In terms of safety, toxicity, cost and manufacturing simplicity, lithium ion batteries with aqueous electrolytes offer significant advantages over their non-aqueous counterparts. Despite these advantages, a low thermodynamic stability window of water (1.23 V, 2.62 V to 3.85 V vs. Li⁺/Li at pH 7) yields a low cell voltage, hence a low energy density system as compared to state-of-the-art non-aqueous Li-ion batteries. However, recent ‘water in salt’ (WiS),¹ ‘water in bisalt’ (WiBS)² and ‘hydrate melt’³ electrolytes widen the electrochemical stability window of water from 1.23V to >3.0V (~1.8 V to 5.0 V vs. Li⁺/Li), which results an energy density >100 Wh/kg when coupled a S-anode to a LiMn₂O₄ cathode.⁴ While these reports highlight impressive overall cell performances,^1-4 gas evolution analyses of H₂ at anode via HER and O₂ at cathode via OER, or any other gas producing parasitic reaction, e.g. Li₂S + 2H₂O → H₂S + 2LiOH @ S-anode, are not adequately addressed. At Liox, we anticipate that H₂ and O₂ generation from water electrolysis, or any other gas generating parasitic reaction would be detrimental to overall cell performance, and severely limit their ability to be practical and safe energy storage systems. To validate this conjecture, we have designed a special hermetically sealed Swagelok cell with known volume, comprising a high precision pressure sensor (Omega) and a three way Valco valve which can easily be connected to a high resolution mass spectrometer for qualitative and quantitative - in situ or ex situ - analysis of evolved gases during cell operation. We have tested a number of cathode and anode active materials – such as LFP, LMO, NMC, NCA, LCO, S, LTO, C-TiO₂, etc. in the hermetically sealed cell. To study the cathode and anode separately, we assembled a series of symmetric cells; either cathode/cathode for OER or anode/anode for HER, and cycled them to monitor pressure changes, and identify and quantify the evolved gases by high precision RGA mass spectrometer. In this presentation, a series of symmetric and full-cell cycling data (>300 cycles) to discern gas evolving parasitic side reactions, pressure changes during cell operation, in situ and ex situ mass spectrometric analysis of evolved gases will be discussed.

References:

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