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Using Li2S to Compensate for the Loss of Active Lithium in Li-ion Batteries
Electrochimica Acta 255 (2017) 212–219
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Research Paper
Using Li2S to Compensate for the Loss of Active Lithium in Li-ion Batteries Yuanjie Zhana,b , Hailong Yua,b , Liubin Bena,b , Yuyang Chena,b , Xuejie Huanga,b,* a b
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China School of Physical Sciences, University of Chinese Academy of Sicences, Beijing, 100190, China
A R T I C L E I N F O
Article history: Received 23 August 2017 Received in revised form 26 September 2017 Accepted 27 September 2017 Available online 28 September 2017 Keywords: Lithium sulfide Active lithium Lithium-Ion Battery Core-shell Cathode pre-lithiation
A B S T R A C T
Lithium-ion batteries with graphite as the anode consume 10% of the active lithium from the cathode to form a solid electrolyte interphase layer during the first cycle, resulting in a reduced reversible capacity. Here, we report using Li2S as a cathode pre-lithiation material to compensate for the loss of active lithium and, consequently, enhance the specific energy of lithium-ion batteries. A Li2S material with a core-shell structure is prepared by mixing Li2S, Ketjenblack (KB) and poly(vinylpyrrolidone) (PVP) in anhydrous ethanol, and the material shows a specific charge capacity of 1053 mAh g1 (631 mAh g1 based on the total weight of cathode pre-lithiation materials, binders and conductive additives). The ability of this material to compensate for active lithium is investigated by coating a typical cathode LiFePO4 as an example, with the core-shell Li2S/KB/PVP via a simple and non-toxic coating method. Our results show that the LiFePO4 (Li2S)/graphite full cell exhibits a specific discharge capacity of 146.7 mAh g1 in the first cycle, which is the same as the specific discharge capacity of the LiFePO4 half cell. XPS analysis reveals Li2S decomposes into lithium ions and sulfur with release of electrons in the first charge. Such a successful extraction of the active lithium from Li2S results in excellent cycling performance with increased specific energy. © 2017 Published by Elsevier Ltd.
1. Introduction Lithium-ion batteries (LIBs) have attracted much attention for applications in mobile phones, electric vehicles, etc. because of their long cycle life and high specific energy [1]. However, during the first charge process of LIBs with graphite as the anode, 10% of the active lithium from the cathode is consumed to form a solid electrolyte interphase (SEI) layer on the anode surface, reducing the specific energy of the existing LIBs [2–5]. To address this problem, many efforts have been devoted to compensating the initial loss of active lithium in the first cycle of LIBs. Stabilized lithium metal powders and lithium silicide particles have been investigated as anode pre-lithiation materials [6–8]. These methods can effectively enhance the specific energy of LIBs. However, the preparation processes involved are complicated and usually expensive, limiting their practical applications. Cathode pre-lithiation materials, usually
* Corresponding Author at: Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China. E-mail address: [email protected] (X. Huang). https://doi.org/10.1016/j.electacta.2017.09.167 0013-4686/© 2017 Published by Elsevier Ltd.
less expensive, were suggested by Armand and co-workers, e.g. azide, oxocarbon, dicarboxylate and hydrazide families, which can be used as sacrificial salts to compensate for the initial loss of active lithium [9]. Some stable lithium salts, such as Li6CoO4 [10] and Li2NiO2 [11], have also been explored as cathode prelithiation materials due to their low Coulombic efficiency in the first cycle. Cui’s group showed that nano-composites of mixed lithium salts and Co, e.g. Li2S/Co, LiF/Co and Li2O/Co composites [12–14], were good candidates to be used as cathode prelithiation materials. In general, an ideal pre-lithiation material should be inexpensive and higher in capacity. Herein, we have designed a core-shell Li2S/KB/PVP nano-composite as a cathode pre-lithiation material. The active lithium in this Li2S can be fully released in 1 M LiPF6 in EC/DMC (1:1 by volume), according to the decomposition reaction (Li2S ! 2 Li + + S + 2 e). This reaction corresponds to a high theoretical specific capacity of 1167 mAh g1 [15]. In addition, the potential barrier of Li2S in the initial charge is as high as 3.5 V [16], which is lower than the cutoff charge potential of many existing cathode materials, e.g. 4.1 V for LiFePO4 [17], 4.2 V for LiCoO2 [18], and 4.9 V for LiNi0.5Mn1.5O4 [19] etc., ensuring the full delithiation of Li2S. Furthermore, the cutoff discharge potential of the existing cathode is higher than 2.5 V to avoid the electrochemical lithiation of sulfur
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(<2.4 V) [20]. Thus, in theory, all the active lithium in Li2S can be irreversibly extracted during the first cycle and used to compensate for the loss of the active lithium consumed to form the SEI. Note that active lithium is difficult to extract from commercial Li2S due to its micromorphology and insulating nature. In this work, the core-shell Li2S/KB/PVP nano-composite was synthesized via a simple coating method. The effectiveness of the core-shell Li2S/KB/PVP was examined in a LiFePO4/graphite battery that is widely used in electric vehicles due to its high thermal stability, low cost, excellent cyclability and high rate capability [21,22]. Li2S with a theoretical specific capacity of 1167 mAh g1 is a very good cathode pre-lithiation material. Normally, Li2S is used as the cathode in ether-based electrolytes for Li-S battery, and it is believed that there is no electrochemical activity in carbonate-based electrolytes [23,24]. However, we show that the core-shell Li2S/KB/PVP nano-composite exhibits a specific charge capacity of 1053 mAh g1 (631 mAh g1 based on the total weight of cathode pre-lithiation materials, binders and conductive additives) and a limited specific discharge capacity between 2.5–3.6 V in carbonate-based electrolytes.
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2. Experimental 2.1. Synthesis of the Cathode and graphite electrode For the preparation of the cathode, e.g. LiFePO4 in this work, deionized water was used as the solvent, and the LiFePO4 powders, conductive additives (Super P), carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) (92:5:1:2) were mixed together to form a uniform slurry. Then, the slurry was coated on aluminum foil. The prepared electrodes were put into the oven at 55 C for 5 h and then cut into discs (14 mm in diameter) using a cutting machine. The loading level of active material in LiFePO4 electrode was 7.8 and 9.1 mg cm2. For the preparation of the graphite electrode, deionized water was also used as the solvent, and the graphite powders, Super P, CMC and SBR (93:2:2:3) were mixed together to form a uniform slurry. Then, the slurry was coated on copper foil. The prepared electrodes were put into the oven at 55 C for 5 h and then cut into discs (14 mm in diameter) using a cutting machine. The loading level of active material in graphite electrode was 4.3 mg cm2. These discs were all put into an oven at 120 C for 6 h.
Fig. 1. (a) Schematic of the synthesis of the core-shell Li2S/KB/PVP material via a simple dissolution in anhydrous ethanol. The corresponding SEM images of (b1–b3) commercial Li2S, (c1–c3) KB, (d1–d3) Li2S/KB, and (e1–e3) Li2S/KB/PVP after drying are shown.
