Structural and Electrochemical Study of the Li–Mn–Ni Oxide System within the Layered Single Phase Region

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Single-crystal LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532) with a grain size of 2-3 μm was compared to conventional polycrystalline uncoated NMC532 and polycrystalline Al 2 O 3 -coated materials in this work. Studies were made to determine how single crystal NMC532 material with large grain size could be synthesized. Ultra high precision coulometry (UHPC), in-situ gas measurements and isothermal microcalorimetry were used to make comparative studies of the three materials in Li-ion pouch cells. All the diagnostic measurements suggested that the single crystal material should yield Li-ion cells with longer lifetime. Long-term cycling tests verified these predictions and showed that cells with single crystal NMC532 exhibited much better capacity retention than cells with the polycrystalline materials at both 40 • C and 55 • C when tested to an upper cutoff potential of 4.4 V. The reasons for the superior performance of the single crystal cells were explored using thermogravimetric analysis/mass spectrometry experiments on the charged electrode materials. The single crystal materials were extremely resistant to oxygen loss below 100 • C compared to the polycrystalline materials. The major drawback of the single crystal material is its slightly lower specific capacity compared to the polycrystalline materials. However, this may not be an issue for Li-ion cells designed for long lifetime applications. Lithium ion batteries with high energy density, long lifetime and low cost need to be developed for applications in electric vehicles and stationary energy storage. The family of Li(Ni x Mn y Co z )O 2 (x + y + z = 1) (NMC) materials with high nickel and low cobalt are used as positive electrode materials in lithium ion cells.1,2 One simple way to increase the energy density of NMC lithium ion cells is to increase their upper cutoff voltage which gives access to higher specific capacity from the positive electrode.3,4 However, increasing the upper cutoff voltage usually decreases the lifetime of cells due to an acceleration of 'unwanted' parasitic reactions between the electrolyte and the delithiated positive electrode surface at high voltages. Such reactions include oxidation of species found in the electrolyte, transition metal dissolution, etc. [5][6][7] In addition, structural reconstruction of the positive electrode surface can occur which can contribute to impedance growth and capacity loss. 3,4 The by-products of oxidation at the positive electrode can migrate to the negative electrode surface and be reduced there. 8,9 Such reactions can lead to the consumption of lithium ions from the electrolyte, (to maintain charge neutrality in the electrolyte), a reduction in lithium inventory, as well as a thickening of the negative electrode solid electrolyte interface (SEI) which together ultimately cause cell-failure.10,11 These processes are accelerated by higher charging potentials and higher temperatures.Methods such as modification of the positive electrode surface with coatings or dopants 12,13 and/or modification of electr...

Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Comparison of Single Crystal and Polycrystalline LiNi_0.5Mn_0.3Co_0.2O₂Positive Electrode Materials for High Voltage Li-Ion Cells

Cameron

et al. 2017

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286

“…Impact of lithium to transition metal (Li/TM) ratio.-The phase diagram of the Li−Mn−Ni-oxide pseudo-ternary system has been extensively studied by by E. McCalla et al 13 and by J. Li et al 10 The phase diagram of the Li−Ni−Mn−Co oxide pseudo-quaternary system has been extensively studied by C. Brown et al 14 In those studies it was demonstrated that the single layered phase region in the Gibbs diagram of the Li-Ni-Mn-O pseudo-ternary and Li-Ni-Mn-Co oxide pseudo-quaternary systems are large regions, in which samples with a Ni: Mn: Co ratio of 5: 3: 2 can be written as Li 1+x M 1-x O 2 (M = Ni 0.5 Mn 0.3 Co 0.2 ). The Li/TM ratio is equal to (1+x)/(1-x).…”

Section: Resultsmentioning

confidence: 99%

“…The details of the synthesis of similar precursors has been described by J. Li et al 10,11 Precursors with three different sizes, labelled as large (L), medium (M) and small (S), were prepared individually by adjusting the pH and stirring rate. Unless specified, precursor L was * Electrochemical Society Fellow.…”

