Film-forming Additives Research Articles

Effect of alkali-metal phosphate additives on the interphase structure of 5-V LiNi0.5Mn1.5O4-based lithium–ion batteries

High-voltage (5-V) lithium–ion batteries (LIBs) have garnered substantial interest due to their superior energy density. Nevertheless, the unstable interphase of high-voltage LIBs can cause electrolyte decomposition, electrode collapse, transition metal dissolution, and lithium dendrite growth during cycling, which impedes their widespread commercial use. To mitigate these challenges, various alkali-metal phosphates were incorporated into a carbonate-based electrolyte as film-forming additives, aiming to improve the electrochemical performance of LiNi0.5Mn1.5O4 (LNMO)-based LIBs. Following a detailed comparison of the electrochemical performances of various alkali-metal phosphates, KH2PO4 and NaH2PO4 were selected for an in-depth investigation in this research. The highly protective passivation film generated by KH2PO4 and NaH2PO4 on the electrode surface provided significant protection by effectively inhibiting continuous electrolyte decomposition. This finding was supported by density functional theory calculations and a variety of electrochemical and characterization tests. The H2PO4− primarily contributed to the formation of the cathode electrolyte interphase film on the cathode surface. KH2PO4 (via the electrostatic shielding effect of K+ions) and NaH2PO4 (through the co-precipitation of Na+ and Li+ ions) both effectively inhibited the growth of lithium dendrites on the lithium metal anode. Additionally, the KH2PO4 and NaH2PO4 additives regulated the amount of HF in the electrolyte through acid–base equilibrium reactions. The electrolytes containing additives significantly enhanced the electrochemical performances of Li||Li, LNMO/Li, LNMO/graphite, and LNMO/Li4Ti5O12 cells. LNMO/Li half-cells with standard electrolyte (STD) supplemented with KH2PO4 and NaH2PO4 additives demonstrated capacity retentions of 92.9 % and 90.0 % after 500 cycles at 5 C, respectively, in contrast to only 79.4 % for the STD electrolyte alone. Furthermore, the LNMO/graphite full cell exhibited an increased capacity retention, reaching 81.1 % (STD+KH2PO4) and 78.29 % (STD+NaH2PO4), up from 70.8 % with the STD, after 100 cycles at 0.5 C. Given their ease of use, affordability, and widespread availability, KH2PO4 and NaH2PO4 additives show significant potential to enhance the electrochemical properties of high-voltage LIBs.

Non-aqueous Liquid Electrolyte Additives for Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) have been recognized as one of the most promising new energy storage devices for their rich sodium resources, low cost and high safety. The electrolyte, as a bridge connecting the cathode and anode electrodes, plays a vital role in determining the performance of SIBs, such as coulombic efficiency, energy density and cycle life. Therefore, the overall performance of SIBs could be significantly improved by adjusting the electrolyte composition or adding a small number of functional additives. In this review, the fundamentals of SIB electrolytes including electrode-electrolyte interface and solvation structure are introduced. Then, the mechanisms of electrolyte additive action on SIBs are discussed, with a focus on film-forming additives, flame-retardant additives and overcharge protection additives. Finally, the future research of electrolytes is prospected from the perspective of scientific concepts and practical applications.

Tackling application limitations of high-safety γ-butyrolactone electrolytes: Exploring mechanisms and proposing solutions

Developing wide-temperature and high-safety lithium-ion batteries (LIBs) presents significant challenges attributed to the absence of suitable solvents possessing broad liquid range and non-flammability properties. γ-Butyrolactone (GBL) has emerged as a promising solvent; however, its incompatibility with graphite anode has hindered its application. This limitation necessitates a comprehensive investigation into the underlying mechanisms and potential solutions. In this study, we achieve a molecular-level understanding of the perplexing interphase formation process by employing in-situ spectroelectrochemical techniques and density function calculations. Our findings reveal that, even at high salt concentrations, GBL consistently occupies the primary Li+ solvation sheath, leading to extensive GBL decomposition and the formation of a high-impedance and inorganic-poor solid-electrolyte interphase (SEI) layer. Contrary to manipulating solvation structures, our research demonstrates that the utilization of film-forming additives with higher reduction potential facilitates the pre-establishment of a robust SEI film on the graphite anode. This approach effectively inhibits GBL decomposition and significantly enhances the battery's lifespan. This study provides the first reported intrinsic understanding of the unique GBL-graphite incompatibility and offers valuable insights for the development of wide-temperature and high-safety LIBs.

