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and irreversible lithium consumption; and third, large volume changes during lithium plating and stripping. [7][8][9] Electrochemical impedance spectroscopy (EIS) is a powerful characterization technique that can provide physical and chemical information about processes occurring at the interfaces in an electrochemical system. EIS has been employed in various areas, including the characterization of batteries in general and Li-ion batteries in particular. [10][11][12] In the latter area, EIS has been largely used as a complementary in situ technique for elucidating the role of (dis)charge rates, interfacial phenomena, as well as cell-reaction and failure mechanisms. [11,13,14] In order to ensure that the maximum amount of information is extracted from the EIS spectra, various fitting models have been explored. The "classic" Randles model is the most-used one. [15,16] It includes a solution resistance, an RC circuit (double-layer capacitor in parallel with a polarization resistance), attributed to the double-layer effect and typically related to the semicircle observed in the Nyquist plot, and a Warburg element, related to the diffusion dynamics. [15,17] The more phenomena-such as charge-transfer reaction at the electrode-electrolyte interface, double-layer effects, and resistance/capacitance growth-are occurring at the interfacial level, the more semicircles (either separated or overlapping) need to be considered when fitting the experimental data. More complex models, such as the transmissionline model, have also been used for the study and analysis of EIS spectra of battery electrode materials, as well as specifically for metallic lithium anodes, but this method has rather limited application compared to the Randles model, due to its complexity and the need for dedicated advanced fitting scripts for the analysis. [18,19] When performing impedance spectroscopy of metallic lithium anodes, mostly thick lithium electrodes have been studied, resulting in Nyquist plots with quasi-ideal semicircle. [20] Typically, this semicircle is assigned to the impedance due to the migration of Li + ions through the interface and is fitted based on the model proposed by Aurbach and coworkers. [21] In this model, each contribution from the equivalent circuit is attributed to a different layer within the composite passivation layer, known as the solid electrolyte interphase (SEI), which is spontaneously formed when lithium gets in contact with the liquid electrolyte. [22,23] In some other Lithium-metal batteries offer substantial advantages over lithium-ion batteries in terms of gravimetric and volumetric energy densities. However, their widespread practical use is hindered by safety concerns, often attributed to the poor stability of the metallic lithium interface, where electrochemical impedance spectroscopy (EIS) can provide crucial information. The EIS spectra of metallic lithium electrodes proved to be more complex than expected, especially when studying thin lithium metal foils. Here, it is identified that charge-transfer impe...

Lithium ion batteries are typically based on one of three positive-electrode materials, namely layered oxides, olivine-and spinel-type materials. The structure of any of them is 'resistant' to electrochemical cycling, and thus, often requires modification/post-treatment to improve a certain property, for example, structural stability, ionic and/or electronic conductivity. This review provides an overview of different examples of coatings and surface modifications used for the positive-electrode materials as well as various characterization techniques often chosen to confirm/detect the introduced changes. It also assesses the electrochemical success of the surface-modified positive-electrode materials, thereby highlighting remaining challenges and pitfalls.

Natrium super ionic conductor (NASICON) materials providing attractive properties such as high ionic conductivity and good structural stability are considered as very promising materials for use as electrodes for lithium-and sodium-ion batteries. Herein, a new highperformance electrode material, Li 0.5 Ni 0.5 Ti 1.5 Fe 0.5 (PO 4) 3 /C, was synthesized via the sol−gel method and was electrochemically tested as an anode for lithium ion batteries, providing enhanced electrochemical performance as a result of nickel substitution into the lithium site in the LiTi 2 (PO 4) 3 family of materials. The synthesized material showed good ionic conductivity, excellent structural stability, stable long-term cycling performance, and improved high rate cycling performance compared to LiTi 2 (PO 4) 3. The Li 0.5 Ni 0.5 Ti 1.5 Fe 0.5 (PO 4) 3 / C electrode delivered reversible capacities of about 93 and 68% of its theoretical one at current rates of 0.1 C (6.42 mA•g −1) after 100 cycles and 5 C (320.93 mA•g −1) after 1000 cycles, respectively. Theoretically, three Li + ions can be inserted into the vacancies of the Li 0.5 Ni 0.5 Ti 1.5 Fe 0.5 (PO 4) 3 /C structure. However, when the electrode is discharged to 0.5 V, more than three Li + ions are inserted into the NASICON structure, leading to its structural transformation, and thus to an irreversible electrochemical behavior after the first discharge process.