The ever-growing demand for advanced rechargeable lithium-ion batteries in portable electronics and electric vehicles has spurred intensive research efforts over the past decade. The key to sustaining the progress in Li-ion batteries lies in the quest for safe, low-cost positive electrode (cathode) materials with desirable energy and power capabilities. One approach to boost the energy and power densities of batteries is to increase the output voltage while maintaining a high capacity, fast charge-discharge rate, and long service life. This review gives an account of the various emerging high-voltage positive electrode materials that have the potential to satisfy these requirements either in the short or long term, including nickel-rich layered oxides, lithium-rich layered oxides, high-voltage spinel oxides, and high-voltage polyanionic compounds. The key barriers and the corresponding strategies for the practical viability of these cathode materials are discussed along with the optimization of electrolytes and other cell components, with a particular emphasis on recent advances in the literature. A concise perspective with respect to plausible strategies for future developments in the field is also provided.

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The large-voltage hysteresis remains one of the biggest barriers to optimizing Li/Na-ion cathodes using lattice anionic redox reaction, despite their very high energy density and relative low cost. Very recently, a layered sodium cathode Na2Mn3O7 (or Na4/7Mn6/7□1/7O2, □ is vacancy) was reported to have reversible lattice oxygen redox with much suppressed voltage hysteresis. However, the structural and electronic structural origin of this small-voltage hysteresis has not been well understood. In this article, through systematic studies using ex situ/in situ electron paramagnetic resonance and X-ray diffraction, we demonstrate that the exceptional small-voltage hysteresis (<50 mV) between charge and discharge curves is rooted in the well-maintained oxygen stacking sequence in the absence of irreversible gliding of oxygen layers and cation migration from the transition-metal layers. In addition, we further identify that the 4.2 V charge/discharge plateau is associated with a zero-strain (de)­intercalation process of Na+ ions from distorted octahedral sites, while the 4.5 V plateau is linked to a reversible shrink/expansion process of the manganese-site vacancy during (de)­intercalation of Na+ ions at distorted prismatic sites. It is expected that these findings will inspire further exploration of new cathode materials that can achieve both high energy density and efficiency by using lattice anionic redox.

Modified Li-rich layered cathode Li(Li 0.2 Mn 0.54 Ni 0.13 Co 0.13 )O 2 has been synthesized by a simple strategy of using surface treatment with various amounts (0-30 wt.%) of Super P (carbon black). Based on detailed characterizations from X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS), it is suggested that the phase transformation from Li 2 MnO 3 -type of structure to spinel-like phase take place at the surface regions of particles during post annealing process at 3506C, leading to increase in both first coulombic efficiency and rate capability, from 78% and 100 mAh?g 21 (charge capacity at 2500 mA?g 21 ) of the pristine material to 93.4% and 200 mAh?g 21 . The evidences of spinel formation and the reasons for electrochemical enhancement are systematically investigated.T o overcome the critical drawbacks of commercial used LiCoO 2 such as limited energy density, high cost, toxicity and etc., an alternative cathode within layered structure is urgently needed to meet rapid development of hybrid electric vehicles (HEV) and electric vehicles (EV) [1][2][3] . To this end, the composite integrated in a nano-scale in form of layered-layered xLi 2 MnO 3 ?(1-x)LiMO 2 (M refers to commonly-used transition metals) has become appealing in a recent decade because the applicable specific capacity in such unique framework can reach as high as 250 mAh?g 214,5 . This is close to the theoretical capacity of layered cathodes. The high reversible capacity is believed to be associated with an initial loss of oxygen from the lattice as well as a corresponding phase transition in the first charging process [6][7][8][9] . However, the essential reasons for the structural stability at a deep delithiated state compared to other layered members such as Li 1-x CoO 2 and Li 1-x MnO 2 (0 , x , 1) are still not clear. Thackeray et al. 4 claimed that the substitution of units (Li 2 MnO 3 ) rather than cations or anions in the structure is the main reason for the substantial stability. In this regard, the control of activated amounts of Li 2 MnO 3 during first charging process is critical for following reversibility but limits the usage of real capacity 10 . On the other hand, although the group of Li-rich cathode materials is easy to achieve high reversible capacity, large irreversible capacity in the first cycle as well as the poor rate capability still gap them from the real applications.To understand principles and mechanisms governed poor rate capability, some research groups have used advanced techniques to lead investigation. Using real time gas analysis, Hong et al.11 observed that the oxygen ions are reversibly involved in the formation of Li 2 CO 3 species through surface regions of particles after the first activation, which could contribute to reversible capacity. However, such kinetic process during formation and decomposition of Li 2 CO 3 may slow down the flow rate of both ions and electrons which could...

A novel composite consisting of graphene-like MoS₂ nanosheets and ultrasmall Fe₃O₄ nanoparticles (≈3.5 nm) is synthesized as an anode for lithium ion battery application. In such composite anode, MoS₂ nanosheets provide flexible substrates for the nanoparticle decoration, accommodating the volume changes of Fe₃O₄ during cycling process; while Fe₃O₄ nanoparticles primarily act as spacers to stabilize the composite structure, making the active surfaces of MoS₂ nanosheets accessible for electrolyte penetration during charge/discharge processes. Owing to the high reversible capacity provided by the MoS₂ nanosheets and the superior high rate performance offered by ultrasmall Fe₃O₄ nanoparticles, superior cyclic and rate performances are achieved by FeFe₃O₄/MoS₂ anode during the subsequent electrochemical tests, delivering 1033 and 224 mAh g⁻¹ at current densities of 2000 and 10,000 mA g⁻¹, respectively.