Maarit Atomic / Molecular Layer Deposition of Lithium Terephthalate Thin Films as High Rate Capability Li-Ion Battery Anodes

Content Provider	Semantic Scholar
Author	Nisula, Mikko Karppinen, Maarit
Copyright Year	2016
Abstract	We demonstrate the fabrication of high-quality electrochemically active organic 9 lithium electrode thin films by the currently strongly emerging combined atomic/molecular layer 10 deposition (ALD/MLD) technique using lithium terephthalate, a recently found anode material for 11 lithium-ion battery (LIB), as a proof-of-the-concept material. Our deposition process for Li12 terephthalate is shown to well comply with the basic principles of ALD-type growth including the 13 sequential self-saturated surface reactions, a necessity when aiming at micro-LIB devices with 3D 14 architectures. The as-deposited films are found crystalline across the deposition temperature range 15 of 200 – 280 °C, which is a trait highly desired for an electrode material but rather unusual for 16 hybrid organic-inorganic thin films. Excellent rate capability is ascertained for the Li-terephthalate 17 films with no conductive additives required. The electrode performance can be further enhanced 18 by depositing a thin protective LiPON solid-state electrolyte layer on top of Li-terephthalate; this 19 yields highly stable structures with capacity retention of over 97 % after 200 charge/discharge 20 cycles at 3.2 C. 21 22 23 The miniaturization of electronic devices demands for energy storage systems of equal 24 dimensions. In order to retain reasonable energy and power densities, all-solid-state thin-film 25 microbatteries based on three-dimensional (3D) microstructured architectures are seen as a viable 26 solution. Compared to 2D thin-film batteries, the increased specific surface area of 3D 27 microstructures provides us with enhanced energy density while the electrodes can still be kept 28 thin enough for short Li diffusion paths and thereby good power density.1–3 Such an approach 29 places an apparent need for a thin-film deposition method capable of manufacturing the electrode 30 and electrolyte materials on high-aspect-ratio substrates. Atomic layer deposition (ALD) is an 31 established thin-film technology for producing conformal coatings on such high-aspect-ratio 32 structures.4,5 It is based on sequential exposure of gaseous precursors on the target substrate where 33 surface-saturation limited reactions allow the layer-by-layer deposition of high-quality thin films 34 with sub-monolayer accuracy.6 However there is an apparent need for broadening the currently 35 rather narrow range of available deposition processes for Li-ion electrode materials. 36 Organic electrode materials would possess several attractive features compared to the current 37 transition-metal based inorganic materials. They are composed of cheap, earth-abundant, 38 environmentally friendly and light elements, and owing to the low molecular mass together with a 39 possibility for multiple redox processes per molecule, organic electrodes display very high 40 theoretical specific capacities of several hundred mAh per gram. Moreover, the redox properties 41 can be tuned by the addition of electron donating/withdrawing functional groups. The biggest 42 obstacles in putting organic electrode materials into practical use in next-generation LIBs are their 43 instability/dissolution in the commonly employed liquid electrolytes and their negligible electronic 44 conductivity which dictates the need for very large amounts of conductive additives resulting in 45 greatly reduced actual capacities.7,8 In an all-solid-state thin-film LIB these obstacles could 46 possibly be circumvented: the dissolution issue would be completely avoided by replacing the 47 liquid electrolyte by a solid one, while the reduced dimensions in thin films should contribute 48 towards mitigating the effect of intrinsically poor electronic conductivity of organics. 49 Owing to the recent progress in combining the ALD technique for inorganic materials with the 50 strongly emerging molecular layer deposition (MLD) technique for organics it has become 51 possible to fabricate inorganic-organic hybrid thin films in a well-controlled atomic/molecular 52 layer-by-layer manner; for a recent review of the combined ALD/MLD technique see ref. 9. A 53 number of ALD/MLD processes with different organic constituents have already been developed. 