Introduction

Borate-based materials (LiMBO3, M = Mn, Fe, and Co) were first reported by Legagneur which have received wide attention in the research area of Li-ion batteries, since LiMBO3 materials provide high theoretical capacity, high thermal stability, and good electrochemical stability because of small volume changes compared to other cathode materials [1,2,3,4]. In addition, (BO3)3− polyanion could afford a high operating voltage along with an improved structural stability due to the inductive effect of B–O bond. Among the borates, LiMnBO3 has a higher operation potential and a higher theoretical specific energy, which is a more promising candidate for lithium-ion batteries. LiMnBO3 has two different crystal structures, the monoclinic phase and the hexagonal phase [5,6,7,8,9]. The two phases of LiMnBO3 have the same theoretical capacity of 220 mAh g−1, while due to their different crystal structure, the average potential for the monoclinic phase is 3.7 V and the hexagonal phase is 4.1 V [10,11,12,13,14].

However, LiMnBO3 has intrinsically low electronic and ionic conductivity which is the main cause of the poor electrochemical performance of LiMnBO3. Additionally, LiMnBO3 is sensitive to moist air, which can induce both partial Mn2+ oxidation and Li+ loss from the crystal framework, and resulting in the reduce of the electrochemical activity. Thus, many strategies have been made to enhance the performance of LiMnBO3 cathode materials, for instance, doping with metal ions, coating with carbon [15,16,17,18]. In these methods, carbon coating may be an effective way, which can improve the structure stability and enhance the cycle performance of LiMnBO3 materials when it encounters with moist air and electrolyte [19,20,21,22,23]. Various polymers are already used as additives or carbon sources for cathode materials, such as polyethylene glycol (PEG), polystyrene, polyvinyl alcohol, polyaniline, and polypyrrole [24,25,26,27,28]. Among these polymers, PEG is widely used in electrochemical and biomedical fields owing to its lubricity, nontoxicity, hydrophilicity, and solubility. As a dispersing agent and surfactant, PEG can be easily adsorbed onto the surfaces of particles by the ordered and uniform chain structure, effectively inhibiting the aggregation of colloidal particles in the process of gel formation, and playing the role of carbon source during sintering [29].

Herein, LiMnBO3/C was prepared using a sol-gel method with PEG as the reductive agent and carbon source. We used PEG-6000 as carbon source, which serves multiple purposes: first, as a dispersant and surfactant, PEG can be easily adsorbed onto the surfaces of particles by the ordered and uniform chain structure during the formation of the gel; second, the formed particles are coated by the carbon produced by PEG decomposition during heat treatment, the in-situ coated carbon controls the growth of LiMnBO3 particles; and last, the carbon coating can improve the conductivity of LiMnBO3 materials and prevent LiMnBO3 from air corrosion. Meanwhile, electrolyte adsorption in amorphous carbon layer also provided a flexible structure against volume expansion/contraction during the cycling progress [30,31,32].

Experimental

Synthesis of LiMnBO3/C

The stoichiometric amount of LiOH·H2O (AR, 95%), Mn(CH3COO)2·4H2O (AR, 99%), and H3BO3 (AR, 99.5%) was dissolved in deionized water. In the same way, the demand amount PEG-6000 (CP) (the weight ratio of PEG and product LiMnBO3/C is 1:2) was dissolved in deionized water. Then, the two solutions were mixed and stirred spiritedly at 80 °C until the homogeneous gel was obtained. Then, the gel was dried at 120 °C for 24 h in a vacuum oven. At last, the obtained powders were sintered at 350 °C for 3 h in argon atmosphere and then ball-milled for 1 h to minimize the particle size, subsequently, at 650–800 °C for 10 h in argon atmosphere to obtain LiMnBO3/C powders.

Synthesis of bare LiMnBO3

The bare LiMnBO3 was prepared in the above synthetic process; the stoichiometric amount of LiOH·H2O (AR, 95%), Mn(CH3COO)2·4H2O (AR, 99%), and H3BO3 (AR, 99.5%) was dissolved in deionized water. Then, the solution was stirred spiritedly at 80 °C until the homogeneous gel was obtained. Then, the gel was dried at 120 °C for 24 h in a vacuum oven. At last, the obtained powders were sintered at 350 °C for 3 h in argon atmosphere and then ball-milled for 1 h to minimize the particle size, subsequently, at 750 °C for 10 h in argon atmosphere to obtain LiMnBO3 powders.

