The Electrochemical Mechanism of Preparing Mn from LiMn 2 O 4 in Waste Batteries in Molten Salt

: The electrochemical reduction mechanism of Mn in LiMn 2 O 4 in molten salt was studied. The results show that in the NaCl-CaCl 2 molten salt, the process of reducing from Mn (IV) to manganese is: Mn (IV) → Mn (III) → Mn (II) → Mn. LiMn 2 O 4 reacts with molten salt to form CaMn 2 O 4 after being placed in molten salt for 1 h. The reaction of reducing CaMn 2 O 4 to Mn is divided into two steps: Mn (III) → Mn (II) → Mn. The results of constant voltage deoxidation experiments under different conditions show that the intermediate products of LiMn 2 O 4 reduction to Mn are CaMn 2 O 4 , MnO, and (MnO) x (CaO) (1 − x ) . As the reaction progresses, x gradually decreases, and ﬁnally the Mn element is completely reduced under the conditions of 3 V for 9 h. The CaO in the product can be removed by washing the sample with deionized water at 0 ◦ C. product obtained after applying 3 V voltage at 750 ◦ C for 12 h of deoxidation is elemental metal manganese.


Introduction
As a new type of energy storage material, lithium-ion batteries have the advantages of high energy, high power density, and long service life [1], and have become the power source for various electronic products and vehicles [2]. Lithium manganate with spinel structure has many advantages and is one of the most promising anode materials for lithium-ion batteries [3][4][5]. With the large-scale application of lithium manganate batteries, the cumulative amount of waste lithium manganate batteries is bound to increase year by year. In 2020, the total amount of retired power batteries of the new energy vehicles in China will exceed 200,000 tons, and it is expected to reach to nearly 800,000 tons by 2025 [6], for which a significant proportion are lithium manganese oxide batteries. Although some scholars are committed to researching materials with self-healing properties to extend the life of lithium-ion batteries [7,8], waste lithium manganate batteries contain metal elements with high added value such as manganese and lithium, and their recycling will produce good economic and social benefits. Lithium-ion battery recycling process is mainly divided into fire and wet recycling technology [9]. Tang et al. used vacuum pyrolysis to recover lithium and cobalt in lithium-ion batteries, and the recovery rates reached over 93% and 99%, respectively [10]. Zhuang et al. used a mixture of phosphoric acid and citric acid as the leaching solution to recover LiNi 0.5 Co 0.2 Mn 0.3 O 2 , the leaching rate of lithium reached 100%, while the leaching rates of nickel, cobalt, and manganese are 93.38%, 91.63%, and 92.00% [11], respectively. The condition of pyrometallurgical recovery is higher temperature, while many of the pollutants such as exhaust gas are produced [12,13]. Then the purification equipment, which causes a sharp increase in cost, and many valuable metals will enter the slag, which then causes a great loss [14]. Wet recycling produces a large amount of acid-base liquid waste [15,16], and the selectivity is too low [17]. Due to the advantages of its concise process, easy access to equipment, cleanliness, conductivity, productivity, high current density, and precise reduction [18][19][20], the molten salt recovery process has been used to recover aluminum, calcium, magnesium, sodium potassium,

Reaction of LiMn 2 O 4 in Molten Salt
When lithium manganate was added to the molten salt at 750 • C, bubbles could be observed. As the reaction progressed, the bubbles gradually decreased and eventually disappeared. It can be seen from Figure 1  When lithium manganate was added to the molten observed. As the reaction progresses, the bubbles grad disappeared. It can be seen from Figure 1 that the compo ing for 0.1h are spinel lithium manganate (LiMn2O4), la nO2) and Mn3O4. LiMn2O4 has the strongest characteristic 0.5h also contains LiMn2O4, LiMnO2, and Mn3O4, b LiMn2O4are significantly weakened, and the peaks of L ened. It shows that LiMn2O4 reacts at 750℃ to produce L leased. The reaction equation of this process is: When the holding time reaches 1h~1.5h, LiMn2O4 an cium manganate (CaMn2O4) and Mn3O4 appear, indicatin decomposition of LiMn2O4 is transformed into CaMn2O formula is: The schematic diagram of the two-step reaction process is shown in Figure 2.  