Article Cite This: ACS Appl. Energy Mater. 2018, 1, 4158−4168
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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* 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
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ABSTRACT: Preactivation 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 preactivate 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 preactivation 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 preactivation, 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 the automotive industry.1−4 To fulfill this requirement, the development of 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 structuresLi2MnO3 (a monoclinic C2/m structure) and LiMO2 (a hexagonal R3̅m 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 that corresponds to the anomalous high capacities. This has been attributed to either the extraction of Li+ ions accompanied by the irreversible loss of oxygen from the lattice or the surface reaction through electrode/electrolyte reduction and/or Li+/H+ exchange.11−13 © 2018 American Chemical Society
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, preactivation is difficult to realize because of the thermodynamic equilibrium of te 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 (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 Received: May 24, 2018 Accepted: August 2, 2018 Published: August 2, 2018 4158
DOI: 10.1021/acsaem.8b00812 ACS Appl. Energy Mater. 2018, 1, 4158−4168
Article
ACS Applied Energy Materials into the Li-rich material. Although these chemical solution methods reported are effective in realizing preactivation, the crystal structures of the cathode materials involved are easily damaged, leading to deterioration of cycle stability. Recently, gas−solid interfacial reaction has been introduced as a gentle method for preactivation, which is advanced in preservation of the crystal structure.18,19 However, the gas treatment process is always complex and uncontrollable compared with the traditional solution methods. Furthermore, previous reportsthat further explore the long-term implication of preactivation in the electrode material during the charge−discharge process are rare. In view of these, we employ stable and neutral glycol as solvent to avoid destroying the structural stability and to innovatively introduce a facile microwave treatment strategy to assist in Li-rich material preactivation. Composition and phase structure analysis reveal that Li ions are extracted from the cathode material without causing structural destruction, which indicates the 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 an unusual continuous increase in discharge capacity in the course of the cycle process at a high rate. Electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) tests indicate that 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 because of the evolution of crystal structure in the cyclic process, which can aggrandize the Li-ion diffusivity and ameliorate the rate performance. This study reveals that preactivation by microwave-assisted treatment can optimize the evolution of material structure during te cyclic process to enhance the cycling stability and rate performance of Li-rich layered materials.
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Figure 1. Schematic illustration of the microwave-assisted preactivation. crystallographic phase of samples and the data was collected using an X’PERT PRO MPD with filtered Cu Kα radiation (λ= 1.54056 Å). 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 aluminum foil. The aluminum foil was then dried at 120 °C 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 glovebox 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 °C. During GITT testing, a constant current flux of 40 mA g−1 for a given time period (10 min) was supplied, followed by 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 1 × 105 Hz to 1 × 10−2 Hz. The Crates mentioned in this text were calculated considering 1 C as 250 mA g−1.
EXPERIMENTAL SECTION
Synthesis of Pristine Material. The material of Li[Li0.21Ni0.131Co0.122Mn0.538]O2 was synthesized by a typical coprecipitation method. Two 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 °C. 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 °C for 24 h. The obtained precursor particles were mixed with precalculated Li2CO3. A 5% excess of lithium was introduced intentionally to compensate the Li loss during the calcination. The mixture was first annealed at 450 °C for 3 h and then at 800 °C 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 of pristine sample was added into a beaker with 150 mL of glycol and stirred for 5 min in order to disperse evenly. The beaker was then put into a microwave oven and treated for several minutes. After that, the samples were filtered, washed, and dried at 80 °C for 12 h. The pristine sample and the treated samples by microwave for 1, 3, and 5 min were marked as MT-0, MT-1, MT-3, and MT-5, respectively. 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
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RESULTS AND DISCUSSION Characterizations of Electrode Material. The morphologies of Li-rich layered oxides before and after microwaveassisted 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. The metal atom ratios Li:Co:Mn:Ni acquiring from ICP-OES are listed in Table 1 to accurately describe each sample. The Co:Mn:Ni ratio is essentially unaltered after microwave-assisted treatment for different times. 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, Li+ ions are not extracted from the LiMO2 component because
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DOI: 10.1021/acsaem.8b00812 ACS Appl. Energy Mater. 2018, 1, 4158−4168
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ACS Applied Energy Materials
Figure 2. SEM images of (a) MT-0, (b) MT-1, (c) MT-3, and (d) MT-5.
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 is similar to 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 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-
Table 1. Relative Amounts of Li, Co, Mn, and Ni (normalized to the Mn content) (ICP data) MT-0 MT-1 MT-3 MT-5
Li
Co
Mn
Ni
1.210 1.186 1.172 1.147
0.122 0.123 0.121 0.122
0.538 0.538 0.538 0.538
0.131 0.130 0.131 0.130
such a reaction would be necessary to dissolve one of the metals as an oxide for charge compensation.16 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 (642.2−42.8 eV).20,21 The Co 2p and Ni 2p peaks shown in Figure 3b, 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
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. 4160
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Figure 4. (a−−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.
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 preactivation on the surface. 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: R3̅m), 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 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 the Li2MnO3 activation. From Figure 4c, d, the (104) peaks of all samples mainly stay in the same location, whereas 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 preactivation, 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 4161
DOI: 10.1021/acsaem.8b00812 ACS Appl. Energy Mater. 2018, 1, 4158−4168
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Figure 5. Initial charge−discharge curves of the MT-0 and MT-3 samples between (a) 2.0 and 4.4 V and (b) 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.
portion of Li2MnO3 has been activated before. The first charge voltage profiles provide further evidence that microwaveassisted 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 that of 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 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
main peaks coincide well with the characteristic of Li2MnO3 activation. Glycol is a relatively stable solution, no matter at room temperature or high temperature (even at boiling point), which is different from the solvents reported for preactivation such as HNO3, Na2S2O8, and (NH4)2SO4 which have harsh chemical properties. But from the characterizations of the electrode material mentioned above, the Li-rich materials realized preactivation successfully. Thus, we believe the violent chemical reaction between active material and solvents is not indispensable to realizing 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, and thus it most probably gains from the MnO2 component, which is generated from preactivation of Li2MnO3 during microwaveassisted treatment.16,37 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 4.5 V 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 the plateau region is lower than that of the pristine sample. This must be due to the fact that a 4162
DOI: 10.1021/acsaem.8b00812 ACS Appl. Energy Mater. 2018, 1, 4158−4168
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Figure 6. (a, b) Cycling performance of the MT-3 sample at 0. 1 and 3 C, respectively. (c) Cycle profiles of MT-3 sample between 0.1 and 1 C. (d) Rate performance of MT-0 and MT-3 samples before and after cycling.
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. Table S2 provides the electrochemical performance of samples preactivated 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, we separately calculated discharge capacities below and above 3.0 V and depicted them in Figure S5. The curves of MT-3 sample demonstrate that the increase of capacity mainly comes from the lower-potential region (
