Cathode-electrolyte Interface Layer Research Articles

Precycling Strategy in Suitable Voltage to Improve the Stability of Interfacial Film and Suppress the Decline of LiNi0.6Mn0.2Co0.2O2 Cathode at Elevated Temperatures.

Layered ternary oxide LiNixMnyCo1-x-yO2 is a promising cathode candidate for high-energy lithium-ion batteries (LIBs). However, the capacity of LIBs is significantly restricted by several factors, including the repeated dissolution-regeneration of the interfacial film at high temperatures, the dissolution of transition metals, and the increase of impedance. Herein, a new precycling strategy in suitable voltage scope at room temperature is proposed to construct a uniform, thermally stable, and insoluble cathode-electrolyte interface (CEI), which helps to maintain stable cycling performances at high temperatures. Specifically, after 5 precycles in the range of 3.85-4.3 V at room temperature, a CEI layer containing numerous inorganic components and oligomers is formed on the surface of LiNi0.6Mn0.2Co0.2O2. Subsequently, the harmful side reactions are effectively suppressed, endowing the cell with an excellent capacity retention of 84.67% after 50 cycles at 0.5C and 55 °C, much higher than that of 65.61% under the conventional film-forming process conditions. This work emphasizes the crucial role of the precycling strategy in regulating the characteristics of CEI layer on the surface of cathode electrode, opening up a new avenue for the high-temperature application of positive electrodes of LIBs.

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.

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.

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.

Mechanism of Bilayer Polymer-Based Electrolyte with Functional Molecules in Enhancing the Capacity and Cycling Stability of High-Voltage Lithium Batteries.

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) are favorable for all-solid-state lithium metal batteries (ASSLBs) to ensure safety and enhance energy density. However, their narrow work windows and unstable electrode/electrolyte interfaces hinder their practical application in high-voltage ASSLBs. Although introducing additives in SPEs has been proven to be effective to address the above issues, it could hardly optimize both cathode and anode interfaces by an individual additive. Herein, heterogeneously double-layer SPEs are constructed with two typical additives (LiPO2F2 and LiFSI), which are used to modify the LiNi0.6Co0.2Mn0.2O2 (NCM)-cathode/electrolyte interface (CEI) and lithium-anode/solid electrolyte interface (SEI), and further understand their respective mechanism in enhancing the capacity and cycling stability of ASSLBs. Specifically, LiPO2F2 not only leads to a uniform CEI layer to prevent the oxidation decomposition of PEO and LiTFSI but also ensures fast Li+ diffusion at high voltage (>3.9 V), improving the rate performances and life spans of the cells. The LiFSI contributes to a stable SEI layer with rich LiF, suppressing the growth of lithium dendrites and maximizing the specific capacity for ASSLBs. Integrating the advantages of the two functional molecules, the optimized ASSLB displays an excellent capacity of 141.4 mAh g-1 at 1C and an outstanding capacity retention of 81.6% after 400 cycles when using the NCM cathode, even reaching 154.2 mAh g-1 at 0.1 mA cm-2 with a high mass loading (6.4 mg cm-2). Additionally, the bilayer SPEs also match well with a LiFePO4 electrode with a high mass loading of 11.0 mg cm-2, displaying a high capacity of 155.7 mAh g-1 at 0.1 mA cm-2.

Heterocyclic polymer supported cathode/Li interface layers to lower the operational temperature of PEO-based Li-batteries