Physical characterization

The structural analysis of the compounds was characterized by X-ray diffraction (XRD, Rint-2000, Rigaku) analysis equipped with Cu-Kα radiation by step scanning in the 2θ range of 10° to 80°. The morphology of the samples was examined by a scanning electron microscope (SEM, JSM-6380 LV) and a Tecnai G12 transmission electron microscope (TEM). Energy-dispersive spectroscopy (EDS) analysis was obtained with the JSM-6380 LV microscope. The carbon content of the sample was confirmed using C-S analysis equipment (Eltar, Germany). Thermal analysis of the sample was determined by TG-DSC equipment (STA-449C, Germany) in argon from room temperature to 900 °C at a heating rate of 5 °C min−1.

Electrochemical tests

The electrochemical performance of LiMnBO3/C was characterized by coin-type cells (CR2025) with lithium metal as the negative electrode. The positive electrodes were prepared by mixing LiMnBO3/C, acetylene black, and polyvinylidene fluoride (PVDF) (the weight ratio is 80:10:10) in N-methylpyrolline onto an aluminum foil and dried at 120 °C for 4 h in a vacuum oven. Then, the coin-type cells were assembled in a glove box filled with high purity argon. The electrolyte was 1 M LiPF6 solution in a mixture of ethylene carbonate and dimethyl carbonate with 1:1 volumetric ratio. The cells were tested in the voltage range of 1.0–4.8 V at various charge-discharge rates from 0.025 to 0.5 C at room temperature. The cyclic voltammetry (CV) tests and electrochemical impendence spectrum (EIS) measurements were carried out on a CHI660D electrochemical work station. The CV measurements were performed on LiMnBO3/C electrodes at the scanning rate of 0.05 mV s−1 from 1.0 to 4.8 V. The EIS measurements were performed in the frequency range of 0.01 Hz–100 kHz with an AC voltage of 5 mV.

Results and discussion

Figure 1 shows the TG-DTA curves of the precursor powder of LiMnBO3/C. There are three weight loss regions in the TG curve. The first weight loss zone before 240 °C corresponds to the release of molecular water. The second weight loss region (240–350 °C) is mainly attributed to the decomposition of Mn(CH3COO)2 and PEG. A continuous weight loss between 350 and 500 °C should be related to the reaction of carbon with residue oxygen. No more weight loss is observed in the temperature range from 700 to 900 °C. According to the result, we choose 650, 700, 750, and 800 °C as the heating temperatures to synthesize the LiMnBO3/C composite.

Fig. 1
figure 1

TG-DTA analysis curve for the precursor recorded from 40 to 900 °C at the heating rate of 5 °C min−1 in Ar atmosphere

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Figure 2 shows the XRD patterns of bare LiMnBO3 and LiMnBO3/C composites synthesized at different temperatures. It is obvious to see that the LiMnBO3/C synthesized at 650 and 700 °C, the hexagonal phase is the dominate phase, but a small amount of monoclinic phase at 34.8° and 40.4°. In addition, the amount of m-LiMnBO3 is reduced with the increasing sintering temperature. In accordance with previous reports, the h-LiMnBO3 is obtained at higher temperature but m-LiMnBO3 is formed at lower temperature [17]. From Fig. 2, it can be seen that the diffraction peaks in the patterns of LiMnBO3/C (synthesized at 750 °C) and LiMnBO3 can be well matched with the h-LiMnBO3. The diffraction peaks of carbon are not detected, which indicates that the carbon generated from PEG is amorphous.

Fig. 2
figure 2

XRD patterns of bare LiMnBO3 and LiMnBO3/C samples synthesized at different temperatures

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Rietveld refinement is performed to analyze the crystal structure and phase content of the samples, and the results are shown in Fig. 3 and Table 1. From Table 1, the calculated weight percent of h-LiMnBO3 and the m-LiMnBO3 of LiMnBO3/C synthesized at 750 °C is about 99.84 and 0.16 wt%, respectively. In Fig. 3, a hexagonal phase (ICSD no. 1511217) with P-6 space group can be observed, and the structure is crystallized well for the reliability factor (R w) is good. The calculated and measured 2θ values are well matched. In accordance with the refinement results, the lattice parameters of h-LiMnBO3 are a = 8.1740 Å, c = 3.1489 Å, V = 182.20 Å3, α = 90°, β = 90°, and γ = 120°, compared well with the reported ones by Zhiping Lin (a = 8.1720 Å, c = 3.1473 Å, V = 182.02 Å3, α = 90°, β = 90°, γ = 120°) [13].

