Electric vehicles (EVs) are poised to dominate the next generation of transportation, but meeting the power requirements of EVs with lithium ion batteries is challenging because electrolytes containing LiPF6 and carbonates do not perform well at high temperatures and voltages. However, lithium benzimidazole salt is a promising electrolyte additive that can stabilize LiPF6 through a Lewis acid–base reaction. The imidazole ring is not eligible for high-voltage applications owing to its resonance structure, but in this research, electron-withdrawing (−CF3) and electron-donating (−CH3) substitutions on imidazole rings were investigated. According to the calculation results, the CF3 substitution facilitates a high electron cloud density on imidazole ring structures to resist the electron releases from bezimidazole in oxidation reactions. In addition, through CF3 substitution, electrons are accepted from the lattice oxygen (O2–) in lithium-rich layer material and O– is converted by an electron released. The O– is then adsorbed with the ethylene carbonate and catalyzed to alkyl carbonate by Ni2+. The −CF3 substituted benzimidazole triggers a further reaction with alkyl carbonate and forms a new polyionic liquid solid electrolyte interphase on the cathode’s surface. Furthermore, the cycle performance tested at 60 °C and 4.8 V showed that the CF3 substitution maintains the battery retention effectively and exhibits almost no fading compared with both the blank electrolyte and the CH3 substitution.
Ni-rich high-energy-density lithium ion batteries pose great risks to safety due to internal short circuits and overcharging; they also have poor performance because of cation mixing and disordering problems. For Ni-rich layered cathodes, these factors cause gas evolution, the formation of side products, and life cycle decay. In this study, a new cathode electrolyte interphase (CEI) for Ni2+ self-oxidation is developed. By using a branched oligomer electrode additive, the new CEI is formed and prevents the reduction of Ni3+ to Ni2+ on the surface of Ni-rich layered cathode; this maintains the layered structure and the cation mixing during cycling. In addition, this new CEI ensures the stability of Ni4+ that is formed at 100% state of charge in the crystal lattice at high temperature (660 K); this prevents the rock-salt formation and the over-reduction of Ni4+ to Ni2+. These findings are obtained using in situ X-ray absorption spectroscopy, operando X-ray diffraction, operando gas chromatography–mass spectroscopy, and X-ray photoelectron spectroscopy. Transmission electron microscopy reveals that the new CEI has an elliptical shape on the material surface, which is approximately 100 nm in length and 50 nm in width, and covers selected particle surfaces. After the new CEI was formed on the surface, the Ni2+ self-oxidation gradually affects from the surface to the bulk of the material. It found that the bond energy and bond length of the Ni–O are stabilized, which dramatically inhibit gas evolution. The new CEI is successfully applied in a Ni-rich layered compound, and the 18650- and the punch-type full cells are fabricated. The energy density of the designed cells is up to 300 Wh/kg. Internal short circuit and overcharging safety tests are passed when using the standard regulations of commercial evaluation. This new CEI technology is ready and planned for future applications in electric vehicle and energy storage.
Self-terminated oligomer additives synthesized from bismaleimide and barbituric acid derivatives improve the safety and performance of lithium-ion batteries (LIBs). This study investigates the interface interaction of these additives and the cathode material. Two additives were synthesized by Michael addition (additive A) and aza-Michael addition (additive B). The electrochemical performances of bare and modified LiNi0.6Mn0.2Co0.2O2 (NMC622) materials are studied. The cycling stability and rate capability of NMC622 considerably improve on surface modification with additive B. According to the differential scanning calorimetry results, the exothermic heat of fully deliathiated NMC622 is dramatically decreased through surface modification with both additives. The electrode surface kinetics and interface interaction phenomena of the additives are determined through surface plasma resonance measurements in operando gas chromatography–mass spectroscopy (GCMS) and in situ soft X-ray absorption spectroscopy (XAS). The binding rate constant of additive B onto NMC622 particles is 1.2–2.3 × 104 M–1 s–1 in the temperature range of 299–311 K, which is ascribed to the strong binding affinity toward the electrode surface. This affinity enhances Li+ diffusion, which allows the electrode modified by additive B to provide high electrochemical performance with superior thermal stability. In operando GCMS reveals that gas evolution due to the electrolyte degradation at the NMC622 surface contributes to safety hazards in the bare NMC622 material. In situ soft XAS indicates the occurrence of structural transformation in the bare NMC622 material after it is fully charged and at elevated temperatures. The NMC622 material is stabilized by incorporating additives. The unique performance of additive B can be attributed to its linear structure that allows superior electrode surface adhesion compared with that of additive A. Therefore, this study presents an optimized working principle of self-terminated oligomers, which can be developed and applied to improve the safety and performance of LIBs.
