Supplementary Materials for

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1 advances.sciencemag.org/cgi/content/full/3/5/e /dc1 Supplementary Materials for Electroplating lithium transition metal oxides Huigang Zhang, Hailong Ning, John Busbee, Zihan Shen, Chadd Kiggins, Yuyan Hua, Janna Eaves, Jerome Davis III, Tan Shi, Yu-Tsun Shao, Jian-Min Zuo, Xuhao Hong, Yanbin Chan, Shuangbao Wang, Peng Wang, Pengcheng Sun, Sheng Xu, Jinyun Liu, Paul V. Braun This PDF file includes: Published 12 May 2017, Sci. Adv. 3, e (2017) DOI: /sciadv section SI. Plating bath and solubility of transition metal oxides. section SII. Mesoporous carbon foam. section SIII. Flexible carbon scaffold. section SIV. Quasi-reference electrode. section SV. Thermodynamic modeling. section SVI. Crystallography of LiCoO2. section SVII. Scanning electron nanobeam diffraction. section SVIII. Flexible battery. section SIX. Electrodeposition of spinel LiMn2O4. section SX. Electroplating of Al-doped lithium cobalt oxide. section SXI. Calculation of energy density of flexible batteries. fig. S1. Schematic illustration of a flexible CNF. fig. S2. High-resolution TEM images of the CNF. fig. S3. High-resolution TEM image and electron diffraction pattern of an electroplated LiCoO2 crystal flake. fig. S4. Crystallographic structures of O3-, O2-, and spinel-phase lithium cobalt oxides and two superstructures with lithium staging and 2 2 periods. fig. S5. Illustration of the SEND technique. fig. S6. Cross-sectional SEM image of ~200-μm-thick LiCoO2 electroplated on an Al foil. fig. S7. Charge/discharge voltage profiles of the CNF anode. fig. S8. Charge/discharge curves of a LiCoO2/CNF flexible battery. fig. S9. Optical images of bending tests.

2 fig. S10. Schematic illustrations of the structure difference between traditional and electroplated flexible batteries. fig. S11. SEM images of a LiCoO2/CNF cathode before and after 1000 bending cycles. fig. S12. Materials and electrochemical characterization of the electroplated LiMn2O4/CNF battery. fig. S13. The Gibbs free energy of the formation from the elements for LiMnO2. fig. S14. The Gibbs free energy of LiMnO2. fig. S15. The potential-ph2o diagram of the LiOH-KOH-MnO-H2O melt system. fig. S16. Materials and electrochemical characterization of the electroplated Aldoped LiCoO2. table S1. Thermodynamic data used for the LiOH-KOH-CoO system at 260 C. table S2. Thermodynamic data of the LiOH-KOH-MnO system at 25 C. table S3. Thermodynamic data of the LiOH-KOH-MnO system at 300 C. References (42 82)

3 section SI. Plating bath and solubility of transition metal oxides. The mixture of KOH and LiOH is milled with a mortar and pestle and placed in a crucible. The operation is conducted in an Ar-filled glovebox. It is then heated at 260 C until the mixture becomes transparent. When CoO is added into the melt, the solution turns blue due to the formation of Co(OH)4 2- (42). The solution is yellow after adding MnO. We measured the solubility of CoO, MnO and Al(OH)3 in the LiOH/KOH eutectic at 260 C to be 15 g, 10 g, and 5 g, per 100 g of the eutectic, respectively. section SII. Mesoporous carbon foam. The mesoporous carbon foam (Part No. XF500) is synthesized and provided by Xerion Advanced Battery Corp. The foam has an ~ 600 nm pore size (measured by a Nova 2200e Porometer, Quantachrome), ~ 85% porosity, and surface area of ~ 50 m 2 /g (measured by BET, Quantachrome). section SIII. Flexible carbon scaffold. The carbon nanofiber (CNF) scaffold is purchased from Applied Science Inc. As illustrated in fig. S1, it is composed of a carbon fiber (~ 20 m diameter) backbone as the mechanical support and carbon nanofibers (~ 200 nm diameter) to provide high surface area for achieving desirable active material loading. This composite scaffold offers great flexibility and maintains excellent mechanical integrity during bending.

