An electrochemical investigation into the lithium insertion properties of LixNiO2 (0 ≤ x ≤ 1)

SOLID

STATE IONICS

An electrochemical investigation into the lithium insertion properties of LixNiO, (0 5 x 5 1)

Abstract The thermodynamic and kin&c properties li)r the lithium inscrtioa in Li, NiOCz have been probed by clcctrochemical techniques. The preparative method used ensured the starling muterial was nominally stoichiometric with a low estimated level of I,i and Vi inter-mixing. The step potential prolocol of the electrochemical vollagc spectroscopy method was adopted to reveal the nature of the lithium insertion and extraclion reactions. The firstcycle dcmonslratcd a poor coulombic cOicicncy, which may bc associated with the structural rcarrangemcnt caused by Ihe initial hexagonal to monclinic phase wmsition. The efficiency improved during the second cycle and showed thai a specilic capacity around I X0 mA h/g (LLU= 0.65) could bc cycled rcversihly. Several sharp differential capacity peaks were dctectcd, which appeared 10 correlate well with the structural phase transitions previously detcrmined from in-situ XRD sludies. The dift’crential capacity protilc indicated that the main cxtr-acrion-insertion peaks were well-dclincd with only minor peak separation consistent with a I-evcrsiblc system demonstrating low overvohagc. The multiple phase transitions were dcmonstratcd to have a propound efect on the nature of the ionic transport properties within the host laltice. Over most of the conccnlration range, the diffusiotl coefticients averaged around 1O-x cm2 s- ‘. values which arc comparable M:ith similar mcasuremcnts carried OLII on composite cleclrodes matlc from the altcmatiw high voltage cathode materials Lix Mn20, and Li.,CoO,.

1. Introduction Due to its ability to insert lithium reversibly at high oxidativc potential, the Li,NiO, (0 5.x 5 I ) compound has attracted interest as a cathodc-active material for lithium ion type batleries [l-3 1. The clectrochemically acfive compound comprises alternative layers of Li and Ni, which both occupy the octahedral sites of a cubic close-packed oxide lattice making a rhombohedral structure with the R%n

space group 141. The normal operating voltage range for the Li,NiO, is between about 3.6 and 4.5 V vs. Li/Li; and is characterized by several voltage plateaus corresponding to multiple phase transitions. Under extremely low rate conditions, a reversible specific capacity of over 211 mA h/g (AX = 0.77) has been reported [SJ, although under mow realistic rates between 110-l 50 mA h/g @Ix = 0.4-0.6) could be reliably attained owr multiple chargedischarge cycles 143. It has also recently been reported that as s values in Li,NiO, decrease, there is a concurrent increasing tendency for oxygen to be

0167-2738/9h/El5.00 Copyright Q I996 Elscvier Scirncc H.V. All rights reserved PII SO167-2738(96)00’62-7

libcratcd from the structure [h]. This observation is then expected to limit the available lithium that can be safely extracted when Li,NiO, is used as the lithium source material in lithium ion cells 161. In contrast to the more commonly studied cathode materials, Li,CoO, and Li.,Mn,O,. the synthesis 01 the nickel compound is reported to be relatively difficult 17-101. For instance, small variations in the precise preparative conditions can result in the formation of non-stoichiometric Li,NiO, in which there is a significant departure in the expected Li and Ni ion distributions within the structure 11 I, 121. This cation mixing phcnomcnon products an electrochemically inferior Lhrm of the material in which the achievable specilic capacity is significantly impaired. Considerable effort has been expended, by several research groups, in attempts to elucidate the mcchanisms for this cation mixing phenomenon by careful examination of the reactions associated with the Li,NiO, formation [7,1 1,121. It has been reported that near stoichiometric Li,NiO,, with low cation mixing, can be prepared by reacting LiZ02 (or LiOH.H,O) and NiO under a controlled ambient atmosphere at around 700°C 17.1 l]. In this study we have undertaken a carefully ccntrolled preparative route to produce a near stochiomctric form of Li*NiO,. A Rietveld-type refinement routine has been used to characterize both the structural integrity of the material and the extent of cation mixing 181. To understand the precise mechanisms for lithium insertion and extraction, WC have then conducted detailed electrochemical measurements to generate both thermodynamic and kinetic information. These measurements were performed on metallic lithium anode cells incorporating a composite l_i,NiO,-cont.ainiiIg cathode. WC have adopted the step potential protocol ol’ the clectrochcmical voltage spectroscopy (EVS) technique, which has been demonstrated previously by this group to be an extremely powerful tool for characterizing alkali mclal insertion reactions [ I3- 1S].

