current coating techniques are greatly hindered by the complicated or high-cost coating processes.^30,31 Consequently, a simple, low-cost, environmental friendly, and easily scaled-up coating technique is urgent to be developed to solve the above problems.

In this work, a one-time sintering process was employed to synthesize ZrO₂-coated LiMn₂O₄ materials. Scanning and high resolution transmission electron microscope (SEM and HRTEM) results clearly demonstrate that the LiMn₂O₄ particles can be successfully coated by ZrO₂ via the one-time sintering process. The electrochemical measurements demonstrate that the RT and HT cycling performances of ZrO₂-coated LiMn₂O₄ materials are greatly enhanced. This one-time sintering process to synthesize ZrO₂-coated LiMn₂O₄ materials is very simple, low-cost, and environmental friendly, which can promote its practical application. Besides, we systematically investigated the key influence factors in the synthesis of ZrO₂-coated LiMn₂O₄ materials via the one-time sintering process, which can provide a valuable reference for synthesizing other coating-type cathode materials for LIBs.

Experimental

Material preparation

The pristine and different amounts of ZrO₂-coated LiMn₂O₄ materials were synthesized by a high-temperature solid-state reaction. The shape and particle size distribution of the raw materials of Mn₃O₄ (Nanjing Tianyuan Magnetic Material Co., Ltd.) and ZrO₂ (Xuan Cheng Jing Rui New Material Co., Ltd.) can be seen in Fig. S1 and S2 in ESI.† The amount of ZrO₂ was calculated based on the weight of the final LiMn₂O₄ material. Therefore, in order to synthesize 1, 3, and 5 wt% ZrO₂-coated LiMn₂O₄, the adding amount of ZrO₂ was 1.19, 3.56, and 5.93 wt% of the weight of Mn₃O₄, respectively.

The one-time sintering process to synthesize 3 wt% ZrO₂-coated LiMn₂O₄ is as follows as an example: industrial grade Mn₃O₄ (100 g) and nano-sized ZrO₂ (3.56 g) were firstly weighed and ball-milled for 1 h, and then battery-grade Li₂CO₃ (25.43 g, Shandong RuiFu Lithium Co., Ltd.) with the molar ratio of Li : Mn = 1.05 : 2 was added to the above mixed materials and further ball-milled for 3 h. Note that the mass ratio of the agate balls to the raw materials is 1 : 1, and the rotational speed of the planetary ball mill is 400 rpm. The obtained mixtures of different amounts of ZrO₂-coated Mn₃O₄ & Li₂CO₃ was added into alumina crucible, then pre-heated at 550 °C for 5 h and calcined at 800 °C for 20 h in O₂ atmosphere. Both of the heating and cooling rates are 3 °C min⁻¹. The calcined samples were crushed down and sifted through a sieve of 325 meshes, thus the final products were obtained. The step for preparing ZrO₂-coated Mn₃O₄ can be ignored in the synthesis of the pristine LiMn₂O₄.

In order to investigate the influence of the blending manners of raw materials on the uniformity of the coating layer on particle surface of the final materials, stoichiometric amount of Mn₃O₄, Li₂CO₃, and ZrO₂ (Li : Mn = 1.05 : 2, molar ratio) are added and ball-milled at the same time. The detailed synthetic process can be seen in the ESI.† The obtained sample is denoted as LMO@ZrO₂. Additionally, 3 wt% ZrO₂-coated LiMn₂O₄ material was also synthesized by the conventional dry coating method to compare the coating effect of two coating methods.

