LiMn_0.8Fe_0.2PO₄ (LM8F) was synthesized using HEBM on MiniMill II apparatus. The starting materials included ammonium dihydrogen phosphate (NH₄H₂PO₄) as the phosphate source, lithium carbonate (Li₂CO₃) as the lithium source, manganese(II) acetate tetra-hydrate (Mn(CH₃COO)₂ • 4H₂O), and iron(III) phosphate (FePO₄) as the sources of manganese and iron, respectively. Glucose powder (C₆H₁₂O₆) was employed as a carbon source for creating a composite material. The stoichiometric calculation for obtaining LM8F was guided by the reaction equation:

Li₂CO₂ + 1.6 Mn(CH₃COO)₂ • 4H₂O + 1.6NH₃H₂PO₄ + 0.4FePO₄ → 2LiMn_0.8Fe_0.2PO₄ + nH₂O + 4CO₂ + 1.6NH₃.

The mixture was subjected to high-energy ball milling at 270 rpm for 8 h. Subsequently, a portion of the milled powder was analyzed using a Q-600 Thermo-gravimetric analyzer for TGA–DSC–DTA analysis. This analysis aimed to investigate the reaction mechanism and structural stability under temperature effects. The conditions included a 60 ml/min argon gas flow and a heating rate of 20 °C/min over a temperature range from 23 °C to 1000 °C.

Post-milling, the powder was calcined at 700°C for 3 h under a nitrogen atmosphere to prevent iron oxidation.

The crystal structure of the calcined powder was studied using X-ray diffraction (XRD) on a panalytical diffractometer operating at 45 kV and 30 mA. The diffraction patterns were collected over a 2θ range of 20° to 80° with a step time of 1 sec. using CuK_α radiation (λ = 0.15406 nm). For molecular property analysis, Fourier transform infrared spectroscopy (FTIR) was conducted using a SHIMADZU instrument, covering a wavenumber spectrum from 400 to 4000 cm^-1 with a KBr disc method. The morphology and microstructure were examined using a field emission scanning electron microscope (FeSEM).

To prepare the carbon-composite version, LM8F-10%C, glucose powder was mixed with the initial materials to achieve a 10% carbon content by weight, aiming for a carbon-coated product. Both the LM8F and LM8F-10%C samples were compacted into cylindrical disks (diameter = 12 mm, thickness = 1 mm) using a uniaxial hydraulic press at 100 kN pressure, and then sintered under N₂ atmosphere at 700 °C for 3 h.

The electrical properties of the sintered samples were analyzed to assess the impact of carbon coating on LM8F’s electrical performance. An LCR meter (WK4310) with a frequency range of 20 Hz to 100 KHz was employed. Both Bode and Nyquist plots were utilized to distinguish between the carbon-coated and non-coated samples. The temperature’s effect on electrical impedance and conductivity was also studied using the same LCR meter, providing insights into the material’s behavior under varying thermal conditions.

This comprehensive experimental approach enabled a detailed exploration of LM8F’s suitability as a cathode material for solid-state lithium batteries, focusing on structural, morphological, and electrical characteristics, with a particular emphasis on the enhancements provided by carbon coating.

The comprehensive examination of the thermal behavior of solid solution LM8F precursors during the calcination process provides critical insights into their stability and decomposition patterns. These insights are essential for refining their synthesis for application as cathode materials in lithium-ion batteries. Figures 1a and 1b effectively illustrate these thermal properties, presenting the thermogravimetric analysis (TGA) and the derivative thermogravimetry (DTG) curves. The TGA curve elucidates the weight changes experienced by the LM8F precursors as a function of temperature, laying the groundwork for understanding their thermal stability. The DTG curve, derived from the TGA data, further refines this understanding by identifying the exact temperatures at which these weight changes occur, offering a more detailed view of the precursor’s calcination behavior.

