Synthesis of MnO-LiF Composite Cathode Material Using NH4F as a fluorine source

doi:10.21203/rs.3.rs-4010416/v1

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Synthesis of MnO-LiF Composite Cathode Material Using NH4F as a fluorine source

https://doi.org/10.21203/rs.3.rs-4010416/v1

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published 08 Jun, 2024

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Non-intercalation cathode materials based on nanocomposites of lithium fluoride and Li-free transition metal compounds have drawn attention as materials for fabricating the cathode of lithium ion batteries owing to their high theoretical specific capacities and high redox voltages. In the present study, a nanocomposite of MnO-LiF was in situ synthesized by a quick annealing at 500 °°C for just 5 minutes using a mixture of ammonium fluoride, manganese acetate, and lithium acetate as precursor. The synthesized composite is investigated by X-ray diffraction, scanning electron microscopy, nitrogen sorption, and charge-discharge measurements. As a lithium-ion battery cathode, it delivers respectively an initial discharge specific capacity of 170 mAh g^-1 and a discharge specific capacity of 137 mAh g^-1 in the 50th cycle at a current rate of C/20, equivalent to a moderate capacity retention rate of 80.6%. The novel synthesis approach proposed offers access to electrochemically active composites of lithium fluoride and Li-free transition metal oxides.

MnO-LiF composite

Cathode

Lithium ion batteries

Driven by the rapid technological progress on electronics and electric vehicles, the requirement for lithium-ion batteries (LIBs) with larger specific capacity/energy density is increasingly stringent. Along with the anode transitioning from graphite (theoretical specific capacity of 372 mAh g^− 1) to silicon (4200 mAh g^− 1) occurring, the demand for cathodes with high specific capacity/energy density rises as a big challenge for LIBs. Lithium intercalation compounds, such as LiCoO₂ [1], LiMn₂O₄ [2, 3], LiNi_xCo_yMn_1−x−yO₄ [4, 5], LiMPO₄ (M = Fe, Mn and VO) [6–9], which are mostly studied cathode materials of LIBs, have now hit their ‘‘bottleneck” in terms of specific capacity/energy density due to only one oxidation state change per transition metal ion upon intercalation and de-intercalation of Li⁺.

Recently, the discovery of a new class of non-intercalation cathode materials based on nanocomposites of alkali metal compounds (i.e., LiF) and Li-free transition metal compounds (i.e., MnO and FeF₂) with transition metal in the intermediate oxidation state, was reported [9–21]. These nanocomposites not only donate lithium ions to the anode but also show high specific capacities and high redox voltages. Among the above-mentioned composites, MnO-LiF, FeO-LiF, and CoO-LiF nanocomposites deliver specific capacities of more than 235, 310, and 206 mAh g^− 1 in the potential range of 1.5–4.8 V at 20 mA g^− 1, respectively [10]. By comparison with the traditional lithium intercalation/de-intercalation reaction, this peculiar electrochemical performance is attributed to a surface conversion reaction. However, these nanocomposites for next generation LIBs are still faced with many challenges. For example, the upper limit voltage as high as 4.8 V is needed to drive the redox reaction and break Li–F bond upon charge [10–12, 14, 21, 22], which is incompatible with the widely used carbonate electrolytes in the LIBs industry, rarely worked above 4.3 V. Additionally, the composites, which were prepared via conventional mechanical grinding, did not display satisfactory properties owing to inhomogeneous spatial distribution of various active material particles, loose interfacial contact between active materials, and their big size.

In this study, the in situ synthesis of a MnO-LiF nanocomposite by a simple procedure based on the thermal reaction of a mixture of ammonium fluoride, manganese acetate, and lithium acetate which was prepared by spray drying at a moderate temperature of 550 °C for limited duration of just 5 minutes in inert atmosphere is suggested. By comparison with mechanical grinding, which is conventionally used for preparing MnO-LiF composites, spray drying route ensures the homogeneous distribution and close interfacial contact between LiF and MnO components. Additionally, Ketjen black® EC-600JD (KB) with extremely large mesoporous area and electroconductibility is used as a carbon source to modify the MnO-LiF composite to increase in the surface electroconductibility and constrict the growth of LiF and MnO particles.

