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Shinichi Takeno, Taiki Suematsu, Ryusei Kunisaki, [Gen Hasegawa](https://orcid.org/0000-0002-9297-6902), [Ken Watanabe](https://orcid.org/0000-0001-7374-7322), [Naoaki Kuwata](https://orcid.org/0000-0002-0736-6967), [Kazutaka Mitsuishi](https://orcid.org/0000-0002-9361-4057), [Tsuyoshi Ohnishi](https://orcid.org/0000-0002-2333-7752), [Kazunori Takada](https://orcid.org/0000-0001-7568-1806), Kohichi Suematsu, Kengo Shimanoe

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[New insight into designing a thick-sintered cathode for Li-ion batteries: the impact of excess lithium in LiCoO<sub>2</sub> on its electrode performance](https://mdr.nims.go.jp/datasets/bbb5b66e-db1e-427d-bb0f-ddb7dcece963)

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New insight into designing a thick-sintered cathode for Li-ion batteries: the impact of excess lithium in LiCoO2 on its electrode performanceJournal ofMaterials Chemistry APAPEROpen Access Article. Published on 06 December 2024. Downloaded on 12/18/2024 10:59:47 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView JournalNew insight intoaDepartment of Molecular and Material ScieEngineering Sciences, Kyushu University, KabNational Institute for Materials Science (JapancDepartment of Advanced Materials ScienceSciences, Kyushu University, Kasuga, Fukuoken.331@m.kyushu-u.ac.jp† Electronic supplementary informahttps://doi.org/10.1039/d4ta07377kCite this: DOI: 10.1039/d4ta07377kReceived 16th October 2024Accepted 6th December 2024DOI: 10.1039/d4ta07377krsc.li/materials-aThis journal is © The Royal Societydesigning a thick-sintered cathodefor Li-ion batteries: the impact of excess lithium inLiCoO2 on its electrode performance†Shinichi Takeno,a Taiki Suematsu,a Ryusei Kunisaki,a Gen Hasegawa, bKen Watanabe, *c Naoaki Kuwata, b Kazutaka Mitsuishi,b Tsuyoshi Ohnishi, bKazunori Takada, b Kohichi Suematsuc and Kengo ShimanoecIncreasing the capacity of Li-ion batteries is one of the critical issues that must be addressed. A thick anddense electrode using an active material sintered disk is expected to have a high capacity because thevolume of the active material is 100% in the cathode. This study focused on LiCoO2, the most well-known active material for the cathode, to improve the properties of the sintered cathode. Weinvestigated the impact of excess Li on various properties. We found that the degree of c-axisorientation in the sintered disk decreased as excess Li increased. In addition, results of 7Li-MAS-NMRsuggest the presence of defects resulting from excess Li when the Li excess reached 5.1% or more. Thedischarge capacity of the LiCoO2 sintered cathode increased as the amount of excess Li increased, anda maximum discharge capacity of 11.2 mA h cm−2 was obtained when the Li excess amount was 7.3%.This result was attributed to the significant improvement in the Li-ion conductivity of LiCoO2 by both thedecrease in the degree of c-axis orientation and the introduction of defects due to excess Li. Notably,introducing defects derived from excess Li enhances the Li-ion conductivity. Thus, tuning the amount ofexcess Li for the LiCoO2 sintered cathode was crucial in enhancing its electrochemical performance asan electrode.IntroductionLi-ion batteries are widely used as power sources for mobileapplications and electric vehicles, and there is a strong demandfor highly capacitive batteries to realize a carbon-neutralsociety.1 The capacity of Li-ion batteries depends on theloading amount of active materials in electrodes. Therefore, toachieve high capacity, efforts to increase the amount of activematerial in electrodes are underway. One approach to increasethe amount of active material is to increase the thickness ofelectrodes. Thick electrodes result in a higher amount of activematerial per unit area. Therefore, thick electrodes can achievehigh capacity. To increase the thickness of electrodes, methodssuch as using foam current collectors to shorten the electronicconduction path,2,3 formation of electrodes using 3D printing,4composite electrodes using active materials and solidnces, Interdisciplinary Graduate School ofsuga, Fukuoka, 816-8580, JapanNIMS), 1-1 Namiki, Tsukuba 305-0044,and