Fig. 3
figure 3

Rietveld refinement XRD data of the LiMnBO3/C synthesized at 750 °C

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Table 1 Results of phase content analysis obtained from X-ray Rietveld refinement
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Figure 4 shows the scanning electron microscopy (SEM) images of the LiMnBO3/C samples. From Fig. 4a–c, we can see that the samples all consist of urchin-like and spherical-like particles. As the annealing temperature is increased, the amount of the urchin-like particles is reduced and the amount of the spherical-like particles is increased; when the annealing temperature is raised to 800 °C, all particles become spherical-like with a particle size of about 1 μm in Fig. 4d. Moreover, to analyze the uniformity of element distribution for LiMnBO3/C, energy-dispersive spectrometry was carried out (Fig. 5). Oxygen, manganese, and carbon are distributed on the various parts of the particles (synthesized at 650 °C), it is clear that carbon are uniformly distributed in the LiMnBO3/C sample. Combined with the XRD results, it can be speculated that the urchin-like particles are m-LiMnBO3 and the spherical-like particles are h-LiMnBO3.

Fig. 4
figure 4

SEM images of LiMnBO3/C samples: 650 °C (a), 700 °C (b), 750 °C (c), and 800 °C (d)

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Fig. 5
figure 5

EDS images of oxygen, manganese, and carbon for LiMnBO3/C synthesized at 650 °C

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The morphology and microstructure of LiMnBO3 and LiMnBO3/C (synthesized at 750 °C) were characterized by SEM and TEM. As shown in Fig. 6a, the bare LiMnBO3 particles with an average size of ~2 μm are aggregated to form secondary particles. In Fig. 6b, the average particle size of LiMnBO3 in the composite is much smaller than that of bare LiMnBO3, due to the confinement of the conductive carbon network. Figure 6c shows the LiMnBO3/C particles (synthesized at 750 °C) are well wrapped with a nano-carbon layer. Figure 6d shows that the interior planar distance between the adjacent lattice fringes is 0.2876 nm, which corresponds to the d-spacing value of the (101) plane of h-LiMnBO3, and the thickness of the carbon coating in the external surface which is beneficial for the electrolyte to permeate into the electrode. Therefore, this structure may be conducive to the insertion/extraction of Li+ from the LiMnBO3 electrode, and the electronic conductivity of LiMnBO3 will be greatly improved.

Fig. 6
figure 6

SEM images of LiMnBO3 (a) and LiMnBO3/C synthesized at 750 °C (b). TEM images of LiMnBO3/C synthesized at 750 °C (c) and (d)

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Figure 7 shows the first charge-discharge curves of the LiMnBO3/C samples synthesized at different temperatures. The cells were all charged at 0.025 C (1 C = 220 mA g−1) rate. The initial discharge capacities of samples synthesized at 650, 700, 750, and 800 °C are 135.4, 187.8, 213.4, and 169.9 mAh g−1, respectively. The sample synthesized at 750 °C has the highest capacity. The samples calcined at 650 and 700 °C show lower capacities owing to the lower crystallinity. The sample sintered at 800 °C also exhibits a lower capacity due to the bigger particle size and agglomeration which will lengthen the Li+ diffusion/conduction path.

Fig. 7
figure 7

First charge-discharge curves of LiMnBO3/C samples at 0.025 C rate, 650 °C (a), 700 °C (b), 750 °C (c), and 800 °C (d)

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Figure 8a, b shows the first charge-discharge curves of LiMnBO3/C (synthesized at 750 °C) and bare LiMnBO3 cell in the voltage range between 1.0 and 4.8 V at different rates. The first discharge capacities of LiMnBO3/C at 0.025, 0.05, 0.1, 0.2, and 0.5 C are 213.4, 170.8, 128.6, 114.8, and 109.7 mAh g−1, respectively. While the bare LiMnBO3 can only deliver 118.5, 105.5, 95.4, 90.8, and 65.6 mAh g−1 at the same condition. Figure 8c shows the cycling performance of LiMnBO3/C (synthesized at 750 °C) at different rates. It can be seen that the LiMnBO3/C sample has displayed better cycling performance than bare LiMnBO3. After 50 cycles, the discharge capacities of LiMnBO3/C are 155.2, 135.2, 101.2, 100.6, and 94.3 mAh g−1, respectively, delivering much higher specific capacities than those of bare LiMnBO3 (Fig. 8d). In fact, the surface of the LiMnBO3 material is very sensitive to the moist air, which tends to induce severe degradation of electrode properties. In our experiment, the in-situ carbon coating from decomposition of the PEG-6000 can prevent LiMnBO3 from contacting with moist air and resist corrosion by the acidic electrolyte during cycling. In addition, the electronic conductivity and ionic conductivity can be also improved.

