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Optimizing the structural evolution of Li-rich oxide cathode materials via microwave-assisted pre-activation Min-Jun Wang, Fu-Da Yu, Gang Sun, Da-Ming Gu, and Zhen-Bo Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00812 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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ACS Applied Energy Materials

Optimizing the Structural Evolution of Li-Rich Oxide Cathode Materials via Microwave-Assisted Pre-Activation Min-Jun Wang, Fu-Da Yu, Gang Sun, Da-Ming Gu, Zhen-Bo Wang* MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No. 92 West-Da Zhi Street, Harbin, 150001, China

Pre-activation can play a promising role in suppressing oxygen loss during the first charging process in Li-rich oxide cathode materials, relieving a series of problems such as large initial irreversible capacity loss, structure transformation due to ion rearrangement, and oxidation/decomposition of the electrolyte. However, the strategies previous adopted are mainly chemical delithiation which has violent effect on the crystal structure and deteriorates the cycle performance. Here, we report a facile and effective microwave-assisted treatment method to pre-activate Li-rich layered oxides without structural distortion. The microwave-treated sample shows a high discharge capacity of about 281 mAh g-1 with a coulombic efficiency of 87% at 0.1 C and exhibits unnormal continuous increase of discharge capacity from 203 mAh g-1 to 218 mAh g-1 during 110 cycles at 1 C as well as presents distinctly improved cycling stability. EIS, GITT and XRD studies reveal that the kinetics of electrochemical reaction on electrode surface and Li ionic migration coefficient in crystal structure become progressively enhanced due to the gradual expansion of interplanar spacing upon cycling. Those results demonstrate that pre-activation by microwave-assisted treatment contributes to optimize the structure evolution of Li-rich material for rate performance in cyclic process, which also provides new thinking in modifying Li-rich layered cathode materials. KEYWORDS: Microwave-assisted pre-activation; Structure evolution; Cycle performance; Li-ion diffusion coefficient; Li-rich oxide cathode material;

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Introduction Facing the fundamental challenges to develop eco-friendly energy storage devices, lithium-ion (Li-ion) batteries as a foremost stockpile equipment have been increasingly urged to be of high energy density in order to satisfy the increasing demands in mobile electronic devices and automotive industry[1-4]. To fulfil this requirements, developing advanced cathode materials with high operating voltage and large specific capabilities is highly significant. Recently, the layered lithium-excess transition metal (TM) oxides, xLi2MnO3·(1-x)LiMO2 (M=Mn, Co, Ni, etc.), as a kind of promising cathode materials with high reversible capacities up to 250 mAh g-1 have attracted wide attention.[5-9] The composition structures of such materials are quite intricate, consisting of two diverse local structures—Li2MnO3 (a monoclinic C2/m structure) and LiMO2 (a hexagonal R-3m structure) at atomic scale. The Li2MnO3 component not only plays a pivotal role in improving the structural stability of Li-rich layered oxides at high potentials but also acts as an active phase for Li-ion extraction[10]. Li2MnO3 becomes activated when charged above 4.5 V and a characteristic voltage plateau appears, which corresponds to the anomalous high capacities. This either has been attributed to the extraction of Li+ ions accompanied with the irreversible loss of oxygen from the lattice, or the surface reaction through electrode/electrolyte reduction and/or Li+/H+ exchange.[11-13] The loss of oxygen during the high-potential electrochemical process gives rise to low initial coulombic efficiency (CE) and the oxygen vacancies generated in this process can aggravate the transition metal ion migration to Li layer and surface structural transformation, leading to poor cycling performance, poor rate performance and potential degradation during extended cycles.[11, 14, 15] Partial delithiation before electrochemical cycling is a promising approach to reduce oxygen release while traversing along the 4.5 V plateau. However pre-activation is difficult to be realized due to the thermodynamics equilibrium of electrode material. Harsh chemical conditions have always been adopted in previous works. Kang et al. treated Li-rich cathode material 0.5Li2MnO3·0.5LiNi0.44Co0.25Mn0.31O2 using HNO3 as activation solution.[16] Li+ ions were pre-extracted from Li2MnO3 component by the reaction of Li+/H+ exchange. Zheng et al. treated Li[Li0.2Ni0.133Co0.133Mn0.544]O2 with Na2S2O8 and ACS Paragon Plus Environment

