Inhibitive role of crystal water on lithium storage for multilayer FeC2O4•xH2O anode materials

The application of iron oxalate (FeC2O4) as an anode material for rechargeable lithium-ion batteries (LIBs) is hindered by its poor thermostability and difficulty in obtaining 100% anhydrous oxalate material. Herein, we fabricated the multilayer FeC(2)O(4)xH(2)O materials with different contents of crystal water by a simple sintering process and analyzed the special role of water molecules. In hydrate iron oxalate, because of the stronger Fe-O ionic bonds and hydrogen bonds, crystal water can provide a stable sustaining force and enlarge interlayered spacing. However, during Li+ (de)intercalation process, water molecules not only can cause the slippage stresses and structural collapse, but also present an obvious inhibitive role on the electrochemical activity of complex nanocomposites and formation of compact organic deposit interfaces, thus resulting in substantial structural deficiencies and poor lithium storage performance. Meanwhile, anhydrate iron oxalate, coupled with excellent structural flexibility and stability, suggests superior long-term stability (a reversible specific capacity of 1070.8 mAh g(-1) and capacity retention of 70.84% after 500 cycles at 0.5 A g(-1)) and satisfactory rate capability (1030, 705 and 602 mAh g(-1) at 0.1, 3 and 5 A g(-1), respectively). In addition, based on the electrochemical analysis, crystal water could stably exist in three hydrous compounds (FeC(2)O(4)2H(2)O, Li2Fe(C2O4)22H(2)O and Fe[H2O](2)) during cycling, which provide a significant opportunity to enhance the electrochemical properties of hydrous oxalate materials.

Automated summary

This study investigated the role of water molecules in both hydrated and anhydrous iron oxalate materials for rechargeable lithium-ion batteries. It was found that water molecules in hydrated materials may lead to structural collapse and hinder electrochemical activity, while anhydrous iron oxalate exhibited superior long-term stability and satisfactory rate capability. The presence of crystal water in hydrous compounds during cycling offers a significant opportunity to enhance the electrochemical properties of hydrous oxalate materials.