Article pubs.acs.org/cm
Differential Electrochemical Mass Spectrometry Study of the Interface of xLi2MnO3·(1−x)LiMO2 (M = Ni, Co, and Mn) Material as a Positive Electrode in Li-Ion Batteries Elias Castel, Erik J. Berg, Mario El Kazzi, Petr Novák,* and Claire Villevieille Paul Scherrer Institut, Electrochemistry Laboratory, CH-5232 Villigen, Switzerland S Supporting Information *
ABSTRACT: Lithium-rich mixed metal layered oxides constitute a large class of promising high-potential positive electrode materials in which higher specific charges are accessed only by activation of the Li2MnO3 domains. During the activation, oxygen is extracted from the oxide and evolves at the electrode−electrolyte interface. Differential electrochemical mass spectrometry was employed to follow volatile species developed during cycling. Although typical Li-ion aprotic carbonate electrolytes already suffer from oxidative decomposition at high potentials, the presence of O2 is here confirmed to enhance its reactivity. During the first cycle, O2 and CO2 evolve and their respective amounts vary as a function of the cycling conditions. However, for ethylene carbonate-based electrolytes, the amount of O2 and CO2 is found to be independent of the electrolyte composition. Moreover, X-ray photoelectron spectroscopy revealed that carbon-based components of the solid layers are dissolved between 3.0 and 4.0 V versus Li+/Li where no gas is evolving. oxygen atoms are extracted from the HE-NCM lattice.16,17 The oxygen extraction is presumed to originate from the oxygen oxidation of Li2MnO3 domains during activation,18,19 as suggested by X-ray diffraction20 and internal cell pressure studies of HE-NCM.21 Although the extracted oxygen evolves partly as a gaseous product, these reactive oxygen species may also participate in subsequent reactions at the HE-NCM−electrolyte interface and contribute to the formation of thick surface “films” via electrolyte oxidation.17,22 A good understanding of the interface processes between electrodes working at very positive potentials and model electrolytes23 during cycling is an imperative, prior to the suggestion of any electrolyte additives24,25 or other strategies26 for improving the cycling stability and calendar life of lithium-ion batteries. To follow the evolution of the interfacial chemistry in HENCM half-cells, we analyzed the gas evolution occurring during the first cycles using differential electrochemical mass spectrometry (DEMS). X-ray photoelectron spectroscopy (XPS) was also employed to understand the evolution of solid surface layers before and after cycling.
I. INTRODUCTION The rapid progress in the portable electronics industry has amplified the need for more efficient power sources.1 Among competing energy storage technologies, rechargeable lithiumion batteries are able to deliver high energy and power densities at reasonable costs.2 Today, most of the efforts are focused on improving the electroactive materials for the positive electrode as they determine the final performances and the warranty characteristics (safety and calendar life) of lithium-ion batteries.3 During the past several decades, binary or ternary oxides from the LiCoO2−LiNiO2−LiMnO2 phase diagram have been presented as alternatives to replace LiCoO2.4,5 With various manganese-rich oxides, high thermal stability and specific charges of 180 ± 20 mAh/g have been reported.3,6 The inherent safety and the electrochemical performance of these manganese-rich oxides were further improved by the substitution of lithium for transition metals, e.g., Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2.7−9 Among these advanced positive electrode materials, the commercial overlithiated nickel cobalt manganese oxide xLi2MnO3·(1−x)LiMO2 (M = Ni, Co, or Mn; x ≈ 0.5) (HED HE-NCM, BASF SE), hereafter called HE-NCM, displays a high and reversible specific charge {∼250 mAh/g [see the electrochemical cycling experiments in the Supporting Information (Figure S1a,b)]}.10,11 Even though the monophasic or biphasic nature (“layered−layered” model) of the HENCM material is still under debate,12,13 there are publications suggesting the presence of Li2 MnO3 domains in this compound.14,15 During the initial charge, up to 4.5−4.6 V versus Li+/Li, lithium ions are deintercalated from the Li2MnO3 domains and © XXXX American Chemical Society
II. EXPERIMENTAL SECTION 1. DEMS Working Electrodes. The working electrodes were prepared by doctor-blading mixtures of 80 wt % HE-NCM active material (BASF SE), 10 wt % Super P carbon black (TIMCAL), and 10 wt % polyvinylidene fluoride (PVDF Kynar Flex, Arkema) suspended in an N-methylpyrrolidone solvent (Sigma-Aldrich) directly Received: June 17, 2014 Revised: July 25, 2014
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dx.doi.org/10.1021/cm502201z | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
Figure 1. Potential and gas evolution profiles of CO2 and O2 as well as cumulative gas amounts measured during galvanostatic cycling of HE-NCM electrodes versus Li in the LC30 electrolyte. Gas evolution onset/range potentials are defined by the dashed lines labeled by a−c as guides for the eye. onto the titanium metal current collectors of the DEMS cell.21,27 The NMP was evaporated under vacuum at 80 °C overnight. The geometrical surface area of the coatings is ∼7.8 ± 0.1 cm2. The working electrodes were finally dried at 120 °C under vacuum and directly transferred into an argon-filled glovebox before finally being assembled into the electrochemical DEMS cells with metallic lithium as the counter electrode.27 The average electrode loading with respect to the oxide was ∼6.4 ± 0.4 mg/cm2. For the comparative study, electrochemical cycling was performed with the following electrolytes: LP30 (Merck, Selectipur), which consisted of 1 M LiPF6 in a 1:1 mixture (w/w) of ethylene carbonate (EC) and dimethyl carbonate (DMC); LP10, which consisted of 1 M LiPF6 in a 3:3:4 mixture (w/ w) of EC, DMC, and ethyl methyl carbonate (EMC); and LC30 (Novolyte Technologies), which consisted of 1 M LiClO4 in a 1:1 mixture (w/w) of EC and DMC. The content of water in these electrolytes (lithium battery grade) was
