1 Introduction

Rechargeable lithium-ion batteries (LIBs) have been widely used in portable electronic devices due to their high energy density, high power density and good cycling stability [1–4]. Currently, LiCoO2, LiNiO2, LiFePO4, LiNi x Co y Mn1−x−y O2 and LiMn2O4 are the most common cathode materials applied in LIBs [5–7]. Among the numerous transition metal oxides, spinel LiMn2O4 has been extensively studied as cathode material for its low cost and non-toxicity [8, 9]. The development of hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs) demands for lithium-ion batteries with high power and safety stability, but the traditional LiMn2O4 material cannot meet the technical requirements. Owing to the dissolution of Mn3+ in 4-V discharge platform, the capacity of LiMn2O4 fades severely during the charge and discharge processes [10–13]. To improve the cycle performance of LiMn2O4, many researches have focused on the substitution of Mn with other transition metal elements to form new spinel cathode material [14, 15]. Among various doped materials, LiNi0.5Mn1.5O4 cathode material has been seen as one of the most prospective lithium-ion battery materials due to its high theoretical capacity (147 mAh·g−1) and high discharge platform [16, 17].

LiNi0.5Mn1.5O4 has been prepared by many methods, including solid-state reaction [18, 19], hydrothermal [20], sol–gel [21, 22] and co-precipitation [23, 24]. Among these methods, the co-precipitation method received extensive attention on account of simplicity in operation and easiness in industrialization [25]. Generally, carbonate and oxalate are used as precipitants for preparing LiNi0.5Mn1.5O4 cathode material by co-precipitation method. The method includes two main steps: (1) the carbonate or oxalate precursor preparation and (2) precursors calcination [26–28]. During calcination process, the carbonate and oxalate precursors decomposition and the gases (CO2) releasing from precursors result in loose structure and poor compactness of final material.

Compared with carbonate or oxalate precipitation method, many spherical cathode materials (LiCoO2, LiNiO2, LiCo1/3Ni1/3Mn1/3O2) with high tap density, uniform particle size and better thermal stability could be synthesized by hydroxide co-precipitation method [29, 30]. However, the high content of Mn2+ and OH− is easy to form fine hydroxide precipitant nucleation [31], and the pure LiNi0.5Mn1.5O4 cathode material with regular spherical morphology is hard to be prepared by hydroxide co-precipitation method. Meanwhile, the different solubilities of manganese hydroxide and nickel hydroxide are easy to result in different ratios of nickel and manganese or impurity phases in the final product [32].

In this paper, spherical LiNi0.5Mn1.5O4 cathode material was successfully synthesized via co-precipitation method with the help of sodium hydroxide as precipitant and ammonia as complexing agent. The impurity phases in the final material were successfully reduced by annealing process. The prepared pure LiNi0.5Mn1.5O4 cathode material with spherical morphology exhibited good electrochemical properties.

2 Experimental

2.1 Synthesis

In order to obtain spherical and pure LiNi0.5Mn1.5O4 powders, spherical precursor Ni0.25Mn0.75(OH)2 was firstly synthesized through co-precipitation method. 1 mol·L−1 NH3·H2O solution as mother liquor was added into the reactor. 2 mol·L−1 NiSO4 and MnSO4 mixed solution (cation ratio of Ni and Mn = 1:3) was fed into a stirred tank reactor. Meanwhile, 4 mol·L−1 NaOH and a certain amount of NH3·H2O solution were injected into the reactor under the conditions of 60 °C and pH = 10. The reaction was protected by N2 atmosphere. The obtained hydroxide precursors were first washed with hot deionized water, and then dried in a vacuum oven under 80 °C for 12 h. Finally, the dry precursors were mixed with an excess of LiOH·H2O (over 5%), and calcined in air at 500 °C for 5 h and then calcined at 900 °C for 15 h in a muffle furnace. To improve crystallinity and reduce impurity phases, annealing experiments were carried out. The obtained materials after calcination on the condition of 900 °C were annealed for 20 h at 500, 600 and 700 °C, respectively.

