Improved electrochemical performance of rare earth doped LiMn1.5-xNi0.5RExO4 based composite cathodes for lithium-ion batteries

Accepted Manuscript Improved electrochemical performance of rare earth doped LiMn1.5-xNi0.5RExO4 based composite cathodes for lithium-ion batteries Pura Ram, Attila Gören, Renato Gonçalves, Ganpat Choudhary, S. Ferdov, Maria M. Silva, Rahul Singhal, Carlos M. Costa, Rakesh K. Sharma, Senentxu LancerosMéndez PII:

S1359-8368(17)30481-X

DOI:

10.1016/j.compositesb.2017.11.054

Reference:

JCOMB 5416

To appear in:

Composites Part B

Received Date: 8 February 2017 Revised Date:

29 November 2017

Accepted Date: 30 November 2017

Please cite this article as: Ram P, Gören A, Gonçalves R, Choudhary G, Ferdov S, Silva MM, Singhal R, Costa CM, Sharma RK, Lanceros-Méndez S, Improved electrochemical performance of rare earth doped LiMn1.5-xNi0.5RExO4 based composite cathodes for lithium-ion batteries, Composites Part B (2018), doi: 10.1016/j.compositesb.2017.11.054. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

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Improved electrochemical performance of rare earth doped LiMn1.5-xNi0.5RExO4 based composite cathodes for lithium-ion batteries Pura Ram1, Attila Gören2,3, Renato Gonçalves2,3, Ganpat Choudhary1, S. Ferdov2, Maria M. Silva3, Rahul Singhal4, Carlos M. Costa2,3*, Rakesh K. Sharma1*, Senentxu

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Lanceros-Méndez2,5,6 Department of Chemistry, Indian Institute of Technology Jodhpur, 342011, Rajasthan,

India

Centre of Physics, University of Minho, 4710-057 Braga, Portugal

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Centre/Department of Chemistry, University of Minho, 4710-057 Braga, Portugal

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Department of Physics and Engineering Physics, Central Connecticut State University,

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New Britain, CT, 06067, USA 5

BCMaterials, Parque Científico y Tecnológico de Bizkaia, 48160 Derio, Spain

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IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain

Abstract

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LiMn1.5Ni0.5O4 with spinel type of structure was synthesized by sol-gel method and doped by different rare-earth elements (Nd, Gd and Dy). It was found that the inclusion of rare-earth element preserves the cubic spinel structure, leads to smaller particle size

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and improves the cyclic performance of the produced batteries. The best battery performance was obtained after doping the spinel structure by Gd. Among the used rare-

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earth elements Gd is the only one with stable oxidation state of 3+ which leads to stabilization of the Mn dissolution. Thus, the improvements observed in battery performance (rate performance and cycle life) are due to the superior structural stability of the doped active materials. It is concluded that the LiMn1.48Ni0.5Gd0.02O4 sample is the one with the best overall performance to act as active material for cathodes in rechargeable lithium-ion batteries. Keywords: A. Nano-structures; B. Electrical properties; B. Physical properties; B. Chemical properties; 1

ACCEPTED MANUSCRIPT 1. Introduction

Environmental issues with respect to global warming and the necessary reduction of CO2 emission exerts great pressure for the development of efficient electrical energy

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storage systems to support the use of energy generated by environmental friendly sources, such as photovoltaic, wind and geothermal [1, 2]. The main energetic challenge is developing efficient energy storage systems, i.e, batteries, for electric vehicles (EV)

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and hybrid electric vehicles (HEV), leading to a strong reduction of CO2 emissions and fossil fuel consumption [3, 4].

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Batteries are the most used energy storage systems due to the larger amounts of energy stored in comparison to related systems, i.e, fuel cells and supercapacitors. The battery type most widely used is the lithium-ion battery, with a market share of 75% [5, 6]. The main components of a lithium-ion battery are the anode, cathode and separator/electrolyte [7]. Each constituent has specific function in the operation of the

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battery: the cathode (positive electrode) is responsible for the cell capacity and cycle life; the anode (negative electrode) should have a low potential in order to provide a

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high cell voltage with the cathode [8]; the separator is placed between electrodes and allows to control the movement and number of the ions between anode and cathode

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during charge and discharge and prevents the short-circuit of the battery [9]. Independently the type of cathode, they are typically composed by a polymer binder, conductive additive and the corresponding active material. In this structure, the polymer binder has the role of binding the active material and conductive additive, the conductive additive increases the electrical conductivity of the cathode and the active material acts as lithium reservoir in the electrode [10]. Typically, carbon fibers [11], silicon nanowires with carbon nanotubes [12] and TiO2 nanoparticles [13] can be used as active materials for the anode electrode. 2

ACCEPTED MANUSCRIPT The active material used for cathode development must be easily reducible; react with lithium in a reversible way; be a good electronic conductor and stable. Few examples of cathode materials for Li ion rechargeable batteries are LiCoO2 [14], [Li, Mn, Ni, Co]O2

LiMSiO4 (M=Mn, Fe, Co, and combinations of them) [18].

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[15], lithium metal polyoxyanion Li3V2PO4 [16], LiMn1.5Ni0.5O4 [17], LiMPO4 and

The manganese spinel cathode LiMn2O4 has been used in the first generation of plug-in hybrid electric vehicles or electric vehicles due to the high operating voltage above 4.0

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V. Despite that, this spinel cathode shows high manganese solubility in the electrolyte, associated with a high concentration of Mn3+ ions, and a high capacity fading [19-21].

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The operating voltage depends on the material type and in case of LiFePO4, LiCoO2 and LiMn1.5 Ni0.5O4 it is 3.4 V, 3.9 V and 4.7 V, respectively, the higher voltage of LiMn1.5 Ni0.5O4 is related to the Ni4+–Ni2+ couple, the Mn4+ ions acting as inactive ions and reducing manganese solubility [4].

