Cathode Electrolyte Interphase Research Articles

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

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

Electrical Double Layer Formation at Intercalation Cathode–Organic Electrolyte Interfaces During Initial Lithium‐Ion Battery Reactions

AbstractInformation on the cathode/organic–electrolyte interface structure provides clues regarding the rate and reversibility of lithium intercalation reactions in lithium‐ion batteries. Herein, structural changes within the LiCoO2 electrode, throughout the interphase region, and in the LiPF6/propylene carbonate electrolyte are observed concurrently by in situ neutron reflectometry. The formation of an electrical double layer (EDL) during the early stages of charging and discharging is investigated and compared with that at an intercalation‐inactive Nb:SrTiO3 electrode. At the intercalation‐inactive interface between Nb:SrTiO3 and the electrolyte, a voltage‐dependent ionic distribution corresponding to the EDL forms on the electrolyte side without the formation of a cathode/electrolyte interphase (CEI) layer. In contrast, at the intercalation‐active LiCoO2/electrolyte interface, a CEI layer forms immediately after cell construction, and the ionic distribution in the electrolyte formed outside the CEI layer scarcely changes upon voltage application. The CEI/electrolyte interface is shielded from potential changes by the electronically insulating CEI; therefore, structural changes in the EDL are restricted. This supports the prevailing understanding that the CEI layer defines the rates of solvation/de‐solvation and adsorption/desorption reactions of lithium ions.

Insight into the Contribution of the Electrolyte Additive LiBF4 in High-Voltage LiCoO2||SiO/C Pouch Cells.

High-voltage pouch cells using an LiCoO2 cathode and SiO/C anode are regarded as promising energy storage devices due to their high energy densities. However, their failure is associated with the unstable, high-impedance cathode electrolyte interphase (CEI) film on the cathode and the solid electrolyte interphase (SEI) film on the anode surface, which hinder their practical use. Here, we report a novel approach to ameliorate the above challenges through the rational construction of a stable, low-impedance cathode and anode interface film. Such films are simultaneously formed on both electrodes via the participation of the traditional salt, lithium tetrafluoroborate (LiBF4), as electrolyte additive. The application of 1.0% LiBF4 enhances the capacity retention of the cell from 26.1 to 82.2% after 150 cycles between 3.0 and 4.4 V at 1 C. Besides, the low-temperature discharge performance is also improved by LiBF4 application: the discharge capacity of the cell with LiBF4 is 794 mAh compared with 637 mAh without LiBF4 at 1 C and -20 °C. The excellent electrochemical performance of pouch cells is ascribed to the contribution of LiBF4. Especially, the low binding energy of LiBF4 with the oxygen on the LiCoO2 surface leads to the enrichment of LiBF4 that forms the protective cathode interface, which fills the blanks of previous research.

Engineering A Boron-Rich Interphase with Nonflammable Electrolyte toward Stable Li||NCM811 Cells Under Elevated Temperature.

Despite the high energy of LiNi0.8 Co0.1 Mn0.1 O2 (NCM811) cathode, it still suffers serious decay due to the continuous solvents decomposition and unstable cathode electrolyte interphase (CEI) layers, especially under high temperatures. The intense exothermic reaction between delithiated NCM811 and flammable electrolyte, on the other hand, pushes the batteries to their safety limit. Herein, these two issues are tackled via engineering the electrolytes, that is, utilizing salts with higher HOMO levels and nonflammable solvents with lower HOMO levels, to reduce the massive decomposition of solvents and improve battery safety under elevated temperatures. Consequently, a thin and boron-rich CEI is generated, which effectively inhibited the side reactions, thus improving the cycling stability and safety. Deviated from the highly concentrated electrolytes which heavily relies on the usage of massive salts, the electrolyte recipe can introduce a robust inorganic-rich CEI but use much less salt (i.e., dilute electrolyte), and thus, offer an encouraging alternative toward practical applications. As such, the NCM811 cathode exhibits a high-capacity retention of 81.2% after 950 cycles at 25°C and 75% after 300 cycles at 55°C. This work provides a universal electrolyte design strategy for designing stable and safe high-temperature electrolytes for the NCM811 cathode.

