Structural, Thermal, and Electrochemical Studies of Novel Li2CoxMn1

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Structural, Thermal and Electrochemical Studies of Novel Li2CoxMn1-x(SO4)2 Bi-metallic Sulphates Aravind Muthiah, Tom Baikie, Mani Ulaganathan, Mark P Copley, Guang Yang, Vanchiappan Aravindan, and Srinivasan Madhavi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08203 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017

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The Journal of Physical Chemistry

Structural, Thermal and Electrochemical Studies of Novel Li2CoxMn1-x(SO4)2 Bi-metallic Sulphates Aravind Muthiah1,2,*, Tom Baikie1, Mani Ulaganathan2, Mark Copley3, Guang Yang2, Vanchiappan Aravindan4*, and Srinivasan Madhavi 1,2* 1

Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, Research

Techno Plaza, 50 Nanyang Drive, Singapore 637553 2

School of Materials Science and Engineering, Nanyang Technological University, Singapore,

639798 3

Johnson Matthey Technology Centre, Reading, RG4 9NH, UK

4

Department of Chemistry, Indian Institute of Science Education and Research (IISER),

Tirupati-517507, India

ACS Paragon Plus Environment

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ABSTRACT

A novel high-voltage solid solution of Li2CoxMn1-x(SO4)2 (x=0 to 1) was synthesized using simple solid state reaction. Single phase monoclinic crystal structure of Li2AxB1-x(SO4)2 was identified and there was no evidence for the presence of orthorhombic polymorph at a sample preparation temperature of 450 ˚C. Interestingly, the bi-metallic sulphates were found to be thermally stable upto 550 ˚C and it was validated through various characterization techniques. Beyond 550 ˚C, the materials start to decompose and form Li2SO4 and α-CoSO4 in the case of Li2Co(SO4)2 while Li2Mn(SO4)2 decomposes to form Li2Mn2(SO4)3. In addition, an important property called sensitivity towards moisture was also characterized which plays a critical role in the handling of sulphate based cathode during the cell fabrication. Thus, the reaction mechanism involved during the absorption of the atmospheric moisture was also investigated carefully. Finally, for the first time electrochemical activity of both Co and Mn systems was discovered where the average redox potential was found to be 5.02 and 4.85 V vs. Li, respectively. For a purely Co or Mn based system, this is the highest voltage reported and is promising for sulphate based polyanionic systems.


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Introduction The discovery of LiFePO4, a positive electrode material that allowed for safe handling and

operation1-2 heralded research into polyanionic cathodes as viable candidate for lithium ion batteries and has resulted in intensive research in these type of compounds over the past two decades3. One of the major issues with these compounds is the additional weight of the polyanionic group3 that reduces the specific capacity which directly affects the energy density of the system. Since the energy density is linearly linked with the operating potential of the cathode, and it is important to increase the operating potential window of such a compound for further enhancement in the cell characteristics. In turn, the potential window of battery materials is mainly based on the redox reaction involved in the compounds during the cell operation. The redox potential in polyanionic systems is governed primarily by the iono-covalency of the M-O bond (M - transition metal) where the higher the ionicity of the bond, greater is the system’s redox potential4. The O-ion component of the XO4n- (X = P, S, W, Si) polyanionic group is subjected to an inductive effect based on the electronegative character of the X ion5-6. A higher electronegativity results in a higher redox potential and this has prompted the intense research activity into sulphate based cathodes 4, 7-8. Among the sulphate-based positive-electrodes, sulphates synthesized through solid-state reactions have shown highest redox potentials for Fe at 3.83 V vs. Li 9, second, only to the ionothermally synthesized fluorosulphate version which shows 3.90 V vs. Li followed by synthesis of the Co, Mn and Ni based compounds

11-12

10

. This was

. Previous reports

9, 11, 13-16

have purely focused on Fe based sulphates as they have been unable to uncover the electrochemical activity of other transition elements except Co in LiCoSO4OH at 4.8 V vs. Li 17. However, the atomistic modelling/Density Functional Theory (DFT) techniques of the