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The spinel cathode LiMn1.5Ni0.5O4 has received great interest due to its advantages as high operating voltage of ~ 4.7 V, high energy density of 650 Wh.kg−1 and theoretical capacity of 146.7 mAh.g−1 [22, 23]. Through the addition of the Ni element,

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LiMn1.5Ni0.5O4 avoids the Jahn-Teller (JT) distortion of LiMn2O4 and can crystallize in two structures: a simple cubic phase denominated by “ordered spinel” (P4332) in which

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Li, Ni and Mn, atoms occupy the 8c, 4a and 12d sites, respectively, whereas the O(1) and O(2) oxygen atoms occupy in the 24e and 8c sites, and a face-centered cubic phase denominated by “disordered spinel” (Fd3m) where Ni and Mn atoms are randomly distributed in the 16d sites, whereas Li and O occupy 8a tetrahedral sites and 32e sites, respectively [23-27]. Cathodes based on this spinel structure show capacity fade resulting from an aggressive side reaction with the electrolyte that occurs at high-voltage operation [17]. One

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ACCEPTED MANUSCRIPT strategy for reducing this issue is the substitution of nickel and/or manganese and/or oxygen in the structure by small amounts of different atoms (cations and anions) that also improve the electronic and ionic conductivity of this spinel structure [17, 23]. The different atoms that have been used for doping this spinel structure include Al, Co, Cr,

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Cu, F, Fe, Ga, Mg, Mo, Nb, Rh, Ru, S, Sm, Ti, V, W and Zn [22, 28-44]. Typically, the doping of LiMn1.5Ni0.5O4 leads to improved cyclic performance and rate capability of the battery [14].

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For LiMn1.5Ni0.5O4, atoms with valence (+3) induce a charge compensation from a corresponding amount of manganese, giving rise to mixed Mn3+ and Mn4+ ions in the

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lattice, which improve rate capability. In this scope, the goal of this work is the synthesis, characterization and battery performance evaluation of LiMn1.5Ni0.5O4 doped with different rare earth elements (REE), i.e, neodymium (Nd), Gadolinium (Gd), classified as light REE, and Dysprosium (Dy), classified as heavy REE, in order to

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evaluate the effects of both rare-earth element and quantity, the latter being performed for Gd. Further, the REE used in this work are the most used in many applications,

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including lasers and magnets, among others.

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ACCEPTED MANUSCRIPT 2. Experimental

2.1. Synthesis of the active materials

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LiMn1.5Ni0.5O4, LiMn1.48Gd0.02Ni0.5O4 and LiMn1.5-xNi0.5RExO4 (RE: Gd, Nd, Dy; x=0.04) were synthesized via sol gel route with analytical grade chemicals. Stoichiometric mixtures of lithium acetate, manganese acetate, nickel acetate,

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gadolinium acetate, neodymium acetate and dysprosium acetate were obtained in stoichiometry ratio from Alfa Aesar, USA. The raw powders were subjected to heating

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treatment to reach phase purity and stable form of the materials. Each constituent was dissolved in 2-ethylhexanoic acid to get a homogeneous solution at a temperature 100150 ºC for 10-30 minutes and at a magnetic stirring rate of 250 rpm. After completely dissolution of each component, these were added into manganese acetate solution. More

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details of the synthesis process are described in [19]. The brownish precursors of the synthesized spinel undergo a two-step heat treatment. The removal of organic impurities and phase purity were obtained at 450 ºC for 4 hours and at 850 ºC for 14 hours in a box

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furnace, Nabertherm GmbH, Germany. The phase pure powder was ball milled for 6 hours with 10:1 ball weight/ material weight ratio in a planetary high energy ball

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milling, Retsch (PM-100 model).

2.2. Powder characterization

The surface morphology and elemental analysis of the samples were studied by Scanning Electron Microscopy (SEM) using Carl Zeiss, Evo special addition. The beam

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ACCEPTED MANUSCRIPT current during surface morphology and energy dispersive X-ray were 80 pA and 200 nA respectively. High resolution transmission electron microscopy (HRTEM) measurements were performed with a TEM Tecnai G2 instrument by FEI with an accelerating voltage of

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200 kV. The physical characterization of pure phase powders was carried out using Xray diffractogram, Brucker D8 advance, in the range two theta angle 10°- 80° with 0.02° per second scan rate in locked coupled mode. The amounts of the as-synthesized phases

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were determined by the method of Rietveld using TOPAS-3 software (Bruker AXS Inc.). The method involved fit the complete experimental diffraction patterns with

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calculated profiles and backgrounds. The phases included in the quantification were identified by the EVA software (EVA Software, DIFFRACplus Release 2006, Bruker AXS). Raman analysis were performed with 2 mW power air cooled LASER excitation source (operating at 532 nm) using an STR 500, Airix Japan, spectrophotometer. BET

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surface analysis was performed in AutoSorb(iQ3), Quantunchrome Instrument, USA. Experiments were carried out in 300 mg samples that were degassed in H2 atmosphere pressure at 300 ºC for 2 hr and the 20 points adsorption and desorption were measured

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in N2 atmosphere.

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2.3. Cathode preparation and half-cell fabrication

The cathode slurry was prepared with LiMn1.5Ni0.5O4, LiMn1.48Gd0.02Ni0.5O4 and

LiMn1.5-xNi0.5RExO4 (RE: Gd, Nd, Dy; x=0.04), carbon black and poly(vinylidene fluoride) (PVDF, Solvay 5130) in a 80:10:10 weight ratio (%), respectively, and Nmethylpyrrolidone (NMP, Fluka). The PVDF was dissolved in NMP to form the binder solution and the other components (active material and conductive additive) were dried mixed. Then, small amounts of solid material (active material and conductive additive)

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ACCEPTED MANUSCRIPT were added to the PVDF binder solution under constant stirring. After adding all solid material, it was left stirring during 2 hours at 1000 rpm to obtain a good dispersion. The slurry was spread uniformly on Al metal grade foil and dried in a furnace at 80 °C for 2 hours. The prepared cathodes were dried overnight at 90 °C in vacuum. Li metal foil

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was used as anode material, Whatman glass microfiber filters (grade GF/A) used as separator and 1M LiPF6 in ethylene carbonate + dimethyl carbonate (EC+DMC, 1:1) from Solvionic as electrolyte. Swagelok type-cells (two or three electrodes) were

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assembled in a home-made argon-filled glove box. Different electrochemical tests were performed with the cells. Galvanostatic cycling was carried out at different C rates (C/8,

respectively,

using

Landt

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C/5, C/2, C and 2C) between a minimum and a maximum limit of 3.5 V and 4.9 V, CT2001A

Instrument.

Electrochemical

impedance

spectroscopy was measured with an Autolab PGSTAT12 instrument in a frequency range from 1 MHz to 10 mHz with an amplitude of 10 mV. The cyclic voltamogram

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(CV) was performed in the range from 3.5 V to 4.9 V at the scan rate 0.1 mV/sec.

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ACCEPTED MANUSCRIPT 3. Results and discussion

LiMn1.5Ni0.5O4 particles doped with different rare earth (RE) elements – Gd, Nd and Dy

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(LiMn1.5-xNi0.5RExO4; x = 0.02 and 0.04) were synthesized via sol gel. The effects of rare-earth doping element and concentration on the crystal structure and electrochemical

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performance were evaluated.

