Lithium Difluorophosphate Research Articles

Elucidating the Limit of Lithium Difluorophosphate Electrolyte Additive for High‐Voltage Li/Mn‐Rich Layered Oxide || Graphite Li Ion Batteries

Li/Mn‐rich layered oxide (LMR) cathode active materials offer remarkably high specific discharge capacity (>250 mAh g−1) from both cationic and anionic redox. The latter necessitates harsh charging conditions to high cathode potentials (>4.5 V vs Li|Li+), which is accompanied by lattice oxygen release, phase transformation, voltage fade, and transition metal (TM) dissolution. In cells with graphite anode, TM dissolution is particularly detrimental as it initiates electrode crosstalk. Lithium difluorophosphate (LiDFP) is known for its pivotal role in suppressing electrode crosstalk through TM scavenging. In LMR || graphite cells charged to an upper cutoff voltage (UCV) of 4.5 V, effective TM scavenging effects of LiDFP are observed. In contrast, for an UCV of 4.7 V, the scavenging effects are limited due to more severe TM dissolution compared an UCV of 4.5 V. Given the saturation in solubility of the TM scavenging agents, which are LiDFP decomposition products, e.g., PO43− and PO3F2−, higher concentrations of the LiDFP as “precursor” cannot enhance the amount of scavenging species, they rather start to precipitate and damage the anode.

In Situ Formation of Stable Dual-Layer Solid Electrolyte Interphase for Enhanced Stability and Cycle Life in All-Solid-State Lithium Metal Batteries.

All-solid-state lithium metal batteries have emerged as a promising solution to overcoming the energy density and safety challenges associated with conventional lithium-ion batteries. Solid polymer electrolytes, particularly those based on poly(vinylidene fluoride) (PVDF) and dimethylformamide (DMF), demonstrate significant potential. However, interfacial side reactions between residual DMF solvents and lithium metal present substantial challenges. In this study, we investigate the in situ formation of solid electrolyte interphase protective layers to mitigate these side reactions. By incorporating F-rich additives, such as fluoroethylene carbonate and lithium difluorophosphate, we successfully establish a dual-layer inorganic SEI structure characterized by an outer LiF layer and an inner Li2O layer. Consequently, our approach extends the cycle life of lithium symmetric batteries to 3000 h. Additionally, the Li||LiFePO4 solid-state battery demonstrates exceptional stability, enduring 400 cycles at a 1C rate with an impressive capacity retention of 84%. This strategic methodology effectively leverages the benefits of residual solvents, ensuring both enhanced battery efficiency and long-term operational stability for PVDF-based all-solid-state lithium metal batteries.

Enhancing the performance of LiNi0.8Mn0.1Co0.1O2-based pouch cells through advanced electrolyte additive systems for high-temperature lithium-ion batteries

Next-generation lithium-ion batteries, which achieve high energy densities and prolonged cycle performance, require cathode materials with high operating voltages and specific capacities. Because of their high capacity and operating voltage, nickel-rich layered oxides like LiNi0.8Mn0.1Co0.1O2 (NMC811) are attractive cathodes; nevertheless, they have drawbacks such capacity loss, poor cycle performance, structural instability, and thermal instability. Enhancing the stability of the NMC811 cathode-electrolyte interphase demands improvements in both cathode materials and electrolytes. This study explores the enhancement of NMC811-based battery systems through electrolyte modification, focusing on mixed additive combinations to evaluate their synergistic effects at elevated temperatures, relevant for tropical climates. A ternary additive system comprising vinylene carbonate (VC), lithium difluorophosphate (LiPO2F2), and p-toluenesulfonyl isocyanate (PTSI) in a LiPF6-based electrolyte showed significant improvements. This system mitigated capacity fading for cells cycled over 3 V–4.2 V at both 25 °C and 45 °C, outperforming the baseline electrolyte. The designed electrolyte stabilized the electrode-electrolyte interface, enhancing cell performance and the cathode's structural integrity. Notably, the graphite/NMC811 pouch cell's cycling performance showed an increase in capacity retention from 82.4% to 90.2% after 150 cycles at C/2 at 45 °C. Moreover, the deactivation of PF5 by PTSI can help to suppress the side reaction from the instability of LiPF6-based electrolytes, especially at elevated temperatures. These findings carried out from this strategic electrolyte additive system can substantially improve nickel-rich LIBs, especially in challenging thermal environments.