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2.2. Synthesis of the Li2S electrode Commercial lithium sulfide powders (Li2S, 99.9% trace metals basis) were purchased from Alfa Aesar. Anhydrous ethanol (99.5%, <0.005% water) and poly(vinylpyrrolidone) powders (PVP, K90 average Mw 360,000) were purchased from SigmaAldrich. The powders and ethanol were used as the starting materials. In a typical procedure, 0.3 g Li2S was added to 20 mL anhydrous ethanol and stirred for 12 h at room temperature. The Li2S solution sat for a few hours, and the supernatant was removed and placed into another bottle by a pipette. Then, 0.15 g Ketjenblack carbon black (KB) and 0.05 g PVP were added into the solution, and the solution was agitated by an ultrasonic needle for 1 h and further stirred for 12 h to form a uniform slurry. Finally, the slurry was uniformly coated on carbon paper by a brush. The prepared electrode was put into an oven at 120 C under vacuum for 6 h and then cut into 14 discs (14 mm in diameter) by a cutting machine. 2.3. Synthesis of the Li2S coated Cathode To balance the cathode and graphite anode in a full cell, 20 mL Li2S/KB/PVP slurry, the optimized amount, was coated on the LiFePO4 electrode with loading level of 12 mg. To better observe the Li released, more Li2S/KB/PVP (40 mL) slurry was coated on the LiFePO4 electrode for half cell test. The prepared electrode was placed on a heating plate at 60 C for 2 h and then heated to 120 C for 10 h. The Li2S synthesis was performed in an argon-filled glove box (Lab Star, Braun, Germany) with moisture and oxygen concentrations below 5 ppm due to the sensitivity of Li2S to moisture and oxygen. The amount of Li2S/KB/PVP (0.5 mg for
20 mL) was calculated by measuring the weight of the LiFePO4 and LiFePO4 (Li2S) electrodes with a high precision balance. 2.4. Electrochemical characterization Coin cells of CR2032 type were assembled in an argon-filled glove box (Lab Star, Braun, Germany) to evaluate the electrochemical properties and mechanisms of the electrodes. The LiFePO4, LiFePO4 (Li2S) and Li2S electrode were used as the cathode, and graphite (full cell) and lithium plates (half cell) were used as the anode. The full cell was balanced according to the capacity of the cathode and graphite with a ratio of 1: 1.1. Lithium hexafluorophosphate (1 M) dissolved in ethylene (EC): dimethyl carbonate (DMC) (1:1 by volume with 1% VC, BASF, Suzhou, China) was used as the electrolyte. A PP/PE/PP was used as the separator. All the cells were galvanostatically charged and discharged between 2.5 V and 3.6 V on a Land auto battery tester (Wuhan Land). All the cells were cycled at 0.05C in the 1 st cycle and 0.2C for the following cycles. The specific capacities and current rates were calculated according to the mass of Li2S for the Li2S cathode and the mass of LiFePO4 for the LiFePO4 or LiFePO4 (Li2S) cathode. All C rates are based on the theoretical capacity of Li2S (1C = 1166 mA g1) and LiFePO4 (1C = 160 mA g1). Cyclic voltammetry with a scan rate of 0.02 mV s1 was carried out on a CHI660C workstation (CH Instruments) in a potential range from 2.5 to 3.8 V versus Li+/Li. 2.5. Materials characterization The CR2032 coin cells of LiFePO4 and LiFePO4 (Li2S) half cell were disassembled in an argon-filled glove box (Lab Star, Braun,
Fig. 2. (a) HRTEM images of the Li2S/KB nano-composite. (b) Enlarged region corresponding to the black square in (a). (c) Initial voltage profile for the core-shell Li2S/KB/PVP half cell in EC/DMC at 0.05C rate.
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Germany) in the 1 st and 10th charge state. The cathode electrodes were thoroughly washed with anhydrous DMC to remove the residual electrolytes and were dried in the antechamber of the glove box under vacuum overnight prior to characterization and analyses. X-ray diffraction (XRD) characterization. The XRD diffraction patterns were recorded on a Bruker D8 ADVANCE X-ray diffractometer (Germany) using Cu Ka radiation (l = 0.15406 nm) from 10 to 60 with the Sample-Saver storage container under purified, protecting argon at a scan rate of 0.1 s per 0.02 . Scanning electron microscopy (SEM) characterization. The morphology and microstructure were characterized by SEM on a HITACHI SU-4800 instrument with the Sample-Saver storage container under purified, protecting argon. The elemental composition and mapping were conducted and performed by energy dispersive X-ray (EDX) analyses on an SEM HITACHI SU-4800 instrument operated at 15 kV. The TEM characterization was performed on a TEM JEOL 2010F instrument with a field emission gun and operated at an acceleration voltage of 200 kV.
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X-ray photoelectron spectroscopy (XPS) characterizations were carried out on a photoelectron spectrometer using Mg Ka radiation (ESCALAB 250, Sigma Probe, Thermo VG Scientific Co. Ltd.). The binding energy calibrations were corrected using the signal of the C 1 s peak (284.8 eV) to eliminate charging of the samples during the analysis. 3. Results and discussion Fig. 1a shows the detailed synthesis method for the core-shell Li2S/KB/PVP material. The particles of commercial Li2S are several micrometers in size, Fig. 1b1–b3, and they were initially dissolved in anhydrous ethanol. Then, KB with an average size of 20–30 nm, Fig. 1c1–c3, was added into the solution, which was agitated by an ultrasonic needle to ensure a homogeneous distribution. The BET of KB is 1131.7 m2 g1, as shown in Fig. S1. The dissolved Li2S particles coat the surface of KB due to its high adsorption during the evaporation of the solution. The resulting KB particles with the surface coated Li2S do not show a morphology change except for an increased size (30–50 nm), Fig. 1d1–d3. The XRD diffractions of the Li2S coated on the surface of the KB exhibit the characteristic Li2S peaks with distinct broadening, as shown in Fig. S2.
Fig. 3. (a) Schematic of the synthesis of the LiFePO4 electrode coated by the core-shell Li2S/KB/PVP via a simple coating method. SEM images of the prepared (b) LiFePO4 and (c) LiFePO4 (Li2S) electrodes, respectively. (d) Typical image of the LiFePO4 (Li2S) electrode and the corresponding EDS mapping for the element distribution, (e) C and (f) S.
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To prepare the Li2S electrode, the PVP powders used as binders were added into the above Li2S/KB solution, and the morphology was shown in Fig. 1e1–e3. Li2S/KB was uniformly coated by PVP due to the strong affinity between the polarity of Li2S and the high polarity of the >C¼O functional groups in PVP, and this also prevented the undesirable growth of individual Li2S particles via a steric separation mechanism [25,26]. Li-S battery is commonly believed to only function in etherbased electrolytes and not in carbonate-based electrolytes except for special structure [24,27]. However, we showed that active lithium can be fully released in the core-shell Li2S/KB/PVP in EC/ DMC. To understand the lithium insertion/extraction of Li2S, a Li2S half cell was initially investigated. The HRTEM images of the Li2S/KB nano-composite and the corresponding enlarged region show that the amorphous core is coated by a crystalline layer with a selected area showing lattice fringes with 0.33 nm spacing, corresponding to the (111) plane of Li2S (Fig. 2a, b). Fig. 2c shows a typical voltage profile for the coreshell Li2S/KB/PVP half cell in EC/DMC in the first cycle. During the initial charge process, there is a voltage hump that probably originated from the phase nucleation of the polysulfides [28,29]. A single delithiation slope starts at 2.7 V and increases sharply to 3.1 V. The active lithium is continuously extracted from Li2S at the voltage plateau until the voltage reaches 3.6 V, resulting in a full specific charge capacity of 1053 mAh g1 (631 mAh g1 based on the total weight of cathode pre-lithiation materials, binders and conductive additives). This value is almost the same as the theoretical specific capacity of 1167 mAh g1 based on the typical reaction (Li2S ! 2 Li+ + S + 2 e). During the initial discharge process, there is almost no discharge capacity in EC/DMC above 2.5 V (only approximately 11 mAh g1) (Fig. 2c), suggesting that the active lithium extracted during the first charge cannot be consumed to convert S into Li2S above this voltage. Thus, the active lithium released from Li2S can be fully used to compensate for the loss of the active lithium from the cathode via various mechanisms, such as the SEI formation in the first cycle and the following loss of active lithium.
