Dalhousie researchers find low-voltage NMC532 cells have higher energy density than LFP, very long lifetime
Researchers led by Professor Jeff Dahn at Dalhousie University in Canada report that low-voltage NMC cells—particularly those balanced and charged to 3.8V rather than ≥4.2V—show better coulombic efficiency, less capacity fade and higher energy density compared to LFP cells and are projected to yield lifetimes approaching a century at 25 °C. An open-access paper on their work is published in the Journal of The Electrochemical Society.
Single crystal Li[Ni0.5Mn0.3Co0.2]O2//graphite (NMC532) pouch cells with only sufficient graphite for operation to 3.80 V (rather than ≥4.2 V) were cycled with charging to either 3.65 V or 3.80 V to facilitate comparison with LiFePO4//graphite (LFP) pouch cells on the grounds of similar maximum charging potential and similar negative electrode utilization.
The NMC532 cells, when constructed with only sufficient graphite to be charged to 3.80 V, have an energy density that exceeds that of the LFP cells and a cycle-life that greatly exceeds that of the LFP cells at 40 °C, 55 °C and 70 °C. Excellent lifetime at high temperature is demonstrated with electrolytes that contain lithium bis(fluorosulfonyl)imide (LiFSI) salt, well beyond those provided by conventional LiPF6 electrolytes.
… Overall, low voltage NMC532 cells exceed LFP cells in lifetime and volumetric energy density. This should warrant use consideration where the energy density of LFP cells is insufficient and the device lifetime is more important than initial costs. This does not immediately dismiss LFP cells as a viable storage technology, as it is believed that initial cost and safety would remain superior.—Aiken et al.
Batteries containing NMC positive electrodes are typically high energy cells, owing to the high specific capacity of the materials and high average voltage. Higher voltage operation leads to more oxidizing conditions, can limit lifetime and can lead to stability and safety concerns in the charged state.
Further, acquisition of the transition metals required to synthesize NMC materials, particularly cobalt, is accompanied by ethical complications and considerable financial expense.
On the other hand, LFP (LiFePO4) cells provide less energy, due to the material having both smaller specific and volumetric capacity, as well as lower average voltage, but have superior stability and safety characteristics compared to NMC.
A large portion of the capacity offered by an NMC positive electrode is accessed by charging to higher voltages, beyond 4.0 V vs Li+/Li. At high voltages, however, positive electrode failure modes, including irreversible phase transitions, electrolyte oxidation and particle cracking, become more likely and can significantly reduce cell lifetime.
As such, NMC containing cells are conventionally designed with 4.3 V vs Li+/Li, or 4.2 V vs graphite as the maximum intended charging voltage and only contain enough negative electrode capacity for charging up to this specified voltage (with some safety margin). This leaves unutilized lithium in the positive electrode but preserves longevity. Any use of the cell to lower charging voltages in this configuration results in unutilized graphite.
LFP, being a two-phase material cannot deliver voltage-metered capacity and LFP containing cells are normally built with exactly enough negative electrode capacity to match the positive electrode capacity (again with some margin). Because LFP is operated at a lower voltage, failure modes like electrolyte oxidation are less likely.—Aiken et al.
The Dalhousie study detail the performance of NMC-containing cells balanced for operation to lower-than-conventional voltages and compares these cells with LFP cells. Similar positive electrode voltages and full negative electrode utilization are achieved in both cell types. The researchers found that NMC-containing cells built in this fashion are superior alternatives that can be used in some applications that typically target LFP-containing cells.
The team noted that in addition to superior lifetime compared to LFP cells, the use of NMC materials in cells balanced to low voltages present a multitude of opportunities for improved Li-ion batteries. These represent opportunities for future work and may include:
Available charging strategies to overcome capacity loss from electrode slippage and further increase lifetime.
Superior impedance characteristics for fast charging.
Better electrochemical compatibility with liquid electrolytes designed for fast charging.
Option to use high nickel, low cobalt (i.e. Li[Ni0.8 Mn0.1 Co0.1 ]O2 ) or cobalt-free materials (i.e. Li[Ni0.95 Mn0.05 ]O2 ) without suffering from structural degradation at higher voltages.
Potential improved performance in blended LFP + NMC positive electrodes.
It is easy to dismiss the results presented here, as it seems obvious that lower voltage operation of NMC532 cells should yield better lifetime than operation at higher voltage, and any lifetime or energy gains may be offset by the low price of LFP materials. Applications that require immense cycle life, such as stationary energy storage, EV batteries serving as vehicle-to-grid storage or battery leasing services can benefit from such cell designs as cost per unit of lifetime energy throughput is definitely superior for low voltage NMC compared to LFP cells. The initial cost imbalance can possibly be alleviated as recycling efforts are improved and lower the economic pressure on certain resources, such as cobalt and nickel.—Aiken et al.
C.P. Aiken et al. (2022) “Li[Ni0.5Mn0.3Co0.2]O2 as a Superior Alternative to LiFePO4 for Long-Lived Low Voltage Li-Ion Cells,” Journal of The Electrochemical Society doi: 10.1149/1945-7111/ac67b5