In situ formed amorphous carbon-coated LiMn₂O₄ cathode with long-term stability for lithium-ion batteries

Abstract

Spinel LiMn₂O₄ is one of the most promising cathode materials due to its green, low-cost, and abundant resources. However, issues such as manganese dissolution, electrolyte decomposition, and inadequate cycling stability have hampered its further application. Herein, we designed a modification strategy for LiMn₂O₄ cathodes using N-methyl-2-pyrrolidone (NMP), water, and ethanol mixed solution as the carbon source to in situ coat LiMn₂O₄ truncated octahedra particles. It is found that the amorphous carbon layer serves as a structure stabilization agent to enhance the electrochemical performance of LiMn₂O₄. Specifically, the coating carbon layer can effectively minimize the acid corrosion, facilitate Li⁺ diffusion, improve the interface between the electrode and electrolyte, and strengthen cycle stability. As a result of these improvements, the optimized 2 mL C/LiMn₂O₄ sample exhibits outstanding long-term performance, with an initial discharge specific capacity of 107.6 mAh g⁻¹ and a capacity retention of 50.56% after 2000 cycles at 10C. This outstanding performance makes our material a promising candidate cathode for lithium-ion battery in future applications.

---

1. Introduction

The concept of low-carbon development has fueled a dramatic increase in lithium-ion battery (LIB) production to meet the demands of energy storage.^1–6 Yet, cathode materials are pivotal in determining the overall lithium-ion battery performance.^7,8 Among various cathode materials, the spinel LiMn₂O₄, which exhibits natural abundance, environmental friendliness, low cost, excellent thermal stability, high operating voltage, and three-dimensional Li-ion diffusion channels, is regarded as one of the most promising candidate LIB cathode materials.^9,10 Nevertheless, the widespread use of spinel LiMn₂O₄ has been hindered by the dissolution loss of Mn and decomposition of the electrolyte.^11,12 Consequently, it is crucially important to develop strategies to inhibit Mn dissolution and electrolyte decomposition while enhancing surface structure stability of LiMn₂O₄.

Surface coating is regarded as one of the most effective strategies to alleviate Mn dissolution and electrolyte decomposition. So far, various materials have been employed to coat LiMn₂O₄ cathodes, such as transition metal oxides,^13–15 fluoride,^16,17 phosphate,^18,19 and carbon materials.^20,21 However, these non-carbon coating materials have poor electronic conductivity, which can limit the electronic transfer to affect the rate performance of LiMn₂O₄. Consequently, carbon materials with high electronic and ionic conductivity have received much attention.^22–26 The carbon coating improves the structural stability of LiMn₂O₄, which mainly brings the following two aspects. On the one hand, carbon coating can provide a continuous electron pathway, thus promoting fast charge transfer, which results in improved rate capability of LiMn₂O₄.²⁷ On the other hand, the carbon coating layer can avoid direct contact between the LiMn₂O₄ cathode and electrolyte, and protects the LiMn₂O₄ from the electrochemical erosion, thereby reducing the dissolution loss of Mn and decomposition of the electrolyte to enhance the cycling stability of LiMn₂O₄.²⁸ However, organic compounds (such as glucose and sucrose) as the carbon source to coat LiMn₂O₄; the thermal decomposition temperature of organic compounds requires ca. 600 °C and an Ar/H₂ atmosphere, which is a challenge to achieve large-scale industrial manufacturing. For example, Lee et al. synthesized carbon-coated LiMn₂O₄ nanoparticle clusters using sucrose as the carbon source and fired at 600 °C for 10 min.²⁹ Sun et al. prepared carbon-coated LiMn_0.85Fe_0.15PO₄ cathode material using sucrose as a carbon source and calcined for 15 h at 700 °C in a furnace purged with an Ar/H₂ (96/4 by vol%) mixture.³⁰ Therefore, it is urgent to develop a simpler method and low-cost carbon sources to prepare carbon coating on the LiMn₂O₄ surface.

Herein, truncated octahedra LiMn₂O₄ were coated by an amorphous carbon layer using N-methyl-2-pyrrolidone (NMP), water, and ethanol mixed solution as the carbon source. The pyrolysis of this carbon source only requires 400 °C and an air atmosphere. To the best of our knowledge, this unique strategy is rarely reported for LiMn₂O₄. The designed the optimized 2 mL C/LiMn₂O₄ cathode material demonstrates outstanding rate capability and capacity retention. This work provides a new horizon for LiMn₂O₄ cathode material application.