The LiFePO4 electrode coated by the core-shell Li2S/KB/PVP was prepared via a simple coating method, as described in Fig. 3a. Fig. 3b and c show the SEM images of the LiFePO4 and LiFePO4 (Li2S) electrodes, respectively. The surface of the LiFePO4 (Li2S) electrode was clearly covered in nano particles, which were assigned as Li2S, compared to the smooth surface of the LiFePO4 electrode. Fig. 3d–f show the typical EDS elemental mapping of sulfur and carbon, which demonstrates the uniform distribution of sulfur and carbon element on the surface of the LiFePO4 (Li2S) electrode. All these results suggest that nano-sized Li2S is uniformly coated on the surface of the LiFePO4 electrode. The electrochemical performance of the LiFePO4 (Li2S) and LiFePO4 half cells were investigated to evaluate the effect of Li2S as a cathode pre-lithiation material. Fig. 4a shows the voltage profiles of the 1 st (0.05C) and 2nd cycle (0.2C) of the LiFePO4 (Li2S) and LiFePO4 half cells. The initial specific charge capacity for the LiFePO4 half cell was 150 mAh g1, and it increased significantly to 195 mAh g1 for the LiFePO4 (Li2S) half cell. An additional curve ranging from 2.6 V to 3.5 V with a specific capacity of approximately 17 mAh g1 was observed for the LiFePO4 (Li2S) half cell, which was a clear indication of the decomposition of Li2S. The specific charge capacity of the LiFePO4 (Li2S) half cell at a platform of 3.5 V oxidation of LiFePO4 was 178 mAh g1, which was 28 mAh g1 higher than that of the LiFePO4 half cell at 150 mAh g1. The extra 28 mAh g 1 was assigned to the decomposition of Li2S. The initial specific discharge capacities for both the LiFePO4 (Li2S) and LiFePO4 half cells were similar, 147 mAh g1 at a platform of 3.4 V. There was almost no additional specific discharge capacity associated with Li2S, indicating that the active lithium extracted from Li2S in the 1 st charge process cannot convert to Li2S due to the discharge potential limit > 2.5 V. Fig. 4(b) shows the cycle performance for the LiFePO4 (Li2S) and LiFePO4 half cells at 0.2C rate. Both samples showed excellent cycling performance without a significant capacity degradation and with 100% Coulombic efficiency. The initial capacity fading in both half cells was attributed to the C rates, i.e., 0.05C in the first cycle and 0.2C in the following cycles. Both samples showed a
Fig. 4. Comparison of the electrochemical performance for the LiFePO4 (Li2S) and LiFePO4 half cells. (a) The 1 st and 2nd cycle voltage profiles for the LiFePO4 (Li2S) and LiFePO4 half cells. (b) The cycle performance for the LiFePO4 (Li2S) and LiFePO4 half cells. The cyclic voltammograms for (c) LiFePO4 (Li2S) and (d) LiFePO4 half cells in the 1 st and 2nd cycle, respectively. The rate for the initial cycle was 0.05C, and that for the following cycles was 0.2C.
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Fig. 5. Comparison of the electrochemical performance for the LiFePO4 (Li2S)/graphite and LiFePO4/graphite full cells. (a) The 1 st and 2nd cycle voltage profiles for the LiFePO4 (Li2S)/graphite and LiFePO4/graphite full cells. (b) The cycle performance of the LiFePO4 (Li2S)/graphite and LiFePO4/graphite full cells. The rate for the initial cycle was 0.05C, and that for the following cycles was 0.2C.
specific discharge capacity of approximately 140 mAh g1 at 0.2C in the following cycles. The excellent cycling performance of LiFePO4 (Li2S) indicates that the core-shell Li2S/KB/PVP coated on the LiFePO4 electrode can stably work in EC/DMC and does not affect the specific capacity or Coulombic efficiency of LiFePO4. The rate performance of the both half cells with no obvious distinction were also shown in Fig. S4. Fig. 4c and d show the slow scan cycle voltammograms for the LiFePO4 (Li2S) and LiFePO4 half cells, respectively, in the 1 st and 2nd cycle. The anodic peak for both half cells in the 1 st cycle was higher than that in the 2nd cycle due to the electrochemical activation of LiFePO4 in the 1 st cycle. However, for LiFePO4, only one anodic and one cathodic peak was observed in the 1 st and 2nd cycle. For LiFePO4 (Li2S), there were clearly two anodic peaks and one cathodic peak in the 1 st cycle. In the 2nd cycle, the anodic and cathodic peaks were similar to that of LiFePO4. The additional small and broad peak for LiFePO4 (Li2S) in the 1 st cycle starts at 2.6 V, which suggests the decomposition of Li2S begins above this voltage. The peak is in agreement with the slope observed in Fig. 4a. The large broad peak can be assigned to the platform at 3.5 V in the voltage profiles, which is similar to that of LiFePO4. Note that the large broad peak for LiFePO4 (Li2S) is centered at 3.66 V, which is 0.04 V lower than that of LiFePO4. That may be due to the extra capacity that comes from the decomposition of Li2S at the platform of 3.5 V, which is observed in Fig. 4a. The broad cathodic peak of LiFePO4 (Li2S) in the 1 st cycle shows the same features as that of LiFePO4, suggesting no reduction from S to Li2S occurs up to 2.5 V. Note that there is no trace of a peak that can be assigned to the decomposition of Li2S during the 2nd cycle, indicating that the Li2S coated on the surface of the LiFePO4 electrode completely decomposes during the 1 st cycle. This is consistent with the voltage profiles shown in Fig. 4a. LiFePO4 (Li2S)/graphite and LiFePO4/graphite full cells were also assembled to better investigate the effect of Li2S as a cathode prelithiation material. Fig. 5a shows the voltage profiles for the 1 st (0.05C) and 2nd cycle (0.2C) for both LiFePO4 (Li2S)/graphite and LiFePO4/graphite full cells. During the initial charge process, there was a slope below the 3.5 V plateau for both samples; however, the slope of the LiFePO4 (Li2S)/graphite full cell was sharper than that of the LiFePO4/graphite full cell, which indicated the decomposition of Li2S, as observed in Fig. 4a. The most striking difference was that the LiFePO4 (Li2S)/graphite full cell exhibited a higher initial specific charge capacity of 174.4 mAh g1, which is 25 mAh g1 higher than that of the LiFePO4/graphite full cell. The extra capacity can only be assigned to the active lithium donated by the decomposition of Li2S. For the LiFePO4/graphite full cell, the specific discharge capacity was 132.2 mAh g1 with a Coulombic efficiency of 90% during the initial discharge process. The decreased capacity was mainly due to the formation of the SEI
on the surface of the graphite, which is well explained in the literature [2–5]. However, the LiFePO4 (Li2S)/graphite full cell exhibited an initial specific discharge capacity of 146.7 mAh g1, which is the same as the theoretical capacity of the LiFePO4 half cell, as shown in Fig. 4a. The significantly increased capacity was attributed to the full compensation of the active lithium by the active lithium released from the decomposition of Li2S during the initial charge process. Fig. 5b shows the discharge capacity retention and Coulombic efficiency for the LiFePO4 (Li2S)/graphite and LiFePO4/graphite full cells at 0.2C. The specific discharge capacity of the LiFePO4 (Li2S)/ graphite full cell was 140 mAh g1, which is the same as that of the
Fig. 6. The S XPS spectra and peak deconvolution of the LiFePO4 (Li2S) electrodes before cycle and after the 1 st and 10th cycle, respectively.
