Layered P2/O3 Intergrowth Cathode: Toward High Capacity and High Power Na-Ion Batteries

One objective in sodium-ion battery (SIB) research is to increase the capacity of the cathode in order to increase the SIB energy densities.  Because of the lower nominal Na content in stoichiometric P2 layered i.e. Na_xMO₂ (x ~ 0.67; M= Mn, Ni, Co), the extractable capacity for the cathode and hence its energy density in a sodium-ion battery (SIB) is generally too low.  In previous work, as an effort to up the reversible capacity, we introduced Li for charge balancing and charge ordering stabilization in order to increase the x Na content and its full removal in Na_aLi_b(Ni_0.25Mn_0.75)O_δ (a+b = 1.2) (1).  However, despite the Li addition to form a single phase P2 layered material, the capacity was still too low because of a limited amount (25%) of redox active divalent Ni (~ 100 mAhg^-1).  Thus, in the present work we increased the redox active Ni(II) content to 0.5 mole stoichiometry (Na/Li = 1.0) in Na_1-xLi_xNi_0.5Mn_0.5O₂ in an attempt to maximize the capacity to a theoretical value of ~ 180 mAhg^-1.  In so doing we caused the unexpected  formation of an intergrowth of P2 and O3 layered phases in this material.  Figure 1 shows the HRTEM of the Na_0.7Li_0.3Ni_0.5Mn_0.5O_2+δsample.  It is noted that the domains have an orientation relationship.  In fact the intergrowth structures are topotactic layers with nanometer thickness. This would suggest that the crystal structure changes of the O3 – P3 layer during electrochemical cycle may be influenced by the adjacent P2 layer. This type of phase stabilization is a well-known phenomenon in artificially grown heterostructures, and as such may stabilize the composite phase to variable Na content, particularly during cycling.

The results of electrochemical high power rate tests in Na half cells have been established.  The rate capability of the x=0.3 sample is superior being 140 mAhg^-1 specific capacity at a rate of 125 mAg^-1.  At the same time, the percentage of P2 is the greatest in the x=0.3 composite thus suggesting that the P2 portion of the composite is responsible for the material’s high-rate.  The improvement is nearly linear with x value.  The x=0 material, which is O3 stacked is the lowest performer;  this cathode material is full of phase changes from O3-P3-P’3-O’3(‘ = monoclinic distortion) during cycling, as manifested by a series of voltage humps, plateaus, and kinks that in turn can adversely affect the cycling behavior particularly at high voltages necessary to extract more Na (2).

In this presentation we will describe and  highlight the synthesis, materials chemistry and its relation to structure-function-property relationships.

References

(1)             D. Kim, et.al., Advanced Energy Materials, 1, 333 (2011)

(2)             S. Komaba et al., Inorg. Chem. 51, 6211 (2012)

Acknowledgments

Funding from the Department of Energy under Contract DE-AC02-06CH11357 is gratefully acknowledged.  The transmission electron microscopy was accomplished at the Electron Microscopy Center at Argonne National Laboratory, a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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