Cathode Electrolyte Interface Research Articles

Fluorinated solvent molecule tuning to improve electrochemical performances of low-concentration electrolyte

The development of low-concentration electrolyte has a favorable application prospect due to its advantages of saving lithium resources. However, the large-scale application is limited by the formation of high-impedance organic electrode/electrolyte interphase (EEI) and the higher desolvation energy of Li+. Herein, a mixed flurorinated solvents of ethyl difluoroacetate (2F) / ethyl trifluoroacetate (3F) (3:7, in volume ratio) is designed to adjust the interaction between Li+ and the solvents, so as to achieve performance improvements of the low-concentration electrolyte system containing 0.5 M LiPF6. Results shows that 2F solvent greatly weakens the interaction between Li+ and the solvents, and 3F solvent further promotes anions to enter the solvation sheath to form Li(2F)3.0(PF6−)2.3(3F)0.5 structure. It promotes the Li+ desolvation, and brings about the formation of a LiF-rich cathode-electrolyte interface film on the surface of LiFePO4 (LFP) cathode due to the HOMO of the 2F solvent in the solvation sheath of Li+ will be increase as the involvement of PF6− in the solvation sheath of Li+. As a result, the cycling stability and rate performance (130 mAh g−1 at 4 C) of the LFP/Li half-cell are significantly improved. This work provides a new strategy to improve the electrochemical performances of low-concentration electrolytes by fluorinated solvent molecule tuning.

Enhancing High Voltage Stability via Fluorination of Functionalized Metal Organic Framework Electrolyte in Lithium Metal Batteries

The electrolyte stability under high‐voltage conditions considerably limits the upper cut‐off potential of solid‐state electrolytes (SSEs) and therefore the energy density of all‐solid‐state batteries (ASSBs). In this work, metal‐organic frameworks (MOFs) were fluorinated to lower the energy level of HOMO orbital and allow access to a 4F‐MOF that exhibit enhanced anodic stability. The composited 4F‐MOF/PEO electrolyte could not only transport Li+ ions in the ordered framework channels, but also provide a remarkable high‐voltage stability up to 5.0 V, shielding oxidative decomposition that would otherwise occur at around 3.9 V for conventional PEO electrolytes. In addition, stable lithium deposition was demonstrated for more than 1,300 hours at 0.1 mA·cm‐2, while reversible charge‐discharge cycling performance was delivered in assembled Li||LiNi0.5Mn1.5O4 (LNMO) ASSBs up to 5.0 V. Post‐mortem X‐ray photoelectron spectroscopy (XPS) investigation on cathodes revealed presence of a LiF‐rich cathode electrolyte interface (CEI), supporting promoted stability towards high‐voltage ASSBs.

Numerical simulation of the residual stress of solid oxide fuel cells with a three-dimensional nonplanar cathode–electrolyte interface

The longevity of solid oxide fuel cells is influenced by internal residual stresses, which may induce deformation or fracturing of components. This study investigates the residual stress distribution at the nonplanar cathode–electrolyte interface by approximating the actual interface with trigonometric functions and developing a three-dimensional (3D) model. The model reveals that the stress patterns at nonplanar interfaces can elucidate the genesis of interfacial cracks. During fabrication, anode contraction results in compressive stress within the electrolyte and tensile stress within the anode, with thermal discrepancies between layers being the primary cause of residual stresses. The reduction process diminishes these stresses, thus enhancing the mechanical integrity of the cell. Mitigating interface nonplanarity is beneficial for minimizing residual stress. At each interface crest, the electrolyte exhibits a local minimum in compressive stress, and a local maximum in shear stress occurs between each crest and trough. Furthermore, decreasing the initial porosity and NiO volume fraction can slightly lessen interlayer thermal discrepancies, with little effect on residual stresses.

