['Article\nSubmicron-Sized Nb-Doped Lithium Garnet for High\nIonic Conductivity Solid Electrolyte and Performance\nof Quasi-Solid-State Lithium Battery\n\nYan Ji 1, Cankai Zhou 1, Feng Lin 1, Bingjing Li 1, Feifan Yang 1, Huali Zhu 2\nZhaoyong Chen 1,*\n1 College of Materials Science and Engineering, Changsha University of Science and Technology,\n\n, Junfei Duan 1 and\n\nChangsha 410114, China; jueﬂy@stu.csust.edu.cn (Y.J.', '); zhoucankai@stu.csust.edu.cn (C.Z.', ');\n18216359528@163.com (F.L.', '); krystalbingjingli@163.com (B.L.', '); yﬀ_0413@126.com (F.Y.', ');\njunfei_duan@csust.edu.cn (J.D.)', '2 College of Physics and Electronic Science, Changsha University of Science and Technology, Changsha 410114,\n\nChina; juliezhu2005@126.com\n\n* Correspondence: chenzhaoyongcioc@126.com\n\nReceived: 23 December 2019; Accepted: 20 January 2020; Published: 24 January 2020\n\nAbstract: The garnet Li7La3Zr2O12 (LLZO) has been widely investigated because of its high\nconductivity, wide electrochemical window, and chemical stability with regards to lithium metal.', 'However, the usual preparation process of LLZO requires high-temperature sintering for a long time\nand a lot of mother powder to compensate for lithium evaporation.', 'In this study submicron\nLi6.6La3Zr1.6Nb0.4O12 (LLZNO) powder—which has a stable cubic phase and high sintering\nactivity—was prepared using the conventional solid-state reaction and the attrition milling process,\nand Li stoichiometric LLZNO ceramics were obtained by sintering this powder—which is diﬃcult\nto control under high sintering temperatures and when sintered for a long time—at a relatively\nlow temperature or for a short amount of time.', 'The particle-size distribution, phase structure,\nmicrostructure, distribution of elements, total ionic conductivity, relative density, and activation\nenergy of the submicron LLZNO powder and the LLZNO ceramics were tested and analyzed using\nlaser diﬀraction particle-size analyzer (LD), X-Ray Diﬀraction (XRD), Scanning Electron Microscope\n(SEM), Electrochemical Impedance Spectroscopy (EIS), and the Archimedean method.', 'The total ionic\n−1, the activation energy\nconductivity of samples sintered at 1200\nwas 0.311 eV, and the relative density was 87.3%.', 'When the samples were sintered at 1150\nC for\n60 min the total ionic conductivity was 3.49 × 10\n−1, the activation energy was 0.316 eV, and\nthe relative density was 90.4%.', 'At the same time, quasi-solid-state batteries were assembled with\nLiMn2O4 as the positive electrode and submicron LLZNO powder as the solid-state electrolyte.', 'After\n50 cycles, the discharge speciﬁc capacity was 105.5 mAh/g and the columbic eﬃciency was above 95%.', 'C for 30 min was 5.09 × 10\n\n−4 S·cm\n\n−4 S·cm\n\n◦\n\n◦\n\nKeywords:\nsolid-state batteries\n\nsolid-state electrolyte;\n\nsubmicron powder; garnet;\n\nlithium-ion conductivity;\n\n1.', 'Introduction\n\nCurrently, lithium-ion batteries are widely used in electric vehicles (EVs), hybrid electric vehicles\n(HEVs), computers, smart grids, wearable devices, etc.', '[1].', 'Traditional lithium-ion batteries have organic\nliquid electrolytes which easily burn and explode under abusive conditions.', 'In addition, with the\ndevelopment of modern society, lithium-ion batteries have gradually moved toward high speciﬁc energy.', 'Researchers have studied cathode materials, such as LiCoO2, LiMn2O4, LiFePO4, LiNi0.6Co0.2Mn0.2O2,\nLiNi0.8Co0.1Mn0.1O2, LiNi0.8Co0.15Al0.05O2, and xLi2MnO3·(1− x)LiMO2, to increase the energy density\n\nMaterials 2020, 13, 560; doi:10.3390/ma13030560\n\nwww.mdpi.com/journal/materials\n\nmaterials\x1e# \x1e&\x01!', '*-\x010+\x1f\x1c/ .', 'Materials 2020, 13, 560\n\n2 of 11\n\nof lithium-ion batteries [2–8].', 'The energy density can be improved by increasing the charging voltage,\nwhich will lead to serious side reactions and safety issues.', 'In order to solve the safety