Comparison between Na-Ion and Li-Ion Cells: Understanding the

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Comparison between Na-ion & Li-ion Cells: Understanding the Critical Role of the Cathodes Stability and the Anodes Pretreatment on the Cells Behavior Ezequiel de la Llave, Valentina Borgel, Kang-Joon Park, Jang-Yeon Hwang, Yang-Kook Sun, Pascal Hartmann, Frederick Chesneau, and Doron Aurbach ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09835 • Publication Date (Web): 08 Dec 2015 Downloaded from http://pubs.acs.org on December 9, 2015

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ACS Applied Materials & Interfaces

Comparison between Na-ion & Li-ion Cells: Understanding the Critical Role of the Cathodes Stability and the Anodes Pretreatment on the Cells Behavior.

Ezequiel de la Llave, Valentina Borgel, Kang-Joon Parka, Jang-Yeon Hwang a, Yang-Kook Suna, Pascal Hartmannb, Frederick- Francois Chesneaub and Doron Aurbach*

Chemistry Department, Bar-Ilan University, Ramat-Gan 5290002, Israel a

Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea b

BASF SE, Ludwigshafen 67056, Germany

*Corresponding authors [email protected] (Doron Aurbach)

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Abstract The electrochemical behavior of Na-Ion and Li-Ion full cells was investigated, using hard carbon as the anode material, NaNi0.5Mn0.5O2 and LiNi0.5Mn0.5O2 as the cathodes. A detailed description of the structure, phase transition, electrochemical behavior and kinetics of the NaNi0.5Mn0.5O2 cathodes is presented, including interesting comparison with their lithium analog. The critical effect of the hard carbon anodes pretreatment in the total capacity and stability of full cells is clearly demonstrated. Using impedance spectroscopy in three electrodes cells we show that the full cell impedance is dominated by the contribution of the cathode side. We discuss possible reasons for capacity fading of these systems, its connection to the cathode structure and relevant surface phenomena.

Keywords: Na ion batteries, Li ion batteries, Li intercalation, Na intercalation, cathodes, LiMn0.5Ni0.5O2, NaMn0.5Ni0.5O2

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1. Introduction The demand for inexpensive and effective energy storage technologies is rapidly increasing. The imminent exhaustion of fossil fuel resources and the environmental consequences related to their use, combined with the rapid development of renewable energy sources, promote the scientific community to develop advanced energy storage systems. Among the several technologies that are suitable for large-scale energy storage, rechargeable batteries appear as one of the most promising options. These electrochemical energy storage systems become appealing due to their flexibility to meet different practical demands, becoming a realistic option for the integration with green energy harvesting technologies as solar panels based on photovoltaic cells and wind turbines. While lithium-ion batteries (LIB) may be our primary candidate as a leading battery technology, concerns about the feasibility of a sustainable lithium supply arise in the scientific community. Even though that is widely distributed in the Earth’s crust, lithium cannot be considered as an abundant element and its natural resources are unevenly distributed.1 Increasing demand for Li commodity chemicals for portable applications, combined with the growth of the electric vehicles industries, will drive up prices.2 Supplies will be even further constrained, if Li batteries will be adopted for large-scale energy storage. Therefore the use of large-scale lithium based energy storage systems would be unavoidably restricted. Sodium-ion batteries (NIB) appear as a promising alternative to the well-established lithium batteries technology.3-4 Sodium natural resources are unlimited everywhere, being sodium the 4th most abundant element on earth.1 Additionally, sodium insertion materials have been extensively studied over the last few years as promising candidates to be used as cathodes in sodium batteries.5-11

