Phase Transition Dominated High-Rate Performances of the High

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C: Energy Conversion and Storage; Energy and Charge Transport

Phase Transition Dominated High-Rate Performances of the High Voltage LiNi Mn O Cathode: Improvement on Structure Evolution and Ionic Diffusivity by Cr Doping 0.5

1.5

4

Jiawen Li, Hailong Wang, Wenhao Dong, Zhongqi Shi, Wenqi Xie, Huali Qiao, Qiaoyan Yu, Min Zhang, Jiabin Hu, Lei Yang, and Jiaying Hong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09054 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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The Journal of Physical Chemistry

Phase

Transition

Dominated

High-Rate

Performances of the High Voltage LiNi0.5Mn1.5O4 Cathode:

Improvement on Structure Evolution

and Ionic Diffusivity by Cr Doping Jiawen Li,† Hailong Wang*,† Wenhao Dong,† Zhongqi Shi, ‡ Wenqi Xie, ‡ Huali Qiao, †Qiaoyan Yu,† Min Zhang,† Jiabin Hu,† Lei Yang,† and Jiaying Hong† †Advanced

Energy Storage Materials and Devices Lab, School of Physics and Electronic-

Electrical Engineering, Ningxia University, Yinchuan, 750021, China ‡State

Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an

710049, China

ABSTRACT: Phase transition can profoundly influence the electrochemical performances of cathode materials for lithium ion batteries. The intricate phase transitions upon electrochemical cycling constrain the high-rate performances of the LiNi0.5Mn1.5O4 cathode. The formation of the rocksalt-like phase causes the diffusion of Li ions asymmetric in the lithiation and de-lithiation reactions. The evolution of multiple cubic phases results in poor diffusivity of Li ions in the LiNi0.5Mn1.5O4. High-resolution XRD scans on the chemically de-lithiated samples reveal that

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the intricate phase transitions are effectively suppressed in Cr doped LiNi0.5Mn1.5O4. The suppression of phase transitions not only enhances the Li ions’ diffusivity inside the lattice, but also stabilizes the charge transfer interfaces by reducing lattice mismatch and alleviating structural stress during the lithiation and de-lithiation. Consequently, the improvement on structure evolution endows the LiNi0.5Mn1.5O4 with enhanced diffusion coefficient of Li ions, larger accessible capacity and improved coulombic efficiency.

1. INTRODUCTION Cathode materials with excellent high-rate performances are imperative for next generation high power Li ion batteries.1 The atomistic scale fundamental structure-property relations determine the intrinsic electrochemical performances of cathode materials. Phase transition in the electrode materials plays a crucial role in the chemistry of batteries, which could profoundly influence the energy density and power density.1-7 Many emerging high power applications such as electric vehicles (EVs) and hybrid electric vehicles (HEVs) demand the cathodes materials with excellent high-rate performances.8-13 The LiNi0.5Mn1.5O4 cathode with cubic spinel structure (space group Fd3𝑚) is a promising candidate for high power applications owing to its high working potential around 4.7 V vs. Li+/Li.14-24 However, subject to the intricate phase transitions upon the lithiation/de-lithiation

reaction,

the

LiNi0.5Mn1.5O4

exhibits

unsatisfactory

high–rate

performances. The formation of the rocksalt-like phase in the de-lithiation reaction has recently been confirmed by the X-ray diffraction (XRD) and scanning transmission electron microscope (STEM) observation, where some transition-metal (TM) ions migrate into the empty octahedral sites, and then hinders the diffusion of Li ions in the subsequent lithiation reaction.25-26 Furthermore, the evolution of multiple cubic phases during the de-lithiation/lithiation reaction causes lattice mismatch, which adds extra barriers for Li ions’ transportation in the spinel


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lattice.9, 14, 27-38 Therefore, eliminating the detrimental phase transitions has been considered as an effective pathway towards excellent high-rate performances for the LiNi0.5Mn1.5O4 cathode. Transition-metal doping has been widely used to improve electrochemical performances of the LiNi0.5Mn1.5O4, and the improvements were generally ascribed to the elimination of impurities, altering cation ordering, manipulating Mn3+ concentration and so on.14,