2 Experimental section

2.1 Material synthesis

The pristine LiMn₂O₄ sample was synthesized via a facile calcination process. Initially, Mn₃O₄ and LiOH·H₂O were purchased from Aladdin Reagent Co. Ltd, China and were mixed in a specific stoichiometric ratio in an agate mortar, with a 5% excess of lithium. The mixture was then heated at 500 °C for 3 h in air in the muffle furnace. The gained black sample was pulverized by hand-grounding using an agate mortar and heated at 750 °C in air for 6 h again. After cooling to room temperature, the final LiMn₂O₄ product was obtained. To prepare a carbon-coated LiMn₂O₄ sample. Firstly, 1 g of LiMn₂O₄ was transferred to the 50 mL beaker and then added to 15 mL of water and ethanol mixed solution (V_water : V_ethanol = 3 : 1). Afterward, the mixture was vigorously stirred for 5 min. Subsequently, 0.5, 2, and 4 mL of N-methyl-2-pyrrolidone (NMP) were dissolved in sequence in the above mixture. Next, the suspension was vigorously stirred for 2 h. This solution was then dried in an oven at 105 °C. After drying, the obtained gel was calcined at 400 °C for 1 h under an air atmosphere to obtain the carbon-coated LiMn₂O₄ product. The resulting powders were denoted as 0.5 mL C/LiMn₂O₄, 2 mL C/LiMn₂O₄, 4 mL C/LiMn₂O₄ with different NMP levels.

2.2 Material characterization

The crystal structure of pristine LiMn₂O₄ and carbon-coated LiMn₂O₄ was analyzed using X-ray diffraction (XRD, Ultima-IV, Rigaku, Tokyo, Japan) with the 2θ range of 10–80° and scanning rate of 3° min⁻¹. The Rietveld refinement based on XRD data was calculated by Fullprof software. Surface morphology and element composition of the materials were investigated by scanning electron microscopy (SEM, Gemini 450, ZEISS, Jena, Germany) with an accelerating voltage of 5.0 kV and transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan) equipped with an energy-dispersive X-ray detector at an accelerating voltage of 200 kV, respectively. The Image J program was used to calculate the average grain sizes of the materials. The surface chemical information and Mn valence state were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fischer, ESCALAB 250Xi, USA) using the monochromatic Al-K_α as the excitation source and C 1s (284.8 eV) for a reference binding energy. The microstructure and atomic-scale information of the samples were analyzed using spherical aberration-corrected transmission electron microscopy (JEM Grand ARM300F, JEOL, Japan). The electrodes of LiMn₂O₄ and 2 mL C/LiMn₂O₄ after 2000 cycles used for XRD and XPS characterizations were disassembled from coin cells.

2.3 Electrochemical tests

The electrochemical performances of the prepared pristine LiMn₂O₄ and carbon-coated LiMn₂O₄ were evaluated by using a CR2025-type coin cell, where pristine LiMn₂O₄ and carbon-coated LiMn₂O₄ electrodes were fabricated by mixing of active materials, carbon black, and polyvinylidene fluoride (PVDF) in a weight ratio of 8 : 1 : 1 dissolved in N-methyl-2-pyrolidone (NMP) solvent to form a slurry in air using a high-energy Micro-Vibration Mill. The slurry was then evenly spread onto an Al foil current collector using a coating blade and dried in a vacuum oven at 120 °C for 12 h to remove residual solvents. The vacuum-dried foils were cut into 14 mm diameter discs using a punching machine to serve as the working electrode. The mass loadings of the active materials per the working electrode sheet area ranged from 1.3 to 2.0 mg cm⁻². The CR2025-type coin cells were assembled in an argon-filled glove box (O₂ < 0.01 ppm, H₂O < 0.01 ppm). The lithium metal sheet (thickness of 0.45 cm) was used as the negative electrode. And the Celgard 2400 polypropylene membrane was used as the separator. The 1 mol L⁻¹ LiPF₆ in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1 in volume) was used as the electrolyte. Galvanostatic charge–discharge tests were conducted using a computer-controlled LAND-CT2001 battery testing system. All electrochemical tests were conducted at room temperature. The cycling voltage window was set at a range of 3.0–4.5 V. Cycling test was carried out at 10C (1C = 148 mAh g⁻¹) for 2000 cycles. The rate capability test was performed at rates of 0.5C, 1C, 2C, 3C, 5C, and 10C. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed on the coin cell configuration using a CHI604E electrochemical workstation (Shanghai Chenhua, China). The CV measurements were conducted at scan rates of 0.05, 0.1, 0.15, 0.2, and 0.25 mV s⁻¹ within a voltage range of 3.6–4.5 V (vs. Li/Li⁺). The EIS tests were carried out with 10 mV perturbation amplitude in the frequency range of 10⁻¹ to 10⁵ Hz in automatic sweep mode.

