Stabilization of Mn4+ in synthetic slags and identification of important slag forming phases

The expected shortage of Li due to the strong increase in electromobility is an important issue for the recovery of Li from spent Li-ion batteries. One approach is pyrometallurgical processing, during which ignoble elements such as Li, Al and Mn enter the slag system. The Engineered Artificial Minerals (EnAM) strategy aims to efficiently recover critical elements. This study focuses on stabilizing Li-manganates in a synthetic slag and investigates the relationship between Mn4+ and Mg and Al in relation to phase formation. Therefore, three synthetic slags (Li, Mg, Al, Si, Ca, Mn, O) were synthesized. In addition to LiMn3+O2, Li2Mn4+O3 was also stabilized. Both phases crystallized in a Ca-silicate-rich matrix. In the structure of Li2MnO3 and LiMnO2, Li and Mn can substitute each other in certain proportions. As long as a mix of Mn2+ and Mn3+ is present in the slag, spinels form through the addition of Mg and/or Al.


Introduction 1.1 Recovery of critical elements from the slag phase
The usage of critically relevant elements, such as Li, will be increasingly in demand due to their use in various technological areas [1].The EU has established a list of critical elements, which includes Li as well as Co [2].In Li-ion batteries (LIBs), these two elements are used as cathode materials (LiCoO2).
The geological supply of such elements is limited, thus the recovery of e.g.Li from the industrial waste stream such as from old LIBs is indispensable [3][4][5].Pyrometallurgical processes [6] are a promising method for the recovery.With this route, various elements (e.g.Co, Ni or Cu) can be recovered directly, while base elements (e.g.Li, Mg, Al or Mn) are either lost as dust or enter the slag phase and form complex compounds.For efficient recovery of critical elements such as Li, it is essential to concentrate the element of interest in a single phase within the slag [1,7].Due to the high O2 affinity of Li, this element should only crystallize in a Li-rich phase.In addition, this phase should have a high Li-content as well as good processing properties (e.g., habitus, crystal size, magnetic properties).The stabilization of Li in a single phase for an efficient recovery of critically relevant elements is the idea behind: Engineered Artificial Minerals (EnAM) [8][9][10] (Figure 1).

Potential EnAMs for efficient recovery
Studies by Elwert et al. [11] shown that the Li-aluminate LiAlO2 would be suitable as a potential EnAM, as the idiomorphic to hypidiomorphic crystals can be separated from the remaining slag via flotation [12].In the presence of especially Mg, Al and Mn, spinel complexes are formed during solidification and solid solutions occurred between spinels (e.g.MgMn2O4, MgAl2O4, MnAl2O4 and Mn 2+ Mn 3+ 2O4), which hampered the formation of LiAlO2 [8,[13][14][15].In addition, this Li aluminate can contain up to 3 wt.%Si, which complicates hydrometallurgical processing [12].As the use of Mn in new Li batteries increases, Mn will become part of a complex slag system along with Li, Mg and Al.In addition, small amounts of Mg and Al in LiMnO2 and Li2MnO3 are expected to promote intralayer diffusion and Mg should also promote interlayer diffusion [16].For this reason, current research is focusing on the stabilization of Li in Li-manganates as a potential EnAM [9].The difficulties here are the redox-sensitive behavior of Mn and the Jahn-Teller effect on the Mn 3+ at temperatures above 1445 K [14,15].Schnickmann et al. [9] showed that it is possible to stabilize a Li-manganate in a complex synthetic oxide slag system (Li, Mg, Al, Si, Ca, Mn).Under normal atmosphere a mixture between Mn 2+ and Mn 3+ was present and pure LiMnO2 has formed.Nevertheless, the formation of spinels and spinel solid solutions could not yet be prevented, as well as the incorporation of Mn 2+ into the Ca-silicate matrix.In a follow-up experiment, pure oxygen (100 % O2) was used to stabilize a higher Mn oxidation stage in the slag.For this experiment, the same batch precursors were used as in Schnickmann et al. [9].This experiment is designed to investigate whether it is possible to stabilize higher Mn oxidation states in synthetic slags and whether Mn 4+ forms a compound with the other elements, especially with Mg and/or Al.

