Research on the doping strategy and electrochemical performance of nickel-rich cathode materials

The commercialisation of lithium-ion batteries (LIBs) has gradually reformed people's daily life since the 1990s. Compared with other cathode candidates, the Ni-rich ternary cathode materials have been continuously developed due to their high energy density and lower price. However, the fast capacity fading and poor thermal stability still restricted the applications of Ni-rich cathodes. In this regard, a comprehensive understanding of the failure mechanism and corresponding modification strategies need to be raised urgently. This thesis will firstly introduce the working mechanism of LIBs and different cathode material categories. Secondly, the Li/Ni mixing, which mainly induce the capacity fading of the Ni-rich cathode materials, will be schematically reviewed. The origin of the Li/Ni mixing has been attributed to (i) the similar bonding environments and ionic radius of Ni2+ and Li+, (ii) relieving magnetic frustration, (iii) reducing system entropy and (iv) lower Ni2+ migration activation energy. This thesis also introduced the methods to characterise the Li/Ni mixing from currently used powder diffraction and electron microscopy techniques to the future potential tools. Meanwhile, the influence of the Li/Ni mixing has also been summarised to (i) the restriction of Li+ diffusion, (ii) driving phase evolution and (iii) stabilising the structure. The possible tunning methods of Li/Ni mixing has also been concluded from the (i) optimising the sintering parameters, (ii) elements substitution and (iii) surface engineering. Besides, the thesis also briefly outlooked the future development of the Li/Ni disordering, covering the accuracy of Li/Ni mixing characterisation and tunning the Li/Ni mixing into an acceptable way to utilise it positively.