This dual analytical approach not only increases the precision of the thermal characterization but also streamlines the interpretation of the calcination dynamics of LM8F precursors. The DTG curve is especially beneficial as it clearly marks significant thermal events, such as decomposition or phase transitions, by showcasing peaks that correspond to rates of weight loss. This method enables a deeper investigation into the thermal stability and decomposition pathways of the precursors, pivotal for comprehending the formation mechanisms of the LM8F phase. Moreover, by linking these thermal events to specific temperatures, researchers can tailor the calcination process to achieve complete conversion of precursors into the desired cathode material, thereby reducing the formation of undesirable by-products. The analysis of LM8F precursors’ thermal behavior is more than a mere step in material synthesis; it is a gateway to understanding the complex chemical reactions that occur during heat treatment. As we delve into the results and discussion sections, we will explore the impact of these thermal characteristics on the final material properties of LM8F, such as crystal structure, purity, and electrochemical performance. Understanding the thermal decomposition and stability of these materials is fundamental to developing more efficient and durable synthesis methods, potentially leading to cathode materials that enhance the performance of lithium-ion batteries. The thermogravimetric analysis is depicted in Fig. 1a. outlines four distinct intervals of mass reduction throughout the heating protocol for the LM8F precursors. The initial stage, occurring between 50 and 90 °C, results in a 3.66% loss in mass, primarily due to the evaporation of water molecules adsorbed on the surface and within manganese acetate [32, 37], leading to its anhydrous form, and possibly the evaporation of residual ethanol. The second interval, between 130 and 160 °C, shows a 9% decrease in mass. In the third interval, from 175 to 209 °C, there’s a 4% reduction in mass. The final and most significant interval, ranging from 291 to 340 °C, exhibits a 18% mass loss, indicative of the loss of crystalline water, decomposition of manganese acetate, degradation of organic phosphonates, and breakdown of carbonates, respectively. The thermogravimetric curve’s plateau between 400 and 750 °C suggests the end of phase transitions and likely marks the start of the LM8F crystallization process around 420 °C. Complementing the TGA data, Fig. 1b presents the differential thermal analysis (DTA) and differential scanning calorimetry (DSC) curves for LM8F precursors. An initial exothermic peak at 91 °C on the DTA curve corresponds to desorption processes. Additionally, a series of exothermic peaks from 151 to 320 °C on the DTA curve, closely paralleled by the DSC curve within the same temperature range, indicates the stages of decomposition as identified by the TGA. An endothermic peak at 420 °C on the DTA curve is associated with the crystallization phase of the sample, in line with the thermogravimetric analysis’s indication of crystallization initiation at a similar temperature.

Figure 1.

(a) Thermogravimetric analysis of LM8F precursor; (b) differential thermal analysis and differential scanning calorimetry curves for LM8F precursors

The successful synthesis of LM8F was confirmed through a series of structural and morphological analyses. X-ray diffraction patterns shown in Fig. 2a revealed the characteristic peaks corresponding to the olivine and orthorhombic structure with space group Pnma of LM8F, in alignment with the standard COD#9015172 [38–40]. This phase purity indicates a successful synthesis process, with no detectable secondary phases, suggesting that the HEBM followed by calcination under a nitrogen atmosphere was effective in achieving the desired stoichiometry and crystallinity.

In the subsequent stage of phase determination, the XRD pattern of the calcined LM8F powder was subjected to Rietveld refinement, with the procedural intricacies depicted in Fig. 2b. Utilizing the FullProf Suite for this refinement revealed a remarkable alignment between the experimentally obtained pattern and the theoretical model generated through the Rietveld refinement process. The lattice parameters for the LM8F powder were meticulously refined to a = 1.0419 nm, b = 0.6086 nm, and c = 0.4734 nm, showing a close resemblance to the values documented in the Crystallography Open Database COD#9015172 [38, 39], which are recorded as a = 1.04145 nm, b = 0.60795 nm, and c = 0.47403 nm. The refinement process culminated successfully with a goodness-of-fit value (χ²) of 0.8, and an R-Bragg factor of 9.6, verifying the accuracy of the analysis. Additionally, the average crystallite size of the LM8F powder, determined to be 34.45 nm using the Scherrer equation, is detailed in Table 1. This rigorous analysis highlights not only the precision in aligning the observed data with the established crystalline structure but also confirms the high purity of the synthesized powder.

Fourier transform infrared spectroscopy analysis further corroborated the structural integrity of the synthesized materials. Peaks observed in the FTIR spectra were consistent with the vibrational modes of PO₄^-3 groups, characteristic of the olivine phosphate structure. The absence of bands associated with impurities further confirmed the purity of the synthesized material. Figure 3 showcases the FTIR analysis of the LM8F powder after undergoing calcination. The spectrum identifies specific vibrational modes across two major frequency ranges: from 400 to 560 cm^-1 and 1000 to 1200 cm^-1. The vibrations detected between 459 and 636 cm^-1 are attributed to the bending movements (including both asymmetric and symmetric) of the O–P–O bonds within the phosphate [PO₄]^-3 groups. The presence of peaks within the 979 to 1141 cm^-1 range signals the stretching vibrations of the P–O bonds in the phosphate groups, encompassing both symmetric and asymmetric types. The spectrum also reveals absorption bands spanning from 1438 to 1589 cm^-1, which correspond to the bending vibrations of water molecules [41, 42]. Additionally, fainter peaks observed in the 2341 to 2360 cm^-1 region are associated with the asymmetric stretching of C–O bonds in CO₂ molecules, peak at 2048 cm^-1 related to O₂, and finally peak at 3406.28 cm^-1 related to Asymmetric Stretch OH^-1. These FTIR spectral features for the LM8F powder are meticulously cataloged in Table 2. Discussing these results, the FTIR spectral analysis provides profound insights into the chemical structure of the calcined LM8F powder. The identification of vibrational modes related to O–P–O and P–O bonds within the phosphate groups validates the presence of the phosphate framework in the LM8F structure, which is fundamental for its application as a cathode material in lithium-ion batteries. The detection of water molecule vibrations suggests some level of hydration post-calcination, which could influence the electrochemical properties of the material. The observed CO₂-related peaks may be indicative of atmospheric CO₂ absorption onto the powder’s surface, highlighting the reactive nature of the calcined material. These spectral characteristics not only affirm the successful synthesis of the LM8F powder but also underscore the importance of controlling the calcination environment to minimize unwanted chemical interactions.

