Effect of lithium borate coating on the electrochemical properties of LiCoO2 electrode for lithium-ion batteries Chimica Techno Acta ARTICLE published by Ural Federal University 2021, vol. 8(1), № 20218101 journal homepage: chimicatechnoacta.ru DOI: 10.15826/chimtech.2021.8.1.01 1 of 6 Effect of lithium borate coating on the electrochemical properties of LiCoO2 electrode for lithium-ion batteries Victor D. Zhuravlev a , Ksenia V. Nefedova a* , Elizaveta Yu. Evshchik b , Elena A. Sherstobitova a , Valery G. Kolmakov b , Yury A. Dobrovolsky b , Natalia M. Porotnikova a , Andrey V. Korchun b , Anna V. Shikhovtseva b a: Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 91 Pervomaiskaya St., Ekaterinburg, 620990, Russia b: Institute of Problems of Chemical Physics, Russian Academy of Sciences, 1 Ac. Semenov ave, Chernogolovka, Moscow region, 142432, Russia * Corresponding author: nefedova@ihim.uran.ru, ksenia_nef@rambler.ru This article belongs to the regular issue. © 2021, The Authors. This article is published in open access form under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract The effect of a protective coating of fused lithium borate, Li3BO3, on the physicochemical and electrochemical characteristics of LiCoO2 has been studied. A cathode material produced by the SCS method us- ing binary organic fuel, glycine and citric acid. The influence of the experiment conditions on the morphology, crystal structure and spe- cific surface of lithium cobaltite was studied. Electrochemical testing of LiCoO2∙nLi3BO3 samples, n = 5 and 7 mass %, has been performed in the cathode Li|Li+-electrolyte|LiCoO2∙nLi3BO3 half-cell using 1M LiPF6 in EC/DMC mixture (1:1) as electrolyte in the 2.7-4.3 V range at normalized discharge current С/10, С/5, С/2. The maximal initial discharge capacity of 185 mAh/g was detected for the samples with 5 mass % Li3BO3. The coulomb efficiency of optimal materials in the 40 th cycle was 99.1%. Keywords lithium-ion batteries lithium cobalt oxide solution combustion synthesis Li3BO3 protective coating Received: 13.10.2020 Revised: 18.11.2020 Accepted: 26.11.2020 Available online: 21.12.2020 1. Introduction Lithium cobaltite LiCoO2 (LCO) is used as a cathode mate- rial since 1990, and in spite of the appearance of such promising cathode materials as LiNi1/3Mn1/3Co1/3O2 (NMC), LiNi0.8Co0.15Al0.05O2 (NCA), LiMn2O4 (LMO) etc. it is still employed as a component of lithium-ion batteries (LIB) with low discharge rates in portable gadgets [1]. Chemical interaction between the electrolyte and the cathode mate- rial leads to non-recoverable losses of lithium cations re- duced service lifetime and accelerated capacity failure of LIB. The manufacturers of LIB materials try to eliminate this effect by decreasing the specific surface of dispersed materials, by using microgranulation processes or apply- ing modifying coatings on cathode material particles, pro- tecting them from the action of acid fluorine-containing components of electrolyte. In particular, Al2O3, ZrO2, ZnO, SiO2, TiO2 and other oxides are used as protective coatings [2-5]. Recently appeared publications reporting the appli- cation of glasses and boron and lithium based oxides as coatings [5-7]. A. Nagasubramanian et al. [6] studied the effect of LiBO2 coating on the electrochemical performance of orthorhombic LiMnO2 cathode. ShuangYuan Tan et al. [7] showed that using glass-coated NMC/Li2O∙2B2O3 can increase the discharge capacity retention of the cathode from 22.5% to 57.8% at −40 °C. Among compounds in the Li2O-B2O3 system, lithium borate Li3BO3 should be noted [8-10]. It has the lowest melting temperature, 715±15 °C [11], which allows applying it as a flux for more refractory compounds, creating dense protective coatings. In addi- tion, Li3BO3 is a lithium ion conductor [10], and its molec- ular mass is smaller than that of LiCoO2 and other cathode materials. Li3BO3 coating also increases the concentration of Li + in the contact layer with electrolyte. Most coating strategies are based on the sol-gel method or impregnation of cathode materials