Electrolyte Decomposition Products Research Articles

Enhanced Surface Stability of K0.27MnO2·0.54H2O Cathode Materials with LiF/LixPFyOz Coating for Potassium-Ion Batteries

Manganese based oxides have significant potential as cathode materials for potassium ion batteries (PIB). Unfortunately, the practical application of manganese-based oxides is hindered by severe electrode surface erosion from electrolyte decomposition products and material expansion and collapse during potassium ion intercalation. Consequently, the surface modification strategy is adopted herein to uniformly cover the surface of K0.27MnO2·0.54H2O (KMO) microspheres with LiF/LixPFyOz coating to effectively protect the electrode surface and significantly inhibit the collapse of the material crystal structure. Additionally, the coating exhibits excellent electron/ion conductivity, enhancing the diffusion efficiency of potassium ions within the electrode material. The existence of LiF/LixPFyOz coating is confirmed by characterization measurements. The LiF/LixPFyOz-coated KMO microspheres (sample LLO@KMO-2) exhibit outstanding cyclic stability (66.3% capacity retention at 50mAg-1) and high discharge capacity (100.2 mAh g-1 at 50mAg-1), demonstrating their excellent cycle stability. The modification strategy of LiPF6-electrolyte-solvothermal coating provides an innovative surface modification method for potassium ion batteries, showcasing its potential for application in future high-performance battery technologies.

Impact of Salts and Linear Carbonates on the Performance of Layered Oxide/Hard Carbon Sodium-Ion Pouch Cells with Alkyl Carbonate Electrolytes

This study examines the influence of electrolyte salts and solvents on the performance of O3 layered oxide NaMn0.39Fe0.31Ni0.22Zn0.08O2/hard carbon sodium-ion pouch cells with polyethylene terephthalate (PET) jellyroll tape. A significant enhancement in cell performance between 2.0 and 3.8 V was observed across various temperatures (20, 40, and 55 °C) by substituting NaPF6 with NaFSI, including reduced impedance growth, minimized gas generation, and supressed jellyroll tape decomposition. Ultra-high precision coulometry revealed that the use of NaPF6 resulted in increased unwanted parasitic reactions associated with tape decomposition, e.g., capacity fade and charge endpoint capacity slippage. Teardown of sodium-ion pouch cells after cycling in DMC-based electrolytes revealed a severe decomposition of the PET tape with NaPF6 but not with NaFSI. Gas chromatography shows significantly more electrolyte decomposition products with NaPF6 as opposed to NaFSI. DEC-based electrolyte showed less capacity fade, less electrolyte decomposition products, and less tape decomposition after cycling than DMC-based electrolyte. The electrolyte additive DTD can prevent parasitic reactions in DMC- and NaPF6-based electrolyte. Overall, the choice of salts and linear carbonates in alkyl carbonate electrolytes plays a crucial role in determining the overall cycling performance of the layered oxide/hard carbon sodium-ion cells with PET jellyroll tape.

Solid–electrolyte interphase formation during Li metal deposition in LiN(SO2F)2-based solvate ionic liquids

AbstractSolvate ionic liquids (SILs) are promising electrolytes for Li metal batteries. In this study, Li plating-stripping reactions in SILs were investigated using electrochemical quartz crystal microbalance measurements and X-ray photoelectron spectroscopy (XPS). SILs were prepared by mixing Li salt and glyme (triglyme or tetraglyme) in a 1:1 molar ratio. During the Li plating-stripping reaction, a reversible mass change was observed in LiN(SO2F)2 (LiFSA) based SILs. In contrast, a considerably higher mass change than the theoretical value calculated from the Faraday current was observed in the LiN(SO2CF3)2 (LiTFSA) based SILs, owing to the accumulation of decomposition products of electrolytes on the electrode surface. XPS depth profiling for the deposited Li suggested the formation of thin solid-electrolyte interphase (SEI) in the LiFSA-based SILs while considerably thicker SEI was formed in the LiTFSA-based SILs. The SEI formed in LiFSA-based SILs effectively suppressed the decomposition of electrolytes. Thus, LiFSA-based SILs are favorable for achieving highly reversible charge–discharge of Li electrodes in Li metal batteries.

