Characterization of LTO//NMC Batteries Containing Ionic Liquid or Carbonate Electrolytes after Cycling and Overcharge

Abstract

Electrochemical stabilities under cycling and overcharge of Li 4 Ti 5 O 12 (LTO) // LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC) batteries with two ionic liquid-based electrolytes and a commercial electrolyte were investigated at 25 and 60 • C. The electrolytes were constituted of ethylene carbonate / dimethyl carbonate mixtures with LiPF 6 , and of imidazolium and pyrrolidinium-based ionic liquids containing the bis(trifluoromethanesulfonyl)imide anion (NTf 2 ) associated to LiNTf 2 salt. The experiments were performed in coin and pouch cells in order to follow the evolution of the stability of both solid, liquid and gas phases. The decomposition products and stability of the electrolytes was assessed by gas chromatography coupled with infrared spectroscopy. The characterization of the electrode surfaces by SEM, XRD and XPS techniques was reported, after cycling and overcharge. The XPS analysis of both electrodes indicated that their surfaces were covered by an IL film in the case of NMC, and by several NTf 2 anion decomposition products in the case of LTO. This work highlights the difference between thermal and electrochemical stability, especially for imidazolium-based electrolytes. Commercial lithium-ion batteries could lead to safety issues related to the use of flammable carbonates as electrolyte solvents.1 To provide safer cells, ionic liquids (IL) are widely studied as they possess negligible vapor pressure and reduced flammability.2 However, though information concerning the thermal stability of IL during combustion is beginning to emerge, 3,4 few data exist concerning their stability in case of short-circuit, overcharge or overdischarge. The aim of this work is to compare cycling and overcharge behavior at 25 and 60• C of two IL-based electrolytes and a commercial electrolyte, in a system using Li 4 Figure 1. For each electrolyte, cycling and overcharge experiments were run in LTO // NMC coin or pouch cells. The evolved gases and the battery components (electrolyte and electrodes) were analyzed after cycling and overcharge at 60• C. Results and DiscussionIn a first set of experiments, the cycling performances of LTO // NMC coin cells (∼ 2 mAh) and pouch cells (∼ 10 mAh) with the three electrolytes were investigated at 25• C and 60 • C. In the case of pouch cells, the battery evolutions were monitored by measuring the volume increase, and analyzing evolved gases. In a second set of experiments, the batteries were submitted to a charge at C/10-rate up to 4.5 V (NMC potential reached 6 V vs Li + /Li) followed by a floating of 20 h at this voltage at 60• C. Meanwhile the currents required to maintain the cell voltage and the cell volumes were measured. The emitted gases were identified by gas chromatography coupled with infrared spectroscopy (GC-IR). The post mortem analyses of the electrode surfaces were carried out by scanning electron microscopy (SEM), Xray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. * Electrochemical Society Student Member.z E-mail: lea.chancelier@univ-lyon1.frCycling ...

Adiponitrile–Lithium Bis(trimethylsulfonyl)imide Solutions as Alkyl Carbonate‐free Electrolytes for Li4Ti5O12 (LTO)/LiNi1/3Co1/3Mn1/3O2 (NMC) Li‐Ion Batteries

“…The linear correlation obtained indicates that the current is limited by diffusion in the solid phase( NMC cathode is limiting the current).T he Li + diffusion coefficient is obtained from the slope of the graph: D(Li + ) = 3.19 10 À10 cm 2 s À1 .T his value is closet ot hat obtained by others for NMC and other different electrolytes: 3.53 10 À10 cm 2 s À1 in EC/DEC-LiPF 6 (1 m) [36] and 3.502 10 À10 cm 2 s À1 in EC/DEC/EMC-LiPF 6 (1 m). The diffusion coefficient is calculated by plotting the value of the maximum current (I p )i ntensity against the square root of the scanning rate.…”

“…The diffusion coefficient is calculated by plotting the value of the maximum current (I p )i ntensity against the square root of the scanning rate. The linear correlation obtained indicates that the current is limited by diffusion in the solid phase( NMC cathode is limiting the current).T he Li + diffusion coefficient is obtained from the slope of the graph: D(Li + ) = 3.19 10 À10 cm 2 s À1 .T his value is closet ot hat obtained by others for NMC and other different electrolytes: 3.53 10 À10 cm 2 s À1 in EC/DEC-LiPF 6 (1 m) [36] and 3.502 10 À10 cm 2 s À1 in EC/DEC/EMC-LiPF 6 (1 m). [37] It can also be seen that the cathodic and anodic peaks are shiftedt owardh ighera nd lower values, respectively,w hich meanst hat the system is only quasi-reversible under these scan rates.…”

Chromatographic Techniques in the Research Area of Lithium Ion Batteries: Current State-of-the-Art

“…Several hydrocarbons besides trifluoro methane were identified and the nature of present hydrocarbons was reported to change with the utilized cationic species. In follow-up studies of the same group, the stability of IL-based electrolytes in lithium titanate (LTO)/NCM-based cells was investigated and compared to standard organic carbonate solvent-based electrolytes [152,153]. The cells were cycled at 60 • C and the emerging gases obtained after 100 cycles.…”

“…Formation AED [224] MS [195,218,225,226] TCD [57,196,227] n/a [228] Cyclic Aging FID [229] MS [230] TCD [229] n/a [231] Overcharge FID [152] FTIR [152,153] MS [232] TCD [153] n/a [233] Thermal Runaway/Abuse Conditions (ARC, etc.) FID [234] FTIR [198] MS [198,235] TCD [234,[236][237][238][239][240] VUV [217] n/a [241,242] Model Aging of LIB Components (Thermal; Radiolysis, etc.)…”

Temperature Dependence of Electrochemical Degradation in LiNi1/3Mn1/3Co1/3O2/Li4Ti5O12 Cells

“…As the conventional operating temperature for LIBs is optimal in between 20 and 45 °C, −10, 30, and 55 °C are selected to investigate the effects under colder and warmer conditions. Based on previous studies, there exists an interplay between the electrochemical performance, the surface chemistry during cycling, and the bulk structure of the compounds—which are all dependent on the operational temperature during cycling. Therefore, chemical information including composition and oxidation state for electrode surfaces and deeper is investigated after galvanostatic cycling for 30 cycles at different temperatures by combining soft X‐ray photoelectron spectroscopy (SOXPES), hard X‐ray photoelectron spectroscopy (HAXPES), and X‐ray absorption near edge structure (XANES) spectroscopy.…”