High-performance poly (ethylene oxide) (PEO)-based solid-state polymer Li-batteries (SSLBs) at low temperature (≤ 25 ℃) is hindered by high interfacial resistance and low ionic conductivity. Herein, we simultaneously construct cathode and anode interface layers via in-situ heat-induced heterocyclic polymerization reaction, where the cathode-electrolyte interface layer (PPL) consists of bromine-doped poly (3,4-ethylene-dioxy-thiophene) (PEDOT), PEO and bis-trifluoromethanesulfonimide (LiTFSI), featuring with mixed ionic/electronic conduction, and the anode-electrolyte interface layer (PVCL) was formed by poly (vinyl carbonate) (PVC), PEO and LiTFSI, having a robust fast ionic conduction. Benefiting from the PEDOT conductivity, high mechanical strength of PVC and good interface affinity aroused by heat-induced reaction, the solid-state PEO-based LiFePO4||Li cell shows low interfacial resistance of 11.02 Ω that decreased by 92% compared to the cell without interface modification, and delivering a discharge capacity of 164.4 mAh g−1 at 0.1 C (25 ℃), as high as 115.9 mAh g−1 at 10 ℃. X-ray photoelectron spectroscopy revealed the enrichment of LiF, Li3N and LiBr on the lithium metal surface after cycling, further enhancing stable cycling Li-batteries. The dual interface-constructed strategy provides a reliable way to resolve the bottle-neck interfacial issues of SSLBs.

Irreversible Mechanical Deformations Associated with the Formation of Cathode-Electrolyte Interface on Lithium Iron Phosphate Cathodes

The demand for rechargeable batteries for energy storage in electrical automobiles is rising presently due to their key role in reducing carbon emissions. However, the increasing demands require improvement in batteries lifetime. interfacial instabilities associated with the formation of cathode-electrolyte interface and intercalation-induced volumetric expansions are the primary reasons behind the capacity fade. Although there have been great efforts to understand structural stress/strains in cathodes, there is a lack of knowledge about the formation mechanism of cathode-electrolyte interface (CEI) layers. To fill the gap, we investigate the role of electrolyte chemistry on the interfacial deformation of lithium iron phosphate (LFP) cathodes by employing digital image correlation (DIC), spectroscopy, and electrochemical techniques. DIC was utilized to study the dynamic physical behavior of the electrode materials and irreversible strains associated with the electrolyte decomposition on the electrode surface [1]. Therefore, the overall objective of this study is to elucidate the mechanical deformations in the LFP cathodes of battery electrodes by investigating the physical response of the electrode and dynamic changes on the electrode – electrolyte interface.To be able to understand the different effects of the electrolyte solution, electrolytes were made of either LiPF6, LiClO4, or LiTFSI salts dissolved in 1 M EC:DMC. When cycled in the LiClO4 and LiTFSI salts containing electrolytes, the volumetric contraction is observed in the electrode because of the extraction of lithium and associated phase transformations during the first delithiation. However, positive strain generation was observed only during the first cycle in LiPF6 containing electrolyte, although delithiation results in reducing the lattice parameters. Control experiments were conducted where the electrode was cycled in LiClO4-containing electrolyte for one cycle and electrolyte was changed to the LiPF6-containing electrolyte for the subsequent cycles. A similar irreversible mechanical behavior was also observed in the control experiments where positive strain generation is recorded during the subsequent first delithiation cycle in LiPF6-containing electrolyte. To further analysis, electrochemical impedance spectroscopy measurements were performed on each system to identify surface resistance of the electrodes in a voltage range between 3 - 4.4 V. Increasing in the surface resistance of the electrodes were associated with CEI layer formations [2, 3]. Chemical composition of the CEI layers were characterized by using x-ray photoelectron spectroscopy. Our study provides a pathway to investigate the formation of the cathode-electrolyte interface on the cathode electrodes by monitoring irreversible physical behavior of the electrode – electrolyte interface during cycling.This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (Award numberDE-SC0021251). References Çapraz, Ö. Ö., Rajput, S., Bassett, K. L., Gewirth, A. A., White, S. R., & Sottos, N. R. (2019). Controlling expansion in lithium manganese oxide composite electrodes via surface modification. Journal of The Electrochemical Society, 166(12), A2357.Liu, Y., & Xie, J. (2015). Failure study of commercial LiFePO4 cells in overcharge conditions using electrochemical impedance spectroscopy. Journal of The Electrochemical Society, 162(10), A2208.Bassett, K. L., Çapraz, Ö. Ö., Özdogru, B., Gewirth, A. A., & Sottos, N. R. (2019). Cathode/electrolyte interface-dependent changes in stress and strain in lithium iron phosphate composite cathodes. Journal of the Electrochemical Society, 166(12), A2707.