powders with salt solutions with subsequent drying and annealing [1-3, 5-7]. However, the application of lithium borates via solutions guarantees neither synthesis of the nominal composition of lithium borate nor the density of the coating after an- nealing at 500 °С. It can be assumed that addition and distribution of LiBO2, Li2B4O7 or Li3BO3 compounds in the cathode bulk with subsequent annealing at melting tem- peratures of the corresponding eutectics may cause posi- tive effect. http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2021.8.1.01 http://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0001-5933-4310 https://orcid.org/0000-0002-5147-1414 http://orcid.org/0000-0002-3562-8805 https://orcid.org/0000-0002-4443-1318 https://orcid.org/0000-0001-9292-0266 https://orcid.org/0000-0002-2163-6863 https://orcid.org/0000-0001-5284-4553 https://orcid.org/0000-0003-1002-8114 https://orcid.org/0000-0003-4058-2968 Chimica Techno Acta 2021, vol. 8(1), № 20218101 ARTICLE 2 of 6 For the production of cathode materials, different mod- ifications of solid-phase and hydrothermal methods are usually employed [12]. In laboratory studies for the ob- taining of the cathode materials, in particular, lithium cobaltite, combustion reactions [13-18] are more often used [19-24]. The attraction of solution combustion syn- thesis (SCS) reactions for industrial application is deter- mined by the following characteristics: 1. cathode material can be produced almost without sew- age; 2. energy consumption for the decomposition of precur- sors is reduced compared to conventional technologies, since the method employs internal exothermal pro- cesses requiring only relatively moderate energy con- sumption for evaporation of reaction solutions and preliminary heating of xerogel before the beginning of redox reaction; 3. dispersion and chemical activity of produced precursor may reduce the time of high-temperature annealing to attain the monophasic; 4. although the initial solutions contain nitrates, combus- tion proceeds with almost complete transformation of nitrogen dioxides into molecular nitrogen; 5. this method allows to control the dispersion of the ma- terial while reducing the costs for milling and lessen- ing the risk of pollution of the material during milling. However, in the LiNO3-Co(NO3)2-glycine (urea) systems, a redox reaction of LCO formation proceeds intensely with outflow of a considerable part of precursor with effluent gases outside the reactor. Under stoichiometric combus- tion conditions, the combustion rate may have an explo- sive character. It is reasonable to expect that the described above effects detected under conditions of laboratory ex- periments will be multiply strengthened if the mass of the material is increased. This problem can be solved by using of less energetic fuel for controlled reduction of SCS rate, for example, sucrose, ammonium acetate starch, citric acid and oxalic acid [24-28]. In this paper, we report structural, morphological, di- mensional and electrochemical characteristics of LCO powders produced in SCS reactions with glycine and citric acid with subsequent coating with fused lithium borate Li3BO3. 2. Experimental 2.1. Starting materials For the combustion synthesis of LCO powders, cobalt(II) nitrate hexahydrate (99%) and cobalt(II) carbonate hy- droxide hydrate CoCO3∙mCo(OH)2∙nH2O (with cobalt con- tent of 55.5%) (Ural Chemical Reagents Plant, Russia) were used as cobalt sources, and lithium carbonate (UNICHIM (Russia), 99%) was used as a source of lithium. Citric acid hydrate H3C6H5O7∙H2O (Citrobel (Russia), 99.8%) and amino acetic acid (glycine) H2N(CH2)COOH (Kamhimkom (Russia), 98.5%) were used as fuel, while double-distilled water served as a solvent for precursor solutions. The synthesis of lithium borate was carried out from boric acid (UNICHIM (Russia), 99.5%) and lithium carbonate (NPF Nevsky Chemist (Russia), 99.5 %). 