Spiers Memorial Lecture: Lithium air batteries - tracking function and failure.

The lithium-air battery (LAB) is arguably the battery with the highest energy density, but also a battery with significant challenges to be overcome before it can be used commercially in practical devices. Here, we discuss experimental approaches developed by some of the authors to understand the function and failure of lithium-oxygen batteries. For example, experiments in which nuclear magnetic resonance (NMR) spectroscopy was used to quantify dissolved oxygen concentrations and diffusivity are described. 17O magic angle spinning (MAS) NMR spectra of electrodes extracted from batteries at different states of charge (SOC) allowed the electrolyte decomposition products at each stage to be determined. For instance, the formation of Li2CO3 and LiOH in a dimethoxyethane (DME) solvent and their subsequent removal on charging was followed. Redox mediators have been used to chemically reduce oxygen or to chemically oxidise Li2O2 in order to prevent electrode clogging by insulating compounds, which leads to lower capacities and rapid degradation; the studies of these mediators represent an area where NMR and electron paramagnetic resonance (EPR) studies could play a role in unravelling reaction mechanisms. Finally, recently developed coupled in situ NMR and electrochemical impedance spectroscopy (EIS) are used to characterise the charge transport mechanism in lithium symmetric cells and to distinguish between electronic and ionic transport, demonstrating the formation of transient (soft) shorts in common lithium-oxygen electrolytes. More stable solid electrolyte interphases are formed under an oxygen atmosphere, which helps stabilise the lithium anode on cycling.

Delineating the Impact of Transition-Metal Crossover on Solid-Electrolyte Interphase Formation with Ion Mass Spectrometry.

Lithium-metal batteries (LMB) employing cobalt-free layered-oxide cathodes are a sustainable path forward to achieving high energy densities, but these cathodes exhibit substantial transition-metal dissolution during high-voltage cycling. While transition-metal crossover is recognized to disrupt solid-electrolyte interphase (SEI) formation on graphite anodes, experimental evidence is necessary to demonstrate this for lithium-metal anodes. In this work, advanced high-resolution 3D chemical analysis is conducted with time-of-flight secondary-ion mass spectrometry (TOF-SIMS) to establish spatial correlations between the transition metals and electrolyte decomposition products found on cycled lithium-metal anodes. Insights into the localization of various chemistries linked to crucial processes that define LMB performance, such as lithium deposition, SEI growth, and transition-metal deposition are deduced from a precise elemental and spatial analysis of the SEI. Heterogenous transition-metal deposition is found to perpetuate both heterogeneous SEI growth and lithium deposition on lithium-metal anodes. These correlations are confirmed across various lithium-metal anodes that are cycled with different cobalt-free cathodes and electrolytes. An advanced electrolyte that is stable to higher voltages is shown to minimize transition-metal crossover and its effects on lithium-metal anodes. Overall, these results highlight the importance of maintaining uniform SEI coverage on lithium-metal anodes, which is disrupted by transition-metal crossover during operation at high voltages.

Improving K-Ion Battery Performance By Using K-Metal-Pretreated Electrolyte Solutions