When the holding time reaches 1~1.5 h, LiMn2O4 and LiMnO2 disappear, while calcium manganate (CaMn2O4) and Mn3O4 appear, indicating that LiMnO2 generated by the decomposition of LiMn2O4 is transformed into CaMn2O4 during this time. The reaction formula is: The schematic diagram of the two-step reaction process is shown in Figure 2.   The square wave voltammetry was used to monitor the electrochemical reduction process of Mn ions in molten salt, and the frequency f was changed to obtain a square wave voltammetry curve as shown in Figure 3a. It can be seen from the figure that the peak a, peak b, and peak c indicate that Mn (IV) in LiMn 2 O 4 is reduced to Mn in three steps. However, the fitting results from Figure 3b show that the linear relationship between f 1/2 and i pc of peaks a, b, and c are poor. Therefore, the electrochemical reduction process of Mn in lithium manganate on the surface of the Pt electrode is an irreversible reaction. Consequently, square wave voltammetry cannot be used to calculate the number of transferred electrons, it can only be used as a qualitative analysis reaction step. The square wave voltammetry was used to monitor the electrochemical reduction process of Mn ions in molten salt, and the frequency f was changed to obtain a square wave voltammetry curve as shown in Figure 3a. It can be seen from the figure that the peak a, peak b, and peak c indicate that Mn (IV) in LiMn2O4 is reduced to Mn in three steps. However, the fitting results from Figure 3b show that the linear relationship between f 1/2 and ipc of peaks a, b, and c are poor. Therefore, the electrochemical reduction process of Mn in lithium manganate on the surface of the Pt electrode is an irreversible reaction. Consequently, square wave voltammetry cannot be used to calculate the number of transferred electrons, it can only be used as a qualitative analysis reaction step.

Open Circuit-Chronopotential
In order to further study the dissolution and deposition process of manganese on the surface of Pt electrode, the open circuit potential of NaCl-CaCl2-LiMn2O4 molten salt was used. Platinum, as the working electrode, applied a voltage of −3.0 V relative to the reference for 30 s and then stopped, the open circuit potential method was used to record the change at this time. That is, the relationship between the potential and time without external electric field as shown in Figure 4. As time increases, the low potential gradually changes to the equilibrium, in this process the curve appears stepped, having three platforms. There appears to be a stable potential when the metal dissolved in the molten salt or during the transition from low-valent ions to high. The first plateau appears around −1.4 V, which corresponds to the dissolution and deposited of the metal Mn, and manganese gains electrons and oxidizes to Mn (II). The second plateau appears around −1.0 V, which corresponds to the oxidation of Mn (II) to Mn (III). At −0.8 V, it is the process of converting Mn (III) to Mn (IV). The oxidation process of manganese is basically consistent with the results of the cyclic voltammetry scan. The potential of it is slightly positive compared with the reduction potential, which is closer to the oxidation peak in the cyclic voltammetry curve.

Open Circuit-Chronopotential
In order to further study the dissolution and deposition process of manganese on the surface of Pt electrode, the open circuit potential of NaCl-CaCl 2 -LiMn 2 O 4 molten salt was used. Platinum, as the working electrode, applied a voltage of −3.0 V relative to the reference for 30 s and then stopped, the open circuit potential method was used to record the change at this time. That is, the relationship between the potential and time without external electric field as shown in Figure 4. As time increases, the low potential gradually changes to the equilibrium, in this process the curve appears stepped, having three platforms. There appears to be a stable potential when the metal dissolved in the molten salt or during the transition from low-valent ions to high. The first plateau appears around −1.4 V, which corresponds to the dissolution and deposited of the metal Mn, and manganese gains electrons and oxidizes to Mn (II). The second plateau appears around −1.0 V, which corresponds to the oxidation of Mn (II) to Mn (III). At −0.8 V, it is the process of converting Mn (III) to Mn (IV). The oxidation process of manganese is basically consistent with the results of the cyclic voltammetry scan. The potential of it is slightly positive compared with the reduction potential, which is closer to the oxidation peak in the cyclic voltammetry curve.

