Magnetic and Transport Properties of Heat-Treated Polyparaphenylene-Based Carbons

Electron spin resonance (ESR), magnetic susceptibility, and transport measurements were recently performed on a set of heat-treated polyparaphenylene (PPP)-based carbon samples, which are of significant interest as novel carbon-based anode electrodes in Li-ion batteries. Attention is focused on the evolution of the carbonaceous structures formed at low heat-treatment temperatures ( T HT ) in the regime of 600° C ≤ T HT ≤ 800° C, where percolative transport behavior is observed. At the percolation threshold, the coexistence of two spin centers with peak-to-peak Lorentzian linewidths of Δ H pp (300K) = 0.5 and 5.0 G is observed. The relatively high ratio of hydrogen. carbon (H/C) near is believed to influence the ESR results through an unresolved hyperfine interaction. Curie-Weiss temperaures are found from measurements of [ I pp (Δ H pp ) 2 ] -1 , where I pp is the peak-to-peak lineheight, yielding results that are in agreement with static susceptibility, χ( T ), measurements. At low T HT , PPP-based materials exhibit a large amount of disorder and this is evidenced by the high density of localized spins, N c , which is obtained from a Curie-Weiss fit to χ( T ) assuming a spin quantum number of S=12 . A model explaining the microstructure and high electrochemical doping capacity of PPP samples heat-treated to 700° C can be related to Li-ion battery performance.