2.2. Synthesis Lithium nitrate combined with cobalt nitrate imparts ex- cessive combustion intensity to SCS reactions. In the methods where glycine used as a fuel / reductant, the combustion rate during synthesis of cathode materials of LIB can be considerably decreased by replacing lithium nitrate by lithium carbonate or lithium citrate [24]. Be- sides, the reduction of the fraction of cobalt nitrate (oxi- dizer) due to its replacement by cobalt citrate also lowers the SCS intensity allowing the yield increasing of the re- sulting material [24]. In this work, we used one-step mode of LCO produc- tion. For this purpose, a 150 ml solution of cobalt nitrate (66.67 g/dm 3 Co) and citric acid (237.5 g/dm 3 ) (solution 1) was placed into a 2 dm 3 reactor, to which 120 cm 3 lithium citrate solution (286.68 g/dm 3 Li2HC6H5O7) was added. Cobalt (II) carbonate hydroxide hydrate and glycine suc- cessively added to the resulting solution (Table 1). The reaction solution was heated on an electrical heat- er with a capacity of 1 kW (the temperature of the heater was 550–600 °С) for dehydration and initiation of SCS reaction. The produced LCO precursor was ground in a ball mill with grinding bodies made of stabilized zirconi- um oxide prior and between the annealing at 650, 800 and 850 °С. The annealing duration at each stage was 10 h. After certification, LCO powders were coated with fused Li3BO3 (LBO) produced preliminarily in a solid-phase reaction using lithium carbonate and boric acid at 560– 600 °С for 35 h with intermediate grinding. 5 and 7 mass % of LBO were added to the initial LCO powder and mixed in a ball mill for 1 h, then they were annealed at 750 °С for 5 h. After the first annealing, the LCO/LBO samples were repeatedly ground and annealed for the second time at 750 °С for 5 h to produce better uniform coating. 2.3. Characterization of powder samples The diffraction patterns of the powders were taken at room temperature with a Shumadzu XRD-700 (Cu Kα1 radiation, 2θ = 10-80°) diffractometer, equipped with PDF2 database. The refinement of the crystal structure according to the Rietveld method carried out using the software package FullProf [29]. SEM images obtained with a JEOL JSM 6390 LA microscope. Specific surface area (S) of powders was determined by BET nitrogen desorption during heating in a SORBI N4.1 (Meta, Russia). The parti- cle size distribution of the obtained powders was deter- mined using a Horiba LA-950V2 laser particle meter. 2.4. Electrochemical measurements The electrochemical properties of the LCO powder samples were studied using two-electrode pouch cells. The compo- site electrodes were prepared by inkjet printing of a ho- Chimica Techno Acta 2021, vol. 8(1), № 20218101 ARTICLE 3 of 6 mogenized mixture of the synthesized material, a conduc- tive additive (acetylene black) and a binder (polyvinyli- dene fluoride dissolved in N-methyl-2-pyrrolidone) (weight ratio of solid components 80:10:10) onto an alu- minum foil. The electrodes were compacted in a rolling mill and then dried under vacuum at 120 °С for 12 h. The area of the prepared electrode was of 2.25 cm 2 . The active material loading of the electrode was about 3-6 mg cm -2 . The electrochemical test cells Li│liquid electro- lyte│LCO were assembled in an argon-filled MBraun LAB Star glove box with O2 and H2O contents <0.1 ppm. Lithi- um foil (99.9 %, Alfa Aesar) was used as a counter elec- trode. Celgard 2300 film was used as a separator. The so- lution of 1 M LiPF6 in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (1:1 vol.) (Sigma Aldrich) was used as the electrolyte. The residual water content in the electrolyte solution did not exceed 30 ppm. Cycling performance and rate capability of the cathode half-cells were examined at 25 °C by galvanostatic charge- discharge curves measured with P-20X80 multichannel potentiostat (“Elins” LLC, Russia) in the voltage range of 2.7–4.3 vs. Li 0 /Li + . Current density varied from 0.1 to 0.5C. 3. Results and Discussion SCS as the chosen LCO obtaining method allowed to in- crease the mass of the obtained product without growing the reaction temperature and rate, i.e. without discharge of the produced powder. Evaporation of the solution led to the formation of a