The next generation of mobile and wireless communication system, i.e. 5G mobile technologies are emerging in our daily life and are expected to reshape modern society in frameworks like Smart City, Smart Grid and Internet-of-Things. Targeting at tremendous improvement in bandwidth, scalability, capacity and end-to-end delay, 5G technologies intrinsically demand energy storage devices with even larger reversible capacity and higher power density in base stations and terminal devices. Rechargeable Li-ion batteries are the core energy storage devices in modern world, which will provide valid support for the upcoming 5G era. Among various battery materials, LiCoO₂ takes a large market share in electronic applications, mostly due to its superior electrochemical properties and supreme volumetric energy density. However, LiCoO₂ has been confined within relatively low charging voltage ~4.3 V for practical applications in past decades, leaving much potential capacity (~30-40% of theoretical value) unexploited.

To meet the approaching market flourish, Li-ion batteries are leading towards even higher voltage and larger reversible capacity. However, high voltage cycle of LiCoO₂ generally accompanies crystal lattice collapse, interfacial side reactions, dramatic volumetric expansion and shrinkage during cycle, and even more, safety concerns. All these factors add up to drastic LiCoO₂ battery failure at high voltage. Particularly, researchers believe that oxygen participation in electrochemical reactions and release from lattice at high voltage is a major reason for the rapid fading of LiCoO₂ at voltage higher than ~4.3 V. To overcome the long-lasting hindrance, foreign atoms doping has been developed as a promising technique to alter electronic structure, phase transition process and favor high voltage cycling. Corresponding investigations have been marching on gradually in past years.

With delicate and optimized trace Ti-Mg-Al codoping strategy, researchers from Institute of Physics CAS, Dr. Jienan Zhang, Dr. Qinghao Li, Prof Xiqian Yu and Prof Hong Li, cooperated with oversea researchers from Brookhaven National Laboratory, SLAC National Accelerator Laboratory and Lawrence Berkeley National Laboratory, achieved stable high voltage cycle of LiCoO₂ and revealed the doping mechanisms. Reversible capacity high as 174 mAh/g after 100 cycles can be realized in voltage range 3.0-4.6 V. The up-to-date highest value makes another benchmark toward further exploiting theoretical capacity of the most prevailing cathode material. Corresponding investigation is published on Nature Energy (Nature Energy，2019，DOI: 10.1038/s41560-019-0409-z), with title of “Trace doping of multiple elements enables stable battery cycling of LiCoO₂ at 4.6 V”.

To realize high voltage cycle stability and understanding fundamental mechanism, researchers combined optimal material design and systematic characterizations. By virtue of EDX/EELS, Mg and Al demonstrate homogeneous distribution, while Ti aggregates at particle surface. The phase transition process was characterized by in-situ XRD, verifying that the mitigation of phase transition from O3 to H1-3 as the origin of enhanced cycle stability. By virtue of synchrotron based FY-STXM performed at SLAC and RIXS conducted at ALS, more fundamental insights can be obtained on the role of doping elements. The 3D distribution of dopants can be visualized by FY-STXM, validating Ti aggregation at particle surface as well as internal grain boundaries. Moreover, Ti doping at particle surface proves beneficial to superior O stability at high voltage, as verified by O-K edge RIXS map and supported by XPS characterization. The experimental findings can be well supported and explained by theoretical calculations on doping elements distribution and charge compensation.

The findings from this work help build up a more comprehensive understanding toward high voltage cycle of LiCoO₂. The specific doping elements distribution and doping induced O stabilization is verified decisive to the improved high voltage cycle of LiCoO₂. More importantly, this work demonstrated the significance of comprehensive modifications from various aspects in material design. Multi-scale and multi-faceted characterisations are the key to gain insights into the roles of the modification elements as well as the fundamental principles of the modification approaches. Furthermore, these findings are not limited to LiCoO₂ system but are inspiring to the development of other types of cathode materials, which is not only fundamentally important but also practically valuable.

This work was supported by funding from National Key R&D Program of China (grant no. 2016YFB0100100), National Natural Science Foundation of China (grant no. 51822211, 11574281) and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (grant no. 51421002).