2. Hxperimental LiXNiO, was prepared by solid state reaction of nickel(U) oxide and lithium peroxide at elevated temperatures. Stoichiometric am0unt.s 01’ nickel( II) oxide, NiO. (Alfa Chemicals, purity 99.9%) and

lithium peroxide, Li202. (Alfa, purity 95%) were thoroughly ground in an argon filled glove-box. This mixture was then pellctixcd and placed in a platinum boat. which was enclosed in a quartz tube equipped with inlet and outlet taps; such that oxygen could be continuously passed over the reactants during the heating step. The furnace was healed at a prcciscly controlled rate of I”C/min to a final temperature of 800°C and held at this tcmpcrature for 48 h. After this calcination period, the furnace was slowly cooled to room tcmpcrature. the pellets were crushed, the material reground, and then rcheatcd to 800°C for a further 24 h. The final material was analyzed by X-ray diffraction using a Siemens D5000 apparatus equipped with a diffracted beam monochromator and using Cu KU radiation. Atomic absorption spectroscopy (lnstrumcntation Laboratory 357 Spectrophotometer) was used to determine the lithium and nickel contents in the prepared material. For electrochemical testing, composite electrodes were prepared in the following manner. A powder mixture comprising 90 w/o Li,NiO, and 10 w/o acetylene black was thoroughly mixed and subsequently added to a 10 w/o solution of ethylene propylene diene tcrmonomer (EPDM) in cyclohexane. The formulation was ad.justed until the final concentration of binder in the composite electrode would bc 3 w/o. The slurry was uniformly coated onto an aluminum foil current collector. The coated films were dried at 120°C under a dynamic V~C~LI~~ and then transferred to an argon tilled glove-box. The electrochemical cell comprised the Li,NiO, containing electrode and a metallic lithium anode. The electrolyte used was 1 M LiPI:, in a 66/34 ratio of ethylene carbonate (EC) to dimethyl carbonate (DMC). The electrode area was 2.4 cm2 and the entire cell assembly was placed inside a hermetically sealed polypropylene ccl1 holder. For electrochemical testing, the cells were thermostatically held at 232 I “C. The bulk of electrochemical data was collected using the EVS method \13151. EVS is a voltage step t.echnique which provides a high resolution approximation to the open circuit voltage discharge-charge curves for the electrochemical system uncler imestigation. The technicluc initially involves the measurement of the cell current resulting from the application of a small potential step, dV to the cell. The current density is continu-

ously monitored until it decays to a prc-set minimum value, Zlim. The cell potential is then slepped by dV again. The value of lli,,, is chosen such that the cell is close to thermodynamic equilibrium at the end of the current transient lo ensure dial voltage corrections due to the cell ZR drop and diffusion overvoltages arc insignificant. By using small potential steps, the voltage-char&c relation gencratcs an accuracc approximation to the thermodynamic properties or the cell between the chosen voltage limits. During each step, the current is integrated to allow calculation or the differential capacity. dQld17, where Q is the charge, or d.rldV, where x is the lithium ion concentration in, for cxamplc, Li,NiO,. In the experiments conducted here Ihe values of clV and flim were _f 10 mV and 50 fiA/cm’ respectively. Kinetic properties, such as the solid state lithium ion diffusion within lhe cathode host, may also be estimated by analyzing the decay of the cell current following each EVS voltage step. This then is proportional to t -‘!’ for line‘ar diffusion in a semiinfinite system [161. This treatment is essentially a solution of the current-time response (for short time periods) for the well known Cottrell equation 1171. Separate experiments were performed to verify the diffusional data using the so-called Galvanostatic Intermittent Titration Technique, GITT 1181. In this method, a small conslant current pulse is applied across a cell, while the transient voltage is measured as a function of time. The change of the steady state voltage following the pulse then delermincs the dependence of the ccl1 voltage on the concentration of the clectroactive species. The EVS and GlTT instrumentation used was an Advantest R6142 Precision Voltage source/sink coupled with a Hcwlctt Packard Model HP.3440 IA D&$tal Multimeter (DMM), for current or voltage monitoring. The experimental instrument control was performed using custom software written in Hewlett Packard HP Instrument Basic in conjunction with a standard HP-IB (IEEE-488) parallel interface bus.

