Tailoring Co3d and O2p Band Centers to Inhibit Oxygen Escape for Stable 4.6 V LiCoO₂ Cathodes

Abstract

High-voltage LiCoO 2 delivers ah igh capacity but sharp fading is ac ritical issue,a nd the capacity decay mechanism is also poorly understood. Herein, we clarify that the escape of surface oxygen and Li-insulator Co 3 O 4 formation are the main causes for the capacity fading of 4.6 VL iCoO 2 . We propose the inhibition of the oxygen escape for achieving stable 4.6 VL iCoO 2 by tailoring the Co3d and O2p band center and enlarging their band gap with MgF 2 doping.T his enhances the ionicity of the CoÀObond and the redox activity of Co and improves cation migration reversibility.T he inhibition of oxygen escape suppresses the formation of Liinsulator Co 3 O 4 and maintains the surface structure integrity. Mg acts as apillar,providing astable and enlarged channel for fast Li + intercalation/extraction. The modulated LiCoO 2 shows almost zero strain and achieves ar ecordc apacity retention at 4.6 V: 92 %after 100 cycles at 1C and 86.4 %after 1000 cycles at 5C.

Hierarchical Doping Engineering with Active/Inert Dual Elements Stabilizes LiCoO2 to 4.6 V

“…Figure S22a S24 in the Supporting Information, it can be observed that the CoO bond of both LCO and LCONP with Ni TM site presented similar length at pristine state and contractive trend at highly delithiated state, implying their stronger covalency and more charge compensation from lattice O at high voltage. [6] In contrast, conspicuous divergence can be found from the model of LCONP with Ni Li site , wherein its longer CoO bond of 2.03995 Å than other samples suggested its increased ionicity between Co and O atoms. For the model of LCONP with Ni Li site at highly delithiated state, the CoO bond length became longer than its pristine state, suggesting its furtherly improved ionicity of CoO bond at high voltage.…”

“…FigureS24in the Supporting Information, it can be observed that the CoO bond of both LCO and LCONP with Ni TM site presented similar length at pristine state and contractive trend at highly delithiated state, implying their stronger covalency and more charge compensation from lattice O at high voltage [6]. In contrast, conspicuous divergence can be found from the model of LCONP with Ni Li site , wherein its longer CoO bond of…”

“…[5] Besides, owing to the overlap of O 2-2p and Co 3+/4+ 3d electronic band, deep delithiation nearing 4.6 V (vs Li/Li + ) would trigger the oxidation of lattice oxygen and formation of oxygen radicals, causing oxygen escape from the surficial lattice of LCO. [6] Even worse, the formed O radicals and dissolved high-valence Co would bring terrible catalytic impact, compelling severe side reactions with the electrolyte. [7] From the inner structure deterioration to the interfacial side reactions of LCO over high voltage, the interiteration effect inside and outside resultantly aggravates the degradation of LCO under 4.6 V (vs Li/Li + ).Various approaches have been tried to stabilize the LCO to 4.6 V. Surface coating with electrochemically inert components like metal-oxide, [8] phosphates, [9] fluorides, [10] and lithium compounds [11] was capable of resisting some irreversible reactions due to their passivated interface reaction with electrolyte.…”

“…[5] Besides, owing to the overlap of O 2-2p and Co 3+/4+ 3d electronic band, deep delithiation nearing 4.6 V (vs Li/Li + ) would trigger the oxidation of lattice oxygen and formation of oxygen radicals, causing oxygen escape from the surficial lattice of LCO. [6] Even worse, the formed O radicals and dissolved high-valence Co would bring terrible catalytic impact, compelling severe side reactions with the electrolyte. [7] From the inner structure deterioration to the interfacial side reactions of LCO over high voltage, the interiteration effect inside and outside resultantly aggravates the degradation of LCO under 4.6 V (vs Li/Li + ).…”

A review – exploring the performance degradation mechanisms of LiCoO2 cathodes at high voltage conditions and some optimizing strategies

“…Then, Co migration from Co layers to Li layers transforms the layered structure into a spinel-like/rock-salt structure. 26 2.2.2. Layer-to-spinel phase transition.…”