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(NH4)2SO4.[17] The discharge capacity and initial CE were improved because the structural rearrangement caused by lithium ion and oxygen anion extraction generated a spinel phase on the surface, which could facilitate Li+ ions to diffuse into the Li-rich material. Although these chemical solution methods reported are effective in realizing pre-activation, the crystal structures of the cathode materials involved are easily damaged, leading to deterioration of cycle stability. Recently, gas–solid interfacial reaction is introduced as a gentle method for pre-activation, which is advanced in preservation of the crystal structure.[18, 19] But gas treatment process is always complex and uncontrollable compared with the traditional solution methods. Furthermore, previous reports are rare to further explore the long-term implication of preactivation in electrode material during charge-discharge process. In view of these, we employ stable and neutral glycol as solvent to avoid destroying the structural stability and innovatively introduce a facile microwave treatment strategy to assist Li-rich material preactivation. Composition and phase structure analysis reveal that Li-ion extracts from cathode material without causing structural destruction, which indicates temperate solution is also practicable for preactivation by introducing suitable reaction condition. After microwave processing, the target materials not only exhibit improved coulombic efficiency and cycling stability, but also display unnormal continuous increase of discharge capacity in the course of cycle process at high rate. Electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) tests indicate the charge transfer impedance and Li+ ionic migration coefficient become beneficial for rate performance after cycles. XRD refinement data suggests that the lattice parameter c becomes larger due to the evolution of crystal structure in cyclic process, which can aggrandize the Li-ion diffusivity and ameliorate the rate performance. This study reveals that pre-activation by microwave-assisted treatment can optimize the evolution of material structure during cyclic process to enhance cycling stability and rate performance of Li-rich layered materials.


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Experimental Synthesis of Pristine Material The material of Li[Li0.21Ni0.131Co0.122Mn0.538]O2 was synthesized by a typical co-precipitation method. 2 mol L-1 aqueous solution of CoSO4·7H2O, NiSO4·6H2O and MnSO4·H2O with appropriate molar ratios was added into a continuously stirred tank reactor at the temperature of 55 oC. In the meantime, an equimolar Na2CO3 solution with desired amount of NH4OH was added into the reactor to keep the pH at 7.50. The coprecipitated particles were collected and dried at 120 oC for 24 h. The obtained precursor particles were mixed with precalculated Li2CO3. 5% excess of lithium was introduced intentionally to compensate the Li loss during the calcination. The mixture was first annealed at 450 oC for 3 h and then at 800 oC for 12 h. Microwave-Assisted Treatment The microwave-assisted treatment method is schematic illustrated in Figure 1 and the detailed process was carried out as follows: 2 g pristine sample was added into a beaker with 150 ml glycol and stirred for 5 minutes in order to disperse evenly. Then the beaker was putted into a microwave oven and treated for several minutes. After that, the samples were filtered, washed, and dried at 80 oC for 12 h. The pristine sample and the treated samples by microwave for 1min, 3min, and 5min were marked as MT-0, MT-1, MT-3, and MT-5, respectively.

Figure 1. Schematic illustration of the microwave-assisted pre-activation. ACS Paragon Plus Environment