Material Synthesis

MnO-LiF samples with MnO to LiF molar ratio of 1∶2 were synthesized by spray-drying and subsequent sintering in Ar atmosphere. In a typical synthesis, stoichiometric amount of ammonium fluoride (NH₄F, Aladdin, 98%), manganese acetate tetrahydrate (Mn(CH₃COO)₂⋅4H₂O, Aladdin, 99%), and lithium acetate dihydrate (CH₃COOLi⋅2H₂O, Aladdin, 99%) were dissolved in deionized water with continuous stirring at room temperature to obtain the precursor solution with 0.02 mol L^− 1 of Mn(CH₃COO)₂, 0.04 mol L^− 1 of NH₄F, and 0.04 mol L^− 1 of CH₃COOLi. Second, a designed amount of KB (Lion Corporation) was mixed into the precursor solution with ultrasonical treatment for 2 h. Third, the resulting dispersion was spray-dried under the spraying pressure of 0.3 MPa and at the inlet and outlet temperatures of 250°C and 110°C, respectively. Finally, MnO-LiF composite with 20 wt% KB was obtained by annealing precursor powder at 550°C in an argon atmosphere for 5 minutes.

Characterization

Powder X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8-Advance X-ray powder diffractometer operated at voltage of 40 kV and current of 40 mA with Cu Kα radiation. The XRD patterns were analyzed with MDI Jade software to identify phase. The morphologies of the samples were observed by a Hitachi S-4800 field emission scanning electron microscope equipped with an energy dispersive spectrometer (FESEM-EDS). The surface areas and pore volumes of samples were determined via nitrogen sorption with a Micromeritics ASAP 2020 adsorption porosimeter at 77 K. The surface area and pore volumes were calculated using the Brunauer-Emmet-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) method, respectively.

Electrochemical Measurements: The electrochemical tests were performed using 2025-type coin half cells with metallic lithium as counter electrode. To prepare the working electrode, the as synthesized MnO-LiF composite, acetylene black (battery grade) and polyvinylidene fluoride binder with a mass ratio 75:10:15 were mixed into a uniform slurry in N-methyl-1,2-pyrrolidone (Aladdin, 99.5%). The slurry mixture was coated onto Al foil and then dried in vacuum at 80°C overnight. The cathode plates with 12 mm circular sections were punched. Typical active material loading in each electrode was 1–3 mg. The cells were assembled in a glove box. The electrolyte solution comprised saturated LiPF₆ in ethylene carbonate/diethyl carbonate/ethyl methyl carbonate (1∶1∶1 by volume). The test of galvanostatic charge/discharge of the battery was carried out under the NEWARE battery test system in the potential range of 1.5 to 4.8 V (vs Li/Li⁺) at room temperature. Cyclic voltammetry (CV) tests were performed respectively using MnO-LiF composite as the working electrode and lithium metal foil as the reference electrode and counter electrode within 1.5–4.5 V (vs Li/Li⁺) at a sweep rate of 1.0 mV s^− 1. Electrochemical impedance spectra (EIS) were measured using CHI760E electrochemical analyzer at a discharged stage over the frequency range of 10^− 2 to 10⁵ Hz with a 5 mV AC signal.

The XRD pattern of the as-obtained MnO-LiF composite is shown in Fig. 1. The standard powder diffraction patterns of LiF phase (card No. 72-1538) and MnO phase (card No. 89-2804) from inorganic crystal structure database are also shown in the bottom of Fig. 1 as references, respectively. The diffraction peaks of MnO-LiF sample in Fig. 1 can be indexed to cubic LiF and cubic MnO, respectively, and no other common impurities such as MnF₂, Li₂O, and Li₂CO₃ have been detected, indicating that the target product was successfully obtained at a moderate temperature of 550°C in very short time (5 minutes). The fast reaction kinetics in annealing process is attributed to the solution based on spontaneous intimate mixing of precursor salts being converted into to a remarkably homogeneous intermediate product by spray drying. There were no typical diffraction peaks of KB probably due to its amorphous nature [23]. The lattice parameters of MnO and LiF in the composite, refined with the MDI Jade program, are listed in Table 1, well consistent with the JCPDS 89-2804 and 72-1538, respectively.

