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Ti2Nb2O9 with a tunnel-type structure is considered as a perspective negative electrode material for Li-ion batteries (LIBs) with theoretical capacity of 252 mAh g–1 corresponding to one-electron reduction/oxidation of Ti and Nb, but only ≈160 mAh g–1 has been observed practically. In this work, highly reversible capacity of 200 mAh g–1 with the average (de)­lithiation potential of 1.5 V vs Li/Li+ is achieved for Ti2Nb2O9 with pseudo-2D layered morphology obtained via thermal decomposition of the NH4TiNbO5 intermediate prepared by K+→ H+→ NH4 + cation exchange from KTiNbO5. Using operando synchrotron powder X-ray diffraction (SXPD), single-phase (de)­lithiation mechanism with 4.8% unit cell volume change is observed. Operando X-ray absorption near-edge structure (XANES) experiment revealed simultaneous Ti4+/Ti3+ and Nb5+/Nb4+ reduction/oxidation within the whole voltage range. Li+ migration barriers for Ti2Nb2O9 along [010] direction derived from density functional theory (DFT) calculations are within the 0.15–0.4 eV range depending on the Li content that is reflected in excellent C-rate capacity retention. Ti2Nb2O9 synthesized via the ion-exchange route appears as a strong contender to widely commercialized Ti-based negative electrode material Li4Ti5O12 in the next generation of high-performance LIBs.

We have applied spectroscopic ellipsometry with binary polarization modulation to study the refractive index n(X) and extinction coefficient k(X) spectra of as-deposited and irradiated with nitrogen ions polymethylmethacrylate (PMMA) and polystyrene (PS) films in 300-1030 nm range. The results of performed investigation confirmed the possibility and estimate restrictions of the ion implantation for local change the refractive index of polymeric materials.

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