Biomass-derived porous carbon-supported single-atomic cobalt toward high-performance aqueous zinc-sulfur batteries at room temperature

Aqueous zinc-sulfur batteries at room temperature hold great potential for next-generation energy storage technology due to their low cost, safety and high energy density. However, slow reaction kinetics and high activation energy at the sulfur cathode pose great challenges for the practical applications. Herein, an innovative biomass-derived carbon-supported single-atomic cobalt material (MMPC-Co) is synthesized as the cathode in Zn-S batteries. Results clearly indicate that the catalysis of single-atom Co sites greatly promotes the transform of the cathode electrolyte interface (CEI) on the cathode surface, while offering accelerated charge transfer rate for high conversion reversibility and large electrochemical surface area (ECSA) for high electrocatalytic current. Furthermore, the rich pore structure not only physically limits sulfur loss, but also accelerates the transport of zinc ions. In addition, the large pore volume of MMPC-Co is able to relieve the stress effect caused by the volume expansion of ZnS during charge/discharge cycles, thereby maintaining the stability of the electrode intact structure. Consequently, the sulfur cathode maintains a high specific capacity of 729.96 mA h g−1 after 500 cycles at 4 A g−1, which is much better than most cathode materials reported in the literature. This work provides new insights into the design and development of room-temperature aqueous Zn-S batteries.

Boosting the capacity and stability of Na3V2(PO4)2F3-2xO2x microspheres, using atomic layer deposition of artificial CEI

Phosphate-based materials [e.g. Na3V2(PO4)2F3-2xO2x; (NVPFO2x;0 < x < 1)] are regarded as a promising intercalation cathodes for Sodium-ion batteries (SIBs) due to their high reversible specific capacity and stability. However, so far only 2 Na ion were demonstrated to be active in these polyanionic cathodes, which limit their capacity. Herein we provide a strategic approach towards electrochemical activation of a 3rd Na ion, which leads to higher capacity, and preserves structural integrity. We synthesize and study a series of NVPFO2x (0 < x < 1) with well-controlled surface morphology and vanadium oxidation state, and study the dependence of the electrochemical behavior on the various composition and morphology. The optimized NVPFO cathode exhibited highest initial specific discharge capacity (131 mA h g−1) indicating the activation of 3rd Na ion. Nevertheless, the material suffers rapid capacity fading within ~50 cycles. We demonstrate how this instability is suppressed profoundly through the formation of a thin artificial cathode electrolyte interface (CEI) layer of TiO2 (ca. 2 nm) on the surface of NVPFO by atomic layer deposition (ALD). This TiO2 coating enabled even further extraction of sodium through higher voltage domain, but more importantly, it facilitated excellent long-term stability over 200 cycles. The structural stability of the cathode material was revealed by post cycling HRSEM, XPS, and XRD study. All these studies demonstrate that the thin TiO2 layer effectively contributes to formation of a robust CEI layer, diminishes the parasitic reaction on the electrode surface, reduces carbonate formation, and restricts the electrolyte decomposition.

Reconstruction of solvation structure at the cathode-electrolyte interface via partial de-solvation in a metal-organic framework

The formation of cathode-electrolyte interphase (CEI) is closely related to the solvation structure in direct contact with the cathode. The de-solvated solvent molecules are subject to the attack of highly catalytic transition metal ions in the cathode materials (such as Ni-rich LiNi0.8Co0.1Mn0.1O2, Ni-rich NCM), especially at high voltage or high temperature, leading to the poor CEI structure that accelerates the deterioration of rechargeable batteries. Considerable strategies have been proposed to solve this issue. Herein, we have constructed a porous Mg-MOF buffer layer on the surface of Ni-rich NCM, which not only greatly reduces the contact between solvent molecules and cathodes to alleviate the extensive interface reactions, but also promote the de-solvation of Li+ solvation structure through strong affinity between solvents and Mg2+. The partially de-solvated structure induces the anion to participate in the solvation structure and contributes to the formation of a uniform LiF-rich CEI with superior mechanical integrity and chemical stability. Therefore, the NCM with Mg-MOF buffer layer exhibits improved cycling stability at both elevated temperature of 45 °C and high voltage of 4.5 V with lithium as anode. Moreover, the modified NCM/graphite full cells also deliver ultra-stable rechargeable capability with a remarkable capacity retention of 87.3% over 1000 cycles as compared to the only 21.7% of pristine NCM based cells.

