Safe ionic liquid-sulfolane/LiDFOB electrolytes for high voltage Li1.15(Ni0.36Mn0.64)0.85O2 lithium ion battery at elevated temperatures

Understanding the Role of Imide-Based Salts and Borate-Based Additives for Safe and High-Performance Glyoxal-Based Electrolytes in Ni-Rich NMC811 Cathodes for Li-Ion Batteries

Herein, the design of novel and safe electrolyte formulations for high-voltage Ni-rich cathodes is reported. The solvent mixture comprising 1,1,2,2-tetraethoxyethane and propylene carbonate not only displays good transport properties, but also greatly enhances the overall safety of the cell thanks to its low flammability. The influence of the conducting salts, that is, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI), and of the additives lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFOB) is examined. Molecular dynamics simulations are carried out to gain insights into the local structure of the different electrolytes and the lithium-ion coordination. Furthermore, special emphasis is placed on the film-forming abilities of the salts to suppress the anodic dissolution of the aluminum  current collector and to create a stable cathode electrolyte interphase (CEI). In this regard, the borate-based additives significantly alleviate the intrinsic challenges associated with the use of LiTFSI and LiFSI salts. It is worth remarking that a superior cathode performance is achieved by using the LiFSI/LiDFOB electrolyte, displaying a high specific capacity of 164 mAh g-1 at 6 C and ca. 95% capacity retention after 100 cycles at 1 C. This is attributed to the rich chemistry of the generated CEI layer, as confirmed by ex situ X-ray photoelectron spectroscopy.

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

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

Abundant Hydrogen Bonds Formed in a Urea-Based Gel Polymer Electrolyte Improve Interfacial Stability in Lithium Metal Batteries

As an emerging and potential replacement system for liquid electrolytes, polymer electrolytes (PEs) exhibit unique capacity in suppressing metal dendrite formation and leakage risks. However, the most used polymer matrix, including polyether, polyester, and polysiloxane, still cannot meet the practical demands for metal electrode compatibility and long lifespan. In this study, gel polymer electrolytes consisting of a polyurea network with abundant hydrogen bonds and deep eutectic electrolyte (DEE) are designed and prepared in-situ. The hydrogen bonding between polyurea chains and polyurea-DEE provides good interfacial stability between PEs and lithium metal. As a result, the assembled Li/LiFePO₄ cells based on this electrolyte deliver a long cycle life with 90 % retention after 500 cycles and 76.5 % retention after 1000 cycles at 1 C. In addition, the flexible design characteristics of polyurea structure permit easy operation for performance optimization by modulating the composition of hard and soft segments, and enhanced ionic conductivity and self-healing efficiency are obtained. This study provides a novel method for preparing advanced polymer electrolytes for lithium metal batteries.

Challenges and Recent Advances in High Capacity Li-Rich Cathode Materials for High Energy Density Lithium-Ion Batteries

Li-rich cathode materials have attracted increasing attention because of their high reversible discharge capacity (>250 mA h g^-1 ), which originates from transition metal (TM) ion redox reactions and unconventional oxygen anion redox reactions. However, many issues need to be addressed before their practical applications, such as their low kinetic properties and inefficient voltage fading. The development of cutting-edge technologies has led to cognitive advances in theory and offer potential solutions to these problems. Herein, a recent in-depth understanding of the mechanisms and the frontier electrochemical research progress of Li-rich cathodes are reviewed. In addition, recent advances associated with various strategies to promote the performance and the development of modification methods are discussed. In particular, excluding Li-rich Mn-based (LRM) cathodes, other branches of the Li-rich cathode materials are also summarized. The consistent pursuit is to obtain energy storage devices with high capacity, reliable practicability, and absolute safety. The recent literature and ongoing efforts in this area are also described, which will create more opportunities and new ideas for the future development of Li-rich cathode materials.

High-voltage liquid electrolytes for Li batteries: progress and perspectives

Since the advent of the Li ion batteries (LIBs), the energy density has been tripled, mainly attributed to the increase of the electrode capacities. Now, the capacity of transition metal oxide cathodes is approaching the limit due to the stability limitation of the electrolytes. To further promote the energy density of LIBs, the most promising strategies are to enhance the cut-off voltage of the prevailing cathodes or explore novel high-capacity and high-voltage cathode materials, and also replacing the graphite anode with Si/Si-C or Li metal. However, the commercial ethylene carbonate (EC)-based electrolytes with relatively low anodic stability of ∼4.3 V vs. Li⁺/Li cannot sustain high-voltage cathodes. The bottleneck restricting the electrochemical performance in Li batteries has veered towards new electrolyte compositions catering for aggressive next-generation cathodes and Si/Si-C or Li metal anodes, since the oxidation-resistance of the electrolytes and the in situ formed cathode electrolyte interphase (CEI) layers at the high-voltage cathodes and solid electrolyte interphase (SEI) layers on anodes critically control the electrochemical performance of these high-voltage Li batteries. In this review, we present a comprehensive and in-depth overview on the recent advances, fundamental mechanisms, scientific challenges, and design strategies for the novel high-voltage electrolyte systems, especially focused on stability issues of the electrolytes, the compatibility and interactions between the electrolytes and the electrodes, and reaction mechanisms. Finally, novel insights, promising directions and potential solutions for high voltage electrolytes associated with effective SEI/CEI layers are proposed to motivate revolutionary next-generation high-voltage Li battery chemistries.

Novel piperidinium-based ionic liquid as electrolyte additive for high voltage lithium-ion batteries

Conventional carbonate-based electrolyte is prone to oxidative decomposition at high voltage (over 4.5 V vs. Li/Li+), which leads to the bad oxidation stability and inferior cycling performance of lithium ion batteries (LIBs). To solve these problems, a novel ionic liquid (IL) N-butyronitrile-N-methylpiperidinium bis(fluorosulfonyl)imide (PP1,CNFSI) was synthesized and explored as the additive to the LiPF6-ethylene carbonate (EC)/dimethyl carbonate (DMC) electrolyte. For the cell performance, the addition of PP1,CNFSI not only inhibits overcharge phenomenon, but also improves discharge capacity, thus enhancing capacity retention capability. Compared to the cell with blank electrolyte, the capacity retentions of adding 15 wt% PP1,CNFSI into the electrolyte were improved to 96.8% and 97% from 82.8% and 78.7% at 0.2 C and 5 C, respectively. The effects of PP1,CNFSI on the LNMO cathode surface were further investigated by electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It reveals that PP1,CNFSI addition drives the formation of solid electrolyte interphase (SEI) film which suppresses oxidative decomposition of the electrolyte and protects the structure cathode material.