3 Results and discussion

3.1 Structural and surface analysis

X-ray diffraction analysis is used to investigate the effect of different NMP contents on the crystal structure of LiMn₂O₄. As shown in Fig. 1a, the diffraction patterns of four samples are well indexed to the standard spinel LiMn₂O₄ (JCPDS 35-0782), confirming that the introduction of NMP would not induce any crystal structure alteration of LiMn₂O₄. Also, no diffraction peaks of carbon were observed, which may be attributed to the carbon layer is inherently amorphous and the contents is low. To better analyze the sample structure, the lattice parameters of LiMn₂O₄ and carbon-coated LiMn₂O₄ are calculated through XRD Rietveld refinement, as shown in Fig. 1c–f. Each fitted curve is well indexed to the measured data (R_wp < 10, CHI² < 10),^31,32 indicating refinement results are reliable. Fig. 1b presents the relationship between lattice parameters and the amount of carbon coating. It can be seen that the lattice parameters of the four samples are basically the same. Therefore, it can be further verified that the carbon coating does not cause structural changes of LiMn₂O₄ bulk phase.

Fig. 1 (a) XRD patterns of LiMn₂O₄ and carbon-coated LiMn₂O₄ with different NMP contents; (b) lattice parameters vs. amount of carbon coated plot; (c–f) XRD Rietveld refinement results of LiMn₂O₄ and carbon-coated LiMn₂O₄ with different NMP contents.

The morphology of LiMn₂O₄ and carbon-coated LiMn₂O₄ with different NMP contents was characterized using scanning electron microscopy. As shown in Fig. 2a–d, both materials exhibited similar truncated octahedra morphologies. Histograms of particle size distribution of LiMn₂O₄ and carbon-coated LiMn₂O₄ are examined on the basis of micrograph analysis (Fig. S1). The average particle size of LiMn₂O₄ and the carbon-coated LiMn₂O₄ sample is similar (ca. 129.27 nm), which indicates the carbon coating does not influence the morphology and particle size of LiMn₂O₄. To verify the existence of the carbon coating layer, EDS elemental mapping has been performed (Fig. 2e). Fig. 2e exhibits Mn and O elements are homogeneously distributed in the central region of the particle, while the C element is highly concentrated in the outer region and shows a high intensity. This provides strong evidence that the carbon layer is coated on the surface of LiMn₂O₄.

Fig. 2 (a–d) SEM images of LiMn₂O₄ and carbon-coated LiMn₂O₄ with different NMP contents; (e) STEM images and corresponding EDS elemental maps of 2 mL C/LiMn₂O₄.

In order to further confirm that carbon is coated on the surface of LiMn₂O₄, high-resolution C 1s XPS spectra are measured as shown in the Fig. 3a. The C 1s spectra of 2 mL C/LiMn₂O₄ (Fig. 3a) exhibit three deconvoluted peaks comprising of C–C, C–O, and C=O located at 284.82, 286.52, and 288.47 eV, respectively,^24,33 which guarantees the carbon coating. Notably, the Mn valence state of LiMn₂O₄ and 2 mL C/LiMn₂O₄ were investigated (Fig. 3b). The Mn 2p spectral peaks were separated into the Mn 2p_3/2, Mn 2p_1/2, and satellite peak due to spin–orbit splitting,^34,35 which displays the mixing of Mn³⁺ and Mn⁴⁺. It is worth noting that the binding energy of Mn 2p_3/2 is similar for LiMn₂O₄ and 2 mL C/LiMn₂O₄, which suggests the carbon coating had less effect on the Mn valence state. The detailed Mn valence state information was obtained, which fitted the Mn 2p_3/2 spectra using the CasaXPS software; the results were shown in Fig. 3c, d and Tables S1, S2. It was found that the concentrations of Mn³⁺ and Mn⁴⁺ in the LiMn₂O₄ sample were 51.50% and 48.50%, respectively, whereas those in the 2 mL C/LiMn₂O₄ sample were 52.54% and 47.47%, respectively. This further suggests that carbon coating does not cause Mn valence state changes of LiMn₂O₄.

Fig. 3 (a) High-resolution C 1s XPS spectra of 2 mL C/LiMn₂O₄; (b) high-resolution Mn 2p XPS spectra of LiMn₂O₄ and 2 mL C/LiMn₂O₄; fitted spectra of Mn 2p_3/2 for (c) LiMn₂O₄ and (d) 2 mL C/LiMn₂O₄.

