Interface stabilization via lithium bis(fluorosulfonyl)imide additive as a key for promoted performance of graphite‖LiCoO2 pouch cell under -20 ○C.

The effects of lithium bis(fluorosulfonyl)imide, Li[N(SO2F)2] (LiFSI), as an additive on the low-temperature performance of graphite‖LiCoO2 pouch cells are investigated. The cell, which includes 0.2M LiFSI salt additive in the 1M lithium hexafluorophosphate (LiPF6)-based conventional electrolyte, outperforms the one without additive under -20 °C and high charge cutoff voltage of 4.3 V, delivering higher discharge capacity and promoted rate performance and cycling stability with the reduced change in interfacial resistance. Surface analysis results on the cycled LiCoO2 cathodes and cycled graphite anodes extracted from the cells provide evidence that a LiFSI-induced improvement of high-voltage cycling stability at low temperature originates from the formation of a less resistive solid electrolyte interphase layer, which contains plenty of LiFSI-derived organic compounds mixed with inorganics that passivate and protect the surface of the cathode and anode from further electrolyte decomposition and promotes Li+ ion-transport kinetics despite the low temperature, inhibiting Li metal-plating at the anode. The results demonstrate the beneficial effects of the LiFSI additive on the performance of a lithium-ion battery for use in battery-powered electric vehicles and energy storage systems in cold climates and regions.

Fire-Preventing LiPF6 and Ethylene Carbonate-Based Organic Liquid Electrolyte System for Safer and Outperforming Lithium-Ion Batteries.

Battery safety is an ever-increasing significance to guarantee consumer's safety. Reducing or preventing the risk of battery fire and explosion is a must for battery manufacturers. Major reason for the occurrence of fire in commercial lithium-ion batteries is the flammability of conventional organic liquid electrolyte, which is typically composed of 1 M LiPF6 salt and ethylene carbonate (EC)-based organic solvents. Herein, we report the designed 1 M LiPF6 and EC-based nonflammable electrolyte including methyl(2,2,2-trifluoroethyl)carbonate, which breaks the conventional perception that EC-based liquid electrolyte is always flammable. The designed electrolyte also provides high anodic stability beyond the conventional charge cut-off voltage of 4.2 V. A graphite∥LiNi0.6Co0.2Mn0.2O2 lithium-ion full cell with our designed EC-based nonflammable electrolyte with a small fraction of vinylene carbonate additive under an aggressive condition of 4.5 V charge cut-off voltage, 0.5C rate, and 45 °C exhibits increased capacity, reduced interfacial resistance, and improved performance and rate capability. A basic understanding of how a high-voltage cathode-electrolyte interface and anode-electrolyte interface are stabilized and how failure modes are mitigated by fire-preventing electrolyte is discussed.

Robust Solid-Electrolyte Interphase Enables Near-Theoretical Capacity of Graphite Battery Anode at 0.2†C in Propylene Carbonate-Based Electrolyte.

The formation of a robust solid-electrolyte interphase (SEI) layer at the surface of a graphite anode by electrolyte control is a key technology for high-performance lithium-ion batteries. Although propylene carbonate (PC) offers a lower melting point than ethylene carbonate, its combination with the graphite anode without additive is a worse choice, owing to co-intercalation of PC and Li+ ion into graphite, exfoliation of graphene sheets, and death of the battery. This study reports a graphite anode with an unprecedentedly high initial coulombic efficiency of 94 %, close to theoretical capacity, and excellent capacity retention of 99 % after 100 cycles in a PC-based electrolyte system, even at an unusually high rate of 0.2 C, which is generally attainable only at a very low rate of below 0.05 C in commercial electrolyte. The SEI stabilization for a graphite anode in PC-based electrolyte provides a new avenue for high-energy and high-performance batteries in widened range of working temperatures. A strong correlation between anode-electrolyte interfacial stabilization and highly reversible cycling performance is clearly demonstrated.