Hard carbon Li2.6Co0.4N composite anode materials for Li-ion batteries

Hard carbon/Li 2.6Co 0.4N composite anode materials for Li-ion batteries

Hao Sun,Xiangming He ?,Jianjun Li,Jianguo Ren,Chang Yin Jiang,Chunrong Wan

Institute of Nuclear and New Energy Technology,Tsinghua University,Beijing 100084,China Received 1April 2006;received in revised form 23June 2006;accepted 23June 2006

Abstract

Hard carbon/Li 2.6Co 0.4N composite anode electrode is prepared to reduce the initial high irreversible capacity of hard carbon,which hinders potential application of hard carbon in lithium ion batteries,by introducing Li 2.6Co 0.4N into hard carbon.Lithiated Li 2.6Co 0.4N provides the compensation of lithium in the first cycle,leading to a high initial coulombic efficiency of ca.100%versus lithium.As-prepared hard carbon/Li 2.6Co 0.4N composite electrode presents initial capacity of 438mA h g ?1.A full cell using LiCoO 2cathode and the composite anode shows much higher initial coulombic efficiency and capacity than those of a cell using LiCoO 2and hard carbon anode.This paves the way to reduce the large initial irreversible capacity of hard carbon.?2006Elsevier B.V .All rights reserved.

Keywords:Hard carbon;Lithium cobalt nitride;Composite anode;Initial irreversible capacity;Li-ion batteries

1.Introduction

In recent years,a great deal of effort has been put into research on high performance anode materials for lithium ion batteries.To replace the conventional graphite due to its limited theoretical capacity,a series of anode materials have been proposed.Among them,hard carbon with the amorphous struc-ture becomes attractive due to its high capacity and low cost.The initial irreversible capacity of hard carbon is quite large,however,leading to very low coulombic efficiency at the first cycle.Several attempts were made to improve the performance of hard carbon by modification of the precursors of hard carbon [1,2].Unfortunately,the initial irreversible capacity of hard carbon still remains to be too large for its practical application.On the other hand,Li 2.6Co 0.4N with the hexagonal structure shows an attractive merit with a high specific capacity over 760mA h g ?1[3,4].However,these Li-rich anodes cannot directly combine with the typical high potential cathodes such as LiCoO 2and LiMn 2O 4to constitute lithium ion batteries.The lithium must be extracted from the structure of Li 2.6Co 0.4N in an initial anodic oxidation by either chemical or electrochemical ways.Furthermore,Li 2.6Co 0.4N can compensate the high

irreversible capacities of the SnSb x ,SnO,SiO x [5–9],Si/graphite [10]and MCMB [11]based electrodes in the first cycle.In this study,hard carbon/Li 2.6Co 0.4N composite anode electrode was prepared to eliminate the initial irreversible ca-pacity of hard carbon.A full cell using LiCoO 2cathode and the composite anode showed improved electrochemical perfor-mance over the cell with hard carbon anode.2.Experimental 2.1.Material preparations

At first,Co powders were cleaned by reduction under 20%H 2/N 2gas for 16h at 600°C.This was essential to remove oxide from the surface of Co;otherwise,it would lead to the formation of Li 2O during the preparation of Li 2.6Co 0.4N [12,13].The mixture of Li 3N (Aldrich,reagent grade)and as-prepared Co powders were then pressed into tablets with 10mm in diameter and 3mm in thickness under 3MPa.The tablets were heated at 800°C for 12h under a N 2stream (99.999%)to produce Li 2.6Co 0.4N [4,14].Hard carbon was from LG Chem of Korea and used as received.The hard carbon/Li 2.6Co 0.4N composite was prepared by thoroughly mixing the hard carbon (75wt.%)and Li 2.6Co 0.4N (25wt.%)in a mortar in a glove box filled with argon gas.

Powder X-ray diffraction (XRD,D/max-RB)using CuK αradiation was used to identify the crystalline phase and

crystal

Solid State Ionics 177(2006)1331–

1334

?Corresponding author.Tel.:+861089796073;fax:+861089796031.E-mail address:hexm@https://www.sodocs.net/doc/4e12739998.html, (X.He).

0167-2738/$-see front matter ?2006Elsevier B.V .All rights reserved.doi:10.1016/j.ssi.2006.06.029

lattice parameters of the powders.The step width was0.02°and data acquisition time per step was0.3s.The samples were protected from O2and H2O by using an air-sealing holder.The samples assembling and measurements were performed under an air-sealing condition.