19, 39-52

Recent studies

have consistently suggested the exceptional improvement effects of Cr doping on the electrochemical performances of LiNi0.5Mn1.5O4,38, 41-42, 44-45, 53-57 which were ascribed to several factors including the highly exposed (100) surface,45 inhibition of impurities and detrimental Mn3+ ions,45, 53, 55 , suppressing the SEI formation and increased electrical conductivity.41, 54, 58 Nevertheless, the structural origin of the improved electrochemical performances remains elusive. In this work, high resolution XRD scans on chemically de-lithiated samples gain insight into the structure evolution that both the formation of the rocksalt-like phase and the transitions between multiple cubic phases are significantly suppressed by Cr doping, which not only enhances the Li ions’ diffusivity inside the lattice, but also stabilize the charge transfer interfaces. Consequently, the diffusion coefficient of Li ions, accessible capacity and coulombic efficiency of the LiNi0.5Mn1.5O4 are enhanced prominently owing to significantly suppressed phase transitions. 2. EXPERIMENTAL SECTION 2.1 Sample preparation. The pristine LiNi0.5Mn1.5O4 and LiNi0.5-xCrxMn1.5O4 (x=0.05 and 0.1) were synthesized by a polymer-assisted method. First, CH3COOLi·2H2O, (CH3COO)2Mn·4H2O, Ni

(CH3COO)2·4H2O

and

(CH3CO2)7Cr3(OH)2

were

stoichiometrically

mixed

with

C2H2O4·2H2O and grounded in a planetary ball mill for 12 hours to obtain a homogeneous mixture. Then, 40mL polymer (PEG-400) was added, and grinding continued for 12 hours.


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Finally, the mixture was calcined at 400 ℃ for 2 hours followed by firing at 800 ℃ for 10 hours to obtain the final products. Chemically de-lithiated samples were prepared through oxidizing the synthesized spinels by the NO2BF4 in acetonitrile solution with constant stirring. The Li content (x) of de-lithiated samples were determined by inductively coupled plasma mission spectrometry (ICPE 9000 SHIMAZU). 2.2 Materials characterization. The crystal structure of obtained samples was examined by XRD (D8 advance, Bruker) equipped with a Cu Kα radiation source (λ=0.154056 nm) in a 2θ range of 10-80° using the step mode (0.01° per step). Obtained diffraction data are further analyzed by Rietveld refinement using GSAS software. SEM observations are carried out using a Hitachi S4800 scanning microscope. Fourier Transformation Infrared Spectrum (FTIR) was recorded with KBr pellets with a Nicolet 6700 spectrometer. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) measurement was conducted to determine the concentration of the Mn3+ ions. Energy-dispersive X-ray spectroscopy (EDS) was conducted using the JEM2100F/HR Transmission Electron Microscope. 2.3 Electrochemical tests. Electrochemical performances of the spinel cathodes were evaluated on coin cells (CR-2025) using lithium foil as anode. The cathode was prepared by mixing the active material (80 wt.%), super-P conductive carbon (10 wt.%) and polyvinylidene difluoride (PVDF) binder (10 wt.%) in N-methyl-2-pyrrolidinone (NMP) to form a homogeneous slurry, which was then coated on an Al foil and dried in a vacuum oven at 120 ℃ for 12 hours. The loading density of the active material is around 3.5 mg. cm-2. Cyclic Voltammetry measurements (CVs) were carried out using an electrochemical workstation (Auto-lab 204) with scan rates varying from 0.1 to 0.5 mV. s-1. Charge/discharge tests were conducted in the range between 3


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and 5 V on a battery tester (MC-8). All electrochemical tests were carried out at 25 ℃ in a temperature chamber. 3. RESULTS AND DISCUSSION