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3.1. Synthesized powder properties

Figure 1 shows the surface morphology of the LiMn1.5Ni0.5O4 powder obtained by SEM and

detailed

TEM

images

of

the

LiMn1.5Ni0.5O4,

LiMn1.48Ni0.5Gd0.02O4,

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LiMn1.46Ni0.5Gd0.04O4, LiMn1.46Ni0.5Ny0.04O4 and LiMn1.46Ni0.5Dy0.04O4 particles.

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Figure 1 - SEM image of LiMn1.5Ni0.5O4 (a) and TEM images of LiMn1.5Ni0.5O4 (b), LiMn1.48Ni0.5Gd0.02O4 (c), LiMn1.46Ni0.5Gd0.04O4(d), LiMn1.46Ni0.5Ny0.04O4 (e) and LiMn1.46Ni0.5Dy0.04O4 (f) powders. Inset TEM images shows the atomic arrangement of the samples.

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ACCEPTED MANUSCRIPT The SEM image of LiMn1.5Ni0.5O4 (Figure 1a), also representative for the rest of the particles, reveals that the powder shows a non-homogenous size, which is confirmed by the TEM images, Figures 1b)-f), showing that the particle size ranges between 50 nm

that these images confirm the crystallinity of the particles.

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and 200 nm. The inserts of the Figures 1b)-f) show HRTEM. It should be mentioned

The particle shape is not spherical, but in polyhedron form, in which the smaller polyhedron are assembles together to form large polyhedron secondary particles. Thus,

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the larger particle sizes are due to agglomeration.

The inset in Figure 1b) shows that the atomic arrangement for LiMn1.5Ni0.5O4 is in a planar

structure.

Figure

1c-d)

for

LiMn1.48Ni0.5Gd0.02O4

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well-defined

and

LiMn1.46Ni0.5Gd0.04O4 reveal a larger agglomeration trend when compared to the other samples, as evidenced by a dull planar structure. Figure 1e-f) for LiMn1.5Ni0.5O4 doped with Nd and Dy shows less agglomerated and finer particle size when compared to the

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other samples. LiMn1.46Ni0.5Dy0.04O4 (Figure 1f) demonstrates a defined structure than the other samples, as confirmed by the clarity and contrast in planar structure. The TEM images for all particles (Figure 1b-f) show the lattice fringes of the (111)

samples.

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plane of cubic spinel LiMn1.5Ni0.5O4, revealing the high crystallinity present in the

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The selected area electron diffraction (SAED) patterns of LiMn1.5Ni0.5O4 and LiMn1.48Ni0.5Gd0.02O4 show the inner and intense circle corresponding to the (111) plane and the next successive rings corresponding to (311) and (400), respectively (see supplementary information S1). The results are similar for the rest of the doped samples. The mapping of the elements was analyzed through SEM assisted EDX and it is shown in Figure 2 a)-c) for LiMn1.5Ni0.5O4 powder and in figure 2d), LiMn1.46Dy0.04Ni0.5O4

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LiMn1.5Ni0.5O4 powder.

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Figure 2 – EDX mapping for LiMn1.5Ni0.5O4 powder: a) Mn element, b) Ni element and

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c) O element and d) Dy element into LiMn1.46Dy0.04Ni0.5O4 powder.

Figure 2 shows the uniform distribution of Mn, Ni and O through-out the nano-regime, the lower intensity of Ni element being due to the lower atomic concentration. Figure 2d) shows the rare-earth doping elemental mapping showing also homogeneous distribution Dy in the samples.

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ACCEPTED MANUSCRIPT Table 1 shows slight differences in the chemical composition of each element (Mn, Ni, O and RE) for the different samples. Further, the elemental composition for the different samples is nearly in the stoichiometric ratio.

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Table 1 – Elemental atomic composition obtained from SEM-EDX for LiMn1.5Ni0.5O4, LiMn1.48Gd0.02Ni0.5O4 and LiMn1.5-xNi0.5RExO4 (RE: Gd, Nd, Dy; x=0.04) Elemental Atomic (%) Sample description

LiMn1.48Ni0.5Gd0.02O4 LiMn1.46Ni0.5Gd0.04O4 LiMn1.46Ni0.5Nd0.04O4

7.67

RE

68.92

0.0

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24.41

O

23.85

6.09

69.21

1.05

23.77

7.65

66.14

2.44

20.11

7.51

68.15

3.23

22.98

6.25

68.67

2.10

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LiMn1.46Ni0.5Dy0.04O4

Ni

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LiMn1.5Ni0.5O4

Mn

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Figure 3 shows the X-ray diffraction patterns of LiMn1.50Ni0.5O4, LiMn1.48Gd0.02Ni0.5O4, LiMn1.46Gd0.04Ni0.5O4, LiMn1.46Nd0.04Ni0.5O4 and LiMn1.46Dy0.04Ni0.5O4. All the patterns

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were successfully refined starting with model of cubic spinel phase and space group Fd3m (JCPDS card No. 35-782) [17, 23]. Small amounts of impurities (below 10%) were detected only in the LiMn1.46Nd0.04Ni0.5O4 and LiMn1.46Dy0.04Ni0.5O4 samples. The secondary phases were evaluated by Rietveld analysis and it was found the presence of Ni6MnO8, NiO, Mn2O3 and MnO2 impurities indicated in the Figure 4 by arrows. These results are in agreement with previous literature in which the presence of impurities is ascribed to oxygen deficiency while keeping the spinel structure during the sintering process [40]. 12

(440) (531)

LiMn1.46Dy0.04Ni0.5O4

(511)

Intensity / a.u.

(222)

(331)

(311)

(400)

(111)

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LiMn1.46Nd0.04Ni0.5O4

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LiMn1.46Gd0.04Ni0.5O4 LiMn1.46Gd0.02Ni0.5O4

LiMn1.5Ni0.5O4

20

30

40

50

2θ CuKα

60

70

80

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Figure 3 - Refined (red: calculated; black: experimental) XRD patterns for LiMn1.5Ni0.5O4, LiMn1.48Gd0.02Ni0.5O4 and LiMn1.5-xNi0.5RExO4 (RE: Gd, Nd, Dy; x=0.04).

