https://doi.org/10.1002/adfm.202311782
Fast charging times in lithium-ion batteries (LIBs) exceeding the standard 0.3 C (3.3 hours charging time) are a pressing goal, but the slow desolvation kinetics of ethylene carbonate-based conventional electrolytes, lithium deposition and dendrite growth at graphite anodes, and fire hazards have hindered this goal.
Recently, the team of Jiyoung Heo from Sangmyung University in South Korea, Young Joo Lee from the Korea Institute of Basic Sciences, and Seung-Wan Song from Chungnam National University reported a novel weakly bound fully linear molecular-based non-flammable electrolyte (WNLE), including 1 m LiPF6-methyl carbonate and 2,2,2-trifluoroacetate and additives, and the LIB charging speed is 10-20 times faster than that of traditional electrolytes. The main advantages of WNLE are a 44% reduction in viscosity, a 62% increase in Li+ diffusion coefficient, a 20% increase in Li+ migration number, and a 17% reduction in desolvation energy, which facilitates Li+ diffusion kinetics and desolvation kinetics near the graphite anode, achieving a dendride-free LIB while also being non-flammable. The WNLE-based 800 mAh industrial graphite//lithium nickelcobalt manganese oxide (high active mass 13 mg cm-2) lithium-ion pouch battery achieves an excellent 700 cycles at 3 C (charging in 20 minutes) with 82% capacity retention and up to ≈100% coulombic efficiency. The formation of a robust solid electrolyte interface layer at the anode and cathode mitigates interface failures, enabling fast charging speeds of up to 7 C and longer cycle life. This new electrolyte formulation is a promising solution and a new opportunity to enable fast-charging LIBs to operate safely for long periods of time in real-world applications.
【Key points】
A lower viscosity compared to conventional electrolytes will be highly beneficial for improving the impregnation and wettability of active material particles in high-loaded, high-density graphite anodes and nickel (Ni)-rich cathodes, allowing for uniform diffusion of Li+ ions and the formation of SEI. The viscosity of a liquid electrolyte is highly dependent on the choice of solvent. The conventional liquid electrolyte consists of 1.0-1.2 m lithium hexafluorophosphate (LiPF6) as a Li+ ionophore solvent in a cyclic EC, and a linear dimethyl carbonate (DMC), diethyl carbonate (DEC), and/or methylethyl carbonate (EMC) co-solvent with a viscosity of 3-4 cP.
In response to the constraints and several problems caused by fast charging, the researchers propose six electrolyte design strategies for fast charging LIBs.
First, linear carbonate molecules were specifically selected, which have a lower viscosity, significantly weaker binding strength to Li+ ions than cyclic ECs, and moderate salt dissociation capacity. Second, linear structural ester molecules were chosen as the solvent class, which have a lower viscosity and higher ionic conductivity than linear carbonates, as evidenced by the effect of methyl acetate or methyl propionate replacing DMC in conventional electrolytes to improve ionic conductivity.
In addition, the researchers also employed ethyl 2,2,2-trifluoroacetate (TFA) because fluorine substitution with electron adsorption (especially alkoxy functionality) and selection of shorter chain ester molecules would favor improved (electro)chemical stability and reduced viscosity, respectively, compared to unsubstituted counterparts. Since TFA has a lower minimum unoccupied molecular orbital (LUMO) energy level than EC and EMC, TFA is subjected to cathodic reduction earlier when the graphite anode forms SEI.
Third, the combination of weakly bonded linear carbonate (EMC) and TFA with Li+ can greatly improve ionic conductivity and low viscosity, and more importantly, reduce the desolvation energy near the graphite anode.
Fourth, non-flammability or low flammability is considered a basic criterion for battery safety. In general, non-flammable TFA is non-flammable when used in larger quantities than flammable EMC, so the ratio of EMC to TFA should be optimized to meet the requirements for fast charging capability and non-flammability.
Fifthly, the concentration of lithium salts is fixed at 1.0 m, which is economical and provides a low viscosity of a high concentration of electrolytes.
Sixth, the electrolyte formulation includes SEI-forming additives such as ethylene carbonate (VC) and hexafluorobutane anhydride (HFA) to compensate for the deficiencies of EMC and TFA on graphite anodes and nickel-rich cathodes, respectively. These methods are all practical strategies for large-scale commercialization.
Based on the above strategy, the researchers designed and developed a new formulation of a non-flammable, low-viscosity electrolyte based on weakly bound molecules (EMC and TFA) (named WNLE) that enables graphite-based LIBs to be quickly charged to 7 C (charged in 8.6 minutes) and cycled at 3 C for long periods of time (20 minutes to be charged), which is 10-20 times faster than traditional charging protocols. The fast charging capability of WNLE, consisting of 1.0 m LiPF6 in EMC and TFA (30:70 by volume ratio), 2 wt% VC and 0.1 wt% HFA additives, was demonstrated to be a Ni0.8Co0.1Mn0.1O2 (NCM811) lithium-ion pouch cell stacked with 10 high-load 2.9 mAh cm-2 NMC811 cathodes and 11 high-load graphite anodes stacked on top of a conventional electrolyte at a high rate of 3 C. Unprecedented fast charging performance and cycle life are obtained. The current electrolyte design strategy based on linear carbonate and linear fluoride ester weakly bound solvents is a breakthrough to achieve fast charging, safety, long cycle life, low cost, and high energy density for next-generation LIBs.