Fig. 8
figure 8

First charge-discharge curves of LiMnBO3/C (a) and bare LiMnBO3 (b) synthesized at 750 °C. Cycle performance of LiMnBO3/C (c) and bare LiMnBO3 (d) synthesized at 750 °C

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Figure 9 shows the rate capability of LiMnBO3/C (synthesized at 750 °C) and bare LiMnBO3. The current rates are in the range of 0.025, 0.05, 0.1, 0.2, and 0.5 C in sequence for 4, 6, 20, 20, and 20 cycles, respectively. The initial discharge capacities of LiMnBO3/C are 200.1, 168.7, 133.4, 129.2, and 112.6 mAh g−1, respectively. After 70 cycles, the current rate returned back to 0.025 C, a capacity of 196.6 mAh g−1 still remained. However, the LiMnBO3 can only deliver a capacity of 78.2 mAh g−1 at 0.2 C and 57.8 mAh g−1 at 0.5 C. The LiMnBO3/C exhibits better rate capability than bare LiMnBO3. The excellent electrochemical performance of LiMnBO3/C composites could be attributed to the intimate imbedding of the particles into the conductive carbon network and electrolyte adsorption in amorphous carbon layer also provided a flexible structure against volume expansion/contraction during the cycling progress. The growth of LiMnBO3 particles is also confined in the sintering process by the carbon network, therefore, lithium ion and electron transport is promoted.

Fig. 9
figure 9

Rate capacity of LiMnBO3/C and bare LiMnBO3 synthesized at 750 °C

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Although, the in-situ carbon coating decreases the capacity loss, the capability rate of LiMnBO3/C is not so excellent, several factors may cause this problem: first, Mn2+ may be dissolved into the electrolyte, as similar as the LiMnPO4; second, the LiMnBO3 particles may be separated from the carbon network by the SEI impedance membrane; and last, the particles may be also not small enough for a fully reversible delithiation/lithiation.

Figure 10 shows the CV curves of the LiMnBO3/C (synthesized at 750 °C) at the scanning rate of 0.05 mV s−1 in 1.0–4.8 V. A wider oxidation peak can be observed around 4.1 V, ranging from 3.4 to 4.4 V in each cycle, while the corresponding reduction peak does not appear in the reduction process. The intensities of the oxidation peak mildly drop at each cycle corresponding to the capacity loss observed in the charge-discharge curves. The peak potentials almost have little change, suggesting reversible insertion/extraction during cycling. Through the previous report, the low electrical conductivity, dissolution of Mn, and small Li-ion diffusivity of LiMnBO3 led to the CV curves with sharp and symmetrical redox peaks difficultly [30].

Fig. 10
figure 10

Cyclic voltammetry curve of LiMnBO3/C (750 °C) at a scan rate of 0.05 mV s−1 in the potential range of 1.0–4.8 V

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The EIS tests (Fig. 11) were comparatively conducted to investigate the electrical conductivity of bare LiMnBO3 and LiMnBO3/C synthesized at 750 °C. The EIS spectrums exhibit a semicircle in the high frequency range and an inclined line in the low frequency range. Using the Z-view software, EIS data can be fitted by an equivalent circuit model, and some parameters are tabulated in Table 2. An equivalent circuit (insert figure) was conducted to refine the spectra. R s represents the resistance of the electrolyte as the intercept impedance on the Z′ axis, R ct is the charge transfer resistance, Z w is the Warburg impedance arising from the Li+ diffusion in electrode, corresponding to the inclined line. CPE represents the double-layer capacitance. It is found that the charge transfer resistance of LiMnBO3/C (153.6 Ω) is much lower than that of bare LiMnBO3 (716.6 Ω), indicating the charge transfer speed of the electrochemical reaction is significantly increased. The exchange current density (j 0) is calculated by the following equation. The results are also shown in Table 2.

$$ {j}_o=\frac{RT}{nFR_{ct}} $$
Fig. 11
figure 11

EIS spectra of LiMnBO3/C (750 °C) and bare LiMnBO3

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Table 2 Parameters obtained from equivalent circuit fitting of experimental data
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Where R is the gas constant, T is the absolute temperature, n is the number of electrons involved in the redox process, F is Faraday’s constant, and R ct is the charge transfer resistance. The exchange current density (j 0) of LiMnBO3/C (1.67 × 10−4 mA cm−2) is about one order magnitude higher than that of bare LiMnBO3 (3.59 × 10−5 mA cm−2), which indicates that the reversibility of the electrode reaction of LiMnBO3/C is much better than that of LiMnBO3.

Conclusions

LiMnBO3/C cathode materials were successfully prepared by a sol-gel method using PEG as the reductive agent and carbon source. LiMnBO3/C synthesized at 750 °C shows excellent electrochemical performance with the discharge capacities of 155.2, 135.2, and 94.3 mAh g−1 after 50 cycles at 0.025, 0.05, and 0.5 C rates, respectively, exhibiting a much better cycling performance than that of bare LiMnBO3. The enhancement of the electrochemical performance of LiMnBO3/C could be attributed to the homogeneous coating of the carbon network, which can not only improves the electronic conductivity of LiMnBO3 but also boosts the Li+ diffusion.