J. Phys. Chem. C 2010, 114, 10999–11008
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Electrode/Electrolyte Interface Reactivity in High-Voltage Spinel LiMn1.6Ni0.4O4/Li4Ti5O12 Lithium-Ion Battery R. Dedryve`re,*,† D. Foix,† S. Franger,‡ S. Patoux,§ L. Daniel,§ and D. Gonbeau† IPREM/ECP, UniVersity of Pau, He´lioparc Pau Pyre´ne´es, 2 aV. Pierre Angot, 64053 Pau cedex 9, France; ICMMO/LPCES, UniVersity Paris XI, 15 aV. Georges Clemenceau, 91405 Orsay cedex, France; and CEA/DRT/LITEN, 17 rue des Martyrs, 38054 Grenoble cedex 9, France ReceiVed: March 24, 2010; ReVised Manuscript ReceiVed: May 11, 2010
High-voltage spinel oxides combined with Li4Ti5O12 result in 3 V lithium-ion batteries with a high power capability; however, the electrochemical performances are limited by electrode/electrolyte interfacial reactivity at high potential. We have investigated electrode/electrolyte interfaces in LiMn1.6Ni0.4O4/Li4Ti5O12 cells by X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectrocopy (EIS). EIS has shown that both electroadsorption and film-formation mechanisms occur at the positive electrode. XPS has revealed that very low amounts of lithiated species are deposited at the surface of the positive electrode, despite the high potential, but that great amounts of organic species are deposited. Interesting results were obtained for the Li4Ti5O12 electrode. Whereas Li4Ti5O12 is usually considered as a passivation-free electrode material, large amounts of organic and inorganic species were deposited at the surface of this electrode. The question of a possible interaction between both electrodes in the formation mechanisms of surface films is discussed. 1. Introduction Lithium-ion batteries are today the usual power source of portable electronic devices and other nomad applications. Research efforts in the field of lithium-ion batteries are now motivated by an increasing need for efficient energy storage systems for renewable energies and urban transportation.1-4 For future Li-ion battery applications such as electric vehicles (EVs) or hybrid electric vehicles (HEVs), safety concerns are the most important point and require the development of new electrode materials. The use of a carbon negative electrode raises problems for high-power applications such as EVs/HEVs because of safety concerns. Indeed, due to the low Li+ intercalation potential of carbon electrodes, during a fast charge metallic lithium may be plated at the surface of the electrode instead of being intercalated. Repeated fast charges may result in dendritic growth of Li0 and lead to internal short circuits. Lithium titanate Li4Ti5O12 is an interesting negative electrode material for high-power applications such as EVs/HEVs.5-7 Its lattice parameters are almost unchanged upon Li+ ion insertion/extraction, which makes Li4Ti5O12 a very robust electrode material with an excellent cycling reversibility. Moreover, it displays an excellent lithium ion mobility and authorizes high charge rates without any problem of dendrite formation. However, its high working potential (1.55 V vs Li+/Li) is a drawback for energy density and requires the use of high potential positive electrodes. LiFePO4 is now considered as a good candidate as positive electrode material for EVs/HEVs or other future applications and has been the subject of an intense international scientific research since it was proposed in 1997 by Goodenough and co-workers.8 Indeed, it is safe, robust, and less expensive than LiCoO2 and sustains high cycling rate capability and high specific capacity. However, whereas LiFePO4 has many advan* Corresponding author. E-mail
[email protected]. † University of Pau. ‡ University Paris XI. § CEA/DRT/LITEN.
