Nanocrystalline (∼50 nm)tion synthesis using citratthe gel at ∼177 ◦C for prisit completes only aboveeter, c, with increasing ATransmission electron miadding 1.0 mol% silver incof magnitude (1.5 × 10−3
the synthesized powdersLiCoO2 cathode (140 mAh
1. Introduction
LiCoO2 is the most widely used cathode material for lithium-ion batteries. Though it has a theoretical capacity of ∼270 mAh g−1
but practically only 120–160 mAh g−1 could be realized because of
the fact that only 0.5 mol of lithium is extractable from one moleof LiCoO2 in the voltage range 2.8–4.2 V [1–6]. In an attempt toextract more lithium per mole of LiCoO2 by applying higher volt-ages (∼4.8 V), the layered rhombohedral structure with Li–Co–Ostacking in the order AB CA BC gives way to the monoclinic formwith the stacking pattern as AA BB CC with little or no electro-chemical performance as a cathode [7–10]. Thus, the structuralstability of LiCoO2 is the most important parameter for obtaininggood electrochemical performance and by stabilizing the structureit might be possible to extract lithium beyond the value realizedat present. Accordingly, several efforts have been directed towardssubstituting cobalt by different transition and non-transition ele-ments such as Ni, Mn, Cr, Fe and Al, Mg, B [8,9,11–23]. Co-dopingof Mn and Ni into the lattice of LiCoO2 results in an increase ofthe operating voltage of the cell to 4.5 V in Li//LiCo1/3Mn1/3Ni1/3O2system but a high capacity loss is observed [13,14]. Substitutionof Co in LiCoO2 with Fe2+ results in inferior electrochemical prop-erties compared to the pristine compound. Alacantara et al. [17]have observed a poor reversibility of the Li//LixFeyCo1−yO2 cellsand attributed it to the poor Li+ diffusivity due to the presence
O2 powders containing 0–10 mol% of Ag have been prepared by combus-rate combustion route. Thermal analyses show a sharp decomposition ofiCoO2. With addition of silver, the decomposition becomes sluggish and. X-ray powder diffraction analyses show an increase in lattice param-
tent suggesting the occupation of Ag within LiCoO2 interlayer spacings.py indicates diffusion of Ag into LiCoO2 grains. It has been observed that
s the room temperature electrical conductivity by more than two orders1). Galvanostatic charge–discharge profiles of coin cells fabricated witha two-fold enhancement in the discharge capacity for 1.0 mol% Ag-added
of Fe2+ in both tetrahedral and octahedral sites of LiCoO2. On theother hand, for doping with Cr3+, it has been observed that as theproportion of Cr3+ increases, the amount of cycleable Li+ reducesdrastically [18]. Other than the transition elements, extensive stud-ies have also been carried out to substitute Co3+ in LiCoO2 withnon-transition elements like Mg2+ [8,9,19], B3+ [20,21], Al3+ [22,23],but only a marginal increase in cell potential from 4.2 to 4.3 V isreported with associated capacity fading. While this capacity fad-
ing (>50% after 10 charge–discharge cycles) is large for Al3+ [22],a somewhat better capacity retention (∼90% after 100 cycles) hasbeen observed for Mg2+ and B3+ as dopants [19–21]. Therefore, itappears that although various dopants, both transition and non-transition ions have been tried, no significant improvement in theelectrochemical performance of LiCoO2 could be achieved. On theother hand, there are only a few reports on the effect of additionalnon-substituting elements e.g., Ag [24]. Having a much larger ionicradius (∼0.13 nm) than Co3+ (∼0.063 nm), it is unlikely that Ag+
would enter the LiCoO2 lattice and substitute Co3+. Instead, beinga highly mobile ion, Ag+ may migrate to the interlayer space sta-bilizing the layered framework of the LiCoO2 structure. Also, Ag isknown to influence the phase transformation and structural stabil-ity of several metal oxides in addition to increasing the electricalconductivity [25,26]. Recently, Yang et al. [27] have reported thatcomposite of Si/Ag anode powders prepared by electroless depo-sition and high energy mechanical milling show a better stabilityas an anode with a high coulombic efficiency of 83.4% and a highercapacity retention (∼800 mAh g−1) after 30 cycles whereas the pris-tine Si shows a sharp loss (∼90%) in charge capacity after a fewinitial charge–discharge cycles. Similarly, Huang et al. [28] have
Fig. 1. TG curves of LiCoO2 gels with varying Ag content.
[37]. Differential thermal analysis (not shown here) shows that thissharp decomposition is exothermic in nature accompanied by evo-lution of a large amount of heat and thus, instantaneous burninggives rise to the as-synthesised LiCoO2 precursor powder. On theother hand, metal-LiCoO2 gels show a sluggish decomposition reac-tion which goes to completion at ∼430 ◦C, suggesting more complexreactions due to the addition of Ag+. The weight loss in the tem-perature range 250–450 ◦C can be attributed to the removal of theorganic moieties. After 450 ◦C, the weight loss becomes negligiblesuggesting the formation of LiCoO2/Ag precursor powders.