problem of\nlithium-ion batteries, researchers have turned their attention to all-solid-state lithium batteries, which\nuse inorganic electrolytes.', 'The non-ﬂammability, long cycling life and wide electrochemical window\nof all-solid-state lithium batteries are considered to provide the high safety and high energy density of\nthe next-generation energy storage systems [9,10].', '−3 S·cm\n\nA solid electrolyte is an important component of all-solid-state batteries.', 'It can not only be used\nas a lithium ionic conductor, substituting for a liquid organic electrolyte, but also can be used to block\ndirect contact between the positive and negative electrodes, like a separator [11].', 'Solid electrolytes\ngenerally contain Li3N, LiPON, perovskite, LISICON, NASICON, garnet, etc.', '[12–17].', 'Some of these\n−1).', 'However, some issues still exist, such as\nsolid electrolytes have high ionic conductivity (~10\ninstability in an ambient atmosphere (Li10GeP2S12, LGPS) and the metal cation being easily reduced\nby lithium (such as Ti4+ in LixLa2/3 -x/3TiO3, LLTO) [18,19].', 'The cubic garnet LLZO was discovered\nby Murugan et al.', '[17] in 2007 and attracted world-wide attention for its advantages, e.g., the simple\n−1) at room temperature, high electrochemical\npreparation process, high ionic conductivity (~10\nwindow (0~6 V vs. Li/Li+), and electrochemical stability of lithium metal.', 'On the other hand, LLZO\nalso has some defects, such as an unstable cubic phase and a low density of ceramics [20,21].', 'Moreover,\na mass of LLZO mother powder is needed to compensate for lithium loss when sintering at high\ntemperatures [21,22].', 'Many solutions have been adopted to solve the above issues.', 'For example,\nAl, Ga, Fe, Ta, Nb, W, Y, and Sb doping were used to stabilize the cubic phase [23–30]; hot pressing\nsintering, plasma sintering, and microwave sintering were adopted to improve the relative density and\nsintering additives [31–33]; and Y2O3, Al2O3, B2O3, CaO, MgO, Li3PO4, and Li4SiO4 were investigated\nto reduce the grain-boundary resistance [34–40].', 'Usually, in order to evaluate the electrochemical\nperformance, LLZO was used as solid electrolyte in all-solid-state batteries [41–43].', '−3 S·cm\n\nIn this study, submicron LLZNO powder with a stable cubic phase was synthesized using\nthe conventional solid-state reaction and prepared by the attrition milling process.', 'The submicron\nLLZNO powder had a high sintering activity, which promoted the sintering process, reduced the\nsintering temperature and time, and reduced the loss of Li during high-temperature sintering.', 'All these\ncharacteristics favored lithium stoichiometry and ionic conductivity.', 'Furthermore, LLZNO ceramics\nwere obtained without mother powder while sintering under reduced temperature and time.', 'The particles-size distribution, phase structure, microtopography, total ionic conductivity, relative\ndensity, and activation energy were characterized and analyzed.', 'The quasi-solid-state lithium batteries\nwith LiMn2O4 as the positive electrode and submicron LLZNO powder as the solid electrolyte were\nassembled and the electrochemical performance are tested and analyzed.', '2.', 'Materials and Methods\n\n2.1.', 'The Synthesis of LLZNO Powder and Ceramics\n\nA process ﬂow chart of the preparation of submicron LLZNO powder and the sintering of LLZNO\nceramics is showed in Figure 1.', 'LLZNO powder was synthesized by the conventional solid-state\nreaction [44].', 'Lithium hydroxide monohydrate (LiOH·H2O, 98%, Xilong Scientiﬁc Co., Ltd., Shantou,\nChina), lanthanum oxide (La2O3, 99.99%, Shanghai Aladdin Bio-Chem Technology Co., Ltd., Shanghai,\nChina), zirconia (ZrO2, 99%, Shanghai Aladdin Bio-Chem Technology Co., Ltd., Shanghai, China), and\nniobium oxide (Nb2O5, 99.99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were used\nas the raw materials and 10 wt% excess of