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Fig. 7. (a) Comparison of the existing cathode pre-lithiation materials in the 1 st cycle. The specific capacity was calculated based on the weight of electrode of cathode prelithiation materials (excluding current collectors). The increase in (b) specific capacity and (c) specific energy of LiFePO4/graphite full cell via the compensation of the lost active lithium using Li2S/KB/PVP. The specific capacity was calculated based on the weight of the cathode electrode (excluding current collectors). The specific energy was calculated based on the total weight of the cathode and anode electrode (excluding current collectors).
half cell and is 12 mAh g1 higher than that of the LiFePO4/graphite full cell, 128 mAh g1 at 0.2C in the following cycles. The Coulombic efficiency of both full cells was almost 100%. After 100 cycles, the capacity of the LiFePO4 (Li2S)/graphite full cell was still approximately 140 mAh g1, which was almost a 100% capacity retention. The specific capacity was obviously enhanced by the active lithium compensation from Li2S. To confirm the irreversible conversion of the core-shell Li2S in LiFePO4 (Li2S) during the 1 st cycle, XPS and XRD analyses of the electrode before cycle and after the 1 st and 10th cycle were performed. The XRD diffraction patterns, Fig. S3, showed no trace of Li2S due to its limited content. Fig. 6 shows the fitted S XPS spectra of the LiFePO4 (Li2S) electrode before cycle and after the 1 st and 10th cycle, respectively. For the electrode before cycle, a pair of S 2p3/2 and S 2p1/2 doublets at 161.2 eV and 162.4 eV were observed and are typical of the Li-S binding energy for Li2S [30]. The pair of S 2p3/2 and S 2p1/2 doublets at 166.2 eV and 167.4 eV represent Li2SO3, which may be attributed to the oxidation of some surface of Li2S before treatment [31]. However, the results for the 1 st and 10th cycle showed a pair of S 2p3/2 and S 2p1/2 doublets at 163.6 and 164.8 eV, respectively, which belong to the S-S bonding [32]. The pair of S 2p3/2 and S 2p1/2 doublets at 168.8 and 170.0 eV, respectively, belong to the sulfate [33]. Thus, S8 was confirmed as the charge product of Li2S according to the following conversion reaction mechanism: Li2S ! S + 2 Li+ + 2 e. Fig. 7a shows the delithiation and lithiation specific capacity of the existing cathode pre-lithiation materials in the 1 st cycle. The core-shell Li2S/KB/PVP in this work has the highest specific delithiation capacity (631 mAh g1) and low specific lithiation capacity (6.6 mAh g1) based on the weight of cathode electrode (cathode pre-lithiation materials, binders and conductive additives), among the existing cathode pre-lithiation materials. Since various types of current collectors are used by different groups, the weight of current collectors was excluded during calculation. Furthermore, compared with other pre-lithiation materials, the preparation of the cathode pre-lithiation materials by dissolving
Li2S in anhydrous ethanol only involves with a simple and environmental-friendly method. Moreover, the fully delithiates voltage of the core-shell Li2S/KB/PVP is less than 3.6 V, which can be readily used on other cathodes, such as LiCoO2, Li2MnO4 and LiNi0.5Mn1.5O4 etc. Fig. 7b shows the increase of the specific capacity (based on the weight of the cathode electrode, but excluding current collectors) of LiFePO4/graphite full cell by the compensation of the 10% lost active lithium using Li2S/KB/PVP. The specific capacity of LiFePO4 (Li2S)/graphite full cell showed a remarkable increase (8.2%), compared to LiFePO4/graphite. The specific energy, Fig. 7c, was calculated based on the total weight of cathode and anode electrode of the full cell (excluding current collectors). The specific energy of LiFePO4/graphite also showed an increase of 5.6% via the compensation of the lost active lithium by using Li2S/KB/PVP. 4. Conclusion In summary, we designed a core-shell Li2S/KB/PVP nanocomposite via simply mixing Li2S, KB and PVP in anhydrous ethanol. The active lithium in this core-shell Li2S/KB/PVP material can be fully released during the first charge in carbonate-based electrolytes and has a specific charge capacity of 1053 mAh g1 (631 mAh g1 based on the total weight of cathode pre-lithiation materials, binders and conductive additives). The limited first specific discharge capacity between 2.5–3.6 V suggests the releasing of lithium from Li2S is irreversible; thus, the irreversible active lithium can be used to compensate for the active lithium lost in the formation of the SEI. The LiFePO4 electrode coated by the core-shell Li2S/KB/PVP was prepared via a simple and non-toxic coating method to verify the ability of the core-shell Li2S/KB/PVP material to act as a cathode pre-lithiation material. Our results showed that the LiFePO4 (Li2S) electrode had a higher specific energy than that without Li2S. Furthermore, there was almost no capacity or Coulombic efficiency deterioration in the carbonatebased electrolytes. The excellent electrochemical performance was described as the following. 1) The LiFePO4 (Li2S)/graphite full cell shows 25 mAh g1 more specific charge capacity and 14 mAh g1
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more specific discharge capacity in the first cycle compared with the LiFePO4/graphite full cell. This was attributed to the compensation of the lost active lithium by Li2S. 2) The specific discharge capacity of the LiFePO4 (Li2S)/graphite full cell is 140 mAh g1, which is the same as that of the half cell and 12 mAh g1 higher than that of the LiFePO4/graphite full cell (128 mAh g1) after the first cycle at 0.2C. 3) The LiFePO4 (Li2S)/ graphite full cell shows a Coulombic efficiency of almost 100% in the following cycles at 0.2C. The core-shell Li2S/KB/PVP is a very good cathode pre-lithiation material for LiFePO4 as observed here and many other cathode materials. Acknowledgments This work is financially supported by the National Program on Key Basic Research Project of China (973 Program, Grant No. 2013CB934002), “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA09010000) and National Key R&D Program of China (2016YFB0100300). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.electacta.2017. 09.167. References [1] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [2] E. Peled, The Electrochemical Behavior of Alkali and Alkaline Earth Metals in Nonaqueous Battery Systems-The Solid Electrolyte Interphase Model, J. Electrochem. Soc. 126 (1979) 2047–2051. [3] P. Verma, P. Maire, P. Novák, A review of the features and analyses of the solid electrolyte interphase in Li-ion batteries, Electrochim. Acta 55 (2010) 6332– 6341. [4] K. Zaghib, G. Nadeau, K. Kinoshita, Effect of Graphite Particle Size on Irreversible Capacity Loss, J. Electrochem. Soc. 147 (2000) 2110–2115. [5] Y. Matsumura, S. Wang, J. Mondori, Mechanism Leading to Irreversible Capacity Loss in Li Ion Rechargeable Batteries, J. Electrochem. Soc. 142 (1995) 2914–2918. [6] M.W. Forney, M.J. Ganter, J.W. Staub, R.D. Ridgley, B.J. Landi, Prelithiation of Silicon-Carbon Nanotube Anodes for Lithium Ion Batteries by Stabilized Lithium Metal Powder (SLMP), Nano Lett. 13 (2013) 4158–4163. [7] C.R. Jarvis, M.J. Lain, M.V. Yakovleva, Y. Gao, A prelithiated carbon anode for lithium-ion battery applications, J. Power Sources 162 (2006) 800–802. [8] Z.H. Wang, Y.B. Fu, Z.C. Zhang, S.W. Yuan, K. Amine, V. Battaglia, G. Liu, Application of Stabilized Lithium Metal Powder (SLMP1) in graphite anode—A high efficient prelithiation method for lithium-ion batteries, J. Power Sources 260 (2014) 57–61. [9] D. Shanmukaraj, S. Grugeon, S. Laruelle, G. Douglade, J.M. Tarascon, M. Armand, Sacrificial salts: Compensating the initial charge irreversibility in lithium batteries, Electrochem. Commun. 12 (2010) 1344–1347. [10] M. Noh, J. Cho, Role of Li6CoO4 Cathode Additive in Li-Ion Cells Containing Low Coulombic Efficiency Anode Material, J. Electrochem. Soc. 159 (2012) A1329– A1334. [11] M.G. Kin, J. Cho, Air stable Al2O3-coated Li2NiO2 cathode additive as a surplus current consumer in a Li-ion cell, J. Mater. Chem. 18 (2008) 5880–5887. [12] Y.M. Sun, H.W. Lee, Z.W. Seh, N. Liu, J. Sun, Y.Z. Li, Y. Cui, High-capacity battery cathode prelithiation to offset initial lithium loss, Nat. Energy 1 (2016) 15008.