Preparation, formation mechanism, and performance of nitrocellulose aqueous coating based on the anti-solvent method

The organic solvent-based nitrocellulose (NC) coatings exhibit a high volatile solvent content, leading to significant environmental pollution. Whereas, existing waterborne NC-based coating have demonstrated poor water resistance and complex application methods. Therefore, this study aims to develop a green and highly water-resistant waterborne NC-based coating system utilizing a simple anti-solvent method with two environmentally friendly plasticizers, namely acetyl tributyl citrate (ATBC) and triethyl citrate (TEC), as film-forming additives. The dynamic light scattering (DLS) studies revealed that the particle size of NC-based nanoparticles increased with an increase in ATBC content, while it remained relatively constant with an increase in TEC content. Moreover, the particle size results demonstrated that the storage stability of aqueous dispersions containing NC/ATBC (NA) was superior to those containing NC/TEC (NT). Additionally, the physicochemical properties of aqueous dispersions-based films (Ndis), including film formation, hygroscopicity, water resistance, transparency, thermal stability, and mechanical properties were evaluated using solvent-based films (Nsol) as control samples. The results demonstrated that the Ndis films exhibited a dense and uniform structure, with physical and chemical properties that were essentially comparable to those of Nsol films. However, the mechanical properties of Ndis films were somewhat influenced by the nanoparticle size, leading to reduced mechanical performance when the particle size of nanoparticles in the aqueous dispersion was excessively large. The equilibrium moisture content for NA and NT films were reduced to 1.119% and 1.872%, respectively, at a relative humidity of 95%, in comparison to pure NC films (3.975%). Simultaneously, water contact angle measurements indicated an increase from 77° for pure NC film to 89° for NA film and 82° for NT film. All films exhibited superior transparency characteristics. Overall, NA films display excellent water resistance and transparency as well as good mechanical properties.

Unveiling the adsorption tendency of film-forming additives to enable fast-charging hard carbon anodes with regulated Li plating

By unveiling the adsorption tendency of EC and FEC additives on defective graphene surfaces and its impact on SEI formation, hard carbon anodes with efficient Li plating regulation can be achieved for fast-charging lithium-ion batteries.

In situ polymerization of 1,3-dioxolane and formation of fluorine/boron-rich interfaces enabled by film-forming additives for long-life lithium metal batteries.

In situ polymerized 1,3-dioxolane (PDOL) is widely utilized to construct solid polymer electrolytes because of its high room-temperature ionic conductivity and good compatibility with lithium metal. However, the current polymerization additives used in PDOL do not effectively contribute to the formation of a robust solid electrolyte interphase (SEI), leading to decreased cycle life. Herein, a film-forming Lewis acid, tris(hexafluoroisopropyl) borate (THB), is demonstrated not only to be a catalyst for the ring-opening polymerization of DOL, but also an additive for the formation of a stable fluorine- and boron-rich SEI to improve the interfacial stability and suppress the Li dendrite growth. Moreover, molecular dynamics simulations and experimental results demonstrate that the introduction of THB can promote the dissociation of lithium salt and release more Li+ while the boron site can effectively restrict the free movement of TFSI- anion, thus increasing Li+ transference numbers (0.76) and ensuring the long-term cycling stability of cells. By using THB-PDOL, a stable cycling of Li‖Li symmetric cell for 600 h at a capacity of 0.5 mA h cm-2 can be achieved. Furthermore, employing THB-PDOL in Li‖LiFePO4 full cell enables a capacity retention of 98.64% after 300 cycles at 1C and a capacity retention of 95.39% after 200 cycles at a high temperature (60 °C). At the same time, this electrolyte is also suitable for the Li‖NCM523 full cell, which also achieves excellent stability of more than 180 cycles. This film-forming Lewis acid additive provides ideas for designing low-cost, high-performance PDOL-based lithium metal batteries.

