88 89 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 at the end of the first charge, and no traces of other crystalline or amorphous secondary phases could be detected. After the first charge, the RScycles reversibly between charged and discharged states with an exchange of ~0.4 Li per formula unit without the further participation of Li 2 O. The local structural evolution of the RS phase remaining after the initial charge is investigated in more detail in Fig. 1b, where the effect of the lattice parameter changes was removed by multiplying the r -scale by the ratio of the lattice parameters determined by Rietveld refinement. The narrow Mn-Mn distributions centred around the expected values for the average RS structure indicate the Mn-framework is well ordered, while the broader, asymmetric and shifted Mn-O distributions indicate a high degree of disorder within the oxygen site.Thus, the RS component can accommodate varying concentrations of lithium thanks to the breathing of the cubic Mn-framework that isotropically contracts and expands to extract and incorporate lithium, accompanied by displacements of oxygen atoms. These observations evidence the electrochemical activation of Li 2 O catalysed by nanostructured RS could act as a Li + -donor and explain the continuous A cathode additive improves lithium-ion batteries Related publication: Diaz-Lopez M., Chater P. A., Bordet P., Freire M., Jordy C., Lebedev O. I. & PralongV. Li2O:Li–Mn–O Disordered Rock-Salt Nanocomposites as Cathode Prelithiation Additives for High-Energy Density Li-Ion Batteries. Adv. Energy Mater. 10 , 1902788 (2020). DOI: 10.1002/ aenm.201902788 Publication keywords: Li-ion batteries; In situ pair distribution function; Cation disordered rock-salts; Sacrificial additives D uring the first charging cycles in lithium-ion (Li-ion) batteries, lithium from the cathodematerial is irreversibly bound at the surface of the anode. This loss of lithium from the cathode reduces the overall cell capacity and limits the battery’s performance. Additives can provide extra lithium ions to the cathode by reacting during the first charge, compensating for lost lithium ions. However, many of these sacrificial cathodematerials produce gases, which can be dangerous. Researchers developed a sacrificial cathode additive, made from a nanoscale mixture of Li 2 O and Li-Mn-O, with outstanding first charge capacities. Theymonitored the sacrificial cathode reaction during the first charging cycles, using in situ Pair Distribution Function (PDF) on the I15-1 beamline at Diamond Light Source. Theymade use of the Diamond Radial In situ X-ray (DRIX) electrochemical cell, which is optimised for fast operando PDF analysis. The low and constant background scattering from the DRIX cell provides excellent total scattering data quality, even on nanostructured or weakly-scattering samples. This design allows for a comprehensive understanding of reactions within battery materials including knowledge of the short-range ordering, nanostructures and amorphous intermediates. ThestudydemonstratedthatthefirstchargecapacitiesofLi 2 O:Li-Mn-OsacrificialsareachievedbytheelectrochemicalactivationofLi 2 O.Adding the sacrificial to the battery cathode improved the first charge capacity of LiFePO 4 and LiCoO 2 by 13%. Due to their lowcost, ease of preparation and compatibility with industrial battery fabrication, Li 2 O:Li-Mn-O sacrificials are highly promising, outperforming currently used additives. The irreversible loss of lithium from the cathode material during the formation of a Solid Electrolyte Interface in the first battery charge notably reduces the capacity of Li-ion batteries, with capacity losses ranging from 7-20 % for graphitic anodes to up to 30 % for silicon (Si). Several prelithiation strategies have been explored which incorporate sacrificial additives acting as Li + -donors into the cell, such as the addition of Li metal to the anode side. Usually, anode prelithiation routes lead to the formation of unstable reaction products and low battery potentials. Thus, the incorporation of small amounts of sacrificial additives (5-10 wt%) to the cathode side constitutes a more attractive route to counteract capacity losses in Li-ion batteries 1 . Several compounds with high initial charge capacity (e.g. Li 2 Mn 2 O 4 , Li 2 NiO 2 andLi 6 CoO 4 ) have shown promising results for cathode prelithiation, although their performance is limited by low specific capacities of <300 mAh g −1 . Here, we report a significant improvement of the capacity of sacrificial cathodes reaching 1157 mAh g −1 in the Li 2 O:Li 2/3 Mn 1/3 O 5/6 nanocomposites, where Li 2/3 Mn 1/3 O 5/6 was previously reported as a cathode material with a strongly disordered and non-stoichiometric manganese oxide (MnO)-type rock-salt (RS) structure 2 . In our work, the