Study and Property Characterization of LiMn₂O₄ Synthesized from Octahedral Mn₃O₄

Abstract

The structure of Mn₃O₄ with an octahedron structure was similar to that of LiMn₂O₄, and the lithium manganate prepared with it had good electrochemical performance. During the preparation of octahedron Mn₃O₄, the effects of the pH regulator, temperature, and reaction pH on its morphology, specific surface area, and other properties were studied in this paper. LiMn₂O₄ was prepared from Octahedron Mn₃O₄ obtained by using better technology. The effects of calcination time and temperature on the physicochemical and electrochemical properties of LiMn₂O₄ were studied. The research results indicated that the optimal synthesis conditions for Mn₃O₄ were as follows: ammonia water was used as a pH regulator and complexing agent, reaction pH was 8, reaction temperature was 80 °C, reaction time was 12 h, and oxygen flow rate was 3 L∙min⁻¹. The LiMn₂O₄ synthesized had a good octahedron morphology when the calcination temperature was 800 °C and the calcination time was 10 h. The first discharge-specific capacity was 121.9 mAh∙g⁻¹ at a current density of 0.2 C, the discharge-specific capacity was 114.1 mAh∙g⁻¹ after 100 cycles, and the capacity retention rate was 93.6%. Therefore, the lithium manganate prepared by using octahedron manganous oxide had good electrochemical reversibility and a good application prospect.

1. Introduction

Battery-grade Mn₃O₄ could be used to prepare cathode materials for lithium-ion batteries and sodium-ion batteries. As one of the main materials for lithium-ion positive electrodes, LiMn₂O₄ has been mainly prepared through the high-temperature calcination of electrolytic MnO₂ in industry [1,2,3,4]. More and more studies have shown that LiMn₂O₄ prepared using Mn₃O₄ has better electrochemical performance compared to electrolytic MnO₂ [5]. Due to the high electrolysis cost, uneven particle size, and poor dispersion of MnO₂, the LiMn₂O₄ crystal size is uneven, resulting in low specific capacity and poor cycling performance. The LiMn₂O₄ crystal synthesized using Mn₃O₄ has a small degree of distortion, uniform morphology, and uniform size. A good morphology is conducive to lithium-ion unembedding, reducing the crystal structure changes caused by unembedding and improving the cycling performance of LiMn₂O₄. Therefore, Mn₃O₄ has the potential to replace electrolytic MnO₂ [6,7,8].

As the most basic cathode material, LiMn₂O₄ has the advantages of green environmental protection, low cost, and good safety performance [9,10,11,12]. However, due to the disproportionation and Jahn-Teller effect [13], Mn dissolves, which reduces the cycle performance of LiMn₂O₄ and limits its large-scale production and application [14,15,16]. By controlling the reaction conditions, the morphology can be optimized to achieve the purpose of improving the cycle performance of the cathode material [17]. Cao et al. [18] used MnO₂ and LiOH to obtain LiMn₂O₄ nanowires under high-temperature calcination in the air environment. At a 0.1 C current, the first discharged specific capacity was 132 mAh·g⁻¹, but the cycle capacity decayed rapidly. Niraj et al. [19] prepared one-dimensional LiMn₂O₄ with a diameter of 10–50 nm using ultrafine α-MnO₂ nanorods. At a current of 0.05 C, the initial discharge capacity reached 130 mAh·g⁻¹, and the retention rate was 98% after 100 cycles. The corresponding size and morphology were synthesized to alleviate the capacity decay of LiMn₂O₄ by controlling the reaction conditions [20]. Octahedral Mn₃O₄ and LiMn₂O₄ had the same spinel structure, which could reduce crystal structure changes during high-temperature roasting, facilitate lithium ion disembedding, reduce the effect of lithium-ion disembedding on crystal structure, and improve the high-temperature performance, cycling performance, and gram specific capacity of LiMn₂O₄ [21,22].

Manganese trioxide (Mn₃O₄) is a natural black manganese ore with a theoretical manganese content of 72.03%, mainly consisting of reddish-brown and brownish-yellow. Mn₃O₄ is a type of spinel-structured Mn²⁺ [Mn³⁺]O₄, where Mn²⁺ and Mn³⁺ are distributed at different lattice positions. Therefore, Mn₃O₄ could be expressed in the form of MnO∙Mn₂O₃ [23]. As an important industrial raw material, Mn₃O₄ has extremely wide applications; it is widely used in industries such as soft magnetic materials, supercapacitors, new energy materials, and pigments [24,25]. Mn₃O₄ has spherical, octahedral, nanorod and, hollow spherical shapes. Mn₃O₄ has a wide variety of morphologies, different sizes, and uneven dispersion, which have a significant impact on the electrochemical and cycling performance of the synthesized positive electrode material [26,27,28]. At present, according to the classification of raw materials, the preparation methods of Mn₃O₄ include the metal manganese method, manganese salt method, high valent manganese reduction method, etc. [29,30]. Wang et al. [31] synthesized Manganese(II) carbonate by using the precipitation method and prepared linear single crystal Mn₃O₄ with a particle size of 40–80 nm through high-temperature calcination. Although MnCO₃ calcination could produce various morphologies of Mn₃O₄, its products would contain impurities such as Mn₂O₃. A high calcination temperature means high cost and high energy consumption. Wang et al. [32] prepared octahedron hollow Mn₃O₄ using KMnO₄ and formamide as raw materials, the reaction was maintained for 12 h at 140 °C in a high-pressure reactor. Cao et al. [33] mixed KMnO₄ and sodium citrate 1:1 and reacted the mixture for 10 h at 180 °C in a high-pressure reactor to produce high-purity octahedron Mn₃O₄. Although this method could obtain various morphologies of Mn₃O₄, it had a long reaction time, low production efficiency, high equipment requirements, and certain limitations in mass production. Ramulu et al. [34] used cellulose from waste tissue as a scaffold to deposit manganese precursors and then calcined them at high temperatures to prepare high-pore hollow microtubules of Mn₃O₄ using morphological methods. Mansournia et al. [35] directly synthesized Mn₃O₄ powder with excellent catalytic performance at room temperature by using manganese(II) chloride and ammonia as raw materials. Researchers also studied some new methods, but these methods had many shortcomings, such as severe agglomeration, low crystallinity, and complex operating procedures [36,37,38,39,40,41]. In this paper, battery-grade Mn₃O₄ was prepared through liquid phase oxidation of manganese sulfate solution. The Mn₃O₄ prepared by using this method had the advantages of low cost, abundant raw materials, easy control of grain morphology, and uniform particle size, which were beneficial for improving the electrochemical performance of LiMn₂O₄.

