The Synthesis of LiMn_xFe_1−xPO₄/C Cathode Material through Solvothermal Jointed with Solid-State Reaction

Abstract

LiMn_xFe_1−xPO₄/C material has been synthesized through a facile solid-state reaction under the condition of carbon coating, using solvothermal-prepared LiMnPO₄ and LiFePO₄ as precursors and sucrose as a carbon resource. XRD and element distribution analysis reveal completed solid-state reaction of precursors. LiMn_xFe_1−xPO₄/C composites inherit the morphology of precursors after heat treatment without obvious agglomeration and size increase. LiMn_xFe_1−xPO₄ solid solution forms at low temperature around 350 °C, and Mn²⁺/Fe²⁺ diffuse completely within 1 h at 650 °C. The LiMn_xFe_1−xPO₄/C (x < 0.8) composite exhibits a high-discharge capacity of over 120 mAh·g⁻¹ (500 Wh·kg⁻¹) at low C-rates. This paves a way to synthesize the crystal-optimized LiMn_xFe_1−xPO₄/C materials for high performance Li-ion batteries.

1. Introduction

Thanks to the research of Good enough and co-workers since 1997 [1], olivine LiMPO₄ (M = Fe, Mn, Co, Ni) have been attracting much attention as cathode materials. Among all the LiMPO₄ compounds, LiMn_xFe_1−xPO₄, retaining high energy density of LiMnPO₄ as well as stability of LiFePO₄, is considered as a promising material for its low cost, nontoxicity, and compatibility with commercial electrolytes [2,3]. Previous research suggests that Mn–Fe inter-doping offers LiMn_xFe_1−xPO₄ material better rate capability than LiMnPO₄ and higher energy density than LiFePO₄ [4]. However, the synthesis of a highly uniform LiMn_xFe_1−xPO₄ solid solution is still challenging. Firstly, synthesis by hydrothermal or solvothermal suffers from the segregation of LiMnPO₄ or LiFePO₄. Due to different chemical activities among various cations [5,6,7,8], the co-precipitation of Mn²⁺ and Fe²⁺ by a soft chemistry method needs careful control of pH value, concentration, raw material, and solvent, even though the Mn²⁺–Fe²⁺ proportion in the co-precipitation generally falls in a limited range. Secondly, the synthesis through solid-state reaction suffers from poor batch uniformity, which might be caused by the nonuniform cation diffusion or phase separation during the solid reaction process. Thirdly, it is not easy to realize the morphology control for olivine cathode materials, which is very important for improving its electrochemical performances. In previous reports of the synthesis of a LiMn_xFe_1−xPO₄ solid solution [9,10], it is not widely and systematically investigated how the LiMn_xFe_1−xPO₄ solid solution forms and whether it experiences a mixture of LiMn_xFe_1−xPO₄, LiMnPO₄, LiFePO₄, or a combination of these introduced by partial phase separation of solid solution. As is known to all, LiMnPO₄ phase exhibits poor properties ascribed to instability of Mn³⁺ [11,12] and a large volume misfit (11.6%) between lithiated and delithiated phase [13]. With Fe substitution, LiMn_xFe_1−xPO₄ solid solution has much better rate capability [14] than LiMnPO₄ of comparable morphology, which is attributed to reduced volume misfit between coexisting phases and a higher stability of crystal structure [15,16]. It is of great importance to have a clear sight into the phase separation of LiMn_xFe_1−xPO₄ solid solution since the electrochemical properties strongly depend on composition. Thus, it is urgent to have a clear and comprehensive investigation on the phase reaction mechanism of LiMn_xFe_1−xPO₄ for better application of this type of cathode material.

Considering that LiMn_xFe_1−xPO₄ material also needs carbon coating to improve the conductivity before commercial application, we joined solvothermal with solid-state reaction together and made use of a novel access method to synthesize a morphology-regulated LiMn_xFe_1−xPO₄/C composite with accurate stoichiometric Mn/Fe composition. Firstly, morphology regulated LiMnPO₄ and LiFePO₄ nano-plates were obtained through a solvothermal method. Then, these precursors were carbon-coated with sucrose as a carbon resource. LiMn_xFe_1−xPO₄/C composite was obtained through a facile heat treatment during the process of carbon coating. The solid reaction process and phase composition were studied by TG-DSC, XRD and SEM. In addition, the rate properties of LiMn_xFe_1−xPO₄/C composites with various value of x were compared.

