Solar Energy Vol. 68, No. 6, pp. 523–540, 2000 2000 Elsevier Science Ltd Ó Pergamon PII: S0038 – 092X( 00 )00028 – 1 All rights reserved. Printed in Great Britain 0038-092X/00/$ - see front matter www.elsevier.com/ locate / solener ELECTROCHROMIC PROPERTIES OF LITHIATED Co-OXIDE (Li CoO )x 2 AND Ni-OXIDE (Li NiO ) THIN FILMS PREPARED BY THE SOL–GELX 2 ROUTE ˇ ˇ ˇ ˇ†FRANC SVEGL , BORIS OREL and VENCESLAV KAUCIC National Institute of Chemistry, SI-1000 Ljubljana, Hajdrihova 19, Slovenia Communicated by CLAES GRANQVIST Abstract—Layered Li CoO and Li NiO thin films (x | 1) were prepared by a peroxo wet chemistry routex 2 x 2 from Li(I), Co(II) and Ni(II) acetate precursors and the addition of H O . Structural changes during the2 2 processing of xerogel to final oxide were followed by X-ray diffraction and infrared spectroscopy. Electrochromic properties were determined with in-situ potentiodynamic, potentiostatic and galvanostatic spectroelectrochemical measurements. Single dipped films with composition Li Co O or Li Ni O0.99 1.01 2 0.94 1.06 2 exhibited stable voltammetric response in 1 M LiClO /propylene carbonate electrolyte after about 60 cycles.4 22The total charge exchanged in a reversible charging/discharging cycle was about 630 mC cm for 22Li Co O and 620 mC cm for Li Ni O oxide films. Galvanostatic measurements showed that0.99 1.01 2 0.94 1.06 2 1 about 1 /2 (x | 0.5) and 2/3 (x | 0.3) of Li ions could be reversibly removed from the structure of Li Co O and Li Ni O films, respectively. Practical applicability of Li Co O and0.99 1.01 2 0.94 1.06 2 0.99 1.01 2 1 1Li Ni O oxide films was studied in electrochromic devices with WO i(H )Li ormolyteiLi Co O0.94 1.06 2 3 0.99 1.01 2 1 1 and WO i(H )Li ormolyteiLi Ni O configuration. The monochromatic transmittance T (l 5 633 nm)3 0.94 1.06 2 s of dark blue coloured devices was extremely low (T | 3%), whereas in bleached state the value reacheds around T | 70%. Ó 2000 Elsevier Science Ltd. All rights reserved.s 1. INTRODUCTION (Van Elp et al., 1991). As-deposited stoichio- metric LiNiO oxide should exhibit coloring and2The electrochromic properties of different cubic 1 would color even more at deintercalation of LiCo- and Ni-oxides have been extensively studied ions. Bleaching of lithiated Ni-oxide materials to since the discovery of the electrochromic effect completely transparent state would be attained at on WO and MoO materials (Deb, 1973). The 13 3 additional intercalation of Li ions into LiNiO2high interest in studying these oxides lies in their structure, leading to the formation of new anodic electrochromism, which makes them suit- Li NiO phase (Granqvist, 1995). All, these2 2able for counter electrodes (CE) in electrochromic properties make LiCoO and LiNiO oxide prom-2 2devices (ECDs). Although Co- and Ni-oxides are ising materials for counter electrodes, which canin the nature of their functioning compatible with have practical application in electrochromic de- the primary cathodic electrochromic layer (EC) vices (ECDs) in a combination with cathodiclike WO or MoO oxide, they still suffer of low3 3 materials like WO .3charge capacity and slow intercalation rate for The electrochemical properties of layered1Li ions in non-aqueous electrolytes (IC) (Gran- Li CoO (0 , x , 1) and Li NiO (0 , x , 1)x 2 x 2qvist, 1995). Work has been done to improve their oxide powders prepared via the solid state electrochromic properties in non-aqueous elec- chemistry processing are well known because of1trolytes with doping the cubic structure with Li their applicability in lithium and rocking-chairions (Rubin et al., 1998). However, even better batteries (Julien and Nazri, 1994). Especially1 results for intercalation of Li ions were obtained Li CoO (0 , x , 1) oxide powders have beenx 2on lithiated Co- and Ni-oxide thin films with widely used for lithium batteries, however, recent- ¯layered