Characterization and Development of High Energy Density & Lifetime Li-Ion Cathode Materials Via Core-Shell Structured Cathodes

Current state of the art cathode materials such as LiCoO₂, Li[Ni_xMn_xCo_(1-2x)]O₂ or Li[Li_(1/3-2x)Ni_(x)Mn_(2/3-x/3)]O₂ ,0≤x≤½, cannot individually meet the needs of next generation cathodes for lithium-ion battery applications.  New electrolyte systems or new cathode chemistries beyond the traditional layered oxides have been extensively researched to meet the demands of next generation materials.  Improving electrolytes with additives has been shown to improve lifetime, but fail to address energy density, while new high potential cathode chemistries exhibit the possibility of increased energy density, but at the expense of the electrolyte and lifetime.   

A complementary approach to electrolyte additives and high potential cathodes is core-shell cathodes using the extensively studied library of layered oxides as candidates for the core and shell.  For high energy density the core of a core-shell material has superior energy density.  Often, high energy density materials exhibit extensive parasitic reactions with the electrolyte, reducing lifetime.  These parasitic reactions might be greatly reduced by encapsulating the core in different cathode compositions that show less parasitic reactions with the electrolyte.  The extent of the parasitic reactions on layered oxides was previously quantified with high precision chargers (HPC) providing precise measurements of coulombic efficiency (CE).

Figure 1 shows the CE and discharge capacity versus cycle number for many common layered oxide cathode materials previously measured with HPC.^1–3  This data was used to screen for candidate materials for the core and shell of core-shell cathodes.

The effect of the shell thickness was examined in this study.  Optimally, the shell coating should be as thin as possible.  If the shell of the particles is too thick then energy density is sacrificed; alternatively, if the shell coverage is thin and incomplete then parasitic reactions between the electrolyte and the core can occur, reducing lifetime.

Cathode materials were developed in a standard two-step process.⁴  The thickness of the shell in the precursor materials was controlled by altering the precipitation time for the core and shell.  The thickness of the shell was determined by modeling the absorption of X-rays by the shell coating observed in the diffraction patterns of core-shell materials.⁵

Figure 2 shows precursors with a Ni_0.5Mn_0.5(OH)₂ core and Ni_0.17Mn_0.83(OH)₂shell developed with a core to shell mole ratio of 1:1 The shell thickness was determined to be 1.77 μm for a 7.9 μm diameter particle.  The precursors were developed into core-shell cathode materials via calcination at various temperatures, times and lithium content.

Discussion will include electrochemical results of coin cells utilizing core-shell cathodes and standard electrolyte formulations tested using HPC. XRD, SEM and EDS results showing the effects of high temperature calcination on the morphology and core and shell composition will also be presented to show the effects of calcination on the core and shell of the cathode materials. 

References

1. A. J. Smith, J. C. Burns, D. Xiong, and J. R. Dahn, J. Electrochem. Soc., 158, A1136 (2011).

2. A. J. Smith, J. C. Burns, S. Trussler, and J. R. Dahn, J. Electrochem. Soc., 157, A196 (2010).

3. Aaron W. Rowe, Eric McCalla, John Camardese, and Jeff R. Dahn, J. Electrochem. Soc.(Submitted).

4. John Camardese, Eric McCalla, Daniel W. Abarbanel, and Jeff R. Dahn, Chem. Mater.(to be Submitted).

5. John Camardese, Eric McCalla, Daniel W. Abarbanel, and Jeff R. Dahn, Chem. Mater. (to be Submitted).