High‐Performance Mg−Li Hybrid Batteries Based on Pseudocapacitive Anatase Ti_1‐xCo_xO_2‐y Nanosheet Cathodes

Abstract

Despite the proposed safety, performance, and cost advantages, practical implementation of MgÀ Li hybrid batteries is limited due to the unavailability of reliable cathodes compatible with the dual-ion system. Herein, a high-performance MgÀ Li dual ion battery based upon cobalt-doped TiO 2 cathode was developed. Extremely pseudocapacitance-type Ti 1-x Co x O 2-y nanosheets consist of an optimum 3.57 % Co-atoms. This defective cathode delivered exceptional pseudocapacitance (maximum of 93 %), specific capacities (386 mAh g À 1 at 25 mA g À 1 ), rate performance (191 mAh g À 1 at 1 A g À 1 ), cyclability (3000 cycles at 1 A g À 1 ), and coulombic efficiency ( � 100 %) and fast charging ( � 11 min). This performance was superior to the TiO 2 -based MgÀ Li dualion battery cathodes reported earlier. Mechanistic studies revealed dual-ion intercalation pseudocapacitance with negligible structural changes. Excellent electrochemical performance of the cation-doped TiO 2 cathode was credited to the rapid pseudocapacitance-type Mg/Li-ion diffusion through the disorder generated by lattice distortions and oxygen vacancies. Ultrathin nature, large surface area, 2D morphology, and mesoporosity also contributed as secondary factors facilitating superior electrode-electrolyte interfacial kinetics. The demonstrated method of pseudocapacitance-type MgÀ Li dual-ion intercalation by introducing lattice distortions/oxygen vacancies through selective doping can be utilized for the development of several other potential electrodes for high-performance MgÀ Li dual-ion batteries.

In Situ Raman Spectroscopy of Li+ and Na+ Storage in Anodic TiO2 Nanotubes: Implications for Battery Design

“…The shape of d­( Q – Q 0 ) d E –1 is typical for a diffusion-controlled intercalation-type battery electrode . The peak positions are slightly shifted to 1.84 and 1.95 V for the reference TiNP possibly due to the difference in the crystallite size and/or morphology variations (Figure b) . The full width at half-maximum (fwhm) of both the anodic and cathodic reflections increased considerably with respect to the TiNT profile, and an additional quasi-rectangular area appears at lower voltages.…”

“…Despite the presence of the three above-discussed distinguishable regions, the plateau region shrinks tremendously and the sloping tail dominates. Considering the broader cathodic peaks and a low-voltage quasi-rectangular region observed in the differential capacity plot, the sloping region is more likely to represent a surface-controlled capacitive-type, probably pseudocapacitive, storage rather than LiTiO 2 formation . The electrochemical Li-ion insertion rate performance of the TiNTs is demonstrated in Figure S2.…”

“…34 The peak positions are slightly shifted to 1.84 and 1.95 V for the reference TiNP possibly due to the difference in the crystallite size and/or morphology variations (Figure 2b). 35 The full width at half-maximum (fwhm) of both the anodic and cathodic reflections increased considerably with respect to the TiNT profile, and an additional quasi-rectangular area appears at lower voltages. The noticeable difference in the d(Q − Q 0 ) dE −1 profile may be attributed to a significant non-diffusion-limited capacitive contribution to the overall storage capacity in addition to the inherent intercalation-type storage.…”

“…Considering the broader cathodic peaks and a low-voltage quasi-rectangular region observed in the differential capacity plot, the sloping region is more likely to represent a surface-controlled capacitive-type, probably pseudocapacitive, storage rather than LiTiO 2 formation. 35 The electrochemical Li-ion insertion rate performance of the TiNTs is demonstrated in Figure S2. The rate performance is in well agreement with the existing reports on anodic nanostructures.…”

“…However, the fwhm of the bands slightly decreases, and the bands experience a negligible but gradual shift to higher frequencies simultaneously with a voltage drop to 0 V. Such stable host lattices with significant crystallinity changes upon ion uptake are typical for capacitive-type charge storage. 9,35 Slight crystallinity rupture observed as the band intensity is lowered shall be credited to the internal stress generated within the lattice due to negligible high-ionic-size Na-ion insertion. In situ observations justify the assumptions obtained from the electrochemical characteristics.…”