rhombohedral (R3m) structure (Wei et al., ly Li NiO (0 , x , 1) oxide became more attrac-x 21992). The band structure calculations indicated tive because of its lower cost, lower maximum that fully stoichiometric LiCoO oxide should be2 charge voltage, higher capacity, better reversibili-highly transparent over the entire solar spectrum ty and higher electrolyte stability (Moshtev et al., 1995). However, it is well known that Li NiOx 2 † (0 , x , 1) oxide with high stoichiometry isAuthor to whom correspondence should be addressed. Tel.: difficult to prepare, because the solid state1386-611-760-200; fax: 1386-611-259-244; e-mail:
[email protected] chemistry preparation at high temperature leads to 523 524 B. Orel et al. 1the formation of disordered structure having Li paper we want to present the results obtained ion vacancies in the lithium layers (Goodenough from the studies of their electrochromic properties et al., 1958). The preparation of nearly stoichio- using different in-situ spectroelectrochemical metric Li CoO (x | 1) oxide powders is easier techniques. Structural changes, which accom-x 2 1due to higher thermodynamic stability of the panied (de)intercalation processes of Li ions layered LiCoO structure (Orman and Wiesman, (in)out of the film structure, were characterised by2 1984). Therefore, the alternative to the solid-state ex-situ X-ray diffraction analysis. These studies high temperature preparation was found in the strongly surpass the scope of this paper, therefore low temperature wet chemistry processing of the we decided to show only the relevant results for initial inorganic salts or metal alkoxides, which this paper and the rest will be soon published in leads to the formation of homogeneous oxide the additional paper. Practical applicability of ¯materials. There have been already few successful prepared layered rhombohedral (R3m) Li NiOx 2 preparations of layered Li CoO (0 , x , 1) (0 , x , 1) and Li CoO (0 , x , 1) oxide thinx 2 x 2 (Zhencheva et al., 1996) and Li NiO (0 , x , 1) films was tested in ECDs in combination withx 2 1 1(Xiao and Xu, 1996) with wet chemistry methods WO films and Li (H ) ormolyte electrolyte.3 from cobalt and nickel organic and inorganic salts. Most of them, however, did not lead to the 2. EXPERIMENTALformation of gels and therefore were not appro- priate for the deposition of thin films with dip- 2.1. Preparation procedure coating technique. Layered rhombohedral Alcoholic Li /Ni- and Li /Co-sols were preparedLi CoO (0 , x , 1) oxide films with ill-definedx 2 21 21from M(II) acetate (M 5 Ni , Co , MC H O *4 6 4stoichiometry were deposited by spray pyrolysis 4H O, Aldrich) and Li(I) acetate2(Chen et al., 1995), laser ablation technique (LiC H O * 4H O, Aldrich) precursors. A mix-4 6 4 2(Antaya et al., 1993) and reactive sputtering (Wei ture of Ni(II) acetate and Li(I) acetate (molar ratioet al., 1992). The film deposition experiences 21 1 r 5 n(Ni ) /n(Li ) 5 2, 1, 0.5) or Co(II) acetatehave shown that the structural and electrochemi- 21 1 and Li(I) acetate (molar ratio r 5 n(Co ) /n(Li )cal properties of layered Li CoO (0 , x , 1) andx 2 5 1) was dissolved with the slow addition of anLi NiO (0 , x , 1) oxide films strongly dependx 2 aqueous solution (30 vol.%) of H O (Fluka),2 2upon the conditions of deposition. Most of these ˇwhich caused highly exothermic reaction (Sveglfilms were studied for the application in thin film and Orel, 1999). The control of reaction wasbatteries. The electrochromic properties were accomplished by cooling the reactive mixturestudied on rf diode sputtered Li CoO (0 , x , 1)x 2 using cold tap water. After the completion of theoxide films which were deposited from stoichio- reaction and the addition of an excess of H O ,2 2metric targets (Goldner et al., 1991). Their prop- the resulting solutions were freeze-dried in vac-erties