Interfaces Studies on LiNi0.82Mn0.07Co0.11O2 (NMC811) with Vinylene Carbonate Additive Using in-Situ Raman and X-Ray Absorption Spectroscopy at Ultra-High Operation Voltage

Interfaces evolutions in lithium-ion battery determines the rate-limiting step for lithiation of most electrode materials by influencing Li+ insertion occurred in the solid phase boundary. To our best knowledge, we will be the first to employ the in-situ characterization techniques including Raman spectroscopy and X-ray absorption spectroscopy (XAS) to probe the cathode electrolyte interface (CEI) formation and its protective effect on NMC cathode with the presence a high concentration of vinylene carbonate (VC). The Raman data showed a brand new peak around 1000 cm-1 appeared after the initial cycle due to the VC polymerization indicating the maturity of CEI layer with a modified electrolyte system, the in-situ XAS data revealed that the surface reconstruction of NMC cathode was significantly suppressed with the VC additive by delta-R analysis from extended X-ray absorption fine structure (EXAFS) and the oxidation state of nickel was relatively stable compared to a baseline electrolyte environment when the operation voltage window was pushed to 4.8V. This work shed light on characterizing materials property across spatio-temporal scales relevant for various additives electrolyte system under in-situ/operando conditions including specific form factors, providing design principles for materials and composites, functional aspects of interfacial operation and degradation pathways. Figure 1

In situ construction of uniform and robust cathode-electrolyte interface toward high-stable P2-Type sodium layered oxide cathodes

As a kind of potential cathode for sodium-ion batteries (SIBs), P2-type Na2/3Ni1/3Mn2/3O2 (P2-NNMO) suffers from the dissolution of transition metals and severe side reactions in conventional electrolytes, which greatly deteriorates its electrochemical performance. Herein, an artificial cathode-electrolyte interface (CEI) is in situ constructed on P2-NNMO cathode based on a spontaneous redox reaction between P2-NNMO material and organic solvent tetrahydrofuran (THF). The uniform and robust CEI layer on P2-NNMO cathode (named P2-NNMO-T) mainly comprises organic Na-based species (R-O-Na, R-CO-Na, and R-O2CO-Na), and converts from an initial thickness of 2 nm into 4.6 nm after cycling. It can serve as a versatile interlayer to facilitate Na+ transport, shield the attack of electrolyte solvents, and suppress the transition metal dissolution effectively, thus boosting the cycling stability and fast charging capability of P2-NNMO cathodes. More importantly, the concept of in situ CEI construction can be extended to other layered oxides beyond SIBs discussed herein.

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.

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.

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.

Scalable conversion of residual lithium into conformal artificial cathode electrolyte interphase via a single-step wet-chemical process for high-voltage Ni-rich cathode operation

Ni-rich layered oxide has become an indispensable cathode for incumbent lithium-ion batteries due to their high energy density and low cost. Despite these advantages, the Ni-rich cathode suffers from surface residual lithium, which causes gas evolution and gelation issue during the electrode fabrication process. Here, we demonstrate a simple wet-chemical and mass-production compatible process that converts undesirable residual lithium into an artificial cathode electrolyte interface (CEI) layer. A scalable wet chemical route effective even in bulk scale is firstly developed, where LiOH and Li2CO3 residual lithium species convert into LiF crystal through NH4F dissolved in ethanol. Pre-existing residual lithium on LiNi0.8Mn0.1Co0.1O2 (NMC811) powder surface reacts with a fluorine source NH4F saturated in ethanol, reincarnating into an artificial LiF CEI layer. A conformally formed CEI layer improves high voltage cycling up to 4.6 V vs. Li/Li+ with ∼ 10 % increase of the initial capacity retention at the 500th cycle compared to that of the control electrode. Suppression of transition metal dissolution along with thermal oxygen evolution from the Ni-rich cathode by the artificial CEI are also demonstrated. Based on validation at 10 g level, a continuous reactor with high-throughput processibility was devised. This study provides a scalable, single-step, recyclable, and inexpensive process to achieve artificial CEI layer on Ni-rich cathode by replacing residual lithium without further post-processes.

