Voltage Hysteresis in Transition Metal Oxide Cathodes for Li/Na‐Ion Batteries

Abstract

essential for the commercial application of Li/Na-ion batteries. The investigated cathode materials mainly include oxides, polyanion compounds, Prussian blue analogues, and organic salts. Among them, layered transition metal oxides contained in oxides are promising candidates due to their suitable operational potentials, 2D ions diffusion channels, and simple synthesis. Meanwhile, it was generally believed that to achieve a fast ions diffusion, the atomic configuration in the crystal structure of layered cathode materials must be well ordered. [7,8] However, this stereotype is now being reconsidered with the discovery of the percolation theory about 3D ions diffusion in a disordered rock-salt (DRX) structure. [9,10] This percolation occurs when Li-ions hop from one octahedral site to another through an intermediate tetrahedral site, where the Liions in the tetrahedral sites are identified as an active Li for the hopping. [11] In this regard, various new types of Li-rich DRX cathode materials with promising capacities (>250 mAh g −1 ) have been discovered. [12][13][14][15][16][17][18][19] Given their unique advantages, these oxide materials have attracted extensive attention from researchers worldwide.Although demonstrating a high capacity, application-wise, these oxide materials also exhibit voltage hysteresis and significant loss of energy density, since the voltage associated with this redox process on charge cannot be recovered on discharge, which represents a key challenge that inhibits exploiting the full potential of these materials. This voltage hysteresis persists throughout the electrochemical cycling, resulting in a continuous decrease of the average discharge voltage. Therefore, the understanding of voltage hysteresis is particularly important because it not only minimizes energy density and drastically reduces energy efficiency but also hinders further development and commercialization of Li/Na-ion batteries. When voltage hysteresis was discovered in LiFePO 4 materials in the early years, experimental limitations and the lack of a theoretical framework prevented researchers from drawing concrete conclusions about the phenomenon. With the development of increasingly sophisticated analytical techniques for studying voltage hysteresis, reassessing the understanding of the origin of voltage hysteresis can now be done from a mechanical perspective and even reveal new discoveries that were previously unavailable due to experimental limitations. [20]

High-entropy oxides: an emerging anode material for lithium-ion batteries

“…Although HEOs exhibit a higher rate and cycle performance than traditional low-entropy transition-metal anodes, the unsatisfactory energy density is inherent due to low ICE 111 and voltage hysteresis 112 from conversion-type anodes. Using HEO anodes with ICE means that the negative/positive capacity ratio of full cells is low, resulting in the energy density of cells at a relatively low level.…”

Interplay between Intermediate Anionic and Cationic Species and the Reflection to Voltage Hysteresis of Oxygen-Redox Battery Electrodes: A Holistic Picture

“…Among these drawbacks, voltage hysteresis is directly associated with the energy efficiency of batteries, creating a formidable challenge for the practical implementation of oxygen-redox materials. 5 Hence, further investigation of the root cause of voltage hysteresis is urgently required.…”

“…Unfortunately, the extra capacity from anionic redox (or oxygen redox) comes at the expense of sluggish kinetics, voltage fade, and hysteresis. Among these drawbacks, voltage hysteresis is directly associated with the energy efficiency of batteries, creating a formidable challenge for the practical implementation of oxygen-redox materials . Hence, further investigation of the root cause of voltage hysteresis is urgently required.…”