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Synthesis Methods and Electrochemical Performance: A Theory on the Valence Disproportionation in LiMyMn2−yO4 (M = Mn, Co) with Interalia Guiding Principles for a Photo-Chargeable Lithium Battery Krishna Rao Ragavendran,*,† Li Lu,‡ Klaus Bar̈ ner,§ and A. K. Arof† †

Center for Ionics, Department of Physics, University of Malaya, Kualalumpur 50603, Malaysia Materials Laboratory, Department of Mechanical Engineering, National University of Singapore, Singapore 117576 § Department of Physics, University of Göttingen, F, Hund Platz 1, 37077, FRG, Germany ‡

ABSTRACT: A physical model, based on valence disproportionation and the electronic instability imposed by the Jahn−Teller condition, is used as a general approach to explain the differences in the electrochemical activities of LiMn2O4 based cathode materials synthesized through different methods. Furthermore, the models also provide interalia insights for a photo-chargeable lithium battery and a physical ansatz to address a fundamental inefficiency problem: the deviation of the experimentally observed electrochemical capacity for LiMn2O4 (∼126 mAhg−1) from the theoretical value (147 mAhg−1). find that these discussions lead to interalia insights for designing a photo-chargeable lithium battery.

1. INTRODUCTION LiMn2O4 first introduced in 1983 by J. B. Goodenough1 and well-known for its interests in applied and fundamental physics2−8 is also known for its capacity fading phenomenon5−8 arising at least from a couple of the most general factors: (a) phase transitions of the Jahn−Teller type5 and (b) Mn dissolution.7,8 Researchers, though mostly through an extensive trial and error approach, have improved upon the electrochemical performance of these materials through treatments such as (a) doping, 9−15 (b) changing the synthesis method,16−19 (c) tailoring the shape20,21 and size,22 etc. A pursuit on the search for a “super cathode material” would demand a systematic research methodology instead of a trial and error method. Quite recently, our group made its preliminary report,23 using trap state spectroscopy, on a mechanism that could account for stable electrochemical cycling in LiCo1/6Mn11/6O4 as compared to LiMn2O4. In this paper, we address the next issue: the physics behind the improvements in the performance of materials achieved by changing the method of synthesis. Cathode materials prepared under different synthesis conditions (method of synthesis,16−19 effect of precursor,27 calcination temperature,28 etc.) exhibit different electrochemical properties due to variation in their trap state structure,29 size,22 shape20,21 effects, etc. The objective of this work is to contain all of these understandings under a general framework of valence disproportionation24−26 and is schematically presented in Figure 1. Furthermore, we © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Synthesis. Two batches of LiMn2O4 were synthesized through (a) the conventional solid state route (SSR) and (b) the wet chemistry route (WCR). For the SSR, LiMn2O4 was prepared by a conventional solid state reaction between Li2CO3 and MnCO3 at 750 °C in air for 6 h, followed by heating at 800 °C for 12 h in air with intermittent grinding. The fuel (starch) assisted solution combustion method was adopted as the wet chemistry route to synthesize the cathode material. The starch assisted combustion method using nitrate precursors is available in the literature.30 However, in our synthesis procedures, we use of LiOH·H2O and Mn(CH3COO)2·4H2O, as opposed to nitrate precursors. This provides at least a couple of advantages compared to the use of the corresponding metal nitrates. In the first place, the storage of the chemicals is much easier compared to the metal nitrates which are deliquescent. Second, lithium hydroxide and manganese acetate assure a neutral pH of the solution, which is reported by us as one of the very important conditions required to obtain well-defined crystal shapes of the cathode material.20 Received: August 14, 2013 Revised: October 8, 2013 Published: October 16, 2013 23547

dx.doi.org/10.1021/jp408127x | J. Phys. Chem. C 2013, 117, 23547−23557

The Journal of Physical Chemistry C

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Figure 1. Schematic representation of the physics in the cathode material synthesized through different methods.