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Among many candidates, layered sodiated transition metal oxides, NaxMeO2 (Me = transition metals), appear as very interesting cathode materials. In the last years, the number of papers related to their use in sodium-ion systems has dramatically increased.12 The interest in these layered oxides probably arose from their high capacities and ease of synthesis.11-12 In fact, their capabilities were demonstrated already in the early 1980s,13-16 but most of the community focused efforts on lithium-ion battery systems, ignoring the potential of sodium intercalation compounds as battery materials. Especially interesting are sodiated layered oxides containing two different transition metals,17-21 some of which could demonstrate specific capacities approaching 200 mAh/g.22-24 There are also publications on full Na ion cells with carbonaceous3,25-26 and conversion reactions based27 anodes. In this work we concentrated on NaNi0.5Mn0.5O2 as promising cathode material for sodium ion batteries, due to the potential application of these materials in commercial cells.19-21 These cathodes were examined in full Na ions cells which employed hard carbon anodes. With the aim to offer a comprehensive view, we compared all the Na-ion systems studied herein to similar Li ion systems. Hard carbon anodes were selected for the present study, based on a previous experience.32 We paid attention to the description of the structure, phase transition and electrochemical behavior of the cathodes, discussing possible capacity fading mechanisms. Also we examined surface phenomena, interfacial charge transfer aspects and differences between lithium and sodium ions insertion processes. We explored the behavior of full cells and compared the contribution on the various components to their overall impedance. Despite extensive work being carried out in these days throughout the world on Na ion battery systems, this paper may provide unique information due to the systematic structural, surface and electrochemical studies and the comparison between the Li and Na ion systems.

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2. Experimental [Ni0.5Mn0.5](OH)2 precursor was synthesized by co-precipitation method as described in the literature.33-34 Appropriate amounts of NiSO4·6H2O and MnSO4·5H2O (cationic ratio of Ni:Mn = 5:5) were pumped into a batch reactor (16 L) which was filled with a certain amount of deionized water, NH4OH solution (aq), and NaOH solution (aq.) in a flowing N2 atmosphere. Concurrently, a NaOH solution (aq., 4.0 mol L-1) and a NH4OH (aq.) were pumped separately into the reactor. The pH, temperature, concentration, and stirring speed of the mixture in the reactor were carefully regulated during co-precipitation reaction. The obtained [Ni0.5Mn0.5](OH)2 powders were washed, filtered, and dried in the vacuum oven at 110 oC for overnight. The [Ni0.5Mn0.5](OH)2 precursor powders were mixed with LiOH·H2O and NaOH, and calcined at 900 oC for 15 h in O2 and at 730°C for 24 h in air to synthesize Li[Ni0.5Mn0.5]O2 and Na[Ni0.5Mn0.5]O2, respectively. Hard Carbon was obtained from Hitachi (Japan), and was used as received. Hard carbon electrodes were prepared by mixing 90 wt. % of active material, 3 wt %. of carbon black and 7 wt. % of Carboxymethyl Cellulose (CMC) binder. The cathodes were made by mixing 80 wt. % of NaNi0.5Mn0.5O2 or LiNi0.5Mn0.5O2, 10 wt. % of carbon black and 10 wt. % of PVdF binder. A planetary centrifugal vacuum mixer was used to shake the vials at 2000 rpm for 10 min under vacuum. The obtained slurry was cast onto an aluminum and copper foils for cathodes and anodes respectively, followed by coating with a doctor blade. The electrodes were dried overnight at 60°C in air and then at 110°C overnight under vacuum as well. The electrode sheets were then rolled, and the thickness of the electrodes was reduced by 10%. The resulting foil was cut into disks of 14 mm in diameter. The weighted cathodes and anodes were dried at 110°C overnight, in order to remove the moisture adsorbed during the rolling process. Composite

