Lithium Bis(2,2,2-trifluoroethyl)phosphate Li[O₂P(OCH₂CF₃)₂]: A High Voltage Additive for LNMO/Graphite Cells

Abstract

The fluorinated phosphate lithium bis (2,2,2-trifluoroethyl) phosphate (LiBFEP) has been investigated as a film-forming additive employed to passivate the cathode and hinder continuous oxidation of the electrolyte. Cyclic voltammetry (CV) and linear sweep voltammetry coupled with online electrochemical mass spectrometry (LSV-OEMS) on a conductive carbon electrode (i.e., a C65/PVDF composite) showed that LiBFEP decreases electrolyte oxidation (CV and LSV) and LiPF 6 decomposition at high potentials. Incorporation of LiBFEP (0.1 and 0.5 wt%) into LiPF 6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7 wt) results in improved coulombic efficiency and capacity retention for LNMO/graphite cells. Ex-situ surface analysis of the electrodes suggests that incorporation of LiBFEP results in the formation of a cathode electrolyte interface (CEI) and modification of the solid electrolyte interface (SEI) on the anode. The formation of the CEI mitigates electrolyte oxidation and prevents the decomposition of LiPF 6 , which in turn prevents HF-induced manganese dissolution from the cathode and destabilization of the SEI. The passivation of the cathode and stabilization of the SEI is responsible for the increased coulombic efficiency and capacity retention. Since their debut in 1991, lithium ion batteries (LIB) have become the universal power source for consumer electronics.1 Larger format LIBs such as those needed to power electric vehicles (EVs), an important future market, have amassed considerable interest; however higher specific energy densities are required for larger format LIBs.1,2 The practical way to increase energy density is to employ cathode materials with increased theoretical capacities and/or high discharge plateaus, and thus high energy (HE) or high voltage (HV) cathodes are required in order for LIBs to meet the demands of the EV market.3 While both HE and HV cathodes have been implemented, current research efforts are focused on overcoming the caveats associated with these materials. The oxidative instability of carbonate-based electrolytes is a central limitation for cells with various cathode chemistries operated above 4.4 V. [3][4][5][6][7] In addition to the instability of the electrolyte, cathodes such as nickel-rich layered oxides (LiNi x Mn y Co z O 2 ), lithium-rich layered oxides (0.6 Li 2 MnO 3 • 0.4 Li(Ni 1/3 Co 1/3 Mn 1/3 )O 2 ), and HV spinel (LiNi 0.5 Mn 1.5 O 4 ) (LNMO) all suffer from structural instability when operated at high potentials.5-11 While the layered oxides are capable of delivering higher practical energy densities, the lack of cobalt in LNMO alleviates the issues of cost and resource limitations.7 As the higher energy densities associated with HE materials can only be obtained at higher cutoff potentials, oxidation of the electrolyte is a universal problem to both HE and HV cathodes. This work focuses on improving the performance of LNMO/Graphite cells.The capacity fading observed in LNMO/Graphite cells is due to continuous oxidation of the electrolyte and transition...

Lithium Bis(trimethylsilyl) Phosphate as a Novel Bifunctional Additive for High-Voltage LiNi1.5Mn0.5O4/Graphite Lithium-Ion Batteries

“…Incorporation of additives into electrolyte formulations can result in the improved performance of LNMO/graphite cells. The specific functions of the additives include scavenging corrosive acid species such as HF or phosphorus pentafluoride (PF 5 ) − or the generation of a passivation film, the surface film on the electrode, − depending on the functional groups in the additives. It was reported that siloxane derivatives such as silyl groups (Si–O) can capture HF or H 2 O to form silicon fluorides (Si–F). ,, Lithium phosphate derivatives such as lithium difluorophosphate and lithium dimethyl phosphate (LiDMP) were reported to improve the cycling and rate performances of NCM/graphite cells by modifying the chemical composition of SEI layers on graphite electrodes. , Moreover, the additives can have more than one mode of performance enhancement, when they possess multiple types of functional groups. ,− …”

Decoupling Interfacial Reactions at Anode and Cathode by Combining Online Electrochemical Mass Spectroscopy with Anode‐Free Li‐Metal Battery

“…For carbonate electrolyte based batteries, the decomposition of electrolyte causes the evolution of gases that lead to the expansion of the cell. [ 3,4 ] The evolution of gases is one of the indications of the instability of electrolytes due to the electrode–electrolyte interfacial reactions. Different approaches designed to investigate the gas evolution along with interfacial reactions include Archimedes’ in situ gas analyzer [ 5 ] and ex situ GC‐MS [ 6 ] via LMBs and Li‐ion batteries (LIBs).…”

“…[ 6–9 ] It has been reported that the interfacial reactions upon charging of Li/NMC and graphite/NMC cells with carbonate electrolytes generate CO 2 , CO, H 2 and C 2 H 4 gases. [ 3,7,9–13 ] Nevertheless, the main source of hydrogen (H 2 ) gas at the early stage of solid–electrolyte‐interphase (SEI) formation is the reduction of residual trace H 2 O at the anode surface with the formula of H 2 O + e − → OH − + ½ H 2 . [ 7,8,14 ]…”

Long-Term Cycling of a Mn-Rich High-Voltage Spinel Cathode by Stabilizing the Surface with a Small Dose of Iron

“…As shown, an excellent performance with a capacity retention of ∼90% is achieved after 300 cycles. This is one of the best performances ever reported at such a high mass loading for the high-voltage LNMO spinel. ,− The CEs are plotted in Figure b; the cell exhibits an average CE of 99.45%, implying that the lithium inventory loss is suppressed. Notably, the cell impedance growth is negligible, as depicted in Figure c.…”