The discharge capacities of LiMn2O4 prepared through other wet chemistry routes such as the hydrothermal route and the sol−gel synthesis are also compared with the above materials in Figure 12. These materials were used for studying the shape effects of cathode materials on the electrochemical performance, and are detailed elsewhere.21 In this section, we provide a brief account of the synthesis details of these wet chemistry cathode materials. A one-step hydrothermal process was used to synthesize the ultrafine LiMn2O4 nanoparticles through a procedure described earlier.22 In the typical process, 0.3 g of KMnO4 was added in 25 mL of deionized (DI) water and stirred by a magnetic bar for 1 h. In the next step, 0.08 g of LiOH and 1 mL of ethanol were added in the solution and the solution was further stirred for another 1 h. After that, the solution was transferred into a 30 mL Telfon-lined stainless steel autoclave. The autoclave was sealed and put in an electric oven at 180 °C for 8 h. After the hydrothermal treatment, the produced samples were collected by filtration and washed with DI water. The nanocrystalline LiMn2O4 samples were finally dried at 100 °C for 12 h for further characterization. LiMn2O4 prepared through the sol−gel method employs lithium hydroxide, manganese acetate, and the organic acid (citric acid or glycine) in the molar ratio 1:2:3. The chemicals were dissolved in a minimum amount of distilled water under a given pH condition and stirred at 80 °C until a gel was formed. LiMn2O4 powder was obtained by calcining the gel at 800 °C for 10 h at a heating/cooling rate of 5 °C min−1. 2.2. Experimental Tools. X-ray diffraction patterns of the materials were recorded using a PANalytical XRD machine with Cu Kα radiation. The XRD pattern (Figure 2) agreed well with the JCPDS data (35-0782) for spinel type LiMn2O4, and hence, the synthesized materials are confirmed for their phase purity. The external crystal shapes of the cathode materials were photographed with a Hitachi, model S 3000H scanning electron microscope at a resolution of 10K (Figure 3). Composite electrodes for electrochemical measurements were made by mixing 80% (weight) cathode material, 10% (weight) carbon black, and 10% (weight) poly(vinylidine fluoride) binder in N-methyl pyrrolidone as solvent, followed by stirring the mixture overnight. The homogeneous slurry was then coated over an aluminum foil and dried overnight at 120 °C in a vacuum (−76 cm Hg). Swagelok type cells were assembled in a glovebox (M-Braun) using two celgard separators, 1 M LiPF6 dissolved in EC/DC 50:50 as the electrolyte, and Li foil as the anode. The oxygen and moisture levels inside the glovebox were maintained at Driving force consequent to the Jahn−Teller stabilization energy

This creates a situation such that the eg electron in Mn3+ is (a) neither available for charge transfer (thereby preventing valence disproportionation and hence Mn dissolution) (b) nor is available within the threshold limits for ejection as a photoelectron. Property (a) is important in the lithium battery context in that it provides an understanding of the cycling stability in doped systems. Cobalt doped lithium manganate is 23551



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Figure 10. Trap state structure in LiMn2O4 and LiCo1/6Mn11/6O4: Cobalt doping in LiMn2O4 traps the Mn3+ eg electron such that it is neither available for (a) emission as a photo-electron (as evidenced from the insensitivity of LiCo1/6Mn11/6O4 to photo-shining) or for (b) charge transfer (valence disproportionation is abhorred and cycle stability is attained) required by Jahn−Teller stability conditions. Adapted with permission from ref 23. Copyright 2013 American Chemical Society.

sol−gel synthesized material (130 mAhg−1) compared to the solid state LiMn2O4 (∼90 mAhg−1).

thus stable toward repeated electrochemical cycling compared to LiMn2O4 (Figure 10). The importance of property (b) will be discussed in the final section on the interalia possibility for a photo-chargeable lithium battery. 3.3.1. Solid State AC-Impedance Measurements on LiMn2O4 (SSR) and LiMn2O4 (WCR). From Figures 11 and 5,

4. SOME APPLICATIONS OF THE ABOVE MODELS TO THE INTERPRETATION OF CYCLE-ABILITY CURVES OF THE LITHIUM BATTERY 4.1. On the Physics Behind the Initial Capacities of Cathode Materials Prepared through Various Wet Chemistry Routes (WCR). From Figure 12, it can be seen

Figure 11. Solid State AC-impedance measurements on sol−gel and solid state synthesized LiMn2O4.