Enhancing structure and cycling stability of single crystal LiNi0.5Mn1.5O4 cathode via multifunctional MXene surface modification

Single crystal LiNi0.5Mn1.5O4 (LNMO) has gained increasing attention due to its cobalt-free composition, low rolling resistance, and high energy density. However, its high voltage operating range renders it susceptible to electrolyte dissolution and side reactions, limiting its large-scale practical application as it exhibits poor cycling stability and rate performance. In this study, a novel composite material, LNMO@MXene, was developed by electrostatic self-assembly of single or few-layered MXene (Ti3C2Tx) onto the surface of LNMO. Among them, LNMO coated with 3 wt% MXene exhibited a discharge capacity of up to 130.86 mAh·g−1 at 1C, and a specific capacity of 109.80 mAh·g−1 at 5C after 500 cycles. After 200 cycles at 55 °C, the specific capacity was 114.4 mAh·g−1 with a retention rate as high as 86.99 %. The electrostatic self-assembly of MXene is a simple and effective coating strategy, which creates a conductive network, increases the capacity through Li+ intercalation, and forms a protective cathode electrolyte interphase (CEI) layer to inhibit interfacial side reactions. The synergistic effect between MXene and LNMO enhances the electrochemical performance of LNMO materials. This coating strategy can also be applied to the design of other cathode materials.

Fluorinated Carbonate‐aided Ionic‐liquid Electrolyte Enables High‐capacity Lithium‐metal Batteries

AbstractIonic liquids (ILs) are promising electrolyte candidates for high‐safety and stability lithium metal batteries. However, their practical application is hindered by high viscosity resulting in a poor wettability of separator and sluggish Li+ transport at room temperature. Herein, a weakly‐solvating solvent of methyl (2,2,2‐trifluoroethyl) carbonate (FEMC) is employed as a co‐solvent in non‐flammable ionic‐liquid electrolyte (ILE) which composed with N‐butyl‐N‐methylpyrrolidinium bis (fluorsulfonyl) imide (Pyr14FSI) and bis (trifluoromethanesulfonyl) sulfonimide lithium salt (LiTFSI), effectively ameliorates the shortcomings of ILs mentioned above without sacrificing its safety character. Compared with pure ILE, the addition of FEMC can distinctly decrease the polarization voltage and electrode‐electrolyte interface resistance. With the assistance of FEMC solvent, the discharge specific capacity of Li||LiFePO4 (LFP) and Li||LiNi0.6Co0.2Mn0.2O2 (NCM622) cells are significantly improved. The origin of this great improvement is contributed to the exceptionally stable and LiF‐rich cathode electrolyte interphase (CEI) layer formed in this electrolyte. Additionally, Li||LiNi0.8Co0.1Mn0.1O2 (NCM811) cells exhibit a remarkable rate capability (105 mAh g−1 at 2 C rate), achieve an initial specific capacity of 202.8 mAh g−1 and outstanding capacity retention of 88.2 % over 100 cycles.

High energy density in ultra-thick and flexible electrodes enabled by designed conductive agent/binder composite

Thick electrodes can increase incorporation of active electrode materials by diminishing the proportion of inactive constituents, improving the overall energy density of batteries. However, thick electrodes fabricated using the conventional slurry casting approach frequently exhibit an exacerbated accumulation of carbon additives and binders on their surfaces, invariably leading to compromised electrochemical properties. In this study, we introduce a designed conductive agent/binder composite synthesized from carbon nanotube and polytetrafluoroethylene. This agent/binder composite facilitates production of dry-process-prepared ultra-thick electrodes endowed with a three-dimensional and uniformly distributed percolative architecture, ensuring superior electronic conductivity and remarkable mechanical resilience. Using this approach, ultra-thick LiCoO2 (LCO) electrodes demonstrated superior cycling performance and rate capabilities, registering an impressive loading capacity of up to 101.4 mg/cm2, signifying a 242% increase in battery energy density. In another analytical endeavor, time-of-flight secondary ion mass spectroscopy was used to clarify the distribution of cathode electrolyte interphase (CEI) in cycled LCO electrodes. The results provide unprecedented evidence explaining the intricate correlation between CEI generation and carbon distribution, highlighting the intrinsic advantages of the proposed dry-process approach in fine-tuning the CEI, with excellent cycling performance in batteries equipped with ultra-thick electrodes.

High-Voltage Instability of Vinylene Carbonate (VC): Impact of Formed Poly-VC on Interphases and Toxicity.