3. Results and discussion 3.1. Structural

and chemical

andysis

The phase purity of the synthesized analyzed by X-ray diffraction and

material was the resultant

diffractogram is shown in Fig. la. The high crystallinity of the material is dcmonstra~ed by the relative sharpness of the peaks. All peaks could bc identified as belonging to the LiNiO, rhombohcdral phase. No other impurity phases could be detected from the analysis. In the expanded pattern shown in Fig. 2h: the clear splitting of the ( 108) and ( 101) peaks, at 2H = 64.4 and 64.7”, is evidenced as expected for the electrochemically active form of the lithium nickel oxide IX]. A modified Rietveld refinement routine (GSAS) was used to assess the cxlent of Li and Ni cation mixing 1191. Fig. la shows the Rietveld relinemen proIile together with lhe calculated patterns and their dirfcrence. The small difference is indicative of the good fit. A sutnmary of the fit data is presented in Table 1. The standard deviations are shown in parcnthcses and refer to the last decimal point. The unit ccl1 compares well with the data from Thomas et al. [20], with the value of the rhombohedral cell volume of 34.01 A”: corresponding roughly to x = 0.09 in Li,.NiO,. The lattice parameters were determined from the best fit to the peak positions, assuming the rhombohcdral R.?m symmetry and indicated a = 2.883 A and c = 14.200 A. consistent with literature values for a near-stoichiometric LilNiO,. The atomic absorption analysis indicated the stoichiomctry or the as-made material (assuming the presence of no residual reactants in the sample) and corresponded to Li,,.,,Ni , .(,,0,. WC: rccognizc that the AA analysis places an accuracy limit of 20.01 on the Li and Ni contents within the sample. However, the values are consistent with the X-ray analysis data presented above. We conclude Tram the data that .from a structural and chemical analysis viewpoint, the preparative route chosen was adequate for producing a nominally stoichiotnetric material with low cation mixing. -3.2. Elccttwcheinic~al data Kepresentativc cells were cycled bctwcen pre-set voltage limits of 3.0 and 4.3 V vs. l,i/Li-. at 23°C under the prevailing experimental EVS conditions. The precise EVS cxperimcntal parameters. described in the experimental section, ensured that the cells were close to thermodynamic equilibrium during the cycling process. The technique approximates to

28

LiNiO2 Lambda

1.5

.l

.2

a-Theta,

aeg

60

.3

.4

.5

.7

.6

.8

.9 XlOE

I

62

64

about a 100 h chap-discharge rate. By essentially removing any unnecessary overvoltage, this method also minimizes the risk of electrochemical oxidation of‘ the electrolyte. The first cycle EVS voltage profile for the as-made LiNiO, material is ~110~11 in Fig. 2. For clarity, the

66

66

1.0 2

70

cell voltage data is plotted as a function of lithium content in the electrode by normalizing the capacity to the lithium composition, x, in Li,NiO,. The compositional figures m’erc based on the integrated charge and the active electrode mass. The onset of the lithium extraction process corn-

Cycle#l 4.2.

+2 -I

4.0

9 5i 2 P = 3

3.0 3.6 3.4 3.2

x in L$NiO, Fig. 2. First cycle EVS ccl1 voltsgc-composition

Table I Structural parameters Li ,.,Ji ! i,1(j2

derived

frwn

X-ray

powder

profile for the Li/Li,NiO,

refnclnznt

fur

Abll

Site

YlU

y/b

e/r

occup~u~~cy

Li Ni(lj Ni(2) 0

30 3~1 3b 6~

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.5 0.2441(6)

I .OY(-) 0.0 16(10) O.Y84(6)

