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New Lithium metal polymer solid state battery for an ultrahigh energy: Nano C-LiFePO4 versus Nano Li1.2V3O8 Pierre Hovington, Marin Lagacé, Abdelbast Guerfi, Patrick Bouchard, Alain Mauger, Christain Julien, Michel Armand, and Karim Zaghib Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b00326 • Publication Date (Web): 25 Feb 2015 Downloaded from http://pubs.acs.org on March 2, 2015

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New Lithium metal polymer solid state battery for an ultra-high energy: Nano C-LiFePO4 versus Nano Li1.2V3O8 P. Hovington1, M. Lagacé1, A. Guerfi1, P. Bouchard1, A. Mauger2, C. M. Julien3, M. Armand4 and K. Zaghib*1 1

2

Institut de Recherche dHydro-Québec (IREQ), 1800 Bd Lionel-Boulet, Varennes, QC J3X 1S1 Canada

Sorbonne Universités, UPMC Univ Paris 06, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), CNRS UMR 7590, 4 place Jussieu, F-75005 Paris,France 3

Sorbonne Universités, UPMC Univ Paris 06, Physicochimie des Electrolytes et Nanosystèmes Interfaciaux (PHENIX), CNRS UMR 8234, 4 place Jussieu, F-75005 Paris, France 4

CIC Energigune, Parque Tecnologico de Alava, Albert Einstein 48, Ed. CIC, 01510 Miñano, Spain *Corresponding author: [email protected]

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Abstract Novel lithium metal polymer solid state batteries with nano C-LiFePO4 and nano Li1.2V3O8 counter-electrodes (average particle size 200 nm) were studied for the first time by in-situ SEM and impedance during cycling. The kinetics of Li-motion during cycling is analyzed selfconsistently together with the electrochemical properties. We show that the cycling life of the nano Li1.2V3O8 is limited by the dissolution of the vanadium in the electrolyte, which explains the choice of nano C-LiFePO4 (1300 cycles at 100% DOD): with this olivine, no dissolution is observed. At high loading and with a stable SEI, in combination with lithium metal, an ultra-high energy density battery was thus newly developed in our laboratory.

Keywords: Lithium metal polymer battery, in-situ SEM, nano C-LiFePO4, nano Li1.2V3O8.

1. Introduction Since the pioneering work of Armand et al. [1] suggesting the use of polymer electrolytes for lithium batteries, major efforts in the 1980's were devoted to develop thin-film, solid-state batteries based on polyether (PE) electrolytes [2]. In parallel with the progress on electrolytes, the development of lithium salts such as LiTFSI or LiFSI, i.e. Li[N(SO2CF3)2], Li[N(SO2F)2] respectively, in combination with amorphous polymer hosts yielded better conductivities [3]. Starting in the early 1990's, Li-ion batteries were developed that still have an advantage, in particular for portable use, but an all-solid Li battery is still very appealing, if it were not only for safety reasons. The Li metal (Lio)-polymer cell (LPC) is low cost through high-speed processing to produce commercial LPCs that contain about 2.5 km of film per battery. In addition, while the current collector for Li-ion batteries is usually copper on the negative electrode, which is expensive, the lithium metal acts simultaneously as the active element and the current collector in


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the LPC. Indeed, the solid polymer avoids the flammability of the liquid organic electrolytes of the Li-ion batteries. Polymer electrolytes with no liquid solvent have very low vapor pressure and are less prone to react with lithium at its melting temperature. Another issue is the degradation of the Li-ion batteries upon increasing the temperature above room temperature. In the case of LPC, this is just the opposite: the battery has to be heated to an operational temperature of 60-80°C to increase the ionic conductivity of the polymer electrolyte. The LPC also avoids the formation of the solid-electrolyte interface (SEI) that must be controlled by the initial formation procedure of Li-ion batteries, which is time consuming and decreases productivity. It also avoids electrode side reactions with the organic electrolytes. The particles that constitute the active element of the positive electrodes of Li-ion batteries are often coated with a protective layer [4], but this increases the cost of the synthesis process. Another attractive property of lithium metal is its high specific capacity (3861 mAh g-1 versus 372 mAh g-1 for graphitic carbon used in Li-ion batteries. There is also more flexibility in the manufacturing process, because the LPC cell can be built either in the charged state (case of Li1.2V3O8 electrode) or discharged state (case of the Nano C-LiFePO4 electrode). A negative feature of the LPC is the formation of dendrites at the surface of the lithium metal. The dendrites are minimized by charging at a low C-rate (the usual convention is used here, according to which nC-rate means cycling in 1/n hours). This property may rule-out some uses such as frequency regulations of smart grids for instance, but is not limiting for many other applications including the electric vehicles; the best example is the “Bluecar®” commercialized by Bolloré, equipped with a LiFePO4/SPE/Li-metal battery. While the driving range (250 km) is longer than similar vehicles with Li-ion batteries, the main constraint, is the need to keep the battery plugged-in on a permanent basis to maintain the operating temperature of 60-80°C when not in use. Improving the performance of these batteries requires optimizing different parameters, such


