Intrinsic Defects-Assisted Phase Transition of Na₂Ti₃O₇ and its Application for High Performance Na-Ion Anodes

Na-ion batteries (NIBs), taking advantage of Na being the fourth most earth-abundant element, have emerged as a promising candidate for such applications. Together with cost-effective electrodes, the use of NIBs can bring a radical decrease in cost compared to the widely used Li-ion batteries, while ensuring sustainability.^[1] However, NIBs are still in a developmental research phase and the exploration of proper electrode materials is necessary.^[2]<br/>Na₂Ti₃O₇, with its large abundance of raw materials, non-toxicity, and high theoretical capacity of 177 mAhg^-1, ^[3,4] has emerged as one of the most attractive anodes for sustainable NIBs. However, previous tests have shown that Na₂Ti₃O₇ suffers from (i) low electrical conductivity and (ii) structural instability, which results in poor electrochemical performance, particularly at high charge/discharge rates.Efforts to improve the cycling stability of sodium titanates have discovered Na₂Ti₆O₁₃ which has properties contradictory to Na₂Ti₃O₇; Na₂Ti₆O₁₃ exhibits high ionic conductivity and structural stability but suffers from low Na storage capacity ^[5]. In this regard, proper hybridization of Na₂Ti₃O₇ and Na₂Ti₆O₁₃ could break the limitations of each structure and offer a composite electrode for high performance NIBs ^[6].<br/>One viable approach to synthesize mixed Na₂Ti₃O₇/Na₂Ti₆O₁₃ anode is a hydrogenation treatment.^[7] When synthesized with hydrogen gas, some of the Na₂Ti₃O₇ spontaneously transforms into Na₂Ti₆O₁₃, which provides high-performance Na₂Ti₃O₇/Na₂Ti₆O₁₃ hybrid anodes without additional synthesis routes. This procedure typically includes two reactions of O and Na removal, i.e., 2Na₂Ti₃O₇ + H₂ → 2Na₂Ti₃O_6.5 + H₂O and 2Na₂Ti₃O_6.5 + H₂O → Na₂Ti₆O₁₃ + Na₂O + H₂. Above reactions imply that O and Na vacancy defects formed during the synthesis condition are closely related to the spontaneous phase transition from Na₂Ti₃O₇ to Na₂Ti₆O₁₃. However, the interplay between defects and phase transition have been neglected to date, and the fundamental driving forces underlying the phase transition remain elusive.<br/>To better understand the role of native defects on the above phase transition behavior, we perform computational analyses on the intrinsic defect chemistry in Na₂Ti₃O₇. In particular, using the combined density functional theory (DFT) calculations with hybrid and PBEsol functionals, we replicate the high-temperature synthesis condition and investigate major defects formed during synthesis. The formation energies of the so-formed major defects are used to discuss the charge compensation behaviors and underlying conductivity mechanism, which can help establishing a doping strategy for anodes with improved performance. Furthermore, the effect of intrinsic defects of Na₂Ti₃O₇ on its atomic structure were studied to elucidate the origin of the spontaneous phase transition to Na₂Ti₆O₁₃, which can potentially guide further optimization of anodes employing phase transitions.<br/><br/>[1] N.Tapia-Ruiz et al., J. Phys. Energy 3.3 (2021): 031503<br/>[2] Chemical reviews 117.21 (2017): 13123-13186.<br/>[3] Advanced Functional Materials 26.21 (2016): 3703-3710.<br/>[4] Nano Energy 18 (2015): 20-27.<br/>[5] Small 12.22 (2016): 2991-2997.<br/>[6] Advanced Science 5.9 (2018): 1800519.<br/>[7] S.I.R. Costa, <b><i><u>Y.S. Choi</u></i></b>, A.J. Fielding, A.J. Naylor, J.M. Griffin, Z. Sofer, D.O. Scanlon, N. Tapia-Ruiz, Chem. - Eur. J. 27.11 (2021): 3875-3886.