An Effective Chemical Prelithiation Strategy for Building a Silicon

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An Effective Chemical Prelithiation Strategy for Building a Silicon/Sulfur Li-ion Battery Yifei Shen, Jingmin Zhang, Yongfeng Pu, Hui Wang, Bo Wang, Jiangfeng Qian, Yuliang Cao, Faping Zhong, Xinping Ai, and Hanxi Yang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00889 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 12, 2019

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ACS Energy Letters

Article type: Letters

An Effective Chemical Prelithiation Strategy for Building a Silicon/Sulfur Li-ion Battery

Yifei Shena, Jingmin Zhanga, Yongfeng Pua, Hui Wanga, Bo Wangb, Jiangfeng Qiana*, Yuliang Caoa, Faping Zhongc*, Xinping Aia*, Hanxi Yanga

a

Hubei Key Laboratory of Electrochemical Power Sources, College of Chemistry and

Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, China E-mail: [email protected]; [email protected] b

Global Energy Interconnection Research Institute North America, San Jose, CA

95134, USA c

National Engineering Research Center of Advanced Energy Storage Materials,

Hunan 410205, China E-mail: [email protected]

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ABSTRACT Prelithiation has important applications to convert non-lithiated cathode or anode materials into a controllably lithiated state that required for developing advanced Li-ion batteries. However, most of the prelithiation reagents developed so far are highly reactive and sensitive to oxygen and moisture, and therefore difficult for practical battery application. In this work, we developed a facile prelithiation strategy using lithium naphthalenide to fully prelithiate sulfur-poly(acrylonitrile) (S-PAN) composite into a Li2S-PAN cathode and to partially prelithiate nano-silicon into a LixSi anode, which leads to a new version of silicon/sulfur Li-ion battery. This LixSi/Li2S-PAN battery can demonstrate a high specific energy of 710 Wh kg-1, with a high initial Coulombic efficiency of 93.5% and a considerable cyclability. Also, this chemical prelithiation approach is mild, efficient and widely applicable to a large range of Li-deficient electrodes, opening up new possibilities for development of low cost, environmentally benign and high capacity Li-ion batteries.

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The increasing requirement for large scale electric storage technologies has triggered the development of advanced rechargeable battery systems that go beyond the currently commercialized Li-ion batteries (LIBs).1-3 Among various “beyond Li-ion technologies”, lithium/sulfur (Li/S) battery has been widely investigated as a promising candidate due to its superior theoretical specific energy of 2600 Wh kg-1, low cost and wide availability of sulfur.4-8 However, the practical specific energy of Li/S battery systems is often relatively poor9-10, because of the low capacity utilization of sulfur and high demand for electrolytes, the latter of which do not contribute to cell capacity. In addition, the poor cycle stability arising from the dissolution loss of its reaction intermediates also has impeded the development of a practically viable Li–S battery. Aside from this cathode problem, the inherent dendritic growth and low Coulombic efficiency of Li anode during cycles also remain a severe challenge for battery applications.11-12 Replacing the Li metal anode by high capacity Li-storable materials is a feasible strategy to avoid Li dendrite growth. In this regard, a few of pioneering work has been done to use Sn and Si anodes for constructing Sn/S and Si/S batteries based on prelithiated sulfur cathodes.13-16 However, due to the formation of solid electrolyte interphase (SEI) and the existence of partly irreversible lithiation at initial cycle, these Sn or Si anodes usually exhibit a huge initial active lithium loss (typically 30%-50%) and therefore require to be paired with excessive amount of lithiated sulfur cathode for compensating the Li consumption, thus leading to a considerable decrease in the available energy densities of these batteries17-19. On the other hand, the sulfur cathode 3 / 22



has to be made in its fully lithiated state (Li2S) so as to provide sufficient Li+ ions for the lithium-free anodes during charge. However, fabricating a usable Li2S cathode is very difficult (Table S1) because of the high reactivity of Li2S with water in the air. If there is a highly efficient and easily applicable method to prelithiate a sulfur cathode from its “fully charged state” (S) into a “fully discharged state” (Li2S), it would be possible to couple the prelithiated S cathode with lithium-free anodes to achieve novel Li-ion battery configurations that utilize the high capacity of sulfur but avoid the lithium dendrite growth as in the case of Li/S battery. Similarly, if the lithium-free anodes such as Sn and Si metals could be simply prelithiated, they may have a greatly decreased initial capacity loss or serve as an alternative to lithium metal anode for developing new Li-ion batteries by coupling with non-lithiated cathode materials such as sulfur, oxygen and their halides. For this reason, chemical prelithiation strategy has attracted considerable attention over the years and a number of methods have been proposed to prelithiate the anodes and cathodes of Li-ion batteries.20-21 As early as in 1998, Owen et al. reported a chemical lithiation method to add lithium to the carbon anode for compensating the initial irreversible capacity loss of the anode.22 Since then, a number of microsized lithium metal powder23-26 or LixSi nanoparticles27-31 were developed as prelithiation reagents to lithiate carbon and Si anodes for achieving a Coulombic efficiency and therefore capacity matching between the anodes and cathodes. These prelithiation reagents are effective but difficult to achieve a delicate control of the lithiation degree of the anodes. In the meantime, several chemical lithiation reagents 4 / 22


