稀有金属(英文版) 2018,37(11),929-935

Lithium cobaltate:a novel host material enables high-rate and stable lithium-sulfur batteries

Wen Ma Qing Xu

GRINM Bohan (Beijing) Publisher Co.,Ltd.,GRINM Group Co.,Ltd.

Subinstitute of Standardization Theory and Strategy, China National Institute of Standardization

作者简介：*Wen Ma,e-mail: raremetals mawen@126.com;

收稿日期：19 July 2018

Lithium cobaltate:a novel host material enables high-rate and stable lithium-sulfur batteries

Wen Ma Qing Xu

GRINM Bohan (Beijing) Publisher Co.,Ltd.,GRINM Group Co.,Ltd.

Subinstitute of Standardization Theory and Strategy, China National Institute of Standardization

Abstract：

Element sulfur is highly attractive due to their potentially low cost and environmental compatibility.However, polysulfides dissolution hinders the lithiumsulfur(Li-S) batteries toward commercialization. To overcome these issues, in this work, lithium cobaltate as a commercial material, for the first time, was devoted to engineering the electrode structure and composition to improve the performance. When incorporated with 80% sulfur powder, the synergetic effects of cobalt atoms and interlayer spaces effectively enable the production of Li-S batteries with a relatively high discharge capacity of 1420 mAh·g^-1 at the low surface current density of 1 mA·cm^-2 and stable capacity retention of 650 mAh·g^-1 at high surface current density of 6 mA·cm^-2. The introduction of lithium cobaltate is a viable approach for successfully developing practical Li-S batteries.

Keyword：

Lithium cobaltate; Polysulfides dissolution; Electrochemical properties; Lithium-sulfur batteries;

Received： 19 July 2018

1 Introduction

Non-renewable resources,such as oil,coal and natural gas,are on the edge of depletion.On the other hand,renewable energies including solar,tidal wind and geothermal energies are instable and vary with geographic conditions,making it challenging to utilize them in specific fields that require stable power supply.In the energy field,massive efforts have been centered on energy conservation and corresponding utilization technology [ 1, 2, 3] .Energy storage devices such as Li-ion batteries (LIBs) are a major part of storage devices for renewable energies.The traditional LIBs,such as LiFePO₄ (170 mAh-g^-1),Li₂TiO₃ (175mAh·g^-1) and LiMn₂O₄ (148 mAh·g^-1),however,are unsatisfactory performance because of low specific capacity that cannot meet the energy demand for practical application [ 4, 5] .Elemental sulfur and silicon are believed to be a promising candidate for cathode and anode material on account of its high theoretical capacity (3579 mAh·g^-1for Li₁₅Si₄ or 1675 mAh·g^-1 for Li₂S) and relatively suitable working potential [ 6, 7, 8] .Unfortunately,there are some problems that obstruct the commercialization of the two materials.For instance,Si anodes suffer from structural degradation,loss of electrical contact and the unstable solid electrolyte interphase (SEI) on the silicon surface [ 7, 9] .Different from the silicon anodes where the dramatic volume change of Si (～300%) during lithiation-delithiation cycling is the exclusive problem,the sulfur cathodes suffer more challenges than silicon anodes due to more complex redox process [ 10, 11, 12, 13] .

Sulfur undergoes a large volumetric expansion of～80%upon full lithiation to lithium sulfide,which can cause pulverization and structural damage at the electrode level because of the difference in density between sulfur and lithium sulfide where the volume density of sulfur is2.03 g·cm^-3 and the lithium sulfide is 1.66 g·cm^-3 [ 4, 14] .Besides volumetric expansion,low conductivity of sulfur and final products of lithium sulfide will cause the poor utilization of active materials [ 15, 16] .Severely,the finial insulating lithium sulfide may cause cathode surface passivation.The dissolution of intermediate lithium polysulfides such as Li₂S₈,Li₂S₆ and Li₂S₄ into ether-based electrolytes is another critical issue [ 17, 18] .During the cycling process,the intermediate products will dissolve readily and lead to continuous loss of active material into the electrolyte [ 19] .The dissolution polysulfides shuttle through the separator and then reach the lithium anode [ 20] .The metal lithium has a very high reductive activity that will promote long-chain poly sulfides (such as Li₂S₈) to short-chain polysulfides (such as Li₂S or Li₂S₂) and middle-chain polysulfides (such as Li₂S₅) [ 21, 22] .The insoluble short-chain polysulfides deposited on the surface of lithium anode,however,dissolved middle-chain polysulfides will migrate back to the cathode,resulting in prolonged charge capacities (low coulombic efficiency) and then rapid capacity decay [ 15, 23] .

