Lithium-ion batteries - ultra-high capacity carbons and thermal safety of lithium-intercalated carbons

Lithium-ion batteries – ultra-high capacity carbons and thermal safety of lithium-intercalated carbons

  1. Ultra-high-capacity carbon

Recently, more interest and research have focused on the development of ultra-high-capacity carbon materials, which are synthesized at relatively low temperature (500–1000 °C) and release a reversible capacity exceeding 372 mA h/g. Researchers have proposed many models and theories to explain the “extra” lithium-ion storage capacity.

Sawai et al. postulate that high-capacity carbons provide high-capacity lithium intercalation spaces, so the mass specific capacity of these carbons should be higher than that of graphite; Yazami et al. propose that multiple layers of lithium are formed on the graphite sheets; Peled et al. attribute the extra capacity to Due to the moderate oxidation of graphite, this is because the cross-section between two adjacent crystallites and between adjacent defects and impurities can accommodate lithium; Sato et al. propose that lithium occupies adjacent sites in the lithium-intercalating carbon; Osaka research team proposes Additional lithium is present in nanoscale holes; Yata et al. discuss the possibility of LiC2 formation in “polyethylene semiconducting” carbon with high-level spacing (~0.400 nm); Matsumura et al. postulate that in addition to intercalated lithium intercalated in graphite In addition, small-grained carbons at the edges and surfaces of graphitic layers can also store considerable amounts of lithium; Xiang et al. suggest that lithium intercalated at the edges of graphitic layers can generate a voltage plateau of about 1 V (vs. Li/Li+).

Dahn and his colleagues conducted a systematic study of lithium-intercalating carbons. They give a comprehensive explanation for the high lithium storage capacity in disordered carbon. For both soft carbon and hard carbon, when the heating temperature does not exceed 800 °C, high capacity is obtained accompanied by a strong hysteresis phenomenon, and the hysteresis capacity is proportional to the hydrogen content in the carbon, so lithium is bound in some way in the hydrogen nearby. Inaba et al. investigated the thermal behavior of low-temperature treated MCMBs during charge-discharge cycles. The hysteresis in the voltage curve is accompanied by a huge heat release. This phenomenon can be explained by the activation energy, which is consistent with the above-mentioned theory for the lithium-hydrogen interaction. As the heating temperature increases, the hydrogen in the carbon is consumed. The capacity available after hydrogen is consumed depends mainly on the crystal structure of the carbon formed. Soft carbon has many disordered disordered structures after heating above 1000°C, so the capacity is lower than 372mA·h/g. As the heating temperature increases, the graphitization effect and the ordered stacking of graphite layers increase, so the capacity increases and approaches 372 mA • h/g. In contrast, the hard carbons obtained at around 1000 °C have little dehysteresis effect, releasing a capacity greater than 372 mA h/g at low potentials of a few millivolts (vs. Li/Li+). Dahn proposed that lithium is “adsorbed” on both sides of the single-layer sheet, arranged like a “fortress in the sky”. If you are interested in reading more about lithium storage knowledge, visit Tycorun Lithium Battery.

  1. Thermal safety of lithium intercalated carbons
Figure 1 - Schematic diagram of the intercalation to intercalation carbon structure transition of Nb2O5/C nanohybrid materials.
Figure 1 – Schematic diagram of the intercalation to intercalation carbon structure transition of Nb2O5/C nanohybrid materials.

Most abusive conditions like short circuits, shocks, needle sticks, overcharge/overdischarge, etc. cause the battery to heat up. Safety issues arise when the battery exceeds a critical temperature and thermal runaway occurs. Therefore, the thermal stability of lithium-ion batteries and the combined use of various battery components are very necessary to understand and improve the safety of batteries. Accelerating calorimetry (ARC) and differential scanning calorimetry (DSC) are commonly used to investigate the origin of thermal runaway in LixC. An acceleration calorimeter is a sensitive adiabatic calorimeter that records the temperature change of a reaction sample as it self-exotherms. In the ARC study, initially, the sample temperature and calorimeter were heated to the initial temperature, and then the self-heating rate of the sample in adiabatic state was monitored. The ARC study demonstrated that the self-heating of LixC6 is determined by at least four factors: the degree of initial lithium intercalation, the electrolyte, the specific surface area of ​​carbon, and the initial heating temperature of the sample. When the metastable SEI is transformed into the stable SEI, a peak is generated on the self-exothermic rate curve, and the intensity of the peak becomes larger as the carbon specific surface area increases.

Richard and Dahn proposed a mathematical model for the heat generation of LixC6 samples in an electrolyte. They used this model to calculate the self-heating rate curve and the DSC curve (DSC is used to measure a fixed heating rate). The DSC study demonstrated that the exothermic reaction occurred for the first time between 12O and 14O°C, which was due to the conversion of the components of the metastable SEI film to LiF and LiC2O3. With further heating, LixC6 will react with the molten PVDF glue through dehydrofluorination at around 200°C. The former reaction mainly depends on the linear relationship between the specific surface area of ​​carbon and the initial irreversible capacity; the latter reaction depends on the PVDF content, the degree of lithium intercalation and the specific surface area of ​​carbon.

To improve the safety of Li-ion batteries, many studies have also focused on the development of flame-retardant electrolytes. An easy route is to add flame retardants to the electrolyte.

Read more: What are the safety concerns of cathode layered materials?

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