4 fig. S1. Schematic illustration of a flexible CNF. Fig. S2a-b shows that the CNFs contain highly aligned graphitic structures with a 0.34 nm interlayer spacing. This allows direct use of the CNF scaffold as an anode (220 mah/g) in the flexible battery. Similarly as graphite, lithium ions can repeatedly intercalate into the interlayers of CNFs during cycling. The lithiation and delithiation voltage profiles of CNFs are shown in fig. S7. The highly conductive CNF scaffold allows a very conformal deposition of LiCoO2 throughout the entire fiber network (fig. S2c-d). As shown in fig. S2e, this LiCoO2/CNF electrode shows a well-defined electrochemical characteristic of LiCoO2.

5 a b c d 100 µm 5 µm e f fig. S2. High-resolution TEM images of the CNF. (a-b) HR-TEM images of the CNF used as the cathode current collector and the anode active material. (c) Cross-sectional SEM and (d)

6 higher-magnification images of the LiCoO2/CNF electrode. (e) dq/dv curve of the LiCoO2/CNF electrode. section SIV. Quasi-reference electrode. It is difficult to find an ideal reference electrode for the molten hydroxide electrolyte (43). However, a quasi-reference electrode with fast ion exchange kinetics can be used. For example, platinum is often used as a quasi-reference electrode, which has a H + /H2 exchange current density of 1 ma cm -2. We perform CV scans on a cobalt wire in the molten LiOH-KOH system and find that the exchange current density of Co/Co 2+ at 260 C is 73.6 ma cm -2, which is sufficiently fast as to serve as a reference electrode. Because the potential of the cobalt reference electrode is related to the Co 2+ concentration, a constant precursor concentration must be maintained during electroplating. In the process of electroplating LiCoO2, LiCoO2 is produced on the working electrode and the Co metal is plated onto the counter electrode. However, neither LiCoO2 nor Co is soluble the LiOH and KOH melt, and thus the Co generated at the counter electrode does not contaminate the LiCoO2 product. We have confirmed that Co metal is not soluble in LiOH and KOH melt by weighing a Co wire after an extensive immersion time (> 6 months) in the bath. section SV. Thermodynamic modeling. LiCoO2-CoO System The acid-base equilibrium in the hydroxide melt is maintained through the reaction: 2OH = H 2 O + O 2 (44-47), where H2O is a Lux-Flood acid and accepts the basic O 2-. In order to understand how the acidity affects the relative stability of several Co compounds and the competing reactions between the LiCoO2 formation and the oxygen evolution, the potential-ph2o diagram containing Co, CoO, LiCoO2, Co3O4, H2O, O2 is calculated based on their Gibbs energy changes with water molar ratio (x H2O ). ph2o, the acidity of the hydroxide melt, is defined as log(x H2O ). Co + 2OH = CoO + H 2 O + 2e [1]

7 3CoO + 2OH = Co 3 O 4 + H 2 O + 2e [2] Co 3 O 4 + 3Li + + 4OH = 3LiCoO 2 + 2H 2 O + e [3] 2OH = H 2 O O 2 + 2e [4] CoO + Li + + 2OH = LiCoO 2 + H 2 O + e [5] Li + OH = LiOH + e [6] For convenience, the Li/Li + redox couple is used as the basis for discussion. A full reaction is constructed by combining the electrochemical half reactions [1~5] and [6]. We obtain the following reaction for the standard potential difference between the half reactions [1~5] and the Li/Li + electrode. Co + 2LiOH = CoO + H 2 O + Li [7] 3CoO + 2LiOH = Co 3 O 4 + H 2 O + 2Li [8] Co 3 O 4 + 4LiOH = 3LiCoO 2 + 2H 2 O + Li [9] 2LiOH = H 2 O O 2 + 2Li [10] CoO + 2LiOH = LiCoO 2 + H 2 O + Li [11] The thermodynamic data of pure solid and gas phases (table S1) are obtained from reference (48). To simplify the modeling, CoO is considered saturated in the hydroxide melt. The solid undissolved CoO is in equilibrium with the dissolved CoO. The chemical potential of the dissolved CoO is calculated based on the equilibrium relationship: μ solid = μ liquid The hydroxide melt is treated as ideal solution. E 1 = E RT 2F E 2 = E RT 2F E 3 = E RT F E 4 = E RT 2F E 5 = E RT F + RT ln(x saturated CoO ) log log x CoO x RT ph LiOH 2F 2 O [12] 1 x 3 CoO x RT LiOH 2F ph 2 O [13] log 1 x RT ph LiOH F 2 O [14] log log P 1/2 O2 x RT ph LiOH 2F 2 O [15] 1 x CoO x RT LiOH F ph 2 O [16] E1 ~ E5 are the potentials relative to the Li/Li + redox couple. The relationships between the potentials and ph2o are plot in Fig. 1D. E1 * ~E5 * are obtained by calculating G zf of equations [7~11]. The phase

