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

Abstract

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...

Quantifying Changes to the Electrolyte and Negative Electrode in Aged NMC532/Graphite Lithium-Ion Cells

“…20 This is certainly an underestimate of the depth from which transition metals might dissolve in the NMC532 particles. If 10% of the transition metals in near-surface layers were to dissolve, then this might impact a depth of 5 nm or so which is consistent with many TEM images of reconstructed surfaces on NMC particles after testing.…”

“…31 Additionally, Li et al found that NMC532/graphire pouch cells with single crystal positive electrode materials, as used in this study, had longer cycling lifetime and better performance than cells with polycrystalline materials. 32 Figure 12 suggests that further reductions in transition metal dissolution and deposition at the negative electrode may have little impact on lifetime, however, more data points are needed, particularly at larger capacity losses, to make an adequate comparison and prediction. Figure 9b showed that the difference in average charge and discharge voltage ( V) increased with cycle number more rapidly as the upper cutoff potential increased.…”

“…These positive electrodes in these cells used single crystal Li [ 20 with 94% NMC532, 4% conducting diluents, and 2% polyvinylidine fluoride (PVDF) binder. The negative electrode consisted of 96% artificial graphite (15-30 μm particle size), 2% carbon black, 2% sodium carbon methylcellulose (NaCMC)/styrene butadiene rubber (SBR) binder.…”

Development of Electrolytes for Single Crystal NMC532/Artificial Graphite Cells with Long Lifetime

“…The negative electrodes had compositions of 95.2% active material, 2% carbon black with the remainder being carboxymethyl cellulose (CMC)/styrene butadiene (SBR) binder. The NMC532 used was called SC532 in an earlier publication by Li et al 22 The positive electrode loading was 21.3 mg/cm 2 and the electrodes were calendared to 3.5 g/cm 3 . The positive electrodes were 94% by weight active material with 4% conducting carbon additives and 2% by weight PVDF binder.…”

“…[10][11][12][13][14][15][16][17][18][19][20][21] Recently, J. Li et al showed that single crystal Li[Ni 0.5 Mn 0.3 Co 0.2 ]O 2 (NMC532) materials in NMC532/artificial graphite cells can have excellent long term lifetime with an electrolyte consisting of 2 wt% prop-1-ene-1,3 sultone (PES) + 1 wt % ethylene sulfate (DTD) + 1 wt% tris (trimethylsilyl) phosphite (TTSPi) in 1 M LiPF 6 in ethylene carbonate: ethyl methyl carbonate (3:7 by weight) (this electrolyte is called PES211 here). 22 During testing to 4.4 V at 40…”