Article pubs.acs.org/cm
First-Cycle Evolution of Local Structure in Electrochemically Activated Li2MnO3 Jason R. Croy,† Joong Sun Park,† Fulya Dogan,† Christopher S. Johnson,† Baris Key,† and Mahalingam Balasubramanian*,‡ †
Electrochemical Energy Storage Department, Chemical Sciences and Engineering Division and ‡X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: X-ray absorption spectroscopy is utilized to determine changes in the local structure of Li2MnO3 resulting from electrochemical extraction and insertion of lithium. Specially prepared electrodes allow for a first-cycle charge close to 100% of theoretical, even in crystalline Li2MnO3 prepared at 850 °C. After full delithiation to ∼5 V, the local Mn environments are similar to those of the freshly prepared Li2MnO3 electrodes but with increased disorder. On discharge, significant reduction of Mn is observed accompanied by rearrangement of Mn resulting in Jahn−Teller distorted, Mn3+ local environments. It is also shown, as expected, that the electrochemical and structural behavior of pure Li2MnO3 is markedly different than that observed for the structurally similar Li2MnO3 component of lithium- and manganese-rich composite cathodes.
■
INTRODUCTION After more than 2 decades as a successful commercial technology, rechargeable lithium-ion (Li-ion) batteries have now been pushed very close to what may be their practical limit.1,2 State-of-the-art, layered-oxide cathode materials (e.g., LiMO2, M = Mn, Ni, Co), while housing considerable stores of lithium (∼280 mAh/g), are structurally unstable on cycling unless capacities are maintained below ∼185 mAh/g. As such, energy densities for this class of cathodes, dictated by capacity and working voltage, are currently limited to ∼65% of the theoretical value.1,2 In order to enhance the stability of these cathodes, the composite class of layered-oxides, denoted as xLi2MnO3·(1 − x)LiMO2 (M = Mn, Ni, Co), originally incorporated a Li- and Mn-rich, “Li2MnO3 component” that was envisioned to act as a stabilizing unit to the LiMO2 regions, as Li2MnO3 was considered electrochemically inactive up to high voltages (∼4.4 V). It is now known that when these electrodes are charged above ∼4.4 V, the Li of the Li2MnO3 component can be extracted and at least partially cycled in a reversible fashion.3 This allows for capacities exceeding 250 mAh/g and seemingly substantial gains in oxide energy densities.4,5 Li2MnO3, which has previously been studied as a possible Li-ion cathode, is now receiving renewed interest because of its connection with the promising class of composite cathodes. This “model” compound, therefore, seems an ideal candidate for studying the complexities of Mn4+ species, cation migration, and the role of oxygen in lithium and manganese oxide battery materials. Pure Li2MnO3 has a monoclinic, C2/m structure in which Li+ and Mn4+ ions reside in the octahedral sites of a cubic closepacked, O3, oxygen lattice.6−8 The structure consists of pure Li © 2014 American Chemical Society
layers alternating with layers occupied by Li and Mn in a 1 to 2 ratio and can be denoted as Li[Li1/3Mn2/3]O2 in layered notation. Stoichiometric Li2MnO3 has typically been thought of as electrochemically inactive;9 however, it is now well-known that a large fraction of the lithium can be extracted from the structure.10 Alternatively, by performing a first discharge, lithium can be inserted into Li2MnO3 with clear evidence of Faradaic charge transfer.11 Mn4+ in an octahedral environment is considered to be stable against migration,12 and clear signatures of octahedral Mn5+ are not found in the experiment.13 The natural questions that arise relate to migration, structural evolution, and charge compensation. Current theories include Li+/H+ exchange, Mn4+/5+ redox couple, oxygen oxidation, oxygen vacancy formation, and Li2O extraction and condensation,13−19 though no consensus yet exists on the electrochemical and structural details of Li2MnO3. One limiting factor has been the inability to electrochemically extract the full capacity from Li2MnO3 electrodes during the first charge. Typically, experimental capacities fall well short of the theoretical ∼460 mAh/g; a trait that has been attributed to synthesis conditions, particle size, and inactivation of the material on charging.15−17,20,21 X-ray absorption has recently been used to look at first-cycle processes taking place in Li2MnO3.15 In that work, the local structures of Li2MnO3 were found to remain largely intact on the first-cycle charge and discharge; wherein, Mn4+ ions remained inactive at all times during the electrochemical Received: September 30, 2014 Revised: November 13, 2014 Published: November 20, 2014 7091