Besides, SiO₂ and TiO₂ were also used as the coating materials to synthesize different amounts of SiO₂- and TiO₂-coated LiMn₂O₄ materials, respectively. The detailed synthetic process can be seen in the ESI.†

Material characterization

The morphologies of the synthesized materials were observed using SEM (Supra 55, Zeiss, Germany) with an energy dispersive X-ray spectroscope (EDX, EDAX, Genesis 60, Germany). The thickness of ZrO₂ coating layer was measured using Auger electron spectroscopy (AES, ULVAC-PHI, AES-PHI 700, Japan). Lattice structure and surface feature of samples were investigated using HRTEM (H-800, Hitachi, Japan). Powder X-ray diffraction measurements (XRD, D/MAX 2500, Rigaku, Japan) using Cu K_α radiation (λ = 0.154 nm) were employed to identify the crystalline phase of the synthesized materials. XRD data were obtained in the 2θ range of 10–80°, with a step size of 0.02°, and a count time of 4 s. From the XRD data, the lattice parameters were calculated by the least-squares method. The thermal and weight changes in the synthetic process were investigated with thermogravimetric and differential thermal analysis (TG-DTA, HCT-1, China). TG-DTA measurements of the mixed materials were performed between 25 °C and 850 °C at a heating rate of 3 °C min⁻¹.

Electrochemical measurement

The prepared powders were mixed with carbon black and polyvinylidene fluoride (PVDF) with a weight ratio of 80 : 10 : 10 in N-methyl-2-pyrrolidinone (NMP) to fabricate the positive electrodes. The obtained slurry was coated onto Al foil, followed by drying at 105 °C for 30 min, and roll-pressed in air. The electrodes were dried overnight at 100 °C in a vacuum oven prior to use. The areal mass loading of the active materials was ∼10 mg cm⁻². The CR2032 coin cells were assembled in an argon-filled glove box with an electrolyte of 1 mol L⁻¹ LiPF₆ in EC-DMC-DEC (1 : 1 : 1 weight ratio) solution and a separator of Celgard 2400. The cells were aged for 6 h before the charge/discharge test performed on a LAND CT2001A test system (Wuhan LAND electronics Co. Ltd., China) in the voltage range of 3.0–4.3 V (vs. Li⁺/Li) at 25 °C and 55 °C. The first 4 cycles were performed at 0.2C (24 mA g⁻¹), and then the following cycles were tested at 1C (120 mA g⁻¹).

Results and discussion

As illustrated in Fig. 1, the one-time sintering process to synthesize ZrO₂-coated LiMn₂O₄ materials includes two main procedures: the mixing and sintering processes. In the mixing process, Mn₃O₄ and nano-sized ZrO₂ are firstly ball-milled for 1 h to prepare ZrO₂-coated Mn₃O₄ materials, and then the stoichiometric amount of Li₂CO₃ is added to the above mixture and further ball-milled for 3 h. The purpose of the above process is to guarantee that most of ZrO₂ particles are coated onto the surface of Mn₃O₄ and would not be coated onto the surface of Li₂CO₃ particles.

Fig. 1 Schematic illustration for the one-time sintering process to synthesize ZrO₂-coated LiMn₂O₄ materials.

Fig. 2 shows the SEM images of the pristine Mn₃O₄ and different amounts of ZrO₂-coated Mn₃O₄ materials without any heat-treatment. As shown in Fig. 2a, the pristine Mn₃O₄ particles have spherical morphology and are composed of the octahedral primary particles with the particle size ranging from 100 to 200 nm. Additionally, we also notice that the Mn₃O₄ particles have a rough surface and there are many particle boundaries among the primary particles. Fig. 2b–d show the SEM images of different amounts of ZrO₂-coated Mn₃O₄ materials. As illustrated from the low-magnification images in Fig. 2b–d, after ZrO₂ coating, the particle boundary on the surface of Mn₃O₄ particles is gradually blurred with ZrO₂ coating amount increasing. In addition, there are no nano-sized ZrO₂ particles existing alone even when the coating amount is up to 5.93 wt%. The inset high-magnification SEM images shown in Fig. 2b–d clearly demonstrate that the surface of Mn₃O₄ is uniformly coated by many ZrO₂ particles with the particle size of about 30 nm. The above results indicate that nano-sized ZrO₂ can be easily coated on the surface of Mn₃O₄ particles through the simple dry coating method without adding any dispersant such as alcohol or water.

