Cathode Electrolyte Interphase Film Research Articles

In Situ Electrochemical Polymerization of Cathode Electrolyte Interphase Enabling High-Performance Lithium Metal Batteries.

Lithium metal batteries (LMBs) with high-voltage nickel-rich cathodes show great potential as energy storage devices due to their exceptional capacity and power density. However, the detrimental parasitic side reactions at the cathode electrolyte interface result in rapid capacity decay. Herein, a polymerizable electrolyte additive, pyrrole-1-propionic acid (PA), which can be in situ electrochemically polymerized on the cathode surface and involved in forming cathode electrolyte interphase (CEI) film during cycling is proposed. The formed CEI film prevents the formation of microcracks in LiNi0.8Co0.1Mn0.1O2 (NCM811) secondary particles and mitigates parasitic reactions. Additionally, the COO- anions of PA promote the acceleration of Li+ transport from cathode particles and increase charging rates. The Li||NCM811 batteries with PA in the electrolyte exhibit a high capacity retention of 83.83% after 200 cycles at 4.3V, and maintain 80.88% capacity after 150 cycles at 4.6V. This work provides an effective strategy for enhancing interface stability of high-voltage nickel-rich cathodes by forming stable CEI film.

Quenching-Induced Ultrathin LiF-Rich Interphases on LiNi0.8Co0.1Mn0.1O2 Cathode

Nickel-rich layered oxides (NCM) are a promising contender material for the cathode electrode of high-energy lithium-ion batteries (LIBs) due to their large reversible capacity and high operating voltage. However, the poor surface/interfacial stability and the dissolution of transition metal ions hinder the commercial application of NCM. To create an artificial cathode electrolyte interphase (CEI) with LiF-rich inorganic phase on the NCM surface, a practical and efficient way of quenching the NCM powder from high temperature in 1,1,2,2-Tetrafluoroethyl 2,2,2-trifluoroethyl ether (HFE) was devised. With this artificial CEI film, the side reactions between NCM and electrolytes are inhibited, and the dissolution of TM ions is retarded. The quenched-NCM achieved fantastic cycling performance and suppressed voltage decay. Our research offers an efficient and worthy approach for improving the surface/interfacial stabilization of nickel-rich cathode materials for high-energy-density LIBs.

Borate-Functionalized Disiloxane as Effective Electrolyte Additive for 4.5 V LiNi0.8Co0.1Mn0.1O2/Graphite Batteries.

Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) is considered the most prominent cathode material to establish a practical high energy density of lithium-ion batteries (LIBs) for future electric vehicles. The energy density of LIBs is greatly determined by the capacity of electrode materials and the operating voltage of the cells. To further improve the cycle lifespan of NCM811 batteries to meet the requirement of driving range for the electric vehicle market, it is vital to design a novel electrolyte additive that can enhance the stability of the cathode/electrolyte interface at a wide range of voltage. Herein, a novel borate functionalized disiloxane compound, 1,1,1,3,3-pentamethyl-3-(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)propyl) disiloxane (PMBPDS), is synthesized as cathode electrolyte interphase (CEI) film-forming additive to improve the cycling performance of NCM811 batteries. Systematic studies reveal that PMBPDS can construct a stable CEI film on the NCM811 surface and efficiently scavenge hydrofluoric acid (HF). The PMBPDS-derived CEI prevents the dissolution of transmission metals in the NCM811 cathode and enhances the capacity retention of NCM811/graphite cells from 68.3 to 70.6% after 200 cycles at 1 C in the voltage window of 3-4.5 V. This work provides more understanding on designing the molecular structure of additive compounds for improving the electrochemical performance of LIBs.

Conductive Robust Interfaces with Fluoro-Borate Based Electrolyte Additive for 4.6V Well-Cycled LiNi0.90Co0.06Mn0.04O2||Li Batteries.

