Voltage fade is a major problem in battery applications for high-energy lithium- and manganese-rich (LMR) layered materials. As a result of the complexity of the LMR structure, the voltage fade mechanism is not well understood. Here we conduct both in situ and ex situ studies on a typical LMR material (Li1.2Ni0.15Co0.1Mn0.55O2) during charge–discharge cycling, using multi-length-scale X-ray spectroscopic and three-dimensional electron microscopic imaging techniques. Through probing from the surface to the bulk, and from individual to whole ensembles of particles, we show that the average valence state of each type of transition metal cation is continuously reduced, which is attributed to oxygen release from the LMR material. Such reductions activate the lower-voltage Mn3+/Mn4+ and Co2+/Co3+ redox couples in addition to the original redox couples including Ni2+/Ni3+, Ni3+/Ni4+ and O2−/O−, directly leading to the voltage fade. We also show that the oxygen release causes microstructural defects such as the formation of large pores within particles, which also contributes to the voltage fade. Surface coating and modification methods are suggested to be effective in suppressing the voltage fade through reducing the oxygen release.Voltage decay is a major problem in applications of high-energy Li- and Mn-rich layer-structured battery materials. Here, the authors report the evolution of redox couples as the origin of the voltage decay and discuss strategies to suppress the problem.

(Invited) Evolution of Redox Couples in Li- and Mn-Rich Cathode Materials and Mitigation of Voltage Fade by Reducing Oxygen Release

The practical application of lithium-ion batteries suffers from low energy density and the struggle to satisfy the ever-growing requirements of the energy-storage Internet Therefore, developing next-generation electrode materials with high energy density is of the utmost significance There are high expectations with respect to the development of lattice oxygen redox (LOR)-a promising strategy for developing cathode materials as it renders nearly a doubling of the specific capacity However, challenges have been put forward toward the deep-seated origins of the LOR reaction and if its whole potential could be effectively realized in practical application In the following Review, the intrinsic science that induces the LOR activity and crystal structure evolution are extensively discussed Moreover, a variety of characterization techniques for investigating these behaviors are presented Furthermore, we have highlighted the practical restrictions and outlined the probable approaches of Li-based layered oxide cathodes for improving such materials to meet the practical applications

Demystifying the Lattice Oxygen Redox in Layered Oxide Cathode Materials of Lithium-Ion Batteries.

With the rapid growth of global electro‐mobility, the demand for cobalt is rapidly increasing because it is currently an indispensable component of the cathode materials in lithium‐ion batteries (LIBs). However, the use of Co raises moral issues caused by its exploitation and the geographic limitations of Co resources may result in risking the supply of LIBs. Thus, the development of cobalt‐free (Co‐free) cathodes has become a focus in the LIBs industry. Currently, Co‐free cathodes of lithium‐rich oxides, nickel‐rich layered oxides and spinel lithium nickel manganese oxide (LNMO) are promising candidates but face severe problems. This review article is the first to synthesize and present the progress of Co‐free cathodes made with Li‐rich oxides, Ni‐rich layered oxides and spinel LNMO, identifying their performance, problems and potential development trends. In particular, the roles of cobalt are further clarified, and the full‐cell performance and related commercialization prospects of the three above‐mentioned Co‐free cathodes are also objectively assessed. The focus of this work is to distill detailed and timely insights from the corresponding research progress on these materials and to demonstrate the associated urgency, challenges and opportunities in this field, thus facilitating a faster industrialization of Co‐free cathodes.

Cobalt‐Free Cathode Materials: Families and their Prospects

Li-rich layered oxides with high capacity are expected to be the next generation of cathode materials. However, the irreversible and sluggish anionic redox reaction leads to the O2 loss in the surface as well as the capacity and voltage fading. In the present study, a simple gas-solid treatment with ferrous oxalate has been proposed to uniformly coat a thin spinel phase layer with oxygen vacancy and simultaneously realize Fe-ion substitution in the surface. The integration of oxygen vacancy and spinel phase suppresses irreversible O2 release, prevents electrolyte corrosion, and promotes Li-ion diffusion. In addition, the surface doping of Fe-ion can further stabilize the structure. Accordingly, the treated Feox-2 % cathode exhibits superior capacity retention of 86.4 % and 85.5 % at 1 C and 2 C to that (75.3 % and 75.0 %) of the pristine sample after 300 cycles, respectively. Then, the voltage fading is significantly suppressed to 0.0011 V per cycle at 2 C especially. The encouraging results may play a significant role in paving the practical application of Li-rich layered oxides cathode.

A Simple Gas–Solid Treatment for Surface Modification of Li-Rich Oxides Cathodes

Stabilizing Li‐rich layered oxides without capacity/voltage fade upon cycling is a prerequisite for a successful commercialization. Although the inhibition of structural and interfacial changes is identified as an effective strategy, the battery community always seeks for a technologically flexible method to make it really competitive among the cathode. Herein, the gradient W‐doping within Li1.2Mn0.56Ni0.16Co0.08O2 (LLMO) is proposed to relieve crystal disintegration and simultaneously enhance interfacial stability because of the formation of Li2WO4 coating layer on the material surface. This is mainly attributed to the scenario that partial Mn replacement by W can stabilize the LLMO structure and regulate the electrochemical activity of Mn element. The W‐doped LLMO (W@LLMO) possesses improved specific capacity and voltage stability (83.2% capacity retention and voltage retention of 94.9% after 200 cycles at 0.5 C). Besides, a practical pouch cell based on the W@LLMO cathode presents sufficient gravimetric energy density (318 Wh kg−1) and cycling stability (capacity retention of 87.7% after 500 cycles at 1.0 C). This study presents an effective method to design robust Li‐rich layered cathodes for next‐generation Li‐ion batteries.

Zhou Xu

Papers

Potassium Prussian blue-coated Li-rich cathode with enhanced lithium ion storage property

Lewis Acidity Organoboron-Modified Li-Rich Cathode Materials for High-Performance Lithium-Ion Batteries

In situ construction of a flexible interlayer for durable solid-state lithium metal batteries

Enhanced Electrochemical Performance of Li1.2[Mn0.54Co0.13Ni0.13]O2 Enabled by Synergistic Effect of Li1.5Na0.5SiO3 Modification