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

Mickdy S Milien,Petra Klose,Ingo Krossing,Brett L Lucht,Witali Beichel,Hubert A Gasteiger,Hans Beyer

doi:10.1149/2.0541811jes

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 LiPF6 decomposition at high potentials. Incorporation of LiBFEP (0.1 and 0.5 wt%) into LiPF6 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 LiPF6, 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.

Highlights

The improvement observed in the presence of LPTB has been attributed to a borate-rich cathode electrolyte interface (CEI) formed from the sacrificial oxidation of LPTB, which inhibits electrolyte oxidation on the surface of the cathode and transition metal dissolution induced damage to the anode solid electrolyte interface (SEI).[13,14,15]
Electrochemical testing.—In order to get insight into the electrochemical activity of LiBFEP, Cyclic voltammetry (CV) were recorded for the blank electrolyte LP30 and the electrolyte with 0.5 wt% of added LiBFEP (Figure 1)
The initial reductive scan of the LP30 reference voltammogram shows the bulk reduction of ethylene carbonate (EC) at 0.7 V, which decreases in intensity in subsequent cycles due to the formation of a protective layer.[38]

Summary

Introduction

The capacity fading observed in LNMO/Graphite cells is due to continuous oxidation of the electrolyte and transition metal dissolution.[3,7] While the former results in the formation of unstable species on the surface of the cathode, the latter results in degradation of the LNMO material (due to loss of manganese) and increased resistance of the SEI (due to deposition of manganese on the anode).[9,12] altering the individual components (electrolyte solvents, electrolyte salts, and cathode material) prior to cell construction have been explored extensively, electrolyte additives have been (LPTB) displayed much better capacity retention after 30 cycles at 55◦C, compared to the baseline electrolyte (1.0 M LiPF6 in EC/EMC (3/7, v/v).[13,14,15] In situ gas analysis as well as ex situ surface analysis suggests that LiBOB is sacrificially oxidized to generate a cathode electrolyte interface (CEI) which decreases manganese dissolution. The good conductivity for electrolytes containing LiBFEP coupled with the film forming properties of similar P-based compounds suggests investigation of LiBFEP in LNMO/graphite cells.

Results

Conclusion