Table 2 presents the lattice parameters, full width at half maxima (FWHM) and the ratio

LiMn1.48Gd0.02Ni0.5O4,

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of intensity of plane (311) to plane (400). The lattice constants for LiMn1.50Ni0.5O4, LiMn1.46Gd0.04Ni0.5O4,

LiMn1.46Nd0.04Ni0.5O4

and

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LiMn1.46Dy0.04Ni0.5O4 are 8.23, 8.24, 8.24, 8.24 and 8.24 Å respectively. Table 2 - XRD structural parameters for LiMn1.5Ni0.5O4, LiMn1.48Gd0.02Ni0.5O4 and

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LiMn1.5-xNi0.5RExO4 (RE: Gd, Nd, Dy; x=0.04). Sample description

Lattice parameters /Å

Unit cell volume / Å3

FWHM / º

I(400) /I(311)

LiMn1.5Ni0.5O4

8.23

556.9

0.29

1.25

LiMn1.48Ni0.5Gd0.02O4

8.24

560.4

0.35

1.14

LiMn1.46Ni0.5Gd0.04O4

8.24

560.4

0.41

1.11

LiMn1.46Ni0.5Nd0.04O4

8.24

558.7

0.28

1.10

LiMn1.46Ni0.5Dy0.04O4

8.24

558.7

0.30

1.28

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ACCEPTED MANUSCRIPT The lattice constant slightly increases with rare-earth doping with respect to LiMn1.50Ni0.5O4. These increases are attributed to the large ionic radius of rare earth elements Nd+3 (0.1 nm) > Gd+3 (0.0938 nm) > Dy3+ (0.0912 nm) compared to Mn3+ (0.064 nm) and Mn+4 (0.054 nm). Further, the changes in lattice parameter can be used

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as an indirect evidence for RE/Mn substitution [40].

The value of FWHM for all samples corresponding to the (111) plane peak was analyzed and it is found 0.287° for undoped LiMn1.50Ni0.5O4 whereas it varies from

x=0.04).

The

absence

of

impurity

phases

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0.281° - 0.411° for LiMn1.48Gd0.02Ni0.5O4 and LiMn1.5-xNi0.5RExO4 (RE: Gd, Nd, Dy; in

LiMn1.48Gd0.02Ni0.5O4

and

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LiMn1.46Gd0.04Ni0.5O4 reveals that Gd+3 is able to replace the octahedral site element in place of the Mn+3/Mn+4 site at 16d. To measure the occupancy of the octahedral site, the peak intensity ratio (400)/(311) was calculated and shown in table 2. The ratio decreases with Gd and Nd elemental doping whereas in the case of Dy the ratio almost match with

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undoped LiMn1.50Ni0.5O4 which supports the octahedral site occupancy [45]. The Raman spectra recorded for all spinel doped and undoped samples show two major vibrational peaks at 625 cm-1 and 488 cm-1 characteristic of the Fd-3m phase (see

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supplementary information S2).

The nitrogen adsorption-desorption isotherms and pore size distribution are shown in

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Figure 4.

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LiMn1.5Ni0.5O4 LiMn1.46Gd0.02Ni0.5O4 LiMn1.46Gd0.04Ni0.5O4

12

LiMn1.46Nd0.04Ni0.5O4

-2

15

LiMn1.46Dy0.04Ni0.5O4

9 6 3 0,2

0,4

0,6

0,8

1,0 -1

0,40

b)

LiMn1.5Ni0.5O4

0,35

LiMn1.46Gd0.02Ni0.5O4 LiMn1.46Gd0.04Ni0.5O4

0,30

LiMn1.46Nd0.04Ni0.5O4

0,25

LiMn1.46Dy0.04Ni0.5O4

0,20 0,15 0,10 0,05 0,00 0

5

10

15

20

25

30

Pore diameter / nm

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Relative Pressure / P.P0

Figure 4 - (a) N2 adsorption - desorption isotherms and (b) pore size distribution for

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LiMn1.5Ni0.5O4, LiMn1.48Gd0.02Ni0.5O4 and LiMn1.5-xNi0.5RExO4 (RE: Gd, Nd, Dy; x=0.04).

LiMn1.5Ni0.5O4, LiMn1.46Nd0.04Ni0.5O4 and LiMn1.46Dy0.04Ni0.5O4 exhibit a meso-porous polyhedral crystal structure having 15 m2/g specific surface area evaluated by multi

table 3.

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point Brunauer-Emmett-Teller (BET) method as is observed in the Figure 4a) and also

xNi0.5RExO4

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Table 3 - BET parameters of LiMn1.5Ni0.5O4, LiMn1.48Gd0.02Ni0.5O4 and LiMn1.5(RE: Gd, Nd, Dy; x=0.04).

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0,0

0,45

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a)

3 -1

18

-1

Volume / ccg @STP

21

Diff. pore volume (10 cm g )@STP

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Sample description

BET parameters Surface area / Average pore m2/g size / nm

LiMn1.5Ni0.5O4

15

3.1

LiMn1.48Ni0.5Gd0.02O4

12

3.0

LiMn1.46Ni0.5Gd0.04O4

21

3.2

LiMn1.46Ni0.5Nd0.04O4

15

3.2

15

35

ACCEPTED MANUSCRIPT LiMn1.46Ni0.5Dy0.04O4

15

3.0

However, the specific surface area for LiMn1.46Gd0.04Ni0.5O4 is 21 m2/g whereas for LiMn1.48Gd0.02Ni0.5O4 is 12 m2/g in which the increase of Gd content reduces the

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particle size, which can be attributed to an agglomeration process. The pore-size distribution, Figure 4b), reveals that the different materials show a wide range of pore sizes from 2 to 35 nm, with an average pore size of 3 nm. Thus, the samples have a non-

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3.2. Electrochemical properties

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homogeneous surface morphology.

Cathodes were prepared from the active materials by previously synthesized by sol gel method and the electrochemical properties evaluated.

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3.2.1. Charge-discharge performance

The charge-discharge profiles of LiMn1.5Ni0.5O4 and LiMn1.46Gd0.04Ni0.5O4 for the 5th cycle at different scan rates and room temperature are shown in Figure 5a) and b),

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respectively. The shape of the charge-discharge curves for the others LiMn1.5Ni0.5O4

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doped powders are similar to the ones presented in Figure 5b, with variations in the charge and discharge value. Independently of the samples, Figure 5 shows three voltage plateaus at ~ 4.0V, 4.6 V and 4.7 V corresponding to Mn3+-Mn4+, Ni2+-Ni3+ and Ni3+-Ni4+ redox processes, respectively [43]. These plateaus can be observed in details through of cyclic voltammetry results (Figure 8). The plateaus at ~ 4.0V and 4.6 V- 4.7 V are attributed to lithium-ion insertion into 16c octahedral and 8a tetrahedral sites of the spinel structure, respectively.