3.2 Analysis of the atomic-level structural

To better confirm the carbon coating on the surface of LiMn₂O₄, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) of LiMn₂O₄ and 2 mL C/LiMn₂O₄ was conducted, as shown in Fig. 4. Fig. 4a and c displays the overall morphology of the probed particle for LiMn₂O₄ and 2 mL C/LiMn₂O₄, respectively. It can be seen that the carbon layer is coated on the surface of the LiMn₂O₄ particle (Fig. 4c). The coating thickness of the carbon layer is about 9.53 nm (Fig. 4d). Detailed crystalline structures between the bulk and the surface of the LiMn₂O₄ and 2 mL C/LiMn₂O₄ were disclosed by the high-resolution HAADF images (Fig. 4b and e). It was found that the bulk and surface structure of LiMn₂O₄ remains the same Mn diamond configuration from the magnified images in Fig. 4b, with the brighter and weaker dots corresponding to Mn1 and Mn2 columns, respectively. Moreover, Fig. 4f and g confirm that the intensity of Mn1 columns is twice that of Mn2 columns due to different stacking density.³⁶ However, no observable contrast in Li and O sites in Fig. 4b. This is because the HAADF image is only sensitive to heavy elements.^37,38 For the carbon-coated LiMn₂O₄ samples, the bulk region possesses the same Mn diamond configuration (Fig. 4e and h). However, the surface region illustrates that the coated carbon layer is amorphous. The carbon layer coated on the surface of LiMn₂O₄, which facilitates the electron conduction and prevents HF from the electrolyte to etch LiMn₂O₄ cathode.

Fig. 4 (a) The low-magnification STEM image and (b) atomic-resolution HAADF image of the LiMn₂O₄ cathode; (c) the low-magnification and (d) medium-magnification STEM image of 2 mL C/LiMn₂O₄ cathode, (e) atomic-resolution HAADF image of 2 mL C/LiMn₂O₄ cathode; The right exhibits surface and bulk region magnified images in the HAADF image from the blue and orange frames, respectively. The surface and the bulk area are demarcated by the golden dotted line. Mn atoms are labeled in orange. (f–h) Line profile corresponding to the blue and purple lines in the magnified image of LiMn₂O₄, and the pink line in the magnified image of 2 mL C/LiMn₂O₄.

3.3 Electrochemical testing

In order to evaluate the effectiveness of carbon coating on the spinel LiMn₂O₄ material performance, electrochemical tests were conducted and are shown in Fig. 5. Fig. 5a shows the long-term cycling performance of LiMn₂O₄ and carbon-coated LiMn₂O₄ with different NMP contents between 3.0 and 4.5 V at 10C at 25 °C. It can be observed that the initial discharge specific capacities of LiMn₂O₄, 0.5 mL C/LiMn₂O₄, 2 mL C/LiMn₂O₄, and 4 mL C/LiMn₂O₄ are 57.7, 91.7, 107.6, and 82.9 mAh g⁻¹, respectively, with corresponding capacity retention rates after 2000 cycles of 39.34%, 44.60%, 50.56%, and 46.56% (Fig. 5a and b). As the coating amount increases, the initial discharge specific capacity and capacity retention rates initially increase and then reduce. This trend can be attributed to the carbon coating layer, which enhances charge transfer between particles, where an optimal carbon coating amount enhances the battery performance. Apart from that, the coulombic efficiency retains over 99% from the 2^nd cycle (Fig. 5a), suggesting the excellent reversibility of the electrochemical reaction for 2 mL C/LiMn₂O₄. These results imply that 2 mL C/LiMn₂O₄ has optimal long-term performance among the modified materials.

Fig. 5 Electrochemical performance of LiMn₂O₄ and carbon-coated LiMn₂O₄ with different NMP contents: (a) long-term cycling performance at 10C for 2000 cycles at 25 °C; (b) comparison of the difference value of initial capacity, 2000th capacity, and capacity retention rates at 10C for 2000 cycles at 25 °C, respectively; (c) rate capability at various C rates from 0.5C to 10C; (d–g) the charge–discharge curves at different current rates; (h) initial charge–discharge curves at 10C.

Fig. 5c presents the rate capability curves of the LiMn₂O₄ and carbon-coated LiMn₂O₄ with different NMP contents. As can be seen in Fig. 5c, the discharge specific capacities of LiMn₂O₄, 0.5 mL C/LiMn₂O₄, 2 mL C/LiMn₂O₄, and 4 mL C/LiMn₂O₄ decrease with increasing rates, whereas the 2 mL C/LiMn₂O₄ exhibits superior performance compared to the other three samples at various rates. The improved high-rate performance of 2 mL C/LiMn₂O₄ can be attributed to the carbon coating layer, which protects the active material from undesirable chemical reactions. Additionally, it is worth noting that all initial charge–discharge of the four samples at low C rate presented two pairs of distinct plateaus platforms (Fig. 5d–g), corresponding to Li-ions extracted/inserted from/into spinel LiMn₂O₄.³⁹ Moreover, the initial charge/discharge special capacity of 2 mL C/LiMn₂O₄ shows slower decay at various C-rates, suggesting that an appropriate carbon coating layer can inhibit the electrolyte decomposition on the electrode surfaces. However, the plateau platforms of the initial charge–discharge of the four samples aren't obvious at 10C high rates (Fig. 5h), which can be attributed to the increase in electrochemical polarization at high current density.

