A smart risk-responding polymer membrane for safer batteries

Abstract

Safety concerns related to the abuse operation and thermal runaway are impeding the large-scale employment of high-energy-density rechargeable lithium batteries. Here, we report that by incorporating phosphorus-contained functional groups into a hydrocarbon-based polymer, a smart risk-responding polymer is prepared for effective mitigation of battery thermal runaway. At room temperature, the polymer is (electro)chemically compatible with electrodes, ensuring the stable battery operation. Upon thermal accumulation, the phosphorus-containing radicals spontaneously dissociate from the polymer skeleton and scavenge hydrogen and hydroxyl radicals to terminate the exothermic chain reaction, suppressing thermal generation at an early stage. With the smart risk-responding strategy, we demonstrate extending the time before thermal runaway for a 1.8-Ah Li-ion pouch cell by 100% (~9 hours) compared with common cells, creating a critical time window for safety management. The temperature-triggered automatic safety-responding strategy will improve high-energy-density battery tolerance against thermal abuse risk and pave the way to safer rechargeable batteries.

Fig. 1.. Mitigation of battery thermal runaway by the SRR strategy.

(A) Thermal runaway incidents of rechargeable Li-ion batteries, including overheating, smoking, fires, and even explosions. (B) Thermal decomposition issues (separator meltdown and electrolyte/cathode breakdown) in a thermal abuse state, releasing a large quantity of highly reactive free radicals (e.g., H^•, HO^• and O^•), causing severe thermal accumulation and final thermal runaway. (C) Schematic of application limitations of P-based flame-retardant additives or cosolvents for Li-ion batteries, including poor passivation at the anode-electrolyte interface and limited cyclic performance. (D) Application superiorities of the TPF membrane for Li-ion batteries at a normal operation state. The TPF membrane with covalently linked diethyl allylphosphonate (DEAP) functional groups substantially enhances the electrochemical stability with anodes and enables a long-term cycle life. (E) Graphical illustration of mitigation of thermal runaway in batteries by the TPF membrane when the battery temperature increases beyond 90°C. P-containing free radicals produced from TPF pyrolysis interfere with the exothermic process by radicals quenching, which mitigates thermal runaway to bring enhanced battery safety. PVDF-HFP, poly(vinylidene fluoride-co-hexafluoropropylene); TMPETA, trimethylolpropane ethoxylate triacrylate.

Fig. 2.. Structure and thermal stability of the TPF membrane.

(A) Polymerization of DEAP and TMPETA monomers by ultraviolet (UV) curing. (B) Optical image of a roll of TPF-Celgard composite membrane. Photo credit: Ying Zhang, Institute of Chemistry, Chinese Academy of Science. (C) Fourier transform infrared (FTIR) spectra of TPF, DEAP, and TMPETA. a.u., arbitrary units. (D) Density functional theory (DFT) calculations of bond energy at different sites of the DEAP functional group at 298.15 K. (E) Free radicals produced at different sites of the DEAP functional group and possible products from radicals quenching. (F) Combustion experiment of polyethylene (PE), TMPETA-PEO (PE oxide), and TPF membranes. Photo credit: Ying Zhang, Institute of Chemistry, Chinese Academy of Science. (G) Shrinkage factors of PE and TPF membranes in the temperature range of 20° to 160°C. (H) Differential scanning calorimetry (DSC) profiles of a carbonate electrolyte consisting of 1 M LiPF₆ in ethylene carbonate (EC)/diethyl carbonate (DEC)/dimethyl carbonate (DMC) (volume ratio: 1:1:1) and charged NCM811 cathode with PE or TPF.

Fig. 3.. TPF-based high-performance Li-ion batteries at room temperature.

(A) Electrochemical stability window of TPF in comparison with an electrolyte containing free DEAP measured by linear sweep voltammetry at 0.1 mV/s. (B) Schematic illustration showing the SEI structure on the surface of different SiO_x-graphite (SiO_x-G) electrodes in contact with the free DEAP solvent (top panel) and TPF (bottom panel). (C and D) Typical charge-discharge voltage profiles and cycling performance of the SiO_x-G//TPF//NCM811 cell at 0.2 C. N/P, negative/positive capacity ratio. (E) Gravimetric energy density of a SiO_x-G//TPF-Celgard//NCM811 cell (inset: schematic of the SiO_x-G//TPF-Celgard//NCM811 cell with the dimensions of different components). Refer to table S1 and fig. S16 for more details. (F) The projected energy density of G//LFP, SiO_x-G//LFP, Li//LFP, G//NCM811, SiO_x-G//NCM811, and Li//NCM811 pouch cells with a design capacity of 10 Ah based on the TPF-Celgard membranes. Refer to tables S2 to S4 for the detailed cell parameters.

Fig. 4.. Thermal stability of pouch cells.

(A) Optical image of a G//NCM811 pouch cell. Photo credit: Ying Zhang, Institute of Chemistry, Chinese Academy of Science. (B) Typical charge-discharge profiles of the G//NCM811 pouch cells using commercial Al₂O₃-Celgard and TPF-Celgard separators. (C) ARC profiles of the charged pouch cells, which record the time and temperature at the occurrence of thermal runaway. For a clear comparison, both of the profiles adopt the same scale. T_s is defined as the self-heating temperature, T_a is defined as the heating accelerating temperature during the thermal runaway process when dT/dt is 1°C s⁻¹ (the dT/dt-T and dT/dt-t curves are shown in fig. S17) (30, 37), and T_max is defined as the maximum temperature during thermal runaway. (D) Thermogravimetry (TG) and DSC profiles of PE and TPF membranes. (E) Timeline of thermal incidents in the 1.8-Ah G//NCM811 pouch cells loaded with different membranes.