Understanding voltage decay in lithium-excess layered cathode materials through oxygen-centred structural arrangement

Abstract

Lithium-excess 3d-transition-metal layered oxides (Li1+xNiyCozMn1−x−y−zO2, >250 mAh g−1) suffer from severe voltage decay upon cycling, which decreases energy density and hinders further research and development. Nevertheless, the lack of understanding on chemical and structural uniqueness of the material prevents the interpretation of internal degradation chemistry. Here, we discover a fundamental reason of the voltage decay phenomenon by comparing ordered and cation-disordered materials with a combination of X-ray absorption spectroscopy and transmission electron microscopy studies. The cation arrangement determines the transition metal-oxygen covalency and structural reversibility related to voltage decay. The identification of structural arrangement with de-lithiated oxygen-centred octahedron and interactions between octahedrons affecting the oxygen stability and transition metal mobility of layered oxide provides the insight into the degradation chemistry of cathode materials and a way to develop high-energy density electrodes.

Unveiling Nickel Chemistry in Stabilizing High‐Voltage Cobalt‐Rich Cathodes for Lithium‐Ion Batteries

“…[44] Interestingly, when the prepared cathodes were first charged to 4.45 V, we observed a small shoulder (at 528-533 eV) for LCO outer surface but not for LCNO (Figure 3j and Figure S20, Supporting Information), which indicates an suppressed oxidation of O 2− in LCNO surface during charge. In O K-edge, the pre-edge corresponds to transition from O core 1s to the unoccupied hybridized band state of O 2p and TM 3d orbitals, indicating the hole states in TMO bonding.…”

Voltage fade mitigation in the cationic dominant lithium-rich NCM cathode

“…To improve the cycling stability of these cathode materials, fundamental problems of the gradual irreversible phase transformation and detrimental voltage fade at around 2.5/3.3 V must be solved before getting into the real application. However, several debates about the evolution of voltage fade exist in the literature [29][30][31][32][33] . Many reported Li-rich Mn-based compounds display different capacity hysteresis between 1st and 2nd cycle at around 2.5/3.3 V where it is considered to undergo an irreversible phase transformation from its initial state to spinel-like phase 17,32 .…”

Controlled Atomic Solubility in Mn‐Rich Composite Material to Achieve Superior Electrochemical Performance for Li‐Ion Batteries

“…The limited solubility of atoms such as Ni and oxygen in the LNMO spinel phase at high temperature (>700 °C) leads to the formation of the Li x Ni y O phase [4] that can react with the Li 2 MnO 3 layered phase. [22] As a result, controlled composite structure can achieve both high energy density and high power capability even with a wide voltage range in the Mn-rich electrode materials. First, the defective LNMOlike spinel phase can substantially improve rate capability in the Mn-rich materials by reducing the structural change even after insertion of large amount of Li below 3 V, where usually Li ions can be inserted into empty 16c sites and then induce severe structural changes caused by a phase transition to the tetragonal spinel phase.…”

“…Considering that the c/a ratio of the layered phase after cycles is related to the degree of cation disordering (or mixing), [21] increased cation disordering in the layered phase in the pristine Q sample can lead to negligible change in the c/a ratio during electrochemical cycles due to high layered structure stability. [22] This result indicates that the degree of the cation disordering barely increases in the Ni incorporated Li 2 MnO 3 -like layered phase in the Q sample during two cycles compared to the S sample because Ni in Li layers in the Li 2 MnO 3 -like layered phase in a pristine Q sample can help in stabilizing the layered structure even with almost full extraction of Li by reducing strong electrostatic repulsion between TM layers (Table S4, Supporting Information). Robust layered structure induced by the increase in cation disordering (Ni incorporation in Li layer) in a pristine material and resilient spinel structure induced by defective structural features such as Ni/Mn disordering with oxygen and Ni vacancies can render the Q sample to enable the Mn-rich spinel-layered composite material to achieve high reversible energy density (Figure 4b) and high power density simultaneously (Figure 4e).…”