semi-sphered dried gel covered with a violet film. The dry outer layer gradually closed the whole semi sphere, inside which a wet gel remained (Fig. 1а) also transforming gradually into dry precursor material. The redox reaction proceeded in the form of a heating wave (Fig. 1b) with low violet flame. There was no dis- charge of the material outside the reactor, and no visible traces of nitrogen dioxide were present. When the com- bustion was completed, a bulky flake black powder of LCO precursor was obtained. The LCO powders obtained after combustion possessed high dispersion and revealed chemically non-equilibrium state due to incomplete crystal lattice formation process- es. This is connected with a short combustion time (the reaction mass was in the high-temperature region for less than 3–5 min); low density of the material (as a result of large amount of gaseous products) hampers the comple- tion of diffusion processes during LCO formation. Usually, the SCS process is supplemented with high-temperature annealing, in this case in the range of 650–850 °С, to re- move carbon-containing impurities and form the LCO crys- tal structure (Table 1, Fig. 2). Upon annealing, the crystal lattice parameters of the produced LCO corresponded to the literature data [30]. Reflections of the fused LBO coat- ing were not recorded, probably due to its glassy charac- ter. The performed sedimentation analysis revealed that LCO represents finely dispersed powders forming agglom- erates with the maximal diameter, Dmax, of <30 µm (Table 2) and the average particle diameter, Dav, of 8.4-11.5 µm. Table 1 LCO crystal lattice parameters after annealing at 850 °С Sample a, Å c, Å V, Å 3 R1 R2 1 2.8165 (2) 14.0604 (21) 96.5921 1.72 0.57 [30] 2.81619 14.05586 96.5382 1.35 0.44 Fig. 1 Formation of the product in SCS reactions Fig. 2 X-ray diffraction patterns of (а) LCO, (b) LCO+5% LBO, (c) LCO +7% LBO Chimica Techno Acta 2021, vol. 8(1), № 20218101 ARTICLE 4 of 6 The fraction of particles less than 5 µm is rather large, from 6 to 30%. The application of borate coating consid- erably changes the particle size distribution in the materi- al; Dav increases to 11-45 µm and depends explicitly on the milling conditions and load. The fraction of particles with a diameter less than 5 µm decreases to 1.1-1.3% (Table 2), which should have a positive effect on the cathode stabil- ity during the interaction with electrolyte. However, the presence of aggregate fractions larger than 30 µm re- quired the classification of powders before applying the electrode mass. Coating of LCO with a layer of fused lithium borate practically does not change the specific surface of the ma- terial (Table 2), remaining equal to 0.8-0.98 m 2 /g. This value is larger than the traditional values for commercial cathode materials, 0.4-0.6 m 2 /g, but possibly has positive impact on the electrochemical characteristics during cy- cling. The advantage of coating by fusion is that fusion and spreading of LBO on the surface of LCO particles and ag- glomerates decreases the coating thickness and increases the probability of connection of small particles, reducing the influence of electrolyte. The sizes of LCO-LBO powders are slightly larger than the base (LCO) particles, it contain a smaller percent of fine fractions due to enhanced sintering into agglomerates and additional annealing time (growth of primary crystal- lites) (Fig. 3). The charge-discharge characteristics of cathode mate- rials based on LCO were studied in the potential range of 2.75-4.3V vs Li/Li + . The charge and discharge rate of the first 10 cycles were 0.1C. Then, at the same charge rate, 10 cycles performed with a discharge rate of 0.2C, 0.5C and again 0.1C. Fig. 4 shows the capacity dependences on the cycle number based on a series of powders: LCO, LCO + 5% LBO and LCO + 7% LBO as samples with the most sta- ble characteristics. The discharge capacity of the sample without coating at the first cycle was 166 mAh/g. A significant increase of the discharge capacity (up to 185 mAh/g) was achieved by using a 5% LBO coating. When