3,3'-(Ethylenedioxy)dipropiononitrile as an Electrolyte Additive for 4.5 V LiNi1/3Co1/3Mn1/3O2/Graphite Cells

Multifunctional Electrolyte Additive for Bi-electrode Interphase Regulation and Electrolyte Stabilization in Li/LiNi0.8Co0.1Mn0.1O2 Batteries

Rechargeable lithium metal batteries (LMBs) with high energy densities can be achieved by coupling a lithium metal anode (LMA) and a LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode. Nevertheless, Li dendrite growth on the LMA surface and structural collapse of the NCM811 material, closely tied with the fragile cathode-/solid-electrolyte interphases (CEI/SEI) and corrosive hydrogen fluoride (HF), seriously deteriorate their performances. Herein, trimethylsilyl trifluoroacetate (TMSTFA) as a multifunctional electrolyte additive is proposed for regulation of the CEI/SEI films and elimination of HF. For one thing, the TMSTFA-derived CEI film rich in C-O species is conductive to Li⁺ transport and structural stability of NCM811 materials, and the TMSTFA-derived SEI film mainly consisting of inorganics (Li₂CO₃ and LiF) and organics (C-O and O-C═O species) can significantly promote Li⁺ homogeneous deposition and impede the Li dendrite growth. For another thing, the undesired reactions of the solvents and LiPF₆ salt are effectively retarded by the TMSTFA additive. Consequently, in the presence of TMSTFA, the capacity retention of Li/NCM811 cell is increased by 17% after 200 cycles at 1C, and the lifespan of symmetrical Li/Li cells is prolonged beyond 600 h at 0.5 mA cm^-2.

Adiponitrile (ADN): A Stabilizer for the LiNi0.8Co0.1Mn0.1O2 (NCM811) Electrode/Electrolyte Interface of a Graphite/NCM811 Li-Ion Cell

Adiponitrile (ADN) is studied as an additive of commercial carbonate electrolyte for a graphite/NCM811 Li-ion cell in this work. Compared with the cell using a baseline electrolyte, the cycling stability of the cells using 0.2, 0.5, 1, and 2 wt % ADN-added electrolytes is significantly improved. Among them, the effect is most prominent when the ADN content is 0.5 wt %, and the capacity retention rate of the cell after 200 cycles at 0.3 C is increased by 17.6%. Scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy analyses confirm that ADN can make the coating layer formed on the cathode and anode surfaces of the cell denser and uniform. The formation of the cathodic coating layer does not obey the usual mechanism of adsorption between the nitrile group and metal ions, but it may be due to the polymerization of ADN initiated by carbocations from the oxidation of solvent molecules (ethylene carbonate/ethyl methyl carbonate) in the electrolyte. The cathodic coating layer greatly decreases the oxidation of the electrolyte so that a stable coating layer (also called solid electrolyte interface/SEI) can be formed on the anode, which is the reason for improvement of the cycling performance of the cell. However, the formation of the anode SEI layer is almost independent of ADN, and it still depends on the common reduction reaction of the electrolyte.

Electrochemical Property Enhancement of LiNi0.5Mn1.5O4 Cathodes at High Temperatures Using 1,1,3,3-Tetramethyldisiloxane

In this work, we propose that 1,1,3,3-tetramethyldisiloxane (TMDS) is beneficial for electrochemical property enhancement of LiNi_0.5Mn_1.5O₄/Li cells at high temperatures. The LiNi_0.5Mn_1.5O₄/Li cells with 1 vol % TMDS revealed capacity retention of 81.2% after a cycling test at 55 °C, while the cells without additive showed capacity retention of only 32.3%. The cells with 1 vol % TMDS also presented a better rate performance, reaching 100 mAh g^-1 under 3C. Physical characterization and theoretical calculations revealed that TMDS formed a thinner and better conductive layer on the LiNi_0.5Mn_1.5O₄ surface and effectively scavenged HF/F^- from the electrolyte, contributing to high stabilization of LiNi_0.5Mn_1.5O₄.

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.