Cyclic Voltammetry
Measured the cyclic voltammetry curve of NaCl-CaCl2 molten salt at 750 °C first, lithium manganate (LiMn2O4) for 1 h, and then performed cyclic voltammetry again. alyzed the electrochemical reduction process of manganese on the surface of the Pt trode. As shown in the dashed line in Figure 5.

Cyclic Voltammetry
Measured the cyclic voltammetry curve of NaCl-CaCl 2 molten salt at 750 • C first, add lithium manganate (LiMn 2 O 4 ) for 1 h, and then performed cyclic voltammetry again. Analyzed the electrochemical reduction process of manganese on the surface of the Pt electrode. As shown in the dashed line in Figure 5.

Cyclic Voltammetry
Measured the cyclic voltammetry curve of NaCl-CaCl2 molten salt at 750 °C first, add lithium manganate (LiMn2O4) for 1 h, and then performed cyclic voltammetry again. Analyzed the electrochemical reduction process of manganese on the surface of the Pt electrode. As shown in the dashed line in Figure 5.  Selected the scan range from −2.4 to 0.5 V. The dashed line in Figure 5 shows the obvious reduction peak at −2.4 V in the reverse scanning direction. This is due to the reduction Crystals 2021, 11, 1066 6 of 16 reaction of Na + in the molten salt, which corresponds to the forward direction. During the scanning process, an oxidation peak appeared at −1.4 V, which was due to the oxidation reaction of the reduced sodium to generate Na + . When the forward scan is close to the 0.5 V potential, the current shows an increasing trend, which is attributed to the current change caused by the loss of electrons of Cl − at this potential, and oxidized to generate Cl 2 and discharge. There is a wide electrochemical window between 0.4 V and −1.2 V, and there is no obvious electrochemical reduction reaction in this range.
The red solid line in Figure 5 is the cyclic voltammetry curve after adding LiMn 2 O 4 to NaCl-CaCl 2 after holding for 1 h. It can be seen that there are two pairs of oxidationreduction peaks, a/a' and b/b'. The reduction potentials of peak a and b are −0.99 V and −1.2 V, respectively, in the reverse direction with the scanning speed is 0.1 V/s. As is shown in Figure 1, the reaction result of adding lithium manganate to NaCl-CaCl 2 molten salt at 750 • C for 1h indicate manganese mainly exists in the form of CaMn 2 O 4 and Mn 3 O 4 . Therefore, the electrochemical reduction process is started by the trivalent manganese in CaMn 2 O 4 . When the potential changes but no electrochemical reduction occurs, the background current is shown as the dotted line. When Mn (III) appears in the molten salt and the potential increased to −1 V, Mn (III) is reduced to Mn (II). At the same time, an electron transfer process occurs on the surface of the electrode, and manganese ions are reduced, which introduces a Faraday current. The coupling effect of the Faraday current and non-Faraday current increases the current density, thereby forming a reduction peak, as shown by peak a in Figure 5. The generated Mn (II) is reduced to elemental manganese, while a two-electron transfer occurs, and the coupling phenomenon of the Faraday current and non-Faraday current occurs again to form a reduction peak b.
Tested the effect of different sweep speeds on the reduction of Mn (III), and drew graphs based on the data of different sweep speeds (0.2~0.5 V/s), as shown in Figure 6. It can be seen from Figure 6 that the cyclic voltammetry curve has two obvious reduction peaks a and b near −1.1 V and −1.38 V. At the same time, there are two obvious oxidation peaks a' and b' above the curve corresponding to the reduction peak. In multi-turn scanning, peak a and b both shift to the negative direction to varying degrees as the scanning speed increases. Peak a' and b' have different degrees of deviation in the positive direction, and the current also changes with the sweep speed. The peak potentials E pc and E pa and the current densities i pa and i pc all change with the different scanning speed v. The curve of i pc -v 1/2 and E pc -v 1/2 is shown in Figure 7.
