Microstructural Insights into Performance Loss of High-Voltage Spinel Cathodes for Lithium-ion Batteries

Spinel-structured LiNixMn2-xO4(LNMO), with low-cost earth-abundant constituents, is a promising high-voltage cathode material for lithium-ion batteries. Even though extensive electrochemical investigations have been conducted on these materials, few studies have explored correlations between their loss in performance and associated changes in microstructure. Here, down to the atomic scale, the structural evolution of these materials is investigated upon the progressive cycling of lithium-ion cells. Transgranular cracking is revealed to be a key feature during cycling; this cracking is initiated at the particle surface and leads to the penetration of electrolytes along the crack path, thereby increasing particle exposure to the electrolyte. The lattice structure on the crack surface shows spatial variances, featuring a top layer of rock-salt, a sublayer of a Mn3O4-like arrangement, and then a mixed-cation region adjacent to the bulk lattice. The transgranular cracking, along with the emergence of local lattice distortion, becomes more evident with extended cycling. Further, phase transformation at primary particle surfaces and void formation through vacancy condensation is found in the cycled samples. All these features collectively contribute to the performance degradation of the battery cells during electrochemical cycling. Key microstructural degrations have been identified for the high-voltage spinel cathode LiNixMn2-xO4 (LNMO) during electrochemical cycling up to 200 cycles, which include transgranular cracking, secondary phase formation, and void formation. These microstructrual changes are extensitvely investigated in multidimensions down to the atomic scale, which provides deeper understanding on the structural change of LNMO upon cycling.image