Stabilized Electrode/Electrolyte Interphase by a Saturated Ionic Liquid Electrolyte for High-Voltage NMC532/Si-Graphite Cells

Maleic Anhydride and (Ethoxy)pentafluorocyclotriphosphazene as Electrolyte Additives for High-Voltage LiCoO2/Si-Graphite Lithium-Ion Batteries

Anhydride additives including maleic anhydride and succinic anhydride are initially selected as additives in the commercial electrolytes for high-voltage lithium-ion batteries with a Si-based anode. The introduction of (ethoxy)pentafluorocyclotriphosphazene as a flame retardant realizes the nonflammability of electrolytes, and the conductivity of electrolytes exceeds 10 mS cm-1 at 25 °C. Maleic anhydride and (ethoxy)pentafluorocyclotriphosphazene jointly contribute to the exceptional performances of 4.45 V LiCoO2/Si-graphite pouch cells at 25 °C. The capacity retention at 1C of 300 cycles reaches 78%, and the discharge capacity ratio of 6C/1C is approximately 83%. These results suggest that this nonflammable electrolyte has good application prospect. Scanning electron microscopy and X-ray photoelectron spectroscopy measurements are implemented to analyze the interface properties of electrodes.

Unprotected Organic Cations─The Dilemma of Highly Li-Concentrated Ionic Liquid Electrolytes

Highly Li-concentrated electrolytes have been widely studied to harness their uniquely varying bulk and interface properties that arise from their distinctive physicochemical properties and coordination structures. Similar strategies have been applied in the realm of ionic liquid electrolytes to exploit their improved functionalities. Despite these prospects, the impact of organic cation behavior on interfacial processes remains largely underexplored compared to the widely studied anion behavior. The present study demonstrates that the weakened interactions between cations and anions engender "unprotected" organic cations in highly Li-concentrated ionic liquid electrolytes, leading to the decomposition of electrolytes during the initial charge. This decomposition behavior is manifested by the substantial irreversible capacities and inferior initial Coulombic efficiencies observed during the initial charging of graphite negative electrodes, resulting in considerable electrolyte consumption and diminished energy densities in full-cell configurations. The innate cation behavior is ascertained by examining the coordination environment of ionic liquid electrolytes with varied Li concentrations, where intricate ionic interactions between organic cations and anions are unveiled. In addition, anionic species with high Lewis basicity were introduced to reinforce the ionic interactions involving organic cations and improve the initial Coulombic efficiency. This study verifies the role of unprotected organic cations while highlighting the significance of the coordination environment in the performance of ionic liquid electrolytes.

Exploring the Promise of Multifunctional "Zintl-Phase-Forming" Electrolytes for Si-Based Full Cells

Li-M-Si ternary Zintl phases have gained attention recently due to their high structural stability, which can improve the cycling stability compared to a bulk Si electrode. Adding multivalent cation salts (such as Mg²⁺ and Ca²⁺) in the electrolyte was proven to be a simple way to form Li-M-Si ternary phases in situ in Si-based Li-ion cells. To explore the promise of Zintl-phase-forming electrolytes, we systematically investigated their application in pouch cells via electrochemical and multiscale postmortem analysis. The introduction of multivalent cations, such as Mg²⁺, during charging can form Li_xM_ySi ternary phases. They can stabilize Si anions and reduce side reactions with electrolyte, improving the bulk stability. More importantly, Mg²⁺ and Ca²⁺ incorporate into interfacial side reactions and generate inorganic-rich solid-electrolyte interphase, thus enhancing the interfacial stability. Therefore, the full cells with Zintl-phase-forming electrolytes achieve higher capacity retentions at the C/3 rate after 100 cycles, compared to a baseline electrolyte. Additionally, strategies for mitigating the electrode-level fractures of Si were evaluated to make the best use of Zintl-phase-forming electrolytes. This work highlights the significance of synergistic impact of multifunctional additives to stabilize both bulk and interface chemistry in high-energy Si anode materials for Li-ion batteries.