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Research Paper
Using Li2S to Compensate for the Loss of Active Lithium in Li-ion Batteries Yuanjie Zhana,b , Hailong Yua,b , Liubin Bena,b , Yuyang Chena,b , Xuejie Huanga,b,* a b
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China School of Physical Sciences, University of Chinese Academy of Sicences, Beijing, 100190, China
A R T I C L E I N F O
Article history: Received 23 August 2017 Received in revised form 26 September 2017 Accepted 27 September 2017 Available online 28 September 2017 Keywords: Lithium sulfide Active lithium Lithium-Ion Battery Core-shell Cathode pre-lithiation
A B S T R A C T
Lithium-ion batteries with graphite as the anode consume 10% of the active lithium from the cathode to form a solid electrolyte interphase layer during the first cycle, resulting in a reduced reversible capacity. Here, we report using Li2S as a cathode pre-lithiation material to compensate for the loss of active lithium and, consequently, enhance the specific energy of lithium-ion batteries. A Li2S material with a core-shell structure is prepared by mixing Li2S, Ketjenblack (KB) and poly(vinylpyrrolidone) (PVP) in anhydrous ethanol, and the material shows a specific charge capacity of 1053 mAh g1 (631 mAh g1 based on the total weight of cathode pre-lithiation materials, binders and conductive additives). The ability of this material to compensate for active lithium is investigated by coating a typical cathode LiFePO4 as an example, with the core-shell Li2S/KB/PVP via a simple and non-toxic coating method. Our results show that the LiFePO4 (Li2S)/graphite full cell exhibits a specific discharge capacity of 146.7 mAh g1 in the first cycle, which is the same as the specific discharge capacity of the LiFePO4 half cell. XPS analysis reveals Li2S decomposes into lithium ions and sulfur with release of electrons in the first charge. Such a successful extraction of the active lithium from Li2S results in excellent cycling performance with increased specific energy. © 2017 Published by Elsevier Ltd.
1. Introduction Lithium-ion batteries (LIBs) have attracted much attention for applications in mobile phones, electric vehicles, etc. because of their long cycle life and high specific energy [1]. However, during the first charge process of LIBs with graphite as the anode, 10% of the active lithium from the cathode is consumed to form a solid electrolyte interphase (SEI) layer on the anode surface, reducing the specific energy of the existing LIBs [2–5]. To address this problem, many efforts have been devoted to compensating the initial loss of active lithium in the first cycle of LIBs. Stabilized lithium metal powders and lithium silicide particles have been investigated as anode pre-lithiation materials [6–8]. These methods can effectively enhance the specific energy of LIBs. However, the preparation processes involved are complicated and usually expensive, limiting their practical applications. Cathode pre-lithiation materials, usually
* Corresponding Author at: Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China. E-mail address: [email protected] (X. Huang). https://doi.org/10.1016/j.electacta.2017.09.167 0013-4686/© 2017 Published by Elsevier Ltd.
less expensive, were suggested by Armand and co-workers, e.g. azide, oxocarbon, dicarboxylate and hydrazide families, which can be used as sacrificial salts to compensate for the initial loss of active lithium [9]. Some stable lithium salts, such as Li6CoO4 [10] and Li2NiO2 [11], have also been explored as cathode prelithiation materials due to their low Coulombic efficiency in the first cycle. Cui’s group showed that nano-composites of mixed lithium salts and Co, e.g. Li2S/Co, LiF/Co and Li2O/Co composites [12–14], were good candidates to be used as cathode prelithiation materials. In general, an ideal pre-lithiation material should be inexpensive and higher in capacity. Herein, we have designed a core-shell Li2S/KB/PVP nano-composite as a cathode pre-lithiation material. The active lithium in this Li2S can be fully released in 1 M LiPF6 in EC/DMC (1:1 by volume), according to the decomposition reaction (Li2S ! 2 Li + + S + 2 e). This reaction corresponds to a high theoretical specific capacity of 1167 mAh g1 [15]. In addition, the potential barrier of Li2S in the initial charge is as high as 3.5 V [16], which is lower than the cutoff charge potential of many existing cathode materials, e.g. 4.1 V for LiFePO4 [17], 4.2 V for LiCoO2 [18], and 4.9 V for LiNi0.5Mn1.5O4 [19] etc., ensuring the full delithiation of Li2S. Furthermore, the cutoff discharge potential of the existing cathode is higher than 2.5 V to avoid the electrochemical lithiation of sulfur
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(<2.4 V) [20]. Thus, in theory, all the active lithium in Li2S can be irreversibly extracted during the first cycle and used to compensate for the loss of the active lithium consumed to form the SEI. Note that active lithium is difficult to extract from commercial Li2S due to its micromorphology and insulating nature. In this work, the core-shell Li2S/KB/PVP nano-composite was synthesized via a simple coating method. The effectiveness of the core-shell Li2S/KB/PVP was examined in a LiFePO4/graphite battery that is widely used in electric vehicles due to its high thermal stability, low cost, excellent cyclability and high rate capability [21,22]. Li2S with a theoretical specific capacity of 1167 mAh g1 is a very good cathode pre-lithiation material. Normally, Li2S is used as the cathode in ether-based electrolytes for Li-S battery, and it is believed that there is no electrochemical activity in carbonate-based electrolytes [23,24]. However, we show that the core-shell Li2S/KB/PVP nano-composite exhibits a specific charge capacity of 1053 mAh g1 (631 mAh g1 based on the total weight of cathode pre-lithiation materials, binders and conductive additives) and a limited specific discharge capacity between 2.5–3.6 V in carbonate-based electrolytes.
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2. Experimental 2.1. Synthesis of the Cathode and graphite electrode For the preparation of the cathode, e.g. LiFePO4 in this work, deionized water was used as the solvent, and the LiFePO4 powders, conductive additives (Super P), carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) (92:5:1:2) were mixed together to form a uniform slurry. Then, the slurry was coated on aluminum foil. The prepared electrodes were put into the oven at 55 C for 5 h and then cut into discs (14 mm in diameter) using a cutting machine. The loading level of active material in LiFePO4 electrode was 7.8 and 9.1 mg cm2. For the preparation of the graphite electrode, deionized water was also used as the solvent, and the graphite powders, Super P, CMC and SBR (93:2:2:3) were mixed together to form a uniform slurry. Then, the slurry was coated on copper foil. The prepared electrodes were put into the oven at 55 C for 5 h and then cut into discs (14 mm in diameter) using a cutting machine. The loading level of active material in graphite electrode was 4.3 mg cm2. These discs were all put into an oven at 120 C for 6 h.
Fig. 1. (a) Schematic of the synthesis of the core-shell Li2S/KB/PVP material via a simple dissolution in anhydrous ethanol. The corresponding SEM images of (b1–b3) commercial Li2S, (c1–c3) KB, (d1–d3) Li2S/KB, and (e1–e3) Li2S/KB/PVP after drying are shown.
