Cathode Electrolyte Interphase Film Research Articles

Cathode‐Electrolyte‐Interphase Film Formation on a LiNiO2 Surface in Conventional Aqueous Electrolytes: Simple Method to Improve the Electrochemical Performance of LiNiO2 Electrodes for Use in Aqueous Li‐Ion Batteries

AbstractPassivation films, the so‐called solid–electrolyte interphase and cathode–electrolyte interphase (CEI), are considered to be essential for the operation of rechargeable batteries because they have a positive impact on electrochemical performance including cyclability. In the field of aqueous Li‐ion batteries (ALIBs) to date, it is generally accepted that these films can only be formed in super‐concentrated electrolytes containing fluorine‐based organic anions such as N(SO2CF3)2 and SO3CF3. This study demonstrates for the first time that a CEI film can be created on a LiNiO2 (LNO) electrode in conventional aqueous electrolytes and improve electrochemical performance such as reversible capacities and cyclability. The results reveal that a CEI film, mainly composed of Li2CO3 and LiOH, is formed on the LNO surface in a saturated LiNO3 aqueous electrolyte containing LiOH beginning at the first de‐intercalation of Li+ ions, suppressing side reactions between the surface and aqueous electrolyte, and improving the recovery rate of structural changes of LNO during the charge/discharge process. As a result, this study suggests that LNO has great potential for application as a cathode material for high energy density ALIBs.

Stabilized Cathode Interphase for Enhancing Electrochemical Performance of LiNi0.5Mn1.5O4-Based Lithium-Ion Battery via cis-1,2,3,6-Tetrahydrophthalic Anhydride

The continuous degradation of carbonate electrolytes and the dissolution of transition metal cations due to parasitic reactions on the cathode-electrolyte interphase (CEI) block the practical application of LiNi0.5Mn1.5O4-based lithium-ion batteries (LNMO-based LIBs) at a high voltage. cis-1,2,3,6-Tetrahydrophthalic anhydride (CTA) has been used as a functional additive in a carbonate baseline electrolyte (BE) for constructing the CEI film to enhance the cyclic stability of LNMO-based LIBs. The LNMO/Li cell with CTA exhibits a preponderant capacity retention of 83.3% compared with those of propionic anhydride (PA) (46.5%) and BE (13.6%) after 500 cycles at the current density of 1 C from 3.5 to 4.9 V. Additionally, the LNMO/graphite full cell with CTA still has a higher capacity retention of 95.46% even after 300 cycles at 1 C. By characterizations, it is reasonably demonstrated that CTA was oxidated to participate in the construction of a CEI film. An unsaturated aromatic group was introduced into the composition of the CEI film along with CTA in the formation process of the CEI film, which further improved the antioxidative activity of the CEI film under the influence of field-effect. Specifically, the CEI film obtains appreciable stability because of its higher antioxidative activity under the influence of field-effect. The stabilized CEI can significantly suppress the parasitic reactions of electrolytes, decrease the consumption of active-Li+, and protect the LNMO cathode structure, thereby enhancing the cyclic compatibility of LNMO-based LIBs with the carbonate electrolytes from 3.5 to 4.9 V.

Optimizing interphase structure to enhance electrochemical performance of high voltage LiNi0.5Mn1.5O4 cathode via anhydride additives

A series of anhydrides have been used as additives in carbonate-based electrolytes for optimizing the interphase of LiNi0.5Mn1.5O4 (LNMO) cathode material to enhance the electrochemical performance of high-voltage (5 V-class) lithium-ion batteries (LIBs). The cyclic stability and rate performance of high-voltage LNMO/Li cells were remarkably enhanced by introducing the anhydride additives into electrolyte. Specifically, the LNMO cathode in the electrolyte with butyric anhydride (BA) additives exhibits high capacity retention of 84.8% after 300 cycles at 1C, and outstanding rate capability of 112 mA h g−1 at 5C. By characterizations, it is rationally demonstrated that the cathode electrolyte interphase (CEI) can be optimized via using different anhydride additives which strongly depend on the molecular structure of anhydride, caused by the electronic and steric hindrance effects during the oxidation process. As a result, the butyric anhydride (BA) derived CEI film exhibits the most suitable microstructure on the LNMO surface, which not only can inhibit the continuous electrolyte decomposition but also facilitate the lithium-ion diffusion coefficient. Moreover, the anhydride can further consume the trace water of electrolyte to stabilize LiPF6 salt and scavenge the erosive HF. Therefore, such an electrolyte additive presents a potential which practical application in high voltage lithium-ion batteries.