Fig. 2 shows the X-ray diffractograms for undoped (A000),1 mol% (A010) and 5 mol% (A050) of Ag-added LiCoO2 samplestogether with that of a standard rhombohedral LiCoO2 [JCPDS fileno. 00-050-0653]. For both A000 and A010, all the peaks can beidentified with those for a rhombohedral LiCoO2 structure. Thecharacteristic splitting of (0 0 6)/(1 0 2) and (0 1 8)/(1 1 0) peaks sug-
P. Ghosh et al. / Materials Chem
observed that Li4Ti5O12/Ag anode powder, prepared by solid statemethod, shows a better capacity retention compared to the bareLi4Ti5O12, showing a capacity loss of 0.32% and 11.8% for the com-posite and bare compounds respectively after 10 cycles. Likewise,Ag-coated LiMn2O4 also shows a superior cycling performancewith the discharge capacity increasing from <20 to 90–100 mAh g−1
[29]. Recently, studies on Ag–LiMn2O4 thin films have shown anenhanced discharge capacity and reduced irreversible capacity lossresulting in an improved cycleability [30]. All these studies indi-cate that Ag addition might have a significant influence on thestructural stability and electrochemical properties of LiCoO2. How-ever, till now, no systematic study on the influence of Ag on thestructural, electrical, and electrochemical properties of LiCoO2 isavailable. Huang et al. [24] have prepared LiCoO2/Ag (9 wt%) com-posite electrodes by adding Ag+ ex situ using planetary milling ofcommercial LiCoO2 with AgNO3 and observed an increase in thecapacity from 138 to 172 mAh g−1 at 1C. Also, the specific capacityof the Ag–LiCoO2 was retained at 126 mAh g−1 after 50 cycles at 10Cresulting in a capacity loss of only 4.88% [24]. In the present work,we report on the synthesis of nanocrystalline LiCoO2/Ag compos-ite powder by combustion technique where Ag is added in situ asa precursor. Replacement of micron sized cathode materials withnanocrystalline materials results in improved lithium intercalationkinetics [31,32]. The influence of Ag on the phase transformationand electrical conductivity are discussed and correlated to the elec-trochemical discharge capacity.
2. Experimental
For the synthesis of LiCoO2/x-Ag powders (x = mol% of Ag, 0 ≤ x ≥ 10), acitrate–nitrate gel combustion process was adopted [33–37]. An aqueous solutionof stoichiometric amounts of lithium nitrate (LiNO3), cobaltous nitrate hexahydrate[Co(NO3)2·6H2O] (s.d. fine Chemicals, 99.5%), silver nitrate [AgNO3] (Merck India,99.5%) and citric acid monohydrate (Merck, 99.0%) was heated at a temperature of∼150 ◦C on a hot plate with constant stirring of the solution by a magnetic needle.Here, citric acid acted both as a fuel for the combustion process as well as a chelat-ing agent for the metal ions (Li+, Co2+ and Ag+) in solution. Gradually the clear darkpurple solution turned into a viscous gel and ultimately burnt into a black mass con-taining the precursor metal oxides for the formation of LiCoO2. The as-synthesizedpowder, thus prepared, was collected and further heat treated in air at 850 ◦C toobtain the rhombohedral phase.
Thermogravimetric analyses (TGA) of the gel samples were done in the temper-ature range 30–1000 ◦C by a NETZSCH, Germany Thermal Analyser (STA 409C) witha heating rate of 10 ◦C min−1. X-ray powder diffractograms were recorded in the 2�range 15–70◦ at a scanning rate of 2o min−1 by a Philips X’Pert X-ray diffractometerwith a CuK� radiation at 40 kV and 40 mA. Rietveld refinement of the powder X-raydiffraction profiles and quantitative phase analysis were carried out using PANalyt-ical Highscore Plus program. Microstructural studies were carried out by a Jeol JEM
2010 High Resolution Transmission Electron Microscope (TEM). The electrical con-ductivity was measured by a two-probe conductivity measurement system fromroom temperature to 150◦ on circular pellets of diameter of ∼10 mm and thick-ness of ∼2 mm, sintered at 800 ◦C. The electrochemical performance was studiedby assembling 2016 coin-type cells. A typical cathode was prepared from a slurryof the synthesized LiCoO2/Ag powder (80 wt%), acetylene black (10 wt%) and PVDFbinder (10 wt%) in n-methyl pyrrolidinone (NMP) solvent. The slurry was coated onan aluminium foil (current collector) and was dried at 110 ◦C in an oven for 12 h.It was then pressed under a pressure of 4 tonne in.−2 for 1 min. Finally, circulardisks of 18 mm in diameter were cut and used as cathode. The cells were assem-bled using Li metal as anode and LiPF6 in EC:DMC (1:1 vol.%) as electrolyte withinan argon filled glove box. Galvanostatic charge–discharge cycles were performedwith Autolab PGSTAT30 galvanostat–potentiostat at a constant current of 0.05 mAduring charging and 0.1 mA during discharging.
