Supporting Information

Ni-Rich LiNi0.88Mn0.06Co0.06O2 Cathode Interwoven

by Carbon Fiber with Improved Rate Capability and Stability

Dong Ren,† Yao Yang,† Luxi Shen, Rui Zeng, Héctor D. Abruña*

Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States.

†D. R. and Y. Y. contributed equally to this work

*Corresponding author: hda1@cornell.edu

Experimental Section

Sample preparation

The Ni0.88Mn0.06Co0.06(OH)2 precursor was synthesized via co-precipitation of NiSO4•6H2O, CoSO4•7H2O and MnSO4•H2O as starting materials, dissolved in distilled water in molar ratios of Ni: Co: Mn = 44:3:3. A 1.5 M aqueous solution of the transition metal sulfates was slowly pumped into a continuously stirred tank reactor. Concomitantly, a 4 M NaOH solution (aq) and a 2.5 M NH4OH solution (aq) were also separately pumped into the reactor. After reaction, the precipitate was filtered and washed several times with water to remove residual ions (Na+, SO42− or other ions). The precipitate was then dried at 100 oC overnight. The resulting hydroxide precursor Ni0.88Mn0.06Co0.06(OH)2 was mixed with LiOH•H2O and calcined at 700 oC for 15 h in oxygen to obtain the pristine LiNi0.88Mn0.06Co0.06O2 (pristine NMC) cathode material.

Pristine NMC interwoven with carbon fibers was achieved as follows. First, 0.5 g of carbon fibers were added to 50 mL of isopropanol with stirring for 15 min. Then, 9.5 g of pristine NMC powders were dispersed in the solution with stirring for 1 h under nitrogen atmosphere. After removing the solvents by filtration, the materials were dried at 80 oC in a vacuum oven for 12 h and then heated at 200 oC for 3 h in a furnace under a nitrogen atmosphere to obtain carbon fiber interwoven NMC (CF-NMC).

Sample characterization

X-ray diffraction patterns were obtained with a Rigaku Ultima VI powder X-ray diffractometer with Cu K radiation (Kα1, λ= 1.5406 Å and Kα2, λ = 1.5444 Å). Diffraction patterns were collected at a scanning rate of 2 o/min, and with a step of 0.02 o. Particle size and size distribution were measured using a Bettersize BT-9300ST Laser Particle Size Analyzer. SEM was performed with a LEO-1550 field emission SEM (FSEM). FEI Strata 400 Focused Ion Beam (FIB) was used to mill the spherical NMC particles into 1 µm cross section at a milling voltage of 30keV. The NMC spherical particle was mounted on a Cu TEM grid by Pt deposition. The final ultrathin cross-sectional TEM specimens (< 20 nm) were prepared with a final milling step of 2 keV to minimize surface damage. Energy dispersive X-ray spectroscopy (EDX) elemental mapping was performed using an Oxford X-Max detector. High-resolution STEM images were acquired on a fifth-order aberration-corrected FEI Titan Themis STEM operated at 300 keV.

Electrochemical tests

Electrochemical measurements were carried out in CR 2032 coin cells assembled in an argon filled glove box (residual water and oxygen were 0.5 ppm and 0.1 ppm, respectively) with lithium metal as the anode. The working cathode was fabricated with 90 wt% active material, 5 wt% super P carbon, and 5 wt% poly(vinylidene fluoride) (PVDF) as binder. For the CF-NMC, the carbon fiber was included in the 5% carbon source and the mass content of carbon fiber in CF-NMC composite was determined by thermogravimetric analysis (TGA, Q500, TA instruments), as shown in Fig. S4. The resulting slurry was coated onto an Al foil current collector. The coated electrodes were dried for 12 h at 110 oC in a vacuum oven. The typical mass loading of the active material was 2.0 mg/cm2. The counter electrode was Li metal. The two electrodes were separated by a polypropylene separator (Celgard 3501). The electrolyte was 1.2 M LiPF6 in a 3:7 ratio of EC (ethylene carbonate) to EMC (ethyl methyl carbonate). Galvanostatic charge/discharge of the coin cells was carried out using an Arbin BT-2000 battery tester with a constant discharge/charge current rate of 0.1 C and a voltage range of 2.8 to 4.3 V vs. Li/Li+ at room temperature. Every experiment was carried out in triplicate (three parallel trials), and the error bar of the measured capacities, among the three trials, was less than 0.25%. We report the measured capacities rounded to the last integral digit; in which case, all results were identical. Thus, we do not include error bars in these cases.

Figure S1. SEM image of pristine NMC particles

Figure S2. SEM image of CF-NMC particles

Cathode

2θ/o (003)

2θ/o (104)

a / Å

c / Å

V / Å3

CF-NMC

18.839

44.49

2.876

14.124

101.17

pristine NMC

18.822

44.48

2.876

14.136

101.25

Table S1 calculated lattice parameters and cell volumes of CF-NMC and pristine NMC

Elements

Molar percentage in (Ni+Mn+Co) (%)

Ni

88.12

Mn

6.02

Co

5.86

Table S2 ICP-AES results of pristine NMC material.

Figure S3. XEDS spectrum corresponding to the XEDS elemental mapping in Figure 4.2. The intensities of the Mn, Co, and Ni Kα peaks are consistent with the Ni-rich component shown in ICP-AES analysis

Figure S4 TG analysis of CF-NMC composite performed in air.

S-2