Voltage Decay of Li‐Rich Layered Oxides: Mechanism, Modification Strategies, and Perspectives

Abstract

Li-rich layered oxides (LLOs) have been considered as the most promising cathode materials for achieving high energy density Li-ion batteries. However, they suffer from continuous voltage decay during cycling, which seriously shortens the lifespan of the battery in practical applications. This review comprehensively elaborates and summarizes the state-of-the-art of the research in this field. It is started from the proposed mechanism of voltage decay that refers to the phase transition, microscopic defects, and oxygen redox or release. Furthermore, several strategies to mitigate the voltage decay of LLOs from different scales, such as surface modification, elemental doping, regulation of components, control of defect, and morphology design are summarized. Finally, a systematic outlook on the real root of voltage decay is provided, and more importantly, a potential solution to voltage recovery from electrochemistry. Based on this progress, some effective strategies with multiple scales will be feasible to create the conditions for their commercialization in the future.

Atomic Manufacturing in Electrode Materials for High-Performance Batteries

“…Voltage decay is also a primary factor that affects battery performance. It will result in a continuous reduction of energy density and increase the difficulty of practical application. , Therefore, comprehending the underlying mechanism and identifying effective solutions at the atomic level are imperative for overcoming this bottleneck. The galvanostatic charge–discharge curves of the Li- and Mn-rich (LMR) cathode exhibit a high discharge capacity but rapid voltage fade and capacity loss concurrently during cycling (Figure f) .…”

“…Benefiting from the reduced voltage hysteresis and polarization, it exhibits higher potential and energy density . The metal-doping strategy at the atomic level decreases the intrinsic antisite defect concentration, eliminating the voltage hysteresis and improving the initial Coulombic efficiency . Therefore, the problem of voltage hysteresis can be effectively solved by adjusting the intrinsic crystal structure of materials through atomic manufacturing.…”

“…58 The metal-doping strategy at the atomic level decreases the intrinsic antisite defect concentration, eliminating the voltage hysteresis and improving the initial Coulombic efficiency. 61 Therefore, the problem of voltage hysteresis can be effectively solved by adjusting the intrinsic crystal structure of materials through atomic manufacturing. Additionally, the distinct electron spin states of metal ions in layer-structured Li-rich cathodes have a significant impact on the oxidation potential of oxygen anions.…”

Structural factors for activating anionic redox in Li-rich Ti-based cathodes

“…These results are consistent with the AR-based voltages reported in experimental studies. 9–12,27 Additionally, Fig. S3,† which represents the net charge variation of anions upon the charge/discharge process, indicated that the four material systems are governed by the AR mechanism, but the presence of (non-)hysteretic anion activity is also suggested.…”

“…3–6 This degradation occurs because Mn 4+ at the octahedral site, which is coordinated with six oxygen ions, initiates oxygen oxidation, resulting in destabilisation of the crystal framework upon charging, because cationic oxidation for further oxidation to Mn 5+ during delithiation is complicated owing to crystal field theory energetics. 7–9 Based on this understanding, a similar model of monoclinic Li 2 TiS 3 , which is recognised as a redox-inactive cathode, was investigated and successfully synthesised to fully exploit the high theoretical capacity (∼339 mA h g −1 ) 9–13 and integrate it as a promising cathode with S-based solid electrolytes. 14–17 Consistent with Li 2 MnO 3 , the chemical state of the Ti ions in the Ti-based Li-rich sulphide was Ti 4+ , and its activity was unfeasible because no redox-capable 3d electrons exist in the valence band.…”