Figure 1. (a) to (c) XRD patterns and Rietveld refinements of the LiNi0.5Mn1.5O4, LiNi0.45Cr0.05Mn1.5O4 and LiNi0.4Cr0.1Mn1.5O4, respectively; (d) comparison of FTIR profiles of all samples. The XRD patterns of the LiNi0.5Mn1.5O4 and LiNi0.5-xCrxMn1.5O4 (x=0.05 and 0.1) are shown in Figure 1a to 1c. Rietveld refinement analysis shown in Table S1 (supporting information) indicates all major peaks of the three samples fit the cubic spinel structure (space group of Fd3 𝑚). Cr doping causes the decrease in the lattice constant due to the smaller ionic radius of Cr3+ (0.61 Å) compared with that of Ni2+ (0.69 Å). Minor peaks of the impurity phase LixNi1-xO can be observed at 43.75° and 63.42° in the LiNi0.5Mn1.5O4 sample (Figure 1a), which is a result of


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the oxygen loss from the spinel lattice during high temperature calcination ( 700 ℃ ).29, 59-60 While, such impurity is absent in the Cr doped samples owing to the strong bonding between Cr3+ and O2-.38, 44-45, 60 Particles of all the three samples possess truncated octahedron shape with average particle size around 300 nm as shown in Figure S1a to S1c (supporting information). FTIR spectra are shown in Figure 1d, the bands at 584 cm-1 and 622 cm-1 are contributed by the Ni-O and Mn-O vibrations, respectively; long range order would significantly increase the intensity of the Ni-O band compared with that of the Mn-O band, and therefore the intensity ratio of the bands at 584 and 622 cm-1 can manifest the degree of cation ordering at the octahedral 16d sites in the spinel lattice.29,

60-62

The ratio estimated is 0.935, 0.945, and 0.87 for the

LiNi0.5Mn1.5O4, LiNi0.45Cr0.05Mn1.5O4 and LiNi0.4Cr0.1Mn1.5O4, respectively. It suggests that the substantial amount of Cr doping could significantly promote random distribution of cations at octahedral 16d sites. It has been reported that the random distribution of Ni/Mn ions in the LiNi0.5Mn1.5O4 could shrink the miscibility gaps among multiple cubic phases upon lithiation/delithiation.29,

63-64

Therefore, it is reasonable to infer that the LiNi0.4Cr0.1Mn1.5O4 with highly

disordered structure would exhibit extended solid-solution phase regions upon lithiation/delithiation. To carefully examine the structure evolution and rule out the potential interference signals from inactive materials such as conductive carbon and current collector, chemically de-lithiated samples with different Li content (x) were directly scanned by the XRD (Figure S2a to S2c, supporting information). Figure 2a to 2c compares the XRD peaks in the range between 13 ° and 25°, a distinct peak appears at around 18.3°in the LixNi0.5Mn1.5O4 (Figure 2a) near the end of de-lithiation. In earlier research work, the migration of TM ions from 16d to 16c sites was firstly proposed owing to additional electron diffraction spots in de-lithiated sample,65 and was


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then directly observed by STEM observations which resulted in the rocksalt-like phase.25-26 XRD simulation predicts minor peaks would exist at around 18 °owing to the rocksalt-like phase;25 meanwhile in-situ XRD of the sample with TM ion occupied 16c sites shows a minor peak at around 18.3 ° .25 The distinct peak at 18.3 ° observed in this work is well-consistent with the earlier reported results, which can be ascribed to the formation of the rocksalt-like phase.25-26 The intensity of this peak is mitigated to the level comparable to the background noise in the two Cr doped samples (Figure 2b and 2c). The formation of the rocksalt-like phase near the end of de-lithiation would cause a substantial loss of capacity in the first lithiation process,26 since some octahedral 16c sites were occupied by TM ions, which would block inserting of Li ions into the spinel lattice as illustrated in Figure 2d.

Figure 2. (a) The rocksalt-like phase appears in the LixNi0.5Mn1.5O4 evidenced by the peak at around 18.3 ° ; (b) and (c) the peak of the rocksalt-like phase is indiscernible in the LiNi0.45Cr0.05Mn1.5O4 and LiNi0.4Cr0.1Mn1.5O4; (d) illustration of the formation of the rocksaltlike phase and its adverse effect on the lithiation reaction.


