Rechargeable batteries: challenges old and new

Abstract

The challenges for rechargeable batteries are cost, safety, energy, density, life, and rate. Traditional rechargeable batteries based on aqueous electrolytes have good rate capabilities but limited energy density because the voltage for a long shelf-life is restricted to 1.5 V. The discovery of fast Na ion conductivity in β-alumina in 1967 introduced the novel concept of a solid oxide electrolyte and molten electrodes: the sodium–sulfur battery operates at 350 °C. Interest in rechargeable batteries with aprotic electrolytes was further stimulated by the first energy crisis in the early 1970s. Since protons are not mobile in aprotic electrolytes, the Li+ ion was the logical choice for the working ion, and on-going work on reversible Li intercalation into layered sulfides suggested the TiS2//Li cell, which was shown in 1976 to have a voltage of V ≃ 2.2 V and good rate capability. However, the organic liquid carbonates used as electrolytes are flammable, and dendrites growing across the electrolyte from the lithium anode on repeated charge/discharge cycles short-circuited the cells with disastrous consequences. Safety concerns caused this effort to be dropped. However, substitution of the layered oxides LiMO2 for the layered sulfides MS2 and reversible intercalation of Li into graphitic carbon without dendrite formation at slow charging rates gave a safe rechargeable lithium ion battery (LIB) of large-enough energy density to enable the wireless revolution. Although carbon-buffered alloys now provide anodes that allow a fast charge and have a higher capacity, nevertheless a passivation layer permeable to Li+ forms on the anode surface, and the Li+ in the passivation layer is taken irreversibly from the cathode on the initial charge. Since the specific capacity of a cell with an insertion-compound cathode is limited by the latter, strategies to increase the specific capacity for a LIB powering an electric vehicle or storing electricity from wind or solar farms include a return to consideration of a solid electrolyte.