Selected the scan range from −2.4 to 0.5 V. The dashed line in Figure 5 shows th obvious reduction peak at −2.4 V in the reverse scanning direction. This is due to the re duction reaction of Na + in the molten salt, which corresponds to the forward direction During the scanning process, an oxidation peak appeared at −1.4 V, which was due to th oxidation reaction of the reduced sodium to generate Na + . When the forward scan is clos to the 0.5 V potential, the current shows an increasing trend, which is attributed to th current change caused by the loss of electrons of Clat this potential, and oxidized to gen erate Cl2 and discharge. There is a wide electrochemical window between 0.4 V and −1. V, and there is no obvious electrochemical reduction reaction in this range.
The red solid line in Figure 5 is the cyclic voltammetry curve after adding LiMn2O to NaCl-CaCl2 after holding for 1 h. It can be seen that there are two pairs of oxidation reduction peaks, a/a' and b/b'. The reduction potentials of peak a and b are −0.99 V and −1.2 V, respectively, in the reverse direction with the scanning speed is 0.1 V/s. As is shown in Figure 1, the reaction result of adding lithium manganate to NaCl-CaCl2 molten salt a 750 °C for 1h indicate manganese mainly exists in the form of CaMn2O4 and Mn3O4. There fore, the electrochemical reduction process is started by the trivalent manganese in CaMn2O4. When the potential changes but no electrochemical reduction occurs, the back ground current is shown as the dotted line. When Mn (III) appears in the molten salt and the potential increased to −1 V, Mn (III) is reduced to Mn (II). At the same time, an electron transfer process occurs on the surface of the electrode, and manganese ions are reduced which introduces a Faraday current. The coupling effect of the Faraday current and non Faraday current increases the current density, thereby forming a reduction peak, as shown by peak a in Figure 5. The generated Mn (II) is reduced to elemental manganese, while two-electron transfer occurs, and the coupling phenomenon of the Faraday current and non-Faraday current occurs again to form a reduction peak b.
Tested the effect of different sweep speeds on the reduction of Mn (III), and drew graphs based on the data of different sweep speeds (0.2~0.5 V/s), as shown in Figure 6. I can be seen from Figure 6 that the cyclic voltammetry curve has two obvious reduction peaks a and b near −1.1 V and −1.38 V. At the same time, there are two obvious oxidation peaks a' and b' above the curve corresponding to the reduction peak. In multi-turn scan ning, peak a and b both shift to the negative direction to varying degrees as the scanning speed increases. Peak a' and b' have different degrees of deviation in the positive direction and the current also changes with the sweep speed. The peak potentials Epc and Epa and the current densities ipa and ipc all change with the different scanning speed v. The curv of ipc-v 1/2 and Epc-v 1/2 is shown in Figure 7.    Figure 7a shows that the current density of peak a raise with the increase of the square root of the sweep speed, but its linear relationship is poor. The potential of reduction peak has a significant deviation, and the peak potential raises with the increase of the scanning speed, which has a very good linear relationship. Therefore, the reduction process of peak a is controlled by diffusion and kinetics, and the reaction is judged to be an irreversible reaction based on the peak shape. Figure 7b shows that the current density of peak b also raises with the increase of the square root of the sweep speed, and the linearity is better. The reduction peak potential has a significant shift of about −1.3 V, and the peak potential raises with the increase of the scanning speed with a good linear relationship. Therefore, the reduction process of peak b is controlled by diffusion. Since the peak potential shifts more obviously with the increase of scanning speed, it is judged that peak b is a quasireversible reaction. According to the reversible reaction calculation Equations (3) and (4), the electron transfer number of peak b can be calculated. The calculated number of transferred electrons of peak b is 2.1 ≈ 2, which corresponds to the reduction process of Mn (II) to elemental manganese.

