Rechargeable Solid‐State Li Batteries Utilizing Polyphosphazene‐Poly(Ethylene Oxide) Mixed Polymer Electrolytes

Abstract

Rechargeable, solid-state Li batteries employing polymer electrolytes are being actively pursued as possible power sources for a variety of applications (1-3). Complexes of Li salts such as LiCF~SO3, LiCI04 or LiBF 4 with poly-(ethylene oxi~e)-(PEO) have been the most widely studied polymer electrolytes in cells utilizing Li anodeand a rechargeable cathode such as TiS~ (I) or V6013 (3).Because of their low Li conductivity, cells employing PEO electrolytes exhibit reasonable rate capabilities only at around lO0~The discovery (4) that the Li + salt complexes of poly[bis(methoxyethoxyethoxide)phosphazene], or MEEP, have significantly higher ambient temperature Li + conductivities than the PEO-based electrolytes offered potential opportunites for the fabrication of polymer electrolyte Li batteries for operation at or near room temperature.However, the liquidlike properties of the MEEP electrolyte, i.e., its tendency to flow, although desirable from conductivity point of view, prevented the preparation of dimensionally stable, thinfilm electrolytes, necessary for the construction of practical solid-state batteries.We report here on a novel approach to utilize the excellent conducting properties of the MEEP-based electrolytes in solid-state Li batteries.We have blended PEO with MEEP to enhance the latter's dimensional stability, enabling the fabrication of thin-film electrolytes with significantly higher conductivity than pure PEO electrolytes.Initial results suggest that this approach can lead to solidstate Li cells with significantly improved discharge rate and cycling capabilities than those possible by the use of presently available polymer electrolytes.MEEP, doped with LiCF3SO 3 or LiClO 4 in a 4:1 mole ratio for the monomer repeat unit to Li +, was prepared according to_published procedures (4,5). PEO (MW = 5 x i0 b) was obtained from Polyscience, Inc. The MEEP and PEO were mixed in 50:50 and 80:20 weight ratios in *Electrochemical Society Active Member. Key Words: Solid-state lithium batteries. 535 acetonitrile. Li salts were added so that the O/Li + ratio in the above mixtures was 8 for the amount of PEO present.Films were cast by pouring the acetonitrile solutions of the above mixtures onto Teflon dishes and slowly evaporating the solutions at room temperature inside an Ar-filled glove box. The resulting films, having a thickness of-4 mil, were dried under vacuum at 100oc for 3 to 4 hours. Conductivity measurements at selected temperatures were carried out using the ac impedance technique with stainless steel ionblocking electrodes between 5 Hz and 200 kHz. ExperimentalLi/TiS 2 cells were constructed with the polymer electrolyte sandwiched between a Li foil anode pressed onto Ni screen and a TiS~ composite cathode. These cathodes of 2 miJ thickness and ~1.5 mAh/cm 2 theoretical capacity were prepared by spreading a mixture of 80 w/o TiS 2, prepared inhouse, and 20 w/o PEO/LiCIO 4 onto one side of a Ni foil. All the cells were constructed cathode-limited. The electrode package was secured between stainless stee...

Poly(ethylene oxide)-based electrolytes for lithium-ion batteries

“…The ionic conductivity of MEEP-LiTf electrolyte was 1-3 orders of magnitude larger than that of PEO systems. Based on advantages of polyphosphazene, the electrochemical properties of MEEP electrolyte, [405][406][407][408][409] MEEP derivatives electrolytes, [410][411][412][413] and MEEP/PEO blend systems 195,414,415 were also studied.…”

Polyphosphazenes Functionalized with Sulfone or Sulfoxide Groups: Synthesis, Characterization, and Possible Polymer Electrolyte Applications

“…Secondary lithium batteries using solid polymer electrolytes (SPEs) have been investigated since 1978, , following the report by Wright of the formation of complexes between poly(ethylene oxide) (PEO) and potassium thiocyanate salts. − In 1984, Blonsky, Shriver, Austin, and Allcock reported the synthesis and study of poly[bis(2-(2-methoxyethoxy)ethoxy)phosphazene] (MEEP, 1 ) as a low glass-transition temperature, amorphous polymer which overcomes many of the shortcomings associated with PEO. , When complexed to LiSO 3 CF 3 , MEEP has an ambient temperature ionic conductivity in the range of 10 -5 S/cm, which is 2−3 orders of magnitude higher than that for PEO. In recent years, more than 30 new polyphosphazenes for ionic conduction have been synthesized in our program, including species with an increased number of oxygen atoms per side unit, others that utilize branched rather than linear oligo(oxyethylene) side groups, examples that use non-ion-coordinating cosubstituents, and polymers which incorporate crown ethers. − MEEP−PEO blends have also been investigated. , Some of these systems improve the ionic conductivity and have higher dimensional stability than MEEP. Furthermore, polyphosphazene-based solid electrolytes have been shown to be stable to lithium metal anodes …”

Preparation and Thermal Properties of Poly(phosphazene-aryl ester) Containing Cyclotriphosphazene Structures

“…Polyphosphazene as an organic-inorganic hybrid polymer is a class of novel macromolecules containing alternate phosphorus-nitrogen single and double bonds with two changing organic side groups to be prepared new specific functions materials. Because the phosphorus-nitrogen bonds are extremely flexible due to the low torsional energy and a large variety of side groups, polyphosphazenes can be made with a wide range of chemical and physical properties, like excellent heat resistance, cold-resistant, water-tolerant, solvent resistance, radicalization resistance and flame retardant performances in many areas [1][2][3][4][5][6][7] .…”