Full exhaustion in specific energy/energy density of state-of-the-art LiNix Coy Mnz O2 (NCM)-based Li-ion batteries (LIB) is currently limited for reasons of NCM stability by upper cut-off voltages (UCV) below 4.3 V. At higher UCV, structural decomposition triggers electrode crosstalk in the course of enhanced transition metal dissolution and leads to severe specific capacity/energy fade; in the worst case to a sudden death phenomenon (roll-over failure). The additive lithium difluorophosphate (LiDFP) is known to suppress this by scavenging dissolved metals, but at the cost of enhanced toxicity due to the formation of organofluorophosphates (OFPs). Addition of film-forming electrolyte additives like vinylene carbonate (VC) can intrinsically decrease OFP formation in thermally aged LiDFP-containing electrolytes, though the benefit of this dual-additive approach can be questioned at higher UCVs. In this work, VC is shown to decrease the formation of potentially toxic OFPs within the electrolyte during cycling at conventional UCVs but triggers OFP formation at higher UCVs. The electrolyte contains soluble VC-polymerization products. These products are formed at the cathode during VC oxidation (and are found within the cathode electrolyte interphase (CEI), suggesting an OFP electrode crosstalk of VC decomposition species, as the OFP-precursor molecules are shown to be formed at the anode.

Bifunctional self-assembled molecular layer enables stable Ni-rich cathodes

Ni-rich layered oxides (LiNixCoyMnzO2, x ≥ 0.8, x + y + z = 1) are attractive cathode material candidates for building high-energy-density batteries owing to their higher specific capacity compared to their lower-Ni-content analogues. However, the high nickel content also brings challenges, such as storage instability in ambient conditions and poor cycle life. In this work, we propose a surface chemistry regulation strategy to simultaneously enhance the air storage stability and electrochemical stability of high-nickel cathode materials. A bifunctional ultrathin layer composed of trimethoxy(1H,1H,2H,2H-perfluorodecyl)silane (PFDTMS) was constructed on the surface of single-crystal LiNi0.83Co0.12Mn0.05O2 (NMC811) particles via molecular self-assembly. The passivating PFDTMS layer forms a superhydrophobic surface on the modified NMC811 particles, effectively mitigating deactivation reactions of NMC811 in air, and ensuring an uncompromised electrochemical performance of NMC811-based electrode after storing in air for two weeks. Furthermore, the self-assembled PFDTMS layer contributes to the formation of an electrochemical stable cathode–electrolyte interphase (CEI) on the NMC811 surface, improving the cycling stability of NMC811 at high cut-off voltages. The PFDTMS-based CEI also alleviates the chemical corrosion of NMC811 by the electrolyte, slows down the dissolution of transition metal ions during long term cycling. These findings present a straightforward, effective, eco-friendly, and cost-efficient approach to tackle the stability challenges inherent to Ni-rich layered cathode materials.

Differing Electrolyte Implication on Anion and Cation Intercalation into Graphite.

Emerging dual-graphite batteries (DGBs) capture extensive interest for their high output voltage and exceptional cost-effectiveness. Yet, developing electrolytes compatible with both the cathode and anode stands to be a tremendous challenge, and how electrolyte impacts anion and cation intercalation into graphite remains inexplicit or controversial. Herein, we have evaluated the performance of graphite anode and cathode in typical ethyl methyl carbonate (EMC) based electrolytes and unveiled their electrode-electrolyte interphase using Cryogenic transmission electron microscopy (Cryo-TEM). The addition of fluoroethylene carbonate (FEC) brings substantial improvement in cycle stability and Coulombic efficiency for both the graphite cathode and anode, but its implication on cation and anion intercalation differs. FEC is involved in anodic side reactions to produce a LiF-embedded solid-electrolyte interphase layer. It is much thinner and more uniform than that formed in the electrolyte without FEC, which is correlated with less graphite exfoliation and enhanced stability. As for the graphite cathode, both basal and edge planes are largely bare, and only few scattered byproducts are found. In addition, we also reveal layer bending and local lattice disordering of the graphite cathode based on multiple Cryo-TEM images, which are speculated to be caused by high lattice strain induced by anion intercalation and local oxidation under high voltage. The absence of cathode-electrolyte interphase (CEI) layers overturns the paradigm of attributing cathodic performance to CEI features and is regarded as a fundamental reason for severe self-discharge of graphite cathode. FEC helps to alleviate graphite exfoliation issues and enhance cycle stability, and we ascribe it to weakened solvation, which means reduced probability of solvent co-intercalation during charging, rather than compositional changes of cathodic byproducts.