Nonaqueous K-ion batteries (KIBs) have attracted much attention as a potential high-voltage and high-power secondary battery due to the low standard electrode potential of K+/K in carbonate ester solutions and small Stokes radius of K+ ions compared with Li cases.1 In studies of new electrode materials and electrolytes for KIBs, half-cell measurements using highly reactive K metal counter electrodes are a standard practice.1 The presence of K metal has been reported to alter both the electrochemical performance and interface chemistry of the working electrode via migration of electrolyte decomposition products across the cell.2-4 Herein, we systematically evaluated the electrolyte decomposition products formed between K metal and electrolytes prepared with KPF6, KN(SO2F)2 (KFSA) or the combined salts (K(PF6)0.75(FSA)0.25) dissolved in ethylene carbonate/diethyl carbonate (EC/DEC).5, 6 To investigate the effect of degradation products on KIB performance, we prepared electrolytes containing KPF6, KFSA or K(PF6)0.75(FSA)0.25 dissolved in EC/DEC. We stored 2 mL of each electrolyte in contact with a freshly cut K metal disk with a diameter of 15 mm for 7 days (Figure 1a-d). Then, the K metal was removed, and all the concentration and composition of electrolytes were adjusted to 1 mol kg-1 K(PF6)0.75(FSA)0.25/EC/DEC by adding either KFSA or KPF6, as shown in Figure 1b-d. We denote the obtained electrolytes based on their treatment by K metal (K-KPF6, K-KFSA, and K-KPF6:KFSA).Using gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS), we identify significant formation of decomposition products derived from carbonate esters, such as oligocarbonates for K-KPF6 (Table 1), while K-KFSA predominantly generates anions combining FSA- with the solvent structures (Table 1). Using three-electrode cells, we delineated positive effects of the K-KFSA and K-KPF6:KFSA electrolytes on graphite negative electrode performance and the negative impact of oligocarbonates in K-KPF6 on K2Mn[Fe(CN)6] positive electrodes.We further evaluated the K-KPF6:KFSA electrolyte in a graphite||K2Mn[Fe(CN)6] full cell. As shown in Figure 2a, the initial charge/discharge curves for the K-KPF6:KFSA electrolyte (purple) cell exhibited a larger reversible capacity (119 mAh g-1 positive) and higher Coulombic efficiency (CE, 74.5 %) than the untreated electrolyte cell shown in red (110 mAh g-1 and 69.6 %). The improved initial CE for the K-KPF6:KFSA electrolyte led to a higher energy density of 277 Wh (kg-active materials)-1 compared with that of 257 Wh kg-1 in the untreated electrolyte. Figure 2b,c displays the CE and capacity retention over 500 cycles. For more than 100 cycles, the K-KPF6:KFSA cell continued to demonstrate a higher CE than the untreated electrolyte (Figure 2b), though both cells eventually showed CEs of approximately 99.9%. Furthermore, the use of K-KPF6:KFSA improved the full cell’s capacity retention (78%) compared with the untreated cell (72%), as shown in Figure 2c. These results suggest that the electrolyte decomposition products formed by K metal treatment can minimize battery capacity loss by promoting initial SEI formation. Indeed, density functional theory calculation and hard X-ray photoelectron spectroscopy indicated the incorporation of the FSA-derived structures into the solid electrolyte interphase at graphite, which is not observed in K metal-free cells. In the presentation, the impact of electrolyte decomposition products on the electrochemical properties of KIBs’ electrodes and their mechanisms will be further discussed.

(Invited) SiO and LiNi0.5Mn1.5O4 Composite Electrodes with Functional Polymer Binders

This study focuses on the optimization of polymer binders in lithium-ion batteries (LIBs) to facilitate the development of high-energy-density storage devices. Despite 30 years of commercialization, the demand for high energy density storage devices has continued to increase, making LIBs a topic of significant research interest. One major avenue of research has focused on high-voltage positive electrode materials such as LiNi0.5Mn1.5O4 (LNMO) and high-capacity negative electrode materials like SiO. However, the practical use of these materials is hindered by issues associated with electrolyte decomposition and large volume changes during the charge-discharge process.To address these challenges, we explored the chemistry of polymer binders blended in composite electrodes to maintain the structure of the electrode and form a passivation/protective layer on the active material surface to suppress side reactions.(1, 2) In particular, we investigated the use of "functional binders" to improve the performance of high-voltage LNMO composite electrodes. We synthesized sulfated alginate binders, which are significantly cheaper and stiffer than commonly used poly(vinylidene fluoride) (PVdF) and uniformly cover the surface of LNMO.(3)The high polarity of the sulfate group also increased the binder's affinity for the electrolyte. During the initial charge-discharge cycle, the electrolyte decomposition products generated on the alginate-covered active materials participated in the formation of a protective passivation layer that suppressed further decomposition during subsequent cycles, resulting in enhanced cycling and rate performances.We also explored the use of partially-neutralized crosslinked sodium polyacrylate (CLPA) binders with flexible monomers to mitigate the effects of volume changes in SiO electrodes.(4)Additionally, we investigated the “maturation” effects on battery performances by storing the composite electrodes with CLPA-based binders in a humid atmosphere for a few days before drying. Our results show that the binder with 20% flexible monomer (CLPA-20) exhibited the highest peel strength and long-term cyclability, and the maturation treatment further improved these properties. Cross-sectional SEM images revealed that the binder and conductive carbon were uniformly dispersed in the composite layer in the matured electrodes, indicating the migration of active materials and binders during the maturation process. Furthermore, surface analysis of the cycled electrode showed that the superior mechanical properties of the electrode endowed by the maturation treatment helped in maintaining the passivation layer on the active material surface, resulting in improved cycling performance.Overall, our findings suggest that optimizing polymer binders can enhance the performance of LIBs by controlling the electrode reaction via binder chemistry. The strategies developed in this study could facilitate the future development of high-energy-density LIBs, making them more accessible and commercially viable for various applications.References (1) R. Tatara, S. Komaba et al., ChemElectroChem, 8, 4345 (2021).(2) H. Isozumi, R. Tatara, S. Komaba et al., ACS Appl. Energy Mater., 3, 7978 (2020).(3) A. Oishi, R. Tatara, S. Komaba et al., ACS Appl. Mater. Interfaces, 14, 51808 (2022).(4) S. Yamazaki, R. Tatara, S. Komaba et al., Mater. Adv., 4, 1637 (2023).

