Cathode Electrolyte Interface Research Articles

A Phosphorofluoridate-Based Multifunctional Electrolyte Additive Enables Long Cycling of High-Energy Lithium-Ion Batteries.

Ni-rich layered oxides are regarded as key components for realizing post Li-ion batteries (LIBs). However, high-valence Ni, which acts as an oxidant in deeply delithiated states, aggravates the oxidation of the electrolyte at the cathode, causing cell impedance to increase. Additionally, the leaching of transition metal (TM) ions from Ni-rich cathodes by acidic compounds such as Brønsted-acidic HF produced through LiPF6 hydrolysis aggravates the structural instability of the cathode and renders the electrode-electrolyte interface unstable. Herein, we present a multifunctional electrolyte additive, bis(trimethylsilyl) phosphorofluoridate (BTSPFA), to attain enhanced interfacial stability of graphite anodes and Ni-rich cathodes in Li-ion cells. BTSPFA eliminates the corrosive HF molecules by cleaving silyl ether bonds and enables the formation of a polar P-O- and P-F-enriched cathode electrolyte interface (CEI) on the Ni-rich cathode. It also promotes the creation of a solid electrolyte interphase composed of inorganic-rich species, which suppresses the reduction of the electrolyte during battery operation. The synergistic effect of the HF scavenging ability of BTSPFA and the stable BTSPFA-promoted CEI effectively suppresses the TM leaching from the Ni-rich cathode while also preventing unwanted TM deposition on the anode. LiNi0.8Co0.1Mn0.1O2/graphite full cells with 1 wt % BTSPFA exhibited an enhanced discharge capacity retention of 79.8% after 500 cycles at 1C and 45 °C. These unique features of BTSPFA are useful for resolving the interfacial deterioration issue of high-capacity Ni-rich cathodes paired with graphite anodes.

Stabilizing Ni-rich LiNi0.8Co0.1Mn0.1O2 cathode by generating ion-conductive polymer coating layer with 1, 3-dioxolane introducing electrolyte for lithium-ion batteries

The employment of electrolyte-film forming additive is a facile and effective strategy to improve the cycling performance of aggressive Nickel-rich layered oxide cathode LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode for high energy lithium-ion batteries (LIBs). Here we report a homogeneous and dense polymer cathode-electrolyte-interface (CEI) film generated via in-situ electrooxidation polymerization of 1, 3-Dioxolane (DOL) in a conventional carbonate-based electrolyte. It is found that the electrolyte modification with 15% DOL is the most effective in balancing the interfacial stability and Li-ion diffusion kinetics of NCM811 cathode, while also achieving its comparable rate capability and greatly improving cycling performance. Morphological and elemental characterizations demonstrate that the 15% DOL-derived CEI layer not only alleviates the impacts of harmful interfacial reactions and hinders the particle disintegration, but also suppresses the irreversible lattice transition of the NCM811 cathode surface to the inactive rock salt phase. Moreover, it is proved that the F-rich interfacial layer displays excellent Li-ion transport kinetics. As a result, the cell exhibits high capacity retention of 85.2% and 78.1% after 200 cycles at cut-off voltages of 4.3 V and 4.5 V at 1 C in the 15% DOL-modified electrolyte. Therefore, creating a stable and effective Li-ion conductive interfacial protective film on the cathode surface by optimizing an electrolyte functional additive is an economical and available approach to develop the highly stabilized Ni-rich cathode for high-energy LIBs applications.

Tuning Electrolyte Solvation Structure and CEI Film to Enable Long Lasting FSI−‐Based Dual‐Ion Battery