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Li2M(SO4)2 class of sulphates have predicted the redox potential to be 5.2 V vs. Li for Co and 4.54 V vs. Li for Mn18. Redox activity of Co is beyond the stability window of conventional electrolytes and hence has not been tested earlier, however Mn which lies well within the stability window was found to be electrochemically inactive4. The Mn dissolution from the crystal lattice in the Mn based compounds has been pointed to as the major issue for its inactivity19. Substitution with Co would certainly alleviate such a Mn dissolution issue. Hence, in this report we mainly focus on developing a novel solid-solution series of Li2CoxMn1-x(SO4)2 (x= 0 to 1) high voltage cathode and detail its structural, thermal and moisture sensitivity properties. In addition, we report for the first time, the electrochemical activity of Mn and Co in a sulphate based cathode. Through a unique combination of high voltage electrolyte based on sebaconitrile 20

and material modification using high energy ball milling, we have demonstrated the activity of

purely Co and purely Mn redox couples at 5.02 and 4.85 V vs. Li in sulphate based cathodes. 2 Materials and Methods 2.1 Synthesis Li2SO4.H2O (99%), CoSO4.7H2O and MnSO4.H2O used to prepare the anhydrous phases were purchased from Sigma Aldrich and used without further purification. Anhydrous phases were prepared by heating the compound in an Argon atmosphere in a tube furnace for 1 h. Li2SO4.H2O was dehydrated at 200 °C, MnSO4.H2O at 300 °C and CoSO4.7H2O at 350 °C. To prepare a pure phase of Li2CoxMn1-x(SO4)2 (x= 0 to 1), stoichiometric ratios of CoSO4, MnSO4 and Li2SO4 were subjected to a solid-state synthesis procedure reported earlier9. The given mol. fraction/percentage of the precursors were mixed with few drops of absolute ethanol and ball milled in a SPEX 8000M high energy ball mill for 24 minutes in air. The resultant powder was then collected and pressed into a 20 mm diameter pellet at a pressure of 8 kN followed by


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annealing the pellet at 450 °C for 12 h with a temperature ramping of 5 °C min.−1 in a box furnace. 2.2 Structural Analysis

Structural characteristics of the as synthesized samples were analyzed by using an X'Pert Powder Panalytical X-Ray Diffraction (XRD) analyzer, equipped with Cu-Kα radiation operated at 40 kV and 30 mA. The data was recorded over the range of 2θ from 10° to 80° at 1.147 °min.−1. Elemental analysis was carried out using Energy Dispersive X-ray (EDX) spectroscopy using a Field Emission Scanning Electron Microscope FE-SEM (Jeol 6340F, Japan). 2.3 Thermal Analysis

Further, the as prepared samples such as Li2Co(SO4)2, Li2Mn(SO4)2, and Li2Co0.5Mn0.5(SO4)2 were subjected to thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) to determine the thermal stability of the compounds. Thermal studies were also coupled with Mass Spectroscopy (MS) to analyze the gas species evolution with respect to the heating temperature. The TGA and MS experiments were carried out under Argon flow (50 ml.min.−1) with ~15-20 mg of powder samples over the temperature range from ambient temperature to 800 °C at a heating rate of 10 °C.min.−1. 2.4 Moisture Sensitivity Analysis

Synthesized samples were subjected to prolonged exposure to a humid atmosphere to determine the effect of atmospheric moisture on the structural stability. The samples were placed in a humidity controlled chamber with a controlled relative humidity of 45-60% over a period of 36 days and subsequently XRD tests were conducted on the sample to track the extent of degradation.


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2.5 Electrochemical Analysis 2.5.1

Electrolyte Preparation

The electrolyte for this work was tested up to 5.6 V vs. Li to explore the operating window of Co outlined as 5.2 V vs. Li in computational studies 18. The most feasible candidate for such a range with minimal modification to commercially used electrolyte components was found to be a cosolvent called sebaconitrile. Sebaconitrile has previously20 been shown to exhibit good electrochemical stability beyond 6 V vs. Li in an EC:DMC (ethylene carbonate: dimethyl carbonate) based electrolyte with a glassy carbon electrode and demonstrated its use with polyanionic cathode Li2NiPO4F. Sebaconitrile was later used to assess performance of another Co based polyanionic cathode, LiCoPO4 and it was found to lead to higher insertion potential of lithium owing to slower kinetics due to the sebaconitrile co-solvent 21. The viability of its use with polyanionic systems therefore made sebaconitrile a suitable candidate to trial with sulphate based cathodes as well. The electrolytes synthesized were of 3 concentrations of increasing sebaconitrile. The composition used was a 1M solution LiPF6 in EC:DMC:sebaconitrile of varying volume percentages; 25:25:50, 15:15:70 and 5:5:90. In addition LiBF4 salt was also tried, but did not show any electrochemical activity and the reasons for this are still being studied. 2.5.2

