Development of Non-Flammable Phosphoric Ester Electrolyte for Lithium Metal Batteries By Mixed Salts of LiPF6, LiPO2F2 and LiNO3
Authors: Nishida Tetsuo , Fukunaka Yasuhiro , Nohira Toshiyuki 
Published: December 2024

From the viewpoint of energy density, Li metal-based batteries, such as Li-S batteries and Li-air batteries, are the most attractive candidates for next-generation batteries. However, as is well known, Li metal negative electrode faces critical problems for practical use. First of all, the deposition and dissolution potential of Li is the lowest and Li itself is very reactive, so it reacts immediately with substances in the electrolyte to form a surface film, so-called SEI (solid electrolyte interphase). In commercially available batteries, the decomposition of the electrolyte is self-terminating due to the SEI. However, degradation of the electrolyte will proceed if the decomposition reaction of the electrolyte is not suppressed by the SEI. Next, the Li deposition is non-uniform, resulting in dendrite formation and causing a short circuit. Furthermore, the most serious problem is that an explosion or other catastrophic incidents could occur in the event of the short circuit due to the extreme flammability of the conventional electrolyte. Therefore, it is essential to design Li metal batteries so that they do not burn even if a short circuit occurs.
Various attempts are underway to increase their safety. The simplest and clearest approach toward safety is to replace the flammable electrolyte with a non-flammable electrolyte, and there has also been vigorous research into flame-retardant electrolytes or additives from this strategy. For example, ionic liquids and organophosphorus compounds are well-known. Although the applicability of various types of ionic liquids to Li metal batteries has been studied, ionic liquids still have challenges such as their high viscosity and cost. Various organophosphorus compounds have been also proposed. Among them, phosphoric esters such as trimethyl phosphate (TMP) and triethyl phosphate (TEP) well known as flame retardant additives for plastics are suitable for electrolyte solvents from viewpoints of their high dielectric constant, high salts’ solubility, relatively low viscosity and wide liquid temperature range. The problem with phosphoric ester electrolytes is essentially the low reductive electrochemical stability and resultant low reversibility of Li deposition and dissolution. Normally, favorable SEI formation is unlikely to be formed in phosphoric ester electrolytes. It is well known that anions of Li salts are involved in SEI formation. Matsuda et al. recently reported that the oxygen evolution reaction during charging occurs efficiently in TEP electrolyte with 1M or 3M LiNO3 at the positive electrode of Li-air batteries (1). The effect of LiNO3 on improving the reversibility of Li deposition and dissolution in phosphoric ester electrolytes is also expected.
The author has previously investigated the effect of current density on the morphological variation of electrodeposited Li (2-3). However, there are few studies dealing with the effects of current density on morphological change of Li electrodeposited in phosphoric ester electrolytes in detail.
In this study, Li electrodeposition was conducted in phosphoric ester electrolytes with lithium difluorophosphate (LiPO2F2) compared to commonly used LiPF6 or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as a salt. The author found that LiPO2F2 is relatively well soluble in phosphoric esters up to about 2 M while it has only 1% solubility in ordinary carbonate ester solvents and improves the reversibility of lithium deposition and dissolution as shown in Fig. 1. In the TEP electrolyte with LiTFSI, unlike LiPO2F2, the appearance of fibrous morphology of electrodeposited Li was observed above a certain current density, as shown in the SEM images of Fig. 2. The morphology was considered to be one reason for lowering the reversibility in the case of LiTFSI or LiPF6.
This study presents the morphological variations of electrodeposited Li depending on current densities and Li metal battery characteristics using phosphoric ester electrolytes based on LiPO2F2/TEP with or without ethylene carbonate, LiPF6 or LiNO3 as film-forming agent or supplemental agent for the conductivity. In addition, the result of the flammability test of the newly developed electrolyte will be shown.
References
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(2) T. Nishida, Y. Fukunaka, T. Homma and T. Nohira, J. Electrochem. Soc.,
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(3) T. Nishida, Y. Fukunaka, T. Homma and T. Nohira, J. Electrochem. Soc.,
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, 090529 (2022).

Figure 1