I .O(-)

mcnences at a cell voltage just less than 3.6 V vs. Li/Li ‘-. There follows a relatively complex and convoluted voltage profile containing several voltage plateaus and inllcctions, consistent with the presence of multiple phases transitions occurring over this range of stoichiometry [7,8]. On the subscqucnt reduction wave: the mztjority of the corresponding lithium insertion features are apparent. The nature of the structural phase changes may be better understood by inspection of the differential capacity data, which will be discussed in detail later. The extent of the voltage hysteresis (i.e., the potential difference between extraction and insertion waves) is small, indicating the relati\/ely low overvoltage associated with the system over this voltage range. The first EVS cycle charge corresponds to a specific capacity of 204 mA h/g (d\- = 0.74) and the first discharge capacity to I83 mA h/g (b = 0.67),

cell cycled bctwcen voltage litnits of 3.0 and 4.3 V vs. Li/Li

‘.

leading to a coulombic efficiency for this cycle of 00%:. These specific capacity figures are somewhat smaller than those reported by Li et al. [8], although this observation may be explained by the low-er voltage limit used in this study (i.e., 4.3 V as opposed to 4.5 V in ref. [8]). Additional capacity may still be accessible above the 4.3 V limit used in our study. This is supported by the non-zero value of our differential capacity data at this cell voltage shown in Fig. 3. It has been previously described that up to around 4.15 V vs. Li/LiT, the extractioninsertion reactions are relatively reversible, demonstrated by the relatively high coulombic efficiency figures reported in the literature 121 J. Beyond about 4.30 V, however, the stability of the Li.NiO, is somewhat questionable, demonstrated by relatively inefficient cycling and exacerbated by safety concerns associated with loss of oxygen from the structure 161. Fig. 3 shows the differential capacity profile for the first cycle. The profile shows several peaks which correlate with the multiple voltage plateaus denoted in Fig. 2. It should bc noted that Fig. 3 may be viewed as a clear spectroscopic fingerprint for the insertion-extraction reactions in lithium the Li,.NiO,. No numerical smoothing of the data was performed (as is often necessary in similar data

30

100 Cycle?Yl

50 Lithium Extraction z

0

>’ 3 -50

-100 I

3.0

3.2

3.4

3.6

Cell

3.8

4.0

4.4

Voltage [v vs Li/Li+]

Fig. 3. First cycle differential capacity plot for the Li/Li,NiO, cell cycled hetwcen annotation clenoles the tliffcrential capacity peaks described iu the tat.

I

4.2

voltage limits of 3.0 and 4.3 V vs. Li/Li

‘. The 1~4

.

4.4

Cycle#2

4.2

3.8 3.6

3.2. 3.0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

x in L$NiO, I;ip. 4. Second cycle EVS cdl volta~c-col~ll~osition Li/Li !-.

prolile for the Li/Li,NiO?

derived from, for instance, constant current cycling regimens) so that the presented data here is basically the raw EVS experimental data set. During the lithium extraction process, we detect five clear

cell cycled

between voltage

limits of 3.0 and 4.3 V vb.

differential capacity pcuks located at 3.6 I, 3.69, 3.80. 4.02 and 4.20 V vs. Lilli ’ . The corresponding lithium insertion peaks are present at 3.49, 3.63. 3.75, 3.98 and 4.16 V vs. L,i/Li ‘-. A less clear feature

is also discernible on the charge wave at 3.71 V and appears as a shoulder on the peak at 3.69 V vs. Li/Li ‘. For reference purposes, all peaks have been denoted on Fig. 3 by text annotation. It has prcviously been demonstrated that sharp peaks in differential capacity plots correspond to structural phase transitions. whereas, minima between peaks correspond to single phase regions 113,221. In addition, broad peaks (corresponding to ‘S-shaped’ voltage-concentration relationships) are normally indicative of varying stoichiornctry within a single phase. Recall from the detailed X-ray difli-action studies carried out by Li ct al. [S] that, during the cxtraction-insertion processes, the Li,NiO, undergoes three distinct phase transitions. These have been identilied as hexagonal to monoclinic (H, +M), monoclinic to hexagonal (M + H ,) and hexagonal to hexagonal (H, + ti,) 181. It was-suggested that the three sharp differential capacity peaks, found during the lithium extraction reactions. supportctl the prescnce of the three phase co-existing regions, at electrode potentials of about 3.66, 4.03 and 4.20 V vs. Li /Li -. The corresponding transitions during lithium insertion were reported at 3.63, 3.98 and 4. IS V vs. Li/Li’ IX]. l’hesc values correlate well with the values found in the work here. Furthermorel the

observations of the much broader extraction peaks, found at 3.61 V and 3.80 V vs. Li/Li-. (and 3.75 and 3.49 V on subsequent lithium insertion), are consistent with the existence of single phase regions of varying stoichiometry. These have been respectively assigned to the presence or the sin@ phase, H, and the single phase, M 181. The second cycle EVS voltage proiile is shown in Fig. 4. The figure indicates that there is a slight improvement (i.e., lowering), over the cycle one data, in the extent of the voltage hysteresis between extraction and insertion waves. This is consistent with a small decrease in the overvoltage for the overall Li/Li,NiO, system. Without the benetit of three electrode cells. incorporating a suitable refcrence elcctrodel we
50.