Engineering, Faculty of Engineeringka, 816-8580, Japan. E-mail: watanabe.tion (ESI) available. See DOI:of Chemistry 2024electrolytes,5 bilayer electrodes,6 electrodes with a conductiveagent/binder composite,7 electrode preparation by a dry elec-trode coating process8 and phase-inversion method9 have beenattempted.Another approach is to increase the volume ratio of the activematerial in the electrode. Park et al. reported an all-in-onemulti-layered cathode–separator–anode monolith structurewith slurry that functions as electrochemically active glue andhas a high capacity of 44.5 mA h.10 Generally, composite cath-odes of active material, a conductive additive, binder, andorganic electrolyte are widely used for Li-ion batteries. There-fore, the amount of active material in the electrode is limited.To overcome this limitation, Yamada et al. proposed a sinteredcathode that consists of a highly densied LiCoO2 disk.11 Sincethe electrode does not contain electrolytes or conductive addi-tives, it can be composed only of active material, resulting inhigh capacity. Furthermore, if the active material can be co-sintered with the oxide-based electrolyte, the sintered high-capacity cathode is suitable for a high-performance cathode ofthe co-sintered solid-state battery.12–15When we design the sintered cathode, there are two keyfactors: the mixed Li-ion and electronic conductivity ofLiCoO2, the most representative active material for thecathode,16 and the interfacial resistance between LiCoO2 andelectrolyte. Regarding interfacial resistance, it has been shownJ. Mater. Chem. Ahttp://crossmark.crossref.org/dialog/?doi=10.1039/d4ta07377k&domain=pdf&date_stamp=2024-12-18http://orcid.org/0000-0002-9297-6902http://orcid.org/0000-0001-7374-7322http://orcid.org/0000-0002-0736-6967http://orcid.org/0000-0002-2333-7752http://orcid.org/0000-0001-7568-1806https://doi.org/10.1039/d4ta07377khttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4ta07377khttps://pubs.rsc.org/en/journals/journal/TAJournal of Materials Chemistry A PaperOpen Access Article. Published on 06 December 2024. Downloaded on 12/18/2024 10:59:47 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinethat the interface resistance between the solid electrolyte,Li3PO4, and LiCoO2 can be reduced to 8.6 U cm2 in thin-lmbatteries.17 In addition, Ohnishi et al. suggested that negli-gibly low LiCoO2/Li3PO4 interface resistance can be achievedby controlling sputtering conditions during interface forma-tion.18 Therefore, even a low surface area is expected to besufficient to achieve low resistance. Therefore, the most crit-ical key is increasing the mixed conductivity of Li ions andelectrons in LiCoO2.As for the mixed conductivity of LiCoO2, utilizing theanisotropic conduction derived from the crystal structure ofLiCoO2 is a promising approach. LiCoO2 exhibits rapid elec-tronic/Li-ion conduction pathways along the c-plane, whileconduction in the c-axis direction is signicantly low.19 Yamadaet al. reported the cathode properties using an oriented sintereddisk of LiCoO2. A sintered cathode with (110)-orientation, thefast Li-ion conduction pathway, has a discharge capacity of102.3 mA h g−1 at 1/3 C with a thickness of 130 mm. Manysimilar studies have also been conducted in epitaxial thin lms.Among them, Kawashima et al. have demonstrated high-speedcharge–discharge of 100 000 C in (104)-oriented epitaxial thinlms.20 On the other hand, despite low electronic/Li-ionconductivity along the c-axis, several research groups have re-ported that c-axis-oriented epitaxial thin lms can operate asbatteries. The grain boundary diffusion of Li ions21 oenexplains these phenomena. Hasegawa et al. suggested thatantisite Li defects, which Li occupies at Co sites, act asa conduction path along the c-axis direction and enhance the Li-ion diffusion based on DFT calculation.22 According to theiridea, controlling not only the orientation of the LiCoO2 sinteredcathode but also the defects caused by excess Li can enhancethe Li-ion conductivity and improve battery performance.The enhancement of battery performance with the excess Liwas previously demonstrated using the battery with liquidelectrolyte and LiCoO2 powder.23,24 However, the mechanism isstill under discussion. Levasseur