Synergistic Impact of Electrolyte Components: Interplay between the Novel Phosphorus-Based Additives and Conducting Salt Advancing NMC811||SiC Cell Performance

A lithium-ion battery (LIB) typically consists of an anode, a lithiated transition metal oxide cathode, and an electrolyte formulation containing inorganic lithium conducting salt in aprotic organic solvents/co-solvents, and functional additives. In-depth research is being conducted in order to develop novel chemistries for the next-generation LIBs. Within the line, silicon-based anodes and nickel-rich cathodes are recognized as two feasible next-generation electrode materials. The long-term galvanostatic cycling performance of resulting cell chemistries relies on the effectivity and robustness of solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) formed during the initial cycles and originate from the galvanostatic cycling conditions, as well as the complex interplay between electrolyte and corresponding electrodes .[1] To enhance the SEI and CEI properties, implementing functional electrolyte additives into baseline electrolyte has been shown to be one of the most effective and cost-favorable strategies. Film-forming additives sacrificially reduce and/or oxidize at corresponding electrodes to form protective layers (SEI and CEI).Previous research has shown that phosphorus-based additives improve electrode |electrolyte interfaces stability and simultaneously enhance cell flame-retardant capability advancing the overall cell performance.[2,3] In this report, we indicated part of the above-mentioned properties originate from the synergy between lithium hexafluorophosphate (LiPF6) which is the state-of-the-art (SOTA) electrolyte conducting salt in LIBs and considered functional additives. Here, we demonstrated the functionality of 2-phenoxy-1,3,2-dioxaphospholane (PhEPi) as a novel film-forming additive[4] and 2,2,4,4,6-pentafluoro-6-(2,2,2-trifluoroethoxy)-1,3,5,2λ5,4λ5,6λ5-triazatriphosphinine (CF3PFFPN) as a new flame-retardant additive, synthesized for the first time by our group. The presence of PhEPi and CF3PFPN at the optimum concentrations in the baseline electrolyte (1 M LiPF6 in EC: EMC 3:7 + 8 % VC) enhances the performance and safety of the resulting NMC811||Si-graphite (20% Si) cell chemistry. Impact of phospholane-based additive was confirmed by substituting LiPF6 with LiBF4 conducting salt and complementary post mortem analysis. This study along with improved cell chemistry, provided a better understanding and deeper insights into the synergistic interplay and impact of considered electrolyte components on the resulting NMC811||Si-graphite cell chemistry. [1] C. Wölke, B. A. Sadeghi, G. G. Eshetu, E. Figgemeier, M. Winter, I. Cekic-Laskovic, Advanced Materials Interfaces 2022, 9, DOI 10.1002/admi.202101898.[2] N. Von Aspern, D. Diddens, T. Kobayashi, M. Börner, O. Stubbmann-Kazakova, V. Kozel, G. V. Röschenthaler, J. Smiatek, M. Winter, I. Cekic-Laskovic, ACS Applied Materials and Interfaces 2019, 11, 16605–16618, DOI 10.1021/acsami.9b03359.[3] N. von Aspern, C. Wölke, M. Börner, M. Winter, I. Cekic-Laskovic, Journal of Solid State Electrochemistry 2020, 24, 3145–3156, DOI 10.1007/s10008-020-04781-1.[4] B. A. Sadeghi, C. Wölke, F. Pfeiffer, M. Baghernejad, M. Winter, I. Cekic-Laskovic, Journal of Power Sources 2023, 557, 232570, DOI 10.1016/j.jpowsour.2022.232570.

