An almost full reversible lithium-rich cathode: Revealing the mechanism of high initial coulombic efficiency

Low initial Coulombic efficiency (ICE) is an important impediment to practical application of Li-rich layered oxides (LLOs), which is due to the irreversible oxygen release. It is generally considered that surface oxygen vacancies are conducive to the improvement of ICE of LLOs. To reveal the relation of oxygen vacancies and ICE, sample PLO (Li-Mn-Cr-O) and its treated product (TLO) are comprehensive investigated in this work. During the treated process, part of oxygen atoms return to original constructed vacancies. It makes oxygen vacancies in sample TLO much poorer than those in sample PLO, and induces the formation of Li-poor spinel-layered integrated structure. Electrochemical measurement indicates the ICE of sample PLO is only 80.8%, while sample TLO is almost full reversible with the ICE of ~97.1%. In term of high-energy X-ray diffraction, scanning transmission electron microscopy, X-ray photoelectron spectroscopy and synchrotron hard/soft X-ray absorption spectroscopy, we discover that the ICE is difficult to be improved significantly just by building oxygen vacancies. LLOs with high ICE not only have to construct suitable oxygen vacancies, but also require other components with Li-poor structure to stabilize oxygen. This work provides deep insight into the mechanism of high ICE, and will contribute to the design and development of LLOs for next-generation high-energy lithium-ion batteries.

Dual-site Doping to Enhance Oxygen Redox and Structural Stability of Li-rich Layered Oxides

Cobalt-free Li-rich layered oxides(LLOs)such as Li_(2)MnO_(3) have attracted extensive attention owing to their high specific capacity and low cost.Nonetheless,numerous problems such as continuous voltage fading and capacity decay have become stumbling blocks in its commercial application.In this study,we propose an effective dual-site doping strategy by choosing Mo as the cation and F as the anion to enhance the capacity and cycling performance.The researchdemonstrates that the cycling stability of LLOs enhances with the doping ratio of Mo,and their capacity increases with the doping ratio of F.It is because Mo as a pillar enhances the structural stability and F doping is conducive to the activation of Li;MnO;.What’s more,dual-site doping also promotes the diffusion of Li;and reduces the internal resistance of the electrode.Due to these improvements,the 5F3M sample still maintains a discharge capacity of 190.98 mAh g;after 100 cycles at 200 mA g^(-1),which is much higher than 165.29 mAh g;of the Pristine sample.This discovery provides a new way to develop advanced layered oxide cathodes for both Na-and Li-ion batteries.