The limitations of current EES devices expose fundamental gaps in our understanding of atomic- and molecular-level processes that govern their operation, performance and failure. This article reviews the development of a new generation of sustainable, affordable and safe EES technologies that approach the theoretical limit for electrochemical storage and deliver electrical energy rapidly and efficiently. Here, the ‘theoretical limit' refers to the limit imposed by thermodynamic considerations, rather than limitations coming from electrochemical system design. First, mechanisms of electrochemical energy storage are discussed, followed by a description of energy storage in asymmetric and hybrid devices, where each electrode in the cell utilizes a different mechanism of charge storage. Hybrids of batteries and ECs represent an important future direction for EES, where there is the promise of achieving high energy and power densities, well beyond what is anticipated from improvements in lithium-ion batteries or carbon-based electrical double-layer capacitors (EDLC). To achieve this goal, there is a need for novel materials and system architectures, which enable electron transfer and ionic transport with multi-electron redox chemistries, as stressed in a recent review5. In appraising the status of this research, it is important to highlight that future opportunities lie in the computationally driven design of new materials and hybrid energy storage devices. Finally, some gaps in the understanding of atomic- and molecular-level processes that govern operation, performance and failure of EES devices are highlighted and pathways to the integration of faradaic and capacitive storage mechanisms are outlined.