6164 | Chem. Commun., 2015, 51, 6164--6167 This journal is©The Royal Society of Chemistry 2015

Cite this:Chem. Commun., 2015,

51, 6164

A Si–MnOOH composite with superior lithiumstorage properties†

Hai Zhong,ab Yanbo Yang,a Fei Ding,b Donghai Wang,c Yunhong Zhoua andHui Zhan*a

The Si–MnOOH composite electrode exhibits very stable cycling

and excellent rate capability, such as 1200 mA h g�1 at 12 A g�1, and

700 mA h g�1 at 20 A g�1. The c-MnOOH component significantly

promotes the alloying/de-alloying reaction between Si and lithium.

High energy/power density Li-ion batteries have been pursuedsince the commercialization of graphite/LiCoO2 LiBs; among allthe cathode/anode combinations, LiBs with Si anodes attract moreattention due to their potential high energy density. Nowadays,silicon triggers much efforts due to its largest theoretical capacity(Li22Si5, B4200 mA h g�1), and relatively lower electrochemicalalloy/de-alloy reaction voltage (B0.4 V vs. Li/Li+).1 However, therepeated large volume changes during Li insertion and extractioncan lead to electrode failure.2 The failure mechanism is believed toinvolve particle pulverization, and electric contact deteriorationwhich will finally cause electrode disintegration and rapid capacityfading.3 Recent research also proves that the solid–electrolyteinterphase (SEI) degradation is another reason for the Si anodefailure.4,5 A number of strategies have been implemented tosuppress the volume change and enhance or maintain theelectric contact between silicon and the conductive agent.Examples typically are Si or Si–C composites with speciallydesigned architecture, such as silicon nanowires, silicon nano-rods, porous silicon and york-shell silicon–C composites,6–9 andthey exhibit the expected capacity performance and cycling stability.However, the relatively complicated multi-step synthesis and veryfluffy, insufficiently compressible product powder bring concernson cost and the electrode fabrication. Other than the specialmaterial processing or special architecture design, binderoptimization or SEI additive has also been proposed to enhance

the properties of the Si anode, and carboxymethylcellulose (CMC)or alginate binder and fluoro-ethylene carbonate (FEC) additivesare now widely used in the research on the Si anode.10–12

Though many composites with good properties have beensuccessfully synthesized, most of them use carbon or conductivepolymers to enhance the electrical contact or buffer the volumeexpansion. Here we report a new Si–MnOOH composite whichshows greatly improved Li-ion storage performance. Hydrothermallyprepared g-MnOOH13,14 was ball milled with silicon to form aSi–MnOOH composite. The obtained Si–MnOOH compositeexhibits an initial charge capacity of 3058 mA h g�1 at a currentof 0.1 A g�1 between 1.5 and 0.01 V. An excellent rate capabilityhas been achieved by the composite electrode, and it shows acapacity of 1200 mA h g�1 at 12 A g�1 and 700 mA h g�1 at 20 A g�1.In addition, the Si–MnOOH composite presents a very stablecycling with more than 1000 mA h g�1 after 1500 cycles. Thesuperior electro-activity toward lithium of the Si–MnOOH com-posite should be attributed to g-MnOOH which has some‘‘catalyzing effect’’ on the alloying/de-alloying reaction betweenlithium and silicon.15

The XRD pattern of the hydrothermally synthesized g-MnOOH isshown in Fig. 1a. It well agreed with the literature data (manganite,space group P21/c(14), JCPDS 41-1379) and all of the diffractionpeaks could be indexed by a monoclinic phase. The sharp peaksindicated good crystallinity and the very notable (11%1) diffractionline suggested a preferred orientation of the product whichcould be further evidenced in SEM observation. A scanningelectron microscopy (SEM) (Fig. 1b) image revealed that theas-synthesized g-MnOOH was one-dimensional (1D) nanomaterial,and was composed of the nanorods with an average diameter of40 nm. The SEM images of silicon and the composite powder areshown in Fig. 1c and d respectively. Bare Si has spherical particleswith a diameter of about 100 nm, and slight particle agglomera-tion could be observed. After the high-energy ball-milling withg-MnOOH, the spherical morphology was mostly reserved inSi–MnOOH and the average particle size did not change much.

To better analyze the electrochemical activity of theSi–MnOOH composite, g-MnOOH alone was first examined in

a Department of Chemistry, Wuhan University, Hubei, 430072, China.