8 stability diagram is further confirmed by experimentally performing CV measurements. table S1. Thermodynamic data used for the LiOH-KOH-CoO system at 260 C. Species Gibbs Energy (KJ/mol) References O (48) Co (48) CoO (48) H 2O (48) H (48) Li (48) LiOH (48) Li 2O (48) KOH (48) K 2O (48) Co 3O (48) LiCoO (49, 50) CoOOH is a metastable compound in the Co-H2O system. As demonstrated in thermal evolution experiments (51, 52) and thermodynamic calculation (53), CoOOH is unstable at 200 C and decomposes to Co3O4. Therefore, CoOOH is not considered in our thermodynamic modeling.

9 fig. S3. High-resolution TEM image and electron diffraction pattern of an electroplated LiCoO2 crystal flake. (a) HR-TEM image of an electroplated LiCoO2 crystal flake and (b) its electron diffraction pattern. section SVI. Crystallography of LiCoO2. LiCoO2 has various crystallographic structures that allow lithium intercalation. For examples, the cubic spinel LiCo2O4 with Fd3m symmetry offers 3D pathways for Li ion diffusion through the face-shared 8a tetrahedra (LiO4) and the interstitial 16c octahedra (CoO6). The layered LiCoO2 consists of alternating LiO6 and CoO6 octahedral layers. Commercial Li-ion battery cathodes usually use layered O3-LiCoO2, which has ABCABC oxygen stacking and R3 m symmetry. Due to the structure similarity, the intergrowth of different phases or the offset of lithium and cobalt atoms in a similar framework structures may lead to complicated electron diffraction patterns. Many important works (25-29, 54, 55) have been reported to address the extra diffraction spots seen in the O3-LiCoO2 material. fig. S4 shows that the reflection of the crystallographic (100) plane family is absent in the simulated diffraction patterns. However, when the layer stacking is shifted, the absent diffraction spots may appear. This suggests that the extra diffraction spots in Fig. 2A are contributed by other non-o3 phases. Compared to the O3-LiCoO2, the O2 structure presents extra diffraction spots at 4.12 nm -1, which is

10 attributed to the ABAC oxygen stacking (33, 56). While in the O3 structure the LiO6 and CoO6 octahedra only share edges, they share both faces and edges in the O2 phase. As a result, the strong electrostatic repulsion between lithium and cobalt cations in the O2 phase leads to slightly lower potential than that of the O3 phase. O2-LiCoO2 is considered as one possible phase that contributes to the extra diffraction spots because its structure agrees well with the experimental electron diffraction pattern. If removing the tetrahedral lithium, the spinel LiCo2O4 shows a similar diffraction pattern in the [111] direction as the O3-LiCoO2 does in the [001] direction. When lithium ions are removed every two layers in O3-LiCoO2 to form a lithium staging structure, the diffraction pattern also shows the similar extra spots (4.12 nm -1 ) as the O2-LiCoO2. If the lithium sites form a 2 2 superstructure (54) after removing a quarter lithium, the diffraction pattern also exhibits similar extra spots at 3.56 nm -1. Thus, it is difficult to determine the crystal structure only by the diffraction patterns. The crystallographic, diffraction, and electrochemical data suggest that during the formation of LiCoO2, non O3-layer stacking and interlayer cation mixing could happen, resulting in metastable phases that contribute to extra reflections in the electron diffraction patterns. The extra reflection can only be detected on the edge or the surface of the electroplated hexagonal crystallite, and these impurity phases are not detectable in XRD measurements. The extra weak reflection or impurity is completely removed after post-annealing.

