LAGP|Li Interface Modification through a Wetted Polypropylene

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LAGP#Li interface modification through wetted polypropylene interlayer for solid state Li-ion and Li-S batteries Dasari Bosubabu, Jeevanantham Sivaraj, Ramakumar Sampathkumar, and Kannadka Ramesha ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00301 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 12, 2019

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ACS Applied Energy Materials

LAGP│Li interface modification through wetted polypropylene interlayer for solid state Li-ion and Li-S batteries

Dasari

Bosubabua,b,

Jeevanantham

Sivaraja,

Ramakumar

Sampathkumara,

K.Rameshaa,b* aCSIR-Central

Electrochemical Research Institute-Chennai Unit, CSIR-Madras Complex, Taramani, Chennai 600113, India bAcademy

of Scientific and Innovative Research (AcSIR), CSIR-CECRI, Karaikudi 630003,

India.

ACS Paragon Plus Environment

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

Abstract The development of solid state batteries is constrained by imperative factors like high reactivity of lithium with solid electrolytes, high grain boundary resistance of solid electrolyte and interfacial resistance between solid electrolyte and electrodes. In the present work we exploited Li1.5Al0.5Ge1.5(PO4)3 (LAGP) as solid electrolyte and noticed a spontaneous reactivity of lithium with LAGP. Further, we observed that an introduction of wetted polypropylene (PP) layer between LAGP and lithium has successfully prevented the undesirable reaction between lithium and surface of solid electrolyte. Besides, it has considerably decreased the interfacial resistance and polarization. This strategy has been applied to both Li-ion and Li-S battery systems and we observed considerable improvement in electrochemical performance of these solid state devices. The lithium ion battery retained 100% capacity and coulombic efficiency after 50 cycles with observed capacity of 190 mAhg-1. After 200 cycles with varying C-rates, 98.5 % capacity retention is observed. Similarly, in Li-S battery, LAGP effectively restricted the polysulfide shuttle and maintained 78% of initial capacity after 200 cycles. Such high capacity retention is the outcome of the PP layer which functions as interlayer protecting LAGP surface against reacting with Lithium metal anode as inferred by postmortem analysis.

Keywords: Li-S battery, Li-ion battery, LAGP, Solid electrolyte, interlayer,


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1. Introduction Lithium-ion batteries are the most efficient and highly marketed rechargeable batteries at present. These batteries have been employed in numerous applications, such as consumer electronics, stationary energy storage systems, electric /hybrid vehicles etc.1–3 However, Liionbatteries encounter setbacks such as capacity loss due to resistive layer at the interface (SEI), electrolyte decomposition at higher voltages and inherent safety issues.4These batteries upon repeated cycles results in lithium dendrite growth followed by cell shortening.5This situation is further aggravated by highly flammable organic liquid electrolytes having low thermal stability and low flame point that might produce explosion or fire.6–8 Besides, lithium-ion batteries, the Li-S batteries are also confronted with thermal runaway issue since they utilize organic liquid electrolytes. Furthermore in Li-S battery systems, during discharge process sulfur is converted to multiple polysulfides and some of them (higher order polysulfide) dissolve in the organic electrolyte which leads to polysulfide shuttling between electrodes.9–11 This shuttle effect creates sequel reactions such as active material loss, capacity decay and self-discharge of the cell. Nevertheless, controlling of this shuttle effect remains highly challenging with the existing organic electrolytes. It might be significantly beneficial to replace organic electrolytes with other electrolytes which exhibit high Li-ion conductivity and good cycle life.12–14 The development of all-solid-state batteries (ASSBs) has become an inevitable progression because solid state electrolytes provide high thermal stability, good electrochemical stability over large electrochemical window, low self-discharge (nanoamps), high cycle life and non-flammability.15–17On the other hand, some major challenges derail the development of solid state batteries including high interfacial resistance between solid electrolyte and electrodes and high grain boundary resistance of solid electrolyte ensuing poor performance.18,19 Amongst a



