ARTICLE Received 17 Dec 2012 | Accepted 18 Apr 2013 | Published 21 May 2013 DOI: 10.1038/ncomms2907 High-performance rechargeable lithium-iodine batteries using triiodide/iodide redox couples in an aqueous cathode Yu Zhao1, Lina Wang1 & Hye Ryung Byon1 Development of promising battery systems is being intensified to fulfil the needs of long-driving-ranged electric vehicles. The successful candidates for new generation batteries should have higher energy densities than those of currently used batteries and reasonable rechargeability. Here we report that aqueous lithium-iodine batteries based on the triiodide/ iodide redox reaction show a high battery performance. By using iodine transformed to triiodide in an aqueous iodide, an aqueous cathode involving the triiodide/iodide redox reaction in a stable potential window avoiding water electrolysis is demonstrated for lithium-iodine batteries. The high solubility of triiodide/iodide redox couples results in an energy density of B0.33 kWh kg À 1, approximately twice that of lithium-ion batteries. The reversible redox reaction without the formation of resistive solid products promotes rechargeability, demonstrating 100 cycles with negligible capacity fading. A low cost, non-flammable and heavy-metal-free aqueous cathode can contribute to the feasibility of scale-up of lithium-iodine batteries for practical energy storage. 1 Advanced Science Institute, Byon Initiative Research Unit (IRU), RIKEN, Hirosawa 2-1, Wako, Saitama 351-0198, Japan. Correspondence and requests for materials should be addressed to H.R.B. (email: [email protected]). NATURE COMMUNICATIONS | 4:1896 | DOI: 10.1038/ncomms2907 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2907 dvancement in clean-energy promoting renewable, safe, (B98% of the theoretical capacity), Coulombic efficiency cost-effective and environmentally friendly energy tech- (499.5%), and cyclic performance (499.5% capacity retention Anology is one of the most significant scientific and for 100 cycles) which is, to the best of our knowledge, the best engineering duties to cut pollution and global warming. result among previous reports using new chemistries and system In particular, development of advanced electric vehicles is one configurations. The energy density reaches B0.33 kWh kg À 1, of the most critical challenges in clean-energy technology to which is higher than that achieved in the conventional lessen air pollution and the dependence on fossil fuels. As a result, batteries. rechargeable batteries have attracted much attention for battery- powered electric motors. According to the Battery Roadmap 2010 Results À À announced by the New Energy and Industrial Technology I3 /I redox reaction in aqueous cathode of Li-I2 batteries. À À Development Organization in Japan, the main target for The I3 /I redox reaction ideally occurs at 0.536 V versus rechargeable batteries is an improvement of the energy SHE (Fig. 1a) via two-electron transfer like the following À 1 À 1 density up to 0.5 kWh kg cell (1.0 kWh l cell) by 2030 (http:// electrochemical equation (1). www.nedo.go.jp/library/battery_rm2010_index.html) to enable À À À o electric vehicles to extend the driving range to a comparable I3 þ 2e $ 3I ; E ¼ 0:536 V versus SHE ð1Þ level with gasoline-powered internal combustion engine vehicles (ca. 500 km). However, achieving more than three times the By using an aqueous half-cell of 1 M of KI solution, the for- energy density raised from the currently employed battery À À À 1 mation of I3 (brown) by the oxidation of I (transparent) and systems (o0.2 kWh kg cell) (ref. 1) is an exceptional challenge, because the current battery technology has almost its reverse reaction could be confirmed via the colour change of reached its performance limitation. Accordingly, new battery electrolyte on a glassy carbon (GC) electrode (see Methods and systems using new chemistries and system configurations are Supplementary Fig. S1), which occurred around 0.57 and 0.49 V versus SHE. needed, which are capable of achieving higher energy density À À than the current ones. The I3 /I redox-couple-based aqueous cathode was prepared with 1 M of aqueous KI by the addition of I2 to adjust Among the new storage systems, non-aqueous lithium-sulfur À the I3 concentration. The solubility of I2 is reasonably (Li-S) and lithium-oxygen (Li-O2) batteries have thus far shown À 1 high in the presence of alkaline iodide, which predominantly the most promising energy density of 0.3 À 0.5 kWh kg À (refs 1,2). However, their poor cycling performance does not transforms I2 to I3 according to the following chemical meet the practical battery criteria, which is mostly due to the equation (2) (ref. 14) parasitic reactions in non-aqueous electrolytes such as the À À I2ðsÞþI $ I3 ; K ¼ 723Æ10 ð2Þ internal shuttling of lithium polysulfides for Li-S batteries2 and instability of non-aqueous electrolytes from superoxide radical2 where K is the equilibrium constant. From