The Effect of Electrolysis Time on the Product
In order to study the effect of deoxidation time on manganese reduction, 0.8 g LiMn2O4 in graphite crucible was electrically deoxidized for 1 h, 6 h, and 12 h at 3 V and 750 °C. Compared the products of the different reaction time during the reduction process. The product under the conditions of 3 V and 1 h is light green and dark gray. According to the XRD results, it can be judged that it is mainly MnO (green), CaMn2O4, (MnO)0.759(CaO)0.241. At this time, the reduction of LiMn2O4 to low-valent manganese has occurred. Figure 8d shows that as time goes by, the low-valence manganese compound gradually gains electrons and reduces. At 2 h, the characteristic peak of metallic manganese has appeared, and the coexistence state of MnO and Mn appears in the product.   Figure 7a shows that the current density of peak a raise with the increase of the square root of the sweep speed, but its linear relationship is poor. The potential of reduction peak has a significant deviation, and the peak potential raises with the increase of the scanning speed, which has a very good linear relationship. Therefore, the reduction process of peak a is controlled by diffusion and kinetics, and the reaction is judged to be an irreversible reaction based on the peak shape. Figure 7b shows that the current density of peak b also raises with the increase of the square root of the sweep speed, and the linearity is better. The reduction peak potential has a significant shift of about −1.3 V, and the peak potential raises with the increase of the scanning speed with a good linear relationship. Therefore, the reduction process of peak b is controlled by diffusion. Since the peak potential shifts more obviously with the increase of scanning speed, it is judged that peak b is a quasi-reversible reaction. According to the reversible reaction calculation Equations (3) and (4), the electron transfer number of peak b can be calculated. The calculated number of transferred electrons of peak b is 2.1 ≈ 2, which corresponds to the reduction process of Mn (II) to elemental manganese.

The Effect of Electrolysis Time on the Product
In order to study the effect of deoxidation time on manganese reduction, 0. Judging from the reaction at different times, the high-valence manganese in LiMn2O4 obtains electrons to be reduced under higher voltage. LiMn2O4 reacts with calcium ions in the surrounding molten and produces CaMn2O4 containing Mn (III) first. During the electrolysis process, due to the influence of the diffusion control reaction rate in the molten salt, the removed oxygen ions gather near the electrode. At the same time, because of the large driving force for electrolytic reduction provided by the voltage, CaMn2O4 is further electrolytically reduced to form (MnO)0.759(CaO)0.241 containing Mn (II). As the diffusion progressed, the concentration of oxygen ions near the electrode decreased and MnO is formed. With the extension of the electrolysis time, the high-valence manganese compound is gradually deoxidized and reduced. The Mn (III) compound disappeared at 6 h and Mn (II) started to be reduced to elemental manganese. A large amount of oxygen ions were removed from CaMn2O4 and diffused slowly, then CaO was formed with surrounding calcium ions. When the electrolysis time was sufficient, the conversion of ionic manganese to elemental manganese was completed. Therefore, the reaction process of reducing LiMn2O4 to manganese metal is: Mn (IV)→Mn (III)→Mn (II)→Mn, which corresponds to the result of square wave voltammetry. At the same voltage and the same temperature, the longer the time of the reaction, the more thorough the electrochemical reaction will proceed, but the time cost in the actual production process cannot be ignored. When the temperature, the longer the time of the reaction, the more thorough the electrochemical reaction will proceed, but the time cost in the actual production process cannot be ignored. When the driving force is large enough, the two-step deoxygenation process may occur simultaneously.