Owing to the outstanding comprehensive properties of high energy density, excellent cycling ability, and reasonable cost, Ni-rich layered oxides (NCM) are the most promising cathode for lithium-ion batteries (LIBs). To further enhance the specific capacity of Ni-rich layered oxides, it is necessary to increase the cut-off voltage to a higher level. However, a higher cut-off voltage can lead to substantial structural changes and trigger interface side reactions, presenting significant challenges for practical applications (cycle life and safety). Herein, to solve above issues, tris(hexafluoroisopropyl)borate (TFPB) is introduced as a high voltage electrolyte additive for LiNi0.90Co0.06Mn0.04O2 cathode. Based on detail in situ/ex situ characterization, this study proves that TFPB forms a protective solid-state interphase (SEI) layer on the Li-anode. Additionally, derivatives of TFPB are easily oxidatively decomposed to create a dense cathode electrolyte interphase (CEI) film on the cathode. This CEI film effectively prevents the continuous oxidation of the electrolyte and mitigates the adverse effects of HF on the battery. Benefit from the protective SEI and CEI layer, the LiNi0.90Co0.06Mn0.04O2||Li battery with a TFPB-containing electrolyte maintains an unprecedented level of performance, with a capacity retention of 89.1% after 100 cycles under the ultrahigh cut-off voltage of 4.6V (vs Li/Li+).

Distinctive interphasial properties and high structural reversibility endowed by B-/CN– electrolyte additive and its superior electrochemical performance for graphite/LiCoO2 pouch cells

The exploration of high energy density batteries requires cathodes with a high specific capacity and high voltage. LiCoO2 (LCO), a prospective cathode material for lithium-ion batteries (LIBs), displays a decent capacity but without a compatible electrolyte to withstand its high voltage. To create an enduring cathode electrolyte interphase (CEI), a novel electrolyte additive, 2-cyanophenylboronic acid (CBB), which possesses both cyano (CN–) and boronic acid (B-) functional group, is firstly put forward to be applied in LCO/Li cells as well as graphite/LCO pouch cell. Data from experimental and theoretical research both reveal that CBB preferentially decomposes on LCO surface, constructing a thin, robust and homogeneous CEI film rich in B-F/B-O species with fast kinetics and low impedance, suppressing the LCO surface transformation upon cycling due to the alleviated Co dissolution. CBB additive can enhance the structural reversibility (in-situ X-ray diffraction), decrease the electrolyte decomposition and eliminate the HF formation, constructing a much smoother surface than base electrolyte. LCO/Li cells with 1% CBB deliver more excellent capacity retention (96%) and a much superior rate capability (145 mAh/g, 5C) than that of base electrolyte (67%, 115 mAh/g). Intriguingly, graphite/LCO pouch batteries containing 1% CBB for 300 cycles deliver more outstanding capacity retention of 80% than that of base electrolyte (2%). This work reveals that CBB can function as an efficient electrolyte additive for high voltage LCO cathode, of which its distinctive CEI modification on the LCO surface provides valuable perspectives for its commercial application towards high energy density LIBs.

Revealing the Subzero-Temperature Electrochemical Kinetics Behaviors in Ni-Rich Cathode.

The sluggish kinetics in Ni-rich cathodes at subzero temperatures causes decreased specific capacity and poor rate capability, resulting in slow and unstable charge storage. So far, the driving force of this phenomenon remains a mystery. Herein, with the help of in-situ X-ray diffraction and time of flight secondary ion mass spectrometry techniques, the continuous accumulation of both the cathode electrolyte interphase (CEI) film formation and the incomplete structure evolution during cycling under subzero temperature are proposed. It is presented that excessively uniform and thick CEI film generated at subzero temperatures would block the diffusion of Li+ -ions, resulting in incomplete phase evolution and clear charge potential delay. The incomplete phase evolution throughout the Li+ -ion intercalation/de-intercalation processes would further cause low depth of discharge and poor electrochemical reversibility with low initial Coulombic efficiency, as well. In addition, the formation of the thick and uniform CEI film would also consume Li+ -ions during the charging process. This discovery highlights the effects of the CEI film formation behavior and incomplete phase evolution in restricting electrochemical kinetics under subzero temperatures, which the authors believe would promote the further application of the Ni-rich cathodes.

Inhibition of transition-metal dissolution with an inert soluble product interface constructed by high-concentration electrolyte

The formation of a compact and stable cathode electrolyte interphase (CEI) film is a promising way to improve the high voltage resistance of lithium-ion batteries (LIBs). However, challenges arise due to the corrosion of hydrogen fluoride (HF) and the dissolution of transition metal ions (TMs) in harsh conditions. To address this issue, researchers have constructed an anion-derived CEI film enriched with LiF and LiPO2F2 soluble product on the surface of LiNi0.5Mn1.5O4 (LNMO) cathode in highly concentrated electrolytes (HCEs). The strong binding of LiF and LiPO2F2 generated an inert LiPO2F2 soluble product interface, which inhibited HF corrosion and maintained the spinel structure of LNMO, contributing to a capacity retention of 92% after 200 cycles at 55°C in the resulting cell with a soluble LiPO2F2-containing CEI film. This new approach sheds light on improving the electrode/electrolyte interface for high-energy LIBs.