AbstractDual‐ion battery (DIB) is a promising energy storage system because it can provide high power. However, the stability and rate performance of the battery depend strongly on the type of salt and solvents in the electrolyte. Herein, the use of lithium bis(fluorosulfonyl)imide (LiFSI) is studied, which has better high‐temperature stability, as salt in the DIB and develop a 3 m LiFSI fluoroethylene carbonate/methyl 2,2,2‐trifluoroethyl carbonate (FEC/FEMC) = 3:7 electrolyte, which stabilizes graphite–lithium DIB with 94.1% capacity retention after 2000 cycles at 5C. The DIB also exhibits excellent rate performance with 100.4 mAh g−1 capacity at 30C, with a utilization of 96.3% compared to capacity at 2C. The outstanding electrochemical performance is attributed to the thin cathode electrolyte interface (CEI) layer and fast FSI− transport kinetics, confirmed by X‐ray photoelectron spectroscopy and activation energy calculation. Superior cycle and rate performances are also obtained from a graphite–graphite full cell. Though, increasing salt concentration to 5 and 6 m leads to sluggish FSI− de‐intercalation reaction and lower capacity, which is attributed to solvent co‐intercalation. The research suggests that the electrolyte plays an important role in ion transport, surface film formation, and stability of DIB.

Resent Progress of LiNi1-x-yCoxMnyO2 for Lithium-ion batteries

The increased use of lithium-ion batteries in larger devices such as electric vehicles and energy storage devices has led to a need for improved battery performance. Researchers are developing cathode active materials with higher energy density, such as lithium phosphate and lithium transition metal compounds. Ternary cathode active materials with high capacity have also been developed, but there are issues with cation mixing and side reactions that can lead to reduced capacity, voltage drop, and even explosions. To address these issues, researchers are focusing on stabilizing and optimizing the cathode-electrolyte interface through methods such as coating with protective layers, cation or anion doping and changing of active materials structure. Herein, we briefly introduce and discuss the recent research with development trend of cathode material’s degradation solution for Li-ion batteries.

Methylboronic acid MIDA ester (ADM) as an effective additive in electrolyte to improve cathode electrolyte interlayer performance of LiNi0.8Co0.15Al0.05O2 electrode

We investigated the effectiveness of using methylboronic acid MIDA ester (ADM) as an additive in an electrolyte to enhance the overall electrochemical and material properties of an LNCAO (LiNi0.8Co0.15Al0.05O2) cathode. The cyclic stability of the cathode material measured at 40 °C (@ 0.2 C) showed an enhanced capacity of 144.28 mAh g−1 (@ 100 cycles), a capacity retention of 80%, and a high coulombic efficiency (99.5%), in contrast to these same properties without the electrolyte additive (37.5 mAh g−1, ~ 20%, and 90.4%), thus confirming the effectiveness of the additive. A Fourier transform infrared spectroscopy (FTIR) analysis distinctly showed that the ADM additive suppressed the EC-Li+ ion coordination (1197 cm−1 and 728 cm−1) in the electrolyte, thereby improving the cyclic performance of the LNCAO cathode. The cathode after 100 charge/discharge cycles revealed that the ADM-containing system exhibited better surface stability of the grains in the LNCAO cathode, whereas distinct cracks were observed in the system without the ADM in the electrolyte. A transmission electron microscopy (TEM) analysis revealed the presence of a thin, uniform and dense cathode electrolyte interface (CEI) film on the surface of LNCAO cathode. An operando synchrotron X-ray diffraction (XRD) test identified the high structural reversibility of the LNCAO cathode with a CEI layer formed by the ADM, which effectively maintained the structural stability of the layered material. The additive effectively inhibited the decomposition of electrolyte compositions, as confirmed by X-ray photoelectron spectroscopy (XPS).

Efficient and scalable encapsulation process of highly conductive 1T-MoS2 nanosheets on Ni-rich LiNi0.83Co0.11Mn0.06O2 cathode materials for high-performance lithium-ion batteries