The optimization of the electrolyte for low temperature LiFePO4-graphite battery

The design of electrolyte is crucial to improve the low temperature electrochemical performance of the LiFePO4-graphite battery. In this paper, by optimizing the solvent composition and introducing film-forming additives in the electrolyte, the low temperature performance of the battery is significantly improved. In detail, the EP (ethyl propionate) solvent with low melting point and low viscosity is added to the original EC (ethylene carbonate)-EMC (ethyl methyl carbonate)-DMC (dimethyl carbonate) solvent systems, which improved the ionic conductivity of the electrolyte, especially at low temperature (−40 °C). On the other hand, the introduction of FEC (fluoroethylene carbonate)/DTD (ethylene sulfate) film-forming agents promotes the formation of SEI (solid electrolyte interface) film, and the SEI is thin and uniform. As a result, the LiFePO4-graphite battery exhibits high capacity and long cycle life under low temperature conditions.

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.

Molecular Design of Film-Forming Additives for Lithium-Ion Batteries: Impact of Molecular Substrate Parameters on Cell Performance

Molecular Design of Film-Forming Additives for Lithium-Ion Batteries: Impact of Molecular Substrate Parameters on Cell Performance

Analysis of the Decomposition of Sulfur-Based Electrolyte Additives in Spent LiNi0.6Co0.2Mn0.2O2||AG Cells

The development and optimization processes of the last decades in the context of lithium ion batteries (LIBs) have led to a variety of electrode chemistries, which enable application-specific targets such as high energy, high power, high safety or long lifetime. Most of these LIB systems rely on a liquid carbonate-based electrolyte, whose basic composition has changed scarcely during this period.1 In this respect, the usage of electrolyte additives is one of the most economical and effective ways to further improve the overall performance, while also impact the application-tailored characteristics of the battery cells at the elctrolyte level without changing the bulk properties.2 These improvements can range from overcharge protection to lowered flammability or interphase formation, among others. In particular, film-forming additives signifcantly influence the performance and life time of LIBs by preventing excessive decomposition of the electrolyte, which is in contact with the positive and negative electrodes, by the formation of a passivation layer. Investigating the electrochemical decomposition of the electrolyte components, especially electrolyte additives, is key to enable a better understanding of the composition from the electrode||electrolyte interface layer and the effects on LIB cell performance.3 Thus, aging mechanisms and parasitic reactions can be elucidated to improve and design new electrolyte additives.Herein, the irreversible decomposition of sulfur-containing electrolyte additives will be addressed. Sulfur-containing additives represent an attractive option for film-forming additives as these compounds endow lower energies in the lowest unoccupied molecular orbital (LUMO) compared to the organic carbonate analogs and therefore are more susceptible to electrochemical reduction.4 However, these class of electrolyte additives find only a limited use in state-of-the-art electrolyte mixtures due to the often reported or suspected carcinogenicity and toxicity towards the human body. Nevertheless, since sulfur-containing additives still play an important role in spent LIBs of recent decades, identification of these hazardous compounds and formed aging products, which should be also classified as potential dangers, is of great interest.1 This contribution focused on the identification of aging products emerging in electrolytes from LIB cells using sulfur-based electrolyte additives of different chemical classes such as sulfates, sulfites, sultones and sulfonates. Ion chromatography and gas chromatography hyphenated to high-resolution acurrate mass spectrometry (HRAM-MS) were used to obtain information on the formation of ionic and volatile compounds after electrochemical operation and to elucidate the corresponding structures. Thus, specific ionic and volatile aging marker molecules could be defined for target-analysis of the original electrolyte additive in LIB material, despite a potentially complete consumption during cycling. This allows improved reverse-engineering of LIB post-mortem analysis and risk assessment. Furthermore, mechanistic conclusions on the decomposition of the investigated electrolyte additives could be drawn, extending literature reports based on X-Ray methods with respect to species information and ultimately contribute to a better understanding of the interphase constitutions.[1] C. Peschel, S. van Wickeren, A. Bloch, C. Lechtenfeld, M. Winter, S. Nowak, Energy Technol. 2022, xxx.[2] S. S. Zhang, J. Power Sources 2006, 162, 1379–1394.[3] J. Henschel, J. M. Dressler, M. Winter, S. Nowak, Chem. Mater. 2019 31 (24), 9970-9976.[4] B. Tong, Z. Song, H. Wan, W. Feng, M. Armand, J. Liu, H. Zhang, Z. Zhou, InfoMat 2021, 3, 1364–1392.

