# Li 2 O:Li-Mn-O Disordered Rock-Salt Nanocomposites as Cathode Prelithiation Additives for High-Energy Density Li-Ion Batteries

Maria Diaz-lopez, Philip Chater, Pierre Bordet, Melanie Freire, Christian Jordy, Oleg Lebedev, Valérie Pralong

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Maria Diaz-lopez, Philip Chater, Pierre Bordet, Melanie Freire, Christian Jordy, et al. Li 2 O:Li-Mn-O Disordered Rock-Salt Nanocomposites as Cathode Prelithiation Additives for High-Energy Density Li-Ion Batteries. Advanced Energy Materials, 2020, 10 (7), pp.1902788. 10.1002/aenm.201902788. hal-03451746

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# Li₂O:Li–Mn–O Disordered Rock-Salt Nanocomposites as Cathode Prelithiation Additives for High-Energy Density Li-Ion Batteries

Maria Diaz-Lopez,* Philip A. Chater, Pierre Bordet, Melanie Freire, Christian Jordy, Oleg I. Lebedev, and Valerie Pralong

The irreversible loss of lithium from the cathode material during the first cycles of rechargeable Li-ion batteries notably reduces the overall cell capacity. Here, a new family of sacrificial cathode additives based on Li₂O:Li₂/₃Mn₁/₃O₅/₆ composites synthesized by mechanochemical alloying is reported. These nanocomposites display record (but irreversible) capacities within the Li–Mn–O systems studied, of up to 1157 mAh g⁻¹, which represents an increase of over 300% of the originally reported capacity in Li₂/₃Mn₁/₃O₅/₆ disordered rock salts. Such a high irreversible capacity is achieved by the reaction between Li₂O and Li₂/₃Mn₁/₃O₅/₆ during the first charge, where electrochemically active Li₂O acts as a Li⁺ donor. A 13% increase of the LiFePO₄ and LiCoO₂ first charge gravimetric capacities is demonstrated by the addition of only 2 wt% of the nano-sized composite in the cathode mixture. This result shows the great potential of these newly discovered sacrificial additives to counteract initial losses of Li⁺ ions and improve battery performance.

A recurring issue limiting the performance of Li-ion batteries is the formation of a solid electrolyte interface (SEI) during the first battery charge. 7–20% of lithium from the cathode material is irreversibly bound at the surface of graphite anodes,[¹] and it can be as high as 30% for Si,[²] in order to form this passivation layer, which results in a loss of capacity. Several prelithiation routes incorporating sacrificial additives into the cell have been explored to alleviate irreversible capacity losses during the first charge. On the whole, sacrificial materials act as Li⁺ donors and offset the loss of active Li⁺ ions during the SEI formation.

The direct contact of anode materials and lithium metal[³,⁴] is a common prelithiation strategy for improving the Coulombic efficiency of the battery, although this is not exempt from several disadvantages mostly related to the use of Li metal in battery manufacture (i.e., incompatibility with ambient environments, common solvents, binders, and thermal processing). These practical challenges, are partly overcome in Stabilized Lithium Metal Powder[⁵,⁶] SLMP, where lithium metal is suspended in hydrocarbon solvents providing stability in dry air. Interestingly, SLMP has been used as a fully lithiated anode in batteries utilizing non-lithiated V₆O₁₃ or LiV₃O₈ cathodes.[⁷] More recently, the also dry air stable LiₓSi–Li₂O core–shell nanoparticles[⁸] have proven an excellent prelithiation reagent and are

potentially compatible with current industrial battery fabrication methods.

As an alternative route to the arduous prelithiation of anodes demanding more reactive lithium sources leading to unstable reaction products and low battery potentials,[³] several prelithiation additives of cathode materials have been studied.

