Correlating Lithium Hydroxyl Accumulation with Capacity Retention in

Subscriber access provided by Loyola University Libraries

Article

Correlating lithium hydroxyl accumulation with capacity retention in VO aerogel cathodes 2

5

Linda W. Wangoh, Yiqing Huang, Ryan Leigh Jezorek, Aoife B Kehoe, Graeme William Watson, Fredrick Omenya, Nicholas F. Quackenbush, Natasha A. Chernova, M. Stanley Whittingham, and Louis F. J. Piper ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02759 • Publication Date (Web): 22 Apr 2016 Downloaded from http://pubs.acs.org on April 25, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 13

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

ACS Applied Materials & Interfaces

Correlating lithium hydroxyl accumulation with capacity retention in V2O5 aerogel cathodes Linda W. Wangoh,† Yiqing Huang,‡ Ryan L. Jezorek,‡,§ Aoife B. Kehoe,¶ Graeme W. Watson,¶ Fredrick Omenya,‡ Nicholas F. Quackenbush,† Natasha A. Chernova,‡ M. Stanley Whittingham,‡ and Louis F. J. Piper∗,† Department of Physics, Applied Physics and Astronomy, Binghamton University, Binghamton, New York 13902, USA, Institute for Materials Research, Binghamton University, Binghamton, New York, 13902, USA, and School of Chemistry & CRANN, Trinity College Dublin, Dublin 2, Ireland E-mail: [email protected]

Abstract

sible for the capacity fade.

V2 O5 aerogels are capable of reversibly intercalating more than 5 Li+ /V2 O5 but suffer from lifetime issues due to their poor capacity retention upon cycling. We employed a range of materials characterization and electrochemical techniques along with atomic pair distribution function (PDF), x-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) to determine the origin of the capacity fading in V2 O5 aerogel cathodes. In addition to the expected vanadium redox due to intercalation, we observed LiOH species that formed upon discharge and were only partially removed after charging resulting in an accumulation of electrochemically inactive LiOH over each cycle. Our results indicate that the tightly bound water that is necessary for maintaining the aerogel structure is also inherently respon-

Keywords Aerogel, V2 O5 , pair distribution function, x-ray photoelectron spectroscopy, lithium ion battery, cathodes

1

Introduction

Vanadium pentoxide gels serve as hosts for a wide array of cations such as Mg2+ , Cu2+ , Zn2+ , Na+ and Li+ , and offer multiple redox potentials that make them attractive as cathodes for Li–ion batteries (LIBs). 1–5 They can incorporate high amounts of lithium with capacities as high as 650 mAh/g for 5.8 Li+ /V2 O5 . 6 Water molecules are required stabilize the large interlayer distance that allows several Li+ to be intercalated. 7,8 However, practical applications of V2 O5 aerogels are hindered by their inability to maintain these high capacities over many cycles. 9–13 The poor capacity retention has been attributed to factors such as vanadium dissolution and volume changes. 11,14,15 Most of these studies, however, have not adequately accounted for the interaction of Li+ with the inherent water molecules.

∗

To whom correspondence should be addressed Department of Physics, Applied Physics and Astronomy, Binghamton University, Binghamton, New York 13902, USA ‡ Institute for Materials Research, Binghamton University, Binghamton, New York, 13902, USA ¶ School of Chemistry & CRANN, Trinity College Dublin, Dublin 2, Ireland § Roy & Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA †

ACS Paragon Plus Environment

1


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

The aerogel structure (similar to the better studied xerogel 16 ) consists of double layers of distorted VO6 octahedra with water molecules (referred to as tightly bound water hereafter) stabilizing the interlayer spacing which varies from 10.5 ˚ A to 14 ˚ A depending on the organic solvent used. 17,18 The presence of tightly bound water has been shown to play an important role in determining the interlayer distance and electrochemical performance of the aerogel. 19 Additional surface adsorbed water can also exist, which is considered to have detrimental effects. 7,20 Surface water on the aerogel can be removed by heating the aerogel to 250◦ C without collapsing the structure. 7 Temperatures above 250◦ C have been shown to result in crystalline V2 O5 , 7 which irreversibly transforms into disordered rock–salt structure and suffers from rapid capacity fade when more than one lithium is intercalated. 21,22 Therefore, a minimum amount of tightly bound water must be retained in order to achieve high capacities associated with the aerogel form. Although the electrochemical and structural properties of aerogels have been extensively studied, 1,23,24 it is still unclear the mechanism by which lithium is inserted in the aerogel. In particular, what other reactions occur during lithium insertion/extraction and how they affect the electrochemistry. The loss of long range disorder in aerogels means that one cannot rely on x–ray diffraction (XRD) to study the Li+ insertion and extraction, typically employed for LIB electrodes. Instead, to probe the local chemical and structural changes, x-ray photoelectron spectroscopy (XPS) and atomic pair distribution function (PDF) techniques are required. PDF provides the likelihood of finding atomic pairs separated by the real space distance r, and is especially useful for materials that have long range disorder like aerogels. 17 It also allows one to monitor changes in the structure with changing lithium content. Combined with density functional theory (DFT) calculations, one can identify contributions from individual bond pairs. Meanwhile, XPS, an element specific technique, can be used to monitor changes in the local chemical environments such as vanadium oxidation state and oxygen