Improved Sigr/NCM523 Cycling Via Triethyl Phosphate-Solubilized Lithium Nitrate Electrolyte

The performance of silicon-graphite (SiGr)/Li and SiGr/NCM523 cells were investigated in electrolytes containing lithium difluoro(oxalate)borate (LiDFOB) and lithium nitrate (LiNO3) in ethylene carbonate (EC), ethylmethyl carbonate (EMC), and different proportions of triethyl phosphate (TEP). While the LiDFOB and LiNO3-containing electrolytes both showed improved cycling stability over industry standards, the 10% TEP electrolyte demonstrated the best capacity retention in half cells and the best overall cycling performance in full cells. Ex-situ surface analysis reveals that the LiDFOB and LiNO3 preferentially decompose on both the anode and cathode surface during the first cycle to generate films consisting of oxalates, fluoroborates, and nitrate reduction products. Therefore, incorporation of two different film-forming electrolyte additives into the electrolyte is significantly more beneficial in formation of a robust SEI and improved cycle life in silicon-containing anodes.

Suppressing Gas Evolution in Li4Ti5O12||LiNi1/3Co1/3Mn1/3O2 Pouch Cells By High Temperature Formation

The increasing demands for a wide range of lithium-ion battery (LIB) applications requires the development of both improved high-energy as well as high-power systems. However, for high-power applications, the performance and thermal stability window of most commercial LIB systems is very limited. To meet these requirements, the spinel-type active material Li4Ti5O12 (LTO) is a very prominent candidate as an alternative to conventionally used graphite as a negative active material for LIBs. However, despite the excellent lifetime and safety properties of LTO [1], its commercialization is still hindered by severe gas evolution during cyclic and calendar aging [2,3].This study demonstrates that elevated temperatures during the formation procedure do not only suppress gas evolution upon subsequent charge/discharge cycling but also have a positive impact on the specific discharge capacity and rate capability. It was shown by same-spot SEM investigations that higher formation temperatures lead to the formation of a homogeneous decomposition layer over the entire electrode surface area. Volume measurements of the cells showed, that an increase in formation temperature is furthermore associated with more gas evolution, suggesting that gas evolution and decomposition layer formation are directly correlating.It is assumed that the formation of this decomposition layer on the LTO electrode surface during the formation procedure essentially serves to reduce or (in case of applying sufficiently high formation temperatures) even suppress surface catalytic gassing reactions during subsequent cell application.In this context, the impact of the electrode surface area on the qualitative and quantitative gassing behavior during cell aging is highlighted. In contrary to the assumption that LTO is solely responsible for gas evolution in LTO-based LIBs, it could be shown that gas evolution is primarily determined by the specific electrode surface area. Accordingly, when LTO is used as negative active material, a protective layer for suppression of gas evolution should be formed not only on the active material particles but also on the inactive surface are, hence, the entire electrode surface area.Therefore, the high-temperature formation approach presented in this study is ideally suited for formation of an effective decomposition layer over the entire electrode. Since neither particle pretreatment nor the addition of film-forming electrolyte additives were necessary to suppress severe gas evolution, the high temperature formation approach could be the cornerstone for a cost-effective and easy commercialization of Li4Ti5O12-based cells.[1] C. Han, Y.B. He, M. Liu, B. Li, Q.H. Yang, C.P. Wong, F. Kang, J. Mater. Chem. A, 2017, 5, 6368–6381[2] K. Wu, J. Yang, Y. Liu, Y. Zhang, C. Wang, J.Xu, F. Ning, D. Wang, J. Power Sources, 2013, 237, 285.[3] S. Wang, J. Liu, K.Rafiz, Y. Jin, Y. Li, Y. S. Lin, J. Electrochem. Soc., 2019, 166, A4150.

