The lithium secondary batteries with higher energy density is strongly required for electric vehicle application. The sulfur cathode and silicon anode show high theoretical capacity (1675 mAh g_sulfur^-1 and 3580 mAh g_silicon^-1). Nevertheless, both active materials change their volume during charge-discharge cycles, which leads to pulverization of electrodes. Therefore, it is difficult to load large amount of active material in the electrode. Limited values of active material results in low capacity in total system of batteries due to the large mass of metal current collector (Al and Cu). To solve the problem, self-supporting, sponge-like paper of carbon nanotubes (CNTs) can be a candidate for non-metal based 3D current collector, which can reduce the mass of current collector. It also accommodates large amount of active material due to high porosity. Previously, we have developed S-CNT cathode and Si-CNT anode [1] and demonstrated high capacity for their individual half cells [1]. In this work, we combined them to achieve higher energy density full cell without any metal current collector.

Submillimeter-long few-wall CNTs produced by fluidized-bed chemical vapor deposition method [2] were used for the 3D current collector. 30-μm-thick CNT paper was fabricated via dispersion and filtration of the CNTs, and the S-CNT cathode was fabricated by sublimation of sulfur into the CNT paper. The Si-CNT anode was fabricated by co-dispersion and co-filtration of Si nanoparticles (50, 100, and 150 nm in diameter) and the CNTs followed by carbon coating by chemical vapor deposition. Lithium was pre-doped to the Si-CNT anode either by contacting it with Li foil in electrolyte or by electrochemically using Li foil as counter electrode. The full cell of the S-CNT cathode (58 mass% S) and the Li_xSi_y-CNT anode (70 mass% Si) showed a high initial discharge capacity and energy density of over 320 mAh/g and 540 Wh/kg based on the total mass of the cathode and anode.

[1] T. Kowase, et al, J. Power Sources, 363, 450 (2017).

[2] Z. Chen, et al., Carbon, 80, 339, (2014)