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as the energy density and a better understanding of the kinetics of plating and dissolution of metal ions, among others. The purpose of the present work is to report in-situ and post-mortem characterization of LPC for two counter-electrodes, namely Li1.2V3O8 (LVO) and Nano C-LiFePO4 (LFP) for comparison, and for different thicknesses of the Lio film to optimize this parameter that affects the weight and energy density of the battery. LVO was proposed in the 1970's [5], and has been the subject of many investigations since. Thackeray’s group extensively investigated LVO, which can reversibly insert up to about 3.8 Li per formula unit with fair cycle life owing to its layered structure in the crystallized state [6]. This material is commonly used as an electrode for LPCs, mostly at temperatures in the range 90-120°C where its electrochemical properties were investigated [7-9]. LFP was first proposed by Goodenough [10]. It was only after some time before LFP was utilized as a positive electrode for a new generation of Li-ion batteries, because of some difficulties. The LFP is poorly conductive, which was solved by carbon coating [11]. Also, the electrochemical properties are very sensitive to any defect and the material must be prepared impurity-free. These obstacles were overcome, and LFP is now commonly used in Li-ion batteries for applications that demand high power, including hybrid vehicles. Several reviews have been devoted to LFP, where the reader will find its physical, chemical and electrochemical properties [12-14]. On the other hand, publications on LFP-LPC are rare [15], despite the fact that they are commercialized (Bathium®, Jameco Electronics-CTC Batteries). The structural properties of the cells during cycling were investigated by in-situ electron microscopy to investigate the deformations and strains associated with Li insertion/extraction, and to improve our understanding of the aging effects that must be overcome to produce LPCs that are even cost effective.

2. Experimental


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The electrochemical properties of LVO vary with the preparation conditions [16]. The LVO powder can be prepared by a solid-state process [6], produces crystalline Li1.2V3O8. However, amorphous LVO is prepared by a low-temperature synthesis method [17]. The amorphous material can accommodate up to 4.5 Li+ ions per formula unit, instead of 3.8 for crystalline Li1.2V3O8. In addition, amorphous LVO has better rate capability than the crystallized counterpart, because of the absence of long-range crystallographic order that reduces the length of the pathways through which Li+ ions diffuse. For this reason, effort is directed to optimize the low-temperature synthesis of amorphous LVO [18-20]. However, the improvement in performance with the amorphous materials comes at the expense of battery life. Due to this severe drawback, we have chosen in the present work to prepare the LVO powder by the commercial (solid-state) process following the procedure published in [6], using LiVO3 and V2O5 as precursors in a 1: 2 molar ratio in a sealed quartz ampoule at high temperature (680°C) for 24 hours. We present in Fig. 1 a micrograph showing the typical plate-like primary particle of LVO ranging from 45 nm to more than 1 µm that was used in this work. The small spherical particles in Fig 1 are from high surface area conductive carbon (Ketjen Blackc).


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Figure 1: Typical SEM micrograph of LVO powder material used to prepare the electrode.

The carbon-coated Nano LiFePO4 powder shown in Fig. 2) (simply referred to as LFP) from HydroQuébec was synthesized by the hydrothermal process described in reference 14. Both of these powders have plate-like active particles of comparable average size, about 300 nm (Fig. 1 see Fig. 2).