Contribution of Electrolyte Decomposition Products and the Effect of Temperature on the Dissolution of Transition Metals from Cathode Materials.

A fundamental understanding of aging processes in lithium-ion batteries (LIBs) is imperative in the development of future battery architectures for widespread electrification. Herein, dissolution of transition metals from cathode active materials of LIBs is among the most important degradation processes. Research has demonstrated that elevated operating temperatures accelerate battery degradation. However, the exact mechanism of transition-metal dissolution at elevated temperatures has still to be clarified. Current literature suggests that the reaction rate of dissolution increases with increasing temperature; moreover, the decomposition of electrolytes results in products that also accelerate dissolution processes. Most studies focus on ex situ analyses of thermally treated full cells. This approach is not appropriate to get detailed insights and to distinguish between different contributions. In this work, with the help of real-time dissolution analysis using an electroanalytical flow cell (EFC) coupled to an inductively coupled plasma mass spectrometer (ICP-MS), we present novel details of the temperature effects on in situ dissolution at the cathode electrolyte interface. With fresh electrolytes, we find increased Mn dissolution even at open-circuit conditions as well as with constant voltage polarization when the electrode sample is heated at constant temperatures between 50 and 80 °C. The release of transition metals also responds in a nuanced manner when applying temperature transients. Utilizing electrolytes preheated at 60 and 100 °C, we demonstrate that decomposition products in the bulk electrolyte have no influence on transition-metal (TM) dissolution when constantly flushing the cell with the thermally aged electrolyte samples. Only when keeping the cathode temperature at 60 °C, the dissolution increases by a factor of 2-3. Our findings highlight the interplay between the cathode and electrolyte and provide new insights into the dissolution mechanism of cathode materials.

Chemical Reaction Networks Explain Gas Evolution Mechanisms in Mg-Ion Batteries.

Out-of-equilibrium electrochemical reaction mechanisms are notoriously difficult to characterize. However, such reactions are critical for a range of technological applications. For instance, in metal-ion batteries, spontaneous electrolyte degradation controls electrode passivation and battery cycle life. Here, to improve our ability to elucidate electrochemical reactivity, we for the first time combine computational chemical reaction network (CRN) analysis based on density functional theory (DFT) and differential electrochemical mass spectroscopy (DEMS) to study gas evolution from a model Mg-ion battery electrolyte─magnesium bistriflimide (Mg(TFSI)2) dissolved in diglyme (G2). Automated CRN analysis allows for the facile interpretation of DEMS data, revealing H2O, C2H4, and CH3OH as major products of G2 decomposition. These findings are further explained by identifying elementary mechanisms using DFT. While TFSI- is reactive at Mg electrodes, we find that it does not meaningfully contribute to gas evolution. The combined theoretical-experimental approach developed here provides a means to effectively predict electrolyte decomposition products and pathways when initially unknown.

