New type polyamides containing disulfide bonds for positive active material of lithium secondary batteries

Abstract

Various polyamides containing a disulfide bond in their main chain were prepared by condensation of a diacid, 3,3′-dithiodipropionic acid (P), 6,6′-dithiodinicotinic acid (N), or 2,2′-dithiodibenzoic acid (B) and a diamine, alkyldiamine (I, II, III, and IV: NH 2-(CH 2) n -NH 2, n = 4, 6, 8, and 12) or cystamine, V: NH 2-(CH 2) 2-S-S-(CH 2) 2-NH 2. Polyamides P-I, P-II, P-III, P-IV, and P-V were insoluble common organic solvents, but soluble in formic acid or trifluoroacetic acid. Polyamides containing N or B as the acid moiety, i.e. aromatic polyamides, were soluble in N,N′-dimethylformamide or dimethyl sulfoxide. The electrochemical behavior of P-II electrode in the organic electrolytes, such as propylene carbonate (PC), PC—1,2-dimethoxyethane (DME) mixture, ethylene carbonate (EC)—DME mixture, which are used as the electrolytes for common lithium batteries was investigated. In the mixed electrolyte, PC—DME (1:1, by volume) a good electrochemical response of P-II electrode based on the redox of disulfide bonds in the polymer was observed. Charge/discharge cycling tests of Li/P-II cells were also performed. At the first discharge, the net capacity of the cell was 97.5 Ah kg −1 (this value was calculated from the weight of loaded P-II on the electrode). Utilization of P-II was 53%. Cell capacity versus cycle number was also investigated.