Article pubs.acs.org/JPCC
Investigation of the Structural and Electrochemical Properties of Mn2Sb3O6Cl upon Reaction with Li Ions Viktor Renman,*,† Mario Valvo,† Cheuk-Wai Tai,‡ Iwan Zimmermann,‡,§ Mats Johnsson,‡ Cesar Pay Gómez,† and Kristina Edström*,† †
Department of Chemistry − Ångström Laboratory, Ångström Advanced Battery Centre, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden ‡ Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden S Supporting Information *
ABSTRACT: The structural and electrochemical properties of a quaternary layered compound with elemental composition Mn2Sb3O6Cl have been investigated upon reaction with lithium in Li half cells. Operando XRD was used to investigate the potential impact of this particular layered structure on the lithiation process. Although the results suggest that the material is primarily reacted through a conventional conversion mechanism, they also provide some hints that the space between the slabs may act as preferential entry points for lithium ions but not for the larger sodium ions. Cyclic voltammetry, galvanostatic cycling, HRTEM, SAED, and EELS analyses were performed to unravel the details of the reaction mechanism with the lithium ions. It is found that two pairs of reactions are mainly responsible for the reversible electrochemical cycling of this compound, namely, the alloying of Li−Sb and the conversion of MnxOy to metallic Mn0 with concomitant formation of Li2O upon lithium uptake. A moderate cycling stability is achieved with a gravimetric capacity of 467 mAh g−1 after 100 cycles between 0.05 and 2.2 V vs Li+/Li despite the large particle sizes of the active material and its nonoptimal inclusion into composite coatings. The electrochemical activity of the title compound was also tested in Na half cells between 0.05 and 2 V vs Na+/Na. It was found that a prolonged period of electrochemical milling is required to fully gain access to the active material, after which the cell delivers a capacity of 350 mAh g−1. These factors are demonstrated to clearly limit the ultimate performances for these electrodes.
1. INTRODUCTION Graphite represents the most popular anodic material for the realization of Li-ion batteries with high energy density, due to its characteristic reaction potential close to the Li+/Li redox couple. Its success so far has been mainly due to a series of favorable characteristics, which together all account for the outstanding electrochemical properties displayed by this anode material. Its stability is nested in its robust layered structure, which undergoes a topotactic reaction with Li ions yielding only a moderate volume change (i.e., 9%) during Li insertion/ deinsertion. The ordered graphene sheets in its structure also provide a high electrical conductivity that promotes the kinetics of the reactions, enhancing the electron transfer. Its gravimetric specific capacity is positively affected by the light weight of the carbon atoms and results in a theoretical value of 372 mAh g−1 for a fully lithiated LixC6 compound (i.e., 0 < x ≤ 1). However, this electrochemical reaction cannot handle more than one e− and Li+ per formula unit, thus limiting the ultimate storage capacity. Accordingly, different elements and compounds have been investigated since the early development of Li-ion cells to obtain reversible redox reactions involving multielectron transfer to boost the capacity for the corresponding anodes. Two major classes of electrochemical reactions have been © 2017 American Chemical Society
identified as alternatives to Li insertion in graphite, namely, Li alloying of metallic (e.g., Sn, Sb, Ag, etc.) and semiconductor (e.g., Si, Ge, etc.) elements, as well as conversion reactions of various metal compounds (i.e., MX, where X = O, P, N, S, etc.). Both classes of reactions have proved particularly efficient in delivering a high number of electrons in their respective processes, thus increasing the storage capacity. However, a series of other crucial issues (i.e., severe volume changes, destruction of the host structures, limited electrical conductivities, etc.) are also unavoidably introduced by the intrinsic nature of these reactions, which force a whole reconstitution of the initial active materials. In this regard, the morphology, structure, texture, and size of the pristine active materials are of the utmost importance since they directly affect the resulting electrochemical performances upon lithium uptake and removal. For metal oxides (MOs) and their use as conversion-based anodes for Li-ion batteries, it has been demonstrated that the crystal structure of the pristine material has a significant impact Received: December 30, 2016 Revised: March 1, 2017 Published: March 2, 2017 5949
DOI: 10.1021/acs.jpcc.6b13092 J. Phys. Chem. C 2017, 121, 5949−5958
Article
The Journal of Physical Chemistry C