Regulating Cathode‐Electrolyte Interphase by Confining Functional Aluminum Compound within Ni‐Rich Cathodes

AbstractThe oxidative capability of Ni4+ and high operation voltage of nickel‐rich LiNi1−x−yCoxMnyO2 (Ni‐rich NCM) cause its continuous and deleterious side reactions with electrolyte and irreversible phase transition, which hinder its industrial application. To mitigate these issues, Al (CF3SO3)3 is proposed as a solid electrolyte additive that can be readily oxidized to regulate the cathode‐electrolyte interphase (CEI) due to the highest occupied molecular orbital‐level of CF3SO3−, meanwhile being confined within the single‐crystalline NCM811. CF3SO3− prior to the electrolyte is oxidized upon increasing voltage to produce sulfur components and involve CEI formation. Concurrently, the released Al3+ ions are combined with reactive oxygen from NCM811 particles and HF from the electrolyte to form Al2O3 and AlF3, respectively. A robust sandwich CEI film containing sulfur and aluminum species is formed, which cannot only prevent decomposition of the electrolyte, but also alleviate the formation of inactive rock‐salt phase on NCM811 surface. Consequently, such CEI leads to high‐performance batteries with a high‐capacity retention of 91.5% after 200 cycles under 0.5 C compared to 72.4% of pristine NCM811. This facile and environmentally benign method provides a new avenue to develop high‐capacity and durable cathodes for lithium‐ion batteries.

Constructing fast-ion-diffusion cathode-electrolyte-interphase to improve LiNi0.8Co0.1Mn0.1O2 cathode cyclic stability with composite polymer conductors

As one of the mainstream cathode materials used in power battery, it’s necessary for the LiNixCoyMn1-x-yO2 (NCM) to perform fast charge and discharge capabilities. However, the unstable cathode-electrolyte-interphase (CEI) at high charging/discharging rates causes cycling performance to degrade rapidly. Modified polyaniline (PANI) and polyethylene glycol (PEG) are adsorbed on layer-structured Ni-rich cathode, forming cathode-electrolyte-interphase CEI film with high electro-chemical stability and mechanical strength layer by coordinating interaction of hydrogen bonds with transition metals. When the carbon nanotubes (CNTs) or the -SO2F functional group in bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) enters PANI skeleton, the composition, morphology and thickness of CEI film would be influenced by the CNTs or -SO2F groups. The improved interface would enhance the cathode-electrolyte interface stability by providing a steady channel for Li+ transport. Hence, the optimized CEI film effectively inhibit the occurrence of side reactions, and the capacity retention of NCM@PANIPEG/LiTFSI||Li cells has been improved from ∼ 55% to ∼ 77% at 45 °C and 2.7–4.3 V (vs. Li+/Li) in 100 cycles at 1 C. This low cost and straightforward preparation process exhibit great application prospects in future industrial applications.

Rescue the cycle life of LiNi0.5Mn1.5O4 cathode on high voltage via glyceryl triacetate as the multifunction additive

The poor cyclic stability is the key point to restricting the commercial application of LNi0.5Mn1.5O4-based lithium-ion batteries (LNMO-based LIBs) on high voltage (5 V-class). Once the working voltage is over 4.3 V, the parasitic reactions on the LNMO interphase will be the trigger for a series of problems such as the continuous decomposition of electrolytes and dissolution of transition metal ions. The glyceryl triacetate (GT) has been used as a functional additive to reconstruct a cathode electrolyte interphase (CEI) film to suppress the parasitic reactions and safeguard the LNMO structure. Compared to the influence of GT and propionic anhydride (PA) on the electrochemical performance of LNMO on high voltage, the LNMO/Li cell with GT exhibits exciting discharge capacity retention of 84.9 % compared with PA (17.6 %) and base electrolyte (BE, 8.6 %) after 500 cycles at 1C. The LNMO/Li cell with GT also has a remarkable rate performance in which capacity retention of 5C/2C is 85.7%, higher than 0.5 wt% PA (75.5%) and BE (62.1%). By characterizations, it is reasoningly demonstrated that the GT not only can be preferentially oxidized to form a robust CEI film on the LNMO surface to prevent the parasitic reaction but also rapidly capture the free transition metal ions and bind trace H2O in the electrolyte to improve the stability of LIBs. Simultaneously, the electrons from the carboxyl group of GT occupy the d-orbital of transition metal from LNMO, further effectively reducing the activity of LNMO. Particularly, the glycerol generated by the reaction of GT with H2O can effectively capture HF, H2O, and other small molecules under hydrogen-bond interaction, thereby improving the stability of LNMO LIBs.

