3O2

Article Cite This: Chem. Mater. 2018, 30, 4909−4918

pubs.acs.org/cm

Probing Thermal and Chemical Stability of NaxNi1/3Fe1/3Mn1/3O2 Cathode Material toward Safe Sodium-Ion Batteries Yingying Xie,†,‡,⊥ Gui-Liang Xu,†,⊥ Haiying Che,‡ Hong Wang,‡ Ke Yang,‡ Xinrong Yang,‡ Fangmin Guo,§ Yang Ren,§ Zonghai Chen,† Khalil Amine,*,†,∥ and Zi-Feng Ma*,‡ †

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Shanghai Electrochemical Energy Devices Research Centre, Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China § X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ∥ Materials Science and Engineering, Stanford University, Stanford, California 94305, United States

Chem. Mater. 2018.30:4909-4918. Downloaded from pubs.acs.org by UNIV OF TEXAS SW MEDICAL CTR on 10/09/18. For personal use only.

‡

ABSTRACT: Because of the low cost and high abundance of sodium, room-temperature sodium-ion batteries have recently been considered as an alternative power source to lithium-ion batteries. In contrast to the electrochemical performance of the batteries, safety has been paid much less attention, but safety is a critical consideration because sodium-ion batteries are intended for large-scale electrochemical energy storage applications. Herein, we have reported a NaNi1/3Fe1/3Mn1/3O2/hard carbon full cell with a good cycling performance and high Coulombic efficiency. The energy density of this pouch cell is close to 95 Wh/kg, and the capacity retention of the NFM full cell attained at 92.6% after 100 cycle numbers. Moreover, we have further used accelerating rate calorimetry, scanning electron microscopy, and operando synchrotron high-energy X-ray diffraction to investigate the thermal/chemical stability of charged NaxNi1/3Fe1/3Mn1/3O2 cathode material at both cell and component level. It is found that the thermal decomposition of desodiated NaxNi1/3Fe1/3Mn1/3O2 is a redox reaction that can be facilitated with the presence of either a reductive environment, such as electrolytes, or a strong oxidative environment that can result from a higher degree of desodiation. The findings presented in this work can guide future development of advanced sodium-ion batteries for practical application.

L

and its analogues.19−21 As a counterpart in lithium-ion battery, layered lithium transition metal oxides (R3̅m in space group) are the dominant cathode materials for lithium-ion batteries, and it has been a straightforward and highly repeatable practice to synthesize high-quality layered materials,22−25 even though the formation mechanism of these materials is still under debate.26,27 Similarly, layered sodium transition metal oxides are widely pursued as promising cathode materials for sodiumion batteries, which were triggered by the belief that sodiumion batteries will operate by a similar electrochemical principle to that in lithium-ion batteries, and that transformative research from lithium-ion to sodium-ion technology is possible

ithium-ion batteries have long been pursued as the most promising high energy electrochemical energy storage technology for portable electronics, electric vehicles, and stationary storage for smart grid applications. It was quickly realized that the full deployment of lithium-ion batteries for these existing and emerging applications would place a substantial amount of pressure on the availability of natural resource like lithium. Therefore, sodium-ion technology has been recently investigated as a supplement to lithium-ion technology,1−5 primarily because of the high abundance and geologically uniform distribution of sodium natural resources. The energy density of sodium-ion battery is largely determined by the cathode materials. Therefore, in the past few years, much effort have been devoted to develop advanced cathode materials, including layered sodium transition metal oxides,6−14,37 polyanion compounds,15−18 and Prussian blue © 2018 American Chemical Society

Received: January 4, 2018 Revised: July 5, 2018 Published: July 7, 2018 4909

DOI: 10.1021/acs.chemmater.8b00047 Chem. Mater. 2018, 30, 4909−4918

Article

Chemistry of Materials

Figure 1. (a) HEXRD and refinement of the synthesized NaxNi1/3Fe1/3Mn1/3O2 powder. (b) SEM image of the synthesized NaxNi1/3Fe1/3Mn1/3O2 powder. (c) Voltage profile of prepared cells cycled to different upper cutoff potentials, and (d) dQ/dV plot of data shown in panel c.

due to the structural complexity with regard to the phase evolution of sodium transition metal oxides during the insertion/extraction of sodium ions in the host materials,32 leading to a substantial change in the energy landscape when lithium ions are replaced with sodium ions. Similar to the lithium ion batteries, electrolyte design and functional additives development is also very effective at promoting the capacities and cycle performance of sodium ion batteries.33,34 These changes can thermodynamically lead to different degradation mechanisms of the cathode materials, and also a compromise of their safety characteristics during the deployment of sodium-ion batteries for large-scale energy storage. Herein, a hierarchical-structured NFM cathode material was synthesized by solid-state reaction between Ni1/3Fe1/3Mn1/3(OH)2 and Na2CO3. It was extremely inspiring that the NFM/hard carbon sodium-ion battery could demonstrate a stable cycle life, and the thermal runaway behavior of the full cell was first measured by accelerating rate calorimetry (ARC), showing much better cycle stability than most of the reported sodium-ion battery. Furthermore, the thermal and chemical stability of the desodiated NFM cathode materials with or without electrolytes were thoroughly investigated in cell and component level by scanning electron microscopy (SEM) and in operando synchrotron high-energy X-ray diffraction (HEXRD). We expect the finding in this work can be served as a good guide to develop high performance sodium-ion battery for practical application.