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2.2. Synthesis of the Li2S electrode Commercial lithium sulfide powders (Li2S, 99.9% trace metals basis) were purchased from Alfa Aesar. Anhydrous ethanol (99.5%, <0.005% water) and poly(vinylpyrrolidone) powders (PVP, K90 average Mw 360,000) were purchased from SigmaAldrich. The powders and ethanol were used as the starting materials. In a typical procedure, 0.3 g Li2S was added to 20 mL anhydrous ethanol and stirred for 12 h at room temperature. The Li2S solution sat for a few hours, and the supernatant was removed and placed into another bottle by a pipette. Then, 0.15 g Ketjenblack carbon black (KB) and 0.05 g PVP were added into the solution, and the solution was agitated by an ultrasonic needle for 1 h and further stirred for 12 h to form a uniform slurry. Finally, the slurry was uniformly coated on carbon paper by a brush. The prepared electrode was put into an oven at 120 C under vacuum for 6 h and then cut into 14 discs (14 mm in diameter) by a cutting machine. 2.3. Synthesis of the Li2S coated Cathode To balance the cathode and graphite anode in a full cell, 20 mL Li2S/KB/PVP slurry, the optimized amount, was coated on the LiFePO4 electrode with loading level of 12 mg. To better observe the Li released, more Li2S/KB/PVP (40 mL) slurry was coated on the LiFePO4 electrode for half cell test. The prepared electrode was placed on a heating plate at 60 C for 2 h and then heated to 120 C for 10 h. The Li2S synthesis was performed in an argon-filled glove box (Lab Star, Braun, Germany) with moisture and oxygen concentrations below 5 ppm due to the sensitivity of Li2S to moisture and oxygen. The amount of Li2S/KB/PVP (0.5 mg for
20 mL) was calculated by measuring the weight of the LiFePO4 and LiFePO4 (Li2S) electrodes with a high precision balance. 2.4. Electrochemical characterization Coin cells of CR2032 type were assembled in an argon-filled glove box (Lab Star, Braun, Germany) to evaluate the electrochemical properties and mechanisms of the electrodes. The LiFePO4, LiFePO4 (Li2S) and Li2S electrode were used as the cathode, and graphite (full cell) and lithium plates (half cell) were used as the anode. The full cell was balanced according to the capacity of the cathode and graphite with a ratio of 1: 1.1. Lithium hexafluorophosphate (1 M) dissolved in ethylene (EC): dimethyl carbonate (DMC) (1:1 by volume with 1% VC, BASF, Suzhou, China) was used as the electrolyte. A PP/PE/PP was used as the separator. All the cells were galvanostatically charged and discharged between 2.5 V and 3.6 V on a Land auto battery tester (Wuhan Land). All the cells were cycled at 0.05C in the 1 st cycle and 0.2C for the following cycles. The specific capacities and current rates were calculated according to the mass of Li2S for the Li2S cathode and the mass of LiFePO4 for the LiFePO4 or LiFePO4 (Li2S) cathode. All C rates are based on the theoretical capacity of Li2S (1C = 1166 mA g1) and LiFePO4 (1C = 160 mA g1). Cyclic voltammetry with a scan rate of 0.02 mV s1 was carried out on a CHI660C workstation (CH Instruments) in a potential range from 2.5 to 3.8 V versus Li+/Li. 2.5. Materials characterization The CR2032 coin cells of LiFePO4 and LiFePO4 (Li2S) half cell were disassembled in an argon-filled glove box (Lab Star, Braun,
Fig. 2. (a) HRTEM images of the Li2S/KB nano-composite. (b) Enlarged region corresponding to the black square in (a). (c) Initial voltage profile for the core-shell Li2S/KB/PVP half cell in EC/DMC at 0.05C rate.
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Germany) in the 1 st and 10th charge state. The cathode electrodes were thoroughly washed with anhydrous DMC to remove the residual electrolytes and were dried in the antechamber of the glove box under vacuum overnight prior to characterization and analyses. X-ray diffraction (XRD) characterization. The XRD diffraction patterns were recorded on a Bruker D8 ADVANCE X-ray diffractometer (Germany) using Cu Ka radiation (l = 0.15406 nm) from 10 to 60 with the Sample-Saver storage container under purified, protecting argon at a scan rate of 0.1 s per 0.02 . Scanning electron microscopy (SEM) characterization. The morphology and microstructure were characterized by SEM on a HITACHI SU-4800 instrument with the Sample-Saver storage container under purified, protecting argon. The elemental composition and mapping were conducted and performed by energy dispersive X-ray (EDX) analyses on an SEM HITACHI SU-4800 instrument operated at 15 kV. The TEM characterization was performed on a TEM JEOL 2010F instrument with a field emission gun and operated at an acceleration voltage of 200 kV.
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X-ray photoelectron spectroscopy (XPS) characterizations were carried out on a photoelectron spectrometer using Mg Ka radiation (ESCALAB 250, Sigma Probe, Thermo VG Scientific Co. Ltd.). The binding energy calibrations were corrected using the signal of the C 1 s peak (284.8 eV) to eliminate charging of the samples during the analysis. 3. Results and discussion Fig. 1a shows the detailed synthesis method for the core-shell Li2S/KB/PVP material. The particles of commercial Li2S are several micrometers in size, Fig. 1b1–b3, and they were initially dissolved in anhydrous ethanol. Then, KB with an average size of 20–30 nm, Fig. 1c1–c3, was added into the solution, which was agitated by an ultrasonic needle to ensure a homogeneous distribution. The BET of KB is 1131.7 m2 g1, as shown in Fig. S1. The dissolved Li2S particles coat the surface of KB due to its high adsorption during the evaporation of the solution. The resulting KB particles with the surface coated Li2S do not show a morphology change except for an increased size (30–50 nm), Fig. 1d1–d3. The XRD diffractions of the Li2S coated on the surface of the KB exhibit the characteristic Li2S peaks with distinct broadening, as shown in Fig. S2.
Fig. 3. (a) Schematic of the synthesis of the LiFePO4 electrode coated by the core-shell Li2S/KB/PVP via a simple coating method. SEM images of the prepared (b) LiFePO4 and (c) LiFePO4 (Li2S) electrodes, respectively. (d) Typical image of the LiFePO4 (Li2S) electrode and the corresponding EDS mapping for the element distribution, (e) C and (f) S.
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To prepare the Li2S electrode, the PVP powders used as binders were added into the above Li2S/KB solution, and the morphology was shown in Fig. 1e1–e3. Li2S/KB was uniformly coated by PVP due to the strong affinity between the polarity of Li2S and the high polarity of the >C¼O functional groups in PVP, and this also prevented the undesirable growth of individual Li2S particles via a steric separation mechanism [25,26]. Li-S battery is commonly believed to only function in etherbased electrolytes and not in carbonate-based electrolytes except for special structure [24,27]. However, we showed that active lithium can be fully released in the core-shell Li2S/KB/PVP in EC/ DMC. To understand the lithium insertion/extraction of Li2S, a Li2S half cell was initially investigated. The HRTEM images of the Li2S/KB nano-composite and the corresponding enlarged region show that the amorphous core is coated by a crystalline layer with a selected area showing lattice fringes with 0.33 nm spacing, corresponding to the (111) plane of Li2S (Fig. 2a, b). Fig. 2c shows a typical voltage profile for the coreshell Li2S/KB/PVP half cell in EC/DMC in the first cycle. During the initial charge process, there is a voltage hump that probably originated from the phase nucleation of the polysulfides [28,29]. A single delithiation slope starts at 2.7 V and increases sharply to 3.1 V. The active lithium is continuously extracted from Li2S at the voltage plateau until the voltage reaches 3.6 V, resulting in a full specific charge capacity of 1053 mAh g1 (631 mAh g1 based on the total weight of cathode pre-lithiation materials, binders and conductive additives). This value is almost the same as the theoretical specific capacity of 1167 mAh g1 based on the typical reaction (Li2S ! 2 Li+ + S + 2 e). During the initial discharge process, there is almost no discharge capacity in EC/DMC above 2.5 V (only approximately 11 mAh g1) (Fig. 2c), suggesting that the active lithium extracted during the first charge cannot be consumed to convert S into Li2S above this voltage. Thus, the active lithium released from Li2S can be fully used to compensate for the loss of the active lithium from the cathode via various mechanisms, such as the SEI formation in the first cycle and the following loss of active lithium.