A tough Janus-faced CEI film for high voltage layered oxide cathodes beyond 4.6 V

Cathode electrolyte interphase (CEI) film is suffering from electrochemical disintegration and physical disturbance caused by particles’ volumetric variation while layered oxide cathodes are operated at high voltage. However, most CEI design strategies only focus on the electrochemical stability of the film. Herein a novel tough Janus-faced CEI film with excellent high-voltage stability and ultrahigh Young's modulus ∼ 30 Gpa is in-situ constructed by succinonitrile (SN) and cyclohexylbenzene (CHB) additives. SN is adsorbed on electrode particles due to the coordinative interaction with transition metals, then CHB is electrochemically polymerized generating an outer layer subsequently. This Janus-faced film not only prevents the decomposition of electrolyte and the irreversible phase transition on particles surface, but also suppresses the particles cracking and Co element dissolution. With the multiply protection of this film, the cyclic performances of LiCoO2 and LiNi0.8Co0.1Mn0.1O2 cathodes are remarkably improved, over 500 & 600 cycles at the high voltages of 4.6 & 4.7 V vs. Li+/Li, respectively. This work not only provides a novel design of functional CEI film, but also throws light on other electrochemical protection fields such as metal corrosion.

Constructing a Stabilized Cathode Electrolyte Interphase for High-Voltage LiCoO2 Batteries via the Phenylmaleic Anhydride Additive

Increasing the charge cutoff voltage (≥4.6 V) is an effective and mainstream strategy to elevate the energy density of LiCoO2 (LCO)-based lithium-ion batteries (LIBs). However, the cycle life is compromised and challenged owing to the vulnerable cathode electrolyte interphase (CEI) and severe structural degradation of LCO at high voltages. In this work, a functional electrolyte additive phenylmaleic anhydride (PMA) is applied to construct a stabilized CEI film with compact and uniform properties, which facilitates preserving the fast Li+ diffusion kinetics during cycling and mitigates the dissolution of Co ions and fragmentation of LCO. As a result, the durability of the LCO battery is significantly improved in the electrolyte with PMA, and the capacity retention is promoted from 55.3 to 78.6% after 300 cycles at 1C at a cutoff voltage of 4.65 V. The electrolyte regulation with a proper additive provides an effective path to enhance the electrochemical performance of LIBs with high energy density and thus realize their practical applications.

Porous Li4Ti5O12-induced cathode electrolyte interphases enable sustainable single-crystalline NCM cathodes

The cathode electrolyte interphase (CEI) layer derived on the cathode surface has a great influence on the coulombic efficiency, energy efficiency, rate performance, self-discharge, and cycling life of the lithium-ion batteries (LIBs). For layered nickel-rich cathodes, the electrochemically-inert residual lithium (e.g., Li2CO3, LiHCO3) combined with the continuous growth of CEI under high voltage (≥4.5 V vs Li/Li+) further increases the difficulty of regulating the activity and stability of LIBs. In this work, single-crystalline LiNi0.5Co0.2Mn0.3O2 (NCM523) is modified with porous Li4Ti5O12 particles through a simple solid synthesis process between residual lithium and metal–organic framework MIL-125(Ti). Here, metal–organic frameworks (MOFs) play an important role to thoroughly derive porous Li4Ti5O12 with a decentralized but not fully covered situation on the NCM523 surface. Moreover, Ti4+-rich Li4−xTi5O12−y would arise when the Li+ and O2– are removed from Li4Ti5O12 under a high voltage, which can promote the growth of cathode electrolyte interphase (CEI) along with enhanced capacity and stability of the NCM523 single crystals. Based on the MOFs materials, a new method of modifying cathodes with promoted CEI film was designed, which could be a reference for the development of high voltage and nickel-rich cathodes.

