N7-(carboxymethyl)guanine-Lithium Crystalline Complex: A Bioinspired Solid Electrolyte

Electrochemical device with components having direct significance to biological life processes is a potent futuristic strategy for the realization of all-round green and sustainable development. We present here synthesis design, structural analysis and ion transport of a novel solid organic electrolyte (G7Li), a compound reminiscent of ion channels, derived from regioisomeric N7-guanine-carboxylate conjugate and Li-ions. G7Li, with it’s in-built supply of Li+-ions, exhibited remarkably high lithium-ion transference number (= 0.75) and tunable room temperature ionic conductivity spanning three decades (≈10−7 to 10−3 Ω−1 cm−1) as a function of moisture content. The ionic conductivity show a distinct reversible transition around 80–100 °C, from a dual Li+ and H+ (<100 °C) to a pure Li+ conductor (>100 °C). Systematic studies reveal a transition from water-assisted Li-ion transport to Li hopping-like mechanism involving guanine-Li coordination. While as-synthesized G7Li has potential in humidity sensors, the anhydrous G7Li is attractive for rechargeable batteries.


Synthesis of I 1 :
Intermediate I 1 is synthesized by the literature procedure of Liu et al 3 .

Ionic conductivity from ac-impedance spectroscopy
Ionic conductivity is obtained from ac-impedance spectroscopy by scanning the sample in the frequency range 0.01-10 6 Hz (signal amplitude: 0.05 V). The pellet (diameter = 10.1 mm; thickness = 1.2 mm) of G7Li powder is sandwiched between two stainless steel electrodes in a home-built cell at ambient condition (25 °C). In order to check the reproducibility of the data and the compound stability, data are collected for two set of heating-cooling cycle on the same pellet.
The G7Li pellet for the conductivity measurement is made at ambient condition (temperature ~25 o C; pressure applied = 50 kg/m 2 ). For the improvement in the interfacial contacts between the surfaces of G7Li pellet and stainless steel (SS) electrodes both surfaces of the pellet are coated with a thin layer of gold. The impedance data typically in the low conductivity regime comprised of a single semicircle. This data could be approximately fitted by a resistance (R1) and CPE1 (= R n-1 C n ) in parallel 4-7 . In the high conductivity regime (i.e. (200-300) o C and around room temperature) the impedance data comprised of a depressed semicircle and a "spike-like" at high and low frequency region respectively. The impedance data in the high conductivity regimes are fitted to a series combination of resistance (R 1 ) and constant phase element, CPE1 in parallel and CPE2 using ZView TM software (Scribner Associates Inc.).
Scheme-S1. Equivalent circuit used for the analysis of the impedance plots for G7Li. Here, R1 is the bulk resistance and CPE1 and CPE2 are constant phase elements.
Fitting the data in the "spike-like" region by a resistance (R2) in parallel to CPE2 is also attempted similar to ceramic conductors [8][9][10][11][12][13] . In this case, the value of n which fitted the data reasonably is found to be low (= 0.5) which resulted in capacitance values  10 -6 F.
The transference number 14 measurements are done using a symmetrical two electrode cell as reported by Evans, J. et al 15,16 . A cell of the type Li(metallic)|G7Li|Li(metallic) is designed with a pellet of G7Li of diameter ~10 mm and thickness ~1 mm (G7Li pellet is sandwiched between two lithium electrodes). The lithium electrodes of similar dimensions as that of G7Li are obtained by punching lithium foil. The whole set up is then inserted into a cell as shown in Scheme-S2. The whole cell assembly is then sealed inside a pouch cell with only the connecting wires projecting outside. The sealing is done inside an argon filled glove-box # Symmetry of A: (i)1-x,-1/2+y,1/2-z (ii) 1-x,1/2+y,1/2-z (iii) -x,-1/2+y,1/2-z (iv) x,1/2-y,1/2+z (v) -x,1-y,1-z where A = acceptor and D = donor.