2. Experimental Section

2.1. Synthesis of LiMnPO₄ and LiFePO₄ Nano-Plates

LiMnPO₄ and LiFePO₄ nano-plate precursors with length less than 100 nm were synthesized by a solvothermal method seen in our previous work [17,18]. Portions of 0.016 mol MSO₄ (M = Mn, Fe) and 0.048 mol LiOH·H₂O were respectively dissolved in 20 mL mixture solvents of ethylene glycol and deionized water (volume ratio 4:1) and then mixed with 0.016 mol H₃PO₄ in a particular feeding sequence. Nano-plates were obtained through solvothermal reaction at 180 °C for 12 h. LiFePO₄ plates with lengths of around 500 nm and LiFePO₄ micro-spheres with diameters over 5 μm were synthesized by other reported solvothermal methods [19].

2.2. Synthesis of LiMn_xFe_1−xPO₄/C Composite

The as-prepared LiMnPO₄ and LiFePO₄ nano-plates in various molar ratios were mixed with 15% of sucrose in weight and milled for 15 min. Then, the mixed powder was calcined in nitrogen flow at 650 °C for 5 h to obtain the LiMn_xFe_1−xPO₄/C composite. To investigate the process of solid-state reaction, we calcined the mixture of LiMnPO₄ and LiFePO₄ nano-plates at different temperatures for various heating times.

2.3. Materials Characterization

X-ray powder diffraction patterns of the composites were characterized on a Bruker D8 Advance X-ray diffractometer (Karlsruhe, Germany) in a Bragg-Brentano configuration with Cu K_α1 and Cu K_α2 radiation (λ = 0.15418 nm). The morphology and element distribution of the composites were inspected with a scanning electron microscope (SEM, JSM-5600LV, JEOL, Tokyo, Japan), a transmission electron microscope (TEM, H-800, Hitachi, Tokyo, Japan), a scanning transmission electron microscopy (STEM, H-800, Hitachi, Tokyo, Japan), and energy dispersive X-ray spectroscopy (EDX mapping, H-800, Hitachi, Tokyo, Japan).

Thermogravimetry-differential scanning calorimetric analyses (TG-DSC) were performed using a NETZSCH STA449F3 (Selb, Germany) in the range of 50–800 °C at a heating rate of 10 °C·min⁻¹ under flowing argon atmosphere.

The electrochemical properties were tested using CR2032 coin-type test cells (Shenzhen Kejing, Shenzhen, China) with lithium metal foil as anode. The cathode was prepared from a mixture of 60% LiMn_xFe_1−xPO₄/C, 10% acetylene black, 20% conductive graphite, and 10% PTFE (polytetrafluoroethylene) in weight. The mixture was cut into rounded slices as a test electrode. The polypropylene film (Celgard 2400, Celgard, NC, USA) was used as separator. Ethylene carbonate/dimethyl carbonate/ethyl methyl carbonate (EC:DMC:EMC = 1:1:1 by volume) solution containing 1 mol·L⁻¹ LiPF₆ was used as the electrolyte. Charge-discharge cycles were carried out on a Land CT2001A battery test system (Shanghai Chenhua Instrument Company, Shanghai, China).

3. Results and Discussion

3.1. Structures and Morphologies Characterization

The XRD patterns of the LiMn_xFe_1−xPO₄/C composites with various values of x (x = 0, 0.2, 0.4, 0.6, 0.8, 1) are shown in Figure 1. An obvious trend of peak shift can be observed in Figure 1 with the increase of x from 0 to 1, while the crystalline peaks for LiMn_xFe_1−xPO₄ (0 < x < 1) all fit with those of LiMnPO₄ and LiFePO₄, indicative of pure olivine phase. It is known that a mixture of LiMnPO₄ and LiFePO₄ will show double peak 29.3° and 29.8°, which can be indexed to LiMnPO₄ (PDF 74-0375) and LiFePO₄ (PDF 81-1173), respectively, so the singe peaks for LiMn_xFe_1−xPO₄ (0 < x < 1) illustrate completed solid-reaction between LiMnPO₄ and LiFePO₄ precursors even under the condition of carbon-coating. Moreover, the shift in the XRD patterns is related with the molar ratio of Mn and Fe in the as-prepared LiMn_xFe_1−xPO₄/C composites.