et al. reported that 7Li MASNMR measurements for LiCoO2 with or without Li-excess,calcined at 900 °C, showed an evident local structural changein the sample with Li-excess.25 They suggested the existence ofLi defects, which are substituted for the Co site (antisite Li), andoxygen vacancies compensate for the antisite Li. This model wasused in the previous calculation by Hasegawa et al. and mayserve as a diffusion path in the c-axis direction.22On the other hand, Murakami et al. examined the state ofexcess Li in LiCoO2 calcined at 800 °C.26 Based on variousinvestigations, they stated that excess Li exists in their sampleas a defect pair of the low spin Co2+ and interstitial Li. Since thisinterstitial Li may also work as a Li-ion conductive carrier, itmay improve Li-ion conductivity. These defect models differdepending on the heat treatment conditions. However, in anycase, defects derived from excess Li are thought to contribute tothe improvement of Li ion conductivity in LiCoO2. Furthermore,it has also been reported that excess Li promotes grain growth27and changes the direction of grain growth,28 which is expectedto bring about unique changes in the orientation and micro-structure of the sintered disk.J. Mater. Chem. AIn this study, we aim to improve the electrode performanceof the LiCoO2 sintered cathode and investigate the effects ofexcess Li on its microstructure and electrical properties.ExperimentalPreparation of the LiCoO2 sintered diskLiCoO2 with excess Li of 0%, 1.0%, 2.0%, 3.0%, 4.1%, 5.1%,6.2%, 7.3%, 8.3%, and 12.8% powder was synthesized by theamorphous malic acid precursor method.12,29 DL-malic acid(C4H6O5, 99%, Fujilm Wako Pure Chemical Corp. Japan),lithium nitrate (LiNO3, 99.9%, Fujilm Wako Pure ChemicalCorp. Japan), and cobalt nitrate (Co(NO3)2$6H2O, 99.5%, Fuji-lm Wako Pure Chemical corp. Japan) were dissolved indistilled water. The pH of the mixed solution was adjusted to 3with aqueous ammonia (28%). The solution was evaporated todryness and heated at 400 °C until the reactive ignition becameunobservable. The powder was calcined at 850 °C for 10 hours,and LiCoO2 powder was obtained. LiCoO2 powder was groun-ded and ball-milled at 450 rpm for 20 hours with isopropanol asthe solvent. Aer ball milling, the solvent was evaporated andground in a mortar. The ne powder was press-formed intoa disk shape and pressed again by cold isostatic pressing. Theobtained disks were sintered at 1000 °C for 15 hours. The diskwas covered with LiCoO2 powder to avoid the evaporation of Liand contamination of other elements during sintering. In allcompositions, the relative density of sintered disks achievedmore than 92%.Material characterizationThe crystal structure of the LiCoO2 sintered disk was evaluatedusing X-ray diffraction (XRD: MiniFlex600, RIGAKU, Japan) withCu Ka as an X-ray source. All samples can be assigned to thelayered rock salt structure (R�3m), as shown in Fig. S1.† Toevaluate the c-axis orientation degree for the sintered disk, wedened the c-axis orientation factor (f003) as the followingequation.f003ð%Þ ¼ 0:66� I104=I0030:66� 100I104 and I003 are the diffraction peak intensity for 104 and 003,respectively. The constant value 0.66 is calculated from the idealI104/I003 based on ICSD 51381.The change in the local structure around Li was evaluated by7Li magic angle spinning NMR (MAS-NMR) using an ECA-400spectrometer (JEOL Ltd, Japan). The resonance frequency ofthe 7Li nucleus was 155.4 MHz. A 1 mol L−1 LiCl aqueoussolution was used as the chemical shi reference at 0 ppm. A3.2 mmMAS probe (HXMAS probe; JEOL) and a 3.2 mm zirconiasample tube were used. The MAS spinning rate was 20 kHz. Thewidth of the p/2 pulse was 2.8 ms. The pulse-recycling periodwas kept longer than 5 s to conrm spin recovery. A single-pulsesequence obtained the NMR spectra. Scanning transmissionelectron microscope (STEM) observation was conducted usinga JEM-ARM200F (JEOL Ltd, Japan). Focused ion beam millingwas used to prepare the specimens for STEM observation.This journal is © The Royal Society of Chemistry 2024http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4ta07377kPaper Journal of Materials Chemistry AOpen Access Article. Published on 06 December 2024. Downloaded on 12/18/2024 10:59:47 