Electrode Preparation, Assembly and Testing

The electrochemical tests were carried out in a half-cell configuration using Swagelok© cell architecture. The high voltage cathode Li2Mn(SO4)2 was taken with Super P Carbon in a weight ratio of 4:1 and ball milled in a high-energy ball mill (SPEX 8000M) for 20 h with a cooling period of 10 min every hour. The milling was done in a tungsten carbide vial as the usage of stainless steel leaked Ni contaminant into the sample. The carbon coated Li2Mn(SO4)2 powder


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was then directly placed in a Swagelok© cell as positive working electrode and lithium foil as the counter and reference electrode. The loading of the active material was 4 mg.cm-2. Two pieces of porous glass microfibre (Whatman Cat. No. 1825–047, UK) were used as the separator and soaked in electrolyte solution. This assembled cell was then subjected to cyclic voltammetry tests using a Solartron Analytical. The tests were carried out at a scan rate of 0.05 mV s-1 between a range of 4.2 and 5.2 V for Li2Mn(SO4)2 while a range of 4.2 and 5.6 V was used for Li2Co(SO4)2. 3

Results and Discussion

3.1 Structural Characterization X-ray diffraction (XRD) patterns of the as prepared Li2CoxMn1-x(SO4)2 samples are shown in Figure 1. The entire series shows the single phase monoclinic crystal structure and there was no evidence of the presence of orthorhombic polymorph as the synthesis temperature exceeded above 400 OC 16.

Figure 1: Left - Powder XRD patterns of Li2CoxMn1-x(SO4)2 solid solution series. The substitution of larger Mn2+ ions by smaller Co2+ ions results in a shift of the Bragg reflections toward higher diffraction angle, indicating a reduction in cell volume. Right Top - Li2Mn(SO4)2,


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Right Bottom - Li2Co(SO4)2 – both having the same monoclinic crystal structure where Green – Lithium ions, Beige – MnO6 octahedra, Magenta - CoO6 octahedra, Yellow – SO4 Tetrahedra A shift towards larger diffraction angles is observed with increasing Co substitution (mole fraction x varying from 0 to 1) in the final composition of the synthesized powders. This indicates a reduction in cell volume as plotted in Figure 2. This trend is expected due to the substitution of the larger Mn2+ ion (Ionic Radius = 0.75 Å)22 by the smaller Co2+ ion (Ionic Radius = 0.65 Å)22 in the presence of high spin ligand SO42-. Thus, the linear reduction in cell parameters with increasing Co substitution in the synthesis is also indicated in the Figure 2. Since, all the three parameters a, b and c obtained from this study show the similar trend, the variation in the crystal structure properties could be considered isotropic.

ß (°)

121.8 121.6 121.4 121.2

a (Å)

4.99 4.98 4.97

b (Å)

8.30 8.20

c (Å)

8.10 8.87

Cell Volume (Å3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8.84 8.81 8.78 370 365 360 355 0.0

0.2

0.4

0.6

0.8

1.0

Atomic fraction of Co (x)


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Figure 2: Variation of the lattice parameters, a, b, c, β, and unit cell volume V with x in Li2CoxMn1x(SO4)2 active materials powders

In addition to XRD analysis, the successful substitution and nominal compositions of the samples were confirmed by Energy-dispersive X-ray spectroscopy (EDX) measurements ( Table 1). The substitution was consistent and in accordance with the stoichiometry used during synthesis process. The confirmation of the successful synthesis of the series and structural characterization was then followed by thermal stability analysis. Table 1: EDX measurements showing the relative percentage concentration of the respective elements Sample

Li2Mn(SO4)2 Li2Co0.1Mn0.9(SO4)2 Li2Co0.3Mn0.7(SO4)2 Li2Co0.5Mn0.5(SO4)2 Li2Co0.7Mn0.3(SO4)2 Li2Co0.9Mn0.1(SO4)2 Li2Co(SO4)2 3.2 Thermal Stability