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ACS Energy Letters

such as n-butyllithium and lithium triethylborohydride were used to prelithiate the sulfur cathode so as to mitigate the huge volume change of conventional sulfur cathode.32-34

However, these prelithiation reagents are extremely sensitive to air and

therefore difficult for practical applications due to safety concern. In addition, such a prelithiated sulfur cathode still suffers from the poor cyclability due to its reaction mechanism in the same way as conventional sulfur cathodes. Here, we report a facile chemical prelithiation approach using lithium naphthalenide (Li-Naph) to fully prelithiate polyacrylonitrile-sulfur composite (S-PAN) for fabricating a cycle-stable Li2S-PAN cathode and to partially prelithiate silicon nanopowder for fabricating a highly efficient LixSi anode. Based on the Li2S-PAN cathode and LixSi anode, we build a rechargeable Si/S Li-ion battery, in which Li+ ions shuttle between the S-PAN cathode and the Si anode during charge/discharge cycles, completely avoiding the problems associated with the dissolution of polysulfide intermediates and the dendrite growth of lithium in conventional Li/S battery. As a result, the Si/Li2S cell demonstrates a nearly 100% capacity utilization of both cathode and anode materials, achieving a high specific energy of 710 Wh kg-1. In addition, this prelithiation process is very mild, highly efficient and easily applicable, possibly providing a new approach for the development of advanced Li-ion batteries. The development of lithium naphthalenide (Li-Naph) as a prelithiation reagent in this work was mainly based on its advantages of easy synthesis, mild reactivity and strong lithiation ability.35-36 As shown in Figure 1a, Li-Naph can be simply prepared 5 / 22



by mixing the stoichiometric amount of lithium and naphthalene in a molar ratio of 1:1 in dimethoxyethane (DME) solvent. As reflected by a rapid change in the color of the mixture solution from transparent to dark green (Figure 1b), a charge transfer complex Li+-Naph- was formed spontaneously by electron donation from lithium to Naph ring, thus producing dark-green colored Naph- radicals.35, 37-39 The formation of Naph- radicals was also confirmed by the Electron Paramagnetic Resonance analysis (EPR) as shown in Figure S1, in which a strong EPR signal representing the existence of radical compound can be observed. To evaluate the redox activity of Li-Naph molecules, cyclic voltammetry was conducted in the Li-Naph solution. As displayed in Figure 1c, the CV feature appears as a pair of reversible redox peaks at 0.32 and 0.52 V (vs. Li/Li+), corresponding to a formal potential of 0.35 V for Naph/Naphcouple. This potential is much lower than the lithiation potential of the sulfur electrode (i.e. ES/Li2S≈2 V, vs. Li/Li+), suggesting that Li-Naph is a sufficient reducing reagent to lithiate elemental sulfur, including S/C composites and S/polymer composites (Figure 1d). This has been confirmed by the experimental result that Li-Naph can easily convert elemental sulfur into a fully lithiated Li2S state under ambient temperature (Figure S2). Apparently, this lithiation reaction proceeds through an electron transfer from Naph- radical to sulfur along with Li+ incorporation into the sulfur matrix to form LixS, meanwhile Naph- is oxidized to naphthalene (Figure 1a). Unlike conventional n-butyllithium reagent, naphthalene has its lone pair of electrons nonlocalized and distributed on its conjugated aromatic rings, leading to stabilization of the Naph- radical (Figure 1e and S3). As a consequence, Li-Naph is relatively 6 / 22


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ACS Energy Letters

stable and appears quite quiet even when exposed to moist air (Figure S4), thereby enabling it for practical battery applications.

Figure1. a) The synthetic reaction of Li-Naph and lithiation reaction of sulfur with Li-Naph. b) Photographs of 1M naphthalene and 1M Li-Naph solution dissolved in DME. c) Cyclic voltammograms of naphthalenide in 0.1M LiTFSI electrolyte at 100 mV s-1. d) Potentials of selected materials related to this paper. e) Mapped electrostatic potential (MEP) surfaces of naphthalene anion and n-butyl anion.

