The thermal instability of the cathode materials in lithium‐ion batteries is an important safety issue, requiring the incorporation of several approaches to prevent thermal runaway and combustion. Systematic studies, using combined well‐defined in situ techniques, are crucial to obtaining in‐depth understanding of the structural origin of this thermal instability in overcharged cathode materials. Here time‐resolved X‐ray diffraction, X‐ray absorption, mass spectroscopy, and high‐resolution transmission electron microscopy during heating are combined to detail the structural changes in overcharged LixNi0.8Co0.15Al0.05O2 and LixNi1/3Co1/3Mn1/3O2 cathode materials. By employing these several techniques in concert, various aspects of the structural changes are investigated in these two materials at an overcharged state; these include differences in phase‐distribution after overcharge, phase nucleation and propagation during heating, the preferred atomic sites and migration paths of Ni, Co, and Mn, and their individual contributions to thermal stability, together with measuring the oxygen release that accompanies these structural changes. These results provide valuable guidance for developing new cathode materials with improved safety characteristics.

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wileyonlinelibrary.com(≈70%) ionic radius than the Li + ion, which results in high energy barrier for Na + ion insertion/extraction and structural degradation of the host during cycling.In LIBs system, it is well known that high capacity can be achieved by utilizing materials based on conversion reactions, such as metal fl uorides, [ 4 ] metal oxides [ 5 ] and metal nitrides, [ 6 ] which differ from the intercalation materials by their reaction mechanism. Among them, metal fl uorides have been extensively studied as potential high capacity cathode materials due to their relatively high working potential and high capacity (500 to 750 mAh g −1 ) versus lithium. Although the theoretical cell potential of metal fl uorides versus sodium is 0.44 V lower than the lithium system, [ 7 ] this shortcoming can be compensated by their high capacity. Thus, it is valuable to examine the feasibility of conversion reactions based on metal fl uorides in sodium ion system. Nishijima et al. [ 8 ] studied the electrochemical performance of FeF 3 versus Na but ended up with a low initial discharge capacity around 150 mAh g −1 and reversible capacity less than 100 mAh g −1 between 1.5 to 4.0 V vs Na + /Na. Li et al. [ 9 ] synthesized a single wall carbon nanotubes (SWNTs) wired FeF 3 ·0.5H 2 O electrode and tested its Na-storage property. It exhibited a reversible capacity of 100 mAh g −1 , and only 74 mAh g −1 were preserved after 50 cycles. Compared with the electrochemical performance of FeF 3 in LIBs system, [ 4b , 4c ] FeF 3 seems not to be fully active in SIBs system. Several other metal fl uorides like TiF 3 , MnF 3 , and CoF 3 were also investigated for sodium storage, but all of them showed poor electrochemical performances. [ 8 ] Previous research suggests that the addition of small amounts of oxygen into the fl uoride structure is effective in improving the material's electronic conductivity and reversibility of (re)conversion reactions with lithium. [ 10 ] It could be expected that introducing oxygen into metal fl uorides may weaken the average bonding strength between cations and anions, which then facilitates the reversible electrochemical reaction between the active material and sodium. However, to the best of our knowledge, there has been no published report on the electrochemical behavior of the metal oxyfl uorides for sodium ion batteries.Here, we report the fi rst experimental results of the electrochemical Na activity of a nanocomposite of carbon and iron oxyfl uoride (FeO 0.7 F 1.3 /C) which was synthesized via a solution based process. The nanocomposite delivers very high reversible Na storage capacity of ≈360 mAh g −1 even after 50 cycles. The origin of such high Na-storage capacity of the nanocomposite Searching high capacity cathode materials is one of the most important fi elds of the research and development of sodium-ion batteries (SIBs). Here, we report a FeO 0.7 F 1.3 /C nanocomposite synthesized via a solution process as a new cathode material for SIBs. This material exhibits a high initial discharge...

Suppressing oxygen release from lithium ion battery cathodes during heating is a critical issue for the improvement of the battery safety characteristics because oxygen can exothermically react with the flammable electrolyte and cause thermal runaway. Previous studies have shown that oxygen release can be reduced by the migration of transition metal cations from octahedral sites to tetrahedral sites during heating. Such site‐preferred migration is determined by the electronic structure of cations. Taking advantage of the unique electronic structure of the environmental friendly Fe, this is selected as substitution element in a high energy density material LiNi0.5Mn1.5O4 to improve the thermal stability. The optimized LiNi0.33Mn1.33Fe0.33O4 material shows significantly improved thermal stability compared with the unsubstituted one, demonstrated by no observed oxygen release at temperatures as high as 500°C. Due to the electrochemical contribution of Fe, the high energy density feature of LiNi0.5Mn1.5O4 is well preserved.

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