Figure 2 shows the SEM and TEM images of the LiMn_xFe_1−xPO₄/C composite with x = 0, 0.4, 1. It is quite clearly seen that, after heat treatment, the LiMn_0.4Fe_0.6PO₄/C composite still retains similar morphology and size distribution like the precursors. Moreover, the crystal growth orientation of LiMn_0.4Fe_0.6PO₄/C composite is preferable along bc-facet, inheriting the orientation of the precursors. The element distribution mappings of Mn and Fe and the EDX spectrum of LiMn_0.4Fe_0.6PO₄ gained from TEM are demonstrated in Figure 3. It can be seen that the distribution of Mn perfectly matches that of Fe, once again certificating a completed solid-reaction between LiMnPO₄ and LiFePO₄. The carbon layer on the surface of as-prepared LiMn_0.4Fe_0.6PO₄/C composite is also confirmed by TEM images (Figure S1), which is approximately 2 nm thick.

To compare Mn–Fe of LiMn_xFe_yPO₄ prepared from this novel method and the solvothermal process, ICP-OES analysis was performed and the result is shown in Figure 4. The solvothermal curve lies below the theoretical curve, which means the proportions of Fe in the solvothermal products are less than the theoretical proportion, and the utilization efficiency of Mn is higher than that of Fe during the solvothermal reaction. Different reactivity of Mn²⁺ and Fe²⁺ during the solvothermal reaction may be responsible for this observation. Similar phenomenon for LiMn_0.9Fe_0.1PO₄ has also been reported [8]. The difference in Mn–Fe between feeding and product seems difficult to eliminate. However, the LiMnPO₄ and LiFePO₄ calcination curve lies closer to the theoretical curve and fluctuates on both sides. In this sense, solid-state reaction is beneficial to accurately control Mn–Fe in comparison with the solvothermal. This helps adjust Mn–Fe-designed products as well as improve the utilization efficiency of raw materials.

In conventional solid-state reaction, it is generally difficult to tune the morphology of the product. The morphology and size retention observed in our study can be explained by two reasons. First, LiMnPO₄ and LiFePO₄ precursors are plate-like and nano-scaled. Plate-like nanoparticles provide a large contact area and short diffusion path for a solid-state reaction. Second, the carbon from sucrose pyrolysis can prevent further growth of LiMn_xFe_1−xPO₄ particles. Figure 5a shows the XRD patterns of BHT (LiMnPO₄ and LiFePO₄ precursor mixture before heat treatment), HT (LiMn_0.4Fe_0.6PO₄ without carbon-coating), and HTC (LiMn_0.4Fe_0.6PO₄ with carbon-coating). In particular, the lines lying around 29.5° can provide evidence of mixture or solid solution. BHT presents dual peaks at 29.3° and 29.8°, which can be indexed to LiMnPO₄ (PDF 75-0375) and LiFePO₄ (PDF 81-1173), respectively. However, HT and HTC both present a single peak at 29.5°, indicating the solid solution behavior of the as-prepared LiMn_0.4Fe_0.6PO₄. Besides, seen in

Table 1. fwhm of HT and HTC at different peaks.

Peak Position/°	17.1	20.7	22.6	23.9	25.5	29.5	32.1
fwhm of HT/°	0.138	0.139	0.143	0.140	0.149	0.132	0.149
fwhm of HTC/°	0.216	0.182	0.241	0.187	0.209	0.193	0.233

, the fwhm (full width at half maximum, criterion of grain size calculation) increase of the HT sample, compared with the HTC sample, proves the inhibition of particle growth when precursors are heated with sucrose. We can also see obvious particle growth and agglomeration of the HT sample in Figure 5b, proving that sucrose has an inhibiting effect on particle growth.

