Abstract:

Sodium-ion batteries (SIBs) have attracted immense attention in large-scale electrical energy storage. During the synthesis of layered cathode materials by the high-temperature solid-phase method, sodium salts, and metal oxides form a layered structure by breaking and reorganizing chemical bonds. However, some sodium salts fail to enter the bulk structure of the materials and remain on the surface of the materials to form alkaline substances, such as NaOH, NaHCO₃, and Na₂CO₃, collectively referred to as residual bases. These residual bases accelerate the dissolution of the transition metal layer in the liquid electrolyte, leading to irreversible degradation of the crystal structure of the cathode material. Moreover, the decomposition of sodium carbonate at high voltages produces CO₂ gas, one of the causes of battery pack expansion poses a safety hazard, and leads to severe sodium ion losses in sodium-ion full batteries, thereby limiting their energy density and cycle life. To solve this issue, this study adopted an innovative experimental scheme to successfully convert the harmful residual alkali components on the material surface into the NaMgPO₄ cladding structure. Differential electrochemical mass spectrometry (DEMS), a technique that has gained immense prominence in recent years, was used to confirm the effectiveness of this residual alkali treatment process. The prepared NaMgPO₄ cladding layer was uniformly covered on the surface of the cathode material with a thickness of about 5 nm and good crystallinity. X-ray diffraction (XRD) analysis showed that the diffraction peaks of the cladding material corresponded exactly to those of the original material, demonstrating that a small amount of cladding would not affect the crystal structure of the material. Furthermore, the presence of the capping layer slightly increases the spacing between the sodium layers, improving the multiplicity performance of the cathode material. The NNM-2 material showed excellent multiplicity performance, achieving a specific capacity of 169 mAh g^-1 during the first turn at a current density of 1C and a capacity retention rate of 74% following 100 cycles. The CO₂ emission was almost eliminated during the charging and discharging process, suggesting a substantial reduction in the residual alkali content.

Key words: sodium-ion battery, residual alkali, NaMgPO₄ coating, electrochemical mass spectrome