Co %

Mn %

2.3 6.6 10.6 15.2 18.8 21.0

20.1 18.5 14.4 10.5 6.1 2.1 -

Experimental Theoretical Ratio Ratio Co: Mn Co: Mn 0:1 0:1 1:8 1:9 3:6.6 3:7 1:1 1:1 7.5:3 7:3 9:1 9:1 1:0 1:0

Previous work16 on this class of bi-metallic sulphates focused on the thermal stability below 500 °C. Very first time, high temperature tolerance (>500 °C) of such class of materials was investigated which gives clear visibility about thermal stability of the samples. These obtained results were in a much higher range than the previously reported work 16. The as prepared Li2Co(SO4)2, Li2Mn(SO4)2, and Li2Co0.5Mn0.5(SO4)2 samples were subjected to simultaneous TGA, DSC and MS tests. The obtained results are shown in Figure 3 and are found to be similar for all three materials, and can possibly be attributed to their similar crystal structures. The TGA curve of all the samples showed a weight loss only after 650, 700 and 750 °C for Li2Co(SO4)2, Li2Co0.5Mn0.5(SO4), and Li2Mn(SO4)2, respectively. This indicates that


9


higher Mn concentration in the system prevents decomposition products from being released at lower temperatures.

DSC

H2 O

SO2

90 Li2Co(SO4)2

90 Li2Co0.5Mn0.5(SO4)2

30

80 100

0 60 Li2Mn(SO4)2

30 0

250

350 450 550 650 Temperature (OC)

Heat Flow (mW)

0 60

80 150

2.5

30

80 100

90

5.0

60

0.0 5.0 2.5 0.0 5.0

Ion Current (10-10A)

TGA 100 Normalized Mass (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.5 0.0

750

Figure 3: Thermal stability analysis of the prepared samples using TGA-DSC-MS

The DSC curve showed three peaks between 550 and 650 °C in all three systems; however, the coupled MS did not reveal any release of gas in this temperature range and thus a phase change of the materials was suspected. In order to investigate this further, a sample of Li2Co(SO4)2 was chosen as a representative crystal structure and subjected to in-situ XRD under a reduced pressure. During the in-situ XRD study, an amorphous phase was obtained beyond 450 °C specifically when heated under a reduced pressure. Hence, to avoid the formation of an amorphous phase, a traditional setup of ex-situ XRD was used. Here, the sample was first heated in a box furnace, cooled to room temperature and then finally subjected to powder XRD. The heating was carried out in stages of 25 °C according to the critical range indicated in the DSC pattern from 550 to 650 °C. The resulting XRD pattern showed the evolution of multiple phases


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as

seen

in

Figure 4.

x - α-CoSO4 | -Li2SO4 Intensity (Arb. Units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O

650 C O

625 C O

600 C O

575 C O

550 C 15

20

25

30

35

40

45

50

55

2θ (degrees)

Figure 4: Thermally induced structural changes in Li2Co(SO4)2 when heated between 550 °C and 650 °C The XRD pattern of the phase finally formed at 650 °C was subjected to Rietveld refinement and yielded the formation of two phases, anhydrous Li2SO4 and α-CoSO4. In Figure 5, comparison of the XRD pattern of the anhydrous CoSO4 precursor with the reference patterns of α and β-CoSO4 obtained from literature23-24 shows that the precursor is β-CoSO4.


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α-CoSO4

Intensity (Arb. Units)

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β-CoSO4

CoSO4 precursor

15

20

25

30

35

40

45

50

55

60

65

70

2θ (degrees)

Figure 5: Comparison of XRD pattern of precursor to reference patterns of α-CoSO4 (Pnma)23 and βCoSO4 (Cmcm)24

The α-CoSO4 (space group Pnma) is structurally different from the β-CoSO4 (space group Cmcm) as shown in Figure 6. α-CoSO4 is isostructural to chalcocyanite 23 and is the more stable of the two polymorphs at temperatures above 432 °C

25

. Therefore, it is proposed that

while β-CoSO4 reacts with Li2SO4 to form Li2Co(SO4)2 at lower temperatures, at high temperature (550 °C) more stable α-CoSO4 is formed which does not recombine with the Li2SO4 upon cooling to reform Li2Co(SO4)2. Li SO + β − CoSO → Li Co(SO ) at 450 °C -------------(1) Li Co(SO ) → Li SO + α − CoSO at >550 °C -------------(2) As a result, the degradation of Li2Co(SO4)2 occurs at a much lower temperature than indicated by the TGA curve.


