Copper sulfide (CuS) is a promising cathode for lithium-ion batteries (LIBs) due to its impeccable theoretical energy density (~ 1015 Wh·kg⁻¹ and 4743 Wh·L⁻¹). However, it suffers from voltage decay leaded energy density loss and low energy efficiency, which hinders its application. In this work, with combined ex-situ/in-situ X-ray diffraction (XRD) and electrochemical analysis, we explore detailed degradation mechanisms. For the voltage decay, it is attributed to a spontaneous reaction between CuS cathode and copper current collector (Cu CC). This reaction leads to energy density loss and active materials degradation (CuS → Cu_1.81S). As for energy efficiency, CuS undergoes a series of phase transformations. The main phase transition processes are CuS → α-LiCuS → Li_2−xCu_xS + Cu → Li₂S + Cu for discharge; Li₂S + Cu → Li_2−xCu_xS → β-LiCuS → CuS for charge. Here, α-LiCuS, β-LiCuS, and Li_2−xCu_xS are newly identified phases. These phase changes are driven by topotactic-reaction-related copper diffusion and rearrangement. This work demonstrates the significance of transition-metal diffusion in the intermediates formation and phase change in conversion-type materials.

Lithium (Li) metal is an ideal anode for the next generation high-energy-density batteries. However, it suffers from dendrite growth, side reactions, and infinite relative volume change. Effective strategies are using porous carbons or surface modification carbons to guide Li deposition into their pores. While the Li deposition behavior is still ambiguous. Here, we systematically determine their deposition behavior in various surface-modified carbons and in different electrolytes via optical microscopy and scanning electron microscopy study. It is found that Li will not spontaneously deposit into the carbon pores, which is significantly dependent on the carbon surface, current density, areal capacity, and electrolyte. Thus, a “lithiophilic” modified commercial hard carbon with Ag is developed as a stable “host” and efficient surface protection derived from the localized high-concentration electrolyte exhibits a pretty low volume change (5.3%) during cycling at a current density of 2 mA·cm⁻² and an areal capacity of 2 mAh·cm⁻². This strategy addresses the volume change and dendrite problems by rationally designed host and electrolyte, providing a broad perspective for realizing Li-metal anode.

Here, by using atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy, we investigate the structural and chemical evolution of Li₃V₂(PO₄)₃ (LVP) upon the high-voltage window (3.0–4.8 V). We find that the valence of vanadium gradually increases towards the core corresponding to the formation of electrochemically inactive Li_3-xV₂(PO₄)₃ (L_3-xVP) phases. These Li-deficient phases exhibit structure distortion with superstructure stripes, likely caused by the migration of the vanadium, which can slow down the lithium ion diffusion or even block the diffusion channels. Such kinetic limitations lead to the formation of Li-deficient phase along with capacity loss. Thus, the LVP continuously losses of electrochemical activity and Li-deficient phases gradually grow from the particle core towards the surface during cycling. After 500 cycles, the thickness of active LVP layer decreases to be ~ 5–20 nm. Moreover, the micromorphology and chemical composition of solid electrolyte interphase (SEI) have been investigated, indicating the thick SEI film also contributes to the capacity loss. The present work reveals the structural and chemical evolution in the cycled electrode materials at an atomic scale, which is essential to understand the voltage fading and capacity decaying of LVP cathode.