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Figure 2:Typical SEM micrograph of LFP powder material used to prepare electrode. The LVO electrode was prepared by mixing the LVO powder with 5wt.% acetylene black and 34 wt.% polyether-based binder. The LFP electrode was prepared by mixing the LFP powder with 5wt% carbon (acetylene black + carbon fiber in equal proportion) and 24 wt.% polyetherbased binder. In both cases, the electrolyte was a polyether-based solid polymer electrolyte (SPE), and the lithium salt was LiTFSI or LiFSI. The cells were examined in a variable pressure SEM (S-3500N, Hitachi, Japan). The same experimental set-up was used recently to investigate nano Si and SiOx-graphite as negative electrodes [21] for Li-ion batteries. The in-situ SEM observations of the electrodes were performed in cross-section view. The details on the electrode preparation and the operational procedures of the SEM are described elsewhere [22-24]. All the measurements were performed at 80°C for the LVO cell, and at 70°C for the LFP cell.


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3. Results and Discussions 3. 1 Thickness of the Li-layer The first step to reduce the weight and the cost of the LPC is to reduce the thickness of the Li layer to be comparable to the amount of Li that is utilized in the electrochemical reactions a small excess for current collection. The experiments were conducted on a Li foil. However, if the Li film is too thin, then cycling creates holes in the Li film that become occupied by SPE due to the applied pressure. The inhomogeneous Li plating is reduced but not completely suppressed when a pressure is applied to the cell, but is one the reason LPC cells are always used under pressure. This is illustrated in Fig. 3 with a 20-µm thick lithium film after cycling using the Hybrid-Electric Vehicle protocol (HEV_P) from the Department of Energy (DOE) and the United States Advanced Battery Corporation (USABC) with an applied pressure 35 psi (2.41 bars)

Lithium Isle SPE Bridge

SPE

SPE Bridge Width

Cathode

Lithium Isle Width

Li

20 um

Figure 3-a SEM Cross-section of a LVO lithium metal polymer battery (Li thickness 20µm) after cycling using HEV protocol (DOE) (40 K pulses) pressed at 35 psi (2.4 bar) illustrating the formation of SPE bridges between islands of lithium.


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Fig. 3b gives a 3-D schematic representation of the phenomenon observed in Fig 3a. We can clearly see a continuous film of Li with some holes filled with SPE (SPE-Bridge). The crosssection view (Fig. 3a) does not clearly show that the Li isles are connected.

Lithium

SPE Cathode Lithium Isle

Bridge

Figure 3-b: 3-D Schematic representation showing the holes filled with SPE in the Li film.


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The surface fraction of Li isles in the Li film as a function of the pressure is shown in Fig. 4a

Figure 4-a: Average number of Li isles per mm2 as a function of the nominal pressure applied to the cell. The symbols are experimental data, the linear broken line is a guide for the eyes. The measured pressure with applied pressure of 55 and 75 psi is 45 psi at the center of the cell which corresponds to the square. Its extrapolation to “zero isle” is at a pressure of circa 180 psi (12 bar.)

Since the pressure is not hydrostatic, the pressure at the centre of the cell is not the same as the applied pressure because of the non-perfect flexural rigidity of the pressure setup, especially at high pressures. Even when the external pressure is set at 55 and 75 psi (3.9 and 5.3 bar), the


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measured pressure in the middle of the cell is 45 psi. A linear extrapolation to zero gives a pressure of 180 psi (12.6 bar) which is impossible to reach in our experiments. Therefore, the non-uniform pressure cannot be avoided, and the presence of some holes in the Li film upon cycling is inevitable. The effect of pressure has also been reported for a LPC with LFP electrode [15]. It is also interesting to note that we have also found that the average distance between SPE bridges (circa 50 µm-200 µm) was not dependant on the pressure (see Fig. 4b), which suggests that it is a property of the Li film.

Figure 4-b: Normalized Distribution of the distances between SPE bridges (Li Isle width) for HEV_P cells cycled at different pressure (5, 15, 35, and 75 psi)

The formation of isles independent of the pressure suggests a reaction different near defects such as grain boundaries at the surface of the Li film, with the Li removed preferentially at these places


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to create holes. These defects generate a non-homogeneous electric field and may reduce the contact with the electrolyte; they are thus a source of degradation of the battery. In the following discussion, the initial thickness of the LVO electrode before cycling was 66 µm, while that of the LFP electrode was 95µm.