Enhancing the Electrochemical Performance of a High-Voltage LiNi0.5 Mn1.5 O4 Cathode in a Carbonate-Based Electrolyte with a Novel and Low-Cost Functional Additive.

Butyric anhydride (BA) is used as an effective functional additive to improve the electrochemical performance of a high-voltage LiNi0.5 Mn1.5 O4 (LNMO) cathode. In the presence of 0.5 wt % BA, the capacity retention of a LNMO/Li cell is significantly improved from 15.3 to 88.4 % after 200 cycles at 1 C. Furthermore, the rate performance of the LNMO/Li cell is also effectively enhanced, and the capacity goes up to 112 mAh g-1 even at 5 C, which is considerably higher than that of a LNMO/Li cell in electrolyte without BA additive (95.4 mAh g-1 at 5 C). Linear sweep voltammetry and cyclic voltammetry results reveal that the BA additive can be preferentially oxidized to construct a stable cathode electrolyte interphase (CEI) film on the LNMO cathode. Subsequently, the BA-derived CEI film can alleviate the decomposition of the electrolyte and the dissolution of Mn and Ni ions from the LNMO cathode as well as maintain the structural stability of LNMO during the cycling process; this leads to outstanding electrochemical performance. Thus, this work provides an effective and low-cost functional electrolyte for high-voltage LNMO-based LIBs.

Operando Fourier Transform Infrared Investigation of Cathode Electrolyte Interphase Dynamic Reversible Evolution on Li1.2Ni0.2Mn0.6O2.

One of the keys to the cycling stability of electrode materials is the formation of a stable interface film on cathode materials, which is called a cathode electrolyte interphase (CEI). For a Li/Mn-rich cathode, especially, the high working voltage will cause an extremely unstable electrolyte environment, becoming a challenge for the stable interface film formation. In this work, an operando-attenuated total reflection-Fourier transform infrared (ATR-FTIR) technique is developed to monitor in real time the dynamic mechanism of CEI formation in a carbonate-based electrolyte with or without the moderate additive tris(trimethylsilyl)borate (TMSB), which could promote the formation and stability of high-quality CEI films when it charges to 4.8 V. It is interesting that the components of CEI are basically generated in the first cycle owing to ethylene carbonate (EC) priority decomposition. Besides, the presence of TMSB can suppress the decomposition of EC in part and modify the stability of the CEI film. This is because TMSB containing an electron-deficient boron atom can easily combine with an electron-rich F- and PF6- forming a polyanion group initially, which will weaken the electrostatic force between the anionic groups and EC to reduce the concentration of EC on the cathode surface and prevent the continuous decomposition of EC at a high voltage. X-ray photoelectron spectroscopy also verifies the presence of polyanion groups and their further participation in CEI formation. This work highlights the dynamical stability of CEI modified by moderate TMSB and the formation mechanism of this dynamical change during cycling characterized by the operando ATR-FTIR technique, which paves the way for a better understanding of the complex and hard-characterized cathode interface reactions.

Impact of evolution of cathode electrolyte interface of Li(Ni0.8Co0.1Mn0.1)O2 on electrochemical performance during high voltage cycling process

Abstract In this work, the electrochemical performance of LiNi0.8Co0.1Mn0.1O2 (NCM811) has been investigated after cycling with various upper cutoff voltages. Noteworthily, electrochemical impedance of NCM811 declined with the increasing cycle number to high voltages. It was found that the decline of charge transfer impedance could be related to the structural and compositional change of cathode electrolyte interphase (CEI) of NCM811 when charging to high voltages, based on the characterization of electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The corresponding mechanism has also been proposed in this study. Specifically, due to the increasing roughness of cathode surface, the bottom of CEI film and cubic phase on cathode surface form a transition region mainly at high voltages, leading to the nonobvious boundary. This newly formed transition region at high voltages could promote the Li ion diffusion from electrolyte to cathode, then reducing charge transfer impedance. Additionally, the decrease of LiF on the surface of the cathode could also make a contribution to lower the interface impedance. This study delivers a different evolution of CEI on NCM811, and the impact of CEI evolution on electrochemical performance when charging to a high voltage.