2. Experiment

2.1. Raw Materials and Instruments

The main reagents used in this experiment included NH₃·H₂O, NaOH, and C₂H₅OH, which were analytically pure and purchased from China, China National Pharmaceutical Group Chemical Reagent Co., Ltd. LiOH·H₂O, C, Li, C₅H₉NO, (CH₂CF₂)n, LiPF₆/(EC + EMC), and diaphragms were purchased from China, Guangdong Zhuguang New Energy Technology Co., Ltd. MnSO₄ was produced in the laboratory from China manganese rhodochrosite after purification by acid leaching with sulfuric acid and meets the standard of battery-grade MnSO₄.

The main instruments used in this experiment were as follows. The constant-temperature blast drying oven (DHG-101-4B) was produced by China Shanghai Langgan Experimental Equipment Co., Ltd. The digital display constant temperature water bath pot (HH-2J) was produced by China Changzhou Langyue Instrument Manufacturing Co., Ltd. The muffle furnace (SX4-10) was produced by China Tianjin Taist Instrument Co., Ltd. The electric mixer (LC-OES-120SH) was produced by China Shanghai Lichen Instrument Technology Co., Ltd. The oxygen concentrator (CR-P5W) was produced by China Keer Medical Technology Co., Ltd. The glovebox (S175) was produced by China Chengdu Delis Industrial Co., Ltd. The battery testing system (CT-4008T-5V) was produced by China Shenzhen Xinweier Electronics Co., Ltd. The button-type battery sealing machine (MSK-110) and manual slicer (MSK-T10) were all produced by China Shenzhen Kejing Zhida Technology Co., Ltd. The X-ray powder diffractometer (D8 ADVANCE) was produced by Brooke Company in Germany; The field emission scanning electron microscope (SU8020) was produced by Hitachi, Japan. The X-ray electron spectrometer (Thermo escalab 250Xi) was produced by Thermo Fisher & Company in the United States Massachusetts City. The laser particle size analyzer (Mastersizer 2000) was produced by Marvin Instruments Limited in the UK Marvin City.

2.2. Experimental Process

The experiment mainly included the following two steps:

The experimental process for preparing battery-grade Mn₃O₄ was as follows. The 1LMnSO₄ solution was loaded into a beaker and placed in a constant-temperature water bath. When the solution was heated to a certain temperature, a pH regulator was added until the preset reaction pH was reached, and an oxidant was continuously introduced. The pH regulators were slowly added to maintain a stable solution pH value during the reaction process. After a certain period of reaction, filtration and washing were carried out. The filter residue was repeatedly washed with 60 °C distilled water until no white precipitate was generated when BaCl₂ was added to the washing solution. The filter cake was dried at 105 °C in a constant temperature blast drying box for 12 h, removed, and ground for later use.

The preparation and electrochemical detection of lithium manganese oxide cathode material were as follows: Mn₃O₄ and LiOH with a certain lithium manganese molar ratio were weighed and ground in an agate mortar for 30 min before being loaded into a crucible. It was roasted in a muffle furnace in an air atmosphere according to the preset heating rate, roasting temperature, and insulation time. After the reaction ended, it was cooled in the furnace to obtain lithium manganese oxide powder. Finally, the physicochemical and electrochemical properties of lithium manganese oxide were tested.

2.3. Performance Characterization

(1)

X-ray diffraction analysis (XRD)

In this experiment, the XRD diffractometer (D8ADVANCE of Brooke Company) was used to detect and analyze the phase of the product. The radiation source was a Cu target, the scanning rate was 6°∙min⁻¹, and the scanning range of 2θ was 10–90°. The diffraction peak of the sample was decided by comparing it with the PDF card of the known substance.

(2)

Scanning electron microscope (SEM)

The scanning electron microscope (SU8020 of Hitachi, Japan) was used to characterize the surface morphology of the samples at 15 kV with different magnifications, and SEM pictures were obtained.

(3)

X-ray photoelectron spectroscopy (XPS) analysis

The Thermo escalab (250Xi electron spectrometer of Massachusetts United States City) was used to analyze the chemical valence state of Mn₃O₄.

(4)

Particle size analysis

The Mastersizer 2000 laser particle size analyzer produced by Marvin Company in the UK was used to perform particle size detection and analysis on materials dispersed in the aqueous phase.