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electrodes were assembled with polyethylene separators using 2325 coin-type cells. All the cell fabrications were carried out in highly pure argon atmosphere in glove boxes. Battery testing was carried out using multichannel battery testers (Arbin Inc.). For all the experiments, the selected electrolyte solutions were 0.5M NaPF6 in Propylene Carbonate (PC) using 2% w/w of Fluoroethylene Carbonate (FEC) as additive, and 0.5M LiPF6 PC:FEC 98:2 w/w, based on a previous experience.32 Cyclic voltammetry (CV) and impedance spectroscopy (EIS) experiments were carried out at room temperature (25 0C) using a VMP Bio-Logic potentiostat with a 1287/1260 FRA systems. Measurements were done with three electrodes coin cells, using the metal (Na or Li) as both counter and reference electrodes. The EIS measurements were carried out with an amplitude of 5 mV around equilibrium in the frequency range of 200 kHz – 0.01 Hz. Scanning electron microscopy (E-SEM) imaging, using a JEOL-JSM 700F instrument was performed to investigate the morphology of the cathode materials The XRD patterns of the materials were obtained by X-ray diffraction with a Bruker D8 Advance X-ray diffractometer using Cu Kα (λ=1.5418 Å) as the source, operated at 40 mA and 40 kV

3. Results and Discussion 3.1. NaNi0.5Mn0.5O2 and LiNi0.5Mn0.5O2 as Cathodes for Na-Ion and Li-Ion Batteries. 3.1.1. Structural Characterization Figure 1 shows the E-SEM images and XRD patterns of the cathodes under study. Powder XRD patterns of NaNi0.5Mn0.5O2 and LiNi0.5Mn0.5O2 materials are depicted in panels (c)

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and (f), respectively. For both cathode materials, all the observed Bragg diffraction peaks are consistent with a single phase, which is isostructural with α-NaFeO2, and could be indexed to a conventional hexagonal (rhombohedral) unit cell, with space group R3̅m. These XRD patterns are in close agreement with those previously reported in the literature,17,28 and are consistent with materials crystallized into an ideal O3-type layered structure. No crystalline impurity was observed in the diffractograms. The morphology of the cathode powders was observed using scanning electron microscopy (SEM) at different magnifications, as shown in figure 1. Panels 1 (a) and (b) for the sodium compound, panel 1 (d) and (e) for the lithium compound show well-crystallized particles of 200 – 500 nm which agglomerate in larger crystallites of 10 to 15 µm in diameter. EDAX measurements (not shown) provided the following output: i) our materials have the proper stoichiometric ratio, within reasonable experimental errors: 1Mn: 1.1Ni: 2.2Na and 1Mn: 1Ni: 2.1Li, for the sodium and lithium compounds respectively. 2) Ni and Mn are equally distributed (no differences in the atomic ratio, obtained from different spots). This structural analysis confirmed that we are working with two well defined cathode materials with similar macroscopic and microscopic structures. In this way, we are able to make properly the comparative analysis in both full and half cells.

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Figure 1. E-SEM images of NaNi0.5Mn0.5O2 (panel a and b) and LiNi0.5Mn0.5O2 (panel d and e) powders. Panel c and f, powder XRD patterns for NaNi0.5Mn0.5O2 and LiNi0.5Mn0.5O2, respectively.

3.1.2. Electrochemical Performance A Figure 2 presents the rate capability of the NaNi0.5Mn0.5O2 (panel a) and LiNi0.5Mn0.5O2 (panel b) cathodes in 0.5M NaPF6 and 0.5M LiPF6 PC:FEC 98:2 electrolyte solutions, respectively at 30 0C in conventional coin-type cells, at different cycling rates. The current was increased stepwise from 12 mA/g (C/10) to 120 mA/g (C/1) and from 9 mA/g (C/20) to 180 mA/g (C/1) in sodium and lithium cells, respectively. The discharge capacity values are computed at each step. In the sodium cells the capacity values were 136, 127, 112 and 94 mAh/g (see fig 2 a), while for lithium cells we found 170, 158, 143 and 100 mAh/g (see fig 2b). At the end of the rate

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test, the current density in both systems was returned to C/10, obtaining capacity values of 122 and 150 mAh/g for sodium and lithium cells, respectively. With increasing current density, the capacity of the lithium cells decreases more pronouncedly than that of the sodium cells. This result suggested that the diffusion of Na+ ions is faster than that of Li+ ions in the O3-type layered oxides frameworks. Even though our cathodes shows both a remarkable coulombic efficiency and good rate capability, the sodium cathode present a constant fading leading to a loss of the initial charge capacity during cycling. In contrast to the sodium cells, the lithium cathodes showed a stable capacity, practically without fading, for more than 100 cycles.