Anhydride type film-forming electrolyte additives for high-temperature LiNi0.6Co0.2Mn0.2O2//graphite pouch cells

LiNi0.6Co0.2Mn0.2O2//graphite full cell, due to its high energy density as the most promising candidate for power batteries received increased attention, but the severe capacity loss under high-temperature conditions prevents their large-scale application. Here, glutaric anhydride (GA) and succinic anhydride (SA) as electrolyte additives are investigated to overcome the high-temperature problem of LiNi0.6Co0.2Mn0.2O2//graphite cells. Due to the unique film-forming features and high-temperature stability, the cells containing 0.5% GA and 0.5% SA can retain capacity retention of 91.2% and 92.4% stored at 60 ​°C for 15 days, corresponding to a negligible capacity loss of 4.6 and 0.8 mAh g-1 after 200 cycles, respectively. In addition, the lifespans of cells under 25 ​°C are also significantly extended by introducing GA and SA into the electrolytes. Electrochemical and spectroscopic techniques results indicate the full cell capacity loss is predominantly caused by the unstable anode solid electrolyte interphase (SEI) layer and a stable SEI film derived from GA and SA is formed on the graphite electrode surfaces. The SEI film can effectively inhibit the consumption of electrolytes and enhance the high-temperature performance of cells. This work paves a way for moderating high energy density cells' work under high-temperature operation by developing special anodic film-forming electrolyte additives.

Recent Progress in Electrolyte Additives for Highly Reversible Zinc Anodes in Aqueous Zinc Batteries

Aqueous zinc batteries (AZBs) are one of the most promising large-scale energy storage devices by virtue of their high specific capacity, high degree of safety, non-toxicity, and significant economic benefits. However, Zn anodes in aqueous electrolyte suffer from zinc dendrites and side reactions, which lead to a low coulombic efficiency and short life cycle of the cell. Since electrolytes play a key role in the Zn plating/stripping process, versatile strategies have been developed for designing an electrolyte to handle these issues. Among these strategies, electrolyte additives are considered to be promising for practical application because of the advantages of low cost and simplicity. Moreover, the resulting electrolyte can maximally preserve the merits of the aqueous electrolyte. The availability and effectiveness of additives have been demonstrated by tens of research works. Up to now, it has been essential and timely to systematically overview the progress of electrolyte additives in mild acidic/neutral electrolytes. These additives are classified as metal ion additives, surfactant additives, SEI film-forming additives, and complexing additives, according to their functions and mechanisms. For each category of additives, their functional mechanisms, as well as the latest developments, are comprehensively elaborated. Finally, some perspectives into the future development of additives for advanced AZBs are presented.

Structure Property and Reaction Mechanism of Boron-Based High-Voltage Electrolyte Additives via First-Principles Calculations

Boron-based additives are currently the mainstream film-forming additives. They generate B–F-containing species and then participate in the formation of the cathode–electrolyte interphase (CEI) film. However, little is known about their oxidative decomposition mechanisms, reaction processes, and the relationship between their molecular structure and interface stability in electrolyte. Here, the reaction mechanisms and processes of trimethyl borate (TMB) with fluorides (F–, PF6–, HF, and PF5) in electrolyte were calculated and studied by combining theoretical and experimental methods. TMB was found to be capable of reacting with fluorides and self-dimerize in a nucleophilic manner. In addition, a comparative study was performed with tris(2-cyanoethyl) borate (TCEB), structurally similar to TMB. TMB and TCEB had the same reaction sites and almost the same energy barriers, and they participated in similar reaction processes. The decomposition of boron-based additives was analyzed through simulations and experiments to fill the gap in the information on the reaction mechanisms and processes of boron-based additives. This work also explored the influence of molecular structure on the mechanism of film formation, thus providing theoretical guidance for the molecular structure design of high-pressure additives and offering ideas for developing electrolyte additives.