To further explore the mechanism of Li⁺ diffusion during the charge–discharge process, cyclic voltammetry (CV) analyses were performed on LiMn₂O₄ and 2 mL C/LiMn₂O₄, as shown in Fig. 6a, b and S2a–c. It can be seen that both CV profiles reveal the doublet redox peaks at around 4.0 and 4.15 V, corresponding to the deintercalation and intercalation of Li⁺, respectively.⁴⁰ The peak positions agree with the plateaus in the charge–discharge curves (Fig. 5d and f). Additionally, before cycling and after 2000 cycles at 10C, 2 mL C/LiMn₂O₄ exhibits higher peak currents, suggesting that carbon coating enhances the lithium storage capacity of LiMn₂O₄. However, after 2000 cycles at 10C, the peak areas of the CV curve of LiMn₂O₄ and 2 mL C/LiMn₂O₄ decrease, which indicates that the capacity reduces significantly. With the growth of the scan rate for CV curves (Fig. S2a and b), the redox peaks become broader and the peak current increases, which results from the irreversible processes in Li⁺ insertion/extraction in/from the spinel structure. However, the carbon coating LiMn₂O₄ has lower electrochemical polarization than that of LiMn₂O₄. Specifically, the relationship of the reduction peak current at bare LiMn₂O₄ and 2 mL C/LiMn₂O₄ with the square root of scan rate is studied (Fig. S2c). Corresponding to lithium-ion diffusion coefficients (D_Li⁺) of LiMn₂O₄ and 2 mL C/LiMn₂O₄ were calculated, the detailed information is shown in the supporting information. The lithium-ion diffusion coefficient of 2 mL C/LiMn₂O₄ was calculated as 5.72 × 10⁻¹⁵ cm² s⁻¹, which is significantly higher than that of LiMn₂O₄ (1.26 × 10⁻¹⁵ cm² s⁻¹), indicating that carbon coating facilitates the diffusion of Li⁺.

Fig. 6 CV curves of the bare LiMn₂O₄ and 2 mL C/LiMn₂O₄ (a) before cycling and (b) after 2000 cycles with scan rates of 0.05 mV s⁻¹; (c) EIS plots of bare LiMn₂O₄ and 2 mL C/LiMn₂O₄ before cycling and the inset shows the equivalent circuits; (d) linear fitting of Z′ vs. ω^−1/2.

To further analyze the effect of carbon coating on the diffusion kinetics of Li⁺, the EIS measurement of LiMn₂O₄ and 2 mL C/LiMn₂O₄ before and after 2000 cycles at a 10C rate was conducted. The Nyquist plots of LiMn₂O₄ and 2 mL C/LiMn₂O₄ before and after 2000 cycles and the equivalent circuit used for fitting the Nyquist plots are shown in Fig. 6c and S2d. Fig. 6c and S2d show EIS plots that consist of high-frequency and medium-region semicircles, and an oblique straight line at low frequency region.⁴¹ The inset of Fig. 6c and S2d shows the equivalent circuits comprising resistors (R) at the high frequency and medium region, constant phase elements (CPE), and a Warburg element (W) at the low frequency region.²⁹ The semicircle in the high-frequency region represents the impedance of the electrolyte between the two electrodes (R₁). The semicircle in the medium frequency region corresponds to the charge transfer impedance (R₂), and the oblique straight line at low frequency region indicates the Warburg impedance associated with the diffusion of Li⁺ ions. To obtain specific impedance values, the fitting of the EIS curve by an equivalent circuit was performed and is listed in Table S3. It can be seen that the charge transfer impedance (R₂) values of 2 mL C/LiMn₂O₄ before and after cycling are significantly lower than those of LiMn₂O₄. This can be attributed to the fact that carbon coating has excellent electron transport, which facilitates Li⁺ transport by improving surface diffusion kinetics.²² To quantitatively evaluate the Li⁺ diffusion coefficient (D_Li⁺), linear fitting of Z′ vs. ω^−1/2 for LiMn₂O₄ and 2 mL C/LiMn₂O₄ before cycling is presented in Fig. 6d, the corresponding calculated process is shown in the SI. As shown in Fig. 6d, the Warburg coefficient (σ) of LiMn₂O₄ and 2 mL C/LiMn₂O₄ is 129.27 and 119.87 Ω s^−1/2, respectively. Therefore, Li⁺ diffusion coefficient (D_Li⁺) of 2 mL C/LiMn₂O₄ was calculated as 2.74 × 10⁻¹⁵ cm² s⁻¹, which is higher than that of LiMn₂O₄ (2.36 × 10⁻¹⁵ cm² s⁻¹), indicating that carbon coating facilitates the diffusion rate of Li⁺.

