Calcium cation enhanced cathode/electrolyte interface property of Li 2 FeSiO 4 /C cathode for lithium-ion batteries with long-cycling life

Hollow Hemispherical Lithium Iron Silicate Synthesized by an Ascorbic Acid-Assisted Hydrothermal Method as a Cathode Material for Li Ion Batteries

High-capacity and high-voltage cathode materials are required to meet the increasing demand for energy density in Li ion batteries. Lithium iron silicate (Li2FeSiO4) is a cathode material with a high theoretical capacity of 331 mAh·g-1. However, its poor conductivity and low Li ion diffusion coefficient result in poor capability, hindering practical applications. Morphology has an important influence on the properties of materials, and nanomaterials with hollow structures are widely used in electrochemical devices. Herein, we report a novel hollow hemispherical Li2FeSiO4 synthesized by a template-free hydrothermal method with the addition of ascorbic acid. The hollow hemispherical Li2FeSiO4 consisted of finer particles with a shell thickness of about 80 nm. After carbon coating, the composite was applied as the cathode in Li ion batteries. As a result, the hollow hemispherical Li2FeSiO4/C exhibited a discharge capacity as high as 192 mAh·g-1 at 0.2 C, and the average capacities were 134.5, 115.5 and 93.4 mAh·g-1 at 0.5, 1 and 2 C, respectively. In addition, the capacity increased in the first few cycles and then decayed with further cycling, showing a warm-up like behavior, and after 160 cycles the capacities maintained 114.2, 101.6 and 79.3 mAh·g-1 at 0.5, 1 and 2 C, respectively. Such a method of adding ascorbic acid in the hydrothermal reaction can effectively synthesize hollow hemispherical Li2FeSiO4 with the enhanced electrochemical performance.

Enhanced Cycling Performance for Lithium-Sulfur Batteries by a Laminated 2D g-C3 N4 /Graphene Cathode Interlayer

Decay in electrochemical performance resulting from the "shuttle effect" of dissolved lithium polysulfides is one of the biggest obstacles for the realization of practical applications of lithium-sulfur (Li-S) batteries. To meet this challenge, a 2D g-C₃ N₄ /graphene sheet composite (g-C₃ N₄ /GS) was fabricated as an interlayer for a sulfur/carbon (S/KB) cathode. It forms a laminated structure of channels to trap polysulfides by physical and chemical interactions. The thin g-C₃ N₄ /GS interlayer significantly suppresses diffusion of the dissolved polysulfide species (Li₂ S_x ; 2<x≤8) from the cathode to the anode, as proven by using an H-type glass cell divided by a g-C₃ N₄ /GS-coated separator. The S/KB cathode with the g-C₃ N₄ /GS interlayer (S/KB@C₃ N₄ /GS) delivers a discharge capacity of 1191.7 mAh g^-1 at 0.1 C after 100 cycles, an increase of more than 90 % compared with an S/KB cathode alone (625.8 mAh g^-1 ). The S/KB@C₃ N₄ /GS cathode shows good cycling life, delivering a discharge capacity as high as 612.4 mAh g^-1 for 1 C after 1000 cycles. According to XPS results, the anchoring of the g-C₃ N₄ /GS interlayer to Li₂ S_x can be attributed to a coefficient chemical binding effect of g-C₃ N₄ and graphene on long-chain polysulfides. Generally, the improvement in electrochemical performance originates from a coefficient of the enhanced Li⁺ diffusion coefficient, increased charge transfer, and the weakening of the shuttle effect of the dissolved Li₂ S_x as a result of the g-C₃ N₄ /GS interlayer.