Surface degradation is a common challenge for many electrode materials. The active surface usually reacts with the molecules in the surrounding environment to form byproducts that hinder the diffusion channels for Li ions and electrons, increase the energy barrier for (de)lithiation reactions, and ultimately shorten the cycle life. Herein, the growth of surface Li ₂ CO ₃ on LiNi _x Co _y Mn _y O ₂ (x = 0.33, 0.6, 0.7, 0.8, x + 2y = 1) cathodes upon storage has been systematically investigated. Ni-rich surfaces are found to result in more Li ₂ CO ₃ growth, based on which three discrete degradation models for layered oxides are proposed. The increase and discretization of the energy barrier for individual particles also explain the State-of-Charge heterogeneity phenomena observed by in situ XRD and the change of cyclic voltammetry curves. By providing a comprehensive picture of surface deterioration of the NCM cathode family, this study enhances the understanding of the degradation mechanism that determines the cycle life of electrode materials.

It is a huge challenge for high-tap-density electrodes to achieve high volumetric energy density but without compromising the ionic transportation. Herein, we prepared compact Li₄Ti₅O₁₂ (LTO) microspheres consisting of densely packed primary nanoparticles. The real space distribution of lithium ions inside the compact LTO was revealed by using scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) to identify the function of grain boundaries for lithium ion transportation during lithiation. The as-prepared LTO microspheres possess a high tap density (1.23 g cm^-3) and an ultra-small specific surface area (2.40 m² g^-1). Impressively, the compact LTO microspheres present excellent electrochemical performance. At high rates of 5C, 10C and 20C, the LTO microspheres show a specific capacity of 146.6, 138.2 and 111 mA h g^-1, respectively. The capacity retention remains at 97.8% at 5C after 500 cycles. The STEM-EELS results indicate that the lithiation reaction of LTO is firstly initiated at grain boundaries during the high rate lithiation process and then diffuses to the bulk area. The abundant grain boundaries in compact LTO microspheres can form a highly efficient conductive network to preferentially transport the ions, which contributes to high volumetric and gravimetric energy density simultaneously.

Nanosized Li₄Ti₅O₁₂ (LTO) materials enabling high rate performance suffer from a large specific surface area and low tap density lowering the cycle life and practical energy density. Microsized LTO materials have high density which generally compromises their rate capability. Aiming at combining the favorable nano and micro size properties, a facile method to synthesize LTO microbars with micropores created by ammonium bicarbonate (NH₄HCO₃) as a template is presented. The compact LTO microbars are in situ grown by spinel LTO nanocrystals. The as-prepared LTO microbars have a very small specific surface area (6.11 m² g⁻¹) combined with a high ionic conductivity (5.53 × 10⁻¹² cm⁻² s⁻¹) and large tap densities (1.20 g cm⁻³), responsible for their exceptionally stable long-term cyclic performance and superior rate properties. The specific capacity reaches 141.0 and 129.3 mAh g⁻¹ at the current rate of 10 and 30 C, respectively. The capacity retention is as high as 94.0% and 83.3% after 500 and 1000 cycles at 10 C. This work demonstrates that, in situ creating micropores in microsized LTO using NH₄HCO₃ not only facilitates a high LTO tap density, to enhance the volumetric energy density, but also provides abundant Li-ion transportation channels enabling high rate performance.

Through a facile sodium sulfide (Na₂S)-assisted hydrothermal treatment, clean and nondefective surfaces are constructed on micrometer-sized Li₄Ti₅O₁₂ particles. The remarkable improvement of surface quality shows a higher first cycle Coulombic efficiency (≈95%), a significantly enhanced cycling performance, and a better rate capability in electrochemical measurements. A combined study of Raman spectroscopy and inductive coupled plasma emission spectroscopy reveals that the evolution of Li₄Ti₅O₁₂ surface in a water-based hydrothermal environment is a hydrolysis–recrystallization process, which can introduce a new phase of anatase-TiO₂. While, with a small amount of Na₂S (0.004 mol L⁻¹ at least), the spinel-Li₄Ti₅O₁₂ phase is maintained without a second phase. During this process, the alkaline environment created by Na₂S and the surface adsorption of the sulfur-containing group (HS⁻ or S²⁻) can suppress the recrystallization of anatase-TiO₂ and renew the particle surfaces. This finding gives a better understanding of the surface–property relationship on Li₄Ti₅O₁₂ and guidance on preparation and modification of electrode material other than coating or doping.