Important slag forming phases
In a complex slag system consisting of Li2O, MgO, Al2O3, SiO2, CaO, and MnO, mainly binary and ternary oxide compounds are formed, followed by a residual melt [8,9].The compounds described below represent possible phases in a crystalline slag with a special focus on Li-rich compounds and the Mn oxides.Compounds between Mn 4+ and Al/Mg have not yet been described in the literature.The ternary compounds are of minor importance for the results presented and are therefore not discussed in detail in this chapter.
The Li2O-MnOx system (1 ≤ x ≤ 2) must be considered for the investigation of possible Mn-containing EnAMs.Due to the redox sensitivity of Mn, the formation of various Li-manganates with different concentrations and Mn speciations are possible.The phase equilibria in Li2O-MnOx system at air is shown in Figure 2. According to the experimental data obtained by Paulsen and Dahn [17], cubic spinel transforms to tetragonal spinel at high temperatures.A miscibility gap between hausmannite (Mn3O4) and t-spinel was proposed.LiMnO2 is stable at temperatures higher than 1223 K, while the Li2MnO3 phase is stable up to 1240 K, approximately.In addition to Paulsen and Dahn [17], Longo et al. [18] have also described such compounds in detail, as well as the influence of temperature, pressure and pH on the formation and stability of such Li-manganates.Below 400 °C, the cubic spinel phases with stoichiometry LiMn1.75O4 and Li4Mn5O12 are found stable.They can also be part of the spinel solid solution, as shown in the calculations of [19] but to achieve equilibrium at these low temperatures would be problematic.The phase Li2Mn3O7 is calculated to be stable, but its stability has not yet been experimentally proven [17,18,20].Between 400 and 800 °C, the Li-spinel (Li(1+x)Mn(2-x)O4) has a large stability field between LiMn2O4 (Mn 3.5+ ) and Li4Mn5O12 (Mn 4+ ) and can coexist with Li2MnO3 (Mn 4+ ).
According to Mishra and Ceder [21], the Jahn-Teller distortion affects the tetragonal spinel LiMn2O4 as well as orthorhombic (e.g., LiMnO2) and monoclinic layered structures [21].In addition, the investigations by Schnickmann et al. [9] showed that in the LiMnO2 system the Li and Mn contents can be exchanged and that this compound can incorporate up to 0.35 wt.% Al.Due to their good flotation properties, Li-aluminates have been investigated as potential EnAMs for a long time.Recently thermodynamic assessments of the Li2O-Al2O3 system using CALPHAD approach have been published by Konar et al. [22] and De Abreu et al. [23].Moreover, De Abreu et al.
experimentally studied the phase equilibria in the Al2O3 rich part of the diagram and made calorimetric measurements of heat capacity for intermediate phases.The calculated phase diagram is shown in Figure 3a.Three stable intermediate phases were proposed: LiAl5O8, LiAlO2 and Li5AlO4.LiAlO2 has already been extensively analyzed and discussed for its potential as an EnAM [8,[22][23][24].Two polymorphs are reported for LiAlO2: low temperature α-phase (trigonal) and the high temperature γ-phase (tetragonal).
Two stable modifications of LiAl5O8 with spinel structure but with different space groups were found in the alumina-rich side of the phase diagram.High temperature modification of spinel is inversed and stable with some homogeneity range.No evidences of LiAl11O17 were found in the microstructures and thus, LiAl11O17 is not treated as stable.
Another important area of Li-rich phases is the Li2O-SiO2 system, with the following stable phases: Li2Si2O5, Li2SiO3, Li4SiO4 and Li8SiO6 (ordered by decreasing Si concentration).According to the latest results, Li6Si2O7 phase is treated as metastable.Li2SiO3 is formed from a Li2O-SiO2 rich melt.Studies by Chakrabarty et al. [25] shown that at different cooling rates an initial dominance of the thermodynamic driving force occurs, followed by kinetic forces.Spinels are the most important Li-free compounds, whereas hausmannite (Mn 2+ Mn 3+ 2O4) is the most important compound in the MnO-Mn2O3 system.Below 1172 °C, the Jahn-Teller effect has an influence on the Mn 3+ position and causes deformation of the crystal parameters.The transformation from tetragonal to cubic is reversible and crystal lattice changes from cubic to tetragonal on cooling.
Hausmannite can be regarded as a low temperature phase as well as a deformed spinel under normal conditions (1 atm; 25 °C) and low oxygen partial pressure.Hausmannite forms from Mn2O3 at 1445 K and air oxygen partial pressure.It can form solid solutions with other spinel compounds.One of the most important spinel in the MnO-Al2O3 system is galaxite (MnAl2O4) [14,15,22].The calculated phase diagrams with thermodynamic parameters optimized by [15] for the Al2O3-MnOx system at air and in presence of metallic Mn are shown in Figure 3b/c, respectively.In the presence of metallic Mn, thus varying oxygen partial pressure p(O2) with temperature, homogeneity range of the spinel phase is narrow and its stoichiometry is close to MnAl2O4.According to crystallographic data the spinel phase is normal at low temperature with tetrahedral sites occupied by Mn 2+ cations, but with temperature increase degree of inversion (fraction of Al +3 in tetrahedral site) increases.According to calculations, which agree with experimental results, the homogeneity range of spinel extends with increase of oxygen partial pressure and at air condition (p(O2) = 0.21 bar) homogeneity range of cubic spinel extends from Mn3O4 to MnAl2O4 (at high temperature).It should be noted that Mn3O4 spinel forming from Mn2O3 by oxygen release has tetragonal structure.The tetrahedral sites are occupied by Mn +2 and octahedral by Mn +3 .
Phase diagrams of other important systems containing the spinel phase, like MgO-MnOx and MgO-Al2O3 systems are also shown in Figure 3d/e, respectively.Thermodynamic parameters obtained by [26] and [27] were applied to calculate the phase relationships for these binary systems.It should be noted that spinel with tetragonal and cubic structures were found in the MgO-MnOx system.Tetrahedral spinel has homogeneity range from Mn3O4 to MgMn2O4, while cubic spinel extends from Mn3O4 to Mg2MnO4.[15,23,26,27].
The gel-combustion method was used to prepare the precursors.For this purpose, common procedures [28,29] were adapted to the experimental setup in the laboratory.The method has also been described by Schnickmann et al. [9].The gel-combustion product was thermally treated with NH₄NO₃ in quartz crucibles up to 480 °C (10 °K/min; Nabertherm LE 1/11/R7, Nabertherm GmbH, Lilienthal, Germany) to remove the residual carbon.In Table 1 the elemental composition of the precursors can be found.[12] and Schnickmann et al. [9] was adapted for this process (Figure 4).During the entire melting experiment, the oxygen supply was 200 l/h.