were tested also in ECDs in combination uum. Freeze-dried residues were afterwards dis-with sputtered WO films and compared to Nb O3 2 5 solved in alcohol (V|100 ml, 95% ethanol orfilms. The deposition of layered rhombohedral methanol) yielding green Li /Ni-sols or brownLi NiO (0 , x , 1) oxide films using differentx 2 Li /Co-sols which were appropriate for the deposi-deposition techniques has led usually to the tion of films by a dip-coating technique. The solsformation of cubic Li Ni O (0 , x , 1) filmsx 12x remained stable in the refrigerator (T|158C)(Wen et al., 1999). The studies of electrochrom- several months. Heat treatment of dry xerogel thinism of lithium nickel oxide films (Li Ni O)x 12x films at different temperatures gave Li /Co- and(x | 0.5) deposited by pulsed laser deposition and Li /Ni-oxide films exhibiting various stoichiome-sputtering showed promising properties for use as try and structural phases.an active counter electrode in ECD devices (Wen The formation of a layered structure ofet al., 1999). Li CoO (x|1) oxide during the thermal de-x 2On the basis of known electrochromic and composition of the xerogel film was favoured,structural properties we decided to synthesise and whereas the formation of layered rhombohedralto characterise lithiated Co- and Ni-oxide thin structure of Li NiO (x|1) oxide film at 5508Cx 2films using wet chemistry processing and dip- was strongly influenced by several preparationcoating deposition technique. We recently pub- ˇparameters (Svegl and Orel, 1999).lished a paper, which describes in details all the steps made in the wet chemistry preparation ] 2.2. Thin film depositionprocedure of layered rhombohedral (R3m) Li NiO (0 , x , 1) and Li CoO (0 , x , 1) Li /Co- and Li /Ni-oxide films were depositedx 2 x 2 ˇoxide thin films (Svegl and Orel, 1999). In this by dip-coating technique on either glass plates Electrochromic properties of lithiated Co-oxide and Ni-oxide thin films 525 coated with an indium-tin oxide (ITO) or tin-fluor Potentiodynamic (cyclic voltammetry-CV), oxide (TFO) electrically conductive thin film potentiostatic (chronocoulommetry-CPC) and gal- (with sheet resistance R |10 V /h) or different vanostatic (chronoamperometry-CA) measure-s metallic substrates (Al, Pt, stainless steel). The ments were performed with an EG&G PAR model substrates were cleaned before deposition with 273 computer-controlled potentiostat-galvanostat, chrome-sulfuric acid, rinsed with deionized water driven by the model 270 Electrochemical Analy- and dried at 608C. The metallic substrates were sis software. A three-electrode system with 1 M additionally cleaned with water solutions con- LiClO in propylene carbonate (PC) electrolyte4 taining non-ionic surfactants (Teol, Teloxid), was used for the electrochemical measurements. followed with rinsing using organic solvents and The electrolyte was purged with pure argon water. The deposition of films was performed on (99.99%, oxygen free) for 15 min prior to analy- 2home-made motor driven dip-coating unit with a sis. The working electrode consisted of 2 cm 21pulling speed of 1–10 cm min . The xerogel Li /Co- or Li /Ni-oxide / ITO glass slide, which 2films were heat-treated at different temperatures was positioned so that exactly 1 cm was in (up to 5508C) from 1 h and up to 24 h in air. The physical contact with the electrolyte. A Pt-rod and thickness of the oxide films produced by a single a modified Ag/AgCl served as the counter and dipping cycle varied from 60 to 250 nm depend- reference electrode, respectively. All working ing on the viscosity of the sols (not measured). electrode potentials (E) were measured against the modified Ag/AgCl reference electrode having 2.3. Measurements techniques standard potential E 510.212 V vs. SHE (stan-0 ˇX-ray diffraction (XRD) analysis of oxide dard hydrogen electrode) (Svegl et al., 1997). powders