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The EDS analysis of the LiNi0.4Cr0.1Mn1.5O4 are shown in Figure S3a and S3b (supporting information), which suggests Ni ions preferentially segregate in the inner part of the particle, while the Cr ions is slightly segregated in the surface region; and this is consistent with previously reported surface segregation of Cr ions in the spinel lattice.42, 49 The strong bonding between Cr3+ and O2- would inhibit the migration of TM ions near the surface region. After electrochemical cycling, the segregations of Cr and Ni ions are similar as before cycling as shown in Figure S3c and S3d (supporting information).

Figure 3. Evolution of multiple cubic phases characterized by the XRD in the (a) LixNi0.5Mn1.5O4, (b) LixNi0.45Cr0.05Mn1.5O4 and (c) LixNi0.4Cr0.1Mn1.5O4. Figure 3a to 3c exhibit the evolution of multiple cubic phases in the LixNi0.5Mn1.5O4, LixNi0.45Cr0.05Mn1.5O4 and LixNi0.4Cr0.1Mn1.5O4, respectively. Variations of the (331), (511) and (440) peaks in the range between 47.5° and 67.5° have been scrutinized to understand the influence of Cr doping on the phase transitions. As shown in Figure 3a, initially all the three peaks of the LixNi0.5Mn1.5O4 shift towards higher degree without peak splitting manifesting a solid-solution phase (phase I) behavior. When x reaches 0.57, peaks’ splitting is perceptible indicating a new cubic phase (phase II) appears, and the phase I coexists with the phase II until x reaches 0.46. The phase II solely dominates until x reaches 0.30, where the phase III appears; and the phase II coexists with the phase III when x between 0.30 and 0.23. Two two-phase


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regions are therefore identified in the LixNi0.5Mn1.5O4 upon de-lithiation. In comparison, Cr doping could significantly suppress the evolution of multiple cubic phases. As shown in Figure 3b, upon de-lithiation of the LixNi0.45Cr0.05Mn1.5O4, the initial solid-solution phase region extends to x = 0.53; and then the phase II emerges as suggested by peak splitting of (511) and (440), and the phase II coexists briefly with phase I (x between 0.53 and 0.48). The phase III emerges when x reaches 0.32, and then the second two-phase region (phase II / III) dominates until x reaches 0.24. Upon de-lithiation of the LixNi0.4Cr0.1Mn1.5O4, the initial solidsolution phase (phase I) dominated region further extends to x=0.43, and only one two-phase region (phase I / II) is identified (x between 0.43 and 0.37), which is evidenced by an obscured splitting of the (440) peak as shown in Figure 3c. No more discernible peak splitting is observed upon further de-lithiation, which suggests the nucleation of the phase III is effectively suppressed in the LixNi0.4Cr0.1Mn1.5O4 upon de-lithiation. Table 1. The region of each cubic phase in the LixNi0.5Mn1.5O4, LixNi0.45Cr0.05Mn1.5O4 and LixNi0.4Cr0.1Mn1.5O4 upon de-lithiation. Phase I

Phase I/II

Phase II

Phase II/III

Phase III

(x)

(x)

(x)

(x)

(x)

LixNi0.5Mn1.5O4

1 ~ 0.57

0.57 ~ 0.46

0.46 ~ 0.3

0.3 ~ 0.23

0.23 ~ 0

LixNi0.45Cr0.05Mn1.5O4

1 ~ 0.53

0.53 ~ 0.48

0.48 ~ 0.32

0.32 ~ 0.24

0.24 ~ 0

LixNi0.4Cr0.1Mn1.5O4

1 ~ 0.43

0.43 ~ 0.37

0.37 ~ 0

/

/

Sample

Table 1 compares the region of each cubic phase in the three spinel cathodes upon delithiation, which clearly shows that Cr doping could extend the solid-solution phase region and


