A new paradigm for safety design of high-specific energy large-capacity commercial lithium-ion power batteries

On November 18, 2022, the team of Academician Ouyang Minggao of Tsinghua University published a research result entitled “Reductive gas manipulation at early self-heating stage enables controllable battery thermal failure” in the journal Joule.

This achievement reveals the mechanism of “reducing gas attack” induced cathode phase transition reaction in the heat accumulation stage before the battery is violently thermal runaway, and realizes the safety of commercial large-capacity and high-specific energy (60 Ah, 280 Wh/kg) lithium-ion batteries by regulating the generation, transport and attack reaction of reducing gas. This research establishes a new paradigm for battery safety design by regulating the material transport or energy release process in the stage of battery thermal accumulation, which provides a new direction for mechanism research and safety design in addition to traditional material design ideas.

The corresponding authors of the paper are Feng Xuning, Wang Li, Ouyang Minggao; The first author is Wang Yu.

Since the invention of liquid organic carbonate electrolyte systems in the 90s, they have supported the rapid development and application of commercial lithium-ion batteries. However, the current commercial organic electrolyte is highly flammable and volatile, which is considered to be an important cause of severe thermal runaway (TR) and accompanying fire and explosion of batteries, causing serious safety hazards.

In order to solve the safety problems caused by the violent loss of battery control, the traditional safety improvement scheme mainly finds more stable electrolyte substitutes through material design, such as solid electrolytes and non-flammable electrolytes. However, improving the thermal stability of a component of the battery alone often has limited effect on the overall safety improvement of the battery, such as the solid electrolyte still cannot avoid the heat generation reaction between the positive and negative electrodes, and the non-flammable electrolyte components can be vigorously exothermic with the negative electrode and lead to thermal runaway of the battery. At the same time, these new materials still have a long development and iteration cycle for practical application.

In this work, the focus of Academician Ouyang Minggao’s team shifted from the stage of severe loss of control that is difficult to suppress to the milder stage of heat accumulation (HA) in the early stage of battery thermal failure, and shifted the design idea of battery safety from the intrinsic thermal stability of the material system to the process of material transport/energy release in the control of heat accumulation stage. By regulating the “reducing gas attack response” during the thermal accumulation stage of the battery, high battery safety is ensured without changing the material system of the commercial battery (Figure 1).

Figure 1: Schematic diagram of the thermal failure path and safety control method mechanism of “reducing gas attack”.

Figure 2: Reducing gas attack reaction heat generation and positive electrode crystal structure changes.

The experiment found that in the test sample where the positive electrode-negative electrode-electrolyte coexisted, a new major thermogenic reaction peak appeared in the low temperature section of the DSC test. This reaction consumes the reducing gas generated by the negative electrode-electrolyte and produces oxidizing products. Further characterization tests found that the presence of reducing gas in a heating environment below 80°C could induce the transformation of the crystal structure of the ternary cathode material from lamellar to spinel (Figure 2).

In addition, the ability of reducing gases to induce heat generation in the positive electrode phase transition is closely related to the dissociation energy of the bonds within gas molecules (Figure 3). Among the many reducing gases produced by electrolyte reduction, unsaturated hydrocarbons such as olefins and alkynes are more dangerous. Experiments show that under the test environment of 2% C3H4 (Ar balance) gas, the starting temperature of the positive electrode thermogenesis peak is advanced from 210 °C to 155 °C in the pure Ar test environment, which is comparable to the starting temperature of the thermogenic peak in the presence of negative electrode-electrolyte (about 155 °C).

Figure 3: The ability of reducing gases to induce positive electrode phase transitions is related to the lowest bond dissociation energy within the molecule.

Figure 4: Reducing gas regulation effectively suppresses thermal runaway of batteries.

Finally, in order to further support the mechanism research results and provide new ideas for battery safety design, four battery safety technical solutions were developed (Figure 1, namely current regulation, temperature control poisoning layer, forced exhaust and cooling separator). Four schemes are aimed at the generation, transfer or attack process of reducing gas, and a variety of thermal abuse tests such as lateral heating, hot box and adiabatic thermal runaway are verified in 1-50Ah commercial high specific energy cells (NCM811-SiC or Gr). The experimental results show that blocking the reducing gas attack reaction in the heat accumulation stage can effectively inhibit the thermal runaway of the battery (Figure 4).

The study revealed the “reducing gas attack” reaction path that leads to the thermal failure of organic electrolyte lithium-ion batteries, and pointed out that reducing gases with lower intramolecular bond dissociation energy are more dangerous in battery thermal failure. Four safety design schemes for controlling reducing gases were developed, and the severe thermal runaway of high specific energy lithium-ion batteries was effectively prevented. This study establishes a new paradigm for battery safety design by regulating reaction timing in the stage of battery thermal accumulation, which provides a new direction for the study of battery thermal failure mechanism and safety design in addition to the original material design ideas. (Source: Science Network)

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