Enhanced Cycling Stability of 4.6 V LiCoO2 Cathodes by Inhibiting Catalytic Activity of its Interface Via MXene Modification

AbstractLiCoO2 plays a key role in energy storage devices due to its high energy density. And the volumetric energy density of LiCoO2 cathode can be significantly improved by increasing the charging cut‐off voltage to 4.6 V. However, the increase in resistance at the LiCoO2 interface, and the damage to the LiCoO2 from the outside to the inside by the HF generated that caused by the decomposition of the organic electrolyte and LiPF6 under 4.6 V conditions are not conducive to structural stability during cycling. Here, it is shown that the decomposition of electrolyte and LiPF6 is effectively mitigated by inhibiting the interfacial catalytic activity of LiCoO2 using an atomically thin layer of MXenes as a interlayer. Density functional theory results suggest that the decomposition energy of LiPF6 is 1.13 and 3.21 eV at the interface of LiCoO2 and MXenes, respectively. Time of Flight Secondary Ion Mass Spectrometry results further indicate that the decomposition products of the organic electrolyte and LiPF6 have a thinner thickness at the interface of MXenes (5 nm) than LiCoO2 (10 nm). This study provides a new and universal strategy for stabilizing the cathode interface to support the development of high energy density lithium‐ion batteries.

Effect of Si Content on Extreme Fast Charging Behavior in Silicon–Graphite Composite Anodes

Commercial Li-ion batteries typically incorporate a small amount of high-capacity silicon (Si)-based materials in the composite graphite-based anode to increase the energy density of the battery. However, very little is known about the effects of Si on the fast-charging behavior of composite anodes. Herein, we examine the effects of the Si/graphite ratio in the composite anode on the fast-charging behavior of full cells. We show that addition of Si increases the rate capability from 1C to 8C and improves the capacity retention in early cycles at 6C due to reduced overpotential in constant current charging cycles. The impacts of Si content on fast-charging aging were identified by Post-Test characterization. Despite realizing benefits of available capacity and reduced Li plating at 6C, silicon–electrolyte interactions lead the time-dependent cell performance to fade quickly in the long term. The Post-Test analysis also revealed the thickening of the electrode and nonuniform distribution of electrolyte decomposition products on the Si-containing anodes, as well as the organic-rich solid electrolyte interphase (SEI), which are the factors behind cell degradation. Our study sheds insight on the advantages and disadvantages of Si/graphite composite anodes when they are used in fast-charging applications and guides further research in the area by designing an optimized composition of Si incorporated in a mature graphite matrix.

Determining the Origin of Lithium Inventory Loss in NMC622||Graphite Lithium Ion Cells Using an LiPF6-Based Electrolyte

X-ray photoelectron spectroscopy (XPS) is often used in interphase investigations of lithium ion batteries (LIBs). Yet, it is unclear, if its results convey an accurate picture of the lithium loss in aged LIBs. Herein, electrochemical and surface analytical approaches were used to separately quantify the contribution of interphase growth to lithium loss in LIBs. For this, LIB pouch cells (NMC622||graphite, 5 Ah) were aged for 400 full cycles at 20 °C or 60 °C. Electrodes were harvested post mortem and subsequent investigations in lithium metal battery cells showed notably higher reversible and irreversible lithium loss after 60 °C than after 20 °C cycling. While the interphases did not notably increase in thickness with aging, the surface area of both electrodes increased, leading to more electrolyte decomposition and larger lithium loss. Along with the surface area increase, more heterogeneous electrolyte decomposition product residues on the negative electrode surface and higher cathode|electrolyte charge-transfer resistances were observed. In conclusion, the applied combination of XPS and nitrogen adsorption can quantify homogenously distributed electrolyte decomposition layers of thicknesses <10 nm, but not thick and heterogeneous decomposition product residues arising with 60 °C cycling. For this, the need for an alternative quantification method is highlighted.

Impact of electrolyte decomposition products on the electrochemical performance of 4 V class K-ion batteries.