LiOH·H2O was added to compensate for the lithium loss in\nthe high-temperature calcination and sintering process.', 'Yttrium stabilized zirconia (YSZ, 4~8 mm in\ndiameter) and isopropanol (IPA) were used as the ball-grinding medium.', 'The ratio of raw material\nto grinding balls was 1:5 and the mixed raw material powder was wet-ball ground at 800 rpm in the\nplanetary ball mill for 6 h. The mixture was dried at 70\nC for 12 h in\nan alumina crucible with ambient air to obtain the cubic-phase LLZNO powder.', 'LLZNO slurry was\nattrition milled (Shanghai ROOT mechanical and electrical equipment Co., Ltd., Shanghai, China, 0.7 L\n\nC for 14 h, then calcined at 950\n\n◦\n\n◦\n\n\x0cMaterials 2020, 13, 560\n\n3 of 11\n\nvolume, 70% ﬁlling rate) at 1000 rpm for 2 h, taking YSZ (0.4 mm in diameter) and IPA as the grinding\nmedium, and the solid-liquid ratio was 1:5.', 'The LLZNO slurry was dried at 70\nC for 14 h to obtain\nsubmicron LLZNO powder, from which green pellets (mass of 3 g, 19 mm in diameter and a thickness\nof about 4 mm) were pressed at 200 MPa under a cold uniaxial press.', 'After that, the green pellets were\nsintered in a muﬄe furnace (Changsha Yuandong Electric Furnace Factory) without mother powder at\n1100–1200\nC for 30–360 min and then cooled down naturally.', 'At the same time, the green pellets were\nput on a platinum wire and placed in a crucible of MgO with the lid on to prevent impurity migration\nand a large amount of volatilization of lithium during the process of high-temperature sintering.', 'For further testing, LLZNO ceramic pellets were polished with 400 and 1000 mesh sandpaper.', '◦\n\n◦\n\nFigure 1.', 'Process ﬂow chart for the preparation of submicron Li6.6La3Zr1.6Nb0.4O12 (LLZNO) powder\nand the sintering of LLZNO ceramics.', '2.2.', 'Fabrication of Composite Cathodes and Assembly of Quasi-Solid-State Batteries\n\nIn order to test the electrochemical performance of submicron LLZNO powder, we prepared a\ncomposite cathode and assembled quasi-solid-state batteries.', 'The composite cathode consisted of a\nLiMn2O4 positive electrode layer and a submicron LLZNO electrolyte layer.', 'The positive electrode was\nfabricated by coating the slurry of a mixture containing LiMn2O4 powder, submicron LLZNO powder,\nacetylene black (Shanghai Hersbit Chemical Co., Ltd., Shanghai, China), and polyvinylidene diﬂuoride\n(PVDF, FR905, Shanghai San ai fu New Material Technology Co., Ltd., Shanghai, China), with a weight\nratio of 7:2:1:1, onto circular aluminum foils (thickness of 20 µm, Shenzhen Kejingstar Technology Ltd.,\nShenzhen, China) as the current collector, and the positive material loading was 1.66 mg/cm2.', 'Then the\ncomposite cathode was fabricated by coating the slurry of a mixture containing submicron LLZNO\npowder and Polyvinylidene Fluoride (PVDF) with a weight ratio of 9:1 onto the positive electrode\nlayer.', 'The composite cathode was punched into disks with 18 mm diameters after compacted by a\nroller press (Shenzhen Kejingstar Technology Ltd., Shenzhen, China), and the density of composite\ncathode was about 2.5 g/cm3.', 'Quasi-solid-state batteries were assembled with two electrode coin\ncells (type CR-2025) in a glove box ﬁlled with argon and with lithium metal foil (15 mm in diameter\nand 1 mm thick, Shenzhen Kejingstar Technology Ltd., Shenzhen, China) as the negative current\ncollector.', 'In addition, 20 µL of a liquid organic electrolyte (1 M LiPF6 dissolved in ethyl carbonate (EC)\nand dimethyl carbonate (DMC) with a ratio of 1:1, CAPCHEM, Shenzhen, China [45]) was added to\nimprove the contact and reduce the interface impedance between the submicron LLZNO electrolyte\nlayer and the anode/cathode [46,47].', 'Compared with lithium-ion batteries, the added amount of liquid\norganic electrolyte was small [48,49].', 'Materials 2020, 13, 560\n\n2.3.', 'Characterization\n\n4 of 11\n\n◦\n\n◦\n\nC with steps of 5\n\nX-ray