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineThe oxidation state of Co ions was evaluated by electron spinresonance (ESR, ESR 5000, Bruker, Germany). LCO sintered diskgrinding powder was obtained and dried in a vacuum at 200 °C,and approximately 50 mg was used for measurement. The eldwas swept from 100 to 600 mT in 60 s. The modulation ampli-tude was 0.2 mT at a modulation frequency of 100 kHz.Microwave power was 10 mW.Fig. 1 (a) The c-axis orientation factor for the sintered LiCoO2 asa function of Li-excess amount. (b) The comparison of the c-axisorientation factor of 0% and 12.8% Li excess LiCoO2 disks before andafter sintering at 1000 °C.Electrical properties and battery performanceThe electric conductivity of the LiCoO2 sintered disk was eval-uated by the DC polarization method with the ion-blockingelectrode. The surface of the sintered disk was polished withthe lapping lm sheet (3 M) up to 280 mm in thickness. The Auelectrode for ion blocking was deposited on both sides of thesintered disk by sputtering. The DC polarization method wascarried out using an HJ1001SD8 (Hokuto Denko Co., Ltd, Japan)in galvanostatic mode. The details of the experimental condi-tions are provided in Fig. S1.†Cathode performance was evaluated using an organicelectrolyte-based cell with a Li-metal anode. Firstly, both sidesof the sintered disk without any carbon or conductive additiveswere polished up to 180 mm in thickness. The Au currentcollector was deposited on one side of the polished disk bysputtering. Then, the sample was transferred to an Ar-lledglovebox, and the test cell was fabricated, as shown inFig. S2.† Here, the loading amount is 82–93 mg cm−2. Thecharge–discharge performance of the fabricated cell was eval-uated by galvanostatic charging/discharging with a constantcurrent of 0.03 C using an HJ1001SD8 (Hokuto Denko Co., Ltd,Japan). The upper and lower cut-off voltages were set at 4.2 and3.0 V (vs. Li+/Li). The non-blocking cell was used to evaluate theLi-ion conductivity for the sintered disk before and aercharging at 4.2 V. A symmetric cell consisting of LijliquidelectrolytejLiCoO2 sintered diskjliquid electrolytejLi, as shownin Fig. S3,† was assembled. The electrical resistance of the cellwas evaluated by the DC polarization method at 25 °C. The Li-ion conductivity was calculated using the slope of the thick-ness dependence of the total resistance for the cell. Experi-mental conditions are provided in Fig. S2.†Results and discussionFirstly, to reveal the effect of excess Li on the crystal orientationof the sintered LiCoO2 disk, XRD was conducted on the surfaceof the LiCoO2 sintered disks. For all samples, the diffractionpeaks can be assigned to NaFeO2-type LiCoO2, as shown inFig. S1.† Fig. 1a shows the dependence of the c-axis orientationfactor (f003) on the Li-excess amount. As the amount of Li excessincreased from 0% to 5.1%, the value of f003 graduallydecreased, although the rapid drop at 3.0% was conrmed.Then, f003 rapidly decreases with the increase in the Li excessamount. In the case without excess Li (stoichiometric compo-sition), f003 was 71%, and the value was the highest among allsamples we tested. On the other hand, in the range from 7.3% to12.8%, these values were almost constant at approximately15%. In this study, since uniform powders are sintered withoutThis journal is © The Royal Society of Chemistry 2024applying pressure, there is no driving force to orient in thedirection of thickness during sintering. Therefore, the mecha-nism of orientation by the crystal growth during sintering isunlikely, and the orientation may be caused by pressing formolding. For plate-like particles, the basal plane is preferen-tially oriented toward applying pressure. Then, the crystalorientation before and aer sintering was examined. Fig. 1bshows f003 before and aer sintering for Li excess amounts of0% and 12.8%, respectively. The f003 of 0% LiCoO2 before andaer sintering is almost the same as that of 67% and 71%,respectively. This result indicates that it is already preferentiallyoriented toward the c-axis at the molding. Similarly, in the caseof 12.8%, the f003 before and aer sintering was 22% and 15%,respectively. The f003 aer molding is directly related to theorientation