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Figure 6: Structural representations of the different polymorphs of CoSO4 modelled using Vesta software26 (Left - α-CoSO4 (Pnma) and Right - β-CoSO4 (Cmcm)) where magenta polyhedra represent the - CoO6 octahedra with the SO4 units shown as yellow tetrahedral .

The Li2Mn(SO4)2 sample on the other hand showed a different mechanism when subjected to temperatures between 550 and 625 °C. Here, the Li2Mn(SO4)2 decomposes into Li2Mn2(SO4)3 and Li2SO4 as indicated in Figure 7. This mechanism, like in the decomposition of Li2Co(SO4)2, does not involve any gas evolution and therefore the mass remains constant as indicated in the TGA plot in Figure 3. Reactions 3 and 4 detail the mechanism of decomposition. Li SO + MnSO → Li Mn(SO ) at 450 °C -------------(3) 2 Li Mn(SO ) → Li SO + Li Mn (SO ) at >550 °C -------------(4)


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Li2Mn2(SO4)3 Li2SO4

Intensity (Arb. Units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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O

625 C O

600 C O

575 C O

550 C 10

15

20

25

30

35

40

45

50

2θ (degrees)

Figure 7: Thermally induced structural changes in Li2Mn(SO4)2 when heated between 550 °C and 625 °C resulting in Li2Mn2(SO4)3 (ref. pattern 27) and Li2SO4 (ref. pattern 28)

Thus, it is seen that the end members of the solid solution series are thermally stable only below 550 °C. Since the other members of the solid solution series are isostructural and show similar DSC patterns, the maximum temperature of stability is applicable to the other materials as well.


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3.3 Sensitivity to Moisture

Sulphates are generally soluble in water4 and are susceptible to moisture during the various stages of materials synthesis, transport and battery fabrication. Thus, this segment sought to utilize a representative sample from the synthesized solid solution series and understand the mechanism of degradation. Li2Co(SO4)2 and Li2Mn(SO4)2 were chosen from the series and placed in a container with relative humidity levels between 45 – 50 % to mimic real world situations. The analysis using powder XRD tests showed extensive degradation of the material within 36 days (Figure 8).

Li2Co(SO4)2 Day 36 Intensity (Counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Li2Co(SO4)2 Day 0 Li2Mn(SO4)2 Day 36 Li2Mn(SO4)2 Day 0

15

20

25 2θ (degrees)

30

35

Figure 8: Moisture induced structural changes in Li2Co(SO4)2 and Li2Mn(SO4)2 in an atmosphere of 4550% Relative Humidity

The final set of degradation products formed gives an idea of the mechanism of degradation. A quantitative Rietveld refinement of the XRD data from a material with 36 days of exposure to a humid atmosphere yielded degradation products as detailed in Table 2 and Table 3.


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Table 2: Decomposition products of Li2Co(SO4)2 when exposure to relative humidity of 45-50 % quantitatively determined using Reitveld refinement of XRD patterns Compound

Li Co(SO ) 2

4 2

Percentage Weight Composition after exposure to humidity 0 days 36 days 100 % -

Li2SO4.H2O

-

39 %

CoSO .H O

-

16 %

CoSO4.6H2O

-

45 %

4

2

Table 3: Decomposition products of Li2Mn(SO4)2 when exposure to relative humidity of 45-50 % quantitatively determined using Reitveld refinement of XRD patterns Compound

Li2Mn(SO4)2

Percentage Weight Composition after exposure to humidity 0 days 36 days 100% 17 %

Li2SO4.H2O

-

33 %

MnSO4.H2O

-

50 %

The initially synthesized Li2Co(SO4)2 was completely reduced to its hydrated precursors phases. This indicates stringent anhydrous protocols need to be adopted while handling sulphate based materials. The predominant degradation products were CoSO4.6H2O and Li2SO4.H2O while a significant proportion of monohydrate of CoSO4 indicates an intermediate stage before complete hydration. Therefore, the mechanism of hydration using a previous study29 on CoSO4.6H2O dehydration is prospected to be as follows: Li Co(SO ) + 2H O → Li SO . H O + CoSO . H O -------------(5) CoSO . H O + 5H O → CoSO . 6H O -------------(6)