Fig. 2 SEM images of (a) the pristine Mn₃O₄ and (b) 1.19 wt%, (c) 3.56 wt%, (d) 5.93 wt% ZrO₂-coated Mn₃O₄ without any heat-treatment. The insets are the corresponding SEM images at high magnification.

As shown in Fig. 3a, c, e and g, low-magnification SEM images of the pristine and different amounts of ZrO₂-coated LiMn₂O₄ materials heat-treated at 800 °C for 20 h (the coating material is ZrO₂, which can be proven in the latter XRD data) illustrate that the spherical morphology is still maintained after long-term high-temperature heat-treatment. The average particle diameter is about 10 μm. Fig. 3b shows that the surface morphology of the pristine LiMn₂O₄ is smooth and clean. In contrast, the surfaces of the coated LiMn₂O₄ become rough and are covered with small particles, as shown in Fig. 3d, f and h. The coating layer becomes more and more obviously with the ZrO₂ coating amount increasing from 1 to 5 wt%, which demonstrates that the coating layer is still maintained even after having been heat-treated at 800 °C for 20 h.

Fig. 3 SEM images of (a and b) the pristine LiMn₂O₄, (c and d) 1 wt%, (e and f) 3 wt%, and (g and h) 5 wt% ZrO₂-coated LiMn₂O₄ obtained by calcining 0, 1.19, 3.56, and 5.93 wt% ZrO₂-coated Mn₃O₄ & Li₂CO₃ mixtures at 800 °C for 20 h.

Lattice structure of the coating layer on LiMn₂O₄ particles was investigated by HRTEM. As illustrated in Fig. 4a, HRTEM images show that there is a film-like layer as we speculate in SEM images. Fig. 4b and d show that the measured interplanar distances of the coating layer of 3 wt% ZrO₂-coated LiMn₂O₄ and nano-sized ZrO₂ heat-treated at 800 °C for 20 h are around 0.218 nm, respectively, which matches well to the (1̄21) plane of ZrO₂ with monoclinic crystal system. Therefore, we can draw a conclusion that the coating layer on the surface of LiMn₂O₄ particles is ZrO₂.

Fig. 4 HRTEM images of (a and b) 3 wt% ZrO₂-coated LiMn₂O₄ and (c and d) nano-sized ZrO₂ heat-treated at 800 °C for 20 h.

Fig. 5b–d shows elemental mapping studies on 3 wt% ZrO₂-coated LiMn₂O₄ particles. It is clear that the presence of Mn, Zr, and O are homogenous within LiMn₂O₄ particles, and the ZrO₂ coating layer is uniformly distributed on the surface of LiMn₂O₄ particles.

Fig. 5 (a) SEM image of 3 wt% ZrO₂-coated LiMn₂O₄ and the corresponding elemental mapping of (b) Mn, (c) Zr, and (d) O.

In order to confirm the extent of inter-diffusion between Zr and Mn ions in the process of high-temperature heat-treatment, EDX was used to measure the Zr/Mn atomic ratio in the particles before and after high-temperature calcination. As illustrated in Fig. 6b, the atomic ratio of Zr/Mn in 3.56 wt% ZrO₂-coated Mn₃O₄ particle is 0.079/99.921. As to the final product of 3.0 wt% ZrO₂-coated LiMn₂O₄ (Fig. 6d), the atomic ratio of Zr/Mn is 0.071/99.929, which is slightly lower than that of 3.56 wt% ZrO₂-coated Mn₃O₄ particle. If all of Zr ions are diffused into the bulk LiMn₂O₄ particles, the atomic ratio of Zr/Mn would be 0.022/99.978. Therefore, we can draw a definite conclusion that the inter-diffusion between Zr and Mn ions is very little.