To build a cycle-stable and fully prelithiated sulfur cathode, we selected S-PAN40-43 as a sulfur host material and then prelithiated it into a fully lithiated state Li2S-PAN by Li-Naph. The use of S-PAN rather than other nanostructured S/C composites was mainly based on the consideration that the sulfur in S-PAN material is chemically bonded to the conductive polymer skeleton44 and therefore undergoes its 7 / 22



charge/discharge (lithiation/delithiation) reactions through Li+ insertion/extraction mechanism without the generation of dissoluble polysulfide intermediates, thereby ensuring high capacity utilization and cycling stability for battery operation in simple carbonate electrolytes.45-46 The chemical prelithiation of sulfur cathode was conducted simply by immersing the PAN-S electrode into Li-Naph solution for a certain time, followed by DME rinse to remove the residual Li-Naph. To optimize the lithiation condition, prelithiated LixS-PAN electrodes with differient immersing time in Li-Naph solution were prepared and tested by charging/discharging them in a lithium half-coin cells. As can be seen from Figure 2a and 2b, with the lithiation time increasing from 0 to 25min, the open circuit voltage (OCV vs. Li+/Li) of LixS-PAN electrodes decreases, while its initial charge (Li extraction) capacity increases, reflecting a gradually increased lithiation degree. After 20 min of prelithiation, the OCV of LixS-PAN electrode decreases to ~1.2 V and its initial charge capacity increases to 703 mAh g-1, which is very close to the theoretical capacity of S-PAN cathode (702.24 mAh g-1 for 42% wt. S content in the composite), indicating that the sulfur molecules in PAN matrix have been fully lithiated to Li2S. Further increase in lithiation time (25 min) will lead to the overlithiation of S-PAN, as reflected by the considerably increased initial charge capacity (>820 mAh g-1) but almost no changed discharge capacity. Hence, we selected lithiation time of 20 min to lithiate S-PAN cathode. Figures 2c and 2d show morphologies of the S-PAN material before and after prelithiation treatment. Li2S-PAN have a similar spherical particle morphology to 8 / 22


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ACS Energy Letters

PAN-S but an enlarged particle size from 233 nm to 319nm (Figure S5) due to the volume expansion caused by the Li+ incorporation. X-ray photoelectron spectroscopy (XPS) provides a direct evidence for the effective prelithiation of the S-PAN electrode. As shown in Figure 2e, the S2p binding energy of pristine S-PAN appears to be a broad peak around 163 eV, which can be deconvoluted into four Lorentzian peaks with binding energies of 164.93, 163.77, 161.95 and 163.11 eV. The former two peaks at 164.93 and 163.77 eV correspond to S2p1/2 and 2p2/3 binding energies in thioether C-Sx-C, respectively, whereas the latter two peaks at 161.95 and 163.11 eV are characteristic of the 2p1/2 and 2p2/3 orbits of sulfur in thioamide N-C=S.

44

After

lithiation, all the S 2p signals of Li2S-PAN electrode shift towards lower bonding energy, indicating the presence of sulfur in a S2- state.47 Meanwhile, a pronounced Li 1s peak appears at 55.31 eV in the XPS spectra of Li2S-PAN, further confirming the incorporation of Li+ ions into the sulfur host. Furthermore, The Li7 NMR spectra reveal that the S-PAN material has been fully converted into Li2S-PAN (Figure 2f). Besides, Raman evidence also ensures a successful lithiation of S-PAN cathode (Figure S5). To evaluate the electrochemical performance of the Li2S-PAN cathode, we fabricated CR 2016 coin cells using lithium metal anodes and 1M LiPF6-PC-EC-DEC electrolyte. Pristine S-PAN electrodes were also tested under the same conditions for comparison. Figure 3a shows typical CV curves of the Li2S-PAN cathode at first three cycles. During the first positive scan, an oxidation peak emerged at 2.25 V, due to the initial delithiation of Li2S-PAN to form S-PAN. In the subsequent cathodic scan, two 9 / 22



reduction peaks appeared at 2.0 V and 1.75 V, characterizing the stepwise electrochemical reduction of sulfur to Li2S in the PAN matrix.40, 48 In the following cycles, the shapes and areas of these redox peaks remained almost unchanged, suggesting a very reversible redox reaction. Notably, the CV response of Li2S-PAN resembles closely that observed for pristine S-PAN electrode, except for the first cathodic scan due to the formation of solid electrolyte interface on the surface of cathode side as well as the activation of chemically bonded sulfur molecules.48

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Figure2. a) The Li extraction voltage profiles of LixS-PAN electrodes prelithiated in Li-Naph solution for a different time. b) The charge and discharge capacities of LixS-PAN cathodes with different prelithiation time. SEM images of c) S-PAN electrode and d) Li2S-PAN electrode. e) XPS spectra of lithium and sulfur in the S-PAN and Li2S-PAN electrodes. f) Solid state nuclear magnetic resonance 11 / 22



spectra (7Li NMR, 7kHz) of commercial Li2S and the prelithiated Li2S-PAN electrode.