Sacrificial salts[⁹] (e.g., azides,[¹⁰] oxocarbons, dicarboxylic acids, or hydrazides) contain oxidizable anions that lose electrons during the first charge, forming Li⁺ donors and gaseous species (e.g., CO, CO₂, or N₂) amounting to ≈70% of the salt

Dr. M. Diaz-Lopez
ISIS Facility
STFC Rutherford Appleton Laboratory
Didcot OX11 0QX, UK
E-mail: maria.diaz-lopez@stfc.ac.uk

Dr. M. Diaz-Lopez, Dr. P. A. Chater
Diamond Light Source Ltd.
Diamond House
Harwell Science and Innovation Campus
Didcot OX11 0DE, UK

Dr. P. Bordet
Institut Néel
CNRS
Grenoble Alpes Université
38000 Grenoble, France

Dr. M. Freire, Dr. C. Jordy
SAFT
111-113 Bd Alfred Daney, 33074 Bordeaux, France

Dr. M. Freire, Dr. O. I. Lebedev, Dr. V. Pralong
Crismat
CNRS
Unicaen
Ensicaen
Normandie Université
14000 Caen, France

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201902788.

© 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/aenm.201902788
weight. In spite of the promising improved potential ranges 3–4.5 V versus Li⁺/Li, the uncontrolled evolution of gases could potentially damage the battery.

Several compounds with high initial charge capacity, where nonreversible applications may be found (e.g., Li₂Mn₂O₄,[¹¹] Li₂NiO₂,[¹²] Li₆CoO₄,[¹³] and Li₂CuOO[¹⁴]), have shown promising results as sacrificial additives and although they offer effective compensation of Li⁺ loss during the first cycle, these materials show low specific capacities of <300 mAh g⁻¹.

Reversible electrochemical conversion reactions between lithium and transition metal oxides have awoken interest as both positive and negative electrodes in Li-ion batteries,[¹⁵,¹⁶] and recently, Li₂O:M,[¹⁷] Li₂S:M,[¹⁸] and LiF:M[¹⁹] nanocomposites have been presented as attractive cathode prelithiation additives, able to store more than 4 times the theoretical specific capacity of existing cathodes ≈500–930 mAh g⁻¹. The best performance is given by Li₂O:M (e.g., 724, 799, and 935 mAh g⁻¹ for M = Co, Fe, and Mn, respectively), increasing the overall capacity of a LiFePO₄ (LFP) cathode with the Li₂O:Co additive by 11%.[¹⁷] Such high capacity of sacrificial cathodes have only been improved in Li₃N reaching 1399 mAh g⁻¹ during the initial charge to 4.2 V.[²⁰] However, Li₃N is highly reactive and incompatible with most widely used solvents in lithium-ion batteries' manufacture.

Recently, we reported the outstanding charge capacity of 350 mAh g⁻¹ in nanostructured Li₄Mn₂O₅[²¹] cathode material with a strongly disordered and nonstoichiometric MnO-type rock-salt (RS) structure, where 2/3 of Mn are substituted by Li and accommodating 1/6 oxygen vacancies. Thus, the chemical formula of Li₄Mn₂O₅ is henceforth given as Li₂/₃Mn₁/₃O₅/₆. Li₂/₃Mn₁/₃O₅/₆ is part of an emerging family of Li-rich cathode materials based on disordered RS structures, displaying higher capacities

(200–350 mAh g⁻¹) than Li-rich layered oxides (150–250 mAh g⁻¹). Besides their superior capacities, disordered RS offers a versatile chemical playground and several disordered RS compositions have been reported over the last few years that can present multivalent transition metals, mixed O²⁻/F⁻ anions, or oxygen redox.[²²⁻²⁷]

Detailed compositional, structural, and electrochemical characterizations showed that the presence of 7 mol% excess of Li₂O in the 0.07Li₂O:0.93Li₂/₃Mn₁/₃O₅/₆ composite[²⁸] (previously reported as single phase Li₄Mn₂O₅) increased the capacity of the composite mixture by 100 mAh g⁻¹, while the capacity of single phase Li₂/₃Mn₁/₃O₅/₆ was reduced to 250 mA h g⁻¹. In this work, we demonstrate that outstanding first charge capacities (>1150 mAh g⁻¹) can be achieved by the increase of the Li₂O content in the Li₂O:Li₂/₃Mn₁/₃O₅/₆ composite, thanks to the electrochemical activation of Li₂O acting as a Li⁺ donor to the RS.