Interphase Optimum via Synergistic Effect of Poly(1,3‐dioxolane) Electrolytes and Lithium Difluorophosphate for Stable Lithium Metal Batteries

Interphase Optimum via Synergistic Effect of Poly(1,3‐dioxolane) Electrolytes and Lithium Difluorophosphate for Stable Lithium Metal Batteries

A novel multifunctional electrolyte system for high performance Li-ion batteries

Lithium-ion batteries have advantages such as environmental protection and sustainable use, but the widespread application of lithium-ion battery-powered vehicles is limited by the inability to quickly charge and extreme ambient temperatures, and reasonable design of electrolyte components can effectively solve these problems. Here, a novel fluoroethylene carbonate (FEC)/ethyl acetate (EA) based electrolyte containing lithium difluorophosphate (LiPO2F2, 2 wt%) and lithium bis(fluorosulfonyl)imide salt (LiFSI, 1.0 M) is first reported. The electrolyte containing these components has the characteristics of low freezing point, low viscosity and high ionic conductivity. Especially, due to the preferential decomposition of FEC, LiFSI and LiPO2F2, uniform multiphase inorganic LiF and Li3N species are formed on the electrode surface with enhanced robustness and ion transfer kinetics. So, the graphite||LiNi0.5Co0.2Mn0.3O2&LiMn2O4 (Gr||LMO&NCM523) batteries in the designed electrolyte maintain high capacity and coulombic efficiency (100 %) after 200 times at 1C charge and discharge current density at 25 °C and 45 °C. At −20 °C, the batteries have a coulombic efficiency of 50 % and capacity of 38.0 mA h g−1 after 200 charging and discharging cycles, showing excellent low-temperature fast dynamics. Our work supplies an efficient way to accelerate the engineering application of Li-ion batteries.

Optimizing the Electrolyte Additive Content in Practical Li-Ion Cells with Si-Dominant Anodes

Due to their high capacity, Silicon (Si) anodes are very promising for the next generation of lithium-ion batteries. However, Si materials suffer from a high volume expansion, which cause cracking and accumulated growth of the solid electrolyte interphase (SEI), resulting in low coulombic efficiencies and lifetime of the Si anodes. Applying additives in the electrolyte is a facile and straightforward approach to improve the composition of the SEI and reduce cracking. For Si-containing anodes, fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are considered the most effective electrolyte additives. The salt lithium difluorophosphate (LiDFP) is known to improve the interphases of cathode materials and graphite when applied as additive, but has been poorly investigated for Si anodes. In our study, we combine VC, FEC, and a variation of LiDFP concentrations to investigate the effect on Si-dominant anodes in detail. We conducted detailed electrochemical testing in half coin cells and Si/NCM pouch cells, including long-term cycling, rate capability tests, potential monitoring of the anode and electrochemical impedance spectroscopy. For the latter two, we applied a novel perforated reference electrode concept, which allows for broad and precise measurement in pouch cells, while avoiding a blocking effect of the lithium ions. Furthermore, post-mortem analysis was conducted to gain detailed insights into the SEI formation and microstructure influenced by the different additive concentrations.

Synergistic Effects of Bisalt Additives in High-Voltage Rechargeable Lithium Batteries.

The stability of high-energy-density lithium metal batteries (LMBs) heavily relies on the composition of the solid electrolyte interphase (SEI) formed on lithium metal anodes. In this study, the inorganic-rich SEI layer was achieved by incorporating bisalts additives into carbonate-based electrolytes. Within this SEI layer, the presence of LiF, polythionate, and Li3N was observed, generated by combining 1.0 м lithium bis(trifluoromethanesulfonyl)imide in ethylene carbonate: ethyl methyl carbonate:dimethyl carbonate in a 1 : 1 : 1 volume ratio, with the addition of 2 wt% lithium difluorophosphate and 2 wt% lithium difluoro(oxalato)borate additives (EL-DO). Furthermore, this formulation effectively mitigated corrosion of aluminum current collectors. EL-DO exhibited outstanding performance, including an average coulombic efficiency of 98.2 % in Li||Cu cells and a stable discharge capacity of approximately 162 mAh g-1 after 200 cycles in a Li||LiNi0.8Co0.1Mn0.1O2 (NCM811) configuration. Moreover, EL-DO displayed the potential to enhance the performance not only of LMBs but also of lithium-ion batteries. In the case of Gr||NCM811 cell using EL-DO, it consistently maintained high discharge capacities, even achieving around 135 mAh g-1 after the 100th cycle, surpassing the performance of other electrolytes. This study underscores the synergistic impact of bisalts additives in elevating the performance of lithium batteries.