A Novel Scheme to Improve the Stability of Conventional‐Concentration Electrolyte at High Voltage

AbstractIt is well‐known that conventional‐concentration electrolytes (CCEs) will undergo serious oxidative decomposition in high‐voltage lithium‐ion batteries, while high‐concentration electrolytes (HCEs) have good oxidation stability. However, HCEs are difficult to scale‐up for industrial applications due to their high viscosity and cost. Herein, based on the anti‐oxidation mechanism of HCE, a novel scheme to improve the stability of CCE at high voltage was proposed. By means of magnetron sputtering LiF on LiNi0.5Mn1.5O4 surface and introducing LiPO2F2 additive, a stable cathode electrolyte interphase (CEI) film is constructed in CCE. AFM results show that the constructed CEI film in CCE has better mechanical strength and adhesion than that formed by HCE. Moreover, the battery performance with the constructed CEI film in CCE, which affords high stability at high voltage, fast‐charging kinetics, and improved cycling stability (135 mAh g−1 after 200 cycles), is comparable to that with HCE.

Revealing the cathode electrolyte interphase on Li- and Mn-rich materials by in-situ electrochemical atomic force microscopy

Solid electrolyte interphase (SEI) grown on electrode surfaces during charge-discharge processes plays a key role in the cycle performance of lithium-ion batteries. In-situ study of cathode electrolyte interphase (CEI) is challenging due to the complicated interfacial reactions on the cathode materials including gas formation and the formation of thin CEI film. Herein, we applied the electrochemical atomic force microscope (EC-AFM) to study the interfacial changes in high energy density Li- and Mn-rich (LMR) materials with an F-rich electrolyte (1 M LiPF6 FEC/FEMC/HFE). The study indicated that the electrolyte formed a uniform and dense passivation CEI film on the LMR material surface at high voltage. The CEI is composed of inorganic LiF substrate as confirmed by X-ray photoelectron spectroscopy (XPS). The assembled battery (LMR||Li) shows an excellent cycle performance and maintains capacity at 85.5% after 100 cycles, compared to the 13.7% retention rate of commercial carbonate electrolyte (1 M LiPF6 EC/EMC/DMC).

Antioxidation Mechanism of Highly Concentrated Electrolytes at High Voltage.

It has been researched that highly concentrated electrolytes (HCEs) can solve the problem of the excessive decomposition of dilute electrolytes at a high voltage, but the mechanism is not clear. In this work, the antioxidation mechanism of HCE at a high voltage was investigated by in situ electrochemical tests and theoretical calculations from the perspective of the solvation structure and physicochemical property. The results indicate that compared with the dilute electrolyte, the change of solvation structures in HCE makes more PF6- anions easier to be oxidized prior to the dimethyl carbonate solvents, resulting in a more stable cathode-electrolyte interphase (CEI) film. First, the lower oxidation potential of the solvation structure with more PF6- anions inhibits the side effects of the electrolyte effectively. Second, the CEI film, consisted of LiF and LixPOyFz generated from the oxidation of PF6- and Li3PO4 generated from the hydrolysis of LiPF6 via the soluble PO2F2- intermediate, can reduce the interface impedance and improve the conductivity. Intriguingly, the high viscosity of HCEs and the hydrolysis of LiPF6 are proven to play a positive role in enhancing the interfacial stability of the electrolyte/electrode at a high voltage. This study builds a deep understanding of the bulk and interface properties of HCEs and provides theoretical support for their large-scale application in high-voltage battery materials.

Coexistence of (O2)n- and Trapped Molecular O2 as the Oxidized Species in P2-Type Sodium 3d Layered Oxide and Stable Interface Enabled by Highly Fluorinated Electrolyte.

The interface stability of cathode/electrolyte for Na-ion layered oxides is tightly related to the oxidized species formed during the electrochemical process. Herein, we for the first time decipher the coexistence of (O2)n- and trapped molecular O2 in the (de)sodiation process of P2-Na0.66[Li0.22Mn0.78]O2 by using advanced electron paramagnetic resonance (EPR) spectroscopy. An unstable interface of cathode/electrolyte can thus be envisaged with conventional carbonate electrolyte due to the high reactivity of the oxidized O species. We therefore introduce a highly fluorinated electrolyte to tentatively construct a stable and protective interface between P2-Na0.66[Li0.22Mn0.78]O2 and the electrolyte. As expected, an even and robust NaF-rich cathode-electrolyte interphase (CEI) film is formed in the highly fluorinated electrolyte, in sharp contrast to the nonuniform and friable organic-rich CEI formed in the conventional lowly fluorinated electrolyte. The in situ formed fluorinated CEI film can significantly mitigate the local structural degeneration of P2-Na0.66[Li0.22Mn0.78]O2 by refraining the irreversible Li/Mn dissolutions and O2 release, endowing a highly reversible oxygen redox reaction. Resultantly, P2-Na0.66[Li0.22Mn0.78]O2 in highly fluorinated electrolyte achieves a high Coulombic efficiency (CE) of >99% and an impressive cycling stability in the voltage range of 2.0-4.5 V (vs Na+/Na) under room temperature (147.6 mAh g-1, 100 cycles) and at 45 °C (142.5 mAh g-1, 100 cycles). This study highlights the profound impact of oxidized oxygen species on the interfacial stability of cathode/electrolyte and carves a new path for building stable interface and enabling highly stable oxygen redox reaction.

