allows for the use of cells with thicker (and thus cheaper to produce) electrodes while simultaneously achieving faster charging capabilities. Additional cost savings could be realized if the use of dry rooms and expensive and moisture-sensitive organic electrolytes could be avoided. Due to high fl ammability of conventional LIBs with organic electrolytes and a fear of a thermal runaway, formation of battery packs in electric vehicles and other applications typically adds ≈40% -75% to the weight, volume, and cost of the individual cells. Such undesirable expenditures could be greatly reduced in ALIBs because of their greatly enhanced safety characteristics. Although typically restricted to lower cell voltages due to the evolution of oxygen gas on the cathode at higher potentials or hydrogen gas on the anode at lower potentials, several approaches could be utilized to increase the voltage and thus a specifi c energy of ALIBs. For example, Wang et al. have recently demonstrated fully functioning high-voltage ALIBs by preventing a direct contact of aqueous electrolytes with a Li anode surface. [ 1 ] Such promises stimulated signifi cant interest in the novel ALIB technology.Several types of lithium intercalation compounds commonly used in organic electrolytes have already been tested with aqueous electrolytes and are found to undergo similar redox reactions. LiFePO 4 (LFP), in particular, has attracted attention for ALIB studies due to its good performance in high-power commercial cells with organic electrolytes and its relatively low electrode potential, which should prevent oxygen evolution in aqueous electrolytes during cell charging. Interestingly, LFP and other intercalation compounds in aqueous electrolytes are found to suffer from unique mechanisms of degradation. For instance, Luo et al. investigated the impact of dissolved oxygen on the stability of LiTi 2 (PO 4 ) 3 in aqueous Li 2 SO 4 solution. [ 2 ] They suggested that in its reduced state Li 3-x Ti 2 (PO 4 ) 3 can react with dissolved oxygen in the electrolyte leading to capacity loss, especially when charged/discharged at a slower rate. [ 2 ] After the removal of oxygen from the electrolyte, they demonstrated greatly improved capacity retention for a battery constructed with carbon-coated electrodes using LiTi 2 (PO 4 ) 3

The development of an aqueous lithium ion battery (ALIB) has the potential to greatly improve the safety and lower the cost of lithium ion battery storage systems, while safeguarding the environment. The problem of thermal runaway and ignition of a flammable electrolyte is absent. Aqueous electrolytes exhibit higher ionic conductivities than their organic counterparts [1], allowing for the use thicker electrodes without suffering mass transport limitations. Combined, this promises to allow for a significant reduction in the amount of inactive components required to assemble a battery cell compared to a typical lithium ion battery cell with an organic electrolyte, helping to improve the final weight, volume and cost of an ALIB storage system [2]. Furthermore, the use of dry rooms and expensive and toxic organic electrolytes is avoided. However, prior to commercialization of ALIBs, several challenges need to be overcome. There have been limited studies into the unique side reactions that occur between active materials and aqueous electrolyte, along with methods for reducing the impact of these side reactions on the battery performance [3]. In addition, creative solutions must be found to improve the voltage of the battery in view of the more limited potential window over which aqueous electrolytes are stable [4]. Here, we share the results of our systematic studies on the interactions of lithium iron phosphate (LFP) with aqueous electrolytes of varying composition. Increasing the lithium salt concentration of the electrolyte was found to significantly increase the cycle stability. Post-mortem analyses using microscopy (SEM, TEM), spectroscopy (XRD, EDS, FTIR, XPS), and depth profiling (TOF-SIMS), as well as electrochemical characterization using EIS and charge-discharge, help to elucidate the origins of the capacity fade and reasons for improved cycling stability with higher salt concentrations [2]. We propose that side reactions between water molecules and LFP lead to electrochemical separation of individual particles in the electrode, and that increasing the electrolyte molarity effectively reduces the concentration of water molecules available for these side reactions. We believe that lessons learned while studying the behavior of LFP in aqueous solutions will likely guide a path toward better understanding and optimizing the behavior of other intercalation materials in ALIBs, as we have already observed similar impacts of the aqueous electrolyte composition on the stability of lithium cobalt oxide (LCO) and lithium nickel manganese cobalt oxide (NMC) [5]. References [1] J.-Y. Luo, W.-J. Cui, P. He, Y.-Y. Xia, Nat. Chem. 2010, 2, 760 [2] D. Gordon, Y. Wu, A. Ramanujapuram, J. Benson, J.T. Lee, A. Magasinski, N. Nitta, C. Huang and G. Yushin, Adv. Energy Mater. 2015, 2, 1501805 [3] Y. Wang, J. Yi, and Y. Xia, Adv. Energy Mater. 2012, 2, 830 [4] C. Wessells, R. Ruffo, R. A. Huggins, Y. Cui, Electrochem. Solid-State Lett. 2010, 13, A59 [5] A. Ramanujapuram, D. Gordon, A. Magasinski, B. Ward, N. Nitta, C. Huang, G. Yushin, Energy and Environmental Science 2016 [in press]