Construction of a stable interface at the Na0.67Ni0.33Mn0.67O2 cathode using LiDFOB electrolyte additives for high-performance sodium-ion batteries

Na0.67Ni0.33Mn0.67O2 (NNMO) exhibits high energy density and reversible capacity and hence is considered a promising cathode material for sodium-ion batteries (SIBs). However, the dissolution of transition metal (TM) ions results in poor cycling performance due to structural destruction. A robust and dense cathode electrolyte interlayer can be formed at the interface between the cathode and the electrolyte to effectively maintain the structure stability of the cathode. Therefore, employing small doses of organic molecules as functional electrolyte additives to form a stable interface between the electrolyte and electrodes is one of the most promising approaches to commercializing SIBs. Herein, lithium difluoro(oxalate)borate (LiDFOB) at different concentrations was added to an electrolyte to improve the rate performance and cycling stability of the cathode during the charge-discharge process by inhibiting structural damage. Results demonstrate that adding 0.8 wt% LiDFOB to a baseline electrolyte is desirable as it decomposes to form a firm and thin cathode-electrolyte interface (CEI) that inhibits the dissolution of TM ions. In addition, it was found to inhibit the decomposition of NaPF6 and decrease the content of hydrogen fluoride (HF), thus preventing the erosion of the surface of cathode materials. The poor cycling stability of NNMO//Na half-cells in the baseline electrolyte led to a capacity retention of only 33.3% after 100 cycles at 1C. In contrast, the NNMO//Na half-cell containing 0.8 wt% LiDFOB showed prominent electrochemical properties, with a high capacity retention of 79.8% by maintaining a specific capacity of 93 mA h g-1 after 300 cycles at 1C. The NNMO//hard carbon full cell containing 0.8 wt% LiDFOB exhibited a specific capacity of 136.59 mA h g-1 and a specific energy density of 432.79 W h kg-1 at a current density of 0.1C. It maintained 99.7% capacity at 1C after 700 cycles. This work is expected to provide useful insights into the addition of LiDFOB as a potential approach to solve the challenge of poor structural stability of NNMO during cycling.