Critical Role of Oxygen Evolved from Layered Li–Excess Metal Oxides in Lithium Rechargeable Batteries

Abstract

The high capacity of the layered Li−excess oxide cathode is always accompanied by extraction of a significant amount of oxygen from the structure. The effects of oxygen on the electrochemical cycling are not well understood. Here, the detailed reaction scheme following oxygen evolution was established using real-time gas analysis and ex situ chemical analysis of the surface of the electrodes. A series of electrochemical/chemical reactions involving oxygen radicals constantly produced and decomposed lithium carbonate during cell operation. Moreover, byproducts, including water, affected the cycle life and rate capability: hydrolysis of the electrolyte salt formed hydrofluoric acid that attacked the surface of the electrode. This finding implies that protection of the electrode surface from damage, for example, by a coating or removal of oxygen radicals by scavengers, will be critical to widespread usage of Li−excess transition metal oxides in rechargeable lithium batteries.

The Role of Oxygen Release from Li- and Mn-Rich Layered Oxides during the First Cycles Investigated by On-Line Electrochemical Mass Spectrometry

“…The gas evolution can be divided into a CO 2 evolution starting at 4.2 V and ending before 4.6 V, followed by a second CO 2 production starting at 4.6 V after the activation plateau and coinciding with the onset of the evolution of O 2 . In agreement with the literature 25,35,37,56 and according to the use of a Li excess in HE-NCM synthesis, we attribute the CO 2 evolution at low voltages mainly to the electrooxidation of Li 2 CO 3 impurities, while the O 2 and CO 2 evolution at voltages higher than 4.6 V are both attributed to oxygen evolved from the HE-NCM lattice. We exclude any possible gas consumption on the Li counter-electrode by comparing the results obtained with LFP as counter-electrode.…”

“…The OEMS measurement shows also a slight decrease in the O 2 and CO 2 signals once the potential decreases below 3.0 V at the end of discharge, which can be attributed to the formation of Li 2 O 2 and Li 2 CO 3 on the HE-NCM surface. 25,56 This newly formed Li 2 CO 3 can then be oxidized in a subsequent charge, which we believe is the reason for the observed CO 2 evolution in the second charge, starting again at 4.2 V. This was already proposed previously. 56 In order to ensure that no gaseous products are consumed on the Li counter-electrode, the same cycling procedure was applied to HE-NCM but using partially delithiated LFP as counter-electrode (green curves in Figure 3; see also in the experimental part).…”

Fluoroethylene Carbonate as Electrolyte Additive for Improving the electrochemical performances of High-Capacity Li1.16[Mn0.75Ni0.25]0.84O2 Material

“…The peak at 286.2 eV is related to C-O, and the broad peak at 288.8 eV is related to C-O and Li 2 CO 3 [41][42][43]. The inorganic decomposition product Li 2 CO 3 , a by-product of electrochemical reaction, suffering from continuous reversible change accompanied with Li + intercalation/deintercalation in lithium-rich manganese-based layered material [44,45], is not obvious in the C1s spectrum of cathode cycled in electrolyte with FEC. The results indicate that the formed SEI film is stabilized in the electrolyte with FEC.…”

Effect of cooling on the structure and electrochemical properties of 0.3Li2MnO3 · 0.7LiNi0.5Mn0.5O2 cathode material

“…This layered oxide is composed of the intergrowth of LiMO 2 (M=Ni, Co, and Mn) and Li 2 MnO 3 phases, which have structural compatibility [5,6]. The Li 2 MnO 3 component which has the structure of C 2/m plays a critical role in stabilizing the structure of the Li-rich layered oxides and enhances the discharge capacity of the electrode at the high voltage above 4.5 V versus Li/Li + accompanying by removal of oxygen and Li + (loss of Li 2 O), which leads to the large initial irreversible capacity loss and poor rate capacity and, furthermore, limits the commercial application of these materials [7][8][9]. So how to improve the electrochemical performances is essential.…”