Publication date: 15th December 2025
One way to improve the safety of lithium-ion batteries is to replace the flammable organic electrolyte with a water-based alternative. However, due to its narrow electrochemical stability window of 1.23 V, water alone is not suitable for Li-ion battery applications. This limitation can be overcome by employing a water-in-salt electrolyte (WISE) since high salt concentration significantly decreases the molecular activity of water, thereby preventing the decomposition of water with the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Therefore, the electrochemical stability window (ESW) of the electrolyte can be widened up to 3 V once high concentration LiTSFI (21 m) is employed.
Unfortunately, experimental evidence shows that HER is still taking place and is reported as the primary source of parasitic reaction responsible for cell failure and poor electrochemical performance. Here, we investigate the ageing mechanism occurring in LiFePO4/TS2 full cell using water-in-salt electrolyte. By using bulk and surface characterisation techniques (XRD, XPS, SEM, HAXPES, XAS, OEMS, etc.), we will shed light on the ageing mechanisms and explain the possible trategies to keep improving water-in-salt system. As an example, we tuned the electrochemical parameters (applied C-rate, cut-off potential, electrolyte concentration) and as can be seen in Figure 1a, the cut-off potential drastically influences the electrochemical curves since more reversibility are obtained from the electrode cycled with a lower cut-off. Additionally, we employed X-ray Absorption Spectroscopy (XAS) at Fe K-edge (Figure 1b) to probe LiFePO4 electrode after a full delithiation in WISE electrolyte. Despite likely HER reaction occurring during the first delithiation, LiFePO4 electrode is properly delithiated but not following a complete linearity imposed by its biphasic nature. Indeed, there is almost no difference between the pristine LiFePO4 and the one cycled at 25%SOC, indicating that the first delithiation process might be linked to parasitic reaction (SEI, or others). Additional surface sensitive techniques such as X-ray Photoelectron Spectroscopy (XPS) and Hard X-ray Photoelectron Spectroscopy (HAXPES) will be used to complete the picture of ageing mechanisms occurring in water-in-salt electrolyte.
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