(5)

Electrochemical performance testing

Preparation of electrode slice. Firstly, polyvinylidene fluoride was dissolved in N-Methyl-2-pyrrolidone to prepare a binder with a concentration of 0.03 g·mL⁻¹. After grinding the positive electrode material, acetylene black, and adhesive evenly in a mass ratio of 8:1:1, they were then coated on carbon-coated aluminum foil. Secondly, they were dried in a constant-temperature blast drying oven for 30 min and then dried in a 110 °C constant-temperature vacuum drying oven for 12 h. Next, the aluminum foil was taken out and made into the positive electrode plate using a manual slicing machine with a diameter of 12 mm punch. Finally, they were weighed and put into the glove box to assemble the battery for testing.

Button battery assembly. The half-cell was assembled in the order of negative case–lithium sheet–electrolyte–diaphragm–electrolyte–positive sheet–gasket–spring sheet–positive case in the glove box filled with argon (water and oxygen contents were less than 0.01 ppm). They were hydraulically sealed using a button-type battery sealing machine and were taken out and left to stand for 12 h before undergoing electrochemical performance testing.

Constant current charging and discharging test. The battery testing system (CT-4008T-5V) manufactured by China Shenzhen Newell Electronics Co., Ltd. was used to conduct constant current charging and discharging tests and rate performance tests on LiMn₂O₄ positive electrode material. The current was calculated based on 148 mAh·g⁻¹ of the LiMn₂O₄ standard specific capacity. The voltage range of the constant current charge–discharge test was 3–4.3 V, and the rate range of the rate performance test was 0.2 C, 0.5 C, 1 C, 2 C, 5 C, and 10 C.

3. Results and Discussion

3.1. Thermodynamic Analysis of Mn₃O₄ Preparation from MnSO₄

Based on relevant thermodynamic data, the E-pH diagram of the Mn-NH₃-SO₄²⁻-H₂O system was drawn, as shown in Figure 1.

From Figure 1, it can be seen that when ammonia was added to a complex with Mn²⁺, it could be directly oxidized in the pH range of 6.7–10.5. Because Mn²⁺ could form stable complex regions with different amounts of ammonia, the pH range of Mn²⁺ oxidation became wider. This was more conducive to controlling the reaction. In the Mn-NH₃-SO₄²⁻-H₂O system at 25 °C, there existed a stable thermodynamic region for Mn₃O₄. Mn²⁺ could be oxidized to Mn₃O₄ in three ways.

(1)

Direct oxidation. When the pH was between 6.76 and 8.24, the voltage (E) was between 0.254 and 0.433, Mn²⁺ could be directly oxidized to Mn₃O₄, and the reaction formula was as follows.

$${6\mathrm{Mn}}^{2+}+{6\mathrm{H}}_2{\mathrm{O}+\mathrm{O}}_2\to 2{\mathrm{Mn}}_3{\mathrm{O}}_4+{12\mathrm{H}}^{+}$$

(1)

(2)

Complex oxidation. When pH was between 8.24 and 10.57, the coordination number of the ammonia manganese complex was different, but the principle of their oxidation to Mn₃O₄ was the same. The reaction formula could be expressed as follows.

$$6[\mathrm{Mn}{\left({\mathrm{NH}}_3\right)}_x{]}^{2+}+{\mathrm{O}}_2+{6\mathrm{H}}_2{\mathrm{O}=2\mathrm{Mn}}_3{\mathrm{O}}_4+x{\mathrm{NH}}_3+12{\mathrm{H}}^{+}$$

(2)

The oxidation potential range was as follows when x = 1, −0.045 < E < 0.343. The oxidation potential range was as follows when x = 2, 0.122 < E < 0.343. The oxidation potential range was as follows when x = 3, −0.086 < E < 0.343. It could be directly oxidized to Mn₃O₄ within the corresponding oxidation potential.

(3)

Precipitation oxidation. When pH was between 10.57 and 14, Mn²⁺ could form Mn(OH)₂ precipitate, the oxidation potential range was between −0.327 and 0.203, and Mn(OH)₂ could be oxidized to Mn₃O_4. The reaction formula was as follows.

$${\mathrm{Mn}}^{2+}{+2\mathrm{OH}}^{-}\to \mathrm{Mn}(\mathrm{OH}{)}_2\downarrow$$

(3)

$${6\mathrm{Mn}\left(\mathrm{OH}\right)}_2{+\mathrm{O}}_2\to {2\mathrm{Mn}}_3{\mathrm{O}}_4{+6\mathrm{H}}_2\mathrm{O}$$

(4)

Among the three oxidation methods, the direct oxidation method had a narrow reaction pH range. After the addition of ammonia and Mn²⁺ complexation, the reaction pH range of the complexation oxidation method became wide, and the oxidation rate of Mn²⁺ slowed down, which was conducted to the nucleation and growth of Mn₃O₄ grains. The precipitation oxidation method needed to filter, wash, and adjust the Mn(OH)₂ slurry, and the reaction process needed to isolate the air to avoid the Mn(OH)₂ on the surface of the solution being oxidized to Mn₃O₄ first.