In-situ electrochemical passivation for constructing high-voltage PEO-based solid-state lithium battery

Polyethylene oxide (PEO) is one of the promising substrates for polymer solid electrolyte. However, when coupled with high energy density nickel-rich layered cathode materials, PEO faces the risk of catalytic decomposition by delithium nickel-rich materials. In this work, the in-situ electrochemical passivation strategy is employed to obtain high voltage PEO-based solid-state lithium battery. Trace commercial liquid electrolyte was added on the surface of LiNi0.8Co0.1Mn0.1O2 (NCM811) electrode, and liquid electrolyte is prior to PEO electrolyte to react with the cathode during charge/discharge process, leading to the formation of cathode-electrolyte interface (CEI) layer. The CEI layer avoids the direct contact between PEO and NCM811, thus effectively prevents PEO from being decomposed by NCM811 under high voltage. As a result, the optimized NCM811||PEO||Li battery displays enhanced electrochemical performance, especially cycling performance. This simple but effective strategy not only simplifies the manufacturing process, but also has instructional guidance for high-voltage PEO-based battery.

Strongly Solvating Triglyme-based Electrolyte Realizes Stable Lithium Metal Batteries at High-Voltage and High-Temperature

The electrolyte in lithium-metal batteries (LMBs) plays a pivotal role in stabilizing the surface of the lithium metal anode (LMA) and high-voltage cathode. However, currently widely studied ether-based electrolytes, although it has good compatibility with LMA due to their lower reactivity, are facing significant challenges related to poor oxidation resistance and cathode/electrolyte interface (CEI) instability when combined with the high-voltage cathode, particularly at elevated temperatures. Therefore, there is an urgent need to design advanced electrolytes capable of maintaining high stability under high-voltage and high-temperature conditions. Here, we have developed an ultra-stable localized high-concentration electrolyte (LHCE) by introducing triethylene glycol dimethyl ether (TG), which exhibits strong solvation ability. Computational and experimental results demonstrate that the designed TG-based dilute high-concentration electrolyte (TG-DHCE) possesses a stable solvation structure, high oxidation resistance, and thermal stability, making it ideal for LMBs. Furthermore, the unique solvation structure of TG-DHCE promotes the preferential decomposition of anions while facilitating the formation of stable, inorganic-rich electrode/electrolyte interfaces on the anode and cathode. Consequently, equipped with TG-DHCE, Li||Li cells (more than 1600 h) and Li||NCM523 cells (80.6%, 250 cycles) show superior stability and also demonstrate remarkable performance even at elevated temperatures (60 °C). Additionally, under the harsh conditions of ultrahigh loading NCM523 cathode (19.7 mg/cm2) and limited Li reservoir (twice excess Li was deposited on Cu, Li@Cu) (N/P=2), Li@Cu||NCM523 cells still maintain stable operation. This work pioneers a new path by choosing strongly solvating solvents, effectively enhancing the cycle life of LMBs under high-voltage and high-temperature conditions.

In Situ Construction of Inorganics-Rich Cathode-Electrolyte Interface toward Long-Life Prussian White Cathode.

Prussian white (PW) is one of the most promising candidates as a cathode for sodium-ion batteries (SIBs) because of its high theoretical capacity, excellent rate performance, and low production cost. However, PW materials suffer severe capacity decay during long-term cycling. In this work, a robust cathode electrolyte interface (CEI) is designed on the PW cathode by employing cresyl diphenyl phosphate (CDP) and adiponitrile (ADN) as electrolyte additives. CDP and ADN possess higher highest occupied molecular orbital energy levels (HOMO) than other solvents, leading to the preferential decomposition of CDP and ADN to construct an inorganics-rich CEI layer in situ on the PW cathode. Benefiting from this CEI layer, the degradation of PW is effectively inhibited during the long cycling. The Na||PW cell achieves an excellent cycling performance with a capacity retention of 85.62% after 1400 cycles. This work presented here provides a feasible strategy for improving the cycling performance of PW by electrolyte modification.