3. 2 Characterization of the LVO cell upon cycling In the following discussion, data were obtained for the LVO cell with a 39 µm-thick Li film and 66 µm-thick LVO film under a pressure of 35 psi before cycling. The in-situ SEM cross-section image is shown in Fig. 5-a before cycling.

lithium

39 µm 20 µm

SPE

Cycling apparatus

LVO Cathode

Figure 5-a: In-situ cross section image of the LVO cell cross-section (~1cm2) before cycling at a discharge rate of C/20 and a charge rate of C/12 +1h floating at 80°C. We also schematically represent the cycling apparatus. The complete video can be seen in the supporting information.

The dimensional changes upon cycling were recorded by video. (Fig 5 b). The high-speed video-1 is shown in video 1 in the supplementary information, where the time t and the voltage V(t) of the cell are indicated at the top left. The cycles consisted of discharge at C/12 rate + charge at C/20


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rate + 1h floating in the voltage ranges 2.3-3.1 V. The changes in the thickness of the different layers registered during the in-situ SEM experiments are presented in Fig. 5-b) .

Figure 5-b: Variations of the thickness of the different layers of the LVO cell upon cycling; the voltage is reported in parallel as a function of time to relate the variations of thickness to the cycling process.

The expansion of LVO upon cycling is negligible; that of the polymer is also very small, except during the first cycle where the SPE contracts by circa 8 µm. The reason, we believe, is that the cathode has some porosity before cycling under pressure, and the SPE fills the pores when the temperature is raised to 80 °C under pressure. After the first cycle, the SPE layer thickness is


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roughly constant, so the SPE no longer plays the role of a volume buffer. It is also interesting to note that the cathode thickness did not change significantly during cycling, indicating the LVO volume change (10%) is compensated by the elasticity of the binder and the voids between the active particles in the cathode. In contrast, the thickness of the Li0 film varies significantly, due to the transfer of lithium between the LVO and Li film. The LVO cell is fully charged before cycling, i.e. we begin the cycling experiments by extracting lithium from the Li0 layer to transfer to LVO. As a consequence, during this first discharge (down to the 100% depth of discharge), we can see in Fig. 5-b that the thickness of the Li film decreases from 39 to 33 µm. Then, upon charging, the thickness of the Li-film increases, as expected. What was less expected, however, is that at the end of the first charge, the thickness of the Li-layer increased to 42 µm (i.e. 3 µm thicker than before cycling), although no significant roughness is detected. This result suggests that lithium removed from the Li-metal during the first discharge deposits on the surface of the Li-metal as a porous and less compact form than the original Li-metal. With further cycling, the Li layer thickness oscillates more or less periodically, because Li transfer involves the same lithium ions in this slightly porous layer. Nevertheless, the thickness at the end of the discharge still increases slowly after multiple cycles, and the increase of the roughness of the surface layer of the lithium film, although small, never stops, reflecting the aging of the battery. The video-1 also gives evidence of the folding of the Li-film, which is increasing upon cycling. This effect is particularly visible on the video after 100 hours of cycling. It is reduced if not suppressed by using a negative electrode containing a current collector of expanded Al metal mesh that is able to accommodate lateral expansion to minimize changes in the plane of the electrode during alloy formation between Al and Li [25]. However, the expanded metal is detrimental to the energy density, a parameter that was a focus in the present work. The other noticeable effect is the dissolution of vanadium, which crosses the polymer electrolyte and creates


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a passivation layer at the interface with lithium. This is illustrated after 600 cycles in Fig. 6 where the thickness of the passivation layer reaches 5.4 µm.


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SP

Li

50 µm

SP

Figure 6: Cross-section of a LVO cell cycled 600 cycles showing an increase SEI layer between the SPE and the Li. This layer in rich in V species

The consequence is an increase of the area specific impedance that reaches 85 Ωcm2 in this case, and ultimately leads to the end of life of this battery at circa 1000 cycles. The kinetics process during cycling is shown in Fig.7.


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Figure 7: Variation of the Li thickness and of the voltage measured during the LVO in-situ experiment of Fig. 5-a during cycling. The Li plating/dissolution rate, the time difference between the end of the charge/discharge, and the decrease/increase in the Li thickness are also reported.