Surface sulfidization of spinel LiNi0.5Mn1.5O4 cathode material for enhanced electrochemical performance in lithium-ion batteries

Stable interfacial structure is crucial for achieving superior electrochemical performances of high-voltage cathode materials for lithium-ion batteries. Herein, surface-sulfidized LiNi0.5Mn1.5O4 cathode materials are synthesized through electrostatic interactions between positively-charged LiNi0.5Mn1.5O4 and negatively-charged sulphur ion. A significant improvement in the rate capability, cycling stability and thermal stability has been achieved in surface-sulfidized LiNi0.5Mn1.5O4 electrode. A discharge capacity of 93.4 mAh g−1 can be still delivered at 2 C after 2500 cycles with a capacity retention of 74.9%, which is far beyond that of the pristine one (45.3% after 1800 cycles). 3D porous structure of sulfidized-layer helps to form a stable cathode electrolyte interphase (CEI) filmon LiNi0.5Mn1.5O4 surface via accommodating interfacial strain between active materials and CEI film. Metal-sulfides on LiNi0.5Mn1.5O4 surface could facilitate electron transfer across the LiNi0.5Mn1.5O4/electrolyte interface, reduce charge transfer resistance and consequently enhance rate capability. The adsorption of SO42− on LiNi0.5Mn1.5O4 surface also helps to enhance LiNi0.5Mn1.5O4/electrolyte interfacial stability. Moreover, the reduced work function induced by surface-sulfidization is considered to suppress decomposition of the electrolyte, improve interfacial stability and improve cycling stability. In terms of the superior electrochemical performances, surface-sulfidized LiNi0.5Mn1.5O4 composites can be utilized as a promising cathode material for high-performance lithium ion batteries.

Enhanced electrochemical performance of LiNi0.5Mn1.5O4 cathode by application of LiPF2O2 for lithium difluoro(oxalate)borate electrolyte

Spinel LiNi0.5Mn1.5O4(LNMO) cathode with high voltage plateau at around 4.7 V (vs. Li/Li+) and high energy density has attracted great attention. However, its application is limited because of the lack of matched electrolyte. Herein, we demonstrate that lithium difluorophosphate (LiPO2F2, LiDFP) as lithium difluoro(oxalate)borate (LiODFB) electrolyte additive significantly improves the electrochemical performance of high voltage LNMO/Li half cells and LNMO/G full cells at room temperature. Capacity retention of LNMO/Li half cell with 4% LiDFP achieves 89.69% after 200 cycles in comparison with 76.31% of that without LiDFP. Even discharging at 10C, the LNMO/Li half cell with 4% LiDFP still delivers a discharge capacity of 76.25 mAh g−1 and maintains at 75.94% capacity retention after 200 cycles. The electrochemical performance of LNMO/G full cells has a similar improvement. The enhanced electrochemical performance of LNMO can be ascribed to the steady and low-impedance cathode electrolyte interphase (CEI) film formed by priority decomposition of LiDFP. Besides, in order to further reveal the mechanism of film-forming additive, we studied on the changes of CEI film during initial cycles using transmission electron microscopies (TEM), X-ray photoelectron spectroscopy (XPS), Fourier Transform Infrared Spectrometer (FT-IR) and electrochemical impedance spectroscopy (EIS) measurements. The results indicate that the LiDFP preferential decomposition product can gradually prevent the contact between the electrolyte and the cathode with the growth of the CEI film, thereby reducing decomposition of the electrolyte. The finally grown CEI film is sufficiently dense to effectively isolate the electrolyte and the cathode, and CEI film formed by LiDFP-containing electrolyte is more conducive to the transmission of Li+,eventually leading to excellent electrochemical performance for the high-voltage LIBs.