with few knowledge barriers. However, reports in the open literature have clearly shown the difficulty in synthesizing the desired materials. By variation of the chemical stoichiometry of the starting materials and the synthesis condition, the final sodium transition metal oxides can exist in two classes (P and O types) of stable layered structures at ambient conditions.6,7 The P type layered oxide has sodium ions occupying the prism sites sandwiched between two TMO6 (TM = Ni, Fe, Co, Mn, Cu, ...) layers, while the O type layered material has sodium ions occupying the octahedral sites between TMO6 layers. Despite P-type materials present high reversible capacity and good cycle stability as well as the wide-diffusion channels for Na+, the lack of a sodium reservoir in P-type cathodes creates a technological barrier to use an anode material without sodium, such as hard carbon. Therefore, owing to its higher Na content, O-type cathode materials have been considered as more promising for practical application. Kim et al.28 have for the first time reported a new layer O3NaNi1/3Fe1/3Mn1/3O2 (NFM) compound, which demonstrated similar charge/discharge voltage profiles as LiNi1/3Co1/3Mn1/3O2 cathode. When coupled with presodiated hard carbon anode, the NayC/Na1−y(Ni1/3Fe1/3Mn1/3)O2 cell had an average voltage of ∼2.75 V, and modest capacity of 100 mAh/g for 150 cycles (1.5−4.0 V). Since that, numerous attention have been paid to Nax[NixFeyMn1−x−y]O2 cathode materials.29−31 However, to the best of our knowledge, unlike LiNi1/3Co1/3Mn1/3O2 cathode for lithium-ion batteries, either half-cells or full-cells with ultralong cycle life and excellent capacity retention using Nax[NixFexCozMn1−x−y−z]O2 as cathode materials has not been reported yet. This could be 4910

DOI: 10.1021/acs.chemmater.8b00047 Chem. Mater. 2018, 30, 4909−4918

Article

Chemistry of Materials

Figure 2. (a) Cycling stability of the NFM full cell at 1 C rate and (b) rate performance of the NFM full cell.

■

RESULTS AND DISCUSSIONS It is widely reported that layered transition metal oxide cathodes can be synthesized through solid-state reaction between transition metal precursors (e.g., Ni1/3Fe1/3Mn1/3(OH)2 and Mn2O3) and sodium salts (e.g., Na2CO3 and NaOH). In this work, a Ni1/3Fe1/3Mn1/3(OH)2 precursor was first synthesized by coprecipitation method. Later, with 10 wt % excess of Na2CO3 to compensate the sodium loss during high temperature calcination, the mixture of Ni1/3Fe1/3Mn1/3(OH)2 and Na2CO3 was then heated to 850 °C for 20 h at air atmosphere with naturally cooling. Figure 1a shows the Rietveld refinement of the obtained product. It can be clearly seen that its XRD pattern can be well fitted by using α-NaFeO2. An excellent fitting result was obtained with a = 2.9684 Å and c = 16.128 Å. The morphology characterization shows that the synthesized NFM powder has a secondary particle size of about 5−7 μm, and they are composed of pallet-like primarily particles with sub micrometer in width and about 100 nm in thickness (Figure 1b). The electrochemical tests have been carried out, and the cycle performance and rate capability of NFM in 2032 type coin cell were reported in our previous work.12 Figure 1c shows the voltage profiles of two Na/NFM cells that were constant-current cycled at C/10 (∼13 mA/g) for the first cycle; the lower cutoff potential for both cells was 2.0 V vs Na+/Na, and the upper cutoff voltages were 4.0 and 4.3 V vs Na+/Na, respectively. It shows that the NFM material delivered a reversible specific charge/discharge capacity of 126.4/123.9 mAh/g when the upper cutoff potential was limited to 4.0 V. Presuming that the synthesized material has the full stoichiometry of NaNi1/3Fe1/3Mn1/3O2, the reversible capacity obtained can be translated into a stoichiometry of desodiated material at 4.0 V as Na0.5Ni1/3Fe1/3Mn1/3O2 (NFM-05). When charged to 4.3 V vs Na+/Na, it delivered a reversible charge/discharge capacity of 165.3/142.4 mAh/g, translating into a stoichiometry of Na0.3Ni1/3Fe1/3Mn1/3O2 (NFM-03) at 4.3 V. It can be seen, the first delithiation voltage curves are identical between open circuit voltage (OCV) and 4.0 V regardless of the upper cutoff voltage. The dQ/dV profiles in Figure 1d illustrated that the charge curves are nearly identical during the first cycle for all voltage ranges below 3.7 V, it is similar to the results report in the Na2/3Ni1/3Fe1/3Mn1/3O2 cathode material.37 Charging the material to 4.3 V vs Na+/Na resulted in an additional anodic peak at 4.1 V, and a cathodic peak at 3.8 V. This finding suggests an irreversible phase transformation reaction during the initial charge to a potential above 4.0 V vs Na+/Na,

resulting in a different charge/discharge loop mechanism. In our previous study,32 we have found that the NFM undergo a reversible O3−P3−P3−O3 phase transformation when cycled between 2.0 and 4.0 V; while once the up cutoff voltage is set to 4.3 V, a more complicated phase transition (O3−P3−O3′− P3′−O3) was observed. This will somehow affect their sodiation/desodiation kinetics and thus compromise their cycle stability. We have further prepared a 1000-mAh pouch cell using the as-synthesized NFM as cathode and hard carbon as anode. The above pouch cell was cycled between 2.0 and 3.8 V to enable reversible phase transition process during cycling and thus extend the cycle life. The energy density of this pouch cell is close to 95 Wh/kg. Figure 2a shows the cycle performance and was measured at 1 C rate, and the first cycle voltage curve of the full cell is illustrated in inset within Figure 2a. It can be seen from Figure 2a, NFM full cell reveals a good cycling performance and high Coulombic efficiency. The capacity retention of the NFM full cell attained at 92.6% after 100 cycle numbers. The rate capability of the full cell is also evaluated at different current densities by varying discharge rates ranging from 0.2C to 3C (Figure 2b). Charge and discharge at high rate current cause defects of SEI film on the anode material surface, which will lead to capacity loss. This is due to the lack of work on the film formation additive on anode materials surface, which currently being developed in full cell, and the next work will be optimized. The anode film formation additive makes the surface have a more stable and dense SEI film, we will further report the research work on the new electrolyte additive. Another important issue for sodium-ion battery is its safety concern. As sodium ion batteries are being dedicated for largescale applications, the electrode materials must be thermally stable because both the possibility of failure in heat management and the impact of unwanted heat can be significantly increased in large units of batteries. Several different thermal analysis techniques, such as differential scanning calorimetry (DSC),38−41 accelerating rate calorimetry (ARC),42−44 and isothermal microcalorimetry (IMC)45,46 are routinely used to investigate the onset temperature and the kinetics of exothermic reactions during thermal runaway of rechargeable batteries. Real-time electron microscopy was also used to probe the local changes in crystallographic and electronic structures as well as morphologies of charged cathode material with increase in temperature.47 However, these thermal analysis approaches and electron microscopies 4911

DOI: 10.1021/acs.chemmater.8b00047 Chem. Mater. 2018, 30, 4909−4918

Article

Chemistry of Materials

Figure 3. (a) Temperature vs time plot of the charged NFM/hard carbon full cell under thermal stability testing with ARC and (b) heating rate vs temperature plot of the charged NFM full cell during thermal runaway testing.