Ni-rich LiNi1-x-yCoxMnyO2 (NCM) is an attractive cathode material that can meet the growing global demand of the Lithium-ion battery market owing to its high energy density and low cost. However, it still suffers from cyclic and thermal instability due to several issues, such as structural deterioration and excessive cathode electrolyte interface (CEI) layer formation arising from side reactions occurring at the NCM particle surface. In this study, molybdenum disulfide (MoS2) nanosheets with a metallic 1T phase are synthesized by chemical exfoliation, functionalized with polyethyleneimine (PEI), and uniformly coated on the surface of Ni-rich NCM particles through electrostatic interactions. As a result, the ceMoS2-PEI layer effectively alleviates the electrochemical performance degradation of NCM caused by irreversible phase transitions, microcrack formation, transition metal dissolution, and thick CEI layer formation by suppressing side reactions due to direct contact with the organic electrolyte or hydrofluoric acid on the surface of NCM. In addition, the ceMoS2-PEI layer provides a sufficient transport pathway for charge transfer and Li+ ion diffusion, thereby mitigating electrode polarization and impedance increase. Consequently, NCM/ceMoS2-PEI electrodes exhibit a high discharge capacity of 150.6 mAh g−1 at 5C and outstanding capacity retention of 96.9 % after 100 cycles at 1C. Moreover, further cycle tests in harsh environments, such as high mass loading and operating temperature, demonstrate that the ceMoS2-PEI layer coating more effectively improves the structural and thermal stability of the Ni-rich NCM in harsh environments.

A Guideline to Mitigate Interfacial Degradation Processes in Solid‐State Batteries Caused by Cross Diffusion

AbstractDiffusion of transition metals across the cathode–electrolyte interface is identified as a key challenge for the practical realization of solid‐state batteries. This is related to the formation of highly resistive interphases impeding the charge transport across the materials. Herein, the hypothesis that formation of interphases is associated with the incorporation of Co into the Li7La3Zr2O12 lattice representing the starting point of a cascade of degradation processes is investigated. It is shown that Co incorporates into the garnet structure preferably four‐fold coordinated as Co2+ or Co3+ depending on oxygen fugacity. The solubility limit of Co is determined to be around 0.16 per formula unit, whereby concentrations beyond this limit causes a cubic‐to‐tetragonal phase transition. Moreover, the temperature‐dependent Co diffusion coefficient is determined, for example, D700 °C = 9.46 × 10−14 cm2 s−1 and an activation energy Ea = 1.65 eV, suggesting that detrimental cross diffusion will take place at any relevant process condition. Additionally, the optimal protective Al2O3 coating thickness for relevant temperatures is studied, which allows to create a process diagram to mitigate any degradation with a minimum compromise on electrochemical performance. This study provides a tool to optimize processing conditions toward developing high energy density solid‐state batteries.

Carbonized Polymer Dots Enhancing Interface Stability of LiNi0.8Co0.1Mn0.1O2 Cathodes

AbstractThe high energy density, low cost, and low toxicity of LiNi0.8Co0.1Mn0.1O2 (NCM811) cathodes has led to their large‐scale mass production. However, the poor interfacial stability between NCM811 and organic electrolytes impairs the long‐cycle performance of lithium ions batteries. In this study, carbonized polymer dots (CPDs) are successfully introduced onto the surface of NCM811 (NCM811@CPDs) via a simple physical mixing process. CPDs with rich surface oxygen functional groups form strong covalent bonds with transition metal (TM) ions at the NCM811 surface, distinctly restraining surface structure degradation, and transition metal ion dissolution. Furthermore, CPDs facilitate the formation of a compact and steady cathode electrolyte interface (CEI) layer during electrochemical cycling. As a result, the NCM811@1 wt% CPDs exhibit enhanced cycling performance with a capacity retention of 89.77% after 100 cycles at 0.5 C compared to 55.39% of the bare NCM811. This facile and effective surface decoration strategy provides valuable guidance for improving the stability and cycling performance of Ni‐rich cathodes.

Unveiling the parasitic-reaction-driven surface reconstruction in Ni-rich cathode and the electrochemical role of Li2CO3