Figure 12. Discharge capacities (at 1C rate), upon repeated cycling, of LiMn2O4 synthesized through different wet chemistry methods.

it can be observed respectively that the Z′ component for LiMn2O4 (WCR) > LiMn2O4 (SSR) and the initial capacity of LiMn2O4 (WCR) > LiMn2O4 (SSR). It is a known fact that the electrical resistivity of a charge ordered configuration (analogous to the situation prevailing in Model I) is higher than that of a charge disordered configuration (analogous to Model II). The sol−gel synthesized material, LiMn2O4 (WCR), which is generally known in the literature to be more homogeneous than LiMn2O4 (SSR) can be modeled to Model I and the solid state synthesized material to Model II. The increased Z′ value for the sol−gel material can thus be attributed to an ordered configuration in the arrangement of Mn3+ and Mn4+ ions. This ordered arrangement also keeps off the nearest neighbor vicinity between Mn3+ (unlike the case with the solid state synthesized material) and hence protects the electrochemically active Mn3+ ion from valence disproportionation, accounting for the higher initial capacity in the

that LiMn2O4 prepared through the wet chemistry routes (WCR) demonstrates much superior electrochemical characteristics compared to the solid state route (SSR). Also, among the WCR methods, the fuel (starch) assisted combustion method shows the highest initial capacity (130 mAhg−1). The hydrothermal LiMn2O4 can be seen to possess better capacity retention properties but lesser initial capacity than the starch assisted material. Other wet chemistry methods such as the citric acid aided sol−gel method (under neutral pH conditions), citric acid aided sol−gel method (under acidic pH conditions), and glycine aided sol−gel method all exhibit a lower initial electrochemical capacity compared to the starch assisted material (Table 1). This phenomenon could be due to the “starch assisted method producing LiMn2O4 with a Mn3+ and Mn4+ arrangement very close to the ideal pattern presented in Model I and this abhors solid state valence disproportiona23552



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subsequent decrease in the capacities, observed in the high rate cycleability curve of LiMn2O4 (Figure 13b), are interesting and require an explanation. The trivial explanation is the possibility of some Mn3+ formation in the SEI layer, contributing to battery electrochemistry after its complete formation, however getting subsequently dissolved after some electrochemical cycling. However, it is not clear how the Mn3+ formed at the SEI layer. An ambitious hypothesis would be “the possibility for the onset of an unknown driving force (kinetic factors originating from high rate cycling)” that triggers a “reverse valence disproportionation reaction” where the Mn2+ and Mn4+ give back the electrochemically active Mn3+! However, this explanation is too ambitious for the present stage and we reserve it for our future pursuits.

Table 1

LiMn2O4 synthesis method starch assisted combustion method hydrothermal method citric acid assisted sol−gel (neutral pH) citric acid assisted sol−gel (acidic pH) glycine assisted sol−gel conventional solid state route (SSR)

initial discharge capacity (mAhg−1) at C/10 rate

discharge capacity (mAhg−1), at C/10 rate, at the end of 15 cycles

130

110

109 119

109 100

118

70

108 84

100 53

tion reaction”. LiMn2O4 prepared through the starch assisted method can thus be considered enriched with the electrochemically active Mn3+ ion, hence leading to higher initial capacities, compared to the other wet chemistry methods. 4.2. On the Initial Capacities of the LiMn2O4 Battery at High Rates and the Shape of the Cycleability Curve. From Figure 13, it can be seen that LiMn2O4 operating at high rates (5C) exhibits a much lower initial capacity, less than 100 mAhg−1, than that operating at lower rates (C/10) where the discharge capacities are much more than 100 mAhg−1. During the cycling process, LixMn2O4 first undergoes charging where x → 0. Then, the cathode material undergoes discharge where x → 1 and the current involved for this process corresponds to the discharge capacity and this is represented as points on the cycleability curves in Figure 13. At a high rate (5C) cycling, the initial charging step disrupts the cathode material lattice (initially close to Model I) and brings about a transition from Model I to Model II, which can be at an equilibrium as represented below.