In order to further understand the morphological changes of the charged NFM cathode with increase in temperature from the material level, we carried out scanning electron microscopy characterization. Figure 4a shows the SEM image of the

have limited capability in revealing the reaction pathway of the decomposition reactions of the electrode materials. Considering the need to get fundamental insights of the reactions, Chen et al. first implemented the in situ HEXRD technique to investigate the decomposition reactions of the lithium-ion cathode in the presence of nonaqueous electrolytes, and were thus able to gain a substantial amount of valuable mechanistic information that is not available from the thermal analysis approaches.33,34,39,40,47,48 Herein, ARC techniques, scanning electron microscopy (SEM) and operando synchrotron HEXRD were used in this work for probing the thermal and chemical stability of the charged layered NaNi1/3Fe1/3Mn1/3O2 (NFM) cathode in realistic conditions to gain fundamental insights of practically working materials in both cell and component level. The thermal runaway behavior of the 1000-mAh sodium-ion pouch cell was determined by using Thermal Hazard Technology (ARC EV+). Figure 3 shows the temperature and heating rate during the whole thermal runaway procedure. To better understand the thermal runaway phenomenon, three characteristic temperatures: T1, T2, and T3 are marked in Figure 3a and 3b. T1 is located at 166.3 °C, and it is the onset temperature of the detected battery self-heating process. This part of the exothermic reaction results from the decomposition of the solid electrolyte interface (SEI). In fact, after each heatwait-seek process, the ARC system will check if the temperature increase rate of the battery reaches the preset target, which is 0.02 K/min. If it does not, there will be another heating stage with precisely controlled increase of 5 K/min. The exothermal (acceleration) stage of the battery begins after T1. During this stage, various parasitic reactions (internal short circuit and the decomposition of the individual components) occurred, forming a chain reaction. Therefore, the exothermic reactions can drive the battery temperature to rise continuously until the catastrophic thermal runaway happens. The onset temperature of the thermal runaway is marked as T2. At this point, the battery temperature increases exponentially. In detail, T2 is defined as the temperature increase rate reaches 1 °C/min, which is 243 °C in this case. After T2, the battery temperature increased sharply and reached the maximum temperature T3 at 312.24 °C in just a few seconds. This temperature is much lower than that of layer cathode materials for lithium-ion battery that can go up to even 800 °C,49,50 indicating that the NFM cathode material presents much better thermal stability and could be a reliable cathode materials for sodium-ion battery.

Figure 4. SEM images of (a) recovered NFM-05, (b) NFM-05 after being heated at dry state, (c) NFM-05 after being heated in the presence of the electrolyte, (d) recovered NFM-03, (e) NFM-03 after being heated at dry state, and (f) NFM-03 after being heated in the presence of electrolyte. The electrolyte was 1.0 M NaPF6 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) solvent (1:1 by volume), and the sample was heated to 400 °C at a rate of 10 °C/min.

recovered electrode NFM-05 after being charged to 4.0 V vs Na+/Na; it can be seen that the morphology of the secondary particles was well maintained. When the dry electrode that was charged to 4.0 V vs Na+/Na was thermally treated at 400 °C in a closed crucible filled with Ar at a rate of 10 °C/min (mimicking the DSC ramping profile), the agglomeration of pallet-like primary particles was also maintained, but a slight increase on the porosity of the secondary particles was observed (Figure 4b). In addition, some fiber-like structures were also seen after the heating of the dry electrode; it is speculated that these structures might result from the melting or reaction of polymeric binder at elevated temperatures. A dramatic morphological change occurred when the electrode was thermally treated in the presence of the nonaqueous electrolyte (Figure 4c). The secondary particle was completely broken down into pallet-like particles; and a growth in the size of the pallet can also be visually observed. Figure 4d shows the SEM image of the recovered electrode NFM-03 after being charged to 4.3 V vs Na+/Na; no obvious morphological change on the secondary particle was observed (Figure 4d), and the same morphology was also maintained after thermal treatment in a dry state (Figure 4e). When NFM-03 was thermally 4912

DOI: 10.1021/acs.chemmater.8b00047 Chem. Mater. 2018, 30, 4909−4918

Article

Chemistry of Materials

Figure 5. (a) Evolution of HEXRD patterns during of heating of dry NFM-05 and (b) comparison of XRD patterns for NFM-05 powder before and after the heating process. (c−e) Areal contour plot of Operando synchrotron HEXRD patterns showing the structural evolution of the powder material.

Figure 6. (a) Evolution of HEXRD patterns during heating of NFM-05 in the presence of a nonaqueous electrolyte and (b) comparison of XRD patterns for NFM-05 powder before and after the heating process. (c−e) Areal contour plot of in situ HEXRD patterns showing the structural evolution of the powder material.