Figure 2. Electrochemical performance of cathodes in half cells. Rate capability and coulombic efficiency of NaNi0.5Mn0.5O2 electrodes in 0.5M NaPF6/PC:FEC 98:2 solution (panel a) and LiNi0.5Mn0.5O2 electrodes in 0.5M LiPF6 /PC:FEC 98:2 solution(panel b). Experiments were carried out at 300C.

In order to provide a clear picture of the electrochemical behavior of the cathodes explored herein, aiming at understanding the difference in the stability described above, galvanostatic charge/discharge profiles of these cells were analyzed. Figure 3 presents the

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voltage profiles of NaNi0.5Mn0.5O2 (panel a) and LiNi0.5Mn0.5O2 (panel b) cathodes in coin cells with NaPF6 and LiPF6 PC:FEC 98:2 electrolyte solutions, respectively. It is clear and known that these profiles represent the cathodes behavior, since the voltage profiles of the metal anodes are flat and reflect low over-potential. The sodium cells were charged to 4.0 V and then discharged to 2.2 V (vs. Na) while the lithium cells were charged to 4.3 V and discharged to 2.7 V (vs Li). The cells were cycled at C/10 rate (12 and 18 mA/g for sodium and lithium, respectively), at a constant temperature of 30 0C. As we mentioned before, figure 3 (a) shows for a typical sodium cell an initial capacity of about 135 mAh/g, which is constantly fading during cycling, leading to a loss of 9% of the initial charge capacity after 25 cycles. This capacity fading slightly decreased, presenting a total capacity retention of 87% and 84% of the initial capacity after 50 and 75 charge/discharge cycles, respectively. In contrast, figure 3 (b) shows that a typical lithium cell still conserves more than 92% of the initial capacity after 75 cycles. The discharge/charge profiles of the Li cells have the typical shape of expected from Lix[MnNi]O2 or Lix[MnNiCo]O2 layered cathode materials. They are sloping, reflecting lithiation/delithiation processes that form solid solutions, with the slope being more flat around 3.7V. These profiles relate to the Ni4+/Ni2+ red-ox system and to a phase transitions from Hex.O3 to Hex.O1 close-packed arrays.29-30 The voltage profiles of the sodium cells show several plateaus in both charge and discharge curves, indicating that the material undergoes several reversible phase transitions during the sodium extraction/insertion. These phase transitions were recently described by Komaba and coworkers,19 as structural changes between Hex.O3 → Mon.O3 → Hex.P3 → Mon.P3 → Mon.P3’ phases. The structural transition from O3 to P3 phases (oxygen stacking sequence transition from ABCABC to AABBCC) is driven by a slab

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gliding mechanism of [Mn0.5Mn0.5]O2 slabs, without breaking of the bonds between the transition metals and oxide ions. Both, ideal Hex.O3 and Hex.P3 structures were further distorted, forming the monoclinic phases denoted as Mon.O3 and Mon.P3. Finally, the cathode material reaches the highly faulted Mon.P3’ layer structure.19 The charge compensation during sodium insertion/extraction is encompassed by the changes in redox state of the nickel ions (Ni2+Ni3+ Ni4+) states. It is worthy to point out that the lithium ions are smaller than the sodium ions (0.76 vs 1.06 Å), so the different plateaus observed with the sodium based cathode materials are probably due to strain effects induced by the transport of the large sodium ions within the solid hosts.

Figure 3. Electrochemical performance of cathodes in half cells. Charge/discharge curves (galvanostatic cycling) for NaNi0.5Mn0.5O2 electrodes in 0.5M NaPF6 PC:FEC 98:2 solution (panel a) and LiNi0.5Mn0.5O2 electrodes in 0.5M LiPF6 PC:FEC 98:2solution (panel b). Experiments were carried out at 300C and C/10 rate.

