Top view of NCM cathodes recovered from NCM/graphite full cells with (g) baseline and (i) 1% ETFB-containing electrolytes after 300 cycles at 25 °C. Microstructure and nanostructure of the NCM cathodes obtained after 400 cycles at 45 °C of NCM/graphite full cells (charge and discharge rate: 0.5C).
Introduction
Lithium-ion batteries
The highly reactive Ni4+ is formed on the surface of the Ni-rich cathodes in a charged state and can cause unwanted oxidative decomposition of the electrolyte. Cation mixing deteriorates the electrochemical performance of Ni-rich cathodes by converting an electrochemically active layered structure into an electrochemically inactive rock salt structure.
LiPF 6 -based electrolytes
Functional electrolyte systems
Understanding the thermal instability of fluoroethylene carbonate in LiPF6-based electrolytes for lithium-ion batteries.
Understanding the thermal instability of fluoroethylene carbonate in LiPF 6 -based
Introduction
Herein, we demonstrate the thermal instability of FEC as a cosolvent in high energy density lithium-ion batteries based on a pitch-coated silicon alloy-graphite (Si-C) anode coupled with LiCoO2 (LCO)-LiNi0.5Co0. 2Mn0.3O2 (NCM) cathode with a high capacity of ~180 mAh g-1. Furthermore, we propose possible mechanisms responsible for the thermal instability of the FEC solvent in the presence of LiPF6, leading to the inferior electrochemical performance of whole cells at elevated temperatures78.
Experimental method
All Si-C samples were obtained from LCO-NCM/Si-C full cells stored for 20 days at 45 °C with EC- or FEC-based electrolyte. The SEM and EDS samples were carefully prepared in a glove box to avoid any possible contamination.
Results and discussion
Cell discharge capacity curve before (solid line) and after storage (dashed line): (c) EC-based electrolyte and (d) FEC-based electrolyte. The deposition of transition metal ions on the anode was confirmed by EDS spectra of Si-C anodes obtained from full cells after storage in EC- or FEC-based electrolyte for 20 days at 45 °C.
Conclusion
Dual-function ethyl 4,4,4-trifluorobutyrate additive for high performance Ni-rich cathodes
Introduction
Li/Li+ and thus prevents the deterioration of the cathode structure, resulting in improved cycling performance.107 The authors have shown that the S=O functional group can delocalize the electrons from the nitrogen nucleus, which can serve as a weak basic site to enhance the reactivity of the nitrogen core. PF5 Lewis acid and the NCO groups can be polymerized into a protective film on the LiNi0.5Co0.2Mn0.3O2 cathode. Furthermore, 1-propylphosphonic acid cyclic anhydride proved to be an efficient additive to improve the capacity retention of a LiNi0.5Co0.2Mn0.3O2 cathode coupled to a graphite anode and reduce the structural degradation of the cathode.108 However, there is a fundamental understanding of the effect of the additives on the interfacial structure and structural stability of Ni-rich cathodes has not yet been achieved. Herein we demonstrate that minimal amounts (1 wt%) of ethyl 4,4,4-trifluorobutyrate (ETFB), used as an electrolyte additive, enable the fabrication of high-performance batteries consisting of a Ni-rich layered cathode (LiNi0.7Co0 .15Mn0 ,15O2, NCM) and a graphite anode, at practical mass loading levels (25.6 mg cm-2 for the NCM cathode and 13.6 mg cm-2 for the graphite anode).
Our study shows that ETFB forms a stable CEI, solves critical problems such as poor mechanical integrity, structural instability, and electrochemical degradation of the NCM cathodes, and ensures the structural stability of the interface formed at the graphite anode. Electrochemical tests confirm that the solid-electrolyte interface (SEI)-forming ability of ETFB in a full cell is greater than that of vinylene carbonate (VC), which is widely used as a standard additive for graphite anode batteries. The beneficial effects of ETFB on the surface chemistry of the NCM cathode and graphite anode were investigated by ex situ X-ray photoelectron spectroscopy (XPS) measurements.
We also propose possible mechanisms by which ETFB, serving as a bifunctional additive, promotes the formation of the CEI on the NCM cathode and the SEI on the graphite anode in a whole cell.112.
Experimental method
To recover the electrodes for ex situ analysis, the NCM/graphite full cells were carefully disassembled in a glove box after each electrochemical measurement. The surface and cross-sectional morphologies of the NCM cathodes were inspected by field emission scanning electron microscopy (FE-SEM, FEI, Nova NanoSEM). The diameter of the NCM cathodes was prepared by an ion milling system (Hitachi High-Technologies Co., IM4000) equipped with an Ar ion beam.
The surface chemistry of the electrodes was investigated by ex situ XPS (ThermoFisher, K-Alpha) with Al Kα (hν = 1486.6 eV) radiation under ultrahigh vacuum. Before use, all samples were prepared in an argon-filled glovebox, sealed with an aluminum bag under vacuum, and then quickly transferred to the vacuum chamber of the XPS instrument to minimize any contamination. Inductively coupled plasma (ICP, Varian, 700-ES) analysis was used to measure the amount of transition metal ions (Co, Mn and Ni) in electrolytes kept in contact with delithiated NCM cathodes at 60 °C for 5 days.
The crystal structure of lithiated graphite anodes recovered from NCM/graphite full cells after storage at 60 °C was determined using XRD (Rigaku, D/MAX2500V/PC) with monochromatic Cu K radiation.