To overcome aforementioned problems,plentiful works had been done in recent years,for instance,designing various porous host material architectures such as carbon nanotube [ 24] ,graphene [ 25, 26, 27] ,meso-and microporous carbons [ 28] ,metal sulfides [ 19] and metallic oxide [ 8, 11, 29, 30] .Among those design technologies,heteroatom doping is regarded as an effective scheme to regulate host material architectures [ 31, 32] .For instance,cobalt atom is the active center that owns strong anchoring sites for polysulfides [ 32] .However,the traditionally heteroatom doping usually needs to be carefully designed and the productivity of active materials is too low.Moreover,the uniform distribution of heteroatom such as cobalt or nitrogen atom among the host materials is quit challenge.Therefore,those methods of heteroatom doping make the processes for massive producing host materials tedious and unsuitable.

Herein,we report a strategy,for the first time,for direct fabrication of plentiful cobalt-embedded sulfur cathodes via using commercial lithium cobaltate (LCO) as host materials.The unique interlayer structure of LCO materials can provide lithium-ion fast transport path among the electrodes,thus resulting in great improvement of the rate performance in lithium-sulfur battery.When combining with 80%sulfur powder,the LCO and sulfur composite(LCO@S) reveals a favorite electrochemical behavior for Li-S battery.The LCO@S cathodes exhibit a high reversible capacity of 1420 mAh·g^-1 at the low surface current density of 1 mA·cm^-2 and stable capacity of 650 mAh·g^-1at large surface current density of 6 mA·cm^-2.

2 Experimental

2.1 Materials

Lithium cobaltate was purchased from Hefei Ke Jing Materials Technology Co,Ltd.Sulfur,polyvinylidene fluoride (PVDF),acetylene black (AB) and TN-methyl pyrrolidone (NMP) were purchased from Aladdin and used as received.

2.2 Synthesis of LCO@S composites

Commercial sulfur powder and LCO were uniformly mixed with a mass ratio of 8:2 via ball-milled for 30 min at 4000r·min^-1 to form uniform powders,and then,the powders were heated to 200℃for 12 h under argon atmosphere.After cooling down,the active materials were obtained.For comparison,AB@S composites were also prepared using the same method.

2.3 Preparation of LCO@S cathodesand electrochemical measurement

Typically,LCO@S composites make slurry with AB and PVDF in a mass ratio of 7:2:1 in N-methyl pyrrolidone(NMP) solvent.The slurry was then coated on the surface of carbon-coated aluminum foil and dried under vacuum at60℃overnight.Electrodes contained～1.2 mg of sulfur loading per square centimeter.For comparison,AB@S cathodes were prepared using similar methods.1 mol·L^-1LiTFSI in a mixture of DOL and DME (1:1 in volume ratio) with 1 wt%lithium nitrate (LiNO₃) additive was as the electrolyte.Argon-filled glove box with the concentrations of moisture and oxygen<0.01×10^-6 was used for assembling the battery.The amount of the electrolyte was strictly controlled for performance evaluation.The microstructures of LCO and LiCoO₂@S were observed by field-emission scanning electron microscope (FESEM,NANOSEI 450,FEI) equipped with energy-dispersive spectrometer (EDS) for elemental analysis.CT2001A cell test instrument (Wuhan LAND Electronic Co.,Ltd.) and CHI660E (Shanghai Chenhua Instrument Co.,Ltd.) electrochemical workstation were employed for electrochemical,cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS),respectively.

3 Results and discussion

3.1 Crystal frame of materials

Figure 1 shows crystal frame of LiCoO₂ in which the solid consists of layers of monovalent lithium cations (Li⁺) lying between extended anionic sheets of cobalt and oxygen atoms,arranged as edge-sharing octahedra,with two faces parallel to the sheet plane.The cobalt atoms are formally in the trivalent oxidation state (Co³⁺) and are sandwiched between two layers of oxygen atoms (O^2-).In each layer(cobalt,oxygen or lithium),the atoms are arranged in a regular triangular lattice.The lattices are offset so that the lithium atoms are farthest from the cobalt atoms,and the structure repeats in the direction perpendicular to the planes every three cobalt (or lithium) layers.Therefore,the unique crystal structure provides plentiful cobalt sites which are beneficial for absorbing poly sulfides and the interlayer space can promote fast lithium-ion transport that is beneficial for enhancing the rate performance of lithiumsulfur battery.