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ACCEPTED MANUSCRIPT From Figure 5a), the discharge capacity of LiMn1.5Ni0.5O4 is 98.5, 101.7, 100.5, 89 and 32 mAh.g-1 at the rates of C/8, C/5, C/2, C and 2C, respectively, due to increased polarization at higher current rates. The low value of the discharge capacity at C/8 is due to the formation of the solid

leaving decomposition products [47].

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electrolyte interface (SEI) layer, as at such a high voltage unwanted electrolyte occurs

Figure 5a also shows for low scan rates (C/8) a large irreversible capacity, which is

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typically observed in the literature for this active material with conventional electrolytes [47].

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For LiMn1.46Gd0.04Ni0.5O4 the discharge capacity is 62.5, 70.6, 75.8, 74.5 and 65.9 mAh.g-1 at the rates of C/8, C/5, C/2, C and 2C, respectively (Figure 5b). The formation of the SEI layer and the irreversible capacity is higher for LiMn1.46Gd0.04Ni0.5O4 when compared with LiMn1.5Ni0.5O4 due to its surface area value.

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Furthermore, the surface area value of this sample is higher than the undoped sample,

AC C

EP

which affects negatively battery performance.

17

ACCEPTED MANUSCRIPT 5,0

4,6

4,6

4,4

4,4

4,2

C/8 C/5 C/2 C 2C

4,0 3,8 3,6

4,0 3,8

3,4 20

40

60

80

100

Capacity / mAh.g

120

140

160

0

80

100

LiMn1.46Dy0.04Ni0.5O4

4,2 4,0 3,8

LiMn1.5Ni0.5O4 LiMn1.46Gd0.02Ni0.5O4 LiMn1.46Gd0.04Ni0.5O4

4,4

Voltage / V

LiMn1.46Nd0.04Ni0.5O4

d)

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LiMn1.46Gd0.04Ni0.5O4

4,4

60

4,6

LiMn1.46Gd0.02Ni0.5O4

4,6

40

-1

LiMn1.5Ni0.5O4

4,8

C/8 C/5 C/2 C 2C

Capacity / mAh.g

4,8

c)

20

-1

SC

0

LiMn1.46Nd0.04Ni0.5O4 LiMn1.46Dy0.04Ni0.5O4

4,2 4,0 3,8 3,6

3,6

3,4

40

60

80

100

120

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20

-1

Capacity / mAh.g

0

20

40

60

80 -1

Capacity / mAh.g

Figure 5 - Charge–discharge curves at C/8 to 2C for the 5th cycle for: LiMn1.5Ni0.5O4 (a) and LiMn1.46Gd0.04Ni0.5O4 (b) cathodes. Charge–discharge curves for all cathodes: at

EP

0

C/8 (c) and 2C (d) for the 5th cycle.

AC C

Voltage / V

4,2

3,6

3,4

5,0

b)

4,8

Voltage / V

Voltage / V

4,8

a)

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5,0

Figure 5 also shows the discharge curves at room temperature for all samples at C/8 (Figure 5c) and 2C (Figure 5d) for the 5th cycle. It is shown that rare earth element doping affects the battery performance of the LiMn1.5Ni0.5O4 cathode material in that depends of its type and amount through of the surface area value. As is observed in the Figures 5c) and d), the voltage plateaus are independent of the doping type and content.

18

100

ACCEPTED MANUSCRIPT The discharge capacity for the cathodes at C/8 (LiMn1.5Ni0.5O4: 99.3 mAh.g-1; LiMn1.48Gd0.02Ni0.5O4:

113.7

mAh.g-1;

LiMn1.46Gd0.04Ni0.5O4:

62.5

mAh.g-1;

LiMn1.46Nd0.04Ni0.5O4: 89.5 mAh.g-1; LiMn1.46Dy0.04Ni0.5O4: 96.9 mAh.g-1) are higher for

the

cathode

LiMn1.48Gd0.02Ni0.5O4:

material

100.3

at

mAh.g-1;

(LiMn1.5Ni0.5O4:

32.8

mAh.g-1;

LiMn1.46Gd0.04Ni0.5O4:

65.9

mAh.g-1;

2C

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than

LiMn1.46Nd0.04Ni0.5O4: 86.2 mAh.g-1; LiMn1.46Dy0.04Ni0.5O4: 69.2 mAh.g-1) due to electrochemical polarization effect.

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It is also observed for the scan rate of 2C, that the 4.7 V discharge voltage plateau decreases and the 4.0 V plateau slowly disappears (Figure 5d).

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The spinel cathode doped samples of LiMn1.48Gd0.02Ni0.5O4, LiMn1.46Nd0.04Ni0.5O4 and LiMn1.46Dy0.04Ni0.5O4 demonstrate a higher discharge voltage in comparison with the undoped sample in which the rare earth element doping affects the discharge capacity value of the LiMn1.5Ni0.5O4 cathode material in that the large surface area of the

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LiMn1.48Gd0.02Ni0.5O4, LiMn1.46Nd0.04Ni0.5O4 and LiMn1.46Dy0.04Ni0.5O4 enhances the electrochemical efficiency through the intercalation and de-intercalation processes of the Li+ ions.

EP

Further, rare-earth doping tends to stabilize the crystal structure and suppress the Mn dissolution as is the case for LiMn1.46Nd0.04Ni0.5O4 and LiMn1.46Dy0.04Ni0.5O4 samples.

AC C

Also, if the amount of doping is low, it results in a stable cycling as it is observed for LiMn1.48Gd0.02Ni0.5O4 sample. Figure 6a) demonstrate the rate performance of the different samples at different C-rates of the discharge value.

19

C/2

C 2C C/8

100 80 60 LiMn1.5Ni0.5O4

40

LiMn1.46Gd0.02Ni0.5O4 LiMn1.46Gd0.04Ni0.5O4

20

LiMn1.46Nd0.04Ni0.5O4

0

LiMn1.46Dy0.04Ni0.5O4

5

10

15

20

25

80 60 LiMn1.5Ni0.5O4 LiMn1.46Gd0.02Ni0.5O4

40

LiMn1.46Gd0.04Ni0.5O4

20

LiMn1.46Nd0.04Ni0.5O4 LiMn1.46Dy0.04Ni0.5O4

0 0

10

20

30

40

Cycle number

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Cycle number

100

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Figure 6 – (a) Rate and (b) cycling performance for the different samples.

