Effects of Flexible Group Length of Phosphonate Monomers on the Performance of Gel Polymer Electrolytes for Sodium-Ion Batteries with Ultralong Cycling Life

Abstract

Using gel polymer electrolytes instead of liquid electrolytes is one of the methods to avoid safety hazards such as leakage, decomposition, and dendrite growth caused by conventional electrolytes. In this work, a series of functional monomers of methyl phosphonate with different flexible-group chain lengths are devised and synthesized. On the basis of these phosphonates, the gel copolymer electrolytes are obtained by in situ thermal copolymerization with trifluoroethyl methacrylate and methyl methacrylate. All gel copolymer electrolytes have excellent thermal stabilities, wide electrochemical stable windows (≥5.0 V vs Na/ Na + ), and adequate ionic conductivities (>1.4 × 10 −3 S cm −1 ), and the sodium ion conduction mechanism is discussed and supplemented by a theoretical calculation. The assembled Na 3 V 2 (PO 4 ) 3 /Na batteries with gel copolymer electrolytes all have better ultralong cycling stabilities than liquid cells with higher capacity retentions above 76% after 5000 cycles. Furthermore, the Na 3 V 2 (PO 4 ) 3 /GPE1−GPE3/Na batteries with shorter lengths of flexible-group chains have much better cycling stabilities and much higher capacity retentions up to 87% after 5000 cycles, and the Na 3 V 2 (PO 4 ) 3 /GPE1−GPE2/Na batteries with shorter lengths of flexible-group chains have better rate performance than GPE4-based batteries with longer chain lengths. In addition, the polarization properties, the interfacial compositions obtained by XPS, and the morphology characterizations of separators after cycling show that the gel polymer electrolytes with short-chain monomers have better interfacial stabilities and better capabilities of inhibiting decomposition of liquid electrolytes. In summary, the influence of multifunctional phosphonate monomers with different chain lengths on battery performance is studied in detail, and the structure−activity relationships of phosphonate-based gel polymer electrolytes are discussed. That provides a reference for the design and preparation of gel polymer electrolytes with better performance.

Non-flammable Gel Polymer Electrolyte for Enhancing the Safety and High-Temperature Performance of Lithium-Ion Batteries

“…To date, various approaches have been introduced for the development of nonflammable electrolytes, such as inorganic electrolytes, flame retardant additives and electrolytes, and solid-state and gel polymer electrolytes (GPEs). − Among them, chemically cross-linked GPEs can effectively immobilize flammable organic solvents within the polymer matrix and ensure good interfacial contacts between the electrolyte and electrodes as well as high ionic conductivity due to the presence of LEs. , Particularly, nonflammable GPEs employing a flame-retardant plasticizer or cross-linked polymer as flame-retardant materials are being extensively explored to enhance the battery safety. − When preparing a nonflammable GPE, organic phosphorus- and fluorinated-based flame-retardant solvents have been studied as plasticizers since they mitigate the risk of ignition of LEs. − For example, phosphorus compounds such as trimethyl phosphate, , triethyl phosphate, tripropyl phosphate, and dimethyl methylphosphonate are commonly employed due to their high fire-extinguishing property and high ionic conductivity. However, these phosphide compounds can trigger deleterious reactions at the graphite anode during cycling, leading to the formation of an unstable solid–electrolyte interface and degradation of the battery performance .…”

“…However, they also have limitations such as high cost and low reductive stability . Among many strategies to address the electrochemical instability, the use of phosphorus compounds as a cross-linking agent, which contain flame-retarding moieties on the polymer backbone, can be a promising method. − …”

In situ synthesis of cross-linking gel polymer electrolyte for lithium metal batteries

“…Functional methacrylates are commonly introduced into PEO to provide reaction sites for radical initiated polymerization [31,32]; it is precisely due to the presence of ester functional groups that polymer electrolytes are endowed with higher electrochemical performance [33]. Compared with traditional PEO-based solid state electrolytes, polycarbonate-based electrolytes are a focus of attention in the field of solid-state polymer electrolytes due to their excellent electrochemical performance and better compatibility with anodes [34][35][36][37].…”