Nickel-rich transition-metal oxides are widely regarded as promising cathode materials for high-energy-density lithium-ion batteries for emerging electric vehicles. However, achieving high energy density in Ni-rich cathodes is accompanied by substantial safety and cycle-life obstacles. The major issues of Ni-rich cathodes at high working potentials are originated from the unstable cathode-electrolyte interface, while the underlying mechanism of parasitic reactions towards surface reconstructions of cathode materials is not well understood. In this work, we controlled the Li2CO3 impurity content on LiNi0.83Mn0.1Co0.07O2 cathodes using air, tank-air, and O2 synthesis environments. Home-built high-precision leakage current and on-line electrochemical mass spectroscopy experiments verify that Li2CO3 impurity is a significant promoter of parasitic reactions on Ni-rich cathodes. The rate of parasitic reactions is strongly correlated to Li2CO3 content and severe performance deterioration of Ni83 cathodes. The post-mortem characterizations via high-resolution transition electron microscope and X-ray photoelectron spectroscopy depth profiles reveal that parasitic reactions promote more Ni reduction and O deficiency and even rock-salt phase transformation at the surface of cathode materials. Our observation suggests that surface reconstructions have a strong affiliation to parasitic reactions that create chemically acidic environment to etch away the lattice oxygen and offer the electrical charge to reduce the valence state of transition metal. Thus, this study advances our understanding on surface reconstructions of Ni-rich cathodes and prepares us for searching for rational strategies.

More than Just a Binder: Versatile Block Copolymer Enhances the Electrochemical Performance of a Nickel-Rich Cathode

Nickel-rich layered cathodes (for example, NCM811) hold great promise for practical applications in high-energy density lithium-ion battery technology. However, a nickel-rich cathode has poor cyclic stability because of the structure degradation induced by repeated cycles and excessive formation of thick cathode–electrolyte interface (CEI) layers. Poly(vinylidene difluoride) (PVDF) is a typical binder used to form a positive electrode, which nevertheless lacks the functionality to suppress structure degradation and control CEI layer formation. A new concept is proposed in this work to use a multi-functional block copolymer of poly(ether imide)-block-poly(dimethyl siloxane) (PEI-b-PDMS; abbreviated as PEMS) as the binder for the nickel-rich cathode. Unlike the conventional PVDF binder, the PEMS binder is capable of inhibiting crack formation within the NCM811 particles to retard structure degradation over cycles because of its elastomeric feature. The PEMS binder also enables tuning the composition of the CEI layer and inhibits the formation of overly thick CEI layers because the carbonate solvent molecules are excluded from the solvation sheath by the PEMS binder. Enhanced electrolyte affinity is exhibited by the electrode with the PEMS binder as evidenced by the contact angle measurement. Improved carbon black dispersion within the electrode is achieved by the PEMS binder because the PEI aromatic chain has strong affinity toward the carbon black particles. Both the cycling and rate performance of the NCM811 cathode is improved with the PEMS binder compared to the PVDF binder. In detail, the capacity retention is increased from 63.9 to 80.6% after 200 cycles at 0.5 C (1 C = 205 mAh g–1) when the PVDF binder is replaced by the PEMS binder. The rate performance at current densities of 2 and 5 C is increased significantly due to enhanced electrochemical kinetics.

Core-Shell Tandem Catalysis Coupled with Interface Engineering For High-Performance Room-Temperature Na-S Batteries.

The sluggish redox kinetics and shuttle effect seriously impede the large application of room-temperature sodium-sulfur (RT Na-S) batteries. Designing effective catalysts into cathode material is a promising approach to overcome the above issues. However, considering the multistep and multiphase transformations of sulfur redox process, it is impractical to achieve the effective catalysis of the entire S8 →Na2 Sx →Na2 S conversion through applying a single catalyst. Herein, this work fabricates a nitrogen-doped core-shell carbon nanosphere integrated with two different catalysts (ZnS-NC@Ni-N4 ), where isolated Ni-N4 sites and ZnS nanocrystals are distributed in the shell and core, respectively. ZnS nanocrystals ensure the rapid reduction of S8 into Na2 Sx (4<x≤8), while Ni-N4 sites realize the efficient conversion of Na2 Sx into Na2 S, bridged by the diffusion of Na2 Sx from the core to shell. Besides, Ni-N4 sites on the shell can also induce an inorganic-rich cathode-electrolyte interface (CEI) on ZnS-NC@Ni-N4 to further inhibit the shuttle effect. As a result, ZnS-NC@Ni-N4 /S cathode exhibits an excellent rate-performance (650 mAh g-1 at 5 A g-1 ) and ultralong cycling stability for 2000 cycles with a low capacity-decay rate of 0.011% per cycle. This work will guide the rational design of multicatalysts for high-performance RT Na-S batteries.

