All-solid-state disordered LiTiS2 pseudocapacitorElectronic supplementary information (ESI) available. See DOI: 10.1039/c7ta03756b

Pseudocapacitive materials offer an opportunity to bridge the energy storage gap between supercapacitor and battery technologies. Herein is chronicled the first report of pseudocapacitance in a system devoid of liquid electrolytes, using the cathode material LiTiS 2 . It is demonstrated that due to extreme crystallite reduction to less than 3 nm, additional charge storage is derived by reducing surface Ti 3+ to Ti 2+ at higher voltages and more reversibly than traditionally shown. Due to facile diffusion pathways in 3-fold coordinated lithium along the TiS 2 surfaces, disordered LiTiS 2 can be used as a singular cathode without conductive additives. The result is a system exhibiting nearly 300 mA h g −1 at a rate of C/2 for 1000 cycles. Whereas active materials in liquid cells typically have size limitations before irreversibilities appear, the high pseudocapacitance demonstrated in this report indicates that active materials used in the solid-state could benefit from size reduction. Hopefully, a new avenue of research stems from this work to investigate mixed conductor nano-domains for solid-state battery/capacitor hybrids. The prospect of a solid-state pseudocapacitor unlocks a series of new applications that offer long shelf life, high temperature capabilities, and enhanced safety. A lithium charge storage mechanism is discovered at the interface of nano-crystallites of LiTiS 2 in the solid state. The surface titanium atoms can be reduced reversibly at higher voltages. Electrochemically, this appears as a pseudocapacitive effect boosting capacity greater than theoretical at high rate with no liquid electrolyte.