Promoting reversibility of layered potassium cathode through interstitial doping

With the increasing needs for large-scale and low-cost energy storage devices, Mn-based layered oxide cathodes have achieved considerable researching interests for potassium-ion batteries owing to their high energy density, abundant resource and low toxicity. However, their developments are challenged by the absence of suitable cathode materials to tolerate large-sized K-ion insertion/extraction and the presence of Jahn-Teller distortion of Mn3+. To address this issue, we present a strategy of embedding boron into interstitial tetrahedral sites to obtain a P3-K0.5Mn0.8Co0.2B0.1O2 cathode. Strong B-O covalent bonds facilitate the construction of robust orthorhombic framework and alleviate the undesired elongation of Mn-O bonds, contributing to excellent electrochemical performance. In addition, boron ions are verified to promote the formation of a homogeneous cathode electrolyte interphase layer, improving interfacial stability and realizing highly reversible cycling in a wide voltage range (1.4–4.3 V). This strategy provides a new pathway towards the development of high-performance cathode materials for potassium-ion batteries.

Surface-interspersed nanoparticles induced cathode-electrolyte interphase enabling stable cycling of high-voltage LiCoO2

The development of high-voltage LiCoO2 (LCO) is crucial for achieving lithium-ion batteries with a high volumetric energy density. However, LCO experiences accelerated degradation at high voltages due to severe interface and structure instability. A uniform and robust cathode-electrolyte interphase (CEI) serves as a vital barrier for protecting the interface. However, the smooth surface of single-crystalline LCO, lacking grain boundaries, poses challenges in generating an effective CEI. Herein, ZrO2 nano-rivets are constructed on the surface of LCO to provide phase boundaries for preferential film-forming sites, inducing the formation of a uniform and robust CEI. The high-quality CEI derived from ZrO2 nano rivets stabilizes the fragile surface of high-voltage LCO and facilitates lithium-ion diffusion. In addition, a Zr diffusion layer is simultaneously built in the LCO bulk. Zr diffusion into the LCO lattice not only effectively suppresses unfavorable phase transitions to stabilize the bulk structure, but also mitigates oxygen charge deficiencies at the highly delithiated state, therefore stabilizing lattice oxygen. Consequently, the modified LCO exhibits excellent capacity retention of 80% after 700 cycles at 4.6 V. This work emphasizes the significance of material surface properties in CEI formation and provides new insights for the design of CEI.

Potential dependent formation of fluorine-rich artificial interfaces for durable dual-ion batteries

The mechanochemically fragile cathode-electrolyte interphase (CEI) cannot protect the anion-storing carbon cathodes against high voltage (>4.5 V) corrosion. The strategy of preforming a fluorine-rich artificial interface is opted here. Two potentially dependent interfaces are derived using Fluoroethylene carbonate (FEC) as an electrolyte additive at high voltage (2.0–5.0 V) and low voltage (0.35–2.00 V). Both high voltage interface (HVI) and low voltage interface (LVI) assist in retaining >87 % capacity over 200 cycles, while the unmodified pristine electrode loses >13 % capacity within 50 cycles only. The interfacial stability enhances more in HVI, which cycles 500 times with 90 % capacity retention, whereas LVI loses >13 % capacity within 200 cycles. The reason is attributed to the fact that the potential-dependent decomposition of FEC follows two separate mechanistic routes, thereby resulting in two chemically dissimilar interfaces (HVI and LVI) that behave differently under the circumstances of PF6− storage. The LVI undergoes dynamic evolution with cycling. The results are impermeably thick CEI, ruptured bulk with a loss in graphitic order, and a weaker electrochemical output. In contrary, the HVI-protected surface subdues electrolyte decomposition to the greatest extent and yields the most durable dual-ion battery.

Guaiacol as an Organic Superoxide Dismutase Mimics for Anti-ageing a Ru-based Li-rich Layered Oxide Cathode.

High-capacity Li-rich layered oxides using oxygen redox as well as transition metal redox suffer from its structural instability due to lattice oxygen escaped from its structure during oxygen redox and the following electrolyte decomposition by the reactive oxygen species. Herein, we rescued a Li-rich layered oxide based on 4d transition metal by employing an organic superoxide dismutase mimics as a homogeneous electrolyte additive. Guaiacol scavenged superoxide radicals via dismutation or disproportionation to convert two superoxide molecules to peroxide and dioxygen after absorbing lithium superoxide on its partially negative oxygen of methoxy and hydroxyl groups. Additionally, guaiacol was decomposed to form a thin and stable cathode-electrolyte interphase (CEI) layer, endowing the cathode with the interfacial stability.