Independently of the sample, Figure 6a) shows very stable discharge capacity value for each scan rate as a function of the cycle number, particularly for scan-rates above C/2. Generically, the coulombic efficiency (CE), related to the reversibility of the process, is

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approximately 90% for all the cathode materials doped with rare earth element. The LiMn1.48Gd0.02Ni0.5O4 sample shows the best rate performance in comparison with the other samples. Also LiMn1.46Nd0.04Ni0.5O4 and LiMn1.46Dy0.04Ni0.5O4 samples show higher discharge capacity value in comparison with undoped sample for scan-rates

EP

0

b)

120

RI PT

C/5

-1

a) C/8

Discharge capacity / mAh.g

120

above C/5. Moreover, when the current returns to the C/8 rate after cycles at various Crates, the discharge capacity of all the samples can return to the initial levels, indicating

AC C

Discharge capacity / mAh.g

-1

ACCEPTED MANUSCRIPT

good reversibility. The improved rate performance of rare earth element doping is being attributed to the high surface area value and stabilization of the structure. Rate capability of the cathode material is influenced by several factors as lithium-ion diffusion through the SEI layer, charge transfer reaction and, ohmic polarization, these contributions will be latter discussed through of electrochemical impedance spectroscopy (EIS), Figure 7, and cyclic voltammetry (CV), Figure 8 [48].

20

50

ACCEPTED MANUSCRIPT The cyclic performances of the all spinel samples at C-rate and room temperature are shown in Figure 6b). It is observed that specific rare earth element doping and amount can improve the cyclic performance of LiMn1.5Ni0.5O4. After 50 cycles, the discharge capacity is 105 mAh.g-1, 84.6 mAh.g-1, 75.7 mAh.g-1, 73.8 mAh.g-1 and 51.9 mAh.g-1

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for LiMn1.48Gd0.02Ni0.5O4, LiMn1.46Dy0.04Ni0.5O4, LiMn1.46Nd0.04Ni0.5O4, LiMn1.5Ni0.5O4 and LiMn1.46Gd0.04Ni0.5O4, respectively, demonstrating that LiMn1.48Gd0.02Ni0.5O4 shows the best cycle stability. Low amount (0.02) of gadolinium (Gd) and neodymium (Nd),

SC

and Dysprosium (Dy) improve the cycling stability of the LiMn1.5Ni0.5O4 spinel cathode.

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For the initial twenty cycles, it was observed instability of the discharge values for LiMn1.48Gd0.02Ni0.5O4 sample in which is related to the daily fluctuation of the ambient temperature affecting battery temperature. The poor cycling performance of LiMn1.46Gd0.04Ni0.5O4 can be presumably attributed to the surface reactivity of the

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sample in contact with the liquid electrolyte [49].

Table 4 shows different parameters calculated from the results in Figure 6b) at C-rate based on the charge-discharge curves at room temperature for the different samples.

EP

The initial charge and discharge capacities (Table 4) indicate enhanced capacity for the doped LiMn1.5Ni0.5O4 cathode electrodes, due to the addition of Gd3+ and Dy3+ in the

AC C

LiMn1.48Gd0.02Ni0.5O4 and LiMn1.46Dy0.04Ni0.5O4 samples that are affected by the surface area value.

The capacity fade after 55 cycles is 4.15% for LiMn1.48Gd0.02Ni0.5O4 and 19% for LiMn1.46Ni0.5Nd0.04O4 and LiMn1.46Ni0.5Gd0.04O4, in comparison to the value of 29.6% observed for LiMn1.5Ni0.5O4.

21

ACCEPTED MANUSCRIPT

Table 4 - Charge and discharge capacities for all samples at C at room temperature. LiMn1.5Ni0.5O4

LiMn1.48Gd0.02

LiMn1.46Ni0.5

LiMn1.46Ni0.5

LiMn1.46Ni0.5

Ni0.5O4

Gd0.04O4

Nd0.04O4

Dy0.04O4

115.1

139.6

90.5

108.1

128.4

105.3

108.5

93.3

122.3

fade First charge capacity

First discharge capacity

2nd and 55th discharge 105; 74.1 capacity (±3) mAh.g-1 Initial capacity loss (%)

8.5

Capacity fade: 55 cycles

97.8; 104

63.8; 51.9

93.3; 75.8

122.0; 84.7

22.2

29.2

13.7

4.8

4.15

19.0

19.0

30.7

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29.6 (%)

64.1

M AN U

(±3) mAh.g

-1

SC

(±3) mAh.g

-1

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Capacity and capacity

EP

Also the capacity fade for LiMn1.46Ni0.5Dy0.04O4 is similar to the one observed for the

AC C

undoped sample. It is observed a higher initial capacity loss for all doped samples except for LiMn1.46Ni0.5Dy0.04O4 in comparison to the undoped sample, indicating that a small fraction of lithium ions is incapable of intercalating back into the host structure due to electrolyte decomposition at high voltage. However, after initial cycling, the cycling efficiency is improved through the formation of protective layer on the electrode surface. Thus, the LiMn1.46Ni0.5Dy0.04O4 (~95%) sample has a higher mean coulombic efficiency than that of the LiMn1.5Ni0.5O4 (~91%) electrode sample.

22

ACCEPTED MANUSCRIPT

3.2.2. Electrochemical impedance spectroscopy and CV analysis of the cathodes

EIS measurements were carried out to further understand the effect of rare earth doping

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into LiMn1.5Ni0.5O4 spinel structure as it was shown that it improves the rate performance and cycling life (Figure 7). Figure 7a) shows the impedance spectra before cycling.

SC

Independently of the samples, the impedance spectra of Figure 7a) is characterized by two semicircles in the high-to-medium frequency region followed by a straight line in

M AN U

the low-frequency region. These two semicircles are described by the contributions of three resistances: the ohmic resistance (liquid electrolyte resistance) given by the intercept of the Zre axis at high frequency; the first semicircle, which is ascribed to surface resistance (Rs, solid-electrolyte interface (SEI) layer); and the second semicircle,

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which is assigned to the charge-transfer resistance process (Rct) [33]. The slope in the low-frequency region represents the Warburg impedance, which is associated with Li+ diffusion in the bulk material [48]. These semicircles (SC1 and SC2) are identified in

a)

2000

AC C

400

EP

the Figure 7a) through an arrow.

b)100

LiMn1.5Ni0.5O4

b)

LiMn1.46Gd0.02Ni0.5O4

LiMn1.46Gd0.02Ni0.5O4

300

1500

LiMn1.46Gd0.04Ni0.5O4

-Z'' / Ω

LiMn1.5Ni0.5O4

LiMn1.46Gd0.04Ni0.5O4

50

LiMn1.46Nd0.04Ni0.5O4 LiMn1.46Dy0.04Ni0.5O4

-Z'' / Ω

-Z'' / Ω

LiMn1.46Nd0.04Ni0.5O4 LiMn1.46Dy0.04Ni0.5O4

200

SC 1

SC 2

1000

0

100

500

0

0 0

100

200

Z' / Ω

300

400

500

0

0

50 Z' / Ω

500

100

1000

1500

2000

Z' / Ω

23

2500

ACCEPTED MANUSCRIPT Figure 7 - Electrochemical impedance spectroscopy (EIS) spectra of the different samples: (a) before and (b) after cycling.

