Operando Lithium Dynamics in the Li‐Rich Layered Oxide Cathode Material via Neutron Diffraction

Abstract

in battery technology have come since its fi rst demonstration, the high energy demands needed to electrify the automotive industry have not yet been met with the current technology. [ 2,3 ] One considerable bottleneck is the cathode energy density. [ 2,3 ] The lithium layered oxides utilize transition metal redox pairs for charge/ discharge compensation during lithium extraction and intercalation offering a theoretical capacity of 270 mAh g −1 for complete lithium extraction. [ 3,4 ] However, practical capacities have so far shown to be ≈200 mAh g −1 due to degradation reactions and large lattice contractions at low lithium content, limiting its capability to meet future demands. One possible cathode material is the Li-rich layered oxide compounds x Li 2 MnO 3 ⋅(1 − x )LiMO 2 (M = Ni, Mn, Co) (0.5 = < x = <1.0) that exhibit capacities over 280 mAh g −1 obtainable by the combination of the typical transition metal redox pair with the additional oxygen redox reaction as the charge compensation mechanism. [ 5 ] In this class of compounds, lithium ions can reside in both lithium layer and transition metal layer of close packed oxygen framework, typical from O3 type layered oxides like LiCoO 2 . Large irreversible capacities are often observed in these materials due to irreversible oxygen loss or side reactions stemming for the electrolyte. [ 6 ] It has been also observed using ex situ NMR (nuclear magnetic resonance) that lithium reinsertion back into the transition metal layer is little to none. [ 7 ] Several different lithium extraction/insertion sites and migration pathways are available, where lithium may be extracted from lithium or transition metal layers and lithium from octahedral coordinated sites to tetrahedral sites to form Li-Li dumbbells. [ 8 ] However, these studies have not revealed the dynamic process of lithium migration for the Li-rich material under operando electrochemical cycling conditions. Neutron scattering has several distinct advantages for battery studies: (1) The sensitivity of neutron to light elements such as lithium and oxygen is signifi cant in order to determine their position in the crystal structure; (2) Compares to the X-ray, the neutron shows larger scattering contrast between neighboring elements in the periodic table specifi cally the scattering lengths, e.g., for transition metals in this case: Ni, 10.3 fm; Mn, −3.73 fm; Co, 2.49 fm; and (3) The deep penetration capability of neutron allows simultaneous observation of the cathode and

Oxygen Release and Surface Degradation of Li- and Mn-Rich Layered Oxides in Variation of the Li2MnO3Content

“…These repulsive forces would furthermore be reduced by the reported reversible oxygen redox, 26,32,33 whereby it is conceivable that the creation of vacancies in the transition metal layer during the first activation cycle is responsible for enabling oxygen redox processes. 38 However, increasing the Li 2 MnO 3 content leads to an increased lithium occupation in the transition metal layer in the pristine material 58 that will be extracted during the first activation charge, leading to a destabilization of the surface at increasingly lower SOCs, as was discussed above.…”

Nucleation of dislocations and their dynamics in layered oxide cathode materials during battery charging

“…In the beginning of the charge at 4.0 V, we observe a continuous displacement field. At 4.2 V the displacement field has changed only slightly, an expected structural response of the nanoparticle to delithiation 20 . When charged to 4.3 V, before the voltage plateau that signifies oxygen evolution 16 , the displacement field is qualitatively different and shows two singularities characteristic of dislocations.…”

“…The local composition of cations and their associated interaction turns out to be crucial for voltage stability and lithium diffusion capabilities 12,18,19 . Various stacking sequences of the transition metal (TM) layers have been identified (stacking faults) 18,20 . X-ray and neutron scattering measurements revealed that the layer spacing (c lattice parameter) expands during charge from 14.25 Å to about 14.40 Å with simultaneous contraction of the a-b plane 20 , resulting in significant volume changes and cracking of secondary particles upon the initial charge to 4.5 V 21 .…”

“…Various stacking sequences of the transition metal (TM) layers have been identified (stacking faults) 18,20 . X-ray and neutron scattering measurements revealed that the layer spacing (c lattice parameter) expands during charge from 14.25 Å to about 14.40 Å with simultaneous contraction of the a-b plane 20 , resulting in significant volume changes and cracking of secondary particles upon the initial charge to 4.5 V 21 . Indeed, the formation of cracks and stress-induced damage in secondary particles has been identified in other layered oxides and is believed to be a leading cause of degradation and debilitating electrochemical performance 3,22,23 .…”

Feasibility of Cathode Surface Coating Technology for High‐Energy Lithium‐ion and Beyond‐Lithium‐ion Batteries

“…[9][10][11] NRO materials, however, suffer from various drawbacks, such as a difficult synthesis procedure with Ni 3+ ions, low safety, unwanted side reactions with the electrolyte on the surface, moisture/air sensitivity, and quick capacity fading. [15,16] Although spinel LiMn 2 O 4 exhibits appealing cathode features in terms of material abundance, cost, and safety, its use is hindered by its low capacity and its quick fading during cycling due to decomposition of the electrode-electrolyte interphase associated with Mn 2+ dissolution and Jahn-Teller distortion of Mn 3+ ions. [15,16] Although spinel LiMn 2 O 4 exhibits appealing cathode features in terms of material abundance, cost, and safety, its use is hindered by its low capacity and its quick fading during cycling due to decomposition of the electrode-electrolyte interphase associated with Mn 2+ dissolution and Jahn-Teller distortion of Mn 3+ ions.…”