4913

DOI: 10.1021/acs.chemmater.8b00047 Chem. Mater. 2018, 30, 4909−4918

Article

Chemistry of Materials

Figure 7. (a) Evolution of HEXRD patterns during of heating of dry NFM-03 and (b) comparison of XRD patterns for NFM-03 powder before and after the heating process. (c−e) Areal contour plot of in situ HEXRD patterns showing the structural evolution of the powder material.

was heated from 243 to 288 °C, the P(003) and P(006) peak shifted to higher 2θ values, and the intensity decreased with temperature, while the P(101), P(012), and P(110) peaks shifted toward lower 2θ values. These results suggest that the material remains a layered structure, but the d-spacing on the a/b axis increased while the d-spacing on the c axis decreased. When the dry NFM-05 sample was heated beyond 288 °C, a new reflection peak emerged at the right side of the P(003) peak at around 1.23° (2θ). The same trend was also observed at the upper end of the P(006) peak. These two new peaks can be indexed to a monoclinic NaxTMO2 (PDF 72-0830, TM = transition metal), as shown in Figure 5b. This result is different from what’s reported for lithium-based cathodes; it is believed that the layered delithiated transition metal oxide is generally transformed into a Li1−xM2O4 type spinel when heated without the presence of the electrolyte.43,44 The root of this contrasting behavior could lie in the difference in the ionic radii of Li and Na: the ionic radius of Na (1.02 Å) is much larger than that of Li (0.86 Å). The Na ions would need to squeeze their way through the oxygen close packed sublattice from Na layers to Mn layers to form a spinel-like structure. This requirement makes the transformation of the desodiated cathode into a spinel structure energetically not preferred. To characterize the thermal stability of NFM-05 in the presence of a reductive medium, 1.0 M NaPF6 with EC/DMC (1:1 by volume) was added to NFM-05 before carrying out the in situ experiment. Figure 6 shows the evolution of the HEXRD patterns during the heating process of the charged NFM-05 in the presence of the electrolyte. Compared to the results shown in Figure 5, new peaks were observed at the beginning of the in situ experiment (room temperature); those peaks appeared at 2θ = 1.66°, 1.94°, 2.04°, 2.2°, and 2.45° and are marked by “#” in

treated in the presence of the nonaqueous electrolyte, the recovered particles totally lost their original morphology; dense, smooth, and spherical particles were formed after the decomposition reaction. This indicates that higher cutoff voltage result in degrading thermal stability of NaxNi1/3Fe1/3Mn1/3O2. The morphology results imply that the desodiated NFM can be thermally unstable, and that a bigger issue can be the coupling between the thermal and chemical instability in the presence of the reductive medium, the nonaqueous electrolyte. However, the morphology results did not provide more insight on the chemical reactions between the desodiated NFM and the electrolyte, and hence operando synchrotron HEXRD was utilized to gain more insights. The evolution of HEXRD patterns during the in situ heating of the dry NFM-05 sample from room temperature to 400 °C with a constant heating rate of 10 °C/min is illustrated in Figure 5a. In particular, Figure 5b shows a comparison between the XRD patterns for NFM-05 before and after the heating process; both patterns were collected at room temperature. The detailed changes in the diffraction peaks can be seen in the zoom-in views shown as Figure 5c-5e. The XRD pattern for NFM-05 before heating can be indexed as a P type hexagonal layered structure (space group R3̅m), as shown in our previous work.32 After the heating process, the P type layered structure was converted into a monoclinic structure (see Figure 5b). Here, P phase will be subscripted with “P” for easier presentation of the HEXRD results; “mon” will stand for monoclinic structure. When heated from 25 to 243 °C, the P(003), P(101), P(012), P(110), and P(113) peaks for the layer material shifted to smaller 2θ values, caused by the thermal expansion of the layered structure. When the sample 4914

DOI: 10.1021/acs.chemmater.8b00047 Chem. Mater. 2018, 30, 4909−4918

Article

Chemistry of Materials

stable O type structure with a higher valence state on transition metal ions. It is also speculated that the composite structure between P and O type domains hinders the transformation of the O type domain into more reactive intermediates such as NiO2-like structures. Gradual structural evolution also occurred in the temperature range from 337 to 400 °C. Over this temperature range, the P type phase disappeared, and new peaks appeared at 2θ = 2.23°, 2.66°, 2.78°, and 4.5°. These new peaks can be well indexed to cubic Fd3m TM3O4 (PDF 13-0162), as shown in Figure 7b. Worth noting is that the evolution of TM3O4 structure did not occur for the dry NFM-05 sample but was observed during the later stage of heating of NFM-05 in the presence of electrolyte. We summarized the above observation in Scheme 1, which illustrates how electrolytes and state of charge (SOC) affect

Figure 6a and 6b. These peaks disappeared gradually at 138 °C. That effect can be attributed to a slow reaction between the NFM-05 and the electrolyte at room temperature. The desodiated process take defects in layered host structure because of the Na ion decreasing. In the present of the electrolyte, the sodium salt in electrolyte supplies more Na ions then they will get into the defect position, lead to a slow reaction between the desodiated NFM and the electrolyte. To the best of our effort, we were not able to identify the structure of the extra peaks and thus leave these peaks nonindexed in this work. Coinciding with the disappearance of the unknown phase at about 138 °C, the peaks corresponding to the P type layered structure became sharper, and the intensity of these peaks also increased (see Figure 6c for the change of the P(003) peak). This implies that the weak unknown peaks were associated with the distribution of defects in the P type structure that reduce the size of continuous domains. The disappearance of the defects can lead to the artificial growth of the domains, which sharpens the peaks. Two new peaks emerged when the temperature was above 138 °C, and they disappeared at a temperature above 293 °C. These new peaks can be seen at θ = 1.45° and 1.66°, both of which can be indexed as (020) and (110) peaks of the MnO2type structure (PDF 73-1539, space group Pnmb (62)). Further increase of the temperature led to the complete conversion of P type and MnO2-like structures into an Mn3O4type structure (PDF 75-0765, space group Pbcm (57)), which was evidenced by the appearance of new peaks at 1.66°, 2.65°, 4.65°, and 6.1°, as marked by asterisks in Figure 6a and 6b. The formation of Mn3O4-type structure is needed for the migration of transition metal cations into the sodium layers, as well as the tetrahedral sites. In addition, this transformation led to the reduction of the valence state of the transition metals, which chemically needs the participation of a reductive medium, such as the nonaqueous electrolyte, to facilitate the reaction. The reaction between the electrolyte and desodiated NFM caused significant migration of transition metal ions in the oxygen framework, resulting in a severe segregation of well separated domains Mn3O4 type structures. Given that the thermal decomposition of desodiated cathode is facilitated by the presence of the reductive electrolyte, this decomposition reaction can also be accelerated by raising the average valence state of the transition metal when charged to a higher potential for a reduced content of Na in the NFM. Hence, the evolution of HEXRD patterns during the heating of a dry NFM-03 sample is also investigated and illustrated in Figure 7. When the NFM sample was charged to 4.3 V versus Na+/Na, the majority of P type structure was converted to a monoclinic layered O′ phase,14 as shown in Figure 7b. Figure 7a and 7c shows that the minor P type structure was maintained up to 337 °C, agreeing in large measure with the observation of the dry NFM-05 sample shown in Figure 5. However, the major O type structure showed phase transformation at a much lower temperature, ∼159 °C (see Figure 7a and 7c for details). The XRD pattern of the emerging phase at 159 °C was very close to that of NiO2 (PDF 85-1977, space group R3̅m). It is believed that the highly desodiated NFM material resulted in a low occupancy of prism sites between the TMO2 layers, reducing the structure stability of P type oxide. Upon the removal of sodium at potentials above 4.0 V, the migration of sodium ions within the prism sites can occur to separate the material into a Na-rich domain, which stays a stable P type structure, and a Na-poor domain, which is a