The impedance curve shape of after cycling (Figure 7b) is similar to the one before

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cycling (Figure 7a) in which the rare-earth doping does not affect its shape.

The total resistance value is the sum of the aforementioned resistances and are listed in Table 5 before and after cycling for the different cathodes It is observed that before

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being similar for the rest of the samples.

SC

cycling the lower resistance value is obtained for the LiMn1.46Ni0.5Dy0.04O4 sample,

Table 5 - Total resistance values obtained before and after cycling for all cathode samples.

Before cycling, Rtotal / Ω

After cycling, Rtotal / Ω

LiMn1.5Ni0.5O4

210

343

LiMn1.48Gd0.02Ni0.5O4

362

337

LiMn1.46Ni0.5Gd0.04O4

203

1685

LiMn1.46Ni0.5Nd0.04O4

340

413

LiMn1.46Ni0.5Dy0.04O4

160

566

AC C

EP

TE D

Sample

After cycling, the total resistance value increases for all samples in comparison with its value before cycling except for the LiMn1.48Gd0.02Ni0.5O4 sample. This increase is due to the SEI formation as it is observed in detail in Figure 7b). It is evident from Figure 7 and Table 5 that LiMn1.48Gd0.02Ni0.5O4 sample presents low resistance value after cycling in comparison to the value before cycling, indicating that this doping suppresses 24

ACCEPTED MANUSCRIPT the growth of SEI, which was likely related to the improvement of the cycling stability (Figure 6). The main reason for this fact is the high surface area of this sample and also that small Gd doping reduces Mn dissolution, which results in more stable SEI layer after cycling.

RI PT

To better understand the polarization effect of the cathodes, the CV curves of the samples, excepting for LiMn1.46Gd0.04Ni0.5O4 sample, are represented in Figure 8. The CV curves are not shown for LiMn1.46Gd0.04Ni0.5O4 due the poor battery performance

SC

and high resistance value (Figure 6). Figure 8a shows the CV curves of the LiMn1.48Gd0.02Ni0.5O4 sample for the first three cycles. Independently of the cycle

M AN U

number, it is observed that the corresponding area for each peak is very similar, showing a low electrochemical polarization.

0,3

0,20

a)

0,2

-0,05

3,6

3,8

4,0

EP

-0,10

4,2

4,4

4,6 +

Potential / V vs Li/Li

4,8

5,0

LiMn1.46Nd0.04Ni0.5O4 LiMn1.46Dy0.04Ni0.5O4

0,0 0,03

-0,1 -0,2

0,02 -1

0,00

Epa1

LiMn1.46Gd0.02Ni0.5O4

0,1

Current / A.g

0,05

Current / A.g

TE D

-1

Scan 1 Scan 2 Scan 3

0,10

LiMn1.5Ni0.5O4

0,01 0,00 -0,01 -0,02 -0,03 3,6

-0,3 3,4

3,8

4,0

3,6

3,8

Epc1

4,2

Potential / V vs Li/Li

+

4,0

4,2

4,4

Epc2

4,6

Potential / V vs Li/Li

+

AC C

Current / A.g

-1

0,15

-0,15 3,4

Epa2

b)

Figure 8- CV curves for the: a) LiMn1.48Gd0.02Ni0.5O4 sample as a function of the scan number and for all samples at the 3th scan except for LiMn1.46Gd0.04Ni0.5O4 sample.

Figure 8a) shows also a typical CV profile with one weak pair of redox peaks at around 4.0 V (Mn3+/Mn4+) and two pairs of well separated strong redox peaks at 4.6 - 4.7 V indicating a two-stage (Ni2+/Ni3+ and Ni3+/Ni4+) Li+ extraction (insertion) from(into) the

25

4,8

5,0

ACCEPTED MANUSCRIPT spinel framework [19]. These redox peaks are also observed for the other samples as it is shown in Figure 8b). The intensity of the peaks of Figure 8b) for all doped samples decreases in comparison with the undoped sample.

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The main characteristics of the CV results for the two pair of redox peaks between 4.6 V and 4.7 V are summarized in table 6. The symbols are defined as follows: Epa: anodic peak potential, Epc: cathodic peak potential. The numbers 1 and 2 denote the redox

SC

couple at lower and higher potential, respectively.

Taking into account the results in table 6, it is observed that ∆Ep1 and ∆Ep2 are higher

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than the ideal condition (∆Ep = 0) which indicates that Ni2+ ↔ Ni4+ is controlled by insertion/disinsertion of Li+ in the cathode electrodes regardless of the sample.

Epa1

EP

Sample

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Table 6 - Electrochemical parameters from CV. Potential values / V

Epa2

Epc1

Epc2

∆Ep1

∆Ep2

4.76

4.82

4.60

4.66

0.16

0.16

LiMn1.48Ni0.5Gd0.02O4

4.73

4.79

4.62

4.68

0.11

0.11

LiMn1.46Ni0.5Nd0.04O4

4.75

4.82

4.61

4.66

0.14

0.16

LiMn1.46Ni0.5Dy0.04O4

4.76

4.84

4.59

4.65

0.17

0.19

AC C

LiMn1.5Ni0.5O4

26

ACCEPTED MANUSCRIPT The lower values of ∆Ep1 and ∆Ep2 for the LiMn1.48Ni0.5Gd0.02O4 sample in comparison with the undoped sample is due to the higher surface area value in which the lithium diffusion behavior is enhanced. Through table 6, the potential difference follows the order LiMn1.48Ni0.5Gd0.02O4 <

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LiMn1.46Ni0.5Gd0.04O4 ≈ LiMn1.5Ni0.5O4 < LiMn1.46Ni0.5Dy0.04O4, indicating that the LiMn1.48Ni0.5Gd0.02O4 sample has the lowest electrochemical polarization, reflecting its increased conductivity and reactivity for extraction-reinsertion of lithium-ions in the

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active material.