Fig. 6 (a) SEM image of 3.56 wt% ZrO₂-coated Mn₃O₄ and (b) its corresponding EDX spectroscopy. (c) SEM image of 3.0 wt% ZrO₂-coated LiMn₂O₄ and (d) its corresponding EDX spectroscopy.

In order to investigate the ZrO₂ coating thickness, the theoretical value was firstly calculated. The calculation method can be seen in Fig. S3 in ESI.† The theoretical coating thickness of 1, 3, and 5 wt% ZrO₂-coated LiMn₂O₄ materials is 12.2, 36.6, and 61.0 nm, respectively. AES was also employed to measure the actual coating thickness of different amounts of ZrO₂-coated LiMn₂O₄ materials. From Fig. 7a, c and e, the peaks indexed to Zr1 can be easily observed on the top surface of 1, 3, and 5 wt% ZrO₂-coated LiMn₂O₄ materials. As the Ar⁺ etching depth increases, the peak intensity of Zr1 decreases, while that of Mn1, Mn2, and Mn3 increases. When the etching depth reaches 192, 374, and 635 nm for 1, 3, and 5 wt% ZrO₂-coated LiMn₂O₄ samples, though the Zr1 peak intensities are weak, they can still be observed, indicating that some Zr ions have diffused into the crystal lattice of LiMn₂O₄.

Fig. 7 AES spectroscopies of (a) 1 wt%, (c) 3 wt%, and (e) 5 wt% ZrO₂-coated LiMn₂O₄. Atomic concentration of Zr and Mn as a function of the etching depth: (b) 1 wt%, (d) 3 wt%, and (f) 5 wt% ZrO₂-coated LiMn₂O₄.

As illustrated in Fig. 7b, d and f, the changing trend of the atomic concentration of Zr and Mn can be divided into three stages: in the first stage (stage-I), the atomic concentration of Zr and Mn changes little, so the etching depth in stage-I can represent the thickness of ZrO₂ coating layer. Therefore, it can be estimated from Fig. 7b, d and f that the coating thickness of 1, 3, and 5 wt% ZrO₂-coated LiMn₂O₄ samples are 32, 42, and 75 nm, respectively. The measured thickness of the coating layer is slightly bigger than the theoretical value; the main reason is that the actual density of ZrO₂ is reduced due to the nano-crystallization of particles. Since ZrO₂ is a granular material, not a film material, so the surface coverage will be incomplete if the coating amount is small. As ZrO₂ coating amount increases, the surface coverage becomes completely, and then the coating thickness will increase proportionally with the increase of ZrO₂ coating amount. Considering that the coating thickness of 3 wt% ZrO₂-coated LiMn₂O₄ sample is only 10 nm bigger than that of 1 wt% ZrO₂-coated sample, and the ratio of the coating thickness between 5 wt% and 3 wt% coating amount is 1.786, which is very close to 5 : 3. So, it is speculated that the surface coverage for 1 wt% ZrO₂ coating amount is not complete. As the ZrO₂ coating amount is ≥3 wt%, the surface coverage becomes completely, which means that the “breakthrough” loading to have a complete coverage should be 3 wt%. In the second stage (stage-II), the atomic concentration of Zr drastically decreases and that of Mn rapidly increases, indicating that Zr ions have diffused into the crystal lattice in the surficial layer of LiMn₂O₄ particles to form a LiMn_2−xZr_xO₄ phase. According to Fig. 7b, d and f, the thickness of LiMn_2−xZr_xO₄ layer for 1, 3, and 5 wt% ZrO₂-coated LiMn₂O₄ samples are estimated to be 92, 158, and 325 nm, respectively, indicating that more ZrO₂ content makes more Zr ions diffuse into LiMn₂O₄ particles. In the third stage (stage-III), the atomic concentration of Zr is very low and changes very little with the etching depth increasing, indicating that it is very difficult for all of Zr ions to completely diffuse into the crystal lattice of LiMn₂O₄ particles. From the atomic concentration of Zr in the inner part of LiMn₂O₄ particles, we can obtain that some Zr ions have been doped to form a LiMn_2−xZr_xO₄ phase (0.01 ≤ x ≤ 0.02).