Functional composite polymer electrolytes with imidazole modified SiO2 nanoparticles for high-voltage cathode lithium ion batteries

Poly(vinylidene fluoride-co-hexafluoro-propylene) doped with imidazole-modified silica nanoparticles (Z-SiO2) is coated in a polyethylene substrate to form a functional composite polymer electrolyte (PVDF-HFP-(Z-SiO2)/PE-based CPEs) and used for high voltage LiNi0.5Co0.2Mn0.3O2 cathode lithium-ion batteries (LIBs). The imidazole-based modified SiO2 nanoparticles are first prepared via a distillation precipitation polymerization. The composite separators with 30 wt% Z-SiO2 nanoparticles prepared via a dip-coating process exhibits a porous and uniformly dispersed morphology and enhanced performance, including excellent electrolyte uptake (310%), high ionic conductivity of 1.03 mS cm−1, and oxidative decomposition voltage up to 4.75 V. More importantly, a stable cathode electrolyte interphase (CEI) layer can be formed, endowing the Li/PVDF-HFP-(Z-SiO2)/PE-based CPEs/LiNi0.5Co0.2Mn0.3O2 (NCM523) cells superior cycling stability and rate capability (169 mAh g−1, 81.9%) under when the charge cut-off voltage increased to 4.5 V, which is higher than that assembled with PE separator (160 mAh g−1, 40.8%). These results demonstrate that the Z-SiO2 nanoparticles not only act as the fillers of the CPEs but also are water/acid scavengers in favor of the formation of a stable CEI film, which promotes the cycling performance and rate capability of high voltage NCM523 cathode and reveals promising prospect for practical applications in LIBs at high voltage operation.

Exploring high-voltage fluorinated carbonate electrolytes for LiNi0.5Mn1.5O4 cathode in Li-ion batteries

Abstract Ethyl-(2,2,2-trifluoroethyl) carbonate (ETFEC) is investigated as a solvent component in high-voltage electrolytes for LiNi0.5Mn1.5O4 (LNMO). Our results show that the self-discharge behavior and the high temperature cycle performance can be significantly improved by the addition of 10% ETFEC into the normal carbonate electrolytes, e.g., the capacity retention improved from 65.3% to 77.1% after 200 cycles at 60 °C. The main reason can be ascribed to the high stability of ETFEC which prevents large oxidation of the electrolyte on the cathode surface. In addition, we also explore the feasibility of electrolytes using single fluoriated-solvents with and without additives. Our results show that the cycle performance of LNMO material can be greatly improved in 1 M LiPF6 + pure ETFEC-solvent system with 2 wt% ethylene carbonate (EC) or ethylene sulfate (DTD). The capacity retention of the LNMO materials is 93% after 300 cycles, even better than that of carbonate-based electrolytes. It is shown that the additives are oxidized on the surface of LNMO particles and contribute to the formation of cathode/electrolyte interphase (CEI) films. This composite CEI film plays a crucial role in suppressing the serious decomposition of the electrolyte at high voltage.

(Pentafluorophenyl)diphenylphosphine as a dual-functional electrolyte additive for LiNi0.5Mn1.5O4 cathodes in high-voltage lithium-ion batteries

(Pentafluorophenyl)diphenylphosphine (PFPDPP) is studied as an effective dual-functional additive for high-voltage LiNi0.5Mn1.5O4 (LNMO) cathodes. The additive can stabilize the LiPF6 salt and form a good cathode–electrolyte interphase (CEI). Hydrolysis of LiPF6 in the electrolyte is investigated by 19F and 31P nuclear magnetic resonance spectroscopy, which reveal that PFPDPP suppresses the decomposition of LiPF6 into LiF, PO3F2−, and PO2F2−. Cyclic voltammetry results reveal that PFPDPP is anodically decomposed at 3.5 V vs. Li/Li+ prior to the oxidation of carbonate electrolytes to form a CEI film. The results of scanning electron microscopy, transmission electron microscopy, and AC impedance analyses show that PFPDPP forms a thin CEI film with low impedance. X-ray photoelectron spectroscopy results also confirm that the addition of PFPDPP suppresses the formation of LiF in the CEI film. Furthermore, the Li|1.0 M LiPF6-EC/DEC (1:1, v/v)|LNMO cell with 0.2 wt% PFPDPP shows a retention capacity of 71% after 300 cycles at a charge–discharge rate of 2C between 3.0 and 5.0 V, whereas the cell without the additive shows a retention capacity of only 53.4% after 300 cycles. Furthermore, self-discharge of the battery is suppressed upon the addition of PFPDPP.