Lithium Extraction

I

-100 ’ 3.0

3.63\/ 4.16V 3.2

3.4

3.6

3.0

4.0

4.2

4.4

Cell VoltageD/ vs Li/Li+] Fig. 5. ScconcI cycle dil’krcntid ca~pcily plot lor the I,i/l.i~~NiO, cell cycled between voltage limits of 3.0 and 4.3 V vs. Li/Li I. The text annotiition drnotcs the dil’fercntial capacity peak-sdcscribetl in the text.

reactions encountered during the initial charge-discharge cycle have largely been removed. The efficiency at this stage is now better than 9%, and a lithium concentration around AX = 0.65 ( 180 mA h/g) could bc cycled reversibly. Subscqucnt chargedischarge cycles showed similar specific material utilization and close to 100%’ coulombic reversibility, thus confirming previously reported findings that Li,,NiOz, at least from a specilic capacily viewpoint. is acceptable as the cathode active material in lithium ion type applications 1241. Fig. 5 shows the second cycle differential capacity profile for the system. One can now detect five clear differential capacity peaks located at 3.57, 3.66. 3.78, 4.01 and 4.20 V vs. Li/Lii. The corresponding lithium ins&on peaks are now present at 3.47, 3.63, 3.75, 3.98 and 4.16 V vs. Li/Li’. From direct comparison with the lirst cycle data, one can obscrvc that the ma,jor changes have been restricted to the peaks situated between about 3.66 and 3.7 1 V on the charge wave. Recall that these peaks halIe previously been associated with the initial (H, + WI) st.ructural re-arrangment occuring within the Li,NiO, material (81. Thus. as lithium is extracted, the symmetry changes concurrently from hexagonal 10 monoclinic. Clearly the sharpness of the peak at 3.66 V on the second cycle has improved and the shoulder at 3.71 V has sharply diminished. One may suspect that the initial structural phase change has induced the rclativcly poor coulombic efliciency value encountered during the first cycle. The differential capacity features at higher voltages i.e., the peaks situated around 4.01 V on charge (3.98 V on discharge) and 4.20 V (4.16 V on discharge) remain sharp and repeatable over several cycles. This is an important observation since, as described previously, the Li,NiO, phase is expected to be relatively unstable at the higher oxidative potentials [61. One may suspect Ihal any apparent elcctrochemically induced instability could result in poorly resolved and nonsymmetrical differential capacity features. Clearly. in this study, this does not appear to be the cast. It can be noted from inspection of Fig. 5, that most of the reversible peaks are separated by about 30 mV, indicative of the general extent or the overvoltage present in the system and consistent with the voltage data presented in Figs. 2 and 4. This value is rclnl-ively low and remains fairly uniform over most

or the voltage range considered. However, note that the reversible peaks round at 3.57 and 3.47 V (on extraction and insertion, respectively) demonstrate a far larger voltage separation. This is substantiated by similar observations frotn the first cycle data. One may speculate that the overvoltage within this single hexagonal phase (H,) is profoundly larger than for the rest of Ihc insertion region, and may rellcct a hindered transport mechanism present within this particular phase. We discuss the transport properties of this region later in this paper. It can also be seen that there are small reversible peaks present on the differential capacity profile at prcciscly 3.71 and 3.70 V vs. Li/Li L on the extraction and insertion waves, respectively. These peaks have been highlighted on Fig. 5 with the aid of arrows. The peaks are less clearly dcfincd on Fig. 3. At present, we cannot reliably identify their origin but postulate that, due to their relative sharpness and small voltage separation ( IO mV), they arc most likely electronic in origin. Recall that the EVS experimental resolution is set primarily by the voltage step sir.e, which in this case was also 10 mV. The voltage separation is therefore within the resolution of the experimcnl. .K!J. Kin&:

datu

Effective diffusion coefficient measurements for the Li,NiOz electrodes were determined by the two electrochemical pulse methods described in the experimental section, and indicated relatively fast reaction kinetics for the lithium exlraction and insertion proccsscs. The values from the two techniques were essentially identical and only Ihe data collected from Ihe EVS experiments will IX presentcd here. Careful collection of the data was pcrformcd to ensure that the Cottrcll fit of the current-time transient for the system was satisfied by the semi-infinite condi.tion (i.e., t < Z’ID, where t is time, 1 = electrode thickness and 11 is the chemical diffusion coefficient) 116,17 ). This procedure enlailed collecting the currenl transient data for relativly short time periods following the voltage step. TI also involved discarding data where a non-Cottrell behavior (within either the semi-infinite or infinite regions) was detected. Effective diffusion coefficients for the electrodes composites were then calcu-

lated from these measurements based on the geometric surface area of the electrodes. WC recognize that these values will be dependent on the intrinsic diffusion coefficients for the electrode materials in question. It should be noted, however, that other electrode parameters (such as the cxtenl of the elcctrolyte/clectrode particle contact within the composite electrode) will also al&t the magnitudes. As these factors are not expected to depend on the state-of-charge of the electrode, the variation in the measured effective diffLGon coefficients are cxpetted to directly reflect the compositional variation of the intrinsic diffusion coeffcient for Li in the electrode material. Fig. 6 shows the variation of the lithium ion diflhsion coefficient for the Li,NiO, composite electrode as a function of the lithium ion concentration during the lithium extraction reaction. The data is derived from the first EVS cycle. Data from subsequent cycles were quantitatively similar. One can detect li-om inspection that, during the initial lithium extraction reaction i.c.: wilhin the H, phase, the magnitude of the diffusion coefficient is relalively low (ca. IO -I0 cm2 s ’ ), and this reflects the relative difficulty for the ionic diffusion within this particular phase [ 101. This observation may, to some

extent, explain the higher overvoltage noted on the differential capacity profiles for this particular phase. since it may confer additional difh~sional overvoltage on the system within this particular concentration range. Over most or the concenlration range considercd. the diffusion coefficient averages around IO -’ cm’ s -I, a value which is comparable with similar measurements carried out on the composite-based Li,Mn,O,, 1251 and Li,CoO, 1261 electrodes. Following the initial rise, it can be noted that there appear to be three distinct local maxima. The positions of the ionic diffusion rate maxima correspond relatively well with the two phase regions reported by Li et al. [8] and described above in the section pertaining to the differential capacity measurements. The first local maximum exists close to the twophase region corresponding LOthe H, -+ M structural phase transition centered around x = 0.80. Following this, the next maximum is located at approximately x = 0.55, which corresponds fairly well to the M + H2 phase transition. The final kinetic maximum is situated at a local lithium concentration of x = 0.35 and correlates well with the center of’ the voltage plateau at 4.2 V vs. Li/LicI which is assocciated with the H, -+ H.; phase transition. It is well known that the kinetic properties for an

-7 ,

0.1

0.2

0.3

0.4

0.5 x

Fig.

6. Thl:

transient

variation

of lhc effective

data of the EVS

mcthotl

diffusion

during

coefficient.

the first cycle

0.7

0.6

0.9

1.0

in LixNi02

13. as LL function lirhiurn

0.6

catradon

of x in Li,lNiiO,. wave.