The SEM, HRTEM, EDX, and AES results show that the LiMn₂O₄ particles can be successfully coated by ZrO₂ via the one-time sintering process. This coating method is very easy to be scaled up and can be used to prepare other coating-type cathode materials. However, the successful synthesis of ZrO₂-coated LiMn₂O₄ via the one-time sintering process is just a specific example. In order to make this method more universal, we systematically studied what factors are essential to guarantee its success.

First of all, the blending manner of raw materials may be one of the key factors. In the preceding experiments, the Mn₃O₄ precursor was coated with nano-sized ZrO₂ in advance, and then Li₂CO₃ was added and further ball-milled for 3 h. Herein, we made an adjustment to the blending manner as below. Three raw materials of Mn₃O₄, Li₂CO₃, and ZrO₂ (the amount of ZrO₂ is 3.56 wt% of the weight of Mn₃O₄) were ball-milled simultaneously, and then the mixture was preheated at 550 °C for 5 h and then calcined at 800 °C for 20 h. The as-prepared material is denoted as LMO@ZrO₂ and deemed as a control sample. SEM images of LMO@ZrO₂ are illustrated in Fig. S4(a and b).† It can be seen that there is very small amount of ZrO₂ observed on the surface of the LiMn₂O₄ particles. However, as illustrated in Fig. S4(c and d),† a large number of ZrO₂ particles can be successfully coated onto the LiMn₂O₄ particles through calcining the mixture of Li₂CO₃ and ZrO₂-coated Mn₃O₄. According to the above comparison results, a definite conclusion can be drawn: only the precursor is coated with the coating material beforehand, can the coating material be uniformly coated onto the surface of the final particles via the one-time sintering process. Besides, it can also be seen from Fig. S4(e and f)† that the coating uniformity of the one-time sintering process is similar to that of the conventional coating method.

The second key factor may be choosing a suitable coating material. According to the diffusion theory, among the various physicochemical properties of the coating materials, the ionic radius may play an important role.³² In order to prove the above assumption, two other metal ions of Ti⁴⁺ and Si⁴⁺ are chosen to synthesize different amounts of SiO₂- and TiO₂-coated LiMn₂O₄ materials via the one-time sintering process. Unexpectedly, the result obtained from Fig. S6–S9† demonstrates that SiO₂- and TiO₂-coated LiMn₂O₄ materials cannot be successfully synthesized by the one-time sintering process, which indicates that it is easy for Si⁴⁺ and Ti⁴⁺ to diffuse into the bulk LiMn₂O₄ crystals at 800 °C. In order to compare the ionic radius difference among Mn³⁺, Mn⁴⁺, Zr⁴⁺, Si⁴⁺, and Ti⁴⁺, their ionic radii are shown in Fig. 8. It is seen that Zr⁴⁺ has a much larger ionic radius than Mn³⁺ and Mn⁴⁺. While, the ionic radius of Si⁴⁺ is much smaller than those of Mn³⁺ and Mn⁴⁺, and the ionic radius of Ti⁴⁺ is between those of Mn³⁺ and Mn⁴⁺. From the comparable results of ZrO₂, SiO₂, and TiO₂ coating layers after heat-treatment at 800 °C, we can conclude that the successful synthesis of ZrO₂-coated LiMn₂O₄ via the one-time sintering process is probably due to the much larger ionic radius of Zr⁴⁺ than those of Mn³⁺ and Mn⁴⁺. Therefore, the ionic radius is also a crucial factor to realize the coating-type materials via the one-time sintering process.