The usual features of the voltage curves for LVO are recovered [26], and discussed elsewhere [6] as follows. The open circuit voltage of Li1.2+xV3O8 drops rapidly to 2.85 V in the region x≤0.8 where the lithium is inserted into the interstitial sites Li(2). Then, the voltage drops less rapidly from 2.85 V to 2.7 V in the region 0.8≤x≤1.7 where the lithium is inserted into the interstitial space of the Li2V3O8 structure, possibly in the St(1) and St(2) tetrahedral sites. At x>1.7, the voltage curve shows a plateau at 2.5 V [20] characteristics of a two-phase system, with the coexistence of Li2.9V3O8 compound with the defect rock salt structure of nominal composition Li4V3O8. According to Fig.7, we find a very good agreement between the theoretical and the experimental Li deposition rates during the first charge: the charge current is Ic=0.09mA


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corresponding to the theoretical deposition rate 7.32nm Li/min, compared to 7.2 nm/min observed experimentally. This is expected since the Li film is not yet porous at this stage. In addition it shows that the coulombic efficiency is close to unity. In the subsequent cycles the formation of the porous surface layer of the Li film decreases the density inside this layer, which results in a faster time dependence of its thickness for the same amount of Li transferred to the lithium film: it increases to 8±1 nm per minute for the three next cycles, 8.9 nm per minute for the subsequent ones. The faster time dependence of the thickness of the Li-film during the discharge with respect to the charge process simply comes from the fact that the discharge is at C/12 rate while the charge is at C/20 rate. The discharge follows the same trends as the charge process, with an increase of the rate of variation of the thickness of the Li-film: the thickness of the film decreases at 13.6±0.3 nm per minute for the first cycles and 14.7±0.3 for the last cycles. One can also observe in Fig. 7 there is also evidence of a time shift between the start of a discharge and the time the Li thickness start to decrease. This time delay is larger than 100 minutes for the first cycles, suggesting that 1 hour float time is not sufficient to restore thermal equilibrium in the battery. This delay reduces to circa 1 hour float in the following cycles, giving evidence of the acceleration of the dynamics upon cycling. This is due to the increasing disorder of the surface layer of the vanadium compound. This disorder was investigated by TEM experiments illustrated in Fig. 8.


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33 nm

Figure 8: TEM micrographs of a 400 cycles cell together with selected area electron diffraction (SAED) of two regions on the same LVO particle showing an amorphous (1) and a crystalline region (2)

Before cycling, the LVO is fully crystallized. The selected area electron diffraction pattern (SAED) at two different areas labelled 1 and 2 of a particle after 400 cycles are shown in Fig. 8. This pattern shows that part 1 is amorphous while part 2 is still crystallized. This disorder weakens the short-range iono-covalent bonding between the ions inside the surface layer. Therefore, increasing disorder facilitates the extraction of the lithium from the surface layer of the vanadium compound, which explains the acceleration of the dynamics of the changes that occur with cycling. This is consistent with the better performance of the amorphous LVO cathode compared to that of the crystallized phase, as mentioned in the introduction. Indeed, Li4V3O8 is known to be less crystalline than Li1.2V3O8, due to the change in the a and b lattice parameters that contract and


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expand, respectively, by 10% upon lithium insertion [6]. This progressive disordering upon cycling ultimately gives rise to amorphization after further cycling. In the first few cycles, the highly crystalline structure is recovered, although a slight modification of the parent structure is observed [6]. We find in the present work that the crystallinity degrades irreversibly upon cycling, which leads to aging of the battery after many cycles. Amorphization is accompanied with dissolution of vanadium into the SPE, as seen in Fig. 9.

Figure 9; Measured Vanadium in the center of the SPE as a function of cumulated discharge capacity (mAh/cm2). The region with no cycling corresponds to the measurements in a half-cell, i.e. no Li film (small squares); therefore no electrochemical activity was present. The concentration of dissolved vanadium is also reported for two full cells prepared with LVO particles of two different average size of 0.3 µm (big triangle) and 1.5 µm (circles). The lines are guides for the eyes.

In this figure, vanadium dissolution in the center of the SPE was measured by quantitative energy


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dispersive analysis. The first rolled layer only contains the positive half-cell (no Li); therefore no electrochemical activity was present (reported has region with no cycling in Fig. 9). This layer did not show any appreciable V-dissolution. On the other hand, during cycling we note a linear increase of the concentration of vanadium dissolved into the SPE as a function of the cumulated discharge capacity. Since it is a surface effect, there is higher dissolution for the smaller particle size (0.3 µm vs 1.5µm).

3.3. Characterization of LFP upon cycling The SEM cross-section image of the cell before cycling is shown in Fig.10.

µm µm µm

Figure 10: In-situ cross section image of the LFP cross-section (~1cm2) before cycling at a charge and discharge rate of C/12 at 70°C. We also present the initial thickness of the anode (46 µm), SPE (29 µm) and cathode (92µm). The complete video can be seen in the supporting information.