variety of Li-ion conducting solid electrolytes, the garnet and NASICON phases are greatly promising due to enhanced Li-ion conductivity, better chemical stability and high electrochemical window with respect to lithium.20–22 However, garnet electrolytes suffer from issues such as huge contact resistance with lithium and instability at ambient atmosphere owing to surface reactivity.23–25In this regard, NASICON structures of the formula LiM2(PO4)3 (M = Ti, Ge, Zr, Sn, Sr, etc.) motivated substantial interest, particularly the Li1.5Al0.5Ge1.5(PO4)3 (LAGP) system, due to high ionic conductivity (10−3 S cm -1 ) at room temperature which is close to that of liquid electrolytes (10−2 S cm -1), wide voltage range (up to 6 V(vs. Li+/Li)) and good chemical stability.26–32 Though LAGP possess good features for all solid state batteries, some reports show that LAGP exhibit instability with Li contact. Cui et al. reported that on contact with Li foil, a black color substance was formed at the LAGP/Li interface and the reaction zone induces creaks in LAGP.33Liu etal. showed that in addition to high interfacial resistance of LAGP, upon lithium contact the germanium in LAGP get reduced from Ge4+ to Ge2+ and Ge0.34,35 It is now mandatory to avoid direct contact of LAGP with lithium. In the present work, we have demonstrated a strategy to protect LAGP surface from Li anode using polypropylene (PP) interlayer. It is found that the PP layer between LAGP and lithium anode can simultaneously impede LAGP corrosion and minimize interfacial resistance. This protected LAGP substantiates remarkable stable performance both in solid state Li-ion and solid state Li-S batteries with negligible capacity decay over 200 cycles. 2. Result and Discussion Figure 1(a) shows the measured XRD pattern of LAGP membrane together with the standard pattern of LiGe2(PO4)3 (JCPDS 41-0034) where 0.5Ge4+ replaced by 0.5Al3+.36,37Apart


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from small amount of AlPO4 (Figure 1(b)) the XRD pattern confirms the formation of NASICON structure of LiGe2(PO4)3which could be evidenced by sharp diffraction peaks (Figure 1(c)). Figure 1 (d) shows the cross-sectional SEM image of LAGP which ratifies that the membrane thickness is ~206.5 µm. The corresponding elemental mapping (Figure1 (e-h)) reveals the uniform distribution of Al, Ge, P and O in the sample. The Figure 1 (i) depicts the energy dispersive X-ray spectra (EDX) of LAGP which confirms the presence of Al, Ge, P and O. The polypropylene (PP) layer is a thermoplastic polymer exhibiting semi-crystalline nature. The crystal structure of PP is analyzed by XRD and the corresponding lattice planes (110), (040), (130) in the XRD confirms that the PP adopts monoclinic structure (Figure S1(a)). The SEM images (Figure S1(b)) of the PP layer revealed a homogeneous inter connected submicron pores distributed in the range of 100-300 nm. Moreover the shapes of pores appeared to be fibrillary and lamellar structures that might be generated by the stretching and annealing of the polymer during synthesis.

Electrochemical characterization A symmetric cell Li│LAGP│Li was fabricated using swagelock type cell by placing Li foil on both sides of LAGP solid electrolyte. A similar symmetrical cell with poly propylene (PP) layer as an interface between Li and LAGP denoted as Li│PP-LAGP-PP│Li is also fabricated. To improve the interfacial contact, PP membranes were wetted with minimal amount of liquid electrolyte (10 µL) containing 1.0 M LiPF6 in ethylene carbonate: dimethyl carbonate: diethyl carbonate (2:1:1 V/V%). Figure 2 (a) represents the Nyquist plots of Li│LAGP│Li and Li│PPLAGP-PP│Li measured using AC impedance analyzer. The impedance spectrum of Li│LAGP│Li (red color) exhibits a semi-circle at lower frequency and resistance value extracted