3. Results and discussion
Fig. 1 shows the TG plots for LiCoO2 precursor gel samples syn-thesized with varying metal content (0.0, 0.5 and 1.0 mol%). Allthe samples show an initial weight loss at temperatures <150 ◦Cdue to loss of absorbed water. The pristine gel sample shows asudden, sharp decomposition step at ∼177 ◦C accompanied by alarge weight loss, after which no significant weight loss is observed
Fig. 2. X-ray diffractograms of (i) pristine LiCoO2 (A000), 1.0 mol% Ag added LiCoO2
(A010) and 5.0 mol% Ag added LiCoO2 (A050) and (ii) standard rhombohedral LiCoO2
(JCPDS file no. 50-0653).
istry a
oO2 ca
Ag (f
00000.00.27.2
13.1
408 P. Ghosh et al. / Materials Chem
Table 1Phase analysis and lattice parameters of LiCoO2 powder: pristine and Ag-doped LiC
gests that the ordered array of cobalt and oxygen atoms remainsunperturbed even after the additional silver phase is introduced.However, in the diffractogram for A050, additional peaks appear at2� = 38.18, 44.42 and 64.5 corresponding to (1 1 1), (2 0 0) and (2 2 0)planes of cubic silver [JCPDS file no. 03-065-2871]. The detailedphase analysis and the lattice parameter of all the samples are givenin Table 1. As the quantity of added Ag is very small in A001, A002(0.2 mol%) and A003 (0.3 mol%), no Ag is detected from the XRDphase analysis for these samples. Also, the fact that no shifting ofthe diffraction peaks of LiCoO2 is observed in the diffractogram of
any of the silver added samples, even for A050, indicates that Agdoes not substitute either Li or Co in the lattice site. This is expectedconsidering the large ionic radii of Ag+ in the tetrahedral (∼0.1 nm)and octahedral (∼0.13 nm) coordination compared to the respec-tive ionic radii of Litet
+ (∼0.059 nm) and Cooct3+ (∼0.063 nm). It is
possible that the added Ag may diffuse into the grains and dur-ing calcination some of the Ag+ ions move towards the surface anddue to the high redox potential of Ag+, get reduced to Ag0. Sincethe Ag–O bond strength is weaker than the Ag–Ag bond, the freeAg0 atoms, having a higher surface energy will eventually form themetallic clusters [26]. On the other hand, it is also possible that afew Ag+ ions, with a high ionic mobility, may enter the interpla-nar spacing of the layered framework of LiCoO2. If that is the case,then it should be reflected as an increase in the lattice parameter cwith Ag addition. Fig. 3 shows the variation of the lattice parameterc with varying Ag content (x, mol%). An initial steep increase in cis observed up to x = 0.5, followed by a near linear increase up tox = 10. These results indicate that addition of Ag stabilizes the lay-ered structure of LiCoO2 and increases the interlayer spacing withrespect to that for pristine LiCoO2 which would facilitate Li+ ionintercalation–deintercalation.
Fig. 3. Variation of lattice parameter, c, with varying quantity of added Ag (mol%).
nd Physics 110 (2008) 406–410
lcined at 850 ◦C
rom XRD phase analysis) (mol%) Lattice parameters (A)
Fig. 4 shows the TEM images of the synthesized pristine andcomposite samples of LiCoO2. Uniform grains of 40–50 nm with ahexagonal shape are observed for pristine LiCoO2 (A000) (Fig. 4(a)).The corresponding selected area diffraction pattern (SADP) given inthe inset of Fig. 4(a), shows characteristic reflections from a hexag-onal crystal lattice. Fig. 4(b) shows the high resolution TEM imageof a single grain of the 0.5 mol% Ag-added LiCoO2 (A005). Existenceof twin boundaries and variation in contrast suggest diffusion ofAg into the grain. The corresponding SADP pattern taken from thisgrain shows a superimposition of reflections from both hexagonaland cubic structures. This suggests that there is probably a partialdiffusion of silver (with a cubic crystal structure) into the LiCoO2matrix.