Characterization techniques
The mineralogical composition of the slags was quantitatively and qualitatively identified using powder X-ray diffraction (PXRD and electron probe microanalysis (EPMA).
The elemental composition of single grains/crystals in the prepared thin sections were determined with EPMA (Cameca SXFIVE FE Field Emission, CAMECA SAS, Gennevilliers Cedex, France) using the Kα lines (Mg, Al, Si, Ca, Mn).For the measurement (15 kV beam diameter 100 -600 nm; Schottky type [32]), the device was calibrated beforehand with certified reference materials (CRM: P&H Developments Ltd; Glossop, Derbyshire, UK and Astimex Standards Ltd; Toronto, ON, Canada).The measured intensities of the emitted X-rays were evaluated using the X-PHI model [33].With the chosen method, the Li content cannot be analyzed quantitatively with the required precision.Therefore the element concentration of Li was determined using virtual compounds according to T. Schirmer [34].
In order to validate the measurement method and obtain consistent results, repeated measurements were carried out, also on different days, using the international standard rhodonite (MnSiO3; Astimex).The low standard deviation (0.03) of Mn indicates that this element content can be analyzed very precisely and is suitable for calculating the Li content via virtual compounds (Table 2).ICDD PDF2 00-024-0734) and the Li-silicate Li2SiO3 (ICDD PDF2: 00-029-0828) is conceivable.
However, the presence of the two phases cannot be clearly verified with this method due to line overlaps.
Schnickmann et al. [9] have already discovered that within the Li-manganate LiMnO2 the elements Li and Mn can replace each other, which can result in a shifting to smaller or larger lattice parameters.The same applies to the replacement of Mn 2+ in Ca-silicate matrix.An exchange of elements in the crystal lattice is only possible as long as the exchangeable elements have approximately the same ionic radii (e.g.[35]).An overview of the recorded diffractograms of the three slags is given in Figure 5.