of the sintered compact. Ceder et al. reported theeffect of the excess Li on the crystal growth from DFT calcula-tions.28 They reported that the c-plane preferentially grows inthe stoichiometric LiCoO2 because the surface energy of (003) isthe lowest. In contrast, the crystal growth becomes isotropicwith excess Li because excess Li increases the surface energy of(003). Therefore, it is considered that the grain growth ofLiCoO2 during the rst calcination is altered by excess Li,resulting in a particle shape-dependent change in the degree ofc-axis orientation under pressing for molding. Unfortunately,we observed no big difference in the particle shape between 0%and 12.8% from the SEM observation shown in Fig. S4.† Thus,further investigation into the orientation mechanism is needed.To reveal the local structure change in LiCoO2 by addingexcess Li, 7Li-MAS-NMR was carried out. Fig. 2a shows 7Li MASNMR spectra for LiCoO2 prepared with different Li-excessamounts. In the 5.1–8.3% Li-excess samples, minor peaksaround 3, −6, and −16 ppm were observed. Those minor peaksJ. Mater. Chem. Ahttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4ta07377kFig. 2 (a) 7Li-MAS-NMR spectra for the sintered LiCoO2 with differentLi-excess amounts. The crystal structure model of LiCoO2: (b) inter-stitial Li and (c) antisite Li, respectively.Journal of Materials Chemistry A PaperOpen Access Article. Published on 06 December 2024. Downloaded on 12/18/2024 10:59:47 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineagreed with the previous results,25 indicating the formation ofLi-excess-related defects. Two different models were proposed,and their heat treatment conditions were different. Levasseuret al. performed high-temperature heat treatment and proposedthe formation of antisite Li-oxygen vacancy couples, as shown inFig. 2b.25 In our case, the sintering temperature was 1000 °C,similar to that reported by Levasseur et al. Thus, we believe thatthe antisite Li and oxygen vacancy couple is formed aersintering.Next, the effect of excess Li on the electronic conductivity ofthe LiCoO2 sintered disk was investigated. Fig. 3a shows theelectronic conductivity of the LiCoO2 sintered disks asFig. 3 The dependence of electronic conductivity on (a) the Li excessamount in LiCoO2 sintered disk and (b) the c-axis orientation factorf003. (c) ESR spectra of LCO with excess Li. (d) The relationshipbetween electronic conductivity and the maximum value of the peakin ESR spectra (Imax).J. Mater. Chem. Aa function of the Li excess amount, and correlations betweencurrent and voltage are shown in Fig. S5.† The dependency ofelectronic conductivity on Li excess amount can be divided intotwo parts. In the range from 0% to 4.1%, the electronicconductivity drastically increased with an increase in excess Li.The conductivity reached 2 × 10−3 S cm−1 when the Li-excessamount was 4.1%. In contrast, the conductivity decreasedwhen the Li-excess amount became larger than 4.1%. Fig. 3bshows the relationship between the electronic conductivity andthe c-axis orientation factor (f003). As seen in Fig. 3b, in the f003range from 60% to 80% without the local structure change, theelectronic conductivity strongly depends on the c-axis orienta-tion degree of the sintered disk. In contrast, the electronicconductivity of the sintered disk, in which 7Li-MAS-NMRdetected the local structure change, decreased despitedecreasing the c-axis orientation degree. This tendency meansthat the excess Li-related defect affects the electronic conduc-tion in the LiCoO2. There are two possible excess Li-relateddefects: the pair of the antisite Li and oxygen vacancy25 andthe pair of the interstitial Li and low spin Co2+.26 In our case,LiCoO2 was sintered at 1000 °C, which was similar to the onereported by Levasseur et al.25 According to their model, thedefect in our LiCoO2 sintered disk is likely to be the antisite Liand oxygen vacancy pair. LCO is a p-type semiconductor, andCo4+, which has unpaired electrons in d orbitals, seems tocontain a charge carrier. Therefore, it is considered that theelectronic conductivity of LCO depends on the amount of Co4+.To evaluate Co4+ in LCO, we conducted ESR measurements.Fig. 3(c and d) show the ESR spectra of LCO of 1.0%, 