and films was made using a Siemens In-situ spectroelectrochemical measurements 5000D X-ray Diffractometer. The sample was were recorded using a HP 8453A diode array mounted on a thin film attachment. The constant spectrophotometer with an EG&G PAR model incident angle CuKa radiation was used and 273 computer controlled polarographic analyser. diffraction data points were recorded every 0.028 Measurements were performed at wavelength of 2Q. l5633 nm or in the ultraviolet-visible-near in- Fourier transform infrared near-grazing inci- frared (UV–VIS–NIR) spectral range between dence angle (FT-IR NGIA) reflection-absorption 300 and 2500 nm. spectra of films deposited on highly reflecting substrates (metal, ITO or TFO) were recorded 21 3. RESULTS AND DISCUSSIONwith 4 cm resolution using p-polarised light (wire-grid polariser, Perkin-Elmer) at incident 3.1. X-ray diffraction, FT-IR and chemical angle of 808 with a Spectratech (UK) reflectance analysis of as-deposited films accessory. For the background measurements a 3.1.1. X-ray diffraction. The XRD patterns forplain ITO coated glass substrate or a smooth the Li /Co- and Li /Ni-oxide thin films are verymetallic plate (Al or Au) was used. similar to each other (Fig. 1). Diffraction lines areThe thickness of films was determined with a characteristic for the formation of a layeredSurface Profiler Alfa Step 2000 with a maximum ¯rhombohedral (R3m) structure (g-NaFeO type)resolution of 5 nm. 2 (X-ray powder diff. file, JCPDS-ICDD, 1993)The average oxidation state of Co and Ni was which is usually formed at high temperaturesdetermined by adding 10 ml of 0.1 M (or 0.01 M) (T .6008C) during solid state reactions (Yoshio etFe(II) (FeSO (NH )SO * 7H O–Mohr’s salt),4 4 4 2 al., 1992). At 5008C highly crystalline Li CoO10 ml of 96% H SO and 10 ml of 85% H PO to x 22 4 3 4 (x|1) thin oxide film was formed with hexag-30–40 mg (or 2–5 mg of oxide film) of sample ˚onal unit cell dimensions: a 52.816 A,while passing Ar stream trough the bottle con- hex ˚c 514.060 A. The formation of layeredtaining the mixture. The solution was back titrated hex Li NiO (x|1) oxide film was successful atwith a standard solution of 0.01 M K Cr O us- x 22 2 7 ˚5558C. The unit cell parameters a 52.816 A,ing diphenylamine as indicator. Total Ni and Co con- hex ˚c 514.060 A were slightly larger than fortent was determined by the titration with 0.1 M hex Li CoO (x|1) film. The crystallites of Li NiOEDTA (ethylenediaminetetraacetic acid) after dis- x 2 x 2 (x|1) film calculated according to the Scherer’ssolution of similar amounts of sample in concen- formula were smaller (|12 nm) than crystallitestrated HCl and neutralisation with NH . Li /Ni3(aq) of Li CoO (x|1) oxide film (|21 nm). Thisor Li /Co ratios were determined by atomic emis- x 2 was reflected in the broadening of the X-ray dif-sion spectroscopy using a Thermo Jarrel Ash fraction lines of the Li NiO oxide film (x|1).ICP-AES Atom Scan 25 analyser. x 2 526 B. Orel et al. Fig. 1. XRD patterns of (a) Li /Co-oxide thin film prepared at 5008C (1/2 h) and (b) Li /Ni-oxide thin film (molar ratio r51) prepared at 5508C (2 h). *, tin-fluor oxide (TFO) substrate. 3.1.2. FT-IR analysis. Transmittance and FT- 3.2. Electrochromic properties IR NGIA reflection-absorption spectra of 3.2.1. In-situ UV–VIS–NIR voltammetric (CV)Li CoO (x|1) and Li NiO (x|1) thin filmsx 2 x 2 spectroelectrochemical measurements. Sol–gelconfirmed the formation of layered structure. The 21 derived yellow-brown Li Co O (mono-0.99 1.01 2appearance of bands at 546 cm in the transmitt- 21 chromatic transmittance of as-deposited filmance spectra and at 685 cm in reflection-absorp- T |73%) and layered grey colored Li Ni Os 0.94 1.06 2tion spectra of Li CoO (x|1) film is characteris-x 2 (monochromatic transmittance of as-depositedtic for transversal (TO) and longitudinal (LO) film T |62%) oxide