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3.2.3. Comparison of the LiMn1.5Ni0.5O4 spinel cathode produced in this work with the literature

It is very useful to compare the electrochemical performance of the samples with related

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dopings reported in the literature for LiMn1.5Ni0.5O4 based cathodes once this is the first time the spinel cathode structure is doped with rare earth elements. Though the powder size and the composition of the cathode electrodes reported in table

EP

7 are not the same for all samples, it is observed that the rare earth doping improves the

AC C

cyclability of the LiMn1.5Ni0.5O4 spinel cathode at room temperature.

27

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ACCEPTED MANUSCRIPT

Table 7 - Comparison of the discharge capacity value for the rare-earth doped LiMn1.5Ni0.5O4 and doping with different atoms reported in the literature.

Cathode

Discharge capacity Size / µm

Co

LiMn1.4Cr0.2Ni0.4O4

Cr

LiNi0.45Cu0.05Mn1.5O4

Cu

LiNi0.5Mn1.5O3.975F0.05

F

127 at C

[50]

3

118 at 5C

[51]

<1

130.8 at 0.15C

[52]

-----

124.5 at 5C

[53]

-----

142 at 0.25C

[54]

~1

136 at C/6

[48]

~ 128 at C/6

[33]

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LiNi0.5Co0.05Mn1.45O4

~ 0.6

Fe

LiMn1.5Ni0.42Ga0.08O4

Ga

LiMg0.05Ni0.45Mn1.5O4

Mg

0.02 – 0.08

117 at C

[34]

LiMn1.4Ni0.55Mo0.05O4

Mo

3-5

122.7 at 0.1C

[35]

LiNi0.525Mn1.425Nb0.05O4

Nb

1-2

102.7 at C

[36]

LiNi0.48Rh0.02Mn1.5O4

Rh

0.08

92.4 at C

[37]

LiNi0.4Ru0.05Mn1.5O4

Ru

0.6 - 1

133.4 at 1C

[55]

LiNi0.5Mn1.5O3.95S0.05

S

0.01

116 at 0.2C

[42]

LiNi0.5Sm0.01Mn1.49O4

Sm

0.3 - 0.5

125 at C

[39]

LiNi0.5Mn1.0Ti0.5O4

Ti

<1

206.5

[40]

LiNi0.4Ti0.1Mn1.5O4

Ti

0.3 - 3.0

134.8 at C

[56]

EP

LiMn1.5Ni0.42Fe0.08O4

AC C

TE D

Al

Ref

/ mAh.g-1

doping LiNi0.45 Al0.10Mn1.45O4

SC

Element

28

ACCEPTED MANUSCRIPT V

0.1 - 0.2

130 at 0.8C

[41]

LiW0.005Ni0.5Mn1.495O4

W

0.5

124.9 at 0.1C

[44]

LiZn0.08Ni0.42Mn1.5O4

Zn

-----

118 at 0.5C

[43]

LiMn1.48Ni0.5Gd0.02O4

Gd

0.05-0.2

104 at C

This work

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Li0.995V0.005Ni0.5Mn1.5O4

Due to their unusual physical and chemical properties and also to the fact that the rare earth elements presented in this work are more abundant on average in the Earth’s crust

SC

than silver, gold, or platinum, and taking into account their improvement in battery performance, the rare earth doping of LiMn1.5Ni0.5O4 is an excellent alternative to

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conventional doping for this spinel structure to solve the problem of capacity fade as it is shown in table 7.

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Conclusion

LiMn1.5Ni0.5O4 with spinel type of structure was synthesized by sol–gel method and doped with different rare earth elements. The rare-earth elements used in this work are

EP

neodymium (Nd), gadolinium (Gd) and dysprosium (Dy) with concentrations of 0.02

AC C

and 0.04 for Gd and 0.04 for the other elements. The particle size of all samples are approximately between 50 nm to 200 nm in that the doping by rare earth elements increases the surface area of LiMn1.48Gd0.02Ni0.5O4 which can be related with the specific distribution of the agglomerated nanoparticles. The rare earth doping does not affect the formation of a cubic spinel structure. The charge–discharge curves show significant high discharge capability values at room temperature with rare earth doping, depending on the amount and type of the rare earth. After 55 cycles at C, the discharge capacity values are 104 mAh.g-1, 84.7 mAh.g-1, 75.8 mAh.g-1, 74.1 mAh.g-1 and 51.9

29

ACCEPTED MANUSCRIPT mAh.g-1

for

LiMn1.48Ni0.5Gd0.02O4,

LiMn1.46Ni0.5Dy0.04O4,

LiMn1.46Ni0.5Nd0.04O4,

LiMn1.5Ni0.5O4 and LiMn1.46Ni0.5Gd0.04O4. Further, the rare earth doping affects the overall electrical conductivity of the cathode electrode. The rare earth doping with Gd and Dy exhibit better electrochemical cycling performance in comparison to pristine

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LiMn1.5Ni0.5O4.

In summary, LiMn1.48Ni0.5Gd0.02O4 is an excellent active material for cathodes in rechargeable lithium-ion battery applications and for high scan-rate (C). This sample

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also shows best electrochemical performance compared with related materials presented

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in the literature.

Acknowledgments

This work was supported by the Portuguese Foundation for Science and Technology

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(FCT) in the framework of the Strategic Funding UID/FIS/04650/2013. The authors thank FEDER funds through the COMPETE 2020 Programme and National Funds through

FCT

under

and

EP

UID/CTM/50025/2013

the

projects grants

PTDC/CTM-ENE/5387/2014 SFRH/BD/90313/2012

and (A.G.),

SFRH/BD/88397/2012 (R.G.) and SFRH/BPD/112547/2015 (C.M.C.). The authors are

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thankful for funding from the FCT under the Indo-Portuguese program of cooperation in science and technology (INT/Portugal/P-02/2013) 2014–2016. The fellowship from MHRD, Govt. of India was acknowledged for Pura Ram. Financial support from the Basque Government Industry Department under the ELKARTEK Program is also acknowledged. The authors acknowledge the MRC, MNIT, Jaipur, Rajasthan, India for TEM, RAMAN characterization facilities for material characterization.

30

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ACCEPTED MANUSCRIPT

References

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EP

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Highlights:

− LiMn1.5Ni0.5O4 was synthesized and doped with rare-earth elements, − The rare-earth elements included neodymium (Nd), Gadolinium (Gd) and Dysprosium (Dy), − Doping with rare-earth element preserves the cubic spinel structure, − Gd and Dy doping improve battery rate performance and cycle life,

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− LiMn1.48Ni0.5Gd0.02O4 is the active material with the best overall cycling performance.