Multifunctional self-reconstructive cathode/electrolyte interphase layer for cobalt-free Li-rich layered oxide cathode

High-capacity cobalt-free lithium-rich manganese-based oxide (LMNO) is a crucial representative of high-energy-density lithium-ion batteries (LIBs). However, the collaboration failure mechanism (CFM) between LMNO and electrolyte always leads to irreversible oxygen loss, harmful electrolyte decomposition, aggravated structural rearrangement, and severe interfacial side reactions, thereby triggering a sustained decrease in electrochemical performance. Therefore, capturing reactive oxygen species and generating the cathode-electrolyte-interface (CEI) layer shows the potential to resolve these typical issues induced by CFM propagation. Herein, an amine-functionalized mesoporous molecular sieve (NASM) additive with active oxygen/water scavenging capability was designed to construct a multifunctional self-reconstructive CEI layer with modified mechanical/electrochemical stability. Meanwhile, the additive-induced anti-fluoridation protective layer was synchronously generated to synergistically regulate the diffusion of lithium ions and electrons during the CEI reconstruction process. Benefiting from these advantages, the LMNO cathode with NASM-containing electrolyte presented outstanding cycle stability, with only ∼0.06% (1 C) and ∼0.07% (5 C) capacity attenuation per cycle during long-term cycling. This additive-induced multifunctional self-reconstructive CEI layer design provides new insights into reducing undesired CFM propagation to achieve a high-stability and high-energy-density LMNO system for advanced LIBs.

A Critical Evaluation of Interfacial Stability in Li-excess Cation-disordered Rock-salt Oxide Cathode

Li-rich cation-disordered rock-salt oxides (DRXs) are promising candidates for next-generation Li-ion cathode materials due to the demonstrated high capacity (> 300 mAh g−1) by utilizing both transition metal (TM) cations and oxygen anions with a much-widened material design space as compared to conventional layered cathodes, which heavily rely on Co and Ni. However, the serious performance deterioration during long-term cycling impedes their practical applications. Although oxygen loss and associated densified surface layer are responsible for the performance deterioration, these issues are part of the scope of cathode electrolyte interface (CEI) which need further study, including its chemical composition, chemical and electrochemical stability, formation mechanism as well as the effects on battery performance. Through careful monitoring of the CEI evolution of Li1.2Mn0.4Ti0.4O2 (LMTO) in conventional carbonated-based electrolyte upon charging and discharging, we identify that the continuously growing CEI layer exhibits gradually increasing LiF as the main composition. The significant change of the CEI layer is closely related to loss of lattice oxygen (O2−), the continuous over-reduction of redox-active TMs (e.g. Mn3+, Mn 4+) from the cathode surface, severe TMs dissolution, and the release of CO2/O2, and irreversible phase transition in surface structures, which eventually cause a fast capacity degradation and voltage decay. This research provides a fundamental interpretation for understanding interfacial stability in this young class of DRX cathode materials.

Dual functional effect of oxygen vacancies and depolarity shield embedded NiCo2O4 cathode in lithium sulfur battery

Lithium–sulfur (Li–S) batteries are considered as promising candidates for next-generation energy-storage devices. However, their reaction reversibility is impacted by severe capacity fading caused by the lithium polysulfides (LiPSs) shuttle effect and consequent loss of active materials in the cathode. Typically, light-weight and nonpolar carbon materials are used as hosts of sulfur but face several issues such as short lifetime, low activity and capacity, which limit the cycling stability and volumetric energy density. Herein, fluorine-doped NiCo2O4 nanofibers with oxygen vacancies are explored as a high-performance host in Li–S batteries. By regulating the ratio of the fluoride ion, the formation of a cathode electrolyte interface (CEI) layer is suppressed during the charge/discharge cycles. Accordingly, the nanofiber electrode exhibits lower polarization and faster reaction kinetics than fluorine-doped NiCo2O4 without oxygen vacancies or NiCo2O4 with a CEI layer. The optimized fluorine-doped NiCo2O4 electrode with oxygen vacancies delivers 628 mAh g−1 at 0.3 C and a low fading rate of 0.052% per cycle over 1300 cycles at 1 C. The present strategy provides insights in the development of host materials that is based on regulating the formation of the CEI layer and the adsorption and conversion of LiPSs.