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In the case of Li2Mn(SO4)2, the most stable hydrated form, MnSO4.H2O was formed along with Li2SO4.H2O as seen in Table 3. Li Mn(SO ) + 2H O → Li SO . H O + MnSO . H O -------------(7) Earlier reports

30-31

have shown the presence of tetra, penta- and hepta-hydrated manganese

sulphate. The reason that these higher states of hydration have not formed like in the case of the Co analogue can be attributed to the conditions of exposure to humidity. The temperature and relative humidity were maintained at 20-25 °C and 45-50 % respectively. It has been noted that for pentahydrates to form lower temperatures of nearly 15 °C with 98 % humidity is required 32. Therefore, at lower temperatures and higher humidity, perhaps the Li2Mn(SO4)2 could decompose to form higher hydrates as well. But in normal atmospheric conditions, the monohydrate is the most favored state as shown in Table 3. Hence, the Li2Mn(SO4)2 absorbs far less water per unit mass when compared to Li2Co(SO4)2 which can hold more water molecules owing to a favored hexahydrate state of the CoSO4. 3.4 Electrochemical Analysis 3.4.1

Cyclic Voltammetry (CV) of Li2Mn(SO4)2

The key step in realizing the electrochemical activity of the Mn analogues of this class of sulphates is the high-energy ball milling process. The as synthesized Li2Mn(SO4)2 was ball milled with Super P carbon for 20 h to prepare the electrode. The sample was analysed using XRD at different stages of ball milling to notice any change to the crystal structure. Figure 9 shows that the major peaks are all intact indicating the crystal structure of the system remains consistent. However, as expected, the peaks have all broadened due to the formation of smaller particle sizes with the continuous ball-milling33. This was further confirmed by scanning electron microscopy (SEM) images which showed that the particle agglomerates over 10 µm in particle


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size were eventually reduced to lesser than 10 µm (Figure 10). This was further accompanied by the formation of needle like structures visible at 20,000x magnification seen in C2, Figure 10. A deeper study of this morphology is yet to be undertaken. Thus, it was concluded that despite the prolonged ball milling over 20 h, the crystal structure was found to be intact.

Figure 9: Comparison of the Li2Mn(SO4)2 ball milled for 6,10 and 20 hrs showed peak broadening when compared to pristine and the reference pattern 12 of Li2Mn(SO4)2


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Figure 10: SEM images comparing the Li2Mn(SO4)2 morphology after ball milling for different duration under two magnifications where A1 - 6 h, 500x, A2 – 6 h, 20 Kx, B1 - 6 h, 500x, B2 – 6 h, 20 Kx, C1 - 6 h, 500x, C2 – 6 h, 20 Kx

Figure 11-13 show the electrochemical activity of Mn in a sulphate based system (Li2Mn(SO4)2) for the first time through a reversible oxidation and reduction cycle consistent over 3 cycles in the voltage window of 4.2 and 5.2 V vs. Li counter electrode. Figure 11 shows the presence of an oxidation peak at 5.12 V and a reduction peak at 4.61 V for the first cycle thereby giving an average redox operating potential of approximately 4.85 V. The oxidation and reduction peaks progressively move toward 5.17 V and 4.60 V respectively at the end of the 3rd cycle indicating increasing polarization. Also of note is the significantly high current in the 1st cycle compared to the subsequent cycles indicating formation of a solid electrolyte interface. This is uniformly found across different concentrations of sebaconitrile in electrolyte. Another point of interest is at 4.73 V and 4.83 V, where small peaks indicate the presence of an alternate redox species. From earlier studies on monoclinic Li2Fe(SO4)2

9, 15-16

where similar artefacts

were observed in CV measurements of the cathode material and were attributed to an orthorhombic polymorph, it is possible that these peaks are due to the formation of a similar polymorph of Li2Mn(SO4)2


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during synthesis however this is still under investigation. As with the 50% sebaconitrile system, similar trends are observed across 3 cycles for varying concentration of sebaconitrle – 70% and 90% as seen in Figure 11 and Figure 12.