Understanding the Capacity Decay of Si/NMC622 Li-Ion Batteries Cycled in Superconcentrated Ionic Liquid Electrolytes: A New Perspective

Silicon-containing Li-ion batteries have been the focus of many energy storage research efforts because of the promise of high energy density. Depending on the system, silicon generally demonstrates stable performance in half-cells, which is often attributed to the unlimited lithium supply from the lithium (Li) metal counter electrode. Here, the electrochemical performance of silicon with a high voltage NMC622 cathode was investigated in superconcentrated phosphonium-based ionic liquid (IL) electrolytes. As a matter of fact, there is very limited work and understanding of the full cell cycling of silicon in such a new class of electrolytes. The electrochemical behavior of silicon in the various IL electrolytes shows a gradual and steeper capacity decay, compared to what we previously reported in half-cells. This behavior is linked to a different evolution of the silicon morphology upon cycling, and the characterization of cycled electrodes points toward mechanical reasons, complete disconnection of part of the electrode, or internal mechanical stress, due to silicon and Li metal volume variation upon cycling, to explain the progressive capacity fading in full cell configuration. An extremely stable solid electrolyte interphase (SEI) in the full Li-ion cells can be seen from a combination of qualitative and quantitative information from transmission electron microscopy, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and magic angle spinning nuclear magnetic resonance. Our findings provide a new perspective to full cell interpretation regarding capacity fading, which is oftentimes linked almost exclusively to the loss of Li inventory but also more broadly, and provide new insights into the impact of the evolution of silicon morphology on the electrochemical behavior.

Ionic Liquid-Type Additive for Lithium Metal Batteries Operated in LiPF6 Based-Electrolyte Containing 2500 ppm H2O

The presence of trace amounts of moisture in the electrolyte can cause hydrolysis of LiPF₆ and deteriorate the stability of lithium metal batteries. Herein, we propose a multifunctional ionic liquid-type additive constituting a 1-methyl-1-butyl pyrrolidium cation (Py₁₄⁺) and an acetate anion (CH₃COO^-) (denoted as IL-AC in this study), which can effectively adsorb the trace moisture and thus prevent the hydrolysis of LiPF₆ via intermolecular interactions. The prepared IL-AC can also remove HF to suppress the dissolution of transition metal ions from cathode materials through the reaction CH₃COO^- + HF → CH₃COOH + F^-. Compared with the baseline electrolyte, the contents of HF and transition metal ions are significantly lower in the electrolyte with 0.5% IL-AC. Upon the addition of 0.5% IL-AC additive and 2500 ppm H₂O, the Li||NCM811 battery shows a capacity of 153.7 mAh g^-1 after 300 cycles, while the Li||LNMO battery possesses stable capacity retention of 93.22% after 500 cycles at 1C and a Coulombic efficiency greater than 99%. Thus, this work provides a convenient and effective method to absorb trace amounts of water and remove HF in the electrolyte and provides a new path for the expensive and tedious process of water removal from the electrolyte in industry.

Concentrated LiFSI-Ethylene Carbonate Electrolytes and Their Compatibility with High-Capacity and High-Voltage Electrodes

The unusual physical and chemical properties of electrolytes with excessive salt contents have resulted in rising interest in highly concentrated electrolytes, especially for their application in batteries. Here, we report strikingly good electrochemical performance in terms of conductivity and stability for a binary electrolyte system, consisting of lithium bis(fluorosulfonyl)imide (LiFSI) salt and ethylene carbonate (EC) solvent. The electrolyte is explored for different cell configurations spanning both high-capacity and high-voltage electrodes, which are well known for incompatibilities with conventional electrolyte systems: Li metal, Si/graphite composites, LiNi_0.33Mn_0.33Co_0.33O₂ (NMC111), and LiNi_0.5Mn_1.5O₄ (LNMO). As compared to a LiTFSI counterpart as well as a common LP40 electrolyte, it is seen that the LiFSI:EC electrolyte system is superior in Li-metal-Si/graphite cells. Moreover, in the absence of Li metal, it is possible to use highly concentrated electrolytes (e.g., 1:2 salt:solvent molar ratio), and a considerable improvement on the electrochemical performance of NMC111-Si/graphite cells was achieved with the LiFSI:EC 1:2 electrolyte both at the room temperature and elevated temperature (55 °C). Surface characterization with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) showed the presence of thicker surface film formation with the LiFSI-based electrolyte as compared to the reference electrolyte (LP40) for both positive and negative electrodes, indicating better passivation ability of such surface films during extended cycling. Despite displaying good stability with the NMC111 positive electrode, the LiFSI-based electrolyte showed less compatibility with the high-voltage spinel LNMO electrode (∼4.7 V vs Li⁺/Li).

