High resolution TEM/SAED structural characterization revealed that Al₂O₃ coatings prevented surface-initiated layered-to-spinel phase transitions in coated materials which were prevalent in uncoated materials. The ability to mitigate degradation mechanisms for Ni-rich NMC and NCA provides insight into a method to enable the performance of high-voltage Li-ion batteries, they concluded.

The energy density of current lithium-ion batteries (LIBs) based on layered LiMO₂ cathodes (M = Ni, Mn, Co: NMC; M = Ni, Co, Al: NCA) needs to be improved significantly… Common high-energy commercial layered-structure cathodes like NMC and NCA achieve specific capacities in the range 150–200 mAh/g with moderate charge upper cutoff voltages (UCV) ~4.2 V; however using higher voltages to realize higher utilization of these materials towards their theoretical capacities ~275 mAh/g causes increased rate of capacity fade and resistance growth (i.e., lower cycle life) symptomatic of cathode-based degradation mechanisms including phase transitions, particle amorphization/pulverization, transition metal dissolution, and electrolyte decomposition.

Electrolyte decomposition at the cathode surface at high voltage, leading to surface passivation and charge transfer impedance growth with negative impact on cycle life, is highly emphasized in the literature; however, the significant contribution of phase transitions to impedance growth has not been as widely recognized, and indeed may be more dominant at higher voltage. … Collectively, the phase transitions create grain boundaries and block Li diffusion pathways and intercalation sites, which cause a charge-transfer impedance increase, and capacity fade. In order to achieve reasonable cycle life utilizing the high capacities gained via high voltage, phase instability is a key problem that must be addressed.

…Here we demonstrate the effects of ALD Al₂O₃ and TiO₂ surface coatings on the structure stability and electrochemical performance of layered Ni-rich NMC81122 (referred to as NMC here onwards) and NCA, in 2 Ah pouch cells.

—Mohanty et al.

For the study, the team fabricated 2 Ah pouch cells (95 × 64 mm format) with ALD-coated and uncoated NMC and NCA cathode materials at ALT Energy’s R&D facility for electrochemical performance and cycling tests with 4.35 and 4.4 UCV, respectively. XALT Energy’s experience is that 95 × 64 mm pouch cell performance projects reliably to larger formats such as 255 × 255 mm.

Broadly, they found that:

1. Al₂O₃ coating enhanced capacity retention of NMC-based cells during low and high rate cycling, whereas TiO₂ coating caused accelerated capacity fade of NMC-based cells during high rate cycling;

2. Al₂O₃ and TiO₂ coatings each enhanced the capacity retention of NCA-based cells during high rate cycling;

3. Al₂O₃ coating remained a distinct phase on NMC and NCA particles after cycling at 4.35 V/4.4 V UCV (for NMC and NCA, respectively), preserving the particle surface from phase transformation, associated with a significantly lower rate of charge transfer impedance growth of both NMC and NCA electrodes during 1 C/−1 C cycling; and

4. changes in thickness and uniformity of TiO₂ coatings on NMC and NCA particle surfaces were observed after cycling at 4.35 V/4.4 V UCV, while the thickness and uniformity of Al₂O₃ coatings remained relatively unchanged under these conditions.

The results … show that ALD surface coating, particularly Al₂O₃ coating, stabilized the NMC and NCA phase structures at particle surfaces, causing slower charge transfer impedance growth in cathode electrodes and higher capacity retention of cells with both NMC and NCA cathodes during high rate cycling. … is work highlights the dominant effects of surface chemistry on active material performance. Continued development of surface coatings promises to open new pathways to tune properties and performance of a wide range of active materials. The new functionality of surface coatings to stabilize surface structure of Ni-rich NMC811 and NCA demonstrated in this study appears to have greater impact than metal composition or distribution, and is critical for the ability to cycle these Ni-rich materials with higher upper cutoff voltages to achieve significantly greater energy densities without sacrifice to cell cost or cycleet al. life.

—Mohanty et al.

XALT is currently building prototype large format 255 mm × 255 mm cells using a scaled-up lot of Al₂O₃ ALD-coated NMC811 material; these cells are projected to have an energy density exceeding 500 Wh/L.

Resources

• Debasish Mohanty, Kevin Dahlberg, David M. King, Lamuel A. David, Athena S. Sefat, David L. Wood, Claus Daniel, Subhash Dhar, Vishal Mahajan, Myongjai Lee & Fabio Albano (2016) “Modification of Ni-Rich FCG NMC and NCA Cathodes by Atomic Layer Deposition: Preventing Surface Phase Transitions for High-Voltage Lithium-Ion Batteries” Scientific Reports 6, Article number: 26532 doi: 10.1038/srep26532