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Materials Characterization The morphology and size of the samples were determined by scanning electron microscopy (SEM) (QUANTA200). Chemical compositions of the synthesized materials were identified by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-OES). X-ray photoelectron spectroscopy (XPS) measurements were performed to ascertain the chemical valence state on a Physical Electronics PHI model 5700 instrument with an Al X-ray source. C 1s line at 284.6 eV was used as baseline to correct the XPS spectra. X-ray power diffraction (XRD) method was used to identify the crystallographic phase of samples and the data was collected using an X'PERT PRO MPD with filtered Cu Kα radiation (λ= 1.54056Å). The Rietveld refinement of powder XRD data was performed using General Structure Analysis System (GSAS) code. Electrochemical Measurements Cathode electrodes were prepared by mixing active material, acetylene black, polyvinylidene difluoride and appropriate quantity of N-methyl pyrrolidone (NMP) solvent homogeneously and pasting the mixture on an aluminium foil. Then the aluminium foil was dried at 120 oC for 12 h to remove the NMP solvent. After that, the electrode sheets were punched into circular disks as cathode. The loading mass of the active material was approximately 4.0 mg cm−2. The coin-type cells were assembled in an argon-filled dry glove-box using lithium metal as anode. The composition of electrolyte consists of 1 M LiPF6 dissolved in ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) at a volumetric ratio of 1:1:1. A NEWARE battery tester was used for Galvanostatic charge–discharge experiments and GITT tests at the temperature of 25 oC. During GITT testing, a constant current flux of 40 mA g-1 for a given time period (10 min) was supplied, followed by an open circuit conditions for 40 min. EIS and cyclic voltammetry (CV) were measured by an electrochemical workstation (CHI660E). EIS was recorded by applying an AC voltage of 5 mV in the frequency range from 105 Hz to 10-2 Hz. The C-rates mentioned in this text were calculated considering 1 C as 250 mA g-1. ACS Paragon Plus Environment

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Results and Discussion Characterizations of Electrode Material The morphologies of Li-rich layered oxides before and after microwave-assisted treatment are characterized by SEM and shown in Figure 2. MT-1, MT-3, MT-5 all indicate morphology feature coinciding with the pristine MT-0 sample, which means microwave-assisted treatment does not have side effect on the spherical dense configuration of active material. (a)

(b)

(c)

(d)

Figure 2. SEM images of (a) MT-0 (b) MT-1 (c) MT-3 (d) MT-5

The metal atom ratios Li: Co: Mn: Ni acquiring from ICP-OES are listed in Table 1 to accurately describe each sample. Co: Mn: Ni ratio keeps essentially unaltered after microwave-assisted treatment in different time. By contrast, the lithium content significantly reduces from 1.210 for pristine sample to 1.186, 1.172 and 1.147 for MT-1, MT-3 and MT-5, respectively. Therefore, lithium ion was extracted from the active material during microwave-assisted treatment and the extension of treatment time is beneficial for removing lithium ion. In consideration of the little distinction of transition metal content,


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Li+ ions do not be extracted from LiMO2 component because such a reaction would have necessary to dissolve one of the metals as an oxide for charge compensation.[16] Table 1. Relative amounts of Li, Co, Mn, and Ni (normalized to the Mn content) (ICP data). Li

Co

Mn

Ni

MT-0

1.210

0.122

0.538

0.131

MT-1

1.186

0.123

0.538

0.130

MT-3

1.172

0.121

0.538

0.131

MT-5

1.147

0.122

0.538

0.130

The valence states of elements on the surface of MT-0, MT-1, MT-3 and MT-5 are identified by XPS. Figure 3a displays two main peaks in the Mn 2p spectra. These peaks are located at similar position for the four samples and can be assigned to the manganese 2p3/2 at 642.7 eV and 2p1/2 at 654.2 eV. Mn 2p3/2 coincides well with binding energy for the standard Mn4+ range (642.2 eV-642.8eV).[20, 21] The Co 2p and Ni 2p peaks shown in Figure 3b and c also do not display any apparent shift among the four samples. Because of spin–orbit coupling, Co 2p spectrum is split into 2p3/2 and 2p1/2, with a main line and a satellite in each component.[22] The Co 2p3/2 core level spectrum is located at 780.6 eV and 2p1/2 is located at 795.8 eV. The differences between the binding energies are 15.2 eV. The satellite peaks of the 2p3/2 level are located at 790.3 eV. These data imply Co ions in the four samples are presented as Co3+.[23, 24] Figure 3c exhibits that the binding energies of Ni 2p3/2 and 2p1/2 peaks are 855.1 and 872.6 eV, respectively. The difference value between this two peaks is 17.5 eV, which indicates the valence state of Ni in the four samples is +2.[25, 26] In general, the valences of Mn, Co and Ni do not change after microwave-assisted treatment. Therefore oxygen must be released from the lattice when lithium is extracted from the Li-rich materials in order to keep valence balance.[17] This process just like the electrochemical activation of Li2MnO3 while traversing along the plateau near 4.5 V in the initial charge. Figure 3d shows the O 1s spectra of the four samples. A peak observed at ~531.6 eV can be assigned to oxygen species on the surface.[22] The peak intensity of the samples after microwave-assisted treatment are