the LBO content of the sample increases to 7%, the discharge capacity drops to 164 mAh/g. The negative effect probably related to the increasing thickness of the coating layer and/or the for- mation of less conducting glasses during the interaction of fused LBO with LCO. All samples retain 94, 93 and 93% of the original dis- charge capacity after 40 cycles when the cycle rate re- turned to the original rate 0.1C. In addition, the coulomb efficiency of these samples is more than 99% throughout all 40 charge-discharge cycles (Table 3). Fig. 5 shows the charge-discharge curves of the first cycle for LCO, LCO+5% LBO and LCO +7% LBO. The type of the charge-discharge curves corresponds to the typical charge-discharge curves obtained for lithium cobaltate- based cathodes [21, 31, 32]. The charge-discharge curves have a plateau at a poten- tial of 3.9 V and two small quasi-plateaus at 4.1 and 4.2 V. These plateaus correspond to the peaks on the cyclic volt- ammograms (Fig. 6). According to the literature data [32- 36], the main peak at 3.9 V is related to the first order transition from LCO to Li0.8CoO2; the two less pronounced peaks at ~4.06 and ~4.17 V are associated with phase transitions to the monoclinic structure and back to the hexagonal structure. Table 2 The results of sedimentation analysis of LCO-LBO powders LCO - LBO Dav, µm Dmed, µm Fraction < 5 µm, % Dmax, µm Fraction >30 µm, % Fraction 30-100 µm, % S, m 2 /g LCO 8.4 8.2 13.5 30 0 0 0.82±0.02 LCO+5% LBO 43 14 1.3 300 24 11 0.86±0.04 LCO+7% LBO 37 16 1.1 300 29 19 0.92±0.03 Table 3 The cyclic performance of LCO powders in the range of 2.75–4.3 V at different rates Discharge capacity, mAh/g (cycle number) Coulomb efficiency, % (cycle number) 0.1С (10) 0.2С (20) 0.5С (30) 0.1С (40) (10) (20) (30) (40) LCO 162 158 145 153 99.5 99.7 99.9 99.1 LCO+5% LBO 185 180 173 172 99.5 99.7 99.9 99.1 LCO+7% LBO 162 158 152 150 99.5 99.7 99.9 99.1 Fig. 3 Morphology of (а) LCO, (b) LCO+5% LBO, (c) LCO +7% LBO powders Chimica Techno Acta 2021, vol. 8(1), № 20218101 ARTICLE 5 of 6 Based on the obtained data, it is possible to conclude that borate coating does not affect the electrode polarization, as the discharge plateau does not change. According to the data obtained by the cyclic voltamme- try method and based on the peaks intensity on the cyclic voltammograms, one may conclude that due to the pres- ence of 5 mass % of LBO coating on the lithium cobaltite surface, it is possible to activate the electrode surface and increase the reversibility of lithium introduction and ex- traction processes into the electrode. Wider peaks for the uncoated sample may be an indication of inhibition of lithiation and delithiation processes as compared to the modified samples. Consequently, the presence of LBO coat- ing may affect the rate of lithium diffusion into cathode material particles. Thus, borate coating increases the stability of cobaltite cycling at elevated rates. When the rate increases to 0.5C, the capacity falls by 4% for coated samples and by 8% for uncoated samples. 4. Conclusions The best electrochemical characteristics, discharge capaci- ty of 185 mAh/g at 0.1C and coulomb efficiency of 99.1% after 40 cycles, were demonstrated by the sample ob- tained in the SCS reaction of cobalt nitrate with lithium citrate and glycine and coated with 5 mass % of LiBO3. Fused LiBO3 coating increases the cobaltate cycling stabil- ity at elevated rates. Acknowledgments The work was performed in accordance with the state as- signments of the Institute of Chemistry of Solids of the Ural Branch of the RAS, No. AAAA-A19-119031890026-6 and No. АААА-А19-119102990044-6, the state assignment of the Institute of Problems of Chemical Physics of the RAS, No. АААА-А19-119061890019-5, and Thematic map No. 0089-2019-0007 «Functional materials for chemical power sources». References 1. Blomgren GE. The development and future of lithium ion batteries. J Electrochem Soc Jpn. 2017;164:A5019–25. doi:10.1149/2.0251701jes 2. Maximov MYu, Popovich AA, Rumyantsev AM. 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