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suppress the nucleation of the third cubic phase in the LixNi0.4Cr0.1Mn1.5O4. The variation of lattice parameter upon the evolution of cubic phases is compared through Figure 4a to 4c. The discontinuous and steep change in lattice parameter would cause severe strain and stress in the LixNi0.5Mn1.5O4, which would add extra barriers impeding the Li ions’ transportation.1, 64, 66 The extension of the solid-solution phase region would reduce lattice mismatch and alleviate structural stress among different phases, which could benefit both the fast diffusion of Li ions and the structure stability.27, 33

Figure 4. The variation of lattice parameter upon de-lithiation in the (a) LixNi0.5Mn1.5O4, (b) LixNi0.45Cr0.05Mn1.5O4 and (c) LixNi0.4Cr0.1Mn1.5O4. Although the transition between multiple cubic phases is considered as a reversible process in the spinel cathode,9,

28, 30-31, 37

the formation of the rocksalt-like phase in the end of the de-

lithiation causes the diffusion of Li ions asymmetric in the lithiation and de-lithiation reactions. To estimate the average diffusivity of Li ions during the de-lithiation/lithiation cycle, Cyclic Voltammetry measurements (CVs) were carried out on electrochemical half-cells, which use the LiNi0.5Mn1.5O4, LiNi0.45Cr0.05Mn1.5O4 and LiNi0.4Cr0.1Mn1.5O4 as cathode, and the Li foil as anode. With the increase of the potential (vs. Li+/Li) scan rate, both the anodic peaks (delithiation) and cathodic peaks (lithiation) gradually become blunt and indistinguishable as shown in Figure 5a, which is a manifestation of the sluggish charge transportation in the LiNi0.5Mn1.5O4.


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While the redox peaks of Cr doped sample are much sharper as shown in Figure 5b and 5c. The linear fit of the peak current (ip) vs. the square root of the scan rates (ν1/2) (Figure 5d) can be used to calculate the diffusion coefficient of Li ions according to Randles-Sevcik equation (equation (1)):67-68

Figure 5. Cyclic Voltammetry measurements (CVs) of the (a) LiNi0.5Mn1.5O4, (b) LiNi0.45Cr0.05Mn1.5O4 and (c) LiNi0.4Cr0.1Mn1.5O4 at different potential scan rates. (d) The plotting of peak current vs. square root of the scan rate for the LiNi0.5Mn1.5O4, LiNi0.45Cr0.05Mn1.5O4 and LiNi0.4Cr0.1Mn1.5O4.

i p  2.69 10 5 n 3 / 2 C Li ADLi 1 / 2 （at 25℃） (1) where, i p is the peak current value (A), n is the number of electrons per reaction species, C Li is the bulk concentration of Li ions in the electrode, A is the surface area of electrode (cm2), DLi is


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the diffusion coefficient of Li ions (cm2. s-1), and  is the potential scan rate (V. s-1). The calculated DLi are 1.17×10-11 cm2.s-1, 2.16×10-11 cm2.s-1 and 3.57×10-11 cm2.s-1 for the LiNi0.5Mn1.5O4, LiNi0.45Cr0.05Mn1.5O4 and LiNi0.4Cr0.1Mn1.5O4, respectively. It is worth to note that the controversial effect of Mn3+ can be ruled out here, since the concentration of Mn3+ of the Cr doped samples (10.33% and 8.37%) are smaller than that (14.94%) of the LiNi0.5Mn1.5O4 as determined by the XPS analysis (Figure S4a to S4c, supporting information), which is also consistent with the earlier reported results.45

Figure 6. Rate capability of the (a) LiNi0.5Mn1.5O4, (b) LiNi0.45Cr0.05Mn1.5O4, (c) LiNi0.4Cr0.1Mn1.5O4. (d) comparison of capacity retention at different discharge rates.