4.1%, and7.3% Li-excess and the relationship between the electronicconductivity and the maximum value of the peak in ESR spectra(Imax). The peak appeared at g z 2.13 for all spectra, and theelectronic conductivity strongly depends on Imax. Mukai et al.reported this peak can be assigned to unpaired electrons forCo4+ in a low-spin state.30 Compared with 1.0% Li excess, themaximum value of the EPR peak of 4.1% excess was larger. Itwas indicated that an increase in Co4+ was due to chargecompensation of the excess Li. On the other hand, in the case of7.3% Li-excess with antisite Li, the maximum value of the peakwas lower than that of 4.1% without antisite Li. These resultsindicate that the antisite Li was compensated for not only byCo4+ but also by oxygen vacancy, resulting in a decrease in theelectronic conductivity with the formation of the antisite Li.There is another possible reason for the decrease in electronicconductivity. That is the formation of an impurity, an electronicinsulator, at the grain boundary. From STEM observation, asshown in Fig. S6,† there is no impurity at the grain boundary.Therefore, the formation of the antisite Li related to the excessLi is a reasonable reason for the decrease in the electronicconductivity with an increase in excess Li.The charge–discharge properties for the LiCoO2 sinteredcathode prepared in the composition of 1.0%, 3.0%, and 7.3%Li-excess are shown in Fig. 4a–c. For all samples, the chargecapacity was more than 120 mA h g−1, and no signicantdifference was observed. On the other hand, the dischargecapacity drastically increased as the amount of Li-excessincreased. Fig. 4d shows the dependence of the dischargeThis journal is © The Royal Society of Chemistry 2024http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4ta07377kFig. 4 The charge–discharge curves for sintered disks of (a) 1.0%, (b)3.0%, and (c) 7.3% Li-excess LiCoO2. The dependence of the dischargecapacity of LiCoO2 sintered disks on (d) Li excess amount and (e) the c-axis orientation factor f003. (f) The Li-ion conductivity for 3.0%, 4.1%,5.1% and 7.3% Li excess LiCoO2 sintered disks. (g) The Li-ionconductivity for 3.0%, 4.1%, 5.1% and 7.3% Li excess LiCoO2 sintereddisks after CCCV charge to 4.2 V. (h) The correlation between thecharge–discharge capacity of the 1st cycle and Li-ion conductivitybefore charging.Paper Journal of Materials Chemistry AOpen Access Article. Published on 06 December 2024. Downloaded on 12/18/2024 10:59:47 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinecapacity for the rst three cycles on the Li excess amount. It isclear that the discharge capacity drastically increases with anincrease in the Li excess amount to 5.1%. In addition, thedischarge capacity is almost constant in the Li excess amountrange of more than 5.1%. 7.3% excess Li exhibits the highestdischarge capacity of 135.8 mA h g−1 and 11.2 mA h cm−2.Fig. S7† shows the relationship between cycle number anddischarge capacities of charge–discharge tests at 0.1 C usingLCO sintered disks with a thickness of approximately 130 mm.The capacity retention was improved by excess Li addition. Asmentioned before, the excess Li affects the c-axis orientationdegree of the disk and forms the anti-site Li defect. Thus, thistendency should be related to both of them. Fig. 4e shows thedischarge capacity of the LiCoO2 sintered cathode as a functionof the c-axis orientation factor f003. Here, the red triangle andthe blue rectangle show the LiCoO2 disk with or without the Li-excess-related defect detected by 7Li-MAS-NMR, respectively.The discharge capacity of LiCoO2 without Li-excess-relateddefects depends on the c-axis orientation degree, indicatingthat the relaxation of c-axis orientation for the sintered diskcauses the initial increase in the discharge capacity. Incontrast, these values do not depend on the c-axis orientationdegree for the samples with Li-excess-related defects. It shouldbe noted that, as seen in region (i) shown in Fig. 4e, althoughThis journal is © The Royal Society of Chemistry 2024the c-axis orientation degree of 3.0% is lower than that of 5.1%,5.1% Li excess exhibits a higher discharge capacity of 125 mA hg−1 than 3.0%. These results clearly indicate that not only the c-axis orientation degree but also the Li-excess-related defectsaffect the electrode performance. From the electronicconductivity measurement results, 5.1% exhibits lowerconductivity than 3.0%. Thus, the higher discharge capacity of5.1% than 3.0% is likely related to the Li-ion conductivity. Toconrm whether the Li-ion conductivity can be increased byintroducing the Li-excess-related defects, the Li-ion conduc-tivities of 3.0% and 5.1% Li-excess LiCoO2 disks were evalu-ated. Fig. 4f compares the Li-ion conductivity of 3.0%,4.1%,5.1%, and 7.3% Li-excess LiCoO2 disks, and correlationsbetween current and voltage are shown in Fig. S8.