An enlightening comparison emerges from the analysis of the dQ/dE vs. E curves presented in figure 4. In such presentations, the response of phase transitions (appearing as plateaus in the voltage profiles) appears as sharp peaks in the dQ/dE vs. E curves. Figure 4 11



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shows the response of the NaNi0.5Mn0.5O2 cathodes (panel a) and the LiNi0.5Mn0.5O2 cathodes (panel b) measured with coin cells containing NaPF6 and LiPF6 PC:FEC 98:2 solutions. The curves relate to the lithiated material (figure 4b) present a pair of peaks at 3.8 V (anodic) and 3.75 V (cathodic) , vs. Li , which do not change the peak potentials due to cycling and their intensity remain nearly stable during 50 cycles. This behavior it is in agreement with the stable capacity, perfect coulombic efficiency and excellent cyclability described above. The sodium cells present a different behavior. There derivative curves show several sets of peaks, corresponding to the plateaus in figure 3a. These peaks slightly and gradually change position and intensity as a function of cycling. These trends are seen very clearly in Fig. 4b, with most intensive peaks below 3V vs. Na, which relate to the phase transition between hexagonal and monoclinic O3 phases.19 This indicates that during cycling the main charge transfer step related to the Hex.O3 → Mon.O3 phase transition, becomes progressively more hindered.

Figure 4. Electrochemical performance of cathodes in half cells. dQ/dE vs. E (voltage profiles derivatives) for NaNi0.5Mn0.5O2 cathodes in 0.5M NaPF6 PC:FEC 98:2 solution (panel a) and LiNi0.5Mn0.5O2 cathodes in 0.5M LiPF6 PC:FEC 98:2 solution (panel b).

In order to understand better their electrochemical response, discharged NaxNi0.5Mn0.5O2 12


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electrodes after cycling we examined ex-situ by XRD measurements. Figure 5 shows the XRD patterns for the cathode powders (black curve), the pristine electrodes before cycling (blue curve), and the electrodes after 50 galvanostatic cycles in both half cells (red curve) and full cells (purple curve). For both cathodes, the pristine electrodes and the powder present the same XRD patterns, revealing that we can measure the composite electrodes as references. In line with the experiments described above, figure 5b shows that the lithium insertion cathodes conserve the main features of the pristine material after 50 cycles in both half and full cells, showing their excellent stability during cycling in LiPF6 PC:FEC 98:2 solution. In turn, figure 5a shows that cycling NaxNi0.5Mn0.5O2 electrodes induces structural changes. Zooming the low angle region of the XRD patterns reveals that after 50 cycles in half cells (red curve), the (003) peak splits into two peaks, what means a formation of two different components. According to Komaba’s work,17 this structural features may reveal the coexistence of Hex.O3 and Mon.O3 phases. A more pronounced effect is observed in full cells (purple curve). In this case, the position of the (003) peak is shifted to lower angles, revealing that after prolonged cycling in full cells, most of the material remains in a hexagonal P3 phase.17 Hence our studies have revealed that NaxNi0.5Mn0.5O2 undergoes irreversible phase transition upon repeated sodiation/de-sodiation processes, which lead to capacity fading. Such processes do not occur with LixNi0.5Mn0.5O2 electrodes. Hence, we attribute the irreversibility to strains developed upon transport of the relatively big Na cations within the solid host. The exact mechanism is being explored in a parallel study, using in-situ techniques (beyond the scope of the present publication).

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Figure 5. XRD patterns of NaNi0.5Mn0.5O2 (panel a) and LiNi0.5Mn0.5O2 electrodes (panel b). black curves: of cathode powders, blue curve: pristine electrodes before cycling, and of electrodes after 50 galvanostatic cycles in both half cells (red curve) and full cells (purple curve).