diﬀraction (XRD, Cu-Kα radiation, λ = 1.542 Å, Bruker D8 ADVANCE, Bruker AXS GmbH,\nKarlsruhe, Germany) was used to determine the phase of the ceramics pellets at room temperature\nC/min.', 'Jade Software was used to match and analyze the phase of\nwithin 10–60\nthe sample.', 'The relative density of ceramics was measured by Archimedes’ method and deionized\nwater was used as the immersion medium.', 'Meanwhile, the theoretical density of LLZNO, calculated\nby the Jade Software, was 5.20 g/cm3, and the relative density was the measured density divided\nby the theoretical density.', 'The particle size and distribution of the powder were determined by\nthe laser diﬀraction particle-size test method (LD, Mastersizer 3000, Malvern Instruments Limited,\nMalvern, UK), and the relative density, refractive index, and absorption rate of the LLZNO powder was\n5.20 g/cm3, 1.4, and 0.1, respectively.', 'The microtopography of the submicron LLZNO powder and cross\nsection of the ceramic pellets was observed by scanning electron microscope (SEM, TESCAN MIRA3\nLMU, TESCAN Orsay Holding, a. s., Brno, Czech Republic).', 'Energy dispersive spectrometer (Oxford\nX-ray Max20, Oxford Instruments plc, Oxford, UK) mapping was used to characterize the distribution\nof each element in the cross section of the ceramic pellets.', 'The total lithium ion conductivity of the\nceramic pellets was measured by an Electrochemical Impedance Spectroscopy (EIS, Gamry Reference\n600+, Gamry Instruments, Warminster, PA, USA) within a temperature range of 25–80\nC, within\nthe frequency of 10 Hz–5 MHz, and with an AC amplitude of 40 mV.', 'The blocking electrode was\nuniformly coated by a thin silver layer on both sides of the ceramic pellets.', 'The activation energy of\nC and calculated based on the\nthe ceramic pellets was measured within a temperature range of 25–80\nArrhenius equation [16].', 'The quasi-solid-state batteries were tested under the battery charge-discharge\n◦\ntester (BTS-5V3A, Neware Technology Co., Ltd., Shenzhen, China) at 25\nC, and current density was\n0.02 mA/cm2.', '◦\n\n◦\n\n3.', 'Results and Discussions\n\nThe XRD pattern of the LLZNO powder is shown in Figure 2b and was identiﬁed as cubic phase\n(PDF 63-0174).', 'The LD result and SEM image of the LLZNO powder after the attrition milling process,\nwhich demonstrated a submicron powder, are showed in Figure 2a and Table 1.', 'The D(10), D(50),\nD(90), and primary particle size of the submicron LLZNO powder were 0.43 µm, 0.59 µm, 0.812 µm,\nand about 0.1 µm, respectively.', 'The value of D(3,2) (0.575 µm) is similar to that of D(4,3) (0.607 µm),\nwhich indicates that the prepared powder had a uniform particle-size distribution.', 'In addition, the\npowder also had a higher speciﬁc surface area (2007 m2/kg), which means that the powder had a high\nsintering activity, which can promote crystal growth and the rapid densiﬁcation of ceramics in the\nsintering process.', 'Figure 2.', '(a) Particle-size distribution of the LLZNO powder after being attrition milled 2 h at 1000 rpm\nand its SEM image and (b) XRD pattern of the LLZNO powder.', 'Materials 2020, 13, 560\n\n5 of 11\n\nTable 1.', 'Laser particle-size test results of submicron-scale LLZNO powder.', 'Preparation\nCondition\n\nAttrition milled\n2 h @ 1000 rpm\n\nD10 (µm)\n\nD50 (µm)\n\nD90 (µm)\n\nD(3,2) (µm) D(4,3) (µm)\n\nSpeciﬁc Surface\nArea (m2/kg)\n\n0.430\n\n0.590\n\n0.812\n\n0.575\n\n0.607\n\n2007\n\n◦\n\nThe XRD patterns of the LLZNO ceramic samples are showed in Figure 3.', 'The phases of all the\nprepared ceramic samples were identiﬁed as cubic phases (PDF 63-0174).', 'The crystal parameters of the\nC × 60 min\ndiﬀerent samples are showed in Table 2.', 'The XRD patterns of the samples sintered at 1200\nC × 360 min (SL5) showed a few impure phase peaks, mainly belonging to LiNbO3\n(SL1) and at 1100\n(PDF 82-0459) and Li7NbO6 (PDF 29-0816), and due to the