Unravelling lithium storage mechanism in high performance MnO2@CNT Li-ion battery anode by in situ X-ray synchrotron scattering.

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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.

Sodium-ion hybrid capacitors (SHCs) have attracted great attention owing to the improved power density and cycling stability in comparison with sodium-ion batteries. Nevertheless, the energy density (<100 Wh·kg–1) is usually limited by low specific capacity anodes (<150 mAh·g–1) and “kinetics mismatch” between the electrodes. Hence, we report a high energy density (153 Wh·kg–1) SHC based on a highly pseudocapacitive interface-engineered 3D-CoO-NrGO anode. This high-performance anode (445 mAh·g–1 @0.025 A·g–1, 135 mAh·g–1 @5.0 A·g–1) consists of CoO (∼6 nm) nanoparticles chemically bonded to the NrGO network through Co–O–C bonds. Exceptional pseudocapacitive charge storage (up to ∼81%) and capacity retention (∼80% after 5000 cycles) are also identified for this SHC. Excellent performance of the 3D-CoO-NrGO anode and SHC is owing to the synergistic effect of the CoO conversion reaction and pseudocapacitive sodium-ion storage induced by numerous Na2O/Co/NrGO nanointerfaces. Co–O–C bonds and the 3D microstructure facilitating efficient strain relaxation and charge-transfer correspondingly are also identified as vital factors accountable for the excellent electrochemical performance. The interface-engineering strategy demonstrated provides opportunities to design high-performance transition metal oxide-based anodes for advanced SHCs.

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