molecules Communication Thermal Decomposition Study on Li2O2 for Li2NiO2 Synthesis as a Sacrificing Positive Additive of Lithium-Ion Batteries Jaekwang Kim, Hyunchul Kang, Keebum Hwang and Songhun Yoon * Department of Nanomaterials Science and Engineering, School of Integrative Engineering, Chung-Ang University, 84, Heukseok-ro, Dongjak-Gu, Seoul 06974, Korea; [email protected] (J.K.); [email protected] (H.K.); [email protected] (K.H.) * Correspondence: [email protected]; Tel.: +82-2-820-5769 Academic Editor: Gregorio F. Ortiz Received: 8 November 2019; Accepted: 14 December 2019; Published: 17 December 2019 Abstract: Herein, thermal decomposition experiments of lithium peroxide (Li2O2) were performed to prepare a precursor (Li2O) for sacrificing cathode material, Li2NiO2. The Li2O2 was prepared by a hydrometallurgical reaction between LiOH H O and H O . The overall reaction during annealing · 2 2 2 was found to involve the following three steps: (1) dehydration of LiOH H O, (2) decomposition of · 2 Li2O2, and (3) pyrolysis of the remaining anhydrous LiOH. This stepwise reaction was elucidated by thermal gravimetric and quantitative X-ray diffraction analyses. Furthermore, over-lithiated lithium nickel oxide (Li2NiO2) using our lithium precursor was synthesized, which exhibited a larger 1 yield of 90.9% and higher irreversible capacity of 261 to 265 mAh g− than the sample prepared by 1 commercially purchased Li2O (45.6% and 177 to 185 mAh g− , respectively) due to optimal powder preparation conditions. Keywords: Li2O2; thermal decomposition; Li2O; Li2NiO2; Li source; Li-ion battery 1. Introduction Within Li-ion (Li+) batteries (LIBs), organic electrolytes are easily decomposed due to the highly negative potential of the anode materials during first few cycles. This decomposition can be favorable in one sense, since the solid electrolyte interphase (SEI) passivates the surface of the electrode and prevents further decomposition of the electrolyte. The stabilized SEI film resulted in reasonable coulombic efficiency and cycle performance [1–4]. However, the electrolyte decomposition is not beneficial with respect to the specific energy of LIBs. Since decomposition consumes electric charges and Li+ during the first few charging cycles, an overload of the positive electrode material is inevitable to compensate the Li+ and electric charges for the SEI formation [5,6]. The carbonaceous materials, such as soft carbons, hard carbons, graphite, etc. have been widely used as anode materials for LIBs for recent device applications such as electric vehicle (EV) and hybrid EV [7–15]. At present, moreover, high-capacity materials are on use (ex. SiO) in commercial LIBs to increase energy density. Unlike to a graphite (>90%), the SiO exhibits a quite low coulombic efficiency about 45% in the initial few cycles because of a charge consumption originated from the irreversible formation of the SEI. However, this high-capacity material have been applied to meet the demands of higher energy density cells. Then, the compensation of charge (or Li+) consumption should become a critical issue in the future market of LIBs [3,16,17]. For this reason, the role of a sacrificing positive electrode additive has been widely discussed. Researchers who studied the sacrificing materials reported that the materials should be of a light weight for better energy density, easily decomposed near the charging potential Molecules 2019, 24, 4624; doi:10.3390/molecules24244624 www.mdpi.com/journal/molecules Molecules 2019, 24, 4624 2 of 8 Molecules 2019, 24, x 2 of 8 ofcharging cathode potential material, of and cathode over-lithiated material, forand maximumover-lithiated irreversible for maximum capacity irreversible to match capacity the consumed to chargesmatch the (and consumed Li+). charges (and Li+). TheThe over-lithiated lithi lithiumum nickel nickel oxide oxide (Li (Li2NiO2NiO2) has2) has been been considered considered as a as promising a promising additive additive sincesince itit isis bothboth over-lithiatedover-lithiated toto produceproduce thethe largelarge irreversible irreversible capacitycapacity duringduring initialinitial cyclescycles andand ofof light light weight [18]. Li2NiO2 is widely synthesized by nickel oxide (NiO) and lithium oxide (Li2O) since weight [18]. Li2NiO2 is widely synthesized by nickel oxide (NiO) and lithium oxide (Li2O) since they they directly react in solid-state in stoichiometric ratio 1:1. However, as the main precursor of Li2NiO2, directly react in solid-state in stoichiometric ratio 1:1. However, as the main precursor of Li2NiO2, Li2O isLi still2O is expensive still expensive to prepare to prepare as it isas obtained it is obtained by direct by direct oxidizing oxidizing of Li of metal. Li metal. Therefore, Therefore, the