initial capacity of single phase Li 2/3 Mn 1/3 O 5/6 RS of 250 mA h g −1 was improved by the increase of the Li 2 O content in 35 and 55 mol% Li 2 O- rich composites with exceptional 898 and 1157 mAh g −1 first charge capacities. The increased capacities correspond to the extraction of 1.55 and 3.04 Li + per Li 2/3 Mn 1/3 O 5/6 formula unit in 35 and 55 mol% Li 2 O composites respectively, amounting to larger quantities than originally present in the RS; this observation indicates a reaction between Li 2 O and Li 2/3 Mn 1/3 O 5/6 . The reaction mechanism was studied by in situ total scattering using the DRIX electrochemical cell [3] semi- crystalline and amorphous phases present during (dis (Fig. 1) at Diamond’s I15-1 (XPDF) beamline. The improved DRIX cell design with a radial geometry allowed for a comprehensive understanding of the electrochemical activation of the nanostructured sacrificial. Figure2showsasolidsolutionresponseofthe0.35Li 2 O:0.65RSnanocomposite with a continuous evolution of the lattice parameters, which mimic the shape of the electrochemical curve. The unit cell of the active RS phase contracts with the extraction of lithium after the first charge at 4.5 V and expands during cell discharge. After the first charge, an irreversible contraction of the RS cell volume is observed, ascribed to a lattice densification motivated by the migration of Mn into cation vacancies created after the extraction of lithium. Such a material’s densification is commonly observed in Li-rich cathode materials.The main Bragg reflections from Li 2 O in the diffraction pattern of 0.35Li 2 O:0.65RS, highlighted with an asterisk in Fig. 2, disappear gradually until they are no longer observed Crystallography Group Beamline I15-1 evolution of the first charge capacity vs Li 2 O content in Li 2 O:RS. Note the increased capacity is only limited to the first charge; only the capacity of the active RS component is retained over the following cycles. The 0.55Li 2 O:0.45RS nanocomposite with the largest capacity has been evaluated as an additive of LiCoO 2 cycled against Li metal in Fig. 3.The addition of 2 wt% of 0.55Li 2 O:0.45RS resulted in a 13 % increase of the first charge capacity, and matching discharge capacities to that of LiCoO 2 with no interference of the sacrificial additive with the electrochemical performance of the cathode. The large initial charge capacity of 0.55Li 2 O:0.45RS allows for the use of a small amount of additive (2wt%vs. 5-10wt%which is commonly used to compensate similar capacity losses with other materials). Moreover, ~55 mol% of the sacrificial is consumed after the first charge and only <1 wt% remains in the cathode, and the smaller volumes of released gas vs sacrificial salts mitigates potentially detrimental effects related to gas evolution during battery cycling. The low cost, ease of preparation, compatibility with industrial battery fabrication and outstanding first charge capacities of Li 2 O:RS nanocomposites make themhighly promising additives for the prelithiation of cathode materials. References: 1. SunY. et al. High-capacity battery cathode prelithiation to offset initial lithium loss. Nat. Energy 1 , 15008 (2016). DOI: 10.1038/nenergy.2015.8 2. Freire M. et al. Investigation of the exceptional charge performance of the 0.93Li4-: XMn2O5-0.07Li2O composite cathode for Li-ion batteries. J. Mater. Chem. A 6 , 5156–5165 (2018). DOI: 10.1039/c8ta00234g 3. Diaz-Lopez M. et al. Fast operando X-ray pair distribution function using the DRIX electrochemical cell. J. Synchrotron Radiat. 27 , 1190–1199 (2020). DOI: 10.1107/S160057752000747X Funding acknowledgement: This work was supported by the ANR Grant No. ANR15-CE05-0006-01DAME. Corresponding authors: Dr Maria Diaz-Lopez, Diamond Light Source,
[email protected] ; Dr Philip Chater, Diamond Light Source,
[email protected] Figure 1: Photograph of a DRIX cell with a schematic representation of the battery stack. The incoming and transmitted X-ray beams are represented in yellow. Figure 2: (a) 0.35Li 2 O:0.65Li 2/3-x Mn 1/3 O 5/6 evolution with cycling characterised by in situ X-ray Total Scattering. From left to right: Electrochemical performance (solid line) overlaid with lattice parameters from a sequential Rietveld refinement (empty circles with error bars), reciprocal and real space (PDF) total scattering data. Bottom figures show the data at selected potential values marked by solid spheres in the electrochemical curve. (b) PDFsat selected potential values rescaled on the r-range by the lattice parameter ratio. Vertical lines indicate the expected distances in the average RS structure for the most strongly scattering atom pairs: Mn-Mn and Mn-O. Figure 3: Improved first charge performance of LiCoO 2 (blue) with 2 wt% 0.55Li 2 O:0.45Li 2/3- x Mn 1/3 O 5/6 sacrificial additive (orange).