3.4 Analysis of cycled cathodes

To assess the crystal stability of carbon-coated LiMn₂O₄, XRD analysis was performed on the pure LiMn₂O₄ and 2 mL C/LiMn₂O₄ before and after cycling. As shown in Fig. 7a and b, it can be seen that the diffraction peaks of pure LiMn₂O₄ and 2 mL C/LiMn₂O₄ after cycling remain similar to those before cycling, apart from the appearance of the C impurity peaks derived from the conductive agent carbon black and Al impurity peaks derived from the Al foil current collector, suggesting that the spinel-type LiMn₂O₄ structure is still maintained. However, it can be clearly observed from Fig. 7a that after 2000 cycles, the intensity of the (111) diffraction peak of pure LiMn₂O₄ is weaker than that before the cycle, indicating that the crystallinity of pure LiMn₂O₄ is decreased. The (111) diffraction peak of 2 mL C/LiMn₂O₄ after 2000 cycles still has high intensity (Fig. 7b), indicating that 2 mL C/LiMn₂O₄ has high cyclic stability. This result indicates that carbon coating can suppress Mn dissolution and alleviate side reactions at the electrode surface, thereby improving the structural stability of LiMn₂O₄.

Fig. 7 Comparison of XRD patterns before cycling and after 2000 cycles at 10C of (a) LiMn₂O₄ and (b) 2 mL C/LiMn₂O₄.

To gain a deeper understanding of the stability of 2 mL C/LiMn₂O₄, XPS tests were also carried out after cycling. In the C 1s spectra of LiMn₂O₄ and 2 mL C/LiMn₂O₄ after 2000 cycles at 10C (Fig. 8a, d), the C–C peak originates from conductive carbon black and carbon coating, the C–O and C=O peaks are derived from the decomposition products of the organic electrolyte and carbon coating.⁴² It can be seen that the intensity of the C–C peak for 2 mL C/LiMn₂O₄ is higher than that of LiMn₂O₄. However, the intensity of the C–O and C=O peaks for 2 mL C/LiMn₂O₄ is lower than that of LiMn₂O₄, indicating that carbon is still coated on the surface of LiMn₂O₄ after 2000 cycles at 10C and can inhibit the decomposition of the electrolyte. In the O 1 s spectra, 2 mL C/LiMn₂O₄ exhibits stronger Mn–O peaks (Fig. 8b and e), suggesting that carbon coating can prevent the release of oxygen from the electrode surface. Comparing F 1s spectra, it can be discovered that the weaker peaks of LiF/Li_xPO_yF_z and C–F on the 2 mL C/LiMn₂O₄ surface were detected (Fig. 8c and f), further verifying that carbon coating can suppress the decomposition of LiPF₆. Therefore, carbon coating can inhibit the decomposition of the electrolyte and prevent oxygen release from the material surface, thus enhancing the interfacial stability and long-term cycling performance of LiMn₂O₄.

Fig. 8 XPS spectra of C 1s, O 1s, and F 1s for (a–c) LiMn₂O₄ and (d–f) 2 mL C/LiMn₂O₄ after 2000 cycles at 10C.

4 Conclusions

In summary, an amorphous carbon layer coating on the surface of the LiMn₂O₄ cathode material is synthesized through a facile calcination process. It was found that the introduction of the carbon layer didn't change the bulk structure, surface morphology, and Mn oxidation state of the LiMn₂O₄. However, the carbon layer serves as a structure stabilization agent to reduce Mn dissolution, improve the interface between the electrode and electrolyte, and inhibit electrolyte decomposition, which enhances Li⁺ migration. The optimized 2 mL C/LiMn₂O₄ sample exhibits the most excellent long-term performance with the initial discharge specific capacity of 107.6 mAh g⁻¹ and the capacity retention of 50.56% after 2000 cycles at 10C, which is much higher than that of LiMn₂O₄ (57.7 mAh g⁻¹, 39.34%). This work provides a simple and efficient modification strategy for developing high-performance LiMn₂O₄ cathode materials.