We report for the first time 35 and 55 mol% Li₂O-rich composites synthesized by mechanochemical routes[²⁸] with exceptional 898 and 1157 mAh g⁻¹ first charge capacities (see **Figure 1**). The large capacities obtained correspond to the extraction of 1.55 and 3.04 Li⁺ per Li₂/₃Mn₁/₃O₅/₆ formula unit in 35 and 55 mol% Li₂O composites, amounting to larger quantities than originally present in the RS active material. This observation evidences an in situ reaction between Li₂O and Li₂/₃Mn₁/₃O₅/₆ during the course of the first charge where up to 84% and 97% of lithium in Li₂O has reacted from 35 and 55 mol% Li₂O composites, respectively. Electrochemically activated Li₂O could act as a Li⁺ donor and explain the continuous evolution of the first charge capacity versus Li₂O content in Figure 1f. While the addition of Li₂O can increase by 300% the theoretical capacity of Li₂/₃Mn₁/₃O₅/₆ in the highest 55 mol% Li₂O composite, the increased capacity is only limited to the first charge;

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Voltage composition profiles of a) Li₂/₃-xMn₁/₃O₅/₆ RS and b,d,e) Li₂O:Li₂/₃-xMn₁/₃O₅/₆ composites. c) Summary of the first charge capacities for the RS and three composites studied. f) Maximum and reversible capacity versus Li₂O concentration, where the theoretical capacity of Li₂/₃-xMn₁/₃O₅/₆ is provided as a reference. Note that the gravimetric capacity values are expressed as a function of the total mass of composite.</figcaption></figure>
and only the capacity of the active RS component is retained over the following cycles. Further increasing the Li₂O proportion did not improve the capacity further in 0.75Li₂O:0.25Li₂/₃Mn₁/₃O₅/₆ which displayed 914 mAh g⁻¹ first charge capacity (see Figure S1 in the Supporting Information), where only 55% of Li₂O reacted during the first charge. Although no intermediate compositions between 55% and 75% of Li₂O were studied in this work, an optimal capacity value could be found within this compositional range. The miscibility of Li₂O and Li₂/₃⁻Mn₁/₃O₅/₆ phases at the nanoscale was demonstrated by high-resolution transmission electron microscopy images of the 0.55Li₂O:0.45Li₂/₃⁻Mn₁/₃O₅/₆ and carbon black (30 wt%) composite in Figure S2 (Supporting Information), which shows an agglomeration of ball-shaped nanoparticles, with no clear cleavage planes, and no obvious surface layers of different compositions.

The reaction mechanisms between Li₂O and Li₂/₃Mn₁/₃O₅/₆ were studied by in situ total scattering. 7% and 35% Li₂O composites were studied over the course of one and two charge/discharge cycles in Figure 2 while for the 55 mol% Li₂O-richest composite, only 20% of the first charge was recorded (see Figures S3 and S4 in the Supporting Information for in situ total scattering and ex situ diffraction and X-ray absorption near-edge spectroscopy (XANES) data for the pristine and charged at 4.5 V samples).

Figure 2 shows a solid solution behavior of the RS with a continuous evolution of the lattice parameters over the 1.5–4.5 V potential window, in agreement with previous in situ XANES[29] and ex situ total scattering[28] studies. The lattice parameter evolution, quantified by the sequential Rietveld refinement of over 100 in situ data sets, mimics the shape of the electrochemical curve. A steeper change in the lattice parameter occurs at lower potential values 2–3.5 V, following by a more gradual increase in the range 3.5–4.5 V (see Figures S6–S8 in the Supporting Information for the evolution of all sequentially refined parameters).