Scheme 1. Schematic Representation Summarizing Thermal Decomposition Process of Charged NFM with Different Condition Depending on the Temperaturesa

a

Circle and triangle symbols represent charged to 4.0 and 4.3 V, respectively. Solid and hollow symbols represent with and without electrolytes, respectively.

the early decomposition of desodiated NFM cathode. The decomposition of desodiated NFM goes through the disordered Mn3O4-type spinel phase, where the valence state of the TM cations is reduced. Without the presence of other media, the reduction can only be achieved through oxygen evolution from the structural lattice, which can extend the thermal stability window. The presence of electrolytes provides an electron-rich environment. Then, the valence state of TM cations is reduced through the electron transfer to the charged NFM. The process is evidenced by the reduced onset temperature for the formation of disordered M3O4-type spinel phase. With higher cutoff voltage that can raise the average valence state of the transition metal, the formation of Mn3O4 also occurred below 400 °C. This supports our speculation that the conversion from desodiated NFM to TM3O4 is a redox reaction that can be promoted by an increase of the reductive environment or the oxidation state of the transition metals. Therefore, further modification such as surface coating, advanced electrolytes or controlling the crystal structure orientation are believed to be able to further increase the thermal stability of NFM cathode material. This could 4915

DOI: 10.1021/acs.chemmater.8b00047 Chem. Mater. 2018, 30, 4909−4918

Article

Chemistry of Materials

scanning electron microscopy (SEM) using a Hitachi S4700 microscope at the Center of Nanoscale Materials (CNM) of Argonne National Laboratory. Chromatic aberration imaging correction was performed at 80 kV. Full-Cell Assembly. 1000-mAh full cells were assembled with the NFM electrodes and hard carbon (Sumitomo Bakelite Co., Ltd.) anodes. The NFM electrodes were prepared from mixed slurry of 90 wt % active material, 5 wt % carbon black (Super P), and 5 wt % of PVDF (Kureha) as a binder in an N-methyl-2-pyrrolidone (NMP) solvent, and all of the cathode and anode slurry were subsequently coated onto an Al foil using a coating machine. The electrode sheets were punched drying in the vacuum oven at 100 °C overnight. The areal densities of cathode and anode were controlled at about 30 and 14 mg cm−2, respectively. Celgard2700 was used as separator. The electrolyte was 1.0 M sodium hexafluorophosphate (NaPF6) with propylene carbonate (PC) and ethyl methyl carbonate (EMC) (1:1 v/v) with 2 wt % fluoroethylene carbonate (FEC). The electrochemical performance of the full cells was tested in a battery test system (LAND CT2001A Model, Wuhan Jinnuo) between 2.0 and 3.8 V at 1 C rate. Accelerating Rate Calorimetry (ARC) Test for Full-Cell. The thermal behavior of the 1000-mAh sodium-ion pouch cells was determined by using Thermal Hazard Technology (ARC EV+). The ARC temperature was ramped at a rate of 5 K min−1 with the protocol of heat-wait-seek mode from 50 °C to approximately 300 °C. Full cells were brought to the desired temperature and then checked for self-heating during a 20 min equilibration. If the self-heating rate (dT/ dt) exceeded 0.02 K min−1, the ARC discontinued the heating process and followed the exotherm. If the self-heating rate was lower than 0.02 K min−1, the temperature continued to incrementally increase until the upper limit was reached. Ex Situ High-Energy X-ray Diffraction (HEXRD). Ex situ HEXRD measurement was carried out at Sector 11-ID-C of the Advanced Photon Source (APS) at Argonne National Laboratory. The wavelength of the X-ray beam was 0.1173 Å. Operando Synchrotron HEXRD. Operando synchrotron HEXRD was performed at beamline 11-ID-C of the APS. The experimental setup was similar to that previously reported.35,36 The samples were housed in a standard stainless-steel high-pressure crucible with an internal volume of 45 μL. A gold-coated copper gasket was used for airtight sealing. The copper gasket bursts through the 1 mm hole in the center of the cap if the internal pressure exceeds 150 bar. A flower-shaped piece of aluminum was specially designed to keep the solid and liquid samples in the detecting zone. Part of the edge was removed to leave more room to host extra gas released from the reaction between the electrode material and the electrolyte. A high-energy X-ray beam (115 keV, λ = 0.117418 Å) was used because it can penetrate the stainless-steel crucible. The sample was heated to 400 °C at a constant heating rate of 10 °C/min. The temperature was measured and recorded before and after each XRD exposure. During the course of heating, the transmitted XRD patterns were collected by a PerkinElmer detector at a rate of one pattern per 10 s. The collected two-dimensional pattern was then integrated into conventional onedimensional data (intensity vs 2θ) using the fit2d program.

guarantee the safe operation of sodium-ion battery for largescale electrochemical energy-storage application.