Li-manganates
Two different Li-manganates (Li2Mn 4+ O3 and LiMn 3+ O2) were found in the slag samples.Li2MnO3 was only detected in slag 2 with EPMA, were 2 wt.%Mg was added.This phase theoretically contains 47.03 wt.% Mn and 11.88 wt.% Li.Within this phase, the average measured Mn content was 47.08 wt.% (min.: 46.88 wt.%; max.: 47.28 wt.%) (Figure 7).In addition, this phase forms idiomorphic crystals between 30 and 100 µm.Based on the crystal shape, it can be determined that Li2MnO3 forms early during solidification.Point measurements and recorded linescans within this phase show that no other elements (e.g.Mg, Ca, Si) have been incorporated.According to the linescan results, the Mn content within the crystal is constant (Figure 8).8).Slag 3 contains 0.83 wt.% Al, which results in a lower Mn concentration (Table 3).The results of the linescans shown that the Mn content within the individual grains is almost homogeneous.In addition, no decrease in element concentration is observed towards the grain boundaries (Figure 9).Table 3: For the two Li-manganates LiMnO2 and Li2MnO3, the average structural formula was calculated from all measuring points.To clarify the possible exchange of Li and Mn ions in the crystal lattice, a structural formula was also calculated for the lowest and highest Mn concentration according to: Li(1-x)Mn(1+0.33x)O2 / Li(1+x)Mn(1-0.33x)O2 and Li(2-x)Mn(1+0.33x)O3 / Li(2+x)Mn(1-0.33x)O3.

Hausmannite and spinel
Lithium-rich hausmannite was found in all three slag samples.Hausmannite consists of a mixture of Mn 2+ and Mn 3+ occupying tetrahedral and octahedral sites, respectively.When Mn +2 is substituted by Li +1 , Mn +4 can appear in the octahedral sublattice to balance the charge without change of oxygen stoichiometry.However, there is not enough crystallographic data for site occupancies of delithiated hausmannite.Additionally, tetragonal Mn3O4 and orthorhombic LiMnO2 are intergrown together.With an average crystal size of 10-30 µm, these crystals are significantly smaller than LiMnO2.In all three slag samples, the incorporation of other elements into the crystal lattice was detected.The incorporation of Li was recorded in all slag samples.The largest amount of Li was incorporated into S1 with 1.59 wt.%, followed by S2 with 0.80 wt.% and S3 with 0.51 wt.%.In addition, an average incorporation of 0.84 wt.% Mg and 0.82 wt.% Al was observed in S3.For the calculation of the spinel solid solution in S3 the following virtual compounds were defined: Mn3O4, MnAl2O4, MgAl2O4 and Li2Mn2O4.This solid solution can be expressed by the following generalized stoichiometric formula: (Li(2x),Mg(x),Mn(1-2x))1+x(Al(2-z),Mn 3+ (z))2O4 and for Li-rich hausmannite in S1 and S2 these following generalized stoichiometric formula: (Li(2x)Mn((1-x))1+x(Mn 3+ )2O4 was applied (Table 4).

Matrix forming phases
The main matrix forming mineral is wollastonite (CaSiO3) followed by rankinite (Ca3Si2O7) in S1 and S2 and larnite (Ca2SiO4) in S3.All three phases form xenomorphic crystals, which indicates crystallization at the end of solidification.Due to approximately equal ionic radii of Mg 2+ , Ca 2+ and Mn 2+ , Ca can be replaced by Mg and Mn in the crystal lattice [35].In S1 0.85 wt.% Mn was incorporated in wollastonite and 1.28 wt.% in rankinite, in S2 0.59 wt.% Mn in wollastonite and 2.22 wt.% in rankinite and in S3 0.75 wt.% Mn in wollastonite and 6.97 wt.% Mn and 0.76 wt.% Mg in larnite (Table 5).