2.2 Characterizations and electrochemical measurements

The crystal structures of prepared particles were measured by X-ray diffractometer (XRD, Rigaku Ru-200, Cu Kα radiation, 40 kV, 100 mA, λ = 0.154056 nm) with scan rate of 2 (°)·min−1 in the range of 10°–80°. The morphology and size of obtained samples were observed by scanning electronic microscopy (SEM, KYKY-EM3900 M). The molar ratios of Ni and Mn of obtained materials were analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES, VARIAN, VISTA-MAPX CCD). The specific surface area of prepared materials was measured by the specific surface area and porosity analyzer (Micromeritics, Gemini VII 2390).

CR2032 coin-type cells were assembled to examine the electrochemical properties of cathode materials. The cathode electrodes were prepared by 80 wt% of active material, 10 wt% of acetylene black and 10 wt% of polytetra fluoroethylene (PTFE). A Celgard 2400 polypropylene microporous membrane was performed as separator. The electrolyte was composed of 1 mol·L−1 LiPF6 mixture of 1:1 (volume ratio) ethylene carbonate (EC) and dimethyl carbonate (DMC). Pure lithium sheets were used as anode electrodes. The cells were assembled in an argon-filled glove box.

Charge–discharge tests were carried out in the voltage range of 3.50–4.95 V with Land-CT2001A instrument. Cyclic voltammetry (CV, scan rate of 0.1 mV·s−1 and voltage range of 3.5–4.9 V) curves and electrochemical impedance spectroscopy (EIS, AC amplitude of 5 mV and frequency range of 0.10 MHz–0.01 Hz) were performed on PARSTAT®2273 electrochemical workstation. The cathode electrodes after charge–discharge cycles were dismantled from cells and dissolved in toluene to obtain charged cathode materials.

3 Results and discussion

Figure 1 shows SEM images of obtained Ni0.25Mn0.75(OH)2 precursors and LiNi0.5Mn1.5O4 cathode materials. As shown in Fig. 1a, b, the obtained Ni0.25Mn0.75(OH)2 precursors are spherical with the average particle diameter of about 6 μm. It is found that the secondary precursor particles are uniformly dispersed with clear boundaries and smooth surfaces. LiNi0.5Mn1.5O4 materials formed after calcining and annealing at 500, 600 and 700 °C are shown in Fig. 1c–h, respectively. The obtained LiNi0.5Mn1.5O4 cathode materials inherit the spherical morphology of precursors. The cathode materials LiNi0.5Mn1.5O4 are compact spherical agglomerations and composed of micro- or nano-particles (primary particles). It also can be seen that the boundaries of primary particles of LiNi0.5Mn1.5O4 cathode material are clearer after annealing at 600 °C (Fig. 1f), which means a better crystallization [33]. The pore volumes of cathode materials LiNi0.5Mn1.5O4 after annealing at 500, 600 and 700 °C are 1.01 × 10−2, 1.31 × 10−2 and 8.04 × 10−3 cm2·g−1, and the corresponding specific surface areas are 8.95, 12.84 and 7.26 m2·g−1, respectively, which indicates that the cathode material LiNi0.5Mn1.5O4 annealed at 600 °C has the largest specific surface area and can more fully contact with the electrolyte. The molar ratios of Ni and Mn of obtained materials with different annealing temperatures were analyzed by ICP, and the results are listed in Table 1. It can be seen from Table 1 that the molar ratios of Ni and Mn in the final products are similar to the designed contents within the error range. Figure 2 shows XRD patterns of spherical LiNi0.5Mn1.5O4 cathode materials annealed at different temperatures. The structures of LiNi0.5Mn1.5O4 cathode materials prepared by hydroxide co-precipitation method belong to Fd3m space group, where Mn ions are present as mainly Mn4+. In addition, Li+ cations, metal cations (Mn4+, Ni2+) and O2− anions occupy in the 8a tetrahedral positions, 16d octahedral sites and 32e sites, respectively. For LiNi0.5Mn1.5O4 cathode material without annealing, weak impurity phase Li x Ni1−x O close to peaks of (311) and (400) can be found. According to the paper reported, the impurity Li x Ni1−x O generated for oxygen deficiency has a bad influence on electrochemical performance of LiNi0.5Mn1.5O4 cathode material [23, 34, 35]. To eliminate impurity phases as much as possible, the annealing experiments at different temperatures were carried out. The results shown in Fig. 2 suggest that the impurity peaks reduce and more pure LiNi0.5Mn1.5O4 cathode material can be obtained by annealing process at 600 °C.