films showed an excellentlattice vibrations of CoO octahedra. The spectra s6 electrochromic response in non-aqueous 1 Mof Li NiO (x|1) showed crystal lattice TO andx 2 21 LiClO /PC electrolyte as compared to their non-LO modes at 412 and 563 cm and additional 4 21 ˇlithiated analogues (Svegl, 1997). The voltammet-bands at 868, 1449 and 1554 cm typical for the 22 ric response of Li Co O and Li Ni Opresence of CO groups which remained in the 0.99 1.01 2 0.94 1.06 23 ˇ films is presented in Fig. 2a,b and Fig. 3a,b. Forfilm structure (Svegl and Orel, 1999). both type of films an intensive anodic voltammet- 3.1.3. Chemical analysis. The results of chemi- ric peak was observed around 10.90 V which is 1cal analysis showed high stoichiometry of as- associated with deintercalation of Li ions from deposited Li Co O thin films. The presence the film structure. The deintercalation process is0.99 1.01 2 21 of very small amount of Co cations was accompanied by the coloring of Li Co O0.99 1.01 2 determined. On the contrary to the band structure film to grey-brown color and Li Ni O oxide0.94 1.06 2 calculations, which indicated that fully stoichio- films to dark grey color. The deficiency of metric LiCoO oxide should be highly transparent positive charge, which appeared at deintercalation2 1(Van Elp et al., 1991), as deposited films showed of Li ions, was compensated by simultaneous 31brown color. The substoichiometry found in as- oxidation of the appropriate amount of Co or 31deposited Li Ni O thin films showed on Ni ions to oxidation state 14. The appearance0.97 1.03 2 1deficiency of Li ions in the film structure. The of cathodic voltammetric peak (at around E|1 1grey coloring of as-deposited Li Ni O films 0.40 V) is associated with intercalation of Li0.97 1.03 2 was in agreement with stoichiometry. ions back into the film structure and the reduction Electrochromic properties of lithiated Co-oxide and Ni-oxide thin films 527 Fig. 2. In-situ UV–VIS–NIR voltammetric (CV) spectroelectrochemical response of Li Co O film (1x dipping, film0.99 1.01 2 thickness d |122 nm, 5008C, 1/2 h) on tin-fluor oxide (TFO) substrate in 1 M LiClO /PC electrolyte. (a) Voltammetric curvesf 4 21 obtained after 1st, 3rd, 10th, 30th, 60th and 100th cycle. Scan rate was 20 mV s . (b) Accompanying changes of monochromatic transmittance T (l5633 nm) detected in the potential range between E511.50 V and E521.00 V vs. modified AguAgCls reference electrode (E 510.212 V vs. SHE).0 528 B. Orel et al. Fig. 3. In-situ UV–VIS–NIR voltammetric (CV) spectroelectrochemical response of Li Ni O film (1x dipping, film0.94 1.06 2 thickness d |115 nm, 5508C, 2 h) on tin-fluor oxide (TFO) substrate in 1 M LiClO /PC electrolyte. (a) Voltammetric curvesf 4 21 obtained after 1st, 2nd, 3rd, 4th, 5th, 10th, 20th, 60th and 100th cycle. Scan rate was 20 mV s . (b) Accompanying changes of monochromatic transmittance T (l5633 nm) detected in the potential range between E511.50 V and E521.30 V vs.s modified AguAgCl reference electrode (E 510.212 V vs. SHE).0 Electrochromic properties of lithiated Co-oxide and Ni-oxide thin films 529 Fig. 4. In-situ UV–VIS–NIR voltammetric (CV) spectroelectrochemical response of Li Ni O film (1x dipping, film0.94 1.06 2 thickness d |115 nm, 5508C, 2 h) on tin-fluor oxide (TFO) substrate in 1 M LiClO /PC electrolyte. (a) Voltammetric curvesf 4 21 obtained after 1st, 2nd, 3rd, 20th and 100th cycle. Scan rate was 20 mV s . (b) Accompanying changes of monochromatic transmittance T (l5633 nm) detected in the potential range between E511.50 V and E521.90 V vs. modified AguAgCls reference electrode (E 510.212 V vs. SHE).0 530 B. Orel et al. 41 41 of the corresponding amount of Co or Ni changes in the structure appear, causing the ions into lower oxidation state. Bleaching of the reduction of electrochromic performance of the ˇfilms into the initial state