Discussion
The aim of this study was to investigate the stabilization of Mn 4+ in the slag and to verify also which Lirich phases are formed.Moreover, it should be investigated whether Mn 4+ forms a compound with Mg or Al.Based on the phase diagrams, the experiment provides an overview of which phases crystallize in a synthetic slag consisting of Li2O, MgO, Al2O3, SiO2, CaO and MnOx.

Influence of Mg and Al on Mn 4+
As can be seen from Figure 3e, the spinel phase in the MgO-Al2O3 system is stable phase from low temperatures and has an extension towards both Al2O3 and MgO concentrations at high temperatures.
The addition of MgO together with Al2O3 will increase stability range of cubic spinel in the Li2O-MnOx-MgO-Al2O3 system.
The tetragonal-to-cubic transformations for the spinel phase in the MgO-MnOx and Al2O3-MnOx systems are driven by the Jahn-Teller distortion of octahedral sites, primarily occupied by Mn 3+ ions.In the cubic phase, assuming a disproportionation of Mn 3+ into Mn 4+ and Mn 2+ and occupation the octahedral sites by Mn +4 and distribution of Mn +2 between tetrahedral and octahedral sites, a general formula for cubic spinel (Al 3+ , Mn 2+ )1 (Al 3+ , Mn 3+ , Mn 2+ , Mn 4+ ,Va)2 (Mn 2+ , Va)2 (O 2-)4 is employed in Al2O3-MnOx system and (Mg 2+ , Mn 2+ )1 (Mg 2+ , Mn 3+ , Mn 2+ , Mn 4+ ,Va)2 (Mg 2+ , Mn 2+ , Va)2 (O 2-)4 in MgO-MnOx system.This formulation accommodates the variation in oxidation states of Mn within the crystal lattice.The cubic spinel phase is further characterized by the inclusion of vacancies (Va) on the octahedral sites and Mn +2 /Mg +2 in interstitial site.The vacancies are introduced into the spinel model to extend the homogeneity ranges towards Al2O3 side and interstitial site to model extension towards MnOx/MgO side.The applied models account for deviations from a perfectly ordered structure to describe disordering in cationic sites and to describe homogeneity ranges of spinel thus enhancing the model applicability in a large range of conditions (compositions, temperature, oxygen partial pressure).
Modelling of tetragonal spinel differs from cubic by introducing Mn +3 into tetrahedral site and by absence of Mn +4 in octahedral site in both systems.Tetragonal phase is stable at lower temperatures and in more narrow composition range than cubic spinel.
The octahedral sites are substantially occupied by Mn 4+ in cubic spinel in the MgO-MnOx system in the whole possible composition range.Low concentration of Mn 4+ is found in the spinel phase with compositions close to stoichiometric MnAl2O4 in the Al2O3-MnOx system even at high oxygen partial pressure.Thus, the introduction of Mg into the spinel phase not only enhances the stability but also effectively increase the Mn 4+ concentration.This suggests a more pronounced role of Mg in maintaining the desired Mn oxidation state within the spinel structure.
The Li addition introduces a notable impact on the Mn 4+ ion concentration on the octahedral sites.
Substitution of Mn +2 for Li 1+ in the tetrahedral sites reduces the charge of cations in spinel.In order to preserve the electroneutrality of this structure the fraction of Mn +4 in octahedral sites occupied by Mn +3 in tetragonal spinel should be increased.The increase of Mn +4 in octahedral site should also reduce Jahn-Teller effect caused by electronic structure of Mn +3 cation.It should be noted that in the stoichiometric spinel LiMn2O4 cation Li +1 completely occupies tetrahedral site and the ratio of Mn +4 /Mn +3 in octahedral site is equal to one.In case of Li content is higher than in stoichiometric spinel, Li +1 partially occupies octahedral site and ratio Mn +4 /Mn +3 becomes larger than one.With the temperature increase spinel becomes enriched by Mn with the shift of composition to stoichiometric spinel LiMn2O4 and even higher content of Mn.Therefore, spinel with high Li content stable at lower temperature, with the temperature increase will decompose to spinel with lower Li content and Li2MnO3 phase (Figure 2).At temperature 1230 K LiMnO2 forms from spinel and Li2MnO3 and at temperatures 1250 K, Li2MnO3 decomposes into LiMnO2 and Li2O.Tetragonal spinel with composition close to stoichiometric one is stable in equilibrium with hausmannite with much lower Li content from one side and from other side it is in equilibrium with LiMnO2.High temperature stability limit of tetragonal spinel is defined by decomposition to hausmannite and LiMnO2 at 1270 K.
According to the phase diagram shown in Figure 3b, no reactions should take place between MnO2 and Al2O3 up to 688 K.The experimental results indicate that Mg and Al react to form spinel (Mg2AlO4) and therefore small amounts of these elements are not a problem for Li-manganate (LiMnO2 or Li2MnO3) forming processes.As long as Mn 2+ and Mn 3+ are present in the slag, spinels with Mg, Al and Mn are formed, which can interfere with the formation of EnAMs.In order to produce large and pure EnAM crystals that can also be efficiently separated from the rest of the slag, it must be ensured that LiMnO2 or Li2MnO3 crystallize out of the melt first.

