First principles calculations and experiments for Cu-Mg/Li hydrides negative electrodes
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First principles calculations and experiments for Cu-Mg/Li hydrides negative electrodes M.H. Braga1,a, V. Stockhausen1, M. Wolverton2, J.A. Ferreira3, J.C.E. Oliveira1,b 1
Engineering Physics Department, FEUP, Porto University, R. Dr. Roberto Frias, s/n, 4200-465,
Porto, Portugal and CEMUCa and CFPb. 2
LANSCE, Lujan Center, Los Alamos National Laboratory, mail stop: H805, NM, 87545, USA.
3
Energy and Geology National Laboratory, LNEG, R. da Amieira, S. Mamede Infesta, Portugal.
ABSTRACT We have studied CuLi0.08Mg1.92 and determined that the compound reacts with hydrogen to form CuLi0.08Mg1.92H5 [1]. Additionally, we have proposed the compound as a negative electrode material which is the main purpose of the present study. Moreover, we have observed that the latter compound acts as a catalyst in the formation of MgH2, LiH, TiH2 [2] and hydrogen desorption. In this work, first principles and phonon calculations were performed in order to establish the reactions occurring at the negative electrode of a Li conversion battery in presence of CuLi0.08Mg1.92H5 and (Li) – solid solution of Mg in Li – approximately Li2Mg3. We have calculated the minimum theoretical specific capacity to be 1156 mAh/g (for an anode with 100% of CuLi0.08Mg1.92H5) and the △Eeq = 0.81 V (vs. Li+/Li) at 298 K. Furthermore, we have determined all the reactions occurring in the referred system and its sequence using Inelastic Incoherent Neutron Scattering (IINS) and X-Ray Diffraction (XRD). INTRODUCTION Li-ion batteries are appealing as they provide higher energy density compared to other rechargeable batteries. The use of Li-metal as negative electrode improves the specific capacity but rises safety issues. In a Nat. Mater. paper, Oumellal et al. [3] investigated the use of MgH2, TiH2 and NiMg2H4 as enhanced negative electrodes in Li batteries. These authors addressed a type of reaction with implications for both Li-ion batteries and fuel cells. It was shown that MgH2 reacts with Li according to the equation MgH2+2Li++2e-Mg+2LiH (△Eeq = 0.56 V Li+/Li). MgH2 electrodes lead to a theoretical maximum discharge capacity of 2034 mAh/g (experimental: 1500 mAh/g), while they possess a reversible capacity of 1125 mAh/g in comparison with 372 mAh/g for graphite (currently used in commercial Li-ion batteries). Although Oumellal et al. succeeded in reducing cell polarization, metal hydride electrodes remain plagued by a large initial irreversible capacity (~30%) and poor capacity retention limited to less than 20-50 cycles regardless of the cycling rate or voltage window. Other Li alloy anodes, like Li22Si5, also suffer from short life-cycle due to the extremely high increase in volume (△Vtheoretical = 316 %). We have studied CuLi0.08Mg1.92 and determined that the compound reacts with hydrogen to form CuLi0.08Mg1.92H5[1]. Moreover, we have observed that the latter compound acts as a catalyst in the formation/hydrogen desorption of MgH2, LiH and TiH2 [2]. Therefore, such a system is a potential candidate for application as negative electrode in Li convers
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