Nancy Twu

NaTiO2: a layered anode material for sodium-ion batteries

We report on layered NaTiO2 as a potential anode material for Na-ion batteries. The material is prepared from only earth-abundant elements, delivers 152 mA h g−1 of reversible capacity at C/10 rate, shows excellent cyclability with capacity retention over 98% after 60 cycles, and high rate capability. Furthermore, in situ X-ray diffraction analysis reveals a reversible O3–O′3 phase transition, including an unusual lattice parameter variation coupled to complicated Na vacancy orderings in a series of 2nd order phase transitions.

A new class of high capacity cation-disordered oxides for rechargeable lithium batteries: Li–Ni–Ti–Mo oxides

Recent successes with disordered Li-excess materials and applications of percolation theory have highlighted cation-disordered oxides as high capacity and energy density cathode materials. In this work, we present a new class of high capacity cation-disordered oxides, lithium-excess nickel titanium molybdenum oxides, which deliver capacities up to 250 mA h g−1. These materials were designed from percolation theory which predicts lithium diffusion to become facile in cation-disordered oxides as the lithium-excess level increases (x > 1.09 in LixTM2−xO2). The reversible capacity and rate capability in these compounds are shown to considerably improve with lithium excess. In particular, Li1.2Ni1/3Ti1/3Mo2/15O2 delivers up to 250 mA h g−1 and 750 W h kg−1 (∼3080 W h l−1) at 10 mA g−1. Combining in situ X-ray diffraction, X-ray absorption near edge spectroscopy, electron energy loss spectroscopy, and electrochemistry, we propose that first charging Li1.2Ni1/3Ti1/3Mo2/15O2 to 4.8 V occurs with Ni2+/Ni∼3+ oxidation, oxygen loss, and oxygen oxidation in this sequence, after which Mo6+ and Ti4+ can be reduced upon discharge. Furthermore, we discuss how oxygen loss with lattice densification can affect lithium diffusion in the material by decreasing the Li-excess level. From this understanding, strategies for further improvements are proposed, setting new guidelines for the design of high performance cation-disordered oxides for rechargeable lithium batteries.

Designing New Lithium-Excess Cathode Materials from Percolation Theory: Nanohighways in LixNi2–4x/3Sbx/3O2

Increasing lithium content is shown to be a successful strategy for designing new cathode materials. In layered Li(x)Ni(2-4x/3)Sb(x/3)O2 (x = 1.00-1.15), lithium excess improves both discharge capacity and capacity retention at 1C. Structural studies reveal a complex nanostructure pattern of Li-Sb and Ni-Sb ordering where the interface between these domains forms the correct local configuration for good lithium mobility. The <1 nm Li-Sb stripe domains and their interfaces thereby effectively act as nanohighways for lithium diffusion.