To exhibit the inheritance of morphology, precursor plates with different morphologies are mixed and heat-treated to obtain the LiMn_0.4Fe_0.6PO₄/C composite. As seen in Figure 6, (a) LiMnPO₄ nano-plates with lengths of less than 100 nm were respectively calcined with (b) LiFePO₄ plates with lengths of around 500 nm or (c) a LiFePO₄ micro-sphere with a diameter of over 5 μm. In Figure 6d, we can see particles with lengths less than 100 nm as well as around 400–500 nm. Meanwhile, in Figure 6e,f, cracked micro-spheres attached with nanoparticles can be observed. The cracks probably result from the fracture of the micro-sphere when calcined at high temperature. LiMn_0.4Fe_0.6PO₄/C samples with hybrid morphology were obtained, simultaneously inheriting the morphologies of both LiMnPO₄ and LiFePO₄ precursors. The XRD patterns presented in Figure 7 indicate a completed solid-state reaction between LiMnPO₄ and LiFePO₄ precursors despite their quite different morphologies.

The XRD patterns of LiMn_0.4Fe_0.6PO₄ samples synthesized without sucrose at various calcination temperature and time are shown in Figure 8. It can be seen from Figure 8a that, when calcined at 250 °C for 5 h, no visible solid-state reaction occurs between LiMnPO₄ and LiFePO₄ for the obvious dual peaks in the XRD pattern. However, the peaks undergo an evolution from dual to unimodal when the calcination temperature rises up to 350 °C. This observation proves that the formation of solid solution can proceed at no more than 350 °C. With calcination temperature sequentially rising up to 450 °C, dual peaks disappear and a solid-state reaction occurs. In addition, the fwhm of XRD patterns decreases as calcination temperature heightens from 450 to 750 °C, indicating that particles grow at high temperatures when calcined without sucrose, consistent with the result shown in Figure 5b. When calcined at 650 °C for a different time, as seen in Figure 8b, a solid-state reaction proceeds fast and occurs within 1 h. Considering the sufficiency of carbon coating, a calcination strategy of relatively higher temperature and longer time (650 °C and 5 h) is necessary.

TG-DSC analyses were performed to determine the process of the solid-state reaction. Conventional solid-state synthesis of LiMn_0.4Fe_0.6PO₄ using LiH₂PO₄, MnCO₃, and FeC₂O₄·2H₂O was performed and is shown in Figure 9a. The weight loss started at 160 °C and finished at 450 °C. Three endothermic peaks at 180 °C, 224 °C, and 410 °C were assigned to the evaporation of dehydrated water from FeC₂O₄·2H₂O, the thermal decomposition of MnCO₃, and the thermal decomposition of FeC₂O₄. Additionally, there are two small exothermic peaks at 575 °C and 688 °C without weight loss in the TG profile, which might be attributed to the crystal transform and the lattice heat of LiMn_0.4Fe_0.6PO₄. However, during heat treatment of LiMnPO₄ and LiFePO₄ nano-plate mixture, seen in Figure 9b, no peaks are observed in TG-DSC curves above 200 °C. This indicates that Mn²⁺ and Fe²⁺ diffusion between LiMnPO₄ and LiFePO₄ phases are dominant during heat treatment since there is no concentration difference of Li⁺ and PO₄³⁻ between the two phases. Mn²⁺ and Fe²⁺ diffusion can easily proceed through particle boundaries at low temperatures without any exothermic or endothermic processes.