decomposition from the high sintering\nactivity of the LLZNO after having been sintered for too long at a high temperature.', 'Moreover, these\nimpure phases decreased the total ionic conductivity of LLZNO ceramics by increasing the resistance\nof the grain boundary.', '◦\n\nFigure 3.', 'XRD patterns of the LLZNO ceramics with diﬀerent sintering conditions.', '◦\n\nAC impedance plots and the enlargement of the LLZNO ceramic pellets under diﬀerent sintering\nconditions are showed in Figure 4a, b.', 'The ﬁtting curve of the sample sintered at 1200\nC for 30 min\n(SL2) is showed in Figure 4c, and it consists of a quasi-semicircle at high frequency and a long diﬀusion\ntail at low frequency.', 'The equivalent circuit model Rb(RgbQgb)(RelQel), in which Rb, Rgb, and Rel are\nresistances originating from the bulk, grain boundaries, and Ag electrodes, is used to ﬁt the plots and\nis shown in Figure 4d.', 'The total ionic conductivity of the ceramics is mainly decided by Rb plus Rgb.', 'The total ionic conductivity and relative density of the LLZNO ceramic pellets are showed in Figure 4e\nand Table 2.', 'The highest total ionic conductivity (5.09 × 10\n−1) of the LLZNO ceramic pellets\nC × 30 min), and its\nwas obtained when sintered at a high temperature and for a short time (SL2, 1200\nrelative density is 87.3%.', 'This indicates that high-performance LLZNO ceramics are obtained when\nsintered at high temperatures only for short sintering times.', 'However, the total ionic conductivity\nand relative density of ceramic pellets decreased and impure phases occurred when the sintering\n−4 S·cm\nC. The lowest total ionic conductivity (0.35 × 10\n−1) and\ntime was prolonged at 1100 and 1200\n◦\nC for 360 min\nrelative density (83.4%) were obtained when the ceramic pellets were sintered at 1100\n\n−4 S·cm\n\n◦\n\n◦\n\n\x0cMaterials 2020, 13, 560\n\n6 of 11\n\n◦\n\n−1) and a higher relative density\nC × 60 min) were obtained when sintered for 60 min from 1100 to\nC, and this result indicates that, in this study, increasing the sintering temperature too much was\n\n(SL5).', 'Meanwhile, a higher total ionic conductivity (3.49 × 10\n(90.3%) of ceramic pellets (SL3, 1150\n1200\ndisadvantageous for obtaining LLZNO ceramics with good performance.', '◦\n\n−4 S·cm\n\n◦\n\nTable 2.', 'Sintering condition, cell parameter, total ionic conductivity at 25\nrelative density of LLZNO ceramics.', 'C, activation energy, and\n\nSample\nName\nSL−1\nSL−2\nSL−3\nSL−4\nSL−5\n\nSintering\nCondition\nC × 60 min\n◦\n1200\nC × 30 min\n◦\n1200\nC × 60 min\n◦\n1150\nC × 60 min\n◦\n1100\nC × 360 min\n◦\n1100\n\nCell Parameter\n\n(Å)\n\n12.8952\n12.8953\n12.9028\n12.8916\n12.8870\n\nTotal Ionic Conductivity\n(10−4 S·cm−1), 25\n\nC\n\n◦\n\n1.58\n5.09\n3.49\n0.51\n0.35\n\nActivation\nEnergy (eV)\n\n0.315\n0.311\n0.316\n0.319\n0.328\n\nRelative\nDensity\n86.7%\n87.3%\n90.4%\n90.3%\n83.4%\n\nArrhenius plots and the linear ﬁtting curve are showed in Figure 5a.', 'The activation energy\nof ceramics samples is showed in Figure 5b and Table 2, and their values are within the range of\n0.31–0.33 eV.', 'This indicates that there was no obvious eﬀect on the activation energy of the ceramics\nwhen the green pellets prepared from the submicron LLZNO powder were sintered.', 'The variation\ntendency of the activation energy was similar to the total ionic conductivity, and the lowest and the\nC × 360 min).', 'highest activation energy was 0.311 eV (SL2, 1200\n\nC × 30 min) and 0.328 eV (SL5, 1100\n\n◦\n\n◦\n\n◦\n\nFigure 4.', '(a, b) AC impedance plots of the LLZNO ceramics with diﬀerent sintering conditions at\nC; (d) equivalent circuit to ﬁt the curves.', '25\n(e) Total conductivity and relative density of the LLZNO ceramics.', 'C; (c) AC impedance plots and ﬁtting curve of SL2 at 25\n\n◦\n\n\x0cMaterials 2020, 13, 560\n\n7 of 11\n\nFigure 5.', 'LLZNO ceramics.', '(a) Arrhenius plots and ﬁtting results and (b) the