3.1.3. Diffusion Coefficients In order to investigate the kinetic behavior of our cathode materials, the average diffusion coefficients were roughly calculated from CV measurements. The electro-analytical response of insertion electrodes was thoroughly explored, especially for Li ions intercalation electrodes.35-36 Using CV at low enough scan rates (in the microvolt/sec level) may reflect indeed the solid state diffusion processes. However, the diffusion coefficient (D) of ions insertion into carbonaceous, oxides and olivine hosts may depend on the intercalation level and the electrode’s potential.35-36 CV measurements provide only average values for D. More rigorous measurements like PITT37 are required in order to enable calculation of D at higher resolution, as a function of the electrode’s potential. Nevertheless, we use CV measurements in order to obtain a rough comparison between the kinetics of the Li and Na insertion cathodes studied herein. Fig. 6 shows the cyclic voltammograms at different scan rates ranging from 0.01 to 0.1 mV/s for NaNi0.5Mn0.5O2 (panel a) and LiNi0.5Mn0.5O2 (panel b) electrodes. For both cathodes, increasing the scan rate broadened the peaks and slightly increased the peak potential separation, as

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expected, due the resistive elements in the system.38 As shown in figure 6, the lithium insertion cathodes’ CVs exhibit only one set of peaks, while the sodium insertion cathodes’ CVs exhibit three sets of peaks (labeled as I, II and III for the anodic peaks; IV, V and VI for the cathodic peaks). These voltammetric responses are fully coherent with the dQ/dE vs. E curves calculated from the electrode’s voltage profiles (figure 4). In order to determine the apparent average diffusion coefficients, the Randles–Sevcik equation was used:39 ⁄ ⁄ ⁄ (1) were the constant k has a value of 2.69×105 C mol−1 V−1/2, where Ip (A) is the peak current, n is the number of electrons involved in the process, A (cm2) the electroactive area A [resulting from the active mass loading and BET surface area], D (cm2 s-1) is the diffusion coefficient, C (mol cm-3) is the ion concentration and v (V s-1) is the scan rate.

Figure 6. Cyclic voltammograms of cathodes at different scan rates. Panel a: NaNi0.5Mn0.5O2 electrodes in 0.5M NaPF6 PC:FEC 98:2 solutions. Panel b: LiNi0.5Mn0.5O2 electrodes in 0.5M LiPF6 PC:FEC 98:2 solution. Inset: maximum peak currents vs. the square root of the scan rate. D can be calculated from the slope of the linear curves thus obtained. The linear relationship between the peak current and the square root of the scan rate for each peak are shown as insets in the figure 6. From the slope of these plots, the average diffusion 15



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coefficient can be determined. The calculated values of D are listed in Table 1. These results show that the sodium insertion cathodes have averagely higher diffusion coefficient than that of the Li insertion cathodes, suggesting that the ionic extraction/insertion processes are faster for the sodium insertion electrodes. These rough findings are in line with the rate capability measurements presented in figure 2, which demonstrated that NaxNi0.5Mn0.5O2 electrodes are faster than LixNi0.5Mn0.5O2 electrodes. Hence, comparing the results of these measurements, presented in figures 2 and 6, indicates indeed that the solid state diffusion processes determine in fact the overall kinetics and hence the rate capability of these electrodes. The results presented herein are coherent with many findings by others. For instance, using first principles calculations, Ceder and co-workers40 showed that for NaCoO2, which is also an O3-type layered oxides, the diffusion barriers for sodium are lower than those for lithium ions diffusion. Komaba showed that the alkali−oxygen bonds are longer in sodiated transition metal oxides, resulting in a weaker electrostatic interaction.41

Table 1. Calculated diffusion coefficients (D) for NaNi0.5Mn0.5O2 and LiNi0.5Mn0.5O2 cathodes obtained from CV results.