According to the research of Jongsoon Kim [20], the thermal stability of fully delithiated Mn_xFe_1−xPO₄ and partially delithiated Li_1−yMn_xFe_1−xPO₄ (0 < x < 1, y ≈ 0.6) is relatively poor. Mn_xFe_1−xPO₄ and Li_1−yMn_xFe_1−xPO₄ would decompose into other phases such as (Mn_xFe_1−x)₃(PO₄)₂, (Mn_xFe_1−x)₂P₂O₇, and LiMn_xFe_1−xPO₄ at different temperatures according to different x-values. The thermal stability of delithiated LiMn_xFe_1−xPO₄ is influenced sensitively by the Fe–Mn content in the structure. Nevertheless, with our analysis of the process of the solid-state reaction of theLiMn_xFe_1−xPO₄ synthesis, we conclude that fully lithiated LiMn_xFe_1−xPO₄ is thermodynamically stable for all x-values ranging from 0 to 1 and temperatures ranging from 450 to 800 °C, and the reaction energy barrier to form a LiMn_xFe_1−xPO₄ solid solution from LiMnPO₄ and LiFePO₄ precursors is quite low. The LiMn_xFe_1−xPO₄ phase separation of LiMn_xFe_1−xPO₄ can be ignored during preparation. Even if LiMnPO₄ and/or LiFePO₄ phases do exist during synthesis, they can react with each other and reunite into a LiMn_xFe_1−xPO₄ solid solution during carbon-coating treatment.

3.2. Electrochemical Performancesof LiMn_xFe_1−xPO₄/CMaterials

The cycling property of LiMPO₄ (M = Mn, Fe) is recognized as excellent [21,22,23], which can also be confirmed from Figure S2. For LiMn_0.4Fe_0.6PO₄/C composite material prepared by LiMnPO₄ nano-plates and LiFePO₄ nano-plates, after 50 cycles at 0.1 C (1 C = 170 mA·g⁻¹), the capacity retention is higher than 98%, exhibiting excellent cycling stability. The half-cells are charged at 0.1 C and discharged at various C-rates to help sufficient delithiation and to remove the side effect of discharging. As shown in Figure 10a, discharge capacity at 5 C is 42.8 mAh·g⁻¹, 72.1 mAh·g⁻¹, 106.3 mAh·g⁻¹, 118.9 mAh·g⁻¹, 114.9 mAh·g⁻¹, and 144.9 mAh·g⁻¹, corresponding to x = 0, 0.2, 0.4, 0.6, 0.8, and 1, respectively. LiMn_0.4Fe_0.6PO₄/C shows an outstanding high rate property of 78 mAh·g⁻¹ at 10 C, reaching up to 50% retention of that at 0.1 C, comparable to other previously reported research [14,24,25]. Figure 10b shows the variation of energy density performing at different discharge rates. At a low discharging rate (0.1 to 0.2 C), LiMn_0.2Fe_0.8PO₄/C shows outstanding high energy density nearly 600 Wh·kg⁻¹, reaching very close to theoretic value of 612 Wh·kg⁻¹. With discharging rate getting higher (0.5 to 2 C), energy density of LiMn_xFe_1−xPO₄/C remains nearly unchanged when x lands in the range of 0 to 0.8, promoting LiMn_xFe_1−xPO₄/C to a role of tolerant material for stable energy storage at low current density. The energy density of LiMn_0.4Fe_0.6PO₄/C at 5 C and 10 C is 393.2 Wh·kg⁻¹ and 235.6 Wh·kg⁻¹, which makes LiMn_0.4Fe_0.6PO₄/C a promising material for high-energy applications.

4. Conclusions

LiMn_xFe_1−xPO₄/C nano-plates with regulated morphology and accurate stoichiometry are synthesized through a novel solid-state reaction of solvothermal-prepared LiMnPO₄ and LiFePO4 nano-plates under the condition of carbon coating with sucrose. For the benefit of carbonated sucrose, a LiMn_xFe_1−xPO₄/C composite inherits the morphology, crystalline structure, and particle size of LiMnPO₄ and LiFePO₄ precursors. Ion diffusion of Mn²⁺ and Fe²⁺ can proceed easily through LiMnPO₄ and LiFePO₄ phases at only around 350 °C to form a thermodynamic stable LiMn_xFe_1−xPO₄ phase. With the optimization of x in LiMn_xFe_1−xPO₄/C, the LiMn_0.4Fe_0.6PO₄/C composite shows excellent high rate discharge capacity of 118.9 mAh·g⁻¹ at 5 C and 78 mAh·g⁻¹ at 10 C, equivalent to 393.2 Wh·kg⁻¹ and 235.6 Wh·kg⁻¹ in terms of energy density. This paves a novel and facile way to synthesize LiMn_xFe_1−xPO₄ material with low cost, high energy density, and stability for lithium ion batteries.