Page 2 of 13

species. Combining these techniques should provide insight into lithium reaction mechanisms within the aerogel. Here, we synthesize and fabricate V2 O5 and employ an array of characterization techniques including XPS, PDF and DFT, to study electrochemically cycled V2 O5 ·0.6H2 O aerogel cathodes with varying amounts of Li+ . V2 O5 ·0.6H2 O serves as a model system as the aerogel structure is maintained (i.e. tightly bound water present) whilst ensuring that the surface and adsorbed water are removed. We characterize the aerogel structure and its evolution in the first lithiation/delithiation cycle. In addition to observing reversible vanadium reduction and oxidation, we find concurrent formation of LiOH that is only partially reversible. This is attributed to lithium ions reacting with the tightly bound water, which is considered to accumulate over many cycles contributing to the observed capacity fade.

2

Experimental Details

Synthesis. V2 O5 ·nH2 O aerogel was synthesized using the freeze drying process as described by Sudant et al. 25 The aerogel was synthesized using vanadium(V) oxytriisopropoxide (Aldrich) precursor, water and acetone. The composition consisted of 2.4 mL of precursor and mixture of 7.2 mL of distilled water and 15.0 mL of acetone for a molar ratio of 1/40/20. The precursor and the water/acetone mixture were chilled in an ice bath for 15 minutes to slow down hydrolysis kinetics. The two were then mixed vigorously for less than a minute resulting in gel formation. After a 1 day aging period, the gel was washed with acetone once per day for 3 days to remove the water and hydrolysis byproduct. After solvent exchange with acetone, washing was then done with cyclohexane once per day for 3 days. The cyclohexane was chosen because of its high melting point (7◦ C). The gel was then transferred into a round bottom flask and frozen with liquid nitrogen in a dewar then transferred into an ice bath. A high vacuum pump was used to decrease pressure until all cyclohexane had sub-


2

Page 3 of 13

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


limated. The aerogel was then dried overnight at 85◦ C in a vacuum oven. Electrochemical cells. The cathodes were prepared by mixing the active material, carbon black and polyvinylidene fluoride (PVDF) in the ratio of 8:1:1 in 1–methyl–2–pyrrolidinone solvent. The slurry formed was cast onto an Al foil current collector before drying. The dried electrodes containing 3-5 mg of active material were placed in a coin cell in a He-filled glovebox with pure lithium foil as the counter and reference electrode and a separator between them. LiPF6 (1 M) in a 1:1 volume ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC) was used as the electrolyte. Characterization. Thermogravimetric analysis (TGA) was performed using a Netzsch TG 209 F1 IRIS apparatus using a heating rate of 5◦ C/min from room temperature to 500◦ C under N2 flow. X-ray diffraction (XRD) was perfomed using a Philips PW3040-MPD powder diffractometer operated at 40 kV (20 mA) with filtered Cu Kα radiation (λ= 1.54178 ˚ A) ◦ ◦ collected from 5 to 60 2θ with a step size of 0.05◦ . The Fourier transform infrared spectroscopy (FTIR) was recorded using KBr pellets between 400 cm−1 and 4000 cm−1 (Perkin– Elmer Model 1600). Ex-situ x-ray photoemission spectroscopy (XPS) of the V2 O5 ·nH2 O aerogel samples was performed using a Phi VersaProbe 5000 system without air exposure. All samples were mounted on Ta foil using conductive tape to obtain good electrical contact. The sample mounting and handling was done in a glove box and samples were transported in inert atmosphere to the XPS chamber where they were immediately transferred to the load lock and pumped down. The core-levels (O 1s, V 2p, Li 1s, C 1s) were measured with a pass energy of 23.5 eV, corresponding to an instrumental resolution of 0.51 eV from analyzing both the Au 4f7/2 and Fermi edge of the Au foil. A flood gun was used to charge compensate insulating samples, with energies aligned to the O 1s (530 eV). Ex-situ PDF measurements were carried out at XPD beamline 28-ID at National Synchrotron Light Source II (NSLS II), Brookhaven