Phosphate-Rich Interface for a Highly Stable and Safe 4.6V LiCoO2 Cathode.

Increasing the upper cut-off voltage of LiCoO2 (LCO) is one of the most efficient strategies to gain high-energy density for current lithium-ion batteries. However, surface instability is expected to be exaggerated with increasing voltage arising from the high reactivity between the delithiated LCO and electrolytes, leading to serious safety concerns. This work is aimed to construct a physically and chemically stable phosphate-rich cathode-electrolyte interface (CEI) on the LCO particles to mitigate this issue. This phosphate-rich CEI is generated during the electrochemical activation by using fluoroethylene carbonate and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyletherare as the solvents. Both solvents also demonstrate high thermal stability, reducing the intrinsic flammability of the commercial organic electrolyte, thereby eliminating the safety concern in the LCO-based systems upon high-voltage operation. This stable CEI layer on the particle surface can also enhance the surface structure by blocking direct contact between LCO and electrolyte, improving the cycling stability. Therefore, by using the proposed electrolyte, the LCO cathode exhibits a high-capacity retention of 76.1% after 200 cycles at a high cut-off voltage of 4.6V. This work provides a novel insight into the rational design of high-voltage and safe battery systems by adopting the flame-retardant electrolyte.

Designing Bidirectionally Functional Polymer Electrolytes for Stable Solid Lithium Metal Batteries

AbstractThe development of high energy density lithium metal batteries has been retarded by the uncontrolled lithium dendrite formation and unstable Ni‐rich cathode–electrolyte interface (CEI). Herein, the bidirectionally functional polymer electrolytes (BDFPE) are designed via direct UV solidification of functional polymer species on electrode surfaces to simultaneously handle the interface issues faced by anodes and cathodes. By constructing the BDFPE, a smooth and dendrite‐free lithium deposition is enabled for Li||Li symmetry cells after 1800 h ultralong cycling at 1 mA cm−2 and 1 mAh cm−2, which are attributed to the fast ion conductivity (5.84 × 10−4 S cm−1), high Li+ transfer number (0.69) of BDFPE and low interfacial resistance between electrode and solid electrolytes. Furthermore, Li||LiNi0.6Co0.2Mn0.2O2 batteries demonstrate a favorable cycling and rate capability, and a stable and phosphate‐based CEI layer is constructed in situ. DFT studies reveal that the functional additives FEC and TEP participate in the interface formation. The finding provides a promising design strategy to accommodate the anode and cathode interfaces for high energy density lithium metal batteries.

High cycling stability graphite cathode modified by artificial CEI for potassium-based dual-ion batteries

Expanded graphite is the most common cathode for dual-ion batteries (DIBs), although graphite cathode suffers from a poor cycle life due to decomposition of the solid cathode electrolyte interface (CEI). Herein, a kind of surface modification strategy is developed to improve the cycling stability of graphite by forming artificial CEI on the cathode surface. Expanded graphite cathode modified by an artificial CEI layer can stably deliver a discharge capacity of 56.1 mAh g −1 after 100 cycles, which is obviously better than that of the pristine graphite cathode. The artificial CEI layer can still maintain a thin and uniform morphology after 100 cycles, which is beneficial to protect the interface between graphite and electrolyte and improve the stable intercalation/deintercalation of ions. • A simple method enables the spontaneous formation of artificial CEI films on the surface of graphite cathodes. • The modified graphite cathode (AS) in KDIBs can achieve a stable discharge capacity of 56.1 mAh g -1 after 100 cycles at 1 C. • The artificial CEI can prevent the decomposition of the electrolyte and effectively improve the cycling stability of KDIBs.