In the pursuit of long-life K-ion batteries (KIBs), half-cell measurements using highly reactive K metal counter electrodes are a standard practice. However, there is increasing evidence of electrolyte decomposition by K metal impacting electrode performance. Herein, we systematically explored the K metal-treated electrolytes KPF6, KN(SO2F)2 (KFSA), and their combination in ethylene carbonate/diethyl carbonate (EC/DEC), referred to as K-KPF6, K-KFSA, and K-KPF6:KFSA, respectively, after storage in contact with K metal. Through mass spectrometry analysis, we identified significant formation of carbonate ester-derived decomposition products such as oligocarbonates for K-KPF6, while K-KFSA predominantly generates anions combining FSA- with the solvent structures. Using three-electrode cells, we delineated the positive effects of the K-KFSA and K-KPF6:KFSA electrolytes on graphite negative electrode performance and the negative impact of oligocarbonates in K-KPF6 on K2Mn[Fe(CN)6] positive electrodes. The interactions between the decomposition products and the electrodes were further evaluated using density functional theory calculations. Full cell measurements using K-KPF6:KFSA showed an improved energy density and capacity retention of 78% after 500 cycles compared with an untreated electrolyte (72%). Hard X-ray photoelectron spectroscopy indicated the incorporation of the FSA-derived structures into the solid electrolyte interphase at graphite, which was not observed in K metal-free cells. Overall, this work indicates further complexities to consider in KIB measurements and suggests the potential application of decomposition products as electrolyte additives.

Laser-Assisted Surface Lithium Fluoride Decoration of a Cobalt-Free High-Voltage Spinel LiNi0.5Mn1.5O4 Cathode for Long-Life Lithium-Ion Batteries.

High-voltage spinel LiNi0.5Mn1.5O4 (LNMO) is a promising next-generation cathode material due to its structural stability, high operation voltage, and low cost. However, the cycle life of LNMO cells is compromised by detrimental electrode-electrolyte reactions, chemical crossover, and rapid anode degradation. Here, we demonstrate that the cycling stability of LNMO can be effectively enhanced by a high-energy laser treatment. Advanced characterizations unveil that the laser treatment induces partial decomposition of the polyvinylidene fluoride binder and formation of a surface LiF phase, which mitigates electrode-electrolyte side reactions and reduces the generation of dissolved transition-metal ions and acidic crossover species. As a result, the solid electrolyte interphase of the graphite counter electrode is thin and is composed of fewer electrolyte decomposition products. This work demonstrates the potential of laser treatment in tuning the surface chemistry of cathode materials for lithium-ion batteries.

Sulfated Alginate as an Effective Polymer Binder for High-Voltage LiNi0.5Mn1.5O4 Electrodes in Lithium-Ion Batteries.

Although the increasing demand for high-energy-density lithium-ion batteries (LIBs) has inspired extensive research on high-voltage cathode materials, such as LiNi0.5Mn1.5O4 (LNMO), their commercialization is hindered by problems associated with the decomposition of common carbonate solvent-based electrolytes at elevated voltages. To address these problems, we prepared high-voltage LNMO composite electrodes using five polymer binders (two sulfated and two nonsulfated alginate binders and a poly(vinylidene fluoride) conventional binder) and compared their electrochemical performances at ∼5 V vs Li/Li+. The effects of binder type on electrode performance were probed by analyzing cycled electrodes using soft/hard X-ray photoelectron spectroscopy and scanning transmission electron microscopy. The best-performing sulfated binder, sulfated alginate, uniformly covers the surface of LNMO and increased its affinity for the electrolyte. The electrolyte decomposition products generated in the initial charge-discharge cycle on the alginate-covered electrode participated in the formation of a protective passivation layer that suppressed further decomposition during subsequent cycles, resulting in enhanced cycling and rate performances. The results of this study provide a basis for the cost-effective and technically undemanding fabrication of high-energy-density LIBs.

Boron‐Based Functional Additives Enable Solid Electrolyte Interphase Engineering in Calcium Metal Battery

AbstractCalcium‐metal batteries have received growing attention recently after several studies reporting successful metal plating and stripping with organic electrolytes. Given the low redox potential of metallic calcium, its surface is commonly covered by a passivation layer grown by the accumulation of electrolyte decomposition products. The presence of borate species in this layer has been shown to be a key parameter allowing for Ca2+ migration and favoring Ca electrodeposition. Here, boron‐based additives are evaluated in order to tune the SEI composition, morphology, and properties. The decomposition of a BF3‐based additive is studied at different potentiostatic steps and the resulting SEI layer was thoroughly characterized. SEI growth mechanism is proposed based on both experimental data and DFT calculations pointing at the formation of boron‐crosslinked polymeric matrices. Several boron‐based adducts are explored as SEI‐forming additives for calcium‐metal batteries paving the way to very rich chemistry leading to Ca2+ conducting SEI.