National Laboratory using high-energy x-rays of 65.6 keV (λ= 0.189 ˚ A) at room temperature. The high energy x-rays were used in combination with a large amorphous-silicon based area detector to collect total scattering data to high values of momentum transfer. The data was corrected for background scattering, Compton scattering and detector effects within PDFgetX2 and Fourier transformed to get G(r), the PDF. 26 The DFT calculations were performed in VASP, 27,28 which utilizes periodic boundary conditions to simulate a theoretically infinite lattice. A plane wave basis set is used to represent valence electrons while core electrons are modeled using localized atomic orbitals, and interactions between the core and valence electrons (V:[Ar]; O:[He]) are described using projector augmented wave (PAW) approach, with a plane wave cutoff of 500 eV used in this study. The hybrid DFT functional HSE06, 29,30 is used to carry out all calculations, which combines 25% Hartree–Fock exchange energy with the PBE functional. 31 HSE06 has been shown to predict electronic structure of oxides more accurately than standard DFT. 32–35 A Van der Waals correction to the DFT energy using the DFT–D3 method of Grimme et al. 36 is also applied due to the layered nature of the aerogel structure. A k-point mesh of 2 x 5 x 2 and is used to model the aerogel simulation cell, which comprises 40 atoms with 4 formula units of V2 O5 and 4 water molecules (V2 O5 ·H2 O), and a k-point mesh of 2 x 6 x 4 was used for the 14 atom crystalline alpha phase of V2 O5 . A force convergence criterion of 0.005 eV/˚ A per atom was used in all cases. For both structures optimization was performed over a range of constrained volumes with the equilibrium cell volume found by fitting the energy–volume curve to the Murnaghan equation of state, 37 which minimizes the associated problem of Pulay stress. 38

3

Results

a) V2 O5 ·nH2 O Powder The FTIR spectrum of the as–prepared aero-


3


gel is shown in figure 1a. The bands at 1619 cm−1 and 3320 cm−1 are due to O–H and OH–H respectively which confirms the presence of water. The vibration mode at 1002 cm−1 is due to V=O bonds. The V–O–V asymmetric bonds at 768 cm−1 and V–O–V symmetric bonds at 542 cm−1 are also observed. The spectra is consistent with a similarly synthesized vanadium 39 oxide aerogel reported by Chaput et al. The TG curve of the aerogel (Fig. 1b) shows a weight loss of 5% up to 220◦ C that can be attributed to surface water removal. A sharp drop in weight (4%) is observed between the temperature range 220◦ C–260◦ C that is related to removing tightly bound water. 7 Above 260◦ C, a relatively linear decrease is observed. A total loss of 14% is observed for the as-prepared sample. We determine that the as–prepared material corresponds to 1.6 H2 O per V2 O5 , consistent with previous reports. 16,20 The water content drops to 0.6 H2 O per V2 O5 and 0.3 H2 O per V2 O5 in the material heated to 220◦ C and 300◦ C, respectively. The water removal is irreversible for n < 0.5. 40 As a result, we consider heating to 220◦ C to be sufficient to remove adsorbed surface water but maintain the desired aerogel form with 0.6 H2 O per V2 O5 remaining in the aerogel. To confirm the aerogel structure is not lost by annealing to 220◦ C, we perform XRD measurements. The as–prepared sample (Fig. 1c) shows two broad peaks similar to aerogels prepared using freeze drying, 41,42 and supercritical drying 43 methods. The broad peak at 6.57◦ corresponds to an interlayer spacing of ∼13.45 ˚ A along the 25 (001) direction. The second peak at ∼26.12◦ has been attributed to interlayer reflections. 25 The interlayer distance is about the same for the sample heated to 220◦ C, which still has tightly bound water. Peak identification of the crystallized 300◦ C sample confirms the collapse of the aerogel structure and formation of orthorhombic V2 O5 (JCPDS 41-1426). Some V3 O7 (JCPDS 27-0940) impurities are also observed in the 300◦ C sample. Finally, to confirm the vanadium oxidation state, XPS analysis was carried out on the as– synthesized V2 O5 ·nH2 O aerogel powder which displayed a dark green color (Fig. 2). The

V-O-V V=O V-O-V

O-H

Transmittance,a.u.

a)

OH-H

4000

3000

2000

1000 -1

Wavenumber, cm

b)

Amorphous

Crystalline

n = 0.6 95 220°C

Weight, %

100

90 85 100

c)

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

Page 4 of 13

200 300 400 Temperature, °C

500

As prepared 220°C 300°C

α-V2O5 V3O7

10

20 30 40 2 theta, degrees

50

Figure 1: a) Infrared absorption spectra of as–prepared V2 O5 aerogel. Indicated are absorption bands for H2 O and vanadium-oxygen bonds. b) Thermogravimetric curve of the asprepared aerogel heated from room temperature to 500◦ C in N2 . c) X-ray diffraction pattern of the as-prepared, 220◦ C and 300◦ C annealed samples. Included is a orthorhombic V2 O5 reference (JCPDS 41-1426). green color is consistent with the presence of V4+ . The V 2p3/2 core level region showed the aerogel powder was nominally in the V5+ oxidation state with some V4+ . The presence of


4

Page 5 of 13

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


V4+ is essential in the gelation process of the aerogel. 25,40,44

V2O5•nH2O

of the powder. Based on the DFT calculated structure, peak (i), (ii) and (iii) are assigned to O–H, V–O and V–V bonds, respectively. The presence of the O–H bonds supports the FTIR data (Fig. 1 a) that also confirmed the presence of water within the structure. Comparison of the powder and pristine electrode show similarly aligned features especially for values of r

Correlating Lithium Hydroxyl Accumulation with Capacity Retention in

Recommend Documents