Enthalpy of formation of Li 1+x Mn 2x O 4 (0 < x < 0.1) spinel phases Damian M. Cupid *, Alexandra Reif, Hans J. Seifert Karlsruhe Institute of Technology, Institute for Applied Materials (IAM-AWP), Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen 76344, Germany A R T I C L E I N F O Article history: Received 24 June 2014 Received in revised form 31 October 2014 Accepted 2 November 2014 Available online 4 November 2014 Keywords: Li 1+x Mn 2x O 4 Spinel Oxide solution calorimetry Enthalpy of formation Lithium-ion battery A B S T R A C T The enthalpies of formation of Li 1+x Mn 2x O 4 (0 < x < 0.1) spinel phases were measured using high temperature oxide solution calorimetry in a sodium molybdate solvent at 701 C. One commercially available LiMn 2 O 4 powder and two Li 1+x Mn 2x O 4 samples synthesized using the sol–gel method were used for the measurements. The enthalpies of formation of the spinels from the binary constituent oxides and from the elements become more exothermic with increasing lithium content and increasing average oxidation state of the manganese cation in the range 0 < x < 0.1. Additionally, the lithium-rich boundary of the Li 1+x Mn 2x O 4 phase for 0 < x < 0.1 was re-investigated using differential thermal analysis combined with thermogravimetric analysis. ã 2014 Elsevier B.V. All rights reserved. 1. Introduction The lithium–manganese–oxygen system is a key materials system for the development of advanced cathodes for lithium-ion batteries. In particular, the Li 1+x Mn 2x O 4 cubic spinels (Fd 3m spacegroup) are attractive because they are considered environ- mentally friendly, are stable against Li extraction, have a high electrode potential of approximately 4 V versus lithium, and because the cost of Mn is lower than that of Co which is required for LiCoO 2 production. The Li 1+x Mn 2x O 4 cubic spinels contain a three-dimensional network of occupied tetrahedral (8a) and vacant interstitial octahedral (16c) sites. During charge and discharge, the lithium ions diffuse from the 8a sites to adjacent 16c sites and from these 16c sites to neighboring 8a sites. The reversible capacity of the cubic spinels originates from the Mn 3+ /Mn 4+ redox pair, in which the oxidation state of the transition metal cation oscillates between +3 and +4 during lithium insertion and extraction. The enthalpies of formation of Li 1+x Mn 2x O 4d spinels have been measured by Wang and Navrotsky [1] using high temperature oxide solution calorimetry and by Idemoto et al. [2,3] using acid solution calorimetry. Both Wang and Navrotsky [1] and Idemoto et al. [2,3] report that the enthalpies of formation of the Li 1+x Mn 2x O 4d spinels become more exothermic with increasing lithium content (increasing x in Li 1+x Mn 2x O 4d up to x = 1/4). According to Wang and Navrotsky [1], this trend results from the oxidation of Mn 3+ to Mn 4+ (an exothermic process) which occurs during Li substitution of Mn on the Mn (16d) sites in the absence of oxygen vacancy production. Interestingly, Idemoto et al. [2] showed that an increase in thermodynamic stability of the Li 1+x Mn 2x O 4d cubic spinels, as indicated by a more exothermic enthalpy of formation, leads to better cycling stabilities and decreased capacity losses. Generally, however, there is a discrep- ancy between the enthalpy of formation data of Wang and Navrotsky [1] and Idemoto et al. [2,3]. For example, Idemoto et al. [3] show enthalpies of formation of LiMn 2 O 4 from the constituent oxides of 62.1 1.2, 65.9 1.0, and 70.5 1.3 kJ/mol for their samples which were annealed at 500 C for 48 h in argon gas, at 700 C for 24 h in 1 atm O 2 , and at 500 C for 48 h in 20 atm O 2 respectively. Wang and Navrotsky report a value of 82.47 1.79 kJ/mol [1]. In that work, Li 2 CO 3 and MnO 2 were mixed, pelletized, calcined at 500 C in air for 12 h and subsequently annealed at 750 C for 24 h in air to produce LiMn 2 O 4 . Both Wang and Navrotsky [1] and another work of Idemoto et al. [2] measured the enthalpy of formation of Li 4/3 Mn 5/3 O 4 from the constituent oxides. The value of Idemoto et al. [2] is 101.7 1.5 kJ/mol whereas that of Wang and Navrotsky is 110.26 2.60 kJ/mol. It must be mentioned that Wang and Navrotsky [1] measured the oxygen content in nominal Li 4/3 Mn 5/3 O 4 (their reported composition was Li 4/3 Mn 5/3 O 3.849 ) and took the deviation from stoichiometric oxygen contents into account in the thermodynamic cycle. The aim of this work is to clarify the discrepancies in the published enthalpies of formation data from Wang and Navrotsky [1] and Idemoto et al. [2,3]. We therefore independently measured the enthalpies of formation of the Li 1+x Mn 2x O 4 spinels using high temperature oxide melt drop solution calorimetry in a newly installed Tian–Calvet high temperature calorimeter. Additionally, * Corresponding author. Tel.: +49 7216 822258; fax: +49 7216 824567. E-mail address: [email protected](D.M. Cupid). http://dx.doi.org/10.1016/j.tca.2014.11.003 0040-6031/ ã 2014 Elsevier B.V. All rights reserved. Thermochimica Acta 599 (2015) 35–41 Contents lists available at ScienceDirect Thermochimica Acta journa l home page : www.e lsevier.com/loca te/tca