The

&la

were

collccled

from

the currenl

insertion system arc governed by the guest-host and guest-guest coulombic interactions encountered within a particular structural phase 125,261. In this study wc detect that the multiple phase trasitions encountered over the particular concentration range have a profound effect on the nature or the ionic transport properties within the host lattice. This is an expected observation since the different structural modifications will confer varying degrees 01 hostguest coulombic interactions on t.he difrusing ions: thereby tending to influence the relative care of ionic motion within the host lattice. Within restricted concentration ranges, comparativc diffusion cocfticient data ha\/e been previously reported on composite Li,NiO, electrodes by Choi at al. [27] using a modified GITT approach. and by Bruce and co-workers IlO] employing values derived from ac impedance measurements. In both approaches t11c geometric surface areas of the composite electrodes have been used for the derivation of the diffusion cocCficient values. without allowance for the composite nature of the insertion electrodes. This allows us to make a relatively direct, yuantitative comparison with the data presented in this work. Choi et al. report that as the lithium content increases (i.e.. as x in Li,NiO, increases within the range 0.5 to O.Sj so the value of lhc difl’usion coefficient decreases from around 10 ’ cm2 s ’ to around 10 9 cm2 s-’ [27]. Both the general variation with lithium concentration and the quantitative values are consistent with the observations shown in Fig. 6. It was concluded that, given the small variation in the lattice constant? c of the Li,$iO? over this lithium concentration range, the transport properties within this particular stojchiometry are governed primarily by the number of available sites within the host lattice and not by the change in lattice parameters [ 271. Bruce and co-workers measured the kinetic properties for Li,NiO, over the lithium concentration range x = 1.00 to 0.75 in Li,NiO, [lo]. In general, it was reported that the maximum values for the diffusio; coelXcients were found to be around IO-” similar to 1hose measured in cm’s . quantitatively this stLldy. Bruce et al. disclose that the sharp increase in the dill’uusion rate. during the initial lithium extraction from the material (i.e.. within the H, phase), is consistent with the L,i.’ transport being

best dcscribcd as occurring by a vacancy mechanism. Furthcrmorc, two distinct local diffusion cocificienr maxima were reported. The conccntrational positions 01 these maxima are strikingly similar to those measured in this study.

4. Conclusions 111order to elucidate the mechanisms of lithium insertion and extraction, we have undertaken detailed electrochemical measurcmcnts on the LiANNiO, system. A careful preparative method was employed to ensure a nominally stoichiometric starting material was used. The extent of cation mixing. which is well known to distinctly affect the electrochemical performance of Li,KiO,. was estimated using a structural rcfincmont procedure [19]. We have adopted the step potential protocol of tbc EVS method, which has been previously demonstrated to be a powerful technique for understanding alkali metal insertion reactions 1.13- 151. The lithium extraction and subsequent insertion processes were characterized by several voltage plateaus in the cell voltage-lithium concentration profile. This is consistent with the multiple phase transitions which are encountered over this particular lithium concentration range. Tbc first EVS cycle, covering the cell voltage range 3.0 to 4.3 V vs. Li/Li’ , revealed that around 18.3 mA h/g (i.e.. & = 0.67 j could bc reversibly inserted. Tbe coulombit efficiency. however, was found to be somewhat low (i.e., 90%). Tt was initially suspected that this observation may be consistent with the literaturereported instability of the Li,NiO, at high oxidative potentials (> 4.0 V vs. Li/Li’),- where there is a likelihood for the material to irreversibly lose oxygen from its structure [61. However, the differential capacity data revealed that the irrcversjble changes were associated with djffercnccs occurring over the voltage range bctwcen 3.66 to 3.71 V, during which there is a concurrent hexagonal to monoclinic phase change. The differential capacity features between 4.0 and 4.3 V appeared to be both well-resolved and symmertical over repetitive charge-discharge cycles, consistent with a generally reversible system. The second cycle rev&cd improved coulornbjc eRicicncy and showed around I80 mA h/g could be

rcversihly inserted. Subsequent cycles showed similar material utilization, thus dcmonscrating the promise of the LisNiO, material in practical lithium ion applications [24].- The second cycle differential capacity prolile dcfincd a clear fingerprint for the lithium extraction-insertion reactions. Several sharp differential capacity peaks were detected, which appeared to correspond well w/ith the structural phase transitions described by Lu et al. derived from their detailed in-situ XRD studies [Sj. The profile indicated the main extraction-insertion peaks to be welldcfincd, with only minor peak separation consistent with a reversible systcrn demonstrating low ovcrvoltage. A small and previously unidentified feature was found in the differential capacity data at around 3.70 V vs. Li/Li ‘-. At present, we are unable to reliably assign the origin of this feature, but believe it to be electronic in nature. The kinetic data indicated that the multiple phase transitions have a profound effect on the nature of the ionic transport properties within the host lattice. Generally, the diffusion coefficient values average around 10. ’ cm* s -I, which arc comparable with similar measurements carried out on the compositebased Li,Mn,O, [25] and Li,CoO, [261 insertion electrodes. Within restricted concentration ranges, the kinetic data appears quantitatively similar to literature reported values [ 10,271.

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