2.2.Cell assembling and electrochemical measurements

The cells were assembled in a glove box filled with argon gas.The cathodes were prepared by the composition of80wt.% active materials,15wt.%acetylene black,and5wt.%PTFE. The separators were a Celguard2400microporous polypropyl-ene membrane.The electrolyte was1M LiPF6EC+DEC (1:1by volume).A lithium metal anode was used to test the composite.The discharge–charge cycling was galvanostatically performed at a constant current density of0.2mA cm?2with cut-off voltages of1.4–0.005V(versus Li/Li+)at20°C for the test of the composite.Charge and discharge of the cell refer, correspondently,to lithium extraction from,and insertion into,

the active hosts.

The cells using LiCoO2(Ruixiang Company,Chang Sha,

China)cathode and composite anode(or hard carbon anode)

were also assembled and cycled in the voltage range of2.0–

4.2V at a constant current density of0.2mA cm?2.

3.Results and discussion

Fig.1shows the X-ray diffraction pattern of Li2.6Co0.4N

powders.They are well in accordance with that of Li2.6Co0.4N

reported in Ref.[15].The Li2O impurity phase(two

theta=33.6)can be observed.The Li2O phrase is probably

formed on the sample surface during the synthesis process,

because the material is very sensitive to O2and H2O.An

alternative mixing gas of1%H2–99%N2instead of a N2stream (99.999%)is proposed during the synthesis and it may reduce

the content of Li2O in the sample.However,it may bring

hydrogen in the structure and decrease the Li–N interaction[4].

Fig.2shows the initial charge and discharge curves of as-

prepared Li2.6Co0.4N electrode.The initial discharge capacity is

186.3mA h g?1,which seems to be correspondent to the

amount of lithium vacancy in Li2.6Co0.4N.Because Li2.6Co0.4N is nearly fully lithiated,so the initial discharge capacity(lithium insertion capacity)can be only linked to the amount of lithium vacancy in Li2.6Co0.4N[3,4].During the following charge process,the cell voltage gradually increases up to1.0V and remains at1.0V plateau afterwards.The Li/Li2.6Co0.4N cell cycled in the voltage range of0.005–1.4V shows a high capacity of912.4mA h g?1.Such a large capacity is ac-companied with an irreversible transformation of Li2.6Co0.4N from a crystal to an amorphous phase at1.0V plateau[16,17]. The amorphous phase arising from the first cycle strongly affects the charge and discharge behavior of Li2.6Co0.4N in the subsequent cycles.The amorphous structure of Li2.6Co0.4N can be exactly maintained upon cycling.On the other hand, Li2.6Co0.4N does not show the satisfactory capacity retention upon long cycles in Fig.3.Although the degradation mech-anism is still ambiguous,it could be due to the instability of the nitrides in the deep Li-extraction and the formation of pas-sivating surface film caused by the decomposition reaction between electrolyte and Li2.6Co0.4N[18]

.

Fig.1.XRD pattern of as-prepared Li2.6Co0.4N

powders.

Fig.2.Initial charge and discharge curves of as-prepared Li2.6Co0.4N

electrode.

Fig.3.Cycling performance of as-prepared Li2.6Co0.4N electrode. 1332H.Sun et al./Solid State Ionics177(2006)1331–1334

Lithium in Li2.6Co0.4N can be a lithium source for the compensation to reduce the initial irreversible capacity of the anode.Due to this initial irreversible capacity,hard carbon cannot be commercially used.Therefore,the composite con-sisting of hard carbon and Li2.6Co0.4N is proposed to eliminate its initial irreversible capacity.

Despite the fact that the open-circuit potential is0.85V for Li2.6Co0.4N electrode,the first lithium extraction capacity (charge capacity)below1.0V is quite low(ca.150mA h g?1, 16%of the total capacity).Lithium extraction from Li2.6Co0.4N primarily occurs at ca.1.0V versus lithium,while lithium can be inserted into hard carbon mainly under0.8V versus lithium. For this reason,a direct lithium transfer from Li2.6Co0.4N to hard carbon is negligible[10].

Compared with the pristine hard carbon electrode,we can observe the initial coulombic efficiency increases from66.3% to100%and the reversible capacity rises from300.2mA h g?1to438mA h g?1for the hard carbon/Li2.6Co0.4N composite electrode(Fig.4).The coulombic efficiency of the composite electrode remains at about100%upon cycling.In fact,the capacity above1.0V from Li2.6Co0.4N plays an important role for the compensation of the initial irreversible capacity.