Constructing the Polymer Molecules to Regulate the Electrode/Electrolyte Interface to Enhance Lithium-Metal Battery Performance.

Lithium-ion batteries, with high energy density and long cycle life, have become the battery of choice for most vehicles and portable electronic devices; however, energy density, safety and cycle life require further improvements. Single-functional group electrolyte additives are very limited in practical applications, a ternary polymer bifunctional electrolyte additive copolymer (acrylonitrile-butyl hexafluoro methacrylate- poly (ethylene glycol) methacrylate- methyl ether) (PMANHF) was synthesized by free radical polymerization of acrylonitrile, 2, 2, 3, 4, 4, 4-hexafluorobutyl methacrylate and poly (ethylene glycol) methyl ether methacrylate. A series of characterizations show that in Li metal anodes, the preferential reduction of PMANHF is conducive to the formation of a uniform and stable solid electrolyte interphase layer, and Li deposition is uniform and dense. At the NCM811 cathode, a film composed of LiF- and Li3N-rich is formed at the cathode-electrolyte interface, mitigating the side reaction at the interface. At 1.0 mA cm-2, the Li/Li cell can be stabilized for 1000 cycles. In addition, the Li/NCM811 cell can stabilize 200 cycles with a cathode capacity of 153.7 mAh g-1, with the capacity retention of 89.93 %, at a negative/positive capacity ratio of 2.5. This study brings to light essential ideas for the fabrication of additives for lithium-metal batteries.

Pentafluorophenyl diethoxy phosphate: An electrolyte additive for high-voltage cathodes of lithium-ion batteries

Increasing the upper limit of the charging voltage in commercial lithium-ion batteries (LIBs) can enable them to have a higher energy density, but during high-voltage charging and discharging, the electrolyte will accelerate the decomposition, and the interfacial adverse reaction among the cathode and electrolyte is serious, which corrodes the cathode active material and consumes excessive electrolyte, and ultimately lead to a shorter service life of lithium-ion batteries. To address this issue, an electrolyte additive with properties suitable for high voltage cathodes, named pentafluorophenyl diethoxy phosphate (FPOP), was synthesized in this work and successfully applied to LiNi0.8Co0.1Mn0.1O2 cathode (NCM-811). Density Functional Theory (DFT) computations reveal that the FPOP additive undergoes preferential oxidation and self-polymerizes on the NCM-811 cathode surface to establishing a safeguarding film at the cathode-electrolyte interface (CEI). This film effectively inhibits detrimental side reactions. The electrochemical performance test data indicated that the retention rate of discharge-specific capacity in batteries was markedly enhanced with 0.1 wt% of FPOP when added to the standard electrolyte. Specifically, after subjecting the batteries to 200 cycles at a current of 0.2C and under voltages ranging from 4.2 V to 4.5 V, the retention of discharge ratio capacity increased by 37.6 %, 39.2 %, 26.9 %, and 37.1 % correspondingly. Morphology characterization and spectroscopic characterization showed that FPOP could not only construct a stable CEI film by oxidation during the first stages of lithium precipitation but also remove HF by strongly binding to the HF in the electrolyte or dissociating H+ and F− from HF, which ultimately enhanced the batteries' capacity to operate effectively at elevated voltage levels. Therefore, incorporating FPOP as an additive in electrolyte presents a cost-efficient approach to achieving elevated energy density.