Extreme fast charge aging: Effect of electrode loading and NMC composition on inhomogeneous degradation in graphite bulk and electrode/electrolyte interface

Empowering extreme fast charging (XFC) requires a comprehensive understanding of its application with advanced anode and cathode materials in lithium-ion batteries. No report exists for the full extent of limitations for the anode with crosstalk effect from paired cathode as well as Li plating due to electrode loading under XFC. In this study, a combination of cell testing and multiple length characterization is used to investigate XFC aging mechanism in cells with a low loading of 1.5 mAh cm−2 and high loading of 2.5 mAh cm−2 for graphite (Gr)/Ni-rich LiNixMnyCo1-x-yO2 (NMCs). Operando XRD mappings show 1.5 mAh cm−2 loadings result in higher strain in graphite for all three cathode types. Among the three NMC cathodes, the graphite from NMC532 and NMC811 cells show comparable strain. Scanning electron microscopy (SEM) images show distinct differences between 6-C-charged anodes in two loadings. Significantly increased electrode thickness can be seen due to more damage in the graphite bulk and accumulation of the electrolyte decomposition products in electrode pores. X-ray photoelectron spectroscopy (XPS) reveals both cathode chemistry and Li plating influence the non-uniform SEI composition on graphite surface. Higher Ni content in NMC811 promotes the higher levels of salt decomposition on the SEI and formation of higher mass of electrolyte aging products.

Glyoxylic acetals as electrolytes for Si/Graphite anodes in lithium-ion batteries

Using silicon-containing anodes in lithium-ion batteries is mainly impeded by undesired side reactions at the electrode/electrolyte interface leading to the gradual loss of active lithium. Therefore, electrolyte formulations are needed, which form a solid electrolyte interphase (SEI) that can accommodate to the volume changes of the silicon particles. In this work, we analyze the influence of two glyoxylic acetals on the cycling stability of silicon-containing graphite anodes, namely TMG (1 M LiTFSI in 1,1,2,2-tetramethoxyethane) and TEG (1 M LiTFSI in 1,1,2,2-tetraethoxyethane). The choice of these two electrolyte formulations was motivated by their positive impact on the thermal stability of LIBs. We investigate solid electrolyte decomposition products employing x-ray photoelectron spectroscopy (XPS). The cycling stability of Si/Gr anodes in each electrolyte is correlated to changes in SEI thickness, composition, and morphology upon formation and aging. This evaluation is completed by comparing the performance of TMG and TEG to two carbonate-based reference electrolytes (1 M LiTFSI in 1:1 ethylene carbonate: dimethyl carbonate and 1 M LiPF6 in the same solvent mixture). Cells cycled in TMG display inferior electrochemical performance to the two reference electrolytes. By contrast, cells cycled in TEG exhibit the best capacity retention with overall higher capacities. We can correlate this to better film-forming properties of the TEG solvent as it forms a smoother and more interconnected SEI, which can better adapt to the volume changes of the silicon. Therefore, TEG appears to be a promising electrolyte solvent for silicon-containing anodes.

Utilizing fast ion conductor for single-crystal Ni-rich cathodes to achieve dual-functional modification of conductor network constructing and near-surface doping