follows the intercalation film (Svegl, 1997). process and reduction of metallic ions. Both 3.2.2. In-situ UV–VIS–NIR galvanostatic (CA)Li Co O and Li Ni O films showed0.99 1.01 2 0.94 1.06 2 spectroelectrochemical measurements. The dein-stable electrochromic response after approximate- 1tercalation of Li ions, which is taking place atly 60 cycles. The integration of cathodic and electrochemical oxidation of Li Co O filmanodic peak area showed that around 625–30 mC 0.99 1.01 2 2222 at constant current pulse (110 mA cm ) leads tocm of charge is reversibly exchanged at (de)in- the changes of the film stoichiometry (x), whichtercalation processes. was calculated from the film active mass and theBy extending the potential range limit to more 1 charge transfer. The changes of the film stoi-negative values (E521.90 V) an excess of Li chiometry have direct influence on the electrodeions can be reversibly intercalated in the structure potential, which is presented in Fig. 6a. Theof Li Ni O films (x|1.7), leading to a totalx 1.06 2 abrupt increase of the potential at the beginning ofbleaching of the film (T |85%) (Fig. 4a,b). Thes 1 excess of Li ions in the Li Ni O film (x|1.7) deintercalation process from open circuit value atx 1.06 2 structure caused formation of an additional new 10.20 V vs. AguAgCl to almost 10.60 V was phase identified by XRD analysis, which was caused by the resistance of the electrochemical structurally very similar to a brucite Ni(OH) cell. In the range between x|0.98 to 0.70 the2 phase (Fig. 5) (Dahn et al., 1990). It has been constant electrode potential value of 10.67 V was 1 established that no additional amount of Li ions observed. The electrode potential starts to increase can be intercalated in the structure of after x|0.7 and attains again constant values of Li Co O film, because the irreversible E510.87 and E510.98 V in the narrow flat0.99 1.01 2 Fig. 5. XRD pattern of Li Ni O oxide thin film (1x dipping, film thickness d |115 nm, 5508C, 2 h) on tin-fluor oxide1.60 1.06 2 f 1(TFO) substrate obtained after additional intercalation of Li ions (x.1) into the films structure at more negative potentials (E521.90 V). F1-hexagonal phase; F2-brucite Ni(OH) like phase. *, tin-fluor oxide (TFO) substrate.2 Electrochromic properties of lithiated Co-oxide and Ni-oxide thin films 531 Fig. 6. In-situ UV–VIS–NIR galvanostatic (CA) spectroelectrochemical response of Li Co O film (1x dipping, film0.99 1.01 2 thickness d |122 nm, 5008C, 1/2 h) on tin-fluor oxide (TFO) substrate in 1 M LiClO /PC electrolyte. (a) Changes of thef 4 1 electrode potential E at (de)intercalation of Li ions presented versus changes in the stoichiometry of the film in the range 220.25x50.99. The applied current pulse was 610 mA cm . (b) The derivative dx / udV u, plotted versus electrode potential E for the cycle shown in (a). (c) Changes of monochromatic transmittance T (l5633 nm).s 532 B. Orel et al. Fig. 6. (continued) voltage regions at x|0.55 and x|0.50, respective- product obtained after the first cycle has the 1 composition Li Ni O . The galvanostaticly. Further deintercalation of Li ions to the 0.81 1.06 2 1 measurements showed that about 2 /3 of Li ionsstoichiometry value around x|0.37 causes an (up to x|0.3) could be reversibly removed fromincrease of the electrode potential to 11.30 V. the structure of Li Ni O film. DeintercalatedFrom here on the electrode potential doesn’t grow 0.81 1.06 2 films (x|0.3) showed dark grey color with mono-so steeply. The differential curve 2dx / udEu5f(E) chromatic transmittance around 20% (Fig. 7c). It(Fig. 6b) shows more clearly the different steps has been shown that additional intercalation ofattributed to several phases which appear at 1Li ions in the Li Ni O film structuredeintercalation process. These phases were iden- 0.81 1.06 2 completely bleaches the film into transparent statetified also by authors studying powder electrodes (T |85%).