Correlating Voltage Profile to Molecular Transformations in

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C: Energy Conversion and Storage; Energy and Charge Transport 2

Correlating Voltage Profile to Molecular Transformations in Ramsdellite MnO and Its Implication for Polymorph Engineering of Lithium Ion Battery Cathodes Prashant Kumar Gupta, Arihant Bhandari, Jishnu Bhattacharya, and Raj Ganesh S Pala J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02708 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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The Journal of Physical Chemistry

Correlating Voltage Profile to Molecular Transformations in Ramsdellite MnO2 and Its Implication for Polymorph Engineering of Lithium Ion Battery Cathodes Prashant Kumar Guptaa†, Arihant Bhandaric†, Jishnu Bhattacharyac* and Raj Ganesh S. Palaa,b* a

Department of Chemical Engineering, Indian Institute of Technology, Kanpur, 208016, India b

c

Material Science programme, Institute of Technology, Kanpur, 208016, India

Department of Mechanical Engineering, Indian Institute of Technology, Kanpur, 208016, India *

Corresponding authors: [email protected], [email protected] †Equally contributed to work

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Abstract: The transition metal oxides are the primary choice for cathode materials in lithium ion batteries. These oxides occur as different polymorphs which have varied structures, stabilities and affinities for Li+ and consequently, it is desirable to develop heuristics for the choice of the optimal polymorph. MnO2 is the most extensively used cathode material for lithium ion battery and it exists in more than ten polymorphic forms. Among them, ϒ-MnO2, the most electrochemically active polymorph is mixture of Pyrolusite and Ramsdellite polymorphs. Here we have focused on the less explored Ramsdellite MnO2 (R-MnO2). Highly crystalline R-MnO2 has been synthesized and the experimentally obtained discharge features are compared with the calculations based on density functional theory followed by cluster expansion to obtain molecular insights into the intercalation process. R-MnO2 shows voltage fading beyond x=0.5 (in LixMnO2). Computational results suggest that change in Li environment from tetrahedral to octahedral beyond x=0.5 (in LixMnO2) causes the voltage fading. This change in Li environment is also correlated with experimentally obtained Mn 2p, O 1s and valence band XPS spectra. The shift in the d-band center, plotted for valence band spectra at different lithium concentration is adduced for the migration of lithium ion from tetrahedral site to less favorable octahedral site beyond x=0.5. Volume expansion of about ~20% at full lithiation is accompanied by Jahn-Teller distortion of MnO6. Further, computations reveal a diffusional energy barrier to be 200 meV in the limit of dilute Li concentration (x=0.125) and 481 meV in the limit of dilute vacancy concentration. A structural rationale for the smaller barrier is developed. The experimental and computational studies provide insight into lithiation mechanism in R-MnO2 and is relevant to rationalize the high performance of intergrowth structure of ϒ-MnO2. We develop a wider implication of the study by correlating the voltage profile with structural changes in MnO2 polymorphs, which gives general design principles to make better cathodes through polymorphic engineering.


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Introduction: Transition metal oxides, which are being extensively used as cathodes in the Lithium ion batteries,1 co-exist in different polymorphic forms due to the variable coordination number, bond angle and bond distance between transition metal and oxygen atoms.2-4 For availing the maximum performance of the material, it is important to develop a rationale to identify the optimal polymorphic form for transition metal oxides. Manganese oxides are among the widely used materials by the battery industries due to its low cost, less toxicity and superior safety characteristics5-8. There are more than 10 polymorphs of MnO2 out of which six polymorphs have been extensively explored for lithium ion battery.8-10 Choosing an optimal polymorph for lithium storage is a trade-off between the structural stability and the maximum amount of lithium insertion. α-MnO2 with its open tunnel (2×2 channels) favors fast ionic diffusion and large storage capacity.11 But these tunnels are contaminated by interstitial defects in the form of cations or small molecules such as NH4+, K+, H2O etc. during synthesis. These impurities provide structural stability at the cost of slower ionic diffusion and less capacity for lithium intercalation, leading to poor cycling performance.11,

12

The thermodynamically most stable

polymorph, β-MnO2, also known as Pyrolusite with its narrow 1×1 channels, has slow Li iondiffusion rates and has low lithium storage capacity rendering the material unsuitable for cathode electrode.8,