activation energy of diﬀerent\n\n◦\n\n◦\n\n◦\n\n◦\n\nC for 60 min, Figure 6c), and ﬁnally, all grains were about 200 µm (SL1, 1200\n\nSEM images of cross sections of the LLZNO ceramics, which were sintered under diﬀerent\nconditions, are showed in Figure 6a–e.', 'We found that the grain size of the ceramics that were sintered\nfor 60 min within a temperature range of 1100 to 1200\nC gradually increased from 1~5 µm (SL4,\nC for 60 min, Figure 6d).', 'A few of the grains were 5 µm and most of the grains were 100~200 µm\n1100\n(SL3, 1150\nC for 60 min,\nFigure 6a).', 'Here, we found a mass of abnormal growth grains (AGGs) [50], as shown in Figure 6a,c,e,\nand a mass of pores were distributed in the AGGs.', 'Meanwhile, the total ionic conductivity was\nlower when the AGGs were bigger.', 'This was due to the submicron LLZNO powder having a high\nsintering activity, which made the crystal grain of the LLZNO ceramics have a high speciﬁc surface\nenergy during the high-temperature sintering process, and promoted rapid grain growth and ceramic\ndensiﬁcation in the sintering process.', 'For the above reasons, the growth rate of the grains was higher\nthan the migration rate of the pores at the grain boundaries when the sintering temperature was higher\nand the sintering time was longer and the pores could not be discharged from the grain boundaries and\nﬁnally stay on the inside of the AGGs.', 'As a result, the bulk impedance of the crystal grains increased,\nand the total ion conductivity was reduced.', 'However, although the submicron LLZNO powder had\nhigh sintering activity, the growth of grains could not be entirely promoted in a shorter sintering time\nand at a lower temperature.', 'Therefore, a mass of grains which stayed in the initial state are shown in\nC × 60 min), and this was disadvantageous for lithium-ionic conduction due\nFigure 6d (SL4, 1100\nto the incomplete surface of the LLZNO grains after the attrition milling process.', 'Eventually, the\nceramic pellets showed a lower total ionic conductivity (0.51 × 10\n−1).', 'A cross-sectional SEM\nimage of the sample sintered at 1200\nC for 30 min (SL2) is showed in Figure 6b.', 'It was found that\nthe grains grew uniformly (~4 µm), their surfaces were smooth without pores, and they bond tightly\nwith other grains.', 'A highest ionic conductivity of 5.09 × 10\n−1 was obtained, which indicates\nthat the submicron LLZNO powder had a higher sintering activity and high total ionic conductivity\nLLZNO ceramic pellets could be obtained by sintered at a high temperature for only a short time.', 'At the same time, the LLZNO ceramic pellets which had a higher total ionic conductivity could also be\nalso obtained when the sintering temperature was properly reduced.', '−4 S·cm\n\n−4 S·cm\n\n◦\n\n◦\n\n◦\n\nFigure 6f shows the SEM image and its EDS mapping, including La, Zr, and Nb in the cross section\nC× 30 min (SL2).', 'The cross section of the sample exhibits a transgranular\nof the LLZNO ceramic of 1200\nfracture and an intergranular fracture, and the elements of La, Zr, and Nb are relatively uniformly\ndistributed, which indicates that the Nb element was successfully incorporated into the LLZO lattice.', 'This is also veriﬁed by the XRD result.', 'However, the non-uniform distribution of Zr, La, and Nb exists\nin the central part of the EDS mapping.', 'This indicates that during high-temperature sintering element\nsegregation and depletion occurred due to the diﬀerent migration rates of the elements.', 'Materials 2020, 13, 560\n\n8 of 11\n\nFigure 6.', '(a–e) SEM images of the cross-sectional microstructures of the ceramics that were sintered by\ndiﬀerent particles sizes under diﬀerent sintering conditions and (f) EDS mapping of LLZNO ceramics\nsection sintered at 1200\n\nC for 30 min.', '◦\n\n◦\n\nThe speciﬁc capacity and coulombic eﬃciency of quasi-solid-state batteries with LiMn2O4 as\nthe positive electrode