Silica and nitrogen-doped carbon co-coated lithium manganese iron phosphate microspheres as cathode materials for lithium batteries

Lithium manganese iron phosphate (LiMn1-xFexPO4, LMFP) combines the advantages of LiFePO4 and LiMnPO4. However, low electronic conductivity and sluggish lithium ion diffusion of the LMFP cathode limits its commercial application. In this work, the LMFP microspheres were co-coated by silica and N-doped carbon for the improvement of electronic and ionic conductivity of LMFP. The hydrolysis of tetraethyl orthosilicate and the polymerization of dopamine can be mutually promoted in one reaction system to realize the simultaneous precipitation of Si and C species on the LMFP surfaces without the addition of acid–base catalysts or buffering agents. After high-temperature treatment in argon, the silica and N-doped carbon co-coated LMFP microspheres were obtained with improved cycling stability (84.4% of capacity retention for 300 cycles at 1 C) and enhanced rate performance (80.0 mAh g−1 at 5 C). Therefore, this work shows a facile and common method for the composite coating of cathode materials.

Construction of Carbon-Coated LiMn0.5Fe0.5PO4@Li0.33La0.56TiO3 Nanorod Composites for High-Performance Li-Ion Batteries

The carbon-coated LiMn_0.5Fe_0.5PO₄@Li_0.33La_0.56TiO₃ nanorod composites (denoted as C/LMFP@LLTO) have been successfully obtained according to a common hydrothermal synthesis following a post-calcination treatment. The morphology and particle size of LiMn_0.5Fe_0.5PO₄ (denoted as LMFP) are not changed by the coating. All electrode materials exhibit nanorod morphology; they are 100-200 nm in length and 50-100 nm in width. The Li_0.33La_0.56TiO₃ (denoted as LLTO) coating can facilitate the charge transfer to enhance lithiation/delithiation kinetics, leading to an excellent rate performance and cycle stability of an as-obtained C/LMFP@LLTO electrode material. The reversible discharge capacities of C/LMFP@LLTO (3 wt %) at 0.05 and 5 C are 146 and 131.3 mA h g^-1, respectively. After 100 cycles, C/LMFP@LLTO (3 wt %) exhibits an outstanding capacity of 106.4 mA h g^-1 with an 81% capacity retention rate at 5 C, indicating an excellent reversible capacity and good cycle capacity. Therefore, it can be considered that LLTO coating is a prospective pathway to exploit the electrochemical performances of C/LMFP.

A new strategy to hydrothermally synthesize olivine phosphates

A new strategy based on the P excess reaction system is innovated to hydrothermally synthesize olivine phosphates (LiMPO4), and even unoptimized samples still show competitive electrochemical performance. Meanwhile, it is found that the presence of NH4+ in the P excess reaction system is detrimental to the efficient hydrothermal synthesis of LiMPO4.

Tuning Li-Ion Diffusion in α-LiMn1-xFexPO4 Nanocrystals by Antisite Defects and Embedded β-Phase for Advanced Li-Ion Batteries

Olivine-structured LiMn_1-xFe_xPO₄ has become a promising candidate for cathode materials owing to its higher working voltage of 4.1 V and thus larger energy density than that of LiFePO₄, which has been used for electric vehicles batteries with the advantage of high safety but disadvantage of low energy density due to its lower working voltage of 3.4 V. One drawback of LiMn_1-xFe_xPO₄ electrode is its relatively low electronic and Li-ionic conductivity with Li-ion one-dimensional diffusion. Herein, olivine-structured α-LiMn_0.5Fe_0.5PO₄ nanocrystals were synthesized with optimized Li-ion diffusion channels in LiMn_1-xFe_xPO₄ nanocrystals by inducing high concentrations of Fe²⁺-Li⁺ antisite defects, which showed impressive capacity improvements of approaching 162, 127, 73, and 55 mAh g^-1 at 0.1, 10, 50, and 100 C, respectively, and a long-term cycling stability of maintaining about 74% capacity after 1000 cycles at 10 C. By using high-resolution transmission electron microscopy imaging and joint refinement of hard X-ray and neutron powder diffraction patterns, we revealed that the extraordinary high-rate performance could be achieved by suppressing the formation of electrochemically inactive phase (β-LiMn_1-xFe_xPO₄, which is first reported in this work) embedded in α-LiMn_0.5Fe_0.5PO₄. Because of the coherent orientation relationship between β- and α-phases, the β-phase embedded would impede the Li⁺ diffusion along the [100] and/or [001] directions that was activated by the high density of Fe²⁺-Li⁺ antisite (4.24%) in α-phase. Thus, by optimizing concentrations of Fe²⁺-Li⁺ antisite defects and suppressing β-phase-embedded olivine structure, Li-ion diffusion properties in LiMn_1-xFe_xPO₄ nanocrystals can be tuned by generating new Li⁺ tunneling. These findings may provide insights into the design and generation of other advanced electrode materials with improved rate performance.