Exploiting the Degradation Mechanism of NCM523 ∥ Graphite Lithium-Ion Full Cells Operated at High Voltage

Layered oxides, particularly including Li[Nix Coy Mnz ]O2 (NCMxyz) materials, such as NCM523, are the most promising cathode materials for high-energy lithium-ion batteries (LIBs). One major strategy to increase the energy density of LIBs is to expand the cell voltage (>4.3 V). However, high-voltage NCM ∥ graphite full cells typically suffer from drastic capacity fading, often referred to as "rollover" failure. In this study, the underlying degradation mechanisms responsible for failure of NCM523 ∥ graphite full cells operated at 4.5 V are unraveled by a comprehensive study including the variation of different electrode and cell parameters. It is found that the "rollover" failure after around 50 cycles can be attributed to severe solid electrolyte interphase growth, owing to formation of thick deposits at the graphite anode surface through deposition of transition metals migrating from the cathode to the anode. These deposits induce the formation of Li metal dendrites, which, in the worst cases, result in a "rollover" failure owing to the generation of (micro-) short circuits. Finally, approaches to overcome this dramatic failure mechanism are presented, for example, by use of single-crystal NCM523 materials, showing no "rollover" failure even after 200 cycles. The suppression of cross-talk phenomena in high-voltage LIB cells is of utmost importance for achieving high cycling stability.

Stabilizing a High-Voltage Lithium-Rich Layered Oxide Cathode with a Novel Electrolyte Additive

We report a novel electrolyte additive, bis(trimethylsilyl)carbodiimide, that effectively stabilizes high-voltage lithium-rich oxide cathode. Charge/discharge tests demonstrate that even trace amounts of bis(trimethylsilyl)carbodiimide in a baseline electrolyte improve the cycling stability of this cathode significantly, either in Li-based half cells or graphite-based full cells, where the capacity retention after 200 cycles between 2 and 4.8 V at 0.5C is enhanced from 40 to 72% and 49 to 77%, respectively. Analyses using physical characterization and theoretical calculations reveal that this additive not only builds a protective film on the cathode but also eliminates detrimental hydrogen fluoride via its strong coordination with hydrogen fluoride or protons.

Trimethyl Borate as Film-Forming Electrolyte Additive To Improve High-Voltage Performances

Enhancing the stability of the interface between the electrode and electrolyte at high voltages is crucial concerning the development of Li-ion batteries with high energy densities. Application of some additives in the electrolyte is not only the simplest but also the most effective way to form a protection layer against the electrolyte decomposition and the electrolyte corrosion to the electrode. Herein, we introduce trimethyl borate (TMB) as an additive of the commercial electrolyte to ameliorate the performance of a LiCoO₂ cell charged to 4.5 V because its addition lowers the oxidation potential of the baseline electrolyte (3.75 V vs 4.25 V). By being oxidized preferentially and thus forming a compact protection layer of about 25 nm thick on the cathode surface, the additive suppresses the electrolyte decomposition and protects the LiCoO₂ cathode against the structural degradation. The capacity retention of the cell after 100 cycles between 2.5 and 4.5 V at 0.1 C increases from 64 to 81% when 2.0 wt % TMB is added into the baseline electrolyte. The X-ray photoelectron spectroscopic results demonstrate the oxidation of TMB on the cathode and therefore the suppressed decomposition of the electrolyte. The results of the X-ray diffraction and Raman spectroscopy show the better structural maintenance of the LiCoO₂ material in the TMB-containing electrolyte. The protection mechanism of the TMB additive was comprehensively studied.

Constructing Unique Cathode Interface by Manipulating Functional Groups of Electrolyte Additive for Graphite/LiNi0.6Co0.2Mn0.2O2 Cells at High Voltage

A novel electrolyte additive, 1-(2-cyanoethyl) pyrrole (CEP), has been investigated to improve the electrochemical performance of graphite/LiNi_0.6Co_0.2Mn_0.2O₂ cells cycling up to 4.5 V vs Li/Li⁺. The 4.5 V cycling results present that after 50 cycles, up to 4.5 V capacity retention of the graphite/LiNi_0.6Co_0.2Mn_0.2O₂ cell is improved significantly from 27.4 to 81.5% when adding 1% CEP to baseline electrolyte (1 M LiPF₆ in EC/EMC 1:2, by weight). Ex situ characterization results support the mechanism of CEP for enhancing the electrochemical performance. On one hand, the significant enhancement is ascribed to a formed superior cathode interfacial film by preferential oxidation of CEP on the cathode electrode surface suppressing electrolyte decomposition at high voltage. On the other hand, the duo Lewis base functional groups can effectively capture dissociation product PF₅ from LiPF₆ with the presence of an unavoidable trace amount of water or aprotic impurities in the electrolyte. Thus this mitigates the hydrofluoric acid (HF) generation that leads to the reduction of transition-metal dissolution in the electrolyte upon cycling at high voltage. The theoretical modeling suggests that CEP has a mechanism of stabilizing electrolyte via combination of -C≡N: functional group and H₂O. The work presented here also shows nuclear magnetic resonance spectra analysis to prove the capability of CEP reducing HF generation and X-ray photoelectron spectroscopy analysis to observe cathode surface composition.