Double-Layer Carbon-Coating Method for Simultaneous Improvement of Conductivity and Tap Density of LiMn0.65Fe0.35PO4/C/KB Cathode Materials

A LiMn0.65Fe0.35PO4/C/KB composite with superior electronic conductivity and high tap density is synthesized by a double-layer carbon-coating method with Ketjen Black (KB) as a carbon additive. On the one hand, as an excellent conductive material, KB usually presents a unique chainlike structure, and due to the strong van der Waals forces, the structure tends to be tightly gathered. Therefore, when KB is used for carbon coating, the primary particles of LiMn0.65Fe0.35PO4 coated by KB could also be tightly aggregated. Therefore, the addition of KB simultaneously improved the tap density (1.6 g cm–3) and conductivity (8.6 × 10–2 S cm–1) of LiMn0.65Fe0.35PO4. On the other hand, the unique structure of KB also brings a large specific surface area, which has powerful surface energy, so it often tends to perform agglomeration when used as an additive. In this work, two-step sintering was used to solve this problem, which can form a double-layer carbon coating composed of amorphous carbon (formed by the carbonization of glucose) in the inner layer and KB in the outer layer. Thus, KB can effectively adhere to the amorphous carbon to prevent the agglomeration of KB. Combining the exploration of the amount of KB added and the double-layer coating method, we showed that the LiMn0.65Fe0.35PO4/C/KB composite with 5% KB additive exhibits a high discharge capacity of 159.3 mAh g–1 at 0.2C and an excellent capacity retention of 96.8% at 1C rate after 500 cycles. In addition, due to the high surface area of KB, sufficient liquid electrolyte can be absorbed to increase the exchange rate of lithium ions at the electrode interface.