11 fig. S4. Crystallographic structures of O3-, O2-, and spinel-phase lithium cobalt oxides and two superstructures with lithium staging and 2 2 periods. Their simulated diffraction patterns along [001] and [111] directions are shown below the structures. section SVII. Scanning electron nanobeam diffraction. Scanning electron nanobeam diffraction (SEND) and diffraction imaging are powerful techniques to analyze complex local microstructure within a crystallite (57). As shown in fig. S5, SEND utilizes a nano-sized or smaller (e.g. 1 nm) electron beam to scan the crystallite and produce diffraction patterns at each scan spot (a semi-convergent beam of 3nm in full-width half maximum is used in our study). The diffraction patterns and relevant information (e.g. diffraction intensity, intensity ratio between

12 different phases) are collected using a slow scan CCD camera and mapped according to their probe positions. HR-TEM images and low-dose SEND are recorded using a JEOL 2100 TEM equipped with a LaB6 gun and operated at 200 kv. fig. S5. Illustration of the SEND technique. Nano-sized electron beam is used to scan a small area. In each step, a diffraction pattern is recorded as shown 1, 2,, m n. The scanning diffraction patterns shown in Fig. 2 are acquired over an area of 30 nm x 30 nm in 11 x 11 pixels, corresponding to a step size of 3 nm. In our SEND experiment, we use 4 times binning (512 x 512 pixels). The typical exposure time for each diffraction pattern is 0.1 s. A total of 121 diffraction patterns are recorded over a period of 6 min without a beam stop. The obtained diffraction images are further processed to extract structural information.

13 fig. S6. Cross-sectional SEM image of ~200-μm-thick LiCoO2 electroplated on an Al foil. section SVIII. Flexible battery. A monolithic LiCoO2/CNF cathode, a separator and a CNF mat are stacked (fig. S8) and assembled into a pouch cell, where the CNF mat is used as an anode. The CNF anode can reversibly intercalate Li ions around 0.2V (vs Li/Li + as shown in fig. S7) and deliver a specific capacity of ~ 220 mah/g. fig. S7. Charge/discharge voltage profiles of the CNF anode.

14 fig. S8. Charge/discharge curves of a LiCoO2/CNF flexible battery. The stacked CNF/separator/LiCoO2 is sealed in a flexible polyester bag. fig. S9. Optical images of bending tests. Optical images of (a) the instrument that is used to test the

15 flexibility of batteries, (b) a battery during the bending test, and (c) a flexible LiCoO2/CNF battery being bent manually while connected to an LED display showing the cell voltage. fig. S10. Schematic illustrations of the structure difference between traditional and electroplated flexible batteries. Schematic illustrations of (a) a traditional battery material agglomerate (including active materials, binder, and conductive agents) prepared by slurry casting methods, (b) a fractured traditional battery electrode caused by flexing, (c) a 3D cathode with active material conformally coated on a 3D current collector, and (d) a flexible battery consisting of such flexible cathode and a CNF anode. c d 100 um 100 um