Fig. 1
figure 1

SEM images of obtained samples: a, b Ni0.25Mn0.75(OH)2, c, d LiNi0.5Mn1.5O4 annealed at 500 °C, e, f LiNi0.5Mn1.5O4 annealed at 600 °C, and g, h LiNi0.5Mn1.5O4 annealed at 700 °C

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Table 1 Molar ratio results of Ni and Mn obtained by ICP analyses
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Fig. 2
figure 2

XRD patterns of spherical LiNi0.5Mn1.5O4 cathode materials after annealing at different temperatures

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LiNi0.5Mn1.5O4 cathode materials prepared at different annealing temperatures were monitored by CV curves in Fig. 3. There are common characteristics of a pair of redox peaks in related to 4.55–4.85 V region, which are consistent with the reversible reaction of Ni2+/Ni4+. For obtained LiNi0.5Mn1.5O4 materials after annealing at 500 and 700 °C, the slight redox peaks associated with the transformation of Mn3+/Mn4+ couple appear in ~4.0 V region. However, the redox peak (4.0 V) of material collected after annealing at 600 °C is weaker. The results corresponding to XRD patterns shown in Fig. 2 can further verify that most of Mn3+ ions have been reduced by annealing process at 600 °C and more pure LiNi0.5Mn1.5O4 cathode material is obtained. Remarkably, the more pure LiNi0.5Mn1.5O4 may have better electrochemical performances.

Fig. 3
figure 3

CV curves of LiNi0.5Mn1.5O4 cathode materials prepared at different annealing temperatures

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Initial charge and discharge capacity and coulomb efficiency of obtained LiNi0.5Mn1.5O4 after annealing at different temperatures with a cutoff voltage range of 3.50–4.95 V at different rates at 25 °C are shown in Table 2. It can be seen from Table 2 that LiNi0.5Mn1.5O4 cathode material obtained after annealing at 600 °C shows better capability than others. The initial discharge capacities of cathode material annealed at 600 °C are 145.0 (0.1C), 126.8 (0.5C), 113.5 (1.0C) and 112.4 mAh·g−1 (2.0C), and the corresponding coulomb efficiencies maintain 95.2%, 95.0%, 92.5% and 94.8% in the first cycle at a rate of 0.1C, 0.5C, 1.0C and 2.0C, respectively. It verifies that the initial irreversible capacity loss is minor, and LiNi0.5Mn1.5O4 cathode material annealed at 600 °C has high discharge capacity, good rate performance and preferable reversibility.

