There are many practical challenges in the use of graphene materials as active components in electrochemical energy storage devices. Graphene has a much lower capacitance than the theoretical capacitance of 550 F g⁻¹ for supercapacitors and 744 mA h g⁻¹ for lithium ion batteries. The macroporous nature of graphene limits its volumetric energy density and the low packing density of graphene-based electrodes prevents its use in commercial applications. Increases in the capacity, energy density and power density of electroactive graphene materials are strongly dependent on their microstructural properties, such as the number of defects, stacking, the use of composite materials, conductivity, the specific surface area and the packing density. The structural design of graphene electrode materials is achieved via six main strategies: the design of non-stacking and three-dimensional graphene; the synthesis of highly packed graphene; the production of graphene with a high specific surface area and high conductivity; the control of defects; functionalization with O, N, B or P heteroatoms; and the formation of graphene composites. These methodologies of structural design are needed for fast electrical charge storage/transfer and the transport of electrolyte ions (Li⁺, H⁺, K⁺, Na⁺) in graphene electrodes. We critically review state-of-the-art progress in the optimization of the electrochemical performance of graphene-based electrode materials. The structure of graphene needs to be designed to develop novel electrochemical energy storage devices that approach the theoretical charge limit of graphene and to deliver electrical energy rapidly and efficiently.

NASICON-type (lithium super ionic conductor) solid electrolyte is of great interest because of its high ionic conductivity, wide potential window, and good chemical stability. In this paper, the key problems and challenges of NASICON-type solid electrolyte are described from the aspects of ionic conductivity, electrode interface, and electrochemical stability. Firstly, the migration mechanism of lithium ion is analyzed from the three-dimensional structure of NASICON-type solid electrolyte, and progress in the research of conductivity and stability is summarized. Then, the effective methods to reduce interface impedance and improve the cycle stability of all-solid-state lithium batteries (ASSLBs) with NASICON-type solid electrolyte are introduced. Finally, solutions to improve the conductivity of electrolytes and deal with electrode/electrolyte interface problems are summarized, and the development prospects of ASSLBs are discussed.

Materials chemistry focuses on all aspects of the production of electrode materials or the properties or applications of materials related to energy storage, which thus plays an important role in the field of energy storage. Electrochemical energy storage includes the conversion reaction between chemical energy and electric energy, with the electric energy being stored in chemical bonds of electrode materials of both battery and pseudocapacitor types. Energy density, power density and safety of these devices, i.e. lithium ion batteries and supercapacitors, are mostly dependent on the electrode materials with high electroactivity, high electron/ion conductivity, and high structural/electrochemical stability. Following the function-directed materials design rule, we can select appropriate elements, chemical bonds, crystal structures, and morphologies of those materials toward high electrochemical performances. In this review, we summarize, from both theoretical and experimental viewpoints of materials chemistry, recent advances in designing electrode materials from element and structure selections to final morphology selection. Electronegativity, atom radius, chemical bonding behavior, and oxidation state have been identified as controllable materials properties to synthesize high-performance electrode materials. This review provides general materials chemistry rules to rationally design electrode materials with improved electrochemical performance.

A kind of La³⁺ inserted δ-MnO₂ was synthesized by a facile process. Benefiting from the larger interlamellar spacing and reduced Zn²⁺ (de)insertion resistance after intercalation, the capability and rate performance is significantly boosted.