Research On The Structural Transformation And Modification Of LiNi_1-xCo_xO₂ Cathode Materials For Lithium Ion Batteries

Single-crystal particles are the research hotpot now,because it could efficiently solve the issue of particle cracking during the cycling of high-Ni layered oxides,and deliver a better cycling stability.However,some surficial residues,including NiO-type rock-salt phase and Li₂CO₃,are inevitably formed at the particle surface due to the high reactivity of Ni³⁺,which always result in an inferior electrochemical performance,and become one of the most serious problems to hinder the practical application.Accompanying with the high Ni content,Li/O loss from the layered structure,as well as the relevant structural evolution,have been extensively considered as a general origin for various detrimental phenomena,such as cationic disordering during high-temperature solid-state synthesis,chemical weathering at the surface during storage,and the capacity fading at high upper voltages?>4.3 V?during electrochemical tests.Herein,multiple macroscopic/microscopic characterization techniques,including in-situ transmission electron microscopy?TEM?,ex-situ X-ray diffraction?XRD?,and X-ray photoelectron spectra?XPS?,are combined to comprehensively investigate the thermal-induced local structural evolution vs Li/O loss in a representative binary high-Ni layered oxide LiNi_0.9Co_0.1O₂.The heterogenous Li/O loss kinetics in the bulk and at the surface are simultaneously tracked based on a rational structural model,revealing a quantitative relationship between Li/O loss and the phase transformation.The local structural evolution within single primary particles monitored by in-situ TEM further uncovers that,Li/O loss at the particle surface is accelerated via the large Li⁺diffusivity at high temperatures,finally leads to a phase transformation process from the bulk to the surface,in which a peculiar“anti-core-shell”structure within single primary particles is observed.The quantitative analysis combined with the direct observation not only demonstrate a feasible route to investigate the Li/O loss kinetics,but also provide valuable insights into the performance improvement of high-Ni layered oxides from the aspect of Li/O loss.Herein,an unprecedentedly clean surface without any surficial residues,composed by a disordered layered phase?⁶-nm thick?,is constructed in a representative LiNi_0.8Co_0.2O₂cathode by Ti gradient doping.It leads to the highest rate capacity reported yet(146 mA h g^-1at 20 C)and the superior cycling stability?a capacity retention of 97.71%after 200 cycles at1C and 25?,and 96.37%after 100 cycles at 5 C and 45??.This strategy could be extended to high-Ni layered oxides with different TM compositions and beyond.