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much lower than that of pristine one, indicating that some oxygen species on the material surface have been removed in treatment process. Another peak at ~529.4 eV represents oxygen atoms of the Li-rich layer oxides’ lattice,

[27]

and it shifts to higher binding energy after microwave-assisted treatment. In

consideration of the unchanged valence states of Mn, Co, and Ni, such variation can be only attributed to the enhancement of binding energy between the oxygen atoms. This is probably because the extraction of oxygen from lattice relieves the electrostatic repulsion between the oxygen anions as shown in Figure 4e and forms peroxo-like O22- species or under-coordinated oxygen atoms.[28-32] The valence states of subsurface elements also have been detected by XPS and depicted in Figure S1. The valence states of Mn, Co, and Ni are mainly keep constant and can be assigned to Mn 4+, Co 3+, Ni 2+. But the peak of lattice oxygen shift to lower binding energy, which must be due to the removal of peroxo-like O22- species generated by pre-activation on the surface.

Mn 2p3/2

Mn 2p3/2 635

640

Mn 2p1/2

Mn 2p1/2

Mn 2p1/2

Co 2p3/2

MT-3

MT-1

Co 2p3/2

Co 2p3/2

MT-0

645 650 655 Binding Energy (eV)

Co 2p3/2

660 775

780

(c)

Co 2p1/2 MT-5

Co 2p1/2

Ni 2p3/2

Ni 2p3/2

Co 2p1/2 MT-0 805

810

Ni 2p1/2 MT-5

(d) O 1s

Ni 2p3/2 MT-3

Co 2p1/2 MT-1

785 790 795 800 Binding Energy (eV)

Ni 2p3/2

850

855

860

Ni 2p1/2 MT-3

Ni 2p1/2 MT-1 Ni 2p1/2 MT-0

865 870 875 880 Binding Energy (eV)

885

Intensity (a.u.)

Mn 2p3/2

(b)

MT-5

Intensity (a.u.)

Mn 2p1/2

Intensity (a.u.)

(a) Mn 2p3/2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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MT-5

O 1s

MT-3

O 1s

MT-1

O 1s

MT-0

527 528 529 530 531 532 533 534 Binding Energy (eV)

Figure 3. XPS spectra of the MT-0, MT-1, MT-3 and MT-5 samples: (a) Mn 2p, (b) Co 2p, (c) Ni 2p and (d) O 1s core levels.

XRD is an effective technique to study crystal structure and phase transformation. The diffraction peaks of the four samples shown in Figure 4a match well with each other and can be indexed to a hexagonal α-NaFeO2 structure (space group: R-3m), excepting a few broad peaks between the 20-25° range which is a feature of Li2MnO3-type structure and can be indexed to a monoclinic phase (space group: C2/m). No uncharted peaks are detected, indicating no impurity phases are introduced during microwave-assisted treatment. The peaks of (006)/(012) and (018)/(110) with a notable split are attributed


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to the existence of highly layered structure.[33, 34] The shapes of peaks between 20° and 25° presented in Figure 4b change from sharp to broad as the time of microwave-assisted treatment increases, which can be ascribed to the further severity of Li2MnO3 activation. From Figure 4c and d, the (104) peaks of all samples mainly stay in a same angle while the (003) peaks of the treated samples shift to higher angle. This indicates that the interplanar spacing of (003) slab becomes narrow, which is probably ascribed to the extraction of oxygen from lattice during pre-activation, followed by the decrease of electrostatic repulsion between the oxygen anions in adjacent (003) slab as shown in Figure 4e.[35] Such transformation is accordance with the variation of lattice parameter during the process of Li2MnO3 electrochemical activation.[36] XRD test confirms that the crystal lattices of Li-rich layered oxides maintain integral layered structure after microwave-assisted treatment and the changes of main peaks

(017) (010) (118) (113)

(104) (015)

(006) (012)

(a)

(b) MT-5

Intensity (a.u.)