Results and discussion
Applying a load voltage cutoff of 4.35 V in the full cell, the voltage of the NCM cathode reached ~4.4 V. The effect of the electrolytes on the electrode-electrolyte interface was investigated by the resistance of the NCM/graphite full cells after 300 cycles at 25 and 45 °C (Figure 3.2c and d). Further evidence of the positive impact of the ETFB additive on the interfacial stability of the NCM cathode materials is provided in Figure 3.3b.
The superior mechanical integrity of the NCM cathode achieved with the ETFB-added electrolyte can be explained by the uniform coverage of the ETFB-derived CEI on the NCM cathode particles. The charge and discharge voltage profiles of the NCM cathode and graphite anode for 300 cycles at 25 °C in a full three-electrode cell containing the base electrolyte are shown in Figure 3.6a and b, respectively. The black dotted boxes show the voltages of the NCM cathode and graphite anode in the full three-electrode cells at the end of the charging process.
The peak intensities of P 2p core levels corresponding to LixPFy, LixPOyFz and phosphate (P-O) were significantly reduced for the NCM cathode containing 1% ETFB compared to the base electrolyte as shown in Figure 3.8c and d. A comparison of the C 1s core level peaks of NCM cathodes pretreated in electrolyte with and without ETFB confirms the formation of ETFB-derived CEI on the cathode (Figures 3.8e and f). A peak attributable to the CF3 moiety was observed around 292 eV in the C 1s core level spectrum of the NCM cathode pretreated with the ETFB added electrolyte.
Conclusion
Cyclic amino-silane-based additive ensuring stable electrode-electrolyte interfaces in Li-ion
Introduction
Computational studies and nuclear magnetic resonance (NMR) measurements revealed that TMS-ON enhances the dissociation of LiPF6 by coordinating the TMS-ON carbonyl with a Li+ ion, alleviates the hydrolysis of LiPF6 by stabilizing PF5, and removes HF. In addition, HF capture by TMS-ON was shown to produce 2-oxazolidinone (ON), which appropriately modifies the interfacial structures of both the NCM cathode and the graphite anode. The interfacial structures of both the cathode and the anode were confirmed by X-ray photoelectron spectroscopy (XPS), and the characteristic nature of the ON-derived CEI was identified by three-dimensional (3D) visualization using time-of-flight secondary ion mass spectrometry (TOF-SIMS).
High-resolution transmission electron microscopy (HR-TEM) and electron energy loss spectroscopy (EELS) clearly demonstrated that the ON-derivative CEI mitigates the unwanted electrolyte decomposition in NCM cathodes and alleviates the irreversible phase transformation of NCM cathodes.
Experimental method
To avoid contamination of the cycling electrodes with air or moisture contamination, an XPS (SIGMA PROBE, Thermo) equipped with an Al Kα (1486.6 eV) X-ray source and directly connected to the glove box was used to measure the surface chemistry of the electrodes . TOF-SIMS (TOF-SIMS 5, ION TOF) was used to analyze the surface chemical structure and for depth profiling. HR-TEM (JEM-2100F, JEOL) and STEM images were acquired at 300 kV to investigate the structural degradation of the Ni-rich NCM cathodes at the atomic level.
EELS was used to assess the electronic structure of the Ni-rich NCM cathodes containing the valence states of transition metals. Microcracks of the secondary Ni-rich NCM particles were observed using cross-sectional scanning electron microscopy (SEM, Verios 460, FEI). The three-parameter Becke hybrid functional combined with the Lee-Yang-Parr correlation functional (B3LYP) was used for the exchange correlation energy.
The spin-polarized calculation was used with version 4.4 of the double numerical plus polarization (DNP) basis set.
Results and discussion
A lower interfacial resistance, consisting of CEI and SEI film resistance (Rf) and charge transfer resistance (Rct), was observed for the NCM/graphite full cell with 0.5% TMS-ON electrolyte after 400 cycles at 45 °C (Figure 4.8f). 1H NMR measurements revealed that 0.5% TMS-ON is completely converted to ON and TMSF in the NCM/graphite full cell (Figure 4.9a). The effect of the electrolyte containing 0.5% TMS-ON on the NCM cathode interface structure was investigated using ex situ XPS.
3D visualization and depth profiles of the CEIs on the NCM cathodes cycled with (b) and (d) the baseline electrolyte and (c) and (e) the 0.5% TMS-ON electrolyte with Cs+ sputtering. The mechanical integrity of the NCM secondary particles was investigated in the baseline and 0.5% TMS-ON containing electrolytes. Magnified STEM images of the surfaces of NCM cathodes cycled with (c) baseline and (d) 0.5% TMS-ON electrolytes.
Cross-sectional SEM images of NCM cathodes cycled with (g) and (h) baseline and (i) and (j) 0.5% TMS-ON electrolytes.
Conclusion
Intragranular cracking as a critical barrier for high-voltage use of layer-structured cathode for lithium-ion batteries, Nat. Understanding Transition-Metal Dissolution Behavior in LiNi0.5Mn1.5O4 High-Voltage Spinel for Lithium-Ion Batteries, J. A New Insight into the Content Effect of Fluoroethylene Carbonate as a Film Forming Additive for Lithium-Ion Batteries, Int.
Comparative study of fluoroethylene carbonate and vinylene carbonate for silicon anodes in lithium-ion batteries, J. Understanding the thermal instability of fluoroethylene carbonate in LiPF6-based electrolytes for lithium-ion batteries, Electrochem. Recent Electrolyte Advances for Interfacial Stability of High-Voltage Cathodes in Lithium-Ion Batteries, RSC Adv.
Surface reconstruction and chemical evolution of stoichiometric layered cathode materials for lithium-ion batteries, Nat.