Alternative Multifunctional Cyclic Organosilicon as an Efficient Electrolyte Additive for High Performance Lithium-Ion Batteries

Three kinds of functional organosilicons electrolyte additives, i. e. octamethylcyclotetrasiloxane (D4), octamethylcyclotetrasiloxane (OMCTS) and 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetra- siloxane (ViD4), are firstly used to produce high-performance LiNi0.6Co0.2Mn0.2O2 cathode for rechargeable Lithium-ion batteries (NCM622/Li half-cell) at a high potential. The electrochemical performance tests at 4.5V suggest that ViD4 is the most promising electrolyte additive than its counterparts. Moreover, the prepared NCM622/Li half-cell with 0.5wt.% ViD4 exhibits an improved energy density of 187.2mAhg−1 and a good discharge capacity retention of 83.6% at 1C rate after 150 cycles at 4.5V, which is relatively higher than that with D4 (81.3%), OMCTS (81.9%) and much better than the bare electrolyte (76.1%). The electrochemical characterizations, density functional theory calculation and surface analysis provide strong evidences that combined effects of the Si-O bond and vinyl functional group in ViD4 can promote the formation of a uniform cathode electrolyte interphase (CEI) film. Moreover, the ViD4-incorporated CEI film effectively enhances the interfacial stability between the NCM622 cathode and the electrolyte by suppressing the continuous decomposition of carbonate-based electrolyte and the exfoliation of these decomposition products. Hence, the ViD4-containing electrolyte shows a promising potential for the application in high energy density lithium-ion batteries.

Improved cyclic stability of layered lithium cobalt oxide at high potential via cathode electrolyte interphase formed by 4-(trifluoromethyl) benzonitrile

The cyclic stability of LiCoO2 as cathode of lithium ion battery under 4.5V is improved by using 4-(trifluoromethyl) benzonitrile (4-TB) as an electrolyte additive. In a standard (STD) electrolyte (1.0mol/L LiPF6 in EC/EMC/DMC, 1/1/1 by volume) containing 0.5wt.% 4-TB, LiCoO2 exhibits an improved cyclic stability when the end-off charge potential is increased to 4.5V. The characterizations, from XRD, ICP, SEM, FTIR, and XPS, demonstrate that a cathode electrolyte interphase (CEI) film is formed on LiCoO2 from 4-TB, which suppresses the successive decomposition of STD electrolyte and prevents LiCoO2 from crystal destruction. Theoretical calculations confirm that 4-TB is adsorbed and oxidized on LiCoO2 preferentially to the carbonates in electrolyte, forming the protective CEI film.

Study of the Cathode–Electrolyte Interface of LiMn1.5Ni0.5O4 Synthesized by a Sol–Gel Method for Li-Ion Batteries

High voltage spinel has been synthesized by a modified Pechini sol–gel method and has been characterized by transmission electron microscopy, X-ray diffraction (XRD), and electrochemical methods. The synthesized materials are porous structures of nanosized crystallites ranging in size from 21 to over 400 nm depending on the sintering temperature used. The XRD patterns of the materials were assigned to the disordered spinel structure of the space group . The Li-ion batteries assembled using the synthesized cathode materials showed significant capacity fade for samples sintered at , while for those sintered at the capacity fade was low. Impedance spectroscopy, Fourier transform IR spectroscopy, and X-ray photoelectron spectroscopy were used to determine the compositions of the cathode electrolyte interphase (CEI). Impedance spectroscopy confirmed the spontaneous formation of the CEI on and that its thickness grows on cycling. After more than 100 cycles, it is found that the CEI film is composed of polycarbonates, polyether, LiF, and salts. The composition of the organic layer was the same regardless of the capacity fade.