From Figure 1, it can be seen that the byproducts such as Mn₂O₃ and MnO₂ could also be generated under the corresponding redox potential in the reaction system. Therefore, when using ammonia as a pH regulator and complexing agent to control the oxidation rate of Mn²⁺ in this experiment, to prepare high-purity and high-performance battery-grade Mn₃O₄, the reaction conditions should be strictly controlled to suppress the generation of side reactants such as MnO₂, Mn₂O_3, and Mn₂(OH)₂SO₄.

3.2. Effect of pH Regulator on Mn₃O₄

When the content of Mn²⁺ was 80 g∙L⁻¹, the reaction pH was 8, the reaction temperature was 70 °C, the reaction time was 12 h, the oxygen flow rate was 2 L∙min⁻¹, and the effects of pH regulators such as NaOH (120 g∙L⁻¹) and ammonia (12%) on the physicochemical properties of Mn₃O₄ were explored.

The microstructure of Mn₃O₄ prepared with NaOH and ammonia as pH regulators is shown in Figure 2. As shown in Figure 2, pH regulators had a significant impact on the morphology of Mn₃O₄. The Mn₃O₄ grains prepared by NaOH were mostly sheet-like, with large voids, no fixed morphology, and poor crystallinity. The Mn₃O₄ grains prepared by using ammonia were complete, most of which were octahedron-shaped, with uniform size and small voids. When ammonia was as a pH regulator, on the one hand, the complex was formed between NH₃ and Mn²⁺; they could slow down the oxidation rate of Mn²⁺ and control the nucleation rate of Mn₃O₄. On the other hand, ammonia manganese complexes existed in the form of ion clusters and had a certain spinel structure, resulting in a regular and uniform grain shape and size of Mn₃O₄.

The manganese content, D50, and surface areas of manganese tetroxide prepared with different pH regulators are shown in

Table 1. Physicochemical parameters of Mn₃O₄ prepared with different pH regulators.

pH Regulator	Mn Content (%)	D50 (μm)	Specific Surface Area (m2∙g−1)
NaOH	71.08	3.68	22.18
NH3∙H2O	71.32	4.67	3.49

.

According to Table 1, the manganese content in Mn₃O₄ prepared with NaOH and ammonia water was 71.08% and 71.32%, respectively. When NaOH served as a pH regulator, Mn²⁺ was converted into Mn(OH)₂ and then oxidized to Mn₃O₄. The generation and oxidation of Mn(OH)₂ occurred simultaneously, resulting in some Mn(OH)₂ being tightly encapsulated by the Mn₃O₄ product layer. The diffusion of oxygen to the surface of unreacted Mn(OH)₂ became a limiting link, resulting in a lower Mn content in the product. When ammonia was used as a pH regulator, the ammonia manganese complex was a soluble complex, which was in full contact with oxygen, so the Mn content in the product was higher. From Table 1, it could be seen that the D₅₀ particle sizes of Mn₃O₄ prepared with NaOH and ammonia water were similar, and they were 3.68 μm and 4.67 μm, respectively. However, there was a significant difference in specific surface area; the specific surface areas were 22.18 m²∙g⁻¹ and 3.49 m²∙g⁻¹, respectively. Using ammonia as a pH regulator to form a complex with Mn²⁺ not only slowed down the nucleation rate of Mn₃O₄ but also helped to oxidize Mn²⁺ into a single, regular morphology. The sheet-like Mn₃O₄ synthesized by using NaOH was stacked and disordered, resulting in a larger specific surface area. A smaller specific surface area was beneficial for improving the activity of the positive electrode material and improving the cycling and rate performance of the battery. Therefore, it was better to choose ammonia water as a pH regulator to prepare Mn₃O₄.

3.3. Effect of Reaction pH on Mn₃O₄

When the content of Mn²⁺ was 80 g∙L⁻¹, the ammonia concentration was 8%, the reaction temperature was 70 °C, the reaction time was 12 h, and the oxygen flow rate was 3 L∙min⁻¹, The effects of reaction pH values of 7, 7.5, 8, 8.5, and 9 on the physicochemical properties of Mn₃O₄ were investigated.

The microstructure of Mn₃O₄ prepared under different reaction pH conditions is shown in Figure 3. It could be seen from Figure 3a–e that the microscopic morphology of Mn₃O₄ was still composed of octahedron particles under different reaction pH, but their particle size and uniformity changed. The aggregation phenomenon of Mn₃O₄ microparticles was severed and they had a few voids and small particles, all around 100 nm when the reaction pH was 7. The particles became larger, the dispersity increased, the octahedron shape was completed, and the average particle size was about 250 nm when the reaction pH was 7.5. The octahedron particles were the largest, the average particle size was about 280 nm, the particle size was uniform, and the boundary between particles was cleared when the reaction pH was 8. When the pH was 8.5 and 9, the agglomeration phenomenon increased, particle boundaries became blurred, voids and particles became smaller, the particle sizes were around 200 nm, and many small particles appeared. When the reaction pH was low, the oxygen oxidation potential was high, and the oxidation difficulty of the ammonia manganese complex increased, slowing down the oxidation rate of the ammonia manganese complex. However, it reduced the precipitation of components for crystal nucleus growth, leading to hindrance of crystal nucleus growth. Therefore, severe aggregation and narrow gaps occurred. When the reaction pH was low, the crystal particles were small. When the reaction pH was high, the formation rate of the ammonia manganese complex was fast, the oxidation potential was low, and the ammonia manganese complex was easy to oxidize, which increased the supersaturation rate of Mn₃O₄ solute and was conducive to the formation of the crystal core. The nucleation rate of Mn₃O₄ was much higher than the growth rate of crystal nuclei, the crystals were dispersed, and a large number of crystal nuclei were formed, making it difficult to grow. Small particles adhered to the surface of large particles. Finally, the nucleation rate and growth rate showed mutual fluctuation.

