Since the 1990s, human life has greatly changed because commercial lithium (Li)-ion batteries (LIBs) were first released by Sony1 and became widely used for small electronic devices, which allow people to use their electronics without power cables, i.e. popularization of portable electronics. More recently, the application of LIBs has been expanding to large systems such as electric vehicles (EVs), advanced portable electronics and large scale stationary energy storage systems, because environmental pollution and limited deposits of fossil fuel issues came to the fore, which makes the development of the next generation of energy sources essential. Unfortunately, it is a great challenge for conventional Li-ion cells because the performance of Li-ion cells has not improved as fast as the increase of demand for high-performance portable electronics and is not satisfactory for emerging market demands such as electric automobiles and aircraft. For example, an EV that is fully operated by battery power requires a high specific energy of ~350 Wh/kg at the three-hour discharge rate with reasonably low cost.2 The specific energy (Wh/kg) of a cell is technically determined by the operating voltage (V) and the specific capacity (Ah/kg) of the cell, however, conventional Li-ion cells composed of a carbon anode and a transition metal oxide cathode can offer only about 100–200 Wh/kg of practical specific energy, due mainly to low specific capacity of the transition metal oxide cathode (theoretical specific capacity of LiCoO2 cathode3: 274 Ah/kg). Even the theoretical specific energy of conventional Li-ion cells (500–600 Wh/kg) is not far from the practical requirement for EV applications, so it seems very unlikely that any Li-ion cell can meet the 350 Wh/kg goal. For these reasons, seeking the next generation of rechargeable cells with higher specific energy has become essential.