Enhancing Cycle Stability of Lithium Iron Phosphate in Aqueous Electrolytes By Increasing Electrolyte Molarity

Abstract

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]

A High-Voltage and Cycle Stable Aqueous Rechargeable Na-Ion Battery Based on Na2Zn3[Fe(CN)6]2–NaTi2(PO4)3 Intercalation Chemistry

“…This increased solvation free energy implies an increased difficulty for the chemical and electrochemical dissolution of Na 2 Zn 3 [Fe(CN) 6 ] 2 when using a concentrated 17 m NaClO 4 electrolyte. 25,28 In addition, we performed an in situ XRD study of Na 2 Zn 3 [Fe(CN) 6 ] 2 electrodes to investigate and determine the structural changes during cycling in 1 and 17 m NaClO 4 electrolyte. When 1 m NaClO 4 was used as electrolyte (Figure 4a and Figure S6), a new cubic phase 20 with 2θ values of 17.10°, 24.26°, 34.60°, and 38.83°attributed to Zn 3 [Fe-(CN) 6 ] 2 was observed during Na-ion extraction.…”

“…20 Using highly concentrated electrolytes appears to be a facile and efficient way to extend the electrochemical stability window of aqueous electrolyte 4,11,21−24 and suppress the electrochemical dissolution of electrode-active materials. 17,25,26 For example, Suo et al 11 first reported a wide stability window of >2.5 V for an aqueous electrolyte solution by use of 9.26 m sodium trifluoromethanesulfonate (NaOTF) electrolyte. Very recently, Nakamoto et al 12,17 revealed that increasing the salt concentration of NaClO 4 aqueous solutions can effectively reduce the electrochemical corrosion of Na 2 MnFe(CN) 6 due to a common-ion effect, which restricts the dissolution of MnFe(CN) 6 2− anions into the aqueous electrolyte and therefore dramatically enhances the cycling stability of the electrode.…”

Influence of Binders, Carbons, and Solvents on the Stability of Phosphorus Anodes for Li-ion Batteries

“…Cycling degraded cells at an elevated temperature should accelerate ionic transport without significantly effecting electrical transport, and thus help to differentiate electrical and ionic degradation mechanisms. 34,35 Figure 6 shows how none of the cells recovered any significant portion of their capacity when cycled afterward at 60 °C. This points to the loss of electrical contact between particles, whether via volume change from lithiation/delithiation of P and/or from large SEI growth that induces electrical isolation of the active material.…”

Enhanced Electrochemical Stability of Molten Li Salt Hydrate Electrolytes by the Addition of Divalent Cations

“…Yushin et al also reported stable charge−discharge cycling of LiFePO 4 electrodes by increasing Li salt molarity in aqueous electrolytes. 18 Despite the facile approach, there is a complex rational mechanism behind the successful demonstration of moderately high voltage ARLBs with molten salt hydrate electrolytes. A high concentration of Li + ions significantly reduces the activity of uncoordinated water, leading to the enhanced oxidative stability of the electrolyte.…”