Synthesis of a polyacrylonitrile/tetrachloro-1,4-benzoquinone gel polymer electrolyte for high-performance Li-air batteries

Enhancing the Performance of Lithium-Oxygen Batteries with Quasi-Solid Polymer Electrolytes

The quasi-solid electrolyte membranes (QSEs) are obtained by solidifying the precursor of unsaturated polyester and liquid electrolyte in a glass fiber. By modifying the ratio of tetraethylene glycol dimethyl ether, QSE with balanced ionic conductivity, flexibility, and electrochemical stability window is acquired, which is helpful for inhibiting the decomposition of electrolyte on the cathode surface. The QSE is beneficial to the interfacial reaction of Li+, electrons, and O2 in the quasi-solid lithium-oxygen battery (LOB), can reduce the crossover of oxygen to the anode, and extend the cycle life of LOBs to 317 cycles. Benefitting from the application of QSE, a more stable solid electrolyte interface layer can be constructed on the anode side, which can homogenize Li+ flux and facilitate uniform Li deposition. Lithium-oxygen pouch cell with in situ formed QSE2 works well when the cell is folded or a corner is cut off. Our results indicate that the QSE plays important roles in both the cathode and Li metal anode, which can be further improved with the in situ forming strategy.

Polyacrylonitrile Porous Membrane-Based Gel Polymer Electrolyte by In Situ Free-Radical Polymerization for Stable Li Metal Batteries

Because of their high ionic conductivity, utilizing gel polymer electrolytes (GPEs) is thought to be an effective way to accomplish high-energy-density batteries. Nevertheless, most GPEs have poor adaptability to Ni-rich cathodes to alleviate the problem of inevitable rapid capacity decay during cycling. Therefore, to match LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811), we applied pentaerythritol tetraacrylate (PETEA) monomers to polymerize in situ in a polyacrylonitrile (PAN) membrane to obtain GPEs (PETEA-TCGG-PAN). The impedance variations and key groups during the in situ polymerization of PETEA-TCGG-PAN are investigated in detail. PETEA-TCGG-PAN with a high lithium-ion transference number (0.77) exhibits an electrochemical decomposition voltage of 5.15 V. Noticeably, the NCM811|PETEA-TCGG-PAN|Li battery can cycle at 2C for 120 cycles with a capacity retention rate of 89%. Even at 6C, the discharge specific capacity is able to reach 101.47 mAh g^-1. The combination of LiF and Li₂CO₃ at the CEI interface is the reason for the improved rate performance. Moreover, when commercialized LFP is used as the cathode, the battery can also cycle stably for 150 cycles at 0.5C. PETEA and PAN can together foster the transportation of Li⁺ with the construction of a fast ion transport channel, making a contribution to stable charge-discharge of the above batteries. This study provides an innovative design philosophy for designing in situ GPEs in high-energy-density lithium metal batteries.

An innovative approach towards the simultaneous enhancement of the oxygen reduction and evolution reactions using a redox mediator in polymer based Li-O2 batteries

For safety concerns, polymer-based Li-O₂ batteries have received more attention than traditional non-aqueous Li-O₂ batteries. However, poor cycling stability, low round trip efficiency, and over charge potential during cycling are the major shortcomings for their future applications. In this work, a soluble redox mediator integrated into a polymer electrolyte provides immediate access to the solid discharged product, lowering the energy barrier for reversible Li₂O₂ generation and disintegration. Moreover, introducing a redox mediator to the polymer electrolyte boosts the ORR during discharge and the OER during the recharge process. The synergistic redox mediator pBQ (1,4 benzoquinone) dramatically reduces the over-potential. A small proportion of pBQ in the polymer electrolyte allows Li₂O₂ to develop in a thin film-like morphology on the cathode surface, resulting in a high reversible capacity of ∼12 000 mA h g^-1 and an extended cycling stability of 100 cycles at 200 mA g^-1 with a cut-off capacity of 1000 mA h g^-1. The remarkable cell performance is attributed to the fast kinetics of para benzoquinone for the ORR and OER in Li-O₂ batteries. The use of a redox mediator in a polymer electrolyte opens a new avenue for practical Li-O₂ battery applications in achieving low charge potential and excellent energy efficiency.

Composite Cathode Architecture with Improved Oxidation Kinetics in Polymer-Based Li-O2 Batteries

The Li-O₂ battery based on the polymer electrolyte has been considered as the feasible solution to the safety issue derived from the liquid electrolyte. However, the practical application of the polymer electrolyte-based Li-O₂ battery is impeded by the poor cyclability and unsatisfactory energy efficiency caused by the structure of the porous cathode. Herein, an architecture of a composite cathode with improved oxidation kinetics of discharge products was designed by an in situ method through the polymerization of the electrolyte precursor for the polymer-based Li-O₂ battery. The composite cathode can provide sufficient gas diffusion channels, abundant reaction active sites, and continuous pathways for ion diffusion and electron transport. Furthermore, the oxidation kinetics of nanosized discharge products formed in the composite cathode can be improved by hexamethylphosphoramide during the recharge process. The polymer-based Li-O₂ batteries with the composite cathode demonstrate highly reversible capacity when fully charged and a long cycle lifetime under a fixed capacity with low overpotentials. Moreover, the interface contact between hexamethylphosphoramide and the Li metal can be stabilized simultaneously. Therefore, the composite cathode architecture designed in this work shows a promising application in high-performance polymer-based Li-O₂ batteries.

Building Better Batteries in the Solid State: A Review

Most of the current commercialized lithium batteries employ liquid electrolytes, despite their vulnerability to battery fire hazards, because they avoid the formation of dendrites on the anode side, which is commonly encountered in solid-state batteries. In a review two years ago, we focused on the challenges and issues facing lithium metal for solid-state rechargeable batteries, pointed to the progress made in addressing this drawback, and concluded that a situation could be envisioned where solid-state batteries would again win over liquid batteries for different applications in the near future. However, an additional drawback of solid-state batteries is the lower ionic conductivity of the electrolyte. Therefore, extensive research efforts have been invested in the last few years to overcome this problem, the reward of which has been significant progress. It is the purpose of this review to report these recent works and the state of the art on solid electrolytes. In addition to solid electrolytes stricto sensu, there are other electrolytes that are mainly solids, but with some added liquid. In some cases, the amount of liquid added is only on the microliter scale; the addition of liquid is aimed at only improving the contact between a solid-state electrolyte and an electrode, for instance. In some other cases, the amount of liquid is larger, as in the case of gel polymers. It is also an acceptable solution if the amount of liquid is small enough to maintain the safety of the cell; such cases are also considered in this review. Different chemistries are examined, including not only Li-air, Li-O2, and Li-S, but also sodium-ion batteries, which are also subject to intensive research. The challenges toward commercialization are also considered.