Fig. 8 The ionic radii of five types of ions: Mn³⁺, Mn⁴⁺, Zr⁴⁺, Si⁴⁺, and Ti⁴⁺.

The third key factor should be the choice of the pre-calcination temperature. If the reaction between Li₂CO₃ and ZrO₂ is ahead of that Li₂CO₃ and Mn₃O₄, the coating material may be Li₂ZrO₃ and not ZrO₂, thus some extra Li₂CO₃ will be consumed in vain and the loss of the specific discharge capacity of LiMn₂O₄ will be more seriously. In order to choose a suitable pre-calcination temperature, TG-DTA was performed to reveal the reaction process between Li₂CO₃ and ZrO₂-coated Mn₃O₄ and the result is shown in Fig. 9. For the mixture of Li₂CO₃ and Mn₃O₄, the reaction temperature range is between 457 °C and 600 °C. While for the mixture of Li₂CO₃ and ZrO₂, the reaction temperature range is between 582 °C and 712 °C. Besides, it is also noticed that when the Mn₃O₄ is coated with ZrO₂ beforehand, the starting reaction temperature between Li₂CO₃ and Mn₃O₄ is postponed by 43 °C to 500 °C. This is because Li⁺ must pass through the ZrO₂ coating layer and then can react with Mn₃O₄ to form LiMn₂O₄. Although there is overlap in the reaction temperature range between the mixtures of Li₂CO₃ & ZrO₂-coated Mn₃O₄ and Li₂CO₃ & ZrO₂, it is probably that Li₂CO₃ would react only with Mn₃O₄ and not with ZrO₂ if the pre-calcination temperature is set at 550 °C.

Fig. 9 TG curves of the raw material mixtures with a heating rate of 3 °C min⁻¹ in air.

Influence of the sintering temperature and time on the crystal structure and performance of the pristine LiMn₂O₄ and different amounts of ZrO₂-coated LiMn₂O₄ materials were investigated in detail. As illustrated in Fig. S10 and S11,† the diffraction peaks of ZrO₂ can always be observed between 550 °C to 850 °C, which shows that the coating layer is indeed ZrO₂ and the coating layers do not disappear even after long-term high-temperature calcination. Fig. S12† shows the influence of sintering temperature on cyclic performance of the pristine LiMn₂O₄, and the optimal synthesizing temperature is 800 °C. Fig. S13† demonstrates that the discharge capacity slightly increases and the capacity retention slightly decreases with the sintering time increasing. Therefore, the synthesizing condition of the pristine LiMn₂O₄ and different amounts of ZrO₂-coated LiMn₂O₄ materials is set at 800 °C for 20 h.

The XRD patterns of the pristine and different amounts of ZrO₂-coated LiMn₂O₄ materials calcined at 800 °C for 20 h are shown in Fig. 10a. The major diffraction peaks of the pristine LiMn₂O₄ are in good agreement with that obtained from JCPDF file no. 35-0782, corresponding to the cubic spinel structure with Fd3̄m space group. For the 1 wt% ZrO₂-coated LiMn₂O₄ sample, there is no diffraction peaks corresponding to ZrO₂ observed in the XRD pattern, this is because the ZrO₂ amount is too low to be detected. As expected, the impurity phase of ZrO₂ is obviously observed with the coating amount increasing to 3 wt% and 5 wt%, which demonstrates that most of ZrO₂ do not react with Li₂CO₃. A suitable pre-calcination temperature can guarantee that Li₂CO₃ does not react with ZrO₂ to form Li₂ZrO₃, which leads to two advantages as below: (i) the weight of the coating layer is not further increased, which will not greatly reduce the specific discharge capacity of the final products; (ii) extra Li₂CO₃ won't be consumed in vain. Therefore, the suitable pre-calcination temperature is the key factor to determine the chemical compositions of the coating materials.

Fig. 10 (a) XRD patterns and (b) the lattice parameters of 1, 3, and 5 wt% ZrO₂-coated LiMn₂O₄ materials.