3.2 Full cells comprising NaNi0.5Mn0.5O2 or LiNi0.5Mn0.5O2 cathodes and hard carbon anodes. 3.2.1. Electrochemical Performance of full cells In order to construct and test full cells, we used hard carbon as the anode material for

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both lithium and sodium insertion cathodes, choosing 0.5M LiPF6 and 0.5M NaPF6 PC:FEC 98:2 as electrolyte solutions, respectively. We refer to our recent paper,32 in which we discuss the rationale behind the selection of both this anode material and the electrolyte solutions. Figure 7 presents both the discharge capacity and the coulombic efficiency for sodium (panel a) and lithium (panel b) full cells at 30 0C, at different rates. The results presented herein arise from the average of at least five independent experiments, for each full cell. At the lowest rate tested (C/30), the lithium and sodium cells exhibit 185 and 120 mAh/g, respectively (calculated per gram of cathode material in the cell). Increasing the rates to C/10 did not show drastic changes in capacity of the sodium full cells, while the lithium cells steeply decreased to a discharge capacity value of 162 mAh/g at C/10 rate (~ 12% less than at C/30) . At C/5 rate a reduction of only 8% in capacity (110 mAh/g) was observed for sodium cells, while the lithium cell lost ~22% capacity (showing only 145 mAh/g). After cycling at the high rates (C/1) both cells return to their previous capacity measured initially at C/10 rates. The coulombic efficiency remained close to 100% in all the experiments. It was shown that with increasing current density, the capacity of the lithium cells decreases more rapidly than that of the sodium cells. These results substantiate our conclusion that the sodium insertion cathodes are intrinsically faster than the lithium insertion cathodes.

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Figure 7. Electrochemical performance of full cells. Rate capability and coulombic efficiency of HC/NaNi0.5Mn0.5O2 in 0.5M NaPF6 PC:FEC 98:2 (panel a) and HC/LiNi0.5Mn0.5O2 in 0.5M LiPF6 PC:FEC 98:2 (panel b). Experiments were carried out at 300.

3.2.2. The Effect of the Hard Carbon Anodes Pretreatment on cells behavior. As mentioned in the experimental section, the full cells were assembled using an excess of the hard carbon anode, so the cell’s capacity is limited by the cathodes. However, the key condition for obtaining maximal capacity of full cells is pretreatment of the anode, namely prelithiation or pre-sodiation. Figure 8 presents curves of discharge capacity vs. cycle number, of full Na-Ion (panel a) and Li-Ion (panel b) cells. This figure clearly shows the effect of prelithiation or presodiation of the anodes on the cell’s capacity. The pretreatment process involved a first step of galvanostatic charge/discharge cycle of the anodes in half cells, at slow rates (C/20). After this first cycle, the anodes are charged until reaching 50% of their total capacity. Then, the cells are opened and the pre-treated anodes are introduced into full cells. The top red curves in both panels show data of pretreated cells, while the bottom blue curves relates to cells with untreated anodes. Cells were tested at 30 0C and C/10 rate. The advantage of using pretreated anodes is spectacular: 115 vs. 65 mAh/g and 162 vs. 92 mAh/g for sodium a lithium cells respectively (figure 8). It seems that anodes pre-treatments are mandatory in order to extract maximal capacities with Li and Na ion batteries, especially when using anodes such as hard carbons, which suffer from relatively high initial irreversible capacity. A big challenge remain, how to develop practical pre-lithiation and pre-sodiation processes that may be suitable and acceptable for the batteries industry.

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Figure 8. Effect of anodes pre-treatment on the discharge capacity on full cells. Panel a: HC/NaNi0.5Mn0.5O2 in 0.5M NaPF6 PC:FEC 98:2. Panel b: HC/LiNi0.5Mn0.5O2 in 0.5M LiPF6 PC:FEC 98:2. Experiments were carried out at 300C and C/10 rate