Boosting the electrochemical performance of LiCoO2 by triple-phase interface via “island-bridge” shaped surface coating

Surface coating as an effective strategy has been extensively utilized to enhance the surface/interface stability of cathode for lithium-ion batteries. As widely recognized, the electrochemical performance of coated cathode is greatly dependent on the properties of coating layer. Rational design on the structure and composition of coating layer becomes crucial. Here, a unique “island-bridge” shaped Zr-based coating layer has been constructed through a facile synthetic protocol. This coating layer composes of several high-crystalline ZrO2 nanoparticles and a thin low-crystalline zirconium hydroxide layer that connects these nanoparticles, forming a typical “island-bridge” structure. The successful formation of such a unique coating layer was demonstrated to be related to the usage amount of Zr source and the calcined temperature. The novel “island-bridge” coating has been successfully applied to modify the surface of LiCoO2, showing excellent capability on improving the electrochemical performance. The unique “island-bridge” structure coating induces the formation of a triple-phase interface, namely island-cathode-bridge interface. Theoretical calculation suggests that such an island-cathode-bridge triple-phase interface can effectively reduce the Li+ diffusion energy, leading to favorable Li+ transportation kinetics and suppressed cathode-electrolyte interface evolution. Furthermore, this “island-bridge” layer can well prevent direct contact between LiCoO2 and electrolyte, enhancing the surface/interface stability, resulting in improved cyclic performance. This work provides new insights into the rational design of surface coating for high-energy density cathodes.

Tuning surface chemistry to reduce the step-like degradation of LiCoO2 at 4.6 V

Elevating the cut-off voltage is an effective route to increase the energy density of LiCoO2 (LCO). However, the highly delithiated LCO faces the issues of poor structural reversibility, O loss, and Co dissolution, etc., especially in the surface region. Herein, the step-like surface degradation (SSD) of pristine LCO (P-LCO) is firstly revealed to be responsible for the rapid capacity decay. To reduce the adverse impact of SSD, a solid electrolyte is coated and annealed to achieve the optimized surface structure chemistry of LCO (SE-LCO), featuring the outermost surface Li3PO4, surface rock-salt layer, and subsurface spinel-like layer. Benefiting from this surface optimization, the SE-LCO not only shows an enhanced but more reversible phase transition to enhance the structure stability, but also promotes the formation of tough cathode electrolyte interface (CEI) to reduce the O loss and Co dissolution issues. As a result, SE-LCO/graphite cell achieves excellent cycle stability with a remarkable capacity retention of 81.2% after 800 cycles in a potential range of 3–4.55 V, which is among the best reported cell performances. This work broadens the cognition for developing more advanced LCO cathodes.

A Timescale characterization of (NMC811)/SiOx-graphite 1Ah pouch cell: Improving cycling stability with carbonate-based electrolytes and triallyl phosphate (TAP) co-additive

The role of 2 wt% triallyl phosphate as an electrolyte additive in (NMC811)/SiOx-graphite 1Ah pouch cells is studied using electrochemical impedance spectroscopy, long-term cycling, rate, timescale characterisation and X-ray photoelectron spectroscopy. Six kinetic processes are identified within the 100 kHz to 0.01 Hz frequency range. Anode processes consistently occur faster than the cathode and at higher frequencies. The triallyl phosphate (TAP) additive improves capacity retention from 70% to 85% after 500 cycles at 45 °C, exhibiting a higher and more stable coulombic efficiency, with ∼100% reduction in DCIR growth. TAP cells produce less gas during cycling. Timescale characterization suggests that the high impedance in the base formulation originates from cathodic charge transfer and cathode electrolyte interface (CEI) films in non-TAP-containing samples. The XPS data confirm the DRT findings, indicating the presence of high-impedance species associated with solvent oxidation and transition metal oxides (TMO) in the CEI of the TAP-free sample. The initial higher impedance of 14%–0.06 Ω observed in TAP cells is due to the formation of a thicker CEI in the TAP samples. Consequently, TAP samples show a ∼10 mAhg−1 lower rate capacity, at higher C-rates of 3C and 5C, while both samples exhibit consistent rate-capacity retention.

Construction of Uniform LiF Coating Layers for Stable High-Voltage LiCoO2 Cathodes in Lithium-Ion Batteries.

