IEEE Access Special Section Editorial

Research output: Contribution to journalEditorial

Abstract

Single crystal LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532) was shown to have superior stability at high voltages and elevated temperatures compared to conventional polycrystalline NMC532 by the authors. Conventional LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) usually offers more capacity than NMC532 when charged to the same upper cutoff voltage so NMC622 is attractive. It is expected that single crystal NMC622 could also provide better performance than typical polycrystalline NMC622 materials. This work explores the synthesis of single crystal LiNi 0.6 Mn 0.2 Co 0.2 O 2 and preferred synthesis conditions were found. A washing and reheating method was used to remove residual lithium carbonate after sintering. The synthesized single crystal NMC622 material worked poorly after the washing-heating treatment without the use of electrolyte additives in the electrolyte. However, with selected additives, single crystal cells outperformed the polycrystalline reference cells in cycling tests. It is our opinion that single crystal NMC622 has a bright future in the Li-ion battery field. The rapid development of electric vehicle industry is increasing the demand for cost-effective lithium ion batteries with longer lifetime and higher energy density. The family of Li[Ni x Mn y Co z ]O 2 (x + y + z = 1) (NMC) positive electrode materials with higher Ni content and lower Co content is attractive due to their high capacity and lower cost. The excellent recent review article on NMC materials by Noh et al. 1 summarizes the advantages and disadvantages of the various NMC grades. In both industry and academia, many valuable efforts have been made to make improved NMC materials. For example, Aru-mugam et al. 2 showed that an Al 2 O 3 coating on LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) can act synergistically with carefully designed electrolytes at 4.4 V. Aurbach et al showed that Al doping of LiNi 0.5 Co 0.2 Mn 0.3 O 2 can provide higher structural stability and less capacity fade. 3 J. Li et al. showed that single crystal Li[Ni 0.5 Mn 0.3 Co 0.2 ]O 2 materials in NMC532/artificial graphite cells have superior performance compared to their polycrystalline counterparts. 4 T. Kimijima et al. 5 showed that single crystal NMC111 could be synthesized using a molten salt method, and a similar method was also applied by Y. Kim et al. 6 to synthesize single crystal NMC811. J. Li et al. 7 introduced a method to synthesize single crystal NMC532 and explored the impact of key synthesis parameters. It was found that the lithium/transition metal molar ratio and sintering temperature played important roles in the synthesis. Excess lithium can provide a flux-type environment which facilitates particle growth. 8 High sintering temperature also assists the particle growth significantly. NMC622 has higher specific capacity and better rate capability compared to NMC532 at the same voltage, 1 which makes it very attractive. In this work, the synthesis of single crystal NMC622 is explored with the goal of attaining optimized electrochemical performance. For conventional polycrystalline NMC material synthesis, different NMC ratios require different synthesis conditions. 1 There are no publications about single crystal NMC622 synthesis that have been reported in the scientific literature. Although Wang et al. 9 reported the synthesis of single crystal NMC622, their synthesized materials were made up of agglomerates with individual grain size less than 1 μm. In our view, single crystal materials should be individual grains of greater than 2 μm. The electrochemical properties of single crystal NMC622 (SC622) were compared with polycrystalline NMC622 (PC622) made with conventional methods. Experimental Reagents used for the synthesis of NMC622 included nickel (II) sulfate hexahydrate (NiSO 4 • 6H 2 O, 98%, Alfa Aesar), manganese sulfate monohydrate (MnSO 4 • H 2 O, 98%, Alfa Aesar), cobalt sul-fate heptahydrate (CoSO 4 • 7H 2 O, 98%, Alfa Aesar), sodium hy-droxide (NaOH, 98%, Alfa Aesar), ammonium hydroxide (NH 4 OH, 28.0-30.0%, Sigma-Aldrich). All aqueous solutions used in the precursor synthesis were prepared with deionized (DI) water which was de-aerated by boiling for 10 minutes. Reagents used for coin cells included 1:2 v/v ethylene carbonate: diethyl carbonate (EC:DEC, BASF, purity 99.99%), fluoroethylene carbonate (FEC. BASF, purity 99.94%), ethylene sulfate (DTD, Sigma Aldrich, purity 98%) and lithium hexafluorophosphate (LiPF 6 , BASF, purity 99.9%, water content 14 ppm). Synthesis of single crystal NMC622 and polycrystalline NMC622.-Ni 0.6 Mn 0.2 Co 0.2 (OH) 2 precursors were made using co-precipitation in a continuously stirred tank reactor (CSTR) (Brunswick Scientific/Eppendorf BioFlo 310). More details of precursor synthesis has been reported by J. Li et al. 10 The dried precursors were mixed with a stoichiometric equivalent of Li 2 CO 3 (from Chemet-all > 99%) by ball milling using a mixer miller (SPEX Certi Prep 8000-D), which had been modified to mix at low speed, for 30 minutes with a beads/material mass ratio of 1.5. Samples with lithium/transition metal molar ratios (Li/TM ratios) of 1.05, 1.1 and 1.15 were prepared. The mixed powders were sintered in air in a box furnace at 800 • C for 3 hours, and further at 900, 925, 940 or 955 • C for 6 hours, with a same heating and cooling rate of 10 • C/min. Sin-tered single crystal materials were "brick"-like chunks and needed to be ground and sieved. Further work is ongoing to eliminate the formation of the large chunks requiring grinding after sintering. Selected synthesized single crystal NMC622 materials were washed with deionized (DI) water to remove the excess lithium car-bonate residue. 2 g of synthesized powders were added to 8 ml of DI water and centrifuged for 10 minutes 3 three times. The washed powders were dried at 80 • C for 12 hours and were further heated in an oxygen environment at 550 • C for 3 hours. The heating after washing was done based on the work reported by Chen and Dahn, 11 who showed that LiCoO 2 exposed to water could be "repaired" by a heat-treatment to 550 • C. Scanning electron microscopy imaging (SEM).-SEM imaging was conducted using a Nano Science Phenom Pro G2 Desktop Scanning Electron Microscope with a backscattered electron detector. Samples were prepared by mounting the powders onto adhesive carbon) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.179.254.153 Downloaded on 2018-08-07 to IP
Original languageEnglish
Pages (from-to)8959-8963
Number of pages5
JournalIEEE Access
Volume6
DOIs
Publication statusPublished - 01 Jan 2018

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