Oxygen-Defective TiNb₂O₇ Anode Synergized with Lithiated Polyacrylic Acid Binder for High-Power Lithium-Ion Storage

The growing demand for renewable energy necessitates the advancement of high-performance energy storage devices. Lithium-ion batteries (LIBs) are the primary energy storage solutions for portable electronic devices and have become predominant in the electric vehicle (EV) market. The existing LIBs use an organic-based liquid electrolyte and a graphite anode. However, the graphite anode has intrinsic limitations related to power density and safety. These limitations arise because lithium-ion intercalation into graphite occurs at a potential very close to that of lithium plating, especially during charging at high rates and low temperatures, which can lead to capacity loss, reduced cycle life, and safety concerns.<br/>TiNb₂O₇ (TNO) emerges as a promising intercalation-type anode material for LIBs due to its high theoretical capacity (≈ 388 mAh g^-1) and operating potential (≈1.6 V vs. Li⁺/Li) - above the decomposition voltage of carbonate-based electrolytes. However, TNO encounters several challenges that require attention for widespread adoption: (i) low electronic conductivity resulting from its wide energy band gap, (ii) time- and energy-intensive synthesis using conventional solid-state methods (typically exceeding 12 hours at 1250 °C), and (iii) the absence of a stable solid electrolyte interface (SEI) layer when discharged to 0.5 V. In our study, we address these issues using energy-efficient and environmentally friendly approaches.<br/>In this study, we prepared oxygen-defective TiNb₂O₇ (OD-TNO) using an ultra-fast Joule heating method. By introducing oxygen vacancies, the unit cell volume expanded due to partial reduction of Ti⁴⁺ and Nb⁵⁺, thereby accelerating lithium diffusion into the TNO structure. The transition from <i>d</i>⁰ to <i>d</i>¹ electronic configuration in Nb⁵⁺ (to Nb⁴⁺) and Ti⁴⁺ (to Ti³⁺) resulted in electronic conductivity in OD-TNO that was five orders of magnitude higher than that of conventional TNO. To form an artificial SEI layer on the OD-TNO particles, we replaced the conventional PVDF binder, which requires the toxic solvent N-methyl-2-pyrrolidone (NMP), with a water-based lithiated polyacrylic acid (Li-PAA) binder. The amorphous nature of Li-PAA not only improved film coverage through hydrogen bonding with surface oxygens in OD-TNO, preventing direct contact between TNO particles and the liquid electrolyte, but also facilitated rapid Li⁺ transport due to its incorporation of Li⁺ within the polymer structure. As a result, the Li-PAA binder enhanced both cycle stability and rate capabilities.<br/>The OD-TNO electrode with a Li-PAA binder exhibits a high initial discharge capacity of ~370.74 mAh g^-1, and remarkable rate capability (~149.40 mAh g^-1 at 10 C). After setting the rate back to 1C, the recovered capacity is 247.12 mAh g^-1 and the capacity remains stable, delivering a capacity of 181.03 mAh g^-1 with a retention of 73.26% at 1C over 800 cycles, demonstrating significantly improved cycling performance. To confirm its practical application potential, a full-cell comprising an OD-TNO anode and a LiNi_0.5Mn_1.5O₄ cathode was investigated. The full-cell demonstrates a capacity of 146.5 mAh g^-1 over 100 cycles with a capacity retention of 95.8%. These results highlight the synergetic effect of OD-TNO and Li-PAA as a practical high-energy and high-power density anode for LIBs.