The reaction formula is: The formation and decomposition of CaMn 2 O 4 in the electrolytic deoxidation and reduction process of LiMn 2 O 4 were studied. Analyzed the experimental result of applying different voltages for a short certain time ( Figure 9) and low voltages for different time ( Figure 10). ystals 2021, 11, 1066 9 The reaction formula is: 2LiMn2O4 + Ca 2+ + 4e -→ CaMn2O4 + 2MnO + 2Li + + 2O 2- The formation and decomposition of CaMn2O4 in the electrolytic deoxidation reduction process of LiMn2O4 were studied. Analyzed the experimental result of apply different voltages for a short certain time ( Figure 9) and low voltages for different t (Figure 10). As shown in Figure 9, when the electrolysis voltage is 0.5 V~1 V, CaMn2O4 and Mn are formed. When the electrolysis voltage increases to 1.5 V~3 V, the characteristic p of Mn3O4 in the product disappears, and the trivalent manganese is transformed into valent manganese, forming (MnO)x(CaO)(1−x) and MnO. The reason for this is in the cess of electrolytic reduction of high-valent manganese to elemental manganese, it ne to rely on energy to promote the process of obtaining electrons. The driving force of process is voltage. The magnitude of the applied voltage determines whether the elec chemical reaction can proceed and the extent of the reaction. The electrochemical reac that can be carried out requires energy to break its thermodynamic conditions, that is equilibrium potential, while the occurrence of the kinetic process requires a certain o potential. When the over-potential is greater, the reaction speed is greater. Accordin the analysis of electrolysis products at different electrolysis voltages in a short time, w the voltage is greater, the driving force of the reaction is greater, the reaction spee faster, and the degree of reaction progress is greater. Therefore, the reduction proces high valent manganese is: LiMn2O4 (III, IV) is reduced and chemically reacts with Ca the molten salt to form CaMn2O4; Subsequently, Mn (III) in CaMn2O4 undergoes a red tion reaction to produce MnO, while CaO is gradually separated, and the x As shown in Figure 9, when the electrolysis voltage is 0.5 V~1 V, CaMn 2 O 4 and Mn 3 O 4 are formed. When the electrolysis voltage increases to 1.5 V~3 V, the characteristic peak of Mn 3 O 4 in the product disappears, and the trivalent manganese is transformed into divalent manganese, forming (MnO) x (CaO) (1−x) and MnO. The reason for this is in the process of electrolytic reduction of high-valent manganese to elemental manganese, it needs to rely on energy to promote the process of obtaining electrons. The driving force of the process is voltage. The magnitude of the applied voltage determines whether the electrochemical reaction can proceed and the extent of the reaction. The electrochemical reaction that can be carried out requires energy to break its thermodynamic conditions, that is, the equilibrium potential, while the occurrence of the kinetic process requires a certain over-potential. When the over-potential is greater, the reaction speed is greater. According to the analysis of electrolysis products at different electrolysis voltages in a short time, when the voltage is greater, the driving force of the reaction is greater, the reaction speed is faster, and the degree of reaction progress is greater. Therefore, the reduction process of high valent manganese is: LiMn 2 O 4 (III, IV) is reduced and chemically reacts with Ca 2+ in the molten salt to form CaMn 2 O 4 ; Subsequently, Mn (III) in CaMn 2 O 4 undergoes a reduction reaction to produce MnO, while CaO is gradually separated, and the x in (MnO) x (CaO) (1−x) that appears during the reaction gradually becomes smaller. The electrolysis products of LiMn2O4 at different voltages in a short period of time have consistent changes. LiMn2O4 (III, IV) has undergone a gradual reduction process from trivalent manganese, bivalent manganese to elemental manganese. At 0.5 V electrical deoxidation for 0.5 h, the products are mainly LiMn2O4 with a small amount of CaMn2O4 and Mn3O4. When the time increases to 1~1.5 h, LiMn2O4 disappears, CaMn2O4 increases, and (MnO)0.614(CaO)0.386 is produced. As the time is increased to 2 h, (MnO)0.614(CaO)0.386 in the product decreased, some characteristic peaks of (MnO)0.759(CaO)0.241 