13

The most active polymorph is ϒ-MnO2, which is an intergrowth structure of

Ramsdellite (2×1 channels) and Pyrolusite (1×1 channels), with the Ramsdellite (R-MnO2) providing higher capacity and Pyrolusite providing structural stability.7 R-MnO2 with 2×1 channels is desirable in terms of fast kinetics and high capacity due to its open structure. Nevertheless, it is always found in an intergrowth structure along with the β-MnO2 which diminishes its performance.14 The amount of β-MnO2 depends upon the synthesis process which changes the properties of the intergrowth material, which was first explained by Chabre and Pannetier.14 λ-MnO2, a metastable form of MnO2, which is derived from the delithiation of spinel LiMn2O4, provides high rate capability due to 3D interstitial network for lithium diffusion, while suffers from the Mn dissolution into the electrolyte over repeated cycling.15 The layered structure δ-MnO2 shows a poor structural stability due to the collapse of layers at high degree of delithiation.8



ϒ-MnO2 has been also found to be a potential material in other energy storage applications such as supercapacitors,2, 16 primary batteries,17 secondary batteries18 and Li-air battery catalysts.14, 19 The complex intergrowth structure limits the fundamental understanding of this material in applications. In order to understand the material properties one must first address its individual components which are Ramsdellite and Pyrolusite. The detailed molecular understanding of lithium intercalation in Pyrolusite from both experiments20 and simulations21 is available in literature, while the study of lithium intercalation in Ramsdellite is limited. Naturally occurring mineral Ramsdellite was first discovered by Ramsdell in 1932 and the crystal structure was reported by Byström in 1949.22,

23

The chemical synthesis of pure

Ramsdellite is difficult due its metastability even at high temperature and pressure.24 The degree of intergrowth of Pyrolusite in Ramsdellite varies with the synthesis conditions. The intergrowth structure is responsible for the De Wolff disorder and the poor X-ray powder diffraction pattern, which is known as micro-twinning, both of which makes the understanding more complex.14, 25, 26

Chabre and Pannetier determined the XRD pattern of ϒ-MnO2 by peak-fitting of four major

structures and further, proposed a model to calculate the percentage of Pyrolusite (Pr) in ϒMnO2.14 Thackeray et al.27 synthesized highly crystalline Ramsdellite with very small percentage of Pyrolusite and showed first discharge capacity of 308 and 220 mAh/g for Li/MnO2 cell with cutoff potential of 0.5 V and 2 V respectively. They have found that the cycling of this material is very poor and capacity fades to just 115 mAh/g after 10 cycles for a potential window of 3.8 and 2.0 V. This loss in capacity with cycling for highly crystalline Ramsdellite is believed to be associated with the volume increase of unit cell on deep cycling.27-29 Thackeray et al. have also studied the effect of temperature on the structural change of R-MnO2 during chemical insertion of lithium into the structure. The gap in the literature lies in the mechanistic explanation of electrochemical lithiation and the molecular insight for voltage fading at higher lithium concentration in R-MnO2. Here, we attempt to study these two aspects to further explain the lithiation behavior of ϒ-MnO2. In this context, we synthesized highly crystalline R-MnO2 by slow rate acid digestion of LiMn2O4 at moderate temperature and obtain the discharge feature experimentally, which is correlated with the first principles-cluster expansion calculations in order to get insight of the Li intercalation process. The computation and experimental results suggests that the change in


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chemical environment of lithium from tetrahedral to octahedral beyond x=0.5 leads to voltage drop. We also observe volume expansion of ~20% which causes Jahn-Teller distortion in MnO6 unit during intercalation process. The voltage fading is rationalized with the core and valence XPS spectra at different lithium concentration. The shift in the d-band center for the valence band calculated from density functional theory endorse the lithium ion transfer from tetrahedral site to less stable octahedral site. Similar trend has been also observed from the experimentally obtained d-band center through valence band XPS spectra. We have also looked into the molecular aspect for fast kinetics of R-MnO2 by calculating diffusion barrier which is found to be 200 meV in the dilute limit of Li concentration and 481 meV in the dilute limit of vacancy concentration. Finally, we develop a general rationale to correlate voltage profile with structural changes in different polymorphs of MnO2. Experimental Details: I.