Fig. 5 shows the temperature variation of electrical conductivityin the range 35–150 ◦C for the pristine LiCoO2 (A000) together withthe Ag added samples, A005 and A010. Room temperature elec-trical conductivity of A000 has been found to be 6 × 10−5 S cm−1.When 0.5 mol% Ag was added, the room temperature conduc-tivity increases only slightly to 7 × 10−5 S cm−1. However, whenthe Ag addition is further increased to 1.0 mol% (A010), the roomtemperature conductivity sharply increases by about two ordersof magnitude to 1.5 × 10−3 S cm−1. With increasing temperature,there is a slow almost linear increase in the conductivity of thepristine LiCoO2 (A000) in the entire temperature range studiedindicating a semiconducting behavior [9]. A similar behavior hasbeen observed for the sample A005 with a small amount (0.5 mol%)of added Ag. However, for sample A010, with increasing temper-ature, the conductivity first slowly decreases up to about 75 ◦Cand then increases sharply up to 150 ◦C. Presence of Ag in theLiCoO2 matrix gives rise to a metallic conduction, the magnitudeof which increases with increase in the Ag content. In the lower
temperature region, this metallic conduction predominates oversemiconduction resulting in a slow decrease in conductivity in A010with increasing temperature. With further increase in temperature,a large number of electronic charge carriers are generated fromthe semiconductor and thus, semiconduction supersedes the effectof metallic conduction and the overall conductivity rises sharplyfor A010. In case of A005, the amount of Ag being very small, thecontribution of metallic conduction is not conspicuous.
For electrochemical performance testing, 2016 type coin cellshave been fabricated and cycled between OCV and 4.05 V at aconstant current of 0.05 mA during charging and 0.1 mA duringdischarging. Fig. 6 shows the first discharge capacity vs. potentialplots for three cells fabricated using the pristine sample (A000) andthe Ag-added samples (A005 and A010) as active cathode materi-als. The discharge capacity of the pristine sample (A000) is foundto be 70 mAh g−1. The observed low discharge capacity of A000may be due to the highly agglomerated nanocrystalline nature ofthe LiCoO2 particles. Such agglomeration causes hindrances to Liintercalation in LiCoO2. Moreover, there may be loss of electricalcontact between the nanocrystalline particles resulting in a poorcharge transfer rate [38]. This was also evident from a low room
P. Ghosh et al. / Materials Chemistry and Physics 110 (2008) 406–410 409
Fig. 4. Transmission electron micrograph of (a) A000 and (b) A005. The selecte
Fig. 5. Temperature dependence of electrical conductivity of A000, A005 and A010.
Fig. 6. Capacity vs. potential plots for A000, A005 and A010.
d area diffraction patterns of the respective samples are shown in insets.
temperature electrical conductivity value of the sample A000. ForA005 (with 0.5 mol% of added Ag), the capacity increases sharplyto 117 mAh g−1. As discussed earlier, presence of Ag increases theinterlayer spacing of LiCoO2 facilitating Li intercalation. Therefore,an increase in capacity is observed. For A010 (with 1.0 mol% of Ag),it further increases to 140 mAh g−1. The increased electrical con-ductivity in this sample leads to an improved charge transfer rateas shown in Fig. 5. Thus, an improved charge transfer rate togetherwith an increased interlayer spacing account for the observed sharpincreases in the discharge capacity of A010.
4. Conclusions
Ag-added nanocrystalline LiCoO2 cathode materials are syn-thesized by citrate–nitrate combustion synthesis technique witha particle size of 40–50 nm. It is found that though Ag does not sub-stitute either Li or Co in the lattice site, it has a significant impact onthe material and electrochemical properties of LiCoO2. The natureof combustion changes from a sharp decomposition to a sluggishone upon introduction of Ag. X-ray powder diffraction analysesshow that silver is present as metallic clusters (Ag0) and not as
Ag–O. The lattice parameter, c, gradually increases from 14.0387 Afor pristine LiCoO2 to 14.0604 A for 10 mol% Ag-added sample. It ispossible that some of the Ag would enter the interlayer spacingsof LiCoO2 thereby increasing c. TEM results also suggest a possiblediffusion of Ag into LiCoO2 grains. As a result, the electrical con-ductivity of the doped samples is increased by about two ordersof magnitude to 1.5 × 10−3 S cm−1, thereby accelerating the chargetransfer process during the charge–discharge of the cells fabricatedwith the Ag–LiCoO2 powder as the active cathode material. Thedischarge capacity of pristine LiCoO2 (70 mAh g−1) is significantlyincreased to 117 mAh g−1 by 0.5 mol% of Ag addition which is fur-ther increased to 140 mAh g−1 for 1.0 mol% of Ag-added sample dueto improved charge transfer rate and easier Li intercalation.
Acknowledgements
The authors wish to thank Director, CGCRI for his kind permis-sion to publish this paper. Kind help from Dr. S. Gopukumar, CECRIand Mr. M.W. Raja, CGCRI for recording the electrochemical data isthankfully acknowledged.
istry a
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410 P. Ghosh et al. / Materials Chem
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