Fundamental degradation mechanism of Ni-rich layered cathodes on Lithium-ion batteries
The main strategy for increasing the discharge capacity of Li[NixCoyMn1−x−y]O2 (NCM) cathodes has been a progressive increase of the Ni fraction; however,
this causes a proportional deterioration of the cathode’s ability to retain its original capacity during cycling. The relatively inferior cycling stability of NCM with x > 0.8 is
attributed to the phase transition near the charge-end. Stress stemming from the H2 to H3 phase transition destabilized the internal microcracks and allowed the
microcracks to propagate to the surface, providing channels for electrolyte penetration and subsequent degradation of the exposed internal surfaces.
Concentration gradient cathode materials for advanced lithium-ion batteries
Multicompositional particulate gradient Li[NixCoyMn1−x−y]O2 (NCM) cathodes in which Ni-rich part at the particle center (to extract maximum capacity) is encapsulated
by a concentration gradient shell with the highly stable outermost Ni-less surface are proposed. NCM cathodes with concentration gradients represent a viable solution
that simultaneously addresses the specific energy density, cycling and chemical stability, and safety issues of Ni-enriched NCM cathodes. Currently, 4G gradient cathode
(TSFCG) have been developed, and 5G development is in progress.
Microstructurally modified cathodes
Primary particle morphology, and subsequently the internal microstructure of secondary particles of a Ni-rich NCM cathode can be redesigned by special treatment.
Structurally redesigned cathode which has the strong crystallographic texture, and unique particle morphology can relieve the anisotropic internal strain generated
during deep charging of the cathode. By protracting the occurrence of microcracks, the cycle life of Ni-rich NCM cathode is significantly improved compared to
conventional cathode having polygonal-shaped primary particles.
Doping strategies (W, Zr, Al, …)
The enhancement in cycling stability by W doping appears to generally apply to all classes of Ni-rich layered cathodes and greatly alleviates the hurdles faced by the
Ni-enrichment strategy, which increases the energy density of the layered cathodes but is also impaired by instability and capacity fading. Various elements such as
W, Zr, Al, etc. have been studied so far, and each element makes a positive contribution to the cathode material through the effect of its own doping.