Fig.1 Crystal frame of LiCoO₂

In conventional approaches for anchoring poly sulfides such as carbon-based host materials (e.g.,acetylene black,AB),as shown in Fig.2,physical adhesion is the main method;thus,the capacity will fade with time when there is no bonding effect between polysulfides and the host materials,resulting in the fast dissolution of poly sulfides after many cycles [ 6, 10] .Therefore,polar host materials such as cobalt atoms are urgently needed for Li-S battery technologies.Interestingly,LCO teems with plentiful cobalt atoms and suitable lithium-ion transfer ability thatcan donor a strongly ability for both absorbing polysulfides and improving rate performance.

Fig.2 Schematics of Co sites to absorb polysulfides:a failing approaches using commercial carbon as host materials that intermediate sulfur species fast dissolve with time and b polar Co sites with strong absorption for imprisoning polysulfides

3.2 Characterization of materials

As shown in Fig.3a-c,the microscopy images of the overall view of LCO powers display a typical morphology,showing uneven distribution and quite smooth surface.Each element (Co and O) in LCO particles is homogeneously distributed (the inset image in Fig.3c).Further,sulfur was incorporated into LCO host materials with mass rate of 8:2 by melt diffusion under 200℃for 12 h.After high-temperature calcination,the structure of LCO is mostly maintained.The morphologies of LCO@S particles,however,display a rough surface,as presented in Fig.3d-f.Element mapping in Fig.3e shows that sulfur coating is the mainly reason that causes the surface folds.EDS results in Fig.3g-i exhibit homogenous sulfur elemental mapping distribution,indicating that sulfur has been uniformly coated on LCO surface.

3.3 Electrochemical properties of LCO@S cathodes

The electrochemical properties of LCO@S cathodes are presented in Fig.4.Figure 4a shows the overlapping curves of CV tests at the scan rate of 0.1 mV·s^-1,indicating a high reversibility of redox reactions and electrode stability.Two pairs of reduction peaks appear,demonstrating typical sulfur cathode redox characteristic.The peak located at～2.3 V represents the transformation from sulfur to long-chain poly sulfides (e.g.,Li₂S₈,Li₂S₆,Li₂S₄) accompanied with active sulfur transfer from solid to liquid phase.The other peak at～2.05 V is ascribed to the further reduction in long-chain polysulfides to loworder polysulfides (e.g.,Li₂S₂ Li₂S) [ 1, 6] .When scanning backward,the two overlapped anodic peaks at around 2.35and 2.38 V are related to the oxidation of low-order polysulfides to high-order polysulfides and eventually transform to elemental sulfur,respectively.Notably,there are no apparent current or potential changes in these CV peaks with repeated scans,proving superior reversibility and stability of the cells.The rates performance of LCO@S and AB@S is evaluated at various surface current densities stepwise from 1 to 6 mA·cm^-2 every 5 cycles,respectively.As shown in Fig.4b,the battery of LCO@S exhibits an excellent capacity of 1420 mAh·g^-1 at low surface current density of 1 mA·cm^-2.For comparison,the AB@S shows gradually attenuated capacity at 1 mA·cm^-2 and only delivers 946 mAh·g^-1 at the fifth cycle.When the surface current density increases to 6 mA·cm^-2,LCO@S still exhibits a stable capacity of 650 mAh·g^-1,while the battery of AB@S shows weak capacity of 197 mAh·g^-1.It strongly indicates that LCO@S owns an excellent rate capability.Importantly,after high-rate measurement,a high reversible capacity of 1364 mAh·g^-1 is received when the surface current density is switched back to 1 mA·cm^-2.Additionally,the long-term cycling performance of LCO@S is further tested at low current densities of1 mA·cm^-2,as shown in Fig.4c.It still delivers a high initial capacity of 1440 mAh·g^-1,and a capacity of 694mAh·g^-1 is retained after 200 cycles with nearly 100%coulombic efficiency.These results effectively demonstrate that LCO@S battery in this work has many distinctive properties,including high capacity and excellent long-term cycling stability.