Figure 4a displays an FE-SEM (Field Emission Scanning Electron Microscopy) image of the calcined LM8F powder, showcasing agglomerated semi-spherical particles. The image vividly demonstrates that these particles tend to cluster together, forming agglomerates with a semi-sphere shape. Within these agglomerates, several structural imperfections, including voids and dislocations, are clearly visible, suggesting a complex microstructure. Figure 4b then presents the particle size distribution of the calcined LM8F powder through a histogram, indicating that the average particle size is approx. 48 nm. Discussing these observations, the FE-SEM analysis of the LM8F powder highlights the semi-spherical morphology and the tendency of the particles to agglomerate. These features are critical in determining the electrochemical performance of the material as a cathode in lithium-ion batteries. Agglomeration can affect the lithium-ion diffusion pathways and, consequently, the rate capability of the battery. The presence of voids and dislocations within the clusters may also impact the material’s structural integrity and its ability to maintain consistent electrochemical properties under cycling conditions. However, the relatively small average particle size of 48 nm is advantageous, as it can potentially enhance the surface area available for lithium-ion interaction, thereby improving the material’s overall electrochemical performance. This balance between morphology, agglomeration, and particle size is crucial for optimizing the LM8F powder for use in lithium-ion battery cathodes [6, 30, 43].

Figure 2.

(a) XRD pattern of LM8F sample and (b) Rietveld refinement of LM8F sample

Figure. 3.

FTIR spectrum for LM8F

Figure 4.

(a) FeSEM image for LM8F and (b) particle size distribution for LM8F

Electrical measurements were performed on both LM8F and LM8F-10%C samples to assess the impact of carbon coating as shown in Fig. 5a. The results showed a significant enhancement in the electrical conductivity of LM8F-10%C compared to the uncoated LM8F sample. This improvement can be attributed to the carbon coating facilitating electron transport across the material, which is crucial for efficient cathode performance in solid-state lithium batteries.

Temperature-dependent impedance measurements indicated a decrease in resistance with increasing temperature for both samples shown in Fig. 5b. However, the LM8F-10%C composite exhibited a more pronounced decrease, suggesting better thermal stability and ion mobility at elevated temperatures. These findings align with the expectation that carbon incorporation should mitigate the intrinsic electronic conductivity limitations of phosphate-based cathode materials.

Figures 6a and 6b depict the dielectric properties of the solid solution LM8F and LM8F-10%C, respectively. The incorporation of carbon results in a noticeable enhancement of the dielectric constant while concurrently reducing the loss tangent, indicating improved stability with temperature. Specifically, the addition of a carbon coating to LM8F not only elevates the dielectric constant but also diminishes the loss factor. The observed increase in the dielectric constant for LM8F-10%C compared to LM8F can be attributed to the interface polarization at the carbon-material interfaces. Carbon’s conductive nature introduces additional polarizable interfaces within the composite, enhancing the material’s ability to store electrical energy, reflected in a higher dielectric constant. This interface polarization, known as Maxwell–Wagner–Sillars polarization, is more pronounced in composites containing conductive fillers like carbon, leading to the observed enhancement in dielectric properties. Conversely, the reason behind the greater loss tangent in LM8F-10%C compared to pure LM8F involves the complex interplay between conductivity and dielectric behavior. While the carbon coating improves the dielectric constant due to increased polarization, it also introduces free charge carriers that can contribute to dielectric loss, especially at lower frequencies. These free carriers, moving under the influence of the applied electric field, can lead to increased energy dissipation, manifested as a higher loss tangent. The balance between enhanced dielectric properties and controlled dielectric loss is crucial, as it influences the material’s efficacy in applications requiring stable dielectric behavior under varying temperature conditions.

Bode and Nyquist plots further confirmed the superior electrical performance of the carbon-composite material. The Nyquist plots shown in Fig. 7a of LM8F-10%C displayed a smaller semicircle in the high-frequency region, indicating a lower charge-transfer resistance compared to LM8F. This observation is consistent with the enhanced conductivity facilitated by the carbon network within the composite material. Bode plot shown in Fig. 7b for both LM8F and LM8F-10C shows a capacitive behavior for the impedance with lower impedance for carbon coated sample. The phase angle for LM8F appears near 90° while for LM8F-10%C was below 90° which means an increase in conductance due to carbon coating.

The synthesis strategy employed in this study successfully produced LM8F with a pure olivine and orthorhombic structure, as confirmed by XRD and supported by FTIR and FeSEM analyses. The addition of carbon not only improved the electrical conductivity but also enhanced the thermal stability and ion mobility of the material, as evidenced by temperature-dependent impedance measurements.

Figure 5.

(a) Electrical conductivity and (b) electrical impedance and phase angle of LM8F and LM8F-10%C varied with temperature

Figure 6.

(a) Dielectric constant and (b) dielectric lose factor of LM8F and LM8F-10%C varied with temperature

Figure 7.

(a) Nyquist plot and (b) bode plot of LM8F and LM8F-10%C