Li-rich phases as a new potential EnAM
A total of four Li-containing phases crystallized out in the slag.These phases include the two Limanganates LiMnO2 and Li2MnO3, as well as the Li-silicate Li2SiO3 and Li-containing hausmannite.
With an average of 0.51 wt.% (S3) to 1.59 wt.% Li (S1), the Li content in hausmannite is too low to be a potential EnAM.The Li +1 is incorporated at the tetrahedral site and replaces Mn 2+ in the crystal lattice.
The incorporation of Li +1 into cubic LiMn2O4 can occur by balancing Mn 4+ on the octahedral site, but it also can occur by deviation from stoichiometry of oxygen by introducing vacancies [36][37][38].There is not enough crystallographic data for hausmannite showing how substitution of Mn +2 by Li +1 is compensated.Since it is not possible to detect small amounts of Mn 4+ with EPMA, more advance technique should be used for further investigated whether Mn 4+ can be incorporated into the crystal lattice of hausmannite for charge compensation.Hausmannite forms in addition relatively tiny crystals and only occurs in small amounts.Due to the partially inhomogeneous mixture of Mn 2+ and Mn 3+ in this phase, further processing would be necessary after successful separation from the slag.With an average of 15.43 wt.% Li, Li2SiO3 would be more suitable as a new EnAM.However, this phase crystallizes towards the end of solidification and thus forms relatively small and needle-shaped crystals, which is an inhibitor for a successful separation of Li.Furthermore, Li and Si must be separated from each other after the recycling step.Therefore, an Li-manganate like LiMnO2 or Li2MnO3 would be more suitable as new potential EnAMs.The Li-manganate Li2MnO3, with 11.88 wt.% Li, could only be found in slag 2, where 2 wt.%Mg was added.No incorporation of other elements could be observed within this phase.
As shown in Figure 2, the Li2MnO3 phase is stoichiometric which corroborates with the absence of solubility of other elements.Furthermore, the Li2MnO3 phase has some advantages as a new EnAM.
Firstly, it is an early crystallite with large, idiomorphic crystals and containing more Li compared to LiAlO2 (10.35 wt.% Li).The stabilization of only Mn 4+ in the slag has the additional advantage that spinel formation with Mn could be suppressed and that the Jahn-Teller effect does not cause deformation.In addition, LiMnO2 (7.39 wt.% Li) can also be considered as a potential EnAM.The low incorporation of Al (0.83 wt.%) does not seem to have any effect on the crystal lattice.According to Kong et al. [16], small amounts of Al in the Li-rich cathode material should promote intralayer diffusion.
Nevertheless, it remains to be determined which of the two Li manganates, LiMnO2 or Li2MnO3, formed first.It is possible that LiMnO2 forms first as a high temperature phase and remains stable during cooling.At low temperatures, according to phase diagram the stable phases are spinel and Li2MnO3.The alternative would be that Li2MnO3 forms first and decomposes to LiMnO2 and Li2O, which reacts with SiO2 forming Li2SiO3.