The analysis results of Mn content, D50 particle size, and specific surface area of Mn₃O₄ prepared with different reaction pH are shown in Figure 4. The Mn content decreased with the increase in reaction pH. When the reaction pH rose, it was necessary to increase the acceleration of ammonia drops to maintain the stability of the reaction pH, and the concentration of ammonium and Hydroxide ions in the solution increased accordingly. On the one hand, some impurity ions generated insoluble hydroxides and entered the impurity phase. On the other hand, it promoted the formation of Mg (NH4)₂(SO4)₂ and Mn₂(OH)₂SO₄ through side reactions. The byproducts were tightly wrapped and entered the precipitation, leading to an increase in sulfur content and a decrease in Mn content.

From Figure 4b, the D50 particle size of Mn₃O₄ gradually decreased, and the specific surface area gradually increased with the increase in reaction pH. When the reaction pH was low, the supersaturation and nucleation rate were also low, the agglomeration sites were few, and the grains attracted and agglomerated with each other to form large particles. When the reaction pH was too high, the solute supersaturation increased, the grains were more dispersed, the particle size was finer, a large number of Mn₃O₄ crystal nuclei were formed, and the agglomeration points increased, resulting in a large number of agglomerated particles and small particle size, due to which the particle size of Mn₃O₄ decreased slightly and the specific surface area increased. Therefore, the nucleation rate of Mn₃O₄ could be controlled, the number of crystal nuclei and aggregation points could be reduced, and the agglomeration of grains into large particles could be promoted by controlling the reaction pH. A comprehensive evaluation of each performance indicated that a pH of 8 was more appropriate.

3.4. Effect of Reaction Temperature on Mn₃O₄

When Mn²⁺ was 80 g∙L⁻¹, the ammonia concentration was 8%, the reaction pH was 8, the reaction time was 12 h, and the oxygen flow rate was 3 L∙min⁻¹. The effects of reaction temperatures of 25 °C, 50 °C, 60 °C, 70 °C, and 80 °C on the physicochemical properties of Mn₃O₄ were investigated.

The microstructure of Mn₃O₄ prepared at different temperatures is shown in Figure 5. The microstructure of Mn₃O₄ underwent significant changes with different reaction temperatures. When the reacting temperature was at a room temperature of 25 °C, the majority of Mn₃O₄ grains were spherical and rod-shaped small particles, at only a few nanometers in size, the agglomeration was serious, and there were large voids within the structure. When the temperature rose to 50 °C and 60 °C, the crystal nucleus grew fast and the grain size became larger. Their particle sizes were about 160 nm and 200 nm, respectively, but the grain size was uneven, a large number of particles were agglomerated into small particles, and there were large gaps between particles. When the temperature rose to 70 °C and 80 °C, the microstructure was all octahedral grains, the grain size was about 280 nm and 300 nm, the grain surface was smooth, they agglomerated into large particles, and the gap between particles was small. When the temperature was low, the nucleation rate was small, the crystal nucleus grew slowly, it was difficult to grow, and a large number of small grains agglomerated. With the increase in temperature, the oxidation potential of oxygen decreased, and the ammonia manganese complex was more easily oxidized. The growth rate of the crystal nucleus was accelerated, the grain shape was more regular, the size was uniform, the thermal movement of particles was intensified, and the particles could be agglomerated into compact large particles.

Some physicochemical properties of Mn₃O₄ prepared at different reaction temperatures are shown in Figure 6. As shown in Figure 6a, the Mn content increased with increasing temperature. On the one hand, as the temperature increased, the Gibbs free energy of the system reaction decreased, promoting the conversion of Mn²⁺ to ammonia manganese complex, increasing the oxidation rate of the complex, and reducing the generation of byproduct basic MnSO₄. On the other hand, as the temperature increased, the oxygen oxidation potential decreased, which could oxidize some basic MnSO₄ and weaken the peroxidation of oxygen, reducing the generation of high-valent manganese oxides. Therefore, the Mn content gradually increased with the increase in temperature.

From Figure 6b, as the reaction temperature increased, the D₅₀ particle size gradually increased and the specific surface area gradually decreased. When the temperature was low, the crystal growth rate was slowed and small grains were formed, so the particle size of the agglomerated particles was smaller. The supersaturation and nucleation rate increased with increasing temperature, which promoted the growth of grains. At the same time, with the increase in temperature, the collision probability of grains increased due to the intensification of thermal motion, and the grains were agglomerated to form large particles. As a result, the particle size increased and the specific surface area decreased.

3.5. Characterization Analysis of Mn₃O₄ Product

According to the requirements of particle size and specific surface area of Mn₃O₄ for lithium batteries, Mn₃O₄ with regular morphology, large particle size, and small specific surface area was selected for characterization and analysis. Based on the above research results, the preparation of Mn₃O₄ under the following conditions was suitable. The Mn²⁺ content was 80 g∙L⁻¹, ammonia water was the regulator, the reaction pH was 8, the reaction temperature was 80 °C, the reaction time was 12 h, and the oxygen flow rate was 3 L∙min⁻¹.

3.5.1. Phase Analysis

X-ray diffraction analysis was performed on the product Mn₃O₄, and its spectrum is shown in Figure 7.