tages, it suffers from its low potential (3.45 V vs Li+/Li) in comparison with layered oxides, particularly in prospect of using Li4Ti5O12 as negative electrode. For example, the combination of Li4Ti5O12 with LiFePO4 results in a 1.9 V Li-ion battery.9,10 The use of high-voltage positive electrode materials like spinel oxides, i.e., LiMn2O4 and its doped variants, allows to enhance the cell energy density.11 Numerous research works have been focused on simple or multisubstitutions of various elements for manganese in LiMn2O4 (for example, Al, Cu, Ti, Cr, Co, and Ni) to enhance the capacity at high voltage or to improve the cyclability.12-16 Potentials up to 5 V vs Li+/Li can be reached in LiMn2-xMxO4 spinel oxides (M ) Ni, Fe).17,18 For the past years intensive efforts have been devoted to the development of LiMn1.5+xNi0.5-xO4 materials.19,20 In these materials, manganese remains essentially in the +IV oxidation state under normal cycling conditions, reducing problems associated with Jahn-Teller distortion of Mn3+ ions in six-coordination sites and Mn dissolution into the electrolyte, which are predominant causes of capacity fading of LiMn2O4. The theoretical capacity of the LiMn1.5IVNi0.5IIO4/Mn1.5IVNi0.5IVO4 redox system is 146.7 mAh g-1 (700 Wh kg-1), with a redox potential of 4.7-4.75 V vs Li+/Li corresponding to NiII/NiIII and NiIII/NiIV couples. The synthesis and composition of these materials have been optimized for the past several years. As a result, it appears very interesting to combine high-voltage spinel oxides with Li4Ti5O12 in order to get a 3 V system with a high power density (charge and discharge), which is useful for EVs/HEVs and power tools for instance.21,22 In recent papers, some of us have depicted the advantage of using high-voltage nickel-manganese spinel oxides LiMn2-xNixO4 versus Li4Ti5O12 through the presentation of optimized electrochemical performances.23,24 However, no unquestionable solution for a stable electrolyte above 4.2-4.5 V vs Li+/Li has been found yet. Indeed, conventional electrolytes consisting of LiPF6 salt dissolved in carbonate mixtures give a reactive electrode/electrolyte interface at high voltages. Despite ab initio calculations predicting that common electrolyte solvents used in Li-ion batteries have
10.1021/jp1026509 2010 American Chemical Society Published on Web 05/28/2010
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J. Phys. Chem. C, Vol. 114, No. 24, 2010
oxidation stability up to 5.5 V vs Li+/Li (propylene carbonate PC, ethylene carbonate EC, dimethyl carbonate DMC), in practice the oxidation potential limits are far below, as illustrated by the work of Kanamura et al.25,26 For instance, the stability of a PC-based electrolyte falls from 5 V (calculation) to 4.2 V vs Li+/Li (experiment). The origin of this difference may be multiple: catalytic effect of the surface of the active material, traces of water in the electrochemical system, influence of lithium salt, etc. This reactivity of the electrolyte toward the high-potential positive electrode results in a rapid self-discharge process (up to 80% per month), which prevents commercial use for the moment.24 We believe that the material itself cannot be significantly further optimized. Improvement of the stability of electrode/electrolyte interfaces is expected to be the key point for large-scale applications of these high-voltage materials. Therefore, in this paper we will focus on the study of electrode/ electrolyte interfaces in the LiMn1.6Ni0.4O4/Li4Ti5O12 system in order to better understand the degradation mechanisms of the electrolyte that are responsible for self-discharge processes and poor electrochemical performances. This study will be carried out with two complementary techniques: X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectrocopy (EIS), which allow a chemical and an electrochemical analysis of the interfaces, respectively. These two techniques, which provide a very different approach of the analysis of the interfaces, are not so commonly used together. 2. Experimental Section 2.1. Materials Synthesis. High-voltage nickel manganese spinel oxide LiMn1.6Ni0.4O4 was prepared by solid state reaction, starting from stoichiometric proportions of the respective carbonates MnCO3, Li2CO3, and NiCO3 · 2Ni(OH)2. Pure powders were obtained after mechanical activation using extensive ball-milling in hexane (PM100 Retsch equipment) and successive thermal treatments at high temperatures (10 h at 600 °C and 15 h at 900 °C) with specific cooling rate, usually 1.0 °C min-1. Particles were then treated in mildly acidic aqueous solution to separate agglomerates and to clean surfaces. The obtained material has a specific surface area of ∼0.7 m2 g-1 and particle sizes between 1 and 20 µm. Pure spinel oxide Li4Ti5O12 was obtained after multisteps energetic ball-milling of TiO2 and Li2CO3 in stoichiometric proportions in hexane, and thermal treatments at temperatures up to 900 °C during several hours, with a final quick treatment under an argon atmosphere, as previously described.27 The obtained product has a specific area of 13.5 m2 g-1 and laser granulometry revealed two populations of particles between 0.1 and 100 µm size. 2.2. Electrochemical Cycling Conditions. Electrochemical tests were realized in 2032 type coin cells, using thin composite electrodes prepared by thoroughly mixing active material (spinel oxides) with Super P carbon black (Timcal) and PVdF binder (poly(vinylidene difluoride), Solef 1015, Solvay) in NMP (Nmethylpyrrolidone). Weight proportions of active material/ carbon/binder are 89/6/5 for the LiMn1.6Ni0.4O4 electrode and 82/12/6 for the Li4Ti5O12 electrode. As-prepared mixtures were then coated onto aluminum foils using a doctor blade with 100 µm gap for the positive electrode and 140 µm gap for the negative one. Electrodes (1.54 and 2.00 cm2 disks for positive and negative electrodes, respectively), with loading of ∼7 mg of active material (∼0.8 mAh cm-2), were dried for 24 h at 55 °C, pressed (6.5 t cm-2), and finally dried again for 48 h at 80 °C under vacuum. Coin cells were assembled in an argon-filled drybox (