Stabilized Anionic Redox by Rational Structural Design from Surface to Bulk for Long-Life Fast-Charging Li-Rich Oxide Cathodes.

On account of high capacity and high voltage resulting from anionic redox, Li-rich layered oxides (LLOs) have become the most promising cathode candidate for the next-generation high-energy-density lithium-ion batteries (LIBs). Unfortunately, the participation of oxygen anion in charge compensation causes lattice oxygen evolution and accompanying structural degradation, voltage decay, capacity attenuation, low initial columbic efficiency, poor kinetics, and other problems. To resolve these challenges, a rational structural design strategy from surface to bulk by a facile pretreatment method for LLOs is provided to stabilize oxygen redox. On the surface, an integrated structure is constructed to suppress oxygen release, electrolyte attack, and consequent transition metals dissolution, accelerate lithium ions transport on the cathode-electrolyte interface, and alleviate the undesired phase transformation. While in the bulk, B doping into Li and Mn layer tetrahedron is introduced to increase the formation energy of O vacancy and decrease the lithium ions immigration barrier energy, bringing about the high stability of surrounding lattice oxygen and outstanding ions transport ability. Benefiting from the specific structure, the designed material with the enhanced structural integrity and stabilized anionic redox performs an excellent electrochemical performance and fast-charging property..

Enhancing the Electrochemical Properties of Nickel-Rich Cathode by Surface Coating with Defect-Rich Strontium Titanate.

Ni-rich layered ternary cathodes (i.e., LiNixCoyMzO2, M = Mn or Al, x + y + z = 1 and x ≥ 0.8) are promising candidates for the power supply of portable electronic devices and electric vehicles. However, the relatively high content of Ni4+ in the charged state shortens their lifespan due to inevitable capacity and voltage deteriorations during cycling. Therefore, the dilemma between high output energy and long cycle life needs to be addressed to facilitate more widespread commercialization of Ni-rich cathodes in modern lithium-ion batteries (LIBs). This work presents a facile surface modification approach with defect-rich strontium titanate (SrTiO3-x) coating on a typical Ni-rich cathode: LiNi0.8Co0.15Al0.05O2 (NCA). The defect-rich SrTiO3-x-modified NCA exhibits enhanced electrochemical performance compared to its pristine counterpart. In particular, the optimized sample delivers a high discharge capacity of ∼170 mA h/g after 200 cycles under 1C with capacity retention over 81.1%. The postmortem analysis provides new insight into the improved electrochemical properties which are ascribed to the SrTiO3-x coating layer. This layer appears to not only alleviate the internal resistance growth, from uncontrollable cathode-electrolyte interface evolution, but also acts as a lithium diffusion channel during prolonged cycling. Therefore, this work offers a feasible strategy to improve the electrochemical performance of layered cathodes with high nickel content for next-generation LIBs.

Quantifying Degradation Mechanisms in a High-Performance Parallel Hybrid Lithium-Ion Supercapacitor Induced by Long Term Cycling at High Current Rates

Understanding the degradation pathways of electrode materials is a key to develop more reliable Li-ion technologies along with an increased energy density and power rate. This study aims to demonstrate the benefits of the combined use of X-ray based characterization techniques and electrochemical assessment for thorough multi-scale analysis to elucidate the aging mechanisms of a Li4Ti5O12/AC//LiMn2O4/AC parallel hybrid lithium-ion supercapacitor. Analyses performed on samples extracted from full stack representative of industrial battery application, show that irreversible modifications are observed at all length scales on both electrodes. At the negative, the disaggregation and corrosion of the LTO active material, as well as AC particle cracking and electrode film delamination have been observed. In the meantime, drastic cracking of the AC and LMO active material along with important micro-strain increase at the crystallite level for LMO as well as Mn3+ dissolution are reported at the positive. The formation of a cathode electrolyte interface (CEI) is also reported. These structural and chemical changes have been identified as precursors to important polarization increase, Li inventory loss and furthermore capacity fading leading thus to device failure.

