Unveiling Capacity Degradation Mechanism of Li‐ion Battery in Fast‐charging Process

Abstract

In this work, capacity degradation in fast-charging process is studied by using a power-optimized graphite-Li-Ni 0.80 Co 0.15 Al 0.05 O 2 (NCA) electrode couple and a three-electrode coin cell as the testing vehicle. It is shown that capacity degradation consists of two distinct stages, (1) rapid degradation in early period and (2) progressive degradation over lifetime, which are associated with the electrochemical reduction of electrolyte solvents on the graphite anode and the structural deterioration of the NCA cathode, and are well correlated to a decrease in Coulomb efficiency. Analyses on the change in differential capacity profile and the recoverability of the lost capacity reveal that capacity degradation mainly originates from structural deterioration of the NCA cathode over lifetime, and it aggravates with the state-of-charge and temperature. Based on this guideline, we show that capacity degradation of the Li-ion batteries in the fast-charging applications can be greatly mitigated by limiting the cell's operation to a relatively low state-of-charge (i. e., a relatively low charging cut-off voltage).

Challenges and Strategies for Fast Charge of Li‐Ion Batteries

“…Although Li plating has been overwhelmingly considered the culprit of fast charge problems, it was found that even if the potential of the graphite anode never drops below 0 V vs. Li/Li + during 10 C‐charging in power‐optimized Gr/NCA cells, the accelerated performance degradation is still unavoidable. Similarly, Mussa et al reported that energy‐optimized batteries (25 Ah, Gr/NMC111) release large amounts of gas, accompanied by severe graphite exfoliation, as the batteries are charged at 4 C. The above results point to the fact that under fast charge, the existing SEI on the graphite anode is unable to protect electrolyte solvents from electrochemical reduction.…”

“…Therefore, effective additives for the layered cathode are mainly centered on the compounds that can react with the alkaline Li residual compounds, such as sulfates & sultones, phosphates & phosphites, and borates . As pointed out in Section 3.1, electrolyte solvents are subject to electrochemical reduction on the graphite anode under fast charge . In this view, the electrolyte additives are of particular significance in mitigating capacity fade and stabilizing coulombic efficiency under fast charge.…”

“…In particular, the white solid residues on the graphite anode were identified by XRD to be MCO 3 , MF 2 (M=Ni and Co), LiOH, Al−Li alloys, and other XRD unidentifiable organic polymers, indicating that severe dissolution of transition metal ions from the NCA cathode occurred during the 2 C CCCV cycling. Also, the M 2+ ion dissolution (in relation to the loss of oxygen) and resulting structural degradation of NCA were independently observed from Gr/NCA coin cells after 200 cycles by 10 C CCCV charging . In fact, the severest safety risk for fast charge would be the heat and gas generation, which triggers cell venting, so that hot flammable gas and electrolyte vapor release and catch fire once they meet an ignition source.…”

“…The capacity fade in Li‐ion batteries under fast charge typically shows a rapid loss in early cycling, followed by a decelerated rate in later cycles . Interestingly, the rapid capacity loss is always accompanied by low coulombic efficiencies even without Li plating due to the electrochemical reduction of the electrolyte solvents . A good charging protocol should account for the chemistry of the batteries, and be able to smartly adjust current with the SOC.…”

“…Therefore, the SOC is a better parameter than voltage to be used for terminating the charging process. For example, Zhang found that the capacity retention of Gr/NCA cells in 10 C CCCV‐charging cycling can be dramatically stabilized by lowering the charging cutoff voltage to 4.0 V from the normal 4.2 V. Similarly, Yang et al demonstrated that high energy Gr/NMC532 pouch cells (9.5 Ah, 170 Wh kg −1 ) can retain 91.7 % of the original capacity after 2,500 cycles at a 10‐min charging rate by limiting the capacity to 80 % SOC and charging the battery at ∼60 °C. The above facts reveal that both the large internal resistance at low SOCs and the chemical and structural instability of the layered cathode materials at high SOCs should be taken into account for an ideal fast‐charging protocol.…”