Furthermore, the lattice parameter a (= b = c) is also calculated from the XRD data and the results are shown in Fig. 10b. It is interesting that the lattice parameter of ZrO₂-coated LiMn₂O₄ materials is slightly smaller than that of the pristine LiMn₂O₄ material. The reason is probably that after heat-treatment at such high temperature of 800 °C for 20 h, a small fraction of Zr⁴⁺ diffuses into the spinel lattice of the bulk LiMn₂O₄ to partial substitute the site of Mn⁴⁺. Ionic radius of Zr⁴⁺ is much bigger than that of Mn⁴⁺, the lattice constant should be bigger if Zr⁴⁺ ions substitute the site of Mn⁴⁺ in the spinel. But the fact is on the contrary. Actually, the lattice constant of spinel LiMn₂O₄ crystal is not only related to the ionic radius, but also related to the M–O bond energy.³³ The Δ_fG^Θ(ZrO₂) and Δ_fG^Θ(MnO₂) is −1042.8 kJ mol⁻¹ and −465.1 kJ mol⁻¹, respectively. The Δ_fG^Θ(ZrO₂) is much more negative than Δ_fG^Θ(MnO₂), which can deduce that bond energy of Zr–O is much stronger than Mn–O and the crystal volume shrinks. Therefore, the structure with Zr⁴⁺ doping is more compact and has smaller lattice constant. Besides, the lattice parameter changes little with ZrO₂ content increasing from 1 wt% to 5 wt%, the reason is that the doping content of Zr⁴⁺ in the inner part of LiMn₂O₄ is almost constant, no matter the ZrO₂ coating amount is 1 wt%, 3 wt% or 5 wt%. It implies that the upper limit of the actual Zr⁴⁺ amounts diffusing into the spinel lattice of LiMn₂O₄ is very small, primarily because the ionic radius of Zr⁴⁺ is much larger than that of Mn⁴⁺.

The electrochemical properties of the pristine LiMn₂O₄ and different amounts of ZrO₂-coated LiMn₂O₄ materials were studied in the voltage range of 3.0–4.3 V at 25 °C and 55 °C. Fig. 11a shows the initial charge–discharge curves of different amounts of ZrO₂-coated LiMn₂O₄ materials at a current rate of 24 mA g⁻¹ corresponding to 0.2C. It can be seen that all discharge curves have two voltage plateaus at approximately 3.95 V and 4.1 V, which is typical for LiMn₂O₄ and its variants. The two voltage plateaus indicate that the extraction (and subsequent re-insertion) of lithium ions from tetrahedral sites occurs in two stages. The pristine LiMn₂O₄ material delivers the highest specific discharge capacity of 124.4 mA h g⁻¹ with the initial columbic efficiency of 97.6%. The specific discharge capacities of ZrO₂-coated LiMn₂O₄ materials decrease with the ZrO₂ coating amount increasing. The initial discharge capacity of 1, 3, and 5 wt% ZrO₂-coated LiMn₂O₄ is 121.5, 118.8, and 115.6 mA h g⁻¹ with the initial columbic efficiency of 96.4, 96.7, and 96.1%, respectively. The RT and HT cycling performance of the pristine and different amounts of ZrO₂-coated LiMn₂O₄ materials is evaluated with the current density of 120 mA g⁻¹ corresponding to 1C and the results are shown in Fig. 11b and c. At 25 °C, the initial specific discharge capacity of 0, 1, 3, and 5 wt% ZrO₂-coated LiMn₂O₄ materials is 121.3, 118.3, 115.2, and 109.6 mA h g⁻¹ with the 400^th capacity retention of 79.2, 84.7, 90.1, and 87.5%, respectively. As the test temperature increases to 55 °C, the pristine LiMn₂O₄ shows severe capacity loss with the capacity retention of 67.0% after 150 cycles. While the specific discharge capacity of 1, 3, and 5 wt% ZrO₂-coated LiMn₂O₄ after 150 cycles is 94.1, 102.8, and 101.7 mA h g⁻¹ with the capacity retention of 79.5, 88.9, and 92.9%, respectively. Compared with the pristine LiMn₂O₄, though the specific discharge capacities of ZrO₂-coated LiMn₂O₄ decrease, their cycling performance is greatly improved. By comparison of the comprehensive electrochemical performance, the 3 wt% coating amount is preferred, because it has a relatively high specific discharge capacity and an excellent cycling performance.