(Reimers and Dahn, 1992) and films prepared by s The results of a thorough investigation ofspray pyrolysis (Chen et al., 1995). The initial structural changes accompanying (de)intercalationamount of extracted lithium was not reversibly processes will be published in the near future.re-inserted at intercalation process (Fig. 6a), and a material having the composition Li Co O is0.93 1.01 2 3.2.3. In-situ UV–VIS–NIR potentiostaticobtained as the end product. This composition is (CPC) spectroelectrochemical measurements. Thenevertheless fully reversible on subsequent (de)in- in-situ UV–VIS–NIR potentiostatic spectroelec-tercalation cycles in the stoichiometry range up to trochemical measurements of Li Co O filmx|0.5. The deintercalation process is accom- 0.99 1.01 2 and Li Ni O film are presented in Fig. 8a–d,panied with the coloring of the film to dark 0.94 1.06 2 and Fig. 9a–d, respectively. The charge ex-brown color and total transmittance change of changed (DQ) at the potentiostatic coloring (atDT5T 2T 573233540% (Fig. 6c).bleached colored 1 E511.50 V) and bleaching (at E521.50 V) ofThe intercalation of Li ions bleaches the film Li Co O film after 60 s was approximatelyback into the initial state. Reversible structural 0.99 1.01 2 22 changes appear at slightly lower potentials. 630 mC cm (Fig. 8a,b). Total change in The behaviour of Li Ni O film was simi- monochromatic transmittance between colored0.94 1.06 2 lar, showing flat voltage regions in slightly differ- brown and bleached pale yellow state was around ent stoichiometry ranges (Fig. 7a,b). The end DT|30% (Fig. 8c). The coloring of Li Ni O0.94 1.06 2 Electrochromic properties of lithiated Co-oxide and Ni-oxide thin films 533 Fig. 7. In-situ UV–VIS–NIR galvanostatic spectroelectrochemical response of Li Ni O oxide thin film (1x dipping, film0.94 1.06 2 thickness d |115 nm, 5508C, 2 h) on tin-fluor oxide (TFO) substrate in 1 M LiClO /PC electrolyte. (a) Changes of the electrodef 4 1potential E at (de)intercalation of Li ions presented versus changes in the stoichiometry of the film in the range 0.35x50.94. 22The applied current pulse was 610 mA cm . (b) The derivative dx / udEu, plotted versus electrode potential E for the cycle shown in (a). (c) Changes of monochromatic transmittance T (l5633 nm).s 534 B. Orel et al. Fig. 7. (continued) film was performed at E511.50 V and bleaching vanostatic coloring and bleaching process both at E521.90 V (Fig. 9a). The total charge ex- devices showed a remarkable change of changed at (de)intercalation processes was similar color from completely dark blue coloring to Li Co O film (Fig. 9b). The total change (T |3%) in the colored state to light yel-0.99 1.01 2 colored in monochromatic transmittance between com- low (T |60%) bleached state forbleached 1 1pletely transparent bleached and grey colored WO i(H )Li ormolyteiLi Co O and to3 0.99 1.01 2 state was DT5T 2T 585245540% completely transparent (T |68%) bleachedbleached colored bleached 1 1(Fig. 9c). The majority of coloring of state for WO i(H )Li ormolyteiLi Ni O3 0.94 1.06 2 22Li Co O and Li Ni O film was at- device. About DQ|630 mC cm were ex-0.99 1.01 2 0.94 1.06 2 tained after about 20s, while bleaching of films changed by ECD device containing took only few seconds. The electrochromic ef- Li Co O film, corresponding to electro-0.99 1.01 2 2 21ficiencies h5DOD/DQ (DOD, change in optical chromic efficiency of h|40 cm C . The density; and DQ, (de)intercalated charge) of electrochromic efficiency of ECD with Li Co O and Li Ni O films were Li Ni O oxide film was a little bit lower0.99 1.01 2 0.94 1.06 2 0.94 1.06 2 2 21 2 21around 9 cm C , which are characteristic values h|25 cm C , because of better intercalation for passive counter electrodes with good intercala- capability of Li Ni O oxide films at bleach-0.94 1.06 2 tion properties (Granqvist, 1995). Both films ing (x.1). All devices showed good reversibility show in colored state continuous absorption of and stability after performing about 1000 gal- solar radiation in the whole solar spectrum (Fig. vanostatic cycles. 