Molecular Design of Asymmetric Cyclophosphamide as Electrolyte Additive for High-Voltage Lithium-Ion Batteries

Abstract

Elevating the charging voltage could greatly promote the energy density of lithium-ion batteries (LIBs) with LiNi x Mn y Co z O 2 cathodes, although challenges arise from severe parasitic reactions and rapid capacity decay at high voltage, especially for nickel-rich cathodes. Herein, by incorporating various useful functionalities into one single molecule, we rationally design and synthesize a new class of five-membered asymmetric cyclophosphamides as electrolyte additives to enable stable cycling of highvoltage LIBs. It is demonstrated that the strong Lewis-base feature of the P− N bond can effectively scavenge the detrimental HF and H 2 O in the electrolyte. Meanwhile, the five-membered ring with an asymmetric amine moiety undergoes ring-opening polymerization to generate a highly robust and thin polymeric cathode−electrolyte interphase. Benefiting from the above merits, the asymmetric cyclophosphamide additive significantly suppresses decomposition of the electrolyte, dissolution of the transitional metals, and structural damage to the cathode, thus markedly improving the cycling stability and Coulombic efficiency of both high-voltage coin and pouch LIBs (up to 4.6 V).

A straightforward approach to improve NCM523/graphite pouch battery performance in a wide temperature range at 4.35 V using film-forming additive N-phenylimidodisulfuryl fluoride (PhFSI)

“…However, challenges persist, such as the degradation of conventional carbonate-based electrolytes at delithiated cathode surfaces, leading to by-product build-up, Li + depletion, and transition metal (TM) ion dissolution, which result in decreased capacity and increased impedance. 6,7 Current research studies are focused on enhancing the stability of the electrode-electrolyte interphase through strategies such as electrode surface coating, material doping, and lm-forming additives. [7][8][9][10][11][12][13][14][15] Coating and doping have been known to improve electrode stability, but their large-scale application is limited by complex preparation and high costs.…”

“…However, challenges persist, such as the degradation of conventional carbonate-based electrolytes at delithiated cathode surfaces, leading to by-product build-up, Li + depletion, and transition metal (TM) ion dissolution, which result in decreased capacity and increased impedance. 6,7…”

In situ polymerization of 1,3-dioxane as a highly compatible polymer electrolyte to enable the stable operation of 4.5 V Li-metal batteries

“…5,6 However, the conventional organic liquid electrolytes continuously react with both Li-metal anodes and high-voltage cathodes, depleting the electrolytes and thickening the electrode/electrolyte interphases. 7–9 Especially for the Li-metal anode side, the inhomogeneous deposition of Li + in organic liquid electrolytes would result in uncontrollable dendritic growth and formation of dead lithium, leading to low Coulombic efficiency (CE), poor cycle life, and even short-circuiting. 10,11 Moreover, the highly volatile and flammable characteristics of organic liquid electrolytes would exacerbate the safety hazards of Li-metal batteries (LMBs).…”

Multifunctional Cyclic Phosphoramidate Solvent for Safe Lithium-Ion Batteries

“…By combining the molecular structures of traditional cyclic carbonates (e.g., EC) and linear P-based solvents (e.g., triethyl phosphate), unique functionalities, such as stable SEI formation and nonflammability (SI, Figure S2), are realized . Additionally, the high capacities for scavenging radicals, Lewis acids, and HF, which are due to the lone-pair-rich P and O atoms and Lewis-base P–N bond in the molecule, improve battery safety, with suppressed exothermic reactions at the anode. , …”

“…(a) C 1s, Li 1s, P 2p, and N 1s XPS spectra of the pristine graphite electrode and those cycled in the given electrolytes. Peaks representing the Li–P–O, Li–O, and Li–N bonds are observed in the Li 1s, P 2p, and N 1s spectra of the samples cycled in the DMAP-based electrolytes, indicating the formation of DMAP-derived Li + -conducting functional SEIs. − (b) Schematic of the DMAP-derived SEI formed on the graphite surface, which aids in the suppression of electrolyte decomposition.…”