The nonlinear evolution of Mn–Mn interatomic distances with Li concentration in 0.07Li₂O:0.93Li₂/₃Mn₁/₃O₅/₆ previously characterized by extended X-ray absorption fine structure,[29] is in good agreement with the refined lattice parameters in Figure 2a. The unit cell of the active RS phase contracts from 4.158(2) to 4.070(2) Å with the extraction of lithium after the first charge at 4.5 V, and expands to 4.126(1) Å during cell discharge. The irreversible contraction of the Li₂/₃Mn₁/₃O₅/₆ cell volume could be ascribed to a densification of the nanostructured RS motivated by the migration of Mn cations into cation vacancies created after the extraction of lithium. Such a material’s densification would in turn explain the irreversible exchange of lithium after the first charge (=0.15 less Li is reincorporated in the 7 mol% Li₂O composite) as a result of a lower number of cation vacancies available for lithium to migrate into. Due to the relatively low scattering power of the Li₂O component by X-rays compared to Li₂/₃Mn₁/₃O₅/₆, this minority phase could not be accurately refined in the 7 mol% Li₂O composite. It was however included in the refinement of the 0.35Li₂O:0.65Li₂/₃Mn₁/₃O₅/₆ in situ data in Figure 2b. Note that in spite of the greater concentration of Li₂O in this nanocomposite, the Li₂/₃Mn₁/₃O₅/₆ component behaves in an analogous way as in 7% Li₂O, albeit a more pronounced contraction of the lattice parameters occurs during the first charge down to 4.035(3) Å. The main Bragg reflections from Li₂O in the diffraction pattern of 0.35Li₂O:0.65Li₂/₃Mn₁/₃O₅/₆, highlighted with an asterisk in

Figure 2b, disappear gradually until they are no longer observed after the first charge (see Figure S7 in the Supporting Information for the refined phase wt%), and no traces of other crystalline or amorphous secondary phases could be detected. After the first charge, Li₂/₃⁻Mn₁/₃O₅/₆ cycles reversibly between charged and discharged states with an exchange of ≈0.4 Li per formula unit without the further participation of Li₂O. In contrast to Li₂O:MO nanocomposites with 100% capacity retention over several cycles,[15,30] the participation of Li₂O in Li₂O:Li₂/₃⁻Mn₁/₃O₅/₆ is irreversible, which explains the large irreversible capacities in Figure 1f.

The local structural evolution of the RS phase remaining after the initial charge is investigated in more detail in Figure 2c, where the effects of the lattice parameter changes were removed by multiplying the r-scale by the ratio of the lattice parameters determined through Rietveld refinement. The narrow Mn–Mn distributions centered around the expected values for the average RS structure indicate that 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 (since Mn is well ordered, the broadness of these peaks could only be ascribed to disorder within the oxygen sites). Thus, Li₂/₃⁻Mn₁/₃O₅/₆ is able to 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. It is worth noting that the evolution of the RS phase is identical to all composites, whichever the initial concentration of Li₂O.

Galvanostatic tests of mechanically milled Li₂O powders without Li₂/₃⁻Mn₁/₃O₅/₆ failed to electrochemically decompose Li₂O. Thus, although the mechanism by which Li₂O becomes electrochemically active is not yet understood, the nanostructured Li₂/₃⁻Mn₁/₃O₅/₆ component is expected to play a key role in catalyzing this reaction. The RS phase could catalyze the irreversible decomposition of Li₂O into Li⁺ (consumed in the cell to form the SEI) as well as O₂ (g) or superoxide radicals (O₂⁻) following the reactions.

$$ \mathrm{Li_2O} \rightarrow 2\mathrm{Li}^+ + 1/2 \mathrm{O_2} \ (\mathrm{g}) + 2\mathrm{e}^- \quad (1) $$

$$ \mathrm{Li_2O} \rightarrow 2\mathrm{Li}^+ + 1/2 \mathrm{O_2}^- + \mathrm{e}^- \quad (2) $$

During the experiments in Figure 2 and Figure S3 (Supporting Information) performed in transparent cells made of quartz glass, no significant bubbling of electrolyte was observed that could result from an uncontrolled evolution of O₂ (g). Thus, if O₂ release occurs, it must be formed at a slow rate through the course of the first charge over a wide potential window lapsing several hours.