Fig. 11 (a) The charge–discharge curves of 1, 3, and 5 wt% ZrO₂-coated LiMn₂O₄ materials with the current rate of 0.2C. The cycling performance of 1, 3, and 5 wt% ZrO₂-coated LiMn₂O₄ materials tested at (b) 25 °C and (c) 55 °C at 1C.

The enhanced cycling performance should be mainly contributed to the ZrO₂ coating layer, which can suppress the Mn²⁺ dissolution from LiMn₂O₄ cathode into the electrolyte. In order to measure the Mn²⁺ dissolution, 0.3 g cathode materials were added into 10 mL electrolyte and stored for 7 days at 60 °C, and then the Mn²⁺ concentration was tested by ICP. As illustrated in Fig. 12, the Mn²⁺ concentration in electrolyte for the pristine LiMn₂O₄ is 0.75 mmol L⁻¹. In comparison, the Mn²⁺ concentrations in electrolyte for 1, 3, and 5 wt% ZrO₂-coated LiMn₂O₄ materials drops quickly to 0.62, 0.25, and 0.2 mmol L⁻¹, which proves that the ZrO₂ coating layer can greatly inhibit the Mn²⁺ dissolution from the active material into the electrolyte.

Fig. 12 Concentration of Mn²⁺ in the electrolyte as a function of the coating amounts of ZrO₂.

In order to investigate the stability of the ZrO₂ coating layer, SEM and XRD were used to characterize the morphology and crystal structure of 3 wt% ZrO₂-coated LiMn₂O₄ electrode post electrochemical testing, and the results are shown in Fig. S14 and S15.† Based on the analysis of SEM and XRD data, we can conclude that the ZrO₂ coating layer on the surface of LiMn₂O₄ particles is very stable to the volume changes brought by the long-term charge–discharge processes. The reason is probably that the coating layer calcined at 800 °C is very rigid and stable.

Conclusions

In summary, different amounts of ZrO₂-coated LiMn₂O₄ materials are successfully synthesized via a one-time sintering process. Three key factors to realize ZrO₂-coated LiMn₂O₄ materials are as follows: (i) the Mn₃O₄ precursor is coated by nano-sized ZrO₂ in advance; (ii) the ionic radius of Zr⁴⁺ is much larger than those of Mn³⁺ and Mn⁴⁺; (iii) the pre-calcination temperature is set in the reaction temperature range between Li₂CO₃ and Mn₃O₄ and lower than that between Li₂CO₃ and ZrO₂. The as-prepared 3 wt% ZrO₂-coated LiMn₂O₄ material exhibits an excellent electrochemical performance with the initial specific discharge capacity of 118.8 mA h g⁻¹ at 0.2C and the capacity retention of 90.1% after 400 cycles at 25 °C and 88.9% after 150 cycles at 55 °C at 1C. The enhancement of the cycling performance is mainly contributed to the ZrO₂ coating layer which can suppress the side reactions between LiMn₂O₄ and the electrolyte. Most significantly, the one-time sintering process to synthesize ZrO₂-coated LiMn₂O₄ materials is very simple, low-cost, environmental friendly, and easy for large-scale industrial production, so as to promote its practical application and provide a valuable reference for synthesizing other coating-type cathode materials for LIBs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (U1407118 and 51272020).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra01421c

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