Controlling Cation Ordering and Oxygen Release in LiNi0.5Mn1.5O4

• LiNi0.5Mn1.5O4 (LNMO) is a promising spinel-type positive electrode (cathode) material for lithium-ion batteries (LiBs), with a theoretical capacity of 147 mAh/g and an operating voltage around 4.7 V (vs. Li/Li+). With both high ionic and electronic conductivity it is regarded as a suitable cathode material for high power applications. LMNO can adopt two different structural arrangements, P4332 (cation ordered) or Fd-3m (disordered) [1]. Generally speaking, it has been observed that batteries incorporating the latter structure exhibit reduced capacity fading during electrochemical cycling. However, synthesis conditions leading to disordering also lead to oxygen deficiency, rock-salt impurities and as a result generate Mn3+ [2,3]. Furthermore, while many literature reports categorise the material as either “ordered” or “disordered”, the material can adopt varying degrees of ordering. Thus, isolating the exact cause of capacity fade is challenging. In this study, in-situ neutron diffraction is performed on disordered and slightly Mn-rich LNMO samples (Mn:Ni ratio of 1.56:0.44) to follow cation ordering-disordering transformations during heating and cooling (see figure). The study shows for the first time that there is not a direct connection between oxygen deficiency and cation disordering. This demonstrates that it is possible to tune disordering in LNMO without inducing oxygen deficiencies or forming the rock-salt impurity phase. Electrochemical testing of samples with different degrees of ordering and oxygen deficiency (i.e. highly ordered, partially ordered and fully disordered) have been performed in LNMO-LTO (Li4Ti5O12) full cells. It was shown that all cells behave similarly during the initial period of cycling even when discharged at 10C rate, however, over time the disordered sample exhibited the best performance.

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NATURVETENSKAP  -- Kemi -- Materialkemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Materials Chemistry (hsv//eng)

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