ethefficient efficient and economic preparation of the Li2O is highly required. The thermal decomposition of and economic preparation of the Li2O is highly required. The thermal decomposition of lithium lithium peroxide (Li2O2) is can be less expensive way to synthesize Li2O, because it only exhales O2 peroxide (Li2O2) is can be less expensive way to synthesize Li2O, because it only exhales O2 during its during its reaction and its precursor (LiOH·H2O) is inexpensive. To the best of our knowledge, the reaction and its precursor (LiOH H2O) is inexpensive. To the best of our knowledge, the systematic systematic study of this thermal decomposition· of Li2O2 has not yet been reported. To provide a better study of this thermal decomposition of Li O has not yet been reported. To provide a better insight insight into this beneficial synthesis method2 2 , the authors have investigated the thermal into this beneficial synthesis method, the authors have investigated the thermal decomposition of decomposition of Li2O2 by applying various decomposition temperatures and reaction times. LiQuantitative2O2 by applying analysis various is performed decomposition on the reaction temperatures products and to reaction elucidate times. the composition Quantitative ratios analysis of is performedseveral lithium on the compounds reaction products obtained to from elucidate the reaction. the composition Moreover, ratios two types of several of Li2 lithiumNiO2 (L2N) compounds are obtainedobtained fromby using the reaction. as prepared Moreover, or commercially two types of Lipurchased2NiO2 (L2N) Li2O, are and obtained their byelectrochemical using as prepared orperformances commercially are purchased compared. Li2O, and their electrochemical performances are compared. 2.2. ResultsResults and Discussion Discussion TheThe morphologies of of the the Li Li2O22O and2 and its thermally its thermally decomposed decomposed products products at 450 at°C 450and◦ 600C and °C are 600 ◦C arecharacterized characterized by the by FE-SEM the FE-SEM images images presented presented in Figure in Figure1. The 4501. The °C reaction 450 ◦C products reaction productsdisplay well- display well-defineddefined primary primary particles particles compared compared to the to theLi2O Li2. 2OHowever,2. However, as-prepared as-prepared particles particles tended tended to to aggregate,aggregate, suggesting that that their their surfaces surfaces were were slightly slightly sintered sintered to produce to produce a massive a massive secondary secondary particle.particle. In In support support of this of this observation, observation, the second the secondaryary particles particles reacted reactedat 600 °Cat displayed 600 ◦C a displayed highly a highlysintered sintered phase and phase it is and difficult it is di toffi cultfind toany find primary any primary particleparticle boundaries. boundaries. Since the Since Li2O2 the was Li 2O2 2 2 2 wassynthesized synthesized by the by thehydrometallurgical hydrometallurgical reaction reaction between between LiOH·H LiOHO Hand2O andH O H 2toO 2generateto generate Li2O2·H2O2·3H2O, contamination with LiOH·H2O or anhydrous LiOH during· vacuum, drying is Li2O2 H2O2 3H2O, contamination with LiOH H2O or anhydrous LiOH during vacuum, drying is inevitable· [19].· Furthermore, it is widely known· that LiOH·H2O and anhydrous LiOH are easily inevitable [19]. Furthermore, it is widely known that LiOH H2O and anhydrous LiOH are easily sintered when transformed to the anhydrous phase and Li2O, respectively,· via the molten phase [20]. sintered when transformed to the anhydrous phase and Li2O, respectively, via the molten phase [20]. Therefore, it is obvious that an unintended Li impurity exists in the prepared Li2O2 and that its Therefore, it is obvious that an unintended Li impurity exists in the prepared Li2O2 and that its presence presence should be attributed to the monolithic phase of a reacted product during the thermal should be attributed to the monolithic phase of a reacted product during the thermal decomposition. decomposition. Figure 1. FE-SEM images of (a) Li2O2 and its decomposed products at (b) 450 ◦C and (c) 600 ◦C. Figure 1. FE-SEM images of (a) Li2O2 and its decomposed products at (b) 450 °C and (c) 600 °C. For further investigation, XRD and TGA analyses were conducted on the prepared materials. For further investigation, XRD and TGA analyses were conducted on the prepared materials. The XRD spectra of the materials prepared at 350, 450, 600 C are presented in Figure2a. The materials The XRD spectra of the materials prepared at 350, 450, 600 °C◦ are presented in Figure 2a. The materials werewere exposedexposed to individual reaction reaction temperature temperature for for 60 60 min. min. The The red, red, navy navy and