As for O₂⁻, the formation of these more reactive radicals is frequently associated to decomposition reactions with the carbonate solvent,[31] leading to the formation of CO₂, H₂O, and crystalline Li₂CO₃ or Li₂O₂ decomposition products (among other phases). However, neither crystalline nor amorphous secondary phases were detected by total scattering (see Figure S9 in the Supporting Information). Given that non-negligible amounts of Li₂CO₃ or Li₂O₂ are expected to form when all Li₂O reacts via this route in the 35 and 55 mol% Li₂O-rich composite and these are not observed, the sole decomposition into O₂⁻
<figure><img src="image_4.png" /><figcaption>**Figure 2.** a) 0.07Li₂O:0.93Li₂/₃-xMn₁/₃O₅/₆ and b) 0.35Li₂O:0.65Li₂/₃-xMn₁/₃O₅/₆ evolution with cycling characterized 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, where errors bars often lie underneath the symbols), 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. c) 0.35Li₂O:0.65Li₂/₃-xMn₁/₃O₅/₆ PDFs at 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, see Figure S5 (Supporting Information) for all of the pair contributions to the PDF.</figcaption></figure>

is unlikely to be responsible for the large capacities measured. Thus, Equation (1) is expected to be the dominant route for Li₂O decomposition.

The 0.55Li₂O:0.45Li₂/₃⁻xMn₁/₃O₅/₆ nanocomposite with the best performance has been evaluated as an additive of LFP and LiCoO₂ cathodes cycled against Li metal in **Figure 3**.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Improved first charge performance of a) LFP (green) with 2 wt% 0.55Li₂O:0.45Li₂/₃⁻ₓMn₁₃O₅/₆ sacrificial additive (purple), and b) LiCoO₂ (blue) with the same amount of additive (orange).</figcaption></figure>

The electrochemical curves of the pristine cathodes and with 0.55Li₂O:0.45Li₂/₃⁻ₓMn₁₃O₅/₆ additive appear more different at 3.5–4 V due to a more predominant contribution to the charge capacity from the additive at the higher potential. The addition of 2 wt% of 0.55Li₂O:0.45Li₂/₃⁻ₓMn₁₃O₅/₆ resulted in a 13% increase of the first charge capacity, and matching discharge capacities to that of pristine LFP and LiCoO₂. Thus, while increasing the initial charge capacity, the sacrificial additive did not interfere with the electrochemical performance of the pristine cathode materials (see Figure S10 in the Supporting Information for cycling stability over 7–10 cycles). Note that the large initial charge capacity of 0.55Li₂O:0.45Li₂/₃⁻ₓMn₁₃O₅/₆ allows for the use of such small amount of additive of only 2 wt% (vs more routinely used 5–10 wt%[11–13,19] to compensate for similar capacity losses). 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 versus sacrificial salts[9] mitigate potentially detrimental effects related to gas evolution during battery cycling.

In summary, we propose a design principle for cathode prelithiation to compensate the first-cycle Li loss in Li-ion

batteries based on nanoscale mixtures of Li₂/₃⁻ₓMn₁₃O₅/₆ and Li₂O. The high prelithiation efficacy demonstrated exploits the irreversible electrochemical activation of Li₂O during the first charge. With such a cathode prelithiation additive, the first charge capacity of LFP and LiCoO₂ has been improved by 13%, while the subsequent discharge capacity matches that of the pristine cathode materials. Due to their low cost, ease of preparation, and potential compatibility with industrial battery fabrication, Li₂O:Li₂/₃⁻ₓMn₁₃O₅/₆ nanocomposites are highly promising additives for the prelithiation of cathode materials. The reported pretreatment could be applied to other Li-cathode materials, and seems extrapolable for the presodiation of Na-ion batteries based on initial tests on Na₂O:Na-Mn-O composites that showed an analogous irreversible electrochemical activation of Na₂O with improved first charge capacities.

**Supporting Information**

Supporting Information is available from the Wiley Online Library or from the author.

**Acknowledgements**

This work was supported by the ANR Grant No. ANR15-CE05-0006-01DAME. The authors acknowledge Jean Justine at the CRISMAT for technical assistance, Dr. Olivier Proux for support during beamtime at the FAME beamline (CRG-ESRF) and Diamond Light Source for time on beamline I15-1 under proposal EE20893.

**Conflict of Interest**

The authors declare no conflict of interest.

**Keywords**

cation-disordered rock salts, initial capacity losses, Li₄Mn₂O₅, lithium batteries, sacrificial

Received: August 27, 2019

Revised: November 11, 2019

Published online: January 12, 2020

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