The thickness of the LFP positive electrode was circa 90 µm, with zero porosity. This high density electrode is estimated to give a range of 350 km (45 kWh battery pack with 150 Wh/kg and 100


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km/h) according to our calculation at the pack level. The thickness of the Li-film metal negative electrode was circa 45 µm. The evolution of this cross-section upon cycling at a charge and discharge rate of C/12 between 2.2 and 3.9 V is shown in video 2 in the supplement, together with the state of charge of the cell. Folds in the Li-film are also observed upon cycling, like in the LVO cell. On the other hand, the video shows the emergence of a region of darker Li at the Li-SPE interface, which was not the observed with LVO. The difference is attributed to the fact that, in its initial state, the cell is discharged. The measurements of the electrochemical properties begin during charge, i.e. the new lithium is first extracted from LFP and transfers to the native surface of the Lio film. The new Li was never exposed to oxygen and then appears darker using backscattered electron (BSE) in the SEM. This surface is thus different from that experienced by the Li-ions arriving on the Lio film during the first charge of the LVO-LPC battery, where the first charge was preceded by a discharge process. The adherence of the Li-ions arriving at the surface of the Lio film is thus different in the two cases, and so is the disorder and porosity of the lithium deposited on the film. The consequence, however, is the same, namely slow Li release kinetics induced by the disorder and porosity upon delithiation, as shown in Fig. 11.


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Figure 11: Variations of the Li thickness and the voltage for the LFP in-situ presented in Fig 10. The symbols are experimental data and the linear broken line is a guide for the eyes. The increase in the Li dissolution kinetics at the end of discharge is clearly seen; which decreases with increasing cycle number. This figure also shows that the kinetics accelerates at the end of discharge, for the three first cycles only. One hypothesis is that at the end of discharge, the Li concentration x inside the LFP exceeds the critical concentration above which the solid solution is stabilized compared to the two-phase region at smaller x. In that case a larger diffusion coefficient of Li inside LFP would explain the results. However Fig.11 shows that the change in the Li thickness associated with this faster kinetics decreases with the cycle number and is already barely detectable at the 3rd cycle, which is in contradiction with the fact that the capacity remains almost unchanged. We are thus led to attribute the faster kinetics at the end of discharge to the fact that most of the lithium deposited on


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the Li-film during the previous charge (minus the irreversible capacity) has already been reinserted into the positive electrode, so that some Li from the well-crystallized Li metal beneath the initial surface of the Li film is extracted with faster dynamics. No Fe dissolution or amorphization of Nano C-LiFePO4 were observed upon cycling. This was expected, because LFP can be cycled for about 30 000 cycles at 100 % depth of discharge without any deterioration even at high C-rates [27-28]. The substitution of graphite by Lio implies the battery must be used at low C-rate only and the consequence is that the experimental time to explore aging over tens of thousands of cycles takes many years, which we consider as impracticable in the laboratory. Indeed, LiFePO4/SPE/Li-metal polymer batteries made by Bathium® equipping the Bluecar of the Bolloré group has a lifetime of 400 000 km [29]. Since the range of the battery of the Bluecar is 250 km on average, 400 000 km amount to 2000 cycles. For an average 100 km drive per day, the distance is achieved after about 11 years. The difference of aging between LFP and LVO that has investigated in this work is best summarized in Fig. 12 where we show the capacity of both cells cycled at the same conditions (cycles at C/3 charge and discharge rates, 1hour float). Near 50% capacity was already lost with LVO after 50 cycles, while the capacity is still maintained at 130 mAh g-1 after 1400 cycles with LFP.


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Figure 12: Cycle life of LVO and LFP polymer cells (cycles at C/3 charge and discharge rates, 1h float).


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4. Conclusions The in-situ SEM characterization of lithium metal polymer batteries with Nano C-LiFePO4 (LFP) and Nano Li1.2V3O8 (LVO) are reported. The LVO battery ages due to the additive effects of a progressive deterioration of the crystallinity of the LVO and the dissolution of the vanadium in the electrolyte, while no such effects were observed in the case of LFP. We also found a difference in kinetics during Li extraction for the LFP since new lithium is first extracted from the LFP and plated on the surface of the Lio film which could contribute positively to stabilize the lithium interface. On the other hand, the soluble species produced with LVO will reach the Li interface on cycling and destabilize it and increase its area surface impedance. Although the energy capacity of LVO is much larger than that of LFP, the much longer cycle life and calendar life of the LFP battery justifies its choice by different companies for different uses, including electric vehicles. In addition, LFP is also a far greener and sustainable material.