E-mail: zhanhui3620@126.comb Tianjin Institute of Power Source, Tianjin, 300381, Chinac Department of Mechanical and Nuclear Engineering, The Pennsylvania State

University, University Park, Pennsylvania, 16802, USA

† Electronic supplementary information (ESI) available: Sample synthesis, experi-mental conditions, electrochemical properties of MnOOH and different Si/MnOOH ratios. See DOI: 10.1039/c4cc09207d

Received 18th November 2014,Accepted 27th February 2015

DOI: 10.1039/c4cc09207d

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a 1 M LiPF6/EC + DMC electrolyte. After the low efficiency in the1st cycle, the coulombic efficiency increased gradually untilfinally stabilized at around 98% (Fig. S1a in ESI†). In addition,the voltage profile (Fig. S1b in ESI†) and the differential dQ/dVplot (Fig. S1c in ESI†) indicated that after the 1st charge/discharge, the electrochemical behavior of g-MnOOH was quitesimilar to that of MnO except a higher reducing voltage.16,17

The result suggests that the redox reaction of g-MnOOH shouldfollow the route g-MnOOH + Li+ + e� 2 MnO + LiOH andMnO + 2Li+ + 2e�2 Mn + Li2O, thus the g-MnOOH phase firstdecomposes into MnO and then a conversion reaction of MnOoccurs. The higher reducing voltage of MnOOH than MnO mayfurther imply the ‘‘catalyzing effect’’ of LiOH, and a similarobservation has also been made on the Ni(OH)2 anode whichshowed a higher reducing voltage than NiO.18,19

The effect of g-MnOOH on the Si anode was investigated byelectrochemical tests. The Si–MnOOH composite with differentg-MnOOH contents was obtained by ball-milling. The optimumSi–MnOOH ratio of 55 : 5 was chosen according to the result inFig. S2 (in ESI†). Elemental mapping in Fig. S3b–d (in ESI†)showed the uniform distribution of Mn and O along with siliconin the Si–MnOOH composite (Fig. S3a in ESI†), confirming thehomogeneous dispersion of g-MnOOH in the Si powder.

The electrochemical properties of the Si–MnOOH compositewere evaluated by galvanostatic charge–discharge tests, and thecapacity is based on the mass of the composite. Fig. 2a displaysthe cycling stability of the Si–MnOOH electrode. Two ‘‘for-mation’’ charging/discharging with a low current of 0.1 A g�1

was applied, and then a current of 2 A g�1 was adopted in thesubsequent cycles. In the 1st cycle, the discharge and chargecapacity was 3907 mA h g�1 and 3058 mA h g�1, respectively,corresponding to a coulombic efficiency of 78.3%. Uponincreasing the current to 2 A g�1, the capacity dropped to2605/2501 mA h g�1 in the 3rd cycle, but recovered to above3000 mA h g�1 in the 7th cycle. A similar observation was madepreviously on nano-Si.20 Actually, the formation process to

completely activate the electrode material has been widelyreported on the electrode with a mesoporous/microporousstructure and is explained by the gradual electrolyte impregna-tion into the electrode.21,22 After the initial formation cycles, amaximum capacity of 3056/3007 mA h g�1 was achieved in the7th cycle. A very slow capacity decay occurred in the subsequentcycles; after 100 cycles, the electrode could release near2200 mA h g�1 capacity. Even after 1500 cycles, the capacitywas still above 1200 mA h g�1 (Fig. S4 in ESI†). The voltageprofiles of the Si–MnOOH electrode in different cycles areshown in Fig. 2b. We can see that except in the first cycle,the voltage hysteresis between the charge and the dischargeplateaus gradually increased with cycling but still remainedvery small within 100 cycles. To better understand the redoxprocess of the Si–MnOOH electrode, a differential analysis wasconducted on the 7th charge/discharge curve and the result isdisplayed in Fig. 2c. The alloying/de-alloying of lithium with Siled to two pairs of redox peaks at 0.29/0.08 V and 0.48/0.24 V vs.Li/Li+. The result is consistent with the previous report on Siand should be attributed to the phase transformation betweendifferent amorphous LixSi.23 The small voltage segregationbetween the oxidizing and reducing branches indicated thegood reaction reversibility of Si–MnOOH. However, we couldnot unambiguously discern the redox peak of g-MnOOH, andthe reasons may be the low proportion of g-MnOOH in thecomposite electrode or the redox peaks of g-MnOOH beingcovered by those of silicon.

The Si–MnOOH electrode exhibited an excellent rate capability.In Fig. 3a, at a low current of 0.1 A g�1, the capacity of Si–MnOOHwas just slightly higher than that of the Si electrode, such as3100 mA h g�1 to 2900 mA h g�1; however, when the currentincreased to 1 A g�1, a capacity of more than 2800 mA h g�1 wasobtained on Si–MnOOH while barely 2000 mA h g�1 on Si. Thecapacity difference was more notable when the currentincreased further. The pristine Si anode almost failed at acurrent of 8 A g�1, and the Si–MnOOH electrode still could be

Fig. 1 (a) XRD pattern of g-MnOOH; (b) SEM image of g-MnOOH synthe-sized by the hydrothermal method; (c) SEM image of the silicon powder;(d) SEM image of the Si–MnOOH composite.

Fig. 2 (a) Plot of charge/discharge capacity and coulombic efficiency vs.cycles of the Si–MnOOH electrode (0.1 A g�1 in the first two cycles, 2 A g�1 inthe rest of the cycles); (b) voltage profiles of the Si–MnOOH electrode indifferent cycles; (c) dQ/dV curve of the Si–MnOOH electrode in the 7th cycle.