5. INTERALIA INSIGHTS 5a. Prospects for a Photo-Chargeable Lithium Battery. The discussions made in section 3.3 throw interalia insights for developing a working hypothesis and subsequent research plan for a possible demonstration of a photorechargeable lithium battery. Though recent advancements35,36 in solar energy harvesting are available within the framework of dye and quantum dot sensitized solar cell materials, and while the idea to utilize the Fujishima−Honda effect37 to photolyze LiMn2O4 into Li and Mn2O4 can be a prospective direction for photo-chargeable batteries, this section provides the initiative on a new direction for possible utilization of the solar energy to drive electrochemical reactions. The battery reaction during charge and discharge can be represented as charge

LixMn2O4 XooooooooY LixMn2O4

(x = 1) Mn =+3.5 discharge (x = 0) Mn =+4

Model I ⇌ Model II

On the basis of charge balance, it can be easily seen that the formal oxidation states of Mn in LixMn2O4 (discharged state when x = 1) and LixMn2O4 (charged state when x = 0) are 3.5 and 4, respectively. This indicates that electrochemical charging of LiMn2O4 requires oxidation (electron ejection) of the Mn levels from +3.5 to +4.

Model II being vulnerable to valence disproportionation lowers the electrochemically active Mn3+ fraction in the cathode material, leading to a decreased value of the initial discharge capacity. The increase in the subsequent discharge capacities for the first few cycles (the circled part in Figure 13b) and the

Figure 13. (a) Discharge capacities at low rates (C/10) of LiMn2O4 synthesized through a citric acid assisted sol−gel method at neutral and acidic pH conditions designated. (b) Discharge capacities at high rates (5C) of LiMn2O4 synthesized through a citric acid assisted sol−gel method at neutral and acidic pH conditions. The initial capacity for the low rate discharge is usually higher than that for the high rate discharge. Adapted with permission from ref 20. Copyright 2011 Elsevier B.V. 23553



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The battery reaction given above can now be pictorially represented in terms of the occupancies of d-orbitals of Mn (Figure 14). Under the influence of a weak ligand field, as

This leads to a situation as represented in Figure 6. In such a case, the propensity for the eg electron to undergo charge transfer will be much higher than the propensity for its emission as a photo-electron. This is because the Jahn−Teller energy stabilization requirement drives the eg electron to undergo the charge transfer, increasing its threshold frequency to be emitted as a photo-electron. This explanation can address the lower photo-electron emissive efficiency in LiMn2O4 with lower crystal quality,29 and vice versa. From the above observations and discussions, we can see that the requirement for Jahn−Teller stabilization creates two possibilities on the Mn eg electron: Thus, two types (pathways) of electron transfer can be envisaged in the cathode material in the current situation: type I belonging to the photo-emission type and type II belonging to the electron transfer driven by Jahn−Teller requirements for electron stability. The competition between these two pathways decides the battery performance and also some novel applications (such as the photo-rechargeable battery); i.e., efficient photo-emission of the eg electron, and hence photo-charging of the battery, depends on the extent to which valence disproportionation in the cathode material can be forbidden. This understanding can be considered as a guiding principle in designing cathode materials for the photo-lithium battery. Interestingly, such cathode materials provide a dual advantage: (a) possibility to harvest the solar energy (b) since the photochargeable cathode material should abhor valence disproportionation as a requirement, electrochemical stability of the material is naturally achieved (Figure 15). An electronic valve effect now needs to be introduced in the material such that it selectively blocks the path-II electron transfer that leads to valence disproportionation but promotes the path-I electron transfer, leading to photo-emission and hence photo-charging. The electronic valve effect could be introduced in the cathode material through a dopant using a suitable guiding principle or through modifying certain key

Figure 14. Energy level representation of the electrochemically active ion in LiMn2O4 during charge and discharge.

