Spinel materials for Li‐ion batteries: new insights obtained by operando neutron and synchrotron X‐ray diffraction

In the last few decades Li‐ion batteries changed the way we store energy, becoming a key element of our everyday life. Their continuous improvement is tightly bound to the understanding of lithium (de)intercalation phenomena in electrode materials. Here we address the use of operando diffraction techniques to understand these mechanisms. We focus on powerful probes such as neutrons and synchrotron X‐ray radiation, which have become increasingly familiar to the electrochemical community. After discussing the general benefits (and drawbacks) of these characterization techniques and the work of customization required to adapt standard electrochemical cells to an operando diffraction experiment, we highlight several very recent results. We concentrate on important electrode materials such as the spinels Li 1 +  x Mn 2 −  x O 4 (0 ≤ x ≤ 0.10) and LiNi 0.4 Mn 1.6 O 4 . Thorough investigations led by operando neutron powder diffraction demonstrated that neutrons are highly sensitive to structural parameters that cannot be captured by other means (for example, atomic Debye–Waller factors and lithium site occupancy). Synchrotron radiation X‐ray powder diffraction reveals how LiMn 2 O 4 is subject to irreversibility upon the first electrochemical cycle, resulting in severe Bragg peak broadening. Even more interestingly, we show for the first time an ordering scheme of the elusive composition Li 0.5 Mn 2 O 4 , through the coexistence of Mn 3+ :Mn 4+ 1:3 cation ordering and lithium/vacancy ordering. More accurately written as Li 0.5 Mn 3+ 0.5 Mn 4+ 1.5 O 4 , this intermediate phase loses the symmetry, to be correctly described in the P 2 1 3 space group.