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6166 | Chem. Commun., 2015, 51, 6164--6167 This journal is©The Royal Society of Chemistry 2015

discharged to around 1600 mA h g�1. The voltage–capacityprofile in Fig. 3b more clearly shows the capacity/energyadvantage of Si–MnOOH over Si. For Si–MnOOH, the capacityonly dropped from 3197 mA h g�1 at 0.1 A g�1 to 2800 mA h g�1

at 1 A g�1, while the voltage gap between charging anddischarging remained almost constant. Even at a current of12 A g�1, a decent capacity of around 1200 mA h g�1 wasdelivered. It is worth noting that the Si–MnOOH electrodepresented an appreciable capacity of 700 mA h g�1 at 20 A g�1.To our knowledge, such capacity has seldom been reported onsilicon-based materials in the ultrafast charge/discharge mode.Actually, even on a ‘‘super thick’’ Si–MnOOH electrode (massloading is 3 mg cm�2, the ratio of active material : acetyleneblack : PAA binder is 6 : 3 : 1, specific area capacity at 1 A g�1 is4.5 mA h cm�2), the improvement in rate capability is still verysignificant (Fig. S5 in ESI†), however, the stable cycling willrequire more work on the binder because of much more seriouselectrode peeling during electrode fabrication and electrode cycling.

As the performance is obtained on the Si–MnOOH compositeprepared by a simple ball-milling, further enhanced capacity/energy performance can be reasonably expected if architecturedesign or component optimization can be involved.

The AC impedance comparison provides some clues for thebetter electrochemical performance of Si–MnOOH than Si(Fig. 3c). After 100 cycles, impedance spectra were recordedon both electrodes. They all showed two semicircles in the highand high-to-medium frequency range, which represents Li+

diffusion through the SEI and charge-transfer reaction, andcan be indexed by RSEI and Rct, respectively, according to theequivalent circuit diagram. The low-frequency Warburg sloperepresents the semi-infinite diffusion of lithium-ion in theelectrodes.24 Comparing the impedance diagrams, we can seethat the Si–MnOOH electrode has smaller semicircles than theSi electrode in the high-frequency and medium-frequencyregions, suggesting the smaller RSEI and Rct for the Si–MnOOHelectrode than the Si electrode. The fitted data in Table S1(in ESI†) indicate that the Rct value of Si–MnOOH is muchsmaller than that of the Si electrode, and the result revealsmuch better reaction kinetics of Si–MnOOH than Si. The veryclose Warburg factor (s) of Si and Si–MnOOH electrodes suggestsa similar Li+ diffusion coefficient of these two electrodes,25 andit is reasonable because Si and Si–MnOOH have very similarparticle size and essentially the same structure (the very lowproportion of MnOOH in the Si–MnOOH composite will notsignificantly affect the structure of bulk materials). We thinkthat the impedance results explain the excellent cycling stabilityand rate capability of the Si–MnOOH composite and may implythe ‘‘catalyzing’’ effect of g-MnOOH on the alloying/de-alloyingreaction of the Si anode; more work is still in progress onthis point.

In conclusion, the g-MnOOH nanorod was prepared througha hydrothermal method, and then ball-milled with Si to form theSi–MnOOH composite. The as-prepared Si–MnOOH compositeshowed a high capacity of about 3900 mA h g�1 and very stablecycling. Additionally, the composite exhibited an excellent ratecapability, such as 1200 mA h g�1 at 12 A g�1, and 700 mA h g�1

at 20 A g�1. The result provides a new strategy to enhance theelectrochemical performance of the Si anode, and the facilepreparation offers a new way to obtain mass-producible Si anodematerial with perfect Li-ion storage properties.

The authors would like to express their sincere thanks to theNational Natural Science Foundation of China (No. 21073138,21273168, 21233004) for the financial support.

Notes and references1 J. X. Song, M. J. Zhou, R. Yi, T. Xu, M. L. Gordin, D. H. Tang, Z. X. Yu,

M. Regula and D. H. Wang, Adv. Funct. Mater., 2014, 24, 5904–5910.2 J. H. Ryu, J. W. Kim, Y. E. Sung and S. M. Oh, Electrochem. Solid-State

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Lett., 2007, 10, A52–A55.4 M. Y. Nie, D. P. Abraham, Y. J. Chen, A. Bose and B. L. Lucht, J. Phys.

Chem. C, 2013, 117, 13403–13412.5 Y. Liu, X. Guo, J. F. Li, Q. L. Lv, T. Y. Ma, W. T. Zhu and X. P. Qiu,

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Fig. 3 (a) Discharging capacity vs. cycle number for the bare siliconelectrode and the Si–MnOOH electrode at different currents in the voltagerange of 0.01–1.5 V; (b) voltage profiles of the bare silicon electrode andthe Si–MnOOH electrode at current densities of 0.1, 0.5, 1, 4, 8, 12, 16and 20 A g�1; (c) AC impedance of the bare silicon electrode and theSi–MnOOH electrode after 100 cycles.

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