Cycling performance and surface analysis of Lithium bis(trifluoromethanesulfonyl)imide in propylene carbonate with graphite

Reconstructing Helmholtz Plane Enables Robust F-Rich Interface for Long-Life and High-Safe Sodium-Ion Batteries

Hard carbon (HC) is the most commonly used anode material in sodium-ion batteries. However, the solid-electrolyte-interface (SEI) layer formed in carbonate ester-based electrolytes has an imperceptible dissolution tendency and a sluggish Na+ diffusion kinetics, resulting in an unsatisfactory performance of HC anode. Given that electrode/electrolyte interface property is highly dependent on the configuration of Helmholtz plane, we filtrated proper solvents by PFBE (PF6 - anion binding energy) and CAE (carbon absorption energy) and disclosed the function of chosen TFEP to reconstruct the Helmholtz plane and regulate the SEI film on HC anode. Benefiting from the preferential adsorption tendency on HC surface and strong anion-dragging interaction of TFEP, a robust and thin anion-derived F-rich SEI film is established, which greatly enhances the mechanical stability and the Na+ ion diffusion kinetics of the electrode/electrolyte interface. The rationally designed TFEP-based electrolyte endows Na||HC half-cell and 2.8 Ah HC||Na4Fe3(PO4)2P2O7 pouch cell with excellent rate capability, long cycle life, high safety and low-temperature adaptability. It is believed that this insightful recognition of tuning interface properties will pave a new avenue in the design of compatible electrolyte for low-cost, long-life, and high-safe sodium-ion batteries.

Unraveling the origin of reductive stability of super-concentrated electrolytes from first principles and unsupervised machine learning

Developing electrolytes with excellent electrochemical stability is critical for next-generation rechargeable batteries. Super-concentrated electrolytes (SCEs) have attracted great interest due to their high electrochemical performances and stability. Previous studies have revealed changes in solvation structures and shifts in lowest unoccupied molecular orbitals from solvents to anions, promoting the formation of an anion-derived solid-electrolyte-interphase (SEI) in SCE. However, a direct connection at the atomic level to electrochemical properties is still missing, hindering the rational optimization of electrolytes. Herein, we combine ab initio molecular dynamics with the free energy calculation method to compute redox potentials of propylene carbonate electrolytes at a range of LiTFSI concentrations, and moreover employ an unsupervised machine learning model with a local structure descriptor to establish the structure-property relations. Our calculation indicates that the network of TFSI- in SCE not only helps stabilize the added electron and renders the anion more prone to reductive decomposition, but also impedes the solvation of F- and favors LiF precipitation, together leading to effective formation of protective SEI layers. Our work provides new insights into the solvation structures and electrochemistry of concentrated electrolytes which are essential to electrolyte design in batteries.

Investigation of a Novel Ecofriendly Electrolyte-Solvent for Lithium-Ion Batteries with Increased Thermal Stability

This study presents tributyl acetylcitrate (TBAC) as a novel ecofriendly high flash point and high boiling point solvent for electrolytes in lithium-ion batteries. The flash point (TFP=217∘C) and the boiling point (TBP=331∘C) of TBAC are approximately 200 K greater than that of conventional linear carbonate components, such as ethyl methyl carbonate (EMC) or diethyl carbonate (DEC). The melting point (TMP=−80∘C) is more than 100 K lower than that of ethylene carbonate (EC). Furthermore, TBAC is known as an ecofriendly solvent from other industrial sectors. A life cycle test of a graphite/NCM cell with 1 M lithium hexafluorophosphate (LiPF6) in TBAC:EC:EMC:DEC (60:15:5:20 wt) achieved a coulombic efficiency of above 99% and the remaining capacity resulted in 90 percent after 100 cycles (C/4) of testing. As a result, TBAC is considered a viable option for improving the thermal stability of lithium-ion batteries.

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.