The LiFePO4 electrode coated by the core-shell Li2S/KB/PVP was prepared via a simple coating method, as described in Fig. 3a. Fig. 3b and c show the SEM images of the LiFePO4 and LiFePO4 (Li2S) electrodes, respectively. The surface of the LiFePO4 (Li2S) electrode was clearly covered in nano particles, which were assigned as Li2S, compared to the smooth surface of the LiFePO4 electrode. Fig. 3d–f show the typical EDS elemental mapping of sulfur and carbon, which demonstrates the uniform distribution of sulfur and carbon element on the surface of the LiFePO4 (Li2S) electrode. All these results suggest that nano-sized Li2S is uniformly coated on the surface of the LiFePO4 electrode. The electrochemical performance of the LiFePO4 (Li2S) and LiFePO4 half cells were investigated to evaluate the effect of Li2S as a cathode pre-lithiation material. Fig. 4a shows the voltage profiles of the 1 st (0.05C) and 2nd cycle (0.2C) of the LiFePO4 (Li2S) and LiFePO4 half cells. The initial specific charge capacity for the LiFePO4 half cell was 150 mAh g1, and it increased significantly to 195 mAh g1 for the LiFePO4 (Li2S) half cell. An additional curve ranging from 2.6 V to 3.5 V with a specific capacity of approximately 17 mAh g1 was observed for the LiFePO4 (Li2S) half cell, which was a clear indication of the decomposition of Li2S. The specific charge capacity of the LiFePO4 (Li2S) half cell at a platform of 3.5 V oxidation of LiFePO4 was 178 mAh g1, which was 28 mAh g1 higher than that of the LiFePO4 half cell at 150 mAh g1. The extra 28 mAh g 1 was assigned to the decomposition of Li2S. The initial specific discharge capacities for both the LiFePO4 (Li2S) and LiFePO4 half cells were similar, 147 mAh g1 at a platform of 3.4 V. There was almost no additional specific discharge capacity associated with Li2S, indicating that the active lithium extracted from Li2S in the 1 st charge process cannot convert to Li2S due to the discharge potential limit > 2.5 V. Fig. 4(b) shows the cycle performance for the LiFePO4 (Li2S) and LiFePO4 half cells at 0.2C rate. Both samples showed excellent cycling performance without a significant capacity degradation and with 100% Coulombic efficiency. The initial capacity fading in both half cells was attributed to the C rates, i.e., 0.05C in the first cycle and 0.2C in the following cycles. Both samples showed a
Fig. 4. Comparison of the electrochemical performance for the LiFePO4 (Li2S) and LiFePO4 half cells. (a) The 1 st and 2nd cycle voltage profiles for the LiFePO4 (Li2S) and LiFePO4 half cells. (b) The cycle performance for the LiFePO4 (Li2S) and LiFePO4 half cells. The cyclic voltammograms for (c) LiFePO4 (Li2S) and (d) LiFePO4 half cells in the 1 st and 2nd cycle, respectively. The rate for the initial cycle was 0.05C, and that for the following cycles was 0.2C.
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Fig. 5. Comparison of the electrochemical performance for the LiFePO4 (Li2S)/graphite and LiFePO4/graphite full cells. (a) The 1 st and 2nd cycle voltage profiles for the LiFePO4 (Li2S)/graphite and LiFePO4/graphite full cells. (b) The cycle performance of the LiFePO4 (Li2S)/graphite and LiFePO4/graphite full cells. The rate for the initial cycle was 0.05C, and that for the following cycles was 0.2C.
specific discharge capacity of approximately 140 mAh g1 at 0.2C in the following cycles. The excellent cycling performance of LiFePO4 (Li2S) indicates that the core-shell Li2S/KB/PVP coated on the LiFePO4 electrode can stably work in EC/DMC and does not affect the specific capacity or Coulombic efficiency of LiFePO4. The rate performance of the both half cells with no obvious distinction were also shown in Fig. S4. Fig. 4c and d show the slow scan cycle voltammograms for the LiFePO4 (Li2S) and LiFePO4 half cells, respectively, in the 1 st and 2nd cycle. The anodic peak for both half cells in the 1 st cycle was higher than that in the 2nd cycle due to the electrochemical activation of LiFePO4 in the 1 st cycle. However, for LiFePO4, only one anodic and one cathodic peak was observed in the 1 st and 2nd cycle. For LiFePO4 (Li2S), there were clearly two anodic peaks and one cathodic peak in the 1 st cycle. In the 2nd cycle, the anodic and cathodic peaks were similar to that of LiFePO4. The additional small and broad peak for LiFePO4 (Li2S) in the 1 st cycle starts at 2.6 V, which suggests the decomposition of Li2S begins above this voltage. The peak is in agreement with the slope observed in Fig. 4a. The large broad peak can be assigned to the platform at 3.5 V in the voltage profiles, which is similar to that of LiFePO4. Note that the large broad peak for LiFePO4 (Li2S) is centered at 3.66 V, which is 0.04 V lower than that of LiFePO4. That may be due to the extra capacity that comes from the decomposition of Li2S at the platform of 3.5 V, which is observed in Fig. 4a. The broad cathodic peak of LiFePO4 (Li2S) in the 1 st cycle shows the same features as that of LiFePO4, suggesting no reduction from S to Li2S occurs up to 2.5 V. Note that there is no trace of a peak that can be assigned to the decomposition of Li2S during the 2nd cycle, indicating that the Li2S coated on the surface of the LiFePO4 electrode completely decomposes during the 1 st cycle. This is consistent with the voltage profiles shown in Fig. 4a. LiFePO4 (Li2S)/graphite and LiFePO4/graphite full cells were also assembled to better investigate the effect of Li2S as a cathode prelithiation material. Fig. 5a shows the voltage profiles for the 1 st (0.05C) and 2nd cycle (0.2C) for both LiFePO4 (Li2S)/graphite and LiFePO4/graphite full cells. During the initial charge process, there was a slope below the 3.5 V plateau for both samples; however, the slope of the LiFePO4 (Li2S)/graphite full cell was sharper than that of the LiFePO4/graphite full cell, which indicated the decomposition of Li2S, as observed in Fig. 4a. The most striking difference was that the LiFePO4 (Li2S)/graphite full cell exhibited a higher initial specific charge capacity of 174.4 mAh g1, which is 25 mAh g1 higher than that of the LiFePO4/graphite full cell. The extra capacity can only be assigned to the active lithium donated by the decomposition of Li2S. For the LiFePO4/graphite full cell, the specific discharge capacity was 132.2 mAh g1 with a Coulombic efficiency of 90% during the initial discharge process. The decreased capacity was mainly due to the formation of the SEI
on the surface of the graphite, which is well explained in the literature [2–5]. However, the LiFePO4 (Li2S)/graphite full cell exhibited an initial specific discharge capacity of 146.7 mAh g1, which is the same as the theoretical capacity of the LiFePO4 half cell, as shown in Fig. 4a. The significantly increased capacity was attributed to the full compensation of the active lithium by the active lithium released from the decomposition of Li2S during the initial charge process. Fig. 5b shows the discharge capacity retention and Coulombic efficiency for the LiFePO4 (Li2S)/graphite and LiFePO4/graphite full cells at 0.2C. The specific discharge capacity of the LiFePO4 (Li2S)/ graphite full cell was 140 mAh g1, which is the same as that of the
Fig. 6. The S XPS spectra and peak deconvolution of the LiFePO4 (Li2S) electrodes before cycle and after the 1 st and 10th cycle, respectively.