Electrode-electrolyte interface mediation via molecular anchoring for 4.7 V quasi-solid-state lithium metal batteries

Solid polymer electrolytes (SPEs), promising electrolyte candidates for lithium metal batteries (LMBs), still suffer from the great challenge of unstable solid-solid interface between the electrodes and SPEs. Herein, SPEs are modulated by trimethyl phosphate (TMP) molecular anchoring to reconstruct the Li+ solvation structure, satisfy the interfacial compatibility of electrolyte-electrode and further unlock ultra-high voltage quasi-solid-state lithium metal batteries. An ultra-long cycle life of over 9000 h is achieved in Li|Li symmetrical cell at 0.1 mA cm−2. Furthermore, multiple highly-sensitive characterization techniques together with molecular dynamics simulations were applied to demonstrate that the in-situ induced stable cathode-electrolyte interface (CEI) rich in LiF, LixPFyOz and organic hybrids is responsible for the structural integrity of Ni-rich cathodes at 4.7 V ultra-high voltage. More importantly, the 91.55% capacity retention after 300 cycles of NCM811|Li full cell can well satisfy the practical application of high-voltage quasi-solid-state Li-metal energy storage systems.

Revealing the mechanisms of electrolyte additive PTS on Ni-rich electrode: Tolerance to high temperature (50 °C) and high voltage (4.6 V)

Ni-rich (Ni≥0.8) LiNixCoyMn1−x−yO2 (NCM) cathode materials have attracted great interest for high-energy-density lithium ion batteries (LIBs). However, layered oxides of Ni-rich CAMs are suffering from the notorious structural and interfacial degradation in the liquid electrolyte, especially at high state-of-charge (voltage) and temperature, which leads to electrochemical performance deterioration and thermal runaway of the cell. This work systematically and comprehensively reveals the underlying mechanism of the electrolyte additive phenyl trifluoromethyl sulfide (PTS) to enhance the electrochemical property of Ni-rich layered oxides at high voltage (∼4.6 V) and high-temperature (∼50 °C). Specifically, PTS observably encourages the electrode stability via cathode-electrolyte interface strengthening and reactive oxygen species deactivating mechanism, which facilitates the NCM811 cathode to adapt to the deep charging state as well as the highly oxidized environment. The binding energy calculation results reveal that the superoxide radical could steadily adsorb on the S radical or Li+ sites by interacting with Li+, thus making the superoxide radical deactivated. With the interface strengthening of PTS, the Ah-level NCM811||Gr pouch cell exhibits excellent cycling stability. This work affords a guide to designing electrolytes for Ni-rich LIBs towards high-voltage and high-temperature.

Stabilizing Interfaces in High-Temperature NCM811-Li Batteries via Tuning Terminal Alkyl Chains of Ether Solvents.

Ether electrolytes are promising for lithium metal batteries. Despite the intensive research in recent years, most state-of-the-art ether electrolytes still cannot form reliable electrode-electrolyte interfaces in NCM811-Li batteries at diluted concentrations, especially in those operating at elevated temperatures. We report a simple but effective strategy to break this bottleneck and stabilize interfaces in high-temperature NCM811-Li batteries in ether electrolytes. We propose that by gradually extending the terminal groups of glycol diethers from methyl groups to n-butyl groups, the comprehensive stability of ether electrolytes is improved. An anion-dominated solvation structure is realized at a concentration of 1 M. Accordingly, the electrode-electrolyte interactions are suppressed, and a thinner, denser, and more inorganic-rich solid- /cathode-electrolyte interface is achieved. Additionally, the phase transition and structural degradation of NCM811 cathode are alleviated. Consequently, in the ethylene glycol dibutyl ether-based electrolyte, the Coulombic efficiency for Li-Cu cells working at 60 °C is boosted to 99.41% with a cycling life of over 200 cycles. The lifespan of high-temperature NCM811-Li cells is prolonged by more than 400% with a stable average Coulombic efficiency of 99.77% at quasi-practical conditions of 50 µm Li, lean electrolyte of 10 µL mAh-1, and medium-high cathode loading of >2.2 mAh cm-2.