† The Li-ionconductivity of 3.0% was 2.7 × 10−6 S cm−1, higher than that of4.1% (8.4 × 10−8 S cm−1). As Fig. 1a shows, f003 of 3.0% is lowerthan that of 4%. Therefore, it was found that the f003 decreasesdue to excess Li, contributing to the increase of Li-ionconductivity. The Li-ion conductivity of 5.1% was 5.2 × 10−6S cm−1, higher than that of 3.0%. Therefore, it was found thatthe Li-excess-related defect enhances the Li-ion conductivity ofthe LiCoO2 sintered disk. This result agrees with the predictioncalculated by DFT, suggesting the antisite Li probably works asthe diffusion pathway across the CoO6 layer.22 Moreover, it wasreported that the Li-ion diffusion coefficient of LiCoO2increased with an increase in the charging state.22,31 Fig. 4gshows the Li-ion conductivity of LiCoO2 at 4.2 V aer chargingunder CCCV mode, and correlations between current andvoltage are shown in Fig. S9.† For all samples, the Li-ionconductivity at 4.2 V was much higher than that beforecharging, reaching 10−4–10−5 S cm−1. In addition, a rate ofincrease in Li-ion conductivity before and aer charging agreeswith the results of the diffusion coefficient.22,31 These resultsstrongly indicate that the increase in Li-ion conductivitythrough the charging process enhances the charging behaviorfor all samples, resulting in similar charge capacities. Fig. 4hshows the correlation between charge–discharge capacity atthe 1st cycle and Li-ion conductivity before charging. Althoughthe charge capacity is almost constant, the discharge capacitydepends on the Li-ion conductivity before charging. Asmentioned, Li-ion conductivity drastically decreases as thedischarge proceeds during lithiation due to the decrease in Livacancy. This trend suggests that to enhance the dischargecapacity through the thick and dense LiCoO2 sintered pellet,the improvement in the Li-ion conductivity for full lithiation ofLiCoO2 is crucial.These results show that excess Li affects the electrodeproperties of the LiCoO2 sintered cathode, and this tendencycan be explained by the increase in Li-ion conduction throughthe LiCoO2 sintered disk. This means it is a crucial design factorwhen the LiCoO2 sintered cathode is applied to the Li-ionbattery. Moreover, in the case of the co-sintered solid-statebattery, the excess Li is added to the electrolyte to prevent Li-loss during sintering at high temperatures. Thus, more precisetuning of the amount of excess Li in LiCoO2 for the cathode ofthe co-sintered solid-state battery will be strongly required torealize the high-performance battery.J. Mater. Chem. Ahttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4ta07377kJournal of Materials Chemistry A PaperOpen Access Article. Published on 06 December 2024. Downloaded on 12/18/2024 10:59:47 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineConclusionThis study investigated the effect of excess Li in the LiCoO2thickly and densely sintered cathode without conductive carbonadditives on the microstructure, the local structure, electricalproperties, and battery performance to enhance the electrodeperformance of thick, sintered LiCoO2 cathodes for Li-ionbatteries. Four key ndings followed.(1) The degree of c-axis orientation of the sintered diskdecreases with an increase in the Li-excess amount.(2) 7Li-MAS-NMR detects the formation of the defect-relatedexcess Li.(3) LiCoO2 with excess Li exhibited superior electrode prop-erties compared to the stoichiometric version.