Figure 11: CV of Li2Mn(SO4)2 carried out with an electrolyte of 1M LiPF6 in EC:DMC:Sebaconitrile (25:25:50) vol.%


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Figure 12: CV of Li2Mn(SO4)2 carried out with an electrolyte of 1M LiPF6 in EC:DMC:Sebaconitrile (15:15:70) vol.%

Figure 13: CV of Li2Mn(SO4)2 carried out with an electrolyte of 1M LiPF6 in EC:DMC:Sebaconitrile (5:5:90) vol%

A comparison of the 3 concentrations of electrolytes over each cycle as shown in Figure 14 however brings out several interesting conclusions. Firstly, in the 1st cycle the system with 70% sebaconitrile has a higher oxidation peak as compared to 50% and 90% systems. This indicates a higher decomposition of electrolyte in the 70% system contrary to the expected higher decomposition in the 50% system which has a higher EC:DMC concentration which is prone to decomposition at high voltages. Secondly, a progressive shift in the voltage at which oxidation and reduction occurs is observed with the oxidation peak moving toward higher voltages while reduction peak moving toward lower voltages. This is a strong sign of polarization and it is observed that across all 3 cycles, the polarization in each cycle is highest for the 90% sebaconitrile system. Finally, comparison of the reduction peak current for the 3 different concentrations indicates that the 90% system shows the highest reduction. This corroborates the finding reported earlier 21


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that increased sebaconitrile content improves insertion/deinsertion efficiency due to the slowing down of kinetics by sebaconitrile.

Figure 14: Comparison of the CV of Li2Mn(SO4)2 for varying concentration of sebaconitrile from 50, 70 and 90 vol% 3.4.2

Cyclic Voltammetry of Li2Co(SO4)2

Figure 15 shows the electrochemical activity of Co in a sulphate based system (Li2Co(SO4)2) for the first time through a reversible oxidation and reduction cycle over 3 cycles in the voltage window of 4.2 and 5.6 V vs. Li counter electrode. In comparison to the Li2Mn(SO4)2 system in Figure 11, the most important observation is the voltage at which the oxidation and reduction occurs at 5.30 and 4.74 V vs. Li respectively, thereby giving an average redox potential of 5.02 V. This is the highest reported potential for a Co only cathode system. The requirement of


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extending the voltage window can also be understood as the high polarization in the system means that the oxidation takes place at 5.3 V and beyond.

Figure 15: CV of Li2Co(SO4)2 carried out with an electrolyte of 1M LiPF6 in EC:DMC:Sebaconitrile (25:25:50) vol%

Figure 15 also reveals the extensive decomposition of the electrolyte in the first cycle through the huge current intensity, going beyond 150 µA.cm-2. In comparison, Figure 15 shows that Li2Mn(SO4)2 showed current intensity much lower around 120 µA.cm-2. This is due to the extended voltage window going up to 5.6 V vs. Li. Also in comparison, the intensity of the reduction current is much lower close to 10 µA.cm-2. This reduction peak further is completely absent in the 3rd cycle indicating an irreversibility within the first 3 cycles. This indicates a need for further studies to modify the structure to fully realise the benefits of such a high voltage system. Nevertheless, the reversibility shown in the first 2 cycles, albeit with high inefficiency, is


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promising for utilizing such polyanionic sulphate based cathodes as high voltage cathode materials. 4

Conclusion

A novel single phase solid solution series of monoclinic Li2CoxMn1-x(SO4)2 (x = 0 to1) was successfully synthesized and the structural characteristics were confirmed using powder X-ray diffraction. Representative samples of the series were subjected to thermal stability and moisture sensitivity tests. It was found that this class of bi-metallic sulphates was found to be stable upto 550 °C after which the materials decompose into their constituent components in the form of Li2SO4 and α-CoSO4 in the case of Li2Co(SO4)2 while Li2Mn(SO4)2 forms Li2Mn2(SO4)3. The moisture sensitivity test yielded a pronounced impact of atmospheric moisture on the chemical stability of the material. Approximately, 90% of the original phase was degraded with hydrated sulphates identified as the major crystalline decomposition products during the handling of these materials. The moisture sensitivity of these materials makes it particularly challenging to realize the practical applications of such sulphate based battery electrode materials. Finally, for the very first time Mn and Co have been shown to be electrochemically active in a sulphate matrix. Their redox potentials at 4.85 V and 5.02 V, respectively, are the first examples of high voltage nearing 5 V in purely Mn and purely Co based systems. The activity demonstrated was achieved through a combination of high energy ball milling as well as addition of a suitable co-solvent, sebaconitrile, to extend the voltage window of conventional electrolytes. AUTHOR INFORMATION Corresponding Author