Fig.3 a-c Different resolution SEM images of LiCoO₂ showing a relative smooth surface (inset in c being elemental mapping);d-f SEM images of LiCoO₂@S of which surface shows deep rill-like folds;elemental mappings of g Co,h O and i S clearly showing that sulfur was successfully coated on surface of LiCoO₂

Figure 5 further shows the galvanostatic charge/discharge profiles of AB@S and LCO@S electrodes at different surface current densities (1,2,3,4,5,6 mA·cm^-2).AB@S electrode exhibits serious polarization,once the surface current density increases to 5 mA·cm^-2,expressed as the charge/discharge profiles which gradually disappear.This phenomenon indicates that the battery of AB@S battery bears low sulfur utilization and redox reaction kinetics.Satisfactorily,LCO@S electrode shows stable and flat charge/discharge profiles (Fig.5b).The plateaus exhibit an excellent capacity even with the surface current density up to 6 mA·cm^-2,suggesting that the LCO@S electrode has efficient kinetic reaction process with a small barrier for promoting redox reaction kinetics and provides high sulfur utilization to achieve an acceptable capacity retention.

Iri order to reveal the internal mechanism,EIS measurements of LCO@S and AB@S cathode were conducted within the frequency range from 100.00 MHz to 0.01 Hz,respectively.As presented in Fig.6,where Z′and Z″are,respectively,the real part and imaginary part of impedance,the Nyquist plots are composed of a depressed semicircle and a straight slopping line in the low-frequency region(1.00-0.01 Hz).To get a better understanding of the change of impedance parameters,the equivalent circuit diagram was analyzed,as presented in Fig.6b.The R_s corresponds to the resistance of electrolyte.The semicircle is represented by R_ct//CPE,which is assigned to the charge transfer resistance (R_ct) and constant phase element (CPE)that is used in the model in place of a capacitor to compensate for non-ideal behavior of electrode,for example rough or porous electrode surface.Warburg impedance(W_o) is used to represent diffusion of the ions within the cathode.According to the quantitative analysis,the changes between LCO@S cathode and AB@S cathode in R_s are not significant,implying that the resistances of the electrolyte are similar.In contrast,LCO@S cathode shows lower R_ct values (24.16Ω) than AB@S cathode (68.17Ω).Those results strongly associate with charge transfer of cathodes.Clearly,LCO cathode exhibits effective enhancement for electrochemical kinetics.As expected,qualitatively monitoring ion diffusion by Warburg-type impedance element shows that LCO@S cathode owns a clear 45°slope in the low-frequency region,whereas AB@S cathode owns a distorted Warburg-type impedance element,indicating an inferior lithium-ion diffusion ability of AB@S cathode.Those results are strong indication of the advantage of LCO@S to facilitate Li-ion diffusion.

Fig.4 a CV curves of LCO@S electrode in voltage range of 1.5-3.0 V with a sweep rate of 0.1 mV·s^-1;b rate performances of LCO@S and AB@S electrodes at different surface current densities;c cycling performances of LCO@S electrode at surface current density of 1 mA·cm^-2

Fig.5 Corresponding discharge and charge voltage profiles of a AB@S and b LCO@S electrode at different surface current densities

Fig.6 a A.C.impedance spectroscopy data of LCO@S cathodes (frequency region of 10,000 MHz to 0.01 Hz and inset showing a magnification of high-frequency region;b A.C.impedance spectroscopy data of AB@S cathodes (inset showing equivalent circuit diagram)

4 Conclusion

In summary,we have successfully developed lithium cobaltate with plenty cobalt sites as multidimensional anchoring sites for absorbing polysulfides,making substantial progress in improving electrochemical properties and therefore resolving the chronic insufficient cycle lives of sulfur cathodes.It is demonstrated that the suitable interlamellar channels of LCO can facilitate lithium-ion diffusion in lithium-sulfur batteries and achieve satisfactory rate performance.When commercial sulfur powder was applied to assemble cells,the LCO@S cathodes could show excellent rate performance and stable capacity retention at different rates.As a result,the concise preparation of sulfur cathodes will arouse the battery community’s interest in fabricating long-life lithium-sulfur batteries.

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