Table 1

Lattice parameters of MnO and LiF in the composite determined from XRD
sample	a/Å	b/Å	c/Å
JCPDS 89-2804	4.4440	4.4440	4.4440
MnO	4.4463	4.4463	4.4463
JCPDS 72-1538	4.0280	4.0280	4.0280
LiF	4.0314	4.0314	4.0314

Figure 1

Table 1

Typical FESEM images of the MnO-LiF composite and its corresponding EDS elemental mapping are shown in Fig. 2. FESEM image (Fig. 2a) shows that the sample is composed of regular and uniform sphere-like primary particles with an average size of about 50 nm that form bigger agglomerates. The EDS elemental mappings of MnO-LiF composite shown in Fig. 2b confirm that Mn, F, O and C elements are quite uniformly dispersed within the composite, firmly demonstrating that MnO and LiF are integrated into a KB matrix and well-interconnected by conductive networks, while the overlap of Mn and F mapping proves compact contact between LiF phase and MnO phase. Therefore, intimate contacts among all three subcomponents of C, MnO and LiF are expected.

Figure 2

To further analyze the structure of the composite, the nitrogen adsorption–desorption isotherms were measured. Figure 3 shows the N₂ adsorption/desorption isotherms for KB and the MnO-LiF samples. Each sample exhibits a pronounced hysteresis loop between the adsorption and desorption isotherms and an obvious turning point, which are adsorption-desorption characteristics of the porous materials. Nevertheless, the turning point in the isotherm for MnO-LiF sample displays a shift to a smaller relative pressure, meaning that the mesopore of KB is filled or blocked by MnO and LiF particles, bringing about a respective decline in surface area and pore volume. A comparison of pore structure parameters calculated from the results of nitrogen adsorption between KB and the MnO-LiF sample is recorded in Table 2. As shown in Table 2, the BET surface area and the pore volume fell from 1290 m² g^− 1 and 2.47 cm³ g^− 1 for KB to 408 m² g^− 1 and 0.78 cm³ g^− 1 for the MnO-LiF sample, respectively, which ulteriorly prove MnO and LiF particles fill or block the pores of KB.

Table 2

Pore volumes and BET surface areas of KB and the MnO-LiF sample
Sample	S_BET (m² g^− 1)	Pore volume (cm³ g^− 1)
KB	1290	2.47
MnO-LiF composite	408	0.78

Figure 3

Table 2

Figure 4 shows the typical charge − discharge curves of MnO-LiF composite at various cycling periods with C/20 rate (1C = 273 mAh g^− 1) in the voltage window of 1.5–4.8 V. The first cycle curve of the MnO-LiF composite displayed an initial charge specific capacity of 245 mAh g^− 1, which is slightly smaller than its theoretical specific capacity of 273 mAh g^− 1 (corresponding to the electrochemical reaction ), and a first discharge specific capacity of 170 mAh g^− 1, higher than that for MnO-1.5LiF composite (130 mAh g^− 1 with a current density of 5 mA g^− 1) prepared by high-energy ball mill [22]. A large irreversible specific capacity loss of 75 mAh g^− 1 was observed during the first cycle, indicating a low coulombic efficiency of 69.4%, which is probably associated with the decomposition of the electrolyte at the high charge voltage. Similar phenomena have often been observed with cells using CoO-LiF, NiO-LiF, NiMn2O4-4LiF and MnO-1.5LiF composites as positive materials in lithium-ion batteries [10–12, 21]. Nonetheless, subsequent testing demonstrated that this low coulombic efficiency was restricted primarily to the initial cycle because coulombic efficiency was gradually increased to 89.7% in the 10th cycle and 89.0% in the 20th cycle from 69.4% in the first cycle. A similar observation has been made on the NiMn₂O₄-4LiF composite by Tomita and co-workers [12]. They found that the coulombic efficiency in the first cycle was 84%, and the efficiency after 100 cycles was 95%. It is worth mentioning that the first charge exhibited a higher charge voltage than that of the subsequent every one, closely associated with the electrochemical activation of MnO via the splitting of LiF to produce a new active phase, most likely “Mn³⁺-O-F” type species, which is in agreement with previous observations [14, 21].

Figure 4

The electrochemical impedance spectroscopy (EIS) tests were conducted on the electrodes over charge–discharge cycles (Fig. 5). We observed in Fig. 5 that compared with the first cycle, the diameter of the semicircle, equivalent to the integrative effect of the electrode charge transfer impedance and the electrode/electrolyte interface impedance, significantly increase after 50 cycles, which results in growth in polarization upon cycling and great decrease in capacity, as shown in Fig. 4.