3.2.3. The Impedance response of full cells Figure 9 shows impedance spectra of full three electrodes Na-ion cells (panel a) and Liion cells (panel b) measured at 30 0C after 10 galvanostatic cycles. Measured when the cells were discharged. Our three electrodes coin type cells setup, allows to measure the impedance of the full cell and also the individual contributions of the cathodes and the anodes. To measure in "full cell" mode we short the sodium reference electrode with the hard carbon anode. To measure the cathode and the anode separately, we use the sodium metal as reference and the anode or the cathode as working or counter electrodes (as relevant). As we discussed many times before,42-43 impedance spectra of composite intercalation electrodes are very complicated. Although they may reflect all the relevant time constants of all the processes which the electrodes undergo, assignment of impedance spectral features of composite electrodes to specific processes and time constants may be impossible. Nevertheless, impedance spectroscopy of composite intercalation electrodes can provide useful comparative and qualitative information about stability and evaluation of dominant factors that contribute to electrodes and cells impedances. 19



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In fact, the information provided by the spectra in figure 9 can serve as a classical example for the usefulness of EIS, for the analysis of complex cells and electrodes, although we cannot analyze the spectra in relation to the various electrodes processes and time constants. The panels of figure 9 show spectra of full cells, anodes and cathodes. Comparing the anode, cathode and full cell spectra of each system clearly shows that the cathodes’ impedance dominate the overall full cell impedance for both Li and Na cells. In general, the overall impedance of the Na cells is smaller than that of the Li cells, fully coherent with the above discussed rate capability and voltammetric measurements (figures 2 and 6). Hence, these spectra reflect the general better transport properties of Na ions intercalation/de-intercalation in these hosts, compared to Li ions. Interestingly, the impedance of the carbon anodes in the sodium cells is much higher than the anodes impedance in the Li cells, in line with our previous studies.32 As we explained therein, the carbon anodes’ impedance is usually dominated by the resistance to Li or Na ions migration through the surface films that always cover these electrodes. We showed that Li ions migration through surface films comprising ionic Li compounds is usually faster than migration of Na ions through surface films composed on ionic sodium compounds.32

Figure 9. Electrochemical impedance spectroscopy (EIS) of full cells in three electrodes cells 20


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after 10 galvanostatic cycles at C/10. Nyquist Plots of - Panel a: HC/NaNi0.5Mn0.5O2 in 0.5M NaPF6 PC:FEC 98:2 solution. Panel b: HC/LiNi0.5Mn0.5O2 working in 0.5M LiPF6 PC:FEC 98:2 solution.

4. Conclusion We demonstrated herein the possibility to compose sodium ion battery prototypes, comprising hard carbon anodes and NaNi0.5Mn0.5O2 cathodes.

Highly important was the

rigorous comparison to similar Li ion cells. In general, the surface films resistance of hard carbon electrodes in Na cells is much higher than that of carbon anodes in Li cells, due to the slower Na ions migration within surface films comprising ionic sodium compounds. In turn, solid state diffusion of Na ions in the Nax[MnNi]O2 host is faster than Li ions diffusion in Lix[MnNi]O2. Thereby, the sodium ions intercalation cathodes have better rate capability than the similar lithium ion electrodes. It was found that pre-lithiation and pre-sodiation of hard carbon anodes is a mandatory treatment in order to extract maximal capacity and retain stability upon cycling, because these electrodes suffer from initial high irreversible capacity. Thereby, it is important that the active metal source needed to complete a first cycle of hard carbon anodes will not come from an excess of cathode material in the cell. Comparing Na and Li ion cells, the former may exhibit a faster kinetics, however, Na ion batteries cannot rival Li ion batteries in terms of energy density. Another open question is cathodes’ stability. While Na ions insertion kinetics in transition metal oxides may be relatively fast, transport of the relatively big Na ions seems to induce irreversible changes in the structure of Nax[MnNi]O2 cathodes. Since Na ion battery technology is developed in connection to load leveling applications, the stability of sodium insertion cathode materials upon cycling is a serious concern. However, the authors believe that based on several decades of intensive work on cathode materials for Li ion batteries, it will be possible to develop hosts for fully reversible sodium ions intercalation, for practical 21



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sodium ion batteries.

Acknowledgement A partial support for this work was obtained for the ISF – Israel Science foundation, in the framework of the INREP project.

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Comparison between Na-Ion and Li-Ion Cells: Understanding the

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