dx.doi.org/10.1021/cm5039792 | Chem. Mater. 2014, 26, 7091−7098
Chemistry of Materials
Article
cycle. In order to account for these observations Li+−H+ exchange is cited as a major contributor to the electrochemistry, facilitating the partly reversible shearing of oxygen layers during charge/discharge as a result of lattice protons. The present report builds on a previous work, wherein structural protons were not found to be significant in the electrochemistry of Li2MnO3,22 to show that the local structures of Li2MnO3 are markedly different after the first cycle and that this evolution is correlated to the electrochemical reduction of manganese from 4+ to 3+ on the first-cycle discharge; specifically, Mn is involved in the electrochemistry of Li2MnO3. This can be shown, in part, by the changes in Mn−O bonding that occurs for cycled electrodes. The key to understanding the different interpretations is in the analysis of the EXAFS data as detailed in the present work. It will also be shown that exceptional capacities, very near theoretical, can be electrochemically extracted, even from crystalline Li2MnO3 when electrodes are prepared with a specific protocol. Therefore, the structural data herein are important for future simulations and experiments regarding the transformation mechanisms of Li2MnO3.
■
6
Li magic angle spinning, nuclear magnetic resonance (MAS NMR) experiments were performed at 7.02 T (300 MHz) on a Bruker Avance III HD spectrometer operating at a Larmor frequency of 44.21 MHz, using a 1.3 mm MAS probe. A rotor synchronized Hahn echo pulse sequence was used at a spinning frequency of 67 kHz. A π/2 pulse width of 1.5 μs was used with recycle delays of 0.2 s. All data were collected at a constant temperature of 283 ± 0.1 K, and spectra were referenced to 1 M LiCl at 0 ppm.
■
RESULTS Figure 1 shows charge and discharge data for two drop-cast cells at 30 °C from the Li2MnO3 powders synthesized at 450
EXPERIMENTAL SECTION
Li2MnO3 powders were prepared by thoroughly mixing the stoichiometrically required amounts of Li2CO3 and MnCO3 followed by firing at 450 °C for 36 h, 500 °C for 24 h, or 850 °C for 24 h. Representative X-ray diffraction patterns for low-temperature Li2MnO3 can be found elsewhere23,24 and in the Supporting Information (Figure SI-1) for the 850 °C sample. Electrochemical tests were performed using 2032-type coin cells containing cathode electrodes prepared by two methods. The first method consisted of making a slurry from acetone and Li2MnO3 powders that were mixed with Super P carbon (20% wt). The slurry was drop-cast on the stainless steel spacers/current collectors of the coin cell assemblies, thoroughly dried, and then assembled into cells as usual. The second method consisted of laminating Al foil current collectors with a slurry containing 84 wt % Li2MnO3 powder, 8 wt % Super P carbon, and 8 wt % polyvinylidene difluoride (PVDF) binder in NMP. The process consisted of either (1) making the slurry by simultaneously mixing the Li2MnO3, Super P carbon, PVDF binder, and NMP, or (2) by premixing the Li2MnO3 and Super P carbon with a mortar and pestle before mixing with the NMP and PVDF binder. The types of cells are labeled according to their preparation as Dropcast, Laminate, and Pre-Mixed Laminate. The electrolyte used for all cells was a 1.2 M solution of LiPF6 in a 3:7 mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). Lithium metal anodes were used in all cells, and cycling experiments were performed at 30 °C. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data were collected in transmission mode at the Sector 20-BM beamline of Argonne National Laboratory’s Advanced Photon Source. Pellets of active material dispersed in boron nitride were prepared for ex-situ measurements from fresh powders and cycled powders, collected after disassembling coin cells in an argon-filled glovebox (