Anion-mediated interphase construction enabling high-voltage solid-state lithium metal batteries

Enhancing the interphase stability of polymer electrolytes with high-voltage cathodes is crucial for the development of high-energy-density lithium metal batteries (LMBs), especially in wearable devices area. Herein, lithium difluorophosphate (LiDFP) is incorporated into the solid polymer electrolyte of poly(ethylene oxide) (PEO) to enhance the formation of a robust cathode-electrolyte interphase (CEI) in high-voltage LiCoO2 LMBs. Through combining electrochemical measurements, spectroscopic techniques and theoretical calculations, the underlying modification mechanism is revealed. Due to its stronger anionic coordination with both polymer electrolyte chain and proton, and more oxidation-active resultant anion-coordinated polymer compared with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), LiDFP suppresses the formation of the strong acid HTFSI deriving from the deprotonated process of the terminal O-H group thus avoiding the further oxidative attack to the polymeric framework. Furthermore, under the synergetic decomposition of HDFP intermediate, the polymer-derived radical helps to reconstruct a chemo-mechanically stable and Li-conductive CEI layer, which consists of abundant LiF/Li3PO4/LixPOyFz inorganic species distributed in lithiated organic macromolecules. The in-situ constructed CEI effectively passivates catalytic sites on LiCoO2 directly against PEO, resulting in well-maintained layered structure during high-voltage cycling. The proposed interphasial chemistry that regulates CEI formation provides directive knowledge of electrolyte optimization for high-voltage solid-state batteries.

Anion‐Dominated Conventional‐Concentrations Electrolyte to Improve Low‐Temperature Performance of Lithium‐Ion Batteries

AbstractLow temperatures (< −20 °C) significantly diminish lithium‐ion battery performance due to freezing issues within commercial electrolytes and the high energy barrier for Li+ desolvation at the interface. Although high‐concentration electrolytes and localized high‐concentration electrolytes enhance Li+ desolvation kinetics featuring anion‐participated solvation structures, their high viscosity and propensity for Li salt precipitation render them unsuitable for low‐temperature environments. This study introduces an anion‐dominated conventional‐concentrations electrolyte (ACCE) created by incorporating Lithium difluorophosphate(LiPO2F2)into a 1 M Lithium bis((trifluoromethyl)sulfonyl)azanide(LiTFSI) Dimethyl carbonate(DMC)/Fluoroethylene carbonate(FEC)/Methyl acetate(MA) electrolyte solution. LiPO2F2, characterized by its poor solubility and strong binding with Li+, demonstrates a pronounced tendency to integrate into the primary solvation sheath of Li+. Moreover, the synergy between LiTFSI and LiPO2F2 establishes a dual anion configuration, unveiling a dual anion‐driven mechanism. This mechanism significantly diminishes the interaction between Li+ and solvent molecules, resulting in reduced desolvation energy under low temperatures. The ACCE exhibits high ionic conductivity of 1.3 mS cm−1 at −50 °C, enabling stable cycling of Li/NCM811 cells at −50 °C, and further allows 0.75 Ah graphite(Gr)/LiNi0.8Co0.1Mn0.1O2(NCM811) batteries dischargeable at −40 °C. This study presents a practical application potential for poorly soluble lithium salts and provides a new avenue for designing electrolytes suitable for low‐temperature applications.

Tuning the Interface Stability of Nickel-Rich NCM Cathode Against Aggressive Structural Collapse via the Synergistic Effect of Additives.