Table 2 Initial charge and discharge capacity and coulomb efficiency of LiNi0.5Mn1.5O4 obtained after annealing at different temperatures with a cutoff voltage range of 3.50–4.95 V at different rates and 25 °C
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Figure 4 shows the charge and discharge profiles of LiNi0.5Mn1.5O4 obtained after annealing at different temperatures with a cutoff voltage range of 3.50–4.95 V at 1.0C and 25 °C. It can be seen from Fig. 4 that all of the discharge and charge curves of the cathode materials annealed at 500 °C (Fig. 4a), 600 °C (Fig. 4b) and 700 °C (Fig. 4c) are similar. Common characteristics of a pair of redox peaks in 4.55–4.85 V region can be found in the discharge curves of all the obtained materials, which are consistent with the reversible reaction of Ni2+/Ni4+. Meanwhile, the charge and discharge capacities of the cathode materials annealed at 500, 600 and 700 °C decrease with the cycle number increasing. The cathode material annealed at 600 °C shows better cycle performance. After 50 cycles, the capacities of LiNi0.5Mn1.5O4 cathode materials annealed at 500, 600 and 700 °C are 101.4, 109.2 and 97.7 mAh·g−1, respectively. The capacity retention of the cathode material annealed at 600 °C (96.2%) is higher than that of materials annealed at 500 °C (87.0%) and 700 °C (95.0%). It is also reported that the impurity Li x Ni1−x O can result in a higher residence and have a bad influence on electrochemical performance of LiNi0.5Mn1.5O4 cathode materials, and the capacity of pure LiNi0.5Mn1.5O4 cathode materials fades more slowly than that of cathode materials with impurity phases [10–13, 23, 34, 35], so the results of electrochemical performance tests agree well with structure analysis (Fig. 2), CV measurement (Fig. 3) and results reported in Refs [10–13, 23, 34, 35]. Figure 5 shows EIS result of prepared cathode materials with different annealing temperatures after 50 cycles at 1.0C, where Z′ is the real part of impedance and Z″ is the imaginary part of impedance. It can be seen from Fig. 5 that EIS of samples consist of a semicircle in the high frequency and a straight sloping line in the low frequency. After 50 cycles at 1.0C, the residence value of the prepared sample annealed at 600 °C is lower than those annealed at 500 and 700 °C, which further indicates that the sample annealed at 600 °C has good ions conductivity and electrochemical performance. Moreover, the prepared LiNi0.5Mn1.5O4 cathode material with best electrochemical performance (annealed at 600 °C) can maintain its morphology during charge–discharge processes. Figure 6 shows SEM image of LiNi0.5Mn1.5O4 annealed at 600 °C after 50 cycles at 1.0C. It can be seen from Fig. 6 that the cathode material keeps the original size and spherical shape, displaying attractive morphological stability. Therefore, the results above indicate that the pure LiNi0.5Mn1.5O4 cathode materials with enhanced electrochemical performance can be prepared via hydroxide co-precipitation method with a suitable controlled annealing temperature.

Fig. 4
figure 4

Charge and discharge curves of LiNi0.5Mn1.5O4 at 1.0C at annealing temperatures of a 500 °C, b 600 °C and c 700 °C with a cutoff voltage range of 3.50–4.95 V at 25 °C

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Fig. 5
figure 5

EIS results of prepared LiNi0.5Mn1.5O4 cathode materials at different annealing temperatures after 50 cycles at 1.0C

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Fig. 6
figure 6

SEM image of LiNi0.5Mn1.5O4 annealed at 600 °C after 50 cycles at 1.0C

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4 Conclusion

In summary, the high-voltage spherical LiNi0.5Mn1.5O4 cathode material for lithium-ion batteries was successfully synthesized by hydroxide co-precipitation method. The prepared LiNi0.5Mn1.5O4 cathode materials are composed of micro- or nano-primary particles. The more pure LiNi0.5Mn1.5O4 cathode material with Fd3m cubic spinel structure and good electrochemical properties can be obtained with the help of an annealing process at 600 °C. For the material prepared under this condition, the initial discharge capacity (0.1C, 145.0 mAh·g−1) is close to its theoretical capacity (147 mAh·g−1). After 50 cycles, the discharge capacity is 109.2 mAh·g−1 at 1.0C with a cutoff voltage range of 3.50–4.95 V at 25 °C and the capacity retention is 96.2%. When the obtained LiNi0.5Mn1.5O4 cathode material discharges under different rates with a cutoff voltage range of 3.50–4.95 V at 25 °C, the discharge capacities are kept at about 145.0 (0.1C), 126.8 (0.5C), 113.5 (1.0C) and 112.4 mAh·g−1 (2.0C) and the initial coulomb efficiencies retain above 95.2% (0.1C), 95.0% (0.5C), 92.5% (1.0C) and 94.8% (2.0C), respectively. The results would contribute to performance improvement and industrial production of LiNi0.5Mn1.5O4 cathode materials for 5 V Li-ion batteries.