appeared at the same time, and the peak intensity of products containing Mn (II) increased and CaMn2O4 decreases. When the electro-deoxygenation is 0.5 h, the composition of the 1.5 V product is the same as that of the 0.5 V product, but 1.5 V produces are more CaMn2O4. When deoxidation time was increased to 1 h, the characteristic peaks of LiMn2O4 and Mn3O4 all disappeared, and the product was still dominated by CaMn2O4. At the same time, characteristic peaks of MnO and (MnO)0.759(CaO)0.141 appeared. Compared with 0.5 V, the product at the same time of 1.5 V contains more Mn (II), and with the extension of electrolysis time, the amount of MnO gradually increases, and the characteristic peak intensity of CaMn2O4 decreases. When the time reaches 2 h, the characteristic peak of (MnO)0.877(CaO)0.123 which contains more MnO appears in the product. The electrolysis When the electrolysis voltage is small, because of the small driving force of the electrochemical reaction, the rate of gaining electrons is small, and the main reaction that occurs is the conversion of LiMn 2 O 4 (III, IV) to Mn (III) and Mn (II). When the electrolysis voltage is large, the electrochemical reaction rate increases, so as to reach the electrochemical thermodynamic conditions for the transformation from Mn (II) to elemental manganese. Therefore, at different voltages at the same time, the low-valence manganese compounds gradually raise with the increase of voltage, and elemental manganese is obtained under high voltage. The electrochemical reaction of the reduction process is Equations (6) Figure 11 shows the morphological characteristics of raw material and intermediate reduction products in the process of LiMn 2 O 4 electro-deoxidation to prepare elemental manganese.

Electro Deoxidation Product Analysis
It can be seen from Figure Figure 11 shows the morphological characteristics of raw material and intermediate reduction products in the process of LiMn2O4 electro-deoxidation to prepare elemental manganese.  It can be seen from Figure 11 that the lithium manganate raw material (LiMn2O4) has a honeycomb structure. The main elements are Mn and O and the oxygen-manganese ratio is about 1.55, which is closer to the oxygen-manganese ratio of the lithium manganate raw material of 4:2. The products of LiMn2O4 electrolyzed at 0.5 V for 1 h are CaMn2O4 and Mn3O4. The surface of substance B in the figure is relatively dense and smooth, and its main elements are Mn, O, and Ca. The ratio of calcium to manganese to oxygen is close to 1:2:4, so that is the intermediate compound calcium manganate (CaMn2O4). Substance C is loose and has porous spherical particles, the main elements are Mn and O, the atomic ratio of oxygen to manganese is about 3:4, and the product is trivalent manganese oxide Mn3O4 during the deoxidation process of lithium manganate.

Electro Deoxidation Product Analysis
The product deoxygenated at 3 V for 12 h was cleaned with distilled water at 20 °C and 0 °C under ultrasonic action. Figure 12 shows the XRD of the product at different cleaning temperatures. There are no other forms of manganese that appear except for elemental manganese. However, Ca(OH)2 appeared in the product treated with distilled water at 20 °C. This is because CaO is easier to dissolve in distilled water at 0 °C. The analysis of the electro-deoxidation product shows that the surface texture of it is uneven as in Figure 13. The particle surface scan results show that the elements are Mn and O. The atomic ratio of manganese at point 1 is 78.82%, and the atomic ratio of manganese at points 2 and 3 reaches 95.91% and 96.75%. It can be seen from the distribution of manganese that the process of manganese oxide gaining electrons from high to low prices starts from the particle surface at the three-phase boundary, and the boundary gradually extends to the inside of the particle over time, and finally elemental manganese is obtained.  