Stabilizing LiCoO2 (LCO) at 4.5 V rather than the common 4.2 V is important for the high specific capacity. In this study, we developed a simple and efficient way to improve the stability of LiCoO2 at high voltages. After a simple sol-gel method, we introduced trifluoroacetic acid (TA) to the surface of LCO via an afterwards calcination. Meanwhile, the TA reacted with residual lithium on the surface of LCO, further leading to the formation of uniform LiF nanoshells. The LiF nanoshells could effectively restrict the interfacial side reaction, hinder the transition metal dissolution and thus achieve a stable cathode-electrolyte interface at high working-voltages. As a result, the LCO@LiF demonstrated a much superior cycling stability with a capacity retention ratio of 83.54% after 100 cycles compared with the bare ones (43.3% for capacity retention), as well as high rate performances. Notably, LiF coating layers endow LCO with excellent high-temperature performances and outstanding full-cell performances. This work provides a simple and effective way to prepare stable LCO materials working at a high voltage.

Enhancing room temperature performance of solid-state lithium cell via a facile solid electrolyte-cathode interface design

Herein, we report the enhanced electrochemical performance of a solid-state cell realized through an engineered solid electrolyte-cathode interface via a simple casting technique. The ceramic-in-polymer solid electrolyte sample with 15 wt% NASICON-type Li1·2Sn0·9Zr0·9Al0·2(PO4)3 ceramic filler in a P(VDF-HFP) matrix (CPSE15) showed the highest room temperature conductivity of ∼1.1 × 10−4 S cm−1 and lithium-ion transference number of ∼0.60. The symmetric Li|CPSE15|Li cell showed a consistent voltage profile with an over-potential of ∼45 mV for 750 h at a current density of 0.1 mA cm−2. CPSE15 slurry was directly cast onto the LiFePO4 (LFP) cathode layer using a doctor-blade coating method to obtain a cathode-electrolyte assembly. Subsequently, a full cell fabricated using this assembly and lithium metal delivered an initial discharge capacity of ∼157 mAh g−1 at 0.2C. Further, the cell exhibited ∼90 mAh g−1 discharge capacity at 1C and sustained 500 charge-discharge cycles at room temperature. The engineered interface in cathode-electrolyte assembly enabled this cell to outperform its conventionally fabricated counterpart with a stacked LFP cathode sheet and freestanding CPSE15 electrolyte membrane, which was attributed to a lower interfacial resistance (∼51%) of the former.

Anion-Induced Uniform and Robust Cathode-Electrolyte Interphase for Layered Metal Oxide Cathodes of Sodium Ion Batteries.

Layer metal oxides demonstrate great commercial application potential in sodium-ion batteries, while their commercialization is extremely hampered by the unsatisfactory cycling performance caused by the irreversible phase transition and interfacial side reaction. Herein, trimethoxymethylsilane (TMSI) is introduced into electrolytes to construct an advanced cathode/electrolyte interphase by tuning the solvation structure of anions. It is found that due to the stronger interaction between ClO4- and TMSI than that of ClO4- and PC/FEC, the ClO4--TMSI complexes tend to accumulate on the surface of the cathode during the charging process, leading to the formation of a stable cathode/electrolyte interface (CEI). In addition, the Si species with excellent electronic insulation ability are distributed in the TMSI-derived CEI film, which is conducive to inhibiting the continuous side reaction of solvents and the growth of the CEI film. As a result, under a current density of 250 mA g-1, the capacity retention of the NaNi1/3Fe1/3Mn1/3O2 (NFM) cathode after 200 cycles in the TMSI-modified electrolyte is 74.4% in comparison to 51.5% of the bare electrolyte (1 M NaClO4/PC/5% FEC). Moreover, the NFM cathode shows better kinetics, with the specific discharge capacity increasing from 22 to 67 mAh g-1 at 300 mA g-1. It also demonstrates greatly improved rate capability, cycling stability, and Coulombic efficiency under various operating conditions, including high temperature (55 °C) and high cutoff voltage (2.0-4.3 V vs Na+/Na).

Electrolyte Design for High-Voltage Lithium-Metal Batteries with Synthetic Sulfonamide-Based Solvent and Electrochemically Active Additives.