16 fig. S11. SEM images of a LiCoO2/CNF cathode before and after 1000 bending cycles. Top view SEM micrographs of (c) a conventional slurry cast LCO electrode and (d) a LiCoO2/CNF electrode after 100 bending cycles. The slurry cast electrode exhibits significant cracking and partial delamination of active materials from the current collector, while no cracks are observed in the CNF electrode. section SIX. Electrodeposition of spinel LiMn2O4. The spinel LiMn2O4 is plated using the same electroplating setup by replacing CoO precursor with MnO (~ 2wt%). fig. S12a-b demonstrate the conformal coating of LiMn2O4 on a CNF scaffold. The as-deposited material shows mixed spinel and tetragonal phases. However, the tetragonal phase can be easily removed by leaching in dilute H2SO4 solution (~ ph3) and annealing at 300 C in air for 1 hour. The XRD analysis (fig. S12d) confirms that the electroplated LiMn2O4 has a pure spinel phase. The electrochemical charge/discharge profile and the dq/dv curve in fig. S12c and S12f exhibit a 4V characteristic plateau of the spinel LiMn2O4. As shown in fig. S12e and S12g, the electroplated LiMn2O4 has an excellent cycle life and rate capability. LiMnO2-MnO System According to the XRD analysis, the cubic spinel phase (Fd3 m, c-limn2o4) and the tetragonal phase of LiMnO2 (I4 1/amd, t-limno2) can coexist in the electroplated material. Compared to the spinel phase, the [Mn2O4] frame in the tetragonal phase is distorted due to the Jahn-Teller effect. During galvanostatic discharge, a spinel phase is converted to a tetragonal phase at x close to 1 for LixMnO2. Therefore, it is possible to see tetragonal and spinel phases coexist in anodic electroplating. Thermochemical data of LixMnO2 are not fully available. As shown in fig. S13, previously reported Gibbs free energy data are scattered in the range of 200 ~ 800 C. To obtain a reliable Gibbs free energy at 300 C, we have evaluated the available data based on thermodynamic calculations. The ab initio calculation is used to calculate the 0 K energy. Please refer to the references (58-61) for the calculation method. Our calculated formation enthalpy for t-limno2 at 0K is -851 KJ/mol, which is close to the enthalpy ( KJ/mol) reported by Wang et al. in calorimetry experiments (62). Thus, it is appropriate to use Wang s t-limno2 enthalpy for our electroplated LiMnO2. Fisher et al. find that the cubic and the tetragonal spinel phases have a very small energy difference (63). This may also explain

17 why the cubic and the tetragonal spinels coexist during electrodeposition. Due to the small energy difference, the following thermodynamic modeling would not be able to differentiate the cubic and the tetragonal phases. We use LiMnO2 as the general chemical formula. 1 µm 100 nm f g fig. S12. Materials and electrochemical characterization of the electroplated LiMn2O4/CNF battery. (a-b) SEM images of the LiMn2O4/CNF cathode prepared by molten salt electrodeposition. (c)

18 The charge/discharge potential profiles, (d) the XRD spectrum, (e) the cycle life, (f) dq/dv curves, and (g) the high rate discharge curves of the electroplated LiMn2O4. Because the heat capacity of LiMnO2 is not available, we consider LiMnO2 as an ideal solution of Mn2O3 and Li2O (equation 17) according to the Neuman-Kopp rule (49, 64, 65), LiMnO 2 = Mn 2 O 3 /2 + Li 2 O/2 [17] The heat capacity of LiMnO2 is calculated as C p (LiMnO 2 ) = C p (Mn 2 O 3 )/2 + C p (Li 2 O)/2 [18] The variation of Gibbs free energy with temperature is given by (50) T 298 G 0 T = H 0 T T S T = H CpdT T S 298 T T Cp 298 T dt [19] The enthalpy and entropy data are summarized in table S2. fig. S14 shows the Gibbs free energy of LiMnO2 at various temperatures KJ mol -1 is used in the potential-ph2o calculation at 300 C. The thermochemical data used for 300 C calculations are listed in table S3. fig. S13. The Gibbs free energy of the formation from the elements for LiMnO2. The data points are obtained from the literatures (62, 66-68)

19 fig. S14. The Gibbs free energy of LiMnO2. The black line is the calculated value of LiMnO2 using estimated heat capacity. table S2. Thermodynamic data of the LiOH-KOH-MnO system at 25 C. Species Enthalpy (KJ mol -1 ) Entropy (J K -1ˑmol -1 ) Gibbs Energy (KJ mol -1 ) References Mn (48) MnO (48) Mn 2O (48) Mn 3O (48) LiMnO (62) table S3. Thermodynamic data of the LiOH-KOH-MnO system at 300 C. Species Gibbs Energy (KJ mol -1 ) References Mn (48) MnO (48) Mn 2O (48) Mn 3O (48) LiMnO Estimated

