In the ongoing quest to develop high-capacity cathode materials for high-energy batteries, the conventional approach has been to rely on lithium-excess materials to operate the cathode at high voltage. This approach, however, has been dogged by numerous unresolved issues, such as voltage fading and structural failure.
Now, an international team from the US and S. Korea is proposing a different approach to high-capacity cathodes: lithium deficiency as opposed to lithium excess. In a paper published in the journal Advanced Sustainable Systems, they reported on their development of a multiphase structure (MPS) NMC cathode material that exploits the benefit of both spinel and layered structures. The prepared cathode material showed high structural, thermal and electrochemical stability with high cycle life and improved rate performance.
|Concept diagram of multiphase structure cathode materials: Li-deficiency resulted in the formation of an additional spinel phase (top, red) apart from the major layered phase (bottom). Cho et al. Click to enlarge.|
To produce the MPS material, they used lithium deficiency in the solid state lithiation of the spray-dried metal hydroxide precursor. Based on a controlled sintering rate to avoid the oxidation of Ni2+, the rapid—and cost effective—lithiation of the precursor with the lithium deficiency acts as a driving force partially to convert the layered material to a spinel phase—yielding a multiphase structure (MPS) cathode material.
The shorter calcination time in the process can also reduce the overall electrical cost of the cathode material by 37%.
Spinel material in a Li-ion battery electrode generally offers better rate capability than a layered oxide material—but this benefit is offset by a lower capacity. This issue, as well as the spinel material’s capacity degradation at elevated temperature, is typically a drawback to the use of spinel material. The MPS addresses this, however.
We have synthesized a multiphase structure cathode material (MPS) where both the layered structure and the spinel structure is present. The structural stability of the defect-spinel structure (Fd-3m) NMC material is significantly outperforming because of the pillaring structure in lithium layer. As such, spinel NMC can sustain up to a very high voltage of 4.5 V and even higher than 4.5 V. Also, the spinel structure contains 3D channels for the Li-ion to transfer during lithiation/delithiation, which eventually improves the rate capability. Spinel NMC thus offers improved cycling stability as well as high rate capability. Yet, comparing to the layered structure, the spinel structure has less number of active sites for Li-ion since transition metal ions stay and act as pillars in the lithium layer. Hence, the specific capacity of Spinel NMC is lower than that of the layered NMC.
In this new concept, we have synthesized a multiphase structure cathode material to exploit the benefit of both the spinel and layered structure, i.e., to gain high structural stability with improved rate performance while keeping the capacity high enough, we have used similar manufacturing process as of current commercial production of NMC layered cathode materials.—Cho et al.
|Electrochemical Performance of MPS and single-phase layered NMC (SPS). a) formation cycle voltage plot; b) rate capability (up to 8C); c) 1C/1C cycle data at room temperature; and d) at 60 °C. Cho et al. Click to enlarge.|
After cycling, the MPS showed significantly reduced phase transition compared to a commercial single-phase layered NMC (SPS). Capacity retention after 250 cycles at room temperature was 75% for MPS, compared to 72% in SPS. MPS also showed high rate capability up to 8C, compared to SPS. At high temperature (60 °C) cycling of 50 cycles, MPS showed high capacity retention of 93.5% compared to 79.1% with SPS.
The researchers attributed the high performance of MPS to the multiphase structure which is structurally more stable at higher voltages.
A patent has been filed on the work (“Multi-phase structured cathode active material for lithium ion battery” (PCT Patent Filing Number/US2015/051426, 2015). The named inventor, Dr. Sung-Jin Cho, is the lead author of the paper, and is an Assistant Professor at North Carolina A&T State University—the applicant on the patent.
S.-J. Cho, M.-J. Uddin, P. K. Alaboina, S. S. Han, M. I. Nandasiri, Y. S. Choi, E. Hu, K.-W. Nam, A. M. Schwarz, S. K. Nune, J. S. Cho, K. H. Oh, D. Choi (2017) “Exploring Lithium Deficiency in Layered Oxide Cathode for Li-Ion Battery” Adv. Sustainable Syst. 1700026 doi: /10.1002/adsu.201700026