■

CONCLUSION In summary, we have reported a NaNi1/3Fe1/3Mn1/3O2/hard carbon sodium-ion pouch cell with a good cycling performance and high Coulombic efficiency. The energy density of this pouch cell is close to 95 Wh/kg. The capacity retention of the NFM full cell attained at 92.6% after 100 cycle numbers. ARC measurement of the charged 1000 mAh NFM full cell revealed that the onset temperature of 166.3 °C in the detected battery self-heating process, it is higher than lithium ion batteries, it indicated the NFM cathode material is reliable for sodium-ion battery commercialization. Furthermore, the chemical and thermal stability of desodiated NFM was thoroughly investigated by operando synchrotron high-energy X-ray diffraction. It was found that the O3 type NFM was converted into a P type layered structure when slightly desodiated to 4.0 V; further desodiation up to 4.3 V leads to the reorganization of Na+ and vacancies in prism sites, leading to a composite between P type and O′ type structures. Compared to Li+, the larger ionic radii of Na+ make the transformation of the desodiated cathode into a spinel structure energetically not preferred, in which Na+ need to squeeze their way through the oxygen close packed sublattice from Na layers to Mn layers to form a spinel-like structure. It is also found that the thermal decomposition of desodiated NFM is a redox reaction that can be facilitated with the presence of either a reductive environment or a strong oxidative environment, resulting from a higher degree of desodiation. We believe this study could serve as a good guide for the development of highperformance sodium-ion battery with long cycle life and high safety.

■

EXPERIMENTAL SECTION

Synthesis of O3 Type NaNi1/3Fe1/3Mn1/3O2 (NFM). NaNi1/3Fe1/3Mn1/3O2 was prepared by a solid-state reaction using transition metal hydroxide Ni1/3Fe1/3Mn1/3(OH)2 and Na2CO3 as precursors. The Ni1/3Fe1/3Mn1/3(OH)2 was prepared by a coprecipitation method previously reported12 with modification. Then, the Ni1/3Fe1/3Mn1/3(OH)2 powder and Na2CO3 (2:1.1 atomic mole ratios) were thoroughly mixed by using a rotatory mixer for 24 h. The mixtures were then calcined at 850 °C for 20 h in air with a heating rate of 3 °C/min and then cooled down to room temperature naturally to obtain the final products. The final products were stored in a glovebox to minimize the impact of moisture. Preparation of NFM Electrode. The NFM electrodes were prepared from mixed slurry of 80 wt % active material, 10 wt % carbon black (Chevron), and 10 wt % of PVDF (Kureha) as a binder in an N-methyl-2-pyrrolidone (NMP) solvent. This slurry was subsequently coated onto an Al foil using a doctor blade. After they were dried in the vacuum oven at 100 °C overnight, the electrode disks (ϕ14 mm) were punched and weighed. The mass loading of active materials on cathodes was controlled at about 3 mg cm−2. Electrochemical Characterization of NFM Electrode. 2032type coin cells were assembled in the glovebox with NFM electrodes, metallic Na foil counter electrodes, and glass fiber separators. The electrolyte of 1.0 M NaPF6 dissolved in propylene carbonate (PC) and ethyl methyl carbonate (EMC) (1:1 v/v) with 2 wt % fluoroethylene carbonate (FEC) was sufficient used. The cells were rested for 10 h before cycling for the electrodes properly wetted, and then cycled with a MACCOR cycler operated between 2.0 V and different upper cutoff voltage of 4.0 and 4.3 V at a constant current density of 13 mA g−1 (C/10 rate). Morphology Characterization. The morphologies of the prepared NFM powder and NFM electrodes were characterized by

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zonghai Chen: 0000-0001-5371-9463 Khalil Amine: 0000-0001-9206-3719 Author Contributions ⊥

Y.Y.X. and G.L.X. contributed to this work equally.