The feasibility for natural graphite to replace artificial graphite in organic electrolyte with different film-forming additives

The feasibility for natural graphite (NG) to replace artificial graphite (AG) in organic electrolytes with different additives are investigated. Although the strong film-forming additives contributes to form robust solid electrolyte interphase (SEI) film on graphite particle surface, great differences in gas evolution, lithium inventory loss and other side reactions are observed. Lithium bis(oxalato)borate (LiBOB) and Fluoroethylene carbonate (FEC) are found more effective and the combination shows to be more promising. In the optimized electrolyte, natural graphite anode exhibits excellent long-term cycling capability. After 800 cycles at high temperature, the capacity retention is comparable to that using artificial graphite. The mechanisms for the capacity-fading of the full cells with AG and NG anode are investigated by ICP, SEM and polarization studies. The results shows that NG electrode consumes more active lithium due to the rough surface and larger volume expansion. The rapid capacity-fading in the initial 100 cycles is related to the instability of the SEI film aroused from large volume expansion. The systematic analysis is inspiriting for the development of high performance lithium ion batteries with reduced cost.

Improved SiGr/NCM523 Cycling via Triethyl Phosphate-Solubilized Lithium Nitrate Electrolyte

The performance of silicon–graphite (SiGr)/Li and SiGr/LiNi0.5Co0.2Mn0.3O2 (NCM523) cells was investigated in electrolytes containing lithium difluoro(oxalato)borate (LiDFOB) and lithium nitrate (LiNO3) in ethylene carbonate (EC), ethylmethyl carbonate (EMC), and different proportions of triethyl phosphate (TEP). While the LiDFOB and LiNO3-containing electrolytes both showed improved cycling stability over industry standards, the 10% TEP electrolyte demonstrated the best capacity retention in SiGr/Li cells and the best overall cycling performance in SiGr/NCM523 cells. Ex-situ surface analysis reveals that the LiDFOB and LiNO3 preferentially decompose on both the anode and cathode surface during the first cycle to generate films consisting of oxalates, fluoroborates, and nitrate reduction products. Therefore, incorporation of two different film-forming additives into the electrolyte is significantly more beneficial in formation of a robust SEI and improved cycle life in silicon-containing anodes.

Synergistic role of functional electrolyte additives containing phospholane-based derivative to address interphasial chemistry and phenomena in NMC811||Si-graphite cells

Herein, we report on the 2-phenoxy-1,3,2-dioxaphospholane (PhEPi) molecule as a novel functional phospholane-based electrolyte additive. The presence of PhEPi in optimal concentration in the baseline electrolyte (EC:EMC 3:7, 1 M LiPF6) significantly enhances the electrochemical performance of the NMC811||Si-graphite cell chemistry. To further optimize the cell's overall performance, two well-known film-forming additives, namely, vinylene carbonate (VC) and fluoroethylene carbonate (FEC), were used as co-additives with PhEPi, and the resulting cells thoroughly characterized by selected electrochemical and spectroscopic techniques. The addition of PhEPi to the VC-containing electrolytes showed the complementary advantages of high specific discharge capacity and prolonged cycle life of the considered cells and outperformed the overall performance of the FEC counterpart. To gain better understanding of the role of each additive on the individual positive and negative electrode interphases, we conducted a systematic investigation of both electrodes, which were pre-cycled against Li-metal electrode. Post mortem spectroscopic investigations of the corresponding interphases provided insights into the synergistic effect of VC and PhEPi in the NMC811||Si-graphite cell chemistry.