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The enhanced diffusion coefficient of Li ions endows the Cr doped spinel cathodes with better rate capability as displayed in Figure 6a to 6d. All the half-cells were firstly charged at a constant rate of 0.2 C (1 C=147 mA·g-1), and then discharged at rates increasing from 0.2 C to 20 C. At the low discharge rate of 0.2 C, all the three cathodes can offer a capacity around 125 mAh·g-1. At the highest discharge rate of 20 C, the LiNi0.5Mn1.5O4 with the lowest DLi exhibits the largest polarization and can only release 92.5 mAh·g-1 (Figure 6a). With significantly enhanced DLi , the LiNi0.45Cr0.05Mn1.5O4 and LiNi0.4Cr0.1Mn1.5O4 could offer 105 mAh·g-1 and 119.6 mAh·g-1 at the 20 C (Figure 6b and 6c), respectively. Figure 6d compares the capacity retention of the three cathodes, where the discharge capacity at the 0.2 C is normalized to 100%. With increase of the discharge rate, the capacity retention of the LiNi0.5Mn1.5O4 declines fast to only 75.7% at the 20 C, while the LiNi0.4Cr0.1Mn1.5O4 can sustain 94% of the initial capacity at the 20 C rate.

Figure 7. (a) Comparison of the discharge capacity cycling at the 5 C rate, and (b) comparison of the coulombic efficiency for 500 cycles. Cyclic tests carried out at the 5 C charge/discharge rate further validate the effect of suppressed phase transitions. As shown in Figure 7a, the discharge capacity of all the three cathodes rises from a low level during the initial few cycles, which is due to the decrease of


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polarization by the relaxation of charge transfer interfaces. The LiNi0.5Mn1.5O4 releases the lowest capacity (98.42 mAh.g-1) at the 1st cycle among the three spinel cathodes. It takes 6 cycles for the LiNi0.5Mn1.5O4 to reach the highest capacity (103 mAh. g-1), and 77.69 mAh. g-1 remains at the last cycle. While, the Cr doped cathodes reach the highest capacity in 3 cycles, and the LiNi0.4Cr0.1Mn1.5O4 exhibits the best cyclic performance with 89.53 mAh. g-1 maintained at the 500th cycle. The results suggest that the intricate phase transitions in the LiNi0.5Mn1.5O4 not only put obstacles for the diffusion of Li ions inside the lattice, but also may influence the charge transfer interfaces of the electrode. The coulombic efficiency shown in Figure 7b indicates the utility of Li ions during cycling, which also manifests the variation of charge transfer interfaces during electrochemical cycling. Both the formation of solid/electrolyte interphase (SEI) layer and the adjustment of electrical contacts (active material/conductive agent/current collector) jointly contribute to the low coulombic efficiency at initial cycles.69-72 Meanwhile, the rocksalt-like phase in the LixNi0.5Mn1.5O4 hinders the 1st lithiation reaction (Figure 2d) and results in the lowest coulombic efficiency (94.7%). The structural relaxation in the electrode could help form the uniform SEI layer and stable electrical contacts, and then the coulombic efficiency could reach the highest value. Both the SEI layer and the electrical contacts are influenced by the stress and strain caused by the phase transitions of the cathode.12, 69, 72-74 It take 9 cycles for the LiNi0.5Mn1.5O4 and 5 cycles for the LiNi0.4Cr0.1Mn1.5O4, respectively, to reach the highest coulombic efficiency (> 99%). As demonstrated in Figure 4, the discontinuous and steep change in the lattice constant of the LixNi0.5Mn1.5O4 would cause more complicated strain/stress conditions in the charge transfer interfaces, which cost longer time and consume more Li ions to complete the structural relaxation. In the full cell using other materials rather than Li foil as the anode, the supply of Li