Figure 1. a) Flammability test results of 1.0 m LiPF6/EMC:TFA (30:70 by volume) electrolyte. b) Comparison of ionic conductivity and viscosity of 1.0 mLiPF6/EMC:TFA (30:70 by volume) and 1.0 mL LiPF6/EC:EMC (30:70 by volume) electrolytes. d) Comparison of the diffusion coefficients of Li+ and PF6- ions and the number of migration of Li+ ions between 1.0 m LiPF6/EMC: TFA and WNLE and 1.0 m LiPF6/EC: EMC (without and with VC+HFA additives) (conventional electrolyte). e) Rate capability of 800 mAh graphite//NCM811 (cathode active mass 13 mg cm-2) lithium-ion pouch batteries using WNLE or conventional electrolytes at different rates from 0.1 C (charged in 10 hours) to 7 C (charged in 8.6 minutes). f) Schematic diagram of WNLE's fast-charging lithium-ion battery, and a summary of WNLE's advantages over conventional electrolytes.
Figure 2. a) 17O NMR profiles of pure TFA, EMC, EMC:TFA (50:50% by volume) mixture, and 1.0 m LiPF6/EMC:TFA (50:50% by volume). 0 m LiPF6/EMC: Change in 17O NMR chemical shift of carbonyl oxygen and ether oxygen in TFA electrolyte as a function of solvent ratio.
Figure 3. a) Geometry of the Li+/EMC:TFA solvent complex optimized at the PBE/TZP level in a covalent first solvated sheath of 4. b) Comparison of the Gibbs energy of desolvation at 298.15 K with the Mulliken charge of the Li+-solvent in a Li+ solvent complex depending on the solvent molecule.
Figure 4. a) Linear-scanning voltammetry of a three-electrode cell using a conventional electrolyte or WNLE. b) Initial charge-discharge curves during two formation cycles of an 800 mAh graphite//NCM811 (cathodic active mass of 13 mg cm-2) lithium-ion pouch cell using conventional electrolyte or c) WNLE. 800 mAh pouch lithium-ion battery e) DCIR variation, f) thickness variation.
Figure 5.a,c) Optical and SEM images of the circulating graphite anode at 3C under conventional electrolyte and b,d) WNLE conditions. e) Lithium 1s XPS spectra and curve fitting results of primitive graphite anode and recirculating graphite anode under (i) conventional electrolyte and (iii) WNLE conditions. f) SEM images of conventional electrolytes and g) cyclic NCM811 cathodes at 3C in WNLE. h) (i) Ni 2p3/2 peaks on the Ni 2p XPS spectrum of the original and cyclic NCM811 cathodes in WNLE and (iii) conventional electrolytes and (iii) WNLE, and h') their curve fitting results.
【Conclusion】
The researchers have developed a low-viscosity, non-flammable, fast-charging WNLE formulation consisting of 1.0 m LiPF6/EMC:TFA with a 30:70 volume ratio with the addition of 2 wt% VC and 0.1 wt% HFA additive) that can fast charge industrial 800 mAh graphite//NCM811 lithium-ion pouch cells at 3 C (within 20 minutes) and achieve long-term stable cycling performance with a high capacity retention rate of >82% after 700 cycles , high coulombic efficiency, and no lithium electrodeposition and dendrite. In addition, it enables fast charging of up to 7 C (charging time of 8.6 minutes). The excellent fast charging performance is due to a unique electrolyte formulation (all linear solvent molecules are weakly bound to Li+) and SEI stabilizing additives, which effectively provide faster diffusion kinetics of Li+ ions in WNLE than conventional electrolytes. In addition, WNLE reduces the desolvation energy of the Li+/EMC:TFA solvent complex in the first solvation sheath, facilitating desolvation kinetics near the graphite anode, enabling the LIB to be recharged quickly without lithium deposition and dendrite growth. Compared to conventional electrolytes, WNLE formulations offer advantages in viscosity, wettability, ionic conductivity over a wide temperature range, and high voltage stability, and form a robust SEI layer at the graphite anode and NCM811 cathode, preventing lithium deposition and dendrite growth at the anode, respectively, and inhibiting metal dissolution at the cathode. As a result, fast charging of graphit-based lithium-ion pouch cells in excess of 1 C is achieved with WNLE, which was not possible before. These results are first-of-its-kind and highest performance. In addition, graphite's non-flammability and ability to prevent SEI formation of lithium dendrites further validates the role of WNLE in battery safety. The proposed novel electrolyte formulation strategy provides new opportunities for the development of various fast-charging electrolyte systems, as well as potential solutions to the problems of slow charging and unsafe LIBs. A similar electrolyte can also be used in lithium metal batteries and other batteries to increase their charging speed.
https://doi.org/10.1002/adfm.202311782
Source: Electrochemical Energy
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