Author contributions

Wangqiong Xu: writing – review & editing, writing –original draft, funding acquisition. Xianrong Li: data curation. Xueqing Kang: data curation. Lijuan Chen: formal analysis. Baiyan Guo: software. Qiling Li: visualization, validation, conceptualization, investigation. Kun Xu: resources. Yiming Cao: resources. Zhe Li: resources. Yongsheng Liu: supervision, resources, methodology.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

All relevant data are included in the paper. The data supporting this study's findings are available from the corresponding author upon reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6ra02942f.

Acknowledgements

This work was supported by the Fundamental Research Project of Yunnan Province (202301AT070239), the Special Basic Cooperative Research Programs of Yunnan Provincial Undergraduate Universities' Association (grant No. 202401BA070001-129), the Special Basic Cooperative Research Innovation Programs of Qujing Science and Technology Bureau & Qujing Normal University (KJLH2023ZD02).

Notes and references

1. J. E. Zhou, Y. L. Li, M. Y. Zou, X. Hu, X. S. Miao, X. X. Xie, X. M. Lin, J. Qian, C. Yang and R. J. Chen, Adv. Funct. Mater., 2025, 35, 2501603.
2. X. C. Ye, X. Y. Fei, M. J. Liu, H. Gao, B. L. Qiu, H. Y. Yin, Z. H. Zhang and L. Y. S. Lee, Adv. Mater., 2025, 37, 2416537.
3. Y. Y. Bi, S. L. Tian, Y. Zhang, A. K. Huang, P. Kumar, Z. Ma, W. Q. Liu and J. Ming, ACS Energy Lett., 2026, 11, 1670–1679.
4. J. Guo, J. Y. Du, W. Q. Liu, G. Huang and X. B. Zhang, Angew. Chem., Int. Ed., 2024, 63, e202406465.
5. Y. Zhang, Z. Y. Hu, Y. Y. Bi, S. L. Tian, H. R. Sun, K. Li, W. Q. Liu, L. S. Sun, W. Liu and D. Wang, ACS Sustainable Chem. Eng., 2025, 13, 5381–5393.
6. Y. F. Cui, Z. B. Zhuang, Z. L. Xie, R. F. Cao, Q. Hao, N. Zhang, W. Q. Liu, Y. H. Zhu and G. Huang, ACS Nano, 2022, 16, 20730–20738.
7. H. J. Yang, L. Yan, J. M. Qin, Y. Wang, N. Hu and T. Zhou, Adv. Funct. Mater., 2026, 36, e15317.
8. J. Guo, H. B. Chen, D. P. Wang, W. Q. Liu, G. Huang and X. B. Zhang, Sci. Bull., 2024, 69, 3237–3246.
9. W. H. Zeng, F. J. Xia, J. Wang, J. L. Yang, H. Y. Peng, W. Shu, Q. Li, H. Wang, G. Wang, S. C. Mu and J. S. Wu, Nat. Commun., 2024, 15, 7371.
10. J. Z. Du, Z. W. Sun, B. H. Peng, X. M. Xu, L. J. Fu, Y. H. Chen, L. L. Liu, X. Liu and Y. P. Wu, Adv. Funct. Mater., 2025, 35, 2421179.
11. S. Q. Zhong, Y. C. Huang, F. C. Zhang, H. Z. Wang, P. W. Liu, J. W. Liu, Z. Q. Li, Y. Z. Li and Z. G. Lu, Adv. Funct. Mater., 2025, 35, 2414602.
12. G. C. Tan, S. Wan, J. J. Chen, H. Q. Yu and Y. Yu, Adv. Mater., 2024, 36, 2310657.
13. R. X. Yin, W. H. Xu and Z. W. Zhao, Desalination, 2026, 620.
14. Q. L. Li, W. Q. Xu, H. L. Bai, J. M. Guo and C. W. Su, Ionics, 2016, 22, 1343–1351.
15. Y. F. He, H. Pham, X. H. Liang and J. Park, J. Power Sources, 2023, 586, 233682.
16. T. Wang, W. Wang, D. Zhu, X. B. Duan, Z. Q. Wet and Y. G. Chen, Chin, J. Inorg. Chem., 2014, 30, 2461–2468.