Notes

The authors declare no competing financial interest. 4916

DOI: 10.1021/acs.chemmater.8b00047 Chem. Mater. 2018, 30, 4909−4918

Article

Chemistry of Materials

■

(15) Kundu, D.; Talaie, E.; Duffort, V.; Nazar, L. F. The emerging chemistry of sodium ion batteries for electrochemical energy storage. Angew. Chem., Int. Ed. 2015, 54, 3431−3448. (16) Ellis, B.; Makahnouk, W.; Makimura, Y.; Toghill, K.; Nazar, L. A multifunctional 3.5V iron-based phosphate cathode for rechargeable batteries. Nat. Mater. 2007, 6, 749. (17) Lee, K. T.; Ramesh, T.; Nan, F.; Botton, G.; Nazar, L. F. Topochemical synthesis of sodium metal phosphate olivines for sodium-ion batteries. Chem. Mater. 2011, 23, 3593−3600. (18) Barpanda, P.; Liu, G.; Ling, C. D.; Tamaru, M.; Avdeev, M.; Chung, S.-C.; Yamada, Y.; Yamada, A. Na2FeP2O7: A safe cathode for rechargeable sodium-ion batteries. Chem. Mater. 2013, 25, 3480− 3487. (19) Wessells, C. D.; Huggins, R. A.; Cui, Y. Copper hexacyanoferrate battery electrodes with long cycle life and high power. Nat. Commun. 2011, 2, 550. (20) Lu, Y.; Wang, L.; Cheng, J.; Goodenough, J. B. Prussian blue: a new framework of electrode materials for sodium batteries. Chem. Commun. 2012, 48, 6544−6546. (21) Yang, D.; Xu, J.; Liao, X.-Z.; Wang, H.; He, Y.-S.; Ma, Z.-F. Prussian blue without coordinated water as a superior cathode for sodium-ion batteries. Chem. Commun. 2015, 51, 8181−8184. (22) Lu, Z.; MacNeil, D.; Dahn, J. Layered Li[NixCo1−2xMnx]O2 cathode materials for lithium-ion batteries. Electrochem. Solid-State Lett. 2001, 4, A200−A203. (23) Yabuuchi, N.; Koyama, Y.; Nakayama, N.; Ohzuku, T. Solidstate chemistry and electrochemistry of LiCo1/3Ni1/3Mn1/3O2 for advanced lithium-ion batteries II. Preparation and characterization. J. Electrochem. Soc. 2005, 152, A1434−A1440. (24) He, Y.-S.; Ma, Z.-F.; Liao, X.-Z.; Jiang, Y. Synthesis and characterization of submicron-sized LiNi1/3Co1/3Mn1/3O2 by a simple self-propagating solid-state metathesis method. J. Power Sources 2007, 163, 1053−1058. (25) Wang, J.; Yao, X.; Zhou, X.; Liu, Z. Synthesis and electrochemical properties of layered lithium transition metal oxides. J. Mater. Chem. 2011, 21, 2544−2549. (26) Aurbach, D. Review of selected electrode−solution interactions which determine the performance of Li and Li ion batteries. J. Power Sources 2000, 89, 206−218. (27) An, S. J.; Li, J.; Daniel, C.; Mohanty, D.; Nagpure, S.; Wood, D. L. The state of understanding of the lithium-ion-battery graphite solid electrolyte interphase (SEI) and its relationship to formation cycling. Carbon 2016, 105, 52−76. (28) Kim, D.; Lee, E.; Slater, M.; Lu, W.; Rood, S.; Johnson, C. S. Layered Na[Ni1/3Fe1/3Mn1/3]O2 cathodes for Na-ion battery application. Electrochem. Commun. 2012, 18, 66−69. (29) Yuan, D.; Hu, X.; Qian, J.; Pei, F.; Wu, F.; Mao, R.; Ai, X.; Yang, H.; Cao, Y. P2-type Na0.67Mn0.65Fe0.2Ni0.15O2 cathode material with high-capacity for sodium-ion battery. Electrochim. Acta 2014, 116, 300−305. (30) Oh, S.-M.; Myung, S.-T.; Yoon, C. S.; Lu, J.; Hassoun, J.; Scrosati, B.; Amine, K.; Sun, Y.-K. Advanced Na[Ni0.25Fe0.5Mn0.25]O2/C−Fe3O4 sodium-ion batteries using EMS electrolyte for energy storage. Nano Lett. 2014, 14, 1620−1626. (31) Hasa, I.; Buchholz, D.; Passerini, S.; Scrosati, B.; Hassoun, J. High Performance Na0.5[Ni0.23Fe0.13Mn0.63]O2 Cathode for SodiumIon Batteries. Adv. Energy Mater. 2014, 4, 1400083. (32) Xie, Y.; Wang, H.; Xu, G.; Wang, J.; Sheng, H.; Chen, Z.; Ren, Y.; Sun, C. J.; Wen, J.; Wang, J.; et al. In operando XRD and TXM study on the metastable structure change of NaNi1/3Fe1/3Mn1/3O2 under electrochemical sodium-ion intercalation. Adv. Energy Mater. 2016, 6, 1601306. (33) Che, H.; Chen, S.; Xie, Y.; Wang, H.; Liao, X.-Z.; Amine, K.; Ma, Z.-F. Electrolyte design strategies and research progress for roomtemperature sodium-ion batteries. Energy Environ. Sci. 2017, 10, 1075−1101. (34) Che, H.; Liu, J.; Wang, H.; Wang, X.; Zhang, S. S.; Liao, X.-Z.; Ma, Z.-F. Rubidium and cesium ions as the electrolyte additive for

ACKNOWLEDGMENTS Research was funded by U.S. Department of Energy (DOE), Vehicle Technologies Office. Support from David Howell and Tien Duong of the U.S. DOE’s Office of Vehicle Technologies Program is gratefully acknowledged. Argonne National Laboratory is operated for the US Department of Energy by UChicago Argonne, LLC, under contract DE-AC0206CH11357. This work was also supported by the Natural Science Foundation of China (21676165, 21573147 and 21506123), the National Key Research and Development Program (2016YFB0901500). The authors also thank the support from Clean VehiclesUS−China Clean Energy Research Center (CERC−CVC2). Use of the Advanced Photon Source (APS) and the Center for Nanoscale Materials, including resources in the Electron Microscopy Center, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

■

REFERENCES

(1) Lu, Z.; Dahn, J. R. In situ X-ray diffraction study of P2Na2/3[Ni1/3Mn2/3]O2. J. Electrochem. Soc. 2001, 148, A1225−A1229. (2) Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research development on sodium-ion batteries. Chem. Rev. 2014, 114, 11636− 11682. (3) Slater, M. D.; Kim, D.; Lee, E.; Johnson, C. S. Sodium-ion batteries. Adv. Funct. Mater. 2013, 23, 947−958. (4) Xie, Y.; Zhang, W.; Gu, S.; Yan, Y.; Ma, Z.-F. Process engineering in electrochemical energy devices innovation. Chin. J. Chem. Eng. 2016, 24, 39−47. (5) Pan, H.; Hu, Y.-S.; Chen, L. Room-temperature stationary sodium-ion batteries for large-scale electric energy storage. Energy Environ. Sci. 2013, 6, 2338−2360. (6) Nayak, P. K.; Yang, L.; Brehm, W.; Adelhelm, P. From lithiumion to sodium-ion batteries: Advantages, Challenges, and Surprises. Angew. Chem., Int. Ed. 2018, 57, 102−120. (7) Delmas, C.; Fouassier, C.; Hagenmuller, P. Structural classification and properties of the layered oxides. Physica B+C 1980, 99, 81−85. (8) Sathiya, M.; Hemalatha, K.; Ramesha, K.; Tarascon, J. M.; Prakash, A. S. Synthesis, structure, and electrochemical properties of the layered sodium insertion cathode material:NaNi1/3Mn1/3Co1/3O2. Chem. Mater. 2012, 24, 1846−1853. (9) Yoshida, H.; Yabuuchi, N.; Komaba, S. NaFe0.5Co0.5O2 as high energy and power positive electrode for na-ion batteries. Electrochem. Commun. 2013, 34, 60−63. (10) Wang, H.; Yang, B.; Liao, X. Z.; Xu, J.; Yang, D.; He, Y.; Ma, Z. F. Electrochemical properties of P2-Na2/3[Ni1/3Mn2/3]O2 cathode material for sodium ion batteries when cycled in differentvoltage ranges. Electrochim. Acta 2013, 113, 200−204. (11) Vassilaras, P.; Toumar, A. J.; Ceder, G. Electrochemical Properties of NaNi1/3Co1/3Fe1/3O2 as a Cathode Material for Na-Ion Batteries. Electrochem. Commun. 2014, 38, 79−81. (12) Wang, H.; Liao, X.-Z.; Yang, Y.; Yan, X.; He, Y.-S.; Ma, Z.-F. Large-scale synthesis of NaNi1/3Fe1/3Mn1/3O2 as high performance cathode materials for sodium ion batteries. J. Electrochem. Soc. 2016, 163, A565−A570. (13) Kubota, K.; Asari, T.; Yoshida, H.; Yaabuuchi, N.; Shiiba, H.; Nakayama, M.; Komaba, S. Understanding the structural evolution and redox mechanism of a NaFeO2-NaCoO2 solid solution for sodium-ion batteries. Adv. Funct. Mater. 2016, 26, 6047−6059. (14) Xu, G. L.; Amine, R.; Xu, Y. F.; Liu, J.; Gim, J.; Ma, T.; Ren, Y.; Sun, C.; Liu, Y.; Zhang, X.; et al. Insights into the structural effects of layered cathode materials for high voltage sodium-ion batteries. Energy Environ. Sci. 2017, 10, 1677−1693. 4917