Differences between cation and anion storage electrochemistry of graphite and its impact on dual graphite battery

The redox-amphoteric nature of graphite is tactically utilized to store cations and anions simultaneously in a dual graphite battery (DGB). The major differences between cation and anion storage electrochemistry of graphite are identified here and their impacts on the electrochemical performances of DGBs are analyzed. All the differences are found to originate primarily from ionic size mismatch (76 p.m. of Li+vs. 254 p.m. of PF6−) that further induces kinetic and thermodynamic limitations upon intercalation. The electrochemical studies reveal that the anion intercalation lags behind the cation because of a higher energy barrier for the interconversion among (PF6)Cx stages, the occurrence of a valley-type region in the discharge profile that delays deintercalation by 500 mV, dynamic changes in organic-rich cathode electrolyte interphase, and sluggish diffusion inside graphite bulk. In contrast, cation intercalation affects DGBs' performance negatively due to higher desolvation barriers and lithium plating on the anode surface at higher current densities. Therefore, the cathode limits the capacity and cycling efficiency, whereas the anode restricts cycle life and power density. This work also explores the strategies to mitigate the degradation pathways by electrolyte modification.

Trimethylsilyl Compounds for the Interfacial Stabilization of Thiophosphate-Based Solid Electrolytes in All-Solid-State Batteries.

Argyrodite-type Li6 PS5 Cl (LPSCl) has attracted much attention as a solid electrolyte for all-solid-state batteries (ASSBs) because of its high ionic conductivity and good mechanical flexibility. LPSCl, however, has challenges of translating research into practical applications, such as irreversible electrochemical degradation at the interface between LPSCl and cathode materials. Even for Li-ion batteries (LIBs), liquid electrolytes have the same issue as electrolyte decomposition due to interfacial instability. Nonetheless, current LIBs are successfully commercialized because functional electrolyte additives give rise to the formation of stable cathode-electrolyte interphase (CEI) and solid-electrolyte interphase (SEI) layers, leading to supplementing the interfacial stability between electrolyte and electrode. Herein, inspired by the role of electrolyte additives for LIBs, trimethylsilyl compounds are introduced as solid electrolyte additives for improving the interfacial stability between sulfide-based solid electrolytes and cathode materials. 2-(Trimethylsilyl)ethanethiol (TMS-SH), a solid electrolyte additive, is oxidatively decomposed during charge, forming a stable CEI layer. As a result, the CEI layer derived from TMS-SH suppresses the interfacial degradation between LPSCl and LiCoO2 , thereby leading to the excellent electrochemical performance of Li | LPSCl | LiCoO2 , such as superior cycle life over 2000 cycles (85.0% of capacity retention after 2000 cycles).

Revealing the surface modification effect on Li-ion insertion into Ni-rich NCM cathode material by cyclic voltammetry

This article presents a new approach to the determination of electrochemical parameters characterizing the degradation behavior of electrodes based on LiNi0.85Co0.10Mn0.05O2 (Ni-rich NCM) using the method of cyclic voltammetry with a linear scan of the electrode potential (CV). To establish the relationship between electrochemical, structural, and morphological characteristics, XRD, synchrotron XRD (SXRD) in the operando mode, and SEM were used. The results of the galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) were used to verify the CV data. The features of the irreversible phase transition in NCM during the first cycle of extraction-insertion of lithium ions and its influence on the further behavior of the material during cycling were studied. It was found that the nature of the slope variation of capacity and coulomb efficiency dependences on the cycle number with an increase in the electrode potential scan rate is a criterion for the ratio of reversible and irreversible components of the electrode capacity degradation. Reversible electrode capacity degradation is due to the difference between the diffusion coefficients of lithium ions in the anodic and cathodic processes, irreversible degradation is due to the structural degradation of the material. For NCM samples with different particle surface conditions (with different composition of Cathode Electrolyte Interphase (CEI)) the values of Li+ ions diffusion coefficient (D) for each stage of the intercalation process were found in the range 4.5·10−14 – 1.2·10−13 cm2·s−1 for the anodic process, 1.7·10−14 – 5.6·10−14 cm2·s−1 for the cathodic process. It has been established that the reversible and irreversible degradation components of electrodes based on Ni-rich NCM in the potential range corresponding to the H2-H3/H3-H2 interphase transition correlate with each other. Therefore, the difference in D values in the anodic and cathodic processes, which correspond to the current peaks of the cyclic voltammogram in this region, is a criterion for the irreversible degradation of the material’s capacity: the greater the difference in the D values for these peaks, the higher the irreversible degradation of the material’s capacity.