Methods Electrode, electrolyte and battery cell preparation. Two different cathode electrodes are made (experimental part): LVO: binder (Polyether (PE)LiTFSI): AB 70:25:5; LFP: binder (PE-LiTFSI): AB:70:25:5 and LFP: binder (PE-LiTFSI): AB¼54:39:7, with average thickness of about 45 and 90 m, respectively, and coated on the carbon-coated aluminum current collector. The solid polymer electrolyte (SPE) is mixture of polyether (Hydro-Québec) and LiTFSI (3M) salt (molar ratio of Li/O 1/30) that are dissolved in acetonitrile(Aldrich). Irgacure (Aldrich) is added as the photoinitiator (1% by weight with PE ) in


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the slurry. After evaporation of acetonitrile, the composite electrolyte and cathode is also dried overnight at 80°C under vacuum. . The solid polymer SPE is used in the binder in the cathode and as the separator between cathode and anode. After punching defined area (3.8 cm2) of cathode anode and SPE, the electrodes and separator are assembled into a (Li/SPE/cathode) cell. The cell is pressed under vacuum and high temperature to produce uniform interfaces between SPE and cathode and lithium metal. The cell is thermo-sealed between metal plastic packaging. The temperature of cell is between 60 to 80°C during the experimental tests. Electrochemical cycling. The cells were equilibrated at open circuit for 3 h before testing. The lithium cells were galvanostatically discharged at various current densities to either a lower voltage cut off of 2.5 V vs. Li/Li+ or a capacity (mAh) cut off, and charging was cut off with an upper voltage of 4. V for LFP. The voltage range was 2.2–3.5V versus Li/Li+ for LVO, using a battery cycler (Bitode, USA). Impedance measurements were performed using a VMP3 potentiostat/galvanostat with EIS/Z capabilities and EC-Lab_ software (Bio-Logic Science Instruments, France).

Competing financial interests The authors declare no competing financial interests.


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References 1. M. B. Armand, J.-M. Chabagno and M. J. Duclot, in Fast Ion Transport in Solids: electrodes, and electrolytes : Proceedings of the International Conference on Fast Ion Transport in Solids, Electrodes, and Electrolytes, Lake Geneva, Wisconsin, U.S.A., edited by P. Vashista, J.N. Mundy annd G.K. Shenoy, North-Holland, New York, 1979, p. 131-136. 2. M. Gauthier, D. Fauteux, G. Vassort, A. Belanger and M. Duval, I. Ricoux, J.-M. Chabagno, D. Muller, P. Rigaud, M. B. Armand and D. Deroo, J. Power Sources 14 (1985) 23-26. 3. M. Gauthier, A. Bé1anger, P. Bouchard, B. Kapfer, S. Ricard, G. Vassort, M. Armand, J. Y. Sanchez, L. Krause, J. Power Sources 54 (1995) 163-169. 4. For a review, see A. Mauger, C. M. Julien, Ionics 20 (2014) 751-787. 5. J.O. Besenhard, R. Schöllhorn, J. Power Sources 1 (1976/1977) 267-276. 6. L.A. Picciotto, K.T Adendorff, D.C. Liles, M.M. Thackeray, Solid State Ionics 62 (1993) 297307. 7. K. West, B. Zachau-Christiansen, J. Electrochem. Soc. 143-3 (1996) 820-825. 8. F. Bonino, M. Ottavoni, B. Scrosati, G. Pistoia, J. Electrochem. Soc. 135-1 (1988) 12-15. 9. Z. Liu, Q. Yao, L. Liu, J. Power Sources 45 (1993) 15-19. 10. A. Manthiram, J.B. Goodenough, J. Solid State Chem. 71 (1987) 349-360. 11 Electrode Materials with High Surface Conductivity, US Patent No. 6,855,273 B2 12. K. Zaghib, A. Guerfi, P. Hovington, A. Vijh, M. Trudeau, A. Mauger, J.B. Goodenough, C.M. Julien, J. Power Sources 232 (2013) 357-369.


