Electrolytes for Li-Ion Batteries DOI: 10.1002/anie.201408648 Superhalogens as Building Blocks of Halogen-Free Electrolytes in Lithium-Ion Batteries** Santanab Giri, Swayamprabha Behera, and Puru Jena* Abstract: Most electrolytes currently used in Li-ion batteries contain halogens, which are toxic. In the search for halogen- free electrolytes, we studied the electronic structure of the current electrolytes using first-principles theory. The results showed that all current electrolytes are based on superhalogens, i.e., the vertical electron detachment energies of the moieties that make up the negative ions are larger than those of any halogen atom. Realizing that several superhalogens exist that do not contain a single halogen atom, we studied their potential as effective electrolytes by calculating not only the energy needed to remove a Li + ion but also their affinity towards H 2 O. Several halogen-free electrolytes are identified among which Li(CB 11 H 12 ) is shown to have the greatest potential. Li-ion batteries play an important role in modern portable electronics, due to their light weight and high energy density. [1–3] The three primary components of a Li-ion battery are the anode, cathode, and electrolyte. Whereas graphite is used as the most popular commercial anode, the cathode is generally composed of metal oxides, layered oxides (such as lithium cobalt oxide), polyanions (such as lithium iron phosphate), or a spinel (such as lithium manganese oxide). [4–7] The Li + ions that move from anode to cathode when discharging and reverse when charging are supplied by the electrolytes. Considerable research is under way to improve the cost, efficiency, durability, and safety of Li-ion batteries by improving the materials of the anodes, cathodes, and electrolytes. [8–15] Here we focus on the electrolytes that act as ion carrier between the anode and cathode when current flows through an external circuit. The current electrolytes consist of lithium salts such as LiAsF 6 , LiBF 4 , LiPF 6 , LiFePO 4 , LiClO 4 , LiN- (SO 2 F) 2 , and LiN(SO 2 CF 3 ) 2 , combined with organic solvents like ethylene carbonate and dimethyl carbonate. [16–19] Although these electrolytes are commercially available and popularly used in Li-ion batteries, they have certain disad- vantages. With the exception of LiFePO 4, the above electro- lytes contain halogens, which are toxic. LiAsF 6 is poisonous, whereas LiClO 4 is explosive. LiBF 4 has inferior ability in forming solid electrolyte interphases at the graphite elec- trode, [20–22] whereas LiN(SO 2 CF 3 ) 2 corrodes the cathode. [23] LiPF 6 decomposes to PF 5 and LiF, the former readily hydrolyzing to form HF and PF 3 O. These two products are very reactive on both the cathode and anode surfaces and impact negatively on the electrodes performance. [24] Recently it has been shown that LiFePO 4 suffers from the same memory effect that has plagued nickel–cadmium and nickel–metal hydride batteries, which gradually lose usable capacity if recharged repeatedly after being only partially discharged. [25] Furthermore, Li-ion batteries have limited performance at elevated temperatures and due to surface phenomena on both electrodes, their life cycle is also limited. [26] The safety features of commercially prepared Li- ion batteries are also insufficient for large size applica- tions. [27, 28] To tackle these problems, several attempts have been made by either introducing new solvents or using different salts and additives. [29, 30] There are three characteristics of electrolytes that need improvement. First, they should be halogen-free to improve safety. Second, since the binding energy between Li + and the anionic part of the salt plays an important role in ion conduction, it should be small so that ions can move easily from one electrode to the other. Third, the affinity of the electrolyte to water should also be low so as to increase battery life. Here we show that an in-depth understanding of the electronic structure and stability of current electrolytes allows us to address the above-mentioned three challenges. We begin by analyzing the electronic structure of the negative ions of the Li salts. These are BF 4  , PF 6  AsF 6  , FePO 4  , ClO 4  , N(SO 2 F) 2  , and N(SO 2 CF 3 ) 2  , as discussed above. We note that the oxidation state of B is + 3, whereas it is + 5 for P and As. Fluorine, on the other hand is electro- negative and needs only one electron to satisfy its electronic shell closure. Consequently, BF 4 , PF 6 , and AsF 6 need one extra electron for shell closing. With the electronic config- uration of Fe 2+ ,P 5+ , and O 2 , FePO 4 also needs one extra electron for electron shell closing. In the case of ClO 4  , four oxygen atoms need eight electrons to close their electronic shells, but Cl can contribute a maximum of seven electrons. Thus, one extra electron is needed to fulfill the electronic shell closure. Electron counting in N(SO 2 F) 2 and N(SO 2 CF 3 ) 2 is slightly more complicated as the oxidation state of N can vary from 3 to + 5 and that of S from 2 to + 6, depending on the nature of the ligand. For example, in N 2 O, N is in the + 1 state, whereas it is in the 3 state in NH 3 . In N(SO 2 F) 2 , N, S, O, and F have oxidation states of + 1, + 4, 2, and 1, respectively. In N(SO 2 CF 3 ) 2 , on the other hand, the oxidation state of N is [*] Dr. S. Giri, S. Behera, Prof. P. Jena Department of Physics, Virginia Commonwealth University Richmond, Virginia (USA) E-mail: pjena@vcu.edu [**] This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engi- neering under Award no. DE-FG02-96ER45579 and the VCU Presidential Research Initiative program. Resources of the National Energy Research Scientific Computing Center supported by the Office of Science of the U. S. Department of Energy under Contract no. DE-AC02-05CH11231 is also acknowledged. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201408648. A ngewandte Chemi e 1 Angew. Chem. Int. Ed. 2014, 53,1–5  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! Ü Ü