The product deoxygenated at 3 V for 12 h was cleaned with distilled water at 20 • C and 0 • C under ultrasonic action. Figure 12 shows the XRD of the product at different cleaning temperatures. There are no other forms of manganese that appear except for elemental manganese. However, Ca(OH) 2 appeared in the product treated with distilled water at 20 • C. This is because CaO is easier to dissolve in distilled water at 0 • C. The analysis of the electro-deoxidation product shows that the surface texture of it is uneven as in Figure 13. The particle surface scan results show that the elements are Mn and O. The atomic ratio of manganese at point 1 is 78.82%, and the atomic ratio of manganese at points 2 and 3 reaches 95.91% and 96.75%. It can be seen from the distribution of manganese that the process of manganese oxide gaining electrons from high to low prices starts from the particle surface at the three-phase boundary, and the boundary gradually extends to the inside of the particle over time, and finally elemental manganese is obtained. Figure 14 shows the XPS spectra of the products of electro-deoxygenation at 750 • C with voltages of 2.6 V and 3.0 V for 12 h. It can be seen from Figure 14b that the spectrum can divide into two different environments of Mn atoms, with binding energies of 641.7 eV and 653.4 eV, respectively. The signal peak with the binding energy at 641.7 eV corresponds to the Mn 2p3/2 orbital. The corresponding valence states of Mn are +2 and +3. MnO and Mn 2 O 3 exist at the same time, the signal peak with a binding energy of 653.4 eV corresponds to the Mn 2p1/2 orbital, and the corresponding substance is MnO, which proves that the valence state changes during the reduction of lithium manganate to metallic manganese. It can be seen from Figure 14d that the fitted curve has two obvious peaks; the binding energies are 638.5 eV and 650.2 eV, corresponding to the characteristic peaks of Mn element of Mn 2p3/2 orbital and Mn 2p1/2 orbital, respectively. This indicates that the product obtained after applying 3 V voltage at 750 • C for 12 h of deoxidation is elemental metal manganese. ratio of manganese at point 1 is 78.82%, and the atomic ratio of m 3 reaches 95.91% and 96.75%. It can be seen from the distributi process of manganese oxide gaining electrons from high to low ticle surface at the three-phase boundary, and the boundary gra side of the particle over time, and finally elemental manganese   Figure 14 shows the XPS spectra of the products of electro-deoxygenation at 750 °C with voltages of 2.6 V and 3.0 V for 12 h. It can be seen from Figure 14b that the spectrum can divide into two different environments of Mn atoms, with binding energies of 641.7 eV and 653.4 eV, respectively. The signal peak with the binding energy at 641.7 eV corresponds to the Mn 2p3/2 orbital. The corresponding valence states of Mn are +2 and +3.

Conclusions
The electrochemical mechanism of the reduction of high valence Mn in LiMn2O4 to Mn elementary substance was studied.
The study of square wave voltammetry and open-circuit-chronopotentiometry shows that the reaction process of LiMn2O4 reduction to manganese in NaCl-CaCl2 molten salt is: Mn (IV)→Mn (III)→Mn (II)→Mn.
Cyclic voltammetry measured that LiMn2O4 produced CaMn2O4 after 1 h in molten salt. The reduction of CaMn2O4 to Mn was completed in two steps: Mn (III)→Mn (II)→Mn.
The results of constant voltage deoxidation under different conditions show that LiMn2O4 first reacts with Ca 2+ to form CaMn2O4, which is then reduced to (MnO)x(CaO)(1−x) containing divalent Mn, and x gradually decreases as the reaction progresses. MnO was completely reduced to Mn element after 9 h of 3 V electrolysis, and the CaO in the product could be removed by cleaning the sample at 0 °C.

Conclusions
The electrochemical mechanism of the reduction of high valence Mn in LiMn 2 O 4 to Mn elementary substance was studied.
The study of square wave voltammetry and open-circuit-chronopotentiometry shows that the reaction process of LiMn 2 O 4 reduction to manganese in NaCl-CaCl 2 molten salt is: Mn (IV)→Mn (III)→Mn (II)→Mn.
Cyclic voltammetry measured that LiMn 2 O 4 produced CaMn 2 O 4 after 1 h in molten salt. The reduction of CaMn 2 O 4 to Mn was completed in two steps: Mn (III)→Mn (II)→Mn.
The results of constant voltage deoxidation under different conditions show that LiMn 2 O 4 first reacts with Ca 2+ to form CaMn 2 O 4 , which is then reduced to (MnO) x (CaO) (1−x) containing divalent Mn, and x gradually decreases as the reaction progresses. MnO was completely reduced to Mn element after 9 h of 3 V electrolysis, and the CaO in the product could be removed by cleaning the sample at 0 • C.