Generating lithium fluoride-abundant interphase on layered lithium-rich oxide cathode with lithium 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide

Layered Li-rich oxides (LLROs) attract much attention due to their high capacities. However, the electrolyte decomposition and interfacial reactions especially driven by the catalysis effect of high-valance transition metal ions under high voltage cause severe performance recession, hindering their practical applications. Lithium fluoride (LiF) is an effective component in both cathode electrolyte interphase (CEI) and solid electrolyte interphase (SEI) films. In this work, lithium 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide (LiHFDF) is employed as a novel electrolyte additive for generating LiF-abundant interphase on LLRO. With the addition of LiHFDF, the capacity retention and Coulombic efficiency of the LLRO are improved noticeably. In addition, the interfacial side reactions and voltage decay issues are mitigated. Transmission electron microscopy and X-ray photoelectron spectroscopy measurements confirm that a compact and thin CEI film with more LiF is formed on the cathode surface for the cell using LiHFDF, which is beneficial for the enhancement of the electrochemical performance. Moreover, differential scanning calorimetry (DSC) investigations reveal that the LLRO electrode with LiHFDF has improved safety compared to that with the blank electrolyte. This work provides an effective LiF-generating electrolyte additive for high-voltage LLROs, which is believed to be positive for other high-voltage cathode materials.

Tris(trimethylsilyl) borate as electrolyte additive alleviating cathode electrolyte interphase for enhanced lithium-selenium battery

Lithium-selenium (Li-Se) batteries have attracted increasing attentions in recent years because of their high energy density and theoretical capacity. One of the keys that influences the cycle stability of Se cathode is the formation of a stable cathode electrolyte interphase (CEI) film between the cathode material and electrolyte. In this work, we report utilizing tris(trimethylsilyl) borate (TMSB) as electrolyte additive to promote the formation of stable CEI film for enhanced Li-Se battery. The TMSB containing electron-deficient boron atoms easily adsorb electron-rich F− and PFx− to form polyanionic groups, which suppress the formation of insulating LiF in the CEI film, promoting the conductivity and stability of the cathode. Furthermore, the TMSB adsorbing PFx− releases more active Li+ for reaction to improve the capacity. These largely improve the compatibility of the electrolyte and the electrode material interface, significantly reducing the interface impedance and increasing the rate capability. As the result, the system with 1 wt% TMSB exhibits a high specific discharge capacity of 462 mA h g –1 at 1 C after 500 cycles, showing 130% improvement for the cathode without TMSB. Our work here confirms that TMSB as electrolyte additive can effectively improve the electrochemical performance of Li-Se batteries.

Selection of Sodium Salt Electrolyte Compatible with Na0.67Ni0.15Fe0.2Mn0.65O2 Cathode for Sodium‐Ion Batteries

The construction of an optimized electrolyte system compatible with layered transition metal (TM) oxides is of great importance to advanced sodium‐ion batteries (SIBs). Herein, a low‐cost iron‐containing manganese‐based layered cathode material of Na0.67Ni0.15Fe0.2Mn0.65O2 (N‐NFM) is prepared through an improved coprecipitation method. Then, the chemical properties of interfaces between the N‐NFM cathode and organic liquid electrolytes based on NaClO4 and NaPF6 are investigated, respectively. Results show that the cathode electrolyte interphase (CEI) film formed in the NaPF6‐based electrolyte is dense and uniform, which inhibits the dissolution of TM ions effectively and provides a low energy barrier for the transport of Na+. Apart from that the CEI film formed in the NaClO4‐based electrolyte contains more organic but less inorganic compounds, resulting in an increase in impedance. In addition, it is believed that the stability of the CEI film is susceptible to the perchlorate with strong oxidizing property. In this role, a small part of the CEI film falls from the cathode surface, accelerating the dissolution of TM ions and leading to the reactivation of electrolyte decomposition.