Considering practical viability, Li-metal battery electrolytes should be formulated by tuning solvent composition similar to electrolyte systems for Li-ion batteries to enable the facile salt-dissociation, ion-conduction, and introduction of sacrificial additives for building stable electrode-electrolyte interfaces. Although 1,2-dimethoxyethane with a high-donor number enables the implementation of ionic compounds as effective interface modifiers, its ubiquitous usage is limited by its low-oxidation durability and high-volatility. Regulation of the solvation structure and construction of well-structured interfacial layers ensure the potential strength of electrolytes in both Li-metal and LiNi0.8 Co0.1 Mn0.1 O2 (NCM811). This study reports the build-up of multilayer solid-electrolyte interphase by utilizing different electron-accepting tendencies of lithium difluoro(bisoxalato) phosphate (LiDFBP), lithium nitrate, and synthetic 1-((trifluoromethyl)sulfonyl)piperidine. Furthermore, a well-structured cathode-electrolyte interface from LiDFBP effectively addresses the issues with NCM811. The developed electrolyte based on a framework of highly- and weakly-solvating solvents with interface modifiers enables the operation of Li|NCM811 cells with a high areal capacity cathode (4.3 mAh cm-2 ) at 4.4V versus Li/Li+ .

Saccharin Sodium Coupling Fluorinated Solvent Enabled Stable Interface for High-Voltage Li-Metal Batteries.

Optimizing the electrode/electrolyte interface structure is the key to realizing high-voltage Li-metal batteries (LMBs). Herein, a functional electrolyte is introduced to synergetically regulate the interface layer structures on the high-voltage cathode and the Li-metal anode. Saccharin sodium (NaSH) as a multifunctional electrolyte additive is employed in fluorinated solvent-based electrolyte (FBE) for robust interphase layer construction. On the one hand, combining the results of ex-situ techniques and in-situ electrochemical dissipative quartz crystal microbalance (EQCM-D) technique, it can be seen that the solid electrolyte interface (SEI) layer constructed by NaSH-coupled fluoroethylene carbonate (FEC) on Li-metal anode significantly inhibits the growth of lithium dendrites and improves the cyclic stability of the anode. On the other hand, the experimental results also confirm that the cathode-electrolyte interface (CEI) layer induced by NaSH-coupled FEC effectively protects the active materials of LiCoO2 and improves their structural stability under high-voltage cycling, thus avoiding the material rupture. Moreover, theoretical calculation results show that the addition of NaSH alters the desolvation behavior of Li+ and enhances the transport kinetics of Li+ at the electrode/electrolyte interface. In this contribution, the LiCoO2 ǁLi full cell containing FBE+NaSH results in a high capacity retention of 80% after 530 cycles with a coulombic efficiency of 99.8%.

Modification of Layered Cathodes of Sodium-Ion Batteries with Conducting Polymers

Layered oxides exhibit interesting performance as positive electrodes for commercial sodium-ion batteries. Nevertheless, the replacement of low-sustainable nickel with more abundant iron would be desirable. Although it can be achieved in P2-Na2/3Ni2/9Fe2/9Mn5/9O2, its performance still requires further improvement. Many imaginative strategies such as surface modification have been proposed to minimize undesirable interactions at the cathode–electrolyte interface while facilitating sodium insertion in different materials. Here, we examine four different approaches based on the use of the electron-conductive polymer poly(3,4-ethylene dioxythiophene) (PEDOT) as an additive: (i) electrochemical in situ polymerization of the monomer, (ii) manual mixing with the active material, (iii) coating the current collector, and (iv) a combination of the latter two methods. As compared with pristine layered oxide, the electrochemical performance shows a particularly effective way of increasing cycling stability by using electropolymerization. Contrarily, the mixtures show less improvement, probably due to the heterogeneous distribution of oxide and polymer in the samples. In contrast with less conductive polyanionic cathode materials such as phosphates, the beneficial effects of PEDOT on oxide cathodes are not as much in rate performance as in inhibiting cycling degradation, due to the compactness of the electrodes without loss of electrical contact between active particles.