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Use of natural graphite based electrodes as insertion anodes in rechargeable magnesium-ion batteries.

Concerning prospective energy storage, lithium-ion batteries (LIBs) are one of the most outstanding technologies due to their high energy density as well as great cycle stability. To increase their operational lifetime, thermal degradation of the most common conductive salt lithium hexafluorophosphate LiPF6 (1) should be avoided. Therefore, it’s crucial to minimize traces of protic impurities resulting from e.g. organic carbonates. In fact, the products of the endothermic equilibrium of LiPF6 are preferably formed at higher temperatures, leading to the formation of hydrofluoric acid (HF) and subsequently to further degradation of electrolyte components. (2)  LiPF6 (s)  LiF (s) + PF5(g)                          [1] PF5 (g) + H2O (l) → O=PF3(g) + 2 HF (l)           [2] Lux et al. (3) followed the HF formation in LiPF6 containing electrolytes by spectroscopic ellipsometry of SiO2 layers. As we reported recently (4), SiO2 probably promotes the degradation of LiPF6 in organic carbonate based electrolytes. Herein, we report about the decomposition of 1M LiPF6 in a binary mixture of ethylene carbonate and diethylene carbonate in a ratio of 40:60 (w/w) at ambient and elevated temperature (cf. table 1). Degradation products are studied by nuclear magnetic resonance spectroscopy (NMR, cf. figure 1), gas chromatography mass spectrometry (GC-MS) and headspace GC-MS (HS-GC-MS). Acid-base and coulometric titration are used to determine the total amount of acid and water content upon aging, respectively. Ultraviolet-visible (UV-Vis) spectroscopy is used to follow the color change of the electrolyte (cf. figure 2). The influence of protic contamination, different housing materials and added active materials on the electrolyte aging is shown. Acknowledgement VOLKSWAGEN VARTA Microbattery Forschungsgesellschaft and Herbert Quandt Foundation are gratefully acknowledged for funding. References 1. K. Xu, Chem. Rev., 104, 10 (2004). 2. C. L. Campion, W. Li, B. L. Lucht, J. Electrochem. Soc., 152, A2327 (2005). 3. S. F. Lux, I.T. Lucas, E. Pollak, S. Passerini, M. Winter, R. Kostecki, Electrochem. Commun., 14, 47  (2012) 4. P. Handel, G. Fauler, K. Kapper, M. Schmuck, C. Stangl, F. Fischer, F. Uhlig, S. Koller, J. Power Sources, 267, 255 (2014). Figure 1

The unabated demand of cost-effective and high performance battery systems compells research groups to look for further improvements of energy storage systems around the world. Lithium-ion batteries seem to be an appropriate solution for this energy issue, but on the other hand extraordinary breakthroughs within the next years are not to be anticipated. Furthermore, lithium-ion batteries still suffer from serious safety concerns. Considering properties such as a rather low reduction potential (-2.36 V vs. SHE), high specific capacity (3380 mAh/cc), low equivalent weight and moreover, sufficient safety due to a non-dendritic metal deposition, magnesium-ion batteries appear to be competitive to lithium-ion batteries [1]. Since organic, aprotic electrolyte systems create a passivation layer on the Mg metal surface preventing a reversible Mg-deposition and dissolution, corrosive electrolytes such as Grignard compounds have to be used [2]. However, research focuses on the development of Mg-battery systems with an operational voltage of 3V Grignard electrolytes do not provide. A strategy to achieve the ambitious 3V goal is the development of intercalation/insertion compounds such as tin or graphite and replacement of corrosive electrolytes by organic ones [3]. From literature it is well known that the Mg-intercalation into graphite is impossible showing even solvated intercalation [4, 5]. However, in this contribution we want to shed light on the intercalation behavior of Mg into graphite with a broad range of electrochemical and spectroscopical analysis methods showing a non destructive and reversible magnesiation and de-magnesiation of a common graphite electrode with organic, aprotic electrolytes (cf. figure 1, figure 2 and figure 3). References: [1] P. Saha, M.K. Datta, O.I. Velikokhatnyi, A. Manivannan, D. Alman, P.N. Kumta. Progress in Materials Science 66, 2014, 1-86. [2] Z. Lu, A. Schechter, M. Moshkovich, D. Aurbach. Journal of Electroanalytical Chemistry, 1999, 203–217. [3] N. Singh, T.S. Arthur, C. Ling, M. Matsui, F. Mizuno. ChemComm, 2012. [4] M. Kawaguchi, A. Kurasaki. Chem. Commun., 2012, 48, 6897–6899. [5] Y. Maeda, Ph. Touzain. Electrochimica Acta, 1988, 1493-1497. Figure 1