Simultaneous B/W dual coating on ultra-high nickel single crystal cathode material for lithium-ion batteries

The ultra-high nickel LiNixCoyMn1-x-yO2 (x > 0.9) cathode material is a prime candidate for powering next-generation electric vehicles. However, its inherent structural instability and complicated interface side-reactions limit its commercialization. Here, the LiNi0.92Co0.04Mn0.04O2 single-crystal cathode material with an average particle size of 1.69 μm is prepared via a simple temperature fluctuation method combined with B and W surface modification. As-formed surface Li–B–O and Li2WO4 lithium ion conductors can successfully enhance the discharge capacity of single crystal LiNi0.92Co0.04Mn0.04O2 from 219.9 to 226.5 mAh g−1 by facilitating lithium ion diffusion. At 4.5 V, the B and W surface modification significantly enhances the cycle retention from 71.8% to 87.1% at 1 C after 100 cycles due to the construction of a uniform boron-rich cathode/electrolyte interface (CEI) layer. The specific composition of the as-formed CEI film is clearly identified. The analysis results of B and W modifications show that the presence of boron-rich CEI and Li2WO4 coating layer protects the particles from electrolyte erosion under high voltage and enhances lithium-ion diffusion on the particle surface. Therefore, the chemical states of Ni within the particle for B- and W-modified samples are distributed more homogeneously after long cycling.

High-voltage electrolyte design for a Ni-rich layered oxide cathode for lithium-ion batteries

LiNi0.8Co0.1Mn0.1O2 (NCM811) is one of the most promising cathode materials in high-energy-density Li-ion batteries (LIBs) because of its high capacity and low cost. However, it still suffers from irreversible capacity fading at high cut-off voltages. This is mainly because high voltage accelerates the hydrolysis reaction of lithium hexafluorophosphate with trace water to generate byproducts such as highly corrosive hydrogen fluoride (HF) resulting in an unstable cathode–electrolyte interface and continuous irreversible phase transitions. Here, we modify a conventional electrolyte by adding the dual additives of tetrabutyl titanate (TBT) and lithium difluoroxalate borate (LiDFOB) to form a stable Ti-, B-, and F-rich interfacial layer to eliminate the unfavorable cathode-electrolyte side reactions and suppress deleterious phase transitions. Additionally, TBT can stabilize the electrolyte by removing H2O/HF. With the synergistic effect of the dual additives, the cycling stability of NCM811 at high voltages is enhanced considerably. The Li∣NCM811 cell with dual additives exhibits a high capacity retention rate of 86% after 200 cycles at 1 C and a high cut-off voltage of 4.5 V. This strategy provides a reference for designing high-voltage electrolytes for LIBs.

Flexible and free-supporting Prussian blue analogs /MXene film for high-performance sodium-ion batteries

Sodium-ion batteries are the most promising alternative to lithium-ion batteries because of their low cost and environmental friendliness. Prussian blue analogs (PBAs) have attracted the attention of researchers because of their open skeleton structure and abundant redox active sites. However, the interstitial water in its crystal structure and poor electron conductivity limit its performance in sodium-ion batteries. A flexible self-supporting electrode is prepared by anhydrous preparation in a ball mill and simple filtration method. The high surface area nanoparticles and the high flexibility MXene layer in the electrode adapt to the rapid Na+ dynamics, alleviating the electrode expansion. The formation of a NaF-rich cathode-electrolyte interface (CEI) layer on the electrode surface inhibits the erosion of active substances by electrolyte, reduces the capacity attenuation of the electrode, and enhances the cyclic stability of the electrode. Electrode without a conductive agent and binder shows excellent electrochemical performance. The discharge capacity of the electrode is 58.3 mAh g−1at 1000 mA g−1 current density, and the capacity retention rate is 59.7% after 1000 cycles. This work has implications for the design of the next generation of low-cost, flexible, and fast charge and discharge energy storage devices.