Nickel-rich layered oxides are envisaged as one of the most promising alternative cathode materials for lithium-ion batteries, considering their capabilities to achieve ultrahigh energy density at an affordable cost. Nonetheless, with increasing Ni content in the cathodes comes a severe extent of Ni4+ redox side reactions on the interface, leading to fast capacity decay and structural stability fading over extended cycles. Herein, dual additives of bis(vinylsulfonyl)methane (BVM) and lithium difluorophosphate (LiDFP) are adopted to synergistically generate the F-, P-, and S-rich passivation layer on the cathode, and the Ni4+ activity and dissolution at high voltage are restricted. The sulfur-rich layer formed by the polymerization of BVM, combined with the Li3PO4 and LiF phases derived from LiDFP, alleviates the problems of increased impedance, cracks, and an irreversible H2-H3 phase transition. Consequently, the Ni-rich LiNixM1-xO2 (x > 0.95) button half-cell cycled in LiDFP + BVM electrolyte exhibits a significant discharging capacity of 181.4 mAh g-1 at 1 C (1 C = 200 mA g-1) with retention of 83.7% after 100 cycles, surpassing the performance of the commercial electrolyte (160.7 mAh g-1) with retention of 53.3%. Remarkably, the NCM95||graphite pouch cell exhibits a remarkable capacity retention of 95.5% after 200 cycles. This work inspires the rational design of electrolyte additives for ultrahigh-energy batteries with nickel-rich layered oxide cathodes.

Long-Term Cycling and Mechanisms of Cell Degradation of Single Crystal LiNi0.95Mn0.04Co0.01O2/Graphite Cells

Extremely high nickel content positive electrode materials have high specific capacity leading to high energy density Li-ion cells. The long-term cycling stability of pouch cells with a single crystal LiNi0.95Mn0.04Co0.01O2 positive electrode material was studied here. Cells with such high nickel content demonstrated excellent cycling when only charged to 4.04 V (about 75% state of charge (SOC)), while they showed more capacity loss when charged to 4.18 V or 100% SOC. Lowering the upper cut-off voltage is in favor of the cycling stability however decreases the cell energy density. The main reason for the capacity loss at 40 °C is due to positive electrode impedance growth, which originated from parasitic reactions between the positive electrode material and the electrolyte, especially when the cells are operated to 4.18 V. There was no noticeable positive electrode particle cracking by scanning electron microscopy and no significant active mass loss even for cells operated to 4.18 V. XRD of cycled positive electrodes indicated no appreciable amount of nickel migrating into the lithium layer, so the impedance growth mainly comes from the positive electrode particle surfaces. Using 1.2 M LiPF6 fluoroethylene carbonate: ethyl methyl carbonate 20:80 electrolyte with 1 wt% lithium difluorophosphate allows cycle life to be extended by reducing impedance growth of the cells.

Functionalized Electrode Additive for Simultaneously Reinforcing Chemo‐Mechanical Properties of Millimeter‐Thick Dry‐Electrode for High‐Energy All‐Solid‐State Batteries

AbstractVoids are widely disseminated in a powder when mixed, and hence the typical dry‐electrode preparation method yields a sparse dry‐electrode because the external pressure applied to the surface of the mixed powder is not evenly distributed. Consequently, particle cracking and void remnants appear in the electrode after calendaring. This study introduces a practically applicable bi‐functionalized electrode additive to simultaneously reinforce the chemo‐mechanical properties of millimeter‐thick dry electrodes. The agyrodite Li6PS5Cl solid electrolyte and lithium difluorophosphate (LiPO2F2) additive have different sizes, as demonstrated by their densification from their elaborate bimodal structure, which is densely packed. Because the additive is filled into the interparticle voids in the electrode sheet during electrode fabrication, a decrease in electrode cracking after calendaring is possible because of the uniform distribution of external pressure. Hence, a thin, highly ionic/electronic‐conductive electrode can be constructed by the addition of LiPO2F2 additive. Furthermore, during the formation, the active material surface‐contacted LiPO2F2 promptly produces a surface layer comprising LiF and LixPFy. Thus, the addition of LiPO2F2 further reduces the degradation of Li6PS5Cl solid electrolytes. As a result, the LiPO2F2 additive simultaneously improves the electrochemical and physicochemical properties of millimeter‐thick dry‐electrodes for high‐energy all‐solid‐state battery systems.