Materials and synthesis:

Lithium manganese oxide (LiMn2O4) has been purchased from Sigma-Aldrich, India. Sulphuric acid (H2SO4) has been purchased from Fisher Scientific. We have synthesized highly crystalline R-MnO2 by the reaction of spinel LiMn2O4 with 2.6 M H2SO4 at 95 oC for 48 hr. The resultant solution has then been centrifuged at 8000 rpm and washed with DI water several times in order to remove unreacted acid. The synthesized particles have been dried at 100 oC overnight. The powder has finally been kept at 250 oC for 3 hr. to remove any moisture content. II.

Characterization:

X-ray diffraction (XRD, PANalytical, Germany) has been performed from 10o to 80o at a scanning speed of 2o per minute to determine the crystal structure. The surface elemental composition and chemical states of components have been analyzed by using XPS technique. We have opened the coin cell inside the glove-box and have washed the electrode with diethyl carbonate purchased from Sigma-Aldrich, India. We have kept electrodes for drying inside the glove-box. X-ray photoelectron spectra (XPS, PHI 5000 Versa Prob II, FEI, USA) have been measured in the binding energy range of 0–1400 eV. The binding energy scale has been calibrated using the C 1s peak at 284.6 eV. Al Kα (25 W, 15 kV) is used as emission source. 5 ACS Paragon Plus Environment


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Morphological characterization has been performed by using Field emission scanning electron microscopy (FESEM, ZEISS Supra 40VP, Germany) operated at 10 kV voltage. FEI Titan G2 60-300 has been used to perform high-resolution transmission electron microscopy (HRTEM). We have used carbon coated copper TEM grids to perform HRTEM. III.

Electrochemical measurements:

The synthesized R-MnO2 is ground in a mortar and pestle prior to the fabrication of electrode. 70 wt% of the active material, 15 wt% of super P as a conducting additive and 15 wt% of polyvinylidene fluoride (PVDF) binder have been mixed in N-Methyl 2-Pyrrolidone (NMP) solution to make a slurry. We have coated the prepared slurry on a carbon coated aluminum foil current collector with the help of doctor blade and dried at 120oC for 12 h in vacuum. The electrodes have been cut in the form of a circular disc of diameter 15 mm appropriate for 2032coin cell. We have assembled the coin cell in the argon filled glove box and used the 750 µm thick lithium foils as counter electrode. 1.0 M LiPF6 in ethylene carbonate and diethyl carbonate EC: DEC (1:1 v/v) has been used as electrolyte solution and trilayer polypropylene membrane (Celgard 2320) has been used as a separator. Approximately 2-3 mg of active material has been loaded on carbon coated aluminum foil. Before the measurements, the coin cells have been aged for

12

hr.

We

have

performed

cyclic

voltammetry

(CV)

using

an

Autolab

potentiostat/galvanostat (302 N, Netherlands) at a scan rate of 0.1 mV s−1. Galvanostatic charge/discharge of assembled coin cells have been measured by using a battery analyzer (MTI Corporation, USA). Computational Methodology: The discharge voltage profile upon lithiation and the energy barrier for Li diffusion in the cathode has been computed from density functional theory simulation. We have considered LixMnO2 as the cathode and lithium metal as anode. I.

Voltage profile during discharge:

The open circuit voltage (OCV) or the equilibrium voltage produced by the battery can be written as30 () = −

()


(1)

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Where, and are the lithium chemical potentials at cathode and anode,

respectively, n is the number of electrons that are transferred during process and F is Faraday constant. The Li chemical potential at cathode will change as a function of Li intercalation (x) in the cathode. However, the Li chemical potential of anode is constant and is same as the Gibbs free energy of the Li metal (GLi). Consider the following lithium intercalation reaction in MnO2: !" #$%& + (& − ( ) ! ⟶ !* #$%&

(2)

Where, ( and & are initial and final mole fractions of Li (0 ≤ 1 , 2 ≤ 1) in cathode. The average intercalation voltage (Vavg) or the OCV can be written as:31 56 = −

∆89 (3) :∆9

Where, the Gibbs free energy change of the reaction, ∆89 = ∆∆?9 The volume term, =∆9 (which is of the order of 10-5 eV) and the entropy term, >∆?9 at 0K are much smaller than the change in internal energy, ∆

Correlating Voltage Profile to Molecular Transformations in

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