Fig.5shows the charge and discharge curves of the hard carbon/Li2.6Co0.4N composite electrode versus lithium at dif-ferent cycles.When75wt.%hard carbon is mixed with25wt.% Li2.6Co0.4N to form a composite electrode,the open-circuit potential drops from1.9V(only in the case of hard carbon as active material)to1.1V versus lithium with the influence of Li2.6Co0.4N.In the first discharge,the electrochemical behavior of the composite is mainly associated to hard carbon because fully lithiated Li2.6Co0.4N has no extra vacancy for the initial lithium intercalation.The first charge potential plateau can clearly be divided into two parts,corresponding to different stages of lithium extraction from Li2.6Co0.4N(1.4–1.0V)and hard carbon(1.0–0.01V).A potential plateau at 1.0

V Fig.5.Charge and discharge curves of the hard carbon/Li2.6Co0.4N composite

electrode versus lithium at different

cycles.

Fig.6.Cycling performance of the hard carbon/Li2.6Co0.4N composite and the

hard carbon

electrodes.

Fig.4.Charge and discharge curves of the hard carbon/Li2.6Co0.4N composite

and the hard carbon electrodes versus lithium at the first

cycle.

Fig.7.Charge and discharge curves for a cell with LiCoO2cathode and the hard

carbon anode.The weight ratio of cathode to anode active material=2.

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H.Sun et al./Solid State Ionics177(2006)1331–1334

corresponds to the phase transformation of Li 2.6Co 0.4N from a crystalline to an amorphous phase.It is obvious that the crys-talline structure of Li 2.6Co 0.4N remains unchanged even after thoroughly mixing.The first cycle profile is different from the others because of the irreversible reaction.The plateau near 1.0V disappears in the following cycles.From the second cycle to the subsequent cycles,the potential-capacity trends show gradually fading that owns to a poor reversibility of Li 2.6Co 0.4N.During cycling,Li 2.6Co 0.4N active material yields charge po-tentials mainly in the realm of 1.0–1.4V versus lithium and discharge potentials of 0.01–0.6V versus lithium.So a re-markable potential hysteresis of the composite electrode between charge and discharge is still obvious.

Fig.6demonstrates the charge capacity as a function upon the cycle number of the electrodes based on the hard carbon/Li 2.6Co 0.4N composite and the pristine hard carbon.The capacity of the composite in the 17th cycle is 290mA h g ?1.The capacity fading is rapid within the first 10cycles and shows comparable capacity during the subsequent cycling,which is probably caused by the degradation of the Li 2.6Co 0.4N.However,the capacity of the composite remains at about 290mA h g ?1in the following cycles,which is mainly the capacity of hard carbon.The cycle stability needs to be further improved.

The performance of a cell using LiCoO 2cathode and hard carbon anode is shown in Fig.7.The cell presents 365mA h g ?1of first charge capacity,followed by 188.8mA h g ?1of dis-charge capacity,leading to a low initial coulombic efficiency of 51.7%.The initial irreversible capacity is too large to put hard carbon into practical use.By contrast,a cell using LiCoO 2cathode and composite anode was assembled to examine the charge and discharge behavior in the voltage range of 2.0–4.2V ,as illustrated in Fig.8.This cell presents 525mA h g ?1of first charge capacity,followed by 428.7mA h g ?1of discharge capacity.The initial coulombic efficiency increases to 81.6%,indicating that the composite of Li 2.6Co 0.4N is effective to im-prove the initial coulombic efficiency.Because the only dif-ference between the above mentioned two different full cells is

the addition of Li 2.6Co 0.4N to the composite anode.From this point of view,a better cycling performance can be achieved while a series of low-cost lithium metal nitride by reducing the Co content and with high cycling stability is adopted [19,20].4.Conclusions

The initial coulombic efficiency of hard carbon can be im-proved by combining with Li 2.6Co 0.4N.The hard carbon/Li 2.6Co 0.4N composite electrode demonstrates a high initial coulombic efficiency of 100%and a relatively large reversible capacity of about 438mA h g ?1versus lithium.A full cell using LiCoO 2cathode shows its initial coulombic efficiency increases to 81.6%from 51.7%after combining hard carbon with Li 2.6Co 0.4N as anode materials.The composite with Li 2.6Co 0.4N paves the way to reduce the large initial irreversible capacity of hard carbon.Acknowledgements

The authors highly appreciate the financial support for this study from LG Chem of Korea.The authors also highly appreciate the comments for the revision from three anonymous reviewers.References

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Fig.8.Charge and discharge curves for a cell with LiCoO 2cathode and the hard carbon/Li 2.6Co 0.4N composite anode.The weight ratio of cathode to anode active material=3.

1334H.Sun et al./Solid State Ionics 177(2006)1331–1334