Figure 3b shows the charge/discharge profiles of Li2S-PAN cathode cycled at a constant current of 100 mA g-1. In good agreement with CV response, the Li2S-PAN electrode exhibits flat charge/discharge plateaus at 2.0/1.8 V. Generally, as reported previously for the Li2S cathode, a high activation voltage (up to 4 V) was needed to activate the Li2S at the first charge,14-15 due to the low conductivity and long Li+ diffusion path of electrically isolative Li2S particles. Nevertheless, our Li2S-PAN sample did not require any noticeable delithiation overpotential, suggesting an activated Li2S-PAN electrode by chemical prelithiation. The charge/discharge capacities in the first cycle are 703.2/675.4 mAh g-1, respectively, corresponding to a high initial Coulombic efficiency of 96.1%. During the following cycles, the voltage profiles kept stable with no discernible capacity decay, demonstrating good cycle stability of the Li2S-PAN cathode. It is worth noting that the reversible capacity of the Li2S-PAN electrode is even higher than pristine S-PAN (~665 vs. ~630 mAh/g), possibly because the prelithiation treatment helped to form a stable SEI film on the sulfur surface, eliminating the generation and dissolution of soluble polysulfide intermediates.49 As shown by the EIS analysis in Figure S6, the prelithiated Li2S-PAN electrode shows lower interfacial resistance than the electrochemically discharged S-PAN electrode. Figure 3c shows the rate capabilities of Li2S-PAN electrode evaluated at varying rates from 50 mA g-1 to 4000 mA g-1. The electrodes demonstrate excellent rate 12 / 22


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ACS Energy Letters

performance with capacities of 611.8, 581.1, 540.0 and 487.4 mAh g-1 at high rates of 500, 1000, 2000 and 4000 mA g-1, respectively. Moreover, when the current was changed back to 50 mA g-1, the electrode almost recovered to its original reversible capacity, indicating excellent structural stability and high rate tolerance of the Li2S-PAN cathode. The cycling performance of the Li2S-PAN cathode is displayed in Figure 3d. After 250 cycles, Li2S-PAN still delivers 630.1 mAh g-1, 90.6% of its initial capacity. Besides, the Coulombic efficiency of Li2S-PAN kept around 100% after the first cycle.

Figure3. a) CV curves at a scan rate of 0.01 mV s-1. b) charge/discharge profiles and c) rate capability at various current rates from 50 to 4000 mA g-1 and d) cycling performance and Coulombic efficiency over 250 cycles at a current of 100 mA g-1 of S-PAN and Li2S-PAN electrodes. The electrolyte is 1M

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LiPF6 in PC/EC/DEC (1:4:5 by volume).

Since Li2S-PAN can function well as a Li-rich cathode, it could be therefore used to couple with Li-free anode such as silicon to construct a Si/S Li-ion battery. In this study, commercial silicon nanoparticles were chosen as anode host because of its high reversible capacity of 3000 mAh g-1 and low working potential of ~0.25 V (vs. Li/Li+)50, which enable a reasonably high cell voltage and high energy density when coupled with Li2S-PAN cathode. However, the large initial irreversible capacity (~1500 mAh/g), due to the formation of solid electrolyte interphase (SEI) during the 1st cycle, has long been a severe challenge for the practical use of Si anode. To solve this problem, we chemically prelithiate the Si anode by use of Li-Naph. Since its redox potential (0.35 V) is just below the SEI formation potential (~0.5 V) and slightly higher than the Li+ insertion potential (0.25 V vs. Li/Li+) of Si, the Li-Naph reagent is ideally suitable for partially prelithiating Si anode to eliminate its initial irreversible capacity loss. Figure 4a shows the voltage profiles of Si anode before and after lithiation. After prelithiation, the irreversible capacity of Si anode was greatly decreased, leading to a high initial Coulombic efficiency of 96.1% (Figure 4a and S7). It is worth noting that Li-Naph is also suitable for using as a prelithaition reagent to lithiate other carbon-based and alloy-type anodes, such as Sb/C, P (Phosphorous)/C, and hard carbon anodes (Figure S8-S10), thus improving their initial Coulombic efficiency significantly. Based on the full prelithiated S-PAN cathode and partially prelithiated nano-Si anode, we built a Si/S Li-ion battery with an optimized cathode/anode capacity ratio 14 / 22