Full‐Cell Presodiation Strategy to Enable High‐Performance Na‐Ion Batteries

AbstractAs promising alternatives to lithium‐ion batteries, sodium‐ion batteries suffer from low coulombic efficiency, which stems from Na‐deficiency and unstable interfaces for both cathode and anode materials. Exhibiting a favorable solvation structure, Na‐naphthalene/tetrahydropyran (Na‐NP/THP) is screened and selected as a full‐cell presodiation solution for hard carbon (HC) anodes and a P‐2 type Na0.67Fe0.1Al0.1Mn0.8O2 (NFAM) cathode, which are both Na‐deficient. Through a wealth of characterizations, it is revealed that the proposed presodiation agent not only boosts the initial coulombic efficiency of both HC and NFAM electrodes effectively, but also facilitates robust solid–electrolyte interphase (SEI) and cathode–electrolyte interphase (CEI) on both electrodes. Consequently, the long‐term cycling and rate performance are greatly improved thanks to the stabilized structures of electrode materials. Furthermore, a proof‐of‐concept NFAM||HC pouch cell based on this presodiation strategy is assembled, showing a high reversible capacity and superior cycling performance. By offering a remedy to a full cell system subjected to severe Na‐deficiency, this work conceptually demonstrates a feasible strategy to fully tap the potential of high‐energy‐density materials in sodium‐ion batteries.

New stachydrine-derived ionic liquids enable the formation of robust cathode electrolyte interphase for improving the performance of LiFePO4 battery

In the present study, several new stachydrine-based room temperature ionic liquids ([Cn-Sta] ILs, n = 2, 3, 4 or 5) are prepared, characterized and applied as an electrolyte additive to significantly improve LiFePO4 battery performance. The Sta-based ILs are structurally homologous with the popular pyrrolidinium ILs combining a chiral ester group close to the N+ cationic center, leading to low symmetry for the absence of cold crystallization, a wide liquid range from about −60 °C (glass transition temperature) to 320 °C (thermal decomposition temperature) and a decent electrochemical stability window (∼4.5 V). These ILs are ready to decompose on the surface of LiFePO4 particles to form LiF-rich and robust cathode electrolyte interphase (CEI) passivation layers, according to an aggregate study of electrochemical and morphological evidence. As a result, the ILs are used as an efficient CEI-forming additive to improve the LiFePO4 battery performance, namely higher capacity and cyclic capacity retention, better high-current rate performance and lower initial resistance.

Manipulating Na occupation and constructing protective film of P2-Na0.67Ni0.33Mn0.67O2 as long-term cycle stability cathode for sodium-ion batteries

P2-Na0.67Ni0.33Mn0.67O2 (NNMO) is promising cathode material for sodium-ion batteries (SIBs) due to its high specific capacity and fast Na+ diffusion rate. Nonetheless, the irreversible P2-O2 phase transformation, Na+/vacancy ordering, and transition metal (TM) dissolution seriously damage its cycling stability and restrict its commercialization process. Herein, Na occupation manipulation and interface stabilization are proposed to strengthen the phase structure of NNMO by synergistic Zn/Ti co-doping and introducing lithium difluorophosp (LiPO2F2) film-forming electrolyte additive. The Zn/Ti co-doping regulates the occupancy ratio of Nae/Naf at Na sites and disorganizes the Na+/vacancy ordering, resulting in a faster Na+ diffusion kinetics and reversible P2-Z phase transition for P2-Na0.67Ni0.28Zn0.05Mn0.62Ti0.05O2 (NNZMTO). Meanwhile, the LiPO2F2 additive can form homogeneous and ultrathin cathode-electrolyte interphase (CEI) on NNZMTO surface, which can stabilize the NNZMTO-electrolyte interface to prevent TM dissolution, surface structure transformation, and micro-crack generation. Combination studies of in situ and ex situ characterizations and theoretical calculations were used to elucidate the storage mechanism of NNZMTO with LiPO2F2 additive. As a result, the NNZMTO displays outstanding capacity retention of 94.44% after 500 cycles at 1C with 0.3 wt% LiPO2F2, excellent rate performance of 92.5 mA h g−1 at 8C with 0.1 wt% LiPO2F2, and remarkable full cell capability. This work highlights the important role of manipulating Na occupation and constructing protective film in the design of layered materials, which provides a promising direction for developing high-performance cathodes for SIBs.