Figure 5

To measure the cycle performance of MnO-LiF composite, galvanostatic charge-discharge cycles were carried out at a charge-discharge rate of C/20 in the voltage range 1.5–4.8 V, as shown by Fig. 6. It can be found from Fig. 6 that the composite delivers respectively a specific capacity of 170, 190, 189, 179, 159, 145, 138 and 137 mAh g^− 1 in the 1st, 2nd, 3rd, 10th, 20th, 30th, 40th and 50th cycle. The capacity retention rate was 80.6% after 50 cycles, comparable with the values found for NiO-LiF composite milled for 72 h and the NiMn₂O₄-4LiF composite [11, 12]. Similar observations of cycle capacity fading were reported by Tomita team in the NiO-LiF and NiMn₂O₄-4LiF systems [11, 12]. The capacity decay of the composite may be directly relative with the decomposition of the electrolyte, dissolution of manganese oxide and the growth of polarization on increase in the number of cycle, as shown in Figs. 4 and 5. As well known, capacity losses of manganese-based cathode materials, such as LiMn₂O₄ and Li1.9Mn0.95O2.05F0.95, have been partly ascribed to the dissolution of Mn ions into the electrolyte, which in turn gives rise to loss of active material from the cathode and increment in impedance of the Li metal anode due to deposite of Mn-containing species [24–26].

Figure 6

Figure 7 displays the rate capability of MnO-LiF composite at different rates from C/20 to C/5. It can be observed that MnO-LiF sample shows a significant decrease in specific capacity from 169 mAh g^− 1 to 104 mAh g^− 1 as the rate increases from C/20 to C/5. We suggest that the sluggish conversion reaction kinetics of MnO-LiF composite may be attributed to the following two major factors : one is bigger MnO-LiF agglomerate particles in comparison with the previously reported FeO-LiF, NiO-LiF, and MnO-LiF composites [10, 11, 19], which were highly dispersed using long time high-energy mechanical milling, suggestive of rise in diffusion path for electrons and lithium ions in sample grains during charge and discharge; and another is that not all the MnO and LiF grains are in close contact with KB, resulting in difficulty of the charge transport in areas with less KB. The future researches will focus on the fabrication of smaller active material particles and the application of more effective conductive additives.

Figure 7

Figure 8 shows the CV of the MnO-LiF composite electrode after the electrode activation. The CV curve was gentle on the whole, and broad peaks were observed. In the oxidation process, a broad anodic peak, ranging from ~ 2.55 to 3.59 V Li/Li⁺, and a weak anodic peak at ~ 3.81 V Li/Li⁺ were observed, which could be associated to manganese redox reaction (Mn²⁺→Mn³⁺→Mn⁴⁺). We found our CV observations were almost consistent with those reported by Jung et al [10] in the case of LiF-MnO nanocomposite, in which the oxidation reaction appeared mainly at 2.5V and 3.75V Li/Li⁺. In the subsequent reduction process, only one main reduction peak at ~ 3.13 V vs Li/Li⁺ and a very gently downward slope from 3.92 to 3.59 V were observed, different from that of the previous report for the LiF-MnO nanocomposite by Jung group [10]. In their work, two reduction peaks respectively at 2.5 and 3.75 V were detected during the lithiation, whereas the intensity of the high potential peak is much weaker than that of the low high potential peak. As far as we know, this reductive feature in our CV experiment is not clear.

Figure 8

In summary, MnO-LiF nanocomposite was synthesized successfully by thermal reaction using NH₄F as a fluorine source at a moderate temperature of 550 °C for minimal duration (5 minutes). The SEM image revealed that the sample is composed of regular and uniform sphere-like primary particles with an average size of about 50 nm. When cycled at a rate of C/20, the composite exhibited an initial specific capacity of 170 mAh g^− 1 and a discharge specific capacity of 137 mAh g^− 1 in the 50th cycle, equivalent to a moderate capacity retention rate of 80.6%. Rate capability for the composite, however, is in need of improvement.

Competing interests The authors declare no competing interests.

Funding information

This work was financially supported by Hunan Provincial Natural Science Foundation of China (No. 2021JJ30264).

Author Contribution

Chen JD synthesized the composite, fabricated the electrode, performed electrochemical tests and data collection, and made some charts. Tang AP guided the experimental design and analysis of experimental results, and wrote the whole manuscript including some tables and figures. All authors participated in the discussion of experimental results.

Data availability

No datasets were generated or analysed during the current study.

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You are reading this latest preprint version

Synthesis of MnO-LiF Composite Cathode Material Using NH4F as a fluorine source

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Abstract

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Introduction

Experimental Section

Material Synthesis

Characterization

Results and Discussion

Conclusions

Declarations

Funding information

Author Contribution

Data availability

References

Additional Declarations

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