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Fig. 7. (a) Comparison of the existing cathode pre-lithiation materials in the 1 st cycle. The specific capacity was calculated based on the weight of electrode of cathode prelithiation materials (excluding current collectors). The increase in (b) specific capacity and (c) specific energy of LiFePO4/graphite full cell via the compensation of the lost active lithium using Li2S/KB/PVP. The specific capacity was calculated based on the weight of the cathode electrode (excluding current collectors). The specific energy was calculated based on the total weight of the cathode and anode electrode (excluding current collectors).
half cell and is 12 mAh g1 higher than that of the LiFePO4/graphite full cell, 128 mAh g1 at 0.2C in the following cycles. The Coulombic efficiency of both full cells was almost 100%. After 100 cycles, the capacity of the LiFePO4 (Li2S)/graphite full cell was still approximately 140 mAh g1, which was almost a 100% capacity retention. The specific capacity was obviously enhanced by the active lithium compensation from Li2S. To confirm the irreversible conversion of the core-shell Li2S in LiFePO4 (Li2S) during the 1 st cycle, XPS and XRD analyses of the electrode before cycle and after the 1 st and 10th cycle were performed. The XRD diffraction patterns, Fig. S3, showed no trace of Li2S due to its limited content. Fig. 6 shows the fitted S XPS spectra of the LiFePO4 (Li2S) electrode before cycle and after the 1 st and 10th cycle, respectively. For the electrode before cycle, a pair of S 2p3/2 and S 2p1/2 doublets at 161.2 eV and 162.4 eV were observed and are typical of the Li-S binding energy for Li2S [30]. The pair of S 2p3/2 and S 2p1/2 doublets at 166.2 eV and 167.4 eV represent Li2SO3, which may be attributed to the oxidation of some surface of Li2S before treatment [31]. However, the results for the 1 st and 10th cycle showed a pair of S 2p3/2 and S 2p1/2 doublets at 163.6 and 164.8 eV, respectively, which belong to the S-S bonding [32]. The pair of S 2p3/2 and S 2p1/2 doublets at 168.8 and 170.0 eV, respectively, belong to the sulfate [33]. Thus, S8 was confirmed as the charge product of Li2S according to the following conversion reaction mechanism: Li2S ! S + 2 Li+ + 2 e. Fig. 7a shows the delithiation and lithiation specific capacity of the existing cathode pre-lithiation materials in the 1 st cycle. The core-shell Li2S/KB/PVP in this work has the highest specific delithiation capacity (631 mAh g1) and low specific lithiation capacity (6.6 mAh g1) based on the weight of cathode electrode (cathode pre-lithiation materials, binders and conductive additives), among the existing cathode pre-lithiation materials. Since various types of current collectors are used by different groups, the weight of current collectors was excluded during calculation. Furthermore, compared with other pre-lithiation materials, the preparation of the cathode pre-lithiation materials by dissolving
Li2S in anhydrous ethanol only involves with a simple and environmental-friendly method. Moreover, the fully delithiates voltage of the core-shell Li2S/KB/PVP is less than 3.6 V, which can be readily used on other cathodes, such as LiCoO2, Li2MnO4 and LiNi0.5Mn1.5O4 etc. Fig. 7b shows the increase of the specific capacity (based on the weight of the cathode electrode, but excluding current collectors) of LiFePO4/graphite full cell by the compensation of the 10% lost active lithium using Li2S/KB/PVP. The specific capacity of LiFePO4 (Li2S)/graphite full cell showed a remarkable increase (8.2%), compared to LiFePO4/graphite. The specific energy, Fig. 7c, was calculated based on the total weight of cathode and anode electrode of the full cell (excluding current collectors). The specific energy of LiFePO4/graphite also showed an increase of 5.6% via the compensation of the lost active lithium by using Li2S/KB/PVP. 4. Conclusion In summary, we designed a core-shell Li2S/KB/PVP nanocomposite via simply mixing Li2S, KB and PVP in anhydrous ethanol. The active lithium in this core-shell Li2S/KB/PVP material can be fully released during the first charge in carbonate-based electrolytes and has a specific charge capacity of 1053 mAh g1 (631 mAh g1 based on the total weight of cathode pre-lithiation materials, binders and conductive additives). The limited first specific discharge capacity between 2.5–3.6 V suggests the releasing of lithium from Li2S is irreversible; thus, the irreversible active lithium can be used to compensate for the active lithium lost in the formation of the SEI. The LiFePO4 electrode coated by the core-shell Li2S/KB/PVP was prepared via a simple and non-toxic coating method to verify the ability of the core-shell Li2S/KB/PVP material to act as a cathode pre-lithiation material. Our results showed that the LiFePO4 (Li2S) electrode had a higher specific energy than that without Li2S. Furthermore, there was almost no capacity or Coulombic efficiency deterioration in the carbonatebased electrolytes. The excellent electrochemical performance was described as the following. 1) The LiFePO4 (Li2S)/graphite full cell shows 25 mAh g1 more specific charge capacity and 14 mAh g1
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more specific discharge capacity in the first cycle compared with the LiFePO4/graphite full cell. This was attributed to the compensation of the lost active lithium by Li2S. 2) The specific discharge capacity of the LiFePO4 (Li2S)/graphite full cell is 140 mAh g1, which is the same as that of the half cell and 12 mAh g1 higher than that of the LiFePO4/graphite full cell (128 mAh g1) after the first cycle at 0.2C. 3) The LiFePO4 (Li2S)/ graphite full cell shows a Coulombic efficiency of almost 100% in the following cycles at 0.2C. The core-shell Li2S/KB/PVP is a very good cathode pre-lithiation material for LiFePO4 as observed here and many other cathode materials. Acknowledgments This work is financially supported by the National Program on Key Basic Research Project of China (973 Program, Grant No. 2013CB934002), “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA09010000) and National Key R&D Program of China (2016YFB0100300). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.electacta.2017. 09.167. References [1] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [2] E. Peled, The Electrochemical Behavior of Alkali and Alkaline Earth Metals in Nonaqueous Battery Systems-The Solid Electrolyte Interphase Model, J. Electrochem. Soc. 126 (1979) 2047–2051. [3] P. Verma, P. Maire, P. Novák, A review of the features and analyses of the solid electrolyte interphase in Li-ion batteries, Electrochim. Acta 55 (2010) 6332– 6341. [4] K. Zaghib, G. Nadeau, K. Kinoshita, Effect of Graphite Particle Size on Irreversible Capacity Loss, J. Electrochem. Soc. 147 (2000) 2110–2115. [5] Y. Matsumura, S. Wang, J. Mondori, Mechanism Leading to Irreversible Capacity Loss in Li Ion Rechargeable Batteries, J. Electrochem. Soc. 142 (1995) 2914–2918. [6] M.W. Forney, M.J. Ganter, J.W. Staub, R.D. Ridgley, B.J. Landi, Prelithiation of Silicon-Carbon Nanotube Anodes for Lithium Ion Batteries by Stabilized Lithium Metal Powder (SLMP), Nano Lett. 13 (2013) 4158–4163. [7] C.R. Jarvis, M.J. Lain, M.V. Yakovleva, Y. Gao, A prelithiated carbon anode for lithium-ion battery applications, J. Power Sources 162 (2006) 800–802. [8] Z.H. Wang, Y.B. Fu, Z.C. Zhang, S.W. Yuan, K. Amine, V. Battaglia, G. Liu, Application of Stabilized Lithium Metal Powder (SLMP1) in graphite anode—A high efficient prelithiation method for lithium-ion batteries, J. Power Sources 260 (2014) 57–61. [9] D. Shanmukaraj, S. Grugeon, S. Laruelle, G. Douglade, J.M. Tarascon, M. Armand, Sacrificial salts: Compensating the initial charge irreversibility in lithium batteries, Electrochem. Commun. 12 (2010) 1344–1347. [10] M. Noh, J. Cho, Role of Li6CoO4 Cathode Additive in Li-Ion Cells Containing Low Coulombic Efficiency Anode Material, J. Electrochem. Soc. 159 (2012) A1329– A1334. [11] M.G. Kin, J. Cho, Air stable Al2O3-coated Li2NiO2 cathode additive as a surplus current consumer in a Li-ion cell, J. Mater. Chem. 18 (2008) 5880–5887. [12] Y.M. Sun, H.W. Lee, Z.W. Seh, N. Liu, J. Sun, Y.Z. Li, Y. Cui, High-capacity battery cathode prelithiation to offset initial lithium loss, Nat. Energy 1 (2016) 15008.
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