using a dedicated In-lens detector for the secondary electrons. “Post-mortem” SEM analysis of the extensively cycled sample required an initial step of cell disassembly in an Ar-filled glovebox and the subsequent washing of the reacted electrode in dimethyl carbonate (DMC) to remove the electrolyte species present on its surface. A piece of the washed electrode was then attached onto a SEM stub having an adhesive carboncontaining tape. The prepared sample was thereafter sealed in an inert atmosphere and quickly transferred into the SEM chamber in order to significantly minimize contact with air. A series of dedicated measurements combining operando XRD/electrochemical experiments were performed using a STOE & Cie GmbH Stadi-P powder diffractometer equipped with a Dectris Mythen 1K position-sensitive detector (∼18 in 2θ). All diffraction patterns for the lithium system were collected using a Cu Kα1 X-ray radiation in transmission mode using the same type of pouch cell design described earlier. The lithium-ion cell was cycled at a slow rate of C/110 (i.e., full utilization of its discharge capacity in 110 h), and the experiment was set up so that one full XRD pattern was collected every hour. Complementary to these operando measurements, ex situ XRD experiments were also performed. The corresponding cycled cells were disassembled in an Ar-filled glovebox (MBraun), and the reacted electrode material was then sealed with polyimide tape (Kapton) to prevent possible oxidation due to air exposure during the measurements. Transmission electron microscopy (TEM) images were collected using a field-emission electron microscope (JEOL JEM-2100F) operated at 200 kV equipped with a Gatan Ultrascan 1000 CCD camera and an imaging filter (Tridiem 863). “Post-mortem” samples of the Li-ion batteries were prepared by carefully scratching the cycled- and DMCprewashed electrode material onto a TEM copper grid (i.e., supporting a holey carbon film) under inert atmosphere in an Ar-filled glovebox. The grid was then transferred from the glovebox to the microscope column by using a JEOL vacuumtransfer holder. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were carried out to study in depth the morphological, structural, and textural features of the resulting nanocomposite and to facilitate a systematic phase identification for the reacted compound. Electron energy-loss spectroscopy (EELS) was also performed to obtain additional information about the chemical environments and valence states of the elements involved in the electrochemical reactions. Core-loss spectra of Mn-L2,3 and LiK edges were collected by the Gatan Tridiem imaging filter in spectroscopy mode with a dispersion of 0.2 eV/channel. The acquisition time depended on the signal-to-noise ratio. Further analysis of the EELS spectra, e.g., background subtractions, was performed by means of the Gatan Digital Micrograph software suite.
on the subsequent electrochemical behavior and cycling stability, despite the fact that these hosts are always destroyed and undergo amorphization during their first discharge.1,2 This can be related to the properties of the nanostructured composite that is simultaneously formed in situ upon incorporation of lithium. Here, we investigate for the first time the electrochemical properties of Mn2Sb3O6Cl, which allows for a combination of both Li-alloying and conversion reactions within the same electrode material. The presence of the stereochemically active lone pair on Sb3+ in combination with the inclusion of a halide ion (i.e., Cl−) is effectively a way to control the dimensionality of the crystal structure, which clearly can play a major role in this context. The principle governing this important aspect is the so-called “Hard−Soft Acid−Base” (HSAB) theory. More extensive elaborations on this theory, as well as examples of dimensionally tailored compounds, can be found elsewhere3,4 along with our previous electrochemical investigation of a related transition metal oxofluoride.5 Due to the preference of Sb3+ to bond to O2− rather than to Cl−, the crystal structure can be tuned in this type of quaternary compounds by adjusting the elemental ratios since Mn2+ readily bonds to both Cl− and O2−. In this case, Mn2Sb3O6Cl adopts a layered structure that is upheld by electrostatic repulsion of the lone-pair electrons of Sb3+. We speculate that the layered voids can provide suitable entry points for Li+ and Na+ and thus facilitate the lithiation or sodiation process, thereby increasing the overall conversion efficiency and ultimately leading to a more homogeneous in situ formation of a nanocomposite with good electrochemical properties.
2. EXPERIMENTAL SECTION Mn2Sb3O6Cl was prepared by a ceramic method using a mixture of MnCl2, MnO, and Sb2O3. The full details of the synthetic procedure are reported elsewhere.6 Electrochemical measurements were performed in polymerlaminated pouch cells. The synthesized powder was ground thoroughly by ball milling. Electrodes were prepared by spreading a dispersion of 75% active material, 15% carbon black (Super P, Timcal), and 10% of CMC (carboxylmethyl cellulose, Sigma-Aldrich) onto a nickel foil. Circular electrodes with a diameter of 2 cm were dried overnight under vacuum at 80 °C in a tubular oven placed in an argon-filled glovebox (O2 and H2O levels