Dual-Additive Approach for Tuning Si-Based Anode Interphase Realized By Operando ATR-FTIR

Dual-additive approach for tuning Si-based anode interphase realized by Operando ATR-FTIRMasoud Baghernejad1, Matthias Weiling1, Felix Pfeiffer1, Diddo Diddens1, Lars Frankenstein2, Christian Lechtenfeld2 1Helmholtz-Institute Münster, IEK-12, Forschungszentrum Jülich GmbH, Corrensstraße 46, 48149 Münster, Germany 2MEET Battery Research Center, Institute of Physical Chemistry, University of Münster, Corrensstraße 46, 48149 Münster, GermanySilicon is a promising next-generation anode material, offering a high gravimetric capacity. However, silicon-based anodes suffer from significant volume expansion during lithiation that can cause particle pulverization and capacity fade, continuous solid-electrolyte interphase (SEI) (re-)formation, and constant electrolyte consumption. To alleviate these issues, the effect of 2-Sulfobenzoic anhydride (2-SBA) as an SEI electrolyte additive was investigated through complementary electrochemical and operando spectroscopic techniques.The electrochemical performance was evaluated through galvanostatic charge/discharge cycling. The results show that the optimum concentration of 2-SBA improves the cycling stability and capacity retention of NMC811||AG+20% SiOx (4.5 – 2.8 V) cells to more than 100 cycles compared to 70 cycles with the baseline electrolyte (BE) before cell failure. Introducing lithium difluorophosphate (LiDFP), a common additive for high-voltage applications, to the electrolyte containing 2-SBA further increased the cycle life to 160 cycles. Interestingly, LiDFP alone in the electrolyte did not lead to improved electrochemical performance or longer cycle life of the cells compared to BE, proving a beneficial dual-additive approach of combining inorganic and organic compounds for sufficient SEI formation on AG+20% SiOx anodes. The analysis of the electrochemical performance concludes that the improved SEI structure formed by 2-SBA and LiDFP mitigates pulverization. In this regard, the SEI structure is further analyzed using operando ATR-FTIR spectroscopy.Operando ATR-FTIR spectroscopy proves to be a powerful technique in battery research, particularly for investigating SEI. The ATR mode's surface sensitivity enables studying the composition of the SEI without bulk signal interference. In this work, we employed the internal reflection mode of the ATR-FTIR spectroscopy, for which the electrode of interest has to be IR permeable. Furthermore, by variation of the incident angle of the IR beam, the penetration depth can be varied to obtain an interphase profile from electrode to bulk electrolyte. In this study, the SEI composition on the silicon anode in the presence of BE and the corresponding additives, 2-SBA and LiDFP, was investigated under real-working conditions. The results elucidate that 2-SBA is accumulated and decomposed via ring-opening on the silicon surface to form a stable SEI layer. However, in the presence of LiDFP and 2-SBA, a hybrid organic and inorganic SEI layer is formed, which proves to enhance electrochemical performance and cycle life. The findings of this study provide valuable structural insights into the silicon particle surface for the development of high-voltage silicon-based lithium-ion batteries with Ni-rich cathodes.

Compositional engineering of composite polymer electrolytes for all solid-state batteries to simultaneously improve reaction kinetics and long-term stability

All-solid-state batteries (ASSB) with Li anodes are urgently required to increase both energy density and safety. However, interfacial instabilities between the solid electrolyte and Li anode (SEI), and between the solid electrolyte and cathode (CEI) limit stable operation at elevated temperatures. In this study, a composite polymer electrolyte (CPE) comprising succinonitrile (SN) and lithium difluorophosphate (LiDFP) is used to significantly increase the cycle life and Li+ transport kinetics. SN successfully enhances the electrochemical performance of CPE by improving the compatibility and interface contact with the Li anode and the Li+ transport kinetics. LiDFP improves the oxidation stability of CPE, suppresses the increase in resistance, and promotes Li + conductivity by forming a favorable SEI and CEI consisting of LiF and Li3PO4. Most of all, the combination of SN and LiDFP result in synergistic effects. Therefore, CPE with SN and LiDFP enables stable cyclability of Li-iron phosphate/Li cell over 1000 cycles with capacity retention of 76 % and high average Coulombic efficiency of 99.5 % at 3C and 60 °C. Understanding novel composite solid electrolytes aids in the design of advanced CPE and even ceramic solid electrolytes for high-performance ASSBs.