It can be seen from Figure 7 that the product Mn₃O₄ was a solid powder, the baseline of the diffraction peak was flat, the peak pattern was narrow, and the peak intensity was high, so the crystallinity of the product was good. The cell size of Mn₃O₄ was a = b = 0.576 nm, c = 0.945 nm, and the crystal volume was 0.314 nm³. The grain size was around 290 nm according to the Scherrer formula [42], which is consistent with the result of the previous electron microscopy scans.

3.5.2. Chemical Element Analysis

The chemical elemental analysis of Mn₃O₄ was performed, the Mn content was detected by using the chemical titration method, and the remaining impurity elements were detected according to the method of the Mn₃O₄ standard for lithium batteries (YB T4736-2019). The results are shown in

Table 2. Results of chemical element analysis.

Element	Content (%)	Element	Element (%)
Mn	71.45	Cu	0.0001
K	0.0012	Zn	0.016
Na	0.0011	Si	0.019
Ca	0.015	Fe	0.0031
Mg	0.013	S	0.031
Co	0.0061	Cl	0.027
Ni	0.0059	H2O	0.3

.

From Table 2, the Mn content and impurity element content could meet the product standard of Mn₃O₄ for lithium batteries.

3.5.3. Valence Analysis

The average valence of Mn₃O₄ was +2.7, and there were two cases. When it existed in the form of MnO∙Mn₂O₃, it exhibited +2 and +3 valence states. When it existed in the form of 2MnO·MnO₂, it exhibited +2 and +4 valence states. XPS detection was performed to analyze the valence states, and the results are as shown in Figure 8.

The characteristic peaks of Mn 2p and Mn 3s appeared in the full spectrum of the XPS map. The characteristic peaks of Mn 2p and Mn 3s were fitted. The fitting curves of Mn 2p showed the peak of Mn²⁺ 2p^1/2, Mn²⁺ 2p^3/2, Mn³⁺ 2p^1/2, and Mn³⁺ 2p^3/2. The fitting curves of Mn 3s showed the peak of Mn²⁺ 3s and Mn³⁺ 3s. ∆E = 5.9 eV for Mn²⁺ 3s and ∆E = 5.6 eV for Mn³⁺ 3s. According to the relevant literature reports [43,44], it could be judged that the prepared Mn₃O₄ could be expressed as MnO·Mn₂O₃.

3.5.4. Particle Size Distribution

The particle size of the product was analyzed, and the result is shown in Figure 9.

As shown in Figure 9, the particle size of Mn₃O₄ was normally distributed. D₁₀, D₅₀, and D₉₀ were 3.669 μm, 6.17 μm, and 9.969 μm, respectively. The particle size distribution of Mn₃O₄ was good, indicating that the particle size was uniform.

3.6. Effect of Roasting Time on LiMn₂O₄

Mn₃O₄ prepared under the above optimal conditions was a manganese source. The molar ratio of lithium to manganese was 0.5, and they were mixed evenly and roasted in a muffle furnace at 800 °C for a certain time. After the reaction was completed, the sample naturally cooled in the furnace. The effects of roasting times of 4 h, 6 h, 8 h, 10 h, and 12 h on the cathode material LiMn₂O₄ were investigated.

3.6.1. The Roasting Time Phase and Morphology

The X-ray diffraction patterns of LiMn₂O₄ prepared under different roasting times are shown in Figure 10.

From Figure 10, the characteristic peaks of the five samples corresponded to the characteristic peaks of the standard card of LiMn₂O₄, basically indicating that these samples were all spinel LiMn₂O₄ with an Fd-3m space group. The diffraction peaks of LiMn₂O₄ prepared at different times were sharp, the baseline was stable, and no impurity peaks appeared, indicating that the product had good crystallinity and high purity. With the increase in roasting time, the characteristic peak became more and more sharp, and the peak value increased, indicating that the crystallinity of the product became better and better.

The crystal structure analysis of LiMn₂O₄ samples was performed, and the results are shown in

Table 3. Lattice parameters of LiMn₂O₄ prepared at different roasting times.

Time/h	A (Å)	V (Å3)	R = I311/I400
4	8.236	558.7	1.020
6	8.234	558.3	1.016
8	8.233	558.1	1.014
10	8.231	557.6	1.013
12	8.234	558.3	1.021

.

As could be seen from Table 3, the lattice constants and cell volume first decreased and then increased with increasing roasting time, and the R-values changed in the same trend. When the reaction time was short and the reaction was incomplete, lithium and manganese could not fully react to form a new phase, and some reactants did not fully react. The reaction was completely carried out until 8 h, the new phase was completely generated with good crystallinity, and the degree of distortion was small. The reaction time was prolonged, small particles fused to grow, and the distortion increased.

The microstructure of LiMn₂O₄ prepared at different calcination times is shown in Figure 11.

From Figure 11, the LiMn₂O₄ particles formed and grew gradually with the increase in roasting time. When the roasting time was 4 h, there were a small number of particles that did not participate in the reaction, so the sample showed small particles and irregular shapes. As the reaction time increased, when it reached 6 h, 8 h, and 10 h, the particles were completed, the size was uniform, the gaps of grains were small, their density was high, and the octahedron had sharp edges and corners. The increase in time made the crystalline phase transformation complete, and the crystallinity increased. When the reaction time reached 12 h, the grains continued to grow after the crystalline phase transformed, and some of the products deoxygenated, which led to the lack of stability inside the material and the increase in distortion. Therefore, the roasting time of 10 h was more suitable.

