**Significance Statement**

^{2+}ions for advanced solid state rechargeable magnesium batteriesis of potential interest.A number of reasons make Mg a good alternative to Li: apart from its electrochemical equivalence of 2.2 Ahg

^{-1}and its electrode potential of -2.37 V

*versus*SHE, Mg is non-toxic, non-explosive, lightweight, its mineral sources are abundant and it is cheap to produce. Furthermore, as the ionic radii of Li

^{+}and Mg

^{2+}are close in magnitude (68 and 65 pm, respectively), in principle the insertion compounds that have already been produced for Li cells may be used in Mg-based batteries.In this work two families of sol-gel derived di-urethane crossed-linkedsiloxane-based frameworks incorporating short poly(oxyethylene) (POE) chains were doped with a wide concentration range of magnesium triflate (Mg(CF

_{3}SO

_{3})

_{2}) (2 ≤ n < ∞,where n is the molar ratio of OCH

_{2}CH

_{2}units per Mg

^{2+}). The resulting xerogels were represented by the notation d-Ut(Y)

_{n}Mg(CF

_{3}SO

_{3})

_{2}with Y = 300 and 600, where Y represents the average molecular weight of the POE chains in gmol

^{-1}. The d-Ut(300)

_{n}Mg(CF

_{3}SO

_{3})

_{2}and d-Ut(600)

_{n}Mg(CF

_{3}SO

_{3})

_{2}materials with n > 6 and n > 5, respectively, are amorphous.The ormolytes with the highest conductivity of the d-Ut(300)

_{n}Mg(CF

_{3}SO

_{3})

_{2 }andd-Ut(600)

_{n}Mg(CF

_{3}SO

_{3})

_{2 }series are the samples with n = 6 (3.9×10

^{-8}Scm

^{-1 }at 26 ºCand 8.7×10

^{-5}Scm

^{-1}at 97 ºC) and n = 100 (2.63×10

^{-7 }Scm

^{-1 }at 20 ºC and 1.4×10

^{-5 }Scm

^{-1 }at 85 ºC), respectively. Since the electrolytes for Mg batteries that have been proposed up to now have many intrinsic problems and although the room temperature conductivity values exhibited by the systems developed in the present study are still low in view of practical application, this work opens new prospects for the development of solid state Mg-ion electrolytes.

Ionics, 2014, Volume 20, Issue 1, pp 29-36.

M. C. Gonçalves, L. C. Rodrigues, M. M. Silva, R. A. Sá Ferreira, L. D. Carlos, J. Hümmer, V. de Zea Bermudez.

Departamento de Química and CQ-VR, Universidade de Trás-os-Montes e Alto Douro, 5001-801, Vila Real, Portugal and

Departamento de Química, Universidade do Minho, Gualtar, 4710-057, Braga, Portugal and

Departamento de Física and CICECO, Universidade de Aveiro, 3810-193, Aveiro, Portugal.

## Abstract

Two siloxane-based di-urethanesil frameworks incorporating poly(oxyethylene) (POE) chains have been synthesized by the sol–gel process and doped with magnesium triflate (Mg(CF_{3}SO_{3})_{2}) with the goal of developing electrolytes for the fabrication of solid-state rechargeable magnesium batteries. In these matrices, short POE chains are covalently bonded to the siloxane network via urethane linkages. The xerogels have been represented by the notation d-Ut(*Y*)_{ }_{n}_{ }Mg(CF_{3}SO_{3})_{2}, where *Y* = 300 and 600 represents the average molecular weight of the POE chains and *n* stands for salt composition (molar ratio of OCH_{2}CH_{2} units per Mg^{2+}). Xerogels with compositions ranging from 2 ≤ *n* < ∞ were prepared. A crystalline POE/Mg(CF_{3}SO_{3})_{2} complex of unknown stoichiometry is formed in the d-Ut(300)_{ }_{n}_{ }Mg(CF_{3}SO_{3})_{2} materials with *n* ≤ 6 and in the d-Ut(600)_{ }_{n}_{ }Mg(CF_{3}SO_{3})_{2} materials with*n* ≤ 5. The organically modified silicate electrolytes with the highest conductivity of the d-Ut(300)_{ }* _{n}*Mg(CF

_{3}SO

_{3})

_{2}and d-Ut(600)

_{ }

_{n}_{ }Mg(CF

_{3}SO

_{3})

_{2}series are the samples with

*n*= 6 (3.9 × 10

^{−8}S cm

^{−1}at 26 °C and 8.7 × 10

^{−5}S cm

^{−1}at 97 °C) and

*n*= 100 (2.63 × 10

^{−7}S cm

^{−1}at 20 °C and 1.4 × 10

^{−5}S cm

^{−1}at 85 °C), respectively. Since the electrolytes for Mg batteries that have been proposed up to now have many intrinsic problems and although the room temperature conductivity values exhibited by the systems developed in the present study are still low in view of practical application, this work opens new directions for the development of solid-state Mg ion electrolytes.