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ions is strictly limited by the cathode reservoir; the more Li ions consumed on the structural relaxation, the lesser capacity is available for the practical utilization. The significantly suppressed phase transitions in the LixNi0.5Mn1.5O4, which gives rise to enhanced Li ions’ diffusivity, improved charge transfer interfaces and higher coulombic efficiency of initial cycles, is therefore crucial for the practical high-rate applications. 4. CONCLUSIONS In this work, we demonstrate that the high-rate performances the high voltage LiNi0.5Mn1.5O4 are significantly influenced by phase transitions upon lithiation/de-lithiation, which not only impede the diffusion of Li ions inside the lattice, but also affect the charge transfer interfaces of the electrode. Cr doping exhibits the outstanding effect on inhibiting the formation of the rocksalt-like phase, which prevents the significant capacity loss at the first cycle. With the increase of Cr doping content, the evolution of multiple cubic phases is effectively suppressed, and the lattice mismatch and structural stress are mitigated, which give rise to enhanced diffusivity of Li ions and improved charge transfer interfaces. Consequently, Cr doped LiNi0.5Mn1.5O4 exhibits excellent electrochemical performances at high rates. The results present here demonstrate that improving the structure evolution is an effective way to achieve excellent high-rate performances of the LiNi0.5Mn1.5O4 cathode.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge. particles’ morphology under SEM observation, full range of XRD patterns of de-lithiated samples, EDS results of the


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LiNi0.4Cr0.1Mn1.5O4, XPS spectra and fitting curves of Mn3+ and Mn4+, and structural information obtained from Rietveld refinement (PDF).

AUTHOR INFORMATION Corresponding author *Email: [email protected] ORCID Hailong Wang: 0000-0003-0725-2326

ACKNOWLEDGMENT This work was primarily supported by the National Science Foundation of China (NSFC51462029). This work was also supported by Technology Leading Talent Program of Ningxia province (KJT2016003), open project of State Key Laboratory for Mechanical Behavior of Materials (20161811), Ningxia Key Research and Development Project (2015DY002), National First-Rate Discipline Construction Project of Ningxia (No. NXYLXK2017A04) and Major Innovation Projects for Building First-class Universities in China's Western Region (ZKZD2017006).

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During Cycling at 5 V. Electrochem. Solid-State Lett. 2004, 7, A216-A220.


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(57) Aklalouch, M.; Amarilla, J. M.; Rojas, R. M.; Saadoune, I.; Rojo, J. M. Sub-micrometric LiCr0.2Ni0.4Mn1.4O4 Spinel as 5 V-Cathode Material Exhibiting Huge Rate Capability at 25 and 55 degrees C. Electrochem. Commun. 2010, 12, 548-552. (58) Liu, D.; Zhu, W.; Kim, C.; Cho, M.; Guerfi, A.; Delp, S. A.; Allen, J. L.; Jow, T. R.; Zaghib, K. High-Energy Lithium-Ion Battery Using Substituted LiCoPO4: From Coin Type to 1 Ah Cell. J. Power Sources 2018, 388, 52-56. (59) Cabana, J.; Casas-Cabanas, M.; Omenya, F. O.; Chernova, N. A.; Zeng, D.; Whittingham, M. S.; Grey, C. P. Composition-Structure Relationships in the Li-Ion Battery Electrode Material LiNi0.5Mn1.5O4. Chem. Mater. 2012, 24, 2952-2964. (60) Liu, D.; Hamel-Paquet, J.; Trottier, J.; Barray, F.; Gariepy, V.; Hovington, P.; Guerfi, A.; Mauger, A.; Julien, C. M.; Goodenough, J. B.; Zaghib, K. Synthesis of Pure Phase Disordered LiMn1.45Cr0.1Ni0.45O4 by a Post-Annealing Method. J. Power Sources 2012, 217, 400-406. (61) Moorhead-Rosenberg, Z.; Huq, A.; Goodenough, J. B.; Manthiram, A. Electronic and Electrochemical Properties of Li1-xMn1.5Ni0.5O4 Spinel Cathodes as a Function of Lithium Content and Cation Ordering. Chem. Mater. 2015, 27, 6934-6945. (62) Kunduraci, M.; Amatucci, G. G. Synthesis and Characterization of Nanostructured 4.7 V Li xMn1.5Ni0.5O4

