Demonstrating Apparently Inconspicuous but Sensitive Impacts on the Rollover Failure of Lithium-Ion Batteries at a High Voltage

Abstract

Layered oxides, such as Li­[Ni0.5Co0.2Mn0.3]­O2 (NCM523), are promising cathode materials for operation at a high voltage, i.e., high-energy lithium-ion batteries. The instability-reasoned transition metal dissolution remains a major challenge, which initiates electrode cross-talk, alteration of the solid electrolyte interphase, and enhanced Li-metal dendrite formation at the graphite anode, consequently leading to rollover failure. In this work, relevant impacts on this failure mechanism are highlighted. For example, a conventional coating of NCM523 with aluminum oxide as a typical high-voltage modification improves kinetic aspects but can only postpone the rollover failure to later charge/discharge cycles. Interestingly, a similar effect on the rollover failure is observed merely after modification of the cell formation protocol, i.e., the first cycles. Further influences of specific test protocols are highlighted and show that the rollover failure even disappears at C-rates above 2C, which can be attributed to a more homogeneous distribution of Li-metal dendrite formation. It is worth noting that a variation of anode porosity can reveal similar effects, as, e.g., variations in anode processing also impact Li dendrite distribution and the appearance of rollover failure. Overall, the rollover failure is a valid but complex phenomenon, which sensitively depends on apparently inconspicuous parameters and should not be disregarded.

Suppressing Electrode Crosstalk and Prolonging Cycle Life in High‐Voltage Li Ion Batteries: Pivotal Role of Fluorophosphates in Electrolytes

“…b) Suppression strategies on material level. Apart from cathode as the initiator, [48,50] particularly the design of electrolytes is an effective strategy aiming at TM scavenging, for example via Li x PO y F z species. [28,49] ChemElectroChem ("EC-free" electrolyte; Sigma-Aldrich, purity: battery grade), (iii) 1 m LiPF 6 in EMC + 1.0 wt% of LiDFP (American Elements; CAS No.…”

“…Even when electrode crosstalk is present, enhanced homogeneity of TM deposition at the anode surface can be also beneficial in terms of a more homogeneous SEI alteration, as it can prevent the detrimental TM‐ and HSAL “island‐like” accumulations, thus minimize the risk of short‐circuits and rollover failure. For example, this can be achieved via a more homogeneous SEI, e. g. with the support of vinylene carbonate (VC) additive, [28] a homogenous separator, e. g. fiber separators, or a higher anode porosity through control of electrode processing [48,57] …”

"Knees" in lithium-ion battery aging trajectories

“…These knees were classified as resistance "pseudo-knees" due to increased electrolyte oxidation on the positive electrode, as evidenced by the strong dependence of the knee severity on discharge rate as well as positive electrode impedance measurements. For electrode design, Ma et al 6 and Klein et al 146 found that positive electrode particle coatings and low positive electrode loadings delayed the knee. Ma et al 6 and Glazier et al 147 also found that the graphite type (i.e., natural or artificial) can substantially impact the knee location; while natural graphite has larger irreversible expansion and thus higher parasitic reaction rates 147 , the root cause of the knee in this case is unclear.…”

“…Rate-independent plating can occur if the ratio of negative electrode loading to positive electrode loading is too low (i.e., n:p < 1); 47 however, rate-dependent lithium plating can occur at low loading ratios if the negative electrode is too thick or the porosity is too low. 19,128,146 Additionally, small changes in the electrolyte can play an outsized role on the lifetime performance. Ma et al 6 demonstrated the sensitivity of the knee location to the electrolyte additive mixture; specifically, high methyl acetate (MA) concentrations (MA is used to increase electrolyte transport capability) and low LiPF 6 concentrations consistently led to earlier knees.…”

“…For instance, Weng et al 149 found that NMC/graphite cells formed with a fast formation protocol that emphasizes time at high SOCs exhibited later knees than cells formed with a slower baseline formation protocol, which was attributed the creation of well-passivating SEI at high potentials to decrease the amount of side reaction product formed during use. 150 Klein et al 146 found that decreasing the upper cutoff voltage in formation from 4.5 V to 4.3 V delayed the knee in NMC/graphite cells, which was attributed to decreased transition metal dissolution to cause plating. In principle, the formation protocol can be optimized to avoid or delay knees for a given use case.…”