exemplified by the oxygen backbone, the energy gap (Δo) separating the t2g and eg states will be less than the pairing energy of the electron in the d-orbital, that the electrons fill into the Mn d-orbital as represented in Figure 14. From Figure 14, it can be argued that the photo-charging of LiMn2O4 requires that the Mn eg electron is ejected as a photoelectron. It will be interesting to cite here our earlier work where we reported that photo-electron emission in LiMn2O4 is higher when the quality of the crystalline sample is high.29 The usual explanation is that in high quality crystalline materials electron−hole recombination is less, and this leads to a higher photo-electron emissive efficiency. An alternative explanation, based on the models dealt with in this paper, can also be provided for materials such as LiMn2O4 where lower quality means that the degree of randomness in the position of Mn3+ and Mn4+ ions is more. This implies that Mn3+ ions can stay as nearest neighbors as represented below: − Mn3 + − Mn3 + − Mn 4 + − Mn 3 + − Mn 4 + − Mn 4 + − Mn3 + − Mn3 + − Mn3 + − Mn 4 + − Mn3 + − Mn3 + − Mn3 + − Mn 4 + −

(Model II)

Figure 15. Pictorial representation depicting the possibilities for a photo-chargeable Li battery: (A) The trap state operating in LiCo1/6Mn11/6O4 neither allows the eg electron for charge transfer nor does it allow it to be emitted as a photo-electron. (B) In the modified LiMyCo1/6Mn12−yO4 compound, the dopant “M” is so selected that it creates an “electronic valve” in the material. The valve allows for the photo-emission but effectively forbids it from charge transfer and hence valence disproportionation and capacity fading. 23554



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theoretical electrochemical capacity of LiMn2O4 is 147 mAhg−1, and the experimental electrochemical capacity is ∼126 mAhg−1. The trivial explanation to account for this deviation is usually built upon the concept of lithium diffusion pathways. However, an explanation can also be provided on the framework of the valence disproportionation model. Unlike LixCoO2, LixMn2O4 does not suffer from Li intercalation, deintercalation problems. Thus, while x in LixCoO2 can only vary from 1 to 0.5 due to structural stability issues, x in LixMn2O4 can vary through the entire range from 1 to 0.33,34 Assuming an ideal diffusion pathway for lithium in LixMn2O4, this material can serve as a prototype for investigations on valence disproportionation models. Figure 17 explains the deviation from theoretical and experimental capacities on the basis of VD models. While ∼126 mAhg−1 capacity is available for useful work, the remaining charge equivalent to ∼21 mAhg−1 is used for typeII electron transfer as a requirement for attaining Jahn−Teller stabilities. The type-II electron transfer brings about valence disproportionation and hence capacity fading. The high initial capacity (∼130 mAhg−1) and capacity retention of LiMn2O4 synthesized through the starch assisted combustion route as compared to all the other varieties of LiMn2O4 (Table 1) can thus, be appreciated. Deviation of the experimental capacities from the theoretical capacity are usually accounted for using Li diffusion pathways.41 Also, while the diffusion model is the classical basis that could only account for the difference in the theoretical and experimental values, the VD model on the other hand is the electronic basis and could account for the deviation in the experimental capacity from the theoretical values and as a consequence could explain the origin of irreversible capacity loss in the material upon repeated cycling. Not limited to mere practical applications, we believe that extensive work on the physical ansatz for the ideal battery will have more important consequences in fundamental physics.

attributes of the pristine cathode material. Figure 16 depicts the operation of an electronic valve in lithium battery cathode

Figure 16. Principle of the electronic valve in lithium battery cathode materials: A cathode material with optimal (a) periodicity and (b) distance between the Mn3+ ions could selectively abhor the valence disproportionation and promote photo-ionization of the eg electron.