DOI: 10.1021/acs.chemmater.8b00047 Chem. Mater. 2018, 30, 4909−4918

Article

Chemistry of Materials improving performance of hard carbon anode in sodium-ion battery. Electrochem. Commun. 2017, 83, 20−23. (35) Chen, Z.; Ren, Y.; Qin, Y.; Wu, H.; Ma, S.; Ren, J.; He, X.; Sun, Y.-K.; Amine, K. Solid state synthesis of LiFePO4 studied by in situ high energy X-ray diffraction. J. Mater. Chem. 2011, 21, 5604−5609. (36) Lin, C.-K.; Piao, Y.; Kan, Y.; Bareño, J.; Bloom, I.; Ren, Y.; Amine, K.; Chen, Z. Probing Thermally Induced Decomposition of Delithiated Li1.2−xNi0.15Mn0.55Co0.1O2 by in Situ High-Energy X-ray Diffraction. ACS Appl. Mater. Interfaces 2014, 6, 12692−12697. (37) Thorne, J. S.; Dunlap, R. A.; Obrovac, M. N. Investigation of P2-Na2/3Mn1/3Fe1/3Co1/3O2 for Na-ion battery positive electrodes. J. Electrochem. Soc. 2014, 161, A2232−A2236. (38) Sun, Y. K.; Kim, D. H.; Yoon, C. S.; Myung, S. T.; Prakash, J.; Amine, K. A Novel Cathode Material with a Concentration-Gradient for High-Energy and Safe Lithium-Ion Batteries. Adv. Funct. Mater. 2010, 20, 485−491. (39) Chen, Z.; Qin, Y.; Ren, Y.; Lu, W.; Orendorff, C.; Roth, E. P.; Amine, K. Multi-scale study of thermal stability of lithiated graphite. Energy Environ. Sci. 2011, 4, 4023−4030. (40) Chen, Z.; Qin, Y.; Amine, K.; Sun, Y.-K. Role of surface coating on cathode materials for lithium-ion batteries. J. Mater. Chem. 2010, 20, 7606−7612. (41) MacNeil, D.; Dahn, J. Test of reaction kinetics using both differential scanning and accelerating rate calorimetries as applied to the reaction of LixCoO2 in non-aqueous electrolyte. J. Phys. Chem. A 2001, 105, 4430−4439. (42) Luo, W.; Dahn, J. The Impact of Zr Substitution on the Structure, Electrochemical Performance and Thermal Stability of Li [Ni1/3Mn1/3−zCo1/3Zrz]O2. J. Electrochem. Soc. 2011, 158, A428− A433. (43) Luo, W.; Li, X.; Dahn, J. Synthesis, characterization, and thermal stability of Li [Ni1/3Mn1/3Co1/3−z(MnMg)z/2]O2. Chem. Mater. 2010, 22, 5065−5073. (44) Zhou, F.; Zhao, X.; Dahn, J. Impact of Al or Mg substitution on the Thermal Stability of Li1.05Mn1.95−zMzO4(M= Al or Mg). J. Electrochem. Soc. 2010, 157, A798−A801. (45) Lu, W.; Belharouak, I.; Liu, J.; Amine, K. Thermal properties of Li4/3Ti5/3O4/LiMn2O4 cell. J. Power Sources 2007, 174, 673−677. (46) Seo, J.; Kim, C.-S.; Prakash, J.; Zaghib, K. Thermal characterization of Li/sulfur cells using isothermal micro-calorimetry. Electrochem. Commun. 2014, 44, 42−44. (47) Chen, Z.; Ren, Y.; Lee, E.; Johnson, C.; Qin, Y.; Amine, K. Study of thermal decomposition of Li1‑x(Ni1/3Mn1/3Co1/3)0.9O2 using in-situ high energy x-ray diffraction. Adv. Energy Mater. 2013, 3, 729− 736. (48) Chen, Z.; Ren, Y.; Jansen, A. N.; Lin, C.-k.; Weng, W.; Amine, K. New class of nonaqueous electrolytes for long-life and safe lithiumion batteries. Nat. Commun. 2013, 4, 1513. (49) Lei, B.; Zhao, W.; Ziebert, C.; Uhlmann, N.; Rohde, M.; Seifert, H. J. Experimental analysis of thermal runaway in 18650 cylindrical liion cells using an accelerating rate calorimeter. Batteries. 2017, 3, 14. (50) Jhu, C.-Y.; Wang, Y.-W.; Wen, C.-Y.; Shu, C.-M. Thermal runaway potential of LiCoO2 and Li(Ni1/3Co1/3Mn1/3)O2 batteries determined with adiabatic calorimetry methodology. Appl. Energy 2012, 100, 127−13.

4918

DOI: 10.1021/acs.chemmater.8b00047 Chem. Mater. 2018, 30, 4909−4918