A Comparison of the Performance of Different Morphologies of LiNi_0.8Mn_0.1Co_0.1O₂ Using Isothermal Microcalorimetry, Ultra-High Precision Coulometry, and Long-Term Cycling

Abstract

Ni-rich positive electrode materials for Li-ion batteries have the dual benefit of achieving high energy density while reducing the amount of Co used in cells. However, limitations in cycle life are still an issue for the widespread adoption of these materials. The benefit of using single crystal materials has been demonstrated for LiNi0.5Mn0.3Co0.2O2 (NMC532), LiNi0.6Mn0.2Co0.2O2 (NMC622), and now LiNi0.8Mn0.1Co0.1O2 (NMC811). This work uses long-term cycling, ultra-high precision coulometry (UHPC), and isothermal microcalorimetry to investigate the effect of particle morphology on the lifetime of NMC811/graphite pouch cells. NMC811 with uncoated single crystal (SC) particles, coated polycrystalline (PC) particles, and a composite “bimodal” (BM) material are studied with electrolyte systems that have shown excellent cycle life in other NMC materials. Results from this work show that SC cells have improved cycle life in long-term cycling, as well as higher coulombic efficiency (CE) and lower charge endpoint capacity slippage as seen in the UHPC measurements. This correlates well with the isothermal microcalorimetry results, in which SC cells show the lowest parasitic heat flow over a range of upper-cutoff voltages. This study suggests excellent lifetimes can be achieved in single crystal NMC811/graphite cells with further electrolyte optimization.

Transition Metal Dissolution and Degradation in NMC811-Graphite Electrochemical Cells

“…3) and will undergo fewer electrolyte side reactions at the cathode. 4,16,27 Under these cycling conditions, the capacity fade occurs predominantly via lithium trapping in the SEI layer, as was found in other studies with these electrodes. 25 However, when the capacity fade based on the electrochemistry and cathode SoC measured using diffraction differ significantly, this indicates that the loss of lithium inventory (estimated based on x XRD ) cannot account for the capacity fade fully and there must be additional contributions.…”

Boosting Reaction Homogeneity in High‐Energy Lithium‐Ion Battery Cathode Materials

“…[ 14 ] Furthermore, Dahn et al reported that a morphology consisting of the single particle cathode materials enhanced the thermal stability compared with that of polycrystalline cathode materials, which was attributed to the reduced surface area. [ 30 ] In terms of single‐crystalline NCM, the absence of an anisotropic volume change guarantees morphological integrity, preventing issues originating from intergranular crack generation.…”

Restraining Oxygen Release and Suppressing Structure Distortion in Single‐Crystal Li‐Rich Layered Cathode Materials

“…In contrast, the polycrystal layered Ni‐rich cathode materials generally displayed the rapid capacity decay especially at high voltage and high temperature during long‐term cycling because of the serious crack generation triggered by the accumulation of heterogeneous stress. [ 8 ] Furthermore, the anode side also suffered from the transition metal (TM) reduction reaction on the surface, which can be attributed to the TM dissolution from polycrystal cathode side, further accelerating the cycling performance fade of pouch cells. [ 7a,8,9 ] Therefore, the design of single‐crystal morphology can be regarded as an effective strategy to improve the structural stability and electrochemical performances by means of unique features, including crack‐free, restrained side reactions, and limited gas generations, homogeneous internal stress, etc.…”