Conclusions and outlook
The presented results indicate that it was possible to stabilize the Mn 4+ containing Li-manganate Li2MnO3 in a synthetic slag.However, the conditions under which Li2MnO3 was formed and how Mn 4+ was stabilized throughout the slag remain to be clarified.A promising approach would be to investigate phase relations (experimentally and theoretically) and to develop thermodynamic databases for a complex system (e.g. a slag system consisting of several elements such as Li, Mg, Al, Si, Ca, Mn, O).
The interpretation of these data could help to understand the processes taking place during solidification.
This could help to optimize the slag system and the cooling curves to maximize the EnAM yield.
Both Li-manganates, Li2MnO3 and LiMnO2 have proven to be extremely pure phases.Accordingly, the added oxygen only had an influence on the regions close to the surface, where Li2MnO3 could find.
Therefore, the viscosity of the slag should therefore be reduced in ongoing experiments so that the oxygen can influence the entire slag in subsequent experiments.To achieve this, small amounts of FeO and Fe2O3 will be added to the precursors (e.g.[39]).With the help of the added iron, more oxygen should enter the slag and mainly Mn 4+ should be stabilized.In addition, more investigations are necessary to verify whether Mn 4+ forms a spinel-like compound with Al or whether the formation of spinels could be suppressed by oxidizing Mn to Mn 4+ .This includes determining at what wt.% Mg and Al inhibit the formation of Li-manganates.In addition, it must be determined how much Mn 4+ can be incorporated into the crystal lattice of the spinels and which phases crystallize first on solidification of the slag -spinels between Mg 2+ , Al 3+ , Mn 2+ and Mn 3+ (as well as Mn 4+ ) or Li-manganates.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figure 1 :
Figure 1: Sketch of the EnAM strategy for efficient Li recovery.The aim is to form a Li-manganate as an early crystallizate from the liquid melt.The other elements (Mg, Al, Si, Ca) should form matrix-forming phases.This early crystallizate (Li-Manganate) should be efficiently separated from the matrix.

Figure 2 :
Figure 2: Adapted phase diagram of the Li2O-MnOx system at air based on experimental data obtained from Paulsen and Dahn [17].

Figure 4 :
Figure 4: used heating program for the experiments.Slow heating of the precursor (2.90 °C/min; 1.49 °C/min) should minimize the loss of Li.By cooling down more slowly, the phases should have enough time to form crystals that are as large as possible.During the entire melting experiment, the oxygen supply was 200 l/h.

Figure 7 :
Figure 7: Overview of the measured Mn concentrations within the Li-manganates in the three slags.The dotted lines indicate the Mn concentration of the pure stoichiometric compounds.In all three slag samples the phase LiMnO2 was found, Li2MnO3 only in slag 2 (2.1).The shown results based on point measurements in different crystals of the respective phase.Measurements on the reference material rhodonite demonstrate that the Mn content can be measured with an accuracy of ± 0.03 %.X marks the mean value.

Figure 8 :
Figure 8: Elemental composition of matrix (CaSiO3) and the two Li-manganates (Li2MnO3) and (LiMnO2) in slag 2. a) The linescan results shown that minimal amounts of Mn 2+ were incorporated into the matrix.Furthermore, there is no decrease or increase in the element distribution towards the grain boundaries.b) BSE(Z) image showing the path of the line scan (A: start, B: end, step size: 5.3 µm).

Figure 9 :
Figure 9: Elemental composition of the matrix (CaSiO3) and Li-manganate(III) (LiMnO2) in slag 2. a) The linescan results shown that minimal amounts of Mn 2+ were incorporated into the matrix.Furthermore, there is no decrease or increase in the element distribution towards the grain boundaries.b) BSE(Z) image showing the path of the line scan (A: start, B: end, step size: 4.4 µm).

Table 2 :
The CRM rhodonite was used to verify the measurement accuracy of Mn.The Mn content can be determined with an inaccuracy of ± 0.03 %.Indication in wt.%.*The iron content was not measured.

Table 4 :
Determined structural formula of lithiated hausmannite slag in 1,2 and 3.The Li-silicate Li2SiO3 was only found in S1 and S2.This phase forms 20 to 50 µm rounded, elongated to needle-shaped crystals.The hypidiomorphic to xenomorphic crystal shape indicate crystallization towards the end of solidification.Ideal crystals of this phase contain an average Si content of 31.22 wt.% and an average Li content of 15.43 wt.%.In S1 the average Si content is 31.66wt.% and in S2 31.68 wt.%.A higher Si content indicate a lower Li content.