materials. Tuning the periodicity and the distance between the Mn3+ ions serves as the electronic valve that selectively abhors the type-II electron transfer and promotes type-I electron transfer. Material design as illustrated in Figure 16 can be expected to suppress the valence disproportionation (and hence capacity fading) and imbibe in the material the virtue of light energy harvesting. At this juncture, it will be interesting to reckon the visible light induced charge storage phenomenon in alkali metal intercalated WO3.38 A difference in the selection rules between the two pathways for electron transfer could provide clues for designing the electronic valve. Thus, while Type-I pathway for the electron should be photo-chemically allowed, the Type-II pathway, driven by Jahn−Teller stability requirements, should be phonon mediated, non-radiative and hence thermally activated. This conjecture can be supported from the well known observation in lithium battery literature that Mn dissolution in LiMn2O4 is accelerated at higher temperatures (∼55 °C)39,40. 5a. The Ideal Battery: Accounting for the Deviation between the Theoretical and Experimental Discharge Capacities of LiMn2O4 Based Cathode Materials. The

6. CONCLUSION The past few decades have witnessed astonishing advancements in the physics of transition metal oxides, with lithium battery materials in the 1970s, high Tc superconductivity in the 1980s, and colossal magneto resistance in the 1990s. Valence

Figure 17. Deviation between theoretical and experimental capacities: An explanation within the framework of the valence disproportionation model. 23555



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scope of this paper and will be advanced in our next report on spintronic cathode materials for lithium batteries

disproportionation is a common phenomenon prevalent among these three types of materials. In the case of phenomena HTSC and CMR valence disproportionation creates mixed valent states which are crucial for realizing such super phenomena. However, with battery materials such as LiMn2O4, valence disproportionation is harmful, as it leads to electrochemical instability. An enhancement or sometimes even a change in the physics of the material activity upon an amendment in the synthesis method adopted is well-known. For instance, Bednorz and Muller had reported in their 1987 Nobel lecture that the HTSC phenomenon was observed only when their materials were synthesized through certain synthesis methods. Concerning the cathode materials for lithium battery applications, it is well-known in the literature that synthesis methods and doping improve their capacity retention properties. The objective of this report is not concerned with optimizing the conditions for cathode materials synthesized through a certain synthesis method: sol−gel, solid state, hydrothermal, etc. Different synthesis conditions exhibit different electrochemical properties, and they have to be understood within a general framework. The physical models advanced in this paper, within the framework of valence disproportionation, provide new and initial starting points to satisfactorily understand why the electrochemical characteristics of the cathode material should differ with synthesis methods. Synthesis methods alter the vicinity between Mn3+ and Mn4+. Certain synthesis methods lead to arrangements with nearest neighbor interactions between Mn3+ and Mn3+, while others keep the Mn3+ species far enough to allow an interaction between them. The synthesis method responsible for the former arrangement leads to valence disproportionation, Mn dissolution, and hence capacity fading. The models advanced in this paper can also be used to explain why doping should improve the capacity retention. In one of the recent papers by Goodenough et al. (J. Power Sources 2012, 199, 214−219), it was reported that undoped LiMn2O4 showed high rates of Mn dissolution as large as 480 ppm, while with the doped LiMn2O4 the dissolution rate was as low as 67 ppm. Our models advanced within the framework of valence disproportionation (Figures 8−10) can explain these observations as well. Furthermore, the models advanced in the paper lead to a new dimension hypothesis on photo-chargeable cathode materials. While research activities on available methods such as semiconductor solar cells, dye sensitized solar cells (DSSCs), and quantum dot sensitized solar cells (QDSSCs) are actively being pursued worldwide, the hypothesis put forward in this paper provides a totally new approach for research activity on electrochemical solar energy harvesting. Advanced cathode material synthesis with optimal periodicity and distance between the Mn3+ ions seems to hold prospects for photochargeable lithium batteries. This paper describes situations that could avoid the interionic charge transfer between Mn+3 ions. However, it has be borne in mind that charge transfer is inevitable and should proceed to stabilize the system from J−T instabilities associated with the eg1 configuration of the Mn+3 ion. If charge (interionic) transfer is prevented, VD is abhorred. But the issue of JT instability remains. Models on intra-ionic charge transfers can get rid of the VD, while also satisfying the requirements imposed by the JT condition. Such discussions are beyond the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. my. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the NANO FUND GRANT (53-02-0301089), Malaysia. REFERENCES

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