(4) The Li-ion conductivity increases with an increase in Li-excess amount.Notably, the highest discharge capacity of 135.8 mA h g−1and 11.2 mA h cm−2 was achieved when the Li-excess amountwas 7.3%. This outstanding battery performance can be attrib-uted to improving the Li-ion conductivity by decreasing the c-axis orientation and introducing the antisite Li defects. There-fore, our presented results strongly highlight the importance oftuning the excess Li in the LiCoO2 sintered cathode for highlycapacitive Li-ion and solid-state batteries.Data availabilityThe data supporting this article have been included as part ofthe ESI.†Author contributionsShinichi Takeno conducted the experiment and draed themanuscript. Taiki Suematsu and Ryusei Kunisaki carried outthe experiment. Ken Watanabe created the idea, conducted theexperiment, and performed supervision and editing. GenHasegawa and Naoaki Kuwata conducted the NMR experiment.Kazutaka Mitsuishi conducted STEM observation. TsuyoshiOhnishi and Kazunori Takada supported the cell fabricationexperiment and performed the discussion. Koichi Sematsu andKengo Simanoe performed the discussion and editing.Conflicts of interestThere are no conicts to declare.AcknowledgementsThis work was supported by Japan Science and Technology(JST), the Advanced Low Carbon Technology Research andDevelopment Program, Specially Promoted Research for Inno-vative Next Generation Battery (ALCA-SPRING) project, GrantNumber JPMJAL1301, Green technologies of excellence (GteX)Program Japan, Grant number JPMJGX23S22, the establish-ment of university fellowships towards the creation of sciencetechnology innovation, Grant Number JPMJFS2132, and JSPSKAKENHI Grant Number 22K04739. We thank the NationalJ. Mater. Chem. AInstitute for Materials Science (NIMS) Battery Research Plat-form for preparing TEM samples.Notes and references1 L. Matthew, L. Jun, C. Zhongwei and A. Khalil, Adv. Mater.,2018, 30, 1800561.2 G. F. Yang, K. Y. Song and S. K. Joo, RSC Adv., 2015, 5, 16702.3 M. Fritsch, G. Standke, C. Heubner, U. Langklotz andA. Michaelis, J. 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See DOI: https://doi.org/10.1039/d4ta07377k New insight into designing a thick-sintered cathode for Li-ion batteries: the impact of excess lithium in LiCoO2 on its electrode performanceElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta07377k New insight into designing a thick-sintered cathode for Li-ion batteries: the impact of excess lithium in LiCoO2 on its electrode performanceElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta07377k New insight into designing a thick-sintered cathode for Li-ion batteries: the impact of excess lithium in LiCoO2 on its electrode performanceElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta07377k New insight into designing a thick-sintered cathode for Li-ion batteries: the impact of excess lithium in LiCoO2 on its electrode performanceElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta07377k New insight into designing a thick-sintered cathode for Li-ion batteries: the impact of excess lithium in LiCoO2 on its electrode performanceElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta07377k New insight into designing a thick-sintered cathode for Li-ion batteries: the impact of excess lithium in LiCoO2 on its electrode performanceElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta07377k New insight into designing a thick-sintered cathode for Li-ion batteries: the impact of excess lithium in LiCoO2 on its electrode performanceElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta07377k New insight into designing a thick-sintered cathode for Li-ion batteries: the impact of excess lithium in LiCoO2 on its electrode performanceElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta07377k New insight into designing a thick-sintered cathode for Li-ion batteries: the impact of excess lithium in LiCoO2 on its electrode performanceElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta07377k New insight into designing a thick-sintered cathode for Li-ion batteries: the impact of excess lithium in LiCoO2 on its electrode performanceElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta07377k New insight into designing a thick-sintered cathode for Li-ion batteries: the impact of excess lithium in LiCoO2 on its electrode performanceElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta07377k