Ni-rich transition metal oxides are regarded as appealing candidate cathodes for next-generation lithium batteries because of their high discharge capacity. Single-crystal is an effective means to restrain the formation of microcracks to improve the cyclic property of LiNi0.8Co0.15Al0.05O2 (NCA) materials. Nonetheless, critical structural and interfacial instabilities make single-crystal NCA cathodes hard to achieve ideal electrochemical performance at high voltage. Herein, the fast ion conductor Li5La3Nb2O12 with higher conductivity and wider electrochemical window was selected as the material to enhance the high voltage performances for single-crystal NCA. Interestingly, density functional theory calculations found that La and Nb have different distribution characteristics in NCA. Therefore, we take advantage of oxidation reaction and element diffusion during high temperature sintering to achieve dual-functional modification of La-rich conductor network constructing and near-surface Nb doping for single-crystal NCA material. Impressively, the modified electrodes exhibit excellent cycling performances with a retention of 82.9% after 500 cycles at 2.7–4.3 V and even outstanding stability under extended cutoff voltage as well as improved rate capabilities. Detailed study reveals that the doped Nb with stronger Nb-O bond can suppress the migration of transition metal ion, thus impeding the distortion of material structure. Meanwhile, surface ion-conductor network layer provides fast Li+ diffusion channels as well as protecting interface layer from electrolyte decomposition products. The facile approach of Li5La3Nb2O12 modification proposed in this manuscript is suitable to solve interfacial instability and structure stability for cathodes but not only for single-crystal NCA, which provides application hopes for high energy lithium-ion batteries.

Atomic Layer Vs. Sol-Gel Deposited Coatings for Long Cycle-Life Li-Ion Battery Positive Electrodes

High nickel content layered oxide positive electrodes for Li-ion batteries suffer from capacity fade due to, among other factors, particle cracking induced by lithiation/delithiation, transition metal dissolution into the electrolyte, and attack from HF. Surface modification has been shown to reduce the severity of these degradation mechanisms by blocking transition metal ions from dissolving, and protecting the surface from the electrolyte decomposition products, as well as physically restraining the particles to reduce cracking. Atomic layer deposition (ALD) is a coating method that promises good uniformity and a high level of thickness control with a wide range of possible coating materials. ALD is a derivation of chemical vapor deposition in which chemical precursors are injected separately into a low pressure heated reaction vessel. Precursors are selected to create a self-limiting reaction at each step, allowing monolayers of precursor to be deposited on each cycle.1 With a large chemical space to explore, the challenge remains to find a coating material that offers substantial protection and lifetime extension, while allowing facile Li+ diffusion that does not substantially impede the capacity and rate capability of the cell.Alumina, Al2O3, being the archetypal ALD coating material has been frequently explored to improve the stability of known Li-ion positive electrode materials.2 Recently Wang et al. found marked stability improvement of Al2O3 coated LiNi0.6Mn0.2Co0.2O2 over even as few as 40 charge/discharge cycles.3 In this work, we present a comparison study of alumina coatings on LiNi0.6Mn0.2Co0.2O2 deposited by ALD versus coatings prepared through a sol-gel method tested to 4.5 V vs Li+/Li (Figure 1). Depositing a range of Al2O3 thicknesses by ALD we see a general trend of increased capacity retention with increased coating thickness. Multiple Al-containing precursors have been investigated for sol-gel coatings and we have found varying results, some giving comparable performances to the ALD coated samples, whereas other sol-gel precursors resulted in poorer performance than the uncoated material. Both types of films have been characterised by low energy ion scattering and x-ray photoelectron spectroscopy to detect the presence of the coatings, as well as electron microscopy with energy dispersive x-ray spectroscopy to characterise the uniformity and thickness of the coatings. R. L. Puurunen, J. Appl. Phys., 97, 121301 (2005).L. A. Riley, S. V. Atta, A. S. Cavanagh, Y. Yan, S. M. George, P. Liu, A. C. Dillon, and S. Lee, J. Power Sources, 196, 3317–3324 (2011).X. Wang J. Cai, Y. Liu, X. Han, Y. Ren, J. Li, Y. Liu, and X. Meng, Nanotechnology, 32, 115401 (2020). Figure 1: Capacity retention after 3 formation cycles of LiNi0.6Mn0.2Co0.2O2 positive electrodes uncoated (black), Al2O3 coating by 50 ALD cycles (blue) and 2 wt% Aluminium tri sec-butoxide sol-gel (red). Cells were cycled at 2.5-4.5 V vs Li+/L at 25 °C at 1C after 3 formation cycles at C/10. The negative electrode was Li metal and the electrolyte was 1.0 M LiPF6 in ethylene carbonate : ethyl methyl carbonate 3:7 v/v with 1 wt% vinyl carbonate. Figure 1