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(74) Rhodes, K.; Meisner, R.; Kim, Y.; Dudney, N.; Daniel, C. Evolution of Phase Transformation Behavior in Li (Mn1.5Ni0.5) O4 Cathodes Studied by In Situ XRD. J. Electrochem. Soc. 2011, 158, A890-A897


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TOC Graphic


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Figure 1. (a) to (c) XRD patterns and Rietveld refinements of the LiNi0.5Mn1.5O4, LiNi0.45Cr0.05Mn1.5O4 and LiNi0.4Cr0.1Mn1.5O4, respectively; (d) comparison of FTIR profiles of all samples. 127x93mm (300 x 300 DPI)


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Figure 2. (a) The rocksalt-like phase appears in the LixNi0.5Mn1.5O4 evidenced by the peak at around 18.3°; (b) and (c) the peak of the rocksalt-like phase is indiscernible in the LiNi0.45Cr0.05Mn1.5O4 and LiNi0.4Cr0.1Mn1.5O4; (d) illustration of the formation of the rocksalt-like phase and its adverse effect on the lithiation reaction. 150x84mm (300 x 300 DPI)



Figure 3. Evolution of multiple cubic phases characterized by the XRD in the (a) LixNi0.5Mn1.5O4, (b) LixNi0.45Cr0.05Mn1.5O4 and (c) LixNi0.4Cr0.1Mn1.5O4. 254x67mm (300 x 300 DPI)


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Figure 4. The variation of lattice parameter upon de-lithiation in the (a) LixNi0.5Mn1.5O4, (b) LixNi0.45Cr0.05Mn1.5O4 and (c) LixNi0.4Cr0.1Mn1.5O4. 254x64mm (300 x 300 DPI)



Figure 5. Cyclic Voltammetry measurements (CVs) of the (a) LiNi0.5Mn1.5O4, (b) LiNi0.45Cr0.05Mn1.5O4 and (c) LiNi0.4Cr0.1Mn1.5O4 at different potential scan rates. (d) The plotting of peak current vs. square root of the scan rate for the LiNi0.5Mn1.5O4, LiNi0.45Cr0.05Mn1.5O4 and LiNi0.4Cr0.1Mn1.5O4. 120x90mm (300 x 300 DPI)


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Figure 6. Rate capability of the (a) LiNi0.5Mn1.5O4, (b) LiNi0.45Cr0.05Mn1.5O4, (c) LiNi0.4Cr0.1Mn1.5O4. (d) comparison of capacity retention at different discharge rates. 108x83mm (300 x 300 DPI)



Figure 7. (a) Comparison of the discharge capacity cycling at the 5 C rate, and (b) comparison of the coulombic efficiency for 500 cycles. 254x93mm (300 x 300 DPI)


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Figure S1 Particles’ morphology under SEM observation for the: (a) LixNi0.5Mn1.5O4, (b) LixNi0.45Cr0.05Mn1.5O4 and (c) LixNi0.4Cr0.1Mn1.5O4.



Figure S2 XRD patterns of chemically de-lithiated samples for the: (a) LixNi0.5Mn1.5O4, (b) LixNi0.45Cr0.05Mn1.5O4 and (c) LixNi0.4Cr0.1Mn1.5O4.


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Figure S3. (a) EDS analysis on the pristine LiNi0.4Cr0.1Mn1.5O4 particle, (b) atomic ratios of Ni and Cr ions from surface to the inner part; (c) EDS analysis on the LiNi0.4Cr0.1Mn1.5O4 particle after 500 cycles electrochemical cycling at the 5C rate, (d) atomic ratios of Ni and Cr ions from surface to the inner part. 90x91mm (300 x 300 DPI)



Figure S4. Determination of the concentration of Mn3+ by the XPS spectra and fitting: (a) 14.94% in the LiNi0.5Mn1.5O4, (b) 10.33% in the LiNi0.45Cr0.05Mn1.5O4 and (c) 8.37% in the LiNi0.4Cr0.1Mn1.5O4. 185x50mm (300 x 300 DPI)


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Phase Transition Dominated High-Rate Performances of the High

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