54 However, the range of the metal components is yet limited, and as far as we know no ALD/MLD 55 processes for lithium-organic thin films have been reported. Here we demonstrate that the 56 ALD/MLD technique indeed is commendably suited for the deposition of organic LIB electrodes; 57 our proof-of-the-concept data are for lithium terephthalate (Li2C8H4O4 or LiTP). The 58 electrochemical activity of bulk LiTP was discovered by Tarascon et al.;10 their data revealed high 59 gravimetric capacity of 300 mAh/g associated with a flat reduction potential at around 0.8 V vs. 60 Li+/Li. As such LiTP is indeed an attractive anode material as it offers considerably higher specific 61 energy compared to other electrode material candidates for thin-film LIBs including TiO2 and 62 Li4Ti5O12. Moreover, computational predictions indicate that the volume change of LiTP during 63 (de)lithiation is relatively small, i.e. ~6 %.11 Hence LiTP would be preferable also over the 64 traditional high-capacity anode materials such as silicon and transition-metal oxides for which the 65 large volume expansion during lithiation results in poor cycle life of the material.12 The major 66 findings of the present study are three-fold. First, we succeeded in fabricating crystalline organic 67 LIB electrode thin films for the first time through a gas-phase deposition technique. Secondly, the 68 films were found to be electrochemically active with excellent rate capability without relying on 69 any conductive additives thus demonstrating that LiTP can perform as a high-rate anode material 70 if the electronic conductivity issue can be overcome. Lastly we show that by applying a protective 71 layer consisting of the lithium phosphorus oxynitride (LiPON) solid-state electrolyte deposited by 72 ALD, the LiTP electrodes can be stabilized even in LiPF6-based liquid electrolytes resulting in an 73 excellent cycle life. 74 The LiTP thin films were deposited using Li(thd) (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) 75 and terephthalic acid (TPA) as precursors. The defining feature of an ALD/MLD process is the 76 self-limiting film growth, that is, after a certain threshold value, the growth-per-cycle (GPC) 77 calculated from the resultant film thickness value (determined in our case by spectroscopic 78 ellipsometry) becomes constant regardless the pulsing times of the precursors. To verify the 79 ALD/MLD-type film growth, the deposition rate for our Li(thd)–TPA process was studied as a 80 function of the precursor pulse lengths at a deposition temperature of 200 °C using 200 ALD/MLD 81 cycles. As shown in Figure 1a, in the case of Li(thd), saturation is achieved with a pulse length of 82 4 s whereas TPA requires a longer pulse length of 10 s. With these optimized pulse lengths the 83 saturation-limited growth rate at 200 °C is ~3.0 Å/cycle. The saturation limited growth was further 84 investigated by depositing LiTP thin films on microstructured silicon substrates consisting of 85 trenches ~50 μm deep and 7.5 μm wide using the aforementioned pulse and purge lengths with 86 400 deposition cycles. As shown in Figure S1 in Supporting Information, essentially conformal 87 films are achieved even with parameters optimized for planar substrates. 88 Using the same pulse lengths, the growth rate was further studied in a temperature range of 200 89 – 280 °C using 400 ALD/MLD cycles. From Figure 1b, no region of constant GPC value, i.e. a 90 so-called ALD window, is observed; instead there is a rather monotonous decrease in GPC with 91 increasing deposition temperature which is actually a common feature for a majority of ALD/MLD 92 processes.9,13,14 The density of the films, as obtained from the X-ray reflectivity (XRR) data, 93 appears to remain essentially constant in the deposition temperature range of 200 – 240 °C (Figure 94 1b). At temperatures higher than this, the decrease in density might arise from thermal 95 decomposition of either of the precursors resulting in inclusion of carbon impurities. Within the 96 uniform density region, the resultant film density of ~1.4 g/cm3 is in a rather good agreement with 97 the ideal density of bulk LiTP (1.6 g/cm3) calculated from its crystal structure proposed in ref. 18. 98 As shown in Figure 1c, instead of the expected linear relationship between the film thickness and 99 the number of deposition cycles, the growth rate increases with increasing number of deposition 100 cycles. The growth rate nears constant after 200 deposition cycles. Simultaneously, an opposite 101 trend is seen in the film density which decreases until reaching a stable value of 1.4 g/cm3 after 102 200 deposition cycles. The film thicknesses up to 100 deposition cycles were crosschecked using 103 X-ray reflectivity (XRR) measurements (Table S1). As the roughness of the samples prevented the 104 use of XRR on the thicker samples, the thickness of the sample with 400 deposition cycles was 105 measured also from a scanning electron microscopy (SEM) cross-section image. 106 Atomic force microscopy (AFM) image taken after 70 deposition cycles (Figure 2a) shows 107 distinct granular shapes with voids in between. The average feature height of 20 nm matches well 108 with the film thickness obtained with ellipsometry. In the AFM image taken after 400 deposition 109 cycles (Figure 2b) the granular features have gained in size and coalesced forming a more 110 continuous film. In between the granules there appear to be deep voids and the overall roughness 111 of the sample is quite high. As such, the nonlinearity of could possibly be explained by the island 112 growth model of crystalline films.15,16 The initial nucleation and growth is not uniform; instead, 113 distinct islands are formed, which grow in size as th
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