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confidence: 99%

Synthesis of Single Crystal LiNi_0.5Mn_0.3Co_0.2O₂for Lithium Ion Batteries

Stone

et al. 2017

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Single crystal Li[Ni 0.5 Mn 0.3 Co 0.2 ]O 2 materials in NMC532/artificial graphite cells have excellent long term charge-discharge cycle lifetime which greatly exceeds that of conventional NMC532 materials. There are a few patents from industry regarding the synthesis of single crystal NMC. In addition, there have only been a few reports in the academic literature showing that single crystal NMC with a grain size of ∼2-5 μm having good electrochemical performance was successfully synthesized, but these workers used complex approaches. This work systematically studies the steps required to synthesize single crystal NMC materials. The key synthesis steps including the impact of the Li to transition metal ratio, sintering temperature, precursor size and sintering time are discussed. This work provides guidance for the synthesis of single crystal NMC positive electrode materials that may be suitable for lithium-ion cells with high energy density and long lifetime. 1, the authors briefly discussed the synthesis method for single crystal materials, while the detailed information regarding the impact of the steps required to synthesize single crystal NMC materials was not discussed. In this work, the key synthesis steps including the impact of lithium to transition metal (Li/TM) ratio (molar), sintering temperature, sintering time and precursor size will be discussed. The optimization of the final synthesis conditions and evaluation of the electrochemical stability of the materials is not the main focus of this paper. However, the initial electrochemical properties, such as reversible specific capacity and irreversible specific capacity, of the best materials in this report are certainly competitive to commercial NMC532 materials. ExperimentalReagents used for the synthesis of single crystal NMC532 included nickel (II) sulfate hexahydrate (NiSO 4 • 6H 2 O, 98%, Alfa Aesar), manganese sulfate monohydrate (MnSO 4 • H 2 O, 98%, Alfa Aesar), sodium hydroxide (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%) and lithium hexafluorophosphate (LiPF 6 , BASF, purity 99.9%, water content 14 ppm).Synthesis of single crystal NMC532.-Ni 0.5 Mn 0.3 Co 0.2 (OH) 2 precursors were prepared via co-precipitation in a continuously stirred tank reactor (CSTR) (Brunswick Scientific/Eppendorf BioFlo 310). 9The details of the synthesis of similar precursors has been described by J. Li et al. 10,11 Precursors with three different sizes, labelled as large (L), medium (M) and small (S), were prepared individually by adjusting the pH and stirring rate. Unless specified, precursor L was * Electrochemical Society Fellow.z E-mail: jeff.dahn@dal.ca used for the synthesis of lithiated samples. The dried precursors were mixed with a stoichiometric equivalent of Li 2 CO ...

“…The layered lithium Ni-Mn-Co oxides Li 1+x (Ni y Mn z Co (1-y-z) ) 1-x O 2 (NMC) are considered to be promising positive electrode materials. [15][16][17][18][19][20][21][22][23] In our previous report 24 and in numerous literature reports, 6,14,16,17,19,[25][26][27][28][29][30][31][32] it has been shown that the electrochemical performance of these lithium-rich layered materials is dependent on their structure and composition. The lithium-rich manganese-rich NMC materials with excess lithium in the transition metal layer can have extraordinary high specific capacity of more than 250 mAh/g with an average potential of ∼3.6 V (vs. Li + /Li) after an irreversible oxygen release activation process at ∼4.5 V (vs. Li + /Li).…”

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confidence: 99%

Study of the Failure Mechanisms of LiNi_0.8Mn_0.1Co_0.1O₂Cathode Material for Lithium Ion Batteries

Downie

et al. 2015

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372

LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) can deliver a high capacity of ∼200 mAh/g with an average discharge potential of ∼3.8 V (vs. Li + /Li), making it a promising positive electrode material for high energy density lithium ion batteries. However, electrochemical tests from half cells and full cells show poor cycling performance when charged to potentials above 4.2 V. The calendar and cycle lifetimes of cells are affected by the structural stability of the active electrode materials as well as the parasitic reactions that occur in lithium ion batteries. In order to explore the major failure mechanisms of the material, half cells (coin cells) with control electrolyte and full cells (pouch cells) with control electrolyte and with selected electrolyte additives were tested over four different potential ranges. Isothermal microcalorimetry was used to explore the parasitic reactions and their potential dependence. In-situ and ex-situ X-ray diffraction and scanning electron microscopy were used to investigate the structural and morphological degradation of the materials over cycling. It was found that the dramatic c-axis change of the active material during charge and discharge may not be the major problem for cells that are cycled to higher potentials. The parasitic reactions that arise from the interactions between the electrolyte and the highly reactive delithiated cathode surface at high potentials are suggested as the main reason for the failure of cells cycled above 4.2 V. It should be possible to further improve the performance of NMC811 at high potentials by modifying the cathode surface and/or identifying and using electrolyte blends which reduce parasitic reactions. High energy density lithium ion batteries (LIBs) that are cheaper, safer, and with longer lifetimes need to be developed in order to meet the increasing demand for applications such as electric vehicles and large scale stationary energy storage. The energy density of LIBs can be increased by increasing the specific capacity and average potential of the cells. The calendar and cycle lifetimes of cells are affected by the structural stability of the active electrode materials as well as the parasitic reactions that occur in lithium ion batteries. The degree of lithium utilization of LiCoO 2 is limited to ∼70% in order avoid the O3 -H1-3 -O1 phase transformation when charged above 4.45 V.1 Parasitic reactions such as electrolyte oxidation at the cathode-electrolyte interface can ultimately cause cell failure.2-5 The rate of the parasitic reactions is related to both the catalytic role of the cathode surface which depends on its composition and surface area, 3,6 as well as on the stability of the electrolyte.2-5 Methods such as the use of electrolyte additives 7-11 and core-shell positive electrode materials [12][13][14] have been developed and studied to reduce the rate and extent of parasitic reactions, and hence increase capacity retention and lifetime of high-voltage Li-ion cells. The layered lithium Ni-Mn-Co oxides Li 1+x (Ni y Mn z Co (1-y-z) ) 1...