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ACS Energy Letters

of 1:1.2. As shown in Figure 4b, this prelithiated Si/Li2S-PAN cell delivers an average voltage of 1.6 V and a first discharge capacity of 619.7 mAh g–1, nearly a full utilization of the theoretical capacities of both cathode and anode. The first-cycle Coulombic efficiency of this cell attains to a very high value of 93.5% and then increases to nearly 100% at the 3rd cycle. In contrast, the full based on pristine nano-Si anode shows a large irreversible capacity loss of 30% during the first cycle. The specific energy of the prelithiated Si/Li2S-PAN cell is realized to be 710 Wh kg-1 (including all the active electrode materials), which is even higher than the theoretical specific energies of currently commercialized C/LiCoO2 (410 Wh kg-1) and C/LiFePO4 (385 Wh kg-1) Li-ion batteries15. Typically, the active electrode weights are about 60% of the total weight of a practical lithium-ion cell 9. Thus the practical specific energy of nearly 420 Wh kg–1 is expected for our LixSi/Li2S-PAN system. The high rate performance of the full cell is revealed by Figure 4c. As the current density increases from 0.2 A g-1 to 0.5 and 1.0 A g-1, the capacities of full cells gradually decrease to 574.1, 486.0 and 398.5 mAh g-1. Even at a high current rate of 2 and 4A g-1, the full cell can still deliver a capacity of 331.0 and 272.3mAh g-1, respectively, demonstrating a good rate capability. In addition to its high energy density and good rate performance, the prelithiated Si/Li2S-PAN battery can be cycled over 50 times with a capacity retention of 82.6% (Figure 4d). Figure 4e and Table S2 compare the specific energy and cycle stability of Li2S-based Li-ion batteries developed in this work and in literatures15-16, 51-54. Compared with the rechargeable batteries reported previously by use of lithium metal-free 15 / 22



anodes and Li2S cathodes, this prelithiated Si/Li2S-PAN battery possess the highest specific energy and most stable cycling performance, demonstrating the advantages of this prelithiation treatment for battery application.

Figure4. a) Charge/discharge profiles of nano-Si and prelithiated Si anodes. b) Charge/discharge profiles of LixSi/Li2S-PAN full cells. c) Rate capability of prelithiated Si/Li2S-PAN battery. d) Cycling performance and Coulombic efficiency of full cells cycled at 100 mA g-1. e) A comparison of specific energy of the Li2S-based Li-ion batteries reported in literatures15-16, 51-54.

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In summary, we demonstrate a facile and efficient prelithiation strategy by use of lithium naphthalenide for prelithiating sulfur cathode and silicon anode, which lead to a new version silicon-sulfur Li-ion battery. Benefiting from structural and electrochemical stabilities of S-PAN material, the fully prelithiated Li2S-PAN cathode can behave as a high capacity, cycle-stable lithium-rich cathode that can be easily paired with Li-metal-free anodes to develop advanced Li-ion batteries. In addition, this prelithiation strategy can also be used to prelithiated Si-based anode in a controllable degree, thus eliminating the large initial irreversible capacity loss usually encountered in Li-metal-free anodes. Based on the prelithiated Li2S-PAN cathode and nano-Si anode, we assembled a low cost Si/S Li-ion battery, which demonstrated a high specific energy of 710 Wh kg-1, a very high initial Coulombic efficiency of 93.45%, and a considerable cyclability, offering a new choice for development of low cost, environmentally benign and high capacity Li-ion batteries. Besides, the chemical prelithiation process using Li-Naph as a prelithiation reagent developed in this work is mild, highly efficient and widely applicable to a large range of cathode and anode materials, possibly making a significant contribution to the commercial application of nanostructured electrode materials that suffered from a low initial Coulombic efficiency or large initial capacity.

ASSOCIATED CONTENT Supporting Information Available. Synthetic details of materials and electrodes, optimized prelithiation process 17 / 22



conditions and structure characterizations. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] E-mail: [email protected] E-mail: [email protected]

ACKNOWLEDGEMENT The authors appreciate the financial support from the National Science Foundation of China (Grant No. 21773177), and the Natural Key Research and Development Program for New Energy Vehicles (No. 2016YFB0100200).

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ACS Energy Letters

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ACS Energy Letters

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An Effective Chemical Prelithiation Strategy for Building a Silicon

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