Additive-Derived Surface Modification of Cathodes in All-Solid-State Batteries: The Effect of Lithium Difluorophosphate- and Lithium Difluoro(oxalato)borate-Derived Coating Layers.

Sulfide-based electrolytes, with their high conductivity and formability, enable the construction of high-performance, all-solid-state batteries (ASSBs). However, the instability of the cathode-sulfide electrolyte interface limits the commercialization of these ASSBs. Surface modification of cathodes using the coating technique has been explored as an efficient approach to stabilize these interfaces. In this study, the additives lithium difluorophosphate (LiDFP) and lithium difluoro(oxalato)borate (LiDFOB) are used to fabricate stable cathode coatings via heat treatment. The low melting points of LiDFP and LiDFOB enable the formation of thin and uniform coating layers by a low-temperature heat treatment. All-solid-state cells containing LiDFP- and LiDFOB-coated cathodes show electrochemical performances significantly better than those comprising uncoated cathodes. Among all of the as-prepared coated cathodes, LiDFP-coated cathodes fabricated using a slightly lower temperature than the phase-transition temperature of LiDFP (320 °C) show the best discharge capacity, rate capability, and cyclic performance. Furthermore, cells comprising LiDFP-coated cathodes showed significantly low impedance. X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy confirm the effectiveness of the LiDFP coating. LiDFP-coated cathodes minimized side-reactions during cycling, resulting in a significantly low cathode-surface degradation. Hence, this study highlights the efficiency of the proposed coating method and its potential to facilitate the commercialization of ASSBs. Overall, this study reports an effective technique to stabilize the cathode-electrolyte interface in sulfide-based ASSBs, which could expedite the practical implementation of these advanced energy-storage devices.

The optimal amount of lithium difluorophosphate as an additive for Si-dominant anodes in an application-oriented setup.

Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are considered the most effective electrolyte additives for improving the solid electrolyte interphase (SEI) of Si-containing anodes while lithium difluorophosphate (LiDFP) is known to improve the interphases of cathode materials and graphite. Here, we combine VC, FEC, and different amounts of LiDFP in a highly-concentrated electrolyte to investigate the effect on Si-dominant anodes in detail. Cycle life tests, electrochemical impedance spectroscopy and rate tests with anode potential monitoring were conducted in Si/NCM pouch cells. The results reveal that adding LiDFP to the electrolyte improves all performance criteria of the full cells, with a concentration of 1 wt.% being the optimal value for most cases. Post-mortem analyses using scanning electron microscopy and x-ray photoelectron spectroscopy showed that a more beneficial SEI film was formed for higher LiDFP concentrations, which led to less decomposition of electrolyte components and a better-maintained anode microstructure.

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.

Electrolyte Optimization to Improve the High-Voltage Operation of Single-Crystal LiNi0.83Co0.11Mn0.06O2 in Lithium-Ion Batteries

Single-crystal Ni-rich layered oxide materials LiNi1−x−yCoxMnyO2 (NCM, 1 – x − y ≥ 0.6) are emerging as promising cathode materials that do not show intergranular cracks as a result of the lack of grain boundaries and anisotropy of the bulk structure, enabling extended cyclability in lithium-ion batteries (LIBs) operating at high voltage. However, SC-NCM materials still suffer from capacity fading upon extended cycling. This degradation of capacity can be attributed to a reconstruction of the surface. A phase transformation from layered structures to disordered spinel/rock-salt structures was found to be responsible for impedance growth and capacity loss. Film-forming additives are a straightforward approach for the mitigation of surface reconstruction via the formation of a robust protection layer at the cathode’s surface. In this work, we investigate various additives on the electrochemical performance of single-crystal LiNi0.83Co0.11Mn0.06O2 (SC-NCM83). The results demonstrate that the use of 1% lithium difluoroxalate borate (LiDFOB) and 1% lithium difluorophosphate (LiPO2F2) additives substantially enhanced the cycling performance (with a capacity retention of 93.6% after 150 cycles) and rate capability in comparison to the baseline electrolyte (72.7%) as well as electrolytes using 1% LiDFOB (90.5%) or 1% LiPO2F2 (88.3%) individually. The superior cycling stability of the cell using the combination of both additives was attributed to the formation of a conformal cathode/electrolyte interface (CEI) layer, resulting in a stabilized bulk structure and decreased impedance upon long-term cycling, as evidenced via a combination of state-of-the-art analytical techniques.