20 The calculated potential-ph2o diagram is shown in fig. S15. The formation potential of LiMnO2 from MnO decreases with the decrease of water concentration, which is similar to the case for LiCoO2. However, there is no predominant Mn3O4 region in the entire range of the water concentration. Without LiOH, Mn3O4 could be electrodeposited around 2V according to equation [22] (69). Although Mn3O4 can be electroplated, once LiOH is added, Mn3O4 is spontaneously converted to LiMnO2 through the chemical reaction [23], because the Gibbs free energy difference of the reaction [23] is only KJ/mol. The formation potential of LiMnO2 is far below the oxygen evolution line, suggesting that LiMnO2 is relatively easier to form in the molten LiOH-KOH system comparing to LiCoO2. Unlike the LiCoO2-CoO system, the crossover of the MnO/LiMnO2 and the MnO/Mn3O4 lines happens in the very acidic region. It suggests that LiMnO2 can be electroplated in aqueous solutions. The above conclusions have been confirmed by recent reports (15, 16), where Mn3O4 is electrodeposited in an aqueous solution and converted to c-limn2o4 by hydrothermal treatment (16) or sintering with LiOH (15). fig. S15. The potential-ph2o diagram of the LiOH-KOH-MnO-H2O melt system. Without LiOH, the blue dash line (22) is the conversion reaction of equation [22]. With LiOH, the predominant area of LiMnO2 is above the black line (21). MnO + H 2 O + 2e = Mn + 2OH [20]

21 LiMnO 2 + H 2 O + e = MnO + Li + + 2OH [21] Mn 3 O 4 + H 2 O + 2e = 3MnO + 2OH [22] Mn3O4 + 2LiOH = 2LiMnO2 + MnO +H2O [23] section SX. Electroplating of Al-doped lithium cobalt oxide. LiAlyCo1-yO2 systems have received great attention because Al-doping can increase the charge/discharge potential (40, 70) and thus the energy density (41, 71, 72) of LiCoO2. In this work, we have also explored the feasibility of electroplating Al-doped LiCoO2 by adding Al(OH)3 to the molten KOH-LiOH-CoO bath. The SEM image in fig. S16a demonstrates that the electroplated Al-doped LiCoO2 has good crystallinity, and the EDX measurement (fig. S16b) shows that there is a significant amount of aluminum in the electroplated material. The XRD pattern in fig. S16c is generally in agreement with that of the intrinsic LiCoO2 (JCPDS card no. # ) except a slight shift of peak positions. Al substitution does not disturb the layered LiCoO2 structure but decreases a and increases c cell parameters. The c cell parameter is found to expand by 0.035Å (the cell parameters are calculated based on the R3 m space group.). fig. S16d shows that the charge curve of Al-doped LiCoO2 shifts to higher voltage comparing to the intrinsic LiCoO2. The observed 50 mv difference in dq/dv curves (fig. S16e) agrees well with previously reported Al-doped LiCoO2 (41, 72, 73), suggesting that aluminum dopants replace cobalt atoms in our electroplated material. The above results have shown that it is possible to electroplate lithium aluminum cobalt oxides in molten salt. This preliminary exploration may also be extended to other transition metal oxide systems.

22 fig. S16. Materials and electrochemical characterization of the electroplated Al-doped LiCoO2. (a) SEM image, (b) EDX spectrum, and (c) XRD pattern of the electrodeposited Al-doped LiCoO2. (d) Charge and (e) dq/dv curves of the electroplated LiCoO2 and the electroplated Al-doped LiCoO2. section SXI. Calculation of energy density of flexible batteries. We scrutinize previous works on flexible batteries and summarize the relevant references (74-82) in Fig. 4F. Often, the energy densities of the batteries in these reports are presented in various ways. In order to compare their energy densities on the same basis, the energy density of the cell after the bending test is re-calculated with the remaining cell energy and the total mass of the cathode and the

23 anode. Some reports only show the electrochemical and mechanical properties of flexible anodes or cathodes (half cells). In this case, we calculate the energy density of a 1:1 matched full cell with a 110 mah/g LiCoO2 cathode (or a 220 mah/g graphite anode) and their reported anodes (or cathodes). The re-calculated energy density may not reveal the exact properties of a realistic battery but provides important and relevant information for quantitative comparison between this work and previous works. For other reports which only show the measured voltage output during the bending test, we did not consider them in Fig. 4F, because the voltage measurement cannot accurately reflect the battery capacity fade.