after 50 cycles of a galvanostatic charge-discharge test at 25\nC are showed in\nFigure 7a.', 'The 1st, 2nd, 10th, 20th, and 50th galvanostatic charge-discharge curves of quasi-solid-state\nbatteries are showed in Figure 7b.', 'The quasi-solid-state batteries showed good cycling performance at\na current density of 0.02 mA/cm2 and a voltage within 3.0–4.3 V. The ﬁrst discharge speciﬁc capacity\nwas 106.4 mAh/g and the coulomb eﬃciency was 93.23%.', 'The 2nd, 10th, 20th, and 50th discharge\nspeciﬁc capacities were 106.8 mAh/g, 105.3 mAh/g, 106.9 mAh/g, and 105.5 mAh/g, respectively.', 'After\n50 cycles of the galvanostatic charge-discharge test, the coulomb eﬃciency was maintained at about\n95% and the capacity retention rate was 99.15%.', 'The capacity of the batteries increased in the early\nstage of the galvanostatic charge-discharge test, which may be caused by the activation of positive\nmaterial.', 'This indicates that submicron LLZNO powder can be used in quasi-solid-state batteries,\nand that the speciﬁc capacity and the cycling stability of quasi-solid-state batteries are relatively good.', 'Here, the electrochemical performance of quasi-solid-state batteries using submicron LLZNO powder\nis only discussed, and further research will be carried out in the future.', '(a) Speciﬁc capacity and coulombic eﬃciency and (b) the 1st, 2nd, 10th, 20th, and\nFigure 7.', '50th galvanostatic charge-discharge curves of quasi-solid-state batteries with LiMn2O4 as the\npositive electrode.', 'Materials 2020, 13, 560\n\n4.', 'Conclusions\n\n9 of 11\n\nIn this study, we synthesized Nb-doped stabilized cubic-phase LLZO powder using the\nconventional solid-state reaction and prepared submicron LLZNO powder using the attrition milling\nprocess.', 'Electrolyte ceramics prepared using submicron LLZNO powder can be sintered without\nmother powder, which reduces the sintering temperature and shortens the sintering time.', 'After being\n◦\nC for 60 min, the total ionic conductivity, relative density, and activation energy was\nsintered at 1150\n3.49 × 10\n−4 S·cm\n−1, 90.4%, and 0.316 eV, respectively.', 'When sintered at 1200\nC for 30 min, we obtained\nthe highest total ionic conductivity of 5.09 × 10\n−1, the relative density was 87.3%, and the\nsmallest activation energy was 0.311 eV.', 'For the quasi-solid-state batteries assembled with submicron\nLLZNO powder, the capacity retention rate was 99.15% and the speciﬁc capacity was 105.5 mAh/g after\n50 cycles at room temperature with a current density of 0.02 mA/cm2.', 'Therefore, we have presented a\nsimple method to reduce the waste of raw materials and energy used when sintering LLZO ceramics.', 'At the same time, the prepared submicron LLZO powder can also be applied in quasi-solid-state\nbatteries, with a good electrochemical performance.', '−4 S·cm\n\n◦\n\nAuthor Contributions: Methodology and conceptualization, Z.C., Y.J., and C.Z.', '; resources, Z.C.', 'and H.Z.', '; data\ncuration, Y.J., C.Z., F.L., B.L., and F.Y.', '; writing—original draft preparation, Y.J.', '; writing—review and editing,\nY.J.', 'and J.D.', '; funding acquisition, Z.C.', 'and H.Z.', 'All authors have read and agreed to the published version of\nthe manuscript.', 'Funding: This work was supported by the National Natural Science Foundation of China (No.', '51874048),\nthe National Science Foundation for Young Scientists of China (No.', '51604042), the Research Foundation of\nEducation Bureau of Hunan Province (No.', '19A003), the Scientiﬁc Research Fund of Changsha Science and\nTechnology Bureau (No.', 'kq1901100), and the Postgraduate Innovative Test Program of Hunan Province.', 'Conﬂicts of Interest: The authors declare no conﬂict of interest.', 'References\n\n1.', '2.', '3.', '4.', '5.', '6.', 'Zhu, C.; Wei, D.; Wu, Y.; Zhang, Z.; Zhang, G.; Duan, J.; Li, L.; Zhu, H.; Zhu, Z.; Chen, Z. 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