Intensity (a.u.)

MT-5

(101)

(003)

coincide well with the characteristic of Li2MnO3 activation.

MT-3 MT-1

10

MT-3 MT-1

MT-0

MT-0

20

30 40 50 60 2-Theta (degree)

70

80

20

(c)

(003)

21 2-Theta (degree)

(104)

22

(d)

MT-5

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MT-5 MT-3 MT-1 MT-0

18.2

MT-3

MT-1

MT-0

18.4

18.6 18.8 2-Theta (degree)

19.0

19.2 44.0 44.2 44.4 44.6 44.8 45.0 45.2 45.4 2-Theta (degree)


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(e)

Figure 4. (a), (b), (c) and (d): XRD patterns of the MT-0, MT-1, MT-3 and MT-5 samples. (e) Schematic illustration for the variation of electrostatic repulsion after microwave-assisted treatment.

Glycol as a relatively stable solution, no matter at room temperature or high-temperature (even at boiling point), is different from the solvents reported for pre-activation such as HNO3, Na2S2O8 and (NH4)2SO4 which have harsh chemical property. But from the characterizations of electrode material mentioned above, the Li-rich materials are realized pre-activation successfully. Thus, we believe the violent chemical reaction between active material and solvents is not indispensable to realize preactivation and a chemically stable solvent is sufficient in a suitable physical conditions (such as microwave treatment).

Electrochemical Performance In Figure 5a, the pristine and MT-3 samples are galvanostatically cycled between 2.0 and 4.4 V at 0.1 C (25 mA g−1). In this process, Li2MnO3 has not been activated and contributes no capacity. Both samples display a slope charge curve corresponding to the transformation of Ni2+/Ni4+ and Co3+/Co4+ in LiMO2 segment. Their charge capacities in this section are similar for about 105 mAh g−1 while the discharge capacity is 84 mAh g−1 for the pristine MT-0 and 92 mAh g−1 for MT-3. The additional capacity in MT-3 mainly comes from the voltage profile between 2.0 and 3.6 V, thus it most probably gains from MnO2 component which is generated from pre-activation of Li2MnO3 during microwave-assisted treatment.[16, 37]


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Figure 5b shows typical initial charge and discharge voltage profiles of the MT-0 and MT-3 samples between 2.0 and 4.8 V. The charge profile in slope region before 4.5V and corresponding electrochemical reaction process are similar to Figure 5a. Near the voltage at 4.5 V, both samples exhibit a distinct long plateau region which is usually attributed to the electrochemical activation of Li2MnO3 component,[38, 39] also an electrolyte/electrode side reaction is expected in this region[18]. The charge capacity of MT-3 in plateau region is lower than pristine sample. This must be due to the fact that a portion of Li2MnO3 has been activated before. The first charge voltage profiles provide further evidence that microwave-assisted process more probably affect Li2MnO3 than LiMO2. A high initial discharge capacity of 268.6 mAh g−1 is obtained for MT-3, more than 256.7 mAh g−1 for MT-0, and the initial coulombic efficiency increases from 76.8 to 86.7%. The increase of discharge capacity and coulombic efficiency reveals that more oxygen reactions in the form of O2-/O- or O22- are triggered to compensate for ionic changes during the charge process instead of the O2-/O2 redox couples which induces O2 to be extracted from lattice.[18] Figure S4 shows that the discharge capacity of MT-3 reaches 280.9 mAh g−1 compared with that of 255.2 mAh g−1 for MT-0 depicted in Figure S3 three laps later. Figure 5c shows the first-cycle CV profiles of MT-0 and MT-3 samples which is obtained in the voltage range of 2.0~4.8 V at a scan rate 0.1 mV s−1. Peak 1 at ~4.2 V is assigned to extraction of Li+ ion from the Li layer accompanying with the reduction of Ni2+/Ni4+ and Co3+/Co4+ in LiMO2 structure.[40, 41] Peak 2 at the potential >4.5 V is ascribed to the electrochemical activation of Li2MnO3 component and the intensity of peak is related to the quantity of oxygen releasing during first charge.[42-44] The peak intensity of MT-3 is lower than MT-0 in step with the shorter voltage plateau near 4.5 V in the initial charge process mentioned above. In the discharge process, peak 3 is related to the process of O2-x reduction, peak 4 and peak 5 are related to Li+ ion insertion into the transition metal layer and Li layer respectively.[41] The peaks of 4 and 5 shift to higher potentials which suggests that Li+ ion possesses greater kinetics to insert into active material after microwave-assisted treatment.[19]