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*[email protected] (M.Aravind), [email protected] (V. Aravindan, to whom correspondence should be addressed) and [email protected] (S. Madhavi). ACKNOWLEDGMENT Johnson Matthey and Nanyang Technological University are thanked for their financial support. VA thank the financial support from the Science & Engineering Research Board (SERB), a statutory body of the Department of Science & Technology, Govt. of India through Ramanujan Fellowship (SB/S2/RJN-088/2016). REFERENCES 1. Padhi, A.; Nanjundaswamy, K.; Goodenough, J., Phospho‐Olivines as Positive‐Electrode Materials for Rechargeable Lithium Batteries. Journal of the Electrochemical Society 1997, 144, 11881194. 2. Hu, M.; Pang, X.; Zhou, Z., Recent Progress in High-Voltage Lithium Ion Batteries. Journal of Power Sources 2013, 237, 229-242. 3. Masquelier, C.; Croguennec, L., Polyanionic (Phosphates, Silicates, Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries. Chemical Reviews 2013, 113, 6552-6591. 4. Rousse, G.; Tarascon, J.-M., Sulfate-Based Polyanionic Compounds for Li-Ion Batteries: Synthesis, Crystal Chemistry, and Electrochemistry Aspects. Chemistry of Materials 2013, 26, 394-406. 5. Gutierrez, A.; Benedek, N. A.; Manthiram, A., Crystal-Chemical Guide for Understanding Redox Energy Variations of M2+/3+ Couples in Polyanion Cathodes for Lithium-Ion Batteries. Chemistry of Materials 2013, 25, 4010-4016. 6. Manthiram, A., Materials Challenges and Opportunities of Lithium Ion Batteries. The Journal of Physical Chemistry Letters 2011, 2, 176-184. 7. Barpanda, P., Sulfate Chemistry for High‐Voltage Insertion Materials: Synthetic, Structural and Electrochemical Insights. Israel Journal of Chemistry 2015, 55, 537-557. 8. Dwibedi, D.; Barpanda, P., Designing Novel Sulphate-Based Ceramic Materials as Insertion Host Compounds for Secondary Batteries. Transactions of the Indian Ceramic Society 2015, 74, 191-194. 9. Reynaud, M.; Ati, M.; Melot, B. C.; Sougrati, M. T.; Rousse, G.; Chotard, J.-N.; Tarascon, J.-M., Li2Fe(SO4)2 as a 3.83 V Positive Electrode Material. Electrochemistry Communications 2012, 21, 77-80. 10. Barpanda, P.; Ati, M.; Melot, B. C.; Rousse, G.; Chotard, J. N.; Doublet, M. L.; Sougrati, M. T.; Corr, S. A.; Jumas, J. C.; Tarascon, J. M., A 3.90 V Iron-Based Fluorosulphate Material for Lithium-Ion Batteries Crystallizing in the Triplite Structure. Nature Materials 2011, 10, 772-779. 11. Reynaud, M.; Ati, M.; Boulineau, S.; Sougrati, M. T.; Melot, B. C.; Rousse, G.; Chotard, J.-N.; Tarascon, J.-M., Bimetallic Sulfates A2M(SO4) 2. nH2O (A= Li, Na and M= Transition Metal): As New Attractive Electrode Materials for Li-and Na-Ion Batteries. ECS Transactions 2013, 50, 11-19. 12. Reynaud, M.; Rousse, G.; Chotard, J.-N.; Rodríguez-Carvajal, J.; Tarascon, J.-M., Marinite Li2M(SO4)2 (M = Co, Fe, Mn) and Li1Fe(SO4)2: Model Compounds for Super-Super-Exchange Magnetic Interactions. Inorganic Chemistry 2013, 52, 10456-10466.


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33. 184.

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TOC Graphic


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Structural, Thermal, and Electrochemical Studies of Novel Li2CoxMn1

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