Building a Flexible and Highly Ionic Conductive Solid Electrolyte Interphase on the Surface of Si@C Anodes by Binary Electrolyte Additives.

Si@C as a high specific capacity anode material for lithium batteries (LIBs) has attracted a lot of attention. However, the severe volume change during lithium de-embedding causes repeated rupture/reconstruction of the solid electrolyte interphase (SEI), resulting in poor cycling stability of the Si-based battery system and thus hindering its application in commercial batteries. Using electrolyte additives to form an excellent SEI is considered to be a cost-effective method to meet this challenge. Here, the classical film-forming additive vinyl carbonate (VC), and the newly emerging lithium salt additive lithium difluorophosphate (LiDFP), are chosen as synergistic additives to improve the electrode-electrolyte interface properties. Final results show that the VC additive generates flexible polycarbonate components at the electrode/electrolyte interface, preventing the fragmentation of Si particles. However, the organic components show high impedance, inhibiting the fast transport of Li+. This defect can be supplemented from the decomposition substances of the LiDFP additive. The derived inorganic products, such as LiF and Li3PO4, can strengthen the reaction kinetics of the electrode, reduce the interfacial impedance, and promote the Li+ transport. Thus, the synergistic effect of VC and LiDFP additives builds an effective SEI with good flexibility and high ionic conductivity and then significantly improves the cycling and rate stability of Si@C anodes. The experimental results show that the utilization of LiDFP and VC additives to modify the Si@C anode interface enhances the capacity retention of the Si@C/Li half-cell after 100 cycles from 68.2% to 85.1%. Besides, the possible mechanism of action between VC and LiDFP is proposed by using the spectral characterization technique and density functional theory (DFT) calculations. This research opens up a new possibility for improvement of SEI, and provides a simple way to achieve high-performance Si-based LIBs.

Studying the Role of Additives in Carbonate Electrolytes for Graphite||NMC811 Batteries with Interface-Sensitive X-Ray Spectroscopies

Li-ion rechargeable batteries are increasingly used as power sources for portable consumer products and electric vehicles. The formation of stable electrode-electrolyte interphases is critical to long-term performance, however, these typically degrade during extended cycling due to ongoing side reactions and electrode cross-talk. Over the past few years, various electrolyte additives have been reported as being able to improve the properties of the interphases formed on electrode surfaces and suppress cell degradation. However, these are often tuned for a specific electrode (e.g. cathode), and may negatively impact on the other electrode (e.g. anode). A key experimental challenge is therefore to reveal how additives affect the interfacial layers formed at both anode and cathode in full cells, their resulting influence on the cross-talk, and on the long-term stability of each electrode. Soft X-ray absorption spectroscopy (soft-XAS) and X-ray photoelectron spectroscopy (XPS) are powerful techniques to investigate the evolution of the short-range local structure and chemical species in complex systems. These properties make soft-XAS and XPS a suitable technique to study phase evolution, chemical species, local structure and stability of the interface formed on the electrodes. Here, we have studied carbonate-based electrolytes with various additives (such as Vinylene Carbonate, Lithium difluorophosphate, and fluoroethylene carbonate) in Graphite||NMC811 full cell. Soft-XAS and XPS have been performed to understand how additives in the electrolyte alter the chemical nature of interface formed on both anode and cathode in early cycles and their influence on extended longer-term degradation related to cross-talk, such as transition metal cross-over. Further, we identified which additives are beneficial for which electrodes and which cross-talk reactions they help to mitigate, this informs the development of electrolyte formulations where different additives are combined to improve battery lifetime and performance. Comparing the chemical information obtained with electrochemical cycling data we determined the connection between the chemical degradation of the electrodes and the reduction in electrochemical performance.