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13. K. Zaghib, A. Mauger, H. Groult, J.B. Goodenough, C.M. Julien Materials 6-3 (2013) 10281049. 14. K. Zaghib, A. Mauger, J. Goodenough, C.M. Julien, ‘Design and Properties of Nano CLiFePO4 Nano-materials for High-Power Applications, in Nanotechnology for Li-ion Batteries, D. Lockwood, ed. (Springer Verlag, Berlin, 2013) Chapter 8, 179-220. 15. K. Striebel, A. Guerfi, J. Shim, M. Armand, M. Gauthier, K. Zaghib, J. Power Sources 119-121 (2003) 951-954. 16. K. Nassau, D.W. Murphy, J. Non-Cryst. Solids 44 (1981) 297-304. 17. G. Pistoia, M. Pasquali, G. Wang, L. Li, J. Electrochem. Soc. 137 (1990) 2365-2370. 18. J. Dai, S.F.Y. Li, Z. Gao, K.S. Siow, J. Electrochem. Soc. 145 (1998) 3057-3062. 19. J. Kawakita, Y. Katayama, T. Miura, T. Kishi, Solid State Ionics 110 (1998) 199-207. 20. J. Xie, J. Li, H. Zhan, Y. Zhou, Mat. Lett. 57 (2003) 2682-2687. 21. P. Hovington, M. Dontigny, A. Guerfi, J. Trottier, M. Lagace, A. Mauger, C.M. Julien, K. Zaghib, J. Power Sources 248 (2014) 457-464. 22. K. Zaghib, P. Hovington, M. Lagacé, A. Guerfi, P. Charest; New Trends in Intercalation Compounds for Energy Storage and Conversion in : K. Zaghib, C.M. Julien, J. Prakash (Eds.), ECS Proceeding Volume PV 2003-20, , The Electrochemical Society, Pennington, NJ (2003), p. 670. 23. K. Zaghib, M. Armand, M. Gauthier, J. Electrochem. Soc., 145 (1998) 3135-3140.


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24. K. Zaghib, M. Armand, M. Gauthier, in: J. Broadhead, B. Scrosati (Eds.), Proceedings of the Symposium on Lithium Polymer Batteries, The Electrochemical Society, Pennington, NJ (1997), p. 250-264. 25. K. Zaghib, M. Gauthier, M. Armand, J. Power Sources, 119-121 (2003) 76-83. 26. L.A. De Picciotto, M.M. Thackeray, G. Pistoia, Solid Stat. Ionics 28-30 (1988) 1364-1370. 27. K. Zaghib, M. Dontigny, A. Guerfi, P. Charest, I. Rodrigues, A. Mauger, C.M. Julien, J. Power Sources 196 (2011) 3949-3954. 28.K. Zaghib, M. Dontigny, A. Guerfi, J. Trottier, J. Hamel-Paquet, V. Gariepy, K. Galoutov, P. Hovington, A. Mauger, H. Groult, C.M. Julien, J. Power Sources 216 (2012) 192-200. 29. French National Chamber of Commerce (2014) http://www.blog-ccfcmtl.com/la-batterie-aulithium-une-revolution-pour-les-voitures-electriques/. Acknowledgement : The authors acknowledge Hugues Marceau for all of the quantitative images analysis of the in-situ LFP experiment and M. Dontigny , D. Clement , J. Hamel, S. Verreault, N. Turcotte, M. Simoneau, C. Baril, L. Gastonguay, B. Kapfer, Y. Choquette and J. Cloutier from IREQ for their technical support. Supporting Information Video1-2: Two videos have been added. The high-speed Video 1 (Video1.avi) The high-speed video-1 shows the evolution of the cell cross-section of the Li1.2V3O8 / polymer / lithium cell upon cycling. The cycles consisted of discharge at C/12 rate + charge at C/20 rate + 1 h floating in the voltage ranges 2.3-3.1 V. The time t and voltage V(t) of the cell during this process are indicated at the top left.


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The Video 2 (Video2.avi) shows the evolution of the cell cross-section of the LiFePO4 / polymer / lithium cell upon cycling at a charge and discharge rate of C/12 between 2.2 and 3.9 V, together with the monitoring of the cell state of charge. This material is available free of charge via the Internet at http://pubs.acs.org.


New Lithium Metal Polymer Solid State Battery for ... - ACS Publications

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