3.6.2. Electrochemical Properties

The LiMn₂O₄ prepared with a lithium manganese ratio of 0.5 and different roasting times was used as the cathode material to prepare the CR2032 button half-cell. Cycling performance was tested at 0.2 C, multiplication performance was tested at different current densities from 0.2 C to 10 C, and the results are shown in Figure 12.

From Figure 12a, the charging and discharging curves both showed a voltage platform around 3.95 V and 4.1 V at a current density of 0.2 C, indicating the existence of two reversible redox reactions for the process. With the increase in roasting time, the first discharge-specific capacity gradually increased. The first discharge-specific capacity of LiMn₂O₄ roasted for 4 h was the lowest at 101.1 mAh∙g⁻¹, while that of LiMn₂O₄ roasted for 10 h was the highest at 121.1 mAh∙g⁻¹.

As shown in Figure 12b, the first charge/discharge-specific capacities of the five samples were 101.1mAh∙g⁻¹, 110.5 mAh∙g⁻¹, 117.5 mAh∙g⁻¹, 121.9 mAh∙g⁻¹, and 115.2 mAh∙g⁻¹ at a 0.2 C current density, and the retention rates of specific capacity were 72.4%, 92%, 92.5%, 93.6%, and 91.5% after 100 cycles, respectively. Among them, the specific capacity decay of the sample was the fastest after roasting for 4 h, and the cycling performance was the best after roasting for 10 h. From Figure 12c, after being charged and discharged at different current densities, the specific capacitance values of the five samples could still return to the specific capacitance values at the corresponding current density of 0.2 C. The sample roasted for 10 h had a specific capacity of about 87 mAh∙g⁻¹ after charging and discharging at a current density of 10 C. When the current density returned to 0.2 C, the specific capacity became 110 mAh∙g⁻¹. It was shown that the LiMn₂O₄ obtained after 10 h roasting had good multiplicative properties and good electrochemical reversibility.

When the roasting time was too short, the reaction of the reactant was not sufficient, the crystallinity of the product was poor, and the crystal cell structure was unstable. Therefore, the first charge–discharge capacity of the product was low, its cyclic decay was fast, and its rate performance was poor. When the roasting time was extended, the reaction was sufficient, the crystal phase transformation was complete, the product particle size was uniform, the structure was stable, it was conducive to the removal of lithium ions, and the high current had little damage to the crystal structure. Therefore, the first charge–discharge had a higher specific capacity, slower capacity decay, and good rate performance. When the reaction time continued to extend, on the one hand, it caused oxygen deficiency in LiMn₂O₄, and on the other hand, small particles melted and grew into large particles, leading to an increase in distortion and structural instability. As a result, specific capacity decreased gradually. In summary, LiMn₂O₄ had good electrochemical properties after roasting for 10 h.

3.7. Effect of Roasting Temperature on LiMn₂O₄

The molar ratio of lithium to manganese was 0.5, and the sample was roasted at a certain temperature for 10 h and then cooled in the furnace. The effects of roasting temperatures of 500 °C, 600 °C, 700 °C, 800 °C, and 850 °C on LiMn₂O₄ were investigated.

3.7.1. The Roasting Temperature Phase and Morphology

X-ray diffraction analysis was performed on LiMn₂O₄ prepared at different calcination temperatures, and the results are shown in Figure 13.

As could be seen from Figure 13, the diffraction peaks of the products synthesized at different temperatures were basically the same, and their characteristic peaks were consistent with those of the spinel structure LiMn₂O₄ standard card, all of which had the Fd-3m space group. There were a few Mn₃O₄ spurious peaks at 500 °C and 600 °C, indicating that some Mn₃O₄ failed to react at lower temperatures. When the temperature reached 850 °C, some impurity peaks also appeared, which was caused by the deoxygenation reaction of LiMn₂O₄ to generate LiMn₂O_4−x. When the temperature was 700 °C and 800 °C, no impurity peak appeared, the baseline was stable, the characteristic peak was sharp, and the peak value of the characteristic peak increased. It was indicated that the material LiMn₂O₄ had high purity and good crystallinity.

The crystal structure analysis of the lithium manganate samples was performed, and the results are shown in

Table 4. Lattice parameters of LiMn₂O₄ under different roasting temperatures.

Temperature (°C)	A (Å)	V (Å3)	R = I311/I400
500	8.246	560.7	1.051
600	8.222	555.8	1.010
700	8.229	557.2	1.011
800	8.231	557.6	1.013
850	8.237	558.9	1.023

.

As can be seen from Table 4, the lattice constant and cell volume of LiMn₂O₄ first decreased and then increased with the increase in temperature, both of which were smaller than the lattice constant of the standard LiMn₂O₄ (the standard lattice constant of LiMn₂O₄ is a = 0.8245 nm, V = 0.5609 nm³). The ratios of (311) to (400) changed in the same pattern. At a low temperature of 500 °C, it failed to melt completely, resulting in intergranular adhesion, larger lattice constants, and a large degree of distortion. There was enough Mn₃O₄ to react with LiOH at 600 °C, so the intergranular boundaries were cleared, and the lattice constants became smaller. As the reaction temperature increased, the energy of reactants increased, the percentage of activated molecules increased, and the number of effective collisions of reacting molecules increased, so the grains kept growing, and the lattice constants and cell volume kept increasing. When the temperature was at 800 °C, the cell volume was small, the R-value was suitable at 1.016, the distortion was small, the cell structure was stable, which was favorable for the deintercalation of lithium-ions, and the sample had a better electrochemical basis.