An Environmentally Benign Electrolyte for High Energy Lithium Metal Batteries

A hybrid electrolyte comprising a high content of H₂O for a lithium metal cell is reported. At high LiFSI salt concentration, the N-methyl-N-propyl-piperidinium bis(fluorosulfonyl) imide (PMpipFSI) electrolyte can tolerate up to 1 M H₂O addition without sacrificing its redox stability on both lithium nickel manganese cobalt oxide (NMC) cathode and lithium metal anode. Molecular dynamics simulations revealed the underpinned mechanism that, at high salt concentrations, H₂O molecules are embedded in the Li⁺, PMpip⁺, and FSI^- bulk as a structural material with a strong solvation with Li⁺ and are orderly distributed at the surface of both electrodes. This electrolyte eliminates the critical moisture controls required for the state-of-the-art (SOA) carbonate/LiPF₆ electrolyte, electrode, separator and cell assembly, thus significantly reducing the cost of the mass production of the batteries.

Tetraethylthiophene-2,5-diylbismethylphosphonate: A Novel Electrolyte Additive for High-Voltage Batteries

In this work, a novel high-voltage electrolyte additive, tetraethylthiophene-2,5-diylbismethylphosphonate (TTD), was synthesized, and the influence of TTD on the electrolyte and its electrochemical performance under different voltages were studied by changing the content of the TTD additive. The results showed that the TTD additive significantly improved the capacity, cycle stability, and rate capability of batteries when charging/discharging at high voltages. After adding 1 % TTD to the basic electrolyte, the capacity retention rate of batteries after 200 cycles at 4.2, 4.3, 4.4, and 4.5 V increased by 20.8, 18.3, 50, and 31.9 %, respectively. In addition, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) results showed that TTD could effectively inhibit the decomposition of the electrolyte and participate in the formation of a uniform, thin, and stable cathode electrolyte interphase (CEI) film on the electrode surface, thereby effectively inhibiting the side reaction between the electrolyte decomposition product and the CEI membrane, and finally improving the high-voltage performance of the battery. The TTD additive may provide a cost-effective solution for high-performance high-voltage electrolytes.

Abuse-Tolerant Electrolytes for Lithium-Ion Batteries

Safety issues currently limit the development of advanced lithium-ion batteries (LIBs) and this is exacerbated when they are misused or abused. The addition of small amounts of fillers or additives into common liquid electrolytes can greatly improve resistance to abuse without impairing electrochemical performance. This review discusses the recent progress in such abuse-tolerant electrolytes. It covers electrolytes with shear thickening properties for tolerating mechanical abuse, electrolytes with redox shuttle additives for suppressing electrochemical abuse, and electrolytes with flame-retardant additives for resisting thermal abuse. It aims to provide insights into the functioning of such electrolytes and the understanding of electrolyte composition-property relationship. Future perspectives, challenges, and opportunities towards practical applications are also presented.

A fluorine-substituted pyrrolidinium-based ionic liquid for high-voltage Li-ion batteries

A fluorine-substituted ionic liquid based on a pyrrolidinium cation and a bis(fluorosulfonyl)imide anion was synthesized using a facile one-step reaction. The resulting ionic liquid is highly pure and when dissolved with LiFSI, the IL-based electrolyte showed good compatibility both in Li and graphite anodes, and superior voltage stability is demonstrated in a LiNi_0.5Mn_0.3Co_0.2O₂ cell.