from the data is 930 Ω cm2 which can be divided into two parts viz., the resistance of LAGP (total resistance = 87 Ω cm2) and the other one is the interfacial resistance between LAGP-Li foil (843 Ω cm2). There exist two symmetric interfaces therefore, the single Li foil-LAGP interfacial resistance is approximately calculated by dividing by a factor of two and is found to be 421 Ω cm2. The observed high interfacial resistance can be attributed to poor solid-solid interfacial contact between lithium and LAGP solid electrolyte. On the other hand the impedance spectrum of Li│PP-LAGP-PP│Li (black color) displays two semi-circles. The semi-circle at higher frequency corresponds to the total resistance of LAGP (87 Ω cm2) and the semicircle at the lower frequency corresponds to the interfacial resistance between Li foil and LAGP (189 Ω cm2). By subtracting the resistance of LAGP (87 Ω cm2) and dividing by a factor of two the Li│PP-LAGPPP│Li can be calculated as 51 Ω cm2. A drastic reduction in the interfacial resistance of Li│PPLAGP-PP│Li symmetric cell is clear evidence that the polypropylene film creates a homogeneous contact between Li foil and LAGP membrane. Moreover the PP layer acts as a physical barrier to direct contact of LAGP surface with the lithium metal foil. For comparison, the ionic conductivity of the PP layer wetted with liquid electrolyte (LE) was measured by constructing a symmetrical cell Li│LE-PP│Li (Figure (S3)). The conductivity of the wetted PP layer was found to be 3.03 x 10-3 S cm-1. Figure 2(b) shows DC polarization study on Li│LAGP│Li and Li│PP-LAGP-PP│Li symmetric cells in the voltage range 0.5V to -0.5V. The positive and negative voltage signifies lithium stripping and plating. The experiments were carried out at different current rates i.e., 50, 100, 150, 200 and 250 µA and the corresponding potential data were collected for duration of 500 sec for every current rate. The Li|LAGP|Li symmetric cell showed large voltage polarization values of 100, 201, 309, 380, 477 mV for the corresponding current rates of 50, 100, 150, 200


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and 250 µA. The observed large polarization values signify an uneven Li+ transport across the Li foil and LAGP interface at the measured current densities. Poor contact between lithium foil and LAGP membrane is supposed to be the prime cause for large currents to accumulate at the interface leading to higher cell polarization. Whereas, the Li│PP-LAGP-PP│Li symmetric cell exhibits relatively stable stripping/plating with lower polarization values of 26, 56, 78, 134, 193 mV respectively at measured current rates. The presence of PP interlayer between LAGP and Li foil in Li│PP-LAGP-PP│Li symmetric cell creates a good interfacial contact and found to be the reason for the observed lower polarization compared to the Li│LAGP│Li cell. The photographic images of LAGP and Li metal foil before and after contact for 30 min are shown as Figure 3 (a) and Figure 3 (b), respectively. The observed black colored spots on the LAGP surface confirm that on contact with lithium metal, LAGP surface spontaneously get reduced.35,38 To further confirm this, after polarization studies the Li│LAGP│Li symmetric cell was opened inside glove box. The observed black colored area around the center of LAGP film ratifies reduction of LAGP surface (Figure 3 (c)). The low magnification SEM imaging of LAGP membrane indicate reacted and unreacted regions with dark and white contrast as shown in Figure 3 (e). Inset shows fragment of pellet where the black color region indicates reacted zone and the white area corresponds to unreacted zone. The SEM image of unreacted zone (Figure 3 (d)) under higher magnification revealed presence of well-defined cubic particles with sharp edges similar to the one observed for pristine LAGP. However, the SEM image of reacted zone exhibited irregular shaped crystals (Figure 3 (f)).



Fabrication and performance evaluation of surface protected LAGP as solid electrolyte in Lithium ion and Li-S cells The electrochemical cycling performance of LAGP solid electrolyte with and without surface protection (PP layer) was evaluated by constructing Li-ion and Li-S cells in argon filled glovebox where O2 and moisture levels were strictly restricted to

LAGP|Li Interface Modification through a Wetted Polypropylene

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