this, it is estimated 3,4 B À and at high potential for Li-O2 batteries. In addition, the that there is 0.1 M of I3 in the given mixture of 0.1 M of I2 and À À insoluble insulating discharge products decrease in the electronic 1 M of aqueous KI. The potential of I3 /I redox reaction can be and ionic conductivities. The alternative to make a less calculated by Nernst equation (3): problematic storage system is aqueous lithium batteries5–8. The RT a À aqueous solution has a high ionic conductivity in the presence of E ¼ E0 À ln I ð3Þ À completely ionized substances, which leads to rapid redox nF aI3 reactions in electrochemical cells. The idea has been to employ o these aqueous electrolytes in a cathode, referred to as the aqueous where E is the redox reaction potential, E is the standard cell cathode9,10, using redox couple reactions that do not deteriorate potential, R is the gas constant, T is the absolute temperature, n is the number of moles of electrons transferred, F is the Faraday the aqueous cathode and electrically conductive current collector, À À constant, and aI À and aI À are the activity of I3 and I , as well as not leave over any solid product and precipitation 3 À À residue at the aqueous cathode/current collector interface. The respectively. Assuming that the activity of I3 and I equals to its aqueous cathode can, therefore, offer negligible polarization and concentration, respectively, the redox reaction potential becomes 0.508 V versus SHE in 0.1 M of I2 and 1 M of aqueous KI. volume expansion. À À A promising aqueous cathode can be determined from the The I3 /I aqueous cathode was directly used for the aqueous redox couples possessing high solubility and a suitable redox Li-I2 battery. The aqueous Li-I2 batteries consist of Li anode potential avoiding the electrolysis of water. The solubility is (Cu mesh/Li metal/organic electrolyte/buffer layer), ceramic proportional to the energy density. In the diagram of redox separator, aqueous cathode, and current collector (Super P car- couple solubility with respect to the standard reduction potential bon/Ti foil) as shown in Fig. 1b. Li metal with 1 M of LiPF6 in À À ethylene carbonate (EC)/dimethyl carbonate (DMC) electrolyte (Fig. 1a) (ref. 11), the triiodide/iodide (I3 /I ) redox couple reaction shows a favourable solubility (over 8 mol l À 1). The was used for the anode. The Super P carbon-coated Ti foil was À À employed as the current collector in the aqueous cathode. The redox potential of the I3 /I couples (0.536 V versus standard hydrogen electrode (SHE)) is also suitable to avoid water water-stable and Li ion conductive Li2O-Al2O3-TiO2-P2O5 electrolysis. Therefore, in this work for the first time, we (LATP) glass ceramic (see Supplementary Fig. S2 for the X-ray À À diffraction (XRD) pattern) separated the two electrodes and present the aqueous cathode operated by the I3 /I redox 15 couples and apply this for a lithium-iodine (Li-I ) battery. The allowed only the Li ion to transfer across it . The aqueous Li-I2 2 battery is operated as follows (Fig. 1c). aqueous Li-I2 battery we demonstrate is noticeably different from either the conventional all-solid-state or non-aqueous electrolyte- Li anode: based Li-I2 batteries, which have performed at extremely low Li $ Li þ þ e À ð4Þ discharge current rate or shown low Coulombic efficiency with 12,13 À À the formation of a lithium iodide (LiI) layer . The I3 /I redox reaction in aqueous cathode is rapid and performed up Aqueous cathode: 12 mA cm À 2 of discharge current rate without serious potential I À þ 2e À $ 3I À ð5Þ drop. The aqueous Li-I2 battery attains superior storage capacity 3 2 NATURE COMMUNICATIONS | 4:1896 | DOI: 10.1038/ncomms2907 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2907 ARTICLE 1.2 2+ – Cu /Cu(CN)2 (sulphate) 1.1 – – Br 3/Br (sodium salt) Br /Br – (aqueous solution) 1.0 2 – 3– 4– 0.9 Ru(CN) 6/Ru(CN) 6 (solubility unknown) IrCl2–/IrCl3– (sodium salt) 0.8 6 6 3+ 2+ Fe /Fe (nitride salt) Mo(CN)3–/Mo(CN)4– (solubility unknown) 0.7 8 8 + – – I3 /I (potassium salt) I /I– (aqueous solution) 0.6 2 I– /I– (lithium salt) – 2– 3 MnO4/MnO 4 (potassium salt) 0.5 Sn4+/Sn2+ (fluoride) – – I3 /I (sodium salt) Potential / versus SHE / versus Potential 0.4 3– 4– Fe(CN) 6/Fe(CN) 6 (potassium salt) 0.3 – 2– TcO4/TcO 4 (solubility unknown) 3– 4– 0.2 W(CN) 8/ W(CN) 8 (solubility unknown) 2– 3– OsBr 6/OsBr 6 (solubility unknown) 0.1 S O2– /S O2– (potassium salt) Ru3+/Ru2+ (solubility unknown) 4 6 2 3 0.0 0123456789101112 Solubility per mol per litre water 0.4 – e 0.4 Oxidation scan 0.3 Discharge –1 – – → – 0.3 0.01–0.25 mV s I3 + 2e 3I + 0.2 Li 0.2 e– 0.1 0.1 0.0 Li – e– → Li+ 3I– – 2e– → I– 0.0 3 –0.1 + Anode – Current (mA) –0.1 Cathode Li Separator e Peak current (mA) Peak –0.2 –0.2 Li+ + e– → Li –0.3 –0.3 Reduction scan Charge –0.4 3.0 3.2 3.4 3.6 3.8 4.0 4.2 0.1 0.2 0.3 0.4 0.5 e– Potential / versus Li+/Li 1/2 (mV1/2 s–1/2) À À Figure 1 | I3 /I redox reaction-based aqueous cathodes.