The increasing demands of energy storage require the significant improvement of current Li-ion battery electrode materials and the development of advanced electrode materials. Thus, it is necessary to gain an in-depth understanding of the reaction processes, degradation mechanism, and thermal decomposition mechanisms of electrode materials under realistic operation conditions. This understanding can be obtained by in situ/operando characterization techniques that provide information on the structure evolution, redox mechanism, solid-electrolyte interphase (SEI) formation, side reactions and Li-ion transport properties under operating conditions. Here, the recent developments in the in situ/operando techniques employed for the investigation of the structural stability, dynamic properties, chemical environment changes and morphological evolution during electrochemical processes are described and summarized in detail. The experimental approaches reviewed in this paper include X-ray, electron, neutron, optical, and scanning probes. Each advanced technique has unique capabilities to study specific properties of electrode materials within specific limitations. The experimental methods and operating principles, especially the in situ cell designs, are described in detail. To illustrate the applicability and uniqueness of each technique, representative studies making use of the in situ/operando techniques are discussed and summarized. Finally, the major current challenges and future opportunities of the in situ/operando techniques are discussed. Several important battery challenges are likely to benefit from these in situ/operando techniques, including the inhomogeneous reactions of This article is protected by copyright. All rights reserved. 4high energy density cathodes, the development of safe and reversible Li metal plating and the development of stable SEI on electrodes.Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))

Ether based electrolytes have surfaced as alternatives to conventional carbonates allowing for enhanced electrochemical performance of sodium-ion batteries; however, the primary source of the improvement remains poorly understood. Here we show that coupling titanium dioxide and other anode materials with diglyme does enable higher efficiency and reversible capacity than those for the combination involving ester electrolytes. Importantly, the electrolyte dependent performance is revealed to be the result of the different structural evolution induced by a varied sodiation depth. A suit of characterizations show that the energy barrier to charge transfer at the interface between electrolyte and electrode is the factor that dominates the interfacial electrochemical characteristics and therefore the energy storage properties. Our study proposes a reliable parameter to assess the intricate sodiation dynamics in sodium-ion batteries and could guide the design of aprotic electrolytes for next generation rechargeable batteries.

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