Abnormal Overcharging during Lithium-Ether Co-Intercalation in a Graphite System: Formation of Shuttling Species by the Reduction of the TFSI Anion

Materializing an ultrafast charging system is one of the crucial technologies for next-generation Li-ion batteries (LIBs). Among many studies aimed at achieving fast charging systems, Li-ether solvent cointercalation into the graphite electrodes in LIB has been identified as a novel concept for achieving high power performance because this system does not consist of the sluggish desolvation step and a resistive solid-electrolyte interface (SEI) layer. Interestingly, while studying the Li-ether solvent cointercalation into graphite electrodes, employing lithium bis-trifluoromethane sulfonimide (LiTFSI) as the Li salt, we observed an abnormal overcharging phenomenon. Here, we screened the specific conditions, under which the abnormal overcharging occurred, and revealed that this abnormal overcharging was attributable to the shuttling mechanism. The formation of shuttling species could have been derived by the reduction of TFSI^- anion. With this understanding of the underlying mechanism, we efficiently suppressed the abnormal overcharging by adding LiNO₃ to the electrolyte. The shuttling and resulting overcharging could be prevented by the synergistic contributions of LiNO₃ and S_xO_y, dissolved in the electrolyte, to the formation of a dense solid LiS_xO_y SEI layer on Li-metal. We expect that this work could be a great reference in analyzing many unsolved phenomena in systems utilizing TFSI^-.

Locally Concentrated LiPF6 in a Carbonate-Based Electrolyte with Fluoroethylene Carbonate as a Diluent for Anode-Free Lithium Metal Batteries

Currently, concentrated electrolyte solutions are attracting special attention because of their unique characteristics such as unusually improved oxidative stability on both the cathode and anode sides, the absence of free solvent, the presence of more anion content, and the improved availability of Li⁺ ions. Most of the concentrated electrolytes reported are lithium bis(fluorosulfonyl)imide (LiFSI) salt with ether-based solvents because of the high solubility of salts in ether-based solvents. However, their poor anti-oxidation capability hindered their application especially with high potential cathode materials (>4.0 V). In addition, the salt is very costly, so it is not feasible from the cost analysis point of view. Therefore, here we report a locally concentrated electrolyte, 2 M LiPF₆, in ethylene carbonate/diethyl carbonate (1:1 v/v ratio) diluted with fluoroethylene carbonate (FEC), which is stable within a wide potential range (2.5-4.5 V). It shows significant improvement in cycling stability of lithium with an average Coulombic efficiency (ACE) of ∼98% and small voltage hysteresis (∼30 mV) with a current density of 0.2 mA/cm² for over 1066 h in Li||Cu cells. Furthermore, we ascertained the compatibility of the electrolyte for anode-free Li-metal batteries (AFLMBs) using Cu||LiNi_1/3Mn_1/3Co_1/3O₂ (NMC, ∼2 mA h/cm²) with a current density of 0.2 mA/cm². It shows stable cyclic performance with ACE of 97.8 and 40% retention capacity at the 50th cycle, which is the best result reported for carbonate-based solvents with AFLMBs. However, the commercial carbonate-based electrolyte has <90% ACE and even cannot proceed more than 15 cycles with retention capacity >40%. The enhanced cycle life and well retained in capacity of the locally concentrated electrolyte is mainly because of the synergetic effect of FEC as the diluent to increase the ionic conductivity and form stable anion-derived solid electrolyte interphase. The locally concentrated electrolyte also shows high robustness to the effect of upper limit cutoff voltage.

Realizing High-Performance Li-Polysulfide Full Cells by using a Lithium Bis(trifluoromethanesulfonyl)imide Salt Electrolyte for Stable Cyclability

Since concentrated electrolytes have attracted great attention for the stabilization of lithium-metal anodes for lithium-ion batteries, the demonstration of a full cell with an electrolyte concentration study has become a research topic of interest. Herein, we have demonstrated a proof of concept, a lithium-polysulfide full cell battery using various lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte concentrations with glass-fiber-based composite and hard carbon as the cathode and anode, respectively. The initial capacity of the lithium-polysulfide full cell is found to be 970 mA h g^-1 at 0.1 C. The capacity is stabilized at 870 mA h g^-1 after 100 cycles with a capacity retention of 88.6 %. An excellent capacity retention of ≈80 % is achieved after long 800 cycles at 0.5 C by using full cell technology. Further, our post-mortem analysis sheds light on the difference in SEI layer formation on hard carbon anodes with changing electrolyte concentration, thereby indicating reasons for the obtainment of a high cyclic performance with 1 m LiTFSI salt electrolyte. The successful demonstration of the long cyclic performance of Li-polysulfide full cells is indeed a step towards producing high performance Li-polysulfide full cell batteries with long cycling using conventional LiTFSI salt electrolyte and commercial anode materials.