8d and Fig. 9d). 4. CONCLUSIONS3.3. Testing of electrochromic devices (ECDs) The practical applicability of Li Co O The work described in this paper showed that0.99 1.01 2 and Li Ni O oxide films was tested in the the wet chemistry processing offers the possibility0.94 1.06 2 electrochromic devices, containing sol–gel de- of preparing Li /Co- and Li /Ni-oxide thin films ] rived WO oxide film (Orel et al., 1998) as an with nearly perfect layered rhombohedral (R3m)3 active electrochromic layer (Fig. 10a,b). At gal- structure at temperatures much lower than those at Electrochromic properties of lithiated Co-oxide and Ni-oxide thin films 535 Fig. 8. In-situ UV–VIS–NIR potentiostatic (CPC) spectroelectrochemical response of Li Co O film (1x dipping, film0.99 1.01 2 thickness d |122 nm, 5008C, 1/2 h) on tin-fluor oxide (TFO) substrate in 1 M LiClO /PC electrolyte. (a) Profile of the appliedf 4 1potential pulse, (b) CPC response at (de)intercalation of Li ions, (c) changes of monochromatic transmittance T (l5633 nm)s and (d) UV–VIS–NIR spectra of as-deposited, colored (at E511.50 V) and bleached (at E521.50 V) Li Co O film in0.99 1.01 2 the spectral range between 300 and 1100 nm. 536 B. Orel et al. Fig. 8. (continued) solid state chemistry processing. These layered applied in EC devices in a combination with 1Li Co O and Li Ni O oxide films are cathodic electrochromic WO film, applying Li0.99 1.01 2 0.94 1.06 2 3 novel, excellent counter electrodes, which can be non-aqueous electrolyte. These oxides are posses- Electrochromic properties of lithiated Co-oxide and Ni-oxide thin films 537 Fig. 9. In-situ UV–VIS–NIR potentiostatic (CPC) spectroelectrochemical response of Li Ni O oxide thin film (1x dipping,0.94 1.06 2 film thickness d |115 nm, 5508C, 2 h) on tin-fluor oxide (TFO) substrate in 1 M LiClO /PC electrolyte. (a) Profile of the appliedf 4 1potential pulse, (b) CPC response at (de)intercalation of Li ions, (c) changes of monochromatic transmittance T (l5633 nm)s and (d) UV–VIS–NIR spectra of as-deposited, colored (at E511.50 V) and bleached (at E521.90 V) Li Ni O film in the0.94 1.06 2 spectral range between 300 and 1100 nm. 538 B. Orel et al. Fig. 9. (continued) Electrochromic properties of lithiated Co-oxide and Ni-oxide thin films 539 Fig. 10. In-situ UV–VIS–NIR spectra (300–2500 nm) obtained at galvanostatic (CA) spectroelectrochemical testing of (a) 1 1 1 1WO iLi (H )ormolyteiLi Co O and (b) WO iLi (H )ormolyte iLi Ni O ECD device. Sol–gel derived WO film3 0.99 1.01 2 3 0.94 1.06 2 3 which was deposited (3 times, film thickness d |350 nm) on tin-fluor oxide (TFO) substrate (sheet resistance R |10 V /h) andf s 1 1heat treated 15 min at 1208C served as cathodic electrochromic layer. Li (H ) ormolyte having ionic conductance of about 23 21d |3*10 S cm and activation energy E |0.25 eV was used (1 mm) for ionic contact between the electrodes.a 540 B. Orel et al. Julien C. and Nazri G.-A. (1994) Solid State Batteries:sing excellent intercalation properties and stability Materials Design and Optimization. Kluwer Academic,in a wide potential range. A certain disadvantage London. of Li Co O films is light yellow coloring in Moshtev R. V., Zlatilova P., Manev V. and Sato A. (1995) The0.99 1.01 2 LiNiO solid solution as a cathode material for rechargeable2a bleached state, yet this does not seriously lithium batteries. J. Power Sources 54, 329–333.influence their practical applicability. The electro- ˇ ˇ ˇ ˇOrel B., Opara-Krasovec U., Lavrencic-Stangar U. and Judens- 1chromic properties of both ECD devices were tein P. 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