The improvement of the energy density of lithium-ion batteries often comes with an increase in safety risks. The current mainstream electrolyte system is based on carbonate solvents, which have low flash points and are flammable and volatile. Under abuse conditions such as overcharging, short circuiting, or high temperature, they are highly prone to cause thermal runaway, leading to battery fires or even explosions, posing serious safety hazards. At working temperatures above 50°C, the side reactions between the electrode and the electrolyte will accelerate sharply, resulting in rapid capacity degradation of the battery and limiting its reliable application in wide temperature ranges. Flame-retardant phosphate ester solvents are considered an ideal alternative to carbonate solvents. However, the long-standing problem of incompatibility with graphite anodes has hindered their commercialization: in conventional or low-salt concentrations, phosphate ester electrolytes cannot form a stable solid electrolyte interface (SEI) on the surface of graphite anodes, and the electrolyte will be co-embedded in the graphite interlayers with lithium ions, causing continuous decomposition and structural damage.
Currently, a team has innovatively synthesized a new type of six-membered ring fluorinated phosphoric acid ester solvent, named 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphosphorinane-2-oxide (HTP). Compared to the traditional five-membered ring phosphoric acid ester, the six-membered ring structure of HTP has lower ring tension, which significantly enhances the thermal stability and chemical stability of the molecule from a thermodynamic perspective, and inhibits side reactions such as ring-opening decomposition. Based on this, the team selected tri-(2,2,2-trifluoroethyl) phosphate (TFEP) as a co-solvent and coordinated with lithium bis(fluorosulfonyl)imide (LiFSI) salt, successfully constructing a low-concentration all-phosphoric acid ester electrolyte system with a lithium salt concentration of only 1.2M. The core advantage of this electrolyte lies in that through unique inter-solvent interactions, it regulates the solvation structure of lithium ions, thereby inducing the formation of a stable and dense SEI film on the graphite anode, overcoming the compatibility problem.
This work is based on a new type of hexacyclic phosphoric acid ester solvent (HTP), and successfully constructed a low-concentration all-phosphoric acid electrolyte. This electrolyte, with its wide electrochemical window, extremely high flash point and excellent flame retardancy, enables a 4.5 V graphite||NMC811 battery to operate stably at up to 100°C, and passes rigorous safety tests such as puncture, breaking through the bottleneck where traditional electrolytes are difficult to balance high safety and high voltage and high-temperature performance.