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Long-term cycling performance of the pristine and microwave-assisted treated samples are evaluated between 2.0 and 4.8 V at a current density of 250 mAh g−1 after activated at 0.1 C for three cycles. The capacity retention during the first 200 cycles is shown in Figure 5d. Compared with the pristine MT-0 (178.9 mAh g−1 for initial discharge capacity at 1C, 81.1% capacity retention after 200 cycles), MT-1 exhibits higher initial discharge capacity of 193.5 mAh g−1 and excellent cycle stability, with 94.1% capacity retention after 200 cycles. Interestingly, the discharge capacity of MT-3 sample not only delivers a high initial value of 202.8 mAh g−1 at 1 C, but also displays unnormal continuous increase in cycling process. The ever-increasing of discharge capacity sustains about 110 cycles and achieves 217.7 mAh g−1 finally. After about 190 cycles, the gap of capacity between MT-0 and MT-3 is 66.2 mAh g−1. The initial discharge capacity of MT-5 is slightly lower than MT-0, nevertheless the capacity increases rapidly in the subsequent cycling similar to MT-3 sample. These results suggest that the Li-rich materials after microwave-assisted treatment demonstrate excellent cycling stability. (b) Voltage versus Li+/Li0 (V)

Voltage versus Li+/Li0 (V)

(a) 4 MT-0 MT-3 3

4

50

100 150 200 -1250 Capacity (mAh g )

300

MT-0 MT-3

3

2

2 0

350

0

50

100 150 200 250 Capacity (mAh g-1)

300

350

250

(c)

2

200

1

100

MT-0 MT-3

0

-100 2.0

5 2.5

3 4

3.0 3.5 4.0 Potential (V vs Li/Li+)

4.5

Discharge Capacity (mAh g-1)

300

Current (mA g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(d) 200 150 MT-0 MT-1 MT-3 MT-5

100

1C

50 0 0

25

50

75

100

125

150

175

200

Cycle Number

Figure 5. The initial charge–discharge curves of the MT-0 and MT-3 samples between (a) 2.0 and 4.4 V. (b) The initial charge–discharge curves of the MT-0 and MT-3 samples between 2.0 and 4.8 V. (c) Cyclic voltammograms of MT-0 and MT-3 samples at a scan rate of 0.1 mV s-1. (d) Cycling performance of the MT-0, MT-1, MT-3 and MT-5 samples at 1 C. ACS Paragon Plus Environment

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Table S2 provides the electrochemical performance of samples pre-activated by different chemical methods reported in previous reports. Although these methods are efficient in improving the initial CE but the cycle performance is a harsh matter to be ameliorated. Microwave-assisted treatment not only possesses higher discharge capacity but also delivers unique excellent cycle performance far beyond other methods. To investigate which part of the discharge process should be responsible for the increased capacity, discharge capacities below and above 3.0 V are separately calculated and depicted in Figure S5. The curves of MT-3 sample demonstrate that the increase of capacity mainly comes from the lower-potential region (3.5V) after ten cycles and their maximal values are 9.88×10-14 cm2 s-1 and 4.56×10-14 cm2 s-1, respectively. The diffusivity value of MT-3 is relatively stable after 50 cycles while MT-0 sample exhibits slight decrease. On the contrary, in low voltage region (

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