17. Y. P. Wang, X. Y. Wang, S. Y. Yang, H. B. Shu, Q. L. Wei, Q. Wu, Y. S. Bai and B. N. Hu, J. Solid State Electrochem., 2012, 16, 2913–2920.
18. S. Zhao, Y. Bai, L. H. Ding, B. Wang and W. F. Zhang, Solid State Ionics, 2013, 247, 22–29.
19. C. B. Qing, Y. Bai, J. M. Yang and W. F. Zhang, Electrochim. Acta, 2011, 56, 6612–6618.
20. M. J. Lee, E. Lho, P. Bai, S. Chae, J. Li and J. Cho, Nano Lett., 2017, 17, 3744–3751.
21. D. Yao, Z. D. Wu, J. J. Song, L. Cai, Y. Ouyang, W. Lei, S. Mathur, F. Q. Wu and Q. L. Hao, ACS Appl. Energy Mater., 2020, 3, 4840–4851.
22. J. F. Wang, W. Liu, S. Liu, J. X. Chen, H. L. Wang and S. P. Zhao, Electrochim. Acta, 2016, 188, 645–652.
23. G. Chen, B. Y. Peng, R. Han, N. X. Chen, Z. Z. Wang and Q. Wang, Ceram. Int., 2020, 46, 20985–20992.
24. V. Selvamani, N. Phattharasupakun, J. Wutthiprom and M. Sawangphruk, Sustainable Energy Fuels, 2019, 3, 1988–1994.
25. M. Molenda, R. Dziembaj, E. Podstawka, L. M. Proniewicz and Z. Piwowarska, J. Power Sources, 2007, 174, 613–618.
26. K. Peng and T. F. Peng, Ceram. Int., 2014, 40, 15345–15349.
27. X. L. Yu, J. J. Deng, X. Yang, J. Li, Z. H. Huang, B. H. Li and F. Y. Kang, Nano Energy, 2020, 67, 104256.
28. J. Y. He, Y. Ren, S. X. Zhuang, S. Y. Jiang, X. X. Pan, G. X. Sun, B. Zhu, Y. F. Wen and X. D. Li, Chem. Commun., 2023, 59, 13050–13053.
29. S. Lee, Y. Cho, H. K. Song, K. T. Lee and J. Cho, Angew. Chem., Int. Ed., 2012, 51, 8748–8752.
30. Y. K. Sun, S. M. Oh, H. K. Park and B. Scrosati, Adv. Mater., 2011, 23, 5050–5054.
31. Y. K. Pan, Y. H. Li, L. Hu, Y. D. Yang, J. C. Guo and S. J. Yang, J. Power Sources, 2025, 650, 237527.
32. P. A. Ferreirs and G. H. Rubiolo, J. Mater. Sci., 2018, 53, 2802–2811.
33. C. Tomon, S. Sarawutanukul, N. Phattharasupakun, S. Duangdangchote, P. Chomkhuntod, N. Joraleechanchai, P. Bunyanidhi and M. Sawangphruk, Commun. Chem., 2022, 5, 54.
34. H. Q. Li, N. Chen, S. J. Zhang, Y. H. Han, X. Gao, H. Q. Chen, Y. X. Bai and W. S. Gao, ACS Appl. Mater. Interfaces, 2025, 17, 21269–21280.
35. E. Kim, J. Lee, J. Park, H. Kim and K. W. Nam, Nano Lett., 2025, 25, 619–627.
36. W. Q. Xu, C. Z. Song, R. J. Qi, Y. H. Zheng, Y. N. Wu, Y. Cheng, H. Peng, H. C. Lin and R. Huang, ACS Appl. Mater. Interfaces, 2022, 14, 55528–55537.
37. S. Y. Yang, P. Yan, W. D. Bao, H. Y. Zhu, X. C. Cai, L. Q. Zhao, Y. Zhang, W. Y. Lin, Y. D. Deng, Y. F. Wu and J. Xie, ACS Energy Lett., 2023, 8, 4278–4286.
38. R. P. Qing, J. L. Shi, D. D. Xiao, X. D. Zhang, Y. X. Yin, Y. B. Zhai, L. Gu and Y. G. Guo, Adv. Energy Mater., 2016, 6, 1501914.
39. K. Chudzik, M. Lis, M. Swietoslawski, M. Bakierska, M. Gajewska and M. Molenda, J. Power Sources, 2019, 434, 226725.
40. S. J. Jiang, S. Wang, Y. J. Li, S. P. Hao, X. M. Xi, S. W. Liu, Y. K. Xiong, J. C. Zheng and P. P. Zhang, Mater. Today Energy, 2022, 29, 101096.
41. G. J. Yan, P. Y. Li, S. Xie, J. J. Li, W. J. Li, J. C. Zheng and M. L. Yuan, Ceram. Int., 2025, 51, 12029–12034.
42. S. Wang, S. J. Jiang, Y. J. Li, Z. L. Tan, S. P. Hao, J. C. Yang and Z. J. He, J. Power Sources, 2023, 579, 233292.

---