The microstructure of LiMn₂O₄ prepared at different roasting temperatures is shown in Figure 14.

From Figure 14, it can be seen that the LiMn₂O₄ grains formed and grew gradually with the increase in temperature. The boundary between the grains was blurred, and many edges and corners had not yet grown completely when the temperature was 500 °C. When the temperature rose to 600 °C and 700 °C, the grains were formed, and they were small and evenly distributed, but there were many voids in the structure. When the temperature rose to 800 °C, the morphology of the particles was a regular octahedron, the grains grew up, the size of the particles was uniform, the boundaries were obvious, the voids became fewer, the particles were closely agglomerated, and the average particle size was about 400 nm. When the temperature was increased to 850 °C, the grains continued to grow, but the grains exhibited a polyhedral growth trend, which was probably due to structural changes caused by the deoxygenation of LiMn₂O₄ at this temperature. The increase in temperature resulted in higher crystallinity and larger grains of LiMn₂O₄, closer aggregation between particles, and a more stable structure. This was beneficial for Li⁺ deintercalation, reducing the impact of deintercalation on grain structure and improving the cycling performance of LiMn₂O₄. However, if the temperature was too high, some oxygen atoms would detach and the crystal structure would collapse, affecting its performance.

3.7.2. The Electrochemical Properties

The LiMn₂O₄ prepared with the lithium manganese ratio of 0.5 and different roasting temperatures was used as the cathode material to prepare the CR2032 button half-cell. The cycling performance was tested at a 0.2 C current, the multiplier performance was tested at different current densities from 0.2 C to 10 C, and the results are shown in Figure 15.

From Figure 15a, when charging and discharging were at a current density of 0.2 C, LiMn₂O₄ prepared at different temperatures exhibited two voltage platforms, indicating two reversible oxidation and reduction processes during the charging and discharging process of the material. The first discharge-specific capacity of LiMn₂O₄ first increased and then decreased with the increase in roasting temperature. Its first discharge capacity was 102.1 mAh∙g⁻¹, 108.4 mAh∙g⁻¹, 113.8 mAh∙g⁻¹, 121.9 mAh∙g⁻¹, and 116.7 mAh∙g⁻¹, and the first charge–discharge coulomb efficiency was above 90%. Due to the increase in temperature, the purity and crystallinity of the material were improved, promoting a more stable structure, reducing the degree of structural distortion, and reducing the impact of lithium-ion intercalation on the structure of lithium manganese.

According to Figure 15b, the capacity retention rates of the five samples after 100 cycles were 86%, 87.5%, 89%, 93%, and 91.3%. The cycling performance of this material first increased and then decreased with temperature, and then it decreased when it exceeded a certain temperature value. Increasing the reaction temperature was beneficial for increasing the crystallinity of particles, stabilizing the structure, and thus improving cycling performance and capacity retention rate. When the temperature was too high, the distortion of the structure increased as the particles became larger, which increased the diffusion distance of lithium ions and led to a decrease in capacity and cycling retention rate.

The rate curves of the LiMn₂O₄ material are shown in Figure 15c. Overall, LiMn₂O₄ could still return to a higher discharge-specific capacity after being charged and discharged at different current densities of 0.2C 10C 0.2C. After charging and discharging at a current density of 10 C, the discharge-specific capacity of the five samples was 79.6, 81.3, 82.9, 86, and 85.09 mAh∙g⁻¹. The discharge-specific capacity of LiMn₂O₄ prepared at 800 °C was 110 mAh∙g⁻¹ when the current density returned to 0.2 C. The above results indicated that the LiMn₂O₄ cathode material obtained by roasting at 800 °C had good electrochemical performance.

4. Conclusions

Battery-grade Mn₃O₄ could be prepared through liquid-phase complex oxidation of MnSO₄. The optimal synthesis conditions were as follows. Ammonia water was used as a pH regulator and complexing agent in the experiment. The reaction was carried out at a pH of 8, a temperature of 80 °C, and a duration of 12 h. The oxygen flow rate during the reaction was 3 L∙min⁻¹. The impurity elements in the sample met the standard for manganese tetroxide in lithium batteries. The D50 particle size was approximately 6.17 μm, and the chemical formula could be represented as MnO∙Mn₂O₃. The cathode material was prepared using octahedron Mn₃O₄ as the manganese source and LiOH as the lithium source, employing the high-temperature solid-state method. The product, LiMn₂O₄, was obtained under the following conditions: a lithium manganese molar ratio of 0.5, a roasting temperature of 800 °C, and a roasting time of 10 h. The resulting LiMn₂O₄ exhibited an Fd-3m spinel structure with good crystallinity, high purity, and a small cell size. When subjected to charge and discharge at a current density of 0.2 C, the first charge–discharge-specific capacity was 125.7 mAh∙g⁻¹ and 121.9 mAh∙g⁻¹, respectively, with a first charge–discharge coulombic efficiency of 96.7%. After 100 cycles, the specific discharge capacity was 114.1 mAh∙g⁻¹, and the capacity retention rate was 93.6%. In the multiplier mode, the discharge-specific capacity was about 87mAh∙g⁻¹ at a 10 C current density and 110 mAh∙g⁻¹ when it returned to 0.2 C. The LiMn₂O₄ had good electrochemical performance.