Highly delithiated LiCoO 2 has high specific capacity (>200 mAh g −1 ); however, its degradation behavior causes it to have poor electrochemical performance and thermal instability. The degradation of highly delithiated LiCoO 2 is mainly induced by oxygen vacancy migration and weakening of oxygen-related interactions, which result in pitting corrosion and fault formation on the surface. In this research, a coupling agent, namely, 3-aminopropyltriethoxysilane (APTES), was grafted onto the surface of LiCoO 2 to form a cross-linking structure. Through the aza-Michael addition reaction, an oligomer formed from barbituric acid and bisphenol a diglycidyl ether diacrylate were reacted with the cross-linking APTES to form an artificial cathode electrolyte interphase (ACEI). The highly delithiated LiCoO 2 containing the ACEI had considerably less degradation on the surface of the bulk material caused by oxygen release. The formation of the O1 phase was prevented in high delithiation and high-temperature operations. This research revealed that the ACEI reinforced the Co−O bond, which is crucial in preventing gas evolution and O1 phase formation. In addition, the ACEI prevents direct contact between the electrolyte and highly active surface of LiCoO 2 , thereby preventing the formation of a thick and high impedance traditional cathode electrolyte interphase. According to the present results, highly delithiated LiCoO 2 containing the ACEI exhibited outstanding cycle retention and capacity at 55 °C as well as low heat capacity release in the fully delithiated state. The ACEI considerably protected and maintained the electrochemical performance of highly delithiated LiCoO 2 , which is suitable for high-energy-density applications, such as electric vehicles and power tools.
Gradually eradicating petrolic use is therefore the correct direction for maintaining the environment of the earth. Developing energy storage and saving energy consumption is the key to maintaining a good life. Currently, the lithium-ion battery is one of the best choices for the use of vehicles. However, the energy density of the lithium-ion batteries on current developments is less than 300 Wh kg -1 , which is restricted by the capacity of electrode active materials.Si has the maximum theoretical capacity (4000 mAh g -1 ) compared with the graphite (372 mAh g -1 ) and Li 4 Ti 5 O 12 (175 mAh g -1 ). However, Si can only be used in small amounts as an additive (less than 10 wt%) in commercial battery owing to the problems of volume expansion [6,7] and electrochemical irreversibility. [8,9] In terms of previous discussions, the repeatable pulverization of Si during cycling is the key in decaying the performance. [10,11] With the volume expansion and the pulverization, the new surface area of Si is continuously generated and contacts electrolyte for more solid electrolyte interphase (SEI) formation, which increases the impedance and consumes lithium ions significantly. Moreover, 300-400% volume change of Si during cycling makes it difficult for battery design. Several researches have been investigated for solving those problems of Si such as carbon coating, [12] element Silicon (Si) has the maximum capacity compared with the conventional graphite, which can dramatically increase the energy density of the battery. However, due to some tremendous drawbacks of Si material such as electrochemical irreversibility and volume expansion on alloy reaction, pure Si cannot be used in large quantities in the anode electrode. In this research, a polymer brush core-shell structure (PBCS) on Si nanoparticle provides three significant functions because of the intramolecular effect of hydrogen bonding with PBCS and the binder delivers a good dispersion in the slurry, a mechanical protection during cycling, and excellent ionic conductivity for highrate tests. The carbonyl groups of polymer brush on Si surface are fabricated to enhance lithium-ion diffusion and the adjustment of attraction and repulsion by intramolecular hydrogen bonding effect with binder in between each Si particles. The PBCS-Si electrode shows the first coulombic efficiency is 87.1%; the retentions are 92.5% (0.1C/ 0.1C) for 200 cycles and 86.2% (0.5C/ 0.5C) for 400 cycles. Operando TXM displays that the PBCS structure significantly protects the nano Si from cracking owing to the high elastic function and intramolecular hydrogen bonding effect of the PBCS. With this novel PBCS-Si material, a high energy density lithium-ion battery can be expected.
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