Tale of Three Phosphate Additives for Stabilizing NCM811/Graphite Pouch Cells: Significance of Molecular Structure–Reactivity in Dictating Interphases and Cell Performance

Abstract

Electrolyte additives have been extensively used as an economical approach to improve Li-ion battery (LIB) performances; however, their selection has been conducted on an Edisonian trial-and-error basis, with little knowledge about the relationship between their molecular structure and reactivity as well as the electrochemical performance. In this work, a series of phosphate additives with systematic structural variation were introduced with the purpose of revealing the significance of additive structure in building a robust interphase and electrochemical property in LIBs. By comparing the interphases formed by tripropyl phosphate (TPPC1), triallyl phosphate (TPPC2), and tripropargyl phosphate (TPPC3) containing alkane, alkene, and alkyne functionalities, respectively, theoretical calculations and comprehensive characterizations reveal that TPPC3 and TPPC2 exhibit more reactivity than TPPC1, and both can preferentially decompose both reductively and oxidatively, forming dense and protective interphases on both the cathode and anode, but they lead to different long-term cycling behaviors at 55 °C. We herein correlate the electrochemical performance of the high energy Li-ion cells to the molecular structure of these additives, and it is found that the effectiveness of TPPC1, TPPC2, and TPPC3 in preventing gas generation, suppressing interfacial resistance growth, and improving cycling stability can be described as TPPC3 > TPPC2 > TPPC1, i.e., the most unsaturated additive TPPC3 is the most effective additive among them. The established correlation between structure–reactivity and interphase-performance will doubtlessly construct the principle foundation for the rational design of new electrolyte components for future battery chemistry.

Using Triallyl Phosphate as Electrolyte Additive to Stabilize Electrode–Electrolyte Interface of LiNi0.5Mn1.5O4/Graphite High Voltage Lithium Ion Cells

“…19−21 They believe that the additive can be oxidized and reduced on the cathode and anode at the same time to form a thicker passivation layer to decrease the decomposition of electrolyte, but no further reaction mechanism is proposed. Recently, Zhao et al 22 analyzed the electrolyte and generated gas of a NCM811/ graphite cell containing TAP electrolyte additive after a longterm cycling and considered that intermediate products C 6 H 10 O , and so on. In this work, TAP as an electrolyte additive was found to improve the cycle life of the LNMO/graphite high voltage lithium ion cell.…”

“…Triallyl phosphate (TAP), as an electrolyte additive, has been used by several research groups to improve the cycling stability of batteries composed of ternary layered materials (LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811), LiNi 0.6 Co 0.2 Mn 0.2 O 2 , LiNi 0.42 Co 0.16 Mn 0.42 O 2 ), and graphite. − They believe that the additive can be oxidized and reduced on the cathode and anode at the same time to form a thicker passivation layer to decrease the decomposition of electrolyte, but no further reaction mechanism is proposed. Recently, Zhao et al analyzed the electrolyte and generated gas of a NCM811/graphite cell containing TAP electrolyte additive after a long-term cycling and considered that intermediate products C 6 H 10 O 4 P • , C 3 H 5 + , C 6 H 10 O 3 P + , and C 3 H 5 O • of TAP oxidation combine with the intermediate product C 3 H 3 O 3 • of EC oxidation to form C 9 H 13 O 7 P, C 6 H 10 O 3 PF, and so on. In this work, TAP as an electrolyte additive was found to improve the cycle life of the LNMO/graphite high voltage lithium ion cell.…”

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

“…To endow the CEI with high ionic conductivity and flexibility, various P-containing compounds (i.e., phosphate and phosphite) have been identified as additives to generate a P-containing organic CEI layer on the cathode surface. − Especially those P-containing compounds containing a dioxolane-like five-membered ring can easily undergo ring-opening polymerization under nucleophilic attack by the surface oxygen sites of the NMC cathode, leading to the formation of a polyphospho­ester-based CEI with superior ionic conductivity and flexibility, which not only inhibits the decomposition of electrolyte but also prevents the dissolution of TMs. ,, Unfortunately, such polyphospho­esters-based CEIs are still not robust enough to resist the attack of reactive species (e.g., HF and PF 5 ) generated by LiPF 6 .…”