Boron-Centered Organic Salts Enabling Na-Ion Supply and Interfacial Protection for Na-Ion Batteries

Sodium (Na)-ion batteries employing hard carbon anodes suffer from a significant irreversible loss of active Na ions (up to 20%) during the initial formation cycle. Conventional Na-ion compensation methods are hindered by issues such as incomplete decomposition of Na-ion supply agents, the generation of harmful byproducts, and electrode degradation. To address these challenges, we utilized unsupervised machine learning to develop an organic Na salt, methylboronic acid sodium salt (CH₃B(ONa)₂), which is coated on cathode particles and effectively delivers over 15% Na-ion compensation. Meanwhile, its decomposition product, sodium metaborate (NaBO₂), in situ formed a protective cathode coating that mitigates transition metal dissolution. Spectroscopic and microscopic studies identified a free radical mechanism of CH₃B(ONa)₂ decomposition reaction and effective inhibition of nickel metal dissolution in cathode due to the presence of NaBO₂. In addition, no side effects were found in the process of Na-ion supply. The initial coulombic efficiency of a hard carbon|P2-Na_0.75Ni_0.25Fe_0.25Mn_0.5O₂ pouch cell increased from 81% to 97%, with a capacity retention of 81.5% over 700 cycles. This dual-function approach significantly enhances cycling stability and capacity retention in Na-ion batteries.